『Original by HaiChuan Translation Team』Applications of Zeolite Molecular Sieve Adsorbents in the Chemical Processing Industry (Part 1)

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English title: Zeolite-Based Molecular Sieve Adsorbents In The Chemical Process Industry. Original article link: Click here. Original author: ankur2061. Translator ID: @YuHuan. Proofreader ID: @MyChemicalRomance. Those who wish to subscribe to the articles published by the “Haichuan Translation Team” are invited to leave a message below saying “I want to subscribe” or send me a message; we will notify everyone as soon as new articles are available. Since the 1960s, molecular sieve adsorbents have been firmly established as a means for carrying out difficult separations, including those involving gas-gas, liquid-liquid, and solution-solute interactions. They are usually supplied in spherical or granular form, and occasionally in powder form as well. Molecular sieve adsorbents can be discontinued and discarded after being used once, but the usual practice is to regenerate them after a period of use so that they can be reused. They are usually stored in cylindrical containers through which the flow to be processed passes. For the regeneration process, this vessel uses two or more beds equipped with appropriate valves, in order to achieve a continuous production process. As a unit operation, adsorbents are unique in several aspects. In some cases, one separation is equivalent to hundreds of mass transfer units. In other cases, molecular sieve adsorbents selectively remove a certain component from a mixture based on differences in molecular size, which is highly unlikely to be accomplished by any other method. Furthermore, pollutants can be removed from the flow, and the impurity concentration can be reduced to almost undetectable levels. Molecular sieve adsorbents can be used in applications involving the adsorption of several grams to several tons. Before focusing on zeolite adsorbents, an important prerequisite is to understand the two basic categories of adsorbents, namely amorphous and crystalline. Amorphous adsorbents: The specific surface area of amorphous adsorbents (silica gel, activated alumina, and activated carbon) is typically in the range of 200–1000 m2/g, but for certain types of activated carbon, this value can be higher (above 1500 m2/g). The difficulty lies in the fact that carbon often lacks mechanical strength in certain high-pressure regions, which limits its use in many practical applications. Some high specific surface area materials also contain a large proportion of very small pores, which makes them unsuitable for applications involving the adsorption of macromolecules. In typical amorphous adsorbents, the pore size distribution can be very wide, ranging from a few nanometers to one micron. Since different adsorption behaviors occur in different pore size ranges, IUPAC (the International Union of Pure and Applied Chemistry) recommends the following classification: Micropores: diameter less than 2 nm; Mesopores: diameter between 2–50 nm; Macropores: diameter greater than 50 nm. This classification is somewhat arbitrary, as it is the size of the pore relative to the size of the molecules being adsorbed that plays a dominant role in the adsorption behavior, rather than the absolute pore size. However, general concepts are useful. In micropores (with pore sizes only slightly larger than those of the adsorbed molecules), the adsorbed molecules can never escape from the force field of the pore walls, even if they are located at the center of the pore. These micropores usually determine the adsorption capacity for molecules small enough to penetrate. Transport in these pores may be severely restricted by steric hindrance effects, thereby giving rise to zeolitic behavior. Mesopores contribute to the adsorption capacity to a certain extent, but their main function is to act as channels, providing pathways for the smaller micropores. Macropores contribute little to the adsorption capacity, but they usually have a significant impact on kinetics. Therefore, their function is similar to that of highways, allowing adsorbed molecules to diffuse to particles with the lowest diffusion resistance. Crystalline adsorbents: Crystalline adsorbents refer to zeolites and zeolite analogs, such as porous aluminum phosphate. Their micropore size depends on their crystal structure, so there is almost no pore size distribution. However, in some zeolites, since exchangeable cations occupy positions within the zeolite’s internal structure that block pore openings, it is sometimes possible to achieve a certain degree of control through ion exchange. The crystals of such materials are extremely small (1–5 micrometers), and they are usually aggregated together using an appropriate binder (usually clay) to form macroporous particles that are large enough to be directly filled into adsorption containers. Therefore, such materials typically exhibit a good bimodal pore size distribution, with the microporous structure within the crystal (tens of nanometers) being connected to a network of macropores on the same order of magnitude as the crystal diameter (~1 micron). Desiccant: Solid desiccants are simply adsorbents with high hydrophilicity and water-absorbing capacity, which can be used to selectively remove moisture from gas (or liquid) streams. Therefore, the main requirements for an effective desiccant are a highly polar surface and highly specific regions (pores). The most widely used desiccants are silica gel, activated alumina, and alumina-rich zeolites (4A or 13X). Zeolites exhibit high hydrophilicity and high absorption capacity at low partial pressures. In situations where a very low humidity or a very low dew point is required, zeolites can serve as highly effective dehydrants. The micropores of 3A zeolite are very small, sufficient to remove most molecules other than water molecules. Therefore, zeolites are highly suitable for drying reactive gases. The main drawback of zeolite desiccants is that their regeneration process requires high temperatures (>300°C); therefore, using zeolites is not economical when only moderate low dew point requirements exist. Temperature swing adsorption (TSA) process: The temperature swing adsorption or temperature change adsorption (TSA) cycle involves a desorption process that takes place at a much higher temperature than the adsorption temperature. Variable temperature adsorption is mainly applied in processes where the concentration of the substance to be separated is low, such as purification processes. Low residual concentration and high operating load are the main characteristics of temperature swing adsorption. The high absorption capacity for low-concentration components means that, for a bed of reasonable size, the cycle time for temperature-dependent adsorption is long, ranging from several hours to several months. Since the bed loaded with adsorbent reacts slowly to changes in gas temperature, such a long cycle time is necessary and beneficial. The components thermally adsorbed in the bed are removed by purging and/or vacuum, after which the bed is restored to the adsorbed state through cooling. Systems with strong adsorption of substances are particularly suitable for TSA. Such applications include “drying,” “desulfurization,” “carbon dioxide removal,” and “pollutant control.” Next time, we will cover the applications of molecular sieve adsorbents.