RU2518579C1 - Method of obtaining carbon adsorbent - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of obtaining carbon adsorbent includes carbonisation of crushed birch wood at 300-800°C in inert medium. After carbonisation carbonisate is kept at final temperature for 30 minutes. After that, carbonisate of birch wood is mixed with betulinol in ratio, equal 4:1, fused at 400-450°C, and activated. Activation is carried out in argon atmosphere in presence of hard potassium hydroxide at temperature rise to 800°C. After activation it is kept at final temperature for 60 min. Product is washed with acid solution, then water at temperature 50°C and dried.

EFFECT: improvement of carbon sorbent characteristics with respect to low-molecular components of gas mixtures.

2 dwg, 1 tbl

Description

The invention relates to the field of sorption technology, in particular, to methods for producing carbon sorbents based on plant materials, and can be used for carrying out short-cycle adsorption of gaseous substances in the pores of a nanometer-sized carbon carrier for their purification or separation, as well as adsorbent storage gases.

There are known processes using a variety of adsorbents and devices for the separation or extraction of various components from a multicomponent gas mixture, for example, natural gas. Some of them are aimed at cleaning the gas from undesirable impurities, others, on the contrary, at cleaning, concentrating and isolating the target component from the mixture. The main technological methods in gas separation processes suitable for purification and extraction, for example, methane from impurities of hydrogen, helium, CO ₂ , are cryogenic separation (RU 2380630, publ. January 27, 2010), multi-cycle adsorption separation at variable pressure - PSA method (US 6592749, publ. July 15, 2003) and membrane separation (RU 2322284, publ. 20.04.2008).

The described methods are highly energy intensive and require significant investment, economically viable when used on an industrial scale. In certain areas of application (chemical, gas, fuel cell manufacturing, medicine, ecology, etc.), industrial methods for separating gaseous mixtures are economically difficult to implement due to extremely high costs and because of the multi-stage extraction with the necessary selectivity and concentration of target components. Therefore, to solve specific problems in many technological fields, methods for the rapid separation of gases by short cycle adsorption of RBA (RPSA) based on separation and / or different rates of adsorption and mass transfer of gas mixture components are implemented.

The solution to the problem of purification and separation of gaseous systems largely depends on the effectiveness of the adsorbents used, suitable for the possible complete separation of one of the components or the required purity of the gas to be purified (RU 2378046, publ. 10.01.2010, US 6660064, publ. 09.12.2003). In order to increase the degree of extraction or concentration of the target components by the CBA separation method, the search for methods for producing carbon materials (CM) with an optimal pore size suitable for purification, separation and storage of fuel gases continues. The need for highly efficient sorbents obtained from relatively inexpensive sources of plant origin, including adsorbents based on activated carbon with high microporosity, is constantly growing.

Known methods for producing highly microporous carbon materials by alkaline activation (KOH, NaOH) of carbonized lignocellulosic precursors selected from the group: wood chips, sawdust, coconut shell, subjected to preliminary activation with a solution of phosphoric acid (US 5416056, publ. 16.05.1995; US 5626637, publ. May 16, 1997), or otherwise activated (US 5710092, publ. 01.20.1998, US 5965483, publ. 12.10.1999). Initially activated, including commercial, carbon products are subjected to secondary chemical activation with an alkaline agent in ratios of 0.5: 1.0-1: 4 at temperatures of 650-1110 ° C with the formation of pores with acid washing and drying in the matrix of the carbon material about 8-20 A in size and not less than 50%, suitable for the accumulation and storage of gaseous hydrocarbons, such as natural gas, in particular methane.

Known methods for producing UM have the disadvantage of using in the first stage of activation additional reagents such as phosphoric acid, which is prone to the formation of difficult to remove polymerization products, and the formation of corrosive and environmentally hazardous liquid wastes.

These inventions are aimed at producing carbon storage adsorbents capable of accumulating up to 150 volumes of natural gas (mainly methane) at a temperature of 25 ° C, but are not intended to separate gas of variable composition into individual components.

Known methods (RU 2311227, publ. 11/27/2007; RU 2310604, publ. 20.11.2007) for producing nanostructured carbon materials with a high specific surface and microporosity by carbonization at 400-800 ° C of lignocellulosic wood processing waste (sawdust, wood dust) and high ash biomass (husk of rice, oats, wheat straw), subsequent alkaline activation at 600-1000 ° C in the presence of carbonates and / or hydroxides of sodium or potassium, washing after activation with an acid solution and drying.

Carbon materials obtained according to the proposed methods are potentially able to find application as sorbents for the purification of liquid and gas media from organic substances, heavy metal compounds, catalysts, but their molecular sieve properties have not been studied. A disadvantage of the known methods for producing carbon materials with high values of specific surface area (S _BET = 1000-3000 m ² / g), total pore volume (V pore 1-2 cm ³ / g) and microporous volume is the use of a large number of stages associated with the use of complex technical design of the method, requiring the use of special catalysts and a specific gas atmosphere.

In addition, the inventions do not demonstrate a wide range of adsorption efficiency of the obtained porous CMs (except for N ₂ adsorption); they are not supported by examples of the use of CMs either as sorbents for energy gases (hydrogen, methane, etc.), or as systems for separating gas mixtures.

The known process (US 2011/0005392 A1, publ. 13.01.2011) of CO ₂ emission regulated by PSA adsorption on a porous carbon material that has a specific surface _area S _BET ≥1500 m ² / g and is capable of adsorbing up to 0.5-0.8 cm ² / g of adsorbent. The process of preparing a carbon adsorbent known for the separation process involves the use of at least one zeolite of a certain structural type (mordenite, ZSM-5, Na-Y, EMC-2) and at least one unsaturated polymerized organic substance, which is subjected to one or more stages of carbonization and then the stage of removal (dissolution) of the zeolite with an acid solution. The process is carried out in several stages: one or more chemical deposition of the olefin or alkyne compound from the gas phase at 500-800 ° C for 1-10 hours on the zeolite; pre-impregnated with a polymerizing agent (furfuryl alcohol); heat treatment of the mixture at 850-1000 ° C for 1-10 hours with a mass content of deposited carbon of 10-25%; dissolution of the zeolite matrix in excess of HF (≤24 h). As a result of subsequent boiling of the isolated products in HCl, filtering, washing with water from inorganic impurities and drying, carbon materials with uniform porosity and texture are obtained, the proportion of micropores in which is at least 50% with a predominant size of 0.5-1.5 nm, characterized by selectivity to the separation of the mixture of CO ₂ / CH ₄ equal to 2.9-3.0.

Significant disadvantages of the method is the multi-stage procedure for obtaining the described carbon adsorbent with the obligatory separation of the intermediate product at each stage, the complex technical design of the method, which requires compliance with controlled conditions at all stages, the high cost and duration of implementation (at least 5 days). In addition, the disadvantages of this method include the use with a high acid module of hydrofluoric and hydrochloric acids and, as a result, the presence of a large amount of liquid toxic waste.

A known method (US 5098880, publ. 24.03.1992) for the preparation of a modified carbon molecular sieve, suitable for separation by PSA of gases with different adsorption rates, in particular oxygen and nitrogen. The process involves sequential processing of the initial microporous carbon carrier with a pore size of 4.5-8 A with two different carbon-containing volatile organic compounds, mainly hydrocarbons (HC) with different molecular sizes (propylene, isobutylene, cyclohexane, etc.). The compounds are selected in such a way that the size of the molecules of the modifying substance in the first stage of processing is greater than in the second, thereby achieving the conditions for controlling the microporosity of the sieve. Thus, under conditions of selective pyrolysis, a carbon surface layer is created from the carrier that is unable to penetrate into the pores of the carrier itself and undergoes destruction of the first hydrocarbon. The intermediate carbon material formed in the first stage undergoes, in the second stage, under pyrolysis conditions, a contact with a second volatile organic compound having a smaller molecular size but not allowing HC to penetrate into the pores of the carbon layer. Two different stages of surface cracking of carbon-containing compounds as a result of pyrolysis at 500-900 ° C provide a modified carbon adsorbent with a controlled penetration of 4-8 A molecules, effective for selective adsorption and separation of a gas mixture.

However, this method requires a balance of interdependent factors to realize the desired modifying effect: strict regulation of the flow rate and concentration of hydrocarbon products, their volatility, compliance with temperature and completeness of cracking, depending, inter alia, on the size and configuration of the reactor. The adsorbents obtained in this way are very expensive.

Closest to the claimed method according to the technical essence and the achieved result is a method (RU No. 2436625, publ. 12/20/2011) for the production of microporous carbon adsorbent based on plant materials, namely from waste birch wood (DB). The method of producing a carbon adsorbent includes carbonization in an inert medium of birch wood, crushed to a fraction of 2-3 mm, in the temperature range 300-800 ° C with a heating rate of 20 ° C / min, exposure at a final temperature of 30 minutes, subsequent activation in an argon atmosphere in the presence of solid potassium hydroxide, taken in a mass ratio of coal: alkali, equal to 1: 3, and raising the temperature to 800 ° C at a rate of 10 ° C / min, holding it for 60 minutes, then washing the product with an acid solution and water at a temperature of 50 ° C to neutral and drying at. The obtained carbon adsorbents have the separation coefficients of the gas mixture H ₂ (He) -CH ₄ Cr = 3.2-3.8.

The disadvantage of this method is the heterogeneous microstructure of the obtained carbon adsorbent, which depends largely on the composition of the carbonized precursor, determined by the pyrolysis temperature.

The objective of the invention is to develop a method for producing a microporous carbon adsorbent, characterized by improved properties for separating a gas mixture of H ₂ (He) -CH ₄ by the method of short-cycle non-heating adsorption.

The technical result of the invention is as follows:

- to improve the uniformity and micro-texture of carbonized coals by modifying the surface with an organic carbon-containing substance;

- to improve the quality characteristics of activated carbon adsorbents to increase the selectivity of the separation of low molecular weight gases.

The technical result is achieved by the fact that in the method for producing a carbon adsorbent, which includes carbonization of in an inert medium crushed birch wood to a fraction of 2-3 mm with a heating rate of 20 ° C / min in a temperature range of 300-800 ° C and holding at a final temperature of 30 minutes, activation of argon in the atmosphere in the presence of solid potassium hydroxide, taken in a mass ratio of coal: alkali, equal to 1: 3, and raising the temperature at a rate of 10 ° C / min at 800 ° C for 60 min, washing the product with an acid solution, water at a temperature 50 ° C to neutral and drying the second medium according to the invention before activation of alkali carbonizate birch mixed with betulinol in a ratio equal to 4: 1, and then melted at 400-450 ° C for 20-30 min.

A comparative analysis of the present invention shows that, in contrast to the prototype, where the carbon sorbent is produced sequentially by carbonization of wood raw materials, and then by alkaline activation of the obtained carbonizates, in the inventive method, an additional stage of mixing and fusion of carbonized materials using the birch bark extraction product as a modifying component is introduced - betulinol (triterpene alcohol).

The porosity of simply carbonized coals is usually disorganized and imperfect, the disadvantages of which stem from the nature of the precursor (natural product) and the production method. For this reason, porosity is more or less represented by pores having a diameter that is not suitable for the usual conditions of separation processes by the CBA method (PSA). The use of a chemical modifier of organic origin with a molecular size that prevents their penetration into the pores of the carbon carrier, provides at a relatively low temperature of thermal decomposition (400-450 ° C) the creation of a porous carbon layer on the surface of the carrier. As a result of the migration of molten betulinol over the surface of the coal material and its thermal destruction, a composite porous structure is formed with the localization of a part of the active carbon fragments near the inlets at the mouth of wide pores, contributing to their narrowing. Thus, in contrast to the prototype, nanoregulation of the porosity of the intermediate carbon composite with a more uniform porous structure is achieved. Subsequent activation of betulinol-modified coals with KOH shifts the pore distribution in the activated carbon material to an area favorable both for the sorption of small molecules and for the separation of a mixture of methane (hydrogen) and helium.

The invention is illustrated by the scheme for producing a carbon adsorbent (see Fig. 1).

The implementation stage of the modifying treatment at temperatures below 400 ° C, especially about 300 ° C, contributes to the partial sublimation of betulinol and the deposition of unactivated carbon on the walls of the reactor. Conducting at temperatures exceeding 450 C, on the contrary, is rapid and uncontrolled thermal decomposition with the formation of carbon deposits - conglomerates on the surface of the sample.

Unlike the prototype, due to the modifying action of betulinol, the inventive method allows to obtain carbon adsorbents from birch wood, which have better molecular sieve properties when separating a mixture of gases He (H ₂ ) -CH ₄ compared with unmodified adsorbents from DB. In terms of their separation coefficient (Кр), used as a criterion for separation ability, they also exceed the normalized separation coefficient (Кр≥2), which is used in CBA methods for gas separation, in the entire investigated range of DB pre-carbonization temperatures.

Figure 2 shows the effect of the carbonization temperature of DBs and the conditions for producing activated PUMs on the separation coefficient of the He (H ₂ ) -CH ₄ gas mixture, where: 1 - Cp for PUMs from DB (based on the prototype), 2 - Cp for PUMs from DB modified betulinol, 3 - normalized Kr for separation of a mixture of gases He (H ₂ ) -CH ₄ .

Thus, from the foregoing, it follows that the additional step of modifying betulinol with carbonizates from birch wood in the claimed combination of features to a certain extent affects the achievement of a technical result, namely, the production of carbon adsorbents with increased separation ability with respect to low molecular weight components of gas mixtures.

Due to this distinguishing feature in the claimed method, it was possible to improve such qualitative characteristics of the obtained carbon microporous sorbent as the adsorption capacity of the sorbent and separation selectivity, depending on the ratio of adsorption / desorption rate and retention time on the adsorption surface.

It was found that the adsorbents obtained have a high degree of selectivity during the separation of the He (H ₂ ) -CH ₄ gas mixture. It was found that the highest separation coefficient (Kp = 3.84–4.11) is possessed by betulinol-modified samples from birch wood previously carbonized in the temperature range 600–800 ° C obtained by KOH activation. In the prototype, the same value Кр≤3,2-3,8.

The method is as follows.

Plant material in the form of sawdust of birch wood is dried to constant weight, crushed to a particle size of 2-3 mm. The sample is heated at a rate of temperature rise of 20 ° C / min in an argon atmosphere to a temperature of 300-800 ° C, incubated for 30 minutes After cooling to room temperature, the resulting carbonized sample is mixed with betulinol powder, taken in a ratio of 4: 1, respectively. The mixture is heated in an inert gas stream at 450 ° C for 30 minutes. Cooled to room temperature, the composite carbon product is mixed with solid potassium hydroxide in a weight ratio of 1: 3. The mixture is placed in a flow reactor and heated at a temperature rise rate of 10 ° C / min in an argon atmosphere to a temperature of 800 ° C, at which the mixture is kept for 1 hour. After cooling to room temperature, the obtained sample was treated with a 1 M HCl solution, then the product was washed with hot water (~ 50 ° C) to a neutral wash water medium. The carbon material is dried in an oven at a temperature of 105-110 ° C to constant weight.

The specific surface area was measured on an ASAP 2420 Micrometrics setup using N ₂ adsorption at 77 K after preliminary training of the samples for 6 h at 300 ° C and a residual pressure of less than 0.001 mm Hg. no contact with the atmosphere after training. Nitrogen adsorption isotherms were measured in the range of relative pressures Р / Ро from 0.005 to 0.995 and their standard processing with calculation of the total surface by the BET method, the total pore volume and micropore volume (with a size of less than 2 nm).

The separation efficiency of the He (H ₂ ) -CH ₄ gas mixture was determined by the chromatographic method on a column with a diameter of 3 mm and a length of 1 m, filled with a CM of 0.25 mm fraction on a Chrom 5 chromatograph. As a criterion for the separation ability of the sorbent in the short cycle separation method by the method of heating without adsorption, the coefficient K _p calculated from the experimental data by the equation was used:

$$K_p=\frac{b}{m_{\mathrm{one}}+{\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="00000003.png"\; state="\{\{state\}\}">m</figure-callout>}}_2}$$

where b is the distance between the output peaks of individual gases He (H ₂ ) and CH ₄ ;

m ₁ and m ₂ - peak width of each of the gases at the base.

The method is confirmed by a specific example:

Example 1. 50 g of dried and ground to a particle size of 2-3 mm sawdust of birch wood is placed in a metal reactor and carbonized at a temperature rise rate of 20 ° C / min in an argon atmosphere to a temperature of 700 ° C, at which the sample is kept for 30 minutes . After cooling, 20 g of the obtained raw coal (carbonizate) are thoroughly mixed with 5 g of betulinol powder, the mixture is heated at 450 ° C for 30 minutes. Cooled to room temperature, the carbon composite product is mixed with solid potassium hydroxide in a mass ratio of coal / alkali equal to 1: 3. The mixture was placed for activation in a flow reactor and heated at a temperature rise rate of 10 ° C / min in an argon atmosphere to a temperature of 800 ° C, at which the mixture was kept for 1 hour. After cooling, the obtained sample is treated with a 1 M HCl solution, then the product is washed with hot water (~ 50 ° C) to a neutral wash water medium. The resulting sample is dried in an oven at a temperature of 105-110 ° C to constant weight. The carbon adsorbent has a specific surface _area S _BET = 1273 m ² / g, a total pore volume of 0.53 cm ³ / g, and micropores in the sample account for at least 70% and selectivity in the separation of the He (H ₂ ) / CH ₄ Cr mixture = 4.11 (see table).

Thus, the claimed technical solution makes it possible to prepare, on the basis of birch wood components (sawdust, birch bark extract), an activated carbon sorbent with improved characteristics that can be used for fast and high-quality separation of a gas mixture consisting of components of low molecular weight compounds He (H ₂ ) and CH ₄ . This invention may find use in processes of high-quality purification and separation of gaseous media, including the purification of natural gas.

Characterization of the texture and adsorption properties of carbon adsorbents obtained by activating KOH at 800 ° C of composites prepared from modified 25% by weight. betulinol samples of birch wood, pre-carbonized in the temperature range 300-800 ° C.

Table No. Sample S _BET , m ² / g Cr mixture of H ₂ (He) -CH ₄ Vsp, cm ³ / g Sorption, g / g H ₂ O Benzene one DB-300 + Betulinol / KOH, 1: 3,800 960 2.83 0.40 0.23 0.34 2 DB-400 + Betulinol / KOH, 1: 3,800 1293 3.03 0.54 0.37 0.49 3 DB-500 + Betulinol / KOH, 1: 3, 800 915 3.43 0.38 0.25 0.35 four DB-600 + Betulinol / KOH, 1: 3, 800 1381 3.84 0.58 0.39 0.51 5 DB-700 + Betulinol / KOH, 1: 3, 800 1273 4.11 0.53 0.39 0.45 6 DB-800 + Betulinol / KOH, 1: 3,800 1163 4.05 0.41 0.41 0.47

Claims (1)

A method of producing a carbon adsorbent from plant materials, including carbonization in an inert medium of birch wood crushed to a fraction of 2-3 mm in the temperature range of 300-800 ° C with a heating rate of 20 ° C / min with exposure at a final temperature of 30 min, activation at 800 ° C for 60 min in an argon atmosphere in the presence of solid potassium hydroxide, taken in a mass ratio of coal: alkali, equal to 1: 3, and raising the temperature at a rate of 10 ° C / min, washing the finished product with an acid solution, water at a temperature of 50 ° C to a neutral environment and su a pulp, characterized in that before activation, the birch wood carbonizates are mixed with betulinol in a ratio of 4: 1, and then fused at 400-450 ° C for 20-30 minutes.

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