Method to Produce LNG

• US 20160061519A1
• Filed: 04/14/2014
• Published: 03/03/2016
• 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
• Granted Patent: US 10,077,937 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• 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 vapour 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 vapour 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), comprising:

passing a dewatered natural gas stream at pressures of between 700 psig and 1200 psig through one or more heat exchangers to pre-cool the natural gas stream;

passing the pre-cooled natural gas stream through a gas column where natural gas liquid fractions and natural gas fractions are separated;

passing the natural gas fractions at pressures of between 700 psig and 1200 psig through a gas treatment unit to remove carbon dioxide gas;

passing the de-carbonized natural gas fractions at pressures of between 700 psig and 1200 psig through one or more heat exchangers to pre-cool the de-carbonized natural gas fractions;

passing the de-carbonized natural gas fractions at pressures of between 700 psig and 1200 psig through a gas expansion apparatus where pressure of the de-carbonized natural gas fractions is lowered to a pressure of less than 100 psig; and

passing the de-carbonized natural gas fractions at a pressure of less than 100 psig through a separator where the de-carbonized natural gas fractions are separated into an LNG stream and a natural gas stream at a pressure of less than 100 psig.

2. The method of claim 1, wherein a high pressure LNG pump is used to divert a reflux stream from the LNG stream to the gas column.

3. A method to produce LNG where a high pressure, pre-treated, pre-cooled natural gas stream from a straddle plant is routed to a gas expansion apparatus, the LNG section comprising:

providing heat exchangers on a high pressure, pre-treated, pre-cooled natural gas LNG feed tine to a gas expansion apparatus;

providing a gas column downstream of the heat exchanger, on the LNG plant feed line to the a gas expansion apparatus;

providing a gas treatment unit downstream of the column, on the LNG plant section feed line to a gas expansion apparatus;

providing a heat exchanger downstream of a gas treatment unit on the LNG plant section feed line to a gas expansion apparatus;

providing a natural gas liquid separator downstream of the gas expansion apparatus; and

providing a high pressure LNG pump for a reflux stream to the column.

4. The method of claim 3, wherein the gas expansion apparatus can be a JT valve or a gas expander turbine.

5. The method of claim 3, wherein further cooling is provide by the vapour fraction of the expanded gas.

6. The method of claim 3, wherein the reflux stream to the column controls the methane concentration of the overhead stream.

7. The method of claim 3, wherein the column bottoms are routed to the distillation column as distillation column reflux and for NGL′

recovery.

8. The method of claim 3, wherein a gas treatment unit removes the carbon dioxide content in the column overhead to a concentration less than 50 ppm.

9. The method of claim 3, wherein a de-carbonated high pressure gas stream is further cooled by the vapour fraction of the expanded gas before entering the expander apparatus.

10. The method of claim 9, wherein the gas to be liquefied is liquid natural gas (LNG).

11. The method of claim 3, wherein the reflux stream is LNG.

12. The method of claim 4, wherein the bottoms of the column are a mixture of NGL'"'"'s from the LNG plant section feed line, carbon dioxide and methane.

14. The liquefied gas production plant of claim 11, including a feed line feeding a LNG pump that transports LNG to storage.

15. The liquefied gas production plant of claim 11, wherein a slipstream of LNG is routed to a high pressure pump for use as reflux on the high pressure column. This reflux stream controls the methane content on the high pressure column overhead stream and hence on the LNG product.

13. The straddle plant LNG section, comprising:

an heat exchanger to farther cool the pre-treated, pre-cooled high pressure feed gas;

a high pressure gas column gas to control the methane content on its overhead stream;

a high pressure LNG pump to feed a reflux stream to the high pressure column;

a carbon dioxide gas treatment unit;

an heat exchanger to further cool the de-carbonated gas stream;

a feed line feeding gas to the gas expansion apparatus;

a gas expansion apparatus;

a LNG receiver to separate the vapour and liquid fractions; and

a compressor to re-compress the vapour fraction.