Study On Optimization Of Sectional Pulsed Vacuum Drying Process Based On Multi-parameter Monitoring

There are many advantages of Pulsed vacuum drying (PVD) technology like high drying efficiency and good product quality, which has the good prospect of application. PVD system with multi parameter monitoring function was designed in this work to the problems such as time-varying with phase during drying process. This system has been applied to the materials of carrot slices and Lycium barbarum drying. The heat and mass transfer under PVD were analyzed, and the parameters of PVD at stepwise variable temperature and pressure difference were optimized separately based on the changes of instantaneous moisture content and temperature during the drying process.The heating system of PVD was rebuilt, and monitoring system with MODBUS communication protocol, the touch screen, multi block microcontroller and intelligent weighing instrument were given for realizing the equipment control and real-time accurate monitoring, DAta display and storage function of the material status (weight, moisture content, and internal temperature of material), machine condition (vacuum degree and the heating temperature), environmental conditions (temperature and humidity) and energy consumption (run time information vacuum pump) simultaneously. The weighting resolving power of this system is0.5g. The performance test showed:the automatic weighing relative error was less than±1%, wet basis moisture content analysis accuracy was less than±1.5%.The range of instantaneous DA of pressure retention phase under PVD was0-0.5g/min when the moisture content of material was more than37%(wet basis), less material quality of dehydrated, and increased obviously of material temperature. Vacuum retention phase is the main dehydration stage of drying process, the extreme value of material DA and cooling amplitude took on before the early of this stage, and duration was shorter. Later, DA oscillation was stable, the material temperature was constant approximate, duration was longer. We can also analyze the stage of drying and dehydration status of material by monitoring the instantaneous temperature and DA. Similar temperature changes were acquired at the same heating temperature and stable range of oscillation period of DA. Material instantaneous moisture content had a significant impact on the instantaneous temperature and dehydration rate of the material.Base on the changes of instantaneous DA and the temperature of the material, the PVD process was divided into:preheating, constant drying, mid-falling rate, slow dehydration period. The optimization of carrot slices and Lycium barbarum segmentation process showed that the technology combination of PVD at stepwise variable temperature and pressure difference had high-efficiency and energy-saving optimization effect compared to the fixed process test both ensured the minimum temperature rising of material.Vacuum drying were suitable for the later drying stage of carrot slices, better drying parameters were heating temperature70℃in the first two pulse period of vacuum preheating, lower heating temperature to65℃when the wet basis moisture content (Mw) was75.0%, lower vacuum value of90.0kPa when Mw≤35.0%.The total drying time could be reduced by11.9%or so, reduce the energy consumption of around22.7%and the running time of the vacuum pump was about29.7%compared with the conventional pulsating drying process.Vacuum drying had significant effects on later drying time of Lycium barbarum, further optimization of the process were heating temperature65℃in the first two pulsation cycle vacuum preheating, lowering the heating temperature to60℃when Mw was65.0%, lower vacuum value of90.0kPa when Mw≤35.0%, change the lower vacuum value to88.0kPa when Mw was25.0%, which could shorten the drying energy consumption about30%, the running time of the vacuum pump was29.7%or so, but the drying time was slightly extended compared with the conventional pulse drying process.The multi-parameter control test showed that the test bench provided not only a platform for optimizing the parameters by monitoring the heat and mass transfer of materials during drying process, but also the technical support for development of PVD technology and equipment research.