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EUROPEAN AND APANESE HEMICAL INDUSTRIES SYMPOSIUM
L
Trichloroethylene and Perchloroethylene Coproduction by Ethylene Process TAKE0 KAWAGUCHI YOSHITAKA SCdZUKl RE191 SAITO his study was undertaken to establish a new method for producing trichloroethylene and perchloroethylene from ethylene. It was experimentally proved that ethylene accelerated the substitutive chlorination in the absence of catalysts. Ethylene is converted into the chlorinated ethanes at 80-130°C in the liquid phase. Relative rates of consecutive chlorination were measured. After separating C2H4C12 and C2HaC13,which are recycled to the reactor, the mixture of 1,1,1,2-, 1,1,2,2CZH2C14 and CzHC15 is dehydrochlorinated at about 450'C under pressure. I n this cothermal cracking, the rates of decomposition are in the order of 1,1,2,2- > 1,1,1,2-CzHzC14 > CzHC15. A small quantity of FeC13 accelerates the cracking of 1,1,1,2-CzHzCl4 and reduces the formation of carbon. Trichloroethylene is commonly produced from acetylene by a process which is simple, but expensive due to the high cost of acetylene. More recently the petrochemical industry has developed a process €or producing trichloroethylene from low-cost ethylene thus implementing the use of ethylene instead of acetylene as a raw material. There are two methods of producing trichloroethylene from ethylene. One is photochlorination (6) or catalytic chlorination (7) of 1,2-dichloroethane (EDC) followed by the dehydrochlorination of tetrachloroethane ; the other is the oxychlorination of ethylene or EDC at high tcmperature (3-5), although it raises serious problems regarding the reactor material and ethylene's high combustion rate. 36
INDUSTRIAL A N D ENGINEERING CHEMISTRY
On the other hand, perchloroethylene is commonly produced with carbon tetrachloride from propylene or 1,2-dichloropropane. Compared to the complicated methods above, our new method involves the direct production of tetrachloroethane and pentachloroethane from ethylene using its catalytic action (70). At almost the same time, Reiche and Jackson ( 9 ) and Le Page and Riding ( 8 ) suggested similar methods although the products and some of the details of the process were different. Our method consists of two processes as shown in Figure 1. T h e chlorination of ethylene precedes thermal cracking. Ethylene is converted to the chlorinated ethanes in the absence of light or a catalyst such as FeC18. The mixture of 1,1,1,2-, 1,1,2,2-tetrachloroethanesand pentachloroethane is dehydrochlorinated into trichloroethylene and perchloroethylene as final products. T h e production ratio of trichloroethylene to perchloroethylene is variable over a wide range. We started this study in 1962, and based on the results, a new plant having a total capacity of 3000 metric tons/month of trichloroethylene and perchloroethylene was built in April 1968 and is still in operation. Experimental Apparatus As part of the experiment, various types of chlorination apparatus were used. Typicai is the one shown in Figure 2 . T h e reactor is made of glass-lined steel and has a n inside diameter of 42 m m and a length of 700
Figure 7. Schematic diagram
Trichloroethylene and perchloroethylene produced from ethylene by new simplified process Figure 2. Chlorination apparatus
mm. The reaction was carried out by the continuous supply of gaseous ethylene and chlorine into the liquid chloroethanes. Thermal cracking involved two types of apparatus. One is the straight silica tubular type (28 mm i.d., 850 mm long) packed with Raschig ring, and the other is the stainless steel U-tubular type (9.2 mm i d . , 3410 mm long) as shown in Figure 3. Results and Discussion
Chlorination of ethylene. Figure 4 shows the catalytic action of ethylene. The experiment was carried out under the constant flow rates of EDC and chlorine a t 6OoC. I n the absence of ethylene, chlorine conversion is no more than 15 mol %. However, chlorine conversion becomes higher with the introduction of ethylene. The lower curve indicates the ratio of consumed chlorine by the substitutive reaction against feed chlorine. I t is evident from this figure that ethylene accelerates the substitutive chlorination of EDC. Figure 5 shows the relation between flow rate and conversion of ethylene and chlorine. The mole ratio of Clz/CzH4 feed was kept constant at 3.81, and the x-axis gives the flow rate of ethylene instead of total gas. At higher gas velocity, the ethylene conversion is low, but the chlorine conversion is kept at 100%. This indicates that the substitutive chlorination apparently proceeds in the range of higher gas velocity. O n the contrary, if the gas velocity is low, the additive chlorination of
Figure 3.
Thermal cracking apparatus
Figure 4. Catalytic action of ethylene VOL 62
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Figure 5. Effect of feed gas velocity
Figure 6. Effects of FeCla and mole ratio of C12/Cfid
Figure 7. Effect of temperature
ethylene increases, and the unreacted chlorine separates out. Therefore, the feed gas velocity should be kept high. This increases the capacity of a reactor. The effect of FeCh and the mole ratio of Clz/CzH4 feed is shown in Figure 6 . The ethylene conversion becomes higher with an increased amount of FeC13. This shows that FeC13 is a catalyst for the additive chlorination of ethylene. However, the increase of FeC13 causes the lowering of chlorine conversion. O n the other hand, when the mole ratio of Clz/CzHd is high, the effect of FeC13 appears more sensitive. I n other words, if we expect the same conversion in both ethylene and chlorine, even with a high mole ratio of Clz/CzHh, it is necessary to reduce the quantity of FeC13 to be comparable with a low mole ratio. With the raising of reaction temperature, the ethylene
conversion becomes lower, but the chlorine conversion is kept constant a t 100% (Figure 7). At higher temperatures, the substitutive chlorination of EDC takes place in preference to the additive chlorination of ethylene. There are two important points regarding the effect of pressure: (a) The ethylene conversion is kept constant even under pressurized operation. T h a t is to say, the ratio of substitution against addition is not changed by pressurizing; (b) The rate of reaction increases remarkably by means of pressurizing. It is well known that the radical reaction is inhibited by oxygen. If oxygen is contained in chlorine gas, the substitutive chlorination is inhibited and unreacted chlorine gas results. T h e metal chlorides that have an adverse effect on the substitutive chlorination are selenium, antimony, and
38
lNDUSTRlAL AND E N G I N E E R I N G CHEMISTRY
TABLE 1.
RELATIVE RATES OF CONSECUTIVE CHLORINATION h a
k3a
C2H4 -+ CZHdClI e CsHaC18
7 1,1,1,2-CzH&14 k4a
ksa Y'
l,l,2,2-C2HzC147
k48
ks
bismuth chlorides. Nickel and chromium, both components of stainless steel, are not preferred although they are not so undesirable as iron. From the above experimental results, the mechanism of chlorination of ethylene is presumed to be :
+ Clz
CHFCH~
C1
CHz=OH*
-t
I
2c1
CH2CH2C1
+ Cl2 CH2ClCHzCl + Cl + Clz + CHzClCH2Cl) (CH-CHz CHzClCHzCl + C1 CHzClCHCl + HC1 CH2ClCHC12 + Cl CHzClCHCl + C12 CHzClCHC12 + e1 CHzClCC12 + HCl 'CHClCHCl2 + HC1 CHzClCCls + C1 CHzClCCl2 + Cl2 CHClzCHC12 + Cl CHClCHC12 + Cl2 CH2CH2Cl
C2HC16 * CzCI6 Complete mixing flow system Reaction temp. 6OoC Photochlorination Cad-Chlorination 0.080 0.081 k3a/kZs 0.089 0.091 kS8/k28 0.033 ta/k~s 0.030 0.065 0 071 k4a/k2s 0.016 0.013 ks/kea
-+
-+
-+
+
-+
TABLE It. EFFECT OF COTHERMAL CRACKING
Reactor Decompn. temp. Space velocity
Silica tube 460 OC 140 hr-1
yo Decomposed Component One 1,1,2,2-CzH2Cla 41.1 1,1,1,2-CzHzCla 8 0 . 3 83.3 CzHCls a Mole ratio = 1 :1 * Composition, mol yo 1,1,1,2-CzHzClr 1,1,2,2-CzHzCla d2HC16
73.7 52.0
-
Twoa 92.5
-
55.0
-
89.4 69.0
Threeb 92.6 80.3 58.0
45 40 15
--t
First, the chlorine radical is formed in the presence of ethylene, and then, the consecutive chlorination takes place radically. Meanwhile, during the formation of EDC from ethylene, in addition to the radical mechanism, the ionic mechanism may be taking place. Table I shows the relative rates of consecutive chlorination. For the experimental apparatus, a complete mixing flow system was used. Moreover, the relative rates of photochlorination were measured to compare with the ethylene-chlorination. As is evident from this table, the relative rates of these two are very similar. If the reaction temperature rises, chlorinated ethanes having high substitutive character show slight increase in the relative rates of reaction. Thermal Cracking. Table I1 shows the effect of cothermal cracking. If the thermal cracking of each component is carried out individually, the ease of decomposition is in the order of CzHCls > lY1,1,2-C2HzC14
AUTHORS Takeo Kawaguchi, Yoshitaka Suzuki, and Reiji Saito are employed by the Toagosei Chemical Industry Co., Ltd. in Nagoya, Japan. Their paper was presented as part of the Symposium on Novel Processes and Technology of the European and Japanese Chemical Industry, 158th ACS Meeting, New York, N . Y., September 7-12, 1969.
> 1,1,2,2-CzHzCld. But, in the mixture of two components (1 :1, mol), this relation changes in the reverse order. Similarly, in the case of cothermal cracking of three components mixture, the ease of decomposition is changed to the order of 1,1,2,2-C2H2C14 > 1,1,1,2CzH2C14> C2HCls. If we presume that CzHCls is apt to be dechlorinated, a hydrogen atom of 1,1,2,2-CzHzCld is apt to be comparatively drawn out by this chlorine radical. Figure 8 shows one of the effects of FeC13. I n the presence of FeC13, the asymmetric chloroethanes decompose, especially in the case of 1,1,1,2-CzHzC14. The preferred amount of FeC13 may be more than 0.02 mol % for the feed material. This is important in industrial application because 1,1,1,2-CzHzCl4 is decomposed in the distillation column made of mild steel, and will cause problems. There is another important effect of FeC13. If more than 0.01 mol % ' FeCla is contained in the reactant, carbon formation is extremely reduced even under a high temperature of 450°C. Toluene, a well-known radical acceptor, is extreme in inhibiting decomposition. But, by adding FeC13, the thermal cracking reoccurs, especially in the case of 1,1,1,2-C2HzC14. These results are shown in Table 111. From Table 111, it is presumed that the thermal VOL. 6 2
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Figure 9. Formation of trichloroethylene dimer
Figure 8. Effect of FeC&for cothermal cracking
cracking takes place not only with the radical mechanism but also with the ionic mechanism, especially in the case of 1,1,1,2-CzH2C14. Figure 9 shows the formation of high-boiling materials on the thermal cracking. Though the high decomposition rate is preferable, the formation of a trichloroethylene dimer-an undesirable by-product-rapidly increases. The dimer consists of pentachlorobutadiene and a small amount of hexachlorobutene. Its forma.tion is not affected by pressure change. Figure 10 shows the Arrhenius plot at the cothermal cracking in the presence of FeC13. Previously, it was confirmed that this reaction takes place in nearly the first-order mechanism for the partial pressure of each component. The rates of reaction, k, are higher than Barton reports (7, 2), because the effects of cothermal cracking and FeC13 are considerable.
TABLE 1 1 1 .
EFFECT OF TOLUENE
Reactor Silica tube Decompn. temp. 450°C Space velocity 140 hr-' Composition of feed material, mol yo 1,1,1,2-C2HzCld 45-50 1,1,2,2-CzHzC14 40-45 CzHC16 5-15
% Decomposed Fee13
To1uen e 1,1,2,2-CzH&14 1,1,1,2-CzH&14 C2HC16
40
None 92,6 80.3 58.0
None
5 mol 7 0 Trace a :ce
irace
0.2 mol 7 0 5 mol 93.3 12.7 100 62.1 91.3 24.1
None
r0
INDUSTWlAL AND ENGINEERING C H E M I S T R Y
Figure 70. Arrhenius plot
Commercial Process
The commercial processes of chlorination and thermal cracking are shown in Figure 11. Compressed chlorine gas is dissolved into the reaction medium, and introduced to the bottom of the reactor with ethylene gas. The mole ratio of chlorine to ethylene is adjustable for a given product ratio. T h e reaction is carried out at 100-130°C and a pressure of 8 atm. For the removal of heat of reaction, the reaction medium is recycled to the external cooler. Chlorine conversion is almost loo%, and ethylene conversion ranges from 95 to 98% by the structural device. Hydrogen chloride exhausted from the reactor is used for the oxychlorination of ethylene. After chlorination, the reaction mixture is passed through the flash column and stripper, and the dissolved hydrogen chloride is removed. Then, the reaction mixture is fractionated. EDC and trichloroethane are recycled from the top of the column to the reactor. From the still, we get the
Figure 71. Flow sheet of commercial process
crude products which contain tetrachloroethane, pentachloroethane, and a small amount of hexachloroethane. T h e mixture of tetrachloroethane and pentachloroethane is introduced into the thermal cracker of the monotubular type and then converted to trichloroethylene and perchloroethylene. The thermal cracking is carried out at 450°C and 9 atm. The undesirable byproduct, trichloroethylene dimer, is less than 0.3 wt %. The work for decoking is not required for more than three months, because the formation of carbon by overcracking is so small. The cracked products are quenched and the hydrogen chloride is separated. And then, unreacted tetra- and pentachloroethanes are recovered and recycled to the cracker. T h e crude products are kjassed through a purification process, which consists mainly of the fractionating columns, and are then purified into products having good quality. Quality of Products
T h e obtained trichloroethylene and perchloroethylene are stabilized by adding a small amount of aliphatic amines, phenols, and/or epoxides. The purity of each product is more than 99.8 wt %, and the quality easily passes existing standards.
TABLE IV.
RAW MATERIALS AND CONSUMPTION
Per 1000 kg products Trichloroethylene Perchloroethylene Raw Materials Ethylene Chlorine By-product HC1 (100%) Utilities Steam (3 kg/cmz gauge) Steam (12 kg/cmZ gauge) Cooling water (32OC) Process water (20 " C ) Fuel oil Refrigeration ( - 30 " C ) Electricity Nitrogen
UTILITIES
862 kg 138 kg 220 kg 1720 kg 830 kg
2400 kg 2100 kg 300 m3 25 m3 35 kg 43 X I O 3 kcal 200 kW hr 65 ma at S.C.
(4) Variable production ratio of trichloroethylene to perchloroethylene (5) Easy combination of this process with the oxychlorination process and/or the methyl chloroform process. Acknowledgment
Economics
Table I V gives the raw materials and utilities consumption for our new plant of 2600 metric tons/month trichloroethylene and 400 metric tons/month perchloroethylene capacity. The battery limit investment was U. S . $2,800,000, based on Japanese construction costs in 1968. Conclusions
The main characteristics of this method can be listed as : (1) Low capital charge (2) Less troublesome operation (3) High yield with good quality
The authors express appreciation to their cooperators in Toagosei Chemical Industry Co., Ltd. who assisted in this work. Acknowledgment is also given to the Toagosei Chemical Industry Go., Ltd. who permitted publication of these studies. REFEREUCES Barton, D. H. R.,J . Chcm. Soc., p 148 (1949). Barton, D. H. R., and Howlett, K. E., ibid., p 2033 (1951). Bohl, L. E., Brit. Patent 904,084 (Aug. 22, 1962). Cass, 0. W., U.S. Patent 2,308,489 (Jan. 19, 1943). Cass, 0. W., U. S, Patent 2,342,100 (Feb. 22, 1944). (6) Coleman, G. H., and Moore, G. V.,U. S.Patent 2,174,737 (Oct.3, 1939). (7) Jung, K., and Zimmermann, A., Ger. Patent 545,993 (Feb.18, 1932). (8) Le Page, N. J., and Riding, F., Brit. Patent 1,056,522 (Jan. 25,1967). (9) Reiche, C. R., and Jackson, J. M., Jr., U. S. Patent 3,344,197 (Sept. 26, 1967). (10) Suzuki, Y.,and Saito, R., Japan Patent 537,422 (July 21,1966).
(1) (2) (3) (4) (5)
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