Improvement of the Thermal Cracking Product Quality of Heavy

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Improvement of thermal cracking products quality of heavy vacuum residue using solvent deasphalting pretreatment Mohammad Hamidi Zirasefi, Farhad Khorasheh, Javad Ivakpour, and Aylin Mohammadzadeh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02297 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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Improvement of thermal cracking products quality of heavy vacuum residue using solvent deasphalting pretreatment Mohammad Hamidi Zirasefia, b, Farhad Khorasheh*, b, Javad Ivakpour*, a, and Aylin mohammadzadehc a

Petroleum Refining Division, Research Institute of Petroleum Industry, PO Box 1485733111, Tehran, Iran.

b

Department of Chemical and Petroleum Engineering, Sharif University of Technology, PO Box 11155-9465, Tehran, Iran. c

Department of Chemical Engineering, Tehran University, PO Box 14155-6619, Tehran, Iran.

Corresponding Authors: *Telephone: +982148255038. E-mail: [email protected] *Telephone: +982166165411. E-mail: [email protected]

Abstract In this work we used the vacuum distillation residue from an Iranian heavy crude oil refinery in a solvent deasphalting (SDA) process using different temperatures (60 to 120 °C), pressures (5 to 9 bar), solvents (n-pentane and ethyl acetate), and with solvent to feed ratio of (3 to 1, 5 to 1 and 7 to 1). The resulting products included deasphalted oil (DAO) and residue (PITCH). The DAO yields in SDA when n-pentane was used as solvent were significantly higher than those when ethyl acetate was used as solvent. The DAO was subsequently processed by thermal cracking at 500 °C and atmospheric pressure to investigate the effect of solvent deasphalting processing conditions on the yield of coke and liquid products as well as the metal and sulfur content of the produced coke. The results indicated that SDA effectively removed up to 45% of vanadium and up to 40% of nickel from the vacuum residue. Simulated Distillation (SimDis) results of the liquid products of thermal cracking of DAO indicated that for both n-pentane and ethyl acetate as the SDA solvent, an increase in the solvent to feed ratio resulted in increased amount of lighter compounds (C5-C20) of the liquid products.

Keywords Solvent deasphalting, Deasphalted oil, Vacuum residue, Delayed coking, Thermal cracking

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1. Introduction Crude oil has been the most important global energy resource in the last decades because of the increasing worldwide energy demand especially from growing economies of China and India.1-3 Due to the depletion of conventional oil resources and increasing prices (in last decades), various technologies for utilizing unconventional oil and low-value crude residues that have not yet been fully exploited are currently being explored.4, 5 The physical separation of higher molecular weight and higher boiling fractions from the different crude oils is first performed in a vacuum distillation process. There are, however, valuable components left in the vacuum residue fractions.6, 7 Due to the reactivity of petroleum fractions at high temperatures, there is a limit to the temperatures that can be used in simple separation processes. The crude oil cannot be subjected to temperatures much above 350oC, irrespective of its residence time in tower, without some thermal cracking. Therefore, to separate the higher molecular weight and higher boiling fractions from heavy crude oil, special processing stages must be used.8-10 The unconventional heavy oil and low-value crude residues typically need to be upgraded by either a hydrogenation or a carbon rejection process. Among many existing upgrading process, solvent deasphalting (SDA), a technology for removing asphaltene-rich pitch and producing higher-value deasphalted oil (DAO) by using light paraffinic solvents, is a promising process because it presents the advantages of low capital cost and flexibility in terms of the controlling of the quality of pitch and DAO products.2, 11, 12 Operation costs are also low in SDA as it is employed under relatively low pressure and temperature conditions. Moreover, the design of the SDA process is relatively simple and scale-up can be performed easily.11, 13 The SDA process consists of two main stages: asphaltene separation and solvent recovery section. Solvent recovery is a key factor for determining the operating cost and feasibility of the SDA process since it requires large quantities of an expensive solvent the recovery of which consumes significant amounts of energy.8, 14 This is the main disadvantage of the SDA process since the amount of solvent used in the extraction stage of the process is four to ten times the amount of vacuum residue (VR). The energy consumed by the solvent recovery stage of the SDA process is the major contribution of the total required energy.11, 14, 15 With SDA process requiring a significant amount of expensive solvents, solvent recovery, an energy-intensive process, should be optimized to improve the efficiency.11 Kerr-McGee’s ROSE (Residue Oil Supercritical Extraction) and UOP’s DEMEX are two commercial SDA processes with significant application worldwide.16-18 The DAO is an acceptable feedstock for both fluid catalytic cracking and, in some cases, hydrocracking. Since it is relatively less expensive to desulfurize the DAO than the heavy vacuum residue, the solvent deasphalting process suggests a more economical route for disposing of vacuum residue from high-sulfur crude. However, the question of disposal of the asphalt remains.8, 19 Asphaltenes (asphalt binder) are commonly used as a road-packing material, low-grade fuel, and as a feed to produce heat and hydrogen by gasification.11, 20, 21 The use of 2 ACS Paragon Plus Environment

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asphaltenes fractions as a refinery fuel is less common because expensive stack gas line clean-up facilities may be needed.8 Solvent deasphalting and delayed coking are used frequently for heavy feedstock conversion. The high demand for petroleum coke, mainly for use in the aluminum industry, has made delayed coking a major heavy feedstock conversion process.8, 9 Coke produced from many heavy crude oils, however, does not meet the sulfur and metals specifications for aluminum electrodes. Coker gas oils are also less desirable feed stocks for fluid catalytic cracking than straight run gas oils. The solvent deasphalting process can be easily applied to most crude oil vacuum residues and heavy feeds.8 Most crude oils contain trace amounts of vanadium, nickel, and other metals which have detrimental impact during downstream catalytic conversion processes. Depending on the origin of crude oil, the concentration of the vanadium varies from as low as 0.1 ppm to as high as 1200 ppm, while that of nickel typically varies from trace to 150 ppm.22- 24 Metal concentrations in bitumen and vacuum residue are typically much higher compared with the virgin crude thus imposing problems for the economical upgrading of these feedstocks.5, 9, 25 The presence of vanadium compounds in the coke product can lead to the formation of vanadium pentoxide compounds during combustion causing a toxicity concern if released directly to the environment from a stack as well as leading to corrosion problems for turbines when used in power generation.22, 25 The majority of the metals are contained within the highly aromatic and highly polar asphaltene fractions in the form of metal porphyrins. Asphaltenes are very complex macromolecules containing saturated rings and condensed aromatic, aliphatic chains, and heteroatoms.4 They correspond to fractions insoluble in paraffinic hydrocarbons, such as n-heptane or n-pentane, but soluble in aromatics such as toluene.7, 26 This fraction has been shown to aggregate/associate significantly in most (if not all) solvents.5 The precipitation of some or the entire asphaltene fraction from the feed also results in considerable removal of nickel and metals. However, this precipitation process is nonselective and results in a considerable loss of product in the form of a contaminated asphaltene fraction.5 The effect of various solvents (such as propane, n-pentane, nheptane) on average molecular weight, viscosity, density, metals content, elemental composition, and structure of asphaltenes has been studied in numerous investigations using different analytical techniques including nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), size exclusion chromatography (SEC), scanning electron microscope (SEM) imaging, freezing-point osmometry, thermogravimetry (TG/DTG), and X-ray fluorescence.4, 13, 15, 27-33 In a recent study, primary porphyrin extracts were obtained from carbon tetrachloride solutions of different heavy oil residues using sulfuric acid.34 It was shown that a higher concentration of porphyrins in sulfuric acid extracts were obtained for residues containing higher nickel and vanadium content and lower ratios of asphaltenes to resins.34 Although sulfuric acid effectively 3 ACS Paragon Plus Environment

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removed sulfur and metals along with the resin and asphaltic compounds, the formation of an emulsion was always a serious problem leading to darker oil and significant amounts of acid sludge.35 An alternative process for metal removal is hydrodemetallization (HDM). It has been shown that the most active HDM catalysts are those prepared from synthetic aluminum oxide or natural aluminum silicate enriched with the oxides of nickel, cobalt, and molybdenum,36, 37 and their catalytic performance has been studied extensively.38, 39 The natural aluminosilicate activated with sulfuric acid solution was found to be best at removing vanadium and nickel.22, 40 Most of crude oils contain trace amounts of nickel, vanadium, and other metals which have damaging impact during downstream catalytic conversion processes. The main objective of this study was to use the vacuum distillation residue of Abadan refinery (Iran) in a solvent deasphalting process at different temperatures (60 to 120 °C), pressures (5 to 9 bar), solvents (n-pentane and ethyl acetate), and with solvent to feed ratio of (3 to 1, 5 to 1 and 7 to 1) to investigate the effects of SDA operating conditions on DAO yield and quality. The deasphalted oil was subsequently processed by thermal cracking at 500 °C and atmospheric pressure to investigate the effect of solvent deasphalting processing conditions on the yield of coke and liquid products as well as the metal and sulfur content of the produced coke.

2. Materials and Methods 2.1 Materials The properties and SARA analyses of a vacuum distillation residue from Abadan refinery that was used as the feedstock in this investigation are presented in Table 1. The specifications of the solvents used for deasphalting are presented in Table 2. Table 1. Properties of vacuum distillation residue from Abadan refinery12

Property

value

ASTM test method

kinematic viscosity at 100 °C (cSt)

572

D2170

kinematic viscosity at 135 °C (cSt)

107

D2170

1.004

D3289

Asphaltenes (mass %)

4.3

ASTM D-2007

Resins (mass %)

22.3

ASTM D-2007

Aromatics (mass %)

50.1

ASTM D-2007

Saturates (mass %)

23.3

ASTM D-2007

pour point (°C)

>40

D97

specific gravity at 25/25 °C

17.41

D189

N content (mass %)

Conradson carbon residue (mass %)

0.5

D5291

S content (mass %)

3.35

D4294

vanadium (V) (ppm)

130

D5863

nickel (N) (ppm)

34

D5863


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Table 2. Solvent specifications

Solvent n-pentane

ethyl acetate

Toluene

Purity 95%

Boiling point (°C) 36.1

Company Merck

99.5%

77.1

Riedel-dehaen

99%

110.6

Merck

Structure

2.2 Experimental Procedures 2.2.1 Solvent deasphalting process The schematic diagram of the experimental apparatus that was used for solvent deasphalting of the feedstock is presented in Figure 1.

Figure 1. Schematic diagram of the experimental apparatus for solvent deasphalting

In this work the vacuum residue was first subject to a solvent deasphalting process. Thermal cracking of the deasphalted oil was subsequently performed. The vacuum residue was initially heated in an oven at 150 °C with stirring to keep the feed uniform. 40 grams of the feed was then 5 ACS Paragon Plus Environment

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poured in to a stainless steel autoclave and was allowed to cool down to room temperature (about 25 °C) to avoid evaporation of the solvent when added to the residue feed. The desired solvent (n-pentane or ethyl acetate) was added to the residue sample with solvent to oil ratio of 3 to 1, 5 to 1, or 7 to 1. After the addition of solvent, a magnetic stirrer was placed inside the autoclave and the reactor was sealed using a soft rubber gasket to keep the reactor leak-free under high pressure operation. The reactor was then placed on a heater and its temperature was monitored using a thermocouple that was placed inside the reactor. The reactor temperature was controlled to within +2 oC of the desired set point using a temperature controller. The reactor was connected to a nitrogen cylinder to maintain the reactor pressure at the desired level. The autoclave was equipped with a safety valve and the reactor pressure was so chosen that the solvent would remain in the liquid phase at the reactor temperature. Once the reactor temperature had reached the desired solvent deasphalting temperature, the reactor content was kept under stirring for an additional 30 minutes for the deasphalting process to be completed. After 30 min both the heater and the stirrer were then turned off and the reactor was placed in a cold water bath. The reactor content would reach ambient temperature in approximately one hour for all experiments. The reactor pressure was then allowed to reach atmospheric pressure by slowly releasing the nitrogen from the autoclave. The reactor was opened and the deasphalted oil was separated and decanted in to a flask that was subsequently placed in a dark and static position for 24 hours to allow for the asphaltic compounds that were suspended in DAO to be deposited at the bottom of the flask. Most of the insoluble compounds in the reactor that are asphaltic particles were deposited on the wall and bottom of autoclave reactor. These compounds were removed by dissolving them in toluene and were added to the suspended compounds recovered from DAO flask and washed with toluene. 2.2.2. Solvent recovery A rotary evaporator was use to separate the solvent from DAO as well as removing toluene from PITCH. The rotary evaporator bath temperature was set to 60 °C with samples placed under a vacuum pressure of 60 mmHg. A small amount of solvent would remain in the DAO and PITCH after rotary evaporation that were subsequently removed by heating the samples in an oven at 150 °C. The samples were then weighed to determine the amount PITCH and DAO recovered from solvent deasphalting process. The DAO was then subject to thermal cracking experiments. 2.3. Thermal cracking of DAO 2.3.1 Delayed coking process Depending on the feed quality and the operating conditions of the coke production units, the quality of coke produced may vary from fuel-grade and anode-grade to needle-grade coke. Fuelgrade coke is used primarily in power and cement plants as fuel application; anode-grade coke is widely used in the aluminum industry for the production of anode electrodes; and high-grade


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needle coke is a premium coke used for production of electrodes for the steel industry. Table 3 shows typical specifications for three grades of coke.6 Table 3. Typical specifications for different grades of coke6 Property

Fuel coke

Bulk density, kg/m3 Sulphur, wt% Nitrogen, ppmw Nickel, ppmw Vanadium, ppmw Volatile combustible material, wt% Ash content, wt% Moisture content, wt% HGI (Hardgrove Grindability Index)

880 3.5-7.5 6000 489 141 12 0.35 8-12 35-70+

Calcined anode coke 720-800 1.0-3.5 200 350 0.5 0.4 0.3 60-100

Calcined needle coke 670-720 0.2-0.5 50 5-7 0.5 0.1 0.1 -

Figure 2. Schematic diagram for thermal cracking experiments apparatus: (1) nitrogen source, (2) regulator, (3) pressure gauge, (5) flow meter, (7) autoclave, (8) furnace, (9) storage tank, (10) condenser, (11) thermometer, (12) flow meter, back, (14) gas scrubber12

A schematic of the apparatus for thermal cracking experiments is shown in Figure 2. For each experiment, 20 g of DAO was poured in the stainless steel autoclave reactor that was then closed and sealed and placed inside a furnace. In all tests, heating at 10oC/min was used to increase the reactor temperature to 500oC. The reactor was maintained at this temperature for an additional 2 hours. Details of the apparatus, the procedures for thermal cracking experiments, methods for


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determination of product yields, and analysis of coke, gas, and liquid products are presented elsewhere.12

3. Results and Discussion 3.1. Effect of extraction temperature, pressure, solvent to oil ratio and solvent type on the properties and yield of products Previous investigations on SDA reported in the literature have focused on extracting DAO under various conditions, targeting maximum utilization of the vacuum residue. Solvents normally used in the SDA process include single components such as propane, butanes and pentanes, as well as mixtures of these components. In most cases the solvent is supplied from LPG products within the refinery and hence their use as SDA solvent is relatively inexpensive.17 The DAO yield using a mixed C4 solvent decreased to 47.2% from 58.0% when deasphalting temperature was increased from 100 to 120oC.16 When n-butane was used, the yield of DAO was higher compared with cases where i-butane or propane was used as solvent. Solvent recovery was found to increase with temperature and decrease with pressure for all the solvents that were tested and the best results were obtained for propane.11, 14, 16, 19, 27 SDA process with mixed C4 solvent and a vacuum residue (VR) feed obtained from a mixed crude oil exhibited increased DAO yield from 51.6% to 56.1% with solvent–VR ratio increasing from 4.5 to 6.5. In addition, a heavier mixed C4 solvent would improve the DAO yield but deteriorate its quality.15, 16, 41, 42 At a constant DAO yield, increasing the amount of solvent in the extractor improved the degree of separation of the individual components and resulted in the recovery of a higher-quality DAO.16, 17 This improvement, however, should be balanced against the additional operating and capital costs.17 An extraction method using a polar solvent (ethyl acetate) was found to be very effective for removal of oil-soluble metals from heavy residual fractions.35 In a separate study, a set of comparative experiments was conducted to observe the extraction performance of five different solvents widely used in refinery operations. N-methylpyrrolidone (NMP) was found to perform best with regards to both the yield and the quality of extract.41 A further study on extraction temperature and solvent-to-oil ratio was also conducted with NMP. Results showed that at fixed solvent-to-oil ratio a higher temperature could improve the yield of extract but deteriorate its quality.41 Results that were reported when a mixed C4 solvent was used for extraction were in contradiction with those from the study using NMP (N-Methyl-2-Pyrrolidone) as solvent. It should be emphasized that the boiling point of the mixed C4 solvent is about -1oC while that for NMP is about 202oC. The two solvents may have a different behavior over the temperature range that was employed (100-120oC). This different behavior may be due to what is reported in the literature as the “cluster behavior” leading to different solubility properties of a solvent over a wide temperature range as a result of certain physical and structural properties of the solvent. 8 ACS Paragon Plus Environment

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With low temperatures, the solvent has a high density and a much higher solubility, leading to a low asphalt yield.42 When n-pentane was used as the extraction solvent for a Vacuum Tower Bottoms in the temperature and pressure ranges of 210–250°C and 42–122 bar, respectively, the DAO yield was found to increase with increasing pressure and with temperature approaching a critical temperature.12 Increasing the extraction temperature reduces the solubility of the heavier components of the feedstock, improving DAO quality but reducing DAO yield. Subsequent increases in the extractor temperature can further improve the quality of the DAO by causing additional rejection of the asphaltene components.15, 17 The effect of operating pressure is opposite to temperature but to a lesser extent. In general, the higher the operating pressure the more DAO is extracted at a specific temperature.17 In the present investigation, solvent deasphalting was performed at different temperatures (60, 90, and 120oC) with solvent to oil ratios of 3 to 1, 5 to 1, and 7 to 1 with either n-pentane or ethyl acetate as solvent. Experimental product yields at different conditions are presented in Table 4. The results for experiments with the lowest solvent to oil ratio of 3 to 1 contained significant errors resulting from the difficulty in the separation of DAO from PITCH after the SDA process. The results for these experiments were not reliable and were not reported in Table 4. Results indicated that for a constant temperature, the yield of DAO slightly increased with increasing solvent to oil ratio (from 5 to7) for both solvents. This increase was more considerable for ethyl acetate compared with n-pentane. It was also found that for a constant solvent to oil ratio, the yield of DAO slightly decreased with increasing temperature for both solvents. With increasing temperature the solubility in the solvent would increase (because of solvent structure and diffusion in vacuum residue) while the solvent density would decrease. Since after the completion of solvent deasphalting the reactor was cooled down to room temperature, some compounds could precipitate out of the DAO during the cooling process. Pressure did not have a significant effect on the DAO yields. The results indicated that the DAO yields when n-pentane was used as solvent were significantly higher than those when ethyl acetate was used as solvent. It’s notable that the amounts of deposited asphalt (after 24 h) in the DAO flask obtained when ethyl acetate was used (typically about 4 g) were significantly greater than those when n-pentane was used as extraction solvent (typically about 0.3 g). Furthermore, the DAO from SDA using ethyl acetate as solvent was observed in an emulsion state which can have an important implication for separation in large scale SDA plant design. This emulsion state (some asphalt compounds are dispersed in DAO) is unstable and after 24 h, the dispersed asphaltenes are deposited. This emulsion state for ethyl acetate’s DAO can be due to the approximately equal density of ethyl acetate and dispersed asphaltene at room temperature.


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Table 4. Product yield (%) from solvent deasphalting of vacuum residue

Solvent n-Pentane n-Pentane n-Pentane n-Pentane n-Pentane n-Pentane Ethyl acetate Ethyl acetate Ethyl acetate Ethyl acetate

S/F ratio (V/V) 5 5 5 7 7 7 5 5 7 7

Extraction Temperature(oC) 60 90 120 60 90 120 90 120 90 120

Pressure (bar) 5 7 9 5 7 9 7 9 7 9

DAO Yield (wt. %) 87.3 85.6 83.2 89 87.5 86.4 74.4 72 78 75.6

PITCH Yield (wt. %) 12.7 14.4 16.8 11 12.5 13.6 25.6 28 22 24.4

3.2. Yield and properties of products obtained from thermal cracking of DAO Coker units are generally designed to operate at low pressure to maximize the yield of distillate products and produce fuel-grade coke as a by-product. Cokers producing anode-grade and needle-grade coke require more severe conditions of temperature and pressure along with a high recycle ratio.43 Coke formation and cracking of heavier molecules to lighter components proceed via free radical mechanisms during thermal cracking and it has been shown that the addition of free radical initiators such as dimethyl disulfide increases the rate of both cracking and coke formation reactions.44 Recent studies have indicated that the yield of liquid products was higher at 1 bar compared with higher pressures, that increasing the reaction temperature at a given reaction pressure would lead to an increase in the yield of liquid products, and that increasing the reaction pressure at a given temperature would lead to higher yields of coke and gases.12, 43 In this work, DAO samples obtained from SDA were subsequently processed under thermal cracking. The results of thermal cracking experiments are summarized in Table 5. The results show that when n-pentane was used as the deasphalting solvent, an increase in the solvent to feed ratio in the extraction process led to a lower coke yield and a higher distillate yield from thermal cracking of the resulting DAO indicating that with increasing solvent to feed ratio in SDA process, more of the lighter compounds of vacuum residue have been transferred into the DAO phase. With ethyl acetate as the solvent, however, an increase in the solvent to feed ratio resulted in an increase in the coke yield. This could be explained by the fact that with increasing solvent to feed ratio when ethyl acetate was used as solvent, the resulting DAO contained more of the heavy compounds that would eventually remain in the product coke upon thermal cracking. Also due to the polarity of ethyl acetate, more of the polar compounds are extracted into the DAO phase (as compared with n-pentane). The presence of these polar compounds could influence the cracking of DAO to coke products. 10 ACS Paragon Plus Environment

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Table 5. Yields of coke and distillate in thermal cracking of DAO from different SDA experiments

Solvent

n-Pentane n-Pentane n-Pentane n-Pentane Ethyl acetate Ethyl acetate Ethyl acetate Ethyl acetate

Extraction S/F ratio (V/V) 5 5 7 7 5 5 7 7

Extraction Temperature(oC)

Extraction Pressure(bar)

90 120 90 120 90 120 90 120

7 9 7 9 7 9 7 9

Coke Yield (wt. %) 20.2 20.8 18.7 18.8 18.1 16.4 19.3 20

Distillate Yield (wt. %) 65 64 65.3 66 68.4 62.4 62.8 63.6

Coke/VB (wt. %)

Distillate/VB (wt. %)

17.3 17.3 16.4 16.2 13.5 11.8 15.0 15.1

55.6 53.2 57.1 57.0 50.9 44.9 49.0 48.1

Coke and distillate products from thermal cracking of the Abadan vacuum residue sample and three DAO samples obtained from the extraction process were analyzed in terms of sulfur and metals (vanadium and nickel). The results presented in Table 6 indicate that when ethyl acetate was used as solvent in SDA, more metals were transferred to oil residue asphalt phase (PITCH) during the extraction process leading to lower vanadium and nickel content of the coke produced in thermal cracking of the resulting DAO as compared with the case with n-pentane as SDA solvent. The results also indicated that the quality of produced coke in terms of metal and sulfur content had improved by increasing the solvent to feed ratio and increasing the temperature in the extraction process. A closer look at the results presented in Table 6 reveals that the amount of metals (especially nickel) is higher than the expected values considering the mass balances. It may be that a portion of nickel in the coke samples has come out from the stainless steel material of the reactor. According to the supplier specifications, the thermal cracking reactor is made of an alloy containing 10% Ni and 18% Cr. With the assumption of similar metal contamination from the reactor body in all coke samples, results show a significant reduction of nickel and vanadium in the produced coke by SDA process. Depending on operating conditions, SDA removed up to 43% of vanadium and up to 32.7% of nickel from the vacuum residue resulting in metals being concentrated in the pitch fraction. Due to the existence of Ni in both the extraction and thermal cracking autoclave reactors, it is expected that the removal of Nickel be more than the reported values. Subsequent thermal cracking of DAO resulted in the remaining metals to be concentrated in the produced coke.


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Table 6 . Product yields and metal and sulfur content of products from the cracking of vacuum residue without removing asphalt and from DAO samples

Sample

Liquid (wt. %)

VB refinery n-Pentane DAO(at 120oC, S/F=7) Ethyl acetate DAO(at 120oC, S/F=5) Ethyl acetate DAO(at 120oC, S/F=7)

62 66 62.4 62.8

Coke (wt. %) 22.4 18.8 16 20

Gas (wt. %)

Sulfur (wt. %) (coke)

Sulfur (wt. %) (liquid)

V(ppm) (coke)

Ni(ppm) (coke)

15.6 15.2 21.6 17.2

5.51 6.03 5.90 5.97

1.98

543 377 361 309

260 213 193 175

Simulated distillation results are presented for the distillates obtained from thermal cracking of selected DAO in Table 7. The results indicate that for ethyl acetate, an increase in the solvent to feed ratio in the SDA process, resulted in increased amounts of lighter compounds (C5-C20) of the distillates produced from thermal cracking of DAO. Also results show that with increasing solvent to feed ratio in extraction process the initial boiling point (IBP) of liquid product is decreased, and under the same operating conditions, liquid product obtained from ethyl acetate showed appreciably higher IBP than liquid products from n-pentane. Table 7. Simulated distillation results for the liquid product produced from thermal cracking of DAO obtained using ethyl acetate and n-pentane as solvent

SDA /coking operating conditions

SDA Temperature (120 oC) SDA Pressure (9 bar) SDA Solvent/feed ratio (7) Coking Temperature (500 oC) Coking time (2 hours at 500 oC) Coking rate (10 oC /min)



Fractions

boiling range (oC)

Cut Wt. %

n-pentane IBP=51 C1-C4 Naphtha (C5-C12) Middle distillates(C13-C20) Gasoil (C20-C30) Reside (C31-C100)

0-35 36-225 225-344 345-450 451-554

0 20 25 30 25

ethyl acetate IBP=91 C1-C4 Naphtha (C5-C12) Middle distillates(C13-C20) Gasoil (C20-C30) Reside (C31-C100)

0-35 36-216 216-344 345-450 451-554

0 13 22 30 35

ethyl acetate IBP=80 C1-C4 Naphtha (C5-C12) Middle distillates(C13-C20) Gasoil (C20-C30) Reside (C31-C100)

0-35 36-216 216-344 345-450 451-554

0 16 25 31 28


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Energy & Fuels

3.3. Reproducibility of experimental data To estimate the error associated with the experiments, the runs at 120°C and 9 bars were repeated. The duplicate results presented in Table 8 indicate a reasonable agreement between the two sets of data. Two experiments were repeated for determining the error in the yield of DAO, Pitch, Coke and Distillate at each condition. That maximum error was 1.2 % for extraction and 1.8% for thermal cracking experiments. Table 8. Results of the duplicate experiments to establish the experimental error

Sets No.

1

2

3

Solvent

extraction Temperature (oC)

Extraction Pressure (bar)

S/F

DAO yield

Pitch yield

Condensate Yield

Coke Yield

(Wt. %)

(Wt. %)

(Wt. %)

(Wt. %)

n-Pentane

120

9

7

86.8

13.2

65.1

19.1

n-Pentane

120

9

7

86

14

66.9

18.5

Ethyl acetate

120

9

5

71.7

28.3

61.8

16.8

Ethyl acetate

120

9

5

72.3

27.7

63

16

Ethyl acetate

120

9

7

75

25

63.0

20.2

Ethyl acetate

120

9

7

76.2

23.8

64.1

19.8

4. Conclusions The following conclusions can be made from this investigation: 1. At a given temperature for SDA, the yield of DAO slightly increased with increasing solvent to oil ratio for both n-pentane and ethyl acetate solvents. This increase was more pronounced for ethyl acetate compared with n-pentane. It was also found that for a constant solvent to oil ratio, the yield of DAO slightly decreased with increasing temperature for both solvents. 2. Pressure did not have a significant effect on the DAO yields in SDA and the DAO yields when n-pentane was used as solvent were significantly higher than those when ethyl acetate was used as solvent. 3. When n-pentane was used as the deasphalting solvent, an increase in the solvent to feed ratio in the extraction process led to a lower coke yield and a higher distillate yield from thermal cracking of the resulting DAO. With ethyl acetate as the solvent, however, an increase in the solvent to feed ratio resulted in an increase in the coke yield. 4. When ethyl acetate was used as solvent in SDA, more metals were transferred to oil residue asphalt phase (PITCH) during the extraction process leading to lower vanadium


Energy & Fuels

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Page 14 of 16

and nickel content of the coke produced in thermal cracking of the resulting DAO as compared with the case with n-pentane as SDA solvent. 5. The quality of produced coke in terms of metal and sulfur content had improved by increasing the solvent to feed ratio and increasing the temperature in the extraction process. 6. For ethyl acetate as the SDA solvent, an increase in the solvent to feed ratio resulted in increased amounts of lighter compounds (C5-C20) of the distillates produced from thermal cracking of DAO. Furthermore results show that for ethyl acetate initial boiling point (IBP) of liquid products decreased with increase in the solvent to feed ratio in SDA process. Acknowledgment The authors are thankful to the Research Institute of Petroleum Industry of Iran (RIPI) and the Iranian Mines and Mining Industries Development and Renovation Organization (IMIDRO) for financially supporting this work.

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Improvement of the Thermal Cracking Product Quality of Heavy

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