DE4142749A1 - Desalinating appts. for sea or brackish water - has electrolysis chambers interlinked with ion conductive dividing plates made of sand and cement - Google Patents

Abstract

The desalinating appts. for sea and brackish water, uses electrolysis with electrolysis chambers interlinked by ion conductive plates made from cement and sand. The electrolysis chambers are interlinked by capillaries for the ion channels with capillary forces at required levels and Na+ and Cl- channels. The electrolysis chamber dividers can also be diaphragms, with ion conductivity, but which prevent reverse diffusion of the reaction product. The electrodes can be integrated into the diaphragms such as a grid. The electrode chambers have very thin diaphragms, with high surface flows, and small gaps between the electrodes and diaphragms to reduce the contact surfaces between the reaction solution and the diaphragms. H2 and Cl2 are developed during electrolysis. The power supply is not only DC, but also rectified AC as a pulsed DC. USE/ADVANTAGE - The assembly is for decentralised use for individual use, where water pipes are to be fed with pure drinkable sea water or only partly desalinated water. The appts. requires low voltages for operation, such as DC supplies or solar energy.

Description

It is known that freshwater can be extracted from seawater after several Methods can win. Let us just mention: distillation, Reverse osmosis, electrodialysis, evaporation and ion exchange chromatography, freezing.

General information and notes can be found e.g. B. in the following Literature:

Franz Kurawski, Our future the sea, Überreuter, Vienna- Heidelberg, 2nd edition 1970 (5.27f, 83),

Joachim Borneff, Hygiene, Thieme, Stuttgart, New York. 4th Edition 1982, pp. 131f.

Leonhard Hütter, water and water analysis, third edition 1979, Diesterweg and Sauerländer, series: Chemistry Laboratory Books Otto Klee, Applied Hydrobiology, Drinking Water, Wastewater Water protection, Thieme 1985.

General basics are the standard textbooks of physics and taken from inorganic chemistry.  

Be detailed and on a scientific-technical level Problems of sea water desalination in the congress tape "Fresh water from salt water ":

Dechema Monographien Volume 47, Part 1 and Part 2, Verlag Chemie GmbH, Weinheim discussed.

In the quoted book "Our future the sea" the Possibility of extracting fresh water from sea water using Electrolysis mentioned, but neither detailed, nor further references are given. In the above There are numerous Dechema monographs cited Indications of electrodialysis procedures, but also none References or statements to pure electrolysis processes.

The currently most commonly used methods of Desalination are distillation and reverse osmosis.

Even if this is very often the case with the distillation process most desalinating water in dry countries pronounced solar radiation is preheated, and also attempts are being made to gradually remove water to be desalinated This is to heat up the heat of condensation Very energy-intensive process. This is due to the relatively low sea water temperatures even in hot countries, the required high temperatures, ultimately from the high temperature difference to be overcome. Besides, that is Evaporation heat (enthalpy) for water with 545.1 cal / g (= 9.82  kcal / mol) at 100 ° C, so that even after reaching z. B. amount of heat to be added to the temperature of 100 ° C considerably high. It should also be mentioned that on the Desalination plants based on the principle of distillation, because of the settling salts are relatively sensitive and maintenance-intensive are.

The relative susceptibility to faults is also the case for the Seawater desalination with reverse osmosis (RO, reversed osmosis) characteristic. Because with this procedure desalinating water using very high pressure (approx. 10-15 bar and more) is pressed through membranes (e.g. polyamide membranes), the water must be cleaned very thoroughly before filtration to mechanical destruction of the membranes prevent. The membranes must also be rinsed thoroughly, to remove micropollutants. Basically, RO- Membranes (reversed osmosis or reverse osmosis, see above) die Risk of extensive microbiological contamination and therefore require elaborate preparation measures (including Chlorination) of the resulting water.

Electrodialysis processes mostly work with ion-selective Membranes and a concentration of saline under simultaneous dilution of the starting solution.

problem

The goal should be energy-saving and environmentally friendly To construct desalination plants. Besides, should these systems are easy to maintain and both on an industrial scale and for individual needs have it operated decentrally. The last point of view is too take into account if, for example, only water supply systems created with pure sea water or only partially desalinated water will.  

Achievable advantages

The desalination of seawater according to the principle of electrolysis works in separate, but electrically conductively connected chambers, particularly when the methods proposed in the patent claims are realized, in terms of energy efficiency than methods based on the distillation or icing method. It seems particularly favorable that the system can be operated with relatively low voltages and only moderately high, that is to say technically manageable, direct currents. In particular, solar cells can be used in a suitable connection. The advantages achieved with the invention also consist in the fact that the manufacturing costs of the system are significantly cheaper than in the established systems and processes. Due to the simple and clear construction, the maintenance and maintenance costs are also very cheap. Use of this system therefore appears to be particularly suitable for developing countries, which usually have plenty of solar energy but lack of water. Since sodium hydroxide solution NaOH or, depending on the version, more or less hypochlorous acid HOCl is generated in the electrolysis chambers, microbiological contamination of the membranes is very unlikely. Since chlorine gas (Cl ₂ ) is also produced in addition to hydrogen (H ₂ ), the latter could be used for additional disinfection (disinfection) of the fresh water or other process water. Overall, the process is to be classified as a microbiologically extremely harmless process not only because of the possible presence of traces of HOCl, but also because of the anodic oxidation process occurring at the anode. For this reason, agar membranes could also be used for days without bacterial contamination.

Further embodiment of the invention

Advantageous embodiments of the invention follow from the above claims. Thus, through a suitable choice of the membranes (material, thickness, permeability for ions, electrical conductivity, etc .; see also section claims), the speed and the differentiation (ie the selectivity with regard to the types of ions that pass through) of the process of "separation" in the process water interfering ions are varied and adapted to predetermined boundary conditions. By using membranes which, in addition to the above-mentioned properties, also allow a small net flow of liquid to the electrolysis chambers under a pressure applied to the diaphragms separating the chambers, the speed of the separation process is increased and the likelihood of back diffusion (of NaOH or HOCl as well of Cl _- - and Na ₊ ions) reduced (see below the description of the test results). The preferred direction of the ions guided in the membranes or the question of whether cations or anions or both ions are preferably equally well guided can also be determined by the construction of the membranes. These parameters are addressed by electroendosmosis or EEO-like processes ("acid" or "basic" groups in the "membrane matrix"), suitable selection of (already known) ions of selective membranes or by modification of the membranes, as described in the patent claims required operating conditions adapted. Of course, any combinations of ion-selective, electroendosmosis-dependent or depending on the salt content of the starting solution or the membranes listed under the claims are also possible. In addition to the possible replacement or "dilution" of the HOCl and NaOH products formed in the electrolysis chambers by the salt solution present in large quantities, in addition to a possibly built up pressure gradient (see above), a possibly disruptive back diffusion of Reduce or completely prevent anions from the cathode chamber or from cations from the anode chamber by additionally installing "auxiliary anodes" in the cathode compartment or "auxiliary cathodes" in the anode compartment. These auxiliary electrodes require their own power supply circuits and, of course, much smaller currents, since only disturbing back diffusions of very small currents would have to be compensated for. These auxiliary electrodes are not to be equated with the membranes mentioned below with already integrated (rake-like) electrodes.

Description of some embodiments

The basic principle of the method is that, due to the electrochemical series of voltages in an aqueous NaCl solution, a current begins to flow in systems with a DC voltage when a voltage of -0.4 V at the cathode and a voltage of +1.4 V at the anode is exceeded becomes. When these values are reached, there is a discharge of H⁺ ions at the cathode and essentially the formation of gaseous hydrogen H ₂ O and a discharge of a stoichiometric amount of Cl⁻ ions at the anode and the formation of Cl ₂ -Molecules that escape essentially as Cl ₂ gas. Depending on the path covered in the aqueous NaCl solution and the pH value, hypochlorous acid, HOCl, is also formed according to the following equation: Cl ₂ + H ₂ ⇆ HOCl + H⁺Cl⁻, where from the H⁺ + Cl⁻ formed on the anode and the Na⁺ + OH⁻ formed on the cathode again (almost) completely dissociated NaCl. Quantitatively, the formation of hypochlorous acid (at the pH values that arise) is of relatively minor importance, especially in the exemplary embodiments proposed in the process applied for. Essentially, this results in the formation of aqueous NaOH solution, expressed simply by the following equation:
2 Na⁺ + 2 Cl⁻ + n H ₂ O ⇆ 2 Na⁺ OH⁻ + 2 H ₂ + 2 Cl ₂ + (n-2) H ₂ O.

Platinum was used as the electrode material (for cost reasons Wire) as well as the combination of iron cathodes and carbon, Graphite anodes used. According to the electrochemical series but also all of the materials are used that are not in the arrangement mentioned in Solution go.

With a suitable choice of voltages (strength and duration) the Order and the quantitative ratio of the ions to be discharged to be controlled.

Another basic principle of the registered process is that the use of diaphragms separates the reaction products formed on the anodes and cathodes (see above). As shown in Fig. 1, firstly the above voltages must be reached on the electrodes and secondly the membranes or diaphragms must meet the following conditions so that an ion current can flow without an exchange of the reaction products formed in the electrode spaces. The diaphragms must conduct the ion current (preferably sodium), but on the other hand must oppose the free passage of liquid at a higher pressure than the maximum osmotic pressure achievable in the respective chambers (as a result of different concentrations which can be achieved in the respective electrode chambers). Instructions for the production of membranes of suitable porosity, conductivity and compressive strength according to the invention are given in the patent claims and below.

First attempts were made with devices as shown in FIG. 1. The membranes were made from a mixture of cement, sand and common salt (in a ratio of 2: 1: 1, among others) and used in household tubing and sealing compounds in (also common) plastic tubs to set up two electrode rooms. After the membranes have hardened, they are watered. After removing the finely ground table salt, the prepared diaphragms have the above-mentioned properties. For the production of fresh and industrial water in large quantities, many small membranes in a frame (temperature and solvent resistant, not electrically conductive, water not permeable, etc., see below) become an (almost unlimited) large membrane area composed.

The carbon anode A in FIG. 1 (although only a single anode was used for the sake of simplicity) is in this arrangement applied to the aqueous NaCl starting solution (see chamber K ₁ in FIG. 1) using the surface tension (see FIG . 1 in Fig. 1) to release and derive the Cl _{2 formed} (cf. arrows in Fig. 1) as quickly and effectively as possible.

In both chambers K ₁ and K ₂ of FIG. 1 there are initially aqueous NaCl solutions of the same initial concentration. We essentially observe a Na⁺ ion current with this configuration. In the anode chamber the Na⁺ and Cl⁻ concentrations decrease in the same direction. (In the cement and agar diaphragms, negatively charged space groups, sulfates, etc. promote the transport or EEO of cations, positively charged residues promote the transport of anions.)

In principle, u. a. also agar membranes (and were both in diaphragm operation and in the form of connections plates between completely separate vessels also tested) which the Na⁺ ion conductivity (and conductivity for other, not tested cations) on the known conductivity for charged molecules in the electric field but also on the Electroendosmosis was based.  

The extension of the two-chamber method shown to the three-chamber method illustrated in FIGS. 2 a) and b), which can be expanded to any (2n + 1) - chamber method, appears to be particularly worth protecting.

In Figure 2a and 2b represent.:
A: anode chamber or reaction chamber A,
K: cathode chamber or reaction chamber K.

Buoyancy forces (AK in Fig. 2a) promote the flow into the reaction chambers A and K when they are pressed down with a small force. This flow can also be increased by other external pressures or negative pressures.

Chamber K ₁ in Fig. 2a contains aqueous NaCl starting solution (reservoir) and at the same time also serves as a "collecting container" for the demineralized water, since the disruptive electrolytes are removed.

If back diffusions occur, the contents of chambers A u. K can be replaced by starting salt solution available in any quantity or by a solution that has already been partially desalinated in chamber K ₁ .

In FIG. 2b, one of the reaction chambers is shown enlarged, with 1 being the wall of the electrode chamber, the cover 2 of the electrode chamber with the diaphragm and the sealing ring 3 represent.

Both the small distance between the electrodes (here cathode K) and diaphragm D ₂ and the small interface between the reaction chamber and chamber K ₁ have a favorable effect.

The “rake” diaphragms described below could be used as diaphragms for both the cathode chamber K and the anode chamber A. The anode in the anode chamber A can also be placed on the diaphragm D ₁ analogously to FIG. 2b.

The diaphragm D ₁ in the anode space A could be constructed thinner (in particular in the case of cement-sand-salt membranes) than that in the cathode space.

Similarly as in Fig. 2a), a three-chamber method (operates without fig.), Which contains a further Diaphrama compared to Fig. 1, and then the two outer chambers correspond to the reaction chambers A and K in Fig. 2a in which.

In principle, the known anion and cation exchange membranes can also be used in the 3-chamber and multi-chamber processes. However, the cost-effectiveness (price, sensitivity to contamination, service life) of these membranes in the arrangements described here would still have to be determined. In particular, the usability of the ion exchange membranes known hitherto with regard to the basic (NaOH) and acidic (HCl, HOCl) reaction products formed has to be critically examined. The combination of different membrane types and diaphragms were also tested (in the two- and multi-chamber electrolysis process described below), which triggered a net liquid flow under higher pressure and therefore reduced the yield of demineralized water, but on the other hand significantly increased the speed of the process. See also variant (I) in FIG. 3 described in the following part.

Two additionally listed variations of a three-chamber method that are worth protecting are shown in FIG. 3.

The processes are designated in FIG. 3 with (I) and (II) and differ from the process in FIG. 1 in that electrolysis is no longer in the chamber K ₁ , but in a separate one, in FIG. 3 with A designated reaction chamber, for example the shape as in Figs. 2a and. 2b, and the diffusion of HCl and HOCl produced in small quantities during the anode process into the reaction chamber K ₁ , which contains the compartment to be desalinated, is largely prevented.

Procedure (I)

D ₁ in FIG. 3 corresponds to D in FIG. 1. With D _2, net diffusion in chamber A is possible; occasional suction prevents diffusion of HOCl in chamber K ₁ . In the simplest case, this arrangement can be realized by cascading several commercially available filter membranes or, better, from self-pressed mineral wool or cotton membranes.

Procedure (II)

The capillary tension initially prevents back diffusion of HOCl into chamber K ₁ . By closing the upper opening of the capillary, HOCl can be "siphoned off" or the reaction can be carried out in a separate reaction chamber analogous to process (I). The strong Cl ₂ development on the anode wire irregularly interrupts the conductive connection between the anode wire and the liquid column in the capillary, which is why, in this process, several Na⁺ or Cl⁻ deliberately conductive capillaries are used simultaneously, or when in use a separate reaction chamber (see above) attaches the anode to the end of the capillary there.

For variants (I) u. (II) in Fig. 3 allows e.g. B. a pH probe, cf. 1 in Fig. 3, the targeted suction at (I) or at (II) when a back diffusion of HCl and HOCl in the chamber K ₁ begins. This regulation R ₁ complicates the registered system only insignificantly. In the case of variant (II), this control mechanism is designated R ₂ .

Another variant of the method shown in Fig. 1 and Fig. 3 (not shown.) Is that the resulting products each in chamber K ₁ in varying degrees HCl and HOCl in a subsequent (ie, after the reduction of the ion Na⁺- to the desired concentration level) electrolysis process in chamber K ₁ (ie anode and cathode are in chamber K ₁ ) in the gaseous elements H ₂ and Cl _{2 are} converted and driven off.

Another variant that is worth protecting in FIGS. 1 and 3 consists in a grid-shaped arrangement of the cathode K (usually made of iron) or the anode A in or on the outside of the corresponding diaphragms facing the cathode or anode space. In this way, the area flow through the membrane can be increased and a possible back diffusion of Cl ^{- can be} reduced. In addition, this "cathode" or "electrode mesh" increases the stability of the diaphragm and allows the production of thinner and larger membranes, which further increases the area current. Although H _{2 is} very diffusion-friendly, the volatility is not so great that there is a risk of significant diffusion of the gaseous hydrogen into the chamber K ₁ (see FIGS. 1 and 3).

Another very advantageous special form of the arrangement of electrodes and diaphragms or membranes just described consists of a two-dimensional rake, the tines of which (as a third dimension) protrude into the membrane or diaphragm (straight or oblique, so that in the latter) If the gaseous elements formed can more easily escape from the membrane when the membranes are arranged vertically) but of course do not push through the membrane surface on the other side, but end just below it. The power supply to these rakes is then of course carried out in well-insulated and chemically stable lines. The diaphragms conduct anions or cations preferentially (corresponding to positive or negative voltages applied). The thickness of the membrane can be reduced due to increasing stability and the area current can be increased further. Diffusion of gaseous elements and concentration compensation (osmotic pressure, osmosis) do not occur. Of course, according to the invention, these membrane-electrode “rakes” can also be used in electrolysis chambers according to FIG. 2.

Energy and reaction time considerations

The speed of the desalination process depends significantly on the current strengths achieved or the surface current strengths. A change in resistance of the Membranes or the liquids in the electrolysis chambers was within the scope of the tests achieved or aimed for Measurement accuracy not demonstrable. With agar membranes there is a higher temperatures, however, the risk of losing the firm consistency. The frame in which membranes become one large-area membrane, of course, may not be electrically conductive, must be watertight and be stable at these temperatures.

Reverse diffusion processes obviously played no role in the experiments, since the decrease in the molar salt concentration within the scope of the available measurement accuracy correlated relatively well with the amount of charge flowed through (or the current strengths achieved and the time spent). This statement applies in any case even up to initial concentrations of about 3-4% NaCl. Should back diffusions e.g. B. from Na⁺ or diffusion of Cl⁻ from chamber K ₂ in chamber K ₁ of FIG. 1, in analogy corresponding to the method in FIG. 2 or FIG. 3, at higher starting concentrations to a significant extent, so far The "static" electrolysis process described can advantageously be replaced by a step-wise, ie by a "quasi-continuous" process. In the case of a quasi-continuous process, the liquids of the chamber A and the chamber K are replaced in FIG. 2, in the process of FIG. 1 (or 3) the liquids of the chambers K ₁ and. K ₂ (or K ₁ , K ₂ and A) in gradually decreasing electrolysis chambers by partially desalted solutions (or only the liquids of the chambers K ₂ in Fig. 1, A and K in Fig. 2 and K ₂ and A in Fig. 3 by usually existing in abundant starting solution).

Apart from ohmic losses, from net losses in the process of FIG. 2 (if in the chambers A and K the concentrated reaction solutions are replaced by partially desalinated solutions from the chamber K ₁ or by starting solution, see paragraph above) or of FIG. 3 (if there is a pure net loss of liquid in chamber K ₁ ) and possibly net losses due to back diffusion, the electrical energy used is directly proportional to the molar concentration change.

It should be explained once again that the application for the Procedures to minimize "back diffusion processes" for reasons electrical neutrality and for reasons of Efforts to balance different chemical Concentrations (osmosis, osmotic pressure), ultimately from Entropy considerations can occur.

Looking at Fig. 1, the resulting disproportion of the number of cations in the discharge of H⁺ ions compared to the number of anions in chamber K ₂ can be caused not only by the flow of cations from K ₁ to K ₂ , but also by the forced migration of anions from the chamber K ₂ into the chamber K ₁ , that is, by backdiffusion of Cl⁻ ions or other anions can be compensated. The reverse applies to the back diffusion of cations from the anode compartment.

Due to the emergence of z. B. NaOH in the cathode space, there is an increase in the osmotic pressure of NaOH and an effort is made to compensate for this concentration difference between the reaction spaces separated by the membranes. This concentration compensation is prevented in the method applied for by the construction of the membranes or additionally by the control systems R ₁ and R ₂ shown in FIG. 3.

Final remarks

Since gaseous Cl _{2 has} unpleasant stinging and irritating effects on the mucous membrane and gaseous hydrogen H _{2 is} relatively volatile and therefore difficult to handle as an energy source, it is most environmentally compatible to react H ₂ and Cl ₂ to HCl and this hydrochloric acid in a process according to FIG. 1 Termination of the process into the reaction liquids from the chamber K ₂ , in the processes according to FIG. 2 or FIG. 3 into the reaction liquids from the chambers A and K or A and K ₂ . The result is essentially a more concentrated starting solution, which can be subjected to evaporation processes in warm countries if it is not desirable to return it to the sea or brackish water for ecological reasons. The further processing just mentioned also serves as an example for the demarcation of the requested process from the usual electrodialysis processes. With these processes namely (as a rule in countercurrent processes of capillary dimension, ie in sensitive and therefore maintenance-intensive processes), there is an immediate repeated repetitive concentration and concentration in closely adjacent areas of the starting solution and thus no actual electrolysis in disjoint areas.

Claims (6)

1. Device for sea and brackish water desalination by electrolysis, characterized in that the electrolysis chambers are interconnected by ion-conducting agar plates. 2. Device for sea and brackish water desalination Electrolysis, characterized in that the electrolysis chambers are connected to each other by ion conduction in capillaries, targeted capillary forces or Na⁺ and Cl⁻ channels are used will. 3. Device for sea and brackish water desalination Electrolysis, characterized in that the electrolysis chambers through diaphragms that conduct ions but the back diffusion of the Prevent reaction products from being connected to each other the diaphragms u. a. can be trained so that the Electrodes are built into the diaphragms (e.g. calculation form). 4. The device according to claim 3, characterized in that Electrode chambers are used that have very thin diaphragms (high surface currents) contain small distances between Have electrodes and diaphragms and the contact, or the Contact surfaces between reaction solution and diaphragms minimize. 5. Device according to one of the preceding claims 1 to 4, characterized in that H ₂ and Cl ₂ are formed in the electrolysis. 6. Device according to one of those mentioned under 1 to 5 Process, characterized in that not only direct currents, but also rectified alternating currents, i.e. H. pulsating Direct currents are used.