GB2583137A

GB2583137A - Method and apparatus for treating a gas mixture

Info

Publication number: GB2583137A
Application number: GB1905562.3A
Authority: GB
Inventors: Guzmann Marcus; Maniut Constantin; Mercer James
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2019-04-18
Filing date: 2019-04-18
Publication date: 2020-10-21
Also published as: GB201905562D0; WO2020211984A1

Abstract

A method 100 for treating a starting gas mixture comprising hydrogen sulphide and carbon dioxide comprises a gas conditioning step 10 to provide the gas mixture at an absorption pressure of between 4 and 10 bar (abs) and an absorption temperature of between -30 and -50°C and a gas separation step 20 comprising a physical absorption 21 at the absorption pressure and temperature level using an absorption liquid to generate at least two fractions, one of which predominantly or exclusively comprises carbon dioxide. A fraction comprising hydrogen sulphide and a fraction comprising hydrogen are preferably formed. The starting gas mixture may be a tail gas of a Claus process 1 or a hydrogenated gas mixture produced from a tail gas treatment unit 2. The gas conditioning step may comprise compressing 11 and cooling 12 the gas mixture. The absorption liquid is preferably methanol.

Description

Method and apparatus for treatinq a qas mixture The invention relates to method for treating a starting gas mixture comprising hydrogen sulphide and carbon dioxide, the starting gas mixture being produced involving a Claus process, and to a corresponding apparatus according to the preambles of the independent claims.

Prior art

Methods and apparatus for treating sour gas mixtures based on the Claus process are known from the prior art. Reference is e.g. made to the article "Natural Gas" in Ullmann's Encyclopedia of Industrial Chemistry, on-line publication 15 July 2006, DOI: 10.1002/14356007.a17_073.pub2, especially chapter 2.4, "Removal of Carbon Dioxide and Sulphur Components, and chapter 2.7, "Recovery of Sulfur." As mentioned in more detail below, sour gas mixtures to be treated accordingly can be obtained from gas mixtures like natural gas or gas mixtures obtained in refinery processes.

Corresponding methods and embodiments are e.g. disclosed in US 4,684,514 A, disclosing a method which removes water concurrently with the condensation of sulphur and which can be operated at high pressure, and in US 2013/0071308 A1, relating to a method and a plant for recovering sulphur from a sour gas containing hydrogen sulphide and carbon dioxide wherein the carbon dioxide is compressed and at least a part of the carbon dioxide is injected into an oil well. The present invention may also be concerned with producing carbon dioxide which can be used accordingly.

The Claus process originally only mixed hydrogen sulphide or a corresponding sour gas mixture with oxygen and passed the mixture across a pre-heated catalyst bed. It was later modified to include a free-flame oxidation upstream the catalyst bed in a so-called Claus furnace. Most of the sulphur recovery units (SRU) in use today operate on the basis of a correspondingly modified process. If, in the following, therefore, shorthand reference is made to a "Claus process" or a corresponding apparatus, this is intended to refer to a free-flame modified Claus process as just described. In the Claus furnace, hydrogen sulphide in the gas mixture which is fed to the Claus furnace is oxidized, preferably quantitatively, to sulphur dioxide which is subsequently, typically in several catalytic stages, converted to elementary sulphur. The latter is condensed and typically withdrawn in liquid form.

So-called oxygen enrichment is a well-known economic and reliable method of debottlenecking existing Claus sulphur recovery units with minimal capital investment. Oxygen enrichment is, however, as described in detail below, not limited to retrofitting existing Claus sulphur recovery units but can likewise be advantageous in newly designed plants. The "term oxygen enrichment" shall, in the following, refer to any method wherein, in a Claus sulphur recovery unit or in a corresponding method, at least a part of the air introduced into the Claus furnace is substituted by oxygen or a by gas mixture which is, as compared to ambient air, enriched in oxygen or, more generally, has a higher oxygen content than ambient air.

Oxygen or oxygen enriched gas mixtures for Claus sulphur recovery units can be, in general, provided by cryogenic air separation methods and corresponding air separation units (ASU) as known from the prior art, see e.g. Haering, H.-W., "Industrial Gases Processing," Wiley-VCH, 2008, especially chapter 2.2.5, "Cryogenic Rectification." However, oxygen or gases enriched in oxygen in comparison to atmospheric air can also be produced using non-cryogenic methods, e.g. based on pressure swing adsorption (PSA), particularly with desorption pressure levels below atmospheric pressure (Vacuum PSA, VPSA).

If a so-called tail gas obtained after the catalytic conversion of sulphur dioxide in the Claus furnace and the catalytic stage(s) subsequent thereto does not meet the required emission levels, particularly due to a non-quantitative conversion of hydrogen sulphide to sulphur dioxide or of the latter to elementary sulphur, further processing is required. This classically involves tail gas treatment in a so-called tail gas treatment unit (TGTU). In a conventional tail gas treatment unit, sulphur dioxide is partially converted to hydrogen sulphide so that the resulting ratio sulphur dioxide and hydrogen sulphide stochiometrically results in a 100% conversion by synproportionation to elemental sulphur. In the Shell Claus off-gas treating (SCOT), in contrast, sulphur dioxide is substochiometrically present in the Claus process, and therefore an excess of hydrogen sulphide is present in the tail gas. After cooling, the hydrogen sulphide-containing tail gas is therefore, in the latter process, contacted with a solvent to remove the hydrogen sulphide, much like in a conventional gas treating plant. The solvent is then regenerated to strip out the hydrogen sulphide, which is then recycled to the upstream Claus sulphur removal unit for subsequent conversion and recovery. Reference is made to chapter 2.7 of the article in Ullmann's Encyclopedia of Industrial Chemistry mentioned hereinbefore.

Particularly in order to adjust the hydrogen content in the tail gas for a successful hydrogenation to hydrogen sulphide, so-called reducing gas generators (RGG) can be arranged in a tail gas treatment unit. A reducing gas generator is classically also operated with air and a fuel gas and represents a further furnace in the whole process.

It can be operated using oxygen enrichment as well.

While several alternatives for tail gas treatment are known from the prior art, they often proof as unsatisfactory, particularly in cases when carbon dioxide is to be recovered from the tail gas.

In an oxygen enriched operation of a Claus process, less or no nitrogen is present in the tail gas, as no such nitrogen is introduced into the Claus furnaces as a part of combustion air. Therefore, such a tail gas generally can be seen as an attractive source of carbon dioxide, e.g. for Enhanced Oil Recovery (EOR) in which the carbon dioxide is used to facilitate oil extraction from oil wells, particularly in the third (tertiary) stage of oil recovery. If a sufficiently high carbon dioxide content can be achieved, as may be the case according to the present invention, carbon dioxide produced accordingly may also e.g. put to food and beverage uses or can be used for desalination or the production of liquid carbon dioxide (LIC).

An object of the present invention is to provide improved methods of this kind, particularly in view of reducing capital and operating expenses.

Disclosure of the invention

In view of the above, the present invention provides a method for treating a starting gas mixture comprising hydrogen sulphide and carbon dioxide, the starting gas mixture being produced involving a Claus process, and to a corresponding apparatus according 35 to the preambles of the independent claims. Advantageous embodiments of the present invention are the subject of the dependent claims and of the description that follows.

Further background of the invention

Before specifically referring to the features and advantages of the present invention, some terms used herein will be defined and briefly explained. Furthermore, the operating principle of a Claus sulphur removal unit operated with oxygen enrichment will be further explained. A Claus process is classically used for desulphurisation of a sour gas mixture. Therefore, these terms will be initially be further defined hereinafter.

The term "sour gas mixture" refers, in the language as used herein, to a gas mixture containing at least hydrogen sulphide and optionally carbon dioxide and other known sour gases in a common an amount of at least 50%, 75%, 80% or 90% by volume, these numbers relating to the content of one of these compounds or to a common content of several ones of these components. Further components besides sour gases may be present in a sour gas mixture as well, particularly water, hydrocarbons, benzene, toluene and xylenes (BTX), carbon monoxide, hydrogen, ammonia and mercaptans. A sour gas mixture of the kind mentioned can particularly be obtained when "sweetening" natural gas or other gas mixtures, particularly including scrubbing processes as known from the art.

The term "desulphurisation" as used herein shall refer to any process including conversion of a first sulphur compound comprising sulphur at a lower oxidation stage, which is contained in a sour gas mixture, to a second sulphur compound comprising sulphur at a higher oxidation stage in a first reaction step, and particularly further including forming elementary sulphur from the second sulphur compound in a second reaction step, the elementary sulphur particularly being obtained in liquid state. The first sulphur compound may be hydrogen sulphide and the second sulphur compound may be sulphur dioxide. The first reaction step may particularly include combusting the first sulphur compound and the second reaction step may particularly include using a suitable catalysis reaction as generally known for the Claus process.

In the language as used herein, a mixture of components, e.g. a gas mixture, may be rich or poor in one or more components, where the term "rich" may stand for a content of more than 75%, 80%, 85%, 90%, 95%, 99%, 99.5% or 99.9% and the term "poor" for a content of less than 25%, 20%, 15%, 10%, 5%, 1%, 0.5% or 0.1° k, on a molar, weight or volume basis. In the field of sour gas treatment, a sour gas mixture with a hydrogen sulphide content of more than 80% is generally referred to as "rich" while a sour gas mixture containing less hydrogen sulphide is generally referred to as "lean." A mixture may also be, in the language as used herein, enriched or depleted in one or more components, especially when compared to another mixture, where "enriched" may stand for at least 1, 5 times, 2 times, 3 times, 5 times, 10 times or 100 times of the content in the other mixture and "depleted" for at most 0.75 times, 0.5 times, 0.25 times, 0.1 times, or 0.01 times of the content in the other mixture.

The terms "pressure level" and "temperature level" are used herein in order to express that no exact pressures but pressure ranges can be used in order to realise the present invention and advantageous embodiments thereof. Different pressure and temperature levels may lie in distinctive ranges or in ranges overlapping each other. They also cover expected and unexpected, particularly unintentional, pressure or temperature changes, e.g. inevitable pressure or temperature losses. Values expressed for pressure levels in bar units are absolute pressure values.

Oxygen enrichment, which was already mentioned hereinbefore, can also eliminate the need for fuel gas co-firing in the Claus furnace which is classically required to maintain the correct temperature for contaminant destruction, for example for destruction of benzene, toluene and xylenes (BTX) in the sour gas mixture. Whether or not a corresponding co-firing is required particularly depends on the hydrogen sulphide content of the sour gas mixture treated and whether a sufficient temperature and a stable flame can be obtained by burning the sour gas mixture alone. As, when using oxygen enrichment is used, oxygen is less diluted with nitrogen, the energy density and therefore the combustion temperature is higher.

The concept of oxygen enrichment entails replacing part or all of the air fed to the Claus furnace by air enriched in oxygen or pure oxygen. Correspondingly, the volumetric flow through the Claus sulphur recovery unit decreases, allowing more of the sour gas mixture to be fed to the system. This results in an increased sulphur production capacity without the need for significant modifications to existing equipment or major changes to the process plant pressure profile.

Oxygen enrichment can also have advantages in plants where the acid gas mixtures obtained are lean and contain benzene, toluene and xylenes. Such plants classically require feed gas and/or combustion air preheating and the use of fuel gas co-firing and have not, historically, been considered for oxygen enriched operation. However, also in such plants, the use of oxygen enriched technology results in a reduction in the physical size of all major equipment items and an associated, significant reduction in capital cost. Particularly, a large reduction in or even elimination of fuel requirements in co-firing in the Claus furnace and other units can be achieved and therefore more fuel, e.g. natural gas, can be used for other purposes or alternatively provided as a product of the whole plant.

A particular advantage of oxygen enrichment is, as also mentioned hereinbefore, that the tail gas downstream a tail gas treatment unit is less "diluted" with nitrogen from the combustion air classically used in the reaction furnace of the Claus process and potentially in the reducing gas generator of the tail gas treatment unit. If little or no additional nitrogen is introduced into the process, the main component of the sour gas mixture after desulphurisation, e.g. carbon dioxide, can be obtained in a simpler and more cost-effective way as no cryogenic separation of nitrogen and carbon dioxide is necessary. This is specifically the case when the carbon dioxide is to be used for purposes like enhanced oil recovery or fracking in which no absolute purity is necessary.

As mentioned before, treatment of a tail gas of a Claus process may involve a removal of hydrogen sulphide, much like in a conventional gas treating plant. That is, a so-called amine unit is typically utilised for removing hydrogen sulphide using chemical based solvents. Solvents for the amine unit are often selected in view of selectivity towards hydrogen sulphide. For an overview, reference is made to F.S. Manning & R. Thompson, "Oilfield Processing of Petroleum: Natural Gas", PennWell Books, 1991, Chapter 7, "Gas Sweetening." For example, Flexsorb solvents are well known due to high selectivity they offer at low pressures. The chemical solvents are usually amine-based systems that rely on chemical reactions to bind the hydrogen sulphide. In other words, classical methods involve using a chemical absorption process in order to remove hydrogen sulphide.

Features and advantages of the invention In contrast to the methods mentioned before, according to the present invention, a physical absorption step is used to remove hydrogen sulphide at a relatively low pressure from the tail gas or from a gas mixture obtained from the tail gas. Particularly, a process based on cold methanol or a different polar organic solvent not chemically reacting with the hydrogen sulphide, preferably a so-called Rectisol process, can be used according to the present invention. The Rectisol process and other physical absorption methods are known per se and e.g. described in G. Hochgesand, Ind. Eng.

Chem. 1970, 62, 7, 37-43, or a plethora of publications subsequent thereto. According to the present invention, the physical absorption method is performed at a pressure level substantially below that classically used for such processes, particularly for the Rectisol process. A "physical" absorption particularly is based on a non-covalent association of molecules to be absorbed and absorbing molecules (absorbent). In contrast to "chemical" absorption, no covalent binding is involved here.

According to the present invention, further tail gas treatment, particularly the hydrogenation of sulphur dioxide, can be dispensed of completely, as components which are classically converted in tail gas treatment can easily be separated off in the inventive method and can e.g. be recycled to the Claus process, particularly together with hydrogen sulphide which is recycled anyway.

In summary, and in the language of the claims, the present invention provides a method for treating a starting gas mixture comprising hydrogen sulphide and carbon dioxide, the starting gas mixture being produced involving a Claus process, wherein the method comprises a gas conditioning step and a gas separation step. A starting gas mixture is, in the language as used herein, produced "involving" a Claus process if it contains at least some compounds which were previously produced or converted in the Claus process. The starting gas mixture may particularly a tail gas of the Claus process remaining after sulphur dioxide was converted at least in the predominant part to elementary sulphur and the elementary sulphur was removed. The starting gas mixture may also be produced from or include such a tail gas. In all cases, the starting gas mixture must not be produced exclusively in the Claus process and may also comprise components from other sources. Not all the tail gas of a Claus process must, on the other hand, be used according to the present invention.

It is to be noted that the "gas conditioning step" and the "gas separation step" are mentioned herein merely for reasons of reference, and that these steps must not be clearly be separated from each other. Particularly, the gas conditioning step and the gas separation step not necessarily have to be performed in different units of a corresponding apparatus and media used in one of these steps can be used in the other as well. Particularly, this refers to heat transfer streams like steam. However, the gas conditioning step is rather a step preparing the starting gas mixture for separation, essentially leaving the gas composition unchanged, while the gas separation step operates under the conditions provided by the gas conditioning step and performs a substantial separation of the starting gas mixture. In the present invention, the gas separation step comprises at least partially separating the hydrogen sulphide from the starting gas mixture.

According to the present invention, the gas conditioning step comprises providing the starting gas mixture at an absorption pressure level of 4 to 10 bar (abs.) and at an absorption temperature level of -30 to -50 °C, and the gas separation step comprises a physical absorption at the absorption pressure level and at the absorption temperature level using an absorption liquid and generating at least two fractions, one of which predominantly or exclusively comprising carbon dioxide.

Generally, in the physical absorption process used according to the present invention, carbon dioxide may more easily dissolve or be absorbed in the absorption liquid used, e.g. in "cold" methanol at the absorption pressure and temperature levels. This means that, when using an absorption column with a certain number of theoretical plates to which the conditioned starting gas is applied at the bottom and is allowed to raise in countercurrent and direct contact with a methanol stream, the absorption liquid loaded with mostly carbon dioxide can be withdrawn from the absorption column at a position below a position at which the absorption liquid loaded with hydrogen sulphide may be withdrawn. From the former, carbon dioxide may be stripped, resulting in the fraction predominantly or exclusively comprising carbon dioxide (also referred to as "carbon dioxide fraction" hereinafter). From the latter, if produced, hydrogen sulphide may be stripped, resulting in a further fraction which contains predominantly or exclusively hydrogen sulphide (also referred to as "hydrogen sulphide fraction" hereinafter).

Hydrogen, like carbon monoxide and other non-polar compounds that may be present in the starting gas mixture as well, preferably stay in the gas phase, the gas phase forming a yet further fraction poor in or free from carbon dioxide and hydrogen sulphide and containing hydrogen (also referred to as "hydrogen fraction" hereinafter).

In summary, therefore, in the physical absorption a further fraction predominantly or exclusively comprising hydrogen sulphide and a further fraction poor in or free from carbon dioxide and hydrogen sulphide and containing hydrogen may be formed.

In this way, using one absorption column with a suitable number of theoretical plates, a separation between a carbon dioxide fraction, a hydrogen sulphide fraction and a hydrogen fraction is possible. Each of these fractions may contain further components, if they are contained in the starting gas mixture. For example, sulphur dioxide may be contained the hydrogen sulphide fraction and this fraction may be recycled to the Claus process wherein the hydrogen sulphide may be converted to sulphur dioxide. This sulphur dioxide, and the sulphur dioxide recycled to the Claus process from the hydrogen sulphide fraction may be converted to elementary sulphur. Therefore, a tail gas treatment by hydrogenation, which is classically used to remove sulphur dioxide can be dispensed of according to the present invention.

The absorption pressure level at which the starting gas mixture is provided, i.e. compressed, in the gas conditioning step is particularly from 5 to 9 bar (abs.), preferably ca. 5 bar (abs.), or preferably ca. 6 bar (abs.), or preferably ca. 8 bar (abs.). The absorption temperature level at which the starting gas mixture is provided, i.e. to which it is cooled, in the gas conditioning may preferably be between -35 to -50 °C, particularly ca. -40 °C or ca. -45 °C. Advantages of this approach include that less energy is required for compression, as compared to classical physical absorption processes, and that the equipment used for the physical absorption does not need to withstand higher pressures, reducing capital expenses.

The gas separation step used according to the present invention, as mentioned, preferably provides a fraction predominantly or exclusively comprising hydrogen sulphide which can be recycled to a point upstream. Depending on the composition of the starting gas mixture, this fraction may comprise further components as well. If a Claus process is used, as is the case according the present invention, this fraction can be at least partially introduced into the Claus furnace, for example. The physical absorption is performed, as mentioned, preferably using a cold polar organic solvent, particularly methanol, a mixture of several different solvents or one or more solvents and additives. However, none of the compounds used in the physical absorption preferably chemically bind with the carbon dioxide or the hydrogen sulphide, at least not covalently. The carbon dioxide and the hydrogen sulphide are preferably driven out of such the solvent or solvent mixture used in the physical absorption, or parts thereof, respectively, particularly in a regeneration unit or step.

As mentioned, the starting gas mixture may be provided as an unhydrogenated tail gas of the Claus process or as a gas mixture produced from the tail gas in a tail gas treatment unit involving a hydrogenation. Particularly, a tail gas treatment unit can be used in order to convert traces or residuals of sulphur dioxide to hydrogen sulphide by hydrogenation, i.e. producing the starting gas mixture from the tail gas in the tail gas treatment unit at least includes hydrogenating sulphur-containing compounds to hydrogen sulphide. As mentioned before, this may include usage of a reducing gas generator in order to adjust the hydrogen content, or may also entail introducing hydrogen from external sources. However, as mentioned below, this may not be necessary according to the invention, as here sufficient amounts of hydrogen may be present already. Preferably, according to the present invention, however, no tail gas treatment by hydrogenation is used, making the invention particularly energy and cost effective. Therefore, according to an embodiment of the invention, the starting gas mixture may include sulphur dioxide and other compounds which are not hydrogenated to hydrogen sulphide. They are thus transferred into the hydrogen sulphide fraction and may be recycled into the reaction furnace of the upstream Claus process.

The present invention is particularly advantageous in connection with oxygen enrichment, particularly because this results in carbon dioxide being easily obtainable from the tail gas. In other words, according to a preferred embodiment of the present invention, a gas mixture comprising a higher oxygen content than atmospheric air or pure oxygen is provided to oxidise hydrogen sulphide in the Claus process. In this connection, the term "pure" oxygen shall also refer to gas mixtures containing more than 90, 95, 98, 99 or 99,5 vol.-% of oxygen. Operating the process under oxygen enrichment may also have further advantages, as also mentioned below.

According to a particularly preferred embodiment of the present invention, the tail gas is produced comprising hydrogen in the Claus process. The hydrogen may be separated off and may e.g. be used in hydrodesulfurization steps generating sour gases which then may be converted in the Claus process. Hydrodesulfurization is a process known from the prior art per se. Reference is made to expert literature.

As was found according to the present invention, when operating a Claus process under oxygen enrichment, its tail gas comprises very substantial amounts, i.e. 5 to 35 vol.-%, of hydrogen, even if no reducing gas generator is present. This hydrogen is at least in part, preferably completely, transferred to the subsequent method steps and therefore the starting gas mixture comprises at least a part of the hydrogen contained in the tail gas. Therefore, in the separation step, this hydrogen can be separated off and it can be used in upstream method steps such that no external hydrogen needs to be provided. While purifying carbon dioxide according to the present invention, the remaining off gas been found to be very rich in hydrogen, with a content of typically more than 90 vol.-%. This hydrogen, being made available purely by the purification of the carbon dioxide, can optionally be utilized in the method and as such avoids the installation of the reducing gas generator or alternatively an external hydrogen source like a steam reformer.

In a preferred embodiment of the present invention, the fraction at least predominantly comprises carbon dioxide can be used as a gas product which is used for enhanced oil recovery. The fraction predominantly or exclusively comprising hydrogen can, as mentioned, preferably used in upstream method steps, i.e. for hydrodesulfurisation, or can be combusted, and the fraction predominantly or exclusively comprising hydrogen sulphide can be recycled.

In the method according to the present invention, conditioning the starting gas mixture upstream of the separation step and/or the separation step comprises a first compression and/or the fraction which predominantly or exclusively comprises carbon dioxide is further compressed in a second compression. The first compression is the one which is performed upstream the physical absorption and which is performed at the pressure used therein. In both cases, compression heat, i.e. compression heat from the first and/or the second compression can advantageously be utilised in generating steam, thereby recovering substantial amounts of energy.

The first compression may be performed to the absorption pressure level used in the physical absorption step. The second compression for the application in enhanced oil recovery may particularly be performed to a pressure as high as 200 bar (g) or more.

Therefore, substantial amounts of compression heat are generated. Through the use of one or more heat exchangers, this heat may be withdrawn. A heat transfer medium may be used to this purpose, absorbing the heat energy from the compression stage(s). This heat transfer medium may be used to preheat water in a steam generation system, actually boiler feed water. Due to the preheating of the boiler feed water, less thermal energy is required to generate steam, reducing the overall energy demand.

In an illustrative example, the embodiment just mentioned may be used in connection with the production of carbon dioxide in an amount of 190,000 Nm3/h (normal cubic meters per hour) at 2 bar (abs.) wherein no substantial amounts of other gaseous or liquid products are provided, as they are preferably recycled. An air separation process may be optimized to minimize energy consumption and includes supply of the process air at one, two or more pressure levels. The main air compressor in this unit may be of the axial-radial type with a high isentropic efficiency in its axial stage.

The steam demand of the main air compressor aspiring ambient air at e.g. 35 °C may be 150 t/h at 25 bar (abs.) at a temperature of 450 °C. This required steam mass flow rate to drive the main air compressor corresponds to a similar feed water volume with a feed condensate temperature of 40.3 °C. Downstream of a heat exchanger used in this illustrative example for heating boiler feed water, the condensate exit temperature level is e.g. at about 165 °C. For such a heating of boiler feed water from 40.3 to 165 °C compression heat can be used. This allows to save around 15% of natural gas required for classical boiler operation.

In another case, the sulphur recovery unit used in this example may produce superheated high-pressure steam at 25 bar (abs.) and 450 °C with a mass flow of about 35,900 t/h, saturated at 42 bar (abs.) and 370 °C. Due to a heat integration between the air separation unit and the sulphur recovery unit according to the present invention, the sulphur recovery unit may produce an increased volume of high-pressure steam. The gain in steam mass flow in the present example could be up to 30 t/h and as such quite significant with reference to a steam flow required to operate the air separation unit. This steam can be at 25.5 bar (abs.) and 450 °C.

Due to the heat integration as mentioned, a larger volume of high pressure steam can be generated. Steam can e.g. be utilised to drive the air separation units or compressors used for compressing streams in the method itself. The gain in steam volume corresponds to approx. 20% of the steam volume required to operate the method itself and a corresponding air separation unit. Oxygen enrichment in desulphurization itself (due to temperature increase in reaction furnace) as well as the heat integration as described results in increased high pressure steam production from the production site. This steam can be used as just stated.

In the present invention, a content of the hydrogen sulphide in the starting gas mixture is preferably 50 ppm to 10 vol.-% and a content of the carbon dioxide in the starting gas mixture is preferably 40 to 90 vol.-%. The starting gas mixture can, as frequently mentioned, particularly be the tail gas of a Claus process or a gas mixture which is obtained from such a tail gas using the processes mentioned before.

A number of different advantageous options is available for the individual processes comprised by the gas conditioning step. This can e.g. comprise compressing the starting gas mixture to the absorption pressure level and cooling the starting gas mixture to the absorption temperature level thereafter.

Particularly, the cooling may be performed using a cooling system in which a nitrogen coolant is expanded and compressed and/or using a fluid stream produced in an air separation unit. The latter can e.g. be an oxygen or oxygen rich stream withdrawn from the air separation unit in order to be fed into the Claus furnace for the oxygen enriched operation envisaged in an embodiment of the present invention.

The injection of water into the Claus furnace results in the benefit that the water molecule is thermally decomposed into hydrogen and oxygen. The oxygen can react with the hydrogen sulphide and results in the same effect such as oxygen enrichment. Furthermore, in case oxygen enrichment is applied, the same desired effect of oxygen enrichment, such as e.g. debottlenecking, can be achieved with less oxygen being introduced, as water is introduced into the reaction furnace (flame) that in situ generates the oxygen being available as reactant.

In an embodiment of the method according to the invention, the starting gas mixture comprises water and the gas conditioning step comprises drying the starting gas mixture. Such a drying can be particularly involve using molecular sieve adsorbers known from the prior art or, if sufficient, condensation cooling. In the drying, the water content of the starting gas mixture is particularly reduced to less than 100, less than 50 or less than 20 ppm by volume.

Particularly, according to the present invention, the cooling may comprise at least two cooling steps and preferably the drying is performed between at least two of the at least two cooling steps.

The present invention, as mentioned, also relates to an apparatus for treating a starting gas mixture comprising hydrogen sulphide and carbon dioxide, the starting gas mixture being produced involving a Claus process. The apparatus comprises a gas conditioning unit and a gas separation unit, wherein the gas separation unit comprises a separator adapted to at least partially separate the hydrogen sulphide from the starting gas mixture.

According to the present invention, the gas conditioning unit is adapted to provide the starting gas mixture at an absorption pressure level of 4 to 10 bar (abs.) and at an absorption temperature level of -30 to -50 °C, and that the separator adapted to at least partially separate the hydrogen sulphide from the starting gas mixture in the gas separation unit is adapted for physical absorption of at least a part of the hydrogen sulphide at the absorption pressure level and at the absorption temperature level.

As to further features and advantages of a corresponding apparatus, reference is made to the explanations relating to the features and advantages of the inventive method and its preferred modifications and embodiments which equally apply here. This also is the case for a particularly preferred embodiment of the inventive apparatus which is adapted to perform an inventive method or an embodiment thereof.

The present invention will further be described with reference to the appended drawings which relate to a preferred embodiment of the present invention.

Short description of the drawings

Figure 1 illustrates a method according to an embodiment of the invention.

Detailed description of the drawings

In Figure 1, a method according to an embodiment of the invention is schematically illustrated and indicated with 100.

In the method 100, a sour gas stream A is supplied to a Claus process 1 which is, for illustrative purposes only, shown as a method step separate from tail gas treatment 2. The Claus process 1 and the tail gas treatment 2 can also be performed in a common apparatus exchanging media and energy. According to a particularly preferred embodiment of the present invention, as mentioned before, the tail gas treatment 2 can also be omitted.

In the method 100, the Claus process 1 is operated using oxygen enrichment, and therefore, an air separation 3 is present as well. The air separation 3 can be performed cryogenically or non-cryogenically. A stream B of pure oxygen or air enriched in oxygen is provided to the Claus process 1 and used as described. A fuel gas stream, which can also be used in the Claus process 1 is not shown. The Claus process 1 is operated essentially as described hereinbefore.

From the Claus process, a tail gas stream C is withdrawn. The tail gas stream C is introduced into the tail gas treatment 2. In the tail gas treatment 2, which can be operated with or without a reducing gas generator, particularly a hydrogenation of sulphur dioxide is performed as described. To this purpose, a hydrogen stream can be provided, which can be a recycle stream as described below.

A stream E, representing a starting gas mixture comprising hydrogen sulphide and carbon dioxide is withdrawn from the tail gas treatment 2 and is subjected to a gas conditioning step 10 and, subsequently thereto, to a gas separation step 20. If no tail gas treatment of this kind is present, the tail gas stream C may also be directly submitted to these steps.

The gas conditioning step 10 comprises providing the starting gas mixture at an absorption pressure level and at an absorption temperature level described in more detail before. In the embodiment shown here, the gas conditioning step 10 first comprises compressing 11 the starting gas mixture to the absorption pressure level and then cooling 12, 14, the starting gas mixture to the absorption temperature level.

In the example shown, the starting gas mixture of stream E comprises water and therefore the gas conditioning step 10 comprises drying 13 the starting gas mixture. In the example shown, at least two cooling steps are used for the cooling 12, 14 and the drying 13 is performed between these cooling steps.

The gas separation step 20 comprises a physical absorption 21 of at least a part of the carbon dioxide and of the hydrogen sulphide at the absorption pressure level and at the absorption temperature level. The starting gas mixture conditioned in the condition step is provided to the gas separation step in the form of a stream F. In the gas separation step 20, a stream G rich in or essentially consisting of hydrogen sulphide is formed. This may be recycled to the Claus process. Furthermore, a stream H rich in or essentially consisting of hydrogen is formed. This may be used as the stream D already mentioned before. A stream I rich in or essentially consisting of carbon dioxide is transferred to a compression step 30.

Compression heat from the compression step 30, as indicated with K, but any other compression heat as well, may be withdrawn and e.g. be used in a steam system 4.

Steam M generated in the steam system 4 from boiler feed water 5 may e.g. be used to operate the air separation 3. From compression 30, a carbon dioxide product stream N is withdrawn which may e.g. be used in enhanced oil recovery.

Claims

Claims 1. A method (100) for treating a starting gas mixture comprising hydrogen sulphide and carbon dioxide, the starting gas mixture being produced including a Claus process, wherein the method comprises a gas conditioning step (10) and a gas separation step (20), wherein the gas separation step (20) comprises at least partially separating the starting gas mixture, characterised in that the gas conditioning step (10) comprises providing the starting gas mixture at an absorption pressure level of 4 to 10 bar (abs.) and at an absorption temperature level of -30 to -50 °C, and the gas separation step (20) comprises a physical absorption (21) at the absorption pressure level and at the absorption temperature level using an absorption liquid and generating at least two fractions, one of which predominantly or exclusively comprising carbon dioxide.
The method (100) according to claim 1, wherein in the physical absorption (21) a further fraction predominantly or exclusively comprising hydrogen sulphide and a further fraction poor in or free from carbon dioxide and hydrogen sulphide and containing hydrogen are formed.
The method (100) according to claim 1 or 2, further including the Claus process (1), wherein the starting gas mixture is provided as a unhydrogenated tail gas of the Claus process (1) or as a hydrogenated gas mixture produced from the tail gas in a tail gas treatment unit (2).
The method according to any of the preceding claims, wherein a gas mixture comprising a higher oxygen content than atmospheric air or pure oxygen is provided to oxidise hydrogen sulphide in the Claus process (1).
The method according to any one of claims 2 to 4, wherein the tail gas is produced comprising hydrogen in the Claus process (1).
The method according to claim 5, wherein the tail gas is produced comprising 5 to 35 vol.-°/0 hydrogen in the Claus process (1), and wherein the starting gas mixture comprises at least a part of the hydrogen contained in the tail gas. 2. 3. 4. 5. 6.
7. The method according to any one of the preceding claims, wherein the gas conditioning step (10) and/or the gas separating step (20) comprises a first compression and/or wherein the fraction predominantly or exclusively comprising carbon dioxide is further compressed in a second compression, compression heat from the first and/or the second compression being utilised in generating steam.
8. The method (100) according to any of the preceding claims, wherein a content of the hydrogen sulphide in the starting gas mixture is 50 ppm to 10 vol.-°/0 and wherein a content of the carbon dioxide in the starting gas mixture is 40 to 90...vol.-%.
9. The method (100) according to any of the preceding claims, wherein the gas conditioning step (10) comprises compressing (11) the starting gas mixture to the absorption pressure level and cooling (12, 14) the starting gas mixture to the absorption temperature level.
10. The method (100) according to claim 9, wherein the cooling (12, 14) is performed subsequent to the compression (12).
11. The method (100) according to claim 10, wherein the cooling (12, 14) is performed using a cooling system in which a nitrogen coolant is expanded and compressed and/or using a fluid stream produced in an air separation unit (3).
12. The method (100) according to any of the preceding claims, wherein the starting gas mixture comprises water and wherein the gas conditioning step (10) comprises drying (13) the starting gas mixture.
13. The method (100) according to claim 12, wherein the cooling (12, 14) comprises at least two cooling steps and wherein the drying (13) is performed between at least two of the at least two cooling steps.
14. An apparatus for treating a starting gas mixture comprising hydrogen sulphide and carbon dioxide, wherein the apparatus comprises a gas conditioning unit and a gas separation unit, wherein the gas separation unit comprises a separator adapted to at least partially separate the starting gas mixture, characterised in that the gas conditioning unit (10) is adapted to provide the starting gas mixture at an absorption pressure level of 4 to 10 bar (abs.) and at an absorption temperature level of -30 to -60 °C, and that the separator is adapted to at least partially separate the starting gas mixture in the gas separation unit (10) at the absorption pressure level and at the absorption temperature level using an absorption liquid and generating at least two fractions, one of which predominantly or exclusively comprising carbon dioxide.