Open-channel monoliths

Open-channel monoliths are better defined. The Sherwood (and Nusselt) number varies mainly in the axial direction due to the formation ofa hydrodynamic boundary layer and a concentration (temperature) boundary layer. Owing to the chemical reactions and heat formation on the surface, the local Sherwood (and Nusselt) numbers depend on the local reaction rate and the reaction rate upstream. A complicating factor is that the traditional Sherwood numbers are usually defined for constant concentration or constant flux on the surface, while, in reahty, the catalytic reaction on the surface exhibits different behavior. 

Figure 2.49 CO conversion on monolith (open symbols) and catalyst deposited in micro channels (closed symbols). Feed 10% steam and 2-10% carbon monoxide, balance argon [82] (by courtesy of G. Germany).

Figure 2.88 CO conversion found for low-temperature water-gas shift at various reaction temperature vs. modified residence time (catalyst weight/carbon monoxide flow). Results from a micro channel stack reactor (closed symbols) are compared with conventional cordierite monoliths (open symbols) [82].

Magnetic resonance imaging permitted direct observation of the liquid hold-up in monolith channels in a noninvasive manner. As shown in Fig. 8.14, the film thickness - and therefore the wetting of the channel wall and the liquid hold-up -increase nonlinearly with the flow rate. This is in agreement with a hydrodynamic model, based on the Navier-Stokes equations for laminar flow and full-slip assumption at the gas-liquid interface. Even at superficial velocities of 4 cm s-1, the liquid occupies not more than 15 % of the free channel cross-sectional area. This relates to about 10 % of the total reactor volume. Van Baten, Ellenberger and Krishna [21] measured the liquid hold-up of katapak-S . Due to the capillary forces, the liquid almost completely fills the volume between the catalyst particles in the tea bags (about 20 % of the total reactor volume) even at liquid flow rates of 0.2 cm s-1 (Fig. 8.15). The formation of films and rivulets in the open channels of the structure cause the further slight increase of the hold-up. 

Many commercial ceramic membranes nowadays come in the form of a monolith consisting of multiple, straight channels parallel to the axis of the cylindrical structure (Figure 3.6). The surfaces of the open channels are deposited with permselective membranes and possibly one or more intermediate support layers. The porous suppon of these multi-channel structures are produced by extrusion of ceramic pastes described above with a channel diameter of a few millimeters. Their lengths are somewhat limited by the size of the furnaces used to dry, calcine and sinter them and also by such practical considerations as the total compact weights to be supported during heat ueatment and the risk of distortion in the middle section. It should be noted that this type of honeycomb... 

A generalization of this concept of a monolithic multi-channel honeycomb structure is described in a patent by Hoover and Roberts [1978]. An integral support of porous ceramic material has a multiplicity of parallel passageways (or open channels). These passageways are substantially uniformly spaced. On the surface of these channels are coated with a permselective membrane layer. The feed stream flows inside the channels. The membrane, being the first layer in direct contact with the process stream, is selective to one or more species in the stream. In the normal cases of properly weued membrane pores, the permeate under a driving force will uansport through the membrane, any... 

Catalysts were prepared on Corning EX20 cordierite, open channel monolithic substrates (nominally 62 square channels per square centimeter). High surface area supports were activated with base or noble metal components. The final composition of the fresh catalysts are shown in Table 1, where the metal content is expressed as grams of metal per liter of catalyst (including substrate). 

Once an open channel monolith was reassembled it was sealed into a demountable catalyst holder and placed in the exhaust of a diesel vehicle which was driven over a prescribed cycle on a chassis dynamometer. The vehicle was a 1977 International Harvester diesel Scout equipped with an indirect injected 3.2L, six cylinder engine. Commercial number two diesel fuel was used for all the vehicle experiments. 

FIGURE 47.5 Representative mass spectrum of leucine-enkephalin obtained using chip with open channel emitter (a) and porous polymer monolith-assisted electrospray process from a microdevice at a constant infusion (b). (Reprinted with permission from Koerner, T. and Oleschuk, R. D Rapid Comm. Mass Spectr., 19, 3279, 2005. Copyright 2005 J. Wiley.) Conditions peptide solution 1.0 p-mol/L in 50% aqueous acetonitrile with 0.5% acetic acid, flow rate of 100 nL/min, applied voltage of 3.5 and 3.0 kV, respectively. 

The macroporous monolith approach, introduced by Frechet and Svec, seemed to address many of the problems associated with open-tubular and particle packed columns. First, the adsorptive capacity of capillary monoliths has been found to be 3-5 orders of magnitude larger than that of both open channel and bead-packed columns [32]. Next, since the polymerization takes place within the fused-silica capillary, the tedious process of packing the capillary columns may be avoided. Furthermore, the limitations in chromatographic efficiency caused by irregularities in particle packing and by the nonuniformity of particle sizes are eliminated. 

Chromatographic monoliths are a specific type of chromatographic stationary phase. In contrast to conventional, particle shaped supports, they consist of a single piece of porous material. The pores form a highly interconnected network of open channels through which the mobile phase flows. Therefore, the main transport mechanism of the molecules to be separated is convection. Because of this, mass exchange between the mobile and stationary phase is very fast, similar to that in the membrane supports. In contrast to the membranes, however, monoUths can be prepared in various shapes and dimensions. Besides convective transport, monoliths also exhibit several other unique features ... 

The column itself can be produced as a single open channel or as a branched system of channels starting from one microchannel and ending in another microchannel. The latter type is often named as COMOSS (collocated monolith support structure). With some of the materials mentioned, the channels can serve as stationary phase itself or can be functionalized by coating, packing, or incolumn polymerization with appropriate chromatographic phases. 

By performing in situ polymerization, monolithic columns can also be produced. This also greatly improves the surface area compared to open channel columns. These monoliths can be functionalized during the polymerization or afterward. 

Diesel particulate filters (DPFs) (see Figure 19.1) are used to remove soot particles from the exhaust stream [1-4]. These filters usually consist of wall-flow monoliths, that is, honeycomb-like structures with 50% of the channels plugged at the gas entry side and the remaining channek plugged at the exit The gas stream enters into the filter through the open channels and is forced to pass through the porous walls where the soot particles get stuck. 

The effect of tortuosity becomes insignificant in 1-D monolithic channeled stractures, as schematically shown in Fig. 1(b). The straight path for ion transport in the electrolyte gives the fast and open channel for ionic conduction while 1-D bulk stmcture provides short and continuous pathways for electron transport. The rapid transport of electroactive species makes the overall process facile, leading to increased energy and power densities. Moreover, as far as the wall of the 1-D stmcture can be controlled to be thin enough, reaction-induced mechanical disintegration of the electrode can be suppressed and at the same time the extremely short... 

Another important constraint comes from the pressure drop across the catalytic bed, which must be kept to a minimum to avoid a loss in engine power and performance. This requirement is satisfied by a very shallow pellet bed of no more than ten pellets deep, a monolithic structure with many open parallel channels, or a pile of metallic screens and saddles. 

The three principal catalyst bed configurations are the pellet bed, the monolith, and the metallic wire meshes. An open structure with large openings is needed to fulfill the requirement of a low pressure drop even at the very high space velocities of 200,000 hr-1. On the other hand, packings with small diameters would provide more external surface area to fulfill the requirement for rapid mass transfer from the g .s stream to the solid surface. The compromise between these two ideals results in a rather narrow range of dimensions pellets are from to 1 in. in diameter, monoliths have 6 to 20 channels/in., and metallic meshes have diameters of about 0.004 to 0.03 in. 

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