Modes zeolite separation

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. 

Dual-temperature ion-exchange purification of concentrated solutions of alkali metal salts from divalent and monovalent ion admixtures on a number of natural and artificial zeolites without auxiliary reagents consumption is studied. Two types of dual-temperature techniques - the temperature-swing mode of separation and the parametric pumping - are used. 

Figure 10. N-hexane ( ) / 2-2 dimethylbutane ( ) separation with the composite zeolite-alumina membrane (fluxes in the permeate as a function of the temperature). A mixture of n-hexane. 2-2 dimethylbutane and nitrogen (5 6 89) was fed in the tube (Fig. 2) with a flow rate of 2 1/h. Sweep gas (N2), countercurrent mode, flow rate 0.5 1/h.

ENSORB [ExxoN adSORBtion] A process for separating linear from branched hydrocarbons, using a zeolite molecular sieve. The adsorbed gases are desorbed using ammonia. It operates in a cyclic, not a continuous, mode. Developed by Exxon Research Engineering Company, and used by that company on a large scale at the Exxon refinery in Baytown, TX. Asher, W. J., Campbell, M. L., Epperly, W. R., and Robertson, J. LHydrocarbon Process., 1969, 48(1), 134. 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

Faulkner and coworkers (27., ), have also studied the interaction of Ru(bpy)32+ with zeolite X. Luminescence lifetime measurements and emission spectra were used to study electron transfer quenching of the electron donors N,N,N, N -tetramethyl-p-phenylenediamine and 10-phenyl-phenothiazine. Lifetime measurements show at least two modes of quenching for the interaction of Ru(bpy)32+ ions with these donors. Products of these electron transfer reactions were isolated and these experiments show that the zeolite can separate the products of light induced electron transfer. 

The pulse chromatographic technique was used to study low coverage adsorption of linear and monobranched alkanes in the CS-C8 range on two different 10-membered ring zeolites, ZSM-22 and ZSM-23. Henry adsorption constants, enthalpies of adsorption, preexponential Actors of the van t Hoff equation and separation fectors were determined. A detailed interpretation of the experimental data confirmed the previously proposed pore-mouth adsorption mode for branched alkanes on zeolite ZSM-22, where these molecules do not have access into the depth of the pores, but rather point their linear part herein. Zeolite ZSM-23 shows adsorption properties between zeolite ZSM-22 and zeolite ZSM-S, which is a shape selective zeolite where normal and monobranched molecules have both access into its pore system. 

Figure 2.19 Left part of panel, (a-h) Qualitative representation of the IR spectroscopic features of weak, medium, and strong A-H- -B or A - -H-B H-bonded complexes. The half-width of the bending modes is (somewhat arbitrarily) assumed to slightly increase upon increasing the hydrogen bond strength. The shaded areas correspond to regions obscured by the skeletal modes of the zeolite framework. Right part of panel, (a -h ) Schematic representation of the correlated evolution of the proton potential as function of the A-H distance in A-H- - -B or A - - -H-B The separation barrier in e -f can be very low, and a potential curve characterized by an asymmetric single flat minimum may be used alternatively. Reproduced with permission from Ref. (11). Copyright 1997 American Chemicai Society.

The bridging hydroxyls in zeolites (3610-3600 cm ) also manifest a pronounced (Voh + Sqh) mode, which is observed at higher frequencies than the combination modes of the silanol groups, namely in the region of4654-4661 cm (1.S3,312,323,411j. Thus, like the overtones, the combination modes of these different bands are well separated (see Figure 2.50). 

The major difficulty in the determination of the mode(s) of coking and of deactivation is to establish the composition of coke. Indeed coke is a complex mixture which is very difficult to separate from the zeolite. A critical review of the methods used for characterizing coke will constitute the first part of this paper whereas the second part will show how the composition of coke as function of various parameters varies. 

For commercial applications, an adsorbent must be chosen carefully to give the required selectivity, capacity, stability, strength, and regenerability. The most commonly used adsorbents are activated carbon, molecular-sieve carbon, molecular-sieve zeolites, silica gel, and activated alumina. Of particular importance in the selection process is the adsorption isotherm for competing solutes when using a particular adsorbent. Most adsorption operations are conducted in a semicontinuous cyclic mode that includes a regeneration step. Batch slurry systems are favored for small-scale separations, whereas fixed-bed operations are preferred for large-scale separations. Quite elaborate cycles have been developed for the latter. 

Dead-end filtration causes excessive crystal accumulation whereas the cross-flow mode allows a more uniform and compact zeolite layer, because the suspension flows tangentially along the support surface. In cross-flow filtration separation systems, a high velocity is typically used for increasing the sweeping action through the membrane. This is not desired for seeding purposes which... 

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