Page 179 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.4 Evaporator performance 177

highest temperature steam and is advantageous for a hot feed or a thermally sensitive concentrated
product that would be damaged/deposit scale at high temperature. Low steam economy with cold feed
can be improved by preheating feed, in stages, with vapor bled from intermediate effects.
In the backward feed, the feed enters the last (coldest) effect, and the product is discharged from the
first effect. Intermediate liquor pumps are essential to transfer liquor to
destination effect at a higher pressure. The backward feed is advantageous for
cold feed or viscous products and usually has the highest steam economy.
Backward feed
When product viscosity is high, but a hot product is not needed, the liquid
from the first effect is sometimes flashed to a lower temperature in one or
more stages, and the flash vapor is added to the vapor of the latter effects.
Thus, for hot feed, forward feeding possesses greater economy while for cold feed, backward
feeding is more economical.
Mixed feed combines the advantages of higher steam economy of backward feed arrangement and
also permits the final evaporation to be done at the highest temperature. This
reduces the number of intermediate liquor pumps and is used for special ap-
plications, for example, when liquor at an intermediate concentration and a
Mixed feed
certain temperature is desired for additional processing.

In a parallel feed arrangement, feed is pumped, and liquor is withdrawn from each effect while the
steam flow is similar to the previous cases. Primarily, this is used when the feed
is substantially saturated and the product is a slurry or a solid; for example, the
saturation of brine to make common salt. They are employed in services with a
Parallel feed
higher tendency to scale and foul. Fresh feed entering the effects provides a
washing effect for the deposits. This arrangement is also useful when more than
one product concentration is required to be produced.
A comparison of the four configurations is presented in Table 6.1.

Use of vapor as a “hot stream” in the plant
Vapor from the multiple-effect evaporator system can also be viewed by the designer as a cheap source
of heat (low pressure steam). Part of vapor from an effect can be utilized elsewhere after it has already
generated i times its own weight of vapor (i is the effect number from where the vapor is withdrawn).
This is an important aspect of improving the general economy of a plant and is fully exploited in the
beet sugar industry. No special provision in the design is necessary if such withdrawals are small. For
substantial withdrawals, additional area has to be provided in subsequent effects to ensure a reasonable
temperature difference.

6.4.2 Vapor recompression
In this scheme, vapor from an effect is compressed to increase its condensation temperature, and the
heat of condensation released at this higher temperature is utilized for heating the same or some other
effect of the same evaporator. The extra heat energy available in vapor recompression thus involves
input energy for compressing the vapor to the required pressure. The compression may be by me-
chanical means (mechanical vapor recompression) or by steam jet ejector (thermocompression).