Article pubs.acs.org/EF
Eight-Lump Kinetic Model for Upgrading Residue by Carbon Rejection in a Fluidized-Bed Reactor Hong-liang Wang,† Gang Wang,*,† Dong-chao Zhang,†,‡ Chun-ming Xu,† and Jin-sen Gao† †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Beijing Branch, China Petroleum Engineering Co., Ltd., 100085, China
‡
ABSTRACT: Inferior residues can be effectively upgraded over a special catalyst in a fluidized-bed reactor to provide feedstock with trace metals and a few asphaltenes for downstream processes. A new eight-lump kinetic model was proposed to describe the reaction behaviors of upgrading residue by fluid carbon rejection. In this reaction system, the residue (>500 °C) was cut into four lumps according to saturates, aromatics, resins, and asphaltenes to expand the model to various feedstock. Moreover, the model took into account gas oils (350−500 °C) secondary cracking reactions. The experimental data were obtained in a fixed fluidizedbed reactor. The kinetic constants were estimated on the basis of Marquardt’s algorithm. The apparent activation energies were calculated through the Arrhenius equation. This model fitted the experimental data well. Furthermore, model simulations were carried out to determine the effects of such parameters on product yields, as reaction temperature, weight hourly space velocity, and along the reactor height, which were helpful for optimizing the yields of desired products.
1. INTRODUCTION The processing of vacuum residues has gained interest due to the increasing demand of light oil fractions.1,2 One of the
are poisons for catalysts, and CCR causes deactivation of catalysts.3,4 Bartholic5 introduced an attractive solution to this problem. It used a FCC-like process to upgrade residues through high temperature, short-time contact with fluidizable solid particles in a fluidized-bed reactor to deposit high boiling components and metals, whereby CCR values and metal content of the residues were reduced to tolerable levels before further processing. The schematic diagram of the process is shown in Figure 1. Earlier studies mainly were devoted to investigating residues with CCR about 10 wt %.6−9 Metals were essentially 88−95% removed, and CCR was more than 70% removed. Recently, our group focused on the residue with 21.78 wt % CCR.10 In that study, more than 98% metals and 85% CCR was removed. Even more remarkable, the ratios of coke yields to CCR of feedstock were less than 0.8, while those generally were 1.5−2.0 for industrial delayed coking.11,12 Although a number of studies have been carried out on the process, the subject of the kinetic model did not get much attention. Building a kinetic model is a feasible way for describing reaction behaviors. Although the reaction taking place is considered to be essentially thermal cracking,9 the reaction behavior of residue in the above-mentioned fluidized process is distinct from that of the delayed coking. The induction period played an important role in the formation of coke in delayed coking.13,14 Nevertheless, it was much less significant in the fluidized process, because there usually involved rapid vaporization, rapid cracking, and rapid coke formation on the surfaces of solid particles.15 Recently, our group was interested in developing a lumping kinetic model with a detailed product distribution for upgrading of high CCR feedstock in a fluidizedbed reactor.10 The model can describe singly dry gas, LPG
Figure 1. Schematic diagram of the residue upgrading process.
challenges to face for upgrading residues is to properly handle contaminants in residue. These impurities, in particular, metals and Conradson carbon residue (CCR), can cause many problems in conversion units, such as vanadium and nickel © 2012 American Chemical Society
Received: February 22, 2012 Revised: June 27, 2012 Published: June 27, 2012 4177
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(liquefied petroleum gas), and coke. Unfortunately, we did not consider feedstock properties. Therefore, the model was not expanded to describe the reaction behavior of other feedstock. One possible solution to the above problem is to split feed into several lumps. This can be categorized as two types. One is that the lumps are gained based on molecular structure characteristics of feed, such as the 10-lump model by Gross et al.16 and the 12-lump model by Peixoto et al.17 This type of lump made the kinetic model extend to the molecular level. It achieved success in FCC while using gas oil as feedstock. However, it is difficult to quantitatively analyze the molecular structure of residue under current analytical techniques due to residue complexity. The other is that the lumps are gained on the basis of group composition of residue. Ferreira et al.18 proposed a model of residue hydrotreating based on the saturates, aromatics, resins, and asphaltenes (SARA). Zhou et al.19 presented an 11-lump kinetic model for describing the reaction behaviors of delayed coking. In the model, residue was cut into saturates, light aromatics, heavy aromatics, light resins, heavy resins, and asphaltenes. Wang et al.20 developed a 14lump kinetic model for FCC of residue by cutting residue into saturates, aromatics, and (resins + asphaltenes). A great deal of research has been carried out on cracking behaviors of SARA of residue.21−26 The results indicated that, although there are differences in molecular structure for the same group from different residues, the kinetic characteristics of them are similar, while the kinetic characteristics of various groups from the same residue are of significant difference. Therefore, the lump based on group composition of residue is a feasible method. It is clear, if we know well the reaction pathways of SARA in residue, that we could predict product distribution of other feedstock. It is, therefore, necessary to develop a model to describe the reaction behavior of various fractions of residue. In this article, the typical experimental results of upgrading residue by carbon rejection in a fixed fluidized-bed reactor were present. An eight-lump model then was presented for describing reaction behaviors during the process. In this model, the residue (>500 °C) was split into four lumps based on SARA in residue to expand the model to other feedstock. The kinetic parameters were estimated. After that, the effects of parameters on product distribution were also investigated with the model.
Table 1. Properties of Feedstocks properties
Orinoco VR
Liaohe VR
Conradson carbon residue (wt %) density (20 °C) (kg·m−3) Ni (μg·g−1) V (μg·g−1) total metals (μg·g−1) sulfur (wt %) nitrogen (wt %) group analysis (wt %) saturates aromatics resins asphaltenes group analysis (>500 °C) (wt %) saturates aromatics resins asphaltenes
21.78 1038.8 138.6 539.2 808.8 4.30 0.66
17.82 993.8 79.4 88.1 275.3 1.10 1.03
12.51 42.69 33.56 11.24
21.13 35.33 37.51 6.03
7.94 38.81 39.20 14.05
16.68 27.91 47.37 8.04
Figure 2. Simulated distillation profiles for the two vacuum residues.
Table 2. Properties of the Catalyst
2. EXPERIMENTAL SECTION 2.1. Feedstock and Catalyst. To gain an insight into the reaction behaviors of inferior residue, two kinds of high CCR residues (Orinoco VR and Liaohe VR) were selected as experimental materials. The CCR values of them were 21.78 and 17.82, respectively. There were significant differences in chemical components of the two kinds of feeds. The former contains relatively high aromatics and n-heptaneinsoluble asphaltenes. The latter contains more saturates and resins. Although there are differences in molecular structure for the same fraction from Orinoco VR and Liaohe VR, the compositions and kinetic characteristics of them are similar. Here, it was assumed that the compositions of the same fraction from the two residues are the same. The Liaohe VR was mainly used for model verification. The two materials were prepared by true-boiling-point distillation of Orinoco and Liaohe extra-heavy crude oil, which was supplied by China National Petroleum Corp. Their main properties were listed in Table 1. The detailed simulated distillation profiles for the two residues were shown in Figure 2. A special catalyst for upgrading high CCR residue was used. It was designed and prepared by the State Key Laboratory of Heavy Oil Processing at China University of Petroleum. The catalyst was designed with an affinity for asphaltenes and metals. Except for its
property
value
property
pore volume (cm3·g−1) surface area (m2·g−1) attrition index (wt %) packing density (g·cm−3) microactivity index composition (wt %) Na2O Al2O3 SiO2 SO3
0.277 86.5 0.7 0.76 17.3
pore size distribution (v %) 1−5 nm 5−10 nm >10 nm particle size distribution (v %) 0−20 μm 20−40 μm 40−80 μm 80−110 μm >110 μm
0.15 83.00 15.00 0.12
value 21.6 18.4 60.0 0 2.81 34.83 28.96 33.4
higher attrition resistance, the catalyst has essentially the same size distribution, density, and fluidization characteristics as FCC catalysts. Moreover, this catalyst has a fairly high pore volume and specific surface area, but a low catalytic activity. The basic properties of the catalyst were shown in Table 2. 2.2. Equipment and Experimental Procedure. The experiments on upgrading residues were conducted in a fixed fluidized-bed reactor. The schematic diagrams of the experimental setup and the physical dimensions of the reactor were presented in Figures 3 and 4, respectively. The experimental setup consists of five sections, feedstock input mechanisms; gases and steam input mechanisms; reaction and 4178
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Figure 3. Schematic diagram for the experimental setup: (1) oxygen; (2) air; (3) constant temperature box; (4) electronic balance; (5) feedstock; (6) oil pump; (7) water tank; (8) water pump; (9) steam generator; (10) preheater; (11) reactor; (12) thermocouple; (13) first-condenser; (14) receiver for liquid products; (15) second-condenser; (16) cold trap; (17) gas collection bottle; (18) water bottle; (19) gas sample connection; (20) drain sump; (21) CO converter; (22) drier; (23) CO2 infrared detector. Experiments were carried out in batches, with a fixed amount of feedstock and varying the time of injection from 19 to 78 s. In each experiment, a certain amount of catalyst was loaded into the reactor. In the beginning, the catalyst particles were fluidized with air to prevent catalyst accumulating at the bottom. As the temperature of the lower part of the reactor reached 300 °C, catalyst was fluidized by steam, and air was displaced at the same time. When the temperature was up to the desired reaction temperature, the feedstock was pumped and mixed with the stream. The mixture was preheated to approximately 330 °C in the preheater and then entered into the reactor. Reactions took place as the feedstock contacted the fluidizing catalyst particles. The oil gas after exiting from the reactor was cooled and separated into liquid and gas products and collected in a receiver and a gas collection bottle, respectively. The water emulsified in the liquid product was separated by centrifugation before being weighed. Coked catalyst was stripped continuously with steam for 30 min to recover entrapped hydrocarbons. After that, the reactor was heated to 680 °C to burn the coke over catalysts with oxygen. The total amount of coke was quantified by a CO2 infrared detector, after the flue gases had passed through a CO converter and a drier. In all experiments, the yields were calculated as weight percentages of the feedstock. The content of sulfur was high in feedstock. A part of the sulfur was removed from the feedstock during reaction, which would affect the material balances. Therefore, the weight loss of sulfur (WLS) was considered in each experiment and was defined as follows:
WLS(%) = (Ws‐feed − Ws‐total liquid)/Wfeed × 100
(1)
The contents of nitrogen and metals were relatively low, so their effects were neglected here. Total material balances on the feedstock were generally 96% or greater, and the reported results were normalized to no loss basis. 2.3. Analytical Methods. Several methods were used for the analysis. The gas product was analyzed using an Agilent 6890N gas chromatograph (by ASTM D1945, D1946, and UOP 539 standard methods) to measure the volume percentages of the components of H2 and C1−C6 hydrocarbons. The obtained data were converted to mass percentages using the state equation of ideal gases. To obtain the weight percentage of gasoline (IBP−200 °C), diesel (200−350 °C), gas oil (350−500 °C), and heavy oil (>500 °C), the liquid product was subject to simulated distillation. The analysis was performed in an Agilent 6890 gas chromatograph, according to the standard method
Figure 4. Dimensioned drawing of the reactor (in mm): (1) inlet tube of feed and steam; (2) bushing of thermocouple; (3) inlet and outlet of the catalyst; (4) filter. regeneration zones; temperature control system; and product separation and collection systems. The reactor involves three zones from top to bottom: the dilute phase zone, the main reaction zone, and the contact zone of feedstock and catalyst. To control the reaction temperature, three thermocouples were placed in the upper, middle, and lower parts of the reactor, respectively, and connected to a feedback temperature controller. 4179
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ASTM D-2887. In this article, light oil (LO) was defined as the liquid (IBP−350 °C); distillate was defined as the liquid (C5−500 °C). The total liquid product includes LO, gas oil, and heavy oil. The boiling ranges of residues were analyzed by high simulated distillation by the standard method ASTM D7169-05. SARA fractions in residue were determined by the SH/T 0509-92. In addition, other properties such as metals contents were determined by inductively coupled plasma emission spectrometry (Ultima2). The total sulfur content was measured according to ultraviolet fluorescence (RPP-2000S). CCR was determined in accordance with the D189-81 ASTM standard test method. The compositions of the catalyst were obtained by X-ray fluorescence (XRF) analyses conducted on a ZSX-100e 4580 instrument. Nitrogen adsorption was used to measure the surface areas and pore volumes of the catalyst. The microactivity index of catalyst was determined in a fixed-bed MAT unit designed according to ASTM D-3907.
clear that the total liquid product from Orinoco VR contained 0.43 wt % n-heptane-insoluble asphaltenes, 4.01 wt % CCR, and 15.7 μg·g−1 total metals (including 2 μg·g−1 Ni and 9.5 μg·g−1 V). As well, the total liquid product from Liaohe VR contained less contaminant. It was reported that the feedstock with 40 μg·g−1 metals (Ni + V) and 8 wt % CCR was an acceptable commercial residue FCC feedstock.27,28 Therefore, the liquid product satisfies the requirements as the feedstock of RFCC. In addition, most of the contaminants were removed from the two feeds, with more than 96% removal of metals, nominally 80% removal of CCR, and about 97% removal of asphaltenes. It indicated that the process has an excellent ability of contaminant removing. Another aim was to maximize the obtained liquid product. Here, the typical product distributions under the above optimum conditions were shown in Table 4, as is seen from which, high liquid yield and low coke yield can be obtained. The total yields of liquid for Orinoco VR and Liaohe VR were 76.27 and 81.95 wt %, respectively. The ratios of coke yields to CCR for the two feed were about 0.73, whereas those ranged from 1.5 to 2.0 for delayed coking.11,12 Calculating by the above ratios, the total liquid yield can be 15% higher than that of delayed coking for the two feeds. The above product distribution of the two feeds caused a great interest. So we desire to know other feeds upgrading results. The product distribution is closely related to concentrations of components and reaction rates of components to various products. For a given feed, the contents of components are constant. Thus, reaction rates of components become the focus of study. The reaction rate of each reaction depends on the reaction environment. In this study, the process has two significant features as compared to delayed coking. First, residue is distributed as a thin film on the particles of the catalyst after it was contacted with high-temperature catalyst.15 This leads to a large gas−liquid interphase form, enhancing the mass transfer from liquid to gas phase. Thus, most of the residue molecular instantaneously enters into the gas phase after preliminary cracking in the liquid film. After that, a gas− solid reaction takes place. It is well-known that the coke mainly formed by condensation reaction of aromatic rings in liquid phase in delayed coking. The residence time of the residue molecular is very short in liquid film as compared to delayed coking. This definitely reduces condensation reaction of aromatic rings in liquid phase and then reduces coke yield. Second, the catalyst used has slight catalytic activity, which enhances the reaction rate of oil gas. Above, the qualitative analysis for reaction environment effect on reaction rates was conducted, as well as the operating conditions, such as temperature, weight hourly space velocity, and influence reaction rate and product distribution. Therefore, it is necessary to study the reaction kinetic of the process to gain insight into each reaction in depth.
3. TYPICAL RESULT OF UPGRADING RESIDUE BY FLUID CARBON REJECTION We previously described the process of upgrading residue by fluid carbon rejection, the aim of which was to remove Table 3. Contaminant Contents in and Removal Rates from the Total Liquid Products contaminant
Orinoco VR
Liaohe VR
CCR content (wt %) removal of CCR (%) asphaltenes content (wt %) removal of asphaltenes (%) total metals content (μg·g−1) removal of total metals (%) Ni content (μg·g−1) removal of Ni (%) V content (μg·g−1) removal of V (%)
4.01 81.59 0.43 96.17 15.7 98.06 2.0 98.56 9.5 98.24
3.61 79.74 0.11 98.18 9.3 96.62 2.3 97.10 2.0 97.73
Table 4. Product Distribution for the Two Feedstocks yields (wt %)
Orinoco VR
Liaohe VR
dry gas LPG gasoline diesel gas oil heavy oil (>500 °C) coke WLS distillate (C5 ∼500 °C) total liquid product
2.28 3.82 18.99 21.40 28.16 7.72 15.38 2.25 68.55 76.27
1.71 2.78 15.04 20.63 34.95 11.33 13.02 0.54 70.62 81.95
contaminants and obtain the maximum liquid yield. Here, the removal rate of contaminants is defined as: ⎛ Wi ,total liquid ⎞ ⎟⎟ × 100 removal(%) = ⎜⎜1 − Wi ,feed ⎠ ⎝ ⎛ Ytotal liquidCi ,total liquid ⎞ ⎟⎟ × 100 = ⎜⎜1 − Ci ,feed ⎝ ⎠
4. EIGHT-LUMP KINETIC MODEL 4.1. Lumping and Reaction Scheme. The cracking of residue involves a series of complicated reactions. In such a case, the lumping strategy, grouping a large number of chemical compounds into groups of pseudocomponents according to their molecular characteristics, is very effective.29,30 For the present study, the relevant kinetic scheme had been proposed by our group for residue upgrading.10 However, the reported seven-lump model considered the residue fraction (>500 °C) as a single lump. Model parameters of the seven-lump model would vary with changes of material ingredients. A reasonable
(2)
To display the treatment effect, the contaminants’ contents and removal rates were given in Table 3. Experiments were conducted under the optimum reaction conditions. The reaction temperature, catalyst-to-oil ratio, and weight hourly space velocity were 460 °C, 6, and 20 h−1, respectively. It was 4180
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the weight loss of sulfur.10 As mentioned above, the whole reaction system involved eight lumps (saturates, aromatics, resins, asphaltenes, gas oil, LO, gas, and coke′). 4.1.2. Reaction Scheme. Although the complicated cracking reactions of residue have been simplified by the eight lumps mentioned above, the reaction pathways among them are still multiple. To simplify the kinetic model further, some assumptions were introduced. First, the cracking and condensation reactions of residue were considered as irreversible reactions.31,33 Second, it was popularly designated as saying there was no interaction between saturates, aromatics, resins, and asphaltenes groups. Finally, the LO lump was supposedly not further cracked, which had been observed in our previous research.10 Moreover, the secondary cracking of gas oil was also observed.10 Thus, an eight-lump kinetic model with 19 reactions was established on the basis of the above analysis to describe the reaction pathways of upgrading inferior residue by fluid carbon rejection. The reaction network was depicted in Figure 5. The model involved parallel cracking of SARA to gas oil, LO and gas plus coke′, with consecutive cracking of the gas oil to LO, gas plus coke′. The advantage of this model is that it can predict product distribution for various residues. 4.2. Model Equations. To develop the model equations for upgrading residues by fluid carbon-rejection in the fluidized-bed reactor, the following assumptions were introduced: (1) The fixed-fluidized-bed reactor used in this study was considered as an isothermal reactor.34 (2) One-dimensional ideal plug flow prevailed in the reactor without radial and axial dispersion.35 (3) Instantaneous vaporization of feedstock occurred after it entered into the reactor.32,36 (4) A first-order reaction was assumed for all reactions.37 (5) The chemical reactions were the rate-controlling step. (6) The catalyst used in this study was essentially considered to be inert, the activity change of which was neglected. For each reaction, generate rates of change of hydrocarbon reactants in the reactor in accordance with:
Figure 5. Kinetic scheme for the eight-lump model.
Table 5. Average Molecular Weight of the Eight Lumps lumps
average molecular weight (g·mol−1)
saturates lump aromatics lump resins lump asphaltenes lump gas oil lump LO lump gas lump coke′ lump
760a 880a 1350a 3650a 400b 165b 31b 400c
a
Molecular weights for saturates, aromatics, resins, and asphaltenes were estimated according to Lin.12 bMolecular weights for gas oil, LO, and gas were estimated on the basis of Wang et al.10 cMolecular weight of coke′ lump was the same as the data from Peixoto and Medeiros,17 Heydari et al.32
solution is to cut feed into several lumps based on chemical composition. 4.1.1. Lumping Scheme. To predict the cracking reactivity of various feeds, the residue (>500 °C) was divided into four lumps according to SARA (saturates, aromatics, resins, and asphaltenes). Generally, the more lumps a model includes, the more kinetic parameters need to be estimated. Thus, the numerous experimental data are required.31 Consequently, it is necessary to establish a simple model that can give the key kinetic information. Although gasoline (IBP−200 °C) and diesel (200−350 °C) were usually considered as two lumps for catalytic cracking,32 it was reported that the fraction (
