RU2211210C1 - Method for preparing tetrafluoromethane and device for its realization - Google Patents

Abstract

FIELD: chemical technology. SUBSTANCE: invention relates to preparing tetrafluoromethane used as a solvent, foaming agent, in production of plastics, as dry etching agent of electronic circuits. Method is performed by interaction of carbon material with fluorinating gas at temperature 700-1200 C. Powder-like graphite with granulometric composition 0.8-2.5 mm is used as carbon material. Process is carried out in continuous regimen at constant control of temperature in the point on distance from center of reaction zone equal to 1.5-2.0 diameter of the reaction zone. Feeding fluorinating gas is ceased at temperature in this point 100-150 C. Then all mass of carbon material is subjected for stirring and process is restored at decrease of the carbon material temperature to 60-70 C. Process is carried out into reactor fitted with cooling jacket and nipple for the carbon material charge, connecting pipes for addition of fluorine and gas self- igniting from contact with fluorine, nipple for removal of technological gas and the dust trap. Reactor is fitted also with agitator of carbon material and the thermocouple pocket. Lower end of the thermocouple pocket is installed on the distance from center of reaction zone equal to 1.5-2.0 diameter of the reaction zone. Invention provides reducing consumption of reagents and liquid nitrogen, enhanced output, stabilized content of limited impurities in the end product. EFFECT: improved preparing method. 6 cl, 1 dwg, 1 tbl

Description

 The invention relates to a technology for the production of organofluorine compounds, and specifically, to the production of tetrafluoromethane by reacting a carbon material with fluorine.

Tetrafluoromethane (CF ₄ , R-14) is a gaseous substance, widely used as a solvent, foaming agent, in the production of foams, as a dry etchant of electronic circuits.

It is known that the interaction of a carbon material with fluorine at temperatures up to 630 ^o С forms mainly solid fluorinated carbons, above 630 ^o С - only gaseous products CF ₄ , C ₂ F ₆ , etc. (J. of Fluorine Chemistry, 46, 1990, 461-477, F. Nakajima et al. "Preparation, structure and reduction of certain intercolation compounds to graphite").

There is a known method of producing CF ₄ by burning degassed soot in a fluorine stream, then crude CF _{4 is} condensed at minus 183 ^° C, pumped out to remove dissolved gases, then passed through several washes filled with 20% KOH. At the same time they are freed from impurities F ₂ CO, SiF ₄ and HF, then they are freed from moisture over P ₂ O ₅ , condensed and fractionated again to separate from homologues (C ₁ F ₆ , C ₃ F ₈ ), then pumped for several hours at a temperature liquid nitrogen - remove dissolved air (Guide to inorganic synthesis. Editor G. Brower, t.1, M., Mir, 1985). Laboratory method.

There is also known a method of producing CF _{4 by the} interaction of carbon with fluorine at a temperature of 700-1200 ^o C, which is created and maintained by supplying a gas self-igniting in contact with fluorine to the reaction zone (RF patent 2117652, 1997) - prototype.

 According to this method, the process is as follows.

In a nickel reactor equipped with a cooling jacket, a pipe for loading carbon material, fittings for supplying fluorine and gas self-igniting from contact with fluorine, a fitting for discharging reaction gas, carbon material is loaded - powdered graphite (up to 1.5 tons), the supply of fluorine and propane is opened. After 3-5 minutes, the propane supply is stopped. The resulting process gas passes through the carbon material from the reactor to the dust collector and is sent to the impurity capture system (pressure in the system does not exceed 0.6 atm), including adsorbers with CPI and zeolite, and then to condensation at a temperature of minus 150-160 ^o С. The filled capacitor is removed from the circuit, the process gas is supplied to the backup capacitor. Condensed gas from the filled condenser is subjected to “training” (pumping) at a temperature of minus 150-160 ^o С from the gases dissolved in it (nitrogen, oxygen, carbon monoxide and others), which are discharged into the ventilation system, after which tetrafluoromethane freed from impurities is packed into receiving containers - transport containers. The process is conducted continuously with a stop of fluorine supply for 1-2 hours per day to cool the contents of the reactor. The method provides for the production of CF ₄ with a basic substance content of not less than 99 vol.%, But has the following disadvantages.

Large dust collector of incandescent small particles of carbon material, accompanied by clogging of dust collecting devices and communications; fluorine slip into the process gas purification system from impurities; periodic collapse of the mass of graphite in the reaction zone, leading to the shutdown of the process and to the unloading of the contents of the reactor to clean clogged gas supply fittings in the reactor; a significant range of content of limited impurities in the final product, up to the content of the above technical requirements; increased consumption of reagents (fluorine, CPI, zeolites) and liquid nitrogen; increased corrosion rate of equipment up to burnout; increased losses of CF ₄ during the "training" of the condensed process gas, low rate of removal of impurities from the condensate; low productivity.

 An object of the invention is to stabilize the process, reduce the cost of reagents and liquid nitrogen, increase productivity, stabilize the content of limited impurities in the final product - technical refrigerant R14.

The problem is solved in that when receiving tetrafluoromethane by a method involving the interaction of a carbon material with fluorine at a temperature of from 700 to 1200 ^o C, which is created by supplying combustible gas to the reaction zone, and purification of the process gas from impurities using sorption, condensation and distillation of impurities from condensed process gas, followed by packaging of the finished product in receiving transport containers, a carbonaceous powder mate is used as a carbon material rial (mainly graphite) with a particle size of 0.8-2.5 mm, the process is conducted with continuous monitoring of the temperature of the carbon material at a distance from the center of the reaction zone equal to 1.5-2.0 diameter of the reaction zone, the process is stopped when the temperature reaches this point is 100-150 ^o С, after which the carbon material is turned in the reactor and the process is resumed when the temperature at the control point drops to 60-70 ^o С; anode gas of a fluorine medium-temperature electrolyzer is used to process the carbon material, water and water-alkaline traps are used to purify the process gas from HF, after which the gas is cooled to a temperature of minus 30 ^o C and sent to an adsorber with zeolite; the solubility of soluble in condensed gas impurities is carried out at a temperature of minus 130 ^o C in the working capacitor of the main technological scheme, the selection of the finished product from the condenser is carried out through a column filled with adsorbent (zeolite).

 The rationale for the proposed method is as follows.

1. A hollow sphere is formed in the carbon material located in the reaction zone of the reactor, the diameter of which depends on the geometric dimensions of the reactor, on the resistance of the layer of carbon material to the gases leaving the reaction zone, and on the rate of fluorine entering the reaction zone. The diameter of the hollow sphere calculated by us turned out to be 0.15-0.20 m and approximately corresponded to the actual size established during repeated manual unloading of graphite from the reactor during emergency shutdowns of the process. With the optimal course of the fluorination process, the constancy of the size of the reaction zone is ensured by the continuous flow of graphite into the combustion zone. The process goes by the equation
C + 2F ₂ = CF ₄ (1)
In the layers of carbon material adjacent to the reaction zone, the temperature decreases and reaches values below 700 ^° C. The reaction gases leave the reaction zone (and this is residual fluorine diluted with tetrafluoromethane and impurities, including HF) through the carbon material and begin to interact with it with the formation of solid fluorocarbons of the type (CF _x ) _n :
2nC + nxF ₂ = 2 (CF _x ) _n (2)
and their thermal decomposition by reaction (3):
(CF _x ) _n = C * + CF ₂ F ₆ + et al. (3)
with the formation of active carbon black (C *, which immediately interacts vigorously with fluorine) and fluorocarbons of various compositions. At the same time, there is a swelling of the particles of the formed solid graphite fluorides with respect to the starting carbon material not less than twice.

 Thus, practically all the processes described in the patents concerning the methods for producing solid fluorocarbons are realized in the layers of carbon material adjacent to the reaction zone (for example, US Pat. No. 2,770,660-1,956; Japanese Patent Application No. 61-35122B, publ. 11.08.86, RF patents 2119448, 1998 and 21707001, 2001).

High temperature reaction gases find their way through separate channels in a layer of carbon material. The carbon material is densified in the layer adjacent to the reaction zone (and in the walls of the channels), which prevents carbon from entering the reaction zone. The lack of graphite in the reaction zone leads to an increase in the size of the reaction cavity due to the burning of carbon along its perimeter, to a decrease in temperature in the reaction zone. Unreacted fluorine may appear in the gas at the outlet of the reactor, which causes accelerated production of CPI with strong local overheating (and equipment corrosion up to burnout), poisoning of adsorbents in the system for trapping impurities from the process gas. The propane supply does not help, but leads to a sharp increase in the content of volatile fluorocarbons with a molecular weight higher than that of CF ₄ . The internal cavity of the reaction zone burns out to critical sizes, the arch of the reaction zone collapses with a mass of carbon material (of the order of 1-1.5 t) located in the reactor, with clogging of the gas supply fittings to the reactor and the reaction gas outlet. The only way out in this case: stopping the process, long-term cooling, unloading of the semi-reacted carbon material from the reactor is very difficult manual labor, it is necessary to hammer a sintered mass like plasticine with a crowbar. Analysis of such a material showed a fluorine content of from 14 to 50 wt.%.

 2. To prevent collapse, it is necessary to stabilize the flow of carbon material into the reaction zone and thereby reduce the burnout of carbon material around the reaction zone to a critical size that causes collapse. This is achieved by using graphite powder with a particle size of 0.8-2.5 mm and an anode gas of a fluorine medium temperature electrolyzer with an HF content of not more than 5-7 vol.%, As well as the action of a tedder in a new device.

We have established the time of continuous operation of the reactor equipped with the agitator in the optimal mode. It is determined by the temperature of the carbon material, measured at a point which is 1.5-2.0 times the diameter of the reaction zone from the center of the reaction zone. And it is 100-150 ^o С. At a higher temperature, the concentration of heavier volatile fluorocarbon compounds (C _n F _{2n + 2} with n = 2, 3, 4 or more) in the gases leaving the reactor increases, and explosions (pops) may occur ) in communications for the removal of gases from the reactor, conditions are created for the collapse of graphite in the reaction zone.

Stopping the process and tedding the carbon material contribute to the cooling and equalization of the temperature of the carbon material, the destruction of the compacted layers of carbon material around the perimeter of the vent channels and the reaction cavity. At a temperature of 60-70 ^o With resume the flow of fluoride. The process in this case resumes without propane. At a temperature below that specified, a propane supply is necessary for starting, which increases the content of C ₂ F ₆ , C ₃ P ₈ , CO ₂ in the process gas.

 The particle size distribution of carbon raw materials (0.8-2.5 mm) is essential. With a smaller particle size, we have significant pulverization of the smallest fraction in a hot state, clogging of the communications of the process gas. In the dust collection system at elevated temperatures, the reactions of formation and decomposition of impurity compounds continue, their content in the gas increases. With a larger particle size, the heterogeneous reaction of fluorine with carbon decreases due to a decrease in the gas-solid contact surface per unit volume of solid.

3. The composition of the fluorine gas supplied to the reactor is of great importance for stabilizing the combustion process. The anode gas of the fluorine medium temperature electrolyzer contains (under normal conditions) 5-7 vol.% HF. It is known (US patent 3872032, 1975) that when the HF content in the fluorinating carbon gas is more than 5 vol. % sharply slows down the rate of formation of solid graphite fluorides. The maximum yield of graphite fluorides occurs when the HF content is at the level of 3-5%. During interaction reactions in the reaction zone, we have fluorine consumption, due to which the volume fractions of fluorine-containing impurities (including HF) increase by at least 2 times. The increased concentration of HF in the exhaust gases has two positive aspects: a) the fluorination temperature of the carbon material in the layers adjacent to the reaction zone decreases, which contributes to the formation of predominantly carbon dicarbonyl fluoride (C ₂ F) _n , which decomposes readily (T _decomp. 375 ^o С), b) the rate of formation of carbon monofluoride slows down, the decomposition of which occurs at a higher temperature (580-600 ^o C) with the release of volatile fluorocarbons several times more in volume than the decomposition of dicarbonofluoride. When the content of HF in the anode gas is more than 7 vol.%, The rate of the main reaction (1) of the formation of CF ₄ decreases.

4. The use of water washing of the process gas followed by washing in an aqueous alkaline solution completely purifies the process gas from HF and, to a large extent, from CO ₂ . The operations of unloading and loading of KhPI are eliminated.

5. Cooling the process gas to minus 30 ^o C helps to remove moisture from it and increase the efficiency of purification from impurities on the zeolite.

6. "Training" at a temperature of minus 130-135 ^o С of the condensed process gas (with the discharge of gases into the working condenser) significantly accelerates the process of distillation of N ₂ , O ₂ , СО impurities soluble in the condensate and allows reducing tetrafluoromethane losses by 5-6%.

 7. The supply of purified (after "training") tetrafluoromethane to the receiving containers through a column of zeolite provides additional purification from moisture, carbon monoxide and hexaforetan.

 To implement the new method, a device is proposed (see the drawing), which is a reactor (1) equipped with a cooling jacket and equipped with a pipe (2) for loading carbon material, fittings for supplying fluorinating gas (3) and propane (4), and a branch pipe (5) for removal reaction products, temperature control point (6) (which can be used as a thermocouple placed in a protective pocket), agitator (7) and lower reactor discharge pipe.

Based on the foregoing, a new process is as follows (see drawing). Graphite powder, previously subjected to sieving, is loaded into the reactor (1) through a pipe (2). Through the supply fittings, the anode gas supply of the fluorine electrolyzer (3) and propane (4) are opened. After 3-5 minutes, the propane supply is closed. The resulting reaction zone is supported by the continuous flow of graphite (by gravity) and the discharge of the resulting process gas through the pipe (5) and the dust collector into the purification and separation system of technical R-14 refrigerant. The process is carried out with constant monitoring of the temperature of the carbon material at the point (6) of the reactor and the content of CO ₂ , C ₂ F ₆ , C ₃ F ₈ in the gas. At a temperature at the indicated point of 100-150 ^o C and the sum of CO ₂ , C ₂ F ₆ , C ₃ F ₈ in a gas above 0.06 vol.%, The process is stopped (the flow of fluorine to the reactor is closed). With the agitator (7), the resulting arch in the combustion zone of graphite is destroyed, as well as the formed and caked channels in the carbon material, and when the carbon material is cooled to 60-70 ^° C, the reactor is started again by supplying the anode gas of the fluorine electrolyzer to work. For this particular reactor, a reduction to this temperature occurs in 8 hours. Based on this, the optimal operating mode of the reactor was established: 16 hours of continuous operation, process shutdown, 8 hours — reactor cooling, including 1 hour — tedding of carbon material. When the 1st reactor is stopped, a standby reactor is launched into the circuit. Thus, three reactors are involved in the circuit: two of them are in operation, the third is for cooling. The gases leaving the rector are sent sequentially to water, water-alkaline (10% KOH) absorbers, then to a heat exchanger with cooling to minus 25-30 ^o С, an adsorber with zeolite, a condenser (temperature minus 150-160 ^o С), after which the exhaust gases are sent to the ventilation system. Upon filling the product with a working condenser, the process gases are sent to the reserve one, which becomes working, and the condenser filled with the product is heated to minus 130 ^o C and pumped out with the discharge of the pumped gases into the working condenser. The finished product (after pumping out impurities) is evaporated and fed through an adsorber with zeolite for filling into receiving transport tanks. When working in optimal mode, relative to the prototype, there is a decrease in fluorine consumption by 12%, carbon material by 7-8%, liquid nitrogen by 12-13%, and productivity increase by 33%.

 Below are examples of the method on the base and new samples of an industrial reactor.

Base sample:
non-sifted graphite, fluorine supply 50 m ³ per hour.

Prototype:
sieved graphite, granular composition 0.8-2.5 mm, fluorine supply 50 m ³ per hour.

 The composition of the final refrigerant product R-14 is presented in the table.

From the table it follows that the proposed method provides a reduction in the content of impurities in tetrafluoromethane relative to the prototype: СО ₂ - by two orders of magnitude, О ₂ + N ₂ , CO, C ₃ F ₈ - by an order of magnitude, Н ₂ О - by 5-7 times, C ₂ F ₆ - twice.

Claims (5)

1. A method of producing tetrafluoromethane by reacting a carbon material with a fluorinating gas at a temperature of 700-1200 ^o C followed by purification of the process gas from impurities by the processes of adsorption, condensation and distillation, characterized in that as a carbon material using powdered graphite with a particle size distribution of 0.8- 2.5 mm, the process is conducted in continuous mode with constant monitoring of the temperature of the carbon material at a distance from the reaction zone equal to 1.5-2.0 diameter of the reaction zone, the supply of fluorinating gas is stopped at a temperature at this point of 100-150 ^o C, produce the tedding of the entire mass of carbon material, and the process is resumed when the temperature of the carbon material is reduced to 60-70 ^o C. 2. The method according to p. 1, characterized in that the anode gas of a fluorine medium temperature electrolyzer with an HF content of not more than 5-7 vol. %
3. The method according to PP. 1 and 2, characterized in that the process gas at the outlet of the reactor is subjected to purification from impurities by sequential sorption on water, an aqueous solution of potassium hydroxide, followed by preliminary freezing out of moisture at a temperature of minus 25-30 ^o C. 4. The method according to PP. 1-3, characterized in that the process gas, condensed at a temperature of minus 150-160 ^o C, is heated to a temperature of minus 130-135 ^o C and pumped from soluble impurities in it with the direction of the exhaust gases into the process gas, subjected to condensation at a temperature minus 150-160 ^o C.  5. The method according to PP. 1-4, characterized in that the selection of purified tetrafluoromethane in the receiving tank is carried out through an adsorber filled with zeolite.  6. A device for implementing the method according to PP. 1 and 2, comprising a reactor equipped with a cooling jacket and a nozzle for loading carbon material, fittings for introducing fluorine and gas self-igniting from contact with fluorine, a nozzle for discharging process gas and a dust collector, characterized in that the reactor is equipped with a tedder of carbon material and a thermocouple pocket, the lower end of which is installed at a distance from the center of the reaction zone equal to 1.5-2.0 diameter of the reaction zone.

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