Commercial adsorbents

Important properties and a number of applications of several commercial adsorbents are summarized in Tables 2—4. 

Fig. 3. Adsorption equiHbrium isotherms for moisture on three commercial adsorbents peUetized 4A zeoHte (—), siHca gel (-), and a typical activated...

Most commercial adsorbents for gas-phase appHcations are employed in the form of pellets, beads, or other granular shapes, typically about 1.5 to 3.2 mm in diameter. Most commonly, these adsorbents are packed into fixed beds through which the gaseous feed mixtures are passed. Normally, the process is conducted in a cycHc manner. When the capacity of the bed is exhausted, the feed flow is stopped to terminate the loading step of the process, the bed is treated to remove the adsorbed molecules in a separate regeneration step, and the cycle is then repeated. 

Although we did not discuss this, you should be able to readily identify commercial adsorbents that can compete with activated carbon in water treatment applications. What are they, what are their properties, and how do unit costs compare In performing the cost analysis, take into consideration the volumes of adsorbents needed to achieve comparable degrees of water treatment. To do this, you should develop a base case scenario. 

ZeoHte catalysts and adsorbents are widely accepted in industry. Commercial adsorbents based on synthetic aluminosihcates zeolite A and X became available in 1948 [4]. Zeolite Y as FCC catalyst became commercially available in 1964 [5]. 

Other applications of commercial adsorbents are given in Table 17.1, taken from the work of Crtttenden(1). Some typical solvents which are readily recovered by adsorptive techniques are listed in Table 17.2, taken from information supplied by manufacturers. 

All such processes suffer one disadvantage in that the capacity of the adsorbent for the adsorbate in question is limited. The adsorbent has to be removed at intervals from the process and regenerated, that is, restored to its original condition. For this reason, the adsorption unit was considered in early industrial applications to be more difficult to integrate with a continuous process than, say, a distillation column. Furthermore, it was difficult to manufacture adsorbents which had identical adsorptive properties from batch to batch. The design of a commercial adsorber and its operation had to be sufficiently flexible to cope with such variations. 

Another feature of the breakthrough method is that the uniform geometry of the packed column permits fairly straightforward analysis of the thermal waves produced due to heats of adsorption, and of their effect on the shape of the sorption fronts. The most obvious advantage of this method, however, is the fact that the results of the breakthrough experiments can be applied rather directly to the design of commercial adsorbers, with relatively little analysis of the data. 

In the fall of 1948, I was measuring the adsorption characteristics of numerous commercial adsorbents and of the natural zeolite, chabazite. Several uses for silica gel in air separation plants were identified. But the more we learned about chabazite, the more intrigued I became by its potential as a commercial adsorbent as well as its possible use in air purification and separation. I envisioned, as others had before me [1-5], major new separation processes based on a series of different pore size zeolites. The stumbling blocks were that (1) chabazite was the only known zeolite with seemingly practical adsorption... 

Stationary phase surface are the primary factor for successful separations. Most commercial adsorbents reflect their surface chemistry in their names (e.g., C18, C8, Phenyl, etc.) while the base material used usually is not specihed, although its properties are very important. 

While water is used as a solvent in a majority of commercial adsorbent and... 

I. Alpay E., Haq N., Kershenbaum L.S. and Kirkby N.F., Adsorption parameters for strongly adsorbed hydrocarbon vapours on some commercial adsorbents. Gas Separation and Purification 10 (1996) pp.25-33. 

Chemically 1-MCP is very interesting, because it is a highly unstable cyclopropene compound. A few years ago scientists thought about cyclopropenes as scientific curiosities without practical value. The 1-MCP proves that they were wrong. Being unstable and a gas as a neat substance, it is supplied commercially adsorbed to solid materials in powder form to make handling easier. When the powder is mixed with a specified amount of water, the 1-MCP gas is released to the gas phase where it interacts with the plants before it decomposes. 

In a much more insightful and detailed study, Huang and Fu 46) reexamined the same effects using a. series of 15 well-characterized commercial adsorbents and found an order of magnitude difference in the uptakes at optimum pH (4.0). 

The attachment of molecules to the surface of a solid by adsorption is a broad subject. This chapter is focused on the adsorption of gases in high-capacity solid adsorbents such as active carbon or zeolites. These commercial adsorbents owe their enormous capacity to an extensive network of nanopores of various shapes (cylinders, slits) with specific volumes in the range from 100 to 1000 cm kg . Applications of adsorption exploit the ability of nanoporous materials to adsorb one component of a gas preferentially. For example, the preferential adsorption of nitrogen from air passed through an adsorption column packed with zeolite creates a product stream of nearly pure oxygen. 

Most commercial adsorbents consist of small microporous or nonporous crystals formed into macroporous pellets or particles. The solutes carried along the col-lunn by the fluid mobile phase must first be transported from the bulk fluid phase to the external surface of the adsorbent and then they must diffuse inside the particles. Within a particle there are two distinct kinds of diffusion phenomena that contribute to the resistances to mass transfer, the macropore (or inter-crystaUine) diffusion through the pellet and the micropore (or intra-crystaUine) diffusion resistance. The relative importance of macropore and micropore diffusion resistances depends on the pore size distribution within an adsorbent particle. Micropores have diameters smaller than 2 nm, macropores diameters greater than 50 nm while mesopores are in the range of 2 to 50 nm. 

Commercial adsorbents are generally produced in bound forms (0.5-6.0 mm diameters) in regular particle shapes (beads, pellets, extrudates, granules, etc.). The purpose is to reduce pressure drops in adsorbers. Clay, alumina, polymers, pitch, etc. are used as binders, which typically constitute 10-20% (by weight) of the final product. The binder phase usually contains a network (arteries) of meso- and macropores (0.5-50.0 pm diameters) to facilitate the transport of the adsorbate... 

An excellent review and detailed coverage on commercial adsorbents and new adsorbent materials has been presented by Yang in his newly published monograph on adsorbents.A very brief overview of existing commercial adsorbents is given here. Commercial sorbents that have been used in large-scale adsorptive separation and purification processes include activated carbon, zeolites, activated alumina, silica gel, and polymeric adsorbents. Although the worldwide sales of sorbent materials are relatively small as compared with other chemical commodities, sorbents and adsorption processes play a very important role in many process industries. The estimated worldwide sales of these sorbents are as follows ... 

Fig. 1 compares the pore size distributions of major commercial adsorbents discussed in this section. Activated carbons have a broad pore size distribution like activated alumina and silica gel. Although activated carbon is thought to be hydrophobic, it does adsorb... 

Table 7.1 lists the typical sorbents used their uses as well as strengths and weaknesses. The four major commercial adsorbents are the following zeolite molecular sieves (zms), activated alumina, silica gel, and activated carbon. The surfaces of activated alumina and most molecular-sieve zeolites are hydrophilic, and will preferentially adsorb water over organic molecules. Silicalite, which is a hydrophobic zeolite, is the main exception. Activated carbon, on the other hand, preferentially adsorbs organic and non-polar or weakly polar compounds over water. The surface of silica gel is somewhere in between these limits and has affinity for both water and organics. Detailed information about each of these classes of adsorbents can be found in Refs. [1,4, 6, 7]. 

Many adsorbents, such as activated carbon and ion-exchange resins, can efficiently separate antibiotics and other small biologically active molecules from the fermentation broth. Unfortunately, these adsorbents also interact with the microbial cells and some of the dissolved nutrients. Thus, the use of ion-exchange resins and activated carbon to remove fermentation products is frequently associated with problems of simultaneous removal of nutrients and side products. Substantial volume reduction occurs but only limited purification can be achieved. Commercial adsorbents and ion-exchange resins are available in various matrices and sizes. Larger particles are preferred for easy separation from the broth but they can be internal mass transfer limited. 

In this section, we focus on the properties, uses, and origins of a few generic types of commercial adsorbents. Neither the list nor the descriptions are comprehensive. The intent is to provide some background information regarding the most important aspects. 

The TPD of CO2 is given in Figure 3. It shows that CO2 is chemisorbed on AD-101 and hence has strong basic centre. Treatment with alkali makes it a weak basic site whereas treatment with acid makes it further very weak site. The commercial adsorbent which is used for removal of antioxidant has a weak site which is close to that of acid treated alumina sample. 

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