Gas phase transfer velocity

The development of the design equation for a countercurrent packed tower absorber or stripper begins with a differential mass balance of component A in the gas phase, in a manner similar to that of Example 2.12. However, this time we do not restrict the analysis to dilute solutions or to constant molar velocity. If only component A is transferred (4 G = VA[ = 1.0), considering the fact that the gas-phase molar velocity will change along the column, and that F-type mass-transfer coefficients are required for concentrated solutions, the mass balance is... 

Radial density gradients in FCC and other large-diameter pneumatic transfer risers reflect gas—soHd maldistributions and reduce product yields. Cold-flow units are used to measure the transverse catalyst profiles as functions of gas velocity, catalyst flux, and inlet design. Impacts of measured flow distributions have been evaluated using a simple four lump kinetic model and assuming dispersed catalyst clusters where all the reactions are assumed to occur coupled with a continuous gas phase. A 3 wt % conversion advantage is determined for injection feed around the riser circumference as compared with an axial injection design (28). 

Mechanical agitation is needed to break up the gas bubbles but must avoid rupturing the cells. The disk turbine with radial action is most suitable. It can tolerate a superficial gas velocity up to 120 m/h. (394 ft/h) without flooding, whereas the propeller is limited to about 20 i7i/h (66 ft/h). When flooding occurs, the impeller is working in a gas phase and cannot assist the transfer of gas to the liquid phase. Power input by agitation and air sparger is 1 to 4 W/L (97 to 387 Btu/[fF-h]) of liquid. 

It is seen that equations (13) and (15) are very similar to equation (10) except that the velocity used is the outlet velocity and not the average velocity and that the diffusivity of the solute in the gas phase is taken as that measured at the outlet pressure of the column (atmospheric). It is also seen from equation (14) that the resistance to mass transfer in the stationary phase is now a function of the inlet-outlet pressure ratio (y). 

More recently, Rosen (R3), Spalding (S5), and Johnson and Nachbar (J4) have considered a simplified approach using the analysis of laminar-flame propagation velocities. According to these investigators, the principal exothermic reactions occur in the gas phase. Some of the heat liberated by these reactions is then transferred back to the solid surface to sustain the endothermic surface-gasification processes. Thus, the temperature profile within the reactive zone is quite similar to that of Rice and Crawford. However, gasification of the solid surface is assumed to be endothermic, while exothermic reactions were considered in the studies discussed previously. 

It is possible to eliminate the mass transfer resistances in Steps 2, 3, 7, and 8 by grinding the catalyst to a fine powder and exposing it to a high-velocity gas stream. The concentrations of reactants immediately adjacent to the catalytic surface are then equal to the concentrations in the bulk gas phase. The resulting kinetics are known as intrinsic kinetics since they are intrinsic to the catalyst surface and not to the design of the pores, or the pellets, or the reactor. 

---