Back pressure regulators are commonly used in membrane separation and membrane filtration processes to carefully control both the permeate pressure and retentate pressure. Depending on the osmotic pressure of the chemistry involved, the differential pressure across the membrane (transmembrane pressure) must be carefully maintained in a narrow range. Many small molecule separations require transmembrane pressures in the range of 1 psi. Such low pressure control can be a challenge, both in the research environment and in industrial separation applications.
Equilibar’s unique dome-loaded back pressure regulators are ideally suited for these demanding challenges. The frictionless dome-loaded design can easily maintain differentials below 1 psig, and is sensitive to set-point changes less than 0.01 psi.
The design of the Equilibar back pressure regulator uses sensitive diaphragms of PTFE or Viton FKM or other materials. These regulators can be made from SS316L, Hastelloy, 100% PTFE, or other exotic materials for resistance to strong acids and other chemicals.
Manual Control using Differential Air Regulators
There are several ways in with the Equilibar back pressure can be configured to control permeate and retentate pressure. In the schematic below, the differential pressure is precisely maintained by a biasing air relay. The dome-loaded back pressure regulators are installed on each stream. The pilot set-point pressure is set explicitly by a simple precision air regulator. The trans-membrane pressure is adjusted by the air biasing relay, which establishes a differential bias from the first pilot pressure.
Electronic Control using High Resolution Regulators
The schematic below shows how electronic pressure regulators can be used to automatically maintain higher resolution control of the trans-membrane pressure. The supervisory controller monitors process pressuring using sensors. PID control algorithms are used to maintain both permeate and retentate pressures at the desired levels. Because both the Equilibar back pressure regulator and the Equilibar QPV electronic pressure regulator have no significant hysteresis or deadband, it is possible to maintain these pressures with high resolution, to 0.1 psid.
Alternate Methods – Gas / Liquid Contacting Membranes with Adjustable U-Tube
The dome-loaded design of the Equilibar offers interesting alternate methods of control for gas/liquid systems. In the schematic below, the trans-membrane pressure is actually maintained by an adjustable liquid-filled manometer. The process gas pressure is used as the basis for the set-point pressure, and the trans-membrane pressure is established by the differential height between the membrane and the surface of the liquid in the U-tube. Because water loses 1 psid with each 27.4″ of elevation rise, the installation of the liquid surface 14 inches below the membrane location would present a transmembrane pressure of 0.5 psig (air greater than liquid).
Differential Pressure Control Using Electronic Pressure Regulators
An electronic pressure regulator can be equipped to control the pressure above or below a pneumatic reference signal. This allows a simplified method for membrane separation differential control that does not require a PID control or external wetted pressure sensors. The electronic command to the dP Modified E/P directly equates to the desired differential pressure.
This “open loop” control can be achieved because of the high resolution of the Equilibar QPV electronic pressure regulator and the low mechanical hysteresis of the Equilibar back pressure regulator.