Sulfur Recovery. Chapter 16 Based on presentation by Prof. Art Kidnay

Transcription

1 Sulfur Recovery Chapter 16 Based on presentation by Prof. Art Kidnay

2 Plant Block Schematic 2

3 Topics Introduction Properties of sulfur Sulfur recovery processes Claus Process Claus Tail Gas Cleanup Sulfur storage Safety and environmental considerations 3

4 Introduction & Properties of Sulfur

5 Sulfur Crystals 5

6 Molten Sulfur 6

7 Sulfur Usage & Prices Natural gas & petroleum production accounts for the majority of sulfur production Primary consumption is agriculture & industry 65% for farm fertilizer: sulfur sulfuric acid phosphoric acid fertilizer $50 per ton essentially disposal cost Chinese demand caused runup in Cleaning up their act, Gordon Cope, Hydrocarbon Engineering, pp 24-27, March 2011 Ref: Updated April 9,

8 U.S. sulfur production 8

9 Sulfur Chemical Structure Pure sulfur exists as S X where X = 1 to 8 The dominant species are S 2, S 6, & S 8 May be in ring structure or open chain Species determined by temperature This composition greatly affects its properties! Octasulfur, S 8 9

10 Sulfur Vapor Species 10

11 Viscosity of Molten Sulfur 5 Pure Sulfur

12 Viscosity of Molten Sulfur H 2 S Partial Pressure, psia Pure Sulfur

13 Viscosity of liquid sulfur 13

14 Properties of sulfur

15 Sulfur Recovery Typically a modified Claus process H 2 S rich stream burned with 1/3 stoichiometric air. Hot gases are then passed over alumina catalyst to produce free sulfur Combustion: H 2 S O 2 H 2 O + SO 2 Claus Reaction: 2 H 2 S + SO 2 2 H 2 O + 3 S Sulfur formation reaction mildly exothermic Sulfur conversion reactors kept above 400 o F (sulfur dew point) The Claus reaction is reversible therefore, 100% conversion can never be achieved Practically, Claus units are limited to about 96% recovery Tail gas units are used to provide improved conversion 15

16 Modified Claus Process Petroleum Refining Technology & Economics 5 th Ed. by James Gary, Glenn Handwerk, & Mark Kaiser, CRC Press, 2007 Converters o F Burner & Reactor above 1800 o F o F for NH 3 destruction GPSA Engineering Data Book, 13th ed., Fig. 22-2,

17 Straight-through Claus Process waste heat boiler acid gas 110 F 8 psig air reaction furnace F gas reheat HP steam boiler feed water (BFW) 600 F LP steam gas reheat 450 F 590 F 375 F Claus #1 Claus #2 LP steam gas reheat 420 F 453 F LP 300 F 350 F steam condenser condens condenser condenser er BFW BFW BFW sulfur sulfur sulfur 400 F Claus #3 407 F condenser 270 F sulfur tail gas 17

18 Temperature Regimes for the Claus Process 18

19 Equilibrium Conversion of H 2 S to S 19

20 Equilibrium Conversion of H 2 S to S Percent Conversion of H2S to Sulfur Claus #3 Claus #2 melting point boiling point 800 Claus # Furnace & waste heat boiler F Temperature, F 20

21 Hydrocarbons in the Claus Process 21

22 Claus Process Use multiple stages to obtain highest conversion Typically three Various flow patterns Straight-through best, used whenever possible Split flow best at low H 2 S feed concentrations (5 to 30 mol% H 2 S) Sulfur recycle < 5% H2S Direct oxidation < 5% H2S 22

23 Claus Process Claus unit feed usually contains H 2 S and CO 2 High concentrations of noncombustible components (CO 2, N 2 ) Lower flame temperature Difficult to maintain stable combustion furnace flame temperatures below 1700 o F Solutions include Preheating air Preheating acid gas feed Enriching oxygen in air Using split-flow process 23

24 Split-flow Claus Process gas reheat gas reheat gas reheat HP steam 2/3 flow Claus #1 Claus #2 Claus #3 acid gas 1/3 flow LP steam LP steam LP steam air condenser condenser condenser condenser boiler feed water (BFW) reaction furnace BFW sulfur BFW sulfur sulfur sulfur tail gas 24

25 Typical Claus Configurations Approximate concentration of H 2 S in feed (mol%) Process variation Straight-through Straight-through + acid gas and/or air preheat Split-flow or acid gas and/or air preheat Split-flow with acid gas and/or air preheat 5-10 Split-flow with fuel added, O 2 enrichment, or with acid gas and air preheat 25

26 Tail Gas Clean Up Three process categories: Direct oxidation of H 2 S to sulfur (Superclaus) 2 H 2 S + O 2 2 S + 2 H 2 O Sub-dew point Claus processes (Cold Bed Adsorption) SO 2 reduction and recovery of H 2 S (SCOT) 26

27 Claus Tail Gas Cleanup Conventional 3-stage Claus units recover 96 to 97.5% of sulfur Remainder was burned to SO 2 and vented Adding fourth stage results in 97 to 98.5% recovery Regulations now require 99.8 to 99.9% recovery Meeting regulations requires modified technology 27

28 Shell Claus Offgas Treating (SCOT) Four step process: Mix feed with reducing gas (H 2 and CO) Convert all sulfur compounds to H 2 S Cool the reactor gas Strip H 2 S using amine 28

29 SCOT Process Claus Tail Gas To Stack H2S Recycle Reducing Gas Air Catalytic Reactor Fuel Furnace Low Pressure Steam H2O Low Pressure Steam Reboiler Mix Claus tail gas with H 2 and CO and heat in inline burner Catalytically convert all sulfur compounds to H 2 S Reactions: SO 2 + 3H 2 H 2 S + 2H 2 O S 2 + 2H 2 2H 2 S COS + H 2 O CO 2 + H 2 S CS 2 + 2H 2 O CO 2 + H 2 S Cool reactor gas (exiting at ~570 o F in waste heat exchanger and water wash to complete cooling. Strip H 2 S from gas & recycle to Claus Typically use MDEA can get to low H 2 S levels in Stack Gas & slip CO 2 so it doesn t build up in the recycle gas 29

30 Alternate Conversion & Sulfur Removal Processes Selectox TM Proprietary catalyst removes need for furnace CrystaSulf Uses modified liquid-phase Claus reaction Elemental sulfur removed by filtration Mid-range process to handle sulfur amounts between 0.1 and 20 tons per day 30

31 Sulfur storage

32 Sulfur Piles Sulfur pile at North Vancouver, B.C., Canada, brought by rail from the province of Alberta Ref: Wikimedia Commons 32

33 Sulfur Blocking Fort McMurray oil sands operations, Alberta, Canada 33

34 Summary

35 Summary Natural gas & petroleum production accounts for the majority of sulfur production Primary consumption is agriculture & industry, 65% for farm fertilizer. $50 per ton essentially disposal cost Sulfur properties depends upon which sulfur species are present Dominant species are S 2, S 6, & S 8 Dominant sulfur recovery process is modified Claus Extent controlled by chemical equilibrium, so can only get 96% conversion Tail Gas Cleanup process required for very low sulfur emissions Sulfur storage Temporary storage as hot liquid Shipping in pellets & long-term bulk storage as blocks 35