5. Basic Claus vs Modified Claus Process
Basic Claus reaction
Introduced in 1883 by English Scientist
Carl Friedrich Claus
𝟐𝑯 𝟐 𝑺 + 𝑶 𝟐 → 𝟐𝑺 + 𝟐𝑯 𝟐 𝑶
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
9. Other Side Reactions:
Reaction Furnace:
𝐻2 𝑆 + 𝐶𝑂2 → 𝐶𝑂𝑆 + 𝐻2 𝑂
𝐶𝐻4 + 2𝑆2 → 𝐶𝑆2 + 2𝐻2S
Reheater (hydrolysis)
𝐶𝑂𝑆 + 𝐻2
𝑂 → H2S + CO2
𝐶𝑆2 + 2𝐻2 𝑂 → 2H2S + CO2
Modified Claus Process for Sulfur Recovery
10. 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
11. 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
14. Other Variations
Oxygen 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
15. 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
16. TGCU (Tail Gas Clean Up) Processes
Process Example Company
1. 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
20. Sulfur & its properties
Solid at ambient temperatures
Solid sulfur at ambient temperature
Gaseous sulfur allotropes
S8
S6
S2
S3, S4, S5, S7 – detected but not fully characterized
Sulfur
Crystalline Amorphous
slowly changes to
rhombic form at
ambient temperaturesRhombic Monoclinic
stable at <
204°F
stable at
>204oF
prepared by rapidly
chilling liquid sulfur
Both exists in octatomic
crystalline structures
presence not desired
28. Result – Hand Calculation
Furnace Temperature & Conversion:
From Equilibrium constant chart
x (mols/hr) (assumed)
Kp (calculated) Equilibrium
Temp (F)
89.52 20.00351623 1875
93.54
30.01536222
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 2,474.45
96.231 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
29. Result – Hand Calculation
y, mols/hr
Stream
Enthalpy
(Btu/hr)
Converter Heat
Balance (Btu/hr)
20.15 2,908,018 2,638,763
21.96 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 F
Converter Outlet Temperature = 603 F
1st Catalyst Converter
30. Result - Hand Calculation
2nd catalytic converter outlet temperature 3rd catalytic converter outlet temperature