Characterization of Petroleum Residues with Varying Cutting Depth

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Characterization of Petroleum Residues with Varying Cutting Depth and Corresponding Vacuum Distillates derived from Laboratory Deep-Vacuum Fractionation of Unconventional Heavy Oil for an Instructive Utilization Guideline Peng Xue, Shi-guang Fan, Xuebing Wang, Kun Chen, He Liu, Wei Xia, Aijun Guo, and Zongxian Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02504 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017

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Characterization of Petroleum Residues with Varying Cutting

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Depth and Corresponding Vacuum Distillates derived from

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Laboratory Deep-Vacuum Fractionation of Unconventional

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Heavy Oil for an Instructive Utilization Guideline

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Peng Xue, Shiguang Fan, Xuebing Wang, Kun Chen*, He Liu, Wei Xia, Aijun Guo, Zongxian

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Wang*

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State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China

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University of Petroleum (East China), 66 Changjiangxi Road, Huangdao Zone, Qingdao, Shan

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dong 266580, China.

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*Corresponding author: Tel: +86-532-86983050, e-mail address: [email protected].

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*Corresponding author: Tel: +86-532-86981851, e-mail address: [email protected].

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ABSTRACT

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Petroleum residues with varying cutting depth and corresponding vacuum distillates have been

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prepared through from laboratory deep-vacuum fractionation using unconventional heavy oil

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(UHO) originated from Venezuela first. They are then sequentially characterized by general

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analysis, process analysis, and pyrolysis analysis with thermogravimetric analysis (TGA),

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derivative thermogravimetric analysis (DTG), and differential scanning calorimetry (DSC).

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Results indicate that a more increase in distillates recovery would be expected in the case of

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UHO than that of other referenced crudes under the same cut temperature and stand a good

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chance to make refineries involved with UHO get comfort by partially offsetting the shortage of

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light distillates. From the view of structural composition characteristics, cuts, even the ones with

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high cut temperature, have a high chance of cracking into compounds with lower molecular

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weight during conversion. Notably in the case of +565 oC petroleum residue (R5), except for the

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endothermic effect, an exothermic effect is also shown in the DSC profile, which means that the

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thermodynamic characteristics of petroleum residues would change from endothermic to

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exothermic as the thermal process exceeds some conversion point, at which coke yield is 45.8 wt%

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in the study.

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Keywords: deep-vacuum fractionation; unconventional crude oil; petroleum residue;

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endothermic/exothermic effect

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1. INTRODUCTION

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New refineries are currently under construction in China1, Venezuela2, and Brazil3 for

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processing rather high or even full amount of unconventional heavy oil, represented by

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Venezuelan extra-heavy oil, to produce liquid transportation fuels, such as gasoline, kerosene,

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and diesel4. Driven by the threat of conventional crudes shortage, quite a few other constructed

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conventional refineries have tentatively processed the unconventional heavy oil, and

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consequently have to evaluate revamps and/or introduce new designed units to cope with the

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problems involved with it4-7. The overarching problem is that the capacity of Residue Fluidized

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Catalytic Cracking (RFCC), Hydrocracking, and Coking limits the process amount of the

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unconventional heavy oil because the yields of straight vacuum gas oil (VGO) and petroleum

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residue generated are higher than that of conventional heavy crudes4, 5, which would definitely

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mean a heavy burden for downstream processes as foresaid. Furthermore, in consideration of the

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properties of petroleum residue2, it would not be hard to conclude that the coking technology

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could survive in process challenges and the coker capacity will ultimately determine the


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maximum amount of Venezuelan extra-heavy oil refineries can utilize. Therefore, maintaining

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the yield of VGO as high as possible through deep-vacuum fractionation becomes a solution8. It

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would ease the coking charge rate and permit a maximum processing of unconventional heavy

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oil5. The feasibility of the processing scheme combined with the deep-vacuum fractionation and

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coking lies on the premise, which is that the VGO distillates and the corresponding deep-cut

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petroleum residues obtained had better be able to be accepted by conventional refinery units

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through proper operation adjustments9. However, the asphaltenes precipitation10, the shorter

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coking tendency11, and the significant thermal performance differences12 of oil streams only

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three of the many challenges involved with unconventional heavy oil might urge conventional

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refineries to lean on an alternative solution that is operating at a relative low gas oil cut-point.

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This method favors lowering the operation risk, but adds to the burden of cokers and reduces

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yields of liquid products5. Therefore, systematic characterization of distillates and resultant

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petroleum residues derived from deep-vacuum fractionation of unconventional heavy oil in

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laboratory is necessarily important not only for acquiring data that could be directly used or for

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references in the design, optimization, and operation of thermal processes13, but also for a

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reasonable prognosis and evaluation of unconventional heavy oil process option.

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The scale utilization of unconventional heavy oil has been through over a decade, that is

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mainly evolved with upgrading process2, 4 for transportation through oil pipes and oil tankers.

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Hence the viscosity, density, and stability in storage are the main concerned properties14, 15. As

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for the refining of unconventional heavy oils, especially for the refining with the purpose of

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efficient and/or prolific producing ultra-low sulfur clean fuel, it is still in infancy. Reports

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revolved around the process of unconventional heavy oil are continuously emerged. Earlier

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studies started that a detailed comparison, under unified and reproducible conditions, of thermal


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behavior of tar sand bitumen are desirable for evaluating effects of varying feedstocks on yield

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and product properties16. Izquierdo17 further stated that detailed characterization of vacuum

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residua is of great importance in the utilization of these unconventional energy sources. The

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properties vary among different feedstocks and they determine different behavior during the

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process17. Rahimiand Gentzis11 complemented investigation of the thermal behavior and

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interaction Venezuelan heavy oil fractions obtained by ion-exchange chromatography and

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prognosis that removal of acidic, basic, and amphoteric fractions from Venezuelan Hamaca

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heavy oil prior to processing is needed to minimize the coke yield. Later Rahimiet al.18 studied

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the coking propensity of narrow cut fractions from Athabasca bitumen by using hot-stage

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microscopy (HSM). Guo et al.19 analyzed the composition of atmospheric residue and vacuum

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residue both derived from Venezuelan crudes, including the structure, and correlated it with the

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coke formation during carbonization. Alvarez et al.20 obtained pyrolysis kinetics of atmospheric

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residue through thermogravimetric analysis (TGA). Furthermore, Chen et al.12 determined

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enthalpy of reactions (including cracking and condensation) of four vacuum residues during the

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thermal process on their papers, in which the thermal characteristics of petroleum residue from

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Venezuelan heavy oil that distinguishes sharply from that of other petroleum residues from

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conventional crudes has been revealed.

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However, previous studies mainly focused on petroleum residues with one certain distillation

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cutting temperature (around 500oC). Our interests thus lie in systematically evaluating varying

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petroleum residues with varying distillation cutting depths, plus corresponding distillates in order

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to obtain a better insight of the characteristics of feedstocks derived from unconventional

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resource, especially the thermal performance, thus, might serve as a instructive guideline for

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design, optimization of process, and the scheme adjustment of thermal process (e.g. coking)


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involved with unconventional heavy oil and derivatives. A description of liquid and solid

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products from thermal process will be reported in subsequent publications.

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2. MATERIALS AND METHODS

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2.1 Feedstocks used in the characterization.

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A Venezuelan heavy oil sample (VNHO) was used as the deep-vacuum fractionation feedstock

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in this work. Selected properties are presented in Table 1. Four fractionations with varying

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distillation cutting depths were performed for varying petroleum residues and corresponding

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vacuum distillate samples following the standardized procedure21.


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Table 1. Selected properties of VNHO, and its series of residues and corresponding vacuum distillates, plus the labels for them Properties Density (g/cm3, 20 ºC) API Gravity Molecular weight a (g/mol) Viscosity (mm2/s, 100 ºC) MCRb (wt%) Heteroatoms composition (wt%) S N O Basic nitrogen H/C ratio fAc fNd fPe n7-Asphaltenes a Determined by VPO (g/mol)

cut4 0.9265 21 290 0.2

cut5 0.9537 17 315 0.5

cut6 0.9821 13 450 2.8

cut7 0.9886 12 524 3.2

VNHO 0.9925 11 620 456 11.5

R1 1.0061 9 850 3327 17.1

R2 1.0221 7 1120 9807 18.9

R3 1.0307 6 1350 58530 23.8

R4 1.0405 4 1490 122688 24.7

R5 1.0669 1 1935 226970 28.7

0.5 0.2 0.6 6586 1.70 0.16 0.27 0.57 0

2.5 0.2 0.5 6803 1.68 0.17 0.37 0.46 0

3.3 0.3 0.6 7559 1.62 0.20 0.27 0.53 0.15

3.5 0.4 0.3 8895 1.61 0.24 0.21 0.55 0.2

3.49 0.2 0.5 1.63 0.23 0.28 0.49 9.30

4.1 0.7 0.4 1.54 0.28 0.25 0.47 10.30

4.3 0.8 0.3 1.53 0.29 0.25 0.46 11.47

4.8 0.9 0.3 1.48 0.31 0.23 0.46 17.93

4.9 1.0 0.4 1.47 0.31 0.28 0.41 18.64

5.1 1.0 0.5 1.43 0.33 0.29 0.38 19.05

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b

Micro carbon residue determined according to ASTM D4530-11

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c

Aromaticity (ratio of aromatic carbons to total carbons).

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d

Naphthenicity (ratio of naphthenic carbons to total carbons).

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e

Aliphaticity (ratio of paraffinic carbons to total carbons).

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Structural parameters (aromaticity, naphthenicity, and aliphaticity) are calculated through Brown–Ladner method, which is based on

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1

H-NMR data.


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The viscosities of oil samples were measured using a Brookfield Viscometer with the referenced procedure22 and then converted into

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kinematic viscosities for the sake of refiners.

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2.2 Deep-Vacuum Fractionation The deep-vacuum fractionation was carried out on a molecular distillation apparatus named as KDL-5 (shown in Figure 1) built at UIC Corporation in Germany. The finalized distillation procedure is divided into three stages: evacuation of the apparatus, fractionation under presented pressure

and

clean

of

apparatus.

Each

stage

is

presented

as

below.

Figure 1. The molecular distillation apparatus named as KDL-5 from UIC Corporation in Germany Evacuation of the Apparatus. Before the evacuation, disassembly parts, like the flask that will accept the distillates, residue and cold trap, were first installed and the oil in the vacuum pump was maintained at sufficient level and free of contamination and/or oil phase separation. Liquid


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nitrogen was introduced into the cold trap as cooling media for preventing vacuum pumps from being contaminated by light components. As the temperature reached the setting point, the rotary vane pump and the oil diffusion pump were sequentially switched on to ensure the absolute vacuum required for the apparatus. Fractionation under the temperature of 150 oC was chosen to ensure that the sticky heavy oil could flow well. The evaporation pressure was as low as 1.0×10-3 mbar. The procedure of fractionation in detail was described in previous report from Wang et al.23 Because the distillates and residues were acquired under deep-vacuum and untraditional evaporation conditions, the operation temperatures of evaporator were achieved through the conversion of the boiling temperatures under atmospheric pressure of oil sample, based on the Framol formula24 and previous deep-vacuum distilling curves when similar crudes were involved. The VNHO was then separated into series of distillates and corresponding residue according to varying fractionation schemes shown in Figure 2 to obtain serial distillates and residues with desired boiling temperature range in the study. All samples were kept in inert atmosphere from oxidation.

Figure 2. The fractionation schemes and the photograph of the varying cuts and residues from fractionations using molecular distillation KDL-5, cut1: distillate (boiling range ~200oC), cut2:


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distillate (boiling range 200~280oC), cut3: distillate (boiling range 280~350oC), cut4: distillate (boiling range 350~420oC), cut5: distillate (boiling range 420~500oC), cut6: distillate (boiling range 500~540oC), cut7: distillate (boiling range 540~565oC), R1: atmospheric residue (boiling range 350oC~), R2: vacuum residue (boiling range 420~oC), R3: vacuum residue (boiling range 500oC~), R4: vacuum residue (boiling range 540oC~), R5: vacuum residue (boiling range 565oC~). Figure 2 presents the fractionation schemes in detail and the photograph of the varying cuts from fractionations using molecular distillation KDL-5 (UIC Laboratory, Germany)， including the corresponding labels for cuts and residues for clear presentation. 2.3 DSC and TGA Experiments All of thermal analysis experiments (DSC and TGA) were carried out on a Linseis STA PT1600 (Linseis Corporation, Germany). A amount of 5±0.1 mg of sample was introduced into a standard aluminum crucible after taring. The apparatus was vacuumed and refilled with high purity nitrogen three times for obtaining inert atmosphere. All DSC and TGA experiments were performed with the prospection of a pure flowing nitrogen gas for the sophisticated system. As suggested by J. V. Dubrawski25 and Alshareefet al.26, the thermal analysis system was heated at a constant rate (15 K/min) to 650 °C for the moderately heat up time and necessarily accurate thermal data.

3. RESULTS AND DISCUSSION 3.1 The distribution of distillation products


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Figure 3. Distillation yield as a function of cut temperature that is up to 565 oC (dash-spot line). The distillation results from Rajeev Kumar et al27 (solid line) and Shixiong Lin28 (spot line) are acquired using TBP distillation and shown for comparison. As being a typical light crude oil originated from the Middle East (oAPI 33.1), Abu Dhabi crude oil studied in the research of Rajeev Kumar et al27 is chosen for comparison. The asphaltenes content (2.9 wt%) and MCR (5.8 wt%) of Gudao crude oil are close to those of Abu Dhabi crude oil. However, the Gudao crude oil generated in Shengli oil field is a conventional heavy crude (oAPI 17.5). Therefore, it is also chosen to be a reference crude in the study. Distillation profile in the Figure 3 shows that VNHO used in the study does not have distillates at all until the cut temperature approaches to 175 oC. The yields of gasoline (~200 oC) and diesel distillates (200~350oC) are 0.8 wt% and 11.7 wt%, respectively. It is suggested that the overall


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yield of light distillates derived from this typical unconventional heavy oil, i.e. VNHO, is quite low, even compared with that (21.0 wt%) of conventional heavy oil from Gudao in China, not speak of comparison with that (51.2 wt%) of crude oil originated from Abu Dhai. The similar issue has also been concerned in the refining of another typical unconventional resource, i.e. Canada tar sand bitumen, which is why quite a few refineries in North America are trying revamping or adding new vacuum units for maximizing the yield of vacuum gas oil distillates5. As seen in Figure 3, in addition to the difference in absolute yield of distillates, the slopes of the distillation profiles in the range from 300 to 565 oC are also different: we achieved 0.23 wt%/oC increase while Rajeev Kumar et al.27 achieved 0.15 wt%/oC increase. The results indicate that a more increase in distillates recovery would be expected in the case of VNHO than that of Abu Dhai crude under the same cut temperature, which means more feedstock is available to the secondary processes when vacuum distillation, deep-vacuum distillation technology in special, is applied to the refining of VNHO and will make refineries involved with it get comfort by partially offsetting the shortage of light distillates. However, the deep-vacuum distillation of VNHO faces challenges, for instance, high viscosity of distillation feedstock. The viscosity of VNHO is 456 mm2/s at 100 oC, which is fairly higher than that of Gudao heavy oil28 and that of Abu Dhai crude. More importantly, as shown in the Table 1, the viscosity of residual samples rises phenomenally, as the cut temperature increases, which is rationalized by the corresponding increase of heteroatoms and asphaltenes content29 in residues. When cut temperature reaches 500 oC, the viscosity of residue (58530 mm2/s) is more than forty times that of Gudao residue (1403 mm2/s). The viscosity changing tendency of feedstocks illustrates that even refineries used to process heavy oil might suffer from the significant property differences between the VNHO and conventional heavy oil, and implies that


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the escape of relatively light distillates from feedstock in distillation units would very probably be thwarted by strong “cage effect” resulted from adhesion force between polar functional groups30. 3.2The characterization of petroleum residues and corresponding vacuum distillates The application of deep-cut technology to VNHO for maximization of vacuum distillates (cuts) could not only promote refinery margins, but also ease the burden of coking process. However, the chemical composition of cuts and petroleum residues varies as the cutting depth of distillation varies, which urges a prudent prognosis in order to determine the cutting depth and/or the sequential process options. 3.2.1 General characteristics of distillates and petroleum residues The analytical data in Table 1 show that the density and molecular weight of petroleum residues gradually increase as the cut temperature increases. When distillates that less than 565 oC were expelled out, the oAPI of the petroleum residue, being termed as R5 in the study, reaches one degree, which means that the density of R5 is coming to a critical point that is almost beyond the limits of oAPI criteria. The molecular weight of R5 is 1935 g/mol, which is evidently higher than that of R1 (350+oC residue ) and even lies in the molecular weight range of asphaltenes31 which means that the components of R5 do not generally favor the volatilization during the process, leading to higher tendency of coking. The density and molecular weight of cuts have similar tendency as the cut temperature increases. The elemental analysis indicates that the total amounts of S, N, and O of petroleum residues are fairly high. Especially, the value is up to 6.60 wt% in R5, which means that the heavier residue, the more functionalized it is, and the more active sites it has. Although the total amounts heteroatoms of cuts are relatively lower than that of petroleum residues, cuts acquired in the study are still considered as not being appropriate direct feedstocks


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for the production of gasoline components with less than 10 µg/g in secondary processes, such as FCC4. As a matter of fact, extra hydrogenation pretreatment and selective hydrodesulphurization aftertreatment are strongly recommended5 when cuts from UHO are used to produce low and/or ultra-low sulfur clean gasoline components, which would definitely and considerably raise the capital investment and operation cost up, resulting in rarely being chosen in the world. From the view of asphaltenes and MCR (as shown in Table 1), cut4 and cut5 are qualified for secondary process, while cut 6 and cut 7 are not5, unless with the aid from hydrotreatment. The data presented in Table 1 shows that H/C atomic ratios of cuts and petroleum residues decrease in the order: cut4 > cut5 > cut6 > cut7 > R1 > R2 > R3 > R4 > R5, leading to increasing aromaticity (fA) in the order: cut4 < cut5 < cut6 < cut7 < R1 < R2 < R3 < R4 < R5. From the view of structural group composition of all UHO-derivatives, it is worthy of noting that cut5 has the highest naphthenics among all the cuts and petroleum residues and R5 possess the highest value of fA+fN (0.62) and lowest value of fP (0.38), which is unusual even in the range of petroleum residues28, 32. 3.2.2 Processing characteristics of distillates and petroleum residues Processing characteristics of feedstock is one of the main issues concerned in refineries, and thus by researchers33, 34, especially in the case that UHO was involved11, 35. Wiehe36 proposed a so-called Pendant-Core model and a correlation between the carbon residue and the content of hydrogen to predict the process behavior of petroleum residues in 1994. Chen and Cao34, and Roberts33 suggested that carbon residue as a fraction of H/C ratio could be an appropriate method to indicate the effect of feedstock structural property on the coking ability. They believed that the relationship between carbon residue and H/C ratio is linear distribution type. Furthermore, Speight37 proposed a distribution map to reveal that it would be better that the relationship


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between carbon residue and H/C ratio is described by regional distribution type. Interesting results are observed as carbon residues of cuts and petroleum residues in the study are piloted with H/C ratios and compared with previous literatures in Figure 4. This figure, adapted from Chen and Cao34, Roberts33, Guo et al.19 and Chen et al.32, shows that the correlation based on our data could be described using linear models (purple solid lines as shown in Figure 4) suggested by Chen and Cao34, when the data are divided into two groups (A and B shown in Figure 4), while the correlation in view of the overall situation is indeed fitting in regional distribution model with an upper limitation (upper dash line) and a lower limitation (lower dash line), as suggested by Speight37.


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Figure 4. MCR of cuts, UHO and petroleum residues as a function of H/C ratio. The data of Chen and Cao31, Guo et al.19, and Chen et al.29 are shown as comparison and background (region filled with dense slash line). It is indicated that the correspondence between MCR and H/C ratio from feedstocks with varying structural characteristics is not one-to-one type. In other words, feedstocks with same H/C ratio might have evidently different MCR in some cases, which is proved by the behavior of UHO and cut 6-7. Even the H/C ratios are a little lower than that of UHO, the MCR of UHO is 10 wt% higher than that of cut 6-7. The shift in MCR shown in Figure 4 for fractions with similar H/C ration could be ascribed to the molecular compositional difference between cut 6-7 and residues. McKenna et al.38 analyzed a heavy vacuum gas oil (VGO) distillates (500-538 oC) originated from Athabasca Bitumen, which is also a typical UHO. The results of exhaustive compositional analysis by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR MS) indicated that the heavy VGO has a carbon number of ~39 and a double bond equivalents (DBE) of 10. The light VGOs (375-400 oC, 400-425 oC, and 425-450 oC) have the same DBE of 7. The progression of DBE when the VGO becomes heavier occurs through the addition of cycloalkane rings39 with more vulnerable C-C bond than that of polycyclic aromatic rings, which is the typical structure in residue40. From the view of structural composition characteristics, cuts, even the ones with high cut temperature, have a high chance of cracking into compounds with lower molecular weight during conversion. On the contrary, petroleum residues, even atmospheric residue, tend to be close to the upper line in Figure 4, which means that they are inclined to form coke during process, rationalized by the abundant higher-order fused-ring cores of asphaltenes40 contained in the petroleum residue and proved by the common


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process choice-delayed coking process tolerating coking issue, whether in upgrader or in refinery, when UHO and derived petroleum residues are involved. 3.2.3

Pyrolysis

characteristics

of

distillates

and

residues

by

TGA/Derivative

Thermogravimetric Analysis (DTG) and DSC

Figure 5. TGA profiles of distillates (cut4-cut7), (A) and petroleum residues (R1-R5), (B). Table 2. TGA/DTG and DSC parameters of distillates (cut4-cut7) and petroleum residues (R1R5) Sample Ti a Tf b ∆T c Tmax d Coke yield e

cut4 168 255 87 230 0

cut5 220 329 109 305 0

cut6 289 387 98 358 3.7± 0.3

cut7 399 482 83 450 4.9± 0.4

R1 189 488 299 440 14.5± 0.2

R2 213 493 280 442 15.1± 0.3

R3 285 497 212 441 21.4± 0.3

a

Temperature of initial weight loss (oC) determined by a referenced method41.

b

Temperature of final weight loss (oC) determined by a referenced method41.

c

Difference between Tf and Ti (oC).

d

Temperature of maximum rate of weight loss (oC).

e

Carbonaceous residue at 500 oC (wt%).

R4 381 488 107 445 22.0± 0.2

R5 390 489 99 452 24.8± 0.3


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Figure 6. DSC curves with corresponding DTG curves of distillates (cut4-cut7).


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Energy & Fuels

Figure 7. DSC profiles with corresponding DTG curves of petroleum residues (R1-R5). A combination analysis of TGA and DSC with derivative DTG is carried out for the pyrolysis characterization, as suggested by Koሷk42, Milosavljevic43, and Chen et al.32. The data from TGA profiles (Figure 5) indicate that indeed all distillates (cut4-cut7) barely tend to coke in thermal


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Energy & Fuels

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process. In the case of cut4 and cut5 no coke residue could be detected, while in the case of cut6 and cut7, even though obvious coke residues of 3.66 wt% and 4.87 wt% are observed, it is still bearable considered that cut6 and cut7 are directly derived from petroleum residue (+500 oC). On the contrary, petroleum residues from UHO all have considerable coke residues, which is consistent with the MCR data (as seen in Table 1). Furthermore, the TGA curves of R1 and R2 evidently show that there are two stages at least in the thermal process of the two petroleum residues, which is further analyzed through DTG curves and discussed later. In general, DTG profiles shift to high temperature end as distillates become heavier. The analytical data in Table 2 indicate that the representative temperatures when the rate of weight loss reaches the maximum point (labelled as Tmax) increase in the order: cut4 (230 oC) < cut5 (305 oC) < cut6 (358 oC)

Characterization of Petroleum Residues with Varying Cutting Depth

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