Advances in Upgrading Process of Petroleum Residue : A Review

Due to the depletion of light crude oil reserves, heavy crude oil and residues are the alternatives to meet ‎the ‎increasing global demand for light oil products. Heavy crude oil and residues are characterized by the presence of heavy ‎hydrocarbon ‎compounds which contain high levels of impurities such as metals, nitrogen, and sulfur-containing compounds. ‎Methods of ‎upgrading are required to increase refining efficiencies and to obtain high-quality products.‎ Upgrading processes can be categorized into three categories; ‎carbon rejection processes, hydrogen addition processes, and a combination of the two. The catalyst can be used with any of these processes for better improvement. Many types of research have been carried out to develop a high-performance process which is stable, high commercial products yield, and low solids formation. In this work, recent advances on petroleum residues upgrading with catalyst, solvents, and thermal cracking were reviewed. Advantages and disadvantages of each process were discussed along with conditions and main features. Nanoparticles catalysts and supercritical fluids based-processes are the trends of upgrading due to the excellent performance of these processes.


I. INTRODUCTION
The global energy demand is increasing due to the increasing of population and the technological development.According to the international energy outlook 2016 (IEO2016), the world energy consumption will increase by 14.6% during the period 2012 -2020 and by 48% to 2040.Over half of this energy will be consumed by the industrial sector [1].Fossil fuels continue to have the largest share and account for more than three-fourths of world energy supply in 2040.The world consumption of petroleum and other liquid fuels will grow up from 90 MBPD to 100 MBPD in 2020 and 121 MBPD in 2040.This increase is driven by economic growth and populations increase of non-OECD countries of Asia, Middle East, and Africa.To face the growing demand for liquid fuels, liquids production is predicted to grow by 31 MBPD from 2012 to 2040.This forecast accounted for different liquid fuels such as conventional oil, shale oil, extra-heavy crude oil, field condensate, and bitumen.Other than conventional oil all others are heavies.Production of tight oil increases from 2.5 MBPD in 2012 to 5.95 MBPD in 2020.US supplies contributes about 90% of this growth and the rest comes from Canada, Russia, and Argentina [1].The world heavy oil reserve was estimated to be 6 trillion barrels and in 40 years' heavy oil will be the major feed stock to refineries [2].Moreover, there are residues produced in oil refineries from atmospheric and vacuum distillation units.Although the temperature of the CDU is set below 350 o C to avoid thermal cracking of oil compounds; some authors defined petroleum residue as the fraction remains at the bottom of the vacuum tower VDU [3].In upgrading processes, heavy oils and petroleum residues of low quality and prices can be transferred into high quality lighter products.
Due to the increase of prices and depletion of conventional oil sources, many studies have carried out to develop upgrading processes for petroleum residues.The main objectives of these processes are to improve the properties of the residue; especially H/C ratio and impurities removal.This can be done by adding hydrogen or removing carbon [4].This process can be catalytic or non-catalytic.It is reported that 725 MMT of petroleum residues were upgraded to light products using various processes [2].

II. CHARACTERISTICS OF PETROLEUM RESIDUE
The characteristics of petroleum can be useful to understand the behavior of oil in recovery/production, transportation, and upgrading processes [5].It is also useful to determine the price of oil.Oil is classified based on its properties.The most important properties are the API gravity which is used to define oil as light or heavy.Heavy oil is very difficult to be recovered and processed in comparison to conventional oil.Most commonly heavy oil is defined to have API gravity less than 20 o [6].API gravity of petroleum residue depends on distillation bottom temperature and the API gravity of vacuum residue varies between 2 o and 8 o [6].
Residue or heavy oil usually contains many impurities such as sulfur, nitrogen, oxygen, and heavy metals.These impurities affect the oil characteristics and the quality of products.However, the level of these impurities depends on the origin of the crude.Residue characteristics based on its origin are shown in table I.   Residual compounds can be classified according to their solubility and adsorption characteristics to coke and solids, pre-asphaltenes, asphaltenes, resins, aromatics (C41-53), and saturates (C38-50) [3].Residues are mostly common named by their predominant compositions such as paraffinic, naphthenic, and aromatic, etc. Petroleum residue has high contents of resins and asphaltenes.These materials can aggregate together and form a colloidal solution.SARA analysis is used to determine the fraction of each compound in the residue.Different solvents and adsorbents are used and the separation process depends on the polarity of each fraction.
Generally, heavy oil has low API gravity, high molecular weight, low H/C ratio, high viscosity, heavy metals, and black color.Residue and heavy oil characteristics have been studied extensively in the literature [7]- [11].

III. UPGRADING PROCESSES OF PETROLEUM RESIDUE
Many processes have been developed for the products improving quality, distribution, and impurities content.These processes can be categorized as; carbon rejection process; hydrogen addition process; and combination of these two processes.Thermal cracking (coking, and visbreaking) are examples of carbon rejection process.Hydrogen addition processes based on addition of materials contains hydrogen such as hydrogen donor from water, methane, and methanol.They are classified according to the type of the reactor used; fixed-bed, moving-bed, ebullatedbed and slurry-bed processes [12].Example for combination processes are deasphalting, gasification, delayed coking, residual fluid catalytic cracking (RFCC), ebullating-bed reactor, slurry phase reactor and fixed-bed hydrotreating.

A. Catalytic Upgrading Processes
Over the past decades many catalytic upgrading processes have been developed for upgrading of heavy residues with and without the addition of hydrogen.Catalytic hydrocracking process is with the addition of hydrogen and catalytic cracking processes in the absence of hydrogen.Many catalysts have been tested and the products were analyzed.
Hydroprocessing of vacuum residue (VR) was investigated with unsupported nickel-tungsten sulfide (NiWS(x)) particles [13].x values were 0, 0.005, 0.01, and 0.02.The results indicate that the activity of NiWS(x) catalyst depends on the degree of sulfidation with NiWS(0.02)showed the highest activity and the liquid yield was 87.0 wt.%, sulfur removal was 86.5%, and the coke formation was 4.0 wt.%.Moreover, the catalyst performance was better than commercial WS2 and MoS2 catalysts.The catalyst recovery and reusability need to be addressed to further decide on the catalyst feasibility for VR upgrading.
Heavy crude oil upgrading was also investigated at reservoir temperature and pressure (523.15K and 5.0 MPa).Three catalysts were studied (iron carbonyls, iron oxide, and metallic iron).From compositional analysis it was found that resin fraction destruction is the main effect of this upgrading process while asphaltene content remained unchanged.For upgrading using Fe3O4 catalyst, the process efficiency depends strongly on the particles size in the range of 20-60 nm [14].The effect of reactions conditions on the products distribution is very important related to the yield.Not only the catalyst features need to be varied but the reaction conditions need to be optimized to obtain high yield of high quality products.In this regard, Kim et al. investigated the slurry phase hydrocracking of VR with dispersed MoS2 catalyst at different temperatures and pressures [15].Their work showed that within 4 hours at 673 K and 9.5 MPa; the gas, liquid, and coke yields were 12, 77, and 11%, respectively.V, Ni, and Fe were lowered by 90% while S was lowered below 60% and N was lowered below 50%.Increasing the pressure over 15 MPa at 673 K will increase the light oil products to 90% while the coke was less than 1%.
Nano-sized catalysts have been studied.They have higher hydrogenation ability in comparison to the bulk catalysts.This is due to the enhanced mass transfer conditions and the good dispersion of heavies.Nano-particles tungsten carbide of 2.8 nm was investigated for hydrocracking of vacuum residue with API gravity of 2.3 o and C/H ratio of 7.7 [16].The reaction was carried out at 673 K for 4 hours and initial hydrogen pressure of 70 bar.Catalyst concentration was 1000 ppm and stirring speed of 1000 rpm.The results indicated that using nano-sized catalyst enhances the catalyst activity, inhibits coke formation, and increases liquid products yield.The comparison of non-catalytic, bulk catalysts and nano-sized catalysts are shown in table IV.Catalyst reusability and regeneration need to be investigated since in the actual process the VR residue may contain metals.Catalytic cracking of Athabasca vacuum residue (AVR) with high viscosity; > 200,000 cP, was studied [17].Two dispersed catalysts were used namely alumina nanoparticles (Al2O3 NPs) and a waste product from drilling industry called drilling cuttings (DC).The upgrading was held in autoclave reactor for 60 minutes at temperatures of 400 o C and 420 o C. The results were compared to a control sample without catalyst.Liquid yield of 90 wt% is achievable.NPs based cracking is characterized by coke formation and high gas fraction yield.On the other hand, cracking based on 10 wt% DC showed less coke formation, and lower energy consumption.This good performance is attributed to the high acidity of alumina and the ability of nanoparticles to maintain good mass transfer.DC is a cheap catalyst and using it has an economic and environmental advantages.The catalyst regeneration and reusability is not investigated to have complete evaluation of the process.
Hydrocracking of vacuum residue (API gravity of 2.3) using nanosheet-structured WS2 catalyst in an autoclave batch reactor was studied [18].At reaction conditions of 400 o C and initial H2 pressure of 70 bar, the nanosheet catalyst of single layer showed higher activity than the bulk catalyst such as WS2 and MoS2.It showed 500% increase in the API gravity.The good performance of the nano sheet catalyst is attributed to its largest specific surface area.
Upgrading process based on slurry-phase reactors has been developed.Go et al. used slurry-phase hydrocracking with MoS2 dispersed catalysts to investigate the upgrading of residue [19].The vacuum residue has API gravity of 5.84 o and kinematic viscosity of 1030 cSt.The reaction was performed at temperature from 385 to 440 °C, initial hydrogen pressure of 80 bar, and concentrations of the dispersed molybdenum from 100 to 2000 ppm.The reaction time was 4 hours.The vacuum residue conversion and product yield were increased with an increase in operating reaction Increasing catalyst concentration can reduce coke formation and improve the quality of products.Product with API of 35 o and sulfur removal up to 80 wt% was obtained.However, increasing the catalyst concentration for VR conversion over 80 wt% has no effect.
Coke is known to be formed as a result of colloidal aggregation of asphaltenes and resins during the upgrading processes of heavy oils.The effect of using artificial materials on the coke formation and products yield distribution was studied [20].Mo-based and Fe-based catalysts were used in slurry-phase hydrocracking reactor with reaction temperature of 430°C and reaction time of 2 hours.Among five different investigated dispersant materials, the PIBSI shows the highest dispersion of asphaltene in heavy feedstock and therefore lower coke formation.This performance is due to the ability of dispersant to lower the rate of reactions of polycondensation and subsequently coke formation.In this way asphaltene dispersants can be considered as coke inhibition materials.
Fixed-bed reactor has been used for catalytic cracking of petroleum residues.Nguyen-Huy et al. used fixed-bed reactor for catalytic cracking of VR in the presence of steam and ZrO2-impregnated macro-meso-porous red mud catalysts [21].The reaction was carried out at 500 o C and 2 hours.Catalyst with macro-porous structure showed higher catalyst activity due to the improved accessibility to active sites which led to high liquid yield (11 wt% more) and higher conversion.Both structures showed good regenerability and stability.Leyva et al. investigated the hydroprocessing of atmospheric residue in fixed-bed reactor using NiMo/c-Al2O3 and NiMo/SiO2-Al2O3 catalysts [22].It was found that NiMo/SiO2-Al2O3 has higher dematalization than NiMo/c-Al2O3.This is due to its high acidity [23], [24].The asphaltene obtained using both catalysts was analyzed and it was found that the one obtained from NiMo/SiO2-Al2O3 catalyst is of smaller molecular size and of lower metals and aromatics content.
Hydroprocessing processes using solvents as hydrogen donor instead of hydrogen have been developed.Golmohammadi et al. studied the cracking of VR with nanoparticles catalysts and in the presence of supercritical water (SCW).Three catalyst were used, CeO2, Co3O4, and MnO2 [25].The performance of the catalysts was in the order CeO2> Co3O4> MnO2.According to XRD results, CeO2 is the only stable catalysts while others oxidation states were reduced.
The SEM results indicate the agglomeration of the catalyst particles together caused catalyst deactivation due to the loss of catalyst surface area so the size of the catalyst particles needs to be optimized to avoid such problem.Kim et al. investigated this process with Mo(CO)6 catalyst [26].Four solvents were used as Hdonor; naphthalene, 1-methyl naphthalene (1-MN), decalin, and tetralin at 673 K and 10 MPa N2 or H2.The coke formation was 23.4,19.0, 12.4, and 4.1 wt% in the presence of naphthalene, 1-methyl naphthalene (1-MN), decalin, and tetralin, respectively.This is lower than thermal cracking coke formation of 29.8 wt%.Further improvement was achieved by addition of hydrogen where the coke formation reduced down to 1.2% in presence of tetralin.H-donor not only facilitates hydrogen transfer but also helps in the solubility of asphaltene.
Ahn et al. studied the catalytic VR with subcritical water [27].MoNaph catalyst was used in autoclave batch reactor.The highest liquid products yield was 55.5 wt.% at 400 o C, 50g VR/0.5g /6g water in 6 hours' reaction time.Further analysis showed that the water use gives comparable results to H2.This can be very beneficial for refinery operations and environment.
For more environmental friendly process, the upgrading of VR with initial boiling point of >550 °C using iron/iron oxide nano-particles in supercritical water (SCW) and formic acid (FA) solution was investigated.The feed ratios were: catalyst/water is 1/20 g/g, FA/water is1/2 v/v and VR/water is 1/2 g/g.The results indicate that the yield and the light liquid products increase with the increase of the iron oxide's oxidation state.On the contrary, coke formation and heteroatoms content decreases as the iron oxide's oxidation state increases.The existence of supercritical water provides hydrogen for hydrogenation and oxygen for the oxidative cracking of heavy constituents.The disadvantage of this process is the CO2 production which affects the environment; a method is needed to deal with this emission [28].One suggested method is to use CO2 to produce syngas.Utilizing waste material for oil upgrading is of environmental and economic benefits.Bimetallic bionanoparticles (bio-NPs) were investigated as a catalyst for heavy oil [29].Pd and Pt ions were used at 5 wt% and 20 wt% metal loading with hydrogen as donor and bacteria as support.The bio-NPs showed a very good performance in comparison with thermal cracking and Ni-Mo/Al2O3 catalyst.The API was 9.1 o , 6.3 o , and 11.1 o for bio-NPs, thermal cracking, and Ni-Mo/AL2O3, respectively.The viscosity reduction was 99.2%, 98.7%, and 99.6% for bio-NPs, thermal cracking, and Ni-Mo/AL2O3, respectively.Moreover, the bio-NPs effectively suppress coke formation.Bio-NPs are promising catalysts which are cheap and environment friendly materials.
Due to its high activity, Ni-Mo supported catalyst was investigated for VR residue upgrading with different supports.Magendie et al. studied VR hydrocracking process based on Ni-Mo catalysts supported over silica grafted Al2O3 at 370 o C [30].The results indicated that the catalyst acidity significantly enhances cracking of asphaltene aliphatic chain.
Screening study was performed to investigate the active site loading effect on the upgrading of Kuwait Export Crude atmospheric residue [31].Fixed-bed reactor was used with Ni-Mo/c-Al2O3 catalyst.The results indicated that the vanadium and asphaltene removal are strongly dependent on the catalyst porosity while nickel removal depends on the Ni-Mo loading.In the other hand sulfur removal is highly dependent on the catalyst porosity and the catalytic sites.However further investigation is required to find the products distribution.Ni-Mo/goethite was also investigated for the upgrading of VR [32].It was found that the highest activity (69.8% liquid products) was achieved with 1%Ni-4.5%Mo/Goethiteat 420 o C, 70 bar initial hydrogen pressure, and 3 hours' reaction time.It is far better than using of mono-metallic catalyst.
The yield of liquid fuels can be increased using partial oxidation.Wang et al. investigated the partial oxidation of VR using Al and Zr-doped α-Fe2O3 catalysts [33].With Fe/Al/Zr atomic ratio of 16:1:1 (FeAlZr-1), the gasoline and diesel yields are 11.5 wt% and 43.8 wt%, respectively.This was far better than without catalyst case (6.6 % gasoline and 14.6% diesel) and un-doped catalysts (9.4% gasoline, 41.8% diesel) and the catalyst has a good regenerability.
Olefins content in oil is causing problems during storage and transportation due to olefins instability and also olefins reduce the products quality and cause environmental issues.Therefore, olefins formed during heavy oil upgrading need to be saturated.Lou et al. studied olefins saturation under methane environment over PdOx/H-ZSM-5 catalyst [34].91.3% cycloalkane selectivity was achievable with 0.27 wt% Pd/ZSM-5 catalyst.This greatly enhances the products quality.So, methane is a promising alternative donor for H2 and could reduce the cost of operation.
To make use of the catalytic processes' formed coke, residue cracking gasification (RCG) has extensive attention.The process integrates VR residue catalytic cracking and the gasification of the coke generated from the cracking.The catalyst is regenerated while syngas is produced [35], [36].Zhang et al. investigated RCG in the presence of synthesized kaolin, silica sand, and commercial FCC catalysts [35].The results showed the highest liquid yield of 80 wt% was achieved with kaolin catalyst under optimized reaction conditions.Gasification of the deposited coke was carried out in the presence of steam and oxygen at 800 o C. The analysis of the produced gas indicated 80% syngas.The regenerated kaolin catalyst also showed stable performance for few times.Few modifications of this catalyst are required to achieve continuous operation.

B. Upgrading Processes with Hydrogen Donors Solvents
Due to the environmental regulations to have green processes, many technologies employed solvents as hydrogen donor have been deployed.Examples for these solvents are supercritical fluids, aromatics, and alkanes.Recent trends of these technologies are highlighted below.
Supercritical methanol (scMeOH) in batch reactor was used for upgrading of Arabian heavy oil at temperature range 653-693 K and reaction time range 0-120 minutes.The product has more saturate and lower levels of aromatics and resins.The coke formation increases with temperature increasing [37].Other work used methanol supercritical method for the upgrading of vacuum residue (VR).Toluene and hydrogen were added with reaction temperature range 350-425 °C and reaction time 30-90 minutes.The optimum yield was found at a 400 °C, 16.7 wt% of VR in methanol for 60 min.The produced light-fraction oil (LFO) has better qualities than the vacuum residue as shown on the table V [38].LFO has lower heteroatoms content and lower asphaltene content.Complete asphaltene removal was achievable at 400 °C and reaction time of 90 minutes.Viet et al. investigated the Hydrocracking of vacuum residue with activated carbon in supercritical aromatic hydrocarbons and normal alkane hydrocarbons as solvents at 400 o C with H2 partial pressures of 3.45 MPa and 6.89 MPa [40].m-xylene and toluene were used as aromatic hydrocarbons and nhexane and n-dodecane were used as normal alkane hydrocarbons.m-xylene resulted in high conversion of 69.2 wt%, low coke formation of 13.5 wt%, and high quality liquid products.The increasing of H2 partial pressure has no effect on the VR conversion.
Many works were done related to VR upgrading with solvents in the presence of activated carbon.Tetralin solvent-treated activated carbon is one of these methods [41].The solvent acts as hydrogen donor diluent for the coke precursors while the activated carbon helps in the dehydrogenation of the tetralin and transfer the hydrogen to the VR.The results showed that at 450 °C, the coke formation is negligible compared to 32.7 wt% in case of VR pyrolysis.By using tetralin as supercritical and activated carbon at 450 °C and 5.33 MPa, gas and light oil fraction was 47 wt% light and the coke was 6 wt%.Condensed aromatics in a refinery can act as tetralin after partial hydrogenation.Heavy crude oil with asphaltene content of 6.6 wt% was upgraded using supercritical water-oxygen (SCW/O2) fluid and activated carbon [42].It was found that at temperature of 723 K and pressure of 30 MPa the volatile products yield was 24.0% and the liquid products yield was 59.5 wt.%.Asphaltene content was reduced below 10% while Ni and V were reduced to 16 wt% and 3 wt%, respectively.This process caused deposition of materials on the reactor wall which limits the possibility of continuous processing and method of removal is required either by burning or by mechanical means.Demetallization of VR with supercritical carbon dioxide (scCO2) was investigated in the presence of modifiers of methanol, ethanol, acetonitrile, acetone, ethyl acetate, nheptane, toluene and o-xylene [43].The lowest metal content was achieved by ethyl acetate, n-heptane and toluene.The results indicated that at 50 o C and 30 МPa and with toluene, deamtalization level above 95 wt% was achieved along with extract yield of 60 wt%.
In order to avoid low temperature poisoning of heteroatoms as well as acting as a catalyst and reducing coke formation, nickel (II) dimethylglyoxime (Ni-DMG) was added to supercritical methanol for upgrading VR [44].The optimum conditions were found to be 1-hour reaction time at temperature of 400 °C and pressure of 30.5-31.2MPa.At these conditions liquid fuel oil (LFO) of 72.2 wt.% with asphaltene content in the range of 1-2 wt.%, the coke content was 20.5 wt.% lower than without Ni-DMG.With H2 initial pressure of 1 MPa, LFO increased to 92.2 wt% and the coke content was 6.2 wt%.Heteroatoms content was significantly reduced by 51.2%, 85.7%, and 60.4% for iron, vanadium, and nickel, respectively.However, no indication was given for the reaction conditions and the individual products yields.

C. Thermal Cracking
Thermal cracking of VR was investigated to optimize the process conditions and the reaction conditions impact on the products quality and distributions [45]- [47].Thermal cracking of four serial petroleum residues was investigated [48].The coking temperature was ranged from 480 °C to 500 °C and the coking pressure was 0.15 MPa.The feed introduction to the Coker was kept between 10-12 hours.The results indicate that using of deeper distilling residue cut decreases the formation of coke.This is due to the stripping of gas oil from the coke as shown in table VI.
The conventional thermal upgrading methods are known for their high energy demand and low products yield [49]. Reducing energy requirements and/or the process yields of these technologies will increase their feasibility.Alfi et al.
studied thermal cracking of heavy asphaltic oil with electron beam irradiation.This resulted in 30% viscosity reduction in comparison to thermal cracking.The process was found effective for oil with high asphaltene content where the electron radiation affects the reaction mechanism by interacting with the aromatic structure of the asphaltene which led to more cracking.Controlling the process conditions during the thermal cracking is very crucial due to their impacts on products yield and quality.Therefore, the reaction conditions need to be optimized.In this regard, Lababidi et al. studied the asphaltene properties during thermal cracking at different conditions [50].The results showed that high reaction severity decreases the asphaltene molecular size and increases the aromaticity.Furthermore, the sulphur existed as disulphide and the metals are concentrated in the core of smaller polyaromatic asphaltene molecules.Understanding of such behaviours can help in predicting the reaction pathways as well as process design.
Hydrothermal cracking of Maya heavy oil was studied at temperature ranges 350-400 o C and H2 initial pressure of 800 psi for 4 hours [51].It was found that the products distribution is of high gas yield and high coke formation which was due to asphaltene conversion.On the other hand the asphaltene and residue conversions increase with increasing temperature and decreasing pressure.For further improvement of the process Mo catalyst was added.Good performance was obtained at 1000 ppm Mo and 400 o C.

IV. CONCLUSION
Upgrading processes of petroleum residues is an important issue these days because the availability of this material in the market with reasonable prices.The upgrading processes for petroleum residue were reviewed and discussed.The following conclusions were derived: a. Catalytic cracking processes are costly so there is a need for an effective process at low operation cost. b.
Catalyst regenrability and reusability is of a greater importance due to the direct relation to the process operating cost.
c. Coke formation can be reduced by dispersion process of asphaltenes, therefore increasing products yield and catalyst deactivation d.
Using of nano-sized catalyst reduces the deactivation of catalysts due to the large surface area.This enhances mass transfer conditions and the good dispersion of heavy materials.However, recovery methods are needed to regenerate the catalyst.e. Supercritical solvents are promising alternatives for hydrogen in hydroprocessing.These solvents are cheap and of less environmental effect compared to the hydrogen.
f. Catalytic upgrading of vacuum residue in the presence of H-donor solvents at high temperature has been found of a great help for hydrogen transfer.
g. Controlling the process conditions during thermal cracking is very crucial due to their impacts on products yield and quality.
h.Using of activated carbon with H-donors helps transfer hydrogen to the VR and thus increase the conversion. i.
Catalytic partial oxidation can be used to increase the liquid fuel yields.j.
Coke formation and metals deposition are the main reasons for catalyst deactivation in catalytic cracking.This needs to be studied carefully since it relates to the process feasibility and reducing catalyst deactivation can be regarded as a challenge.

TABLE I :
[3]PERTIES OF VACUUM RESIDUE[3]heteroatoms contents are greater than the acceptable level.Typical ranges of residual heteroatoms are shown in tables II and III.The quality and quantity of residual heteroatoms is very crucial for upgrading processes. Residual

TABLE IV :
[16]ARISON OF PERFORMANCE FOR NANO-SIZED CATALYST AND CONVENTIONAL PROCESSES[16]