CAL CHEM CORPORATION
To: CHE Juniors Date: Winter Quarter
File: CHE 333
From: CHE faculty
Subject: Gas Separation Membrane Experiment
Figure 1. Hollow-fiber module used for air separation.
Gas separation with polymer membrane is becoming an important component of separation technology1. Examples of commonly used membrane separations are enrichment of nitrogen from air, hydrogen separation in ammonia plants and refineries, removal of carbon dioxide from natural gas, and removal of volatile organic compounds from mixtures with light gases. Gas separation membranes are often packaged in hollow fiber modules depicted in Figure 1. As air flows under pressure into the module through the bores of the hollow fiber, some of the air gases permeate through the wall of the fibers into the shell of the hollow fiber. The gas in the shell side of the fibers leaves the module as the permeate stream. Since oxygen, water, and carbon dioxide are more permeable than nitrogen and argon, the gas in the fiber bore is enriched as it moves from the feed to the residue end of the module.
Figure 2. Schematic of a membrane with thickness t used to separate O2 from N2.
The flux NO2 of oxygen across the membrane shown in Figure 2 is given as
NO2 = (xP yp) (1)
where is the permeance of the membrane to oxygen, x is the mole fraction of oxygen on the upstream, or high pressure P, side of the membrane, and y is the mole fraction of oxygen on the downstream, or low pressure p, side of the membrane. The ratio of permeance to membrane thickness is called the permeability of the membrane to oxygen. The permeability can be viewed as a mass transfer coefficient that connects the flux with the driving force for transport, which is the partial pressure difference between the upstream and downstream sides of the...