Large, commercial, thermal desalination plants achieve high efficiencies by reusing the heat released when water vapor condenses to evaporate additional water. In large desalination plants, the processes of evaporation >> condensation >> heat recovery >> additional evaporation are done within vacuum vessels with heat transfer across expensive cupronickel or titanium heat exchangers. AILR’s Diffusion-Gap Distillation (DGD) also achieves high efficiencies through evaporation>>condensation>>heat recovery>>additional evaporation. However, the DGD process avoids the need for vacuum vessels and expensive metallic heat exchangers.
The novel feature through which DGD achieves high performance is to locate hot, evaporating surfaces that are wetted with brine very close to cooled, condensing surfaces, the gaps between the surfaces being less than 3 mm (about 1/8th inch). This close positioning of evaporating and condensing surfaces allows a high flux of water vapor to flow from the evaporating surface to the condensing surface even when the evaporating surface is only a few degrees warmer than the condensing surface. Thus, high fluxes of water vapor driven by small differences in temperature, which characterizes large commercial thermal desalination plant, are achieved without vacuum vessels or metallic heat exchangers.
As shown in the neighboring figure, the DGD system consists of two or more plates that have brine entering at the bottom and leaving at the top. Thin, wicking surfaces are positioned in the gaps between the plates. The brine that flows up within the plates is heated as water vapor condenses on the outer surface of the plates. The source of this water vapor is the brine, which after being preheated in the plates is further heated by an external source (shown as steam in this figure) and then is delivered to the top of the wicks. The cooled brine flowing off the wicks (downward purple arrow) and the condensate flowing off the plates (downward blue arrows) are collected in separate troughs.
In a February 2016 test, a five-plate DGD prototype initially distilled water from a 6.5% NaCl solution at a GOR of 14 kilograms product per kilogram of steam input. At the end of the 185-hour test the prototype distilled water from a 16% NaCl solution at a GOR of 6. A PowerPoint presentation with additional information on the February 2016 can be downloaded, as can a AILR's pending patent application that describes the DG process in more detail.
AILR is now working with Apricus to field test a 200 to 500 liter per day DGD system operating on Apricus' solar collectors. A one-page brochure can be downloaded that introduces the Apricus/AILR collaboration.