Gas flow, reversal

At the bottom of the vortex, there is substantial turbulence as the gas flow reverses and flows up the middle of the cyclone into the gas outlet tube. As indicated above, if this region is too close to the wall of the cone, substantial reentrainment of the separated solids can occur. Therefore, it is very important that cyclone design take this into account. 

The RFSG has a pair of regenerative packed-bed heat exchangers at the air inlet and product gas outlet, with foe gas flow reversing typically every 20 minutes. The heat exchangers are modelled using a one-dimensional flnite diflerence technique, after Schmidt and Willmott [4]. 

The idea of utilizing the centrifugal force for separation of liquid from gas also finds application in gas-oil hydrocyclone separators (Fig. 2.9). In order to create a centrifugal stream inside these separators, an attached unit - a hydrocyclone - is installed on their lateral surface (Fig. 2.9, a, b, c). A hydrocyclone is usually a vertical device with a fiat tangential input and directing branch pipe for gas flow reversal in the top part, and a liquid cross-flow section in the bottom part. The technological reservoir is made in the form of a horizontal separator with various devices for additional separation of liquid from gas that are typical for oil-gas separators. 

Erosion has often been experienced at the nozzles used in CEB combustors (Fig. 1). A dominant mechanism leading to erosion is the pressure-induced gas flow reversal that will be discussed below in Sec. 6. Solids which have entered into the nozzle during a period of flow reversal are entrained out once the gas flows at high velocity in the outward direction again. The entrained sohds in the high-velocity flow in the nozzle hole may cause severe erosion of the wall of the hole. A... 

Figure 12 Solids flowing into the nozzle shaft during gas flow reversal.

Fig. 23. Two types of hollow-fiber modules used for gas separation, reverse osmosis, and ultrafiltration applications, (a) Shell-side feed modules are generally used for high pressure appHcations up to - 7 MPa (1000 psig). Fouling on the feed side of the membrane can be a problem with this design, and pretreatment of the feed stream to remove particulates is required, (b) Bore-side feed modules are generally used for medium pressure feed streams up to - 1 MPa (150 psig), where good flow control to minimise fouling and concentration polarization on the feed side of the membrane is desired.

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

Historically the most common gas disperser for cross-flow plates has been the bubble cap. This device has a built-in seal which prevents liquid drainage at low gas-flow rates. Typical bubble caps are shown in Fig. 14-20. Gas flows up through a center riser, reverses flow under the cap, passes downward through the annulus between riser and cap, and finally passes into the liquid through a series of openings, or slots, in the lower side of the cap. 

Additional samples were prepared from the three resins and were heated at temperatures between 940° and 1100°, under different inert gas flow rate and with different heating rates. The samples have different microporosities and show different capacities for lithium insertion. The results for all the carbons prepared from resins are shown in Fig. 32, which shows the reversible capacity plotted as a function of R. The reversible capacity for Li insertion increases as R decreases. This result is consistent with the result reported in reference 12,... 

Reverse air cleaning filters Filters that become self-cleaning by dislodging the impacted dust when the gas flow is reversed. 

Some small cooling circuits have reversing refrigerant flow (i.e. cooling/ heat pump) and may work at reduced gas flow for capacity control. Under such conditions it may not be possible to maintain the minimum velocity to carry oil back to the compressor, and it will settle in the circuit. Arrangements must be made to increase or reverse the gas flow periodically to move this oil. 

Data collected during these process variable scans indicate that more C02 than expected was being formed (Figure 4). Selectivity to C02 reached a maximum of 5-10% at about 90-95% conversion. At higher conversions, the C02 level is reduced by reverse shift and subsequent methanation of CO or by direct methanation of C02. This selectivity to C02 can be eliminated by cofeeding small amounts of C02 (3-5%). Since multiple C02 absorbers are required in the commercial SNG plant, one or more could be relocated downstream of the methanation step. This could offer some economic advantages since C02 absorption would now occur at higher concentration and pressure and at lower total gas flow. 

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