Hydrocarbon gas processing

• US 6,516,631 B1
• Filed: 08/10/2001
• Issued: 02/11/2003
• Est. Priority Date: 08/10/2001
• Status: Expired due to Term

• Current Assignee: Mark A. Trebble
• Original Assignee: Mark A. Trebble
• Inventors: Trebble, Mark A.
• Primary Examiner(s): CAPOSSELA, RONALD C
• Application Number:
  US09/682,268
• Publication Number:
  US 20030029190A1
• Time in Patent Office: 550 Days
• Field of Search: 626/30, 626/31
• US Class Current: 62/630
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• 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 2200/72 Refluxing the column with at least a part of the totally condensed overhead gas
• 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 2215/62 Ethane or ethylene
• 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 3/0209 Natural gas or substitute natural gas
• 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: Y02C 20/40 of CO2

Abstract

A cryogenic natural gas liquids recovery process which includes the use of a demethanizer and a deethanizer includes a step of recycling a portion of the deethanizer overhead to the demethanizer.

Claims

1. In a cryogenic natural gas liquid extraction process wherein a gas stream comprising methane, carbon dioxide, C₂ components and heavier hydrocarbons is first separated in a demethanizer into a volatile fraction comprising primarily methane and a relatively less volatile fraction comprising C₂, carbon dioxide and heavier hydrocarbon components;

and wherein the less volatile fraction is then separated in a deethanizer into a fraction comprising primarily C₂ and which may comprise carbon dioxide and a fraction comprising heavier hydrocarbon components, the improvement comprising the step of removing a portion of the C₂ fraction as a vapour and recycling the portion back to the demethanizer or to any point upstream of the demethanizer, or otherwise using the portion in a plant implementing this process.

3. The improvement of claim 1 or claim 2 wherein the process further comprises at least one side reboiling step and the recycled portion of the deethanizer overhead fraction is returned as a vapour to the demethanizer above a reboiler reentry point.

4. The improvement of claim 3 wherein the recycled portion of the deethanizer overhead fraction is returned as a vapour to the demethanizer below the upper feed point.

5. The improvement of claim 4 wherein the recycled portion of the deethanizer overhead fraction is returned as a vapour to the demethanizer at a point immediately below the upper feed point.

6. The improvement of claim 3 wherein the recycled portion of the deethanizer overhead fraction is compressed to a higher pressure and treated in one or more heat exchange steps and expanded to the same pressure as the demethanizer tower and returned to the demethanizer tower.

7. The improvement of claim 3 wherein the recycled portion of the deethanizer overhead fraction is returned as a vapour to the demethanizer tower by combining the recycled portion of the deethanizer overhead fraction with the said second feed stream.

8. The improvement of claim 3 wherein the proportion of the deethanizer overhead fraction which is recycled to the demethanizer is varied by varying the amount of cooling performed in a deetanizer overhead condenser.

9. The improvement according to claim 1 or 2 wherein the recycled portion of the C₂ product stream is not recycled but is otherwise utilized in the process.

2. In a cryogenic natural gas liquid extraction process wherein a gas stream comprising methane, carbon dioxide, C₂ components and heavier hydrocarbons is separated into a volatile fraction comprising primarily methane and a relatively less volatile fraction comprising C₂, carbon dioxide and heavier hydrocarbon components, in which process:

(a) said gas stream is treated in one or more heat exchange steps and at least one separation step to produce at least a first feed stream that has been cooled under pressure to condense substantially all of it, and at least a second feed stream that has been cooled under pressure;

(b) said substantially condensed first feed stream is supplied to a demethanizer at a mid-column feed point;

(c) said cooled second feed stream is divided into a third feed stream which is expanded and at least partially condensed at temperature colder than the first feed stream, and thereafter supplied to said demethanizer at an upper feed point, and a fourth feed stream which is substantially condensed and provided to the demethanizer as a top reflux;

(d) said first, third and fourth feed streams are fractionated in said demethanizer at said lower pressure to produce the volatile fraction from the top of the demethanizer and the relatively less volatile fraction comprising of primarily C₂, carbon dioxide and heavier hydrocarbons which is condensed and recovered from the bottom of the demethanizer;

(e) the relatively less volatile fraction is fractionated in a deethanizer to separate the relatively less volatile fraction into a fraction comprising primarily of C₃ and heavier hydrocarbons and a fraction comprising primarily of C₂ and which may also comprise carbon dioxide;

the improvement comprising the step of recycling a portion of the deethanizer overhead fraction to the demethanizer, or to any point upstream of the demethanizer.