Reactive membranes

In correspondence to the just-given introduction to the state of the art of feasible product analysis of reactive distillation and reactive membrane separation processes, the following three major sections of this chapter are structured as follows ... 

Section 4.4 extends the proposed methodology to reactive membrane separation systems being controlled by vapor-liquid mass transfer and finite chemical reaction kinetics, simultaneously. For this general case the term kinetic arheo-trope is introduced for the singular points obtained in phase diagrams. 

The possible products of a reactive membrane separation process are influenced by the mass transfer characteristics of the applied membranes. In the following section it is shown how the concepts and tools, being developed for reactive vapor-liquid separation, can also be used to analyze the feasibility of membrane separators. 

It should be noted that the equivalent continuously operated process of the batch reactive membrane process is the membrane reactor depicted in Fig. 4.26(b). Here, the spatial coordinate z replaces the time coordinate of the batch process. Feasibility analysis has the task of estimating the retentate composition which is attainable at infinite reactor length. 

In analogous manner, residue curve maps of the reactive membrane separation process can be predicted. First, a diagonal [/e]-matrix is considered with xcc = 5 and xbb = 1 - that is, the undesired byproduct C permeates preferentially through the membrane, while A and B are assumed to have the same mass transfer coefficients. Figure 4.28(a) illustrates the effect of the membrane at nonreactive conditions. The trajectories move from pure C to pure A, while in nonreactive distillation (Fig. 4.27(a)) they move from pure B to pure A. Thus, by application of a C-selective membrane, the C vertex becomes an unstable node, while the B vertex becomes a saddle point This is due to the fact that the membrane changes the effective volatilities (i.e., the products xn a/a) of the reaction system such that xcc a. ca > xbbO-ba-... 

Fig. 4.28. Residue curve maps for reactive membrane separation ...

Since water is the byproduct, and also has an undesired inhibitory effect on catalyst activity, it must be separated efficiently from the reaction mixture. To achieve this, both conventional reactive distillation and reactive membrane separation are considered as process alternatives. In the latter process, a Knudsen-membrane is applied. Consequently, the mass transfer matrix [/c] has a diagonal structure and the diagonal elements are the Knudsen-selectivities - that is, the square-roots of the ratios of the molecular weights Mr. 

Similar to the reactive distillation process, at Da —> °° the composition space is again divided into two subspaces which have either pure THF or pure water as attractors (Fig. 4.30(c)). However, as a very important feature of the reactive membrane... 

Fig. 4.30. Residue curve maps for reactive membrane separation 1,4-BD — THF + Water p= 5 atm Knudsen-membrane. (a) Da = 0 (b)...

The locations of the singular points in reactive membrane separation processes depend on both the separation effects (distillation/membrane separation) and the reaction effect. Singular points can be generally obtained as steady-state solutions of Eq. (88) ... 

Reactive membrane separation [k]-matrix is a non-scalar matrix. This case was intensively studied by Huang et al. [20] ( kinetic arheotrope ). 

Similar to reactive distillation (as discussed in Section 4.2.2.1), the concept of the PSPS is useful to analyze the singular points of a reactive membrane separation process. By introducing the new transformed variables... 

PSPS of reactive membrane separation with diagonal [/c]-matrix... 

For a more generalized analysis of the qualitative influence of membranes on the singular points, the reactive membrane separation process is now considered with a nondiagonal [/c]-matrix. The condition for a kinetic arheotropes is given by... 

Fig. 4.31. Potential singular point surfaces and stable node bifurcation behavior of reactive membrane separation at different mass transfer conditions B + C< > A Keq = 5 ccba = 5.0, acA = 3.0.

Figure 4.34 shows the PSPS for the reactive membrane separation process with application of a Knudsen-membrane. In comparison with reactive distillation, the membrane turns the vertical hyperbola into a horizontal hyperbola. In particular, the membrane shifts the stable node branch towards the THF-vertex such that THF-rich products can be attained in the considered Knudsen-membrane reactor. 

Fig. 4.34. Potential singular point surface and bifurcation behavior of reactive membrane separation using a Knudsen-membrane l,4-BD— THF + Water p= 5 atm.

A singular point of reactive membrane separation should be denoted as kinetic arheotrope because it is a process phenomenon rather than a thermodynamic phenomenon. The condition for arheotropy can be elegantly expressed in terms of new transformed variables, which are a generalized formulation of the transformed composition variables first introduced to analyze reactive azeotropes. 

The determination of feasible products is very important for conceptual process design and for the evaluation of competing process variants. In this chapter, methods have been discussed to identify feasible products as singular points of residue curve maps (RCM). RCM-analysis is a tool which is well established for nonreactive and reactive distillation processes. Here, it is shown how RCM can also be used for reactive membrane separation processes. 

Table 4.4. Classification of singular points in reactive distillation and reactive membrane separation processes.

In addition, a reactor may perform a function other than reaction alone. Multifunctional reactors may provide both reaction and mass transfer (e.g., reactive distillation, reactive crystallization, reactive membranes, etc.), or reaction and heat transfer. This coupling of functions within the reactor inevitably leads to additional operating constraints on one or the other function. Multifunctional reactors are often discussed in the context of process intensification. The primary driver for multifunctional reactors is functional synergy and equipment cost savings. 

Although there is no commonly accepted definition of a membrane reactor (MR), the term is usually applied to operations where the unique abilities of membranes to organize, compartmentalize, and/or separate are exploited to perform a (bio)chemical conversion under conditions that are not feasible in the absence of a membrane. In every MR, the membrane separation and the (bio)catalytic conversion are thus combined in such a way that the synergies in the integrated setup entail enhanced processing and improved economics in terms of separation, selectivity, or yield, compared to a traditional configuration with reactor and separation separated in time and space. When the membrane itself carries the catalytic functions, it is mostly referred to as a reactive membrane. ... 

Suzuki, F. and Yanagimachi, R. (1989). Changes in the distribution of intramembraneous particles and filipin-reactive membrane sterols during in vitrol capacitation of golden hamster spermatozoa. Gamete Res. 23 335-347. 

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