Reactive Distillation Potential

The RD s mthesis of methyl acetate by Eastman Chemicals is considered to be the textbook example of a task integration-based process s3mthesis (Stankiewicz and Moulijn, 2002 Stankiewicz, 2003, 2001 Li and Kraslawski, 2004 Siirola, 1996a) (see figure 1.1). 

2 Technical Challenges in the Process Design and Operation of Reactive Distillation 

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Thus, there are a number of potential advantages for reactive distillation ... 

In this research Initiative, we have examined the potential of reactive distillation (9) for terb a/j-butanol dehydration to isobutylene using solid acid catalysis. Advantages to employing reactive distillation for reaction (1) include a) the mild operating conditions required (<120°C), b) quantitative tBA conversions per pass, and c) the option to use lower purity/lower cost, tBA feedstocks. 

TABLE 1 Reported Existing and Potential Applications of Reactive Distillation... 

The most important examples of reactive separation processes (RSPs) are reactive distillation (RD), reactive absorption (RA), and reactive extraction (RE). In RD, reaction and distillation take place within the same zone of a distillation column. Reactants are converted to products, with simultaneous separation of the products and recycling of unused reactants. The RD process can be efficient in both size and cost of capital equipment and in energy used to achieve a complete conversion of reactants. Since reactor costs are often less than 10% of the capital investment, the combination of a relatively cheap reactor with a distillation column offers great potential for overall savings. Among suitable RD processes are etherifications, nitrations, esterifications, transesterifications, condensations, and alcylations (2). 

Muller D, Schafer JP, Leimkuhler HJ. The economic potential of reactive distillation processes exemplified by silane production. Proceedings of VDI-GVC, DECHEMA and EFCE Meeting, Section Thermal Separations, Bamberg, Germany, 2001. 

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. 

CEA quickly selected reactive distillation as its reference process for the iodine section (Goldstein, 2005), because of its simplicity and potential efficiency. In reactive distillation, iodine stripping from the HI/I2/H20 mixture produced by the Bunsen section is performed in the same column as HI gas phase decomposition, taking advantage of iodine condensation into the liquid phase to displace the thermodynamically limited decomposition equilibrium. 

Figure 4.33 illustrates the PSPS and bifurcation behavior of a simple batch reactive distillation process. Qualitatively, the surface of potential singular points is shaped in the form of a hyperbola due to the boiling sequence of the involved components. Along the left-hand part of the PSPS, the stable node branch and the saddle point branch 1 coming from the water vertex, meet each other at the kinetic tangent pinch point x = (0.0246, 0.7462) at the critical Damkohler number Da = 0.414. The right-hand part of the PSPS is the saddle point branch 2, which runs from pure THF to the binary azeotrope between THF and water. 

Fig. 4.33. Potential singular point surface and bifurcation behavior for reactive distillation 1,4-BD —> THF + Water p = 5 atm.

In this chapter, unifying concepts for analyzing and understanding the dynamics of integrated reaction separation processes with rapid chemical reactions are introduced. The text is based on some recent studies [11-13], and extends the concepts introduced earlier for reactive distillation processes [23] to other integrated reaction separation processes. The class of processes to be considered is rather broad. It includes reaction processes where simultaneous separation is used to enhance a reaction, for example, by shifting inherent equilibrium limitations. Various process examples of this kind are provided in this book. The chapter also includes separation processes with potentially reactive mixtures. In this case, a chemical reaction can be either an unwanted side effect or it can be used directly to achieve a certain separation, which is not possible under nonreactive conditions (see e.g. Ref. [10]). The latter represents a reaction-enhanced separation. 

However, we have noticed in previous chapters that the more coupled the process is, the more potential difficulties we have with operation and control. The reason has to do with snowballing, trapping of components, and propagation of composition and thermal disturbances. These issues are definitely still present in reactive distillation, making the control system design for these systems a challenging problem. 

Systems that have the most potential for reactive distillation are those where the reaction is reversible, heat of reaction is not excessively large, and the products have the correct volatilities in relation to the reactants. Those systems reach chemical equilibrium (i.e., reaction stops) unless the reactants are in large excess or the products are continuously removed. An example system has been reported in the literature by Eastman Chemical (Agreda et al., 1990) for the production of methyl acetate from methanol and acetic acid. The discussion about process operation and the control strategy shown in the paper certainlv adhere to the plantwide control principles we have outlined in this book. 

FTocesses for the production of tertiary amyl methyl ether (TAME) Brockwell et ah, Hyd. Proc., 70(9), 133 (1991)]. Highly endothermic reactions may require intermediate reboilers. None of these heat management issues preclude the use of reactive distillation, but must be taken into account during the design phase. Comparison of heat of reaction and average heat of vaporization data for a system, as in Fig. 13-97, gives some indication of potential heat imbalances [Sundmacher, Rihko, and Hoffmann, Chem. Eng. Comm., 127, 151 (1994)]. The heat-neutral systems [-AH (avg)]... 

Distillation columns with multiple conventional side reactors were first suggested by Schoenmakers and Buehler German Chem. Eng., 5, 292 (1982)] and have the potential to accommodate gas-phase reactions, highly exo- or endothermic reactions, catalyst deactivation, and operating conditions ontside the normal range snitable for distillation (e.g., short contact times, high temperatnre and pressure, etc). Krishna (chap. 7 in Sundmacher and Kienle, eds.. Reactive Distillation, Wiley-VCH, 2003). 

Consequently, the HI at the bottom of the reactive column is I2 rich and is fed back to Section 1 as I2 supply for the Bunsen reaction. H2 is bled off the column top as a compressed gas for storage or use. The environment of HIx and high temperature and pressure required in the heat exchanger in the reactive distillation process make it potentially one of the most corrosive environments within the S-I cycle. 

Potential applications of reactive distillation to other systems, including those with unfavorable thermodynamics, are discussed in recent articles... 

Reactive residual curves can be used to estimate the potential products that can be obtained from a reactive distillation column. The main drawback of this concept... 

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