Ind. Eng. Chem. Res. 1996, 35, 931-937
931
A Detailed Quantitative Study of 1,2-Dichloroethane Cracking to Vinyl Chloride by a Gas Chromatographic Pyrolysis Device Joseph A. Incavo Occidental Chemical Corporation, Technology Center, 2801 Long Road, Grand Island, New York 14072
The pyrolysis of 1,2-dichloroethane (EDC) is investigated by a novel apparatus which incorporates a microcracking furnace into a gas chromatograph equipped with a capillary separation column. The device allows for the vaporization and pyrolysis of the liquid feed, in addition to sampling and analysis of the furnace effluent, to be accomplished in a single experimental run. This design circumvents the problems normally encountered when sampling a two-phase mixture containing toxic and corrosive materials. Moreover, it provides the most complete analysis of 1,2-dichlorethane pyrolysis reported to date. Results demonstrating the influence of conversion parameters such as pyrolysis temperature and residence time on product distribution are presented. It is also shown that kinetic parameters for such pyrolysis reactions can be measured with the GC pyrolysis apparatus. The simple and versatile apparatus could also be used to quantitatively study the pyrolysis of other liquid feedstocks such as naphthas, condensates, and other chlorinated hydrocarbons. Introduction A major industrial route to vinyl chloride monomer (VCM) is the pyrolysis of 1,2-dichloroethane (ethylene dichloride or EDC): 500 °C
ClCH2CH2Cl 9 8 Ni alloy H2CdCHCl + HCl + byproducts Production of VCM by this cracking reaction is known to be significantly influenced by ppm levels of additives acting as promoters or inhibitors (Barton, 1949, Ashmore et al., 1982; Kolesnikov et al., 1985). However, the introduction of any additive into the feed of a large scale polymer-grade operation warrants careful consideration. For example, if additional byproducts were passed on to VCM, the quality of the poly(vinyl chloride) end product could suffer. Coke-producing side products such as benzene, acetylene, and 1,3-butadiene (Graff and Albright, 1982) are especially important since furnace down time is always costly. Greater production of byproducts could in turn place increased loads on separation columns and incinerators, which create environmental concerns. It is therefore necessary to investigate not only the degree of VCM enhancement that results from the addition of an initiator but also the quantitative changes in byproduct composition that occur. EDC cracking is typically carried out at 50-60% conversions, and the large amounts of feed remaining in the product result in a two-phase mixture at ambient conditions. This fact makes performing a detailed quantitation of the furnace effluent very challenging. Not only are corrosive and toxic gases generated but the products partition between the two phases, which complicates sampling. Consequently, many studies pertaining to the effect of initiators on the EDC cracking reaction have simplified the analytical requirement by quantitating only selected reaction products. Eberly (1956) investigated yield enhancers by determining only the HCl formed by passing the effluent through a train 0888-5885/96/2635-0931$12.00/0
of aqueous traps and titrating with NaOH. Other groups employed gas chromatography using packed columns (Ashmore et al., 1982; Holbrook et al., 1971; Kolesnikov et al., 1985; Zalinyan et al., 1985). These were complex designs which required dual separation columns and some means of preventing condensation. Holbrook et al. (1971) carried out EDC pyrolysis in a furnace at high vacuum and injected the effluent through a gas sample loop. The stream was analyzed on two separate packed columns for vinyl chloride, 2-chlorobutadiene (chloroprene), methane, ethylene, and acetylene. Lashmanova et al. (1991) reported on the determination of 1,3-butadiene in the product by cold trapping the heavier components and absorbing the uncondensed gases in an alkali solution analyzed for butadiene. A more recent analytical system (Wright, 1991) employs a heartcutting GC method using two capillary columns to achieve similar separations, but the technique applies to determing only trace impurities in final product VCM. Obvious differences between this analysis and the present are that HCl is absent from the samples and that an assay of vinyl chloride is not achieved. The current study describes the design and use of a novel apparatus capable of providing pyrolysis and inline analysis by means of a modified gas chromatograph. Use of this microcracking device in conjunction with a high resolution capillary column allows for the most extensive quantitative analysis of the EDC pyrolysis reaction reported to date. The performance of the system is tested by evaluating the effects of known initiators on VCM yield. Applications which take advantage of the ability to control the variables of pyrolysis temperature and residence time are also presented. Experimental Section System Design: Pyrolysis Apparatus Description. The modified GC used in this study, hereafter referred to as the GC pyrolysis apparatus, is shown in Figure 1. A liquid EDC sample is injected by the © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996
Figure 1. Schematic diagram of the gas chromatographic pyrolysis apparatus used in this study: (1) liquid autosampler, (2) transfer line, (3) split/splitless injection port, (4) temperature control unit, (5) pyrolysis coil, (6) guard column, (7) PLOT capillary column, (8) needle valve, (9) sodium carbonate cartridge, (10) VCM Drager tube, (11) GC oven, (12) zero dead-volume tee. F1 and F2 refer to points where volumetric flow rates are measured.
autosampler (1) and flash vaporized in a back-pressure regulated injection port (3) operating in the split mode. The EDC vapor is transported to the pyrolysis chamber through a heated stainless-steel transfer line (2) by means of a flowing stream of helium carrier gas. Pyrolysis occurs in the heated coil (5) at a temperature e600 °C regulated by a control unit (4). The effluent passes through a deactivated fused silica guard column (6) and is adsorbed at the porous layer of the capillary column (7) at 40 °C. The reaction products are chromatographed on the open tubular capillary column and detected by a flame ionization detector. A zero deadvolume tee (12) is used to divert a portion of the pyrolysis effluent to vent. The hazardous off-gases are scrubbed with in-line cartridges prior to venting. The first cartridge (9) contained granular Na2CO3 to neutralize HCl. The latter cartridge (10) converts VCM to Cl2 over a Cr(VI) complex which is indicated on otoluidine (Drager tube no. 6728031). The remainder of the effluent is chromatographed and destroyed via combustion in the flame of the detector. The average residence times (τ) in the pyrolysis coil (5) were calculated according to:
τ ) V/v ) πd2l/4(F1 + F2) where V ) internal volume of a pyrolysis coil, πd2l/4; υ ) volumetric flow rate through a pyrolysis coil, F1 + F2; d ) internal diameter of pyrolysis coil tubing; l ) length of pyrolysis coil tubing; F1 ) volumetric flow rate at second split (measured after the Drager tube (10)); F2 ) volumetric flow rate through a GC column (measured at FID). Insulated 1/16 in. × 0.03 in. i.d. stainless steel tubing was used as the transfer line (2) and was routed against the heated block of the injection port to maintain elevated temperatures. All connections were made with zero dead-volume fittings. The pyrolysis coil consisted of 1/16 in. o.d. × 0.03 in. i.d. 316-L stainless steel tubing shaped into a 3 in. coil. Various lengths were used depending on the desired residence time, ranging from 12 to 30 in. Considerable effort was expended in programming the temperature control unit to achieve (1 °C at 500 °C. The pyrolysis coil
rested on the insulated baseboard located above the oven but below the metal exterior so that the coil did not protrude out of the instrument housing (enabling the liquid autosampler to be mounted on the instrument). The exit of the pyrolysis coil was connected to a zero dead-volume tee (12) residing in the GC oven (11). A 1 m × 0.53 mm i.d. segment of deactivated fused silica was used as a guard column. This guard column was connected directly to the tee (12) using a fused silica adapter with a 0.9 mm sleeve (Valco Instr.) and was connected to the porous layer open tubular (PLOT) chromatography column (7) using a glass press-fit connector (Restek Inc.). Hastelloy tubing (1/8 in. o.d.) was used to direct flow through the side port of the tee (12) to a Hastelloy needling valve (8). Flow rates through this secondary split ranged from 0 to 30 mL/ min depending on the degree of cracking required and the injection volume used. After extended operation for 3-4 months, particulates of black coke were visible at the head of the guard column, and plugging was observed in one instance for a 1/16 in. pyrolysis coil when operated for 10 weeks. This coking was manifested in a progressive increase in chromatographic retention times and increased conversions due to restricted flow (i.e., increased residence time). This problem was avoided thereafter by replacing the pyrolysis coil after a few weeks of service. Gas Chromatograph Description. A modified Hewlett-Packard 5890A gas chromatograph with Pascal Chemstation was used for all experiments. Injections were made with a 7673A liquid autosampler fitted with a nanoliter adapter accessory, enabling reproducible injection volumes of 0.1-0.2 µL. The split/ splitless injector port was held at 250 °C and 2 psig head pressure, and contained a 4 mm quartz insert. A 30 m × 0.53 mm i.d. GS-Q PLOT column was used (J&W Scientific) and the carrier gas was helium. The oven temperature program used was as follows: 40 °C for 5 min; 10 °C/min to 240 °C for 10 min. A standard H2/air flame ionization detector at 275 °C with helium make-up gas was used for all experiments. Peak identification for byproducts on the GS-Q column was confirmed by gas chromatography/mass spectrometry (GC/MS). These analyses were performed on samples obtained from a laboratory-scale cracking furnace. This furnace effluent was diluted in nitrogen to prevent condensation and subsequently captured in Tedlar gas bags. Scanning at g40 m/z provided a chromatogram free from the HCl peak but sacrificed detection of the lower m/z hydrocarbons. These peaks were identified by matching retention times with gas standards. Calibration was accomplished using certified gas standards containing methane, acetylene, ethylene, chloromethane, and chloroethane (Air Products Inc.) and a 100 ppm VCM standard in nitrogen (Supelco Inc.). Gas mixtures of these compounds were also prepared manually in Tedlar gas bags using large volumetric syringes (100-1000 mL capacity). Normalized results were calculated in all instances using FID integrated areas corrected by relative FID response factors recorded on the instrument. Linear response was demonstrated with vinyl chloride standards in nitrogen. Precautions were taken not to exceed
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 933 Table 1. Effect of Various Additives on VCM Yield As Measured by the GC Pyrolysis Apparatusa additive carbon tetrachloride
chlorine hexachloroethane chloroprene (2-chlorobutadiene)
spike level (µg/g)
VCM yield without additive (wt %)
VCM yield with additive (wt %)
classification
conditionsb
1000 1000 1300 1000 1300 1000 1000 1000
47.40 ((0.61) 9.91 ((0.35) 36.10 (( 0.47) 9.91 ((0.35) 25.45 ((0.47) 47.40 ((0.61) 47.40 ((0.61) 9.91 ((0.35)
50.90 ((0.53) 11.79 ((0.68) 41.78 ((0.21) 12.0 ((0.61) 33.36 ((0.47) 49.00 ((0.63) 46.90 ((0.53) 9.26 ((0.66)
promoter promoter promoter promoter promoter slight promoter possible inhibitor possible inhibitor
1 2 3 2 3 1 1 2
a All values are means of four trials with standard deviations shown. b Conditions: (1) 410 °C coil temperature, 2.1 s residence time, reagent-grade EDC. (2) 435 °C coil temperature, 0.1 s residence time, technical-grade EDC. (3) 500 °C coil temperature, 0.6 s residence time, technical-grade EDC.
the capacity of the PLOT column which was displayed by peak tailing and a negative deviation from linearity. The detection limits achieved depended largely on the split ratios used and on the amount of chlorine in the analyte, which typically ranged from 0.001 to 0.004 wt %. Since HCl did not respond to the FID under these conditions, HCl wt % was calculated from the measured VCM wt % assuming a 1:1 molar ratio. Other capillary columns were evaluated for the separation of the pyrolyzed EDC samples. A 7.0 µm film, 60 m × 0.53 mm Rtx-1 column (Restek Inc.) was evaluated in addition to a 0.5 µm film, 100 m × 0.25 mm Petrocol DH column (Supelco Inc.). With CO2 cryofocusing at 0 °C, separation of 1,3-butadiene and vinylacetylene was achieved with both of these methylsilicone columns. However, neither column could withstand the high levels of HCl. A 30 m × 0.53 mm Al2O3 PLOT column deactivated with KCl (Chrompack Inc.) was also evaluated. This column provided excellent resolution of the C1-C4 hydrocarbons, but the phase reacted with EDC, probably via abstraction of a proton from the acidic phase. Method precision was measured by computing standard deviations of repetitive injections under cracking conditions. The standard deviations ranged from 0.11% to 2.6%, and all data sets contained 4-6 repetitive injections. The highest precisions were achieved when the pyrolysis temperature fluctuations were minimized and when the pyrolysis chamber had been conditioned with approximately five injections prior to collecting data. This conditioning of EDC pyrolysis chambers is well documented (Barton, 1949, Holbrook, 1971) and probably serves to coat the steel wall with a carbon overlayer. For all experiments involving evaluations of additives, neat EDC was injected immediately prior to the spiked sample. Neat EDC was then reinjected at the end of this sequence to check for drift that may have occurred over the course of the experiment. Reagent-grade 1,2-dichloroethane (Fisher, certified ACS) was used in addition to a technical-grade EDC which contained benzene (1637 ppm) and trichloroethylene (8826 ppm). The carbon tetrachloride, chlorine, vinylidene chloride, cis-1,2-dichloroethylene, trans-1,2dichloroethylene, benzene, trichloroethylene, and 1,1,2trichloroethane were purchased from Aldrich Chemical Co. and used without further purification. A gas mixture used for identification of 1,3-butadiene, vinylacetylene, and chloroethane was obtained from Scott Specialty Gases. Chloroprene was prepared by the addition of 2,3-dichlorobutene to a 20% solution of KOH in water at 70-80 °C, isolated as a crude wet distillate, and dried over 3A molecular sieves. All solutions of
EDC containing liquid additives were prepared by weighing the additive into volumetric flasks and diluting to known volumes. Solutions of chlorine dissolved in EDC were prepared by slowly bubbling Cl2 gas through the solution and measuring the corresponding weight change. The solutions were immediately transferred to septum-capped vials and injected by autosampler. Results and Discussion System Validation: VCM Yield. To assess the validity of the GC pyrolysis apparatus as a microreactor for the cracking of 1,2-dichloroethane, additives with known effects on VCM yield were evaluated. Table 1 shows VCM conversions in the presence and absence of selected additives. The results indicate that carbon tetrachloride is indeed promoting VCM formation, which is consistent with previous studies performed on both plant (Brindus et al., 1984) and laboratory (Eberly, 1956) scales. Chlorine is also found to be a strong initiator using the GC pyrolysis apparatus, in agreement with numerous previous findings (Barton, 1949, Ashmore and Owen, 1982, Holbrook, 1971, Longhini, 1984). Chloroprene is believed to act as a free-radical scavenger (Kolesnikov et al., 1985), and this inhibiting effect may be occurring in the present apparatus as well. The same promoting effect found by Sonin et al. (1971) with hexachloroethane is also demonstrated with the GC pyrolysis apparatus. These results establish that the mechanism of transporting EDC vapor through a microreactor with an inert carrier gas as is performed here does not alter the well-known reactivity patterns of 1,2-dichloroethane pyrolysis with respect to VCM yield. Figure 2a shows the increase in VCM yield obtained as a function of CCl4 contained in the feed EDC. The slope of this linear relation indicates a VCM enhancement of +0.44 wt %/100 ppm CCl4. The measured decline in EDC concentration of -0.72 wt %/100 ppm also exhibits linearity. The difference of +0.28 wt %/100 ppm compares favorably with the formation of +0.25 wt % HCl calculated from the measured VCM concentration (assuming a 1:1 molar ratio), indicating that a mass balance is achieved in these experiments. Figure 2b shows that chlorine exhibits a contrasting behavior with respect to the efficiency of VCM promotion in EDC cracking. The initial 300 ppm Cl2 produces a strong enhancement of 1.67%/100 ppm, whereas the slope lessens to 0.2-0.3 thereafter. As with CCl4 initiation, the EDC drop at each incremental addition
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Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996
Figure 2. VCM yields of pyrolyzed EDC at various levels of added promoters: (top) carbon tetrachloride; (bottom) chlorine. Conditions are provided in Tables 2 and 3, respectively.
of promoter is approximately tracked by the sum of the VCM and HCl gains. The detailed reaction mechanisms by which these changes occur are reported elsewhere (Barton, 1949; Holbrook et al., 1971; Ashmore et al., 1982) and are beyond the scope of the present study. Byproduct Formation. Figure 3 shows representative chromatograms for reagent-grade EDC pyrolyzed at various temperatures and fixed residence time. It is apparent that ethylene is the primary byproduct at low cracking temperatures. Lesser amounts of methane and chloromethane are also formed. (The 1,1-EDC is present as an impurity in the uncracked reagent-grade EDC and is not a product of cracking.) As conversion increases, numerous other byproducts are formed: acetylene, vinylidene chloride, and chloroprene are the most prominent. Methane and ethylene are again detected and four other minor peaks have appeared: butadiene/ vinylacetylene, methylene chloride (dichloroethane), cis1,2-dichloroethylene, and trans-1,2-dichloroethylene. As EDC is further consumed, many other chlorinated C1C4 byproducts are formed. Mono- and dichloroacetylenes appear as well as chloroethane, (chlorovinyl)acetylene, methylacetylene, and trace amounts of propylene, trichloroethylene, benzene, and 1,1,2-trichloroethane. Most of the byproducts produced steadily increase in amount as the coil temperature increases. In contrast, a few byproducts exhibit further consumption after growth, such as chloroprene, methane, and chloromethane. One aspect of the thermal cracking occurring in the GC pyrolysis apparatus is notably dissimilar to typical
larger scale EDC pyrolysis. Residence times of 6-8 s are normally required to obtain 50% cracking (EDC consumption) at 500 °C. To achieve this conversion at 500 °C with the present system, residence times of about 1 s were required. Holbrook et al. (1971) found a correlation pertinent to this issue which links higher surface/volume ratios (S/V) of the reactor to increased reaction rates for EDC pyrolysis. This relationship provides a likely explanation for the rapid reaction observed in the present study since the 1/16 in. o.d. × 0.03 in. i.d. pyrolysis coil employed possesses a very high S/V ratio (52 cm-1). To further investigate this behavior, cracking was performed with a pyrolysis coil of S/V ) 19 cm-1 constructed from the same 316-L steel alloy. A larger residence time of 4.3 s was required to achieve 50% cracking depth at 500 °C, thus confirming this S/V dependence in the GC pyrolysis apparatus. Holbrook et al. (1971) also found that ethylene is the major byproduct of the EDC pyrolysis reaction at high S/V conditions. This is consistent with the present findings, as is shown in Figure 3. The formation of ethylene by surface-induced dechlorination of 1,2dichloroethane appears to be significant in the present system as well. It should also be noted that this type of in-line pyrolysis was initially attempted in the injection port of a standard gas chromatograph (HP 5890A), and the maximum conversion achieved was 1% due to limitations imposed by the injection port on residence time and temperature. The introduction of a pyrolysis coil between the injection port and analytical column does increase system dead volume. The consequence is longitudal peak broadening, which is visible at longer retention times (see Figure 2). This effect is not severe, however, because the band is focused at the head of the capillary column by means of EDC condensation and adsorption onto the high surface area porous polymer. Effect of Promoters on Byproduct Formation. In addition to providing quantitative information about how initiators effect VCM and EDC yield, the GC pyrolysis apparatus is also capable of determining numerous byproducts. Table 2 shows the changes in byproduct compositions resulting from CCl4 addition to the feed EDC. Some byproducts such as methane and ethylene show an initial drop and then level off, whereas 1,1-EDC shows an initial increase before remaining constant. Methyl chloride and dichloromethane show changes only when higher levels of CCl4 are present (between 650 and 1300 ppm). The remainder of the compounds show trends that increase or decrease progressively with added CCl4. The ratio (R) of concentrations at 1300 ppm to those in neat EDC was computed to compare relative changes in the amounts of byproducts formed. It is apparent that CCl4 addition causes the largest enhancements with vinylidene chloride (R ) 1.7), trans-1,2-dichloroethylene, chloroacetylene, chloroprene (R ) 1.5), acetylene, butadiene/ vinylacetylene (R ) 1.4), and dichloromethane. A trace amount of (chlorovinyl)acetylene is produced at 1300 ppm CCl4 but is absent in the pyrolyzed neat EDC. Significant drops in concentrations of chloroethane (R ) 0.4), ethylene (R ) 0.7), methyl chloride and methane (R ) 0.8) are also evident. The results of experiments performed with chlorine addition are shown in Table 3. Chloroprene (R ) 2.6), trans-1,2-dichloroethylene (R ) 2.2), acetylene (R ) 1.4) and chloroacetylene showed the largest enhancements, whereas significant inhibition occurred with vinylidene
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 935
Figure 3. Gas chromatograms of pyrolyzed EDC at various coil temperatures: (A) 350, (B) 400, (C) 450 °C. Residence time ) 12 s.
chloride (R ) 0.2), methyl chloride (R ) 0.6), methylene chloride (R ) 0.5), ethane (R ) 0.5), propylene (R ) 0.7), and methane (R ) 0.7). As was found with CCl4 addition, (chlorovinyl)acetylene is absent in pyrolyzed neat EDC, yet 0.014% is present at 1300 ppm Cl2. Reaction Kinetics. The GC pyrolysis apparatus is configured with a tee (12) preceding the separation column to divert a portion of the coil effluent through a needle valve (8) to vent (see Figure 1). This design enables a variation of the pyrolysis coil flow rate while still maintaining a modest flow to the capillary GC column. Adjusting the flow through the needle valve
(8) thus allows for variation in the pyrolysis coil residence time. The results of an experiment illustrating this feature are shown in Figure 4. Both the rate of decay of EDC and the rate of production of VCM were measured by gradually increasing the reactor residence time at constant temperature. Such data are useful in the determination of kinetic parameters such as activation energies and rate constants. The ease with which this data can be acquired is apparent; all of the data contained in Figure 4 was recorded within an 8-h day. Since the GC system is equipped with a liquid autosampler and a data station capable of sequencing injections,
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Table 2. Detailed Analysis of Pyrolyzed EDC at Various Levels of CCl4 Additiona neat methane ethylene acetylene ethane propylene methyl chloride methylacetylene chloroacetylene vinyl chloride acetaldehyde butadiene/vinylacetylene chloroethane dichloroacetylene methylene chloride vinylidene chloride trans-1,2-dichloroethene 1,1-ethylene dichloride cis-1,2-dichloroethene (chlorovinyl)acetylene chloroprene ethylene dichloride benzene trichloroethylene HCl-calculated
325 ppm
650 ppm
1300 ppm
mean
StdDev
mean
StdDev
mean
StdDev
mean
StdDev
R (1300 ppm/neat)
0.018 0.63 0.18 nd nd 0.029 0.001 0.004 36.10 nd 0.016 0.004 nd 0.016 0.23 0.009 0.043 0.026 nd 0.23 39.89 0.15 1.47 20.96
1.43E-3 0.17 3.65E-3
0.014 0.46 0.19 nd nd 0.029 0.001 0.005 37.27 nd 0.016 0.002 nd 0.016 0.26 0.010 0.050 0.027 0.001 0.25 38.14 0.15 1.47 21.64
1.30E-4 9.72E-3 7.56E-3
0.014 0.43 0.21 nd nd 0.029 0.001 0.006 39.06 nd 0.017 0.001 nd 0.016 0.30 0.011 0.051 0.030 0.001 0.29 35.23 0.15 1.47 22.68
1.22E-3 7.91E-3 1.85E-3
0.014 0.45 0.26 nd nd 0.021 0.002 0.007 41.78 nd 0.022 0.001 nd 0.021 0.40 0.014 0.048 0.038 0.001 0.34 30.71 0.14 1.47 24.26
3.21E-3 6.73E-3 5.42E-3
0.8 0.7 1.5
3.00E-4 7.05E-4 6.37E-4 0.21
0.7 1.3 1.7 1.2
1.04E-3 5.82E-4
1.4 0.4
4.77E-3 1.61E-2 6.59E-4 5.52E-3 3.67E-3 0.00 2.53E-3 0.36 1.03E-3 4.89E-3 0.123
1.4 1.7 1.7 1.1 1.4
7.51E-3 3.62E-4 1.49E-4 0.47 4.53E-4 1.68E-3 1.55E-3 1.62E-2 6.99E-4 3.14E-3 3.91E-3 1.83E-2 0.66 3.56E-3 5.61E-3 0.273
1.38E-3 1.22E-4 2.82E-4 0.49 6.07E-4 4.10E-4 8.97E-4 6.58E-3 2.11E-4 2.08E-3 7.03E-4 0.00 9.08E-3 0.79 9.69E-4 9.77E-3 0.282
5.83E-3 1.42E-4 9.75E-5 0.13 7.67E-4 4.07E-4 1.77E-3 5.11E-3 2.09E-4 7.70E-4 1.38E-3 0.00 2.87E-3 0.21 9.69E-4 6.93E-3 0.077
1.5 0.8 1.0 1.0 1.2
a All values are means of four trials with standard deviations shown. All values reported in ppm ) µg/g; pyrolysis coil temperature ) 500 ( 1 °C; residence time ) 1.2 s; nd ) none detected.
Table 3. Detailed Analysis of Pyrolyzed EDC at Various Levels of Cl2 Additiona neat methane ethylene acetylene ethane propylene methyl chloride methylacetylene chloroacetylene vinyl chloride acetaldehyde butadiene/vinylacetylene chloroethane dichloroacetylene methylene chloride vinylidene chloride trans-1,2-dichloroethene 1,1-ethylene dichloride cis-1,2-dichloroethene (chlorovinyl)acetylene chloroprene ethylene dichloride benzene trichloroethylene HCl-calculated
325 ppm
650 ppm
1300 ppm
mean
StdDev
mean
StdDev
mean
StdDev
mean
StdDev
R (1300 ppm/neat)
0.13 5.47 0.11 0.021 0.004 0.089 0.002 0.002 25.45 0.41 0.33 0.021 nd 0.024 0.23 0.005 nd 0.024 nd 0.041 51.61 0.16 1.37 14.78
1.17E-2 0.15 1.14E-2 1.77E-2 2.07E-3 4.73E-3 2.34E-4 2.08E-4 0.47 7.89E-2 4.11E-3 3.12E-3
0.095 5.08 0.17 0.011 0.002 0.058 0.001 0.004 30.87 0.33 0.45 0.024 nd 0.016 0.03 0.005 nd 0.026 0.003 0.074 43.55 0.17 1.50 17.93
4.44E-3 0.014 3.87E-3 1.54E-3 1.42E-4 1.57E-3 1.36E-4 1.72E-4 0.36 1.49E-2 1.26E-3 1.73E-3
0.091 4.97 0.18 0.011 0.002 0.052 0.001 0.003 31.92 0.32 0.45 0.026 nd 0.013 0.032 0.007 nd 0.025 0.009 0.085 42.03 0.17 1.47 18.53
3.01E-3 0.18 9.29E-3 7.50E-4 1.94E-4 7.59E-4 3.31E-4 4.02E-4 1.15 2.29E-3 1.27E-3 4.20E-4
0.091 4.68 0.18 0.011 0.003 0.053 0.002 0.003 33.36 0.37 0.46 0.029 nd 0.013 0.047 0.012 nd 0.027 0.014 0.10 39.94 0.17 1.45 19.37
5.28E-3 0.16 2.28E-2 1.22E-3 1.45E-3 2.23E-3 3.72E-4 6.03E-4 1.73 4.62E-2 3.38E-3 1.47E-3
0.7 0.9 1.6 0.5 0.7 0.6 0.8 1.6 1.3 0.9 1.4 1.3
4.75E-4 1.04E-2 2.73E-3
0.5 0.2 2.2
2.59E-3 1.49E-3 1.84E-2 2.59 1.04E-3 1.17E-3 1.002
1.2
3.52E-3 3.34E-2 8.42E-4 2.18E-3 5.61E-3 0.71 3.41E-3 2.84E-2 0.272
2.09E-3 1.55E-3 4.81E-4 3.15E-4 8.52E-5 1.01E-3 0.57 1.58E-3 1.56E-3 0.207
2.56E-4 4.42E-3 8.40E-4 4.18E-4 7.43E-4 1.44E-2 1.66 5.82E-4 3.52E-3 0.665
2.6 0.8 1.0 1.1 1.3
a All values are means of four trials with standard deviations shown. All values reported in ppm ) µg/g; pyrolysis coil temperature ) 500 ( 1 °C; residence time ) 0.6 s; nd ) none detected.
operator intervention is required only when changing coil temperature or vent flow. Conclusions A modified gas chromatographic apparatus has been devised to measure the product and byproduct concentrations in the pyrolysis reaction of 1,2-dichlorethane, by performing the pyrolysis and in-line chromatography in a single step. It has been shown that the device is capable of measuring changes in VCM yield that correspond to expected trends when known promoters (CCl4, Cl2, C2Cl6) are present in the feed EDC. Numer-
ous byproducts were identified and quantitated in the reaction product stream using a single chromatographic column. The reactivity exhibited in the microreactor was characteristic of high S/V furnaces: the reaction proceeds rapidly, and ethylene is the major byproduct. It is also shown that the GC pyrolysis apparatus is capable of measuring changes in reactant/product concentrations as a function of reaction temperature and reactor residence time. The methodology could also be applied to the quantitative monitoring of pyrolysis reactions using other liquid feedstocks as well, such as naphthas, condensates, and various chlorinated hydrocarbons.
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 937
Figure 4. Concentrations of EDC and VCM as a function of residence time at constant temperature. Pyrolysis coil: 1/16 in. o.d. × 0.03 in. i.d. × 30 in. long, held at 475 ( 1 °C.
Acknowledgment The author thanks Drs. K. Seper, R. Stanton, and W. Schwartz for helpful discussions and G. Hermann for assistance with GC/MS experiments. Literature Cited Ashmore, P. G.; Owen, A. J. Chlorine-catalyzed Pyrolysis of 1,2Dichloroethane. Part 2. Unimolecular decomposition of the 1,2dichloroethyl radical and its reverse reaction. J. Chem. Soc., Faraday Trans. 1 1982, 78, 677. Ashmore, P. G.; Owen, A. J.; Gardner, J. W.; Smith, B.; Sutton, P. R. Chlorine-catalyzed Pyrolysis of 1,2-Dichloroethane. Part 1. Experimental Results and Proposed Mechanism. J. Chem. Soc., Faraday Trans. 1 1982, 78, 657. Barton, D. H. The Kinetics of the Dehydrochlorination of Substituted Hydrocarbons. Part I. Induced Dehydrochlorination. J. Chem. Soc. 1949, 148.
Brindus, N.; Anca, E.; Chiroiu, N. Initiating the Thermal Decomposition of 1,2-Dichloroethane. Rom. Pat. RO84803, 1984. Eberly, K. E. Cracking Ethylene Dichloride. U.S. Patent 2 755 315, 1956. Graff, M. J.; Albright, L. F. Coke Deposition from Acetylene, Butadiene and Benzene Decompositions at 500-900 °C on Solid Surfaces. Carbon 1982, 20, 319. Holbrook, K. A.; Walker, R. W.; Watson W. R. The Pyrolysis of 1,2-Dichloroethane. J. Chem. Soc. B 1971, 577. Howlett, K. E.; Barton, D. H. The Kinetics of the Dehydrochlorination of Substituted Hydrocarbons. Part II. The Mechanism of the Thermal Decomposition of 1,2-Dichloroethane. J. Chem. Soc. 1949, 155. Kolesnikov, V. A.; Lashmanova, N. V.; Efremov, R. V.; Danov, S. M. Influence of Impurities on Pyrolysis of 1,2-Dichloroethane. Zh. Prikl. Khim. 1985, 58, 383. Lashmanova, N. V.; Kolesnikov, V. A.; Efremov, R. V.; Danov, S. M.; Komlev, Y. V. Effect of Feedstock Quality on Concentration of 1,3-Butadiene in Vinyl Chloride Obtained in the Pyrolysis of 1,2-Dichloroethane, Khim. Promst. 1991, 6, 328. Longhini, D. A. Vinyl Chloride Production. U.S. Patent 84 674 192, 1984. Sonin, E. V.; Englin, A. L.; Pimenov, I. F.; Fedotova, I. I. Gasphase Reactions for Producing and Transforming Chloroethanes. V. Dehydrochlorination of 1,2-Dichloroethane Initiated by Hexachloroethane. Zh. Fiz. Khim. 1971, 45, 1121. Wright, D. W. Assaying for Trace Impurities in High Purity Vinyl Chloride Using HRGC & MDGC. Presented at the Pittsburgh Conference, Chicago, IL, March 1991. Zalinyan, V. P.; Airapetyan, A. S.; Maslyukova, D. F.; Gevorkyan, M. G. Gas Chromatographic Method of Separating Chlorinecontaining Mixtures Chem. Abstr. 85:153390.
Received for review August 10, 1995 Accepted December 19, 1995X IE9505017
X Abstract published in Advance ACS Abstracts, February 15, 1996.
