Wastewater Evaporator

Evaporation is the production of vapor from a liquid. It is similar to the industrial process of drying and distillation, yet different and distinguishable. The evaporation process produces a separate and distinctly different liquid and vapor from the original liquid, where as drying produces a separate and distinctly different solid and vapor from the original liquid. Although CASTion’s CAST flash vacuum distillation system is an evaporative process, it differs in that the various components of the vapor are additionally separated; this does not occur in evaporation.

Wastewater evaporators are usually used in industry for the following operations: (1) solvent extraction, (2) product concentration and (3) waste reduction. All three operations are used on industrial process solutions and are essentially the same in practice. The process solution by definition must consist of at least one solvent and one solute. Various combinations of solvents, solutes and non-soluble constituents are normally encountered. The vapor generated from the original solution is typically referred to as the solvent. This vapor may be discharged as waste or condensed to a liquid form. In the liquid form the condensed vapor is typically referred to as condensate. The remaining liquid will then consist of some portion of the original liquid solvent and the non-evaporated solute and other constituents. This liquid is typically referred to as the concentrate.

(1) Evaporative solvent extraction is the recovery of valuable solvent from a process solution. The condensate is considered the product, and the concentrate is usually considered waste.

(2) Evaporative concentration is the concentrating and recovery of valuable process solutions. The concentrate is considered the product and the vapor, often not condensed, becomes the waste.

(3) Evaporative reduction is the reduction of liquid waste. Usually, both the concentrate and the vapor (condensate) are considered waste. This is generally an expensive process and used only when the cost of disposal can justify it.

Both wastewater evaporators and CAST are best suited for applications in which either the concentrate and/or the condensate has an intrinsic recoverable value. In the past, the valuable components were typically contained in the concentrate, with the condensate, usually water, being discharged to drain. However, with the rising cost of water, sewer discharge rates, water rationing and increasingly more stringent environmental protection discharge requirements, water can no longer be considered expendable. Additionally, process chemicals, having an intrinsic value which was originally deemed insufficient to warrant recovery, often require expensive and complex treatment for removal prior to sewer discharge. Incomplete removal may have a catastrophic environmental impact, subjecting the operator to criminal charges, costly fines and possible shut down. Hence, industries requiring large quantities of process water, such as the metal finishing industry, are actively seeking alternative waste treatment technologies to aid in their water consumption and pollution reduction programs.

CAST and wastewater evaporators offer the ability to recover the valuable chemical components and solvent, usually water, from many industrial process solutions, often eliminating the cost of waste treatment, waste disposal, sewer discharge, lost chemistry and expended water. This makes both CAST and wastewater evaporator based systems attractive alternatives to conventional wastewater treatment technologies.

The metal finishing industry, for example, uses very large quantities of water in the rinsing of components. The rinse process produces aqueous-based waste solutions containing environmentally hazardous material, typically, heavy metals as salts. Heavy metals posses an intrinsic recoverable value; however, the actual salts, as used in electroplating have a much greater value. The recovery of these salts in an immediately reusable form and the reuse of the rinse water are commonly referred to as “closed looping”.

“Closed-loop operation” implies that there is no sewer discharge or waste products produced under normal operating conditions. Generally, the ultimate goal in any waste reduction/recovery scheme is a closed-loop process. In the metal finishing industry, as in many other industries, both wastewater evaporators and CAST systems offer closed-loop operation. Additionally, both wastewater evaporators and CAST systems can be utilized in waste reduction/recovery programs.

As an example, of a closed-loop operation would be wastewater treatment from an electroplating operation. Rinse water is drawn from the first rinse tank and processed by either the wastewater evaporator or CAST system. Reusable plating chemistry and water are returned to the plating tank as “make-up”.  “Make-up” is usually defined as the additional products added to the plating tank to maintain a constant liquid level and chemistry. “Make-up” compensates for the loss of plating solution due to the mechanical carry-over of solution on processed parts, referred to as “drag-out”, natural evaporation and electro-deposition. The rinse water is returned to the final rinse tank for reuse in the process. Hence, no waste is generated.

Wastewater evaporators are classified by the mode of evaporation, type of heat transfer surface and operational principal. Evaporation is divided into film, nucleate boiling, and flash modes. Heat transfer surface are divided into jacketed, coil, tube-and-shell and plate types. Heat transfer surface type and orientation is a function of the mode of evaporation and the operational principle. Host heat transfer surface are oriented horizontally or vertically. Operational principles are based on the mode of evaporation and are usually divided into natural, forced circulation and mechanically aided. They are further complicated by multiple staging of effects and mechanical and thermal vapor compression schemes. The actual mode of evaporation, heat transfer surface and operational principle are functions of the products to be processed and the desired goals.

Commercially available wastewater evaporator systems are typically based on climbing film/horizontal tube, falling film/horizontal tube, natural circulation/horizontal tube or forced circulation flash evaporators.

In climbing and falling film horizontal wastewater evaporators, the liquid to be evaporated is distributed over the surface of horizontally oriented tubes. The liquid forms a thin film on the heat transfer tubing, creating a large surface area for evaporation. In falling film applications, excess liquid falls from tube to tube by gravity, an effect which may be augmented by spraying. In climbing film applications, the film is forced over the tube surfaces by the higher velocity vapor being generated. The heat required for evaporation is supplied by a fluid media passing through the heat transfer tubing, usually hot water or steam. The vapor generated may be utilized as the heat media for an additional evaporator, multiple effects, or mechanically compressed and passed through the original heat transfer surface.

This type of wastewater evaporator is commercially available in a sprayed falling film/horizontal tube configuration with electrically driven mechanical vapor compression and a single- or multiple-effect climbing film/horizontal tube configuration. Higher energy efficiencies are derived from incorporating the mechanical vapor compression. However, the increase in energy efficiency is offset by the increase in the cost of construction, system complexity, increased maintenance and decreased reliability. Additionally, both climbing and falling film/horizontal tube arrangements intrinsically suffer from salting and scaling of the heat transfer surfaces, limited operating viscosity ranges, large floor space requirements, and very complex and expensive construction.

In natural circulation horizontal tube wastewater evaporators, the liquid to be evaporated is contained in a boiling chamber. The heat transfer tubing is immersed in the liquid. Evaporation is based on nucleate boiling. The heat required for evaporation is supplied by a fluid media passing through the interior of the tube. The vapor generated may be utilized as the heat media for an additional wastewater evaporator, multiple effects.

The natural circulation horizontal tube wastewater evaporator is commercially available in two different configurations, the main differences being the method of heating the liquid to be evaporated and cooling or condensing the vapor. The first unit is based on recovering the waste heat generated by a refrigeration cycle. The refrigerant is used to condense the vapor. The heat transferred to the refrigerant is then retransferred to the liquid to be evaporated in the boiling chamber. The second system utilizes a more conventional approach: high pressure steam is passed through the heating coils and coolant, usually water supplied from a cooling tower, is passed through the condenser. This system can be arranged as a two-effect device to increase energy efficiency.

Wastewater evaporators based on nucleate boiling are not recommended for use with foaming solutions. This limits their application. Additionally, they tend to suffer from carry-over, the entrainment of process solution by the raising vapor as it exits the boiling liquid surface, and splashing. Carry-over is prevented by the use of mist separators. Mist separators are subject to fouling, corrosion and flooding. Inspection, cleaning and replacement of the separators is required. The excessive buildup of carry-over in a separator referred to as flooding, decreases the evaporator efficiency and contaminates the vapor passing through the separator. Splashing of process liquid due to excessive boiling may also cause flooding. Nucleate boiling systems also suffer from heat exchanger scaling and deterioration due to poor fluid circulation and high film temperatures at the exchange surfaces.

Note that systems based on refrigeration cycles have high energy efficiencies; however, refrigeration equipment requires costly electrical energy to operate, thus yielding operational costs similar to those of a single pass standard steam heated wastewater evaporator. Additionally, refrigeration equipment suffers from poor reliability, requires regular extensive maintenance and is complex and expensive in construction.  The low pressure steam and cooling towers is most often recommended for industrial applications due to its simplicity, efficiency, reliability, minimal capital investment and common understanding of the technologies.

In forced circulation flash wastewater evaporators, the fluid to be evaporated is gravity-fed from the process vessel to a centrifugal pump.  The pump pressurizes the fluid.  The fluid to be evaporated is gravity-fed from the process vessel to a centrifugal pump the pressurized fluid is sprayed into the evaporation chamber where the vapor flashes off the spray cone, eliminating nucleate boiling. The vapor is condensed by an external heat exchanger/condenser, or it can be utilized as the heating media in addition effects. Additionally, the heat of vaporization can be recovered from the condensing vapor and used to preheat the process solution. By using a vacuum, significant energy savings can be realized thorough lower boiling points of the liquids injected.  This technique can produce energy efficiencies similar to those of large and complex multi-effect systems, yielding better than 7.0 lbs. of vapor per 1.0 lb. of steam used (approximately $0.01/gal. processed).