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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.2 ATMOSPHERIC EVAPORATORS
3.2.4 Technology/Equipment Description
The equipment described in this section is manufactured by four firms that responded to the Vendors Survey. Three of these companies represent approximately 90% of the total number of atmospheric evaporators purchased by the respondents of the Users Survey (not counting home-made units or in cases where the manufacturer was not given).
The manufacture of atmospheric evaporators most often found in the Users Survey data is Poly Products Corporation. They manufacture four different models: ET-II Junior, ET-II, ET-III and ET-III-W. The ET-II Junior, ET-II, and ET-III are progressively larger units of the same design. The ET-III-W has a modified design, intended for use with solutions that have a high solids content. Each model has the same basic appearance and operating principles (refer back to Exhibit 3-3). During operation, pumped solution is circulated at approximately 10 to 45 gpm from the process tank or transfer tank to the unit and is gravity drained back to the tank. In the evaporator, solution is sprayed onto 700 to 1,000 ft2 of evaporative panels to humidify the blower air that is forced through these panels. The air is provided by a blower sized from 1/2 to 1-1/2 hp, depending on the model. The humid air then passes through a chevron mist eliminator to remove entrapped chemical solution droplets and is ducted to the outdoors. Poly Products offers an optional mesh pad mist eliminator to further decrease chemical emissions. The ET units are manufactured of molded polyethylene. The largest ET unit occupies a floor space of 48 in. x 34 in. All models have maximum air temperature limits of 104oF and solution limits of 160oF (ref. Poly Products file).
Techmatic Inc. manufactures the MAX-EVAP™. There are four models available: MAX-EVAP, Super MAX-EVAP, MAX-EVAP Cr and Super MAX-EVAP Cr. The MAX-EVAP and Super MAX-EVAP are general purpose units with advertised evaporation rates of 50 to 60 gph and 80 to 90 gph, respectively. The bodies of these units are manufactured from polyethylene. The MAX-EVAP Cr and Super MAX-EVAP Cr are designed specifically for hexavalent chromium plating solutions. These two models are manufactured from steel and are lined with Koroseal (PVC). As with the operation of the Poly Products line, solution is circulated from the process tank or transfer tank to the unit and is gravity drained back to that tank. In the evaporator, solution is sprayed onto 5 to 10 ft3 of polyethylene packing. Air is drawn through the unit by the shop's existing ventilation system or air movement is provided by an optional blower. The exhaust air passes through a "vertical extruded mist eliminator" (not defined by manufacturer) to remove chemical mists. The MAX-EVAP unit occupies a floor space of 63 in. x 21 in. and has a height of 31 in. (blower option measures 55 in. high) (ref. Techmatic file).
Kinetic Recovery manufactures a unit with a somewhat different design than the two previously discussed units. With the Kinetic Recovery unit (Exhibit 3-6) the process solution is pumped from its tank up to the liquid distributor of the evaporator which is located at the top of the cylinder. The liquid trickles over a tower packing down to the bottom drain of the cylinder and returns to the process tank. A blower, located at the bottom of the cylinder, blows air (300 cfm) upward through the packing to the top of the evaporator. When the air goes through the packing (pressure drop 0.3 in. of superfacial vapor velocity of 1.5 fps) it becomes saturated with vapor from the process solution. The cylindrical design improves air flow distribution by reducing dead space. On the top of the cylinder there is a vent transition which contains a mist eliminator (12 in. flexi-chevron). There are two view ports located on the unit, one at the liquid distributor and another below the packing. The basic Kinetic Recovery unit is manufactured from polypropylene and they offer construction in PVDF. The PVDF unit operates with a maximum solution temperature of 180oF.
NAPCO manufactures atmospheric evaporators with two basic designs: (1) cross-flow air pattern and (2) vertical air flow pattern. Both types of units are normally constructed of polyethylene, but a variety of plastics are also available, depending on the customers needs. The basic NAPCO unit consists of a blower, recirculation pump, high surface area packing and demister baffling. Numerous options are available, including heating coils, temperature controls, level controls and make-up solenoids and pumps. Their standard size units range from 10 gpm process feed/1,200 cfm air flow (NAPVAP Jr) to 30 gpm process feed/4,000 cfm air flow (NAPVAP Sr). NAPCO has been marketing their equipment to the plating industry since 1985 and have sold approximately 160 units to 100 different plating shops.
For each of these four commercial evaporators, the heat used for evaporation comes from the plating or transfer tank (unless a heating coil option is specified). 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. Most tank heating systems are designed to provide a quick heat-up and are underutilized after the operating temperature is maintained. Therefore, auxiliary tank heating equipment is usually not necessary when an atmospheric evaporator is installed (unless a transfer tank is also installed). However, each potential installation should be evaluated before purchasing evaporative equipment. Nomographs and methods of calculating tank heating requirements are available in electroplating engineering references (e.g., ref. 341).
The atmospheric evaporators previously discussed are used primarily for chemical recovery, but are occasionally applied to waste concentration. Some evaporators are designed specifically for waste concentration. Two such units include the Technotreat Wastewater Evaporator and the Samsco Water Evaporator.
The Technotreat unit consists of an enclosed carbon steel (stainless steel available) tank equipped with electric immersion heaters (stainless steel or titanium). Liquid waste is fed into the evaporator by a feed pump from a drum or storage tank. The solution level in the evaporator is maintained by a level switch which activates the feed pump. The steam is vented by an assisting electric blower. As water is evaporated, the waste is concentrated. Oil floats to the top and is drawn off periodically. Sludge and viscous materials are removed through a bottom drain. The Technotreat unit comes in 100 and 200 gpd capacities. The range of costs is approximately $10,000 to $15,000 for the standard materials of construction (ref. 360).
The SAMCO Water Evaporator combines direct heating and air flow. The unit consists of an enclosed tank (carbon steel or 304 or 316 stainless steel) that contains a serpentine gas-fired heat exchanger and blower (530 cfm to 2,000 cfm) that draws air in through both the burner and an opening in the top of the tank. In operation, solution is fed to the tank in either a batch or continuous mode. The solution is heated to boiling (approximately 212oF) by the heat exchanger. The action of the blower draws air across the surface of the heated liquid, sweeping away water vapor as it breaks to the surface. The moisture-saturated air and the flue gases leave the tank via separate passages and are joined together at the blower entrance. The two air streams are mixed in the blower and released up a stack. Free oils that float on the surface of the liquid overflow a trough into an external waste receptacle. Solids that settle onto the sloping bottom of the tank are removed via a clean-out port. The SAMCO Water Evaporator comes in three capacities: 15 gph, 33 gph and 63 gph. The area dimensions of the largest unit are 65 in. x 68 in.
Installation of an atmospheric evaporator is a relatively easy task with the exception of the ventilation ducting. The basic units are shipped pre-piped and preassembled. For ventilation, ductwork is usually routed directly to the outdoors through a roof penetration. Alternatively, the exhaust of the evaporator can be connected by ductwork to an existing ventilation system. In this case, an evaporation system blower is unnecessary and the air would be drawn through the evaporation chamber rather than being pushed or blown through. As discussed in Section 3.2.3, this configuration is occasionally used with hard chromium plating.
Installation and operation of an atmospheric evaporator may require a new air permit or revisions to an existing air permit.