CLIAMS:1. A process for Sulphur Recovery comprising the steps of:
(a) conducting combustion of acid gas and sour water stripper gas comprising H2S, NH3, hydrocarbons and moisture with combustion air having oxygen content from 25-75%(by volume)in the reaction furnace to obtain an effluent stream comprising H2S, SO2, H2O, CO2, COS, CS2, N2 , Sx at temperature in the range of 1200-1500 degree Celsius;
(b) directly cooling a part of the effluent stream obtained from the reaction furnace in a sulphur condenser to temperature below 170 degree Celsius ; and
(c) directly recycling the stream obtained from step (b) in the reaction furnace by using a motive stream in an ejector or a blower and quenching the reaction furnace effluent below 1500 degree Celsius.

2. The process as claimed in claim 1, wherein the acid gas comprises 50-94% (by volume ) H2S, and the sour water stripper gas comprises 25-67 % (by volume) NH3
3. The process as claimed in claim 1, wherein the recycle comprises up to 45% of the reaction furnace effluent stream.
4. The process as claimed in claim 1, wherein the motive steam flow varies from 5-50% of the recyle flow .
5. The process as claimed in claim 1 for increasing capacity from 30% to 100%.
6. The process as claimed in claim 1 , wherein saving in the energy consumption up to 65%.
7. The process as claimed in claim 1 for additional increase in sulphur recovery up to 1.0 %.
8. The process as claimed in claim 1, wherein the sulphur condenser uses boiler feed water and generates either low pressure steam or hot boiler feed water .
,TagSPECI: FIELD OF THE INVENTION
The present invention relates a process for capacity enhancement of an oxygen enriched Claus sulphur plant. The invention in particular relates recycling a part of the effluent from the Claus reaction furnace after cooling the hot effluent by using a sulphur condenser. Introducing of the cold recycle effluent to the Claus reaction furnace by help of either ejector or a blower reduces the Claus reaction furnace temperature below the maximum temperature of the refractory present in the Claus reaction furnace.
BACKGROUND OF THE INVENTION
Gas sweetening units are widely used to recover H2S from natural gas and refinery off gas in natural gas purification plants and oil refineries. Claus sulfur recovery plant is used to convert H2S to elemental sulphur. The H2S concentration in feed gas to Claus sulphur plant is generally 90-94% (by volume).But some gas sweetening plants use selective solvents for removal of both CO2 and H2S . In such cases, the acid gas contains relatively more CO2 and less H2S which ranges from 30-50% (by volume).Besides CO2 and H2S,theacid gas contains other components like NH3,hydrocarbon ranging from CH4 to C6H14,H2O, N2,HCN,mercaptanes, BTX. The Claus sulphur plant also takes feed from sour water stripper plant. The sour water stripper gas contains H2S ranging from 0-33% ( by volume) , NH3 ranging from 0-67% (by volume) and water. The Claus sulphur plant is designed and operated by using combustion air from atmosphere. In refineries, the sulphur content in the crude oil changes widely and the product specification becomes stringent due to mitigation of environmental norms. Because of these reasons, refineries need to increase the capacity of the existingClaus sulphur plants.
As the acid gas throughput to the Claus sulphur plant increasesabove the design capacity of the plant, the plant faces several operating problems. The operation of the Claus sulphur plant and the tail gas treating plant in the down stream of the Claus sulphur plant depends on the back pressure. The back pressure increases for several reasons, like increase in process gas flow to the Claus sulphur plant, deposition of solid sulphur on condensers, catalyst bed and demister of tail gas coalescer. Increase in acid gas flow in the Claus sulphur plant decreases the residence times in the reaction furnace and Claus converters. Consequently,the conversion of H2S to sulphur decreases. Reduced conversion in Claus sulphur plant affects the performance of the down stream tail gas treating plant and finally increases the SOx emission through the stack. Therefore, overall sulphur recovery is reduced at higher throughput to the Claus sulphur plant .
The capacity of the Claus sulphur plant is increased by reducing the nitrogen content in the combustion air which is fed to the Claus reaction furnace. However, the temperature of the effluent of the Claus reaction furnace increases as the nitrogen concentration in the combustion air decreases or the oxygen concentration in the said combustion air increases. The temperature of the effluent of the reaction furnace is restricted to the maximum allowable temperature of the refractory used inside the Claus reaction furnace.
Reference is now drawn to prior art documents US 6,508,998, EP0252497, US 4,756,900, US 4,888,162which disclose various methods of arresting temperature rise during oxidation in reaction furnace used in the Claus sulphur plants.
US 6,508,998 discloses temperature moderation of an oxygen enriched Claus sulphur plant in various stages of cooling and reheating and a stream is taken from downstream of waste heat boiler and treated before it is recycled to the reaction furnace.
EP0252497 discloses injection of sulphur dioxide into the reaction furnace zone or immediately downstream thereof in an oxygen-enriched Claus process to reduce oxygen consumption.
US 4,756,900 discloses recycle of effluent stream taken from one of the stages after waste heat boiler.
US 4,888,162 disclosesintroduction of water into the reaction furnace to moderate oxygen induced higher temperature.
The inventors of the present invention have found the need of improved way of temperature adjustment wherein no more than a single stage of temperature adjustment and recycle are employed.

OBJECTS OF THE INVENTION
The principal object of the present invention is to derive a process employing a recycle sulphur condenser for temperature adjustment and recycle of the cold effluent stream of the reaction furnace to the reaction furnace.
Another object of the invention is enhancement of capacity and improvement of sulphur recovery of the Claus sulphur plant.

SUMMARY OF THE INVENTION
The present invention provides a process for sulphur recovery comprising the steps of:
1. A process for Sulphur Recovery comprising the steps of:
(a) conducting combustion of acid gas and sour water stripper gas comprising H2S, NH3, hydrocarbons and moisture with combustion air having oxygen content from 25-75%(by volume)in the reaction furnace to obtain an effluent stream comprising H2S, SO2, H2O, CO2, COS, CS2, N2 , Sx at temperature in the range of 1200-1500 degree Celsius;
(b) directly cooling a part of the effluent stream obtained from the reaction furnace in a sulphur condenser to temperature below 170 degree Celsius ; and
(c) directly recycling the stream obtained from step (b) in the reaction furnace by using a motive stream in an ejector or a blower and quenching the reaction furnace effluent below 1500 degree Celsius.
In an embodiment of the present invention the recycle is up to 45% of the effluent stream coming from the reaction furnace.
In another embodiment of the present invention the motive steam flow varies from 5-50% of the recyle flow .
In another embodiment of the present invention sour water stripper gas comprises H2S, NH3, and moisture
In yet another embodiment of the present invention effluent stream of the reaction furnace comprises H2S, SO2, H2O, CO2, COS, CS2, N2., S2,,S4,S8.
In an embodiment of the present invention the effluent recycle stream is cooled to a temperature below 170 degree Celsius in a recycle sulphur condenser.
In an embodiment of the present invention the cold effluent stream from recycle sulphur condenser is pressurised up to 0.9 Kg/cm2g by using either an ejector or a blower.
In an embodiment of the present invention the capacity of the Claus sulphur plant increases up to 100% of the normal capacity of the plant.
In an embodiment of the present invention the sulphur recovery of the Claus sulphur plant is improved up to 1% above the normal sulphur recovery of the plant.

BRIEF DESCRIPTION OF DRAWINGS
The above and other aspects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments, taken in conjunction with the drawing, wherein:
Figure 1 is a schematic representation of the process of theClaus sulphur plant.
Figure 2 is a schematic representation of the process of theClaus sulphur plant with present invention

DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to increasing the capacity of the Claussulphur plant by using O2 along with air from atmosphere. The oxygen concentration in the combustion air is 19.7% (by volume) and can be increased beyond 75%.(by volume) Oxygen enrichment can be done by injecting vaporized oxygen into the combustion airline or by injecting vaporized oxygen directly to main burner of the reaction furnace. Increase of oxygen concentration in the combustion air reduces the nitrogen concentration in the effluent of reaction furnace. The capacity of the Claus sulphur plant is increased upto 30-32% by using oxygen enriched air without any major modification of the plant because the refractory of the reaction furnace can withstand the increased temperature due to change in environment.
Increase of O2 concentration in combustion air increases the temperature of reaction furnace effluent for a specified acid gas and sour water stripper gas compositions. For typical 90% (by volume) H2S in the acid gas stream and 37% (by volume) NH3 in SWS stream, the temperature of reaction furnace effluent is about 1438degree Celsius. The temperature of reaction furnace effluent exceeds 1850 degree Celsius if the concentration of O2 maintains at about 45 %( by volume).The refractory of reaction furnace cannot withstand at this high temperature. The Claus reaction is exothermic in nature and it gets affected at the high temperature. The Clausreactionis described as
2H2S +SO2 = 3/x Sx + 2H2O + Heat
However, high temperature is required for destruction of ammonia and thermal cracking of H2S to hydrogen and sulfur. The reaction are described as followed
2NH3 = N2 + 3H2- Heat
H2S = H2 + 1/x Sx - Heat
Hydrogen produced by the above reactions reacts with oxygen to form water. The reaction of hydrogen and oxygen is exothermic and is not favoured at high temperature. Therefore the high temperature favours formation of hydrogen in reaction effluent.
In case of high CO2content in the acid gas, the temperature of the reaction furnace effluent is not increased. COS and CS2 are formed in large quantity and they affect the downstream operation . It also increases the load of hydrogenation reactor of tail gas treating plant . The reactions for formation of COS and CS2 are favoured at high temperature and the reactions are defined as follow
CO2 + H2S = COS + H2O - Heat
COS + H2S = CS2 + H2O - Heat
The process of the present invention will now be described in more detail with reference to the accompanying Figure 2.
According to the present invention, the temperature of the reaction furnace effluent is monitored at desired condition so that the problem mentioned above is not occurred. The present invention involves quenching of the reaction furnace effluent by recycling the cold stream taken from the reaction furnace itself. The present invention provides use of either combustion air or air with vaporized oxygen up to 75%. (by volume). The temperature of the reaction furnace is maintained below 1500 degree Celsius. The process involves taking a stream from reaction furnace and varying the recycle flow up to 45%(by volume) of the total flow of the reaction furnace effluent to adjust the reaction furnace temperature. The hot stream from the reaction furnace is cooled in a sulphur condenser which cools the recycle stream below 170degree Celsius. During cooling, the sulphur vapour present in the recycle stream is condensed and the liquid sulphur from the sulphur condenser runs down to sulphur pit. Low pressure steam is generated from recycle sulphur condenser. The sulphur condenser can also be used to preheat boiler feed water during cooling the hot reaction furnace effluent. To build up pressure for injecting the recycle stream to upstream of the burner of the reaction furnace,either an ejector or a blower is employed.
When the ejector is employed, low pressure or medium pressure steam is used as a motive fluid. The quantity of the motive fluid varies from 5-50% of recycle flow. Increase of motive fluid decreases the temperature of reaction furnace effluent.
The present invention provides increase in the capacity of the Claus sulphur plant up to 100% with maximum H2S concentration in the acid gas 90-94% (by volume) and NH3 in sour water stripper gas 67% (by volume) . The present invention also provides additional increase in the sulphur recovery of sulphur plant up to 1.0%.
The present invention uses simulation of the Claus sulphur plant to describe the use of recycle stream. The tables 1,2 and 3 present the first approach of the present invention. The acidgas contains 91% (by volume) H2S and SWS contains 37% (by volume) NH3. The oxygen level is increased to 74% and recycle is maintained 42% of the reaction furnace effluent. The temperature of reaction furnace effluent is retained at 1499 degreeCelsius .The motive fluids maintainedis 21% of the recycle stream flow.
The invention is now described by way of the following non-limiting examples.

EXAMPLE-I
A Claus sulphur plant is studied for processing the acid gas of 230.42kmole /hr and sour water stripper gas of 69.8 kmole/hr with air as reference of the present invention .The schematic presentation of the Claussulphur plant is shown in Figure 1. The plant configuration consists of reaction furnace, waste heat boiler , first condenser, first reheater, first Claus converter, second condenser, second reheater, second Claus converter and third condenser. The plant uses the combustion air available by the combustion air blower. The temperature of the reaction furnace is 1438 degree Celsius. The flow of the reaction furnace effluent to waste heat boiler is 934.4 Kmole/hour. The hot stream passes through the first condenser to remove sulphur. The cold stream from the first condenser is heated by the first reheater and is fed to the first Claus converter consisting of Claus catalyst. Sulphur is formed due to reaction of H2S and SO2and the sulphur is removed in the second condenser. The process gas is further heated by the second reheater and is fed to the second Claus converter for further reaction of H2S and SO2 to sulphur. The process gas is finally cooled to remove sulphur and is sent to tail gas treating plant. The total sulphur produced from the Claus plant is 7353kg/hr .The sulphur recovery of the Claus sulphur plant is 96 %. The stream summary,energy produced and energy consumed in the Claus sulphur plant are generated by simulator and are given in Table 1, 2 and 3.
 
Table 1: Stream Summary of the Claus sulphur plant

Description Acid Gas SWS
Gas
Combustion Air Reaction Furnace
Effluent Waste Heat Boiler Effluent First Condenser
Effluent First
reheater
Effluent
Steam No 1 2 3 4 5 6 7
Components
H2S 210.06 27.51 46.21 46.21 46.21 46.21
CO2 0.44 0.85 0.85 0.85 0.85
CH4 0.02 0.00 0.00 0.00 0.00
C2H6 0.33 0.00 0.00 0.00 0.00
H2 0.09 28.11 28.11 28.11 28.11
CO 0.00 0.27 0.27 0.27 0.27
H2O 19.15 16.01 28.32 268.45 268.45 268.45 268.45
O2 125.87 0.00 0.00 0.00 0.00
N2 475.17 487.36 487.36 487.36 487.36
COS 0.02 0.02 0.02 0.15
SO2 22.99 22.99 22.99 22.99
S2 78.11 78.11 3.37 3.37
S6 2.02 2.02
S8
CS2
NH3 0.33 26.25
Total Flow (Kmol/hr) 230.42 69.78 629.36 934.39 934.39 857.62 857.76
Temperature
(DegreeCelsius) 90 90 105 1438.4 295.4 188.8 290
Pressure (Kg/cm2g) 0.87 0.90 0.91 0.65 0.58 0.55 0.53

Table 2: Stream Summary of the Claus Plant

Description First Converter Effluent Second Condenser
Effluent Second reheater effluent Second Converter Effluent Third
Condenser
Effluent Total sulphur Tail Gas
Steam No 8 9 10 11 12 13 14
Components
H2S 21.21 21.21 21.21 5.05 5.05 5.05
CO2 0.87 0.87 0.87 0.87 0.87 0.87
CH4
C2H6
H2 28.11 28.11 28.11 28.11 28.11 28.11
CO 0.27 0.27 0.27 0.27 0.27 0.27
H2O 293.44 293.44 293.44 309.59 309.59 309.59
O2 0.00 0.00 0.00 0.00 0.00 0.00
N2 487.36 487.36 487.36 487.36 487.36 487.36
COS
SO2 10.49 10.49 10.49 2.41 2.41 2.41
S2 0.23 0.89 0.89 0.00 0.13 114.89 0.13
S6 1.94 0.42
S8 4.02 2.93
CS2
NH3
Total (Kmol/hr) 847.94 842.64 842.64 837.01 833.79 114.89 833.79
Temperature
(Degree Celsius) 344.7 161.4 200 234.1 131.7 131.7
Pressure (Kg/cm2g) 0.49 0.46 0.44 0.4 0.37 0.37

Table 3: Energy produced in the Claus sulphur plant
Heat Duty ( MMKcal/hr)
Waste Heat Boiler 10.1
First Condenser 1.23
Second Condenser 1.35
Third Condenser 0.725

Table 4: Energy consumed the Claus sulphur plant
Heat Duty ( KWH)
First Reheater 814
Second Reheater 297
Combustion Air Blower 1320

Example - II
In the present invention, the Claus sulphur plant of Example –I is studied with recycling of reaction furnace effluent. In the study, the Claussulphur plant configuration consists of reaction furnace, waste heat boiler , first condenser, first reheater, first Claus converter, second condenser, second reheater, second Claus converter, third condenser , recycle sulphur condenser and ejector. The Claus sulphur plant treats 93% more the acid feed gas and 72% more sour water stripper gas by recycling stream. The plant uses the combustion air and vaporised oxygen toincrease oxygen concentration in the combustion air . The concentration of oxygen in the combustion air is 74% (by volume) .The temperature of the reaction furnaceincreases to 1499 degree Celsius. The flow of the reaction furnace effluent to waste heat boiler isclose to the flow of effluent to waste heat boiler given in Example – I .About 42% of the reaction furnace effluent is sent to the recycle sulphur condenser . The cold stream at 162 degree Celsius is returned to the reaction furnace by using an ejector . The ejector uses low pressure steam as motive fluid. The sulphur recovery from the Claus sulphur plant is 97 %. The tail gas is further treated in the tail gas treating plant. The stream summary , energy produced and energy consumed of the Claus sulphur plant with recycle of reaction furnace effluent are generated by the simulator used in Example-I and are given in Table 5,6 ,7,8 and 9.

Table 5: Stream Summary of the Claus Plant with recycle of reaction furnace effluent
Description Acid Gas SWSGas
Combustion Air Reaction Furnace
Effluent Waste Heat boiler Effluent First Condenser
Effluent First
reheater
Effluent
Steam No 1 2 3 4 5 6 7
Components
H2S 404.76 55.02 75.01 75.01 75.01 75.01
CO2 0.89 1.71 1.71 1.71 1.71
CH4 0.05
C2H6 0.66
H2 0.17 51.06 51.06 51.06 51.06
CO 0.53 0.53 0.53 0.53
H2O 38.29 12.03 7.06 480.65 480.65 480.65 480.65
O2 355.31
N2 118.41 245.29 245.29 245.29 245.29
COS 0.03 0.03 0.03 0.03
SO2 37.35 37.35 37.35 37.35
S2 91.99 91.99 3.68 3.68
S6 3.06 3.06 0.12 0.12
S8
CS2
NH3 53.18 0.00 0.00
Total (Kmol/hr) 444.82 120.23 480.77 986.67 986.67 895.42 895.42
Temperature
(Degree Celsius) 90 90 105 1499.6 295.4 193.5 273
Pressure (Kg/cm2g) 0.87 0.90 0.91 0.65 0.58 0.55 0.53

Table 6: Stream Summary of the Claus sulphur plant with recycle of reaction furnace effluent

Description First Converter Effluent Second Condenser
Effluent Second reheater effluent Second Converter Effluent Third
Condenser
Effluent Total sulphur Tail Gas
Steam No 8 9 10 11 12 13 14
Components
H2S 36.57 36.57 36.57 8.73 8.73 8.73
CO2 1.73 1.73 1.73 1.73 1.73 1.73
CH4
C2H6
H2 51.06 51.06 51.06 51.06 51.06 51.06
CO 0.53 0.53 0.53 0.53 0.53 0.53
H2O 519.08 519.08 519.08 546.93 546.93 546.93
O2
N2 245.29 245.29 245.29 245.29 245.29 245.29
COS
SO2 18.12 18.12 18.12 4.19 4.19 4.19
S2 0.38 1.32 1.32 0.01 0.22 223.21 0.22
S6 2.98 0.72
S8 5.89 5.01
CS2
NH3
Total (Kmol/hr) 881.63 873.70 873.70 864.20 858.68 223.21 858.68
Temperature
(Degree Celsius) 344.7 172.6 180 233.5 131.7 131.7
Pressure (Kg/cm2g) 0.49 0.46 0.44 0.4 0.37 0.37

Table 7: Stream Summary of the Claus sulphur plant with recycle of reaction furnace effluent

Description Oxygen to reaction furnace Recycle effluent from reaction furnace Effluent from recycle condenser Steam to Ejector
Steam No 15 16 17 18
Components
H2S 54.31 54.31
CO2 1.23 1.23
CH4
C2H6
H2 36.98 36.98
CO 0.39 0.39
H2O 348.05 348.05 150.00
O2 1300.00
N2 177.62 177.62
COS 0.02 0.02
SO2 27.05 27.05
S2 66.61 0.67
S6 2.22 0.02
S8
CS2
NH3
Total (Kmol/hr) 1300.00 714.48 646.34 150.00
Temperature
(Degree Celsius) 45 1499.6 162 147
Pressure (Kg/cm2g) 0.65 0.61 0.58 3.5

Table 8 :Energy Produced in the Claus sulphur plant with recycle of reaction furnace effluent
Heat Duty ( MMKcal/hr)
Waste Heat Boiler 12.03
First Condenser 1.38
Second Condenser 1.48
Third Condenser 0.77
Recycle Sulphur Condenser 9.38

Table 9: Energy Consumed the Claus sulphur plant with recycle of reaction furnace effluent
Heat Duty ( KWH)
First Reheater 632
Second Reheater 55
Combustion Air Blower 1153

The process disclosed in the present invention provides following advantages:
• The capacity of Claus sulphur plant is increased up to 100 %.
• The sulphur recovery is increased up to 1% over the normal operation of the Claus sulphur plant with air.
• The energy saving in the air blower and process gas reheater I and II is around 65% .
• The minimum capital investment for revamp of operating Claus sulphur plant with the present invention.
• The shutdown period for revamp of Claus sulphur plant with the present invention is minimum.
• Design of grass rout Claus sulphur plant with the present invention requires low capital cost in comparison to the conventional design of the Claus sulphur plant with air .