Issues Affecting new Overall performance of one’s Bubble Column Reactor
Superficial gas velocity : Increase in superficial velocity increases gas holdup, the effective interfacial area and the overall mass transfer rate. Further, an increase in superficial gas velocity decreases the size of bubbles, thus increasing the interfacial area of contact between the gas and liquid. The effect of superficial gas velocity is negligible on the mass transfer coefficient (KL), but significant on mass transfer area (a). For chemical reaction controlled gas-liquid reaction (slow or very slow gas-liquid reaction) a certain minimum KLa must be achieved to overcome the effect on mass transfer. It is recommended to keep the superficial gas velocity less than or equal to 10 m/s.
Shallow Gas Acceleration
Backmixing: Studies have shown that gas flow in bubble column operates in the plug flow manner without any backmixing while a considerable amount of backmixing in the liquid phase is observed. Backmixing in the liquid phase decreases the concentration of liquid reactant and thus the rate of chemical reaction. Consequently, it decreases the overall rate of reaction. Use of packings, trays or baffles reduces liquid backmixing and thus reduces dilution of reactants by products. Hence, this technique provides higher concentrations of reactant and a higher rate of chemical reactions, (?rA) = (?1/Vl) (?dNA/dt), but decreases the liquid holdup (Vl). The combined effect on the rate of consumption of limiting reactants in reaction (?dNA/dt) must be checked before using any internal in the bubble column. In the case of highly exothermic gas-liquid reactions, backmixing in the liquid phase is desirable to control the nature of the reaction or its temperature. In such cases, placing packings to improve the overall rate makes temperature control very difficult because it increases the heat duty required and decreases the heat transfer coefficient.
Characteristics out-of Gasoline Phase and you can Water Phase: Brand new real functions of one’s fuel stage don’t have any impact on the fresh new show of the line, however, the ones from the h2o phase, elizabeth.g., surface pressure, viscosity, are profound. An increase in the fresh new viscosity regarding drinking water stage or a decrease on facial skin stress advances the active interfacial urban area and therefore advances the mass import rate. The existence of electrolyte throughout the drinking water stage effects the new show significantly. An enthusiastic electrolytic provider gets faster ripple dimensions and consequently increased effective interfacial urban area and higher rate out of mass transfer.
Pulsation: The performance of bubble columns can be improved by pulsation. For very low superficial velocity of gas (0.8 to 2.4 cm/s), the value of KLa can be increased by as much as a factor of 3 by pulsation.
Inclusion of Packing or Packaged Ripple Line: Including loading reduces the axial combination of liquids. And therefore, a packed ripple line can be used in which h2o backmixing was unwanted. Gas-h2o responses where reasonable fuel shrinkage takes place in new reactor have quite low shallow gasoline velocities, and that subsequently bring about an awful price regarding size import in the top of the an element of the reactor. In such cases, position packings in the higher region is beneficial. The latest packing escalates the active interfacial city and you will gas holdup so you’re able to an extent. Additionally, it brings a higher rate regarding bulk import. But not, a packed bubble line isn’t used if the good strong dirt exists on system while the catalyst otherwise as reactant. Loading is even not prominent to own very exothermic or endothermic reactions, because the manufactured towers has actually poor heat import coefficients. Reactions instance manufactured towers bring bad heat transfer coefficient. Desk 21-5 reveals Tempe hookup the benefits and you may cons between a bubble line and you may a determined container reactor, and you may Dining table 21-6 listings industrial examples of bubble line reactor.