Ind. Eng. Chem. Res. 1996, 35, 3803-3807
3803
RESEARCH NOTES Coke Produced in the Commercial Pyrolysis of Ethylene Dichloride into Vinyl Chloride Isao Mochida,*,† Tadanori Tsunawaki,† Chiaki Sotowa,† Yozo Korai,† and Kazuo Higuchi‡ Institute of Advanced Material Study, Kyushu University, 6-1 Kasuga, Fukuoka 816, Japan, and TOSOH Corporation, 4560 Kaisei-cho, Shin-nanyo, Yamaguchi 746, Japan
A variety of cokes formed in the reactor and transfer lines of a commercial ethylene dichloride pyrolysis process producing a vinyl chloride monomer were characterized by optical and electron scanning microscopies to determine the mechanism by which they were formed and to find means for suppressing this coke formation. Typical columnar pyrolytic carbon was found on the reactor wall, which was anisotropic carbon of granular appearance. Deposits of carbon found in the transfer lines of product and feed varied in amount and composition, depending on the location of the deposition. The latter carbon appeared to be formed through a mechanism similar to refluxing carbonization of reactive species such as chloroprene and acetylene, which are easily oligomerized, can precipitate on the wall, and finally yield carbon after the repeated dissolution and precipitation. The temperature and kinds of reactive species in the product and feed may define the extent of adhesion, coalescence, and growth of carbon primary granules and may induce their softening during carbonization. Introduction Pyrolysis of ethylene dichloride (EDC) into a vinyl chloride monomer (VCM) is a well-established commercial process practiced on a large scale. Operations are carried out at reasonable conversions, and very high selectivity of VCM is achieved. However, major problems are coke formation, which reduces heat transfer in the reactor and heat exchanger, and plugging the gas flow in the transfer lines, which increases pressure drop. Hence, it is necessary to rather frequently decoke the reactor and transfer lines by combustion decoking. This decoking interrupts the continuos operation and requires labor-intensive maintenance. In the present study, the authors describe the nature of the coke produced at different stages of a commercial process unit, aiming to clarify the mechanism of its formation and to find a way to suppress its formation. Experimental Section The commercial cracker was operated at 500 °C under high pressure of EDC. Figure 1 illustrates the commercial EDC pyrolysis process flow diagram. The cokes analyzed in this paper were obtained at B-1, at the outlet of cracker; B-2, in the transfer line (product line) at the outlet of the heat exchanger; B-3, in the transfer line at the inlet of the quencher; B-4 and B-5, on the shell surface (feed line) of the transfer tube in the heat exchanger; and B-6, in the feeding transfer line just before the cracker, respectively. The feed EDC consisted of 50% fresh EDC and 50% recycled EDC recovered from the product. The cokes were examined by optical (Olympus BH-2) and scanning electron mi* Corresponding author. † Kyushu University. ‡ TOSOH Corporation.
S0888-5885(96)00024-3 CCC: $12.00
Figure 1. Process diagram of EDC pyrolysis.
croscopies (JEOL JSM-5400). The elemental composition of B-4 coke was determined by conventional elemental analysis and also estimated by EDAX measurement (JEOL JED-2001). Results Structural Characterization of Cokes. The optical and scanning electron micrographs of coke B-1-B-6 are shown in Figures 2-7. B-1 Coke. Figure 2a shows a scanning electron micrograph of the outer surface of B-1 coke. This coke consisted of granular agglomerates about 40 µm in size, which also included smaller granules of about 5 µm. Figure 2b shows a scanning electron microscope of the cross-sectional surface of B-1 coke. The cross-sectional surface had basically the same appearance as that of the outer surface. Figure 2c shows an optical micrograph of a polished surface of B-1 coke oriented perpendicular to the cracker wall. Typical features of © 1996 American Chemical Society
3804 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 2. SEM micrographs of (a) surface and (b) cross section and optical micrograph of (c) cross section of deposited carbon at the outlet of cracker (B-1). B-1 coke consisted of anisotropic granules and had a typical columnar structure.
columnar pyrolytic carbon were indicated in this figure (Bokros, 1969), where apparently the columns grew outward from the surface of the wall. A unit of the granule may correspond to a column. It is of vlaue to note that a typical column pyrolytic carbon is produced at as low as 500 °C during the cracking of EDC. This is significantly lower than the pyrolytic columnar carbons reported by Otani (1981) in the cracking of cis1,2-dichloroethylene. However, in the commercial process the run period is much longer. No fibrous carbon (Baker and Harris, 1978) was found in the pyrolytic carbon nor on the reactor surface. B-2 Coke. Figure 3a shows the outer surface of B-2 coke. This coke also consisted of granular agglomerates having a variety of shapes. The surface appeared very porous between the granules. Figure 3b shows the cross section of B-2 coke. The very flat surface was a featureless column (or cut surface of sphere) and contained numerous voids among the columns. Figure 3c shows an optical micrograph of a polished surface. B-2 coke showed evidence of fusion of small isotropic
Figure 3. SEM micrographs of (a) surface and (b) cross section and optical micrograph of (c) polished surface of deposited carbon at the outlet of heat exchanger (B-2). B-2 coke consisted of isotropic granules, which fused with each other at some locations and provided many voids.
granules partially at some locations and completely at other locations. B-3 Coke. Figure 4a shows the micrographic surface of B-3 coke. This coke was also composed of aggregates of small granules. Some of them appeared to be fused and coalesced with each other. Figure 4b is the crosssectional surface of B-3 coke and indicates coalescence of granules with a concomitant loss of the voids. Optical microscopy showed coalesced isotropic granules that were connected by thin walls and surrounded by widely developed pores (Figure 4c). B-4 Coke. Figure 5a shows the surface of B-4 coke, which consisted of partly coalesced granules of a variety of sizes. Unlike the previous cokes, a number of very small granules were observed. The cross section of the coke was a featureless plate with pores, indicating the coalescence to form the plate underneath the surface (Figure 5b). Optical microscopy showed a featureless isotropy with a number of small pores (Figure 5c).
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Figure 4. SEM micrographs of (a) surface and (b) cross section and optical micrograph of (c) polished surface of deposited carbon at the inlet of quencher (B-3). B-3 coke carried isotropic granules, which fused completely to lose voids in contrast with B-2 coke.
B-5 Coke. Figure 6a shows the surface of B-5 coke. Independent spheres of 5-25 µm diameter appeared to be gathered together to form the surface. However, its cross section showed layers of isotropic plates that were separated from each other by the definite boundary (Figure 6b). No granules were observable within the plate. Coalescence was completed in the plate underneath the surface. B-6 Coke. Figure 7a shows that B-6 coke consisted of granules about 5 µm in which small microgranules were fused together. Helical or layer-accumulated structures were observable on the surface of the microgranules. The cross section of the coke was completely isotropic with a number of pores in visible size (Figure 7b). Further Carbonization of Coke. Further carbonization of the cokes produced in the transfer lines provided carbon yields of 80-90% regardless of the location of its production. The appearance and optical features were unchanged after carbonization, as shown in Figure 8. It is suggested that the carbon network
Figure 5. SEM micrographs of (a) surface and (b) cross section and optical micrograph of (c) polished surface of deposited carbon on the tube of heat exchanger, low-temperature side (B-4). There were many granules on the surface, but the cross section showed a layer of isotropic plate having a number of small pores.
developed sufficiently during its production even though the temperature was as low as 200-300 °C. Elemental Analysis and EDAX Measurement. The elemental composition of B-4 coke was determined by conventional elemental analysis and also estimated by EDAX measurement (Table 1). It was found that chlorine was included in the coke, estimated to be about 4-5% by both methods. Although EDAX can only determine the surface composition, this result is expected to be representative of all of the coke. Hence, hydrocarbons containing chlorine probably contributed to the coke formation. Discussion The present study identified the basic structural characteristics of cokes produced in commercial EDC pyrolysis processes. One form is anisotropic pyrolytic
3806 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 1. Conventional Elemental Analysis and EDAX Measurement of B-4 Coke elemental analysis EDAX a
C
H
N
Cl
ash (wt%)
92.9 94.8
2.6
0.2
3.9a 5.2
0.4
By difference.
Figure 7. SEM micrograph of (a) cross section and optical micrograph of (b) polished surface of deposited carbon at the inlet of cracker (B-6). B-6 coke was an isotropic and very porous carbon.
Figure 6. SEM micrographs of (a) surface and (b) cross section of deposited carbon on the tube of heat exchanger, high-temperature side (B-5). B-5 coke had many spherical carbons on the surface and accumulated layers of isotropic plates in contrast with B-4 coke.
carbon, which is produced in the cracker reactor, and the others are isotropic granules agglomerated into dense carbon bodies in the transfer lines before (feed line) and after (product line) the cracker. The former type is a typical columnar carbon, of which formation is noted at as low as 500 °C, which is much lower than previously reported for hydrocarbon pyrolyses (Bokros, 1969). However, the growth may have been much slower. Radical formation from EDC at this temperature may be the principal cause of this coke formation. Acetylene and butadiene, which are byproducts from EDC, and more polymeric products are believed the intermediates in pyrolytic carbon formation (Palmer and Cullis, 1965). Some of these reactive species are indeed found in the cracked products of EDC. We do not exclude the possibility that other reactive species such as chloroprene and methallyl chloride, found in the recycle feed, may also contribute the formation of pyrolytic carbon. It is suggested that the deposition of pyrolytic carbon takes place at as low as 500 °C once adequate radical species are produced. It is of value to point out that very few other types of carbon are found on the wall of the cracker. The second type carbon is produced at temperatures as low as 200-300 °C. The precursor of this coke may be high boiling pyrolysis products formed at high temperatures in the cracker that are subsequently
Figure 8. Optical micrograph of polished surface of carbonized B-2 coke. Carbonized B-2 coke showed the same appearance as B-2 coke (Figure 3); isotropic granules fused with each other at some locations.
polymerized and carbonized in the transfer lines. However, carbon produced in EDC feed before the cracker cannot come from very heavy products because the recycle feed distilled. Hence, we concluded that much of the carbon is produced from reactive species, which have boiling points around 300 °C. Carbon of this kind is characterized by isotropic spherical granules, although the extent of adhesion or coalescence of granules appears to depend on the temperature or location in the process. The most reactive substances may produce granules with less adhesion, as observed in B-2 coke at high temperature. Lower temperature or low reactive substances exhibited extensive adhesion and coalescence. We believe that this is due to high solubility and softening by swelling.
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The unique features of these carbons have not been observed in other carbons formed in conventional carbonizations in gas, liquid, or solid phases. The present authors have previously reported similar dendritic features in carbons produced by the reflux carbonization method for acenaphthylene, although that carbon is anisotropic (Mochida et al., 1977, 1980). Reactive species produced in the cracker may form polycyclic aromatic hydrocarbons of low solubility and high melting point. Such species may stay on the wall of the transfer lines by repeating precipitation and dissolution for times as long as a few months at 200300 °C. Such a situation is a kind of refluxing carbonization. Although the temperature is low, the boiling points of the major components in the product are in that range. Hence, the refluxing of a major product allows the carbonization of the precipitated fraction to form isotropic granules. The extent of softening, the solubility of the precipitated product, and the dissolving power of the matrix may determine the adhesion of the granules as a function of the temperature. The temperature may also define the reactivity of carbonizing substrate, the solubility in the matrix, and the vaporizing tendency of the matrix. The unique nature of this type of coking in a continuous flow process is that a very minor component can be the cause of a plugging problem. Based on a simple material balance, reactive species of about 80 ppm can
plug the transfer line or reactor easily within 2-3 months if all of it is carbonized in a localized area. The elimination of these reacftive species or the use of a powerful solvent to dissolve the carbonaceous precursors may be able to reduce the amount of coke accumulated. Work based on this approach is in progress. Literature Cited Baker, R. T. K.; Harris, P. S. Chemistry and Physics of Carbon; Marcel Dekker Inc.: New York and Basel, 1978; Vol. 14, p 83. Bokros, J. C. Chemistry and Physics of Carbon; Marcel Dekker Inc.: New York, 1969, Vol. 5, p 1. Mochida, I.; Miyasaka, H.; Fujitsu, H.; Takeshita, K. Anisotropic Mesophase of Novel Features Found in the Refluxing Carbonization of Acenaphthylene. Carbon 1977, 15, 191. Mochida, I.; Miyasaka, H.; Fujitsu, H.; Takeshita, K.; Takahashi, R.; Marsh, H. Microscopic and Chemical Study of Anisotropic Mesophase Possessing Novel Features. Fuel 1980, 59, 349. Otani, S. Preparation of Low Temperature Pyrolytic Carbon from cis-1,2-Dichloroethylene. Carbon 1981, 19, 468. Palmer, H. B.; Cullis, C. F. Chemistry and Physics of Carbon; Marcel Dekker Inc.: New York, 1965, Vol. 1, p 265.
Received for review January 10, 1996 Revised manuscript received July 1, 1996 Accepted July 15, 1996X IE9600248 X Abstract published in Advance ACS Abstracts, August 15, 1996.
