kinetic studies of rapid oil shale pyrolysis

8/14/2019 Kinetic studies of rapid oil shale pyrolysis

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Kinetic studies of rapid oil shale pyrolysis:2. Rapid pyrolysis of oil shales in a laminar-flowentrained reactor

Ming-Shing Shen, Alain P. LUI*, Lawrence J. Shadle, Guo-Qing Zhangt

and Gary J. Morris?Morgantown Energy Technology Center, Morgantown, WV 26507, USAtDepartment of Mechanical Engineering, West Virginia University, Morgantown, WV26506, USAReceived 26 March 1991; revised 31 May 1991)

Rapid pyrolysis of Kentucky New Albany shale was conducted in a laminar-flow entrained reactor (LFER)to obtain a fundamental understanding of thermal reactions, which occur during high-heating-rate retortingprocesses. The reactor configuration was designed to simplify operation and allow accurate modelhng.Temperature characterization and flow visualization in the LFER were conducted to provide the datanecessary to proceed with a kinetic study on the rapid pyrolysis of oil shale. The reactor with gas preheaterswas constructed to achieve high particle heating rates and to feed oil shale fine particles generated from

beneficiation. The rapid pyrolysis of raw and beneficiated oil shales was carried out in nitrogen attemperatures between 700C and 85OC, with gas preheat temperatures up to 98OC. For each temperature,the sampling probe was set at different positions along the length of the reactor tube to obtain differentresidence times. A pyrolysis kinetic model of LFER has been developed to calculate the particle heat-up rateand residence time under each set of conditions. Comparisons are presented to evaluate the effects ofbeneficiation, temperature and heating rate as well as residence time. The effects of particle size and gasenvironment on heat transfer rates and conversion yields were also studied, and the results were used tovalidate the heat transfer model and to evaluate the imnact ofdevolatil ization behaviour on shale combustionin a circulating fluidized-bed reactor.

Keywords: oil shale; pyrolysis; kinetic)

Oil shale research at the Morgantown Energy Tech-

nology Center has been in high-heating-rate oil shalepyrolysis using flash lamp and entrained flow reactors tosimulate the rapid heat-up attained in fluidized-bedcombustion and retorting processes. Flash lamp pyrolysisstudiesle3 on small micrometre-sized shale particlesprovided mechanistic insight into the nature of thermalreactions that occur during rapid retorting processes. Theoverall conversion of shale kerogen to liquids plus gasesis higher for flash pyrolysis conditions than forslow-heating processes such as in Fischer Assay andthermogravimetry research. The rate of the initial phaseof kerogen decomposition in flash pyrolysis is extremelyrapid. Complete conversion of the kerogen in oil shale

can be achieved in a millisecond time frame; however, alarge portion of the product is gas. Compared toslow-heating processes, flash lamp pyrolysis products: (1)contain more unsaturated gas species (benzene, tolueneand xylenes) and heteroatomic liquid species; (2) arehigher in heavy molecular weight liquids; and (3) havegreater gas to liquid ratios.

We found that the controlling reaction pathways thatgovern liquid quality are determined by the heating rate(incident flux), while the overall conversion and the extentof cracking, polymerization or condensation reactions

Presented at Eastern Oil Shale Symposium, 6-S November 1990.Lexington, KY, USA* Present address: EC&G Washington Analytical Services Center, Inc..Morgantown Operations, Morgantown Energy Technology Center,3610 Collins Ferry Road, Morgantown, WV 26507, USA

0016-2361/91/11127748c 1991 Butterworth-Heinemann Ltd.

are determined by the peak temperature (net flu~)~. We

found that the liquids from retorting at elevatedtemperatures were mainly high molecular weightaliphatic components. The process conditions that affectthe nature of these products are also expected to affectthe nature of the yields and product distribution.

We found that shale pyrolysis mechanisms are heatingrate dependent: the number of kerogen decompositionpathways are more diverse at high-heating rates than atlow-heating rates. The optimum conditions for producingliquids from rapid retorting of oil shale is at a slowerheating rate than could be attained in a flash lamp, thus,leading to the development of an entrained reactor.

Many studies on the rapid devolatilization of coal have

been conducted using entrained reactors4P9; however, theresulting kinetic rates from these and other types of rapidheating experimentsloP13 vary by several orders ofmagnitude. Several researchers4.5.7.9.12 report that thekinetic rates for coals of different rank were within afactor of 2 to 5 when determined in the same reactor,while the rates obtained for the same coal rank, but fromdifferent research groups, varied by several orders ofmagnitude. Thus, coal type is not the reason for thevariations reported. The inconsistency in these rapidheating experiments has been cited as caused byvariations in estimations of particle temperaturehistory-. Temperature-dependent variations in thethermal properties and the physical structure of coal areoften cited as one potential cause for the uncertainty inthe temperature histories of coal in rapid heating

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Kinet ic s tudies of rapid pyrolysis : M S. Shen et al.

Table Composition of samples

Amount (wt%)

Colorado New Albany Beneficiated

Composition (dry)Organic matterMineral matter. LTA

or8

LiHNS (total)Ash

Moisture (X as recovered)Particle density (g cm-3)Fischer Assay (I t - )Mean particle diameter

(pm)

21.00 13.8279.00 86.1817.02 11.14

5.37 0.161.98 1.260.58 0.320.79

59.790.21 1.181.89 2.04

135.60 52.5070.00 79.00

5.0179.43

35.9764.0327.76

0.143.090.794.61

56.691.691.66

149.204.20

experiments .-9,11 While the physical structure and thusthe physical properties of coal particles are dominatedby the formation of a melt, often referred to as metaplast,which flows, swells and becomes glassy at various stagesduring heatingi4, the physical structure in oil shale isdominated by the mineral matter. This importantdistinction makes the estimation of thermal and physicalproperties, such as thermal conductivity, heat capacity,density and diameter, much more reliable, at least whenthe shale particles contain a normally representativedistribution of minerals and kerogen.

To study the causes and effects of oil yield enhancementcaused by rapid heating, a 5 cm inside diameter,laminar-flow entrained reactor (LFER) was constructedto achieve heating rates approaching those anticipated ina fluidized bed. Fast heat-up and subsequent rapidquench in this system enable the study of the initial stagesof retorting. The primary objective is to determine thekinetics for shale devolatilization at short residence timesand high-heating rates. Measuring kinetic parameters atrapid heating conditions is necessary since there is somevariability in the low-heating rate data, and sinceextrapolation for rapid heating rate applications can berisky. Rapid heating of oil shale involves complex massand heat transfer effects and chemical reactions that aredependent on a wide range of parameters, particularlyheating rate, final temperature, particle size, shale typeand grade. The kerogen in oil shale is such a complicatedheterogeneous mixture of organic compounds that thereported pyrolysis kinetics are undoubtedly an averagefor many different reactions that give the oil product.

We present a critical assessment of the factors involvedin determining shale pyrolysis rates using the LFER.Enough similarities exist to other analyses for theevaluations made here to be applicable to other rapiddevolatilization experiments.

EXPERIMENTAL

Samples

Experiments were conducted with Colorado shale,New Albany shale and beneficiated New Albany shale inthe LFER. The raw shales were crushed once through ahammer mill, sieved and then aerodynamically cleanedand separated with the turbo classifier at the FluidizationResearch Center of West Virginia University. Proximateand ultimate analyses are presented in Table 1.

The beneficiated New Albany shale was prepared bythe Mineral Resources Institute, Alabama, using finegrinding followed by froth flotation. The elementalanalyses suggested that the beneficiation resulted in athree-fold enrichment over the raw shale. The particledensities were measured using mercury porosimetry, and

the pressure required to fill the interstitial particle volumewas determined using non-compressive, non-porous glassparticles ground to similar sizes according to thetechnique recommended by Gan and co-workers 5.

Entra inedflow reactor

Figure 1 shows a schematic diagram of the reactorset-up. The geometry of the reactor system was simplifiedin design to minimize the effect of mixing zones onresidence time, and to facilitate modelling the Bow. Oilshale particles were fed from a syringe pump feeder,entrained in a nitrogen carrier gas (primary gas), andintroduced to the reaction zone using an injection probe.The syringe pump feeder was modified to accommodateoil shale fine particles generated from beneficiation. Oneof the reasons that most experimentation in entrainedreactors is conducted with particle sizes >40 pm is thatstatic forces that result in cluster formation tend to bemuch lower in larger particles5v798. It is likely thatresearchers who failed to observe the expected particlesize dependence4 caused by differences in heat transferrates were feeding large clusters rather than discreteparticles.

To disrupt these clusters, a porous gas distributor wasinserted in the bottom of the solids container and anauxiliary gas line was introduced below the distributorto permit fluidization of the solids. An auxiliary gas ventoutlet was installed at the top of the solids reservoirchamber to vent the nitrogen gas required for fluidizationin excess of that needed for primary gas injection to theentrained reactor.

During operation, the solids feed bed was slumpedseveral times prior to fluidization by pressurizing thesolids reservoir and releasing the pressure using a valve inthe vent line. This aided the breakdown of particle clumpsand allowed even fluidization. The result was a feederthat provided a continuous flow of well-dispersedparticles rather than clusters of particles.

The injection probe outlet was located at the inlet ofa Lindberg three-zone tube furnace that was maintainedat reaction temperature. Preheated gas (secondary gas)was introduced to the reactor zone through a flowstraightener; the gas contacted the entrained oil shale atthe probe exit to heat the shale and primary gas to thedesired temperature. A collection probe was used thattraversed the reaction zone to quickly quench the entireproduct stream. Both the injection probe and collectionprobe were water-cooled, providing a well-definedresidence time within the reactor4. The position of thecooled collection probe was adjusted to control residencetimes. Immediately downstream from the collectionprobe was a 0.5 pm ceramic thimble filter used to collectthe solid residue.

While this configuration permits complete collection ofall particles, it has the disadvantage that condensibleliquids may deposit back on the spent shale. In fact, thereis some evidence that this happens. Thus, the observedweight loss is only the light oil that is not condensed andthe heavy oil that deposits on the walls of the

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Kinetic studies of rapid pyrolysis. M.-S. Shen et al.

SOLIDS CARRIER GAS

20 PSI4 A-4 I I -

First- Preheater L

-_.

t . Vibrator-n

SecondPreheater

HEATING GAS

Fi gure 1 Schematic diagram of laminar-flow entrained reactor

water-cooled collection probe. The amount of oil thatcondensed on the spent shale and the filter wasdetermined by simply washing with an organic solvent,toluene. The extract was distinguished from what wetypically consider as bitumen, because it could be washedout in a Soxhlet apparatus in 2 h, while bitumen wouldcontinue to be extracted from shale after 24 h.

In an entrained reactor, the environment temperatureand the entraining gas flow are critical parameters indetermining the shale particle temperature and residencetimes. Shale particle temperature and shale flow rate mustbe determined to measure the kinetics of oil shalepyrolysis. In the LFER, the temperatures of the furnacewalls and the entraining gas were independentlycontrolled to match reactor gas and wall temperatures.Flow visualization tests ensured that proper inlet designand operating conditions were sufficient to preventdispersion of shale particles and to ensure uniformtreatment conditions for all particles.

Shale was fed into the reactor at a rate of -4 g h- .Nitrogen was used as the primary and secondary gaswith 0.4 and 13.0 1 min- at standard temperature andpressure, respectively. The gas profiles were characterizedusing the techniques of Flaxman and Hallett and theoperation was established to be steady laminar asdescribed elsewhere16. The sum of the primary gas andthe shale represented a thermal load of < 10% of that

ST Adjusting

available in the secondary gas stream, and so ample heatwas available to ensure that there were no heat limitationsor requirements to make up significant portions of theheat in the reaction zone. The furnace temperature wasset to be isothermal using a cluster thermocouple prior toeach test run as presented previously16. The oil shaleparticles were pyrolysed at 976, 1026, 1077 and 1125 Kin a nitrogen atmosphere. For each temperature, separatetests at different residence times were conducted by

setting the sampling probe at different positions alongthe length of the reactor tube.

For each set of conditions, the organic conversion ratewas determined by using an ash tracer techique. Theamount of solid spent shale was collected and weighedand its ash content was determined. The equation for theash tracer method for calculating weight loss is

1)

where AW=weight loss per cent (daf basis), A,=proximate ash in dry oil shale and A = proximate ash indry spent shale.

The results were modified from a daf basis to a dmmfbasis by correcting for the mineral matter as determinedfrom the low-temperature ash (LTA). However, volatilesthat evolve from the minerals during pyrolysis, such asCO, from the decomposition of carbonates or H,O fromhydrated minerals, will report as weight loss in this typeof analysis.



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Kinetic studies of rapid pyrolysis: M. -S. Shen et a I.

The ash recovery efficiency for all of these experimentswas >98%. The reproducibility of these measurementswas tested by comparing the results of replicate tests atthe same conditions. The average standard deviationfrom four sets of duplicate measurements of the weightloss was k 1.4%, which corresponds to a precision of

k 3.9% at the 95% confidence limit.

Particle trajectory model

The reactor was designed to provide short reactiontimes without the complications caused by mixing,solid-solid interactions or solid-wall interactions. Thegeometry of the reactor system was made simple tominimize the effect of mixing zones on the residence timeand also to facilitate the modelling of the flow velocityand temperature profiles. Since there was a dilute flow,it was reasonable to consider a single particle as thecontrol volume:

article volume < 1

gas volume 10000 >

The particle was approximated as a homogeneous,spherical particle. Fluid and particle properties8-21,including gas density, thermal conductivity, viscosity,and gas and shale heat capacity, were determined asfunctions of particle temperature. The arithmetic mean ofparticle temperature and the bulk gas temperature weretaken as the temperatures of the fluid boundary layer:

T=Tp+Tpf -I 2)L

Thermal mixing of the primary and secondary gasstreams was assumed to be instantaneous.

Particle heat-up was modelled as the sum of theconvection plus radiative heat input minus the heat ofreaction; as a first approximation, the conversion rate ofoil shale pyrolysis was assumed to follow the Arrheniusfirst-order rate expression discussed previously16. Noattempt was made to model the cooling rate of thesample, which was expected to be approximately the sameas the heating rate, based on similar heat transfermechanisms. The gas must first cool by contacting thecold walls but the smaller diameter in the collection probe(relative to that of the reactor) provides better gas contactand relatively high gas convection rates. It is acceptedthat some reaction could have taken place in thecollection probe during the particle quench but this wasconsidered a relatively small error.

Convective and radiative heat transfer relationshipsand the heat of reaction for the control volume were usedto calculate the particle temperature during the periodimmediately following injection into the LFER:

where pp = particle density (g cm - 3), C, = particle heatcapacity (J cm -3 K-l), VP particle volume (cm3),h, = convection heat transfer coefficient (J cm- K s- ), d, = particle diameter (cm), cp = particle emissivity(0.9 for oil shale), F,_, =shape factor between theparticle and wall and pK = organic density (g cmm3).

Equation (3) was solved numerically to find the particletemperature, Tp, as a function of time, t.

The distance that particles travel, 2, was also afunction of time. Particle acceleration was modelled asthe sum of the drag force and gravity:

d2Zd,=CD;;d,(Vg- I,)+G

P4)

where C, = drag coefficient and G =acceleration ofgravity.

Equation (4) was solved numerically for distance Z asa function of time.

RESULTS AND DISCUSSION

Particle trajectory

Typical particle velocity profiles in the reaction zone,calculated from Equation (4), are shown in Figure 2 forColorado shale. The profiles for New Albany shale andbeneficiated New Albany shale were similar. The primaryjet was introduced at a higher linear velocity than thesecondary stream so that initial particle velocity washigher than gas velocity. Because instantaneous equi-libration of the primary and secondary gas velocities wasassumed, drag forces decelerated the particle until itapproached the gas velocity.

A minimum velocity is predicted at -0.03 s. Theassumption of instantaneous velocity equilibration mustbe investigated further, since flow visualization testsindicated that the central jet was kept intact and thusmay have penetrated into the flow profile of the secondarygas.

The particles then accelerated with the gas along thereactor centreline as the laminar velocity profiledeveloped. The particle velocity reached a stady-statevalue at -0.3 s when the gas flow reached a fullydeveloped state.

After this point, the particles were fully entrained andflowing at near the velocity of the gas at -90 cm s-l,while the terminal velocity was much less at only10-20 cm s-l. New Albany shale equilibrated at slightlyhigher velocity than Colorado shale.This was becausethe New Albany shale had a slightly larger particle sizeand higher density. For beneficated New Albany shale,

50 _II I I I I I

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0

Time set)

17

Figure 2 Predicted particle velocity profile in the reaction zone forColorado shale. Temperature (K): a, 976; b, 1026; c, 1077; d, 1125



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l l OO-

G l OOO-e

5 900-

5

& aoo-

f- 700-

m4 600-r

L? 500-

400-

I300

I I I I I I I0.0 0.1 0. 2 0.3 0.4 0. 5 0.6 0.7

Time set)

Figure 3 Predicted particle temperature profile in the reaction zonefor New Albany shale. Temperature (K): a, 976; b, 1026; c, 1077; d, 1125

the initial particle velocity was higher but equilibrated atlower velocity because of its fine particle size and lowerdensity, reducing the contribution of its terminalvelocity.

Typical temperature history profiles for shale particlesat the radial centre of the reaction zone are displayed inFigure 3. The initial increase in particle temperature ofthe Colorado shale was greater than that of New Albanyshale. This was also because the New Albany shale hada slightly larger particle size and higher density. For thefine, beneficiated shale particles, the initial temperaturerise was very fast indeed. The beneficiated shale particlesreached the final temperature in - 1 ms because of thesmall particle sizez2.

Analysis of pyrolysis rate data

When analysing the weight loss data in terms of adecomposition reaction with first-order dependence onthe concentration of shale organic matter, the weight losswas normalized to the maximum yield. In Figures 4 and5. the maximum yield was assumed to be the asymptoticyield, w* at the highest test temperature. In a singlefirst-order reaction model, the maximum yield isindependent of temperature, particle size and heating

rate.Although this is not completely consistent with existingevidence on oil shales, this type of analysis was deemedappropriate for comparative purposes. In any case, theasymptotic yield was 13.2% for the New Albany shale.In the absence of other effects, this corresponded to avolatile yield of nearly 95% of the organic matter in aperiod of < 1 s. At the present time, the distribution of~olatiles into gases and liquids is not known quantita-tlvely. The presence of clays that are known crackingcatalysts would favour the formation of gas over liquids.

The rate data for the New Albany shale exhibited ageneral consistency with literature rate data. Theexperimental data compared with the model curve fits areshown in Figure 4. The experimental data agreed with themodel fits within the experimental confidence limits,having a relative standard error of 3.9%; however, thetemperature dependence appeared too low and systematicvariations existed in the fits. The maximum yield for thebeneficiated New Albany shale**, 23.7%, was muchhigher than that for the raw New Albany shale. The small

Kinetic studies of rapid pyrolysis: M.-S. Shen et al .

beneficiated shale particles heated quickly to reach thefinal temperature, but the depletion of reactive startingmaterial was notably slower than that for the raw shale,especially at the lower temperatures22. The temperaturedependence was greater for beneficiated shales, beingmore in line with extrapolations from the literature at

lower reaction temperatures.When the weight loss data is analysed based on total

mass of organic material lost, the beneficiated andthe raw New Albany shales exhibited comparabledevolatilization rates ~ at least at the lowest temperatures.This suggests that vaporization of the products may becontrolling the initial devolatilization rate. Such avaporization process would be more important in thericher particles of the beneficiated shales because of thelarger heat requirements and mass fluxes.

The conversion rate of Colorado shale was alsonormalized to its asymptotic yield at 14.7%. Themaximum volatile yields corresponded to -70% of the

organic matter Figure 5). Preliminary data on tolueneextraction of the spent shale residues indicated that heavyoils condensed on the solid particles either in the gasstream or in the collection thimble. The extraction yieldswere greater in the Colorado shale than in the NewAlbany shale. The fit between experimental data and

5 0. 6

2 0.7

E

' s 0. 68

; 0. 5

0.1

0.0

1

-10 5 10 15 20 25 30 35 40 45 50 55

Di stance (cm

Figure 4 Comparison of experimental data and curves calculatedusing the kinetic parameters for New Albany shale. Temperature (K):A, 976; 1026: +, 1077; 0, 1125 -

3 0. 6 -

2 0.7 -

k. -g

0.6 -

I i 0 .5 -

:s 0.4 -

E 0. 3. g

3 0. 2

0.1- 1

0 5 l b 15 20 25 30 35 40 45 50 55

Di stance ( cm

Figure 5 Comparison of experimental data and curves calculatedusing the kinetic parameters for Colorado shale. Temperature (K): A,976: 1026; +, 1077; 0, 1125



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Kinetic studies of rapid pyrolysis: M. -S. Shen et a I.

calculated curves was not quite as good as for the NewAlbany shale, possibly because of the complications frommineral carbonate decomposition and heavy oil materialcondensation. The temperature dependence of theColorado shale was intermediate between that of the rawand beneficiated New Albany shales. This ordering is

consistent with the suggestion that the early stages ofrapid shale devolatilization are controlled by the rate ofvaporization where there is a dependence on oil shalegrade.

In addition, the fact that the eastern shale respondedfavourably to the rapid heating experiments indicatedthat the pyrolysis mechanism in an aromatic shale followsa set of parallel reaction pathways such that aromaticsnot immediately evolved undergo retrogressive reactionsto form char. On the other hand, the conversionefficiencies in the western shale were not significantlyhigher than those obtained at slow-heating rates,probably because of the lower coking tendency of these

highly aliphatic kerogens. This would suggest that a setof parallel reaction pathways may not be necessary toexplain rapid pyrolysis rate data for the western oil shales.

Particle size considerations

The optimum particle size for these raw shales wasfound to be 170 by 200 mesh 23 At this size fraction, therewas little segregation of components and the particlesretained a representative mixture of kerogen andminerals. This minimizes the possibility that individualparticles of kerogen or minerals will skew the data. Inaddition, the once-through grinding technique minimizes

the potential that kerogen will become fluid because ofheat and compression during grinding, and thencedeposit as a coating on the external surfaces of particles.This situation has been observed when repeated finegrinding to a small particle size (< 100 pm) was desired.Obviously, the kerogen conversion in such externallycoated particles would experience unnaturally large heatand mass transfer rates.

Model calculations indicated that particle size was amost important parameter in determining particletemperature history in the LFER. Recent attempts tomeasure coal particle temperatures in this type of reactorhave indicated that the heat-up times are faster than

models estimate. This has been assumed to be becausethe heat transfer coefficient was underestimated;however, recent evidence obtained using temperature andparticle size measurements on single particles suspendedin an electrodynamic balance is inconsistent with thisexplanation . One implication from this work is thatthe heat transfer calculations in all studies that assumespherical particles will grossly overestimate the volumeof irregularly shaped, non-spherical particles. Since thespherical particle assumption minimizes the surface areato volume ratio, the calculations result in higher thermalinertia and thus lower heat-up rates. For instance, a cubehaving a characteristic length (largest dimension) equal to

that of a sphere has a nearly equal surface area but onlytwo-thirds the volume, and so it has about a 40% higherarea to volume ratio.

The spherical-volume equivalent size of the shaleparticles are reported in Table I as determined using theCoulter counter technique and assuming an ellipsoidgeometry. Scanning electron microscopy (SEM) photo-micrographs of the shales and subsequent image analysis

indicated that the sphericity (ratio of the spherical-equivalent diameter to the spherical diameter) was0.96-0.97; however, the SEM is a two-dimensionaltechnique and the third dimension was assumed to be thesame as the smallest measured length. In fact, thethickness of these particles could have been significantly

smaller than assumed, especially for the raw shale inwhich the particle size was equal to or larger than thethickness of the bedding layer23.

If the net heat flux is comparable to the rate ofdevolatilization, decomposition takes place duringheat-up. Transient heat transfer equations have beensolved24 using the Biot number, a dimensionlessparameter, to determine the relative resistance for heattransfer externally (to a particle) and internally (withina particle). For example, if a Colorado oil shale(135.6 1 t- ) is heated in a hot nitrogen environment(lOOOC), the error in assuming that the particletemperature is uniform is < 5% when the particle size is

< - 125 pm. A characteristic heating time for theintraparticle heating of shale particles of 70 pm is - 5 ms,while that for external shale heating is -80 ms. Thesecalculations indicate that for small non-reacting shaleparticles, the rates of heat transfer to the particles aremuch higher than the rates of heat transfer within theparticle. On this basis, an intraparticle temperaturegradient is insignificant.

Experiments on Colorado oil shale with differentparticle sizes were conducted to evaluate mass and heattransfer effects. The results indicate that as the meanparticle diameter reduced from 70 to 63 to 60 pm,the conversion (weight loss fraction based on shale)

increased from 12.0 to 17.2 to 18.7% at a reactortemperature of 800C. At 85OC, the conversion increasedfrom 14.7 to 18.1 to 19.7%, respectively. This magnitudeis not inconsistent with the effects expected because ofexternal heat transport to the particle. Smaller particleshad longer residence times because of slower particlevelocities. Smaller particles heat up more quickly thanbigger particles at the same residence time. Differencesarise between experimental data and model calculationsfor the small particles only. These could be a result ofseveral things. First of all, the model assumes a singleparticle as a control volume, but there might be a clusterformation. The model also assumes the primary gas

heated up instantaneously and does not take into accountthe heat transfer of primary gas around the particle.Furthermore, these particle sizes may be smaller than thethickness of the bedding layer, causing some segregationof kerogen and mineral components, which would, inturn, affect the heat and mass transfer rates, and, thence,the kerogen conversion.

Additional heat tramfer considerations

Experiments on Colorado oil shale with helium gaswere also conducted to evaluate mass and heat transfereffects. The results indicated that using helium as thecarrier gas resulted in an increased weight loss of 6% at800C and an increase of 14.2% at 850C. Helium hasa smaller density, lower viscosity and higher heatconductivity than nitrogen. The smaller density andviscosity will generate a shorter residence time, while thehigher heat conductivity will produce a higher heatingrate. The higher heating rate and shorter residence timewould enhance the pyrolysis conversion rate. The trend



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Kinetic studies of rapid pyrolysis: M.-S. Shen et al.

of experimental results was in agreement with the modelcalculation results. However, the weight loss ratepredicted by the model was greater than that ofexperimental results, since the nitrogen gas was used asa secondary gas in these experimental tests.

The maximum potential conversion obtained in helium

or with the smaller particles was larger than thatgenerated for 70 pm particles in nitrogen. The LFERcomputer model was used to conduct a sensitivityanalysis on particle size and gas atmosphere. The modelcan account for the differences in the time-temperatureprofile and, therefore, the changes in the conversion rate.The model can accurately simulate the data for timedependence of conversion at a single temperature.However,the maximum conversion had to be adjustedto match the experimental values for the different particlesizes or gas atmospheres. The ultimate organicconversion (or the maximum recoverable organic fractionin the shale studied) had to be varied for the different

particle sizes and gas atmospheres. This suggests that thechanges in heating conditions altered the extent ofretrogressive reactions such that higher heating rates andmass transfer rates favoured higher conversion levels evenat these short residence times (0.54.6 s).

Liquid product composition

Part of the pyrolysis products from LFER was retainedon the spent shale, and this part was Soxhlet extractedwith toluene. The toluene extract was analysed usingFT-i.r.. In the toluene extract of the New Albany spentshale, the relative aromatic concentration appeared to

be high. There were also many C=O and C-Ostructures. These observations demonstrated that theLFER pyrolysis liquid products had similar chemicalcharacter to asphaltenes (hexane insolubles). The fractionof asphaltenes in toluene solubles increased withtemperature. This was attributed to the formation ofcondensed and non-condensed polycyclic aromaticcompounds from less complex aromatic compounds asthe pyrolysis temperature increased. For a giventemperature, the relative amount of oils in toluenesolubles increased with decreasing residence time. For agiven residence time and temperature, the fraction of oilsin toluene solubles was in the order:

Colorado shale > beneficiated New Albany

> New Albany shale

The oils from LFER samples were analysed by FT-i.r..It was observed that ester-type structures were ratherprominent in LFER oils from New Albany shale, whileColorado shale oils contained a weak ester peak only at850C. This may be attributed to the higher (almosttwice) organic oxygen content of New Albany shale thanColorado shale. For the New Albany shale, the relativeconcentration of the C=O bonds to aliphatic CHsincreased with temperature. This may have resulted fromreduced decarboxylation of the ester groups underhigher heating rates that prevailed in the reactor. In aslow-heating-rate process, such as a Union B Retort, theester structures would be readily thermolysed. For thebeneficiated New Albany shale, the C=O bondstretching mode appeared at higher frequency indicatingthat the R group in the ester RCOOR was more alkylthan aryl in nature. The relative concentration of C=O

bonds to aliphatic CH groups did not change significantlywith temperature.

CONCLUSIONS

The techniques applied to investigate rapid oil shale

pyrolysis have provided an insight into the reactionmechanisms. The conversion of oil shales has beendemonstrated to be an extremely rapid process, withmaximum yields being achieved in < 1 s at temperaturesnear 800C. The conversion of the beneficiated shaleexhibited a greater temperature dependence than the feedshale. The weight loss data suggested that vaporizationmay play an important role in the initial stages of rapiddevolatilzation. If vaporization is a rate-controllingprocess, the implications will be quite important in thedesign of processes in which shales of various grades arerapidly devolatilized. The rate data for the raw shalesindicated that eastern and western shales exhibited

different reaction mechanisms: the conversion efficiencyin the eastern shale depended strongly on heating ratewhile the western shale did not. Thus, different reactionmodels may be necessary to describe the rapiddevolatilization of these two shales.

The LFER experiments require further improvement,especially with regard to the treatment of the particle sizeand shape determination, the heat transfer and velocitycalculations in the injection region of the reactor, particlecluster formation in the reactor, determination of theproduct yield, and separation of solids and vapours.Further validation of the particle trajectory model isdesired and measurements of particle residence times,

temperatures, velocities and size are required. Inaddition, further work is required on gas and oilgeneration/separation to determine carbon distributionin char, gases and liquids. The effect of particle size andheat transfer rates (using gases of different thermalconductivity) on rates and yields should be studied inmore detail, and the results used to validate the heattransfer model.

Kinetic analysis of the data is being expanded toinclude an analysis of the shale conversion as a numberof parallel first-order reactions in which the measuredproducts are lumped together, and the overall activationenergy can be estimated as a function of conversion.

In this method, the relationship between the log of thetime required to reach a fixed level of conversion and thereciprocal temperature is used to determine the activationenergy at that level of conversion. This analysis providesa straightforward determination of the error limits onactivation energy and bounds the activation energywhereas other approaches do not. It also provides amethod for determining the fraction of each componentin the reacting system in cases where the activation energyvaries with conversion.

REFERENCES

1 Shadle, L. J.. Gaston, M. H. and Rosencrans, R. D. Proc. ofthe 1985 Eastern Oil Shale Symposium, Kentucky Center forEnergy Research Laboratory, Lexington, 1985

2 Shadle, L. J., Rosencrans, R. D., Shen, M. S. cr al. Proc. of theInternational Conference on Oil Shale and Shale Oil, ChemicalIndustry Press, Beijing, 1988

3 Shadle, L. J., Hobbs, G. R., Shen, M. S. et al. Flash Pyrolysisof Green River Shale, Technical Note. DOE/METC-88/4080,NTIS/DE88001077, 1987, p. 34



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Perry, R. H. and Chilton, C. H. Chemical EngineersHandbook, 5th Edn, McGraw-Hill Book Co., New York, 1973Camp, D. W. Proc. of the 1987 Eastern Oil Shale Symposium,Kentucky Energy Cabinet Laboratory, Lexington, 1987Bauahman. G. L. Svnthetic Fuels Data Handbook, 2nd Edn,Cameron Engineers,*Inc., Denver, 1978Johnson, D. R., Smith, J. W. and Young, N. B. StratigraphicVariation of Oil Shale Enthalpy of Retorting Through the GreenRiver Formation on the Colorado C-a Tract, LETC/RI-79/9,1979Shaw, R.J. Specific Heat of Colorado Oil Shale, USBM Reportno. 4151, 1947Shen, M. S., Shadle, L. J. and Zhang, G. Q. Proc. of the 1990Oil Shale and Tar Sand Contractors Review Meeting,Morgantown Energy Technology Center, Morgantown, 1990Grimm, U. and Shadle, L. J. Chemical Separations. 11.Applications, Litarvan Literature, Denver, 1986Holman, J. P. Heat Transfer, 5th Edn, McGraw-Hill, NewYork, 1981Szladow, A. J. and Given, P. H. Ind. Eng. Chem. D es. D eo.1981, 20, 27


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