[TokiyoRio]

It says in my textbook that "Temperature control is unlikely to be effective for condensers, unless the liquid stream is subcooled". For Reboiler and Vaporizer control, it says later on that "As with condensers, temperature control is not effective, as the saturated vapor temperature is constant at constant pressure".

My question is that if you have a two component mixture, why can you not use temperature control on the product stream as the difference between the dew and bubble temperatures are non-negligible?

[johnsmith101]

It just states that it's ineffictive. Plenty of multi-component operations utilize column overhead vapor temperature to the condenser and/or bottoms product temperature to "infer" product quality.

If I'm reading your statements correctly, you should never design the condenser / reboiler duties to control the condenser / reboiler outlet temperatures though. If you have a mixture of two or more components, the outlet temperature from condenser / reboiler will be somewhere between the bubble point and Dew point of the mixture (small range with little change with duty adjustments).

[pinknemo13]

If I understand your question and I'm not sure I do, because you don't know how much of the liquid component is hitting the temperature sensor. Yes it's true, it reads a temperature between the gas temperature and the condensed liquid even if that's infinitesimal. An atmospheric wet bulb temperature measurement is done with the second bulb sitting in a saturated wad of material wrapping the thermometer. It's then swung through the (unsaturated) air at high velocity but keeping the (saturated) wad wet. If the wad started to dry out, is then as much as a heat transfer question as a mass transfer question.

Perhaps a sketch would help clarify.

[bravesix]

A condenser is designed for a maximum pressure and temperature. Normal operation of a condenser should include an outlet gas maximum temperature set point and a fixed inlet and outlet coolant/refrigerant temperature. The outlet gas temperature is a critical indicator of the efficiency of a condenser.