Problems that involve the inter-nals of a distillation towerhave received a respectableamount of attention in thepractical engineering litera-

ture.* But one cannot say the same forones involving the tower overhead.This lack is surprising, because thereis considerable potential for flawed de-sign and for operating difficulties in-volving elevated equipment associ-ated with distillation columns.

Consider, for instance, the followingthree real-life examples. Although thespecifics differ widely, these instancesshared the following attributes:• In each case, a minor flaw kept the

equipment from achieving any-where near its full potential

• A key to the correct diagnosis con-sisted of knowing the ultimateequipment capacity and recognizingthat the equipment performancewas well short of its capability

• Another key to correct diagnosis layin performing a hydraulic analysisthat combined basic calculationswith field tests and observations

• In each case, the correct diagnosisled to simple, inexpensive modifica-tions. Sledge hammers were notused (see box)

Case No. 1:AERATION DESTABILIZESREFLUX FLOWIn this facility, the reflux to a chemi-cal-plant distillation tower flowed bygravity from a 30-in. (inside diameter)vertical accumulator through a vortexflowmeter and a flow control valve.The level in the accumulator was notautomatically controlled.

During initial operation, the columnreflux flowrate was very erratic. Theoperators dampened this behavior bykeeping the control valve wide openand running the reflux accumulator atapproximately 10% liquid level.

Over time, however, the refluxflowrate dropped off, so the liquidlevel in the accumulator rose. The nor-mal reflux flowrate was reestablishedby stroking the control valve two orthree times. However, this mode of op-eration destabilized the column andwas an operating nuisance.

Figuring out the causeThe control valve was examined sev-eral times during brief outages. Each

time, the valve was found to be cleanand in good condition.

Then the fabrication drawings forthe accumulator were reviewed. Itwas noted that at the 10%-liquidlevel, the entering feed dropped about6 ft into a shallow pool of liquid at thebottom of the vessel (Figure 1). Theliquid level was only about 18 to 20 in.above the liquid outlet nozzle.

Descending like a waterfall, the liq-uid entrained vapor as it penetratedthe shallow pool of liquid, creatingvery fine vapor bubbles. Some of thesebubbles then became entrained in thedischarging liquid, and a portion ofthose in turn became trapped down-stream, at the control valve inlet.

Cover Story Part 2

56 CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2004

Understanding how the equipment works — andkeeping elementary physics in mind — are the keysto eliminating effects such as siphons and unsteady

vapor-collapse rates in reflux accumulators,condensers and other overhead components

TOWER FLAWS TYPIFY A MORE BASIC PROBLEM

I n his preface to a 1997 book, “A Working Guide to Process Equipment” [1], authorNorman Lieberman states: “The general knowledge as to how process equipment re-ally functions is disappearing from the process industries. This is not only my opinion,

but the general view of senior technical managers, in many large corporations.”Speculating about the root cause of this trend, Lieberman points that chemical engi-

neers, the traditional guardians of process know-how, are spread thin over today’s hugebody of knowledge. This state of affairs dilutes their understanding of process equip-ment. Minor, seemingly unimportant design flaws slip by, later turning into major hid-den bottlenecks that remain unresolved and limit production for decades. When the bot-tlenecks become entirely intolerable, a very expensive solution is often implemented: asledge hammer is brought in to crack the nut.

In one case involving distillation [2], a flashing feed entering a downcomer bottle-necked an entire olefins plant for 17 years. This bottleneck survived three failed fix at-tempts. The fourth, “sledge hammer,” attempt would have replaced all the column traysby packing. A last-minute investigation correctly diagnosed the problem and imple-mented a successful and inexpensive solution.

The main text of this article presents three additional instances where minor flaws ledto major bottlenecks, involving distillation-tower overhead systems. o

*See, for instance, References [1] and [3].

DISTILLATION:

Diagnosing InstabilitiesIn the Column OverheadHenry Z. Kister, Fluor

James F. Litchfield, Consultant

These trapped bubbles limited theflowrate through the valve.

The reflux pipe leaving the bottomof the accumulator had a diameter of2 in. This size was generous for drain-ing non-aerated liquid, but too smallfor liquid that was aerated. Drainingthe latter requires rundown lines thatare sized for self-venting flow; in otherwords, flow in which liquid descendswhile any entrapped vapor bubblesdisengage upward.

An excellent correlation by Simpson[7] and Sewell [8] for self-venting flowis Figure 4.5 in Reference [3]. Basedon that correlation, a 2-in. line candrain up to 7 gal/min of aerated liquid.

Because the reflux flowrate in thiscase was 12 gal/min, the balance (5gal/min) would build up in the refluxaccumulator, raising the liquid level.At the same time, gas trapped at thevalve would reduce the flow areathrough the valve and line, loweringthe reflux flow rate. Stroking thevalve vented the trapped bubbles andsiphoned the accumulating liquid outof the accumulator.

Plant operating personnel recalledthat the accumulator had initially op-erated at levels much higher than10%. At these higher levels, operationhad been far more erratic. The expla-nation for that behavior is as follows:

At those higher liquid levels, the“waterfall” height would diminish.This diminution and the greater pooldepth would keep vapor bubbles from

reaching the accumulator outlet. Theliquid at the bottom of the accumula-tor would degas, reverting to non-aer-ated liquid. In this condition, the liq-uid would easily siphon out, and theaccumulator level would rapidly drop.But thereupon, the waterfall wouldagain aerate the bottom liquid, andthe aerated liquid flow would resume.The back-and-forth switches betweenaerated liquid flow and siphoningcaused the initial erratic behavior.

The cure was simpleThe accumulator happened to have an8-in. hand-hole, located 15 in. abovethe bottom tangent line. The problemwas solved by rerouting the 2-in. feedline so as to enter the drum by passingthrough the hand-hole cover, asshown in Figure 1. The rerouted linewas configured in such a way that theportion inside the vessel extended tothe drum centerline and then bent up-wards, discharging upwards against aflat horizontal deflector baffle. Thisbaffle redirected the incoming liquid,spreading it sideways. This arrange-ment eliminated the waterfall andaeration, and fully restored the stabil-ity of the reflux.

Case No. 2:SIPHONING IN DECANTEROUTLET PIPESIn this process unit, the decanter inFigure 2a separated a light liquidphase (for recycling back to a reactor)

from a heavy liquid phase that thenwent to a distillation tower and otherdownstream equipment for final-prod-uct purification. The decanter was ahorizontal drum, 4 ft in diameter by 8ft long, that provided well in excess of1 h of residence time for the phaseseparation.

The maximum liquid level in the de-canter was set by the 3-in. nozzle forlight-phase drawoff, which was lo-cated in the decanter head, 6 in. belowthe top of the decanter. The heavyphase flowed through a block valveand an isolation control valve, andthen upward through a seal loop. Theelevation at the top of the seal loopwas an inch or two lower than the ele-vation of the light phase draw nozzle.A 1-in. pressure-balance line con-nected the top of the seal loop to thedecanter vapor space. After leavingthe decanter, both phases flowed totheir respective surge tanks at gradelevel, which was about 50 ft below thedecanter elevation.

ProblemsWith the block and isolation valveswide open, the decanter proved to besusceptible to siphoning through theseal loop, creating erratic flow in thissystem. In fact, the seal loop had si-phoned as much as 70% of the de-canter liquid.

Because of the erratic behavior, thedecanter was unable to operate at itsdesign temperature. What’s more, it

CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2004 57

FIGURE 1. A simple reroutingof pipe solved a problem,converting a waterfall into amodest fountain

FIGURE 2. The addition of a small siphon breakerdrum to a seal loop eliminated recurring siphoningand erratic flow

did a poor job of handling liquid surgesfrom upstream heat exchangers.

Initial modificationsIn the first try at solving these prob-lems, the size of the pressure balanceline was increased from 1 to 2 in. andthat of the seal loop pipe from 2 to 4 in.These changes helped, but the erraticflow persisted.

Operators were then able to lessenthe siphoning by closing the seal-loopblock valve halfway. But operation inthis mode was not desirable, becausewhenever a feed surge occurred, someof the heavy phase was carried overinto the light phase. Furthermore,closing the valve had little effect inhelping the decanter to reach the de-sign temperature.

Making a hydraulic balanceA hydraulic balance over the relevantequipment was made. The findingswere illuminating.

With the pressure-balance linedoing its job, the static pressure PSLat the top of the seal loop would equalthe static pressure PD in the decantervapor space. The small, 1-to-2-in. ele-vation difference (hHP + hLP - hSL) be-tween the liquid level in the decanterand in the top of the seal loop gave, asexpected, enough driving force to over-come the friction head losses in theseal loop at normal flows. Calculationsconfirmed that the flow resistancethrough the seal loop, including theopen valves, was extremely small.

The 50-ft elevation drop from the top

of the seal loop to grade exerted strongsuction at the top of that loop. With ap-propriate pressure balancing, enoughvapor from the top of the decanterwould have become entrained in therundown line liquid to raise friction inthe rundown line and make the pres-sure at the top of the seal loop thesame as that in the vapor space of thedecanter. But if there were no pressurebalancing whatsoever, the suction atthe top of the seal loop would havecaused the liquid flow to rapidly in-crease and siphon out the decanter.

The observation that siphoning wastaking place meant that the pressure-balance line was not fully achieving itsintended function. Increasing the linesize from 1 to 2 in. had helped, but hadnot remedied the situation completely.

The throttling of the liquid valve be-tween the decanter and seal loop in-creased the pressure difference be-tween the two, which dampenedsurging by further increasing vaporflow to the seal loop. However, the hy-draulic balance had made it apparentthat better vapor balancing andsiphon breaking were required.

The full cureThe seal loop was replaced by a 1-by-4-ft vertical drum (Figure 2b) thatprovided good siphon breaking andpressure equalization with the de-canter. The siphoning was broken bydrawing the heavy phase from a sidesump, into which liquid could onlyenter by overflowing a chordal weir. A3-in. line was installed to balance thepressures between the top of the drumand the decanter vapor space.

Two other changes improved thedecanter operation further. To mini-

mize turbulence and short-circuitingin the decanter, the internal feed pipein that vessel was increased in diame-ter from 2 to 4 in., and the feed wasdischarged against the head, as seenin Figure 2b. And for better control ofthe light-phase thickness, a weir wasinstalled in the light phase just up-stream of the outlet nozzle.

As a result of these modifications,the siphoning was eliminated and thedecanter outlet flow was no longer er-ratic. Most important, stabilization ofthe flow allowed the decanter to oper-ate at its design temperature, whichwas 10 to 20ºF lower than the pre-mod-ification temperature, leading to majorimprovement in phase separation.

Case No. 3:POOR HOOKUP OF HOT-VAPOR BYPASS PIPESThis installation consisted of the over-head for a new debutanizer column sep-arating C3 and C4 hydrocarbons fromgasoline. The column overhead vaporwas totally condensed by a battery offour submerged condensers (Figure 3a).The reflux drum was elevated. Noncon-densables, if any, from the condenserswere vented to the drum using 1-in.vent lines (not shown). The tower pres-sure was intended to be controlled by ahot-vapor bypass, hooked up as shownin that figure.

But when the tower was put into ser-vice, it experienced severe pressurefluctuations. Maintaining a constantcolumn pressure was impossible, whichbottlenecked the tower throughput.

Diagnosing the problemFor successful pressure control by ahot-vapor bypass, correct piping is

58 CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2004

FIGURE 3. In this example, the harmfulpressure fluctuations could have beenavoided in the first place by not ignoringthe distillation-design literature

mandatory. Bypass vapor must enterthe vapor space of the reflux drum(Figure 3b), the bypass line should befree of pockets where liquid can accu-mulate, and any horizontal pipe runsshould drain into the reflux drum.Most important, liquid from the con-denser(s) must enter the reflux drumwell below the liquid surface. The bot-tom of the drum is the most suitablelocation, but instead, extending theliquid line downward to near the bot-tom of the drum (Figure 3b) is an ac-ceptable alternative. These principlesand recommendations were first pub-lished almost fifty years ago [4, 5].Since then, they have been stronglyendorsed by key recent sources ad-dressing methods for distillation-col-umn pressure control [3, 6].

The initial piping design for thedebutanizer (Figure 3a) defies thoseprinciples, and violates the recom-mended practices for hot-vapor bypasspiping. Subcooled liquid mixes withvapor at its dewpoint; the vapor col-lapses at the point of mixing; the rate ofvapor collapse varies with changes insubcooling, overhead temperature, andcondensation rate. It is the variationsin this collapse rate that induce thepressure fluctuations experienced, andthe consequent control-valve hunting.

Problems similar to this were re-peatedly described in the early litera-ture mentioned above [4, 5]. Butstrangely enough, the incorrecthookup that is shown in Figure 3akeeps reappearing in modern distilla-tion designs.

Switching to the sound designTo remedy the situation at the debu-tanizer, the liquid and vapor lineswere separated. The vapor line wasmodified so that it introduced thevapor into the top of the reflux drum.As for the liquid line, it was extendeddownward, terminating a few inchesabove the bottom of the reflux drum.Figure 3b shows the modified system.

These changes fully solved the prob-lem: the tower pressure no longer fluc-tuated. What’s more, one could feelthe differences in temperature be-tween the top part of the reflux drum,

which contained hot vapor, and thebottom part, which contained sub-cooled liquid, simply by (cautiously)touching the drum. n

Edited by Nicholas P. Chopey

CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2004 59

adlinks.che.com/3646-37

AuthorsHenry Z. Kister, a senior fel-low and director of fractiona-tion technology at Fluor Corp.(Aliso Viejo, Calif.; Phone: 949-349-4679; e-mail: Henry.Kister@fluor.com), has over 25 yearsexperience in design, trou-bleshooting, revamping, fieldconsulting, control and startupof fractionation processes andequipment. Previously, he wasBrown & Root’s staff consul-

tant on fractionation, and worked for ICI Aus-tralia and Fractionation Research Inc. (FRI). Au-thor of the textbooks “Distillation Design” and“Distillation Operation,” plus 70 published arti-cles, he has taught the IChemE-sponsored “Practi-cal Distillation Technology” course 260 times. Arecipient of Chemical Engineering’s 2002 Awardfor Personal Achievement in Chemical Engineer-ing, he is also a member of that magazine’s Edito-rial Advisory Board. He holds B.E. and M.E. de-grees from the University of NSW in Australia. AFellow of IChemE and a member of AIChE, heserves on the FRI Technical Advisory and DesignPractices Committees.

James F. Litchfield, a con-sultant residing in Ventura,Calif. (email: litchja@-aol.com), has 37 years of ex-perience. He was with CF-Braun/Brown & Root Braunfor 34 years, including 15 inresearch and the last two as astaff consultant. For over adecade, he was the firm’stechnical representative tothe Particulate Solids Re-

search Institute. His expertise is in chemical en-gineering design and multiphase flow; he has op-timized or upgraded numerous phase separators(for two or three phases), inlets to vacuum andatmospheric columns, and lines for two- orthree-phase flow. He holds a B.S. from the Uni-versity of California at Berkeley, and an M.S.from the University of Idaho, both in chemicalengineering.

References1. Lieberman, N. P., and Lieberman, E. T., “A

Working Guide to Process Equipment,” Mc-Graw-Hill, New York, 1997.

2. Kister, H. Z., Hower, T. C., Freitas, P. R. deM., and Nery Souza Neto, J. “Problems andSolutions in Demethanizers and C2 Splitterswith Interreboilers,” 8th Annual EthyleneProducers Conference, New Orleans, La.,February 25–29, 1966.

3. Kister, H.Z., “Distillation Operation,” Mc-Graw-Hill, 1990.

4. Whistler, A.M., Locate Condensers atGround Level, Pet. Ref. 33 (3) , p. 173, 1954.

5. Hollander, L., Pressure Control of Light-EndsFractionators, ISA J. 4 (5), p. 185, 1957.

6. Chin, T. G., Guide to Distillation PressureControl Methods, Hydrocarbon Proc., 58(10), p. 145, 1979.

7. Simpson, L.L., Sizing Piping for ProcessPlants, Chem. Eng., p. 192, June 17, 1968.

8. Sewell, A., Practical Aspects of DistillationColumn Design, Chem. Engineer, 299/300, p.442, 1975.

An earlier version of this article was presentedat the 2003 annual meeting of AIChE, in SanFrancisco last November.)