PACKED COLUMNS FOR DISTILLATION

The selection of a suitable packing material is based on the same arguments as for absorption towers considered in Chapter 12, although for industrial units the most usual 

This is a simple method of representation which has been widely used as a method of design. Despite this fact, there have been few developments in the theory. Murch 71 gives the following relationships for the HETP from an analysis of the results of a number of workers. Columns 50-750 mm diameter and packed over heights of 0.9-3.0 m with rings, saddles, and other packings have been considered. Most of the results were for conditions of total reflux, with a vapour rate of 0.18-2.5 kg/m2s which corresponded to 25-80 per cent of flooding. The relationship is  

It may also be noted that the mixtures considered were mainly hydrocarbons with values of relative volatilities only slightly in excess of 3. In equation 11.143  

G = mass velocity of vapour (kg/m2 s of tower area), dc = column diameter (m), 

Ellis(36) presented the following dimensionally consistent equation for the HETP (Zt) of packed columns using 25 and 50 mm Raschig rings  

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Fig. 21. Random packing elements for distillation columns (a), Raschig ring (metal) (b). Bed saddle (ceramic) (c), Intalox saddle (ceramic) (d), PaH ring...

Introduction Packed columns for gas-liquid contacting are used extensively for absorption, stripping, and distillation operations. Usually the columns are filled with a randomly oriented packing material, but for an increasing number of applications the packing is very care-... 

Industrial analysis of hydrocarbon gases 25 years ago was limited almost to Orsat-type absorptions and combustion, resulting in crude approximations and inadequate qualitative information. The more precise method of Shepherd (56) was available but too tedious for frequent use. A great aid to the commercial development of hydrocarbon gas processes of separation and synthesis was the development and commercialization of high efficiency analytical gas distillation units by Podbielniak (50). In these the gaseous sample is liquefied by refrigeration, distilled through an efficient vertical packed column, the distillation fractions collected as gas and determined manometrically at constant volume. The operation was performed initially in manually operated units, more recently in substantially automatic assemblies. 

For gas absorption, the equipment possibilities are generally packed columns plate distillation towers, possibly with mechanical agitation on every plate deep-bed contactors (bubble columns or sparged lagoons) and mechanically agitated vessels or lagoons. Packed towers and plate distillation columns are discussed elsewhere. Generally these... 

In addition, most situations that involve mass transfer require material balances, but the pertinent area is ambiguous. Examples are packed columns for absorption, distillation, or extraction. In such... 

At rl. l,1.1.2.2,3.3,5,5-nonafluoro-5-iod()pentane (5 10 g, 0.028 mol) was added dropwisc to stirred SbFjT I, (7 g, 0.028 mol) in a flask fitted with a short, packed column and distillation head heat of reaction caused the temperature to rise to 50 60 C. The mixture was stirred for 30 min then healed to distill the fluorinated product. The fluorinated product was washed with dil sodium bisulfite, dried (MgSO, ) and distilled yield 6.5 g (71 %) bp 47 C. 

A procedure was developed by Hiatt (1983) and Dreisch and Munson (1983) to identify and quantify 1,1-dichloroethane in fish tissue samples by GC/MS, employing a fused-silica capillary column (FSCC) and vacuum distillation (extraction). An advantage of the vacuum extraction is that the system does not require elevated temperatures or the addition of reagents, which could produce unwanted degradation products (Hiatt 1981). The FSCC provides a more attractive approach than packed column for chromatographic analysis of volatile organic compounds, because FSCC can be heated to a higher-temperature (350°C) than that recommended for packed column thereby... 

An alternative to a plate-type unit is the packed column. In distillation columns larger than 4 feet in diameter, trays or plates have been found generally more economical for alcohol production. But in small columns, the cost of fabrication, installation and maintenance of plates often makes a packed unit less expensive and more workable. 

The reactions were carried out in isopropyl alcohol. This compound was selected as solvent because the natural lifetime of the aromatic radical anion with respect to protonation (1, 3) is longer than in methyl alcohol or ethyl alcohol. The isopropyl alcohol was obtained from Mathe-son, Coleman, and Bell and was freshly distilled over sodium metal through a glass-packed column for each set of runs. Anthracene from Matheson, Coleman and Bell, pyrene from Aldrich Chemical Co., m-terphenyl and p-terphenyl from City Chemical Corp. were purified as described (2). 9,10-Dimethylanthracene obtained from K and K Laboratories, Plainview, N. Y., was recrystallized from isopropyl alcohol solution. 

Packed column for preparative low-temperature distillation (Stage)... 

Sulzer Brothers, Ltd., "Sulzer Separation Columns for Distillation and Absorption—Packings, Columns, Plants, Winterthur, Switzerland. 

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. 

Geelen and Wijffels (19) investigated the reaction of vinyl acetate with stearic acid in a distillation column to form vinyl stearate and acetic acid. A modified form of the McCabe-Thiele diagram was employed to obtain the number of theoretical plates for a given conversion. This was tested experimentally using a bubble-cap Oldershaw column for the distillation reactor followed by a packed column for separation of top products (vinyl acetate acetic acid). Theoretical considerations agreed reasonably well with experimental findings. 

PCBs do not volatize easily. For example, at atmospheric pressure, the distillation range of PCBs chlorinated at 42% (Aroclor 1242) is from 325° to 336°C. For Aroclor 1254, it is from 365 to 390°C and for Aroclor 1260, from 385 to 420°C. This is another factor favoring the use of capillary columns which are better suited than packed columns for eluting non volatile compounds. 

It is important to know the total pressure drop Ap of the irrigated packed bed when designing packed columns for gas/liquid systems in counter-current flow of the phases. In absorption as weU as desorption processes, the total pressure drop Ap of the packing determines the blower capacity and thus the major part of the operating costs of the process. In rectihcation, the sum of the top pressure pT and the total pressure drop Ap gives the bottom pressure pw, which determines the bottom temperature tw- The bottom temperature, in turn, determines the effective temperature difference, at a given heating medium temperature, at which the reboiler in the distillation column must be operated. 

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