Mechanical Technology May 2016

⎪ Heating, cooling, ventilation and air conditioning ⎪

Heat exchanger desalination

Roger Rusch (left), CEO of Industrial Water Cooling (IWC), talks about the suitability of

his company’s SONDEX heat exchanger desalination solution for Africa, a technology that can utilise waste heat from engines or other heat

rejecters to produce drinkable water by evaporating seawater under vacuum.

T here are a number of valid rea- sons for South Africa’s growing water crisis such as climate change, lower rainfall, crum- bling water-pipe infrastructure, multiple minor and major leaks across urban and rural communities, theft of water and more. One solution to the seriously diminish- ing fresh water supplies in South Africa is to tap into the largest water resources available: the ocean, and the desalination of such water. Desalination is a well-established method of removing salt from water in order to produce process water, ultra-

SONDEX’s fresh water distillers utilise the waste heat from diesel engine jacket cooling water or other heat sources to produce pure drinkable water by evaporating seawater under high vacuum. This system produces up to 150 t of drink- able water per day. heat from diesel engine jacket cooling water or other heat sources to produce pure drinkable water by evaporating sea water under a high vacuum, thereby enabling the feed water to evaporate at temperatures below 48 °C. Steam can also be used as the heat source instead of the hot jacket water. This technology eliminates the need for an additional, costly energy source.” The Fresh Water Distiller is based on a two-stage design in a vertical con- figuration. It is made up of two custom- designed titanium plate heat exchanger packs acting as evaporators and one titanium plate heat exchanger pack act- ing as a condenser. These plate packs are located in two separate chambers under different vacuums. In stage one, the first evaporator plate pack, located in chamber one at 70% vacuum, turns the seawater into vapour, thereby separating the salt. This vapour is then directed through a large diam- eter pipe down into stage two, which is located in the second chamber working at 90% vacuum. The hot brine obtained from stage one is redirected to the sec- ond plate pack, which is also located in Chamber two. When the vapour reaches chamber two, it is condensed and its heat released into the second plate pack with the hot brine inside. Acting as an evaporator, the second plate pack turns the hot brine into vapour, leading to more salt separation. The third plate heat exchanger pack, How to use heat exchanger packs to desalinate ocean water

pure or potable water. This is ac- complished through using membranes (reverse osmosis and nano-filtration) or thermal processes (multi-effect distil- lation, evaporation and crystallisation). The reason desalination hasn’t yet become more popular is cost limitations. Desalination is an expensive process. Salt dissolves very easily in water, form- ing strong chemical bonds, and those bonds are difficult to break, requiring large amounts of energy. A global company at the forefront of developing next-generation, fresh water distillation technology is Danish-based SONDEX. To bypass the considerable expense, SONDEX has developed a process that uses heat exchangers to recycle excess or waste energy generated

in an industrial process. This energy is then redirected to power a separate thermal distillation plant. In thermal distil- lation, boiling water is turned into vapour, which leaves the salt behind. The vapour is then condensed back into drinkable water as it cools. This process requires more energy when compared to the other common method of desalination, reverse osmosis, where seawa- ter is forced through a semi-permeable mem- brane that separates the salt from water. Roger Rusch, CEO of IWC, an official supplier

SONDEX has developed a process that uses heat exchangers to recycle excess or waste energy generated in an industrial process.

of SONDEX heat exchangers in Africa, says: “SONDEX’s fresh water distiller utilises the waste

30

Mechanical Technology — May 2016