Method and system for obtaining hydrogen from a feed mixture which contains hydrogen and hydrocarbons

Abstract

To obtain hydrogen from a gaseous C2minus feed, it is cooled from a first to a second temperature level at a first pressure level forming one or more condensates. A gaseous remainder is cooled to a third temperature level and subjected to a counterflow absorption at the first pressure level, obtaining a top gas rich in hydrogen and methane and a sump liquid. The former is heated and subjected to pressure swing adsorption at the first pressure level, forming a product stream rich in hydrogen and depleted in or free from methane. The condensate(s) and/or the sump liquid is/are expanded to and fed into a low pressure demethanizer at the second pressure level. The counterflow absorption is carried out using fluid taken from the demethanizer at the second pressure level, compressed in gaseous form to the first pressure level and cooled to the third temperature level.

Claims

1. Method (100, 200) for obtaining hydrogen from a feed mixture containing hydrogen, methane and hydrocarbons with two carbon atoms and low in or free from other hydrocarbons, wherein fluid of the feed mixture is cooled from a first temperature level to a second temperature level at a first pressure level, while one or more condensates are precipitated out of the fluid of the feed mixture, leaving a residual gas, fluid of the residual gas is further cooled to a third temperature level and subjected to a counterflow absorption, thereby obtaining a top gas rich in hydrogen and methane and a sump liquid, fluid of the top gas is heated and subjected to pressure swing adsorption (9) at the first pressure level, to form a product stream which is rich in hydrogen and depleted in or free from methane, and fluid of the condensate or condensates and/or of the sump liquid is expanded from the first pressure level to a second pressure level and is fed into a low pressure demethanizer at the second pressure level, characterised in that the counterflow absorption is carried out at the first pressure level using fluid which has been taken from the low pressure demethanizer at the second pressure level, compressed in gaseous form to the first pressure level and cooled to the third temperature level.

2. Method (100, 200) according to claim 1, wherein the feed mixture contains 55 to 90, particularly 60 to 90, mol % of methane.

3. Method (100, 200) according to claim 2, wherein the fluid taken from the low pressure demethanizer and used in the counterflow absorption predominantly or exclusively contains methane and is used as liquid reflux in the counterflow absorption.

4. Method (100, 200) according to claim 1, wherein the feed mixture contains 30 to 55, particularly 30 to 40, mol % of methane.

5. Method (100, 200) according to claim 4, wherein the fluid taken from the low pressure demethanizer and used in the counterflow absorption contains methane and ethylene and is at least partly freed from the ethylene in the counterflow absorption.

6. Method (100, 200) according to claim 4, wherein an ethane-rich reflux is used in the counterflow absorption, by means of which hydrocarbons with two carbon atoms are washed out both from the fluid of the residual gas and from the fluid taken from the low pressure demethanizer and used in the counterflow absorption.

7. Method (100, 200) according to claim 6, wherein the counterflow absorption is carried out using a two-part separating unit having a first absorption section and a second absorption section, in which the fluid of the residual gas is fed into a lower region of the first absorption section and the fluid taken from the low pressure demethanizer is fed into an upper region of the first absorption section, a gas is transferred from an upper region of the first absorption section into a lower region of the second absorption section and the ethane-rich reflux is fed into an upper region of the second absorption section.

8. Method (100, 200) according to claim 7, wherein the two-part separating unit is configured as a two-part absorption column (5) in which the first absorption section is arranged underneath the second absorption section in a common outer casing.

9. Method (100, 200) according to claim 1, wherein the fluid of the residual gas is enriched in hydrogen and depleted in methane before the pressure swing adsorption (9).

10. Method (100, 200) according to claim 9, wherein the fluid of the residual gas is enriched in hydrogen and depleted in methane by further cooling and precipitation of a liquid containing more methane than hydrogen from the fluid of the residual gas.

11. Method (100, 200) according to claim 1, wherein the first temperature level is at 35 to 57 C. and/or the second temperature level is at 60 to 80 C. and/or the third temperature level is at 95 to 100 C., particularly at 97 to 99 C.

12. Method (100, 200) according to claim 1, wherein the first pressure level is at 20 to 35 bar, particularly at 27 to 29 bar, and/or the second pressure level is at 10 to 25 bar, particularly at 12 to 15 bar.

13. Apparatus for obtaining hydrogen from a feed mixture containing hydrogen, methane and hydrocarbons with two carbon atoms and low in or free from other hydrocarbons, with means that are configured: to cool fluid of the feed mixture from a first temperature level to a second temperature level at a first pressure level, so that one or more condensates are precipitated out of the fluid of the feed mixture, leaving a residual gas, to further cool fluid of the residual gas to a third temperature level and subject it to counterflow absorption, thereby obtaining a top gas rich in hydrogen and methane and a sump liquid, to heat fluid of the top gas and to subject it to pressure swing adsorption (9) at the first pressure level, thereby forming a product stream which is rich in hydrogen and depleted in or free from methane, and to expand fluid of the condensate or condensates and/or of the sump liquid from the first pressure level to a second pressure level and to feed it into a low pressure demethanizer at the second pressure level, characterised by means which are configured to carry out the counterflow absorption at the first pressure level using fluid which is taken from the low pressure demethanizer at the second pressure level, compressed in gaseous form to the first pressure level and cooled to the third temperature level.

14. Apparatus according to claim 13 which, for the purpose of carrying out the counterflow absorption, comprises a two-part separating unit having a first absorption section and a second absorption section, means being provided which are configured to feed the fluid of the residual gas into a lower region of the first absorption section and to feed the fluid taken from the low pressure demethanizer into an upper region of the first absorption section, to transfer gas from an upper region of the first absorption section into a lower region of the second absorption section and to feed an ethane-rich reflux into an upper region of the second absorption section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a method which is not according to the invention, in the form of a schematic process flow diagram.

(2) FIG. 2 shows a method according to one embodiment of the invention, in the form of a schematic process flow diagram.

(3) FIG. 3 shows a method according to one embodiment of the invention, in the form of a schematic process flow diagram.

(4) In the Figures, corresponding elements have been given identical reference numerals and their description has not been repeated, for the sake of clarity.

DETAILED DESCRIPTION OF THE DRAWINGS

(5) FIG. 1 shows a method, not according to the invention, for obtaining hydrogen from a feed mixture containing hydrogen, methane and hydrocarbons with two carbon atoms which is low in or free from other hydrocarbons, in the form of a schematic process flow diagram, generally designated 300.

(6) The feed mixture, which contains hydrogen, methane and hydrocarbons with two carbon atoms and is low in or free from other components, is supplied, in the embodiment shown, in the form of a stream a to the warm side of a heat exchanger 1, which, in contrast to the embodiment shown, may also be in the form of a plurality of heat exchangers, heat exchanger sections or blocks.

(7) The heat exchanger 1 may for example be operated with C2-refrigerants such as ethylene in the form of the streams x, y and z, wherein the stream x for example is provided at a temperature of 57 C., the stream y for example is provided at a temperature of 80 C. and the stream z for example is provided at a temperature of 100 C.

(8) After removal from the heat exchanger 1 at an intermediate temperature level the stream a is fed into a first precipitation container 2. As a result of the cooling in the heat exchanger 1 a liquid condensate is precipitated in the first precipitation container 2, and can be drawn off in the form of a liquid condensate stream b. Uncondensed fluid of the stream a is passed through the heat exchanger 1 again in the form of the stream c, where it is cooled further, and finally fed into a second precipitation container 3. A liquid condensate stream, here designated d, and a gaseous stream, here designated e, are also taken from the second precipitation container 3. The stream e is cooled again in the heat exchanger 1 and then fed into a third precipitation container, here designated 101.

(9) A liquid condensate stream r taken from the third precipitation container 101 is transferred, together with the liquid condensate streams b and d from the first precipitation container 2 and the second precipitation container 3, into a separating unit 110, which comprises a demethanizer section 111 and an absorption section 112. Both the demethanizer section 111 and the absorption section 112 are operated at the pressures that are conventional for a low pressure demethanizer, for example at 12 to 14 bar in the embodiment shown.

(10) The demethanizer section 111 is operated with a sump evaporator 113 in which, for example, ethylene can be used as the heat medium. A stream f which predominantly contains hydrocarbons with two carbon atoms and is low in or free from other components can be drawn off from a sump of the demethanizer section 111. From the top of the demethanizer section a stream s is drawn off, liquefied in another heat exchanger 102 and added as reflux to the absorption section 112.

(11) In addition, a stream t is fed as reflux into the absorption section 112, which has been removed in gaseous form from the third precipitation container 101 and expanded in a turboexpander 103. By using the reflux in the form of the stream s any remaining hydrocarbons with two carbon atoms which have not gone into the liquid phase and hence into the stream r in the third precipitation container 101 can be precipitated from the stream t. In this context, a stream u is also used, which is removed from the absorption section 112 at the top end, also liquefied in the heat exchanger 102 and then fed into an upper region of the separating unit 110.

(12) To enable the streams s and u to be liquefied, considerable amounts of cold are required which can only be provided by the use of a turboexpander 104 which expands a stream v drawn off from the upper region of the separating unit 110. However, as a result of the expansion of the stream v, this stream from which hydrogen is to be obtained is at a pressure which is still significantly above the pressure at which the separating unit 110 is operated.

(13) To obtain pure hydrogen, for which purpose pressure swing adsorption 9 is provided, it is therefore necessary to re-compress the stream v in one or more compressor stages 106. Precooling 107 is carried out beforehand. As mentioned several times, the operation of the compressor stage(s) 106 in particular has proved decidedly energy-intensive and also problematic in terms of the maintenance of the apparatus required.

(14) During pressure swing adsorption 9 a hydrogen-rich product stream l and a so-called tail gas stream m are obtained from the stream v. The tail gas stream m, which advantageously contains essentially all the methane from the streams a and v and smaller amounts of hydrogen, is compressed in a compressor 10 to a suitable pressure, for example to a pressure as required for use in firing, and also discharged.

(15) FIG. 2 shows a method according to one embodiment of the invention in the form of a schematic process flow diagram which is generally designated 100.

(16) In contrast to the method 300 illustrated in FIG. 1, in the method 100 shown in FIG. 2 only two precipitation containers 2 and 3 are used. The stream e drawn off in gaseous form from the top of the second precipitation container 3 is fed into a separating unit designated 5, the operation of which will be described hereinafter.

(17) Like the liquid condensate streams b, d and r in the method 300 shown in FIG. 1, the liquid condensate streams b and d in the method 100 shown in FIG. 2 are also transferred into a corresponding separating unit, here designated 4. In contrast to the separating unit 110 in the method 300 shown in FIG. 1, the separating unit 4 in the method 100 shown in FIG. 2 is a pure distillation column, i.e. functionally an apparatus corresponding to the demethanizer section 111 of the separating unit 110. A sump evaporator of the separating unit 4 (not specifically designated) may be heated, for example with ethylene, like the sump evaporator 113 of the separating unit 110 in the method 300 shown in FIG. 1. Here, too, a corresponding stream f which predominantly or exclusively contains hydrocarbons with two carbon atoms is drawn off from the sump of the separating unit 4.

(18) From the top of the separating unit 4, which is configured as a low pressure demethanizer, a stream designated g is drawn off, compressed in gaseous form in a compressor 6 and then cooled in the heat exchanger 1. As a result of the above-mentioned compression, the fluid of the stream g, which is upstream of the compressor 6 at the pressure of the low pressure demethanizer 4 and would not be capable of liquefaction at this pressure at the temperatures of the heat exchanger 1, reaches a pressure that is sufficient to enable liquefaction. Therefore, in contrast to the prior art, no expansion of large fluid volumes is necessary to be able to provide temperatures below that which the heat exchanger 1 is able to provide. For advantages thereof, reference is made to the explanations above. After cooling in the heat exchanger 1 to a temperature of about 98 C. the stream g is also fed into the separating unit 5.

(19) If, in the method 100 shown in FIG. 2, comparatively small amounts of methane and comparatively large amounts of hydrogen are provided in the stream a, because the stream a originates, for example, from the steam cracking of feedstocks predominantly containing ethane, the stream g contains comparatively large amounts of ethylene, but comparatively little methane. If the stream g were to be fed exclusively into the separating unit 5 there would therefore be a danger of comparatively large losses of ethylene into a top stream i, which is removed from the top end of the separating unit 5.

(20) Therefore, in the method 100 which is illustrated in FIG. 2, a reflux stream h which predominantly or exclusively contains ethane and has been cooled to the third temperature level in the heat exchanger 1 is added to the separating unit 5. A corresponding stream h may be removed, for example, from a C2 splitter to which the stream f is supplied, or the stream h may consist at least partly of ethane which has been supplied externally. If the stream h is not already at a sufficient pressure for use in the separating unit 5, its pressure is increased by means of a pump 7.

(21) As a result of the operation of the separating unit 5 as described, the ethylene of the stream g, which goes at least partially into the gaseous phase in the separating unit 5, can be washed back, thereby minimising ethylene losses into the stream i.

(22) The stream i thus still consists predominantly or exclusively of methane and hydrogen. It is heated in the heat exchanger 1 and subjected to heating to about 20 to 25 C. in a unit 8, without any further pressurisation. The stream i is then fed into the pressure swing adsorption 9, in which the streams l and m described earlier with reference to the method 300 illustrated in FIG. 1 are formed.

(23) The method 100 illustrated in FIG. 2 has proved particularly advantageous, as, in contrast to the re-compression of the stream v, which has to be carried out in the method 300 illustrated in FIG. 1, no such re-compression is required. This can particularly be put down to the fact that there is no need for any additional cooling to liquefy a top stream g from the separating unit 4 (in contrast to the streams s and u in the method 300 illustrated in FIG. 1). The stream i, according to FIG. 2, is already at a pressure that is suitable for use in the pressure swing adsorption 9. The top stream of the separating unit 4 simply has to be compressed in a relatively small amount by means of the (cold) compressor 6. The operation thereof proves to be significantly more favourable as much smaller amounts have to be compressed therein. Moreover, in the embodiment shown, a further absorption medium is provided by the use of the stream h, which again does not require any additional cooling for its preparation in the method 100.

(24) Although FIG. 2 shows a method 100 in which a stream h is used which predominantly or exclusively contains ethane, in certain cases it may also be sufficient to simply feed the streams e and g into the separating unit 5. This is particularly the case when the stream a contains amounts of methane which are sufficient for the backwashing of ethylene in the separating unit 5. This is particularly the case when the stream a originates from the steam cracking of liquid feedstocks or gaseous feedstocks such as propane or combined feedstocks (consisting, for example, of naphtha, ethane and LPG). In this case, the methane content of the stream g is sufficient to enable adequate backwashing of hydrocarbon with two carbon atoms in the separating unit 5 even without the additional use of the stream h.

(25) FIG. 3 shows a variant of the method 100 shown in FIG. 2, which is generally designated 200. This is a method according to another embodiment of the invention.

(26) In contrast to the method 100 shown in FIG. 2, here the stream i is passed through another heat exchanger 11 and cooled therein to a temperature of approx. 152 C., for example. Then the stream i, which has been at least partially liquefied by cooling in the further heat exchanger 11, is transferred into a precipitation container 12, from the sump of which a methane-enriched and hydrogen-depleted stream o is removed and from the top of which a hydrogen-enriched and methane-depleted stream n is removed. The two streams are heated in the heat exchanger 1, the stream n being treated in the same manner as the stream i in method 100, which is shown in FIG. 2. As a result of the already considerable enrichment of the stream n with hydrogen, this stream n can be treated in the pressure swing adsorption 9 with significantly better yields, thus making it easier to produce pure hydrogen in the form of the stream l. After being heated in the heat exchanger 1 the stream o is cooled further in a unit 13 and combined with the stream m downstream of the compressor 10.