RU2100483C1 - Process of water treatment with sodium hypochlorite and flow electrolyzer to produce sodium hypochlorite - Google Patents

Abstract

FIELD: electrochemistry, treatment of water with sodium hypochlorite produced at consumption site by electrolysis of underground mineralized water carrying sodium chloride. SUBSTANCE: electrolysis is carried out under flow mode with mineralized water supplied by gravity flow. Proposed flow electrolyzer has tank with inlet and outlet branch pipes and chambers to inject and discharge solutions, vertical laminated electrodes installed in tank between mentioned chambers grouped in cassettes placed in parallel and spaced apart with reference to each other. Each cassette has two monopolar and intermediate bipolar electrodes positioned with minimal interelectrode gaps. Lower edges of laminated electrodes are placed at some distance from bottom of tank to form passage for flow of sodium chloride under laminar mode. EFFECT: increased efficiency and productivity of process. 4 cl, 2 dwg

Description

 The invention relates to water purification, and in particular to methods for disinfecting drinking and wastewater, pools and other water bodies using aqueous solutions of chlorine and other chlorine-containing compounds, in particular sodium hypochlorite, and can be used in water treatment of drinking water.

 Among the known oxidative methods of water treatment, the leading place belongs to chlorination. The relative availability and low cost of liquid chlorine led to its widespread use in water treatment practice.

 However, technological (rather complicated technology, special safety measures during transportation, storage and use, etc.), economic (maintenance of reagent facilities and additional staff of maintenance personnel), environmental (chlorine toxicity and the high probability of formation of toxic, mutagenic during water treatment and carcinogenic halogen-containing organic compounds) aspects of the use of liquid chlorine stimulated the search for a more economical technical solution.

 At present, chlorine-containing reagents are used as a substitute for liquid chlorine, which are safer and less toxic than liquid chlorine. One of these reagents is sodium hypochlorite.

 Comparative studies of the quality of drinking water prepared using liquid chlorine and sodium hypochlorite showed that the water quality indicators standardized by GOST 2874-82 are quite close, and the treated water meets the requirements of the specified GOST.

 In addition, the use of sodium hypochlorite as an oxidizing agent and disinfectant can improve the environmental situation due to the elimination of hazardous and toxic chlorine storage facilities in the city and most of the reagent facilities of the water treatment plant.

 One of the most preferred methods of using sodium hypochlorite as an oxidizing agent and disinfectant is a method of treating water with sodium hypochlorite, obtained at the place of its application by electrolysis of sodium chloride solutions (Usoltsev V.A. Sokolov V.D. et al. Water treatment using sodium hypochlorite. The journal "Water supply and sanitary equipment", Stroyizdat, N 11, 1994).

 In particular, there is a known method of treating water with sodium hypochlorite, obtained at the place of consumption by electrolysis of artificially prepared solutions of sodium chloride. (Medriiz G.L. Equipment and devices for water disinfection. Water supply and sanitary equipment. Stroyizdat, N 2, 1993).

 However, the use of artificially prepared solutions of sodium chloride causes either complex control over the electrolysis process and the dosage of sodium hypochlorite in the treated water in the case of using technical sodium chloride with a variable composition, or the use of specially prepared rather expensive sodium chloride, which significantly increases the cost of the water treatment technology. When treating drinking water, a sodium hypochlorite solution suitable for its disinfection must meet increased requirements for purity and impurity content, which in cases of using artificially prepared solutions of sodium chloride is achieved using purified sodium chloride and specially prepared water or subsequent purification of sodium hypochlorite, which significantly increases the cost of treating drinking water.

 Closest to the proposed one is a method of treating water with sodium hypochlorite, obtained at the place of its consumption using natural sodium chloride solutions, for example, underground saline water or sea water, as a feedstock for electrolysis. In this case, electrolysis is carried out in a non-continuous mode (Medriiz G.L. Equipment and devices for water disinfection. Water supply and sanitary equipment. Stroyizdat, N 2, 1993, p. 7-8). However, seawater cannot be used without pre-treatment for the production of sodium hypochlorite, suitable for the treatment of drinking water, due to its contamination, which leads to increased costs for water treatment. The use of natural underground solutions of sodium chloride, in particular underground saline water, to produce sodium hypochlorite does not currently have practical use due to the lack of an economical technological scheme.

 For the production of sodium hypochlorite, non-diaphragm electrolyzers of periodic or continuous action are used, designed for the electrolysis of concentrated solutions containing at least 15 g / l sodium chloride (ed. St. N 1793008, patent N 733521), periodic electrolysis plants of the type ЭН, commercially available by the plant "Communal" (GL Medriiz Equipment and devices for water disinfection. Water supply and sanitary equipment. Stroyizdat, N 2, 1993).

 The use of batch electrolyzers complicates the technological scheme of water treatment due to additional control of the parameters of the output product; therefore, it is more preferable to use continuous electrolyzers.

 Closest to the proposed is a vertical diaphragm-free bipolar electrolyzer (ed. Certificate of the USSR N 733521), including a container with inlet and outlet pipes and cameras for input and output of solutions located, respectively, in the lower and upper parts of the container, horizontal in height partitions with holes for the flow of the solution, along the edges of which plate electrodes are fixed. The electrodes are fixed at one end along the edges of the holes of one partition, and the other ends of the electrodes are passed through the holes of the subsequent partition and are fixed to the next highest partition. In this electrolytic cell, all the required flow rate of the sodium chloride solution, necessary to obtain a given amount of sodium hypochlorite, flows through the gaps between the electrodes, so these gaps are chosen quite large. In this case, a rational consumption of electricity for electrolysis is possible only when using electrolytes with high electrical conductivity, that is, concentrated solutions containing at least 15 g / l sodium chloride.

 From the theory of electrolysis it is known that the practical yield of sodium hypochlorite is not more than 15% of the content of sodium chloride in the solution. Thus, the use of concentrated solutions of sodium chloride in many cases seems irrational, increasing the cost of water treatment, especially in the case of using artificially prepared solutions of sodium chloride.

 In addition, in this electrolyzer, a pressure supply of sodium chloride solution is carried out, which does not ensure the stability of hydrodynamic processes, which also increases the cost of water treatment due to additional control.

 The objective of the invention is the creation of a method of treating water with sodium hypochlorite and a flow electrolyzer to implement a method in which a natural solution of sodium chloride would be used to produce sodium hypochlorite in order to reduce the cost of water treatment technology while ensuring environmental safety.

 The problem is solved in that in the method of water treatment, comprising introducing into the water to be treated a solution of sodium hypochlorite produced at the place of consumption by electrolysis of underground saline water containing sodium chloride, according to the invention, saline water containing from 1.5 to 15 g is used / l sodium chloride and produced at the place of production of sodium hypochlorite, electrolysis is carried out in a flow mode with a conversion factor of sodium chloride to hypochlorite equal to 10 ± 2% of the extracted mineral lysed water is pumped into the tank, from which its gravity supply to the electrolysis cell with a given flow rate is provided.

 The authors theoretically discovered and experimentally confirmed that the electrolysis of weakly concentrated solutions of sodium chloride (1.5 to 15 g / l) is energetically favorable when the conversion coefficient of sodium chloride hypochlorite is close to 10%, in particular within 10 ± 2%. a non-linear dependence of the coefficient of conversion of sodium chloride into hypochlorite on the amount of energy spent on electrolysis is manifested. Accordingly, with an increase in the conversion coefficient of sodium chloride to sodium hypochlorite at a given concentration of sodium chloride solution, the process becomes energetically disadvantageous. At the same time, such a low conversion coefficient can be set only with cheap raw materials for obtaining a solution of sodium chloride, which in the present invention is underground mineralized water produced at the place of consumption of sodium hypochlorite. If the conversion coefficient decreases below the indicated value, the efficiency of the electrolysis process decreases due to excessive consumption of saline water.

 The gravity flow of mineralized water to the electrolyzer and flow mode are aimed at establishing a stationary electrolysis mode, which is especially important for maintaining the conversion rate of sodium chloride to hypochloride close to 10%, i.e. for the energy-efficient electrolytic production of sodium hypochlorite from mineralized water with the proposed sodium chloride content. The use of underground mineralized water produced at the site of production of sodium hypochlorite reduces, compared with known methods, the cost of transportation and preparation of solutions of sodium chloride. The low cost of produced mineralized water allows for its gravity supply to the electrolyzer and flow mode of electrolysis.

 The groundwater of one deposit has constant physical and chemical characteristics: chemical composition (including sodium chloride concentration), temperature, pressure, etc., which allows us to simplify the control system for the electrolysis parameters and the solution supply system, ensuring electrolysis in flowing gravity flow mode. Accordingly, at the outlet of the electrolyzer there will be a sodium hypochlorite solution of a given concentration, suitable for use without additional control. At the same time, there is no need for special control equipment, maintenance of water treatment plants is simplified. The ecological purity of groundwater allows it to be used to produce sodium hypochlorite, suitable for the treatment of drinking water without additional purification. According to the research of the authors, there are especially in the North-Western region of Russia, large reserves of underground saline water with a sodium chloride content of 1.5 to 15 g / l, which makes it possible to industrialize the proposed method.

 The problem is also solved by the fact that in a flowing electrolyzer to obtain a solution of sodium hypochlorite from a solution of sodium chloride, which includes a container with inlet and outlet pipes and cameras for input and output of solutions and monopolar and bipolar plate electrodes placed in the tank between these chambers installed vertically and in parallel according to the invention, the electrolyzer is equipped with spaced apart cassettes in which a plate is installed with an interelectrode gap of 1 to 3 mm These electrodes, monopolar electrodes are placed at the edges of the cassettes and are connected in parallel, while the lower edges of the plate electrodes are separated from the bottom of the container, forming a channel for the sodium chloride solution to flow under them, and the container is equipped with a means directing the input stream of the sodium chloride solution under the plate electrodes and means directing the sodium hypochlorite solution to the outlet pipe above the upper edges of the plate electrodes.

 It is known that when an electric current passes through a solution of sodium chloride, sodium hypochlorite is formed and hydrogen is released, which rises in the form of bubbles, entraining a solution of sodium chloride. The lifting force of these bubbles increases the flow rate of the sodium chloride solution in the interelectrode gaps, thereby pulling it from the near-bottom flow into the interelectrode gaps between the plate electrodes, which are significantly less than the distances between the electrode cassettes. In this case, an electrolysis product, sodium hypochlorite, will be collected in a stream above the plate electrodes.

 The smaller the interelectrode gap, the more energy efficient the process of electrolysis of sodium chloride. However, the minimum value of the interelectrode gaps is limited by the condition that the sodium chloride solution flows through them, as well as by the technological capabilities of making flat surfaces of plate electrodes. It was experimentally found that in the selected interval of interelectrode gap values, a sodium chloride solution flows between the plate electrodes with the lowest possible hydraulic resistance and, in addition, the specified interelectrode gap can be reached without the risk of contact of the electrode surfaces due to technological defects. The presence of channels that are much wider than the interelectrode gaps between the electrode cassettes combining the plate electrodes, as well as the channel along the bottom of the tank, which ensures a uniform supply of sodium chloride solution to all electrode cassettes with the plate electrodes, allows passing through the electrolyzer in the laminar mode with the least hydraulic resistance the entire flow rate of the sodium chloride solution required to obtain a given amount of sodium hypochlorite.

 It is advisable that the inlet pipe was made in the upper part of the tank, and the output in its lower part, the means guiding the input and output streams are made in the form of input and output vertical partitions installed in the tank parallel to the plate electrodes, forming the input and output chambers, while the lower edge of the inlet partition is located above the bottom of the vessel not lower than the level of the plate electrodes, and its upper edge is higher than the level of the plate electrodes, the lower edge of the outlet partition is capacitively fixed to the bottom sti, and the upper edge is located not lower than the upper level of the plate electrodes, but below the upper edge of the input partition.

 According to the authors, the proposed constructive solution of a flow-through electrolyzer is the most technologically advanced and provides the least hydraulic resistance to flow, a uniform supply of a solution of sodium chloride to all electrode cassettes and a uniform discharge of sodium hypochlorite.

 To avoid overflow in an emergency, the tank is equipped with an additional drain pipe located under the upper edge of the inlet partition.

 In FIG. 1 shows a flow chart of the treatment of water with sodium hypochlorite; in FIG. 2 flow-through electrolyzer to obtain sodium hypochlorite from a solution of sodium chloride.

 A method of treating water with sodium hypochlorite is as follows. Mineralized underground water containing sodium chloride 1.5-15 g / l, from the well 1 (Fig. 1) is supplied through a pipe 2 to a distribution tank 3. From a reservoir 3, mineralized water flows by gravity to a flowing electrolyzer 4. A predetermined flow rate of mineralized water, supplied to the flowing electrolyzer 4, is installed by valves 5 and is controlled by a flowmeter 6. In the electrolyzer 4 there is an electrolytic decomposition of an aqueous solution of sodium chloride, the result of which is the formation of sodium hypochlorite and the allocation in portly. A solution of sodium hypochlorite of a given concentration from a flowing electrolyzer 4 by gravity enters the buffer tank 7, from where it is fed by a pump 8 to the storage tank 9. From the tank 9, a solution of sodium hypochlorite by gravity enters the entry points for water disinfection.

 In FIG. 2 shows a scheme of a flowing electrolyzer 4 for producing sodium hypochlorite from a weakly concentrated chloride solution, in particular, mineralized water with a sodium chloride content of from 1.5 to 15 g / l. Flow electrolyzer 4 contains a container 10 with inlet and outlet pipes 11 and 12 located on its opposite walls, respectively. Electrode cassettes 13, perpendicular to the sodium chloride stream and parallel to each other, are installed in the container 10, each of which consists of a group of vertical plate electrodes 14. The plate electrodes 14 are connected in a bipolar circuit with current supply to the end electrodes 14 of each electrode cassette 13. The electrode cassettes 13 are connected in parallel to a common stabilized power supply 15.

 To ensure the most energy-efficient operation of the electrolyzer, the magnitude of the interelectrode gaps between the plate electrodes 14, depending on the concentration of the sodium chloride solution, is selected from the interval from 1 to 3 mm. The authors found that with gaps less than 1 mm, the hydraulic resistance in the interelectrode space will be too large, which prevents the sodium chloride solution from flowing through these gaps. If the gaps are higher than the specified limit, the electrolysis process is inefficient, since most of the energy applied to the electrodes will not be consumed for electrolysis, but for heating a weakly concentrated solution, which is mineralized water with a sodium chloride content of from 1.5 to 15 g / l. In addition, with a smaller interelectrode gap, due to errors in the manufacture of flat electrodes 14, it is possible to touch oncoming flat surfaces of the electrodes 14.

 The effective electrode surface area, the number of plate electrodes 14 in each cassette 13, as well as the number of electrode cassettes 13 in the tank 10 are calculated by known methods from the accepted operating characteristics of flow electrolysers (current density, voltage across the electrode gap), as well as from the nominal operating current values and voltage.

 The distance between the electrode cassettes 13 is set from the condition for eliminating edge electrochemical effects in order to ensure the independent operation of each electrode cassette 13.

 The electrodes 14 are installed at a distance from the bottom of the tank 10, forming a channel 16 for the flow of sodium chloride solution under the plate electrodes 14. The area of the passage section of the channel 16 is determined by known methods from the conditions for ensuring flow in the laminar mode, as well as a uniform supply of sodium chloride solution to all electrode cassettes 13.

 The inlet pipe 11 is made in the upper part of the tank 10, and the output 12 in its lower part. Opposite the indicated nozzles 11 and 12 in the vessel 10, the input and output partitions 17 and 18, respectively, parallel to the plate electrodes 14. are installed. The partitions 17 and 18 form chambers 19 and 20, respectively, of the input of a solution of sodium chloride and the output of a solution of sodium hypochlorite. The lower edge of the input partition 17 is located above the bottom of the capacitance 10 not lower than the lower level of the plate electrodes 14, and its upper edge is located near the cover 21 of the tank 10 above the upper level of the plate electrodes 14.

 The output partition 18 is fixed with a lower edge on the bottom of the tank 10, while its upper edge is not lower than the upper level of the plate electrodes 14, but below the upper edge of the input partition 17.

 The tank 10 is equipped with a drain pipe 22 located under the upper edge of the inlet partition 17.

 There are various ways of fixing the plate electrodes 14 in the container 10. For example, the extreme electrode plates in the electrode cartridge 13 can be provided with shanks (not shown), with which they are suspended at the thickened edges (not shown) of the container 10. The middle electrode plates are mounted between the extreme and among themselves with studs. At the same time, at the bottom of the tank there is a crossbar with grooves (not shown) in which the lower edges of the electrode plates are installed.

 Other variations of the invention may be proposed without departing from its scope as defined by the appended claims.

 The proposed flow-through electrolyzer operates as follows.

 Mineralized water containing 1.5-15 g / l sodium chloride flows by gravity from the reservoir 3 through the inlet pipe 11 into the input chamber 19, directing the mineralized water through the channel 16 along the bottom of the tank 10 under the plate electrodes 14. Flowing under the lower edges of the plate electrodes 14, mineralized water enters the interelectrode gaps of the cassettes 13 connected in parallel to a stabilized power supply panel 15.

 When an electric current passes through mineralized water containing sodium chloride flowing in the electrode gaps, electrolytic decomposition of an aqueous solution of sodium chloride occurs with the release of free hydrogen.

Водород в виде пузырьков поднимается вверх в межэлектродных зазорах, увлекая за собой минерализованную воду. Подъемная сила этих пузырьков увеличивает скорость течения минерализованной воды в межэлектродных зазорах, тем самым затягивая ее из околодонного потока. При этом в потоке над пластинчатыми электродами 14 будет собираться продукт электролиза гипохлорид натрия. Раствор гипохлорита натрия, переливаясь через верхнюю кромку выходной перегородки 18, поступает в выходную камеру 20, откуда через патрубок 12 самотеком поступает в буферный резервуар 7.A simplified chemical reaction can be shown as follows:

Hydrogen in the form of bubbles rises up in the interelectrode gaps, dragging mineralized water along with it. The lifting force of these bubbles increases the flow rate of mineralized water in the interelectrode gaps, thereby drawing it out of the near-bottom flow. At the same time, the sodium hypochloride electrolysis product will be collected in a stream above the plate electrodes 14. The sodium hypochlorite solution, overflowing through the upper edge of the outlet baffle 18, enters the outlet chamber 20, from where it flows by gravity into the buffer tank 7 through the pipe 12.

 The authors theoretically discovered and experimentally confirmed that the electrolysis of weakly concentrated sodium chloride solutions of 1.5-15 g / l is energetically favorable when the conversion of sodium chloride to sodium hypochlorite is close to 10%. Therefore, the presence of much wider than the interelectrode gap of the channels between the electrode cassettes 13, combining the plate electrodes 14, as well as the channel 16 under the plate electrodes 14, providing a uniform supply of a solution of sodium chloride to all electrode cassettes 13, allowing t pass through the cell in laminar flow with the lowest hydraulic resistance to flow all mineralized water required to produce a given amount of sodium hypochlorite.

The tests performed showed the possibility of creating a flow electrolyzer with the following main parameters:
Active chlorine productivity, kg / h not less than 1.5;
The consumption of mineralized water, m ³ / h 1,0 9,0;
Current on electrodes, A 250 400;
Voltage at the electrodes, V 22;
Power consumption per 1 kg of active chlorine, kW / kg A.h. no more than 7.

 Since the groundwater of one deposit has constant physicochemical characteristics: chemical composition (including the concentration of sodium chloride), temperature, pressure, etc., and also due to the flow-through electrolysis mode under the gravity-fed supply of saline water, the sodium hypochlorite solution at a given concentration will be at the outlet suitable for consumption without additional control. This eliminates the need for special control equipment, simplifies the maintenance of water treatment plants, which reduces the cost of the entire method of treating water with sodium hypochlorite.

Claims (4)

 1. A method of treating water with sodium hypochlorite, comprising introducing into the treated water a solution of sodium hypochlorite produced at the place of consumption by electrolysis of underground saline water containing sodium chloride, characterized in that saline water containing 1.5 to 15.0 g / l is used sodium chloride and produced at the site of production of sodium hypochlorite, electrolysis is carried out in a flow mode with a conversion factor of sodium chloride to hypochlorite (10 ± 2)%, while the produced mineralized water is pumped into ezervuar from which it Samotechnaja provide a predetermined flow rate of the electrolyser.  2. A flow-through electrolyzer for producing a sodium hypochlorite solution from a sodium chloride solution, including a container with inlet and outlet pipes and solution input and output chambers and mono- and bipolar plate electrodes placed vertically and parallel to each other located in the container between these cameras, characterized in that the electrolyzer is equipped with spaced apart cassettes, in which electrodes are installed with an interelectrode gap of 1-3 mm, monopolar electrodes are placed at the edges of the cassettes and are connected in parallel, while the lower edges of the electrodes are separated from the bottom of the container, forming a channel for the sodium chloride solution to flow under them, and the container is equipped with a means directing the input stream of the chloride solution under the plate electrodes, and a means directing the sodium hypochlorite solution to the outlet over the upper edges plate electrodes.  3. The electrolyzer according to claim 2, characterized in that the inlet pipe is made in the upper part of the tank, and the output in its lower part, the means guiding the input and output flows are made in the form of input and output vertical partitions installed in the tank parallel to the plate electrodes forming the inlet and outlet chambers, while the lower edge of the inlet partition is located above the bottom of the tank not lower than the lower level of the plate electrodes, and its upper edge is higher than the level of the plate electrodes, the lower edge of the outlet burnout ki is fixed on the tank bottom, while the upper edge is not located below the upper level plate electrodes but below the upper edge of the front baffle.  4. The electrolyzer according to PP.2 to 4, characterized in that the tank is equipped with an additional drain pipe located under the upper edge of the inlet partition.

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