Packings loading point

For ordered, or structured, packings, pressure-drop estimation methods have been reviewed by Fair and Bravo [Chem. Eng. Progr, 86(1), 19 (1990)]. It is not common practice to use the packing factor approach for predicling pressure drop or flooding. For operation below the loading point, the model of Bravo et [Hydrocarbon... 

The first term is a film number the second is the Fronde number. The equation apphes to ring packings only operating below the load point and correlates all literature data to about 20 percent. 

Strigle [94] proposed this term to better describe the performance of a packed column at or near the previously described loading point. Kister [93] evaluated the limited published data and proposed using the MOC at 95% of the flood point. The flood point can be estimated by Equation 9-20 or from the plots in References 90 and 93. The data are reported to be within 15-20% of the prediction [93]. See Figure 9-22 for the identification of MOC on the HETP vs. Cg chart For more accurate information... 

Pressure drop through gauze and sheet metal structured packings [115] applies for the region below the loading point and cannot predict the flood point because liquid holdup vs. gas velocity is not included. The latest version of the equation is in Reference 108 ... 

This is only slighdy lower than good practice, but because the operating point is at the lower loading point, continue with this selection. A smaller diameter tower might calculate to be a better choice. However, extreme caution must be used in designing too close to limits with packed towers. Very litde of the data are exact, and often the range is not known. 

Piret et al. measured liquid holdup in a column of 2J-ft diameter and 6-ft packed height, packed with graded round gravel of lj-in. size, the total voidage of the bed being 38.8%. The fluid media, air and water, were in countercurrent flow. The liquid holdup was found to increase markedly with liquid flow rate, but was independent of gas flow rate below the loading point. Above the loading point, an increase of liquid hold-up with gas flow rate was observed. 

There are several ways of calculating the pressure drop across a packed column when gas and liquid are flowing simultaneously and the column is operating below the loading point. 

In many industrial applications of packed columns, it is desirable to know the volumetric hold-up of the liquid phase in the column. This information might be needed, for example, if the liquid were involved in a chemical reaction or if a control system for the column were being designed. For gas-liquid systems the hold-up of liquid Hw for conditions below the loading point has been found(48) to vary approximately as the 0.6 power of the liquid rate, and for rings and saddles this is given approximately by ... 

Fig. 8.22. Experimental dimensionless RTD for katapak-S packing (from Ref. [22]) far below loading point (dashed line) at loading point (dotted line) beyond loading point (full line).

Flv, Reit and Gat are given by Eqs. (8.3), (8.8), and (8.9), respectively a, , C1>Lot and C2>li0 are constants for each packing, and are tabulated in Table 8.2. The Reynolds number Re, is based on the liquid velocity at the load point. The Billet and Schnltes correlation predicted most of thair load point data within about 10 percent (79). It applies for both random and structured packings. 

Packed lowers, 687-708 allowable velocity in, 697-700 cost of components of, 708-713 flooding point in, 697-702 liquid distribution in, 691-692 loading point in, 692-694 pressure drop in, 692-697 relative merits of plate and, 706-708 Packings ... 

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