FIGURE 13.6-1. Process flow diagram for cross-flow model. 13.6. CROSS-FLOW MODEL FOR GAS SEPARATION BY MEMBRANIS 13.6A Derivation of Basic Equations A detailed flow diagram for the cross-flow model derived by Weller and Steinet (:/3, W4) is shown in Fig. 13.6-1. In this case the longitudinal velocity of the high-press'te or reject stream is large enough that this gas stream is in plug flow and flows parallel to the imembrane