Reactor volume membrane reactors

Fig. 14.7 Model based prediction of FPS and reformer efficiency is shown along with hydrogen recovery from the fixed volume membrane reactor...

Concerning function integration, for example, micro-flow membrane reactors can exhibit similar process intensification, as shown already for their large-scale counterparts [75]. Separation columns for proteomics, immobilizing enzymes, utilize the large surface-to-volume ratios. Surface tension differences can guide and transport liquids selectively. 

Application of the largest dendritic catalyst 8 (Figure 4.15) in a continuous process showed activity over 15 exchanged reactor volumes (Figure 4.16). The decrease in activity caused by wash out was calculated to be only 25% (retention of ligand 98.1%). The drop in activity was therefore ascribed to the decomposition of the palladium catalyst. Addition of membrane material to batch catalysis experiments did not change the conversion showing that this was not the cause of decomposition. 

A residence time is defined as the time needed to fully replace the reaction volume of the continuous operating, membrane reactor system. 

It is obvious from the above discussion that porous and dense membranes form two different cases, each with its own advantages and disadvantages. Dense membranes, (permeable only to one component) operating at optimum conditions, can be used to obtain complete conversions. However, because the permeation rate is low, the reaction rate has also to be kept low. Porous membranes (permeable to all components but at different permselectivities) are limited under optimum conditions to a maximum conversion (which is not 100%) due to the permeation of all the components. The permeation rates through porous membranes are, however, much higher than those through dense membranes and consequently higher reaction rates or smaller reactor volumes are possible. 

Fig. 13.7 Required reaction time for 95% conversion in the pervaporation-assisted esterification of tartaric acid with ethanol at different membrane area per reactor volume [35].

Retention factors were calculated using the equation retention factor = 1 + ln[a/(n -P i)]/x), where a is the amount of dendrimer inside the reactor after the experiment b the amount of dendrimer that went through the membrane x the number of reactor volumes flushed with substrate solution. 

Fig. 6. Allylic alkylation in a continuous flow membrane reactor using dendritic ligand 5c (flow rate 50mLh reactor volume 20 mL, Koch MPF-60 NF membrane, molecular weight cut-off = 400 Da slight increases are due to pump failures) (19b).

As discussed in this volume, the use of membrane reactors (Bernstein, et oL), monoliths (Hickman and Schmidt), optimized catalyst distribution in pellets (Gavriilidis and Varma), and supercritical conditions (Azzam and Lee) are examples of engineering developments that may provide improvements over existing processes. 

Although these systems involve two variables, their steady-state solutions can be calculated in general and a more complete mathematical analysis of dissipative structures is possible. From a practical point of view it is interesting to note that systems obeying equations of the form (2) may be found in artificial membrane reactors.22 Examples are presented by D. Thomas in this volume. 

The enzyme membrane reactor (EMR) is an established mode for running continuous biocatalytic processes, ranging from laboratory units of 3 mL volume via pilot-scale units (0.5-500 L) to full-scale industrial units of several cubic meters volume and production capacities of hundreds of tons per year (Woltinger, 2001 Bommarius, 1996). The analogous chemzyme membrane reactor (CMR) concept, discussed in Chapter 18, Section 18.4.5, is based on the same principles as the EMR but is far less developed yet. 

Under certain conditions, scale-up of membrane reactors is straightforward. Provided that (i) the reactor contents are well mixed so that the reactor is operated as a CSTR, and that (ii) the membrane is configured for filtration in the tangential mode, the pertinent design criterion, besides constant residence time T in the reactor, is constant fluidity F of the substrate/product solution through the membrane at all reactor scales. Fluidity is defined by Eq. (19.36) (V = ultrafiltered volume, AP = transmembrane pressure, t = filtration time, and A = membrane area). 

The field of chemical process miniaturization is growing at a rapid pace with promising improvements in process control, product quality, and safety, (1,2). Microreactors typically have fluidic conduits or channels on the order of tens to hundreds of micrometers. With large surface area-to-volume ratios, rapid heat and mass transfer can be accomplished with accompanying improvements in yield and selectivity in reactive systems. Microscale devices are also being examined as a platform for traditional unit operations such as membrane reactors in which a rapid removal of reaction-inhibiting products can significantly boost product yields (3-6). 

Moreover, this catalytic reaction could be employed in a continuously operated membrane reactor [105,106]. A stirred membrane reactor module equipped with a solvent-stable Koch MPF-50 membrane [107] was operated at 40 atm. After exchange of a few reactor volumes a steady conversion is achieved, e.g., 30% cyclohexene conversion for the example shown in Fig. 9 [32], corresponding to a catalytic activity of 1200 TO h 1. Over 30 exchanged reactor volumes, corresponding to a time of operation of 30 h, a productivity of a total of 29 000 turnovers was observed. 

The achievable fluxes through membranes, J, were designated in [35] as area time yields (ATY, in molm-2s). Figure 12.4 provides an estimation of the current state regarding the possibility of matching the two processes. For a wide range of membranes under consideration, the required ratios of membrane area to reactor volume (Am/Vr) are between 10 and 100 nr1. These values allow to estimate that the diameter of applicable cylindrical tubular reactors should be between 0.04 and 0.4 m. These appear to be reasonable values for industrial applications, and indicate that matching of the two processes under consideration is achievable with currently available membranes. 

When dense Pd-based membranes are used, permeabilities are low. To increase the membrane surface per unit volume of reactor the use of spiral tubes or helix-shaped Pd membranes has been proposed [39]. 

Fig. 13.10 Change in space-time yield during the electrochemical HDH of 200 mM DBP in paraffin oil media using a Nation 117 membrane reactor. Ratios of the waste volume (cm3) to the cathode geometric surface area (cm2) are indicated in figure. Cathode Three-layer Ti mesh-supported Pd (25 cm2, 2 mg Pd cm-2). Anode Three-layer Ti mesh-supported Pt (25cm2, 2mgPtcm 2). Controlled current density 10 mAcm-2. Catholyte 200 mM DBP in paraffin oil (50-1,000cm3). Anolyte O.5MH2SO4 aqueous solution (50-1,000cm3). Flow rate 100 ml min-1. Temperature 18.5 0.5°C...

In addition to this, that an interesting novel emulsion membrane reactor concept overcomes the difficulties of the large solvent volume otherwise required for the reduction of poorly soluble ketones [30]. 2-Octanone was reduced by a carbonyl reductase from Candida parapsilosis to (S)-2-octanol with > 99.5 % ee and total turnover number of 124 - the 9-fold value of that obtained in a classical enzyme reactor. 

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