Crude Oil Scheduling In Refinery Operations - Core

NomenclatureBinary VariablesXPpt1 if parcel p is connected to SBM/jetty discharge line during period tXTit1 if a tank i is connected to SBM/jetty discharge line during period tYiut1 if a tank i feeds CDU u during period tZii′t1 if crude transfers between tanks i and i′ during period t0-1 Continuous VariablesXFpt1 if a parcel p first connects to the SBM/jetty during period tXLpt1 if a parcel p disconnects from the SBM/jetty at time tXpit1 if a parcel p and tank i both connect to the SBM line at tYYiut1 if a tank i is connected to CDU u during both periods t and (t 1)COut1 if a CDU u has a changeover during period tContinuous VariablesTFpTime at which parcel p first connects to SBM/jetty for unloadingTLpTime at which parcel p disconnects from SBM/jetty after unloadingFPTpitAmount of crude transferred from parcel p to tank i during period tFTUiutAmount of crude that tank i feeds to CDU u during period tFUutTotal amount of crude fed to CDU u during period tFCTUiuct The amount of crude c delivered by tank i to CDU u during period tVCTictAmount of crude c in tank i at the end of period tVitCrude level in tank i at end of period tfictConcentration (volume fraction) of crude c in tank i at the end of period tDCvDemurrage cost for vessel vSCtSafety stock penalty for period tZTitVariable to denote the number of times tank i exchanges crude withanother tank in a given period tFCTTii′ctThe amount of crude c transferred from tank i to tank i′ during period tFTTii′tTotal amount of crude transferred between tank i to tank i′during tAFTTii′tAbsolute amount of crude transferred between tank i to tank i′ during tChapters 7-9 (Part II)SubscriptsiStorage tanksxi

NomenclaturecCrude TypesuCrude Distillation Units (CDUs)vVesselstTime periodss, s′Slotsp, p′, p″ParcelsSuperscriptsUUpper limitLLower limitSetsPSSet of pairs (parcel p, slot s) such that p can connect to SBM line during sTSSet of pairs (period t, slot s) such that slot s is in period tPISet of pairs (parcel p, tank i) such that i may receive crude from pIUSet of pairs (tank i, CDU u) such that i can feed CDU uICSet of pairs (tank i, crude type c) such that i can hold cPCSet of pairs (parcel p, crude type c) such that p has crude cPVSet of pairs (parcel p, vessel v) such that p is the last parcel of vSSSet of pairs (slots s and s′ with s′ s) such that a tank receiving crude in smay settle brine up to the beginning of s′ParametersETApExpected time of arrival of parcel pFPTpiULimit on the rate of crude transfer from parcel p to tank iFTU iuL / ULimits on the rate of crude charge from tank i to CDU uFU uL / ULimits on the crude processing rate of CDU uL/UxccuLimits on the composition of crude type c in feed to CDU uxkkuL / ULimits on the concentration of key component k in feed to CDU uVi L / UAllowable limits on crude inventory in tank ixticL / ULimits on the composition of crude type c in tank iTCDTotal crude demand in the scheduling horizonDM uTotal crude demand for CDU u in the scheduling horizonCDutCrude demand for CDU u in period tCPcuMarginal profit ( /unit volume) from crude c in CDU uxii

NomenclatureCOCCost (k ) per changeoverSSPtSafety stock penalty ( / unit volume below desired safety stock) for periodtSStDesired safety stock (kbbl) of crude inventory for period tSWCvDemurrage or Sea waiting cost ( /unit time)STDvStipulated time of departure as mentioned in the logistics contract ofVLCC vETUpEarliest possible unloading period for parcel pVPpSize (m3 or bbl) of parcel pNPTotal number of parcels to unload during the horizonNSTotal Number of slots in the scheduling horizonDtStart time of period tDDt Dt D(t–1) or length of period tSTMinimum time for crude settling and brine removalBinary VariablesXPps1 if parcel p is connected to the SBM during slot sXTis1 if tank i is connected to the SBM during slot sYius1 if tank i feeds CDU u during slot s0-1 Continuous VariablesXFps1 if parcel p first connects to the SBM during slot sXLps1 if parcel p disconnects from the SBM at the end of slot sYTis1 if tank i is delivering crude during slot sZTis1 if tank i is idle or settling during slot sCOus1 if CDU u has a changeover at the end of slot sContinuous VariablesTLsEnd-time of slot sSLsLength of slot sRLPpsTime for which p connects to the SBM during sRLTisTime for which i connects to the SBM during sRLUisTime for which i feeds crude during sRLZisTime for which i is idle or settles brine during sRUiusTime for which i feeds u during sRPpisTime for which p unloads crude into i during sFPTpisAmount of crude transferred from p to i during sxiii

NomenclatureFTUiusAmount of crude that i feeds to u during sFUusTotal amount of crude feed to u during sFCTUiucsAmount of c delivered by i to u during sVCTicsAmount of c in i at the end of sVisCrude level in i at the end of sficsConcentration (volume fraction) of c in i at the end of sDCvDemurrage for vessel vSCtSafety stock penalty for period txiv

LIST OF FIGURESFigure 3.1Schematic of oil unloading and processing33Figure 3.2Operation schedule for the motivating example37Figure 3.3Operation schedule for the motivating example of Lee et al.(1996)40Figure 4.1Schematic representation of parcel creation48Figure 5.1Flow chart for the solution algorithm67Figure 6.1Key component concentration in CDU feeds at varioustime periods for example 283Figure 7.1Schematic of oil unloading using SBM and processing inCDUs95xv

LIST OF TABLESTable 3.1Candidate and optimal schedules for the motivating example39Table 3.2Profit Comparison for candidate and optimal schedules39Table 3.3Key component concentration in the feed to CDU for differentallowable concentration limits (Cases 1 & 2) for chargingtanks in Lee et al. (1996) motivating example and feedcomposition discrepancy in Case 242Table 3.4Charging schedule of CDUs obtained using Li et al. (2002)approach for the motivating example44Table 4.1Constraints for different refinery configurations62Table 5.1Details of individual iterations for the motivating exampleusing the proposed algorithm70Table 6.1Crude arrival information for example 175Table 6.2Crude types and processing margin for example 175Table 6.3Initial crude inventory and storage capacities of storage tanksfor example176Table 6.4Initial crude levels, capacities of storage tanks for examples 2676Table 6.5Initial crude composition in storage tanks for examples 2-676Table 6.6Tanker arrival details, crude demands and key componentconcentration limits on CDUs for Examples 2 to 677Table 6.7Crude types, marginal profits and key component details forexamples 2-678Table 6.8Economic data and limits on crude transfer amounts forexamples 2-678Table 6.9Model performance and statistics for illustrated examples79Table 6.10Operation schedule for Example 2 of Li et al (2002) obtainedvia our approach80Table 6.11Jetty allocation Schedule for Example 2 of Li et al (2002)obtained via our approach80xvi

List of TablesTable 6.12Operation schedule for Example 282Table 6.13Solution details for Example 283Table 6.14Operation schedule for Example 3 (contd.)85Table 6.14Operation schedule for Example 386Table 6.15Operation schedule for Example 487Table 6.16Berth allocation details for Example 487Table 6.17Operation schedule for Example 589Table 6.18Comparison of schedules without and with tank transfers forExample 690Table 9.1Tanker arrival details, crude demands and key componentconcentration limits on CDUs114Table 9.2Storage tank capacities and initial inventory of crude stock115Table 9.3aEconomic data and limits on crude transfer amounts116Table 9.3bCrude types, key component concentrations and marginalprofits116Table 9.4Performances of the continuous and discrete models on theexamples117Table 9.5Statistics of the continuous and discrete models for theexamples118Table 9.6Operation schedules for Example 1 obtained from thecontinuous and discrete (Case2) models120Table 9.7Operation schedules for Example 2 obtained from thecontinuous and discrete (Case2) models123Table 9.8Operation schedules for Example 3 obtained from thecontinuous model and discrete (Case2) models124xvii

Chapter 1INTRODUCTION1.1 Introduction to Refining industryPetroleum is the second largest consumable on the planet, second only to water. Themodern society is dependent on petroleum products more than any other naturalresource for the comforts and convenience. The petroleum business involves manyindependent operations, beginning with the search for oil and gas and extending to thedelivery of finished products, with incredibly complex manufacturing processes in themiddle. Each of these processes has unique objectives and demands. Unlike discretemanufacturing, petroleum manufacturing does not start with bill of materials. A bill ofmaterials is a list of all the materials that go into making a particular product. In caseof petroleum processes feed stock and intermediates can come from a number ofdifferent sources with significantly different qualities, and yet be processed andcombined to form precise products, meeting all regulatory specifications as well asshipping times. Petroleum refining is a typical continuous process plant that has acontinuous flow of material going in and coming out. Crude oil is basic raw materialfor producing most of petroleum products. Petroleum refining involves separatingcrude oil into its constituents, purifying them, and converting them into marketableproducts. In the refinery, crude enter the crude distillation unit and separated intocomponent streams. Some of these streams are desirable. The undesirable streams areeither subject to a series of treatments and undergo specific unit operations andprocesses in separate units such as crackers, reformers and alkylation units to yielddesirable products or blended with desirable streams into finished products.1

Chapter 1: IntroductionUndesirable streams may also be sold off or used as low-cost fuels. A general refineryconfiguration includes crude handling facilities, distillation units, vacuum distillationsunits, catalytic reforming units, fluid catalytic/Hydro cracking units, hydro treaters,visbraker/delayed coker units and off-site storage/blending facilities to store/processthe finished products/intermediate streams. Distillation, vacuum distillation andcatalytic reforming units are normally grouped as primary processing units. Fluidcatalytic/hydro crackers, hydro treaters, visbraker/delayed coker units are grouped assecondary units where heavier hydrocarbon streams are converted to more useful fuelstreams. Quality of the finished petroleum products is very important and has to bestrictly complied. Since crude is the basic raw material for producing these products,quality of the crude oil is very crucial. Based on long range forecast of productdemand, crude oils are often planned, purchased, and have a delivery schedule set longbefore they arrive at the refinery. In addition, crudes originated from different sourceshave different qualities and product patterns, and even some times crudes from thesame origin differ in quality. So all the products cannot be produced from every typeof crude, and this difference in yield/quality poses different set of constraints on thedown stream treatment/processing units resulting in processing bottlenecks, lowerthroughputs, excessive productions etc. Crude assay provides detailed yields/propertiesof different cuts based on their boiling range. Impurities present in crudes limit theirprocessing to produce acceptable products. Impurities can be in the form of high sulfurlevels, higher metal content, higher levels of basic nitrogen or naphthenic acids etc.Impure crudes are relatively cheaper but fewer refineries are equipped with therequired technology that allows processing these crudes. Capital investment inacquiring, updating to the latest technologies is quite high and return on investment isnot very attractive. The operating conditions, processing requirements, available2

Chapter 1: Introductiontechnology, and production demands forces refiners to select such crudes which can beprocessed neat or by blending among the available basket of crudes. Storage orprocessing segregation of crude types is a common feature in refinery. Segregation isbased on key component levels, impurity levels in crudes and processability to yieldspecific product pattern. Driven by market requirements, refinery operates in differentmodes, each mode producing a different set of products. Sometimes the streams fromdifferent modes are blended to meet product quality specification. Mode switch ischanging the feed to distillation unit from one type of crude segregation to other. Everymode switch produces some off-spec production and needs reprocessing and modeswitches are inevitable in long run to meet demands. Normally refineries with multiCDU configuration segregate processing and avoid mode switches unless inevitable.Within segregated mode operation, change in feed composition to processing unitresults in perturbed operation in the unit resulting some production loss. Given a set ofcrudes, considerable effort/expertise is required to identify better crude combinationsfor processing in order to meet customer commitments and to boost gross profitmargins. Thus, the refining industry operates under uncertain product demand, anduncertain manufacturing/plant capabilities leading to deviations in actual performanceto operating targets set by the monthly refinery plan.1.2 Need for SchedulingEvery industrial manufacturing business aspires to have maximum profit/return oninvestment. For being a market leader, the company needs to have a good globalcoverage, efficient consumer services, lower production costs and reduced inventorylevels. Apart from these multiple objectives, the dynamic nature of demands anduncertainties involved in a refinery makes the life of higher management more andmore difficult. The data flow into the system, in terms of sale targets/forecasts,3

Chapter 1: Introductionmaterial/product inventories, manufacturing costs and deadlines (product deliverydates, crude arrival time, etc), is also enormous. All these factors must be taken intoaccount for planning and scheduling of operations.With a substantial increase in computational power of modern day computers,there has been renewed interest in finding solutions to these planning/schedulingproblems keeping in view as many factors as possible. The next-generation decisionsupport systems, which employ Advanced Planning and Scheduling (APS) techniques,coupled with supply chain management considerations have been in course ofdevelopment. With the advent of e-business age, when the dynamics of supply chainsand collaborations are redefining ways to conduct business, the core of success stillrelies on seamless flow of information and material across various business ‘nodes’ in‘networked’ economies. In such a high velocity environment, the goal is to optimizethe operations to have maximum gains and planning/scheduling forms the foundationof this process.In recent years, refining has become an extremely competitive businesscharacterized by fluctuating demands for products, ever-changing raw material prices,and the incessant push towards cleaner fuels. To survive financially, a refinery mustoperate efficiently. From an operational perspective, a plant would operate the best in asteady state with consistent feed stocks and product requirement and all units operatingat full capacity. Any change is undesirable, as it may lead to off-spec products, reducedthroughputs, increased equipment wear and tear, uncertainty, and more work.Nevertheless, in the current competitive environment, profit depends on agility, i.e.,the ability to exploit short-term opportunities to fill demand at higher profit margins.Processed crude compositions have the greatest influence on refinery margins.4

Chapter 1: IntroductionTherefore, refiners tightly control the quality of crude charge and use advancedtechnologies to plan and schedule crude oil changes.The increasing reliance on petroleum products is motivation to find better waysto process, and deliver products while maximizing the margins, minimizing the waste,and improving the profitability within the constraints set by the nature of process andenvironmental regulations. Managing the crude oil operations is vital for bettervisibility of downstream processes. Since day-to-day operation enhancements areprecursors to high throughput and lower operating expenses of a refinery, the focus inthis work is on short-term scheduling of crude oil operations. By short termscheduling, it is understood that monthly/weekly targets for the production facilitiesare known a priori and the objective is to achieve the maximum from the system so asto optimally utilize resources. Before the problem of scheduling is attempted, there is aneed for understanding the terminologies involved in refinery business andrequirements of a "good schedule".1.3 TerminologyCrude handling facilities in refinery includes crude unloading facilities, storagefacilities and processing facilities. The raw material, crude is commonly transportedusing sea route in small cargo ship called ‘crude tanker’ or ‘crude vessel’ that canarrive near to the shore for mooring. The mooring station is called ‘Jetty’. Bearing inmind the cost of transportation adding to the cost of raw material and looking at themargins of processing, it is preferable to get different small crude packets normallytermed as ‘crude parcel’ in one VLCC (very large crude cargo) ship. VLCC cannotcome near to the shore requires special facility mooring called SBM (Single buoymooring). In both cases a pipeline connects the mooring station to the storage tanks atrefinery site. The pipeline connecting the SBM to refinery storage tanks is called5

Chapter 1: Introduction‘SBM line’. Crude is stored in floating roof tanks to avoid the vaporization losses.Crude is normally pumped to a tank and pumped out of tank using different nozzlesconnected at some height from the bottom of the tank and crude below this nozzleheight will be stagnant and is called ‘Heel’ of the tank. This portion facilitates thesediments and brine to settle for periodic removal after every receipt of the crude. Thetank operation can be standing gauge or running gauge. Standing gauge operationallows only one operation either receiving or delivering at any one point of time whereas running gauge allow

Scheduling considerations prevalent with crude oil operations in a petroleum refinery have been addressed in this work. Scheduling of crude oil operations involves unloading crude oil from vessels to storage tanks and charging various mixes of crude oils from tanks to each distillation unit subject to capacity, flow, and composition limitations.