Method to produce LNG

• US 10,077,937 B2
• Filed: 04/14/2014
• Issued: 09/18/2018
• Est. Priority Date: 04/15/2013
• Status: Active Grant

• Current Assignee: 1304338 Alberta, Ltd., 1304342 Alberta, Ltd.
• Original Assignee: 1304338 Alberta, Ltd., 1304342 Alberta, Ltd.
• Inventors: Millar, MacKenzie, Lourenco, Jose
• Primary Examiner(s): Jules, Frantz
• Assistant Examiner(s): King, Brian
• Application Number:
  US14/784,931
• Publication Number:
  US 20160061519A1
• Time in Patent Office: 1,618 Days
• CPC Class Codes: C10L 3/106 of water
• CPC Class Codes: F25J 1/0022 Hydrocarbons, e.g. natural gas
• CPC Class Codes: F25J 1/02 requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
• 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 2200/74 Refluxing the column with at least a part of the partially condensed overhead gas
• CPC Class Codes: F25J 2200/76 Refluxing the column with condensed overhead gas being cycled in a quasi-closed loop refrigeration cycle
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2215/04 Recovery of liquid products
• CPC Class Codes: F25J 2220/66 Separating acid gases, e.g. CO2, SO2, H2S or RSH
• 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 2270/04 Internal refrigeration with work-producing gas expansion loop
• CPC Class Codes: F25J 2290/60 Details about pipelines, i.e. network, for feed or product distribution
• 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

Abstract

A method to produce LNG at straddle plants. In contrast to known methods, there is provided a slipstream of a high pressure, pre-treated, pre-cooled natural gas stream to a straddle LNG plant section. The slipstream is further cooled, and processed in a high pressure column to a methane content of 85% or 85 plus by mole. The processed stream is further treated to remove carbon dioxide. The de-carbonated high pressure stream is further cooled in a heat exchanger by a counter-current vapor fraction of the expanded gas before entering an expander apparatus. The processed, treated and cooled gas is expanded into a separator. The produced LNG fraction is pumped to storage. A portion of the LNG fraction is used as a reflux stream to the high pressure column. The cold vapor fraction from the separator flows through counter-current heat exchangers, giving up its coolth energy before being re-compressed into the high pressure transmission gas pipeline.

Claims

1. A method to produce liquid natural gas (LNG) using an LNG plant feed stream, the LNG plant feed stream comprising a high pressure, pre-treated, pre-cooled natural gas stream from a straddle plant, the method comprising:

cooling the LNG plant feed stream in at least one heat exchanger;

removing natural gas liquids (NGLs) from the cooled LNG plant feed stream using a gas column downstream of the at least one heat exchanger;

removing carbon dioxide gas from a vapor stream of the gas column using a gas treatment unit downstream of the gas column;

cooling the vapor stream using one or more heat exchangers downstream of the gas treatment unit to produce a cooled de-carbonated stream;

expanding the cooled de-carbonated stream in a gas expansion apparatus;

providing an LNG separator downstream of the gas expansion apparatus to separate the expanded cooled de-carbonated stream into an LNG stream and a natural gas stream; and

providing a high pressure LNG pump for pumping a portion of the LNG stream to the gas column of the straddle plant as a reflux stream.

2. The method of claim 1, wherein the gas expansion apparatus is a JT valve or a gas expander turbine.

3. The method of claim 1, wherein cooling in one or more heat exchangers is provided by the natural gas stream from the LNG separator.

4. The method of claim 1, wherein the reflux stream to the gas column controls a methane concentration of an overhead stream of the gas column.

5. The method of claim 1, wherein the gas column produces a bottoms stream that is routed to the gas column as a gas column reflux and for NGL recovery.

6. The method of claim 1, wherein the gas treatment unit removes the carbon dioxide gas in an overhead stream of the gas column to a concentration of less than 50 ppm.

7. The method of claim 1, wherein the vapor stream downstream of the gas treatment unit is cooled by the natural gas stream from the LNG separator before entering the gas expansion apparatus.

8. The method of claim 2, wherein the gas column produces a bottoms stream that is a mixture of NGLs from the LNG plant feed stream, carbon dioxide and methane.

9. The method of claim 1, further comprising a straddle plant feed gas stream that feeds a distillation column, and wherein the LNG plant feed stream is obtained by diverting a portion of the straddle plant feed gas stream upstream of the distillation column.

10. The method of claim 9, wherein the NGLs from the gas column are expanded and injected into the distillation column as a distillation column reflux stream.