Bedding structures

Fiber-Bed Scrubbers Fibrous-bed structures are sometimes used as gas-liquid contactors, with cocnrrent flow of the gas and hqnid streams. In such contactors, both scrubbing (particle deposition on droplets) and filtration (particle deposition on fibers) may take place. If only mists are to be collected, small fibers may be used, but if solid particles are present, the use of fiber beds is limited by the tendency of the beds to phig. For dnst-cohectiou service, the fiber bed must be composed of coarse fibers and have a high void fraction, so as to minimize the tendency to plug. The fiber bed may be made from metal or... 

These are broadly in line with the comments of others, except that some hold the view that to obtain the densest possible bed structure the impinging particle must hit the accumulating bed with sufficient speed to forcibly displace those particles already in position [130,131,152-154]. The slurry is displaced into the... 

The Waters system uses a plastic cartridge which is inserted into a device (the Z-module) that subjects the column to radial compression, ie pressure is applied along the radial axis of the column tube. The flexible wall of the column then moulds itself into the voids that are present in the wall regions of the column. This method is claimed to produce an improvement in the packed bed structure, better column performance and longer useful column life. 

So far, there have only been a few modeling studies to try to link local fluid flow to bed structure. Chu and Ng (1989) and later Bryant et al. (1993) and Thompson and Fogler (1997) used network models for flow in packed beds. Different beds were established using a computer simulation method for creating a random bed. The model beds were then reduced to a network of pores, and either flow/pressure drop relations or Stokes law was used to obtain a flow distribution. 

Fig. 14. CFD predictions of turbulent flow (meshed REU geometry, air pathlines with spheres hidden for clarity, contours of turbulent kinetic energy k) for configuration B2. Copyright 2001 From Turbulent Resistance of Complex Bed Structures by J. Tobis. Reproduced by permission of Taylor and Francis, Inc., http //www.taylorandfrancis.com.

At this stage it seemed clear that to improve near-wall heat transfer modeling would require better representation of the near-wall flow field, and how it was connected to bed structure and wall heat transfer rates. Our early models of full beds of spheres at N — 4 were too large for our computational capacity when meshed at the refinement that we anticipated to be necessary for the detailed flow fields that we wanted. We therefore developed the WS approach described above in Section II.B.3. 

In Fig. 18, flow path lines are shown in a perspective view of the 3D WS. By displaying the path lines in a perspective view, the 3D structure of the field, and of the path lines, becomes more apparent. To create a better view of the flow field, some particles were removed. For Fig. 18, the particles were released in the bottom plane of the geometry, and the flow paths are calculated from the release point. From the path line plot, we see that the diverging flow around the particle-wall contact points is part of a larger undulating flow through the pores in the near-wall bed structure. Another flow feature is the wake flow behind the middle particle in the bottom near-wall layer. It can also be seen that the fluid is transported radially toward the wall in this wake flow. 

Alliger (U.S. Patents 3,659,402,1972, and 3,905,788,1975) describes fiber-bed structures which are not random, but are rather built up from flat mesh sheets offset angularly from one layer to the next and then compressed and bonded. Such bonded beds of relatively coarse hydrophobic fibers both are remarkably flushable, to prevent fouling by insoluble solids, and have surprisingly high collection efficiency per unit pressure drop for submicrometer particles, approaching that of irrigated fine hydrophobic fiber filters such as described by Fair (U.S. Patent 3,135,592, 1964) and Vosseller (U.S. Patent 3,250,059, 1966). 

The effect that the quality of the bed structure has on the chromatographic properties of columns packed with particles has been well known for a long time [1]. Similarly, the efficiency of capillary electrophoretic separations reaches its maximum for a specific capillary diameter, and then decreases steeply for both larger and smaller size [ 117]. Therefore, any improvement in the efficiency of the polymeric monolithic columns for the isocratic separations of small molecules is likely to be achieved through the optimization of their porous structure rather than their chemistry. 

First of all, the physical structure of the packed bed in the conversion system is defined. The fuel bed structure can be divided into three phases, namely the interstitial gas phase, the intraparticle solid phase, and the intraparticle gas phase. By means of this terminology it is easier to address certain mass and heat transport phenomena taking place on macro and micro scale inside the packed bed during the thermochemical conversion, see Figure 8. 

Due to the very differentiated and complex phenomenology inside a thermochemically degrading fuel bed, it is very important to have a clear and differentiated terminology for the physical structure of the fuel bed, herein referred to as fuel-bed structure, so that the many specific thermochemical conversion phenomena can be more precisely adressed inside the packed bed. 

In conclusion, the fuel-bed system comprises three structures, which are interstitial gas phase, intraparticle gas phase and intraparticle solid phase, see Figure 18. In a comprehensive partial differential theory of the conversion system these three structures need to be considered. The fuel-bed structure is independent of conversion concept (see definition below) applied that is, the three structures shown in Figure 18 will be the same for all categories of packed fuel-bed systems. 

The long-term goal in the science of thermochemical conversion of a solid fuel is to develop comprehensive computer codes, herein referred to as a bed model or CFSD (computational fluid-solid dynamics). Firstly, this CFSD code must be able to simulate basic conversion concepts, with respect to the mode, movement, composition and configuration of the fuel bed. The conversion concept has a great effect on the behaviour of the thermochemical conversion process variables, such as the molecular composition and mass flow of conversion gas. Secondly, the bed model must also consider the fuel-bed structure on both micro- and macro-scale. This classification refers to three structures, namely interstitial gas phase, intraparticle gas phase, and intraparticle solid phase. Commonly, a packed bed is referred to as a two-phase system. 

A major problem associated with loading methods could be the inconsistency in bed structure, i.e. mean and local voidage properties, from fill to fill. Taking into consideration the fact that pressure drop is greatly influenced by the bed voidage and that pressure drop is critical for gas-phase systems, the loading of particles is of great importance, especially in gas-phase reactors (Afandizadeh and Foumeny, 2001). 

FIGURE 5.3. The nanopore test bed structure containing a SAM of functionalized OPE 1. 

For a bed with Group A particles, bubbles do not form when the gas velocity reaches Umf. The bed enters the particulate fluidization regime under this condition. The operation under the particulate fluidization regime is characterized by a smooth bed expansion with an apparent uniform bed structure for Umf < U < Umb, where Umb is the superficial gas velocity at the minimum bubbling condition. The height of the bed expansion in terms of a can be estimated by [Abrahamsen and Geldart, 1980a]... 

As opposed to the relatively uniform bed structure in dense-phase fluidization, the radial and axial distributions of voidage, particle velocity, and gas velocity in the circulating fluidized bed are very nonuniform (see Chapter 10) as a result the profile for the heat transfer coefficient in the circulating fluidized bed is nonuniform. 

A realistic treatment of mass transfer between the gas and solid phases requires consideration of the bed structure comprising the bubble phase and the emulsion phase (see 9.4). Considering bubbles containing species A passing through a fluidized bed where species A is in depletion, the mass transfer, or the mass interchange coefficient from the bubble phase to the emulsion phase, K, can be related to the difference in the concentration of species A in the bubble phase, CAib, and that in the emulsion phase, CA,e, by [Kunii and Levenspiel, 1968]... 

The mixing results of the bimodal intersecting mixer show that microstructured analogs of packed-bed structures can mix efficiently. Small micro channels resemble... 

Several protocols can be used to fabricate packed bed structures for use in CEC. In this chapter, we will discuss the packing techniques and column fabrication protocols that have been used for packing particulate material. We concentrate, therefore, on the different approaches used to deliver chromatographic particles into the capillary column. We present an overview of the different packing protocols available to the practitioner, as well as of the CEC column fabrication method, as performed in our laboratory. Our own experiences, practices, and views regarding packing procedures are also provided, when appropriate. 

The study of problems concerning the flow of fluids through beds of granular material has many applications. It is indispensable to the theory of industrial filtration and water purification. The flow of ground water is controlled by soil structure and the most elementary phases of granular bed structure are fundamental to soil mechanics and soil physics. 

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