Continuous phase circulation velocities

As noted above, small bubbles, a uniform gas holdup radial distribution and an appropriate liquid circulating velocity can intensify mass transfer between the gas phase and the continuous phase and improve the production efficiency in EL-ALRs. In order to reduce the bubble size and obtain a more uniform radial distribution of the local gas holdup and the liquid and bubble rise velocities, and regulate the liquid circulating velocity appropriately as well, in this work, a novel internal is used and mounted in the riser column to improve hydrodynamics and mass transfer. The hydrodynamic behavior and mass transfer characteristics of an EL-ALR with the new designed internal are investigated. 

In many multiphase (gas-liquid, gas-solid, liquid-liquid and gas-liquid-solid) contactors, a large degree of circulation of both discrete and continuous phases occurs. This circulation causes a good degree of mixing and enhances heat and mass transfer between fluid and walls. The degree of circulation depends on a number of parameters such as the size of equipment, the nature of the phases involved, velocities of various phases, nature of the internals within the equipment and many others. 

Joshi O) has shown that the average continuous phase circulation velocities in multiphase contactors are 1 to 2 orders of magnitude larger than the net superficial continuous velocities. 

Up — Uc represents the resultant slip velocity between the particulate and continuous phase. Some other commonly used drag coefficient correlations are listed in Appendix 4.2. For fluid particles such as gas bubbles or liquid drops, the drag coefficient may be different than that predicted by the standard drag curve, due to internal circulation and deformation. For example, Johansen and Boysen (1988) proposed the following equation to calculate Cd, which is valid for ellipsoidal bubbles in the range 500 < Re < 5000 ... 

For instance, for a riser gas holdup, of 0.2 and a reactor height, of 10 m, the average liquid circulation velocity is 6.26 m/s. That is, liquid circulation velocity is very high. If a bubble is introduced in the downcomer, it is entrained downward if the liquid velocity is higher than the bubble rise velocity. It may be pointed out that the presence of gas on the downcomer side reduces the pressure driving force. However, the circulation continues even if part of the downcomer section is occupied by the gas phase. This is because (Figure 11.23b) part of the downcomer is bubble free. is given by... 

Reynolds numbers of about 20 (in stagnant drops), gains strength as the separation point shifts at higher Reynolds numbers. The increase in transfer coefficient in the continuous phase, even for drops not fully circulating, may thus be attributed to the combined effect of the increased disturbance intensity around the separation point and in the wake, to the forward movement of the separation point, and to higher velocities. 

Stokes law is an analytic solution of the Navier-Stokes equation for the simplified flow case with solid particles and creeping flow. If the particles are fluid and in the absence of surface-active components, internal circulation inside the particle will reduce the drag. (Note that this is not necessarily valid for small fluid particles, but these are irrelevant in gravity separation.) The viscosity correction term for this case is given in Eq. (9). From this equation it can be seen that, for large viscosity differences between the dispersed and continuous phases, the settling will approach the Stokes velocity or 3/2 Stokes velocity (the two limiting... 

The bioreactor geometry effects in an ELALR can be quite complex and dynamic. As the area ratio increases, the liquid circulation velocity decreases. Hence, the gas-phase circulation time decreases and gas holdup increases. The increase in gas holdup leads to an increase in the interfacial area. Some bubble dynamics are reflected in the growth, but, due to the lower bubble-bubble interactions in ELALRs, the increase is fairly continuous, but at a relatively slow rate. For example, Joshi et al. (1990) showed that by increasing the area ratio from 0.25 to 1.0 using a 10-m ELALR at 0.3kW/m yielded a negligible increase... 

Mass transfer during drop formation can be quite significant. After formation the drop falls (or rises) through the continuous phase at its terminal velocity. Small drops (<2 mm), those in the presence of surfactants, or those for which the continuous-phase viscosity is much less than the drop viscosity behave as rigid spheres with little internal circulation. For this situation the continuous-phase coefficient can be obtained from correlations such as Eq. (2.4-38) indeed, much of the data for this correlation were obtained from evaporation rates of pure liquid drops in a gas. If no circulation is occurring within the drop, the mass transfer mechanism within the drop is that of transient molecular diflusion into a sphere for which solutions are readily available (see Section 2.3). 

Figure 1.13 Internal circulations (indicated by the dashed lines) within segmented flow segments. Segments are white continuous phase is the gray area, (a) Circulation over the whole length of the segment. This occurs within liquid segments suspended in an air continuous phase, (b) In a liquid-liquid system, circulation occurs at the front ofthe segment. The volume fraction ofthe circulation zone is dependent on certain parameters. Higher segment velocities increase the volume fraction ofthe circulation.

This circulation zone can also be increased by using a lower viscosity continuous phase. Low interfacial tension also increases the size. High interfacial tension and viscosity can lead to no circulation at all. (c) At high segment velocities, counter-rotating circulation can be initiated towards the rear of the segment. Circulation zones are always set up in the continuous phase between the segments, irrespective of the other parameters [96]. 

Slurry reactors (bubble towers) are fluidized with continuous flow of gas. The particles are smaller (less than 0.1 mm) than in the liquid fluidized systems (0.2-1 mm). In some operations the liquid and solid phases are stationary, but in others they circulate through the vessel. Such equipment has been used in Frscher-Tropsch plants and for hydrogenation of fatty esters to alcohols, furfural to furfuryl alcohol, and of glucose to sorbitol. Hydrogenation of benzene to cyclohexane is done at 50 bar and 220-225°C with Raney nickel of 0.01-0.1 mm dia. The relations between gas velocities, solids... 

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