GB2441827A

GB2441827A - Desalination plant

Info

Publication number: GB2441827A
Application number: GB0618171A
Authority: GB
Inventors: William Alexander Courtney
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-09-15
Filing date: 2006-09-15
Publication date: 2008-03-19
Also published as: GB0618171D0

Abstract

A device for purifying a liquid or separating liquids (e.g. desalination of brine) having different boiling points by distillation comprises a heated upper evaporation chamber 1 enclosing a trough 2 holding a liquid and a lower condensation chamber 4 in thermal contact with the base of the trough. A passage allows vapour produced in the evaporation chamber to flow into the lower chamber where it condenses on the base of the trough and is preferably collected in a drip tray 8, the latent heat released by condensation helping to warm the impure liquid. After the system has warmed up and steady state conditions are established, the evaporation chamber and its contents has a warm end 5 and a cool end 6. In order to ensure that the pressure inside the evaporation chamber is in balance with the external air pressure, air may be added by a pump 7 to the water vapour. Preferably the device is heated by solar radiation and a canopy of lenses may be placed above the upper chamber to focus the energy on the upper chamber. Further, variations in vapour pressure and specific volume may be used to displace a surface against a load, enabling the device to do external work.

Description

Desalination Plant Technical Field This invention relates to improvements in devices used to purify or separate liquids by distillation. According to the present invention there is provided a device for purifying a liquid or separating liquids having different boiling points at the same atmospheric pressure, by the process of distillation, comprising a heated upper evaporation chamber enclosing a trough holding the impure lire liquid(s) and a lower condensation chamber, in good thermal contact with the base of the trough, with the two chambers being connected by a passage, allowing vapour produced in the upper chamber to flow into the lower chamber and condense on the external base of the trough, characterised by the lower chamber extending to cover the length of the trough in the upper chamber, with latent heat released by the condensation process helping to warm and evaporate the impure liquid(s) and creating a horizontal temperature gradient, with the highest temperature being at the end of the device where the vapour flows from the upper chamber to the lower chamber. Brief description of the drawings Figure 1 is a schematic diagram depicting vertical cross section along the length of a solar powered version of the invention, used for distillation of brine to potable water. Figure 2 depicts a variation on the invention that uses pressure drops caused by condensation to motivate a second pump, which acts in series with the pump 7 of Figure 1 , to compress the air plus residual vapour back to atmospheric pressure. Figure 3 is a schematic diagram depicting a West to East vertical cross section of a solar powered version of the invention. Figure 4 illustrates how parallel solar powered desalination devices 1 and 2 may be grouped under a common Fresnel lens canopy. Figure 5 illustrates how at the cool end of the system, additional shading can be added to the trough, to ensure that the condensing vapour always has access to cooler water above it. Figure 6 depicts a vertical cross section similar to that depicted in Figure 4, but with steam carrying pipes being used as the primary heat source for the brine. Oisciosure of the invention The invention is related to improvements in devices used for the purification of liquids or separation of liquids having different boiling points at the same atmospheric pressure. The inventive step will be illustrated by revealing versions of the invention suitable for distilling brine to potable water. These illustrations are not intended in any way to limit the scope of the invention. Figure I is a schematic diagram depicting vertical cross section along the length of a solar powered version of the invention, used for distillation of brine to potable water. In Figure 1, a closed transparent roofed evaporation chamber 1 encloses a brine filled trough 2. Solar energy 3 provides the primary source of energy used for evaporating the trough water. Steam (water vapour) created by the evaporation process circulates round to an underlying condensation chamber 4. The steam gradually condenses out as it comes into contact with an overlying area of the brine filled trough that is at a lower temperature. The latent heat released when the vapour condenses is transferred to the brine, aiding the evaporation process. After the system has warmed up and steady state conditions are established, the evaporation chamber and its contents has a warm end 5 and a cool end 6. In order to ensure that the pressure inside the evaporation chamber is in balance with the external air pressure, air is added to the steam. At the cool end, air dominates the mixture, at the warm end; the mixture is predominantly steam at or just below its saturation pressure for the temperature of the steam. In contrast, the total pressure inside the condensation chamber falls from about atmospheric pressure at the warm end, where the steam plus air mixture enters, to low pressure, at the far end, under the cool end of the evaporation chamber. A pump 7 compresses the air and any residual water vapour back to atmospheric pressure and injects it into the space above the brine at the cold end of the evaporation chamber. Some drop in pressure must take place along the length of the condensation chamber in order to maintain circulation, but an excessive pressure drop would be unproductive. In order to reduce the pressure gradient, the lower chamber tapers towards its cool end. A drip tray 8 catches the potable water condensing from the external base of the trough. The potable water is drawn off through a down pipe 9. A second pumplO draws fresh brine into the warm end of the trough via a conducting walled pipe that rests inside a gully in the in the drip tray. The fresh brine picks up thermal energy from the condensate water, so that it enters the trough at a temperature only slightly below the temperature of the local water in the trough. Concentrated brine is drawn off through a down pipe 11 which passes through a heat exchanger 12, used for pre-warming the fresh brine. The device is well lagged to reduce heat losses. A plurality of glazing layers may be used, to further reduce heat losses. The glazing may include antireflecting light interference layers, to reduce radiant energy reflection losses. Optionally, an array of solar powered photovoltaic cells 13 is used to provide electricity for the pumps. If the invention is used for the fractional distillation of two liquids, for example, the distillation of ethanol out of water, the single drip tray is replaced by a chain of drip trays, running from the warm to the cool end of the lower chamber. The pressure and specific volume changes experienced by the condensing vapour in the lower chamber may be exploited to enable the device to incidentally do external work. This feature will now be revealed by an illustrative example, which is not intended in any way to limit the scope of the invention. Figure 2 depicts a variation on the invention that uses pressure drops caused by condensation to motivate a pump 1, which acts in series with the pump 7 of Figure 1, to compress the air plus residual vapour back to atmospheric pressure. An iris or other type of variable closure 2 is periodically closed, preventing vapour entering the condensation chamber. This causes the pressure at the warm end of the condensation chamber to drop as vapour continues to condense out. This draws up the piston or diaphragm of the pump 3, opening a valve 4 at the cool end of the condensation chamber and drawing in air plus vapour into the pump. When die iris valve opens, the external pressure on the pump piston increases, compressing the contents of the pump. This reduces die amount of work needing to be done by the second pump. Skilled engineers will be able to use existing knowledge to devise mechanisms for synchronising die movements of the vapour flow iris or door and die movement of the pump piston. Figure 3 is a schematic diagram depicting a West to East vertical cross section of a solar powered version of the invention, at right angles to the cross section depicted in Figure 1. The evaporation chamber is split into a plurality of parallel channels, in this example there are seven channels, numbered 1 to 7. By using a canopy 8 made up of cylindrical Fresnel thin lenses, (i.e., micro prisms) the sun can be tracked throughout the day without the complexity of the moving mirrors commonly associated with large solar power units. At the instant shown, the central channel 5 is receiving the most intense solar radiation. All of the channels, which effectively act as a common trough are in good thermal contact with a common underlying condensation chamber 9. The water in the channels to the sides of channel 5 becomes progressively cooler. Thermal energy can only be transferred from vapour condensing in the lower chamber, to the water above, if the water in the above section of the trough is at a lower temperature. The version of the invention revealed in Figure 3 assists in the heat transfer process, by always offering one or more relatively cool channels. Roll up silvered blinds 10 and 11 reflect solar energy onto the channels of the trough when the sun is at a low angle. Internal glass walls 12 and 13 combine with external glass walls 14 and 15 to create semi-sealed cropping zones 16 and 17 where comfortable levels of humidity can be maintained, even in arid desert climates. The cropping zones are only illuminated by scattered sunlight from the sky. In effect, the inner glazed zone, occupied by the trough, acts like a giant heat pump, shunting heat away from the cropping zones. Photosynthesis will convert about 10% of the solar energy falling on to the leaves in to chemical energy inside the plants. Evaporation of water from the leaves of the plants provides further cooling. This invention will allow people to live comfortably in the desert, without the need for air conditioning. Figure 4 illustrates how parallel solar powered desalination devices 1 and 2 may be grouped under a common Fresnel lens canopy. Additional internal shading 3 can be provided for pedestrians, cyclists and horse riders. Figure 5 illustrates how, at the cool end of the system, additional shading can be added, to ensure that the condensing vapour always has access to cooler water above it. The trough water 1 is separated from the underlying vapour 2 by a corrugated trough base 3. The corrugations add stiffness to the base and increase the contact area between d e vapour and the brine. Matt black shades, for example 4, are placed above the troughs in the corrugations. Silvered shades, for example 5 are placed above the peaks of the The silvered shades are angled, to reflect solar radiation up to the matt black shades. This geometry of shades, having different reflectivities encourages a streamline flow of convection currents, as depicted by the arrows. Any form of heat that can raise the temperature of the warm end of the evaporation chamber to about 100[deg.]C can be used to drive the steam plus air mixture from the evaporation chamber to the condensation chamber. Examples of heat sources include electrical resistance nichrome heating wires and pipes carrying steam. Figure 6 depicts a vertical cross section similar to that depicted in Figure 4, but with steam carrying pipes 1 - 4 being used as d e primary heat source for die brine. Vertical baffles, for example 5, may be added to encourage regulated convection currents. Figure 6 also illustrates three different design features that can be used to enhance the stiffness of the base of the trough while encouraging regulated liquid convection current flow. Areas of the trough base under the sinking brine are modified to be at a slightly lower temperature than below the raising brine. The general principle underpinning each of the three features is to slightly reduce the local thermal conductivity of the trough base under the sinking brine. Feature 6 is the use of reinforcing box cross members, 7 is the use of solid supports and 8 is the local thickening of the base along the peaks of the corrugations. The scope of the invention is extended to cover at least the following fluids as the source of vapour in the evaporation chamber: (i) Liquids being heat treated or sterilised, for example liquid sewage. (ii) Drying semi-solid sludge, for examples, solids rich sewage or effluent from paper making mills, (iii) Fluids for which the temperature and humidity conditions of the evaporation chamber act as a stimulant for chemical or biological change, for example the generation of methane, with any gaseous end product being collected after compression by the pump at d e cool end of the condensation chamber, (iv) Water rich crops or other organic material being dried out.

Claims

Desalination Plant Claims

1. A device for purifying a liquid or separating liquids having different boiling points at the same atmospheric pressure, by the process of distillation, comprising a heated upper evaporation chamber enclosing a trough holding the impure lire liquid(s) and a lower condensation chamber, in good thermal contact with the base of the trough, with the two chambers being connected by a passage, allowing vapour produced in d e upper chamber to flow into the lower chamber and condense on the external base of the trough, characterised by the lower chamber extending to cover the length of the trough in the upper chamber, with latent heat released by the condensation process helping to warm and evaporate die impure liquid(s) and creating a horizontal temperature gradient, with the highest temperature being at d e end of the device where the vapour flows from the upper chamber to the lower chamber.

2. A device according to d e first claim, with air or other gas being added to the vapour in the upper chamber, so that the pressure in the upper chamber can balance the external atmospheric pressure, with one or more pumps being added to compress the gas and residual vapour in the lower chamber at the low temperature end of the device and pump the mixture back into the upper chamber.

3. A device according to die first claim with variations in die vapour pressure and specific volume in the lower chamber being used to displace a surface against a load, enabling the device to do external work.

4. A device according to die first claim, with one of die means of heating being solar energy.

5. A device according to claim four, with shades added inside the trough to prevent die base of the trough being heated directly by solar energy.

6. A device according to claim four, with a canopy of lenses being placed above the upper chamber, to focus solar energy on a plurality of separate channels of a common trough, with different channels receiving the bulk of the focused solar energy, as the apparent position of the sun moves throughout the day.

7. A device according to claim six, with transparent walls being added to d e canopy of lenses, creating a closed zone under the canopy, having a total area that exceeds the total plan surface area of the troughs.