Low pressure olefin recovery process

• US 7,429,686 B2
• Filed: 07/20/2007
• Issued: 09/30/2008
• Est. Priority Date: 07/02/2004
• Status: Active Grant

• Current Assignee: Kellogg Brown & Root LLC (KBR Incorporated)
• Original Assignee: Kellogg Brown & Root LLC (KBR Incorporated)
• Inventors: Verma, Vijender K., Hu, Jichuan
• Primary Examiner(s): Griffin; Walter D.
• Assistant Examiner(s): CAMPANELL, FRANCIS C
• Application Number:
  US11/780,682
• Publication Number:
  US 20070260103A1
• Time in Patent Office: 438 Days
• Field of Search: 585/809, 585/802, 585/800, 585/810, 208/24, 208/37
• US Class Current: 585/809
• CPC Class Codes: C07C 11/04 Ethylene
• CPC Class Codes: C07C 11/06 Propene
• CPC Class Codes: C07C 7/005 Processes comprising at least two steps in series
• CPC Class Codes: C07C 7/04 by distillation
• CPC Class Codes: C10G 70/043 by fractional condensation
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2205/50 using absorption, i.e. with selective solvents or lean oil, heavier CnHm and including generally a regeneration step for the solvent or lean oil
• 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 2215/62 Ethane or ethylene
• CPC Class Codes: F25J 2230/30 Compression 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 2270/02 Internal refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/08 Internal refrigeration by flash gas recovery loop
• 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 3/0219 Refinery gas, cracking gas, coke oven gas, gaseous mixtures containing aliphatic unsaturated CnHm or gaseous mixtures of undefined nature
• CPC Class Codes: F25J 3/0238 separation of CnHm with 2 carbon atoms or more
• CPC Class Codes: F25J 3/0252 separation of hydrogen production of hydrogen containing gas in general C01B3/00, e.g. separation of hydrogen or hydrogen containing gases form gaseous mixtures at low temperatures C01B3/506
• CPC Class Codes: Y02P 30/40 Ethylene production

Abstract

A low-pressure olefins recovery process and plant are described. The feed gas is compressed and distilled at a primary distillation pressure. The overhead stream is chilled at a pressure less than 30 kg/cm2 (430 psia) to partially condense the overheads. The primary distillation tower is refluxed with at least a portion of the condensate. The overhead vapor is further chilled and partially condensed and the condensate is fed to a demethanizer. The remaining vapor is cooled in a cold section and the resultant liquid is phase-separated and expanded to provide refrigeration for the cold section. The expanded vapor from the cold section is recycled to the process gas compressor. The bottoms streams from the primary distillation zone and the demethanizer are fractionated into respective streams consisting essentially of ethylene, ethane, propylene, propane, C4'"'"'s, and C5+.

Claims

1. A process for recovering olefins from a feed stream, comprising:

supplying the feed stream at a primary distillation pressure, including if required compressing the feed stream in a primary compression stage;

distilling the feed stream at the primary distillation pressure in a primary distillation zone to obtain a primary overhead vapor stream enriched in ethylene and one or more ethylene-lean bottoms streams;

chilling the primary overhead vapor stream at a pressure less than 24.5 kg/cm² in a first cooling stage to recover a first partial condensate stream and a first-stage vapor effluent;

refluxing the primary distillation zone with at least a portion of the first partial condensate stream;

further chilling the first-stage vapor effluent to recover at least a second partial condensate stream and a second-stage vapor effluent;

feeding the at least second partial condensate stream and any remaining portion of the first partial condensate stream to a demethanizer to recover a methane-rich overhead stream and a bottoms stream essentially free of methane and lighter components;

fractionating the bottoms streams from the primary distillation zone and the demethanizer into respective streams consisting essentially of hydrocarbons selected from the group consisting of ethylene, ethane, propylene, propane, C4'"'"'s, C5+ and combinations thereof;

further chilling the second-stage vapor effluent in a cold section and phase-separating the resulting mixed vapor-liquid stream in one or more stages to obtain additional condensate and a vapor tail gas stream essentially free of ethylene, wherein the additional condensate is vaporized to a relatively lower pressure to provide refrigeration for the condensation and to form a low pressure recycle vapor stream.

2. The process of claim 1 further comprising passing the feed stream in contact with a catalyst, adsorbent or combination thereof to remove at least one contaminant selected from the group consisting of acetylene, arsenic, mercury, carbonyl sulfide, nitrogen oxides, oxygen, and combinations thereof.

3. The process of claim 1 further comprising passing the primary overhead vapor stream in contact with a catalyst, adsorbent or combination thereof to remove at least one contaminant selected from the group consisting of acetylene, arsenic, mercury, carbonyl sulfide, nitrogen oxides, oxygen, and combinations thereof.

4. The process of claim 1 further comprising compressing the primary overhead vapor stream in a secondary compression stage to a discharge pressure effective to provide reflux for the primary distillation tower.

5. The process of claim 4 wherein the primary distillation pressure is from 7 to 21 kg/cm² and the discharge pressure from the secondary compression stage is greater than the primary distillation pressure and less than 28 kg/cm² (400 psia).

6. The process of claim 5 wherein the discharge pressure from the secondary compression stage is from 3.5 to 7 kg/cm² greater than the primary distillation pressure.

7. The process of claim 5 wherein the primary distillation pressure is from 7 to 11 kg/cm² and the discharge pressure from the secondary compression stage is from 10.5 to 17.5 kg/cm².

8. The process of claim 4, further comprising recycling the overhead stream from the demethanizer into the primary overhead vapor stream upstream from the secondary compression stage.

9. The process of claim 8, wherein the demethanizer consists essentially of an unrefluxed stripper column.

10. The process of claim 1, wherein the primary distillation zone comprises a depropanizer and the process further comprises fractionating the bottoms stream from the depropanizer in a debutanizer to obtain respective streams consisting essentially of C4'"'"'s and C5'"'"'s and heavier hydrocarbons, and fractionating the bottoms stream from the demethanizer in a deethanizer, a C2 splitter and a C3 splitter to obtain respective streams consisting essentially of ethylene, ethane, propylene and propane.

11. The process of claim 10, comprising exporting a portion of an overhead stream from the deethanizer.

12. The process of claim 10, comprising exporting a portion of the first-stage vapor effluent.

13. The process of claim 10, wherein the deethanizer is refluxed with a side draw from the C2 splitter.