Upgrading vacuum distillation residue of oil refinery using microwave irradiation

Journal Pre-proof Upgrading Vacuum Distillation Residue of Oil Refinery using Microwave Irradiation Zarrin Nasri

PII:

S0255-2701(19)30212-0

DOI:

https://doi.org/10.1016/j.cep.2019.107675

Reference:

CEP 107675

To appear in:

Chemical Engineering and Processing - Process Intensification

Received Date:

23 February 2019

Revised Date:

22 September 2019

Accepted Date:

28 September 2019

Please cite this article as: Nasri Z, Upgrading Vacuum Distillation Residue of Oil Refinery using Microwave Irradiation, Chemical Engineering and Processing - Process Intensification (2019), doi: https://doi.org/10.1016/j.cep.2019.107675

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Upgrading Vacuum Distillation Residue of Oil Refinery using Microwave Irradiation

Zarrin Nasri

Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), P.O. Box 33535111, Tehran, Iran E-mail address: [email protected], [email protected]

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Graphical Abstract

Picture of optimum upgraded vacuum residue in a centrifuge tube

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Highlights

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Comparison between of vacuum residue of oil refinery (on the right) and optimum upgraded product in this research (on the left)

 Iranian vacuum residue upgrading using microwave is studied for the first time.  The effects of catalyst, power level, sensitizer, hydrogen source are investigated.  Also, desulphurization agent and process time effects are investigated.  The studied catalysts are iron, nickel, copper, MoS2, iron oxide.  The performed tests include temperature, API, asphaltene, viscosity.

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ABSTRACT Vacuum distillation residue upgrading of Tehran oil refinery, Iran, using microwave was performed, and the effects of different parameters on temperature, viscosity, API1 gravity, and asphaltene were evaluated. The various catalysts effects (iron, nickel, copper, MoS2, Fe2O3), microwave power level at three levels (50, 70, 100%), activated carbon at four levels (5, 10, 15, 20 wt%), iron catalyst at four levels (0, 10, 15, 20 wt%), NaBH4 as hydrogen source at four levels (0, 3, 6, 10 wt%), desulphurization agent at four levels (0, 2, 5, 10 wt%) and process time at four levels (15, 30, 45, 60 min) were investigated. The vacuum residue specifications include API gravity: 8.79, viscosity (60 0C): 16391 cSt, and asphaltene: 13.3 wt%. The results showed that Fe was the best catalyst. Power level, activated carbon, Fe catalyst, and process time presented

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very positive effects on the process. 3 wt% NaBH4 was the optimum amount, but desulphurization agent had no positive impact on upgrading. At the best conditions (power level: 100%, Fe catalyst: 20 wt%, activated carbon: 20 wt%, NaBH4: 3 wt%, 1 hr process time) asphaltene decreased by 94.22%, viscosity reduced by

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99.53% and API gravity increased by 88.56%.

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Keywords: Vacuum residue, Heavy oil, Upgrading, Microwaves.

1. Introduction

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The issue of upgrading heavy oil and vacuum distillation residue to increase their value has become very important in recent years. Vacuum residue upgrading units have been considered as a critical process in

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modern oil refineries. There are large amounts of oil residues in the oil refineries, including atmospheric and vacuum residues, which can be upgraded into valuable light fuels [1]. A vacuum distillation column typically

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produces 45% vacuum residue and 54.2% vacuum gas oil. The atmospheric residue is typically 53%, and vacuum residue is 25% of crude oil as refinery feed [2]. The vacuum residue resulting from heavy crude oil

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is 2 to 3 times more than that from medium and light crude oil [3]. The conventional upgrading processes of vacuum residue perform with chemical, thermal, and mechanical methods, which are very costly, produce side effects and require very complex systems. The problems with conventional methods of upgrading vacuum residue increase the tendency of researchers to apply modern technologies. Using MW irradiation for heating heavy molecules, leads to improvements in their physical and chemical properties. The benefits of microwave technology include fast heating, volume heating, superheating, hot spots, and selective heating.

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American Petroleum Institute

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The microwave irradiation is converted into thermal energy by the reorientation of dipole and conduction [4]. The application of microwave irradiation as a heating source provides an improved heat distribution, rapid heating rate, and increases the production rate in comparison with conventional heating [5]. Herbst et al. (1990) [6] applied microwave to strip used catalysts of fluid catalytic cracking. Gunal and Islam (2000) [7] evaluated asphaltene rheology alteration with using electromagnetic and ultrasonic. Klepfer et al. (2001) [8] presented a process for cracking of plastic materials using microwaves. Jackson (2002) [9] studied the effects of microwave frequency to upgrade heavy oil. The variables were additives and microwave frequency. In their work, the additives were activated carbon, iron powder, iron oxide, NaOH, methanol, and

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molybdic acid. The best performance was related to the case of using only activated carbon as an additive. The results showed that activated carbon addition resulted in oil with the pipeline specifications. Al-Mayman and Al-Zahrani (2003) [10] evaluated the cracking of gas oil by using the electromagnetic field. Bjorndalen and Islam (2004) [11] studied the effects of microwaves and ultrasonic on crude oil during production. One

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of the problems of horizontal wells is plugging due to asphaltene precipitation. Sharivker and Honeycutt (2004) [12] studied the desulphurization process of oil using microwaves. The additives were including

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sensitizer, hydrogen source, catalyst, and desulphurization agent. The feed was fuel oil. In their work,

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activated carbon was as a sensitizer, iron powder was as a catalyst, NaOH, KOH, CaCO3, and NaHCO3 were used as desulphurization agents. Britten et al. (2005) [13] upgraded five samples of heavy oil from Alberta using microwave irradiation. They mixed the samples with heavy and light naphtha as the hydrogen source

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and studied effects of microwave on asphaltene content. Gunnerman (2006) [14] investigated the upgrading of heavy oils using microwave and ultrasonic to reduce sulfur and molecular weight. Greff and Babadagli

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(2011) [15] studied the effects of metal ions on asphaltene cracking. Three types of metal particles were studied: iron nano powder, iron oxide nano powder, and copper micro powder. They used ultrasonic to

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prepare a stable suspension of nano metal particles in oil and concluded that with an increase in catalysts of nano and micro metals using microwaves, the viscosity of heavy oil effectively decreased. Huda and Nour (2011) [16] investigated separation of water in crude oil emulsions using microwaves and compared the results with the conventional methods. Mohammed et al. (2012) [17] used microwaves to reduce sulfur contents of heavy oil. They applied solvent extraction-oxidative using heptane and methanol. Greff and Babadagli (2013) [18] studied the effect of microwaves on heavy oil recovery from oil sands. They performed experiments with nano nickel and nano iron as catalysts. The results showed that the performance 3

of nickel was better than that of iron. Li et al. (2014) [19] applied nano carbon as a catalyst to upgrade heavy crude oil using microwaves. They reported that nano catalysts could effectively upgrade heavy crude oil at low temperatures. They investigated the effect of carbon size on the process. The heavy oil was from a Chinese field. Miadonye and Mac Donald (2014) [20] used microwaves to upgrade heavy oil with different additives. They applied palladium oxide as a catalyst, and activated charcoal and serpentine as sensitizers. They concluded that activated charcoal was a better sensitizer of microwaves. The results showed that viscosity increased during the process resulting from recombination reactions domination over cracking reactions. Strohm (2014) [21] performed the upgrading of heavy hydrocarbons using microwaves. They used iron as a catalyst and concluded that conversions without and with applying the iron catalyst were 20 and 42

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wt%, respectively. Al-Gharabli et al. (2015) [22] investigated the extraction of oil from oil shale using microwaves. They used some solvents such as ethanol, methanol, and acetone to extract oil at different temperatures. They concluded that microwaves were effective for extraction oil from oil shale. They

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extracted 23% oil using methanol solvent at 100 0C and 10 min process time. Ansari et al. (2015) [23] investigated heavy oil and vacuum residue upgrading using hydrodynamic cavitations and microwaves. They

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studied the effect of activated carbon on the process with microwaves. They concluded that without activated carbon, there was no conversion even after 45 min irradiation. The addition of 4% activated carbon resulted

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in a 13% conversion of vacuum residue. Singh et al. (2015) [24] studied coal conversion using microwaves. Johnson et al. (2016) [25] performed bitumen extraction using microwaves. They used dichloromethane and

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methanol as solvent. Tsodikov et al. (2016) [26] investigated the effect of microwaves on upgrading residues. They used carbon as a sensitizer. Activated carbon has dielectric loss several orders of magnitude than

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organic substances. Mozafari and Nasri (2017) [27] investigated the effects of various parameters on upgrading heavy crude oil using microwaves. Nasri and Mozafari (2018) [28] performed the statistical

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analysis and process optimization of heavy crude oil upgrading using microwaves. The Box-Behnken design and response surface methodology were applied to determine input parameters levels and to model the responses, respectively. In this research, the vacuum residue of Tehran oil refinery in Iran was upgraded using microwaves. The effects of various parameters such as sensitizer, power level, type and amount of catalyst, hydrogen source, process time, and desulphurization agent on properties of vacuum residue such as temperature, API gravity, asphaltene content, and viscosity were systematically investigated. In the investigations reported in this area, 4

the numbers of input and output parameters are more limited. In this study, the systematic investigation of the effects of the parameters on the upgraded vacuum residue properties led to a product with better properties in comparison with the other researchers’ results. To our knowledge, no similar studies have been reported in the literature. In addition, this study for Iranian vacuum residue has been performed for the first time.

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Materials and Methods Materials

2.1. The vacuum distillation residue used in this research was from Tehran oil refinery, Iran. The basic characteristics of it have been shown in Table 1. The chemicals used in this research consisted of activated

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carbon as a sensitizer, iron (Fe), copper (Cu), molybdenum disulfide (MoS2), iron oxide and nickel as catalysts, sodium borohydride (NaBH4) as a hydrogen source, sodium hydroxide (NaOH) as a desulphurization agent. All of these chemicals were provided from Merck, Loba Chemie, and Rang Chem. companies with analytical grades.

Experimental Setup

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2.2.

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Table 1 The basic characteristics of the vacuum residue

Fig. 1 illustrates the diagram of the experimental setup. The frequency and power of microwaves generator

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were 2450 MHz and 1000 W, respectively. The system is adjustable with 10 power levels from 10 % to 100% according to Table 2. The reactor made of Pyrex glass, 5 mm thickness and 500 cm3 capacity was used

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for the experiments.

Fig. 1. The diagram of the experimental setup

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Table 2 Power levels of microwaves system used in the experiments

A condenser was vertically connected to the reactor for both condensing and recycling the rising vapors into

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the reactor. So, the system was in total reflux state. A Pyrex glass circular tube was mounted in the middle of the condenser to inject nitrogen gas and to purge the reactor. So, the reactions were performed in a nitrogen atmosphere. Two other condensers were attached to the vertical condenser to condense further the light vapors and to collect the condensates in a container, respectively. The cooling fluid was circulated into three condensers by a circulating pump.

2.3. Process procedure The experiments were performed according to the following steps: 5

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20 lit (0.02 m3) barrels containing the vacuum distillation residue from Tehran oil refinery were placed in a 100 lit (0.01 m3) barrel containing hot water (60 0C) as a bath for at least 5 hr until the vacuum residue fluidization. Afterward, it was poured into one lit (1000 cm3) metal cans to be stored.

2)

The required amount of the vacuum residue (120 g) was charged to the reactor.

3)

The required amounts of additives including sensitizer, catalyst, hydrogen source, desulphurization agent were charged into the reactor.

4)

The reactor was fixed in a specified location on a Teflon stand in the middle of the microwave cavity, just in front of the generator. Since the reactor was connected to the vertical condenser outside the microwave cavity, its location was exactly constant in all the experiments. The vertical condenser and the other attachments including two condensers, nitrogen gas inlet, etc. were

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5)

connected at the top of the reactor. 6)

All the connections were checked, and then the coolant circulation pump was switched on at least 20

7)

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min before the test.

Nitrogen gas was directly flowed into the reactor from the nitrogen cylinder for about 10 min before the

8)

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test; in order to purge the system. So, the tests were carried out in the nitrogen atmosphere. The reactor and its contents were exposed to microwave irradiation for a specified time at a total reflux

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state.

The surface temperature was measured and recorded by a non-contact thermometer.

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10) The system was allowed to cool to the ambient temperature for at least 2 hr. 11) When the reactor was cooled, the amount of required chloroform was added to the reactor and was

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thoroughly mixed to obtain a fully diluted suspension. 12) The diluted suspension was centrifuged at 7000 rpm for 15 min to separate the formed coke and the

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additives.

13) The diluted upgraded vacuum residue with chloroform was separated by decanting. 14) Chloroform was separated by vacuum evaporator, and the upgraded product was obtained. 15) The upgraded product was analyzed to measure API gravity, density, viscosity, and asphaltene content. 2.4. Methods of analysis The viscosity was determined using glass capillary viscometers, according to ASTM D445 test method [29]. The asphaltene content was determined based on the n-heptane insoluble fraction of petroleum according to 6

ASTM D-4124 test method [30]. The API gravity of vacuum residue and the products were determined by ASTM D-70-3 and ASTM D-1217 [31,32]. The surface temperature of the reactor was measured with an IRPyrometer (model No. CT029021). 2.5. Experimental design The effects of operational parameters were studied according to the experimental design presented in Table 3. The operational parameters included catalyst type (iron, nickel, copper, MoS2, Fe2O3), microwave power level at three levels (50, 70, 100%), activated carbon as sensitizer at four levels (5, 10, 15, 20 wt%) and in two cases of iron catalyst (10 and 20 wt%), NaBH4 as hydrogen source at four levels (0, 3, 6, 10 wt%), iron catalyst at four levels (0, 10, 15, 20 wt%) and in two cases of activated carbon (10 and 20 wt%),

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desulphurization agent at four levels (0, 2, 5, 10 wt%) and process time at four levels (15, 30, 45, 60 min).

Table 3 The experimental design of independent variables

Results and discussion The effect of catalyst type

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3. 3.1.

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Fig. 2 shows the effect of catalyst type on temperature, asphaltene reduction, viscosity reduction, and API gravity increase, respectively. In these experiments, the other parameters including microwave power level:

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100%, process time: 30 min, activated carbon as a sensitizer: 10 wt%, sodium borohydride as hydrogen source: 3 wt%, and the catalyst: 10 wt% were considered constant. The applied catalysts included iron,

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MoS2, copper, iron oxide, and nickel powders.

According to Fig. 2(a) the two highest recorded temperatures are related to the catalysts of iron powder and

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nickel and are 314 and 291 0C, respectively. The order of temperature increase is as follows:

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Cu < Fe2O3 < MoS2 < Ni < Fe

One of the microwave effects on catalytic reactions is selective heating. As a result, the components with high dielectric constants tend to absorb microwave radiation, whereas less polar substances are poor absorbers. This characteristic of microwave heating is beneficial for selective heating of catalysts. In catalytic reactions under microwave irradiation, two different forms of hot spots can be created. Macroscopic hot spots are measurable, and microscopic hot spots are on a molecular level. Macroscopic hot spots are no isothermal and can be measured by using optical pyrometers (fiber optic or IR pyrometers). Microscopic hot 7

spots have molecular dimensions and are not measurable. It should be noted that microwave radiation effects at the molecular level are not well understood. It is estimated that the dimensions of these hot spots are in the region of 90– 1000 µm. The temperature of hot spots can be 100–200 K above the bulk temperature [33]. So, microwave irradiation in an inhomogeneous system leads to the non-uniform temperature distribution. Local superheating produces hot areas, resulting in a temperature gradient in solids and also increases contact frequency of the reactants. It is challenging to measure and estimate the temperature distribution generated by microwave heating in solids. Consequently, local temperatures are much larger than the measured values. The measurement usually produces an average temperature. It is necessary to mention that classical temperature sensors in microwaves are not applicable, because the

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metal wires connected to the sensors would be distorted by induced strong electric currents. Optical fibers thermometers can be a solution. However, the measurements are limited to below 250 0C. For higher temperatures, the surface temperature can be measured by using an infrared camera or a pyrometer [33].

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In this research, the reactor surface temperature was measured by IR Pyrometer. Since the reactor contents possessed a non-uniform temperature distribution, the temperatures were measured at different points of the

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reactor surface, and the maximum temperature was recorded.

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Table 4 shows the specifications of the materials used in this research. It should be noted that the dielectric data related to water are found in the literature, but for the other materials, it is scarce. The parameters of this

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table are as follows. The relative permittivity of martial represents an interaction between electromagnetic

is relative permittivity,

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which

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waves and dielectric media and is obtained from the following equation [33].

(1)

is dielectric constant, and

is dielectric loss factor. Permittivity is

typically related to dielectric materials. However, metals are considered to have an effective permittivity with real relative permittivity equal to one. The dielectric constant contributes to the stored energy of material by polarization [34]. The storage of electromagnetic energy is expressed by the real part, whereas the thermal conversion is proportional to the imaginary part. The dielectric constant decreases with

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increasing temperature. Materials with loss factor between 0.01 and 1 generally are adequately heated at microwaves. The loss factor is dependent on temperature, frequency, and moisture content [35]. Another important parameter is the penetration depth. The penetration depth (Pd) is defined as the distance from the surface of a dielectric material at which the incident power drops to 37% [33]. Microwave energy can penetrate up to 50 cm for materials with a low loss factor, whilst for materials with a high loss factor, such as water, it just penetrates a few centimeters [35]. The penetration depth within metals is substantially small [33]. On the other hand, microwaves are a form of electromagnetic radiation with wavelengths ranging from about

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one millimeter to one meter. The electromagnetic field consists of electric and magnetic fields perpendicular to each other in space. Electromagnetic heating of materials can be classified into two categories of induction heating and dielectric heating. Induction heating is used to heat materials with high electrical conductivity.

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Dielectric heating is used to heat-insulating materials or materials with low electrical conductivity. In the induction heating, the magnetic field is larger than the electric field. While, in dielectric heating, the electric

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field is dominant [10].

and interfacial polarization [47].

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Microwave heating has occurred via three mechanisms including dipolar polarization, electrical conduction,

1- Dipolar polarization is a process by which heat is produced in polar molecules. Microwaves can

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effectively increase the temperature of a dielectric material. A dielectric material is electrical insulation and consists of small dipoles. When these materials are exposed to an external electric field, no electrical current

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exists, but microwave radiation oscillates polar molecules and creates a new arrangement within the electric

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field, resulting in a rearrangement and reorientation of charged particles, called polarization. 2- The electrical conduction mechanism produces heat via electrical resistance. The electrical conductors reflect microwaves. The electrical field of microwaves oscillates the electrons or ions in a conductor and leads to an electric current. This current encountered internal resistance, heating the conductor. 3- The interfacial polarization mechanism is a combination of conduction and polarization methods. This is related to heating systems including a conductor material dispersed in a dielectric medium. Examples of this are particles of metals in oil. 9

The catalysts used in this study include iron, nickel, copper, molybdenum disulfide, and iron oxide. It should be noted that the choice of these catalysts is based on the references in this field [1,9,18,48-53]. In this regard, the catalysts applied in this study can be divided into two groups. The first group is including metals, which are electrical conductors comprises iron, copper, and nickel. The second group is semiconductors containing iron oxide and molybdenum disulfide. The catalyst used in the microwaves should have two main properties. The first, the catalyst should have adequate dielectric or magnetic dissipation to absorb microwave energy, so that it can be heated to the required temperature for reaction. In the catalytic process under microwave irradiation, the catalyst is used as

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an energy converter. The second, the catalyst should have the desired catalytic properties such as activity, selectivity, and long life [10].

a) In the first group, the catalysts used in this study are metals, which reflect microwaves. In this case, the

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material is not effectively heated. In response to the electric field of microwave irradiation, electrons move freely on the surface of the material, and the electron current can heat the material via resistive heating [54].

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Joule heating, resistive heating, or ohmic heating is a process by which an electric current passes through a conductor and generates heat [38]. The increasing temperature in metals due to microwave irradiation is

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enhanced with increasing thermal conductivity and power dissipation distribution and decreases with increasing density and heat capacity [44].

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As shown in Fig. 2(a), the order of temperature increase in metal catalysts is iron, nickel, and copper, respectively. This is in accordance with increasing electrical resistance, reducing the thermal conductivity

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and the density of the catalysts according to Table 4. The absorbed heat by activated carbon is reflected in the vacuum residue by metallic catalysts and leads to cracking of the components.

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The other issue is the electrical discharge of metals exposed to microwave irradiation. According to Fig. 2(a), the temperature is higher in iron and nickel metal catalysts in comparison with the other catalysts. This issue can also be analyzed with electric discharge. The electric discharge is electricity release and transmission in an electric field through a medium such as a gas [38]. The metallic catalysts perform two functions. The first is the transformation of microwave energy to produce surface electric discharge, resulting in reactions. The second is a catalytic property which increases reactions of radicals on the surface of the catalyst [55]. Hydrocarbons cracking reactions is well known in an electric arc or electric discharge. In the presence of a 10

catalyst, more valuable products can be formed from free radicals produced in an electric discharge. When the thickness of metal is small compared to the penetration depth, the surface is hot. Under these conditions, electrons are released from the material and are accelerated in the electric field [8]. This electron energy is sufficient to crack the chemical bonds in the molecules constituting free radicals [12]. In the present study, concerning the abovementioned issues, improved results were governed in terms of temperature increase, viscosity reduction and API gravity increase, which could be attributed to the rare and remarkable magnetic properties of the iron catalyst. It should be noted that strong electrical discharge in this research occurred only in the presence of Fe

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catalyst, at the beginning of microwave irradiation, leading to high conversion. In the experiments, the Fe catalyst was recognized as an excellent material for electrical discharge using microwave irradiation. In order to compare the results with the other references, it can be referred to the following items. Electric

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discharge in metal and solvent mixture including Mg, Zn, Cu, Fe, Ni were studied under microwave irradiation, and it was concluded that Fe and Ni powders were rapidly heated and represented strong electric

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discharge [40]. Wan et al. (2000) [55] examined the effect of various metal catalysts such as Cu, Ni, Fe on the methane conversion reaction using microwaves. They concluded that among pure metals, iron provided

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electric discharge under microwave irradiation, enhancing the conversion of methane and selective formation of aromatics. Strohm (2014) [21] investigated upgrading heavy fossil fuels using microwaves. They studied

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dielectric discharge of different catalysts, including Fe powder, Fe filling, and NiO and observed that Fe powder produced dramatic discharging. Leadbeater and Khan (2008) [56] used Ni and Fe catalysts on

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desulfurization of crude oil using microwaves. They found out that it is possible to perform hydrodesulphurization reactions using microwave heating in conjunction with iron powder as a catalyst.

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b) In the second group, the catalysts used in this study, contain Fe2O3 and MoS2, which are semiconductors. The heating mechanism of these materials exposing to microwaves irradiation is dielectric heating and differs from metals. In dielectric heating the electric field is dominant. The dielectric properties determine their ability to absorb microwave energy [10]. As shown in Table 4, molybdenum disulfide has a higher electrical resistance and therefore possesses more capacity to absorb microwave energy compared to iron oxide. In Fig. 2 (a), it is also observed that the increasing temperature with using molybdenum disulfide catalyst is higher in comparison with iron oxide. 11

Based on the results of Fig. 2(b) the effect of different catalysts on asphaltene reduction is approximately the same and varies from 31.30 to 35.12 %. It can be observed in Fig. 2(c) that there is a significant difference between the effects of the used catalysts on viscosity reduction. The highest viscosity reduction has been found for iron powder and equals 76.43%. MoS2, Cu, and Fe2O3 increased viscosity by 22.4, 21.23, and 36.68%, respectively. Ni has just reduced the viscosity by 4.25%. According to Fig. 2(d) the iron catalyst has the most effect on API gravity increase, which is 23.43%. In summary, it can be concluded that the iron catalyst has the best results from the viewpoint of temperature increase, viscosity reduction, and API gravity increase. It should be noted that the cracking behavior of

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asphaltenes is complex. The application of a free radical chain mechanism for complex hydrocarbon mixture is more complicated. Taking into account the complexity of the large molecules involved, as well as the complex reactions occurring during the upgrading of vacuum residue, interpreting output data is difficult. Asphaltenes are sometimes become colloidal during cracking and lead to increased viscosity. Organic

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compounds can react by changing their structure without a change in molecular weight, such as

isomerization. They can also be subjected to chain cutting to generate lower molecular weight products, or

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during addition reactions produce higher molecular weight products. Chain cutting can occur either at the

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chain-end, creating a specific product, or at a random position along the chain to provide a different range of lower molecular weight products [57].

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In order to compare the results with the other references, it can be referred to the following items. Some experiments were performed with iron nanopowder, iron chloride, and iron oxide, and reported that iron powder had the best viscosity reduction [51]. Ansari et al. (2015) [23] investigated upgrading heavy oil and

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reported that, in some cases, the density and viscosity behavior of the product was inversely proportional to

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the original sample. In other words, the density decreased, but viscosity increased, suggesting that different viscosity and density behaviors could be due to their compounds. Each of the compounds with high molecular weight produces different combinations of molecular chains. In addition, the carbon-iron compound under microwave irradiation has already been proven to be effective as a catalyst [33].

From the abovementioned results, it can be concluded that among the applied catalysts, the iron powder is the most appropriate catalyst for upgrading the vacuum residue using microwaves, leads to increasing the

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light components and reducing the heavy components. Using this catalyst, asphaltene contents and viscosity reduced by 31.3% and 76.43%, respectively, and API gravity increased by 23.43%. Fig. 2. The effect of catalyst type on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase (Power Level: 100 %, Time: 30 min, Carbon Sensitizer: 10 wt%, NaBH4: 3 wt%, Catalyst: 10 wt%)

3.2. The effect of power level Fig. 3 shows the effect of microwave power level on parameters of temperature, asphaltene reduction, viscosity reduction, and API gravity increase. In these experiments, the other parameters were considered constant (process time: 30 min, activated carbon: 10 wt%, sodium borohydride: 3 wt%, iron catalyst: 10

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wt%). According to Fig. 3 (a) with increasing power level from 50% to 100%, temperature increases from 218 to 314 0C. Fig. 3 (b) shows that asphaltene reduction increases from 28.05% to 31.30%. Based on the results of

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Fig. 3 (c), the effect of power level on viscosity reduction is significant. With an increase of power level from 50% to 70%, the viscosity reduction increases from 16.35% to 72.35%. Increasing the power level from 70%

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to 100% has reduced viscosity by 76.43%. The reduction of viscosity is due to the cracking of heavy

to 23.43% at a 100% power level.

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hydrocarbons and asphaltenes. Fig. 3 (d) shows that API gravity increases from 19.07% at a 50% power level

From this section, it can be concluded that increasing the power level has a very desirable effect on the

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studied parameters. This effect is particularly significant in increasing viscosity. The experiments show that at low power levels, a small electric discharge is produced on the catalyst surface, so only small

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concentrations of radical species are generated. As microwaves power level increases, radical species increase, resulting in more products [55]. Microwaves affect polar and heavy molecules, crack heavy

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hydrocarbons and produce lighter molecules resulting in improvement of the vacuum residue properties. In this study, chloroform has been added to dilute the product in order to centrifuge and separate any possible impurities such as catalysts, activated carbon, and coke. It should be noted that due to high concentration of product, diluents are required for centrifuge. Other researchers also used a diluent such as toluene [58], dichloromethane [59-62], chloroform [1, 63, 64], Tetrahydrofuran (THF) [65] and benzene [66]. Chloroform was evaporated from the mixture. During evaporation, the weight of the mixture was measured. When the 13

weight of the mixture was reduced to the weight of the added chloroform, it was considered that all chloroform was separated from the mixture. Therefore the results have been reported based on chloroformfree. Fig. 3. The effect of power level on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase (Time: 30 min, Carbon Sensitizer: 10 wt %, NaBH4: 3 wt%, Fe Catalyst: 10 wt%)

3.3. The effect of sensitizer Fig. 4 shows the effect of activated carbon as sensitizer at two levels of Fe catalyst (10 and 20 wt%) on

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various parameters such as temperature, asphaltene reduction, viscosity reduction, and API gravity increase, respectively. In these experiments, the other parameters, including power level: 100%, process time: 30 min, sodium borohydride: 3 wt% were considered constant. The experiments were performed at four levels of

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activated carbon including 5, 10, 15, and 20 wt%.

It can be observed in Fig. 4 (a) that with increasing activated carbon, the temperature has continuously

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increased. In the case of 10 wt% Fe catalyst, the temperature increases from 224 0C (activated carbon: 5 wt%) to 336 0C (activated carbon: 20 wt%). In the case of 20 wt% Fe catalyst the increase is from 324 0C

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(activated carbon: 5 wt%) to 345 0C (activated carbon: 20 wt%). The comparison between the two cases shows that the effect of increased iron on temperature is higher at lower amounts of activated carbon (5

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wt%).

Due to this fact that oil is non-polar, the effect of microwave irradiation is low. According to Table 4, the

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dielectric constant of oil is 2-3, resulting in poorly absorbing the microwave. When vacuum residue is mixed

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with appropriate catalysts and sensitizers such as activated carbon, these powders can be applied as lossy additives to induce losses within solids with low dielectric losses. Hence the upgrading process will be more productive. In fact, microwaves deals with polar components and cause them to rotate and thus increase the temperature. The microwave sensitizer adsorbs the microwave radiation emitted from the magnetron and creates heat energy [4]. Increased heat transfer from the activated carbon heated by microwave heating, resulted in higher process temperature [5]. One of the most effective mechanisms to upgrade vacuum residue

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is rising temperature. The higher the amount of activated carbon results in the higher absorption of microwave energy and so higher vacuum residue temperature. There is experimental evidence that some reactions under microwave heating occur faster than conventionally heated reactions at the same temperature. These rate enhancements could be attributed to the localized superheating or hot spots. In fact, most reports of substantial rate enhancements by microwaves concern heterogeneous reactions. In these circumstances, localized heating of a microwave-absorbing solid or catalyst could lead to an increased reaction rate [33]. Chemat and Poux (2001) [67] studied a reaction with classical and MW heating under homogeneous and

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heterogeneous conditions. When the reaction was conducted in the homogeneous phase at similar temperature, identical reaction rates and similar yields and selectivity were observed for both heating modes. In contrast, in the presence of carbon, improved yield and selectivity were obtained under MW irradiation compared with conventional heating. They ascribed this phenomenon to localized superheating ("hot spots")

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on the carbon surface [3].

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As shown in Fig. 4 (b) increasing activated carbon from 5 wt% to 20 wt% increases asphaltene reduction at two levels of Fe catalyst (10 and 20 wt%). In the case of 10 wt% Fe catalyst, increasing activated carbon

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causes asphaltene reduction alters from 32.59 wt% (activated carbon: 5 wt%) to 35.73 wt% (activated carbon: 20 wt%). In the case of 20 wt% Fe catalyst, the increase in activated carbon causes an increase in

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asphaltene reduction from 32.88 wt% (activated carbon: 5 wt%) to 44.87 wt% (activated carbon: 20 wt%). The comparison between the two cases shows that the effect of increased Fe catalyst on asphaltene reduction

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is significant at higher amounts of activated carbon (15 and 20 %). With an increase in activated carbon content, the effect of Fe catalyst on asphaltene reduction has significantly increased. It also can be

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observed that the reduction of asphaltene is sharp at initial addition of activated carbon and thereafter decreases slowly with increasing activated carbon.

Hydrocarbons are transparent to

microwaves due to low dielectric constant, but compounds containing sulfur and nitrogen such as asphaltenes have more dielectric constant. The materials with higher dielectric constant could absorb more microwave radiation, convert it to heat and transfer the heat.

15

Since the chemical reactions involving the breaking of C–C bonds are highly endothermic, requiring considerable energy inputs [45]. So, the reaction temperature is a controlling variable for upgrading vacuum residue with a high percentage of asphaltene [68]. In the presence of more microwave absorbent, a higher amount of heat energy is generated to reach a higher temperature, and this results in more cracking of vacuum residue [5]. It should be noted that in this study, Fe/C combination exhibited electric discharges under microwave irradiation during some experiments. This is according to the other references [40]. In addition, in the literature, activated carbon is used as a catalyst to upgrade heavy oils and plays an important role in effective

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conversion of heavy hydrocarbon compounds into lighter fractions. Fukuyama et al. (2004) [69] used activated carbon as a catalyst for heavy oil upgrading. They reported that activated carbon has the affinity to heavy hydrocarbon compounds and concluded that activated carbon showed the adsorption selectivity to

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asphaltenes in vacuum residue.

Based on the results of Fig. 4(c) with increasing activated carbon from 5 wt% to 20 wt%, a significant

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increase in viscosity reduction is observed. In the case of 10 wt% Fe catalyst, with increasing activated carbon, viscosity reduction has occurred from 18.37 % to 89.81%. This reduction is especially noticeable at

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carbon increase from 5 wt% to 10 wt%. In the case of 20 wt% Fe catalyst, increasing the amount of activated carbon increases viscosity reduction from 55.84 wt% (activated carbon: 5 wt%) to 93.73 wt% (activated

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carbon: 20 wt%). The comparison between two cases of Fe catalyst shows the effect of Fe catalyst is more significant at lower amounts of activated carbon. It can be concluded that the best conditions for viscosity

96.39%.

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reduction are activated carbon: 15 wt% and Fe catalyst: 20 wt%, resulting in a viscosity reduction of

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It can be observed in Fig. 4(d) that increase in activated carbon at both levels of Fe catalyst (10 and 20 wt%) increases API gravity, but the effect of increased Fe catalyst is more significant at more activated carbon contents. In the case of 10 wt% Fe catalyst, with increasing the activated carbon from 5 wt% to 20 wt%, API gravity increase has been continuously enhanced, from 20.15% (at 5 wt% activated carbon) to 39.02% (at 20 wt% activated carbon). In the case of 20 wt% Fe catalyst, API gravity increase has been continuously increased from 11.57% (activated carbon: 5 wt%) to 52.26% (activated carbon: 20 wt%). It can be observed

16

that higher amounts of activated carbon (20 wt%) and iron ( 20 wt%) are the best conditions, leading to an API gravity increase of 52.26%. API gravity increase could be attributed to higher molecular weight hydrocarbons and asphaltene molecules cracking to lower molecular weight hydrocarbons and reduced intermolecular resin bonds. At low amounts of activated carbon, less microwave radiation was absorbed by sensitizer, resulting in a lower extent of heating and cracking occurred within reactor. At higher amounts of activated carbon, higher amount of heat energy was generated to reach a high temperature, and this resulted in more cracking of vacuum residue [4]. Selective heating under microwaves has been exploited in heterogeneous reactions to heat selectively polar

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catalysts, resulting in significantly improving reactions. The temperature of the catalyst can be higher than the temperature of the bulk, which implies that such a process might be more energy-efficient than conventional processes [70]. Under the action of microwave irradiation, the threshold reaction temperature (i.e., the lowest temperature at which the reaction proceeded) was substantially reduced; this was explained

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by ‘‘hot spots’’ formed [33].

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Another significant application of microwave irradiation involves the modification of the reaction selectivity [70]. During microwave irradiation, the catalyst active sites are heated to the required temperatures. In

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general, because the bond-breaking step occurs on the catalyst surface, the reaction is enhanced. Since the temperature of the bulk reactants is lower, side reactions may occur to a lesser extent. So, the selectivity of

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the reaction can be significantly improved. As a consequence, if two reactions are possible for the same system, one reaction could occur to a greater extent than the other in microwave fields [33]. It is considered

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that superheating and hot spots by microwave irradiation are responsible for changes in reaction selectivity [33,70]. Some examples of increased reaction selectivity by microwave irradiation can be found in the

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literature [33,70,71]. De La Hoz et al. (2004) [70] reviewed the reaction selectivity in organic reactions under microwave irradiation. In summary, it can be concluded that activated carbon has a significant effect on all investigated parameters. It causes an increase in temperature, asphaltene reduction, viscosity reduction, and API gravity increase. So, the best conditions for all response parameters are 20 wt% activated carbon and 20 wt% Iron.

17

Fig. 4. The effect of carbon sensitizer on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase (Power Level: 100 %, Process Time: 30 min, NaBH4: 3 wt%)

3.4. The effect of sodium borohydride as a hydrogen source Fig. 5 shows the effect of sodium borohydride as a hydrogen source on parameters of temperature, asphaltene reduction, viscosity reduction, and API gravity increase, respectively. The other parameters, including power level: 100%, process time: 30 min, activated carbon: 10 wt% and Fe catalyst: 10 wt% were considered constant. The experiments were carried out at four levels of sodium borohydride containing 0, 3,

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6, and 10 wt%. As shown in Fig. 5 (a), with increasing sodium borohydride as an additive, the temperature has decreased. The reason for this can be found out in the low dielectric loss of sodium borohydride. It can be observed in

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Fig. 5 (b) that increases in sodium borohydride contents decrease asphaltene reduction. Asphaltene reduction varies from 40.19 wt% in the absence of sodium borohydride to 14.44 wt% at sodium borohydride of 10

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wt%. According to Fig. 5 (c), an increase of sodium borohydride decreases viscosity reduction. The viscosity reduction alters from 85.72% in the absence of sodium borohydride to 59.02% in the sodium borohydride

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content of 10 wt%. As shown in Fig. 5 (d), the increase of sodium borohydride as a hydrogen source has a positive effect on the API gravity increase. It changes from 19.84% in the absence of sodium borohydride to

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23.43% by adding of 3 wt% sodium borohydride. Afterward, API gravity decreases to 20.71%. In summary, the increase in sodium borohydride leads to the decreasing of temperature and asphaltene and

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increasing viscosity. On the other hand, there is an optimal value of 3% sodium borohydride for increasing

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the API gravity. There are two different aspects in the increasing of NaBH4 content, firstly on its dielectric properties and secondly on producing hydrogen. According to Table 4 the dielectric constant of sodium borohydride is 2.5, which is much less than of the value for the activated carbon. So it is not the absorber of microwaves, leading to a decrease in temperature and has a negative effect on asphaltene and viscosity reduction. On the other hand, cracking reactions are regulated by hydrogen production and leads to a 23.43% increase in API gravity.

18

Fig. 5. The effect of hydrogen source on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase (Power Level: 100 %, Process Time: 30 min, Carbon: 10 wt%, Fe Catalyst: 10 wt%)

3.5. The effect of Fe catalyst Fig. 6 shows the effect of iron catalyst at two levels of activated carbon (10 and 20 wt%) on parameters of temperature, asphaltene reduction, viscosity reduction, and API gravity increase, respectively. The other parameters, including power level: 100%, process time: 30 min, and sodium borohydride: 3 wt% were considered constant. The experiments were carried out at four levels of Fe catalyst, including 0, 10, 15, and

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20 wt%. According to Fig. 6 (a) increase in Fe catalyst leads to an increase in temperature. The highest temperature is at an iron catalyst level of 20 wt% and equals 330 0C and 345 0C for two levels of activated carbon of 10

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and 20 wt%, respectively. The comparison between the effects of iron catalyst at two different levels of activated carbon in Fig. 6 (a) shows that the effect of increasing the iron catalyst on increasing temperature is

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more significant in lower amounts of activated carbon (10%). In other words, the increasing temperature is equal to 60 °C and 31 °C in the active carbon addition of 10% and 20%, respectively.

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It should be noted that the measured temperature is related to the surface of the components. The local temperature of activated carbon as a sensitizer and so the vacuum residue in its vicinity is higher than this

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amount. Activated carbon particularly absorbs microwaves, so the local temperature will increase. The iron catalyst particles reflect microwave, therefore increase absorption of microwave energy by vacuum residue,

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in which catalyst particles are dispersed.

Fig. 6 (b) shows the effect of iron catalyst on asphaltene reduction. It can be observed that the increase in Fe

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catalyst has a desirable impact on asphaltene reduction. At both levels of activated carbon (10 wt% and 20 wt%), the maximum asphaltene reduction is observed at the maximum level of iron catalyst (20 wt%) and is equal to 35.99% and 44.87%, respectively. Fig. 6 (c) shows the effect of iron catalyst increase on viscosity reduction. It can be observed that in the case of 10 wt% activated carbon, with an increase in catalyst amount from 0 to 15 wt%, the viscosity reduction has been continuously increased; whilst, from 15 to 20 wt% it has decreased. In the case of 20 wt% activated 19

carbon, with increasing the catalyst from 0 to 15 wt%, viscosity reduction has been continuously increased; afterward, with increasing the catalyst content from 15 to 20 wt%, an insignificant decrease of about 1% in viscosity is observed. The comparison between two cases of activated carbon shows that the increase in the iron catalyst in the higher amount of activated carbon (20 wt%) has a more consistent trend, and has continuously reduced the viscosity. The presence of Fe metal particles increases the cracking of carbon-sulfur bonds in asphaltene molecules. As a result, the amount of asphaltene decreases, and amounts of saturates and aromatic components increase. So, the viscosity of vacuum residue decreases.

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Fig. 6 (d) shows that the increase in iron at both amounts of activated carbon (10 wt% and 20 wt%) increases API gravity, but the effect of iron in a higher amount of activated carbon is more significant. In the case of 10 wt% activated carbon, the highest API gravity increase is related to the iron catalyst amount of 20 wt%

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and equals 36.04%. In the case of 20 wt% activated carbon, API gravity increase is from 23.2% in the absence of Fe catalyst to 52.26% in the case of 20 wt% catalyst.

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It can be concluded from the results of this section that the best conditions for all parameters are activated

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carbon 20 wt% and iron 20 wt%.

Fig. 6. The effect of Fe catalyst on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase

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(Power Level: 100 %, Process Time: 30 min, NaBH4: 3 wt%)

3.6. The effect of desulfurization agent

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Desulphurization additives can be consist of CaCO3, CaO, MgO, Mgo-CaO, NaOH, KOH, NaHCO3 [12]. With considering Sulphur compounds in vacuum residue as R-S-H, in which R can be an aromatic or

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aliphatic group, the desulphurization reactions by using NaOH can be defined as follows [72]: R-S-R + NaOH

R-S-Na+ + ROH

R-S-Na+ + ROH

RONa+ + RSH

ROH + H2 R-S-H + H2 R-O-Na+ + H2O

RH + H2O RH + H2S ROH + NaOH 20

Fig. 7 shows the effect of sodium hydroxide as desulphurization agent on parameters of temperature, asphaltene reduction, viscosity reduction, and API gravity increase, respectively. The other parameters including power level: 100%, process time: 30 min, activated carbon: 20 wt%, catalyst: 20 wt%, sodium borohydride: 3 wt% were considered constant. The experiments were carried out at four levels of sodium hydroxide containing 0, 2, 5, and 10 wt%. As shown in Fig. 7 (a), with increasing sodium hydroxide, the temperature is decreased. The temperature varies from 345 0C in the absence of sodium hydroxide to 323 0C at sodium hydroxide of 10 wt%. The reason for this can be found out in the low dielectric loss of sodium hydroxide. According to Fig. 7 (b), the

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increase of sodium hydroxide decreases asphaltene reduction. Asphaltene reduction alters from 42.64 wt% in the absence of sodium hydroxide to 22.19 wt% at sodium hydroxide of 10 wt%. The sulfur reduction has not been directly measured. However, studies show that increasing asphaltene contents increases the amount of

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sulfur [73]. So, in this research, asphaltene reduction has been considered as sulfur reduction.

According to Fig. 7 (c), the increase of sodium hydroxide decreases viscosity reduction. The viscosity

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reduction varies from 93.37% at the absence of sodium hydroxide to 84.87% in the sodium hydroxide content of 10 wt%.

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As shown in Fig. 7 (d), the increase of sodium hydroxide as a desulphurization source decreases API gravity increase. It changes from 52.26 % in the absence of sodium hydroxide to 21.6% in the case of 10 wt% of

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sodium hydroxide.

According to the results of this part, this can be concluded that sodium hydroxide is harmful in improving the

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amount of asphaltene, viscosity, and API gravity of the upgraded residue and therefore has no positive effect on the response parameters. In order to explain this, it can be explained that an increase in NaOH, has two

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different aspects, one in terms of its dielectric constant, and the other in terms of desulphurization of vacuum residue. According to Table 4, the dielectric constant of NaOH is approximately 3, which is a small number compared to the dielectric constant of activated carbon. As a result, less microwave radiation was absorbed by the materials in the reactor, and this resulted in lower heating and a decrease in temperature; therefore lower cracking has been occurred within the reactor, leading to the adverse effects on asphaltene and viscosity reduction and API gravity increase [4].

21

Sharivker and Honeycutt (2004) investigated the upgrading of fuel oil using microwaves. They applied activated carbon as a sensitizer, iron as a catalyst and NaOH, KOH, CaCO3 as desulphurization agents. Although they used the desulphurization agents in all experiments, their effect in upgrading fuel oil had not been individually determined. Fig. 7. The effect of desulfurization agent on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase (Power Level: 100 %, Process Time: 30 min, Carbon: 20 wt%, NaBH4: 3 wt%, Fe: 20 wt%)

3.7. The effect of process time

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Fig. 8 shows the effect of process time on parameters of temperature, asphaltene reduction, viscosity reduction, and API gravity increase, respectively. The process time effect was investigated at four levels of 15, 30, 45, and 60 min. The other parameters, including the power level: 100%, activated carbon: 20 wt%,

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sodium borohydride: 3 wt% and Fe catalyst: 20 wt% were considered constant.

It can be observed that with increasing process time from 15 min to 60 min, the temperature changes from

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294 to 396 0C, asphaltene reduction increases from 22.62 to 94.22 wt%, viscosity reduction increases from 68.52 to 99. 53%, and API gravity increase is enhanced from 27.7% to 88.56%. Fig. 8 (a) indicates that the

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heating rate is decreased from 14.9 at the first 15 min to 2.4 in the last 15 min. Also, according to Fig. 8 (b), the viscosity reduction rate is decreased from 4.57 at the first 15 min to 0.018 in the last 15 min. The heating

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rate is calculated by dividing the temperature changes achieved within 15 min reaction time. At higher temperatures, large organic molecules of asphaltenes in vacuum residue crack into smaller

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molecules. It can be concluded from this section that increasing the process time has desirable effects on all the studied parameters. This effect is particularly significant in reducing asphaltene and increasing API

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gravity. It should be noted that for process time of 60 min, there was no need for the addition of chloroform to the experiment mixture before centrifugation since the mixture dilution was appropriate. The final product, in this case, had fluidity and was similar to the conventional crude oil entering an oil refinery.

Fig. 8. The effect of process time on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase (Power Level: 100 %, Carbon: 20 wt%, NaBH4: 3 wt%, Fe Catalyst: 20 wt%)

22

3.8. Coke formation In the conventional methods of upgrading vacuum residue, the residence time of heavy molecules on the surface of catalysts is prolonged, leading to increasing the possibility of polymerization reactions [73]. The studies show that during cracking of vacuum residue using microwave irradiation, due to selective heating and the direct heating of catalyst, the reactions occur faster than conventionally heated reactions at the same temperature, leading to the reduction in the coke formation. It should be noted that during microwave irradiation, reactivation and regeneration of catalysts can be

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performed by catalytic superheating or by selective heating of active sites. In addition, carbon is used as a catalyst to upgrade heavy oils [74]. Fukuyama et al. (2004) [69] used active carbon for heavy oil upgrading and observed that it has the affinity to heavy hydrocarbons and adsorption selectivity to asphaltenes,

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resulting in restricting the coke formation during the hydrocracking reaction of vacuum residue.

In this research, the coke and the other additions were separated from the chloroform phase by centrifuge.

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The solid phase consisted of the formed coke and the other additions. The amount of coke and the additions including activated carbon, catalyst, and sodium borohydride were determined as toluene insoluble of

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separated solids. The solid product weight (coke) was determined by subtracting the additions weight from the total weight of toluene insoluble solids [49,59,61]. The maximum amount of coke was obtained in this

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study equal to 12.5 wt% of vacuum residue.

It should be noted that in the other references, the amount of formed coke have been reported 21.9%

ur

(without catalyst) and 3-19% (with catalyst) [65] , 10-15 % [75], 15% [69], 1-15% [66]، 19.3 % (without

Jo

catalyst) and 5-11% (with catalyst) [64], 12 % [76], 18.9% (without catalyst) and 8-14% (with catalyst) [1]. From an economic point of view, it is worth noting that microwave irradiation in comparison with conventional technologies is a fast and efficient heating process. Microwave equipment to upgrade oil shale, bitumen, waste tires and plastics to valuable components have been manufactured by some companies such as Global Resource Corp [77] and Focus Technology Co. Ltd, [78]. In addition, microwave technology has been used to stimulate heavy oil in oil recovery by some companies such as EMR Microwave Technology Corp. [79], Electromagnetic Oil Recovery Inc. [80], and Dongying Landdrill Oilfield Supply Co. Ltd. [81]. 23

3.9. Repeatability of the experiments Some experiments were repeated to check the repeatability. It may be expressed in the term of relative standard deviation (RSD). RSD were determined 2%, 2.48%, 1.96%, and 1.47% for temperature, asphaltene, viscosity, and API gravity measurement, respectively. In summary, in this paper, the effect of various parameters including microwave power level, process time, type and amount of catalyst, microwave sensitizer, solid hydrogen source, desulphurization agent on different characteristics of vacuum residue including temperature, asphaltene amount, API gravity, viscosity, were investigated. The parameters examined were as follows:

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1) The effect of various types of catalysts including, iron, nickel, copper, MoS2, iron oxide. 2) The effect of the microwave power level at three different levels (50, 70, 100%).

3) The effect of activated carbon as a microwave sensitizer at four levels (5, 10, 15, and 20 wt%) and in two

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cases of iron catalyst (10 and 20 wt%).

4) The effect of iron catalyst (as the most appropriate catalyst) at four levels (0, 10, 15 and 20 wt%) and in

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two cases of activated carbon (10 and 20 wt%).

5) The effect of sodium borohydride as a hydrogen source at four levels (0, 3, 6, and 10 wt%).

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6) The effect of desulfurization agent at four levels (0, 2, 5, and 10 wt%). 7) The effect of process time at four levels (15, 30, 45, and 60 min).

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The most significant results of this research are as follows: 1) Among the used catalysts (including iron, nickel, copper, MoS2 and iron oxide), iron is the most

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appropriate catalyst to upgrade vacuum residue using microwaves and increases light components. 2) The increased power level has a very positive effect on the studied parameters.

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3) The increase in activated carbon has a very desirable effect on all the studied parameters, increasing the temperature, decreasing asphaltene, decreasing viscosity, and increasing API gravity.

4) The increase of iron catalyst has a desirable effect on the studied parameters.

5) The optimum amount of sodium borohydride as a hydrogen source is 3 wt%. 6) The desulfurization agent has no positive effect on the investigated parameters. 7) With increasing process time, the properties of vacuum residue are improved.

24

At the best conditions, asphaltene reduction was 94.22%, viscosity reduced by 99.53% (from 16391 cSt to 77.74 cSt (at 60 0C)) and API gravity increased by 88.56% (from 8.79 to 16.57). The product specifications in this situation were such that the product was wholly fluid and similar to the crude oil entering an oil refinery.

4. Conclusions Upgrading vacuum residue of an oil refinery was performed using microwave irradiation in this research. The applied vacuum residue is from Tehran oil refinery, Iran. Effects of type and amount of catalyst,

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microwave power level, activated carbon, solid hydrogen source, process time, desulphurization agent were evaluated. The objective parameters included temperature, asphaltene content, viscosity, and API gravity. Five catalysts including iron, nickel, molybdenum disulfide, iron oxide, and copper were investigated.

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The results showed that iron is the best catalyst to upgrade vacuum residue. In addition, increased catalyst amount, activated carbon, process time, microwave power level, had significant and positive effects on

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objective parameters and resulted in decreasing asphaltene, viscosity, and increasing temperature and API gravity.

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Based on the results, the best conditions for vacuum residue upgrading were power level: 100%, iron powder catalyst: 20 wt%, activated carbon as microwave sensitizer: 20 wt%, sodium borohydride as hydrogen

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source: 3 wt% and 1 hr process time. In these conditions, asphaltene reduction was 94.22%, viscosity

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reduced by 99.53% and API gravity increased by 88.56%.

Conflict of Interest

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I would like to express my thanks to Iran National Science Foundation (INSF) for financial support of this research.

Acknowledgments

25

I would like to express my thanks to Iran National Science Foundation (INSF) for financial support of this research and I also wish gratefully to acknowledge Tehran oil refinery for providing vacuum residue.

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ur

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Jo

81)M.A. Shi, Oil Electromagnetic Heater, Dongying Landdrill Oilfield Supply Co., Ltd. , 2008.

30

Jo ur

na l

Pr

e-

pr

oo

f

Figure captions

31

f oo pr ePr na l Jo ur

Fig. 1. The diagram of experimental setup

32

f 216

200

165

150

130

100

31.30

30 25 20 15

Pr

10

50

34.22

32.74

35.12

34.56

pr

250

35

5 0

0 MoS2

Cu

Catalyst Type

(a)

Fe2O3

Ni

Fe

na l

Fe

MoS2 Cu Fe2O3 Catalyst Type

(b)

Jo ur

Temperature (C)

300

e-

291

oo

40

314

Asphaltene Reduction (%)

350

33

Ni

100

23.43

40

API Increase (%)

20 60

15.74

15

4.25

13.32

10

0

MoS2

Cu

-22.40

-21.23

Fe2O3

7.81

Ni

e-

Fe -20 -40

5

Pr

-36.68

-60

12.23

pr

20

oo

f

76.43

0

Fe

Catalyst Type

Ni

(d)

na l

(c)

MoS2 Cu Fe2O3 Catalyst Type

Fig. 2. The effect of catalyst type on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API Increase (Power Level: 100 %, Time: 30 min, Carbon Sensitizer: 10 wt%, NaBH4: 3 wt%, Catalyst: 10 wt%)

Jo ur

Viscosity Reduction (%)

80

25

34

35

200 150

25 20 15 10

100

5

50

0

0 70 Power Level (%)

100

50

70

Power Level (%)

(a)

(b)

na l

50

f

oo

218

Pr

250

30

Jo ur

Temperature (C)

300

31.30

29.75

28.05

pr

314

e-

312

Asphaltene Reduction (%)

350

35

100

30 76.43 23.43

25

70

50 40 30

15

5

10 0

e-

10 16.35

20.01

19.07

Pr

20

20

pr

60

oo

72.35

API Increase (%)

Viscosity Reduction (%)

80

0

50

70

100

50

70

100

Power Level (%)

Power Level (%)

(d)

na l

(c)

Jo ur

Fig. 3. The effect of power level on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API Increase (Time: 30 min, Carbon Sensitizer: 10 wt %, NaBH4: 3 wt%, Fe Catalyst: 10 wt%)

36

f

90

Fe = 10 wt %

Fe = 10 wt %

Fe = 20 wt %

Fe = 20 wt % 50

400

250

224

200 150

40 35

32.88 32.59

35.99

25

15 100

e-

10 50

35.73

32.53

31.30

30

20

f

300

oo

325

314

41.78

pr

324

45

336

5 0

10

15

20

Carbon Sensitizer (wt %)

(a)

5

10

15

Carbon Sensitizer (wt %)

(b)

na l

5

Pr

0

Jo ur

Temperature (C)

350

44.87

345

340

Asphaltene Reduction (%)

330

37

20

Fe = 10 wt %

Fe = 10 wt %

Fe = 20 wt %

120

Fe = 20 wt % 60

58.60 60

55.84

oo

76.43

49.30

50

39.02

40

36.04

31.17

30

23.43

20.15 40

pr

89.81 83.65

80

f

93.37

API Increase (%)

Viscosity Reduction (%)

52.26

96.39

100

e-

20

11.57

18.37 20

0

Pr

10

0

5

10

15

20

Carbon Sensitizer (wt %)

10

15

20

Carbon Sensitizer (wt %)

(d)

na l

(c)

5

Jo ur

Fig. 4. The effect of carbon sensitizer on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase (Power Level: 100 %, Process Time: 30 min, NaBH4: 3 wt%)

38

284

300 250 200 150

40.19

40 35

32.10

31.30

30

pr

311

25 20

14.44

15 10

50

5

Pr

100

e-

314

oo

45

Asphaltene Reduction (%)

330

0

0 3 6 NaBH4 (wt %)

10

(a)

0

3 6 NaBH4 (wt %)

(b)

na l

0

Jo ur

Temperature (C)

350

f

50

400

39

10

85.72

60

59.02

50 40

20 15

5

Pr

10

20.71

e-

10

30 20

oo

62.00

20.45

19.84

pr

70

23.43

25

76.43

80

API Increase (%)

Viscosity Reduction (%)

90

f

30

100

0

0 0

3 6 NaBH4 (wt %)

0

10

3

6

10

NaBH4 (wt %)

Jo ur

na l

(c) (d) Fig. 5. The effect of hydrogen source on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API Increase (Power Level: 100 %, Process Time: 30 min, Carbon: 10 wt%, Fe Catalyst: 10 wt%)

40

Carbon = 10 wt %

Carbon = 20 wt %

Carbon = 10 wt % 50

250 200 150

40 35

32.53 31.06

30 25 20 15

100

34.87

35.99

31.30

e-

10

35.73

oo

270

44.87

43.14

45

pr

317

314

345

Asphaltene Reduction (%)

314

330

50

5 0

0

10

15

20

Fe Catalyst (wt %)

0

10

15

Fe Catalyst (wt %)

(b)

na l

(a)

Pr

0

Jo ur

Temperature (C)

340

336

350

f

400

300

Carbon = 20 wt %

41

20

84.18 76.43

80 70

52.26

79.99

50

API Increase (%)

66.61 58.60

60 50

43.91

30

36.04

23.20 23.43

40 20

30

39.02

40

22.28

e-

Viscosity Reduction (%)

90

60

93.37

94.64

Carbon = 20 wt %

f

89.81

Carbon = 10 wt %

oo

100

Carbon = 20 wt %

pr

Carbon = 10 wt %

20

10

0

Pr

10

8.24

0

0

10

15

20

Fe Catalyst (wt %)

10

15

20

Fe Catalyst (wt %)

(d)

na l

(c)

0

Jo ur

Fig. 6. The effect of Fe catalyst on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API gravity Increase (Power Level: 100 %, Process Time: 30 min, NaBH4: 3 wt%)

42

350 345 345

45 342

42.64 38.22

40

340

320 315

f oo

323

325

30

22.84

25 20

22.19

pr

330

Asphaltene Reduction (%)

335

35

15

e-

10 5

310

0

305 2

5

10

NaOH (wt %)

2

5

NaOH (wt %)

(b)

na l

(a)

0

Pr

0

Jo ur

Temperature (C)

340

43

10

f

60 52.26 50 90.29

90 88 86

36.99

40 30

e-

84.87

45.7

pr

92

API Increase (%)

Viscosity Reduction (%)

94

94.48 93.37

oo

96

21.6

20

84 82 80 0

2

5

10

(c)

0

0

2

5

10

NaOH (wt %)

na l

NaOH (wt %)

Pr

10

(d)

Jo ur

Fig. 7. The effect of desulfurization agent on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API Increase (Power Level: 100 %, Process Time: 30 min, Carbon: 20 wt%, NaBH4: 3 wt%, Fe: 20 wt%)

44

100 396

200 150 100

40 30

10

0 30

45

60

Process Time (min)

(a)

22.62

0

15

30 45 Process Time (min)

(b)

na l

15

44.87

50

20

50

oo

60

pr

250

70.10

70

Pr

300

294

80

e-

350

90

360

Asphaltene Reduction (%)

345

Jo ur

Temperature (C)

400

94.22

f

450

45

60

100

60

70

60.23

60

52.26

50 40 30

27.70

20

20 0

Pr

10

e-

40

oo

68.52

80

pr

80

93.37

88.56

90

99.53

API Increase (%)

Viscosity Reduction (%)

100

99.26

f

120

0

15

30

45

60

Process Time (min)

15

30

45

Process Time (min)

(d)

na l

(c)

Jo ur

Fig. 8. The effect of process time on upgrading vacuum residue of oil refinery (a) Temperature (b) Asphaltene Reduction (c) Viscosity Reduction (d) API Increase (Power Level: 100 %, Carbon: 20 wt%, NaBH4: 3 wt%, Fe Catalyst: 20 wt%)

46

60

Table

Table 1 - The basic characteristics of vacuum residue

API gravity

8.79

0

16391 cSt

Viscosity (at 80 0C)

1910 cSt

Asphaltenes

13.3 wt%

Jo

ur

na

lP

re

-p

ro of

Viscosity (at 60 C)

47

Table 2 power levels of microwave system used in experiments Irradiation time in 30 second

10%

3

20%

6

30%

9

40%

12

50%

15

60%

18

70%

21

80%

24

90%

27

100%

30

Jo

ur

na

lP

re

-p

ro of

Power level (%)

48

Table 3 Experimental design for upgrading vacuum residue of oil refinery

Time (min)

Sensitizer (wt%)

Sodiumborohydride (wt%)

Desulphurization agent (NaOH, wt%) -

100

30

10

3

50 70 100

30

10

3

100

100

Parameter Fe

MoS2

Cu

Fe2O3

Ni

10 -

10 -

10 -

10 -

10

catalyst type

10

-

-

-

-

Power level

10

-

-

-

-

Sensitizer amount (1)

20

-

10

-

-

30

30

30

5 10 15 20 5 10 15 20 10

3 3 -

0 3 6 10

0 30

20

3

10

30

20

100

15 30 45 60

20

3

0 2 5 10 -

3

Jo

ur

100

3

na

30

49

-

-

-

Sensitizer amount (2)

-

-

-

Sodiumborohydride

-

-

-

Catalyst amount (1)

-

-

-

-

Catalyst amount (2)

20

-

-

-

-

Desulphurization agent

20

-

-

-

-

Process Time

15

lP

100

10

-

-

re

100

ro of

100

Catalyst (wt%)

-p

Power Level (%)

20 0 10 15 20

Table 4 The specifications of the materials used in this research

< 100

Iron

< 44

Nickel

< 10

Copper

< 10

Molybdenum disulfide

< 10

Iron(III) oxide

<5

Sodium hydroxide Sodium borohydride

Pellet Powder

Asphalt

-

Crude oil

-

Loss factor (imaginary part)

14.4 ][36 1.4 ][39

9.57 ][36

10 – 30 ][41 1.4-1.8 ][39 3 ][42 [43] 2.7 ][39 2-3 , , 45[39 ]46

penetration depth (µm)

Resistivity (Ω m)

Thermal Conductivity (W/mK)

3.2 ][40 2.7 ][40 1.3 ][40

3.5 E -5 ][37 10 E -8 ][38 6.99 E -8 [38] 1.7 E -8 ][38

119-165 ][38 80.4 ][38 90.9 ][38 401 ][38

0-30 ][41

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

0.15 ][44

1.8-2.1 [38] 7.87 ][38 8.91 ][38 8.96 ][38 5.06 ][38 5.25 ][38 2.13 ][38 1.07 ][38 0.87 ][44

0.005-0.006 [45]

N.A.

0.15 ][44

0.73 ][44

N.A.*

][43

N.A.

Jo

ur

na

lP

re

* Not Available

50

Density (g/cm3)

Cp (J/mol K)

ro of

Activated Carbon

Dielectric constant (real part)

-p

Material

Size (µm)

N.A. 25.10 [38] 26.07 [38] 24.44 [38] 103.9 [38] 59.5 [38] N.A. 1675 [44] 2250 [44]