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TechnicalPaper

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TP1194EN.doc Feb-12

Refinery Process Fouling Control

Abstract

Fouling of refinery process equipment is a commonproblem resulting in severe economic penalties, aswell as significant safety and environmental con-cerns.

GE has developed the technology, the applicationsexpertise, and the global industry experience toeffectively treat and minimize fouling throughoutthe refinery. Typical problem areas include crude

preheat exchangers and furnaces, hydrotreaterexchangers and reactor beds, FCCU slurry ex-changers, and thermal cracking process ex-changers and furnaces.

This paper will cover the following topics related torefinery process fouling:

Fouling mechanisms

Exchangers cleaning, bypassing, pretreating

Economic penalties

Environmental and safety impact Antifoulant chemistries

Crude (crude blends) quality

Analytical tools

Monitoring tools and techniques

Value of a comprehensive treatment program

Background

Since the early 1980s, chemicals have been usedto help reduce fouling. Early on, the benefits ofchemical treatment were difficult to demonstratedue to limitations in monitoring and program justi-fication, combined with a developing understand-ing of fouling mechanisms.

Modern refineries strive for reliability and pro-cessing flexibility, with longer run lengths and min-imal equipment fouling, to allow the processing oflower quality feedstocks, which offer favorableeconomic incentives.

For more than 30 years, GE Water & ProcessTechnologies has invested extensively in the iden-tification and understanding of fouling mecha-nisms, as well as how to minimize the fouling inprocess equipment. The result of these efforts hasbeen the development of proven chemical treat-ment programs that are unique to each applica-tion.

Multiple factors impact fouling including equip-ment design; flow rates; temperatures; crackingunits operational severity; fluid characteristics;caustic addition, can impact crude preheat foulingand coker furnace fouling; and upstream process-es, such as desalter operation. Consequently, eachprocess unit and fouling control program isunique. GE has developed state-of-the-art analyt-

ical tools and sophisticated monitoring techniquesto identify and track process fouling, as well asquantify the impact and economic benefits of achemical treatment program.

Heat exchangers are used to recover heat fromproduct streams to preheat the feedstock, mini-mizing the furnace fuel needed to raise the feed tothe required process temperature. Fouling in theexchanger train and the related reduction of heattransfer can cause significant energy loss and in-crease operating costs. Fouling can also become

so severe that unit capacity and production limitsare reached.

In a crude preheat exchanger system, the hotpreheat section is usually the area of greatestconcern. The hottest exchangers or exchangerswith the highest heatflux typically show the high-

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est fouling rates. This is also the case for the hy-drotreater feed/effluent exchangers and the slurryexchangers.

Intermediate cleanings can be applied, althoughthroughput reductions are typically required if anonline clean is carried out or even a shutdown isnecessary to clean an exchanger that cannot beisolated and bypassed. This represents significanteconomic penalties, if not planned in the normalrefinery operation.

Mechanical changes and spiral metal inserts,which vibrate as fluid flows through the tube havebeen used to control the fouling rate. However,other problems such as hydraulic limitations havebeen encountered.

Introduction

Heat exchangers are used to recover sensibleheat from process streams leaving reactors, distil-lation columns etc. to preheat the feedstock andminimize the external energy demand to providethe required heat for the process. Of the total pro-cessing cost, energy is a major part, particularly indistillation processes, which can directly influencerefinery profitability. Controlling, or minimizing,heat exchanger fouling can dramatically loweroverall operating costs.

Typically, refiners acknowledge and accept the

gradual diminishing performance from their heattransfer equipment, as fouling inevitably builds.However, when fouling becomes so severe thatthroughput restrictions occur, the economic pen-alty is no longer acceptable. Units are then shut-down, or the equipment is by-passed for cleaning,incurring high maintenance costs and productionlosses, as well as increased environmental andsafety concerns.

Economic Impact of Fouling

Fouling of refinery process heat exchange sys-tems can cause significant economic penalties.Minimizing operating costs, sustaining throughput,and maintaining equipment reliability are primarygoals in todays difficult economic climate, withmore difficult crude slates and blends, stringentregulatory mandates, and greater environmentaland safety focus than ever before.

Many factors contribute to the economic impactof refinery process fouling, including those de-scribed below. A correctly designed and appliedchemical treatment program can help significant-ly reduce costs and penalties related to equip-ment fouling.

Energy Cost: Fouling in exchanger networks willreduce the amount of heat recovered from prod-uct streams. This in turn, requires more heat to besupplied by the furnace to compensate for thelower feed temperature. Also, the higher loadingof the furnace often reduces furnace efficiencyand increases the fouling or coking tendency ofthe furnace. Both effects further increase fuel de-mand and energy costs.

Maintenance Cost: High fouling rates can lead toexcessive equipment cleaning requirements andcosts. Mechanical cleaning, using high pressure

jetting, is typical. It requires equipment openingand often heat exchanger bypassing. In caseswhere the exchangers cannot be bypassed, a unitshutdown may be required to clean the equip-ment and a substantial production loss needs tobe added to the total cost of cleaning. Openingthe equipment also increases the risk of damageand additional repair costs, as well as safety andenvironmental concerns.

Throughput Loss: Fouling of the preheat ex-changers will increase the heat duty of the fur-

nace up to the point that the maximum furnacecapacity is reached. As that limit is approached,the unit throughput will need to be cut back grad-ually in order to unload the furnace, and again,significant production losses are incurred.

Process stream fouling can lead to increasedpressure drop in the exchangers and furnace. Insome cases the fouling is so severe that the hy-draulic limits are reached, the throughput is re-duced, and significant processing incentives arelost. In the case of a hydrotreater, reactor fouling

will also cause pressure drop problems resulting insimilar consequences.

Feed Flexibility: The fouling rate can be greatlyinfluenced by the crude type or blend. Somecrudes contribute to such severe fouling problemsthat they are not considered for processing in cer-tain refineries. These opportunity crudes offer anattractive economic incentive to the refinery thatcan process them.

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Loss Opportunity: Modern refinery operationstrives to maximize the run-length between shut-downs and plan shutdown activities so as to min-imize downtime and production losses. Unitflexibility and equipment reliability are key factorsin avoiding unscheduled shutdowns, which might

hinder contract fulfillment or the ability to capital-ize on opportunity crudes.

Equipment fouling in the distillation column, canlead to poor product separation and quality, withdramatic economic consequences.

In a hydrotreater, severe fouling can impact con-version generating off-spec, lower value products,and can also result in throughput reductions.

In an FCCU, severe slurry fouling may force therefinery to run a lower main fractionation tower

bottoms temperature, dropping valuable productsinto the less valuable slurry.

Safety: Regular equipment opening for cleaningincreases the risk of leakages, specifically duringthe start-up. Although difficult to quantify, theeconomic penalties associated with a fire or ex-plosion, and moreover personnel injuries, can beenormously high.

Fouling Mechanisms

Fouling deposits can be categorized into two ma-jor types, inorganic and organic. To properly con-trol fouling, the differences between these twocategories must be thoroughly understood andaccounted for when identifying the fouling mech-anisms involved and designing an appropriatechemical treatment program.

Inorganic: Corrosion of process equipment willform ferrous-based corrosion products, such asFeS (iron sulfide) or Fe2O3, (ferric oxide) which willdeposit in exchangers, mainly in areas with lowervelocities.

Solid, inorganic contaminants in crude oils or re-processed streams, such as sand and silt, can alsodeposit in the exchanger and cause hydraulic orthermal obstructions. As the crude is heated in thepreheat train, the viscosity of the oil is lowered,and the deposition of solids increases. Althoughmost of the salts in the crude should be removedvia the desalting process, some inorganic salts

can remain and cause deposition in the preheattrain.

A caustic solution is sometimes injected into thedesalted crude for distillation tower overhead cor-rosion control. However, this practice can also en-hance the potential for fouling in downstream

exchangers and coking in downstream processfurnaces.

In an FCC slurry system, catalyst fines can con-tribute to significant fouling.

Additionally, inorganic salts in the effluent side ofthe hydrotreater feed/effluent exchangers canbecome a serious problem.

Organic: Organic fouling in a crude unit resultsfrom the precipitation of organic componentswhich become insoluble in the system, such asasphaltenes, and high molecular weight hydro-carbons (i.e. paraffins). The asphaltenes can be-come unstable because of the blending ofincompatible crudes and/or the heating of the flu-id. The precipitated organic molecules can sit atthe metal surface and dehydrogenate, formingcoke. The coke formation can also result fromthermal degradation of hydrocarbons due to longheating periods.

Organic fouling on the feed side of the hydrotreat-er feed/effluent exchangers and reactors systemsis usually caused by polymerization reactions ini-tiated by fouling precursors that are present in thefeed such as unsaturates, carbonyls, highly reac-tive nitrogen compounds, and metals to name afew. Two polymerization mechanisms have beenidentified, free radical and non-free radical re-actions.

The most common mechanism is free radicalpolymerization, where unsaturated components,such as olefins and diolefins, react to form longerchain molecules. The molecule chain length in-

creases to the point that solubility is exceeded anddeposition occurs. This mechanism is catalyzed bythe presence of active metals, like corrosion prod-ucts, and oxygen, which can form peroxides. The-se peroxides are unstable molecules requiringonly a low level of energy to form peroxy radicals,which act as initiators for further polymerizationreactions. Once the initiators are formed, thepropagation reaction will be enhanced by higher

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levels of unsaturated molecules, temperature, andresidence time.

Non-free radical polymerization mechanism oc-cur mainly as a result of condensation reactionsinvolving components such as carboxylic acid andpyrolle nitrogen.

In an FCCU slurry system, similar polymerizationreactions can take place. Furthermore, heavy mo-lecular weight polynuclear aromatics (PNA) canagglomerate and further degrade on the tube sur-face and contribute to fouling. These heavy aro-matic compounds result from the cleaving ofaliphatic side chains off of naturally occurring as-phaltenes compounds and from recombinationreactions of high molecular weight cracked hy-drocarbons within the transfer line and in thequench zone of the main fractionator.

Chemical Treatment Capabilities

Following the identification and accessing the rel-ative importance of each fouling mechanism oc-curring in a heat exchanger system, the chemicaltreatment program can be designed to obtainmaximum inhibition efficiency.

Dispersants: Dispersants are designed to limit theparticle size of solids in the system. Various dis-persants have different efficacies, depending onthe components to be dispersed. Dispersant

chemistries are available that address depositionproblems such as coke particles, asphaltene pre-cipitation, organic or inorganic deposition. Disper-sants will prevent smaller particles fromagglomerating to form larger particles which de-posit more easily. Similarly, they also prevent thesmall particles from being attracted to alreadyexisting deposits in the system. Maintaining ahigh fluid velocity also helps to keep small parti-cles from settling onto the process equipment.Some dispersants can also be surface activeproviding a surface which hinders a particles abi l-

ity to lay down on the metal surface.

Corrosion Inhibitors: Corrosion inhibitors are de-signed to minimize the contact between the metalsurface and the corrosive fluid in order to mini-mize the formation and deposition of corrosionproducts in the system.

Metal Coordinator: A metal coordinator (deacti-vator) will modify the metal ions by complexing,

thus reducing the catalytic activity of the metal, sothat initiation of polymerization reactions is mini-mized. The GE metal coordinator is unique andproven to be very effective on both iron and cop-per.

Polymerization Inhibitors: Free radical poly-merization inhibitors are designed to react imme-diately with any radical formed in the system toform a new stable molecule which will no longercontribute to the propagation reactions. Theseinhibitors will reduce the free radical polymeriza-tion of olefins and some of the sulfur compoundsand stabilize unstable feedstocks.

Non free radical polymerization inhibitors reducethe condensation polymerization reactions of car-boxylic acids and some of the nitrogen com-pounds.

The most effective and economical treatmentprogram is one that addresses only those mecha-nisms that cause the fouling problems and at thesame time provide sufficient flexibility to handlethe typical processing variations.

Tools Used to Design the ChemicalTreatment Program

The selection and design of a successful chemicaltreatment program relies on tools specifically de-veloped to identify the root cause and quantify theimpact of the fouling problem.

Deposit Analysis: Deposit samples are taken fromthe unit during equipment opening, prior to highpressure jetting.

Sampling should be done to ensure an averagecomposition is obtained, unless the local differ-ences are intentionally analyzed. The shutdownprocedure prior to equipment opening should beclearly described, such as rinsing, steam-out etc.in order to provide the correct interpretation to

the analytical results.

A thermographic analysis is performed to definethe organic and volatile inorganic part of thedeposit as well as the ash level remaining. Amethylene chloride extraction is typically appliedto identify the entrained hydrocarbon anddegraded oil fraction in the sample. The non-extractables represent mainly the coke and inor-ganic fraction of the sample. Besides metal analy-

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sis (Fe, Na, Ca, Mg, Cu, etc.) elemental analysis forC,H,N and S is also conducted. The deposit analy-sis can give an indication if the major mechanismis inorganic, organic or a combination.

Feedstock Characterization:

A feedstock can be analyzed to identify specificconcern levels for components that reflect a spe-cific fouling mechanism.

Besides the physical property data, analysis ofsalts, filterable solids, asphaltenes, sulfur, mercap-tans, basic nitrogen, neutralization No., bromineNo. and metal levels are performed to character-ize the feedstocks.

Pressurized Hot Wire Test: A relatively rapidmethod to evaluate the efficacy of different anti-foulant products and combinations is the Pressur-ized Hot Wire Test. The feedstock sample is firstequally divided into several sample cells. An elec-trical resistance wire is then submerged in the flu-id. The sample cells are the pressurized and thewire is then electrically charged for a specifiedperiod (Figure 1), depending on the fouling ten-dency of the fluid.

Figure 1: Pressurized Hot Wire Tester

Four tests can be conducted simultaneously un-der the same conditions including the blank. Both

the wire deposit and the solids formed in the fluidare compared relative to each other for quantityand nature. Figure 2 shows a picture of the wiresafter the test, and clearly illustrates the efficacydifferences between the treatment programs incomparison with a blank.

Figure 2

Hot Liquid Process Simulator: The Hot Liquid Pro-cess Simulator (HLPS) is a dynamic simulation testdevice which can be used in different operationmodes, simulating a heat exchanger, a furnace oreven a hydrotreater reactor pressure drop prob-lem. This simulation method is much more time

consuming than the Pressurized Hot Wire Test. Aschematic is shown in Figure 3.

Figure 3: H.L.P.S. - Simulator Schematic

The fluid is circulated at a controlled flowrate viaan electrically heated core, which provides therequired heat to simulate the process. The inlettemperature is maintained constant and depend-ing if the core or outlet temperature is controlled,the outlet or core temperature respectively is

monitored to evaluate the fouling rate. Again, dif-ferent treatment programs can be evaluated asshown in Figure 4. Figure 5 illustrates two coresshown with different deposits.

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Figure 4: Crude Preheat Fouling Simulation Test

Figure 5

Monitoring Techniques

The development and selection of the appropriatemonitoring technique for each heat exchangersystem is crucial in order to be able to evaluatethe performance of the chemical treatment pro-gram and quantify the benefits achieved. Monitor-ing should not only define the actual state of atreated system, but also compare the data withthat obtained during an untreated processingperiod. The selected technique(s) depend on theaccuracy and availability of process data.

Three direct indicators of heat exchanger trainperformance are the furnace inlet temperature(FIT), throughput, and pressure drop. The processconditions change so often that comparing theactual measured furnace inlet temperatures is notpossible and a normalization technique is required

to evaluate the performance of a treatment pro-gram.

A unit survey is required to collect the necessarybasic information to define the appropriate moni-toring and calculation methods. This surveyshould provide the correct system configurationand exchanger mechanical data with indicationsof the available flow and temperature measure-ments and their locations.

Techniques such as regression analysis and statis-tical programs are used to evaluate the overalltrain performance without detailing each individ-ual exchanger. The same techniques can be usedto evaluate the pressure drop over the exchangertrain or reactor bed. Furthermore, simple heattransfer calculations (Q and U) can be done onexchangers where phase change is not a factor.

Additionally, GE has developed a rigorous PC-based calculation program, called Heat-RatePro*, which calculates the actual status of eachindividual exchanger and recalculates the furnaceinlet temperature under standardized conditions(NFIT). In addition, the program also performs sim-ulation calculations in order to define the opti-mum cleaning program and calculate the impacton the whole train if selective exchanger cleaningwas applied. The program uses the actual plantdata to check the heat balance per exchangerand over the whole train. Data corrections can be

applied to ensure the adiabatic nature of heattransfer in heat exchangers is respected and con-sistent, accurate data is used for the calculations.The fouling factors and heat transfer coefficientsare also calculated per exchanger. Comparing thenormalized furnace inlet temperature for a treatedand untreated run period will quantify the perfor-mance of the chemical treatment, where the foul-ing rate or slope of the NFIT curve is comparedduring both runs (Figure 6).

Figure 6: Treatment Savings - No Cleaning

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All parameters measured and calculated areregularly evaluated versus the preset objectives.

Treatment Evaluation andEconomical Justifications

The methods applied to evaluate the treatmentprogram and the parameters used to quantify thebenefits are dependent on the original treatmentobjectives and monitoring methods selected whenthe treatment program was designed. Some of thebenefits can be quantified quite easily; however,others are not but still offer significant value. Ex-amples of these benefits are: increased safety,reduced risk of leakage, avoiding unplanned shut-downs, operating stability and flexibility, lesscleaning and less toxic waste generation, less su-pervision required, less outside contracting per-

sonnel on-site, built-in engineering support etc.

The return on investment for a treatment pro-gram, based on the quantifiable savings is definedas:

R.O.I. = Total savings - Treatment cost

Treatment cost

Typical ROIs of 50 to 300% are obtained andsome even exceed 1000%.

The quantification of the obtained total savings is

based on one or more of the following savings:

Energy cost: The energy cost associated withfouling can be one of the major driving forces toinitiate energy saving projects. In order to calcu-late the energy cost, either the normalized dutyacross the exchanger network or normalized fur-nace inlet temperatures at the start of run (SOR)and at the end of run (EOR) must be used.

Using an MRA, a normalized duty or FIT can beeasily obtained at the beginning and end of run.

The following is an example on how to calculatethe energy lost due to fouling.

Given:

QSOR 7000 MMBTU/D

QEOR 5600 MMBTU/D

Run Length (RL) 365 daysFurnace Efficiency (FE) 85 %

Fuel Cost 4 $/MMBTU

Assuming a linear fouling rate the heat transferloss can be calculated as follows:

Qloss= ((b*tf2+ 2*a*tf)/2 + (b*t02+ 2*a*t0)/2)/FE

a = (QSORQEOR) b = (QEOR- QSOR)/(RL)

a = 1400 MMBTU/D b = -3.84 MMBTU/D2

Qloss= 300,588 MMBTU/year

Energy Cost = Qloss*Fuel Cost = 1.2 MM$/year

Maintenance cost: The direct cleaning costs arecompared for a treated and equivalent untreatedperiod. In the case of online cleaning, the numberof exchanger cleanings is multiplied by the costper exchanger to estimate the cleaning costs.Damage to the heat exchanger can occur duringexcessive cleaning operations. In particular when

the bundles need to be pulled for cleaning, the riskthat repair costs may be involved is significantlyhigher.

Throughput: In todays refinery operation withlimited margins on crude oil processing, it isimportant to maximize the utilization rate of theunit in order to remain profitable. Any throughputrestriction can be viewed as a direct profitabilityrestriction. However, the crude processing incen-tive is directly market related and shows largevariations with time.

Throughput restrictions occur for many reasons.

Due to hydraulic limitations or capacity limitationsof the furnace as more duty is required when thefurnace inlet temperature declines, the refineryneeds to reduce gradually the throughput in theunit from a certain time during the run in order tomaintain column operating parameters.

Example: After six months operation, a 100,000BPD refinery needed to gradually reduce thethroughput. By the end of the year, the refineryaveraged 5% in throughput loss for the last 6

months. . The refinery operating incentive to pro-cess this crude is 10$/bbl. The reduction in operat-ing income, due to production loss, is calculated tobe:

COF = 100,000 BPD * 0.05/2 * 180 days *$ 10 /bblCOF = 4.6 MM$/yr

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Similarly, incentive losses can be calculated inthose cases where the throughput needs to bereduced during on-line cleaning or for the periodthat an extra shutdown is required to clean theequipment.

Crude quality: Some refiners are reluctant toprocess lower quality crudes in view of the antici-pated problems, of which crude preheat foulingoften is the major problem. These opportunitycrudes however can offer a significant economicincentive to the refinery. A correctly designedtreatment program can cost effectively enableprocessing of these crudes without significantproblems.

Example: Assuming a refinery with a capacity of100,000 BPD processes a low quality crude equalto 20% of the total capacity. The extra incentivewould be $0.5/bbl opportunity crude, after sub-tracting the cost of chemical treatment and anyother additional operating expenses. The operat-ing income for the refinery would increase as fol-lows:

100,000 BPD * 0.2 * 365 days *$ 0.5 /bbl =US$ 3,650,000/yr

Conclusions

Fouling of refinery process equipment results froma number of different mechanisms; unique fluid

characteristics; operating practices; unit configu-ration; as well as ever changing operational pa-rameters, primarily temperature and pressure.Regardless of the cause, process fouling exactssignificant economic and operational penalties.Chemicals, if correctly selected and applied, cancost effectively reduce fouling in several criticalareas throughout the refinery. . The appropriatetools need to be used to understand and quantifythe fouling phenomena and design the most ef-fective treatment program. Clear treatment objec-tives need to be set prior to the treatment start up

based on reliable plant data and an agreed moni-toring schedule where key performance parame-ters are clearly defined. The research efforts haveled to the creation of unique treatment chemis-tries, which allow processing of variable feed qual-ity enabling refiners to increase their margins.Chemical treatment programs have been provento be very effective, which is supported by real lifecase histories. Treatment programs are justifiedbased on the preset treatment objectives, which

are then closely monitored to evaluate the actualperformance. Chemical treatment programs showa return of investment of 50 to 300% of whichsome even exceed 1000%. A lot of spin-off bene-fits are also obtained with the treatment pro-grams, which sometimes can be more importantthan the quantifiable benefits.

A successful antifoulant treatment program is theresult of the co-operation of the refinery with anexperienced and knowledgeable chemical suppli-er both striving to achieve mutual goals.

References1. Antifoulants: a proven energy-savings

investment by: R.M. Wilson and J.J.Perugini;Betz Process Chemicals, Inc. The Woodlands,Texas AM-85-52 National Petroleum RefinersAssociation

2. Predicting Crude Oil Fouling Tendency byD.E. Fields, R.F. Freeman and B.E. Wright; BetzProcess Chemicals,Inc. The Woodlands, TexasEnergy Progress (Vol 8, No. 4)

3. Chemical, Mechanical Treatment OptionsReduce Hydroprocessor Fouling by: B. C.Groce; Betz Process Chemicals, Inc. The Wood-lands, Texas Oil & Gas Journal, January 29,1996