Liquid Flash T Increase

Lili Lyddon

January 9, 2007

There has been a question concerning the temperature coming out of the rich amine flash in ProMax. Often when a higher pressure liquid stream is flashed to a lower pressure some vapor is generated and some cooling occurs, however, this is not always the case. Instead, when the pressure of a rich amine stream is reduced, a small amount of dissolved hydrocarbon and acid gas is flashed off and the temperature heats up by a couple of degrees. For example, the rich amine from the absorber is flashed from 1014.7 psia to 100 psia, and the temperature increases from 157.982 to 160.159 oF, which is about a 2 oF temperature increase. A small increase in temperature is predicted by ProMax for all the amines, and the larger the pressure drop the greater the temperature increase. The steam tables demonstrate that when a subcooled (compressed) liquid has the pressure reduced to the saturation pressure, the liquid heats up. This phenomenon can be explained as follows: As vaporization occurs energy is needed to supply the heat of vaporization. The energy available from the enthalpy change which occurs when the compressed liquid is throttled can either go into sensible heat effects (increasing the temperature) or go into vaporizing some of the liquid (heat of vaporization) with no temperature change. If enough vaporization occurs even more energy is needed. This energy must come from the sensible heat effects and the temperature decreases. Thus when a compressed liquid is throttled the temperature can either increase or decrease, depending on the amount of energy needed for vaporization compared to the enthalpy change in the liquid. Even when only hydrocarbons are present a temperature rise may occur when a higher pressure solvent stream is flashed.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)