Observation of Heavy Oil Vaporization under Rapid Heating

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Energy & Fuels 1998, 12, 1174-1180

Observation of Heavy Oil Vaporization under Rapid Heating Hiroshi Nagaishi,* Kouji Ikeda, and Kunihiro Kitano Hokkaido National Industrial Research Institute, 2-17-2-1, Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan

Edward W. Chan Syncrude Canada Ltd., Edmonton Research Centre, 9421-17 Avenue, Edmonton, Alberta T6N 1H4, Canada

Murray R. Gray Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada Received January 27, 1998

The pyrolysis and coking of samples of Athabasca bitumen and its pentane-insoluble fraction were observed under rapid heating conditions. Samples were suspended on a thin rod and heated with radiant heaters under vacuum. Images were recorded using a CCD camera system. Samples were heated for 2 s to give inside temperatures as high as 673 K in a 1 mm droplet. The heating rates at the exterior surface of the samples exceeded 680 K/s. Important variables included the composition, shape, and size of the sample. Temperature measurements depended on the location of the thermocouple in the sample and the size of the thermocouple. The observations showed that the liquid phase rapidly evolved a cloud of vapor, which was bright and luminous under the radiant heaters. The rate of formation and expansion of the cloud depended on the volatile content of the sample. The samples were liquid under the observation conditions, but even under vacuum, the liquid did not foam or froth significantly once vapor evolution had begun. These observations suggest that pyrolysis and coking of liquid droplets and films can be modeled using the simple geometry of the liquid phase, rather than expanded vapor-liquid foams.

Introduction To provide useful energy and minimize the environmental impact, fossil fuels should be processed more efficiently. Oil sand bitumen is a major fossil fuel resource that is currently under utilized, in part due to the cost of processing. The estimated reserves1 are considered comparable to those of petroleum in the Middle East, but its production as an energy source is much lower than that of petroleum. More than 90% of the world’s oil sand reserves are in western Canada. At present, about 80% of the oil sand bitumen produced in Canada is converted into synthetic crude oil by the fluid coking process2-4 and by delayed coking, while another 20% is treated by hydroprocessing.5,6 Cracked oils derived from both processes are subjected to further hydrotreating before supply to downstream refineries. (1) Oil Gas J. 1984, 82 (20), 66-67. (2) For example, see: Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.; Dekker: New York, 1991; pp 539-544. (3) Kamiya, Y. RAROP Heavy Oil Processing Handbook; RAROP: Japan, 1991; pp 7-8. (4) Kamiya, Y. RAROP Heavy Oil Processing Handbook; RAROP: Japan, 1991; pp 15-16. (5) Kamiya, Y. RAROP Heavy Oil Processing Handbook; RAROP: Japan, 1991; pp 61-62. (6) For example, see: Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Dekker: New York, 1994; pp 186-191.

The conversion of the heavy fractions of the bitumen is accomplished by thermal reactions. In the coking processes, the valuable products are evolved in the vapor phase; therefore, the fundamental mechanism of interest is pyrolysis of the oil to give volatile products and a solid coke residue. The yields from this pyrolysis will depend on both the properties of the organic compounds in the feeds and the reaction conditions. Secondary reactions of the cracked components are particularly sensitive to details of the reactor design and the local environment at the reaction site. Clarification of the mechanism of pyrolysis is essential in order to improve the conventional conversion technologies and support innovation toward new conversion technologies which reduce environmental impact. Investigations on the initial pyrolysis of fossil fuels have already been reported.7 In general, models were assessed on the basis of their applicability to experimental data for the yield of various product fractions. Observation and photography of the pyrolysis behavior could give new insights and check the validity of the models proposed so far.8 In a detailed design of a (7) Albright, L. F.; Crynes, B. L.; Corcoran, W. H. Pyrolysis: Theory and Industrial Practice; Academic Press: New York, 1983; pp 69-87, 133-201.

10.1021/ef980017r CCC: $15.00 © 1998 American Chemical Society Published on Web 09/26/1998

Observation of Heavy Oil Vaporization

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Table 1. Elemental Analysis of Sample Used Bitumen pentane-insoluble

C

H

N

O

S (diff.)

H/C

82.6 77.7

10.5 7.9

0.5 1.1

2.3 4.2

4.1 9.1

1.5 1.2

reactor for pyrolysis, the kinetics of reactions must be combined with analysis of mass transfer, convection, vaporization, and carbonization of the coke. In a rapid heating system, however, such kinetic analysis can be difficult due to factors such as very rapid reactions, giving only a short time for measurements, and the need for rapid cooling of vapor products to avoid secondary reactions. The kinetic data usually combine both intrinsic chemical reaction kinetics and the coupled effects of transport processes. Consequently, in situ measurements or observations are valuable as alternate experimental methods. The behavior and geometry of the liquid phase during the pyrolysis of viscous liquids, such as bitumen, can be defined by direct observation. For example, bitumen could form coke having an expanded structure,9 especially during the initial stage of rapid heating. Foaming of the liquid by gas bubbles, followed by partial solidification of the surfaces by carbonization, would give a highly expanded coke structure. The mass and heat transfer characteristics of such an expanded foam would be dramatically different from a quiescent liquid droplet. Consequently, direct observation will provide a basis for modeling the heat and mass transfer processes during rapid pyrolysis. This study used direct observation of heavy oil droplets during heating and pyrolysis in order to understand the behavior of the liquid phase during vaporization and reaction. Two materials were studied: Athabasca bitumen and the pentane-insoluble fraction or asphaltenes of the bitumen. The droplets were heated as rapidly as possible by a constant addition of radiant energy. Due to the difficulty in controlling the temperature during such rapid heating, the maximum temperature was not controlled. Experiments were undertaken at reduced pressure to avoid the effect of natural convection as much as possible. Experimental Section Materials. Athabasca bitumen was supplied by Syncrude Canada Ltd. from its mining and extraction operation at Mildred Lake, Alberta. The pentane-insoluble fraction or asphaltenes constituted 16.7 wt % of the oil sand bitumen as received. Elemental analysis results of the samples used are listed in Table 1. The microcarbon residue of the bitumen determined according to the ASTM Method D 4530 was 13.8%, and it contained 54% of the residue fraction by weight (524 °C + fraction) distilled following the ASTM D 1160 procedure. Volatility of Bitumen Samples. Thermogravimetric analysis (TGA) of the samples was done using an improved apparatus (Figure 1) for simulation of distillation (SD-TGA),10 using an ULVAC (Shinku-Riko) TGCC 7000 thermogravimetric conduction calorimeter. The sample (200 mg) was placed in a platinum sample container inside a furnace with nitrogen flowing at 200 cm3/min under atmospheric pressure (100 kPa) and at 30 cm3/min under reduced pressure (0.4-53 kPa). The (8) Ranz, W. E.; Marshall, W. R., Jr. Chem. Eng. Prog. 1952, 48, 141-146. (9) Moszkowicz, P.; Witzel, L. Chem. Eng. Sci. 1996, 51, 4075-4086. (10) Hasegawa, Y.; Yoshida, T.; Narita, H.; Maekawa, Y. J. Fuel Soc. Jpn. 1987, 66, 855-860.

Figure 1. Schematic diagram of the thermogravimetric conduction calorimeter used. pressure was controlled in the latter case by a vacuum controller. The voltage difference between the thermocouple measuring the sample temperature (TC1) and the one measuring the temperature of the cover surrounding the sample container (TC2) was constant at 300 µV. This maintained a constant temperature difference between the sample and the cover. In the analysis, therefore, the temperature was controlled to simulate the distillation method (ASTM D 2892), which is used for crude oil and comparable to the standard distillation method (ASTM D 2887). The heating rates in the thermogravimetric analysis were constant at about 11-13 K/min until the weight loss was mostly completed and leveled off at about 873 K. Over 873 K, the heating rate was accelerated, up to a final temperature of 1073 K. Observation during Rapid Heating. Schematic diagrams of the experimental apparatus for rapid heating are shown in Figure 2(a-c). To examine the temperature difference between the inside and outside of a sample droplet, two different temperature measurements were done, respectively, as shown in Figure 2a. Samples of 1-2 mm diameter, shaped as spherically as possible, were fixed directly on the tip of a chromel-alumel (CA) thermocouple (TC) or wire for inside or outside temperature measurement, respectively. Both the wire and the thermocouple, with a diameter of 0.5 mm, were inserted into a 1.4 mm alumina tube. The pentane-insoluble (PI) fraction was melted once to make a spherical droplet, because the material is a solid at room temperature. A thermocouple of platinum versus platinum-13% rhodium (Pt/ Rh), with a diameter of 0.1 mm, was also used to obtain a rapid temperature response. The sample was heated rapidly by irradiation by four spot heaters having gold mirror surfaces and halogen lamps (24 V; 75 W) as shown in Figure 2b. Each heater had a focal distance of 15 mm and could focus on a spot of about 5 mm around the sample. The heater performance for each one was, for instance, such that an alloy tip of 18 Cr-8 Ni stainless steel (0.5 × 3 × 3 mm, heat capacity 0.51 kJ/kg K at 400 K, heat conductivity 16.5 W/m K) located on the focus could be

1176 Energy & Fuels, Vol. 12, No. 6, 1998

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Figure 3. (a) Weight-loss curve by SD-TGA (b) for bitumen and (c) its pentane-insoluble fraction. measurement, the electromotive force change was transferred into temperature using the calibration curves made from the calibration tables for thermocouples of CA and Pt/Rh, respectively.

Results and Discussion Figure 2. (a) Sample support system and location of temperature measurement. (b) Four-spot heater system and actual image observed of the system. (c) Chamber put heater system for visual observation. heated at 160 K/s of heating rate. The heating system was set inside a 16 dm3 sealed chamber as shown in Figure 2c. The pressure in the chamber was reduced to 1.6 kPa. The chamber was purged to remove oxygen and filled up to atmospheric pressure with nitrogen of 99.99% purity, then the pressure was reduced again to 1.6 kPa for experiments at reduced pressure. The sample droplet was heated by operating the spot heaters at a constant potential of 24 V each, for a duration of 2 s. During the heating and the subsequent cooling period, images of the droplet were obtained by the ELMO CCD camera (1:3.1, f ) 24 mm lens) through a optical filter (Neutral Density 8 light reduced to 1/8) to reduce reflection. The images were recorded on a videotape to allow replay as still views at 33 ms intervals. The electromotive force changes of the thermocouples were measured at 309 points/s and recorded automatically by a computer during the heating. After the

Volatility of Bitumen Samples. The profiles of weight loss with temperature in the TGA analysis of bitumen and the pentane-insoluble fraction are shown in Figure 3a. The difference of the weight loss at each temperature indicates the amount of components released at the temperature from the sample container in the TGA apparatus. Under atmospheric pressure, the temperature is the apparent boiling point of the components released. When the pressure is reduced, the temperature is, in general, shifted to lower temperature without a change in the amount of components having the same boiling point, if there are no reactions and any interactions due to reducing the pressure. The differential curve from the weight loss profile for bitumen is shown in Figure 3b. One peak was observed at ca. 753 K at atmospheric pressure (100 kPa). Another peak appeared at lower temperatures when the pressure was reduced from 100 to 0.4 kPa. This peak shifted to lower temperatures and increased as the pressure was



Figure 5. Expansion of luminous part and directions for data analysis. Figure 4. Visual observation of volatilization during heating at 1.6 kPa.

reduced. The peak at ca. 753 K, on the other hand, shifted very little as a function of pressure, although its height was reduced as the low-temperature peak emerged at low pressure. The shift in the low-temperature peak with pressure was consistent with the removal of volatile components of the bitumen, which would distill off at lower temperatures as the pressure decreased. The minor shift in the peak at ca. 753 K to a lower temperature as the pressure was reduced to 0.4 kPa could be due to more effective removal of volatile cracked fractions from the residual liquid during pyrolysis. The data for the pentane-insoluble fraction (Figure 3c) support these conclusions. No peak was observed in the lower temperature region at any pressure. The observed peak for the pentane-insoluble fraction was in the same range as that of bitumen, ca. 753 K, and a major shift of the peak was not observed at lower pressure. The pentane extraction of the bitumen would remove almost all volatile material; therefore, the pentane-insoluble fraction would exhibit pyrolysis only, without distillation. In contrast, both vaporization of distillable components and pyrolysis of nondistillable residue would be observed for bitumen. Observation during Rapid Heating. The images in Figure 4 show the changes during the heating of ca. 1 mm droplets of bitumen and the pentane-insoluble fraction. The time shown in the figure is measured from the time when the heaters were turned on. A small, weak luminous zone was observed in the center of the field of view for both samples after 200 ms, likely due to volatilization. The luminous zone expanded outward rapidly between 200 and 300 ms and became much more distinct. After 500 ms, the rate of expansion gradually decreased. The luminous zone was almost spherical for

both samples. The luminous material was not analyzed for chemical composition, but it was likely due to reflection of light from the expanding cloud of vapor released from the sample droplet. Combustion of vapors was unlikely because the container had been purged with nitrogen. The luminosity from the pentaneinsoluble samples was qualitatively weaker than that of the bitumen, which was consistent with the smaller fraction of distillable components in the sample. The bitumen samples gave luminous whisker shapes above the main spherical domain, for example, in the image at 400 ms. This behavior could be due to natural convection, even at low pressure. Figure 5 shows the outline of the expanding luminous zones observed in Figure 4, in time steps that were 33 ms apart. The difference in the shapes of these vapor clouds between bitumen and the pentane-insoluble fraction could be caused mainly by the amount of distillable material and its heat capacity. Both of these factors would affect convection in the observation chamber. The data of Figure 6a and b show the expansion rates of the luminous zones as a function of time change with time, in the different directions shown in Figure 5. The data were scattered for both pentane-insoluble and bitumen samples; however, a distinct difference in the trend of expansion rate versus time can be recognized in the average expansion rate, plotted in Figure 6c. The expansion rates for both samples were initially high and decreased during the first 300 ms of heating. The vapor cloud from bitumen expanded more slowly at longer times, while for pentane-insoluble samples, the rate increased after about 300 ms. The initial rate for pentane-insoluble samples was higher than that for bitumen, although its starting point of expansion was later than that for bitumen. In this paper, the sample was heated by the constant irradiation for each experiment even if the sample


Nagaishi et al.

Figure 7. Volatilization of bitumen under normal pressure.

Figure 6. Comparison of expansion rate for different directions: (a) pentane-insoluble fraction, (b) bitumen, (c) average of all directions.

property was different. The temperature profile of the sample supported could be changed with volatilization progress for the same sample and also changed in different samples. To measure heavy oil pyrolysis and to compare the difference of the behaviors, correct relations of temperature and behaviors would be necessary. However, correct measuring and controlling methods of temperature are still not firm for this observation method. A volatilization mechanism by vaporization and pyrolysis, therefore, could be speculated from the expansion behavior. Vaporization of distillable components would tend to reduce the temperature of the bitumen sample by two mechanisms. The energy required to vaporize the components would give a lower rate of heating of the sample. Once the cloud of vapor forms, it tends to reflect light, thereby reducing the irradiation of the sample at the center of the container. Both mechanisms would give slower heating of the bitumen sample in comparison to the pentane-insoluble sample, which would account for the differences in the initial expansion rates. The thin cloud of vapor from the pentane-insoluble sample would reflect less of the light; therefore, this sample would heat more rapidly

after 300 ms and generate volatiles by pyrolysis reactions. Clarifing the validity of these assumptions would require much more detailed observations to correct for temperature and concentration in the luminous zone and its relationship to weight change of the sample due to volatilization and pyrolysis. Distinction between vaporization and pyrolysis was not easy. Vaporization, however, could be affected by pressure more than pyrolysis. The effect of pressure on the observed behavior was examined at atmospheric pressure. Heating of a bitumen droplet of ca. 2 mm at atmospheric pressure is illustrated in Figure 7. Natural convection was much more significant at atmospheric pressure, based on the immediate upward motion of the luminous cloud. The downward motion in the bottom frame was due to falling liquid from the sample, due to the larger sample size. The results in different atmospheres with different sized samples indicated that the present experimental method would not be useful for investigation of the pressure effect on vaporization and pyrolysis. However, the results suggested that the visual observation method, with monitoring of the correct temperature and weight loss due to vaporization and pyrolysis, could be used to develop a model for bitumen behavior under rapid heating, even under different pressures, if the observation would be carried out, for instance, in microgravity conditions.11 Temperature Measurement. The temperature profiles obtained at different measurement points and by different thermocouples are shown in Figure 8. The heating method was the same in all cases: 2 s of heating at a constant 24 V to each bulb. The temperature measured by the 0.5 mm chromel-alumel (CA) thermocouple in the absence of sample increased to ca. 773 K, giving a heating rate of 570 K/s. Natural cooling (11) Okajima, S.; Kumagai, S. Preprinted from Fifteenth Symposium (International) on Combustion, Tokyo, 1974; pp 401-407.



Figure 8. Temperature profile of sample supported.

after the end of the heating period dropped the temperature to 473 K in 7 s. The signal-to-noise ratio was improved by the presence of a sample in contact with the thermocouples. The thermocouples turned from silver to black color in the preliminary heating tests; therefore, reflection was not significant. The inside of a 1 mm bitumen droplet was heated almost as rapidly as the thermocouple without sample, as measured by the 0.5 mm CA thermocouple. The beginning of the temperature rise was delayed by 200300 ms due to volatilization of the lighter components. The interior of a 2 mm droplet of bitumen only reached a maximum temperature of 373 K; therefore, liquid was present throughout the experiment. The outside temperature of a 1 mm bitumen sample gave a heating rate of about 680 K/s, which was faster than the case without a sample. At 873 K, the temperature exceeded the upper voltage of the equipment and went over the scale. The difference between the inside and outside temperatures of the 1 mm sample showed the importance of the thermal conductivity of the bitumen. The data from the 0.1 mm Pt/Rh thermocouple showed, however, that the apparent temperatures were sensitive to the size of the thermocouple. The high-sensitivity, low-mass thermocouple showed an apparent heating rate of 1500 K/s during the first 300 ms. Furthermore, the temperature profile showed such details as a change in heating rate due to vaporization of the sample to form the luminous cloud (see Figure 3). The sensitivity of the results to the thermocouple suggests that the temperature response and measurement point should be checked carefully for each sample. The thermocouple response and mass are clearly major factors in any attempt to model the heating process. Behavior of a Large Pentane-Insoluble Droplet. The images in Figure 9 illustrate the heating of a larger sample of the pentane-insoluble fraction, with a diameter of 2 mm, at a pressure of 0.4 kPa. Melting and boiling of the droplet was observed after the start of heating, at 70 ms. The diameter of the droplet expanded due to boiling, then began to hang downward at about 130 ms. At 300 ms, a luminous cloud caused by volatilization was observed to surround the droplet at the tip of the needle and continue to expand. The expanded luminous part became faint by 400 ms, then another expansion began at 990 ms before the first one disappeared completely and continued even at 1090 ms. Such cyclic behavior was observed several times until the end of the measurement at 1930 ms. Subsequent observation was not possible due to the darkness of the

Figure 9. Illustration of irregular behavior of large droplet pentane-insoluble fraction.

sample. Similar cyclic behavior was observed in experiments using a very small quantity of bitumen, with a diameter ,1 mm, on the surface of the needle supporting the sample (see Figure 2a), although the cycling period was shorter. These samples likely formed a surface skin of carbonized material surrounding a liquid core. Continued heating of the exterior would push bubbles of vapor out at intervals, rather than continuous evolution. The data of Figure 8 for the interior of the 2 mm particle suggest that liquid will persist on the interior of a larger droplet and make this phenomenon much more important than with a 1 mm sample. We speculate that pyrolysis of carbonized matter formed during the heating was predominantly observed in such cases. Pentane-insoluble samples may be carbonized on the needle because of their high molecular weight. Their viscosity is much higher compared with bitumen at the same temperature, even if sample melts by heating. With the increase in temperature, pyrolysis and carbonization, especially on surface of the droplet, progress simultaneously. A heated sample would rapidly form harder carbonaceous matter by carbonization on the surface of the sample, without dropping from the tip of the needle due to a viscosity decrease with a temperature increase. This behavior could be observed frequently in a 2 s interval for 2 mm pentane-insoluble samples because the amount is more than that of the 1 mm sample. If this hypothesis is valid, the case of less bitumen as mentioned above could be explained as follows: a small amount of bitumen never drops down from the needle, even when its viscosity decreases by heating. Therefore, carbonaceous matter pyrolizing and carbonizing remains on the needle without disappearing from the observed view. For a much larger sample like 1 and 2 mm of bitumen, melting and boiling on the surface could take place much more easier than for pentane-insoluble samples. As a result, most of the


sample was supposed to disappear by splashing and dropping due to the low viscosity during the observation. In the present condition, reflection by the luminous part was strong in most cases so simultaneous observation of such a splashing and dropping behavior in the luminous part and vaporization behavior was impossible. However, after the experiments ended, scattered small samples have been sometimes observed on the heater cover of the bottom lamp. Although the heating rate for this method was extremely different from that of the thermogravimetric analysis, the carbonization process by pyrolysis would be observed as a carbonaceous matter remained after the thermogravimetric analysis. Thus, the amount of sample strongly affected the temperature profile and the behavior observed. Although the bright luminous zone dominated the images from the rapid heating experiments, the initial images showed that foaming or frothing of the liquid phase was not significant. Only the 2 mm pentaneinsoluble samples showed an increase in volume due to boiling, and in this case, the change was modest. If droplets did not form foam and expanded structures at 1.6 kPa due to the evolution of volatiles and cracked products, then such behavior would be even less likely at atmospheric pressure. This conclusion is important for modeling of pyrolysis and coking processes that subject droplets or liquid films to rapid heating. In

Nagaishi et al.

these cases, the physical dimensions of the reacting liquid phase and its transport properties will not change abruptly with the appearance of the vapor phase. Expanded coke structures9 could not form during coking and carbonization in these processes. Conclusions Rapid radiative heating of bitumen and pentaneinsoluble samples under vacuum showed that volatiles were evolved as a spherical cloud around the sample. Boiling and bubbling of the samples during the early stage of heating was minimal; therefore, the geometry of the sample for heat transfer and diffusion was defined by the liquid droplet. Parameters such as initial sample shape, sample size, composition, and method of measuring temperature had a significant effect on the experimental results. Control of these variables would be required in order to allow modeling of the heating and reaction of the samples. The visual observation and the photography of the pyrolysis behavior could give new insight, and check the validity of the models proposed so far, which were assessed mostly on the basis of their applicability to experimental data for the yield of various product fractions. EF980017R

Observation of Heavy Oil Vaporization under Rapid Heating

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