Mechanical Vapor Recompression Evaporation

Mechanical vapor recompression evaporation (MVRE) has been used in industry for decades. It has served to desalinate sea water, concentrate juices, and treat wastewater on large industrial scales. The technology has evolved somewhat, specifically in Europe where water and energy conservation are more regulated.

 

As it stands, MVRE is under-utilized in the treatment of industrial wastewaters typically found in metal fabricating and finishing industries. Until recently, businesses have relied on technologies such as membrane filtration, atmospheric evaporators, chemical treatment & flocculation, and others. Increasing energy costs, rapidly decreasing fresh water resources and growing sensitivity towards the environmental impacts of industrial management practices are the driving forces in the development of more sustainable technology.

 

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. 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. Given the applicability of MVRE to both water recycling and the treatment of wastewater, this versatility will be implied throughout the remainder of this article.

 

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 useable to heat the original mixture in a heat transfer apparatus. It is this resulting temperature difference produced by compressing the steam that allows 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 watts per gallon are consumed by the process, compared to 2,400 watts per gallon required by atmospheric wastewater evaporators.

 

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 much as 1,000 gallons of wastewater per hour, and smaller, more compact units are capable of processing 18 gallons per hour or 400 gallons per day. The units generally operate unattended and create zero emissions other than the concentrated waste. Units equipped with a clean-in-place subsystem run a descaling cycle that takes only a few hours a month.

 

Evaporators such as the Écopur by Proceco 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 just above atmospheric pressure, and is then condensed. To ensure the highest quality distillate, a multi-stage separator eliminates

entrapped 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 Écopur 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 allowing particles to settle in the concentration tank rather than the heat transfer surfaces.

 

Membrane filtration (i.e. ultra filtration, reverse osmosis) and MVRE both serve to separate water and concentrate waste streams. Beyond this common objective, each technology has its place in waste treatment with limitations as follows:

 

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

 

Membrane systems often don’t fare well with oily emulsions or when wastewater temperatures are above 140°F.

 

Unlike reverse osmosis, MVRE can handle incoming wastewater concentration of solids of 5 percent and solid particles up to 1,000 μm in size. Attempting to recycle water from wastewater with reverse osmosis usually requires pretreatment like sand filters, carbon filters, and ultra-filtration.

 

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 upward of 600,000 mg/L. In comparison, membrane technologies can make clean water from sources no higher in TDS than approximately 35,000 mg/L.

 

MVRE units are generally constructed of stainless steel 316 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 above 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.

 

 

An aircraft engine manufacturer uses MVRE alongside of their parts washer. The evaporator continuously purifies the rinse baths to low conductivity levels (10-30 μ/cm). Rinse water is used for wash water make-up to create a zero discharge system.

 

A surface treatment company specializing in conversion coatings for aluminum sheet and galvanizing of steel structures switched over from a chemical treatment process for their wastewater to MVRE. The wastewater stream included a mixture of wash and rinse water from cleaning processes used prior to surface treatment as well as some heavy metals. With MVRE, the wastewater was concentrated to 3 percent of the original volume.

 

An aerospace company uses MVRE to recycle all of their 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 μ/cm. PC