WO2020211982A1

WO2020211982A1 - Method and apparatus for treating a gas mixture

Info

Publication number: WO2020211982A1
Application number: PCT/EP2020/025173
Authority: WO
Inventors: Jian CHI; Oliver Neuhaus; Marcus Guzmann; Constantin Maniut; James Mercer
Original assignee: Linde Gmbh
Priority date: 2019-04-18
Filing date: 2020-04-20
Publication date: 2020-10-22
Also published as: GB201905565D0; GB2583139A

Abstract

The present invention relates to a method (100) for treating a starting gas mixture comprising carbon dioxide, hydrogen and water, the starting gas mixture being produced including a Claus process (1), wherein the method comprises forming at least one product fraction predominantly or exclusively containing carbon dioxide. According to the present invention, the starting gas mixture is enriched in carbon dioxide and hydrogen and depleted in water in a first group (10) of method steps forming an intermediate gas mixture, the first group (10) of method steps including compressing (11), cooling (12, 13), condensating and drying (13), and in that the intermediate gas mixture is at least partially submitted to a second group of method steps (20), the second group (20) of method steps including subjecting at least a part of the intermediate gas mixture to a partial liquefaction (21, 22, 23) forming a liquid fraction and a gaseous fraction, the liquid fraction being at least partially used in forming the at least one product fraction. A corresponding apparatus is also part of the present invention.

Description

Method and apparatus for treating a gas mixture

The invention relates to a method for treating a starting gas mixture comprising carbon dioxide, hydrogen and water, 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, relating to 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 in the classical tail gas treatment units mentioned hereinbefore, 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.

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 (at least) carbon dioxide, hydrogen and water, the starting gas mixture being produced involving a Claus process, and to a corresponding apparatus according 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 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%, 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 be 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 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

Like in classical methods the present invention preferably also uses a tail gas from a Claus process as a starting gas mixture for a separation process that particularly produces one or more product fractions predominantly or exclusively comprising carbon dioxide. Such a fraction can be used as a further attractive product of the method, e.g. for purposes of enhanced oil recovery or for food and beverage uses as mentioned below. Such an advantageous use provides a further incentive to operate a Claus process including oxygen enrichment, independently from the question whether a sulphur capacity increase is intended or not. As mentioned, no cryogenic separation of nitrogen and carbon dioxide from each other is necessary to provide such a fraction when oxygen enrichment is performed. This is particularly the case when substantially pure oxygen is used in the oxygen enrichment, i.e. oxygen with an oxygen content of more than 90 mol-%, particularly more than 95 mol-% or 99 mol-%.

The combination of oxygen enriched desulphurization in combination with carbon dioxide generation in a corresponding fraction has been found particularly attractive in terms of total cost of ownership if a classical tail gas treatment unit is partially eliminated and substituted by inventive method steps. In the inventive method, the removal of water and non-condensable gases, e.g. hydrogen, argon and (traces of) nitrogen are part of the carbon dioxide processing instead of the tail gas treatment.

This important process redesign obviates the requirement for additional off gas processing units as not required anymore.

According to the present invention, classical tail gas processing units such as a sulphur dioxide hydrogenation step, a corresponding catalytic stage and a sulphur condenser may still be part of the method, depending of what variant of tail gas treatment is performed. In order to provide hydrogen for such hydrogenation, a reducing gas generator of the kind known from the prior art may be used, or, alternatively, a fraction containing hydrogen may be formed in the inventive method, wherein such hydrogen can be used for hydrogenation instead of hydrogen generated in a reducing gas generator. In a further alternative, hydrogen can be provided from external sources.

The present invention, in summary, provides a method for treating a starting gas mixture comprising carbon dioxide, hydrogen and water, the starting gas mixture being produced including a Claus process. The starting gas mixture may be a tail gas of the Claus process which is subjected to any treatment that is usual for a tail gas treatment unit, e.g. processes forming hydrogen sulphide from sulphur dioxide. The tail gas may also be particularly poor in sulphur components, and the method is preferably used with oxygen enrichment in the Claus process and even more preferably used with pure or substantially pure oxygen being supplied to the Claus process. The starting gas mixture may be also be formed using recycle streams from the inventive method. The starting gas mixture may also comprise other components like, but not limited to, carbon monoxide, carbonyl sulphide and carbon disulphide.

A starting gas mixture is, in the language as used herein, produced“including” 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 be a tail gas of the Claus process, as mentioned. The term“Claus process” is intended to refer to any type of process that produces sulphur from a sulphur compound and furthermore produces a gas mixture including carbon dioxide, sulphur dioxide and/or hydrogen sulphide. The term“Claus process” is not limited to the classical Claus processes described above in relation to the expert literature. It may include any number of catalytic stages, e.g. one, two, three or more catalytic stages. The starting gas mixture may also be produced from or include such a tail gas, particularly after a hydrogenation and further processing steps. In all cases, the starting gas mixture is not necessarily produced exclusively in the Claus process and may also comprise components from other sources. Not all the tail gas of a Claus process is, on the other hand, necessarily used according to the present invention as a starting gas mixture.

According to the present invention, the method comprises forming at least one product fraction predominantly or exclusively containing carbon dioxide, this fraction being later herein also referred to a“carbon dioxide product.” According to the present invention, particularly carbon dioxide fractions with relative high purity may be formed, including optionally a liquid fraction which can be used for food and beverage purposes and a gaseous fraction which is compressed to a pressure level suitable for enhanced oil recovery. Said fractions may be produced in parallel or alternatively to each other. Further fractions may additionally or alternatively be generated. The inventive method comprises that the starting gas mixture is enriched in carbon dioxide and hydrogen and depleted in water in a first group of method steps forming an intermediate gas mixture, the first group of method steps including compressing, cooling, condensating and drying, and in that the intermediate gas mixture is at least partially submitted to a second group of method steps, the second group of method steps including subjecting at least a part of the intermediate gas mixture to a partial liquefaction forming a liquid fraction and a gaseous fraction, the liquid fraction being at least partially used in forming the at least one product fraction.

The present invention is particularly advantageous if the Claus process is operated using oxygen enrichment, i.e. if the Claus furnace is supplied with oxygen that is contained in a fluid stream which is enriched in oxygen when compared to atmospheric air or which is substantially pure oxygen. In this case, as mentioned, a carbon dioxide product can be provided without cryogenic separation of nitrogen and carbon dioxide from each other. Oxygen enrichment in a Claus process may therefore form part of the present invention. Particularly, according to the present invention, the Claus process is used with high purity oxygen as indicated below.

The separation of the carbon dioxide is part of an integrated process according to the present invention in which the adjustment of the operation of the Claus process with preferably more than 90 mol-% oxygen, preferably more than 95 mol-% oxygen, most preferably more than 99 mol-% oxygen purity, will include positive aspects like that the presence of nitrogen will be reduced to trace levels or preferably eliminated from the starting gas mixture deriving from the Claus process and optionally the hydrogenation. As such, the concentration of carbon dioxide in the starting gas mixture used according to the present invention can be increased to values of at least or exceeding 55 to 70 mol-%. Furthermore, the presence of sulphur species can be reduced using such oxygen enrichment to levels below 1000 vppm, preferably below than 500 vppm, most preferably less than 200 vppm, such components mainly including hydrogen sulphide and carbonyl sulphide.

The present invention involves a compact configuration process to achieve carbon dioxide separation using a combination of two main steps as partially indicated above. In such way, the recovery rate of the carbon dioxide equals or even exceeds 99,8%, similar to a recovery rate achieved using a chemical solvent (e.g. monoethanolamin) but eliminating all typical negative aspects like usage of large amounts of low pressure steam and dealing with its chemistry and the complications derived from that.

Generally, according to the present invention, the starting gas mixture which is used according to the present invention may, in the dry part, i.e. in the part not being water, comprise 50 to 80 mol-% carbon dioxide, preferably 55 to 70 mol-% carbon dioxide.

The rest may, for the main part, be formed by hydrogen. As mentioned, trace levels of other components may also be present, according to the present invention. The water content may be 5 to 15 mol-%.

According to the present invention, the starting gas mixture is compressed at a clearly defined pressure range and a separation of highly concentrated carbon dioxide in its liquid form is performed. Such a compression which is performed after the enrichment in carbon dioxide and hydrogen in the first group of method steps indicated above is performed to 30 to 40 bar (abs.) in one or more compression steps, the pressure levels achieved preferably being between 35 and 38 bar (abs.) levels. A corresponding compression enables for an advantageous control of the water content and efficiency in drying the starting gas mixture in the first group of method steps. Secondly, the compression allows for a control of the liquefaction energy and carbon dioxide recovery in the second group of method steps. This is explained in the following. The cooling in the first group of method steps is preferably performed to a temperature level of 12 to 20 °C, particularly ca. 15 to 18 °C and the drying in the first group of method steps is preferably performed to a residual water content less than 50 vppm, preferably less than 20 vppm. The drying can be performed using absorbers, e.g. molsieve absorbers known in the art.

Particularly, during said compressing the water content in the starting gas mixture may be reduced to 3,000 to 6,000 vppm, preferably to 4,000 to 5,000 vppm. This is a result of the compression performed. The cooling in the first group of method steps is at least partially performed subsequent to the compressing in the first group of method steps, i.e. intercoolers may be present. Herein, the water content in the starting gas mixture is further reduced to 800 to 1 ,200 vppm, particularly 950 to 1 ,000 vppm. This process step eliminates the requirement for either a separate chiller or overdesign of the drier system subsequent thereto to match a more elevated water content. The drying in the first group of method steps is preferably performed subsequent to the steps just mentioned. To be able to liquefy carbon dioxide and achieve a high purity of at least or exceeding 99.1 mol-%, preferably more than 99.9 mol-%, which may be the purity of the product fraction(s) predominantly or exclusively comprising carbon dioxide which are formed according to the present invention, the water content needs to be reduced to the vppm values indicated for the drying step above. As a reduction of the water content in the upstream steps is realised according to the present invention, a conventional drier system can be installed to achieve this purpose.

At the advertised pressure range (see above), and after drying, the intermediate gas mixture obtained accordingly, or a part thereof, is subjected to the partial liquefaction in the second group of method steps. This includes initially routing the gas through a cooling or condensing stage wherein the gas is preferably cooled against streams formed subsequent thereto (see below).

In the partial liquefaction in the second group of method steps, the intermediate gas mixture gas is partially liquefied and carbon dioxide is thereby concentrated in its liquid form. In the partial liquefaction, cold energy of the liquid carbon dioxide stream which may be evaporated thereby can be used. As using such recovered cold energy alone, the ratio of liquid to gas, i.e. of the liquid fraction to the gaseous fraction, will typically not exceed 3 (0.75/0.25), a further cooler may be used. After further cooling in this further cooler, the ratio mentioned may be increased towards 3,3 (0.77/0.23) or more. The duty of the additional cooler is relatively low, being estimated at ca. 4 to 6% of the entire compression duty required. The additional cooler may be used only to start up and to maintain the liquefaction of carbon dioxide during the process.

Achieving a ratio of the liquid fraction to the gaseous fraction of 3 or 3.3 or even more is particularly desirable to maintain efficiency and to be able to keep a recycle stream to the liquefaction stage, which is further described below, at the minimum flow possible. Any reduction in that ratio will lead to an increased rate of the recycle stream. Increasing the concentration of nitrogen (or other non-condensable like Argon, hydrogen or oxygen) have the same negative impact on the overall performance. Therefore, in other words, a ratio of the liquid fraction to the gaseous fraction formed during the partial liquefaction in the second group of method steps is at least 3, preferably at least 3.3 (expressed as a molar percentage). The actual forming of the liquid phase, which is part of the liquefaction downstream of the condensing, may preferably be done in a single stage liquid/gas separator or a corresponding separation column. The concentration of liquid carbon dioxide at this point typically exceeds 99.1 mol-%. The carbon dioxide at this stage typically still contains ca. 3,700 vppm of hydrogen and about as much argon. In case a higher carbon dioxide purity is required (at least 99.8 mol-%), a single stage separator can be replaced with a rectification column.

The liquid phase may in a part firstly be flashed to lower pressures than the in order to achieve temperatures of at least -45 °C, preferably -50 °C, most preferably -55 °C and may then be routed back through the condensing step in the liquefaction in order to provide cold via indirect heat exchange, where the corresponding part may also be fully be evaporated. Such a full evaporation can also be performed in the cooling to which the starting gas mixture is submitted in the first group of method steps, or evaporated gas can be routed through this, in order to provide cold.

The method according to the present invention particularly includes that at least a part of the gas phase formed in the partial liquefaction in the second group of method steps is subjected to a physical absorption step forming a higher polar fraction comprising carbon dioxide and a less polar fraction comprising hydrogen. For sake of clarity, also the higher polar fraction may contain (some) hydrogen and the less polar fraction may contain (some) carbon dioxide. In any case, however, the content of hydrogen is higher in the less polar fraction than in the higher polar fraction and the content of carbon dioxide in the less polar fraction is lower than in the higher polar fraction. Particularly, the higher polar fraction may be poor, in the sense elucidated above, in hydrogen. In this physical absorption step, known physical solvents like polyethylene glycol dimethyl ether (DEPG), propylene carbonate (PC), N-methyl-2-pyrrolidone (NMP) or methanol can be used in a suitable temperature range. Due to the low temperature of the gaseous fraction which is formed according to the present invention, which is e.g. -45 to -50 °C, and due to its elevated pressure in the range indicated above, for illustrative purposes the process description is limited to methanol that may be preferred for these conditions, but the invention can equally be used with other solvents. The higher polar fraction containing substantial amounts of carbon dioxide is preferably recycled to the suction side of the first compressor stage used in the second group of method steps. The main advantage here is that the carbon dioxide concentration of the feed gas will increase towards a higher value, preferably 70 to 80 mol-%, e.g. ca. 73 mol-%, improving the efficiency of the condensing step. Thus, the overall recovery rate of carbon dioxide can be increased.

The gaseous phase from the partial liquefaction enters the physical absorption step at conditions highlighted above where the carbon dioxide is removed with an efficiency of at least or exceeding 99%. From the carbon dioxide rich stream cold energy can be recovered in the partial liquefaction. The higher polar fraction may be recovered at low pressures of ca. 1.1 to 1.3 bar (abs.) and will preferably be, as mentioned, routed back to the method, e.g. for use in hydrogenation. The recovery of hydrogen in the less polar fraction may be ca. 98%. Being a high-pressure low temperature stream (ca. -45 to - 50 °C), it can be further reused in the tail gas treatment in order to reduce the usage of hydrogen. Before that, the cold energy of this stream can be recovered as well in the partial liquefaction, further reducing the duty of the additional cooler, if installed.

Depending on the amount of impurities in the starting gas mixture, the concentration of hydrogen may exceed 93.8 mol-% in the less polar fraction. The carbon dioxide content in the less polar fraction may e.g. be 0.6 to 1 mol-%.

Summarizing the above, at least a part of the higher polar fraction may be recycled to the partial liquefaction in the second group of method steps and/or at least a part of the less polar fraction may be used as a fuel gas and/or as a hydrogen source in a hydrogenation. The method may comprise a startup operation mode, the startup operation mode comprising a further cooling step between the compression and cooling step and the phase separating step, as explained above. The compression and cooling step preferably comprises a compression to a suitable pressure level of 30 to 40 bar (abs.) or in a more preferred range as indicated above. A carbon dioxide content of the liquid fraction formed in the partial liquefaction in the second group of method steps is preferably higher than 99,1 mol-%.

According to the present invention, a part of the liquid fraction formed in the partial liquefaction in the second group of method steps may be further processed obtaining food-grade carbon dioxide, and in the partial liquefaction in the second group of method steps heat may be transferred, as mentioned, to a further part of the liquid fraction formed in the partial liquefaction in the second group of method steps, forming evaporated carbon dioxide. The evaporated carbon dioxide may be compressed to a pressure level suitable for enhanced oil recovery.

The high purity carbon dioxide obtained according to the present invention can generally be used for oil and gas applications including e.g. enhanced oil recovery, fracking applications, and so-called carbon capture and storage (CCS). In these cases, further a compression stage to compress the carbon dioxide to pressures as high as required for injection, exceeding e.g. 200 bar (abs.) may be required. Furthermore, liquid carbon dioxide (LIC) in technical or beverage grade may be provided. In case beverage grade carbon dioxide is required, at this stage further purification steps for further reduction of sulphur components may be required. Also, the capacity of the additional cooling system may have to be increased accordingly. The increase of capacity is dependent by the amount of liquid taken out.

Though oxygen is required for the oxygen enrichment and thus contributes to capital expensed and operating expenses, the carbon dioxide can be generated by far cheaper as in comparable methods according to the present invention. In an illustrative example, a sour gas composition shall comprise 60 mol-% hydrogen sulphide and 29 mol% carbon dioxide on dry basis which is to be treated in a Claus process, and a corresponding unit shall be designed with a capacity of 5,000 tons per day sulphur. In this case, a 100% oxygen enrichment plus cryogenic carbon dioxide purification, as done according to the present invention, results, at a volume of 2.750 megatons per day carbon dioxide in costs of less than 30 USD/ton (basis 10% interest rate). A classical process including methyl ethanolamine based amine carbon capture, at a carbon dioxide volume of 2.800 megatons per day results in costs of more than 45 USD/ton (basis 10% interest rate).

The starting gas mixture to be processed by the carbon dioxide purification process according to the present invention has been assessed. It has been found that due to the oxygen enrichment of the desulphurisation step, the hydrogen concentration in the carbon dioxide raw gas is increasing. Any compression step in the method of the present invention generates substantial amounts of compression heat. 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 invention 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 used in providing oxygen for oxygen enrichment in the Claus 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.

The present invention also includes an apparatus for treating a starting gas mixture comprising carbon dioxide, hydrogen sulphide and water, the starting gas mixture being produced including a Claus process, wherein the apparatus comprises means adapted to forming at least one product fraction predominantly or exclusively containing carbon dioxide.

In the inventive apparatus, means are provided which are adapted to enrich the starting gas mixture in carbon dioxide and hydrogen sulphide and to deplete the starting gas mixture in water forming an intermediate gas mixture in a first group of method steps including compressing, cooling, condensating and drying, and in that means are provided which are adapted to at least partially submit the intermediate gas mixture to a second group of method steps, the second group of method steps including subjecting at least a part of the intermediate gas mixture to a partial liquefaction forming a liquid fraction and a gaseous fraction, the liquid fraction being at least partially used in forming the at least one product fraction.

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 starting gas mixture A comprising carbon dioxide, hydrogen and water is produced including a Claus process 1. A recycling stream B explained below may also be used in forming the starting gas mixture A. A known tail gas treatment 2 may also be used in the method 100.

The method 100 generally comprises forming a fraction predominantly or exclusively containing carbon dioxide. In the method 100, the starting gas mixture A is enriched in carbon dioxide and depleted in sulphur dioxide and water in a first group 10 of method steps forming an intermediate gas mixture C, the first group 10 of method steps including compressing 11 , cooling 12, 13, condensating and drying 14. The

intermediate gas mixture C is at least partially submitted to a second group of method steps 20, the second group 20 of method steps including a liquefaction 21 , 22, 23 as further explained below.

The first group 10 of method steps includes subjecting the starting gas mixture to a compression step 11 , optionally with intercooling 12. Condensates are formed in this compression step 11 but these are not shown here for reasons of conciseness, and the first compression step 11 and the optional intercooling are shown as a common block 11 , 12. The compression 11 can be performed in several compression stages in a corresponding compressor.

At least a part of the compressed gas remaining after the compression 11 and the optional intercooling 12 is subjected to a cooling step 13 forming further condensate (again not shown). At least a part of the remaining gas is subjected to a drying step 14, obtaining a dried gas mixture, and at least a part of the dried gas mixture is used as the intermediate gas mixture C which is subjected to the second group 20 of method steps. For specific concentrations, pressure and temperature levels, specific reference is made to the explanations already given above.

The partial liquefaction 21 , 22, 23 includes a compression and cooling step 21 , followed by an optional further (balance) cooler 22 and a gas/liquid separation 23. Ultimately, in the partial liquefaction 21 , 22, 23, a liquid, carbon dioxide rich fraction D and a gaseous fraction E are formed.

A first part D1 of the liquid fraction is, in the example shown, preferably flashed to a lower pressure, used for cooling in a condenser of the partial liquefaction 21 , 22, 23, and further used for cooling in the cooling step 13, thus being evaporated. It can subsequently be compressed in a step 26 and used e.g. for enhanced oil recovery. A second part D2 of the liquid fraction is, in the example shown, subjected to a further processing 25, particularly for fracking or beverage use, and then withdrawn from the method 100.

The gaseous fraction E is subjected to a physical absorption step 24 of the kind mentioned, producing a higher polar fraction F and a less polar fraction G. The physical absorption step 24 can be embodied as known from the art, particularly including a regeneration system. A solvent as explained above may be used therein.

The higher polar fraction F, containing substantial amounts of carbon dioxide, is for cooling purposes routed through the condenser used in the compression and cooling step 21 and then preferably used as the recycle stream B. The less polar fraction, containing substantial amounts of hydrogen, is also for cooling purposes routed through the condenser and may then be used for hydrogenation or other purposes, e.g. in the tail gas treatment 2 (not shown).

Claims

1. A method (100) for treating a starting gas mixture comprising carbon dioxide, hydrogen and water, the starting gas mixture being produced including a Claus process (1), wherein the method comprises forming at least one product fraction predominantly or exclusively containing carbon dioxide, characterised in that the starting gas mixture is enriched in carbon dioxide and hydrogen and depleted in water in a first group (10) of method steps forming an intermediate gas mixture, the first group (10) of method steps including compressing (11), cooling (12, 13), condensating and drying (13), and in that the intermediate gas mixture is at least partially submitted to a second group of method steps (20), the second group (20) of method steps including subjecting at least a part of the intermediate gas mixture to a partial liquefaction (21 , 22, 23) forming a liquid fraction and a gaseous fraction, the liquid fraction being at least partially used in forming the at least one product fraction.

2. The method (100) according to claim 1 , wherein the starting gas mixture

comprises, in the part not including water, 0 to 80 mol-% carbon dioxide.

3. A method (100) according to claim 1 or 2, wherein the compressing (11) in the first group (10) of method steps is performed to a pressure level of 30 to 40 bar (abs.) in one or more compression steps, wherein the cooling (12, 13) in the first group (10) of method steps is performed to a temperature level of 12 to 20 °C, and wherein the drying (13) in the first group (10) of method steps is performed to a residual water content less than 50 vppm.

4. A method (100) according to claim 3, wherein during the compressing (11) in the first group (10) of method steps the water content in the starting gas mixture is reduced to 3,000 to 6,000 vppm, wherein the cooling (12, 13) in the first group (10) of method steps is at least partially performed subsequent to the compressing (11) in the first group (10) of method steps, further reducing the water content in the starting gas mixture to 800 to 1 ,200 vppm, and wherein the drying in the first group (10) of method steps is performed subsequent thereto.

5. The method (100) according to any of the preceding claims, wherein a ratio of the liquid fraction to the gaseous fraction formed during the partial liquefaction (21 , 22, 23) in the second group (20) of method steps is at least 3.

6. The method (100) according to any of the preceding claims, wherein at least a part of the gas phase formed in the partial liquefaction (21 , 22, 23) in the second group (20) of method steps is subjected to a physical absorption step (24) forming higher polar fraction comprising carbon dioxide and a less polar fraction comprising hydrogen.

7. The method (100) according to claim 6, wherein at least a part of the higher polar fraction is recycled to the partial liquefaction (21 , 22, 23) in the second group (20) of method steps and/or wherein at least a part of the less polar fraction is used as a fuel gas and/or as a hydrogen source in a hydrogenation.

8. The method (100) according to any of the preceding claims, wherein the partial liquefaction (21 , 22, 23) comprises a compression and cooling step (21) and a phase separating step (23) subsequent thereto.

9. The method (100) according to claim 8, wherein comprising a startup operation mode, the startup operation mode comprising a further cooling step (23) between the compression and cooling step (21) and the phase separating step (23).

10. The method (100) according to claim 8 or 9, wherein the compression and cooling step (21) comprises a compression to a pressure level of 30 to 40 bar (abs.).

11. The method (100) according to any of the preceding claims, wherein a carbon dioxide content of the liquid fraction formed in the partial liquefaction (21 , 22, 23) in the second group (20) of method steps is higher than 99,1 mol-%.

12. The method (100) according to any of the preceding claims, wherein a part of the liquid fraction formed in the partial liquefaction (21 , 22, 23) in the second group (20) of method steps is further processed (25) obtaining food-grade carbon dioxide, and wherein in the partial liquefaction (21 , 22, 23) in the second group (20) of method steps heat is transferred to a further part of the liquid fraction formed in the partial liquefaction (21 , 22, 23) in the second group (20) of method steps, forming evaporated carbon dioxide.

13. The method (100) according to claim 12, wherein the evaporated carbon dioxide is compressed to a pressure level suitable for enhanced oil recovery.

14. An apparatus for treating a starting gas mixture comprising carbon dioxide,

hydrogen and water, the starting gas mixture being produced including a Claus process (1), wherein the apparatus comprises means adapted to forming at least one product fraction predominantly or exclusively containing carbon dioxide, characterised in that means are provided which are adapted to enrich the starting gas mixture in carbon dioxide and hydrogen sulphide and to deplete the starting gas mixture in water forming an intermediate gas mixture in a first group (10) of method steps including compressing (11), cooling (12, 13), condensating and drying (13), and in that means are provided which are adapted to at least partially submit the intermediate gas mixture to a second group of method steps (20), the second group (20) of method steps including subjecting at least a part of the intermediate gas mixture to a partial liquefaction (21 , 22, 23) forming a liquid fraction and a gaseous fraction, the liquid fraction being at least partially used in forming the at least one product fraction.