Resistance to gas flow

Hydraulic Gradient Hydraulic gradient, the head of hquid necessary to overcome the frictional resistance to hquid (froth) passage across the plate, is impoiTant for plate stabihty inasmuch as it is the only liquid head that varies across the length of passage. If the gradient is excessive, the upstream portion of the plate may be rendered inoperative because of increased resistance to gas flow caused by increased liqmd head (Fig. 14-34). In general the empirical criterion for stable operation is /j > 2.5/j/,g. 

However, disadvantages inclnde weight, a relatively high resistance to gas flow, and the size of the apertnres is not directly controlled. Movement of the shot or balls dnring a deflagration or detonation conld lead to failnre of the flame arrester (HSE 1980). 

As an extra safeguard, on new compressors a fabric liner may be fitted inside the mesh strainer to catch fine dirt which will be present. Such liners must be removed at the end of the running-in period, as they create a high resistance to gas flow. 

Genetic or other factors that induce stomatal closure will reduce plant sensitivity to oxidant pollutants. Generally, the sensitivity of plants at the time of exposure is controlled primarily by factors that affect the stomatal aperture. The internal resistance to gas flow may also influence leaf sensitivity. Factors that affect sensitivity during growth usually cause physiologic changes in the plant that tend to make it more resistant to the added stress of oxidant. Many of these stresses may alter membrane physiology and make the membranes either more or less sensitive to oxidant stress. 

The characteristics considered most desirable for good packing performance are a high surface area, a uniform distribution of liquid, and a low resistance to gas flow. 

For a packed column, the principle requirements for the packing is that it should (1) provide a large surface area between the gas and liquid (2) promote uniform liquid distribution on the packing surface (3) promote uniform vapour gas flow across the column cross-section and (4) have an open structure to give a low resistance to gas flow. Many diverse types and shapes of packing have been developed to satisfy these requirements and details of the most recent ones can be found on the web sites of column internals vendors, e.g. Refs. 9 or 10. 

The tendency in ceramic packing is towards high surface area, low gas flow resistance perforated saddles. These saddles give rapid dehydration with low resistance to gas flow. 

Fig. 8.1. Photograph of catalyst pieces, courtesy Haldor Topsoe A/S www.haldortopsoe.com Rings, daisy (star) rings and pellets are shown. The daisy rings maximize catalyst area and minimize resistance to gas flow. In operation (700-900 K), the catalyst consists of a molten vanadium-alkali metal sulfate/pyrosulfate layer on a porous solid silica substrate. The outside diameter of the largest piece (far left) is 20 mm.

Daisy rings have the largest surface area and lowest resistance to gas flow. Pellets have the smallest surface area and highest resistance to flow. Simple rings are intermediate. 

The manifold and the pumping system must be made and connected in such a way as to keep the resistance to gas flow as low as possible. Thus, large tubing and large stopcocks are required between the manifold and the pumping system, and the traps should be made using... 

The length of the column is usually about 5 to 6 ft and rarely exceeds 10 ft. With longer columns, the resistance to gas flow becomes quite large, and rather high inlet pressures will be required in order to obtain a reasonable flow rate. 

The adsorbent particles are normally used as beads, extrudates, or granules (-0.1 -0.3 cm equivalent diameters) in conventional H2 PSA processes. The particle diameters can be further reduced to increase the feed gas impurity mass transfer rates into the adsorbent at the cost of increased column pressure drop, which adversely affects the separation performance. The particle diameters, however, cannot be reduced indefinitely and adsorption kinetics can become limiting for very fast cycles48 New adsorbent configurations that offer (i) substantially less resistance to gas flow inside an adsorber and, thus, less pressure drop (ii) exhibit very fast impurity mass transfer coefficients and (iii) minimize channeling are the preferred materials for RPSA systems. At the same time, the working capacity of the material must be high and the void volume must be small in order to minimize the adsorber size and maximize the product recovery. Various materials satisfy many of the requirements fisted above, but not all of them simultaneously. 

Packed tower scrubber 1-5 Gas velocities to 4000 m/min, high resistance to gas flow (pressure drop)... 

Absolute filter 0.01-0.1 High efficiency, but high resistance to gas flow... 

The modelling of gas permeation has been applied by several authors in the qualitative characterisation of porous structures of ceramic membranes [132-138]. Concerning the difficult case of gas transport analysis in microporous membranes, we have to notice the extensive works of A.B. Shelekhin et al. on glass membranes [139,14] as well as those more recent of R.S.A. de Lange et al. on sol-gel derived molecular sieve membranes [137,138]. The influence of errors in measured variables on the reliability of membrane structural parameters have been discussed in [136]. The accuracy of experimental data and the mutual relation between the resistance to gas flow of the separation layer and of the support are the limitations for the application of the permeation method. The interpretation of flux data must be further considered in heterogeneous media due to the effects of pore size distribution and pore connectivity. This can be conveniently done in terms of structure factors [5]. Furthermore the adsorption of gas is often considered as negligible in simple kinetic theories. Application of flow methods should always be critically examined with this in mind. 

Because of the resistance to gas flow in the connection between a vacuum chamber and the pump, the effective pump volume speed (S) at the chamber is usually less than the volume flow rate of the pump (Sp) at its inlets. The relationship between S and Sp is given by [3]... 

Special consideration must be given to physical characteristics in the choice of catalyst support. Since asbestos fibers are not porous to any extent, catalysts supported on asbestos are held in the inter-fibrilar spaces. When asbestos is used, precautions must be taken to prevent the felt-like mass from compacting and increasing the resistance to gas flow. For this purpose, asbestos-supported catalysts are used in numerous thin layers, each layer on a separate perforated plate. Despite its resistance to temperature effects, asbestos is not favored as a support and the more rigid materials preferred. 

Procedure. A quantity of 2 lbs. of coal of 4-8 Tyler mesh or 1/4-3/8-in. size was charged to the feed hopper and heated rapidly with steam to just below the softening temperature of the coal (350°C. for Pittsburgh-seam coal). This temperature was previously (3) determined as the value at which a fixed bed of coal offered an increase in resistance to gas flow. The heated coal was fed from the hopper at a controlled rate by a screw feeder into the 20-ft. long treater where it fell freely through a countercurrent flow of steam containing oxygen to a receiver,... 

Such slabby stone does result in slightly higher resistance to gas flow than a similar size of more cubical stone. This necessitates the use of fans with increased suction, but the extra power requirement is more than offset by the above benefits. 

Shaped catalyst bodies with optimized geometries (e.g., wagonwheels, honeycombs) offer lower resistance to gas flow and lower the pressure loss in reactors. The mechanical and thermal stabihty of catalysts and supports is being improved. New support materials such as magnesite, silicon carbide, and zircon (ZrSi04) ceramics with modified pore structures offer new possibilities. Meso- and macropores can be incorporated into solids to accelerate transport processes, and the question of porosity will increasingly be the subject of interest. 

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