Packed-Column Operation

Packed columns are used primarily in gas absorption and liquid extraction and in air-water contact operations such as humidification and water cooling, which we take up in Chapter 9. They are found less frequently in distillation operations where their use is confined mostly to small-scale processes involving high-efficiency packing. 

The analysis of packed-column operations has the same three basic goals we have seen in similar contexts, i.e., design, prediction of equipment performance, and, to a lesser extent, parameter estimation from experimental data. We may, for example, wish to calculate the height of a column required to achieve a certain degree of separation or level of pimfication. Or it may be required to assess the effects of changes in feed concentration or flow rate on column performance. In all these problems, the same basic model equations are applied and manipulated in appropriate ways to extract the desired information. 

The principal mathematical feature shared by all steady-state packed-column operations is the distribution of the concentration variables in space, principally in the direction of flow. The model must therefore be composed of mass balances taken over a difference element of each phase, which are then converted into ODEs and integrated to obtain concentration profiles and other useful information. Algebraic (integral) mass balances also appear and can often be combined with the ODE balances to obtain important results. 

We have already, in Illustrahon 2.3, alerted the reader to the existence of this multitude of mass balances, and we have occasion now to obtain a broader picture of their derivation and various applications. To do this, we return to the example of the gas scrubber taken up there and reexamine it in greater detail. 

---

It is seen that, for GC packed columns operated under the conditions assumed, the two factors contributing to dispersion by resistance to mass transfer are of the same order of magnitude. Consequently, equations (20) and (21) cannot be simplified and must be used in their existing form for all optimization procedures using packed GC columns. If the conditions differ significantly from those assumed, then by using the same procedure the possibility of modifying expressions (20) and (21) can be reexamined. 

In a packed column, liquid and vapor flow counter-currently and separation between the liquid and vapor phases takes place continuously. In contrast, in a column with trays, separation occurs in stages. In a packed column, vapor does not bubble through the liquid as in the columns with trays. For this reason, and due to the absence of the vapor-flow orifices, packed columns operate at a much lower pressure drop. In addition, because liquid and vapor contact in a packed column is less agitated than in a trayed column, packed columns are less likely to foam. 

In a packed column, operating at approximately atmospheric pressure and 295 K, a 10% ammonia-air mixture is scrubbed with water and the concentration of ammonia is reduced to 0.1%. If the whole of the resistance to mass transfer may be regarded as lying within a thin laminar film on the gas side of the gas-liquid interface, derive from first principles an expression for the rate of absorption at any position in the column. At some intermediate point where the ammonia concentration in the gas phase has been reduced to 5%. the partial pressure of ammonia in equilibrium with the aqueous solution is 660 N/nr and the transfer rate is ]0 3 kmol/m2s. What is the thickness of the hypothetical gas film if the diffusivity of ammonia in air is 0.24 cm2/s ... 

Equation 12.18 allows us to predict the concentration profile through a packed column operating at total reflux conditions. The resulting profile is known as a residue curve, since they were first developed by considering the batch vaporization of a mixture through time8,9. Such... 

Ong SA, Toorisaka E, Hirata M, Hano T (2006) Decolorization behavior of azo dye with various co-substrate dosages under granular activated carbon-biofilm configured packed column operation. ARPN J Engin Appl Sci 1 29-34... 

SAWISTOWSKI, H. Chem. Eng. Sci. 6 (1957) 138. Flooding velocities in packed columns operating at reduced pressures. 

H. Krehenwinkel and H. Knapp, Pressure drop and flooding in packed columns operating at high pressures, Chem. Eng. Technol., 10 (1987) 231-242. 

Bypassing-Controlled Trayed or packed columns operate with countercurrent flow and can achieve many equilibrium stages in series by good distribution of gas and liquid, and careful control of details. Other devices such as sprays are vulnerable to bypassing and are limited to one equilibrium stage. 

Summary. The above discussion identifies the following regimes for packed column operation. 

The object of this study was to characterize the flow and to measure the hold-up and axial dispersion coefficients of the fluid phases (water-nitrogen) in a countercurrent gas-liquid packed column, operating under a pressure up to 1.5 MPa. 

In industry, many installations operate under pressure for technical and/or economic reasons. Although many hydrodynamic studies have been reported for liquid-gas contactors at atmospheric pressure, there are few concerning elevated pressure. The aim of this study is to investigate the influence of pressure on fluid flow in a packed column operating with a gas-liquid countercurrent. 

The study under pressure with countercurrent flow has shown that the operating pressure has no effect on hold-up. In fact for a liquid flow rate of 4.63 kg/rrPs and constant gas velocity of 0.037 m/s, liquid hold-up is 0.21 for pressure ranging between 0.1 and 1.3 MPa. This result is in agreement with the conclusions of Van Gelder el al. [1] concerning the influence of pressure on liquid hold-up in a packed column operating with cocurrent flow. Larachi el al. [4] report the same observations in a fixed bed column with cocurrent flow. 

Capillary columns operate at carrier gas flow rates of about 1 to 2 ml/min and the mass spectrometer can accept all the effluent from the column without loss of performance. Packed columns operate at much greater flow rates (around 30 ml/min) and in order to maintain the vacuum an enriching device, or separator, must be used to prevent all of the effluent entering the ion source. Three types of separator are in common use and each relies on the difference in physical properties of the carrier gas and the sample. 

Similar to tray columns, packed columns operated at high gas velocities causes backmixing, and low gas velocities reduce the mass transfer rate. If the gas velocity is too high, the column will flood. In addition, at low liquid flow rates the packing will not wet completely, resulting in a reduction in mass-transfer. Another problem is the tendency for the liquid to channel. To minimize this effect, redistributors have to be installed every 5 to 10 m (16.4 to 30.5 ft) [23] to even out the liquid flow. 

The term appear is used as the solvent profile itself is not actually changed, only the profile as presented on the recorder or printer. The effect of the detector time constant can be calculated and the results from such a calculation are shown in figure 14. The undistorted peak, that would be monitored by a detector with a zero time constant, is about 4 seconds wide. Thus, for a GC packed column operating at 20 ml/min this would represent a peak having a volume of about 1.3 ml. 

The approximate efficiency of a packed column operated at its optimum velocity (assuming the inlet/outlet pressure ratio is small) is given by the Van Deemter equation,... 

For high vacuum distillation, Eckles et al. [150] suggest using a thin film or conventional batch process for industrial type installations however, there are many tray and packed columns operating as low as 4 mm Hg, abs Eckles... 

The optimized pressure concept can be extended. Figures 2.29 and 2.30 show nomograms for well packed columns operated at their van Deemter curve minima. The nomograms show the interrelationship between column length, pressure, number of theoretical plates, particle size of the stationary phase and breakthrough time. Two of these five parameters may be selected at random, the other three being geared to the optimum flow rate. 

Packed columns operate in the counterflow mode, and thus it is not really appropriate to utilize stagewise concepts for their analysis and design. Despite this, the HETP approach [Equation... 

The effluent obtained at the outlet is directly esterified around 160 to 180 0, a temperature at which the by-product acrylic add can also be converted in a packed column operating at atmospheric pressure. A mixture of acrylate, excess alcohol and water is recovered at the top, and another mixture at the bottom, consisting of sulfuric add, ammonium bisulfate and heavy organic by-products, such as 3-aikoxypropionates, which can be thermally decomposed to allow the use of the ammonium sulfate as fertilizer. 

Air containing 5 mol% NH3 at a total flow rate of 20 kmol/h enters a packed column operating at 293 K and 1 atm where 90% of the ammonia is scrubbed by a counter-current flow of 1500 kg/h of pure liquid water. Estimate the superficial gas velocity and pressure drop at flooding, and the column inside diameter and pressure drop for operation at 70% of flooding for two packing materials ... 

A wastewater stream of 0.038 m3/s, containing 10 ppm (by weight) of benzene, is to be stripped with air in a packed column operating at 298 K and 2 atm to reduce the benzene concentration to 0.005 ppm. The packing specified is 50-mm plastic Pall rings. The airflow rate to be used is five times the minimum. Henry s law constant for benzene in water at this temperature is 0.6 kPa-m3/mol (Davis and Cornwell, 1998). Calculate the tower diameter if the gas-pressure drop is not to exceed 500 Pa/m of packed height. Estimate the corresponding mass-transfer coefficients. The diffusivity of benzene vapor in air at 298 K and 1 atm is 0.096 cm2/s the diffusivity of liquid benzene in water at infinite dilution at 298 K is 1.02 x 10 5 cm2/s (Cussler, 1997). 

The main drawback of chimney trays is that they consume more column height than alternative drawoff devices, resulting in a more expensive arrangement. Chimney trays are also relatively high-pressure-drop devices, which is a major disadvantage in packed columns operating in deep vacuum. 

---