distillation

Atwo-phase survey was recentlycompleted by Fluor of all the casehistories related to malfunctions in

refinery towers that have been document-ed over the last 50 years. Altogether, 400case histories were found in the literature.The first phase identified the most com-mon root causes of problems in refineryfractionators (towers), but did not exam-ine the troublespots in each specific ser-vice. This phase yielded generalguidelines for trouble-free design, but didnot address issues related to each specificfractionator.

In the second phase, case histories oftower malfunctions are analysed specifi-cally for each of the major refinery frac-tionators. Each case history teaches alesson. Together, these lessons are the besttool for understanding the potential trou-blespots in each service, and for drawingguidelines for trouble-free design of eachservice.

I have previously described the Fluorsurvey methodology in Distillation Opera-tion (McGraw-Hill, New York,1990). Allthe case histories used as a basis for thesurvey were extracted from the publishedliterature. There were 900 total cases, ofwhich 400 were for refinery towers. Inabout one quarter of these, the specificservice was not stated or the service wasone that did not have enough casesreported on to permit detailed analysis.This left about 300 cases for the main

refinery fractionators,and these form thebasis for the currentanalysis.

As with other Fluorsurveys, certainground rules wereapplied to limit thescope. Only specificincidents wereincluded. For exam-ple, a statement suchas “leakage fromchimney trays inrefinery vacuum tow-ers can be reduced byseal-welding” doesnot constitute a casehistory. On the otherhand, a statement such as “one vacuumtower experienced severe chimney trayleakage at low-rate operation. Seal weld-ing tray sections reduced leakage toacceptable levels” does.

Also, incidents of corrosion and foul-ing were included only if a feature uniqueto the column design, operation, or con-trol contributed to their occurrence. Forinstance, an incident where the wrongcorrosion inhibitor or antifoulant wasapplied does not qualify as a case historyin this survey. A case where fouling wascaused by insufficient liquid flow, maldis-tribution, or poor process control, does.

Finally, optimisation case studies(where capacity was raised or pressuredrop lowered by replacing trays by pack-ings) are outside of the scope of the sur-vey. The objective of the current survey isto identify the issues that make towers fallshort of achieving these design capacities.

There is some overlap in the tabulationof cases for each fractionator. Forinstance, a coked chimney tray case studywill be listed once under “coking” andanother time under “intermediate draws”.This means that adding the individualmalfunctions may yield a number greaterthan the number of malfunctions report-ed for the service. Table 1 lists the mainfractionators surveyed and the concisenumber of cases reported for each service.

It clearly shows that the vacuum tower isby far the most troublesome refinery ser-vice, which is where the survey begins.

Vacuum tower malfunctionsThe 86 case histories reported for the vac-uum tower is almost double the numberreported for the atmospheric crude tower,which is the next most troublesome refin-ery tower. When a vacuum tower per-forms poorly, valuable distillate is lost tothe resid, and poor distillate quality poi-sons FCC catalyst. The wash section ofthe fractionator is the most critical sec-tion and also one where most of the mal-

Trouble-free design of refinery fractionators

A review of factors most frequently the cause of distillation towers falling shortof design objectives. Analysis of case histories provides guidelines for identifying

potential troublespots in the most important fractionators

Henry Z KisterFluor Corporation

MASS TRANSFER: DISTILLATION

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Number of cases

1. Vacuum towers 862. Atmospheric crude fractionators 453. Debutanisers 374. FCC main fractionators 33 5. Deethanisers 236. Depropanisers, C3/C4 splitters 227. Alky main fractionators/isostrippers 178. Coker main fractionators 159. Naphtha splitters 11

10. Deisobutanisers 811. Amine towers 8

Table 1

Fractionator malfunctions

Figure 1 Uplifted packing in wash section of a vacuum tower

No. Description Cases1 Damage 272 Coking 213 Intermediate draws 174 Misleading measurements 105 Plugging 9– Installation mishaps 9– Abnormal operation (startup,

shutdown, commissioning) 9

8 Maldistribution 6– Weeping 6

10 Condenser 4

Table 2

Top causes of vacuum towermalfunctions

functions were reported. The wash sec-tion of the vacuum tower is therefore themost troublesome tower section in therefinery (Figure 1, on previous page).

Table 2 shows the common malfunc-tions reported in vacuum towers. With 27case histories, damage tops the list. Mostof this damage can be readily prevented.Table 3 shows the most common causes.Foremost are water-induced pressuresurges, which account for one third of thereported damage incidents. In three of thenine cases reported, the source of waterwas poor draining of stripping steamlines. In another three, pockets of waterlying in the piping of spare pumpsentered the hot tower when these pumpswere connected to the hot tower.

A lesson in this case is that many, pos-sibly most, vacuum tower damage inci-dents can be prevented by design andoperating procedures that adequatelydrain the tower steam lines in wet towers,and that positively prevent water fromspare pump piping from entering thetower. A joint designer/refiner “hazop”should focus on these troublespots.

The next source of damage in Table 3,insufficient mechanical strength, is alsoreadily preventable. It should be recog-nised (as can be readily seen from Table 2),that damage is a major issue in a vacuumtower, and that heavy duty internalsdesign should be used. Although theheavy duty design would not be able towithstand a major pressure surge, it wouldweather the smaller pressure surges. Somegood heavy duty design practices havebeen described by Shieveler [Shieveler G H,Use heavy-duty trays for severe services; ChemEng Progr, Aug 1995].

Special attention should be paid to gridinstallation and tightening. In two of thefive cases, poorly fastened grids disinte-grated in service. Through-bolting hasbeen far more effective than J-bolting forkeeping grid together, and should be rou-tinely specified.

Spray distributors and their headers areprone to damage (Table 3). Again, thisdamage can be easily prevented by sounddesign, good installation, and thoroughinspection and testing. Water testingspray nozzles and headers can readilydetect damage (Figure 2). Header damagecan be prevented by using standard

flanges and gaskets and properly allowingfor thermal expansion in the headerdesign.

Damage due to high base level con-tributed three out of the 27 damage casehistories. This again is an issue that can beat least alleviated by good level monitor-ing, alarms, and well-designed trip sys-tems. Another three damage-relatedcase-histories were caused by packingfires. This type of damage is more difficultto prevent due to the difficulty of clean-ing the packings, especially when coked.Nonetheless, much progress has beenreported in developing preventive mea-sures, and is discussed in two excellentpapers by Bouck and Markeloff [Bouck DS, Vacuum Tower Packing Fires; API Operat-ing Practices Symposium, 27 April 1999.Markeloff R, Packing fires; FRI Technical Advi-

sory Committee, San Antonio, Texas, Nov2001].

Coking of the wash section (Figure 3) isa close second in Table 2 with 21 reportedcase studies. Table 4 gives a breakdown ofthe causes. Excess stages and vaporisationoccur in wash beds that are either too tallor contain packings that are too efficient.In either case, the additional stages inten-sify the vaporisation of the wash oil, leav-ing little liquid to reach and wet the lowersections of the bed. These lower sectionsof the bed dry and coke. Poor modellingand simulation is another cause of coking.Golden et al stress that the heavy ends ofthe crude must be correctly characterisedin the simulation and that the feed entryto the tower must be modelled by a seriesof flash steps that correctly represent thephysical sequence of steps between the

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No. Description Cases1 Water-induced pressure surges 92 Insufficient mechanical strength 53 Broken nozzles or headers of

spray distributors 4

4 High bottom liquid level 3– Packing fires 3

Table 3

Causes of damage in vacuumfractionators

Figure 2 Broken, non-standard flange in the spray header supplying wash oil to thewash bed of a vacuum tower

Figure 3 Coking of fouling resistant grid in the wash section of a vacuum tower

heater outlet and flash zone [Golden S W,Vacuum Tower Troubleshooting; AIChE SpringMeeting, 1994].

When these principles are overlooked,the simulation underestimates wash oilvaporisation, leading to the drying up andcoking reported in four cases.

In three other reported coking inci-dents, it was stated that the wash flowratewas insufficient but no specific reasonwas given. It is likely that in those casestoo, either the number of stages wasexcessive, or the modelling/simulationwere poor, or both. In three other cases,coking was produced by either a mislead-ingly low coil outlet temperature signalthat caused excessive firing, or a faultylevel measurement on the overflash

chimney tray that caused entrainmentinto the wash bed. Three cases werereported where maldistribution of vapouror liquid led to coking.

Intermediate draw malfunctions is aclose third of the most common vacuumtower malfunctions, with 17 reportedcase histories (Table 5). Foremost is leak-age from total draw chimney trays. Leak-age of the HVGO chimney tray representsgood distillate degraded into resid withno beneficial effects whatsoever, as leak-ing liquid poorly distributes in the washbed and does little washing. Leakage ofLVGO into the HVGO section lowers theHVGO boiling point and can reduce heattransfer, even limiting vacuum on thetower.

This leakage needs to be avoided withtotally seal-welded chimney trays. Specialtechniques, as recommended by Lieber-man, are effective and need to be incor-porated to avoid tray buckling due tothermal expansion [Lieberman N P, ProcessDesign for Reliable Operation, 2nd ed; Gulf Pub-lishing, Houston, Texas, 1988].

Level measurement on chimney trayshas been troublesome in three reportedcases. This may lead to overflow orentrainment. While overflow is equiva-lent to leakage, entrainment from theoverflash chimney tray can induce cok-ing. Leaking trapout trays were reported

to be troublesome in three cases. Withtrapout trays being used only in tray tow-ers, this issue is experienced mainly invacuum lube towers that have trays andnot packing. The remaining case historiesdescribe coking and excessive hydraulicgradients.

With 10 case histories, misleading mea-surements are in the 4th spot in Table 2.Three of these 10 are the troublesomechimney tray level measurements previ-ously mentioned. Other troublesomecases have been reported with short coiloutlet thermocouples (two cases), ambi-ent changes affecting vacuum measure-ments with ordinary gauges (two cases),bottom level, heater fuel flow rate, andreflux to a packed bed distributor. Thelessons from these case histories to instru-ment specifications are self-explanatory.

With nine case histories, plugging (asdistinct from coking) is in the 5th spot inTable 2. Of the nine cases, five were plug-ging of spray headers. One case wasreported of plugged packing, pluggedquench pipe, plugged instrument lineand plugged ejector. In two cases, theplugging was by corrosion products. Mea-sures found effective for alleviating plug-ging in the wash spray headers, which isone of the most common troublespots,are to provide good wash oil filtration andto specify an all-stainless-steel wash oil

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No. Description Cases1 Excess stages and vaporisation 4– Poor modelling and simulation 43 Insufficient wash, reason not

reported 3

– Misleading measurement 3– Liquid, vapour maldistribution 3

Not reported 4

Table 4

Causes of coking in vacuum fractionators

line downstream of the wash oil filter.Sharing the 5th spot in Table 2 is installa-tion mishaps. Three case histories wereassociated with spray header installation,and another three with grid or packingassembly. Other cases describe problemswith tower out-of-roundness, using car-bon steel bolts where stainless steel wasspecified and poor installation of strip-ping trays.

Also sharing the 5th spot in Table 2 isabnormal operation incidents. Five ofthese describe incidents during startupwhere a pocket of water entered the hottower and created a pressure surge. Poorblinding/unblinding contributed two casehistories, one case is related to pressur-ing/depressuring and one to flushing.

Maldistribution problems, other thanthose attributed to coking, plugging ordamage, are in the 8th spot in Table 2with a surprisingly low number of casehistories (six). Of the reported six, fourwere vapour maldistribution, three ofthese originating in the flash zone andone in the previously mentioned traychimney. The other two cases reportedliquid maldistribution problems. Fluor’sexperience has been that maldistribution,especially of vapour from the flash zone,has been far more troublesome than sug-gested by the low spot of this item inTable 2.

Vapour horn design and good distribu-tion of liquid to the wash bed are centralfor achieving trouble-free performance ofthe wash bed. An expert hydraulic analy-sis, often with the aid of computational

fluid dynamics (CFD) is essential. Improv-ing the vapour horn design has eliminat-ed operating problems and improved theperformance of several vacuum towers.

A surprisingly high number of casehistories place tray weeping in the equal8th spot in Table 2. This is not an issuewith most vacuum towers that are allpacked, but appears to be a major issuewith the trayed vacuum towers, mostlyin lube service. In two of these, leakageat a draw tray made it impossible to drawsufficient product. In two others, weep-age at pumparound trays starved thepump and reduced heat transfer. In twomore, poor separation between side-cutsresulted. Blanking valves, using highturndown valves, and in the case of theside cuts, drawing from seal-weldedchimney trays, was the solution.

Condenser issues complete Table 2.Condenser problems raise pressure in thetower and thus reduce distillate recovery.Two of the four cases reported dealt withexcess lights, one with ejector plugging,and one with flash equilibrium at the pre-condenser.

Crude tower malfunctionsThe three most common atmosphericcrude tower malfunctions (Table 6) areplugging, intermediate draw malfunc-tions and damage.

There were nine plugging incidentsreported in atmospheric crude towers:Four in the wash section or gasoilpumparound, three in the top section ortop pumparound, and two in the strip-ping section. No cases of plugging werereported in the middle of the tower. In thewash section, the most common cause ofplugging was entrainment from the flashzone or from the vapour overhead of apreflash drum. In the top of the tower, theplugging was by scale and corrosion prod-ucts, corrosion inhibitors, and salting out.Five of the nine incidents resulted inplugged trays, two in plugged downcom-ers. In one case, a packed bed plugged, inanother, a liquid distributor to the pack-ing plugged.

The large number of intermediate drawincidents is well in line with Fluor’s expe-rience: chimney trays and downcomertrapouts make or break fractionators. Ofthe nine, seven took place with down-comer trapouts, two with chimney trays.Four of these involved choking or restric-tion in the outlet liquid line, while in twoothers, leakage at the drawoff restrictedthe recovery of a side cut.

Four of the nine damage incidentsreported were due to water-induced pres-sure surges. Two of these were caused byundrained stripping steam lines, one by awater pocket in a spare pump, and one byplugged drainholes in the bottom sealpan. One case of damage resulted fromexposing column internals to cold water

and air premature upon shutdown,another from a packing fire at theturnaround. The cause of damage in theother three cases was not reported.

Seven abnormal operation (startup/shutdown/commissioning) incidentswere reported. Four of these resulted infour of the damage incidents listed, oneled to an explosion, another to a fire andone to a chemical release. Three of thepressure surges listed in Table 6 underDamage resulted from poor dehydrationduring startup or pump switchover. Poorblinding and unblinding led to onereported case of explosion and another ofchemicals release.

The next four entries in Table 6 are wellbelow the top four, and have three tofour reported malfunctions. Four installa-tion mishaps were reported, all involvingtrays or chimney trays. Leaks of apumparound exchanger, a pump seal,and resid to atmosphere were the threereported leak cases. Controlling liquidflow to the wash section has been a spe-cial challenge, contributing three morecase histories.

Trouble-free designs properly dis-entrain the vapour in the flash zone andpreflash drum and properly desalt thecrude in order to minimise plugging inthe fractionator. Specifying fouling-resis-tant hardware in the wash zone andupper trays is good practice. Downcomertrapouts and chimney trays are the mostimportant internals for ensuring trouble-free operation. They need to be designedand inspected carefully, not just left to

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No. Description Cases1 Plugging 9– Abnormal operation 93 Liquid maldistribution to

packing liquid 6

4 Intermediate draws 55 Water-induced pressure surges 46 Pressure control 3– Vapour maldistribution 3

Table 7

Top causes of FCC main fractionator malfunctions

No. Description Cases1 Control 102 Vapour cloud release 5– Installation Mishaps 54 Feed arrangement, tray towers 4– Reboiler draw arrangements 4

Table 8

Top causes of malfunctions indebutanisers, incl stabilisers and

depentanisers

No. Description Cases1 Leaking total draw chimney trays 62 Level measurement on total draw

chimney trays 3

– Leaking trapout tray 34 Chimney tray coking 2– Excessive hydraulic gradients on

chimney trays 2

6 Others 1

Table 5

Intermediate draw malfunctionsin vacuum towers

No. Description Cases1 Plugging 9– Intermediate draws 9– Damage 94 Abnormal operations 75 Installation mishaps 46 Condenser Problems 3– Poor control of wash 3– Leaks 3

Table 6

Top causes of atmospheric crudetower malfunctions

others, as these will make or break thefractionator.

Prevention of water entry, by ensuringadequate drainage on stripping steam linesand eliminating dead pockets inside thetower, is central for damage prevention.Proper inspection of equipment is a mustto prevent the installation mishaps. Propercontrol of the liquid flow rate to the washsection is the prime control considerationin the tower.

Main column malfunctionsThere are some similarities with regard toFCC main fractionator malfunctions(Table 7) and atmospheric crude towermalfunctions, but there are also majordifferences. Two malfunctions top thelist: plugging and abnormal operation

incidents.There were nine plugging inci-dents reported in FCC main fractiona-tors. Of these, four were salting-outincidents that plugged trays near the topof the tower, and were overcome byonline water washes. Three were inci-dents in which grid in the slurry sectioncoked up due to vapour or liquid-maldis-tribution. One incident was plugging dueto catalyst carryover into upper packedsections, and another was plugging of aline draining the main feed line, bothduring startup.

There were also nine abnormal opera-tion (startup/shutdown/commissioning)incidents reported, two of which werepreviously described. These two inci-dents, plus two others, occurred duringliquid circulation and dehydration. Inthree of these four, a pressure surge andmajor damage resulted, the other was thecatalyst carryover. The remaining fiveincidents include poor unblinding caus-ing a toxic release; switching over oxygenand nitrogen purge gas causing explo-sions; trip failure on the reflux drum caus-ing liquid carryover and majorcompressor damage; a major leak due tothe thermal shock while opening or clos-ing the valve in the tower inlet; and astartup pressure control problem resultingfrom steam condensation.

Packings are used more frequently in

FCC main fractionators than in atmo-spheric crude fractionators, so it comes aslittle surprise to find liquid maldistribu-tion to packings in a prominent spot inTable 7. Of the six reported incidents, twoinvolved liquid maldistribution to theslurry pumparound section, the others tovarious fractionation sections. Intermedi-ate draws in FCC main fractionator havebeen troublesome in five reported casehistories, more in chimney trays than indowncomer trapouts. Finally, four casesof water-induced pressure surges werereported, three of which led to majordamage.

Two other malfunctions are also shownin Table 7: Vapour maldistribution, allcases dealing with grid in the slurry sec-

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No. Description Cases

1 Reboiler draw and return arrangements 6

2 Excessive tower base level 4– Control 4– Component accumulation 4– Side draw arrangements 4

Table 9

Top causes of malfunctions indeethanisers (absorbers and

strippers)

No. Description Case1 Reboiler draw and return

arrangements 7

2 Tower flooding by excess base level 6

3 Vapour cloud release 5

Table 10

Top causes of malfunctions inC3/C4 splitters and depropanisers

(excl those in alky units)

tion, and pressure control. Plugging andcoking can be alleviated by providing ade-quate on-line wash facilities near the topof the fractionator, by using plugging-resistant trays there, and by preferringshed decks or disk and donut trays to gridin the wash section. Shed decks and diskand donut trays are far less sensitive tovapour or liquid maldistribution thangrid, and therefore far less prone to cok-ing during upsets.

Startups and shutdowns are majorissues in FCC main fractionators, and agood design of these needs to hazop whatcan go wrong and take preventive mea-sures. Intermediate draws and liquid dis-tributors are the weakest links in theinternals design, and need to be designedand inspected carefully, not just left toothers. Finally, pressure controls as well asliquid flow control to the wash sectionare major considerations in these frac-tionators.

Debutaniser malfunctionsDue to similar functions, stabilisers anddepentenisers have been lumped togetherwith debutanisers. Over 70% of the cases,however, were contributed by debutanis-ers.

Table 8 shows that the most commonmalfunctions experienced in debutanisersare widely different from those experi-enced in the vacuum, crude and FCC frac-tionators. Topping the list with 10 casehistories is controls, an item that showedlow down (if at all) on the main fraction-ator malfunctions list. Of the 10 cases, fivereported difficulties with pressure and con-denser controls. In all five, a total con-denser was used with partial flooding ofthe condenser. In two of the five, the prob-lem was induced by presence of non-con-densables. Composition control or theassembly of a control system contributedthe other control case histories.

Vapour cloud release and installationmishaps share the second spot in Table 8.Three of the five case histories of vapourclouds ended in explosions, and one morein a fire. Some of these were accompaniedby injuries and heavy damage. Line frac-ture (two incidents), poor blinding (twoincidents), and freeze-ups in leakingvalves (two incidents) were some of thecontributing factors. Hazops of debutanis-ers should consider some of the lessonslearned from previous vapour cloudreleases to positively eliminate furtheraccidents.

With four case histories, poor feedarrangements closely follow, leading to acapacity bottleneck or an efficiency loss inthe feed region. Also with four case historiesare reboiler draw arrangements, includingvapour entrainment choking the reboilerdraw lines and liquid leaking from a trapouttray to a once-through thermosiphonreboiler, thus “starving” the reboiler.

There is a distinct link between thefeeds and reboiler draw arrangements.Both constitute “points of transition”, ie,where a stream enters or leaves the tower.These points of transition are some of themajor troublespots in a tower. The lessonfor debutanisers is that all points of transi-tion need to be critically examined forpotential bottlenecks, both at the design(or debottleneck) and when trouble-shooting.

Deethaniser malfunctionsThe strippers and absorbers are included inthe deethaniser malfunctions. Topping thelist (Table 9) with six case histories arereboiler draw and return arrangements.Three of the six cases report excessive pres-sure drop in the process inlet or outletpipes of a kettle reboiler. The high-pressuredrop either caused the tower base liquidlevel to rise above the reboiler return inlet,or back liquid up on the chimney tray feed-ing the reboiler to the top of the chimneys.

Insufficient heat during coke drumswitchover was reported in two cases, oneof them due to weeping from the draw trayto a once-through thermosiphon reboiler.Four case histories were reported of baselevel exceeding the reboiler return. Two ofthese were due to high-pressure drop inthe kettle piping (those previously men-tioned), the other two due to absence of orto poor level indication. As with debu-tanisers, control issues are also importantin deethanisers, and account for four casehistories. Also, with four case histories,component accumulation in deethanisersis a problem.

Either ethane or water or both accumu-late and can lead to cycling, capacity bot-tlenecks, and in the case of water, alsocorrosion. Finally, choking of side drawswith entrained gas bubbles has been aproblem in four case histories.

The lessons learned from this documen-tation are that the points of transition indeethanisers (the side draws as well as theregion below the bottom tray, includingthe reboiler draw and return lines) requirethorough design, review, and inspection,and must not just be left to others. Preven-tion of component accumulation and care-ful review of the control systems are alsoprime considerations that make the differ-ence between a troublesome and trouble-free deethaniser.

Splitter malfunctionsMalfunctions in C3/C4 splitters also includedepropanisers other than those in alkyunits, which are uniquely different (Table10). Similar to deethanisers, reboiler drawand return arrangements lead the list withseven reported cases. Again, the main prob-lems have been excess pressure drop ininlet and outlet lines of a reboiler causingbase liquid level to exceed the reboilerreturn inlet (two cases); leaking draw tray

or pan causing liquid to bypass a once-through thermosiphon reboiler (twocases); a reboiler tube leak and slug flow atthe reboiler outlet pipe.

Slightly behind, with six case histories, istower flooding by excess base level. Two ofthese resulted from the type of reboilerproblems previously discussed. False levelindica tions led to two others, and frothingor foaming at the tower base led to theremaining two. Clearly, the lessons learnedare that troubleshooting and trouble-freedebottlenecks of C3/C4 splitters should focuson the reboiler piping and the bottomsump.

Similar to debutanisers, depropanisersand C3/C4 splitters have experienced a highnumber of vapour cloud releases, mostlydue to line rupture (three cases), but alsodue to poor blinding or plugging/freeze upsof valves. Some major blasts resulted. Thevapour cloud lessons described underdebutanisers extend to depropanisers andC3/C4 splitters.

Other fractionatorsFor other refinery fractionators, the num-ber of case studies reported was less than20, a sample too small for a detailed anal-ysis. Nonetheless, some observations aresignificant and require more detail, includ-ing coker main fractionators, alky unitmain fractionators/isostrippers, naphthasplitters, deisobutanisers and amineabsorbers/regenerators.

A total of 15 malfunction case historiesof coker fractionators have been report-ed. Of the 15, seven described fouling bycoking or carryover of coke, while fiveothers described damage due to water-induced pressure surges. There is nodoubt that coking and water-inducedpressure surges are the major issues withthese fractionators.

A total of 17 malfunctions have beenreported for alky unit mainfractionators/isostrippers. Of these, fourdescribed plugging, mostly by scale or cor-rosion products; three described explo-sions, either due to vapour cloud release ordue to HF carryover in the hydrocarbonsand a violent reaction in a caustic beddownstream; and three others describedaccumulation of either ethane or water inthe overhead system.

A total of 11 naphtha splitter malfunc-tions have been reported. Four of thesereported plugging, mainly by scale and cor-rosion products; three reported reboilerissues; two were a result of poor installa-tion; and two reported control problems.Control problems with other refineryfractionators have also been reported. Forexample, of the eight total deisobutanisermalfunctions that have been reported, sixinvolved control problems. Five of thesewere temperature control issues that canbe particularly troublesome with narrow-boiling mixtures.


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Eight case histories were specificallyreported for refinery amine absorbers. Ofthese, six reported foaming, and threereported scale and corrosion productscatalysing the foam and causing plugging.Seven other case histories of amineabsorbers and/or regenerators were report-ed without stating whether they camefrom refineries or natural gas plants. Ofthese, foaming was the issue in five. Thereis no doubt that foaming, and to a lesserdegree fouling with scale and and corro-sion products, is the prime issue in aminetowers.

Lessons learnedThe vacuum tower is by far the most trou-blesome refinery fractionator. Damage,wash bed coking, and intermediate drawsare prime trouble spots. Water-inducedpressure surges are the leading cause ofdamage. Most of the damage incidents inthe vacuum towers are preventable.Hazoping the possibility of water entry,using heavy duty mechanical designs,inspecting, testing and correctly design-ing spray headers, paying attention tolevel measurement, and applying specialdesigns and procedures to prevent pack-ing fires can drastically reduce damageincidents.

Wash bed coking can be alleviated byusing short beds of relatively inefficientpacking by correctly simulating the washzone and by avoiding insufficient wash.Intermediate draw malfunctions can bealleviated by seal-welding draw trays andwater-testing them at the turnaround andby paying attention to reliable level mea-surement on these trays. Other measuresthat promote trouble-free operation aregood instrument specifications, good fil-tration of the wash oil followed by stain-less steel piping downstream of the filters,and good distribution of vapour and ofthe wash oil to the wash bed.

Plugging, intermediate draws, damageand abnormal operation incidents are themost troublesome malfunctions in atmo-spheric crude towers. Plugging is mostcommon near the tower top, at the washzone, or in the stripping section. Chokingand leakage are the prime intermediatedraw issues. Water-induced pressuresurges are the most common cause ofdamage. Plugging problems can be allevi-ated by eliminating the source of foulingand/or by using plugging-resistant inter-nals. Downcomer trapouts and chimneytrays need to be designed and inspectedcarefully, not just left to others.

Prevention of water entry by properdraining of stripping steam lines and bysound dehydration procedures at startupis critical. Proper inspection of equip-ment and adequate control of liquidflowrate to the wash section are alsoimportant for promoting trouble-freeoperation.

Plugging, abnormal operation issues,liquid maldistribution to packings andgrid, intermediate draws and water-induced pressure surges are the key trou-ble spots in FCC main fractionators. Themain causes of plugging in these fraction-ators are salting out near the top and cok-ing of grid beds in the slurry section.These can be alleviated by online waterwashes and by avoiding grid in the slurrysection, respectively. Tower dehydrationand liquid circulation are important start-up operations that can turn troublesomeand lead to pressure surges or catalyst car-ryover into the less fouling-resistantregions.

Control issues, especially pressure con-trols, are the primary source of problemsin debutanisers. Vapour cloud releaseshave led to explosions and major damagein debutanisers. Installation mishaps,tower feed entry arrangements, andreboiler draw arrangements have alsobeen major trouble spots. Critical reviewof the control system, especially the pres-sure/ condenser controls, learning frompast vapour-cloud accidents, usinghazops to minimise the possibility ofvapour cloud releases, and sound designof feed entry piping and of tower basearrangements are the key for trouble-freedebutanisers.

In deethanisers, the points of transi-tion (the side draw arrangements and theregion below the bottom tray, includingthe reboiler draw and return lines) areprime trouble spots and are key to trou-ble-free design and operation. Preventionof water and ethane accumulation in thistower is also important.

Reboiler draw and return arrange-ments, and tower base level are the key totrouble-free depropanisers and C3/C4

splitters. Depropanisers also are prone tovapour cloud releases, and lessonslearned from past vapour cloud incidentsshould be incorporated in the design andoperation of depropanisers and C3/C4

splitters.Trouble-free operation of coker frac-

tionators focuses on preventing cokingand water-induced pressure surges; ofalky main fractionators focuses on plug-ging, vapour cloud and component accu-mulation prevention; of deisobutaniserson composition control; and of amineabsorbers and regenerators on foamingand plugging prevention.

Henry Z Kister is a Fluor Corporation fellow and director of fractionation technology at Aliso Viejo, California, USA.He has over 25 years’ experience in design,control and startup of frationation processesand equipment. He obtained his BE and MEdegrees from the University of New SouthWales, Australia.

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distillation

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