Hydrodynamic Considerations in an External Loop Airlift Reactor with a Modified Downcomer Samuel T. Jones † and Theodore J. Heindel* Department of Mechanical Engineering, Iowa State UniVersity, Ames, Iowa 50011-2161 Gas holdup and superficial liquid velocity in the downcomer and riser are studied for an external loop airlift reactor with a downcomer-to-riser area ratio of 1:16. Two downcomer configurations are investigated over a range of superficial gas velocities (0.5 e U G e 20 cm/s) using three aeration plate open area ratios (A ) 0.62, 0.99, and 2.22%). These results are compared to a bubble column operated with similar operating conditions. Gas holdup in both the riser and downcomer are found to increase with increasing superficial gas velocity. Results show that riser gas holdup varies slightly with downcomer configuration, while a considerable variation is observed for downcomer gas holdup. The superficial liquid velocity varies considerably for the two downcomer configurations and is a function of superficial gas velocity and flow conditions in the downcomer. Observed variations are independent of aeration plate open area ratio. Introduction Studies involving external airlift loop reactors (EALRs) have indicated that reactor geometry is a key factor in determining gas holdup and liquid velocity in the downcomer and riser. 1-10 When EALRs are used as biological fermentors, gas holdup and liquid velocity in the riser and downcomer become key hydrodynamic factors that determine if there will be dead zones in the reactor. If the liquid velocity is too slow, dead zones may result and biological growth will cease, reducing the overall reactor productivity. Thus, prior to using an EALR in biological applications, the effect of reactor geometry on EALR hydro- dynamics must be fully understood. Previous investigators have reported that EALR performance depends on such parameters as superficial gas velocity, cross- sectional area ratio of the downcomer and riser, type of gas sparger, horizontal connector geometries, and liquid physical properties. 1-10 Most of these previous works have focused on airlift reactors with a downcomer-to-riser area ratio greater than 1:9. As the downcomer-to-riser area ratio decreases, there exists a point when some of the EALR hydrodynamics, such as riser gas holdup, may more closely resemble the hydrodynamics observed in bubble column reactors. For EALR with a down- comer-to-riser ratio greater than 1:9 this is not observed. This current study focuses on a fixed downcomer-to-riser area ratio of 1:16 to determine the effect of reducing the downcomer-to- riser ratio on EALR hydrodynamics. Experimental Procedures A schematic of the EALR used in this study is shown in Figure 1 and consists of a 2.4 m acrylic riser (0.10 m internal diameter) and a 2.4 m acrylic downcomer (0.025 m internal diameter). The downcomer and riser sections are connected via two 0.13 m long horizontal acrylic tubes (0.025 m internal diameter) located at H ) 0.05 and 1.27 m, where H is the reactor height above the aeration plate. The gas phase is injected at the riser base through one of three stainless steel aeration plates having open area ratios of A ) 0.62, 0.99, and 2.22%. For each aeration plate, the change in open area ratio is accomplished by changing the number of uniformly distributed 1 mm diameter holes. A gas plenum is located below the aeration plate and filled with large glass beads to promote uniform gas distribution into the riser. The riser and downcomer top sections are joined together with a ball valve as they enter the column vent, providing two possible reactor configurations where gas may or may not be allowed to pass through the upper section of the downcomer. Likewise, a gate valve is located in the middle of the downcomer section so that when closed, liquid flow through the downcomer is stopped and the reactor vessel approximates a semibatch bubble column. All tests are completed at local barometric pressure and room temperature (18-22 °C). The gas phase is compressed air, and the liquid phase is unconditioned tap water. All measurements * To whom correspondence should be addressed. Fax: (515) 294- 3261. E-mail: theindel@iastate.edu. † E-mail: sjones@iastate.edu. Figure 1. Experimental external airlift loop reactor (EALR) schematic. Ind. Eng. Chem. Res. 2010, 49, 1931–1936 1931 10.1021/ie901311r  2010 American Chemical Society Published on Web 01/07/2010