Mixing downcomer liquid

Liquid mixing time decreases sharply for an initial increase in the gas sparging rate and approach an asymptotic value that is determined by the height and diameter of the downcomer and the liquid properties [5]. A higher liquid velocity shortens the gas residence time and results in a decrease of gas holdup and interfacial area. The radial profile of the liquid is parabolic. These are disadvantageous for mass transfer. The mounting of internals in a fixed bed is often used to improve the radial profile of the liquid velocity. This motivates us to mount internals in an EL-ALRs to improve the radial profile of the gas holdup and the liquid velocity and to intensify turbulence. 

Downcomer mixing. The AIChE model assumes that liquid along the downcomer length is perfectly mixed. A recent study using a tracer injection technique (174) showed that in large trays, the downcomer liquid is unmixed. A computational analysis (174) showed that efficiency reduction due to an unmixed downcomer is relatively small, but will intensify in the presence of liquid stagnant regions on the tray. 

From the mass transfer point of view, airlift fermenters should be designed and operated in such a way that carryover of air from the riser into the downcomer is kept as low as possible. Gas in the downcomer liquid contributes little to oxygen transfer. It reduces the effective density difference between the contents of the riser and downcomer, however, which reduces the liquid circulation rate and also impairs the mixing performance of the fermenter. In fermentations where even a momentary lack of oxygen can very seriously affect productivity, some air is essential to maintain aerobic conditions and sustain fermentation in the downcomer. 

The physical model used for both of these efficiencies is shown in Figure 10-12. The gas streams and the downcomer liquids are assumed to be perfectly mixed. Murphree also assumed that the liquid on the tray... 

In the pilot plant column, the downcomers were adjacent to one another, separated by a weir. This arrangement caused a semicircular liquid path which tends to induce a more complete liquid mixing. 

Trays operate within a hydraulic envelope. At excessively high vapor rates, liquid is carried upward from one tray to the next (essentially back mixing the liquid phase in the tower). For valve trays and sieve trays,. i capacity limit can be reached at low vapor rates when liquid falls through the rray floor rather than being forced across the active area into tlic downcomers. Because the liquid does not flow across the trays, it rass.scs contact with the vapor, and the separation efficiency drops dramatically. ... 

The column contains a total of N-p theoretical trays. The liquid holdup on each tray including the downcomer is M . The liquid on each tray is assumed to be perfectly mixed with composition x,. The holdup of the vapor is assumed to be negligible throughout the system. Although the vapor volume is large, the number of moles is usually small because the vapor density is so much smaller than the liquid density. This assumption breaks down, of course, in high-pressure columns. 

The tray may flood. Water and hydrocarbon mixing on the tray deck, stirred up by the flowing gas, creates an emulsion. The emulsion does not separate as readily as clear liquid from the gas. Premature downcomer backup, followed by tray deck flooding, result. 

Liquid flow may be directed onto the tray across a low entrance weir to even out the flow, or the downcomer skirt clearance alone may be relied upon for this function. The liquid is agitated into a frothy mix as it flows across to the outlet weir. The outlet weir is normally 1-4 in. high to provide a liquid seal and leads to a downcomer that occupies about 8-20 /o of the tower cross-sectional area. 

Liquid from the tray above descends a downcomer to the tray below. In the downcomer, the vapor in the liquid leaving the active area de-entrains. The downcomer inlet panel deflects the liquid sideways to the active area. Mass-transfer contacting devices on the active area intimately mix the liquid and rising vapor. The aerated liquid leaves the tray by entering the downcomer. In the space above the active area, the entrained liquid falls out of the vapors and returns to the tray deck. Fig. 1 shows the function of the tray zones. 

Picket fence weirs are used in low-liquid-rate applications (Fig. 8). Picket fence weirs can serve two purposes at low liquid rates. First, they reduce the effective length of the weir for liquid flow increases the liquid height over the weir. This makes tray operation less sensitive to out-of-level installation. Second, pickets can prevent liquid loss (blowing) into the downcomer by spraying. This occurs at low liquid rates when the vapor is the continuous phase on the tray deck. Picket fence weirs should be considered if the liquid load is less than 1 gpm per inch of weir (0.0267 ft /sec/ft, 0.00248 m /sec/m). At liquid rates lower than 0.25 gpm per inch of weir (0.00668 ft / sec/ft, 0.000620 m /sec/m) even picket fence weirs and splash baffles have a mixed record in improving tray efficiency. Operation at liquid rates this low strongly favors the selection of structured packing. 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

The multi-chamber column of Klein, Stage and Schultze [38] also has interna] downcomers. The vapour is distributed on each plate by a number of holes arranged in a circle, the intention being to produce small bubbles giving a thorough mixing of the liquid. Stage has also developed a bubble-cap column provided with. so-called... 

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