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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.2 ATMOSPHERIC EVAPORATORS
3.2.1 Overview
Atmospheric evaporators are the most widely used method of chemical recovery in the plating industry. Of the 318 plating shops responding to the Users Survey, 71 (or 22.3%) have employed atmospheric evaporators for chemical recovery. Some shops have purchased or built two or more units for different recovery applications, resulting in a total number of 91 units used by survey respondents. The literature cites an extreme case, where a Virginia shop employs 14 atmospheric evaporators and has achieved zero discharge of wastewater effluent (ref. 33). Most of the atmospheric evaporators used by survey respondents were commercial units although approximately 5% were built in-house. By comparison, 26% of the electrowinning units were built in-house. The low percentage of home-made atmospheric evaporation units is most likely due to the low capital cost and successful track record of the commercial units.
Atmospheric evaporators are also used by some shops to concentrate liquid plating wastes prior to hauling them off-site for treatment/disposal, thereby reducing transportation costs and in some cases treatment/disposal costs. A total of 3 shops responding to the survey used atmospheric evaporators for this purpose.
An atmospheric evaporator is a device that evaporates water to the atmosphere. The commercial atmospheric evaporator used for recovery in the plating shop consists of a pump to move the solution, a blower to move the air, a heat source, an evaporation chamber in which the solution and air can be mixed, and a mist eliminator to remove any entrained liquid from the exit air stream (see Exhibit 3-3) The evaporation chamber is usually filled with packing material or finned panels to increase the air to water interface. In operation, the temperature of the solution being evaporated is elevated and the heated solution is introduced into the evaporation compartment. Air from the plating room is then blown through the compartment where it accepts the water vapor, and is then vented out of the chamber.
Commercial units are advertised to have evaporation rates of 10 to 90 gph, depending on the size of the unit and operating conditions (e.g. solution temperature). Often actual evaporation rates are considerably less because the atmospheric conditions within most plating shops do not match the ideal conditions under which the manufacturers rate their systems. To meet higher evaporative requirements, it is feasible to utilize multiple atmospheric evaporators in series. However, the use of atmospheric evaporators is generally limited by energy costs to applications where the required evaporation rate is 50 gph or less. Beyond this point, vacuum evaporators (Section 3.3) are more cost effective, considering both capital and operating costs.
The key attributes of atmospheric evaporators include: (1) low capital cost; (2) simple operation and low maintenance; (3) very high recovery rates can be achieved (usually 90% to nearly 100%); (4) no additional reagents are needed; and (5) no sludges or only small quantities of sludges are generated (when used in recovery application). The major negative aspects of this technology are: (1) high energy requirement (i.e., requires constant solution heating and during the winter months there is a heat loss due to venting the exhaust to the outside); (2) the air that is vented by these devices must be discharged to the outside (due to contaminants present and its high humidity) and may be a regulated source of air pollution; (3) because moisture is exhausted to the atmosphere it cannot be reused as rinse water as with vacuum evaporators1; (4) evaporators return contaminants to the bath and may reduce bath life; and (5) spray/fog rinsing over the bath or fume suppressants (floating chemical type or plastic balls) are not compatible with atmospheric evaporators since they reduce the head room in the plating tank and limit the return of rinse water/drag-out.
Most commercial atmospheric evaporator units have the same principals of operation. To achieve chemical recovery, solution from a heated plating tank is fed to and concentrated by the evaporator and returned to the plating tank. This approach reduces the volume of solution in the plating tank, thereby "making room" for the recovery rinse water/drag-out to be added to the plating bath. Often two or more recovery rinse stations are used to minimize the overall rinse water requirements of the process and increase the recovery rate of plating chemicals. Less frequently, atmospheric evaporation is applied to ambient or low temperature baths. In this case, the recovery rinse water may be fed to the evaporator from a heated transfer tank, which increases the overall evaporative capacity of the system. The latter application is often restricted by the maximum temperature that can be applied to the solution, since heat sensitive components of the bath could be destroyed.
Although some commercial atmospheric evaporators have their own heat source, nearly all units employed for plating chemical recovery in use today use the heat in the plating tank (and/or heated transfer tank) as the energy source for evaporation. Any solution above room temperature that is pumped to the evaporator will be returned at a lower temperature. The temperature difference is primarily due to the heat that goes into evaporation, which is (ref. 358):
- 540 calories/g of water evaporated, or
- 2,137 Btu/l (8,090 Btu/gal), or
- 626 watts/l (2,371 watts/gal), or
- 0.02137 "gas company" therms/l (0.08090 therms/gal), or
- 0.0637 boiler hp/gal
The heat taken from the plating bath must be replaced by the tank heating system (e.g., immersion heaters or steam system) in order to maintain the operating temperature of the bath. During winter months, when a plating shop is heated, the room's ambient air that is exhausted by the evaporator (typically 300 to 3,000 cfm) must be replaced. These two elements make up the bulk of the non-labor operating costs for atmospheric evaporation.
The humidity and temperature of a plating shop will significantly affect the evaporative capacity of an atmospheric evaporator, especially for low to moderate solution temperature applications. If the air in the plating shop is very humid (e.g., 90% relative humidity) before entering the evaporation chamber, it will hold a limited amount of additional water and the evaporation rate will be affected. From thermodynamic tables, it is known that warmer air holds more moisture than cooler air. For example, one pound of air will hold 0.015 pounds of water at 70oF. By comparison, that same pound of air will hold 0.220 pounds of water at 120oF. In all cases, the air entering the evaporation chamber is heated to a maximum temperature equal to a point less than the solution temperature (the closeness of the air and solution temperatures will depend on the evaporator design and resultant heat transfer efficiency). With high temperature solutions, the air temperature will reach 100oF or more. At this temperature level, the original moisture content of the air is relatively small in comparison to the new capacity of the air, hence relative humidity plays a lesser role.
The above example also indicates that preheating of the plating shop air before introducing it to the evaporation chamber would improve performance. Although air heating systems are discussed in the literature (e.g., ref. 299), no commercial units designed for plating shops were identified with this feature. The strategy of manufacturers of commercial units is to maximize air flow and increase the water/air contact area rather than increase air temperature.
Platers may be tempted to use outside air for make-up to their atmospheric evaporator, especially during winter months in colder climates, to reduce the loss of heated indoor air. This strategy generally does not work. Although the outside air may be dryer than the inside air, its low temperature will have an overriding impact on the evaporation process. The low temperature of the air will prevent it from reaching a sufficiently high temperature in the evaporation chamber to attain a reasonable water holding capacity.
In many cases, the types of commercial units used for concentrating wastes before off-site disposal are the same types of units used for recovery. However, there are also available specially designed waste concentration units. These devices usually have a direct heat source and operate at much higher temperatures than the recovery units. Higher temperatures can be used with wastes since there is no concern for the integrity of the chemical components. Because higher temperatures are used, the materials of construction for these units differ from the recovery units. Evaporation of water from wastes may be viewed by regulatory agencies as thermal treatment and they may require a RCRA permit for the operation of these units, depending on the interpretation of the application. Regulatory aspects of evaporators are not discussed in this report, but should be closely investigated before purchasing and operating evaporation equipment.