Air separation method and apparatus

Patent metadata

Field	Value
Publication number	US-9518778-B2
Application number	US-201213726679-A
Country	US
Kind code	B2
Filing date	Dec 26, 2012
Priority date	Dec 26, 2012
Publication date	Dec 13, 2016
Grant date	Dec 13, 2016

Abstract

A method and apparatus for separating air in which production of the liquid products can be selectively varied between high and low production rates by varying the pressure ratio across a turboexpander used in imparting refrigeration with the use of a branched flow path. The branched flow path has a system of valves to selectively and gradually introduce a compressed refrigerant air stream into either a booster compressor branch having a booster compressor to increase the pressure ratio during high modes of liquid production or a bypass branch that bypasses the booster compressor to decrease the pressure ratio during low modes of liquid production. A recycle branch is connected to the booster compressor branch to allow compressed air to be independently recycled from the outlet to the inlet of the booster compressor during turndown from the high to the low liquid mode of liquid production to prevent surge.

First claim

We claim:

1. A method of separating air in an air separation plant comprising:
rectifying compressed, purified and cooled air in a distillation column system of the air separation plant that is configured to produce at least one liquid product and imparting refrigeration into the air separation plant with the use of a constant speed turboexpander not directly coupled to a single compressor of the air separation plant on a common pinion, the refrigeration imparted by forming a compressed refrigerant air stream within the air separation plant, expanding the compressed refrigerant air stream in the constant speed turboexpander to produce an exhaust stream, and introducing the exhaust stream into the distillation column system of the air separation plant;
varying production of the at least one liquid product by selectively introducing the compressed refrigerant air stream into either a booster compressor branch of a branched flow path having a constant speed booster compressor to further compress the compressed refrigerant air stream and thereby to obtain a higher pressure ratio across the constant speed turboexpander and a higher rate of the liquid product production or a bypass branch of the branched flow path, bypassing the constant speed booster compressor, thereby to obtain a lower pressure ratio across the constant speed turboexpander and a lower rate of the liquid product production;
the compressed refrigerant air stream introduced into the booster compressor branch by gradually diverting the compressed refrigerant air stream from the bypass branch to the booster compressor branch, activating the constant speed booster compressor and circulating a recycle stream flowing within a recycle branch of the branched flow path from an outlet of the constant speed booster compressor to an inlet of the constant speed booster compressor until booster compressor branch pressure at the outlet of the constant speed booster compressor exceeds bypass pressure within the bypass branch whereupon flow of both the recycle stream and the compressed refrigerant air stream within the bypass branch is suspended; and
the compressed refrigerant air stream introduced into the bypass branch by gradually diverting the compressed refrigerant air stream from the booster compressor branch to the bypass branch while circulating the recycle stream in the recycle branch until the bypass pressure exceeds the booster compressor branch pressure whereupon the constant speed booster compressor is deactivated and set in a low pressure mode of operation;
wherein the constant speed turboexpander maintains a specific pressure ratio and a high efficiency during both high liquid production and low liquid production but not at peak efficiency during either high liquid production or low liquid production.

2. The method of claim 1 , wherein:
the compressed refrigerant air stream is partially cooled in a main heat exchanger used in cooling the air; and
the branched flow path is connected to a warm end of the main heat exchanger.

3. The method of claim 1 , wherein when the constant speed booster compressor is deactivated, passing a purge air stream, composed of purified air, through the constant speed booster compressor to prevent ambient air from entering the constant speed booster compressor.

4. The method of claim 1 , wherein:
a liquid stream is removed from the distillation column system and divided into a first subsidiary liquid stream and a second subsidiary liquid stream;
the at least one liquid product comprises the first subsidiary liquid stream;
the second subsidiary liquid stream is heated within the main heat exchanger to form a heated product stream; and
during the decreasing of production of the at least one liquid product, air flow rate of the air supplied to the air separation plant is decreased to maintain product flow rate of the heated product stream constant.

5. The method of claim 4 , wherein:
the distillation column system comprises a higher pressure column and a lower pressure column operating at a lower pressure than the higher pressure column, configured to further refine a crude liquid oxygen column bottoms produced in the higher pressure column and connected to the higher pressure column in a heat transfer relationship so that a nitrogen-rich vapor column overhead produced in the higher pressure column is condensed through indirect heat exchange with an oxygen-rich liquid produced in the lower pressure column, thereby providing liquid nitrogen reflux to the higher pressure column and the lower pressure column;
the liquid stream is an oxygen-rich liquid stream composed of an oxygen-rich liquid column bottoms produced in the lower pressure column;
the oxygen-rich liquid stream is divided into the first subsidiary liquid stream and the second subsidiary liquid stream;
the second subsidiary liquid stream is pumped to produce a pressurized liquid product stream and warmed within the main heat exchanger to produce the heated product stream;
a further compressed air stream is formed within the air separation plant;
the further compressed air stream is liquefied in the main heat exchanger through indirect heat exchange with the pressurized liquid product stream, thereby to produce a liquid air stream; and
at least part of the liquid air stream is reduced in pressure and introduced into at least the lower pressure column.

6. The method of claim 5 , wherein the exhaust stream is introduced into the higher pressure column.

7. The method of claim 5 , wherein:
at least part of the air, after having been compressed and purified is divided into first and second subsidiary steams;
the first subsidiary stream is further compressed to form the compressed refrigerant air stream; and
the second subsidiary stream is further compressed to form the further compressed air stream.

8. The method of claim 5 , wherein:
at least part of the air, after having been compressed and purified, is further compressed and divided into first and second subsidiary streams;
the first subsidiary stream forms the compressed refrigerant air stream; and
the second subsidiary stream is further compressed to form the further compressed air stream.

9. The method of claim 5 , wherein:
at least part of the air, after having been compressed and purified, is further compressed and divided into first and second subsidiary streams;
the first subsidiary stream forms the further compressed air stream; and
the second subsidiary stream is further compressed to form the refrigerant air stream.

Assignees

• Rauch Jeremiah J
• Sarigiannis Catherine B
• Warta Andrew M
• Dowd Sophia J
• Praxair Technology Inc

Inventors

• Rauch Jeremiah J
• Sarigiannis Catherine B
• Warta Andrew M
• Dowd Sophia J

Classifications

• F25J2220/40

  Separating high boiling, i.e. less volatile components from air, e.g. CO2, hydrocarbons · CPC title

• F25J3/04824

  Stopping of the process, e.g. defrosting or deriming; Back-up procedures · CPC title

• F25J3/04175

  at a pressure of substantially more than the highest pressure column · CPC title

• F25J3/04781 Primary

  Pressure changing devices, e.g. for compression, expansion, liquid pumping · CPC title

• F25J3/04296

  Claude expansion, i.e. expanded into the main or high pressure column · CPC title