FI90947C - Continuous reaction of gases with liquids - Google Patents

Description

90947 i

Continuous Reaction of Gases with Liquids The present invention relates to the ability to continuously effect reactions between gases and liquids. More specifically, it discusses improved systems, equipment, and methods for the continuous production of liquid products from gas-liquid reactions. The systems and apparatus of this invention are indeed particularly suitable for performing gas-liquid reactions in a constant flow system to produce the desired homogeneous gaseous or liquid product at a rate obtained, i. to provide the chemical as desired for use in a continuous process.

Preferred systems are those that produce a liquid product that does not have significant Mari insoluble solids. Examples of some particularly suitable gas-liquid reactions include the following: 1. reaction of chlorine gas with an aqueous solution of liped to obtain solutions of alichloroacid and / or hypochlorite salts; 2. reacting the organic liquid with a gas such as hCl, Cl 2, SO 3, O 3 or NO 2 to give the corresponding liquid organic derivative; 3. reaction of an aqueous solution of chlorite or chlorate with gases such as O3, NO2, Cl2 »HCl or SO2 to obtain chlorine dioxide as a gas or in solution.

In systems and equipment for the continuous operation of gas / liquid reactions, it has traditionally been popular to direct gas and liquid feed streams countercurrently. Significant long-term examples of this dominant trend include performance equipment in use, such as packed columns, bellows columns, scrubbers, and the like.

A rare example of the continuous production of a liquid chemical reagent by reacting a gas with a liquid in a constant parallel flow system is shown in Figure 2.

U.S. Patent Nos. 2,889,199 and 2,965,443 to Osborne et al., In which chlorine gas is reacted with an aqueous slurry of calcium hydroxide to prepare a calcium hypochlorite bleach solution. However, the reactor used in these patents does not comprise anything other than an ordinary pipeline extension or loop (Ref. No. 16 in the drawings). Other patents for gas / liquid mixing for said liquid disclose myGs discloses parallel flow apparatus and systems, such as U.S. Patent 2,606,150 to Thorp and U.S. Patent 3,997,631 to Matsuoka et al. NSmS patents describe liquid jet vacuum cleaners. or ejectors for mixing ozone gas with the liquid to be treated, e.g. water. Similar ejectors have also been used in various systems and equipment to induce gas / liquid reactions, e.g., U.S. Patent 4,483,829 (Louthan); 1,808,956 (Ketterer); 2,020,850 (Myhren and others); and 2,127,571 (Pardee, Jr.). However, U.S. Patents 4,483,826 and 1,808,956 are directed only to batch or sub-batch-20 operations, while the continuous systems disclosed in U.S. Patents 2,202,850 and 2,127,571 are based on the pfiSos countercurrent flow principle for gas and liquid bay fluid. in spite of the local parallel flows which take place in the ejector units disclosed in said patents. In addition, the equipment and systems of the latter two references are unnecessarily complicated, expensive and cumbersome to use, as they both involve multiple iterations of all the different pieces of equipment used therein, especially ejectors, pumps, valves, separators, etc.

The main object of the invention is to provide a simple means for carrying out a chemical reaction continuously under conditions obtained from a gaseous and liquid starting material to produce a homogeneous gaseous or liquid product. The advantages and disadvantages of achieving this goal are of particular importance in terms of economy and ease of delivery of the product for use in industrial manufacture or handling, when said 90947 3 product is difficult or expensive to transport or store. Another object is to provide a simple, compact in-line parallel flow device that provides vigorous mixing and a hydrated gas / liquid mixer, thereby accelerating the desired gas / liquid reaction and minimizing the required residence time. A further object is to provide a complete system for the efficient performance of such gas / liquid reactions continuously with reliable and consistent results. Still other objects and advantages of the present invention will become apparent from the detailed descriptions and specific embodiments thereof set forth herein.

The above objects and advantages are achieved when the desired gas / liquid reaction is carried out using a completely parallel, in-line flow system using a cholera head connected in series in the following order: 1. a liquid-operated in-line flow mixing device; adapted to form a thorough gas / liquid mixture; 2. A closed, regenerative, turbine pumping apparatus equipped with a single or multi-blade mechanically operated multi-vane running pyramid and its bellows stator adapted to cause intense turbulence and interaction in the gas / liquid mixture as it fills the blowing and simultaneously raises it. single atmospheric sharks; and 3. a closed product receiving tank and a gas / liquid separation tank equipped with a pressure sensor and a liquid surface sensor which effectively maintains a sufficient gas space therein.

Said sailid 3 is preferably provided with pipes from its gas space and lower liquid space, which connect to gas and liquid ignition lines respectively leading to said flow mixing device 1, thus allowing a part of the gas and / or liquid passing through said device 4 to be recirculated material. The MyOs self-contained in-line flow mixer 1, which has been found to be most suitable for initiating the desired gas / liquid reaction, is a venturi jet mixer, while the regenerative pumping device 2 found to be most effective in accelerating the completion of the final stage of turbine reaction or a liquid ring compressor, with a turbine pump generally being preferred unless the liquid stream is only a small fraction of the volume of the gas stream.

In general, then, the present invention rapidly provides a chemical reaction in the presence of gaseous and liquid feedstocks to continuously produce a liquid product 15 by a process comprising the steps of: (a) continuously feeding a liquid and / or gaseous feedstock at a pressure well above atmospheric pressure; to form a thorough gas / liquid reaction mixture and remove said mixture from said zone at a pressure not substantially below atmospheric pressure, although lower than at least one of the feedstocks of said feedstocks; (b) a gas / liquid reaction mixture removed from the feed (a) to a general area of a mechanically operated fluid injection zone in which at least one multi-bladed rotary impeller rotates at least one side chute adjacent to said side jet to cause a strong gas / liquid interaction applying a pressure of at least one atmosphere and removing said mixture therefrom at an absolute pressure of at least two atmospheres; and (c) feeding the mixture removed from (b) to a closed product 35 kerosene and gas / liquid separation zone maintained at an absolute pressure of at least about two atmospheres and having a spacious gas space in the yia portion thereof.

90947 5

Of course, in many gas / liquid reactions, it may prove desirable to recycle a portion of the gas and / or liquid separated from the gas / liquid separation jet of step (c) back to the first stage of the entire process, such as the in-line parallel flow mixing jet of step (a). Thus, in addition to the improved yield of the potential product and the better utilization of the starting materials, the simplified recycling part of this system provides a great 10 processing flexibility, i. the ability to achieve production speed while maintaining very good quality control.

The close cooperation and critical independence of the three main parts of the equipment connected to the 15-gauge gas / liquid reaction system can be better assessed from the following limited analysis performed by each part of the functions separately.

Nain: 20 (1) A liquid pressure mixing device not only performs the thorough formation of a gas / liquid mixture, but mytts sfs it into a regenerative turbine pump evenly at a pressure of at least about atmospheric pressure and preferably above atmospheric pressure; 25 (2) A regenerative turbine pump equipped with a multi-bladed running gear and operating in conjunction with side-chute elements receives a smooth supply of mixed and rapidly reacting gas and liquid and effectively fills it into the blade while simultaneously increasing its pressure. an atmospheric shark so as to remove it at a pressure of at least about two atmospheres from the ceramic and gas / liquid separation silicon; and (3) the product is equipped with a liquid-35 surface and a pressure measurement that allows gas / liquid separation to occur there at a pressure of at least two atmospheres, thus simplifying the continuous flow of the product, such as mybs partial liquid and liquid. and / or the desired degree of circulation of the gas components.

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From the descriptions of the basic equipment described above and the brief functional analyzes, it can also be seen that the agitator 1 and the pumping unit 2 operate efficiently as reactors 5 in this system, i. they actually provide the main zones and lies for the desired gas / liquid reaction. Due to the compact nature of the two devices and the large blade flow volumes at which they operate efficiently, the total residence time of the gas / liquid reaction mixture 10 as it passes through the blade is only a few seconds, i. typically about 1-5 seconds.

Despite the short residence times in women, very good results have been obtained in terms of both the conversions and yields of the starting materials and the quality of the desired product, after carrying out easy gas / liquid reactions with the current system and equipment. Eslm. 90% conversions are often achievable even without any recirculation, probably due to the strong gas / liquid mixing and the highly hydrated, yet uniform mixing phase liquid flow achieved in efficient reaction zones and this system equipment. Likewise, the yield of the desired product, based on the modified starting materials, is generally greater than 90%, indicating that waxes or no undesired by-products (such as chlorites or chlorates instead of hypochlorites) are formed.

This system and apparatus are highly preferred for use with highly reactive gas / liquid pairs, with 30 degree thermodynamics preferred even in standard lamp conditions. Of particular interest and suitability for the practice of this invention are the silent reactions of aqueous solutions of compounds (i.e., salts or hydroxides) with gases such as chlorine to produce hydrated liquid and / or gaseous reaction products. For example, chlorine gas can be reacted with sodium hydroxide solution to form sodium hypochlorite solutions that are useful in various disinfection and bleaching treatments. Similarly, hypochlorous acid (HOCl) can be produced in a similar manner to obtain a 90947 7 mile reactive lipid to chlorine ratio. Alternatively, the HOCl solution can be produced by reacting chlorine gas with solutions of carbonate or hypochlorite salts.

In addition, gaseous product streams containing an active substance such as chlorine dioxide can be obtained by reacting an aqueous solution of the hypochlorite salt with a reactive gas such as NC> 2 or O3.

The accompanying drawing is a simplified schematic representation of a typical flow diagram of a continuous process for carrying out a gas / liquid reaction according to the invention.

This flow chart schematically shows certain points in the apparatus and shows how the apparatus can be combined and used as a system to continuously produce a uniform flow of a liquid reagent, such as an aqueous solution of dilute sodium hypochlorite bleach, by continuous reaction of chlorine gas with a dilute lipid solution.

Referring now to the drawing, the main equipment of the apparatus is a venturi jet mixer 14, a multi-stage regenerative turbine pump 16 and a product collection tank 18 connected in series with fluid handling pipes 15 and 17. The pump 16 is driven by an electric motor 24 capable of rotating 25 shafts mounted, at least about 1000 rpm. speed. The reservoir 18 is provided with a pressure relief valve 20 and holes 21 to maintain the desired overpressure of the souls and a liquid level transmitter 22 to ensure sufficient gas space for the souls. The hole 21 leads to a scrubber 30 or other satisfactory cleaning device (not shown) to remove harmful gases.

Liquid source material, i.e. dilute lipid solution, is introduced longitudinally at a suitable pressure into the venturi 13 14 of the jet mixer 35 14 via the injector nozzle 12 along the injection line 11, while gaseous chlorine is injected at a lower pressure along the supply line 10 into the pressure equalization chamber of the jet mixer 14. Due to the low boiling point of chlorine, it can also be fed as a cold liquid and allowed to evaporate rapidly just as it enters said pressure equalization chamber. The feed lines 10 and 11, which connect the chlorine-5 to the bay 5 and the main batch 6, respectively, are provided with suitable feeds 7 and 9 for obtaining the feed rates and serving of the feedstocks.

After passing through the venturi jet mixer 14 and the turbine pump 16 10, most of the chlorine has reacted to form hypochlorite and the resulting reaction mixture is passed through a tube 17 to a product collection tank 18 where unreacted gas (chlorine) is collected in the liquid portion of said tank. If an automatic process feedback yield of the process is desired, a continuous meter such as a pH analyzer 32 may be placed on the side line of the product supply line 17. The signal from this analyzer 32 is then continuously fed to a receiver 34 having a preselected pH reference point to which said signal is compared. a correction can be made to the relative gain of the chlorine feed rate gain valve. Such reciprocating feedback systems are ideally suited for use in this system because of their exceptionally short residence times.

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To recirculate the unreacted gas back to the beginning of the process, a liquid kneading pipe 23 which feeds the flow supply valve 25 is installed between the gas space of the tank 18 and the gas supply line 30 10 leading to the venturi jet mixer 14. In addition, for maximum flexibility, the portion of the tank 18 below its gas space may be provided with an optional fluid recirculation line 27 leading back to the liquid supply line 11 of the vent valve 29 of the venturi jet mixer 14. Finally, the tank 18 is also equipped with a (liquid liquefaction portion 28) leads from the system to the valve 30 of the liquid product, depending on when the receiver of the liquid surface controlled by the receiver 22 is received.

Il 90947 9

As a result of the repressurization of the reaction mixture provided by the regenerative turbine pump 16, it is very easy to recycle the feed material from the product size 5 gas and gas / liquid separation tank 18 to the beginning of the process.

Either gas or liquid jet streams (or both) can be used for optimum results with maximum flexibility. Of course, the advantage that must be noticed when the gaseous starting material is clarified directly from the gas / liquid separator is generally greater when the liquid product is prepared, while the direct liquid recirculation usually freezes when the main product is gas.

In the case of Slina, when chlorine is reacted with dilute aqueous sodium hydroxide solution to produce sodium hypochlorite bleach, the coupling reaction proceeds as follows: 2 NaOH (aq.) + Cl2 (g) ----> NaOCl (aq.) + NaCl (aq.)

Stoichiometrically, this requires a total of 0.51 g of NaOH per reactant 0.45 kg of Cl 2, giving theoretically 0.477 kg of NaOCl and 0.377 kg of NaCl. In K3y production, it is better to have an excess of plenol NaOH relative to the stoichiometric, because a high pH sodium hypochlorite solution with good stability is obtained (i.e., a pH of about 5-15% above zero is generally obtained in countries with a concentration of zero to 11-13. 25 NaOH: a). Thus, in the preparation of the sodium hypochlorite solution according to the present invention, it is recommended to use 0.545 to 0.59 kg of NaOH per 0.454 kg of chlorine, which promotes better utilization of the chlorine blowing agent.

It is also desirable for the product to have a liquid white pulp containing less than 10% palno% NaOCl in order to maintain large pond removal operations and deposits in the system. The reaction ampere thus released must be raised to a maximum of 1 ° C (i.e. zero 40 ° C) of the liquid whitewash product, which switches from zero to 10% by volume of NaOCl, even if the sodium hydroxide solution to be cooled is substantially cooled. C.

Thus, a bleach solution which contains zero 1-6% by weight

NaOCl, represent ideal products for production in a continuous production system and are fortunately generally used in an area which is very important in a wide range of industrial processing processes.

White The preferred reaction conditions in this system are quite reasonable and generally similar to the usual outdoor openings in gold, the gas / liquid mixtures made there being kept for the most part in the heat of the air. Thus, the initial fluid pressure of the initial lllk-10 applied to the flow mixer 14 at the beginning of the first lie is generally about two atmospheres of absolute flux and is sufficient to burn the reactant mixture from the flue 16 to added to give the flake of the ground reaction mixture to one whole atmosphere. Said turbine pump is preferably capable of re-fumigating the reaction mixture, which is removed in the fume of the product collection tank 18, which is approximately as high as the initial 20 propulsion fluid pressure provided by the agitator 14.

The use of such overflow pressures appears to be a preferred factor that promotes both uniformity and completion of the desired reaction in the system. In particular, the blade of the gas / liquid reactant jet mixer 14 and the pumping unit 16 is stabilized and remains fairly even and stable in a woman with abundant airborne conditions. It is also believed that mass transfer and reaction rates in general generally increase with pressurization, e.g., as a result of the solubility of the added gas in the liquid and the similar effects of the earth. Accordingly, the preferred pressure for the initial moving liquid, such as the contents of the myOs product collection tank 18, is about 2.8 to about 5.6 bar, while the preferred pressure in the reaction mixture 35 exiting the mixer 14 and fed to the pump 16 is about 1.4 -2.8 bar.

90947 11

As set forth in the foregoing description and the accompanying discussion, an outlined flow chart of the accompanying drawing is provided to concretely illustrate the powder coating plates of the present invention and to name certain embodiments of good devices for their successful use. In addition to those of the main alternatives already presented in detail, it will be apparent to those skilled in the art that several other small variations and substitutions are simultaneously possible.

Thus, it is advantageous to repeat both of them from the main reactor unit (i.e. the jet mixer 14 and the pump 16) in the el-horizontal position. Eslmerkiksl if the pump 15 16 is provided with an inlet opening, it may be more advantageous to use a closure mixer in a vertical in-line channel connecting it to the inlet of said pump. Likewise, other mechanisms may be used in place of the pH analyzer 32 (and the co-operating receiver 20 34) to apply the automatic feedback to the system. Eslmerkiksl similar instrumentation based on the measurement of another property of the product (such as oxidation-reduction potential) can be used instead in many cases, e.g. when producing solutions of hypochlorites, hypochlorous acid, etc.

The following specific use example is included to illustrate further details and aspects of the work related to the successful use of the present invention, but said example may be considered to provide critical limitations on the scope of the invention.

Example 35 This example illustrates a system substantially the same as that described in the accompanying drawing to produce an aqueous bleach solution containing about 2% by weight NaOCl 2.

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Referring to the said drawing, 379 liters per minute of 0.6 molar NaOH solution (24 g NaOH per liter) are injected at a pressure of 2.8 bar in a 5.1 cm pipeline onto a nozzle 12 in a jet mixer 14 otherwise equipped with 7.6 cm pipelines. , while a total of 7.8 kg of chlorine gas per minute is injected at a pressure of about 0.7 bar into a filling chamber surrounding said nozzle 12 in the pump 14. The resulting chlorine-slurry reaction mixture is removed from the venturi mixer and passed through 10 tubes at a pressure of about 1.2 bar in a three-stage process. , to the inlet of a low NPSH turbine pump 16 having a turbine running gear at each stage and driven by a 20 hp, 1800 rpm engine. Each treadmill has about 20 blades and is tightly fitted to the channel ring staat-15 market elements. (A minimum of six blades are required per running gear, and depending on the size of the motor blade, it is preferable to have about 10 and 30 blades.)

The reaction mixture is removed from the pump 16 at a pressure of about 3.5 bar 20 and is injected with a siliconGG 18 having a volume of more than 758 liters and equipped with a liquid surface converter set to hold the liquid / gas interface at about 30 and 70% of the tank height and pressure transducers, set at a pressure of about 3.2-3.5 bar.

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The separate liquid and gas phases formed in said sails were examined and found that the resulting liquid bleach solution had a pH of almost 12 and a concentration of about 1.97% by weight of NaOCl, while unreacted chlorine gas accumulated in it at a rate of about 0.45. kg per minute. These figures show that about 94% of the chlorinated chlorine had reacted and that the yield of NaOCl relative to the reacted chlorine was over 98%.

35 This fraction continued to operate substantially at the same time in the system except that the supply of fresh chlorine to the mixer 14 was reduced from 0.4 kg / min to 7.4 kg / min, while 0.4 kg / min of unreacted chlorine was circulated from the tank 16 via line 13 and It was fed to mixer 14 together

II

90947 13 with fresh chlorine mentioned. This mode of operation el elucidated the quality of the liquid bleached and not the rate of accumulation of unreacted chlorine, which demonstrated the ease of recirculation of unreacted chlorine and the achievement of its efficient utilization in this system.

Having described the invention, including its basic principles, illustrative embodiments, and various useful modifications and variations, it is intended that the scope of the appended claims be limited only by their own equivocal and defined conditions and not by way of example of certain details or objects set forth herein. definition or arbitrary inclusion.

Claims (15)

14 A solid apparatus for continuously carrying out a chemical reaction between two gaseous and liquid feedstocks flowing in the same direction to form a desired liquid product, characterized in that it comprises the following parts of the apparatus which are tightly connected in series: (a) a pressurized, liquid-operated, in-line unidirectional mixing device having separate inlets for gas and liquid near its inlet end, and further means for continuously feeding gas and liquid, respectively, to said outlet and outlet; (b) an encapsulated, regenerative turbine pumping device, if at least one is at least one mechanically aspirated multi-bladed impeller adapted to push the gas / liquid mixture forward evenly through the discharge device at the outlet of the discharge device and at the same time add the liquid mixture; (C) a liquid conveying pipe between the outlet of said flow mixing device and said encapsulated regenerative turbine pumping device; and (d) an encapsulated, pressurized, pressure-tight product collection container equipped with a pressure control device and a liquid-level height adjustment device to maintain a desired overpressure and sufficient gas space therein, with an outlet containing said container. a pipe connecting said outlet to the outlet of the regenerative pumping device (b), said tank 30 also being provided with liquid recirculation pipes connecting its gas space and the lower liquid space to said parallel flow mixing device (a). Apparatus assembly according to claim 1, characterized in that said device for continuously injecting a liquid into the liquid inlet of the flow mixing device (a) is capable of injecting a liquid at a pressure of at least about 2.8 bar. II 90947 15 Apparatus assembly according to claim 2, characterized in that the flow mixing device comprises a venturi jet mixer. Apparatus assembly according to claim 1, characterized in that the regenerative turbine pumping device (b) is a multi-stage unit, of which at least two stages are provided with a multi-bladed impeller, the drive device of which is adapted to rotate over 1000 impellers. Device assembly according to Claim 4, characterized in that each impeller has at least 10 blades. 15 Device assembly according to Claim 4, characterized in that the pumping device is a self-priming regenerative turbine pump. A process for the continuous production of a continuous stream of dilute hypochlorite bleach solution of uniform quality by reacting chlorine gas with a dilute aqueous solution of lye, characterized in that it comprises: continuous feeding to an pressurized liquid-operated, in-line, parallel flow mixing zone to form a thorough gas-liquid reaction mixture, and removing said mixture substantially below the said mixing zone; (B) In step (a) of the reaction mixture sySttåmisen marsh 35 mechanically kåytetyn directly regenerative liquids enter to transfer-relaxed and re-pressurizing pumppausvy6hykkeen front portion, wherein the vy6hykkeesså våhintåån yhtää multi-paddle juoksupyoråå pyfiritetåån side channel of the stator låhellå to provide 16 powerful gas / liquid interaction and erittåin swirling the reaction mixture flow pumppausvydhykkeen låpi and simultaneously raising the pressure of the reaction mixture in at least one atmosphere and removing the mixture at an absolute pressure of at least two atmospheres; (c) feeding the mixture removed in step (b) to a closed product collection and gas / liquid separation zone maintained at an absolute pressure of at least about two atmospheres and having a large gas space at the top thereof; (D) collecting the resulting liquid bleach solution as a product in the lower part of said separation zone and collecting unreacted chlorine gas in the gas space; and (e) recirculating a portion of the liquid collected in at least one of said collection zones in step (d) back to the front of the parallel flow mixing zone of step (a). Process according to Claim 7, characterized in that the concentration of the dilute aqueous sodium hydroxide solution and the amount at which it is reacted with chlorine gas are adjusted to produce a liquid bleaching solution containing about 1 to 6% by weight of sodium hypochlorite. A method according to claim 7, characterized in that a portion of both the unreacted chlorine gas collected in the separation zone (d) and the liquid bleaching solution is recycled to the front end of said parallel zone (a). A method according to claim 7, characterized in that steps (a) and (b) are performed in only a few seconds. A method according to claim 7, characterized in that the pressure maintained in the separation zone of step (c) is between about 2.8 and 5.6 bar and is not less than the pressure of the pressurized pressurized liquid used. 90947 17 as the driving force in the parallel flow mixing zone in step (a). A continuous process for producing a reaction between gaseous and liquid starting materials, characterized in that it comprises: the starting materials are fed at a pressure substantially above atmospheric pressure, thus forming a thorough gas / liquid reaction mixture; (b) removing the gas / liquid reaction mixture from the rear end of said mixing tank at a pressure lower than the pressure at which at least one of the starting materials is injected in step (a), but not substantially below atmospheric pressure; (c) transferring the reaction mixture removed in step (b) directly to the front of a mechanically operated, regenerative, liquid-forwarding and repressurizing pumping zone in which at least one multi-blade rotating impeller rotates at high speed. the flow of the highly porous reaction mixture through the pumping zone and at the same time raising the pressure of the reaction mixture to at least one atmosphere and removing the mixture at an absolute pressure of at least about two atmospheres; (d) directing the pressurized mixture removed in step (c) to a pressurized, pressure-resistant product collection and gas / liquid separation zone maintained at at least one atmosphere under controlled overpressure and equipped with a liquid surface. when maintaining a large volume of liquid in its lower part; (E) collecting the desired liquid product in the lower part of said gas / liquid separation zone while the unreacted gases accumulate in said gas space; and 18 (f) partially recirculating the gas accumulated in the gas space and / or the liquid accumulated in the lower part of the separation zone of step (d) to the front end of the parallel flow-mixing zone of step (a). 5 A process according to claim 12, characterized in that said liquid starting material is introduced in step (a) at a pressure of at least about 2.8 bar and said hydride in step (d) is maintained at an irradiated pressure higher than the pressure in which the liquid starting material is fed in step (a). A process according to claim 12, characterized in that the gaseous starting material is selected from the group consisting of Cl 2, HCl, SO 3 and NO 2, and the liquid starting material is an organic liquid. Process according to Claim 12, characterized in that the gaseous starting material is Cl 2 and the liquid starting material is an aqueous solution of an alkali metal hydroxide or salt.