Method for operating gas storage devices, and device for carrying out the method

Abstract

The invention relates to a method for operating a gas storage device, wherein existing natural gas storage facilities or natural gas storage devices are used to store hydrogen as a stored gas; in the event of a mixture of hydrogen and natural gas, the removed stored gas is separated in a gas separation system; the gas separation process is carried out in a pressure swing adsorption system (E02) having a connected membrane system (E03); the pressure swing adsorption system (E02) is supplied with the stored gas; the membrane system (E03) is then subjected to the desorption pressure originating from the pressure swing adsorption system (E02); the permeate gas is conducted from the membrane system (E03) back to the pressure swing adsorption system (E02) after the pressure has been increased by means of a compressor (E04); and the permeate pressure and the transmembrane flow, and thus the gas composition and the residual hydrogen content in the retentate of the membrane system (E03), are set by means of one or more valves (V03a, V03b, V03c). The invention further relates to a device for carrying out the method.

Description

This is particularly the case when hydrogen is used as storage gas in an existing natural gas reservoir or an existing storage facility in order to use the reservoir or the existing natural gas storage facility as a hydrogen storage facility. Where the term “storage gas” is used below, this denotes, in the broadest sense, a gas which contains, in addition to hydrogen, other organic or fossil gases, such as natural gas or other residues from storage facilities. The term “storage gas” does not mean that this gas arises or is processed in the immediate vicinity of, or in a direct relationship with, a gas storage facility. At https: / / www.geostockgroup.com / en / four-ways-to-store-large-quantities-of-hydrogen / , the view is expressed that hydrogen would not mix with a gas present in the reservoir. “Estimation of Diffusion Losses of Hydrogen During the Creation of its Effective Storage in an Aquifer”, Daniil Pavlovich Anikeev et al,, SPE-206614-MS, 2021, in particular page 11, first paragraph, likewise expresses the view that hydrogen does not mix with natural gas or, owing to its lower density, even separates from it and accumulates at the highest point of the reservoir. Contrary to assumptions in the art, tests conducted by the Applicant have shown that, surprisingly, the introduced hydrogen, which constitutes the storage gas, does indeed mix with the residual natural gas. As a result of this mixing, the storage gas also penetrates more easily into less permeable regions of the reservoir. This results in an increasing discharge of natural gas. However, this also means that the mixed gases have to be separated again. It must additionally be taken into account that, for reasons of time and / or cost, the cushion gas, namely the gas remaining in the storage facility as a pressure reservoir, may, at the beginning of H2 storage operation, consist, for example, of one half hydrogen and one half natural gas. The production of a gas mixture is therefore to be expected over a longer period. The effects mentioned above occur both during the conversion of a natural gas reservoir into a hydrogen storage facility and during the conversion of a natural gas storage facility into a hydrogen storage facility, as well as over an extended operating period of a hydrogen storage facility itself. Contrary to the common view that hydrogen and natural gas separate in the reservoir or that the two gases do not mix in the first place, field tests conducted by the Applicant have shown that the hydrogen mixes with the natural gas both dispersively and diffusively. During operation of a hydrogen storage facility in a natural gas reservoir, as well as during the conversion of a reservoir into a storage facility or of a natural gas storage facility into a hydrogen storage facility, non-steady conditions with respect to the gas composition are to be expected over an extended period. It is also known from storage operation that the pressure conditions and volumetric-flow conditions may fluctuate greatly depending on the filling level and the injection and withdrawal rates demanded by the market. The present method is intended substantially for the operating phase of a hydrogen storage facility in a natural gas reservoir. However, it is not excluded that the method according to the invention may already be used during a phase in which a natural gas storage facility is being converted into a hydrogen storage facility and natural gas cushion gas is to be replaced with hydrogen cushion gas. This does not mean that a unit according to the invention has to be operated in the immediate spatial vicinity of a storage facility. The unit may also receive gas via pipelines from one or more storage facilities, including remotely located storage facilities. In any event, a unit which treats the gas mixture has to be highly flexible with respect to input parameters such as pressure and gas composition. Methods and units for separating gases are, of course, already known. However, known units are designed for steady processes, for example in refineries, in which the volumetric flow, pressure conditions and gas composition are not subject to large fluctuations. If such known units were adapted to the circumstances described above, they would have to be designed disadvantageously large in order to cope with all production conditions. Moreover, the controllability and achievability of the desired separation effect for all production conditions of such prior-art gas separation systems are not provided. In order to provide an economically expedient configuration with high technical efficiency, a pressure swing adsorption unit is combined according to the invention with membrane separation, wherein the pressure in the unit is influenced, thereby allowing the separation effect to be achieved even when the input parameters fluctuate greatly. The invention therefore relates in particular to a method for operating a gas storage facility, wherein existing natural gas reservoirs or natural gas storage facilities are used for storing hydrogen as storage gas, wherein the withdrawn production gas is separated in a gas separation unit when a mixture of hydrogen and natural gas is present, wherein the gas separation is carried out in a pressure swing adsorption unit having a connected membrane unit, wherein the pressure swing adsorption unit is supplied with gas at the storage gas pressure and the membrane unit is subsequently subjected to the desorbate pressure originating from the pressure swing adsorption unit, wherein the permeate gas is conveyed from the membrane unit back to the pressure swing adsorption unit after a pressure increase by means of a compressor (E04), wherein the permeate pressure and the transmembrane flow, and thus the gas composition and the residual hydrogen content in the retentate of the membrane unit, are adjusted using one or more regulating valves. In a further development, it is provided that, when the storage pressure is low, a pressure increase by means of a booster is provided for the membrane unit in order to increase the pressure differences for operating the pressure swing adsorption unit and the membrane unit. In a further development, it is provided that the permeate pressure is adjusted using regulating valves, namely a permeate valve in the connection between the membrane unit and the compressor, a bleed valve in the connection between the pressure side and the suction side of the compressor, and one or more regulating valves in the compressor which influence the quantity of gas drawn in. In a further development, it is provided that the pressure swing adsorption unit (E02) is supplied directly with gas at the storage gas pressure or at a preset pressure, wherein a pressure reduction unit is arranged upstream of the pressure swing adsorption unit in order to adjust the storage gas pressure to a permissible pressure of the unit design. In a further development, it is provided that various devices for preconditioning the storage gas are arranged upstream of the pressure swing adsorption unit, which include, inter alia, drying units, particle filtration units, condensate removal units, pressure reduction units, preheating units and hydrocarbon removal units. In a further development, it is provided that the gas treatment units are operated in the immediate vicinity of the gas storage facility or are supplied with hydrogen from the gas storage facility via a pipeline or a gas network. In a further development, it is provided that, in addition, the retentate pressure or the quantity of retentate gas is adjusted using a further regulating valve in the retentate gas line. This measure is taken in particular when the production conditions exhibit a large variance with respect to the withdrawn gas composition, when the storage pressure range exhibits large amplitudes, or when the control possibilities using the regulating valves have been exhausted. In a further development, it is provided that the absolute permeate gas pressure directly downstream of the membrane unit is adjusted to from 0.5 bar to 11 bar, in particular to between 1 bar and 6 bar. In a further development, it is provided that the pressure of the permeate is influenced using the regulating valves and that the partial pressures of hydrogen and the hydrocarbons in the membrane unit are thus also influenced, wherein the partial pressures affect the transmembrane flow of hydrogen and the transmembrane flow of the hydrocarbons. In a further development, it is provided that, when an excessively high hydrogen content is detected in the retentate, the permeate valve is opened and / or the bleed valve is closed and / or the regulating valve in the compressor is adjusted in order to bring about an increased transmembrane flow, wherein, because hydrogen permeates faster than hydrocarbons, the increase in the transmembrane flow is proportionally greater for hydrogen than for methane or hydrocarbons, wherein the proportion of hydrogen in the retentate is reduced because proportionally more hydrogen than methane permeates from the retentate side to the permeate side, and wherein, in the event of an excessively low hydrogen content in the retentate, the control is carried out in the opposite direction. In a further development, it is provided that the partial pressures are influenced by closing the second regulating valve, whereby a change in the transmembrane flow is brought about such that more hydrogen enters the permeate. In a further development, it is provided that, in the case of particularly low desorbate quantities, the functionality of the membrane unit is maintained by backing up the permeate, and the methane is not recycled via the permeate but instead enters the retentate. In a further development, it is provided that the permeate is admixed to the storage gas via a gas mixer as a constituent of the feed gas to the pressure swing adsorption unit. In a further development, it is provided that the permeate gas is recompressed via a compressor stage to the current pressure of the storage gas so that it can be admixed to the storage gas in the gas mixer. In a further development, it is provided that the compressor stage is controlled via the suction pressure on the suction side, wherein a lower suction pressure is brought about by closing the permeate valve and / or closing the bleed valve and / or adjusting the regulating valves in the compressor, in particular in the case of small permeate quantities and low proportions of hydrocarbons in the storage gas. In a further development, it is provided that, when the storage gas pressure decreases, the compressor output is adapted and reduced to the lower required feed pressure of the permeate into the gas mixer, and that, when the proportion of hydrocarbons in the gas withdrawn from storage increases and the desorbate quantity correspondingly increases, the pressure on the permeate side is lowered by opening the permeate valve and / or closing the bleed valve and / or adjusting the regulating valves in the compressor, such that the membrane unit can process larger gas quantities. In a further development, it is provided that an increase in the pressure on the suction side of the compressor stage (E04) is brought about by opening the permeate valve, whereby the compressor stage can compress larger permeate gas quantities. In a further development, it is provided that an increase in the permeate gas quantity and a reduction in the permeate gas pressure are achieved by closing the bleed valve (V03b). In a further development, it is provided that an increase in the pressure on the suction side of the compressor stage (E04) is brought about by adjusting the regulating valve in the compressor (V03c), whereby the compressor stage can compress larger permeate gas quantities. A further aspect of the invention relates to a device for separating natural gas and hydrogen when operating a gas storage facility, in particular for carrying out the method described above, wherein existing natural gas reservoirs or natural gas storage facilities are used for storing hydrogen as storage gas, wherein the withdrawn storage gas is separated in a gas separation unit when a mixture of hydrogen and natural gas is present, characterised in that a pressure swing adsorption unit having a connected membrane unit is provided for separating the gases, wherein a gas-carrying connection for the permeate gas from the membrane unit to the pressure swing adsorption unit is provided, wherein regulating valves for controlling the permeate pressure and the gas composition of the retentate are arranged in a fifth gasconducting connection. In a further development, it is provided that a second regulating valve is provided in the fifth gas-carrying connection for the retentate gas from the membrane unit. In a further development, it is provided that a gas mixer is arranged in a first gas-conducting connection for the storage gas, wherein the gas-carrying connection for the permeate gas opens into the gas mixer, and the gas mixer is configured to mix the storage gas and the permeate gas to form the feed gas for feeding into the pressure swing adsorption unit. In a further development, it is provided that devices for gas conditioning (E06) are arranged in the first gas-carrying connection which connects the gas storage facility and the gas mixer. In a further development, it is provided that a compressor stage for compressing the permeate gas is arranged in the sixth gas-conducting connection for recirculating the permeate gas. The invention is explained by way of example with reference to a drawing, in which: Figure 1 shows, in a highly schematic manner, a block diagram of a device according to the invention together with the method sequence; Figure 2 shows the natural gas content in the gas as a function of the storage pressure. The device is shown schematically in Figure 1. The device according to the invention comprises one or more gas storage facilities E01 which are filled with a gas mixture consisting substantially of hydrogen and gaseous hydrocarbons (natural gas). The storage gas is supplied through a first gas-conducting connection S01 to a gas mixer E05 and passes from there, as feed gas, through a second gas-conducting connection S02 to a pressure swing adsorption unit E02. The gas-conducting connection S01 may also be a pipeline which connects one or more remotely located gas storage facilities to the device according to the invention, or, more generally, a hydrogen network. A gas treatment stage E06 of any type may be provided upstream of the gas mixer E05, in which stage the gas is preconditioned. Preconditioning is understood below to include, inter alia, the known methods of removing high or higher hydrocarbons, gas drying, condensate removal, particle filtration and gas pressure reduction. The pressure swing adsorption unit E02 comprises one or more adsorption stages and / or adsorbers, control valves and customary process accessories. Through a fourth gas-conducting connection S04, gas passes from the low-pressure side of the pressure swing adsorption unit E02 to a membrane unit E03, the gas comprising the desorbate gas of the pressure swing adsorption unit E02 with a mixture of hydrogen and hydrocarbons. A so-called booster E07, which increases the feed pressure for the membrane unit E03, may be arranged in the fourth gas-conducting connection S04. In addition, a buffer vessel may form part of the booster, or a separate buffer vessel, which is not shown, may be provided in order to equalize the pressure for the membrane unit. Through a third gas-conducting connection S03 from the high-pressure side of the pressure swing adsorption unit E02, the product gas containing the purified hydrogen is conveyed to the hydrogen network, to another hydrogen consumer or to a hydrogen storage facility. The membrane unit E03 comprises, in particular, one or more membrane stages and / or membrane modules, fittings and accessories. A fifth gas-conducting connection S05 leads from a high-pressure side of the membrane unit E03 to a natural gas network or to another hydrocarbon consumer or hydrocarbon storage facility and carries the retentate gas of the membrane unit containing the enriched hydrocarbons. A sixth gas-conducting connection S06 leads from a low-pressure side of the membrane unit E03 to the gas mixer E05. The permeate gas of the membrane unit E03, containing a mixture of hydrogen and hydrocarbons, is thereby supplied to the gas mixer E05 and mixed with the storage gas from the gas-conducting line S01 to form the feed gas for the pressure swing adsorption unit E02. The permeate gas from the membrane unit E03 is conveyed through a compressor stage E04 which consists of one or more compressors comprising one or more compression stages together with the corresponding control valves and accessories. Furthermore, the device according to the invention may comprise one, several or all of the devices from the following group: • a first regulating valve V01 positioned in the connection carrying product gas, • a second regulating valve V02 in the connection carrying the retentate gas, • one or more or all of the third regulating valves V03a, V03b and V03c in the connection carrying the permeate gas, • a first measuring device X001 for measuring the gas composition in the feed gas or storage gas, • a second measuring device X002 for measuring the gas composition in the product gas, • a third measuring device X003 for measuring the gas composition in the retentate gas, • a first pressure measuring device P004 for measuring the pressure in the retentate gas or desorbate gas, • one or more measuring devices F005 for measuring the quantity of the product gas stream or storage gas stream. The method according to the invention provides for initially withdrawing from a storage facility E01 a storage gas comprising a hydrogen-natural gas mixture. This storage gas is conveyed to a gas mixer E05 and is mixed there with the permeate gas from the membrane unit E03. These two gases form the feed gas which is conveyed to the pressure swing adsorption unit E02. In the pressure swing adsorption unit E02, the feed gas is separated into the hydrogen-enriched product gas and the hydrogen-depleted desorbate gas. The product gas is fed through a line S03 into a corresponding hydrogen gas network or is stored again. Measuring devices F005 for measuring the quantity of the product gas, measuring devices X002 for measuring the gas composition and at least one valve V01 may be provided in the line S03. The desorbate gas is supplied through the line S04 to the membrane unit E03 and is separated there into the hydrocarbon-enriched retentate gas and the permeate gas consisting of hydrogen and hydrocarbon. The permeate gas is conveyed through the line S06 to the gas mixer E05, wherein the pressure of the permeate gas is increased by means of the compressor stage E04 when necessary. This is necessary in particular in the case of comparatively low permeate gas quantities. According to the invention, the pressure swing adsorption unit may be supplied directly at the storage pressure or storage gas pressure if this pressure is within a range corresponding to the pressure range of the unit design. If the storage gas pressure is too high, that is, if it exceeds the maximum pressure of the unit design, a pressure reduction stage may be arranged upstream of the pressure swing adsorption unit. The membrane unit is subjected to the desorbate pressure originating from the pressure swing adsorption unit. According to the invention, equipment-related and energy-related costs for increasing the pressure are thereby avoided. According to the invention, stable and controllable operation of the device in combination with a gas storage facility is achieved by detecting the product gas quantity using the measuring device F005 and, on the basis of this measured value, adjusting the withdrawn quantity of purified hydrogen to the specified value by adapting the valve position of the valve V01. Furthermore, according to the invention, the gas composition in the feed gas or storage gas is measured using the measuring device X001. Alternatively or additionally, the gas composition in the product gas is measured by means of the measuring device X002. The valve switching times of the pressure swing adsorption unit E02 are controlled with the aid of these measured values. This means, for example, that when increasing methane contents are measured in the feed gas, the adsorbers have to be switched over earlier or the cycle times have to be shortened because the adsorbers become loaded more quickly. As is known, pressure swing absorption is operated in cycles, in which a change takes place whenever the absorber is loaded in order to release the absorbed gas. The lower the methane content in the gas is (particularly at the beginning of withdrawal), the longer the cycles can be and can be adjusted accordingly. In addition, according to the invention, the gas composition in the retentate gas is detected using the measuring device X003. With the aid of this measured value, the desired gas composition in the retentate gas is adjusted by controlling the valve positions of the regulating valves V03a, V03b and V03c. According to the invention, the pressure in the retentate gas and / or desorbate gas may furthermore be measured using the pressure measuring device P004. With the aid of this measured value, compliance with a pressure range predefined in the process design may be ensured by means of the valve position of the valve V02. The provision of a combination of pressure swing adsorption with a membrane unit and one, several or all of the control operations outlined above produces a surprising technical effect in that, with low equipment expenditure, such a unit operates efficiently even under the permanently changing conditions of storage operation. In order to circulate only a small quantity of hydrogen and repeatedly supply it to the pressure swing absorber, it is expedient to measure the methane content of the feed gas and to adapt the cycle time as a function thereof. During a typical withdrawal process from an underground storage facility filled with hydrogen and natural gas, the pressure gradually decreases along a ramp from a maximum storage pressure to the minimum permissible storage pressure. The gas composition in the withdrawn gas is subject to various fluctuations which depend on a series of factors, such as the withdrawal rate, the type of storage facility, geological conditions and previous operating processes. According to the experience of the inventors, the withdrawal process usually begins with a high hydrogen content which then continuously decreases to a lower value. Such an exemplary progression is shown in Figure 2. The withdrawal ramp begins at 65 bar and a natural gas proportion of approximately 4%. During withdrawal and the associated pressure reduction, the natural gas proportion initially increases relatively slowly. As withdrawal continues, the natural gas proportion increases at an ever greater rate. At the end of withdrawal, at approximately 35 bar, the natural gas proportion in the withdrawn gas is already greater than 12%. However, linear increases are also known. In the energy-saving direct supply according to the invention of the pressure swing adsorption unit at the storage pressure, and also when a pressure reduction stage is arranged upstream, the difficulty in applying the method lies in the fact that the driving force, namely the pressure, available for operating the pressure swing adsorption unit and the membrane unit decreases, while the demand for this driving force increases as a result of the increasing natural gas proportion. Despite this disadvantageous relationship, the unit according to the invention can be operated over a broad range of production conditions using the disclosed method when the control operations according to the invention are applied. The normal storage operating gauge pressure available for operating the unit is within a range of between 10 and 320 bar, in particular between 10 and 150 bar, and frequently also between 20 and 80 bar. The hydrogen content to be achieved in the product gas is greater than 95%, in particular greater than 98% and preferably greater than 99.999% when fuel-cell applications are intended. The hydrogen purity is adjusted during the design phase of the pressure swing adsorption process and, during the operating phase of the unit, by adjusting the valve switching times of the pressure swing adsorption unit E02. The control valve V01 serves to control the quantity and depressurizes the product gas toward the downstream hydrogen consumer, the operating pressure of which is normally below the lowest value of the storage pressure range. The absolute pressure of the desorbate gas is normally within the range of between 2 bar and 30 bar, but usually between 4 bar and 11 bar. The desorbate pressure range has a substantial influence on the separation performance of the pressure swing unit and the membrane unit and is selected during the design phase of the method. During operation of the unit, the position of the valve V02 controls the desorbate pressure or retentate pressure. The valve depressurizes the natural-gas-enriched retentate gas toward a natural gas consumer, the operating pressure of which is normally below the lowest value of the desorbate or retentate pressure range. The adjusted residual hydrogen content in the retentate gas is normally less than 10%, but usually less than 2%. The specified hydrogen proportion or natural gas proportion in the retentate gas is adjusted by the position of the valve V03a through influencing the permeate pressure directly downstream of the membrane unit E03. Furthermore, instead of the valve V03a, particularly when a compressor stage E04 is provided, or in addition thereto, a valve V03b or V03c may be provided. The valve V03b is a so-called bleed valve which is connected in parallel with the compressor stage E04 and can return gas from a region downstream of the compressor stage E04, in relation to the permeate stream, to a position upstream of the compressor stage E04. It thus connects the suction side to the pressure side of the compressor. Alternatively or additionally, the compressor stage E04 may integrally comprise the valve V03c as a regulating valve, wherein gas from the high-pressure side of the compressor itself is returned, within the compressor stage, to a position upstream of the compressor through the valve V03c. The quantity drawn in by the compressor is thereby influenced. If the production conditions relating to the withdrawn gas composition or the storage pressure range differ very widely, the position of the valve V02 may, where appropriate, also be used within a limited range to adjust the retentate gas composition. Otherwise, adjustment to a constant pressure with the aid of the position of the valve V02 in the retentate gas or desorbate gas is sufficient. The absolute permeate gas pressure directly downstream of the membrane unit E03 is normally between 0.5 bar and 11 bar, but usually between 1 bar and 6 bar. According to the invention, gas separation is thus intended to be possible under changing pressure and gas composition conditions, which, on the one hand, produces the highest possible purity of the gas streams with the highest possible yield. By applying the pressure adjustment values and pressure adjustment devices specified above, the device and the method may be used for stable operation in order to achieve constant gas compositions in the product gas and retentate gas, even when the storage gas pressure and storage gas composition are subject to fluctuations having a large amplitude. This is additionally achieved with the lowest possible energy expenditure. The surprising effect is attributable to a particular combination resulting from the nature of the two separation methods and the selection of optimized control operations. During the course of a withdrawal process, the natural gas proportion in the storage gas increases and directly causes an increase in the natural gas proportion in the desorbate gas and an increase in the desorbate gas quantity. For the membrane unit which treats the desorbate gas, these two factors compensate for one another to a certain extent. On the one hand, the membrane has to process an increased desorbate gas quantity. However, the hydrogen proportion to be separated is lower, which facilitates the separation task for the membrane unit to a certain extent. In any event, the pressure conditions of the membrane unit have to be adjusted to the fluctuations. This is achieved primarily by adjusting the regulating valves V03a, V03b, V03c, which adjust the permeate pressure and, consequently, the transmembrane flow and the gas composition in the retentate. The regulating valves V03a, V03b, V03c influence the pressure of the permeate and thus also the partial pressures of hydrogen and the hydrocarbons or methane in the membrane unit. The partial pressures affect the transmembrane flow of hydrogen and the transmembrane flow of the hydrocarbons. If, for example, a desorbate having a particular gas composition and gas quantity is introduced into the membrane unit and an excessively high hydrogen content is detected in the retentate using the third measuring device X03, opening the third regulating valve V03a produces an increased transmembrane flow. Since hydrogen permeates at least one order of magnitude faster than methane or hydrocarbons, the increase in the transmembrane flow is proportionally greater for hydrogen than for methane or hydrocarbons. Consequently, the proportion of hydrogen in the retentate is reduced because proportionally more hydrogen than methane permeates from the feed or retentate side to the permeate side. The same control effect may be achieved by closing the bleed valve V03b and / or by adjusting the regulating valve V03c in the compressor such that the quantity of gas drawn in through S06 is increased. If the hydrogen content in the retentate is too low, the control operation and method described above may be carried out in the opposite direction. The separation performance may additionally be changed using the second regulating valve V02, which adjusts the desorbate pressure of the pressure swing adsorption unit. It should therefore be used only when the control possibilities provided by the valves V03a, V03b, V03c have been exhausted. A pressure increase or closing of the second regulating valve V02 on the retentate side has an effect analogous to opening the third regulating valve V03a or supports the effect thereof. The partial pressures are also influenced in this case. Closing the second regulating valve V02 also produces a change in the transmembrane flow and acts on the hydrogen flow, such that an increased quantity of hydrogen enters the permeate. A further purpose of the valves V03a, V03b, V03c is to maintain the functionality of the membrane E03 by backing up the permeate when the desorbate quantities are particularly low, particularly at the beginning of withdrawal, so that the methane is not recycled through the permeate but instead enters the retentate. The permeate contains hydrogen and methane and is admixed to the storage gas through the gas mixer E05 as a constituent of the feed gas. Because the pressure decreases across the two separation stages from the desorbate to the permeate, the permeate has to be recompressed by means of the compressor stage E04 to the current pressure of the storage gas so that it can be admixed to the storage gas in the gas mixer E05. Customary compressors cannot be controlled over the entire range of occurring differences in gas quantity. According to the invention, the compressor stage is therefore controlled by means of the suction pressure on the suction side if speed control is insufficient for the desired control range or is not permissible at all. A lower suction pressure may be produced by closing the permeate valve V03 and / or closing the bleed valve V03b and / or adjusting the regulating valve V03c, whereby the compressor stage draws in smaller gas quantities. This is necessary particularly in the case of small permeate quantities resulting from low proportions of hydrocarbons in the storage gas. The procedure has two advantages. On the one hand, only the small permeate proportion has to be recompressed. On the other hand, the storage gas pressure changes, namely decreases, such that the compressor output can be adapted to the lower feed pressure required for feeding the permeate into the gas mixer. Since compressors in such units are responsible for a considerable proportion of the energy consumption, the unit can thereby be operated in an energy-efficient manner. Conversely, if the proportion of hydrocarbons in the withdrawn gas increases, the desorbate quantity increases correspondingly. By opening the permeate valve V03a and / or closing the bleed valve V03b and / or adjusting the regulating valve V03c, the pressure on the permeate side decreases and the membrane unit can process larger gas quantities. Opening the third regulating valve V03a, namely the permeate valve, simultaneously produces an increase in the pressure on the suction side of the compressor stage E04, whereby the compressor stage can compress larger permeate gas quantities, as was to be achieved by the disclosed control method. The same effect of increasing the permeate gas quantity may be achieved by closing the bleed valve V03b and / or correspondingly adjusting the regulating valve V03c in the compressor. The suitable control ranges for the valves V03a, V03b, V03c with respect to controlling the permeate quantity and the interaction between the membrane unit E03 and the compressor stage E04 are selected predominantly during the design phase. During the operating phase, by contrast, the positions of the regulating valves V03a, V03b, V03c are used predominantly to control the gas composition in the retentate. Correct selection of the design parameters of all process units and control devices results in correct adjustment of the pressure values in the feed gas and permeate gas of the membrane unit E03 and appropriate operation of the compressor stage E04 at varying permeate quantities. Overall, however, intelligent control as described above is then required in order to adjust the pressure differences correctly. Furthermore, it is surprising that such a procedure using a pressure swing absorber and a membrane stage is particularly well suited to hydrogen storage in porous storage facilities because any hydrogen sulfide which may be produced already enters the desorbate during pressure swing adsorption and passes from there to the separated CH4, where it causes comparatively few problems. The hydrogen sulfide in the discharged hydrogen is conveyed to the pressure swing adsorption process and is dried again or enriched in the circuit and, if necessary, chemically removed, for example using activated carbon or metal oxides. This serves to prevent accumulation in the circuit. List of Reference Signs E01   gas storage facility E02   pressure swing adsorption unit E03   membrane unit E04   compressor stage E05   gas mixer E06   gas preconditioning E07   booster S01 first gas-conducting connection S02 second gas-conducting connection S03 third gas-conducting connection S04 fourth gas-conducting connection S05 fifth gas-conducting connection S06   sixth gas-conducting connection V01   first regulating valve V02 second regulating valve V03a third regulating valve (permeate valve) V03b bleed valve V03c regulating valve in the compressor X001 first measuring device X002 second measuring device X003 third measuring device P004 fourth measuring device F005 quantity measuring device

Claims

1. Method for operating a gas storage facility, wherein existing natural gas reservoirs or natural gas storage facilities are used for storing hydrogen as storage gas, wherein the withdrawn storage gas is separated in a gas separation unit when a mixture of hydrogen and natural gas is present,characterized in thatthe gas separation is carried out in a pressure swing adsorption unit (E02) having a connected membrane unit (E03), wherein the pressure swing adsorption unit (E02) is supplied with the storage gas and subsequently the membrane unit (E03) is subjected to the desorbate pressure originating from the pressure swing adsorption unit (E02), wherein the permeate gas from the membrane unit (E03) is conveyed, after a pressure increase by means of a compressor (E04), back to the pressure swing adsorption unit (E02), wherein the permeate pressure and the transmembrane flow and thus the gas composition and the residual hydrogen content in the retentate of the membrane unit (E03) are adjusted using one or more valves (V03a, V03b, V03c).

2. Method according to claim 1, characterized in that, at a low storage pressure of the membrane unit (E03), a pressure increase by means of a booster (E07) is provided in order to increase the pressure differences for operating the pressure swing adsorption unit and the membrane unit.

3. Method according to claim 1 or 2, characterized in that the permeate pressure is adjustable using the regulating valves permeate valve (V03a) in the connection between the membrane unit (E03) and the compressor (E04), and / or bleed valve (V03b) in the connection between the pressure side and the suction side of the compressor (E04) and / or one or more regulating valves (V03c) in the compressor (E04) which influence the quantity of gas drawn in.

4. Method according to one of the preceding claims, characterized in that devices (E06) for preconditioning the storage gas are arranged upstream of the pressure swing adsorption unit, which are one, several or all of drying units, particle filtration units, condensate removal units, pressure reduction units, preheating units, hydrocarbon removal units.

5. Method according to one of the preceding claims, characterized in that the pressure swing adsorption unit (E02) is supplied directly with the storage gas pressure or with a reduced storage gas pressure, wherein a pressure reduction unit is arranged upstream of the pressure swing adsorption unit (E02) in order to adjust the storage gas pressure to a maximum pressure of the unit design.

6. Method according to one of the preceding claims, characterized in that, in addition, the retentate pressure or the retentate gas quantity is adjusted using a second regulating valve (V02) in the retentate gas line (S05).

7. Method according to one of the preceding claims, characterized in that, in addition, the retentate pressure or the retentate gas quantity is adjusted using a further regulating valve (V02) in the retentate gas line. This measure is taken in particular when the production conditions exhibit a large variance with respect to the gas composition discharged from storage or the storage pressure range exhibits large amplitudes or the control possibilities using the regulating valves V03a, V03b and / or V03c are exhausted.

8. Method according to one of the preceding claims, characterized in that the absolute permeate gas pressure directly downstream of the membrane unit (E03) is adjusted to 0.5 bar to 11 bar, in particular between 1 bar and 6 bar.

9. Method according to one of the preceding claims, characterized in that, using the regulating valves V03a, V03b, V03c, the pressure of the permeate is influenced and thus also the partial pressures of hydrogen and the hydrocarbons in the membrane unit are influenced, wherein the partial pressures affect the transmembrane flow of hydrogen and the transmembrane flow of the hydrocarbons.

10. Method according to one of the preceding claims, characterized in that, when an excessively high hydrogen content is detected in the retentate, the permeate valve (V03a) is opened and / or the bleed valve (V03b) is closed and / or the regulating valve in the compressor (V03c) is adjusted in order to bring about an increased transmembrane flow, wherein, since hydrogen permeates faster than hydrocarbons, the increase in the transmembrane flow becomes proportionally greater for hydrogen than for methane or hydrocarbons, wherein the proportion of hydrogen in the retentate is reduced, since proportionally more hydrogen permeates from the retentate side tothe permeate side than methane, wherein, in the event of an excessively low hydrogen content in the retentate, the control is carried out in the opposite direction.

11. Method according to one of the preceding claims, characterized in that, by closing the second regulating valve (V02), intervention is made in the partial pressures, wherein a change in the transmembrane flow is brought about, so that hydrogen increasingly enters the permeate.

12. Method according to one of the preceding claims, characterized in that, in the case of particularly low desorbate quantities, the functionality of the membrane unit (E03) is maintained by backing up the permeate and the methane is not recycled via the permeate but enters the retentate.

13. Method according to one of the preceding claims, characterized in that the permeate is admixed to the storage gas via a gas mixer (E05) as a constituent of the feed gas to the pressure swing adsorption unit (E02).

14. Method according to one of the preceding claims, characterized in that the permeate gas is recompressed via a compressor stage (E04) to the current pressure of the storage gas so that it can be admixed to the storage gas in the gas mixer (E05).

15. Method according to one of the preceding claims, characterized in that the compressor stage (E04) is controlled via the suction pressure on the suction side, wherein a lower suction pressure by closing the permeate valve (V03a) and / or by closing the bleed valve (V03b) and / or by adjusting the regulating valves in the compressor (V03c) is, in particular in the case of small permeate quantities, in the case of low proportions of hydrocarbons in the storage gas.

16. Method according to one of the preceding claims, characterized in that, when the storage gas pressure decreases, the compressor output is adapted and reduced to the lower required feed pressure of the permeate into the gas mixer and, when the proportion of hydrocarbons in the gas discharged from storage increases and the desorbate quantity correspondingly increases, the pressure on the permeate side is lowered by opening the permeate valve (V03a) and / or by closing the bleed valve (V03b) and / or by adjusting the regulating valves in the compressor (V03c), so that the membrane unit (E03) can process larger gas quantities.

17. Method according to one of the preceding claims, characterized in that an increase in the permeate gas quantity and a reduction in the permeate gas pressure are achieved by closing the bleed valve (V03b).

18. Method according to one of the preceding claims, characterized in that an increase in the pressure on the suction side of the compressor stage (E04) is brought about by adjusting the regulating valve in the compressor (V03c), whereby it can compress larger permeate gas quantities.

19. Method according to one of the preceding claims, characterized in that an increase in the pressure on the suction side of the compressor stage (E04) is brought about by opening the permeate valve (V03a), whereby it can compress larger permeate gas quantities.

20. Device for separating natural gas and hydrogen when operating a gas storage facility, in particular for carrying out the method according to one of the preceding claims, wherein existing natural gas reservoirs or natural gas storage facilities are used for storing hydrogen as storage gas, wherein the withdrawn storage gas is separated in a gas separation unit when a mixture of hydrogen and natural gas is present, characterized in that a pressure swing adsorption unit (E02) having a connected membrane unit (E03) is present for separating the gases, wherein a sixth gas-carrying connection (S06) for the permeate gas from the membrane unit (E03) to the pressure swing adsorption unit (E02) is present, wherein control valves (V03a, V03b, V03c) for controlling the permeate pressure and the gas composition of the retentate are arranged in the fifth gas-conducting gas connection (S05).

21. Device according to claim 20, characterized in that a second regulating valve (V02) is present in the fifth gas-carrying connection (S05) for the retentate gas from the membrane unit (E03).

22. Device according to claim 20 or 21, characterized in that a gas mixer (E05) is arranged in a gas-conducting connection (S01) for the storage gas, wherein the gas-carrying connection for the permeate gas (S06) opens into the gas mixer (E05) and the gas mixer (E05) is configured for mixing the storage gas and the permeate gas to form the feed gas for feeding into the pressure swing adsorption unit (E02).5       23. Device according to one of claims 20 to 22, characterized in that devices for gasconditioning (E06) are arranged in the first gas-carrying connection which connects the gas storage facility and the gas mixer.

24. Device according to one of claims 20 to 23, characterized in that a compressor stage (E04) for compressing the permeate gas is arranged in the gas-conducting connection10          (S06) for recirculating the permeate gas to the pressure swing adsorption unit.

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