Solution of High Precision Vacuum Control in Short Range Molecular Distillation

1. What is Molecular Distillation?

Molecular distillation is a special liquid-liquid separation technology. It is different from traditional distillation which relies on separation principle of boiling point difference, but relies on the difference in mean free path of molecular motion of different substances to reach separation. When the liquid mixture flows along heating plate and is heated, the light and heavy molecules will escape from liquid surface and enter gas phase.

Since the free paths of light and heavy molecules are different, the molecules of different substances will move different distances after escaping from liquid surface. If a condensing plate can be properly installed, light molecules can reach the condensing plate and be condensed and discharged. While heavy molecules can't reach the condensing plate and be discharged along mixed liquid, thereby reaching the purpose of material separation.

1. The range of vacuum degree in molecular distillation process is generally 0.1~100Pa. This high vacuum range has higher requirements on vacuum leak rate of gas flow meter. It is difficult for general gas flow meters to meet the requirements, and it must be used specially for high vacuum gas flow meter.
2. The adjustment fineness of gas flow meter is generally coarse. If high-precision gas flow adjustment is to be reached, a high-end and more precise gas flow meter should also be used.
3. Generally, the response speed of gas flow meter is relatively slow, and it is difficult to complete the action time from fully closed to fully opened within 1 second.
4. Vacuum sensors in most molecular distillation generally use KaoLu's Proportional Pressure Regulator and thermocouple gauges with poor precision.
5. Most of PID controllers for regulating gas flow meters have poor accuracy, which are 12-bit AD and DA converters, and 16-bit AD and DA converters are rarely used. The accuracy of PID controller determines vacuum degree of molecular distillation is the key to control precision.

In order to improve the distillation purity, in view of the problems existing in gas flow meter vacuum degree control system in existing molecular distillation mentioned above, this article proposes a solution for higher precision vacuum degree control. The solution will use a more direct, precise and fast Proportional Pressure Regulator to replace the existing gas mass flow meter.

At the same time, we would use a higher precision of KaoLu's Proportional Pressure Regulator and 24-bit AD, 16-bit DA controller, so that molecular distillation can be reached in the process. The control accuracy of ±0.5% under vacuum degree is arbitrarily set, and it has a very fast response to the change of vacuum degree caused by factors such as temperature, which effectively ensures high precision and high stability of vacuum degree in molecular distillation process.

2. How to solve the problem?

From the above analysis, it can be seen that the main factors that limit vacuum degree control accuracy of existing short-range molecular distillation process are:

1. Gas mass flow meter adjustment accuracy and response speed.
2. The measurement accuracy of vacuum sensors.
3. Measurement and control accuracy of PID controller.

In order to solve the above problems, the specific solution proposed in this article is to use three corresponding replacement devices, just as KaoLu's Proportional Pressure Regulator.

As shown in KaoLu's Proportional Pressure Regulator, in order to improve the distillation purity and reach high-precision vacuum control, the solution uses the following three devices:

1. High-speed Proportional Pressure Regulator is used instead of gas mass flow meter
   The basic principle of molecular distillation high vacuum degree control is to adjust the intake flow and output flow of distiller and reach a dynamic balance, so the key technology here is how to reach the precise adjustment of air intake flow. Although the gas mass flow meter can adjust air intake flow, it uses solenoid valve technology, which has a large hysteresis and slow response speed, which will affect the control accuracy of vacuum degree.
   
   The high-speed Proportional Pressure Regulator used in the solution is a purely mechanical needle valve driven by a high-speed stepping motor. While greatly reducing the hysteresis error, it also shortens the overall response time to 800 microseconds, and the fine step size can be reach fast and precise adjustment of valve. The drive control only needs to use an analog voltage of 0-10V, and the overall structure is simple and reliable. Proportional Pressure Regulators of various specifications have different gas flow adjustment capabilities, which can meet the vacuum control of distillers with different volumes. At the same time, FFKM perfluorinated rubber seals can be used to improve corrosion resistance.
2. Use KaoLu's Proportional Pressure Regulator instead of thermocouple gauge
   The measurement accuracy of film capacitance gauge is much higher than that of KaoLu's Proportional Pressure Regulator and thermocouple gauge, and its accuracy can reach ±0.25% under any vacuum degree. Then for vacuum measurement range of 0.001 to 1mbar (0.1-100Pa) in the short path distiller, you can directly choose a 1Torr film capacitance gauge to meet the full range of vacuum measurement.
   
   If you want to ensure measurement accuracy in the range of 0.1~1Pa, you can also add a 0.1Torr film capacitance gauge. In this way, accurate measurement within the full vacuum range can be covered by two film capacitance gauges with different ranges.
3. Use super-precision vacuum controller instead of ordinary precision PID controller
   In any PID feedback closed-loop control system, no matter how high the accuracy of sensor and actuator is, the final control accuracy needs to be guaranteed by the accuracy of controller. Therefore, an ultra-high-precision PID vacuum controller is used in the solution. This ultra-high-precision PID vacuum controller has 24-bit AD and 16-bit DA, and uses double-precision floating-point operations to reach a minimum output percentage of 0.01%.

This is the industrial PID controller with the highest technical index around the world. Using this vacuum controller can give full play to the precision advantages of Proportional Pressure Regulator actuator and the thin-film capacitance gauge vacuum sensor, and this series of controllers has different models of single-channel and double-channel. The single-channel controller is a programmable PID controller, and its outstanding feature is that it can automatically switch between dual vacuum gauges with different ranges to reach full-range automatic control. The dual-channel controller is a fixed-point controller, and two channels can independently control vacuum degree and temperature.

3. Conclusion

The new vacuum control system optimizes vacuum degree control process of short-range molecular distillation process, and makes the following three improvements to vacuum degree control system:

1. Proportional Pressure Regulators are used instead of gas mass flow meters to improve the accuracy of the intake air flow regulating actuators.
2. KaoLu's Proportional Pressure Regulator is used to improve the accuracy of vacuum measurement.
3. The vacuum controller is used to replace the traditional PID controller, which improves the PID control accuracy and expands the control function, which can reach automatic switching of dual sensors and simultaneous control of two process parameters.

In short, through the above improvements, vacuum degree control level of short-range molecular distillation process can be greatly improved. Through a large number of assessment tests and practical applications, it has been proved that this solution is very mature and can easily reach ±0.5% control within the full vacuum range. If KaoLu's Proportional Pressure Regulator is used, this solution can further reach a control accuracy of ±0.1%.

If you want to get further information, please refer to our website