Simulation of Mass Transfer Equipment

Mass transfer is one subject that is unique to chemical engineering. Typical mass transfer problems include diffusion out of a polymer to provide controlled release of a medicine, diffusion inside a porous catalyst where a desired reaction occurs, or a large absorption column where one chemical is transferred from the liquid phase to the gas phase (or vice versa). The models of these phenomena involve multicomponent mixtures and create some tough numerical problems. 

When modeling mass transfer equipment, there are two key points to remember (1) thermodynamics is important and (2) convergence is difficult. The corollary is that you have to compare your thermodynamic predictions with experimental data. Also, you may start with ideal thermodynamics and obtain a solution. This solution can then be used as the initial guess when the thermodynamic model is more realistic. Process simulators do not always work, so you need to be flexible about how you approach a problem. 

Introduction to Chemical Engineering Computing, by Bruce A. Finlayson Copyright 2006 John Wiley Sons, Inc. 

The options for the vapor phase are relatively easy. Some of them are used in Chapters 2 and 3, because once you choose an equation of state, the vapor phase activity coefficient can be determined. The options for the activity coefficient of the gaseous phase are ideal gas, Redlich-Kwong or Redlich-Kwong-Soave, Peng-Robinson, plus a few specialized ones (i.e., HF hexamerization and Hayden-O Connell). 

In the liquid phase, the simplest option is an ideal liquid, with an activity coefficient equal to 1.0. That choice leads to Raoult s law, which may suffice for similar chemicals. Other models include regular solution theory using solubility parameters (although not in Aspen Plus), NRTL, Electrolyte NRTL, UNIFAC, UNIQUAC, Van Laar, and Wilson. Characteristics of the models are  

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For case 1, a screening unit of high flexibility and availability is needed. The extraction volume can be small. For case 2 the amount of extract should be apt to determine the course of the extraction with time. It has proved that about 100 to 500 g of solid substrate is appropriate, meaning that the extraction vessel volume should be about 1000 cm If a gas cycle is added, a parameter study can be carried out on extraction and precipitation. For case 3 the equipment should be able to carry out the total process or to simulate all the process steps in sequence in the same manner as intended in a production process. This means that for purposes of screening and a parameter study the precipitation of caffeine in a decaffeination process can be carried out by adsorption on active charcoal or by absorption in water. But for demonstration of process principles, the individual steps have to be carried out by the same unit operation and with the same type of mass transfer equipment as intended for use in a large scale process. Otherwise, no indication of the joint operation can be obtained. For information on scale up, the extractor volume must be at least one order of magnitude larger than in the laboratory type of equipment. 

In recent years, we explored in this new area on the closure of the differential turbulent mass transfer equation by proposing the two-equation — Ec model and the Reynolds mass flux (fluctuating mass flux) u f model. Our approach has been successfully applied to various chemical processes and equipments, including distillation, absorption, adsorption and catalytic reaction. The interfacial behaviors of mass transfer were also studied extensively by both simulations and experiments. 

The present book is devoted to both the experimentally tested micro reactors and micro reaction systems described in current scientific literature as well as the corresponding processes. It will become apparent that many micro reactors at first sight simply consist of a multitude of parallel channels. However, a closer look reveals that the details of fluid dynamics or heat and mass transfer often determine their performance. For this reason, besides the description of the equipment and processes referred to above, this book contains a separate chapter on modeling and simulation of transport phenomena in micro reactors. 

Design of extraction processes and equipment is based on mass transfer and thermodynamic data. Among such thermodynamic data, phase equilibrium data for mixtures, that is, the distribution of components between different phases, are among the most important. Equations for the calculations of phase equilibria can be used in process simulation programs like PROCESS and ASPEN. 

When appropriate material systems are not available for model experiments, accurate simulation of the working conditions of an industrial plant on a laboratory- or bench-scale may not be possible. Under such conditions, experiments on differently sized equipment are customarily performed before extrapolation of the results to the full-scale operation. Sometimes this expensive and basically unreliable procedure can be replaced by a well-planned experimental strategy. Namely, the process in question can be either divided up into parts which are then investigated separately (Example 9 Drag resistance of a ship s hull after Froude) or certain similarity criteria can be deliberately abandoned and then their effect on the entire process checked (Example 41/2 Simultaneous mass and heat transfer in a catalytic fixed bed reactor after Damkohler). 

When the absorption involves a dilute solute in the gas phase, there is resistance in the gas phase as well. Then simulation also requires identical values of the true gas-side mass-transfer kc- In the laboratory model, the values of itc may be adjusted to that of the industrial equipment either by varying the gas flow rate (D2) or by stirring the gas phase (D2, D6, B16, H8). 

The nonequilibrium stage in Figure 14.1 may represent either a single tray or a section of packing in a packed column. In the models described in this chapter the same equations are used to model both types of equipment and the only difference between these two simulation problems is that different expressions must be used for estimating the binary mass transfer coefficients and interfacial areas. 

The requirements of a process design or research engineer who wishes to use rigorous multicomponent mass transfer models for the simulation and design of process equipment. 

A glib generalization is that the design equations for noncatalytic fluid-solid reactors can be obtained by combining the intrinsic kinetics with the appropriate transport equations. The experienced reader knows that this is not always possible even for the solid-catalyzed reactions considered in Chapter 10 and is much more difficult when the solid participates in the reaction. The solid surface is undergoing change. See Table 11.6. Measurements usually require transient experiments. As a practical matter, the measurements will normally include mass transfer effects and are often made in pilot-scale equipment intended to simulate a full-scale reactor. Consider a gas-solid reaction of the general form... 

With both staged equipment and differential contactors, availability oradequate phase-equilibrium models and rate expressions would allow application of existing correlations and simulation algorithm). For example. knowledge of metal-extraction kinetics in terms of interfacial species concentrations conld be combined with correlations of film mass transfer coefficients in a particular type of equipment to obtain the inlerfacial flux as a fuuction of bulk concentrations. Correlations or separate measurements of inierfacial area and an estimate of dispersion characteristics would allow calculation of extraction performance as a... 

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