Pollution Prevention and Control Technologies for Plating
Section 3 - Chemical Recovery
3.3 VACUUM EVAPORATORS
Vacuum evaporators are one of the earliest technologies used in
the plating industry for chemical recovery. However, vacuum evaporators
are currently used less frequently than some other recovery technologies,
such as atmospheric evaporators (see Section 3.2).
This is primarily due to the fact that the average vacuum evaporation
unit costs approximately ten times more than the average atmospheric
unit. Also, the vacuum units have more sophisticated and expensive
operational and maintenance requirements. Of the 318 plating shops
responding to the Users Survey, 23 shops (or 7.2%) have employed
vacuum evaporators (30 total units of which approximately 80%
were still in operation at the time of the survey) for chemical
recovery, whereas, 71 shops (or 22.3%) have used atmospheric evaporators
(86 total units). Another six shops (or 1.9%) use vacuum evaporators
as end-of-pipe technologies to concentrate their wastes prior
to off-site hauling and disposal. This section discusses recovery
applications of this technology and Section 6.4.4 addresses end-of-pipe
A vacuum evaporator is a distilling device that vaporizes water
at low temperatures when placed under a vacuum. The following
explanation of the kinetic theory of liquids and in particular
vapor pressure helps in understanding this phenomena.
Liquids as well as gases are in constant motion in varying degrees,
depending upon the chemical composition of that matter and the
temperature and pressure applied to it. Molecules near the surface
have a tendency to escape into the surrounding atmosphere. In
open systems, most of these molecules do not return to the liquid
and the substance is said to vaporize. In a closed system, molecules
return to the liquid in proportion to their concentration in the
gaseous phase. Eventually a steady state is reached where the
quantities of molecules leaving and returning to the liquid are
equal. The vapor is then said to be saturated and the pressure
exerted by these escaping molecules is referred to as vapor pressure
(ref. 361). Since the kinetic energy of all molecules increases
with increasing temperature, so does the vapor pressure. When
a liquid reaches the temperature at which its vapor pressure becomes
equal to that of the atmosphere above it, boiling occurs. This
is the rapid evaporation from all parts of the liquid mass, with
bubbles of vapor forming in the interior and rising to the surface.
Liquids with appreciable vapor pressure may be caused to boil
over a wide range of temperatures by decreasing or increasing
the pressure of the atmosphere above it (ref. 362). For example,
water boils at 212oF at sea level, but will boil at room temperature
if the pressure above it is reduced to about 0.4 psia (ref. 361).
Vacuum evaporators depend on the fact that water, when introduced
into a vacuum, tends to boil off, or vaporize. The rate of vaporization
is directly related to the level of the vacuum and the temperature
of the solution. In operation, heated solution is introduced into
the vacuum chamber, the boiling point of the solution is reduced
by the vacuum and the resultant vapor (distilled water) is removed
from the chamber. The vapor can be either discharged or can be
condensed for return to the process (e.g., as rinse water).
Vacuum evaporation systems are relatively complex and are therefore
more expensive to construct and maintain than the more simple
atmospheric systems discussed in Section 3.2.
There are several types of vacuum evaporators used in the plating
industry: rising film, flash type, and submerged tube. Generally,
each consists of a boiling chamber which is under a vacuum, a
liquid/vapor separator and a condensing system. Site-specific
conditions and the mode of operation influence the selection of
one system over another.
Two techniques have been applied successfully to reduce steam
demand for evaporation; both involve reusing the heat value contained
in the vapor from the separator. The most common technique is
to use a multiple-effect evaporator. Essentially, these are vacuum
evaporators in series with different boiling points, made possible
by varying the pressure between effects (subsequent effects have
lower pressures). The driving force of a multiple effect system
is the pressure drop from the first to the last effect. The solution
to be concentrated is fed into the boiling chamber of the first
effect and external heat is introduced to volatilize the water.
The water vapor is then condensed at a different vacuum level
and the energy is used to heat the subsequent vacuum chamber.
Therefore, the same energy is used several times in multiple stages.
The second technique is to use a mechanical compressor. With this
equipment, the water vapor from the separator enters the suction
of the compressor where its temperature and pressure are increased.
The vapor is then desuperheated and enters the reboiler. Thus
the latent heat of evaporation, normally lost to the condenser
is recycled by the compressor, providing a temperature difference
across the heat exchanger. The needed energy then comprises only
the power for the pressure increase to provide the temperature
There are a number of advantages accruing to vacuum systems. Among
them are the fact that they are essentially independent of the
requirement to heat and move large volumes of air, thus reducing
the air pollution problem, at least when compared to atmospheric
systems. Further, they are operated at relatively low temperatures,
which could be of considerable importance in systems that handle
temperature-sensitive products. Additionally, vacuum systems are
advantageous with alkaline cyanide solutions which would build
up carbonates more rapidly with atmospheric evaporators because
the latter type aerates the solution.
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