Method for separating a hydrocarbon mixture, separating plant and steam cracking plant

Abstract

A method for separating a hydrocarbon mixture, which is obtained at least in part by steam cracking and which contains at least hydrocarbons having one, two and three carbon atoms, including ethane and ethylene, a first fraction initially being obtained from the hydrocarbon mixture by separating off other components at least in part, said fraction containing the predominant part of the hydrocarbons having two or more carbon atoms previously contained in the hydrocarbon mixture or the predominant part of the hydrocarbons having two or fewer carbon atoms previously contained in the hydrocarbon mixture, further fractions subsequently being obtained from the first fraction. A fraction containing ethane is separated off in an amount which reduces the ethane content in the first fraction to less than 25%, the fraction containing ethane being low in or free from other hydrocarbons having two carbon atoms.

Claims

1. A method for separating a hydrocarbon mixture comprising: steam cracking a feed to produce a hydrocarbon mixture comprising at least hydrocarbons having one, two and three carbon atoms, including ethane and ethylene; separating the hydrocarbon mixture in a demethanizer to produce a first fraction comprising at least 50% of the hydrocarbons having two or more carbon atoms; separating the first fraction in a separation unit downstream of the demethanizer to produce a fraction R comprising ethane and a C2 fraction, wherein the fraction R comprising ethane comprises less than 10% of other hydrocarbons having two carbon atoms, wherein the C2 fraction comprises at least 50% ethane and ethylene but less than 25% ethane; separating the C2 fraction in a C2 separation unit to produce an ethane fraction and an ethylene fraction; and separating at least part of the fraction R comprising ethane in a deethanizer.

2. The method according to claim 1, further comprising: specifying a maximum acceptable ethane content which can be tolerated in a separation system for the method of separating the hydrocarbon mixture; and adapting the amount of ethane in the fraction R comprising ethane in accordance therewith.

3. The method according to claim 1, wherein the C2 fraction comprises less than 20% ethane.

4. The method according to claim 1, wherein the fraction R comprising ethane comprises less than 1.5% ethylene.

5. The method according to claim 4, wherein at least a portion of the fraction R comprising ethane is passed to the steam cracking.

6. The method according to claim 1, further comprising: hydrogenating the hydrocarbon mixture or the first fraction.

7. A method for separating a hydrocarbon mixture comprising: steam cracking a feed to produce a hydrocarbon mixture comprising at least hydrocarbons having one, two and three carbon atoms, including ethane and ethylene; separating the hydrocarbon mixture in a deethanizer to produce a first fraction comprising at least 50% of the hydrocarbons having two or less carbon atoms; separating a fraction S comprising ethane, wherein the fraction S comprising ethane is separated in parallel with or downstream of the deethanizer, wherein the fraction S comprising ethane comprises less than 10% of other hydrocarbons having two carbon atoms, wherein separation of the fraction S comprising ethane in parallel with the deethanizer comprises separating from at least a portion of the hydrocarbon mixture separated from the deethanizer the fraction S comprising ethane and a remaining fraction comprising less than 25% ethane, and combining the remaining fraction with the first fraction to produce a C2? fraction, and wherein separation of the fraction S comprising ethane downstream of the deethanizer comprises separating the fraction S comprising ethane from the first fraction and producing a C2? fraction comprising less than 25% ethane; separating the C2? fraction in a demethanizer to produce a C2 fraction comprising at least 50% ethane and ethylene; and separating the C2 fraction in a C2 separation unit to produce an ethane fraction and an ethylene fraction.

8. The method according to claim 7, further comprising: specifying a maximum acceptable ethane content which can be tolerated in a separation system for the method of separating the hydrocarbon mixture; and adapting the amount of ethane in the fraction S comprising ethane in accordance therewith.

9. The method according to claim 7, wherein the first fraction after the fraction S comprising ethane is separated comprises less than 20% ethane.

10. The method according to claim 7, wherein the fraction S comprising ethane comprises less than 1.5% ethylene.

11. The method according to claim 10, wherein the fraction S comprising ethane is passed to the steam cracking.

12. The method according to claim 7, further comprising: hydrogenating the hydrocarbon mixture.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A schematically shows the sequence of a method for producing hydrocarbon products in accordance with the prior art.

(2) FIG. 1B schematically shows the sequence of a method for producing hydrocarbon products in accordance with the prior art.

(3) FIG. 2 schematically shows the sequence of a method for separating a hydrocarbon mixture in accordance with an embodiment of the invention.

(4) FIG. 3 schematically shows the sequence of a method for separating a hydrocarbon mixture in accordance with an embodiment of the invention.

(5) FIG. 4 schematically shows the sequence of a method for separating a hydrocarbon mixture in accordance with an embodiment of the invention.

(6) FIG. 5 schematically shows the sequence of a method for separating a hydrocarbon mixture in accordance with an embodiment of the invention.

(7) Corresponding elements are provided with identical reference signs in the drawings, and for brevity are not described more than once.

DETAILED DESCRIPTION OF THE DRAWINGS

(8) FIG. 1A shows in the form of a schematic stream chart the sequence of a method for producing hydrocarbon products by steam cracking and subsequent separation of an obtained cracked gas into fractions in accordance with the prior art.

(9) The central part of the method is a steam cracking process 10, which can be carried out using one or more cracking furnaces 11 to 13. Only the operation of the cracking furnace 11 is described in the following; the further cracking furnaces 12 and 13 may operate in a corresponding manner.

(10) The cracking furnace 11 is loaded with a stream A as a furnace charge, which may in part be a fresh charge provided from sources outside the system, and in part be a recycled stream obtained within the method itself. The other cracking furnaces 12 and 13 may also be charged with corresponding streams. Different streams may also be fed into different furnaces 11 to 13, one stream may be divided between a plurality of cracking furnaces, or a plurality of substreams may be combined into a combined stream which is for example supplied to one of the cracking furnaces 11 to 13 as a stream A.

(11) As a result of the steam cracking in the steam cracking process 10, a crude gas stream B is obtained, sometimes already referred to at this stage as a cracked gas stream. The crude gas stream B is processed in a series of processing steps (not shown) of a processing process 20, for example subjected to oil quenching, prefractionated, compressed, cooled further and dried.

(12) The correspondingly treated stream B, the actual cracked gas C, and thus the hydrocarbon mixture separated in the context of the present invention, are subsequently subjected to a separation process 30. The separation process 30 is implemented in a corresponding separation system. A number of fractions are obtained, and are denoted in accordance with the carbon number or the predominantly contained hydrocarbons, as described above. The separation process 30 shown in FIG. 1A operates by the demethaniser-first principle; a further separation process by the deethaniser-first principle is shown in FIG. 1B.

(13) In the separation process 30, a C1 or C1minus fraction (denoted by reference sign C1) is initially separated off in gaseous form from the cracked gas C, in a first separation unit 31 (the demethaniser), and may also further contain hydrogen if not already removed previously. This fraction is typically used as a fuel gas. This leaves a liquid C2plus fraction (reference sign C2+), which is transferred into a second separation unit 32 (the deethaniser). In the present application, the C2plus fraction is denoted as the first fraction if a demethaniser-first method is used.

(14) In this second separation unit 32, a C2 fraction (reference sign C2) is separated off, in gaseous form, from the C2plus fraction. The C2plus fraction may for example be subjected to a hydrotreatment process 41 to convert acetylene contained therein to ethylene. Subsequently, the C2 fraction is separated out into ethylene (reference sign C2H4) and ethane (reference sign C2H6) in a C2 separation unit 35 (also referred to as a C2 splitter). This ethane can be subjected to the steam cracking process 10 again in one or more cracking furnaces 11 to 13 as a recycled stream D. In the example shown, the recycled streams D and E are added to the stream A. The recycled streams D and E and the stream A may also be passed into different cracking furnaces 11 to 13.

(15) As described, if fresh charges which contain a non-negligible amount of ethane are used, the proportion of ethane in the cracked gas C increases in particular. In existing systems, which are set up for use of exclusively low-ethane fresh charges, the described separation units are not configured for such large amounts of ethane.

(16) A liquid C3plus fraction (reference sign C3+) is left behind in the second separation unit 32, and is passed into a third separation unit 33 (the depropaniser), in which a C3 fraction (reference sign C3) is separated off from the C3plus fraction and for example subjected to a further hydrotreatment process 42, so as to convert methylacetylene in the C3 fraction into propylene. Subsequently, the C3 fraction is separated out into propene (reference sign C3H6) and propane (reference sign C3H8) in a C3 separation unit 36. This propane can be subjected to the steam cracking process 10 again in one or more cracking furnaces 11 to 13 as a recycled stream E, separately or together with other streams. A liquid C4plus fraction (reference sign C4+) is left behind in the third separation unit 33, and is passed into a fourth separation unit 34 (the debutaniser), in which a C4 fraction (reference sign C4) is separated off from the C4plus fraction. A liquid C5plus fraction (reference sign C5+) is left behind.

(17) If exclusively gaseous furnace charges are used, it is possible that no C3plus, C4plus or C5 plus hydrocarbons or much smaller amounts thereof will occur, making it possible to dispense with the last separation units.

(18) Needless to say, all of the fractions shown may also be subjected to suitable post-treatment steps. For example, 1,3-butadiene may be separated out from the C4 hydrocarbon stream, if obtained. Further, additional recycled streams may be used, which may be subjected to the steam cracking process 10 analogously to the recycled streams D and E.

(19) FIG. 1B shows in the form of a schematic flow chart the sequence of an alternative method for producing hydrocarbons by steam cracking in accordance with the prior art. In this case too, the central part of the method is a steam cracking process 10, which can be carried out using one or more cracking furnaces 11 to 13. By contrast with the method of FIG. 1A, in this case the cracked gas C is subjected to a separation process 30 by the deethaniser-first principle.

(20) In this case, in the separation process 30, a C2minus fraction (reference sign C2?) is initially separated off, in gaseous form, from the cracked gas C, in a first separation unit 37, and predominantly contains methane, ethane, ethylene and acetylene, and may also further contain hydrogen if not already removed previously. In the present application, the C2minus fraction is denoted as the first fraction if a deethaniser-first method is used.

(21) The C2minus fraction may be subjected as a whole to a hydrotreatment process 43 so as to convert acetylene contained therein into ethylene. Subsequently, a C1 fraction is separated off from the C2minus fraction in a C2minus separation unit 38, and used further as above. This leaves a C2 fraction, which is separated out into ethylene and ethane in a C2 separation unit 35 as above. In this case too, this ethane can be subjected to the steam cracking process 10 again in one or more cracking furnaces 11 to 13 as a recycled stream D. In this case too, a liquid C3plus fraction may be left behind in the first separation unit 37, and is treated in the separation units 33 to 36 and optionally the hydrotreatment unit 42, as described previously for FIG. 1.

(22) In this case too, an increased proportion of ethane in the cracked gas is problematic, since the described separation units in systems for exclusively low-ethane fresh charges are not configured for such large amounts of ethane.

(23) A plurality of further method alternatives, which differ in particular in the preparation of the cracked gas C and/or the separation process used, are known to the person skilled in the art, for example from the aforementioned article Ethylene in Ullmann's Encyclopedia of Industrial Chemistry.

(24) FIG. 2 shows an approach for solving the problem of the increased proportion of ethane in the cracked gas when fresh charges which contain a non-negligible amount of ethane are used, in accordance with an embodiment of the invention.

(25) The method shown in FIG. 2 is illustrated in the form of a schematic flow chart, and is based on the method shown in FIG. 1A by the demethaniser-first principle. To illustrate the universal applicability of the method according to the embodiment shown, the processes and devices used to produce the cracked gas C have been dispensed with.

(26) A corresponding cracked gas C can, however, be obtained analogously to the method shown by way of example in FIG. 1A. In particular, a cracked gas C of this type comes from one or more cracking furnaces 11 to 13 which are loaded at least in part with a fresh charge as a furnace charge, which comprises a non-negligible amount of ethane.

(27) As described, if exclusively gaseous furnace charges are used, it is possible that no C3plus, C4plus or C5plus hydrocarbons or much smaller amounts thereof will form, and so the processes and devices used for separating corresponding fractions have not been shown. If corresponding hydrocarbons do occur, processes and devices of this type may also be provided in the method shown in FIG. 2.

(28) Instead of a cracked gas C, a fraction of a corresponding cracked gas may be used which results from separation into hydrocarbons having four or more carbon atoms on the one hand and hydrocarbons having three or fewer carbon atoms on the other hand.

(29) The method shown in FIG. 2 in accordance with an embodiment of the invention differs from the method shown in FIG. 1A in particular in the use of an additional separation unit 51 in the separation process 30.

(30) In the additional separation unit 51, for example a distillation column, which has the features specified above, a liquid fraction containing ethane is separated off, but in a demethaniser-first method of the type shown here still contains higher-boiling components, in particular C3plus and higher hydrocarbons. The content is dependent on the furnace charge, as stated, and is relatively low for exclusively gaseous furnace charges. This fraction predominantly containing ethane is denoted by reference sign R in FIG. 2.

(31) This fraction R, which predominantly contains ethane and the higher-boiling components, is also low in other hydrocarbons having two carbon atoms. It is subsequently transferred into a further separation unit 52, in which the higher-boiling C3plus components and the ethane are separated from one another. Part of the ethane or part of a corresponding ethane-rich fraction may also be fed into the separation unit 52 from the previously described separation unit 35 (shown in dotted lines). As a result, energy can be saved and the separation capacity of the separation unit 52, which would be available in any case, can be made use of. In the separation unit 52, which is typically in the form of a distillation column, this ethane fraction can be released from the separation unit 35, in particular at the head. Further, as is illustrated by a dashed arrow, further ethane can be fed into it.

(32) An ethane fraction obtained in the separation unit 52, for example a head product of a corresponding distillation column, can be drawn off, united with further streams, and fed back into a cracking process, as illustrated previously using the stream D.

(33) The further stream obtained in the separation unit 51, here denoted as C2, is the proportion of the C2plus fraction left behind after the fraction R predominantly containing ethane (and comprising the C3plus hydrocarbons) is separated off, and can be adjusted to any desired ethane content by way of the separation. Thus, the separation units arranged downstream from the separation unit 51 (as shown in FIG. 1A) do not need to be changed when a corresponding cracking system is converted from low-ethane to (more) ethane-rich fresh charges.

(34) The function of the additional separating unit 51 thus corresponds in part to the function of the separation unit 32 of a conventional method (cf. FIG. 1A), but in the separation unit 51, instead of a pure C3plus fraction, a C3plus fraction comprising a non-negligible proportion of ethane (specifically the repeatedly mentioned fraction containing ethane) is separated off. The function of the separation unit 52 also corresponds in part to the function of the separation unit 32 of the conventional method, but with the difference that in the separation unit 52 comparatively pure ethane is separated from the C3plus hydrocarbons. In functional terms, this is a deethaniser, which would be present in any case.

(35) FIG. 3 is another, alternative drawing of the method shown in FIG. 2. As can be seen from FIG. 3, the separation units 31, 35, 51 and 52 are implemented in corresponding systems in the form of distillation columns.

(36) FIG. 4 again corresponds to FIG. 2 and shows an embodiment of the invention in which the hydrocarbon mixture or cracked gas C is separated by the deethaniser-first principle. As described, this means that, in a first separation unit 37 provided therein, a fraction which contains the predominant proportion of the hydrocarbons having two or fewer carbon atoms contained in the cracked gas C is formed from the cracked gas C. However, when (more) ethane-rich fresh charges are steam cracked, this C2minus fraction has ethane contents which are just as high as before, and which potentially could not be tolerated in conventional separation systems if they are set up for separating cracked gases which are obtained by steam cracking low-ethane fresh charges.

(37) A separation unit 33 set up for processing the C3plus hydrocarbons is merely shown schematically here, but may also be present along with downstream devices.

(38) Also, therefore, in an additional separation unit 53, a fraction S containing ethane is initially separated from the first fraction, in this case from the initially present C2minus fraction downstream from the separation unit 37, specifically in an amount which reduces the ethane content in the remaining C2minus fraction downstream from the separation unit 53 to a tolerable level, e.g. to less than 25%. Unlike previously, in this case this fraction containing ethane does not contain any significant amounts of heavier components, in other words C3plus hydrocarbons.

(39) As shown previously in FIG. 1, the C2minus fraction is supplied to a separation unit 38, which operates as described in relation to FIG. 2B. However, because of the upstream separation unit 53, this has to process a much smaller amount of ethane.

(40) The hydrogenation unit 43 is preferably arranged as shown in FIG. 4 (cf. different arrangement from FIG. 1), in such a way that it only has to process the C2minus stream downstream from the additional separation unit 53.

(41) As an alternative to the arrangement shown in FIG. 4 of the additional separation unit 53, downstream from the separation unit 37, it may also for example be provided in parallel with the separation unit 37 or downstream from the separation unit 38 for processing the C2 stream which occurs there.

(42) FIG. 5 shows the first of these alternatives, specifically the arrangement in parallel with the separation unit 37. The separation unit 37 is shown in detail together with the additional separation unit 53.

(43) In the example shown, the separation unit 37 comprises a first separation device 371 and a second separation device 372 (also known as a double column).

(44) The cracked gas C is supplied to the first separation device 371. The first separation device 371 is operated using a liquid return stream 371a. A bottom product, which still contains substantially all of the components of the cracked gas C, precipitates in the bottom of the first separation device 371. This is drawn off as a stream 371b and fed into the second separation device 372. At the head of the first separation device 371, a head stream 371c is drawn off, which is a C2minus stream, of which the ethane content can be set inter alia by way of the amount of the return stream 371a.

(45) In the second separation device 372, a bottom product 372a in the form of a C3plus fraction low in or free from other hydrocarbons is obtained. In an upper region of this second separation device 372, a stream 372b is drawn off and passed on in part (after condensation in a condenser, not shown) to the first separation device 371 as the liquid return stream 371a, and transferred in part into the separation unit 53, which is for example in the form of a distillation column as described previously.

(46) As described previously, the separation unit 53 is formed so as to separate off the fraction S containing ethane. The fraction S containing ethane (and possibly further containing residues of C3plus hydrocarbons) is the bottom product of this separation unit 53. By contrast, a stream 53a, which also contains other hydrocarbons having three carbon atoms as well as ethane (which is not precipitated in the bottom of the separation unit 53), is drawn off from the head of this separation unit. This stream can thus be united with the head stream 371c drawn off at the head of the first separation device 371.

(47) The united streams 53a and 371c correspond to a C2minus stream, such as is obtained in the separation unit 53 in accordance with FIG. 4.