Method for producing a separation product containing predominantly hydrocarbons with two carbon atoms

Abstract

The invention relates to a method (100) for the recovery of a separation product which contains predominantly hydrocarbons with two carbon atoms, with the use of a separation feedstock which contains predominantly methane, hydrogen and hydrocarbons with two carbon atoms, wherein the methane content of the separation feedstock is up to 20%, and the separation feedstock is provided in a gaseous state. It is provided that, at a first pressure level, the separation feedstock is partially condensed in a single step by cooling from a first temperature level to a second temperature level, thereby obtaining precisely one first liquid fraction and precisely one first gaseous fraction; at least one part of the first gaseous fraction is partially condensed in a single step through further cooling from the second temperature level to a third temperature level, thereby obtaining precisely one second liquid fraction and precisely one second gaseous fraction; at least one part of the second gaseous fraction at the second pressure level is subjected to a contraflow absorption in the contraflow to an absorption liquid containing predominantly methane, thereby obtaining precisely one third liquid fraction and precisely one third gaseous fraction; the first, the second and the third liquid fraction are at least partially combined and, at least partially, at a second pressure level above the first pressure level, subjected to a low-temperature rectification, thereby obtaining a sump liquid and an overhead gas; at least one part of the overhead gas at the second pressure level is partially condensed in a single step through further cooling from the second temperature level to the third temperature level, thereby obtaining a fourth liquid fraction and a fourth gaseous fraction; and the absorption liquid containing predominantly methane is formed through further cooling of at least a part of the fourth gaseous fraction to a fourth temperature level. A corresponding plant also forms the subject matter of the invention.

Claims

1. A method for the recovery of a separation product which contains predominantly hydrocarbons with two carbon atoms comprising: providing a separation feedstock which contains predominantly methane, hydrogen and hydrocarbons with two carbon atoms, wherein the methane content of the separation feedstock is up to 30%, and the separation feedstock is provided in a gaseous state, partially condensing the separation feedstock at a first pressure level in a single step by cooling from a first temperature level at −20 to −35° C. to a second temperature level at −75 to −80° C., and recovering one first liquid fraction and one first gaseous fraction partially condensing at least a part of the first gaseous fraction at the first pressure level in a single step through further cooling to a third temperature level, thereby obtaining one second liquid fraction and one second gaseous fraction, subjecting at least one part of the second gaseous fraction at the first pressure level to a contraflow absorption with an absorption liquid containing predominantly methane, thereby obtaining one third liquid fraction and one third gaseous fraction, combining, at least partially, the first, the second and the third liquid fraction and, at a second pressure level above the first pressure level, at least partially subjecting the combined liquid fractions to a low-temperature rectification, thereby obtaining a sump liquid and an overhead gas, partially condensing at least a part of the overhead gas at the second pressure level in a single step through further cooling to a fourth temperature level, thereby obtaining a fourth liquid fraction and a fourth gaseous fraction, cooling at least a part of the fourth gaseous fraction to a fifth temperature level to form at least a portion of the absorption liquid containing predominantly methane.

2. The method according to claim 1, in which the third temperature level is at −100 to −105° C., and/or the fourth temperature level is at −95 to −100° C., and/or the fifth temperature level is at −140 to −155° C.

3. The method according to claim 1, in which the first pressure level is at 32 to 37 bar, and/or the second pressure level is at 35 to 40 bar.

4. The method according to claim 1, in which, for the contraflow absorption, an absorption column is used, which comprises a sump region and an absorption region separated from the sump region by a liquid barrier, which is arranged above the sump region, wherein the liquid barrier is constituted in such a manner that it allows liquid which collects in a lower region of the absorption region on the liquid barrier, to drain downwards into the sump region and, in this context, prevents a rising upwards of gas from the sump region into the absorption region.

5. The method according to claim 4, in which the separation feedstock cooled from the first temperature level to the second temperature level is fed into the sump region as a two-phase mixture, wherein, within the latter, the first liquid fraction is separated from the first gaseous fraction.

6. The method according to claim 5, in which the first gaseous fraction or its part further cooled to the third temperature level is fed at the sump end as a two-phase mixture into the absorption region, wherein, within the latter, the second liquid fraction is separated from the second gaseous fraction.

7. The method according to claim 6, in which the third liquid fraction is combined with the second liquid fraction, the liquid barrier and released via the liquid barrier into the sump region, where it is combined with the first liquid fraction.

8. The method according to claim 1, in which the first, the second and the third liquid fraction or their combined parts are compressed by means of a sump pump and transferred into a rectification column used for the low-temperature rectification.

9. The method according to claim 8, in which, for the partial condensation of the overhead gas or of its part, an overhead condenser of the rectification column which is cooled with the use of low-pressure ethylene is used.

10. The method according to claim 1, in which at least a part of the third gaseous fraction at the first pressure level is partially condensed in a single step through further cooling to the fifth temperature level, thereby obtaining precisely one fifth liquid fraction and precisely one fifth gaseous fraction.

11. The method according to claim 10, in which, for the cooling of the third and of the fourth gaseous fraction or their parts, at least one heat exchanger is used, which is cooled with the use of at least one part of the fifth liquid fraction and the fifth gaseous fraction.

12. The method according to claim 10, in which, for the cooling of the separation feedstock, at least one heat exchanger is used, which is cooled with the use of at least one part of the fifth liquid fraction and of the fifth gaseous fraction and with high-pressure and medium-pressure ethylene.

13. The method according to claim 1, in which, for the cooling of the first gaseous fraction, at least one heat exchanger is used, which is cooled with the use of at least one part of the fifth liquid fraction and of the fifth gaseous fraction and with low-pressure ethylene.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a process according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(2) In FIG. 1, a method according to a particularly preferred embodiment of the invention is illustrated in the form of a schematic process-flow diagram and designated as a whole as 100. The explanations relating to the method 100 apply in a similar manner to a corresponding plant, so that, when reference is made to method steps, the corresponding explanations correspond at the same time to components of the plant and vice versa.

(3) In the method 100, a gaseous mixture containing predominantly methane, hydrogen and hydrocarbons with two carbon atoms which can previously be subjected, for example, to a hydration for the conversion of previously contained acetylene, and which is formed from the cracked gas of a steam cracking method not illustrated here, is provided in gaseous form as a separation feedstock.

(4) The separation feedstock in the form of a substance flow a is cooled in a heat exchanger 1 against a hydrogen fraction (substance flow b), methane fraction (substance flow c), high-pressure ethylene (substance flow d) and medium-pressure ethylene (substance flow e) to a pressure level of approximately 34.9 bar (designated here as “first pressure level”), starting from a temperature level at approximately −23° C. (“first temperature level”) to a temperature level of approximately −78° C. (“second temperature level”), in this context, partially condensed in a single step and then guided at a pressure level of approximately 34.7 bar (accordingly, still the first pressure level) into the sump region 21 of an absorption column 2 of the C2 absorber. There, the condensate occurring (“first liquid fraction”) is separated from the gaseous phase f (“first gaseous fraction”).

(5) The first liquid fraction is enriched with hydrocarbons with two carbon atoms. Because of the single-stage partial condensation, a comparatively large quantity of methane is separated from the separation feedstock in the first liquid fraction. This is larger than it would be in a conventional multi-stage partial condensation, as illustrated, for example, in EP 3 029 017 A1 with reference to FIGS. 1 and 2. The first gaseous fraction contains substantially all the components of the separation feedstock, but is, in particular, depleted with regard to hydrocarbons with two carbon atoms. By comparison with conventional multi-stage partial condensations, it also contains less methane for the reasons mentioned.

(6) In the illustrated example, the first gaseous fraction is withdrawn completely from the sump region 21 of the absorption column 2 in the form of a substance flow f and further cooled in a heat exchanger 3 against the already mentioned hydrogen and methane fraction (substance flows b and c) and against low-pressure ethylene (substance flow g), to a temperature level of approximately −103° C. (“third temperature level”) and, in turn, partially condensed. Because of the use of the substance flows b and c, the third temperature level is lower than would be attainable with low-pressure ethylene alone. After this, the substance flow f is recycled, still as a two-phase mixture, back into the absorption column 2 above a liquid barrier 22, which divides the sump region 21 of the absorption column 2 from an absorption region 23 disposed above it. The liquid barrier 22 allows a downward flow of liquid accumulating in the lower region of the absorption region 23 and prevents a rising upwards of gas from the sump region 21 into the absorption region 23.

(7) In order to overcome the pressure loss in the heat exchanger 3, the latter is arranged geodetically above the absorption column 2. The absorption column 2 operates at a pressure level of approximately 34 to 35 bar, that is, also at the first pressure level already mentioned several times.

(8) In the absorption column 2 or respectively its absorption region, a phase separation of the substance flow f or respectively of the correspondingly cooled first gaseous fraction, takes place. The liquid phase (“second liquid fraction”) accumulates above the liquid barrier 22 and is combined there with charged absorption liquid (“third liquid fraction”) trickling downwards from above. The gaseous proportion (“second liquid fraction”) remaining in the case of the phase separation of the substance flow f or respectively of the correspondingly cooled first gaseous fraction rises upwards into the absorption region and, in this context, is subjected to a contraflow absorption in the contraflow to an absorption liquid containing predominantly methane, in the form of a substance flow n.

(9) In the contraflow absorption, a liquid fraction (the “third liquid fraction” already mentioned) is formed, which combines with the second liquid fraction, as already mentioned. The second and third liquid fraction drain in combination via the liquid barrier 22 into the sump region 21 of the absorption column 2, where they are combined with the first liquid fraction. The gaseous fraction remaining (“third gaseous fraction”) in the case of the contraflow absorption rises upwards and is withdrawn from the absorption column 2 in the form of a substance flow o.

(10) From the sump region 21 of the absorption column 2, the combined first, second and third liquid fraction is withdrawn by means of a sump pump 4 at a temperature of approximately −79° C. (that is, still at the first temperature level) from the absorption column 2, more precisely from the sump region 21, and pumped (substance flow h) into a rectification column 5, the so-called de-methaniser. Through the action of the sump pump 4, a pressurisation to approximately 38 bar occurs. In the rectification column 5, the hydrocarbons with two carbon atoms, that is, the “separation product” mentioned several times, at a pressure level of approximately 35 bar (“second pressure level”) are separated from methane and lighter components and leave the rectification column 5 via the sump as sump liquid in the form of a substance flow i. In general, the rectification column 5 operates at the second pressure level, especially at approximately 35 to 36 bar, its sump is evaporated off in a sump evaporator 52 with high-pressure propylene. The substance flow i, that is, the separation product, can be warmed in the heat exchanger 1 and supplied to a further separation step for the separation of hydrocarbons with two carbon atoms from one another.

(11) Overhead gas of the rectification column 5 is cooled in the form of a substance flow k in a heat exchanger 6 with the use of low-pressure ethylene, which is present at a temperature level of approximately −101° C., to a temperature level of approximately −98° C. (“fourth temperature level”) and partially condensed. The heat exchanger 6 is built into the head of the rectification column 5, so that the occurring condensate (“fourth liquid fraction”) flows back into the rectification column 5 as a reflux in the form of a substance flow l, without a pump only through gravity. Because here, only low-pressure ethylene is used, the fourth temperature level is disposed above the third temperature level which is provided by the heat exchanger 3. The remaining gas (“fourth gaseous fraction”) comprises predominantly methane and leaves the rectification column 5 at the head in the form of a substance flow m. The majority of this substance flow m is further cooled in the form of a substance flow n in a heat exchanger 7 to a temperature level of approximately −152° C. (“fifth temperature level”), during this course, predominantly condensed, and then, as already mentioned, supplied as reflux to the absorption region 23 of the absorption column 2.

(12) The overhead product of the absorption region 23 of the absorption column 2 (that is, the third gaseous fraction) is also cooled in the form of a substance flow o, which is present at a pressure level of approximately 34.4 bar (that is, the first pressure level), in the heat exchanger 7 to the fifth temperature level of approximately −152° C. and partially condensed. In a separation container 8, the condensate occurring, the so-called methane fraction (“fifth liquid fraction”), is separated from the gaseous phase, the so-called hydrogen fraction (“fifth gaseous fraction”). The methane fraction, here initially still designated with p, is first depressurised to an appropriate pressure level, for example, of a heating gas network, and then warmed in the heat exchangers 7, 3 and 1.

(13) For the cold-balancing of the heat exchanger 7, liquid methane is removed from the rectification column 5 above a liquid tray 51 and supplied in the form of a substance flow q to the methane fraction of the substance flow p, after it has been cooled in the heat exchanger 7 to the fifth temperature level of approximately −152° C. Similarly, a small part of the substance flow m can be fed in the form of a substance flow r to the substance flow p. The combined flow formed from the substance flows p, q and r is still designated as a methane fraction and is illustrated in the form of the already mentioned substance flow c.

(14) The gaseous phase with approximately 90 mole percent hydrogen from the separation container 8, is warmed, like the methane fraction of the substance flows p and respectively c, in the heat exchangers 7, 3 and 1, against the warm substance flow a.