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  • 2. The simulation, synthesis and design of reactiveand azeotropic distillation. Such topics still consti-tute a gap in the knowledge of distillation tech-nology.

    3. Investigation of complex conRgurations for batchdistillation processes.

    4. Use of optimization methods for obtaining opti-mal conRguration and design of batch and con-tinuous distillation processes.

    5. Online optimization and control of columns.

    See also: II/Distillation: Batch Distillation; Theory of Dis-tillation; Vapour-Liquid Equilibrium: Correlation and Pre-diction; Vapour-Liquid Equilibrium: Theory.

    Further Reading

    Diwekar UM (1995) Batch Distillation: Simulation, Opti-mal Design and Control. Series in Chemical and Mech-anical Engineering. Washington, DC: Taylor & Francis.

    Doherty MF and Buzad G (1992) Reactive distillation bydesign. Transactions of the Institution of Chemical En-gineers 70: part A.

    Gmehling J and Onken U (1977) Vapor}Liquid Equilib-rium Data Collections, DECHEMA Chemistry Dataseries, vol. 1. Frankfurt:

    Henley EJ and Seader JD (1981) Equilibrium-Stage Separ-ation Operations in Chemical Engineering. New York:Wiley.

    Holland CD (1981) Fundamentals of Multicomponent Dis-tillation. New York: McGraw-Hill.

    King CJ (1980) Separation Processes, 2nd edn. New York:McGraw-Hill.

    Kister HZ (1992) Distillation Design. New York:McGraw-Hill.

    Perry RH, Green DW and Maloney JO (1984) PerrysChemical Engineers Handbook, 6th edn. New York:McGraw-Hill.

    Schweitzer PA (1979) Handbook of Separation Techniquesfor Chemical Engineers. New York: McGraw-Hill, TheKingsport Press.

    Treybal RE (1980) Mass Transfer Operations, 3rd edn.New York: McGraw-Hill.

    Packed Columns: Design and Performance

    L. Klemas, Bogota, ColombiaJ. A. Bonilla, Ellicott City, MD, USA

    Copyright^ 2000 Academic Press

    Use of Packing in Distillation

    Use of packing in mass transfer has its origins in theearly 1800s for simple applications such as alcoholdistillation, and in sulfuric acid plant absorbers. Glassballs, coke or even stones were used as packing ma-terials. Nevertheless packings for distillation were notestablished until the 1930s with the use of regularshape materials such as ceramic Raschig rings andBerl saddles, as well as the availability of distillationcalculations such as the McCabe}Thiele and Pon-chon}Savarit methods. Early in the second half of thecentury, the use of packing for distillation wentthrough a transformation, producing the second-generation packings (see Table 1). Regular and im-proved shape of packings, such as pall rings, becameavailable with larger open areas that permitted a sub-stantial increase both in capacity and column efRcien-cy. In the 1960s Sulzer introduced the wire-meshpackings with very high efRciency (low height equiva-lent to a theoretical plate, HETP), resulting in a newtransformation in the use of packings. In the 1970s

    and 1980s all major mass-transfer equipment manu-facturers developed structured packings. Comparedto the traditional tray columns spectacular improve-ments in plant capacity were achieved, but also someprojects were pitfalls, when the expected beneRts didnot materialize. Manufacturers started realizing thatliquid distributors had to be improved, but there wasno coherent understanding, nor correlations, thatcould lead to a safe distributor-column system design.Many manufacturers returned to trays, producingnew improved designs, using the area under thedowncomer for vapour Sow: these trays are offeredwith new names that indicate their increased vapourSow capacity (MaxySow, Superfrack, etc.). The needfor good distribution and its effect on the columnefRciency are now well understood, allowing safedesign and efRcient applications for random andstructured packings in large industrial columns.

    General Concepts

    Distillation separation is based in relative volatilitythat makes it possible to concentrate the more volatilecomponents in the vapour phase while the less vol-atile ones remain in the liquid phase. Distillationcolumns are countercurrent vapour}liquid mass-transfer devices, where the required separation andpuriRcation of components is achieved.

    II / DISTILLATION / Packed Columns: Design and Performance 1081

  • Figure 1 Number of stages required vs. relative volatility at several product purities.

    Table 1 Evolution of packing

    First generation,before 1950

    Second generation,1950}1970

    Third generation, after1970

    Random packings Rashing rings Intalox (Norton) IMTP (Norton)Lessing rings Pall Ringsa CMR (Koch Glitsch)Saddles Chempakb

    Fleximax (Koch Glitsch)Nutter Ring (Nutter)

    Grids C-Grid (Koch Glitsch)c

    EF-25 (Koch Glitsch)c

    Structured packing Wire-mesh typed Sulzer BX and CYMellapack (Sulzer)Flexipack (Koch Glitsch)Gempack (Koch Glitsch)Intalox (Norton)Montz packing (Montz)

    aDeveloped by BASF, still marketed (or variations of it) by most packing manufacturers.bDeveloped by Leva, marketed by Nutter.cVariations of these grids are now offered by most packing manufacturers.dDeveloped by Sulzer, they are now offered by other manufacturers.

    The main variable inSuencing the column designrequirements is the relative volatility, . Figure 1illustrates the effect of on the column perfor-mance:

    As increases, the number of theoretical stages(NTS) required to achieve a Rxed product qualitydecreases, since NTS is proportional to 1/ln(). As decreases and approaches 1, the number of stagesrequired increases approaching inRnity. At anygiven , the minimum number of stages required toachieve a given separation corresponds to a totalreSux operation. At total reSux all overhead va-pours are condensed and returned to the column as

    reSux, so that there is no net product. The min-imum reSux sets the limiting slope of the operatingline, required to achieve a given separation.

    At constant , the NTS increases as the productpurity increases. The increase is proportional to thelogarithm of the key components purity ratio.

    It can be also demonstrated that:

    At constant product purity, the minimum reSuxdecreases as increases.

    At constant product purity, the minimum numberof stages decrease as increases.

    At constant , the minimum reSux decreases as theproduct purity decreases.

    1082 II / DISTILLATION / Packed Columns: Design and Performance

  • Figure 2 Packed tower illustration. (Photo courtesy of SulzerChemtech.)

    At constant , the minimum number of stages in-creases as the product purity increases.

    All these statement say that deRnes the separationdifRculty. For values around 1.1 and lower, separ-ation by distillation becomes very difRcult, requiringvery large and expensive columns. For "1 the mix-ture is azeotropic and would require the addition ofselective entrainers if azeotropic or extractive distilla-tion is to be applied.

    Packed Column Description

    Figure 2 illustrates a tower with structured packing.In addition to the packing itself, packed columnsrequire other internals to assure the performance ofthe packing. These internals are:

    Liquid feed pipes to deliver the Suid to the liquiddistributors, as seen at the top of the tower and atthe intermediate distributor.

    Liquid collection and mixing as shown below thetop bed.

    Liquid draw-off sump and pipe as shown below thetop bed.

    Liquid redistributors, as presented between the twobeds.

    Vapour feed pipes as shown at the vapour inletnozzle, at the bottom of the tower.

    Packing support plates resting on beams and level-led rings welded to the vessel.

    Hold-down plates.

    Incorrect design or incorrect installation of any ofthese elements can lead to tower failure. One of themost critical element, and often the culprit of towerfailures, is the liquid distributor.

    Packing Selection

    Figures 3 and 4 illustrates random and structuredpackings. There are many parameters to be con-sidered in the selection of packings; in some cases,there are one or two considerations that dictate theselection, such as capacity for a revamp, which couldfavour structured packing. There are also some con-siderations or applications, such as high-pressuredistillation, that could make structured packing aquestionable choice. Table 2 gives some general guid-lines on packing selection.

    Pressure Drop in Packed Beds

    The dry-bed pressure gradient is given by the follow-ing equation:

    Pd"C1gu2g [1]

    II / DISTILLATION / Packed Columns: Design and Performance 1083

  • Figure 3 Random packings: (A) IMTP. (Photo courtesy of Norton Chemical Process Products Corporation.) (B) Nutter Ring.(Photo courtesy of Sulzer Chemtech.) (C) Cascade Mini-Rings (CMR) and Fleximax. (Courtesy of Koch}Glitsch Inc.) (D) PallRings metal and plastic. (Courtesy of Koch}Glitsch Inc.)

    HNote: in this correlation the original term 100.3g was replaced by100.024g since the original correlation predicts too high a pressuredrop.

    Leva extended the correlation to irrigated beds:

    Pi"C110u1gu2g.Robbins developed the following set of general pres-sure-drop correlations:

    P"C2G2f 10C3Lf

    #0.4(Lf/20 000)0.1(C2G2f 10C3Lf)4 [2]

    where:

    Gf"G(0.075/g)0.5(Fp/20)0.5100.024 g

    (for pressures over 1 atm)H

    1084 II / DISTILLATION / Packed Columns: Design and Performance

  • Figure 4 Structured packings: (A) Wire gauze structured packing. Close view, packing and wiper bands. (Photo courtesy ofKoch}Glitsch Inc.) (B) Two structured packing layers rotated 903. (Photo courtesy of Koch}Glitsch Inc.) (C) One structured packingelement for small towers. (Photo courtesy of Sulzer Chemtech.) (D) Structured packed bed for a small tower. (Photo courtesy ofKoch}Glitsch Inc.) (E) Packed bed for a large tower built in sections. (Photo courte

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