To optimize a chemical reaction, chemists must find the best combination of compounds and introduce these in precise proportions into the reaction chamber. This reactor may be kept at a certain pressure and temperature and a catalyst may be added to accelerate the reaction. The input of the reaction gases must be accurately measured at all times, even while pressurizing the reactor. An overshoot in flow (outside the scale of the mass flow meter) should be avoided, because this will introduce inaccuracy.
Bronkhorst has developed a specialized combination of electronic pressure and thermal mass flow controllers for automated pressure control of reactor vessels. This standard solution can be applied for low flow lab reactor systems as well as for high flow industrial applications like in hydrogenation processes in the food industry industry or at chemical plants.
At the inlet of the reactor, an EL-FLOW Mass Flow Controller (MFC) provides the process gas delivery, while an EL-PRESS Electronic Pressure Transmitter (EPT) measures the reactor pressure. At the outlet of the system there is a flow restriction which could simply be a (needle) valve or, as shown in the illustration, a MASS VIEW Mass Flow Regulator (MFR) with local display. The reactor pressure is controlled by giving a setpoint to the pressure transmitter.
In the illustration this is achieved by a script programmed into a PC and delivered via RS232. The integrated PID-controller of this pressure meter (Master) controls the valve position of the MFC’s control valve (Slave). When building up the pressure in the reactor, the maximum inlet flow is restricted by the MFC, thus preventing a flow peak. By using the ‘slave factor’ option, the maximum flow can be adjusted. When the process pressure has reached the desired value, it is kept constant while the required amount of reaction gas is controlled with a constant flow.
It is also possible to pre-set the total amount of reaction gas allowed into the system by using a batch control function. Once the total amount is reached, the set-point for the MFC can be programmed to be reset to zero, thus switching off the gas supply, independent of the process pressure.