FIELD OF THE INVENTION
The present disclosure relates to a process and apparatus for enhancement of the Claus
Sulphur unit (SRU) capacity by using oxygen enriched stream of waste nitrogen from nitrogen
plant, oxygen and steam.
BACKGROUND AND PRIOR ART
Generally, the sulphur recovery units employ Claus process for conversion of toxic hydrogen
sulfide gas to elemental sulphur. The sulphur plant uses reaction furnace, heat recovery
section consisting of sulphur condensers, process gas heaters and Claus converters for
conversion of hydrogen sulphide and sulphur dioxide to elemental sulfur in presence of solid
catalyst.
In a Claus process sulphur recovery unit, the first step is to oxidize hydrogen sulphide to
sulphur dioxide in the reaction furnace. In this section other components like ammonia and
hydrocarbon also oxidized to form nitrogen oxide, carbon dioxide and small amounts of
carbon disulphide and carbonyl sulfide. Hydrogen sulphide also reacts with sulphur dioxide to
form elementary sulphur in the reaction furnace but this reaction is made faster in the Claus
reactor in presence of activated Alumina. The sulphur vapour produced in the reaction furnace
and Claus reactors is separated from the gaseous phase in the sulphur condensers.
Conventional plant uses the normal atmospheric air in the Claus sulphur recovery unit.
A number of process are known in the prior art which enhance the capacity of the process. In
one of the invention, Giech US patent number 4,138,473 discloses the sulphur recovery
process using pure oxygen for combusting feed gas rather than the normal air in the reaction
furnace. It brings down the process gas quantity in the downstream of reaction furnace. But it
increases the reaction furnace temperature.
In another invention, Chen et. al. (US patent number 4,632,818) discloses the process which
uses oxygen enriched air and a liquid sulphur stream in the reaction furnace to moderate high
temperatures which is obtained due to use of oxygen enriched air. This process obviously
increases sulphur load to the downstream condensers and associated pipelines. Use of liquid
sulphur and its introduction to reaction furnace needs additional devices which may cause
both operational and maintenance problems.
3
In the 1983 publication by Linde of Union Carbide titled ‘Claus plant oxygen enrichment’, it
is noted that the oxygen enrichment limitation exists for rich hydrogen sulphide stream due to
the temperature limit in the reaction furnace and waste heat boiler at the downstream. US
patent number 4,279,882 discloses a sulphur recovery process which uses a series of catalytic
reactors rather than a reaction furnace. A temperature modifying recycle stream is set forth.
This process is economical only for dilute hydrogen sulphide applications. It needs a recycle
blower operating at high temperature.
US patent number 4,552,747 discloses improvement of oxygen enriched Claus sulphur plant
by recycling effluent from first condenser back to the reaction furnace to moderate high
temperature. The invention did not disclose extent of capacity enhancement by increasing
oxygen concentration from 30 to 90% and using recycle of 5 to 60% of combustion effluent
stream. It also explains impact of sulphur mist in recycling the process gas to the reaction
furnace. Probability of sulphur ingression may cause additional reaction like hydrogenation of
sulphur in the reaction furnace.
US patent number 5,139,764 discloses a process for processing ammonia containing sour gas
by using at least 90% pure oxygen. Process also uses additional acid gas which does not
contain ammonia. The process considers two reaction zones in the reaction furnace. The first
zone is operated at very high temperature and there is no temperature moderation in the first
zone. But the second zone moderates the temperature. Generally, oxygen enrichment process
needs temperature control for the complete operation of the reaction furnace to avoid
formation of COS and CS2 as well as thermal effect of refractory.
US patent number 4,756,900 discloses a process for splitting the effluent from waste heat
boiler after the reaction furnace and recycling a portion thereof using a separate sulphur
condenser and a mechanical blower to moderate the high furnace temperatures.
US patent number 4,552,747 describes a process for moderating the high temperature in
reaction of furnace induced by oxygen enrichment by passing the effluent through a condenser
and then using a mechanical blower to recycle the cold effluent to the reaction furnace.
In another invention, US patent number 6,508,998 describes a similar process but uses
eductor to recycle the cold effluent stream.
All these above-mentioned inventions describe different approaches for moderating the high
reaction furnace temperatures while oxygen enrichment is done to enhance the sulphur plant
capacity.
4
There are number of ways in sulphur plant to increase the capacity of the plant known in the
prior art. For example, utilizing oxygen with plant air reduces nitrogen content of the
combustion air and allows more feed to the plant. Another method uses direct injection of
oxygen to the reaction furnace along with air in a separate line. To mix oxygen and air, some
methods consider mixing device. Other applications include different sources of oxygen
enriched air including oxygen enriched air from nitrogen plant. Steam is used to control the
reaction furnace temperature. Steam along with air or oxygen or oxygen enriched air injection
to reaction furnace causes two phase resulting problem in reaction furnace operation. Two
phase system needs additional cost for pipelines as well as special design of the burner of
reaction furnace.
Further, high temperature causes damage of refractory and also reverses the reaction where
sulphur reacts with hydrogen to form hydrogen sulphide. It reduces sulphur recovery
conversion in the reaction furnace. Also, the SRUs known in the prior art are prone to back
pressure.
The present disclosure addresses one or more problems as discussed above and other
problems associated with the art by providing a process and an apparatus which utilizes
oxygen enriched stream from nitrogen plant, oxygen and steam in an efficient cost-effective
way for enhancement of the Claus Sulphur plant (SRU) capacity.
For example, the present disclosure overcomes to short coming of prior art by increasing
throughput of SRU with steam, oxygen and waste nitrogen beyond the feasible limits of the
prior art. It provides stable flame in the reaction furnace which provides complete combustion
of all impurities in the feed acid gas and feed sour water stripped gas in the reaction furnace.
In the present invention, use of oxygen enriched air sourced from nitrogen plant and pure
oxygen provides an approach for enhancing the capacity of sulphur plant. It moderates the
temperature by injecting low pressure steam either along with the combustion air or by mixing
with the feed acid gas stream. The invention also engages a specialized device for proper
mixing of waste nitrogen, air and pure oxygen at a minimum distance from main burner of the
sulphur plant.
Further, the use of oxygen in combustion air brings down back pressure of the plant. The back
pressure is further reduced when water injected in the sulphur plant is removed in the
5
downstream tail gas treating plant. Therefore, operation of both the SRU and its downstream
plant is improved.
The present disclosure minimizes the capital cost of the plant by reducing size of the
equipment’s in the sulphur plant and the downstream tail gas treating plant.
SUMMARY
The present invention relates to a process and apparatus for recovery of sulphur from gaseous
streams containing hydrogen sulphide, carbon dioxide, ammonia and other components like
hydrocarbons in a Claus Sulfur Recovery Unit (SRU). The process includes introducing pure
oxygen, oxygen enriched air from nitrogen plant and low-pressure steam into the reaction
furnace.
In a particular aspect, the process relates to high level recovery of elemental sulphur from
gaseous mixture of low pressure (LP) steam enriched acid gas and sour water stripper (SWS)
gas comprising hydrogen sulphide. The process involves introducing the low pressure (LP)
steam enriched acid gas, sour water stripper (SWS) gas and low pressure (LP) steam enriched
combustion air to the reaction furnace and passing through the Claus reactors.
The utilization of low cost waste nitrogen from nitrogen plant consisting of 30 to 35% (v)
oxygen, pure oxygen and air which is mixed and combined with low pressure (LP) steam and
introduced into the reaction furnace enhances the capacity of sulphur recovery unit by 100% ,
keeping the reaction furnace temperature close to the maximum tolerable temperature of 1500
oC.
The present invention also relates to an apparatus for sulphur recovery in a Claus Sulfur
Recovery Unit (SRU). The apparatus comprises
(a) reaction furnace, for combustion of hydrogen sulphide, hydrocarbons and ammonia,
having:
- an acid gas supply line and low pressure (LP) steam line directed to an acid gas
heater which is further routed to the reaction furnace;
- a sour water stripper (SWS) gas supply line directed to the reaction furnace; and
- a combustion air supply line comprising a mixing device followed by an air heater
directed to the reaction furnace;
(b) Claus reactor downstream of the reaction furnace comprising:
6
- heat exchangers to recover heat from hot gases,
- reheaters to heat up the cold gases and
- catalytic reactors to convert hydrogen sulphide to sulphur by reaction with sulphur
dioxide
wherein the mixing device is for proper mixing of air, pure oxygen and oxygen enriched air
from nitrogen plant,
wherein the composition of the gas leaving from reaction furnace ensures hydrogen sulphide
to sulphur dioxide ratio at 2:1 throughout the flow.
BREIF DESCRIPTION OF ACCOMPANYING FIGURES
The invention will be further understood and appreciated from the following detail description
and from figure 1 and figure 2, which illustrate a preferred embodiment of the invention and
comparative study if the conventional process with the invention.
Figure 1. shows the Claus sulphur recovery unit configuration which uses the plant air. The
air blower supplies air to the main burner of the reaction furnace. The process uses acid gas
heater, main burner, main combustion chamber, waste heat boiler (WHB), condensers,
reheaters and converters to convert hydrogen sulphide in feed acid gas and feed sour water
stripper gas (SWS Gas). The Boiler feed water (BFW) is provided in WHB and condensers.
Figure 2. shows the present disclosure with the Claus sulphur recovery unit configuration
which uses the waste nitrogen from nitrogen plant, oxygen from liquid oxygen storage, and
low-pressure (LP) steam with additional equipment. The additional equipment is mixing
device and air heater when low pressure steam is injected in the oxygen enriched air. The
condition when low pressure steam is injected to the feed acid gas line, does not need air
heater.
DETAILED DESCRIPTION
The present disclosure relates to a process and an apparatus for removal of hydrogen sulfide
from an acid gas or amine regenerator off gas and sour water stripper by oxygen enriched
waste nitrogen, oxygen from liquid oxygen storage and low-pressure steam in a Claus Sulfur
Recovery Unit (SRU).
In accordance with the teachings of the present disclosure, the process involves complete
combustion of hydrocarbons in acid and sour gas stripper. By following teachings of present
7
disclosure, it is possible to reduce pressure of SRU and to process additional feed acid gas and
feed sour water stripper gas in SRU.
According to an embodiment, the present disclosure relates to a process for sulfur recovery
from an acid gas and sour water stripper gas containing H2S in a Claus Sulfur Recovery Unit
(SRU), the said process comprising the steps:
a. introducing into a reaction furnace a mixed stream of:
• acid gas (AG) feed;
• combustion air feed;
• sour water stripper (SWS) gas feed;
b. processing a tail gas received from a last condenser in a downstream tail gas treating
unit and recycling remaining H2S of the tail gas to a front end of the sulfur recovery
unit (SRU);
wherein, the combustions air is an oxygen enriched air comprising waste nitrogen, pure
oxygen and atmospheric air.
According to an embodiment of the present disclosure, the acid gas (AG) comprises mixed
low pressure (LP) steam.
According to an embodiment of the present disclosure, the acid gas (AG) which is mixed with
low pressure (LP) steam and introduced through an acid gas-heater.
According to an embodiment of the present disclosure, the combustion air comprises mixed
LP steam.
According to an embodiment of the present disclosure, the combustion air is prepared in a
mixing device and introduced through an air heater.
According to an embodiment of the present disclosure, the feed acid gas utilized having H2S
range of 30 to 95% and the sour water stripper gas utilized having H2S range of 30 to 90%.
According to an embodiment of the present disclosure, the mixed stream of step (a) is heated
to the process gas temperature above its dew point to avoid condensation and to produce a hot
gas after combustion.
8
According to an embodiment of the present disclosure, the combustion of mixed stream of
step (a) in the reaction furnace converts H2S to SO2.
According to an embodiment of the present disclosure, the hot gas from reaction furnace is
cooled, removing sulfur vapour as liquid sulfur and introducing the gas into two or more
Claus reactors after heating converts H2S and SO2 to elementary sulfur.
According to an embodiment of the present disclosure, condensation of vapour from the
cooled gas in a condenser downstream of each Claus reactor takes palace and then reheating
the cooled gas after each condenser before processing in the next Claus reactor;
According to an embodiment of the present disclosure, the hot gas leaving from the reaction
furnace is passed through two or more Claus reactors, preferably 2 to 3 Claus reactors.
According to an embodiment of the present disclosure, the composition of the hot gas leaving
from the reaction furnace ensures hydrogen sulphide to sulphur dioxide ratio at 2:1 throughout
the flow.
According to an embodiment of the present disclosure, the cooled gas is reheated to
temperature of 180 to 160 oC.
According to an embodiment of the present disclosure, the percentage of LP steam added to
acid gas is 20 to 30% (v).
According to an embodiment of the present disclosure, the concentration of oxygen is from
20% to 66.6% (v).
According to an embodiment of the present disclosure, the maximum concentration of oxygen
in waste nitrogen is 35 % (v).
According to an embodiment of the present disclosure, the percentage of LP steam added to
acid gas is in the range of 20 to 30% (v).
According to one of the embodiments of the present disclosure, the expression “mixed low
pressure steam” is understood for the present disclosure as when the steam is mixed with other
gases such as an acid gas, sour water stripper gas or combustion air.
9
According to an embodiment of the present disclosure, the reduction of process gas
temperature in the reaction furnace is 1.4oC for use of every Kmol of LP steam with acid gas.
According to an embodiment of the present disclosure, the air flow rate in combustion air to
reaction furnace varies from 100% to 35% (v).
According to an embodiment of the present disclosure, the use of pure oxygen reduces by 3%
and hence reduces the operating cost of the unit.
According to an embodiment of the present disclosure, the capacity of the sulphur recovery
unit increases 50% to 100% over the sulphur recovery unit being operated with the normal
air.
According to one of the embodiments, the process relates to high level recovery of elemental
sulphur from gaseous mixture of low pressure (LP) steam enriched acid gas and sour water
stripper gas comprising hydrogen sulphide respectively. The process involves introducing the
low pressure (LP) steam enriched acid gas, sour water stripper gas and low pressure (LP)
steam enriched combustion air to the reaction furnace and passing through the Claus reactors.
The utilization of low cost waste nitrogen from nitrogen plant consisting of 30 to 35% (v)
oxygen, pure oxygen and air which is mixed and combined with low pressure (LP) steam and
introduced into the reaction furnace enhances the capacity of sulphur recovery unit by 100% ,
keeping the reaction furnace temperature close to the maximum tolerable temperature of 1500
oC.
In one of the embodiments, the process is provided for recovery of sulphur from hydrogen
sulphide by Claus reaction in the Claus catalytic reactors. Low pressure (LP) steam, used in
the reaction furnace, acts as a diluent and it moderates the reaction furnace temperature. In the
absence of the diluent, the oxidation of gaseous streams enriched with hydrogen sulphide and
oxygen enriched air causes high temperature in the reaction furnace. The high temperature
affects the refractory of reaction furnace. The hot gas or combustion gas produced in the
reaction furnace comprises hydrogen sulphide, sulphur dioxide, carbon dioxide, water,
nitrogen, carbon disulphide, carbonyl sulphide and elemental sulphur. This gas is cooled to
remove elemental sulphur species and the heat of the gas is recovered by steam production in
waste heat boiler.
10
In another embodiment, the present invention also relates to an apparatus for enhanced
sulphur recovery in a Claus Sulfur Recovery Unit (SRU) wherein the apparatus comprises:
(a) Reaction furnace, for combustion of hydrogen sulphide, hydrocarbons and ammonia,
having:
- an acid gas supply line and low pressure (LP) steam line directed to an acid gas
heater which is further routed to the reaction furnace;
- a sour water stripper (SWS) gas supply line directed to the reaction furnace; and
- a combustion air supply line comprising a mixing device followed by an air heater
directed to the reaction furnace;
(b) Claus reactor downstream of the reaction furnace comprising:
- heat exchangers to recover heat from hot gases,
- reheaters to heat up the cold gases and
- catalytic reactors to convert hydrogen sulphide to sulphur by reaction with sulphur
dioxide
In yet another embodiment of the process and apparatus, the mixing device is utilized for
proper mixing of air, pure oxygen and oxygen enriched air from nitrogen plant,
In yet another embodiment of the process and apparatus, the composition of the gas leaving
from reaction furnace ensures hydrogen sulphide to sulphur dioxide ratio at 2:1 throughout the
flow.
In another embodiment of the process and apparatus, the temperature of the reaction furnace
which is maintained in the range of 1300 oC to 1500 oC, preferably in the range of 1400 oC to
1500 oC.
According to the further aspect of the process and apparatus, the diluent that is low pressure
(LP) steam is condensed out in the downstream quench column. Water removal means low
process gas flow to the tail gas treating unit which is integrated at the downstream of sulphur
recovery unit.
According to a further aspect of the process and apparatus, decrease in nitrogen concentration
in the process gas that is leaving the reaction furnace allows more hydrogen sulphide enriched
gaseous streams into the reaction furnace.
11
In one of the embodiments, the process and apparatus of the invention enhances the capacity
of the process by 50 to 00%.
The present invention utilizes waste nitrogen to enrich oxygen level in the combustion air.
The concentration of the oxygen in waste nitrogen is from 30% (v)to 35% (v) but by injecting
pure oxygen, the level of oxygen in the combustion air increases, preferably to 66.6% (v).
This allows more gaseous streams with hydrogen sulphide into the reaction furnace.
Decrease in process gas flow from the sulphur recovery unit (SRU) improves the operation,
reduces energy requirement in the downstream tail gas treating unit engaged in recovery of
unreacted sulphur compounds from the tail gas. The disclosed process describes overall
sulphur recovery of 99.9%.
Accordingly, the process and the apparatus of the invention increase the capacity of SRU by
100% where SRU processes the acid gas and sour water stripper gas containing hydrogen
sulphide. The present disclosure shows that the use of waste nitrogen along with oxygen and
steam leads to the minimization of the process gas flow in the SRU and reduction of reaction
furnace temperature.
In one of the embodiments of the process and apparatus, the combustion air is prepared by
complete mixing of waste nitrogen with oxygen, in this regard, the process uses a mixing
device before reaction furnace.
In another embodiment, the process and the apparatus of the present disclosure uses a mixing
device for complete mixing of waste nitrogen and oxygen before its entry to the main burner
of reaction furnace.
In another embodiment of the process and apparatus, low pressure (LP) steam is injected
either into the feed acid gas line or into the oxygen enriched air-line. The process or the
apparatus uses a heater to increase the temperature of acid gas above its dew point when low
pressure steam is injected into the acid gas.
The steam moderates the reaction furnace temperature. Further, the use of waste nitrogen
which gives oxygen enriched air from nitrogen plant minimizes requirement of oxygen. The
capacity of SRU is increased by this new process and apparatus up to 100% over the normal
capacity of the unit designed for operation with plant air. The process and the apparatus uses
specialized device to mix oxygen enriched air from nitrogen plant and oxygen for the desired
12
combustion of hydrogen sulphide, hydrocarbons and other combustible components in the
feed gas to the sulphur plant.
According to one of the embodiments, the process and the apparatus use dedicated heater to
prevent two phases formation in the acid gas when the low-pressure steam is injected into the
acid gas.
In another embodiment, the process and the apparatus of the present disclosure uses low
pressure (LP) steam which is injected either into the feed acid gas line or into the oxygen
enriched combustion air-line. After mixing, the mixed stream is heated before its entry to the
main burner of the reaction furnace. The process uses the acid gas-heater to heat the feed acid
gas or the mixed feed acid gas. The process and the apparatus uses air-heater when the lowpressure steam is mixed with the oxygen enriched air.
In one of the embodiments, the process and apparatus uses low pressure (LP) steam to
maintain the reaction furnace temperature below 1500 oC for processing 50% to 100%
additional feed acid gas and feed sour water stripper gas in the SRU. The maintaining of
reaction furnace temperature below 1500 oC ensures use of conventional and low-cost
refractory in the reaction furnace and thus, reduces cost of the plant.
In another embodiment, the process and the apparatus of the present disclosure maintains
oxygen concentration below 67% (v), and nitrogen concentration below 32% (v) in the
mixture comprising of waste nitrogen, oxygen and plant air.
In another embodiment, the process and the apparatus of the present disclosure maintains
hydrogen sulfide to sulphur dioxide ratio at 2:1 in the reaction furnace for maximum
conversion of H2S to elementary sulphur. However, high temperature in the reaction furnace
provides cracking of H2S to sulphur and hydrogen which gives better conversion of H2S to
sulphur. Generation of hydrogen reduces hydrogen requirement in the downstream tail gas
treating plant for conversion of sulphur bearing compounds like SO2 and polysulphur to H2S.
In an embodiment of the present disclosure, the process and the apparatus uses waste nitrogen
from nitrogen plant which is conventionally vented to atmosphere. It reduces operating cost
for processing H2S in acid gas and sour water stripper gas.
The present disclosure enriches waste nitrogen which contains maximum 35% (v) oxygen.
The oxygen concentration of waste nitrogen is increased upto 67% (v) by injection of oxygen
13
from liquid oxygen storage section. It minimizes nitrogen content in the combustion air and
consequently increases capacity of the sulfur plant.
In an embodiment of the present disclosure, use of low-pressure (LP) steam along with the
oxygen enriched combustion air or with the feed acid gas maintains reaction furnace
temperature below 1500°C.
In an embodiment, the mixing device used in the present disclosure ensures complete mixing
of waste nitrogen and oxygen before its use in the main burner of reaction furnace. It provides
stable flame in the combustion chamber resulting in complete combustion of undesirable
components like hydrocarbons and traces of ammonia in feed stream.
In an embodiment of the present disclosure, use of steam causes water condensation in the
combustion air-line to the main burner of the reaction furnace. Accordingly, the present
disclosure provides dedicated feed acid gas heater or air heater to increase the temperature of
mixed gas above its dew point. Therefore, it prevents malfunction of the main burner.
In an embodiment, the present disclosure provides increase in capacity of the SRU from 50%
to 100% over the capacity of the unit operated by the plant air.
In yet another embodiment, the present disclosure allows processing of feed acid gas having
H2S range of 30 to 95% (v) and the sour water stripper gas having H2S range of 30 to 90% (v).
In yet another embodiment, the present disclosure provides revamp of running SRU and
downstream tail gas treating plat at minimum investment and shutdown period.
In yet another embodiment, the present disclosure reduces the capital cost of SRU and its
downstream tail gas unit because of smaller size of the equipment.
In yet another embodiment, the present disclosure utilizes waste nitrogen in the process
stream, which is otherwise vented out in refineries. The present process and apparatus not
only increase SRU capacity but also reduces the overall cost of incurring liquid oxygen/other
oxygen sources on regular basis.
In another embodiment, the present disclosure utilizes liquid oxygen from the storage and
waste nitrogen from nitrogen plant along with the acid gas, sour gas and combustion air.
14
In yet another embodiment of the present disclosure, the mixing device is provided for the
proper mixing of oxygen, waste nitrogen and steam thereby allowing SRU burner to operate
with stable flame.
In another embodiment, the present disclosure utilizes mixing device for proper mixing of air,
oxygen and waste nitrogen before main burner.
In yet another embodiment, the present disclosure provides the combination of waste nitrogen
along with oxygen and combustion air for capacity enhancement. This combination, to reduce
the overall operating cost, is not known in the prior art.
In another embodiment, the present disclosure provides the capacity enhancement of 100%
overcoming the problems in lower capacity enhancement.
In yet another embodiment, the use of waste nitrogen and low-pressure (LP) steam produced
in SRU makes this invention economically beneficial.
In another embodiment, the LP steam in the acid gas line which is routed to acid gas preheater
to avoid condensation.
In one of the embodiments, combustion of acid gas mixed with LP steam, H2S enriched sour
water striper (SWS) gas and oxygen enriched combustion air in the reaction furnace is carried
out such that a conversion of 1/3rd of H2S to SO2 is achieved and H2S:SO2 ratio of 2:1 is
maintained.
In one of the embodiments, combustion of acid gas, plant air and H2S and enriched and NH3
enriched sour water striper gases into reaction furnace for converting 1/3rd of to H2S to SO2
and maintaining 2:1 ratio of H2S:SO2.
In one of the embodiments, the process and the apparatus involves cooling the hot gas or
combustion gas from reaction furnace and to remove sulfur vapor as liquid sulfur before its
introduction to Claus reactors.
In one of the embodiments, the process and the apparatus involves subjecting the cooled
combustion gas to two or more Claus reactors after heating for converting H2S and SO2 to
elementary sulphur.
15
In one of the embodiments, the process and the apparatus involves condensing vapor in the
condenser at the downstream of each reactor and warming the cooled gas after each condenser
before processing in each Claus reactor.
In one of the embodiments, the process and the apparatus involves processing a tail gas from a
last condenser in downstream tail gas treating unit for recycling remaining H2S to the front
end of the sulphur recovery unit and maintaining minimum 99.9% (wt) overall sulphur
recovery.
In one of the embodiments, the process and the apparatus wherein the H2S content in acid gas
varies from 30 % to 95 % (v) and in sour gas 30% to 90% (v).
In one of the embodiments, in absence of LP steam and oxygen enriched air, the temperature
of reaction furnace is about 1409°C.
In one of the embodiments, the percentage of LP steam added to acid gas is in the range of
20% to 30% (v).
In one of the embodiments, the oxygen concentration in the combustion air varies from 20%
to 66.67% (v).
In one of the embodiments, the waste nitrogen flow to mixing device is 0 to10% (v).
In one of the embodiments, the air flow rate in combustion air to reaction furnace varies from
100% to 35% (v).
In one of the embodiments, the oxygen concentration in waste nitrogen varies upto 35% (v).
In one of the embodiments, the use of pure oxygen reduces by 3% and hence reduces the
operating cost of the unit.
In one of the embodiments, a special mixing device is engaged to mix air, waste nitrogen and
pure oxygen for generating a homogenous mixture of combustion air in short distance from
main burner.
In one of the embodiments, concentration of oxygen varies from 90% to 100% (v) depending
on the source.
In one of the embodiments, the capacity of the sulphur recovery unit increases 50% to 100%
of the acid gas and sour water stripper gas to sulphur recovery unit operated with plant air.
16
In one of the embodiments, reduction of process gas temperature in the reaction furnace is 1.4
oC for use of every Kmol of LP steam with acid gas.
In one of the embodiments, a dedicated acid gas preheater is provided to increase the process
gas temperature above its dew point and to prevent any malfunctioning of main burner.
In one of the embodiments, present invention provides revamp of running SRU and
downstream tail gas treating unit at minimum investment and shut down period.
In one of the embodiments, present invention reduces the capital cost of sulphur recovery unit
and downstream tail gas treating plant for grass root unit owing to smaller size equipment.
Further salient features and enhancements of the process and apparatus are discussed in the
examples provided below.
The invention will be further understood and appreciated by the following example with
reference to figure 1 and figure 2.
Examples
Eleven cases were evaluated for the efficiency in recovery of sulfur for an acid gas and a sour
water stripper gas fed to the main burner of reaction furnace. The acid gas consists of 90.3%
(v) of H2S and 8.5% (v) of moisture, besides trace amount of CO2 and hydrocarbons in acid
gas. Low pressure (LP) steam is injected in all the cases from case 2 onwards to moderate
flame temperature. The oxygen and waste nitrogen were added into combustion air through a
mixing device for case 2 to case 11 and oxygen concentration is maintained 66.7% in the
enriched air. For study on effect of LP steam on flame temperature and process gas flow from
reaction furnace, the acid gas was getting continuous increased flow of LP steam for case 2 to
case 6. In case 2, the reaction furnace temperature reached close to 1500°C which is
maximum allowable temperature of reaction furnace. To control the temperature below the
maximum limit, LP steam with increased flow was injected. The result showed decrease in
temperature. Study concluded the reduction of temperature by 1.4°C for each Kmol of
addition of steam.
For case 7 to case 11, steam and acid gas flow were adjusted to maintain higher throughput
and controlling the temperature at 1500°C. The results of this study showed capacity
enhancement of upto 100% of initial acid gas flow and sour water stripper flow shown in Case
1. There was an increase in process flow from reaction furnace by 3.73% as shown in Case
10. Designers of sulphur recovery unit keep margin of 25% in reaction furnace hydraulics.
17
The Case 10 concluded the enhancement upto 100% with marginal but acceptable increase in
hydraulic load. Keeping acid gas treatment on priority, the capacity of the unit can be
increased under same hydraulic load by adjusting sour water stripper flow at 50%
enhancement. The overall sulphur recovery of all the cases was around 96% wt. which was
similar to that for Claus sulphur recovery unit consisting of two reactors.
Thus, the utilization of combustion air comprising waste nitrogen in all the case studies
showed benefit in consumption of pure oxygen and air. Waste nitrogen flow was kept 9 % (v)
for the benefit of 3% reduction in pure oxygen. Both waste nitrogen and pure oxygen reduced
air flow upto 87 % which saved the power required for air blower. The case studies
maintained all the process parameters of SRU close to the normal operation of conventional
unit and it confirm that no major modification required for capacity enhancement of a running
unit. Table 1 shows the results of case studies.
Table 1
Case 1 Case2 Case3 Case4 Case5 Case6 Case7 Case8 Case9 Case10 Case11
Acid Gas kmole/hr 228.52 342.78 342.78 342.78 342.78 342.78 399.91 434.19 445.61 457.04 457.04
H2S rich Sour Water Stripper
Gas kmole/hr 26.28 39.41 39.41 39.41 39.41 39.41 45.98 49.92 51.24 52.55 36.79
NH3 rich Sour Water Stripper
Gas kmole/hr 32.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Steam added to Acid Gas kmole/hr 0.00 80.00 85.00 90.00 95.00 100.00 90.00 100.00 100.00 100.00 100.00
Steam added into Acid Gas %wt 0.00 23.34 24.80 26.26 27.71 29.17 22.51 23.03 22.44 21.88 21.88
CAPACITY ENHANCEMENT
FACTOR
Acid Gas 1.00 1.50 1.50 1.50 1.50 1.50 1.75 1.90 1.95 2.00 2.00
H2S rich Sour Water Stripper
Gas 1.00 1.50 1.50 1.50 1.50 1.50 1.75 1.90 1.95 2.00 1.40
NH3 rich Sour Water Stripper
Gas 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
H2S in AG mixed with Steam % Mole 90.30 73.21 72.36 71.52 70.71 69.91 73.71 73.75 74.09 74.09
H2O in AG mixed with Steam % Mole 8.47 25.79 26.65 27.50 28.33 29.14 25.28 25.24 24.90 24.90
Acid Gas Temperature oC 90.00 94.60 94.84 95.08 95.30 95.53 94.47 94.55 94.46 94.36 94.36
Air Temperature oC 100.61 67.60 67.60 67.60 67.60 67.60 67.60 67.60 67.60 67.60 67.60
Oxygen Enriched Air to Reaction
Furnace kmole/hr 594.8400 212.5500 213.0400 213.2800 213.7300 214.2000 247.6800 269.3000 275.9600 282.9700 269.3300
% of Waste Nitrogen 0.0000 9.0576 9.0579 9.0580 9.0577 9.0579 9.0577 9.0579 9.0578 9.0579 9.0577
% of Oxygen 0.0000 56.6309 56.6311 56.6312 56.6313 56.6312 56.6311 56.6313 56.6310 56.6311 56.6313
% of air 100.0000 34.3105 34.3105 34.3103 34.3096 34.3105 34.3104 34.3104 34.3104 34.3104 34.3107
% Reduction in air flow 0.0000 87.7401 87.7118 87.6980 87.6723 87.6449 85.7138 84.4667 84.0826 83.6783 84.4649
Composition of Oxygen Enriched
Air
H2O mol
fraction 0.0480 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200
O2 mol
fraction 0.1996 0.6665 0.6665 0.6665 0.6665 0.6665 0.6665 0.6665 0.6665 0.6665 0.6665
N2 mol
fraction 0.7524 0.3135 0.3135 0.3135 0.3135 0.3135 0.3135 0.3135 0.3135 0.3135 0.3135
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Reaction Furnace Process Gas
Temperature Deg C 1408.74 1496.33 1489.37 1483.92 1477.97 1471.73 1499.34 1497.90 1499.66 1501.60 1488.38
REACTION FURNACE
PROCESS GAS FLOW Kmole/hr 886.41 694.84 700.04 705.20 710.45 715.67 807.10 878.87 899.03 919.48 887.52
CHANGE IN PROCESS GAS
FLOW % wt
-21.61 -21.03 -20.44 -19.85 -19.26 -8.95 -0.85 1.42 3.73 0.12
Sulphur Recovery from SRU %wt 96.20 96.24 96.20 96.15 96.09 96.03 96.19 96.27 96.28 96.30 96.27
Although the invention has been described in terms of preferred embodiment giving specific
temperature, process flow and other operating conditions as required for design and operation
of sulphur recover unit, the invention is not limited thereby but all use made of the invention
as defined by the claims appended hereto.
The advantages of the disclosed invention are thus attained in an economical, practical and
facile manner. While preferred embodiments and example have been shown and described, it
is to be understood that various further modifications and additional configurations will be
apparent to those skilled in the art. It is intended that the specific embodiments herein
disclosed are illustrative of the preferred and best modes for practicing the invention and
should not be interpreted as limitations on the scope of the invention.

We Claim:
1. A process for sulfur recovery from an acid gas and sour water stripper gas containing H2S
in a Claus Sulfur Recovery Unit (SRU), the said process comprising the steps:
c. introducing into a reaction furnace a mixed stream of:
• acid gas (AG) feed;
• combustion air feed;
• sour water stripper (SWS) gas feed;
d. processing a tail gas received from a last condenser in a downstream tail gas treating
unit and recycling remaining H2S of the tail gas to a front end of the sulfur recovery
unit (SRU);
wherein, the combustions air is an oxygen enriched air comprising waste nitrogen, pure
oxygen and atmospheric air.
2. The process as claimed in claim 1, wherein the acid gas (AG) comprises mixed low
pressure (LP) steam.
3. The process as claimed in claim 2, wherein the acid gas (AG) mixed low pressure (LP)
steam is introduced through an acid gas-heater.
4. The process as claimed in claim 1, wherein the combustion air comprises mixed LP steam.
5. The process as claimed in claim 4, wherein the combustion air is prepared in a mixing
device and introduced through an air heater.
6. The process as claimed in claim 1, wherein the feed acid gas having H2S in the range of 30
to 95% and the sour water stripper gas having H2S range of 30 to 90%.
7. The process as claimed in claim 1, wherein the mixed stream of step (a) is heated to the
process gas temperature above its dew point to avoid condensation and to produce a hot
gas after combustion.
8. The process as claimed in claim 1, wherein the hot gas is passed through two or more
Claus reactors, preferably 2 to 3 Claus reactors.
9. The process as claimed in claim 1, wherein the composition of the hot gas leaving from
the reaction furnace ensures hydrogen sulphide to sulphur dioxide ratio at 2:1 throughout
the flow.
10. The process as claimed in claim 1, wherein the hot gas is cooled in a condenser
downstream of each Claus reactor and then reheating the cooled gas after each condenser
before processing in the next Claus reactor.
20
11. The process as claimed in claim 1, wherein the cooled gas is reheated to temperature range
of 180 to 160 oC.
12. The process as claimed in claim 1, wherein the percentage of LP steam added to acid gas
is in the range of 20 to 30% (v).
13. The process as claimed in claim 1, wherein the concentration of oxygen is in the range of
20% to 66.6% (v).
14. The process as claimed in claim 1, wherein the maximum concentration of oxygen in
waste nitrogen is 35 % (v).
15. The process as claimed in claim 1, wherein the reduction of the process gas temperature in
the reaction furnace is 1.4oC for use of every Kmol of LP steam with acid gas.
16. The process as claimed in claim 1, wherein the air flow rate in combustion air to reaction
furnace varies in the range from 100% to 35% (v).
17. The process as claimed as in claim 1, wherein the use of pure oxygen reduces by 3% the
operating cost of the unit.
18. The process as claimed in claim 1, wherein the capacity of the sulphur recovery unit
increases from 50% to 100%.
19. An apparatus for enhancement of sulphur recovery in a Claus Sulfur Recovery Unit (SRU)
wherein the apparatus comprises:
(a) a reaction furnace, for combustion of hydrogen sulphide, hydrocarbons and ammonia,
having:
- an acid gas supply line and low pressure (LP) steam line directed to an acid gas
heater which is further routed to the reaction furnace;
- a sour water stripper (SWS) gas supply line directed to the reaction furnace; and
- a combustion air supply line comprising a mixing device followed by an air heater
directed to the reaction furnace;
(b) a Claus reactor downstream of the reaction furnace comprising:
- heat exchangers to recover heat from hot combustion gases,
- reheaters to heat up the cold gases and
- catalytic reactors to convert hydrogen sulphide to sulphur by reaction with sulphur
dioxide.
21
20. The apparatus as claimed in claim 19, wherein the mixing device is utilized for mixing of
air, pure oxygen and oxygen enriched waste nitrogen from a nitrogen plant.
21. The apparatus as claimed in claim 19, wherein the composition of the gas leaving from
reaction furnace ensures hydrogen sulphide to sulphur dioxide ratio at 2:1 throughout the
flow.
22. The apparatus as claimed in claim 19, wherein the temperature of the reaction furnace is
maintained in the range of 1300 oC to 1500 oC
23. The apparatus as claimed in claim 19, wherein the temperature of the reaction furnace is
maintained in the range of 1400 oC to 1500 oC.