Temperature-sensitive materials are a class of materials whose physical properties change dramatically and discontinuously with temperature. Many different industries require drying treatment of temperature-sensitive materials, such as the manufacturing of various additives, life sciences, tissue engineering, liquid chromatography, drug delivery and research and development. Other temperature-sensitive materials include fruits, vegetables and other plants.
The ideal method for drying heat-sensitive and/or oxygen-sensitive materials is vacuum drying, as it has the advantages of removing moisture at a lower temperature and minimizing possible oxidation reactions. In addition, vacuum drying also has the advantage of a faster drying speed and can be carried out in an oxygen-deficient processing environment.
The drying of temperature-sensitive materials needs to be carried out in a vacuum drying device. To avoid damaging the products, the temperature and vacuum degree inside the vacuum drying device need to be precisely controlled, among which the control of vacuum degree is particularly important. This is because in the vacuum drying process based on the principle of saturated vapor pressure, the level of vacuum degree determines the upper limit of the water evaporation temperature.
At present, some commonly used vacuum drying ovens are equipped with digital temperature control, but most of them adopt manual vacuum control valves. In response to some customers' requests for upgrading and transforming the drying oven to automatically control the vacuum degree, this article will introduce a precise vacuum degree control solution. The solution elaborates on several different control methods and also includes solutions for simultaneous control of temperature and vacuum degree to meet the automation control needs of vacuum drying equipment for different customers.
Ii. Technical Solution
Vacuum drying processes and equipment come in various forms. Generally speaking, they can be mainly classified into the following three types:
(1) Sealed cavity type.
(2) Fill with inert gas.
(3) Containers of different volumes.
Pressure regulation methods for closed cavities
For vacuum drying equipment with a closed cavity structure, the precise control of the internal vacuum degree of the drying equipment can be directly achieved by controlling the pressure. The specific pressure control device is only connected to the components on the right side of the middle drying oven (back pressure valve and vacuum pump), and adopts an external vacuum gauge method in combination with a PID controller to adjust the vacuum degree.
Among them, the back pressure valve is a vacuum type back pressure valve, which is driven by an electrical proportional valve. The external vacuum gauge, PID controller and electrical proportional valve form a closed-loop control circuit, through which the vacuum degree inside the drying oven can be controlled according to any set value, or the vacuum degree curve can be controlled according to the set program.
The flow regulation method during the inflation process
For some oxygen-sensitive materials, vacuum drying sometimes requires the introduction of inert gas for oxygen displacement. In this case, the control of the vacuum degree of the drying oven needs to be achieved by adjusting the intake and exhaust gas flow rates to reach a dynamic balance.
The intake flow rate is regulated by an electric needle valve, and the exhaust flow rate can be adjusted by a vacuum back pressure valve or an electric ball valve. The regulation of the intake and exhaust gas flow is achieved through a closed-loop loop composed of a vacuum gauge and a 2-channel PID controller. Similarly, from this, the vacuum degree inside the drying oven can be controlled according to any set value, or the vacuum degree curve can be controlled according to the set program.
Method for large-diameter pressure regulating valves in large-sized cavities
For large-sized cavities that require vacuum drying of a large number of products, the key to vacuum degree control is to use large-diameter regulating valves. That is, the vacuum back pressure valve (or electric ball valve and electric butterfly valve) shown in Figure 1 all need a large diameter to achieve a high pumping speed and rapid control of vacuum degree. At present, various large-diameter control valve technologies have matured and there are standard products available.
Simultaneous control method for temperature and vacuum degree
For the vast majority of vacuum drying equipment, it is necessary to control both temperature and vacuum degree simultaneously. In the technical solution proposed in this paper, a 2-channel high-precision PID controller is adopted. The two channels can be respectively used to simultaneously control temperature and vacuum degree, that is, by connecting the temperature sensor and the vacuum degree sensor, two independent closed-loop circuits are formed for control.
The two-channel PID controller can not only be connected to the 0-10V voltage signal output by the vacuum degree sensor, but also to the input signals of various temperature sensors (thermocouples, platinum resistors, thermistors, etc.). Especially with the adoption of 24-bit A/D analog-to-digital and 16-bit D/A digital-to-analog converters, this two-channel PID controller can achieve very high control accuracy.
Through the above technical solution, the requirements for automatic control of temperature and vacuum degree during the drying process of temperature-sensitive materials can be fully met, and a very high control accuracy (absolute pressure 10Pa to 0.1MPa, control accuracy better than 2%) can be achieved.