1. Prof. Mohammad Asif Room 2B45, Building 3
Department of Chemical Engineering King Saud University Separation Processes – I CHE 316
Prof. Mohammad Asif
Room 2B45, Building 3
Tel:

2. MEMBRANE MODULE CASCADES
The purpose is to increase the degree of separation.
A single membrane module or a number of such modules arranged in parallel or in series without recycle constitutes a single-stage membrane separation process.
The extent to which a feed mixture can be separated in a single stage is limited and is determined by the separation factor, α. To achieve a higher degree of separation than possible with a single stage, a cascade of stages can be used.
In a cascade arrangement, assuming the upstream pressure drop to be negligible, the permeate must be pumped, if a liquid, or compressed, if a gas, to be sent to the next stage. In the case of gas permeation, compression costs can be high. Thus, membrane cascades for gas permeation are often limited to just two or three stages.
Compared to a single stage, the two-stage stripping cascade is designed to obtain a more purer retentate, whereas a more purer permeate is the goal of the two-stage enriching cascade.

3. Two-stage enriching cascade
Two-stage stripping cascade

4. Case 1: Single membrane Stage
An example of application of the enrichment cascades is given here for the removal of carbon dioxide from natural gas (assumed to be methane) using cellulose acetate membranes in spiral-wound modules (approximated cross-flow) using α* = 21. Results of computer calculations are given in Table for a single stage, a two-stage enriching cascade. In all three cases, the feed is 20 million (MM) scfd of 7 mol% CO2 in methane at 865 psia and the retentate is 98 mol% in methane. For each stage, the downstream membrane pressure is 25 psia. For both cases, stream A is the feed, stream B is the final retentate, and stream C is the final permeate.
Case 1: Single membrane Stage
Case 1 achieves a 90.2% recovery of methane.
Case 2 increases that recovery to 98.7%.
Case 2: Two-stage enriching cascade