Cross-flow configuration

A reactor is termed a radial or panel-bed reactor when gas or vapor flow perpendicular to a catalyst-fiHed aimulus or panel. These are used for rapid reactions to reduce stresses on the catalyst or to minimize pressure drops. Similar cross-flow configurations also are used for processing soHds moving... 

Process Description Microfiltration (MF) separates particles from true solutions, be they liquid or gas phase. Alone among the membrane processes, microfiltration may be accomplished without the use of a membrane. The usual materi s retained by a microfiltra-tion membrane range in size from several [Lm down to 0.2 [Lm. At the low end of this spectrum, very large soluble macromolecules are retained by a microfilter. Bacteria and other microorganisms are a particularly important class of particles retained by MF membranes. Among membrane processes, dead-end filtration is uniquely common to MF, but cross-flow configurations are often used. 

Figure 3.7 Alternated 90° turned adjacent platelets forming a stacked cross-flow configuration. Image of thermally bonded devices (top). Complete mounted multi-plate stack micro reactors (bottom) [45].

To help understand the performance of membranes, brief explanations of a few terminologies are in order. Permeability of a membrane is determined by dividing permeate flux by the transmembrane pressure. It indicates the membrane s throughput per unit area (flux) per unit pressure difierence. An important factor afiecting flux and retention ability of the membrane is the direction of the feed flow relative to the membrane surface. In through-flow configuration, the feed flow is perpendicular to the membrane surface. In cross-flow configuration, the feed stream flows parallel to the membrane... 

Fig. 1.6 Illustration of a planar-stack, solid-oxide fuel cell (SOFC), where an membrane-electrode assembly (MEA) is sandwiched between an interconnect structure that forms fuel and air channels. There is homogeneous chemical reaction within the flow channels, as well as heterogeneous cehmistry at the channel walls. There are also electrochemical reactions at the electrode interfaces of the channels. A counter-flow situation is illustrated here, but co-flow and cross-flow configurations are also common. Channel cross section dimensions are typically on the order of a millimeter.

In a cross-flow configuration, one stream with one fluid enters from the left and two streams carrying the same other fluid are fed from above and below [112]. The latter have much larger flow rates as the first stream so that this layer is hydro-dynamically compressed when all three streams enter the outlet channel on the right side of the cross. The channel which carries the stream to be focused narrows to a kind of nozzle when approaching the T-junchon... 

This device consists simply of a T-type junction where two pressure-driven flows are contacted (see Figure 1.169) [48]. After a short passage, such a bi-laminated stream is exposed to injections from both sides by adjacent channels, which yields a cross-flow configuration. From there on, the injected flow passes a long main channel and finally reaches a reservoir. 

Recently, the cross-flow configuration was also investigated for a closed reactor, that is, constant volume (Grachev et al, 1995). In this case, the amount of gas-phase reactant decreases due to chemical reaction, which would decrease the gas pressure. However, the gas is heated during reaction, which would increase the gas pressure. Depending on the reactor volume, for typical reaction conditions, the gas pressure was shown to have a maximum either at full or intermediate conversion. This conclusion is important, since in practice a decrease in gas pressure is usually associated with reaction completion. 

From a cross-flow point of view it may be of interest to mention the phosphoric acid fuel cell with the so-called DiGas system (Fig. 9), which is an air-cooled cross-flow configuration for use in utility-power stations [39]. The process air stream is diverted into two types of channels into individual cells with relatively small cross-sectional area, and into cooling plates (approximately one for every five cells) with a lai ge cross-section. Bipolar plates were molded from a mixture of graphite and phenolic resin, with a Pt-on-carbon cathode and a Pt anode combined with colloidal PTFE on a graphite-paper backing. 

Table 1 shows the types of separations achievable with MF, UF and RO membranes when operated in cross-flow configuration. For MF or UF application, the choice of membrane materials includes ceramics, metals or polymers, whereas for RO at the present only polymer membranes are predominantly used. Although cross-flow filtration is practiced in all the above three types of membrane applications, the description of membrane... 

Nonuniform TMP values over the filtration surface area may cause substantial (up to 50%) reduction in the product recovery in the permeate. A novel approach to improving the flux and/or product recovery utilizes the concept of a uniform transmembrane pressure. lf This is achieved by varying the permeate side pressure with an independent recirculation pump to adjust the TMP to a constant value. A schematic of the UTP and conventional cross-flow configuration is shown in Figs. 11 and 12, respectively. The TMP profiles for the two operational modes are shown in Fig. 13. Flux improvements up to 500% have been achieved compared with the conventional cross-flow mode in many important food, beverage and biotechnology applications. 

If the (oi - oj) curve does not reach the value for the desired (a -aS), then the globule capacity is insufficient to achieve the required separation in a single extraction unit. If fresh emulsion is delivered to all extractors of a multistage scheme, commonly called a cross-flow configuration, then the information in Figure 2 remains applicable. In an alternate scheme for the of 20 and oj of 1, one extractor could reduce the feed to a o of 10 (requiring a Te of about 0.12 or 0.5 minutes). 

Instabilities of a fluid-fluid interface can be prevented by patterning the internal surface of a microchannel to create hydrophilic and hydrophobic paths [43]. Capillary forces can then be used to create predsely controlled vertical interfaces or virtual walls between two immiscible fluids with different wetting behavior [140]. Such pinned interfaces result in large interfadal areas for effident mass transfer between the two phases and have previously been used for microfluidic extraction in co-flow and cross-flow configurations ]93]. Kenis et al. have guided misdble liquids in... 

Membrane filters of early design can foul quickly owing to the concentration polarization effect. To control this effect, the configuration of a membrane filter is designed as a cross-flow mode (Belfort, 1986). In cross-flow configuration, the main direction of the suspension flow is perpendicular to the permeate flow. As permeate flow passes through the membrane, a portion of the particles is deposited at the surface of the membrane to form a cake layer. The cake thickness varies with the operating time and so does the rate of permeation. Thus the time-dependent behavior represents a major factor of the cross-flow membrane filtration. 

If the gases supplied to the cells are connected manifold externally to the stack, then the crossflow configuration is the only option and gas inlets and outlets for the fuel and oxidant can be located on the four sides of the stack. Figure 2.7 shows the cross-flow configuration adopted by CFC Solutions. 

Reverse osmosis, nanofiltration, and ultrafUtration are typically used in the cross-flow configuration, where the feed stream flows across or tangential to the membrane surface. The constant flow across the membrane surface minimizes the buildup in concentration of salts at the membrane surface for reverse osmosis and nanofiltration products, and inhibits the formation of a gel or particulate layer for ultrafUtration and microfUtration products. The permeate passes through the membrane, and the concentrate or retentate retains the dissolved and suspended solids rejected by the membrane. 

Rigorous analytical treatment is not possible for the cross-flow configuration even for Newtonian fluids. In a recent study, Tanner (1993) has obtained an approximate numerical solution for the creeping flow of power-law fluids. In particular, he has obtained drag as a function of the flow behavior index (n < 1). Table 3 shows a summary of his approximate and numerical (boundary element method) results, in the form of a dimensionless drag force F% defined as... 

The objective of designing hollow fiber modules is to minimize non-ideal effects like pressure losses and concentration polarization. Usually, there is trade-off between reducing pressure losses and reducing concentration polarization. The effect of co-current, counter current and cross-flow configurations on the mass transfer was elaborated. 

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