By Emilio Turchi
Summary: As a physical evaporation process, vacuum evaporation is dependable for many wastewater treatment operators. Water containing PVA glues is one application that may benefit from this process with a final goal of zero discharge. One European company shares some of its experience with the process.
Motivated by higher disposal costs, a leading company in glue and adhesive production last year installed the first part of a plant designed to treat wastewater and recover the subsequent dissolved PVA (polyvinylacetate) glues.
The plant as a whole will be able to treat about 15 tons of wastewater a day. The initial phase consists of a heat pump vacuum evaporator with forced circulation to pre-concentrate the waste. The final phase aims to reach the required concentration through the heat pump-scraped vacuum evaporator (see Figure 1).
The company carries out the mixing of the basic adhesive ingredients in suitable reactors. The final product is sold in liquid form to the wood and furniture industry as well as the graphics, textiles and automotive industries.
Expectations and results
Some months after start-up, analytical results confirmed the initial expectations of the end product quality—a constant density solution to be mixed with fresh glue as demanded by the market. The distillate recycling for the equipment rinsing allows wastewater discharge to be reduced to zero. This recycling means all industrial wastewater will be treated and used again in the process and no longer released into the sewer system or surface waters. As a result, the company eliminated an entire series of problems and requirements linked to controls on discharge, in addition to converting the waste from a high cost into an income. The analytical data are reported in Table 1. The waste has a typical milky appearance with a chemical oxygen demand (COD) higher than 44,000 parts per million (ppm), for which the traditional purification systems are not particularly effective.
In the past, the company disposed of wastes with a high annual cost and significant storage problems (See Table 2). Currently, part of this cost item has been eliminated and, together with the limited plant management costs, it is able to sell the concentrated product on a stable basis, guaranteeing an annual income. To effect a simple comparison, Table 3 shows the financial figures. The important result—which should be underlined in regards to the installed vacuum evaporator—is the return on investment in less than one year.
The evaporators/concentrators exploit the combined effects of vacuum and a heat pump to obtain the cool evaporation (35°C or 95°F) of aqueous solutions. The evaporators are made of a ferritic austenitic alloy—SAF 2507—that’s resistant to possible pitting corrosion phenomena due to the presence of a high chloride concentration. The low-temperature evaporation plant (see Figure 2) is equipped with scraping blades inside the boiling chamber that keep the heat exchange walls clean. The lower part of the boiling chamber is made by a heating jacket that provides necessary heat for evaporation of the liquid.
The boiling chamber feed occurs through a pneumatic valve while the discharge of the concentrate, produced by the evaporation cycle, is carried out through a diaphragm pump. The steam produced inside the boiling chamber enters the heat exchangers located on the top of the boiler and then condenses (see Figure 3). The vacuum system sucks about 528 gallons a day of condensed distillate together with possible incon-densable gases.
The low-temperature evaporation plant (see Figure 4) is equipped with a circulation pump, which takes the liquid from the boiling chamber and sends it to the shell and tube heat exchangers outside the boiling chamber, which provide the heat necessary for boiling. The heated waste goes back to the boiler where the water turns from liquid to steam.
The steam produced enters the heat exchangers located on the top of the boiler where it condenses. The vacuum system sucks about 3,170 gallons a day of condensed distillate together with possible inconden-sable gases. The boiler feed and concentrate discharge operations occur through pneumatic valves. The distillate, which is about 90-95 percent of the treated volume, overflows from the vacuum tank (see Figure 5).
The heat pump installed on the evaporators consists of a frigorific circuit (that supplies both heating and chilling, i.e., calories and frigories) using hydrofluorocarbon (HFC) R-407C—an environment-friendly refrigerant fluid. The liquid of the heat pump is heated, due to the effect of compression, by the main compressor and releases its condensation heat to the wastewater inside the heat exchangers. A finned battery completes the condensation and the sub-cooling of the refrigerant liquid; the heat subtracted from the refrigerant liquid is released to the environment while the refrigerant liquid, in liquid form, is stored in an auxiliary receiver/exchanger to pass further through an expansion valve.
The refrigerant liquid, due to expansion, evaporates while cooling and condenses the steam that has formed in a shell and tube heat exchanger. The heat pump cycle finishes with the return of the refrigerant liquid to the compressor. The heat pump only needs electric power for operation of the compressor and it doesn’t cause any emission of substances into the atmosphere.
The vacuum system consists of a pump and an ejector. The heat pump takes the water from the vacuum tank and sends it to the ejector to generate low pressure in the boiling chamber. Inside the vacuum tank there’s a coil connected to the main frigorific circuit, which cools the stored distillate and therefore keeps the efficiency of the ejector high.
All the evaporators have a PLC and they’re available in different building materials, according to the liquid to be treated. These range from the standard one with stainless steel 316L—an extra-low carbon variation that avoids carbide precipitation caused by welding and offers excellent corrosion resistance where welding is extensive—to more specialized ones (for aggressive liquids) with steels having a high nickel percentage and materials such as PTFE. The main features of evaporators are:
- Mobility and extremely reduced dimensions
- Low consumption
- Possibility to obtain high concentrations
- Total automation to be able to work round the clock
- Easy installation
Vacuum evaporation has proven to be a highly reliable physical separation process. It can be applied to different liquid systems and doesn’t necessarily require particular pre-treatments in all cases. Separation occurs according to the different volatility of the components of the liquids to be treated. The vacuum evaporation technology applied to the treatment of water containing PVA glues proves to be effective and advantageous. This is because the chemical-physical characteristics of the distillate are constant, even if there’s a variability of the waste and it has resulted in the transformation of waste produced at considerable cost. The development of a zero discharge plant as a stage in obtaining ISO 14000 environmental certification is in line with European community policy, which encourages and funds projects for reducing waste and reusing water.
About the author
Emilio Turchi is marketing and area sales manager at Led Italia Srl, of Pordenone, Italy. He has been with Led Italia since 1989 and has held various positions within the company from process designer to sales support manager. Turchi can be reached at +39-0434516311, +39-0434516310 (fax), email: [email protected] or website www.leditalia.com