Products of rapidly heated oil sand

Products of rapidly heated oil sand Peter

C. Stangeby

and Paul L. Sears

Institute for Aerospace Studies, Ontario, Canada, M3H 5T6 (Received 30 April 1980)

University

of Toronto,

4925

Dufferin

Street,

Downsview,

Previous work on direct oil sand heating showed that hydrocarbon removal of wQO% could be achieved below a temperature of ~550% It was also found that the rate of heat-up did not affect.the total yield. In the present work, the influence of various parameters, including heating rate, on the yield distribution of products from directly heated oil sand and separated bitumen was examined using gas chromatography. It is found that the heating rate has a very dramatic influence on the distribution of products. Fast heating rates cause a strong shift to lighter species in either inert or chemically reactive atmospheres. It is possible to achieve a very wide range of distribution of products by employing suitable values of heating rate and final temperature. Increasing hydrogen pressure is found to have a significant influence on the yield distribution up to ~3 MPa, after which point, little further change is observed.

A number of possible methods exist for the separation of usable hydrocarbons from the complex material of the Athabasca oil sands. One such possibility is the direct heating of the oil sand, with resultant cracking and distillation of the organic material. However, the heating of a large body of oil sand is necessarily slow and any fluid products may have to travel a large distance through the solid material to escape. The products actually observed may thus be the result of reactions occurring to raw oil sand at a variety of temperatures. Furthermore, the cracking of the products as they leave the sand may also distort the results. In the present study, the products of oil sand pyrolysis in situations which permit the very rapid heating of a sample of the sand, and also the rapid removal and cooling of gaseous products, are examined. Previous work’ has established that the amount of bitumen vaporized from a small oil sand sample reached a maximum of =90x at any temperature greater than 55O”C, virtually regardless of other experimental conditions. These conditions included as variables, ambient gas and its pressure, temperature, heating rate, and time at final temperature. Because the weight loss of oil sand samples is complete by 55O”C, it is evident that it is unnecessary to heat the material beyond this temperature unless the higher temperatures can be achieved before the hydrocarbon material has volatilized. The previous study indicated that the volatilization was complete in a period of the order of one second, so heating rates of the order of 1000 K s-l are required to observe the effects of higher temperatures than 550°C on the raw oil sand. Research elsewhere on coa12S3 has shown that a more favourable product distribution can be obtained when coal is heated rapidly as compared with slow heating. A shift toward higher yields of aromatic liquids, predominantly benzene, is noted. The present experiments were undertaken to establish the effect of heating rate on the product distribution from Canadian Athabasca oil sand and in particular to identify conditions for favourable yields.

0016-2361/81/02012546$2.00 01981 IPC Business Press

EXPERIMENTAL Two separate flow line apparatuses were used for this work. For a pressure of one atmosphere a conventional glass vacuum line was employed, but a stainless-steel line was required for the high-pressure work. While the basic layouts of both lines are similar, their construction and operation differ considerably and are, therefore, described separately. The high-pressure design is based on one used elsewhere2. Low pressure

A schematic

line

diagram

of this apparatus is shown in is regulated at a pressure by a low-pressure regulator. The

Figure I. Gas from a cylinder

slightly

above ambient

Reactron

chamber

‘monometer

Figure 7 struction.

Schematic diagram of low-pressure system. Elements and operation explained in text

FUEL,

1981,

Glass con-

Vol 60, February

125

Products

of rapidly heated oil sand: P. C. Stangeby Q

and P, L, Sears

Power connection

Brass ro d Gasin

-

-Nylon

Reaction

chamber

I

Sample i n SS mest ,

Side

‘r

view

Gasout

Figure

2

Reaction

chamber

in low-pressure

glass system

pressure used is typically w3 kPa above ambient. The gas enters the system by tap A and passes through a flowmeter, into the reaction chamber, and down into a cooling coil. A tap C is used to regulate the gas flow which may subsequently be exhausted via taps D and F, or directed into a 1 1 storage bulb via D and E. This bulb is iitted with a mount for a rubber septum. D is a 3-way glass tap which may be used to connect any two of its three lines, or to connect all three. F is a large glass’tap used to isolate the vacuum pump. All other taps are Rotaflo teflon/glass S8740-16 or S5740-216. Samples are mounted in the reaction chamber by the support shown in Figure 2. Two $ inch (6.35 mm) brass rods are mounted in a large rubber stopper which is fitted into a conical joint socket at the upper end of the chamber. The lower ends of these rods are milled and provided with tapped holes which permit the attachment of a small brass plate to the flat area of each rod. The sample holder, which is a piece of stainless-steel mesh, is held in place by these plates. A thermocouple mount is also supported by the brass rods, and consists of two pieces of nylon sheet clamped to the rods by means of bolts. A brass reinforced teflon strip attached to this then supports the thermocouple wires so that the thermocouple is just in contact with the stainless-steel mesh. Very thin (0.025 mm) chromel-alumel thermocouples are employed, and adjustment of their position is achieved by movement of an adjusting screw which passes through a captive nut bonded to the brass strip and then bears on

126

FUEL, 1981, Vol 60, February

the nylon surface. The thermocouple leads are supported in slots at the lower end of the teflon and taped to it higher up. To leave the reaction chamber, the leads run between the rubber stopper and the glass socket. A reference thermocouple in ice and water, and a Sanborn 296 chart recorder complete the thermocouple circuit. This system can easily follow temperature rise rates of lo4 K s-l. The stainless-steel mesh is heated by passing a current through it via the brass rods. A 90 A current gives a heating rate of ~6000 K s-l, and * 33 A is required to maintain a temperature of 800°C. The first current is supplied by direct connection of a 12 V battery across the mesh, and the latter by the insertion of a resistor into this circuit. An electrical timer controls the duration of the high current by firing a sequence of SCR (siliconcontrolled rectifier) switches. This system permits very rapid heating of the stainless-steel mesh, and then the indefinite maintenance of an elevated temperature. When hydrogen is in use as the gas in the system, a blast blanket is supported in front of the line, in case the inadvertent admission of air leads to an explosion. Operation of the system

A sample of oil sand (ca. 5 mg) is spread over the centre portion of a weighed piece of stainless-steel mesh (25 x 21 mm), which is then folded twice to contain the sample. The result is a 25 x 7 mm sample holder, which is weighed and mounted as shown in Figure 2. Care is required to ensure that the oil sand is spread as evenly as possible over the unsupported part of the sample holder, but does not extend as far as the brass rods. Failure to do this results in uneven heating. When the sample is in place in the line and all electrical connections have been made, the heating circuit is adjusted to reach and maintain the desired temperature. This adjustment required some judgement, as the thermal loading provided by the oil sand increases the time taken to attain a given temperature and this necessitates slightly variable timer settings. Timer setting and the maintenance current resistor must be well-matched to provide a rapid temperature rise to a steady temperature. The sample bulb is then evacuated and the remainder of the line is flushed with the gas in use. Valve C is used to throttle the gas flow, set at ~40 ml s-l, which passes through the pump to the exhaust. The sample is now heated. On closing the electrical circuit, the gas flow is switched to the storage bulb. As this is evacuated, this makes no immediate difference to the flow rate, which is adequate to ensure the very rapid removal of volatile materials, from the vicinity of the mesh into the cooling coil. It is evident that convection is completely swamped by the bulk gas flow. Product gases are carried into the storage bulb and the gas flow is maintained after the heating current is switched off to flush the line clear and raise the bulb pressure to that of the source regulator. Tap E is then closed, and the gas supply shut off. The sample holder is removed and reweighed to give sample weight loss and samples of the gas in the storage bulb are taken by syringe and analysed by gas chromatography. Initially, it was hoped to collect heavier materials in the cooled section of line, to dissolve them in a suitable solvent, and then to analyse these by g.c.; however, the only materials collected in this section were very heavy

Products

G

Flowmeter

C ( Gas

collection

cylinders

High pressure

line schematic

of rapidly

heated

oil sand:

P. C. Stangeby

and P. L. Sears

test section are maintained at =9O”C by means of electrical heating tapes. The electrical circuit for the test section heating current is similar to that employed for the low-pressure apparatus except that 24 V of heavy duty batteries are used for the rapid heating current, producing temperature rise rates of * 600 K s -’ and mechanical relay switching is used instead of SCR switching. It is estimated that 1000 A flows through the circuit during heatup. Two, four or six volts are used to maintain an elevated temperature, fine adjustment being made by varying the length of tube heated. The switching of both high and low currents is controlled by a timer. Typically the high current is on for cu. one second, before being switched to the low. Operation

Figure 3 Schematic diagram of high-pressure system. Stainlesssteel construction. Elements and operation explained in text

tars, which did not yield any g.c. results. For some samples it was necessary to insert a glass wool plug into the bottom of the reaction chamber to prevent the large quantities of tar produced from blocking the tap at C. High-pressure

line

This line is shown in Figure 3. It is constructed primarily of $ inch (6.35 mm) o.d. stainless-steel tubing, with walls 0.035 inch (0.9 mm) thick. A pressure-regulated gas supply enters via a check valve, A, and a handoperated valve, B. A bleed line and valve C to exhaust are followed by a solenoid valve D which is connected to a pressure switch downstream in such a way as to close valve D in the event of a drop in the pressure sensed at the switch. This is intended to shut off the high-pressure gas flow should the line rupture in the test section. The test section of the line lies downstream of the pressure switch. It is mounted in two steel supports, one of which holds the tube firmly and provides an electrical connection and the other allows it to slide freely when it expands on heating. These two mounts are fixed to a wall. A third steel device is clamped firmly to the tube at a position between the others to provide another electrical connection to the tube. A large current passed between these connections (which lie ==20 cm apart) serves to heat the tube and thus the sample within it. The test section of the line has a wall thickness of 0.02 inch (0.5 mm). A chromel-alumel thermocouple is spot-welded to the tube at the point where the sample is to be deposited. This is connected to the chart recorder. Another check valve, E, permits the flow of gas into the next section of line, but prevents its return should the test section rupture. Approximately six feet of line lies between E and a needle valve, F, which is used to regulate gas flow. A two-way valve, G, directs the flow either through a flowmeter to exhaust, or to a storage and sampling facility consisting of a vacuum pump, two stainless-steel cylinders and pressure (O-2 MPa) and vacuum gauges. A connection from this section leads to a gas chromatograph. All of the lines, taps and cylinders downstream of the

A weighed sample of oil sand (cu. 15 mg) is deposited in the test section by means of a small aluminum boat on the end of a steel rod. Bitumen samples (cu. 3 mg) are deposited with a glass capillary bonded to the end of a metal tube. A glass wool plug is inserted below the heated section of the line. All joints in the line are then remade and all taps below D as far as G are fully opened. G is positioned so as to allow gas to flow from the test section to the exhaust, and the collection cylinders and associated line are evacuated. At this stage, D is closed. A flow of hydrogen is established from the regulated supply through A and B, and then through C to the exhaust. B is used to permit only a very slow flow of gas, while C is fully open. D is now opened, using a manual override switch and C is slowly closed, directing the gas flow through the system between D and G. When air has been flushed from the line, F is closed, and the pressure between B and F builds slowly to the regulator pressure. Too rapid pressurization of the system blows the oil sand along the tube. When equilibrium has been reached, B is opened fully and the pressure switch is adjusted so that a small drop in pressure will close D. F is then opened sufficiently to permit the desired gas flow rate to occur. A blast blanket is suspended in front of the apparatus and the sample is then heated, the gas flow being diverted into the collection cylinders. The pressure in these cylinders is allowed to rise to 1 MPa at which point the flow is shut off. The timing of these operations is adjusted to ensure complete collection of the gaseous hydrogenation products. These are then analysed by gas chromatography. This procedure is then repeated using 400 kPa oxygen as the gas in the line, and filling only one collecting cylinder to 300 kPa with product gases. This mixture is then analysed for carbon dioxide to give the amount of carbon remaining in the sample after hydrogenation. Gus analyses A Varian 3700 Gas Chromatograph is used for all work reported here. Hydrocarbons are measured by the response of flame ionization detectors which are assumed to have a rectilinear response based on the carbon content of the carrier stream. Detectors are calibrated by measuring the output produced by samples of analysed gas mixtures. Carbon dioxide is measured with thermal conductivity detectors calibrated with a COJnitrogen mixture. Hydrocarbons up to hexane are separated using a 5 foot x $ inch (152.4 cm x 3.18 mm) phenylisocyanate on SO/l00 mesh Porasil C column, with nitrogen carrier gas ‘. This column is operated isothermally flowat 15mlmin-

FUEL,

1981,

Vol 60, February

127

Products of rapidly heated oil sand: P. C. Stangeby and P. L. Sears I

I

I

I

l

1

l

Oil sand samples Both medium- and high-grade oil sand samples were tested. The results for the two samples were similar, therefore, only the high-grade sample results are reported. Further details are available elsewhere4. The samples were supplied by the Alberta Research Council Sample Bank and the high-grade material was from the lower bench of the formation at the Great Canadian Oil Sand Co. site in the Athabasca deposit. Analysis (Dean-Stark): bitumen, 14.3; water, 0.9; solids, 83.9 wt %; discrepancy, 0.4%.

RESULTS Low-pressure

I

LOO

600

I

I

-1

000

T ('Cl Figure 4 Yield of CH, (0) and CzH4 (u) as a function of peak temperature and very fast heating rate, ~2000 K s-1. In 1 atm helium

at 30°C if only hydrocarbons below C, are being measured, otherwise it is operated at 30°C for 1 min followed until by a 15°C min -’ rise to 60°C which is maintained completion of the run. For hydrocarbons from benzene to naphthalene a pair of 18 inch x$ inch (457 mm x 3.18 mm) 5% OVlOl on Chromosorb G HP 100/120 columns are used with a 15 ml min-’ nitrogen carrier gas flow in each. These are run at 30°C for a min followed by a 15°C mini rise to 120°C which is maintained until completion. Differential detector arrangements eliminate most of the drift arising from column bleed. Carbon dioxide is separated on a 6 foot x $ inch (182.9 cm x 3.18 mm) 100/120 mesh Poropak Q column, using hydrogen carrier at 20 ml min-’ and isothermal operation at 30°C. Detector output is measured by a CDS 111 integrator and data system. For the low-pressure study a 0.5 cm3 syringe is used to sample the product gas. The total amount of each hydrocarbon collected is deduced from the sample chromatogram, a recent calibration chromatogram, and the measured volume and pressure of the collected gas. The high-pressure line is connected more directly to the gas chromatograph by means of a 6-port sampling valve mounted in the g.c. oven. This also has a 0.5 cm3 sample volume. Amounts of hydrocarbons and carbon dioxide are calculated from measured pressures, temperatures and volumes. Collection volumes are measured by filling with water, which is then either weighed or measured volumetrically. It is not necessary to have accurate measurements of the volumes of the nominally 0.5 cm3 syringe and sampling valve, as the same volumes are used for both calibration and experimental samples. It was not possible to completely separate all hydrocarbon gases up to C, with any one column. The use of the phenylisocyanate column grouped propylene with the butanes and butylenes with the pentanes.

128

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1981,

Vol 60, February

line

Weight loss data are reported as a percentage of the nominal bitumen and water content of the oil sand sample. Hydrocarbon yield data are given as the carbon content of the hydrocarbon produced expressed as a percentage of the nominal bitumen content of the sample. On this basis, complete carbon recovery would be indicated by a total equal to the carbon content of the bitumen, -85%. In the low-pressure work, it was not feasible to investigate the production of hydrocarbons above ca. C,, or to directly measure the char remaining on the spent sand sample. However, weight loss data for the samples give an indication of the latter quantity. In all this work the production of large amounts of heavy tarry material was evident, and as this presumably contained virtually all the carbon not accounted for, it represented the majority of the bitumen in the sample. As in the earlier study’ it was found that total weight loss of samples was essentially complete, =90x, by a temperature of 500°C and was not greatly influenced by ambient gas composition or heating rate. However, the composition of the product yield was significantly influenced by the heating rate. Figure 4 gives results of methane and ethylene yield for very fast heating, = 2000 K s- ’ in a low-pressure inert atmosphere (1 atm helium); Figure 5 shows the results for slower rates. Yields of methane and ethylene increase with temperature but for heating rates < 200 K s-i the yields of these two gases do not exceed 4%, even at 950°C whereas rapid heating can give over 4% as methane and over 10% as ethylene at temperatures as low as 700°C. Raising the heating rate to 200 K s ’ has a measurable effect on these yields, and a further increase to 400 K s- ’ produces a large rise, though not to levels comparable with the very rapid heating results. The composition of the very rapid heating products is of some interest. The relative abundances of the components are approximately the same at all temperatures; the order of abundances is: C,H,

> CH, > C,H,

> 2-C,H,

> C,H,

Other products are relatively minor. For a heating rate of 150 K s-l the range of hydrocarbon concentrations is much smaller, and the order is typically: CH, > C,H,

> C,H,

> C,H,

> C,H,

> l-C,H,

> 2- C,H,

The high-heating rate causes a marked increase in the production of all gaseous hydrocarbons, but also appears

Products

: 1, L

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21

s

3 ,.0 n

1

Ti ._ > 0.8

91 0.6 I

0,L

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and P. L. Sears

monstrated by the results for benzene, toluene and xylenes. In contrast to the low-pressure results, high yields of ethylene are produced at the higher temperatures. This is expected because the low-pressure work is run under conditions which are hydrogen deficient (even in 1 atm hydrogen gas) but the high-pressure conditions provide a large amount of readily available hydrogen. Another difference between the two sets of conditions involves the time during which the gaseous products remain in a hot zone. In the low-pressure work, cooling is immediate and very rapid, but in the high-pressure apparatus the gas requires ~0.5 s to leave the heated tube, providing an opportunity for product cracking and hydrocracking. A carbon-balance was made by burning off the residual char left in the tube and measuring the CO,, see Tuhlr 1. In contrast with the coal studies2*3 where it was often not possible to close the carbon balance, balance was found except for peak temperatures below 500°C. In the latter case it appeared that heavy material was volatilized but did not reach the g.c. Slow heating (40 K s-i) of the same material under otherwise similar conditions gave the results shown in Figure 7. It will be noted that the production of hydrocarbons levels at = 6OO”C, in contrast to the situation for fast heating where yields continue to increase. The

1.6

1.

of rapidly heated oil sand: P. C. Stangeby

\

..i

\

a

0

L

I-I

I

I

I

800

600 T

1C

L(

1°C)

Figure 5 Yield of CH4 (0) and CzH4 (m) as a function of peak temperature and for a slower heating rate than in Figure 4. Numbers indicate heating rate in K s-1. In 1 atm helium. Note scale change from Figure 4

to favour alkenes over alkanes. A small amount of acetylene appears on rapid heating to temperatures > 550°C. Similar low-pressure runs but in a hydrogen atmosphere showed a definite but not large increase in the yields of the various light products. No significant reduction in the amounts of unsaturated compounds was noted. To make a simple check for catalytic effects two tests were made: (1) the stainless-steel sample holder was replaced with tungsten foil, but no significant changes in product distribution were noted: and (2) to check for possible catalytic effects of the sand particles, experiments were made with pure separated oil sand bitumen, but again no significant changes in results were observed. High-pressure experiments were confined to hydrogen atmospheres for the earlier study had shown a suppression in total yield in high-pressure, inert atmospheres. Figure 6 gives the results for fast heating, 700 K s-l, in 10 MPa H,. As previously, complete conversion of the bitumen in the oil sand to hydrocarbons occurs for all temperatures above ~600°C. Higher temperatures favour a simplification in the product distribution and a general reduction in molecular weight. The aromaticity of the products also tends to increase, and side-chains are removed as de-

3C 0 zr z .P > 2(

10

B

\

0

0

d2TzrcEI.._

I al

I___L

600 T

800 (‘Cl

Figure 5 Yield of light hydrocarbons as a function of peak temperature and for fast heating, a700 K s-l. In 10 MPa Hz. 0, CH4; 0, CzHg; n, CgHg; 0, naphthalene

FUEL,

1981,

Vol 60, February

129

Products of rapidly heated oil sand: P. C. Stangeby and P. L. Sears Table

1 High-grade

oil sand, fast heating (=700

K s) in 10 MPa hydrogen

C3H6/

L

CH4

C2H6

C2H4

C3b

C4W

C4H10

W1o/

CsH12

C6H6

C6H14

C7Hs

Xyl.

Naph.

Other

Char

Total

0.1 2.9 4.1 4.4 trace

0.5 4.1 4.4 0.7 -

7.6 6.4 9.0 12.8 27.7 16.2

8.1 32.3 19.0 15.3 10.3 10.4

69.5 11.0 7.8 9.3 5.5 5.3

88.6 111.0 98.0 96.6 101.2 99.3

(wt %I

470

2.0

0.2

600 670 720 780 840

10.8 14.3 16.6 27.5 45.4

13.4 19.8 22.6 16.3 5.2

I

L

_

0.3

0.3

-

-

-

1.3 0.7 0.6 0.2 0.1

13.0 12.3 8.2 trace trace

7.9 4.1 1.9 -

3.0 1 .o 0.1 -

2.8 -

1.9 2.5 4.1 13.7 16.7

I

I

I

LOO

600

1

I

18

I

.

I

I

I 800

T (‘Cl Figure 7 Yield of light hydrocarbons as a function of peak temperature and for slower heating than used in Figure 6, =40 K s-l. In 10 MPa Hz. 0, CH4; 0, C2H6; n, C6H6; 0, naphthalene

amounts of char remaining are similar to those with fast heating, but total recoveries of carbon are reduced by m 40%. These results are expected because volatilization of the bitumen is virtually complete in a few seconds at 550°C; heating rates of e.g., < 200 K s- I, never actually heat the bitumen much beyond 55O”C,and the vapourized material is carried into cooler sections of the apparatus before the temperature rises further. Figure 8 shows the effects of hydrogen pressure variation on the yields of the hydrocarbon products. The data refer to rapid heating in the high-pressure apparatus, with one exception, which is the PwO line, the data for which come from the low-pressure study, and thus used a higher heating rate. However, it may be seen that they are consistent with the rest of the Figure. Figure 8 shows that the changes induced in the product yields as the hydrogen pressure rises, are largely complete before 4 MPa. Further increase in pressure has little, if any, effect, therefore, it may be that =3 MPa is the optimum pressure for any applications of this work. The rate of removal of the volatile products from the heated feedstock is undoubtedly important because the longer these products spend in a hot environment, the greater will be the secondary reactions, e.g. cracking and/or hydrocracking. Both the low- and high-pressure systems used in the present study achieve short dwell times for the volatiles at elevated temperature, of the order of 100s of milliseconds, thus minimizing the influence of secondary reactions. Nevertheless, the data generated in the present work incorporate effects due to (1) rapid heating of the feedstock and (2) some secondary reactions. Further research is underway directed toward separating these two processes by varying the dwell-time of the volatile products. CONCLUSIONS The principal finding of this study is that heating rate has a dramatic effect on the distribution of products from oil

130

FUEL,

1981,

Vol 60, February

l-l, pressure

1kPa)

Yield of light hydrocarbons as a function of Hz pressure. Figure8 Rapid heating, a700 K s-l. 0, CH4; 0, C2H4; n, C6H6; 0, naphthalene

sand and bitumen. This strong influence of heating rate on yield distribution contrasts with the essentially negligible influence of heating rate on total yields’. Faster heating rates cause a strong shift to lighter species in either inert or chemically reactive atmospheres. It appears to be possible to achieve a very wide range of distribution of products by employing appropriate values of heating rate and final temperature. ACKNOWLEDGEMENT This work was carried out with support by the Alberta Oil Sands Technology and Research Authority. REFERENCES Stangeby, P. C. and Sears, P. L. in ‘Oil Sand and Oil Shale Chemistry’, (Eds. 0. P. Strausz and E. M. Lown), Verlag Chemie Int., 1978, pp. 101-118 GraN, R. A., Dobner, S. and Squires, A. M. Fuel 1976, 55, 139, 113; also Science 1975, 189,793 Steinberg, M., Sheehan, T. V. and Lee, Q. Am. Chem. Sot. DIV. Petroleum Chem., Preprint 1975, 20(2), 542 Stangeby, P. C. and Sears, P. L. ‘Analysis of the Products of Rapid Devolatilization/Hydrogenation of Oil Sand’. Report to the Alberta Oil Sands Technology and Research Authority, August 1978