RU2113874C1 - Method for destruction of highly toxic organic compounds - Google Patents

Abstract

FIELD: thermal decomposition of organic substances; may be used in destruction of wastes. SUBSTANCE: method includes pyrolysis without presence of oxygen-containing gases with subsequent cleaning of produced pyrolysis gas with solid alkali adsorbent. Pyrolysis is conducted in one stage under cyclic conditions up to preset value of decomposition of organic substance with use of regenerative heat exchange on fixed heat-accumulating attachment. Pyrolysis gas after cleaning with solid adsorbent is additionally cleaned with organic solvent and after burning, liberated heat is returned to pyrolysis process for heating of fixed attachment. EFFECT: higher efficiency. 2 dwg , 2 tbl

Description

 The invention relates to methods for thermal decomposition of organic compounds and can be used for waste disposal in the chemical and several other industries; for the destruction of various classes of toxic substances (OM).

 Methods of thermal decomposition of organic compounds based on combustion have several disadvantages: insufficient environmental purity, insufficient reliability, high level of capital and operating costs.

According to [1], when burning 1 ton of mustard gas at the installation of AO "Lurgi", 4.4 tons of standard fuel, 3.2 tons of NaOH, 6250 kWh of electricity are consumed. 94,000 nm ^{3 of} contaminated SO ₂ and HCl flue gases are formed, the purification of which is complicated by the parallel absorption of CO ₂ and results in 83 nm ^{3 of} contaminated SO ₂ and HCl flue gases, the purification of which is complicated by the parallel absorption of CO ₂ and results in 83 nm ^{3 of} salt containing Na ₂ SO ₃ , NaCl and NaCO ₃ . It is noted that in situations leading to the cessation of fuel combustion in the furnace, system pollution and the release of undecomposed compounds into the atmosphere are inevitable. In this regard, to increase reliability, it is recommended [2] to accumulate flue gases in gas tanks and to discharge them into the atmosphere after a control analysis. The implementation of such a recommendation with a flue gas volume of 94,000 nm ³ / t will require a significant increase in capital costs.

Insufficiently high decomposition rates of 99.995% - 99.997% [2], despite the use of high temperatures of 1000 ^o C - 1100 ^o C, significantly complicate the selection of materials that are stable under these conditions and reduce the reliability of the furnace [3].

 It was found [4] that combustion plants similar to the installation of JSC Lurgi pollute the environment with highly toxic oxygen-containing compounds such as dioxins and furans. The formation of these compounds is associated with the presence of free oxygen in gases at all stages of the process and therefore cannot be eliminated in the framework of methods based on combustion.

 The closest in technical essence is the method of thermal decomposition of heteroatoms-containing organic compounds without the use of oxygen-containing gases, based on two-stage pyrolysis with water vapor [2].

 In FIG. 1 shows a process diagram.

In the first stage, at a pyrolysis temperature t _p = 500 - 600 ^o C, a partial decomposition of the compound occurs in the simultaneous adsorption of the resulting acid gases by a circulating solid alkaline adsorbent. In the second stage, a deep decomposition of the compound is achieved at t _p = 1150 - 1500 ^o C in an apparatus with electric heating made of heat-resistant ceramics. The final purification of the pyrolysis gas occurs on activated carbon and zeolites. The use of pyrolysis without oxygen-containing gas eliminates a number of disadvantages of the method [1]. For example, pyrolysis of 1 ton of mustard gas according to the equation S (CH ₂ -CH ₂ C) ₂ + 4H ₂ O = H ₂ S + 2HCl + 4CO + 6H ₂ gives 1840 nm ^{3 of} pyrolysis gas to be purified from H ₂ S and HCl, or ≈ 50 times less than at the installation of JSC "Lurgi". In addition, there is no CO ₂ in the gas. All this dramatically reduces the cost of gas purification, for example, the consumption of alkali in 2 - 2.5 times. Most of the salt waste is obtained in solid form, which reduces the amount of salt flow. The absence of free oxygen in the pyrolysis gas eliminates the formation of dioxins and furans.

 At the same time, the method [2] has drawbacks, some of which may limit or completely exclude its use, especially for the decomposition of highly toxic compounds.

During the tests described in [2], it was found that due to the low thermal conductivity of heat-resistant ceramics, temperature gradients arise in the walls of the tubes of the reactor of the second pyrolysis stage, leading to their cracking and, consequently, to depressurization of the apparatus. The low thermal conductivity of ceramics can also cause high costs for second-stage equipment and seriously complicate the creation of industrial equipment with a sufficiently high productivity. The separation of the pyrolysis process into two stages and the circulation of the solid adsorbent in the first stage under conditions of incomplete decomposition of compounds further reduces the reliability of the process. The possibility of using the combustion heat of purified pyrolysis gas, which is potentially sufficient to compensate for the endothermic thermal effect of the pyrolysis process ≈ 10 • 10 ⁶ kcal / t of substance, which is compensated by electricity, was not found. As a result of the analysis of various options, the accumulation of pyrolysis gas in a gas tank and, after a control analysis, burning on a candle was adopted in [2].

 The purpose of the invention is to develop a method for thermal decomposition of heteroatom-containing organic compounds based on their pyrolysis without oxygen-containing gases and at the same time free from the considered disadvantages of the method [2].

 This goal is achieved by developing a single-stage cyclic pyrolysis process without oxygen-containing gas with a regenerative heat exchanger on a fixed nozzle, heated mainly by the heat of combustion of the purified pyrolysis gas; the development of a two-stage process for the purification of pyrolysis gas with adsorption on the first stage of acidic components by a solid alkaline adsorbent and adsorption of high molecular weight pyrolysis products with an organic solvent on the second; development of a process for burning purified pyrolysis gas, which ensures the use of combustion heat for pyrolysis; increased process reliability and long continuous operation of the pyrolysis reactor.

 The practical use of the proposed process is illustrated by the examples given.

Example 1 (see figure 2). Liquid tributylphosphite (C ₄ H ₉ O) ₃ P (TBP) from vessel 2 is supplied with compressed nitrogen from recipient 1 at a rate of 1000 kg / h to the middle of the pyrolysis reactor 3, which operates in a four-operation cycle at t _p = 800 ^o C The first operation (I) - supply and pyrolysis of TBP and purification of pyrolysis gas, duration 28 minutes, the second operation (II) - purging the volumes of the combustion chamber 6 and reactor 3 with purified pyrolysis gas, duration 2 minutes, the third operation (III) - burning purified pyrolysis gas in the combustion chamber and heating the nozzle of the reactor 5, duration 28 mi n, the fourth operation (IV) - purging the volumes of the combustion chamber 6 and the pyrolysis reactor 3 with purified pyrolysis gas, after which the cycle repeats.

For the initial heating of the reactor 3 from the tank 12 through the gas holder 13 in the combustion chamber 6 serves propylene. Air blower 14 is supplied to the reactor. By adjusting the air-gas ratio, the combustion temperature in the combustion chamber is maintained at ≈ 1000 - 1100 ^o C. The resulting flue gases pass through the nozzle 5, heat it and leave the reactor with a temperature of 800 ^o C and are released into the atmosphere through valve 3.

Once on the nozzle, the TBP evaporates and its vapor, moving from top to bottom on the nozzle 5, is heated and pyrolyzed by heat exchange with it. The pressure at the top of the reactor is approximately 1000 mm H ₂ O. At the beginning of operation I, the temperature of the entire nozzle is ≈ 800 ^o C and pyrolysis proceeds over its entire height with a residence time of 10 s. The degree of decomposition of the TBP in this case is ≈ 100% (estimated by extrapolating the experimental data obtained in the packed pyrolysis reactor in the range of t _p = 700 - 1050 ^o C and τ _p = 1-3 s using the equation α = 1-exp (-K • t), where K is the experimental value of the thermal decomposition rate constant of TBP at 800 ^o C, which is 5.5 s ^-1 ).

At the end of operation I (Fig. 2 temperature curve "b"), the active nozzle layer Δh ₂ = 0.5-0.7 m is located in the lower part of the reactor, in front of the nozzle safety layer. After the safety layer, the degree of decomposition of TBP reaches α = 99.99999 (residence time τ _p = 3 s). At the end of operation I, a sample of pyrolysis gases is taken at the outlet of the reactor for analysis on TBP. Thus, during the entire time of operation I, safe conditions for the treatment of pyrolysis gas in the subsequent stages are ensured for any toxicity of the decomposable compound.

The pyrolysis gas obtained during operation I has an average composition, vol. %: H ₂ ≈7, CH ₄ ≈20, C ₂ H ₄ ≈48.5, C ₂ H ₂ ≈14, (P ₂ O ₃ + P ₂ O ₅ ) ≈10, aromatic and high molecular weight compounds - 0.5 .

After the pyrolysis reactor 3, the pyrolysis gas (400 nm ³ for step I) is cooled due to heat losses in the duct to 700 ^o C and enters the purification unit from (P ₂ O ₃ + P ₂ O ₅ ) 7. Adsorption of the main amount (P ₂ O ₃ + P ₂ O ₅ ) flows in the lower layers of the nozzle of apparatus 7 at t ≈ 700 ^o C. Under these conditions, condensation or transition to the solid phase (P ₂ O ₃ + P ₂ O ₅ ) is excluded. In the upper layers of the nozzle, the partial pressure (P ₂ O ₃ + P ₂ O ₅ ) is reduced, and at t = 400 - 250 ^o C the deposition of oxides on the nozzle also does not occur. In apparatus 7, moving from bottom to top through a nozzle layer of pieces of quicklime (CaO) with sizes of 10-15 mm, the gas is purified from phosphorus oxides to a residual content of 1 mg / m ³ . Quicklime is loaded in batches, and the spent lime is discharged with the help of lock feeders 8. Lime consumption for operation I ≈ 250 kg with a degree of use of ≈ 70%.

The pyrolysis gas purified from phosphorus oxides with a pressure of 600 mmH ₂ O and a temperature of 250 ^o C is fed to the scrubber 10 for cleaning aromatic and high molecular weight compounds, irrigated with solar oil ≈ 10 kg during operation I, where the gas is cooled to 30 - 35 ^o With water passing through a coil laid on plates. A solution of aromatic and macromolecular compounds in solar oil of approximately 20 kg during operation I is accumulated in a container 11 and periodically burned during operation III.

The purified pyrolysis gas during operation I accumulates in the gas tank 13, the capacity of which is 600 m ³ and a pressure of 400 mm H ₂ O.

 When, as a result of heat consumption by the pyrolysis process, the temperature along the nozzle height 5 will be characterized by curve "b", operation I is completed by closing valves A and B. To remove the remaining TBP pyrolysis products, operation II is used - purging the volumes of the combustion chamber and pyrolysis reactor with purified pyrolysis gas. As a result of purging, the remaining part of the OB vapor in the reactor passes through the safety layer of the nozzle, completely decomposing (α = 99.99999%). A purge time of ≈ 2 min is carried out by closing valve B. Operation II ends when the volumes of the combustion chamber 6 and pyrolysis are filled with purified gas. Since the gas moves almost in the ideal displacement mode in the volume filled with the nozzle, the required gas volume is also almost equal to the sum of the volumes of the combustion chambers 6 and pyrolysis, and the time of operation II is equal to the time of supplying this gas volume - 2 min.

For operation III, with continued supply of purified pyrolysis gas through valve G, close valves D and G and open valves E and Z. Blower 14 and gas blower 15 operate continuously throughout all operations. By controlling the air-gas ratio, the combustion temperature in the combustion chamber 6 is maintained at ≈1100 ^° C. The resulting flue gases pass through the nozzle 5, heat it, provide combustion of the carbon deposited during operation I, and leave the reactor at a temperature of 800 ^° C. During operation III, the pressure at the top of the pyrolysis reactor is ≈ 1000 mm H ₂ O, in front of the valve is 3 ≈ 100 mm H ₂ O, and atmospheric pressure is at the end of the discharge pipe.

 After completion of operation III, i.e. obtaining a temperature profile (Fig. 2 curve "a"), to remove flue gases, carry out step IV — purging the volumes of the combustion chamber 6 and reactor 5 with purified pyrolysis gas (2 min). To do this, close valve E and open valve D with continued gas supply through valve G. After completion of step IV, open valve B, close valves G and 3, open valves A, B and G, i.e. resume the supply of TBP, and the cycle repeats.

The spent lime, discharged during operation I from apparatus 7, is a mixture of CaO with phosphate salts of Ca, which does not contain organic impurities, since the adsorption process is carried out in the temperature range of 700 - 600 ^o C. After cooling, the spent lime can be packaged in paper bags and buried.

возможен в диапазоне t_п = 700 - 750^oC, т.к. распад молекулы начнется по связи C-P, энергия которой ≈65 ккал/моль, на 10 ккал/моль меньше энергии связи C-O ≈ 75 ккал/моль в молекуле ТБФ.The second waste product - flue gases, as experiments have shown, will be a mixture of H ₂ , CO ₂ , O ₂ containing ≈ 0.1 mg / m ³ (P ₂ O ₃ + P ₂ O ₅ ) and ≈ 10 mg / m ³ (NO + NO ₂ ). Other phosphorus-containing organic compounds can be decomposed in a similar manner. Some differences in the operation mode of the installation will be associated with different ratios of organic and inorganic parts of the molecule, the presence of a different set of heteroatoms, different rates of thermal decomposition, etc. For example, pyrolysis

possible in the range t _p = 700 - 750 ^o C, because The decay of the molecule will begin via the CP bond, whose energy is ≈65 kcal / mol, 10 kcal / mol less than the binding energy CO ≈ 75 kcal / mol in the TBP molecule.

 At the stage of gas purification with CaO, in addition to phosphate salts, in this case, monofluorophosphates and other similar fluorine-containing calcium salts will be obtained, which are also non-toxic and can be buried.

 Due to the smaller proportion of the organic part of the sarin molecule compared to TBP, the installation load will be possible to increase by 50%.

распад молекулы начнется по связи C-S, энергия которой 55 ккал/моль, и с достаточной полнотой будет протекать при температурах t_п = 650 - 700^oC. При этом в пиролизном газе будут содержаться соединения типа нитрилов, т.к. образование цианистых соединений происходит при t_п > 750^oC. При сжигании газов пиролиза нитрилы будут разлагаться до элементарного азота. Группа, содержащая фосфор, кислород и серу, при t_п = 650 - 700^oC не будет разлагаться и на стадии очистки, связываясь с CaO, будет давать смесь, содержащую фосфорнокислые соли кальция и тиофосфаты.In the pyrolysis of VX gases -

The decomposition of the molecule will begin via the CS bond, whose energy is 55 kcal / mol, and will proceed with sufficient completeness at temperatures t _p = 650 - 700 ^o C. In this case, compounds of the nitrile type will be contained in the pyrolysis gas, since the formation of cyanide compounds occurs at t _p > 750 ^o C. When burning pyrolysis gases, nitriles will decompose to elemental nitrogen. The group containing phosphorus, oxygen and sulfur, at t _p = 650 - 700 ^o C will not decompose and at the stage of purification, binding to CaO, will give a mixture containing phosphate salts of calcium and thiophosphates.

из емкости 2, необходимое давление в которой поддерживается подачей азота из реципиента 1, со скоростью 2000 кг/ч подают в среднюю часть реактора пиролиза 3. В связи с низкой энергией связи S-C≈ 55 ккал/моль пиролиз с достаточной скоростью будет протекать в диапазоне t_п + 550 - 650^oC. На стадии очистки газа от кислых компонентов H₂S и HCl оказывается необходимым использовать адсорбент, содержащий Na, например натровую известь, т. к. в случае использования чистого CaO образующийся CaCl₂ может обусловить разжижение смеси солей за счет поглощения атмосферной влаги.Example 2. Mustard

from the vessel 2, the necessary pressure in which is maintained by the supply of nitrogen from the recipient 1, at a speed of 2000 kg / h is fed into the middle part of the pyrolysis reactor 3. Due to the low binding energy SC ≈ 55 kcal / mol, pyrolysis will proceed at a sufficient rate in the range t _p + 550 - 650 ^o C. At the stage of gas purification from acidic components of H ₂ S and HCl, it is necessary to use an adsorbent containing Na, for example, soda lime, because in the case of using pure CaO, the resulting CaCl ₂ can cause dilution of the salt mixture for due to atmospheric absorption in AGI.

The rest of the process is carried out as described in example 1. It should be noted that the mustard pyrolysis, as well as the pyrolysis of other OM, is proposed to be carried out at a pressure in the apparatus slightly higher than atmospheric pressure, while it is generally recommended [1], [4] to have a somewhat below atmospheric. The decision is not required for the method under consideration. All the considered processes of thermal decomposition of organic compounds containing heteroatoms can be carried out under a pressure below atmospheric. For this, it would only be necessary to install a gas blower that would supply gas from the apparatus 10 to the gas holder 13, creating a pressure drop of ≈ 2000 mm H ₂ O. At the same time, at the top of the pyrolysis reactor there would be 500 mm H ₂ O below atmospheric pressure. Nevertheless, the processes based on pyrolysis, it is advisable to carry out under a pressure slightly higher than atmospheric, in view of the following.

In this example, as a result of pyrolysis of 1000 kg of mustard gas, a total of 700 nm ^{3 of} pyrolysis gas is obtained. Even a small air leak will lead to a significant concentration of free oxygen and can cause the formation of dioxins and furans, gas fire or explosion. At the same time, the pyrolysis mode virtually eliminates the possibility of a breakdown of undecomposed OB into the atmosphere even in emergency situations.

For example, at the end of operation I, valve B did not close, and for some reason, valve E turned out to be open and gas blower 14 turned off. A vapor stream OB flows through the nozzle to valve E and beyond. Since the temperature of the nozzle is ≈ 1100 ^o C (curve "b", Fig. 2), the complete decomposition of the OBs of the classes considered will occur in 0.01 - 0.02 s and completely safe gas will enter the valve E and then. As long as the temperature of the entire packing layer does not drop below 800 ^o C (10-15 minutes), a breakdown of the undecomposed OB will be impossible. If the vapor flow of OB is directed downward, then decomposition will occur at the lower safety level of the nozzle. The proposed method of pyrolysis allows you to organize reliable control of the tightness of the reactor vessel and the integrity of the lining by controlling the temperature of the shell over its entire surface.

 It has already been noted above that the degree of decomposition provided during operation I determines the safety of all further operations with the pyrolysis gas.

 In the table. 1 shows the performance of thermal decomposition of heteroatoms-containing organic compounds at a plant capacity of 500 kg / h.

 Thus, the option of working with pressure slightly higher than atmospheric is more appropriate from all points of view. The above are some indicators of the considered methods of thermal decomposition of organic compounds.

The data table. 1 show that the proposed method is the least capital- and energy-intensive, capital costs ≈ 4 times lower than in methods [1] and [2], with virtually no need for an external energy source, while the energy consumption for the installation [2] is 2.4 • 10 ⁹ rubles. / g and the fuel consumption at the installation [1] is 1.3 • 10 ⁹ rubles / g. The cost of acquiring alkali will be reduced by 6 • 10 ⁹ rubles / g. At lower costs, the proposed method provides a deeper, by 3 - 4 orders of magnitude, decomposition of compounds at lower temperatures of 300 - 600 ^o C compared with the methods [1], [2]. The ability to work at low temperatures increases the reliability of the process and provides the use of cheaper and less scarce materials for the manufacture of equipment. In comparison with the method [1], in addition to the aforementioned, the discharge of contaminated gases and salt effluents is sharply reduced and the content of dioxins and furans in these effluents is completely eliminated. The simplicity of the design of the main apparatuses, the high intensity of regenerative heat transfer make it possible to create apparatuses of large unit power and thereby further improve the technological parameters of thermal decomposition units of organic compounds.

 In the table. 2 shows the exciliation of the equipment of FIG. 2.

Claims (1)

 A method of destroying highly toxic organic compounds by pyrolysis without access of oxygen-containing gases, followed by purification of the resulting pyrolysis gas with a solid alkaline adsorbent, characterized in that the pyrolysis process is carried out in one step in a cyclic mode to a predetermined value of the degree of decomposition of the organic compound using regenerative heat transfer on a stationary heat-accumulating nozzle, and the pyrolysis gas after purification with a solid adsorbent is additionally purified with an organic solvent m and after combustion, the generated heat is returned to the pyrolysis process to heat the fixed nozzle.

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