Simulation of pyrolysis in oil shale particles

Mathematics and Computers North-Holland

in Simulation

SIMULATION

OF PYROLYSIS

S.M. FETERIS

and 0. SITNAI

CSIRO,

93

30 (1988) 93-98

Division of Mineral Engineering,

IN OIL SHALE PARTICLES

Clayton, Victoria, Australia

Models of heat and mass transfer in the pyrolysis of Julia Creek oil shale have been Results for single particles are compared with those for clusters of developed. as would be experienced particles and the effect of periodic temperature fluctuations, in mixing of solids in a retort, is discussed.

1.

INTRODUCTION Oil

shales

have

chemistry

complex

unique

of

mineral

processing.

shale

place

during

retorting

spent

shale

(0.1 - 6.0 mm)

leaving

hot

solids

part

of

the

In

the

retort

on the heating Various been

derived may

Wallman

the

design

and

for

organic

Multiple

and

combustion

retort

play

at

is burnt

recycled

to

the

important

role

and

inorganic

reactions

800

about

"C.

as

bed

a heat

the take

particulate

The

in the fluidized

retort

in

an

combustor

and

supplying

carrier

Szekely

in particulate

et

pyrolysis

of

the

solids

particle

that

t21.

of the

the

and

affecting have

Applications

pyrolysis

mechanism

been

for

oil

shale

compounds

effect

of

fast

reaction

the

slow

heating

depend rate.

A general

skeleton

from

by a non-isothermal

should

transfer

proposed.

The inorganic

oil

therefore

the heat

kinetics oil

of

rate

is beneficial

are

shales

t31.

particle

heating

review

Many

technique

for

the

oil

can

be used

selected

models

of coal.

of this Julia

the pyrolysis

particle

[l I.

al.

of coal

reaction

objectives

inside

on the parameters

for diffusion

the

retorting

place

and

work Creek

of oil

clusters

are

are

oil

shale. also

to devise shale. Some

described,

a generalised

model

In this

several

effects and

paper

which

of the periodic their

heating

importance

for

of single

the

reactor

discussed.

UNSTEADY-STATE A

is

[61 confirmed

al.

'C the

take

reactions

studies

studied

overall

described

particles

2.

by

as in the flash

simulate

reactions

a resistance

et

500

of the particle

early

[3-S]

The

are

ash

for

published

introduce

yield

to

models

about

which

solids.

the

rate

from

authors but

shale

to the raw shale

heat

has

at

compositions

single

convection

HEAT

particle

and radiation

037%4754/88/$3.50

TRANSFER model fluxes

of

MODEL the

FOR SOLID

unsteady

to the particle

0 1988, IMACS/Elsevier

PARTICLES

heat with

Science Publishers

transfer

is

based

the conduction

on

the

and heat

B.V. (North-Holland)

balance

accumulation

of

the in

94

S. M. Feteris, 0. Sitnai / Simulation

the

particle

into

M

as

shown

concentric

Based

schematically

shells

the energy

balance

equation

equation

shell

A

and

overall

heat

of

the

of

+

Tfc

the

sin(2

(21

- B,(T,-

n

(Tf - TM'

are

T,_,)),

- BM(TM B

and

II t/tc)

coefficient

(3)

- TM_,)),

=

n

(4)

7 3 (n -1)-/n&.

for

l
for

transient

heating

for periodic

appears

in

the

heating.

shell

Bi and

includes

contributions

of

fluxes. program

was

[81.

of

For

gas

was

tested

of

the

the

applied

for

to

the

numerical

the approximation

particle

shells

to

was

and

sizes

(M) implying surface

solution

errors

under

that

the

of particle

hot

an

in the

stream a model

physical

basis

particle

diffusivity.

for

range

similar

cluster Consistent

the

in

the

bed

carrier

to were

10

rate

with mm

of

diameter

the

the

the known

controlling

or that

time

(as

2 the

of

so that

and other

Biot

set

of

analytical

the

model

mechanism

Number

was

is the

(Bi) for

strongly

or clusters

previous

in the

of 0.1 for

model.

for

In the

voidaqe,

results

the shell

Heisler present

errors

engineering

pellets

adjusted

of

of heating

depends

are

the

model

been the

thereby

relative and

0.1 with

improves work

the

ratio

comparable

physical

size

shale

of the

devised

modifying

of cluster

Bi as the cluster

The

l.E-4

raw

mixing

has

generalised

with

calculations.

the cold

on the

for a range

to

In the

relative

in

Fo).

set

solution

the

rate

Number,

were

shells.

relevant

the

expressed

Fourier

solids

the

M=lO

integration

theoretical

reactor

for

of particle

accuracy value

Figure

number

- 0.15

properties

of heat

and

to

0.02

fluidized

in

temperature

increase

for heating

a constant

shown

approximation

with

scale

are

temperature

of the physical

In a large

maintaining

test

bounds

error

selected

Therefore

the

dimensionless

The Bi

the errors

the

f0

constant

number

from

respectively.

a

=T

Heisler

absolute

the

- T

AM(Bix

=

transfer

results

decreasing

by

symmetry:

is small.

variables

Bi/M

n

simulation

to

transfer

The

and

in the particle.

transfer:

the system

3 7 2 = n /(n- - (n - l)-)

The solution

insensitive

particle

is divided

(M):

and radiation

ACSL

equations.

heat

spherical

describing

An(Tn+,

= (3a/x2)

0 = TfO

Tf

the convective

solution

gradient

particle

dT/dr)dr)

=T

n

Tf

and

The

heat

the

conditions: T

The

temperature

state

with

(l/r2 d(r2

= (3a/x2)

shell

dTM/dt

initial

convenience

(1)

equations

dTn/dt

For

For

(n):

and for the outermost

where:

the radial

for unsteady

for solids

= k

the difference/differential the inner

1.

= k A dT/dr

dsC dT/dt

for

Figure

to approximate

on the Fourier dQ/dt

and

in

of pyrolysis in oil shale particles

sizes

two

particles streams.

(Figure

1) on

properties the was

increases.

of

thermal achieved

by

SM.

Figure 1.

Feteris, 0. Sitnai / Simulation

Model Scheme

0. Figure 2.

Figure

3

shows

FOURIER

Model Verification

the

strong

2. 3. 4. 5. 8. 7. 8. 9. 10.

1.

effect

of

the particle

cluster

temperature of the cluster at a constant gas temperature of 500 OC. the cluster consisted of a single particle with diameter of 2 mm. exhibit

temperature

95

of pyrolysis in oil shale particles

gradients

only

for

particle

sizes

of

NUMBER

size

on

a.t/x*

-

the

average

In the figure, at R/r=1 Whereas single particles

several millimeters,

particle

clusters of several centimeters, which can be present even in a "well mixed" fluidised bed reactor, are large enough to create pronounced temperature gradients in the solids.

In this

case the controlling mechanism for the heat transfer is the heat conduction in the cluster. During the mixing of the cold and hot streams of solids in the retort, particles can a

encounter

fluctuating

temperature

environment.

The

significance

of

this

effect

was

evaluated for periodic fluctuation of the gas temperature, with amplitude 300 OC about a mean of 500 oc.

One of the cases (Figure 4) shows that the periodic heating affects the surface

layers of particles but not the average particle temperature, because of the reversibility of the heating process.

3.

PYROLYSIS MODEL OF THE OIL SHALE RETORTING. The model devised by Campbell et al. [71 has been modified for Julia Creek oil shale.

The

proposed

model

for

the reaction mechanism

is schematically

shown in Figure

5.

It

consists of four chemical reactions of the first order relative to the mass of the key reactant, and a diffusion process

for the liquid oil.

Diffusion resistance for the oil

vapours is assumed to be negligible. The

first

decomposition

reaction

of

the

primary

organic

matter

of

the

oil

shale

(Kerogen/Bitumen) is a high activation energy reaction similar to pyrolysis of any other fossil

fuel

organic

matter.

The

cracking

reactions of

the

primary

oil are well known

96

SM.

Figure 3.

Feteris, 0. Sitnai / Simulation

of pyrolysis in oil shale particles

Heating Pates of Clusters

Figure 4.

because of their importance in the oil refining industry. oil

is

assumed

to be

proportional

to the heating

Periodic Heating of Particle

The diffusion rate for the liquid

rate of

the particle.

reactions in the gas phase are controlled by the "quench time", in

the gas

phase

before the

The cracking residence

time

temperature of the volatile products leaving the retort is

quickly reduced to avoid any further loss of the oil for the particle heating consists of the convection and heat conduction within the particle.

product.

The heat transfer mechanism

and radiation transfer to the particle

The heating rate is then calculated, depending on a

specific case of the particulate solids/gas system. heating model is employed as described above. coefficient

i.e. the

For the large clusters the unsteady

In parametric studies the overall heating rate

(Kh) was varied in the range 0.05 to 0.20 (l/s) which corresponds to heating

rates achieved in large scale moving bed and fluidized bed reactors. The mass balance equations for the oil production are: dmKB/dt = -k,

(5)

mKB

dmOILL/dt = (1-y) C, (-dmKB/dt)

(6)

- (k2+k3) mOILL

dmOILE/dt = k2 mOILL (1 - k4 tq )

(7)

dmOILG/dt = y C, (-dmKB/dt) (1 - k4 tq) where:

the residence

time of

gases

in the

reactor is denoted

(8) by tq - the quench time,

KB stands for the Kerogen/Bitumen component, OILL for liquid oil in the shale particle, and oil outside the particle is either in liquid state (OILE) or in gas phase state (OILG). 4.

PYROLYSIS SIMULATION. Some effects of residence time and heating rate for solids and quenching time for the

product vapours are shown in Figures 6 and 7.

The simulation results in these figures show

S. M. Feteris, 0. Sitnai / Simulation

,

PARTICLE

’

the

strong

higher the a

effect

effect

large should

of

of

on the

of

streams

the

design

rate

time

the

at

oil

higher

product

,

( I ,

be

must

,

an

in

yield

uneven

temperature

oil

generation

but

later

OF THE

SIMULATION

STUDIES.

increase

rate

oil

at

tq>O.,

yield

This

is

has

a

in the reactor

further

cracking.

where

mixing

temperatures.

but

increase the

when

The

reactor

of

period

is critical

reactor.

to avoid

(see Figure

initial

rate

of solids

in a

distribution

initially on

the

the bed

the bed

fluctuations in the

in

to prevail

at t = 0. q

7, Kh=O.lO),

temperatures.

vapours

bypass

expected

Generation

(Figure

Either

reactor.

temperature

to the

,

The heating

rates

at the higher

the

products can

result

can of

periods of

of

of a pyrolysis

of particles

effect

heating

Oil

6.

on the oil yield.

medium

is pronounced

residence

detrimental The

pyrolysis.

At

cracking

solids

the

is not

temperature

final

the

heating

surprisingly,

Figure

or the pyrolysis

be shallow,

model,

,

TIMEt SEC?,. 1

Scheme

retorting.

of secondary

Periodic

hence

of the

function

bearing

large

Reaction

5.

temperatures

strong

, I ,

GAS PHASE

Figure

91

of pyrolysis in oil shale particles

of

Perhaps

8), as indicated

by

only

end of

the of

towards

rate

the

of

the

heating

irreversible

and

cracking

reactions. 5.

APPLICATION The

system with that

simulation

behaviour the the

aim

Further, preferably achieved.

of a

have

shown

consistently

results

provision

runs

of Campbell

for

a

intensive

short under

high

with

oil

residence

[71.

yield mixing,

time and

the proposed

experimental

et al.

solids

5 seconds,

that

of

For

requires

reaction

experience. the design that

the

oil

when

product

quenching

The model

of

in

scheme

heat the

reactor

consideration carrier gas

the product

describes

predictions

of a pyrolysis

careful

particularly

effective

mechanism

be

solids

phase vapours

must and

the

agree it was

given are be

well shown

to

the

involved. specified,

gases

must

be

S.M. Feteris, 0. Sitnai / Simulation

TIME

Figure

7.

Oil

of pyrolysis in oil shale particles

( SECS. 1

Generation

TlME

Figure

kh = 0.10

8.

Oil

l SEW.

Generation,

)

Periodic

Heating

NOTATION a A Bi C ",I d FS h k ki

thermal diffusivity = k/cd, area Biot Number = h l/k heat capacity stoichiometric coefficient particle diameter particle density Fourier Number = a t/x2 heat transfer coefficient heat conductivity chemical rate constant for i-th reaction

C)

m2/s m2 kJ/kg

K

m

1 m. Q' r R t

kg/m3

t tq

kJ/m2 s K kJ/m s K

TC

l/s

Zf Y

thickness mass of j-th component heat radial distance particle radius time quench time cycle time temperature temperature of gas = R/M shell thickness vapour fraction of oil

m kg kJ m m S S S K

K m

REFERENCES [II t21 t31 141 [51

[61

[71

[81

Szekely, J., Evans, J.W and Sohn, H.Y., Gas-Solid Reactions. Academic Press (1976) of Thermal Decomposition of Pulverised Coal Badzioch, S. and Hawksley, G-W., Kinetics Vo1.9, No.4, (1970) 521-530. Proc.Des. Ind.Eng.Chem. & Develop., Particles. The Effects of Particle Size on Oil Shale Pyrolysis Kinetics, AICHE J.M., Forgac, Spring Nat.Meet. in Anaheim. CA. (1984) W.E., Report Invest., US Bur.Mines, No.4744. (1950) Hubbard, A.B. and Robinson, and Modelling Investigation Of Gregg, M.L., Cambell, J.H. and Taylor, J.R., Laboratory a Colarado Oil-Shale Block Heated to 900 'C, Fuel, Vo1.60, (1981) 179-188. Oil Shale Retorting Kinetics, in: B.G., P.H., Tamm, P.W., and Spars, Wallman, Tar Sands and Related Materials. ACS Symposium Stanffer, H.C. (ea.), Oil Shale, (1981) 93. Series 163. Oil Shale Retorting: Campbell, J.H., Koskinas, G.J., Stout, N.D. and Coburn, T.T., Size and Heating Rate on Oil Evolution and Intraparticle Oil Effects of Particle In Situ, Vo1.2, No. 1, (1978) 1-47. Degradation. Temperature Charts for Induction and Constant-Temperature Heating, Heisler, M.P., Trans.ASME, Vo1.69, (1947) 227-236.