Page 197 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.6 Evaporator design 195

Design input
Process design of evaporator with N effects starts with known
(i) feed rate (F), feed conditions (x F ,T F ) and desired product concentration (x P )
th
(ii) Pressure (P N )in N stage (last effect).
(iii) System properties e calculation procedure and/or tables for saturation temperature of solvent
(water: steam tables), BPE, enthalpy of solution and solution properties (may be available in the
form of Figs. 6.20 and 6.22 as already discussed).
(iv) Steam supply pressure and temperature (P steam and T steam ). Usually T steam will have a few
degrees of superheat.
(v) Evaporator type and configuration and the estimated heat transfer coefficient of each effect (U 1 ,
U 2 , . U N ). This is usually known from past experience of design and operation. It is important
to note that for evaporators, U i is based on the temperature difference of condensing vapor/
steam in the steam chest (T s (P i 1 )) and the (saturation) temperature of vapor leaving ith effect.
In case of the first effect, T s (P i 1 ) ¼ T s (P steam ).

Process design output for the chosen type of multiple-effect evaporator includes:
(i) Heating steam requirement e estimated from mass and energy balance
(ii) Heating surface requirement e estimated from heat transfer equations
(iii) Estimated temperature in each effect
(iv) Amount of vapor leaving the last effect and going to the condenser
The design solution involves trial and error.
Design objective
Meeting the final product concentration with high steam economy is the foremost objective. The
multiple-effect evaporator design aims to arrive at a design with (nearly) same heat transfer area for
each effect. This is an indirect way to economize on fabrication cost and minimize inventory cost by
holding common spares.

Design deliverables
To summarize, process design output gives the heat transfer areas (A i ,i ¼ 1, .,N) and also includes
values of all variables shown on Fig. 6.23, whose values are not specified as design input.

Design algorithm

i) Note the inputs: N, F, x F ,T F ,P N, U 1 ,U 2 , . U N
ii) Find the value of T s (P N ) from steam table
iii) L N ¼ F x F /x P
iv) V total ¼F L N N 1
P
v) Assume: V i: i ¼ 1, .,N 1; Calculate V N ¼ V total V i ; Initial guess can be V i ¼ (F L N )/N,
for i ¼ 1,...,N. i¼1
vi) L 1 ¼ Fe V 1 and L i ]L i 1 V i for i ¼ 2, .,N 1
vii) BPE i ¼ BPE(x i ), for i ¼ 1, .,N e this can be found from the Duhring plots (Fig. 6.20) that are
linear and pass through the origin.