Gas permeation is increasingly considered for high capacity applications. This contribution discusses the commonly employed membrane module types for flat sheet membranes and suggests a new module concept. The applications considered are the separation of CO2 from flue gas and hydrocarbon dewpointing of natural gas. Rigorous models for predicting the operating performance of the module types are described. Pilot plant experiments were conducted to validate the model for envelope type modules. Simulation studies were carried out to predict the performances of the different module types for the two examples and assess their advantages and disadvantages.