Porous sorbent

At a much earlier stage in the research and development cycle, fluidized-bed processes use porous sorbents containing copper oxide (82), cerium oxide (83), and other metal oxides (84). 

The Brunauer type I is the characteristic shape that arises from uniform micro-porous sorbents such as zeolite molecular sieves. It must be admitted though that there are indeed some deviations from pure Brunauer type I behavior in zeoHtes. From this we derive the concept of the favorable versus an unfavorable isotherm for adsorption. The computation of mass transfer coefficients can be accompHshed through the construction of a multiple mass transfer resistance model. Resistance modehng utilizes the analogy between electrical current flow and transport of molecular species. In electrical current flow voltage difference represents the driving force and current flow represents the transport In mass transport the driving force is typically concentration difference and the flux of the species into the sorbent is resisted by various mechanisms. 

Highly porous sorbents were obtained by dehydrochlorination of Rovil fiber in LiOH solution in dimethylsulfoxide at 160°C followed by activation with carbon dioxide at 950°C (Fig. 4.4). 

WC Lee. Protein separations using non-porous sorbents. J Chromatogr B 699 29-45, 1997. 

CEC has recently become an alternative to HPLC. A capillary is filled or its internal wall covered with a porous sorbent. The free volume remaining in the capillary is filled with an electrolyte. High voltage (on the order of ten kV) is applied across the length of the capillary. Sample plugs are introduced at one end. Sample components are carried to the other end due to electro-osmosis and - in the case of ions - also electrophoresis. In CEC the more important effect is electro-osmosis, which is essentially a flow mechanism of the electrolyte solution without the need for applied pressure. The separation of the sample components occurs mainly due to phase distribution between the stationary phase and the flowing electrolyte. Thus CEC is very similar to HPLC in a packed capillary except that the flow is not pressure driven and that ionic analytes undergo electrophoresis additionally to phase separation. 

The analyte may interact two-dimensionally with the sorbent surface through adsorption due to intermolecular forces such as van der Waals or dipole-dipole interactions [53]. Surface interactions may result in displacement of water or other solvent molecules by the analyte. In the adsorption process, analytes may compete for sites therefore, adsorbents have limited capacity. Three steps occur during the adsorption process on porous sorbents film diffusion (when the analyte passes through a surface film to the solid-phase surface), pore diffusion (when the analyte passes through the pores of the solid-phase), and adsorptive reaction (when the analyte binds, associates, or interacts with the sorbent surface) [54]. 

Figure 2.20. Micro-, macro-, and mesopores in a porous sorbent. (Reprinted with permission from Ref. 56. Copyright 1996 Barnebey Sutcliffe Corporation.)...

For porous sorbents, most of the surface area is not on the outside of the particle but on the inside pores of the sorbent (Figure 2.20) in complex, interconnected networks of micropores (diameters smaller than 2 nm), mesopores (2 to 50 nm), also known as transitional pores, and macropores (greater than 50 nm) [57], Most of the surface area is derived from the small-diameter micropores and the medium-diameter transitional pores [56], Porous sorbents vary in pore size, shape, and tortuosity [58] and are characterized by properties such as particle diameter, pore diameter, pore volume, surface areas, and particle-size distribution. 

On the other hand, the lack of internal pore structure with micropellicular sorbents is of distinct advantage in the analytical HPLC of biological macromolecules because undesirable steric effects can significantly reduce the efficiency of columns packed with porous sorbents and also result in poor recovery. Furthermore, the micropellicular stationary phases which have a solid, fluid-impervious core, are generally more stable at elevated temperature than conventional porous supports. At elevated column temperature the viscosity of the mobile phase decreases with concomitant increase in solute diffusivity and improvement of sorption kinetics. From these considerations, it follows that columns packed with micropellicular stationary phases offer the possibility of significant improvements in the speed and column efficiency in the analysis of proteins, peptides and other biopolymers over those obtained with conventional porous stationary phases. In this paper, we describe selected examples for the use of micropellicular reversed phase... 

A useful literature relating to polypeptide and protein adsorption kinetics and equilibrium behavior in finite bath systems for both affinity and ion-ex-change HPLC sorbents is now available160,169,171-174,228,234 319 323 402"405 and various mathematical models have been developed, incorporating data on the adsorption behavior of proteins in a finite bath.8,160 167-169 171-174 400 403-405 406 One such model, the so-called combined-batch adsorption model (BAMcomb), initially developed for nonporous particles, takes into account the dynamic adsorption behavior of polypeptides and proteins in a finite bath. Due to the absence of pore diffusion, analytical solutions for nonporous HPLC sorbents can be readily developed using this model and its two simplified cases, and the effects of both surface interaction and film mass transfer can be independently addressed. Based on this knowledge, extension of the BAMcomb approach to porous sorbents in bath systems, and subsequently to packed-, expanded-, and fluidized-bed systems, can then be achieved. 

In fig. 1.31 the lUPAC-recommended classification is given for the most common types of hysteresis loops they are refinements of the general type IV in fig. 1.13. In practice, a wide variety of shapes may be encountered of which types HI and H4 are the extremes. In the former, the two branches are almost vertical and parallel over an appreciable range of V, whereas in the latter they remain more or less horizontal over a wide range of p/p(sat). Types H2 and H3 are intermediates. Many hysteresis loops have in common that the steep range of the desorption branch leads to a closure point that is almost independent of the nature of the porous sorbent and only depends on the temperature and the nature of the adsorptive. For example, it is at p/p(sat) = 0.42 for nitrogen at its boiling point (77 K) and at p/p(sat) = 0.28 for benzene at 25°C. 

A very good review article based on a panel study of status, future research needs, and opportunities for porous sorbent materials was published several years ago. It was pointed out that very significant advances have been made in tailoring the porosity of porous sorbent materials in terms of size and shape selectivity. Relatively little progress has been achieved in terms of chemoselectivity of sorbents based on specific interactions between adsorbate molecules and functional groups in the sorbents. Incorporation of active sites into sorbents is of high priority in the development of sorbents. 

Mass-separating agent intra-particle diffusion in a porous sorbent. Energy-separating agent application of an electric field across a liquid phase to accelerate the charged particles relative to neutrals. 

Sorbent booms are specialized containment and recovery devices made of porous sorbent material such as woven or fabric polypropylene, which absorbs the oil while it is being contained. Sorbent booms are used when the oil slick is relatively thin, i.e., for the final polishing of an oil spill, to remove small traces of oil or sheen, or as a backup to other booms. Sorbent booms are often placed off a shoreline that is relatively unoiled or freshly cleaned to remove traces of oil that may recontaminate the shoreline. They are not absorbent enough to be used as a primary countermeasure technique for any significant amount of oil. 

The capacity of a sorbent depends on the amount of surface area to which the oil can adhere as well as the type of surface. A fine porous sorbent with many small capillaries has a large amount of surface area and is best for recovering light crude oils or fuels. Sorbents with a coarse surface would be used for cleaning up a heavy crude oil or Bunker. Pom-poms intended for recovering heavy Bunker or residual oil consist of ribbons of plastic with no capillary structure. General purpose sorbents are available that have both fine and coarse structure, but these are not as efficient as products designed for specific oils. 

In the two examples mentioned above, the porous sorbents are already in a compressed state of fine elementary particles. Therefore we extended our work to measuring and calculating isotherms on Cab-o-Sil, Aerosil, Sil-bead, and a number of carbon blacks whose radii were previously determined by electron microscopic or adsorption methods.A few examples are shown in Fig. 7. The theoretical curves for the S.C. type are shown in the figure by the solid lines calculated from the previously known values of and the... 

Mass Transfer in Fixed Beds of Porous Sorbents... 

MASS TRANSFER IN FIXED BEDS OF POROUS SORBENTS... 

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