2011 BRIMSTONE SULFUR SYMPOSIUM. Fundamentals 2011: Ammonia Destruction in SRU Furnaces

Transcription

1 2011 BRIMSTONE SULFUR SYMPOSIUM Fundamentals 2011: Ammonia Destruction in SRU Furnaces Peter Clark, Alberta Sulphur Research Ltd. Paul d Haene, DANA Technical Services Jim Jenkins, Shell Contents: 1. History 2. Common Industrial Approaches 3. Chemistry 4. Operation examples / issues 5. Concluding comments / new technology

2 AMMONIA DESTRUCTION IN THE CLAUS FURNACE O 2 (N 2 ) The Old Picture Combustion chamber 2500 o C 1300 o C WHB 600 o C flame acid gas (H 2 S / CO 2 ) zone SWSG H 2 S / NH 3 / H 2 O Anoxic zone Products 2 NH 3 + 3/2 O 2 N H 2 O 2 NH 3 + O 2 N 2 + H H 2 O 2 NH 3 + O 2 N H 2 + H 2 O Exothermic 2 NH 3 N H 2 Endothermic More O 2 must be added to the combustion chamber

3 2 NH 3 + SO 2 N H 2 O + 1/2 S 2 + H 2 Does NH 3 react with SO 2? EFFECT OF RATES OF OXIDATION ON AMMONIA DESTRUCTION O 2 (N 2 ) Combustion chamber 2500 o C 1300 o C WHB 600 o C acid gas flame zone SWSG H 2 S / NH 3 / H 2 O Anoxic zone Products Oxidation rates: H 2 S >> NH 3 CH 4 Ea (kcal. mol -1 ) (Calculated from ASRL data) : H 2 S, 18.3; CH 4, 46.4; NH 3, 62.0 Consequences: H 2 S + 3/2 O 2 SO 2 + H 2 O (excess) 4 NH SO 2 2 N H 2 O + 3/2 S 2

4 OVERVIEW OF NH 3 CONVERSION MECHANISMS IN THE CLAUS FURNACE N 2 + H 2 (100%) N 2 + H 2 (< 10%) dry 1200 o C dissociation suppressed by H 2 O and H 2 S wet, 1200 o C (30 vol% H 2 O) O 2 oxidation ( > 1200 o C) N 2 + H 2 O NH 3 SO 2 (distinct reaction) ( > 800 o C) [Ea 27 kcal. mol -1 ] N 2 + S 2 + H 2 O (30 100%) 800 o C >1200 o C 4NH 3 + 3SO 2 2N 2 + 6H 2 O + 3/2S 2 OVER-OXIDATION OF H 2 S DUE TO RELATIVE COMBUSTION KINETICS

5 AMMONIA DESTRUCTION FURNACE DESIGN TO (AG) H 2 S / H 2 O MAXIMIZE CONVERSION BY SO 2 SWSG (Air) Steam NH 3 / SO 2 Conversion Air H 2 S + 3 / 2 O 2 SWSG (Air) SO 2 + H 2 O H 2 O Sulfur condensers [2 H 2 S + SO 2 3 / 2 S 2 + H 2 O] 4 NH3 + 3 SO 2 2 N H 2 O + 3 / 2 S 2 4 NH SO 2 2 N H 2 O + 2 H 2 S 4 NH 3 + SO 2 2 N 2 + H 2 O + H 2 S + 4 H 2 In theory, the temperature of the furnace can be controlled by limiting the SO 2 make but this strategy leads to increase H 2 content in the process gas (low S 8 yield). In general, any SO 2 -rich system should enhance NH 3 conversion.

6 AMMONIUM SALT DEPOSITION IN CLAUS PLANTS Ammonium salts Comments NH 4 H S H 2 S + NH 3 Unstable > 100⁰C NH 4 H SO 3 NH 3 + H 2 SO 3 H 2 O + SO 2 H 2 SO 3 is unstable > 100⁰C NH 4 H S 2 O 3 NH 3 + H 2 S 2 O 3 H 2 O + S 2 O 2 S 2 O 2 is not stable NH 4 H SO 4 NH 3 + H 2 SO 4 H 2 O + SO 3 H 2 SO 4 is stable but SO 3 is not expected in the Claus furnace Plant deposits contain (NH 4 ) 2 SO 4 (NH 4 HSO 4 ) [analysis?] Laboratory studies reveal deposits contain (NH 4 ) 2 SO 4 and (NH 4 ) 2 S 2 O 3 Where do the SO 3 and S 2 O 2 come from?

7 CLAUS CHEMISTRY AND DEPOSITION OF AMMONIUM SALTS H 2 S / SO 2 (g) H 2 O / S 8 (g) H 2 S / SO 2 [H 2 S x O y ] H 2 O + S 8 Al 2 O 3 H 2 S x O y are formed as intermediates in the Claus reaction and include H 2 S 2 O 3 and H 2 SO 4 H 2 S / SO 2 (g) NH 3 [NH 4 HS x O y ] H 2 O / S 8 (g) H 2 S / SO 2 [H 2 S x O y ] H 2 O + S 8 Al 2 O 3 In laboratory experiments, NH 3 (100 ppmv) in H 2 O were found to strip H 2 S x O y from Claus catalyst at 320⁰ 120⁰C leading to deposition of NH 4 HS 2 O 3 and NH 4 HSO 4 in cooler parts of the system. Conclusion: The only requirement for ammonium salt deposition in a Claus plant is unconverted NH 3

8 NEW DIRECTIONS IN AMMONIA DECOMPOSITION ASRL STUDIES Catalyst Assisted Decomposition SWSG /Air (AG) NH 3 / SO 2 Conversion Claus converters Air SO 2 Production SWSG / Air Liquid S 8 Catalyst has dual role: conversion of any residual NH 3 and protection of WHB / ferrule system from flame impingement. Catalyst formulation: Top secret Catalyst conversion

9 WHY DO WE DESTROY NH 3? H 2 S (H 2 O) Claus plant H 3 PO 4 / H 2 O NH 4 H 2 PO 4 (NH 4 ) 2 H PO 4 (NH 3 ) 3 PO 4 Ammonium phosphates [fertilizer - $500 / ton for DAP] SWSG [NH 3 / H 2 O / H 2 S] Both NH 3 and water (largely) would be removed from H 2 S Could be shipped to DAP producer in solution Phosphoric acid scrubbing is an old process NH3 (SWSG) must be heavy metal (As) free.

10 Typical Feed Characteristics Amine Acid Gas >70% vol. H 2 S Sour Water Stripper Offgas 1/3 vol. NH 3 1/3 vol. H 2 S 1/3 vol. H 2 O Combined Feed Gases 5-15% vol. total NH 3 ~25% vol. total NH 3 maximum?

11 Typical Design Practices Design Guidelines Temperature >2400 F (1300 C) Residence Time >1 Second Design Configurations Single Burner Single Zone Two Zone Two Burner Single Zone

12 Typical Operating Practices Single Burner Single Zone Will Require Rich Amine Acid Gas May require Oxygen Enrichment

13 Single Burner Single Zone Example of Operating Plant Oxygen Enrichment

14 Single Burner Single Zone Example of Operating Plant Operational Aspects Burner = BOC Tangential Oxygen Enrichment Level = Up to 100% COPE Reaction Furnace Operating Target 2500 F (1370 C)

15 Typical Operating Practices Single Burner Two Zone Perhaps Most Typical in The Americas May Also Be Used With Oxygen Enrichment

16 Single Burner Two Zone Example of Operating Plant Amine Acid Gas Bypass For Temperature Control Optional

17 Single Burner Two Zone Example of Operating Plant Operational Aspects Burner = Conventional (Not Modern High Performance) Front Mounted No Oxygen Enrichment Hot Gas Bypass Reheats Reaction Furnace Operating Target 2350 F (1290 C) Zone 1

18 Single Burner Two Zone Example of Operating Plant Six Delta Thermocouples Zone One Four Delta Thermocouples Zone Two Unit Feed On Pressure Control No Amine Acid Gas Preheater %Ammonia In Total Feed %Ammonia In Stream To Burner AVG MAX

19 Typical Operating Practices Two Burner Single Zone Not Typical in The Americas Higher CAPEX Operational Reliability Hydrocarbon Concerns

20 Two Burner Single Zone Ammonia Burner Temperature Control Oxidizing Co-Firing Fuel Gas Oxygen Enrichment

21 Operation Outside Typical Guidelines Combined Feed Gases ~25% vol. total NH 3 maximum? Temperature >2400 F (1300 C) Example of Operating Plant Good Experience Not So Good Experience

22 Example - Operation Outside Typical Guidelines Example of Operating Plant Operational Aspects (Single Burner Two Zone) Burner = Conventional (Not Modern High Performance) Front Mounted No Oxygen Enrichment Hot Gas Bypass Reheats Temperature Measurement Accuracy Flame Color Amine Acid Gas Bypass Above Industry Standards? Five Year Catalyst Cycle Ammonia Ammonia CAG % Total Feed % To Burner % To Zone 2 AVG MAX Zone 1 Temp = 1750 F (955 C) Zone 2 Temp = 2140 F (1170 C) *E2T Measurements

23 Operation Outside Typical Guidelines Example of Operating Plant Operational Aspects (Single Burner Two Zone) Burner = Conventional (Not Modern High Performance) Front Mounted No Oxygen Enrichment Steam Reheats Good Historical Operation Abnormally High ph in SCOT Quench Following a Refinery Load Change

24 Operation Outside Typical Guidelines

25 Operation Outside Typical Event Conclusions Guidelines 2200 F (1204 C) Minimum For This Unit (Zone 1) Unexpected Rise in Quench ph May Indicate Ammonia Slip May Not Be True For Every Unit Ammonia Deposits in Claus Ammonia Slip To SCOT Risks SCOT Recycle Line Plugging May Trip The Unit