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How does VTE desalination work?

Vertical Tube Evaporation (VTE) desalination is a distillation process that uses heat from steam of any source to evaporate saline water. A Vertical Tube Evaporator operates similarly to a brine concentrator by distributing saline water to the inside surface of an array of vertical metal tubes under vacuum. The water flows either down or up the inside tube surface in a thin annular layer while it is heated by steam condensing on the outside surface. The thin layer enhances the efficiency of evaporation because water has low thermal conductivity. A thin layer allows heat to be rapdidly transferred from the heated tube surface, through the layer, and to the water/vapor surface where it evaporates. Conversely, steam condensing on the outside surface of the metal tube forms a thin layer of water that trnafers heat to the metal, then quickly runs down the tube surface keeping the condensation process efficient. Saline water and vapor exit the tube into an evacuated separation chamber. There vapor is routed to a condenser and cooled to distilled water. The saline water falls by gravity and is recirculated until it has been evaporated down to a target concentration.

How does VTFE differ from VTE technology?

The Vertical Tube Foam Evaporation (VTFE) process uses a small quantity of surfactant added to the saline water being evaporated to enhance the efficiency of evaporation by promoting the controlled formation of water/vapor bubbles inside the evaporator tubes. This helps keep the layer of saline water at the tube surface thin and helps the water spread on the tube surface by reducing surface tension. The surfactant stays in the saline water as it concentrates, it does not pass to the vapor or distilled water product. The surfactant concentration has to be controlled to avoid exessive foaming.

How does VTE desalination compare to other desalination technologies in use?

VTE desalination is a thermal distillation process which is radically different from the membrane based Reverse Osmosis (RO) process. RO uses high pressure to force water molecules through microscopic pores in a membrane while leaving the larger and charged salt molecules behind. All thermal distillation processes use heat to evaporate saline water, capture the vapor and condense it to distilled water by cooling. RO has been developed into a very energy efficient process by partly recapturing energy from the high pressure pumps, however the energy to drive the pumps is generally electrical, which is expensive energy. Both thermal distillation processes in common use, Multi Effect Distillation (MED) and Multi Stage Flash (MSF) recapture thermal energy from a high temperature evaporation process to drive lower temperature processes that evaporate water under vacuum. Distillation processes are inherently less energy efficient than membrane processes, however the type of energy used (thermal vs. electrical) can be much less costly, as most electrical energy is generated by conversion from thermal energy. MED and MSF are cost effective compared to RO when a low cost heat source is available such as waste heat from a power plant or industrial process. A multi Effect VTE plant is a variant of MED technology which can evaporate saline water to a higher concentration due to the tube design allowing a higher recovery of distillate and a much lower volume discharge of brine.