Method for producing activated carbon

Abstract

It is disclosed a method for producing highly microporous activated carbon, comprising the steps of: providing a plant-based material; reducing hemicelluloses of the plant-based material to provide a hemicellulose-depleted plant-based material; carbonizing the hemicellulose-depleted plant-based material to provide a carbonized hemicellulose-depleted plant-based material; activating the carbonized hemicellulose-depleted plant-based material to provide highly microporous activated carbon.

Description

[0001] METHOD FOR PRODUCING ACTIVATED CARBON

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method for producing activated carbon. More particularly, the invention relates to a method for producing activated carbon from a plant-based material.

[0004] BACKGROUND OF THE INVENTION

[0005] Most carbon containing materials, including industrial waste, can be carbonized in thermochemical conversion process. Obtained high carbon materials can be further converted into activated carbons (AC) through chemical or physical activation processes. It has been reported a wide range of applications of ACs due to its superior adsorbent capabilities. For example, ACs are used for industrial purification processes in waste water treatment and gas cleaning. ACs can also be used as a catalyst support or as a catalyst itself. ACs are further widely used in healthcare as antitoxins due to their capability of alleviating the effects of poisons or over- consumption of stimulants.

[0006] WO 2009 / 011590A1 discloses a process for producing a chemically activated carbon from a mixture of wood particles, comminuted carbonaceous vegetable material selected from kernel or shell material. The process comprises the steps of mixing the mixture with a chemical activating agent, pelletising, heat treating, and carbonising.

[0007] BRIEF DESCRIPTION OF THE INVENTION

[0008] In an aspect, the present invention provides a method for producing highly microporous activated carbon comprising the steps of:

[0009] - providing a plant-based material,

[0010] - reducing hemicelluloses of the plant-based material to provide a hemicellulose-depleted plant-based material,

[0011] - carbonizing the hemicellulose-depleted plant-based material to provide a carbonized hemicellulose-depleted plant-based material,

[0012] - activating the carbonized hemicellulose-depleted plant-based material to provide highly microporous activated carbon.

[0013] It was surprisingly found that activated carbon with an improved performance may be produced by reducing the content of hemicelluloses in a raw material used in the manufacture of activated carbon. Reduction of hemicelluloses, i.e. sugars, of a plant-based raw material provides a highly porous activated carbon material with an increased surface area, an increased adsorption capacity, and homogeneous porous size structure. The carbon content of hemicelluloses is high. However, reduction of hemicelluloses provides an advantage in that blocking of the pores of the plant-based raw material caused by the presence of hemicelluloses is prevented. Thereby, porosity and adsorption capacity of activated carbon may be increased.

[0014] The invention provides an improved method for producing activated carbon in increased yields. The raw material in the method may be a side stream from industrial processes providing an economic and highly sustainable method.

[0015] In another aspect, the invention provides highly microporous activated carbon having a pore size distribution of microporous structure is over 90%.

[0016] DETAILED DESCRIPTION OF THE INVENTION

[0017] The raw material used in the present invention for manufacture of activated carbon is a plant-based material. In an embodiment, the plant-based material is a ligneous plant material. Main components in the ligneous plant material are cellulose, hemicelluloses and lignin. Hemicelluloses are polysaccharides formed from different monosaccharides (e.g., galactose, mannose, xylose). In an embodiment, the ligneous plant material is a wood material, specifically softwood or hardwood, such as spruce, larch or any mixture thereof. Softwood refers to wood from coniferous trees, such as spruce, larch and pine species. Hardwood refers to wood from deciduous trees, such as birch, aspen, and eucalypt species. The main hemicellulose in softwood is galactoglucomannan while xylan is the main hemicellulose component in hardwood. In larch, the main hemicellulose is arabinogalactan.

[0018] In the following, the method of the invention is described in more detail by referring to a ligneous plant material as a plant-based material, however, without limiting the invention thereto.

[0019] In an embodiment, the ligneous plant material is sawdust. In an embodiment, the sawdust is spruce sawdust, larch sawdust, or beech sawdust, or any mixture thereof. In an embodiment, the sawdust is spruce sawdust.

[0020] In the method of the invention, the ligneous plant material is subjected to a step in which the content of hemicellulose of said material is reduced. Thereby, a highly porous activated carbon produced from the resultant hemicellulose-depleted ligneous plant material is advantageously achieved.

[0021] In an embodiment, the reduction of hemicelluloses of the ligneous plant material is performed by extracting the ligneous plant material with an aqueous liquid. In the extraction, at least a part of the hemicelluloses of the ligneous plant material transfer into the aqueous liquid to provide a hemicellulose-depleted ligneous plant material. The extraction may be performed several times. In an embodiment, the extraction is performed two times.

[0022] In an embodiment, the aqueous liquid in the extraction step is water.

[0023] In an embodiment, the extraction step is performed at a temperature of at about 160°C to about 210°C. In another embodiment, the extraction step is performed at about 170°C to about 180°C.

[0024] In an embodiment, the extraction is carried out under a pressure ranging from about 5.5 bar (g) to about 9 bar (g). In another embodiment, the extraction is carried out at about 7.5 bar (g).

[0025] In an embodiment, the extraction is performed for about 5 min to about 60 min. In another embodiment, the extraction is carried out for about 10 min to about 40 min. In a further embodiment, the extraction is carried out for about 20 min to about 30 minutes.

[0026] In an embodiment, the method of the invention comprises a step reducing moisture of the hemicellulose-depleted ligneous plant material to provide a hemicellulose-depleted ligneous plant material with a reduced moisture content, prior to carbonization of the hemicellulose-depleted ligneous plant material. Reduction of moisture content provides a material with a low amount of water, e.g., facilitating the transfer of the hemicellulose-depleted ligneous plant material to the carbonization step. The hemicellulose-depleted ligneous plant material with a reduced moisture content may be conveniently pressed to a compact form, e.g. pellets or briquets.

[0027] Thus, the moisture of the hemicellulose-depleted ligneous plant material is reduced to provide an economic and efficient method for further processing of the hemicellulose-depleted ligneous plant material. In an embodiment, the moisture content of the hemicellulose-depleted ligneous plant material is content is about 15% to about 30%. In another embodiment, the moisture content is about 20% to about 25%.

[0028] The moisture of the hemicellulose-depleted ligneous plant material is reduced by at least one of the following: pressing the hemicellulose-depleted ligneous plant material, heating the hemicellulose -depleted ligneous plant material, and subjecting the hemicellulose-depleted ligneous plant material to vacuum.

[0029] The hemicellulose-depleted ligneous plant material, optionally having a reduced moisture content, is then carbonized to provide a carbonized hemicellu- lose-depleted ligneous plant material. Carbonization of the hemicellulose-depleted ligneous plant material is a process in which the carbon content of said material is increased. In the method of the invention, carbonization of the hemicellulose-depleted ligneous plant material is carried out by thermal treatment. The thermal treatment provides decomposition of cellulose, hemicellulose and lignin thereby increasing carbon content of the thermally treated material. Decomposition of cellulose, hemicellulose and lignin begins at about 200°C. Lignin exhibiting a more complex structure may require thermal treatment up to about 700°C for decomposition.

[0030] Carbonization of the hemicellulose-depleted ligneous plant material is carried out at a temperature of about 200°C to about 900°C. In an embodiment, carbonization is carried out at about 300°C to about 700°C.

[0031] The carbonized hemicellulose-depleted ligneous plant material is activated to provide activated carbon. Activation of the hemicellulose-depleted carbonized ligneous plant material is carried out at a temperature of about 400°C to about 1200°C. In an embodiment, activation is carried out at about 600°C to about 900°C. Activation is carried out in a manner conventionally used in the production of activated carbon. In an embodiment, carbonization and activation steps are performed in a single-stage or a two-stage process. The term "a single-stage process" means that carbonization and activation of the material take place simultaneously. The term "two-stage process" means that activation is followed by carbonization.

[0032] The activation of the carbonized hemicellulose -depleted ligneous plant material is performed by water vapour, carbon dioxide and chemical activation, such as with phosphoric acid, without limiting the activation thereto.

[0033] The activated carbon produced by the method of the invention has a surface area of at least 400 m2 / g. In an embodiment, the surface area is at least 700 m2 / g. In another embodiment, the surface area is at least 850 m2 / g.

[0034] The highly microporous activated carbon produced by the method of the invention has a pore size distribution (PSD) of microporous structure of at least 90%. In an embodiment, the PSD is at least 93%. In another embodiment, the PSD is at least 98%.

[0035] If desired, the highly microporous activated carbon produced by the method of the invention may be granulated or pelletised.

[0036] In addition, the highly microporous activated carbon may be reactivated for re-use. Reactivation process includes a thermal process in which the contaminants are removed for the used activated carbon. Highly microporous activated carbon produced be method of the invention was characterized by as follows:

[0037] Carbon yield

[0038] The carbon yield for each sample was calculated according to the following equation:

[0039] Yield (%) = mass of carbon / mass of feedstock x 100%

[0040] Total carbon

[0041] The total carbon content was measured using Skalar Primacs MCS instrument. Dried samples were weighted in quartz crucibles and combusted at 1100°C in pure oxygen. The formed CO2 gas was analyzed by an 1R analyzer. Citric acid was used as a standard substance. The total carbon content was calculated based on the initial mass of the sample. The precision of the measurements was in the range of 5%.

[0042] Elemental analysis

[0043] Elemental analysis was measured for hydrogen, nitrogen, sulfur, and oxygen contents with a Flash 2000 CHNS-0 Organic elemental analyzer supplied by Thermo Scientific. The sample was provided in the amount of 1.5 mg to 3.5 mg and dried for 1 h at a temperature of 105°C. The sample was then placed in the analyzer and mixed with 10 mg of vanadium pentoxide (V2O5) to enhance burning. The sample was then combusted at 960°C for 600 s using methionine as the standard for hydrogen and nitrogen. The standard used for oxygen was BBOT (2,5-(Bis (5-tert-butyl-2-benzo-axazol-2-yl) thiophene). Plain tin cups were used as a bypass (3 pcs) when starting the measurements.

[0044] Surface areas and pore size distributions (PSD)

[0045] The specific surface area and pore distributions were determined from the adsorption-desorption isotherms using nitrogen as an adsorbate. The determinations were performed with a Micromeritics ASAP 2020 instrument. Portions of each sample (100 mg to 200 mg) were degassed at low pressure (2 gmHg) and at a temperature of 140°C (413 K) for 2 h to clean the surfaces and to remove any adsorbed gas. The adsorption isotherms were obtained by immersing the sample tubes in liquid nitrogen at -196°C (77.15 K) to achieve constant temperature conditions. Gaseous nitrogen was added to the samples in small doses and the resulting isotherms were obtained. The specific surface areas and total pore volumes were calculated from the adsorption isotherms according to the BET method (Brunauer, S. et al., Adsorption of gases in m ultimo lecular layers, Journal of the American Chemical Society 60(2), 309-319 (1938)). The pore size distribution was calculated using the NLDFT (density functional theory) algorithm assuming slit-formed pores (Lastoskie, C. et al., Pore size heterogeneity and the carbon slit pore: A density functional theory model, Langmuir 9(10), 2693-2702 1993). The total pore volumes were in the BET calculated at a P to Po ratio of 0.985 and in the case of the NLDFT calculation, as the total volume of pores. The % distribution of the pore volumes was calculated from the individual volumes of the micropores, mesopores, and macropores.

[0046] The specific surface areas are measured with a precision of ±5% (Hack- ley, V.A., et al., "Real-world" precision, bias, and between-laboratory variation for surface area measurement of a titanium dioxide nanomaterial in powder form, Journal of Nanoparticle Research 15, 1-8 (2013)).

[0047] Fourier-transform infrared (FTIR1 analysis

[0048] Fourier-transformed infrared analysis was performed with an ATR- FT1R spectrometer (Perkin Elmer Spectrum One) with a diamond / ZnSe crystal. The data was normalized.

[0049] Field emission scanning electron microscopy (FESEM1

[0050] The micro structures of the raw materials, the intermediates, and the AC were revealed via the FESEM images, which were obtained using a Zeiss Sigma field emission scanning electron microscope (FESEM) in the Centre of Microscopy and Nanotechnology at the University of Oulu, operated at 5 kV.

[0051] In another aspect, the invention provides highly microporous activated carbon having a pore size distribution (PSD) of microporous structure is over 90% In an embodiment, the PSD is at least 93%. In another embodiment, the PSD is at least 98%.

[0052] The following examples are presented for further illustration of the invention without limiting the invention thereto.

[0053] Example 1. Hemicellulose-depleted ligneous plant material

[0054] Spruce sawdust was used as a ligneous plant raw material. Spruce saw dust was extracted with hot water at a temperature of 180 °C and a pressure of 1500 psi with Dionex ASE 300 system (Thermo Fisher Scientific). The extraction time was 30 min. The hemicellulose-depleted spruce sawdust was collected from the ASE 300 system. The collected spruce sawdust was dried in an oven at a temperature of 105°C overnight to provide a hemicellulose-depleted spruce sawdust.

[0055] In a similar manner, hemicellulose-depleted larch sawdust was produced from a larch sawdust raw material. The hemicellulose-depleted spruce sawdust and hemicellulose-depleted larch sawdust were analyzed for the content of total carbon (TC), N, H, S and 0.

[0056] The results are shown in Table 1.

[0057] Table 1. Analysis of hemicellulose-depleted samples

[0058] "S" = spruce; "L" = larch; "1" = once extracted

[0059] The results show that the extraction of the spruce and larch sawdust samples with water to reduce hemicelluloses did not significantly change the elemental composition of the samples. The total carbon content was slightly lowered in the spruce sample.

[0060] Example 2

[0061] 10 g of a hemicellulose-depleted spruce sawdust sample with a moisture content of about 60% prepared in example 1 was placed into a fixed-bed steel reactor inserted into a tubular oven for carbonization and activation.

[0062] The sample inside the reactor was supported on both sides by quartz wool. The temperature of the oven was raised from room temperature to 800°C at a heating rate of 6.5°C / min to provide carbonized sample. At 800°C, the carbonized sample was steam activated by feeding water into the reactor at a load of 6 cm3h’1for 2 h. The reactor was flushed with an inert gas (nitrogen) during the whole process to avoid oxidation of the sample.

[0063] Activated carbon from the hemicellulose-depleted larch sawdust was produced in a similar manner as described for spruce sawdust.

[0064] The activated carbons produced above from the hemicellulose-depleted spruce sawdust and larch sawdust samples were analyzed by measuring yield, total carbon content (TC), and content of N, H, S and 0. The results are shown in Table 2. Table 2. Analysis of the carbonized and activated materia

[0065] The results of Table 2 show that there are no major differences between the species of the starting material used in carbonization and activation.

[0066] Specific surface areas (BET) and pore size distributions (PSD) of the activated carbons produced above from the hemicellulose-depleted spruce sawdust and larch sawdust samples are given in Table 3.

[0067] Table 3. Nitrogen physisorption of the activated samples

[0068] The characterization of the physisorption showed that surface areas over 800 m2 / g for spruce and over 750 m2 / g for larch can be reached. For both spruce and larch samples, multiple extracted samples showed a lower level of surface area. On the other hand, the pore size distributions calculated with the NLDFT model for carbon show that multiple extractions seemed to produce a higher mi- croporous carbon structure. The result may show a correlation between the hemicellulose content and the pore size distribution.

[0069] It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

CLAIMS1. A method for producing highly microporous activated carbon, comprising the steps of:- providing a plant-based material,- reducing hemicelluloses of the plant-based material to provide a hemicellulose-depleted plant-based material,- carbonizing the hemicellulose-depleted plant-based material to provide a carbonized hemicellulose-depleted plant-based material,- activating the carbonized hemicellulose-depleted plant-based material to provide highly microporous activated carbon.

2. The method of claim 1, wherein the plant-based material is a ligneous plant material, such as a wood material, specifically softwood or hardwood, such as spruce, larch or any mixture thereof.

3. The method of claim 2, wherein the ligneous plant material is sawdust, specifically spruce sawdust or larch sawdust or beech sawdust, or any mixture thereof, more specifically spruce sawdust.

4. The method of any one of the preceding claims, wherein the reduction of hemicelluloses of the plant-based material is performed by extracting the plantbased material with an aqueous liquid.

5. The method of claim 4, wherein the aqueous liquid is water.

6. The method of claim 4 or 5, wherein the extraction is performed several times, specifically two times.

7. The method of any one of claims 4-6, wherein the extraction is performed at a temperature of at about 160°C to about 210°C, specifically about 170°C to about 180°C.

8. The method of any one of claims 4-7, wherein the extraction is performed at a pressure of about 5.5 bar (g) to about 9 bar (g), specifically about 7.5 bar (g).

9. The method of any one of claims 4-8, wherein the extraction is performed for about 5 min to about 60 min, specifically about 10 min to about 40 min, more specifically about 20 min to about 30 minutes.

10. The method of any one of the preceding claims, wherein prior to carbonization of the hemicellulose -depleted plant-based material, the method comprises a step of reducing moisture of the hemicellulose-depleted plant-based material to provide a hemicellulose-depleted plant-based material with a reduced moisture content.

11. The method of claim 10, wherein the moisture of the hemicellulose- depleted plant-based material is reduced by at least one of the following: pressing the hemicellulose-depleted plant-based material, heating the hemicellulose-depleted plant-based material, and subjecting the hemicellulose-depleted plantbased material to vacuum.

12. The method of claim 10 or 11, wherein the moisture content of the hemicellulose-depleted plant-based material with a reduced moisture content is about 15% to about 30%, specifically about 20% to about 25%.

13. The method of any one of claims 10-12, wherein the hemicellulose- depleted plant-based material with a reduced moisture content is pressed to pellets or briquets.

14. The method of any one of the preceding claims, wherein the carbonization is carried out at a temperature of about 200°C to about 900°C, specifically about 300°C to about 900°C.

15. The method of any one of the preceding claims, wherein the activation of the carbonized hemicellulose-depleted plant-based material is performed by at least one of the following: water vapour, carbon dioxide, and chemical activation, such as with phosphoric acid.

16. The method of any one of the preceding claims, wherein the activation is carried out at a temperature of about 400°C to about 1200°C, specifically about 600°C to about 900°C.

17. The method of any one of the preceding claims, wherein the steps of carbonization and activation are performed in a single-stage process or a two-stage process.

18. The method of any one of the preceding claims, wherein the activated carbon has a surface area of at least 400 m2 / g, specifically at least 700 m2 / g, more specifically at least 850 m2 / g.

19. The method of any one of the preceding claims, wherein the activated carbon has a pore size distribution of microporous structure is at least 90%, specifically at least 93%, more specifically at least 98%.

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