Page 186 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

184 Chapter 6 Evaporators

6.5.2 Vent systems
Noncondensable gases are invariably present in the vapor as a result of air in-leak, dissolved air in feed
or reaction during evaporation. When the vapor is condensed in a succeeding effect, the concentration
of noncondensable gases increases and impedes heat transfer. Therefore, noncondensable gases should
be vented well before their concentration reaches 10%. Since gas concentrations are difficult to
measure, the usual practice is to overvent and an appreciable amount of vapor can be lost in poorly
designed systems.
Venting is usually done from the upper part of the steam chest of one effect to the steam chest of the
next, which is at a lower pressure. This line is provided with a throttling valve to limit the flow and
ensure that the excess vapor in one vent performs useful evaporation at a steam economy only about
one less than the overall steam economy. When there are large amounts of noncondensable gases
present as in beet-sugar evaporation, it is desirable to pass the vents directly to the condenser to avoid
serious losses in heat-transfer rates. In such cases, it can be worthwhile to recover heat from the vents
to preheat the entering feed in separate heat exchangers.
The noncondensable gases eventually reach the condenser (unless vented from an effect above
atmospheric pressure to the atmosphere or to auxiliary vent condensers) and will be joined by the
dissolved air in case of direct contact condenser. A water-jet-type condenser or a separate vacuum
ejector may be used to remove these gases. Reciprocating pumps or water-ring (Hytor) pumps may
also be used if high-pressure cooling water or steam is unavailable.
Salt removal
In crystallizing evaporators, salt crystals are to be removed along with a minimum quantity of mother
liquor. In installations with sufficiently high headroom, the body is often located above the barometric
height, and the lower part of the body provides a settling zone, where salt concentration builds up. Large
diameter drains, at convenient locations drain the salt by gravity in short periodic cycles with only a
small risk of plugging. In situations, where the headroom is limiting, and the salt removal rate is up to
1000 kg/hr, a salt trap vessel is connected at the bottom of the effect where the salt settles. The trap vessel
is periodically isolated from the effect and emptied. The air in the trap must be displaced fully with feed
liquor before reconnecting the trap, or else this creates problem in condensation of vapor.

6.6 Evaporator design
A properly designed evaporator must, at a minimum:

• effectively transfer heat at a high rate with the minimum surface area so that it is economical for
installation, operation, and maintenance
• effectively separate vapor from liquid concentrate
• meet solvent evaporation capacity
• meet product quality (concentration)
• be energy efficient by effective use of steam in multiple-effect evaporation or vapor
recompression, wherever possible
• minimize fouling of heat transfer surface
• be constructed of materials which minimize corrosion