Thermal Evaporation and Crystallizer

Thermal process engineering is a competence field where thermo dynamical aspects, as heat and mass transfer, and chemical
kinetics are evaluated contemporarily. The scope is the transformation of matter and energy as well as the product formulation. In
the zero liquid discharge field and in many other processes, the application is named Thermal Separation.

Gemwater can provide the three types of thermal separation which can be sum-up as:
a) Distillation: to separate liquids from gases (or vapors)
b) Evaporation: to separate liquids from liquids
c) Crystallization: to separate solids from liquids.

This classification is of course approximate, since gases/ vapors, liquids and solids may exist at the same time.
For simplicity’s sake, we will call “evaporator” the generic thermal separation device.
In a typical installation, an evaporator will increase the concentration of one of the components, the high-boiling one by thermally remove the other component, the low-boiling.
The scope is to recover one, or both of them, as re-usable substances.
Let’s make an example of thermal separation:
An aqueous solution of sulphates, combined with different anions, is available as the result of the necessary pretreatment. We know the composition of the salts, say Na2SO4 and (NH4)2SO4. These solutes have different solubilities; sodium sulphate has a solubility peak at approximately 33°C as decahydrate salt, while ammonium sulphate has direct solubility; the sodium sulphate solubility is low at, say, 15 °C, while the ammonium sulphate has a higher solubility.
So we may precipitate Na2SO4 .10H2O while keeping (NH4)2SO4  in solution.
See the solubility curves below :

In some cases we have single effect evaporators, where live steam is used once and vapours are condensed. Other types of evaporators may exist.
If the vapor pressure allows it, we may reuse the outcoming vapors to heat a second evaporator: this is   called a double effect evaporator.
The number of effects may be higher, and it is relatively common to find quadruple, quintuple effects and more.
In the simplest case, steam produced in an external boiler is condensed in the evaporator heat exchanger. We can call them “ Steam Evaporator “.
By definition, an evaporator releases vapors - another name for steam - and it is therefore possible to use these vapor, duly compressed, to heat the liquid to be evaporated - a sort of heat pump.
Such a device is defined recompression or thermocompression evaporator. Depending on the way recompression is achieved, we may have thermal recompression or  mechanical recompression
In small units the recent trend is to use intermediate gases, usually HCFC, used in a refrigeration
cycle uses as heat pump.

Submerged (or direct fired) evaporators have a (usually gas-fired) burner operating in the liquid mass. These models are now obsolete due to the very poor thermal efficiency, the maintenance cost due to corrosion at high temperatures, and the environmental problems due to the mixing of fumes to vapors.
A somehow simple example of crystallization is the concentration of exhaust oil from some source.
As an example, the oil-water emulsion is concentrated from a typical 10% w/w oil to 50% – 60%, thus allowing the emulsion to be incinerated without the support of external fuel.  

In case of crystallizing liquors, where solids are present, the evaporator will be of the forced circulation (FC) type, providing controlled flow through the heat exchanger, thus allowing a more precise control of all the evaporator parameters.
Sometimes these systems require tests and trails to adjust the process parameters to the solution to be treated. In those cases Gemwater designs and provides PILOT UNIT for this purpose.