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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2007; 2: 294307Published online 22 August 2007 in Wiley InterScience(www.interscience.wiley.com) DOI:10.1002/apj.023

Research Article

Optimize distillation column design for improved reliabilityin operation and maintenance

Karl Kolmetz,1

* Wai Kiong Ng,1

Siang Hua Lee,1

Tau Yee Lim,1

Daniel R. Summers2

and Cyron Anthony Soyza3

1Sulzer Chemtech, Regional Headquarters, 10 Benoi Sector, SG-629845, Singapore2Sulzer Chemtech, 4019 S. Jackson Avenue, US-Tulsa, OK 74107, USA3Phoenix Training and Development Centre, Johor Bahru, Malaysia

Received 24 June 2005; Accepted 20 September 2006

ABSTRACT: A distillation tower design is normally made in two steps; a process design, followed by a mechanical

design. The purpose of the process design is to calculate the required stream flows and number of required theoretical

stages. Required steam flows could include reflux rate, side draws, and the heat duties (number of pump arounds and

the condenser and reboiler). The purpose of the mechanical design is to select the tower internals, column diameter

and height.

The process and mechanical designs can be completed very quickly utilizing cook book procedures that manyEngineering Procurement and Construction (EPC) firms have established. Often the cook book designs can be

optimized for improved profitability, operations and maintenance.

The best way to review profitability is the life cycle cost, which is the initial capital cost of the plant along with the

first 10 years operating and maintenance cost. The life cycle cost includes a reliability factor, which is very important

in designing any process plant equipment. Improved reliability has a very large impact on return on investment (ROI).

Several factors should be considered when designing distillation equipment;

1. Correct distillation equipment for process conditions

2. Correct equipment selection for expected run length

3. Correct process control strategy to achieve stable operations

4. Fouling/corrosion/polymerization potential

5. Thermal stability, chemical stability and safety

6. Maintenance reliability, accessibility and simplicity of repair

7. Evaluation of the most cost effective solution for minimum life cycle cost

This review will include general distillation design guidelines applicable to any process along with specifics for the

natural gas processing, refining, petrochemicals, and the oleo chemicals industries. 2007 Curtin University of

Technology and John Wiley & Sons, Ltd.

KEYWORDS: distillation column; maintenance reliability

INTRODUCTION

There are many separation processes and each one has

its best application. They include distillation, crystal-lization, membrane, and fixed bed absorption systems.Occasionally the best system may be a combination ofthese systems.

The choice of the best application should be based onthe life cycle cost. The life cycle cost is the initial capital

*Correspondence to: Karl Kolmetz, Sulzer Chemtech Pte Ltd.,Regional Headquarters, 10 Benoi Sector, SG-629845, Singapore.E-mail: [email protected] for, 2nd Best Practices in Process Plant ManagementNikko Hotel, Kuala Lumpur, Malaysia March 1415, 2005.

cost of the plant along with the first 10 years operating

and maintenance cost. The life cycle cost should include

a reliability factor, which is very important in designing

any process plant equipment, reactors or separation

equipment. Improved reliability has a very large impact

on return on investment (ROI). Many life cycle cost

only review energy, but not solvent, adsorbent, or

catalyst cost because of accounting rules and this can

lead to skewed economic decisions.

Distillation may be the most economical and is the

most utilized globally to obtain improved purity prod-

ucts. Distillation is the separation of key components

by the difference in their relative volatility, or boiling

2007 Curtin University of Technology and John Wiley & Sons, Ltd.


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Asia-Pacific Journal of Chemical Engineering OPTIMIZE DISTILLATION COLUMN DESIGN FOR IMPROVED RELIABILITY 29

points. It can also be called fractional distillation or frac-tionation. Distillation is favored over other separationtechniques such as crystallization, membranes or fixedbed systems when;

1. The relative volatility is greater that 1.2,2. Products are thermally stable,3. Large rates are desired,

4. No extreme corrosion, precipitation or sedimentationissues are present,5. No explosion issues are present,6. Low scale up cost factors capacity can be doubled

for about 1.5 additional cost,7. Suitable for heat integration.

THE CORRECT DISTILLATION EQUIPMENTFOR THE PROCESS CONDITIONS

There are many types of processes that are groupedtogether and called distillation. Most have similarities,but some have noticeable differences. A partial list ofthe distillation grouping includes;

1. Distillation2. Absorption3. Stripping4. Extractive distillation5. Reactive distillation6. Azeotropic distillation7. Batch distillation

There are several choices of distillation equipment foreach of these operations. The choice of which to uti-

lize depends on the (1) pressure, (2) fouling potential,(3) liquid to vapor density ratio, (4) liquid loading, andmost important (5) life cycle cost. Distillation equip-ment includes many categories of equipment. The twomajor categories are trays and packing, but each of thesecategories has many divisions.

Tray divisions include;

1. Baffle trays2. Dual flow trays3. Convential trays4. High capacity trays

5. Multiple downcomer trays6. System limit trays

Packing divisions include;

1. Grid packing2. Random packing3. Convential structured packing4. High capacity structured packing

There are both process and economic argumentsfor the best choices in equipment selection. Typicallystructured packing is better than random packing for

fouling service because it has no horizontal surfaces,but if the process has high maintenance concerns,random packing may be chosen to reduce the life cyclecost. An example of this is caustic towers in Ethyleneplants.

General rules of thumb

The first general rule of thumb is to review the commonindustry practice for your particular process. This willgive you a guide in which to start your selection process,but in a competitive environment the lowest initial costmay be the most widely utilized, but may not be the bestoverall option. Rules of thumb will have exceptions andmay only apply about 90% of the time.

Packing rules of thumbPacking should be utilized when;

1. Compounds are temperature sensitive2. Pressure drop is important (vacuum service)3. Liquid loads are low4. Towers are small in diameter5. Highly corrosive service (use plastic or carbon)6. The system is foaming7. The ratio of tower diameter to random packing size

is greater than 10.

Tray rules of thumbTrays should be utilized when;

1. Compounds containing solids or foulants

2. Many internal transitions3. Liquid loads are high4. Lack of experience in the service5. Vessel wall needs periodic inspection6. Multiple liquid phases including water

Tower rules of thumb1. Maintain 1.2 m at the top for vapor disengagement,2. Maintain 2 m at the bottom for liquid level and

reboiler return,3. Limit tower heights to 60 m because of wind load and

foundation concerns,

4. The length to diameter ratio should be less than 30,5. Reflux drums should be horizontal with a liquidresidence time of 10 min.,

6. Gas/liquid separators are vertical,7. If the reflux drum has a second liquid phase, such as

water, the second phase should have a linear velocityof 150 mm/sec and not smaller than 400 mm,

8. Utilize a water boot for small amounts of wateraccumulating in a reflux drum

9. Optimum pressure vessel length to diameter ratio is 310. Choose materials of construction to reduce corrosion

issues,

2007 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2007; 2: 294 307DOI: 10.1002/apj


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96 K. KOLMETZ ET AL. Asia-Pacific Journal of Chemical Engineering

11. Maximize operating flexibility for seasonal or marketconditions.

Pressure

Pressure normally has a large effect on the parametersof surface tension and density ratios. Density ratio is

the ratio difference between the vapor and the liquiddensities. Structured packing can be utilized if thedensity ratios are large. If the density ratio is below 50,a back mixing effect can occur, where the liquid carriesthe vapor downward. The resultant stage efficiency(HETP) in a packed column is lower than expected andtrays may be the most economical solution. Both packedand trayed columns have reduced capacity factors as thepressure increases.

Fouling potential

Designing mass transfer equipment for fouling servicerequires first an understanding of the fouling mech-anism, the process in which the fouling occurs, andbehavior of the process when the fouling is present.An understanding of these items needs to be developedin advance of designing mass transfer equipment forfouling service.

The challenges of operating fouling columns canresult in;

1. Increase energy consumption due to heat transfer andefficiency issues.

2. Reduced column capacity, which may lead to pro-duction loses.

3. Increased down time for cleaning and disposing offouling wastes

4. Potential need for the use of chemical additives

Vapor to liquid density ratio

When structured packing was first introduced, thevapor to liquid density ratio was not understood, andstructured packing was applied in areas of low vapor

to liquid density with unexpected results. In one casean Alky Unit DeIsoButanizer was revamped from traysto rings with less performance, the original trays werethen reinstalled. Several propylene and ethylene splitterswere revamped to structured packing, and then had traysre-installed.

Trayed column are also affected by the vapor toliquid density ratio. The down comer capacity is directlyaffected by the ability of the liquid vapor mixture abilityto separate into their respective phases. At low vaporto liquid density ratios this can be difficult if the downcomers are not sized properly.

Liquid loading

In low liquid loaded systems packing may be the bestchoice because of the mass transfer characteristics ofpacking. The mass transfer in packing applications takesplace on a thin film of liquid that is spread over thesurface area of the packing. If the liquid rate is highthis boundary layer will increase, reducing the mass

transfer. Trays should be considered by high liquidloaded applications.

In low liquid loaded systems trays can have highresidence times leading to undesired affects such asfouling, discoloration, polymerization, and sedimenta-tion. In addition trays in low liquid loaded systems havedifficulty maintaining a good weir loading and distribu-tion across the tray, resulting in lower than expectedtray efficiencies.

Life cycle cost

Life cycle cost should include total operating cost forthe first 10 years of operation. Accounting rules whichlist some items as capital cost and other items asoperating expense need to be totaled or a skewed lifecycle cost can be generated. A partial list would include;

1. Capital2. Catalyst3. Solvents4. Energy5. Maintenance6. Industry average on stream factor (95% 20 days

per year)

For distillation the largest life cycle cost wouldbe energy and maintenance concerns. Distillation istypically the single largest consumer of utilities in achemical plant or refinery, and also the largest producerof finished product in most facilities. For energy cost areview of tray and packing efficiencies is warranted. Formaintenance cost a review of reliability and simplicityis warranted.

CORRECT EQUIPMENT SELECTION FOR

EXPECTED RUN LENGTH

Hydrocarbon producers are exploring avenues to extendthe on-stream time between outages for maintenance.Key equipment that can determine the end of runincludes: catalyst life, cyclone erosion, and compressorand tower fouling. Critical equipment that has beenshown to be a limiting factor can be duplicated toextend run length: for example parallel pumps, reactorsand reboilers. This is a successful method to extendon-stream time, though it is expensive and in fact, attimes cost prohibitive. Incorporating design guidelines



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that increase the on-stream time of the key piecesof equipment is a better economic decision for mostplants.

Currently refiners are planning 4-year run lengths andethylene producers are getting greater than 5-year runlengths. These targets present challenges for distillationcolumn design. Potential problem areas include refin-ing vacuum wash oil beds, ethylene plant quench and

saturator towers, and butadiene and other polymer pro-ducing distillation columns. Each of these applicationshas some common characteristics. A review of success-ful and not so successful designs can help develop keydesign criteria. Design guidelines developed from suc-cessful and not so successful applications can improvethe on-stream time of each of the applications.

Industry review of tower incidents

One way to approach the expected run length issue is to

review the tower incidents that have been reported in theindustry. There are over 900 published cases of towerincidents in the literature. Attached is a list of towerincidents that was found in the literature. (1) Listed arethe top five issues in distillation malfunctions.

1. Fouling, plugging and coking issues2. Tower bottoms and reboiler return issues3. Packing liquid distributors issue4. Intermediate draws5. Assembly mishaps

Below is an explanation of each item.

1. Fouling, plugging and coking issuesa. Cokingb. Precipitation saltsc. Scale, corrosion productsd. Solids in feeds

Fouling, plugging and coking issues are typicallyfounda. Packing beds and distributorsb. Trays, active areas and down comersc. Draw linesd. Instrument lines

e. Feed lines

2. Tower bottoms and reboiler return issuesa. High liquid levelsb. Impingement by vapor inletsc. Vapor maldistributiond. Water induced pressure surgese. Leaking reboiler drawf. Gas entrainment in liquid bottoms

3. Packing liquid distributors issuesa. Distributor overflow

b. Pluggingc. Fabrication mishapsd. Feed entry problemse. Damagef. Poor hole patterng. Poor irrigation quality

4. Intermediate drawsa. Leakage at drawb. Restriction of vapor choking of draw linec. Plugging

5. Assembly mishapsa. Packing liquid distributorsb. Packing assemblyc. Tray panelsd. Internal misorientation at feeds and draws

Recent list of refinery fractionatormalfunctions

A recent list of refinery fractionator malfunctions wasdeveloped. (2) There are over 400 published cases ofrefinery tower incidents. They included;

1. Vacuum towers 862 Atmospheric crude towers 453. Debutanizer towers 374. FCC main fractionators 33

5. DeEthanizer towers 236. DePropanizer towers 227. Alky main fractionators 178 Coker main fractionators 159. Naphtha splitters 11

The main point here is there are plenty of publishedcases, and it is better to learn from others mistakes.The largest number of cases is for the vacuum tower.The top causes of vacuum tower malfunctions include;

1. Damage 272. Coking 213. Intermediate draws 174. Misleading measurements 105. Plugging 96. Installation mishaps 97. Abnormal operation (start up, shut down) 98. Maldistribution 69. Weeping 610. Condenser 4



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The top causes of damage in vacuum tower include

1. Water induced pressure surges 92. Insufficient mechanical strength 53. Broken nozzles or headers of spray distributors 44. High bottoms level 35. Packing fires 3

A lesson to be learned is that possibly one third of thecauses of damage in vacuum towers can be prevented bydesign and operating procedure that adequately preventwater from entering the tower. A joint design/operationshazard and operability review (HAZOP) should focuson the listed potential problem areas.

CORRECT PROCESS CONTROL STRATEGY TOACHIEVE STABLE OPERATIONS

Pressure control challenges

Because humans are less sensitive to pressure thantemperature, we measure pressure in large units. In theideal gas law PV =nRT, pressure is measured in unitsof 1 bar and temperature in units of degrees Kelvin,therefore temperature measures will be much moreaccurate than pressure measurements. Control strategiesthat rely strongly on pressure will be less stable thanthose that rely on temperature.

Of the 37 listed DeButanizer malfunctions, the mostcommon malfunctions are widely different from those in

vacuum, crude and FCC fractionators. Ten of the thirty-seven were in process control, and five of them werewith pressure and condenser controls. The challenge ofDeButanizer condenser is with the noncondensables thatthe previous towers might not totally remove.

Level instrumentation challenges

Level instrumentation is much more difficult than manypeople perceive. Acceptable industry standard methodshave greater than 10% inaccurateness. This is due to

density differences in the tower bottoms and the levelleg or sight glass. The tower bottoms will be frothy andat a higher temperate than the level leg. Because theprinciple of level measurement is Bernoullis Equation(density times gravitational force times height) thedensity has a direct effect on the measurement. Thedensity is a function of the temperature and the frothaeration, both of which are reduced in the sight glassand level leg. For a hot system the level in the towercan be as much as 10% higher, and for a cycrogenicsystem the level can be lower than the sight glass dueto the temperature effect.

Olefins unit application example

An example of the phenomena by one of the authorswas in an ethylene furnace steam drum. Because thesteam drum has a low level shutdown, which also shutdown the furnace, the operations group wanted to runthe drum at a high liquid level to allow increased theoperator response time. Operations decided to keep the

drum level at 80%.This drum operated at a 100 bar system pressure

which has resulted in very high temperatures. Opera-tions noted that there was a loss of efficiency in thesteam turbines that utilized the high pressure steam.Tests were run to determine the carry over of the steamdrums by measuring the sodium levels in the steam. Itwas determined by the sodium test, that the drums werefull at 80% as verified by the photographs (Fig. 1) ofthe tide marks in the steam drums at the next down turn.The steam drum level was lowered to a measured 65%to reduce liquid carryover.

Refinery unit application example

It is not unusual for operation to run a piece ofequipment at higher levels if there is a low levelshutdown, or if the process feeds a multistage pump.Caution needs to be taken and this phenomena need tobe understood or a tower reboiler return can be blowinginto the liquid level, resulting in entrainment to the firsttray. If there is a steam sparger in the tower bottoms asfound in refinery atmospheric crude towers, care must

be taken to insure the sparger is above the liquid level.In one example a refinery atmospheric crude tower

was revamped and the steam sparger was lowered.During crude feed changes the tower bottoms level canbe higher than normal resulting in the steam sparger

Figure 1. Picture of water mark on the steamdrum. This figure is available in colour online atwww.apjChemEng.com.



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Figure 2. Steam Turbine Fouling. This figure is available incolour online at www.apjChemEng.com.

being beneath the bottoms level. When this occurred,the diesel draw became dark, resulting in lost product.

FOULING/CORROSION/POLYMERIZATIONPOTENTIAL

The most suitable mass transfer equipment for foulingservice may also be the least efficient for mass transfer.Grid packing and shed decks can handle nearly everyknown fouling service, but they have low efficiencies

when compared to sieve trays random, and structuredpackings (Fig. 2).

Packing

For packed towers the key fouling factors revolvearound liquid distribution and packing residence time.The longer the residence times the less suitable. Low-pressure drop, smooth surface, low residence timepackings perform best in fouling service. The order ofpreference is:

1. Grid2. Structured packing3. Random packing

Packing distributor concerns

In fouling service, distributors are areas where residencetime is increased and fouling phenomena can occur.In high-fouling services trough v-notch or other type

Figure 3. Example of Trays in Fouling Service. This figure isavailable in colour online at www.apjChemEng.com.

of trough distributors are recommended over pan typedistributors.

TraysThe industry prefers trays in fouling service because ofthe long history of success trays have had in foulingservice applications. The first continuous distillationcolumn with bubble cap trays was developed in 1813and structured packing was developed in 1964. Thedatabase and application know how is much larger withtrays. The best trays to use in fouling services are dualflow trays and large fixed opening devices (Fig. 3).Moveable valve trays are less resistant to foulingbecause the valves are areas where a polymer can seed

and propagate. Solids can pack in small crevices aroundmovable valves making them immovable.

Dual flow trays

Dual flow trays are the trays of preference for heavyfouling services, but have low stage efficiency. Dualflow trays have no down comers, where products offouling phenomena can accumulate. Stagnation in adown comer, or even on a tray deck, due to back mixing,can result in polymer formation.

The vapor and liquid transfers up and down thecolumn thru the holes on the tray deck. This is anadvantage if the fouling is in the vapor state as the underside of the tray is continually washed. The continuousagitation of the liquid on the topside of the trayscombined with continuous underside wetting/washingaction makes this tray suitable for fouling services. Thechallenge of the dual flow tray is maldistribution inlarger diameter towers.

Two types of dual flow trays are available; standarddeck and rippled deck. The standard deck has is aflat plate, and the rippled deck has sinusoidal waves.



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Levelness is of extreme importance to dual flow traysbecause the liquid will migrate to the low level onthe tray and start channeling. Dual flow trays have atendency for harmonic tray vibrations; rippled deckshave an excellent record in fouling service except forone recent documented case.

Fixed opening trays

If mass transfer efficiency and fouling resistance areboth needed, then a fixed opening tray such as an SVGis the preferred choice. This fixed opening device is araised opening on the try deck that is sufficiently largeto allow vapor to keep the tray deck non fouled, whileproviding higher stage efficiency.

Specially chemical application example

Methyl-Meth-Acrylate (MMA) is polymerized into Poly

MMA, which is sometime called acrylates; clear plasticsheets sometimes used a glass substitute, nail polishand floor wax. In the manufacture of MMA the towersnormally require shutting down about every 6 monthsfor cleaning.

In distillation service where there is a high probabil-ity of polymerization, like MMA, dual flow trays maybe the trays of choice. The challenge of dual flow traysis maldistribution, the vapor traveling up one side ofthe column and the liquid down the opposite side. In awindstorm the top of a column can move as much as6 inches, and build a hydraulic instability within the col-

umn, which a dual flow tray cannot correct within itself.

THERMAL STABILITY, CHEMICAL STABILITYAND SAFETY

There are several incident of thermal stability, chemicalstability and safety incidents that need review

Thermal stability

Thermal stability is an issue when dealing with many

speciality chemicals. The need to reduce the tower bot-toms temperature to reduce degradation or polymer-ization can shift the process design toward packing,falling film reboilers and special over head condensersto reduce the tower pressure drop.

Specialty chemical example ethanolaminedistillation general overview

The reaction of ethylene oxide with ammonia rendersa mixture of mono-, di-, and tri-ethanol amines. The

maximum production of mono-ethanolamine from thereactor is typically 70% (Fig. 4). Beyond this maximumrestriction on mono-ethanolamine, the plant may bedesigned for a wide range in product distribution. Thismeans that the plant has very high degree of flexibil-ity and production may be adapted to changing marketdemands.

Ethanolamine distillation general overview

Column top operating pressure for the di- and tri-ethanolamine distillation is typically 1 to 2 mbar andcolumn bottom operating pressure is in the range of10 to 12 mbar. To achieve this low-pressure drop andstill retain high separation efficiency at typically verylow specific liquid loads, wire gauze structured packingis usually specified. Liquid loads can be as low as0.2 m3/m2 hr (Fig. 5).

Low pressure drop gauze packings in distillation

columns create the lowest possible operating temper-ature, preventing deterioration of product quality, whilereducing column shell diameter. The high separationefficiency, leading to;

1. Low energy consumption through reduced refluxrates

2. High product purity, reduced column height3. No organic wastes from the products of polymer-

ization.

Special design of the top condenser provides ex-tremely low-pressure drop of vacuum distillation.

Falling film reboilers permit use the of low steamtemperature, avoiding product quality deterioration andlosses as compared to a high heat flux system.

Chemical stability

In several applications a small amount of the feedstream can accumulate in a distillation column and have

Figure 4. Ethanolamine production. This figure is availablein colour online at www.apjChemEng.com.



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Figure 5. Distillation tower. This figure is available in colouronline at www.apjChemEng.com.

chemical stability issues. In an ethylene plant propylenesplitter tower, Methyl acetylene and propadiene canconcentrate in a section of the distillation tower belowthe feed. At high concentrations, above 40%, thisproduct can auto decompose resulting in large pressureincreases with potential damage to the equipment.

In butadiene plants a small amount of vinyl acety-lene is always present. Extractive distillation is typicallyused to recover the valuable 1, 3 butadiene. If not oper-ated correctly, the vinyl acetylene can accumulated toan auto ignition level resulting in pressure vessel failureand consequential damages.

Column safety

One issue for column safety is packing fires. Packinghas been known to ignite and burn when the tower isopened for maintenance. FRI and others have producedguidelines for reducing the likelihood of tower packingfires.

MAINTENANCE RELIABILITY, ACCESSIBILITYAND SIMPLICITY OF REPAIR

Maintenance reliability, accessibility and simplicity ofrepair issues many times are developed in actual fieldexperiences. The field experience is fed back to the traydesigner to incorporate best practices. This is an areawhere an experienced team can bring huge value to aprocess.

Maintenance reliability trayvibrations fatigue stress cracking

There is a phenomenon that occurs on tray devicesthat is quite unusual yet very destructive (Fig. 6).

Several sets of trays, including dual flow trays, haveexhibited a behavior of harmonic vibrations resultingin failure of the tray decks. Trends and observationsfrom failure of the tray decks due to harmonic vibrationsinclude;

Low loadings or turndown conditions Result in severely cracked & broken decks

Oscillate or Hum at 2040 cycles/second Will occur regardless of tray strength First noted by ICI 30+ years ago Diameters between 7 and 15

Usually effects 1-pass trays

There have been experiences with over eight appli-cations in the past 5 years where this phenomenon hasoccurred.

At normal operating conditions, a tray has a netupwards force put on it by the process Fluids. Any holeor Gap will normally result in vapor escaping ratherthan liquid leaking through the tray.

At Turndown however, the net fluid force may beNeutral. In this case, any disturbance can be magnifiedby the tray deck.

When these two forces are near to balancing thenvibration damage can possibly occur. Waddington statedit best in 1973, (The) vibration mechanism is dueto pressure pulsations generated in association withsynchronous bubble formation across a large part of thetray area. Increasing tray deck (Fig. 7) thickness and/orchanging material type has not eliminated this problem.However, only less damage was noted. After a longertime span, the trays still can tear themselves apart.

Adding extra beams, shear clips and truss lugs appearto have helped one column absorb oscillations. Thetheory behind this is that the trays natural frequencywas changed.

Figure 6. Cracked flow tray. This figure is available in colouronline at www.apjChemEng.com.



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Dry Tray Pressure Drop

Liquid Head

Tray

Froth

Figure 7. Tray deck. This figure is available in colour onlineat www.apjChemEng.com.

To avoid this phenomenon a Vibration Factor (V/Vc)is proposed and keeping V/Vc less than 0.8 or greaterthan 1.6, which has been shown to avoid the vibrationrange.

V =Hole Velocity

Vc =23(LV)HCL FP/(C(V0.5))

C =2.0 CH gCW/12.0

Most applications will have values much greater than1.6. This applies to both fixed & movable openingdevices. In other application increasing the tower load-ing will shift the tray operation away from the vibrationranges. If that is not possible, a second alternative wouldbe to replace the trays with reduced open area on thetray.

Demister pads

Demister pads (Fig. 8) are very easy to design andinstall, but tend to be high maintenance issue items.Typical entrainment removal of 99% can be obtainedwith 150 mm (6 inches) of mesh pads. There havebeen numerous failures in demister pad systems due topressure surges (Fig. 9). The pad may foul with materialand fail due to pressure drop increases.

EVALUATION OF THE MOST COST EFFECTIVESOLUTION FOR MINIMUM LIFE CYCLE COST

The best way to review profitability is the life cycle cost,which is the initial capital cost of plant along with thefirst 10 years operations and maintenance cost. The lifecycle cost includes a reliability factor, which is veryimportant in designing any process plant equipment.Improved reliability has a very large impact on ROI.

Life cycle cost should include total operating costfor the first 10 years of operation. Accounting rules

Figure 8. Typical new demister pad. This figure is availablein colour online at www.apjChemEng.com.

Figure 9. Typical demister pad issues. This figure is availablein colour online at www.apjChemEng.com.

which list some items as capital cost and other itemsas operating expense need to be totaled or a skewedlife cycle cost can be generated. A partial list wouldinclude;

1. Capital

2. Catalyst3. Solvents4. Energy5. Maintenance6. Industry average on stream factor (95% 20 days

per year)

For distillation the largest life cycle cost would beenergy and maintenance concerns.

Factors that increase life cycle costSeveral factors that increase life cycle cost include;



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1. The need for very high or very low temperatures,less that 40 C or greater than 250 C

2. Small concentrations of high boiling contaminantsmust be removed high energy.

3. High operating flexibility for seasonal or marketconditions

4. Low stage efficiency5. Exotic materials of construction

6. Low instrumentation reliability

In distillation the two large life cycle cost drivers arestage efficiency, which is actually energy usage andoperational flexibility.

Stage efficiencyThere are certain rules of thumb in distillation thatapply to stage efficiency behavior. Some of these are:

a. Increased pressure increases tray efficiencyb. Decreased pressure increase packing efficiency

c. Increased viscosity decreases tray and packing effi-ciency

d. Increased relative volatility decreases tray efficiency

Many things influence stage efficiency. The first andforemost is the type device employed for the service.Next is the system itself including the pressure, L/Vratio, relative volatility, and physical properties.

The choice of device is important from the viewpointof capacity, but many times a higher capacity device

will inherently have a lower level of efficiency perfor-mance. Generally, higher capacity devices exhibit lowerefficiency. The reason for this is that the contact timebetween the liquid and the vapor is greatly reduced athigher throughput.

Design of trays to improve efficiencies and

capacities

Trayed Columns utilize a pressure and temperaturedifferential to separate the products. For most trayedcolumns, the weir holds a certain amount of liquidlevel on each tray (Fig. 10). The vapor must overcomethis liquid head to move up the column. On the traythe vapor and liquid are contacted and then above thetray they are separated. Any deviation that restricts thevapor and liquid from contacting and then separatingwill deteriorate the columns ability to meet designspecifications.

Items that lead to improvements in tray efficiency

include;1. Path flow length2. Deck opening size3. Elimination of stagnant zones4 Down comer outlet devices/froth promoters

5. Weir heights

Path flow lengthThe longer the path flow length, the higher the trayefficiency. At short path flow lengths, less than 300 mm

Time 0.5 s 0.9 s 1.5 s

baffle bar

Figure 10. Liquid flow on trays. This figure is available in colour online atwww.apjChemEng.com.



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a tray will achieve the point efficiency. Longer flow pathlengths can actually allow a try to achieve efficiency inexcess of 100%.

Opening sizeThere is an optimum bubble size, therefore an optimumopening size. Too small or too large can effect the size

of the bubble, leading to loss of efficiency. Here is thenormal trade off between capacity and efficiency.

Elimination of stagnant zonesParallel flow across a cordial surface can lead tostagnant areas. Liquid directional push valves can helpeliminated the stagnant zones.

Down comer outlet devices/froth promotersThe clear liquid exiting the down comer becomesfroth on the tray (Fig. 11). Items that assist this froth

generation improve efficiency.

Weir heightsThe weir height has an effect on the tray efficiency(Fig. 12). Recommendations are not to exceed 100 mmor 1/6 of tray spacing, and 50 to 75 is suggested for allservices except vacuum services.

Exotic materials of constructionThe choice of materials of construction can have aprofound effect on the performance of a unit if corrosion

sets in. The engineer is constantly striving to producean economical design with the least expensive materials.However, there are minimum specifications on the typesof materials to be used in common services to ensureminimal corrosion or stress cracking. Some of these are:

Hydrocarbons (no H2S)temp >40 C

Carbon steel (A-569)

Hydrocarbons (no H2S)temp 30 to 40 C

Killed carbon steel

Hydrocarbons (no H2S)

temp 100 to 30

C

3 1/2 Nickel steel

(SA-203)Hydrocarbons (no H2S)

temp


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C10 FREE

COCONUTOIL FA

10,000 kg/h

C12 99%4,851 kg/h

C14 99%1,616 kg/h

Max. 0.4%C14 FA

Figure 13. Case one conventional two-column system. This figure is available in colouronline at www.apjChemEng.com.

Divided wall columns are preferred when;

1. Middle boiling component (B) is not in excess2. Desired purity of middle boiling component (B) is

higher that can be achieved with a side draw.3. Product specifications and relative volatility distri-

bution are uniform.

The main challenges of divided wall columns in theoleo chemical industry include;

1. Limited flexibility2. Potential corrosion problems3. Limited familiarity

Fractionation columns with added side strippers area well proven way to satisfy the increased demands

of capacity and efficiency. The availability of secondgeneration structured packing reduces the pressure drop,while making it possible to increase capacity andmaintain current product purities. New columns canbe designed much more compactly, while revamps canimprove on capacity and purity. Revamping an existingcolumn and adding a side-stripper may allow switchingfrom a two to a three-product production scheme.

Advantages of a Conventional Two Column System(Fig. 13):

1. Established industrial practice

2. Easy to operate3. Low pressure drop as required packing height islimited

4. Flexible in feedstocks and products5. Low energy consumption6. Stable operations7. Small column diameter

Disadvantages of a Conventional Two Column Sys-tem

1. Higher overall capital cost2. Large space requirements

C10 FREECOCONUTOIL FA

10,000 kg/h

C1299%4,851 kg/h

C1499%1,616 kg/h

Max. 0.4%

C14 FA5,149 kg/h

Figure 14. Case two divided wall column sys-tem. This figure is available in colour online atwww.apjChemEng.com.

Advantages of divided wall column:

1. Lowest overall capital cost2. Compact Design, lower space requirements

Disadvantages of Divided Wall Column:

1. Limited feedstock flexibility (design for one singlefeedstock)

2. Additional reflux divider required3. Increased operational and maintenance complexity4. More sensitive to fouling and corrosion/difficult

maintenance5. Increased pressure drop6. Increased column diameter

Advantages of single column with side stripper(Fig. 15):



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1. Low capital cost2. Reasonable energy consumption3. Ease of installation4. Stable flexible operation5. Good performance if middle distillate is in lower

concentration6. Decreased operational and maintenance complexity7. Possibilities to revamp single column and add new

side strippers

Disadvantages of single column with side stripper:

1. Increased pressure drop2. Increased column diameter3. Slightly larger space requirements

A comparison of the three cases (Table 1) is basedon individually optimized detailed process simulationsand consequent plant designs, incorporating differentaspects like the impacts on the required number of

theoretical stages, packing heights, pressure drop etc.on capital and operating cost.

CONCLUSIONS

An engineers function is to find the most economicalsolution to a complex problem. The optimum designguidelines as presented here describe a way to achievethis end through consideration of the life cycle cost. Theauthors trust that the information contained here in hasbeen of help in determining the lowest life cycle costof their separations needs.

REFERENCES

Kister HZ. Distillation Design. McGraw-Hill Book Company Inc.:New York, 1992.

C10 FREE

COCONUTOIL FA

10,000 kg/h

C1299%4,851 kg/h

C1499%1,161 kg/h

Max. 0.4%C14 FA5,149 kg/h

Figure 15. Case 3 single column with side stripper. This figure is availablein colour online at www.apjChemEng.com.

Table 1. Comparison of three the cases.

Two column system case 1 Divided wall column case 2 Column with side stripper case 3

Purity Same Same SameCapacity (%) 100 100 100Column diameter (m) 2.1/1.5 2.8 2.7/2.3 +1.5Overall capital cost 100 70 80Energy consumption (kW) 1690 (100%) 2115 (125%) 1882 (108%)

Life cycle costa 100 94.8b 87.6

a Based on USD 170 per tons of fuel gas and current capital investment cost indexes.b Without consideration of operational flexibility (processing different feedstock).



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Kister HZ. Recent Trends in Distillation Tower Malfunctions.Distillation Topical Conference, Spring National AlChE Meeting,AlChE: New Orleans, LA; MarchApril, 2003.

Kister HZ. Trouble Free Design of Refinery Fractionators. PetroleumTechnology Quarterly, Q4, 2003.

Kolmetz K, Zygula T. Resolving process distillation equipmentproblems. Prepared for The 5th Annual Regional OlefinsConference, Johor Bahru, Malaysia, October 31st November3rd, 2000.

Kolmetz K, Sloley AW, Zygula TM, Ng W, Faessler PW. Designguidelines for distillation columns in fouling service. The 16th

Ethylene Producers Conference, Section T8005 Ethylene PlantTechnology, Advances in Distillation Technology for Ethylene

Plants. American Institute of Chemical Engineers: New Orleans,LA, 2004.

Kolmetz K, Sloley A, Zygula T, Faessler PW, Ng WK, Senthil K,Lim TY. Designing distillation columns for vacuum service. The11th India Oil and Gas Symposium and International Exhibition ,Grand Hyatt, Mumbai, 67 September 2004.

Seader JD, Henley EJ. Separation Process Principles. John Wiley:New York, 1998.

Summers DR. Best Practices in Tray Design, unpublished, 23 Jan2001.

Summers DR. Harmonic Vibrations Cause Tray Damage, 2003.

AIChE Annual Meeting, Distillation Equipment and ApplicationsI, San Francisco, CA, Paper 307g, November 18, 2003.


improved reliability in operation and mantenance

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