Turndown is the capability of the system to operate at reduced capacity (Miller and Söcker 1989). In gas separation, membrane systems are highly capable of maintaining product purity even though the capacity is reduced down to 10 % of the initial design. This is done by either reducing feed pressure, increasing permeate pressure, or by isolating modules from the system. The first two methods can be used for short-term operation and the latter when operating at significantly reduced capacity for extended periods. No previsions are required in the design phase.

Absorption units can maintain both recovery and product purity at throughputs of ca. 30–100 % of design by adjustment of the gas streams and solvent flow rates.

Also PSA units can maintain both recovery and product purity at throughputs of ca. 30–100 % of design by adjustment of the cycle time. Between 0 % and 30 % of design product rates, purity can be maintained, but recovery is reduced unless special provisions are made in the...