Purification, adsorption

Zeolites have been used in the industrial adsorptive purification of aromahc petrochemicals since the early 1970s. The application of zeolites to aromatic adsorptive purification and extraction is a particularly suitable fit because of three major factors. The first is the inherent difficulty involved in separating certain aromatic components by distillation. Petrochemical production requires individual components be obtained in very high purity, often in excess of 99.5%. While distillation is the most popular method of separation in the petrochemical industry, it is not well suited for the final step of producing high purity single component streams from close boiling multi-component aromatics-rich mixtures. 

The simulated moving bed operational mode involves four distinct functional zones, the adsorption, purification, desorption and buffer zones. These zones are described in detail in other parts of this book. We now examine the function of each zone as it applies to p-xylene adsorption and which can be extrapolated to the other aromatics separations. 

Industrial-scale adsorption processes can be classified as batch or continuous. In a batch process, die adsorbent bed is saturated and regenerated in a cyclic, operation. In a continuous process, a countercurrent staged contact between lire adsorbent and die feed and desorbent is established by cidier a true or a simulated recirculation of die adsorbent. The efficiency of an adsorption process is significantly higher in a eoiuinuous mode of operation than in a cyclic batch mode. For difficult separations, batch operation may require 25 times more adsorbent inventory and twice die desorbent circulation rate than does a continuous operation. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, wtiereas a continuous mode allows mure degrees of freedom with respect to these functions, and thus a better overall operation. 

Proper integration of advanced reformer design with adsorption purification systems can reduce overall feed and fuel requirement below 400 Btu per standard cubic foot of hydrogen. 

Adsorptive purification of the type we are talking about here (sometimes called physisorptiori) involves a relatively weak interaction between the sorbate molecule and the surface active sites in the sorbent. The association depends mostly on van der Waals forces, which are primarily caused by electronic and/or electrostatic interactions between electron-rich or electron-poor regions of the sorbate molecule and receptive sites on the sorbent surface. Molecules bound by such weak forces can be removed by various solvent extraction techniques in contrast, molecules that actually form covalent bonds with the surface (chemisorption) usually cannot. 

ADSORPTIVE PURIFICATION AGENTS—DESCRIPTION/PREPARATION/PROPERTIES... 

ADSORPTIVE PURIFICATION AGENTS—DESCRIPTION/PREPARA TION/PROPERTIES... 

As with powdered activated carbon, amorphous silica hydrogels are used in conjunction with bleaching clays in the adsorptive purification of fats and oils. Silica hydrogels possessing average pore diameters greater than about 60 A exhibit high... 

ADSORPTIVE PURIFICATION PROCESS GENERAL DESCRIPTION 323 To Steam Condensate Oil... 

The adsorptive purification process obeys the Freundlich equation, which can be expressed mathematically as... 

Adsorption systems are used to remove water and carbon dioxide from feed air. Two or more adsorption beds are used in a swing arrangement to accomplish the required purification. An energy requirement of 5-7% of the air compressor power is expended in the operation of an adsorptive purification system. But heat exchanger optimization and cold box simplification can be achieved with this type of system and regeneration heat can be recovered and air losses completely eliminated. The energy penalty is overcome by the advantages of an adsorption system and, as a result, most modern air separation plants use adsorptive purification for the feed air. 

Gas activity. The inert gas (Ar) above lead level in the reactor usually contains impurities such as polonium, some amount of hydrogen from oxygen content control systems and tritium. Purification of radioactive cover gas can be made by hydrogen oxidation (to water), water separation and adsorption. Purification from polonium can be made by alkaline extraction. All these methods are considered to develop. 

The basic principles of some generally accepted gas cleaning processes for solvent removal are given in Table 22.1.2. For more than 70 years adsorption processes using activated carbon, in addition to absorption and condensation processes, have been used in adsorptive removal and recovery of solvents. The first solvent recovery plant for acetone was commissioned for economic reason in 1917 by Bayer. In the decades that followed solvent recovery plants were built and operated only if the value of the recovered solvents exceeded the operation costs and depreciation of the plant. Today sueh plants are used for adsorptive purification of exhaust air streams, even if the return is insufficient, to meet environmental and legal requirements. 

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