Page 523 - Industrial Wastewater Treatment, Recycling and Reuse

Zero Liquid Discharge Solutions 493

Vapors
Steam

Condensate

Feed

Figure 13.1 Falling film evaporator.

• Useful for heat sensitive chemical solutions, due to lower residence time
and lower holdup (see Figure 13.1).

Limitations
• Falling film evaporators are not effective for liquids having higher
suspended solid concentrations and higher viscosity.
• Inadequate wetting can lead to tube-fouling in the evaporator’s distribu-
tion system. Thus, design of distributor system is critical.
• Frequent cleaning in place (CIP) is required and involves actual experi-
ence to confirm cleaning frequency.

13.3.1.2 Forced Circulation Evaporators
When the occurrence of boiling at the heating surfaces must be avoided in
order to minimize fouling problems or crystallization, force is used to
increase the flow velocity, as in the forced circulation evaporators. The
advantages and disadvantages of forced circulation evaporators are briefly
outlined below:

Advantages
• The forced circulation evaporator is capable of processing suspended
slurry, saturated solution, and viscous liquids, and therefore, it can
accommodate a wide range of process liquids (see Figure 13.2).