Refinery-wide scheduling for optimization of multiple unit-operations in the supply, production, and demand chains in fuels, lubes, asphalts and petrochemicals industries

We present a refinery-wide scheduling optimization problem found in every process industry with mixing, transforming and separation where the transforming and separation activities in the crude-oil refinery industry are referred to as reacting/converting processes and distilling/fractionating processes, respectively. For this study we highlight only the quantity and logic aspects or phenomena of the problem (what we term logistics) using mixed-integer linear programming (MILP). The quantity and quality details of the problem (what we term quality) using nonlinear programming (NLP) are not described. The NPL attributes such as densities, components, properties and conditions require special processing subtypes such as blenders, splitters, separators, reactors, fractionators and black-boxes [1]. In the proposed refinery-wide scheduling approach, the goal is to investigate the limits or capabilities of an enterprise-wide optimization (EWO) by solving simultaneously the supply, production, and demand chains using MILP. This finds time-varying setups of unit-operations and their connections that construct the flowsheet operations to further NLPs by fixing the MILP results. Such EWO setups maybe used in detailed calculations within other edges such as depooling of aggregated tanks by considering their actual topology and operations [2] or decision-automation for production of lubes and asphalts using sequence-dependent switchovers [3]. Figure 1 shows the oil-refinery flowsheet using the unit-operation-port-state superstructure (UOPSS) [4], [5] in a discrete-time modeling. The rectangular shapes () with the cross-hairs are continuous-process types of unit-operations with modes. The triangular shapes are pool unit-operations and the diamond shapes are called perimeters and represent the points where resources or materials enter and leave the flowsheet i.e., sources and sinks. The circles without cross-hairs are inlet ports and with cross-hairs are outlet ports where ports are the flow interfaces to the unit-operations. The lines with arrow-heads (→) connecting outlet-ports to inlet-ports are called external streams or in graph theory they are called arcs or directed-edges. The lines without arrow-heads are internal streams and refer to inlet ports connected to unit-operations and unit-operations connected to outlet ports. The rectangular shapes without cross-hairs are pipeline unit-operations and the upside-down triangular shape is a parcel unit-operation which is modeled as a batch-process and represents the flow of material from one place to another and may represent road, rail and marine modes of transportation [6]. The dotted line boxes surrounding several of the shapes implies that the physical unit has more than one procedural operation associated with it (has multiple modes of operation) and together forms what we call a projectional unit-operation i.e., a physical unit times a procedural operation equals a projectional unit-operation. Each unit-operation and external stream have both a quantity and a logic variable assigned or available and represent either a flow or holdup if quantity and either a setup or startup if logic. Continuous-processes have flows and setups and pools have holdups and setups where perimeters only have a logic setup variable. The internal streams have neither explicit nor independent flow and setup variables given that their flows are uniquely determined by the aggregation of the appropriate external streams and their setups are taken from the setup variables on the unit-operation they are attached to. Although Figure 1 shows an almost complete and integrated oil-refinery excepting the crude-oil and product blending and storing areas, we highlight only the Crude-oil Distillation Unit (CDU), Vacuum Distillation Unit (VDU) and Delayed-Coking Unit (DCU). The other included major process units known as the Catalytic Reforming Unit (CRU), Fluidized Catalytic Cracking Unit (FCCU), Hydrocraking Unit (HCU), Gas Processing Units (GPU1 and GPU2), Alkylation Unit (AKU), Hydrogen Producing Unit (HPU) and Steam Producing Unit (SPU) are not discussed further. The two-mode CDU with operations of maximum gasoline (MAXGSL) and maximum diesel (MAXDSL) is of interest given that we have included a pipeline with significant holdup where two segregated crude-oil mixes labeled MAXGSL and MAXDSL are transported from the crude-oil blending and storing area located several kilometers away from the oil-refinery typically at an off-site terminal. Following the pipeline modeling found in Zyngier and Kelly [6] and given 30 one-day discrete time-periods, a pipeline with two modes (MAXGSL and MAXDSL) can be arbitrarily shutdown at any time (i.e., has zero or no flow) in any time-period and when flowing has a constant nominal flowrate. As can be seen in Figure 2 for the Gantt chart resource row "PIPELINE,MAXGSL,o," in time-period 3 or day 3 there is a flow out of the pipeline of the MAXGSL crude-oil mix even though the flow into the pipeline in time-period 3 is MAXDSL i.e., see Gantt chart row "PIPELINE,MAXDSL,i,". In the next time-period (time-period 4) there is a flow of MAXGSL into the pipeline but instead MAXDSL flows out of the pipeline. This is directly related to the fact that a pipeline is modeled as a first-in-first-out (FIFO) queue or renewable resource. There are two other necessary logic features of the CDU's feed operation and they are what we call multi-product and standing-gauge tanks. The multi-product logic is implemented for TANKCDU2 which can store MAXGSL and MAXDSL but not at the same time or in the same time-period. This means that when the optimizer decides to switch the tank's material service from one to the other, the holdup or heel in the tank must be at a certain amount specified by the user and in this case, it is set to zero. The standing-gauge logic, also known as a dead-tank, ensures that if there is flow into the tank then there can be no flow out of the tank and vice-versa. This is configured for the three crude-oil tanks and is apparent from the Gantt chart where the grey trend lines below the black horizontal bars indicates the flow in (port-state "i,") and flow out (port-state "o,"). The standing-gauge can be generalized to restricting that if there is flow in then the flow out be at least some number of time-periods after and this can be used to model mixing- or certification-delays for example. Figure 3 highlights the operation of the VDU which receives atmospheric residue or reduced crude-oil (ATR) from both the upstream CDU and from an off-site oil-refinery imported by rail. In this situation, the VDU feed ATR is transported by what are known as unit-trains which will usually have around 100 tanker rail-cars per train. There is a round-trip or travel-time in this case of 4-days where on the first day the ATR is loaded, on the second day it is hauled from the source oil-refinery or terminal to the destination oil-refinery, on the third day it is unloaded and on the fourth day it travels back to the source and this is called industrial-shipping as opposed to liner or tramp shipping. On the Gantt chart row labeled "TANKVDU,i," we see the extra amount of ATR being received into TANKVDU from the parcel unit-operation representing the unit-train deliveries of ATR. Figure 4 displays the Gantt chart for the DCU operation which converts the vacuum residue labeled as VR into pyrolysis naphtha (PN) and distillate (PD) as well as light gases such as methane, ethane (C1C2), propane, propylene, butanes and butylenes (C3C4) including a solid coke material (COKE). The interesting aspect of the DCU is the production of coke which is modeled as a semi-batch arrangement using pool unit-operations. The two physical coke drums COKEDRUM1 and COKEDRUM2 have two operations or modes of FILL and DRAW where the FILL operation can only charge material and the DRAW mode can only discharge material. This means that if COKEDRUM1 is filling then it cannot be drawing which implies that COKEDRUM2 must be drawing. Conversely, if COKEDRUM1 is drawing then it cannot be filling and therefore COKEDRUM2 must be filling. We can observe this behaviour or functionality on the Gantt chart where we see that when COKEDRUM1 is in the FILL mode COKEDRUM2 is in the DRAW operation and vice-versa.