Two-stage rectification for the separation of hydrocarbons

• US 4,600,421 A
• Filed: 04/18/1985
• Issued: 07/15/1986
• Est. Priority Date: 04/18/1984
• Status: Expired due to Fees

• Current Assignee: Linde AG (Linde Plc)
• Original Assignee: Linde AG (Linde Plc)
• Inventors: Kummann, Paul
• Primary Examiner(s): Sever, Frank
• Application Number:
  US06/724,729
• Time in Patent Office: 453 Days
• Field of Search: 62/23, 62/24, 62/25, 62/27, 62/28, 62/29, 62/31, 62/32, 62/33, 62/34, 62/36, 62/38, 62/39, 62/42, 62/44
• US Class Current: 62/621
• CPC Class Codes: C07C 7/04 by distillation
• CPC Class Codes: F25J 2200/04 in a dual pressure main column system
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• CPC Class Codes: F25J 2200/74 Refluxing the column with at least a part of the partially condensed overhead gas
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2210/04 Mixing or blending of fluids with the feed stream
• CPC Class Codes: F25J 2210/12 Refinery or petrochemical off-gas
• CPC Class Codes: F25J 2230/60 the fluid being hydrocarbons or a mixture of hydrocarbons
• CPC Class Codes: F25J 2240/02 Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
• CPC Class Codes: F25J 2245/02 Recycle of a stream in general, e.g. a by-pass stream
• CPC Class Codes: F25J 2270/90 External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0219 Refinery gas, cracking gas, coke oven gas, gaseous mixtures containing aliphatic unsaturated CnHm or gaseous mixtures of undefined nature
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0238 separation of CnHm with 2 carbon atoms or more
• CPC Class Codes: F25J 3/0242 separation of CnHm with 3 carbon atoms or more
• CPC Class Codes: Y02P 20/10 Process efficiency

Abstract

To separate C₂₊ or C₃₊ hydrocarbons from hydrocarbon-containing gaseous mixtures by fractional condensation and rectification, the rectification is conducted in two columns, the second column being at a pressure at least 5 bar lower than the first. One of the condensates is subcooled before being passed, for purposes of peak cooling, to the upper zone of the high pressure column, substantial energy savings are realized.

Claims

1. In a process for the separation of C₂₊ or C₃₊ hydrocarbons from a hydrocarbon-containing raw gas under pressure, by recitification wherein the raw gas is cooled and expanded and the resultant condensates are introduced into a rectification column wherein the C₂₊ or C₃₊ hydrocarbons, respectively, are separated from the lower-boiling components, the improvement which comprises feeding the condensate fractions produced during cooling and expansion of the raw gas into a first rectifying column operated under a higher pressure to yield a bottoms fraction and an overhead product, the latter containing components boiling lower than the C₂₊ or C₃₊ hydrocarbons, expanding the bottoms fraction and then feeding resultant expanded bottoms fraction into a second rectifying column operating under a lower pressure than the first column, to yield C₂₊ or C₃₊ hydrocarbons bottoms product and an overhead fraction, compressing the latter overhead fraction and recycling resultant compressed overhead fraction into the first rectifying column.

2. A process according to claim 1, wherein the overhead fraction of the second rectifying column is heated prior to being compressed, and is recooled prior to being recycled into the first rectifying column.

3. A process according to claim 2, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

4. A process according to claim 1, wherein overhead fraction of the second rectifying column after being compressed, is passed into the raw gas to be cooled.

5. A process according to claim 4, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

6. A process according to claim 1, wherein the overhead fraction of the second rectifying column, after being compressed, is cooled and then passed into cooled raw gas from which condensate has been separated.

7. A process according to claim 6, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

8. A process according to claim 1, wherein components boiling below C₂₊ or C₃₊ are discharged from the process under pressure and wherein the first rectifying column is operated at a pressure above the discharge pressure.

9. A process according to claim 8, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

10. A process according to claim 1, wherein the first rectifying column is operated under a pressure of between 20 and 40 bar.

11. A process according to claim 10, wherein the second rectifying column is operated under a pressure of between 8 and 25 bar, with the operating pressure of the first rectifying column being at least 5 bar above the operating pressure of the second recitfying column.

12. A process according to claim 11, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

13. A process according to claim 10, wherein the second rectifying column is operated under a pressure of between 10 and 18 bar, with the operating pressure of the first rectifying column being at least 5 bar above the operating pressure of the second rectifying column.

14. A process according to claim 13, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

15. A process according to claim 10, wherein the second rectifying column is operated under a pressure of between 10 and 18 bar, with the operating pressure of the first rectifying column being at least 5 bar above the operating pressure of the second rectifying column.

16. A process according to claim 15, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

17. A process according to claim 10, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

18. A process according to claim 1, wherein the second rectifying column is operated under a pressure of between 8 and 25 bar, with the operating pressure of the first rectifying column being at least 5 bar above the operating pressure of the second rectifying column.

19. A process according to claim 18, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

20. A process according to claim 1, said raw gas being subjected to multistage cooling comprising a first cooling stage, separating a first condensate, introducing said first condensate to the first rectifying column, cooling the uncondensed proportion in a second cooling stage, separating a second condensate, subcooling resultant second condensate, feeding the subcooled second condensate into the upper zone of the first rectifying column, engine-expanding uncondensed gas from said second cooling stage, and passing resultant cooled engine-expanded gas into a zone of the first rectifying column below the feed point of the subcooled second condensate.

21. A process according to claim 1, wherein C₂₊ hydrocarbons are being separated.

22. A process according to claim 1 wherein the temperature levels at the top and bottom of the second rectifying column are between 230°

and 300°

K.