Pressure drop in packed towers

Figure 1337. Corn-elation of flow rates, typical pressure drop behavior, and packing factors of random packed beds. [Eckert, Foote, and Walter, Chem. Eng. Prog. 62(1), 59 (1966) Eckert, Chem. Eng. 14 Apr. 1975)]. (a) Correlation of flow rate and pressure drop in packed towers, (b) Typical pressure drop data 2 in. porcelain intalox saddles, with F = 40, in a bed 30 in. dia by 10 ft high, (c) Packing factors, F, of wet random packings-...

The primary factors that affect pressure drop in packed towers are (1) fluid-flow rates, (2) density and viscosity of the fluids, and (3) size, shape, orientation, and surface of the packing particles. Figure 16-17 illustrates the effects of fluid rates at constant operating pressure, and Fig. 16-19 shows how increased gas velocity due to reduction in operating pressure can affect the pressure drop. 

Many methods have been proposed for estimating pressure drop in packed towers. Most of these methods are based primarily on experimental data obtained with countercurrent flow of water and air through various types of packed towers. Because of the empirical nature of these correlations and the fact that the effects of some of the variables are not included, it is always best to predict pressure drops in the design of packed towers on the basis of experimental data obtained with equipment operating under conditions equivalent to those involved in the design. If such data are not available, approximations can be made by using the methods outlined in the following discussion. 

Constants for estimating pressure drop in packed towers by Eq. (22)... 

Example 6 Estimation of pressure drop in packed tower. A column 2 ft in... 

Figure 7-14. Correlation of flow rate and pressure drop in packed towers. Source Eckert [15].

Therefore, the gas mass velocity is 1372 lb/hr ft for 60% flooding condition with a gas mass flow of 1.28 x 10 lb/hr. It is determined that 3 towers are required each one 17.6 ft in diameter. The height of a transfer unit is a 11.4 ft and the total height of a packed tower is 22.8 ft. Pressure drop in the tower is 0.41 lb/in2. A summary of the composition of various streams is given in Table IV and stripper characteristics are given in Table III. 

Pressure drop in packed distillation tower as fimction of gas rate and operating pressure. 

Specially shaped packings for tower internals were introduced to improve the gas-liquid contact while reducing the pressure drop in the towers. These were partition rings, raschig rings, intalox saddles, berl saddles, etc., made of ceramic materials. 

Piche, S.R., Larachi, F, and Grandjean, B.P.A. (2001c), Improving the prediction of irrigated pressure drop in packed absorption towers, The Canadian Journal of Chemical Engineering, 79(4) 584-594. 

Tower packings shall be selected after considering the chemical resistance to scrubbing liquor at the actual operating conditions of maximum temperature and concentration the surface area and void firaction per unit volume when dumped randomly in the towers the expected pressure drop in the tower and the actual crushing strength of the tower packings. 

Pressure Drop. Although the pressure drop through packed towers is usually small at atmospheric pressure, it may become a limiting factor in vacuum distillations. 

To permit the use of vapour recompression due to the reduced pressure-drop produced by tower packings, as compared to that developed by trays in the separation of closeboiling materials. 

The key properties of mixtures of air and water vapor are described in Section 9.1. Here the interactions of air and water in packed towers under steady flow conditions will be analyzed. The primary objectives of such operations may be to humidify or dehumidify the ait as needed for particular drying processes or other processes, or to cool process water used for heat transfer elsewhere in the plant. Humidification-dehumidification usually is accomplished in spray towers, whereas cooling towers almost invariably are filled with seme type of packing of open structure to improve contacting but with minimum pressure drop of air. 

The correlation of Eckert (Fig. 13.37) combines a pressure drop relation and safe flow rates insofar as staying away from the flooding point is concerned. A flooding line corresponds to pressure drops in excess of 2 in. water/ft. In use, a pressure drop is selected, and the correlation is applied to find the corresponding mass velocity G from which the tower diameter then is calculated. Another correlation recommended by a manufacturer of packings appears in Figure 13.40. Example 13.16 compares these correlations for a specific case they do not compare any more closely than could be expected from the scatter of flooding data. 

The main factors favoring packed columns are (1) very corrosive applications, where plastic or ceramic packings are favored over trays, which are almost always constructed of metal (2) low pressure drop requirement, which is easier to achieve with packings than with trays (3) small-diameter columns, because trays require access for inspection and maintenance and (4) foaming systems, which are easier to handle in packed towers. 

Pressure drop at flooding point in packed towers... 

For additional information and methods for estimating pressure drop and flooding velocities in packed towers, see R. H. Perry and D. Green, Chemical Engineers Handbook, 6th ed., Sect. 18, McGraw-Hill Book Company, New York, 1984. 

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