Page 182 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

180 Chapter 6 Evaporators

6.4.3 Heat recovery systems
The following heat recovery schemes are considered during design to increase the economy
• Heatexchangebetweenoutgoing(productandevaporatorcondensate)andincomingstreamstoensure
the minimum possible temperature of outgoing and maximum possible temperature of incoming
fluids. Thisoften requires additional liquid-liquid exchangerswhichare justifiable only inlarge plants.
• Condensate flash system in which the condensate from each except the first effect is flashed in
successive steps to the pressure of the heating element of the succeeding effect. Some amount of
condensate flash may also be used for feed preheating. Product flash tanks may be used in
backward or mixed feed evaporator.
• In the forward feed evaporators, the principal means of heat recovery is by using feed preheaters
heated by vapor blend from each effect. A feed preheated by last effect vapor also reduces
condenser water requirements.
6.4.4 Evaporator selection
The selection of evaporator type best suited for a particular service is governed by (A) heat transfer
considerations, (B) characteristics of feed and product, such as (high) viscosity or (high) solid content,
throughput, fouling tendency and foaming tendency, (C) product quality and (D) corrosion.
However, for simple applications, several types may serve equally well. In such cases, the choice
may be dictated by factors like capacity, small batch production, past plant experience, available space,
operating manpower requirement, utility requirement, maintenance required, and/or cost.
Typical guidelines for selecting evaporator type and configuration are listed below and a
comparison between the different types is presented in Table 6.2.

➢ Batch or stirred batch evaporator preferred for low capacity or multiproduct batch production.
Although it may require more cleaning time, it is typically a low maintenance system. For a high
capacity system, a continuous process is usually used.
➢ Tubular evaporators are the first choice where applicable and are usually the choice for very large
2
systems operating above 0.7 kg/cm (g) and handling suspended solids.
These require less floor space and have fewer gasket limitations as compared to plate evaporators.
➢ Vertical short tube evaporators occupy lower headspace, and rising-falling film type requires
smaller floor space.
➢ Plate evaporators are often the choice for heat-sensitive products and viscous products.
They require lower headroom, less expensive building and installation costs and are also easier to clean.
They allow flexibility as modules can easily be added or removed.

➢ Film evaporators assure a high product quality for heat-sensitive products since they offer the
dual advantage of low residence time and low-temperature difference
• Falling film evaporators (plate or tubular), provides the highest heat transfer coefficient but
are not suitable for heavy fouling products and products with viscosities above 300 cP.
• Falling film plate evaporator ensures the shortest residence time.
• Agitated thin film type is uneconomic for large scale and is used only for a highly valued product
➢ Plate-and-frame and agitated thin-film evaporator are preferred for products that are highly
temperature-sensitive, very viscous, high fouling, or have high solids content.
➢ Forced circulation evaporators can be operated up to 5000 cP viscosity and significantly reduce
fouling. Although capital and operating costs are high, a higher heat-transfer coefficient offsets the
capital cost in some cases. In the case of crystallizing evaporators, the requirement for producing
crystals of a definite uniform size usually limits the choice to forced circulation evaporators.