Vocational Training Report
ON Manufacturing of Caustic Soda Flakes
Heavy Chemicals Division, Tamilnadu Petroproducts Ltd, Manali
By: Hemant Gaule (U04CH115) B.Tech-IV, Chemical Engineering
Acknowledgements The The peri period od of my voca vocati tion onal al trai traini ning ng at the the Heav Heavy y Chem Chemic ical alss Divi Divisi sion on of Tamil amilna nadu du Petroproducts Limited was a great learning experience for me. It helped me relate the theoretical aspects of Chemical engineering to the practical working and functioning of various phases of production of caustic soda flakes. This, however, could not have been without the efforts and cooperations of the employees and mentors at TPL. This report is credit to the contributions and generous aids provided by the members of TPL, and hence I am obliged to say a word of thanks to everyone who helped me with their guidance throughout the training program. I am thankful to Dr. R. Krishnan, Technical Head, Heavy chemicals division for his generous help throughout the training. I woul would d like like to than thank k Mr. Mr. R. Jose Joseph ph Premk Premkum umar ar,, Execu Executi tive ve (Tra (Train inin ing) g) for for givi giving ng me the the opportunity to undergo this training program. I would like to thank Mr. M. Yerumalai, Yerumalai, Technical Technical Head, Brine House for his h is help and support. I am thankful to Mr. V. V. Malayan, Operator, Brine House for all his help and an d co-operation. I express my thanks to Mr. Mr. M, Vijayamohan, Chief of safety, safety, HCD, for his generous guidance. I would also like to thank all the employees at HCD, TPL for all their support and co-operation.
HEMANT GAULE
Contents Chapter 1 General Outline Chapter 2 Process Description 2.1 Primary Brine Treatment 2.1.1 Anthracite Filters 2.1.2 Precoat Filters 2.2 Secondary Brine Treatment 2.2.1 Polishing Filters 2.2.2 Depleted Brine Dechlorination 2.3 Electrolyzers 2.4 Caustic Concentration 2.4.1 Caustic Evaporation Plant 2.4.2 Fusion Plant
Chapter 3 Demand, supply and manufacturers in India Chapter 4 Line Sizing Chapter 5 Instrumentation Chapter 6 Pumps and valves Chapter 7 Motors, RPM, make types; Chapter 8 Environmental Issues Summary
1.General Outline The plant is designed to produce close to 100 TDP of caustic soda by Ion Exchange Membrane electrolysis process. This process has replaced the Mercury Cell process as it produced huge amounts of mercury as waste, which was highly toxic to the environment. Chlorine and Hydrogen gases are liberated as side products. The raw materials are raw salt, water and power. The final products are a) 32.5 wt% wt% caustic caustic soda (concentrate (concentrated d to 48% in in existing existing mercury mercury cell) cell) b) b) liqu liquid id chl chlor orin inee c) hyd hydrogen ogen gas gas The plant consists of sections for raw material treatment, electrolysis, product treatment and utility supply. The chemical process, however, may be broadly classified into three sections:
Table 1General Outline
Sect Sectio ion n No. No. I
II III
Sect Sectio ion n name name Brine treatment section a) Primary Primary brine precipitati precipitation on and and chemical chemical prepara preparation tion b) Primar Primary y brin brinee filt filtrat ration ion c) Slud Sludge ge tre treat atme ment nt d) Seconda Secondary ry brine brine puri purific ficati ation on e) Brin Brinee dec dechl hlor orin inat atio ion n Electrolysis (32% caustic, H2, Cl2 as products) Caustic concentration a) Caustic Caustic Evaporation Evaporation Plant (32% caustic caustic to 48% 48% caustic caustic lye) lye) b) Fusion Fusion Plant Plant (48% causti causticc lye to to 98% caustic caustic flakes flakes))
2. Process description 2.1 Primary brine purification The depleted brine after vacuum dechlorination and sodium sulfite treatment is sent to, the saturators. Two saturators are provided; one working and one stand by. However, when one is in service, a small quantity of depleted brine is sent through the other saturators to prevent clogging of dip pipe as well as to remove the impurities by leaching. During the electrolysis, some amount of water from brine will be transported to the cathode compartment along with the sodium ions. This water loss from brine is made up by adding DM water tom the depleted brine before tit enters the saturators. The concentration of crude brine is controlled by mixing some amount of lean brine with saturated brine from the saturators inside a strainer tank. Raw salt contains many insoluble matters, which get settled at the bottom of the saturators. These insolubles are removed periodically from the bottom of saturator. The bottom discharge is collected in a saturator recovery pit. When the insolubles are settled, the clean brine is recovered back to the saturator by the raw salt sludge pump. The crude crude brine brine contai contains ns impuri impuritie tiess like like calcium calcium,, magnesi magnesium, um, sulphat sulphates, es, heavy heavy metals metals etc. etc. the calciu calcium, m, magnesium and sulphates are eliminated by treating the crude brine with the sodium carbonate, sodium hydroxide hydroxide and barium carbonate respectivel respectively y. The reactions reactions are carried out in two reactors. reactors. The sodium carbonate and barium carbonate are added to the first reactor and sodium hydroxide in the second rector. To speed up the reaction and to prevent settling of precipitators, both reactors are provided with agitation. The first reactor is agitated with air and the second one with mechanical agitator (since any air bubble trapped in the brine affects the settling efficiency of clarifier, in the second reactor air agitation is not used.). Following reaction occur in the reactors: Na2SO4 CaSO4 MgCl2 Ca+ +
+ + + +
BaCo3 BaCO3 2NaOH Na2CO3
BaSO4
+ Na2CO3 BaSO4 + CaCO3 Mg(OH)2 + 2NaCl CaCO3 + 2 Na+
The flow rate of 10 % Na2CO3 and 20 % NaOH supplied to the reactor reactor are controlled to get an excess sodium hydroxide content of 150 – 200 mgpl and excess carbonate content of 250 – 300 mgpl in the brine at the outlet of second reactor reactor. Barium carbonate carbonate slurry concentration concentration is kept at around 16 wt%. The brine recovered recovered from various operations are collected in a pit and supplied to the first reactor at constant flow rate. For the stable operation of the clarifier, it is advisable not to allow major variations in the flow rate o f recovered brine.
Crude brine overflowing from the second reactor enters a flocculants mixing tank where 0.05 wt% flocculants solution so that the concentration of flocculants in brine is maintained at around 5 ppm. (This quantity may further be reduced as per the actual performance of the clarifier). The brine then overflows into the clarifier where the residence time is long. In the clarifier, all the reactions are completed and the precipitates get settled at the bottom. The precipitates are periodically removed from the bottom and sent to the vacuum vac uum drum filter using clarifier sludge pump. The clarifier is a water seal type, where a layer of water is always maintained on the surface of brine to prevent heat loss from brine due to radiation and to prevent convection current (thereby settling is improved). If the
water water layer layer decrea decreases ses,, the sealin sealing g water water temper temperatu ature re increas increases. es. Then the clarif clarifier ier is to be suppli supplied ed with with additional sealing water supply at the top. DM water is used as the sealing water. The clarifier brine overflows into the CB tank. The suspended solid of this brine is about 20 ppm or o r less. SLUDGE FILTRATION AND BRINE RECOVERY 1. Clarifier underflows, precipitates underflow and the backwash from polishing filter are collected in the sludge pit. 2. The polishing filter backwash, clarifier underflow are filtered using vacuum filter and the filtrate is collected in recovered brine pits. 3. The backwash brine from Anthracite filters, various tank overflows and the filtrate from vacuum drum filter are collected in the recovered brine pits. 4. From the pit recovered brine is continuously fed to the brine circuit at a constant flow rate using recovered brine pumps.
DM Water
10% Na2CO3
FIC
Depleted Brine
16% BaCO
20% NaOH
Floxin
pH
Strainer
Mixing Reactor I
Saturator
Mixing Reactor II
Mixing Tank
Clarifier LIC
FIC Filtered Brine Tank Filtered Brine Pump
Polishing Filter
Body Feed Tank
Na2SO3 Prep Tank
Clarified Brine Tank
Backwash pump Clarified Brine Pump LIC
FIC
AIC To Secondary Brine System Polished Brine Tank Polished Brine Pump
Filter Backwash
Fig 1 Primary Brine Treatment
Table 2(a) Parameters PRIMARY BRINE PURIFICATION NO.
ITEM
STANDARD
1
Saturator brine concentration Agitation air flow rate to reactor I Flocculant solution flow rate Crude brine pH Water seal layer temperature of clarifier (V.112)
305 gpl
2 3 4 5
UPPER LIMIT 315 gpl
LOWER LIMIT 270 gpl
500 NM3/HR 1-2 mg/l
5 mg/l
1 mg/l
9 .5 Ambient
1 0 .5 Ambient + 15 Deg.C
9 -
CHEMICAL CHEMICA L PREPARA PREPARATION TION 6 7 8
Agitation air flow to V.114 A/B Agitation air flow to V.116 A/B Agitation air flow to V.117 A/B
50 NM3/hr
100 NM3/hr
-
50 NM3/hr
100 NM3/hr
-
50 NM3/hr
100 NM3/hr
-
PRIMARY BRINE FILTRA FILTRATION TION 9 10 11
12
13
14
Clarified brine tank 70% level Filtered brine tank 70% level Clarified brine 74 M3/hr flow rate to anthracite filters Brine flow rate to 37 M3/hr individual anthracite filters Differential 0.5 ksc pressure across the anthracite filters Turbidity of Less than 10 filtered brine ppm
8 0%
40%
8 0%
40%
80 M3/hr
-
45 M3/hr
-
1.5 ksc
-
10 ppm
-
2.1.1 ANTHRACITE FILTERS The clarified brine from is fed to the primary brine filters to reduce suspended solids content in brine to les than 10 ppm. Out of three filters, two will be working in parallel and one will remain as stand by b y. Anthracite is used as filtering medium. The filtered brine is collected in a filtered brine tank. During the operation, when one filter becomes saturated, the pressure drop across it increases. The differential pressure across the anthracite filters is monitored by individual differential pressure indicators. when the pressure drop across the filter increases then the stand by filter is taken in line and the saturated filter undergoes backwashing. Filtered brine sent from using the backwash pump is used for backwashing. Backwashing carried out in 3 steps: 1) Surf Surfac acee wash washin ing g 2) Back Backwa wash shiing 3) Resett Resetting ing (prepar (preparati ation on ) Step -1: SURFACE WASHING: This is provided to loosen the sludge cake at the surface of filter medium. Step -2: BACKWASHING: During backwashing, the filtered brine is introduced from the bottom of the filter and all the precipitators trapped in the filter medium are removed and sent to the recovered brine pit. Step -3: RESETTING: (Preparation) After the backwashing step, the clarity of filter outlet is established b y passing the filtered brine from top to bottom of filter (resetting) for a small duration. All the steps are carried out either automatically from the DCS or manually from local control station. The selection of the operation shall be through a local/auto selection from DCS. This local/auto selection shall be done b y the engineers on the DCS panel. During each stage of regeneration steps, step break provision is provided to stop the operation. Unless the step break step is activated, the operation goes to the next step automatically, by pass provision also provided for each step of operation. In case of malfunctioning of any value, mishap alarm app ears, indicating valve malfunctioning.
Table 2 (b) Valve Valve Opening O pening Time STEP
VALVE OPENING
1. SURFACE WASHING XC 123 XC 125 2. BACKWASHING XC 121 XC 125 3. RESETTING XC 126 XC 124 4. NORMAL RUNNING XC 127 XC 122
TIME
FLOW RATE
5 MIN
40 M3/HR
15 MIN
80 M3/HR
5 MIN
40 M3/HR
--37 m3/hr (MAX 40 m3/hr /filter)
VALUE DETAILS 1) XC 121 2) XC 122 3) XC 123 4) XC 124 5) XC 125 6) XC 126 7) XC 127
Backwash inlet (6”) Running outlet (4”) Surface wash inlet (4”) Preparation outlet (4”) Backwash outlet (6”) Preapration inlet (4”) Running inlet (4”)
Table 2 (c) Quantity of Anhtracite in each filter and size distribution Size (mm) Top
Bottom
0.6 - 1.2 3.0 – 4.0 5.0 – 10.0 11.0 – 15.0 16.0 – 30.0 4.0 – 50.0
Height (cm) 600 75 100 100 130 630
Cu . m 4 .6 0.55 0.73 0.73 0.94 2.70
2.1.2 PRECOAT FILTERS The filtered brine from is fed to the polishing filters to reduce suspended solids to less than 0.5 ppm. This is a candle type precoat filter. Two such filters are provided – one operating and while the remaining one stays in stand by mode after backwashing. backwa shing. The polished polished brine after after adjusting adjusting the pH to 9 after adding 32 wt% HCl HCl is sent to the polished brine storage tank. DM water is added to the 32 % HCl in small quantities to prevent crystallization of brine at the point of e ntry of HCl. The pH is adjusted to 9 to facilitate converting all the suspended impurities to their ionic forms (chelate resins can absorb only the ionic impu rities). Each operati operating ng filter filter is a cylindr cylindrica icall vessel vessel MS rubber rubber lined. lined. Inside Inside this this vessel vessel,, a perforated plate having 73 holes is provided in the upper portion of the vessel. Carbon filter candles are suspended from the holes of the perforated plate. Each candle has 3 carbo elements. Total no. of elements present is 219 in each filter. The filter elements used in this unit are porous cylinders SCHUMACHER “CARBO 40”. This material is composed of pure carbon with an average pore size of 110 micron. The filter aid used is - Alpha cellulose (BWW 40) FILTERATION Filtration cycle
: Pressure drop : 2 Kg/cm2 (max.) Operating time: 48 Hrs
Whatever expires first i.e., pressure drop exceeds 2 kg/cm2 or operating time 48 hrs filter is isolated. Filtration area Design temp. Design pressure Capacity
41 m2 80 Deg.C 5 kg/cm2 80 m3/hr (normal)
SPECIFICATION OF INLET BRINE Fluid Flow rate
: :
Saturated NaCl Brine 80 m3/hr (normal)
COMPOSITION OF BRINE NaCl - NaClO3 - Na2SO4 - Na2CO3 -NaOH -CaCO3 and Mg(OH)2 -Cl2 (free chlorine) --
300 – 310 g/l 20 g/l (max.) 5 g/l (max.) 300 mg/l (max.) 300 mg/l (max.) 5 mg/l (max.) 0
BaSO4 has to be prevented as it can lead to the clogging of the carbo-cylinders. Density pH – value Temperature Suspended solids
-
1.190 kg/m3 9.5 – 11 (max.) 70 Deg.C normal 5.0 mg/l (max.) 10.0 mg/l
QUALTIY OF POLISHED BRINE Suspended solids: Less than 1 mg/l suspended solids in the treated brine.
SUMMARY OF OPERA OPER ATION The operation can be divided into 4 steps. I II III IV
Preparation of precoat/body feed Precoat Filtration Washing
I PREPARATION OF PRECOA PREC OAT T The precoat tank (V.142) is filled with polished brine up to 80%. The required alpha cellulose (41 kgs) is added. Start the agitator a gitator (A.142) and run it for 10 min. MIXING BY CIRCULATION The mixing aims to make a uniform alpha cellulose concentration in the precoat tank and filter vessel. The precoat suspension from the precoat tank (V.142) is sent to the filter vessel using precoat pump (P.142), through three air vent valves (V.9, V.10, V.11) at the upstream side of the filter. The brine liquid is circulated back to the precoat tank. Precoat pump (P.142) (P.142) discharge flow rate is kept kep t 103 m3/hr. m3/hr. II PRECOATING In precoating the precoat layer is formed on the surface of the filter elements. In order to do this, the precoat outlet valve (V.15) is opened (after completion of precoat mixing by circulation) and then the upstream air vent valves V.9, V.10, V.10, V.11 V.11 are closed. Precoat tank level is monitored. The precoat suspension is pushed from the outside to the inside through the filter elements and is returned to the precoat tank. The filter aid (alpha cellulose) deposits on the surface of the filter elements. Precoat layer formed. The precoat layer contributes to achieve a fine filtration quality by preventing filter elements from blinding. A FLOW STOP OR A SUDDEN FLOW CHANGE AFTER THIS STEP CAN CAUSE BLINDING OF THE FILTER FILTER ELEMENTS Flow rate changed from precoat (103 m3/hr) to filtration (80 m3/hr) in acceptable time. III FILTRATION a) PREFILTRA PREFILTRATION: TION: The filtrate recycle valve is opened (V (V.16). the polished brine which is coming out of the filter is pumped into filtered brine inlet valve V.1 and V.2 (P.121 A/B discharge) opened and V.3 – precoat inlet valve is closed.
The The valv valves es V.1, .1, V.2 shou should ld be open opened ed and and V.3 shou should ld be clos closed ed slow slowly ly and and simultaneously. simultaneously. The prefiltration starts when the above mentioned steps are carried out. The prefiltration i.e., the circulation of brine between filter and filtered brine tank, is intended to secure full and fine filtration efficiency. efficiency. Flow rate of brine is changed by b y control valve (V.5). (V.5). Setting of flow: 8 0 m3/hr. Once the above steps are over, polished brine is sent to polished brine tank (V.143). (V.143). When the filtered brine has passed through the precoat layer formed on the surface of the filter elements from the upstream side to downstream side, the fine impurities are trapped and removed. The control of filtration process made by - filt iltrat ration ion tim timee (48 (48 hrs) hrs) - The The diff differ erent entia iall pres pressu sure re inlet inlet and outlet outlet of filte filterr vess vessel el (2 kg/cm2 kg/cm2)) whichever is earlier. earlier. Once a filter filter is isolated, isolated, the next filter filter should be lined up. Accordingly Accordingly the stand by filter should be precoated and kept ready for lining up. b) DOSING DOSING BODY BODY FEED FEED SUSPEN SUSPENSIO SION N When process has entered precoating, start body feed. BODY FEED METHOD Body feed method is one for mixing filtered brine feed with alpha cellulose by body feed pump. The body feed suspension is filtered together with filtered brine. This method gives advantages to prevent filter elements from blinding and to extend the filteration time. IV WASHING Normally the washing process is carried out four times. The washing steps to be made each time are as follows: The upstream air vent valves V.9, V.10, V.11 are mounted below the perforated plate at 120 Deg. gap around the vessel. During the over flow through sight glass, open the upstream air vent valves V.9, V.10, V.11 one by one in turn, in order to wash evenly the surface under the perforated plate.
Table 3 VALVE DETAILS V1
Filtered brine inlet isolation (Manual)
V2
Filtered brine inlet actuated type (ON/OFF)
V3
Precoat inlet
V4
Mud discharge (Backwash outlet)
V5
Inlet flow control valve
V6
Downstream air vent valve
V7
Plant air inlet isolation valve
V8
NRV Plant air
V9
Upstream air vent valve
V 10
Upstream air vent valve
V 11
Upstream air vent valve
V 12 (ON/OFF)
Filter
outlet
to
V.143
polished
brine
V 13
Outlet isolation valve (manual)
V 14
Backwash inlet (or) washing brine inlet
V 15
Precoat outlet
V 16
Recycle to V.121
V 17
Precoat outlet, air vent overflow to sludge
V 18
Filtered brine to Body feed tank filling
V 19
Precoat recycle to V.142 isolation
V 20
Precoat Precoa t recycle to V.142 (ON/OFF)
V 21
Backwash pump discharge to V.142 filling
tank
Table 4 location and operating range of filters and accessories No.
Item
Location
Standard
Upper limit
Lower limit
PARAMETERS
SECONDARY SECONDA RY BRINE FILTRA FILTRATION TION
1.
2.
3.
Brine flow FIC-140 A/B rate to polishing Filter Differential PDI-140 A/B pressure across polishing filter Polished brine LIA-143 tank level
4.
Polished brine pH
5.
SS in polished brine
6.
Body feed flow rate
AICA-141
74 m3/hr
80 m3/hr
Less tha than n 2 2 kg/cm2 kg/cm2
70%
9
80%
40
10
8
Less than 0.5 ppm ppm P-141 A/B
SECONDARY BRINE PURIFICATION
Less Than 0.5 ppm
7.
Polished brine temperature
TICA-151
60 Deg.C
65 Deg.C
8.
Polished brine flow rate to Chelate resin tower Super purified brine pH
FIA-151
74 m3/hr
80 m3/hr
AIC-151
4
5
9.
10.
Super purified brine hardness (Ca & Mg)
AI-152
Less than 20 ppb as Ca
20 ppb as Ca
11.
Super purified brine hardness (Sr)
Sample Analysis
Less than 100 ppb as Sr
100 ppb as Sr
55
2.2 Secondary Brine Purification Unit The secondary brine unit consists of two chelate resin towers. The tower is a cylindrical MS rubber lined vessel having a perforated plate in the lower portion of the vessel. Around 264 nos. of strainers are fitted in the perforated plate. The strainers are made up of PPO (Poly phenylene oxide). Four cubic meter of DIOION-r-II is kept filled over the strainers. The height of the resin will be around 1.43 mts from the perforated plate. Brine inlet distribution consists of four poly propylene pipes fitted at the top dish end. HCL injection distribution pipe made up of poly propylene wrapped by poly propylene cloth is fitted just above the resin surface. There are 3 sight glasses fitted in the resin tower vessel. This will indicate the resin level during the various stages of regeneration. The resin level during HCL injection will be in the lower most level due to shrinkage of resin. After the second backwash step, the resin level will be in the top sight glass level, due to expansion expan sion of the resin particles. The polished brine after adjusting the temperature to 60 Deg.C is sent to the CHELATE resin columns. Temperature of polished brine to Chelate resin columns is controlled by regulating LP steam flow rate to polished brine heater (E.151). in the chelate resin columns, the hardness of brine is reduced to less than 20 ppb by absorption with chelate resin. Two columns are provided in series and are operated in a merry go round mode. The two ion exchange vessels are normally operated in series while in service. The brine solution passes from one ion exchange vessel (primary) (primary) to the second second ion exchange vessel (secondary). When the ion exchange resin in the primary vessel is exhausted the primary vessel is passed and total brine flow diverted diverted to the secondary vessel. vessel. The primary primary vessel is then regenerated. regenerated. After regeneration, the brine flow is again diverted to pass through the regenerated vessel, which is taken back in service as secondary ion exchanger. When the hardness at the outlet of the first resin column in the series reaches more than 100 ppb or 24 hrs service (whichever is earlier) it is isolated for regeneration. REGENERATION REGENERATION INVLOVES FOLLOWING MAJOR STEPS: 1 BACK BACKW WASH: ASH: DM DM wate waterr is int intro rodu duce ced d from from the the bot botto tom m of the the che chela late te res resin in col colum umn n to make the resin loose and remove any suspended solids or broken resin to outside the column. 2 ELI ELIMINA MINATI TIO ON BY HCL HCL:: HCl HCl dilu dilutted to to abou aboutt 4 wt% wt% is fed fed to to the the colu column mn at at its its top top exposure of the entire resin to the acid causes exchange of all the hardness and heavy heav y metals with H+ ions. After the elimination any excess of HCl in the column is washed o ut with DM water. 3 REPL REPLAC ACEM EMEN ENT T OF OF H+ H+ WIT WITH H Na+ Na+ BY BY NaO NaOH: H: Sod Sodiu ium m hydr hydrox oxid idee dilu dilute ted d to abou aboutt 5 wt% is introduced from the bottom of the resin column. Exposure of the entire resin to NaOH
causes exchange of H+ ion with Na+ ions. After the replacement, any excess NaOH is washed out with DM water. 4 BRI BRINE REP REPLAC LACEMEN EMENT T: Befor Beforee the chel chelat atee resi esin colu colum mn (T.151 .151 A/B A/B)) is put int into operation the fluid in the column is replaced with secondary brine. The regenerated resin is put into service as secondary column in the series. All the secondary brine brine purifi purificat cation ion plant plant operat operation ionss are carrie carried d out automa automatic ticall ally y by a PLC. PLC. However However,, it is interfaced with a DCS for indications. EFFLUENT: The effluent generated during the backwashing which is acidic in nature are collected in a waste water pit (V.152) (V.152) neutralized with NaOH and pumped to the main effluent effluent treatment plant using waste water pumps (P.152 A/B0. the total quantity of effluents generated per regeneration is approx. 65 cu.m.
Table 5 Parameters of Secondary Brine (Resin ( Resin Tower Tower Outlet) N aC l Ca & Mg Sr SiO2 pH Temperature Suspended solids Ba NaClO3 Na2SO4 I Al Fe Mn Ni Total organic content (TOC)
300-310 gpl Less than 20 gpl 0.06 ppm 4ppm 9 .0 Around 60 deg. C 1 ppm 0.4 5 gpl 20 gpl 5 gpl 0.2 ppm 0.1 ppm 0.2 ppm 0.2 ppm 0.01 ppm 10 ppm
AIC LIC
Polished Brine Heater
TIC
SuperPurified Brine Head
Secondary Brine Tank
Chelate Resin Tower
Polished Brine Pump
Feed Brine Pump
To Cl2 First Cooler AIC PIC To Chlorine Section
HC
TIC Brine Dechlorination Tower Vacuum Pump Drain Separator Recovered Cl 2
Electrolyzer By Pass
Depleted Brine Tank
LIC Depleted Brine Pump
Dechlorinated Brine Tank
10% Na2CO3
Saturator LIC
Dechlorinised Brine Pump AIC
Fig 2 Secondary Brine Treatment Treatment & Dechlorination
pH
20% NaOH
2.2.2 Depleted Brine Dechlorination Section To carry out effective dechlorination, chlorinated brine of which the pH is 4-5 under normal operating conditions is acidified to a pH of about 2 by adding 32 wt% HCl in the line before entering the depleted tank. The acid addition under automatic pH control drives the hydrolysis reaction to the left. Cl2
+
H2O
----------
HOCl +HCl
Most of the free chlorine exceeding the equilibrium solubility under acidic brine conditions escapes to the main chlorine gas header. The brine is then sent to the dechlorination tower. In the dechlorination tower, operated at about 230 torr vacuum by a dechlorination vacuum pump, free chlorine is stripped from the chlorinated brine by lowering the chlorine gas equilibrium partial pressure. At the tower outlet, the active chlorine content in the brine will be as low as 20 ppm. This dechlorinated brine after the tower outlet is called “lean brine”. The dechlorinated brine is collected in a dechlorinated brine tank. The condensate collected from the hydrogen gas cooler is also added in this tank. The lean brine is neutralized with 209 wt% NaOH to a pH of 8. The 10wt% Na2SO3 solution is added to the lean brine to eliminate the active chlorine content completely according to the following reaction. NaClO +
Na2SO3
----------
N aC l +
Na2SO4
The élan brine with no active chlorine is then sent to the saturators for further saturation with the raw salt. A part of the lean brine is purged out to maintain the chlorate level (NaClO 3) less than 27 gpl. The 10 wt% Na2SO3 solution is prepared by dissolving Na2SO3 powder in DM water. Two tanks are provided for preparation and supply of Na2SO3 solution. The solution is supplied to the lean brine using metering pumps. The HCl required for acidification of brine is collected in the HCl day tank and supplied to various users by HCl feed pump. To ensure the availability of HCl to all users, one pressure control system is provided in the HCl supply ring. The free chlorine removed from the depleted brine at the dechlorination tower is sucked by a vacuum pump through recovered chlorine coolers. In recovered chlorine coolers, the chlorine gas is cooled to about 50 deg C to reduce its volume. The condensate is returned to depleted brine tank. The vacuum system consists of two liquid ring vacuum pumps (one working and the other standby) one gas separator and one cooler. The cooled chlorine gas from the recovered chlorine gas cooler is sucked by vacuum pump under a controlled pressure of about 230 torr and then sent at a slightly negative pressure to wet chlorine gas main header for further treatment. NOTE: If alkaline brine comes in contact with ceramic packing inside the vacuum tower, it will pick up silica from it which is not desirable for membrane cell operation.
Na2SO3 Addition. It is recommended to add 10 wt% Na2SO3 solution in the line two times the quantity required for treating the chlorine in dechlorinated brine. Flow rate of 10 wt% Na2SO3 solution is calculated as follows.
Vss
=
0.0346
X
C NaClO
X
Vdb
Vss : actual flow rate of 10 wt % Na2SO3 solution (l/h) C NaClO : NaClO concentration in dechlorinated brine after vacuum dechlorination (ppm) Vdb : dechlorinated brine flow rate Depleted brine composition
N aC l Na2SO4 NaClO3 Temperature Flow rate
------
200-220 gpl less than 10 gpl less than 27 gpl 80-90 deg C 58 m3/hr
Chlorate removal system Chlorate is an undesirable by-product in the electrolyzers; the production rate is subjected to the current efficiency level of the electrolyzers in the operation. A build up of NaClO3 level in the brine loop towers the NaCl saturation limit because the solubility of NaClO 3 is higher than that of NaCl and can increase the NaClO3 content in the product caustic. Therefore the NaClO3 level in the feed brine is generally ge nerally maintained at 10 ± 5 g/l. in order to maintain this level, periodically brine will be purged from dechlorinated brine pump discharge to sea disposal.
2.3 Electrolyzer BASIC PRINCIPLE NaOH is produced by b y the following principle. Electrochemi Electrochemical cal reaction reaction in an Ion-ExchangeIon-Exchange-Membr Membrane ane Electroly Electrolyzer zer by utilization utilization of super purified brine. Sodium chloride is dissolved in the brine solution in the anode compartment according to the following equation. NaCl
Na+
+
Cl-
The principal anodic reaction involves oxidation of the anion Cl- to produce gaseous chlorine 2Cl-
Cl2
+
2e-
The cation Na+ in the anode compartment is transferred with water to the cathode compartment throug through h the ion-exc ion-exchan hange ge – membra membrane. ne. Water ater is electr electroly olyzed zed in the cathode cathode compar compartme tment nt according to the following equation. 2H2O +
2e-
2H+
+
2OH-
H2
+
2OH-
The primary cathodic reaction is the reduction of the cation H+ to produce gaseous H2 and to regenerate hydroxyl ions. The sodium cation Na+ combines with OH- to form NaOH Na+
+
OH-
NaOH
Overall electrochemical reaction 2NaCl +
2H2O
2NaOH
+
Cl2 +
H2
Demineralised water is fed in recycle NaOH line in order to regulate the NaOH strength in cathode compartment. Depleted brine is discharged with chlorine from the anode compartment. Caustic soda produced in the cathode compartment is discharged with hydrogen from cathode compartment. Recycled caustic diluted with DM water is fed into the cathode compartment.
REACTIONS AND SIDE REACTIONS Reaction in the cathode chamber The main reaction on the cathode surface is H2O reduction.
H2O
+
e-
½ H2 (g)
+
OH-
--- (1)
1/F mol of OH- is generated in the cathode side and (1/EK) mol of OH- is back migrated to the anode side through the ion exchange membrane where EK is a current efficiency in the cathode side. Reaction in the anode chamber
The main reaction on the anode surface is Cl- oxidation i.e. Cl-
½ Cl2 (g)
+
e-
--- (2)
The electrochemical side reaction is O2 (g) generation by H2O oxidation i.e. H2O
½ O2 (g)
+
2H+
+2e-
--- (30
The first first chemical chemical side reaction in chlorine chlorine dissolutio dissolution n without without dissociati dissociation on according according to the equation 4 Cl2 (g)
Cl2 (aq)
--- (4)
The second chemical side reaction is free chlorine dissolution in water according to equation 5 Cl2 (aq)
+
H2O
HOCl (aq)
+
H+
+
Cl-
--- (5)
The third chemical side reaction is dissociation with the hypochlorus acid, HOCl according to the equation 6 HOCl (aq)
+
OCl-
OCl-
+
H+
--- (6)
The fourth chemical side reaction is CLO3- generation according to the equation 7 2HOCl (aq)
+
OCl-
ClO3
+
2H+
+
2Cl-
--- (7)
The fifth chemical side reaction is neutralization between H+ ions generated by the side reactions (eq. 3, 5, 6 & 7) and OH- ions back migrated from the cathode side i.e. H+
+
OH-
H2O
--- (8)
The sixth chemical side reaction is neutralization between H+ ions and Na2CO3 and NaHCO3 in the feed brine according to the following reactions.
Na2CO3 NaHCO3
+ +
2HCl H Cl
2NaCl + N aC l +
H2O H2O
+ +
CO2 CO2
SPECIFICATION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Type: DCM 405 x 2 No. of unit cells: 5 x 2 Anode effective area : 15.15 M2 x 2 Anode : DSA Cathode : Activated cathode Rated current : 54.24 KA Curr urrent ent densi nsity : 3.29 .29 KA/m2 /m2 (rate ated : 3.59) Cell voltage : 3.18 V Dimensions : 2560 W x 1360 H x 1058 L Weight (empty) -- 4. 4.9 mt Weight (operating) -- 7.4 mt Cathode cur current ef efficiency : 95 95.5 % Powe Powerr cons consum umpt ptio ion n at at rec recti tifi fier er term termin inal al : 225 2250 0 (DC KWH/metric ton of NaOH) Caustic conc. : 32.5 % Cell operation temp: 90 °C (Max.) Life of anode coat oating: 8 yrs (m (min) Life of of cat cath hode coati ating : 8 yr yrs (m (min) Memb Membrrane ane and and gas gasket ket life ife ; 4 yrs (min) min)
STRUCTURE The DCM 405 x 2 consists of DAM – 405 monopolar electrolyzers, i.e., two DCM – 405 are combined with one set of tie rods structurally and connected electrically in series with one set of under cell bus bars. An insulation element is positioned between DCM – 405 electrolyzers. Each DCM – 405 is electrochemically divided into 5 unit cells and structurally consists of 2 anode elements, 1 half anode element, 2 cathode elements, one half cathode elements and between the elements membranes are positioned. CIRCUIT OF ELECTRIC CURRENT The electric current flow for each electrolyzer is as under. The Electric current from the positive side of the rectifier flows through 8 under cell bus bars and enters into each anode element of the first set of DCM 405 x 2 twin electrolyzer. electrolyzer. Then it flows to the cathode element through through ion exchange membrane membrane and to the under cell bus bars.
From the under cell bus bars, the current, flows to the anode element, of the second half of DCM 405 x 2. There it follows similar current path as in the case of first half and goes to the next electrolyzer through inter cell bus bars. Finally from the cathode side of the last electrolyzer, current flows back to the negative terminal of rectifier.
PROCESS DESCRIPTION DESCRIPTION 1 Electrolyzer ci circuit The electrolyzer circuit has 44 no.s of DCM 405 x 2 electrolyzers and a rectifier unit. The circuit is arranged in two rows. Both rows are connected to the rectifier unit with power supply bars and one row is connected to the other with cross over bus bars at the opposite side of the rectifier unit. Each electrolyzer in a row is connected with each other through inter cell bus bars. 2 Fluid stream (Super purified brine) Super purified brine from the brine treatment section is fed into the feed brine manifold of each DCM 405 X 2 electrolyzer through a header pipe and each branch pipe. Brine is distributed to each anode anode chamber chamber.. A restri restricti ction on orific orificee is provid provided ed at the inlet inlet of each anode chamber chamber respectively. respectively. Brine flow rate to each DCM 405 X 2 electrolyzer is adjusted by b y a brine flow gauge mounted on each branch pipe. 3 Anode chamber Super purified brine fed into anode chamber is decomposed electrically into Cl- and Na+ by direct current in the anode chamber. The Cl- ions are oxidized at the anode surface and from chlorine gas. Depleted brine and wet chlorine gas at 80 – 90 ° C form a two phase system s ystem and overflow out of the anode chamber ch amber.. The two phase flows from each anode chamber and are collected into the manifold of each electrolyzer and flows by gravity into depleted brine tank (V.201) from the depleted brine tank chlorine gas goes to chlorine gas treatment section and depleted brine pumped to brine treatment section. Reaction: Sodium chloride is dissociated in brine solution in the anode compartment according to the following equation: N aC l
Na+
+
Cl-
Principle of anodic reaction involves oxidation of the anion Cl2Cl-
Cl2
+
2e-
The sodium ions in the anode chamber pass through the membrane and enter into the cathode chamber together with water. water. Cathode chamber In the cathode chamber water is decomposed electrolytically into into H+ and OH- by direct current. The H+ ions are reduced on the cathode surface and form Hydrogen gas. The OH- ions combine with Na+ ions and form caustic soda. The two phase stream of NaOH + H2 at 80-90 Deg.C overflows out of each cathode chamber and are collected into the manifold; they flow by gravity to NaOH circulation tank (V.202). (V.202). From the NaOH circulation tank, Hydrogen gas and NaOH separated. Hydrogen gas goes to the Hydrogen circulation pump (P.202 (P.202 A/B). This caustic soda is separated into two streams. a) 32.5 wt% wt% caustic caustic head head tank tank for conc. In In Mercury Mercury cell cell up to 48%. 48%. b) Recycle Recycle NaOH NaOH to the the electro electrolyz lyzer er.. Recycled caustic is fed to the recycle caustic manifold of each DCM 405 X 2 electrolyzer through a header pipe and each branch pipe and distributed to each cathode chamber. Caustic flow gauge mounted on each branch pipe is used for adjusting the caustic flow rate to each electrolyzer. The caustic strength is generally kept about 32.5 wt% at the outlet of the electrolyzer by controlling feed quantity of DM water which is added into the recycle caustic stream.
CAUSTIC SODA HEAT EXCHANGER A caustic soda heat exchanger is installed in the recycled caustic steam to control electrolysis temperature by cooling down or by heating up. This heat exchanger functions as a heater to warm up the electrolyte electrolyte in the electrolyze electrolyzerr during during the plant startup. startup. As the membranes membranes become colder and the electrolyzers become less energy efficient due to increase in cell voltage, this heat exchanger functions as a cooler. The gas pressure control in the anode and the cathode chambers are very important for stable electrolyzer operation. The hydrogen gas pressure is controlled to avoid membrane rubbing with the rough cathode surface leading to membrane damage. A differential pressure indicator is provided in the main header to monitor the differential pressure between both chambers. In case of large surge in gas pressure, the interlock system will work to protect the electrolyzers. Moreover a hydrogen gas vent stack is installed top protect the circuit against extremely high press pressure uress of the hydrog hydrogen en gas and two chlori chlorine ne gas seal seal pots pots against against extremely extremely high/low high/low pressures of chlorine gas respectively. The hydrogen vent stack is also used to vent hydrogen gas to atmosphere during startup.
Operating conditions for electrolyzer 1. Curren Currentt incr increas easee and and decrea decrease. se. Sudden current load change will abruptly affect the bubbling ratio in electrolyte and could cause the overflow stream to sprout or stop. Also membrane vibration due to gas pressure fluctuation and the membrane exposure to pressure zone will shorten the membrane life. Therefore the current load should be increased or decreased only gradually at the rate of 1 KA/min. 2. Feed Feed bri brine ne flo flow w rate rate a) Curren Currentt load increase increase.. Before Before increa increasin sing g the curren currentt load, the feed brine brine flow rate is increased to the value corresponding to the projected current load. b) Curren Currentt load load decreas decrease. e. The current current load load is decrease decreased d before before decreasi decreasing ng the feed feed brine brine flow rate to the projected current load. In either case of increase/decrease in current load, the NaCl strength in depleted brine is analyzed and checked if it is more than 180 gpl in order to prevent membrane damage. 3. DM Water ater supp supply ly rate rate.. The DM water supply rate to the recycled caustic soda will be controlled to maintain the caustic soda strength in the catholytic constant. a) Current Current feed increase. increase. Before Before increasing increasing the current, current, the the DM water flow flow rate to the valve valve corresponding to the projected current load is increased. b) Curren Currentt load load decreas decrease. e. The curren currentt load load is decrease decreased d before before decreasi decreasing ng the DM water flow rate corresponding to the projected current load.
2.4.1 Caustic Evaporation Plant The purpose of the plant is to concentrate the 32% caustic coming out of the electrolyzers to 48% caustic lye. This concentration is achieved by three multi effect evaporators. These evaporators concentrate the 32% caustic to 36%, 41% and eventually 48% caustic lye. Multi effect evaporators are usually fed by 3 possible feed systems: 1. Forwar Forward d feed system: system: where where feed and the heating heating medium medium both both enter the first first evaporator, in other words, co-currently. co-currently. 2. Back Backwa ward rd feed feed syst system em:: In this this syst system em the the feed feed enter enterss count counter er-c -cur urre rent ntly ly with with respect to the heating medium. 3. Mixed feed feed system: system: In this feed feed system system the feed feed enters from from the first first evaporator evaporator,, and and the the fres fresh h heat heatin ing g medi medium um may be intr introdu oduce ced d anywh anywher eree betw between een the the evaporators. The caustic caustic evapor evaporati ation on plant plant at TPL employs employs the backwar backward d feed feed system system.. The evapo evaporat rator orss are are shel shelll and and tube tube type. type. The caus causti ticc flow flowss in the the tube tube side side and and stea steam m circulates on the shell side. Glass wool insulated steel lines are used for circulation of steam. The caustic is carried in stainless steel lines up to a concentration of 36%, after which glass wool insulated Titanium lines are used for it. Steam is used as the heating medium. 32% caustic from the electrolyzers is preheated to 90° C by the 48% caustic coming out of the CEP plant. Steam coming from 2nd evaporator is fed into the 1st one, and caustic is concentrated to 36%. It is sent to a vapor separator, which is connected to a steam ejector, which separates the vapors from the caustic which are cooled and sent for recovery. The vapors separated from the 3rd evaporator are used to pre-heat the 36% caustic before it enters the 2nd evaporator; it is heated up to 110° C. Out coming steam from from the rd nd 3 evaporator is circulated inside the 2 one and the caustic is concentrated to 41%, and the temperature to 120° C. The vapor is separated and the used up steam is re-used in the 1st evaporator. The steam and caustic from the 3rd evaporator pre-heat the 41% caustic to 160° C for more efficient concentration. Fresh steam enters the evaporator and concentrates the caustic to 48% caustic lye. The out coming caustic is cooled down by using its heat to pre heat the caustic feeds to the other evaporators. It is finally cooled by cooling water lines to 45° C caustic lye. This is either sold as a product or sent for further concentration in the fusion plant.
Cold water In
°
Water Out Steam Ejector
EV I
°
Heat Exchangers
41% NaOH, 160 C
°
Vapor Separator Condensate to BH
36% NaOH
°
EV II
To Recovery
Steam Inlet
EV III
Vapor Separator
Vapor Separator 32% Fresh Causti
41% NaOH 120 C °
Heat Exchangers
Heat Exchangers Pum
Pum
Int Water Tank
36% NaOH, 110 C
Pum 48% NaOH, 180 C °
48% NaOH, 100 C °
Pum
Fig 3 Caustic Evaporation Plant (CEP)
41% NaOH, 45 C (Product) °
2.4.2 Fusion Plant The purpose of this plant is to concentrate c oncentrate the 48% caustic from the CEP plant to 98% Caustic flakes. The plant consists of three different multi effect evaporators. The temperature of the caustic goes as high as 420° C in this plant. Normal steel apparatus may get corroded in such a highly alkaline and hot environment. Hence all lines and inner lining of the equipments are made up of Nickel. The plant consists of: 1. PrePre-Co Conce ncent ntra rato torr (PC (PC)) 2. Rising Rising Film Film Evap Evaporat orator or (RFE) (RFE) 3. Fallin Falling g Film/F Film/Final inal Evapora Evaporator tor (FF (FFE) E) 48% caustic liquor is fed by pump to the PC from top, operating on the product side under a pressure of approximately 65 torr (87 mbar). During a single pass through the PC, the caustic is concentrated up to approximately 55.4-55% caustic. The PC is heated by the vapors coming out of FFE. Before entering PC, a 5% sugar solution is added to the caustic. Sugar acts as a corrosion corrosion inhibitor. inhibitor. This forms a layer of Nickel oxide on the lines. This layer prevents prevents further oxidation of Ni. The quantity of sugar to be added will be adjusted to the plant capacity each time. The NaOH 55% pump conveys the dilute acid to the pre heater at about 90 ° C. The pre-co pre-concen ncentr trate ated d causti causticc liquor liquor is passed passed throug through h a double double tube tube type type heat exchanger heated by condensed steam off the RFE. The outlet NaOH temperature is about 140-145 ° C. when passing the steam heated RFE, caustic soda is concentrated under a pressure of approximately 200 torr (267 mbar) up to 76% NaOH. The 76% caustic leaving the RFE is dipped into barometric seal pot. The PC and the RFE work under vacuum, which in both cases is produced by the vacuum ejector pump. The vacuum of 65 torr is required for the PC is produced by the 3 step injection group. The vapors from the PC are condensed in the mixing condenser via a barometric dipping tube. The condensed vapors and he cooling water are carried in to the waste water pit. For the RFE, a vacuum vacuu m of 200 torr is produced by b y the 2 step injection group. The 76% NaOH pump carried the caustic soda to the molten salt heated FC, where it is evaporated under normal pressure pressure up to a min of 98.5%Naoh. 98.5%Naoh. The distributing distributing device is fixed to the lower part of the separator and by this device; the caustic soda can be switched to either placer or drum filling station. In order to exclude atmospheric oxygen from penetrating into the system and to avoid corrosion the distributing system is superposed with nitrogen. The filling tubes to the flaker and to the drum filling station have been provided with a molten salt heating and the tube for NaOH 55-76% with steam heating.
The well cooled flakes are then filled into 50 kg bags by means of the bagging scale. The balance to the nominal capacity of 100 tpd (37-20, maximum 50 tpd) of caustic melt can be fed via existing distributing devices to the drum filling unit. Control High Pressure Steam and Condensate System. RFE is heated with high pressure steam concentration of the leaving NaOH solution is determined by it outlet temperature, temperature, therefore therefore a thermometer thermometer is installed in the outlet tube. Above the temperature controller TIC, the temperature of 160 ° C is adjusted which corresponds to the concentration of NaOH to 76% at 200 torr. From there corresponding command variable is given to the steam pressure control valve TCV in the steam tube, thus increasing or diminishing the steam throughput. The steam leaves RFE at the lower end. A level transmitter LT built in keeps the condensate which is under pressure constantly on a certain level. Control valve LCV is arranged in the flow direction after following caustic pre heater and receives its command variable from level control LIC. This arrangement arrangement guarantees guarantees an effective effective heat transfer transfer in rising rising film evaporator evaporator and exploitation of the condensate cooling in the caustic pre heater.
Design Description The dehydration line consists of the following three stages: 1.
Preconcentrator.
It is designed as a tube bundle FFE. In the concentration head, the caustic solution is distributed as follows. Above the tube plate, concentration tubes are provided with vertical slits through which caustic solution flows evenly through the tubes and down the inside surface of the tubes forming an unbroken film along the inside wall. The PC works at 65 torr vacuum on product side. The vacuum is produced by steam ejector vacuum pump. The PC caustic leaves the PC at 55% concentration and reaches the vapor separator installed beneath the PC. The vapors produced in the PC are removed by the vacuum pumps and leave the PC through a demister consisting of very fine nickel wire meshing which permits the separation of even the smallest caustic droplets carried along by the vapor. In the subsequent mixing condenser the vapors are condensed. The PC is heated with vapors from the FC. 2.
Rising Film Evaporator .
The caustic enters the lower part of the RFE with concentration of NAOH at 55% 55 % fills it up and and dist distri ribut butes es into into the the indi indivi vidu dual al tubes tubes of the the tube tube bundl bundle. e. The The risi rising ng film film evaporator is on the product side under a vacuum of 200 torr. Heating is done with steam in reverse direction, i.e. the steam entrance is located at the upper part of the tube bundle and the condensate leaves the lower part.The NaOH solution to be evaporated thus enters the tubes at the bottom surrounded b y steam. Due to condensation enthalpy of the steam set free, the evaporation process in the tubes is initiated and the liquid rises to the top according to the air lift pump principle, or
is driven through the tubes by the existing vapor respectively. The liquid/steam mixture reaches the vapor chamber which is located directly above the tube bundle. Immediately above the upper plate, a buffer plate is mounted acting as a liquid separator. By exploitation of the different densities of both the phases, here an intense liqu liquid id sepa separa rati tion on is achi achiev eved ed.. The The rema remain inin ing g liqu liquid id evap evapor orat ated ed to the the desi desire red d concentration is collected in the area around the upper tube bottom and leaves through a nozzle. The vapors deviated at the baffle plate reach the demister made of fine nickel wire meshing. Caustic droplets carried along are separated by this knitmesh and drop back into the liquid collecting chamber. The vapors leave the vapor room and reach the mixing condenser via a duct, where they’re condensed. The non condensable gases are sucked off by the vacuum pump. The heating or blanketing room of the tube bundle is equipped with a venting nozzle at its upper end. At the lower end there are 2 level control nozzles for the level transmitter.
3. Final Final Concen Concentra trator tor.. It consists of individual tube elements, concentrator tubes and the vapor separator. The caustic is evaporator in the falling film concentrator at atmospheric pressure by means of heat supplied from outside. The PC caustic is fed to the caustic collector by means means of the the caus causti ticc tran transf sfer er pump. pump. Caus Causes es even even dist distri ribu buti tion on to the the indi indivi vidu dual al concentrator elements where the caustic forms an unbroken film over the whole inside surf surfac acee flow flowss from from top top to botto bottom. m. Prop Propel elle lerr inse insert rtss buil builtt into into the the cente centerr of each each concentrator tube prevent the caustic film from being blown off the inside wall surface due to rapid evaporation taking place. The water contained in the caustic is evaporated while flowing down the concentrator tube. The caustic film, now with a concentration of 98.5% leaves the concentrator element and is fed by a collecting element to the seal pot. Each element is provided with an individual heating jacket in which the heat transfer medium flows in the reverse direction to the caustic to be concentrated. The heat transfer medium is fed through these jackets and led off through collection. Each heating jacket is provided with throttle valves on the outlet side which permit even distribution of the heat transfer medium to all concentrated elements. The collectors of the heating jackets of the individual concentrator elements are provided with steam trace heating, to prevent the heat transfer medium from freezing during start-up. Shortly before heat transfer medium reaches operating temperature steam tracing is turned off and vented. Thus formation of inadmissibly high temperature in the steam tracing system is prevented. The vapors resulting from evaporation stream, together with the caustic, flow form top to bottom of the concentrator tube. Then flow velocity, together with the effect of the above mentioned propeller inerts, cause the caustic film to adhere to the inside wall surface of the concentrator tube. At the bottom of the concentrator tubes, the vapors flow into the horizontal collection channel which leads them to the separator. Caustic droplets possibly carried along by the vapors are separated by the gravitation at considerably reduced flow velocity. The vapors roughly clean, then leave the separator through a mattress type demister made of fine nickel wire meshing. This demister offers largest possible surface contacts, greatest number of deflection and still causes the least possible pressure drops. The fine
mist is transformed into large caustic droplets which due to gravity fall back into the collector and are led together with the concentrated caustic to the caustic outlet nozzle. The vapors now almost completely cleaned are led to the PC which removes their the ir heat.
Sucrose System All equipment components and piping coming in contact with the concentrated caust caustic ic are are made made up of pure pure Nicke Nickell (low (low carb carbon on Nick Nickel el)) becau because se of its its corro corrosi sion on resistance. Oxygen acts highly corrosively, particularly combined with high temperature. Therefore, oxygen must be kept out of the equipment components by all means. If corrosion occurs, oxidized nickel would be there in the caustic melt. Penetration of atmospheric oxygen into the equipment is prevented by designing the plant as air tight as possible and by additionally blanketing the distributing device of the final concentrator with nitrogen. However, the caustic solution itself brings down the oxygen into the equipment in the form of Sodium Chlorate (NaClO3). This type of oxygen is eliminated by adding sugar solution to the feed caustic as corrosion inhibitor. The sugar solution reacts with NaClO3 to form Na2CO3 which is harmless from the point of view of Ni corrosion. Depending on the chlorate content of feed caustic and feed quality, granulated sugar is added to the feed as an aqueous solution solution prior to its entry into the NaOH piping. 0.05 kg of sugar per ton of 100% NaOH is added, whereby the nickel pickup of caustic melt is reduced to about 1 ppm. Food quality granulated sugar is use and pumped into the feed liquor as a 5% aqueous solution before being led to the concentrator. At startups, restarts or during any interruptions in the operation, the nickel pickup drops within 24 hours from startup value (approximately 20-30 ppm) to the operating value. The sugar solution solution is prepared prepared in the dissolving dissolving vessel together with the condensate condensate coming off the condensate tank resulting in a 5% sugar solution. The complete sugar solution is led into the NaOH piping before the PC by means of a dosing pump.
Molten Salt Heating System The heating system using molten salt as a heating medium mainly consists of a salt tank and an immersion pump, forced circulation heater with a built in burner, the sir preheater with incorporated fuel gas stack an d salt piping system.
Concept of the Heat Transfer Circuit. The heat transfer salt is molten in the salt tank i.e. kept hot and in liquid phase during interruption of operation. During operation salt is circulated by means of an immersion pump from bottom to top by heating coils of the heaters. The heat produced by the burner is transferred to the salt circulation in the heater coil. From the heater the molten salt goes to the FC. There it heats up and concentrates the caustic, and is circulated back to the salt tank. A by-pass valve regulates the quantity of molten salt in circulation through FC, permitting a part of salt quantity to be returned directly into the salt tank if required.
The whole salt piping system is arranged such that in case of interruption of operation of salt pump (as in case of power failure) the whole salt contents can freely flow back into the salt tank installed at the lowest point of the plant. Heat Transfer Salt. The salt used for final concentration is a mixture of three salts : 1. KNO3 (53%) 2. NaNO2 (40%) 3. NaNO3 (7%) The advantages of using this mixture are: • It has excellent heat transfer properties • Pressureless operation to 500° C • Non-flammable • Non-corrosive The disadvantages of this salt mixture are: • It has a high crystallization point. This is countered by making sure that whole system is heatable and can be easily emptied. Behavior of heat transfer medium. • Provided normal operating conditions, the heat transfer salt can be used for many years. • A certain oxidation of NaNO2 occurs in presence of atmospheric oxygen, causing a rise in point of crystallization. This is prevented by blanketing the salt tanker with Nitrogen. • In case molten salt is heated over a larger period, (for instance quantity of salt circu circula lati ting ng thro through ugh the the heat heater er is insu insuff ffic icie ient nt due due to faul faulty ty oper operat atio ion) n),, the the component NaNO2 may decompose to Na2O, O2 and N2, which also lead to higher point of crystalli crystallization zation of the salt mixture. mixture. Such conditions conditions are, however, however, prevented by temperature control of the wall o f temperature of the salt heater. Despit Despitee above above mentio mentioned ned precaut precaution ionary ary measur measures, es, the heat transfer transfer salt salt has to be regenerated after several years of operation.
Steam, 330 C °
48% NaOH, 50 C °
76% NaOH, 160 C °
PIC TIC 5% Sugar Soln
Steam 55% NaOH, 90 C
LIC
76% NaOH, 160 C
55% NaOH, 150 C
°
°
°
Int Tank 48% Pump
°
55% Pump
Flaker Drum
Oil H
Air
Used Salt, 390 C °
Mixing Condenser
Cold Water Pit
Seal Pot Molten Salt, 420 C
76% Salt Pump Pump
°
Cold Water Pit
Salt Tank
Fig 4 Fusion PLant
98% Caustic Flakes, 45 C
Chapter 3 Demand, supply and manufacturers in India In India the chloralkali industry is over 100 years old. Its products, chlorine and caustic soda are two basic chemicals. Chlorine is used for disinfecting drinking water bleach bleaching ing of textil textiles es and pulp, pulp, manufa manufactu cture re of insect insectici icides des,, pestic pesticide ides, s, weedic weedicide ides, s, pharmaceutic pharmaceuticals, als, pyro/plast pyro/plastics ics and refrigeran refrigerants, ts, industrial industrial solvents, solvents, rocket fuels and treatment of cooling water for electricity generation etc. chlorine production in our country in 1995-96 was 1.61 Mn tons. Though it is highly toxic and hazardous, it has become indispensable for mankind. Caustic soda is used for manufacture of paper p aper and pulp, textiles, soap and detergents, aluminium, rayon /viscoelastic fibre, inorganic c hemicals, dyestuffs petroleum refining and pharmaceuticals etc caustic production in our country was 1.308 Mn tons. There are three basic processes for caustic soda production: 1. Diaphr Diaphragm agm cell cell proces processs (1865) (1865) 2. Merc Mercury ury cell cell proce process ss (189 (1892) 2) 3. Memb Membra rane ne cell cell pro proce cess ss (197 (1970) 0) Each method represents a different method of keep ing the chlorine produced at the anode separate form the hydrogen produces, directly or indirectly at the cathode. Membrane process has the following advantages over the diaphragm and mercury cell process. 1. 25% lower lower energy energy consump consumptio tion n 2. Poll Pollut utio ion n free free oper operat atio ion n 3. Highe Higherr prod produc ucti tion on pur purit ity y 4. Easier Easier and more more flex flexibl iblee opera operatio tion n 5. Lowe Lowerr operat operatio ion n tempe tempera ratu ture re India is self-sufficient in caustic soda. The present installed capacity is distributed amongst different processes as under.
Table 6 membrane cell capacity as 31.3.2002 Sr. No. 1. 2. 3. 4. 5. 6. 7. 8.
Companies Gujarat Alkalies, Vadodara Grasim Inds, Nagoda Standard Alkalis, Kumool Modi Alkali, Alwar Chemfab alkalis, pondicherry Century chemicals, kalyan Hukumchand jute mills, amlai Atul products, valsad
Capacity (1000 MTPY) 170 85.5 53 19.8 16.5 55.7 15.6 11.2
9. 10. 11. 12. 13 14. 15.
Shree gopal, yamunanagar B&T, balarshah Punjab alkalis, naya mangal Tata chemicals Kadakja alkalis, ankleshwar Search chemical Indus., Jhagada DCM Con solidated, Bharuch
13.2 13.2 33 26.4 18.5 16.5 49.5
Indian chloralkali products expect to see caustic soda exports to increase from 90,000 tonne in 2001-02 to 210,000 tonnes onwards to close to 3.5 Mn tonnes/year of new capacity and incremental expansions will be added in 2003-04 will almost double the Indian chloralkali capacity. Live growth in domestic alumina production is one factor that could prompt even higher exports long-term. India has no option but to export 20-25% of the total caustic soda production. A number of new alumina units are at the planning stage, but it will take some time before they can be counted as secure caustic soda consumers. Another factor pushing up Indi Indian an expo export rtss of caus causti ticc soda soda stem stemss from from the the duti duties es char charge ged d on the the impo import rted ed technol technologi ogies. es. Indian Indian caustic caustic soda soda manufac manufactur turers ers face face power power from from 95 cent/KW cent/KWhr hr,, compared with 10-15 cents/KWhr in Saudi Arabia, and 20-25 cents/KWhr in the US. To compet competee intern internati ational onally ly,, Indian Indian produce producers rs have switche switched d to membra membrane ne cell cell technology and invested in captive power plant generation to ensure reliable low cost supply. However, the current import of 25% on both membrane cell plant and captive power plant in India made it a little uneconomical to implement both options together. together.
Chapter 4
Line sizing The chloralkali plant at TPL uses mainly five types of lines in the process; namely 1. Mild Mild Stee Steell Rubbe Rubberr Line Lined d (MSR (MSRL) L) 2. Stai Stainle nless ss Steel Steel (SS) (SS) 3. Fibr Fibree Reinf Reinfor orced ced Pla Plast stic ic 4. Titani tanium um li lines nes 5. Ni plat plated ed or or entir entirely ely Ni lines. lines. The first three types can be generally used about anywhere in the process, except for lines transporting high temperature or highly acidic/ corrosive media. The fourth type, i.e., Nickel lines are used wherever the media being transported is highly alkaline caustic or that at a high temperature. Nickel is used for this purpose because of its resistance against high temperature and alkalinity. Titanium lines are used where high strength and certain resistivity towards heat and alkalinity is desired. The line size, however, varies according to the following factors: 1. Type ype of trans transpor porte ted d media media 2. The The drivi driving ng for force ce of the the medi mediaa 3. Flow Flow rate rate of the the medi mediaa This implies that, for instance, that a media of high viscosity or rich in suspended impurities will require a line a larger diameter. Also, if the media is being transported by a pump, a line of smaller diameter will be favorable. Based on such factors, the size of lines connecting various parts across the plant c an be tabulated as follows:
Table 7 (a) Line Sizing Transported media
Line from
Line to
Line size, MOC
Primary Brine treatment
DM water
DM water
Saturator
16’‘, SS
Depleted Brine
Depleted brine tank
Saturator
16’‘, MSRL
Saturated Brine
Saturator
Mixing reactor-I
32’‘, SS
16% BaCO3
Mixing reactor -I
2’‘, MSRL
Mixing reactor -I
2’‘ MSRL
26% NaOH
Barium chloride storage tank Sodium chloride storage tank Caustic head tank
Mixing reactor –II
2’‘ MSRL
Mixing tank outflow
Mixing reactor -II
Mixing tank
8’‘, MSRL
0.05% Floxin
Floxin tanks- I&II
Mixing tank
1’‘, MSRL
Mixing tank outflow
Mixing tank
2 m to clarifier
16’‘, SS
Rest
Open channel
10% Na2CO3
Clarified brine
clarifier
Clarified brine tank
8’‘ SS
Clarified brine
Clarified brine tank
Anthracite filters
8’‘ FRP
Filtered brine
Anthracite filters
Filtered brine tank
8’‘FRP
Filtered brine
Filtered brine tank
Polishing filters
8’‘ FRP
Body feed tank
Precoat filters
1’‘ FRP
Precoat filters
1’‘ FRP
Polished brine tank
1’‘ FRP
Body feed (alpha cellulose) Na2SO3 HCl
Na2SO3 preparation tank HCl storage tank
Polished brine
Polished brine tank
Polished brine
Primary brine treatment facility Polished brine heat eater
Poli Polish shed ed brin brinee hea heate terr
8’‘, 8’‘, FRP FRP
Chela elate resin tower
8’’ 8’’, FRP
Chelate re resin to tower
Secondary br brine ta tank
8’’, FR FRP
Super Super purifi purified ed brine brine
Seconda Secondary ry brine brine tank tank
8’’, FRP
Super purified brine Depleted brine
Super purified brine head tank Electrolyzer
Super Super purif purified ied brine brine head tank Electrolyzer Depleted brine tank
8’’, FRP
Recycled caustic
Caustic storage tank
Electrolyzer
6’’, Ni plated SS
Heated polished brine Super pu purified br brine
Secondary Brine treatment Secondary Brine and Dechlorination
8’‘, FRP(/PVC)
8’’, FRP
Feed brine
Electrolyzer
8’’, MSRL
Depleted caustic
Super purified brine head tank Electrolyzer
NaOH drain
18’’, MSRL
H2+32% Caustic
Electrolyzer
Depleted brine
Depleted brine tank
10% Na2SO3
Sod. Sulfite Head tank
Caustic concentration 15’’, MSRL plant Brine dechlorination 8’’, FRP tank Dechlorinated brine 2’’, MSRL tank
Table 7 (b) Line Sizing Upto 36% caustic Above 36% caustic Steam lines Cooling water lines
Caustic Evaporation Plant 2’’, SS 2’’, Ti 2’’, SS, Glass wool insulated 2’’, MSRL
Chapter 5
Instrumentation The instruments used in the caustic soda plant work on the following principle. •
Flow Control : flow control is done with the help of the “I to P” apparatus. This produces a pneumatic pressure in the tube in accordance with the current that it receives, thereby controlling the amount of fluid that passes through. Other devices used for this purpose are rotameter, control valves etc. o
o
o
o
o
o
Mass Flow Meter – Works Works on the “Caroli’s principle” Caro Caroli li’’s prin princi cipl ple: e: When When the the flow flow of a liqu liquid id flow flowin ing g in a tube tube varie varies, s, the the frequently frequently with which it vibrates/shakes vibrates/shakes,, the tube also changes. This change can be used to calculate the mass flow/density of the fluid. Electromagnetic flow meter: (Anthracite filters) Prin Princi cipl ple: e: elect electrom romagn agnet etic ic plat plates es are are kept kept or eith either er side sidess of the the line line,, and and electromagnet electromagnetic ic waves are transmitte transmitted. d. The consequent consequent emf induced varies with the flow rate of the fluid. Hence, the induced emf can be used to indicate the flow rate. Oval Meter: This is used for flow metering the oil used as fuel in boilers. This consists of oval leads inside the lines which rotates as the oil flows and their rotations are used to detect the flow rate if oil. Rotameter: these are extensively used throughout the plant. A rotameter rotameter contains a floating device which stays afloat due to the force of the flowing fluid. The elevation gained by the float is used to predict the flow rate of the fluid. This is used for metering the flow of HCl. Critical Control valve. This is a device used to control the flow of a fluid, if it goes above a particular value. I to P (Current to Pressure) flow controller: if its current input changes, this device produces a consequent change in pneumatic pressure over that liquid, thereby controlling its flow. It is used in dechlorinised brine tank.
o
•
Shut Off Valves: Valves: These are used to completely stop/restart the flow automatically can be controlled directly from the control room, without manual involvement. It can’t be used to control the flow rate, however, ho wever, but only to stop/restart.
Thes Thesee are are used used in every every line, line, to stop stop the the flow flow auto automa mati tica call lly y, say say in case case of emergencies o
o
o
o
•
Level Measurement/Indication. o
o
o
o
•
Hydrogen Shut Off Valve: This is used for the supply lines in the boiler. It is equipped with a leak detector which measures pressure of hydrogen on either side of valve. If the pressure is same on either side, it indicates no leak. But if the pressure pressure on either side differs, differs, then this is an indication indication of leak. Similarly Similarly,, there are oil, and water control valves which work on the same principle. Solenoid Valve for Oil Control. It opens and lets the oil inside the burner when relay is received. Positioner: used in the Electrolyzer unit and it displays the position of the control valve after it has been operated on. Photoc Photocens ensor or.. This This is a specia speciall type type of control control used in the salt salt heater heater.. Since Since hydrogen and oil are used as fuel in the heater, it checks the spark generated for ignition. As soon as it’s sensor detects the spark created by the arc, it sends the power in relay, cutting it off and putting it in standby mode.
Level Indicators: (Indicate the level of the liquid inside a vessel) These work on the Bernoulli’s principle as well, and indicate the level of the fluid inside the vessel under a given pressure. Ultrasonic Sensors: transmits ultrasonic waves towards the fluid level and the time lag after which they are received can be used to calculate the level. Bubbler System: this simply indicates weather the fluid level is high or low, by meas measur urin ing g the the pres pressu sure re of the the gas above above it. it. This This is a form form of appr approxi oxima mate te measurement. Displacement Type Level Transmitter. This is primarily used in the whole of fusion plant. It consists of a float inside the tube which on moving up/down deflects a “torque-tube” and its consequent displacement displays the level.
Calcium Analyzer : This is used to measure the quantity of Calcium and Magnesium present in brine, right before it enters the membrane cells. It uses three reagents: 1. Pota Potass ssium ium Hydr Hydroxi oxide de 2. Ethy Ethyle lene ne Gly Glycol col 3. Phen Phenol olph phth thal alei ein n
These are mixed with a sample of brine. The mixer contains a magnetic stirrer and is connected to a colorimeter via fiber optic lines. The color of the sample changes on mixing the reagents and the intensity of the change is detected by the colorimeter, to calculate the amounts of calcium and magnesium in ppm. It can be caliberated from time to time using a standard reagent.
•
Temperature Indication. This is done by the following instruments. o Radiation Temperature Detectors: these are extensively used in the plant, also in the membrane cells for indication of the temperature of vessel contents. This works on the principle of thermal radiation, that every hot body emits some thermal radiation proportional to the fourth power of its temperature, and this instrument uses the intensity of the radiation to indicate the temperature of the body. Flame Temperature Measurement: the device consists of a Ultra-violet sensor, o which compares the color of the flame with standard colors to determine the temperature. For instance, a flame of higher temperature would be more reddish, sometimes whitish in color, and so on. Naturally, this is used only wherever the temperature temperature of a flame flame is required to be measured. measured. It has a relatively relatively high range; of up to 3000ºC. it is used in the burner of the fusion plant. plan t.
•
Pressure Indication: Pressure Pressure transmitte transmitters: rs: These are diaphragm diaphragm type pressure transmitters transmitters and work o on Bernoulli’s principle, i.e. indicate pressure when height of the liquid in a column is already known. o Pressure Switch: these are used to measure the pressure of a flowing fluid. Consists of a diaphragm or a metal sheet lodged inside the line. A switch is placed in the direction opposite to the flow. flow. As the fluid slows, it exerts some pressure on the diaphragm or the metal sheet, which pushes the switch that it is in contact with. The switch displays the pressure it has received.
Following table shows a list of the instruments, their range and accuracy for the primary brine treatment section of the plant.
Table 8 Instrumentation Sr No. 1 2 3 4 5 6
7
8
Description POLISHED BRINE pH FEED BRINE pH EFFLUENT pH DECHLORINATED BRINE pH SATURATED BRINE DENSITY D.P. IN ANTHRACITE FIKTERS BRINE FLOW TO ANTHRACITE FILTERS ANTHRACITE
Range 0-14 pH
Accuracy ±0.2 pH
0-14 pH 0-14 pH 0-14 pH
±0.2 pH ±0.2 pH ±0.2 pH
144-364 gpi
± 5%
0-25 0-2500 00 mm ± 1% WC 0-50 m3/hr
± 1%
0-100 m3/hr
± 1%
9 10 111 12 13 14
15 16 17 18 19
20 21
22 23 24 25 26
27 28
FILTER BACKWASH FLOW RESIN TOWER 0-100m3/hr BRINE FLOW Na2CO3 TANK-A 0-42 0-4200 00 mm LEVEL WC Na2CO3 TANK-B 0-42 0-4200 00 mm LEVEL WC FLOXIN TA TANK-A 0-18 0-1800 00 mm LEVEL WC FLOXIN TA TANK-B 0-18 0-1800 00 mm LEVEL WC CLARIFIED 0-47 0-4720 20 mm BRINE TANK WC LEVEL FILTERED BR BRINE 0-39 0-3900 00 mm TANK LEVEL WC BODY FEED 0-29 0-2900 00 mm TANK LEVEL WC PRE-COAT TA TANK 0-29 0-2900 00 mm LEVEL WC POLISHED BRINE 0-47 0-4700 00 mm TANK LEVEL WC SECONDARY 0-62 0-6250 50 mm BRINE TANK WC LEVEL FEED BRINE 0-32 0-3200 00 mm TANK LEVEL WC DECHLORINATED 0-34 0-3400 00 mm BRINE TANK WC LEVEL SODIUM SULFATE 0-20 0-2000 00 mm TANK-A,B LEVEL WC CLARIFY BR B RINE 0-80 DEG TEMP POLISHED BR 0-100 DEG TEMP VACUUM PUPM 0-100 DEG SUCTION TEMP CAUSTIC 0-100 DEG STOR STORAG AGE E TANK ANK TEMP 20% NAOH TANK 0-20 0-2000 00 mm LVL WC 32%TANK FLOW 0-25 m3/hr
± 1% 1% ± 1% ± 1% ± 1% ± 1% ± 1%
± 1% ± 1% ± 1% ± 1% ± 1%
± 1% ± 1%
± 1% ± 2 DEG ± 2 DEG ± 1% ± 1%
± 1% ± 1%
29
STEAM INLET 0-16 KSC PRESSURE
± 1%
Chapter 6
Pumps and Valves The chlor-alkali plant employs the following types of pumps: • • •
Centrifugal pump Reciprocating pump Metering pump
•
The 4 wetting parts; front casing, impeller, stuffing box,/back casing and sleeve usually differ in MOC according to the application of the pump. The rest of the parts are made of the same MOC for almost every application. The following materials of construction are used for pumping various v arious materials: •
Pure brine: Stainless steel
•
Pure brine + chlorine gas: titanium
•
Acids
•
o
H2SO4: Alloy 20
o
HCl: polypropylene, Ceramic, Fibre reinforced plasic
NaOH: 3% Nickel casting or just Nickel
•
Molten salt: alloy of Cobalt and Molybdenum
•
For any other non-corrosive non-corrosive medium at normal temperature: Cast iron, Mild steel, Stainless steel may be used.
Up to clarified brine tank, no valves or pumps are used, the flow of the substances is completely due to gravity. gravity. After clarified brine tank, the clarified brine is circulated using butterfly valve; having MOCs of either Teflon or MSRL. A level indicating valve is also used in the line from clarified brine tank. The backwater from clarified brine tank is pumped out by backwater pump, having the MOC as Ti. The filtered brine from the brine filters is pumped using filtered brine pump. More than 30 valves are used for polishing
filters. Pneumatic control valves are used to control the flow of brine in this region. As16% slurry is pumped as slurry, slurry, for this purpose, ODS pumps are used (pneumatically controlled). For fusion plant, all the lines are Ni lines, and all the pumps used are centrifugal pumps, except for the sugar pumps, which is a reciprocatory pump. For the molten salt line, wetted parts of pumps are made up of an alloy of molybdenum and cobalt, as the circulatory medium s highly corrosive, and also because this MOC is highly suitable for high temperature. TPL uses VFD (variable frequency drive), which is a mode to save energy. Using this, they are able to run all motors at about 2100 rpm, which can be varied using VFD according to required flow rates.
Chapter 7
Motors RPM make types Electric motors have a tremendous impact on overall energy use. Between 30 to 40 percent of all fossil fuels burned are used to generate electricity, and two-thirds of that electricity is converted by motors into mechanical energy. The two basic parts of an induction motor are the stationary stator located in the motor frame and the rotor that is free to rotate with the motor shaft. The basic simplicity of this design ensures high efficiency and makes them easily adaptable to a variety of shapes and enclosures. By varying the design of the basic squirrel-cage motor, almost any characteristic of speed, torque, and voltage can be controlled by the designer. The speed of an ac induction motor depends on the frequency of the supply voltage and the number of poles for which the motor is wound. The term pole refers to the manner in which the stator coils are connected to the three incoming power leads to create the desired rotating magnetic field. Motors are always wound with an even number of poles. The higher the input frequency, the faster the motor runs. The more poles a motor has, the slower it runs at a given input frequency. The synchronous speed of an ac induction motor is the speed at which the stator magnetic flux rotates around the stator core at the air gap. At 60 Hz the following synchronous speeds are obtained:
Table 9. RPM for No. of Poles Number of poles 2 4 6 8 10 12
RPM 3,600 1,800 1,200 900 720 600
Providing the motor is properly constructed, the output speed can be doubled for a given number of poles by running an ASD supplying the motor at an output frequency of 120 Hz. Induction motors are made with slip ranging from less than 5% up to 20%. A motor with a slip of 5% or less is known as a normal-slip motor. A normal-slip motor is sometimes referred to as a 'constant speed' motor because the speed changes very little from no-load to full-load conditions. A common four-pole motor with a synchronous speed of 1,800 rpm may have a no-load speed of 1,795 rpm and a full-load speed of 1,750 rpm. The rate-of-change of slip is approximately linear from 10% to 110% load, when all
other factors such as temperature and voltage are held constant. Motors with slip over 5% are used for hard to start applications. Historically a variety of terms have been used to describe a system that permits a mechanical load to be driven at user-selected speeds. These terms include, but are not limited to: Variable-Speed Drive Variable-Frequency Drive Adjustable-Frequency Drive Adjustable-Speed Drive The term variable implies a change that may or may not be under the control of the user. Adjustable is the preferred term since this refers to a change directly under control of the user. The term frequency can only be applied to drives with an ac output, while the term speed is preferred since this includes both ac and dc drives. Thus, the term most commonly accepted is Adjustable-Speed Drive (ASD). Most Most ASD ASD units units consist consist of three three basic basic parts. parts. A rectif rectifier ier that that convert convertss the fixed fixed frequency frequency ac input voltage to dc. An inverter inverter that switches the rectified dc voltage to an adjustable frequency ac output voltage. (The inverter may also control output current flow, flow, if desired.) The dc link connects conne cts the rectifier to the inverter. inverter. A set set of controls directs the rectifier and inverter to produce the desired ac frequency and voltage to meet the needs of the ASD system at any moment in time. The advantages of ASDs do not stop with saving energy and improving control. ASD technology can now be applied to manufacturing equipment previously considered too expensive or uneconomical. Such applications are often unique to a particular industry and its equipment, or even to a particular facility. Cost benefits, such as those obtained from improved quality, quality, may be desirable for each application. The motors at the Heavy Chemicals Division of TPL are powered by a pair of recently installed DG sets. These have greatly helped meet the power requirements of the chlor chlor-al -alkal kalii plant plant and have also also ensure ensured d un-int un-interr errupte upted d power power supply supply to the fragil fragilee membrane cells which in turn avoided generation of effluents due to power shortages.
Chapter 8
Environmental Issues I
Impact Identification.
Introduction. Environmental consequences which are generally known as impacts, is creation of a new set of Environmental conditions, adverse or beneficial inducted by the action or the set of actions under considerations. The impact analysis of different factors of the operation of the plant is shown below. The impacts during operational phase: 1. Soci Socio o Eco Econo nomi micc Imp Impact acts. s. Replacement of the mercury cells by the membrane cells has helped abate the problem of effluent disposal, which no more contains mercury. The plant operation requires a total power of 4.5 lac units/day, which it gets from two sources, partially from Tamilnadu Electricity Board, and the rest from its owned DG sets. The installation of the DG sets helped meet the power requirement of the chlor-alkali plant and also ensured un-interrupted source of power supply to the fragile membrane cells which avoided the generation of effluents due to power shut-down. And hence newer installations directly imply more number of job opportunities. This will help improve the status status of the peripheral peripheral local communities communities whose living standards will go up cons consid ider erabl ably y due due to the the highe higherr incom incomee level levels, s, bett better er medi medical cal faci facili liti ties es and and communi communicati cation on advance advancess and access access to better better educati education on facili facilitie ties. s. Due to margina marginall increase in employment opportunities associated with the modernization, development is bound to occur. On the above scores, the impact will be marginally positive in this action. 2. Air Qu Quality a) Impact Impact on air air quality quality due due to plant plant emissi emission. on. Ambient air ambient air quality levels wit respect to SPM, SO2, NOx are well within the prescribed limit by CPCB and MOEF in the core and buffer zone. The outlet compositions of the stacks existing within the premises of the plant are tabulated in table Note: As H2 is used as fuel in boiler there’ll be no SO2 emissions.
The emissions of SPM, SO2, NOx, acid mist and Cl2 from utilities as well as scrubbe scrubbers rs are well well within within the limits limits.. These These stacks stacks are monito monitored red regula regularly rly enforc enforcee preventive measures in case of any an y excess. \
No.
1.
2.
3. 4. 5. 6.
Stack Attached To Caustic Fusion Plant (Concentrator Stack) Caustic Fusion Plant (Salt Heater) Steam Boiler DG Cell Set WAD Plant 1 WAD Plant 2
Table 10 Stack Data Fuel Stack Stack Dia Type Height m m H2 20 0 .8
Temperature °C 200 °C
Volume of gas Nm3/hr 3 MT/hr steam
H2
25
0.38
200 °C
2000
H2 Fuel Oil
35 8 16 16
0.15 0.3 0.3
200 °C 30 °C 15 °C
500 1100 1100
The following control measures have been incorporated\ for controlling the emissions. 1. Each HCl HCl unit is filled filled with with a tail tail gas scrubber scrubber for removi removing ng traces traces of HCl (acid (acid mist) present. 2. Excess of NH3 from from the NH4Cl NH4Cl plant plant is washed washed with with DM water in mother mother liquor liquor scrubber packed with poly propylene packing to remove excess of NH3 3. Cl2 Cl2 wast wastee gase gasess are are remo remove ved d by neut neutra rali liza zati tion on and and abso absorp rpti tion on of caus causti ticc soda/milk of lime towers. Due to enforcement of various control measures, impact on air quality due to existing plant operations are well within sustainable limits. a) Impact Impact on air air quali quality ty due due to DG DG sets. sets. Initially fuel oil was employed for the operation of DG sets which was later replaced by environmentally friendly fuels like LSHS, LSWR, and LNG etc. as liquid fuels are used. The impacts due to SPM are negligible. SO2 and NOx are major pollutants whose impacts are to be contained during the operational phase of the plant. The impacts on air quality due to the following fuels are considered. (i) fuel oil with 4 % sulfur conte ntent (ii) LHS wi with 1. 1.9 % sulfur co conten tent (iii (iii)) LNG LNG with with 0.00 0.001 1 % sulf sulfur ur cont conten entt b) Impact on air air quality quality due to heavy fuel oil. In the hot flue gases let out in the stack, SO2 and NOx are the major pollutants. pollutants. The quantum of SO2 and NOx e mission from the plant will be as below;
SO2 emission (i) (i) Total otal fuel fuel (HFO) HFO) cons consum umpt ptio ion n = 3800 3800 kg/ kg/hr (ii) S co content (max) in in HF HFO = 4.0% (iii) (iii) S load load prod produce uced d assum assuming ing 100% combus combustio tion n = 4% 4% of 3800 = 152 152 kg/hr kg/hr (iv) (iv) Quant Quantit ity y of SO2 SO2 gas gas prod produce uced d = 152*2 152*2 = 304 304 kg/h kg/hrr = 84.4 84.4 kg/ kg/se secc
Considering that 3 DG sets (2 running and 1 standby) will give an output of 30 MW the emission rate of SO2 from each stack is 42. 2 g/sec NOx emission The NOx emission from the stack will be restricted to 1100 ppm which will lead to emission rate of 61.6 g/sec
c) Impact Impact on air air qualit qualityy due due to to LNG LNG LNG is the most environmentally friendly fuel with S content as low as 0.001 % which will lead to almost negligible SO2 emission. The NOx emission will also be 61.6 g/sec Air Quality Modeling The emission rates calculated above have been subjected to the PTDIS model to asses the impact on the air quality due to dispersion effects. Inputs to the model are given below: Stack height : Stack Diameter: Stack temperature: Exit Velocity : Emission rate For SO2 :
65.5 m 1.7 m 453 K 12.22 m/sec 42.2 g/sec
PTDIS Model It is an interactive program that estimates the short term concentration directly down wind of a point source at a distance specified by the user. The effect of estimation vertical dispersion by mixing heat can be included. The model model uses uses Briggs Briggs-P -Phume hume reside residency ncy method method and Pasqui Pasquilli lli-Gr -Grif iffor ford d Disper Dispersio sion n methods for this. The various inputs required to run this model are: • Number of sources • Number of receptors • Emission rate, physical stack height and diameter, stack gas temperature and stack gas velocity for each value entered. • Number of meteorological scenarios. • For each each scenar scenario, io, the meteor meteorolo ologic gical al inform informati ation on requir required ed is wind wind veloci velocity ty,, ambient temperature and mixing heights.
The PTDIS model was run for respective wind velocities (1.5 m/s, 2 m/s, 6.5 m/s and 8.0 m/s) and stability classes 2 and 4 taking into account the natural stability. stability. Analysis of dispersion model
The ground level concentrations have been worked out as mentioned earlier for each of the gas from the stack and predicted in a consolidated form for each parameter namely SO2, NOx in tables 2 for short term concentrations and corresponding 8 hourly and 24 hourly values. For SO2, the max short term concentration, 8 hourly and 24 hourly values due to the use of HFO are 339.0, 99.0, and 66.0 respectively at a downwind distance of 1 km where LSHS is used. The SO2 maximum 8 hourly and 24 hourly values will be 150.0, 45.0 and 30.0 at a distance of 1 km. For NOx the maximum short term concentration 8 hourly and 24 hourly values due to the stack are 351.0, 105.3, and 10.20 mg/m3 respectively at a downwind distance of 1.0 km for 3 stacks. In reality, GLC will be less due to changing wind direction, combine d effects of alternating alternating sources and other Environmental Environmental factors. Even after addition of projected projected ground level concentrations to the existing background loads when 2 GD sets are in operation the ambient air quality will be maintained within prescribed limits both in core and buffer zones. With introduction of LSHS and LNG as fuels, levels of SO2, NOx are relatively reduced.
No.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Table 2 Level Concentration For SO2 (HFO as fuel) in mg/m3 Distance Short Term 3 stacks 8 hourly Concentration (1 stack) 0 .5 0 34.00 102.00 30.60 1 .0 0 110 330.00 99 2 .0 0 48.40 145.2 43.56 3 .0 0 23.70 71.1 21.53 4 .0 0 13.90 41.7 12.51 5 .0 0 9.09 27.27 8.18 6 .0 0 6.61 19.83 5.95 7 .0 0 5.29 15.87 4.96 8 .0 0 4.53 13.59 4.08 9 .0 0 4.04 12.12 3.64 10.00 3.67 11.01 3 .3 15.00 2.58 7.74 2.32 20.00 2.02 6.06 1.82
3. Impact on water quality
24 hourly
1.21 1.55 2 .2 2.42 2.72 3.17 3.97 5.45 8.34 14.22 29.04 66.00 20.40
The entire water requirement of 1980 m3/day will be met from the metro water supply or from ground water sources located within the factory area. The auxiliary cooling water will be recycled and only make up water will be supplied. The domestic waste water generated which is about 80 m3/day will be treated with the existing sewage treatment plan. As such no waste water will be discharged from the process. Occasionally waste water containing oil and grease generated due to washing will also be treated in the ETP. Due to various Environmental control measures environmental aged the impact on water quality will be less.
Environmental Control Measures General The adver adverse se affe affect ctss of the the vari various ous impa impact ctss due due to the the mode modern rniz izat atio ions ns duri during ng the the operational phase of the plant. These can be ameliorated and brought to sustainable limits by planning and adap ting necessary measures.
1. Land Us Use There was no agricultural land spoilage while setting up of the plant. As no forest land or hutment hutmentss exist exist in the core core zone, zone, the questio question n of rehabi rehabilit litati ation on and corres correspon pondin ding g compensatory afforestation does not arise. Socio Economic factors The plant is amidst an industrial area. Hence it enjoys all the conveniences of being close to other industries. The Manali industrial area is situated about 30 kms away from the metro Chennai. Consequently, the plant provides employment opportunities to people of many sections of society. society. Air Quality. Ambient air quality is well within the limits of CPCB and MOEP. Also, LNG is being used as fuel instead of Heavy Heav y Fuel Oil, so less sulfur dioxide is released as pollutant gas. Chlorine Neutralization of Vent Vent Gases. (W (Wad ad Section) Chlorine waste gases generated from chlorine storage and vent gases during start up are neutralized in two absorption towers where caustic lye is circulated. This system has additional caustic soda polishing column to ensure no chlorine escapes to the atmosphere. The vent gases being released into the atmosphere are been continuously monitored by an analyzer and being shown in the control room. Further, a sodium hypo system has also been been introd introduced uced where where chlori chlorine ne is comple completel tely y remove removed d as sodium sodium hypochl hypochlori oride de by effective neutralization with caustic soda. The sodium hypochloride produced is saleable and does not contribute con tribute to any solid or liquid waste. Capacity of Sodium Hypo System
:
1 MT/hr Chlorine
Around 10 KL/day of sodium hypochloride solution is produced per day in this section and dispatched out as a by-product. Water Quality Water quality is maintained well within the prescribed limits as a result of treatment procedures described below: Waste water is treated using the principle of attraction while process waste water is disposed into the sea after pH neutralization. The total quantity of waste water from different sections is given below: Process Domestic
: :
290 m3/day 80 m3/day
The treatment procedures for the two types of effluents are described below:
Method of Treatment of Sewage Effluent Effluent waster from canteen contains oily matter and solid food particles that are bio degradable. The water is treated biologically for recovery and reuse in the canteen waste water treatment plant. Solids in the waste water are retained by a mesh screen and then sent to the oil removal pit, where the mater is relieved of its oil content. From the oil removal pit, the waste water flows from the collection tank by gravity. The water is then pumped to the equalization tank, which serves to smooth the variation in quality of effluent coming from the collection tank. A feed pump delivers waste water from equalization tank to the aeration tan, which is provided with two aerators. Reduction of Biological Oxygen Demand (BOD) takes place in the aeration tank, where microorganisms are allowed to grow for this purpose. From the aerator tank, water flows to the settling tank, where microbial sludge is separated. Clean water overflows to the treated water tank, from where it is pumped for reuse. Part of sludge is sent to sludge pits for disposal. Effluent Treatment Process System
The process effluents come from the following sections: 1. IEM plant floor washing 20m3/day 2. Brine section 90 m3/day 3. Boiler blowdown 5 m3/day 4. Caustic Soda Evaporation 5 m3/day 5. DM water plant 100 m3/day 6. Cooling water 45 m3/day 7. Hydrogen Unit 15 m3/day 8. Ammonium Chloride 5 m3/day 9. DG set (floor washing) 5 m3/day _______________
290 m3/day _______________ The non-mercury bearing effluent stream is collected in the main effluent pit. 12″ dia HDPE pipes laid underground connects effluents generated from different sections to collection pits/ main effluent pit. Since Since cleaner cleaner technolo technology gy is used, used, the effluent effluentss from from the plant will not contain contain any hazardo hazardous us chemica chemicals. ls. pH correc correctio tion, n, if requir required, ed, will will be done and pumped pumped to final final disposal point along with the treated effluent from the canteen waste water treatment plant. The combined treated effluents will continue to be pumped into the sea. Water Quality Qu ality Monitoring: It is required to carry out continuous monitoring of treaded effluent quality in and around factory premises The quality of treated effluents in and around factory premises is being continuously monitored. The treated effluent quality is monitored at the outlet of individual in-plant effluent treatment as well as from the outlet after neutralization. Automatic sampler collectors are installed to collect samples continuously. From each point minimum eight hours composite samples will be collected and analyzed for the relevant pollutants as indicat indicated ed in the respec respectiv tivee treatm treatment ent scheme scheme.. Regula Regularr sampli sampling ng is conduct conducted ed at the discharge point of the diffuser system to detect any deterioration of the treated effluent quality.
Environmental Management Plant It is imperative to device in advance a proper management plant for Environmental control during operation phase of the plant so these measures can be enforced promptly. The HCD at TPL has a well lad out management plan and proper implementations of vario various us cont contro roll measu measure ress have have helped helped to keep keep the the Envi Enviro ronm nment ental al qual qualit ity y with within in sustainable limits. The main environmental factors which are to be included have been given due consideration in the management plan are the following. 1. Ambient Ambient air quality quality for for the monitor monitoring ing of SPM, SPM, SO2 and and NOx emissio emissions ns to asses asses the efficiency of control measures incorporated. 2. Ecol Ecolog ogic ical al fac facto tors rs 3. Ambient Ambient noise noise in the plant plant area area and the surround surrounding ing buffer buffer zone. a. Water analysis analysis due due to dischar discharge ge of treated treated effluents effluents.. b. Safe Safety ty and and healt health h measu measure res. s. Regular systematic and sustained program schedule for implementation and monitoring of various control measures are to be devised in advance with clear cut guidelines for various plants for keeping a continuous surveillance on the environmental quality of the
area. It is noteworthy to highlight here that a full fledged control scheme to minimize the impacts on environmental factors has already been adapted.
Statutory Fulfillments The monitoring schedules will be planned to aim at regular and systematic study of various pollution levels with respect to air and water qualities, noise levels, etc., to ensure that they conform to the standards laid down by the Central and State Pollution Control Board limits. Stack emissions are and shall be monitored regularly to check the efficacy of the control equipments. All the stipulations in respect of methodologies and studies of various parameters by MOEF and State Pollution Control Board should be followed. Wherever excess levels are noticed, prompt control measures will have to be effected to bring the quality parameters under control. Soci Socioo-ec econ onom omic ic stat status us with with resp respec ectt to heal health th,, educ educat atio ion n occu occupa pati tion onal al heal health th,, communication welfare measures etc are reviewed at least once in three years to bring them on par with the population growth, social development etc. HCD at TPL has adapted environmental control and management as a corporate policy to ensure that all the parameters parameters for various various environmental environmental components components such as air, air, water, water, noise, soil, etc conform strictly to standards of Central and State Pollution Control Board. The general manager will over see the environmental management system directly, duly supported by the concerned departmental executives.
Environmental Management Cell This cell will oversee the measures to be taken under the Environmental Management Plan to ensure the impacts arising from the pollutional parameters of the power plant are within prescribed limits. For this purpose the cell will comprise of a monitoring group and a pollution control equipment maintenance group. The environmental management cell will oversee the following aspects. a) Conducting Conducting environ environmental mental audits and reporti reporting ng to TNPCB b) Sendin Sending g periodi periodical cal monitori monitoring ng report reportss as prescribed prescribed by TNPCB TNPCB and other other statut statutory ory authorities. c) Recommending Recommending in advance advance necessary necessary measures measures to improve improve the the environmental environmental aspect aspects. s. d) Conducting Conducting safety audits audits and programs programs and safety awarenes awarenesss among workers/st workers/staff. aff. e) Cond Conduc uctt annu annual al heal health th audi audits ts to dete detect ct any any heal health th prob proble lems ms prom prompt ptly ly in the the workers/staff.
Ambient Air Quality and Stack Monitoring. For ambient ambient air qualit quality y monitor monitoring ing,, necessa necessary ry instru instrumen ments ts such such as high high volume volume samples samples are availab available. le. Stack Stack monito monitorin ring g is conduct conducted ed regula regularly rly in the existi existing ng unit unit available stack monitoring kits, SO2 Analyzer, Automatic Recording emitter, Weather data etc. these will be adapted a dapted by respective instruments having high accurac y. Water Quality Qu ality Monitoring
It is required to carry out monitoring of treated effluent quality in and around factory premises. The treated effluent quality should be monitored at the outlet of individual in plant effluent treatment units as well as from the outlet after neutralization. Automatic sample collectors are installed to collect the samples continuously from each point. Minimum 8 hours of composite samples shall be collected and analyzed for relevant pollut pollutant antss as indicat indicated ed in the respect respective ive treatm treatment ent schemes schemes descri described bed.. The efflu effluent ent samples are collected and analyzed daily. Regular sampling is conducted at the discharge point of the diffuser system to detect any deterioration of the treated effluent quality. The marine ecosystem will also be continuously monitored. Soil Quality Monitoring . Monito Monitorin ring g program program for the soil will will help help in the improv improveme ement nt in physica physicall and chemical chemical properties properties of the soil correspondi corresponding ng improvement improvement in ground water quality and water table levels. Thus, monitoring measures may provide useful guidelines and for taking remedial measures in time for the result and any further development in the activities in the area. Environmental Control Cost Towards pollution control, a capital investment of Rs. 287 lacs has been made of which which Rs. Rs. 207 207 lacs lacs has been been spen spentt for for poll pollut utio ion n cont contro roll sche scheme mess to cont contro roll wate water r pollut pollution ion.. During During the moderni modernizat zation ion program program,, Rs. 40 lacs lacs were were spent spent toward towardss air pollution control equipment and Rs. 30 lacs was spent on Green Belt Development. Besides, the annual expenditure for operation and management is about Rs. 69 lacs for air pollution and Rs. 10 lacs for ETP. ETP.
Summary The heavy chemicals division of tamilnadu petroproducts limited produces about 100 MT/day of Caustic Soda Flakes (98%), along with hydrogen and chlorine as by-products. the mere raw materials it uses are water, electricity and raw salt. The plant is powered by a set of 3 Deisel generator Sets, 2 always operational and one on standby. The plant makes the use of Membrane Membrane Cell technology technology in the electrolyzers, electrolyzers, which relatively more eco-friendly, cheap and efficient in terms of quality of the product. Despite of a few power cuts due to heavy rains, it was observed during the training period that the plant was operating at par with the demand of caustic soda flakes. It makes good use of the gases it generates as by-products in generating power or other chemical substances. TPL takes complete care of all the safety norms required to be followed to ensure the safety of its employees, workers and the surrounding environment. The training was a healthy learning experience.
