Distillation rate-based analysis

This appendix shows how the Aspen Plus simulator can be used to do detailed rate-based analysis of distillation using the Maxwell-Stefan approach outlined in Sections 15.7 and 16.8. Lab 10 should be done before this lab. NOTE If you have convergence problems, reinitialize and try running again. 

Mueller I., C. Pech, D. Bhatia, and E. Y. Kenig, Rate-based analysis of reactive distillation sequences with different degrees of integration, Chem. Eng. Sci., 62, 7327-7335 (2007). 

In turn, the effect of interactions on the control of I3 depends both on the recycle structure and the manipulated variable. Distillate rate D2 gives more interactions compared with the case where the manipulated variable belongs only to S4 (either D4 or Q4). The use of manipulated variables from different units should not be a surprise when these are, dynamically speaking, close enough, as it is the case with S2 and S4. In the base-case and alternative B the effect of the S4 variables on I3 is enhanced by closing the other loops, while in alternatives A and C this effect is hindered. However, at this point there is not a clear distinction between the base-case and alternatives. A dynamic analysis is needed. 

In a typical experiment, a solution of 1-48 g (0-01 mole) of phthalic anhydride in 8-05 ml (0-1 mole) of pyridine was pyrolyzed at 690° in dry, high-purity nitrogen flowing at the rate of 2-7 1/hr. Contact time was 20-2 sec. The products were distilled to recover 6-34 ml of pyridine. The distillation residue weighed 2-12 g, of which 0-06 g was removed for analysis by mass spectrometry. The remainder was dissolved in ether and separated into nitrogen bases (1-44 g) and hydrocarbons (0-62 g) by extraction with dilute hydrochloric acid. Analysis by gas chromatography, by comparison of retention times with authentic samples, gave the results shown in Table 9. 

Cyclohexene was dried and vacuum-distilled before use. It was degassed by three freeze-punp-thaw cycles and stored over activated molecular sieves in a glass bulb. It was introduced into the reactor via vapor phase transfer through a high vacuum manifold (base pressure <10 Torr). After 30 mins, the epoxide yield was quantified on an HP 6890 GC/MS equipped with a J W Scientific DBl capilary column. At the end of each experiment, Ti analysis was performed (15) and epoxide/Ti ratios were calculated. For kinetics experiments, silica powder containing the /ert-butylperoxotitanium complex was prepared in an in situ reactor and the reaction initiated by addition of olefin. The IR spectrum of the gas phase above the silica was recorded at timed intervals. Pseudo-first-order rate constants... 

In this chapter, we describe an algorithm for predicting feasible splits for continuous single-feed RD that is not limited by the number of reactions or components. The method described here uses minimal information to determine the feasibility of reactive columns phase equilibrium between the components in the mixture, a reaction rate model, and feed state specification. This is based on a bifurcation analysis of the fixed points for a co-current flash cascade model. Unstable nodes ( light species ) and stable nodes ( heavy species ) in the flash cascade model are candidate distillate and bottom products, respectively, from a RD column. Therefore, we focus our attention on those splits that are equivalent to the direct and indirect sharp splits in non-RD. One of the products in these sharp splits will be a pure component, an azeotrope, or a kinetic pinch point the other product will be in material balance with the first. 

Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfadal mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples. 

The heterogeneities of most concern to us are those that involve the presence of more than one phase. The analysis of multiphase systems can be important to the design and operation of many industrial processes, especially those in which multiple phases influence chemical reactions, heat transfer, or mixing. For example, phase-equilibrium calculations form the bases for many separation processes, including stagewise operations, such as distillation, solvent extraction, crystallization, and supercritical extraction, and rate-limited operations, such as membrane separations. 

In packed columns, it is conceptually incorrect to use the staged model even though it works if the correct height equivalent to a theoretical plate (HETP) is used. In this chapter we will develop a physically more realistic model for packed columns that is based on mass transfer between the phases. After developing the model for distillation, we will discuss mass transfer correlations that allow us to predict the required coefficients for common packings. Next, we will repeat the analysis for both dilute and concentrated absorbers and strippers and analyze cocurrent absorbers. A simple model for mass transfer on a stage will be developed for distillation, and the estimation of stage efficiency will be considered. After a mass transfer analysis of mixer-setder extractors. Section 16.8 and the appendix to Chapter 16 will develop the rate model for distillation. 

We saw in Section 15.6 that a Fickian mass-transfer analysis can lead to logical inconsistencies when extended to three or more conponents. Thus, a fundamental rate analysis of multiconponent distillation must be based on the Maxwell-Stefan mass-transfer model extended to nonideal multiconponent systems fSection 15.7.7T Since the significant detail required for these calculations is beyond the scope of an introductory textbook, the methods are summarized in enough detail to explain what the commercial simulator does (Lab 13 in appendix to Chapter 161 but not in enough detail to write a program of your own. Readers interested in the conplete details are referred to Taylor and Krishna (1993) and Aspen Plus r2QlQT... 

In order to automate the analysis, these methods frequently combine immobilized enzymes with flow or sequential injection techniques. These methods may include a separation step such as solid-phase extraction, gas diffusion, or pervaporation. The latter is a nonchromatographic separation technique, which selectively separates a liquid mixture by partial vaporization through a nonporous polymeric membrane. Separation is not based on relative volatilities as in distillation, but rather on the relative rates of permeation through the membrane. 

There are four key points about the basic equation that are relevant for membrane separations. First, the separations are inherently based on rate of transport. They depend on diffusion. In this sense, they are like absorption and adsorption. They are much less like the analysis of staged distillation, which depends largely on thermodynamics. 

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