GB2066293A - Steam raising for desalination processes; electrolyser; voltage generator/converter - Google Patents

Abstract

Saturated steam suitable for sea-water desalination distillation processes is raised by decomposing distilled water from any distillation process 3 in electrolyser 2 into hydrogen and oxygen gases which recombine in burner 4 forming superheated steam which is converted to a greater tonnage of saturated steam by mixing in desuperheater 5 with another part of distilled water from 3. Electrolyser 2 has separate cathodic and anodic half-cells, includes an integral acyclic voltage generator/converter 1, has close to nil ohmic resistance, operates at between 1.229 and 1.47 volts. It comprises conductive plate(s) having cathodic and anodic surfaces on both faces served with falling films of water electrolyte. After passing over these surfaces the electrolyte is cool and regains heat in heat exchanger 6 otherwise wasted in 3, before circulation to surfaces oppositely charged in the other half cell. Generator/converter 1 consists of permanently magnetised steel disc spinning in a slot cut in each anode-cathode plate and is driven by polyphase synchronous electric motor 1a. <IMAGE>

Description

SPECIFICATION Steam raising for desalination processes The invention relates to desalination of sea water by distillation processes and to industrial electrochemistry, linking these two fields by providing an electrolytic means of steam raising for use in conjunction with Multi Stage-Flash (MSF) and Multiple-Effect-Film Boiling (MEFB) distillation processes.

The invention avoids the present practice of using steam bled from turbines and increases the output of fresh water relative to the steam input (the performance ratio).

The performance ratios of some six hundred MSF and MEFB desalination plants in service in several countries are widely different according to their design, ranging from 20 tonnes of fresh water per tonne of input steam down to 3, with most of them at a level of 6 to 8. The input steam is usually at a temperature of 100 -120 C used down to 30 -35 C. Taking for example typical figures of 7 for the performance ratio and 11 0,C for the steam, the latent heat put in is about 320 X 103KJ per tonne of fresh water produced.For a population of say 40,000 consuming 0.25 tonne of fresh water per head per day as is common in most towns, then the total latent heat put in is about 320 ) < X 107KJ in 1,429 tonnes of saturated steam, equivalent in electrical units to 889,000 kilowatt hours per day at a rate of 37 megawatts, virtually one kilowatt of steam power per head of population, non-stop day and night.

Thus, any kind of sea water distillation process, be it MSF or MEFB or any other, requires a constant input of 50,000 steam horsepower for 40,000 people if its performance ratio (PR) is 7 as is typical. At a PR of 14, the steam input is 25,000 h.p. and at a PR of 21 it is 16,666 h.p. After twenty years of development. MSF has achieved a PR of 12.5 (in year 1978) and 15 is claimed in the literature to be feasible. MEFB is now out of its early stages of development and is likely to achieve much higher performance ratios than MSF in the next few years with 20 feasible and with perhaps 40 as a foreseeable future possibility and ultimate best, never likely to require a steam input of less than 8,750 horsepower for 40.000 people (6.53 megawatts; 156.7 megawatt hours per day or 56 x 107KJ per day) in 250 tonnes of steam.

It seems that desalination will always require power at such high figures and can never be other than expensive. The figures given above are derived from "Steam Plant Aspects of Sea Water Distillation" by Silver, R.S. (published by the Institution of Mechanical Engineers, London, 1978) and from many papers presented at Desalination Symposia.

In the prior art of desalination, the only means of obtaining sufficient steam power at an acceptable (although still expensive) price has been to attach MSF or MEFB plants to thermal coal, oil or nuclear power-generating stations, bleeding the steam from turbines at 100 -120 C (pressure 1-2 bars), so making the generating stations wholly or partly backpressure stations. That steam is lost to powergeneration and might have been used to better advantage in low-pressure turbine stages.

Although now widely adopted for want of any better means, such dual-purpose plants reduce the efficiency of the electrical side and therefore increase the size and fuel consumption of the boilers for the electrical side, all quite substantially by in many cases up to a quarter. A further disadvantage of dual-purpose plants is that they severely restrict the choice of siting, forcing a generating station to the coast for the convenience of a desalination plant.

It is therefore industrially worthwhile to devise a more efficient and less complicated means of raising hundreds of tonnes per day of saturated steam at about 100 -120 C (pressure 1-2 bars) suitable for MSF or for MEFB as a substitute for steam bled from turbines and that is one object of this invention. A second object is to provide good means of separating MSF or MEFB from the immediate vicinity of a power-generating station so that each may be sited to best advantage independently of the other with only a power-line connecting them. A third object is to permit power-generating stations to operate at their maximum efficiency of about 37% as fully condensing stations at vacuum and not as back-pressure stations at about 28% to 32% efficiency as now required where associated with MSF or MEFB.A fourth object is to permit 90% efficient hydro-electric generating stations inland to be employed for desalination work instead of 28% to 37% efficient thermal generating stations on the coast. A fifth object is to permit a national power supply grid served by any number of generating stations to be employed for desalination work instead of only one specially sited.

The principal novelty of the invention by means of which all the foregoing objects may be achieved lies in an electrolyser different in several ways from the prior art, designed to decompose an aqueous solution of an acid or a base in distilled water into hydrogen and oxygen gases using much less voltage and therefore much less electrical energy than in the prior art of water electrolysis. The design is intended to avoid the well known undesirable energy consumption arising from the over-voltage required to overcome electrical resistances prevailing in the prior art caused by polarisation, by electrode kinetics, by gas bubbles in the electrolyte and by the electrolyte itself. By reducing the resistances and the voltage needed, this electrolyser makes the gases much less expensive in energy and in money.As a useful result, it now becomes for the first time economically feasible to burn the gases together to form directly (without loss of energy) very high temperature low pressure super-heated steam which is convertible (again without loss of energy) in a conventional distilled water spray desuperheater into a much larger tonnage of low pressure saturated steam at lower temperature suitable for distillation of sea water by MSF or by MEFB processes, with the distilled water required by the electrolyser and by the desuperheater being provided by the distillation process.

Combination of electrolysis with distillation for desalination is novel. It would not be worthwhile if using any electrolyser of the prior art as will be discussed.

It is well known from experiment and by calculation that the decomposition voltage of any aqueous solution of acids or bases in distilled water releasing hydrogen and oxygen is 1.229 volts at 25'C and at atmospheric pressure, of which 0.401 volt is employed on the surfaces of the anode to release oxygen and 0.828 volt is employed on the surfaces of the cathode to release hydrogen. These voltages are independent of any additional voltages (overvoltages) needed to overcome electrical resistances elsewhere in an electrolyser.

A lowering of the operating temperature to near freezing (not practicable in the prior art) or a raising of the operating pressure by several atmospheres both increase the decomposition voltage by only a few millivolts, relatively quite insignificant amounts even when added together.

The expansion from a small volume of water into a large volume of the two gases is accompanied by a change of entropy as in a refrigerator with a rapid drop in temperature which, if not thermally compensated, would quickly freeze the electrolyte. If exact thermal compensation for the expansion were to be applied electrically through a precisely adjusted resistance of the electrolyte, which is impossible, then the thermo-neutral voltage (steady temperature operation) would be 1.47 volts-that is an increase of 0.241 volt, which is 16.4% of the total voltage which would be needed to maintain temperature while electrolysing. However, as in this invention, thermal compensation may much more usefully be applied as heat from any convenient external source.The gases released always carry away from the electrolyser all the energy put in-that is electrolysing energy plus heat energy in equals gas energy out, an efficiency of 100%.

No matter what voltage may be needed to operate a practical electrolyser having internal resistance, a steady direct current of 124,000 amperes flowing for one day releases 0.1119 tonne of hydrogen at the cathode and 0.8881 tonne of oxygen at the anode. When burnt together, these stoichiometric gas quantities directly produce 1.0000 tonne of pure superheated steam at about 2,500 C, much more simply and much more efficiently than could any boiler and that steam can easily be converted to useful saturated steam at any desired lower temperature such as 100"-120"C by mixing it with distilled water.

At any applied voltage below 1.229, no current passes in any water electrolyser and no gases are released. At any voltage above 1.229, the current passing and the gas quantities released depend on the resistances imposed by the electrolyte which in this design are close to nil so that a few millivolts more than 1.229 suffice to cause an amperage many times greater than in prior art electrolysers without giving rise to unacceptable heat.

The aqueous solution to be decomposed is conventionally and conveniently considered as H20 dissociated into H + and OH - ions by the addition of KOH or NaOH which are dissociated into K+ and OH- or into Na and OH-.

In reality, it is a highly complex mixture of ions, atoms and molecules of O. 02, O3, H, H2, OH, O2H, H2O, H30, H202, H204, K or Na (plus those of numerous contaminants), all as short-lived unstable liquid and vapour species, all in rapid thermal motion in every direction, all interchanging electrons, combining and breaking apart, all colliding with and obstructing each other, so that the arrival rate of H + at the cathode surfaces and of OH- at the anode surfaces is greatly slowed down.

The immersion of charged electrodes in a mixture ot this nature gives rise to many severe hindrances to the passage of H + and OH- ions which are compelled by innumerable collisions and by innumerable electrostatic fields in the mixture to follow length random paths in three dimensions, losing horizontal velocity at every hindrance, even stopping and reversing direction. There is experimental evidence that it takes about an hour for an ion to move two centimetres horizontally in the mixture. To make matters worse, the working surfaces are blanketed by layers of other charged species and by bubbles of gases; much of the volume of the mixture is occupied by bubbles; the mixture is depleted of available H + ions near the cathode and depleted of OH- ions near the anode, the opposite to what is wanted; membranes between the electrodes to prevent mixing of evolved hydrogen and oxygen present considerable resistance to the passage of the ions. Every hindrance requires to be overcome by applying more voltage to obtain a sufficient amperage and the extra work done generates unwanted heat.

Until a few years ago, 2.0 to 2.5 volts per cell was the operating voltage for several makes in industrial use in several countries.

Recent designs are stated to operate at about 1.85 volts per cell. Although better, this is still 0.621 volt (50%) higher than the decom position voltage and 0.38 volt (26%) higher than the thermo-neutral voltage. At 1.85 volts per cell, any electrolyser generates excess heat equivalent to a rate of 124,000 amperes at 0.38 volt per tonne of gases produced per day and must therefore be provided with parasitic external cooling machinery consuming additional power at about 1.5 times that rate to prevent the electrolyte from boiling.

They operate at about 60 -80 C, cannot make use of waste heat from an external source such as a distillation process and require further parasitic external plant to separate the gases from the electrolyte and to dry the gases. Thus they consume 16.4% plus 26% plus about 1.5 times 26% (total about 80%) more electrical power and energy than does this electrolyser having close to nil electrical resistance to the passage of H + and OH ions on to the electrode surfaces and operating close to freezing, for identical tonnage output of the gases.

Stated the other way round, this electrolyser is designed to produce hydrogen and oxygen from dissociated distilled water at little more than half the coast of any electrolyser requiring 1.85 volts per cell.

To avoid the causes of most of the hindrances noted above, may inventive steps are: (a) not to immerse the electrodes.

(b) to have separate cathodic and anodic half cells.

(c) to feed electrolyte to the cathodic and anodic working surfaces as falling films.

(d) to permit partially discharged electrolyte to fall clear of the cathodic and anodic working surfaces.

(e) to circulate partially discharged electrolyte fallen clear of the cathodic working surfaces directly to the working surfaces of the anode.

(f) to circulate partially discharged electrolyte fallen clear of the anodic working surfaces directly to the working surfaces of the cathode.

(g) to discard the difficult voltage-consuming slow mass migration of positive and negative ions in opposite directions through the mass of an electrolyte and to substitute easy pumped circulation of electrolyte containing those positive and negative ions in the opposite directions stated in (e) and (f) above.

(h) to employ only one pair of half cells in a complete electrolyser supplied with the whole amperage at between 1.229 and 1.47 volts. as close as possible to 1.229, about 1.25.

The arrangement is advantageous in several ways in bringing H easily into contact with the working surfaces of the cathode, in bringing OH- easily into contact with the working surfaces of the anode and in the removal of hydrogen and oxygen gases from the working surfaces and in other respects: (a) There is no overcrowding and resistances to ionic movement are close to nil.

(b) The gases evolve without forming bubbles in electrolyte.

(c) The working surfaces are washed continuously.

(d) The cathode is served with electrolyte depleted of OH - and the anode is served with electrolyte depleted of H + .

(e) There is no electrolytic current through the electrolyte, there are negligible IR (voltage) losses and negligible 12R (heat) losses.

(f) There is no possibility of mixing of gaseous hydrogen and oxygen and no diaphragms or membranes are required.

(g) A pair of half cells may be of any convenient dimensions and the electrodes and the containers may be of simple shape and construction for any output of gases.

(h) Input voltage being low, insulation difficulties do not arise.

(i) Cathode surfaces and anode surfaces need not be of the same areas. Thus optimum current densities may be applied, differing at cathode and anode.

(j) Electrolyte circulation rates may be varied for optimum performance.

(k) Electrolysis may be at any desired pressure and at a temperature close to freezing.

(I) Close to 16.4% of the sensible heat which is in present practice thrown away at about 30 -35 C from the bottom end of a distillation process such as MSF or MEFB can be regained, upgraded and reinserted via this electrolyser as latent and sensible heat at 100t-120 C at the top end of that distillation process with a gain in effect of some 6 to 8 times or more, depending on the performance ratio.

(m) Unlike a boiler which only inserts latent heat into a distillation process, this electrolyser inserts all the heat in the steam. both latent and sensible, all usable down to 30'-35"C, an increase of about 14% compared with steam bled from turbines.

Statement (e) above is emphasised. In this electrolyser, the electron current induced by a voltage generator in the conductors leading to the electrodes goes no further than the working surfaces of the cathode and anode, entirely different from any electrolyser of the prior art. There is no electrolytic flow of electricity between cathode and anode through electrolyte, neither inside the electrolyser nor inside external pipework conveying electrolyte to and from heat exchangers. This being so, there are almost no ohmic resistances to give rise to IR or 12R losses, except only on the electrode working surfaces themselves and this is overcome with a few millivolts more than 1.229 volts. The electrons given up by the OH- ions are in this arrangement passed directly into the H + ions through the shortest practicable external conductors also having almost no ohmic resistances.

Fig. 1 is a general arrangement outline diagram showing this electrolyser interconnected with any desalination distillation process in such a way that they assist each other to work to best advantage individually and therefore to best advantage in combination.

1 is a direct-current voltage generator generating about 1.25 volts, driven by alternating current motors 1 a 2 is this electrolyser, shown as two half-cells on either side of the generator, one producing hydrogen and one producing oxygen in stoichiometric tonnages.

operating at a temperature close to freezing and at a pressure of about one to two bars to suit any interconnected distillation process 3 which may be MSF or MEFB having a performance ratio of any amount up to perhaps twenty although six to eight is more common with existing plants. 4 is an enclosed burner for recombining the hydrogen and oxygen from 2 to form superheated steam at about 2,5009C containing all the electrical and waste heat energy which was put into 2. 5 is an enclosed desuperheater supplied with some of the distilled water from the bottom end of 3 for converting the superheated steam from 4 into a much larger tonnage of saturated steam at about 100"-120"C also containing all the energy which was put into 2 (plus a small energy contribution from the distilled water which was at about 30 -35 C), all of which energy goes into the top end of 3 and comes out at the bottom end of 3 in distilled water and in waste brine at 30"-35"C. 6 is a surface heat exchanger regaining for re-use some of the waste sensible heat in the distilled water leaving the plant to the extent of about 16.4% of the total energy needed in electrolyser 2, waste heat energy which is then upgraded back to 2,500 C via the cold electrolyser 2 and burner 4, thus taking advantage via MSF or MEFB of a free energy gift from Nature in the form of the expansion and cooling change of entropy at about 1.25 volts in the electrolyser, not only once but possibly up to twenty times depending on the performance ratio of the distillation process. 7 is a supply of distilled water from the bottom end of 3 to electrolyser 2 to replace the quantity converted temporarily to hydrogen and oxygen.

The distilled water for electrolyser 2 and for desuperheater 5 is not consumed or lost, it circulates rising and falling in temperature as it passes through the interconnected processes changing in steps from cool water to cool gases to superheated steam to saturated steam to water and so on in a continuous heat cycle receiving, conveying and giving up to the sea water brine inside the distillation process all the energy put in, down to 30"-35"C, as in the following example.

One tonne of superheated steam leaving burner 4 (at about 2,500go and pressure of 1 to 2 bars) contains 15.746 X 103KJ. Of this, 1 3,392 x 103KJ was put in at between 0 and 25GC electrically (at 124,000 amperes and 1.25 volts) and 2,354 X 103KJ was sensible heat regained from waste via exchanger 6 (equivalent to 124,000 amperes at 0.22 volt).Assuming, as an example. that the distillation process needs saturated steam at 110go (containing only 2,692 x 103KJ of latent and sensible heat per tonne) this is producible by mixing together the one tonne of superheated steam and 5.127 tonnes of distilled water spray at 35 > C (containing a further 748 x 103KJ of regained sensible heat) circulated continuously from the bottom end of 3 to desuperheater 5 and back into the top end of 3.

By these means, the total steam energy entering the top end of a distillation process is 16.494 x 103KJ contained in 6.127 tonnes of saturated steam at 11 0'C for every 0.1119 tonne of hydrogen burnt with 0.8881 tonne of oxygen. stoichiometric gas quantities obtained by an input of 13,392 x 103KJ of electrical energy.

The prime electrical energy is 81.2%, the waste heat energy regained in the electrolyser is 14.3% and the waste heat energy regained in the desuperheater is 4.5%.

Stated the other way round, the steam energy input to the distillation process is 123.2% of the electrical energy input to the electrolyser, a gain so far of 23.2%.

The layout typified in Fig. 1 and the figures given above may be applied to a wide range of desalination requirements. Thus, for a population of say 10,000 consuming 2,500 tonnes of fresh water daily (550,000 U.K.

gallons or 660,000 U.S. gallons) employing a distillation process having a good performance ratio for today of twenty, the 81.2% electrical input is 75,900 kwh (27.3 X 107KJ) at 3.163 megawatts; the steam input obtained is 93,500 kwh (33.65 X 107KJ) in 125 tonnes of dry saturated steam at 110 C; the electrolysing direct current is 2.53 X 106 amperes; the hydrogen released is 1 7.639 m3 per minute (2.284 tonnes per day); the oxygen released is 8.805 m3 per minute (18.118 tonnes per day); the burner recombines the gases at 26.444 m3 per minute (20.401 tonnes per day, which is also the water circulation rate to the electrolyser); the water circulation rate to the desuperheater is 104.6 tonnes per day; the 14.3% waste heat regained in the electrolyser is 4.81 x 107KJ; the 4.5% waste heat regained in the desuperheater is 1.51 X 107KJ; at a current density on both surfaces of the electrolyser electrodes of one ampere per square centimetre (twice the current density commonly prevailing in the prior art) the areas of the cathode and anode plates employed are 127 m2 in each half-cell with working surface areas of 253 m2 in each half-cell. The heating rate inside the electrolyser is 53 kw, about a tenth of the cooling rate of 557 kw.

The electrolyser takes a form different from the prior art and is designed so that the path length of any individual electron through copper from anode surface to cathode surface shall be as short as possible. By doing this, the cross-sectional area of the copper is minmised and the assembly of electrodes and voltage generator is compact. An electrolysing current such as 2.53 X 106 amperes is divided between any convenient number of copper electrode plates each of any convenient size, such as say 126 plates (each 1 square metre) in parallel in each half-cell, each plate carrying about 20,000 amperes internally and about 10,000 amperes on each of its two surfaces. Cathodes are in series with anodes as in any other design but much more closely connected, with no external cables and no terminals to cause voltage losses.

Each such copper electrode plate is arranged in this design to be a cathode on both of its surfaces for about half of their areas and to be an anode on both of its surfaces for their remaining areas. The voltage generator consists of one thin large diameter permanently magnetised steel disc spinning in a narrow slot cut in each cathode-anode plate.

The disc is mounted on and driven by a shaft common to two conventional polyphase synchronous electric motors taking power from a generating station or from a supply grid at say 3.163 megawatts (2,120 h.p. each) as in the example above for a population of 10,000.

Fig. 2 is a diagram showing for clarity only one of these copper cathode-anode plates 1.

2 is a slot cut in plate 1. 3 is a permanently magnetised steel disc spinning in slot 2 at right-angles to plate 1. Disc 3 is magnetised radially so that its rim 4 presents only one polarity to both copper ends 5 of slot 2. The opposite polarity of disc 3 is close to the centre of rotation of the disc, is remote from 5 and does virtually no work in opposition to the work done by rim 4.

This arrangement is a novel form of lowvoltage homopolar (acyclic) voltage generator different from all others in that it does not have any wire windings or any voltage collecting brush-gear and uses only one magnetic polarity. It is capable of passing an amperage many times greater than any prior art acyclic generator without giving rise to unacceptable heat. The copper plate becomes charged positively and negatively as shown by the plus and minus signs, so providing cathodic and anodic surfaces on both faces of a single conducting plate, these charged surfaces being connected together through the copper lying between 6 and 7 adjacent to spinning magnetic rim 4.These short lengths of copper (about two cm.) are integral parts of plate 1, parts of the same one piece oi conducting metal and they take the place of lengthy external copper cables used with prior art electrolysers to connect anodes to cathodes through an external voltage generator.

The arrangement can be seen to comply with the well-known right-hand generator rule, the conductors being in this case stationary with the magnetic flux from the rim in rapid motion cutting the conductors at right angles at 5. When electrolysing, half the electron current (say 10,000 amperes) flows to the left from anode to cathode between 6 and 7 at the top of slot 2 and the other half current flows between 6 and 7 at the bottom of slot 2, in the same direction to the left.

Any number of copper cathode-anode plates in parallel can be given identical positive and negative charges on their surfaces by the rim of a single spinning permanently radially magnetised disc when arranged as in diagram Fig.

3, the disc passing through a slot in each plate. The length of any slot does not affect the voltage generated, nor do the height, width or thickness of the plates. The plates are individually charged and are not connected together.

A disc 2.0 cm. thick 2.0 metres diameter spinning at 1,470 r.p.m. with a polar strength on its rim of 0.40 webers per sq. metre maintains a potential difference of 1.25 volts between the positively and negatively charged areas. The permanent polar strength of the rim (and the potential difference maintained) can be increased or decreased by touching the spinning rim momentarily with stronger magnetic poles of the same polarity as the rim or the opposite.

The motors may be started and stopped on no-load when no electrolyte is being circulated. The amperage from anodes to cathodes (i.e. electrons flowing from OH- ions to H ions) may be increased and decreased in controlled steps between zero and maximum by feeding electrolyte film-wise, as explained above, to more or fewer of the charged electrode surfaces, the motor speed, the rim speed and the potential difference remaining steady at all loads, the motors taking more or less power from the alternating current mainssupply, running on say 3,000 volts A.C.

By combining conventional polyphase synchronous motors with the disc and slotted conductors, a useful novel means is provided for converting high-voltage low-amperage alternating current to low-voltage high-amperage direct current.

Fig. 4 shows a convenient assembly of the electrolyser with its integral voltage generator and its driving motors, all being adaptable in sizes, powers and details to a wide range of gas-production and steam-raising duties. The weight and thrust of the disc may be taken on the motor bearings through a common drive shaft without couplings. The electrodes may be varied in height and width to suit their positions on the disc and this allows the drive shaft to be short, with the motors close to the electrodes. The disc and the slots may be placed off-centre in the width of the electrode plates to provide current densities larger or smaller on cathode surfaces than on anode surfaces as may be best. 1 is the disc driven by motors 1 a. 2 are the cathode-anode plates.

3 are the slots cut in plates 2. 4 are soft pipes of polythene (or of rubber or other suitable material) which perform several functions.

They hold plates 2 about one centimetre apart; they act as gas-tight gaskets along the top of plates 2. down the sides of slots 3 and along the bottom of plates 2; plugged at corner 5, they distribute electrolyte along the top of plates 2 through a multiplicity of needle holes. After flowing as film down the surfaces of plates 2. the electrolyte falls clear into a sump at the bottom of end-covers 6 from where it is circulated by pumps 7 to pipes 4 at the top of plates 2 in the oppositely charged half-cell, being reheated on the way in heat-exchanger 6 shown on Fig. 1.The electrolyte enters the top of each half-cell at about 25 C and leaves at the bottom of each half-cell at about 0'C. The hydrogen and oxygen gases evolved on the surfaces of plates 2 collect inside end-covers 6 from where they may be separately piped to burners 8 to form superheated steam. The surfaces of plates 2 may be electroplated after assembly with nickel, chromium, silver, platinum or other inert conducting metal to prevent corrosion and to catalyse the evolution of the gases from the OH- and H - ions.

A current of electrons begins on the anodic surfaces and ends on the cathodic surfaces.

As each OH- ion in the electrolyte film flowing down a positively charged anodic surface touches that surface, it gives up one electron to the metal and releases one atom of oxygen gas. The electron flows upwards or downwards in the anodic part of the plate, then horizontally above or below the slot, then downwards or upwards in the cathodic part of the same plate, on a negatively charged surface of which it enters an H + ion, releasing one atom of hydrogen gas. Thus half the total flow of electrons (i.e. half of say 20,000 amperes per plate) flows upwards and the other half flows downwards, so no crosssection of the plate carries more than say 10,000 amperes.

As in any water electrolyser, the gases are released in stoichiometric proportions by weight (i.e. 2.016 of hydrogen to 16.000 of oxygen). The Na or K ions and all other species remain dissolved in the distilled water and wash off the electrode surfaces. The distilled water must be free of NaCI to avoid release of chlorine gas on the anodic surfaces instead of oxygen gas.

In the prior art of sea-water distillation, steam bled from turbines returns its sensible heat to the boilers and employs about 14% of its latent heat for brine-heating not for brineevaporation. Redesign of the internal flowpaths in MSF and MEFB plants of input steam, brine and condensate to make use of the sensible heat provided by this invention will increase a performance ratio, that is more fresh water will be produced per tonne of input steam it is better steam for distillation purposes.

The means described in this specification will permit all five objects stated in the sixth paragraph to be achieved.

Apart from steam-raising, the electrolyser provides an economical means of obtaining hydrogen and oxygen gases which have many industrial uses. Hydrogen is being increasingly used as a piped energy carrier preferable in some cases to overhead electricity power transmission lines.

As a means of converting alternating current to direct current, the disc and slotted conductors are usefully employable in other electrochemical processes such as electroplating and metal refining.

Claims (6)

1. An electrolyser for decomposition of dissociated distilled water having separate cathodic and anodic half-cells producing hydrogen and oxygen gases employing less than the thermo-neutral voltage of 1.47 volts, the electrodes in the half-cells being not immersed in but being served with that water electrolyte as falling films on their surfaces, the electrolyte falling clear after passing over those surfaces and being then circulated to the surfaces of the oppositely charged electrodes in the other half-cell, the electrolyte cooling in passing over the electrode surfaces and being reheated in a heat-exchanger by waste heat from an associated heat using process, the operating voltage being generated by the rim of a radially permanently magnetised disc spinning in slots cut in the electrodes so that about half the surface area of each electrode is positively charged and the remaining surface area is negatively charged, the rate of gas release and the electrical load being controlled by circulating more or less electrolyte to more or less of the electrode surfaces in the half-cells, the rim speed of the voltage generating disc being held constant at all loads by employing polyphase synchronous driving motors taking power from an electricity generating station or from an electricity supply grid at controlled frequency.

2. An electrolyser and integral voltage generator substantially as in Claim 1.

3. An electrolytic, electromagnetic and thermodynamic assembly for use in conjunction with a distillation process for producing fresh water from sea water, such as the processes known as Multi-Stage-Flash and as Multiple-Effect-Film-Boiling, comprising an assembly as in Claims 1 and 2 followed by a burner for recombining hydrogen gas and oxygen gas to form superheated steam followed by a distilled water spray desuperheater to convert that superheated steam into a greater tonnage of saturated steam at the temperature and pressure required by that distillation process, with a part of the hot distilled water produced by that distillation process being recirculated to the electrolyser and to the desuperheater and with part of the waste heat in that hot distilled water and or in the hot waste brine regained by the cold electrolyte of the electrolyser in a surface heat exchanger and re-used in the distillation process.

4. An improvement to desalination distillation processes by regaining, upgrading and reusing waste heat substantially as in Claim 3.

5. Electrolytic production of hydrogen gas and of oxygen gas by means of an electrolytic electromagnetic assembly as in Claims 1 and 2 used in conjunction with any heat-using process discharging heat to waste by transferring part of that waste heat in a surface heat exchanger to distilled water electrolyte cooled in the electrolyser.

6. A rotary converter for converting highvoltage low-amperage alternating current into low-voltage high-amperage direct current comprising polyphase synchronous motors driving a radially permanently magnetised disc, the magnetic flux from the rim of the disc cutting the ends of slots in conducting plates in which the disc is spinning.