granulated sorbents from wood waste

7
ISSN 0361-5219, Solid Fuel Chemistry, 2007, Vol. 41, No. 2, pp. 100–106. © Allerton Press, Inc., 2007. Original Russian Text © I.N. Malikov, Yu.A. Noskova, M.S. Karaseva, M.A. Perederii, 2007, published in Khimiya Tverdogo Topliva, 2007, No. 2, pp. 46–53. 100 Depending on the fields of application, carbon sor- bents (active carbons) are manufactured in the follow- ing three forms: granulated, crushed, and powdered [1]. Granulated carbon sorbents are highest quality and most expensive sorbents, which are used in moving-bed adsorption processes. These processes impose rigid requirements on the strength of the sorption material, which is subjected to increased abrasion in the course of operation. The main method for the commercial pro- duction of granulated active carbons is the traditional extruder granulation of coal dust or coal-char dust with binding agents. Wood tar and coal tar are most com- monly used, either individually or as mixtures in certain ratios, as the binding agents [2]. However, the limited resources and the expensiveness of these tars adversely affect the cost efficiency of this process for the prepara- tion of active carbon. The technology for the preparation of granulated sorbents from wood wastes involves a number of stages, including the preparation and carbonization of a raw material and the activation and cooling of the fin- ished product [3]. To obtain a molded material, addi- tional stages of the pulverization of a carbonized raw material, the granulation of carbon dust with binding agents to produce granules of a required size, and the drying and thermal treatment of the granules are intro- duced between the carbonization and activation. Figure 1 shows the flow diagram of the manufacture of a gran- ulated sorbent based on wood waste. The quality of a granulated sorbent not only depends on the conditions of carbonization and activation stages, as in the manufacture of crushed sorbents, but also essentially depends on the granulation and drying parameters: the degree of grinding of the starting mate- rial, the concentration of a binding agent, and the dry- ness factor of the resulting granules. The stage of gran- ulation involves both coal preparation and proper gran- ulation. The preparation consists in coal grinding to a degree that provides good contact between dust and a binder, which is responsible for the strength properties of the granulated sorbent: a low degree of grinding of the material impairs its granulation ability because of a decrease in the adhesion of individual particles due to an insufficient contact surface, whereas an extremely high degree of grinding results in a dramatic decrease in dust wettability, which impairs its granulation ability. The degree of grinding is responsible for the density of the resulting granules and, finally, for the pore structure and the strength of the sorbent. The direct granulation of wood wastes cannot be performed because of, on the one hand, the impossibil- ity of finely crushing them and, on the other hand, the high concentrations of volatile substances and high moisture contents. In the subsequent thermal treatment of granules from a moist starting material, great vol- Granulated Sorbents from Wood Waste I. N. Malikov a , Yu. A. Noskova b , M. S. Karaseva b , and M. A. Perederii b a Shakhty Institute, South Russia State University, Shakhty, Rostov oblast, Russia b Institute of Fossil Fuels, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119071, Russia e-mail: [email protected] Received July 20, 2006 Abstract—An integrated study of various wood-utilization wastes as raw materials for the production of car- bon adsorbents was performed. A technology was developed, and granulated sorbents from various wood wastes were prepared and characterized. The regularities of pore-structure formation and the physicochemical properties of sorbents were found depending on the used raw material, binding agents, granulation conditions, and the process parameters of the thermal treatment of granules. DOI: 10.3103/S0361521907020085 W ood waste Crushing Carbonization Grinding Sieving Granulation Drying Pyrolysis Activation Cooling Sieving Packaging Sorbent Fig. 1. Flow diagram of the production of granulated sorbents.

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Page 1: Granulated sorbents from wood waste

ISSN 0361-5219, Solid Fuel Chemistry, 2007, Vol. 41, No. 2, pp. 100–106. © Allerton Press, Inc., 2007.Original Russian Text © I.N. Malikov, Yu.A. Noskova, M.S. Karaseva, M.A. Perederii, 2007, published in Khimiya Tverdogo Topliva, 2007, No. 2, pp. 46–53.

100

Depending on the fields of application, carbon sor-bents (active carbons) are manufactured in the follow-ing three forms: granulated, crushed, and powdered [1].Granulated carbon sorbents are highest quality andmost expensive sorbents, which are used in moving-bedadsorption processes. These processes impose rigidrequirements on the strength of the sorption material,which is subjected to increased abrasion in the courseof operation. The main method for the commercial pro-duction of granulated active carbons is the traditionalextruder granulation of coal dust or coal-char dust withbinding agents. Wood tar and coal tar are most com-monly used, either individually or as mixtures in certainratios, as the binding agents [2]. However, the limitedresources and the expensiveness of these tars adverselyaffect the cost efficiency of this process for the prepara-tion of active carbon.

The technology for the preparation of granulatedsorbents from wood wastes involves a number ofstages, including the preparation and carbonization of araw material and the activation and cooling of the fin-ished product [3]. To obtain a molded material, addi-tional stages of the pulverization of a carbonized rawmaterial, the granulation of carbon dust with bindingagents to produce granules of a required size, and thedrying and thermal treatment of the granules are intro-duced between the carbonization and activation. Figure

1 shows the flow diagram of the manufacture of a gran-ulated sorbent based on wood waste.

The quality of a granulated sorbent not only dependson the conditions of carbonization and activationstages, as in the manufacture of crushed sorbents, butalso essentially depends on the granulation and dryingparameters: the degree of grinding of the starting mate-rial, the concentration of a binding agent, and the dry-ness factor of the resulting granules. The stage of gran-ulation involves both coal preparation and proper gran-ulation. The preparation consists in coal grinding to adegree that provides good contact between dust and abinder, which is responsible for the strength propertiesof the granulated sorbent: a low degree of grinding ofthe material impairs its granulation ability because of adecrease in the adhesion of individual particles due toan insufficient contact surface, whereas an extremelyhigh degree of grinding results in a dramatic decreasein dust wettability, which impairs its granulation ability.The degree of grinding is responsible for the density ofthe resulting granules and, finally, for the pore structureand the strength of the sorbent.

The direct granulation of wood wastes cannot beperformed because of, on the one hand, the impossibil-ity of finely crushing them and, on the other hand, thehigh concentrations of volatile substances and highmoisture contents. In the subsequent thermal treatmentof granules from a moist starting material, great vol-

Granulated Sorbents from Wood Waste

I. N. Malikov

a

, Yu. A. Noskova

b

, M. S. Karaseva

b

, and M. A. Perederii

b

a

Shakhty Institute, South Russia State University, Shakhty, Rostov oblast, Russia

b

Institute of Fossil Fuels, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119071, Russiae-mail: [email protected]

Received July 20, 2006

Abstract

—An integrated study of various wood-utilization wastes as raw materials for the production of car-bon adsorbents was performed. A technology was developed, and granulated sorbents from various woodwastes were prepared and characterized. The regularities of pore-structure formation and the physicochemicalproperties of sorbents were found depending on the used raw material, binding agents, granulation conditions,and the process parameters of the thermal treatment of granules.

DOI:

10.3103/S0361521907020085

Wood waste

Crushing Carbonization Grinding

Sieving Granulation Drying Pyrolysis Activation

Cooling Sieving Packaging

Sorbent

Fig. 1.

Flow diagram of the production of granulated sorbents.

Page 2: Granulated sorbents from wood waste

SOLID FUEL CHEMISTRY

Vol. 41

No. 2

2007

GRANULATED SORBENTS FROM WOOD WASTE 101

umes of the released vapor–gas products break a gran-ule, which has not managed to acquire strength proper-ties. Therefore, it is technologically reasonable toinclude the stage of the preliminary carbonization of astarting material followed by crushing and the granula-tion of raw-coal dust.

The raw waste lumber, a mixture of various woodspecies or barks, was carbonized in an electricallyheated stationary furnace under the following condi-tions, which were found in thermogravimetric studies:a heating rate of

10°

K/min, a final temperature of

700°ë

, and an isothermal exposure for 30 min. Theresulting raw charcoal was crushed in a jaw crusher,pulverized in a vibrating mill (the dust particle size was<100

µ

m), and thoroughly mixed with binding agentsin certain ratios. The resulting mixture was granulatedin a laboratory extruder-type screw granulator equippedwith an electric drive, two cutters (fixed and movable),and a spinneret with 4.5 mm orifices.

Sulfite waste liquor (hydrolysis waste; the class oflignosulfonates), which possesses surface-active andbinding properties, as a commercial 54% aqueous solu-tion (OST 81-79-74) and low-sulfur petroleum pitch(the solid residue of catalytic cracking, which exhibitssintering properties in thermal treatment) with a soften-ing temperature of ~140

°

C were used as binding

agents. The pitch and raw charcoal were powderedbefore mixing. For the better granulation of the result-ing mixture of dust materials, a certain amount of waterwas added, and the contents were stirred to a paste-likestate.

The compositions of raw charcoal and bindingagents were optimized in preliminary experiments inorder to prepare a maximally strong sorbent at a highporosity of granules. As a result, the compositions ofraw granules were prepared for the subsequent thermaltreatment. Table 1 summarizes the composition of theraw granules.

The stage of drying raw granules affects the subse-quent processes and the quality of the sorbent: a highmoisture content of granules subjected to pyrolysis canadversely affect the strength and porosity of the carbon-izate. It was found that an optimum moisture content ofgranules is

W

10%; the raw granules were dried tothis residual moisture content before thermal treatment.Next, the granules were subjected to pyrolysis fordecomposing the binding agents and strengthening thegranules to be activated.

Pyrolysis was performed in a fixed-bed reactor as aquartz retort heated in a vertical electric oven with auto-matically regulated heating rate and final carbonizationtemperature. A weighed portion (~100 g) of dried gran-ules was placed in a zone with a stable temperature,which was monitored at two points. The vapor–gasproducts formed in the course of carbonization wereremoved from the reaction volume to a condensationsystem using a constant flow of air. The condensationsystem consisted of a water cooler, a receiver for liquidproducts, and a gas meter. The conditions of carboniza-tion were refined at various final temperatures but equalheating rates of ~5–7 K/min. The conditions of pyroly-sis were refined (with sample SDG) in the range 600–800

°

C in order to determine the temperature conditionsthat provide enhanced degradation of both a woodmaterial and binding agents. Table 2 summarizes theprocess conditions and the quality indices of thermallytreated granules.

Table 1.

Sample designations and the composition of rawgranules

Waste, sample

Composition of raw granules, wt %

raw coal sulfite waste liquor

petroleum pitch

Wood mixture, SDG

80 10 10

Bark mixture, 1SKG

80 10 10

Bark mixture, 2SKG

60 40 –

Table 2.

Conditions of pyrolysis and the properties of thermally treated granules

Sample,experiment

no.

Conditions Carbonizate quality index

T

f

,

°

C time, min

χ

p

, g/cm

3

Π

, %

V

Σ

, cm

3

/g

W

s

, cm

3

/g

V

ma

, cm

3

/g

SDG-1 600 120 0.23 68.1 0.93 0.10 0.83

SDG-2 700 140 0.36 82.4 0.84 0.09 0.75

SDG-3 800 160 0.42 96.2 0.64 0.05 0.59

1SKG-1 700 140 0.39 94.0 0.89 0.09 0.80

1SKG-2 800 160 0.44 98.4 0.68 0.07 0.61

2SKG-1 700 140 0.34 78.2 0.62 0.09 0.53

2SKG-2 800 160 0.38 79.1 0.59 0.08 0.51

Note:

T

f

is the final temperature of carbonization;

χ

p

is the packed density;

Π

is the abrasion strength;

V

Σ

is the total pore volume;

W

s

isthe sorption pore volume; and

V

ma

is the macropore volume.

Page 3: Granulated sorbents from wood waste

102

SOLID FUEL CHEMISTRY

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No. 2

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MALIKOV et al.

Using wood granules as an example, we found that600

°

C is an insufficient temperature for the develop-ment of the strength properties of granules. In thepyrolysis of all of the test samples of granulated mate-rials, an increase in the treatment temperature wasaccompanied in an increase in the density of the mate-rial and a decrease in reactivity (Table 2). At the sametime, differences in the strength and porosity of ther-mally treated granules depend to a greater extent on thebinding agent rather than the starting material. Woodand bark granules prepared with the use of the samebinders were similar in terms of these characteristics,whereas thermally treated bark granules with differentbinder compositions were considerably different: thestrength of granules with sulfite waste liquor (with nopitch added) was much lower than the strength of theother granules.

The products of the pyrolysis of all of the three sam-ples at 800

°

C exhibited the highest strength character-istics; however, a considerable shrinkage of the mate-rial was found, which was accompanied by an increasein the packed density; a decrease in the porosity; and,as a consequence, a decrease in the reactivity of gran-ules. Higher temperatures or longer activation timeswere required for the development of the pore structureof these granules.

The experimental results allowed us to determine anoptimum temperature of pyrolysis, at with the strongestand most reactive granulated carbonizate can beobtained from the mixtures of various wood species andvarious barks: a pyrolysis temperature of 700

°

C at aheating rate of

10°

K/min. Three experimental batchesof thermally treated granules (samples SDG-2,

1SKG-2, and 2SKG-2) were produced under the opti-mum conditions of all of the process stages in order torefine the activation process.

The activation conditions for granulated carbon-izates from the experimental batches were refined in anelectrically heated fluidized-bed quartz reactor at a con-stant ratio between steam and an inert gas and at an acti-vation temperature of 700

°

C but at various activationtimes. To study the course of the process, sampleSDG-2 was progressively activated at a constant tem-perature for 30, 60, 90, and 120 min. Table 3 summa-rizes the activation conditions of the carbonized gran-ules of three test samples and the properties of theresulting granulated sorbents. For comparison, thequality indices of commercial active carbons (AG-3and SKT) are also given in Table 3.

The results of the progressive activation of woodgranules (SDG-2) allowed us to determine that a 30-min treatment resulted in an additional pyrolysis ofgranules accompanied by strengthening; in this case,the granules were activated to an insignificant extent. Ata maximum activation time of 120 min (a 61% degreeof burn-off), the structure was developed to a very deepextent. This resulted in the degradation of a system ofsorption pores with a transformation to macropores anda sharp decrease in the strength of the resulting sorbent.These results allowed us to exclude both minimum andmaximum activation times in the development of acti-vation conditions for the other two samples. Figures 2and 3 graphically illustrate the results of the progressiveactivation of sample SDG-2.

The above data indicate that the course of the activa-tion process is independent of both the type of the start-

Table 3.

Results of the activation of carbonized granules from wood wastes

Sample,experiment

no.

Activation time, min

α

*, %

χ

p

, g/cm

3

Π

, %Pore volume, cm

3

/g

V

Σ

W

s

V

ma

SDG-2 Carbonizate – 0.36 82.4 0.84 0.09 0.75

1 30 14 0.31 87.2 1.00 0.17 0.83

2 60 30 0.25 83.1 1.13 0.27 0.86

3 90 47 0.19 75.2 1.69 0.35 1.45

4 120 61 0.14 60.9 1.95 0.28 1.67

1SKG-2 Carbonizate – 0.39 94.0 0.89 0.09 0.80

1 30 18 0.32 96.5 1.05 0.16 0.89

2 60 26 0.29 84.3 1.26 0.29 0.97

3 90 44 0.22 72.4 1.35 0.33 1.02

2SKG-2 Carbonizate – 0.34 78.2 0.62 0.09 0.53

1 60 21 0.27 74.4 0.98 0.29 0.69

2 90 41 0.20 65.3 1.46 0.44 1.02

AG-3 – – 0.24 75.0 0.92 0.32 0.60

SKT – – 0.55 71.0 0.75 0.45 0.30

* Degree of carbon burn-off upon activation.

Page 4: Granulated sorbents from wood waste

SOLID FUEL CHEMISTRY

Vol. 41

No. 2

2007

GRANULATED SORBENTS FROM WOOD WASTE 103

ing material and the binder used for the production ofgranules, and it is analogous to the course of this pro-cess in the production of crushed sorbents [3]. The acti-vation limit is a treatment time taken to obtain a sorbentwith the most favorable combination of a sufficientlyhigh strength and a maximally possible volume of sorp-tion pores. For all of the three samples, this time variedwithin the limits 60–90 min. In the case of activation for90 min, the pore volume was maximally developed;however, a decrease in the strength was considerable.Therefore, either a lower or an upper limit of the recom-mended activation time interval should be chosendepending on the purpose of the sorbent (requirementsimposed on strength and porosity). Samples SDG and1SKG, which were prepared by activation over thespecified time interval, were analogous to the commer-cial active carbon AG-3 in terms of quality indices.

The sorbent prepared with the use of only sulfitewaste liquor as a binding agent (sample 2SKG) exhib-ited unique characteristics. The strength of granules inthis sample was lower than that in the other two sam-ples because of the absence of a sintering component(pitch) from the binding agent. The strength of thissample dramatically decreased upon activation for 90min; however, the system of sorption pores was deeply

developed in this case. Because of this, the structure ofsorption pores in the resulting sorbent was analogous tothe structure of the commercial active carbon SKT-3,which is one of the most expensive and highest quality(although its strength is not high) domestic active car-bons.

Under the optimum conditions found for all of thestages of the process, the experimental batches of gran-ulated sorbents were produced from the mixtures ofvarious wood species and various barks for extensivetests in order to determine the promising areas of theirapplication. The process was performed in accordancewith the above multistage procedure: the carbonizationof raw materials to obtain raw coal was performed in astationary pyrolysis unit; the production of granuleswas performed in an extruder-type granulator; thepyrolysis of raw granules was performed in a three-chamber rotary muffle furnace; and the activation wasperformed in a fluidized-bed furnace. With consider-ation for the results of the production of experimentalbatches of crushed sorbents from wood wastes [3], theduration of activation was 15 min for all of the samples.

We studied the quality indices, technical analyses,and elemental compositions of the resulting sorbents(SDG, 1SKG, and 2SKG) from the experimental

0.3

0.2

0.1

0 30 60 90 120

Activation time, min

60

40

20

0

Packed density, g/cm

3

Degree of burn-off, %

1

2

Fig. 2.

Dependence of the (

1

) degree of burn-off and (

2

)packed density of a sorbent on activation time.

0.4

0.3

0.2

0.1

0 30 60 90 120

Activation time, min

2.4

2.0

1.6

1.2

0.8

W

s

,

cm

3

/g

V

Σ

,

cm

3

/g

1

2

Fig. 3.

Dependence of the (

1

) total pore volume and (

2

)sorption pore volume on activation time.

Table 4.

Preparation conditions and quality indices of granulated sorbents

No. Sample

Process conditionsQuality index

pyrolysis activation

T

,

°

C

υ

, K/min

T

f

,

°

C

τ

, min

χ

p

, g/cm

3

Π

, %

V

Σ

, cm

3

/g

W

s

, cm

3

/g

V

ma

, cm

3

/g

1 SDG 700 5–7 700 15 0.23 78.0 1.25 0.32 0.86

2 1SKG 700 5–7 700 15 0.25 77.3 1.30 0.31 0.99

3 2SKG 700 5–7 700 15 0.22 70.2 1.37 0.36 1.01

4 AG-3 – – – – 0.24 75.0 0.92 0.32 0.60

5 SKT – – – – – 66.0 0.90 0.44 0.46

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MALIKOV et al.

batches. Table 4 summarizes the preparation conditionsand quality indices of the sorbents. It can be seen thatthe sorbents meet the requirements imposed on com-mercial active carbons, which were taken as analogs, interms of quality indices (strength and porosity). Thegranulated sorbents from a mixture of oak and pinewoods (SDG) and a mixture of barks (1SKG) exhibitedstrengths higher than that of the commercial active car-bon AG-3 and pore structures similar to that of AG-3.The sorbent 2SKG from a mixture of barks exhibited astrength higher than that of the commercial active car-bon SKT; however, it ranked below SKT in the limitingvolume of sorption space. In this case, all of the sor-bents in the experimental batches from wood wastesexhibited higher macropore volumes than those of theircommercial analogs.

We monitored changes in the technical analyses andelemental compositions of the granulated sorbents ongoing from parent to carbonized (and then to activated)granules using a single sample (1SKG) becausechanges in the composition of the base material (woodor bark) at these stages of treatment were found previ-ously in the production of crushed sorbents [3]. In thiscase (Table 5), the transformations of binding agents,which were the constituents of granules, were consid-ered in order to detect their adverse effect on the qualityof sorbents.

In Table 5, it can be seen that, in the course of thestages of thermal treatment on changing from raw tocarbonized and then activated granules, the sulfur andnitrogen contents of the granules decreased. After thefinal stage (activation), the concentrations of sulfur andnitrogen corresponded to the concentrations of thesecomponents in a crushed sorbent from bark. This can beexplained by the fact that sulfur and nitrogen, whichwere introduced with binding agents, were removed

with the volatile products of the thermal treatment ofgranules. The ash content of the granulated sorbent wassomewhat higher than that of the crushed sorbent.However, the overall picture of changes in the technicalanalyses and elemental compositions in the course ofthe production of granulated sorbents remained thesame as in the production of crushed sorbents: on goingfrom a starting raw material to a carbonizate (and thento a sorbent), volatile, hydrogen, and oxygen contentsdecreased, whereas ash and carbon contents increased.The high ash content of granulated sorbents from barkrestricts their areas of application excluding food indus-try, alcoholic beverage industry, pharmaceutical indus-try, etc., where the possible washout of sorbent ash intosolutions can adversely affect the quality of the result-ing products.

The granules undergo compaction and shrinkage inthe course of thermal treatment at all of the stages.Because of this, the granule size can vary considerably.The extruder method allows us to obtain cylindricalgranules; in this case, the cylinder diameter is regulatedby the size of spinneret orifices in the granulator,whereas the length is specified in accordance with therequirements of the adsorption process in which the fin-ished sorbent will be used. The fractional compositionis given in terms of granule diameter as the geometricparameter most strongly changed in the course of ther-mal treatment. With the use of sample SDG as an exam-ple, we studied changes in the fractional composition ofgranules at all of the stages of the production of granu-lated sorbents (Table 6).

In the course of the preparation of the granulatedsorbent, the apparent size of sorbent granules notice-ably decreased at the stages of thermal treatment andactivation. In parent granules, the fraction of size 3.0–4.5 mm was 96% and granules smaller than 1 mm wereabsent, whereas the diameter of granules graduallydecreased after thermal treatment (pyrolysis and activa-tion), and a representative fraction of size 2–3 mmappeared. It is likely that smaller fractions were mainlyformed because of abrasion rather than shrinkage inthermal treatment processes. The tentatively deter-mined yield of the initial fraction of granules thatpassed all of the stages of thermal treatment was ~53%;this provides an opportunity to specify roughly thespinneret diameter for granulation based on the knowngranule size of the finished product.

Table 5.

Technical analysis and elemental composition of granules at the stages of treatment

Stage ofgranule

treatment

Technical analysis, % Elemental composition (daf), wt%

W

a

V

daf

A

d

C H N O

d

Raw material 6.9 40.9 18.2 65.5 3.2 0.2 0.7 30.4

Carbonizate 6.4 32.7 21.1 71.9 1.1 0.1 0.6 26.3

Sorbent 3.2 22.5 33.2 79.9 0.8 <0.01 0.2 19.1

Std

Table 6.

Changes in the fractional composition of granulesin the preparation of sorbents

GranulesFraction (mm), %

<1 1.0–2.0 2.0–3.0 3.0–4.5

Initial 0 1 3 96

Thermally treated 0 7 25 68

Activated 2 11 36 51

Page 6: Granulated sorbents from wood waste

SOLID FUEL CHEMISTRY

Vol. 41

No. 2

2007

GRANULATED SORBENTS FROM WOOD WASTE 105

We also evaluated the structure parameters andadsorption characteristics of the experimental batchesof granulated sorbents from wood-utilization wastes inorder to determine the priority areas of their applica-tion.

The structure parameters of the sorbents were stud-ied by the processing of the isotherms of nitrogenadsorption on these samples at 77 K. The adsorptionisotherms were measured on a Gravimat 4303 auto-mated vacuum gravimetric unit with a beam microbal-ance with a sensitivity of 1 µg at a sample weight of nohigher than 1 g [4]. The BET specific surface areas, themesopore specific surface areas Sme, the limitingadsorption in micropores W0, the characteristic energyof adsorption E0, and the micropore halfwidth x0 werecalculated from the isotherms. Samples SDG and2SKG of the carbon sorbents most strongly different inporosity were taken for this study. The experimentalresults (Table 7) indicate that the granulated wood-based sorbents exhibit a mixed pore structure. Thebark-based sorbent (2SKG) exhibited more developedmicroporosity and mesoporosity and a smallermicropore diameter than those of the wood-based sor-bent (SDG).

The sorbents were tested using the molecular-probemethod [5] in order to determine the adsorption capac-ities for various adsorbates (Table 8). A typical set of

substances from phenol (with a small molecular size) tomethylene blue, which consists of bulky globules, wasused. All of the substances are standard compounds inthe determination of sorbent quality, and they simulatethe typical categories of organic pollutants in industrialwastewater.

Data on the adsorption of substances with differentmolecular sizes from aqueous solutions on granularsorbents provide an opportunity to classify these sor-bents with respect to their purpose by analogy withcommercial activated carbons with similar or compara-ble pore-structure parameters. Both of the sorbentsexhibited close capacities for both substances withsmall molecules (phenol, benzene, and iodine) andmethylene blue with a very large molecular size. Thisfact suggests a wide pore-size distribution in the struc-ture of granulated sorbents, and it is responsible for awide range of their adsorption properties from the sep-aration and purification of gas mixtures (by analogywith the commercial activated carbon AG-3) to therecuperation of hydrocarbon vapors.

We tested the sorbent 2SKG in the trapping of thehydrocarbon vapor of gasoline fractions (gasolineextraction from natural gas) in a setup, which wasdescribed elsewhere [6]. We used n-heptane as a hydro-carbon that simulated the ë7 gasoline fraction in amodel mixture with natural gas. The concentrations ofn-heptane in a gas at the adsorber inlet and outlet weredetermined by chromatography. In the course of tests(Table 9), we determined the dynamic adsorptioncapacity of a sorbent bed for heptane vapor to a break-through concentration, which was equal to 10% of theinitial concentration. For comparison, Table 9 showsthe results obtained with the best imported silica gelKC-Trockenperlen H. These data indicate that theadsorption capacity (per unit weight) of the granulatedcarbon sorbent 2SKG for n-heptane vapor is higherthan that of the reference silica gel by a factor of almost

Table 7. Pore-structure parameters of crushed and granulat-ed sorbents

Sorbent

Pore-structure parameters

SBET,m2/g

Sme,m2/g

W0,cm3/g

E0,kJ/mol

x0, nm

SDG 550 140 0.30 22.4 0.98

2SKG 680 240 0.32 25.2 0.78

Table 8. Adsorption capacities of granulated sorbents for reference substances

Sorbent Total porevolume, cm3/g

Adsorption capacity for

benzene, cm3/g phenol, mg/g iodine, % methylene blue,mg/g

SDG 1.25 0.32 45 63.6 230

2SKG 1.37 0.36 56 71.3 265

Table 9. Gasoline extraction from natural gas on the sorbent SKG

Sorbent Average gasflow rate, m3/h

Concentration ofn-heptane vapor inthe initial gas, g/m3

Breakthroughtime, h

Dynamic adsorption capacity for n-heptane

g/(100 g) g/(100 ml)

2SKG 20 3.42 1.5 21.4 5.1

Silica gel* 19.8 2.60 5.6 7.7 5.4

* KC-Trockenperlen H silica gel.

Page 7: Granulated sorbents from wood waste

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SOLID FUEL CHEMISTRY Vol. 41 No. 2 2007

MALIKOV et al.

3. However, the dynamic adsorption capacity per unitvolume of the sorbent was somewhat lower in the car-bon sorbent than in the silica gel because of a lowpacked density of the carbon sorbent.

The pore structure of the sorbents is responsible forthe absorption of substances from gas or liquid phasesdue to van der Waals forces. The case in point is physi-cal adsorption, which is determined by the characteris-tic adsorption energy (E0), which depends on the porestructure of the sorbent. In the absorption of a numberof substances (e.g., metal ions), the pore structure of thesorbent does not play the most important role. In thiscase, the chemical nature of the surface, which dependson the presence of base and acid oxides, is of para-mount importance. The absorption of substances fromsolutions due to bonding by surface oxides occurs by anion-exchange mechanism and depends on the characterand amount of these oxides in the sorbent. As a charac-teristic of the ion-exchange activity of a sorbent, thestatic exchange capacity for barium ions, which isdetermined by a chemisorption method, was taken.

To study the efficiency of the test sorbents in theextraction of metal ions from aqueous solutions, wealso evaluated their sorption capacity for iron, which isthe most widespread impurity in natural water. For thispurpose, a sorbent sample was places in a solution ofFe(SCN)3 with a fixed iron concentration, and the con-tents were stirred for a specified time. Thereafter, theresidual concentration of Fe was determined by photo-colorimetry and the sorption capacity of the sorbent foriron ions was calculated. Table 10 summarizes theresults of the measurements of the static exchangecapacities of sorbents for barium ions and the sorptioncapacities for iron ions. As can be seen, the granulatedcarbon sorbent from wood is characterized by a higherexchange capacity and sorption capacity for Fe ions,

whereas the granulated carbon sorbent from bark ischaracterized by a more developed pore structure. Thatis, the difference between the capacities of the woodand bark sorbents for metal ions cannot be explained bythe degree of pore structure development. Of all of thecompared quality indices, these sorbents exhibited thegreatest difference in mineral contents; the mineralcontent of bark carbon sorbents was higher than that ofwood sorbents by a factor of 6–8. It is believed that anincreased ash content of bark sorbents, which impartsbasic properties to these sorbents, dramaticallydecreases the cation-exchange properties of the sor-bents.

The study of the structure parameters, adsorptionproperties, and ion-exchange properties of granulatedsorbents from wood wastes and a comparison withcommercial analogs allowed us to recommend possibleareas of their application. The granulated sorbent fromwood, which is characterized by high ion-exchangeproperties, is best suitable for ion-exchange processesfor the extraction of metal ions from (galvanic) processwater and natural water. Low ash content and the pres-ence of supermicropores and mesopores in the structureof this sorbent suggest that it is applicable as a catalystsupport for a number of processes. It is reasonable touse the mesoporous granulated sorbent from bark (sam-ple 2SKD), which is a structural analog of the commer-cial activated carbon SKT, for the recovery of high-molecular-weight hydrocarbon vapors.

REFERENCES1. Aktivnye ugli. Elastichnye sorbenty. Katalizatory. Osu-

shiteli. Khimicheskie poglotiteli. Katalog (Active Car-bons; Elastic Sorbents; Catalysts; Drying Agents; Chem-ical Absorbents: A Catalog), Cherkassy: NIITEKhim,1996.

2. Mukhin, V.M., Tarasov, A.V., and Klushin, V.M.,Aktivnye ugli Rossii (Active Carbons of Russia), Mos-cow: Metallurgiya, 2000.

3. Karaseva, M.S., Malikov, I.N., Noskova, Yu.A., andPerederii, M.A., Khim. Tverd. Topl., 2006, no. 5, p. 50.

4. Krasil’nikova, O.K., Artamonova, S.D., and Volosh-chuk, A.M., Khim. Tverd. Topl., 2005, no. 4, p. 65.

5. Dubinin, M.M. and Zaverina, E.D., Dokl. Akad. NaukSSSR, 1963, vol. 92, p. 111.

6. Molchanov, S.A., Zolotovskii, B.P., and Kislenko, N.N.,Pererabotka Nefti i Gaza, 2002, no. 2, p. 37.

Table 10. Efficiency of sorbents in the extraction of metalions from aqueous solutions

SorbentSorption

pore volume, cm3/g

Exchangecapacity,

(mg Ba)/g

Sorptioncapacity,(mg Fe)/g

SDG 0.32 130 534

2SKG 0.36 82 283