Page 178 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

176 Chapter 6 Evaporators

6.4.1 Multiple-effect evaporators
In multiple-effect evaporators, the effects are arranged in order of decreasing operating pressure that
results in lowering of the boiling temperature of the liquid in succeeding effects. This enables vapor
generated in one effect (at a higher temperature) to be used as the heating medium for the next effect,
thus utilizing the latent heat of vapor generated in all effects except the last. Steam from an outside
source is required only for the first effect. Accordingly, the steam economy improves with an
increasing number of effects. On some large duty evaporation systems, it is economical to utilize as
many as seven effects. At the same time, for any particular duty, increasing the number of effects
significantly increases the capital cost. The optimum number of effects is decided by an economic
balance between saving in steam and added investment cost, as discussed in Section 6.6.2. In general,
when the evaporation load is above 1400 kg/hr, multieffect evaporation is considered.
Feeding arrangements
There are different configurations of multiple-effect evaporators, depending on the flow arrangement
of feed and heating vapor. As shown in Fig. 6.14, these include forward feed, backward feed, mixed
feed, and parallel feed. In all cases, the effects are numbered (Roman numerals in this book) in the
direction of steam flow irrespective of the feed flow direction. The inter-effect transfer pumps are only
shown in the figures and the feed and product pumps required in all cases are not shown. A comparison
of the four configurations is discussed below and summarized in Table 6.1.

(A) Vapour to (B) Vapour to
condenser condenser
I II III I II III
Steam Steam

Condensate
Condensate Feed
Feed Product Product

(C) (D)
Vapour to Vapour to
condenser condenser
I II III I II III IV
Steam
Steam
Condensate
Condensate
Feed Feed Feed
Product Product Product Product Feed
FIGURE 6.14
Different configurations of multiple-effect evaporators e (A) Forward feed, (B) Backward feed, (C) Parallel
Feed, (D) Mixed feed.

In the forward feed configuration, the dilute feed and the heating steam enters the first effect, and
the liquor flow is parallel to the steam flow. A feed pump is required to introduce
the feed to the first effect (often at about atmospheric pressure) and also to
withdraw a product from the last effect mostly operating under vacuum. Inter-
Forward feed mediate liquor transfer pumps are not needed as the flow occurs due to pressure
differential between successive effects. Control valves on these transfer lines
control the liquor level in the effects. This configuration exposes feed to the