(JP6205925-A)      A multistage process for sepn. of fluids using permeable membranes includes: (a) feeding a fluid (1) to at least one 1st stage membrane unit (5) at a pressure sufficient to produce a permeate stream (6) and a non-permeate stream (7); (b) feeding the non-permeate stream to at least one 2nd stage membrane unit (8) at a pressure sufficient to give second permeate (9) and non-permeate streams (13) which forms the prod.; (c) feeding the second permeate stream via a recycle compressor/pump (11) to at least one 3rd stage membrane unit (12) at a pressure sufficient to give third permeate and non-permeate streams, the compressor/pump being fitted with a control system which adjusts the output so as to maintain a substantially constant pressure on the permeate side of the 2nd stage membrane unit (8); (d) feeding the non-permeate stream from the 3rd stage membrane unit to either the feed inlet or the inlet of the 2nd stage unit, for recycle; (e) using a sensor (26) to monitor the flow rate, pressure or purity of the 2nd stage permeate stream (9) and provide a signal (28) to a controller (21) which, in turn, sends a control signal (29) to the control unit (30) which adjusts the recycle compressor/pump; and (f) adjusting the output of the feed compressor/pump (2) to correspond with demand for the prod..    USE/ADVANTAGE -   Multistage membrane sepn. process for efficient sepn. of fluid mixts., esp. gases, esp. useful in prodn. of high purity N2 without use of deoxo systems. Gives improved control of multistage membrane sepn. systems, esp. during turndown situations and low demand periods. Maintains sepn. efficiency at low prod. demand or in variable demand conditions. Process is esp. useful for 3-stage systems with at least 1 recycle compressor.    (JP2789548-B2)      A multistage process for sepn. of fluids using permeable membranes includes: (a) feeding a fluid (1) to at least one 1st stage membrane unit (5) at a pressure sufficient to produce a permeate stream (6) and a non-permeate stream (7); (b) feeding the non-permeate stream to at least one 2nd stage membrane unit (8) at a pressure sufficient to give second permeate (9) and non-permeate streams (13) which forms the prod.; (c) feeding the second permeate stream via a recycle compressor/pump (11) to at least one 3rd stage membrane unit (12) at a pressure sufficient to give third permeate and non-permeate streams, the compressor/pump being fitted with a control system which adjusts the output so as to maintain a substantially constant pressure on the permeate side of the 2nd stage membrane unit (8); (d) feeding the non-permeate stream from the 3rd stage membrane unit to either the feed inlet or the inlet of the 2nd stage unit, for recycle; (e) using a sensor (26) to monitor the flow rate, pressure or purity of the 2nd stage permeate stream (9) and provide a signal (28) to a controller (21) which, in turn, sends a control signal (29) to the control unit (30) which adjusts the recycle compressor/pump; and (f) adjusting the output of the feed compressor/pump (2) to correspond with demand for the prod..    USE/ADVANTAGE -   Multistage membrane sepn. process for efficient sepn. of fluid mixts., esp. gases, esp. useful in prodn. of high purity N2 without use of deoxo systems. Gives improved control of multistage membrane sepn. systems, esp. during turndown situations and low demand periods. Maintains sepn. efficiency at low prod. demand or in variable demand conditions. Process is esp. useful for 3-stage systems with at least 1 recycle compressor.