Reactive distillation lines

Fig. 5.2-36 Design of the reactive distillation line of a ternary mixture liquid equilibrium . stoichiometric lines... 

For instantaneous chemical reactions a reactive distillation line represents the concentration profile of the liquid within a column. From its knowledge processes of reactive distillation can be designed (Stichlmair and Frey 1998) as is demonstrated at the example of the reversible instantaneous reaction a + b< c. 

Figure 2.2. Representation of stoichiometric and reactive distillation lines for the reactive system A-B-C undergoing the reaction A+B C. Remark tt denotes the pole at which stoichiometric lines coincide, XiT, = VijT,Vj. Legend dashed line stoichiometric line dotted line phase-equilibrium hne continuous line chemical equilibrium line. System feature ...

A third concept is introduced when chemical reaction and phase equilibrium phenomena are superimposed in the unit reactive distillation lines. The following sequence of phase equilibrium— chemical reaction steps is adopted for the evolution of reactive distillation lines,... 

The starting point of this sequence is the reactive mixture xi on the chemical equilibrium line. This liquid mixture is in phase equilibrium with the vapor yl, which is totally condensed to x. Since this mixture is apart from the chemical equilibrium line, it reacts along the stoichiometric line to the equilibrium composition X2- As can be seen in figure 2.2, the difference of the slope between the stoichiometric and liquid-vapor equilibrium lines defines the orientation of the reactive distillation lines. This difference in behavior allows one to identify a point, at which both the phase equilibrium and stoichiometric lines are collinear and where liquid concentration remains unchanged. This special point (labelled A in figure 2.2) is conventionally referred to as reactive azeotrope and is surveyed in section 2.4. 

Stichlmair and co-workers (Frey and Stichlmair, 19996 Stichlmair and Frey, 1999) study the RD design from a thermodynamic-driven point of view. Their approach is based on the existence of reactive distillation lines and potential reactive azeotropes cf. 2.4). 

Description i) the possible products (or feasible separation regions) are determined from the knowledge of the existing concentration profiles within the column, which in turn are represented by reactive distillation lines ii) the column mass balance is included and Hi) possible separation borders are identified. 

A thorough analysis of the behavior of reactive distillation lines with respect to the volatility values of the involved components and the feed staging suggests the following... 

RD is a feasible alternative for systems where the desired products are the nodes in a reactive distillation line diagram and their boiling points differ considerably,... 

Each of the corners represents one pure component. The solid black dot indicates a first minimum-azeotrope. The curved surface illustrates the chemical equilibrium of reaction (1). As the second azeotrope, marked with a circle, lies on that surface, it is a so-called reactive azeotrope. The distillation information is included in the figure as well along the surface of the chemical equilibrium several reactive distillation lines are shown qualitatively. The arrows point towards the direction of decreasing temperatures. 

Bessling, B., Schembecker, G., and Simmrock, K. H. (1997). Design of Processes with Reactive Distillation Line Diagrams. Ind. Eng. Chem. Res. 36, 3032-3042. 

The higher boiling aqueous product fraction flows downwards through the lower distillation section, 10, to a reboiler, 15, where it is heated by an electrical heater. A portion of this higher-boiling aqueous product is withdrawn via an exit line, 15, as shown, and the remainder of the aqueous distillation reaction product is returned to the reactive distillation column, 10, by a reboiler return line. 

Figure A.2 Construction of the distillation lines for nonreactive (left) and reactive mixtures (right).

Bubble-cap plates (Figure 2.3.2-28) have a high liquid capacity and can be used for reactive distillations which require long residence times. The height of the liquid on the plate can be up to 0.5 m. In some applications it is advantageous that the liquid remains on the plates when the plant is off-line. Bubble-cap plates have a particularly wide operating range. 

At the same time, the transformation eliminates the reaction term in the balance equation. The operating line for the rectifying section of a reaction column is formally identical to the operating line of a non-reactive column. An infinite reflux ratio gives an expression that is formally identical to the one for calculating conventional distillation lines [5, 6]. Accordingly, we will refer to lines that have been calculated by this procedure as RD lines. These analogies are found for all the relationships that are important in distillation [7, 8]. 

These analogies become particularly clear if we look at the synthesis of methyl acetate (MeAc) from methanol (MeOH) and acetic acid (HAC) as an example. Essentially, we see diagrams that are similar to the distillation line diagrams of non-reactive systems. As a result of the transformation, the four pure substances lie at the comers of a square and the non-reactive binary systems lie along the edges. 

The concepts of equilibrium stages and of transfer units are, in principle, equivalent in case of parallel operating and equilibrium lines. The more the slopes of these lines differ the more superior are rate-based models to equilibrium-stage models. Further advantages of rate-based models are a better simulation of reactive distillation and absorption processes. At the present state of the art, however, equilibrium stage models are the standard tool for the simulation of distillation columns. 

Identify the azeotropes. Initially, it is very helpful to obtain estimates of the temperature, pressure, and composition of the binary, ternary,..., azeotropes associated with the C-component mixture. For all of the ternary submixtures, these can be determined, as described above, by preparing residue curve or distillation-line maps. When it is necessary to estimate the quaternary and higher-component azeotropes, as well as the binary and ternary azeotropes, the methods of Fidkowski et al. (1993) and Eckert and Kubicek (1997) are recommended. When the C-component mixture is the effluent from a chemical reactor, it may be helpful to include the reacting chemicals, that is, to locate any azeotropes involving these chemicals as well as the existence of reactive azeotropes. This information may show the potential for using reactive distillation operations as a vehicle for crossing distillation boundaries that complicate the recovery of nearly pure species. 

The munber of theoretical and reactive stages is determined from the distillation line and from the intersection of the distillation line and chemical equilibrium manifold (GEM) and represents the boimdary of the forward and backward reactions) (Giessler et al., 1999). Since there are multiple pairs of X and product composition that satisfy the mass balance, the method sets one of the product composition as reference point and solves for the other two (for a 3-component system) by using material balance expressions. Thus, two of the components compositions and X lie on the same line of mass balance (LMB) in the diagram and allow the estimation of the ratio D/B at a certain reboil ratio only by exploring the ratio of the line segments (figure 3.1f>). 

The PRAz is characterized by the fact that the residue lines curve toward it but never converge to it (Ung and Doherty, 1995c) and for practical purposes it imposes the limit beyond which distillation can not proceed (Ung and Doherty, 1995c). This reactive mixture is located halfway up the hypothenuse and behaves as a severe pinch point. As depicted in figure 5.7, the singular points of the reacting system define five reactive distillation boundaries (QAz MNAz), (QAz iC4), (QAz MeOH), (QAz nC4) and (QAz PRAz). 

Jimenez L., Wanhschafft O. and Julka V. (2000). Synthesis of reactive and extractive distillation units using distillation line diagrams. Computer-Aided Chemical Engineering 8, 985-990. 3.2.2... 

Many industrial reactive distillation systems do not use stoichiometric amounts of reactants. An excess (10-20% above the stoichiometric amount) of one of the reactants is fed to the reactive column. There may be kinetic reasons for using an excess in some systems. These include suppressing undesirable side reactions, reducing catalyst requirements, and increasing conversion. However, even in the absence of kinetic reasons, the use of an excess of one of the reactants makes the control problem easier because the fresh feed flowrates of the components do not have to be precisely balanced in the reactive column. Achieving this exact balance may require the use of expensive and high maintenance on-line composition analyzers in some systems. In addition, the variability of product quality may be larger in the neat operation process because there arc fewer manipulated variables available and there is only one column to contain disturbances. 

Figure 6.10 Effect of design variables on vapor boilup rate Vg in reactive distillation column with nominal steady-state values (vertical line).

Volatile reactive liquids present few problems since they generally can be distilled from the vacuum line into a tube equipped with a serum bottle cap (Fig. 9.26). Nitrogen is then admitted to the tube, and the sample is taken with a syringe. Another scheme involves an inlet with a capillary tube or ampule breaker.33 35 This method is potentially useful for vacuum line work, since it is relatively simple to fill and seal off a sample tube attached to the vacuum line. 

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