Liquid circulation velocity

Gas holdup and liquid circulation velocity are the most important parameters to determinate the conversion and selectivity of airlift reactors. Most of the reported works are focused on the global hydrodynamic behavior, while studies on the measurements of local parameters are much more limited [20]. In recent years, studies on the hydrodynamic behavior in ALRs have focused on local behaviors [20-23], such as the gas holdup, bubble size and bubble rise velocity. These studies give us a much better understanding on ALRs. 

Previous workers have studied the influence of the ratio of the cross-section area of the downcomer to the riser [4,5], the reactor height [6,7], the gas-liquid separator configuration [8], and the distributor type and location [9]. All these affect the flow characteristics and mass transfer. Most previous works focus on global parameters, such as the liquid circulation velocity [10-13] and the average gas holdup in the riser [14-16]. Although much work has been carried out on EL-ALRs, the proper design and scale-up of an EL-ALR is still difficult because any variation in the physical properties of the gas or the liquid and the reactor structural feathers can have a considerable effect on the hydrodynamics... 

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. 

Fig. 6 presents the experimental results on the gas holdup profiles with and without the internal at the axial position of 74 cm for different superficial gas velocities. The internal causes extra flow resistance, which in turn decreases the liquid circulation velocity and increases gas holdup [5]. In addition, the experimental results show that the radial profiles of the local gas holdup with an internal are flatter than those without the internal. Therefore, a properly designed internal can have dual function of increasing the gas holdup and improving its radial profile. 

In the case of steam condensation, a film heat transfer coefficient hj is used. In the case of liquid circulation, velocities will be very low because of the large cross-sectional area. 

A loop reactor is used for the bioconversion of methane to produce biomass used, e.g., as fish meal. This is a large-diameter pipe operated at high liquid circulation velocity with the 02/CH4 feed injected at several locations along the reactor. Cooling of the exothermic aerobic fermentation is accomplished by external heat exchangers. Static mixers are used to maintain gas dispersion in the liquid. 

Actually, r L is proportional to the liquid circulation velocity. The expression of Jadhav and Pangarkar is less easily applicable than the other relations given in Table 6, because knowledge of r[Pg.482]

They found that optimal conditions of the position of the stirrer exist in the range of 0.25 < HJd < 0.75 for both turbine and propeller stirrers. Their work suggests that beyond a Reynolds number of 20,000, the power number becomes constant. An increase in solid content increases the power consumption, while an increase in gas velocity reduces the power consumption. Kurten and Zehner (1979) examined the effect of gas velocity on the power consumption for suspension of solids and found that because of simultaneous aeration, a higher power input is required for suspension in the presence of gas. This is mainly due to the reduced liquid circulation velocity in the presence of gas bubbles. Most recently, Albal et al. (1983) evaluated the effect of liquid properties on power consumption for both two- and three-phase systems. They found that power consumption per unit volume increases with solid concentration. The influence of particle size on power consumption increases with the solids concentration. For an unconventional arrangement of a stirrer, they also found different Ne-Re curves for glycerine and CMC solutions. 

The critical stirrer speed can also be predicted on the basis of the liquid flow generated by the impeller. For a six-bladed disk turbine, the average liquid circulation velocity in the bulk can be expressed as... 

In the presence of sparging, the average liquid circulation velocity is reduced by 30% (Joshi et al., 1982). This reduction should be factored in the calculation of Ns. 

The average liquid circulation velocity is given by the following equation. 

The values of K for bubble columns can probably be calculated from the above correlation if the comparison between bubble columns and mechanically agitated contactors is based on the same value of average liquid circulation velocity, V. ... 

Circulation velocity of liquid Circulation velocity of liquid along column axis of bed... 

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... 

A simple measure to correlate liquid circulation velocity with the superficial gas... 

The effective bioreactor height influences the circulation path, which has many hydrodynamic implications as implied by the liquid circulation velocity defined by Blenke (1979) ... 

The bioreactor height is often presented in the form of the riser aspect ratio (Ar/i/r). The effect of riser aspect ratio is expected to depend on the gas flow and gas circulation rate. An increase in the riser aspect ratio does not significantly change the liquid circulation velocity. Therefore, the circulation path length and circulation time increases with increasing riser aspect ratio, which allows the gas phase to achieve its equilibrium bubble diameter more efficiently. 

More importantly for operation, the gas residence time in the separator controls disengagement such that decreasing the gas residence time in this region would lead to an increase in the downcomer gas holdup, which could decrease the hydraulic pressure differential and liquid circulation velocity (Siegel et al., 1986). The negative effect would be if the gas residence time in the separator is too long such that gas easily disengages and causes a decrease in gas holdup and gas-liquid mass transfer (Al-Masry, 1999). 

---