Multiple effect distillation (MED) and multi stage flash (MSF) are the two primary thermal technologies for water desalination. At the end of 2011, MED and MSF accounted for more than 30% of worldwide water production capacity. MED has two main advantages over MSF: better heat transfer and fewer effects needed to achieve a given performance ratio (mass of distillate produced per unit mass of input steam).
In this study, a two-stage lab-scale multi effect distillation (MED) unit with horizontal copper tubes was designed and fabricated to study scaling behavior as a function of water chemistry. This unit is one piece of a lab-scale biomass slow pyrolyzer-MED system in which the thermal and electrical energy needed to operate the MED unit come solely from biomass. This system is intended for small-scale use on farms to produce desalinated irrigation water using energy from local waste biomass. Even though steam from an external source is typically used in the first effect of MED units, design simplicity and laboratory space limitations caused us to choose hot water for sensible heat in the first effect. Design specifications include using 570 kg/hr of hot water to provide the primary heat within the first effect and 80 kg/hr of brackish feed water (1,000-3,000 ppm total dissolved solids) to produce about 20 kg/hr of fresh water. We used polycarbonate sheeting to make the front covers of the effects to enable real-time observation of unit operation.
We will use this custom-built unit to determine the optimal temperature, pressure, and flow rate at which the least scaling occurs for several different water chemistries. Scaling, mostly calcium carbonate and calcium sulfate formations, will be measured using a salt balance across the unit. Four thermocouples have been installed within the second effect of the MED in order to investigate temperature changes and hot spots resulting from scaling. We will also evaluate the effectiveness of several treatments (dilute hydrochloric and sulfuric acid, EDTA, mechanical cleaning, etc.) to remove scaling from copper heat transfer surfaces.
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