Multiple recirculation cells

CARPT experiments in bubble columns demonstrate that while liquid circulation is always present in the column, stationary multiple recirculation cells with height equal to column diameter are not. Secondary cells may exist in the entry region at lower gas velocities (bubble flow regime). The entry length is then limited to 1 to 1.5 column diameter. Similar recirculation profiles were reported for solids in fluidized beds (Moslemian etal., 1989). 

It Is demonstrated that nonlinear Interactions between the transport processes lead to the existence of multiple steady flows In certain operating regions. Concentration transients In the growth of AlAs/GaAs Interfaces are simulated and It Is shown that the presence of flow recirculation cells widens the Interface. 

In practice, the recirculation cells are often eliminated by increasing the inlet flow rate, a procedure that also improves film thickness uniformity. However, because of nonlinear interactions among buoyancy, viscosity, and inertia terms, the transitions between the flow patterns may be abrupt, and multiple stable-flow fields may exist for the same parameter values (24,195,... 

Prediction of liquid recirculation based on the early inviscid models of Freedman and Davidson (1969) and Whalley and Davidson (1974) is only applicable to shallow tanks stirred with gas plumes. Extending the so-called principle of minimization of maximum vorticity to tall columns, lead Joshi and Sharma (1979) to conclude that multiple circulation cells of height equal to column diameter are present (Figure 3a). 

A single liquid recirculation cell (up in the middle, down at the wall) exists at high gas superficial velocities (churn turbulent regime). The same is observed for solids in fluidized beds operated at large multiples of Umf. 

To overcome the weaknesses of the column fermenter, alternatives have been proposed. A tapered column fermenter can maintain a high airflow rate at the lower section of the fermenter where the cell concentration is high. Several sieve plates can be installed in the column for the effective gas-liquid contact and the breakup of the coalesced bubbles. The cylindrical column can be divided into multiple stages with stirrers. This configuration is analogous to stirred-tank fermenters connected in series. To enhance the mixing, the fermentation broth can be recirculated by using a pump. 

ITT systems have been used at aU process scales. With the appropriate cleaning validation to demonstrate removal of protein and debris from previous use, tangential flow Alters can be cleaned and used multiple times. ITT is usually not appropriate for clarihcation of a very shear-sensitive cell hne or a low viability process as the turbulent flow required to recirculate the cells in the system leads to more cell breakage, resulting in release of more impurities in the clarified product. 

Connection of the multiple couples with the ammeter and voltmeter was made through a high-speed scanner with the galvanic cell and recirculating flow stem as shown in Fig. 10. 

Integration of cell culture and microfabrication to form CCA or CCA-lifce systems has advanced rapidly in the last four years. The use of microfabricated devices should allow relatively high throughput studies that are inexpensive, conserve scarce reagents and tissues, and facflitate automated collection and processing of data. One example is the device built by Takayama and coworkers to imitate the behavior of the vascular system. They have fabricated a device with 320 mechanical actuators to maintain and control automated cell culture within microfabricated channels (Sharchi Takayama, personal communication). While this system mimics aspects of the whole body, it does not use multiple cell types with recirculating flow. 

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