The type and quality of the adsorbent material is critical and is used in desiccant dryers to absorb and remove moisture from compressed air. Therefore, the type and quality of the adsorbent material is critical. Common adsorbents are of the following types:
1. Silica gel
The chemical formula of silica gel is SiO?·xH?O. It is a solid silicon dioxide that easily absorbs moisture. It is non-toxic, odorless, and translucent with a porous crystal surface structure. It has a strong adsorption capacity for polar substances (such as water, methanol, etc.); the specific surface area of silica gel is 500-600㎡/g, and it can absorb moisture in the gas up to 50% of its own weight. Silica gel has a large adsorption capacity when the moisture content of the gas is large. The disadvantage of silica gel is that the particles will break when encountering liquid water, especially when there is pressure, so it is generally not used alone in compressed air dryers.
2. Activated alumina
Activated alumina is formed by thermal decomposition of various aluminum hydroxides. Its chemical formula is Al?O?·nH?O. It has great surface hardness and compressive strength. It is not easy to break under static pressure and not easy to wear under alternating pressure. Therefore, it is very suitable for dehydration and drying of compressed air; the specific surface area of activated alumina is 250-300㎡/g, and the adsorption capacity is large, which can be used for drying of high humidity gas. Activated alumina has good mechanical properties and heat resistance, and its pressure dew point after drying can be ≤-40°C, which can meet the requirements of most industries and is the most widely used adsorbent in compressed air dryers.
3. Molecular sieve
Molecular sieve is a crystal of cage-shaped hole skeleton composed of silicon (aluminum) network area (SiO4AlO4). The molecular sieves used for compressed air drying are mainly 3A, 4A and 5A. Its main feature is that it has a very single surface pore size, a specific surface area of 800-1000㎡/g, and can be deeply dried when the moisture content of the gas is not high. The compressed air treated with molecular sieve has a pressure dew point of -70°C. When dealing with gases with high water content, alumina or silica gel should be used for pre-drying, and then molecular sieves should be used to eliminate residual moisture.
Adsorbent material
Adsorption dryers can only have higher performance and better energy-saving effects when high-quality adsorbents are applied. The selection of adsorbent materials in adsorption dryers can be based on the following criteria.
1. Adsorption capacity
Adsorption capacity refers to the maximum amount that can be adsorbed per unit mass (or unit volume) of an adsorbent at a certain temperature and adsorbate concentration.
Generally, an adsorbent with a large specific surface area has a large adsorption capacity and a strong adsorption capacity. The specific surface area of the adsorbent includes the outer surface area and the inner surface area of the particles, and the inner surface area is always much larger than the outer surface area. Only porous substances with a highly porous structure and a huge exposed surface area can provide a huge specific surface area.
The adsorption capacity is not only related to the surface area of the adsorbent, but also related to the pore size, pore size distribution, molecular polarity and the properties of the functional groups on the adsorbent molecules. Large adsorption capacity can reduce the amount of adsorbent required to process a unit of fluid.
2. Compressive strength
Adsorbents need to withstand the load of continuous cycles during the operation of the desiccant dryer; due to this load, low-quality adsorbents are easily decomposed. This will cause the sorbent to rapidly lose its water holding capacity. Additionally, it generates small dust particles that can clog valves and downstream dust filters as well as mufflers. During repairs, this dust can also pose a health and environmental hazard. This means that high-quality adsorbents need to have high compressive strength.
3. Adsorption selectivity
Adsorbents must have good adsorption selectivity, because different adsorbents have different preferential adsorption capacities due to their different compositions and structures. Only with good selectivity can the gaseous mixture be purified economically and effectively.
4. Has certain adsorption and separation ability
The purpose of using the adsorbent is to realize the separation and purification requirements of the mixed gas (liquid) body in the process. The adsorbent should have the ability to separate and purify the gas (liquid) body under a certain condition.
5. Water resistance
The adsorbent will degrade when in contact with liquid water, equipped with waterproof adsorbent material at the bottom of the adsorption desiccant dryer container, which can improve the performance of the adsorbent and prolong the service life of the adsorbent.
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