HYDRAULICS 24 HYDRAULICS & PNEUMATICS January/February 2024 www.hpmag.co.uk LEFT: HRS Unicus Series scrapedsurface evaporators are used to maintain thermal efficiency example, in the energy and power sector (historically the largest user of ZLD technologies), access to clean water suppliers is an increasing concern. Pollution control is also a major driver, and the US EPA considers zero discharge as the preferred treatment option for fly ash and bottom ash transport water, and wastewater from flue gas mercury control systems2. Elsewhere, growing awareness of the toxic effects of petrochemicals and petrochemical waste products is driving efforts by the industry to clean up its waste processing systems. ZLD methods are already widely used in industrial wastewater treatment to recover useable and profitable minerals and by-products from waste streams, and the success of such systems is encouraging their take up by other businesses. In 2015 the Indian government issued a draft policy requiring all textile plants generating more than 25 cu. m. of wastewater effluent a day to install ZLD facilities2. ZLD technology has been utilised in various markets around the world, including Europe, Australia, Canada, the Middle East and Mexico, but the biggest markets, and the biggest potential for expansion, can be found in the United States, China and India2. The benefits and challenges of ZLD In general terms, the use of ZLD reduces water pollution and augments water supply, but this is sometimes offset by high costs and energy consumption and in the past these factors have limited the uptake of the technology. Wastewater reuse minimises the volume and environmental risk of discharged wastewater, but also alleviates the pressures associated with the abstraction of freshwater, but these benefits have to be balanced against the economic and energy costs of implementing ZLD systems. As water scarcity and environmental pollution around the world intensifies, ZLD becomes more feasible and widespread, and the relative costs of ZLD technology versus the alternatives (assuming alternatives even exist) are lowered. Increasing the efficiency of ZLD Separating all of the water out of the product requires large amounts of energy. It takes roughly six times more energy to evaporate water (latent heat) at its boiling point than the energy needed to actually bring it to that boiling point (sensible heat). For that reason, ZLD processes often start with a separation process based on (reverse osmosis) membranes. Membrane separation does not require phase change / boiling. Electrical energy (pumping) is used to push water through the pores of the membrane and separate it from the dissolved solids. Membrane can only work to bring the product up to a certain concentration. To achieve complete separation, evaporation / crystallization processes are needed for completing the process. As explained before, evaporation (due to the latent heat) is highly energy consuming. Therefore, it is wise to choose an evaporation process that involves ways of energy optimization, the most popular being: Multistage evaporation: using the latent heat of the evaporated water as energy source in a next evaporation stage reduces the overall consumption of the boiler to the evaporation plant. Thermal Vapour Recompression (TVR): evaporated steam is mixed with boiler steam. The reuse of the evaporate steam reduces the energy demand. Mechanical Vapour Recompression (MVR): An MVR compressor (driven by an electrical motor) can be used to compress the evaporated steam, thus increasing its pressure, and use this steam as the energy input for the process. MVR compression is very efficient in terms of energy consumption. Due to the factors outlined above, (multistage) vapour compression plants remain the main method employed for ZLD processing globally, with evaporation Depending on the product to be concentrated, HRS says it can select from a series of technologies for designing the most optimal ZLD process. Energy optimisation methods (multistage, TVR, MVR) can be combined with several types of heat transfer technologies (plate evaporators, corrugated tube evaporators, scraped surface evaporators). Whatever the technology applied, the overall process can be separated into three steps: 1. Evaporation / concentration: The product is concentrated to just below its maximum concentration (saturation). The evaporation plant is usually a multistage evaporator setup. 2. Cooling: if the maximum solubility curve is steep (large concentration at high temperature, low concentration at low temperature), the product obtained in step 1 is cooled, provoking immediate precipitation of dissolved solids. 3. Crystallisation: Crystallisation / sedimentation of the solids produced in step 2 occurs in specially designed crystallisation tanks. A supernatant layer of concentrated solution remains after this stage and is returned to step 1 for reprocessing. For products without a steep solubility curve, it is necessary to concentrate inside the evaporator to above the maximum solubility. This means that the step 1 process is equipped with a final evaporator stage (finisher) that is specially designed to work with suspended solids. The fluid with suspended solids is then transferred directly to the crystallization tanks in step 3. The brine cooler and evaporator finisher work with solids in suspension and often this means dealing with fouling products. A typical HRS evaporator / finisher will use Unicus scraped surface evaporators that are self-cleaning and maintain optimal evaporation rates. Typically, our R series scraped surface coolers are used for cooling the saturated brines that are sent to the crystallisation tanks. The result is an efficient process which can work continuously without requiring scheduled downtime.
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