Computer-aided simulation and design of nanofiltration processes.
The modelling of membrane filtration processes is often performed by applying black-box models or short-cut methods, because of the complexity of the molecular interactions on and inside the membrane. The assumptions made for short-cut methods can be applied with accuracy to reverse osmosis processes, whereas the simulation of nanofiltration can lead to unreliable results that sometimes deviate from real conditions to a great extent. A steady-state process simulation, NF-PROJECT, based on input information from membrane characterization, was developed (isothermal operation). The individual separation characteristics of each membrane element are calculated in an iterative sequence, illustrating the successive reduction in permeability and rejection between the elements arranged inside the pressure vessel. The simulation provides information on the increasing feed concentration and osmotic pressure, the hydraulic pressure loss, the deterioration of the flow conditions in the vessel, and the joint performance of the membrane elements to be analyzed. Taking an example from a practical application, a two-stage nanofiltration pilot plant was simulated, the results of which are presented in this article. Examples of optimization potentials are illustrated for the target criteria of economic efficiency (specific energy costs), permeate quality, and flow.