Ethylene recovery
Patent Information
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ BV
- Filing Date
- 2025-02-11
- Publication Date
- 2026-08-06
AI Technical Summary
Existing ethylene recovery processes, particularly those using two-stage distillation systems, are costly, complex, and inflexible, requiring continuous stable operation to maintain product specifications.
A method involving a membrane filtration unit is used to separate ethylene from a mixture of olefins, producing a permeate stream with a higher molar percentage of ethylene, which is then combined with a distillation column to recover residual ethylene, optionally using flash drums to pre-separate heavier components and prevent operational issues.
The membrane filtration unit reduces operational costs and complexity, allowing for flexible operation and higher ethylene recovery efficiency, with permeate streams achieving up to 90% ethylene purity.
Smart Images
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Abstract
Description
[0001] ETHYLENE RECOVERY
[0002] Field of the Invention
[0003] This invention relates to a method and system for recovering ethylene from a mixture of olefins.
[0004] Background of the invention
[0005] Olefins are typically produced by ethylene oligomerisation and olefin metathesis. However, not all of the ethylene is used in the olefin production process, and there is therefore a need to recover ethylene from the olefin product stream.
[0006] In one existing process, this is achieved by using a two-stage distillation system in which the olefin feed stream is provided to a first, high pressure, distillation column (via an upstream heating and flash separation process) . The overhead product stream from the first distillation column is directed to a recovered ethylene product stream, and the bottoms product stream from the first distillation column is provided as a feed stream to a second, low pressure, distillation column. A portion of the overhead product stream from the second distillation column is provided as a feed stream to the fist distillation column, while the bottoms product stream from the second distillation column is removed as the heavy olefin product.
[0007] The operation of distillation columns is expensive and complex. In addition, distillation column operation is inflexible, requiring continuous stable operation to maintain product specifications. It is an aim of the current invention to overcome at least some of the disadvantages of the known system .
[0008] Summary of the Invention
[0009] According to an aspect of the invention there i s provided a method of recovering ethylene from a mixture of olefins , compri sing : providing a feed stream compris ing a vapor phase mixture of olefins to a membrane filter unit ; pas sing the feed stream through the membrane filter unit to produce a permeate product stream and a retentate product stream, wherein the permeate product stream comprises a higher molar percentage of ethylene than the retentate product stream; directing a lea st a portion of the permeate product stream to a recovered ethylene product stream; and directing a lea st a portion of the retentate product stream to a feed stream of a distillation column .
[0010] The present invention i s advantageous as the membrane f iltration unit is potentially les s costly to run than the distillation column of the existing system .
[0011] Optionally the distillation column may be operated to produce an overhead vapor product stream; and a least a portion of the overhead vapor product stream may be directed to the recovered ethylene product stream . This allows ethylene present in the retentate product stream, and separated by the distillation column , to be recovered together with the ethylene in the permeate product stream .
[0012] Alternatively, a least a portion of the overhead vapor product stream from the distillation column may be directed to the feed stream of the membrane f ilter unit to further purify the overhead vapor product stream from the distillation column.
[0013] The method may comprise: providing a primary feed stream comprising a liquid phase, wherein the primary feed stream comprises a mixture of olefins; heating the primary feed stream; passing the heated primary feed stream through a first flash drum to produce a first flash drum vapor product stream and a first flash drum liquid phase product stream; directing at least a portion of the first flash drum vapor product stream into the feed stream of the membrane filter unit; and directing at least a portion of the first flash drum liquid phase product stream to a feed stream of the distillation column. Passing the primary feed stream through the first flash drum is beneficial as this pre-separates heavier olefin components which could plug the nanofiltration unit or otherwise lead to operational issues. The distillation column able to recover residual ethylene that is not separated by the membrane filter unit.
[0014] In one example, the method comprises: cooling the first flash drum vapor product stream; passing the cooled first flash drum vapor product stream, through a second flash drum to produce a second flash drum vapor product stream and a second flash drum liquid phase product stream; directing at least a portion of the second flash drum vapor product stream into the feed stream of the membrane filter unit; and directing at least a portion of the second flash drum liquid phase product stream to a feed stream of the distillation column. Passing the cooled first flash drum vapor product stream through the second flash drum is beneficial a this separates out liquids which can reduce the operational performance of the membrane f ilter unit .
[0015] Optionally the method may comprise directing at least a portion of the second flash drum liquid phase product stream into the retentate product stream .
[0016] The membrane filter unit may optionally comprise a nanof iltration unit . Multiple stage s of nanof iltration units may be arranged so that the permeate and / or retentate of an upstream nanofiltration stage i s filtered by a downstream nanof iltrations stage . This beneficially increases the purity (mol percent ) of the recovered ethylene product stream .
[0017] The feed stream to the membrane filtration unit may comprise a mixture of ethylene and olefins .
[0018] In one example , the permeate product stream may comprise a molar percentage of ethylene greater than or equal to 90% .
[0019] Optionally the retentate product stream may comprise a molar percentage of ethylene les s than or equal to 50 % .
[0020] According to another aspect of the invention there is provided a system for recovering ethylene from a mixture of olef ins , comprising a membrane filtration unit and a distillation column , wherein the system i s configured so that permeate from the membrane f iltration unit is directed to a recovered ethylene product stream, and retentate f rom the membrane filtration unit is directed to a feed stream of the di stillation column . Brief Description of the Drawings
[0021] Figure 1 schematically illustrates a system for recovering ethylene from a mixture of olefins; and
[0022] Figure 2 schematically illustrates a modified system to that shown in Figure 1.
[0023] These drawings depict one or more implementations in accordance with the present teachings, by way of example only, not by way of limitation. In the figures, like reference numerals refer to the same or similar elements.
[0024] Detailed Description of the Drawings
[0025] Figure 1 schematically illustrates a system 10 for recovering ethylene from a mixture of olefins. A primary feed stream 12 comprising a liquid phase mixture of olefins and ethylene is provided to a heater 14. In this example, the primary feed stream 12 has a temperature of 96°C, a pressure of 112 BarG, and comprises about 60-65 mol percent ethylene, and 30-35 mol percent other olefins.
[0026] The primary feed stream 12 is heated by the heater 14 and passed through a letdown valve 15 to produce a mixed phase heated primary feed stream 16. In this example, the mixed phase heated primary feed stream 16 has a temperature of 135°C and a pressure of 112 BarG.
[0027] The heated primary feed stream 16 is passed through a first flash drum 18 to produce a first flash drum vapor product stream 20 and a first flash drum liquid phase product stream 22. The first flash drum liquid phase product stream 22 is passed through a letdown valve 23 to form a feed stream 30 of a distillation column 32. In this example, the feed stream 30 has a temperature of 127 °C, a pressure of 11.8 BarG, and comprises about 20-25 mol percent ethylene, and 75-80 mol percent other olefins.
[0028] The first flash drum vapor product stream 20 is cooled by cooler 28 and passes through letdown valve 25 to produce a mixed phase cooled first flash drum vapor product stream 24 which is provided as a feed to a second flash drum 26. In this example, the mixed phase feed 24 to the second flash drum 26 has a temperature of 90°C and a pressure of 25 BarG.
[0029] The mixed phase feed 24 is passed through the second flash drum 26 to produce a second flash drum vapor product stream 34 and a second flash drum liquid phase product stream 36. The second flash drum liquid phase product stream 36 is passed through a letdown valve 37 to form feed stream 39 which is directed to a feed stream 44 of the distillation column 32. In this example, the feed stream 39 has a temperature of 76°C, a pressure of 11.8 BarG, and comprises about 35-40 mol percent ethylene, and 60-65 mol percent other olefins.
[0030] The second flash drum vapor product stream 34 is mixed with return stream 46 (described in greater detail below) before being provided as a vapor phase feed stream 48 to a membrane filter unit 38. In this example, the membrane filter unit 38 comprises a nanofiltration unit.
[0031] In this example, the vapor phase feed stream 48 to the nanofiltration unit 38 has a temperature of 89°C, a pressure of 25 BarG, and comprises about 75-80 mol percent ethylene, and 20-25 mol percent other olefins The vapor phase feed stream 48 is passed through the nanofiltration unit 38 to produce a vapor phase permeate product stream 40 and a mixed phase retentate product stream 42. In this example, the permeate product stream 40 has a temperature of 90°C, a pressure of 11 BarG, and comprises about 90-95 mol percent ethylene, and 5-10 mol percent other olefins . A higher purity ethylene (mol percent) permeate product stream 40 can be achieved by adding additional nanofiltration stages. Furthermore, it is expected that a higher purity ethylene (mol percent) permeate product stream 40 will be achievable with improvements in nanofiltration technology in the coming years .
[0032] In this example, the retentate product stream 42 is passed through a letdown valve 43 to produce a product stream 45 having a temperature of 90°C, a pressure of 25 BarG, and comprises about 15-20 mol percent ethylene, and 80-85 mol percent other olefins. The permeate product stream 40 therefore comprises a higher molar percentage of ethylene than the product stream 45.
[0033] The product stream 45 is mixed with the second flash drum mixed phase product stream 39 to produce the feed stream 44 to the distillation column 32. The distillation column 32 is operated to produce a bottoms product stream 50 which is removed by pump 52 as a liquid phase heavy olefin product stream 54. In this example, the heavy olefin product stream 54 has a temperature of 153°C, a pressure of 12 BarG, and comprises less than 0.001 mol percent ethylene, and about 99.9 mol percent other olefins . A bottoms stream 56 is removed from the distillation column 32 and returned to the distillation column 32 as stream 58 via heater 60.
[0034] Operation of the distillation column 32 produces an overhead vapor product stream 62 which is cooled by cooler 64 to produce a mixed phase cooled product stream 66. The mixed phase cooled product stream 66 is passed through an accumulator 68 to produce a liquid phase accumulator product stream 70 and a vapor phase accumulator product stream 72. The liquid phase accumulator product stream 70 is returned as reflux to the distillation column 32 by pump 74. In this example, the liquid phase accumulator product stream 70 returned to the distillation column 32 has a temperature of 43°C, a pressure of 11.8 BarG.
[0035] The vapor phase accumulator product stream 72 is passed through a knock out drum 76 to produce a knock out drum product stream 78 which is compressed by a compressor 80 to form vapor phase return stream 46 which is fed to the nanofiltration unit 38 together with second flash drum vapor product stream 34. Thus, a portion of the overhead vapor product stream 62 is provided to the vapor feed stream 48 of the nanofiltration unit 38. In this example, the vapor phase return stream 46 has a temperature of 87°C, a pressure of 25 BarG, and comprises about 55-60 mol percent ethylene, and about 40-45 mol percent other olefins .
[0036] The permeate product stream 40 is passed through a high pressure compressor knock out drum 82 to produce a product stream 84 which is compressed by a high pressure compressor 86 to provide a recovered ethylene product stream 88. Consequently at least a portion of the permeate product stream 40 is directed to the recovered ethylene product stream 88. Any bottom product stream 83 from the high pressure compressor knock out drum 82 is sent to a drain header (not shown) .
[0037] Figure 2 schematically illustrates a modified system 100 to the system 10 shown in Figure 1. The system 100 is similar in most respects to the system 10 described above. However, in system 100 the overhead vapor product stream produced by the distillation column 32is not directed to the feed stream of the nanofiltration unit 38.
[0038] In the system 100, the primary feed stream 12 comprising a liquid phase mixture of olefins and ethylene is provided to the heater 14 and passed through letdown valve 15 as before to produce a mixed phase heated primary feed stream 16.
[0039] The heated primary feed stream 16 is passed through the first flash drum 18 to produce the first flash drum vapor product stream 20 and the first flash drum liquid phase product stream 22 which is passed through letdown valve 23 to form the feed stream 30 of the distillation column 32.
[0040] The first flash drum vapor product stream 20 is cooled by cooler 28 and passed through letdown valve 25 to produce a mixed phase cooled first flash drum vapor product stream 24 which is passed through the second flash drum 26 to produce the second flash drum vapor product stream 34 and the second flash drum liquid phase product stream 36. The second flash drum liquid phase product stream 36 passed through letdown valve 37 to produce feed stream 39 which is directed to a feed stream 44 of the distillation column 32 .
[0041] The second flash drum vapor product stream 34 i s pas sed through the nanofiltration unit 38 to produce a vapor phase permeate product stream 40 and a liquid pha se retentate product stream 42 which i s pas sed through a letdown valve 43 to form a mixed phase retentate product stream 42 which is mixed with the second flash drum mixed phase product stream 39 to produce the feed stream 44 to the distillation column 32 .
[0042] The distillation column 32 is operated to produce a bottoms product stream 50 which is removed by pump 52 a s a liquid pha se heavy olefin product stream 54 .
[0043] A bottoms stream 56 is removed from the di stillation column 32 and returned to the distillation column 32 as stream 58 via heater 60 .
[0044] The overhead vapor product stream 62 produced by the distillation column 32 is cooled by cooler 64 to produce a mixed phase cooled product stream 66 . The mixed phase cooled product stream 66 is pas sed through an accumulator 68 to produce a vapor phase accumulator product stream 72 .
[0045] The vapor phase accumulator product stream 72 i s mixed with the permeate product stream 40 from the nanof iltration unit 38 to form ethylene product stream 178 . The ethylene product stream 178 is compres sed by a high pres sure compres sor 180 and a portion of the compres sed ethylene product stream 178 is recovered as recovered ethylene product stream 188 . The remaining portion of the compre s sed ethylene product stream 184 i s cooled by cooler 186 and expanded back to distillation column pressure before being provided back to the distillation column 32 to reduce the temperature and provide reflux.
[0046] In both of the systems 10, 100 described above, first 18 and second 26 flash drums are employed upstream of the membrane filter unit 38. In another example system (not shown) only the first flash drum 18 is used and the first flash drum vapor product stream 20 is not cooled by cooler 28 before entering the membrane filter unit 38.
[0047] It will be understood that the operational pressures, temperatures and feed compositions described above are examples only and that the systems 10, 100 may be designed to operate with different settings and feed compositions. The membrane filter unit 38 may operate at a temperature of about 135°C and a pressure of about 45 BarG. Depending on the particular application, higher temperature operation of the membrane filter unit 38 may be beneficial in dependence on the operating conditions of any downstream process or system. Similarly, where recompression is needed downstream of the membrane filter unit 38, a higher operating pressure of the membrane filter unit 38 is beneficial to reduce recompression power requirements .
[0048] While many possible variations of the system 10, 100 for recovering ethylene from a mixture of olefins have been described above, it will be clear to the skilled person that additional variations and modifications can be made without departing from the scope of the invention as claimed in the appended claims .
Claims
C L A I M S1. A method of recovering ethylene from a mixture of olefins, comprising: providing a feed stream comprising a vapor phase mixture of olefins to a membrane filter unit; passing the feed stream through the membrane filter unit to produce a permeate product stream and a retentate product stream, wherein the permeate product stream comprises a higher molar percentage of ethylene than the retentate product stream; directing a least a portion of the permeate product stream to a recovered ethylene product stream; and directing a least a portion of the retentate product stream to a feed stream of a distillation column.
2. A method as claimed in Claim 1, comprising: operating the distillation column to produce an overhead vapor product stream; and directing at least a portion of the overhead vapor product stream to the recovered ethylene product stream.
3. A method as claimed in Claim 1, comprising: operating the distillation column to produce an overhead vapor product stream; and directing at least a portion of the overhead vapor product stream to the feed stream of the membrane filter unit .
4. A method as claimed in any preceding Claim, comprising :providing a primary feed stream comprising a liquid phase, wherein the primary feed stream comprises a mixture of olefins; heating the primary feed stream; passing the heated primary feed stream through a first flash drum to produce a first flash drum vapor product stream and a first flash drum liquid phase product stream; directing at least a portion of the first flash drum vapor product stream into the feed stream of the membrane filter unit; and directing at least a portion of the first flash drum liquid phase product stream to a feed stream of the distillation column.
5. A method as claimed in Claim 4, comprising: cooling the first flash drum vapor product stream; passing the cooled first flash drum vapor product stream, through a second flash drum to produce a second flash drum vapor product stream and a second flash drum liquid phase product stream; directing at least a portion of the second flash drum vapor product stream into the feed stream of the membrane filter unit; and directing at least a portion of the second flash drum liquid phase product stream to a feed stream of the distillation column.
6. A method as claimed in Claim 5, comprising directing at least a portion of the second flash drum liquid phase product stream into the retentate product stream.
7. A method as claimed in any preceding Claim, wherein the membrane filter unit comprises a nanofiltration unit.8 . A method as claimed in any preceding Claim, wherein the feed stream to the membrane filtration unit comprises a mixture of ethylene and olefins .9 . A method as claimed in any preceding Claim, wherein the permeate product stream comprises a molar percentage of ethylene greater than or equal to 90% .10 . A method as claimed in any preceding Claim, wherein the retentate product stream compri ses a molar percentage of ethylene les s than or equal to 50% .11 . A system for recovering ethylene f rom a mixture of olefins , compri sing a membrane filtration unit and a distillation column , wherein the system i s conf igured so that permeate f rom the membrane filtration unit is directed to a recovered ethylene product stream, and retentate from the membrane filtration unit i s directed to a feed stream of the distillation column .