A membrane separation model for tubular module reverse osmosis and ultrafiltration processes was developed in this work. The membrane area of a process can be calculated by this model and the stream matrix of a process can be determined by a process simulation program. In this work the UNICORN simulation program was used. The membrane separation model calculates permeate flux and rejection of the solute in small increments of the membrane tube over the entire range of the tube and the process. Calculation of the permeate flux and rejection can be performed by polynomial equations fitted to the experimental data, or by equations based on mass transfer models. The finely porous model and the statistical mechanical model were used. Some experimental data are also needed for determining the parameters of the mass transfer model equations by parameter fitting. The membrane separation model was tested by the simulation of reverse osmosis of aqueous ethanol and acetic acid solutions, and ultrafiltration of aqueous sodium carboxymethylcellulose (CMC) and poly (vinylpyrrolidone) (PVP) solutions. Multistage recycle separation processes with four or five recycle stages were used as test processes. The values of the calculation parameters (i.e., iteration accuracy of the recycle stream, iteration acceleration factor and the number of increments of the membrane tube) giving reliable results with minimum computing time were determined.