Temperature measurement under vacuum conditions
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In the jacketed reactor with stirring, the material inside is a solid-liquid mixture; steam is passed through the jacket to raise the temperature and evaporate the liquid. When the temperature reaches a certain level, it is necessary to create a vacuum of -0.08 MPA in order to continue evaporating the liquid. When there is little liquid left, the supply of steam is stopped to continue the evaporation process until very little liquid remains in the reactor. During the vacuum creation process, the temperature keeps dropping. The thermometer is located inside a sleeve, which contains heat transfer oil; the sleeve does not come into contact with the material. Is the temperature indication related to the actual temperature of the material inside the tank?
It’s definitely related to the material; at this point, the temperature indicated is due to the heat absorbed during solvent evaporation, which causes the temperature to keep dropping. If the amount of solvent is very small, the temperature of the solid should be higher than the temperature measured at this time. But it won’t be much higher either. Your steam has been turned off; there is no heat source anymore. The evaporation of the remaining amount of solvent at the end can also help to reduce the temperature.
Under this vacuum, the boiling point of the solvent is lower than the indicated temperature.
When the solvent is about to evaporate completely, the temperature inside the vacuum tank is higher than that of the solvent. Because the evaporation of a small amount of solvent is no longer sufficient to offset the residual heat in the reactor.
Does that mean the solid’s temperature is higher than the displayed temperature?
Give it a push for myself; I hope more people will see it
What you are measuring now is the temperature of the gas phase, while the temperature of the material is in the liquid phase; there is a difference, and it’s quite significant. It is recommended that you use a bottom-valve thermometer to measure the temperature of the liquid phase, or that you consider replacing the reactor with one that comes equipped with a temperature sensor port on the side.
It is already known that the device has such a flaw; now it seems that the temperature value doesn’t hold much significance, as adjustments to the temperature have no effect on the reaction outcome
The gas phase temperature measured in this way is lower than the temperature of the solid-liquid mixture inside the reactor. When the process is under heating, the temperature difference is not significant; however, after heating stops and evaporation relies solely on vacuum, the gas phase temperature becomes slightly higher than the surface temperature of the solid-liquid mixture... For more accurate measurements, it is best to place the temperature sensor on the stirring blades!
As you said, is the temperature displayed under vacuum higher than the actual temperature of the material? The material is not a solid either; it’s just viscous when evaporated
Yes, in a vacuum state, the temperature of the gas phase is slightly higher than that of the surface of the evaporation medium. The reason for the lower temperature at the surface of the evaporation medium is that evaporation removes heat; due to heat conduction, this temperature is slightly lower than that of the gas phase； Conversely, when the process is still in the heating and vacuuming stage, the same factors cause the temperature of the gas phase to be slightly lower than that of the evaporating medium... In the two processes, differences in operating conditions result in distinct physical and chemical phenomena... which are exactly the opposite in each case