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When the gas adsorption isotherm encounters the denitration catalyst

2019-05-25 14:21:32


The adsorption isotherm curve is the relationship between the concentration of solute molecules in the two phases when the adsorption process reaches equilibrium at a certain temperature at a certain temperature. In other words, the effects of temperature, pressure, gas (adsorbate) and surface (adsorbent) are constant, then the amount of adsorption on a particular surface is constant. Because the amount of gas adsorbed by the solid surface is a function of temperature, pressure, affinity or energy, at a constant temperature, the amount of adsorption per unit weight of adsorbent can be plotted as equilibrium pressure at a constant temperature. The curve for pressure changes is the adsorption isotherm at a specific gas-solid interface.

Experimental gas adsorption isotherms are generally divided into six categories, as shown in the figure, where type IV is classified according to IUPAC, and my isotherm exhibits significant adsorption at low relative pressure (relative pressure is defined as equilibrium vapor pressure divided by Saturated vapor pressure), then stationary. Type I isotherms are generally considered to be the result of microporous adsorption or monolayer adsorption due to the interaction of strongly adsorbed adsorbates, possibly chemisorbed, involving adsorbates and adsorbents Chemical bonding between the surfaces. It is worth noting that pores are classified as micropores (less than 2 nm), mesopores (between 2 and 50 nm) and macropores (higher than 50 nm) based on their diameter (or width).

In general, non-polar gases (N2, Ar) are often used to characterize these porous solids, where chemisorption is not produced. Therefore, typical pre-adsorption isotherms are directly expressed as microporous properties of the material. However, for mesoporous materials with pore sizes close to the micropore range, my isotherm can also be observed. In particular, in the case of adsorbing N2 at 77K or Ar adsorbed in cylindrical pores of 77 and 87K, my isotherm must be stable below the relative pressure of about 0.1 to determine that the material is completely microporous. Therefore, when my isothermal line is not balanced below the relative pressure of 0.1, the sample may exhibit an appreciable amount of mesopores or even mesopores. However, the shape of this type of isotherm may indicate a certain degree of widening of the mesoporous size distribution, as materials with highly uniform cylindrical pores may exhibit identifiable steps on the adsorption isotherm (these isotherms) Classified as type IV).

The adsorption on most macroporous solids is carried out in a multi-layered form such that the amount of adsorption gradually increases with increasing relative pressure, although the multi-layer accumulation close to the saturated vapor pressure may be very significant. This unrestricted multi-layer formation process produces Type II and Type III isotherms. In this case, the adsorption and desorption branches of the isotherms will coincide; that is, no adsorption-desorption hysteresis depends on a given solid. The surface properties may be evident in the single layer formation stage (type II) or the adsorption isotherm may bulge over the entire pressure range (type III). A type III isotherm can be observed when the lateral interaction between the adsorbed molecules is stronger than the interaction between the adsorbent surface and the adsorbate.

Adsorption on mesoporous solids is typically carried out by multiple layers of adsorption followed by capillary condensation (type IV and V isotherms). Therefore, the adsorption process is initially similar to the adsorption process on macroporous solids, but at higher pressures, the adsorption amount rises very steeply due to capillary condensation in the mesopores. After filling these holes, the adsorption isotherms tend to be stable. Capillary condensation and capillary evaporation typically do not occur at the same pressure, which leads to the appearance of hysteresis curves. However, early studies have found capillary condensation in mesoporous OMMs. - Evaporation can also be reversible. In fact, the difference between type IV and V is similar to the difference between type II and type III. In addition, the vi isotherm is known for its stepped nature of the adsorption process. Uniform multi-layer adsorption of uniform non-porous surfaces. Nitrogen adsorption at liquid nitrogen temperature does not allow for the complete form of this isotherm, while argon adsorption under liquid argon can be achieved.



Most of the catalysts used in the selective catalytic reduction denitration (SCR) process are porous media, and the internal pores affect the mass transfer efficiency of the catalytic material and the microscopic specific surface area of the catalyst, large specific surface area and excellent pore structure and help to improve The rate of diffusion of the gas in the pores enhances the reactivity of the catalyst. The study indicates that due to capillary condensation, the concentration of NH3 and SO3 in the pores of the catalyst is much larger than that in the bulk phase. NH4HSO4 is preferentially formed in the micropores with small pore size of the catalyst, and the smaller the inner diameter of the pores, the The more difficult the formation of NH4HSO4 is decomposed, the accelerated poisoning deactivation of the catalyst is accelerated. Therefore, regulating the pore structure distribution of the catalyst and increasing the ratio of the mesopores and macropores of the catalyst are beneficial to reducing the SO3 production rate and equilibrium concentration, improving the sulfur resistance performance and prolonging the service life thereof.

Cellular denitration catalysts can be classified as ceramic products, and the pore structure of such catalysts depends on the addition of pore formers. In general, ceramic pore-forming agents are mainly divided into two categories: one is heating and removing pore-forming agents, including natural fibers, high molecular polymers, organic acids, starches, dextrin, sawdust, urea, naphthalene, amino acid derivatives, Organic pore-forming agents such as PVA, PMMA, PCB, PVB, etc., also include inorganic pore-forming agents such as ammonium carbonate, calcium carbonate, ammonium hydrogencarbonate, ammonium chloride and various types of carbon powder; the second type is made by heating. The pore agent is generally immersed in water, acid or alkali to form a porous structure after sintering of the substrate. Through the addition of different proportioning pore-forming agents, pore structures with different pore sizes and pore volumes can be produced. The BET method was used to establish the gas adsorption isotherm curve. The pore structure of the catalyst was experimentally studied. The pore structure and distribution of the catalyst were diagnosed by data analysis of the catalyst adsorption isotherm. The structure characteristics of the catalyst and the sulfur and denitrification resistance were investigated. The chemical nature of the "structural-effect relationship." After optimization of the process, micropores, mesopores and macropores with different proportions are obtained to achieve optimal catalyst channel distribution and performance matching.

Since 2016, the province's NOx removal standards for non-electrical industries have been upgraded, and SCR technology has gradually been applied to non-electrical industries. However, the flue gas components of non-electrical industries are unique, and the denitration catalysts for the core components of SCR technology are New technical requirements were put forward. Since the ratio of different micropores, mesopores, and macropores also has unique adsorption and desorption characteristics for each gas, the pore-forming technology of the denitration catalyst in the non-electrical industry has become the key.



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