SULPHUR RECOVERY UNIT Unit No. 29
By: Nishant Baranawal
TOPICS OF DISCUSSION 1 .Introduction to SRU
Purpose Commercial Processes
2. SRU at BORL
Sulphur Recovery Block Flow Description for SRU Process Chemistry
SRU TGTU
Process Variables
TOPICS OF DISCUSSION 1 .Introduction to SRU
Purpose Commercial Processes
2. SRU at BORL
Sulphur Recovery Block Flow Description for SRU Process Chemistry
SRU TGTU
Process Variables
Purpose of SRU Overhead gases from SWS I , SWS II and ARU contain H2S &
NH3 H2S can’t be disposed off to atmosphere atm osphere
Extremely dangerous, irritant at higher conc. Combustion gives gives SO2 which has a dispoasal limit limit (CPCB standard -80 microgm/m3 of air)
H2S
can be treated to produce Sulphur NH3 also has a disposable disposable limit of 100microgm/m3 of air(CPCB standard) NH3 is burnt in SRU to N2
Commercial Processes Claus
process - widely accepted for Sulphur recovery Low recovery – Approx. 70% Process was upgraded to increase recovery upto ~99% Two prevailing commercial methods –
Maximum Claus Recovery Concept Claus Section with extension to Tail gas treatment unit
Sulphur Recovery Unit At Bharat Oman Refineries Ltd.
Introduction to SRB Sulphur
R ecovery Block contains following unit Sour W ater Stripper – I Sour W ater Stripper – II A mine R egeneration Unit Sulphur R ecovery Unit Tail Gas Treating Unit
SRB Block Flow Diagram FLUE GAS (<10ppmH2S)
H2S RICH GAS 475.6 kg/hr NH3 RICH GAS 1197 kg/hr
S.W.EX CDU S.W.ex DCU S.W.ex ARU Non Hydroprocessing S.W.ex TGTU Design : 125 T/hr
TAIL GAS
FUEL GAS
SWS I
STRIPPED WATER (< 50 ppm H2S ) SOUR water Ex HCU/DHT
H CI R S H
S A G
SWS II
SOUR water Ex NHT
Hydroprocessing Design : 48 T/hr STRIPPED WATER (< 50 ppm H 2S )
TGTU
H2S
SOUR GAS 90*C r
2
inci
h/ g k 0 7 6 1
RICH AMINE FG & LPG ATU
A G DI C A
S
HCU H2S 481 ppm
C * 0 4 h/ t .5 4 1
CONDENSATE SWS
ARU 470 T/hr
LEAN AMINE OSBL 45*C ( <10 ppm H2S) 450 T/hr
SRU 2 Trains 2X180 TPD
H2S 2.5 ppm
B/D LIQ. SULFUR 2X180 TPD
Sulphur Properties Its
melting point is 120oC so liq. S temp. in pipeline should be above 120oC Sulphur becomes very viscous at temp. above 155oC; flow will be hampered Auto ignition temp. is 184 oC Sulphur fires generate SO2
SRU at BORL Major Outlines Based
on the TGTU technology developed by M/s ENGINEERS INDIA LIMITED along with SINI, Italy Punj Loyd has taken contract on LSTK basis Capacity – 2*180 MT/day Combination of Claus process and the extension of Tail Gas Treatment Unit Feed gas is a mix of acid gas from ARU and sour gas from SWS I & II SRU consists of one thermal reactor (Main Combustion Chamber) and two catalytic Claus converters
Continued… Claus
off gas which contains H2S, SO2 and traces amount of COS and CS2, is subjected for hydrogenation reaction in TGTU H2S formed is recovered by amine absorption Tail gas is incinerated Unit has storage, weighing and loading facilities for liq. S at site itself
Block Diagram For SRU
Claus Section
Incinerator Section
Sulphur Storage and Loading
Process Chemistry for SRU Main Combustion Chamber Combined stream of acid gas & sour gas is fed to the burner of Main Combustion Chamber Following reactions are occuring H2S + 3/2 O2 SO2 + H2O + Heat (1) 2H2S + SO2 2NH3 + 3/2 O2
2H2O+ 3/n Sn + Heat 3H2O + N2
(2) (3)
H2S H2 + 0.5S (4) Controlled combustion of H2S takes place(eq 1) O2 is limiting and air flow is controlled by an Air Demand Analyser at downstream of Claus converter II Eq 2 is the ‘CLAUS REACTION’ and is slightly endothermic at MCC temp. and exothermic at Converter temp.
Claus Reaction Kinetics
Continued.. H2S
Dissociation – 6-7 % of total H2S feed Sulphur formed takes different allotropic forms 3S2 S6 + Heat 4S2 S8 + Heat Some
other reactions that are occuring CH4 + 2O2 CO2 + 2 H2O (6) CO2 + H2S COS + H2O (7) COS + H2S 2H2S
Presence
2H2
CS2
+ H2O (8)
+ S2
(9)
of hydrocarbons in feed also increases COS & CS 2 formation with increase in air requirement
Continued… NH3
combustion requires higher temperature MCC Adiabatic flame temp. – 1350 – 1450 0C At higher NH3 conc., formation of solid nitrogenous salts such as ammonia sulfate and sulfides which get deposited in the downstream units To ensure complete NH3 destruction, acid gas is burnt in two zones which maintains flame temp. above 1350 0C
Continued… CLAUS CONVERTERS Less temperature favors claus reaction in the catalytic converters Converter temp. should be above S dew point Small quantities of CS 2 and COS are formed There
destruction improves S recovery 1st converter temp is higher(around 300*C) to ensure maximum destruction COS + H2O H2S + CO2 CS2 + 2H2O 2nd
2H2S + CO2
Converter temp. subsequently is lower
Continued… Liquid Sulphur Degassing Liquid sulfur absorbs hydrogen sulfide and forms hydrogen polysulfudes H2S + (x-1)S ↔ H2Sx Solubility
of H2S - H2Sx in liquid sulfur depends on the partial pressure of H2S and temperature of liquid sulfur
Increase
in temp. favors H2S in liq. S Total H2S is in the range of 540ppmw to 590ppmw within temp. range of 125 – 140 0C Liq. Sulphur goes to pit and H 2S above S is swept with steam ejectors
Continued… Process
is further accentuated by mechanical agitation and presence of catalyst like CATDEGAS H2S levels of < 10 ppmw can be achieved Moreover, sweeping ensures that H2S content is below 3.7%(LFL)
Process Chemistry For TGTU Remaining
S in Claus off-gases is recovered as H2S Tail gas at min. temperature of 280oC enters Hydrogenator where H2S production takes place as SO2 + 3H2 H2S + 2H2O S + H2 H2S CO + H2O H2 + CO2 COS + H2O H2S + CO2 CS2 + 2H2O 2H2S + CO2 COS + 4H2 H2S + CH4 + H2O CS2 + 4H2 2H2S + CH4
Continued … All
reactions are exothermic Hot gases are cooled in Waste Heat Boiler and sent to water removal tower MDEA solvent selectively absorbs H2S H2S + (C2H5)2CH3N (C2H5)2CH3NH+ + HSH2S is stripped off in regenerator and sent to Claus train Tail gas from absorber is incinerated alongwith NH3
Stream Data for BORL Component
Acid Gas
Kmol/hr(%) H2S
404.75 + 15.36 (ARU) (TGTU) (91.1)
Sour Gas from H2S Rich Gas NH3 Rich Gas SWS I from SWS II from SWS II
Kmol/hr(%)
Kmol/hr(%)
Kmol/hr(%)
7.22 (35.22)
46.43 (90.51)
1.34 (1.98)
CO2
0.89 (0.2)
-
-
NH3
0.67 (0.14)
7.17 (35.19)
0.05 (0.097)
45.28 (66.88)
H2O
38.29 (8.3)
6.15 (30)
4.86 (9.47)
21.03 (31.12)
HC as C1,C2
0.89 (0.2)
-
-
Total flow
460.90
20.50
51.30
-
67.7
Continued… Component
Process Gas from MCC Train A kmol/hr(%)
Tail Gas from both trains to TGTU Kmol/hr(%)
Tail Gas to incinerator kmol/hr(%)
H2S
46.21 (4.95)
10.1 (0.606)
0.20 (0.02)
CO2
0.85 (0.09)
1.74 (0.104 )
1.97 (0.18)
H2
28.11 (3.01)
56.22 (3.37)
41.75 (3.82)
CO
0.27 (0.03)
0.54 (0.033)
0.05 (.005)
H2O
268.4 (28.72)
619.18 (37.13)
72 (6.6)
N2
487.36 (52.16)
974.72 (58.45)
974 (89.31)
COS
0.015 (0.002)
0.00
-
SO2
22.99 (2.46)
4.82 (0.29)
-
S2
78.11 (8.36)
0.26 (0.02)
-
S6
2.02 (0.216)
-
-
Operating Conditions Feed Stream Variation Feed
to SRU includes acid gas, H2s rich gas & NH3 rich gas 1 – Load Variation Acid Gas from ARU is major feed and there is a turndown for it Sour gas from SWS may vary from 0-100% Increase in NH3 concentration requires more O2 Formation of solid salts like Ammonium sulphate, Ammonium bisulphate,Ammonium sulphide is possible at low temperature
2 – Composition Variation Hydrocarbons
Increased hydrocarbon increases O2 consumption Formation of CS2, CO, COS Carbon formation over Claus catalyst also occurs
Continued… Steam
Diluting agent Causes flame instability
Entrained Liquid
Flame stability affected Corrosion of Acid Gas lines
3 – Pressure & Temperature
Increased temp. of feed indicates more steam content High Pr. of feed may lead acid flare routing of gases
Process Variables 1 - Main Combustion Chamber Pressure
fluctuation can be due to feed variation High Pressure leads to high Temp. due to more feed Quench steam is used for temp. above 1450 oC For low temp. below 1300 oC, NH3 is routed to incinerator High Pr. due to blockage of a downstream unit will route the feed gases to acid flare Further, there is provision of routing process gases from condenser I to incinerator
Continued… 2- Air/Acid Gas Ratio Air
is required to burn 1/3rd of total H2S, all NH3 and HCs for efficient plant operation For monitoring air flow, Air Demand Analyser is located at downstream of Condenser III It checks for H2S/SO2 ratio, which must be at or above a min. value Below this ratio, there is excess air, more SO2 formation which may lead to catalyst Sulphation Chances of SO3 formation and hence, corrosion
Continued… 3 – Claus Converters Inlet temp. of process gas to Claus converter is crucial for S recovery 1st Claus converter temp. is around 300 oC to ensure COS, CS2 destruction and better rate of reaction Increased temp. of converters decreases S recovery Temp. below dew point of S may lead to S deposition over catalyst, so that should be avoided
Continued… 4 - Incinerator Fuel
gas firing maintains the temp. O2 content in outlet flue gas should be 2 % mol (wet basis) At high temp., incinerator has a trip Tail gases however are sent to atmosphere for some minutes to restore temp.
Comparison With BPCL Unit Parameter
BPCL RMP SRU
BORL SRU
Capacity
2*70 T/day
2*180 T/day
Process Involved
MCRC sub dew point process
Claus with TGTU
Recovery (design basis)
99.5 %
99.9%
