Monolithic reactors channel flow

Taylor flow have many similarities with monolith reactors. A lot of work on Taylor microchannel flow has been aimed at understanding and improving the conditions within monolithic reactor channels [23]. Because of its low axial mixing properties, Taylor flow can be used in high-throughput screening [4]. Even microfiltration efficiencies have been found to improve in the Taylor flow regime [24]. 

Monolith reactor This type of reactor is used extensively for the abatement of automobiles exhaust emissions. The gas flows continuously through the reactor, whereas the catalyst is a continuous phase consisting of a ceramic support and the active phase, which is dispersed onto the support. The support is structured in many channels and shapes that achieve large catalytic surface at small volume. A typical application of monolith reactors is the exhaust gas cleaning. 

A brief review of the development history of monolith reactor models for TWC applications can be found in Koltsakis and Stamatelos (1997). Various workers have looked at 1-, 2- and 3-dimensional models considering both the whole monolith and just a single channel. A multidimensional model for the whole monolith is required for investigating the effects of a flow maldistribution across the front face of the monolith, but is probably unnecessary when the flow is uniform. Other workers have studied multidimensional single channel models, where the gas flow within the channel is modelled in detail. In general, for a model to be useful in practice, some compromise has to be made between having a reasonable runtime versus detail/complexity, both in terms of the chemical kinetics and the description of the flow field within the channels of and across the monolith. 

In many situations, the monolith reactor can be represented by a single channel. This assumption is correct for the isothermal or adiabatic reactor with uniform inlet flow distribution. If the actual conditions in the reactor are significantly different, more parallel channels with heat exchange have to be simulated (cf., e.g. Chen et al., 1988 Jahn et al., 1997, 2001 Tischer and Deutschmann, 2005 Wanker et al., 2000 Young and Finlayson, 1976). In this section we will further discuss effective single channel models. 

A recent example is the optical fiber monolith reactor, reported by Lin and Valsaraj (208). They used a monolith for photocatalytic wastewater treatment with the channels of the monolith completely filled with flowing liquid. The monolith structure was used merely as the distributor of the optical fibers, but the benefits of monolith, such as low-pressure drop and excellent mass transfer characteristics for multiphase systems, were not fully exploited. 

When comparing film flow monolithic reactors with conventional catalytic packed reactors, one can conclude that the critical hydrod)mamic characteristics (hydraulic capacity, pressure drop, and volumetric mass transfer rates) are similar, but monoliths have distinct advantages greater flexibility, easier scale-up, the susceptibility of fhe surface to coating procedures, and advances in control of flooding—all allowing the use of very small channels and therefore efficienf cafalysf ufilizafion. 

The main features of monolith reactors (MR) combine the advantages of conventional slurry reactors (SR) and of trickle-bed reactors (TBR), avoiding their disadvantages, such as high pressure drop, mass transfer limitations, filtration of the catalyst, and mechanical stirring. Again, care must be taken to produce a uniform distribution of the flow at the reactor inlet. Scale-up can be expected to be straightforward in most other respects since the conditions within the individual channels are scale invariant. 

The use of monoliths as catalytic reactors focuses mainly on applications where low pressure drop is an important item. When compared to fixed beds, which seem a natural first choice for catalytic reactors, monoliths consist of straight channels in parallel with a rather small diameter, because of the requirement of a comparably large surface area. The resulting laminar flow, which is encountered under normal practical circumstances, does not show the kinetic energy losses that occur in fixed beds due to inertia forces at comparable fluid velocities. Despite the laminar flow, monolith reactors still may be approached as plug-flow reactors because of the considerable radial diffusion in the narrow channels [1]. 

The route from reactant to product molecule in a monolith reactor comprises reactant transport from the bulk gas flow in a channel toward the channel wall, simultaneous diffusion and reaction inside the porous washcoat on the channel wall, and product transport from the wall back to the bulk flow of the gas phase. 

Laminar flow is the usual flow regime met in monolith reactors, given that the typical Reynolds number has values below SOO. The radial velocity profile in a single channel develops from the entrance of the monolith onward and up to the position where a complete Poiseuille profile has been established. The length of the entrance zone may be evaluated from the following relation [3] ... 

Equation 2 expresses whether radial diffusion, which in the case of laminar flow is due to molecular diffusion, is fast enough to outlevel radial concentration profiles. This approximation usually holds for monolithic reactors because of the rather small channel diameter. The corresponding axial dispersion coefficient can be calculated [1] from the following ... 

Considerations along the above lines lead to analogous correlations for the Sherwood number for the description of mass transfer in a single channel. The application of the rather simple Nusselt and Sherwood number concept for monolith reactor modeling implies that the laminar flow through the channel can be approached as plug flow, but it is always limited to cases in which homogeneous gas-phase reactions are absent and catalytic reactions in the washcoat prevail. If not, a model description via distributed flow is necessary. 

Almost similar results were obtained experimentally by Votruba et al. [19], who studied evaporation of water and hydrocarbons from porous monoliths. These results predict Nu and Sh values clearly lower than does Eq. (13), and moreover suggest that Nu or Sh values would fall under their theoretically predicted lower limit at a low Reynolds number [16,20]. It is not unlikely that the discrepancy is due to a maldistribution of flow over the different monolith channels, as a result of the low pressure drop, similar to the effect signalized for fixed beds at low Reynolds numbers [7]. Experimental work [4], which was carried out with an inert fixed bed in front of the monolith reactor to assure an even distribution, gave data that come quite near to the results of Hawthorne, Eq. (13) [2]. 

In this chapter, after a general description of possible flow patterns in monolith channels, the main features and properties of monoliths will be discussed. Following this, the monolith reactor will be compared to some other conventional reactors that are widely used. Next, applications of monolith reactors in catalytic gas-liquid processes will be summarized. Finally, some ideas concerning the future needs in this field will be presented. 

Optimum performance of the monolith reactor requires uniform and stable distribution of gas and liquid over the cross section of the monolith. Because the monolith consists of many small channels, it may be difficult to obtain a good distribution of the gas and liquid flows within the monolith. This is very important for the monolith reactor, since an uneven inlet distribution would be propagated throughout the reactor. On the other hand, if the inlet distribution is appropriate, no nonuniformity will occur along the reactor. 

One unique feature of the monolith reactor is the possibility of having an internal recirculation of the gas flow without the use of a pump [5,8-10]. This self-recirculation is possible due to the very low net pressure drop across the monolith. In a monolith reactor with downflow operation in slug flow regime, the fluids are not driven through the channels by an external pressure, but pulled through by gravity. This corresponds to a total superficial velocity of about 0.45 m sec . When liquid is added to the channel at a lower rate, gas will be entrained to make the total velocity 0.45 m sec. ... 

The scale-up of monolith reactors is expected to be much simpler. This is due to the fact that the only difference between the laboratory and industrial monolith reactors is the number of monolith channels, provided that the inlet flow distribution is satisfactory. In slurry reactors, scale-up problems might appear. These are connected with reactor geometry, low gas superficial velocity, nonuniform catalyst concentration in the liquid, and a significant back-mixing of the gas phase. 

The flow distributions over the cross section of the reactor can also be a problem in packed-bed reactors. An additional source of trouble is connected with the flow maldistribution inside the packed-bed reactors caused by inhomogeneity of packing. In monoliths, however, the flow within the channels is stable, provided that the flow is properly distributed at the reactor inlet. 

Ariga et al. [48] have investigated the behavior of the monolith reactor in which Echerichia coli with P-galactosidase or Saccharomyces cerevisiae was immobilized within a thin film of K-carragcenan gel deposited on the channel wall. The effects of mass transfer resistance and axial dispersion on the conversion were studied. Those authors found that the monolith reactor behaved like the plug-flow reactor. The residence-time distribution in this reactor was comparable to four ideally mixed tanks in series. The influence of gas evolution on liquid film resistance in the monolith reactor was also investigated. It was shown that at low superficial gas velocities, the gas bubble may adhere to the wall, which decreases the effective surface area available for the reaction. The authors concluded that the reactor was very effective in the reaction systems accompanied by gas evolution, such as fermentations. 

In this chapter, first, the existing correlations for three-phase monolith reactors will be reviewed. It should be emphasized that most of these correlations were derived from a limited number of experiments, and care must be taken in applying them outside the ranges studied. Furthermore, most of the theoretical work concerns Taylor flow in cylindrical channels (see Chapter 9). However, for other geometries and flow patterns we have to rely on empirical or semiempirical correlations. Next, the modeling of the monolith reactors will be presented. On this basis, comparisons will be made between three basic types of continuous three-phase reactor monolith reactor (MR), trickle-bed reactor (TBR), and slurry reactor (SR). Finally, for MRs, factors important in the reactor design will be discussed. 

Scaling up three-phase monolith reactors from pilot plant to industrial size is easy in some areas and more difficult in others. Since there is no interaction between the channels, the behavior within the monolith channels is independent of scale. Adding more parallel channels will not affect the flow, the mass and heat transfer, or reactions in each channel, as long as the flow distribution is uniform. Also, both the pilot plant reactors and the industrial reactors are adiabatic due to the absence of radial mixing. 

Low pressure drop. Because of the shape of the void space through which the fluid flows, i.e., noncircular channels that are straight in the direction of the flow, the pressure drop across a BSR is comparable to that of a monolithic reactor. This feature is most profitable in processes operating at low pressure and high space velocities, such as catalytic removal of NO, SO, or volatile organic compounds (VOCs) from flue gases. The pressure drop can be manipulated by means of the voidage (see next item). 

In addition to the preparation of packed beds and monoliths, wall coating is an alternative method forthe introductionofcatalysts into continuous flow systems, due to the short diffusion distances obtained within micro reaction channels. An early example of this was demonstrated by Yeung and co-workers [59]. who employed a stainless-steel micro reactor [channel dimensions = 300 pm (width) x 600 pm (depth) x 2.5 cm (length)] coated with an NaA zeolite membrane, followed by a layer of... 

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