KINETIC STUDIES ON RAPID OIL SHALE PYROLYSIS

Ming-Shing Shen and Lawrence J. ShadleU.S. Department of Energy

P.O. Box 880

Morgantown, WV 26505-0880

Guo-Qing ZhangDepartment of Mechanical Engineering

West Virginia UniversityMorgantown, WV 26506

ABSTRACT

Oil shale pyrolysis was investigated under con

dition of high-heating rates to obtain a fundamental

understanding of the processes influencing oil pro

duction and product quality/composition. A 2-inch

I.D., laminar-flow entrained reactor (LFER) with gas

preheaters was constructed to achieve high particle

heating rates. Flow visualization and temperature

characterization in the LFER were conducted to

provide the data necessary to proceed with a kinetic

study on the rapid pyrolysis of oil shale. Calibra

tion curves depicting the flow regimes confirmed

that the existing inlet and reactor design can be

operated without particle dispersion caused by tur

bulence. A thermocouple tree was successfully util

ized to identify the conditions necessary to match

wail and gas temperatures. The rapid pyrolysis of

these oil shales was carried out in nitrogen at

temperatures between 700 and 850C, with gas pre

heat temperatures up to 980C. For each tempera

ture, the sampling probe was set at different

positions along the length of the reactor tube to

obtain different residence times. Model calcula

tions were used to evaluate the residence time and

heat up rates under each set of conditions. For

each position, the organic conversion rate was

determined by using the ash tracer technique and

modified from dry ash free basis to dry mineral

matter free basis. Using a global first-order

Arrhenius relationship, the kinetic parameters were

determined.

INTRODUCTION

Recent interest in oil shale research at METC

has been in high-heating rate oil-shale pyrolysis

using flash lamp and entrained flow reactors.

Previous researchers agree that high-heating rate

(IO3

to10' C/sec) and short residence time

(0.5 to 5 sec) pyrolysis of oil shale results in

significantly higher yields of liquid and gaseous

hydrocarbon products than that of conventional slow

heating rate(1 to 50C/min) longer residence time

(minutes to hours) . Flash lamp pyrolysisstudies1"3

on small micron sized shale particles have

provided mechanistic insight into the nature of

thermal reactions which occur during rapid retorting

processes . The overall conversion of shale kerogen

to liquids plus gases is higher for flash pyrolysis

conditions than for slow heating processes such as

Fischer Assay and thermogravimetryresearch.1 The

rate of the initial phase of kerogen decomposition

in flash pyrolysis is extremely rapid. Complete

conversion of the kerogen in oil shale can be

achieved in the millisecond time frame; however, a

large portion of the product is gas. Compared to

products contain more unsaturated gas species, BTX

(benzene, toluene, and xylenes), and heteroatomic

liquid species, they are higher in heavy molecular

weight liquids, and have greater gas/liquidratios.2

We also found that the controlling reaction pathways

which govern liquid quality are determined by the

heating rate (incident flux) while the overall con

version and the extent of cracking, polymerization,

and/or condensation reactions are determined by the

peak temperature (net flux) We found that the li

quids from retorting at elevated temperatures were

mainly high-molecular weight aliphatic components.*

These are primary products which can be thought of

as intermediates during a rapid heating rate

process. As such, the process conditions which af

fect the nature of these intermediates are also ex

pected to affect either the nature of the yields or

product quality. From these studies, we determined

that the optimum conditions for the production of

liquids from rapid retorting of oil shale is at a

slower heating rate than could be attained in the

flash lamp, probably approaching that of an

entrained reactor.

109

A 2-inch I.D., laminar-flow entrained reactor(LFER) with gas preheaters was constructed toachieve high-particle heating rates. The primaryobjective is to determine the kinetics for shaledevolatization at short residence times and high-

heating rates. In an entrained reactor, theenvironment temperature and the entraining gas floware critical parameters in determining the shaleparticle temperature and residence times. Shaleparticle temperature and shale flow rate must bedetermined to measure the kinetics of oil shalepyrolysis. in the LFER, the temperatures of the

furnace walls and the entraining gas were inde

pendently controlled to match reactor gas and wall

temperatures. Flow visualization tests ensured that

proper inlet design and operating conditions were

sufficient to prevent dispersion of shale particles

and to ensure uniform treatment conditions for allparticles.5

The well-characterized LFER was used to study

the potential oil yield enhancement due to rapid

heating. The short residence times which can be

achieved by fast heat up and subsequent rapid quench

in this system enabled the study of the initial

stages of retorting. The measurements of kinetic

parameters at rapid heating rate conditions were

necessary since there was much variability in the

low-heating rate data, and extrapolation for rapid

heating rate application can be risky. Rapid

heating of oil shale involves complex mass and heat

transfer effects and chemical reactions which are

dependent on a wide range or parameters, particu

larly heating rate, final temperature, particle

size, shale type, and grade. The kerogen in oil

shale is such a complicated heterogeneous mixture of

organic compounds that the reported pyrolysis

kinetics are undoubtedly an average for many differ

ent reactions that give the oil product.

EXPERIMENTAL PROCEDURE

Samples

Experiments were conducted with Colorado shale

and New Albany shale in theLFER. The optimum

particle size for these shales wasfound to be

170 by 200 Atthis size fraction there was

little segregation ofcomponents and the particles

retained a representative mixture of both kerogen

and minerals. The samples were cleaned and

separated with the turbo classifier at the

Fluidization Research Center of West Virginia

University. The proximate and ultimate analyses are

presented in Table 1.

Table 1. Composition of Samples

Amount We.ight PercentConstituent Colorado New Albany

Composition, % dryOrganic Matter 21.02 13.82Mineral Matter, LTA 79.00 86.18r 21.96 11.14

cBl 5.37 0.16H 1.98 1.26

N 0.58 0.32S (Total) 0.19 5.01Ash 59.79 79.43

Moisture, % as recv'd 0.21 1.18Particle density, g/cc 1.89 2.04Fischer Assay, gal/ton 32.0 12.4

The mean particle diameter was obtained from

scanning-electron-microscope. Particle dimensions

were modified by a shape factor so to obtain the

volume-equivalent spherical diameters. The

diameters for Colorado shale and New Albany shale

were 70u. and 79u., respectively. The corresponding

shape factors were 0.96 and 0.97, respectively.

Entrained-Flow Reactor Description

A schematic diagram of the reactor setup is

shown in Figure 1. The geometry of the reactor sys

tem was made simple in design (1) to minimize the

effect of mixing zones on residence time, and (2) to

facilitate modeling the flow. Oil shale was

entrained by a nitrogen carrier gas (primary gas)and introduced to the reaction zone using an injection probe. The injection probe outlet was locatedat the inlet of a Lindberg three-zone tube furnacemaintained at reaction temperature. A preheated gas

(secondary gas) was introduced to the reactor zonethrough a flow straightener and contacted the

entrained oil shale at the probe exit to heat theshale and primary gas to the desired temperature. Acollection probe was used that traversed thereaction zone to quickly quench the entire productstream. Both the injection probe and collectionprobe were water cooled, providing a well-defined

110

Oil Shale with N2Heat Wire

Thermocouple Tree

Figure 1. Schematic Diagram of the Entrained Reactor.

residence time within the reactor.1 The position of

the cooled collection probe was adjusted to control

residencetimes.8

Immediately downstream from the

collection probe was a 0.5 \i, ceramic thimble filter

used to collect the solid residue.

Gas Flow Profiles

Flow visualization studies were performed in a

2-inch I.D. quartz tube cold model using techniques

modified from Flaxman and Hallett.9 Air at room

temperature was the working fluid, and the primaryjet was made visible using tobacco smoke as a tracer

to visually observe the flow regimes within the

reactor. A flow straightener with a ceramic honeycomb design was positioned in the secondary gas

stream at the reactor inlet to provide a flat-flow

profile. These experiments were conducted by intro

ducing smoke into the primary gas stream of the LFERmodel and observing the smoke pattern as the primarygas mixed with the secondary gas stream and flowed

down the tubular reactor. Laminar, unsteady state,

and turbulent flow regions were identified using

this technique.

In order to extrapolate these cold-flow studies

to reaction temperatures, the results were presented

in terms of the ratio of the average velocities of

the primary and secondary flows Up/U, versus the

secondary flow's Reynolds number Re(S), where

(Dp 71 R^

'

Q*S TC (Rj, - R7fc )

.

2PsUs

temperature, the thermocouple with a larger bead hada higher temperature than that with a smaller bead.This was because radiative heat transfer from thewall is more important for large particles whileconvective heat transfer from the hot gas is moreimportant for small particles.

A mathematical model was used to calculate the

wall and gas temperatures based on the temperature

measurements of the three thermocouples. The

details of and results from this model have been

presented elsewhere.5

Pyrolysis Conditions

Shale was fed into the reactor at a rate of

about 4 grams per hour. Nitrogen was used as the

primary and secondary gas with 0.4 and 13.0 liters

per minute at STP, respectively. The oil shale par

ticles were pyrolyzed at (I) 976K, (II) 1,026K,

(III) 1,077K, and (IV) 1,125K in a nitrogen atmo

sphere. For each case, the sampling probe was set

at different positions along the length of the reac

tor tube to obtain different residence times. For

each position, the organic conversion rate was

determined by using an ash tracer technique. The

results were modified from dry ash free basis to drymineral matter free basis by correcting for the

mineral matter as determined from the low-

temperature ash (LTA) . The equation for the ash

tracer method for calculating weight loss is:

I_ dW , . w_w* dt

l w*' (5)

(4)100 * 100 -A

AW = 100 - [100 - K 1 t a"!where

AW = Weight loss percent dry ash free basis.A = Proximate ash in dry oil shale.A = Proximate ash in dry spent shale.

This assumes that minerals generated the same amount

of high-temperature ash before and after retorting.

The ash recovery efficiency for all of these experi

ments was greater than 98 percent.

METHOD OF KINETIC ANALYSIS

From an engineering standpoint, the conversion

rate of oil shale pyrolysiscan satisfactorily be

described by the following overall first-order rate

expression:

where W is the weight loss at time t. This weight

loss includes C02 from mineral carbonates; on the

other hand, oil which condenses on the solid residue

is not included. W* is the maximum amount of

devolatilization. k is the rate constant which is a

function of particle temperature, and

k = A exp (-E./RTP) (6)

where

A = The frequency factor (sec*1) .

E. = The activation energy (k cal/mole) .

Tp = The particle temperature which is a function oftime.

The reactor was designed to provide short reac

tion times without the complications due to mixing,

solid-solid interactions, or solid-wall interac

tions. The geometry of the reactor system was made

simple in order to minimize the effect of mixing

zones on the residence time and also to facilitate

the modeling of the flow velocity and temperatur

profiles.

Since there was a very dilute flow

.particle volume_,

1 . . ,(c: < ,. ---) , it was reasonable to

gas volume 10,000

consider a single particle as the control volume.

The particle was approximated as a homogeneous,

spherical particle.Fluid10

and particle proper

ties11, 12, 13 including gas density, thermal

conductivity, viscosity, and gas and shale heat

capacity were determined as functions of particle

temperature. The arithmetic mean of particle tem

perature and the bulk gas temperature was taken as

the temperature of the fluid boundary layer, e.g.

T + T

T = -E 2 (7)

Thermal mixing of the primary and secondary gas

streams was assumed to be instantaneous.

Convective and radiative heat transfer relation

ships and the heat of reaction for the control vol

ume were used to calculate the particle temperature

during the period immediately following injectioninto the LFER:

112

dT^ = C 1 fidt2 dp (V - V )2 + G4 PP g p (9)

pfp-w(V-tp> rtdPdpK

AH ( ") V (8)

where

pp = particle density (g/cm3)

Cp = particle heat capacity (cal/cm3K)V = particle volume (cm3)hc = convection heat transfer coefficient

(cal/cmKs)

dp = particle diameter (cm)Ep = particle emissivity (0.9 for oil shale)Fp. = shape factor between the particle and wall

pK = organic density (g/cm3)

Particle heat up was modeled as the sum of the con

vection plus radiative heat input minus the heat of

reaction. Equation (8) was solved numerically to

find the particle temperature, Tp, as a function of

time, t.

The distance that particles travel, Z, was also

a function of time. Particle acceleration was

modeled as the sum of the drag force and gravity.

where

CD = drag coefficientG = acceleration of gravity

Equation (9) was solved numerically for distance, Z,as a function of time.

The values of E, and A were obtained by repeatedapplication of the least-squares fit of equa

tions (5) through (9) to the experimental data.

RESULTS AND DISCUSSION

Reactor characterization

The extraction of meaningful kinetic rate datafrom reactor experiments requires identical treatment times for all particles. This in turn demandsthat dispersion of the central jet of oil shale bekept at a minimum. Figure 2 shows photographs oftypical flows produced in the reactor tube. As the

Figure 2. Flow Visualization in the Entrained Reactor.

113

flow increased, the central jet began to break up,first in unsteady laminar fashion, then becomingfully turbulent with rapid dispersion. For thisstudy the reactor was operated to provide bothlaminar flow and short residence times without anycomplications caused by mixing, dispersion, orparticle-wall interaction, m the steadylaminar-flow region, the central jet remained intactwithout significant spreading or dispersion throughout the furnace length.

Flow regimes, characterized visually at different flow ratios and Reynolds numbers are plotted inFigure 3. The curve, distinguishing the steady andunsteady laminar regions, gives the maximum Re(S)for which a steady laminar flow can be achieved as afunction of Up/U.. The observed flow profiles pro

vided ample flexibility in altering the relativeflow rates while maintaining good laminar flow. Itwas found that the inlet must be followed by a flowstraightener with sufficient pressure loss to pro

duce a nearly uniform flow. These conditions were

met by Kobayashi,8 who confirmed that coal particles

remain near the axis. These results apply directly

to a heated reactor as long as isothermal conditions

prevail.

Temperature Trajectory

The particle velocity change in the reaction

zone, calculated from Equation (9), is shown in

Figure 4 for Colorado shale and New Albany shale.

The primary jet is introduced at a higherline?*"

velocity than the secondary stream. Theinitial

particle velocity decrease was very sharp for both

Colorado and New Albany shales, but these

equilibrate in less than 50 milliseconds. The

particle velocities equilibrated and attained speeds

of 50 to 60 cm/sec, about 10 times the particle

terminal velocity. Thus, the particles were fully

entrained in the gas flow. New Albany shale has a

slightly higher terminal velocity because of its

higher density.

The heat up of shale particles in the reaction

zone are displayed in Figure 5. The initial par

ticle temperature increase of Colorado shale was

greater than that of New Albany shale. This was du<

3.5 -

\\\

3.0 -

\

\

2.5 -

\

\

2.0 -SteadyLaminar

\ Unsteady0\ Laminar

s Fully^ Turbulent\

1.5 -O Nitrogen (Steady Laminar)

\

A Fully Turbulent

1.0 - *5 TS-_

i i i I

500 600 700 800 900 1000 1100 1200 1300 1400 1500

Re(s)

Figure 3. Flow Regimes in Laminar Flow Entrained

Reactor.

60

Colorado Shale

Time (sec)

1125.2K

1076.9K102615k976.3K

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 9

Time (sec)

New

\Albany Shale

1125.2Ki- 1076.9Kl^ 1026.3K

^ 976.3K1 1 1 1 1 i

Figure 4. Change in Predicted Particle Velocity inthe Reaction Zone.

114

12001125.2K

1100 _ ; 1076.9K

1.0 r

0.9

0.8

0.7

? 1125.2K1076.9K

a 1026.3K976.3K

?

0.6

0.5A

0.4

0.3?

/A

0.2 / s'm.0.1 / //

0.0II

,

0 10 20 30 40 SO 6

Distance (cm)

Figure 7. Comparison Between Experimental Data andCurves Calculated Using the Overall KineticParameters for Colorado Shale.

experimental data and calculated curves was not

quite as good as for the New Albany shale, because

of the complications from mineral carbonate decom

position and heavy oil material condensation. Anyfuture corrections to the volatile yields for both

the heavy oil condensation and carbonate decomposition are expected to result in a net increases in

the reaction rate constant for kerogen decomposi

tion.

First-order rate constants for retorting

Colorado shale and New Albany shale are compared

with those of other works in Figure 8. Except for

the work done on Chinese oilshale11

(same tempera

ture range as this work) , the other work was done in

lower-temperature ranges. However, the rate of con

version of organic matter in Chinese oil shale is

lower than that in this work. By extrapolating the

rate constant of this work tolower-temperature

ranges, New Albany shale comparedwell with other

eastern shales. However, like othereastern shales,

both E. and A values of New Albanyshale was lower

compared to Colorado shale. Thishas been attrib

uted to the more aromaticnature of the kerogen of

easternCompared to eastern shale, the

reaction rate of Coloradoshale was higher in

high-

temperature range but lower inlow-temperature

range. That is, Coloradoshale did not pyrolyze

significantlyin less than one second

until the

particletemperature reached

above 650C.

The data for oilshale was also analyzed using a

simplifyingassumption that the particles

attained

constantreaction temperature,

ie. isothermalkxnet-

ics, and constant velocity. The kineticparameters

obtained were quite different from those reported

here; the rates were 2 to 3 orders ofmagnitude

lower. This reflects the importance of accounting

for the initial heat-up of particles when performing

a kinetic analysis of shale retortingover this time

regime .

10*

TEMPERATURET,C

800 700 600 500 400 300

10 -7

TYPE \OF GRADE

LEGEND SHALE (gpt)1. MAIER/Z1MMELEY (1924) UTAH (SOLDIER SUMMIT) 40.02. HUBBARD/ROBINSON (1950) COLORADO (RIFLE) 26.7, 52.6, 754. DIRICCO/BARRICK (1956) COLORADO (RIFLE) 24.210. JOHNSON, ETAL (1975) COLORADO 30.011. BRAUN/ROTHMAN (1975) COLORADO (RIFLE) 26.7, 52.6, 7515. CAMPBELL, ET AL (1978) COLORADO (ANVIL POINTS) 22.018. SHIH/SOHN (1980) COLORADO 39.519.RAJESHWAR(1981) COLORADO20. WALLMAN. ETAL (1981) COLORADO (ANVIL POINTS) 27.521. ROSTAM-ABADI (1982) MICHIGAN (ANTRIM) 7.022. RAJESHWAR/DUBOW(1982) COLORADO 26.0, 72.023.WANG/NOBLE (1983) COLORADO 59.324. JOSHI/LEE (1983) COLORADO 26.425.JOSHI/LEE(1983) OHIO (CLEVELAND) 6.026. ELDER/REDOY (1983) OHIO (CLEVELAND) - \27. SHEN, ETAL (1984) KENTUCKY (SUNBURY) 9.3 \28. YANG/SOHN (1984) CHINESE (LIAONING) 13.529. SOHN/YANG (1985) MICHIGAN (ALPENA)30. SOHN/YANG (1985) MICHIGAN (PORT HURON)31. SOHN/YANG (1985) AUSTRAUAN

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4

103/T,K

Figure 8. Arrhenius Plots for Various Oil ShalePyrolysis Results.

116

REFERENCES

1. Shadle, L. J., Gaston, M. H., and Rosencrans,R. D., "Pyrolysis of Oil Shale in a Flash LampReactor," 1985 Eastern Oil Shale Symposium, Lex

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3. Shadle, L. J., et al., "Flash Pyrolysis of GreenRiver Shale," Technical Note, DOE/METC-88/4080(DE88001077), October 1987, pp. 34.

4. Shadle, L. J., unpublished data, 1988.

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12. Camp, D. W. , "Enthalpy Relations for Eastern OilShales," 1987 Eastern Oil Shale Symposium,Lexington, Kentucky, November 18-20, 1987.

13. Baughman, G. L. , "Synthetic Fuels DataHandbook," Second Edition, 1978, CameronEngineers, INC., Denver, Colorado.

14. Yang, H. S., and Sohn, H. Y. , "Kinetics of OilGeneration From Oil Shale From Liaoning Provinceof China," Fuel, 63, 1511, 1984.

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117