Design and application of a bench scale evaporator for the treatment of coal power plant waste water

Abstract

The potential of scale formation during the application of Multi-Effect Distillation (MED) technology for the treatment of Reverse Osmosis (RO) retentate was investigated in this study. An improved bench scale single effect evaporation unit was designed using the Solidworks Simulation Package and appropriate materials. Operating conditions were determined to be 80°C and 0.48 bar with the design constructed to maximise evaporation. Once assembled, RO-retentate was introduced to the evaporator at an evaporation rate of 4 L/h (22.3 L/m²/h), and the variation of the concentrations of scale forming elements determined. The evaporation observed translates to 34.5% water recovery. Speciation using PHREEQ-C modelling software showed that the concentrations of scale forming ions increased during evaporation. Scale prediction and prevention were explored using French Creek software, with anti-scaling agents being used to minimise scaling. The dosage of the anti-scaling agents inside the MED was determined using French Creek software. The anti-scaling agents yielded confirmation of their capability of keeping ions in suspension and preventing them from forming scale. When measuring the performance of the anti-scaling agents on this basis, AMPS (Acrylic Acid-2-Acrylamide-2-Methylpopane Sulfonic Acid Copolymer) fared better than PBTC (2-Phosphonobutane-1,2,4-Tricarboxylic Acid). However, HEDP (1-Hydroxy ethylidene-1,1-Diphosphonic Acid) yielded concentrations of scale forming ions lower than when no anti-scaling agent was dosed. This may be ascribed to the nature of complexes that HEDP forms with the scale forming ions. The application of MED technology combined with the use of anti-scaling agents to treat inland waste waters can see the MED evaporator serving a dual purpose. The primary purpose is to serve as water producing/recycling technology that can treat saline solutions to recycle as much fresh water as possible. The MED reject will be suitable for further treatment using other separation technology to ultimately produce fresh water and salts. The secondary purpose of the MED technology when applied to a power plant could be to dissipate some of the abundant heat present. The MED technology possesses the potential of alleviating the amount of heat energy the cooling towers must dissipate to some extent, thus lowering the amount of cooling water needed, and this should be investigated. Thus, a combination of membrane technology and thermal technology could theoretically be utilised in order to adhere to the zero liquid effluent discharge policy Eskom has, whilst lowering the amount of fresh water needed for cooling purposes. Additionally, the laboratory evaluation of anti-scaling agents prescribed by scale modelling software was investigated. The considered water contained high levels of scale forming agents. The selected test method to determine anti-scaling agent induction times was found to be inefficient and an improvement on the current method was proposed. The possible causes for the inefficiency of the selected induction time test method was mainly ascribed to an insufficient heating apparatus setup.