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所谓吸附,就是物质浓度在两相界面上自动发生变化的现象。当气体分子运动到固体表面时,由于气体分子与固体分子之间的相互作用,气体分子便会暂时停留在固体表面上,形成气体分子在固体表面上浓度的增大,即气体分子被固体表面吸附。研究表明,吸附现象不仅发生在固-气交界面上,在液-气界面、固-液面上同样也会发生。技术上将能发生明显吸附现象的体系叫做“吸附体系”,并将具有一定吸附能力的材料称为“吸附剂”,将被吸附的物质称为“吸附质”。
 
在压缩空气的吸附脱水过程中,常用的吸附剂有硅胶、活性氧化铝和分子筛等,而均匀混合在压缩空气中的水蒸气就是吸附质。在吸附再生式干燥机中活性氧化铝等固体吸附剂与压缩空气中的水蒸气共同组成了一个吸附体系,压缩空气中的干空气在该吸附体系中担当了承载吸附质的任务,其本身并不参与吸附过程。而吸附操作的逆过程为“解吸”或“脱附”过程,我们通常称为“再生”。
 
根据固体表面和气体分子之间作用力的性质,吸附作用可分为物理吸附和化学吸附两种。前者也称范德华吸附,它是由分子间作用力和静电作用力引起的,后者是由化学键的形成引起的。化学吸附的作用力强于物理吸附作用力,而且选择性也高。
压缩空气的吸附干燥以物理吸附为主。当干燥的压缩空气与吸附剂接触时,空气中的水分子扩散到吸附剂上并因范德华引力而被吸附。与此同时,被吸附的水分子因本身的热运动及外界气态分子碰撞,有一部分离开吸附剂表面返回气相,即发生脱附。当单位时间内水分子的吸附量与脱附量相等时,就达到了一个动态吸附平衡,虽然吸附与脱附过程均在进行但速度相等。此时,单位质量吸附剂所吸附的水蒸气量称为吸附剂的“静态吸附量”,常用g(水)/kg(吸附剂)、或%表示。
 
吸干机一般采用硅胶、活性氧化铝、分子筛作为干燥剂。硅胶:较易受水分潮解。通常用于压力露点为-20℃;活性氧化铝:吸附性能强、稳定,遇到水分不潮解,具有高抗碎强度和抗磨蚀性,适用范围较广,通常压力露点为-40℃;分子筛:由于在相对湿度20%以下有较好的干燥性能,常常作为深度干燥的干燥剂,通常用于压力露点为-70℃。

The so-called adsorption is the phenomenon that the concentration of matter changes automatically on the two phase interface. When the gas molecules move to the surface of the solid, the gas molecules will temporarily stay on the solid surface due to the interaction between the gas molecules and the solid molecules, forming the increase of the concentration of the gas molecules on the solid surface, that is, the gas molecules are adsorbed on the solid surface. The study shows that the adsorption phenomenon occurs not only at the solid gas interface, but also on the liquid gas interface and the solid liquid surface. The system that can have obvious adsorption phenomena is called "adsorption system", and the material with certain adsorption capacity is called "adsorbent", and the adsorbed substance is called "adsorbate".
 
In the process of adsorption and dehydration of compressed air, the commonly used adsorbents are silica gel, active alumina and molecular sieve, and the water vapor, which is evenly mixed in the compressed air, is the adsorbate. In the adsorbent regenerative dryer, the active alumina and other solid adsorbents together with the water vapor in the compressed air constitute an adsorption system. The dry air in the compressed air plays the task of carrying the adsorbate in the adsorption system, and it does not take part in the adsorption process itself. The reverse process of adsorption operation is "desorption" or "desorption" process. We usually call it "regeneration".
 
According to the nature of the interaction between solid surface and gas molecule, adsorption can be divided into two types: physical adsorption and chemical adsorption. The former is also known as Fan Dehua adsorption, which is caused by intermolecular forces and electrostatic forces, and the latter is caused by chemical bonds. Chemisorption is stronger than physical adsorption, and selectivity is high.
The adsorption drying of compressed air is mainly physical adsorption.  When the dry compressed air is in contact with the adsorbent, the water molecules in the air diffuse to the adsorbent and are adsorbed by Fan Dehua attraction. At the same time, the adsorbed water molecules are separated from the surface of the adsorbent and return to the gas phase because of their thermal movement and the impact of the external gaseous molecules. When the adsorption amount of water molecules is equal to the amount of desorption in the unit time, a dynamic adsorption equilibrium is achieved, although the adsorption and desorption processes are all at the same speed. At this point, the amount of water vapor adsorbed by the unit mass adsorbent is known as the "static adsorption capacity" of the adsorbent, commonly expressed as G (water) /kg (adsorbent) or%.
 
The adsorption dryer generally uses silica gel, activated alumina and molecular sieve as desiccant. Silica gel: more vulnerable to moisture deliquescence. Usually used for pressure dew point of -20 C; active alumina: strong and stable adsorption properties, no moisture deliquescence, high crushing strength and abrasion resistance, wide application range, and usually pressure dew point at -40 C; molecular sieve: because of better drying performance under relative humidity of less than 20%, it is often used as dry drying. The agent is usually used for the pressure dew point at -70 C.

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