A CMS membrane module includes plurality of hollow fiber CMS membranes that are enclosed within an open cylindrical shell whose ends are embedded in tubesheets. The shell is concentrically disposed within an open cylindrical pressure vessel whose open ends are covered by and secured by end caps. The shell includes a feed fluid inlet formed therein between the tubesheets and a retentate outlet in between one of the tubesheets and an adjacent end cap. A retentate seal is formed between the shell and the pressure vessel at a position between the tubesheets. A permeate seal is formed between the pressure vessel and the tubesheet that is adjacent a permeate port of the module. A structure made up of the CMS membranes, shell, tubesheets, and seals is slidable within the pressure vessel and not fixed in place in relation to the pressure vessel and end caps.