ARTICLE pubs.acs.org/EF
Stability Change of Asphaltene in the Residue during Slurry-Phase Hydrocracking Wenan Deng,*,† Hui Luo,† Jingjie Gao,‡ and Guohe Que† † ‡
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, Shandong 266555, People’s Republic of China Shandong Sunway Petrochemical Engineering Company Limited, Qingdao, Shandong 266555, People’s Republic of China ABSTRACT: Asphaltenes play a key role in the stability of the residue during processing. When asphaltenes reach their solubility limit in the residue, they would begin to aggregate, so that a new phase, called the mesophase, would separate from the oil phase, which eventually leads to coke formation. To relate coking characteristics, the changes in stability of the residue were studied during a slurry-phased hydrocracking reaction. The results indicated that the coke formation is obviously restrained by H2 and a catalyst such that the coke induction period is prolonged and the coke content is also reduced significantly compared to the coke content only in the presence of H2. The colloidal stability parameters (CSPs) determined by means of flocculation onset titration and the colloidal stability function (CSF) calculated on the basis of saturate, aromatic, resin, and C7-asphaltene (SARA) composition of the residue have a similar variation trend, which could be related to coking characteristics. First, the coking onset and the maximum asphaltene content in the residue were in correspondence with the turning point in the downward trend of the stability of the residue. The stability of the residue deteriorated significantly during the coke induction period, and the decreased trend tended to smooth after the coking onset point. Second, it is confirmed that the downward trend of stability is inhibited effectively in the presence of H2 and a catalyst, so that the ability of the residue against thermal disturbance is enhanced to reduce the coke formation. The changes in structure parameters of asphaltenes also showed that the catalyst could inhibit or delay the excessive condensations of asphaltenes to reduce the coke formation.
1. INTRODUCTION Gasoline and middle distillates, especially diesel, are becoming the demanded products; nevertheless, the demand of heavy ends is decreasing drastically. To solve these problems, high conversion of petroleum residue is a focus for refiners. Recently, slurryphase residue hydroprocessing has been attracting more attention in upgrading inferior residue,15 because hydrogenation and thermocracking are combined rationally in this technology, which could increase the liquid yield of distillates and widen the feedstock and product distribution. In the slurry-phase hydroprocessing process, the dispersed catalyst is added into the reactor together with feed and hydrogen, which could promote cracking and restrain coke formation during the catalytic reactions. Simultaneously, the catalyst could become a carrier of cokes and decrease the coke deposition on the reactor wall. However, the coke formation is one of the key factors influencing the long running period of the plant when the slurry-phase hydroprocessing is operated at high temperatures (430470 °C). Crude oil or residua is assumed a colloid structure, in which asphaltenes co-aggregate with the heavier resins of higher polarity and form a mixed micelle and the remaining fractions, including the lighter resins, aromatics, and saturates, act as the dispersing media.6 This colloidal structure is relatively stable under normal circumstances. However, asphaltenes or heavier resins, which are precursors of coke formation, will form reactive radical centers during pyrolysis and initiate condensation reactions, leading to di- and oligomers.7,8 When the concentration of these di- or oligomers exceeds their stability limits, the colloid structure would be destroyed and a new phase, called the r 2011 American Chemical Society
mesophase, separates from the oil phase, which eventually leads to the coke formation.9,10 Coke deposition is known to be enhanced by product instability with the precipitation of asphaltenes.11 For this reason, prevention or minimization of asphaltene precipitation will play a key role for the problem of coke formation. Many studies have evaluated the asphaltene stability in crude oil or residua using of the concept of the solubility parameter.1218 The main method in the determination of the solubility parameter is turbidimetric titration. The principle of this method is to make solutions of crude oil in toluene and then add a titrant (i.e., n-heptane) until precipitation occurs. Several methods for the detection of the precipitation point have been proposed in the literature, such as the Oliensis spot test using examination of drops applied on filter paper, microscopic examination of solutions, optical transmission and light scattering by particles, viscosimetry, fluorescence spectroscopy, particle size analysis, ultraviolet/visual spectroscopyattenuated total reflectance (UV/visATR) probe, etc. However, it is difficult or inconvenient to detect the precipitation through these methods, because crude oil absorbs light very efficiently over a wide range of wavelengths. A convenient method for the detection of the precipitation point could be obtained by the mass-fractionnormalized conductivity according to the study results by Fotland et al.19,20 When the precipitation occurs, the massfraction-normalized conductivity reaches the maximum, making the precipitation point normally easy to detect. Zhang et al.21 and Received: August 1, 2011 Revised: October 12, 2011 Published: October 13, 2011 5360
dx.doi.org/10.1021/ef201114t | Energy Fuels 2011, 25, 5360–5365
Energy & Fuels
ARTICLE
Table 1. Properties of LHAR specific gravity at 20 °C
0.9817
(g cm3)
viscosity at 100 °C
314.8
(mm2 s1)
CCR (wt %)
13.4
elemental analyses
ash (wt %) SARA (wt %)
0.050
C H
86.8 11.6
saturate
31.1
N
0.39
aromatic
26.1
S
0.85
resin
40.1
nickel (μg g1)
88.0
C7-asphaltene
2.70
vanadium (μg g1)
2.2
(wt %)
Wang et al.22 investigated colloidal stability variation of residue oil during thermal reaction, adopting this method, and took the ratio of added n-heptane to residue oil at the asphaltene precipitation onset point as the colloidal stability parameter (CSP) of the residue oil. However, the stability characteristic of the residue during slurry-phased hydrocracking and its influence on the coke formation have near been studied. In this study, changes in the stability of residue oil during the slurry-phased hydrocracking reaction were calculated by use of flocculation onset titration, which was detected by means of mass-fraction-normalized conductivity, and the relationship between the stability and coke formation of residue oil during the reaction was studied.
2. EXPERIMENTAL SECTION 2.1. Raw Material. Liaohe atmospheric residue (LHAR) was taken as the feedstock, and its typical characteristics were shown in Table 1. Nickel sulfate (NiSO4 3 6H2O) was adopted as a catalyst precursor. Ammonium sulfide solution [(NH4)2S] was taken as a sulfurizer for sulfurization. 2.2. Dispersion of the Catalyst. The aqueous solution of the catalyst precursor was added to the feedstock with stirring in a small slurry blender, and then the solution of the sulfurizer was added using the same method; finally, the water was removed with bubbling of nitrogen gas. The details for the dispersion process were describes in the literature.23 2.3. Hydrocracking Reaction. The hydrocracking reaction of the residue was carried out in batch mode using an autoclave with an online sampler. In a typical experiment, the reactor was charged with 350 g of feedstock prepared according to the above method, pressurized with hydrogen to 7.0 MPa at room temperature, and heated to the reaction temperature (430 °C) at about 8 °C/min. Oil samples were collected at various temperatures and times by the online sampler, for example, at the temperatures of 200, 250, 300, 320, 350, 380, 400, and 430 °C during the heating process and at the times of 0, 10, 20, 30, 40, 50, and 60 min after the temperature reached 430 °C. About 30 mL of oil sample was withdrawn at a time, and four oil samples were withdrawn from each batch to ensure that the temperature and pressure of the reaction system had no obvious change. Each oil sample was diluted with a certain amount of toluene and then centrifuged in a LD5-2A centrifuge at the rotational speed of 3000 revolutions/min. The light components of the supernatant liquid (
