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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.3 VACUUM EVAPORATORS
3.3.4 Technology/Equipment Description
3.3.4.1 General
3.3.4.2 Submerged Tube Evaporators
3.3.4.3 Rising Film (Climbing Film)
3.3.4.4 Falling Film
3.3.4.5 Wiped Film Evaporator
3.3.4.6 Flash Evaporators
3.3.4.7 Thermal Compressor Evaporators
3.3.4.8 Heat Pump Evaporator
3.3.4.9 Mechanical Vapor Recompression (MVR)
3.3.4.10 Multiple Effect EvaporatorsThis subsection discusses commercially available vacuum evaporation equipment that is manufactured and/or sold by vendor survey respondents. This is intended to provide the reader with information and data on a cross section of available equipment. Mention of trade names or commercial products is not intended to constitute endorsement for use.
There is a wide range in design of vacuum evaporators, although the majority of these devices work on the principles described in Section 3.3.1. Vacuum evaporators are built by various manufacturers for different applications. Exhibit 3-12 classifies vacuum evaporators according to the way water is vaporized. This design element helps to differentiate between some of the commercial equipment available to the electroplater. It should be noted that not all manufacturers of plating evaporation equipment are represented in this exhibit.
As with any technology group, the vacuum evaporation industry has developed their own terminology for their equipment and its components. Some of their commonly used terms are defined in Exhibit 3-13.
The following subsections describe the types of vacuum evaporators that are applicable to the plating industry. Where information is available, specific commercial units are briefly described.
3.3.4.2 Submerged Tube Evaporators
The submerged tube evaporators, which includes the short and long tube vertical (LTV) types and the horizontal tube type, are termed natural circulation evaporators, because no pump or other recirculation device is employed. These units, which are older, but still widely used types of vacuum evaporators, are sometimes referred to as calandria type evaporators. With the basic design (short tube type), a vertical tube bundle is placed inside a vertical cylindrical evaporator shell. The tubes or tube sheets, usually two to six feet in length, span the body diameter. The liquid level in the body is typically maintained such that 50% of the tube sheets are immersed. Liquid circulates through the tubes at a rate many times greater than the feed rate. The liquor travels up through the tubes and down a central pipe called a "downcomer." Steam or water vapor condenses on the outside surface of the tubes and the liquor is heated and boiled inside the tubes. The circulation of the liquid is achieved because of the difference in specific gravity between the liquor and vapor in the tubes plus a vapor lift effect. This combined phenomenon is known as the thermosyphon effect and it is the design basis for all natural circulation evaporators, which includes the falling film types (ref. 376, 422, 423). In some cases, an agitator, located inside or beneath the downcomer, is used to increase circulation in salting-type applications.
Generally, the submerged tube evaporators are less expensive to purchase than rinsing film or flash units of equal capacity. Steam or thermal demand is the same as for rising film (ref. 376). The evaporators find application for processing mildly scaling liquors and relatively viscous solutions (ref. 422).
LICON Inc., a manufacturer of electroplating evaporation equipment and a Vendors Survey respondent, manufactures single effect and double effect submerged tube evaporators. These devices have been applied to the concentration of chromium (Cr+3 and Cr+6), zinc chloride, nickel chloride, nitric acid, and sulfuric acid bearing waters as well as mixed wastestreams (ref. LICON file).
QPS manufactures the Wastesaver® submerged tube evaporator, which is available with either single, double or triple effects. Their newer units have a pumpless liquid transfer system that reportedly eliminates problems commonly associated with mechanical liquid transfer equipment (e.g., pumps, seals, impellers, etc.). These units are manufactured with capacities ranging from 25 gph to 1,500 gph. The basic units are manufactured from stainless steel with titanium offered as an option.
3.3.4.3 Rising Film (Climbing Film)
The basic rising film evaporator consists of an evaporator body, separator and condenser. The evaporator body is a shell-and-tube heat exchanger. Liquid feed enters the bottom of the heat exchanger, it is preheated until it reaches the boiling point and it then moves up the tube. The vapor generated occupies the center of the tube and the liquid is forced to the tube wall. As the fluid travels up the tube, more vapor is formed resulting in a higher central core velocity. The upward velocity of the vapor forces any remaining liquid to the tube wall and continues to provide an upward motion. As the process continues, the higher vapor velocities result in thinner and more rapidly moving liquid films. This design provides a high heat transfer coefficient and relatively short residence time (ref. 373, 375, 376).
Evaporation is typically accomplished at pressures of 1.3 to 7.5 psia (67 to 388 mm Hg absolute), thereby lowering the boiling point to 110o to 180oF (43o to 82oC). The wastewater leaves the body and enters the separator where the water vapor is separated from the heavier plating solution. The plating solution is either returned directly to the bath or held in an integral reservoir. The vapor leaving the separator is condensed in a shell-and-tube heat exchanger and the distillate is directed to the rinse tanks (ref. 376).
Commercially available rinsing film evaporators used in the plating industry are manufactured by LICON/Aval and Corning. Several existing plating applications of rinsing film evaporators identified in the Users Survey were manufactured by the Pfaudler Company.
Liquid enters the top of the evaporator and a liquid film is formed by gravity, which then flows down the heat transfer surface. During evaporation, vapor fills the center of the channel and as the momentum of the vapor accelerates, the film becomes thinner. Also, the solution accelerates in velocity as it descends inside the tubes because of gravity and the drag of the vapor. Since the vapor is working with gravity, a falling film evaporator produces thinner films than a rising film evaporator for any given set of conditions. This gives rise to shorter residence times and a further improvement over the rising film types in heat transfer. With these devices, liquid is usually separated from the vapor in the bottom liquid chamber of the body.
The falling-film evaporator is particularly useful in applications involving heat sensitive chemical solutions. This is due to a low "driving force" or temperature difference between the heat-transfer medium and the liquid (ÆT's less than 15oF compared to 25oF or more for the rising film) (ref. 375, 377).
No commercial electroplating applications of the falling film evaporator were identified during the Users or Vendors Surveys, although they presumably exist due to the widespread commercialization of these devices (ref. 373, 377).
Feed is introduced at the top of the evaporator and is spread by wiper blades on to the vertical cylindrical surface inside the unit. Evaporation takes place as the thin film moves down the evaporator wall. The heating medium is usually high pressure steam. Use of the wiped film evaporator is limited primarily to highly viscous liquids and the stripping of solvents. The high number of moving parts, such as the rotor and wiper blades, may result in higher maintenance costs than other types of evaporators (ref. 375).
LICON Inc. manufactures a wiped film evaporator (Stratavap) with capacities from 5 to 700 gph, but no plating shop applications were identified in the Users or Vendors Surveys for this device or other wiped film evaporators.
Unlike with thin film types (e.g., falling film or wiped film), with flash evaporators, vaporization does not occur on the heat exchanger surfaces. Instead, liquor flashes as it enters a separator, crystallization takes place, and a suspended slurry results. Since evaporation does not take place on a heat transfer surface, the tendency for scale to deposit is significantly reduced. The flash evaporation system can be used in single or multiple effects.
The LICON Inc. Flashvap is sold as an end-of-pipe industrial waste concentrator.
3.3.4.7 Thermal Compressor Evaporators
The thermal compressor evaporators are not, by themselves, a separate category of evaporator. Rather, they are evaporators, such as a rising film type, that uses a steam jet ejector or thermocompressor in order to increase steam economy. They can be designed with either single or multiple effects, although the thermocompressor is normally used on a single effect evaporator or only on the first effect of a multiple effect evaporator. Typically, the addition of a thermocompressor will provide an improved steam economy equal to the addition of another effect, but at lower cost. They should be considered only when high pressure steam is available. Because of their smaller size in comparison to an additional effect, they are favored in applications where space limitations exist. A disadvantage of these units is that the condensate is sometimes contaminated with product traces and may have to be treated, rather than reused as rinse water.
No applications of thermal compression evaporators were identified during the Users Survey or Vendors Survey.
A heat pump is a device that upgrades a heat source to a higher temperature, thus rendering it more useful. With conventional evaporator/heat pump operation, a refrigerant, upon boiling, absorbs the heat that would otherwise be rejected in a condenser. The refrigerant vapor is compressed to a pressure adequate to permit the vapor to be condensed in the calandria, thereby providing the heat needed for evaporation. The condensate from the calandria is flashed into the condenser, thereby completing the cycle (ref. 373). The heat pump eliminates the waste of single and double effect designs, but does cost electrical power to operate the heat pump. Therefore, it is not applicable to plating shops where waste heat is available. Also, it is generally confined to small flows (< 100 gph) due to the range of heat pumps available.
LICON Inc. manufactures the Fridgevap (3 to 100 gph) heat pump evaporator, in which the solution is evaporated at around 100oF (40oC). This unit finds application where heat sensitive chemicals are involved.
Calfran, Int. manufactures a line of heat pump evaporators that they term COLD VAPORIZATION™. These include the PTU series (immersion coil design) and STU series (reaction vessel type) for applications of 1,000 gpd or less and greater than 1,000 gpd, respectively and the VTU series designed for low solids feed streams (75 to 1,000 gpd). Their basic materials of construction include 316 stainless steel heat exchangers and PVC shells. Their units are also available in all stainless steel design and heat exchangers are available in titanium and Hastelloy.
3.3.4.9 Mechanical Vapor Recompression (MVR)
The MVR evaporator is the highest priced evaporator type used in the electroplating industry and it is also the most energy efficient. The MVR evaporator is similar to a conventional single-effect evaporator, except the vapor released from the boiling solution is compressed (adds energy) in a mechanical compressor. This compressed water vapor condenses and gives up its latent heat, which is used to vaporize more water from the liquid that is being concentrated. The following example from the literature shows the potential operating cost savings from using the MVR evaporator (ref. 375).
Exhibit 3-14 shows an evaporator with a liquid boiling point of 212oF (atmospheric pressure). All of the water vapor that is boiled off passes to a compressor. In order to keep the energy input to the system as low as possible, the pressure boost across the compressor is limited. In the majority of cases, this pressure boost will correspond to a saturated temperature rise in the region of 15oF or less. In this example, there is a pressure boost of 4.5 psi across the compressor. Assuming that there is a pressure loss of 0.5 psi in the system, the effective pressure on the steam side of
Exhibit 3-14. Mechanical Vapor Recompression Evaporator/Condensor Schematic the evaporator is 18.7 psia. This compressed water vapor condenses and gives up its latent heat, which is used to vaporize more water from the liquid that is being concentrated. The latent heat of vaporization of water at atmospheric pressure is 970 Btu/lb. Note that it only requires a theoretical energy input of 18 Btu/lb to raise the water vapor from 14.7 to 19.2 psia. The theoretical steam economy, therefore, is 970/18 = 54. When compressor efficiency is taken into account, this figure is brought down to between 32 and 35 which is another way of saying that the MVR system is equivalent to an evaporator with 32-35 effects (see definitions in Exhibit 3-13). However, when the electricity cost for the compressor drive is taken into account, the MVR system then becomes the economic equivalent of just under a 19 effect evaporator.
The MVR has another definite advantage over steam. The condensate is available at high temperature and is ideal for evaporator feed preheating, particularly if the condensate rate is as high as 90% of the feed rate, i.e., a 10:1 concentration ratio within the evaporator. There are many such evaporators in operation where the sole energy input to the system is through the compressor with steam requirements limited to approximately 15 minutes during start up (ref. 375).
An example of a commercial MVR evaporator used by the plating industry is the LICON Inc. Aquavap. This evaporator has an auxiliary flash stage and is capable of achieving concentrations of 500,000 mg/l or more. Evaporative capacities for the Aquavap range from 50 to 600 gph. Existing plating applications include: concentrations of zinc phosphate rinses (multiple units totaling 1,800 gph), concentration of RO reject (300 gph), and end-of-pipe wastewater concentration (50 to 600 gph) (ref. LICON Inc. file).
3.3.4.10 Multiple Effect Evaporators
Multiple effect evaporators are not a specific type of evaporator, but rather a design element employed to improve the energy efficiency of the evaporation process.
Most evaporators used in the plating industry are single-effect units. Single-effect evaporators operate with one boiler or evaporator section. The water vapor is condensed or exhausted to the atmosphere. Approximately 1.1 pounds (0.5 kg) of steam is consumed in evaporating each pound of water from the plating solution (ref. 376).
Exhibit 3-15 shows the utility requirements for single-effect evaporators as a function of liquid flow rates to the evaporator. The electrical demand is associated with power requirements of the vacuum pump, recirculation pump, and feed pump. As a rule, the cooling water rates are based on a temperature rise of 25oF (14oC) across the condenser (ref. 376). For example, from Exhibit 3-14, if the wastewater flow rate to the evaporator is 80 gal/hr (303 l/hr), the steam rate is 730 lb/hr (331 kg/hr) for 15 lb/in2 gauge (1,536 mm Hg absolute) steam. The electrical demand is 2.9 kWh and the cooling water rate is 56 gal/min (212 l/min). For atmospheric evaporators where no cooling water is used, the steam rate would be at least 20 percent higher (ref. 376).
A general application of a double-effect evaporator, is shown in Exhibit 3-16. The basic principle is to use the heat given up by condensation in one effect to provide the reboiler heat for another effect. In the system shown in Exhibit 3-16, approximately 50 percent of the wastewater is concentrated in the first effect using steam. The vapor from the separator of the first effect enters the second-effect reboiler and condenses to provide the thermal energy required to reach the final concentration of the plating solution.
The steam and cooling water rates for the double-effect unit in Exhibit 3-17 are approximately 50 percent of those required for the single-effect unit.
Exhibit 3-17. Double-Effect Evaporation for Chemical Recovery Some platers using double-effect units achieve an additional benefit by recovering two different plating baths simultaneously. However, care should be taken in employing this arrangement however, because there is a possibility of cross-contaminating baths (ref. 376).
Multiple effect evaporation, when used in the plating industry, is most often applied to submerged tube evaporators, rinsing film and flash types (ref. 376). An alternative method for reusing the heat value contained in the vapor from the separator is to employ a mechanical compressor (see Section 3.4.2.9).