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Technical Paper

Refinery Process Control

Abstract Modern refineries strive for reliability and pro- cessing flexibility, with longer run lengths and min- Fouling of refinery process equipment is a common imal equipment fouling, to allow the processing of problem resulting in severe economic penalties, as lower quality feedstocks, which offer favorable well as significant safety and environmental con- economic incentives. cerns. For more than 30 years, GE & Process GE has developed the , the applications has invested extensively in the iden- expertise, and the global industry experience to tification and understanding of fouling mecha- effectively treat and minimize fouling throughout nisms, as well as how to minimize the fouling in the refinery. Typical problem areas include crude process equipment. The result of these efforts has preheat exchangers and furnaces, hydrotreater been the development of proven chemical treat- exchangers and reactor beds, FCCU slurry ex- ment programs that are unique to each applica- changers, and thermal cracking process ex- tion. changers and furnaces. Multiple factors fouling including equip- This paper will cover the following topics related to ment design; flow rates; temperatures; cracking refinery process fouling: units operational severity; fluid characteristics;  Fouling mechanisms caustic addition, can impact crude preheat fouling and coker furnace fouling; and upstream process-  Exchangers – cleaning, bypassing, pretreating es, such as desalter operation. Consequently, each  Economic penalties process unit and fouling control program is unique. GE has developed state-of-the-art analyt-  Environmental and safety impact ical tools and sophisticated monitoring techniques  Antifoulant chemistries to identify and track process fouling, as well as quantify the impact and economic benefits of a  Crude (crude blends) quality chemical treatment program.  Analytical tools Heat exchangers are used to recover heat from  Monitoring tools and techniques product streams to preheat the feedstock, mini- mizing the furnace fuel needed to raise the feed to  Value of a comprehensive treatment program the required process temperature. Fouling in the exchanger train and the related reduction of heat Background transfer can cause significant energy loss and in- crease operating costs. Fouling can also become Since the early 1980’s, chemicals have been used so severe that unit capacity and production limits to help reduce fouling. Early on, the benefits of are reached. chemical treatment were difficult to demonstrate due to limitations in monitoring and program justi- In a crude preheat exchanger system, the hot fication, combined with a developing understand- preheat section is usually the area of greatest ing of fouling mechanisms. concern. The hottest exchangers or exchangers with the highest heatflux typically show the high-

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TP1194EN.doc Feb-12

est fouling rates. This is also the case for the hy- Many factors contribute to the economic impact drotreater feed/effluent exchangers and the slurry of refinery process fouling, including those de- exchangers. scribed below. A correctly designed and applied chemical treatment program can help significant- Intermediate cleanings can be applied, although ly reduce costs and penalties related to equip- throughput reductions are typically required if an ment fouling. online clean is carried out or even a shutdown is necessary to clean an exchanger that cannot be Energy Cost: Fouling in exchanger networks will isolated and bypassed. This represents significant reduce the amount of heat recovered from prod- economic penalties, if not planned in the normal uct streams. This in turn, requires more heat to be refinery operation. supplied by the furnace to compensate for the lower feed temperature. Also, the higher loading Mechanical changes and spiral metal inserts, of the furnace often reduces furnace efficiency which vibrate as fluid flows through the tube have and increases the fouling or coking tendency of been used to control the fouling rate. However, the furnace. Both effects further increase fuel de- other problems such as hydraulic limitations have mand and energy costs. been encountered. Maintenance Cost: High fouling rates can lead to Introduction excessive equipment cleaning requirements and costs. Mechanical cleaning, using high pressure Heat exchangers are used to recover sensible jetting, is typical. It requires equipment opening heat from process streams leaving reactors, distil- and often bypassing. In cases lation etc. to preheat the feedstock and where the exchangers cannot be bypassed, a unit minimize the external energy demand to provide shutdown may be required to clean the equip- the required heat for the process. Of the total pro- ment and a substantial production loss needs to cessing cost, energy is a major part, particularly in be added to the total cost of cleaning. Opening distillation processes, which can directly influence the equipment also increases the risk of damage refinery profitability. Controlling, or minimizing, and additional repair costs, as well as safety and heat exchanger fouling can dramatically lower environmental concerns. overall operating costs. Throughput Loss: Fouling of the preheat ex- Typically, refiners acknowledge and accept the changers will increase the heat duty of the fur- gradual diminishing performance from their heat nace up to the point that the maximum furnace transfer equipment, as fouling inevitably builds. capacity is reached. As that limit is approached, However, when fouling becomes so severe that the unit throughput will need to be cut back grad- throughput restrictions occur, the economic pen- ually in order to unload the furnace, and again, alty is no longer acceptable. Units are then shut- significant production losses are incurred. down, or the equipment is by-passed for cleaning, incurring high maintenance costs and production Process stream fouling can lead to increased losses, as well as increased environmental and pressure drop in the exchangers and furnace. In safety concerns. some cases the fouling is so severe that the hy- draulic limits are reached, the throughput is re- Economic Impact of Fouling duced, and significant processing incentives are lost. In the case of a hydrotreater, reactor fouling Fouling of refinery process heat exchange sys- will also cause pressure drop problems resulting in tems can cause significant economic penalties. similar consequences. Minimizing operating costs, sustaining throughput, Feed Flexibility: The fouling rate can be greatly and maintaining equipment reliability are primary goals in today’s difficult economic climate, with influenced by the crude type or blend. Some more difficult crude slates and blends, stringent crudes contribute to such severe fouling problems regulatory mandates, and greater environmental that they are not considered for processing in cer- and safety focus than ever before. tain refineries. These “opportunity” crudes offer an attractive economic incentive to the refinery that can process them.

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Loss Opportunity: Modern refinery operation can remain and cause deposition in the preheat strives to maximize the run-length between shut- train. downs and plan shutdown activities so as to min- imize downtime and production losses. Unit A caustic is sometimes injected into the flexibility and equipment reliability are key factors desalted crude for distillation tower overhead cor- in avoiding unscheduled shutdowns, which might rosion control. However, this practice can also en- hinder contract fulfillment or the ability to capital- hance the potential for fouling in downstream ize on “opportunity” crudes. exchangers and coking in downstream process furnaces. Equipment fouling in the distillation , can lead to poor product separation and quality, with In an FCC slurry system, catalyst fines can con- dramatic economic consequences. tribute to significant fouling. In a hydrotreater, severe fouling can impact con- Additionally, inorganic salts in the effluent side of version generating off-spec, lower value products, the hydrotreater feed/effluent exchangers can and can also result in throughput reductions. become a serious problem. In an FCCU, severe slurry fouling may the Organic: Organic fouling in a crude unit results refinery to run a lower main fractionation tower from the precipitation of organic components bottoms temperature, dropping valuable products which become insoluble in the system, such as into the less valuable slurry. asphaltenes, and high molecular weight hydro- carbons (i.e. paraffins). The asphaltenes can be- Safety: Regular equipment opening for cleaning come unstable because of the blending of increases the risk of leakages, specifically during incompatible crudes and/or the heating of the flu- the start-up. Although difficult to quantify, the id. The precipitated organic molecules can sit at economic penalties associated with a fire or ex- the metal surface and dehydrogenate, forming plosion, and moreover personnel injuries, can be coke. The coke formation can also result from enormously high. thermal degradation of hydrocarbons due to long heating periods. Fouling Mechanisms Organic fouling on the feed side of the hydrotreat- Fouling deposits can be categorized into two ma- er feed/effluent exchangers and reactors systems jor types, inorganic and organic. To properly con- is usually caused by polymerization reactions ini- trol fouling, the differences between these two tiated by fouling precursors that are present in the categories must be thoroughly understood and feed such as unsaturates, carbonyls, highly reac- accounted for when identifying the fouling mech- tive nitrogen compounds, and metals to name a anisms involved and designing an appropriate few. Two polymerization mechanisms have been chemical treatment program. identified, “free radical” and “non-free radical” re- actions. Inorganic: of process equipment will form ferrous-based corrosion products, such as The most common mechanism is “free radical” FeS (iron sulfide) or Fe2O3, (ferric ) which will polymerization, where unsaturated components, deposit in exchangers, mainly in areas with lower such as olefins and diolefins, react to form longer . chain molecules. The molecule chain length in- creases to the point that is exceeded and Solid, inorganic contaminants in crude oils or re- deposition occurs. This mechanism is catalyzed by processed streams, such as sand and silt, can also the presence of active metals, like corrosion prod- deposit in the exchanger and cause hydraulic or ucts, and , which can form peroxides. The- thermal obstructions. As the crude is heated in the se peroxides are unstable molecules requiring preheat train, the viscosity of the oil is lowered, only a low level of energy to form peroxy radicals, and the deposition of solids increases. Although which act as initiators for further polymerization most of the salts in the crude should be removed reactions. Once the initiators are formed, the via the desalting process, some inorganic salts propagation reaction will be enhanced by higher

Page 3 levels of unsaturated molecules, temperature, and thus reducing the catalytic activity of the metal, so residence time. that initiation of polymerization reactions is mini- mized. The GE metal coordinator is unique and “Non-free radical” polymerization mechanism oc- proven to be very effective on both iron and cop- cur mainly as a result of condensation reactions per. involving components such as carboxylic and pyrolle nitrogen. Polymerization Inhibitors: “Free radical” poly- merization inhibitors are designed to react imme- In an FCCU slurry system, similar polymerization diately with any radical formed in the system to reactions can take place. Furthermore, heavy mo- form a new “stable” molecule which will no longer lecular weight polynuclear aromatics (PNA) can contribute to the propagation reactions. These agglomerate and further degrade on the tube sur- inhibitors will reduce the free radical polymeriza- face and contribute to fouling. These heavy aro- tion of olefins and some of the sulfur compounds matic compounds result from the cleaving of and stabilize unstable feedstocks. aliphatic side chains off of naturally occurring as- phaltenes compounds and from recombination “Non free radical” polymerization inhibitors reduce reactions of high molecular weight cracked hy- the condensation polymerization reactions of car- drocarbons within the transfer line and in the boxylic and some of the nitrogen com- quench zone of the main fractionator. pounds. Chemical Treatment Capabilities The most effective and economical treatment program is one that addresses only those mecha- Following the identification and accessing the rel- nisms that cause the fouling problems and at the ative importance of each fouling mechanism oc- same time provide sufficient flexibility to handle curring in a heat exchanger system, the chemical the typical processing variations. treatment program can be designed to obtain maximum inhibition efficiency. Tools Used to Design the Chemical Treatment Program : Dispersants are designed to limit the particle size of solids in the system. Various dis- The selection and design of a successful chemical persants have different efficacies, depending on treatment program relies on tools specifically de- the components to be dispersed. veloped to identify the root cause and quantify the chemistries are available that address deposition impact of the fouling problem. problems such as coke particles, asphaltene pre- cipitation, organic or inorganic deposition. Disper- Deposit Analysis: Deposit samples are taken from sants will prevent smaller particles from the unit during equipment opening, prior to high agglomerating to form larger particles which de- pressure jetting. posit more easily. Similarly, they also prevent the small particles from being attracted to already Sampling should be done to ensure an average existing deposits in the system. Maintaining a composition is obtained, unless the local differ- high fluid also helps to keep small parti- ences are intentionally analyzed. The shutdown cles from settling onto the process equipment. procedure prior to equipment opening should be Some dispersants can also be surface active clearly described, such as rinsing, steam-out etc. providing a surface which hinders a particle’s abil- in order to provide the correct interpretation to ity to lay down on the metal surface. the analytical results. Corrosion Inhibitors: Corrosion inhibitors are de- A thermographic analysis is performed to define signed to minimize the contact between the metal the organic and volatile inorganic part of the surface and the corrosive fluid in order to mini- deposit as well as the ash level remaining. A mize the formation and deposition of corrosion methylene chloride extraction is typically applied products in the system. to identify the entrained hydrocarbon and degraded oil fraction in the sample. The non- Metal Coordinator: A metal coordinator (deacti- extractables represent mainly the coke and inor- vator) will modify the metal ions by complexing, ganic fraction of the sample. Besides metal analy-

Page 4 sis (Fe, Na, Ca, Mg, Cu, etc.) elemental analysis for C,H,N and S is also conducted. The deposit analy- sis can give an indication if the major mechanism is inorganic, organic or a combination. Feedstock Characterization:

A feedstock can be analyzed to identify specific concern levels for components that reflect a spe- cific fouling mechanism. Figure 2 Besides the physical property data, analysis of salts, filterable solids, asphaltenes, sulfur, mercap- tans, basic nitrogen, neutralization No., bromine Hot Liquid Process Simulator: The Hot Liquid Pro- No. and metal levels are performed to character- cess Simulator (HLPS) is a dynamic simulation test ize the feedstocks. device which can be used in different operation modes, simulating a heat exchanger, a furnace or Pressurized Hot Wire Test: A relatively rapid even a hydrotreater reactor pressure drop prob- method to evaluate the efficacy of different anti- lem. This simulation method is much more time foulant products and combinations is the Pressur- consuming than the Pressurized Hot Wire Test. A ized Hot Wire Test. The feedstock sample is first schematic is shown in Figure 3. equally divided into several sample cells. An elec- trical resistance wire is then submerged in the flu- id. The sample cells are the pressurized and the wire is then electrically charged for a specified period (Figure 1), depending on the fouling ten- dency of the fluid.

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

The fluid is circulated at a controlled flowrate via Figure 1: Pressurized Hot Wire Tester an electrically heated core, which provides the required heat to simulate the process. The inlet

temperature is maintained constant and depend- Four tests can be conducted simultaneously un- ing if the core or outlet temperature is controlled, der the same conditions including the blank. Both the outlet or core temperature respectively is the wire deposit and the solids formed in the fluid monitored to evaluate the fouling rate. Again, dif- are compared relative to each other for quantity ferent treatment programs can be evaluated as and nature. Figure 2 shows a picture of the wires shown in Figure 4. Figure 5 illustrates two cores after the test, and clearly illustrates the efficacy shown with different deposits. differences between the treatment programs in comparison with a blank.

Page 5 to evaluate the performance of a treatment pro- gram. A unit survey is required to collect the necessary basic information to define the appropriate moni- toring and calculation methods. This survey should provide the correct system configuration and exchanger mechanical data with indications of the available flow and temperature measure- ments and their locations. Figure 4: Crude Preheat Fouling Simulation Test Techniques such as regression analysis and statis- tical programs are used to evaluate the overall

train performance without detailing each individ- ual exchanger. The same techniques can be used to evaluate the pressure drop over the exchanger train or reactor bed. Furthermore, simple heat transfer calculations (Q and U) can be done on exchangers where phase change is not a factor. Additionally, GE has developed a rigorous PC- based calculation program, called “Heat-Rate Pro”*, which calculates the actual status of each individual exchanger and recalculates the furnace inlet 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 impact on the whole train if selective exchanger cleaning was applied. The program uses the actual plant data to check the heat balance per exchanger Figure 5 and over the whole train. Data corrections can be applied to ensure the adiabatic nature of heat transfer in heat exchangers is respected and con- Monitoring Techniques sistent, accurate data is used for the calculations. The fouling factors and heat transfer coefficients The development and selection of the appropriate are also calculated per exchanger. Comparing the monitoring technique for each heat exchanger normalized furnace inlet temperature for a treated system is crucial in order to be able to evaluate and untreated run period will quantify the perfor- the performance of the chemical treatment pro- mance of the chemical treatment, where the foul- gram and quantify the benefits achieved. Monitor- ing rate or slope of the NFIT curve is compared ing should not only define the actual state of a during both runs (Figure 6). treated system, but also compare the data with that obtained during an untreated processing period. The selected technique(s) depend on the accuracy and availability of process data. Three direct indicators of heat exchanger train performance are the furnace inlet temperature (FIT), throughput, and pressure drop. The process conditions change so often that comparing the actual measured furnace inlet temperatures is not possible and a normalization technique is required Figure 6: Treatment Savings - No Cleaning

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All parameters measured and calculated are Assuming a linear fouling rate the heat transfer regularly evaluated versus the preset objectives. loss can be calculated as follows:

2 2 Treatment Evaluation and Qloss = ((b*tf + 2*a*tf)/2 + (b*t0 + 2*a*t0)/2)/FE

Economical Justifications a = (QSOR – QEOR) b = (QEOR - QSOR)/(RL)

The methods applied to evaluate the treatment a = 1400 MMBTU/D b = -3.84 MMBTU/D2 program and the parameters used to quantify the benefits are dependent on the original treatment Qloss = 300,588 MMBTU/year objectives and monitoring methods selected when Energy Cost = Qloss *Fuel Cost = 1.2 MM$/year the treatment program was designed. Some of the benefits can be quantified quite easily; however, Maintenance cost: The direct cleaning costs are others are not but still offer significant value. Ex- compared for a treated and equivalent untreated amples of these benefits are: increased safety, period. In the case of online cleaning, the number reduced risk of leakage, avoiding unplanned shut- of exchanger cleanings is multiplied by the cost downs, operating stability and flexibility, less per exchanger to estimate the cleaning costs. cleaning and less toxic waste generation, less su- Damage to the heat exchanger can occur during pervision required, less outside contracting per- excessive cleaning operations. In particular when sonnel on-site, built-in engineering support etc. the bundles need to be pulled for cleaning, the risk that repair costs may be involved is significantly The return on investment for a treatment pro- higher. gram, based on the quantifiable savings is defined as: Throughput: In today’s refinery operation with limited margins on crude oil processing, it is R.O.I. = Total savings - Treatment cost important to maximize the utilization rate of the Treatment cost unit in order to remain profitable. Any throughput restriction can be viewed as a direct profitability Typical ROI’s of 50 to 300% are obtained and restriction. However, the crude processing incen- some even exceed 1000%. tive is directly market related and shows large The quantification of the obtained total savings is variations with time. based on one or more of the following savings: Throughput restrictions occur for many reasons. Energy cost: The energy cost associated with Due to hydraulic limitations or capacity limitations fouling can be one of the major driving to of the furnace as more duty is required when the initiate energy saving projects. In order to calcu- furnace inlet temperature declines, the refinery late the energy cost, either the normalized duty needs to reduce gradually the throughput in the across the exchanger network or normalized fur- unit from a certain time during the run in order to nace inlet temperatures at the start of run (SOR) maintain column operating parameters. and at the end of run (EOR) must be used. Example: After six months operation, a 100,000 Using an MRA, a normalized duty or FIT can be BPD refinery needed to gradually reduce the easily obtained at the beginning and end of run. throughput. By the end of the year, the refinery The following is an example on how to calculate averaged 5% in throughput loss for the last 6 the energy lost due to fouling. months. . The refinery operating incentive to pro- cess this crude is 10$/bbl. The reduction in operat- Given: ing income, due to production loss, is calculated to be: QSOR 7000 MMBTU/D COF = 100,000 BPD * 0.05/2 * 180 days *$ 10 /bbl QEOR 5600 MMBTU/D COF = 4.6 MM$/yr Run Length (RL) 365 days Furnace Efficiency (FE) 85 % Fuel Cost 4 $/MMBTU

Page 7 Similarly, incentive losses can be calculated in are then closely monitored to evaluate the actual those cases where the throughput needs to be performance. Chemical treatment programs show reduced during on-line cleaning or for the period a return of investment of 50 to 300% of which that an extra shutdown is required to clean the some even exceed 1000%. A lot of spin-off bene- equipment. fits are also obtained with the treatment pro- grams, which sometimes can be more important Crude quality: Some refiners are reluctant to than the quantifiable benefits. process lower quality crudes in view of the antici- pated problems, of which crude preheat fouling A successful antifoulant treatment program is the often is the major problem. These “opportunity” result of the co-operation of the refinery with an crudes however can offer a significant economic experienced and knowledgeable chemical suppli- incentive to the refinery. A correctly designed er both striving to achieve mutual goals. treatment program can cost effectively enable processing of these crudes without significant References problems. 1. “Antifoulants: a proven energy-savings Example: Assuming a refinery with a capacity of investment” by: R.M. Wilson and J.J.Perugini; 100,000 BPD processes a low quality crude equal Betz Process Chemicals, Inc. The Woodlands, to 20% of the total capacity. The extra incentive Texas AM-85-52 National Petroleum Refiners would be $0.5/bbl opportunity crude, after sub- Association tracting the cost of chemical treatment and any 2. “Predicting Crude Oil Fouling Tendency” by other additional operating expenses. The operat- D.E. Fields, R.F. Freeman and B.E. Wright; Betz ing income for the refinery would increase as fol- Process Chemicals,Inc. The Woodlands, Texas lows: Energy Progress (Vol 8, No. 4) 3. “Chemical, Mechanical Treatment Options 100,000 BPD * 0.2 * 365 days *$ 0.5 /bbl = Reduce Hydroprocessor Fouling” by: B. C. US$ 3,650,000/yr Groce; Betz Process Chemicals, Inc. The Wood- lands, Texas Oil & Gas Journal, January 29, Conclusions 1996 Fouling of refinery process equipment results from a 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 exacts significant economic and operational penalties. Chemicals, if correctly selected and applied, can cost effectively reduce fouling in several critical areas throughout the refinery. . The appropriate tools need to be used to understand and quantify the 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 have led 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 proven to be very effective, which is supported by real life case histories. Treatment programs are justified based on the preset treatment objectives, which

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