Modification of FCC slurry oil and deoiled asphalt for making high-grade paving asphalt

Lingrui Cui,Jun Xu,Mannian Ren,Tao Li,Dianhua Liu,Fahai Cao,*

1 Large Industrial Reactor Engineering Research Center of Ministry of Education,East China University of Science and Technology,Shanghai 200237,China

2 Sinopec Luoyang Company,Luoyang 471003,China

Keywords:Paving asphalt FCC slurry oil Ageing resistance Fuel Petroleum Chemical reaction

ABSTRACT With the rapid development of modern industry,high-grade paving asphalt is massively required to meet the demands for modern transportation.As one of additives,natural asphalt is indispensable since it can improve the performance of paving asphalt in all aspects.However,the application of non-renewable natural asphalt is increasingly restricted by its limited reserves.It is imperative to find alternative approaches to produce high-grade paving asphalt.Fluid catalytic cracking(FCC)slurry oil is an ideal soft component for producing paving asphalt due to its high content of aromatics and resins.However,its bad ageing resistance limits its application to only low-grade paving asphalt.In the present work,a novel approach for producing high-grade paving asphalt was investigated using chemically modified FCC slurry oil and deoiled asphalt (DOA).The FT-IR and NMR results showed that dehydrogenation and condensation reaction occurred during the ageing process.From a series of aliphatic alcohols and aldehydes,propanal was selected as a proper modifier to improve the ageing resistance of FCC slurry oil.The propanalmodified slurry oil possessed more substituted aromatic units and less aromatic hydrogen atoms than other modified slurry oils,thus showing better ageing resistance.With the increase of length of aliphatic chains in modifier,the modified slurry oil contained more and longer alkyl substituent group on aromatics.Compared with the cross-linked oil (slurry oil modified by cross-linking agent),modified slurry oil possessed similar ageing resistance but higher flowing ability.Also,the effect of operation conditions on the kinematic viscosity of modified slurry oil were investigated.Blended with modified slurry oil,the penetration ratio of asphalt product increased from 53.7 to 66.2,which met the standard of 70#paving asphalt.Both the microscopic observations and FT-IR results indicated that modification process effectively reduced the oxidation degree of asphalt product,thus increasing the ageing resistance.Consequently,with aid of this process,high-grade paving asphalt was readily produced from low value oil from downstream products of refinery,instead of the depleting natural asphalt.

1.Introduction

With the rapid economic development and urbanization growth,both passenger and freight volume have rapidly increased year by year,requiring the high-quality pavements to meet the demands of modern transportation [1].Compared to the other transport ways,roadways constitute the most accessible and widespread means of transportation in the world[2].During the period from 2009 to 2019,the total length of highway experienced an increase of 26.5% in China,in accordance with the rapid increase of GDP in the same period.As a primary component of pavements,paving asphalt is widely used in urban roads and highways.Accordingly,pavement asphalt suffers an immense consumption,putting forward higher demand to chemical industry.

In the practical applications,road paving asphalt experiences a wide range of static and dynamic stresses at varying temperatures and under different environmental conditions.The lifetime of the road surface is quite limited by the aging of the paving asphalt.The ageing can significantly change the rheological properties of asphalt and cause asphalt hardening,leading to the embrittlement and reduced lifespan of pavements.To solve this problem,nonvulcanized nature and synthetic polymers are widely applied.Besides,reclaimed rubber,pulverized waste tire and special naturally occurring asphalt are also added.These materials improve the surface durability,adhesive property,deformation resistant at great load and freeze resistance of asphalt [3,4].Compared to the other additives,Trinidad lake asphalt(TLA)possesses superior road performances,including the exceptional resistance to deformation,low thermal sensitivity and outstanding tenacity under climatic extremes [5].It can assist the petroleum asphalt against rutting and permanent deformation,fatigue cracking,low-temperature cracking,durability and moisture damage [6].As a nonrenewable material,TLA is mined from a 100 acre ‘‘lake”located on the island of Trinidad in the West Indies.It has been applied worldwide,such as in USA,UK,Germany,Japan and China [7].However,currently,the application of TLA is limited due to its high price and large dosage in paving asphlat [8].The limited production has driven up the price of it to 500–750 USD?t-1.The high blend ratio further increases the cost of pavement products [9].For instance,the ratio(TLA/asphalt binder)of 30%by weight is recommended in Germany and Netherlands,and it is increased to 50%in UK,which accounts for more than half the total cost,much higher than that of other additives.Above all,seeking a technology avoiding using TLA to produce high-grade paving asphalt is becoming more and more urgent for the petrochemical refinery [10,11].In brief,if natural TLA able to be replaced by some cheaper and more abundant petroleum products,tremendous profits can be achieved.

Fluid catalytic cracking(FCC)slurry oil,a residual oil discharged from catalytic cracking unit,is a high-boiling-point and highdensity byproduct from the FCC unit,which is mainly processed into heavy fuel oil for combustion or supplementary feed for delayed coking in the refinery [12].On the other hand,FCC slurry oil is abundant in aromatics and resin,which are considered as‘‘effective components”in asphalt products due to their ability to form stable colloid structure.Thus it is an ideal ‘‘soft”component and can be blended with other‘‘hard”components to produce paving asphalt.In addition,FCC slurry oil is much cheaper than the other chemical raw materials,such as furfural extract oil [13,14].Take China as an example,with the increasing mixing proportion of residual oil in the FCC process,more than 7.5 million tons FCC slurry oil was produced each year,more than half of which was combusted as fuel oil [15,16].Even if only 20% of slurry oil could be used for blending asphalt,more than 2.5 million tons CO2emission could be reduced every year,which is environment friendly and sustainable [17].In recent years,the asphalt production technologies involving using FCC slurry oil have attracted plenty of interests of petrochemical industry [18,19].

Although FCC slurry oil has shown great potential to be a soft component for paving asphalt,its high chemical reactivity at high temperature results in inferior ageing resistance and poor stability,leading to the low quality of asphalt products.For decades,great efforts have been made to promote the ageing resistance of FCC slurry oil and quality of asphalt products by using additives into slurry oil.These additives can be classified into two categories.One is polymer elastomer,including styrene–butadienestyrene(SBS) and ethylene vinyl acetate (EVA),crumb rubber and selfmade tackifier [20].Without reacting with other components in the asphalt,these polymers effectively improve the mechanical performance of asphalt products by reducing fatigue cracking,reflective cracking and increasing durability of the pavement[21,22].However,the high cost of polymers seriously limits their applications.Poor processibility of rubber and other nonindustrial additives also prevent them from large scale usage.Another one is crosslinking agent,such as 1,4-benzenedimethanol and terephthalic aldehyde [23,24],which can react with the polycyclic aromatics in FCC slurry oil.With the aid of crosslinking agent,a three-dimensional macromolecular network is formed under acidic condition.Blended with the crosslinked slurry oil,the ageing resistance of asphalt products could be improved significantly.However,the molecular weight of cross-linked FCC slurry and content of heavy components are intensively increased.The later,such as heavy aromatics and asphaltene,can significantly reduce the penetration of asphalt.Paradoxically,the more the cross-linked FCC slurry is used,the less the ‘‘hard”components is consumed.Commonly,most ‘‘hard”components used in blending process are refinery by-products.For instance,deoiled asphalt (DOA),as one of the primary byproducts in the solvent deasphalting (SDA) process,is widely applied for producing paving asphalt.For this reason,the usage of cross-linked slurry oil brings the overstock of DOA,which reduces the economic benefit of SDA process.Consequently,it is imperative to develop a novel approach which can consume plenty of FCC slurry oil and DOA simultaneously.

In our previous work,a novel high-grade paving asphalt producing process was reported,in which FCC slurry oil was modified with a small molecular aldehyde and thus blended with DOA[25].In this work,chemical modification of FCC slurry oil was carried out,and the modifier compounds were selected from a series of small aliphatic alcohols and aldehydes.The effects of modifier on the kinematic viscosity of FCC slurry oil and penetration ratio of asphalt products were first investigated.The characteristics of molecular structure for various modified slurry oils were studied by FT-IR,1H NMR and13C NMR methods.The effects of operation conditions on kinematic viscosity of FCC slurry oil and penetration ratio of asphalt products were then studied.Also,the primary physical properties and ageing resistance of asphalt products were tested.The ageing degree of asphalt products were investigated with the aid of FT-IR analysis.

2.Experimental

2.1.Materials

Methanol (99.5%),ethanol (99.7%),propanol (99.5%),1,3-propanediol (98%),formaldehyde (99%,),acetaldehyde (99%),propanal (97%),p-toluenesulfonic acid (PTS) (96%) andn-heptane(97%)were purchased from Macklin reagents and used as obtained.

DOA is collected from the spraying granulation processes,and FCC slurry oil was supplied from an FCC unit,both of which are from in Luoyang Petrochemical Co.Ltd.The basic properties and SARA (saturate,aromatic,resin and asphaltene) analysis of FCC slurry oil and DOA are listed in Tables 1 and 2,respectively.

Table 1 The basic properties and SARA analysis of FCC slurry oil and DOA.

Table 2 The basic properties of DOA

2.2.Laboratory experiments

FCC slurry oil 30 g,was heated at 60 °C and put into a 50 ml batch reactor.Then 0.3 g catalyst (PTS) was added in.After cooled to room temperature and kept for 10 min,the modifier was added into the reactor with a defined mass ratio.With stirring at 500 r?min-1,the slurry mixture was heated to the set temperature under the protection of N2for certain hours.After the reaction,the modified slurry oil was blended with DOA particles with a defined ratio to produce paving asphalt.Afterwards,the performances of asphalt product were evaluated.

The cross-linked FCC slurry oil was also prepared as reference sample.Similar to the former one,30 g FCC slurry oil,0.3 g PTS and 1.5 g terephthalic aldehyde were reacted at 190 °C under the protection of N2for 2.0 h.By blending cross-linked oil with DOA particles with a defined ratio,another asphalt product was prepared and its properties were also tested.An asphalt sample was also prepared by blending original FCC slurry oil and DOA particles with a blenging ratio of 1.00 as a parallel experiment.

2.3.Characterization of FCC slurry oil

2.3.1.Elemental analysis

The elemental analysis was performed by a Vario EL III (German) using the dynamic combustion method.The mass percentages of carbon,hydrogen,nitrogen,and sulfur were determined according to ASTM D5291.

2.3.2.MALDI-TOF MS

MALDI-TOF MS measurements were performed using an AB Sciex 4800 TOF mass spectrometer equipped with an ionic source.A 100 μl solution of 3 mg?ml-1resin in toluene was deposited onto a flat stainless-steel plate and dried in air.

2.3.3.FTIR spectroscopy

To characterize the functional groups in FCC slurry oil molecules,FTIR spectroscopy measurements were performed using the Tensor 27 infrared spectrometer from Bruker Optics (Stockholm,Sweden) Company,with a KBr beam splitter.

2.3.4.1H and 13C NMR spectroscopy

All of the spectra were recorded at room temperature on a Bruker Avance 400 MHz super conducting Fourier NMR spectrometer.The sample for1H NMR spectroscopy contained 15 mg of resin in 0.6 ml of CDCl3,with 0.025 mol?L-1CH2Cl2as the internal standard.13C NMR measurements were performed on nearly saturated solutions of resin in a CDCl3solution containing 0.15 mol?L-11,4-dioxane as the internal standard and 0.03 mol?L-1chrome-(III)acetylacetonate as the relaxation agent.

2.3.5.Kinematic viscosity

The modified slurry oil was put into vacuum oven at 100°C and-90 kPa for 1 h before the viscosity measurement,by which slurry modifier agent was removed.Then kinematic viscosity of variouss FCC slurry oils was measured by the kinematic viscometer at 95°C according to the standard GB/T 11137–1989.

2.3.6.TLC-FID

The determination of SARA fractions was carried out by using an IATROSCAN MK-5 thin layer chromatograph (Iatron Laboratories Inc.).The separation was achieved by using a three-stage solvent development sequence [26].

2.4.Preparation of asphalt products

The blending ratio of the FCC slurry oil to DOA was determined by the penetration of asphalt.The higher is the blending ratio,the higher is the penetration of asphalt.According to the GB/T 15180–2000 standard,the penetration of 70# paving asphalt is required between 60 and 80 (0.1 mm).On that basis,the penetration of asphalt products in this work was regulated from 69.0 to 71.0(0.1 mm).

The modified slurry oil 100–150 g was put into vacuum oven at 100 °C and -90 kPa for 1 h,by which un-reacted slurry modifier agent was removed.After that,the processed modified slurry oil was added into a 500 ml aluminum tank.When the temperature reached 150 °C,a shear mixer was dipped into the tank and the rotation speed was set to 100 r?min-1.When the temperature reached 180 °C,a specific mass of DOA particles were fed in with a feeding rate of 5 g?min-1,then blended for 30 min with a shear rate of 150 r?min-1.The mass ratio of FCC slurry oil to DOA particles was recorded as the blending ratio.After that,the asphalt product was cooled down to room temperature and kept for 12 h before measured.

2.5.Tests of asphalt products

2.5.1.Penetration test

According to the standard GB/T 4509–2010,the needle penetration test of asphalt was measured by penetrating a needle upright into the asphalt sample tenths of millimeter in 5 s with a load of 100 g under 25 °C,by which the grades and ageing resistance of asphalts were classified.

2.5.2.Ductility test

The ductility of asphalts was tested according to the standard GB/T 4508–2010.The asphalt was molded into standard briquette,then elongated at a speed of 5 cm?min-1under 25 °C until the asphalt thread was broken.The length at fracture was the ductility of asphalt.

2.5.3.Softening point test

The softening point of asphalt material is the temperature at which the substance achieves a specific degree of softening under stated conditions.According to the standard GB/T 4507–1999,it was measured using the ring-and-ball apparatus dipped in distilled water with a heating rate of 5 °C?min-1.

2.5.4.Ageing procedures of asphalt products

The short term ageing resistance of asphalts was tested by the thin film oven test (TFOT) according to the standard GB/T 5304–2001.In a typical procedure,ca.50 g asphalt sample was spread out over a metal plate,then rotated at the speed of 5.5 r?min-1under the purging of hot air at 163°C for 5 h.After ageing,the softening point,needle penetration and ductility of aged asphalts were measured to evaluate the ageing resistance.The ratio of penetration after to before ageing was recorded as penetration ratio,which evaluated the aging resistance and high-temperature stability of asphalt products.

3.Results and discussion

3.1.Ageing mechanism of FCC slurry oil

FCC slurry oil has great potential to produce asphalt products as a‘‘soft”component,but with low ageing resistance,while the ageing mechanism of FCC slurry oil is not clear yet.Here we would further study the mechanism of ageing of the FCC slurry oil.

The composition of original and aged slurry oil are shown in Table 3.It is found that content of saturates remains constant after ageing,the content of aromatics is decreased significantly while the resin and asphaltene are increased.Combined with the increase ofMnfrom 287.3 g?mol-1to 301.1 g?mol-1(Table 4),the FCC slurry oil becomes heavier after the ageing process.

The FTIR spectra of original and aged FCC slurry oils are shown in Fig.1.After ageing,the characteristic peak at 1600 cm-1ascribed to aromatic C=C becomes stronger,indicating an increase of aromatic content in aged slurry oil.In addition,the intensity of absorptions between 750 cm-1and 875 cm-1also grows stronger,which are from the out-of-plane C-H deformation vibrations(oopC-H deformation vibrations),showing that the aged slurry oil has relatively more substituted aromatic units than the original one.

The1H NMR spectra of original and aged FCC slurry oils were collected and shown in Fig.2(a).The area of proton peaks in1H NMR spectra were integrated (Table S1),where HA,Hα,Hβand Hγ refer to the integral value of aromatic proton and protons in α-CH3,β-CH3and γ-CH3,respectively.Compared with original slurry oil,the content of Hα and Hβincreases while the Hγ content decreases,showing shorter alkyl side chains existing in the aged slurry oil.Furthermore,the aromatic hydrogen decreases significantly from 36.0% to 31.5%,indicating that aged slurry oil has less aromatic hydrogen.The13C NMR spectra of aged FCC slurry oils were also collected (Fig.2(b)).The area of C atoms peaks in13C NMR spectra were integrated (Table S2),where CS,CA,CARH,CARBand CARCrefer to the integral value of aliphatic carbon,aromatic carbon,protonated aromatic carbon (the aromatic carbon bonded to the hydrogen),bridged aromatic carbon (the mutual aromatic carbon of two conjoint aromatic rings)and aromatic carbon substituted by alkyl side chain.After ageing,slurry oil possesses higher content of aromatic carbon but less protonated aromatic carbon.For the content of un-protonated aromatic carbon (129–165),which is used to analyze aromatic structure of slurry oil,the content of CARBand CARCincreases after ageing,indicating that the aged oil contains more aromatic fused-rings and side chains.Thus,the aged slurry oil has more aromatics with higher degree of condensation and substitution than the original oil.

Also,it is found that dehydrogenation condensation reaction occurs during the ageing process.Firstly,the aromatic C—H bond is broken under high temperature and dehydrogenation reaction occurred on the aromatic ring,which results in the decrease of aromatic hydrogen.Then the condensation reaction occurs in the dehydrogenated aromatics,generating higher condensed aromatics.Thus,the slurry oil becomes heavier during the ageing,in which the aromatics are converted to the heavy components(Table 3).Since asphalt is colloidal,its stability depends on the rational group composition of each component.After ageing,the increased heavy components in aged slurry oil would reduce the stability of asphalt products,leading to lower ageing resistance and high-temperature properties.To improve the ageing resistance of FCC slurry oil,the dehydrogenation condensation reaction should be inhibited,in which aromatic hydrogen is considered as an active group.Thus,the content of aromatic hydrogen in slurry oil could be applied to evaluate its ageing resistance.In consequence,the lower is the content of aromatic hydrogen,the higher is the ageing resistance of slurry oil.

Table 3 The group composition of FCC slurry oil determined by TLC-FID/%

Table 4 Molecular weight of original and aged slurry oils determined by MALDI-TOF-MS

Fig.1.FTIR spectra of original and aged FCC slurry oil.

3.2.Selection of FCC slurry oil modifier

Due to the high chemical reactivity at high temperature,FCC slurry oil needs to be modified to enhance its ageing resistance.A proper modifier should not only improve the ageing resistance and high temperature stability of FCC slurry oil,but also avoid forming macromolecules.

Compared to the cross-linkers,proper modifier should possess these properties,like low molecular weight,simple molecular structure and relatively low reactivity.Above all,the modifier can inactivate FCC slurry oil just as cross-linking agent.By controlling the modification conditions and reactivity of modifier,the molecules smaller than those in the cross-linked oil can be formed.Thus,borrowing the knowledge and experiences from some crosslinking agents,such as 1,4-benzenedimethanol and terephthalic aldehyde,here aliphatic alcohols and aldehydes with 1–3 of carbon atoms were initially tried.

Fig.2.1H NMR spectra (a) and 13C NMR spectra (b) of original and aged FCC slurry oil.

In reference to the crosslinking operation conditions,FCC slurry oil was modified with various modifiers under the similar conditions.Afterwards,kinematic viscosity was characterized.Based on the mechanism of crosslinking reaction,some groups in slurry oil were substituted by the electrophilic reagent,which led to an increase of molecular weight [27].As electrophilic reagents,the modification also increased molecular weight of slurry oil and finally led to an increase in viscosity.Thus,the reaction index(RI),which is defined as the ratio of oil viscosity after to before modification,is used to evaluate the reaction activity of different modifiers.The higher was the RI,the higher was the molecular weight of modified oil,and the better was the modifier.After ageing,both the molecular weight and viscosity of slurry oil increased dramatically,since quite a number of light aromatics were converted into heavy components (Table 3).Thus the viscosity increment of aged oil could reflect the slurry oil’s ageing degree and the change of viscosity before and after ageing could reflect the ageing resistance of slurry oil.To improve the ageing resistance,the viscosity of modified oil should be increased slightly after ageing.The penetration ratio of asphalt products was also tested,by which the aging resistance and high-temperature stability of asphalt products could be evaluated.The higher was the penetration ratio,the higher was the ageing resistance of the asphalt and slurry oil.Furthermore,the modified slurry oil should present less ageing activity than original oil.Thus,the content of active groups(aromatic hydrogen)was used to evaluate the ageing resistance of slurry oil.The lower was the content of aromatic hydrogen,the better was the ageing resistance of slurry oil.

3.2.1.Effects of the functional group in modifier on ageing resistance of modified oil

Fig.3.Viscosity increment of aged FCC slurry oil with different modifiers.

RI of each modifier is shown in Table 5.The results indicated that aliphatic aldehydes had higher RI values than the alcohols with similar aliphatic length,showing that aldehydes had higher activity than alcohols.The viscosity increment of aged FCC slurry oils changing with modifier type was investigated (Fig.3).Compared with the original FCC oil,the viscosity increment of modified slurry oils after ageing decreased dramatically from 95.3 mm2?s-1(Table 1) to no more than 23.0 mm2?s-1,indicating that ageing resistance of FCC slurry oil was improved after modification.The viscosity increment of aged slurry oil modified by aldehydes was lower than those modified by alcohols,showing higher ageing resistance of aldehyde-modified slurry oil.In addition,the penetration ratio of asphalt blended with‘‘aldehyde oil”had higher penetration ratio.With the aid of modifier,the active groups in slurry oil were reduced and the reactivity of slurry oil changed much lower.After modification,more stable colloidal structure involving hard components in the asphalt products was built up,leading to the improvement of ageing resistance.Therefore,aldehyde is better than alcohol as modifier.

Table 5 Reaction Index (RI) of different modified slurry oil

Table 6 Average structural information of modified and cross-linked FCC slurry oils

Table 7 Analysis of absorptions in FTIR spectra of modified slurry oil

The group composition of FCC slurry oils before and after modification was measured with Fourier-transform IR (FT-IR) spectroscopy,1H NMR and13C NMR spectra.Based on the results of characterization,structural parameters of composition in FCC slurry oils were analyzed (Table S5).The information of average molecular structural for modified slurry oils is shown in Table 6.

The FTIR spectra of slurry oils modified by various modifiers are shown in Fig.4.The results are analyzed and listed in Table 7.Strong absorptions between 750 cm-1and 875 cm-1are from the out-of-plane C—H deformation vibrations (oop C—H deformation vibrations).1,4-disubstituted and 1,2-disubstituted aromatic compounds have these absorptions.Thus aldehyde-modified slurry oils possess more substituted aromatics than alcohol-modified oils.The1H NMR spectra of modified oils are shown in Fig.5(a)and the content of different protons integrated from spectra are shown in Table S3.The results reveal that the modified slurry oil has less aromatic proton and more aliphatic protons,indicating that aromatic hydrogen is partially substituted by alkyl side chains during modification process.Compared with alcohol-modified slurry oils,‘‘aldehyde”oils had lower content of HAbut more aliphatic hydrogen,showing that more protons in aromatic compounds were substituted,or these slurry oils had fewer hydrogen protons.Similar results are obtained from13C NMR spectra (Fig.5(b)) and integral value of C atoms peaks (Table S4).The alkyl-substituted aromatic carbon (138–150) in ‘‘aldehyde”oils were higher than that of ‘‘alcohol”oils,showing more alkyl side chains existing in aldehydemodified oils.The substitution degree of slurry oil reflects the number of aromatic carbons that have a methyl or a chain that substitutes the proton.As shown in Table 6,the substitution degree of aldehyde-modified oil are 0.353,0.363 and 0.381,higher than that of ‘‘alcohol”modified samples with similar aliphatic length (0.352,0.359 and 0.370),showing more alkyl branches existing in the aromatics of oil modified with aldehyde,in consistence with FT-IR and NMR spectra results.With more alkyl substituted side chains,the ‘‘aldehyde”oils contained much less aromatic hydrogen,thus with higher ageing resistance.Combined with viscosity increment and penetration ratio results,aldehydes as modifiers have better ability to improve the ageing resistance of slurry oil than alcohols.

3.2.2.Effects of length of aliphatic chains on ageing resistance of modified oil

As shown in Table 5,the RI values of aldehydes increased with carbon number.With the increase of length of aliphatic chains in aldehydes,the viscosity increment of aged modified oil decreased while the penetration ratio of asphalt products increased.With largest RI value (1.71),least viscosity increment (7.1 mm2?s-1) and highest penetration ratio (0.653),propanal showed best reaction activity and propanal-modified slurry oil exhibits the highest ageing resistance.

Fig.4.FTIR spectra of differently modified FCC slurry oils.

The FTIR spectra of slurry oils modified with different aldehydes were shown in Fig.4.The content ratio of aromatic C=C/aliphatic CH2is obtained by the intensity ratio of the adsorption at 1600 cm-1to 2921 cm-1,which is used to characterize the average length of alkyl side chains substituted on aromatics[28].The value of‘‘propanal”slurry oil(0.94) is lower than that of slurry oil modified by formaldehyde and acetaldehyde (1.52 and 1.13),showing that average length of alkyl side chains substituted on aromatics in modified slurry oil increases with the length of aliphatic chains.It is reported a linear relationship exists between the intensity ratio of bands in the range from 1460 cm-1to 1380 cm-1and the molar ratio of methylene to methyl groups (n(CH2)/n(CH3)) in molecules[29].Therefore,the trend for change of methylene group relative to methyl group during the modification process could be found by comparing the A1460/A1380values of modified slurry oils.As shown in Table 7,the A1460/A1380ratio increased with the length of modifier’s aliphatic chains,which indicates that slurry oil modified by modifier with longer aliphatic chains have highern(CH2)/n(CH3) value and longer alkyl side chains,in agreement with the results of A1600/A2921.With the increase of modifier’s aliphatic length,the content of HAin modified slurry oil decreased while the content of alkyl hydrogen increased (Table S3),especially Hα and Hγ.It shows that the slurry oil modified by long aliphatic modifier has more and longer side chains and less aromatic hydrogen.Similar to the1H NMR results,the content of aliphatic carbon in modified slurry oil increased with modifier’s aliphatic length,while the aromatic carbon decreased.As for the average molecular parameters of modified oil,the number of aromatic H in oils decreased with modifier’s aliphatic length while the aliphatic H,especially β-and γ-hydrogen increase,indicating that ‘‘propanal oil”possesses lower content of aromatic hydrogen and has higher ageing resistance,in consistence with the viscosity and penetration results.Thus,based on the above results,propanal was regarded as the proper modifier for the FCC slurry oil modification.

Table 8 Molecular weight of various slurry oils determined by MALDI-TOF-MS

Fig.5.1H NMR spectra (a) and 13C NMR spectra (b) of FCC slurry oils after different modifications.

3.2.3.Comparison between modified and cross-linked slurry oil

As a mature technology,the crosslinking process could effectively inactivate slurry oil and improve slurry oil’s ageing resistance with the aid of the cross-linker.Compared with crosslinking agent(terephthalic aldehyde),aged slurry oil modified by propanal shows similar viscosity increment (Table 5) and asphalt product exhibits close penetration ratio (Fig.3).However,the average molecular weight (Mn) of treated slurry oil decreased significantly from 305.2 g?mol-1to 293.7 g?mol-1(Table 8).In addition,the content of heavy components(resins and asphaltene)in modified slurry oil was also lower than that of the cross-linked oil (Table 9).Thus,the penetration of asphalt blended with modified slurry oil would decrease much less than that one blended with cross-linked oil,which leads to the decrease of blending ratio from 1.20 to 1.02 [25].

Next,the molecular structure of both treated oils are compared to find the differences between modification and crosslinking reactions.From the FT-IR spectra,strong intensity of the characteristic adsorptions between 750 cm-1and 875 cm-1are found for both propanal-modified and cross-linked slurry oils,indicating that both oils have multiple substituted aromatics.However,‘‘propanal”oil exhibits highern(CH2)/n(CH3) value and lowernC=C/nCH2value,having more longer alkyl side chains than cross-linked oil.Compared with modified slurry oil (45.8%),the content of aromatic carbon in cross-linked oil (51.4%) is much higher.In the crosslinking reaction,aromatic hydrocarbon in cross-linked oil is substituted by terephthalic aldehyde,resulting in a significant increase of aromatic content [27].Although aromatic content of modified oil is lower,the content of alkylsubstituted aromatic carbon (CARC) of cross-linked and modified oils are the same (7.2%).Combined with the aromatic carbon with similar substitution (about 0.380),as shown in Table 6,the active groups in FCC slurry oil are reduced for both two reactions,supported by the improved ageing resistance of both treated slurry oils and their asphalt products.In addition,the aromaticity of‘‘propanal”oil (0.610) is much lower than that of cross-linked slurry oil (0.668).Furthermore,the condensation degree of aromatic carbons (γ) in propanal-modified oil is 0.044,which is also lower than that of cross-linked oil (0.147).Therefore,larger aromatics are generated in the cross-linked slurry oil,consistent with the enhanced molecular weight and kinematic viscosity.

3.3.Physical properties of the resulting asphalt products

The effects of modification operation conditions,including reaction temperature,pressure,time and modifier content,on the kinematic viscosity of modified slurry oil and penetration ratio of asphalt products are studied and the results are shown in Figs.S1–S4.The highest kinematic viscosity and penetration ratio are obtained at a reaction temperature of 180 °C,a reaction pressure of 2.0 MPa,a reaction time of 5 h and a modifier content of 3.0% (mass).The physical properties of asphalt products are also tested and the results are shown in Table 10.

Compared with the original asphalt,the softening point of asphalt product blended with modified slurry oil increases from 46.9 to 48.2 °C,indicating that modified asphalt contains more heavy components,due to the larger molecular weight(Table 8)and higher content of resin and asphaltene (Table 9).To balance the group components and keep constant penetration of asphalt products,the blending ratio of modified slurry oil is only increased from 1.00 to 1.02.However,the molecular weight of modified slurry oil is smaller than that of cross-linked oil,so the dosage of slurry oil is only slightly increased [25,30],which increases relative dosage of DOA,bringing plenty of economic benefit to refinery.When blended with modified slurry oil,the penetration ratio of asphalt products increases significantly from 53.7 to 66.2,while the softening point increment after TFOT decreases from 2.7 to 1.8°C,which shows much enhanced ageing resistance and perfectly meets the standard requirements of 70#paving asphalt.With the aid of modifier,the active groups in slurry oil are inactivated and the reactivity of slurry oil becomes much lower.Thus,the colloidal stability of asphalt products is enhanced,leading to the improvement of ageing resistance and increase of penetration ratio.

Table 9 The group composition of FCC slurry oil determined by TLC-FID (%)

Table 10 Properties of asphalt blended with modified,cross-linked and original slurry oil

Table 11 The CI and SI of asphalt samples before and after aging

Fig.6.IR spectra of asphalt production pre and post TFOT.(a) Blended with original oil;(b) blended with modified oil.

The structural change of components in asphalt products produced by modified and original slurry oil before and after ageing is investigated by IR spectra.As shown in Fig.6(a),the original asphalt has two new characteristic adsorptions at 1654 cm-1and 1024 cm-1after ageing,ascribed to C=O (carboxyl) and S=O (sulfoxide),respectively.The appearance of carbonyl adsorption shows that a series of oxidation reactions of asphalt occur during ageing process.The enhancement of the sulfoxide adsorption shows that the sulfur oxidation also occurs during aging process.Combined with other weak adsorptions,the functional groups in the asphalt are oxidized to carboxyl,sulfoxide and some other polarity groups after aging [31],which enhances the interactions among components in asphalt and make the asphalt become harder and stiffer.

In contrast to the original asphalt,a stronger absorption peak at 3044 cm-1is found for the asphalt blended with modified slurry oil,which is ascribed to the stretching vibration peak of C-H bond(Fig.6(b)),indicating that the asphalt blended with modified slurry oil possesses higher content of alkyl side chains grafted on aromatics than the original oil-blended asphalt.After ageing,the change of IR spectra of modified asphalt sample is basically the same as original one,in which C=O (carboxyl) and S=O (sulfoxide) IR absorptions become stronger.However,the enhancement degrees of carbonyl peak and sulfoxide peak in modified asphalt are lower than that of original sample.Carbonyl index(CI)and sulfoxide index(SI)of different asphalts are calculated for quantitatively analyzing oxidation degree,respectively (Table 11) [31,32].Compared with the original one,the asphalts blended with the modified oil show a similar trend after ageing,in which both CI and SI increase.However,the carbonyl content of modified asphalt is relatively lower,indicating that modification process is able to reduce the oxidation reaction and inhibit the generation of the carbonyl group.Apparently,the modified asphalt shows a higher ageing resistance,in agreement with the tested physical property.

4.Conclusions

In this work,a novel approach for producing high-grade paving asphalt has been developed,in which FCC slurry oil and DOA particles were blended instead of using natural asphalt.The results of FT-IR and NMR spectra show that dehydrogenation and condensation reaction occur during the ageing.Based on the least kinematic aged viscosity increment (7.1 mm2?s-1),highest penetration ratio(0.653)and lowest content of aromatic hydrogen(31.0%),propanal has been found to be the best modifier.It is revealed that the propanal-modified slurry oil possesses least aromatic hydrogen and most alkyl-substituted aromatic units,thus has the highest ageing resistance.Both the physical properties tests and FT-IR results of asphalt indicate that our modification process is able to effectively reduce the oxidation degree and enhance the ageing resistance of asphalt products.Therefore,by this novel approach,high-grade paving asphalt can be produced by FCC slurry oil and DOA,showing great potential for reducing the consumption of the natural asphalt resources and adding value to the FCC slurry oils.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful for the financial support by Sinopec Innovation Foundation (118009-3).The authors also thank Sinopec Luoyang Company for providing oil samples and technical supports.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.05.040.