Reactive reboiler

As an alternative method, Qi et al. [14] proposed to use a reactive condenser (see Fig. 4.1) to predict possible top products of a countercurrent reactive distillation column. The feasibility analyses of the reactive condenser and the reactive reboiler are analogous to the flash-cascade approach. The latter authors used transformed... 

The dimensionless mass balances for the reactive condenser and the reactive reboiler are obtained as [14] ... 

Starting from Eqs. (8), it can be easily shown that Eqs. (10) are also valid for the singular points of the reactive reboiler. 

Table 4.1. Steady-state conditions of reactive condenser and reactive reboiler.

For example 2, Figs. 4.4(a) and (b) show the bifurcations of all singular points with respect to the Damkohler numbers of the reactive condenser and the reactive reboiler, respectively. As can be seen from the feasibility diagram in Fig. 4.4(c), at Damkohler numbers Dac > 0.830, two possible condenser products - that is, the top products of a fully reactive distillation column, are predicted. The kinetic azeotrope in the reactive reboiler is always the possible bottom product of a column. 

Fig. 4.3. Bifurcation diagrams for reactive condenser (a) and for reactive reboiler (b), and feasibility diagram (c) for ellipse-type system.

Venimadhavan et al. [4, 7] studied the effect of the reaction kinetics on the singular point bifurcations of this system in a reactive reboiler. These authors reported one kinetic pinch point at Dar = 0.166. Below, we describe the PSPS and locate the singular points by intersecting the PSPS with the kinetic surfaces. The VLE parameters and the kinetics are taken from Venimadhavan et al. [4],... 

Fig. 4.7. Reactive reboiler. Intersections of potential singular point surface with reaction kinetic surfaces at four different Damkohler numbers Da, MTBE synthesis at 8.11 x 105 Pa.

As can be seen from Fig. 4.7, the kinetic tangent pinch point at the critical Damkohler number Dar = 0.166 has an important role for the topology of the maps. This is also reflected by the feasibility diagrams given in Fig. 4.8(a-c). In Fig. 4.8(c), the stable node branch at positive Damkohler numbers are collected from the singular point analyses of the reactive condenser (Fig. 4.8(a)) and the reactive reboiler... 

For this system, Venimadhavan et al. [7] have studied the bifurcation of the singular points in a reactive reboiler, while Chadda et al. [13] demonstrated the flash-cascade approach. In the present investigation, the same thermodynamic properties and kinetic expression were used (see Tab. 4 in Ref. [7]). 

Here, a batch membrane separator is considered, as depicted in Fig. 4.26(a). The difference between this process and the reactive reboiler which was considered in Section 4.2 is that a membrane is introduced above the vapor phase. For the further analysis, the following assumptions are made ... 

Figure 4.27 shows residue curve maps for the reactive reboiler at three different Damkohler numbers. In the nonreactive case (Da = 0 Fig. 4.27(a)), the map topology is structured by one unstable node (pure B), one saddle point (pure C), and one stable node (pure A). Since pure A is the only stable node of nonreactive distillation, this is the feasible bottom product to be expected in a continuous distillation process. 

First, the role of reaction kinetics is analyzed considering RD processes for the simple reversible reaction Aj o Aj in an ideal binary mixture. The educt Aj is assumed to be the reaction component with the higher boiling point, so the product A2 is obtained in the distillate. The reaction can be carried out in an RD column sequence with an external recycling loop (Fig. 5.1), a non-RD column on top of a reactive reboiler (Fig. 5.2), or a full RD column (Fig. 5.3). More possible configurations are analyzed elsewhere [1]. 

The integration of the recycling reactor directly into the distillation column leads to the process configuration shown in Fig. 5.2, in which the reaction takes place within the column total reboiler. On top of the reboiler a fully non-RD section is installed. This process can be seen as a simple hybrid RD column with only one reactive tray. Comparing the curves in Fig. 5.4 and Fig. 5.6, the operational characteristics of the two processes, recycling system and distillation column with reactive reboiler, are identical at 93 = 00 and R = co. 

For counter-current RD columns with a single chemical reaction taking place, the attainable bottom compositions x can be interpreted as singular points of the mass balances of the reactive reboiler depicted in Fig. 5.13a... 

The kinetics of a chemical reaction have a significant influence on the products that can be attained from a RD process. The attainable products of counterinfinite reflux ratio can be obtained as singular points of a reactive reboiler batch process (bottom product) or a reactive condenser batch process (distillate product). The compositions of both products are located on a unique singular point curve. This curve is independent of any special type of reaction kinetics. However, the locations of the top and bottom products on this curve depend on the structure of the rate equation and on the intensity of the reaction (Damkbhler number) in the considered reaction system. 

Column configurations RD, reactive distillation column SD, simple distillation column. Including a reactive reboiler. 

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