Separation method and apparatus

A separation method comprising:

• separating a compressed gaseous mixture within a cryogenic rectification plant by purifying the compressed gaseous mixture, cooling the compressed gaseous mixture by indirect heat exchange with mixture component streams after having been compressed and purified and rectifying the gaseous mixture within a separation unit having at least one column to produce the mixture component streams;
• discharging a liquid stream from the separation unit enriched in one mixture component of the gaseous mixture;
• dividing at least part of the compressed gaseous mixture after partial cooling thereof during the indirect heat exchange into a first subsidiary stream and a second subsidiary stream and withdrawing the first subsidiary stream and the second subsidiary stream from the indirect heat exchange at higher and lower temperatures, respectively;
• combining the first subsidiary stream and the second subsidiary stream after withdrawal from the indirect heat exchange to produce a combined stream;
• expanding at least part of the combined stream with the performance of work within a turboexpander to supply refrigeration to the cryogenic rectification plant and introducing at least part of an exhaust of the turboexpander into the separation unit;
• varying flow rates of the first and second subsidiary streams and controlling temperature of the combined stream such that the exhaust stream is at least at about a saturation temperature by controlling the flow rates of the first and second subsidiary streams;
• varying pressure of the at least part of the compressed gaseous mixture to in turn vary the refrigeration supplied by the turboexpander and production rate of the liquid stream such that increasing the pressure of the at least part of the compressed gaseous mixture in a high liquid mode of production increases the production of the liquid stream and decreasing the pressure of the at least art of the compressed gaseous mixture in a low liquid mode of production decreases the production of the liquid stream;
• during the high liquid mode of production, controlling the flow rates of the first subsidiary stream and the second subsidiary stream such that a flow rate of the first subsidiary stream is greater than that of the second subsidiary stream;
• and
• during the low liquid mode of production, controlling the flow rates of the first subsidiary stream and the second subsidiary stream such that the flow rate of the first subsidiary stream is less than that of the second subsidiary stream.