WO2021032319A1 - Method and system for processing natural gas - Google Patents

Abstract

The invention relates to a method (100) for processing natural gas which contains methane, ethane, higher hydrocarbons and nonhydrocarbons, wherein a membrane separation insert is provided using at least one portion of the natural gas, which membrane separation insert is enriched with methane and ethane and depleted of the higher hydrocarbons in comparison to the natural gas and contains at least one portion of the nonhydrocarbons from the natural gas, and wherein the membrane separation insert is subjected to a membrane separation step (20) in which a permeate fraction, which is depleted of methane and ethane and enriched with the nonhydrocarbons in comparison to the membrane separation insert, and a retentate fraction, which is enriched with methane and ethane and depleted of the nonhydrocarbons in comparison to the membrane separation insert, are formed. According to the invention, the membrane separation step (20) is carried out using a glass-like membrane, the provision of the membrane separation insert comprises a pressure change adsorption step (10), to which at least one portion of the natural gas is subjected and in which a high pressure gas, which is enriched with methane and ethane and is depleted of the higher hydrocarbons in comparison to the natural gas and contains at least one portion of the nonhydrocarbons from the natural gas, and a low pressure gas, which is depleted of methane and ethane and enriched with the higher hydrocarbons in comparison to the natural gas, are formed, wherein at least one portion of the high pressure gas is used as the membrane separation insert. The present invention also relates to a corresponding system.

Description

description

Process and plant for processing natural gas

The invention relates to a method for processing natural gas and a corresponding system according to the preambles of the independent claims.

State of the art

Natural gases are gas mixtures of hydrocarbons and other components, hereinafter referred to as "non-hydrocarbons". The hydrocarbons mainly include methane, but also higher alkanes such as ethane, propane, butane and pentane. The non-hydrocarbons include, in particular, acid gases such as carbon dioxide and sulfur compounds as well as hydrogen, nitrogen, helium and neon.

Membrane separation steps can be used to separate corresponding non-hydrocarbons from natural gases. In particular, it is possible to use vitreous membranes in appropriate membrane separation steps. If these membranes are used in unconditioned natural gas, the service life, selectivity and capacity of the membrane can be negatively influenced by the typical content of hydrocarbons with three or more carbon atoms.

The extraction of helium from natural gas using appropriate membranes is described, for example, in WO 2017/020919 A1, EP 3034466 B1, EP 3238808 B1 and EP 3498668 A1. In addition, reference is made to specialist literature such as the article "Noble Gases" in Ullmanns Encyclopedia of Industrial Chemistry, online publication March 15, 2001, DOI: 10.1002 / 14356007.a10_045.pub2 and H.- W. Häring (ed.), Industrial Gases Processing, Wiley -VCH, 2006, especially Chapter 4, "The Noble Gas Helium".

The present invention has the task of specifying improved concepts for processing natural gas using membrane separation steps, in particular when using vitreous membranes. Disclosure of the invention

Against this background, the present invention proposes a method and a system for processing natural gas with the respective features of the independent patent claims. Refinements are the subject of the dependent claims and the following description.

Features and advantages of the invention

As mentioned before, hydrocarbons with three or more carbon atoms have a negative effect on the selectivity, capacity and service life, especially of vitreous membranes. In order to minimize the investment and maintenance costs of a separation step carried out using a suitable membrane, complete removal of hydrocarbons with three or more carbon atoms from the natural gas upstream of the membrane separation step is optimal. In addition, as a result of the complete removal, maximum selectivity can be achieved in the membrane separation step, whereby, for example, the operating costs of multi-stage membrane systems are minimized.

However, the content of hydrocarbons with three or more carbon atoms for the further use of the natural gas downstream of the membrane separation step should usually not be changed to the stated range. In other words, a complete removal of hydrocarbons with three or more carbon atoms is disadvantageous from the aspect of the subsequent use of the natural gas. Furthermore, losses of products of value, such as those that occur when hydrocarbons with three or more carbon atoms are separated off, are not acceptable.

Temperature swing adsorption (TSA) for removing higher hydrocarbons from natural gas are proposed in US 5,557,030 A, US 2013/0291723 A1, WO 2014/021900 A1 and DE 102006011031 A1, among others.

If activated carbon is used as an adsorbent in such processes, hydrocarbons with five or more carbon atoms can be almost completely removed become. However, there are strong fluctuations in the calorific value of the purified natural gas, since the next shorter hydrocarbon than the hydrocarbon to be completely removed is completely retained at the beginning of each adsorption cycle and then fed back into the natural gas as a peak in a relatively short time during regeneration. These high fluctuations are generally not acceptable for feeding into a natural gas pipeline. When silica gel is used as an adsorbent, typically only hydrocarbons with six or more carbon atoms can be effectively removed.

With no temperature change regenerated adsorption process known to the inventors, it is possible to completely remove all hydrocarbons with three or more carbon atoms, as would be optimal for downstream membrane processes in which vitreous membranes are used.

According to WO 2015/116793 A1, natural gas can, for example, also be subjected to oil scrubbing in order to separate off corresponding hydrocarbons. With this technology, the higher hydrocarbons contained in natural gas are absorbed with the help of oil. The washing oil can consist of a short-chain or a long-chain hydrocarbon.

Using a long-chain washing oil with a relatively low vapor pressure has the advantage that relatively little washing oil is lost to the gas to be cleaned, but a relatively large amount of energy has to be expended in the regeneration by boiling. This variant is not an advantageous option for the pretreatment of a gas upstream of a vitreous membrane, since the gas is saturated with long-chain hydrocarbon components of the washing oil, which severely affects the functionality of the vitreous membrane or makes it impossible, even if the corresponding components are only in extremely low concentrations in the natural gas remain.

If a short-chain washing oil with a relatively high vapor pressure is used, a great deal of it goes into the gas phase, so that a relatively large amount of fresh oil has to be made available, which is not economical. Thermal processes in which heavy hydrocarbons are condensed out by direct cooling with the aid of a refrigerant are also known. This technology is very robust, but does not allow a sharp separation of individual hydrocarbon fractions. Only the dew point of the gas can be set. As a result, only a limited depletion of heavy hydrocarbons can be achieved. In addition, it is usually necessary to add chemicals to the application in order to prevent water from freezing out, for example. A lot of energy is required to provide the required cooling capacity.

In other process variants, the natural gas can be expanded isenthalpically from high to low pressure via a throttle, which cools it down. As with direct cooling, only the dew point of the gas can be set. In order to achieve low dew points and thus sufficient separation of hydrocarbons with three carbon atoms, high pressure differentials are necessary, which usually cannot be achieved or can only be achieved with a high compression effort.

Higher hydrocarbons can also be separated from the natural gas with rubber-like membranes. A complete and selective separation of individual hydrocarbon fractions cannot be achieved with this technology. In addition, the use of rubber-like membranes typically leads to a high loss of methane.

With none of the established processes it is therefore possible to completely remove hydrocarbons with three or more carbon atoms from natural gas and thus to ensure the optimal use of vitreous membranes.

The present invention solves the problems mentioned at least partially in the proposed method for processing a natural gas containing methane, ethane, higher hydrocarbons and non-hydrocarbons. An essential aspect of the present invention consists in the use of pressure swing adsorption or a corresponding pressure swing adsorption step upstream of the use of the vitreous membrane in a corresponding membrane separation step. The pressure swing adsorption for processing natural gas is basically already known from DE 103 03233 A1. In the pressure swing adsorption step, any suitable adsorbent can be used within the scope of the present invention, in particular an adsorbent which is in the form of a bed of activated aluminum oxide. The bed can be designed in the form of a single bed or multi-bed bed. The surface, the bulk density and the chemical composition as well as other parameters can be selected in the most suitable manner and / or taking into account other aspects.

The pressure swing adsorption step described with the specified adsorbent allows, in contrast to the other processes mentioned, an almost complete removal of hydrocarbons with three (and more) carbon atoms. The outflowing high-pressure gas from the pressure swing adsorption, freed of hydrocarbons, is ideally conditioned for the use of vitreous membranes. A positive side effect is the greatly reduced dew point of the high pressure gas, so that it remains gaseous during the further treatment steps. A low-pressure or adsorbate stream from pressure swing adsorption contains the separated hydrocarbons with three or more carbon atoms as well as water and part of the carbon dioxide, if contained in the feed gas, in enriched form.

In other words, within the scope of the present invention, using at least part of the natural gas by means of the pressure swing adsorption step, a membrane separation insert is provided which, compared to the natural gas, is enriched in methane and ethane and depleted in the higher hydrocarbons and which contains at least some of the non-hydrocarbons from the natural gas . The term “depletion” here and in the following is also intended to include essentially complete removal (in the sense of “depletion to zero”) so that the membrane separation insert is in particular also (essentially) free of higher hydrocarbons. Concrete numerical values are explained below. The "higher hydrocarbons" include, according to the language used here, in particular hydrocarbons with three or more, in particular three, four and five carbon atoms, as they are usually found in natural gas. The non-hydrocarbons include in particular carbon dioxide, helium and / or neon, but at least the latter noble gases. Carbon dioxide is optionally contained in natural gas. In the context of the present invention, the membrane separation insert is subjected to a membrane separation step in which a permeate fraction that is depleted in methane and ethane compared to the membrane separation insert and enriched in the non-hydrocarbons, as well as a retentate fraction that is enriched in methane and ethane compared to the membrane separation insert and in the non-hydrocarbons is depleted, are formed. In this way, valuable products, in particular helium, can be separated from the natural gas.

According to the invention, the membrane separation step is carried out using a glass-like membrane. The provision of the membrane separation insert comprises, as mentioned, a pressure swing adsorption step to which at least part of the natural gas is subjected and in which a high-pressure gas that is enriched in methane and ethane compared to the natural gas and contains at least a part of the non-hydrocarbons from the natural gas, as well as a low-pressure gas , which is depleted in methane and ethane compared to the natural gas and enriched in the higher hydrocarbons, with at least a portion of the high pressure gas being used as the membrane separation insert. Advantages have already been expressly explained above.

In the proposed method, the pressure swing adsorption step is advantageously operated with an inlet pressure of 30 to 40 bar, the high pressure gas can in particular be formed at the same pressure level, the low pressure gas is advantageously formed at a pressure level of 1.3 to 1.5 bar. In purely physical terms, the pressure swing adsorption step can be operated at pressures below 10 bar and above 40 bar. In particular, the high pressure gas is also the membrane separation insert. The membrane separation insert can be compressed if necessary and is advantageously fed to the membrane separation step at a pressure level of 30 to 100 bar. The retentate fraction can in particular be formed at the same pressure level as the membrane separation insert. This results in particular advantages because these pressures represent the optimal operating window for the pressure swing adsorption step and the membrane separation step.

In the context of the present invention, in particular at least part of the low-pressure gas can be compressed to the pressure level of the retentate fraction. The Low-pressure gas, which contains the separated higher hydrocarbons as well as water and carbon dioxide (if contained in the feed gas) in enriched form, can thus be re-injected with the aid of a compressor, with certain components being able to be added to the retentate stream from the membrane separation step. In this way, the original hydrocarbon composition of the natural gas used can be restored. Any losses of hydrocarbons for possible downstream processes can be avoided in this embodiment of the present invention.

According to the present invention, at least part of the low-pressure gas or its compressed part can be cooled to a temperature level of greater than 0 to less than 40 ° C. after compression. In this way, components condensable at these temperatures, such as water and heavy hydrocarbons, can be separated in liquid form.

The present invention enables simple adjustment of the dew point of the natural gas used. The concentration of higher hydrocarbons in the low-pressure gas results in a significantly higher hydrocarbon and water dew point (if they contain water) after compression than with the same pressure in the natural gas used. This means that at a comparatively high temperature level (ideally cooling water temperature at approx. 30 ° C) a considerable part of the hydrocarbons with four or more carbon atoms (and water) contained in the original natural gas flow can be condensed out and at the same time, due to the high temperature level, a Formation of solids (ice, hydrates) is avoided. In the original natural gas flow, condensation would only be possible at significantly lower temperatures. With this conditioning process, other process steps, for example for water removal, can thus be avoided.

In other words, according to a particularly advantageous embodiment, the present invention includes that at least some of the higher hydrocarbons are recovered from the low-pressure gas or from its compressed and cooled part, with at least some of the recovered hydrocarbons being able to be added to the retentate fraction. In one embodiment of the invention, natural gas containing water can be used, the high pressure gas being depleted in water compared to the natural gas and the low pressure gas being enriched in water compared to the natural gas. In this case, the recovery of at least a part of the higher hydrocarbons from the low-pressure gas or from its compressed and cooled part can comprise the formation of an aqueous phase, a liquid organic phase and a gaseous phase (which can for example also have non-organic components such as carbon dioxide).

After the mentioned condensation at moderate temperatures, a three-phase current can be obtained if water is contained in the natural gas. If this is fed to a three-phase separator, the aqueous phase can be separated from the hydrocarbon-containing phase.

In the context of the present invention, at least part of the liquid organic phase and at least part of the gaseous phase can be added to the retentate fraction. The gas phase from the separator can therefore be mixed again with the retentate stream. The resulting mixture typically has a water dew point of -20 ° C or less.

For the exact setting of a hydrocarbon dew point or calorific value, the liquid organic phase or a part thereof can be fed to the retentate fraction upstream of the feed of the gaseous phase. The upstream feed takes place in particular so that the fastest possible phase transition can be guaranteed. The remaining hydrocarbon-containing liquid phase can be used as a product of value. At least part of the liquid organic phase can be fed to a further separation. By means of suitable separation processes (rectification, distillation), the hydrocarbon-rich liquid phase can be conditioned in a targeted manner before it is fed in, in order to achieve maximum added value.

In the process according to the invention, the natural gas can contain any molar proportion of methane, any molar proportion of ethane, and any molar proportion of higher hydrocarbons up to the saturation point, and any molar proportion of non-hydrocarbons. The proportion of the respective components in the High-pressure gas can also be any purely physically. However, in order to generate the greatest possible advantage for the membrane separation step, the molar fraction of the higher hydrocarbons defined above in the high pressure gas upstream of the membrane separation step should be as small as possible, ideally 0 mol percent.

The high pressure gas from the adsorption separation step is fed to a membrane separation step. Glass-like membranes are used in the membrane separation step. Glass-like membranes have polymers which have an amorphous, glass-like state below the glass transition temperature and above which can change, in particular, to a rubber-elastic state. In particular, they are defined in such a way that they are only operated at temperatures below the glass transition temperature of the polymer used.

The invention also relates to a plant for processing a natural gas, the features of which are expressly referred to in the corresponding independent patent claim. Regarding features and advantages of this system and particularly advantageous configurations which enable a corresponding system in particular to carry out a method in a variant explained above, reference is expressly made to the explanations relating to this.

Fundamental advantages of the method according to the invention and the proposed plant include, summarized again, that in comparison to the previously used method for conditioning natural gas for use with glass-like membranes, an exact separation cut can be made with this (i.e. a selective and complete separation of hydrocarbons with three and more carbon atoms is possible). Therefore, the feed gas of the membrane separation step does not contain any components that are detrimental to membrane performance and service life. By means of direct or indirect cooling of the natural gas, the content of hydrocarbons with three or more carbon atoms in the natural gas can also be reduced depending on the set temperature, but there is no exact cut, so that residual contents of the corresponding components remain, which affects the performance and The service life of the vitreous membranes is limited. If one compares the energy expenditure that has to be expended to cool the natural gas in order to achieve a hydrocarbon dew point comparable to that which the one at Has gas depleted from hydrocarbons with three carbon atoms from the process described here, this is significantly higher than the compression energy for the recompression of the low-pressure gas from the pressure swing adsorption step.

In addition to the conditioning of natural gas for use with vitreous membranes, the hydrocarbon dew point, the water dew point and the calorific value of the discharge gas can be set flexibly with the method according to the invention and advantageous embodiments.

The invention is explained in more detail below with reference to the accompanying drawing, which illustrates an embodiment of the invention.

Description of the drawing

A method according to an embodiment of the invention is illustrated in FIG. The explanations apply to a corresponding system in the same way.

In the method 100, a natural gas containing methane, ethane, higher hydrocarbons and non-hydrocarbons is provided in the form of a feed stream A.

At least part of the natural gas of the feed stream A is subjected to a pressure swing adsorption step 10, in which a high-pressure gas B, which is enriched in methane and ethane compared to the natural gas and contains at least a part of the non-hydrocarbons from the natural gas, and a low-pressure gas C, which compared to the natural gas is depleted in methane and ethane and enriched in the higher hydrocarbons.

The high pressure gas B is used, at least in part, as the membrane separation insert of a membrane separation step 20 which operates using a vitreous membrane. In the membrane separation step 20, a permeate fraction D, which is depleted in methane and ethane compared to the membrane separation insert or the high-pressure gas B and enriched in the non-hydrocarbons, and a retentate fraction E, which compared to the membrane separation insert in methane and ethane is enriched and depleted in the non-hydrocarbons. The permeate fraction is used to provide a, for example, helium-containing, hydrogen-containing and / or neon-containing non-hydrocarbon product G, and the retentate fraction E is used to provide a hydrocarbon product F.

In the illustrated embodiment, the low-pressure gas C is compressed to the pressure level of the retentate fraction E using a compressor 1 and then cooled in a heat exchanger 2. In the embodiment illustrated here, in which the natural gas of the feed stream A contains water, a three-phase mixture is formed, which is fed into a three-phase separator 3. In the latter, an aqueous phase H, which can be discarded, is formed, as well as a liquid organic phase I and a gaseous phase K.

The liquid organic phase I can, in particular, be separated in further separation steps (not illustrated) or only divided in terms of quantity, with a portion L being fed to the retentate stream E in order to set its dew point and calorific value. The feed takes place upstream of the feed of the gaseous phase K. A residue M of the liquid organic phase I can be discharged from the process 100 as a product of value.

Claims

Claims 1. A method (100) for processing a natural gas that contains methane, ethane, higher hydrocarbons and non-hydrocarbons, using at least a portion of the natural gas to provide a membrane separation insert which is enriched in methane and ethane and depleted in the higher hydrocarbons compared to the natural gas is and contains at least a portion of the non-hydrocarbons from the natural gas, and wherein the membrane separation insert is subjected to a membrane separation step (20) in which a permeate fraction which is depleted in methane and ethane compared to the membrane separation insert and enriched in the non-hydrocarbons, and a retentate fraction which is enriched in methane and ethane compared to the membrane separation insert and depleted in the non-hydrocarbons, characterized in that the membrane separation step (20) is carried out using a glass-like membrane, that the provision of the membrane separation insert zes comprises a pressure swing adsorption step (10) to which at least part of the natural gas is subjected and in which a high-pressure gas that is enriched in methane and ethane compared to the natural gas and contains at least part of the non-hydrocarbons from the natural gas, and a low-pressure gas that is compared to the Natural gas is depleted in methane and ethane and enriched in the higher hydrocarbons, with at least a portion of the high pressure gas being used as the membrane separation insert. 2. The method (100) according to claim 1, wherein the pressure swing adsorption step (10) is operated with an inlet pressure of 7 to 100 bar, the high-pressure gas being formed at a pressure level of 0 to 100 bar, the low-pressure gas at a pressure level of 1, 3 to 1.5 bar is formed, the membrane separation insert being fed to the membrane separation step (20) at a pressure level of 20 to 100 bar, and the retentate fraction being provided at a pressure level of 0 to 100 bar. 3. The method (100) according to claim 2, wherein at least part of the low-pressure gas is compressed to the pressure level of the retentate fraction. 4. The method (100) according to claim 3, wherein at least a part of the low-pressure gas or of the compressed part thereof is cooled to a temperature level of 0 to 40 ° C. after compression. 5. The method (100) according to claim 3 or 4, wherein at least some of the higher hydrocarbons are recovered from the low-pressure gas or from its compressed and cooled part. 6. The method (100) according to claim 5, wherein at least some of the recovered hydrocarbons are added to the retentate fraction. 7. The method (100) according to claim 5 or 6, wherein the natural gas contains water, the high pressure gas being depleted in water compared to the natural gas and the low pressure gas being enriched in water compared to the natural gas. 8. The method (100) according to claim 7, wherein recovering at least a portion of the higher hydrocarbons from the low pressure gas or from its compressed and cooled portion comprises forming an aqueous phase, a liquid organic phase and a gaseous phase. 9. The method (100) according to claim 8, wherein at least part of the liquid organic phase and at least part of the gaseous phase of the retentate fraction are added. 10. The method (100) according to claim 9, wherein the liquid organic phase or part thereof is fed to the retentate fraction upstream of the feed of the gaseous phase. 11. The method according to claim 10, wherein at least part of the liquid organic phase is fed to a further separation. 12. The method (100) according to any one of the preceding claims, wherein the non-hydrocarbons comprise one or more components naturally occurring in natural gases that are not composed of carbon and hydrogen atoms exist, especially helium, neon, argon, hydrogen, oxygen and carbon dioxide. 13. The method (100) according to any one of the preceding claims, in which one or more vitreous membranes are used in the membrane separation step which comprise a polymer which has a vitreous, amorphous state below a glass transition temperature. 14. Plant for processing a natural gas that contains methane, ethane, higher hydrocarbons and non-hydrocarbons, the plant having means which are set up to use at least a portion of the natural gas to provide a membrane separation insert which is enriched in methane and ethane compared to the natural gas is depleted of the higher hydrocarbons and contains at least some of the non-hydrocarbons from the natural gas, and means which are set up to subject the membrane separation insert to a membrane separation step (20) in which a permeate fraction which is depleted in methane and ethane compared to the membrane separation insert and is enriched in the non-hydrocarbons, and a retentate fraction, which is enriched in methane and ethane compared to the membrane separation insert and depleted in the non-hydrocarbons, is formed, characterized in that the membrane separation step (20) for the use of a glass-like en membrane is set up that means are provided for providing the membrane separation insert, which are set up to carry out a pressure swing adsorption step (10) to which at least a part of the natural gas can be subjected and in which a high-pressure gas that is enriched in methane and ethane compared to the natural gas and contains at least some of the non-hydrocarbons from the natural gas, as well as a low-pressure gas that is depleted in methane and ethane compared to the natural gas and enriched in the higher hydrocarbons can be formed, and that means are provided in the plant which are set up for to use at least a portion of the high pressure gas as the membrane separation insert.