WO2004024622A1

WO2004024622A1 - A process for the recovery of sulphur from gas streams containing hydrogen sulphide

Info

Publication number: WO2004024622A1
Application number: PCT/GB2003/003734
Authority: WO
Inventors: Amer Sarssam; Peter Frank Cross
Original assignee: Amer Sarssam; Peter Frank Cross
Priority date: 2002-09-12
Filing date: 2003-08-28
Publication date: 2004-03-25
Also published as: GB2392907B; GB2392907A; GB0221119D0; AU2003259380A1

Abstract

This invention relates to a process for the recovery of elemental sulphur from a gas stream containing hydrogen sulphide using Claus and other reactions with oxygen or oxygen enriched air. The process features an oxidation system with in-situ heat removal which may be followed by one or more catalytic stages.

Description

A PROCESS FOR THE RECOVERY OF SULPHUR FROM GAS STREAMS CONTAINING HYDROGEN SULPHIDE

Description

The removal of hydrogen sulphide and other contaminants features in many industrial processes such as the refining of crude oil, conversion of solid fuels and the conditioning of natural gas. This is done so that useful products can be made which meet internationally accepted specifications. The hydrogen sulphide stream removed, a highly toxic gaseous mixture, is usually sent to a sulphur recovery plant in order to recover the bulk of the sulphur from the waste gas and to substantially destroy the other contaminants.

The usual route for recovering sulphur from waste hydrogen sulphide streams is the Modified Claus process also commonly known as the Claus process. That process, which has seen improvements over the years, continues to be the most popular and economic route for bulk recovery of high grade elemental sulphur. The main chemical reactions of the Claus process are given below:

H2S + 3/2 02 SO2 + H2O (1)

2H2S + S02 <«— r 3S + 2H2O (2)

Equations 1 and 2 can be combined to give:

H2S + 1/2 02 ^~^—T S + H2O (3)

The oxidation of hydrogen sulphide (1) and the sulphur synthesis reaction (2) are exothermic. For simplicity, a univalent sulphur atom is shown in the equations above. Other oxidation, reduction and dissociation reactions also take place which include reactions of and between oxygen, hydrogen sulphide, carbon disulphide, carbonyl sulphide, nitrogen, hydrogen, ammonia, phenol, aromatics, hydrocarbons and other contaminants.

One of the important subsidiary reactions relate to the destruction of ammonia, the reduction in carbon based side products that affect sulphur quality and the minimisation of side products containing sulphur.

A reaction which generally favours the recovery of sulphur and helps with the use of any hydrogenation step often positioned down-stream of the Claus reactors is the dissociation of hydrogen sulphide as generally represented by equation (4).

H2S «<_ H2 + S (4)

Dissociation of H2S in reaction (4) is significant at high temperatures, a factor that is particularly relevant to the use of oxygen in Claus plants. In the usual configuration of the Claus process using air and/or oxygen cooling of the hot gases from the thermal oxidation step is somewhat slow and the opposite reaction i.e. from right to left becomes important. Thus only a small overall dissociation of the H2S takes place. A common configuration of the Claus process involves a combustion or thermal stage, with a residence time of typically one second, where reaction (1) takes place. Other reactions also take place such as reaction (2) and also a number of side reactions. Further conversion to sulphur requires a catalyst. Normally alumina and titania based catalysts are used although other catalysts with selective properties can be used to facilitate specific reactions.

Fig 1 is a schematic diagram of a conventional Claus process plant flow-sheet showing a combustion chamber or thermal stage (10) where feed gas containing H2S (3) is burnt in air or enriched air (5). Combustion products (11) are then cooled in a waste heat boiler (15) and sulphur formed is condensed out in the sulphur condenser (20). The process gas stream is then heated to a suitable temperature in a re-heater (25) and passed over a catalyst in a catalytic reactor (30) to produce more sulphur which is then recovered in a condenser (35). Further sulphur production can be achieved by the process of additional re-heating (40), catalytic stage (45) and sulphur condensation (50). This procedure can be repeated but economics usually dictate that two or three catalytic stages may present an optimum configuration to balance out the overall sulphur recovery efficiency with plant cost and complexity.

Re-heating of the process gas before the catalytic stages can be achieved in a number of ways. One of the established techniques is to burn fuel gas or part of the process gas in air in order to raise the overall bulk temperature. The air required for that duty is a small part of the total air demand. Other methods of re-heat such as use of steam or electrical power are also common.

The conditions in the thermal stage result in a number of side reactions and the process gas leaving the Claus furnace is therefore a complex mixture of hydrogen sulphide, sulphur dioxide, sulphur, carbonyl sulphide, carbon disulphide, water, nitrogen, hydrogen, carbon monoxide and traces of other compounds. Prior art exists to minimise the impact of these side products on the overall process and on sulphur recovery efficiency and product purity.

The use of oxygen to supplement or even to replace combustion air has been discussed in literature extensively and is in practice at many Claus type plants. Enrichment of air with oxygen has the benefit of reducing the size of plant equipment within the Claus process because of the reduction in the amount of nitrogen passing through. For existing plant, use of oxygen to replace part or all the air results in increasing the capacity of the plant, usually at minimal extra cost. Many installations have been converted or built to take advantage of this. The advantage in reducing capital usually outweighs the actual cost of oxygen that must be imported, stored or produced at site.

Prior art exists where techniques are used to enable the use of oxygen to either supplement or in some cases to replace air as the combustion medium. The usual approach in a Claus plant is to burn the acid gas in a refractory lined thermal reactor also known as the combustion chamber. The main technical difficulty in using oxygen is the high temperature that results from reaction equation 1.

Thermal limits exist which dictate the maximum practical temperature which the thermal stage can accommodate and that is in the region of 1600 degrees Celsius and is dictated by the mechanical properties of the refractory lining the reaction furnace must have to protect its metallic body. Normally, this means that oxygen concentration in air to be introduced into the thermal stage cannot exceed about 65%. The exception to that is the case of a dilute feedstock containing a non-combustible such as CO2 where the maximum allowable temperature limit is not breached even with pure oxygen. However, in the great majority of applications, especially in oil refineries, H2S concentration can be 90% or thereabouts. For such concentrations use of undiluted oxygen in such a manner would result in temperatures well in excess of the capability of materials in normal use.

The thermal stage in a Claus plant is generally supplied with a bumer and an ignitor. A complex burner management and safety system is provided. Burners which use oxygen are generally more complex than those using air and are, arguably, more susceptible to damage due to the significantly higher temperatures which would prevail with oxygen. Burners are normally made of refractory metals such as high grade stainless steel.

Several designs have been proposed or practised in the Claus process to add and combust oxygen and oxygen enriched air, such as:

1. Enrichment of ambient air with oxygen into the main burner, using conventional burner design. About 2&% oxygen concentration is the limit set by conventional burner / pipeline metallurgy.

2. Use of specialised burners at higher oxygen concentrations. Oxygen concentration is limited by refractory maximum allowable temperature, in the order of 1600 Celsius.

3. Use of dual combustion and cooling to limit the temperature of the system. The intermediate cooling allows the full temperature rise over two steps as is disclosed for example in EP-A-237 216 and EP-A-237 217.

4. Use of side stream combustion system followed by cooling to limit the temperature rise in the combustion chamber as is disclosed for example in British Patent 8702132.

5. Use of a gas recycle system to moderate the temperature rise in the thermal stage as disclosed for example in EP-A-165 609.

Most technical developments have tried to address the excessive temperature increase resulting from the use of oxygen in place of air.

Thus it can be seen that known techniques to address the use of commercial grade or pure oxygen can be grouped as follows:

A - Dilution of the oxygen stream

B - Dilution of the process gas stream

C - Intermediate cooling of the hot gas stream.

The above options and variations have differing drawbacks including operating complexity, safety considerations, capital investment, plant space requirements, instrumentation and control to name but a few. A common drawback to all the combustion processes is the need for a refractory lined combustion chamber which has economic, operational, environmental and regulatory disadvantages. Ideally, the answer to the problem lies in the ability to replace air, to any desired extent, with oxygen and at the same time have a simple and less costly process configuration without metallurgical constraints and preferably without the need for sophisticated and expensive burner and burner management system. This is what the present invention claims to achieve.

According to the present invention there is provided a method of treating a feed gas stream containing hydrogen sulphide and other contaminants, comprising :-

1. Pre-heating to achieve reaction starting temperature.

2. Oxidation of the feed gas stream using oxygen or enriched air.

3. In-situ cooling at conditions in which free oxygen will continue to react with the process gases, but at which partial thermal quench of the resulting products occurs

4. Extraction of produced sulphur.

5. Optional further conversion of H2S to sulphur and sulphur product extraction using catalytic converters.

The reaction between oxygen or enriched air and H2S bearing gas streams are assured to proceed to completion when the gas temperature reaches the kindling temperature. At and above that temperature oxidation and other reactions initiate immediately and proceeds to completion. Such a process requires no mechanical ignitor / burner system. It would normally be necessary to raise the temperature of the feed gas to 150 - 300 degrees Celsius, depending on feed gas composition, in order to kick-start or kindle the oxidation reaction.

It has been ascertained by the inventors that by introducing the process gas and oxygen directly into a reactor system, which operates at or suitably above the kindling temperature, oxidation and other reactions can proceed immediately. Metal and containment temperatures are kept within allowable metallurgical limits, by virtue of the cooled surrounds, in spite of the much higher bulk temperature of the reacting gases.

In the case of a Claus plant the bulk of the oxidation heat is commonly removed in a waste heat boiler to raise steam (Fig 1 stream 15). For this system, a practical and innovative solution would be to introduce the oxygen directly into the waste heat boiler which then acts as containment reactor as well as cooler. Boiler tubes are not as susceptible to the high gas bulk temperature. In such a configuration, oxidation reactions can proceed in full but with the advantage of the containment temperature not rising significantly above the cooling temperature of the outside cooling medium such as that of boiler feed water at steam raising conditions. A number of methods can be used to reach the temperature needed for the oxidation reactions to start including the use of a suitable tube metal wall temperature, using an oxidation promoting catalyst within the tubes and the use of external preheat.

Cooling mediums other than boiler feed water / steam can be used as long as the resulting metal wall temperature in the oxidise/cooler is within limits. Evaluations show that mild steel which is the most common and least expensive material for such a service would be acceptable. To preheat the feed gas stream, a number of methods could be employed including use of steam or other heating medium in a heat exchanger or indeed use of electric heater. Other methods can be used such as mixing with a hot stream, use of gas or heat from the oxidiser/cooler or indeed passing the feed gas through the heat recovery section of the oxidiser/cooler. It is also practicable to use an oxidation catalyst to achieve the desired level of pre-heat with air or oxygen. The design of the process plant would of course consider the temperature levels needed as well as questions of start-up, shutdown and operational variations.

The present invention overcomes the various disadvantages of existing practices and prior art by permitting the introduction of oxygen up to 100% concentration without the need to provide intermediate cooling, gas recycling or to split the process. The invention leads to a novel but much simpler system offering savings in capital investment, reduction in operating costs, reduced utilities consumption, improvements in plant layout, reduction in space requirements, accurate process control in addition to a number of process and chemical reaction gains. The invention also leads to a major reduction in plant start-up and shut-down times normally necessitated by refractory temperature management. For a chemical factory such as an oil refinery with severe environmental restrictions this would be particularly important. The advantages of such an invention are therefore considerable.

Good design practice would ensure the correct technique of bringing the oxygen and H2S bearing acid gas together within the oxidiser/cooler. One such method would be to introduce the oxygen through a manifold directly into the tubes of the oxidiser/cooler. This ensures that the highly exothermic oxidation reactions take place inside the cooled containment. A small residence time is accommodated within the oxidiser/cooler to ensure that the desired reaction equilibria are attained.

It has been postulated by researchers that equation 4 can be made to drive the reaction to the right by effecting the cooling stage quickly. The practice of the present invention will result in a faster cool down than normally encountered in Claus plants. This would result in a greater degree of dissociation leading to greater sulphur yield and the production of hydrogen.

Fig 2 shows an embodiment of the present invention where pre-heated feed gas containing H2S (13) is routed to the oxidiser/cooler where it is contacted with oxygen or oxygen enriched air (14). In the oxidiser/cooler (17 ) heat removal is effected by the production of steam from boiler feed water. Oxidation and other reactions take place within the oxidiser/cooler system. Partially cooled reaction products enter the sulphur condenser (20). The remaining process gas stream having been cooled is then heated to a suitable temperature in a re-heater (25) and passed over a catalyst in a catalytic reactor (30) to produce more sulphur which is then recovered in a condenser (35). Further sulphur production can be achieved by the process of additional re-heating (40), catalytic stage (45) and sulphur condensation (50). This procedure can be repeated but economics usually dictate that two or three catalytic stages may present an optimum configuration to balance out the overall sulphur recovery efficiency with plant cost and complexity. In an existing unit where a thermal oxidation step already exists it would practicable to achieve the desired preheat temperature conditions by using the existing thermal reactor or furnace as a pre-heater thus minimising mechanical modifications to the plant.

Examples of the Invention

Example 1

This example features a common situation of recovering sulphur from a refinery acid gas stream.

The feed to a Claus sulphur recovery plant contains:

Component Mole percent

H2S 90

CO2 4

CH4 1

H2O 5

Using an oxidiser/cooler and two catalytic stages, plant characteristics are determined for a process in accordance with the present invention. This is compared with a plant design based on combustion with air (A) and also one using a two-step combustion with oxygen (B). Plant throughput has been assumed to be a 100 tonnes per day (tpd) of sulphur contained in the feed.

Feed Sulphur, (tpd) 10C )

Produced Sulphur, tpd 95 - 96

Overall Sulphur recovery, % 95 - 96

Process (A) Process (B) This invention

Thermal stage One two None required

Burner system Yes Yes None required

Maximum temperature, deg C 1300 1500 2000

Waste heat recovery size 100 70 55

Catalytic reactors size 100 50 50

Condensers & re-heaters size 100 50 50

Investment cost 100 70 55

It can be seen from the above example that the present invention yields significant advantages both technical and economic.

Example 2

In order to examine the feasibility of converting an existing Claus sulphur recovery plant to the present invention the following example was evaluated. A conventional 100 tonnes per day (tpd) Claus sulphur recovery plant using air burning was used as basis for comparison. Basis of comparison

Capacity 100 tpd

Number of catalytic stages 2

H2S concentration in the feed gas 90%

Results

Oxygen to existing thermal stage 20 % (can also be air or enriched air)

Oxygen to waste heat recovery 80 %

Incremental plant throughput 150%

Conversion cost, million US Dollars 0.3

Thus calculations show that this invention provides a novel and very cost effective route to increasing plant throughput in an existing facility.

Claims

1. A process for the recovery of sulphur from a hydrogen sulphide containing stream, which process features: a. Non catalytic reaction with oxygen, where part of the hydrogen sulphide is converted to sulphur dioxide and sulphur is formed. b. Oxidation medium with or without air carrier, i.e. oxygen concentration can be as high as 100% c. H2S concentration in the feed can be as high as 100%, simultaneously with 100% oxygen d. No need for a combustion burner / ignitor e. Single step process without the need for secondary process cooling, stream dilution or stream temperature moderation. f. Reactions with in-situ simultaneous heat removal at conditions in which free oxygen will continue to react with the process gases.

2. A process, as in claim 1, where a process gas stream is thermally reacted with oxygen or oxygen enriched air

3. A process, as in claim 1, where the oxygen can be introduced in full without the need to provide intermediate cooling, dilution or otherwise moderate the temperature of the oxidation reactions.

4. A process, as in claim 1, where a combustion burner/ignitor is not necessary

5. A process, as in claim 1, where a furnace to combust part of the H2S is not necessary

6. A process, as in claim 1, where high temperature reactions can be sustained and contained using various in-situ heat removal systems

7. A process, as in claim 1, where the dissociation reaction of H2S results in an increase in overall sulphur production by rapid cooling of the oxidation products.