Process and apparatus for producing an oxygen product gas and a nitrogen product gas using ion transport membrane assemblies. The apparatus comprises at least two ion transport membrane assemblies and a turboexpander downstream of one of the ion transport membrane assemblies. In the process, an oxygen- and nitrogen-containing gas is introduced into a first of the ion transport membrane assemblies to produce oxygen-depleted gas and oxygen product gas. The oxygen-depleted gas is divided, with a first portion being expanded in the turboexpander and a second portion introduced into a second of the ion transport membrane assemblies. A nitrogen-rich product gas and additional oxygen product gas are withdrawn from the second ion transport membrane assembly.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with government support under Cooperative Agreement No. DE-FC26-98FT40343 between Air Products and Chemicals, Inc. and the U.S. Department of Energy. The United States Government has certain rights in this invention.