Optimum gas velocity

Calculation of column diameter (for packed columns, this is usually based on flooding conditions, and, for plate columns, on the optimum gas velocity or the liquid-handling capacity of the plate)... 

Plot a graph of //versus Ti and calculate the optimum gas velocity, the corresponding minimum plate height and the maximum plate number. 

However, high gas velocity, may lead to problems in bed operation. It increases the entrainment loss of fluidizing catalyst particles. It also may give rise to excessive reaction in the particle-disengaging space, which sometimes will lead to reactor instability or to decreased selectivity (as discussed in later sections). Attrition or erosion of the reactor is more likely at higher gas velocity. Thus, there is an optimum gas velocity for fluid beds, which for most catalysts is in the range of 20-80 cm/sec, usually 40-60 cm/sec. 

Optimum gas velocity, OGV average carrier gas velocity, mopt> in WCOT columns corresponding to min and maximum column efficiency. 

However, for the relative efficiences of the slurry pump and the compressor in the ratio of 1 to 2, the approach of Dziubinski and Richardson [1985] yields the optimum gas velocity to be 0.2 m/s, which is much smaUar than the value of 2.86m/s for maximum drag reduction. 

Fig. 24 presents some computed results for the three--phase FT process and various catalyst concentrations. Here also the space-time-yield goes via a maximum value in dependence on the inlet gas velocity. The optimum gas velocity, i.e., that at which the STY is highest, increases with the reaction rate, i.e., catalyst content in the slurry. Of course, under nonisothermic conditions when the temperature is allowed to increase slightly the optimum gas velocity rises slightly as well. It is interesting to note that for 20 % wt. Fe in slurry which corresponds to the conditions... 

In Fig. 17 the dependence of Ry a on ug is shown for few values of k2 Cg. It can be seen that the maximum is very flat, hence, in this range Rp a is not sensitive to ug. As high gas velocities imply low conversions of gaseous reactant, operation of the system at lower gas flow rates is more favorable. From the curves of Fig. 17 one can discern that 90 % of the maximum space-time-yield can be reached at only half the optimum gas velocity (ug =0.5 ug p ). However, this conclusion cannot be generalized Uee Fig. 12 It is, therefore, recommended that each individual reaction system be analyzed separately. 

Example 4 Optimum Gas Velocity for Fischer-Tropsch Slurry Reactor... 

The optimum gas velocity through the absorber, hence the vessel diameter The height of the vessel and its internal members or the number of contacting trays... 

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