DE102016011356A1 - Process and plant for producing a natural gas substitute and a carbon dioxide product - Google Patents

Abstract

The invention relates to a process (100) for producing a natural gas substitute and a carbon dioxide product in which a separation insert is provided at a first pressure level and a first temperature level using a feed gas mixture containing methane and carbon dioxide, the separation insert at the first pressure level below a second temperature level the first temperature level is cooled and partially condensed to obtain a first liquid fraction and a first gaseous fraction, the first liquid fraction or a portion thereof is expanded to a second liquid fraction and a second gaseous fraction to a second pressure level above the first pressure level, the first gaseous fraction or a part thereof is used to form the Erdgassubstituts, the second gaseous fraction or a part thereof is returned to form the separation insert, and the second liquid fraction od part of it is used to form the carbon dioxide product. A corresponding system is also an object of the present invention.

Description

The invention relates to a process for the preparation of a natural gas substitute and a carbon dioxide product and a corresponding plant.

State of the art

Non-fossil sources of methane, such as biogas, are widely used to generate energy. However, as described for example in "Feed-in of biogas into the natural gas network experience reports of the first biomethane plants", Energie | Wasser-Praxis 4/2009, natural gas can not be fed directly into natural gas networks due to its composition. Biogas is typically a mixed gas whose main component is methane. Depending on the type of raw materials used, raw biogas contains up to 2/3 methane, 1/3 carbon dioxide and smaller amounts of hydrogen sulphide, nitrogen and oxygen.

For the treatment of biogas to natural gas substitutes, different methods are known from the prior art. For example, the biogas may be subjected to pressure swing adsorption, in which carbon dioxide is absorbed, for example, on a carbon molecular sieve. Further, physical and chemical gas scrubbing methods using water, alkalis, amines, selexol and the like are also known.

Although the present invention will be described primarily with reference to biogas, it is understood that it can also be used to process otherwise obtained gas mixtures of similar or comparable composition as biogas.

The object of the present invention is to improve corresponding processes and, in particular, to utilize the carbon dioxide contained in a corresponding gas mixture.

Disclosure of the invention

Against this background, the present invention proposes a method for producing a natural gas substitute and a carbon dioxide product and a corresponding system with the respective features of the independent claims. Preferred embodiments are subject of the dependent claims and the following description.

Before explaining the features and advantages of the present invention, some basics and the terms used will be explained.

Liquid and gaseous mixtures, as used herein, may be rich or poor in one or more components, with "rich" for a content of at least 50%, 75%, 90%, 95%, 99%, 99.5%, 99, 9% or 99.99% and "poor" for a content of at most 50%, 25%, 10%, 5%, 1%, 0.1% or 0.01% on a molar, weight or volume basis. The term "predominantly" can correspond to the definition of "rich". Liquid and gaseous mixtures may also be enriched or depleted in one or more components as used herein, which terms refer to a corresponding content in a starting mixture from which the liquid or gaseous stream was obtained. The liquid or gaseous mixture is "enriched" if it is at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times or 1000 times, "depleted" when it contains not more than 0,9 times, 0,5 times, 0,1 times, 0,01 times or 0,001 times the content of a corresponding component, relative to the starting mixture. In the present case, for example, "methane" or "carbon dioxide" is mentioned, including a mixture understood that is rich in the corresponding component. However, it can also be the respective clean gas.

A liquid or gaseous mixture is "derived" from or formed from another liquid or gaseous mixture (also referred to as the starting mixture), or if it contains at least some of those contained in or obtained from the starting mixture Components has. A mixture formed in this sense may be formed from the starting mixture by separating or branching off a partial stream or one or more components, enriching or depleting one or more components, chemically or physically reacting one or more components, heating, cooling, pressurizing and the like. However, "providing", for example, a separation insert for a subsequent separation process, may also simply represent passing a corresponding mixture in a suitable conduit and delivering to the separation process.

The present application uses the terms "pressure level" and "temperature level" to characterize pressures and temperatures, thereby indicating that corresponding pressures and temperatures in a given plant need not be used in the form of exact pressure or temperature values to realize the innovative concept. However, such pressures and temperatures typically range in certain areas for example ± 1%, 5%, 10%, 20% or even 50% around an average. Corresponding pressure levels and temperature levels can be in disjoint areas or in areas that overlap one another. In particular, for example, pressure levels include unavoidable or expected pressure drops. The same applies to temperature levels. The pressure levels indicated here in bar are absolute pressures.

Advantages of the invention

In the context of the present invention, it has been recognized that fractions can be obtained by suitably coordinated steps, namely compression, cooling and subsequent expansion, from the gas mixtures mentioned at the beginning, for example biogas, which are particularly suitable for providing a natural gas substitute and a carbon dioxide product and thereby significantly improve the processes known from the prior art and briefly mentioned at the outset.

In particular, the present invention is also suitable for gas mixtures that have rather low carbon dioxide partial pressures of less than 25 mole percent. The composition of a gas mixture processed in the context of the present invention is, for example, 20 to 50 mole percent carbon dioxide and 50 to 79 mole percent methane. As mentioned, corresponding gas mixtures such as biogas may contain minor amounts of sour gases and other gas components. Application scenarios of the present invention are, for example, biogas upgrading plants in which the concentration of, carbon monoxide, nitrogen, oxygen and especially hydrogen is rather low. The presence of so-called non-condensable components, such as hydrogen, can reduce the efficiency at higher concentrations. In particular, in the context of the present invention, the content of non-condensable components and of nitrogen and / or oxygen and / or carbon monoxide is less than 7 mol percent.

A corresponding gas mixture is referred to herein as "feed gas mixture". The feed gas mixture is subjected to the production of the desired products, namely the natural gas substitute and the carbon dioxide product, partially or completely as a separation insert a separation process explained below.

Overall, the present invention proposes a process for producing a natural gas substitute and a carbon dioxide product, wherein a separation insert is provided at a first pressure level and a first temperature level using a feed gas mixture containing methane and carbon dioxide, for example the mentioned biogas. For the concept of "providing" or "forming" a corresponding separation insert, reference is made to the above explanations and definitions. Again, that a corresponding separation use in particular using other gas mixtures, in the context of the present invention, in particular using recycled streams or recycled gas mixtures, can be formed.

In the context of the present invention, the separation insert provided in this way is cooled at the first pressure level to a second temperature level below the first temperature level and partially condensed to obtain a first liquid fraction and a first gaseous fraction. The first liquid fraction is enriched in comparison with the separation insert in the sense explained above in carbon dioxide, the first gaseous fraction contains by corresponding depletion little or no more carbon dioxide.

The first gaseous fraction can therefore, as also explained below, be used in the formation of the natural gas substitute and, for this purpose, is in particular still subjected to a suitable subsequent or reprocessing. In this case, for example, physical laundry units and a subsequent absorption of carbon dioxide may be provided in order to allow a subsequent liquefaction. The present invention has the particular advantage that the content of carbon dioxide to be removed here is only slight, because already by the coordinated steps of compression, cooling and the subsequent expansion, a substantial removal of carbon dioxide was achieved. Contents of methane or carbon dioxide in the corresponding fractions are given below in detail.

The present invention has as further advantages that only one plant for processing a corresponding gas mixture is required and therefore advantages in terms of operating and production costs can be achieved. There is no additional compression of carbon dioxide required. Conventionally used washing water units are not needed in the present invention, which can reduce biological contamination, in particular bacterial growth. The present invention enables good control and management of the contaminants, especially long chain olefins and acids, which are removed in appropriate washing units. In the context of the present invention, a high-purity natural gas substitute can be produced; the quality of the liquid carbon dioxide can be ensured by additional purification steps. Overall, the present system requires one low installation space and is also suitable for comparatively low carbon dioxide concentrations in the feed gas mixture.

As can be seen from the concentrations given below, the first liquid fraction contains, in particular, still appreciable amounts of methane, which should advantageously be recovered and used in the production of the natural gas substitute.

Therefore, the present invention now proposes to relax the first liquid fraction or part thereof to a second liquid fraction and a second gaseous fraction to a second pressure level above the first pressure level. In this relaxation, a part of the first liquid fraction or its relaxed here part evaporates, in particular methane passes into the gas phase. In this "flashing" of the first liquid fraction or its part, therefore, the second liquid fraction compared to the first liquid fraction is further enriched in carbon dioxide. In this way, in the form of the second liquid fraction, a carbon dioxide fraction can be formed, which in particular has the contents of carbon dioxide explained below, and which can be subjected to further separation steps to produce the carbon dioxide product.

The present invention provides, as already mentioned, not to form the separation insert exclusively using the methane and carbon dioxide-containing feed gas mixture, for example the biogas. In particular, it is provided in the context of the present invention to recycle the second gaseous fraction or a part thereof to form the separation insert. The first gaseous fraction or a part thereof, however, as already mentioned, according to the invention used to form the Erstgassubstituts, the second liquid fraction or a part thereof to form the carbon dioxide product.

In particular, forming the carbon dioxide product in the present invention may include cryogenic rectification of the second liquid fraction or its portion used to form the carbon dioxide product at the second pressure level and a third temperature level above the second temperature level. As part of a corresponding cryogenic rectification can be produced in this way, for example, high-purity carbon dioxide. Overall, the present invention provides a more cost effective method of recovering a natural gas substitute that is particularly suitable for lower carbon dioxide partial pressure gas mixtures, as previously discussed.

Advantageously, the forming of the separation insert in the context of the present invention comprises one or more compression steps to the first pressure level, wherein the second gaseous fraction or its part returned to form the separation insert is also fed to the one or more compression steps. In particular, a compression step is selected for the return, which corresponds to the pressure level at which the second gaseous fraction is present, ie in particular the second pressure level or a pressure level in the region of the second pressure level. In this way, additional compression and expansion steps can be saved. For example, the second gaseous fraction or its part recirculated to form the separation insert can be supplied to a compression stage of a multi-stage compressor. It is also possible to use a plurality of compressors, in particular a so-called booster and a booster, wherein the return also takes place at a suitable location.

Advantageously, the first pressure level, i. H. the pressure level at which the separation insert is provided at a pressure level of 20 to 25 bar. In particular, in this case, a pressure level of up to about 22 bar can be used. The second pressure level, d. H. the pressure level at which the first liquid fraction or its part is expanded, is advantageously at 10-15 bar, for example, a pressure level of about 14.2 bar can be used. Advantageously, while a pressure difference between the first and the second pressure level 5-10 bar, for example, a pressure level difference can be at about 7 bar.

The second, used in the context of the present invention temperature level is advantageously from -45 to -50 ° C and / or the third temperature level is advantageously from -35 to -40 ° C. For example, the second temperature level may be -48 or -50 ° C, the third temperature level is in particular at about -38 ° C.

In the context of the present invention, the formation of the natural gas substitute advantageously comprises a physical wash of the first gaseous fraction or its part used to form the natural gas substitute. Upstream or in such a physical wash, in particular, a cold recovery can be carried out, wherein the heat supplied in this case is withdrawn from the separating insert and / or streams formed therefrom. For this purpose, in particular, as also explained below, for example, ammonia and / or carbon dioxide refrigerant circuits can be used.

In the physical wash thereby advantageously a carbon dioxide-containing fraction is recovered, which or part of which is also attributed to the formation of the separation insert. In this way, the carbon dioxide contained in the second gaseous fraction can still be recovered and ultimately converted into the carbon dioxide product.

Advantageously, a ratio in which the first liquid fraction and the first gaseous fraction are formed in the partial condensation to each other, at 25 to 65 to 35 to 65. Corresponding condensation ratios can be achieved in particular by the use of said pressures and temperatures, but they depend in particular also from the composition of the respective feed gas mixtures or recycled fractions.

Advantageously, a ratio in which the second liquid fraction and the second gaseous fraction are formed when the first liquid fraction is expanded relative to one another is 6 to 94 to 8 to 92. The values given are each to be understood as proportions on a molar basis.

A carbon dioxide content in the first liquid fraction can be enriched in this way to a value of 90% to 95% and in the second liquid fraction of 95% to 99% and above that in the first liquid fraction.

The present invention also extends to a plant for producing a natural gas substitute and a carbon dioxide product. For details of a corresponding system, reference is made to the corresponding independent claim. In particular, such a system is set up to carry out a method, as explained above. The corresponding features and advantages are therefore expressly referred to.

The present invention will be further described below with reference to the accompanying drawings which show a preferred embodiment of the present invention.

Short description of the drawing

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

Detailed description of the drawing

1 illustrates a method according to an embodiment of the invention, here in total with 100 is designated. Are below method steps of the method 100 described, these also apply in a corresponding manner to the system parts or apparatus provided in a corresponding system. Will therefore be subsequent to a procedure 100 With reference made, the corresponding explanations also apply to a system according to an embodiment of the invention.

In the process 100 becomes a booster 101 a gas stream c is supplied, which in the example shown is formed from a feed gas stream a, for example biogas, and a recirculated gas stream b, which is explained below. The crude gas stream c may, for example, comprise an amount of 8200 standard cubic meters per hour. The gas stream b comprises, for example, an amount of 2507 standard cubic meters per hour. A correspondingly obtained, precompressed gas stream d is a post-compression 102 fed. The compression takes place, for example, to a pressure level of about 22 bar.

A correspondingly obtained, re-compressed compressed gas stream e is a hydrogen sulfide removal 103 fed. The compressed gas stream f purified in this way is passed through a dryer 104 led here and freed of residual water, which in particular from the hydrogen sulfide removal 103 can come. A correspondingly obtained, dried gas stream g becomes an optional further treatment 105 , For example, the removal of volatile organic hydrocarbons fed.

After purification, cooling takes place 106 of the purified gas stream h. This cooling is carried out in particular to the previously described second temperature level of, for example, about -48 ° or -50 ° C. In this way, a two-phase current i is formed. The vapor / liquid ratios that occur here have been explained previously. In the example shown, for example, 30 molar proportions of the feed gas mixture used are liquefied, which includes, for example, a liquefied amount of about 2464 standard cubic meters per hour of carbon dioxide.

With dashed arrows each cooling circuits are illustrated and for the sake of clarity, not separately explained. For cooling in the cooling step 106 For example, a carbon dioxide refrigeration cycle 107 further provided is the use of an ammonia refrigeration unit 108 possible.

The two-phase mixture of the stream i comprising the "first liquid fraction" explained above and the "first gaseous fraction" explained above becomes a two-stage separation 109 supplied and separated in this phase. As not separately illustrated herein, the first liquid fraction or a portion thereof is thereafter reclaimed to a liquid fraction, previously referred to as a "second liquid fraction", and a gaseous fraction, previously referred to as a "second gaseous fraction", to a second pressure level relaxed above the first pressure level. In the example shown here, this is done, for example, to about 14.2 bar, so that a pressure difference of about 7 bar is overcome. In this way, again results in a liquid / vapor ratio, in the example shown, for example, with a liquid content of 92% to 94%. In this way, the content of the second liquid fraction of carbon dioxide can be increased. The flow of material j, which is in the cryogenic rectification 110 can be brought to a carbon dioxide content of about 98 mole percent, for example, compared to the first liquid fraction, which may have a content of 93 mole percent.

The second liquid fraction or a portion thereof is used to form the carbon dioxide product and, in the form of a stream j, into a cryogenic rectification 110 transferred. In contrast, the second gaseous fraction formed during the expansion can, for example, in the form of a stream k in the compression 102 to be led back.

In the cryogenic rectification 110 For example, the second liquid fraction or its portion fed therein can be further increased in purity. In the cryogenic rectification 110 For example, carbon dioxide can be produced with a purity of 99.99% and in the form of a stream I into a tank system 111 be transferred. The cryogenic rectification or a rectification column used in the low-temperature rectification is also connected to a corresponding cooling circuit, as mentioned, illustrated by dashed arrows. The cryogenic rectification takes place, for example, at a temperature level of -38 ° C. The temperature of the second liquid fraction should be in balance with an operating pressure at which the cryogenic rectification 110 is carried out. In principle, the second pressure reduction can take place up to 11 bar, but it depends on the carbon dioxide content of the feed gas mixture. The pressure depends on the available refrigerant in a corresponding cryogenic cycle and the temperatures that can be generated with it, which in the example shown, for example, at about -50 ° C. The advantage of a corresponding low-pressure distillation is that even traces of higher hydrocarbons (eg ethane very good and propane only limited) can be reduced in the product.

Generally, it can be said that the efficiency of carbon dioxide liquefaction in the range of 25 to 50 mole percent in a feed gas mixture is less dependent on the partial pressure of carbon dioxide in the feed gas mixture. If the concentration decreases, an amount of recirculated flow can be adjusted by deliberately returning carbon dioxide to the separation unit.

The first gaseous fraction resulting from the cooling of the separation insert may be in the form of a mass flow m for cold recovery 112 be supplied. A here partially mentioned stream n is a physical gas scrubbing 113 fed. The amount of stream m may, for example, about 5683 standard cubic meters include the heating in the cold recovery 112 takes place for example at 15 ° C. The physical wash in the washing step 113 can be done using known methods. The purity of the resulting natural gas substitute, which in the form of a stream o from the physical wash 113 can be carried out, for example, be 94.6 mole percent and thus comes close to that known natural gas. In particular, a corresponding stream can also be mixed with a top gas from the low-temperature rectification (not shown), which in the example shown would result in a stream of about 3280 standard cubic meters per hour with a purity of about 92% methane.

In the physical wash 113 separated and recovered carbon dioxide can be recycled in the form of the already mentioned material flow b and in this case, for example, a suction side of the booster 102 be supplied. A quantity of the material stream b can also be used to set a carbon dioxide content in the stream c in order to be able to control a plant accordingly. As mentioned, the natural gas substitution carried out in the form of the stream o contains predominantly methane, is present at high pressure, and can be used either in gaseous or liquid form. Therefore, a carbon dioxide content of approx. 50 ppm should not be exceeded for the liquefaction. To achieve this, at least two physical laundry units can be used 113 and subsequent carbon dioxide absorption may be provided. The present invention has the particular advantage that the content of carbon dioxide to be removed here is only slight.

Overall, the mass balance of the present system depends on the removal of condensate and low waste streams closed, which are not separately illustrated here.

Claims (13)

Procedure ( 100 for producing a natural gas substitute and a carbon dioxide product, in which - using a methane and carbon dioxide-containing feed gas mixture Separation insert is provided at a first pressure level and a first temperature level, - the separation insert at the first pressure level cooled to a second temperature level below the first temperature level and partially condensed to obtain a first liquid fraction and a first gaseous fraction, - the first liquid fraction or a Part thereof is depressurized to obtain a second liquid fraction and a second gaseous fraction to a second pressure level above the first pressure level, - the first gaseous fraction or a part thereof is used to form the natural gas substitute, - the second gaseous fraction or a part thereof Forming the separation insert is returned, and - the second liquid fraction or a portion thereof is used to form the carbon dioxide product. The method of claim 1, wherein forming the carbon dioxide product comprises cryogenic rectification of the second liquid fraction or its portion used to form the carbon dioxide product at the second pressure level and a third temperature level. Procedure ( 100 ) according to claim 1 or 2, wherein the feed gas mixture is formed using biogas and / or synthesis gas or comprises biogas and / or synthesis gas. Procedure ( 100 ) according to claim 1 or 2, wherein the forming of the separation insert comprises one or more compression steps to the first temperature level, wherein the second gaseous fraction or its part returned to form the separation insert is supplied to the one or more compression steps. Method according to one of the preceding claims, wherein the first pressure level at 20 to 25 bar (abs.) And / or at which the second pressure level at 10 to 15 bar (abs.) And / or in which a pressure difference between the first and the second pressure level is 5 to 10 bar (abs.). Method according to one of claims 2 to 5, wherein the second temperature level is -45 to -50 ° C and / or wherein the third temperature level is -35 to -40 ° C. The method of any one of the preceding claims, wherein forming the natural gas substitute comprises physically scrubbing the first gaseous fraction or its portion used to form the natural gas substitute. A method according to claim 7, wherein in the physical wash a fraction containing carbon dioxide is recovered, or part thereof also being recycled to form the separation insert. A method according to any preceding claim, wherein a ratio in which the first liquid fraction and the first gaseous fraction are formed in the partial condensation to each other is 25 to 65 to 35 to 65. A method according to any one of the preceding claims, wherein a ratio in which the second liquid fraction and the second gaseous fraction are formed when heated to each other is 6 to 94 to 8 to 92. A process according to any one of the preceding claims, wherein a carbon dioxide content in the first liquid fraction is 90 to 95 mole percent and in the second liquid fraction is 95 to 99 mole percent and above that in the first liquid fraction. Plant for producing a natural gas substitute and a carbon dioxide product having means adapted for To provide a separation insert at a first pressure level and at a first temperature level using a feed gas mixture containing methane and carbon dioxide, To cool the separation insert to a second temperature level below the first temperature level at the first pressure level and to partially condense to obtain a first liquid fraction and a first gaseous fraction, To relax the first liquid fraction or a portion thereof to a second liquid fraction and a second gaseous fraction to a second pressure level above the first pressure level, To use the first gaseous fraction or a part thereof for the formation of the natural gas substitute, - attributed the second gaseous fraction or a part thereof to form the separation insert, and To use the second liquid fraction or a part thereof to form the carbon dioxide product. Plant according to claim 12, arranged for carrying out a method according to one of claims 1 to 11.

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