Constant Gas Flow Rate

FIGURE 7A.7 Plot of (Pq/Pq) against for various impellers. (Adapted from Middleton 2000 with permission from Elsevier. 1997, Elsevier.) 

Flow regimes in two-phase (gas-liquid) stirred tank reactor 

FIGURE 7A.8 Hydrodynamic regimes in two-phase (gas-liquid) stirred tank reactor. 1, flooding of the impeller 2, gas dispersion above the impeller 3, gas circulation above the impeller with marginal dispersion helow the impeller 4, gas circulation both above and below the impeller 5, recirculation of the gas resulting in the formation of secondary loops besides main discharge streams from the impeller. (Reproduced from Middleton 2000 with permission from Elsevier. 1997, Elsevier.) 

For applications of a stirred tank as a gas-liquid reactor, it is obvious that the minimum effective speed of the impeller is N. For practically all the 

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Bubble-Tube Systems The commonly used bubble-tube system sharply reduces restrictions on the location of the measuring element. In order to ehminate or reduce variations in pressure drop due to the gas flow rate, a constant differential regulator is commonly employed to maintain a constant gas flow rate. Since the flow of gas through the bubble tube prevents entiy of the process liquid into the measuring system, this technique is particularly usefiil with corrosive or viscous liquids, liquids subjec t to freezing, and hquids containing entrained solids. 

The main relationships between the agitation intensity of the dispersion and the total mass-transfer rate are summarized qualitatively for constant gas flow rate by Fig. 1 (G9) wherein interaction effects among the bubbles are indicated by dashed lines. Intermediate phenomena not shown, such as the direct and feedback effects between coalescence and mass transfer (G5, G9), should also be considered. 

The results shown in Figure 2 illustrate that a satisfactorily constant gas flow rate over a period of time in excess of one hour could be achieved readily. 

For a constant gas flow rate, a decrease in gas density leads to an increase in mean gas velocity and/or average gas flow area. The increases in both these quantities prove to be beneficial to atomization quality. In the former case, it accelerates the liquid flow through the injector orifice so that the liquid is discharged at a higher velocity. In the latter case, it reduces the area available for the liquid flow so that the liquid is squeezed into thinner films and ligaments as it flows through the injector orifice. 

In certain situations in which it is impracticable to use selective absorption or high gas flow rates as a means of securing low residence times and product concentrations, a possible alternative is to use a pulsed discharge in which the effective residence time of the gas molecules is dictated by the pulse duration and the time interval between successive pulses. By this means, very short effective residence times can be achieved and typical data, again for the ammonia—hydrazine reaction, are shown in Figure 9, in which the energy yield of hydrazine is plotted vs. the discharge duration for a constant gas flow rate. The reactor was... 

X 10 N/m. Choose room-temperature operation, T = 30°C = 303.16 K. Initial gas volume = nRgT/P = (267.75 x 8314 x 303.16)/ .8 x 10 = 3749 m. Since this volume is very high compared to the reactor volume of 10 m, choose semibatch mode of operation with continuous supply of feed gas and removal of residual gas to maintain the reactor pressure. In view of this, a slurry of Ca(OH)2 in water is prepared and CO2 is bubbled through it for 4 h. The mother liquid phase will be initially saturated with Ca(OH)2. As the reaction proceeds, CaCOj precipitates out. The mother liquor can be separated from the solid CaC03 precipitate by filtration. For ease of operation and control, assume constant gas flow rate. 

Constant gas flow rate with increasing speed... 

FIGURE 7A.6 Plot of versus (a) Constant impeller speed and (b) constant gas flow rate. (Reproduced from Chapman et al. 1983b with permission from The Institution of Chemical Engineers. 1983, The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.)... 

Chapman et al. (1983b) have given the general form of variation of Ap with (Fig. 7A.6a and b) for both the cases of constant speed of agitation and constant gas flow rate. The interpretation of the behavior of Ap in Figure 7A.6a for a disc turbine (DT) is as follows (Chapman et al. 1983b). 

Figure 7A.6b shows typical versus plot at constant gas flow rate for a DT. It can be observed that the power number in the presence of gas exhibits two points of inflection. At low impeller speeds or high the gas passes vertically upward in the... 

For the chromatographic separation, a constant gas flow rate is required. For isothermal separations (i.e., constant temperature throughout the separation), a constant flow rate can be obtained by constant column inlet pressure. The pressure of carrier gas from the gas flask is reduced by the reduction valve (with pressure meters) attached to the gas flask in addition, pressure control is provided at the gas chromatograph. 

The overall collection efficiency is affected by the selective separation of the dust according to particle size in accordance with a separation curve. For constant gas flow rate, the overall collection efficiency increases if the following influencing quantities increase settling velocity of the dust particles, agglomerating tendency of the dust, dust content of the gas to be dedusted (within limits). 

Assuming (1) complete mixing in the gas and liquid phase, (2) constant gas flow rate, (3) absorption with slow reaction and (4) neglecting convective transport of A (ul (c/ lo " c/ l) 0) the following relation between the conversion of A and B can be derived... 

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