Contents Acknowledgement Acknowledgement....................... ................................ ......... ...................... ................................ .......... ..................... ................................ ........... .................... .............................. ............ .. ......... 2 INTRODUCTIO INTRODUCTION N OF UREA ................... ............................... .............. .................... .............................. ............ .. ........................ ................................ ........ ..................... ............................... .......... 4 PHYSICAL PHYSICAL AND CHEMICAL CHEMICAL PROPERTIES PROPERTIES .......................... ................................ ...... .................... .............................. ............ .. ........................ ................................ ........ .......... 5 USES OF UREA ............... ......................... ................. ....... ....................... ................................ ......... .................... ................................ ............ .................... ............................... ............. ................ ................ 6 PROCESS PROCESS TECHNOLOGY TECHNOLOGY............... .......................... ................. ...... .................... .............................. ............ .. ....................... ................................ ......... ..................... ............................... ............ .. 7 ONCE THROUGH THROUGH PROCESS......... PROCESS................... .................... ............. ... .................... .............................. ............ .. ........................ ................................ ........ ..................... .......................... ..... 8 PARTIAL PARTIAL RECYCLE ................. ............................ ............... .... ...................... ................................ .......... ..................... ................................ ........... ..................... ................................ ........... ....... 8 TOTAL RECYCLE OR LIQUID CARBAMATE SOLUTION
RECYCLE PROCESS:- ............................ .............. .................. .... .............. .......... .... 9
STRIPPING PROCESS PROCESS .................... ............................... ............. ....................... ................................ ......... ....................... ................................ ......... .................... ............................... ............. . 10 THEORY THEORY OF STRIPPING .................... .............................. ............ .. ..................... ............................... ............ ....................... ................................ ......... .................... ......................... ..... 10 EFFECT OF PROCESS PROCESS VARIABLES VARIABLES ................... ............................... .............. .................... .............................. ............ .. ....................... ................................ ......... ................... ................... 11 EFFECT OF H2O/CO2 RATIO ................... ............................. ............. ... ...................... ................................ .......... .................... .............................. ............ .. .................... ........................... ....... 12 EFFECT OF PRESSURE PRESSURE AND TEMPERATURE ...................... ................................ .......... ..................... ................................ ........... ....................... ................................ ......... ... 12 EFFECT OF RESIDENCE RESIDENCE TIME.................... ................................ ............ ..................... ............................... ............ ....................... ................................ ......... .................... ......................... ..... 12 BIURET IN UREA ............. ........................ ................... ........ ....................... ................................ ......... .................... ................................ ............ .................... ............................... ............. ........... ........... 13 PROCESS PROCESS DESCRIPTION DESCRIPTION FOR UREA SYNTHESIS ....... ................... ...................... ............. ... ..................... ................................ ........... .................... .............................. .......... 13 HIGH PRESSURE SECTION SECTION ............................... ................................. ..................... ............................... ............ ....................... ................................ ......... .................... ......................... ..... 14 CO2 COMPRESSOR COMPRESSOR....................... ................................ ......... ....................... ................................ ......... .................... ................................ ............ .................... ............................... ............. .... 14 COMPRESSION COMPRESSION DETAILS...... DETAILS................ ..................... ................ ..... ..................... ............................... ............ ....................... ................................ ......... .................... ............................ ........ 15 AMMONIA AMMONIA RECOVERY RECOVERY ....................... ................................ ......... ...................... ................................ .......... ........................ ................................ ........ .................... .............................. ............ .. .. 15 UREA SYNTHESIS SYNTHESIS ................... ............................. ............. ... ...................... ................................ .......... ..................... ................................ ........... ..................... ................................ ........... .......... 15 CONSTRUCTIO CONSTRUCTION N OF UREA REACTOR REACTOR .................. .............................. .............. .. .................... .............................. ............ .. ........................ ................................ ........ ............. ............. 16 OVER FLOW FROM REACTOR REACTOR ..................... ................................ ........... .................... .............................. ............ .. ........................ ................................ ........ ................... ................... 17 AMMONIA AMMONIA STRIPPING STRIPPING ......................... ................................ ....... ...................... ................................ .......... ........................ ................................ ........ .................... .............................. ............ .. .. 17 STRIPPING PROCESS PROCESS ................ .......................... ................ ...... ...................... ................................ .......... ........................ ................................ ........ ..................... ............................... ............ ..... 17 CONSTRUCTION CONSTRUCTION OF STRIPPER..................... ................................ ........... .................... .............................. ............ .. ....................... ................................ ......... ................... ................... 18 CONDENSATIO CONDENSATION N AND SEPERATION SEPERATION ..................... ................................ ........... ..................... ............................... ............ ....................... ................................ ......... .............. .............. 18 MEDIUM MEDIUM PRESSURE SECTION SECTION ............... ......................... ................. ....... ..................... ............................... ............ ....................... ................................ ......... .................... ....................... ... 19 MEDIUM MEDIUM PRESSURE SEPARATOR.................... ................................ ............ .................... ............................... ............. ........................ ................................ ........ .................. .................. 19 MEDIUM MEDIUM PRESSURE DECOMPOSER DECOMPOSER ................. ............................. ............... ... ..................... ............................... ............ ....................... ................................ ......... .............. .............. 20
MEDIUM MEDIUM PRESSURE UREA HOLDER HOLDER ..................... ................................ ........... ..................... ............................... ............ ....................... ................................ ......... .............. .............. 21 MEDIUM MEDIUM PREESURE CONDENSATION CONDENSATION..................... ................................ ........... ................... .............................. ............. .. ....................... ................................ ......... ........... ........... 21 MEDIUM MEDIUM PRESSURE ABSORBER ABSORBER ............ ...................... .................... .......... ..................... ............................... ............ ....................... ................................ ......... .................... .................... 22 ABSORPTION ABSORPTION PROCESS PROCESS ................. ........................... ............... ..... ..................... ............................... ............ ....................... ................................ ......... .................... ............................... ............. . 22 DESCRIPSITIO DESCRIPSITION N OF MEDIUM PRESSURE ABSORBER ........................ ................................ ........ ..................... ................................ ........... .................... ...................... .. 24 MEDIUM MEDIUM PRESSURE AMMONIA AMMONIA CONDENSAT CONDENSATION ION ........................ ................................ ........ ..................... ............................... ............ ....................... ......................... .. 24 AMMONIA AMMONIA RECEIVING RECEIVING .................... ............................... ............. ....................... ................................ ......... ....................... ................................ ......... .................... ............................... ............. . 25 MEDIUM MEDIUM PRESSURE ABSORBER AND INERT WASHING ................. ........................... ............... ..... ..................... ................................ ........... ................. ................. 26 LOW PRESSURE SECTION.......... SECTION.................... ...................... ............ .................... .............................. ............ .. ....................... ................................ ......... .................... .............................. .......... 27 LOW PRESSURE SEPARATOR ..................... ................................ ........... .................... .............................. ............ .. ........................ ................................ ........ ..................... ........................ ... 27 LOW PRESSURE DECOMPOSER DECOMPOSER..................... ................................ ........... .................... .............................. ............ .. ........................ ................................ ........ ..................... ..................... 28 LOW PRESSURE UREA SOLUTION SOLUTION HOLDER HOLDER ............ ....................... .................... ......... .................... ............................... ............. ....................... ................................ ......... .... 29 LOW PRESSURE CONDENSATIO CONDENSATION N .................... ................................ ............ ..................... ............................... ............ ....................... ................................ ......... ................. ................. 29 CARBAMATE CARBAMATE SOLUTION SOLUTION TANK ................. ............................. ............... ... .................... .............................. ............ .. ....................... ................................ ......... ..................... ....................... 30 LOW PRESSURE ABSORPTION AND LOW PRESSURE INERT............... INERT......................... ................. ....... ..................... ................................ ........... ............ ............ 31 WASHING WASHING .................... .............................. ............ .. ....................... ................................ ......... .................... ................................ ............ .................... .............................. ............ .. .................... .................... 31 VACUUM AND EVAPORATIO EVAPORATION N SECTION............... SECTION.......................... ................. ...... .................... .............................. ............ .. ........................ ................................ ........ ........ 32 FIRST VACUUM CONCENTRATI CONCENTRATION ON .................... ............................... ............. .................... .............................. ............ .. ........................ ................................ ........ ................ ................ 32 FIRST VACUUM SAPARATION SAPARATION ......................... ................................ ....... ..................... ............................... ............ ....................... ................................ ......... .................... ....................... ... 33 SECOND SECOND VACUUM CONCENTRATO CONCENTRATOR R .................. .............................. .............. .. .................... ............................... ............. ........................ ................................ ........ ............. ............. 33 SECOND SECOND VACUUM SEPARATOR ........................ ................................ ........ ..................... ............................... ............ ....................... ................................ ......... .................... .................... 34 FIRST VACUUM SYSTEM ................ .......................... ................ ...... ..................... ............................... ............ ........................ ................................ ........ .................... .............................. ............ 35 SECOND SECOND VACUUM SYSTEM ............................ ................................ .... ..................... ............................... ............ ........................ ................................ ........ .................... .......................... ...... 35 FINAL CONDENSATIO CONDENSATION N...................... ................................ .......... ..................... ............................... ............ ....................... ................................ ......... .................... ............................... ............. . 36 UREA MELT PUMPING.................... .............................. ............ .. ...................... ................................ .......... ........................ ................................ ........ .................... .............................. ............ .. .. 36 PRILLING SECTION.......... SECTION..................... ..................... ............ ...................... ................................ .......... ........................ ................................ ........ ..................... ............................... ............ ........ 37
MEDIUM MEDIUM PRESSURE UREA HOLDER HOLDER ..................... ................................ ........... ..................... ............................... ............ ....................... ................................ ......... .............. .............. 21 MEDIUM MEDIUM PREESURE CONDENSATION CONDENSATION..................... ................................ ........... ................... .............................. ............. .. ....................... ................................ ......... ........... ........... 21 MEDIUM MEDIUM PRESSURE ABSORBER ABSORBER ............ ...................... .................... .......... ..................... ............................... ............ ....................... ................................ ......... .................... .................... 22 ABSORPTION ABSORPTION PROCESS PROCESS ................. ........................... ............... ..... ..................... ............................... ............ ....................... ................................ ......... .................... ............................... ............. . 22 DESCRIPSITIO DESCRIPSITION N OF MEDIUM PRESSURE ABSORBER ........................ ................................ ........ ..................... ................................ ........... .................... ...................... .. 24 MEDIUM MEDIUM PRESSURE AMMONIA AMMONIA CONDENSAT CONDENSATION ION ........................ ................................ ........ ..................... ............................... ............ ....................... ......................... .. 24 AMMONIA AMMONIA RECEIVING RECEIVING .................... ............................... ............. ....................... ................................ ......... ....................... ................................ ......... .................... ............................... ............. . 25 MEDIUM MEDIUM PRESSURE ABSORBER AND INERT WASHING ................. ........................... ............... ..... ..................... ................................ ........... ................. ................. 26 LOW PRESSURE SECTION.......... SECTION.................... ...................... ............ .................... .............................. ............ .. ....................... ................................ ......... .................... .............................. .......... 27 LOW PRESSURE SEPARATOR ..................... ................................ ........... .................... .............................. ............ .. ........................ ................................ ........ ..................... ........................ ... 27 LOW PRESSURE DECOMPOSER DECOMPOSER..................... ................................ ........... .................... .............................. ............ .. ........................ ................................ ........ ..................... ..................... 28 LOW PRESSURE UREA SOLUTION SOLUTION HOLDER HOLDER ............ ....................... .................... ......... .................... ............................... ............. ....................... ................................ ......... .... 29 LOW PRESSURE CONDENSATIO CONDENSATION N .................... ................................ ............ ..................... ............................... ............ ....................... ................................ ......... ................. ................. 29 CARBAMATE CARBAMATE SOLUTION SOLUTION TANK ................. ............................. ............... ... .................... .............................. ............ .. ....................... ................................ ......... ..................... ....................... 30 LOW PRESSURE ABSORPTION AND LOW PRESSURE INERT............... INERT......................... ................. ....... ..................... ................................ ........... ............ ............ 31 WASHING WASHING .................... .............................. ............ .. ....................... ................................ ......... .................... ................................ ............ .................... .............................. ............ .. .................... .................... 31 VACUUM AND EVAPORATIO EVAPORATION N SECTION............... SECTION.......................... ................. ...... .................... .............................. ............ .. ........................ ................................ ........ ........ 32 FIRST VACUUM CONCENTRATI CONCENTRATION ON .................... ............................... ............. .................... .............................. ............ .. ........................ ................................ ........ ................ ................ 32 FIRST VACUUM SAPARATION SAPARATION ......................... ................................ ....... ..................... ............................... ............ ....................... ................................ ......... .................... ....................... ... 33 SECOND SECOND VACUUM CONCENTRATO CONCENTRATOR R .................. .............................. .............. .. .................... ............................... ............. ........................ ................................ ........ ............. ............. 33 SECOND SECOND VACUUM SEPARATOR ........................ ................................ ........ ..................... ............................... ............ ....................... ................................ ......... .................... .................... 34 FIRST VACUUM SYSTEM ................ .......................... ................ ...... ..................... ............................... ............ ........................ ................................ ........ .................... .............................. ............ 35 SECOND SECOND VACUUM SYSTEM ............................ ................................ .... ..................... ............................... ............ ........................ ................................ ........ .................... .......................... ...... 35 FINAL CONDENSATIO CONDENSATION N...................... ................................ .......... ..................... ............................... ............ ....................... ................................ ......... .................... ............................... ............. . 36 UREA MELT PUMPING.................... .............................. ............ .. ...................... ................................ .......... ........................ ................................ ........ .................... .............................. ............ .. .. 36 PRILLING SECTION.......... SECTION..................... ..................... ............ ...................... ................................ .......... ........................ ................................ ........ ..................... ............................... ............ ........ 37
nowledgement Ack nowledgement Urea production is based on Snam Progetti's self stripping process. CO2 enters the centrifugal compressor and leaves it at a pressure of about 160 atm. Liquid NH3 from NH3 plant is collected in Ammonia Receiver Tank. From here it is compressed to 25 atm pressure. Part of this ammonia is sent to Medium Pressure Absorber the remaining part enters the high pressure synthesis loop after it is compressed to pressure of 240 atm. Liquid ammonia and gaseous carbon dioxide react in Reactor at a pressure of 150 kg/cm2 and 188 deg C to form ammonium carbamate, a portion of which dehydrates to form urea and water. The reaction products leaving the reactor flow to th e steam heated falling film Stripper which operates at the same pressu re as the Reactor. Reactor. The unreacted carbamate gets stripped off as NH3 and CO2. Urea solution leaving the bottom of the Stripper still contains some amount of carbamate. Purification of urea takes place in the medium and low pressure sections operating at 18atm and 4.5atm pressure respectively. Decomposition of carbamate takes place in medium and low pressure decomposers. The concentration of solution leaving the low pressure section is 72% of urea. Vacuum concentra tors are provided to conce ntrate the solution to 99.8% in two stages operating at 0.3 and 0.03atm pressure respectively. Urea melt from the concentration section is pumped to the top of the prilling tower and sprayed by means of rotating prill bucket. The fine droplets, while descending through the tower, come into contact with cold air and solidify to form prills. Product urea from the bottom of Prill ing Tower is sent to storage or bagging. bagging. In the high pressure section gases leavin g the Stripper thoroughly mixed with the recovered carbamate from the medium pressure section and gets condensed to ammonium carbamate from the medium pressure section and gets condensed to ammonium carbamate in two carbamate condensers operating in series. The heat of reaction is utilised to generate 4.5atm steam. The carbamate thus formed is recycled back to Reactor. Ammonia and CO2 present in the decomposed gases of medium and low pressure sections are recovered in a series of condensers and absorbers. T races of gases present in the condensate from Vacuum Section are removed in a distillation tower. Waste water from the distillation tower is sent to effluent system. The recovered ammonia solution is recycled recycle d back to the the LP Section and ammonium carbamate f rom LP Section is recycled back to the MP Recovery Section.
INTRODUCTION OF URE A Urea synthesis is of historical importance, it is being the first organic compound to be synthesized from inorganic compound in laboratory. WH OLER in 1828 obtained urea from ammonium carbonate. Previous to this urea had been separated from urine and experiment from WHOLER showed that organic chemical can be separated from an inorganic chemical. Commercial production of urea was started in 1920 b y I.C. FORBEN in Germany based on Ammonium carbonate process. Since then Considerable ingenuity was us to overcome process difficulties such as: corrosion problems, recovery of off gases and economic process routes which resulted in present day develo pment. The various commercial urea manufacturing process of today use reaction between liquid ammonia and CO 2 gas to form carbamate and subsequent dehydration of carbamate to form urea and water. Ammonia and CO 2 are usually available from same site, as CO2 is a by product of ammonia plant using hydro carbon as feed stock. Four streams of urea plant having capacity to produce 1320X4mt/day of urea ferti lizer .The technology is based on SNAM PROGETTI, Italy. Liquid ammonia and gaseous carbon dioxide s made availabl e by ammonia plant and sent to reactor after compression and pumping .In reactor ammonia and carbon dio xide react to form carbamate which is further dehydrated to form urea. The output of the reactor is sent to the stripper where urea is further concentrated by removing an dehydrated carbamate using ammonia for stripping .Urea solution leaving the stri pper will contain some carbamate which is further purified in medium pressure se ction and low pressure sections. Vapour containing ammonia and carbon dioxide obtained from these vessels are converted to carbamate and recycled to reactor. Concentration of urea solution is important because increase in temperature encou rages biuret formation which in poisonous to crop, Therefore concentrated under vacuum concentrator. After concentration urea melt is pumped to urea prill tower where it is sprayed by rotating prill bucket and fine droplets of urea are solidified by means of natural air draft. At bottom of prill tower, urea prills are collected and sent to product handling plant by means of scraper and belt conveyor.
PHYSIC AL AND CHEMIC AL PROPERTIES Molecular weight
60.05
Boiling point
Decomposes at atmospheric Pressure before boiling 1.335 2531 578 cal/gm 70.1 2.16 cp 47 Kcal/kg 59.95 Kcal/kg 0.197 Kcal/kg
0
Density at 20 C Heat of combustion Heat of solution in water Critical humidity 0 Viscosity at 150 C Crystallisation heat Fusion heat Thermal conductivity
0
Specific heat: 0
At 20 C
cal/ gm/ C :
0.321
At 98.4 C
:
0.158
0
:
0.194
0
:
0.224
At 220 C
:
0.288
0
At 120.5 C At 160.3 C 0
Solubility in water by wt %: Temperature 0
20
40
60
80
100
120
Urea in gm per 1000 gm of water
105
163
246
396
725
2244
62
0
Urea is a white crystalline chemical product with m.p. of 132.6 C .It is readily soluble in water. On heating beyond its m.p. it decomposes giving CO 2, NH3 and other com0
plex compound of carbon, nitrogen and oxygen. At 160 C it decomposes to yield ammonia, biuret, and higher condensation product. The longer urea is held above its m.p. further reaction proceeds. Urea in fact is the diamide of carbonic acid with a chemical formula represented by:
USES OF URE A Main applications of urea are as follows: (1) As a fertilizer in agriculture. (2) As cattle feed. (3) As an industrial raw material for a number of industrial/household
product like urea formaldehyde, melamine and urea furfural. Due to high n itrogen content of urea the demand for fertilizer grade urea is rising rapidly. Urea today account for a large percentage of nitrogenous fertilizer. (4) Urea is used as cattle feed in western countries. Sheep and cattle are capable
of digesting urea up to about 40%. (5) Urea also finds extensive use in preparation of adhesive, textile, anti shrink
compound in exchange resin and as intermediate in preparation of pigments.
PROCESS TECHNOLOGY Urea is produced by synthesis of liquid ammonia and gaseous carbon dioxide. Ammonia and carbon dioxide react to form carbamate , a portion of which dehydrates to form urea and water. The fraction of carbamate that dehydrates is determined by the ratio of various reactions and various reactants, the operating pressure, temperature and residence time in reactor .The reaction of ammonia and carbon dioxide to produce urea takes place in two stages at elevated pressure and temperature.
2NH3 (l)
+
Liquid Ammonia
CO2 (g)
NH2 CONH4
Gaseous
o
+ 38.1 Kcal/gm mol C
Carbamate
Heat
Carbon dioxide
NH2COONH4
NH2CONH2
Ammonium Carbamate
urea
+
H2O
-7.1Kcal/gm water
heat
The first reaction is highly exothermic & therefore heat is liberated as rea ction occurs with excess NH 3. The CO 2 conversion to carbamate is almost 100%. Provided solution pressure is greater than the decomposition pressure. The decomposition pressure is the pressure at which carbamate will decomposes back in to CO 2 and NH3.
NH2COONH4 Urea
2NH3 Ammonia
+
CO2
Carbon dioxide
Decomposition pressure is the function of NH 3 concentrate in feed and the solution temperature and increased if either temperature or NH 3 recycle is increased. It is desirable if operating pressure is quite high enough to prevent carbamate from decomposition in to NH 3 and CO2. This will maximize CO 2 conversion to urea and towards reaction II. The second reaction is endothermic therefore heat is required for reaction to start. The heat of this comes from formation of carbamate. This reaction is function of
temperature and concentration in feed, the solution effluent from the reaction being a mixture of urea solution. Ammonia, water and carbon dioxide is extremely corrosive in nature. The subsequently stages of process consist of decomposition of u nconverted carbamate recovery of resulting ammonia and carbon dioxide for recycle concentration and prilling of urea solution.
ONCE THROUGH PROCESS This reaction is simple in operation, in this process ammonia and CO 2 are fed to autoclave and effluent solution is stripped of unconverted ammonia and carbon dioxide. The solution is processed further for concentration and for production of urea plant. The off gases concentrating ammonia and carbon dioxide are neutralised with acid for production of over ammonia fertilizer, because this process requires an inves tment for other plant to use more expensive ammonia. The capital cost of urea producing unit will go up. Although the initial cost of urea pant is low and conversion efficiency is high. The once through process is no longer an economic proposition. Besides the exceedingly huge demand of high nitrogen fertilizer has push ed the once through process in back ground.
P ARTI AL RECYCLE In this process the CO 2 from off gases is absorbed in mono ethanol amine. Excess ammonia is recovered by condensation and recycle whereas CO 2 is regenerated and rented. This process gives a better temperature control of reaction. But suffer from the defect that CO 2 is lost MEA used for CO 2 absorption get perpetually degraded.
TOT AL RECYCLE OR LIQUID C ARB AM ATE SOLUTION RECYCLE PROCESS:Today all conventional total re cycle process are much the same. All similar reactor 2
conditions, reactor pressure of about 200kg/cm and a temperature of about 185o
190 c. They maintain a NH 3 /CO2 mole ratio of about 4:1 in synthesis section. A co nversion of about 64% per pass is obtained. Reactor effluent solution is let down and decomposition of carbamate is carried out by heating with steam. The off gases are condensed and recycled as solution to rea ctor. The excess ammonia used is condensed and fed back to reactor Major variation however exists in recycle system in temperature and pressure levels. The various solution recycle processes endeavour-
To maintain heat recovery A> To minimize the amount of carbamate recycled and amount of water recycled back B> To minimize power requirement C> To maximize ammonia recovery D> To minimize investment Put together such amount to find a balance between consumption, maintenance and investment.
STRIPPING PROCESS Efforts to find addition driving force beyond the usual addition of heat. The stripping is introduced from bottom which travels upwards. Medium pressure steam condensing on shell side of stripper supplies heat for decomposition of carbamate. Currently per day consumption does not require injection of ammonia vapours at stripper bo ttom. Only higher molal ratio of NH 3/CO2 is maintained in reactor. Liberated in stri pper ammonia is adequate to carry out stripping efficiency since the reactor strip per condenser system operates at nearly same temperature. It is possible to feed recycle carbamate either by gravity flow which however necessitate the installation of carbamate condenses at which a much higher elevation so as to give the require heat or to use a recycle injector using a part of feed reactant as a motri fluid . Snam Progetti was first to develop the use of such recycle ejector and locate condenser at ground level. Since a portion of unconverted carbamate is decomposed and recycle to reactor at synthetic load on down stream decomposition and recycle section is comparatively much less resulting in lower utility consumption.Further since the sy nthetic pressure in stripping process in lower it is possible to employ, steam drives centrifugal CO2 compressor which has lower maintenance process. Low in pressure steam is generated in carbamate condenser which has resulted in lower consumption of steam and cooling water.
THEORY OF STRIPPING The theory of stripping in s based on Henrys law. The concentration of compound s of every component in solution which in equilibrium with vapour phase is directly proportional as l to partial pressure of component in vapour phase .By changing pa rtial pressure of component , concentration of solution can be changed. This can be illustrated as follows:
CA
:
concentration of component A in solution
CB
:
concentration of component B in solution
PA & P B
:
Partial pressure of component A & B vapour phase
P
:
Total partial pressure
In stripper we introduce excess NH 3 along with reactor effluent .This NH 3 is released as vapour by heating in stripper and this increases partial pressure. Pressure of a mmonia over the solution as total pressure remains same. Partial pres sure of CO 2 reduces to a lower value. In accordance with Henrys law carbamate decomposes to increase partial pressure of CO 2 in vapour phase to approach equilibrium concentr ation. Stripping action is thus carried out and heat of decomposition is supplied from an external source by condensation of medium pressure steam. Partial pressure of either of component can be changed and this will result in decomposition of ca rbamate.
EFFECT OF PROCESS V ARI ABLES Cabamate formation takes place with liberation of heat and urea formation takes place with absorption of heat. The former reaction is rapid and later is slow. The equilibrium conversion to urea will be favoured under following conditions: (1) Higher NH3 concentration (2) Less H2O concentration (3) Higher temperature (4) Higher pressure (5) Increased residence time
EFFECT OF H2O/CO2 R ATIO Water is by product of urea formation. One mole of water is formed when one mole of urea is produced. In presence of excess water, equilibrium reaction shift in reverse direction and yield of urea is poor. However has to add for recycling unconverted NH 3 and CO2 back to reactor. Lower the amount of water in the reactor, higher the yield of urea to low water concentration in low pressure recovery system and result is higher carbamate concentration and this causes pumping problem and clogging in piping system. Excess water in reactor also reduces effective volume for urea formation and additional energy is required to get rid of this. Study shows that presence of one mole of excess water per mole of carbamate equilibrium yiel d of urea to half.
EFFECT OF PRESSURE AND TEMPER ATURE As per Le Chatliers principle higher pressure favours carbamate formation. As the operating condition of carbamate formation is almost instan taneous and reaction tends to completion. Provided reaction heat is removed simultaneously. Lower temper ature favours carbamate formation being an exothermic reaction. In case of urea formation higher temperature is favourable because reaction is endothermic. The rea ction is such that when temperature increases the conversion increases. Maximum 0
equilibrium conversion is achieved at around 190 to 200 C. Reactant are highly corrosive at higher temperature. Operating pressure is totally dependent on temper ature at which conversion takes place in liquid phase. So equilibrium pressure increases when temperature rises.
EFFECT OF RESIDENCE TIME Urea conversion reaction is slow and takes place in 20 min to attain equilib rium considering slowness of reaction. Urea reactor is so designed that residence time should be more than 20 min. The higher residence time favours equilibrium conversion and normally reactor are designed for residence time of 30 min to 1 hour depending on other operating parameters. 68% urea conversion takes pla ce with residence time of 30 min at a mole ratio of 4:1 and temperature at 188C whereas to achieve 60% co nversion with a mole ratio of 2.8:1 at 181C, almost a residence time of 55 min is r equired.
Residence time in urea reactor plays an important part on equilibrium conversion where operating parameter including mole ratio are not favourable for a good yield. The higher residence time contribute to some extend to achieve a better yield. But this is done by providing higher reactor volume which increases capital investment.
BIURET IN URE A A problem facing by every urea manufactures is formation of biuret. In production process it is not a desirable substance at is toxic to plant. It should not exceed more than 1.5% in urea as fertilizer control order. When urea solution is heated in absence of free ammonia an objectionable ingredient called biuret is formed according to following reaction:
2NH2CONH2
NH2CONHCONH2
+
NH3
The formation of biuret is favoured by higher temperature, higher concentration of urea solution, lower ammonia content and higher residence time.
PROCESS DESCRIPTION FOR URE A SYNTHESIS The technology used is urea plant is SNAM PROGETTI from Italy. The whole process has been divided in four main sections: (1) High pressure section (2) Medium pressure section (3) Low pressure section (4) Prilling section
In high pressure section, reactor, high pressure stripper and carbamate condenser are main equipments used. In medium pressure section, medium pressure decomposer, medium pressure absorber, ammonia absorption tower and inert washing tower are the main equipments. In low pressure section, low pressure decomposer, low pressure absorber and carbamate solution tank. In prilling section urea melt is converted in prills by natural draft in prilling tower.
In prilling section it is prilled from top and granules of urea are obtained which is sent to bagging plant, water treatment plant which is required for environment reasons.
HIGH PRESSURE SECTION The liquid ammonia is pumped at high pressure through a ejector which drive the carbamate from carbamate separator in to reactor. CO 2 mixed with small quantity of air is compressed in a two stage CO 2 compressor and is also fed to reactor. The liquid ammonia and CO 2 react together here. The product formed in carbamate which dehydrates in reactor itself, to form urea and water. The oxygen in air forms a passive oxide layer on inside of vessel surface to prevent corrosion by carbamate and urea. The reaction products from reactor overflow to high pressure stripper where the unconverted carbamate is decomposed back into ammonia and carbon dioxide. Heat of decomposition is supplied by medium pressure steam a dmitted in stripper shell side. The condensate obtained is sent to high pressure decomposer shell side. Urea sol ution thus obtained flows out to medium pressure solution through level control valve. The vapour produced on decomposition in high pressure stripper enter high pressure carbamate condenser through a mixer along with carbamate solution from medium pressure section. Here they condense to form carbamate again and flow to high pre ssure carbamate separator.
CO2 COMPRESSOR CO2 is compressed in a twin case centrifugal compressor entering reactor. CO2 is r eo
ceived at battery limit rate of 40601 kg/hr at 1.4 atm and 40 C. Its moisture is recovered by a knock out drum. Air is added to outlet at rate of 382 kg/hr. The gas then enter compressor. The compressor consists of two casing low pressure casing and high pressure casing. Both casing consist of two stage with inter coolers. By successive compression and inter cooling. The moisture present in CO 2 is removed. The gas 0
is pressured from 1.4 atm to 160 atm and 130 C. It then flows to reactor at rate of 39722Kg/hr of dry CO 2.
COMPRESSION DET AILS Compression used in ammonia plant is a turbo driven compression. This compression casing is made of carbon steel. Compressor has two casing: 1. Low pressure casing 2. High pressure casing Both casing has two stages. Thus it is four stages turbo driven case CO 2 compressor. After each stage the temperature of casing increases due to compression. Thus after each stage inter stages cooler are used. These coolers are simply shell and tube type heat exchanger. The outlet CO 2 rate is 39722Kg/hr dry CO 2 + air, Pressure 0
160 atm. and temperature 130 C.
AMMONI A
RECOVERY
Ammonia for urea synthesis comes from ammonia plant at 23 atm . within plant battery limit. Total 30694Kg/hr ammonia first enters in ammonia recovery tower where 0
its temp raises up to 35 C counter current contact with vapour from receiver. Liquid ammonia is then stored in ammonia receiver. Ammonia from ammonia receiver is sent to NH3 booster pump at rate of 52058Kg/hr. A small amount of NH 3 4334kg/hr is sent to medium pressure pump, and remaining is sent to succession of NH 3 feed pump, which is a high pressure pump. Thus booster pump work to boost up succession pressure pump, Because discharge of high pressure ammonia pump is the only reciprocating pump in the ammonia pump. This pump is mo tion driven pump. This increases pressure for liquid ammonia from 25 atm. to 240 atm.
URE A SYNTHESIS Urea synthesis section that is the UREA SYNTHESIS REACTOR is the heart of urea plant, as urea is formed only in reactor. Remaining all section of plant is for concentration of urea. Liquid ammonia and gaseous CO 2 at high pressure enter the reactor bottom and react to form carbamate which further decomposes to form urea an d water. Ammonia at high pressure serves as motion fluid in ejector and drives carbamate from high pressure carbamate separator to reactor. Compositions of carbamate on mixing with NH 3 are:
NH3
70.39%
CO2
17.88%
H2O
11.73%
FLOW RATE
121393Kg/hr
CONSTRUCTION OF URE A RE ACTOR Urea reactor in urea plant is a tubular plug flow type reactor .this is a vertical reactor, made up of carbon steel. It is internal is lined with 7mm thick stainless steel. The feed inlet is of disperse type so hole are provided for both NH 3 and CO2 inlet.
Total number of holes for NH3 -> 350 Total number of holes for CO2
->
200
Total 10 number of sieve plates are provided over length of 22.5 m from top .These plates gives mixing to liquid and gas and avoid the back mixing in the reactor thus maintaining plug though reactor. Data of the plug reactor are:
Design pressure Operating pressure Height Shell thickness
2
170kg/cm 2 160kg/cm 40m 67mm
OVER FLOW FROM RE ACTOR The over flow from over flow line is taken out from reactor at a length of 2.5m from top. The over flow stream from reactor is sent to high pressure stripper from where carbamate is decomposed. The compositions of overflow stream are as follows:
NH3 CO2 UREA WATER FLOE RATE TEMPERATURE PRESSURE
AMMONI A
33.88% 13.34% 33.8% 18.98% 161115kg/hr Vary throughout the reactor 156 atm.
STRIPPING
In stripping process carbamate is decomposed with medium pressure steam. In presence of excess ammonia in a falling film type stripper, unconverted carbamate present in reactor outlet solution of urea is decomposed in stripper. 0
Urea solution from reactor at 156 atm. and 188 C enter the tube side of stripper operating a 147 atm. The high pressure stripper is of falling film type heat excha nger. The stripper tubes are provided with liquid dividers are called FERRULES. These ferrules have three equispaced on their periphery. The urea solution enter s at top tube channel and from a static liquid head over ferrules. This have drive urea solution through holes and in to tubes where a thin film is created on tube inner surface. The ferrules not only ensure uniform.
STRIPPING PROCESS In stripper introduce excess ammonia along with reactor effluent. The ammonia released as vapour by heating in stripper. This ammonia vapour i ncreases partial pressure of NH3 over the solution. as total pressure remain same, the partial pressure of CO2 reduces to the lower value in accordance with the HENRYS LAW carbamate de-
composes to increase the partial pressure of CO2 in vapour phase to approach equ ilibrium concentration. Stripping reaction is thus carried out heat of decomposition is supplied from an external source by condensation of medium pressure steam. Co ndensate from a stripper shall go out to a steam condensate separator and is send to a medium pressure decomposer shell, through level control valve. Stripped urea solution is collected in bottom of stripper and goes to medium pressure decomposer through a level control valve. The valve in stripper is important because the higher level increases the residence time of urea solution in stripper thereby increasing biuret content. Compositions of urea solution coming out of the stripper are as follows:
NH3 CO2 H2O UREA FLOW RATE
24.3% by wt 5.8% by wt 24% by wt 45.9% by wt 118627 kg/hr
The top end of stripper tube is highly susceptible to carbamate corrosion thus to protect it from exposure to high temperature, nitrogen is periodically injected to mai ntain nitrogen blanket in upper shell.
CONSTRUCTION OF STRIPPER The stripper is a falling film type heat exchanger. It has shell and tube type arrangements. Falling film is formed by using ferrules. Ferrules not only ensure uniform a rrangement though all tubes but also aids in continuous film on tubes inner surface. Thus no tube on any of its portion is starved of liquid thereby preventing it from getting overhead it from getting overhead and subsequent carbamate corrosion.
CONDENS ATION AND SEPER ATION The vapours from stripper are condensed in high pressure condenser. The carbamate thus formed is recycled back to reactor. Vapours from stripper obtained by decomposition of unconverted carbamate. These first enter the carbamate mixer at 147 0
atm. and 190 C. These vapours contain mainly of ammonia and carbon dioxide mixed with carbamate solution from high pressure carbamate pump (high speed centrifugal pump). They take suction from medium pressure carbamate pump. It also maintain s a minimum flow through high pressure carbamate pump by flow control valve in di s-
charge line. If carbamate flow to high pressure section decreases which maintain pump though put by recycling more carbamate to high pressure condenser. A very high condensate, flushing connection is provided on line going to kettle. The Comp osition is as follows:
NH3 CO2 H2O FLOW RATE
46.9% 20.1% 33.98% 35713 kg/hr
MEDIUM PRESSURE SECTION Urea solution from bottom of high pressure stripper now enters the medium pressure decomposer after pressure reduction through a level control valve. During e xperiment much of remaining cabamate flashes from NH 3 and CO2 vapour, there by concentrating urea in solution. This urea solution is further led down in pressure by level control valve and enters low pressure section. It consists of three parts- the top most part is medium pressure separator, the middle is medium pressure decomposer and bottom is medium pressure urea solution holder.
MEDIUM PRESSURE SEP AR ATOR The vapours from high pressure decomposer are condense in an medium pressure condenser using ammonium carbamate solution from low pressure section with tempered cooling water on tube side. The cabamate solution overflows from medium pressure condenser in to medium pressure absorber where excess ammonia, inert, a little amount of carbon dioxide separates to form vapour. These vapours are purified in top section absorber with reflux ammonia. Ammonia with i nert gases leaving top of medium pressure absorber are mostly condensed in ammonia condenser, with cooling water on tube side. From ammonia condenser both liquid and ga ses are sent to ammonia receiver, along with incoming liquid ammonia. The inert gases sa turated with ammonia, leaving the receiver enter the ammonia recovery tower. Here ammonia is further condenses with direct contact with cold ammonia from battery limit and flow down to ammonia receiver. The inert with resi dual ammonia from tower are sent to medium ammonia absorber where later gets absorbed in cool co ndensed and recycle to medium pressure absorber as ammonia water.
MEDIUM PRESSURE DECOMPOSER The urea solution coming out of medium pressure stripper and containing 45.9% by wt urea is further concentrate to 63.28% urea by wt. In medium pressure deco mposer from stripper is led down through it. There is a flushing connection and a sa mpling point. The reactor drain lines also meet the stripper outlet line. It is motor operated valve which act as a quick closing isolation valve of medium pressure section. 0
The urea solution from stripper at 147 atm. and 120 C is let down to 18 atm through level control valve and enter at top of medium pressu re separator which has the following composition by wt.:
NH3 CO2 UREA H2O FLOE RATE
24.3% 5.8% 45.9% 24% 118627kg/hr
As a result of pressure let down some solution flashing producing vapour of NH 3, CO2 and H2O. The heat of vaporization is taken from urea solution where the tem perature 0
0
falls from 210 C down to 140 CS. This solution is taken distributed over bed of rasching rings in medium pressure separator. The vapours rising from medium pressure decomposer come in to intimate contact with urea solution on raschig ring bed. Thus more carbamate decomposed by hot vapours. The vapour from medium pressure decomposer after imparting heat for carbamate decomposition in medium pres0
sure separator, flows out at rate of 37113kg/hr at 18 atm. and 145 C to medium pressure condenser. The composition of vapour is as follows by wt:
NH3 CO2 H2Os
73.36% 16.09% 10.55%
The solution thus enriched in urea flow down and is collected on top tube sheet of medium pressure decomposer. The decomposer is falling film type of heat exchanger. The tubes are tight fitted with ferrules which have four tangential holes on their periphery of size 4mm. The carbamate enter bottom of medium pressure decomposer shell and rises to top imparting sensible heat in solution. The car bamate thus decomposes liberating NH3 and CI2 vapours which rise up to packed bed in me-
dium pressure separator. The enriched urea solution falls down in medium pressure urea solution holder at bottom and collected there. 4532kg/hr of vapour and 473kg/hr inert from medium pressure carbamate separator enter medium pressure holder. These vapours have following composition by wt.:
NH3 CO2
97.97% 2.03%
MEDIUM PRESSURE URE A HOLDER These vapours enter the medium pressure urea holder above the liquid level. The i nert in vapour contain oxygen which acts as passivating agent in medium pressure sec0
tion. The medium pressure uses holder let out urea solution at 18 atm . and 156 C having following compositions by wt.:
NH3 CO2 UREA H2O
7.02% 1.16% 63.28% 28.54%
The pressure is then reduced from 18 atm. to 4.5 atm. And Solution then enters the low pressure decomposer, it then control the level in urea solution holder. Two level glasses are provided in medium pressure urea solution holder to monitor level physically.
MEDIUM PREESURE CONDENS ATION The vapours from medium pressure decomposer are condensed in medium pressure condenser with help of recycle carbonate solution from low pressure section. The vapour from medium pressure decomposer enter bottom of medium pressure co ndenser which operate at 18 atm. Before entering they are mixed with 14160 kg/hr of carbonate solution. This solution contains following composition by wt.:
NH3 CO2 H2O
61.38% 13.18% 25.44%
Carbonate from carbonate solution pumped by carbonate solution pump and enter 0
medium pressure condenser at 25atm and 40 C through sprayer. The cone sprayer helps proper mixing of carbonate solution with vapour. The sprayed liquid mixtures with vapours from medium pressure decomposer and enter condenser shell bottom 0
at rate of 51273kg/hr at 18atm and 130 C. The composition of mixture by wt is:
NH3 CO2 H2O
43.79% 8.52% 47.69%
The mixture enters the shell bottom of high pressure condenser. This is a shell and tube heat exchanger. The vapour s condense in liquid carbonate medium and top of shell.
MEDIUM PRESSURE ABSORBER Absorption in medium pressure section is carried out in medium pressure ab sorber. The vapours of ammonia, carbon dioxide and inert is in medium pressure condenser. Outlet solution are absorbed and rectified in medium pressure absorber so that v apours leaving contain only ammonia and inert. 51213kg/hr of liquid and vapour pressure leaving medium pressure condenser at 0
17.5 atm. and 80 C absorber. The medium pressure absorber operates at 17.5atm. It consists of two sections. The top section is called rectification section. It consists of four bubble cap Trays numbered from bottom to top. The bottom portion is called absorber.
ABSORPTION
PROCESS
Ammonia reflux is introduced on tray in rectification section. The liquid and vapour mixture from medium pressure condenser enter the absorber portion of medium pressure absorber. After entering it flows down to bottom medium pressure absorber and comes out of a sprayer. A liquid level is also maintained above sprayer. The vapour pressure of ammonia, carbon dioxide and water comes out of sprayer and get absorbed in liquid above it. This liquid level is made up of carbamate from medium pressure condenser. Pure NH 3 and NH3 solution from the rectification sec-
tion of medium pressure ammonia absorber. Ammonia and water present in solu tion help to absorb the CO 2 content of vapour and form carbamate. Thus CO 2 in vapour which rise to rectification section of medium pressure absorber is reduced to minimum. The carbamate solution in medium pressure absorber provides suction to high 0
pressure carbamate pump at 17.5atm and 72 C and has following composition by wt.:
NH3 CO2 H2O FLOW RATE
46.84% 20.5% 32.66% 35013 kg/hr
Level control in medium pressure absorber bottom is very important. Firstly be cause of it provide suction to high carbamate pump. Secondly if level is not sufficient, v apours from sprayer will be absorbed in solution to little extent. T his will increase the load on rectification section and enhance the change of CO 2 escaping to medium pressure absorber over head off gas line. Medium pressure absorber level is co ntrolled with high pressure condensate purge connection for both tapings. Flow from medium pressure condenser to medium pressure absorber is regulated and so in medium pressure absorber level. Five number of sight glasses are provided besides for physical verification. A high level in medium pressure absorber is dangerous because this may result in liquid may carry over in medium pressure absorber off gas. To safe guard against extraordinary high level an absorber drain is provided. This drain line is equipped with a control valve which let down to solution to carbonate solution tank in low pressure section. High and low pressure condensate flashing connections are provided on sides. Medium pressure absorber is also provided with a drain line to closed drain system with high pressure condensate connection. This is used for raining the vessel before undertaking any maintenance jib. The vapour s rising above the liquid level, before level medium pressure absorber m uch have only ammonia and inert. This is so because ammonia form vapours has to be condensed and returned to ammonia receiver. Presence of CO 2 will choke takes and heat e xchanger in these condensers will reduce. Also ammonia carbamate in the ammonium condenser will not only corrode them but the ammonia receiver as well. It will also cause pumping problem in ammonium booster pump by blo cking the section line from ammonia receiver. To remove possibility of CO 2 slip the vapours pass through the rectification zone. It consists of four bubble cap trays numbered from bottom to
top. The top tray receives pure reflux ammonia at 33c at rate of 4 334kg/hr. Tray number 3 receives 2247kg/hr of ammonia solution from medium pressure ammonia absorber. It has following composition by wt:-
NH3 63.75% H2O 34.25% The vapours come in intimate contact with ammonia solution from top. This solution absorbs the CO2 which react with ammonia to form carbonate. Temperature control is important in medium pressure absorber. Each tray is provided with a temperature tapping. Decrease in temperature profile from first tray indicates that all tray s are functioning properly. It implies that CO 2 is being progressively consumed to form car0
bamate. The overhead gas temperature should be minimum and around 42.5 C.
DESCRIPSITION OF MEDIUM PRESSURE ABSORBER The medium pressure absorber consists of four bubble trays. Each tray has 154 number of bubble caps. Vapour liquid mixture enter through an enter pipe which run down inside column and divided equally in to four perforated arms. These perforated pipes are submerged in carbamate solution, From where unabsorbed gas rises. The rising vapours pass through the distributor to the rectification section. A liquid seal of 50mm is maintined in each bubble tray fourth and third trays are feed with pure reflux and a queous NH3. Respectively solution which eliminated residual CO 2 and H2O from NH3 leaving medium pressure absorber. Flushing water th
connections are provided on 4 , 2
nd
st
and 1 tray for washing carbamate deposits over
the trays.
MEDIUM PRESSURE AMMONI A CONDENS ATION The ammonia in medium pressure absorber off gas is condensed in ammonia condenser. The ammonia condenser consists of two shells and the heat exchanger in series. Vapours enter shell side and cooling water enters shell side. Both ammonia condensers operate at 17.5atm. At this pressure it is possible to condense ammonia with 0
help of available cooling water at 36 C. The vapour from medium pressure absorber first enter ammonia condenser. Much of ammonia vapours condense here. The u ncondensed vapours along with condensed ammonia flow to second ammonia co ndenser. Here further condensation of ammonia take s place. Thus out of a total of
22841 kg/hr of pure ammonia and 473 kg/hr of inert entering ammonia condenser, 18332 kg/hr is condensed. The remaining 4519kg/hr of ammonia stays in vapour phase along with 473 kg/hr of inert. Both liquid ammonia and vapours from ammonia 0
condenser enter the ammonia receiver through separator line at 17.2atm and 140 C. Ammonia receiver can be isolated from by two isolation valves in liquid line and one valve in vapour line. Cooling water first enter condenser shell, then goes to ammonia condenser. Cooling water outlet from is required by control valve. A line is taken from the cooling water and outlet line. It is connected to cooling water circulation pump, discharge line of medium pressure condenser to supply cooling water when necessary. High pressure condenser flushing connection is provided in ammonia co ndenser to ammonia receiver.
AMMONI A
RECEIVING
Liquid ammonia from plant battery limit and recovered ammonia from ammonia condenser and medium pressure section is collected and stored in a ammonia receiver. Ammonia receiving system consists of a receiver portion and an ammonia r ecovery tower. The tower is installed on receiver as an integrated part. The receiver 0
0
holds ammonia at 17.2 atm and 35 C. Liquid ammonia at 23atm pressure and 12 C temperature comes to plant through isolation valves. It enters through a filter. The filter can be bye passed also. This first filter is connected to vent connections. Then the incoming liquid passes through flow totaliser. This send impulse to a control panel, this can be isolated sand bye passed. 30694 kg/hr of liquid ammonia enter the ammonia receiver through level control valve. A portion of this ammonia , enter the top of ammonia recovery tower. The tower is a packed column which operates at 17atm vapour consisting of amm onia and inert rise from the ammonia receiver and enter bottom of ammonia recovery tower. The vapour on rising come in intimate contact in packed bed with fresh liquid ammonia falling from top. The ammonia vapours thus condense giving away heat of condensation to liquid ammonia. The liquid ammonia then flows down to ammonia receiver. The vapours 0
from ammonia recovery tower containing 147kg/ hr of inert at 1.7atm and at 35 C flow to medium pressure ammonia absorber. The recovered liquid ammonia from
ammonia condenser flows down to ammonia receiver at a value o f 18322 kg/hr at 0
17.2atm and 49 C.
MEDIUM PRESSURE ABSORBER AND INERT W ASHING Vapours from top of ammonia recovery tower are scrubbed and washed in medium pressure ammonia absorber and inert gas washing tower. This equipment consists of an absorber portion. It is a single pass shell and tube fa lling film heat exchanger.An inert washing tower is mounted on its top. This tower is equipped with three trays. Cooled condensate is introduced on top trays of inert washing tower. Ammonia vapour and inert enter the bottom tube channel of m edium pressure absorber. Cooling water is circulated on its shell side. The vapour s form ammonia recovery tower enter bottom tube channel of medium pressure a bsorber consist of 1477 kg/hr of inert. Cooled condensate introduced in inert washing tower above comes on top tube. Shell tubes are fitted with ferrules. The cold co ndensate comes down through tangential holes of ferrules along the walls of tubes. It forms weak ammonia solution on absorbing ammonia vapours. This weak ammonia solution flows down the tubes. In this process it absorbs the ammonia vapours rising from bottom tube channel to medium pressure ammonia abso rber. The resultant heat of absorption is removed by cooling water on shell side. The ammonia solution thus form flow out of tubes and is collected in bottom of medium pressure ammonia absorber at rate of 2247kg/hr. This solution has following composition:
AMMONIA WATER
65.75% 34.25%
This is pumped by ammonia solution pump to medium pressure absorber provided in this line central level in medium pressure ammonia absorber bottom. The vapour from medium pressure absorber enter the bottom tray of inert washing tower condensate is cooled by cooling water in steam condensate. Cooler enter top tray of i nert washing tower. The residual ammonia Vapour come in intimate contact with cool condensate in each of three trays and get absorbed. Thus ammonia is complete washed from the vapour leaving only trays.
LOW
PRESSURE SECTION
The urea solution from medium pressure decomposer bottom enters the low pressure decomposer, after expansion through a level control valve. As a result of expa nsion most of remaining carbamate undergoes decomposition. Thus urea solution further concentrated and sent to vacuum section through a level control valve. The vapours enter the low pressure condenser shell and get absorbed in an aqueous carbmate solution from waste water solution. Low pressure condenser has cooling water on tube side. The liquid thus formed goes to carbamate solution tank from where it is recycled back to medium pressure condenser. The inert gases from tank containing ammonia is absorbed in cooled condensate in low pressure ammonia absorber, washed in inert washing tower and sent to vent stack. The liquid flows down to tank. The urea solution flowing out of medium pressure urea solution holder and havi ng 63.28% by wt. Urea is further purified by 71.12% by wt. Urea in low pressure decomposer. The low pressure decomposer consists of three parts:
The top part low pressure separator The middle is low pressure decomposer and Bottom is low pressure urea solution holder.
LOW
PRESSURE SEP AR ATOR
The urea solution from medium pressure solution holder at 18atm and 156c has following compositions:
NH3 CO2 UREA H2O FLOW RATE
7.02% 1.16% 63.28% 28.54% 86040 kg/hr
It is let down to 4.5atm through level control valve and enter s at low pressure separator. The reduction in pressure causes some of unconverted carbamate solution to
flash and generates vapours of NH 3, CO2 and H2O. The heat of vaporization is sup0
0
plied by solution itself, whose temperature consequently falls from 156 C to 128 C. The solution is then distributed over a bed of rasching ring in low pressure separator. The hot vapours from low pressure decomposer come in to intimate contact with urea solution on rasching ring bed. The heat exchange between rising vapourS and down going solution results in decomposition of carbamate still present in solution.
LOW
PRESSURE DECOMPOSER
The vapours from low pressure separator flow out to low pressure condenser. The composition of this vapour steam is as following:
NH3 CO2 H2O FLOW RATE INERT
49.15% 4.07% 46.77% 9480 kg/hr 52 kg/hr
The solution thus enriched in urea flow down and is collected on top tube shell of low pressure decomposer is a falling film type of heat exchanger. The tubes are filled with ferrules or liquid dividers. These ferrules have tangential holes on their periphery. The urea solution flow down in test tube of low pressure decomposer and form a level over ferrules. The liquid head thus formed drives urea solution through holes of ferrules along inner wall of tubes. In this manner thin film is created on inner surface of tube. 54kg/hr at 4.5atm saturated steam is supplied to shell side of low pressure decomposer. The urea solution film on tube inner surface take heat from 4.5atm steam and remaining carbamate in urea solution decomposes into NH 3 and CO2. These hot vapours then rise to packed bed in low pressure separator and carry on composition.
LOW
PRESSURE URE A SOLUTION HOLDER
The urea solution finally flows out of low pressure decomposer and is collected in low pressure urea solution holder. The urea solution contain following composition:-
NH3 CO2 UREA H2O FLOW RATE
1.8% by wt 0.8% by wt 71.2% by wt 26.28% by wt 70560 kg/hr
Level in low pressure urea solution holder is controlled and let down the urea sol u0
tion at 4.5atm & 138 C to evaporation section which operate under vacuum.
LOW
PRESSURE CONDENS ATION
Vapours from low pressure separator are condensed in low pressure condenser with help of recycle carbonate from distillation tower reflux pump. Low pressure condenser is a shell and tube type heat exchanger. Vapours and recycle carbonate enter shell side vapours at rate of 9486 kg/hr are mixed at first with recycle carbonate sol ution from distillation tower reflux pump. The composition of vapours by wt is as follows:
NH3 CO2 H2O INERT
49.15% by wt 4.07% by wt 46.77% by wt 52 kg/hr
This recycle carbonate line is provided with low pressure condensate flushing co nnection. The recycle carbonate has following composition by wt:
NH3 CO2 H2O
34.87% by wt 18.55% by wt 46.55% by wt
The recycled carbamate is distributed through a sprayer in low pressure separator to low pressure condenser. This ensures the proper mixing of vapour and liquid. Mi xture then enters the low pressure condenser shell at bottom and rise to top. T he vapours condense in liquid medium while rising. The heat of condensation is carried away by cooling water on tube side. Most of vapours get condensed in low pressure condenser. The uncondensed vapours, inert and carbamate together overflow out low pressure condenser. Air vapours and inert disengage from carbamate solution and flow out to arbamate solution tank from separator top. The carbamate solution flows down to carbamate solution tank from separator bottom. The carbamate solu0
tion at outlet of low pressure condenser at 4atm and 40 C has following composition by wt.:
NH3 CO2 H2O FLOW RATE INERT
44.61% by wt 8.68% By wt 46.7 by wt 13900 kg/hr 52 kg/hr
C ARB AM ATE SOLUTION T ANK The carbamate solution from low pressure condenser is finally collecting carbonate 0
solution tank at 4atm and 40 C. Carbamate solution from low pressure ammonia absorber and inert washing tower is also connected here. The carbonate solution tank stores carbonate solution having following composition:
NH3 CO2 H2O
43.29% by wt 8.52%by wt 37.69%by wt
A steam sprayer is provided at bottom of carbonate solution tank. Low pressure steam is introduced in sprayer whenever the carbonate solution temperature falls down especially during long shutdown. In this way carbonate is prevented from cry stallizing. The carbonate solution tank provides suction to medium pressure carbonate solution pump. The suction line is provided with a low pressure condensate flushing connection. A cooled condensate connection from steam condensate cooler is also
provided to suction line to build up level in carbonate solution tank whenever r equired.
LOW
PRESSURE ABSORPTION AND LOW PRESSURE INERT
W ASHING The vapours and inert from carbonate solution tank are absorbed in low pressure ammonia absorber and subsequently washed in low pressure inert washing tower before being vented out. Low pressure ammonia absorber is a shell and tube type Heat exchanger. Low pressure inert washing tower consisting of three trays are mounted directly mounted on carbonate solution tank. Cooled condensate from steam cooler flows on top tray of low pressure inert washing tower at following co nditions:-
FLOW RATE 260 kg/hr PRESSURE 31 atm. 0 TEMPERATURE 40 C The condensate flow rate is controlled which also reduces the pressure of that sy stem. This flow rate includes cold condensate flow suction line. Cooling water flows in a shell side of low pressure ammonia absorber from bottom to top. The vapour from carbonate solution tank enter bottom of low pressure ammonia absorber. Cooled condensate entering low pressure inert washing tower flows down to three trays. The inert rising from carbonate solution tank enter low pressure inert washing tower . The residual NH 3 and CO2 are washed off by the cooled condensate on trays forming a weak carbonate solution. The inert then go out at rate of 52 kg/hr. The weak ca rbonate solution then flow down into tubes of low pressure ammonia absorber from top through ferrules. The solution flow down in form of a thin film along walls. It comes in contact with vapour rising from carbonate solution tank. The NH 3 and CO2 vapours are absorbed in weak carbonate solution which then flows out and is co llected in a carbonate solution tank. The heat of absorption is disugrated to cooling water on shell side of low pressure ammonia absorber.
V ACUUM AND EV APOR ATION SECTION Concentration urea solution becomes 70.7% after low pressure decomposition reaction. This solution is further concentrated in vacuum system with a pressure of 3 atm. and 0.3atm. Concentration is done at vacuum concentration, it may be done with low temperature of system.
FIRST V ACUUM CONCENTR ATION Low pressure solution from low pressure urea holder having 71.10% by wt. Urea is concentrated at about 9.5% by wt. Urea in vacuum concentrator which operates at 0.3atm.Low pressure urea solution forming out from low pressure urea solution 0
holder is led down from 1.4atm and 139 C to 0.3atm through level control valve du ring this pressure reduction much of water in urea solution flashes. The heat of v aporisation of water is supplies low pressure urea solution itself, thereby reducing 0
0
temperature 138 C to 100 C. The low pressure urea solution them goes to bottom of st
1
vacuum concentrator through a three way valve. Low pressure condensate flus h-
ing and drain connection are provide in this line. A three way valve is provided in low pressure urea solution line from low pressure urea solution urea holder. This may be used to stop low pressure urea solution flow to first vacuum concentrator and divert it to urea solution tank in case of shutdown of vacuum evaporation section. A control valve is provided to regulate the flow of low pressure urea solution to urea solution tank. Line from 3 way valve is joined by a recovery line measure flow going in to I vacuum concentrator. It is a climbing film shell and tube type heat exchanger opera ting at 0.3atm. Low pressure solution enters the bottom channel of 1
st
vacuum con-
centrator and travel upward forming a film on inner surface of tubes. The low pre ssure saturated steam on shell side supplies heat to low pressure urea solution film in tubes. The water contained in solution start vaporising taking heat from steam. Finally urea solution comes out from top of first vacuum. Concentrator and flashes in to first vacuum separator.
FIRST V ACUUM S AP AR ATION st
st
Urea solution from 1 vacuum concentrator is separated from vapours in 1 vacuum separator. The urea solution coming out of 1
st
vacuum concentrator also contains va-
pours of water, ammonia and carbon dioxide. Urea solution enters tangentially into
NH3
7% by wt
CO2 3.11 by wt UREA 0.49 by wt H2O 89.41 by wt FLOW RATE 19515 kg/hr circular hood. This led to separation of vapours containing mostly water and little ammonia and carbon dioxide from urea solution. The urea solution after imping est
ment falls down to bottom of 1 vacuum separator by g ravity and vapour rise to top. The urea solution then flows out to 2
nd
0
vacuum concentrator at 0.3atm and 128 C. It
has following composition:
st
Three sight glasses are provided in bottom of 1 vacuum separator for visual inspection. The vapour contain following composition :
NH3 CO2 UREA H2O FLOW RATE
0.02%by wt 0.01%by wt 94.97%by wt 5%by wt 57255 kg/hr
SECOND V ACUUM CONCENTR ATOR st
First vacuum separator from vacuum 1 separator in concentrated from 94.97% by wt urea to 99.7% by wt in second vacuum concentrator which operates at 0.03atm. 0
first vacuum urea solution at 0.3atm and 128 C from first separator flows out to second vacuum concentrator which operate at 0.03atm, thereby creating pressure gradient. Second vacuum concentrator is a climbing film shell and tube type exchanger. First vacuum urea solution flows in tube side and low pressure saturated steam in shell side. First vacuum urea solution enter bottom of second vacuum concentrator tube channel and climbs up forming a film in tube inner surface. The remaining w a-
ter, ammonia and carbon dioxide in first vacuum urea solution vaporize at this low pressure at 0.03atm. During its upward climb the heat of vaporization is supplied by low pressure saturated steam on shell side. Thus first vacuum urea solution get con0
centrated to 99.7 % by wt Urea and flows out to second vacuum separator at 140 C. The lines from first vacuum separator to second vacuum separator form a lute. This lute is provided with a drain connection. Low pressure condensate and low pressure steam flushing connection are also provided to lute to clear this line if it get s check with urea.
SECOND V ACUUM SEP AR ATOR The urea melt from second vacuum concentrator flashes into second vacuum separator where it is separated from water, ammonia and CO 2 vapours. The solution va pour mixtures the top of separator tangentially which facilitates the separation of two phases. There is circular hood all around. The urea melt after separation falls down 0
by gravity into a holder at 0.03atm and 140 C. It has following composition by wt.:
UREA WATER FLOW RATE
99.7% 0.3% 54350 kg/hr
Level control in holder is very important as it provided suction on melt urea pump. As holder is under a low pressure of 0.03atm at a minimum liquid head must be mai ntained above suction pump to provide necessary NPSH. This is ensured by maintai n0
ing level in holder. A high level will increase res idence time of urea melt at 140 C and thus more biuret will be formed. The vapour separated from melt urea rise to top at 0
0.03atm and 140 C. It has following composition-
NH3 CO2 UREA H2O
0.41% by wt 0.2% by wt 6.51% by wt 92.85% by wt
A low pressure flushing condensate is provided to top dome of second vacuum sep arator with a spray nozzle for better flushing. The vapour outlet nozzle from second vacuum separator is provided with a washing tore with holes. Low pressure condensate is provided to washing tower.
FIRST V ACUUM SYSTEM First vacuum separator controls the vacuum pressure in first vacuum concentrator and first vacuum separator. Water, ammonia and carbon dioxide vapour from first 0
vacuum separator at 0.3atm and 130 C flow to shell side of first condenser. This is a horizontal shell and tube heat exchanger. Cooling water from outlet of first co ndenser flows in to tube side of first condenser. Low pressure condensate flushing connections are provided at several points on shell side of first condenser. The above maintained vapours get condensed here to quite an extent. The condensate then flow down to waste water tank. The uncondensed vapours are ejected out to second condenser. These vapours enter shell side of second condenser where they condense with the help of cooling water in tube side. The operating pressure in second condenser is higher than first condenser.
SECOND V ACUUM SYSTEM Second vacuum system controls the vacuum pressure in second concentrator. The vapour from second vacuum separator at 0.03atm and are first boosted by steam jet ejector to a higher pressure. It is required to boost the pressure here as it is difficult to condense the vapour at low pressure of 0.03atm with help of cooling water at available temperature. Low pressure stea m is used as motive fluid in steam jet ejector. The vapours are discharged to shell side of condenser. Cooling water flows in tube side and help in condensing the vapour. The condensate flows out of the waste water tank. Low pressure condensate flushing connections are provided t several point on shell side along the length of first condenser to remove urea deposits. These deposits reduce heat transfer. The uncondensed vapours from first condenser are driven to second condenser by first stage ejector. Low pressure steam acts as motive fluid. The vapour and steam enter the shell side of second condenser. Cooling water flowing on tube side condense these vapour. The condensate flows down to waste water tank. The uncondensed vapours from second condenser are ej ected out to final condenser by second stage ejector.
FIN AL CONDENS ATION The uncondensed vapours from first vacuum system and second vacuum system are finally ejected to final condenser which operates at slightly higher pressure than atmospheric pressure. The vapour enters the shell side. The cooling water flowing on tube side condense the vapours. The condensate flows down the waste water tank. The non condensed vapour flows out to pot where entrained liquid separate s and flows out. The non condensable are verged out to atmosphere from pots top.
URE A MELT PUMPING Urea melt from holder having 99.7% by wt. Urea is pumped by melted urea pump. Melted urea pumps are centrifugal pump located sufficiently below the holder so as to provide the required NPSH. Holder collects urea melt from second vacuum separ ator and provides suction to urea melt pump. The common suction line from holder to melted
urea pump is steam jacketed so as to any possibility of crystallisation of urea
melt. This line is then divided into branches with separate isolation valves to provide suction to both melted urea pump. Low pressure steam and connection are provided to each of suction lines.
UREA H2O
99.7% by wt. 0.3 % by wt.
The urea melt is then pumped to a pressure of 15 atm and discharged to the top of prilling tower. The discharged line from each pump is provided with a pressure condensate flushing connection. The common discharge line is steam jacketed is pr ovided in discharged line which controls the level in holder.
PRILLING SECTION Melt urea from the discharged of melted urea pump is sent to top of prilling tower where urea prills are made. Urea melt enter the prilling section from both urea units 11 and 21 through steam jacketed lines. Before entering the prilling bucket both these urea melt lines join t ogether. Two three way valves are provided individually on both the lines. These three way valves will normally allow urea melt to flow to prilling bucket. But in case when prilling has to be diverted, these ways valve will divert the urea melt to urea solution tank. Both these three ways valves are operated by manual switches. As long as prilling is being done the urea melt return lines to urea solution tank . Which are continuously kept flushed with washing stream. At that time washing stream to line from prilling bucket stand close. It will open when urea melt is diverted to urea solution tank. Prilling tower is natural draft type of tower. Melt urea at rate of 54350kg/hr from 0
each unit and at about 15 atm and 149 C is pumped into a prilling bucket at the top of prilling tower. The prilling bucket is housed in the ceiling of prilling tower. Only one prilling bucket is kept in line and other is kept spare. Pri lling bucket is driven by an electric motor. Speed of prilling bucket can be varied to control prill stze. It is i ncreased to reduce the size. A high temperature switch is provided at top of prilling 0
bucket which will sound an alarm at 130 C. This in turn will imply that level in prilling bucket is going high. Thus its speed must be increased to avoid overflow. Melt urea comes out of holes of rotating prilling bucket in form of fine droplets. These drop lets are distributed uniformly throughout the cross section of prilling tower by centrifugal force imparted by rotation of prilling bucket. These droplets then start falling down the height of prilling tower. Ambient air enter s the bottom of prilling tower through louvers and rise upwards. During its rise the air comes in contact with urea melt 0
0
droplets. Thus the urea melt droplets at 149 C first cool down to 132.7 C where they 0
solidify into prills. On travelling down further they are sub cooled to about 60 C. The heat generated during urea solidification and c ooling is dissipated to incoming a mbient air. In this process the air get heated becomes lighter a nd thus rises upwards creating natural draft. The solidified prills falls on rake floor at bottom of prilling tower. They are then separated by a rotating scrapper though a slit onto prilling tower belt conveyor. From conveyor the urea prills falls on the lump separator. The
