Flooding, column

Fig. 24. Generalized method using log scales for estimating packed column flooding and pressure drop, AP, in kPa/m g = gravitational constant, 9.81 m/s t = kinematic viscosity in mm /s (= cSt) E, G have units of kg/(m s) are in kg/m and the packing factor, F, in can be found in...

Flooding and Loading Since flooding or phase inversion normally represents the maximum capacity condition for a packed column, it is desirable to predict its value for new designs. The first generalized correlation of packed-column flood points was developed by Sherwood, Shipley, and Holloway [Ind. Eng. Chem., 30, 768 (1938)] on the basis of laboratory measurements primarily on the air-water system. 

Flooding across a column section reflects itself in an increase in pressure drop and a decrease in temperature difference across the affected section. Product quality is also impaired, but it is hoped that the other indicators will allow correction of the situation before major change in product quality. When a column floods, the levels in the accumulator and bottom often change. It can occur that the accumulator fills with liquid carried over while the reboiler runs dry. 

DEL = Column flooding correlating factor FPL = Tray flow path length, ins. 

A column 0.6 m diameter and 4 m high is, packed with 25 mm ceramic Raschig rings and used in a gas absorption process carried out at 101.3 kN/m2 and 293 K. If the liquid and gas properties approximate to those of water and air respectively and their flowrates are 2.5 and 0.6 kg/m2s, what is the pressure drop across the column In making calculations, Carman s method should be used. By how much may the liquid flow rate be increased before the column floods ... 

An important limit to extraction column behaviour is of course that of column flooding. How could this effect also be incorporated into the simulation ... 

Flooding is an excessive accumulation of liquid inside a column. Flood symptoms include a rapid rise in pressure drop (the accumulating liquid increases the liquid head on the trays), liquid carryover from the column top, reduction in bottom flow rate (the accumulating liquid does not reach the tower bottom), and instability (accumulation is non-steady-state). This liquid accumulation is generally induced by one of the following mechanisms. 

Let us look at an efficiency measurement. Efficiency, N, is usually reported in plates, a dimensionless term that is a throwback to the days of open column, flooded plate distillations. The more plates in the distillation column, the more equilibrations have occurred, and the better the separation that was produced. In an HPLC column, the larger the plate count, the sharper the peaks are, and the smaller the amount of overlap that occurs between them. 

Determination of theoretical stage number and stage efficiency, or more simply HETS, has been established for any extraction process. The next item of order is the packed extractor column flooding limit. Just as fractionation columns must be sized for vapor and liquid, liquid-liquid extraction columns must be sized for flood limits. 

Equations (7.26) and (7.27) are based on 80% extractor column flood. (Note the 0.8 factor in both equations.) Please note that Vc and VD are the superficial full-column internal diameter velocities. Superficial simply means that you calculate these velocities as though the respective liquid phase were the only material in the full-column cross-sectional area... 

The maximum capacity of a tray column is usually limited by the onset of flooding, which occurs when liquid excessively accumulates inside the column. Flooding is... 

In this strategy the feed mixture is charged in the reboiler (at the beginning of the process) to its maximum capacity. For a given condenser vapour load Vc, if the reflux ratio R (which governs the distillate rate, LD, kmol/hr) and the solvent feed rate F (kmol/hr) are not carefully controlled the column will be flooded. To avoid column flooding Tran and Mujtaba (1997) developed the following necessary and sufficient condition ... 

Note that for both strategies the reboiler holdup must not exceed the maximum capacity (constraint) at any time (path) within the entire operation period to avoid column flooding and this imposes a path constraint in the optimisation problem as discussed below. 

Takriff and others have presented a dimensional corellation of (Ug,F) /" VS (ND)(D/T) with Vf as a parameter, which allows prediction of the column flooding gas rate as a function of liquid flow and agitation parameters. Their correlation is presented here as Figure 10.42, where Ug,F is the flooding velocity, m/s, and Vf is the forward liquid velocity, m/s, with both velocities based on the area of the stage divider opening. 

M.S. Takriff, Column Flooding, Gas Holtkip and Interstage Backmixing of an Aerated Multistage, Machanically Agitated, Compartmented Column, PhD Dissertation, University of Arkansas, 1996. 

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