Method for separating a hydrocarbon mixture containing hydrogen, separating device, and olefin plant

Abstract

A method for separating a hydrogen-containing hydrocarbon mixture (C2minus), which in addition to the hydrogen essentially contains hydrocarbons with two carbon atoms and methane, using a distillation column (10). Fluid (a, c, e) of the hydrocarbon mixture (C2minus) is cooled stepwise at a first pressure level, during which time first condensates (b, d) are separated from the fluid (a, c, e). Fluid (e) from the hydrocarbon mixture (C2minus) which remains gaseous after this is fed at the first pressure level into a C2 absorber (7), to which a liquid reflux (r) is added at the top, while a second condensate (f) is drawn off from the sump of the C2 absorber (7) and a gaseous top stream (g) containing predominantly methane and hydrogen is drawn off at the top of the C2 absorber (7). Fluid of the above-mentioned gaseous top stream (g) from the top of the C2 absorber (7) is cooled to a third temperature level and transferred at the first pressure level into a hydrogen separator (8) in which a methane-rich third condensate (i) is separated from the fluid of the gaseous top stream (g), leaving behind a gaseous, hydrogen-rich stream (h). Fluid of the first condensates (b, d) and fluid of the second condensate (f) is depressurized from the first pressure level to a second pressure level below the first pressure level and fed into the distillation column (10) which is operated at the second pressure level. Fluid (e) of the third condensate (i) which is separated in the hydrogen separator (8) from the fluid of the gaseous top stream (g) from the top of the C2 absorber is used as the reflux (r) added at the top of the C2 absorber (7) and transferred from the hydrogen separator to the C2 absorber solely by gravity. The invention also relates to a corresponding separating unit and a corresponding olefin apparatus.

Claims

1. Method for separating a hydrogen-containing hydrocarbon mixture (C2minus), which in addition to the hydrogen essentially contains hydrocarbons with two carbon atoms and methane, using a distillation column (10), wherein the hydrocarbon mixture (C2minus) or a part of the hydrocarbon mixture (C2minus) is cooled stepwise, at a first pressure level, from a first temperature level, via two or more intermediate temperature levels, to a second temperature level, first condensates (b, d) being separated from the hydrocarbon mixture (C2minus) or said part of the hydrocarbon mixture (C2minus) at each of the intermediate temperature levels, a part of the hydrocarbon mixture (C2minus) or said part of the hydrocarbon mixture (C2minus) which remains gaseous at the second temperature level and which is not obtained in the form of the first condensates (b, d) is fed at the first pressure level into a C2 absorber (7), to which a liquid reflux (r) is added at a top, wherein a second condensate (f) is drawn off from a sump of the C2 absorber (7) and a gaseous top stream (g) comprising predominantly methane and hydrogen is drawn off at the top of the C2 absorber (7), the gaseous top stream (g) from the top of the C2 absorber (7) or a part of the gaseous top stream (g) is cooled to a third temperature level and transferred at the first pressure level into a hydrogen separator (8) in which a methane-rich third condensate (i) is separated from the gaseous top stream (g) or said part of the gaseous top stream (g), leaving behind a gaseous, hydrogen-rich stream (h), and the first condensates (b, d) or a part of the first condensates (b, d) and the second condensate (f) or a part of the second condensate (f) is depressurized from the first pressure level to a second pressure level below the first pressure level and fed into the distillation column (10) which is operated at the second pressure level, wherein in the distillation column (10) at least a liquid stream (o) essentially consisting of hydrocarbons with two carbon atoms, and a liquid stream (m) essentially consisting of methane are obtained and drawn off from the distillation column (10), characterised in that the reflux (r) added at the top of the C2 absorber is formed from fluid of the methane-rich third condensate (i) which is separated in the hydrogen separator (8) from the gaseous top stream (g) from the top of the C2 absorber (7) and transferred from the hydrogen separator (8) into the C2 absorber (7) solely by the effect of gravity, wherein the C2 absorber is an absorption column not comprising a sump evaporator or an absorption column comprising a sump evaporator which is not operated, and the C2 absorber comprises installments provided for a stepwise or a constant phase contact.

2. The method according to claim 1, wherein a quantity of the fluid of the third condensate (i) used as reflux (r) is selected so as to be approximately equivalent to a quantity of the liquid stream (m), consisting essentially of methane, which is drawn off from the distillation column (10).

3. The method according to claim 1, wherein fluid from the liquid stream (m) which is drawn off from the distillation column (10) is used at least to cool the fluid (a, c, e) of the hydrocarbon mixture (C2minus) from the first temperature level, via the intermediate temperature levels, to the second temperature level.

4. The method according to claim 1, wherein fluid from the gaseous, hydrogen-rich stream (h) from the hydrogen separator (8) is used to cool the fluid (a, c, e) of the hydrocarbon mixture (C2minus) from the first temperature level, via the intermediate temperature levels, to the second temperature level, and to cool the fluid of the gaseous top stream (g) from the top of the C2 absorber (7) to the third temperature level.

5. The method according to claim 1, wherein the hydrocarbon mixture (C2minus) is obtained from a cracked gas obtained by means of a steam cracking process (50).

6. Separating unit (100) which is designed to separate a hydrogen-containing hydrocarbon mixture (C2minus), which in addition to hydrogen essentially contains hydrocarbons with two carbon atoms and methane, and comprises at least one distillation column (10), a C2 absorber (7) and a hydrogen separator (8), as well as a steam cracking process designed to cool the hydrocarbon mixture (C2minus) or a part of the hydrocarbon mixture (C2minus), at a first pressure level, stepwise from a first temperature level, via two or more intermediate temperature levels, to a second temperature level and to separate first condensates (b, d) from the hydrocarbon mixture (C2minus) or said part of the hydrocarbon mixture (C2minus) at each of the intermediate temperature levels, to feed, at the first pressure level, a part of the hydrocarbon mixture (C2minus) or said part of the hydrocarbon mixture (C2minus) which remains gaseous at the second temperature level and which is not obtained in the form of the first condensates (b, d) into the C2 absorber (7), to add a liquid reflux (r) to the C2 absorber at a top, and to draw off a second condensate (f) from a sump of the C2 absorber (7) and a gaseous top stream (g), predominantly containing methane and hydrogen, at the top of the C2 absorber (7), to cool the gaseous top stream (g) from the top of the C2 absorber (7) or a part of the gaseous top stream (g) to a third temperature level and transfer the gaseous top stream (g) or said part of the gaseous top stream (g), at the first pressure level, into the hydrogen separator (8) and therein to separate off a methane-rich third condensate (i) from the gaseous top stream (g) or said part of the gaseous top stream (g), leaving behind a gaseous, hydrogen-rich stream (h), and to depressurize the first condensates (b, d) or a part of the first condensates (b, d) and the second condensate (f) or a part of the second condensate (f) from the first pressure level to a second pressure level below the first pressure level and to fed it into the distillation column (10), to operate the distillation column (10) at the second pressure level, and to obtain in, and to draw off from, the distillation column (10) at least a liquid stream (o) essentially consisting of hydrocarbons with two carbon atoms, and a liquid stream (m) essentially consisting of methane, characterised in that fluid communication is provided which is designed to form the reflux (r) added at the top of the C2 absorber from fluid (e) of the methane-rich third condensate (i) which is separated in the hydrogen separator (8) from the gaseous top stream (g) from the top of the C2 absorber (7) and is transferred from the hydrogen separator (8) into the C2 absorber (7) solely by the effect of gravity wherein the C2 absorber is an absorption column not comprising a sump evaporator or an absorption column comprising a sump evaporator which is not operated, and the C2 absorber comprises installments provided for a stepwise or a constant phase contact.

7. The separating unit (100) according to claim 6, which is designed for carrying out the method according to claim 1.

8. Olefin apparatus which is designed to carry out a steam cracking process (50) using at least one cracking furnace (51-53), wherein the steam cracking process (50) is designed to recover a hydrocarbon mixture (C2minus) consisting essentially of hydrocarbons with two carbon atoms as well as methane and hydrogen, from fluid of a cracked gas (C) of the at least one steam cracking process (50), characterised by at least one separating unit (100) according to claim 6, which is designed to separate the hydrocarbon mixture (C2minus).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a method for producing hydrocarbons in the form of a schematic flow diagram.

(2) FIG. 2 shows a separating unit for separating a hydrocarbon mixture according to the prior art.

(3) FIG. 3 shows a separating unit for separating a hydrocarbon mixture according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(4) In the Figures, corresponding elements have been given identical reference numerals and are not explained repeatedly, in the interests of clarity.

(5) FIG. 1 shows the course of a method of producing hydrocarbons according to the prior art in the form of a schematic flow diagram. A steam cracking process S is provided which can be carried out using one or more cracking furnaces S1 to S3. Only the operation of the cracking furnace S3 is described hereinafter; the other cracking furnaces S1 and S2 may operate in a corresponding manner or may be omitted.

(6) The cracking furnace S3 is charged with a stream A as the furnace feed, and this may be at least partially a so-called fresh feed which is provided from sources outside the apparatus, and at past partially a so-called recycle stream which is obtained in the method itself, as explained below. The other cracking furnaces S1 and S2 may also be charged with corresponding streams. Different streams may also be fed into different cracking furnaces S1 to S3, one stream may be divided between several cracking furnaces S1 to S3 or several partial streams may be combined to form one combined stream which is fed for example as stream A into one of the cracking furnaces S1 to S3.

(7) As a result of steam cracking in the steam cracking process S a raw gas stream B is obtained which has occasionally is already at this point referred to as a cracked gas stream. The raw gas stream B is treated in a series of treatment stages (not shown) of a treatment process 20, subjected to a so-called oil quench, for example, pre-fractionated, compressed, cooled further and dried.

(8) The correspondingly treated stream B, the actual cracked gas C, is then subjected to a separation process 30. In this process a number of fractions are obtained which, as explained hereinbefore, are named according to the carbon number of the hydrocarbons that they predominantly contain. The separation process 30 shown in FIG. 1 operates according to the principle of Deethanizer First.

(9) The skilled man will be familiar with numerous other process variants, for example from the article Ethylene in Ullmann's Encyclopedia of Industrial Chemistry mentioned hereinbefore, which differ particularly in the preparation of the cracked gas C and/or the separation process used. It is expressly pointed out that the invention can also be used in Demethanizer First processes, for example.

(10) In the separation process 30 a C2minus fraction, which may predominantly contain methane, ethane, ethylene and acetylene and, in particular, still hydrogen, is first separated in gaseous form from the cracked gas C in a separating unit 31. The C2minus fraction as a whole is subjected to a hydrotreatment process 41, to convert the acetylene present into ethylene. Then methane CH4 and hydrogen H2 are separated one after the other or together from the C2minus fraction in a C2minus separating unit 32 and used as a fuel gas, for example. The present invention relates particularly to the separating unit 32 which is illustrated in partial schematic view in the following Figures as well.

(11) A C2 fraction remains which is separated in a C2 separating unit 32 into ethylene C2H4 and ethane C2H6. The latter may also be subjected again to the steam cracking process S as a recycle stream D in one or more cracking furnaces S1 to S3. In the embodiment shown the recycle streams D and E are added to the stream A. The recycle streams D and E and the stream A may also be conveyed into different cracking furnaces S1 to S3.

(12) In the separating unit 31 a liquid C3plus fraction remains which is transferred into a separating unit 33 (the so-called depropanizer). In the separating unit 33 a C3 fraction is separated from the C3plus fraction and subjected to a hydrotreatment process 42, in order to react methylacetylene contained in the C3 fraction to form propylene. Then the C3 fraction is separated in a C3 separating unit 34 into propene C3H6 and propane C3H8. The latter may also be subjected again to the steam cracking process S as a recycle stream E in one or more cracking furnaces S1 to S3, separately or with other streams.

(13) In the separating unit 33 a liquid C4plus fraction remains, which is transferred into a fourth separating unit 34 (the so-called debutanizer). In the separating unit 34 a C4 fraction is separated off in gaseous form from the C4plus fraction. A liquid C5plus fraction remains.

(14) It will be understood that all the fractions described can also be subjected to suitable after-treatment steps. For example, 1,3-butadiene may be separated from the C4 fraction. Also, additional recycle streams may be used which may be subjected to the steam cracking process S analogously to the recycle streams D and E.

(15) FIG. 2 shows a separating unit for separating a hydrocarbon mixture according to the prior art. The separating unit is generally designated 200 and is designed to separate a hydrocarbon mixture which consists essentially of hydrocarbons with two carbon atoms, methane and hydrogen (i.e. a C2minus fraction). The C2minus fraction is fed into the separating unit 200 in the form of a stream a.

(16) The separating unit 200 comprises a first heat exchanger 1, a second heat exchanger 2, a third heat exchanger 3 and a fourth heat exchanger 4. The stream a is first passed through the first heat exchanger 1 and cooled therein. Then it is fed into a first liquid separator 5. The cooling in the first heat exchanger 1 is carried out so that a liquid condensate is separated off in the first liquid separator 5. This condensate is drawn off at the bottom of the first liquid separator 5 as stream b. The further use of the stream b is described hereinafter.

(17) A fraction of the stream a remaining in gaseous form in the first liquid separator 5 is passed through the second heat exchanger 2 as stream c and then fed into a second liquid separator 6. Here, too, a liquid condensate is separated off at the bottom and is drawn off in the form of the stream d. A fraction of the stream c still remaining in gaseous form is cooled as stream e in the third heat exchanger 3 and fed into a C2 absorber 7. A liquid condensate is also separated off in the sump of the C2 absorber 7 and is drawn off as stream f. In addition, a stream m, the origin of which will be described hereinafter, is added at the top of the C2 absorber 7. A top gas drawn off from the top of the C2 absorber 7 is passed through the fourth heat exchanger 4 in the form of the stream g and then fed into a hydrogen separator 8.

(18) At the bottom of the hydrogen separator 8 a methane-rich condensate is separated off, which is drawn off as the stream i and passed through the fourth to first heat exchangers 4 to 1 in the reverse order and direction. A stream h of a hydrogen-rich top gas from the top of the hydrogen separator 8 is also passed through the fourth to first heat exchangers 4 to 1 in this order and direction.

(19) The drawing is particularly simplified in that it does not show cross-connections between the pipes in which specific streams, for example the streams i and h, are carried. For example, a cross-connection of this kind may make it possible to mix a particular amount of the stream h with the stream i. Nor does the drawing show pipes which are used essentially to start up a corresponding apparatus. For example, in a separating unit 200, it may be envisaged not to add the stream m at the top of the C2 absorber 7 during start-up but to pass it through the fourth to first heat exchangers 4 to 1 in the same way as the stream i.

(20) The separating unit 200 further comprises a distillation column 10 which is operated with a sump evaporator 11 (not described in detail), the heat exchanger of which is operated for example with a propylene stream coming from other parts of the apparatus. The distillation column 10 further comprises a top condenser 12, the operation of which is described hereinafter.

(21) As a result of the successive cooling of the streams a, c and e, the condensates obtained accordingly, which are obtained in the form of the streams b, d and f, contain different amounts of hydrocarbons with two carbon atoms and methane. In particular, the stream f has a higher methane content than the stream d and the stream d has a higher methane content than the stream b.

(22) The streams b, d and f are therefore fed into the distillation column 10 at different heights; the distillation column 10 comprises feed devices suitable for this purpose between the plates, which are shown here in highly diagrammatic form.

(23) From the top of the distillation column 10 a gaseous stream k is drawn off and liquefied in a condensation chamber of the top condenser 12. The liquefied stream is separated into a liquid phase and a gaseous phase in a region 13 at the top of the distillation column 10. The gaseous phase goes into the gas space of the distillation column 10 and combines with more top gas at the top of the distillation column 10. A gaseous stream l can be drawn off from the region 13 and combined with the stream i mentioned previously, downstream of the fourth heat exchanger 4. The stream l predominantly contains methane.

(24) A corresponding liquid methane-rich stream m can also be drawn off from the region 13. The stream m is supplied to the warm end of the fourth heat exchanger 4 by means of a pump 9 (so-called cold pump) and cooled in the fourth heat exchanger 4. Then, as mentioned previously, it is added at the top of the C2 absorber 7. The top condenser 12 of the distillation column 10 can be charged with a stream n from other parts of the apparatus, as refrigerant. This may be an ethylene stream, for example.

(25) In the sump of the distillation column 10, a liquid condensate is separated off, which consists essentially of hydrocarbons with two carbon atoms (and is therefore a so-called C2 fraction). The condensate is drawn off in the form of the stream o, heated in the first heat exchanger 1 and then fed into another separating unit, for example. As mentioned at the beginning, pumps are problematic to operate for cryogenic media such as liquid methane and require greater attention during operation. In the separating unit 200 shown in FIG. 2, this applies particularly to the methane pump 9.

(26) In the separating unit 200 which is shown in FIG. 2, the quantity of the stream m fed in at the top of the C2 absorber is regulated on the basis of a fill level of a corresponding condensate in the region 13 of the distillation column 10. A pressure of the evaporated refrigerant (stream n) in the top condenser 12 is regulated on the basis of a temperature measured in the distillation column 10. The amount of refrigerant fed in the form of the stream n is regulated on the basis of a fill level in the top condenser 12.

(27) FIG. 3 shows a separating unit 100 according to one embodiment of the invention. The separating unit 100 comprises the essential components of the separating unit 200 shown in FIG. 2. These will not be described again.

(28) However, in contrast to the separating unit 100 shown in FIG. 2, the pump 9 is omitted here. Moreover, the stream n is not added at the top of the C2 absorber 7 but combined with the stream i from the sump of the methane separator 8 to form a stream p. The stream p, instead of only the stream i as before, is passed through the fourth to first heat exchangers 4 to 1 in the order described previously.

(29) However, before it is combined with the stream m, a partial stream r is branched off from the stream i, in particular, and added at the top of the C2 absorber 7 instead of the stream m (cf. separating unit 200 of FIG. 2). If, as schematically shown in FIG. 3, at least the feed point for the stream l into the C2 absorber 7 is arranged below a removal point for the stream i from the hydrogen separator 8, condensate occurring in the hydrogen separator 8 can be passed as a reflux in a restricted flow through a corresponding valve (not given a reference numeral) onto the C2 absorber by the effect of gravity. The quantity of the condensate from the hydrogen separator 8 added to the top of the C2 absorber in the form of the stream r advantageously corresponds to the quantity of methane drawn off from the distillation column 10 in the form of the stream m and is replaced by it in order to generate cold in the fourth heat exchanger 4. As only a low pressure level is needed for correspondingly supplying the fourth heat exchanger 4, the methane pump 9 can be omitted.