2 Types of packing

Column packing for industrial separation processes is produced from various materialsand is supplied in multifarious shapes and sizes. It may be dumped at random or stacked inregular geometric patterns, and it must ensure a large area of contact between the gas andthe liquid phases and a uniform phase distribution.

The current economic situation favours the adoption of packed columns in rectification,absorption, and liquid-liquid extraction processes. Consequently, the demand for packing inmass transfer equipment for the chemical and allied industries has greatly increased and hastriggered off many new developments in the last few years. As has already been mentioned,packed columns have also become widely accepted in ecological engineering, e. g. in air andgas scrubbing processes and in water treatment. The main unit operation involved is masstransfer, but heat transfer by direct contact is also a significant factor.

The only new high-performance packings to which consideration has been given in thisbook are those for which process engineering performance data are available. Hence, certaingaps may exist in the information presented. Nevertheless, the theoretical fundamentals andthe fluid dynamics and mass transfer models that have been derived from the resultsobtained on all packings investigated are sufficiently accurate for application in industrialpractice.

The main applications dealt with here are systems in which the phases are in counter-current flow.

The analysis and design of packed columns for thermal separation processes can be diffi-cult in many cases. This applies not only to the actual scale-up of laboratory and pilot plantresults but also to the uncertainty involved in many cases by the procedures adopted.

Most hydrodynamic calculations for packed columns are uncomplicated and lead toresults that can be satisfactorily applied in practice. However, results obtained by mass trans-fer calculations, e. g. for the determination of column height, are often associated with adegree of uncertainty, and dubiety can thus arise. An example arises in applying the dataderived from special absorption tests to solve rectification problems. Despite the valuablecontributions made by some research workers, this task was still considered to be insolubleuntil a few years ago. Since the physical laws relating to mass transfer are fundamentally thesame in all cases, it ought to be possible to describe thermal separation processes in termsthat are valid for all systems.

To this aim, the author has performed comprehensive experiments on numerous individ-ual types of packing of various shapes and sizes. The results obtained with different systemsand in columns of different dimensions have been systematically analyzed. They have beencompared with those obtained in a previous model developed by the author about ten yearsago for predicting the mass transfer efficiency of packed columns by means of a relationshipof general validity. The model necessitates prior determination of the liquid holdup. It hasbeen verified by measurements performed by the author or cited in the literature, and itallows all test results obtained with various rectification systems, including data from absorp-tion studies, to be brought to a common denominator.

A model for liquid-liquid systems in packed columns has also been checked againstexperimental results.

Packed Towers in Processing and Environmental Technology. Reinhard BilletCopyright © 1995 VCH Verlagsgesellschaft mbH, WeinheimISBN: 3-527-28616-0

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24 2 Types of packing

The trend in thermal separation techniques decidedly favours high-performance packingwith the outstanding process engineering characteristics specified by the chemical and alliedindustries. The packing systems selected for consideration here can be regarded as represen-tative of the progress made in this field.

The scientific studies required to determine the packing characteristics and to derive datafor industrial-scale columns were performed in the author's laboratories and external pilotplants.

2.1 Packing dumped at random

Many conventional beds of random packing have caused difficulties and problems inlarge-diameter columns, and their shortcomings account for the reservations that are stillheld against them. A frequent difficulty has been to attain uniform distribution of the liquidover the entire cross-section at the feed inlet or the head of the column, i.e. to ensure thatthe packing is adequately wetted. The risk of maldistribution also exists in the layer of pack-ing close to the column shell. Thus the likelihood of channelling in many beds of randompacking introduces an uncontrollable and fortuitous element in the separation of mixtureswith certain physical properties and poses the threat of poor efficiency in columns of largediameter. Consequently, special plates and, in some cases, geometrically arranged beds ofpacking were formerly given preference over random packing in an attempt to realize highcapacities and efficiencies. However, great changes in the design of random packing havelargely altered this situation in recent industrial practice.

Modern types of random packing merit particular attention from the economic aspects ofoptimizing performance and minimizing materials consumption and production costs. Theyfeature a low pressure drop per theoretical stage, which is an absolutely essential asset forsaving energy and avoiding thermal decomposition of the product stream in separation pro-cesses. The examples presented in Fig. 2.1 have been restricted to types for which processengineering performance data were available from studies in the author's research facilities.

The plastics Nor-Pac ring was the first modern type of high-performance packing to beintroduced in industrial practice. It was followed by other latticework types, e.g. Hiflowrings and saddles and Hackette, Din-Pac, Envi-Pac, and VSP rings. They have one feature incommon, viz. their latticework structure, but differ from one another in their basic geometryand the associated characteristic data, i.e. the effective void fraction, the surface area perunit volume, the packing density, and the bulk density.

In the subsequent stages of widespread acceptance in practice, latticework packing set thepace. Systematic studies had shown that it represented an optimum, and it was accordinglyrecommended for a number of applications. Close runners-up were the plastics and metalcascade mini-rings, known under the tradename CMR. New designs of saddles include theSuper-Torus. Characteristic examples of packing that were investigated in the author's labora-tories are illustrated in Fig. 2.1 together with the relevant geometrical data, i.e. the nominalsize d, the number N per unit volume, the surface area per unit volume a, and the void frac-tion e. The studies embraced a relatively wide range of metallic, ceramic and plastics pack-ing of various geometries and dimensions, and the results allow the optimum packing to beselected for a given separation task.

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2.2 Packing stacked in geometric patterns 25

Nominal packing size 50 mm

Fig. 2.1. Examples of packing for random beds including data on the number per unit volume, thearea per unit volume, and the relative void fraction

2.2 Packing stacked in geometric patterns

Packing stacked in a regular pattern permits the realization of a minimum pressure dropper theoretical stage, and is therefore most suitable for minimizing energy consumption inseparation processes that necessitate many stages. It also permits the lowest possible columnbottom temperature in the separation of heat-sensitive mixtures. The capital investment costsfor geometrically arranged packing is normally higher than that for dumped. However, stud-ies on separation process economics have shown that the greatest contribution towards the

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26 2 Types of packing

total costs is made by energy, in which case preference would be given to arranged packing.In fact, the demand for energy-saving separation equipment has even diverted trends frommass-transfer trays towards columns with systematically arranged packing.

The benefits offered by low-pressure-drop packing are by no means confined to rectifica-tion and are now widely recognized in absorption processes - for the removal of pollutantsand recoverable products from off-gas streams - and in desorption. One of the factors thathas initiated this trend has been the increased severity of the legislation imposed in indus-trial countries on the prevention of air and water pollution.

The design aim of all new types of stacked packing has been to minimize the pressuredrop per unit efficiency at high loads. The predecessors of most modern designs are SulzerChemtech gauze and sheet-metal packings. Various reports have confirmed the successfulperformance in practice of stacked packing, even in columns of comparatively large diame-ter. The provisos are that the beds have been carefully installed and that the liquid phase hasbeen uniformly distributed. Maldistribution, which is often observed in conventional beds ofpacking and which greatly impairs the efficiency, can be largely avoided by devices for redis-tributing any liquid that may channel down the walls of the column. Normally, specializedknowledge on how best to distribute the liquid is part and parcel of manufacturing know-how. The wide variety of thermal separation tasks in the process industries acts as an incen-tive for manufacturers of stacked packing to further modify existing designs (cf. Fig. 2.2).

High-performance structured gauze packing has proved successful in many fields of thechemical and allied industries, e. g. the production of heavy water and the distillation ofamines and glycols, higher fatty acids and alcohols, and methanol. A factor that restrictstheir widespread acceptance is their capital investment costs. For instance, they are uneco-nomical at pressure drops higher than 10 mm WG per theoretical stage. This costs factorentails a gap between the applications for stacked beds of packing and those for dumpedbeds.

New and cheaper, yet more effective arranged packing had to be developed to close thisgap. Again the first step towards meeting the requirements was made by Sulzer. The proto-type that they introduced on the market was the Mellapak, which has since been succesfullyused for many years in industrial columns with diameters of up to eight metres. Other com-panies followed suit with designs of outstanding merit, e.g. Glitsch structured sheet-metalpacking, Montz Bl-type metal and Cl-type plastics packings, and Raschig Ralu Pak 250 VCmetal packing.

It is beyond the scope of this book to go into full details on all the developments thathave been made in the last few years. The aim is more to outline the current state of the artand the direction of present trends and to illustrate these in the light of some examples ofmodern, high-performance, randomly dumped and systematically arranged beds of packing.

Fig. 2.2 illustrates the geometry of some modern stacked packing and includes data onthe geometry, e. g. the surface area per unit volume a and the void fraction 8. The corre-sponding process engineering data presented in Chapter 3 were derived from experiments inthe author's pilot plants and were supplemented by manufacturers' data. Obviously, the listgiven in Fig. 2.2 is by no means complete; examples of packings with excellent process engi-neering characteristics that have not been included are the Rombopak and Norton types.

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2.3 Geometric packing parameters 27

Mellapok 250 Y

a-250. e = 0.96

Nontz packing Bl - 300

a = 250

Nontz C 1^300

a^ 202 e = 0 , 9 7 8

- 7 m 3 ] , £ [ m 3 / m 3 ]

Fig. 2.2. Examples of geometrically arranged packing including data on the area per unit volumeand relative void fraction

2.3 Geometric packing parameters

The examples shown in Figs. 2.1 and 2.2 represent a mere cross-section of the multi-farious packings offered for process and ecological engineering. It can be seen that eachpacking has its own peculiar geometry and surface structure. Each must be subjected tocomprehensive experiment in order to determine the performance characteristics upon whichreliable fluid-dynamics and mass-transfer models can be based.

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28 2 Types of packing

Column packing with a high volumetric efficiency obviously necessitates excellent hydro-dynamic design and surfaces that greatly promote separation. A quantity to which the per-formance parameters are usually related is the ratio a of the surface area of the packing tothe volume of the column, i.e.

(2-1)

The effective void fraction or porosity 8 in a bed of packing depends on the surface areaAp of the packing and the thickness s of the basic material, which is usually in the form ofsheet metal, metal foil, or plastics film. The volume of material required for the bed is givenby

V < — s A (2-2)

and the effective void fraction of the bed, by

vc-vPVr

(2-3)

A simple relationship between 8, a, and s can be obtained by combining Eqns (2-1) to(2-3). Thus

8 > 1 - — as (2-4)

20 60 100 140 180 220 260 300 340 380 420 460 500

Area per unit volume a [ m2/m3]

Fig. 2.3. The relationship between the relative void fraction, the area per unit volume, and the aver-age wall thickness of packing

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2.3 Geometric packing parameters 29

In principle, Eqns (2-1) to (2-4) are valid for all beds of packing, but the informationthat they yield on the efficiency, capacity, and pressure drop is purely qualitative. Otherparameters are required to evaluate the performance, and they must allow for the geometryand texture of the packing. Together with 8 and a, they govern the phase distribution andthe area of contact between phases.

Eqn (2-4) reveals the limits within which the effective void fraction 8 for a desired sur-face area per unit volume a can be altered by appropriate selection of the practically realiz-able thickness s of the material. It can be seen from the nomogram presented in Fig. 2.3 thatthe effective void fraction increases as the thickness of the material decreases.

The packing density TV and thus the area a per unit volume in random beds may besomewhat greater in large-diameter columns (subscript T) than in pilot columns (subscript P)

Fig. 2.4. Flue gas desulfurization plant in one of BASF AG's power stations. It consists of twoabsorption columns of 9.4 m diameter and 35 m total height. The beds of 50-mm plastic Hiflow ringsare packed to an effective height of c. 8 m

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30 2 Types of packing

of comparatively small diameter. The differences in the area per unit volume and the rela-tive void fraction can be obtained from the following equations:

(2-5)

ET = 1 - (1 - EP) ^~ (2-6)J\p

A photograph of a twin-line absorption plant is shown in Fig. 2.4. This example clearlydemonstrates that columns with random beds of modern packing can also give good resultsin plants of large capacity.

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