Distillation Column with Reactive Reboiler

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 conceptual design it is reasonable to represent the process operation in a classical McCabe-Thiele diagram (Fig. 5,7). A relative volatility of a = 10 was assumed for the sake of better visualization. The specified distillate composition (assume = 0.95) fixes the operating point of the condenser. The reboiler oper- 

5 Importance of Reaction Kinetics for Cataiytic Distiiiation Processes lOS 

For n = 1, y can be calculated from (5.15). depends on the kinetic parameters (Da, n) as well as on thermodynamic parameters K, a) of the system. Fig. 5.8 illustrates how the minimum reflux ratio varies with the Damkohler number for different reaction orders n. Qualitatively, decreases hyperbolically with increasing Da number. Multiple pinch points are possible for negative reaction orders within a certain Da window due to multiple intersections of the reboUer operating line with the VLF line. At l mm = the critical Damkohler number, Da n, — tP (5.12) is recovered. At Da = the smallest possible reflux ratio R it is obtained, which only depends on thermodynamic parameters besides the product specification xP 

The Fenske equation yields the minimum number of stages at Da = and R = 

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Rg. 5.2. Distillation column with reactive total reboiler... 

Going one level up in aggregation we consider the structures of complete columns. Conventionally, RD columns are classified in hybrid and non-hybrid units, depending on the distribution of reactive trays inside the column. Thus, hybrid columns are distillation columns with a reactive core and nonreactive sections (rectifying and/or stripping sections). Non-hybrid RD columns denote columns where all trays including condenser and reboiler are reactive (Giittinger, 1998). Hybrid columns are commonly... 

Continuous benzene alkylation was conducted in a reactive distillation column of the type illustrated in Figure 1. The process unit comprises the following principal elements a double column of solid catalyst 32, packing columns above and below the catalyst bed, a liquid reboiler 42 fitted with a liquid bottoms product takeoff 44, a condenser 21 fitted with a water collection and takeoff, and a feed inlet... 

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. 

The initial distillate cut is the lightest and, as the distillation progresses, the liquid remaining in the reboiler becomes continuously richer in the heavier components, and subsequent distillate cuts become increasingly heavier. The residue remaining in the reboiler after the last distillate cut is the heaviest cut. A multicomponent feed mixture may be separated in one batch distillation column into a number of products with specified purities. Given the required number of trays and reflux ratio, a batch distillation column could, in principle, separate a normal feed mixture (one that is not reactive or azeotrope forming) into its pure constituents. 

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]. 

Reactive distillation. To avoid conversion of product into unwanted components or to drive equilibrium reactions forward, reactive distillation is used to distill out one of the products immediately as it is formed. A conventional equilibrium process consists of a reactor and a distillation column. One of the products is removed in the column, and the bottom is recycled (see Fig. 6.22). In reactive distillation the column is located on top of the reactor, obviating the need for a reboiler (see Fig. 6.23). Reaction of an organic chlorine compound with an aqueous caustic solution can be carried out in a reactive distillation column, which will combine functions such as dissolution of... 

Can equipment sets be combined (e.g., replacing reactive distillation with a separate reactor and multi-column fractionation train installing internal reboilers or heat exchangers) to reduce overall system volume ... 

Fig. 5.9 shows the geometric representation of these balances in the McCabe-Thiele diagram. The distillate composition xP is located at the intersection of the condenser operating line and the operating line of the RD section. By specifying xP, the number of reactive trays can be estimated from the classical staircase construction. From the intersection of the operating line of the column section with that of the reboiler line, the bottom composition is determined. 

The reactor and column Cl are identical to those used in the pressure-swing process. The methanol-containing distillate Di from the top of the reactive column is fed to stage 6 of a 12-stage extraction column. Water is fed on the top tray at a rate of 1050 kmol/h and a temperature of 322 K, which is achieved by using a cooler (heat removal = 1.24 MW). The column is a simple stripper with no reflux. The column operates at 2.5 atm so that cooling water can be used in the condenser (reflux-dmm temperature = 326 K). Reboiler heat input is 5.96 MW. The overhead vapor is condensed and it is the C5 product stream. 

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