Negative pressure distillation
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I’m a newcomer and have a question. In a closed negative-pressure distillation column, the pressure at the top of the column is 7 KPaA. The gas from the top of the column passes through a condenser and then to a reflux tank; the gas in this reflux tank is connected to a vacuum pump that creates a vacuum. The pressure in the reflux tank is 6 KPaA. The liquid at the bottom of the tank is returned to the column using a centrifugal pump. Is the pressure at the outlet of the centrifugal pump also negative? The pressure indicated on the gauge back at the top of the column is 45 KPaA. I would like to ask the experts: can the outlet of a centrifugal pump be under negative pressure? Why can the pressure of the liquid medium be negative?
It is necessary to consider the entire process principle as a whole; within a closed process system, such pumps can operate as long as their operating conditions meet those required for normal pump operation. As for the negative pressure that is generated, it should be a negative pressure condition created through methods such as condensation or vacuum pumps. It’s a guess, for reference only. :P
The outlet of a centrifugal pump certainly is not under negative pressure – isn’t there a pressure gauge installed? There is a liquid column from the reflux tank to the inlet of the centrifugal pump; the static pressure generated by the height of this liquid column can counteract the negative pressure created by the vacuum pump. As for the pressure gauge in the upper tower showing 45 kPa, you should check whether half-pipe operation is being used, as a pipeline filled with liquid cannot generate negative pressure
I think the same way; the negative pressure in front of this tower should be relatively close to the location of the tower. It seems unlikely that there could be a vacuum condition inside the tubes filled with liquid. As for whether the outlet of a centrifugal pump can be at negative pressure, that’s something I’d like to know too. Is it necessary for the pump’s outlet to be at positive pressure?
The outlet of a centrifugal pump should be able to operate at negative pressure; as long as it is higher than the system pressure, it can meet the required flow rate. However, I have never seen this done – it must make monitoring difficult. As long as the temperature of a liquid is below its boiling point at that pressure, it remains a liquid.