Straight Parallel Channels

Reactant gas enters through the inlet port of the gas feeder, takes multiple parallel flow paths through the channels, and exits through the outlet port. One major advantage of this design is the lower pressure drop because of the parallel nature of the flow. However, the gas distribution through the 

Different flow channel design configurations, (a) Straight parallel channels, (b) Single serpentine curvilinear bends, (c) Single serpentine channel with square bends, (d) Dual 2-mm serpentine parallel channels, (e) Dual 1.2-mm serpentine parallel channels, (f) Dual 1-mm serpentine parallel channels, (g) Four 0.5-mm serpentine parallel channels. 

Straight parallel channel gas flow-field design, (a) Straight parallel flow channels, (b) Gas distribution primarily by diffusion in the gas diffusion layer. 

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The cordierite extruded monoliths, having 400 square cellsAn, were similar to those used in automobile catalytic converters. However, instead of using an alumina washcoat as in the catalytic converter, these catalyst supports were loaded directly with 12 to 14 wt.% Pt in the same manner as the foam monoliths. Because these extruded monoliths consist of several straight, parallel channels, the flow in these monoliths is laminar (with entrance effects) at the flow rates studied. 

With nonadiabatic reaction control, heat must be transported through the fixed bed to the integrated heat exchange surfaces. At the usual mass flow rates of G > lkgm-2s-1, this heat transport takes place mainly by convection, i.e. the fixed bed must allow for a cross flow transverse to the main flow direction. Monolith structures with straight parallel channels are thus unsuitable for nonadiabatic reaction control. 

Monolith forms can have very high specific surfaces combined with a very low pressure loss. Monoliths with straight, parallel channels, such as used for automobile exhaust control have only very poor radial heat transport properties. Crossed corrugated structures are considerably more favorable for isothermal reaction control. They have a very high radial thermal conductivity which is almost independent of the specific surface area the latter can be varied over a wide range by means of the channel dimensions. 

Master wafer (4 in. in diameter) carrying a microstructure of straight, parallel channels see Note 1 and 2). The channel structure is 25 pm high, 500 pm wide, and about 2 cm long see Fig. lb). 

The three-dimensional flow configuration and geometry of straight parallel channels and serpentine channels with different channel sizes are depicted in Figure 10.12. The current densities as well as the activation losses depend on the primary contact surface area of the gas channels adjacent to the gas diffusion layer. 

Computational model and mesh straight parallel channels, (a) Computational model for the bipolar plate with straight parallel channels, (b) Computational mesh for straight parallel channels. 

In the as-synthesized MFI-crystals the tetrapropylammonium (TPA) ions are occupying the intersections between the straight (parallel) and the sinusoidal channels of the zeolite, thus providing an efficient pore filling. The detailed structure of as-synthesized MFI-TPA has been elucidated by X-ray single crystal analysis (ref. 3). Also the combination tetrabutyl-Ztetraethylammonium can be applied as template in MFI-synthesis. A 1 1 build-in is found then (Fig. 1). When only tetrabutylammonium is available as template, the MEL (ZSM-11) lattice is formed with another distance between the channel intersections. 

A large variety of zeolites (e.g., ZSM-12, -22, -23, -48, AIPO4-5, -8, -11, and VPI-5) contain systems of parallel channels with diameters on the order of the molecular dimensions. Molecular propagation in this type of adsorbent represents a special case of diffusion anisotropy, since the main elements of the diffusion tensor referring to the plane perpendicular to the direction of the channel system are equal to zero. In the first PFG NMR diffusion measurements of methane in ZSM-12 and AIPO4-5, only a lower limit on the order of I0 m"s could be determined for diffusivity in the channel direction (160). This value is two orders of magnitude below the diffusivity of methane in the straight channels of zeolite ZSM-5 (see Fig. 14). Since the channel diameters of ZSM-12... 

Honeycomb monoliths (Fig. 6.1) are structured catalyst supports consisting of parallel straight capillary channels. Nowadays, they are widely used for the catalytic exhaust converter in the automobile industry and end-of-pipe gas cleaning. The gas-only application of monoliths stems from the fact that the pressure drop is low using the surface area of the catalyst as a criterion, the pressure drop in a monolith is an order of magnitude lower than in randomly packed beds. The channels are about a millimeter in diameter, and on the wall (-100 pm) a wash-coat of catalytic material (-50 pm) is applied. 

A plentitude of different flow field structures have been described in the literature starting from simple arrays of fence posts via arrays of straight parallel chaimels to complex serpentine type stmctures. In addition foam type structures, expanded metals and woven designs have been proposed as well. Under ordinary circumstances, gas flow will be maintained in the channels of the flow field while transport to the reaction site is by diffusion through the porous gas diffusion media next to the catalyst layer. 

Masuda H, Yamamoto A, Sasaki K, Lee S, Ito K (2011) A visualization study on relationship between water-droplet behavior and cell voltage appeared in straight, parallel and serpentine channel pattern cells. J Power Sources 196 5377-5385... 

Jiao, K., B. Zhou, and P. Quan. 2006b. Liquid water transport in straight micro-parallel-channels with manifolds for PEM fuel cell cathode. Journal of Power Sources 157 226-243. 

A monolith or parallel passage reactor contains a solid construction with parallel channels, usually with a cross sectional area of a few square mm each. They can be manufactured either by drilling parallel holes in a solid block, or by a ceramic process starting with a pile of layers that look like corrugated cardboard, made of clay, which is then baked. The latter type is used as a carrier for catalysts, and applied where a low gas pressure drop is essential, e.g., in automobile exhaust gas purification. The mass transfer rates in such monoliths can be calculated easily from the equations describing mass (and heat-) transfer in straight tubes, see eqs. (4.24), (4.28) and (4.29). 

To overcome the drawbacks of the straight parallel flow field design, a serpentine flow field pattern has been developed. As shown in Fig. 1.11, in the serpentine flow field, the reactant gas flows mainly along the flow channel, which leads to a uniform gas distribution inside the plate. In addition, the gas pressure drop from the inlet to the outlet is large, which favors water removal. 

Flow field layout plays an important role in flow field design, as it affects both reactant gas distribution and water removal. Several flow field layouts have been developed, according to their flow channels the pin-type flow field [95,96], the straight parallel flow field [97,98], the serpentine flow field [98,99], and the interdigitated flow field [100-103], as shown in Fig. 2.19. All these patterns have their own characteristics in terms of reactant gas distribution, water, and heat management, and their advantages and disadvantages have been extensively reviewed by Li and Sabir [86]. 

A straight parallel design involves a number of straight parallel flow channels that run from the inlet port of the gas feeder to the outlet port as shown in Figure 10.13. 

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