Article Cite This: Energy Fuels 2018, 32, 1348−1357
pubs.acs.org/EF
Vacuum Residue Thermal Cracking: Product Yield Determination and Characterization Using Thermogravimetry−Fourier Transform Infrared Spectrometry and a Fluidized Bed Reactor Yuanjun Che, Junhui Hao, Jinhong Zhang,* Yingyun Qiao, Dawei Li, and Yuanyu Tian* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, 266580, China S Supporting Information *
ABSTRACT: To make full use of heavy oil by thermochemical conversion, the thermal behaviors of vacuum residue (VR) were investigated first by thermogravimetry−Fourier transform infrared spectrometry (TG−FTIR) and then via a laboratory-scale fluidized bed reactor (FBR). The TG−FTIR results showed that the changes in the absorbance of volatiles during thermal cracking were consistent with the weight loss in the derivative thermogravimetric curve. The dynamic information about the release profiles of the typical gaseous products such as CO, CO2, CH4, C2H4, light aromatics, and aliphatic olefins revealed the cleavage of varied structures and functional groups of VR at different temperatures. Moreover, the peaks for the maximum releasing rate on the evolving profiles of gaseous products became narrower and sharper, and the yield at maximum releasing rate for the gaseous species increased with increasing the heating rate. The pyrolysis experiments in a FBR with silica sand as a heat carrier showed that alkenes were the dominant gaseous products, with light olefin selectivity higher than 53%. The coke/ Conradson carbon residue ratio was lower than that in the delay coking process. Furthermore, analysis of liquid oil using gas chromatography/time-of-flight mass spectrometry showed that 1-alkenes was the most abundant decomposition product and the selectivity of total olefins from C6 to C22 was 62.74%. gas evolved during heating from 30 to 900 °C and found that the release behaviors of gaseous products for oil sand were different because different functional groups ruptured in the corresponding temperature region. Heating rate is another important factor for the heavy oil thermal cracking process. AlHarahsheh et al.13 observed that both activation energy and pre-exponential factor for oil shale increased with increasing heating rate due to heat and mass transfer effects. Trejo et al.14 analyzed the cracking of asphaltenes using TG and found that slow heating rates would produce more coke, whereas products of fast heating rate were mostly liquids and gases. Previous researchers have studied the effects of reaction temperature and heating rate on the thermal cracking of heavy oil, but the fundamental knowledge of the influences of the two process parameters on gaseous products released during the thermal cracking process of VR has not been fully investigated. Thermogravimetry−Fourier transform infrared spectrometry (TG−FTIR) is a well-recognized technique in the thermal cracking research of coal, biomass, and waste material.15−18 The online FTIR can provide important information on the thermal degradation behaviors of fossil fuel due to its effectiveness and sensitivity for detection of the gaseous products. Therefore, the present research first investigates the effect of temperature and heating on VR thermal behavior using TG−FTIR. Moreover, the products of VR thermal cracking can be typically divided into noncondensable gas, pyrolysis oil, and coke, which may be used as fuels or a feedstock for petrochemicals and other applications. Hence, it is meaningful
1. INTRODUCTION Petroleum residues usually refer to the heavy fractions generated in petroleum refining, such as atmosphere residue (AR) and vacuum residue (VR). The typical residues are high in metal content and coke/Conradson carbon residue (CCR).1 Thus, the conversion of these heavy petroleum residues into light distillates is challenging. Thermal cracking processes such as delayed coking, visbreaking, and fluid coking have been widely used for the conversion of VRs.2−4 Maximizing liquid yield together with minimizing the capital investment and operating costs has been an advantage for thermal cracking processes in upgrading of crude oils. Besides, thermal cracking is also a simple and effective method to remove sulfur and metal element from heavy oils. Thus, the thermal cracking reaction is important for pretreatment of the heaviest fractions of petroleum. Thermochemical conversion of residues occurs according to a free radical mechanism.5,6 To understand the performance of the thermal cracking process and improve the process design, it is important to grasp the mechanisms and characteristics of the thermal cracking reaction. Previous researchers have paid extensive attention to studying the mechanism of cracking by model compounds,7,8 but failed to clarify quantitatively and qualitatively how many model compounds are present in heavy oils. Besides, thermogravimetric analysis (TGA) has been widely used to study the thermal degradation and kinetic parameters of heavy oils.9,10 Many researchers have evaluated the effect of temperature on changing characteristics of the mass loss during heavy oil thermal cracking. Elbaz et al.11 have reported that decomposition of heavy fuel oil consisted of three main stages: the low temperature region, the fuel deposition region, and the high temperature region. Jia et al.12 analyzed the © 2018 American Chemical Society
Received: November 1, 2017 Revised: December 28, 2017 Published: January 4, 2018 1348
DOI: 10.1021/acs.energyfuels.7b03364 Energy Fuels 2018, 32, 1348−1357
Article
Energy & Fuels Table 1. Properties of Shengli Vacuum Residue
a
density(20 °C), kg/m3
viscosity (100 °C), mm2·s−1
CCRa, wt %
C, wt %
H, wt %
S, wt %
N, wt %
O, wt %
H/C
980
900
15.70
87.00
12.00
0.26
0.45
0.22
1.66
CCR is Conradson carbon residue.
Figure 1. Schematic diagram of the fluidized bed experimental apparatus. on the mass and heat transfer. Samples were then heated from ambient temperature to 800 °C at various heating rates (80 °C/min, 160 °C/ min, 320 °C/min and 640 °C/min) in N2 flow (100 mL/min). A transfer line made of polytetrafluoroethylene was used to connect the TGA with the FTIR, with the line temperature kept at 200 °C to minimize secondary reactions. The FTIR was equipped with a 100 mm path length infrared gas cell (cell volume 38.5 mL) whose temperature was also maintained at 200 °C to avoid volatile condensation. The spectral region of the FTIR was 4000−500 cm−1 with a resolution 4 cm−1, and the scanning times were 16 scans per sampling. 2.2.2. Kinetic Analysis. The Friedman procedure, a model-free method, has been widely used to determine activation energy based on an isoconversional. Generally, the description of the kinetics of the VR thermal cracking reactions is to use a first-order reaction for the overall weight loss of volatiles evolution.21 The differential form of the reaction mechanism function of the first-order reaction is
to know the product properties of the pyrolysis gas and oil. Nevertheless, the TG−FTIR apparatus does not allow the product collection; thus, the exact product yield could not be obtained. Fortunately, thermal conversion using a fluidized bed reactor (FBR) allows the separation of pyrolysis products and the direct investigation of distributions and compositions of the yielded solid, liquid, and gas. In addition, previous studies commonly used the calcium aluminate and kaolin catalyst as the heat carriers to study the upgrade of VR in an FBR.19,20 However, these bed materials had some catalytic activity and were used to study the catalytic cracking effects of VR. In contrast, silica sand almost possesses no acidic sites, which means that the catalytic cracking effects can be excluded. Thus, it is appropriate to use silica sand as bed material for studying the thermal cracking behaviors of VR. The purpose of this work is to investigate the thermal behavior, pyrolysis product yields, and compositions of VR under various conditions. First, the weight loss behavior, kinetic parameters, and gas evolution characteristics of thermal cracking of VR were investigated using TG−FTIR. Then, VR was thermally cracked in a laboratory-scale FBR to collect and analyze the thermal cracking products including noncondensable gas, pyrolysis oil, and coke. The combined use of these two complementary techniques would give a deep understanding of the thermal decomposition of the VR.
f (α) = 1 − α
(1)
The reaction rate for thermal cracking of VR can be expressed by
⎛ E ⎞ da ⎟f (α) = A exp⎜ − ⎝ RT ⎠ dt
(2)
where t is the time, α is the extent of conversion, A is the preexponential facto, E is the apparent activation energy, and R is the gas constant. The α in eq 1 can be calculated from
α=
2. EXPERIMENTAL SECTION 2.1. Materials. A VR was provided by Shengli oil field (Shandong Province, China). Its properties are shown in Table 1. The VR is rather heavy, as shown by its high density, viscosity, and CCR. The heat carriers used in the experiments were silica sands. The result of XRF indicated that the content of SiO2 in silica sand was 99.5 wt %. The specific surface area, packing density, and particle size of the sand were
