Cocurrent flows

20 and 3.21 illustrate the location of the operating line relative to the equilibrium line when the transfer is from phase V to phase L, and from phase L to phase V. 

As in the case for countercurrent flow, there is a minimum L VS ratio for cocurrent mass-transfer operations established from the fixed process compositions Yv Yv and X, for transfer from V to L X, X2, and T, for transfer from L to V. 

Repeat Example 3.8, but for cocurrent contact between the gas mixture and the silica gel. 

Therefore, Ts (min) = 6.09 x0.488 = 2.957 kg/s and Ls = 2.0 x 2.957 = 5.920 kg/s. Notice that the mass velocity of the silica gel required for cocurrent operation is about 11 times that required for countercurrent operation. This example dramatically illustrates the fact that the driving force for mass transfer is used much more efficiently in countercurrent than in cocurrent operation. 

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FIgura 7.7 1-1 shells approach pure countercurrent flow, whereas 1-2 shells exhibit partial countercurrent and partial cocurrent flow. 

Fig. 15. Four-bed PSA system cycle sequence chart (64). EQ, equalization C D A, cocurrent depressurization C D T, countercurrent depressurization R, repressurization A, cocurrent flow T, countercurrent flow. Courtesy of American Institute of Chemical Engineers.

Countercurrent or Cocurrent Flow If the flow of the streams is either completely countercurrent or completely cocurrent or if one or both streams are isothermal (condensing or vaporizing a pure component with negligible pressure change), the correct MTD is the logarithmic-mean temperature difference (LMTD), defined as... 

Air-flow control in some services can prevent these problems. Cocurrent flow of air and process fluid during winter may be adequate... 

Countercurrent flow of gas and sohds gives greater heat-transfer efficiency with a given inlet-gas temperature. But cocurrent flow can be used more frequently to diy heat-sensitive materials at higher inlet-gas temperatures because of the rapid coohng of the gas during initial evaporation of surface moisture. 

In some cases in which it is desirable to cool the product before removal to the outside atmosphere, the discharge end of the cyhnder is provided with an additional extension, the exterior of which is water-spray-cooled. In cocurrent-flow calciners, hot gases from the interior of the heated portion of the cylinder are withdrawn through a special extraction tube. This tube extends centrally through the cooled section to prevent flow of gas near the cooled-shell surfaces and possible condensation. Frequently a separate cooler is used, isolated from the calciner by an air lock. 

The majority of spray dryers in commercial use employ cocurrent flow of gas and solids. Countercurrent-flow diyers are used primarily for diying soaps and detergents. Their classif ng ability is useful in these applications. Air flow is upward, cariying entrained fines from the top of the chamber. The coarse product settles and is removed... 

Packed-bed scrubbers m be construc ted for either vertical or horizontal gas flow. Vertical-flow units (packed towers) commonly use countercurrent flow of gas and liquid, although cocurrent flow is sometimes used. Packed scrubbers using horizontal gas flow usually employ cross-flow of liquid. 

Packed Tubes Cocurrent flow of immiscible hquids through a packed tube produces a one-stage contact, characteristic of hne mixers. For flow of isobutanol-water through a 0.5-in diameter tube packed with 6 in of 3-mm glass beads, Leacock and Churchill [Am. Jn.st. Chem. Eng. J., 7, 196 (1961)] find... 

The pipe has also been used for the transfer of heat between two immiscible liquids in cocurrent flow. For hydrocarbon oil-water, the heat-transfer coefficient is given by... 

Gas/Liquid Mass Transfer This topic has been widely investigated for gas absorption in packed beds, usually countercurrent. One correlation for cocurrent flow in catalyst beds is by Sato et al. (First Pacific Chemical Engineering Congre.s.s, Pergamon, 1972, p. 187) ... 

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

Although two fluids may transfer heat in either counter-current or cocurrent flow, the relative direction of the two fluids influences the value of the LMTD, and thus, the area required to transfer a given amount of... 

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

Weekman and Myers (W3) measured wall-to-bed heat-transfer coefficients for downward cocurrent flow of air and water in the column used in the experiments referred to in Section V,A,4. The transition from homogeneous to pulsing flow corresponds to an increase of several hundred percent of the radial heat-transfer rate. The heat-transfer coefficients are much higher than those observed for single-phase liquid flow. Correlations were developed on the basis of a radial-transport model, and the penetration theory could be applied for the pulsing-flow pattern. 

Use cocurrent flow of organic feedstock and S03/air to suppress undesired consecutive reactions. 

The bubbles play the role of the gas phase. The role of the liquid is played by an emulsion phase that consists of solid particles and suspending gas in a configuration similar to that at incipient fluidization. The quasi-phases are in cocurrent flow, with mass transfer between the phases and with a solid-catalyzed reaction occurring only in the emulsion phase. The downward flow of solids that occurs near the walls is not explicitly considered in this simplified model. 

An investigation into the applicability of numerical residence time distribution was carried out on a pilot-scale annular bubble column reactor. Validation of the results was determined experimentally with a good degree of correlation. The liquid phase showed to be heavily dependent on the liquid flow, as expected, but also with the direction of travel. Significantly larger man residence times were observed in the cocurrent flow mode, with the counter-current mode exhibiting more chaimeling within the system, which appears to be contributed to by the gas phase. 

In the first pass, both the hot and cold fluids flow in cocurrent flow through the heat exchanger, whereas in the second pass, the cold fluid now flows countercurrent to the hot shellside fluid. Half the heat exchange area is therefore in cocurrent flow and half in countercurrent flow. 

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