Efficient design and optimisation of two-stage NF processes by simplified process simulation

The modelling of membrane filtration processes is often conducted by applying black-box models or short-cut methods because of its complexity due to molecular interactions on and inside the membrane. The assumptions made for the short-cut-methods are applicable for reverse osmosis, whereas the simulation of nanofiltration processes can lead to unreliable results, which sometimes deviate greatly from real conditions. A steady-state process simulation NFPROJECT, which is based on input information from membrane characterisation, was developed (isothermal operation). The individual separation characteristics of every membrane element are calculated in iteration, which leads to reduction of permeability and rejection for every further element arranged in series 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 along the feed side in the vessel and the combined performance of the membrane elements to be analysed. Serving as an example from practical applications, a two-stage nanofiltration pilot plant was simulated, the results of which will be presented. Examples of design and optimisation potentials will be illustrated for the target criteria of economic efficiency (specific energy costs), permeate quality and flow.

In order to accommodate fluctuations in feed concentrations or demands on permeate volume flow, the hydraulic pressure in the first filtration stage )p1 can be adjusted. The impact on hydraulic pressure on a two-stage NF system was studied in the following analysis concerning permeate flow and concentration and specific costs of power consumption (energy for pressure and recirculation pumps) (Fig. 7). The partial recoveries were kept constant at N1 = 0.75 and N2 = 0.85 as well as the feed concentration, which was set at a constant value of 3.5 mS/cm. Fig. 7 shows that the permeate volume flow is almost a linear function of the hydraulic pressure in the first filtration stage. The permeate flow rises from 0.4 m3/h at )p1 = 7 bar to 2.6 m3/h at )p1 = 17 bar. As a result of the increasing permeate fluxes, the rejection for sodium chloride to 200 µS/cm at )p1 = 17 bar, noting that concentration reduction gradient decreases with increasing hydraulic pressure.