Sulphur Recovery Unit amd it's working procedure at ONGC

The Sru (Surface Recovery Unit) at ONGC plays a critical role in the upstream oil and gas production process, specifically in the treatment and recovery of hydrocarbons from production streams. It is designed to handle the mixture of oil, gas, water, and other impurities extracted from underground reservoirs. The Sru system separates and processes these components, allowing for the efficient recovery of oil and gas, while also managing water and other by-products.

At ONGC, the Sru units are essential in ensuring that the oil and gas meet market standards for purity and are safe for transport. The recovered gas and oil are sent for further processing or transportation, while the water is either re-injected into reservoirs or disposed of following environmental guidelines.

Sru facilities are equipped with advanced technologies to maximize recovery efficiency and reduce emissions, aligning with ONGC's commitment to sustainable and responsible energy production. These units are closely monitored and maintained to ensure their safety, reliability, and environmental compliance. In the larger context, Sru systems help ONGC optimize resource utilization, reduce wastage, and enhance operational efficiency across its fields, contributing significantly to India's energy security.

1. Avinash Chaudhari EE(P) Sulfur Recovery Unit

2. GSU-SRU OVERVIEW SRU ( Locat solution) GSU MDEA Regeneration LEAN MDEA SOUR GAS SWEET GAS ACID GAS VENT GAS CHEMICALS AIR Sulphur slurry (5-15% V/V) to PFS RICH MDEA

3. Salient Features 6 SRU Trains were commissioned in three Phases: Phase - I (Tr.61. 62 , 63) ---- 1988 (Revamped in Jan’22) Phase - II (Tr. 64, 65 ) ---- 1992 Phase - III (Tr. 66 ) ---- 1996 Design PPM - 3000 to 52000 PPM (0.3-5.2%) H2S in Acid Gas Design capacity- - 7000 nm3/ hr /train. No of trains* - 06+01 Incinerator *Tr.65(March’22) and Incinerator(Nov’22) not in operation by order of HO. Prevailing Sulphur Production- 06-07 MT per day.

4. Content Reaction Chemistry Chemical required for process Parameters of LOCAT and range of operability Absorber Column Internals Gaseous effluent monitoring Liquid Effluent Continuous Emission Monitoring System(CEMS) Incinerator Pressure Filtration System(PFS) Major issues in SRU Contract Details

5. REACTION CHEMISTRY Acid Gas dissolves in water as per the reaction in absorber section of column. OH - + H 2 S ⇌ H 2 O + HS - OH - + HS - ⇌ H 2 O + S 2- (aq.) S 2- (aq.) ion gets converted into elemental sulphur by reacting with Fe 3+ ion in LOCAT in exothermic reaction. S 2- (aq.)+ 2Fe 3+ S + 2Fe 2+ (Reduction) Regeneration of metal ions occurs in annulus part of the absorber column by air. O 2 + H 2 O + 2Fe 2+ ⇌ 2(OH) - + 2Fe 3+ (Oxidation) H 2 S (Gas) + O 2 (Gas) S (Solid) + H 2 O (Liquid) (Overall reaction)  

6. CHEMICALS REQUIRED Chemical Function Source Availability Approx. Weekly Consumption KOH To increase the pH of LOCAT solution(7.5-8.5) Open market In 25 kg bags. (Solution is prepared in house) 11 MT 310-M (G202C) To prevent Precipitation of metal ions GNFC In barrels 3000 KG 310-C (G101N) Active metal ions GNFC In barrels (18000ppm) 3000 KG *Surfactant (G302S) To help in settling of free sulphur GNFC In barrels 105L *Defoamer To prevent foaming Open market In barrels 35L *Biocide (ARI400) To prevent bacterial growth in Locat solution ARI In barrels 50 L

7. LOCAT PARAMETERS Important parameters of LOCAT that need to be maintained on daily basis. pH Total Iron content Chelon Oxidation Reduction Potential(ORP) Alkalinity Thiosulphate Specific gravity Air flow LOCAT Temperature

8. pH of LOCAT Function: To make sure of dissolution of acid gas in LOCAT solution. Normal Range: 8.0-8.5 Reason for deviation from normal value: Formation of Carbonic acid due to presence of CO 2 in acid gas lowers the pH of LOCAT solution. CO 2 + H 2 O ⇌ H 2 CO 3 and Formation of thiosulphate ion 2 HS - + 2O 2 S 2 O 3 -- + H 2 O Effect of Deviation: Low pH prevents the dissolution of acid gas in LOCAT solution and also reduces catalyst life by biological activities. Very high pH favours the side reactions like formation of thiosulphate ions. Note: KOH dosing is directly related to acid gas intake in absorber column. LOCAT chemical analysis is done on regular basis, so if any train made HSB in between or gas feed varied then KOH dosing need to be changed accordingly.

9. H 2 S DISSOCIATION IN LOCAT H 2 S ⇌ H + + HS -

10. Total Fe(310-C) in LOCAT Function: Active ingredient for formation of elemental sulphur. S 2- + 2Fe 3+ S + 2Fe 2+ (Reduction) Normal Range: 400-500 mg/l Reason for deviation from normal value: Due to precipitation/loss of LOCAT solution. Effect of Deviation: Less formation of elemental sulphur due to less Fe metal ions(Fe 3+ ). Thus resulting in high vent ppm.

11. Chelon (310-M) in LOCAT Function: It increases the solubility of Fe ions in water and prevent undesirable precipitation of Fe ions. Normal Range: 100% of Fe (mg/l) Reason for deviation from normal value: Small amount of chelon destroyed by oxidation/biological degradation. Effect of Deviation: Less Chelon in LOCAT solution results in precipitation of Fe metals ions and thus prevents the reaction of H 2 S with LOCAT.

12. Oxidation Reduction Potential(ORP) Function: It is indirect measure of ratio of Fe 3+ to Fe 2+ in LOCAT solution. Normal Range: -100 to -250 mV Reason for deviation from normal value: Poor regeneration of Fe 3+ metal ions due to insufficient air flow. Poor absorption of acid gas. Effect of Deviation: Low ORP indicates poor absorption of acid gas.

13. Oxidation Reduction Potential(ORP) ORP(mV) +50 to -50 -50 to -250 -250 to -325 -325 to -400 LOCAT status Fully oxidized(initial fill) Preferred Process range Slightly over reduced Highly over reduced

14. Thiosulphate and Specific Gravity Function: Helps in initialization and catalysing. Normal Range of Thiosulphate: 12.5-250 g/kg Normal Range of Specific gravity: 1.00-1.35 Reason for deviation from normal value: It increases over the period of time. Loss due to drainage of slurry. Effect of Deviation: High amount of thiosulphate results in increase in specific gravity of LOCAT solution and hence can affect the dissolution of acid gas. Note: No extra dosing of thiosulphate is required as it produce with the help of side reaction on continuous basis. For the fresh LOCAT preparation, a small amount of thiosulphate is added.

15. Compressed Air and LOCAT Temperature The o xidizer air rate should be at least 1.3 times of acid gas. Operating at a low air flow will result in over reduction of chemical . Running with high air flow has no adverse effect except for a possible increase in thiosulphate production. The LOCAT solution temperature in absorber should be kept between 16 C to 50 C. At high temperature, thiosulphate production is more in LOCAT. At lower temperature than acid Gas inlet temperature, hydrocarbon in acid gas will condense and may cause foaming.

16. LOCAT Chemical analysis

17. Absorber Column Internals

18. Absorber Column Internals

19. Absorber Column Internals(Scrapper) Scrapper(Torque Set Point=12.50 Nm, rpm= 0.54): To prevent bridging of sulphur inside walls of conical section.

20. Absorber Column Internals Air Blasting: Provide pulsating air jet to prevent bridging of sulphur in lower section of absorber column.

21. SRU AND PFS OVERVIEW

22. MDEA and Effluent Drainage MDEA: Small amount of MDEA carried over with DM water, from GSU, collected in inlet KOD of SRU and then to SRU Sump. With the help of Sump pump, it can be lined up back to GSU. There are three Liquid Effluent streams in SRU. a) Storm Water: Goes to open storm water channel. b) Plant Floor Wash (Phase-2): Contains some LOCAT chemicals and sulphur particles. It goes to NETP. c) Contaminate Rain Water Sewer(CRWS)(Phase-1): Can be transferred to NETP with Phase 2 floor wash or through OWS to WWTP. d) Chemical Sewer(Spent LOCAT): It goes to SCTP for treatment and disposal.

23. Gaseous Eff luent Monitoring Online H2S analyzer Permissible Limit: <30 ppm h2s CEMS(Continuous Emission Monitoring System) -02 nos. of probes -First probe for Flue Gas parameters SO2/ NOx/ CO monitoring -Other is used for SPM parameter monitoring, on continuous basis. Permissible limits for SRU (and Incinerator) (as per new CC&A) PM-  150 mg/Nm3 SO2- 104.8 mg/Nm3 NOx- 350 mg/Nm3 CO-   150 mg/Nm3

24. Incinerator

25. PFS OPERATION The operation of PFS consists of following cycles: Cylinder up Feeding Pre-squeezing Cake wash Final squeezing Air drying Cylinder down Cake discharge and cloth wash

26. PFS OPERATION 1. Feeding/Filtration 2. Diaphragm Pressing I 3. Cake Washing 4. Diaphragm Pressing II 5. Cake Air Blowing 6. Cake Discharge Simultaneous cloth washing Slurry Filter Cloth Filtrate Air Cake Water Wash filtrate Wash liquid

27. Major issues in SRU 1.Problems inside Absorber Section Air sparger choking Acid gas sparger choking Malfunctioning of absorber LTs. Scrapper tripping Floating Sulphur High vent PPM 2.Frequent choking in slurry line. 3.Frequent choking of pump suction strainers. 4.NETP not accepting any floor wash liq. 5.Overflow of vessels.

28. 6.Limited operation capability of Tr.64 and Tr.66 and unavailability of Tr.65 and Incinerator. 8. Sulphur storage space constraint due to slow sale process. 9. Contractual issues Slow rate of Sulphur bagging. Non deployment of full manpower on daily basis. 9. PFS Maintaining 100% availability of both the machine. Choking issues associated with slurry handling. Operation of semi automatic bagging machine. Slow bagging of Sulphur from Sulphur yard. Feedback problems of ABVs installed in slurry line. Major issues in SRU

29. Name of Contractor : M/s Demi Engineering Contract period: 03 Years (15.04.2023 to 14.04.2026) Total manpower: 25 SRU contract is divided into 03 components: 1. SRU Turnaround jobs (08 unskilled worker) 2. Sulphur bagging, preparation of KOH and shifting of chemical barrels (08 unskilled worker) 3. Operation of PFS(09 skilled worker) Contract Details

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