Scale-up of systems

The need for large volumes of material often leads to a scale-up of systems. With UF, for example, hollow fibre systems and stirred cells have been compared for the concentration and fractionation of HS. Kwak and Nelson (1977) showed that retention increased with pressure, decreased with ionic strength and was not concentration dependent. HoUow fibres proved ineffective for UF fractionation compared to flat sheet membranes due to clogging and indiscriminate retention effects (Kiichler et al (1994)). 

Another example of the impact of dissolving solids on reaction performance on the industrial scale is provided by Yamazaki et al. (1989). This example highlights the difficulty of scale-up of systems in which the mass transfer rate influences the product distribution and selectivity of complex reactions and may control the overall reaction rate. In this example the rate-determining step is the dissolution of K2CO3 particles in dipolar solvents. 

Hydrothermal Synthesis Systems. Of the unit operations depicted in Figure 1, the pressurized sections from reactor inlet to pressure letdown ate key to hydrothermal process design. In consideration of scale-up of a hydrothermal process for high performance materials, several criteria must be considered. First, the mode of operation, which can be either continuous, semicontinuous, or batch, must be determined. Factors to consider ate the operating conditions, the manufacturing demand, the composition of the product mix (single or multiple products), the amount of waste that can be tolerated, and the materials of constmction requirements. Criteria for the selection of hydrothermal reactor design maybe summarized as... 

Scale- Up of Electrochemical Reactors. The intermediate scale of the pilot plant is frequendy used in the scale-up of an electrochemical reactor or process to full scale. Dimensional analysis (qv) has been used in chemical engineering scale-up to simplify and generalize a multivariant system, and may be appHed to electrochemical systems, but has shown limitations. It is best used in conjunction with mathematical models. Scale-up often involves seeking a few critical parameters. Eor electrochemical cells, these parameters are generally current distribution and cell resistance. The characteristics of electrolytic process scale-up have been described (63—65). 

Direct Scale-Up of Laboratory Distillation Ljficiency Measurements It has been found by Fair, Null, and Bolles [Ind. Eng. Chem. Process Des. Dev., 22, 53 (1983)] that efficiency measurements in 25- and 50-mm (1- and 2-in-) diameter laboratory Oldersbaw columns closely approach tbe point efficiencies [Eq. (14-129)] measured in large sieve-plate columns. A representative comparison of scales of operation is shown in Fig. 14-37. Note that in order to achieve agreement between efficiencies it is necessaiy to ensure that (1) tbe systems being distilled are tbe same, (2) comparison is made at tbe same relative approach to tbe flood point, (3) operation is at total reflux, and (4) a standard Oldersbaw device (a small perforated-plate column with downcomers) is used in tbe laboratoiy experimentation. Fair et al. made careful comparisons for several systems, utibzing as large-scale information tbe published efficiency studies of Fractionation Research, Inc. 

The scale-up of the Scheibel column is still considered proprietary, and therefore the vendor (Glitsch Process Systems Inc.) should be consulted for the final design. From pilot tests in 0.075-m diameter column, industrial columns up to 3 m in diameter and containing 90 actual stages have been provided. 

Chemical reactions obey the rules of chemical kinetics (see Chapter 2) and chemical thermodynamics, if they occur slowly and do not exhibit a significant heat of reaction in the homogeneous system (microkinetics). Thermodynamics, as reviewed in Chapter 3, has an essential role in the scale-up of reactors. It shows the form that rate equations must take in the limiting case where a reaction has attained equilibrium. Consistency is required thermodynamically before a rate equation achieves success over tlie entire range of conversion. Generally, chemical reactions do not depend on the theory of similarity rules. However, most industrial reactions occur under heterogeneous systems (e.g., liquid/solid, gas/solid, liquid/gas, and liquid/liquid), thereby generating enormous heat of reaction. Therefore, mass and heat transfer processes (macrokinetics) that are scale-dependent often accompany the chemical reaction. The path of such chemical reactions will be... 

This involves knowledge of chemistry, by the factors distinguishing the micro-kinetics of chemical reactions and macro-kinetics used to describe the physical transport phenomena. The complexity of the chemical system and insufficient knowledge of the details requires that reactions are lumped, and kinetics expressed with the aid of empirical rate constants. Physical effects in chemical reactors are difficult to eliminate from the chemical rate processes. Non-uniformities in the velocity, and temperature profiles, with interphase, intraparticle heat, and mass transfer tend to distort the kinetic data. These make the analyses and scale-up of a reactor more difficult. Reaction rate data obtained from laboratory studies without a proper account of the physical effects can produce erroneous rate expressions. Here, chemical reactor flow models using matliematical expressions show how physical... 

Garside, J. and Jancic, S.J., 1979. Measurement and scale-up of secondary nucleation kinetics for the potash alum-water system. American Institute of Chemical Engineers Journal, 25, 948. 

Crystallization Process Systems brings together essential aspects of the concepts, information and techniques for the design, operation and scale up of particulate crystallization processes as integrated crystal formation and solid-liquid separation systems. The focus of the book, however, is on crystallization only dealing with related unit operations as far as is necessary. It is therefore... 

The scale-up of mixing data has been treated with a variety of approaches, some to rather disastrous results. The principles are now W ell established, and it is a matter of truly understanding the par ticular systems that poses... 

Apart from the improvement and scaling up of known systems such as the lead accumulator or the nickel/cadmium cell, new types of cells have also been developed. Here, rechargeable lithium batteries and nickel-systems seem to be the most promising the reason for this will be apparent from the following sections [3]. 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

In general, the only problem in the scale-up of distillation is to obtain reliable vapour-liquid equilibrium (VLB) data. Computer programs cover the vast majority of physical systems and data banks for VLB are included in these programs. However, VLB data are not available for... 

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