optimize distillation columnsdfrey1/ench445/0005benn.pdfa distillation column can use either trays...

Chemical Engineering Progress May 2000 19

DISTILLATION

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OptimizeDistillationColumns

istillation is the dominant pro-cess for separating large multi-component streams into high pu-rity products. So, the chemicalprocess industries’ ongoing

quest to improve energy utilization, reducecapital costs, and boost operating flexibilityis spurring increasing attention to distillationcolumn optimization during design. Design-ers often approach column optimization in aniterative manner, heavily relying on vendorexperience and information. A good under-standing of mass-transfer and pressure-dropfundamentals, as they relate to optimization,will enable the column designer to indepen-dently judge vendor offerings and effectivelydetermine the optimal equipment design.

This article will address the following op-timization goals: (1) maximizing theoreticalstages per height of section or column, (2)minimizing pressure drop per theoreticalstage, and (3) maximizing the operationalrange, turn-down, or turn-up.

A distillation column can use either traysor packings. Their mechanisms of masstransfer differ, but the key for both is a goodapproach to equilibrium through the genera-tion of large amounts of interfacial area. Thisinterfacial area results from the passage ofvapor through the perforations of trays, orthe spreading of liquid on the surface ofpackings.

First, we will discuss the underlying phe-nomena for trayed columns and the designapproaches that can be used to meet the threeoptimization goals. Then, we will address themechanisms and approaches for packedcolumns. Finally, we will consider the selec-tion of trays vs. packing.

DApplication of mass-transferand pressure-drop fundamentals can lead to improved designs for both trayed and packed columns.

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20 May 2000 Chemical Engineering Progress

DISTILLATION

n a trayed column, liquidflows down the columnthrough downcomers andthen across the tray deck,while vapor flows upwardthrough the liquid inventory

on the tray. Tray designs can be di-vided into cross-flow and parallel-flow types. Figure 1 illustrates theconceptual differences. Cross-flowtrays are the most common and leastexpensive, but parallel-flow trays, ifproperly designed, can provide an ef-ficiency that is 10% or more higher.Figure 1 depicts a single-pass cross-flow tray. By “pass,” we mean thenumber of downcomers per tray. Ascolumn diameter increases, the ratio

Part I: Trayed columns

I

Douglas L. Bennett and Kenneth W. Kovak,Air Products and Chemicals, Inc.

■ Figure 1. Types of trays.

Parallel-Flow Trays Single-Pass Cross-Flow Trays

Sieve Tray Bubble Cap

Fixed Valves

Floating Valve

■ Figure 2. Types of tray-deckmass-transfer devices.


of weir length to throughput decreas-es; so, for larger dia. columns, multi-pass trays often are used to increaseweir length and achieve about thesame liquid inventory on the tray.

The performance of a tray also de-pends upon the type of tray deck. Thesimplest is a sieve tray — it has aperforated tray deck with a uniformhole diameter of from less than a mil-limeter to about 25 mm. Trays withvalves, which can be fixed or floating,also are very common; bubble capsstill are used, but infrequently andusually only for extreme turndown.Figure 2 shows representatives ofeach family.

Tray efficiency depends uponcolumn throughput. As seen in Fig-ure 3, there is a relatively flat, sta-ble operation region characterizedby a gradual increase in efficiencyas vapor hole velocity and liquidinventory rise with throughput. Oneither side of this stable region,performance drops off. The drop-off at low rates results first fromweeping and then more significant-ly from dumping. At high rates,heavy entrainment decreases effi-ciency and then performance dra-matically drops when flooding oc-curs. Because our optimizationgoals often will require operationat the extremes of the stable re-gion, it is helpful to understand thecontrolling mechanisms in thesethree regions.

Weeping and dumpingWeeping and dumping are related

but different phenomena. Duringweeping, a minor fraction of liquidflows to the tray below through thetray perforations rather than thedowncomer. This downward-flowingliquid typically has been exposed torising vapor; so, weeping only leadsto a small reduction in overall trayefficiency, to a level rarely worsethan the tray point efficiency. In con-trast, during dumping, a substantialportion of liquid flowing down thecolumn passes through a region ofthe perforated tray deck. Often, mostof this liquid has not been exposedto the rising vapor; therefore, per-formance degrades significantly —frequently resulting in overall trayefficiency being significantly lessthan local point efficiency.

Weeping and dumping differ intheir underlying mechanisms. Forlarge-perforation sieve trays, vaporand liquid can flow in an approxi-mately steady, countercurrent mannerthrough a perforation. More likely,however, especially for sieve trayswith smaller perforations, weeping istransient, resulting in spurts of liquidleaving a nonbubbling perforation.The spurting occurs when there is alocal and instantaneous downwardpressure imbalance over the perfora-tion. The cause of this imbalance canbe associated with the bubbling fre-quency or, because the flow on a

■ Figure 3. Tray performancevs. throughput.

Stable Operation

Column Throughput

Tray

Effi

cien

cy

HeavyEntrainment

Weeping

DumpingFlooding

NomenclatureAB = area of perforated portion of tray

deckAH = total hole areaD = molecular diffusivityDH = sieve-tray perforation diameterFrL = liquid Froude number

= [ρVVH2/ρL g hL]0.5

E = ratio of liquid entrainment massflow to upward column vapor massflow

hD = tray pressure-drop component associated with vapor flowingthrough perforation, expressed inheight of clear liquid

hFe = effective froth height as defined byBennett et al. (2)

hL = liquid inventory on tray, expressedin height of clear liquid

hT = total tray pressure drop expressedin height of liquid

hσ = tray pressure-drop component associated with formation ofbubbles expressed in height of liquid

h2φ = height of two-phase region on thetray, see Figure 4

g = acceleration due to gravityKS = density-corrected superficial gas

velocity over bubbling surface =[ρV/(ρL - ρV)]0.5VS

L = liquid mass-flow rate through column

m = slope of equilibrium curveTS = tray spacingV = vapor mass-flow rate through

columnVH = velocity of the vapor through

perforation holeVS = velocity of vapor over bubbling

surface of the tray

Greek lettersβ = constant defined in Bennett et

al. (1) = 0.5 [1 + tanh (1.3 ln(hL/DH) - 0.15)]

ρ = densityµ = viscosityηPT = point efficiencyηSECT = tray section efficiency, ratio of

number of theoretical stages tonumber of trays in a section

φe = effective froth density, hL/hFe,as defined in Bennett et al. (2)

φ2φ = hL/h2φ

SubscriptsL = liquidML = liquid molarMV = vapor molarV = vaporOpt = optimum

large-scale tray is very complex, theimbalance can stem from local densi-ty and height variations of the frothwaves traveling on the tray. The re-sult is weeping regions that tend tomove around on the tray deck.Dumping is much more extreme andoccurs because, at the intended traythroughput, there is insufficientvapor pressure drop to retain enteringliquid on the tray deck. Thus, signifi-cant quantities of liquid flow througha portion of the tray that has little ifany vapor flow. Flow over the outletweir can be zero. Normally minorphenomena, for example, the hy-draulic gradient or tray-inlet liquidflow maldistribution, can have a sig-nificant impact on the minimum traypressure drop required to preventdumping and where on the tray thedumping occurs — and, therefore, onthe level of performance degradation.

Stable operationIt is easiest to discuss this region

by considering the simplest tray deckdesign, a perforated plate. In thiscase, vapor flows upward though theperforations and enters a two-phaselayer of height h2φ. Vapor momentumis at its maximum as the vapor accel-erates through the perforation. Theexchange of this vapor momentumwith the liquid inventory on the traydeck is critical to the nature of thetwo-phase zone. The two-phase layercan be spray-like or froth-like. Ben-nett et al. (1) have shown that theratio of the liquid inventory, hL, tothe perforation diameter, DH, is keyto this momentum exchange. WhenhL/DH exceeds about 2, vapor mo-mentum is exchanged with signifi-cant quantities of liquid; the two-phase mixture is largely liquid-con-tinuous and behaves as a froth withreasonable mass transfer. WhenhL/DH is under about 1, vapor mo-mentum is exchanged with little liq-uid; the two-phase region is largelyvapor-continuous with significant up-ward liquid and vapor velocity com-ponents. The resulting flow regime isspray-like with poor mass transfer.

Spray-like flows should be avoided ifat all possible — if they cannot beavoided in a tray design, a packedcolumn often is a better choice.

Because spray-like conditionsshould be avoided, we will concen-trate in this article on froth flow. Fig-ure 4 illustrates some of the main pa-rameters of interest. The tray deck isspaced a distance of Ts from adjacenttrays. The bulk of liquid is containedwithin a liquid-continuous region nearthe tray deck that has a height aboutequal to the effective froth height, hFe,as defined by Bennett et al. (2). Asmaller portion of liquid inventory iscontained as droplets in the vapor-con-tinuous region above the liquid-contin-uous region. The velocity of these liq-uid droplets is related to the vapor mo-mentum through the perforation andthe ratio hL/DH; if the vertical compo-nent of velocity is sufficient, dropletswill be carried to the tray above.

Most of the liquid inventory andinterfacial area occur within theliquid-continuous region; therefore,this part of the total two-phase re-gion is most important for bothpressure drop and mass transfer.Smaller perforation sizes, by in-creasing hL/DH, promote the ex-change of momentum of the vaporto the liquid and, thus, the deceler-

ation of vapor velocity within theliquid-continuous layer. The result-ing lower average vapor velocitywithin this layer increases vaporresidence time; lower values of DHand higher values of hL enhancemass-transfer efficiency within thestable operation region.

Heavy entrainment and flooding

At high throughputs, correspond-ing to the top of the stable operationregion, significant quantities of liquiddroplets reach the tray deck aboveand pass through to the upper tray.This recirculation, called entrain-ment, degrades the composition pro-file in the column. If the downcomercan handle this additional liquid traf-fic, the column can tolerate signifi-cant entrainment and operate in a sta-ble manner, but with a lower numberof theoretical stages. At high valuesof entrainment, the column controlsystem may no longer allow stableoperation and the column behaviorcan enter a condition that is bestcalled “operational” flood. (Somehave called this jet flood, but thisterm is misleading because jettingoften is used as a synonym for thespray-like regime and an operationalflood can occur for either spray-like

DISTILLATION


■ Figure 4. Froth flow structure.

TS

hL

DH

hFe

h2φ

Vapor

or froth-like conditions.) The occur-rence of operational flood dependsupon the control system and the sen-sitivity of the overall tray efficiencyto entrainment (for example, parame-ters λ, L/V, and ηPT). Not all columnsand control systems will lead to anoperational flood at high entrainment.

There is a critical distinction be-tween operational flood and hy-draulic flood. Hydraulic flood resultswhen the downcomer, at a given col-umn throughput, becomes fully load-ed with liquid and entrained vapor,and this mixture within the down-comer begins to impede flow overthe outlet weir. The added resistanceincreases tray liquid inventory andpressure drop. This, in turn, raisesthe two-phase mixture height in thedowncomer area, further impedingflow over the outlet weir, and boost-ing pressure drop even more. Finally,at hydraulic flood, all the liquid thatenters the column no longer canleave the column; liquid is accumu-lated above the flood point and pres-sure drop increases rapidly. In con-trast, in operational flood, all the liq-uid entering the column section stillleaves the column, even though pres-sure drop can be very high and effi-ciency is very poor or unstable.

Some mass-transfer andpressure-drop fundamentals

We will use the recent correla-tion for sieve-tray efficiency re-ported by Bennett et al. (3). Theyaddress point efficiency, entrain-ment, mixing within the froth,weeping, and cross-flow and paral-lel-flow tray types. Their correla-tion for point efficiency is given inEq. 1 above. This equation under-scores many of the trends that wewill quantify with further analysis. It

uses a Reynolds number, ρVVHhFe/µV,that other studies have shown to havean impact on bubble size. Larger val-ues of the vapor velocity through theperforation, VH, yield higher interfa-cial area. As expected, the ratiohL/DH plays a significant role. Theratio of hole area to bubbling surfacearea, AH/AB, also is important: small-er values enhance efficiency. The de-nominator of the first term within themajor brackets is the correction re-quired when liquid-phase resistanceis important.

When the objective is to mini-mize the pressure drop per theoreti-cal stage, pressure drop also mustbe calculated; we will use themethod of Bennett et al. (2) forthese calculations:

hT = hL + hD + hσ (2)where hT is tray pressure drop, hL isliquid inventory, hD is pressure dropof the vapor flowing through theperforation, and hσ is pressure dropassociated with bubble formation.

Bennett et al. (3) report that, fortheir composite database for cross-flow trays, weeping did not appearto substantially degrade performanceas long as the Froude number:

(3)

At high vapor rates, entrainmentbecomes significant and decreasestray performance. To take this intoaccount, we will use the correlationfor entrainment given by Bennett etal. (3):

(4)where h2φ is given by Eq. 5 below.

We will account for the degrada-tion in tray efficiency caused byentrainment, but also will assumethat hydraulic flood or operationalflood will not be encountered.

The approach usedThere are no generalized correla-

tions that apply to all types of tray-deck designs. So, our approach willbe to use the broadly based correla-tions developed for sieve trays to de-velop some optimization rules andthen to discuss the implications ofusing other types of trays on theserules. The optimization goals are: (1)maximizing theoretical stages persection or column height, (2) mini-mizing pressure drop per theoreticalstage, and (3) maximizing the opera-tional range, turn-down, or turn-up.

We chose single-variable pertur-bations, assuming a constant liquid-to-vapor mass flow ratio, L/V, foreach example. To test our answers,we selected properties consistentwith a number of model systems:methanol and water (170 and 310kPa), low-pressure C6/C7 (27 and 165kPa), and high-pressure iso/normalbutane (2,000 and 2,800 kPa). Withthese systems, the vapor density, ρV,varied from 1 to about 100 kg/m3, theliquid density, ρL, from 370 to 750kg/m3, and ρV/(ρL - ρV) from 0.015 to0.36. We also looked at a full rangeof typical tray-geometry parameters,

E = 0.00335TS

h 2φ

– 1.10 ρLρV

0.5

φ2φβ

FrL =ρV VH

2

ρL g h L

0.5

≥ 0.5


η PT = 1– exp – 0.0029

1 + mρMVρML

DV (1 – φe)

DLAH

AB

ρV VH h FeµV

0.4136h L

DH

0.6074 AH

AB

– 0.3195

(1)

h 2φ = h Fe + 7.79 1 + 6.9h L

DH

– 1.85 KS2

φe gAH

AB

(5)

values of L/V, and vapor throughputscorresponding to a density-correctedsuperficial gas velocity, KS, from0.025 to 0.06 m/s. In this manner, wewere able to quantify the general im-pact of changing individual geometryparameters. More details are con-tained in Bennett et al. (4).

The optimization resultsThe ratio of section efficiency to

pressure drop, ηSECT/hT, is the numberof stages per unit liquid height ofpressure drop, higher values beingmore energy efficient. In Figure 5a,typical values of this ratio are plottedvs. the perforation diameter. Smallervalues of DH enhance efficiencythrough promotion of froth overspray, and through better mass trans-fer resulting from smaller bubbles. Atvery small values of DH, tray pressuredrop rises substantially due to bub-ble-formation pressure drop. In Fig-ure 5b, we see a similar plot indicat-ing that a relatively gentle optimumoccurs at large values of open area.As the percent open area goes up,pressure drop declines at a faster ratethan efficiency, thereby giving an in-crease in ηSECT/hT. As the fractionopen area continues to rise past thispoint, mass transfer drops as theReynolds number decreases. This plotignores weeping. Further analysisshows, however, that this optimumgenerally occurs when weeping is ex-pected to begin, which for cross-flowtrays is at approximately FrL = 0.5.This gives:

(6)When maximizing ηSECT/hT,

there also is an optimal weir height.This optimum can exist because, atlarger values of outlet weir, pres-sure drop increases at a higher ratethan efficiency. At very low valuesof outlet weir height, liquid inven-tory is less, which cuts tray effi-ciency, but tray pressure drop de-creases less rapidly, because hD be-

gins to dominate. Figure 5c gives aplot of calculated optimum weirheight vs. effective froth density,where effective froth height isgiven by:

(7)The parameter ηSECT/TS is the

number of theoretical stages perheight of column. Higher values

mean that more stages can beachieved within a given columnheight. In Figure 6a, typical valuesfor this parameter are plotted vs.fraction open area. As fraction openarea decreases, vapor velocitythrough the perforation increases,giving more interfacial area. At ex-tremely small values of open area,vapor velocity through the perfora-tions is very large — this promotesentrainment and thus degradesoverall tray efficiency. In Figure

h Fe =h L

e– 12.55 K s0.91 with KS in m/s

AH

AB Opt

=AH

AB FrL = 0.5

= 2 KS1

g h L

DISTILLATION


■ Figure 5. Distillation energyefficiency.

Optimum Weir Height, m

Effe

ctiv

e Fr

oth

Hei

ght,

m

0 0.005 0.01 0.015 0.02 0.025 0.031

0.1

0.2

0.30.4

0.5

0.6

Fraction Open Area

Sect

ion

Effic

ienc

y/Pr

essu

re D

rop

0 0.05 0.10

iso/normal Butane

MEOH/Water

C6/C7

0.15 0.20 0.250

5

10

20

15

Hole Diameter, mmb.

c.

Sect

ion

Effic

ienc

y/Pr

essu

re D

rop

0 5 10

iso/normal Butane

MEOH/Water

C6/C7

15 20 2510

15

20

25a.

6b, ηSECT/TS is plotted vs. perfora-tion hole diameter. Small perfora-tions give better mass-transfer effi-ciency. The minimum hole size fora tray, however, depends uponstructural considerations and thefouling characteristics of the distil-lation system.

It is intuitively obvious that, if therise in liquid entrainment is reason-able, installing more trays will in-crease the theoretical stages per col-umn height. Figure 6c illustrates cal-culated values for ηSECT/TS vs. calcu-lated entrainment rate. These calcula-

tions assume that effective frothheights are less than tray spacing(and ignore questions about the im-pact of entrained liquid on tray hy-draulics). This assumption is notvalid at the very small tray spacingimplied by the high entrainment val-ues given in Figure 6c, but these re-sults do show that significantamounts of liquid entrainment canoccur prior to reaching a maximumvalue of ηSECT/TS. If tray efficiencyper column height is important, oper-ating trays with a significant amountof entrainment, for example, at a

mass-of-entrained-liquid-to-mass-of-vapor-throughput ratio of 0.1 to 0.2,is advantageous. Of course, thedowncomer must be appropriatelydesigned to accommodate this.

Summary of rulesTo maximize the number of theo-

retical stages for a given sectionheight:

For sieve trays:1. Keep the fraction open area

low, for example, in the range of 5%.2. Use the smallest practical per-

foration diameter. This value willdepend upon the degree of systemfouling, the ability to clean fouledtrays, and the tray deck materialand thickness.

3. If practical, select a tray spac-ing that yields high entrainment — atray spacing corresponding to an en-trained-liquid-to-vapor-flow ratio ofabout 0.2 is reasonable, if confidencein the entrainment rate exists anddowncomer capacity is adequate.

4. Consider parallel flow trays,if the cost increase is justified.Bennett et al. (3) show that optingfor such trays generally leads to anenhancement of 10% or more in thenumber of theoretical stages.

For other types of trays:Trays with small fixed valves

can be used with success. Such de-signs have a relatively large openarea, but a smaller perforation sizecompared to larger fixed or float-ing valves. In addition, the vapormust flow around the impact re-gion of the valve and this decreas-es upward momentum and entrain-ment. Selecting a tray spacing thatresults in a reasonable level of en-trainment also is effective. Paral-lel-flow valve trays would lead tomore theoretical trays within agiven column height, but valve de-signs that do not disturb the liquidflow pattern on the tray should beselected. Trays using very smallbubble caps also could be advanta-geous, because the vapor is forced toinitially flow downwards to the trayfloor and this can enhance efficiency.


■ Figure 6. Maximizing thenumber of stagesper column height.

Calculated Entrainment, Mass Liquid/Mass Vapor

Theo

retic

al S

tage

spe

r Met

er

0 0.5 1 1.5

300

200

100

0

Hole Diameter, mm

Theo

retic

al S

tage

s pe

r Met

er

0 0.05 0.10

iso/normal Butane

iso/normal Butane2,758 kPa

MEOH/Water

C6/C7

0.15 0.20 0.250

5

10

20

15

Fraction Open Area

a.

b.

c.

Theo

retic

al S

tage

s pe

r Met

er

0 0.05 0.10

iso/normal Butane

MEOH/Water

C6/C7

0.15 0.20 0.251

1.5

2.0

2.5

3.0

3.5

Sieve trays normally have a better ef-ficiency than cap trays; so, unless ex-treme turndown is particularly impor-tant, the added cost of small bubble-cap trays is not justified.

To minimize the pressure drop pertheoretical stage:

For sieve trays:1. Use the largest fraction open

area that will not result in weeping.Based on the literature for cross-flowtrays, this maximum occurs around:

(8)(8)

The open literature does not addressthis for parallel-flow trays.

2. Use the smallest practical perfo-ration diameter.

3. Specify a low outlet weir, gen-erally less than 50 mm.

4. Consider parallel-flow trays, ifthe cost increase is justified.

For other types of trays:The pressure drops of trays with

fixed valves and bubble caps oftencan be larger than those for sievetrays, if the sieve trays are designedat maximum values of the fraction ofopen area. The increase in hydraulicresistance for valve and bubble-captrays also boosts tray pressure dropand can lead to dumping of liquidthrough the valves or caps, frequentlyat the tray entrance. Techniques havebeen developed to mitigate this ef-fect, but more pressure drop usuallyresults. Tray deck features withsmaller openings will promote small-er bubbles and better efficiency.Lower outlet weirs and parallel-flowtrays can offer efficiency advantages.

To maximize the operating range:For sieve trays:1. Reducing the fraction open area

decreases the onset of weeping andincreases entrainment. In general,however, our study has shown thatlower open areas will maximize theoperating range.

2. Decreasing the tray perforationdiameter to the minimum practicalsize will decrease entrainment andwill not adversely impact weeping.

3. Maximizing the tray spacingwill raise maximum throughput andnot adversely impact weeping, butalso will lower the number of stageswithin a given column height.

4. Decreasing the liquid inventoryby increasing the number of traypasses or lowering the weir heightwill enhance turn-down and generallywill cut entrainment for a given col-umn throughput. Reducing the outletweir height below 25 mm, however,offers little advantage. The lower liq-uid inventory also will decrease trayefficiency. The spray-like regimemust be avoided.

5. Some high-capacity tray designswith downcomers that allow liquid todrop onto the tray deck below can re-sult in weeping through the trayopenings under the downcomer. Forsuch designs, the turn-down capabili-ty of the tray can be reduced by anunacceptable amount of bypass to thetray below.

For other types of trays:Sieve trays, when properly de-

signed to maximize operating range,can have an operating range of goodand stable performance of about afactor of two or more, if larger trayspacing and higher pressure drop atpeak rates are acceptable. Floating

valves, because their minimum flowarea varies with vapor throughput,can have a wider operating range —about a factor of three — with alower maximum pressure drop thansieve trays, but often with a lower ef-ficiency and a higher cost. Small-sizefixed-valves generally will have agreater maximum range than large-size fixed or floating valves when traycount per section height is also im-portant and a larger pressure drop atmaximum rates is acceptable. Theirability to turn down, however, is notas great as floating valves. Small-sizebubble caps, although expensive, canhave substantial turn-down capabilityand, with special design considera-tions to prevent liquid/vapor bypassat extreme turn-down, can give anoperating range greater than a factorof five. CEP

AH

AB FrL = 0.5

= 2 KS1

g h L

DISTILLATION


Literature Cited1. Bennett, D. L., A. S. Kao, and L. W.

Wong, “A Mechanistic Analysis ofSieve Tray Froth Height and Entrain-ment,” AIChE J., 41 (9), p. 2,067(Sept. 1995).

2. Bennett, D. L., R. Agrawal, and P. J.Cook, “New Pressure Drop Correla-tion for Sieve Tray DistillationColumns,” AIChE J., 29 (3), p. 434(May 1983).

3. Bennett, D. L., D. Watson, and M. A.Wiescinski, “New Correlation forSieve-Tray Point Efficiency, Entrain-ment, and Section Efficiency,” AIChEJ., 43 (6), p. 1,611 (June 1997).

4. Bennett, D. L., K. W. Kovak, and D.Zak, “Optimization of Sieve-TypeDistillation Trays,” presented atAIChE Annual Meeting, Los Angeles(1997).

D. L. Bennett is General Manager, Advanced

Gas Separation, for Air Products and

Chemicals, Inc., Allentown, PA ((610)

481–7788; Fax: (610) 706–7420; E-mail:

[email protected]). He has worked for the

company for 24 years, during which time he

has held a number of senior-level technical

and management positions. He has authored

over 60 publications, many in the

separations area. He has been granted 12

patents, and has several others pending. He

received a BA in applied science as well as

BS, MS, and PhD degrees in mechanical

engineering, all from Lehigh Univ. A member

of AIChE, he is a Director of the Institute’s

Separations Div. He also serves on the Board

of Directors of Fractionation Research, Inc. In

1992, he received the Chairman’s Award, the

highest and most-prestigious award given by

Air Products, for his contributions to

cryogenic technology.

K. W. Kovak is Technology Manager, Cryogenic

Systems — Air Separation, for Air Products

and Chemicals, Inc., Allentown, PA ((610)

481-6464; Fax: (610) 481–2177; E-mail:

[email protected]). In his 22 years at the

company, he has held various process

engineering, management, and technology

positions. He has over 20 years of

experience in the design, manufacture, and

operation of cryogenic processing systems,

particularly for cryogenic distillation. He

received BS and MS degrees in chemical

engineering from Manhattan College. He is a

member of AIChE and of the Institute’s

Separations Div.

he wide range of com-mercial packings andtheir variations in ge-ometry introduce sig-nificant empiricism intothe design of packed

columns. Application of fundamentalmass-transfer and pressure-drop prin-ciples, however, still can lead to theidentification of general optimizationrules. As we did for trayed columns,here we will develop rules for the fol-lowing optimization goals: (1) maxi-mizing theoretical stages per height

of section or column, (2) minimizingpressure drop per theoretical stage ofseparation, and (3) maximizing theoperating range of the column that re-sults in reasonable performance.

Columns can be divided into twomajor groups based on the type ofpacking used — either random (ordumped) packing, or structured (orordered) packing. Random packingsoften are either cylindrical (ring)shaped, or half cylinder (saddle)shaped. We will call a single ring orsaddle a packing element. Figure 1 il-

lustrates some of the common typesof random packing. Both rings andsaddles typically have features thatare formed into the packing element.These features and other details canprovide surface area within the struc-ture, increase resistance to deforma-tion, and also prevent elements fromnesting.

Within a packing type, elementsare available in a variety of sizes andmaterials of construction; elementsfabricated from a given material lookessentially alike except in their size.


Part II: Packed columns

TDouglas L. Bennett,Air Products and Chemicals, Inc.

Berl Saddle

Raschig Ring

Intalox MetalTower Packing

IMPT

Pall Ring

Nutter Ring

Cascade MiniRingCMR

■ Figure 1. Types of random-packing elements.

This means that they are geometrical-ly similar and differ only by a charac-teristic dimension, δP. We will findthis very useful as we develop ourtheoretical understanding. For ran-dom packings, we will define δP asthe nominal packing size or diameter.As an example, a 25-mm Pall Ringhas a nominal diameter and value forδP of 25 mm.

Mass transfer occurs at the inter-face between vapor and the liquid filmon the packing surface. The masstransfer is significantly impacted bythe specific area of the packing, aS,and this is controlled by the packing’snominal size. As long as the diameterof the column is very large comparedto δP, aS is inversely proportional toδP. The constant relating the two de-pends upon the packing type and howthe elements are arranged. For exam-ple, if we assume that the packed col-umn is filled with thin-walled cylin-drical elements stacked in an orderedfashion with 60° pitch and the ele-ments are just touching, the value for

aS can be calculated to be about7.25/δP. Packing elements installed ina random or dumped fashion orientthemselves in an irregular manner,and this impacts the constant relatingaS and δP. The constant also can de-pend upon the ratio of δP to the verti-cal height of the element. In general,for both random rings and saddles, aS= 5.7/δP within a band of about ±10%.

The second general category ofpacking commonly is referred to nowas structured packing. It can be madeof woven or solid material, usuallymetal, which typically is corrugatedand bundled into segments that areplaced layer by layer into the columnshell. Most commercially availablestructured packings have perforationsor texturing through stamping to helppromote mixing and liquid spreading,as well as to modestly increase sur-face area. Figure 2 illustrates the gen-eral geometry characteristics of thetype of structured packing often usedtoday.

Pressure-drop and mass-transfer

characteristics can be controlled bychanging the corrugation angle, γ,the fold angle, α, and the height ofthe corrugation. We will define thecharacteristic dimension of struc-tured packing, δP, to be the heightof the corrugation. Most such pack-ings have a corrugation angle of45° and a fold angle of 90°; theheight of the corrugation, therefore,is the primary geometry parameterused by the column designer foroptimization.

The constant relating aS to δP de-pends upon α. If we neglect the sheetthickness and any change in surfacearea resulting from surface texture orperforations, aS = (2/sin (α/2))/δP.Thus, for the typical fold angle of90°, aS = 2.8/δP. This shows that thespecific area for structured packing isless than that for random packings fora given value of δP. Keep in mind,however, that we define δP as the cor-rugation height for structured packingand the nominal packing-element di-ameter for random packing.

DISTILLATION


Primary FlowDirection of Vapor

Primary VaporFlow Direct

Fold Angle

CorrugationAngle

Primary FlowDirection of Liquid

X

X

δP

α

γ

X

X

X

X

■ Figure 2. Parameters for structured packing.

Some mass-transfer fundamentals

Mass transfer within a packed col-umn typically is described in terms ofHETP, the height equivalent of a the-oretical plate — that is, the height ofpacking required for a theoreticalstage of separation. Figure 3 showsthe general relationship betweenHETP and column throughput. Per-formance remains relatively constantand stable except at both low andhigh rates. At low liquid rates, sheetsof liquid are not stable and rivulets donot spread sufficiently to wet the en-tire packing surface; so, mass transferis poor. As column throughput in-creases, there is a large stable regionwhere packing is well wetted andHETP is relatively constant. At thehigher-rate portion of the stable re-gion, mass transfer improves (HETPgets lower). This improvement iscaused by an increase in interfacialarea due to liquid waves and entrain-ment. At higher rates, performancedecreases rapidly due to substantialliquid entrainment in the vapor beingcarried up the column. This entrain-ment degrades the composition pro-file and can result in liquid flow andvapor flow redistribution.

Optimization of the packing for(1) maximizing theoretical stages perheight of section or column, (2) mini-mizing pressure drop per theoreticalstage of separation, and (3) maximiz-ing the operating range of the columnthat results in reasonable performanceoften requires the column to operateat the extremes of the stable operatingrange. We need, therefore, an under-standing of the phenomena that con-trol each of the three regions: poorwetting, stable operation, and heavyentrainment.

Poor wettingLiquid flows from the distributor

onto the packed section as a series ofstreams. Good wetting, resulting in asubstantial fraction of the packingsurface area being wet, requires thatpacking elements or layers of struc-tured packing distribute these

streams into a combination ofrivulets and films that fully migratethroughout the packed column cross-section. The liquid flow rate per wet-ted perimeter controls liquid-film hy-drodynamics. We will designate Γ asthe average value for liquid flow rateper wetted perimeter, assuming all ofthe liquid is uniformly distributed.By mass balance, Γ = L/aS, where Lis the liquid mass flux flowing downthe column. For a given packing typeand distillation system, a minimumvalue for Γ is required for the pack-ing to be substantially wet. We willdesignate this minimum value as ΓC;if Γ is below this value, wetting willbe poor, resulting in poor mass trans-fer and a large HETP. ΓC would beexpected to be a function of contactangle and physical properties, andmay depend upon packing surface-texture details. It should be a con-stant, however, for a given distilla-tion system (composition and pres-sure) and packing type.

Stable operationMost basic mass-transfer text-

books (for example, Ref. 1) developequations for continuous distillationassuming a downward flowing liq-uid phase in contact with an upwardflowing vapor phase. A rearrange-ment of these equations leads to Eq.1 for the HETP vs. the appropriatemass-transfer coefficients (see boxbelow), which can be rearranged toEq. 2 (see box) where VS is the su-perficial velocity of vapor up thecolumn. The term in the bracket isthe increase in HETP resulting fromliquid-phase resistance. Frequently,however, vapor-phase resistancedominates. In such cases, thisbracketed term is approximatelyunity and assuming that kV can becorrelated using the format of Eq. 3we get: Eq. 4 (see box on next page).Packed column data can be used toarrive at values for p and n, but, byanalogy to heat transfer and wetted-wall-column mass transfer, p is ex-


Stable Operation

Column Throughput

HET

P

HeavyEntrainment

PoorWetting Enhanced

Mass Transfer

■ Figure 3. Packingperformance.

HETP = G 1ρMV k V ai

+ mρML k L ai

ln λλ – 1 (1)

HETP = 1ai

VS

k V1 + m

ρMVρML

k V

k L

ln λλ – 1 (2)

pected to be about 0.8 and n about0.3. For the stable region, wherepacking is assumed to be well wetted,ai should be inversely proportional tothe characteristic dimension of thepacking, δP. We find, then, that for agiven distillation system, HETP onlyweakly depends upon vapor flow(about VS

0.2), but strongly dependsupon the characteristic dimension ofthe packing (about δP

0.8). This weakdependency of HETP on VS is whythe concept of HETP is so useful.

We also will need a correlation forpressure drop. One based on a signifi-cant amount of experimental data isthe Eckert Generalized Pressure DropCorrelation for structured packing,this approach defines two axis:

(5)and

(6)

This correlation is graphically repre-sented by a plot of Y vs. X along withlines of constant pressure gradient. Akey test of the validity of this ap-proach is whether a single value ofthe packing factor, FP, adequatelyagrees with all of the constant-pres-sure gradient lines. Kister and Gill (2)evaluated structured-packing pres-sure-drop data and found that a con-stant value of FP for structured pack-ing could not be found; they, there-fore, redrew the constant-pressuregradient lines for structured packing.The difference in the random-packingand structured-packing constant-pres-sure gradient lines becomes signifi-

cant for values of X exceeding 0.20,and for values of pressure gradientmore than 0.5 in. of water pressuredrop/ft of packing height. FP is purelyan experimental constant determinedby minimizing the error between thedata and the correlation, and is afunction of packing type and charac-teristic dimension.

In Figure 4, we have plotted FPvalues from Kister (3) vs. δP for sev-eral commercial packings. We findthat, for a given value of δP, FP forPall Rings is less than half that ofRaschig Rings. This is attributable tothe openness of the Pall Rings com-pared to Raschig Rings. The otherrandom packings show an additionalone-third to one-half reduction in FP,resulting from further increases inperforations of the element and, forsome elements, a reduction in ele-ment height that allows elements to

stack in a more open manner. Forstructured packing, for equal valuesof δP, FP is about one-fourth of theminimum value for random packing.But, because the definition of δP forrandom and structured packing differ,a comparison of values of FP atequivalent specific area is more ap-propriate. We find that structuredpacking values for FP are about one-half the lowest value for randompackings at equal specific area.

For low and moderate loading,pressure drop is directly proportionalto the value for FP; therefore, thepressure drop for structured packingis about one-half that of the lowest-pressure-drop random packings atequivalent values of specific area andequal column throughput.

With reasonable accuracy, a curve-fit of published values for FP is:

FP = C3 (δP)-1.1 with δP in inches(7)

Alternatively, because aS is inverselyproportional to δP,

FP = C4 (aS)1.1 with aS in ft2/ft3

(8)The values of C3 and C4 are given inTable 1 and are important when welook at optimization for differenttypes of packings.

The Eckert-type correlation doesnot allow easy identification of the

X = LV

ρVρL

0.5

Y = VSρV

ρL – ρV

0.5

FP0.5 µL

ρL

0.05

DISTILLATION


Characteristic Dimension, δP, in.Pa

ckin

g Fa

ctor

, FP

Raschig RingPall RingIMPTCMR

Nutter RingStructured PackingStructured Packing atequivalent specific area

1,000

100

10

1.0 100.11

■ Figure 4. Packing factordependency oncharacteristicdimension.

k V δP

DV= C1

ρV VS δPµV

p µV

ρV DV

n(3)

HETP =C2ai

VS δP

DV

1 – p µV

ρV DV

p – nln λλ – 1 (4)

functionality that relates pressuredrop to flow and geometry parame-ters. An explicit correlation would behelpful for our optimization effort.Liquid, as it flows down the packedcolumn, raises pressure drop by: (1)increasing the roughness, (2) takingup space, which, in turn, boostsvapor velocity at a given massthroughput, and (3) providing liquiddroplets that become entrained in thevapor core. Because the first mecha-nism dominates except near flood,we will use a correlation format sim-ilar to that used to calculate pressuredrop for fully turbulent vapor flow inrough tubes:

(9)

The pressure gradient ∆P/∆Z is ex-pressed in units of height of liquidper height of packing, εP is the heightof liquid-flow-induced roughness,and g is acceleration due to gravity,which is required as we are express-ing pressure drop in terms of liquidheight. A and B are constants that wewill borrow from rough tube data. Wewill assume that εP can be approxi-mated from the average height of theliquid film; this can be calculatedfrom a simple force balance, yielding:

(10)

where Γ is the average mass-flow rateof liquid per wetted surface. From be-fore, we know that Γ = L/aS, and, be-cause aS and δP are inversely propor-tional, Γ ∝ L δP. Substitution gives:Eq. 11 (see box below). Based onrough-tube pressure-drop data, B isabout 0.25. The constant A1 is ex-pected to vary for different types of

packing elements but, for geometri-cally similar packings, should beconstant for elements fabricatedfrom the same type of material. Thevariation in A1 values for differenttypes of packing is identical to thevariation in the constants C3 and C4that relate the packing factor to δPand aS. Manufacturing techniquesfor metal, plastic, and ceramic mate-rials differ, however, resulting insome differences in element geome-try even if elements are part of thesame packing type. Thus, A1 willvary depending upon material offabrication. This expression for thepressure gradient is not a completerepresentation of pressure drop, butstill has significant similarities withthe Eckert-type pressure-drop corre-lation, especially in the nonloadedregion (specifically at values of Xless than 0.5, where pressure drop islargely a function of Y2). We expectthat, for this non-heavily-loaded re-gion, there is a weak dependency onliquid mass flux (about L0.08). Also,for geometrically similar packings,FP is approximately inversely pro-portional to the characteristic dimen-sion (about δP

-1.15). These trends willbe useful as we draw conclusionsabout the optimization of packedcolumns.

Heavy entrainmentWe also will need an understand-

ing of packed column flooding. Ascolumn throughput increases beyondthe stable operating region, pressuredrop begins to rise much more quick-ly, due to more liquid entrainment invapor and a greater vapor velocity re-sulting from higher liquid holdup. Inaddition, mass transfer begins to dropas increased liquid entrainment flowsup the column. At very high through-put rates, substantial quantities of liq-uid flow up portions of the column.Because packing is relativity open tocross-flow, liquid and vapor tend toredistribute themselves; so, local val-ues of L/V vary substantially, result-ing in a sharp dropoff in column masstransfer. In packed columns, there isno discrete flood point, but operationof a column anywhere within thisvery heavily loaded region is undesir-able. A force balance between the up-ward pressure drop force on a liquidfilm and the downward gravity forcegives at flood:

(12)

If we define fLFlood to be the liquidvoid fraction at flood, then fLFlood = aSδF. Because aS is inversely propor-tional to δP, we have the interestingrelationship:

(13)

for any particular packing type.This force balance, coupled with

the equation for δF, and our under-standing that Γ is proportional to L

∆P∆Z Flood

∝ fLFlood

δP∆P∆Z Flood

= A2 δF

δF =3 µL Γ

ρL ρL – ρV g

0.33

∆P∆Z = A

εP

δP

B ρVρL

VS2

g δP


∆P∆Z = A1

3 µL L

δP2 ρL ρL – ρV g

0.33B ρVρL

VS2

g δP (11)

Table 1. Constants for Eqs. 15 and 16.

Type of packing C3 C4

Raschig Rings 140 1.35*Metal Pall Rings 62 0.66Intalox Metal Tower Packing 39 0.37*Cascade MiniRings 42 0.34Nutter Rings 35 0.34*Hiflow Rings 34 0.33Structured Packing (α =90°, γ =45°) 8 0.137

Note: published values used for aS when available; when unavailable (indicated by an asterisk), the approximate relationship, aS = 5.7/δP was used.

δP, predicts that the pressure drop re-sulting in flooding is proportional toδP

-0.8. It also indicates that film thick-ness plays some role. In support ofthis force-balance approach, we canuse the empirical Kister and Gill cor-relation for packing flooding (4), Eq.14 in box below. This correlationdoes not agree with the dependencyon liquid rate (the dependency onδF) predicted by the force balance,but, because we have shown that FPis proportional to δP

-1.1, the Kisterand Gill correlation for floodingdoes indicate that the pressure-dropgradient at flood is proportional toδP

-0.77, which is identical to ourforce-balance-approach prediction.

The approach usedWe now can combine these equa-

tions derived from mass-transfer andpressure-drop fundamentals and lookat their implication on column opti-mization. Due to decreasing cost andseveral performance advantages,structured packing is becoming morepopular. We, therefore, will addresscolumn optimization for structuredpackings and then discuss, for eachoptimization goal, the approach forrandom packings.

To minimize HETP:We will assume that gas-phase re-

sistance dominates and packing isfully wet such that ai is proportional

to δP-1.0. We also will assume that the

values for p and n are 0.8 and 0.3, re-spectively. Therefore, we get Eq. 15(see box below).

Most vendors of structured pack-ing have standardized on corrugationangle and fold angle, namely, γ = 45°and α = 90°; therefore, in general,metal structured packings from dif-ferent vendors are very similar withthe possible exception of surface de-tails. These surface differences canhave some impact on the liquid resis-tance, but we are assuming for thisanalysis that vapor-phase resistancedominates. For structured packingwhen α = 90° and γ = 45°, this equa-tion for HETP shows that, for a givenseparation system and columnthroughput, the only geometry vari-able that significantly impacts HETPis δP. Smaller values of δP givegreater values of specific area, aS, andlower section heights or more masstransfer within a given section height.This equation predicts that HETP de-pends upon δP

1.2 (or aS-1.2), which is

very close to the results from pub-lished data.

To minimize pressure drop pertheoretical stage of separation:

We observe the relationship givenin Eq. 16 (see box below) from theequations for pressure drop andHETP.

As discussed before, within a

packing family, for example, PallRings or structured packing with uni-form corrugation and fold angles, A1and C5 are constant. Therefore, withina packing family and for a given dis-tillation system:

(17)

Reducing the vapor throughput willdecrease pressure drop, but will sub-stantially increase capital investmentand is rarely justifiable. Selecting alower value of aS to give sufficientHETP, but at an acceptable pressuredrop, is the typical optimization.

To maximize the stable operatingrange:

For packed columns, the range ofstable HETP performance is boundedat high throughputs by high pressuredrop and poor mass-transfer perfor-mance as flooding is approached, and atlow throughputs by poor performanceresulting from inadequate wetting. Thecharacteristic dimension impactspacked column flooding in two ways.First, pressure drop for a given vaporthroughput is approximately inverselyproportional to δP. Second, as δP getssmaller, the pressure drop that will re-sult in flood is approximately inverselyproportional to δP. The net result is thatfor geometrically similar packing:

KVFlood ∝ δP (18)For a given distillation system and

design L/V, there is a minimum ΓC.We can show that:

(19)

Maximizing the range for stableoperation is equal to maximizingKVFlood/KVMin, which depends uponδP

2. Larger values of δP will result ina significantly larger operating rangeat the expense of reduced mass-trans-fer performance.

For high turn-down service, dis-tributor design poses a further com-plication. Close work with the ven-dor, along with thorough full-scaletesting, at least with water, often isimportant (5,6).

KVMin∝ δ P

– 1.0

∆P∆Z

HETP ∝ aS2.35 VS

1.8 L0.08

DISTILLATION


∆P∆Z

HETP = A13 µL L

δP2 ρL ρL – ρV g

0.083 ρVρL

VS2

g δP/

C5 δPVS δP

DV

0.2 µV

ρV DV

0.5ln λλ – 1

(16)

∆P∆Z Flood

= 0.115 FP0.7 with FP in inches of water (14)

HETP = C5 δPVS δP

DV

0.2 µV

ρV DV

0.5ln λλ – 1 (15)

Summary of optimization rules

To maximize the number of theoret-ical stages for a given section height:

For structured packing:1. Use packing with a high specific

area, and run the column near theupper range of stable operation. Becareful of packings with very highspecific areas, especially for low-pres-sure distillation when low liquidthroughputs are common and partialwetting may occur — this reduces theadvantage of using a higher specificarea to decrease HETP. Recognizethat operating range will be decreasedsignificantly as specific area increases.

2. Use packings with a corrugationangle of 45°.

3. Be concerned about the need forredistribution, which is related moreto theoretical tray count in a sectionthan section height.

For other types of packing:Use random packing elements

with small values of δP, and run thecolumn near the upper range of stableoperation. When looking at differenttypes of random packing, those thathave larger specific areas for a givencharacteristic dimension will tend tohave better values of HETP. In addi-tion, because pressure drop generallyenhances mass transfer, considerpacking types with somewhat higherpressure drop (for example, highervalues of C3 and C4) — but this candecrease capacity and require a largerdiameter column. The cost of high-specific-area random packings oftenis much higher than that of structuredpacking — therefore, when maximiz-ing the number of theoretical stageswithin a given section height, it ishard to beat structured packing.

To minimize the pressure drop pertheoretical stage:

For structured packing:We wish to minimize aS

2.35 VS1.8

L0.08.1. Reducing the vapor velocity

will decrease pressure drop per theo-retical stage, but will increase columndiameter — the additional capital in-vestment rarely is justified.

2. Decreasing the specific area aSwill reduce pressure drop per theoret-ical stage. If an equal number ofstages are required, column heightwill increase.

3. Using structured packing with a30° corrugation angle can give amodest improvement in pressure dropper theoretical stage, but also will re-sult in a higher HETP. This usually isnot cost-justified; increasing the char-acteristic dimension while keeping a45° corrugation angle often is a bettersolution.

For other types of packing:Rules 1 and 2 also apply to ran-

dom packings. There are many typesof random packing, and this resultsin a relatively wide range of pressuredrop for a given characteristic di-mension. This is shown by the rangein values for constants C3 and C4.The total pressure drop stems frombluff-body losses and shear-stresslosses at the surface. Bluff-body loss-es generate turbulence in the mainvapor core, while sheer-stress lossesresult in turbulence at the packingsurface. Because most of the masstransfer in a packed column occursnear the packing surface, packingswith lower bluff-body losses usuallygive better mass-transfer perfor-mance for a given pressure drop.Such packings result in significantcontact area, but generally are moreopen or preferentially lie in a mannerthat results in a more open pattern.These packings have lower pressuredrop for a given specific area or char-acteristic dimension. This means thatpackings with lower values of con-stants C3 and C4 are preferred. Struc-tured packing, however, has a signifi-cantly lower pressure drop for agiven specific area than randompackings — therefore, structuredpacking usually is preferred whenminimizing pressure drop per theo-retical stage.

To maximize the operating range:For structured packing:1. The primary variable that im-

pacts operating range is the character-istic dimension or the packing specific

area. Larger values of δP will increaseoperating range, but will result in ahigher HETP and a taller column for agiven number of theoretical stages.

2. Some increase in range is possi-ble by changing the corrugationangle to yield less pressure drop for agiven vapor throughput — for exam-ple, using a 30° corrugation angle in-stead of the more typical 45°. Thischange, because it reduces pressuredrop, will increase capacity withoutsignificantly changing ΓC. BecauseΓC has not changed, the drop-off inperformance at low rates due to poorwetting still will be the same and op-erating range is enhanced. Structuredpacking with a 30° corrugationangle, however, will have a higherHETP and less vapor mixing — thatmay be a concern for column sec-tions requiring a large number of the-oretical stages.

3. Full wetting is promoted by cap-illary forces; so, packings made ofwoven materials are particularly at-tractive when the column is operatingwith very low liquid rates. This is par-ticularly true for high-vacuum distilla-tions or when poor wetting fluids suchas water are distilled or when a verylarge operating range is required.

4. The operating range of inter-nals, especially liquid distributors andredistributors, often can be the con-trolling factor. The requirement foruniformity of the local L/V is particu-larly important for column sectionswith significant theoretical-stagecount.

For other types of packing:As with structured packings, in-

creasing δP increases the maximumoperating range for random packings,but at the expense of intrinsicallypoorer HETP. Packings that havelower pressure drop for a given valueof δP also will have inherently higheroperating range — such packings arethose with lower values for constantsC3 and C4. Finally, packing elementswith surface texturing or materialsthat tend to better wet with the sys-tem will enhance the maximum al-lowable operating range.


Trays vs. packing Clearly, both trayed and packed

columns will continue to have signifi-cant roles to play in distillation. Un-derstanding the factors involved inoptimizing each provides a basis forchoosing between trays and packing.Here are some general guidelines:

1. When the service is nonfouling,either type of column can be designedto yield comparable theoretical stagecounts per section height. Packinghas the disadvantage of requiring re-distributors if large stage counts with-in a column section are required.

2. Fouling can pose problems forboth trays and packing.

3. Packed columns often can bedesigned with greater stable operatingrange than sieve trays. Valve and bub-ble-cap trays can have a stable oper-ating range equal to or even greaterthan that of columns with random ornormal corrugated structured pack-ing. Structured packing made of ex-pensive woven materials can have avery broad operating range. The de-sign of internals for a stable operatingrange of more than two or three ismuch more difficult for packings thanfor trays.

4. For applications requiring lowpressure drop per theoretical stage,packing has a significant inherent ad-vantage, because the interfacial areafor packed columns is generatedthrough liquid spreading on the pack-ing surface — this is a low-pressure-drop phenomenon compared to themechanism required to generate highmass-transfer efficiency within atrayed column.

5. For low-pressure distillationapplications, liquid flow rates tendto be very low. The resulting modestliquid inventory fosters spray-likeconditions with trays that, in turn,

promote high entrainment and lowtray efficiency. This can be mitigat-ed with small perforations, but analternative is to opt for packedcolumns if liquid rates are sufficientto obtain good wetting. An addedadvantage for packing is its inher-ently lower pressure drop, which isparticularly important for low-pres-sure applications.

6. Trayed columns are intrinsicallylower cost than packed columns, be-cause far less surface is needed fortrays than for packings, and trays re-quire far-lower-cost internals thanpackings. CEP

DISTILLATION


Nomenclatureai = interfacial area per packing volumeaS = surface area of packing/volume of

packingA = unspecified constantA1 = unspecified constantA2 = unspecified constantB = constant estimated at 0.25C1 = unspecified constantC2 = unspecified constantC3 = constant relating FP to δP, expressed

in inchesC4 = constant relating FP to aS, expressed

in ft2/ft3

C5 = unspecified constantD = molecular diffusivityfLFlood = liquid void fraction at floodFP = packing factorg = acceleration due to gravityG = vapor molar fluxHETP = height equivalent to a theoretical

platekL = liquid-phase mass-transfer coefficientkV = vapor-phase mass-transfer coefficientKV = density-corrected superficial velocity

within column, [ρV/(ρL - ρV)]0.5VS

L = liquid mass-flow rate through columnm = slope of equilibrium curve∆P/∆Z= packed-column pressure-drop

gradient, expressed in height ofliquid/height of packing

p = constant estimated at 0.8n = constant estimated at 0.3V = vapor mass-flow rate through columnVS = superficial velocity of vaporX = x-axis on Eckert Generalized

Pressure Drop CorrelationY = y-axis on Eckert Generalized

Pressure Drop Correlation

Greek lettersα = structured-packing fold angle, see

Figure 2γ = structured-packing corrugation angle,

see Figure 2δF = liquid film thicknessδP = characteristic dimension: for random

packing, nominal packing size; forstructured packing, corrugationheight

εP = roughness attributable to liquidwaves

Γ = average mass-flow rate of liquid perwetted surface

ΓC = minimum mass-flow rate per wettedsurface

λ = ratio of slope of equilibrium line, m,to operating line, L/V

µ = molecular viscosityρ = density

SubscriptsL = liquidV = vaporML = liquid molarMV = vapor molarFlood= at floodMin = minimum column throughput for

region of stable operation

Literature Cited1. Treybal, R. E., “Mass Transfer Opera-

tions,” McGraw-Hill, New York, p. 250(1968).

2. Kister, H. Z., and D. R. Gill, “Distilla-tion and Adsorption,” presented atIChemE meeting, Birmingham, U.K.(Sept. 1992).

3. Kister, H. Z., “Distillation Design,” Mc-Graw-Hill, New York, p. 495 (1992).

4. Kister, H. Z., and D. R. Gill, “PredictFlood Point and Pressure Drop for Mod-ern Random Packings,” Chem. Eng.Progress, 87 (2), p. 32 (Feb. 1991).

5. Olsson, F. R., “Detect Distributor De-fects Before They Cripple Columns,”Chem. Eng. Progress, 95 (10), p. 57(Oct. 1999).

6. Bennett, D. L., and K. A. Ludwig,“Understand the Limitations ofAir/Water Testing of Distillation Equip-ment,” Chem. Eng. Progress, 90 (4), p. 72 (Apr. 1994).

D. L. Bennett is General Manager, Advanced

Gas Separation, for Air Products and

Chemicals, Inc., Allentown, PA ((610)

481–7788; Fax: (610) 706–7420; E-mail:

[email protected]). For more background

on the author, see p. 26.

optimize distillation columnsdfrey1/ench445/0005benn.pdfa distillation column can use either trays...

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