Questions regarding vacuum drying
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This post was last edited by daqiaojsq on 2015-6-23 21:53. I have a question for everyone: when drying solids using intermittent vacuum drying, once a certain level of vacuum is achieved, the vacuum is maintained for 2 hours to carry out low-temperature drying. If the pumping capacity of the vacuum pump increases (while the vacuum level remains unchanged), will the drying rate increase? Is there any theoretical basis to explain this?
The exhaust volume should not affect the drying rate. The drying rate depends on the drying area and the temperature difference; with a constant vacuum level and the gases generated being removed promptly, it is not greatly affected by the amount of gas extraction
I can tell you for sure that it’s not possible. You need to understand the essence of drying in a vacuum – it simply means that evaporation can occur even when using a heat source of low quality. The essence of evaporation is the phase change that takes place when a substance absorbs heat; therefore, the amount of heat absorbed is what matters. And the key to determining how much heat is absorbed lies in the surface area of the material that needs to be dried. But undoubtedly, that surface area remains fixed. Do you understand now?
I performed several dryings at temperatures below 45 degrees with organic compounds containing crystalline water; previously, a double cone was always used. A rotary vibrating table can be used regardless of the density.
The lower the vacuum level, the lower the saturated vapor pressure, and the easier it is for water to vaporize
Based on your description, the evaporation rate must have increased! The reason is as follows: The pumping speed of the vacuum pump increases while the vacuum level remains unchanged, which indicates that there is more gas inside your dryer. Where does this gas come from? It can only come from something evaporating! There are too many factors involved; from what you say, that’s the situation.
I agree with the analysis on the seventh floor; if the pumping rate increases while the vacuum level remains unchanged, it must be that the evaporation rate has increased. Of course, factors such as system leaks must be ruled out. The increase in evaporation rate is due to the fact that as the pumping rate increases, the saturated vapor pressure drops, allowing more water to evaporate. Additionally, a decrease in the saturated vapor pressure leads to a slight increase in the temperature difference for heat transfer from the vessel walls to the material, which also contributes to an increased evaporation rate. However, using this method to increase the evaporation rate is not necessarily cost-effective; generally, the drying area is increased, or the heat transfer temperature difference is raised, along with improved stirring.