Definition of a membrane: an intermediate phase that separates two phases from each other and sets different resistance to the transport of different chemical components.
Properties of a technical filtration membrane: a separating layer that retains larger molecules or particles and allows smaller molecules and solvents to pass through.
Product: the desired product can be the concentrate (concentration of a valuable material) or the permeate (removal of large molecular impurities).
Cross-flow filtration (tangential flow filtration, TFF): in contrast to classical filtration (cake filtration, dead-end filtration), there is a strong cross-flow along the membrane on the pressure side.
The cross-flow keeps the boundary layer thin and avoids a top layer. Nevertheless, the concentration of the retained component at the membrane surface is always somewhat higher than in the feed stream. The concentration of dissolved components at the membrane is adjusted so that the convective transport to the membrane (due to the permeate flow) is equal to the diffusive return transport. Compare the magnitudes of overflow and permeate velocity. Even the best membrane is limited in its permeate performance by the feasible cross-flow.
Basic process: The most commonly used membrane processes are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (UO). These are all pressure-driven processes. It ranges from typically 1 bar for microfiltration to 40 bar and more for reverse osmosis.
Batch or Conti:
The batch process is most frequently used. The following diagram shows the basic principle of a simple batch system consisting of feed tank, pump, diaphragm module, pressure maintenance valve and heat exchanger.
A feed solution or suspension flows over a membrane under pressure (cross-flow filtration, tangential flow filtration, TFF). The pressure is adjusted by the pressure control valve. The resulting heat is removed by a heat exchanger. The containers are pressureless. The membrane is chosen to selectively retain the desired components. Depending on the application, the product can be concentrate or permeate. Possible operating modes of the plant are concentration and diafiltration (washing). During concentration, permeate is removed so that the volume in the feed container decreases and the concentration of the retained component increases. In diafiltration, the same volume as permeate is produced fresh washing solution is added to the feed container, so that the concentration of the retained component remains the same while smaller molecules are washed out. For more information, see also Calculation of membrane processes.
Advantages of the batch process:
Flexible with different product properties, product change, throughput, concentration factor or operating mode (diafiltration / concentration).
Operates at the lowest average feed concentration, even with a simple system, so that the smallest product losses are avoided.
Low investment costs
Higher energy consumption (only with very large plants relevant)
Additional buffer tank required
Continuous plant: the pressure-generating feed pump is so small in throughput that the remaining retentate = concentrate has the desired concentration factor directly after removal of the permeate stream. There are usually several membrane loops, each with a circulation pump, which at the operating pressure of the plant must have only a small differential pressure to create the necessary circulation in each loop. What is achieved in the batch process over the course of time must be achieved in the continuous process over the course of the location (from inlet to outlet of the plant). In order to obtain the favourable concentration progression of a batch system, a large number of membrane loops each with its own circulation pump must be installed in a continuous system. Calculations also for the continuous plant see calculation membrane method. Due to the many circulation pumps and loops, the continuous system is more expensive in investment and very inflexible in application. Typically, it is built for large plants and for a single, fixed application (e.g. seawater desalination) or if it has to be integrated into a further continuous process.