Emissions/Environmental Pollutants

SO2 Historical Trends (1900 – 2000) & Effects

Overview of Gas Processing Industry

Basic Claus vs Modified Claus ProcessBasic Claus reaction Introduced in 1883 by English

Scientist Carl Friedrich Claus

highly exothermic difficult to control Low sulfur recovery efficiency overheating of the reactor

Modified Claus

introduced in 1938 by a German

company, I.G Farbenindustrie A.G

improvement on the basic Claus process

free flame oxidation ahead of catalyst

bed

catalytic steps revision

high SRE ranging from 90-99.9%

basis of most sulfur recovery units in use

today

Process Scheme

Modified Claus Process for Sulfur Recovery

Principal steps:

Combustion Step

Catalytic Steps

1. Combustion (Reaction Furnace) Temperature is usually between 1700oF & 2500oF Complete combustion of all hydrocarbons present Conversion of 1/3 H2S in feed to SO2

∆H @ 77oF = -223 100 Btu {1}

∆H @ 77oF = 20 400 Btu {2}

Modified Claus Process for Sulfur Recovery

2. Claus Reaction (Catalytic Converter) Catalyst is activated alumina Reaction proceeds at temperature < 700oF

∆H @ 77oF = -41 300 Btu {3}

Overall reaction:

∆H @ 77oF = -254 400 Btu {4}

Modified Claus Process for Sulfur Recovery

Other Side Reactions:

Reaction Furnace:

S

Reheater (hydrolysis)

H2S + CO2

2H2S + CO2

Modified Claus Process for Sulfur Recovery

Process Control:

Optimum conversion of H2S to sulfur is governed by:

constant 2:1 stoichiometric ratio of H2S to SO2

achieved by varying furnace air flow rate

deviation from ratio results in decreased SRE

Modified Claus Process for Sulfur Recovery

There are two basic process approaches depending on H2S concentration in feed stream: Straight through * Split flow

Claus Process Technology

H2S Conc in Feed, mol%

Process Variation

55 – 100 Straight through

30 – 55 Straight through plus acid gas/air preheat

15 – 30 Split flow or straight through with feed and/or air preheat

10 – 15 Split flow with acid gas and/or air preheat

5 – 10 Split flow with fuel added or with acid gas and air preheat or direct oxidation

< 5 Sulfur recycle or variation of direct oxidation or other sulfur recovery processes

Straight through Claus process

Claus Process Technology

Split-Flow Claus Process

Claus Process Technology

Other VariationsOxygen Enrichment use of pure oxygen instead of air higher and stable flame temperatures low H2S concentration feed and smaller equipment use used in combination with other variations

Acid Gas Enrichment applied ahead of SRU to achieve richer acid gas stream a solvent that selectively absorbs all the H2S from the feed gas stream is used The straight through process can then be used for sulfur recovery

Claus Process Technology

Tail Gas: Contains N2, CO2, H2O, CO, H2, unreacted H2S and SO2, COS, CS2, sulfur vapor,

etc. Limits overall sulfur recovery efficiency to 96-97% Tail gas is incinerated or treated in TGCU depending on local EPA regulation

Incineration - < 5000 ppmv H2S < 2500 ppmv SO2

TGCU Processes (Tail Gas Clean Up) higher H2S conversion efficiencies (>99.9 %) further reduction in SO2 amount vented out

Claus Tail Gas Handling

TGCU (Tail Gas Clean Up) Processes

Process Example Company1. Sub-dewpoint Processes Cold Bed Adsorption

(CBA)BP Amoco, Black & Veatch

2. Direct Oxidation SuperClaus, MODOP Jacobs Engineering & Mobil

3. SO2 Recovery Well-man Lord Luigi Bamag(oxidize to SO2, absorb and recycle to Claus)

4. H2S Recovery BSR Parsons(reduce to H2S, absorb & recycle to Claus)

SCOT MDEA

Shell UOP

video clip

Other Sulfur Removal Processes

Small scale/batch processes

< 20lbs of sulfur recoveryscavenger processes e.g.

Iron Sponge Zinc Oxide Chemsweet Sulfa-check

Medium scale recovery 0.2 – 25 LTD of elemental sulfur Includes Lo-Cat 11 & CrystaSulf

Sulfur Recovery Process Applicability Chart

Sulfur & its properties

Solid at ambient temperatures

Solid sulfur at ambient temperature

Gaseous sulfur allotropesS8

S6

S2

S3, S4, S5, S7 – detected but not fully characterized

Sulfur

Crystalline Amorphous

slowly changes to rhombic form at

ambient temperatures

Rhombic Monoclinic

stable at < 204°F

stable at >204oF

prepared by rapidly chilling liquid sulfur

Both exists in octatomic crystalline structures

presence not desired

Sulfur Vapor Specie Distribution

Design a Sulfur Recovery Plant with:– Capacity ≈ 80 LTD – Sulfur Recovery of > 99%– Using Modified Claus Process

Problem Statement – DEVCO PLANT

Feed Conditions Comp mol frac mols/hrTemp = 120oF H2S 0.9 198.45Flow Rate = 220.5 mols/hr CO2 0.04 8.82Pressure = 8 psig = 22.7 psia H2O 0.05 11.03Air blower discharge Temp = 180oF C2 0.01 2.21

1 220.50

Hysys Model

Promax Model

Result - Promax

Sulfur Recovery Furnace Temperature% oF

450 95.76178703 2280.772453455 96.60018709 2291.263676460 97.17586448 2301.65494464 97.32328731 2309.89643465 97.32088486 2311.946916470 97.15766914 2322.140318475 96.84752858 2332.2359480 96.46512268 2342.234455485 96.04302214 2352.136788490 95.59630649 2361.943749495 95.13319688 2371.656223500 94.65849743 2381.2751505 94.17521901 2390.801295510 93.68535555 2400.235741

Air Flow Rate

(mols/hr)

Air Flow Rate Vs SRE & Furnace Temperature

Result - Hysys

Optimum air flow rate:

Sulfur Recovery Efficiency:

Sulfur in entering H2S = 0.9*220.5*32 = 6350.4 lb/hr

Sulfur Recovered = 6304 lb/hr

% SRE = 0.9926999.269 %

Result - Hand Calculation

% Conversion:

Result – Hand Calculation

Furnace Temperature & Conversion:

From Equilibrium constant chart

x (mols/hr) (assumed)Kp (calculated) Equilibrium

Temp (F) 89.52 20.00351623 1875

93.5430.01536222

2150

96.231 40.00228095 2387.5

x (mols/hr)Equilibrium Temp (Fig

Flame Temperatute (F)

89.52 1875 2,479.68

93.54 2150 2,474.4596.231 2400 2,395.94

From Plot:

x = 96.2 mols/hr

This is amt of H2S that actually reacted

(96.2/132.3)*100%

% Conversion in furnace = 72.71 %

Furnace Temp = 2390 F

Result – Hand Calculation

y, mols/hr

Stream Enthalpy (Btu/hr)

Converter Heat Balance (Btu/hr)

20.15 2,908,018 2,638,76321.96 2,795,414 2,766,609

23.13 2,573,013 2,792,969

From Plot,

y = 22.18mols/hr

% Conversion = (22.18*(2/3 H2S in feed) = 16.76 %

At y = 22.18 mols/hr, Kp = 3166.17

@ Kp = 3166.17 T = 603 FConverter Outlet Temperature = 603 F

1st Catalyst Converter

Result - Hand Calculation2nd catalytic converter outlet temperature

3rd catalytic converter outlet temperature

Heat Duty

Promax: Hysys:

Hand Calculation:

Blocks Duty (Btu/hr)Blower 194,943Waste Heat Boiler 12,988,780Condenser 2,820,970Reheater 1,030,360Condenser 2 1,865,160Reheater 2 367,693Condenser 3 845,212Reheater 3 337,984Condenser 4 423,763

20,874,865

Unit Operartions Duty (Btu/hr)Burner 7,012,000Cooler 12,160,000Splitter 1 40,340Claus 1 691,700Splitter 2 1,309,000Heater 347,300Claus 2 1,686,000Splitter 559,200Heater 373,100Claus 3 122,200Splitter 4 673,900

24,974,740

Duty Btu/hr

FurnaceWaste Heat

BoilerSulfur

Condenser Reheater Catalytic

ConverterSulfur

Condenser Stage 1 14,660,438.61 13,983,571.92 2,627,601.19 626,856.55 2,840,839.57 1,291,525.71 36,030,833.55

Stage 2 _ _ _ 340,712.94 2,061,235.20 645,827.98 3,047,776.12

Stage 3 _ _ _ 220,953.57 1,775,630.90 494,858.12 2,491,442.59

41,570,052.25

Result Summary

  HYSYS PROMAX HAND CALCSRE, % 99.27 97.52 97.41        Air Flow Rate, mols/hr 472.50 464.00 509.33        Furnace/Burner (MMBtu/hr) 7.01 _ 14.66        Waste Heat Boiler (MMBtu/hr) _ 12.99 13.98        Total Heat Duty (MMBtu/hr) 24.97 20.87 41.57        Typical Tail Gas Composition (ppm)      H2S (5000 ppmv) 2100 3382.1 1386.7941SO2 (2500 ppmv) 1000 1564 701.1880