EP2953706A1

EP2953706A1 - Gas separation having a membrane separation unit

Info

Publication number: EP2953706A1
Application number: EP14700253.9A
Authority: EP
Inventors: Johannes Szivacz; Johannes WINTERSPERGER
Original assignee: Axiom Angewandte Prozesstechnik Ges mbH
Current assignee: Axiom Angewandte Prozesstechnik Ges mbH
Priority date: 2013-02-05
Filing date: 2014-01-08
Publication date: 2015-12-16
Also published as: EP2762220A1; WO2014121964A1; DK2762220T3; EP2762220B1

Abstract

The invention relates to a method for increasing the percentage of a gas A in a gas mixture exiting as a retentate from a membrane separation unit, the feed gas of the membrane separation unit comprising a fraction of a gas A and a fraction of a gas B and where applicable other gases, characterised in that a separating membrane is inserted in the membrane separation unit, through which the gas B passes faster than the gas A, and the separating membrane is flushed on the permeate side by means of a gas stream that comprises the gas B in a lower percentage than the feed gas or does not comprise gas B at all. The invention further relates to a device for carrying out such a method.

Description

GAS SEPARATION WITH A MEMBRANE SEPARATOR

The present invention relates to a method and a device for increasing the percentage of a gas A in a gaseous mixture emerging as a retentate from a membrane separation unit.

Weak gases are fuel gas mixtures with a reduced calorific value (eg below 8.5 MJ / m ³ ), the combustible gases (usually methane or other gaseous hydrocarbons, such as hydrogen and / or carbon monoxide) are in lean gas with non-combustible components, such as. As nitrogen (from the air), water vapor or carbon dioxide so strong that an independent, stable oxidation (combustion) can be implemented only with great technical and financial effort. Low gases are mostly free of or low in oxygen. For direct combustion, it is therefore necessary to increase the percentage of oxygen in the lean gas until the resulting gas mixture of oxygen and lean gas is combustible.

Weak gases are produced, for example, during the biological-enzymatic, anaerobic decomposition of organic material such as sewage sludge and manure (biogas, biogas) or landfills (landfill gas). Dome gases from the mining industry are also known as lean gas with low calorific value. Similarly, the product gas produced in wood gasification is often referred to in the literature as lean gas (LCV - low calorific value gas). Low gases also occur in mining or disused mines, especially in the extraction of mine gas (when the methane content of the excavated mine gas is, for example, 20 vol .-%). Such a lean gas would theoretically combustible by itself from the methane content, but contains no oxygen. When admixing sufficient oxygen for combustion, for example in the form of air, the methane concentration necessarily falls below 17.5 vol .-%, which must be used for the combustion of this gas special burner. An example of this is the so-called EFLOX burner, with which a combustion of a gas mixture with between 2.5 and 17.5 vol .-% methane without additional fuel is possible

Lately, increasingly weak gases are also available as a waste product (offgas) of biogas upgrading plants, which off-gas may not be discharged largely untreated into the atmosphere. The concentration of the combustible component CH ₄ in the offgas was due to the rapid technological development of the various treatment technologies of originally 5-10 vol .-%, at which concentration of EFLOX burner very well functional, lowered to 0.5 vol .-%, at which concentration the EFLOX burner stops working.

A general disadvantage of lean gas is that it is thermally difficult to recycle due to its low methane content, for the power and energy production Zündstrahlmotoren must be used. Conventional gas engines require for the purpose of power generation a combustible gas mixture, engines of CHP can be operated with a fuel gas containing up to 38 vol .-% CH ₄ economically, micro gas turbines to 30 vol .-%. It is therefore possible the power generation of mine gas or comparable gas types with a minimum CH ₄ concentration of about 30 vol .-%. With CH ₄ concentrations below this limit, power generation or the operation of a gas engine is only limited, with high costs associated with high technical complexity or not at all possible.

On the other hand, the weak gas described above with a methane concentration of, for example, less than 17.5% by volume or even less than 2.5% by volume or less than 0.5% by volume can not be readily vented into the atmosphere, since methane Due to its high impact (25 times as effective as C0 ₂ ) it contributes around 20% to the anthropogenic greenhouse effect, but the time it takes for methane to stay in the atmosphere is significantly shorter at 9 to 15 years than at C0 ₂ . The global mean methane content of the atmosphere has increased from pre-industrial times (1750) from around 600 ppb to 1, 750 ppb in 1999. In the period from 1999 to 2006, the methane content of the atmosphere remained largely constant, but since 2007 has again increased significantly to over 1800 ppb.

In order to minimize the release of methane into the environment, the legislator in Germany, for example, has set up financial support measures for biogas plants, in which however the maximum amount of methane that can be emitted in the exhaust gas is limited to 0.2% by volume of the methane mass produced in the biogas process. This means that, for example, a biogas upgrading plant with a very high recovery of 99.5% by volume (ie 99.5% by volume of the methane entering the biogas upgrading plant is fed back to the product gas for further use, eg fed into the gas grid). at a CH ₄ concentration of 53 vol .-% in the feed (crude gas) and a CH ₄ concentration of 98 vol .-% in the product gas has a CH ₄ concentration of 0.57 vol .-% in the offgas.

CH ₄ 53.00 132.50 0.57 0.66 98.00 131, 84

C0 ₂ 47,00 1 17,50 99,43 1 14,81 2,00 2,69

Total 250.00 1 15.47 134.53

In the example given, the difference between the allowed and the actually expected CH ₄ emission is as follows.

All results rounded up

As you can see in the given example, the permissible emission value is exceeded by twice. In order to reduce the methane concentration listed in the above example to the required value of 0.2 vol .-% of CH ₄ -Inputs, the exhaust gas is usually thermally or catalytically oxidized. For this purpose, as already mentioned up to a methane concentration of up to 2.5% by volume, so-called FLOX burners, ie burners with flameless oxidation, and up to a methane concentration of up to 0.5% by volume are known as RTO burners (Regenerative Thermal Oxidation) or catalytic afterburning (KNV). A disadvantage of the RTO Benner is the deteriorating ratio of volume to the surface, since the heat losses increase as in any heat engine in this ratio and thereby decreases the efficiency, therefore, in order to carry out the afterburning, either biogas, natural gas or Liquid gas can be added as additional fuel or the CH ₄ content in lean gas can be increased again.

It would therefore be desirable to increase the percentage of methane or of a gas A in a gas mixture as a retentate of a membrane separation unit in that the membrane separation unit fed feed gas comprises a gas A, for example methane, and a gas B, for example C0 ₂ , and a depletion of gas B, for example C0 ₂ , is provided via the separating membrane in the retentate, wherein a proportion of oxygen is preferably also provided in the retentate. Furthermore, it would be desirable if the permeate of the membrane separation unit may be discharged as the legal requirements appropriate exhaust gas, if appropriate, directly into the atmosphere.

Object of the present invention is a two gases A and B and optionally also other gases comprising feed gas, such as lean gas, by means of a Separate membrane separation unit such that the permeate of the membrane separation unit may pass as the legal requirements corresponding exhaust gas into the atmosphere and at the same time the retentate readily, for example, without the addition of an additional fuel, a meaningful, for example thermal, recovery can be supplied. In general, there is the problem with lean gases that such gas mixtures for the operation of conventional recycling units, such as gas engines or burners, can not be used directly. A feed gas which can be used according to the invention can contain, for example, 1% by volume of methane and 99% by volume of CO ₂ , but virtually no oxygen. Such a gas mixture can not be easily discharged as exhaust gas into the atmosphere, on the other hand, it is not self-combustible and can at best be thermally exploited by means of support burners or it must be aftertreated by its content of methane due to legal requirements as described above, for example oxidized.

The above object is achieved in that in a method for increasing the percentage of a gas A in a emerging from a membrane separation unit as a retentate gas mixture, wherein the feed gas of the membrane separation unit is a proportion of a gas A and a proportion of a gas B and optionally still further gases, in the membrane separation unit, a separation membrane is used, through which the gas B passes faster than the gas A, and the separation membrane is permeatseitig purged with a gas stream, which comprises the gas B in a lower percentage than the feed gas or at all not included. In a separation process for biogas, in which case the gas A is methane and the gas B is C0 ₂ , can according to the invention by adjusting (eg feed pressure and / or flow) of the corresponding gas streams of feed gas and purge gas, for example in a volume ratio of between 1 to 0 1 to 1 to 2, for example between 1 to 0.15 to 1 to 0.35, preferably in a ratio of between 1 to 0.20 to 1 to 0.30, more preferably in a ratio of about 1 to 0, 25, a permeate are obtained, which corresponds to the legal requirements (less than 0.2% by volume of methane based on the methane input in the separation process) and therefore can be discharged as exhaust gas directly into the atmosphere. However, the method of the present invention can be applied to any separation method as long as a separation membrane is used in the membrane separation unit through which the gas B passes faster than the gas A, and the separation membrane is purged permeate side with a gas stream containing the gas B in a smaller percentage comprises as the feed gas or not at all included. By the gas B in the gas mixture of the feed gas, which gas B passes through the separation membrane faster than the gas A, the volume flow of the retentate (product gas) decreases because the Gas B preferably passes through the membrane into the permeate or offgas. The purge gas for the separation membrane in this case comprises only a small or no partial pressure of gas B Due to the difference of the partial pressures (little B m purge gas, much B in the feed gas) B passes from the feed gas into the permeate, through the higher passage rate of B through the separation membrane As already mentioned, the volume flow of the retentate simultaneously decreases, which also increases the concentration of A in the retentate. Due to the purge gas, the permeated gas mixture is transported away at an accelerated rate on the permeate side, as a result of which the partial pressure difference also increases or is kept constant. For the purge gas may also be provided a compressor or compressor. In the utilization or treatment of mine gas as lean gas (gas A CH ₄ and gas B C0 ₂ ) and the use of air as purge gas, as already described above for biogas, under suitable process conditions, the methane content in the permeate be kept so low that the permeate has less than 0.2% by volume of methane, based on the methane input, and thus complies with the legal requirements, so that it can be discharged into the atmosphere without prior post-oxidation. Also, the permeate C0 _{2 is} transported away by the purging air, the partial pressure difference between the feed gas (up to more than 99% by volume C0 ₂ ) and permeate is thereby further increased and thus C0 _{2 is} increasingly removed from the retentate. Furthermore, oxygen can pass from the purge gas through the membrane into the retentate, whereby the oxygen content of the retentate increases or even an oxygen content is provided in the first place. By suitable process management, a retentate can thus be obtained which, despite theoretically too low methane content, can be burned due to the increased oxygen content. The process according to the invention can also be carried out at a content of gas A (for example methane) in the feed gas of less than 3% by volume, in particular less than 2.0% by volume, more preferably less than 1.5% by volume become.

Preferably, in the process according to the invention, the partial pressure difference of, for example, gas A and gas B between permeate and retentate in the membrane separation unit is increased, preferably by pressurizing the feed gas, for example by connecting a compressor or compressor upstream of the membrane separation unit. When connecting a compressor or compressor to pressurize the feed gas, the partial pressure of the gas components is increased retentate side, on the largely pressureless permeate side of the partial pressure of the gas components remains unchanged, so that increases the partial pressure difference and thus the separation efficiency of the membrane separation unit. When using hollow fiber membranes in the present process, the fibers are acted upon both internally and alternatively externally with feed gas, depending on where the pressure loss is procedurally better (ie smaller). This means that in the process according to the invention, when using hollow-fiber membranes, either weak gas can be conducted in the fibers or the weak gas is passed externally. Preferably, for large quantities of lean gas to be utilized, the pressure upstream of the membrane separation unit is built up; for small quantities, suction is ensured by a compressor or compressor preferably arranged on the permeate side.

With a by the inventive method of methane and oxygen from the scavenging air enriched retentate, which was obtained from lean gas as feed gas, suitable recovery units, such as gas engines or burners, can be operated more cost-effectively, more stable and in a broader range of methane concentration without further ado.

According to a preferred embodiment of the present invention is in the feed gas and purge gas, more preferably in feed gas, purge gas, retentate and permeate the membrane separation unit at least one parameter, eg the partial pressure, the concentration, the pressure, the flow rate or even the chemical nature, at least one gas component A and / or B, eg of methane and / or C0 ₂ , measured, compared with a desired value and on the determined values by means of, for example, a suitable adjusting device each provided in the feed gas, purge gas, retentate and / or permeate stream compressors, and / or control valves controlled.

It is also advantageous if a control valve is provided on the retentate side in an optional supply line to a utilization unit after the membrane separation unit, the control valve also having a sensor for determining at least one parameter, e.g. the partial pressure, the concentration, the pressure, the flow rate or even the chemical nature, at least one gas component can be assigned in the retentate, which control valve can also be actuated by said central control device.

According to a further aspect, the present invention relates to a device for separating a feed gas by means of gas permeation into retentate and permeate with at least one membrane separation unit (1) having a gas inlet line (2), a retentate outlet line (3) and a permeate outlet line (4) in that the membrane separation unit (1) has a flushing gas inlet line (5) on the permeate side.

In such a device is preferably provided that the membrane separation unit (1) in the gas inlet line (2) has provided a compressor or compressor (6). As a result, as already mentioned above, the partial pressure difference of individual gas constituents can be shifted.

It is particularly preferred if, in the device according to the invention, in the gas inlet line (2) and in the flushing gas inlet line (5), particularly preferably in the gas inlet line (2), in the flushing gas inlet line (5), in the retentate outlet line (3) and in the permeate outlet line ( 4), in each case gas sensors are provided which are connected to an adjusting device (9) for controlling or regulating the compressors or compressors (6), (7) and / or (8) or optionally existing control valves in these lines.

According to a further embodiment of the present invention, a control valve (10) is provided in the retentate outlet line (3), which is preferably connected to the adjusting device (9).

The present invention will be explained in more detail with reference to the following figure, without being limited thereto. If percentages are mentioned in the present invention, unless expressly stated otherwise, always meant% by volume.

1 shows a device according to the invention with a membrane separation unit 1 with a gas inlet line 2 for feeding the feed gas together with a compressor 6 arranged therein, a retentate outlet line 3 with a control valve 10 preferably arranged therein, a permeate outlet line 4 together with a compressor 7 arranged therein and a flushing gas inlet line 5 together In the gas inlet line 2, in the purge gas inlet line 5, in the Permeatausgangsleitung 4 and in the retentate output line 3 respectively gas sensors are provided (shown in phantom), which with the adjusting device 9 for power control or regulation of the compressor or compressors 6, 7th and 8 or for opening and closing the control valve 10 are connected.

Such a device was in an example with a feed gas comprising 1, 0% CH ₄ and 99% C0 ₂ and a pressurization of 140 millibars in a total amount of 0.995 m ³ . The purge gas was air, about 79% N ₂ and about 21% O ₂ in a total of 0.387m ³ . The product gas (retentate) had a composition of 24.8% O ₂ and 1 1, 6% CH ₄ , balance N ₂ and C0 ₂ , in a total amount of about 0.06 m ³ that offgas (permeate) was with 0.24% CH ₄ and 5% O ₂ , balance N ₂ and C0 ₂ , determined in a total amount of 1, 32 m ³ . From the results it is apparent that it is possible by the method according to the invention or with the device according to the invention to increase the oxygen content in product gas to a value which is above the oxygen content in the purging gas in the case of virtually oxygen-free feed gas. At the same time the methane content is increased so that the product gas can be burned without support flame.

Feed purge gas

Nm ³ / h Nm ³ / h

%%

Total 0.9949 0.3869

CH ₄ 01, 00 0.0099 0 0.0000

C0 ₂ 98.56 0,9806 00,03 0,0001

N ₂ 0 0.0000 78.00 0.3056

o ₂ 0 0.0000 21, 00 0.0812

Control 99.56 0.9906 99.03 0.3869

Product offgas

Nm ³ / h Nm ³ / h

%%

Total 0,0584 1, 3239

CH ₄ 1 1, 65 0.0068 00.24 0.0032

C0 ₂ 00.03 0.0000 74.44 0.9855

N ₂ 63.53 0.0371 20.28 0.2685

o ₂ 24.79 0.0145 05.04 0.0667

Control 100,00 0,0584 100,00 1, 3239

Claims

Patent claims

1 . A method for increasing the percentage of a gas A in a gas mixture emerging as a retentate from a membrane separation unit, wherein the feed gas of the membrane separation unit comprises a proportion of a gas A and a proportion of a gas B and optionally further gases, characterized in that Membrane separation unit is a separation membrane is used, through which the gas B passes faster than the gas A, and the separation membrane permeatseitig is purged with a gas stream comprising the gas B in a lower percentage than the feed gas or not at all.

2. The method according to claim 1, characterized in that the partial pressure difference between permeate and retentate in the membrane separation unit is increased by pressurizing the feed gas.

3. The method according to claim 1 or 2, characterized in that the retentate is utilized in one of the membrane separation unit retentate side downstream unit.

4. The method according to any one of claims 1 to 3, characterized in that in the feed gas and purge gas, more preferably in feed gas, purge gas, retentate and permeate the membrane separation unit at least one parameter at least one gas component A and / or B measured compared with a target value and via the determined values in each case in the feed gas, purge gas, retentate and / or permeate stream provided compressors, compressors and / or control valves are controlled.

5. A device for separating a feed gas by means of gas permeation into retentate and permeate with at least one membrane separation unit (1) with a gas inlet line (2), a Retentatausgangsleitung (3) and a Permeatausgangsleitung (4), characterized in that the membrane separation unit (1) permeatseitig a Flush gas inlet line (5).

6. Apparatus according to claim 5, characterized in that in the gas inlet line (2) a compressor or compressor (6) is provided.

7. Device according to one of claims 5 or 6, characterized in that in the gas inlet line (2), in the retentate outlet line (3), in the Permeate output line (4) and / or in the purge gas inlet line (5) each have a compressor or compressor (7) and / or (8) or optionally control valves is or are provided.

8. Device according to one of claims 5 to 7, characterized in that in the gas inlet line (2) and in the purge gas inlet line (5), particularly preferably in the gas inlet line (2), in the purge gas inlet line (5), in the retentate outlet line (3 ) and in the Permeatausgangsleitung (4), each gas sensors are provided which with an adjusting device (9) for controlling or regulating the compressor or compressors (6), (7) and / or (8) or optionally existing control valves in these Lines connected.

9. Apparatus according to claim 8, characterized in that in the Retentatausgangsleitung (3), a control valve (10) is provided, which is preferably connected to the adjusting device (9).

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