Page 177 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.4 Evaporator performance 175

The scheme shows feed mixed with recycled product from a tank (1) being fed by a pump (2) to the
evaporator (4) via preheating heat exchanger (3). The vapor and liquid from evaporator separate in the
separator (5). Vapor leaving the separator condenses in the condenser (8). The system vacuum is
created by the liquid-ring pump (9) pulling out the noncondensable from the condenser shell. The
product, under vacuum, is drawn by a pump (6), part of which is recycled back with feed.
Evaporative crystallizer enables product formation as crystalline solid instead of thick liquor.
Crystals of salts like sodium chloride and ammonium sulfate whose solubility does not change much
with temperature are economically produced in this equipment. These are also used for crystallizing
solutes, whose solubility decreases with increasing temperature (solutes with inverted solubility
curves). Most evaporative crystallizers use an external heat transfer surface. In some cases, further
cooling may lead to the formation of additional crystals. Uniformity of crystal size is ensured by using
forced circulation. Problems of salting, scaling, and fouling are more common in crystallizing
evaporators and these are minimised by providing sufficient submergence and suitable body geometry.
Evaporators without heating surfaces
The submerged-combustion evaporator comprises a tank, where the liquid is heated by direct contact
with combustion gases. A burner and gas distributor is immersed into the liquid, thus eliminating the
need for heat transfer surface. This makes the equipment suitable for severely scaling and corrosive
liquids and for nonheat-sensitive thermally stable materials, where contamination by combustible
gases is acceptable. Since the vapor is mixed with large quantities of noncondensable gases, it is
impossible to reuse the heat in this vapor. Therefore, these units are economical, where the fuel cost is
low. High entrainment loss is a concern. These evaporators cannot be used when control of crystal size
is important.
Disk or cascade evaporators consist of a rotating horizontal shaft on which disks perpendicular to
the shaft or bars parallel to the shaft are mounted. These are primarily used in the pulp and paper
industry for a final concentration of the black liquor before it is burnt in the boiler and to recover heat
and entrained chemicals from boiler stack gases. The assembly is partially immersed in the thick black
liquor, so liquor film is carried into the hot-gas stream as the shaft rotates.

6.4 Evaporator performance
The thermodynamic efficiency of an evaporator is very low. This is because the useful work, equal to
the heat that would be liberated by mixing the product and the liquid solvent to reconstitute the feed
(heat of mixing) is very low compared to the energy input to create the vapor phase (the latent heat of
vaporization). One may recall that thermodynamic efficiency for distillation is also very low due to the
same reason.
Evaporator performance is, therefore, not measured by thermodynamic efficiency but by “steam
economy,” also known as “economy.”
Economy for an evaporator is defined as kg solvent evaporated per kg steam used. This can be
improved by the use of (A) multiple effects and (B) vapor recompression. In this chapter, we refer to
the heating medium as “steam” and the product of evaporation as “vapor.” Thus, in multiple-effect
evaporators, one effect produces vapor, which becomes steam for the next effect. Customarily,
steam refers to water vapor and vapor refers to vapor from any liquid, not necessarily water.