Membrane and Desalination Technologies - TCE Moodle Website

532 J.P. Chen et al.
corrosivity standards. The water can be stabilized by chemical treatment or by blending
with other potable water.
2.2. Working Mechanisms
Distillation is a phase separation method. The saline water is heated to produce water vapor,
which is then condensed to produce freshwater (Fig. 12.3). The distillation processes are
generally operated on the principle of reducing the vapor pressure of water within the unit to
permit boiling to occur at lower temperatures, without the use of additional heat. Distillation
units routinely use designs that conserve as much thermal energy as possible by interchanging
the heat of condensation and the heat of vaporization within the units. The major energy
requirement in the distillation process is to provide the heat for vaporization to the saline feed.
The “distillation process” used for desalination can be analyzed as “evaporation,” which is
commonly adopted for concentration of substances in chemical and biochemical industries
(11). The basic equation for solving for the capacity of an evaporator can be written as
q ¼ U A DT ¼ U A ðTsT1Þ; (1)
where q (W) is the rate of heat transfer, U (W/m 2 K) is the overall heat-transfer coefficient,
A(m 2 ) is the heat-transfer area, and DT (K) is the difference in temperature of the condensing
steam (Ts, K) and that of the boiling liquid (T1, K) in the evaporator.
The evaporation process can be determined by a heat and material balance shown in
Fig. 12.4. The feed to the evaporator F (kg/h) has a solids content of XF (mass fraction),
temperature of TF (K), and enthalpy of hF (kJ/kg). The concentrated liquid leaving the
evaporator (L, kg/h) has a solids content of XL (mass fraction), temperature of TL (K),
and enthalpy of hL. The vapor (V, kg/h) is given off as pure solvent having a solids content
of Yv = 0, temperature of TV (K), and enthalpy (Hv). The vapor (V) is in equilibrium with the
Feed (F): T F , X F , h F
Steam (S): T S , H S
Vapor (V): T V , Y V , H V
Condensate (S): T S , h S
Concentrated liquid (L): T L , X L , h L
Fig. 12.4. Heat and mass balance for single-effect evaporator.