Article pubs.acs.org/EF
Narrow Fraction Model with Secondary Cracking for Low-Severity Thermal Cracking of Heavy Oil Junwei Yang, Dongxiao Chen, Guoping Shen, Jiazhi Xiao,* and Chaohe Yang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, People’s Republic of China ABSTRACT: Thermal reaction experiments of vacuum residuum were conducted in a micro-batch reactor at 410−480 °C. The product yield variation with time indicated that the secondary cracking reaction mainly occurs in the heavy fraction of the liquid product. A narrow fraction model for low-severity thermal cracking of heavy oil, with the secondary reaction taken into account, was developed. To simplify the reaction network, a pseudo-lump was introduced, which was defined as the fraction generated from the secondary cracking reaction of the heavy fraction (420−540 °C). Furthermore, a sequential method to estimate rate constants in complex models was employed, and the kinetic parameters were estimated by Arrhenius’ law. The model-predicted lump yields agreed well with the experimental values. The ratio of secondary cracking/primary cracking was calculated. The result indicated that, even at the initial stage of the reaction, the ratio in the heavy distillate reaches up to 15−36%; therefore, the secondary cracking reaction cannot be ignored even in low-severity thermal cracking.
1. INTRODUCTION Upgrading petroleum residues into light products is crucial to satisfy the increasing demands of transport fuel and petrochemicals. However, the typical residues are rich in metals and asphaltenes. Metals act as poisons for catalysts, and asphaltenes cause deactivation of the catalyst1 by coke formation. Delayed coking2,3 is a main process of thermal cracking, in which the feedstock can be any undesirable heavy stream containing high asphaltene and metal content. A common feed for this process is vacuum residue, and it can also accept fluid catalytic cracking slurry oil and bitumen. In delayed coking processing,4 the heat required to complete the coking reaction is supplied by a coking furnace, while coking itself takes place in drums. The feed to the coke drums is heated from 300−400 to 490−510 °C in the coking furnace, which is a core equipment in this process. As the heavy oil begins to crack at 350 °C,5 there is a part of the thermal cracking reaction in the tube of furnace. That is necessary to be taken into account in the design of the coking furnace, because the apparent velocity and residence time in the tube are all critical parameters for the design and operation of the coking furnace.6 However, the product distribution of the thermal cracking reaction affects these parameters directly; therefore, it is important to establish an accurate model for the product distribution in thermal cracking of heavy oil, which is suitable for the calculation of apparent velocity and residence time in the furnace tube. The thermal cracking behavior of heavy oil has been extensively studied,7−12 and many kinetic models were proposed for different processes and conditions. For most of the lumping kinetic models, it could probably be divided into two categories. One is generally lumped by the products of the process, e.g., visbreaking, delayed coking, etc. Most of the reported models can be classified in this category.13−18 The other is the narrow fraction model,19−21 which is usually employed in the simulation of the reaction process. For the former case, it mainly focused on the product distribution of © 2012 American Chemical Society
the process, e.g., gas, gasoline [initial boiling point (IBP)−150 °C], gas oil (150−350 °C), coke, etc. However, this lumping scheme is too rough for the calculation of apparent velocity and residence time, because the boiling range of the product lumps are too wide to calculate their physical properties accurately. In view of this, Filho et al.22 proposed a parallel reaction model for the residue cracking furnace. Xiao et al.23 developed a narrow fraction model with 12 lumps. However, these parallel reaction models cannot predicate the lump yields of the heavy fraction in the liquid product accurately, because it is assumed that the products have no secondary reaction, which is not suitable for the thermal cracking behavior of heavy oil. The aims of this work were therefore to study the secondary cracking behavior of the liquid product during low-severity cracking of heavy oil and to develop a narrow fraction model, with the secondary reaction taken into account. The experimental data were obtained in the laboratory by thermal cracking of the vacuum residue in a batch reactor.
2. EXPERIMENTAL SECTION Experiments were conducted in a 20 mL stainless-steel batch reactor. A schematic diagram of the experimental setup is presented in Figure 1. This apparatus consists of a micro-batch reactor, a temperature control system, and a product collection system. The reactor was heated by a metal bath and quenched by ice water. The temperatures of the reactor and metal bath were measured by two thermocouples, and special software was developed to control and record the temperature profile. The oil gas after the reaction was cooled, separated into liquid and gas samples, and then collected in a receiver and a gas collection bottle, respectively. The sample from the reactor was collected at approximately 120 °C, and the reaction temperature was 410−480 °C; thus, there was a little lighter fraction in the reactor. Furthermore, the material balance was always above 99%, and the loss may be caused by the remains and volatilization in the pipeline. Received: March 16, 2012 Revised: May 3, 2012 Published: May 4, 2012 3628
dx.doi.org/10.1021/ef3004625 | Energy Fuels 2012, 26, 3628−3633
Energy & Fuels
Article
3. KINETIC MODEL 3.1. Model Description. The thermal cracking product contains gas and liquid fractions. It is necessary to establish a kinetic model with detailed product distributions to describe reaction pathways. Therefore, the liquid product was classified into 11 lumps according to the boiling point. Most of studies reported24−26 that the thermal cracking of heavy oil is amenable to the first-order reaction, and the secondary cracking reaction mainly occurs in the heavy fraction of the liquid product. Figure 2 shows the curve between lump
Figure 1. Schematic diagram of the experimental setup: 1, gas collector; 2, liquid product receiver; 3, electric heater; 4, reactor; 5, temperature control system; 6, computer; 7, condenser; 8, molten tin bath; and 9, thermocouple. Time from the temperature of the beginning cracking reaction of 350 °C to the reaction temperature of 410−480 °C was found to be about 1−2 min. The effect of the non-isothermal reaction at initial and final stages on reaction results is taken into account, which is described in the section 3.3. To study the thermal cracking behaviors of an inferior residue, Saudi Arabia VR, containing 25 wt % Conradson carbon residue (CCR), 15.7 wt % n-heptane asphaltenes, and about 215 μg g−1 of metals, was used in the experiments. The main properties of the feedstock were listed in Table 1.
Figure 2. Lumps yields yi versus reaction time at a temperature of 440 °C: 540 °C, 480−540 °C; 480 °C, 450−480 °C; 450 °C, 420−450 °C; and 420 °C, 390−420 °C.
Table 1. Properties of the Vacuum Residue property density at 20 °C (g cm−3) carbon (wt %) hydrogen (wt %) sulfur (wt %) nitrogen (wt %) saturates (wt %) aromatics (wt %) resins (wt %) n-heptane asphaltene (wt %) naphthenic aromatics (wt %) polar aromatics (wt %)
value
property
yields yi and the reaction time at a temperature of 440 °C. It indicates that lumps from 420 to 540 °C occur as a secondary cracking reaction obviously. Furthermore, a pseudo-lump Cm was introduced to simplify the reaction network. This pseudolump was defined as the fraction generated from secondary cracking of heavy lumps (420−540 °C). Then, the assumptions of this narrow fraction model (Figure 3) are as follows: (1) All
value
1.037
CCR (wt %)
25
84.31
4684
9.8 5.3 0.35 5.4 50.5 28.4 15.7
kinetic viscosity at 100 °C (mm2 s−1) molecular weight (g mol−1) n-pentane asphaltene (wt %) Ca (μg g−1) Fe (μg g−1) Mg (μg g−1) Na (μg g−1) Ni (μg g−1)
1033 23.1 2.2 5.3 0.3 3.3 49.7
30.1
Pd (μg g−1)
