Mechanical Vapor Recompression Evaporation

Mechanical vapor recompression evaporation (MVRE) has been used in the manufacturing industry for decades for such purposes as desalinating sea water, concentrating juices and treating wastewater on large industrial scales. During that time, the technology has evolved somewhat, specifically in Europe where water and energy conservation are more regulated.

As it stands, MVRE is underutilized in the treatment of industrial wastewaters that are typical of metal fabricating and finishing industries. Until recently, businesses have relied on technologies such as membrane filtration, atmospheric evaporators, chemical treatment and flocculation, and others. Increasing energy costs, rapidly decreasing freshwater resources, and growing sensitivity towards the environmental impact of industrial management practices are the driving forces in the development of more sustainable technology.

Distillation Process Recovers Water

MVRE is essentially a distillation process designed to reduce wastewater volumes by recovering most of the water in the waste. Typically the waste is concentrated by as much as 20:1, and the concentrated waste can then be hauled away by local authorities, or dried and buried in a landfill. The recovered water or distillate is of good quality and can be used in a variety of industrial applications, from rinse water to boiler feed water, or it can be simply disposed of into the sewer.

Distillation in itself is a separation process, separating components in a mixture by making use of the fact that some components vaporize more readily than others. The vapors produced by heating the mixture contain the more volatile components of the original mixture, and so a separation process occurs.

MVRE takes the process one step further by using a compressor to increase the pressure of the water vapor, or steam, produced. An increase in vapor pressure increases the condensation temperature of the steam, rendering it usable to heat the original mixture in a heat-transfer apparatus. It is this resulting temperature difference produced by compressing the steam that enables a highly efficient heat transfer to occur. As the steam condenses in the heating chamber, it releases its latent heat of vaporization to further heat the original mixture, which in turn produces more steam. This recycling of heat is what renders MVRE so efficient. As little as 225 w per gal are consumed by the process, compared with 2,400 w per gal required by atmospheric wastewater evaporators.

Water More Volatile than Other Waste

In wastewater processing, the water portion of the waste is more volatile than the other components of the mixture. The steam is then condensed to produce clean water and the remaining, less-volatile components concentrate in the boiling chamber. Over time, the concentrated waste reaches a saturation point and must be discharged for waste disposal.

Commercially available units can process as many as 1,000 gal of wastewater per hr, and smaller, more compact units are capable of processing 18 gal per hr or 400 gal per day. The units generally operate unattended and create zero emissions other the concentrated waste. Units equipped with a clean-in-place subsystem run a descaling cycle which takes just a few hours a month.

Evaporators such as the Éco-Smart from Proceco Ltd. use a roots compressor to create a partial vacuum in the boiling chamber, reducing the boiling temperature of the liquid by as much as 30°F. The vapor is compressed to slightly above atmospheric pressure and then condensed. To ensure the highest quality distillate, a multi-stage separator eliminates trapped liquid droplets from the vapor phase. An additional heat exchanger uses some of the sensible heat remaining in the distillate to preheat incoming wastewater, increasing the overall efficiency of the process. The Éco-Smart uses no pumps or heaters other than the compressor to heat and move the fluid, relying strictly on natural circulation created by the vacuum distillation process. Its vertical heat exchanger arrangement reduces fouling by enabling particles to settle in the concentration tank rather than on the heat transfer surfaces.

Separate Water, Concentrate Waste

Membrane filtration (i.e., ultrafiltration, reverse osmosis) and MVRE both serve to separate water and concentrate waste streams, and each technology has its place in waste treatment with limitations, including:

· Membrane systems generally present a lower up-front investment, but can be more expensive to operate when one considers membrane life and maintenance cost.

· Organic/polymeric membrane systems don’t fare well in high-temperature or harsh chemical environments.

· Unlike reverse osmosis (RO), MVRE can handle incoming wastewater concentration of solids of 5 percent and solid particles as large as 1000µm. Attempting to recycle water from wastewater with RO usually requires pretreatment like sand filters, carbon filters and ultrafiltration (UF).

· Both technologies require that non-emulsified oil and grease be removed prior to processing.

· One of the most important advantages of MVRE over membrane filtration is its ability to make clean water from saturated or even crystallizing brines, achieving total dissolved solids (TDS) levels of more than 600,000 mg/L. In comparison, membrane technologies can make clean water from sources no higher in TDS than about 35,000 mg/L.

· MVRE units are generally constructed of 316 stainless steel for all parts in contact with the wastewater. This grade of stainless steel has a fairly good tolerance to corrosion except where chlorides are concerned. In principle, wastewaters with chloride concentrations greater than 5 mg/L are likely to exhibit severe corrosion with this grade of stainless steel due to the high concentration ratios in the final stages of evaporation. In such cases, more corrosion-resistant and costly materials are required.

· MVRE is not intended for operation with volatile, flammable or potentially explosive substances. During the process, any substance more volatile than water will condense with the steam, reducing overall distillate quality.

Wastewater applications for MVRE include oily emulsions, parts washing, surface treatment rinse water, dye penetrate (FPI) rinse water, vibratory finishing, quenching, die casting and extrusion, DI regeneration, and UF permeates.

MVRE Used in Variety of Cases

One example of the technology in action is an aircraft engine manufacturer that uses MVRE alongside its parts washer. The evaporator continuously purifies the rinse baths to low conductivity levels (10-30 µs/cm), and rinse water is used for wash water makeup to create a zero-discharge system.

An automotive parts manufacturer also uses MVRE in conjunction with UF to recycle water from metal working fluids and parts washing solutions. The UF permeate is distilled with MVRE to recycle the water content, and the concentrated oil (retentate) is sold for its calorific value.

As yet another example, an aerospace company uses MVRE to recycle all of its rinse water used in rinsing dye penetrants in their crack detection process. The wastewater containing fluorides and hydrocarbons produces water for reuse with a conductivity level of less than 5 µs/cm.

Robert Burns is president of Proceco Ltd. and has worked in the parts cleaning industry for more than 30 years.