Adsorption expansion and elastic properties of wood charcoal

Ca,b,in 1968, Vol. 6, pp. 561-570. Pergamon Pr~rs. Printedin GreatBritnirl

ADSORPTION

EXPANSION WOOD R. 1. FUZOUX,

AND ELASTIC CHARCOAL

I?. Z. SALREB

PROPERTIES

OF

and F. S. SAID

Department of Physical Sciences, American University in Cairo National Research Centre, Dokki, Cairo, U.A.R. (Received22 .MUJ 1967) Abstract-The expansion of 18 different charcoal rods due to the adsorption of methanol has been determined together with Young’s modulus of elasticity of the methanol-free and methanolimbibed charcoals, and the results are found to support the theories of adsorption swelling and elasticity proposed by BANGHAM et al. The adsorption extension, coefficient of thermal expansion and elasticity constant of the carbons are, in general, anisotropic, especially when carbonization is at the lower temperatures. Specific surface areas calculated from the application of the PickettAnderson-Dellyes equation to the adsorption ofmethanol, and from the corresponding adsorptionexpansion data, are in good agreement with the areas obtained from low temperature nitrogen adsorption.

1. INTRODUCTION

of gases and vapours by certain rigid porous solids is accompanied by dimensional changes of the adsorbent. BANGHAM and co-workers+4) showed that the expansion of wood charcoal on adsorption of a vapour is proportional to the surface free energy lowering caused by the adsorption. Thus if = is the reduction in the surface free energy, calcuIated by the integration of the Gibbs isotherm according to BANGHAM and RAZOUK,‘~) and x is the linear expansion per cent, then:

THE ADSORPTION

x=X77

(1)

where X is a constant which is related to the elastic properties of the charcoal. On the assumption that the expansion is a purely mechanical effect due to the tangential force exerted as a film pressure over the internal surface, BANGHAM and ~&GGs@) found that X is related to Young’s modulus of eIasticity Ea by the equation: h = 100 p C/Ed,

(2)

where p is the density of the adsorbent, and X its specific surface area. This equation was tested CARBON 614-I Z

in the case of coals by comparing Young’s modulus calculated from equation (2) with directly measured values, and agreement was satisfactory.‘@ The proportionahty between the relative expansion and surface free energy lowering was confirmed by MCINTOSH et al. for charcoalc’) and for porous glass.‘s) Furthermore RAZOUK and EL-GOEBE~LY’~) found approximately the same vahre of X/X when different gases and vapours were adsorbed on the same charcoal. However, YATES found minor variations in the constant of proportionality when different adsorbates are used, and later work by QUINN and MCINTOSH revealed that the constant of proportionality between x and 7~varies considerably among their three different adsorbates on porous glass. YATES”~) suggested that on account of the isotropic expansion of adsorbents described by MEEHAN”~) and by LAKHANPAL and FLOOD,“~) it would be more correct to relate the expansion with bulk modulus K, and he developed an expression analogous to equation (Z), namely: h = 200P Cj9K.

561

(3)

562

R. I. RAZOUK,

F. 2. SALEEB and F. S. SAID

The data obtained by YATES using porous glass supported this expression. FLOOD and co-workers(r*-16) derived thermodynamically expressions describing the adsorption extension on the assumption of a Polanyi compressed film theory. The equations developed indicate that the relatively large adsorption extensions are due mainly to the large stress and stress gradient changes within the adsorbent which accompany relatively small changes in the pressure and pressure gradient of the adsorbed gas. FLOOD and LAKHANPAL”~)obtained a general expression for the dimensional changes resulting from adsorption which agreed satisfactorily with experimental results, and which, under special conditions, reduced to a direct proportionality of expansion to the surface free energy lowering or the spreading pressure of adsorbed film, Throughout the work on adsorption expansion, except with coals, the dimensional changes were considered isotropic. Recent observations during the preparation of charcoal from various kinds of wood have shown that blocks of wood do not contract on charring to the same extent in different directions, and that charcoal rods undergo anisotropic expansion on adsorption. In certain cases, the radial elongation on sorption is approximately double the axial value. This anisotropic behaviour limits the applicability of the theories of elasticity based on dimensional changes as developed by F~o0~'14-16' and by BANGHAM.‘@ But it also provides new direct means of testing the proportionality relation between adsorption extension and surface free energy lowering, by determining from the anisotropic expansion data certain properties of the adsorbent which should be the same independent of direction. Both the specific surface area and the heat of wetting are examples. On the other hand, Young’s modulus of elasticity and the temperature coefficient of expansion of the gas-free adsorbent should depend on the direction along which the measurement is taken.

The present work comprises the results of experiments on expansion adsorption obtained when methanol is adsorbed on charcoal prepared by carbonization of 5 varieties of wood at temperatures between 500 and I 100’33, together with measurements of their elastic moduli, linear thermal expansions, and low temperature nitrogen adsorption. The results of heat of wetting experiments will be described in a following article. 2. EXPERIMENTAL A, A#mztus The adsorption of methanol at 25°C and of nitrogen at -195°C was determined by means of conventional volumetric apparatus. The expansion of charcoal rods on sorption was measured by a metal extensometer(*’ giving results reproducibIe to better than 0.002 per cent. Young’s modulus of elasticity was determined experimentally on compression of the methanolfree and methanol-imbibed charcoals by measuring the contractions resulting from applying loads on the movable jaw of the same extensometer used in adsorption expansion measurements. The coefficient of thermal expansion of the evacuated charcoal rods was also determined, using the same extensometer, allowance being made for the linear thermal coefficient of expansion of the brass of the extensometer.

Five varieties of wood (Casuarina equisetifolia, I ; Eucalyptus resinfera, II ; Acacia arabica, III ; Morus alba, IV; and Gossypium barabadense, V) were cut in longitudinal sections and carbonised at 5OO”C, (A), 65O”C, (B), 8OO”C, (C), 9OO”C, (D), lOOO”C, (E), and IlOO%, (F), in limited supply of air. Some charcoal rods were also prepared in transverse sections, and they are designated by double subscripts. Methanol and nitrogen were spectroscopically pure.

ADSORPTION

EXPANSION

3. RESULTS A. Adsor-tion The

AND DISCUSSION

of methanol

adsorption

charcoals

is rapid

of hysteresis. same

adsorption bolic region axis

and reversible

isotherm.

in shape which in

the

Adsorption carbonization

on the various with no sign

Rods of the same kind of wood cut and then carbonized

conditions, but

give The

rise

to

a small pressure

neighbourhood

of

slightly

temperature

up

saturation.

with

rise

to 900°C

at higher temperatures. isotherms

of methanol.

isotherms

found

fit

0.2

of

are

to

best

the

PICKETT(~‘)-ANDERSON(~*)-DELLYES(~~) equation for a definite

number

at saturation

:

of adsorbed

PC1-P”/P3_

s (PO -

1

amount

adsorbed

and C a constant

0.6

vapour

expressed

related

tion in the same manner

0.L

c-

j

expressed

capacity

layers formed

1p

smc stc

P)

where p/p0 is the relative monolayer

and

563

1 shows typical

are para-

the

CHARCOAL

Figure

same

show

towards

OF WOOD

then decreases markedly

under

the

isotherms

occasionally

is convex increases

PROPERTIES

The

of methanol

axially and radially, the

AND ELASTIC

0.8

PIP0 FIG. 1. Typical adsorption isotherms of methanol at 25°C.

PO

pressure, in

g/g,

S,

(4) S the the

in the same units,

to the heat of adsorpas the corresponding

R. I. RAZOUK,

564

F. Z. SALEEB and F. S. SAID

constant in the BET isotherm. This equation represents the results satisfactorily up to relative pressures of 0.8 to 0.9. Typical curves are shown in Fig. 1. The specific surface area of the different charcoals has been calculated using 18.1 Aa as the area occupied by one molecule of methanol.(S) Table 1 gives the specific surface areas of the various charcoals together with the corresponding values of n. The value of n is found to decrease with rise of carbonization temperature from 500 to 9OO”C, whereas the specific surface area increases, indicating the development of finer pores by the heat treatment, Further rise of temperature is accompanied by the destruction of the fine pore network probably through graphitization, and the specific surface area diminishes then considerably whereas n increases. B. Adsorjtion of nitrogen Low-temperature nitrogen adsorption is found to be slow, specially in the initial stages of adsorption, and the equilibrium is then established after 24 hours. All the isotherms are very steep and exhibit plateaus covering the relative pressure range from about 0.02 to saturation.

i

E 5

0t 0.2

0.4

0.6

0.6

PIpo FIG. 2. Typical adsorption isotherms of nitrogen at

- 195°C.

adsorption isotherms are shown in Fig. 2. The monolayer capacity is taken as the value represented by the plateau. Using 16.2 Aa as the area occupied by a molecule of nitrogen, the calculated specific surface areas Typical

are found to be always slightly higher (up to 17 %) than the corresponding values calculated from the adsorption of methanol (Table 1). C. Adsorption exfiansion The adsorption of methanol by the different charcoals is accompanied invariably by expansion, and no contraction could be detected at the lower equilibrium pressures, in contrast to the observations of many authors. The adsorption expansion is found to be sensitive to the temperature of carbonization. It diminishes regularly with rise of temperature and ultimately no expansion occurs on sorption when the charcoal is pre-heated at 1100°C. In general the adsorption expansion of charcoal rods cut axially and radially is not the same, the latter being usually greater. Similar anisotropic swelling was observed to accompany the adsorption of vapours by coal, the expansion in the direction perpendicular to the bedding plane being greater. The ratio of the radial to axial expansions at the monolayer adsorption is 1.83 and 1.57 for Charcoal I carbonised at 500°C and 900°C respectively, and 1.18 and 0.96 for Charcoal III carbonized at 500 and 800°C respectively. Thus the difference in expansion along the two directions depends on the origin of the charcoal and its pre-heattreatment. This explains the isotropic nature of the expansion described by MEEHAN for yellow pine charcoal(l3) and by FLOODfor rods made by extruding charcoal fines.“4) The expansions at the monolayer capacity (x,) and on immersion (x0) are given in Table 1. It is to be noted that the expansion at the monolayer is roughly half the expansion on immersion On the assumption that the expansion is due to the pressure exerted by a two-dimensional surface film(1-4) a plot of expansion (x) vs. reciprocal amount adsorbed (l/S) would correspond to a T-A diagram where A is the area occupied by a molecule. Typical plots are shown in Figs. 3 and 4. The curves are all of the same type and they closely resemble the T-A

TABLE

DD

D

c

B

A*

A

500 800 800 500 800 500 650 800 1000

1100 500 800

500 500 650 800 900 303 303 338 381 396 396 290 380 286 286 386 371 291 321 165 342 400 39

319 319 373 402 415 415 -

315 424 288 288 454 430 310 341 170 385 437 41

292 375 281 286 379 372 281 323 168 337 394 -

303 303 342 379 394 394 -

1.40 1.40 1.20 1.15 1.10 1.10 __ 1.50 1.15 1.50 1.50 1.15 1.15 1.55 1.30 1.40 I .20 1.15 3.00

Number of layers (P.A.D.) n

AND

0.83 0.31 0.65 0.77 0.26 0.25 0.80 0.34 0.67 0.33 0.25 0.02

0.48 0.88 0.40 0.25 0.21 0.33 -

PROPERTIES

1.04 1.75 0.80 0.44 0.36 0.56 1.90 0.55 1.32 1.43 0.46 0.45 1.87 0.80 1.48 0.60 0.43 0.04

Immersion &I

(%I

Expansion

ELASTK

Monolayer &l

1. ADSORPTION, EXPANSION

Specific surface area Carboniza(m”/g) -------~tion rrcoal temperature N, CH,OH Adsorption (“C) Adsorption Adsorption expansion (P.A.D.) data

-

7.14 4.83 9.52 19.86 24.39 14.29 3.47 17.84 5.18 4.41 20.41 18.52 3.48 9.01 2.79 12.99 21.28 22.73

‘v (Cal/g)

OP WOOD

0.33 1.68 0.49 0.42 1.92 1.74 0.33 0.85 0.17 1.22 2.02 2.16

0.67 0.46 0.90 1.87 2.29 1.35

JL 0.24 0.12 0.30 0.67 0.68 0.25 0.65 0.27 0.46 0.18 0.10 0.71 0.31 0.21 0.67 0.20 0.80 0.86 1.13

0.14 0.11 0.16 0.36 0.42 0.23 0.46 0.12 0.37 0.10 0.09 0.58 0.27 0.10 0.55 0.20 0.65 0.80 1.oo

Calculated Measured Measured from adsorpin in tion expan- methanol air E ill0 E a** sion data

Young’s modulus of elasticity (dyne/cm2 x 10-il)

CHARCOAL

5.8

17.1 8.4 6.9

2 r?

$

g

or

5

T3

17.1 17.9 7.6 7.2 21.3 9.1

g 0 LZ

F g 2 fl

16.4 7.6 6.8 6.4 7.3 6.4 17.8 2.8

u

2

5,

!s

? Z g

8

g

:!

8

12.0

x 106

Coefficient of thermal expansion a per degree

9

Fz

R. I. RAZOUK,

566

0

10

20

F. Z. SALEEB and F. S. SAID

30

LO

50

60

70

80

90

100

11.5(g/g)

FIG. 3. Typical curves of adsorption expansion vs. reciprocal amount adsorbed. Axial expansion of Charcoal I.

0

0

10

20

30

h0

60

60

70

l/S (g/g, FIG. 4. Adsorption expansion vs. reciprocal amount adsorbed for axial and radial expansion of charcoal.

ADSORPTION

diagrams Below

EXPANSION

of condensed

a certain

increases

of

l/S the

linearly

expansion

with

diminishing

values of 1 /S as is usually encountered densed

films.

of the

curve

Extrapolation to

zero

the

of

corresponds

to zero surface

this condition

surface

by long chain surface

could

be

adsorption-expansion between areas

these

It is interesting

surface

and

by

equation

of the same even though

investigation

equation. equation

is to be expected

and

the

Attempts

in

from

by combining

the

Pickett~-Andersonto develop

a simple

of state for adsorbates

obeying

the latter isotherm were not successful. However, it is possible

to express

71 as a function

of p/p0

for simple values of n when this is expressed

surface Pickett-

the ratio of two integers. results

(Table

charcoal

cut along

the area abscissa at

the v vs. l/S curves obtained

general

from the intercepts

intersects

of cut

the expansion

is

of the

The shape of the x vs. I/S curve obtained the present

Dellyes

these

This

the same point.

to the specific

as 80 per cent.

curves for charcoals

on liquid

to note that the same specific

the x vs. l/S curves or radially

is excellent

expansion

the two directions

isotherm

agreement the

may differ by as much

567

CHARCO.iL

shown in Fig. 4, where the extrapolation

Gibbs

the specific of

OF WOOD

under

from

The

means

area is obtained

axially

Assuming

pressure,(20)

calculated

the

which

molecule

alcohols

data.

values

calculated

Anderson-Dellyes 1).

pressure.

by a methanol

at zero areas

density

to be 21 AZ, i.e. equivalent

area occupied substrates

adsorption

part

enables

PROPERTIES

linear

with con-

of this linear

expansion

evaluation

the area occupied

ELASTIC

films on liquid substrates.

value

almost

AND

of calculation

grams from isotherms 2, 3 and

co using

C = 53.6,

and n =

Figure

for which II =

pansion

using’ the proper

represented

calculated

by the circles

and the agreement

S,

1, 1.5 and =

1.5 for Charcoal

results

the

m vs. I/S dia-

the values

experimental data

5 represents

of typical

as

0.0849,

III,.

from value

The

the

ex-

of X are

in the same

figure,

is remarkable.

D . Elastic proper ties Although

the simple

theory

of elasticity

of

BANGHAMand MAWS(~) and of FLOOD”*-~~) and

-

the

calculation

adsorption to charcoals yet

of

moduli

expansion

elasticity

from

data do not strictly

of

apply

which undergo anisotropic

valuable

information

from such calculations. the determination

might

Equation

of Young’s

swelling,

be

obtained

(2) shows that modulus

by the

simple BANGHAM and MAGGS theoryc6) requires the evaluation combining

of C/h. This is made possible

equation

Gibbs isotherm,

(1)

with

the

by

integrated

giving:

(5)

FIG. 5. rr vs. l/S diagrams for systems obeying the Pickett-Anderson-Dellyes isotherm for n = I, 1.5, 2, 3 and to. Circles represent experimental values obtained from adsorption expansion data of Charcoal III,.

where

M is the molecular

weight

of the adsor-

bate. The values of X/X for the various charcoals are given in Table 1. It is clear that this ratio is particularly sensitive to the temperature of

568

R. I. RAZOUK,

F. Z. SALEEB TEMPERATURE

“C

100

1

2

and F. S. SAID

200

3 KILOGRAM

I PER

5

6

cm2

FIG. 6. Typical stress-strain and differential thermal expansion curves measured axially and radially. carbonization, and that it differs for the same kind of charcoal cut in longitudinal and in transverse sections. The values of Young’s modulus calculated by equation (2) from expansion-adsorption data are given in Table 1. The same table includes also the values of Young’s modulus measured directly on compression of air-exposed and methanol-imbibed charcoals. The stress-strain relation is linear within the applied pressures. Typical results are shown in Fig. 6. In calculating the stress per unit area a correction has to be made because the stress is not applied to the whole cross sectional area of the charcoal rod, since part of the pore volume consists of visible sap ducts. As a good approximation, MAGGP~) proposed that the apparent modulus has to be multiplied by the ratio of true density to block density. The latter was measured by mercury displacement and

the former was taken as the density of graphite. The measured modulus for the methanolimbibed charcoal is, in general, greater than the corresponding value for the methanol-free charcoal, presumably because in the former case the charcoal under compression contracts from its extended form. This value is therefore closer to the modulus calculated from adsorptionexpansion data according to the theory of elasticity Of RANGHAM and MAGGS.(@ The agreement between the two values for 18 charcoals is surprisingly satisfactory in view of the assumptions involved in the calculation, and strongly supports this simplified theory of elasticity. The values of the moduli vary between 0.1 and 1.0 x 10” dyne/cma as compared with 1.0 x 1011 dyne/cm2 found by ANDREW et al.(az) in case of molded carbons. Furthermore, it is found that the modulus increases with rise of

ADSORPTIOi’4

EXPANSIOX

AND ELASTIC

carbonization temperature in agreement the resuhs of the same authors.

with

E. Thermal exkansion In view af adsorption swelling and its dependence on temperature, the coefficient of linear thermal expansion of carbons has a definite meaning only when it is determined under vacuum. Experiments on the effect of temperature on the length of charcoal rods were carried out using the same extensometer, ft is found that the differential thermal co&icient of the charcoal and the brass of the extensometer is constant in the range of temperature between 25°C and 300°C. Figure 6 illustrates typical results for 2 rods of the same charcoal cut axially and radially. The absoiute temperature coefficients of thermal expansion of

i

a 500

600

7.

OF WOOD

CHARCOAL

569

charcoals corrected for the expansion of brass, are given in Table I. Rise of temperature of carbonization decreases the thermal expansion in agreement with the results of OKADA on different kinds of molded carbon rods.uQj The coefficients vary between 21.3 and 2.6 x 10-S OC-r as compared with 2.3-2.5 x IO-6 “C-x recorded for molded carbon rods prepared at 1000”C(2~) and 2.3 x 10-e ‘C-1 for graphite parallel to the direction of extrusion.(24) It is found that there is an approximate inverse proportionality between the coefficient of thermal expansion and Young’s modulus in agreement with Gruneisen formuIa : Thermal

expansion = y x C,/v,

where C, is the specific heat at constant volume, ZJ~the specific volume at absolute zero, y a

I

700 TEMPERATURE

FIG.

PROPERTIES

900

800

lOCKI

(“C 1

T@cal curves of the effect of temperature of carbonization on certain proper& of charcoal. Case of Charcoal t’.

R. I. RAZOUK,

570

F. Z. SALEEB

parameter which is usually a ‘small whole number and x is the compressibility (l/K) : x = -

I/v. du/dp

= 3 (I ;

2 pL) ,

where p is Poisson’s ratio. In absence of reliable values of CL,rough agreement The effect of temperature

is satisfactory. of carbonization

on

the various properties of charcoal is made clear in Fig. 7 which represents typical results for Charcoal

V.

4. CONCLUSION

The

simple

expansion

theory

of BANGHAM et al. for

elasticity of _ porous solids explains satisfactorily the experimental results obtained with a number of wood charcoals,

adsorption

and

even though the expansion adsorption

The specific surface area calculated on the basis of expansion data using methanol as adsorbate is independent of the anisotropy of the charcoal and agrees well with the value obtained from low temperature nitrogen adsorption. might

be anisotropic.

REFERENCES BANGHAM D. H. and FAKHOURY N., Proc. Roy. Sot. A130, 81 (1930). BANGHAMD. H. and FAKHOURYN., J. Chem. SOL 1324 (1931). BANGHAM D. H., FAKHOURY N. and MOHAMED A. F., Proc. Roy. Sot. A138, 162 (1932) ; A147, 152 (1934). BANGHAM D. H. and RAZOUK R. I., Proc. Roy. Sot. A166, 572 (1938).

and F. S. SAID

5. BANGHAM D.

H.

and

RAZOUK R.

I.,

Trans.

Faraday Sot. 33, 1459 (1937). 6. BANGHAMD. H. and MAGGS F. A. P., Proc. Conz on the Ultrafine Structure of Coals and Cokes, p. 118. B.C.U.R.A., London (1944). 7. HAINES R. S. and MCINTOSH R., J. Chem. Phys. 15, 28 (1947). 8. AMBERG C. H. and MCINTOSH R., Can. J. Chem. 30, 1012 (1952). 9. RAZOUK R. I. and EL-GOBEILY M. A., J. Phys. Colloid Chem. 54, 1087 (1950). 10. YATES D. J. C., Advances in Catabsis 9,481 (1957). 11. QUINN W. H. and MCINTOSH R., Proceedings of

the Second International Congress of surface Act&i&, Vol. 2, p. 122. Butterworths, London (1957). 12. YATES D. J. C., Proc. Roy. Sot. A224, 526 (1954). .13. , MEEHAN F. T., Proc. Roy. Sot. A115, 199 (1927). 1Lk.LAKHANPAL M. L. and FLOOD E.- A., ban. j. Chem. 35, 887 (1957). 15. FLOOD E. A. and HEYDING R. D., Can. J. Chem.

32, 660 (1954).

16. FLOOD E. A. and LAKHANPAL M. L., Proceedings of the Second International Congress of Surface Activity, Vol. 2, p. 131. Butterworths, London (1957).

l7 PICKETTG., J. Amer. Chem. Sot. 67, 1958 (1945). 18: ANDERSONR. B., J. Amer. Chem. Sot. 68,686 (1946). 19. DELLYESR., J. Chim. Phys. 60, 1008 (1963). 20. DEO A. V., KULKARNI S. B., GHARPUREY M. K. and BISWASA. B., Indian J. Chem. 2, 43 ( 1964). 21. MAGGS F. A. P., Trans. Faraday Sot. 42B, 284 (1946). 22 ANDREW J. F., OKADA J. and WOBSCHALLD. C., Proceedings of the Fourth Carbon Conference, p. 559. Pergamon Press, Oxford, (1960). Proceedings of the Fourth Carbon 23. OKADA J., Conference, p. 547. Pergamon Press, Oxford (1960). 24. BURDKICKM. D., ZWEIG B. and MORELANDR. E., J. Res. Nat. Bur. Standards 47, 35 (1951).