Adsorbent 1. Overview A solid substance that can effectively adsorb some of these components from a gas or liquid.
Adsorbents generally have the following characteristics: large specific surface, suitable pore structure and surface structure; strong adsorption capacity for adsorbate; generally no chemical reaction with adsorbate and medium; easy to manufacture, easy to regenerate; good machinery Strength etc.
Adsorbents can be classified according to pore size, particle shape, chemical composition, surface polarity, etc., such as coarse and fine pore adsorbents, powder, granular, strip adsorbents, carbonaceous and oxide adsorbents, polar and non-polar Polar adsorbent, etc.
Commonly used adsorbents include various activated carbon adsorbents that use carbonaceous materials as raw materials, and metal and non-metal oxide adsorbents (such as silica gel, alumina, molecular sieve, natural clay, etc.).
The main indicators for measuring the adsorbent are: adsorption capacity for different gas impurities, abrasion rate, bulk packing density, specific surface area, crushing strength, etc. Used to filter out toxic gas, refining petroleum and vegetable oil, prevent viruses and mold, recover gasoline from natural gas, and decolorize sugar and other colored substances.
2. Types of adsorbents The commonly used adsorbents in the industry are: silica gel, activated alumina, activated carbon, molecular sieve, etc. In addition, there are adsorption materials developed for the selective adsorption of certain components. The success of gas adsorption separation depends greatly on the performance of the adsorbent, so the choice of adsorbent is the primary problem in determining the adsorption operation.
1. Silica gel is a hard, amorphous chain and network structure of silicic acid polymer particles, the molecular formula is SiO2.nH2O, is a hydrophilic polar adsorbent. It is the treatment of aqueous solution of sodium silicate with sulfuric acid to form a gel, which is washed with water to remove sodium sulfate and then dried to obtain glass-like silica gel. It is mainly used for drying, separation of gas mixtures and petroleum components, etc. The silica gel used in industry is divided into two kinds of coarse pores and fine pores. Under the condition of saturated relative humidity, the adsorption capacity of coarse pore silica gel can reach more than 80% of the weight of the adsorbent, while under low humidity, the adsorption capacity is much lower than that of fine pore silica gel.
Activated alumina is made by heating dehydration of aluminum hydrate. Its properties depend on the structural state of the initial hydroxide. Generally, it is not pure Al2O3, but partially hydrated amorphous porous structure material, not only amorphous. Gel, and crystals of hydroxide. Because of its high activity on the surface of the capillary channel, it is also called activated alumina. It has a strong affinity for water and is an adsorbent for the deep drying of trace amounts of water. Under certain operating conditions, its drying depth can reach below dew point -70 ℃.
2. Activated carbon is made by carbonizing and activating carbon-containing raw materials such as charcoal, fruit shells, and coal. The activation method can be divided into two categories, namely the chemical activation method and the gas activation method. The chemical activation method is to add zinc chloride, potassium sulfide and other chemicals to the raw materials, and heat it in an inactive atmosphere for carbonization and activation. The gas activation method is to heat the activated carbon raw material in an inactive atmosphere, usually after the volatile components are removed below 700 ℃, pass steam, carbon dioxide, flue gas, air, etc., and carry out the reaction in the temperature range of 700 ~ 1200 Its activation. Activated carbon contains many capillary pore structures so it has excellent adsorption capacity. Therefore, it is used in water treatment, decolorization, gas adsorption and other aspects.
3. Zeolite molecular sieve Also known as synthetic zeolite or molecular sieve, its chemical composition formula is:
[M2 (â… ) M (â…¡)] O.Al2O3.nSiO2.mH2O
In the formula, M2 (â… ) and M (â…¡) are monovalent and divalent metal ions, most of which are sodium and calcium. N is called the silicon-to-aluminum ratio of zeolite. Silicon mainly comes from sodium silicate and silica gel, and aluminum comes from In sodium aluminate and Al (HO) 3, etc., the colloid produced by the reaction between them and sodium hydroxide aqueous solution will become zeolite after drying, generally n = 2 ~ 10, m = 0 ~ 9.
The characteristic of zeolite is that it has the function of molecular sieve, it has uniform pore size, such as 3A0, 4A0, 5A0, 10A0 fine pores. The 4A0 zeolite with 4A0 pore size can adsorb methane and ethane, but not the n-alkanes with more than three carbons. It has been widely used for gas adsorption separation, gas and liquid drying, and separation of normal isoparaffins.
4. Carbon molecular sieve is actually a kind of activated carbon. It is different from the general carbonaceous adsorbent in that its pore size is evenly distributed in a narrow range, and the pore size is equivalent to the diameter of the separated gas molecules. The specific surface area of ​​micropores generally accounts for more than 90% of the total surface area of ​​carbon molecular sieves. The main distribution form of the pore structure of carbon molecular sieve is: the diameter of the large pores communicates with the outer surface of the carbon particles, the transition pores branch out from the macro pores, and the micro pores branch out from the transition pores. In the separation process, the large pores mainly serve as transport channels, and the micropores serve as molecular sieves.
Methods for preparing carbon molecular sieve from coal are carbonization method, gas activation method, carbon deposition method and impregnation method. Among them, the carbonization method is the simplest, but to make high-quality carbon molecular sieves, these methods must be used comprehensively.
Carbon molecular sieves have achieved success in the field of air separation to produce nitrogen, and have broad prospects in other gas separations.
3. The physical properties of the adsorbent The good adsorption performance of the adsorbent is due to its dense pore structure. The physical properties related to the pores of the adsorbent are:
a. Pore volume (VP): The volume of the micropores in the adsorbent is called the pore volume, usually expressed as the volume of the micropores of the adsorbent per unit weight of the adsorbent (cm3 / g). The pore volume is the effective volume of the adsorbent, It is the value calculated from the saturated adsorption amount, that is, the volume of the adsorbent that can hold the adsorbate, so the pore volume is better. The pore volume (Vk) of the adsorbent is not necessarily equal to the pore volume (VP), and the micropores in the adsorbent have adsorption effect, so the VP does not include coarse pores. The volume of all pores included in Vk is generally larger than VP.
b. Specific surface area: that is, the surface area of ​​the adsorbent per unit weight, the common unit is m2 / g. The surface area of ​​the adsorbent has hundreds to thousands of square meters per gram. The surface area of ​​the adsorbent is mainly the surface of the pore wall, and the outer surface of the adsorbent is very small.
c. Pore size and pore size distribution: In the adsorbent, the shape of the pores is extremely irregular, and the pore sizes are also different. Holes with a diameter of several Angstroms (A0) to tens of Angstroms are called fine holes, and holes with a diameter of more than hundreds of Angstroms are called coarse holes. The more pores, the larger the pore volume and the larger the specific surface, which is beneficial to the adsorption of adsorbate. The role of coarse pores is to provide a path for adsorbate molecules to enter the adsorbent. The relationship between coarse pores and fine pores is just like the streets and alleys. Foreign molecules can quickly reach the depth of the adsorbent through the coarse pores. Therefore, coarse holes should also occupy an appropriate proportion. The coarse pores and fine pores in the adsorbent such as activated carbon and silica gel are formed during the manufacturing process. Zeolite molecular sieves form crystals with a diameter of several micrometers during synthesis, in which only uniform pores are formed, and coarse pores between crystals are formed during molding.
Pore ​​size distribution is the relationship between pore size and corresponding pore volume. From this, the pore characteristics of the adsorbent are characterized.
d. Apparent gravity (dl): also known as apparent gravity.
The volume of adsorbent particles (Vl) consists of two parts: the volume of the solid skeleton (Vg) and the pore volume (Vk), namely:
Vl = Vg + Vk
Apparent gravity is the ratio of the weight (D) of the adsorbed particles to the volume occupied by them (Vl).
The pore volume (Vk) of the adsorbent is not necessarily equal to the pore volume (VP), the micropores in the adsorbent have a role, so the VP does not include coarse pores. The volume of all pores included in Vk is generally larger than VP.
e. True gravity (dg): also known as true gravity or the weight of adsorbent solids, that is, the ratio of the weight of adsorbent particles (D) to the volume of solid skeleton Vg.
Assuming that the weight of the adsorbed particles is based on one gram, according to the definition of apparent gravity and true gravity:
dl == l / Vl; dg = l / Vg
The pore volume of the adsorbent is then:
Vk = l / dl – l / dg
f. Bulk density (db): also known as filling density, that is, the weight of adsorbent filled per unit volume. This volume also includes the voids between the adsorbed particles, and the bulk density is an important parameter for calculating the volume of the adsorbent bed.
The above gravity units are usually expressed in g / cm3, kg / l, kg / m3.
g. Porosity (εk): the ratio of the pore volume in the adsorbed particles to the particle volume.
εk = Vk / (Vg + Vk) = (dg-dl) / dg = 1-dl / dg
h. Porosity (ε): the ratio of the gap between the adsorbed particles and the entire volume of the adsorbent.
ε = (Vb-Vl) / Vb = (dl-db) / dl = 1-db / dl
Fourth, other adsorbents are also called absorbents. This kind of substance can make the active ingredients adhere to the surface of its particles, and make the liquid trace compound additives into solid compounds, which is conducive to uniform mixing. Its characteristics are strong adsorption and stable chemical properties.
Adsorbents are also generally divided into organic and inorganic substances, organic substances such as wheat germ powder, defatted corn germ powder, corn cob fragments, coarse bran, soybean fine powder, and highly absorbent grains. Including silica, vermiculite, calcium silicate, etc.
The most representative adsorbent is activated carbon. Its adsorption performance is quite good, but the cost is relatively high. It was used in the Songhua River incident to adsorb toluene in water. Secondly, there are molecular sieves, silica gel, activated aluminum, polymer adsorbents and biological adsorbents, etc.
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