1099 † To whom correspondence should be addressed. E-mail: sadighi_sepehr@yahoo.com Korean J. Chem. Eng., 27(4), 1099-1108 (2010) DOI: 10.1007/s11814-010-0172-0 RAPID COMMUNICATION 4-Lump kinetic model for vacuum gas oil hydrocracker involving hydrogen consumption Sepehr Sadighi * ,† , Arshad Ahmad*, and Mehdi Rashidzadeh** *Universiti Teknologi Malaysia, Faculty of Chemical and Natural Resources Engineering, 81310 UTM Skudai, Johor Bahru, Malaysia **Catalysis Research Center, Research Institute of Petroleum Industry, P.O. Box 14665-137, Tehran, Iran (Received 9 September 2009  accepted 4 November 2009) Abstract −A 4-lump kinetic model including hydrogen consumption for hydrocracking of vacuum gas oil in a pilot scale reactor is proposed. The advantage of this work over the previous ones is consideration of hydrogen consumption, imposed by converting vacuum gas oil to light products, which is implemented in the kinetic model by a quadratic expression as similar as response surface modeling. This approach considers vacuum gas oil (VGO) and unconverted oil as one lump whilst others are distillate, naphtha and gas. The pilot reactor bed is divided into hydrotreating and hydrocracking sections which are loaded with different types of catalysts. The aim of this paper is modeling the hy- drocracking section, but the effect of hydrotreating is considered on the boundary condition of the hydrocracking part. The hydrocracking bed is considered as a plug flow reactor and it is modeled by the cellular network approach. Initially, a kinetic network with twelve coefficients and six paths is considered. But following evaluation using measured data and order of magnitude analysis, the three route passes and one activation energy coefficient are omitted; thus the num- ber of coefficients is reduced to five. This approach improves the average absolute deviation of prediction from 7.2% to 5.92%. Furthermore, the model can predict the hydrogen consumption for hydrocracking with average absolute de- viation about 8.59% in comparison to those calculated from experimental data. Key words: Vacuum Gas Oil, Hydrotreating, Hydrocracking, Lump Kinetic Model, Hydrogen Consumption INTRODUCTION Crude oils contain a large fraction of heavy products for which only few outlets exist. Indeed, the world demand for light and middle distillate continually increases, while at the same time, the available crude oil becomes heavier [1]. Therefore, upgrading of heavy crude oil fractions to more useful lighter products is indispensable. Hydro- cracking is one of the most important processes in a modern re- finery to produce low sulfur diesel. The versatility and flexibility of the process makes it economically attractive to convert different types of feedstock into various yields including gas, LPG, naphtha, kerosene and diesel, leading to its widespread applications. Among all the commercially proven technologies for heavy fraction hydro- cracking, those using fixed-bed reactors in series charged with dif- ferent functionalities are very favorable. But, the main disadvantage of fixed-bed reactors is the loss of catalyst activity over time as a result of catalyst deactivation which reduces drastically the length of run [2]. In the particular case of vacuum gas oils (VGO), a pre- vious HDT stage, first stage, for removing nitrogen, sulfur and metal compounds as well as saturation of polynuclear aromatics (PNA) to preserve catalyst from fast deactivation is required [3]. During the HDT process a portion of the hydrogen, dependent on HDS and HDN reactions, is consumed and most of the heavy sulfur and ni- trogen compounds are converted to lighter products. Therefore, it can be concluded that a part of the desirable products and con- sumed hydrogen are the share of HDT in the first stage which should be considered during kinetic modeling of hydrocracking reactions in the second stage. Typical of industrial processes, optimal operation is required to guarantee profitability, and such a task necessitates the use of pro- cess models. These models are used to predict the product yields and qualities, and are useful for sensitivity analysis, so that the effect of operating parameters such as reactor temperature, pressure, space velocity, as well as others on product yields and qualities can be understood. The models can also be used for process optimization and control, design of new units and selection of suitable hydroc- racking catalysts [4]. However, the complexity of hydrocracking feed makes it extremely difficult to characterize and describe its kinetics at a molecular level [5]. One way of simplifying the prob- lem is to consider the partition of the species into a few equivalent classes, the so-called lumps or lumping technique, and then assume each class is an independent entity [6]. This approach is attractive for kinetic modeling of complex mixtures because of its simplicity [7]. Mosby et al. [8] reported a model that describes the performance of a residue hydrotreater using lump first-order kinetics. The pro- posed model divides residue into lumps that are “easy” and “hard” crack. This lumping scheme was used by Aboul-Gheit [9] to deter- mine the kinetic parameters of vacuum gas oil (VGO) hydrocrack- ing, expressing composition in molar concentration. In his four-lump kinetic model, VGO was converted to gases, gasoline, and middle distillates. The model had eight kinetic constants that were esti- mated by experiments performed in a fixed-bed plug flow micro