Study of thermal regeneration of spent activated carbons: Thermogravimetric measurement of various single component organics loaded on activated carbons

Chemfcol Engrneenng Scrence 1978 Vat

33 pp 271-279

PergamonPress

Pnnted m Great Bntun

STUDY OF THERMAL REGENERATION OF SPENT ACTIVATED CARBONS THERMOGRAVIMETRIC MEASUREMENT OF VARIOUS SINGLE COMPONENT ORGANICS LOADED ON ACTIVATED CARBONS MOTOYUKI Institute of Industnal

SUZUKI, DRAGOSLAV and KUNITARO Sctence, Umverslty (Recerued

10 Murch

M MISIC, KAWAZOE

OSAMU

of Tokyo, 7-22 1 Roppongl,

KOYAMA

Mmato ku, Tokyo, Japan

1977, accepted 16 May 1977)

Abstract-Thermogravity analysis of the activated carbons loaded with 32 chfferent single component orgamcs showed that TGA curves could be classified mto three dlstmct groups with regard to then shapes The organrcs that belong to Group (I) are rather volatile and TGA curves can be explamed by eqmhbrmm desorptlon model Group (II) orgamcs are relatively easy to decompose and TGA curves were Interpreted m terms of first-order cracking kinetics The parameters mcluded m these models were obtamed from the measured TGA curves by utllizmg half desorbed temperature T,,, and reciprocal slope of TGA at T,,,, AT Group (III) consists of phenol, &naphtol, lrgnm etc and gave high residuals on activated carbons after heatmg up +o 800°C This suggests that these orgamcs are the ones that are critIcal to the ordinary thermal regeneration method A rough classlficatlon of orgamcs into these groups was done by using the bollmg point and the aromatic carbon

INTRODUCTION

Adsorption of organic pollutants by activated carbon has become one of the reliable methods m the field of water treatment The economic feaslblhty of this process 1s highly dependent on regeneration costs of the spent carbon The most often apphed regeneration method for spent carbons 1s the thermal regeneration Although such a regeneration process 1s commercmlly accepted the lack of fundamental studies makes the choice of design parameters and operating condltlons of regeneration reactors far from an optunum Sometimes a loss of 7% tn adsorption capacities of a regenerated carbon corresponds to more than 50% of the total cost of water treatment by a carbon adsorption system111 Hence, the fundamentals of the regeneration need to be studied more extensively m order to be able to specify the condltlons for mmunal carbon loss and the maxima1 restoration of adsorption characterlstlcs of the spent activated carbons Wet spent carbons, taken out from adsorption columns, undergo during the thermal regeneration the followmg processmg steps (1) drying at about 105”C, (2) heating up to 800°C and (3) gasification of residual orgames by oxldlzmg gas, such as steam or carbon dioxide Since a spent carbon 1s usually loaded with many kinds of orgamcs, origmally present m the water treated, the second step IS thought to be a comphcated process conslstmg of thermal decomposltlon, thermal crackmg, desorptlon of decomposltlon products, and partial crackmg followed by polymerlzatlon of the residuals These processes should be stu&ed m detail, smce the gaslficatlon followmg them 1s greatly affected by the residual and coke The present work 1s focused on elucldatlon of the CES Vol 33 No

‘4-B

271

behavlour of the orgamcs adsorbed on the activated carbon during a temperature rise period m an mert atmosphere Weight change of activated carbons loaded with known amounts of different single component orgames were measured by an electrobalance The orgamcs studied are classified according to the pattern of the weight change-temperature diagrams (1 e the thermal gravity analysis (TGA) curves) EXPERIMENTAL Apparatus Schematic diagram of the apparatus used for the experimental studies IS shown in Fig 1 Shunadzu electrobalance eqmpped with a quartz reactor of 50 mm dla was used The reactor was surrounded by an electic tubular furnace The furnace temperature was controlled to follow a lmear rise from 100” to 8OO’C In most of the experiments, a rise of 6’Clmm was employed The temperature was controlled by a temperature programmer (Chino) Activated carbon sample was m a platinum or quartz basket attached to the electrobalance inside the quartz reactor The weight change dunng the course of temperature rise was recorded by a X-Y plotter (Yokogawa) Durmg the expenmental runs, mtrogen gas was introduced to the bottom of the reactor, through a 5 cm long preheated section packed with cehte spheres, and to the body of the electrobalance to prevent contammatlon of the balancing section Traces of oxygen m nitrogen gas from a laboratory grade cylinder were removed by passmg of nitrogen through a tube furnace with copper fillings kept at 260°C Nitrogen flow rate to the bottom of the reactor was kept at 200 cm3/mm, which IS low enough that the effect of flow rate on the measurement of the weight change was neglwble Preparation of actwated carbon samples The organlcs

M %JZUKl et al

272 Electrabalance De-OX0

unit

1

I

---

H I

PHENOL PEG

4000

Temperature

1

Thermocouple

Fig 1 Expenmental apparatus

hsted m Table 1 were loaded on granular activated carbon A (bltummous coal base) AU of the orgamcs were of special grade obtamed from Wako Chemical Co PEG 400, 1000 and 4000 were provided by Gas Chromatography Kogyo Ltd Activated carbons were screened to obtam particles of No 14/20 mesh, boiled m water to remove carbon fines, dried at 110°C m a thermostat and kept m a desiccator The carbon particles loaded with liquid organlcs were prepared by lmmersmg a known amount of carbon mto a hqmd orgamc, allowmg a sufficient time for adsorption, separating the carbon partrcles, and drying them at room temperatures From the weight change of the sample, the amount of orgamcs retained m the carbon was determmed Phenol, PEG 400, and solid orgamcs were dissolved in dlstllled water and batch adsorption from aqueous solutions was performed to prepare loaded samples Change of concentration of solution gave the amount of orgamcs loaded on the carbon After the adsorption carbon samples were kept overnight in a thermostat at 110°C to remove water To check the effect of loading of organics, phenol and PEG 4000 solutions with different mitral concentrations were used to obtain activated carbons contammg different amounts of organics granular activated carbons from different Also, sources B (hgmte base), C (coconut shell base), and D (petroleum base) were tested with phenol and PEG 4000 to test the effect of possibly different carbon structures on TGA curves The properties of these activated carbons are gven in Table 2

0

200

400 TEMPERATURE

600

800

(“Cl

Fig 2 Temperature wtxght change curves for phenol and PEG 4000, effect of uutml loadmg very sundar regardless

of the mltlal amount adsorbed,

40 Vagm carbons without orgamcs also showed slight change of weight dunng the temperature rise as shown m Fig 2 The amount of orgamcs held on the carbon, q IS defined as a difference between the weight of a Ioaded carbon sample and the weight of the virgm carbon before adsorption per unit weight of the mitral vlrgm carbon sample When residual amounts of 8OO”C, qsoo, are plotted against q. (Fig 3) for cases shown m Fig 2, It can be noticed that qsooIS proportional to q. for the range tested Then the plot of q/q, vs temperature can be considered to be characterlstlc for each organic 0 15

RESULTSANDDISCUSSION Effect of the m&al amount of organxs adsorbed. qO. on TGA curves Carbon samples having different loads of phenol and PEG 4000 were heated up to 800°C and their TGA curves are presented m Fig 2 There are sign&ant differences m the shapes of the curves for phenol and PEG 4000 However, the curves for each organic are

INITIAL

Fig

3

AMOUNT

ADSORBED. 4,

(g/g)

Residual amount of 800’C and initial amount adsorbed for phenol and PEG 4000

1:; 130 146 182 120

k:::,n! octanol corimarin benzophenone vanilin

250 205 4 164 1 118 0 157 5 194 5 290 305 9 285

179 1 310

308

:03 110 6 114 182 285

174 270

98 4

ii

Boiling Point, OC

a) Sodiumdodecylbenzensulfonate b) Tetraethleneglycol c) Polyethyleneglycol

320 122 116 88

1:: 126 170 326 194 s4OC ~1000 s4000 106 122

I!62 100 142 212 a4 78 92 106

Molecular Weight

::?!1 PEG%0 PEG 1000 PEG 4000 Benzaldehyde p-oxybenzaldehyde lignin humlc acid methyleneblue benzoicacid caproicacid butyricacid

n-pentane n-hexane n-heptane n-decane n-pentadecane cyclohexane benzene toluene p-xylene phenal B-naphtol fluoroalucinol p-h#'oxydiphenyl

Adsorbate

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 20 28

105 07 35 45 26 58 150 04 01 02 04 03

:03 0 61 0 68 0 37 0 49 0 50 0

i 10 0 12 0

i

Oe)

geoo/90

II II

I

II II

I I I III III III III II II

I

I I + II

Organic

347 243 222 la0 234 228 372 385

375 350

375 375

460

182 162 255 186

155 152 175 270

195 115

180 182 213 360

157 260

88: 190

88;

100

236

190 159 185

Z

200 155

9 56 5 52

a 93

8 11 10 8

39lf)

70

l!l:

97:

n

9864 11 3

d) loadedby umnerslonand drying over mght at room t@IWratUre e) 0 denotesqeoo i 0 01 g/g carbon f) obtainedby applyingunpropermodel

several 0 111 0 181 0 111 0 092 0 102 several several several 0 434 0 118 00398 0 035 0 084 0 267 0 500 0 425 0 322 0 348 0 460 0 404 0 210 0 175

e

Table 1 Propemesof orgaws and the results of the TGA expemnents

191

102

12 0

z

27 7 25 7 25 7

5 34f)

-

-

6

-

66 6

6

2. 6, 7 6

5

1:

TGA

cou BP VA

cCf C60 C8G

: CSA

CBA OBA LI HU

PEGS

TEG

!s

:i

::0 Cl5 CH B T

:65

l

Key for Figs 5, 6, 11

e

%

4

cn

M

274

et al

SUZUKI

Table 2 PropertIes of actwated carbons Carbon

A

Origin

BI turn1 nous Coal (CAL)

Sieve Opening, mesh Particle g/cm3 Nl

c

0

(DARCO)

Coconut She1 1 (Takeda 911)

Petroleum Pitch (Kureha Beads)

Xl 4/20

#14/20

x14/20

density,

trogen surface area, m2/g

Pore volume, cm’/g Macro pore cm3/g

volume.

pore cm”/g

volume,

Micro

8 Lignite

Most probable pore radius.

micro fi

0 80

0 67

0 82

1140

530

1200

0 91

0 89

0 72

0 85

0 67

0 39

0 21

0 15

0 46

0 45

0 56

0 51

8

22

8

10

Types of TGA curves Since q/q, vs temperature plots were found to be characterlstlc for an orgamc, the results of TGA for all the other orgamcs were plotted m this manner From the shapes of these curves, it was found that the curves could be classified mto three groups according to then shapes These three typlcal shapes are illustrated m Fig 4 Group (I) gves relatively concave q/q0 vs temperature curves and consists of volatile orgames such as hexane, heptane, benzene, etc Group (II) showed rather convex q/q,, vs temperature curve and PEG and DBS belong to this group Group (III) consists of phenol, B-naphtol, hgnm, and humlc acid These orgamcs gave a gradual change of q with temperature nse, and contam relatively high fractlonal residue at 800°C (4 aoolqo) The q/q0 vs temperature (TGA) curves for the orgames which belong to Group (I) are shown m Fig 5 These orgamcs gave negligible qN-pentadecane showed somewhat strange behavlour at higher temperatures, which might suggest that the desorptlon 1s followed by thermal crackmg which becomes dommant above 450°C Figure 6 shows the TGA curves of the orgamcs classlfied in Group (II) and Group (III)

All the experimental

results

are summazed

TGA curves and PEG 4.000 on different actwated carbons Figure 7 shows q/q,, vs temperature plots for phenol and PEG 4000 adsorbed on four different activated carbons Only carbon B showed a slight devlatlon from the other carbons This may be attributed either to the slgmficant difference of macro pore size distibution

0

2

-

0

200

600

400

TEMPERATURE (“C)

FE

5 Tvmcal TGA curves of orgamcs of group I, keys refer to __ Table I

TEMPERATURE

Fig 4 Phenomenologlcal

classdicatlon

m Table

1

of thermo-gravity-curves

of adsorbed orgamcs on actwated

carbons

Study of thermal regeneration

A

of spent actwated carbons

275

06 0

,” =

04

0

200

400 TEMPERATURE

600

800

(“C)

Fig 6 Typical TGA curves of orgamcs of groups II and III

1 0

08 L ,”

0

6

0

4

0

2

. o-

o A (Bituminous

coai

x B (Lignite) 0 C (Coconut a

shell)

D (Petroleum

pitcl

D D

200

Fig

7

Comparison

of thermo-grawty-curves

600

400 TEMPERATURE

800

(“Cl

of phenol and PEG 4000 adsorbed on activated different origins

or due to the large amount of ash m carbon B Carbons C and 0, respectively, are of coconut shell base, and of petroleum pttch base, and are expected to have different surface characterlstlcs from carbon A However, these carbons gave almost identical results as far as these experiments are concerned Smce the devlatlon of the TGA curves by carbon B IS also not too large, the effect of the difference between the activated carbons does not gve a drastic change to the desorptlon process characterlstlcs of the adsorbed orgamcs during the temperature rise period Interpretation of TGA curves Thermal desorptlons or chemical reactions under a linear rise of temperature have been widely studied from the standpoint of clanfymg heats of adsorption on catalysts, or kinetic studies of heterogeneous reactlons The TGA curves obtained here m&t be partly explained m terms of the theories based on simple physlochemical characterlstlcs of adsorbed orgarucs Two of the simplest phenomena expected to occur during the temperature rise period are (a) thermal desorptlon of volatile orgamcs mltlally adsorbed on the activated carbon, and (b) decomposmon of orgamcs on the adsorbent

carbons of four

surface, and then some volatde fragments vaportzed

produced

are

(a) Thermal desorptlon model As a first approxlmatlon, equthbrmm desorptlon with Langmulr Isotherm 1s consldered When an activated carbon loaded with q grams of an orgamcs per gram of carbon IS m eqmhbnum, the temperature dependence of q is given as follows

The equlhbnum pressure P IS not defined here, but during the temperature rise period m an mert stream it 1s assumed as a first rough approximate, to be constant Equlhbrmm constant K IS written m terms of lsostenc heat of adsorption, Q,,,, as K = %exp Then by consldermg

(Q,,,/RT)

that q.. IS temperature

(2)

Independent,

M

276 eqns

Suzu~r

(1) and (2) gve

4-140 AL= 40 1+ WK,P) exp ( - Q,.,/RT)

(3)

When comparmg eqn (3) with the experlmentally obtamed curves, the temperature at CJ/CJ~= ;, T,,*, and the slope of q/q0 vs T (temperature) at Tllz are easy to use for interpretation of the data TliZ IS easily obtamed from eqn (3) as

et al

DTG decompositmn kmetlcs, and suggested to use chromatographlc peak posmon and the half width of such a peak for obtauung activation energies These methods are not convenient to use for TGA rurves since they involve graphical dlfferentiatlon of the curves The basic equation employed here 1s dq ==-k

03)

q

where k is given by the Arrhemus T1’z = ln(K,P)

- ( Q,.oIR) + In [2(qJqo)

- l]

(4)

Also

equation

k = k, exp ( - EIRT)

(9)

By setting that m = dT/dt, we obtam from eqns (8) and (9)

that [qq,,,=

-&= -&--(&)(2-E)

dq

-= 4

(51 where AT 1s defined (3)-U) gves

m Fig

AT

8 Combmatlon

4

Integration c=exp

RT,I,

‘=T,,,=~-WL)

assuming

of eqns

-$exp(-E/RT)dT

of the above equation

[2

g

E,(EIRT)

gives

- yexp

(- E,RTJ]

Qm,

that q. = q_, eqn (6) becomes

&pp

where E,(RT)

0) IS0

(b) Thermal crackmg model When an adsorbed organic 1s easier to pyrolyse on the activated carbon surface than to desorb, a kmetlc equation must be employed Smce there IS no reported work on the crackmg of adsorbed orgamcs, the first order kmetics 1s assumed as a first approximation For nonIsothermal kmetlcs Freeman and Carroll[2] and Van Heck[3] derived d&Terentlal thermal analysis (DTG) curves based on n-order kinetics, and showed how to apply theu method for interpretation of experimental data Also, Seewald[4] has given solution for a first order

=

- [exp ( - x)/x] dx IX

and x = EIRT In order to apply a s1mlla.r approach, as m the case of thermal desorptlon model, for the comparison with the experimental data, 6 = AT/Tllz was derived from eqn (11) as (13) where +(x) = 1 - x exp (x)

*

T1/2 TEMPERATURE

Fig 8

(12)

Defimtlon of T,, and AT of experImental thermo-gravity-curves

E,(x)

(14)

Study of thermal regeneration x and E,(x) were defined previously The fun&on #(x) vs x 1s shown M Fig 9 Then 5 1s obtamed from expenmentaily determined Till and AT The acuvatton energy, E, IS evaluated utthvng Fig 9 The pre-exponenteal factor, k,, IS readtly obtamed as and

k,=g =y

exp(E/RT,,d

ew WRTl,2)

[Q&$--)I-’ (15)

Companson of the two models with the expenmental results Thermal desorptron model (eqn 3) gves rather convex curve srm&tr to the curves of Group (I) m Ftg 5 The orgamcs of Group (I) are volatrle and expected to be desorbed at lower temperatures than the decomposttton temperatures The TGA curves of Group (I) orgamcs compare reasonably wtth eqn (3) However, one needs to know 4.. , the saturatton adsorption capactty, m order to apply eqn (6) As a rough estimate, eqn (7) can be used to calculate QSO Instead of eqn (6) by assummg that qO = q.. IS satisfied after keeping carbon samples m a destccator The results of these caicuiatrons are summartzed m Table 1 The tsostenc heats of adsorptron obtained from the TGA curves are compared wtth latent heat of vaporrzatron and they show that the tsostertc heat 1s of the same order of magtutude as the latent heat Usually, the tsostertc heat of adsorptron is expected to be about twice as large as the heat of vaportzatton for hydrocarbon gases adsorbed on activated carbons In our experiments, the assumptton of q,, = q- may have Introduced a large error m esttmatmg Q,., If q,, IS larger than qm then It IS obvtous from eqn (6) that the caiculated Q,, becomes smaller than the true value Also, attamment of eqmltbrmm may be unfulfilled and the eqmhbrmm pressure, P, 1s possibly a function of temperature The equthbrutm desorptton model IS too simple to determine Q,,, precrsely. but It IS suffictent to explam the shape of q/q0 vs temperature curves for Group (I) or-

of spent activated

curves

for n-hexane

277

and PEG 400 whtch,

respectively, of Group (I) and (II) The equrhb-

are the representatrves

r-rum desorptton model together with the parameters deternnned from T,,, and AT, by assummg go = q.. , for each case are shown by dotted lines Apparently, eqn (3) sattsfies well the data for n-hexane but does not agree with the results for PEG 400 The crackmg desorptron model (eqn 11) gtves curves stmtiar to the curves of type (II) m Ftg 4 Expertmental

data obtamed from orgamcs of Group (II), however, gave a small but defimte amount of restduals at hrgh temperatures Namely, q/q,, decreases wtth the increase of temperature, but tt does not level at zero The most plaustble explanation may be that carbon deposit or some polymenzed fragment a byproduct of the thermal crackmg reactton remams on the activated carbon surface throughout and after the crackmg reactron On thts basis, It LS assumed that a carbon deposit 1s produced proportionally to the extent of crackmg reaction, the theory dertved 1s eastiy apphcable by usmg q; = qO- qaoo, and q’ = q - qW, respecttvely, Instead of qO and q , when obtauung T,,l and AT from the expertmental curve Then eqn (13) IS vabd for T,,* and AT obtamed m such a manner This procedure 1s apphed for the orgamcs which belong to Group (II), and the resultant acttvatton energy, E, and the pre-exponenttal factor, k,, are hsted n-r Table 1 1.0 0.8 A

0

S

0.6 04 02 0 0

200

400 TEMPERATURE

games A typtcal example of the comparison IS shown m Fig 10 In this figure, eqn (3) IS compared wtth q/q,, vs T

10-l

carbons

Fig

10 Comparison

of models

10

7

lo*

x

Ftg 9 Dragrams for obtannng x =

J?/RT,~

from q(x)

600 (“C)

with measured

TGA-curves

800

278

M

SUZUKI et al

The effect of heatmg rate (the programmed rate of temperature rise),, m, 1s included m eqn (11) As It was shown for PEG 400 m Table 3, the effect of heating rate, obviously, 1s accounted for by the simple model adopted for the orgamcs of Group (II) The actlvatlon energy obtained here seems to be smaller when compared with that of a homogeneous thermal crackmg, which suggests that the thermal craclung 1s possibly catalysed by activated carbon surface Cilassrficntlon of the mvestrgated orgamcs It 1s of a practical importance to establish a way to find out to which of the three groups an orgamc belongs By mspection of Table 1, It can be noted that the boiling point of an organic IS one of the factors that affect the easiness of desorption Also, the existence of aromatlc rmgs m molecular structure might be slgnticant m determining the residuals on carbon surface Other possible factors that influence the behavlour of orgamcs may be oxygen content, molecular weight and other chemical functlonal groups contained m a molecule The orgamcs were tested m various ways and it was found that the aromatic content and the botig point are the two mam factors that characterize the behavlour of an organic desorbmg durmg a temperature rise regme Figure 11 1s the plot of the aromatic carbon content, 4, vs the boiling pomt, tB, for the orgamcs examined m this study The aromatlc carbon content 1s defined as follows ~ = number of aromatic carbons m a molecule total number of carbons m a molecule (16) The ordinate, 9, corresponds to the degree of bondmg between adsorbed molecule and the carbon surface In Fig 11, hollow and solid cucles, respectively, correspond

to the orgamcs of Group (I) and (II) while crosses refer to Group (III) Apparently, these three groups have theu distinct regions m the @ vs Q plot As expected, low bollmg point orgamcs constitute Group (I) and the “dlvlslon” hne 1s specified by t, = 220°C for 4, = 0, and tE = 170°C for Q, = 1 Also, when the content of aromatic carbons m an orgamc 1s higher than 0 8, and tB 1s above 17O”C, It IS most hkely that the organic belongs to Group (III) These orgamcs are not easily removed by heatmg only The Fig 11 provides a simple crlterlon for prediction of behavlour of orgamcs frequently met m the lndustnal waste water treatment CONCLUSIONS

TGA of activated carbons loaded with vaflous smgle component orgamcs showed that the TGA curves could be classified mto three dlstmct groups as presented in Fig 4 Orgamcs that belong to Group (I) are rather volatile and TGA curves can be interpreted m terms of the thermal desorptlon model given by eqn (3) Group (II) consists of relatively high boding point orgamcs that are easily decomposed, such as PEG The TGA curves for this group were compared successfully with the first-order crackmg model given by eqn (10) The parameters included m these two models can be easily obtained from the measured TGA curves by means of eqns (6) and (7) for the thermal desorptlon model and eqns (13) and (15) for the thermal crackmg model utlhzmg T,,, and AT, as defined m Fig 8 Group (III) that consists of phenol, /3-naphtol, hgmn, humlc acid, methylene blue, and other phenohc orgamcs, gave high residuals on activated carbons after heating up to 800°C This suggests that these orgamcs, when adsorbed on activated carbons, are not easily regenerated by means

Table 3 Effect of heating rate on the TGA curves for PEG 400 Heating

Rate,

“C/ml n

T~/zr

6

“C

AT.

OC

kcal iiz=iF

e,

ko.

set-’

375

85

25 7

1 2x106

12

392

90

25 5

1 2x106

18

411

95

26 0

1 2x106

100

200 Boiling

300 point

unknown

(“C)

l+g 11 Boihng pomt-@ (aromatlc carbon fraction) plots of the orgamcs examined Hollow cycles, sohd cucles and correspond to group I, II and III Keys refer to Table 1 @ values of hurmc acid and hgmn are unknown

crosses, respectively

Study of thermal regeneration of spent activated carbons

of thermal

methods only, and probably some pretreatment IS needed (for example such as alkalr-washing)

Also whether the residual carbon deposit from those orgamcs may be effectively removed by passage of steam or other oxldmng gases will be of concern m future studies A rough classdicatlon of orgamcs mto the three groups was possible by using the boding point and aromatic carbon content, defined by eqn (16) and presented m Fig 11 When orgamcs m a water to be treated by an activated carbon are known, the effectiveness of the thermal regeneration of the spent activated carbon can be es% mated by checkmg the content of the orgamcs which belong to Group (III)

first order rate constant of thermal l/set pre-exponential factor, eqn (9), l/set heatmg rate, deglsec eqmhbnum pressure, atm lsostenc heat of adsorption, Kcal/mole amount adsorbed, g/carbon = q - qsoo,g/carbon

279

crackmg,

deg

mltml amount adsorbed, g/carbon = qo - q8o0, g/carbon

residual amount at 8OO”C,dcarbon gas constant, temperature, “K temperature at q/q0 or q’/q:, = l/2, “K = (d(q/q,)/dT)-’ or (d(q’/&)/dT)-’ at T,,+, “K-’ time, set = E/RT

Acknowledgement-This work was supported m part by a Scientific Research Grant from the Mnustry of Education, Japan (No 911503)

NOTATION

E E,(x) K &

acttvation energy of thermal Kcal/mole deg exponential integral, eqn (12) eqmhbrlum constant, l/atm constant, eqn (Z), llatm

= ATI T,,, defined by eqn (14) aromatIc carbon content, eqn (16)

RIWERENCES

crackmg,

[ll Hutchins R A, Chem Ennna Pronr 1973 69 48 [21 Freeman E S and Carroll-B-, J whys Chem 1958 62 394 131 _ Van Heek K H , Jiintnen H and Peters W . Chem Inn Tech 1967 71, 113 [4j Seewald H , Untersuchungen zur Sorptzon orgunrscher Stolpe an Aktwkohlen Bergbau-Forschung GmbH (1974)