Packing height specifications

Having determined the column packing height on the basis of equilibrium calculations and whatever mass transfer correlations, the capacity of the column to handle the required vapor and liquid flows hydraulically must be checked. The specific items of concern in this regard are the pressure drop across the packing and the tendency of the column to flood. These are the considerations that determine the required column diameter. 

Develop complete specifications for the distillation column diameter, spacing of trays, type and size of packing, height of contacting zone, and so on, as reported in the present section. 

The following specific steps are required to estimate the packing height by the von Sleekar-Wilke shortcut procedure. 

For these conditions the predicted pressure drop is about 0.23 in. HjO per foot of packed height the total pressure drop is 20 x 0.23 = 4.6 in. H2O (8.6 mm Hg). The actual pressure drop would probably be close to the higher value found from Fig. 22.4, which is based on direet measurements for the specific packing used. (The generalized correlation in Fig. 22.6 is in closer agreement with Fig. 22.4 when GJG p 1.)... 

Packed-bed specific pressure drop, inches water per foot of packed height... 

Table 2-29. Calculation of no. of theoretical stages per m packing height max. allowable gas velocity = at the column head of the random packing and specific pressure drop Apj by Beck [2.118].

Specific pressure drop Ap per m packing height, given in bar... 

Figure 2-75 shows qualitively the dependency of the height specific pressure drop, A/ = A/7/Z of the gas phase and the related liquid contents V//(V e) on the gas velocity and the cross-sectional area related sprinkle density, B = Vj/Ag, with counterflowing phases (where V/ and 1 are the volumetric flow of the liquid phase and the packing volume, e is the relative void fraction and Ag and Z are the packing cross-sectional area and height, respectively). 

The problem at hand is to determine the mass transfer coefficient and the air flow needed in an industrial cooling tower that is meant to refrigerate 2000 kg/s of water that enters at 27°C and is cooled down to 20°C to be reused as cooling agent. The ambient air is at 21°C with an RH of 60% and leaves the column with 90% humidity at 22°C. The specific contact area, a, is 250 m and the cross-sectional area of the column, S, is 25 m (see Figure 3.19). The packing height, Z, is 5 m. 

The specific surface of the packing ranges between 350 and 800 m m" Despite this high surface density, the packing material occupies only about 10% of the volume. The HETP-value, i.e. the packing height equivalent to one theoretical tray, is between about 170 and 500 mm. 

The following formula for calculating the packing height is suggested on the basis of Billet s [15] introduction of the term specific pressure drop Ap/tit. ... 

Nakov [165] investi ed the effective sui ce area of a pofypiopylene honeycomb block packing with specific surface area 18S.S mVm% volume 88.3 TB /va , height of the blocks 31 mm, and diameter of the circle inscribed in the orifice 18.8-19.3 mm. The surface of the packing is preliminaiy treated with sand to increase its wettability. The investigations are carri out using the method of van Krevelen et al. [282]. The viscosity and the doisily varied fiom 1 to 8.3 mPa.s and from 1000 to 1260 kg/m. The experimental data are presented in Fig. 58. 

The experimental data for the pressure drop of the packing P15-23S, Table 31, are presented in Fig. 66 [1 S3]. The data show that such a packing with a specific surface area equal to 182 m /m is able to operate at liquid superficial velocities to 0.11 m /(m s) (396 m /(m h)) and more. Tlie gas velocity reaches about 3.S m/s at pressure drop about 20 mm water column per m packing height. That is, the packing is able to operate at extremely hij gas and liquid superficial velocity at low pressure drop. 

In orfer to determine the form of equation (36), experimental data are needed. The experiments me carried out in a packed column with dimensions of the cross-section 224 x 172 mm. The packing height is 700 mm, its specific mea -359 m /m, and its void fraction - 0.83 m /m. The height of foe elements is 50 mm. The investigation is carried out at different concentrations of sodium... 

The packed height is calculated by selecting Flow sheeting options and then Design specification as shown in Figures 7.53 through 7.55. The simulated column diameter is shown in Figure 7.56. 

Computation of Tower Height The required height of a gas-absorption or stripping tower depends on (1) the phase equilibria involved, (2) the specified degree of removal of the solute from the gas, and (3) the mass-transfer efficiency of the apparatus. These same considerations apply both to plate towers and to packed towers. Items 1 and 2 dictate the required number of theoretic stages (plate tower) or transfer units (packed tower). Item 3 is derived from the tray efficiency and spacing (plate tower) or from the height of one transfer unit (packed tower). Solute-removal specifications normally are derived from economic considerations. 

The height of a theoretical unit (Hqq) can be obtained from the aetual process conditions whenever possible. Tower packing manufacturers have information on a wide variety of specific packing systems that enable the determination of Hqc-Alternatively, this information can be estimated from values of K a using he following relationship ... 

Figures 9-63A and -63B illustrate for a specific packing the hydraulic flood and mass-transfer efficiency limitations. The differences in crimp height can influence the results. Figure 9-63B shows the effect of a higher flow parameter taken using larger columns the system apparendy was approaching its critical, but the cause of the performance is not yet known.

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