Tray reactive distillation

Figure 16.28 Comparison of TACs for EtAc production using reactive distillation (RD), single reactive tray reactive distillation (RD-SRT), and side reactor configuration (SRC).

The rate-based models suggested up to now do not take liquid back-mixing into consideration. The only exception is the nonequilibrium-cell model for multicomponent reactive distillation in tray columns presented in Ref. 169. In this work a single distillation tray is treated by a series of cells along the vapor and liquid flow paths, whereas each cell is described by the two-film model (see Section 2.3). Using different numbers of cells in both flow paths allows one to describe various flow patterns. However, a consistent experimental determination of necessary model parameters (e.g., cell film thickness) appears difficult, whereas the complex iterative character of the calculation procedure in the dynamic case limits the applicability of the nonequilibrium cell model. 

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

In the first case, product purities are controlled indirectly by controlling front positions. In distillation columns the front positions are easily controlled with cheap, reliable and fast online temperature measurements on sensitive trays inside the column [27]. A similar procedure was recently proposed for moving-bed chromatographic processes with UV rather than temperature measurement [37]. However, the performance of such an approach is usually limited. Exact product specifications cannot be guaranteed because of this indirect approach. Furthermore, in combined reaction separation processes the relationship between the measured variable and the variable to be controlled is often non-unique, which may lead to severe operational problems as shown for reactive distillation processes [23], It was concluded that these problems could be overcome if in addition some direct or indirect measure of conversion is taken into account. 

Thus, the key result from the tray-by-tray calculation is that the column design must ensure complete alcohol consumption in the reactive zone, only lauric acid and water are allowed in the top vapor stream. The column behaves more as a reactive absorber than reactive distillation. A higher number of equilibrium stages... 

Reactive distillation is a technology that simultaneously performs fractional distillation and chemical reaction. Fig. 1 is a schematic representation of a reactive distillation tray column for a reaction of the type... 

In reactive distillation, chemical reactions are assumed to occur mainly in the liquid phase. Hence the liquid holdup on the trays, or the residence time, is an important design factor for these processes. Other column design considerations, such as number of trays, feed and product tray locations, can be of particular importance in reactive distillation columns. Moreover, since chemical reactions can be exothermic or endothermic, intercoolers or heaters may be required to maintain optimum stage temperatures. Column models of reactive distillation must include chemical reaction... 

Today the main process application for bubble cap trays is in reactive distillation columns or in chemical absorption columns in either case, it may be necessary to control very carefully the residence time of the liquid to complete a reaction step. For example, bubble-cap trays are used for the methyl acetate column described earlier and published by Agreda et al. An abridged version of the Bolles treatment of bubble-cap tray design is given in the fifth edition of Perry s handbook." ... 

Bubble-cap trays are considered obsolete, but still used in some occasions, for example at extremely low liquid load, or for conducting reactive distillation, where high holdup and longer residence time are necessary. They are very sensitive to fouling and expensive in maintenance. 

Whereas homogeneously catalyzed reactive distillation can be carried out in conventional tray columns (sometimes modified to ensure sufficient residence time of the reactants), a heterogeneous catalyst has to be fixed in the reactive section with the help of special internals. These internals have to combine good wetting characteristics to achieve a good contact between the Hquid and vapor phases with a large amount of catalyst that is readily accessible by the liquid in order to avoid macro-kinetic influences. 

Fig. 3.7. Composition profiles of a reactive distillation column equipped with 10 reactive trays, and 10 non-reactive trays in the stripping and rectifying section each for high and low feed-rates. The feed position was located above the reactive section ([18], reprinted from Chem. Eng. Sci., Vol 57, Beckmann et al., Pages 1525-1530, Copyright 2002, with permission from Elsevier Science)...

The (non-reactive) distillation columns linked to the side reactors can be much smaller in diameter than the RD column and no specially designed trays (e. g., with higher weirs or additional sumps) or proprietary devices such as Katapak-S are necessary. The side-reactor concept is particularly attractive when the conversion requirements are not as stringent as assumed in the MeOAc case study above. 

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