Method and apparatus for treating a gas mixture

The invention relates to a method for treating a starting gas mixture comprising carbon dioxide, sulphur dioxide and water, the starting gas mixture being produced involving a Claus process operated with oxygen enrichment including a tail gas treatment, 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 The latter is condensed and typically withdrawn in liquid form.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, sulphur dioxide and water, the starting gas mixture being produced involving a Claus process operated with oxygen enrichment including a tail gas treatment, 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 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

As mentioned above, different options exist for the treatment of a tail gas of a Claus process, which also depend on the specific method used upstream. The present invention includes at least some components of a tail gas treatment. The present invention may e.g. include a sulphur dioxide hydrogenation step involving a classical reducing gas generator and a catalytic stage subsequent thereto, as well as a physical solvent/amine system for removing hydrogen sulphide. If the Claus process is operated with an excess of hydrogen sulphide, the elements of a quench and amine section of which are used in such cases can be present.

The present invention therefore proposes to use an at least partially treated, particularly a hydrogenated tail gas from a Claus process operated with oxygen enrichment as a starting gas mixture for a separation process that particularly produces a fraction predominantly or exclusively comprises carbon dioxide. This fraction preferably can be used as a further attractive product of the method, e.g. for purposes of enhanced oil recovery or other purposes like food and beverage uses, desalination or the production of liquid carbon dioxide (LIC). Such an advantageous use provided 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.

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 even if a classical tail gas treatment unit is not or at least not completely eliminated, as this simplifies carbon dioxide recovery. In the inventive method, the removal of sulphur compounds and of non-condensable gases, e.g. hydrogen, argon and (traces of) nitrogen may in part be part of the carbon dioxide processing instead of the tail gas treatment, however. This important process redesign may result in substituting at least some of the further off gas processing units as not required anymore.

According to the present invention, particularly tail gas processing units such as a sulphur dioxide hydrogenation step, a corresponding catalytic stage and a sulphur condenser, a quench column, an absorbing unit (including a regeneration system) for removal of hydrogen sulphide may preferably present. The present invention therefore proposes using a tail gas treatment including at least one of a conversion of hydrogen sulphide to sulphur dioxide, a synproportionation of hydrogen sulphide and sulphur dioxide to yield elementary sulphur, and an elimination of hydrogen sulphide from the tail gas. The starting gas mixture obtained thereby is used according to the present invention. The method steps used to produce the starting gas mixture are preferably also part of the present invention.

The present invention, in summary, provides a method for treating a starting gas mixture comprising carbon dioxide, sulphur dioxide and water, the starting gas mixture being produced including a Claus process operated using oxygen enrichment yielding a tail gas, and including a tail gas treatment of the tail gas, the tail gas treatment comprising at least one of the method steps just mentioned. The starting gas mixture may particularly result from a hydrogenating treatment in a tail gas treatment unit. However, the starting gas mixture may be formed using recycle streams from the inventive method as well and, for forming the starting gas mixture, particularly hydrogen sulphide which was previously contained in the tail gas mixture, may be converted to sulphur dioxide. The starting gas mixture may also comprise other components like, but not limited to, carbon monoxide, hydrogen, carbonyl sulphide and carbon disulphide. Particularly, the starting gas mixture may also comprise hydrogen sulphide.

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, which, as mentioned, underwent a tail gas treatment. 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 is produced from such a tail gas, according to the present invention, wherein virtually all tail gas treatment steps known from the prior art may be included. Reference is made to the expert literature cited in the outset. 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.

According to the present invention, the method comprises forming a fraction predominantly or exclusively containing carbon dioxide, this fraction being later herein also referred to a“carbon dioxide product.” The inventive method comprises that the starting gas mixture is enriched in carbon dioxide and depleted in sulphur dioxide and water in a first group of method steps forming an intermediate gas mixture, the first group of method steps including compressing, cooling, condensing and drying steps, 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 an absorbing step in which liquid carbon dioxide is used to absorb sulphur dioxide from the intermediate gas mixture, and the second group of method steps further including liquefying and rectifying at least a part of a gaseous overhead product being produced in the absorbing step.

As mentioned, in performing such an inventively modified kind of separation, the classical tail gas treatment of a Claus process is at least in part performed upstream of the method steps performed according to the present invention. While the Claus process can, in terms of the energy balance, be considered to represent a heat exporter (due to waste heat of the Claus furnace, for example), classical Claus tail gas treatment is heat consumer which consumes large parts, if not all, of the energy that is exported by the actual Claus process. In the inventive method, waste heat can be used likewise or for other purposes. The present invention is used in connection with a Claus process which 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. Oxygen enrichment in a Claus process does therefore form part of the present invention. Using oxygen enrichment, particularly including a high level of oxygen, the starting gas mixture comprises carbon dioxide in a concentration sufficient to perform the inventive separation while, if a corresponding starting mixture comprises a lower concentration of carbon dioxide, a classical e.g. amine based carbon capture can be more economical to produce a carbon dioxide product fraction. With a tail gas treatment being present in the method according to the present invention, the starting gas mixture being which is produced from the tail gas of the Claus process may, in an illustrative example, contain more than 60 mol-%, preferably more than 70 mol-%, most preferably more than 90 mol-% carbon dioxide, the rest of the main components being water, carbon monoxide, sulphur dioxide, hydrogen and traces of carbonyl sulphide and carbon disulphide in combination. In more general terms, sulphur dioxide may be present in a concentration of 0.0001 to 5 mol-%, e.g. ca. 2 mol-% and water may be present in a concentration of at least 5 mol-%, e.g. from 5 to 20 mol-%.

According to a particularly preferred embodiment of the present invention, the first group of method steps includes subjecting the starting gas mixture to a first cooling step at a first pressure level and from a first temperature level to a second temperature level, forming a first condensate and a first gaseous remainder, subjecting at least a part of the first gaseous remainder to a compression step from the first pressure level to a second pressure level, forming a second condensate and a second gaseous remainder, subjecting at least a part of the second gaseous remainder to a further cooling step to a third temperature level, forming a third condensate and a third gaseous remainder, subjecting at least a part of the third gaseous remainder to a drying step obtaining a dried gas mixture, and using at least a part of the dried gas mixture as the intermediate gas mixture which is subjected to the second group of method steps.

The starting gas mixture is, in other words, in the first group of method steps firstly cooled and then compressed using an adequate compressor system designed for this type of media. In the initial cooling step, a condensate (the“first” condensate) is formed which already contains carbon dioxide and sulphur dioxide dissolved in water. The carbon dioxide content is already increased at this point. The subsequent compression of the gaseous remainder (“first” gaseous remainder) with availability of at least or exceeding 30 ton per hour is preferably realized using several, e.g. three, compression stages with intercooling. During this phase, more condensate is generated, additional sulphur dioxide being also removed in the liquid condensate and the carbon dioxide concentration further raises. According to the present invention, the first gaseous remainder is enriched in carbon dioxide and depleted in sulphur dioxide and water in relation to the starting gas mixture, and the second gaseous remainder and the dried gas mixture are each enriched in carbon dioxide and depleted in sulphur dioxide and water in relation to the starting gas mixture and also in relation to the first gaseous reminder.

According to a preferred embodiment of the present invention, the starting gas mixture is provided at a first pressure level which is 0 to 1 bar (g), e.g. at a slight overpressure of 50 mbar (g). The second pressure level may be 20 to 30 bar. The second pressure level preferably is at least 22 bar (g) but due to efficiency reasons is preferably not above 27 bar (g).

Preferably, the first temperature level, at which the starting gas mixture is provided, is 50 to 300 °C, e.g. at least 240 °C, at least or about 150 °C or at least or about 60 °C and the second temperature level and the third temperature level each are 0 to 20 °C, preferably below 10 °C.

Before being supplied to the second group of method steps, which include a cryogenic treatment, water must be removed almost completely. This is achieved by cooling and drying steps known from the art per se. While the third gaseous remainder may still comprise up to 5,000 vppm water, preferably up to 2,000 vppm water, more preferably up to 1 ,000 or 600 vppm water, the dried gas mixture preferably only comprises less than 20 vppm water according to the present invention.

The purpose of the second group of method steps, including a liquefaction, is firstly to increase the carbon dioxide content level to at least 95 mol-%, more preferably more than 98,5 mol-% or even more preferably 99,9 mol-%, which is the content of carbon dioxide in the fraction“predominantly or exclusively containing carbon dioxide” which is formed according to the present invention. Before the liquefaction is started, the purification of the carbon dioxide must be completed, as elucidated below.

According to the present invention, preferably the absorbing step used in the second group of method steps is an absorbing step using an absorption column in which a liquid bottom product and a gaseous overhead product is formed, at least a part of the gaseous overhead product being liquefied, and the liquid carbon dioxide used in the absorbing steps being formed from a part of the liquefied gaseous overhead product. Preferably, exactly one such absorbing step and/or exactly one absorption column using liquid carbon dioxide are utilised in the second group of method steps. Besides the absorbing step, the second group of method steps further includes, as mentioned, liquefying and rectifying at least a part of a gaseous overhead product being produced in the absorbing step.

The absorbing step reduces the amount of sulphur dioxide and traces of carbonyl sulphide/carbon disulphide present in the intermediate gas mixture. For this, the intermediate gas mixture is preferably routed through a highly efficient direct contact liquid carbon dioxide wash unit, referred to as an“absorption column” hereinbefore. Absorption columns usable according to the present invention are known from the art per se. Liquid carbon dioxide used in the absorption step is preferably high purity liquid carbon dioxide generated after the liquefaction, as described below.

At this stage, the liquid carbon dioxide coming in contact with the comparatively warm intermediate gas mixture has a double effect: Firstly, the temperature of the

intermediate gas mixture is reduced to 0 °C or less, more preferably even -20 °C or less, and secondly the sulphur dioxide concentration is greatly reduced. The liquid carbon dioxide/sulphur dioxide mixture with virtually all the carbonyl sulphide and/or carbon disulphide will then concentrate itself in the sump of the carbon dioxide absorption column, yielding the bottom product thereof. This is preferably withdrawn and treated in a waste treatment step or system as described below, as it still contains more than 80 mol-%, particularly about 90 mol-%, carbon dioxide.

The bottom product therefore comprises mostly carbon dioxide and sulphur dioxide, but also carbon monoxide, hydrogen and carbonyl sulphide or at least traces thereof, if present in the starting gas mixture. Due to the high concentration of carbon dioxide, an additional handling of the this bottom product is advantageous and will be described below and will be referred to as“waste” treatment.

In summary, in the waste treatment step, preferably a fraction which is depleted in carbon dioxide and enriched in sulphur dioxide as compared to the liquid bottom product is formed, this fraction being at least in part recycled to the Claus process. Furthermore, preferably a further fraction is formed in the waste handling step, the further fraction being enriched in carbon dioxide and depleted in sulphur dioxide as compared to the liquid bottom product and at least in part being recycled to the second absorbing step. Details will be explained below.

The overhead product of the absorption step preferably only comprises carbon dioxide and other“non-condensable” components of the starting gas mixture like hydrogen and carbon monoxide, while the overhead product may particularly be poor in or (virtually) free from sulphur dioxide and other components like carbonyl sulphide and carbon disulphide.

A second absorption step would serve the purpose of further reducing sulfur components presented in the overhead product is thus not required. This is particularly the result of the use of a tail gas treatment upstream of the inventive method. The overhead product or a part thereof is therefore preferably subjected to the liquefaction without further (previous) purification treatment, as indicated below.

At this stage additional streams may be used in the absorption step, particularly a gaseous carbon dioxide stream from a recycling compressor unit subsequent to the waste treatment system used for treating the bottom product, as described in more detail below.

In other words, besides at least a part of the intermediate gas mixture or a part thereof and the liquid carbon dioxide, the absorption step may also supplied with a stream enriched in carbon dioxide and depleted in sulphur dioxide produced in the waste treatment system using at least a part of the bottom product.

The waste treatment system, which is used for treating the bottom product from the absorption step preferably includes a low-pressure rectification system to separate mainly the liquid carbon dioxide by the gaseous sulphur dioxide. Before entering the low-pressure rectification, the pressure of the corresponding stream may be is reduced to 5 to 6 bar (g), e.g. to 5.5 bar (g) and temperature drops automatically as a result of the isenthalpic process to -40 to -50 °C, e.g. to ca. -46 °C. As mentioned, at least a part of the liquid bottom product is treated in the waste handling step, forming a fraction which is recycled to the Claus process. Two phases are generated in the waste handling step, due to the expansion already, which are a liquid phase and a gaseous phase. Both these streams , i.e. the liquid and the gaseous phase, are preferably forwarded to a further separation system, e.g. a low- pressure rectification system mentioned, the liquid phase being the liquid feed at the top of a rectification column in such a system and the gaseous phase being used as the boil-up. In addition, the reboiler unit of the rectification system will generate enough boil-up to assure a proper sulphur dioxide distillation.

A liquid phase generated in this low-pressure rectification preferably contains carbon dioxide and a combination of sulphur dioxide, carbonyl sulphide and carbon disulphide, e.g. up to 40 mol-% carbon dioxide, preferably up to 30 mol-% carbon dioxide, more preferably up to 20 mol-% carbon dioxide, and sulphur dioxide, carbonyl sulphide and carbon disulphide in correct balance. The gaseous phase preferably contains more than 97 mol-%, preferably more than 98 mol-%, most preferably more than 99.9 mol-% carbon dioxide and only minor amounts of sulphur dioxide.

The liquid phase of this further separation system contains significant amounts of sulphur dioxide and the liquid phase is therefore preferably recycled to the Claus process. Accordingly, the oxygen concentration in the feed to the Claus process is preferably adjusted (decreased) in order to adjust the balance of sulphur

dioxide/hydrogen sulphide in the catalytic stage for Claus reaction (sulphur formation).

The gaseous phase of this further separation system has a particularly high carbon dioxide concentration and comprises only low amounts of sulphur dioxide and traces of hydrogen and carbon monoxide. The cold energy is recovered and further compressed in a recycle compressor to the operation pressure of the absorption step and the gaseous phase is preferably used as a boil-up source therein. Generally, the fine adjustment of the allowed sulphur dioxide, carbonyl sulphide/carbon disulphide impurity levels is dependent by the capacity of a corresponding recycle compressor and increases exponentially in case the specification of the sulphur components nears trace levels, i.e. values of less than 1 vppm or the analytical detection limits.

The feed stream sent to liquefaction, being the overhead product of the absorption or a part thereof, has a carbon dioxide concentration of at least 63 mol-%, preferably at least 73 mol-%, most preferably at least 93 mol-% and particularly up to 98 mol-%, depending on the amount of impurities defined by non-condensable hydrogen and carbon monoxide. Liquefaction and further purification of the overhead product from the absorption step comprises, according to the present invention, a liquefaction and a further purification by rectification, resulting in a carbon dioxide product containing more than 99.9 mol-% of carbon dioxide. In case the purity of carbon dioxide is increased to such a concentration, the carbon dioxide product may be used for desalination or if a use in food or beverage products The final carbon dioxide product may also enter a further treatment step, if necessary for the intended use.

A gas phase remaining in the liquefaction step still may contain carbon dioxide that is preferably recovered. Other components like hydrogen and carbon monoxide may also be present in this stream and have a certain heat value that is preferably reused in the Claus process. The gas may have low temperature of e.g. ca. -42 °C and a high pressure of e.g. ca. 24.6 bar (g), representing conditions that are ideal for the use of a methanol wash system as a separation step, which is therefore preferably used. After the separation step, two gaseous streams are available, one low pressure and low temperature highly carbon dioxide concentrated stream and a synthesis gas stream at high pressure and low temperature gas stream with hydrogen and carbon monoxide to be reused in the Claus process.

Preferably, according to the present invention, heat is transferred in the first group of method steps to a part of the liquefied and purified gaseous overhead product, i.e. the carbon dioxide product, forming an evaporated carbon dioxide product. The evaporated carbon dioxide product may be compressed to provide a compressed carbon dioxide product, representing the fraction comprising predominantly or exclusively carbon dioxide as mentioned above. Compression may e.g. be performed to 200 to 300 bar (g), e.g. ca. 240 bar (g) and the compressed stream may be delivered for use in enhanced oil recovery. Other applications can be included, e.g. usage of carbon dioxide for liquid carbon dioxide production and supply including the beverage market.

In any cases, the fraction predominantly or exclusively comprising carbon dioxide according to the present invention may at least in part be produced as or processed to a liquid fraction. That is, such a fraction may be withdrawn in liquid form or may be liquefied from a gaseous fraction. Any further processing, like conventional in the art for producing carbon dioxide for food and beverage uses, may also be included.

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 relates to an apparatus for treating a starting gas mixture comprising carbon dioxide, sulphur dioxide and water, the starting gas mixture being produced including a Claus process operated with oxygen enrichment yielding a tail gas and including a tail gas treatment of the tail gas, the tail gas treatment comprising at least one of a conversion of hydrogen sulphide to sulphur dioxide, a

synproportionation of hydrogen sulphide and sulphur dioxide to yield elementary sulphur and an elimination of hydrogen sulphide from the tail gas, wherein the apparatus comprises means adapted to forming a fraction predominantly or exclusively containing carbon dioxide.

According to the present invention, means are provided which are adapted to enrich the starting gas mixture in carbon dioxide and deplete the starting gas mixture in sulphur dioxide and water forming an intermediate gas mixture in a first group of method steps including compressing, cooling, condensing and drying steps, 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 an absorbing step in which liquid carbon dioxide is used to absorb sulphur dioxide from the intermediate gas mixture, and the second group of method steps further including liquefying and rectifying at least a part of a gaseous overhead product being produced in the absorbing step.

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, sulphur dioxide 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. The Claus process 1 yields a tail gas which is treated in a tail gas treatment 2, the tail gas treatment 2 comprising at least one of a conversion of hydrogen sulphide to sulphur dioxide, a synproportionation of hydrogen sulphide and sulphur dioxide to yield elementary sulphur and an elimination of hydrogen sulphide from the tail gas. A tail gas incinerator 3 to destroy rests of hydrogen sulphide in the tail gas of the Claus process 1 may also be used in the method 100 as a backup.

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, condensing and drying 14 steps. 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 an absorbing 21 step in which liquid carbon dioxide is used to absorb sulphur dioxide from the intermediate gas mixture C, the liquid carbon dioxide being formed from carbon dioxide contained in the intermediate gas mixture after treatment in the absorbing steps 21. The second group of method steps 20 further includes liquefying 22 and rectifying 23 at least a part of a gaseous overhead product being produced in the absorbing step 21 , as described above and further detailed below.

More specifically, the first group 10 of method steps includes subjecting the starting gas mixture to a first cooling step 11 at a first pressure level and from a first

temperature level to a second temperature level, forming a first condensate and a first gaseous remainder, and subjecting at least a part of the first gaseous remainder to a compression step 12 from the first pressure level to a second pressure level, forming a second condensate and a second gaseous remainder. The first and second

condensates are not shown here for reasons of conciseness, and the first cooling step 11 and the compression step 12 are shown as a common block 12, 13.

At least a part of the second gaseous remainder remaining after compression 12 is subjected to a further cooling step 13 to a third temperature level, forming a third condensate (again not shown) and a third gaseous remainder. At least a part of the third gaseous remainder 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 10 of method steps.

For specific concentrations, pressure and temperature levels, specific reference is made to the explanations already given above.

The absorbing step 21 used in the second group 20 of method steps in the method 100 preferably is performed in an absorption column in which a liquid bottom product D and a gaseous overhead product E is formed. In the absorption step 21 , liquid carbon dioxide is used, shown as a stream H1 , the liquid carbon dioxide being formed from a part of the gaseous overhead product E which is treated in the liquefaction 22 and rectification 23 steps which are also part of the second group 20 of method steps and which is withdrawn therefrom in form of a stream H.

The bottom product which is formed in the absorbing step 21 is submitted to a waste treatment step 24. In the latter, as described in more detail above, and preferably by an expansion and a subsequent low pressure rectification, a stream I rich in sulphur dioxide which is recycled to the Claus process 1 , and a stream K which is rich in carbon dioxide and which submitted to a recompression step 25 and then recycled to the absorption step 21 are formed. In the liquefaction step 22, the overhead product E of the absorption step 21 is liquefied. The liquefied stream, which is now denoted F, is then subjected to the rectification step 23 where the stream H mentioned before is produced in form of a liquid stream rich in carbon dioxide. The arrangement of the steps 22 and 23 can also be different and the stream H can also be produced at a different position. A part H2 of this stream, which is not used as described before, is, in the example shown, submitted to an energy recovery step 26 and then heated in the cooling step 11 in the first group 10 of method steps. Thereafter, a compression in a compression step 40 is performed. The compressed stream H3 may be used for the purposes described above. A further stream L less rich in carbon dioxide than the stream H is withdrawn from the liquefaction and purification step 23 in gaseous form and subjected to a washing step 30, particularly including a methanol wash. Herein, a high pressure stream M rich in carbon monoxide and hydrogen and a low pressure stream N rich in carbon dioxide are formed. Both streams are routed through the energy recovery step 26. The stream M is e.g. used for firing purposes and the stream N is e.g. used as the recycling stream B.