Consider a batch fluidized bed dryer operated under negative pressure (sucks air through a blower in the exhaust line), during the constant rate period, air loses its energy adiabatically to create space for evaporation. The exit air temperature will be constant for constant rate of drying and should not be less than Adiabatic saturation temperature/wet bulb temperature. However air experiences a pressure drop when it passes through the particle bed leading to drop in air pressure simultaneously. How to deal with both the pressure drop which causes throttling and energy exchange simultaneously to determine the new outlet temperature.
I think that you are missing the bypass of inlet gas in the bubbles. The dense phase will be in equilibrium but the dispersed phase is not. You would get equilibrium only if you operated at the minimum fluidising velocity.
The answer is: yes, of course. The wet-bulb is approximately equal to the adiabatic saturation temperature--approximately, in a perfect world where the Lewis number is unity and everything is nicey nicey. In the real world there are situations where it's hard to measure the wet-bulb or the dew-point or even the dry-bulb and local air pressure. It can also be difficult to maintain a steady-state process long enough and smooth enough to accurately measure what's happening.
Hi Pavan and all.
Above the inversion temperature, gases cool on expansion (strictly, at constant enthalpy). So, for a tall fluidized bed of heavy particles, that is, operating under a high pressure drop, it could, theoretically, happen. If you want to avoid the effect (or, at least, reduce it) increase the bed cross section for the same weight of solids. That is, go to shallower beds.
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