Page 187 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.6 Evaporator design 185

There are three principal elements in evaporator design e heat transfer, vapor-liquid separation,
and energy utilization. Among these, heat transfer is the most important factor since the heat transfer
surface represents the largest component of evaporator cost.

6.6.1 Single-effect evaporation

The surface area of a continuous single-effect evaporator is obtained from the heat transfer equation as
q
A ¼ (6.1)
UDT eff
Where q is obtained from energy balance neglecting condensate subcooling and radiation losses. It is
important to have a clear understanding of the basis of definitions of DT eff and its corresponding U.
The effective temperature driving force for heat transfer (DT eff ) is given as e

DT eff ¼ T con T (6.2)
Where T con is the temperature at which the heating vapor (steam) condenses and T is the bulk liquid
temperature, which is estimated by adding the boiling point elevation (BPE) of the liquid leaving the
effect to the boiling point (T s ) of pure solvent (water in most cases) at the operating pressure (P) in the
effect, i.e.,

T ¼ T S þ BPE (6.3)
BPE is a function of the solute concentration x in the thick liquor leaving and is available from
literature and T S is found from steam table as saturation temperature corresponding to pressure P.
Feed to the evaporator can be at a lower temperature than the effect but quickly attains the bulk
liquid temperature (T) due to well-mixed liquid phase in evaporators. The heating steam enters at a
temperature T steam with a few degrees of superheating and quickly cools to its saturation (condensing)
temperature (T con ). Therefore the effective temperature driving force for heat supply to the effect is
considered as DT eff ¼ T con T, and the heat transfer coefficient (U) is defined with respect to this
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temperature driving force. In an evaporator, the average boiling point of liquid is higher than the
boiling point corresponding to the pressure in the vapor space due to the effect of hydrostatic head.
This reduces the DT across the heating surface, thereby causing a decrease in evaporator capacity.
Fig. 6.17 shows the parameters of material and energy flows around a single effect with the
notations F, L, V, S corresponding to feed, liquor, vapor, and steam mass flow rate, respectively. Solute
concentration (x,x F in % w/w), enthalpy per unit mass (h) and temperature (T) and pressure (P) values
are marked therein. Subscripts L, V, steam, and con denote thick liquor, vapor leaving, supply steam,
and condensate generated. h steam is obtained from the steam table at steam supply pressure P steam and
temperature T steam . Enthalpy of feed and thick liquor are respective functions of solute concentration
(x F and x) and temperature. These are found from enthalpy charts (see Fig. 6.22) for different solute-
solvent systems. In absence of such charts or experimental data, information can be obtained from
solution rules with or without considering the heat of solution.

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Earlier practice was to define U corresponding to the apparent temperature difference DT app ¼ (T con T s ). One must be
careful while employing data reported in literature and check whether the value of reported U is based on true or apparent
temperature difference as DT app DT eff .