Ultrafiltration of Kraft black liquor was carried out by using an asymmetric membrane in a stirred batch cell, modified to work on a continuous mode. Spiegler-Kedem (SK) model from irreversible thermodynamics was used for the estimation of different membrane-solute parameters, like solute permeability (Pm) and reflection coefficient (σ). The Pm and σ so calculated from the above model were used to study the variation of these parameters with other process variables, like bulk concentration, pressure difference and stirrer speed. Finally, a simulation model was developed with the objective to predict permeate flux and rejection, which coupled the film theory, osmotic pressure model and SK model. The simulation results obtained from this study were validated with the experimental data using cellulose acetate membrane of 5,000 Da MWCO. Reasonably good agreements between the predicted and experimental values were observed and the average absolute deviation (AAD) for the prediction of flux and rejection using SK model was found to be 6.3%.