Environmental compliance and energy costs have created an ever-increasing concern to industry. Production and environmental managers should be aware of innovative techniques to recover valuable resources using energy-efficient methods. This enables a company to stay ahead of environmental legislation and have a positive impact on the company’s operation.
Vacuum evaporators have been used by industry for many years to treat wastewaters from various processes. Typically, they have been single-stage evaporators using various forms of energy to evaporate the water.
Samsco applies double effect vacuum wastewater evaporators to wastewater treatment by using the latent heat of vaporization energy developed in the first-stage vacuum chamber to evaporate wastewater in a second chamber, without additional energy input (refer to Figure 1). This means the energy in the second stage is effectively free and the cost of evaporation is halved.
The WasteSaver™ system operates in the following way, (see Fig. 1). Steam heat is applied to the first effect vessel’s heat exchanger. As the waste solution boils, water vapor is drawn up through a two-stage mist elimination pad (to prevent entrainment of droplets). This vapor is then conveyed to the heat exchanger tube bundle in the second vessel by the effect of the greater vacuum in that chamber. The condensate from the second effect heating bundle is vacuum drawn up into the system’s distillate tray and is removed by the EVAC evacuation system in which vacuum is drawn on the system utilizing an eductor pump water loop which induces the vacuum in both boiling vessels. The wastewater in the two chambers can be from the same source or the system can be operated as separate treatment chambers with two different waste streams.
This dual effect process enables evaporation of wastewater at half the operating cost of a traditional vacuum evaporator.
Another aspect of this design is that the process liquids are moved throughout the system without the use of pumps. The fluids are moved either by being drawn into a chamber (Distillate EVAC or Condensate EVAC) by suction created by the eductor or by being expelled from the chamber with compressed air. This system offers significant advantages over a typical pump driven system, especially in regard to maintenance requirements.
The vacuum is drawn on the top of the second effect vacuum chamber, so the first effect chamber experiences less of a vacuum, (around 2 psi less). The heat applied in the first chamber is 20 oF to 40 oF higher than that in the second chamber (depending on process requirements).
With these factors in mind, selection of which chamber would best handle a particular chemistry is possible based on the temperature stability of the chemistry and the desirability of recovering the chemistry.
Continuous monitoring of the distillate with conductivity meters in both the steam supply and the process distillate ensures process control and system integrity. Current technology using PLC controls enables real time performance monitoring with programmable quality parameters. If distillate quality falls below a pre-determined level, water is diverted back to the evaporator inlet. If this happens repeatedly, the unit will pause and alert a technician for intervention.
Optimum operating parameters can be pre-evaluated in a bench scale Pilot Study conducted in Samsco’s laboratory with a small sample of the wastewater, or onsite with a scaled down pilot unit. Valuable information can be derived from the pilot study using a WasteSaver double effect evaporator on a specific wastewater application. This enables evaluation of the equipment and its benefit in the user’s process and facility.
As discharge limits for POTW’s become increasingly stringent, knowledge of the various in-house wastewater treatment options and their impact on the facility, will be important for management to evaluate. Any manufacturer of wastewater treatment equipment should provide a detailed evaluation of their particular system and its engineering and environmental impact on the user’s facility.
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