Crossflow filtration (also called tangential flow filtration) is a separation or purification technique which is often used in the (bio)pharmaceutical industry and relies on the feed flow travelling across the surface of a membrane as opposed to dead-end filtration, in which the feed is led through a membrane. The latter causes the formation of “filter cakes”, which can clog your filter. In crossflow filtration, this cake is washed away as it is a continuous process, avoiding clogging and thus loss of efficiency.
Crossflow filtration relies on the pressure difference between the retentate side and permeate side of the membrane. This transmembrane pressure might be affected by viscosity, increase in flow, or other parameters, which will in turn affect efficiency. Equilibar precision pressure controllers are specifically suitable for this type of application, as they can efficiently keep a stable pressure in a relatively small range (0,5% for liquids; 0,2% for gases).
In order to combat this drop in efficiency, Equilibar dome-leaded back pressure regulators can be applied in almost any situation. Equilibar regulators keep a constant pressure while being downstream of a process. Depending on the process requirements, a single set-up or dual set-up can be realized making use of either 1 or 2 regulators; a single set-up could be used for both retentate pressure control and permeate pressure control.
By using a dual back pressure regulator set-up in crossflow filtration – 1 valve for each stream – the pressure on both the permeate and the retentate side can be controlled with high precision, as well as the transmembrane pressure. Because these back pressure regulators have a frictionless dome-loaded design, differential pressures even below 100 mbar can be maintained and set-point changes of less than 1 mbar are possible.
Equilibar back pressure regulators can be used for a variety of crossflow filtration applications. These applications can vary from reverse osmosis to nano- or microfiltration to the purification of water.
The schematic figure below visualizes a possible set-up using a dual Equilibar back pressure regulator: The fluid to be filtered (or retentate) is led from a reservoir tank through a feed pump to the filter membrane. Optionally, the pressure between the pump and the membrane can be measured through a pressure transmitter. By means of filtration, the desired permeate will exit the process – while the pressure is closely monitored by an Equilibar back pressure regulator – into the storage tank on the right. From the filter membrane, the retentate is led back to the reservoir tank, also through use of an Equilibar back pressure regulator. A single set-up would be similar, using just one of the two regulators depending on the specific process requirements.
Equilibar pressure regulators are particularly useful in this regard as they can handle two-phase flow. Because transmembrane pressure can be a sensitive issue in this kind of application, 2 Equilibar back pressure regulators are used in this set-up. Here they are both piloted by the same gas bottle (of compressed air or N2). However, the 1st Equilibar regulator (on the left) is controlled in a closed loop manner by using an external pressure transmitter, which means that the dual valve controller will control the pressure based on the reading of the pressure transmitter. The 2nd Equilibar regulator is controlled in an open loop, with the pressure sensor being a part of the dual valve controller.
The FD series regulators are made specifically for food and biopharma applications and are available in multiple material options such as stainless steel, Hastelloy, and other materials. Diaphragms are available in PTFE, Buna-N, but also in USP class VI PTFE, EPDM among others.
For single-use crossflow filtration applications, Equilibar SD series is the ideal solution because of its high turn down and precision. The single-use regulators consist of a stainless steel support housing with wingnuts for easy (dis)assembly as well as body and diaphragm inserts that are made from USP Class VI materials, avoiding sterilization of components and serving any application that has a need of using disposable components.
In the schematic above, the reference pressure in both Equilibar back pressure regulators is electronically controlled by dual valve controllers. The dome of the back pressure regulator can also be piloted using manual pressure reducing regulators in situations where this is more desirable.