Air separation apparatus

Patent metadata

Field	Value
Publication number	US-10113792-B2
Application number	US-201615292856-A
Country	US
Kind code	B2
Filing date	Oct 13, 2016
Priority date	Dec 26, 2012
Publication date	Oct 30, 2018
Grant date	Oct 30, 2018

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. An air separation apparatus comprising:
an air separation plant having a main air compressor, a purification unit connected to the main air compressor, a main heat exchanger in flow communication with the purification unit to cool the air, a distillation column system connected to the main heat exchanger and configured to rectify the air and thereby to produce at least one liquid product and a constant speed turboexpander connected to the distillation column system so that an exhaust stream generated by the constant speed turboexpander is introduced into the distillation column system, thereby to impart refrigeration to the air separation plant, the constant speed turboexpander not directly coupled to a single compressor of the air separation plant on a common pinion;
the air separation plant also having a branched flow path positioned between the pre-purification unit and the constant speed turboexpander to receive a compressed refrigerant air stream to vary production of the at least one liquid product and having a booster compressor branch including a constant speed booster compressor to further compress the compressed refrigerant air stream and thereby obtain a higher pressure ratio across the constant speed turboexpander and a higher rate of production, a bypass branch, bypassing the booster compressor, thereby to obtain a lower pressure ratio across the constant speed turboexpander and a lower rate of production, a recycle branch connecting an outlet of the booster compressor to an inlet of the booster compressor and connected at opposite ends to the booster compressor branch for flow of a recycle stream from the outlet to the inlet of the booster compressor thereby to prevent surge within the booster compressor, and a system of valves to permit selective introduction of the compressed refrigerant air stream into either the booster compressor branch or the recycle branch;
the system of valves including a first flow control valve located within the booster compressor branch upstream of the inlet of the booster compressor, a second flow control valve located within the bypass branch, a third valve located in the recycle branch and two valves located in the booster compressor branch and the bypass branch, respectively positioned downstream of the outlet of the compressor and the recycle branch and upstream of the second control valve and configured to prevent a reversal of flow in the booster compressor branch when bypass branch pressure within the bypass branch exceeds that of booster compressor branch and the reversal of flow in the bypass branch when booster compressor branch pressure at the outlet of the booster compressor exceeds that of the bypass branch; and
a programmable control system configured to generate control signals to control valve opening of the first flow control valve, the second flow control valve and the third valve and to activate the booster compressor and responsive to selective user input to selectively introduce the compressed refrigerant air stream into the booster compressor branch and the bypass branch, the programmable control system programmed such that:
when the compressed refrigerant air stream is introduced into the booster compressor branch, the first flow control valve gradually opens and the second flow control valve gradually closes to gradually divert the compressed refrigerant air stream from the bypass branch to the booster compressor branch and thereby introduce the compressed refrigerant air stream into the booster compressor branch, the booster compressor is activated, the third valve initially is set in an open position to allow flow of the recycle stream and thereafter, is reset from an open position to a closed position when the booster compressor pressure exceeds the bypass pressure; and
when the compressed refrigerant stream is introduced into the bypass branch, the first flow control valve gradually closes and the second flow control valve gradually opens to gradually divert the compressed refrigerant air stream from booster compressor branch to the bypass branch and thereby introduce the compressed refrigerant air stream into the bypass branch, the third valve is reset in from the closed position to the open position and the booster compressor is deactivated when the bypass pressure exceeds the booster compressor branch pressure.

2. The apparatus of claim 1 , wherein:
the constant speed turboexpander is positioned between a location of a main heat exchanger having an intermediate temperature between warm and cold ends thereof and the distillation column system; and
the branched flow path is positioned between the pre-purification unit and the main heat exchanger upstream of the constant speed turboexpander to receive a compressed refrigerant air stream.

3. The apparatus of claim 1 , wherein the branched flow path has means for passing a purge air stream, composed of purified air, through the booster compressor after the booster compressor is deactivated to prevent ambient air from entering the booster compressor.

4. The apparatus of claim 1 , wherein:
a conduit having an intermediate outlet connects the distillation column system to the main heat exchanger so that a liquid stream is removed from the distillation column system, is divided into a first subsidiary liquid stream discharged from the intermediate outlet and a second subsidiary liquid stream introduced into the main heat exchanger;
the at least one liquid product comprises the first subsidiary liquid stream;
the at least on liquid flow control valve is connected to the intermediate outlet;
the main heat exchanger is configured to heat the second subsidiary liquid stream to form a heated product stream; and
the main air compressor has inlet guide vanes that are able to be adjusted to control air flow rate through the main air compressor and thereby decrease the air flow rate during the low mode of production to in turn maintain product flow rate of the heated product stream constant.

5. The apparatus 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;
a pump is positioned within the conduit to pressurize the second subsidiary liquid stream and thereby to produce a pressurized liquid product stream that is warmed within the main heat exchanger to produce the heated product stream;
means for forming a further compressed air stream is positioned between the pre-purification unit and the main heat exchanger;
the main heat exchanger is configured to liquefy the further compressed air stream and thereby form a liquid air stream;
the main heat exchanger is in flow communication with at least the lower pressure column to introduce at least part of a liquid air stream into the lower pressure column; and
an expansion valve is positioned between the main heat exchanger and the lower pressure column to reduce pressure of the at least part of the air stream prior to introduction into the lower pressure column.

6. The apparatu

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

• F25J3/04412

  in a classical double column flowsheet, i.e. with thermal coupling by a main reboiler-condenser in the bottom of low pressure respectively top of high pressure column · CPC title

• F25J3/04024

  of purified feed air, so-called boosted air · CPC title

• F25J2240/42

  the fluid being air · CPC title

• F25J3/04175

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

• F25J2230/40

  the fluid being air · CPC title