Membrane plug flow

Hollow-fiber microporous silica membrane Plug-flow reactor 55% increase in cyclohexane conversion (the conversion in the membrane reactor approaches 100% whereas it is 45% in the conventional one) Koutsonikolas, Kaldis, ZaspaUs, and SakeUaropoulos (2012)... 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

Industrial hazardous wastewater can be treated aerobically in suspended biomass stirred-tank bioreactors, plug-flow bioreactors, rotating-disc contactors, packed-bed fixed-biofilm reactors (or biofilters), fluidized bed reactors, diffused aeration tanks, airlift bioreactors, jet bioreactors, membrane bioreactors, and upflow bed reactors [28,30]. 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

It is expected that the conclusions reached in the analysis of the series reaction will also be valid for methane partial oxidation. The first objective of this study was to verify this expectation. The second objective of the study was to determine how much faster than methane formaldehyde must permeate for the membrane reactor to begin to outperform a plug-flow reactor. 

In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of... 

In order for a membrane reactor to produce yields of HCHO greater than in a plug flow reactor, the membrane must be permselective for this species. The more permselective the membrane is to formaldehyde the better the membrane reactor performs until the formaldehyde is approximately one thousand times more permeable than methane. At this limit, the concentration of HCHO is essentially equal on both sides of the membrane at all times. No further improvement is possible by increasing the diffusivity of the formaldehyde further because there is... 

Partial oxidation of methane in the membrane reactor configuration shown in Figure 1 will not lead to higher yields of desired products than a plug flow reactor unless the diffusivity of the intermediate product, formaldehyde, is approximately four times that of methane. Presently available membranes that can withstand partial oxidation temperatures do not satisfy this criterion. 

If product inhibition occurs, either a stirred-tank reactor in batch or a plug-flow reactor should be used. In these two reactors, the product concentration increases with time. Alternatively a reactor with integrated product separation (membrane, solvent, etc.) is preferable. 

There is, however, another way of looking at a tubular reactor in which plug flow occurs (Fig. 1.15). If we imagine that a small volume of reaction mixture is encapsulated by a membrane in which it is free to expand or contract at constant pressure, it will behave as a miniature batch reactor, spending a time r, said to be the residence time, in the reactor, and emerging with the conversion aA/. If there is no expansion or contraction of the element, i.e. the volumetric rate of flow is constant and equal to v throughout the reactor, the residence time or contact time... 

Manousiouthakis, V. (1999) Conversion targets for plug flow membrane reactors. Chemical Engineering Science, 54, 2979-2984. 

In addition to these experiments, a simplified isothermal 1-D dispersed plug-flow reactor model of the membrane reactor was used to carry out theoretical studies [47]. The model used consisted of the following mass balance equations for the feed and sweep sides ... 

Again, the simple isothermal 1-D plug-flow reactor model provides a good basis for quantitative descriptions. This model allows to explore the potential of using series connections of several membrane reactor segments. The corresponding mass balance for a component i and a segment k can be formulated as follows ... 

Fig. 12.18. Comparison of the optimized reduced amounts that should be dosed and the corresponding internal compositions for a fixed-bed reactor (discrete dosing, top) and a membrane reactor (continuous dosing, bottom). A triangular network of parallel and series reactions was analyzed using an adapted plug-flow reactor model, Eq. 48. One stage (left) and 10 stages connected in series (right) were considered. All reaction orders were assumed to be 1, except for those with respect to the dosed component in the consecutive and parallel reactions (which were assumed to be 2) [66].

Figure 10.2 Schematic diagram of a plug-flow macroscopic model for a packed-bed membrane shell-and-tube reactor...

The flow conditions (e.g., co-cunent vs. counter-current and plug flow vs. perfect mixing) on both sides of the membrane can have significant effects on the conversion [Itoh et al., 1990] and will be discussed in Chapter 11. 

A non-isothermal plug-flow membrane reactor on both sides of the membrane has been developed and applied to the methane steam reforming reaction to produce synthesis gas at high temperatures according to [Oeitel et al., 1987]... 

Applying an isothermal and plug>flow membrane reactor (on both sides of the membrane) to the above reactions, Itoh and Xu [1991] concluded that (1) the packed-bed inert membrane reactor gives conversions higher than the equilibrium limits and also performs better than a conventional plug-flow reactor without the use of a permselective membrane and (2) the co-current and counter-current flow configurations give essentially the same conversion. 

In those limited cases where the diffusion equation for the membrane/support composite is solved in conjunction with the governing mass transport equations for the tube and shell sides [Sun and Khang, 1988 and 1990 Agarwalla and Lund, 1992], Equation (10-5) applies with = 0 for catalytically inert membranes. In addition, either Equations (10-36) for plug flows or Equation (10-54) for perfect mixing needs to be solved for the... 

Using a developed plug-flow membrane reactor model with the catalyst packed on the tube side, Mohan and Govind [1986] studied cyclohexane dehydrogenation. They concluded that, for a fixed length of the membrane reactor, the maximum conversion occurs at an optimum ratio of the permeation rate to the reaction rate. This effect will be discussed in more detail in Chapter 11. They also found that, as expected, a membrane with a highly permselective membrane for the product(s) over the reactant(s) results in a high conversion. 

Wu and Liu [1992] attempted to approximate the reactions involved during ethylbenzene dehydrogenation under an industrial setting by considering many possible side reactions. They included in their isothermal model for plug flows on both sides of the membrane the following five main side reactions with their corresponding reaction rate expressions ... 

Series and series-parallel reactions in a packed-bed plug-flow membrane reactor have been analyzed by Lund and his co-investigators [Agarwalla and Lund, 1992 Lund, 1992 Bernstein and Lund, 1993]. First consider the following consecutive catalytic reactions ... 

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