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Coke and Byproduct Formation during 1,2-Dichloroethane Pyrolysis in a Laboratory Tubular Reactor Alessandro G. Borsa,† Andrew M. Herring,† J. Thomas McKinnon,*,† Robert L. McCormick,† and Glen H. Ko‡ Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401, and Mitsubishi Chemical Research and Innovation Center, One Broadway, Suite 600, Cambridge, Massachusetts 02142
A laboratory quartz tube reactor apparatus was used to study coke and byproduct formation in 1,2-dichloroethane, also known as ethylene dichloride (EDC), pyrolysis. The effects of metal coupons, feed purity, and temperature on the amount of coke deposited and exit gas-phase compositions were analyzed. Coke formation on nickel, chromium, iron, and stainless steel metal coupons was investigated by scanning electron microscopy/energy-dispersive X-ray spectroscopy and ion-coupled plasma atomic adsorption spectroscopy. Based on scanning electron micrographs, different metals have little effect on the nature of coke formed, and it appears that coke is formed by tar droplet formation in the gas phase with subsequent impingement on surfaces. FeCl2 formation and migration along the reactor accompanied coke formation on the metal coupons. It was determined that the presence of metals coupons increases EDC conversion as well as coke formation. Two distinct types of coke are formed: hard coke is formed in the hot zone of the reactor and soft coke is formed at the exit. Increases in reaction temperature, CCl4 in the feed, and presence of FeCl2 all increase both types of coke formation, but hard coke in particular. A total of 0.3 wt % Cl2 in the feed resulted in 60% EDC conversion at 380 °C. This also reduced by more then half the total amount of hard coke formed, as compared to the lowest amount of hard coke formed by all other runs, but increased the total byproduct formation. EDC feed obtained from a commercial vinyl chloride monomer manufacturing plant produced 28% EDC conversion at 480 °C and large amounts of coke. Chloroprene was the only chemical species that strongly correlated with total coke formation. Introduction Vinyl chloride monomer (VCM) is commercially produced by thermally decomposing 1,2-dichloroethane (also known as ethylene dichloride or EDC) in largescale pyrolysis furnaces. These furnaces operate at approximately 50-60% EDC conversion, 99% VCM selectivity, with gas residence times of 10-30 s, gasphase temperatures of up to 500 °C, and pressures of 10-20 atm. An EDC pyrolysis furnace consists of a large insulated firebox, lined with refractory bricks and heated by a large number of wall- and/or bottommounted burners to the desired operating temperature. EDC flows through stainless steel tubes were positioned inside the firebox to form a mixture of VCM, EDC, HCl, and byproducts. The effluent gases are quenched to minimize byproduct formation. The global reaction that describes this process is
EDC w VCM + HCl
(1)
However, the actual chemical path that EDC takes to form VCM involves hundreds of free-radical reactions, as discussed elsewhere.1,2 Out of all of the possible freeradical reactions, however, only a limited number of them account for most of the molar flux of the system, but it is the minor reactions that have the most interest from a chemical standpoint and that cause most of the problems in industrial reactors. Even though less than * Corresponding author. E-mail:
[email protected]. † Colorado School of Mines. ‡ Mitsubishi Chemical Research and Innovation Center.
1% of EDC mass is lost to byproduct-forming side reactions, the small fraction of byproducts causes severe inefficiencies in the pyrolysis process itself as well as in downstream processes. Coke, one of the byproducts of major concern, is deposited throughout the process from the reactor coils inside the firebox to where the gases are quenched. Coke formation causes three principal process inefficiencies. First, the layer of coke deposited on the tube walls decreases the heat-transfer coefficient, requiring a higher temperature in the firebox to maintain EDC conversion at the desired level. Second, the coke layer slowly decreases the cross-sectional area of the tubes and causes an increase in pressure drop. Finally, coke particles entrained in the gas need to be removed from the liquid stream after the gas-quenching step to avoid plugging and other problems in downstream unit operations. It is known that ppm levels of impurities can act as promoters or inhibitors of EDC pyrolysis reactions.3-5 However, before it is possible to take advantage of the effects of such impurities to increase the efficiency of VCM production, it is important to take into account how impurities in the feed might affect byproduct formation. For example, if promoters increase coke and other byproduct formation, any increase in EDC conversion could be offset by increased maintenance costs and furnace downtime. Increased production of byproducts could increase loads on separation columns and incinerators. Limited information is available in the literature with regard to coke formation in EDC pyrolysis. Work
10.1021/ie0006460 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/19/2001
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reported by Borsa et al.6 presents a detailed study of industrial plant coke samples and concludes that coke is mostly formed in the gas phase as tar droplets in the high-temperature region of the pyrolysis furnace and is subsequently deposited on tube walls. This work also concluded that Fe, in the form of FeCl2, was a promoter of tar droplet formation in the gas phase and hence coke. Less information is available in the literature on how coke formation is affected by impurities in EDC feed. Work presented by Incavo7 looks at the effect of such impurities as carbon tetrachloride, molecular chlorine, and chloroprene on EDC conversion in a stainless steel reactor with a very large surface-to-volume ratio. This researcher quantified the promoting effect of chlorine and carbon tetrachloride on EDC conversion, but he did not look at how coke formation is affected. The residence times used, 0.1-2 s, are considerably less than commercial process conditions, and the iron from the stainless steel material used for the pyrolysis coil most likely had considerable gas-phase catalytic effects. Zychlinsky et al.8 presented results from laboratory experiments in which 99.965, 99.707, and 98.655 wt % pure EDC charges as well as those same EDC charges with 0.1-0.2 wt % carbon tetrachloride added were pyrolyzed in a quartz tube reactor where the amount of coke formed was measured along with EDC conversion. The pure EDC charges all contained varying amounts of the following hydrocarbons: vinyl chloride, chloroethene, 1,1-dichloroethene, 1,2-dichloroethene, 1,1-dichloroethane, chloroform, benzene, trichloroethene, and tetrachloroethene. The authors concluded that both coke formation and EDC conversion are very sensitive to EDC feed purity and that carbon tetrachloride promoter increased EDC conversion as well as coke formation. The authors concluded that the increase in coke was caused by an increase in conversion and not directly by CCl4 and suggest that coke could be formed from EDC or an EDC radical. A 14C labeling technique was used to try to shed light on which compounds might be coke precursors. Their results show that 50-60% of the carbon from chloroprene (C4H5Cl) is converted to coke. It was also determined that, under EDC pyrolysis conditions, benzene was not important in coke formation and that acetylene had only a small effect on coke formation. Conditions used in their experiments were slightly different from common industrial conditions (residence times of 5-10 s and temperatures of 540550 °C), and little quantitative information on coke formation is presented. In later work, the same group9 presented results from a stainless steel laboratory reactor with which they investigated the effect of various chlorinated hydrocarbons on EDC conversion but did not look at coke formation. Finally, the same workers10 presented results, from an investigation of coke formation over stainless steel coupons during EDC pyrolysis, obtained with the use of a thermal gravimetric apparatus. It was found that coke formation on the coupons was accompanied by FeCl2 formation. While coking was observed for steel, little coke formation was noted in the presence of ceramics or quartz under identical conditions. In this paper, we present laboratory experiments used to investigate the effects of initiators and different metals on EDC conversion, coke formation, and other byproduct formation. The main purpose of these experiments is to quantify the effects of such variables on coke formation in order to determine which steps can be
Figure 1. Laboratory reactor schematic.
taken to reduce coke and other byproduct formation in the industrial process. Experimental Section Pyrolysis Reactor Apparatus. The main component of the experimental apparatus is a 100 cm long and 2.2 cm i.d. quartz tube which is heated at constant temperature by a three-stage thermostatically controlled electric resistance furnace (Sola Basic Heavy Duty Lindberg). A schematic diagram of the experimental apparatus is shown in Figure 1. EDC feed is pumped from a glass bottle by a highprecision ISCO model 2350 HPLC pump through a Pyrex vaporizer. EDC feed is kept under positive nitrogen pressure. The vaporized EDC is sent either through a bypass line, directly to the cold trap, or through the reactor. The reactor pressure is monitored with a MKS Baratron pressure transducer. The reactor temperature profile is measured with a 175 cm Omega type K thermocouple that can be moved along the central axis of the reactor inside a 0.15 cm i.d. quartz sheath. The product gases are sent either to the cold trap or to the sampling train. In the sampling train, unreacted EDC and HCl gas are trapped in deionized water. The gases exiting the sampling train are analyzed by an online gas chromatograph (GC). When the exit gases are not analyzed, they flow directly to the waste destruction train where they are thermally decomposed at 900 °C with excess O2 and neutralized in a concentrated sodium hydroxide bath. Preparation. Before each run, the quartz reactor and the thermocouple quartz sheath were thoroughly cleaned by placing them in a concentrated nitric acid solution for 1-2 days, followed by a water wash. A fresh quartz wool plug was prepared for each run and placed at the exit of the tube in approximately a 15 cm long section. In about half of the runs, metal coupons were placed inside the quartz reactor at different locations along the axis of the tube in order to investigate the effects of 316 stainless steel, 99.9% pure iron, 99.8% pure nickel, 99.8% pure chromium metals on coke and byproduct formation. The stainless steel and iron coupons were cut from a thin foil into rectangular shapes (2 × 1 cm). The nickel coupons were cut from a 2 mm thick plate into irregular shapes with the largest dimension of 1-2 cm, and the chromium material was received in the form of irregular chips with the largest dimension of 1-2 cm. One run was performed in the presence of FeCl2 pellets. In this run approximately 1 g of the pellets was placed
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in a small ceramic boat and positioned in the center of the reactor tube. Before each run the EDC feed bottle was rinsed with pure EDC before filling the bottle with fresh feed. This was done under the hood in such a way as to minimize feed contact with air before being placed under positive nitrogen pressure. Feed charges of various purities were prepared by adding small amounts of benzene, carbon tetrachloride, and chlorine to 1283 g of 99.98% anhydrous EDC. HPLC grades, 99.9+ pure, were used to prepare 0.1 wt % benzene and 0.1-0.4 wt % carbon tetrachloride feeds. The chlorine feed (0.35 wt %) was prepared by using 99.5% pure chlorine gas. A small amount of pure EDC (13 g), kept in an ice bath, was saturated with chlorine by slowly bubbling it through EDC, and the amount of dissolved chlorine was determined gravimetrically. In one run, EDC feed supplied directly from a commercial VCM production facility was used. Procedures. All experiments were conducted with EDC liquid flow rates of 1 mL/min (residence times of about 20 s), under ambient pressure (0.82 atm), and temperatures ranging from 380 to 495 °C. Each experimental run lasted 12 h. The residence time and temperatures were chosen so as to mimic industrial plant conditions. The inlet pressure was monitored throughout the run as an indicator of downstream plugging. Insulation at the exit of the reactor was modified to maintain the temperature of the quartz wool plug at between 160 and 120 °C so as to prevent condensation of EDC but at the same time allow deposition of all tar and coke material from the gas phase. At four equally spaced time intervals during the run, the exit gases were analyzed. This analysis consisted of two parts. In the first step, the gases that passed through the sampling train were analyzed by online GC. In the second step, the HCl and unreacted EDC, collected in the sampling train, were analyzed by titration and offline GC, respectively. The coke deposited on the metal coupons was analyzed via scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX). A JEOL model 840 SEM equipped with a Tracor-Northern 5500 EDX system was employed. Selected coupons were also digested in aqua regia, and the resulting solution was analyzed by ion-coupled plasma atomic absorption spectroscopy (ICP-AAS). Axial coke deposition profiles were determined by burning the deposited coke and analyzing the CO2 gas collected. In this process, pure oxygen was metered into the reaction tube using a Tylan mass flow controller. A small moveable electric resistive furnace was positioned along the tube so that at one time only a 5 cm section of coked tube was heated. The effluent gas was collected in a Tedlar bag for offline analysis using GC with thermal conductivity detection. The combination of the elapsed burn time, the oxygen flow rate, and the CO2 concentration allowed for a reliable method of determining the spatially resolved carbon deposition profile. GC. The GC used for analyzing the gas-phase online, the liquid EDC, and the coke burnoff samples is an HP 5890 series II with a dual column and detector system. An Alltech capillary column (30 and 0.53 mm i.d.) connected to a flame ionization detector (FID) was used to analyze the online gas samples as well as the liquid EDC samples. A Supelco stainless steel, 60/80 Carboxen packed column (15 ft and 1/8 in i.d.) was used in conjunction with a thermal conductivity detector (TCD)
to determine the carbon dioxide concentration in the coke burnoff experiments. The GC was calibrated against several commercial standards to obtain response factors for the following chlorinated hydrocarbon species: 1,1,2-trichloroethane, tetrachloroethane, benzene, 1,4-dichlorobutane, 3-chloro1-butene, 1-chloro-2-butene, cis-2-chloro-2-butene, cis1,4-dichloro-2-butene, trans-1,4-dichloro-2-butene, 2-chloro-1,3-butadiene, hexachlorobutadiene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenezene, and 1,4dichlorobenzene. A Scott Specialty Gas standard containing 2010 ppm VCM in helium was used repeatedly to obtain an average response factor for VCM. The EDC response factor was calculated by using a 99.8% pure EDC liquid feed and a calibration standard of 200 µg/ mL EDC in methanol. A gas standard containing 33 mol % CO, 34 mol % CO2, and the balance H2 was used to calibrate for CO2. Results The experiments performed can be divided in two main groups: a first group in which metal coupons were placed inside the quartz reactor and a second group in which no metal coupons were used. EDC feed composition, metals used, maximum temperature, and % EDC conversion for each experimental run is listed in Table 1. The experimental runs of the second group are considered to be metal-free except for one in which iron was introduced in the form of FeCl2 pellets. During each run the temperature profile and % EDC conversion as well as the amount of coke deposited and the exit gasphase composition were measured. % EDC conversion is defined as
% EDC conversion ≡ (moles of HCl/moles of EDC) × 100 (2) EDC conversion was measured at four equally spaced time intervals during each run, and it was found that conversion did not change with time. All temperature profiles have similar shape as well as inlet and exit temperatures. A typical temperature profile is shown in Figure 3. As a matter of convenience, the authors refer to the temperature of each run as being the maximum temperature where the profile is approximately flat. Because of the very long time required for each run (about 3 days), duplicate runs for each data point were not performed. The carbon and chlorine balances for each run were closed to better than (10% and (5%, respectively. It was determined that most of the variability in the experimental results can be attributed to either metal contamination in the quartz tube or error in the temperature measurements or a combination of the two. For the runs with no metals present, a set of three runs (runs 5, 13, and 20) can be considered to have the same conditions, and their % EDC conversion results very by (5% EDC conversion. To a first approximation, the uncertainty in % EDC conversion is around 10%. Experimental conditions of run 12 are comparable to runs 5, 13, and 20, but run 12 resulted in a much larger % EDC conversion. The quartz tube of that run was not properly cleaned. This was determined after the run when a clearly visible residue of iron oxide was visible on the quartz tube reactor after the coke deposit was burned off. Run 13 appears to have a larger % EDC conversion as compared to runs 5 and
Ind. Eng. Chem. Res., Vol. 40, No. 11, 2001 2431 Table 1. Summary of Experimental Runs run
EDC feed
metals
max temp (°C)
% EDC conversion
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
99.98% EDC 0.3 wt % C6H6 99.98% EDC 99.98% EDC 99.98% EDC 99.98% EDC 99.98% EDC 99.98% EDC 0.1 wt % CCl4 0.1 wt % CCl4 0.1 wt % C6H6 99.98% EDC 99.98% EDC 0.1 wt % CCl4 0.32 wt % Cl2 0.35 wt % Cl2 0.35 wt % Cl2 plant EDC 99.98% EDC 99.98% EDC 0.30 wt % Cl2
Cr, Ni, Fe Cr, Ni, Fe Cr, Ni, Fe stainless steel none none none none none none none none none none none none none none FeCl2 none none
497 482 480 480 480 471 472 491 482 482 481 481 481 477 482 465 393 481 477 480 383
35
Table 2. ICP Results on Coupons from Run 2 coupons CrA CrB FeA FeB NiA NiB
Fe analysis (ppm)
Ni analysis (ppm)
102.5 286.4
205.6 71.8 1650.0 1513.6
554.5 1020.6
Cr analysis (ppm)
nd nd nd nd
20, and it is suspected that the quartz tube of that run was not sufficiently cleaned, although no visibly apparent iron oxide residue was detected after this run. Metal Coupons. The first set of experiments consists of runs in which iron, nickel, chromium, and stainless steel coupons were used to investigate the effect that different metals have on coke formation. Three sets of metal coupons were placed at the inlet, in the center, and at the exit of the reactor. Several SEMs of metal coupons were analyzed, and it was found that in all runs crystal-like structures formed on the metal surface. EDX analysis of this surface indicated that the crystal-like structures are most likely FeCl2 crystals. Figure 2 micrographs were taken from coupons that were subject to high severity cracking conditions: 495 °C for 12 h (run 1). As a result, these coupons were more heavily coated with coke than others. Figure 2a shows a micrograph of a nickel coupon from run 1 in which three distinct layers are present: coke on the top, iron chloride in the middle, and nickel at the bottom. Figure 2b shows large rounded features that suggest that coke material is first formed as a liquid and subsequently becomes solid. Metal coupons from run 2 were also analyzed via ICPAAS to determine the propensity of metals to migrate under coking conditions. ICP-AAS results are given below in Table 2. The set of coupons labeled A were positioned at the inlet of the reactor and the ones labeled B at the exit. In both cases the nickel coupon came before chromium, which, in turn, came before iron. The amounts of iron detected on coupons B, which are double that of coupons A, show that iron is migrating down the reactor and depositing on the chromium and nickel coupons. The amount of nickel detected is unchanged on the iron ones and actually decreases on the CrB coupon. Chromium is not detected on other coupons.
45 61 43 41 41 48 50 52 50 66 53 45 100 94 69 28 58 47 60
total coke (g) × 103
chloroprene mole fraction × 105
1.4 0.7 0.8 3.9 6.5 6.3 2.5 13.9 5.9 2.2 91.7 68.5 5.9 3.4 9.6 2.8 1.7
0.9 0.7 1.8 1.8 1.4 1.2 8.5 1.6 39.2 34.9 2.2 0.4 1.2 1.2
Overall, the most significant effect of metal coupons present in the reactor was to increase the EDC conversion and coke formation as compared to runs with no metal coupons present. As the surface area of the metal coupons increased, conversion and coke formation also increased. Run 5, with no metals present, resulted in a 43% EDC conversion. Run 4, with large stainless steel coupons present, resulted in a conversion of 61% and, under visual inspection, produced considerably more carbonaceous material. Coke Deposition. From the first run it became visually apparent that two distinct types of coke were being deposited: “hard coke” in the hot zone of the reactor and “soft coke” in the cool zone at the exit of the reactor where the quartz wool plug is located. The hard coke, similar to pyrolytic graphite, came off the tube in large rigid flakes, whereas the soft coke came off the tube as fine granular material. In most runs, a gap was present on the reactor tube between where the hard coke deposition stopped and the soft coke started. This gap tended to disappear at higher conversion. Under optical magnification, hard coke deposited on the quartz sheath, used for temperature measurements, positioned along the axis of the reactor presented ripples perpendicular to the flow direction. To quantify the effects of feed composition, temperature, and metals on coke formation, coke deposition profiles were measured by burning off the carbonaceous material, in 5 cm long sections, as described above. Coke deposition profiles show a large step increase in material deposited at 80 cm from the inlet of the quartz tube, where soft coke starts, as can be clearly seen in Figure 3. The sum of hard coke and soft coke is considered as an accurate measure of the total coke formed. Figure 3 shows three coke deposition profiles. One is the profile from run 17, with chlorine in the feed, and the other is that from a run with FeCl2 pellets present (run 19). The third one is an average of all of the rest. This profile was computed by averaging each tube section from different runs in order to compare the shape of all coke deposition profiles. It was found that, except for chlorine and FeCl2 runs, hard coke deposits always reached a maximum toward the middle of the reactor. The maximum in the average coke profile is located in the same
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Figure 3. Coke deposition profile for the following runs: run 17 with Cl2 (+), run 19 with FeCl2 (]), and an average of all other runs ([). The profiles for the chlorine and FeCl2 runs were scaled up and down by factors of 2 and 5, respectively. The temperature profile of run 5 is also reported as representative of all profiles (b). A step increase in coke deposition occurs at the transition from the heated to the unheated portion of the reactor.
Figure 4. Coke index for runs 17 and 21 with Cl2 (b), for run 18 with plant EDC feed (]), and for all other runs (excluding runs 15 and 16; [).
Figure 2. Coke layer on the top (D), chloride layer in the middle (C), and metal substrate on the bottom (B) of a nickel coupon (a) and round dropletlike features in the coke on a chromium coupon (b). The coupons are from run 1.
position where the average temperature has a maximum. In the FeCl2 run, a ceramic boat was placed at a position of 51 cm and that position coincides with a step increase in the coke deposition profile. This step change was also quite evident under visual inspection. Coke Index. We have defined a coke index as a simple means of evaluating the tradeoff between EDC conversion and coke formation. The coke index is defined as
coke index ≡ total amount of carbon deposited/ total amount of EDC used/% EDC conversion × 108 (3) Figure 4 presents a plot of coke index versus % EDC conversion. As can be seen from this figure, the coke index is strongly correlated with conversion, regardless of temperature and feed impurities, with two significant
exceptions. In this figure, the data points that are closest to the lower right-hand corner of the graph represent the most efficient operating conditions: relatively high conversion and low coke formation. The opposite is true for data points in the upper left-hand corner. Chlorine in the feed shifts this dependency considerably to the right, and plant EDC feed shifts it in the opposite direction. The experimental runs that contained chlorine in the feed produced low amounts of coke at a relatively high conversion. Chlorine in the feed allowed for a 100 °C lower maximum experimental temperature in order to produce EDC conversions of 60-70%. However, when EDC feed from the industrial plant was used in the experiment, a relatively high amount of coke was formed at an EDC conversion of only 28%. Runs 9 and 10, which have 0.1 wt % CCl4 in the feed, also produced large amounts of coke. The data point with the highest coke index is from run 12. The quartz tube of that run had considerable iron contamination, as determined after the run, which considerably affected coke deposition and conversion. Exit Gas-Phase Composition. When the online and offline GC analysis results were combined, reactor exit gas-phase compositions were calculated. The results were used, first, to investigate effects that experimental
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Figure 5. Reactor exit stream composition. Iron, chlorine, CCl4, and C6H6 effects on (a) acetylene, chloroprene, and 1,1,2-trichloroethane gas compositions and on (b) C6H6, chlorobenzene, and 1,4-dichlorobenzene gas compositions. C6H6 is not reported for the run containing C6H6 in the feed.
conditions have on single byproduct species and, second, to see if there are species that can be correlated to coke deposition. The effects of metals and initiators such as benzene, carbon tetrachloride, and chlorine on exit gasphase composition are shown in Figure 5a,b. The runs that are compared, except for the chlorine run, all have the same temperature, and the conversion varies from 48% to 60%. It was determined that in all of the experimental runs, except for the chlorine runs, 1,1,2trichloroethane, which is present in the 99.98% anhydrous EDC feed, is actually consumed somewhat. In Figure 5b benzene, chlorobenzene, and 1,4-dichlorobenzene mole fractions are plotted. Carbon tetrachloride in the feed promotes the formation of benzene as compared with the other runs. The benzene mole fraction for the
benzene-containing run is not reported. The formation of chlorobenzene is favored in the chlorine run. The relative proportion of chlorinated benzenes to benzene in the chlorine run is significantly different from the rest. In the chlorine runs the aromatic byproducts are almost equally distributed between benzene and the chlorinated benzenes, whereas in the other runs the aromatic byproduct is predominantly benzene. When coke deposition results are combined with exit gas-phase composition results, it was calculated that the overall weight fraction of the feed lost to byproducts (both solid and gaseous) varies from a minimum of 0.01% to a maximum of 0.05%. It was determined that although chlorine in the feed resulted in the lowest amounts of coke it also resulted in a high overall
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Figure 6. Exit chloroprene mole fraction vs total coke excluding runs 7, 14, and 19.
byproduct formation because of the relatively large amount of gaseous byproduct formation. Plant EDC feed resulted in both high coke formation and the highest overall byproduct formation. In all of the runs, except for the two chlorine runs at low temperature, 10-45% of the total byproducts is composed of coke. In the two low-temperature chlorine runs, however, less than 2% of the total byproducts resulted in coke. To investigate a mechanistic linkage between gasphase components and coke formation, we assembled mole fraction plots of chloroprene, acetylene, benzene, chlorobenzene, and 1,4-dichlorobenzene against total coke. All of the plots, except for chloroprene, did not show any correlation. Figure 6 shows that chloroprene, however, correlates linearly with the amount of coke formed. Discussion All experiments presented in this work were performed at ambient pressure, whereas industrial EDC pyrolysis reactors operate at around 20 atm pressure. Pressure has a significant effect on molecular weight growth reactions and, consequently, could have a considerable effect on the coke formation mechanism. However, these experimental results give valuable insight in the tradeoffs between coke formation and such variables as % EDC conversions, initiators, and byproducts in the EDC feed. The results also support the mechanism of coke formation proposed by Borsa et al.6 in a previous paper in which commercial plant coke samples were analyzed. Results from this investigation support the hypothesis that coke formation in 1,2-dichloroethane pyrolysis is not predominantly catalyzed by metal surfaces as in other hydrocarbon pyrolysis processes. Results from this study indicate that coke is formed from the impingement of gas-phase tar droplets onto hot surfaces that subsequently undergo dehydrogenation. However, the presence of metals in the pyrolysis environment indirectly promote coke formation by supplying FeCl2 to the gas phase where it most likely catalyzes the formation of high molecular weight tarlike material. This is also supported by conclusions from a detailed analysis of industrial plant coke samples reported by Borsa et al.6
SEMs of coke-covered metal coupons show that early on in the experimental run crystal-like structures are formed on the surface of the metal and that coke is then deposited on the top of the crystal layer. EDX analysis of the crystals indicate the presence of iron chloride. This is also in agreement with findings by Zychlinski et al.8 which conclude that, in the presence of iron, coke formation is always accompanied by FeCl2 formation. In all of the SEM results no evidence was found of filamentous carbon growth. The high chlorine content of the coke, persistently measured on all coupons, with relative amounts of iron and chlorine corresponding to FeCl2, indicates that iron chloride is probably participating in the gas-phase tar droplet formation and is subsequently incorporated in the coke matrix. ICP-AAS results indicate that iron migrates down the reactor. The iron coupons are first chlorided, and the resulting iron chlorides volatilize. However, it is found that chromium does not migrate and that although nickel migration cannot be excluded it is certainly occurring to a much lesser extent than with iron. It was determined by visual inspection as well as quantitatively that more coke formed downstream of iron chloride pellets placed in the reactor as compared to runs without the pellets. This is evidence that FeCl2 is promoting coke formation and indicates that metal surfaces containing iron, although not promoting coke formation via surface reactions, are nonetheless promoting coke formation by providing a source of FeCl2 to the gas phase. Morphological features of coke formed in these experiments are very similar to features described by Borsa et al.6 in coke samples collected from an industrial reactor. Ripples observed on coke formed in the experimental runs, although of much smaller scale as compared to those observed on the industrial plant coke samples, are also perpendicular to the flow direction. This indicates that coke is initially deposited as a highly viscous fluid material and is shaped by the flowing gases above it before it solidifies. Another similarity between the industrial plant coke and the experimental coke is that in both cases two morphologically different types of coke are formed: hard coke and soft coke. The formation of soft coke occurs at a much lower temperature as compared to the temperature at which hard coke is formed. This indicates that a considerable amount of material is exiting the reactors in the form of higher molecular weight material that condenses out at lower temperature to form the soft coke. Finally the rounded features found via SEM and shown in Figure 2b are also very similar to features found in industrial coke SEM analysis6 and indicate that the material deposited was a liquid at some time during its formation. All of these morphological features support the hypothesis that, in EDC pyrolysis, coke is formed primarily by gas-phase tar droplet impingement and condensation on surfaces. However, the particular chemical mechanisms that govern tar droplet formation in this system are not known. In this experimental study, the coke index was found to be a useful tool for comparison of coke formation across all experimental runs. Overall, it was determined that the coke index increases linearly with an increase in % EDC conversion. In the case of the chlorine runs, this dependency is shifted considerably toward higher % EDC conversion. Chlorine in the feed acts as a strong promoter of pyrolysis reactions. The relatively weak ClCl bond enables chlorine radicals to form at much lower
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temperatures and initiate the pyrolysis reactions. As a result, higher conversions and lower coke formation rates at lower operating temperatures were attained. Although, overall, larger amounts of byproduct formation occur with chlorine addition, the large reduction in coke formation is a very desirable result for industrial operation. When industrial plant EDC was used as the feed, EDC conversion decreased from 50% to 30%, with temperature being equal, whereas coke formation was about the same as that for runs with 50% EDC conversion. Impurities in the industrial plant EDC feed are acting as strong inhibitors of EDC conversion but at the same time result in high coke formation. Industrial VCM production plants have considerable room for improvement by optimizing the byproduct composition distribution in their EDC feed. Gas-phase composition information, studied in this work, is most valuable in conjunction with quantitative coke deposition measurements. This is particularly true if attempts are made to model coke formation. Advances in computer speed, computational algorithms, and property estimation techniques are making it possible to perform detailed chemical kinetic modeling of the chlorinated system up to C4 species.1 However, trying to model, at a detailed level, the formation of much larger chlorinated species, which lead to coke formation, quickly becomes an intractable problem. For this reason, correlations between smaller chemical species concentrations and coke formation become the only practical way of predicting coke formation. Chloroprene was the only chemical species that was found to correlate with the amount of coke formed and appears to be a C4 species that can be used to model coke formation in a detailed chemical kinetic model of EDC pyrolysis. Exit gas-phase compositions for runs with similar % EDC conversion were analyzed to see what effect impurities in the feed had on byproducts. The experimental runs analyzed have the same temperature except for the chlorine run, which has a 100 °C lower temperature. The high surface area of stainless steel present in the metal run results in more than double the acetylene concentration as compared to the other runs. It also considerably increased the formation of chloroprene. The acetylene concentration for the chlorine run instead is actually considerably lower as compared to the rest of the runs. This is a result of the lower experimental temperature. Chlorine present in the feed also increased the formation of chlorobenzenes. In this case an approximately equal distribution of benzene and chlorobenzenes was formed because of the greater availability of chlorine atoms in the reaction system. The other runs form mostly benzene. In the chlorine runs the major gas-phase byproduct was 1,1,2trichloroethane. The highest amount of 1,1,2-trichloroethane was measured in the chlorine run with the lowest temperature. The combination of lower temperature and large availability of chlorine atom favors the formation of 1,1,2-trichloroethane, whereas at higher temperatures it would tend to decompose. Conclusions Metal Coupons. It was found that the specific material of the metal coupons used did not change the nature of the coke formed and that the larger the surface of metal used, the higher the conversion and coke formation. Coke formed in the laboratory is similar to that formed in the industrial plant, and metal surface
catalysis appears to be not of major importance. Instead, we hypothesize that volatile FeCl2 is formed from the exposed metal and can catalyze coke formation in the gas phase. Two types of coke consistently formed in the laboratory runs and are similar to the two types of coke formed in the industrial pyrolysis reactor: hard and soft coke. Coke Deposition. The coke index was a useful tool to compare coke formation rates across experimental conditions. It was found that for all ofthe runs, except for the chlorine and EDC plant runs, coke formation rates are only dependent on % EDC conversion. Chlorine addition resulted in lower operating temperature, the lowest coke formation rates, and higher % EDC conversions and therefore could be a way of reducing coke formation in the industrial process. Chlorine addition, however, resulted in increased overall byproduct formation. The plant EDC feed, instead, lowered % EDC conversion and increased the total amount of coke formed. Therefore, there is considerable room for improvement in plant operations by optimizing the feed composition. Exit Gas-Phase Composition. From analysis of exit gas-phase compositions, it was determined that chlorine runs produce significant amounts of 1,1,2-trichloroethane. The same compound was slightly consumed by the other runs. The stainless steel run produced higher levels of acetylene and chloroprene. The chlorine runs produced equal amounts of benzene and chlorinated benzenes, but the CCl4, pure EDC, and stainless steel runs produced mostly benzene. Finally, it was determined that the chloroprene mole fraction correlates with coke formation, but the acetylene, benzene, chlorobenzene, and 1,2-dichlorobenzene mole fractions do not. Acknowledgment We gratefully acknowledge the support of the Mitsubishi Chemical Co., Mizushima, Japan, and the National Science Foundation National Young Investigator Program. Literature Cited (1) Borsa, A. G.; McKinnon, J. T. Elementary Reaction Modeling of 1,2-Dichloroethane Pyrolysis; Comparison with Experimental and Industrial Plant Data. Ind. Eng. Chem. Res. 2001, submitted for publication. (2) Ranzi, E.; Dante, M.; Faravelli, T.; Mullick, S.; Bussani, G. Mechanistic modeling of chlorinated reacting systems. Chim. Ind. 1990, 911. (3) Barton, D. H. R. The Effects of Impurities on 1,2-Dichloroethane Pyrolysis. J. Chem. Soc. 1949, 148-154. (4) Barton, D. H. R.; Howlett, K. E. Effects of Impurities on the Pyrolysis of 1,2-Dichloroethane. J. Chem. Soc. 1949, 155. (5) Ashmore, P. G.; Gardner, J. W.; Owen, A. J.; Smith, B.; Sutton, P. R. Chlorine-catalyzed Pyrolysis of 1,2-Dichloroethane. Part 1. Experimental Results and Proposed Mechanism. J. Chem. Soc., Faraday Trans. 1982, 78, 657-676. (6) Borsa, A. G.; Herring, A. M.; McKinnon, J. T.; McCormick, R. L.; Yamamoto, S.; Teraoka, Y.; Natori, Y. Characterization of Coke Formed in Vinyl Chloride Manufacture. Ind. Eng. Chem. Res. 1999, 38 (11), 4259-4267. (7) Incavo, J. A. A Detailed Quantitative Study of 1,2-Dichloroethane Cracking to Vinyl Chloride by a Gas Chromatographic Pyrolysis Device. Ind. Eng. Chem. Res. 1996, 35, 931-937.
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(8) Zychlinski, W.; Boettger, G.; Rodewald, D. Coke Formation in Thermal Decomposition of 1,2-DichloroethanesConclusions from Radiotracer Experiments. Chem. Ing. Tech. 1990, 42 (8), 325 (German). (9) Zychlinski, W.; Simmermann, G. A Laboratory Pressure Apparatus and Its Use for the Study of the Thermal Decomposition of 1,2-Dichloroethane. Chem. Ing. Tech. 1994, 66 (2), 197, 198, 200, and 201 (German).
(10) Zychlinski, W.; Mielke, I. Influence of Iron(II) Chloride on Coke Formation in 1,2-Dichloroethane Pyrolysis. Chem. Ing. Tech. 1995, 67 (10), 1346 (German).
Received for review July 6, 2000 Revised manuscript received January 17, 2001 Accepted March 12, 2001 IE0006460
