New Evaporator Technology Cuts Mill Effluent Flows, Process Water Makeup

A new type of falling film, mechanical vapor recompression (MVR) evaporator that employs a heat exchanger constructed of polymeric materials rather than traditional metal (or polymer coated metal) is being investigated and commercially applied in the pulp and paper industry. The relatively low cost polymeric materials used in these units effectively provides much larger surface areas for evaporation, which translates into lower temperature differentials to achieve specific evaporation capacity.

Results from tests being conducted at Stora's pulp and paper mill in Gruvon, Sweden, show that the unit, manufactured by Hadwaco Ltd. Oy of Finland, is a viable solution for reduction of end of pipe effluent flows. Installed in the mill's ECF bleach plant filtrate line, the unit has a capacity to evaporate 80,000 gpd, with a concentrate flow of 3,150 gpd. The unit has a fairly low power consumption of 31-kwh/1,000 gal.

The Stora tests are now being expanded to include a full-scale bleach effluent treatment unit. Aided with funding from the European Commission's Innovation Program, the evaporator project is designed to run for two to three more years at the Stora Paperboard pulp mill in Gruvon. The project consortium consists of representatives from Spain, the Netherlands, Finland, and Sweden. The goal of the project is to develop a new generation of TEF (totally effluent free) pulp and paper mills, based on the new evaporator technology, to be commercialized within the next two years.

A system employing the evaporator technology has also been operating for several years in a water makeup/recovery application at the Arab Paper Manufacturing Co. mill in Saudi Arabia, and results so far confirm the system's expected performance in the mill's process water and effluent streams. Installed as a turnkey operation, the system vessels were delivered in four prefabricated, completely operational modules that were joined together at the site.

The Saudi Arabia system is comprised of two evaporator units, both with two modules. It includes a complete set of instrumentation and a control system using PLCs and a PC operator interface. The installation is designed so that any one of the four modules can be taken offline for cleaning and repair, while the remaining units maintain the water supply to the mill. The system can evaporate 320,000 gpd, with a concentrate flow of 35,000 gpd and a power consumption similar to that of the Swedish installation.

The evaporator is currently being evaluated in various chemi-thermomechanical and other mechanical groundwood pulping applications as well as recycled paper mills, primarily in effluent discharge service. Other evaluations show that the technology can also be used as a "kidney" type of purge system in paper machine white water loops. In North America, the unit has recently been installed in pilot plant operations at several sulfite pulp mills, for water recovery and COD improvement trials. According to Hadwaco, the evaporator technology could allow sulfite mills to "further" their pulping operations while meeting stringent requirements of the EPA's recently promulgated Cluster Rules.

In addition to the paper industry, full scale Hadwaco evaporators have recently been installed in several landfill leachate treatment plants in Finland, Italy, and Spain. In some cases the unit is handling the entire leachate stream, and is allowing all three operations to meet very stringent requirements in their respective countries. A most recent installation is being used for purification of rinse water to a plant for the treatment of aluminum profile surfaces. The purification process at this aluminum plant is based on a closed-loop system whereby purified rinse water is reused, and chrome is efficiently separated and brought to a deposit. The capacity of the plant, at Mkel Alu Ltd in central Finland, is about 14,000 gpd.

The unit is also being installed in a steel mill under construction in South Africa, to treat process water in a closed system operation. Other installations include treatment of a wiper laundry's permeate in the Netherlands, protein recovery for animal feed operations at a Finnish slaughter house, and drinking water desalination in Malta.

Process Technology

As with all MVR evaporators, the driving force for evaporation in the Hadwaco unit is the latent heat (heat of the evaporation/condensation) of vapor. When vapor evaporated from a liquid is compressed, it can be simultaneously used as a heat source. Heat from the condensing vapor is transferred through the heat transfer element to the liquid circulating on the concentrating side. The flow diagram in Figure 1 below illustrates the unit's operating principle.

Click here to see Figure 1. Operating principle of Hadwaco evaporator unit.

The amount of mechanical energy required to be added to the vapor by the fan depends on the design temperature difference across the heat transfer surface (i.e., the pressure ratio of the compressor). Since the Hadwaco system operates on a relatively small pressure differential, the operational cost of the unit is lower than most evaporative technologies. In full-scale units, 1 kW of energy typically generates evaporation corresponding to the heat energy of 70-100 kW (240,000-340,000 Btu/hr). Alternately stated, this same 1 kW of electrical energy generates an evaporated quantity of 27-36 gal of water.

Suitable low cost, high quality polymeric materials for the heat exchanger are generally available for most process effluents. As mentioned above, this allows the effective use of much larger surface areas for evaporation. The larger the surface area, the lower the temperature differential necessary to achieve a desired evaporation capacity. The unit's design allows the use of a low speed fan as a vapor compressor, eliminating the need for costly, high speed, high maintenance mechanical compressors.

The unit operates under vacuum that enables the feed liquor to boil at relatively low temperatures (130o-140o F) and at moderate dissolved solids concentrations. These selections of operating conditions provide the benefits of low cost operation (30-38 kwh/1,000 gal of recovered water) and low corrosion potential.

Operating Principle

As shown in Figure 1 above, wastewater passes through two parallel heat exchangers into the unit. A circulation pump transfers the heated (or cooled) wastewater to the upper portion of the vessel where it is evenly distributed on the heat transfer element through a liquid distributor.

A portion of the circulated wastewater evaporates on the outer surface of the heat transfer element. The resulting vapor flows through a fan type of compressor to increase the heat/pressure energy, after which the vapor flows to the inner surface of the heat transfer element where it condenses. Latent heat is transferred to the wastewater side of the heat transfer element, and clean condensate is collected for discharge or reuse in the process. The concentrated wastewater falls to the bottom of the vessel where it is removed by the concentrate pump for disposal.

Vacuum in the system is created with a standard liquid ring vacuum pump. The pump also removes any uncondensable vapor and gases. Heat from this stream is recovered by a plate heat exchanger.

For more information: David Thomas, Hadwaco Inc. Tel: 770-457-4429.

By Ken L. Patrick