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Can you offer insight into the reasons for such low heat transfer efficiencies?

Q: Our evaporator operates in vacuum of 600 mm Hg. The tube-side fluid is 26% phosphoric acid, which is being concentrated to 48%. The shell-side fluid is steam at a pressure of 3kgf/cm². The evaporator system consists of a series arrangement with respect to feed but has fresh stem to each evaporator. The heat exchanger consists of graphite tubes and it is a forced circulation type. The evaporator is showing an efficiency of around 60% only and is prone to frequent failures. We suspect the presence of solids(predominantly Gypsum) being one of the reasons. Can you give us more insight into the possible reasons for such low efficiencies?

Additional Data:

1. Evaporator is downstream equipment for Phosphoric acid produced using Di hydrate process
2. Vacuum is maintained by liquid seal ring vacuum pumps
3. Equipments present between evaporator and vacuum pump are mist eliminator and fluorine absorption section
4. Frequent failures of circulation pumps in evaporators

A:

Although I am not familiar with the specific equipment design for the wet phosphoric acid process, the forced circulation evaporator design may have insufficient circulation and/or surface area.  This can manifest itself in two ways: 1) velocity and wall shear stress are not high enough to reduce fouling potential.  2)  Wall temperature may be high enough to exceed solubility limits of solids with reverse solubility temperature behavior (carbonates for instance).

In general the best approach to systems that may be prone to fouling is to maximize velocity (and therefore wall shear stress) within the constraints of pressure drop and erosion/corrosion and minimize wall temperature.  This approach typically will produce a higher power pump and larger exchanger, but will increase the mean time between failures.  The Reynolds number of the fluid in the exchanger should also be checked to assure turbulent flow since the viscosity is a strong function of both temperature and concentration of the acid.

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