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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.3 VACUUM EVAPORATORS
3.3.1 Overview
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 applications.
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 difference.
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.