At the prescribed temperature and pressure, gaseous and liquid products of reaction are formed and extracted for analysis.
A back pressure regulator is often used to maintain pressure at the reactor exit; however, this task can be challenging when the application involves two-phase flow (simultaneous flow of liquids and gases). Traditional control valves and back pressure regulators are not designed to process more than one phase at a time.
A common method of addressing this challenge is to install a high pressure separator vessel to remove the liquids ahead of the pressure controller. For gram-scale systems, the liquid can simply be accumulated until completion of the trial. However, larger pilot-scale systems often require a liquid level control system to extract the liquid, adding unwanted complexity and inconvenience. Moreover, in many applications, the chemical composition of reactants is altered when the separation is conducted above the vapor pressure of components.
Solution: Unlike traditional control valves, a dome-loaded back pressure regulator with multiple orifices is capable of processing liquid and gas at the same time while maintaining highly stable pressure. These devices do not rely on springs or traditional valve seats, but rather use a direct 1:1 gas pressure on top of the diaphragm to maintain stability across a 100,000:1 flow range. Because the design controls flow across multiple parallel orifices, certain orifices tend to be dominated by the gas phase while other orifices attract the denser liquid phase.
These dome-loaded regulators work by creating a threeway force balance over each of the multiple orifices. As the back pressure regulator operates, low pressure at the outlet pulls the diaphragm over the orifice to block flow. The valve inlet pressure must slightly exceed the gas setpoint pressure before the diaphragm will lift to allow flow. Because each orifice has its own force balance, the resulting control is exceptionally stable and precise, even when liquids and gases are flowing at the same time.
To control a reactor at 100 bar, for example, a nitrogen supply and manual pressure regulator can be used to supply 100 bar pressure on top of the flexible diaphragm. Many automated reactor test systems use closed-loop controllers and vary the set-point pressure to the regulator by use of an electronic nitrogen pressure regulator.
Equilibar Precision Pressure Control is one manufacturer that offers multi-orifice dome-loaded back pressure regulators in a wide variety of configurations specifically designed for control of laboratory reactors. Common applications for two-phase flow include petrochemical catalyst research and fuel cell testing systems involving condensation.
Equilibar regulators are available in different body materials to accommodate harsh chemistries and temperatures. Selecting the right diaphragm is also important. For many chemical reactions, a PTFE diaphragm reinforced with glass fibers provides the ideal combination of chemical resistance, temperature resistance, and pressure rating. For higher pressure applications, diaphragms of PEEK, polyimide, or Hastelloy are suggested.
For more information, see https://www.equilibar.com/back-pressure-regulators/severe-services/two-phase-flow/
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