Selectivity and mass transfer

Den, W., Sompuram, S. R., Sarantopoulos, S., and Sharon, J. (1999) A bidirectional phage display vector for the selection and mass transfer of polyclonal antibody libraries. J. Immunol. Meth. 222, 45-57. 

The esterification of 1-octanol with hexanoic acid is a good system for studying reactive stripping, as it not only represents many industrial processes, but also illustrates the interdependencies of activity, selectivity, and mass transfer in reactive separations. 

POST-TREATMENTS AFEECTING AFFINITY, SELECTIVITY AND MASS TRANSFER PROPERTIES... 

Ion-pairing or chelating reagents can be added to the donor phase, which permits SLM extraction of various metal ions. Different carrier molecules or ions can be incorporated in the membrane phase to enhance selectivity and mass transfer. Various trapping reagents in the acceptor phase prevent analytes to be extracted back into the membrane. There are several reviews in this field [47-49]. 

Further, with SLM extraction, the pH of the donor solution and the acceptor phase can easily be fine-tuned to obtain extraction which is selective for certain groups of compounds, as described above. Alternatively, a carrier can be added in the membrane to increase the selectivity and mass transfer of the compounds of interest. When the primary arm of adding a carrier is to increase the mass transfer of the analytes by analyte-carrier interactions in the membrane, it is important to choose other conditions carefully so that selectivity is stiU retained. 

The pertraction efficiency and selectivity can also be increased by adding specific carrier to the membrane phase. Various carrier molecules or ions can be incorporated in the membrane phase to enhance the selectivity and mass transfer. Most carriers used for this were originally developed for solvent extraction 28]. However, many new carriers were designed only for liquid membrane pertraction. Characteristic features of a good carrier for SLM pertraction are ... 

However, various carrier molecules or ions have been used in the membrane phase and many reagents in the strip phase for trapping, to enhance selectivity and mass transfer the pertraction, are often not selective enough. Such selective extraction could be obtained by utilizing soluble antibodies in the strip phase. 

The search for a suitable adsorbent is generally the first step in the development of an adsorption process. A practical adsorbent has four primary requirements selectivity, capacity, mass transfer rate, and long-term stabiUty. The requirement for adequate adsorptive capacity restricts the choice of adsorbents to microporous soUds with pore diameters ranging from a few tenths to a few tens of nanometers. 

In addition to the fundamental parameters of selectivity, capacity, and mass-transfer rate, other more practical factors, namely, pressure drop characteristics and adsorbent life, play an important part in the commercial viabiUty of a practical adsorbent. 

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

In order of priority, the factors that govern the selection of industrial dryers are (/) personnel and environmental safety (2) product moisture and quahty attainment (J) material handling capabiHty (4) versatiHty for accommodating process upsets (5) heat- and mass-transfer efficiency and (6) capital, labor, and energy costs. 

Second, it is necessary to take account of non-Newtonian behaviour in the design of process plant and pipelines. Heat and mass transfer coefficients are considerably affected by the behaviour of the fluid, and special attention must be devoted to the selection of appropriate mixing equipment and pumps. 

The selection of reactor type in the traditionally continuous bulk chemicals industry has always been dominated by considering the number and type of phases present, the relative importance of transport processes (both heat and mass transfer) and reaction kinetics plus the reaction network relating to required and undesired reactions and any aspects of catalyst deactivation. The opportunity for economic... 

Improved control over heat and mass transfer as well as residence time by micro-channel processing often allows one to increase the reaction temperature of cryogenic processes without losing selectivity. It often leads to improved selectivity. 

Estimation of parameters. Model parameters in the selected model are then estimated. If available, some model parameters (e.g. thermodynamic properties, heat- and mass-transfer coefficient, etc.) are taken from literature. This is usually not possible for kinetic parameters. These should be estimated based on data obtained from laboratory expieriments, if possible carried out isothermal ly and not falsified by heat- and mass-transport phenomena. The methods for parameter estimation, also the kinetic parameters in complex organic systems, and for discrimination between models are discussed in more detail in Section 5.4.4. More information on parameter estimation the reader will find in review papers by Kittrell (1970), or Froment and Hosten (1981) or in the book by Froment and Bischoff (1990). 

The high surface-to-volume ratio can also significantly improve both thermal and mass transfer conditions within micro-channels in two ways firstly, the convective heat and mass transfers, which take place at the multi-phase interface, are improved via a significant increase in heat and mass transfer area per unit volume. Secondly, heat and mass transfers within a small volume of fluid take a relatively short time to occur, enabling a thermally and diffusively homogeneous state to be reached quickly. The improvement in heat and mass transfer can certainly influence overall reaction rates and, in some cases, product selectivity. Perhaps one of the more profound effects of the efficient heat and mass transfer property of micro-reactors is the ability to carry potentially explosive or highly exothermic reactions in a safe way, due to the relatively small thermal mass and rapid dissipation of heat. 

Intermediate pore zeolites typified by ZSM-5 (1) show unique shape-selectivities. This has led to the development and commercial use of several novel processes in the petroleum and petrochemical industry (2-4). This paper describes the selectivity characteristics of two different aromatics conversion processes Xylene Isomerization and Selective Toluene Disproportionation (STDP). In these two reactions, two different principles (5,j6) are responsible for their high selectivity a restricted transition state in the first, and mass transfer limitation in the second. 

Modem catalysts have to be very active and very (100%) selective, that is, they have to catalyze the desired reaction in the temperature window, where the equilibrium conversion is the highest possible and the reaction rate is high enough to permit suitable process economics. To engineer the reaction, one has to obtain first the intrinsic reaction rate, free of heat- and mass-transfer limitations. In many cases this is very difficult, because in the core of the catalytic process there are several physical and chemical steps that must occur and which may preclude the reaction running in the kinetic regime. These steps are as follows ... 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

Rieckmann and Volker fitted their kinetic and mass transport data with simultaneous evaluation of experiments under different reaction conditions according to the multivariate regression technique [116], The multivariate regression enforces the identity of kinetics and diffusivities for all experiments included in the evaluation. With this constraint, model selection is facilitated and the evaluation results in one set of parameters which are valid for all of the conditions investigated. Therefore, kinetic and mass transfer data determined by multivariate regression should provide a more reliable data basis for design and scale-up. 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

The criteria for adsorbent selection are covered by the following five main performance characteristics selectivity, capacity, mass transfer rate and long-term stability and zeolite type [2]. A brief description of each follows. 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

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