The Vacuum Process
Patent pans of the Furnival type and the later “machine pans” all aimed to make maximum use of the heat energy of the steam from brine boiling at atmospheric pressure. However, more efficient use of this heat energy can be made by taking advantage of the fact that the boiling point of a liquid varies with the superimposed pressure and, at a lower pressure, brine will boil at a lower temperature.
At atmospheric pressure the steam emanating from boiling brine is not hot enough to boil brine in a second vessel – but would be if the second vessel was totally enclosed and at a low enough pressure for it to boil. Likewise the steam vapour from the second vessel would not be hot enough to boil the brine in a third vessel if at the pressure of the second vessel – but would be if the third vessel was at an even lower pressure. Each of these vessels is called an “effect” and such an arrangement with three vessels is “triple effect”. This is the basic principle of the “vacuum process”
The definition of the boiling point of a liquid is the temperature at which its vapour pressure equals the superimposed atmosphere .Hence the boiling temperature and pressure of the last vessel of a multi-effect evaporator is fixed by the vapour pressure of the cooling water which itself is dependent on its temperature. A lower final effect pressure will be obtained on a cold day and also at a higher barometric pressure.
The first effect of the multi-effect evaporator is heated with steam which must provide the heat to raise the temperature of the brine up to its boiling point (Specific heat) and once at the boiling point, it must supply the heat necessary to convert the water into steam (Latent Heat). Saturated brine boils at 109 degrees C and the steam must be superheated to about 126degreesC at 20 psi pressure. Under these conditions a multi-effect evaporator will operate with the first effect vessel at a pressure above atmospheric.
In the early days of vacuum evaporation the evaporator vessels were of cast iron and unsuitable as pressure vessels. This limited the pressure range and hence the number of effects it was practical to operate between the lowest vacuum of the final effect and the maximum allowable pressure in the first effect. The early evaporators were limited to three effects and a maximum steam pressure of about 25psi. In the last fifty years with improvements in design and the introduction of vessels of welded Monel-lined steel, it has been possible to extend the pressure range and increase the number of effects, at first to four, and present day salt plants operate with six effects. Nowadays a typical first effect vessel will operate at pressures of around 50psi.
Parallel with the development of vacuum evaporator in the last century has been an increase in steam economy by the combining of a vacuum plant with electrical power generation. This permits the economic generation of high pressure steam for driving steam turbine generators and then using the lower pressure ‘pass-out’ steam for brine evaporation.