Page 180 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

178 Chapter 6 Evaporators

Table 6.1 Comparison of multiple effect evaporator arrangements.
Arrangement Forward feed Backward feed Parallel feed Mixed feed
Entry of First effect Last effect Each effect Intermediate effect
· Feed First effect First effect First effect First effect
· Steam
Liquor flow Parallel to steam Opposite to steam Independently in Forward feed for
flow flow each effect some effects and
backward feed for
rest
Product Last effect First effect Each effect First effect
withdrawal from
Advantages Needs no inter- - Higher steam - Simplest in - Eliminates
effect transfer economy construction some
pump - Suitable for - Feed washes intermediate
viscous deposit/scales liquor pumps
products - More than one - Allows product
product draw from the
concentration highest
obtained temperature
stage
Limitations Low steam - Needs inter Low steam Low steam
economy for cold effect transfer economy economy
feed pump
Applications - Hot feed - Cold feed - Used as - Used when
- Product with - High product crystallizing liquor at
scaling viscosity evaporator intermediate
tendency at a (e.g., Brine concentration
higher evaporation to is also a
temperature make common product
- Product salt)
sensitive to a
higher
temperature
General Greater economy Greater economy
observation for hot feed for cold feed

Thermocompression (TC) is attractive when steam is available at pressures much higher than can
2
2
be used in evaporators, (>3 kg/cm (g)) and preferably over 7 kg/cm (g)).
Usually, a portion of the steam evaporated from the product is compressed
by a steam ejector and returned to the steam chest. The ejector also serves
Thermocompression
as a reducing valve. Due to its low first cost and ability to handle large
volumes of vapor, this method is an attractive option to improve the
economy of evaporators that must operate at low temperatures, and hence,
cannot be operated in the multiple-effect configuration. TC units do not operate well outside the design