Effect of Carbonisate Particles on the Properties of Active Carbons

35

Effect of Carbonisate Particles on the Properties of Active Carbons

Bronislaw Buczek* AGH University of Science and Technology, Faculty of Energy and Fuels, al. Mickiewicza 30.

30-059 Cracow, Poland.

(Received 10 August 2012; revised form accepted 23 November 2012)

ABSTRACT: In this article, the effect of particle geometry on the properties ofactive carbons, which are obtained by treating steam with hard coal-tarcarbonisate, is reported. Different shapes of carbonisates were activated bysuperheated steam at 1123 K in a monolayer reactor with burn-off rates between45.3% and 72.0%. Steam activation leads to a faster development of ring-shapedparticles, and the kinetics of the process influence surface areaBrunauer–Emmet–Teller, surface mesoporosity and the micropore volume. Themicropore and mesopore structures of active carbons are evaluated usingnitrogen adsorption/desorption isotherms and the corresponding parameters ofthe Dubinin–Radushkevich equation are presented. Micropore-size distributionis calculated using the Horvath–Kawazoe method. The results of these texturalinvestigations showed that a more uniform micropore structure and bettermechanical properties are present in ring-shaped active carbon particles than thatwere reported earlier.

1. INTRODUCTION

Active carbons are produced by mild oxidation of chars or carbonisates, which are obtained fromvarious carbonaceous raw materials. The history and nature of the raw material, as well as theprocess conditions, determine the properties of the active carbon produced. Active carbons, inthe form of granules, extrudates, powders (Bansal and Goyal 2005), spheres (Banghel et al.2011) and cylindrical shapes (Wang et al. 2008), can be obtained from suitable precursors byeither physical or chemical routes. The final active carbon is produced by treating raw materialswith oxidizing gases. This reaction is mostly a two-stage process. In the first stage, i.e.carbonization, a solid residue with a very low adsorption capacity is obtained. In the secondstage, i.e. the process of physical activation, a carbonaceous adsorbent of an extended porousstructure is formed. It is to be noted that both stages of the production significantly affect thespecific area, as well as the degree of surface development, of the porous structure of activecarbon particles (Jankowska et al. 1991).

2. THEORETICAL BACKGROUND

The activation process involves a partial gasification of the carbonisate using gases containingoxygen atoms in their molecules. The carbonisate gradually reacts with the oxidizing gases, whichresults in the formation of gaseous products. The texture of the carbonizate and the reactionsurface are hence developed. Mixtures of oxygen, steam and carbon dioxide are used as oxidizing

*Author to whom all correspondence should be addressed. E-mail: [email protected] (B. Buczek)

agents. The activation energy of these gases indicates that they are useful oxidants. Of equalimportance is the comparison of the relative rates of reaction between carbon and O2, H2O, CO2,which significantly differ from each other (Table 1).

36 Bronislaw Buczek/Adsorption Science & Technology Vol. 31 No. 1 2013

TABLE 1. Activation and Dissociation Energies and the Relative Rate of the Reaction between Carbon andthe Oxidizing Substances (Machorin and Gluchomanjuk 1983)

Reaction of carbon Activation energy Dissociation Dissociation energy Reaction rate atwith different (kJ/mol) reaction (kJ/mol) 1073 K andoxidants 0.1 MPa

C + 1/2O2 = CO 209 ÷ 243 1/2O2 → O 246.9 1 × 105

C + H2O = CO + H2 334.8 H2O → H2 + O 485.5 3C + CO2 = 2CO 359.9 CO2 → CO + O 527.4 1

Because of its exothermic nature, the reaction of oxygen with carbon is out of control. Someareas are overheated and, as a result, faulty pores are formed, mainly on the outer surface of thecarbon particles. By contrast, oxidants such as steam and carbon dioxide require a supply ofexternal heat, which in turn diminishes the reaction rate. Endothermic reactions allow effectivecontrol of the activation process. Results of various experiments have shown that H2O and CO2

react with carbon at a moderate rate, i.e. at 1023–1223 K.It has been proposed that several factors such as viscosity, diffusion coefficient and the size of

molecules not only facilitate the transfer of steam into the interior of the carbonisate, but alsofacilitate the transfer of its reactive products away from the carbon dioxide. This should improvethe structure of the carbon within the entire volume of the sample, but the reaction rate, however,is three times more rapid for H2O than for CO2. This effect diminishes the kinetics region of thereaction. The compensation of both these effects is a probable cause of considerable differencesin the formation of the porous structure on the adsorbents when using both oxidizing agents(Machorin and Gluchomanjuk 1983). The basic reaction of the processes proceeding in thecarbon–steam system is the formation of carbon monoxide and hydrogen:

C + H2O ⇔ CO + H2 + 131.3 kJ/mol (1)

In addition, the following reactions were also observed:

C + 2H2O ⇔ CO2 + 2H2 + 90.2 kJ/mol (2)

C + CO2 ⇔ 2CO + 172.5 kJ/mol (3)

CO2 + H2 ⇔ CO + H2O + 41.2 kJ/mol (4)

The last reaction is called the water–gas shift reaction, or the carbon oxide conversion, and it isthe source of CO2 found in gaseous products. The equilibrium of reaction (4) does not involve anyvolume changes and therefore it does not depend on pressure. The relationship between thetheoretical equilibrium composition of the gas formed in reactions (1)–(4) and the temperature ispresented in Figure 1.

Effect of Carbonisate Particles on Active Carbons 37

50H2

CO

CO2

H2O

40

30

20

873 973

Temperature (K)

Con

cent

ratio

n (%

)

1073 1173 1273

10

0

Figure 1. Composition of the gas phase formed during the reaction of carbon with steam (Buczek 2010).

Reactions (1) and (4) are inseparable and proceed simultaneously when carbon is activatedwith steam. Under equilibrium conditions, the products contain both carbon oxide and hydrogen.The course of the reaction is temperature dependent. At temperatures higher than 1273 K, theproducts only contain hydrogen and carbon oxide, and therefore, the reaction between carbonand steam is described entirely by equation (1). When the temperature diminishes, theconcentrations of CO and H2 are in turn reduced and the contents of H2O and CO2 increase. Thisreaction (2) enables the formation of carbon dioxide and hydrogen at a temperature below 1083 K.The concentration of carbon dioxide is therefore greater than that of carbon oxide at temperaturesbelow 913 K.

Performing a thermodynamic analysis of the reaction between steam water and carbon makesit possible to find not only the direction of this process, but also the effect of the accompanyingand secondary reactions, the equilibrium of the gas composition, as well as the resulting thermaleffects. However, what is still unknown to us is the kinetics of the process, as well as the real ratios

between the components of the final products, which have resulted from the distance and theequilibrium of each individual reaction. The kinetics of the steam gasification of a carbonaceoussubstance consist of the following steps:

(i) Diffusion of water steam from a gas phase into the outer surface of the particles;(ii) Its diffusion in the pores of the particle;(iii) Sorption of water molecules at the inner surface;(iv) Proper chemical reaction;(v) Desorption of reaction products and(vi) Diffusion of reaction products from the surface to the gas phase.

Steps (i)–(vi) proceed either consecutively or simultaneously. If heat effects were to accompanythese steps, however, the reaction surface would rise and accordingly there will some increase inporosity. Taking into account the adsorbent shaping, it can be inferrred that the mechanism of thisprocess and its kinetics are extremely complex. As these processes unfold and, owing to the sizeof these particles, heat and mass transfers vary along the radial and axial directions. This alwaysresults in non-uniformity of physical and chemical properties within the particles (Buczek 1992;Robau-Sanchez et al. 2003).

The aim of this study was to assess the properties of active carbon obtained in the reaction ofsteam with the coal-tar carbonisate of cylindrical and ring-shaped particles, subjected to the sameconditions of the activation process.

3. MATERIALS AND METHODS

3.1. Preparation of the Carbonisate

The carbonisate was obtained by spinning a mixture of hard coal and wood tar, which is extrudedout using a 4.0-mm spinneret. The mixture was then dried at a temperature of up to 453 K andcarbonized at 873 K.

The technical and chemical analyses of the carbonisate are as follows: moisture content,Wa = 4.1; ash, Aa = 8.2; volatiles, Va = 14.7; carbon, Ca = 74.5; hydrogen, Ha = 2.17; sulphur, Sa

= 0.62; nitrogen, Na = 2.06. Mechanical processing of the carbonisate (using a spinneret) gave theparticles a cylindrical shape (4.0 × 4.0 mm). The particle rings had an inner diameter of 0.8 mmand an external size similar to that of the cylinders. The cylinder-shaped carbonisate particles weredenoted as BD0 and the ring-shaped particles as were denoted as D0.

3.2. Water Steam Activation of BD0 and D0 Carbonisates

The activation process was carried out in a monolayer reactor, whose schematic plan is presentedin Figure 2. The process conditions were as follows: flow of superheated steam 0.01556 g/cm2 at1123 K and at various time intervals, for which burn-off rate varied in the range of 45.3–72.0%.Steam activation of the differently shaped carbonizates gave two series of active carbons, namelyBD1–BD4 and D1–D4. Figure 3 presents the kinetics of the steam activation for both the activecarbon particles. Figure 4 shows the changing external surface ring and the cylinder-shaped activecarbons, as observed for the same process conditions. High levels of erosion of the externalsurfaces of cylinder-shaped particles can have damaging effects on the mechanical and otherproperties.


4. RESULTS AND DISCUSSIONS

The porous structure of the stem activation products was analyzed by low-temperature nitrogenadsorption–desorption isotherm. The isotherm was determined by the volumetric method using aSorptomatic 1900 apparatus. The measurements were performed at 77.5 K relative pressures p/p0

between 0.00001 and 0.999.


5 2 3 4

1 7 6 9 8

~220V

~220V

10

~22

0V

Figure 2. Monolayer reactor used for the steam activation process. The numbers in the figure denote the following:1 = reactor; 2 = cooling space; 3 = sample pre-heating; 4 = connecting channel; 5 = entry samples; 6 = furnace; 7 = basket;8 = temperature control and recording; 9 = temperature metre; 10 = gas mixer.

75

70

65

60

55

50

451500 1800 2100 2400 2700

Time (s)

Bur

n-of

f (%

)

3000 3300 3600

80

Figure 3. Influence of time on the burn-off active carbons with ring ( ) and cylinder ( ) shapes.

All obtained isotherms (Figures 5 and 6) show the Langmuir character of rising adsorption inthe low pressure ranges, as well as the appearance of a hysteresis loop, which proves that themicropores and, to a lesser degree the mesoporous structure, were developed under activationconditions.

Dubinin–Radushkevich equation was applied to the isotherms obtained to determineparameters characterizing the microporous structure (W0, B) and the characteristic adsorptionenergy (E0) (Dubinin 1979). Using the known micropore volume and the amount of adsorbed


Figure 4. Comparison of D2 (left side) and BD2 (right side) active carbons with similar burn-offs.

400

350

300

Ads

orpt

ion,

a (

cm3 /

g N

TP

)

250

200

150

100

50

00 0.2 0.4 0.6

Relative presure, p/p0

0.8 1

BD1-adsBD1-desBD2-ads

BD2-desBD3-adsBD3-des

BD4-adsBD4-desBD0-ads

Figure 5. Adsorption–desorption isotherms for active carbons of the BD series.

nitrogen, a formal value of micropore surface area (SDR) can be calculated. To determine themicropore volume from the nitrogen adsorption isotherm, the Horvath and Kawazoe method(Horvath and Kawazoe 1983) was used and from its distribution, a half-slit-pore size (xHK) wasdetermined. The surface area of the mesopores (Sme) was calculated using the Dollimore–Healmethod (Dollimore and Heal 1964). The pore volume (Vp) was calculated as an amount ofadsorbed nitrogen with p/p0 = 0.98 and specific surface area (SBET) calculated from theBrunauer–Emmet–Teller (BET) equation (Roque-Malherbe 2007) and Sing (1995). The resultsof the analyses and calculations, along with the BET data and pore volume, are summarized inTable 2.


TABLE 2. Microporous Structure and Mesopore Surface Area of the Active Carbons

Active carbon W0 B × 106 E0 SDR xHK Sme SBET Vp

cm3/g K−2 (kJ/mol) (m2/g) (nm) (m2/g) (m2/g) (cm3/g)

BD0 a – – – – – – 14 0.009BD1 0.256 0.658 23.6 715 0.463 45 595 0.303BD2 0.313 0.850 20.8 881 0.475 77 765 0.408BD3 0.346 1.015 19.0 976 0.485 111 837 0.863BD4 0.368 1.236 17.2 1035 0.495 158 927 0.546D0 a – – – – – – 11 0.006D1 0.282 0.785 21.6 795 0.486 49 636 0.332D2 0.313 0.836 20.9 883 0.487 72 779 0.393D3 0.320 1.029 18.9 902 0.493 96 812 0.419D4 0.374 0.180 17.6 1054 0.485 150 962 0.547

a Non-microporous coal carbonizates.

400

350

300

Ads

orpt

ion,

a (

cm3 /

g N

TP

)250

200

150

100

50

00 0.2 0.4 0.6

Relative presure, p/p0

0.8 1

D1-adsD1-desD2-ads

D2-desD3-adsD3-des

D4-adsD4-desD0-ads

Figure 6. Adsorption–desorption isotherms for the active carbons of the D series.

With an increase in burn-off, a decrease in the value of the characteristic energy, E0, occurs forboth the D and the BD series of the activation products. This decrease is linked to the increasingsize of the micropore (xHK), as seen predominantly with the BD series. This is furthermoreconfirmed by the changes in volume distribution of the micropores, as a result of their size. Forthe BD series, a fall in the maximum value and a broadening of the curve occur as the volume ofbigger pores increases (Figure 7).


7.00

∆V/∆

d (c

m3 n

m−1

g−1 )

6.00

5.00

4.00

3.00

2.00

1.00

0.000.4 0.5 0.6

XHK (nm)

0.7 0.8 0.9 1

Figure 7. Micropore distribution for BD1 (solid line) and BD4 (dotted line) active carbons.

In the case of the D-product series, only the volume of micropores decreases, whereas their size(xHK) remains approximately the same. This demonstrates that a more uniform microporousstructure can be obtained as a result of the activation of the ring-shaped particles.

5. CONCLUSIONS

These studies show how the shape and the dimensions of the particles affect the properties of theactive carbon obtained from the reaction of steam with a coal-tar carbonisate. Steam activation causesa more rapid development of the texture of the ring-shaped particles, increasing the kinetics, as wellas the specific surface area of the mesopores. The dimensions of the micropores in the ring-shapedparticles of variable burn-off remain practically unchanged. This corresponds to the sieve molecularproperties of the active carbons and thus proves that the course of the reaction is almost symbiotic tothe kinetics region. Moreover, the greater the contribution of the volume of micropores, the greaterthe adsorption capacity of small molecular adsorbates (N2). The ring-shaped active carbons havebetter mechanical resistance (Figure 4) than the cylinder-shaped active carbons. Because of thedifferent shapes of the particles, the pressure drop in the gas flow, as it passes through the ring-shapedbed in an industrial adsorber, will probably be lower than that for cylindrical particles.

ACKNOWLEDGEMENTS

The author is grateful to the AGH University of Science and Technology (Project No.11.11.210.244) for the financial support of this work. The author is also grateful to Professor A.S. Wronski for his helpful suggestions and rewriting of this work.

REFERENCES

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Stanley-Wood and R.W. Lines, editors, The Royal Society of Chemistry, Cambridge, U.K.Buczek, B. (2010) Effect of shape particle char on properties of active carbon [Wplyw ksztaltu ziarna

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Effect of Carbonisate Particles on the Properties of Active Carbons