Packed towers flow parameter

Proper packed-tower design requires prevention of entrainment of liquid droplets in the exiting gas stream. To accomplish this, the tower diameter must be such that the superficial liquid mass flow rate does not exceed certain defined operating limits This parameter is defined in Eq I in terms of the inlet liquid flow rate, liquid density. and tower cross-sectional area. 

C4 = packed drop correlation constants D = packed tower diameter, ft Flg = flow parameter (Equation 7-67)... 

To determine the tower diameter, consider the conditions at the bottom of the absorber where the maximum gas and liquid flow rates are found. Run the packed-tower design program of Appendix D using the data from Example 5.2 and packing parameters from Chapter 4. From Table 4.1 for 50-mm metal Hiflow rings ... 

A countercurrent absorption tower is illustrated in Fig. 6.1-6. The tower may contain either trays or packing. For purposes of this preliminary discussion, it is assumed that a single solute is being absorbed. Since the molar flow rates of the liquid and gas streams, Lu and G, respectively, vary over the length of the tower due to the gain of material by the liquid and ioss from the gas, it is convenient to base the material balance on flow parameters that can be considered constant. The flow rates used ate usually the solute-free liquid, designated Lit, and the solute-free (inert) gas, G u, however, other flow parameters may be used. The Kremser arid Souders and Brown design equations, for example, ate bas on the lean solvent rate and the rich gas rate. 

Over the years, the Sherwood, et al universal flooding correlation, proposed for random dumped tower packings operated in countercurrent flow, has been modified to provide a generalized pressure drop correlation [15]. Leva first modified this correlation to include parameters of constant pressure drop [16]. The abscissa of this correlation is known as the flow parameter ... 

This flow parameter is the square root of the ratio of liquid kinetic energy to gas kinetic energy. The ordinate of this correlation includes the gas flow rate, the gas and liquid densities, the a/e ratio (which is characteristic of the particular tower packing shape and size), and a liquid viscosity term. Lobo et al proposed the use of a packing factor to characterize a particular packing shape and size [17]. They determined that the a/e ratio did not adequately predict packing hydraulic performance. Eckert further modified this correlation and calculated the packing factors from experimentally determined pressure drops [18]. 

Because Intalox structured packing 2T has an absorption efficiency greater than 1-in. metal Pall rings, this packing will be evaluated. At a fixed liquid rate, the mass transfer coefficient will increase at the 0.75 power of the gas rate for this gas-film-controlled absorption (see Chapter 3). At 10,800 CFM air flow, the mass transfer coefficient for Intalox structured packing 2T will be more than sufficient to handle the absorption of an additional 50% of acetic acid vapor with the same mass transfer driving force and packed depth. The inlet air has a density of 0.0728 Ib/ft, while the inlet liquid has a density of 62.2 Ib/ft and a viscosity of 0.81 cps. Because only 232 Ib/h of acetic acid vapor for three trains must be removed by the scrubber, the physical properties of the gas and liquid streams do not change from top to bottom of this tower. The flow parameter at the bottom of the scrubber will be ... 

By experimentation with systems such as those listed in Table 7-1, the maximum operational capacity for Intalox Metal Tower Packing was determined. This capacity was expressed as a function of the flow parameter and system properties by Dolan and Strigle [18]. The flow parameter is a ratio of the square roots of the liquid kinetic energy to the vapor kinetic energy ... 

Fig. 10.2-4. Correlation for estimating tower cross-section. The figure plots the capacity factor on the ordinate vs. the flow parameter on the abscissa for random packing (a) and for structured packing (b).

Once the number of transfer units has been found, the height of the tower is determined from the product of the number and the height of each transfer unit (HTU). The HTU is determined by physical parameters such as the droplet size, the flow patterns in the tower, and the effect of any packing. These all affect the rate of mass transfer, which is addressed in Chapter 9. Very often the rate of mass transfer cannot be estimated from first principles, and it is necessary to estimate the height by determining the number of transfer units achieved and then dividing the actual height of the column employed by the number of transfer units, i.e. ... 

The flow rate of both phases, viscosity, density, surface tension, and size and shape of the packing determine the value of a . These same factors affect the value of the mass transfer coefficients Ky and Kx. Therefore, it is expedient to include a in the mass transfer equation and define two new quantities KyU and Kxa. These quantities would then be correlated with the solution parameters as functions of various chemical systems. If A is the absorption tower cross-sectional area, and z the packing height, then Az is the tower packing volume. Defining Ai as the total interfacial area ... 

Packed distillation towers can often be operated over a moderate range of flow rates at nearly constant separation efficiency. Data for isooctane-toluene separation at total reflux are shown in Fig. 22.25. The three Intalox metal (IMTP) packings numbered 25, 40, and 50 correspond to nominal sizes of 1,1.5, and 2 in., respectively. As the capacity parameter increases, both the liquid rate and the vapor rate increase, which explains why HETP is nearly constant. The gas film has the controlling resistance to mass transfer, and Hoy increases with the 0.3 to... 

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