Liquid-phase adsorptions adsorptive characteristics

Liquid-phase adsorption characteristics examined by atomic force microscopy (AFM) were compared for two pyridine base molecules, pyridine and d-picoline. on (010) surfaces of two natural zeolites, heulandite and stilbite. These adsorption systems formed well-ordered. two-dimensioncJ (quasi-)hexagonal adlayers. The 2D lattice structures of the ordered adlayers w ere dependent on the adsorbate/substrate combinations. Although there existed certain habit in the orientation of the 2D lattice unit vector of the adsorbed phase with respect to the substrate(OlO) lattice vectors, the molecular arrays w ere incommensurate with the substrate atomic arrangements. 

This chapter addresses the fundamentals of zeolite separation, starting with (i) impacts of adsorptive separation, a description of liquid phase adsorption, (ii) tools for adsorption development such as isotherms, pulse and breakthrough tests and (iii) requirements for appropriate zeolite characteristics in adsorption. Finally, speculative adsorption mechanisms are discussed. It is the author s intention that this chapter functions as a bridge to connect the readers to Chapters 7 and 8, Liquid Industrial Aromatics Adsorptive Separation and Liquid Industrial Non-Aromatics Adsorptive Separation, respectively. The industrial mode of operation, the UOP Sorbex technology, is described in Chapters 7 and 8. 

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

Preparative chromatographic processes are of increasing importance particularly in the production of fine chemicals. A mixture of compounds is introduced into the liquid mobile phase, and this then flows through a packed column containing the stationary solid phase. The contacting scheme is thus differential, but since the adsorption characteristics of the compounds in the mixture are similar, many equivalent theoretical stages are required for their separation. Chromatographic processes are mostly ran under transient conditions, such that... 

Another approach to continuous reaction chromatography is the countercurrent moving-bed chromatographic reactor (CMCR). In this type of reactor the stationary (solid) phase travels in the opposite direction to the liquid phase. In practice this is performed by introducing the stationary phase from the top of the reactor. The stationary phase flows downwards under the influence of gravity while the liquid phase is pumped upwards from the bottom. A schematic presentation of such a system is shown in Fig. 7. Depending on the adsorption characteristics of the different components, they can travel in the direction of the liquid or the solid phase resulting in their separation. 

Liquid phase zeoHtic separation includes two main events adsorption and desorption. Adsorption of an adsorbate (liquid component being adsorbed by solid) onto zeolitic adsorbent is dictated by the characteristics of the adsorbate-adsorbent interaction. A zeolitic adsorbent is a crystalline porous solid having particular characteristics (see Chapter 2). When immersed in a liquid mixture, the porous... 

Ion exchange shares many characteristics with adsorption, such as mass transfer from the fluid to the solid phase there are, however, some significant differences. Specifically, although both processes can be characterized as sorption processes, the sorbed species are ions in ion exchange, whereas electrically neutral substances are sorbed hi adsorption. Moreover, in ion exchange, the ions removed from the liquid phase are replaced by ions from the solid phase. So, there actually occurs an exchange of ions and not only a removal... 

The adsorption of surfactants at the liquid-solid surface is affected by the nature of the solid surface (surface charge, polarity and non-polarity), the molecular structure of surfactant molecules (head group charge and characteristics of hydrophobic tail) and the characteristics of the liquid phase (pH, electrolyte concentration, presence of additives and temperature). 

In this section peptide separation by gel permeation, normal-phase adsorption, ion-exchange, and liquid-solid adsorption HPLC will be considered. Compared to reversed-phase HPLC, these elution modes have attracted considerably less interest for peptide separations. This is due to a number of factors arising from the diverse characteristics of the solutes... 

The right combination of the two immiscible liquid phases, mobile and stationary, can lead to highly selective separations at ordinary temperatures for both volatile and nonvolatile solutes. An important characteristic is that th partition ratio ideally is independent of concentration the elution bands are therefore more symmetrical and less subject to tailing than those observed in adsorption chromatography. Consequently, better resolution is usually possible. The principal problem is that of stabilizing the stationary phase the stationary and mobile phases are not completely insoluble in each other, even if one is aqueous and the other a hydrocarbon. 

Adsorption from solution involves the transfer of soluble species from the liquid phase to the surface of an adsorbent, a transfer which is governed by specific system dynamics and which results in a characteristic equilibrium distribution, or phase partitioning, of a solute. In the case of microporous adsorbents such as activated carbon, the uptake of solute consists of the four basic steps illustrated schematically in Figure 4 I) advective or bulk... 

In this section, methods of separation and purification which depend on the partitioning of material between two phases will be considered. If one phase is a solid, the method is called adsorption, and if the process is carried out in such a way as to multiply the efficiency of a single adsorption, it is commonly called chroviatography. If the material is distributed between two liquid phases or a gas and a liquid phase, the process is called extraction. These methods are particularly valuable for separating mixtures, since they discriminate between compounds on the basis of physical properties different from those which were important for the methods previously described. Thus, two solid compounds which cannot be separated conveniently by recrystallization because of their very similar solubility characteristics may often be separated by chromatography. 

The nature of the solvent in liquid-phase alkyne hydrogenations and the extent to which it can influence the competitive adsorption factors needed to attain selectivity should also be considered. The semihydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts of varying metal load gave 1-octene with a selectivity of 100% over a wide range of metal loads when the reaction was run in heptane.38 n-propanol, however, reaction selectivity increased with decreasing metal load. Apparently the alcohol interacted with the catalyst to modify the active sites and influenced the relative adsorption characteristics of the acetylenic and olefinic species. This can affect reaction selectivity particularly if reactant diffusion assumes some importance in the reaction. 

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