RU2224178C1 - Plasma-chemical method of decontamination of gaseous and liquid halogen organic agents and wastes containing such agents - Google Patents

Abstract

FIELD: methods of ecologically safe decontamination of halogen organic agents, such as ozone decomposing refrigerants and polychlorinated biphenyls belonging to super-ecotoxicants such as dioxins and dibenzofurans; chemical, petrochemical and other industries. SUBSTANCE: halogen organic agents are preliminarily heated to temperature not exceeding limit of their thermal stability, after which they are mixed with jet of superheated steam and are evaporated. Steam-and-gas mixture thus obtained is directed to electrically insulated part of reactor into plasma jet of nitrogen. Pyrolysis and oxidation of halogen organic agents occur in reactor at temperature not below 1500 C during 2-10 mc at excess of oxygen contained in steam for complete oxidation of carbon and at excess of hydrogen for binding halogens into halogen hydrogens. Then pyrolysis products are quickly cooled down by aqueous alkaline solution and halogen hydrogens are neutralized excluding formation of secondary high-toxic compounds such as dioxins, dibenzofurans, fluorine phosgenes and chlorine phosgenes, etc. Neutralizing alkaline solution may be used repeatedly at addition of deposited salts and required amount of alkali. Gaseous and liquid components are separated and cooled after hardening and neutralizing. EFFECT: enhanced reliability, efficiency and ecological safety. 4 cl, 1 dwg, 2 tbl

Description

The invention relates to environmentally friendly methods of processing gaseous and liquid organohalogen substances and waste containing them, which are subject to mandatory neutralization. Such substances include ozone-depleting chladones, pesticides, polychlorinated biphenyls - synthetic oils used in electrical equipment, as well as halogen-containing industrial wastes.

The invention can be used in the chemical, petrochemical, electrical and other industries.

A known method for the destruction of chemical waste (European patent No. 0469737, MKI F 23 G 5/08, 7/00 of 02/05/1992), which consists in burning them in a plasma jet having a temperature in the range from 2750 ° C to 3750 ° C and containing at least 70% oxygen. The input of waste into the stream is made in the form of small droplets formed during the spraying of waste under the influence of a conveying gas. The total amount of oxygen entering the reaction zone is proposed to be maintained 30% above the stoichiometric value corresponding to the complete oxidation of the waste. In this case, the temperature in the combustion zone should not be lower than 1450 ° C, and the residence time in the reactor should be at least 2 ms. The combustion products at the outlet of the reactor are rapidly cooled to 300 ° C and lower due to atomization of water in them, which circulates in a closed circuit and is gradually saturated with acid products dissolving in it. With the periodic withdrawal of this quenching agent from the installation, it is neutralized with alkali. The gas components of the reaction products that are not absorbed by quenching water are sent to a scrubber, where the remaining acid gases are removed, and the rest of the gas phase, which does not contain toxic components, is released into the atmosphere.

The main disadvantage of this method is the use as a plasma-forming working fluid of a gas containing a high concentration of oxygen. When the plasma torch operates on such a mixture, which has a high oxidizing ability, one should expect a sharp decrease in the reliability and service life of the plasma torch due to increased erosion of the electrodes in the zone of the supporting spots of the arc, where local temperatures reach values close to the melting temperature of the electrode materials.

In addition, the use of gas with a high oxygen content is expensive and unsafe when operating a high-temperature plasma chemical installation.

The use of this method for the disposal of highly viscous polychlorinated biphenyls (PCBs) and ozone-depleting chladones with a high halogen content is also problematic. PCBs under normal conditions are very difficult to spray even with pneumatic nozzles. Therefore, droplets of PCBs entering the reactor will have a sufficiently large size and their evaporation, pyrolysis and oxidation will require a very long time, not comparable with 2 ms, and this will lead to the need to increase the size of the reactor, significant heat loss to its wall and supercooling of products reactions, resulting in the formation of highly toxic compounds such as dioxins and phosgene.

To neutralize halogen-containing wastes, the pyrolysis of which proceeds with the formation of halogens and hydrogen halides, does not require an excessively large amount of oxygen. In this case, it is more important to increase the temperature in the reaction zone for the decomposition of these wastes into simple substances. However, an increase in the temperature of the plasma jet when using a plasma-forming gas with a high oxidizing ability, as mentioned above, is associated with a decrease in the reliability and service life of the plasma torch.

In addition, during the neutralization of halogen-containing wastes, quenching water circulating in a closed circuit will be saturated with hydrohalic acids, which will lead to intensive corrosion of technological equipment.

A method is proposed (US Patent No. 4438706, MKI F 23 G 7/04 of 03/27/1984) and equipment for the destruction of waste containing evaporating organic materials. When implemented, vaporized waste and preheated air or oxygen are introduced into the plasma air stream entering the reactor. Evaporation of waste and preheating of the air is carried out by using the heat of the pyrolysis products coming from the reactor to the heat exchangers, which simultaneously serve as quenching devices. Part of the air flow heated in the heat exchanger is sent to the reactor to protect its inner wall, which is a ceramic pipe. The surface of this pipe has numerous openings along the entire length, passing through which air creates a protective curtain that protects the wall from the thermal and corrosive effects of pyrolysis products. The pyrolysis products cooled in the heat exchangers are sent to a gas scrubber and then released into the atmosphere.

The main disadvantage of this method is the use of heat exchangers for hardening pyrolysis products. The decrease in gas temperature with this method of quenching occurs at a low rate. At the same time, highly toxic compounds such as dioxins, fluorine and chlorophosgens, benzpyrene, etc. can form in the pyrolysis products. The inner walls of both heat exchangers will be exposed to corrosive substances formed in the reactor and having a high temperature.

A method for pyrolytic decomposition of waste, preferably liquid (US Patent No. 4,448,877, MKI F 23 G 5/10, 5/12 of 02.24.1987), partially free from the disadvantages of the above methods, is proposed. This method provides for the supply of waste into the arc chamber of the plasma torch, the mass-average gas temperature of which reaches 5000 ° C and higher, and therefore the molecules of the substances contained in the waste are destroyed to atoms and ions. These products are then cooled in a reaction chamber to a temperature of 900-1200 ° C. to form recombined products including synthesis gas and soot solid particles.

The pyrolysis products are quenched at a temperature of 80 ° C using spray jets of an aqueous solution of alkali to simultaneously neutralize them and to wet the soot particles. The synthesis gas is separated from the recombined products and burned, and the spent alkaline solution together with soot particles is discharged into the sewer.

In this method, the supply of liquid waste is carried out using a pump into the gap between the axially arranged electrodes. In this case, as the authors note, the quality of the spray does not matter. There is also the option of feeding waste behind the electrodes. This option, according to the authors, is not profitable due to a significant reduction in the residence time of the pyrolysis products in the reactor.

It should also be noted that the viscosity level of liquid waste is limited by the characteristics of the pump. At the same time, air is supplied in the annular gap between the electrodes for aerodynamic stabilization of the arc in an amount of 1-2% of the value corresponding to the air flow during the stoichiometric process of oxidation of pyrolysis products, which makes this method mainly pyrolytic.

The waste supply system includes two reservoirs, one for waste, and the other for an auxiliary substance (non-toxic organic liquid, for example ethanol), which is supplied to the plasma heater at the time of installation start-up as a working fluid for its output to the specified thermal regime (during 3 minutes), as well as when turned off to flush it.

The presence of two tanks and an auxiliary substance (ethanol) leads to a complication of the supply system and additional costs (material and energy). Limitations on the viscosity of the waste do not allow us to consider this method as universal, and the supply of waste together with a small amount of air into the arc chamber of the plasma heater creates a number of problems, namely:

- the surface of the electrodes is subjected to intense erosion as a result of exposure to fluorine and chlorine-containing compounds formed during the pyrolysis of waste and having extremely high chemical activity;

- when changing the composition of the waste fed to neutralization, additional testing of the modes of the arc heater is necessary in order to search for optimal modes that meet the conditions of its reliable operation;

- a large amount of soot appears in the pyrolysis products, which impedes the processes of quenching and neutralization of gaseous pyrolysis products.

It should also be noted that in the case of highly viscous waste, the proposed option for introducing neutralized substances behind the electrodes is not feasible. With this input, it is not possible to achieve the desired spray quality using jet nozzles. The use of pneumatic nozzles is impossible due to the small amount of air introduced into the installation during the implementation of the pyrolytic method.

The reactor proposed by the authors of this method is a stainless steel tank with an internal refractory lining of kaolin fiber material with a volume of 2 m ³ . The residence time of the pyrolysis products is about one second at a temperature of 900-1200 ° C. In these conditions:

- one should expect intense corrosion of the walls of the reactor during the interaction with them of hydrogen halides and other halogen compounds;

- the temperature regime adopted in the reactor does not guarantee the absence of highly toxic secondary substances in the pyrolysis products, such as dioxins, fluorine and chlorophosgens, benzpyrene, and others;

- a large reactor volume significantly increases the dimensions, weight characteristics, metal consumption and installation cost.

The prototype for the proposed plasmochemical method for the neutralization of gaseous and liquid organohalogen substances and waste containing them is the method described in the patent (Russian Patent RU No. 2105928 C1, 6 F 23 G 7/00 of 02/27/1998). It is devoid of many of the disadvantages of the above methods for the destruction of various chemical wastes and most of all corresponds to the conditions of industrial processing of halogen-containing substances.

Its essence lies in the fact that organohalogen substances and the wastes containing them are preheated to a temperature not exceeding the limit of their thermal stability, after which they are sprayed with a stream of hot air at a temperature above the boiling point of the waste, and the resulting vapor-air mixture is sent to an air plasma jet, where carry out pyrolysis at a temperature of at least 1500 ° C, a residence time in the reaction zone of 2-10 ms, with an excess of air necessary for the complete oxidation of carbon. Next, the pyrolysis products are quenched and neutralized with an aqueous alkali solution.

Preheating makes it possible to neutralize high-viscosity liquid waste. The vapor-air mixture is introduced into the reactor behind the plasma torch electrodes. In this case, the corrosive effect of the pyrolysis products on the electrodes is completely excluded, and the plasma heater using air as a plasma-forming gas operates at the nominal mode corresponding to the optimal operating conditions. Thanks to the preliminary heating, evaporation and mixing of the waste with air before entering it into the reactor, it is possible to reduce the necessary residence time of the reacting mixture in the high-temperature zone of the reactor to 2-10 ms and, accordingly, significantly reduce its dimensions.

The preheating temperature should not exceed the limit of thermal stability of the neutralized substances. Otherwise, as a result of low-temperature pyrolysis, a significant amount of soot and corrosive substances can form. This can lead to failure of the system for supplying neutralized substances to the reactor.

The temperature of the pyrolysis products at the outlet of the reactor in front of the quenching zone is set not lower than 1500 ° C, which ensures the absence of dioxins, fluorine and chlorophosgens and other highly toxic substances in the exhaust gas.

The effectiveness of this plasma-chemical method was confirmed by the neutralization of various ozone-depleting chladones, polychlorinated biphenyls and chemical wastes in a pilot plant.

However, during the operation of this installation, intended for the processing of various organohalogen substances, using the considered method revealed its disadvantages. These include the following:

1. The use as an oxidizing agent of air containing about 23 wt.% Oxygen is not beneficial for the neutralization of substances containing a significant amount of carbon, for example polychlorinated biphenyls containing more than 40% carbon. In this case, a large amount of air must be supplied to their oxidation, in the composition of which about 77 wt.% Nitrogen. For the pyrolysis process, nitrogen is the ballast. Its heating has to spend a large amount of thermal energy.

2. During the pyrolysis of organohalogen substances, along with carbon oxidation, the process of binding of halogens to hydrogen halides occurs. Full and reliable implementation of this process is possible only with an excess of hydrogen entering the reaction zone. The considered plasma-chemical method does not provide for the fulfillment of this condition.

3. When mixing in the inlet part of a small-sized reactor neutralized substances and air with a plasma jet having a temperature much higher than 5000 ° C in the core of the stream and being conductive, there is a danger of touching the conductive core of the plasma jet with the reactor wall and shorting an arc discharge to it. In this case, the wall will begin to melt and collapse.

4. The electrodes in arc plasma heaters, in which air is used as a plasma-forming gas, have a shorter service life than electrodes operating in a neutral medium, for example, in a nitrogen medium. Therefore, the use of air as a working gas in plasmatrons designed for continuous continuous operation in industrial plants is not beneficial, since these plants must often be stopped to replace the electrodes.

5. During long-term processing of organohalogen substances containing a large amount of halogens (for example, polychlorinated biphenyls containing more than 50% chlorine), the aqueous alkaline solution rapidly saturates with the appropriate salts and precipitates in the neutralization system, which leads to the need to stop a process for removing said salts.

When designing industrial plants intended for the processing of various halogen-containing substances, it turned out that the method of plasma chemical neutralization, adopted as a prototype, due to the above disadvantages cannot ensure long-term and reliable operation of the plants with the necessary efficiency of the neutralization process. In this regard, the considered method requires significant changes, allowing it to be used in an industrial environment.

The objective of the invention is to remedy all of the above disadvantages related to the prototype. This is achieved by introducing into the plasma-chemical method the following fundamental changes:

1. Instead of air, water vapor containing almost 89 wt.% Oxygen and hydrogen is used as an oxidizing agent for the pyrolysis of neutralized substances, the amount of which with a large margin ensures the process of complete binding of halogens to hydrogen halides.

2. Nitrogen is used as a plasma-forming gas in the plasmatron, which can significantly increase the life of the electrodes and reduce the cost of operation of the plasma torch.

3. To eliminate the possibility of destruction of the reactor from the impact on its wall of the arc discharge, its inlet part is electrically insulated from all units of the installation connected to it.

4. The system neutralization system is closed without draining the spent alkaline solution into the external environment. A continuous centrifuge is used to separate the precipitated salts, and the circulation of the alkaline solution in a closed loop is carried out by monitoring the alkali content in the solution and strengthening it with alkali to the required concentration.

Thus, the essence of the invention lies in the fact that not air, but water vapor is used as an oxidizing agent of the neutralized substances, which, due to a significant reduction in the required oxidizing agent consumption, can significantly reduce energy consumption for the pyrolysis process while fully supplying it with the necessary amount of oxygen and hydrogen. Nitrogen is used as a plasma-forming gas, which significantly increases the service life of the plasma torch electrodes and reduces its cost of operation. The upper part of the reactor is electrically insulated from all parts of the unit connected to it, which eliminates the danger of shorting the arc discharge of the plasma torch to the walls of the reactor, leading to the destruction of the wall. The system for quenching and neutralizing the pyrolysis products is carried out closed, working with the continuous release of salts precipitated in an alkaline neutralizing solution, as well as with the necessary strengthening of the solution with alkali and adding water to it, which allows the unit to operate for a long time without draining the spent alkaline solution into the external environment.

The drawing shows a schematic diagram of the proposed plasma-chemical method of neutralizing gaseous and liquid organohalogen substances and waste containing them, which can be used for industrial applications.

Before the neutralized substances are fed into the reactor, they are heated in the heater (1) to a temperature not exceeding the limit of their thermal stability. Overheating of water vapor is carried out in the heater (3) to the same temperature. Mixing of these components takes place in a vortex mixer (2). The resulting mixture enters the inlet of the reactor (6), which is isolated with the help of electrical insulators (5) from all other units of the installation. A stream of nitrogen plasma is also fed into the inlet of the reactor from a plasma torch (4). The pyrolysis products from the reactor (7) are sent to the quenching and neutralization unit (8), where they undergo a sharp cooling and neutralization of hydrogen halides with an aqueous alkaline solution with the formation of the corresponding salts.

The mixture of gaseous and liquid products from the quenching unit enters the separator (9), in which the gas and liquid phases are separated. Gaseous products with residual vapor and liquid droplets are fed into a water-cooled heat exchanger (15), and then into a cyclone-droplet separator (16). In these devices, the final separation of the liquid from the gas and their further cooling takes place.

Environmentally friendly gaseous products are discharged into the external environment through a sanitary treatment absorber (17).

All spent alkaline solution from the separator, heat exchanger and cyclone-droplet separator is discharged into a receiving tank (11). A device (10) is installed on the drain line, which records the concentration of alkali in the spent alkaline solution. If the alkali concentration drops below the permissible value, using the regulator (20) an additional amount is added to the tank (19). From the tank (11) using a centrifugal pump (12), the spent alkaline solution is fed to a centrifuge (14), where the precipitated salts are separated and loaded into a transport container (13) for delivery to disposal. The filtered solution is poured into a container (19), to which, if necessary, water is added using the regulator (21). The alkaline solution of the required concentration from the tank (19) with the help of a metering pump (18) enters the hardening and neutralization unit (8).

The proposed plasma-chemical method for the neutralization of gaseous and liquid organohalogen substances and the wastes containing them was used when the ozone-depleting freon 12, a mixture of freon 13 and 14 with air simulating industrial wastes from one of the chemical industries, and Sovtol 10, consisting of 90% pentachlorobiphenyl, were destroyed at the pilot plant C ₁₂ H ₅ Cl ₅ ) and 10% trichlorobenzene (C ₃ H ₃ Cl ₃ ).

Table 1 shows the main parameters of these processes in some modes.

The advantage of the proposed plasma-chemical method for the disposal of organohalogen substances over the method adopted for the prototype can be seen by comparing the main parameters of the process for the disposal of Sovtol 10, which was carried out on the same experimental plasma-chemical installation, but using different oxidizing agents (air and water vapor) and plasma-forming gases (a mixture of air with nitrogen and nitrogen). Table 2 shows the main parameters of the processes of neutralization, implemented at close values of the power of the plasma torch and the same mass flow rates of plasma-forming gases. In the compared modes, there was some difference between the coefficients of excess oxygen and the temperatures of the pyrolysis products in the reactor. But these differences are small and can be considered insignificant.

The main conclusion that follows from a comparison of the parameters given in table 2 is that replacing air during the disposal of sovtol 10 with water vapor leads to a significant (1.57-fold) decrease in specific energy consumption.

The proposed plasma-chemical method for the neutralization of gaseous and liquid organohalogen substances and the wastes containing them was used in the design and construction of an industrial plasma-chemical plant designed to destroy the waste from a polytetrafluoroethylene plant. The waste from this plant contains up to twelve different ozone-depleting chladones and other fluorine and organochlorines. Currently, the proposed method is undergoing pilot testing at this facility. It confirms the reliability, efficiency and environmental safety of the disposal of these industrial wastes using the method that is the subject of this invention.

Claims (4)

1. Plasma-chemical method of neutralizing gaseous and liquid organohalogen substances and waste containing them, including preheating these substances to a temperature not exceeding the limit of their thermal stability, evaporating them and mixing with an oxidizing agent, feeding them to a plasma jet, quenching and neutralizing an aqueous alkaline solution of pyrolysis products characterized in that superheated water vapor is used as the oxidizing agent and the vapor-gas mixture is supplied to the electrically insulated inlet part of the reactor in a nitrogen stream oh plasma. 2. The plasma-chemical method according to claim 1, characterized in that the evaporation and mixing of the organohalogen substances and the waste containing them with water vapor is carried out at a temperature above the boiling point of the neutralized substances. 3. The plasma-chemical method according to claim 1, characterized in that the pyrolysis of organohalogen substances and waste containing them is carried out at a temperature of at least 1500 ° C and with an excess of oxygen contained in water vapor. 4. The plasma-chemical method according to claim 1, characterized in that the quenching and neutralization of the pyrolysis products is carried out continuously with an aqueous alkaline solution without discharging the spent solution into the external medium while separating the precipitated salts from it and strengthening it with alkali to the required concentration.

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