Determining the Flooding Point

The evaluation of experimental data also took into account numerous pubHcations of the 90s, which include some experimental results of studies carried out at larger plants, with ds = 0.6,1,1.2,1.6,1.8 and 3 m, see Chap. 2 [6, 7,66-69, 80] or at higher pressures, see Chap. 2 [39, 68, 69, 75]. 

In order to predict the liquid hold-up h° pi at the flooding point, it is necessary to ascertain the phase flow ratio at the flooding point Xq = Vl/Vv,fi. 

Based on experimental data, it was possible in the course of this work to derive a new equation, which describes the Uquid hold-up at the flooding point hp p only based on the phase flow ratio at the flooding point Xq = Vl/Vv.fi, independent of the type, material and size of the packing or the physical properties. 

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A generalised pressure drop correlation for determined the flooding point has been proposed by Eckert (1988), and is displayed in Fig. 16.8. The ordinate is expressed as... 

Temperature gradients are an effective, low-cost method of determining the flood point, but the method s success depends on the existence of a sufficiently large temperature gradient under normal operating conditions. If the normal tray-to-tray temperature difference is small, as in close separations, the flooded temperature profile will not vary a great deal from the normal profile, and temperatiire profiles will be poor indicators of flooding. 

In order to determine the flood point, either vapor or liquid flow rate or both are raised. Most commonly, both are raised, because otherwise column material balance is affected and one product will have poor purity before flooding conditions are reached. The following techniques are commonly used for raising vapor and liquid rates during flood testing ... 

Although results of the preliminary test may suffer from overshooting or undershooting, they are likely to determine the flood point within 10 percent, and often within 5 percent. The results of this test are used to determine a good starting point for the slow test. The preliminary test technique was found effective both for improving accuracy and reducing time requirements for flood point determination (186). 

The dimensioning (diameter) of extraction columns is performed as follows After determining the optimum plate number and the corresponding optimal solvent quantity for a given process, the ratio of the volumetric flow rates of the continuous and disperse phases Vcmal ikp fixed, and one defines an area-specific volumetric throughput (Equation 2.3.4-11) and, in a pilot column, determines the flood point as a function of the pulsation a f, where a is the amphtude (typically 8 mm) and/is the frequency (typically ca. 60 Hz) ... 

In the case of larger packing elements, at moderately negative and normal pressure, the Hquid flow is practically turbulent, which means that Eqs. (2-46) and (2-42) are mostly applicable. Hence, it is possible to determine the flooding point without having to ascertain any experimental hold-up data at the flooding point. 

Whether for a distillation, absorption, or stripping system the material balance should be established around the top, bottom, and feed sections of the column. Then, using these liquid and vapor rates at actual flowing conditions, determine the flooding and maximum operating points or conditions. Then, using Figures 9-21B, -21E, or -21F, establish pressure drop, or assume a pressure drop and back-calculate a vapor flow rate, and from this a column diam-... 

To use the flooding point diagram, first it is necessary to decide whether the drops produced in the extractor are circulating or oscillating. The mean diameter di,2 (see Eq. 9.1) is used for the characteristic drop size. If the flow rate ratio is known from the thermodynamic design, the superficial velocities of both phases can be determined at the flooding point. The minimum column cross-sectional area and diameter necessarily follows directly from the superficial velocity at the flooding point with Eq. 9.19. 

The method of calculation introduced in this chapter not only allows an exact determination of the column diameter for nonpulsed sieve tray columns, but also allows a good estimation of the diameters of pulsed and stirred extractors. For the latter, however, more exact specific equations exist for the flooding point, see for example [1,4]. 

Preselection of a suitable extractor type on the basis of available information and testing at pilot scale Establishment of suitable operating conditions as, for example, the pulse intensity of pulsed extractors or the speed of rotation of centrifugal extractors Determination of the flooding point... 

The so-called bubble point of a membrane - a measure ofthe membrane pore size - can be determined by using standard apparatus. When determining the bubble point of small, disk-shaped membrane samples (47 mm in diameter), the membrane is supported from above by a screen. The disk is then flooded with a liquid, so that a pool of liquid is left on top. Air is then slowly introduced from below, and the pressure increased in a stepwise manner. When the first steady stream of bubbles to emerge from the membrane is observed, that pressure is termed the bubble point. ... 

The upper limit of the velocity in a sieve-tray column is determined by the flooding point or by the velocity at which entrainment becomes excessive. Flooding occurs when the liquid in the downcomer backs up to the next plate, and this is determined mainly by the pressure drop across the plate and the plate spacing. Near the flooding point, most of the pressure drop comes from the term in Eq. 

For example, the cost of a distillation column can be assembled from the cost of elements vertical cylindrical vessel, plus internals (trays or packing), reboiler, condenser, and reflux drum. The height of the shell can be determined from the number of trays and inter-stage height. The column diameter can be found by hydraulic calculations based on the flooding point. In this way, the volume of the cylindrical part can be easily evaluated. The volume of auxiliary vessels, as drum and reboiler, can be estimated from the residence time, typically of 10 minutes. 

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