FIELD OF INVENTION
The present invention relates to a process and a system process that re-utilizes waste nitrogen gas coming from nitrogen plant for the recovery of sulphur from a gas stream containing hydrogen sulphide.

BACKGROUND OF THE INVENTION
The recovery of elementary sulfur from hydrogen sulfide (H2S) containing gas streams is known in the art as disclosed in the article “Fundamental of Sulfur Recovery by the Claus Process” by B. Gene Goar published in the 1977 Gas Conference Report. The conventional thermal and catalytic Claus process is widely practiced and accounts for a major portion of total production of sulfur. In a Claus type sulfur recovery plant (SRU)  the first step is thermal oxidation of a fraction of H2S in the acid gas feed to SO2. This is typically carried out in the thermal reaction zone (Claus furnace) by the addition of air to acid gas which contains mainly H2S but may contain CO2 and possibly hydrocarbons. In the oxidation step  in addition to SO2  the products also include elementary sulfur and water. The down stream catalytic reactors have effective Claus catalyst. The conversion achieved in each reactor is equilibrium limited. Typically reactions occurring in such Claus plant can be summarized by the following equations

H2S + 3/2 O2 H2O + SO2 (Oxidation)

2H2S + SO2 2 H20 + 3S (Claus reaction)

H2S +1/2 O2 H2O + S (Overall)

The use of air as the source of elementary or free oxygen in the thermal reactor introduces large quantity of N2 in to the process. Equipment must be sized into handle this volume of inert gas. By increasing the concentration of O2 in the oxygen source  the quantity of N2  which must be processed  can be reduced. Such oxygen enrichment can significantly increase the capacity of an existing Claus plant and or reduce the capital investment for a new plant.

Oxygen enrichment in the operation of a Claus sulfur plant to increase the capacity of H2S treated in such a plant has also been disclosed in the article ‘Oxygen Use in Claus Sulfur Plants’ by M.R.Gray and W.Y.Svreck  published in the 1981 Gas Conditioning Conference Report. It was disclosed more specifically that oxygen be added to the air feed to the burner of a reaction furnace in a Claus Sulfur plant to increase the amount of H2S which is combusted to sulfur dioxide for later catalytic conversion to elementary liquids sulfur product. The maximum capacity increase which can be achieved with oxygen enrichment is determined by the pressure drop through the plant  the reactor space velocity and the temperature of the reaction furnace and the various catalytic zones  particularly the refractory materials used in the furnace of the Claus plant.

In the 1983 publication by Linde of Union Carbide entitled “Claus Plant Oxygen Enrichment”  it is noted that oxygen–enrichment limits exist for rich hydrogen sulfide streams due to temperature limits in the furance or waste heat boiler of the Claus plant.

US 4 481 181 discloses a Claus plant  which uses oxygen enrichment. A process is provided for recovery of H2 from H2S utilizing the partial combustion of H2S with oxygen to produce sufficient heat to effect thermal decomposition of a substantial portion of the unoxidised H2S. H2S is introduced along with oxygen into a reaction zone in a molar or volume ratio of greater than 10:1. The exothermic combustion reaction  which yields sulfur and water  is used to affect the final raise in temperature and to maintain the temperature within reaction zone above 1027 oC. Promptly quenching the mixture with cooler gas instantaneously reduces the temperature of the gas to below 927 oC and effectively blocks any significant recombination of elementary sulfur. It would otherwise occur in the time usually required to operate even a fast heat exchanger. The available heat in the gases  which result from quenching  is used to raise the temperature of the incoming stream to about 827 oC.

US 4 279 882 discloses a Claus plant using oxygen enrichment. According to the invention  there is provided a novel Claus process which eliminates the thermal reactor  including its combustion chamber and heat exchanger  and which is applicable to the treatment of gas streams containing from about 1 to 100 % by volume of H2S. The process is particularly suited to small plants having output of 20 tones per day of sulfur or less. A stoichiometric amount of oxygen is preferably employed in the mixture of H2S and oxygen. H2S is catalytically converted to SO2 and sulfur at a temperature of 370 oC.

US 3 822 341 discloses a Claus plant  which uses oxygen enrichment. The process involves reacting oxygen and H2S with cooling so that the effluent is at temperature between 315 o C to 400 o C. Those reaction gases are then reacted in a Claus reactor. Liquid sulfur is removed from the gases by lowering the temperature of the gases in a condenser.

US 4 279 882 discloses a sulfur recovery process  which uses only a series of catalytic reaction beds rather than a reaction furnace  as in the traditional Claus plant. A temperature modifying recycle stream is set forth in the patent  wherein a stream is returned to the feed in order to control the temperature in the catalytic zones. This process is economical only for dilute H2S feed gas specifications. It requires a recycle blower operating at high temperature.

US 4 632 818 and EP 0 220 619 B1 disclose a process from recovery of sulfur from H2S containing gas using oxygen-enriched air in a Claus reaction furnace. The process is directed to an improved method of the temperature moderation in the reaction furnace (538 o C to 927 oC) by introducing elementary sulfur into the Claus reaction zone as a temperature moderator or reaction effluent quench. The process is relevant for feed gas H2S contents of 60 %v or more. Preferably  H2S to O2 ratio is ~2.5 : 1 but would decrease if the feed gas contains hydrocarbons. Preferable combustion zone temperature is 1316 oC while the combustion zone effluent is quenched to 760 oC by injecting liquid sulfur (17 g/gm mole of effluent)

US 4 756 900 discloses a Claus plant  which uses oxygen enrichment. In the process  the combustion effluent from a waste heat boiler is divided into a first and second streams so that the sulfur in the first stream is condensed and then the stream is passed on to later Claus reaction stages. Sulfur in the second stream is condensed and then this stream is introduced in the Claus reaction furnace to moderate the temperature. The temperature of the effluent of the second condensation zone is 115 oC – 148 oC.

U.S. Patent 4 798 716 discloses a Claus plant  which uses oxygen enrichment. In the process  the temperature of the Claus reaction furnace is moderated by returning a portion of the dried condenser effluent as diluents to combustion chamber of the reaction furnace. The process uses either pure oxygen or oxygen enriched air in the reaction furnace.

U.S. Patent 4 138 473 discloses a Claus plant  which uses pure oxygen. In this process  the gaseous output of the final converter is combusted with oxygen in a final catalytic converter to convert any remaining H2S to SO2. The H2O  SO2 and CO2 mixture emerging from the converter is treated to remove H2O and CO2. A concentrated SO2 is returned the Claus furnace. This process requires a large amount of water to absorb SO2 and also requires heat inputs in the stripping of SO2 form ti aqueous solution.

In all of the above referred documents  there is no mention of utilizing a waste nitrogen gas coming from nitrogen plant for the recovery of sulphur from a gas stream containing hydrogen sulphide. Thus  there is still need to develop a process and an apparatus for an efficient recovery of sulphur from a gas stream containing hydrogen sulphide which re-utilizes a waste nitrogen gas coming from nitrogen plant.

OBJECTS OF THE INVENTION
The object of the present invention is to provide a process and a system process that re-utilizes waste nitrogen gas coming from nitrogen plant for the recovery of sulphur from a gas stream containing hydrogen sulphide.

SUMMARY OF THE INVENTION
Accordingly  the present invention provides a process for re-utilizing waste nitrogen gas coming from nitrogen plant  said process comprising the step of reacting the waste nitrogen gas with a stream comprising hydrogen sulphide so as to obtain elemental sulphur  steam and vent gas stream.

In an embodiment of the present invention  the stream comprising hydrogen sulphide is acid gas stream.

In another embodiment of the present invention  the acid gas stream further comprises one or more of ammonia  carbonyl disulphide  water  hydrocarbons and carbon disulphide.

In yet another embodiment of the present invention  the acid gas stream comprises 50-94 vol% of hydrogen sulphide.

In still another embodiment of the present invention  the waste nitrogen gas coming from nitrogen plant comprises a minimum of 28 wt% of oxygen.

In a further embodiment of the present invention  the reaction between the waste nitrogen gas and the stream comprising hydrogen sulphide is performed in a Claus type sulphur recovery plant.

In one embodiment of the invention  the process increases the capacity of Claus sulphur plant by 15% -32%.

In another embodiment of the invention  the temperature of the process gas in the main combustion chamber of the plant is increased to 1300-1500 o C.

In another embodiment of the invention  complete destruction of ammonia and combustion hydrocarbons are done in the main burner of the plant.

In another embodiment of the invention  conversion of H2S to sulphur in main burner increase overall sulphur recovery of the plant by 0.05% - 1.0%.

In another embodiment of the invention  reduction in backpressure in the plant due to less nitrogen in the process gas gives smooth and trouble free operation of the plant.

In another embodiment of the invention  stable flame in main burner of the plant is achieved even at H2S concentration of the feed acid gas less than 50%.

In another embodiment of the invention  implementation of process for running Claus type sulphur plant takes minimum modification and minimum shutdown period.

In another embodiment of the invention  flexibility in Claus type sulphur recovery plant operation by using either oxygen-enriched Waste Nitrogen gas or normal air is maintained.

In another embodiment of the invention  Surge vessel & Blower are used to divert the Waste Nitrogen to SRU Burner.

The present invention also provides a system for re-utilizing waste nitrogen gas coming from nitrogen plant  said system comprising a Claus type sulphur recovery plant for reacting the waste nitrogen gas with a stream comprising hydrogen sulphide so as to obtain elemental sulphur  steam and vent gas stream and a sub-system connecting the nitrogen plant to the Claus type sulphur recovery plant for transferring the waste nitrogen gas thereto.

In an embodiment of the present invention  the sub-system connecting the nitrogen plant to the Claus type sulphur recovery plant comprises a conduit connectable between an outlet of the nitrogen plant and an inlet of the Claus type sulphur recovery plant; a storing means for temporarily storing the waste nitrogen gas coming from nitrogen plant; and a control means for controlling the quantity and the rate at which the waste nitrogen gas is supplied to the Claus type sulphur recovery plant.

In another embodiment of the present invention  the sub-system connecting the nitrogen plant to the Claus type sulphur recovery plant further comprises a blower.

In yet another embodiment of the present invention  the Claus type sulphur recovery plant (SRU) comprises a main Burner  main combustion chamber  Claus reactors  heat recovery facilities like waste heat boiler and condensers  and incinerator for processing of acid gas/feed gas.

In still another embodiment of the present invention  the Claus type sulphur recovery plant comprises a main burner for mixing the acid gas/feed gas with the waste nitrogen gas coming from nitrogen plant and oxidation of the contents of the feed gas  main combustion chamber for conversion of hydrogen sulfide to sulfur dioxide  series of Claus reactors for the production of elementary sulphur  waste heat boiler for cooling the processed gas and sulphur condensors for the recovery of sulphur.

In an embodiment the invention  the system is structure so as to be operable on waste nitrogen gas coming from nitrogen plant as well as atmospheric air and the operation is switchable from atmospheric air to waste nitrogen gas coming from nitrogen plant and vice versa.

In an embodiment the invention  the system is structure so as to be operable on waste nitrogen gas coming from nitrogen plant as well as air enriched with oxygen and the operation is switchable from oxygen enriched air to waste nitrogen gas coming from nitrogen plant and vice versa.

According to one embodiment of the invention the process for re-utilizing waste nitrogen gas coming from nitrogen plant comprising the step of mixing the waste nitrogen gas with a stream comprising hydrogen sulphide  conversion of about 1/3rd amount of H2S to sulphur dioxide  reacting the resultant sulphur dioxide with the balance hydrogen disulfide to produce elementary sulphur and water in vapor phase  cooling of processed gas  and recovery of sulphur.

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 Surge vessel and Blower.

DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process for removal of hydrogen sulphide (H2S)  Carbonyl sulphide  and carbon disulfide from amine regenerator off gas by utilizing Waste Nitrogen  which is otherwise vented to atmosphere  from Nitrogen plant as a source of Enriched air. In accordance with the teachings of the present invention  the process involves complete destruction of ammonia (NH3) and improved combustion of hydrocarbons present in acid gas and sour water stripper gas in nitrogen gas coming from nitrogen plant in main burner of the plant. By following the teachings of the present invention it is possible to ensure smooth and trouble free operation of Claus sulphur recovery unit and sulphur recovery units at higher sulphur recovery efficiency.

The present invention provides a process for removal of hydrogen sulphide (H2S)  carbonyl sulphide  and carbon disulfide  amine regenerator off gas called acid gas treated in Claus type sulphur plants by using nitrogen gas coming from nitrogen plant which inherently has high oxygen concentration. Claus sulphur plant comprises of main burner  main combustion chamber  and Claus reactors  heat recovery facilities like waste heat boiler and condensers  and incinerator for processing of acid gas and sour water stripper gases.

The present invention will now be described in greater details with reference to the sole figures of the accompanying drawings which show schematic diagram of the oxygen enrichment and Claus type sulphur plant.

The present invention overcomes the shortcoming of the prior art by increasing throughput of Claus plant with oxygen enrichment beyond the known feasible limits of the prior art. The invention provides better throughput of reaction components in the Claus type sulphur plant by reducing nitrogen in process gas. This is achieved by Using waste Nitrogen from Nitrogen plant.

The process is demonstrated in a commercial Claus type sulphur plant (SRU). The plant processes H2S rich acid gas. The main burner provides complete mixing of air or oxygen enriched Waste Nitrogen with acid gas for oxidation of all hydrocarbons  residual sulphur compounds ammonia and one third of the total hydrogen sulphide present in acid gas.

In the main burner and main combustion chamber  one third of the hydrogen sulfide in the acid stream is burnt to form sulphur dioxide (SO2). The resulting sulphur dioxide is then reacted with the balance of H2S to form elementary sulphur (S) and water in vapor phase in the first Claus reactor and subsequently in the Claus reactors operated at low temperature.

The Claus reaction started in the main combustion chamber. The hot process gas is cooled in a waste heat boiler and subsequently sulphur is removed as liquid sulphur from the sulphur condenser. The cold gas is heated in reheaters and passed through Claus reactors and condensers in the downstream of the first condenser.

The process disclosed in the present invention provides a trouble free operation. As the nitrogen gas coming from nitrogen plant is enriched in oxygen enriched  the operation is smooth and trouble free and provides less pressure drop in the plant and stable flame in the main burner. Injection of nitrogen gas coming from nitrogen plant having enriched oxygen content in combustion air line instantaneously reduces backpressure and high temperature in main combustion chamber.

The process disclosed in the present invention also provides a quick change over from air mode operation to nitrogen gas coming from nitrogen plant mode operation. Sometimes the process encounters low acid gas flow. Continuation of operation is done immediately by switching over from nitrogen gas coming from nitrogen plant having oxygen enrichment operation to normal operation with plant air. Change of operation mode does not call for any precautionary steps neither for main burner and main combustion chamber nor the down stream equipments of main combustion chamber.

A stable flame and good color flame is observed in main burner even at lean acid gas flow. Relatively less nitrogen in combustion air is the reason for good flame stability and high temperature is the reason for greenish color of the flame.

The prior art plants are prone to backpressure problem and due to which it is difficult to increase acid gas flow to the plant beyond certain limits. In the present invention the increased oxygen concentration in nitrogen gas coming from nitrogen plant increases the acid gas flow by reducing backpressure in the plant.

In the present process a steep increase in main combustion chamber temperature is observed. The total steam production from waste heat boilers and other condensers is increased due to processing of more acid gas by the plant under oxygen enrichment condition.

The present process provides improvement in sulphur recovery. High sulphur recovery is confirmed from SO2 and H2S level in flue gas.

The invention is now described by way of non-limiting example.
An existing commercial Claus plant of capacity 14.7 tones per day was revamped for testing the process. Figure -1 depicts overall schematic diagram of the new system.

The SRU plant included Main Burner  Main Combustion Chamber  three Claus reactors  Heat recovery facilities like waste heat Boiler and Condensers and incinerator. Also  a Surge vessel and Blower are used to divert the Waste Nitrogen from the nitrogen plant to SRU plant.

For monitoring the oxygen flow  an online oxygen analyzer was provided in the combustion air pipe.

The switch over between normal air and Waste Nitrogen is done gradually by observing the system parameters like main combustion temperature  backpressure  Claus Converter temperatures etc. The Waste Nitrogen line is hooked at a point upstream of the Combustion air Flow control valve on the normal Air supply line to the Main Burner. The discharge pressure of both the blowers at the battery limit of the SRU is 0.9 Kg/cm2g. The switching of modes is done by slow opening and closing of blowoff valves of both the blowers simultaneously. By this operation the system can be brought to steady state in a few minutes.

Table – 1 and Table -2 show the increase in capacity before and after test run. Results show increase in steam flow for this process and reduction in air flow.
Table 1: Waste Nitrogen mode
S.No Acid gas + Sour gas flow  units
(kg/hr) Back pressure  units (kg/cm2 g) Waste N2 flow  units MCC temperature  DegC H2S concentration  %V
1 360 0.202 473 1305 78.2
2 511 0.277 558 1360 82.54
3 648 0.344 636 1388 88.4
4 684 0.348 646 1389 86.68
5 720 0.410 720 1392 88.2

Table 2: Normal Air mode
S.No Acid gas + Sour gas flow  units
(kg/hr) Back pressure  units (kg/cm2 g) Normal Air flow  units MCC temperature  DegC H2S concentration  %V
1 360 0.336 707 1238 78.2
2 511 0.394 782 1243 82.6
3 648 0.495 1045 1265 88.2
4 684 0.510 1100 1270 86.5
5 720 0.530 1372 1265 88.2

The oxygen concentration in the oxygen-enriched air was ~29%v during test run. The main combustion chamber temperature recorded was above 1350 degC.

Increase in oxygen concentration of the oxygen-enriched air showed instantaneous reduction in the back pressure when the same acid gas flow was maintained in the plant. This was due to less nitrogen in the combustion air.

The process gas from main combustion chamber was cooled in the waste heat boiler and first condenser. After that the process gas undergoes reheating followed by Conversion and Condensation. This cycle is repeated three times. Firstly the process gas was heated in the line burner before passing the gas to first converter where the bed temperature is maintained at ~343 degC. The process gas is subsequently passed through Condenser-2 where the Sulphur is condensed. The process gas is again heated in a reheater and fed to second Converter  where the bed temperature is maintained at around ~270 degC and subsequently the process gas is passed through Condenser-3. The process gas is again heated in reheater-2  passed through Converter-3  where the bed temperature is maintained at 218 degC and lastly through Condenser-4. The process gas emerging from last Condenser is called Tail gas. The Tail gas has sulphur species in the form of unconverted H2S  SO2  Sx. These are oxidized in an Incinerator before venting them to atmosphere.

The overall sulphur recovery of the plant was more than 96%.

Overall operation during oxygen enriched air mode was found smooth and trouble free. Stable flame in main burner  less back pressure  more sulphur formation in main combustion chamber were few factors for better operation.

The laboratory analysis was carried out for analysis of feed acid gas  tail gas  enriched air and stack gas to regulate the operation. The concentration of H2S in the feed acid was found more than 90%v during test run in most of the time.

We claim:

1. A process for re-utilizing waste nitrogen gas coming from nitrogen plant  said process comprising the step of reacting the waste nitrogen gas with a stream comprising hydrogen sulphide so as to obtain elemental sulphur  steam and vent gas stream.
2. The process as claimed in claim 1  wherein the stream comprising hydrogen sulphide is acid gas stream.
3. The process as claimed in claim 1  wherein the acid gas stream further comprises one or more of ammonia  carbonyl disulphide  water  hydrocarbons and carbon disulphide.
4. The process as claimed in claim 3  wherein the acid gas stream comprises 50-94 vol% of hydrogen sulphide.
5. The process as claimed in claim 1  wherein the waste nitrogen gas coming from nitrogen plant comprises a minimum of 28 wt% of oxygen.
6. The process as claimed in claim 1  wherein the reaction between the waste nitrogen gas and the stream comprising hydrogen sulphide is performed in a Claus type sulphur recovery plant.
7. A system for re-utilizing waste nitrogen gas coming from nitrogen plant  said system comprising a Claus type sulphur recovery plant for reacting the waste nitrogen gas with a stream comprising hydrogen sulphide so as to obtain elemental sulphur  steam and vent gas stream and a sub-system connecting the nitrogen plant to the Claus type sulphur recovery plant for transferring the waste nitrogen gas thereto.
8. The system as claimed in claim 7  wherein the sub-system connecting the nitrogen plant to the Claus type sulphur recovery plant comprises:
a conduit connectable between an outlet of the nitrogen plant and an inlet of the Claus type sulphur recovery plant;
a storing means for temporarily storing the waste nitrogen gas coming from nitrogen plant; and
a control means for controlling the quantity and the rate at which the waste nitrogen gas is supplied to the Claus type sulphur recovery plant.
9. The system as claimed in claim 8  wherein the sub-system connecting the nitrogen plant to the Claus type sulphur recovery plant further comprises a blower.