CA1043819A - Process for the preparation of tetrachloro-ethylene from tetrachloromethane - Google Patents

Abstract

PROCESS FOR THE PREPARATION OF TETRACHLOROETHYLENE
FROM TETRACHLOROMETHANE
Abstract of the disclosure:

A process for the preparation of tetrachloroethylene by pyrolysis of tetrachloromethane in contact with substantially graphitized carbon and at elevated temperatures is provided.

Description

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The present invention provides a process for the preparation of tetrachloroethylene from tetrachloromethane.

Tetrachloroethylene is used in large amounts in the field of dry-cleaning. On the other hand, large amounts of tetrachloro-ethylene are also employed for the preparation of C2-fluoro-chloro-carbon compounds.
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It is known for a long time that tetrachloroethylene may be obtained by pyrolysis of tetrachloromethane according to the - equation (t) (1) 2 CC14~ C2C14 ~ C12 . .
Some industrial manufacturing processes utilize this mechanis~
in the following manner: carbon tetrachloride is reacted in an adiabat~c reaction simultaneously with Cl to C3-hydrocarbons or their chlorine derivatives and chlorine, at final temperatures - of from 500 to 700C. The foll~wing equation (2) is an example ` o~ such a reaction:

; (2) 2 CC14 + C2H4C12 1 C12 ~ 2 C2C14 + 4 HCl Also direct pyrolysis of tetrachloromethane has been proposed - for the preparation of tetrachloroethylene. ~or example~ in .~ - U.S. Patent No. 1,930,350, there is described a process where-in tetrachloroMethane vapors are passe~ at 600 to 1500C over electrically heated resistance materials. However, this patent does not indicate any examples showing conversion rates or ., . ' ,~' i.. . .. , . ...... , .~ . .. , , . . , ,. ... , , . . , , , . . . I , . . . ...

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selectivity data ol the reaction. As electrically heated resistance materials, carbon or silicium carbide are cited quite generally without indicating their specific properties.

On the other hand, it is kno~nn from numerous published papers that in the pyrolysis of tetrachloromethane, especially at temperatures above 60o C, there are formed besides perchloro-ethylene a number of by-products, for example hexachloroethane, hexachlorobutadiene, hexachlorobenzene or elementary carbon.
T~ese by-products not only reduce the yield of the perchloro-ethylene process~ but complicate also the work-up of the reaction products, especially in the case of separation of carbon which rapidly clogs the pyrolysis stove or the work-up devices.

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Moreover, some literature references indicate that the sepa-ration of carbon, proceeding according to the following equation (3)~ oceurs already in the temperature range of from 300 to 800 C and is catalyzed by active charcoal:
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(3) CCl4 - ) C ~ 2 Cl2 Because of this separation of carbon it is easy to understand that pyrolysis of tetrachloromethane to tetrachloroethylene at temperature~ above 600C hitherto has not been utilized for a manufacturing process in the industry.
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Not only because of this catalytic separation of carbon often described in the literature, but also because of the reverse reaction of reaction (3) forming tetrachloromethane from chlorine and carbon, according to the literature it was not to be expected that materials substantially on the basis of carbon can be used as wall lining for the pyrolysis process. Indeed, for example, Blac~wood and Cullis, A~str. J. Chem. ~ (1970)~
3 and Kirk-Othmer Encyclopedia of ~hemical Tachnology~ 2nd edition, vol. 5, page 134, indicate that at temperature~ above 600 C, tetrachloromethane is obtained from coal and chlorine.

-_ 3 _ --HOE 74~ 055 -.~343~9 Furthermore, tests carried out prior to this invention have proved that many kinds of coal and coke not only catalyze the separation of carbon or are attacked b~ hot chlorine, but also they must be excluded as heated resistance medium because of poor mechanical and electric properties.
, Surprisingly, it has now been found that under special reaction conditions, the formation of soot in the pyrolysis of tetra-chloromethane can be suppressed. These special reaction con--ditions furthermore allow to achieve extraordinary high selec-tivity and conversion rates of tetrachloroethylene, as well asa long life of the reactor material.
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The present invention provides a process for the preparation of tetrachloroethylene by pyrolysis of tetrachloromethane, optionally in admixture with other chloro-carbon compounds~ in contact with materials heated by direct passage of electric current, in a temperature range of from 600 to 1500C, which comprises pyrolyzing from 1 to 50 kg of gaseous tetrachloro-methane per liter of reaction volume and hour, optionally in admixture with other chloro-carbon compounds, in contact with substantially graphitized carbon, at temperatures of from 750 to 950C and pressures of from 0.1 to 5 bars, subsequently cooling rapidly and then separating the chlorine formed from ~ the tetrachloroethylene obtained.

- By roaction volume, there i~ to be understood the free reactor volume~ that is~ the reaction space between the electrode surfaces. The volume proper to the graphitized carbon being heated by direct passage of electric current is therefore not deducted from the reaction volume.

An advantageous technological embodiment of the reaction device is the followings The pyrolysis stove is a vessel having an inlet and an outlet for the gas. It contains a packing of graphite or graphitized artificial coal particles, which are . electrothermally heated to 750 to 950C, preferably to 800 -.~ .
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.~ ` ' .. ' ~ ' ' ' ' ''.'' 900C, and maintained at this temperature during the pyrolysis.

The arrangement of the packing and the electrodes, especially the kind of coal use~, are important for the obtention of high seléctivity and conversion rates, and for long operation times.

The packing must be easily permeable to the gas current, it must have a large surface, a relatively high and homogeneously distri~uted electric and a good heat conductivity. These pro-perties of the packing are for example ensured in the case where the particles are ball-shaped or in the forn~ of fragmènts of a relatively narrow screening fraction, for example of a mesh size of 5-10 mm, 10-15mm, 15-25 mm.
- , Chemically speaking, the material of the packing and the electrodes consists of pure or nearly pure carbon, substantial~
ly in the form of graphite. ~oreign elements or compounds of foreigen elements such as they are frequently contained, for example~ in active charcoal or other coals~ for exa~ple zinc~
iron~ magnesium~ aluminum, silicon or noble metals, should not be present or only in insignificant amounts. Furthermore~
the material must be of a certain degree of solidity.
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An important characteristic of an appropriate ~nd of coal i~
the fact that a specific ~sctric resistance of, for example, from 10 to 100 ohms-mm2/m, as well as a heat conductivity of~ for example, fro~ 1 to 200, preferable from 10 to 100 kca]/
m-h~degree is ensured.
25 A suitable graphite or artificial coal is obtained for example from cokes having only a very low content of ashes with addition of coking binders by heating at temperatures of from 2500 to 3000 C with exclusion of air. Such substantially graphitized artifidal coals are already available on the market.

The stove is preferably heated using alternating or three-. phase current, the electrodes having voltages of from 10 to .-~ .

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1000 volts, depending on the size and construction of the stove. The temperature is controlled in known manner, for example by a combination of variable and high-potential trans-formers.

The electrodes and the packing may be arranged, for example, in the following manner:

When single-phase alternating current is used, the packing is in a cylindrical vessel in which the electrodes are introduced either from above or from below, or the wall of the vessel is - used as one of the electrodes, the other being in the form of a central graphite rod.

In the case of large-scale plant capacities, the stove is advantageously operated by threephase current, for example by shunting units each ~onsisting of three of the above single-phase stoves, or by operatin~ sets of three electrodes immersedin the packing directly by threephase current.

The reaction in the process of the invention may for example be carried out in the following manner: `
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Tetrachloromethane is introduced into the pyrolysis reactor in gaseous form, that is, before entering the pyrolysis zone as such it is heated to a temperature o~ from 77C, the j boiling point at normal pressure, to 750C, which causes the pyrolysis to start already to a small extent, depending on the temperature.

Reaction rates and selectivities acceptable for industrial manufacturing conditions are obtained by operating at tempera-` tures of from 750 to 950C, preferably from 800 to 900C.
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I It is not required that the tetrachloromethane introduced into ! the pyrolysis stove be pure; it may contain also other chloro-carbon compounds~ such as hçxachloroethane or hexachlorobenzene.

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The pressure during the pyrolysis is from 0.1 to 5 bars, prefèrably from ~ to 3 bars.
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In order to avoid formation of large amounts of hexachloro-ethane after co~pleted pyrolysis by reaction of the tetra-chloroethylene formed with chlorine according to equation (4) (4) C2Cl4 ~ Cl2 ~ C2~l6 and thus a considerably reduced selectivity, it is necessary to drop the temperature of the reaction mixture as rapidly as possible and also to separate the chlorine from the tetrachloro-ethylene. This is for example achieved by introducing the products into a quenching column where they are rapidly cooled : 20 to temperatures below 100C by means o~ liquid tetrachloromethane or liquid reaction products. From this quenching column, chlorine is taken off at the top, and tetrachloroethylene formed~ unreacted tetrachloromethane as well as small amounts of by-products are taken off at the bottom. The reaction products are then worked up in known manner, for example in a sequence of distillation columns.

Under the above optimum reaction conditions, at a throughput of from 1 to 50 kg of tetrachloromethane , for example, from 1000 to 10 000 g/l'h Or tetrachloroethylene per liter of 3 reaction volt~e and hour, and simultaneousl~ the equivalent amount of chlorine are obtained.

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The conversion rates are for example from 40 to 80 %, the selecti~ity above 90 ~ in the case where the amount o~ hexa-chloroethane is considered to belong to the by-products, and above 97 ~, when the hexachloroethane is considered to be a recycled product which, reversing e~uation (4), is converted to further tetrachloroethylene and chlorine after being recycled into the reactor.

Besides formation of hexachloroethane, there is also formation of small amounts of hexachlorobenzene or hexachlorobutadiene as by-products, which amounts, under optimum reaction con-ditions, are less than 1 c~ of the conversion rate. There is no formation of soot, or it is so insignificant that operation periods of more than 1000 hours without any clo~ging are attained. Also corrosion of the graphite material does not occur or only to an insignificant extent.

The tetrachloroethylene obtained according to the process of the invention, after a suitable distillation wor~-up, is especially pure; for example, it is free from trichloroethylene which often is contained in products of otller processes in disturbing amounts.

Chlorine, the second product of the process of this invention, may be obtained in liquid or gaseous form, and it may be - reused in other chlorination processes.
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Very advantageous is the combination of the pr~cess o~ the ; 25 invention with the high-pressure chlorolysis for the preparation of tetrachloromethane described in Chemie Ing. Techni~ 45 ~-(1973), page 1019. This combined operation allows for example to reuse chlorine as well as by-products of the process of `
the invention, such as hexachloroethane, hexachlorobutadiene or hexachlorobenzene, for the preparation of the tetrachloro~
methane starting product in a ~uantitative manner. Furthermore, this combination furnishes the starting material in an i especially suitable ~orm and very cheaply.

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~LV~3~9 By especially suitable form, there is to be understood t~e reaction mixture having a temperature of from 400 to 600~
and containing also hydrogen chloride and chlorine besides tetrachloromethane, or a hot mixture already liberated fr~
C12 and HCl, which, however, may stili contain large am~nts of other chloro-carbon compounds, for example hexachloro-ethane and/or hexacl~robenzene.

The following examples illustrate the invention.
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1. In the central ~ection of a vertically positioned quartz tube having an inner width o~ 40 mm and a length of about 400 ~nm, there is a packing of artificial coal grains of a screening fraction of about 5 to 10 mm, distributed over a length of about 250 mm. The grains are obtained by mechanical crushing of a mass of commercial, substantially graphitized artificial coal containing only very small amounts of ashes and having a specific electric resistance of about 10 to 30 ohms-mm /m in the mass. The heat con-ductivity of the artificial coal is from 50 to 100 kcal/m-- h-degree, and the volume of the graphite packing, that i.5, the calculatory reaction volume, is about 315 ml.

The upper and the lo~-er end of the quartz tube serve as guides for 2 graphite electrodes which are immersed in the packing. Their diameter is about 35 mm, and about 30 mm in an immersion depth of about 15 rt1m. The space between the electrodes and the quartz electrode guide is made gastight by means of asbestos/~aterglass.
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A voltage of from 10 to 20 volts is established at the electrodes by means of a high-potential transformer and water-cooled pole pieces. The amperage is controlled by means of a series-connected variable transformer run at a starting voltage of about 220 volts. The temperature in the packing is controlled by means of a thermocouple being introduced into the packing via the perforated upper - electrode and a quartz temperature core.

Packing and electrodes consist of the same graphite material.
At the upper and lower end of the reaction zone, two 3 connection pieces, ofrset by 180 are fitted to the ~u~rtz tube by blowing, the upper piece serving as reactor inlet and the lower one as outlet.
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The entire reactor is insulated against heat dissipation by means of a quartz wool layer having a thickness of 5 cm and a subsequent layer con~isting of fire-brick.

At a temperature of 860 ~ and a pressure of 1 bar, 2310 g/h of gaseous tetrachloromethane, preheated to 300-400 are fed to this reactor, I~hich amount corresponds to a charge of 7.34 kg per liter of reaction volume and hour.
, In a quenching column directly connected to the pyrolysis stove, the reaction products having a temperature o~ about 850C are quenched to tempera-tures o~ below 100 C by means - of liquid tetrachloromethane or liquid reaction products (see below). Simultaneously, the rapid separation of the chlorine from the other reaction products takes place in this quenching column. At the top o~ this column, a mi~ture of about 710 g o~ chlorine and about 40 ~ of CCl4 per hour is taken off. At the bottom, 1~3 g/h o~ a li~uid - mixture of carbon chloride compound~ is ta~en off, which is composed as follows: Tetrachloroethylene 814 g, tetra chloromethane 700 g, hexachloroethane 34 g, hexachloro-hutadiene 4 g, hexachlorobenzene 1 g (according to gas chromatography analyses and the further work-up by fractional distillation).

After an operation period of 1000 hours, there is }lo deposit of soot, neither in the products nor on the packing in the reactor. Only the reactor outlet takes on a delicate reddish-brown to black layer which, however, does not disturb the operations at all.

From the analysis of the products, the ~1lowing data can be calculated;
, conversion tetrachloromethane: 68 selectivity tetrachloroethylene: 96 ~
space-time yield tetrachloroethylene: 2580 g/l-h .. ' :

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The consumption of electric energy is from 40 to 45 kwh per 100 kg of tetrachloroethylene.

2. The operations according to E~ample 1 are modified in such a manner that instead of tetrachloromethane a gaseous mixture Or 2100 g of tetrachloromethane and 210 g of hexa-chloroethane is fed to the reactor.

About 68~ g of chlorine and a~out 30 g of CCl4 are taken - off per hour at the top of the quenching column, and, at the bottom, 159~ g of a liquid product mixture composed as follows: 850 g of tetrachloroethylene, 705 g of tetra-chloromethane, 35 g of tetrachloroethane, 4 g of hexa_ chlorobutadiene, 1 g of hexachlorobenzene.

As in Fxample 1, no precipitation of soot is observed.
Relative to tetrachloroethylene, the space-time yield is 2700 g/l-h, and the consumption of electric onergy is 40 kwh per 100 kg of tetrac~oroethylene.

3. The operations of Example 1 are modified in such a manner that instead o~ tetrachloromethane a gaseous mixture of 1540 g of tetrachloromethane and 112 Nl (Nl _ normal liters, that is, at 0C and 760 mm Hg) of hydrogen chloride having a temperature of about ~00C are fed to the reactor heated at 850C
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The following quantities per hour are obtained in the work-up section in this case:

chlorine: 535 g, hydrogen chloride: 112 Nl, tetrachloro- :
ethylene: 620 g, tetrachloromethane: 367 g, hexachloro-ethane: 14 g, hexachlorobutadiene: 1 g, hexachlorobenzene:
0.7 g.

29 From these products, the fo]lowing clata are calculated:

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conversion tet~achlor~methane; 76 c~
selectivi~y tetrachloroethylene: 98 oh space-time yield tetrachloroethylene: 1970 g/lh

4. The operations of Example 1 are modified in such a manner that instead of tetrachloromethane a gaseous mixture of 1540 g of tetrachloromethane and 56 Nl of chlorine and 56 Nl of hydrogen chloride having a temperature of about 300 C is fed to the reactor heated at about 850 C.

In this case, the following quantities per hour are ob-tained in the work-up section:

chlorine: 430 g, hydrogen chloride: 56 Nl, tetrachloroethy-lene: 446 g, tetrachloromethane: 615 g, hexachloroethane:
46 g, hexachlorobutadiene; 1 g~ hexachlcro~enzene: I g.

From the~se products, the fol]owing data are calcula~ed:

conversion tetrachloromethane: 60 h selectivity tetrachloroethylene 93 %
space-time yield tetrachloroethylene: 141S g/l h

5. ~he operations of Example 1 are modified in such a manner that the pressure is raised from 1 to 2 bars, and the charge of tetrachloromethane is increased from 2310 g/h to about 5000 g/h. In this case, the following quantities are obtained in the work-up section per hours:

~chlorine: 921 g, tetrachloroethylene: 985 g, tetrachloro 2$ methane: 2850 g, hexachloroethane: 2l~0 g, hexachlorobuta-diene: 2 g, hexachlorobenzene: 1 g.

From these products, the following da~a are calculated:
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conversion tetrachloromethane: 43 ~
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selectivity tetrachloroethylene: 85 % (he~achloroetl-lane ~ being considered as by-product) or 99.7 /~ (hexachloro-ethane being considered as recycled product), space-time yie]d tetrachloroethylene: 3~20 g/l-h :~
, 5 6. The operations of Example 1 are modified in such a manner that the temperature in the reactor is raised from about : 860C to about 930C.

.In this case, the following quantities per hour are obtai.ned .
in the work-up section:

chlorine: 780 g, tetrachloroethylene: 872 g, tetrachloro methane: 600 g, hexachloroethane: L~4 g, hexachlorobutadiene:
8 g, hexachlorobenzene: 5 g. -From these products~ the following data are calculated:

conversion tetrachloromethane: 74 ~
selectivity tetrachloroethylene: 95 ~ :
space-time yield tetrachloroethylene: 2770 g/l'h 7. The operations of Example 1 are modified in such a manner .
that the temperature in the reactor is decreased from .. :
about 860 to about 760C.
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In this case, the following quantities per hour are ob- .
tained in the work-up section:
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chlorine: 556 g, tetrachloroethylene: 622 g, tetrachloro-methane: 1040 g~ hexachloroethane: 89 g, hexachlorobuta-diene: 1 g, hexachlorobenzene: 0.5 g.

Walls and outlet of the reactor remain free from any . deposit~ that is, there is no ~ormation of ~oot. FroM
the products, the following data are calculated:
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3~9 conversion tetrachloromethane: 55 selectivity tetrachloroethylene: 9t ~
space-time yield tetrachloroethylene: 1980 g/l-h .

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the preparation of tetrachloroethylene in which 1 to 50 kg of gaseous tetrachloromethane per liter of reaction volume and hour is pyrolyzed in contact with substan-tially graphitized carbon which has been heated by direct passage of electric current to a temperature of from 750 to 950°C, the pressure is maintained at 0.1 to 5 bars, the resultant mix-ture is then rapidly cooled and the product is treated to separate chlorine from the tetrachloroethylene.

2. A process as claimed in claim 1 in which the carbon is heated to a temperature of from 800 to 900°C.

3. A process as claimed in claim 1 in which the reaction is carried out under a pressure of from 1 to 3 bars.

4. A process as claimed in claim 1, claim 2 or claim 3 in which the mixture is rapidly cooled by quenching with cold liquid tetrachloromethane or with cold liquid reaction products.

5. A process as claimed in claim 1, claim 2 or claim 3 in which the tetrachloromethane contains at least one other chlorohydrocarbon compound.

6. A process as claimed in claim 1, claim 2 or claim 3 in which the tetrachloromethane contains at least one member of the group of chlorine and hydrogen chloride.

7. A process as claimed in claim 1, claim 2 or claim 3 in which the substantially graphitized carbon material has an electric resistance of from 10 to 100 ohms.mm2/m and heat con-ductivity of from 10 to 100 kcal/m.h. degree.

8. A process as claimed in claim 1, claim 2 or claim 3, in which the graphitized carbon is in the form of ball-shaped particles.