Computers and Chemical Engineering 33 (2009) 1413–1426 Contents lists available at ScienceDirect Computers and Chemical Engineering journal homepage: www.elsevier.com/locate/compchemeng Scheduling of loading and unloading of crude oil in a refinery using event-based discrete time formulation Georgios K.D. Saharidis a,∗ , Michel Minoux b , Yves Dallery a a Laboratoire Génie Industriel, Ecole Centrale Paris, Grande Voie des Vignes, 92295 Chatenay-Malabry Cedex, France b Laboratoire d’Informatique de Paris 6, Université Pierre et Marie Curie Place Jussieu, 75005 Paris, France article info Article history: Received 25 July 2008 Received in revised form 4 February 2009 Accepted 20 February 2009 Available online 6 March 2009 Keywords: Scheduling Crude oil Mixed integer linear programming abstract One of the most critical activities in a refinery is the scheduling of loading and unloading of crude oil. Better analysis of this activity gives rise to better use of a system’s resources, as well as control of the entire supply chain. It is important that the crude oil is loaded and unloaded contiguously, primarily for security reasons (e.g. possibility of system failures) but also to reduce the setup costs incurred when flow between a dock and a tank or between a tank and a crude distillation unit is reinitialized. The aim of the present paper is to develop an exact solution approach, widely applicable to most refineries where several modes of blending and several recipe preparation alternatives are used. A novel time formulation is proposed for the scheduling of the system under study called event-based time representation where the intervals are now based on events instead of hours. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Production scheduling defines which products should be pro- duced and which products should be consumed in each time instant over a given period (typically 30 days); hence, it defines which run- mode to use and when to perform changeovers in order to meet the market needs and to satisfy the demand. The study presented in this paper focuses on the scheduling of loading and unloading of crude oil in intermediate storage tanks, between docks and Crude Distillation Units (CDUs) and/or Vacuum Distillation Units (VDUs). A refinery is a system composed of docks, pipelines, a series of tanks to store the crude oil (and prepare the different blends), CDUs, VDUs, production units (such as reforming, cracking, alkylating and hydrotreating), blenders and tanks to store the raw materials and the final products. Once the quantities and the types of crude oil required are known, schedulers must schedule the loading and unloading of tanks. The problem that arises then is how to schedule the transfer of crude oil from the docks to the tanks and from the tanks to the CDUs/VDUs, minimizing the setup cost of the system. There are several different ways that a refinery can receive crude oil: (a) through the use of a pipeline, (b) through the use of tankers or (c) through a combination of both pipelines and tankers. In a typical refinery system, after crude oil is loaded into the tanks it must remain there for a few hours in order to be separated from the seawater. Tanks typically have the capacity to hold hundreds ∗ Corresponding author. Tel.: +306977404429. E-mail address: saharidis@gmail.com (G.K.D. Saharidis). of thousands of cubic meters of crude oil. The crude oil must be stored there before being sent to a CDU/VDU. It is very important that the crude oil is loaded and unloaded contiguously, both for security reasons and to reduce the setup cost incurred when flow between a dock and a tank or between a tank and a CDU/VDU is reconfigured. The system under study in this paper is composed of a series of tanks to store crude oil, docks to accommodate the boats deliver- ing the crude oil, blenders and CDUs/VDUs. Moreover, this system is also composed of pipelines, which connect the docks with the tanks and the tanks with the CDUs and VDUs (see Fig. 1). Finally, electric pumps for loading and unloading the crude oil, as well as mixers for the preparation of the blends required by the CDUs and VDUs, are two other important components of the system. The objective is to schedule the loading and unloading of crude oil in the tanks to min- imize the setup cost. Scheduling must take into account operational constraints and constraints relating to the use and the availability of resources (see Section 3.2). It must also take into account dimen- sions of the system and the capacity of each resource, as well as the selected recipe preparation alternative and mode of blending. There are several modes of blending and various recipe prepara- tion alternatives, which make the problem of loading and unloading of crude oil more complicated. The modeling of these two aspects requires the introduction of a high number of decision variables and constraints into the problem. In a typical refinery there are 1–10 tanks for the storage of the crude oil. The storage capacity of each tank can range from 80,000 m 3 to 150,000 m 3 . Crude oil flows from the docks to the tanks, and its flow depends on the unload- ing capacity of the boats, ranging from 1000 m 3 /h to 5000m 3 /h. 0098-1354/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compchemeng.2009.02.005