Liquid Phase Process Characterization

The concept of a highly automated scale-up process is enticing. One vision of such a process for homogeneous (liquid phase, non-catalytic) reactions starts with little more than a list of reactants, solvents, and desired products. Given this information as well as constraints imposed by economic, environmental, safety, and practical factors, a highly automated system could include both software and hardware components to generate an optimal reactor design. The necessary software components would  

convert proposed mechanisms into reaction rate expressions  

Micro Instrumentation for Throughput Experimentation and Process Intensification - a Tool for PAT 

Edited by M. V. Koch, K. M. VandenBussche and R. W. Chrisman Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31425-6 

insert these rate expressions into a model of the experimental reactor system, including at least material balances and perhaps energy and momentum balances  

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Older methods use a liquid phase process (Figure 10-11). ° New gas-phase processes operate at higher temperatures with noble metal catalysts. Using high temperatures accelerates the reaction (faster rate). The hydrogenation of benzene to cyclohexane is characterized by a highly exothermic reaction and a significant decrease in the product volume... 

Significant development has occurred within the industry over the last several years with respect to liquid-phase processes. One example of this process that is reasonably close to commercialization is that developed by Air Products. A pilot unit has been operated for several years at their La Porte, Texas location. The process is characterized briefly as using an inert hydrocarbon reaction medium in the liquid phase to absorb the synthesis heat of reaction conventional copper-zinc catalyst is fed to the reactor system as a slurry. This type of process appears to be particularly well suited to substoichiometric feeds (hi earbon eontent), such as those produced by partial oxidation or coal gasification. The Air Products process has been extensively deseribed in patent literature [14]. Kinetie data and liquid-phase reaetion systems have also been extensively diseussed by Lee in Methanol Synthesis Technology [15]. 

In chemical laboratories, small flasks and beakers are used for liquid phase reactions. Here, a charge of reactants is added and brought to reaction temperature. The reaction may be held at this condition for a predetermined time before the product is discharged. This batch reactor is characterized by the varying extent of reaction and properties of the reaction mixture with time. In contrast to the flasks are large cylindrical tubes used in the petrochemical industry for the cracking of hydrocarbons. This process is continuous with reactants in the tubes and the products obtained from the exit. The extent of reaction and properties, such as composition and temperature, depend on the position along the tube and does not depend on the time. 

Column reactors can contain a draft tube - possibly filled with a packing characterized by low pressure drop - or be coupled with a loop tube, to make the gas recirculating within the reaction zone (see Fig. 5.4-9). In recent years, the Buss loop reactor has found many applications in two- and three-phase processes About 200 Buss loop systems are now in operation worldwide, also in fine chemicals plants. This is due to the high mass-transfer rate between the gas and the liquid phase. The Buss loop reactor can be operated semibatch-wise or continuously. As a semibach reactor it is mostly used for catalytic hydrogenations. 

The determination of adsorption isotherms at liquid-solid interfaces involves a mass balance on the amount of polymer added to the dispersion, which requires the separation of the liquid phase from the particle phase. Centrifugation is often used for this separation, under the assumption that the adsorption-desorption equilibrium does not change during this process. Serum replacement (6) allows the separation of the liquid phase without assumptions as to the configuration of the adsorbed polymer molecules. This method has been used to determine the adsorption isotherms of anionic and nonionic emulsifiers on various types of latex particles (7,8). This paper describes the adsorption of fully and partially hydrolyzed PVA on different-size PS latex particles. PS latex was chosen over polyvinyl acetate (PVAc) latex because of its well-characterized surface PVAc latexes will be studied later. 

Reynolds numbers calculated for the in vivo hydrodynamics are considerably lower than those of the corresponding in vitro numbers, both for bulk and particle-liquid Reynolds numbers. Remarkably, bulk Reynolds numbers in vivo appear to have about the same magnitude as particle-liquid Reynolds numbers characterizing the flow at the particle surface in vitro using the paddle apparatus. In other words, it appears that hydrodynamics per se play a relatively minor role in vivo compared to the in vitro dissolution. This can be attributed to physiological co-factors that greatly affect the overall dissolution in vivo but are not important in vitro (e.g., absorption and secretion processes, change of MMC phases,... 

Relaxation dispersion data for water on Cab-O-Sil, which is a monodis-perse silica fine particulate, are shown in Fig. 2 (45). The data are analyzed in terms of the model summarized schematically in Fig. 3. The y process characterizes the high frequency local motions of the liquid in the surface phase and defines the high field relaxation dispersion. There is little field dependence because the local motions are rapid. The p process defines the power-law region of the relaxation dispersion in this model and characterizes the molecular reorientations mediated by translational displacements on the length scale of the order of the monomer size, or the particle size. The a process represents averaging of molecular orientations by translational displacements on the order of the particle cluster size, which is limited to the long time or low frequency end by exchange with bulk or free water. This model has been discussed in a number of contexts and extended studies have been conducted (34,41,43). 

However, atom motions cannot be unambiguously imaged by time-resolved optical spectroscopic methods as they do not directly measure the structural dynamics but instead characterize energetic properties. Consequently, novel methods that enable the direct measurement of molecular motions during chemical processes are needed. Furthermore, chemical reactions often occur in solution and, consequently, it is desirable that such methods are applicable to chemical processes in the liquid phase. 

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... 

Sublimation. Accdu to definition ziven in Perry (Ref 13, p660), it covers the physical changes encountered by a substance in passing from a solid phase to a gas and back to a solid phase. It is characterized by the absence of the liquid phase and is used for purification of volatile substances like iodine, camphor, etc. Detailed description of the process is given in Refs 12, 13, 15 18). 

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