Application of nano-nickel catalyst in the viscosity reduction of Liaohe extra-heavy oil by aqua-thermolysis

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 35, Issue 2, April 2007 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2007, 35(2), 176−180

RESEARCH PAPER

Application of nano-nickel catalyst in the viscosity reduction of Liaohe extra-heavy oil by aqua-thermolysis LI Wei1,*, ZHU Jian-hua1, QI Jian-hua2 1

Faculty of Chemical Science and Engineering, China University of Petroleum (Beijing), Beijing 102249, China

2

Henan Shenma Chlor-Alkali Chemical Industry Limited Co, Pingdingshan 467242, China

Abstract: Nano-nickel catalyst was prepared in methylcyclohexane-water-n-octanol-AEO9 micro-emulsion system, and used in the viscosity reduction process of Liaohe extra-heavy oil by aqua-thermolysis. It is observed that the nano-nickel can catalyze the aqua-thermolysis reaction of extra-heavy oil at 280 °C. The experimental results demonstrate that compared with the original crude oil sample, the mean molecular weight of the upgraded sample decreases, the content of sulfur changes from 0.45% to 0.23%, the content of resin and asphaltene reduces 15.83% and 15.33%, respectively. Based on the GC-MS analysis results, a reaction pathway is proposed that involves hydrogen transfer from methylcyclohexane to the extra-heavy crude oil resulting in the formation of toluene. During the cooling process after upgrading reaction, the W/O emulsion is formed in the presence of the surfactant AEO9. As a result, with respect to the original crude oil, the viscosity of upgraded sample is changed from 139800 mPa⋅s to 2400 mPa⋅s at 50 °C, an approximately 98.90% reduction by the synergetic effects of upgrading, emulsification and diluting. Key Words: nano-catalyst; aqua-thermolysis; viscosity reduction; upgrading; hydrogen transfer

As the world’s supply of light, sweet crude oil is depleted, the stocks of heavy oils and bitumen become more and more important as a component in supplying the demand for fuels and petrochemical feedstock. Heavy oil and bitumen are abundant in the world. Among about 10 trillion bbl of remaining oil reserves, about 70% is heavy oil and bitumen[1]. However, the contents of sulfur, nitrogen, oxygen and metals, as well as the viscosity and density of heavy crude oil are higher than those of conventional crude oil. All the properties retard exploitation, transportation and processing of heavy oil. Therefore, viscosity reduction and upgrading heavy oil would be the premise for the heavy oil utilization[2−5]. Steam stimulation (gorge and disgorge) is the most popular and effective technology to recover heavy oil in the world. In the steam injection process, the steam was used as the heat carrier to raise the temperature of viscous oil and to make the viscosity of heavy oil lower, therefore, it could reduce the flow resistance of heavy oil through the porous media of reservoir, and increase the yield and production rate of heavy oil. In fact, there are chemical reactions between steam and heavy oil. Hyne et al.[2] used the term “aqua-thermolysis” to describe the chemical interaction of high temperature and high pressure water steam with the

reactive components of heavy oil and tar sands bitumen. According to the literature[3,4], the primary reason of aqua-thermolysis process may be explained, as the interaction of high temperature and high pressure water steam and heavy oil, could break the C−S bond in heavy oil, reduce the content of resin and asphaltene, and the viscosity of heavy oil. Clark et al.[6] found that when aqueous metal species was added into the process, the viscosity of heavy oil was further reduced in comparison with steam-only experiments. All of the first row transition metal species could reduce the oil viscosity. In China, it has been proved that aqueous metal species could catalyze aqua-thermolysis of heavy oils in situ[7]. Fan et al.[8] investigated the catalytic effect of oil soluble naphthenate on aqua-thermolysis, the experimental result showed that the naphthenate could catalyze the aqua-thermolysis. But some problems in the previous investigations still exist. First, the catalytic effects of metallic species would be limited since water soluble catalyst or oil soluble catalyst only dissolve in water or oil phase but do not dissolve in another phase. Second, it was insufficient for increasing the H/C ratio of the heavy oils to obtain hydrogen only from H2O by aqua-thermolysis. In other words, it is necessary to prepare catalysts that could contact with water and oil simultaneously and catalyze the

Received: 2006-11-18; Revised: 2007-01-30 * Corresponding author. Tel: +86-10-89734601; E-mail: [email protected] Copyright2007, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All right reserved.

LI Wei et al. / Journal of Fuel Chemistry and Technology, 2007, 35(2): 176−180

aqua-thermolysis under lower temperatures, and select an effective and cheaper hydrogen donor which could offer hydrogen to enhance the H/C ratio of the heavy oil. Nano-metal catalysts do not dissolve in water or oil, but these catalysts can be highly dispersed in water or oil and adequately contact with them. The oil phase used to prepare nano-metal, such as cyclohexane, methylcyclohexane or tetraline, could serve as the hydrogen donor simultaneously. The dehydrogenation of cyclohexane, methylcyclohexane usually takes place under a temperature higher than 400 °C in the absence of a proper catalyst[9]. This temperature is obviously higher than that for aqua-thermolysis reaction and steam stimulation, i.e. 200 − 320 °C. Chen and Cacciola[10,11] showed that Ni, Pt etc. have benign activity, selectivity, and stability for the dehydrogenation of cyclohexane and methylcyclohexane. And nickel is a typical hydrogenation catalyst. Thus it is hopeful to catalyze the dehydrogenation of cyclohexane or methylcyclohexane, and then transfer the hydrogen to heavy oil to achieve the upgrading of heavy oil.

1

Experimental

agent of the surfactant, were mixed using the magnetic stirrer until well-proportioned solution was formed. Then a specified quantity of Ni(NO3)2 was added into the solution and mixed by means of the magnetic mixer until Ni(NO3)2 was completely dissolved. Subsequently, by adding specified amounts of LiBH4, the Ni ions in the water pools of the microemulsion were reduced. In order to complete reduction of the Ni ion in the microemulsion, the molar ratio of [LiBH4]:[Ni] was approximately 5:1. Finally, the microemulsion with 20 gNi/L was obtained. 1.3

The specimen for TEM detection was prepared by depositing a 5 µL microemulsion containing the metal nickel onto a gold grid coated with carbon. Then, the specimen was dried in a vacuum oven at 200 °C in nitrogen atmosphere for 24 h to remove water, organic solvent and surfactant. The size of the catalyst particles in the treated specimen was measured by H700H type transmission electron microscope. 1.4

1.1

Preparation of samples

Reagents

Except AEO9, which is of industrial grade, all the reagents used were of analytical grade. The main reagents were methylcyclohexane, n-octanol, Ni(NO3)2 , LiBH4 and AEO9. The heavy oil sample used in experiments was taken from Shuguang zone in Liaohe oil field. The physical and chemical properties of the heavy oil are listed in Table 1. Table 1 Physical and chemical properties of Liaohe extra-heavy crude oil Item Density (20 °C), ρ / g⋅cm−3 Viscosity (50 °C), µ / mPa⋅s Water content, w / % Sulfur content, w / % Initial boiling point, t / °C Saturated, w / %

Data 1.0006 139800 2.4 0.45 213 20.43

Aromatic, w / %

22.05

Resin, w / %

54.52

Asphaltene, w / %

1.2

Characterization of the catalyst

3.0

Preparation of the catalyst

According to Song’s work[12], the microemulsion was prepared with methylcyclohexane, water, surfactant AEO9 and n-octanol with the mass ratio of 50:10:1:0.2. The preparation of catalyst in micromulsion was carried out in an atmospheric environment at 30 °C, methylcyclohexane, water, surfactant AEO9 and n-octanol, that was an auxiliary

Upgrading reactions of heavy oil were carried out in a 500 mL stainless-steel batch reactor equipped with a magnetic stirrer. In the experiment, the reactor was charged with 100 g extra-heavy oil, 10 mL prepared microemulsion nano-nickel catalyst, and 50 g water. The air in the spare space of reactor was replaced with nitrogen. The reaction mixture was heated to 280 °C, and the relative pressure was 6.4 MPa, then was kept for upgrading reaction for 24 h. When the reaction was completed, the heating was stopped and the reaction mixture was cooled to room temperature. In order to prevent the oil and water mixed, first the water under the bottom of reactor was pumped out, and then dumped the oil in the reactor; thereby 1# sample was obtained. The recovery of extra-heavy oil was 95%. In order to evaluate the catalytic effect of nano-nickel, the reactor was charged with 100 g extra-heavy crude oil, 50 g water, and 10 mL microemulsion without nano-nickel catalyst. The experimental conditions and program were the same as in the preparation of 1# sample, thus the 2# sample was prepared. In preparation of 2# sample, the recovery of extra-heavy oil was 94%. The 3# sample was prepared by directly mixing 100 g extra-heavy oil and 10 mL microemulsion without nano-nickel catalyst. The original extra-heavy crude oil was named as the 4# sample. 1.5

The evaluation of oil samples

By using the EA1108 CHN-O elemental analyzer, elements C, H in oil samples were determined. The element S 2# sample was determined by HORIBA SLFA-2800 X-ray

LI Wei et al. / Journal of Fuel Chemistry and Technology, 2007, 35(2): 176−180

fluorescence. The viscosity of oil samples was measured by HADV-II+PRO viscometer. The content of asphaltene was analyzed according to RIPP-7 method. The content of resin was determined by alumina chromatograph. The content of water was measured by using SYD260 type water analyzer. The mean molecular weight of oil samples was determined by using KNAVER osmoscope via VPO method. The fractions with boiling point lower than 160 °C were identified by Agilent 6890-573N GC-MS analyzer.

2

Results and discussion

2.1

Catalyst particle size

The TEM pattern of nano-nickel catalyst deposited on carbon is displayed in Fig. 1. From Fig. 1, it is found that the particles of catalysts are in spheroidal form, and the mean particle size can be estimated as 6.3 nm.

Fig. 1 TEM pattern of nano-nickel catalyst

2.2

Evaluation of upgrading effect for heavy oil

The evaluation results of water contents (wϕ), viscosity µ50 at 50 °C, resin content (wR), asphaltene content (wA), sulfur contents (wS), the H/C molar ratio in mixture of resin and asphaltene, and mean molecular weight (M) for sample1#, 2#, 3# and 4# are listed in Table 2. Table 2 Evaluation results of samples Sample wϕ / % µ50, mPa·s

1# 30.3 2400

2# 31.2 9800

3# 7.0 15000

4# 2.4 139800

wA / %

45.89

wR / %

2.54

2.66

2.68

3.0

wS / %

0.23

0.32

0.43

0.45

H/C (mol ratio)

1.46

1.44

1.42

M

422

47.1

506

48.7

616

54.52

1.43 674

From Table 2, it is known that the contents of water for sample 1# and 2# are 30.3%, 31.2%, respectively, obviously

higher than those for samples 3# and 4#. The reason may be that during the reaction, the temperature increases, the microemulsion in the reactor is broken, and the surfactant AEO9 and nano-nickel catalysts are released from the microemulsion. When the upgrading reaction is completed, with the decrease of the system temperature, surfactant AEO9 can make oil and water mix and form steady W/O emulsion, which further result in higher content of water in oil sample. Compared with sample 2#, the viscosity at 50 °C of sample # 1 is reduced by 75.5%. This result suggests that the nano-nickel catalyst can catalyze aqua-thermolysis of extra-heavy oil. Comparing the viscosity of sample 3# with sample 4#, it is found that the viscosity of sample 3# is decreased by 89.3% because of dilution effect of microemulsion. From Table 2, it is known that compared with sample 3#, the viscosity of sample 1# and 2# could be decreased by 84.0% and 34.7%, respectively. Therefore, the viscosity reduction of sample 1# cannot only be the contribution of dilution effect of microemulsion and emulsification of W/O emulsion, the catalytic upgrade effect of nano-nickel catalyst on extra-heavy oil sample will prompt the viscosity reduction of sample 1#. As a whole, the viscosity reduction of sample 1# will depend on the synergestic effect of dilution effect, emulsification effect and catalytic upgrading effect. The content of resin and asphaltene in samples is in the order 4# > 3# > 2# > 1#. It shows that the aqua-thermolysis between extra-heavy oil sample and high temperature and high pressure water can reduce the content of resin and asphaltene in the sample. In comparison to sample 2#, the content of resin and asphaltene of sample1# is lower, which suggests that nano-nickel catalyst can catalyze aqua-thermolysis reaction and reduce the content of sample resin and asphaltene. From Table 2, it is found that the sulfur contents of samples with aqua-thermolysis, i.e., samples 1# and 2#, are obviously lower than those of the samples without aqua-thermolysis, namely samples 3# and 4#, and the sulfur reduction extent of sample 1# is higher than that of sample 2#. Choosing thiophene and tetrahydrothiophene as aromatic and aliphatic model sulfurs in extra-heavy oil, Clark et al.[13] investigated the reaction between the model sulfur compounds and high temperature and pressure steam, and proposed a possible reaction pathway. In Clark’s theory, as an intermediate compound, CO can be observed during the aqua-thermolysis reaction, and final products are H2S, CO2 and hydrocarbons since the water-gas shift reaction takes place. H2 could be formed via water-gas shift of intermediate CO. The formed H2 may be consumed by hydrogenation partly, and other H2 perhaps took part in hydrodesulphurization reaction, and decreases the sulfur content of the sample. The bond energy of C−S is smaller

LI Wei et al. / Journal of Fuel Chemistry and Technology, 2007, 35(2): 176−180

than that of C−C, and the electronegativity of S is more than that of C. In the molecules of extra-heavy oil, sulfur has negative electricity, whereas carbon has positive electricity. H+ usually attacks sulfur atom and OH− attacks carbon atom during the reaction. With the electron cloud excursion of C−S, the C−S bond energy reduced[7]. With the effect of high temperature and pressure steam in the aqua-thermolysis reaction, the bond between carbon and sulfur will first be broken. In the preparation of sample 1#, the aqua-thermolysis reaction is catalyzed by the nano-nickel catalyst, and the sulfur content of sample decreases obviously. With the dilution effect of methylcyclohexane and n-octanol in microemulsion, the viscosity of extra-heavy oil sample is reduced, and thus the degree of dispersion of nano-nickel catalyst is improved, and further decreases the sulfur content of sample. The H/C molar ratio in the mixture of resin and asphaltene of sample 1# and 2# are higher than those of sample 3# and 4#. Clark[14] showed that the ring opening, desulfurization, denitrogenation, hydrogenation and steam shift reaction may occur between the high temperature and pressure steam and extra-heavy oil during the aqua-thermolysis, and finally H2 was produced. The produced H2 may take part in the hydrogenation and/or hydrodesulphurization of extra-heavy oil, and enhance the H/C molar ratio in a mixture of resin and asphaltene of samples 1# and 2#. At 160 °C, samples 1# and 2# were distilled, and the fraction was left to settle and water separated. The obtained upper fraction was analyzed by GC-MS, and the results are displayed in Fig. 2. 1

1 #

1

2 2

3

4

(1) (2) From Table 2, it is found that the mean molecular weight of sample 1# is the lowest among all the samples; second, the mean molecular weight of sample 2# was lower than that of sample 3#, and finally the mean molecular weight of original sample 4# was the highest. The result demonstrates that the nano-nickel catalyst can catalyze the aqua-thermolysis of extra-heavy oil under 280 °C, break the C−S with smaller bond energy, partly convert components of extra-heavy oil with high molecular weight into smaller molecular weight components, intensify the aqua-thermolysis of the extra-heavy oil, and finally, achieve the partly upgradation of extra-heavy oil.

3

#

2

1

hydrocarbons with 8 − 12 carbon atoms was 5.6% in the fraction from sample 1#. Similarly, it is known that the content of methylcyclohexane is about 98%, and the content of hydrocarbons with 8 − 12 carbon atoms is about 2% in the fraction from sample 2#. However, it can be seen from Table 1 that the initial boiling point of original extra-heavy oil is 213 °C, which means that the fractions with boiling point lower than 160 °C did not exist in the original extra-heavy oil sample. Therefore, the following reactions may occur during the catalytic aqua-thermolysis of extra-heavy oil. The possible reactions can be expressed in the form of Eq. (1) and Eq. (2).

5

6

7

8

Time t / min)

Fig. 2 The chromatogram of fractions by distillation sample 1# and 2# at 160 °C

By using the NIST library, it is found that the peak “1” in Fig. 2 denotes methylcyclohexane, and the peak “2” denotes toluene. Thus, it is proved that the fraction of sample 1# containes toluene, whereas the fraction of 2# sample does not contain toluene. By calculating the peak areas via integration, it is estimated that the content of methylcyclohexane is 86%, the content of toluene is 8.4%, and the content of

Conclusions

It can be concluded as follows: Nano-nickel catalyst could be prepared by using microemulsion method. The characteristic results show that the catalyst particles are in spheroidal form and the mean particle size is 6.3 nm. During the aqua-thermolysis upgrading process of extra-heavy oil, some of C−S in the extra-heavy oil are destroyed, sulfur content is reduced and component with high molecular weight is partly converted into smaller molecules. Thus, the mean molecular weight of extra-heavy oil with upgrading treatment is reduced. During catalytic aqua-thermolysis process of extra-heavy oil, the methylcyclohexane can be dehydrogenated and converted into toluene. The hydrogen produced can transfer into the extra-heavy oil, and enhance the H/C molar ratio of the mixture of resin and asphaltene. It is proved that the presence of nano-nickel catalyst is necessary and effective. The nano-nickel catalyst can catalyze the aqua-thermolysis

LI Wei et al. / Journal of Fuel Chemistry and Technology, 2007, 35(2): 176−180

reaction of extra-heavy oil, and reduce the viscosity, the content of resin and asphaltene of extra-heavy oil effectively. By catalytic aqua-thermolysis of extra-heavy oil, it is possible to achieve the effect of partial upgrade for Liaohe extra-heavy oil.

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