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MATERIALS HANDLING


LEFT: HRS heat exchangers being used for low-temperature evaporation


using surplus heat from heaters and CHP engines. Heat exchangers from HRS are capable


of recovering heat from other sources, such as the heat left from the CHP engine or from pasteurisation, and using this as the basis for an evaporation system can further improve overall process effi ciency. Additionally, using a vacuum in the system to reduce the boiling point reduces the amount of energy required even further.


Combining systems into a multiple-


Think you can’t evaporate with low temperatures? Think again, says Matt Hale


hen treating wastewater, sludge and digestate, it can often be benefi cial to also reduce the volume of material


EVAPORATION W


INNOVATION IN


or increase the solids content. Mechanical dewatering via a centrifuge or belt press to separate solids and liquids is standard for the water treatment and anaerobic digestion sectors – but what if you want to further reduce the water content of the remaining liquid fraction? T e two main reasons to remove or


reduce water from effl uent and waste are either to reduce the volume of material to cut storage, handling and disposal costs, or to produce materials with distinct properties (such as liquid and solid fractions of digestate), which can then be stored and used in the most appropriate way. Traditionally, there have been two ways to achieve this: drying and evaporation. Drying usually requires large quantities of heat and energy, is costly and ineffi cient,


42 www.engineerlive.com


and is unsuitable for materials with high water content. Conventional evaporation techniques require temperatures in excess of 100°C, which is usually provided by boiling water or steam. However, due to the low temperatures used in wastewater treatment and aerobic digestion, such energy is not usually available without additional fuel inputs, making evaporation a high-energy option. However, for many types of effl uent,


low temperature evaporation can be a very energy-effi cient method of water removal. Where process temperatures are 85-90°C, low temperature evaporation combines the use of a vacuum to reduce the boiling point of the liquid to be removed, together with traditional high temperature evaporators, based on heat exchanger technology. Where the temperature of the effl uent or digestate falls below the necessary temperature, it can often be increased via heat exchangers,


eff ect evaporator allows larger quantities of water to be removed for the same initial heat input. Each evaporator is held at a lower pressure than the previous one: because the boiling temperature of water decreases as pressure decreases, the vapour boiled off in one vessel can be used to heat the next – only the fi rst vessel requires an external source of heat, which can be taken from another process elsewhere or generated specifi cally for the purpose. T e type of heat exchanger used will depend on the nature of the products being treated. For materials with low or medium viscosities, such as wastewater and effl uent with low concentrations of organic solids, using the HRS K Series as an evaporator module provides high heat transfer rates with good resistance to fouling. For more challenging and viscous materials, such as thicker effl uents, digestate and solids with higher dry matter concentrations, the HRS Unicus Series contains a self-cleaning scraper mechanism that reduces fouling and maintains heat transfer rates (and therefore operational effi ciency). Both series are commonly used in the type of multi-eff ect evaporation system described above, and can also be used in other types of evaporator. T e ultimate use of these heat exchangers


for low temperature evaporation can be found in the HRS Digestate Concentration System (DCS), which uses multiple evaporation eff ects to increase the solids in liquid digestate three- to four-fold.


Matt Hale is with HRS Heat Exchangers. www.hrs-heatexchangers.com


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