packed columns: design and performanceindex-of.co.uk/tutorials-2/packed columns- design and...

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) Perry’sChemical 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 Chempak�b

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"C1�gu2g [1]


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 100.3�g was replaced by100.024�g since the original correlation predicts too high a pressuredrop.

Leva extended the correlation to irrigated beds:

�Pi"C110�u1�gu2g.

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


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 courtesy of Norton Chemical Process Products Corp.)


Figure 4 Continued

Table 2 Packing selection guidelines (trays included as a reference)

Application in distillation Random packing Structured packing Traditional trays High-capacity trays

Pressure drop/theoretical stage 2 1 3 3Maximum capacitya 2 1 3 2Efficiency at high pressure 2 4 2 1Efficiency at low pressure 2 1 2 3Efficiency at low liquid ratec 2 1 3 4Efficiency at high liquid rated 3 4 2 1Low residence time 2 1 4 4High residence time 3 4 1 1Heat transfer 2 1 2 2Foaming systems 2 2 3 3Non-metallic servicesb 1 2 4 4Fouling systems 4f 2f 1e 1e

Efficiency in high � systems 2 4 1 1Inspection and maintenance 3 4 1 1Low cost 2 4 1 3

Application rating: 1, best; 2, good; 3, fair; 4, poor.aEfficiency may be reduced at high capacities.bAs may be required based on corrosion protection considerations, such as ceramic.cSystems below 5 gallons min�1 ft�2.dSystems over 15 gallons min�1 ft�2.eApplies to sieve trays, specially dual-flow, not to valve trays.fIt would require a fouling-resistant distributor, which may result in reduced efficiency.

Figure 5 Bed �P vs. rates. (Permission from Gulf PublishingCompany.)

Lf"L(62.4/�1)(Fp/20)0.5�0.1l (for Fp over 15)

C2"7.4�10�8 and C3"2.7�10�5

For the case of dry packing Lf"0, the pressure-drop equation reduces to:

�P"C2G2f "C2(0.075/20)FpG

2/�g. [3]

Figure 5 presents a family of pressure drop-lines atconstant liquid Sow as a function of the vapour Sow.The constant liquid rate lines start parallel to the

dry-column line (which is a function of the dragonly). Equation [3] allows calculation of the packingfactor, Fp, by measuring the slope of the dry-packingpressure-drop data. As the vapour rate increases, theslope of the constant liquid rate lines increase; thisincrease is also proportional to the liquid rate. Theinitial departure from the dry-line slope indicatesinteraction between the vapour and liquid, and rep-resents a loading point. EfRcient mass-transferoperations can be achieved only above the loading


point. For any given liquid rate, as the vapour ratefurther increases, the pressure-drop line slope in-creases rapidly until the line becomes near vertical. Atthis point the Sow and �P are unstable, and the bed isSooded; the vapour Sow does not allow the liquid toSow down the bed and there is massive entrainmentof liquid in the vapour phase and mass transfer is nolonger viable.

For most packings, bed Sooding occurs between1 and 2 inches of water-pressure drop per foot ofpacking. Pressure drop at Sooding seems to be a func-tion of the packing size. Kister cited Zenz and laterStrigle and Rukovena observations indicating thatSooding (�Pfl) is higher for smaller size packings, andproposed a correlation to determine the pressure dropat Sooding as a function of the packing factor.

�Pfl"0.115(Fp)0.7 [4A]

We also obtained by regression from data publishedby Strigle:

�Pfl"0.146F 0.75p inch liquid ft�1 or [4B]

�P"0.146SgF0.75

p inch H2O ft�1 [5]

Pressure drop at incipient loading may be estimated:

�Pl"0.072SgF0.75

p [6]

and pressure drop at maximum efRciency loadingmay be estimated by:

�Pe"0.082SgF0.75

p [7]

All the above correlations have been regressed formetallic random packings (Pall Rings and IMPT�).

For column design, it is well-accepted practice toassume Sooding at 1 inch of water per foot of packingpressure drop and design the packing for an operationat 80% Sood. However, when reliable packing-factorinformation is available, the use of the calculated�Pfl, using one of the eqns [4A], [4B] and [5], isa more accurate approach.

Caution: Presence of foam, even incipient foam, hasa great impact on a packing column pressure dropand performance and should be avoided. Amines,insoluble Rne solids (such as corrosion products),high-viscosity organic liquid (0.5}1 cP or higher) andimmiscible liquids are known to foam. For thesesystems, or other systems known to be prone to foam,continuous or intermittent dosing of antifoamagents may be required to maintain an efRcientpacked-column operation. Nevertheless, uncontrol-

led antifoam injection is known to aggravate foamingproblems. Filtration of liquids and adsorption ofcontaminants on activated carbon has proven valu-able to control foaming in some systems such asamines.

Flooding Correlations

Several generalized Sooding and pressure-drop cor-relations have been proposed for commercial pack-ings. Sherwood, Shipley and Holloway presentedthe Rrst correlation between a ‘Sow parameter’ XdeRned as:

X"(L/G) (�g/�l)0.5 [8]

and a ‘Sooding parameter’ Y deRned as:

Yf"(u2g/gc)(a/�3)(�g/�l)�0.2"(G2

f /gc)(a/�3)�0.2/(�g�l).

[9]

Sherwood and co-workers correlated dumped andstacked random packing data and found that Yf isaround Rve times higher for stacked than for dumpedpacking, which means that mass velocity at Sood isover two times higher for stacked packing. This wasthe precursor idea for the later development of ‘struc-tured’ packings.

Lobo and Friend presented a similar correlation ofY and X with indication of pressure-drop lines andSooding line.

Leva proposed a similar correlation with the sameSow parameter given by eqn [8] and modiRed theSooding parameter Yf"(G2

f /gc)(a/�3)�0.2 (�w/�l)2/�l.According to this correlation, minimum loadingYm occurs at about one-third of Yf which means thatloading starts at 50% of the mass Sow rates corre-sponding to the Sooding point.

Eckert observed that the packing geometrical pro-perties factor (a/�3) did not represent correctly thepacking in the Sooding correlations. He introduceda packing factor, Fp. The value of Fp is determinedexperimentally from pressure-drop data. The newSooding parameter became:

Yf"(G2f /gc)Fp�0.2(�w/�l)2/(�g�l) [10]

and is correlated to the same Sow parameterX"(L/G) (�g/�l)0.5.

The most recent proposed correlation was present-ed by Strigle (see Figure 6):

Y"CsF0.5

p (�/Sg)0.05"CsF

0.5p �0.05 [11A]


Figure 6 Striegle pressure drop chart. (Permission from Gulf Publishing Company.)

Figure 7 Modified flooding parameter as a function of the flow parameter.

Y is the vapour Sow parameter and is a function ofvapour capacity factor Cs"ug(�g/(�l!�g))

0.5, thepacking factor and the kinematic viscosity �"�/Sg.Note that at Sooding Y"Yf. Y is plotted in a linearordinate as a function of the Sow parameter X ina logarithmic abscissa and a family of constant �Plines. No Sooding line is shown. The advantage of thelinear ordinate is that it is easier to interpolate thanthe older log}log charts.

Figure 7 presents the Sooding lines of packings asa function of the packing factor Fp and the Sowparameter X. The ordinate is the modiRed Soodingparameter YHf , deRned as follows:

YHf "Yf/F0.5

p "Cs�0.05 [11B]

YHf is plotted as a function of the Sow parameter X,eqn [8], at constant packing factors.


Table 3 Random packing design parameters

Packing metal Nominal size Packingfactor (Fp)

Specific surfaceft2 ft�3 (a)

Void ft3 ft�3 (�) Bulk density(lb ft�3)

Pall Rings 0.625 81 103 0.918 39.91 56 61 0.953 23.11.5 40 39 0.971 14.32 27 30 0.969 14.13.5 18 18 0.972 13.9

CMR� 0 60 103 0.957 20.961 38 76 0.968 15.511.5 33 57 0.961 18.662 26 44 0.970 14.292.5 21 38 0.974 12.543 14 32 0.979 10.224 12 23 0.985 7.365 8 15 0.989 5.46

IMTP� No 15 51 88.7 0.961 17.9No 25 41 69.8 0.970 14.1No 40 24 46.9 0.969 14.6No 50 18 31.2 0.981 9.3No 60 16 25.3 0.982 8.7No 70 12 17.5 0.984 8.1

Nutter Rings� 0.7 N/A 69 0.978 11.01.0 30 51 0.978 11.11.5 24 38 0.978 11.32.0 18 29 0.979 10.82.5 16 25 0.982 9.03.5 13 20 0.984 8.3

Comparing Packed Column vs. TrayTower Capacity

Table 5 presents packing capacities, calculated fromthe above relations, compared to tray Sooding capac-ities at several tray spacings.

Packed Tower Diameter

Figures 6 or 7 can be used to determine the columndiameter. Using Figure 7 the procedure is as follows:

1. Determine the value of the abscissa X"L/G(�g/�l)

0.5.2. Obtain from the manufacturer the selected pack-

ing Fp value, or from Tables 3 or 4.3. Determine the ordinate YHf "Cs�0.05 from Fig-

ure 7.4. Calculate the capacity factor at Sood Cs from

the YHf value, the gas velocity at Sood ug"Cs(�l!�g)

0.5/�g and the Sooding gas mass velocityGfl"ug�g.

5. Determine the column cross-sectional areaAc"V/(0.8Gfl), based on 80% of the G Soodingrate. This is standard design practice for newcolumn sizing, and allows for normal Sow Suctu-

ations that occur in actual operations and forprocess control requirements.

6. Determine the column diameter Dc"12(4Ac/�)0.5.

Turndown and Minimum Wetting Flow

In general, the turndown of a packed tower is limitedto the turndown of the liquid distributor, which is itsability to reduce liquid load and still maintain ahomogeneous distribution. Most standard liquiddistributors can operate efRciently at 50% of its de-sign liquid load; turndown as low as 25% can beachieved.

To operate efRciently as mass-transfer devices,packing should be homogeneously wetted to assureuse of the total surface. Minimum recommendedvalues of liquid irrigation depend on the packingmaterial and surface wettability, as follows:

Random packingCeramic 0.2 gallons min�1 ft�2

Surface-treated orrusted metals

0.5 gallons min�1 ft�2

Glass, glassed ceramicand stainless steel

1.0 gallons min�1 ft�2

Plastics 1.5}2.9 gallonsmin�1 ft�2


Table 4 Structured packing design parameters

Packing 453Crimp angle

Size Packing factor Specific surfaceft2 ft�3 (a)

Void fraction (�) Bulk density(lb ft�3)

Mellapack� (Sulzer) 125Y 10 35 0.989 5.09250Y 20 78 0.987 5.61350Y 23 107 0.983 7.8500Y 34 155 0.975 10.92

Sulzer BX (Gauze) BX 21 150

Gempack� (KochGlitsch)

4A 55 138.1 0.942 173A 23 91.4 0.962 9.92A 15 67 0.972 6.31A 9 35 0.977 4.7

Intalox� (Norton) 1T 28.0 95.2 0.980 10.142T 20.0 65.3 0.984 8.233T 15.0 51.9 0.987 6.554T 13.5 40.6 0.986 6.755T 12.0 27.0 0.991 4.5

Montz B1-100 30B1-200 20 61 0.94B1-250 76B1-300 33 91

Table 5 Relative capacity of packing and traysa

Tray spacing Ratio of packing to tray capacity according topacking factor (Fp)

10 20 30 40 50 60

36 inches 1.15 0.96 0.87 0.81 0.76 0.7324 inches 1.45 1.22 1.10 1.03 0.97 0.9318 inches 1.90 1.60 1.44 1.35 1.27 1.2212 inches 2.41 2.03 1.84 1.71 1.62 1.55

aTray capacity based on the column full cross-sectional area,without discounting any area for downcomers (which implies high-capacity trays). For conventional trays the ratio of packing capa-city/tray capacity will be higher. Tray capacity taken from thegeneralized correlation of tray flooding proposed by Fair JR andMatthews RL (Petroleum Refiner 37(4): 153). The packing capa-city taken from the generalized correlations presented by RFStriegle Jr and Figure 6).

Structure packingsSurface-treated metals 0.2 gallons min�1 ft�2

Plain surface metals 0.5 gallons min�1 ft�2

Type of Liquid Distributors

Liquid distributors can be gravity or pressure feddepending on how the liquid is introduced to thedistributor. Pressure distributors are limited to heattransfer and some simple mass-transfer operations,mainly in stripping or absorption. For distillation,

especially for high-purity separations, only gravitydistributions are used. Table 6 illustrates the maintype of distributors and the main factors to be con-sidered for selection:

� Pipe oriTce headers (POH) (Figure 8) consist ofa pipe ladder arrangement with calibrated oriRcesdrilled in the pipe laterals in a uniform layout.POH can be pressure or gravity fed.

� Pan distributors (PAN) (Figure 9) consist of a Sathorizontal plate (tray) with uniformly spaced calib-rated oriRces that allow the passage of liquid to thepacking below. Round or rectangular risers (chim-neys), located within the oriRce pattern, distributethe vapour to the packing above. The riser layoutshould be uniform and should not interfere withthe uniformity of the oriRce layout. PAN distribu-tors are always gravity fed.

� Narrow trough distributors (NTD) (Figure 10Aand 10B). This distributor is composed of a seriesof narrow (3}4 inches) parallel troughs fed by oneor more larger troughs (parting boxes) oriented at903 from the narrow troughs. The narrow troughsdistribute the liquid to the packing below, throughcalibrated oriRces drilled at the bottom or at thewall. NTD are always fed by gravity.

� Spray nozzle header (SNH) (Figure 11). They aresimilar to POH but spray nozzles are used insteadof oriRces. The density of nozzles in the SNH islower than the density of oriRces in the POH. TheSNH relies on the liquid cone leaving the nozzle for


Table 6 Guidelines for distributor selection

Gravity-fed distributors Pressure-fed distributors

POH PAN NTD POH SNH

Uniformity 1 1 1 2 3High-purity fractionation 1 1 1 3 3Maximum drip points per area 2 1 1 2 2For large diameter towers (over 10 ft) 1 3 1 1 1

Leakage potential C Ha C C CFor high liquid rates 2 1 2 2 1For high vapour rates 1 3 1 1 1Residence time C A B C CSolids handling 3 3 2b 2 1Turndown 1 1 1 1 3Easy installation and levelling 1 3 2 1 1Cost B A A C C

1, Good; 2, fair; 3, poor; A, high; B, medium; C, low.a Unless it is seal-welded.b Very good if a V-notch is provided at the top of the trough wall for liquid flow. Nevertheless, the quality and turndown of the distributorare affected.

Figure 8 POH distributor. (Courtesy of Norton Chemical Pro-cess Products Corp.) Figure 9 PAN distributor.

further spreading. This results in either an overlapor a gap of the cone projection over the packedbed, and deteriorates the uniformity of the distri-bution. SNHs can handle very large liquid ratesand are very efRcient for heat transfer.

Liquid Mixing, Redistribution andMaximum Bed Height

Initial liquid distribution is essential to achieve goodpacked tower efRciency. Hoek suggested that ata given Sow rate, each packing has its natural distri-bution determined by its radial spreading coefRcient.Although this effect does spread the initial liquiddistribution, this effect is not sufRcient to correctpoor initial distribution. Radial concentration gradi-ents already established at the top of the bed cannotbe compensated by additional packing. The result ispermanent efRciency loss.

In general, if a good distribution is established atthe top of the bed, the packing will develop its naturaldistribution and maintain it for bed depths of 10 NTSor more. Columns requiring more than 10 NTS persection should be subdivided into several packingbeds to maintain coefRcient HETP values. Liquidredistribution, and often mixing, are required be-tween these bed sections.

Distributor Design Parameters

Distributor Liquid Level and Hole Diameter

The basic distributor design equation relates the totaloriRce open area, the liquid head and the volumetricSow:

Q"Cona0(h!hd)0.5 [12]


Figure 10 NTD distributor: (A) Photo courtesy of Norton Chemical Process Products Corp. (B) Photo courtesy of Koch}Glitsch Inc.

Figure 11 SNH distributor. (Courtesy of Norton Chemical Pro-cess Products Corp.)

where Q is the volumetric Sow rate, Co the oriRceSow coefRcient, n the number of oriRces, a0 the openarea of one oriRce, h the liquid head overthe oriRce, and hd the vapour-pressure drop acrossthe distributor given in head of liquid. The value ofCo varies between 0.5 to 0.8 and is near 0.6 for mostcommercial distributors. Using this value, eqn [12]becomes:

Q"4.0nd2(h!hd)0.5

and:

n"0.25Q/d2(h!hd)0.5 [13]

The minimum recommended oriRce diameter, to pre-vent plugging, is 3/8 inch for carbon steel and 1/8inch for stainless steel. The minimum recommendedliquid level at minimum Sow is 2 inches. If a 50%turndown is speciRed, the required liquid level atnormal liquid load becomes 8 inches.

Uniformity of the Drip Point Layout

Density of liquid drip points is not enough to assurea good distributor quality. The distribution must behomogeneous; the same amount of liquid should irri-gate the packing at any fraction of the tower cross-sectional area. Areas near the tower wall shouldreceive the same amount of liquid as areas near thecentre.

Other Considerations

A number of factors need to be considered whenselecting and designing packing and distributors.

Ratio Tower to Packing Size

The minimum recommended ratio of the tower dia-meter to the packing size is 8. In the case of structuredpackings, this ratio applies to the ratio of the towerdiameter to the crimp size.

Fouling

Some solids are usually present even in ‘clean systems’because of corrosion products, especially after main-tenance shutdowns, when rust and debris can remainin the tower. The tower shell metallurgy should beadequate to prevent formation of scale or corrosionproducts that can plug distributors. Distributors withsmall oriRces should be protected with Rlters in allliquid lines entering the tower. In other cases, solidsare expected to be present because of the processitself. In these cases the distributor should be designedto handle the solids. A NTD distributor with V-notches for liquid overSow is adequate to handle


Figure 12 Vapour distributor. (Courtesy of Sulzer Chemtech.)Figure 13 Distributor testing facilities. (Photo courtesy ofKoch}Glitsch Inc.)

some slurries. SNHs can also handle slurries but theirapplication is limited to heat transfer.

Vapour Distribution Requirements

Vapours entering the tower have a kinetic energyproportional to their velocity, which is converted intopressure as the vapour turns to start Sowing upwardin the tower. The resulting radial pressure proRle isnot uniform; areas of higher pressure would allowhigher vapour up-Sow. This is especially critical forlow-pressure drop packings such as structured pack-ings. Vapour radial velocity proRles are corrected bypressure drop and by diffusion devices. The followingis the recommended practice for vapour distribution:

� Low vapour inlet velocity (velocity head below 0.5inches of water): no inlet distributor required, pro-vide as minimum 11

2 column diameters, or 36inches between the top of the vapour inlet nozzleand the bottom of the bed.

� Intermediate vapour inlet velocity (velocity headbetween 0.5 and 1.5 inches of water): provide aninlet vapour diffuser directing vapour Sow downthe tower. This type of device can be a horizontalpipe with the bottom half cut as shown at thebottom of the column in Figure 2. Vertical bafSescan be provided for better vapour distribution. Thepurpose of these bafSes is to stop the horizontalvelocity component of the vapour.

� High vapour velocity (velocity heads above 1.5inches of water): provide an inlet vapour diffuser,as described above, plus a small riser chimney traywith a pressure drop of a minimum of 2 inches ofwater. The pressure drop can be created by oriRcesat the bottom of the risers. A vapour distributor, asshown in Figure 12 is a good alternative to thevapour diffuser in critical systems.

Distributor Testing

Water test of assembled distributors at the manufac-turer’s workshop is always a good practice for allhigh-efRciency distributors. The test should deter-mine liquid rate gradients under the distributor,liquid level in the distributor itself at design and

turndown liquid rate, liquid level gradient in thetrough and uniformity of the drip point layout. Theseparameters should be compared to the distributordesign parameters and adjustments made to the dis-tributor if necessary. Figure 13 shows a distributortesting facility.

Packing Performance in Distillation

Factors to Consider in Determining the ColumnDesign HETP

The height equivalent to a theoretical plate (HETP) isdetermined by the following main three factors:

Intrinsic geometric shape and size of the packingThis factor determines the surface per unit of volume,and the packing capacity of establishing effective va-pour liquid interfacial surface. It is a well-known factthat, for any packing, the smaller its particle size, thelarger its surface : volume ratio, and the lower theHETP value. All the other factors being equal, numer-ous available data tend to indicate that the expectedreference packing HETPo may be correlated as fol-lows:

HETPo"Kp/Ff

p [14]

where Fp is the packing factor. The constants Kp andf for different types of commercial packing are corre-lated as in Table 7 for a reference system.


Table 7 HETP correlation factors for a reference system, re-gressed by the authors for eqn [14]

Packing Constant Kp Exponent f

Structured packingsSulzer Mellapak� 126 0.73Koch Flexipac� 100 0.69Koch}Glitsch Gempak� 120 0.76Average of above structuredpackings

106 0.70

Random metallic packingsKoch}Glitsch CMR� 73 0.43Norton IMPT� 198 0.69Pall Rings 250 0.69Average above random packings 110 0.50

Figure 14 HETP vs. loads. (Courtesy of Sulzer Chemtech.)

System properties Numerous investigations havetried to correlate experimental HETP data with thedistillation system Suid physical properties. The bestand most consistent correlations tend to conRrmthat the HETP is proportional to reference HETPo

and a factor proportional to the system physical

properties:

HETP"HETPo(��/Sg�)n/(��/Sg�)n0 [15]

when the n exponent best Rt is between 0.15 and0.21. Replacing HETPo from eqn [14] into eqn [15],and using the reference system we obtain:

HETP"(2.0Kp/Ffp) (��/Sg�)0.2 [16]

In addition, theoretical considerations suggest thatthe HETP is related to "m/(L/V), the ratio of theslopes of the equilibrium line and operating line, bythe correlation:

HETP" ln()/(!1)HTU

where HTU is the height of a transfer unit. Then:

HETP"(2.0Kp/Ffp) (��/Sg�)0.2 f () [17]

Packing loading Figure 14 shows the pilot plantperformance of Sulzer/Nutter ring No. 2.5 in


Figure 15 Effect of number of drip points and liquid irrigationrate on maldistribution. (Permission from Chemical EngineeringProgress.)

isobutane}n-butane separation. Note that althoughonly the vapour rate appears in the abscissa, actuallyboth the vapour and the liquid rates increase in thesame proportion since the chart was developed attotal reSux. All packings present similar curves insmall size experimental columns. The initial HETP ishigh (low efRciency) owing to the low loads thatresult in liquid maldistribution, poor packing wettingand little interaction between the vapour and theliquid (this left section of the curve is not shown inFigure 14). Nevertheless, the HETP continuously de-creases as the loads increase. At a point, correspond-ing to the loading point of the packing, the HETPbecomes constant over a range of loads. This rangerepresents the operating range of the packing. As theloads continue to increase, the HETP shows a dipcorresponding to high interaction between the Suids,followed by a rapid increase in the HETP caused byrecirculation of liquid within the bed. This corres-ponds to the initial Sooding of the bed.

Maldistribution

Liquid maldistribution has a very large effect on col-umn distillation performance. Liquid maldistributionis originated by uneven liquid Sow from the distribu-tor to the top section of the packing. Some degree ofmaldistribution cannot be avoided and it is related tothe following factors.

Drip points density (total drip points/column cross-section area) In principle, a smaller number of drippoints equates to a higher initial maldistribution. Thiscould be solved by constructing distributors witha high number of drip points. However, there arephysical and mechanical limits that make it difRcultto build distributors with more than 20 drippoints ft�2. It has also been demonstrated that if thedistributor deck is not levelled, the resulting maldis-tribution effect may increase as the number of drippoints is increased above an optimal number. Theoptimal number of drip points is related to the liquidirrigation Sow as follows (Figure 15):

Liquid irrigationg ) m�1 ) ft�2

0.25 0.5 1.0 2.0 4.0

Optimum number drippoints per square foot

5 8 13 21 32

Furthermore, the drip points themselves may createadditional maldistribution if they are not evenly dis-tributed across the entire column cross-sectional area.Poor construction making holes of variable diametersor unlevelled installation of the distributor will also

induce additional maldistribution. Operational prob-lems such as plugging of the distributor deck areaswill cause large sectors to be dry, thus producinga macroscopic or sectorial maldistribution.

Maldistribution and spreading factor Initial maldis-tribution produces a condition of uneven liquid/vapour Sow ratio across the column cross-sectionalarea. Some areas or spots are underirrigated and someare overirrigated. The column packing does spreadthe liquid resulting in some correction or attenuationof the initial maldistribution. The overall weightedmaldistribution is attenuated better in small diametercolumns than in larger columns. This is determinedby the nondimensional number (Zb/CD2

c), where Zb isthe bed height in feet, Dc the column diameter ininches, and C is the spreading factor in ft ) in�2 units(see Figure 16). The spreading factor is related to thepacking particle size and the liquid irrigation.

The lost column efRciency is proportional to theliquid maldistribution, and this effect is ampliRed bythe number of theoretical stages required to achievethe separation. Figure 17 presents a useful correlationfor the calculation of the column efRciency in packeddistillation columns.

Liquid distributor quality The liquid distributor in-trinsic maldistribution, Md (related to its design andmanufacture) should be measured at the factory bya water test measuring the liquid Sow under eachsubsection of the column cross-section. The smallerand more numerous the test area subdivisions,the more precise will be the maldistribution


Figure 16 M vs. Zb/CD2c. (Permission from Chemical Engin-

eering Progress.)

Figure 17 Efficiency vs. NTS.

measurement. The mathematical expression of themaldistribution is:

Md"100[((Li/Lav!1)2)/n]0.5 [18]

for each point area subdivision from i"1 to i"n.The following is the correlation between distributorquality and its maldistribution:

Qd (%)"100/[1#(Md/100)2] [19]

If the distributor quality is 90%, the actual measuredmaldistribution should not exceed 33%. A 95% qual-ity implied a maximum measured maldistributionof 23%.

Total maldistribution Additional maldistributioncan originate from operational factors related tolevelness and obstructions. The total initial maldis-tribution, Mo, can be calculated by:

Mo"(M2)0.5 [20]

Assuming a maximum operational maldistributionMop"15%, and using a 90% distributor quality(Md"33%), the total effective operating maldis-tribution at the top of the packing isMo"(332#152)0.5"36.2.

The effective bed attenuated maldistribution is cal-culated by the following equation:

Mbz"Mo/[1#0.16Mo(Z/CD2c)] [21]

With this Mbz value, the bed efRciency Ez may beobtained from Figure 17. The calculated bed efRcien-cy should be used to correct the packing HETP andobtain the bed operating HETPop:

HETPop"HETP Ez/100 [22]

HETPop"[(2.0 Kp/Ffp)(��/Sg�)0.2 f ()]Ez/100.

[23]


The bed effective number of theoretical stages can becalculated by:

NTS"Z/HETPop. [24]

For a column requiring more than 10 NTS, it is ingeneral advantageous to subdivide the packing in twoor more beds and limit the NTS per bed to around 10.The lower the NTS per bed, the higher the resultingbed efRciency. The limiting factor of subdividing thecolumn into a large number of redistributed beds, isthe extra column height (or the effective packed heightloss for column revamps) necessary to accommodateeach redistributor, and the resulting increased cost. Fornew columns, the optimal number of beds is the onethat results in the required performance at minimumcost, for revamps, it is often the one that results in themaximum available overall NTS. The best choice ineach case is determined by an optimization.

Future Developments

With the ability to accurately design and predict theperformance of packings in distillation, it is expectedthat the use of packings in distillation will becomebetter accepted, not only for plant revamps but alsofor grass roots applications. The design and evalu-ation of liquid distributors needs to be better under-stood by users and equipment manufacturers; stan-dard methods for distributor quality rating should beimplemented based on the basic concepts presented inthis contribution. Readers interested in further ex-ploring the column design methods outlined inthis article may download a free demo of BDSIMat url http://www.geocities.com/&combusem/BDSIM.HTM

Nomenclature

Ac Column cross-sectionalarea

ft2

�P Packing pressure drop in ft�1

C Packing spreading factor ft in�2

Co OriRce Sow coefRcientC1, C2, C3 Constants in pressure drop

correlationsd OriRce diameter inDc Column diameter inEz Bed efRciency %Cs Vapour capacity factor,

deRned byCs"ug(�g/(�l!�g))

0.5

ft s�1

�g Gas density lb f�3

�l Liquid density lb f�3

ug Vapour velocity ft s�1

h Liquid head overdistributor oriRce

inches ofliquid

hd Vapour pressure dropacrossliquid distributor

inches ofliquid

G Vapour Sow mass velocity lb ft�2 h�1

Gf Vapour Sow mass velocity(in Robbins equation)

lb ft�2

V Vapour Sow lb h�1

L Liquid mass Sow lb ft�2 h�1

Lf Liquid mass Sow lb ft�2 h�1

Li Liquid mass Sow at point i gallonsmin�1 ft�2

Lav Liquid average mass Sow gallonsmin�1 ft�2

Fp Packing factorKp HETP correlation factorn Number of measured points

(in distributor testing)HETP Height equivalent of a

theoretical platein

HTU High of a transfer unit in�l Liquid viscosity cPSg Liquid speciRc gravityX Flow factor"

(L/G) (�g/�l)0.5

Y Vapour Sow parameter. AtSooding Y"Yf

Yf,YHf Flooding parameters,deRned by eqns [10] and[11]

a Packing surface area ft2 ft�3

a0 Open area of one drip point� Relative volatility� Void fraction� Surface tension dynes cm�1

gc Gravitational constant 32.2 ft s�2

NTS Number of theoreticalstages

R ReSux ratioRm Minimum reSux ratioMd Maldistribution originated

by the distributor design%

Mo Total initial maldistribution%Mbz Effective bed

maldistribution%

Q Liquid Sow gallonsmin�1

Qd Distributor quality %Zb Bed height ft� Kinematic viscosity

constant"�l/Sg

Ratio of the equilibriumcurve slope to the operatingline slope


See also: I/Distillation: Historical Development; Model-ling and Simulation; Theory of Distillation; Tray Columns:Performance; Tray Columns: Performance; Vapour-LiquidEquilibrium; Correlation and Prediction; Vapour-LiquidEquilibrium: Theory.

Further Reading

Bonilla J (1993) Don’t neglect liquid distributors. ChemicalEngineering Progress 83(3): 47.

Eckert JS (1961) Chemical Engineering Progress 57(9): 54.Fair JR and Matthews RL (1958) Petroleum ReTner 37(4):

153.Klemas L and Bonilla J (1995) Accurately assess packed-

column efRciency. Chemical Engineering Progress91(7): 27.

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

Leva M (1954) Chemical Engineering Progress 50(10):51.

Lobo WE et al. (1945) Transaction of the American Insti-tute of Chemical Engineers 41: 693.

Robbins LA (1991) Chemical Engineering Progress, May,p. 87.

Sherwood TK, Shipley GH and Holloway FA (1938) Indus-trial and Engineering Chemistry 30.

Strigle RF Jr (1994) Packed Tower Design and Applica-tions. Houston: Gulf Publishing.

Strigle RF Jr and Rukovena F (1979) Chemical EngineeringProgress 75(3): 86.

Zenz FA (1953) Chemical Engineering, August, p. 176.

Pilot Plant Batch Distillation

M. A. P. de Carvalho andW. R. Curtis, The Pennsylvania State University,PA, USA

Introduction

Laboratory distillation encompasses an operatingrange from millilitres in bench-top devices to pilotunits with the capacity for producing several hundredkilograms of product per day. While the design ofbench-top assemblies is generally geared towards theachievement of a speciRed purity grade of the desiredproduct, quantitative predictions are not usually feas-ible for such equipment and their construction reliesa great deal on ingenuity and craftsmanship. Fordedicated applications, glassware companies offeroff-the-shelf equipment. This article will thereforefocus on the pilot-scale units, where the analyticalprinciples of mass and heat transfer can be appliedto the operation, design and optimization of theequipment.

The section on theory presents analytical descrip-tions of batch distillation for three different ap-proaches in order of decreasing complexity. It startswith a comprehensive model for a nonadiabatic, non-zero hold-up, nonconstant molar overSow, nonidealmulticomponent column. The second model present-ed neglects stage hold-ups and assumes adiabaticstages and constant molar overSows to arrive at a setof equations describing the transient behaviour of theequipment, which can be solved for a binary systemusing a simple spreadsheet. If constant relative vola-tility and operation at minimum reSux are further

assumed, the derivation of a third model is possible,where the transient states within the equipment aregiven by direct analytical expressions.

The design of a batch column can be a challengingtask because batch distillation presents unique con-siderations that are not addressed in most of theavailable literature, which is concerned with continu-ous operation. The section on design is a collectionof advice and criteria for the design of batchcolumns. SpeciRc information is given about equip-ment for batch distillation and accompanying instru-mentation and safety circuitry. Details are drawnfrom a pilot-scale column that is installed in PennState University’s Department of Chemical Engineer-ing. The section on column operation extends thescope of the two preceding sections by providinginformation on establishing operating strategies andoperating protocols for batch runs. Much of thisinformation is based on hands-on experience ac-quired with the column described in the subsection onequipment.

The last section is a synopsis of numerical tech-niques that have been developed in recent years tofacilitate the optimization of the operation and designof batch columns. Inherent difRculties associatedwith the implementation of these numerical tech-niques into computer codes prevents their widespreaduse in equipment operation and design. However, it islikely that these techniques will be integrated intocommercial simulators in the near future and be read-ily available to users with little knowledge of pro-gramming. The aim here is to introduce the reader tothe topic, rather than to offer extensive coverage,

1098 II / DISTILLATION / Pilot Plant Batch Distillation

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