System and Method for Separating Wide Variations in Methane and Nitrogen

• US 20170023294A1
• Filed: 07/22/2015
• Published: 01/26/2017
• Est. Priority Date: 07/22/2015
• Status: Active Grant

• Current Assignee: BCCK Holding Co.
• Original Assignee: Butts Properties Ltd.
• Inventors: Butts, Rayburn C.
• Granted Patent: US 9,816,752 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: F25J 2200/04 in a dual pressure main column system
• CPC Class Codes: F25J 2200/08 in a triple 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 2230/32 Compression of the product stream
• CPC Class Codes: F25J 2235/60 the fluid being (a mixture of) hydrocarbons
• CPC Class Codes: F25J 2270/02 Internal refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2280/02 Control in general, load changes, different modes ("runs"), measurements
• CPC Class Codes: F25J 2290/50 Arrangement of multiple equipments fulfilling the same process step in parallel
• 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 3/0257 separation of nitrogen from air F25J3/04, production of nitrogen in general C01B21/00
• CPC Class Codes: F25J 3/029 of helium

Abstract

A system and method for removing nitrogen and producing a high pressure methane product stream from natural gas feed streams having wide variations in nitrogen and methane content are disclosed. Optional add-on systems may be incorporated into the nitrogen and methane separation to produce an NGL sales stream to reduce excess hydrocarbons in the nitrogen vent stream, or to recover helium. The system and method of the invention are particularly suitable for use with feed streams in excess of 50 MMSCFD and up to 300 MMSCFD and containing up to 100 ppm carbon dioxide. Typical power requirements for compressing the methane product stream to produce a suitably high pressure stream for sale are reduced according to the systems and methods of the invention.

Claims

1. A system for removing nitrogen and for producing a high pressure methane product stream from a first feed stream comprising nitrogen, methane, and other components, the system comprising:

a first splitter wherein the first feed stream is divided into a second feed stream and a third feed stream;

a first fractionating column wherein the second feed stream and the third feed stream are separated into a first overhead stream and a first bottoms stream;

a second fractionating column comprising a condenser and a reboiler, wherein first overhead stream is separated into a second overhead stream and a second bottoms stream;

a first heat exchanger for cooling the first feed stream upstream of the first splitter and for cooling the second feed stream upstream of the first fractionating column through heat exchange with the first bottoms stream and the first overhead stream;

an external reboiler for cooling the third feed stream upstream of the first fractionating column through heat exchange with the first bottoms stream;

wherein the methane product stream comprises the first bottoms stream and second bottoms stream;

wherein the second overhead stream is a nitrogen vent stream; and

wherein a duty for the condenser and a duty for the reboiler for the second fractionating column are independent of each other.

2. The system of claim 1 wherein the third feed stream is selectively fed into the first fractionating column at a tray location below a tray location for feeding the second feed stream into the first fractionating column.

3. The system of claim 2 further comprising a second splitter for dividing the first overhead stream into a fourth feed stream and a fifth feed stream upstream of the second fractionating column and wherein the fifth feed stream feeds into the second fractionating column at a tray below a tray location for feeding the fourth feed stream into the second fractionating column.

4. The system of claim 3 further comprising a second heat exchanger for cooling the first overhead stream upstream of the second splitter and for cooling the fourth feed stream upstream of the second fractionating column through heat exchange with the second overhead stream and second bottoms stream.

5. The system of claim 4 further comprising a third splitter for dividing the first bottoms stream prior to the first heat exchanger into a first part and a second part;

a pump for pumping the second part to increase a pressure of the second part; and

wherein the first and second parts pass through the first heat exchanger.

6. The system of claim 5 further comprising a fourth splitter for dividing the second bottoms stream into at least two portions, wherein one of those portions is a refrigerant stream for the condenser in the second fractionating column prior to passing through the second heat exchanger.

7. The system of claim 6 further comprising a subcooler, wherein the refrigerant stream cycles through the subcooler prior to and after heat exchange in the condenser.

8. The system of claim 1 further comprising a second splitter to divide the first feed stream into first and second portions upstream of the first heat exchanger;

a third fractionating column comprising a reboiler wherein an NGL feed stream is separated into a third overhead stream and a third bottoms stream;

wherein the first splitter comprises a phase separator wherein the first and second portions of the first feed stream are divided into the second feed stream, third feed stream, and the NGL feed stream;

wherein the first portion of the first feed stream provides heat to the reboiler of the third fractionating column prior to the separator;

wherein the first heat exchanger cools the second portion of the first feed stream upstream of the first splitter;

wherein the third bottoms stream is an NGL sales stream; and

wherein the methane product stream further comprises the third overhead stream.

9. The system of claim 6 further comprising a third fractionating column wherein the second overhead stream is separated into a third overhead stream and a third bottoms stream;

a mixer for combining the third bottoms stream with the refrigerant stream prior to passing through the condenser; and

wherein the third overhead stream is warmed in the second heat exchanger prior to venting as the nitrogen vent stream.

10. A method for removing nitrogen and for producing a high pressure methane product stream, the method comprising the steps of:

providing a first feed stream comprising nitrogen and methane;

dividing the first feed stream into a second feed stream and a third feed stream in a first splitter;

feeding the second feed stream and third feed stream into a first fractionating column;

separating the second feed stream and the third feed stream in the first fractionating column into a first overhead stream and a first bottoms stream;

feeding the first overhead stream into a second fractionating column comprising a condenser and a reboiler;

separating the first overhead stream in the second fractionating column into a second overhead stream and a second bottoms stream;

cooling the first feed stream upstream of the first splitter and cooling the second feed stream upstream of the first fractionating column in a first heat exchanger through heat exchange with the first, bottoms stream and the first overhead stream;

cooling the third feed stream upstream of the first fractionating column through heat exchange with the first bottoms stream in an external reboiler;

wherein the methane product stream comprises the first bottoms stream and second bottoms stream;

wherein the second overhead stream is a nitrogen vent stream; and

wherein a duty for the condenser and a duty for the reboiler for the second fractionating column are independent of each other.

11. The method of claim 10 further comprising selectively feeding the third feed stream into the first fractionating column at a tray location below a tray location for feeding the second feed stream into the first fractionating column.

12. The method of claim 11 further comprising dividing the first overhead stream into a fourth feed stream and a fifth feed stream in a second splitter upstream of the second fractionating column;

and

selectively feeding the fifth feed stream into the second fractionating column at a tray below a tray location for feeding the fourth feed stream into the second fractionating column.

13. The system of claim 12 further, comprising cooling the first overhead stream upstream of the second splitter and for cooling the fourth feed stream upstream of the second fractionating column in a second heat exchanger through heat exchange with the second overhead stream and second bottoms stream.

14. The system of claim 13 further comprising dividing the first bottoms stream into a first part and a second part prior to the cooling step in the first heat exchanger;

and pumping the second part to increase a pressure of the second part.

15. The system of claim 14 further comprising dividing the second bottoms stream into at least a first portions and a refrigerant stream and passing the refrigerant stream through the condenser prior to the cooling step in the second heat exchanger.

16. The system of claim 15 further comprising cycling the refrigerant stream through a subcooler prior to and after passing through the condenser.

17. The system of claim 10 further comprising dividing the first feed stream into first and second portions in a second splitter upstream of the first heat exchanger and wherein the first splitter comprises a phase separator wherein the first and second portions of the first feed stream are divided into the second feed stream, third feed stream, and an NGL feed stream;

feeding the NGL feed stream into a third fractionating column comprising a reboiler;

separating the NGL feed stream in the third fractionating column into a third overhead stream and a third bottoms stream;

passing the first portion of the first feed stream through the reboiler of the third fractionating column to provide heat to the reboiler prior to the separator;

wherein the first heat exchanger cools the second portion of the first feed stream upstream of the first splitter;

wherein the third bottoms stream is an NGL sales stream; and

wherein the methane product stream further comprises the third overhead stream.

18. The system of claim 15 further comprising

feeding the second overhead stream into a third fractionating column;

separating the second overhead stream in the third fractionating column into a third overhead stream and a third bottoms stream;

combining the third bottoms stream with the refrigerant stream prior to passing through the condenser; and

warming the third overhead stream in the second heat exchanger prior to venting as the nitrogen vent stream.