Method and plant for producing ethylene

Abstract

Proposed is a process for producing ethylene wherein using a dehydrogenation of ethane a process gas containing at least ethane, ethylene and compounds having a lower boiling point than ethane and ethylene is formed, wherein using at least a part of the process gas a separation input is formed and subjected to a low-temperature separation (6) in which the separation input is cooled and in which one or more condensates are separated from the separation input, wherein the condensate(s) are at least partly subjected to a low-temperature rectification to obtain a gaseous first fraction and a liquid second fraction, wherein the gaseous first fraction contains at least the ethane and the ethylene in a lower proportion than in the separation input and the compounds having a lower boiling point than ethane and ethylene in a higher proportion than in the separation input. It is provided that the first fraction is at least partly subjected to a pressure swing adsorption (7) by means of which a third fraction containing predominantly or exclusively ethylene and ethane and a fourth fraction containing predominantly or exclusively methane and carbon monoxide are formed. A corresponding plant (100) likewise forms part of the subject matter of the present invention.

Claims

1. A process for producing ethylene wherein using a dehydrogenation of ethane a process gas containing at least ethane, ethylene and compounds having a lower boiling point than ethane and ethylene is formed, wherein using at least a part of the process gas a separation input is formed and subjected to a low-temperature separation (6) in which the separation input is cooled and in which one or more condensates are separated from the separation input and wherein at least a part of the condensate(s) are subjected to a low-temperature rectification column to obtain a gaseous first fraction and a liquid second fraction, wherein the gaseous first fraction contains at least the ethane and the ethylene in a lower proportion than in the separation input and the compounds having a lower boiling point than ethane and ethylene in a higher proportion than in the separation input, characterized in that the gaseous first fraction is at least partly subjected to a pressure swing adsorption (7) by means of which a third fraction containing predominantly or exclusively ethylene and ethane and a fourth fraction containing predominantly or exclusively methane and carbon monoxide are formed.

2. The process according to claim 1, wherein in the low-temperature separation (6) for separating the condensate(s) the separation input is cooled to a temperature level of −40° C. to −100° C.

3. The process according to claim 2, wherein the low-temperature rectification is performed using a liquid reflux which is formed by condensing a part of the gaseous first fraction.

4. The process according to claim 1, wherein in the low-temperature separation (6) for separating the condensate(s) the separation input is cooled to a temperature level of −20° C. to −40° C.

5. The process according to claim 4, wherein the separation input is compressed to a pressure level of 25 to 35 bar before the cooling and the low-temperature rectification is operated at a pressure level of 15 to 25 bar.

6. The process according to claim 4, wherein the separation input is compressed to a pressure level of 20 to 25 bar before the cooling and the low-temperature rectification is operated at a pressure level of 9 to 16 bar.

7. The process according to claim 4, wherein the cooling of the separation input comprises transfer of heat to the liquid second fraction.

8. The process according to claim 7, wherein the cooling of the separation input comprises transfer of heat to a refrigerant.

9. The process according to claim 5, wherein the condensate(s) or the part thereof subjected to the low-temperature rectification is or are further cooled to a temperature level of −30° C. to −30° C. by transfer of heat to the gaseous first fraction.

10. The process according to claim 6, wherein the condensate(s) or the part thereof subjected to the low-temperature rectification is or are cooled to a temperature level of −40° C. to −100° C. by transfer of heat to the gaseous first fraction and a refrigerant.

11. The process according to claim 9, wherein the gaseous first fraction is decompressed before heat from the condensate(s) or the part thereof subjected to the low-temperature rectification is transferred thereto.

12. The process according to claim 11, wherein after the transfer of the heat the gaseous first fraction is supplied to the pressure swing adsorption (7).

13. The process according to claim 4, wherein a proportion of the separation input which remains gaseous during the formation of the condensates is also at least partly supplied to the low-temperature rectification.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is more particularly elucidated below with reference to the appended drawings which illustrate inter alia preferred embodiments of the present invention.

(2) FIG. 1 shows a plant for producing olefins according to one embodiment of the invention.

(3) FIG. 2 shows a low-temperature separation for use in a plant according to one embodiment of the invention.

(4) FIG. 3 shows a low-temperature separation for use in a plant according to one embodiment of the invention.

(5) FIG. 4 shows a low-temperature separation for use in a plant according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(6) In the figures that follow, functionally or structurally equivalent elements are indicated with identical reference numerals and for the sake of simplicity are not repeatedly elucidated. When plants and plant parts are described hereinbelow the elucidations concerning these also apply correspondingly to the process steps implemented by means of these plant parts and vice versa.

(7) In FIG. 1 a plant for producing olefins according to one embodiment of the invention in the form of a greatly simplified plant diagram is illustrated and collectively referred to as 100. Notwithstanding that a plant 100 for ODH of ethane (ODH-E) is described below, the present invention is also suitable, as mentioned, for use in a non-oxidative dehydrogenation of ethane. In this case the elucidations which follow apply correspondingly.

(8) In the plant 100 an ethane-rich fresh input a which may contain small amounts of methane is mixed with an ethane-rich, recycled material stream b and supplied to one or more ODH-E reactors 1. The reactor(s) is/are further supplied with a diluent c (for example steam, nitrogen or carbon dioxide) and oxygen d.

(9) The principal reaction of ODH-E is:
C.sub.2H.sub.8+½O.sub.2.fwdarw.C.sub.2H.sub.4+H.sub.2O  (1)

(10) Side reactions that occur are especially the formation of carbon monoxide, carbon dioxide and also of acetic acid.

(11) A gas mixture withdrawn from the reactor(s) 1 in the form of a material stream e, referred to here as “process gas”, is subjected to a cooling 2 and water is removed in the form of a material stream F. Any acetic acid present is likewise removed from the process gas and discharged with the material stream f. The material stream f may subsequently be subjected to further processing to obtain acetic acid (not shown).

(12) The cooled process gas in the form of a material stream g is subjected to a compression 3 and compressed, generally in a multi-stage compressor. Between the compressor stages the process gas is withdrawn in the form of a material stream h to remove carbon dioxide in the form of a material stream k. This may be effected for example by an amine scrub, potash scrub or lye scrub in a carbon dioxide removal unit 4. Also employable are other processes such as membrane processes or a combination of different processes. The process gas freed from carbon dioxide is passed back to the compression 3 in the form of a material stream I.

(13) The compressed process gas is supplied to a drying 5 in the form of a material stream m and the accordingly dried process gas is supplied to a low-temperature separation 6 in the form of a material stream n. Details of a low-temperature separation 6 are illustrated in the form of exemplary embodiments in FIGS. 2, 3 and 4.

(14) In the low-temperature separation 6 the process gas/a separation input formed therefrom is cooled via one or more temperature levels and one or more condensates are separated from the process gas, wherein the condensate(s) is/are at least partly subjected to a low-temperature rectification to obtain a gaseous (“first”) fraction and a liquid (“second”) fraction, wherein the gaseous first fraction contains at least the ethane and the ethylene in a lower proportion than in the separation input and at least the compounds having a lower boiling point than ethane and ethylene in a higher proportion than in the separation input. The process gas is thus subjected to a staged cooling and condensate generated is supplied to a rectification column.

(15) At the top of the rectification column the mentioned gaseous first fraction is withdrawn in the form of a material stream o. To form the material stream o it is also possible to use a proportion of the process gas remaining in gaseous form in the mentioned staged cooling. Due to the mentioned small amounts of the methane usable as reflux in the low-temperature rectification this material stream o still contains considerable amounts of ethane and ethylene.

(16) To reduce the losses of product (ethylene) and reactant (ethane) a pressure swing adsorption 7 for recovering ethylene (and also ethane) is proposed in the plant 100 shown in FIG. 1. Here ethane and ethylene are adsorbed from the material stream o by a suitable adsorbent. Ethane and ethylene are preferably adsorbed in contrast with the light gases nitrogen and argon which are in particular introduced into the plant 100 as impurities in the oxygen d but also in contrast with oxygen, carbon monoxide, methane and other light gases such as hydrogen.

(17) Ethane and ethylene are discharged again at lower pressure in the form of a (“third”) fraction containing predominantly ethane and ethylene and may be discharged in the form of a material stream p. The material stream p may in particular be recycled to the compression 3. The material stream o freed from ethylene and ethane and now labelled q, i.e. a (“fourth”) fraction containing predominantly or exclusively components having a lower boiling point than ethane and ethylene, is still under high pressure and can be utilized for obtaining mechanical work and thermal energy or as an export stream for other applications. The material stream o may also be utilized for refrigeration recovery before the adsorption.

(18) The basic idea of the present invention consists in a combination of a low-temperature separation 6 with a pressure swing adsorption 7. Different embodiments may be envisaged here. In one embodiment illustrated in FIG. 2 a cooling of the process gas to an extremely low temperature level is undertaken, so that the content of ethane and ethylene in the material stream o which is subsequently subjected to the pressure swing adsorption is reduced. In further embodiments illustrated in FIGS. 3 and 4 a cooling of the process gas to a markedly higher temperature level is undertaken, so that the content of ethane and ethylene is reduced to a corresponding extent only in the pressure swing adsorption.

(19) Remaining in the low-temperature separation 6 is the liquid second fraction which may be withdrawn from the low-temperature separation 6 in the form of a material stream r. This liquid second fraction contains predominantly or exclusively ethylene and ethane. In the example shown the material stream r is supplied in a splitter 8 which likewise comprises a low-temperature rectification. An ethylene product containing predominantly or exclusively ethylene is obtained in the splitter 8 and withdrawn in the form of a material stream s. A fraction containing predominantly or exclusively ethane likewise formed in the splitter 8 may be recycled to the reactor(s) in the form of the material stream b.

(20) If a heavy fraction of hydrocarbons having three or more carbon atoms exists, said fraction may be removed upstream or downstream of the low-temperature separation 6. Depending on the amount these heavy hydrocarbons may also be removed from the material stream b.

(21) FIG. 2 illustrates a low-temperature separation for use in a plant for producing olefins, such as may be employed for example as a low-temperature separation 6 in the plant 100 shown in FIG. 1. This low-temperature separation which corresponds to the previously elucidated first embodiment comprises a staged cooling. The material streams n, o and r already depicted in FIG. 1 are also shown here to illustrate the integration of the low-temperature separation shown in FIG. 2 into a corresponding plant 100. The depiction of the respective elements is not true to position and not true to scale.

(22) The process gas is supplied to the low-temperature separation in the form of the material stream n. The process gas is successively passed through heat exchangers 201 to 204 and therein cooled to ever lower temperature levels. To this end the heat exchangers 201 to 204 may be cooled with ethylene streams (not shown). Also employable for cooling and likewise not shown seperately is the material stream o, i.e. the gaseous first fraction elucidated repeatedly above.

(23) Downstream of the heat exchangers 201 to 204 the process gas/a biphasic mixture formed in each case by cooling in the heat exchangers 201 to 204 is in each case transferred into separators 205 to 208 where in each case a condensate is separated from the process gas. The condensates are introduced into a rectification column 209 at a height corresponding to their composition of matter. A smaller proportion of the process gas of the material stream n may also be introduced directly into the rectification column 209 (not shown here).

(24) A bottoms evaporator 210 of the rectification column 209 is heated using propane for example, a tops condenser 211 is cooled using low-pressure ethylene for example. The rectification column 209 is operated such that predominantly components having a lower boiling point than ethane and ethylene undergo enrichment at its top and the higher boiling compounds undergo enrichment at its bottom. In this way a portion of the material stream o, referred to here as o1, may be withdrawn from the top of the rectification column 209 and the material stream r may be withdrawn from the bottom of the rectification column 7. A proportion of the process gas remaining in gaseous form in the separator 208, illustrated here in the form of a material stream o2, may likewise be used in the formation of the material stream o.

(25) The temperature of the process gas/downstream of the heat exchanger 201 is for example about −30° C., the temperature downstream of the heat exchanger 202 for example about −50° C., the temperature downstream of the heat exchanger 203 for example about −75° C. and the temperature downstream of the heat exchanger 204 for example about −99° C. The bottoms evaporator 210 is operated at a temperature level of for example about −17° C., the tops condenser 211 at a temperature level of for example about −97° C.

(26) Correspondingly low temperatures are achievable by use of an appropriate chiller, for example with the refrigerant ethylene. In this way the proportion of ethylene in the gaseous first fraction, i.e. the material stream o, can be substantially reduced. The use of a pressure swing adsorption 7 for removal of ethylene may nevertheless be energetically and economically advantageous depending on the composition of the process gas/of the material stream n.

(27) In this case the pressure swing adsorption recovers ethane and ethylene and expanders which would otherwise be installed, where required, for the production of peak refrigeration from the material stream o may be dispensed with. The material stream o may continue to be utilized for heat integration and the ethylene-free residual gas stream q (see FIG. 1) may be recovered as in the preceding case. In addition, the pressure swing adsorption may be made substantially smaller than when no cooling to correspondingly low temperature levels is undertaken.

(28) FIG. 3 illustrates a low-temperature separation for use in a plant for producing olefins, such as may likewise be employed for example as a low-temperature separation 6 in the plant 100 shown in FIG. 1. This low-temperature separation which operates with the previously elucidated higher temperature levels comprises a multistage cooling to a markedly higher temperature level than elucidated for FIG. 2. Here too, the material streams n, o and r depicted in FIG. 1 are shown to illustrate the integration of the low-temperature separation shown in FIG. 3 into a corresponding plant 100. Also shown here in addition are the pressure swing adsorption 7 and the material streams p and q.

(29) The process gas is introduced into the low-temperature separation at a temperature level of for example about −9° C. and at a pressure level of for example about 30 bar and passed through a first heat exchanger 301 which may be cooled using the material stream r, i.e. the liquid second fraction. After cooling in the first heat exchanger 301 the process gas is then cooled to a temperature level of for example about −35° C. in a second heat exchanger 306. Formed in this way is a biphasic stream which is introduced into a separation vessel 302. A liquid fraction and a gaseous faction are formed in the separation vessel 302. The gaseous fraction is decompressed and introduced into a rectification column 305. The decompression into the second separation vessel 304 is effected from the pressure level of for example about 30 bar to a pressure level of for example about 19 bar at which the rectification column 305 is also operated. The heat exchanger 306 may be operated for example with low-pressure propylene or a corresponding C3 refrigerant.

(30) The liquid fraction is withdrawn from the first separation vessel 302 and cooled in a third heat exchanger 303 to a temperature level of for example about −38° C. and decompressed into a second separation vessel 304 to the pressure level at which the rectification column is operated. This forms a liquid fraction and a gaseous fraction which are introduced into the rectification column. The liquid fraction is used as reflux onto the rectification column 305. The gaseous fraction from the first separation vessel 302 is also introduced into the rectification column.

(31) The rectification column 305 is operated using a bottoms evaporator which may be operated with low-pressure propylene for example. The material stream r may be withdrawn from the rectification column 305 at a temperature level of for example about −21° C. From the top of the rectification column 305 the material stream o is withdrawn at a temperature level of for example about −45° C., decompressed further and passed through the second heat exchanger 303.

(32) The material stream o, i.e. the gaseous second fraction, is subsequently introduced into the pressure swing adsorption 7. For details of this and the further treatment of the material streams p and q reference is made to the elucidations in respect of FIG. 1.

(33) FIG. 4 illustrates a low-temperature separation for use in a plant for producing olefins, such as may likewise be employed for example as a low-temperature separation 6 in the plant 100 shown in FIG. 1. This low-temperature separation constitutes a variant of the low-temperature separation illustrated in FIG. 3 which differs essentially in terms of the pressures and temperatures used. The elements shown are therefore labelled with identical reference numerals.

(34) Here too a cooling to the elucidated comparatively high temperature levels is effected in the heat exchangers 301 and 306. However, the separation input for the low-temperature separation is supplied at for example about 22 bar and the low-temperature rectification is performed at for example about 13 bar. The liquid fraction withdrawn from the first separation vessel 302 is here cooled to a temperature level of for example about 97° C. in the third heat exchanger 303 by additional use of a suitable coolant in the form of a material stream. In this way the material stream r may be withdrawn from the rectification column 305 at a temperature level of for example about −35° C. From the top of the rectification column 305 the material stream o is withdrawn at a temperature level of for example about −97° C., decompressed further and passed through the second heat exchanger 303.