FR2853895A1 - Salt water distillation plant has vertical plates separated by slightly inclined bars with upper surface channels to collect condensation - Google Patents

Abstract

The plant consists of a series of vertical parallel rectangular plates in the form of alternating panels of woven fabric and cellular panels with vertical ducts. Cold salt water is passed up the cellular panels and heated at the top before flowing down through the fabric panels, producing condensation on the cellular panels. The gaps between the panels contain slightly inclined bars (20) with channels in their upper surfaces to collect the condensed water. The top and bottom edges of the fabric panels are enclosed in horizontal collectors that are connected to manifolds, and the lower collectors are connected to at least one suction pump that extracts air and water, creating a reducing pressure in the stages between the inclined bars, and consuming an estimated 0.5 - 1 kWh per cubic meter of water desalinated water produced.

Description

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1. Preamble.

  To provide drinking water in hot arid regions, we offer a fresh water distiller associated with solar panels, of economical construction. The still could be used, below 1000, to recover otherwise lost thermal power.

  Distillers are characterized by the gross distillation yield, the ratio of the heat of vaporization of the fresh water produced, ie approximately 0.7 kWhlkg, to the heat supplied. Flat solar panels, inclined in the favorable direction, give in the arid region about 5 kWh per day and square meter, up to a temperature of about 700. The simplest stills, with a tilted window above a tank of salt water, produce about 1 liter of fresh water per day and square meter, with a yield of the order of 0.14. In more sophisticated stills, hot water cools with evaporation and the vapor diffuses towards a condensing surface, cooled by a salt water circuit, thus preheated before additional heating and passage to the evaporator. The vapor flow being all the more intense as the partial air pressure is lower, there is a succession of degassed stages, at decreasing pressures with temperature. By using fossil fuels, the temperature reaches about 130 "and the output 20. In each stage, it does not seem that we have aimed for a countercurrent exchange allowing the homogenization of temperature differences, for maximum efficiency The floors consist of metal containers with internal collectors and are connected by pipes.

  The author has already proposed distillers at atmospheric pressure alternately comprising vertical surfaces of evaporation and condensation, the small difference of which must allow on the one hand a gO vapor diffusion as intense as possible, on the other hand the establishment of '' a temperature regime close to the perfect counter-current. This would allow the use of low-cost materials (fabrics for evaporation and polypropylene honeycomb plates for condensation) economically compensating for the low diffusion of vapor in the presence of air. These simple stills could be especially useful for small installations. The wedges between the surfaces were provided zs vertical. They must not give rise to mixtures between the flows very close to salt water and fresh water flowing in drops with a thickness of the order of 2 mm on the polypropylene plates.

  We aim here to keep a simple realization and materials at low prices but by organizing, on the height of the plates and fabrics, multiple stages of pressure to intensify the exchanges and the -30 production of fresh water, without requiring collectors between The floors. The still will be able to work without applying pressure differences, but with a lower yield.

2. Ipvention.

  According to the present invention, considering a distiller with closely spaced vertical surfaces, alternately evaporating fabrics (2) and condensing plates (1), layers of 3 5úpregsion are created by interposing, in height, strips (20 ) with a slight slope which: - by their compressive strength, allow the mutual support of the surfaces and the maintenance of their differences, despite the application of the external atmospheric pressure (multiplied on the support by the ratio of the total surface to the surface of the strips), - glued against the condensation plates with a slight slope and provided with an upper repair lip 40 (21), collect towards an edge the condensed fresh water on the upper floor, - pressed against the evaporation fabric by inserting a fine mesh (22), despite the pressure of support, the flow of salt water, which takes place with a relatively high driving pressure between two stages and causes residual air.

  As an order of magnitude, the differences between plates would be of the order of 3 to 4 mm and the - strips, with a support height of 6mm, would be arranged every 40 mm, on total heights of l 'order of 2m, therefore with a number of stages of 50 providing an excellent yield. The polypropylene condensing plates (1) must withstand the crushing pressure against the strips.

3. Pe $ cription.

  The description is based on the following Figures:

  / a Fig 1. Amplified scale section, with collectors in the upper and lower parts, Fig 2. Amplified scale section on an edge strip, Fig 3. Elevation for the upper part, Fi9 4. Installation diagram with distiller and solar panel, Fig 5. Section of a solar panel, ft Fiq 6. Graph of the calculated heat flow in a stage according to temperatures and degassing.

  The alveolar sheets of extruded polypropylene 1 and 1 'are arranged vertically, their channels traversed by salt water introduced "cold" at the bottom and heated by the condensation of steam on the external walls, before receiving an external heating supplement . These plates alternate with rectangles 2 of permeable fabric on which flows the "hot" salt water introduced -o at the top and partly evaporating in the descent. The parallelepiped assembly will be referred to as a "package".

  According to the Figures there 3, the collection at the top and bottom of the plates 1 is carried out in water boxes 3 and 3 ′ covering the entire package of plates by their skirts 4 and 4 ′. These boxes are made of a relatively flexible material such as reinforced rubber, with a relatively thick bottom or L reinforced with a sheet to withstand the external pressure but thinner skirts, which surround the upper periphery of the pack of plates. This allows to accommodate small variations in the overall thickness of the package, subject to varying pressure differences.

  The rectangles 2 made of fabric are stretched vertically between their collectors 6 at the top and 6 'at the bottom, each consisting of a sheet 8, 8' of weldable waterproof canvas, folded in a U on the edge of the fabric and stitched. The tightening of the seams 7, 7 ′ allows a certain pressure-flow ratio at the top, and at the bottom an effective suction of the residual salt water called brine, accompanied by residual air. The collector 6 'at the bottom, in depression with respect to the stage which surmounts it, is held in shape by a rigid inclusion, for example a bar 9 in cellular plate whose cores have been pierced with holes.

  The manifolds 6, 6 ′ end at a lateral end by tight connections to 3D flexible tubes such as 10, 10 ′ which, after a short path intended to allow deflections due to pressure variations, lead to general manifolds 11 at the top and at the bottom which are tubes with holes, with low pressure losses.

  These rectangles 2 are stretched horizontally towards the vertical edges of the package by clamping glued between edge rules 11 and 11 '. The fabric is turned over at the end on a thin rod 12 in 4 which abuts against these rules which bear external reliefs against the edges of the plates.

  To prevent the collectors 6 from sliding down, these contain rods 15 of rectangular section which, placed crosswise, rest on plates 16, 16 ′ glued on either side of the top of the plates 1 and on which the collectors 6 rest. At their ends, these plqque> constitute abutments to the edge rules 11, 11 'and contribute to receiving the support of the water boxes 3.3'. As a heat exchange between the collectors and the plates is unfavorable, the latter must be relatively insulating. A sufficiently strong plastic material is used, for example made of solid polypropylene, the heat loss then being admissible. On the edges of the package, on the side of the tubes 10, 10 ′ passing through the ends of the collectors 6, 6 ′, the plates 16, 16 ′ carry recesses intended not to crush these tubes, and additional plates glued to plug the orifice remaining.

  On the other side, the orifice between these plates is blocked by glued plates 18.

  Each quasi-horizontal strip between stages, denoted 20, consists of: Wo - a strap 20, made of relatively insulating material, for example a dense plastic foam, - a thin strip 21 of plastic material projecting upward to constitute above the strap a condensed water flow channel, - a thin strip of plastic mesh 22 to allow the flow of water and a certain amount of air, despite the compression of the crimps on the fabric. The mesh of mesh is chosen to ensure a pressure drop distributing the pressures suitably on all the stages. The strips are glued on the plates 1, 1 'with a slight slope, so that the water flows towards one side. sweet collected. On this side, against the edge rules 11, 11, there are descent tubes 25 with orifices of small diameter opposite the strips. These tubes are in particular formed by cutting from a cellular plate. They end up in a sealed manner at the bottom of the SD tubes 26 descending under the still over a sufficient height so that the receptacles 27 for receiving fresh water can be opened at atmospheric pressure.

  The salt water is taken, for example, at sea by a pump 30 and, via a filter 31, is injected into the water box 3 ′ to circulate from bottom to top in the plates. It ends up in the box 3 then passes through the plates 50 of the solar panel before returning to the top collector 11 and 2S to flow onto the fabrics. It is then taken up by the collectors 6 ′ and the general collector 11 ′ under the effect of the suction by the pump 40.

  The pump 40 extracts both the salt water ("brine") arriving at the bottom of the tissues 2 and air which was dissolved in the inlet water or comes from leaks. The total pressure in the stages between the strips 20 thus decreases downwards like the vapor pressure, as the salt water cools. The brine and the extracted air pass through a decanting vessel 34 o a free level of water is established under an atmosphere of low pressure air, then to the water pump 40. A float in the vessel activates an air extraction pump 36 or a water pressure ejector to maintain the level between two limits.

  On the two walls of the package corresponding to the edge rules 11, 11 are extended flexible flexible sheets such as 38 to complete the seal. The atmospheric pressure causes a general mechanical compression on the pack of plates, supported in height and in width by the alveolar plates and in thickness by the strips between stages and the corresponding zones in crushing in the plates. The honeycomb plates must have sufficient strength.

  According to a previous patent of the author, the solar panels are in black honeycomb plates (D 50 with collectors in flexible fabric whose diameter can be sufficient for panels of large width. The plates are posed on benches in sandy soil suitably inclined, and insulated upwards by a layer of air 51 under a transparent sheet 52 placed on wedges and grids, and ballasted.

  A distillation step, with its distiller and its solar panels, providing fresh water and more concentrated salt water, can operate in isolation but the production of fresh water is then limited to a small fraction of the salt water flow. , for example 1/15 for an evolution out of 400.

  To limit the flow rate taken at sea, it is possible to conduct the brine leaving the pump 40 to other distillation steps or else mix part of the brine with make-up water, then with a certain thermodynamic loss. Highly concentrated salt water can be sent to a salt marsh.

  To facilitate degassing, it is possible to distill over a lower temperature interval, thereby admitting a higher outlet temperature, but then, to limit the heat required for the solar panels, the first water entering a preheater is preheated. -current, from the last outgoing water. This exchanger can be constructed using cellular plates.

4., Calculations.

  In the horizontal heat exchange, the temperature drop between falling water in the fabric and rising water in the channels is the sum of three terms: - about 10 for the difference between sea water and fresh water of the vapor pressure saturating (osmotic pressure) depending on the temperature, a term which increases if the salinity increases, in the course of several steps.

  - the drop by conduction in the layers of water and in the condensation wall, z proportional to the heat flux and calculated as a function of the thicknesses and conduction, - the drop in air diffusion more vapor, in laminar situation due to the low thickness, with proportionality to the heat flux, and calculated as a function of the difference between the exchange surfaces and partial pressures of vapor and air, by known formulas (Fick, Schirmer).

  The sum of the downflows of salt water and fresh water is equal to the up flow zr of water. The gross distillation yield is equal to the temperature variation over the height, divided by the horizontal temperature drop. The overall yield will take into account the energy consumed for water pumping and for auxiliaries.

  A calculation program provides the heat flux in a countercurrent stage of thickness "a" for the temperatures at the walls ("ta" and "tb 'on fabric at the top and bottom of the stage, difference" dt "36 between falling and rising water) and the total air pressure plus vapor, expressed as a vapor pressure for a higher saturation temperature "ts". The value ts-ta is thus a degassing benchmark.

  It is noted that the heat fluxes vary little according to the temperature level, in a range 30 -70 for example, but especially as the temperature differences, which makes it possible to calculate the still for a single typical stage, with the condition of ensure the degassing thus fixed on the height. The temperature drop by diffusion is reduced by low values of the difference "a" between tissue and plate (slight advantage for a = 3mm, compared to a = 4mm), low values of ta-tb therefore many stages , and low values for ts-ta. For the difference a = 3.5 mm, a reference temperature ta = 500, and dt = 40 (30 for conduction and diffusion, the fluxes varying as dt-1), Figure 7 5ro gives the fluxes calculated as a function of ts-ta and ta-tb.

  In Figure 7 is marked a point corresponding to ts-ta = 10 and tatb = 20, giving a flux of 1086 W / m2 condensing 0.45 g / sm2.

  The calculations also concerned the pressure drops appearing in the water circuits, in particular for the intake of water at sea and for reading the heights of runoff in the still.

  The water intake rate is reduced by the partial recycling mentioned. It is estimated that the pumping and degassing energy will be 0.5 to 1 kWh per m3 of fresh water produced.

5. Performances.

  We give for example a temperature variation of 40, for salt water heated from 200 to 600 in the condenser, then brought to 640 in the solar panels. The distillation yield is then 40/4 = 10- The intended distiller alternately comprises fabrics of thickness 1 mm and plates of thickness 2 mm, separated by spaces of 3.5 mm, which gives 2 surfaces 40 of exchange for 10 mm, or 200 m2 per m3. For the supposed flow of 1086 W / m2 we therefore obtain 90 g / s per m3. Assuming an effective insolation of 8 hours per day or 28,800 seconds, this corresponds to approximately 2,500 kg of water per day per m3. The solar heat used is 0.7 x 2500/10 = 175 kWh per day, which corresponds to 35 m2 of solar panels for the 1 m3 distiller. For this production of 2500 kg per day, 135 m2 of honeycomb plate with a mass of the order of 0.45 kg / m2 / S9 is therefore used, ie 61 kg. Approximately, with the other elements, the materials represent approximately 150 kg of usual plastic material.

  In such an apparatus, the production of water is proportional to the product of the collection surface and the surface of the exchange plates in the still. For prices given per square meter of capture and distillation, the economic optimum is obtained by equalizing the prices of the two functions. it seems clear that vis-à-vis the example chosen, with 100 m2 of exchange plates plus fabrics, and 35 m2 of solar panels whose construction is very simple, it will be advantageous to relatively increase the surface of the panels by reducing the yield. The advantage of storing hot water for the night time operation of the still will be assessed.

  Evaluations of the prices of the materials used and of their implementation, whether artisanal or industrial, show that the investment may be lower than those usually made for the production of fresh water. The mechanical energy required is lower than that of other processes, which generally require more than 3 kWh / m3, and solar energy is free.

Claims (5)

Revepdications   1. Salt water distiller comprising a set of parallel vertical rectangular surfaces, made up alternately of fabrics (2) on which descends salt water supplied hot at the top, and so-called cellular plates (1) integrating vertical conduits in which rises salt water introduced cold at the bottom and, taken up at the top, receiving additional heat before being conveyed to the top of the tissues, the intermediate spaces thus giving rise to a transfer of vapor and to a condensation of fresh water on the honeycomb plates, characterized in that these spaces, laterally closed by vertical edges, contain strips (20) with a slight slope glued against the honeycomb plates and the upper relief of which constitutes a water flow channel condensed towards rapins one of the borders.   A10 2. A still according to claim 1, characterized in that the sliders (20) are pressed against the fabrics (2) by means of a layer (22) comprising vertical passages.   3. Distiller according to one of the preceding claims, characterized in that the fabrics (2) penetrate into the horizontal saltwater collectors at the top (6) and at the bottom (6 ') formed from a folded weldable waterproof canvas and sewn loosely on the fabrics, the bottom collector subjected to suction 45 'being lined internally with a structure (9) to maintain against crushing and each collector being connected to a general collector, supply (34) at the top or evacuation (33) at the bottom.   4. Distiller according to one of the preceding claims, characterized in that the plates (1) lead up and down to water boxes.   5. Distiller according to one of the preceding claims, characterized in that the movements, 0 of water are induced by at least one pump (50) placed in suction on the lower collectors of the tissues and thus collect both air and water, creating a downward pressure in the stages between the strips (20) depending on the temperatures.