Adsorbents for Separation Solid Material

When the product is a solid, such as titanium dioxide used as an additive to paint and to paper to improve opacity and covering, the descriptive parameters also include the crystalline and amorphous nature of the solid, the average diameter of the crystals, 

Let us consider a zeolite adsorbent for selective separation in an important oil refinery stream that contains aromatic compounds. 

Petroleum refineries produce a stream of valuable aromatic compounds called the BTX, or benzene-toluene-xylenes (Ruthven 1984). The Cg compounds can be easily separated from the Ce and C compounds by distillation, and consist of ethyl benzene, o-xylene, m-xylene, and / -xylene. Ethyl benzene is the starting material for styrene, which is used to make polystyrene / -xylene is oxidized to make terephthalic acid, and then condensed with ethylene glycol to make polyester for fibers and films. The buyers of / -xylene are the manufacturers of terephthalic acid, such as BP-Amoco, who in turn sell to the fiber manufacturers such as DuPont and Dow. These are big and sophisticated companies that have strong research and engineering capabilities, and are used to have multiple suppliers. The eventual consumers of adsorbents are the public who consider polyester as one of the choices in fabric and garments, in competition with other synthetic and natural fibers. Their purchases are also dependent on personal income and prosperity. In times of recession, it is always possible for a consumer to downgrade to cheaper fibers and to wear old clothes for a longer period of time before new purchases. 

The process to separate the four Cg aromatics by distillation is very difficult because of their closeness in boiling points (table 11.3), so that only o-xylene can be separated by distillation. / -Xylene has a unique high melting point, and can be separated by cryogenic crystallization, but this is an expensive process that requires refrigeration. What is desired is an economic separation process that singles out jo-xylene among these four compounds. 

The principal types of industrial adsorbent can be divided into amorphous and the crystalline types. The former includes activated carbon, silica gel, and activated alumina the latter includes zeolites and their aluminum phosphate, AIPO4 (or ALPO), analogs. Yang (2003) wrote that, since the invention of synthetic zeolites in 1959, adsorption has become a key separation tool in the chemical, petrochemical, and pharmaceutical industries. Adsorptive separation of different molecules can be achieved by three mechanisms equilibrium adsorption differences, diffusion kinetics differences. 

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