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Ind. Eng. Chem. Res. 2005, 44, 4084-4091
Carbonaceous Material Deposition from Heavy Hydrocarbon Vapors. 1. Experimental Investigation Wenxing Zhang and A. Paul Watkinson* Department of Chemical and Biological Engineering, University of British Columbia, 2216 Main Mall, Vancouver, Canada V6T 1Z4
The unwanted deposition of carbonaceous layers can compromise the efficient operation of processing equipment, particularly for the upgrading of heavy hydrocarbons. An experimental investigation was undertaken to study deposition from the vapor in a tubular test section downstream of a bench-scale continuous bitumen coking reactor. Results of the effects on deposition of coker reactor temperature, steam addition, vapor velocity, and heating or cooling of the produced vapor are presented. Evidence suggests that physical condensation of heavy hydrocarbon species rather than vapor-phase chemical reaction is the primary cause of deposit formation. Entrained droplets of partially converted bitumen also contribute to the deposit. Morphological and chemical characteristics of the deposits are described. Preliminary data are presented on the aging of the amorphous hydrocarbon deposit into coke. 1. Introduction Unwanted buildup of carbonaceous deposits on equipment is a general problem during the processing of many hydrocarbons. Coking at elevated temperatures is a common chemical process for upgrading of bitumen from oil sands and residues in oil refineries, where deposit formation can occur. In fluid coking, vapors leave the coking zone of the reactor, pass through a cyclone to remove coke particles, and then are scrubbed before entering a fractionator. As these vapor-product streams leave the coking reactor, undesired deposition occurs on surfaces of the cyclones and their exit tubes, leading to pressure drop increases, and eventual premature shutdown of the unit at great economic penalty. To operate such equipment most efficiently, or change designs rationally, knowledge of the mechanisms by which deposits are formed is essential. Numerous investigators1-6 have studied vapor-phase fouling phenomena in cracking coils and downstream quench coolers or transfer line exchangers (TLEs) using ethane, propane, naphtha and gas oils, etc. as feedstocks. Three primary coke formation mechanisms have been delineated: the catalytic mechanism, the freeradical mechanism, and the droplets condensation/tar deposition mechanism.7-10 The catalytic mechanism, which produces filamentous coke with nickel and iron acting as a catalyst, is mainly important during equipment startup or when the fresh surfaces are exposed to the gas phase at high temperatures. The free-radical mechanism involves reactions on the solid coke and among gaseous entities in the vapor phase, including free radicals, olefins, acetylenics, etc. The droplet condensation mechanism applies when heavy polynuclear aromatics, which are either present in the feed or formed as a result of secondary reactions, condense directly on the wall or in the bulk, and subsequently collect on the wall. This mechanism is mainly important when cracking heavier feedstocks, such as gas oils, * To whom correspondence should be addressed: Tel: 1-604-822-2741; Fax: 1-604-822-6003. E-mail: apw@ chml.ubc.ca.
vacuum residue, and bitumen, and where gases are cooled. In industrial processes where both cracking and vapor cooling occur, it is possible that all three mechanisms are involved; however, one of them may have a dominant role under particular conditions. The coke formation mechanisms valid for light gaseous hydrocarbons may not apply in the coking of heavy feedstocks as the deposition phenomena and mechanisms are closely related to feedstock properties, and the temperature conditions may differ. Despite increasing utilization of heavy hydrocarbons to meet energy demands, there are very few studies on the topic of deposit formation applicable to bitumen coking equipment. Mallory et al.11 investigated the role of the vapor phase in fluid coker cyclone fouling and found that raising the vapor-phase temperature above the temperature at which it was formed can significantly increase both the yield and rate of coke formation. However, the key experimental conditions used in their study, as summarized later, differ from the conditions in industry. To obtain information for industrial operators to manipulate conditions with the intent of minimizing deposition at similar conditions, and to understand how such deposits form, further study of fouling behavior in cracking heavier hydrocarbons such as pitch (+524 °C residue) and bitumen has been undertaken. The first stage of this research was summarized in Watkinson et al.,12 where tests of coker temperature, and vapor temperatures downstream of the coker are reported, along with deposit properties. This work concluded that physical condensation was a key mechanism for cyclone exit tube fouling. In this paper, results are presented of a more extensive experimental investigation of deposition in a long tubular test section downstream of a pilot bitumen coker. The effects of different operating variables on coke deposition are investigated to identify the primary deposit formation mechanism in this particular system. Deposit characterization is used to further elucidate the formation process. Aging experiments are implemented on selected deposit samples, and the implications of these findings are discussed.
10.1021/ie049055q CCC: $30.25 © 2005 American Chemical Society Published on Web 04/08/2005
Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 4085
Figure 1. Flowsheet of the pilot-plant unit. Legend is as follows: 1, coker; 2, primary and secondary steam generators; 3, cyclone and short exit tube; 4, long test tube; 5, condenser; 6, liquid accumulator tank; 7, scrubber; and 8, afterburner.
2. Experimental Methods Figure 1 shows the flowsheet of a bench-scale coking unit, which was modified from that in the research of Watkinson et al.12 The coker unit consists of a 7.5-cmdiameter × 1.0-m-length reactor equipped with an atomization system, and a 15-cm diameter freeboard section with internal filters to remove coke particles and droplets from the product vapor. Auxiliary equipment consists of a bitumen storage tank and feed pump, and primary and secondary steam generators. Test sections for deposition measurement include an external cyclone with short exit tube (17 cm), and an additional 90-cmlong Type 304 drawn stainless-steel vertical tube test section, which was added for this study. Downstream of the test sections are a condenser and liquid accumulator tank, a scrubber, and an afterburner system. During the experiments, the equipment was preheated to a desired temperature in a nitrogen flow. The heated bitumen (at ∼150 °C), together with atomizing nitrogen, was introduced into the reactor through an atomizing nozzle at the bottom. Water and a small stream of nitrogen (acting as a means to monitor the blockage of the steam feeding line) were fed to a steam generator to produce feeding steam. The mass flow rates of bitumen, primary steam, and nitrogen (including primary and atomizing nitrogen) were ∼0.3 kg/h, 0.3 kg/h, and 0.2 kg/h, respectively. The average vapor residence time (τR) in the empty reactor was ∼15 s. To prevent the liquid droplets or carbonaceous particles from being entrained into the test section, the cracked gas stream was forced to flow through two parallel filters, which were mounted in the freeboard section. Filter materials of differing pore size were used to investigate effects on the deposition rate; however, for most of experiments, one layer of 3M ceramic fiber with a filter rating of 10 µm was used as the filtration
medium. This was wrapped tightly around the perforated metal tubes. With the 10-µm filter in place, deposition is assumed to arise from the vapor phase, rather than from coke particles. A system for injection of secondary steam or nitrogen was added upstream of the cyclone for use in selected experiments to investigate dilution effects. The vapors passed through the cyclone system and its short exit tube, and then into the new 90-cm-long test section, where the experimental conditions could be independently controlled. After cooling, the condensable products in the hot streamsnamely, hydrocarbon and waterswere collected in the accumulator vessel and the off-gas passed through a NaOH scrubber and then an afterburner before being exhausted. All the temperatures were recorded each minute by a data logging system. The feed rate of water to the steam system was monitored through the weight change in the water tank, and that of the bitumen was confirmed by measuring the bitumen level in the feed tank every 10 min. During the course of most runs, the reactor pressure slowly increased from 0 psig to 10-20 psig over the 6-h experiment, because of the accumulation of carbonaceous materials on the filters. To keep the nitrogen flow rate steady, the nitrogen tank outlet pressure was regulated constantly, to maintain the pressure drop across the flow meters and, thus, a steady flow rate. After 6 h of operation, power to all of the furnaces and heating tapes was shut off. After shutting off the bitumen feed, Varsol was pumped through the bitumen feed line for ∼3 min to wash out residues and avoid blockage. A small flow of nitrogen was maintained through the system until the equipment was completely cooled. The entire system was then dismantled for cleaning and collecting deposits. The deposits in the
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Table 1. Properties of Atmosphere Topped Bitumen Value property
sample ATB-A
sample ATB-B
>525 °C (vol %) gas oil (vol %) >750 °C (vol %) density (g/mL) carbon content (wt %) hydrogen content (wt %) sulfur content (wt %) H/C elemental ratio MCR (%)
57 43 17 1.02-1.04 83.1 10.24 4.97 1.47 19.3
53.3 46.7 1.01 84.0 10.7 4.45 1.53 12.5
Table 2. Summary of the Main Experimental Conditions parameter feeding flow rates bitumen primary steam secondary steam total nitrogena coking reactor conditions TR τR coke particles 90-cm long test tube dt (I.D.) vj z τt tube material run time a
value/comment 0.3 kg/h 0.3 kg/h 0-0.55 kg/h 0.2 kg/h 500-570 °C (535 °C for most runs) 15 s absent from reactor (empty reactor)
Figure 2. Extent of filtration effects on the deposition rate in the long test tube. Conditions were as follows: feedstock, ATB-A; TR ) 535 °C; Tin ) 535 °C; and a constant and uniform outside wall temperature of Two ) 530 °C.
5.8-15.7 mm (9.0 mm for most runs) 2.2-16.5 m/s (usual value is 6.9 m/s) 50-415 ms (usual value is 130 ms) stainless steel 6h
Atomizing and primary.
cyclone and exit tube system, and in the long tubular section, were carefully collected using a plastic brush, and the weight difference before and after cleaning was taken as the deposit weight in each section. Although deposits were heavier toward the cooler end of the long test section, local deposition rates were not determined. Results are expressed as a deposition yield, in terms of grams of deposit per kilogram of bitumen feed (g deposit/ kg bitumen feed), or a rate, because the bitumen feed rate was constant at 0.3 kg/h. The cyclone and exit tube, and the long test section, were all reused after cleaning without any additional surface preparation. Most results presented here are for deposition in the long tube only; trends of process variable effects were similar for the cyclone and short exit tube section. Two samples of atmosphere topped bitumen (denoted as ATB-A and ATB-B) from Syncrude Canada Ltd., were used during the investigation. The properties of the two bitumens were similar (Table 1); however, the microcarbon residue content (MCR), which is considered to be proportional to the amount of coke precursors present in the feed, is notably higher in sample ATB-A. The main experimental conditions are summarized in Table 2: 3. Results and Discussion 3.1. Filtration Extent Effects. Vapors produced by bitumen fluid coking contain a very wide spectrum of species, which include water vapor from the addition of steam, gases such as nitrogen carrier gas, hydrogen, low-molecular-mass hydrocarbons, and condensable vapors from C5 up to heavy gas oil species. Unconverted or partially converted feed material with normal boiling points in excess of 750 °C may also be present as liquid droplets. Experiments were undertaken to confirm the extent to which liquid droplets or particles entrained from the
Figure 3. Effect of vapor temperature on the deposition rate in the long test tube. Conditions were as follows: feedstock, ATB-A; TR ) 535 °C; and a constant and uniform wall heat flux of q0 ) (1157 W/m2.
reactor contributed to the deposition rate in the test section. The filter support tube was normally wrapped with a ceramic filter medium with a pore size of 10 µm. Two additional means of filtration were examined: using a filter cloth with a pore size of 150 µm, and operating without a filter cloth. In the latter case, the largest droplets/particles entrained into the test section were considered to be 3160 µm in size, which is the punched hole diameter of the perforated filter support tube. Some experimental conditions were repeated to confirm the reproducibility of experimental data. Figure 2 shows the comparison of the deposition rate in the long test tube with different upstream means of filtration. Reactor vapor temperature was maintained at 535 °C, and the wall temperatures of the long test tube remained uniform and constant at 535 °C. The feedstock was bitumen ATB-A. Increasing the pore size resulted in a steady increase of average deposition rate, which suggests that, particularly with the larger pore size, entrainment of liquid droplets/coke particles from the reactor contributes to the deposit. However, the difference in deposition yield is small, given the reproducibility of the experiments. Increasing the filter pore size by a factor of 15 (from 10 µm to 150 µm) increased
Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 4087 Table 3. Comparison of the Major Conditions between Laboratory Studies and Industrial Operation vapor residence time in cracking reactor, τR steam percentage in produced stream vapor residence time in test section surface material
present pilot-plant study
Mallory et al.11
typical industrial fluid coker
∼15 s ∼65 mol % ∼0.05-0.45 s stainless steel
∼55 s ∼0 mol % ∼0.4-4.4 s quartz
∼14 s ∼51 mol % ∼0.14 s stainless steel
the deposition rate by ∼40%. The deposition data in the cyclone and short exit tube section showed a similar trend. The observation that coarser filter pore size increases deposition does not necessarily confirm that liquid droplets are the primary cause of deposition with the 10-µm filter. Species from large vapor molecules to aerosols through to small micrometer-sized droplets could contribute to fouling. However, there seems to be no evidence that coke particles pass through the filter and cause deposition. 3.2. Effects of Vapor Temperature. The effects of vapor temperature on deposition was studied, through using positive heat (input) or negative heat (withdrawal) in the long test tube, to heat or cool the vapor from the temperature at which it was formed, i.e., 535 °C. The dual purpose of these experiments was to clarify the conflicts, with respect to vapor-temperature effects, and generate results to calibrate and validate a simulation model of the process. Figure 3 shows the vapor-temperature effects on the deposition rate in the long test tube. Vapor temperature into the long tube (Tin) could be regulated and the variation along the tube (Tout - Tin) was (90 °C, corresponding to a constant and uniform wall heat flux of (1157 W/m2 at the fixed mass flow rate. The average vapor temperature was calculated as T h vap ) (Tin + Tout)/ 2.0. The feedstock was bitumen ATB-A. The experimental trend clearly shows that decreasing the vapor temperature results in an increase in deposit formation. The overall deposition rate increases dramatically after the average vapor temperature is below ∼510 °C. Above the reactor temperature of 535 °C, over a wide range of average vapor temperatures (535-680 °C), the deposition rate remains constant. Prior data12 for cyclone plus short exit tube deposition over a narrower range of temperatures also indicated that lower vapor temperatures promote deposition. The aforementioned results clearly suggest physical condensation as the dominant mechanism of deposition in this system. Mallory et al.11 found that increasing the vapor-phase temperature generally resulted in significant increases of vapor-phase coke. However, their experiments for the lowest test section residence times showed higher deposit formation when the vapor was cooled. Differences between the experimental conditions, as shown in Table 3 (particularly at residence times), together with the complicated coking behavior in bitumen cracking, contribute to the differences in overall results. However, the present equipment provided a better match to the industrial fluid coker vapor residence time, and gives confidence that the vapors entering the test section in the present work are more representative of those in industrial fluid cokers. In the aforementioned experiments, changes were made only to the conditions in the long tube test section. Conditions in the upstream parts of the apparatussat the coker, cyclone, and short cyclone exit tubeswere held constant; hence, the deposition yields in this portion of the equipment should be the same. Over a span of 12 runs, the deposition yield in the cyclone plus
Table 4. Vapor Velocity and Residence Time in Tubes with Different Diameters tube diameter, I.D. (mm)
superficial vapor velocity in the tube (m/s)
residence time, τR (ms)
5.8 9.0 15.7
∼16.4 ∼6.9 ∼2.2
∼55 ∼130 ∼409
short exit tube averaged 0.129 g deposit/(kg bitumen feed), with a standard deviation of 0.044. This level of reproducibility of deposition data is considered acceptable. 3.3. Reactor Temperature Effects. The coker reactor vapor-phase temperature was varied over a range of 500-570 °C, to compare the deposition from vapors generated at different temperatures. The long tube wall temperature was maintained constant and uniform at 500 °C, and the vapor inlet temperature into the long tube was controlled at 535 °C. The feedstock was ATB-B, because ATB-A was out of stock. With increasing coker temperature, the deposition rate in the downstream test section increases exponentially, presumably because of the increased vapor yield and the presence of heavier components, which are vaporized in the reactor. A plot of natural logarithm of the deposition rate (ln md) versus the inverse of temperature (1/TR), as done in the Arrhenius form for rate constants, produces a straight line with a pseudoactivation energy of 49 kcal/mol (Figure 4). Quantification of the increase in the conversion of the bitumen with coker temperature, and the yields of highermolecular-mass (condensable) species, is necessary to rationalize the effects on deposition. The aforementioned trend of increased deposition rate with coker temperature was also reported in the work of Watkinson et al.12 Figure 5, in which deposition yield is plotted versus the difference between coker and test section vapor temperature, shows the consistent pattern of sets of experiments where each temperature was varied. This trend might be expected where physical condensation causes deposit formation. This result suggests that it might be beneficial in industry to maintain a lower reactor temperature, to reduce the vapor-phase cyclone fouling. However, this approach also risks decreasing the yield of valuable products from the coker itself. 3.4. Secondary Steam and Nitrogen Effects. The beneficial effects of steam addition to decrease coke deposition at high temperature are well-known in the ethylene industry. The role of secondary steam addition on the deposition from vapors produced under bitumen coking conditions was investigated. Different quantities of secondary steam or nitrogen at a regulated temperature could be injected into the inlet of the cyclone section (as shown in Figure 1) without affecting the velocity or vapor residence time in the upstream coking reactor. The temperature in the coking reactor was set at 535 °C. The feedstock used was ATB-B. Because ATB-B has a relatively lower MCR content (Table 1), the wall temperature of the 90-cm long test tube was purposely set at a lower temperature
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Figure 4. Effect of coking reactor temperature on the deposition rate in the long test tube. Conditions were as follows: feedstock, ATB-B; TR ) 500-570 °C; Tin ) 535 °C; and Two ) 500 °C.
Figure 5. Plot of the average deposition rate versus the difference between the average vapor temperature in the test section and the reactor temperature (∆T ) T h vap - TR).
(500 °C), to collect more deposit and observe more significant effects of secondary steam/nitrogen. The vapor inlet temperature into the long tube was 535 °C. The volumetric flow rate of secondary steam was varied over a range of 0-11.4 L/min (STP). In one run, an equivalent volumetric flow rate of secondary nitrogen replaced the secondary steam and the deposition results were compared to determine if the beneficial effects of steam were due only to dilution, or whether steam itself had some chemical effects. Figure 6 shows the effects of secondary steam or nitrogen on deposition rate in the long test tube. The deposition rate was decreased with the injection of either steam or nitrogen. An increase of 11.4 L/min (STP) of steam to a total vapor flow of 20.9 L/min (STP) reduced deposition by ∼50%. As will be shown below in Section 3.5, increases in the velocity at the same concentration of hydrocarbon vapors do not change the deposition rate; hence, the addition of inert gases presumably changes deposition through concentration effects. Within the range of experimental error, at the same volumetric injection rate, the use of steam and nitrogen yielded the same deposition rate. This suggests that addition of either steam or nitrogen is simply a physical effect in this system, i.e., they decrease the deposition rate through dilution of the fouling precursor
Figure 6. Effect of secondary steam/nitrogen on the deposition rate. Conditions were as follows: feedstock, ATB-B; TR ) 535 °C; Tin ) 535 °C; and Two ) 500 °C.
Figure 7. Effect of tube diameter (vapor velocity/vapor residence time) on the deposition rate in the long test tube. Conditions were as follows: feedstock, ATB-B; TR ) 535 °C; Tin ) 535 °C; and Two ) 500 °C.
concentration. This effect was further studied in a mathematical model of the deposition process described in a companion paper. Industrial experience with the injection of steam at a temperature below that of the vapor reportedly resulted in an increase in deposition. The injection of cooling steam could promote deposit formation via enhanced physical condensation, despite the dilution effect. Steam, which, on the other hand, is hotter than the vapor, has the combined beneficial effect of heating and diluting the hydrocarbon vapor. Simulation of these combined effects using a mathematical model could provide guidance for industrial application. 3.5. Vapor Velocity and Residence Time Effects. A set of experiments was conducted for the purpose of understanding the effects of vapor velocity and residence time in the test section on the deposition rate. Stainlesssteel tubes with different diameters were installed in the long test section, and the vapor velocity (vj z) and residence time (τt) were thereby varied with the same volumetric flow rate of product stream from the coker reactor. Table 4 gives the detailed information of tube diameters, residence time, and vapor velocities of these experiments. Figure 7 shows the effects of tube diameter on deposition rate in the long test tube with conditions of
Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 4089
Figure 8. Scanning electron microscopy (SEM) image of the deposit collected on the filter.
TR ) 535 °C, a constant and uniform outside wall temperature of Two ) 500 °C, and Tin ) 535 °C. The feedstock was ATB-B. Over an almost 8-fold range of velocity, there is no significant difference in terms of the deposition rate within the limits of experimental error. This suggests that vapor residence time does not have a role in the present work. Mallory et al.11 reported that increasing vapor residence time resulted in higher coke deposition rates, in contrast to the present results. As noted previously, experimental differences (see Table 3) between the two studies preclude establishing the exact causes of the contradiction. Mathematical modeling based on the physical condensation mechanism will simulate the relationship between deposition rate and tube diameter, and further discussion will be presented in a companion paper. 4. Deposit Characterization 4.1. Scanning Electron Microscopy (SEM). The deposits collected in the reactor, filter, and test sections from different runs have been selected for scanning electron microscopy (SEM) observation. Representative samples collected on the reactor freeboard filter and in the long tube test section are shown in Figures 8 and 9, respectively. Typical deposits on the filter (Figure 8) generally show clusters of hollow spheres (cenospheres). Cenosphere formation is well-known in the combustion literature for heavy fuel oils,13 and, in the present situatio, centospheres could form as follows. When bitumen is injected into the fluid coker through the atomization nozzle, the temperature increase that is due to heating from the surroundings leads to cracking reactions and possibly local formation of a more viscous or solid film on the surface of the droplets. With an accumulated film permeable to volatiles, the latter will continue to evaporate from the droplet, eventually leaving a remaining hollow sphere, which collects on the filter cloth as a coke layer. The deposits formed in the cyclone and long tube test sections generally have the same morphology. In macroscopic appearance, amorphous deposits were observed in every case. Figure 9 is a photomicrograph of amorphous deposit collected in the long test tube with the following experimental conditions: TR ) 535 °C, T h vap ) 450 °C, and feedstock ATB-A. In the micrograph, spherical and near-spherical aggregates, either existing in
Figure 9. SEM image of amorphous deposition collected in the long test tube.
individual form or connected together through protuberances, are observed throughout the sample, suggesting the possible contribution of physical condensation. These droplets might be formed via the condensation of fouling precursors directly on the cooler wall or in the bulk with transport to the surface where part or all of the material adheres. The viscosities of tar droplets formed and the wetting characteristics, combining with the wettability of the wall, will determine the morphology of the deposit either as a hard smooth layer (low viscosity and high wettability) or an amorphous structure (higher viscosity and low wettability). It has been reported14 that, in pyrolytic processes, amorphous deposits are generally formed from heavier feedstocks (such as gas oil, etc.) and more ordered deposit structures are observed with lighter feedstocks (such as ethane). Current observations are consistent with these findings. To our knowledge, spherical clusters are not generally found in industrial cyclone exit tube deposits, perhaps because of aging under conditions far removed from those of the present tests. The presence of spherical particles in the deposit cannot be exclusively attributed to the physical condensation, because tar droplets could also be formed by chemical reaction.8 However, in the present system, other experimental evidence such as vapor temperature and vapor velocity/residence time effects, do not support a major role of chemical reaction in deposit formation. Therefore, physical condensation seems to be the major contributor for the coke formation in the present investigation. 4.2. Deposit H/C Ratios. Deposits were analyzed for carbon and hydrogen content, using microanalytical methods. The H/C atomic ratio in the deposit indicates how coke-like is the deposit. Figure 10 shows the H/C ratio of samples collected in the long tube section, corresponding to the runs described in Figure 3 in different temperature regimes. It shows a clear downward trend of deposit H/C ratio with increasing vapor temperature, which means that the deposits formed at higher temperatures are richer in carbon and depleted in hydrogen (H/C ratio of ∼0.35), whereas those collected at lower temperature are more similar to “green coke”, with a H/C ratio of ∼0.60.
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Figure 10. H/C ratio of the deposit formed at different temperature regimes in the long test tube.
Figure 11. Effect of aging time on H/C ratio at a fixed aging temperature (550 °C).
5. Deposit Aging Experiments Numerous studies4,10,14-16 have mentioned aging of the fresh deposits; i.e., a fresh deposit is expected to experience further dehydrogenation, subsequent specific volume contractions, and morphological modifications at the elevated temperature of the equipment. However, systematic studies of aging are lacking. In the present investigation, a 7.8-cm-long, 12.7-mm-diameter (outer diameter, O.D.) stainless-steel tube equipped with Swagelok end-caps, and a 7.6-cm-long glass test tube liner, was used as a batch reactor. An amorphous deposit sample from the long tube test section with a H/C ratio of ∼0.60 was placed into the test tube liner; nitrogen then was used to purge and blanket the sample in an inert atmosphere. The top end-cap was attached and tightened to seal the system, before it was placed in a muffle oven for aging. The aging process was monitored intermittently over hours or days via measurement of the H/C atomic ratio and using SEM analysis. Figure 11 shows the variation of the H/C ratio with aging time at a fixed temperature of 550 °C. After 2 days, the H/C ratio of the deposit had declined from its initial value of 0.6 to a value of 0.26. The dehydrogenation process seemed to evolve relatively quickly in the beginning and then slowed, and the H/C ratio eventually leveled off. The H/C ratios after 2 days and 8 days are similar, which suggests that the aging
aging temperature (°C)
before aging
after aging
∆ (H/C)
500 550 600 650
0.595 0.595 0.595 0.595
0.367 0.320 0.209 0.116
0.228 0.275 0.386 0.479
reaction, in terms of hydrogen release, essentially ended within ∼2 days at 550 °C. However, the morphology may continue to change after this point. Because aging is assumed to be a chemical reaction process, the aging temperature is expected to be an important factor in determining the reaction severity. Therefore, aging temperatures were studied for a fixed period (1 day), to monitor the change in the H/C ratio. Results in Table 5 clearly demonstrate that higher aging temperatures yield lower H/C ratios. At 650 °C, the value of the H/C ratio had declined from 0.6 to 0.12 in 1 day. Photomicrographs did not show significant changes in morphology, as the spherical particles shown in Figure 9 were still detected in the deposit samples after aging. However, some physical changes seem to accompany the aging process, because, after aging, the original amorphous deposit was loosely stuck together but easily crushed. The aging experiments demonstrate that the fresh deposits indeed experience some dehydrogenation over time at high temperature, although volume contractions and morphological modifications cannot be directly confirmed in these initial tests. After aging, H/C ratios approach values of ∼0.2, which is typical of deposits in the industrial cyclone gas outlet tube system, although morphology differences persist. The aforementioned experiments do not match industrial operations, because the aging times are very short (the initial deposit of industrial coke can remain in place for more than 1 year), the composition of surrounding atmosphere differs, and significant shear stresses exist at the surface of the industrial deposit, because of high velocity flow. Hence, differences in morphology between aged lab deposits and those from industry may be expected. 6. Concluding Remarks An experimental investigation of carbonaceous deposit formation from the vapor phase in bitumen coking was undertaken, using a bench-scale continuous unit. Different operating parameters and conditions were investigated to both clarify conflicting information on the deposit formation mechanism and identify possible variables by which operators can alleviate deposit formation. Characterization techniques and aging experiments were performed on selected samples to elucidate more information about the coke formation history and mechanism. Experimental results with three different freeboard filter pore sizes suggest that, in addition to vapors, the entrainment of fine droplets could contribute to the overall deposit formation. The deposit yield increases moderately with order-of-magnitude increases in filter pore size. Decreasing the downstream vapor temperature below that at which it was formed in the coking reactor leads to increased deposition. The relationship between vapor
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temperature and deposition rate suggests that physical condensation of heavy components from the vapor phase dominates deposition formation in the present system. The condensable components of deposit precursors have not been identified specifically; however, the H/C atomic ratio of the fresh deposit is in the range of 0.35-0.6. With the coking reactor at a temperature of 535 °C, vapor-phase reactions apparently do not contribute to deposit formation in the downstream test section, up to an average temperature of T h avg ) 680 °C. With increasing reaction temperature in the coker, deposition in the downstream test section increases exponentially under otherwise identical conditions, suggesting a chemical reaction process for the production of the deposit precursors. Deposition rates decrease as the difference between the test-section vapor temperature and the coker temperature increase. Secondary steam or nitrogen additions at the vapor temperature show equally beneficial effects in alleviating deposit formation, apparently via a simple dilution effect through which steam or nitrogen addition reduces the concentration of condensable components in the vapor stream. Thus, significant steam additions are required to show an effect. If steam is added at lower temperature, this benefit is expected to be lost. With coking conditions fixed, vapor residence time in the downstream test section was varied by changing the diameter of the deposit test section. Increasing the vapor residence time from 50 ms to 415 ms, by changing the tube diameter, shows no significant changes in coke deposition rate, relative to the experimental errors. This result is in contrast with some prior results in the literature,11 in which higher vapor residence time produced more deposit. Characterization techniques shed insight on the deposit morphology and properties. Scanning electron micrographs show hollow sphere clusters in deposits collected on the reactor filter. Small spherical or nearspherical particles were observed throughout the amorphous deposit collected in the test section, an evidence of physical condensation. The composition of the amorphous deposit collected in the test section is dependent on the formation temperature; i.e., the deposit collected at higher temperature shows lower H/C ratios than deposits collected at lower temperature. Aging experiments conducted on a fresh deposit that was formed at relatively low temperature confirmed that dehydrogenation occurs during aging. At 550 °C, the H/C ratio decreases sharply in the initial 2 days of aging, attianing values typical of industrial deposits; however, no further hydrogen loss was monitored beyond that, over an 8-day period. The extent of dehydrogenation after 1 day increased as the aging temperature increased over the range of 500-650 °C. No significant morphology changes were observed. Acknowledgment The authors are grateful to the financial support of Natural Science and Engineering Research Council of Canada (NSERC) and Syncrude Canada Ltd. Valuable discussions with Ian Rose and Craig McKnight from Syncrude Canada Ltd., is greatly acknowledged. Experimental work by Gorden Cheng is appreciated. Nomenclature dt ) tube diameter (inner diameter) (m) md ) deposition rate (g deposit/(kg bitumen feed))
Tin ) vapor-phase temperature at the entrance of the long test tube (°C) Tout ) vapor-phase temperature at the exit of the long test tube (°C) TR ) coking reactor temperature (°C) T h vap ) average vapor phase temperature in the long tube; T h vap ) (Tin + Tout)/2.0 (°C) Two ) outside wall temperature of the long test tube (°C) vj z ) average vapor velocity in the long test tube (m/s) Greek Letters τR: vapor residence time in the reactor (s) τt: vapor residence time in the long test tube (s)
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Received for review September 28, 2004 Revised manuscript received February 22, 2005 Accepted March 3, 2005 IE049055Q
