A valuable tool for environmental protection

In the surface finishing industry various spent aqueous solutions and rinse waters are formed. Traditionally all waste water was mixed during collection and treated in a chemical waste water treatment plant. According to the philosophy of production integrated environmental protection the waste water streams have to be separated and separately regenerated. Using vacuum evaporators it is possible to concentrate solutions for reuse in the process. If reuse is not possible, the amount of material to be exposed is drastically reduced by vacuum evaporation. Thus the cost of disposal will be reduced as well.

Heat Pump

The evaporation of water is a very costly process, consuming ca. 0.8 kWh/l for evaporation. On condensation the same energy is to be removed by cooling. The over all energy consumption can be drastically reduced by the use of a heat pump. The principle of the heat pump is to generate a temperature gradient in a separate loop of a cooling medium, which effects evaporation and condensation. The temperature difference of the cooling medium is generated by compression and relaxation, respectively. A compressor condenses the cooling medium, which heats up and condenses in a heat exchanger effecting the evaporation of the product by the heat of condensation. In the next step the cooling medium expands through a valve cooling by expansion. In a second heat exchanger the cooling medium is heated again by the heat of condensation of the water condensing there. At last the cooling medium reaches the compressor again which closes the loop. The reuse of the heat of evaporation during condensation saves a large part of the over all energy consumption of the process. Due to the mentioned technique Korrodest vacuum-evaporators consume only 0,15 kWh/l, if Freon R 407 C is used.

Evacuation

To use the working temperature of the heat pump the system has to be evacuated to 50-100 mbar. At this pressure aqueous solutions boil at ca. 35°C. The low working temperature enables a careful concentration of the product. Therefore it is possible to concentrate thermally instable solutions, e. g. various electroplating solutions or cyanides. As additional positive effect the tendency for incrustation is reduced as well. All Korrodest -vacuum evaporators are equipped with service-free venturi-type vacuum pumps, which use the condensed water as propellant.

Description of the vacuum evaporator

The devices are assembled in stainless steel frames and after connection to the electrical line they are ready to use. Besides the mentioned aggregates the vacuum evaporator consists of a cylindrical stainless steel container, which is
divided into a boiling and a condensation chamber. Due to the corrosiveness of the product the kind of stainless steel or a coating is selected. The capacity of the vacuum evaporators varies between 3 and 650 l/h.

Besides the concentration units special units for drying are available. Instead of heat exchanger pipes a plain jacket for heating is used instead for drying equipment. The crystallised material can be extracted manually by the front door.

Applications

Vacuum evaporation is applicable for concentrating, if aqueous solutions without remarkable concentrations of low boiling solvents have to be treated. Examples of usage are the return of process solutions in electroplating and chromating realising the ideas of production integrated environmental protection. The amount of evolution to be discarded is reduced by concentration the emulsion and grinding waste water. The concentration of emulsions and grinding waste water reduces the amount of solution to be discarded.

Economy

If the return of solutions back into the process is realised, in economical evaluations the elimination of chemicals and discarding costs has to be considered.

In case of the reduction of the volume to be discarded cost savings can be calculated much easier. The ratio of the volumes of original solution and concentrate after vacuum evaporation determines the profitability of the process. If the ratio of concentration is ten for example, the discarding costs are cut by a factor of ten as well. The costs of the energy consumed have to be included in the calculation. Usually the quality of the condensate can be accepted for discharge or reuse.

Description of **Korro*dest*** -Vacuum-evaporators

vacuum evaporators are established for the treatment of aqueous solutions deteriorated by hazardous substances.

The typical working parameters are: pressure ca. 60 mbar; boiling temperature ca. 35 °C. Practically boiling temperatures of 35 to 45 °C are to be expected.

Products containing low boiling point solvents cannot be destined. If the product is foaming, a dosing pump with defoamant can be used.

Function

On start the boiling chamber of the Korrodest vacuum evaporator is evacuated. At the same time product is sucked into the boiling chamber until the working level is reached by the aid of the vacuum. After reaching the working pressure (below 100 mbar) the product is heated by the pipe-heat exchanger in the boiling
chamber. In case of extremely corrosive products like hydrochloric acid an outside heat exchanger may be chosen. Reaching the boiling temperature water is evaporated from the product. Losses due to evaporation are compensated for by a level control valve. The product concentrates to the required value. At this concentration the concentrated product is either pumped off completely or partly in order to perform a quasi continues operation. If the volume pumped off is determined correctly, the evaporation process is disturbed only slightly.

Heat - Pump System

To evaporate 1 kg of water ca. 625 Wh are consumed, the same energy will be evolved on condensation. To guarantee a most cost effective operation Korrodest vacuum evaporators are equipped with an integrated heat pump. The heat pump utilises the heat of condensation, which is extracted from the water vapour, to heat the product up. The big advantage is the effective, over all energy consumption of approx. 140 Wh per litre of condensate. This technique is more effective by a factor of 4.5. compared to evaporation to the atmosphere.

The Heat Transfer Process

The gaseous heat transfer medium heats up due to compression by one or more compressors. The heated gas is led through a tube type heat exchanger, which is surrounded by the product, increasing the product
temperature until the product starts to evaporate. The heat transfer medium itself condensates on heat exchange. As it is still under pressure, it will cool on passing an expansion valve. The pressure difference across the expansion valve is typically 12 bar. Compressor release the transfer medium evaporates. The heat of evaporation is transferred from the water vapour at the second heat exchanger causing the water vapour to condensate. The distilled water is collected outside the chamber for use. To restart the loop the heat transfer medium is compressed again. The conditions for the heat transfer circle are optimal at 35 – 40 °C. Therefore aqueous solutions have to be operated at a reduced pressure of 50 – 100 mbar resulting in the mentioned boiling temperatures.

Starting phase

At the start, when the product is still cool, no water vapour is available to detract the evaporation heat of the heat transfer medium. As a result the compressor cannot be filled with a sufficient amount of gas. This could damage the compressor. To avoid these problems a special bypass is incorporated to guarantee a sufficient supply of the compressor with R 407 c vapour. The use of the bypass eliminates the former electric preheating and reduces the heating time.

Basic considerations

Korrodest vacuum evaporators are completely filled with cooling medium in the factory and completely tested. Everything is ready to plug and no mechanical or electrical installations or piping necessary. As compressor the Maneurop for small capacities and DWM-Copeland for higher capacities are used. Both are leading, world-wide known fabricates.

Controls

Korrodest vacuum evaporators are designed for completely automatic operation. All operation relevant data are shown on the panel in clear English language. The Korrodest vacuum evaporators are equipped with a Mitsubishi PLC control. Other controls (Siemens, Allen & Bradley etc.) are available as an option.

The control cabinet incorporates all controls and panels according to IP 54. The equipment applies to CE, VBG 4 and 20 and DIN VDE regulations respectively. The control valves are operated by compressed air.

Vacuum System

The vacuum system consists of a liquid ring - Vacuum pump made in Plastic, coupled with an Gas streamer. This vacuum system is able to reach pressures of about 40 mbar. The advantage over rotary disk vacuum pumps is the simplicity of the components and their insensitivity against water. The reliability is best.

In the collection tank a small (0,4 bar) overpressure is formed, which enables the condensate to flow out of a pipe. From the Size "2000" all the Evaporators are fixed with an integrated Distillate pump which pumps the "produced" to other Plants or Buffertanks. The vacuum pump uses condensed water only, no fresh water is needed. As the pump produces heat, a compressor cooling equipment is installed controlled by a thermostat.

Picture: Korrodest - K 5000

Concentrate Pump/Level Control

The concentrate pump is used to pump the concentrate out of the equipment after reaching the required concentration. The concentration is controlled either by density or a timer. As the concentrate pump either the centrifugal pump with a special gasket or a compressed air membrane pump is used. The latter is especially useful in case of abrasive products.

The integrated level control opens the product inlet valve to suck in fresh product, gives alarm if the upper and lower level limits are exceeded and switches off the equipment, if necessary.

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