Crystals, adsorption

Separation Processes for PX. There are essentially two methods that are currendy used commercially to separate and produce high purity PX (/) crystallization and (2) adsorption. A third method, a hybrid crystallization /adsorption process, has been successfiiUy field-demonstrated and the first commercial unit is expected in the near future. 

Hybrid Crystallization/Adsorption Process. In 1994, IFP and Chevron announced the development of a hybrid process that reportedly combines the best features of adsorption and crystallization (59,99). In this option of the Eluxyl process, the adsorbent bed is used to initially produce PX of 90—95% purity. The PX product from the adsorption section is then further purified in a small single-stage crystallizer and the filtrate is recycled back to the adsorption section. It is reported that ultrahigh (99.9+%) purity PX can be produced easily and economically with this scheme for both retrofits of existing crystallization units as well as grass-roots units. A demonstration plant was built at Chevron s Pascagoula refinery in 1994. 

Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

A separation is diluted when the distillate or the bottom product is less than 5 wt% with respect to the feed. The distillation (simple, extractive or azeotropic) might not be the most economical, but other methods, such as liquid-liquid extraction, stripping, crystallization, adsorption, or membrane permeation, should be tried. The decision depends on the mixture composition and the nature of the components. 

Perform difficult zeotropic separations later, but before azeotropic separations. Examine other options, such as extractive distillation, L-L extraction, crystallization, adsorption, or molecular sieves. 

Crystallization and adsorption are both widely used to perform the separation distillation is not used (except for orthoxylene separation) because of too small differences between the boiling points (Table 10.1). Despite the still high importance of crystallization, adsorption becomes the most widely used technique because of its high efficiency. The adsorbents which are used for selective adsorption of paraxylene are X or Y zeolites exchanged with adequate cations. Liquid phase Simulated Counter Current adsorption, which is the most efficient process, is generally used (1). In addition to the complexity of this process, the choice of an adsorbent selective for paraxylene is the critical point. 

Mechanical activation of the reaction mixtures by ball-milling before crystallization has been applied to the preparation of high-silica MWW crystals. Adsorption of water and toluene was used to characterize of zeolite products. 

Explain in your own words the terms separation process, distillation, absorption, scrubbing, liquid extraction, crystallization, adsorption, and leaching. (What are they and how do they work )... 

Worcester EM, NakagawaY, WabnerCL, Kumar S, Coe EL. Crystal adsorption and growth slowing by nephrocalcin, albumin, and Tamm-Horsfall protein. Am.J.Physlol. 1888 255 F1179-F1205. 

The design of the SCAC instrument has been described in detail previously 11] and here only a brief account of the experimental technique is given. A schematic of our single-crystal adsorption calorimetry experiment is shown in Fig. 9.1. 

Fig. 9.1 The key components of the single-crystal adsorption calorimeter (SCAC). Reprinted with permission from [1]. Copyright 1996 Elsevier...

Stuck A, Wartnaby CE, Yeo YY, Stuckless JT, Al-Sarraf N, King DA (1996) An improved single crystal adsorption calorimeter. Surf Sci 349 229... 

Below, we describe two examples that illustrate electrochemical applications of STM. Other topics that can be studied by solution-phase STM include surface reconstructions of single crystals, adsorption of molecules, and anodic dissolution [59,60]. 

Separation processes are not only of great importance in refineries, but also in the chemical, petrochemical, gas processing, and pharmaceutical industries. Although the reactor can be regarded as the heart of a chemical plant, in most cases, 60-80% of the total cost is taken up by the separation step. This step involves one or more thermal separation processes such as distillation, extraction, absorption, crystallization, adsorption, membrane processes, c/c., which are used to obtain the products at the required purity. 

A dual-solvent fractional extraction process can provide a powerful separation scheme, as indicated by the examples given above, and some authors suggest that fractional extraction is not utilized as much as it could be. In many cases, instead of using full fractional extraction, standard extraction is used to recover solute from a crude feed and if the solvent-to-feed ratio is less than 1.0, concentrate the solute in a smaller solutebearing stream. Another operation such as crystallization, adsorption, or process chromatography is then used downstream for solute purification. Perhaps fractional extraction schemes should be evaluated more often as an alternative processing scheme that may have advantages. 

Irreversible processes. Diffusion, thermal diffusion, gravitational, crystallization, adsorption, biological, laser-based photochemical, electrolysis. 

On the other hand, for a given electrolyte, the distribution of the adsorption states up to 0.8 V vs. RHE depends on the degree of the surface order of the platinum single crystal. Adsorption states at higher potentials are related to the presence of wide two-dimensional ordered (100) domains [11,62]. The maximum development of these states was obtained after cooling a flame-treated sample in an oxygen-free atmosphere [3,11,62,63]. 

We again point out that there are a number of experimental values of Eads for CO atop buUc Pt(l 11) suii ce. Early studies measured the Eads to be 1.43 eV [42], 1.40 [43] and 1.55 eV [45]. King et al. [46], by single crystal adsorption microcalorimetry, have recently measured a value of 1.90 0.07 eV for the low-coverage adsorption of CO on Pt(l 10) surface. Although this value refers to a different metal bulk surfece, adsorption energies of many small molecules on Pt do not vary much with facial structure. The calculated Eads (= 1-92 eV) of CO on Pt o cluster is closer to the most recent experimental value [46]. 

Borroni-Bird C E, Al-Sarraf N, Andersson S and King D A 1991 Single crystal adsorption microcalorimetry Chem. 

Borroni-Bird C E and King D A 1991 An ultrahigh vacuum single crystal adsorption microcalorimeter Rev. Scl. Instrum 62 2177-85... 

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