Air separation method and apparatus

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 turboexpander connected to the distillation column system so that an exhaust stream generated by the turboexpander is introduced into the distillation column system, thereby to impart refrigeration to the air separation plant, the 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 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 booster compressor to further compress the compressed refrigerant air stream and thereby obtain a higher pressure ratio across the turboexpander and a higher rate of production, a bypass branch, bypassing the booster compressor, thereby to obtain a lower pressure ratio across the 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.