Hydrogenation of bitumen in situ in supercritical water flow with and without addition of zinc and aluminum

J. of Supercritical Fluids 72 (2012) 100–110

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Hydrogenation of bitumen in situ in supercritical water flow with and without addition of zinc and aluminum Oxana N. Fedyaeva, Anatoly A. Vostrikov ∗ Kutateladze Institute of Thermophysics SB RAS, 1, Acad. Lavrentiev Av., Novosibirsk, Russia

a r t i c l e

i n f o

Article history: Received 9 May 2012 Received in revised form 29 August 2012 Accepted 30 August 2012 Keywords: Supercritical water Bitumen Hydrogenation Desulfurization ZnO and Al2 O3 nanoparticles

a b s t r a c t The conversion of bitumen (the gross-formula CH1.56 N0.01 S0.007 ) in supercritical water (SCW) flow at 400 ◦ C, 30 MPa with and without addition of zinc and aluminum shavings into bitumen has been studied. For the conversion without addition of Zn and Al the yield of volatile and liquid products was 3.1 and 47.3%, respectively, in relation to the weight of bitumen. As a result of a chemical interaction of H2 O molecules with bitumen, oxygen atoms appeared in these products and conversion residue; the amount of hydrogen in them being increased. When Zn or Al was added, the conversion and hydrogenation of bitumen significantly increased owing to hydrogen evolution during the oxidation of metals by SCW. This oxidation via the synthesis of ZnO and Al2 O3 nanoparticles was accompanied by on-site heating of reactants. Moreover, when adding Zn and Al into bitumen, the yield of the volatile products increased up to 15.3 and 38.2%, respectively. The addition of Zn resulted in the yield increase of the liquid products up to 62.3%, of the resins up to 33.5%, the content of oxygen in the products being increased too. While the addition of Al resulted in the yield decrease of the resins up to 7.5% and the yield increase of the oil up to 36.1%, no oxygen atoms in the structure of the liquid products being detected. A portion of sulfur was removed from bitumen via the SCW conversion with addition of Zn in terms of the ZnO + H2 S = ZnS + H2 O reaction. These and other peculiarities of the conversion and hydrogenation of bitumen in situ are reported in the present paper. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Supercritical water (SCW: T > 374 ◦ C, P > 22.1 MPa) is a universal reaction medium for the conversion of biomass [1–4], fossil fuels [5–7], and different wastes [8–10] into petrochemical feedstocks due to its properties, such as low viscosity, low dielectric constant and high density at rather high temperatures. At T > 374 ◦ C, a hydrogen-bond net characteristic of liquid water breaks down and does not restore under any powerful compression, i.e. water passes into a gaseous state where it becomes an effective solvent for organic substances and gases [11,12]. The conversion of natural and synthetic bitumens in SCW can be another technique for the production of light hydrocarbons (HC). Research in this field is focused on decrease of crude oil reserves and continuous increase of consumer demand of light HC. Bitumen, owing to a low H/C atomic ratio, cakes when heated in vacuum at 500 ◦ C, losing less than 5 wt.% [13,14]. During the SCW conversion of bitumen, the following changes are observed:

∗ Corresponding author. Tel.: +7 383 330 80 94; fax: +7 383 330 80 94. E-mail address: [email protected] (A.A. Vostrikov). 0896-8446/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2012.08.018

- dissolution of bitumen in SCW increases the conversion rate [13]; - thermolysis of bitumen high molecular components in SCW increases the yield of light HC [14]; - burning of bitumen dissolved in SCW with O2 addition can proceed without soot formation to compensate for heat expenses for its conversion [15,16].

By now, low-temperature (T ≤ 480 ◦ C) supercritical water conversion of vacuum residue [17,18], oil sand bitumen [19,20], high-temperature coal tar [21], petroleum [22] and coal-tar [23] asphaltenes has been already investigated. A greater yield of light hydrocarbon fractions with the H/C atomic ratio higher than that of the raw material has been obtained. However, researchers’ opinions concerning the mechanisms and the role of SCW in the conversion differ. The authors [17–23] assume that the effect of SCW is associated with dissolution of the HC raw material, while the H/C atomic ratio increase in liquid products is proceeds from the thermolysis and condensation reactions and intermolecular rearrangements of hydrogen. The authors [21,23] also believe that H2 O molecules are hydrogen donors preventing radicals from being recombined and promoting the formation of liquid HC. The results [17–23] show that at SCW conversion of the HC raw material, hydrogenation via chemical participation of H2 O molecules is

O.N. Fedyaeva, A.A. Vostrikov / J. of Supercritical Fluids 72 (2012) 100–110

101

Table 1 Characteristics of raw bitumen. Ultimate analysis (wt.%)

Gross-formula

C

H

N

S

O

85.83

11.16

1.34

1.67

–

CH1.56 N0.01 S0.007

insignificant and thus, additional methods of hydrogenation are required. To hydrogenate the conversion products in [24], H2 , CO and the H2 /CO2 mixture was added into supercritical water, in [25,26] CO was obtained in situ by means of decomposition of HCOOH. This hydrogenation was carried out for a number of model compounds [24], coals [25] and bitumen [26] by means of the water gas shift reaction: CO + H2 O = CO2 + H2 . In [24] it was observed that the rate of hydrogenation (400 ◦ C) when adding CO and the H2 /CO2 mixture was higher than when adding H2 . It proves that intermediates, formed by the water gas shift reaction, were more effective for the hydrogenation than H2 [24]. The authors [27–29] carried out the hydrogenation (445 ◦ C) of the products of SCW conversion of coals and model organic compounds in situ when adding zinc powder into an autoclave. It is their opinion that the hydrogenation of organic matter occurs as a result of hydrogen evolution during the oxidation of zinc by SCW. The present authors found out that H2 , CO, organic substances and metal oxide nanoparticles were formed during the oxidation of bulk zinc [30,31] and aluminum [32] samples by SCW with and without CO2 addition. The reactions of zinc and aluminum oxidation by SCW are described as follows [30,32]: mnZn + mn(H2 O) = m(ZnO)n + mnH2 , (H = −2.2 MJ/kgZn )

(1)

mnAl + 1.5mn(H2 O) = 0.5m(Al2 O3 )n + 1.5mnH2 , (H = −17.6 MJ/kgAl )

(2)

It is hoped that our experimental data on the hydrogenation of bitumen in situ in SCW flow with addition of zinc and aluminum shavings reported in the present paper are of great practical value.

Group composition (wt.%) Oil

Resin

Asphaltene

59.2

35.5

5.3

2. Experimental 2.1. Reagents and experimental procedure The object of our investigation was bitumen of Omsk Oil Refinery (Russia). Elemental and group compositions of bitumen are given in Table 1. As can be seen, bitumen contains heteroatoms of sulfur and nitrogen and does not contain oxygen ones. Zn and Al shavings with thickness ≈0.2 mm were added into bitumen. They were prepared without contact with atmospheric oxygen from metal of a high-purity grade and then mixed with definite amounts of bitumen heated up to 40 ◦ C in nitrogen atmosphere. Fig. 1 presents the scheme of the experimental setup. The main parts of the setup are a tubular reactor (internal diameter 24 mm; length 700 mm) made of stainless steel 12Cr18Ni10Ti, an analogue of the AISI 321; a heat exchanger; the vessel for collection of the products; a mass spectrometer vacuum unit. The reactor is equipped with the system measuring the water flow rate and the device for computer registration of pressure and temperature. Heating of the reactor and the heat-exchanger was carried out by outer resistance heaters and controlled by a temperature programmer. Temperature was measured by chromel–alumel thermocouples with the accuracy of 0.1 ◦ C. Pressure was measured by membrane strain sensors, whose accuracy was 0.25%. A definite amount of bitumen or mixture of bitumen with metal shavings was loaded into the reactor onto the partition (Fig. 1) made of porous stainless steel. After the reactor was sealed, all the volumes with air were evacuated by a first-stage vacuum pump. It prevented atmospheric oxygen from participating in bitumen’s and metal’s oxidation. Then, the reactor and the heat-exchanger were uniformly heated up to 400 ◦ C. The rate of heating to 350 ◦ C was 10 ◦ C/min, in the range of 350–380 ◦ C it was 5 ◦ C/min, and up to 400 ◦ C it was 2 ◦ C/min. At 400 ◦ C supercritical water with the flow rate 5.0 ± 0.2 g/min was supplied through the bottom end into the reactor. When the operating pressure (30 MPa) was attained, the

Fig. 1. Schematic diagram of the experimental setup: (1) vessel with distilled water; (2) high-pressure plunger pump; (3) strain gauge; (4) damping vessels; (5) flow meter; (6) thermocouples; (7) heat exchanger; (8) resistance heaters; (9) membrane pressure gages; (10) reactor; (11) quartz beaker; (12) vessel for collection of products; (13) thermostat; (14) prechamber; (15) mass spectrometer control unit; (16) vacuum pump.

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Table 2 Experimental conditions and results for bitumen conversion in SCW flow with and without addition of zinc and aluminum. Reagents

mB (g)

mM (g)

t (min)

mW (g)

mMxOy (g)

Bitumen/SCW Bitumen/SCW/Zn Bitumen/SCW/Al

165 134 148

– 137 51

75 60 60

480 ± 10 280 ± 5 350 ± 5

– 172 94

valve for discharging reactants into the vessel for collection of the products was opened to such an extent that the pressure in the reactor was 30.0 ± 0.5 MPa. The time of SCW pumping through the reactor t at 30.0 ± 0.5 MPa (Table 2) was chosen on the basis of the conditions of complete oxidation of metals by supercritical water. To calculate the t values we used the kinetic equations for the oxidation of Zn [30] and Al [32]. In the experiment without metal addition, the time t was increased because of increasing the weight of bitumen mB loaded into the reactor (Table 2). The weight of metal shavings mM and bitumen mB in the mixture was calculated on the basis of the increased H/C atomic ratio (from 1.56 for raw bitumen (Table 1) up to ≈2.0 for the products and conversion residue) because of hydrogen evolution during the reactions (1) and (2). In each of the three experiments (Table 2), the volume of bitumen or mixture of bitumen with metal shavings was ≈0.45 in relation to the volume of the reactor that provided for dissolution of bitumen in SCW. The experiments were completed when supply of SCW into the reactor was cut off and the heating of the reactor and heatexchanger was stopped. When cooled, the reactants continued discharging from the reactor into the vessel for the products until the excess pressure was kept in it. The water from the hot reactor became a liquid in the vessel for the products cooled by ice. At T ≈ 80 ◦ C the reactor was disconnected from all the systems and the bitumen conversion residue was removed from the reactor for the weight measurement. Hexane and ethyl acetate were used for removing the bitumen conversion residue from the reactor walls and metal oxides. After washing with solvents (until they became colorless) metal oxides were filtered, dried and their weight mMxOy was measured (Table 2). After drying, metal oxides turned into a pale gray fine-dispersed powder of agglomerated nanoparticles. The bitumen conversion residue was separated from solution by solvent evaporation with the use of a rotary evaporator.

2.2. Analytical methods X-ray diffraction (XRD) analysis of oxidized metals was performed by a ThermoARL difractometer with Cu K␣ radiation ( = 0.15418 nm). The structure of oxidized zinc samples was analyzed with the high resolution transmission electronic microscope JEM-2010, equipped by the energy dispersive X-ray system (EDX). The structure of oxidized aluminum samples was analyzed with the high resolution transmission electronic microscope JEM-2200FS with EDX operated at 200 kV. Composition of the volatile conversion products was measured by the quadruple mass spectrometer MS-7303 mounted in a vacuum chamber. For this purpose a certain amount of gas was sampled from the vessel into a previously vacuumed prechamber; then the gas was discharged into an open ionic source of the mass spectrometer via a capillary. The gas was being let into the prechamber and mass-spectra recorded until the pressure of individual compounds in the prechamber was not less than partial pressure of saturated water vapor. The method of mass spectrometric analysis is described elsewhere [33]. The amount of each substance in the volatile conversion products was determined on the basis of the vessel’s volume, gas pressure and relative contents of individual substances. After degassing of the liquid products and water during mass spectrometric analysis, their separation was carried out. It should be noted that after the SCW conversion of bitumen without metals in the quartz beaker (Fig. 1) mounted in the vessel, apart from the HC fraction, the yellowish emulsion was formed. Organic substances from the emulsion were extracted by ethyl acetate. We failed to extract them by chloroform. After the SCW conversion of bitumen with addition of metals in the quartz beaker, there appeared a distinct interface between the water and the HC fraction which made the procedure of products separation less complicated, i.e. no extraction was needed. After separation, the weights of the liquid products mL and water from the vessel mW (Table 2) were measured. Ultimate analysis of raw bitumen, liquid products, and conversion residue was made with the use of an elemental analyzer Vario EL III. Oxygen content was calculated as a difference between 100% and content of C, H, N, and S. The accuracy of determining the weight content of each element was 0.1%. The averaging was performed using 5 samples. The results of the ultimate analysis are given in terms of the gross-formulae in Table 3. As the atomic weight of

Table 3 Parameters of conversion and hydrogenation of bitumen in SCW flow with and without addition of zinc and aluminum, and the gross-formulae of products and conversion residue. ˇ[C] (%)

ˇ[H] (%)

ˇ[N] (%)

ˇ[S] (%)

 C (%)

 M (%)

ı (%)

 (%)

Gross-formula

Bitumen/SCW 3.1 ± 0.2 Volatile Liquid 47.3 ± 1.5 Residue 51.0 ± 1.1 ˙ 101.4 ± 1.9

2.8 ± 0.2 46.0 ± 1.5 49.6 ± 1.1 98.4 ± 1.9

4.0 ± 0.3 51.7 ± 1.9 46.0 ± 1.5 101.7 ± 2.4

2.4 ± 0.2 40.9 ± 2.5 54.8 ± 4.0 98.1 ± 4.8

7.4 ± 0.6 47.3 ± 2.6 37.5 ± 1.9 92.2 ± 3.3

1.1 ± 0.2

–

1.7 ± 0.3

98.8 ± 0.2

CH2.24 N0.004 S0.019 O0.047 CH1.75 N0.01 S0.006 O0.01 CH1.45 N0.01 S0.006 O0.03 –

Bitumen/SCW/Zn Volatile 15.3 ± 0.3 Liquid 62.3 ± 1.4 Residue 25.6 ± 0.6 ˙ 103.2 ± 1.6

12.4 ± 0.3 59.9 ± 1.4 25.4 ± 0.6 97.7 ± 1.6

32.8 ± 0.6 68.3 ± 1.6 24.7 ± 0.9 125.8 ± 1.9

3.6 ± 0.4 63.3 ± 3.8 31.2 ± 2.2 98.1 ± 4.5

2.4 ± 0.2 62.3 ± 3.4 21.2 ± 1.1 85.9 ± 3.6

1.2 ± 0.2

11.7 ± 0.7

13.5 ± 1.4

52.2 ± 2.9

CH4.09 N0.012 S0.001 O0.058 CH1.78 N0.01 S0.008 O0.02 CH1.52 N0.02 S0.006 O0.01 –

Bitumen/SCW/Al 38.2 ± 0.6 Volatile Liquid 44.1 ± 1.1 Residue 21.6 ± 0.6 ˙ 103.9 ± 1.4

33.8 ± 0.6 43.5 ± 1.1 21.3 ± 0.6 98.6 ± 1.4

64.0 ± 0.7 50.5 ± 1.6 20.3 ± 0.6 134.8 ± 1.8

30.4 ± 2.0 44.1 ± 2.7 23.5 ± 1.8 98.0 ± 3.8

35.9 ± 1.5 35.2 ± 2.2 18.2 ± 0.9 89.3 ± 2.8

0.6 ± 0.1

12.5 ± 0.5

21.7 ± 1.4

62.4 ± 2.3

CH2.96 N0.012 S0.008 O0.026 CH1.81 N0.01 S0.006 CH1.48 N0.01 S0.006 O0.01 –

Equation

(4)

(4)

(4)

(4)

(5)

(5)

(6)

(7)

–

Products

˛ (%)

(3)

O.N. Fedyaeva, A.A. Vostrikov / J. of Supercritical Fluids 72 (2012) 100–110

sulfur is much more than that of the other elements, the stoichiometric coefficient for sulfur is recalculated with the accuracy of no less than 0.001. Group composition of raw bitumen and liquid conversion products was determined by the following procedure. The product was covered by hexane with a 20-fold excess by mass and held during 24 h. Then, precipitated asphaltenes were filtered, washed with hexane, dried and their weight was measured. Measurement accuracy of contents of asphaltenes was 1.5%. The part of the liquid products – malthenes (resin and oil) soluble in hexane was fractionated by adsorption chromatography on silica gel. Adsorption column was filled with silica gel whose mass was 15 times as much than that of the investigated sample. After a silica gel layer became compact under the effect of hexane with a 3–4-fold excess by volume, malthenes (≈2 g) were placed onto the column. Then, the column was kept during 1 h for the adsorption front to be distributed. The content of oil (eluted with n-hexane) and resin (successively eluted with an alcohol-benzene mixture (1:1, v/v)) was determined by weighing after the solvent was evaporated. The discrepancy in the fraction yields in repeated measurements is no higher than 5–7%. The gas–liquid (GL) chromatograms of oil extracted from raw bitumen and liquid products were obtained by the Hewlett-Packard model 5890 series II chromatograph in the temperature mode from 80 to 320 ◦ C with the rate of 3 ◦ C/min (a DB-1 capillary quartz column, length 50 m, internal diameter 0.25 mm, film 0.25 ␮m). The formulae for calculation of parameters defining the SCW conversion and hydrogenation of bitumen are listed below. The relative yields of the liquid ˛L and volatile ˛g products and conversion residue ˛R were calculated as follows: ˛L (%) = ˛R (%) =

m  L

mB

m  R

mB

× 100,

˛g (%) =

m  g

mB

ˇ[X],L (%) = ˇ[X],R (%) =

[X]B

 [X]  R

[X]B

ˇ[X],g (%) =

× 100,

[X]g [X]B



× 100, (4)

where [X] in nominator and denominator is the weight of one of the elements [C], [H], [N] or [S]. Weight fractions of water decomposed by the reaction with hydrocarbons  C and the reaction of zinc or aluminum oxidation  M were calculated as follows:



C (%) =

 M (%) =

m∗C

mW + m∗C + m∗M m∗M

mW + m∗C + m∗M

ı(%) =

[H]L + [H]g + [H]R − [H2 ] − [H]B [H]B



× 100,

(6)

where [H]L , [H]g , [H]R , and [H]B are the weights of hydrogen in the liquid and volatile products, conversion residue and raw bitumen, respectively; [H2 ] is the weight of molecular hydrogen. The weight fraction of hydrogen formed by water decomposition and consumed during the hydrogenation of bitumen (the efficiency of hydrogenation of bitumen) was:



 (%) =

1−

[H2 ] [H]R + [H]L + [H]g − [H]B



× 100.

(7)

It should be noted that systematic errors when determining the amount of the products and conversion residue (Table 3) occurred in all the experiments. These errors were caused by losses of organic substances in the valves and connection lines, and also by incomplete removal of the conversion residue of bitumen from the agglomerated particles of metal oxides. The values of these losses characterize the ˙ˇ values given in Table 3. Losses of sulfur appeared to be maximum (≈7–8%) and those of carbon and nitrogen did not exceed 2–3%. Losses of hydrogen were compensated for by decomposition of H2 O molecules. Losses of sulfur under these conditions can be as a result of a sufficiently high solubility of H2 S in water (0.378 wt.% at 25 ◦ C [34]) and the interaction of H2 S with reactor walls [35,36]. Higher values of standard deviation for sulfur and nitrogen compared with hydrogen and carbon (Table 3) can be accounted for by a low amount of sulfur and nitrogen in raw bitumen.

3.1. The SCW conversion of bitumen without addition of metals



× 100,



(3)

Here mL , mg , mR are the weights of the liquid and volatile products and conversion residue, respectively; mB is the weight of bitumen loaded into the reactor. Weight fractions of elements in the liquid and volatile products and conversion residue were calculated as follows: L

The degree of bitumen hydrogenation at SCW conversion was:

3. Results and discussions

× 100,

× 100.

 [X] 

103



× 100,

 × 100.

(5)

Here the weight of water decomposed by the reaction with HC was calculated as m*C = (18/16) × ([O]L + [O]g + [O]R ), where [O]L , [O]g , and [O]R is the weight of oxygen in the conversion products and residue; mW is the weight of water in the vessel for the products after the experiment; the weight of water decomposed by the reaction of metal oxidation was calculated as m*M = (18/16)mM , where mM is increase in the weight of metal loaded into the reactor as a result of oxidation (mM = mMxOy − mM ).

The values of the yield (˛) of the products and conversion residue, the percentage of elements (ˇ) in them, the weight fraction of decomposed water (), the degree (ı) and the efficiency () of bitumen hydrogenation calculated by means of Eqs. (3)–(7) and the gross-formulae of the products and conversion residue are listed in Table 3. It can be seen that at 400 ◦ C the yield of the volatile products is insignificant (˛g = 3.1%) while the yield of the liquid products is almost half as great as the weight of raw bitumen (˛L = 47.3%). The H/C atomic ratio (see the gross-formulae) for the liquid and volatile products increases and for the conversion residue it decreases compared to H/C atomic ratio for raw bitumen (Table 1). The yield of oil, resin and asphaltene calculated as the ratio of their weight to the weight of bitumen is given in Table 4. Oil prevails in the composition of liquid conversion products. The yield of oil, resin and asphaltene in relation to their amount in raw bitumen is 59.6, 33.2, and 3.8%, respectively. Similar results were obtained in [19] when investigating the SCW conversion of oil sand bitumen in a semi-continuous reactor under non-isothermal conditions. The yield of oil is greater than that of resin and asphaltene because of their different solubility in SCW. It is in agreement with the data [37,38] from which it follows that solubility of saturated HC in SCW is higher than that of polyaromatic compounds. Significant quantitative changes in group and element composition of the products Table 4 The yield of oil, resin, and asphaltene calculated as ratio of their weight to the weight of bitumen (wt.%). Reagents

Oil

Resin

Asphaltene

Bitumen/SCW Bitumen/SCW/Zn Bitumen/SCW/Al

35.3 28.0 36.1

11.3 33.5 7.5

0.2 0.8 0.5

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O.N. Fedyaeva, A.A. Vostrikov / J. of Supercritical Fluids 72 (2012) 100–110

Fig. 2. Molecular-weight distribution of n-alkanes and isoprenoids in the oils extracted from raw bitumen and the products of bitumen SCW conversion with and without addition of zinc or aluminum.

and conversion residue proceed from the thermolysis reactions accompanied by the breakage of hydrocarbon chains and separation of aliphatic substitutions from aromatic nuclei, and also from the dehydrogenation reactions and intermolecular rearrangement of hydrogen. Molecular-weight distributions in a series of n-alkanes and iso-alkanes (isoprenoids) presented in Fig. 2 account for these processes. According to the data on GL chromatography, the ratio of isoprenoids/n-alkanes after SCW conversion of bitumen does not change and is equal to 0.16. It is to be noted that raw bitumen is a residue of the thermal destruction of natural hydrocarbons of petroleum. Hence, changes in molecular-weight distributions (Fig. 2) are the consequence of SCW thermolysis of bitumen components. It is evident that during SCW conversion it is the processes of breakage of long-chain HC that prevail, it resulting in significant increase in the share of n-alkanes mainly to the right of maximum (C15 C28 ) and decrease in the share of C29 C35 n-alkanes. In raw bitumen oxygen is not detected (Table 1) but it appears in the volatile and liquid products and conversion residue (see the gross-formulae in Table 3) as the consequence of participation of H2 O molecules in radical reactions. In the volatile products oxygen is mainly detected in CO2 and CO (Table 5). The bulk of oxygen is concentrated in the conversion residue (Table 3). Furthermore, the amount of hydrogen atoms (see the ˙ˇ[H] value in Table 3) in the conversion products and residue is greater than in raw bitumen. Thus, these results are in agreement with the data [20–23] on chemical participation of H2 O molecules in hydrogenation of HC matter. The weight fraction of decomposed water appears to be equal to  C = 1.1% (Table 3) and the fraction of hydrogen formed via water decomposition in relation to hydrogen amount in raw bitumen is ([H]W /[H]B ) × 100 = 3.1%. Bitumen hydrogenation consumes  = 98.8% in relation to [H]W amount and the degree of bitumen hydrogenation is equal to ı = 1.7% (Table 3). From the results of mass-spectrometric analysis of the volatile conversion products (Table 5) it is evident that the yield of hydrocarbons decreases in the following consequence: alkanes > cycloalkanes > alkenes > cycloalkenes > arenes. Carbocyclic hydrocarbons are cyclopentane and cyclohexane; hydrocarbons C7 C9 are mainly methyl- and ethyl-substituted derivatives of C5 C6 . The presence of alkenes in the volatile products is the consequence of both their presence in raw bitumen and the disproportionation reactions which is in good agreement with

the data [39] on thermal destruction of n-alkanes. The amount of aromatic HC is insignificant because of a relatively low temperature of conversion. It corresponds to the data [13,14] from which it follows that the maximum yield of xylene, toluene and benzene at SCW conversion of bitumen is observed at 560, 560 and 660 ◦ C, respectively. The dip of C5 H12 amount in a series of C9 C15 alkanes whose origin is not clear to us can be seen in Table 5. Decrease in the amount of HC beginning with C6 can be accounted for by decrease in pressure of saturated vapor of heavy HC in the vessel from where samples of gas are taken for mass spectrometric analysis. On the basis of the results obtained, it can be concluded that the products of bitumen SCW conversion result mainly from thermolysis. The thermolysis of bitumen components both in bulk and solution generates radicals T, ◦ C

R R−→R• + R• , which can react with other molecules including H2 O or recombine: R• + R1 R1 → R R1 + R1 • ,

(8)

R• + H2 O → RH + OH• ,

(9)

R• + H2 O → ROH + H• ,

(10)

R• + R1 • → R R1 .

(11)

The higher is the content of aliphatic hydrocarbons in raw bitumen and the lower is their molecular weight; the higher is solubility of bitumen in SCW and, consequently, the higher is the rate of reactions (9) and (10). It is obvious that dissolution of aliphatic HC along with H/C atomic ratio decrease for the residue result in increase of the yield of the liquid conversion products. The reactions (9) and (10) cannot provide for a sufficient degree of hydrogenation and, therefore, it is necessary to use an extra hydrogenation to increase the yield of liquid HC. 3.2. The SCW conversion of bitumen with addition of zinc Addition of Zn into bitumen provides for in situ generation of hydrogen by the reaction (1). This leads to qualitative and quantitative changes in the composition of conversion products (Tables 3–5). From the data given in Table 3 it follows that in comparison with bitumen conversion without metal addition, the yield

O.N. Fedyaeva, A.A. Vostrikov / J. of Supercritical Fluids 72 (2012) 100–110

105

Table 5 The yield of volatile products calculated as ratio of their weight to the weight of bitumen (mg/g). Products

Bitumen/SCW

Bitumen/SCW/Zn

Bitumen/SCW/Al

H2 CO CO2 H2 S Alkanes Methane Ethane Propane Butane Pentane + 2,2-dimethylpropane Hexane + 2,2-dimethylbutane Heptane Octane + 2,2-dimethylhexane 4,4-Dimethylheptane Alkenes Propene-1 Butene-1 + butene-2 + 2-methylpropene-1 3,3-Dimethylbutene-1 2,3,4-Trimethylpentene-1 Cycloalkanes Cyclopentane Cyclohexane + methylcyclopentane Cycloheptane 1-Ethyl-1-methylcyclopentane + 1,2,3-trimethylcyclopentane + 1,4-dimethylcyclohexane Cycloalkenes Cyclohexene + 1-methylcyclopentene-1 1-Methylcyclohexene-1 + 1,5-dimethylcyclopentene 2,3,4-Trimethylcyclopentene-1 + 1,2-dimethylcyclohexene-1 Aromatic hydrocarbons Benzene Toluene Ortho-, meta-, para-xylenes + ethylbenzene Naphthalene Thiophenes Thiophene 2,5-Dihydrothiophene 2,4-Dimethylthiophene

0.02 0.61 1.06 1.04

13.77 4.83 5.75 0.01

14.66 7.24 8.21 5.93

0.94 1.04 1.57 5.15 0.41 2.00 1.92 0.88 0.12

16.99 17.39 22.91 29.61 4.20 3.56 0.53 0.26 0.01

32.04 36.95 31.38 48.99 3.12 18.07 20.13 13.85 1.21

− 1.75 0.23 0.19

− 10.83 − −

10.79 12.45 6.56 2.81

4.28 1.43 1.08 0.75

6.56 3.58 0.32 0.08

9.31 14.14 12.38 15.47

0.49 0.42 0.17

0.92 0.28 0.05

19.52 6.80 –

0.02 0.11 0.13 –

0.02 0.18 0.16 –

4.56 4.12 4.00 0.16

0.07 0.15 0.18

0.51 0.53 0.01

0.62 − 0.70

Other compounds Gross-formula

3.08 CH1.71 N0.132 O0.136 S0.024

8.46 CH1.85 N0.073 O0.177

16.84 CH1.54 N0.311

of the liquid and volatile products increases by 1.3 and 4.9 times, respectively, and H/C atomic ratios for the products and residue increase as well. A fraction of water decomposed during oxidation of zinc is equal to  M = 11.7% and there occurs an almost eightfold increase of the degree ı of bitumen hydrogenation. The efficiency of bitumen hydrogenated by hydrogen evolved during the reaction (1) is  = 52.2% because almost half of hydrogen evolves as H2 (Table 5). For the SCW conversion of bitumen with addition of zinc, the portion of the volatile unsaturated HC decreases and molecularweight distribution in a series of aliphatic and carbocyclic HC changes (Table 5). The yield of lower alkanes increases more than by 15-fold. We attribute this to the evolution of hydrogen and heat during the SCW oxidation of zinc. Hydrogen inhibits the reaction (11) of recombination of HC radicals while on-site increase in temperature (on the surface of zinc shavings) [31] owing to heat evolved during the reaction (1) accelerates the cracking of long-chain HC. It testifies to the following results of GL chromatography of alkanes (Fig. 2). First, owing to heat evolution the shares of n-alkanes and isoprenoids with the length of HC chain smaller than C20 increase. Second, since thermal stability of isoprenoids is lower than that of n-alkanes because of a lower value of C C bond energy for tertiary carbon atoms [40], the isoprenoids/n-alkanes ratio decreases from 0.16 up to 0.10. Increase in O/C atomic ratio for the volatile conversion products up to 0.058 when adding Zn (Table 3) can be explained not only by increase in temperature [41] but also by catalytic effect of ZnO nanoparticles on the reaction (10). Decrease in [CO2 ]/[CO] mole

ratio from 1.11 to 0.76 is apparently caused by shift of equilibrium of the water gas shift reaction because of ≈600-fold increase in H2 concentration in the reaction mixture (Table 5). Complete oxidation of zinc by SCW occurs during the conversion of bitumen. Fig. 3 shows images of an oxidized zinc sample obtained by means of the transmission electron-microscope JEM2010. From the EDX analysis data it follows that ZnS along with amorphous carbon (Fig. 3a), which was not removed by organic solvents, were found in the sample. In Fig. 3b both equiaxial individual and equiaxial coalescence particles with the size of 50–100 nm are shown. Step growth of crystals is observed in Fig. 3c. ZnO nanoparticles contain zinc clusters with the size up to 10 nm (Fig. 3d and the emphasized part in Fig. 3c). The morphology of synthesized ZnO nanoparticles is similar to those synthesized during Zn oxidation by SCW with and without CO2 addition [31]. The XRD pattern of oxidized zinc is given in Fig. 4. According to the XRD analysis data, hexagonal ZnO (JCPDS #36-1451) is the major component of the sample. Apart from ZnO in the sample we also detected ≈1.2% of metal zinc (JCPDS #04-0831) and ≈0.5% of cubic ZnS (JCPDS #05-0566). Both the presence of ZnS in the sample of nanostructured ZnO and almost 100-fold decrease in the amount of H2 S in the volatile products of bitumen conversion with zinc addition (Table 5) proceed from the following reaction ZnO + H2 S = ZnS + H2 O.

(12)

It should be noted that the reaction (12) is widely used for H2 S removal from natural gas [42] and synthesis gas [43–45]. Based on the sulfur balance of the conversion products and residue, it

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Fig. 3. Images of oxidized zinc sample obtained by means of the transmission electron-microscope JEM-2010.

is evident that 12.6 wt.% of sulfur of bitumen passes into ZnS while the greater share of sulfur remains in the liquid conversion products (Table 3). This occurs because sulfur in bitumen is present mainly in the structure of aromatic fragments which under these conditions are not subjected to thermal destruction [14,46]. As can be seen from Tables 3 and 4, the addition of Zn into bitumen results not only in increase of the liquid products yield but also in changing the weight ratio in their composition. The yield of oil decreases from 35.3 to 28.0% and that of resin, in the contrary, increases from 11.3 to 33.5% in relation to weight of bitumen. Moreover, the yield of resin as to their amount in raw bitumen is 94.4%, and that of oil is only 47.3%. Simultaneously with increase in the amount of resin there occurs a threefold decrease in the O/C atomic ratio for the residue and a twofold increase in the O/C ratio for the liquid products. The H/C atomic ratio for the resin increases from 1.03 (without zinc) up to 1.60 (for resin of raw bitumen H/C = 0.92) as well. The H/C values were calculated based on the data on the percentage of resin in raw bitumen and the liquid conversion products (Tables 1 and 4) on the assumption that oil is characterized by

Fig. 4. XRD pattern of oxidized zinc sample.

the gross-formula of Cn H2n . Hydrogenation of aromatic nuclei conjugated with the reaction (1) can be responsible for increase of H/C atomic ratio in resin. It is found out [28] that at conversion in the system of model organic compounds–zinc powder–water, in the case of aromatic compounds, the bigger the aromatic nuclei are, the higher is the degree of hydrogenation, but the extent of the reaction is relatively low. Saturation of aromatic nuclei by hydrogen increases solubility of resin in SCW and decreases their amount in the conversion residue. It causes decrease of O/C atomic ratio for the residue and increase of O/C ratio for the liquid conversion products. Also, the reaction (11) is inhibited owing to hydrogen evolution by the reaction (1), thus promoting increase in the yield of resin as well. We do not exclude conversion of a certain amount of oil into resin in terms of the reaction (10). The rise in the yield of polar compounds during the SCW conversion of hydrocarbon feedstocks [21,47] is confirmed by this assumption.

3.3. The SCW conversion of bitumen with addition of aluminum The processes conjugated with the reaction (2), i.e. with the addition of Al into bitumen differ from those with addition of Zn. The yield of residue decreases to ˛R = 21.3% (Table 3). The yield of liquid products decreases by 1.4 times but the yield of volatile products increases by 2.5 times. The total weight of the products and conversion residue (see the ˙˛ value in Table 3) exceeds the weight of bitumen loaded into the reactor by 3.9% owing to the decomposition of H2 O molecules via the reactions with HC and aluminum. The fraction of water decomposed by the reactions with HC appears to be equal to  C = 0.6% and it is only half of the  C value obtained in the experiment with zinc addition. Oxygen atoms in the liquid products are not detected (see the gross-formula in Table 3). The O/C atomic ratios for the conversion residue in the experiments with Al and Zn are the same and for the volatile products at conversion with Al this ratio is twice as less. In the volatile products oxygen is

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present in CO2 and CO only (Table 5), the amount of oxygen in them being 72.8 wt.%. The increment of hydrogen in the products and conversion residue in relation to its amount in bitumen is 34.8% (Table 3). As can be seen from Table 3, the degree of hydrogenation ı = 21.7% and the efficiency of hydrogenation  = 62.4% appear to be substantially higher than those of hydrogenation obtained for bitumen conversion with Zn addition. Different results for these conversions can be explained as follows. First, both the temperature of reactants in the reactor where the reaction (2) occurs and the rate of the hydrogenation increase. Second, inhibition of the reaction (10) occurs in the presence of Al2 O3 nanoparticles. Heat evolution by the reaction (2) is 8 times as much than that by the reaction (1). Under these conditions the average power of heat evolution by the reaction (2) is ≈250 W. We assume that this can result in rise in temperature of reactants in the reactor by no less than 50 ◦ C. The effects of self-heating of Al samples and accelerating of the reaction (2) were observed by us earlier [32]. The rate and degree of decomposition of HC increase along with temperature increase. Consequently, the yield of volatile products and the share of alkenes in them increase (Table 5). From [39] it follows that rise in temperature of n-alkanes from 400 up to 450 ◦ C results in increase of the share of alkenes in the decomposition products. Besides, when taking into account that Al2 O3 is a catalyst for the processes of dehydration of alcohols [48,49], it may be assumed that the oxide formed during the oxidation of Al by SCW affects the composition of conversion products. This effect consists in the increase of the yield of alkenes (Table 5) while there are no oxygen-containing compounds in the liquid conversion products (Table 3). From Table 4 it follows that in the experiment with Al the yield of resin is less than that with Zn and without metals addition by 4.5 and 1.5 times, respectively. This is sure to be the consequence of inhibition of the reaction (10) resulting in the formation of resin. Addition of Al compared with that of Zn causes increase in the yield of benzene, toluene, and xylenes by 228, 23 and 25 times, respectively (Table 5). This is caused by a more extensive thermolysis of resin compounds because of a higher temperature of reagents. Both separation of aliphatic substitutes from the aromatic nuclei and their hydrogenation during thermolysis result in increase in the yields of oil (Table 4) and in further increase of the relative amount of short-chain (
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Fig. 5. XRD pattern of oxidized aluminum sample.

are similar to the particles formed at Al oxidation by SCW and the SCW/CO2 mixture described earlier [32]. Therefore, the SCW conversion of bitumen with Al is characterized by a higher yield of oil (36.1%) and the volatile hydrocarbons (34.6%), and a smaller yield of resin (7.5%) in the absence of oxygen in the liquid conversion products. It is caused by a higher temperature of conversion due to heat evolution by the reaction (2) and a higher efficiency of bitumen hydrogenation conjugated with the reaction (2). 3.4. Economic and environmental aspects of SCW conversion of bitumen with addition of metals The high heating values for the volatile (HHVg ) products, the liquid (HHVL ) products and the bitumen conversion residues (HHVR ) are given in Table 6. The values of HHV were calculated on the basis of the elemental composition of the products and residues (see the gross-formulae in Table 3) as follows [34]: HHV = 2.326 × [146.58C + 568.78H + 29.4S − 51.53(O + N)], (13) where C, H, S, N, and O are the weight percentages of the elements. The HHV˙ values are also given in Table 6. They were calculated on the basis of the relative yields ˛ (Table 3) for all the groups of substances as follows: HHV˙ = ˛g HHVg + ˛L HHVL + ˛R HHVR . The HHVB for raw bitumen calculated by the formula (13) was equal to 44.0 MJ/kg. As can be seen from Table 6, HHVB is higher than HHV˙ for the SCW conversion of bitumen without addition of metals, which is the consequence of partial oxidation of bitumen by SCW in terms of the reaction (10). For the SCW conversion of bitumen with addition of metals, HHV˙ is higher than HHVB , because of hydrogen weight increase brought about by the formation of H2 and hydrogenation of the conversion products (Table 3). Lower HHV˙ value for the conversion of bitumen with Zn compared with that of with Al is due to a higher amount of oxygen in the products of bitumen conversion with Zn (Table 3). The maximum HHVL was obtained when Al was added into bitumen due to a higher value of the hydrogenation degree ı (Table 3). From the practical point of view, the process of hydrogenation of bitumen in situ in SCW flow with the addition of Zn or Al, apparently, requires recycling, i.e. the reduction of metals from oxides. The required energy for this process related to HHV˙ was calculated as follows: ω (%) =

 m q  M M mB HHV˙

× 100,

(14)

where qM is the expenditure of energy consumed for the reduction of 1 kg of metal from oxide; mM and mB are the weights of metal

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Fig. 6. Images of oxidized aluminum sample obtained by means of the transmission electron-microscope JEM-2200.

and bitumen (Table 2), respectively. The ω values (Table 6) were calculated for the following ranges of specific expenditure of energy qZn = 7.7–11.5 MJ/kg and qAl = 40.8–61.2 MJ/kg. These ranges were chosen taking into consideration the following reasoning. According to [50], for the production of zinc and aluminum by means of the electrolytic reduction, the expenditure of energy do not exceed 11.5 and 61.2 MJ/kg, respectively. Since for the hydrogenation of bitumen, metals of a high purity grade are not required, the reduction is rendered possible by a low energy-consuming carborethermal method that is based on the reaction: Mx Oy + C = xM + yCO [51–53]. For the carborethermal reduction, the expenditures of energy are ≈1.5 times lower than for the electrolytic reduction. From the comparison of the ω values (Table 6) it follows that the hydrogenation of bitumen with Zn can is more preferable than with Al owing to a lower value of the expenditure of energy (ωZn /ωAl ≈ 0.6). Electric power required for the reduction of metals can be generated using the mixture of SCW with gases at the outlet from the

reactor as a working substance of a combine cycle gas turbine. Raw bitumen, when partially or completely oxidized by SCW/O2 fluids [15,16], can be used as a basic or additional fuel for power generation. Carbonaceous residue being formed by a partial oxidation of bitumen [13,14] can be used as a reducing agent for the carborethermal reduction of metals. The use of renewable sources of energy for the reduction of metals [51,52] is likely to increase the economic efficiency of the bitumen SCW conversion. The processes of bitumen conversion can be ecologically and technologically clean when using water in a supercritical state. SCW is not only a clean solvent and reagent, but it acts as a heat and mass carrier as well. Since the rate of combustion of hydrocarbons dissolved in SCW corresponds to the rate of gas-phase reactions [15,16], it is obviously possible to provide for autothermicity of the bitumen conversion in a continuous mode when burning bitumen directly in SCW. Harmful oxides and aerosols are neither formed nor emitted in closed-circuit processes at low temperatures.

Table 6 The high heating values for the conversion products and residues and the part of energy required for the reduction of zinc and aluminum. Reagents

HHVg (MJ/kg)

HHVL (MJ/kg)

HHVR (MJ/kg)

HHV˙ (MJ/kg)

ω (%)

Bitumen/SCW Bitumen/SCW/Zn Bitumen/SCW/Al

44.3 54.9 50.3

44.4 44.0 45.6

41.2 43.1 42.5

43.4 ± 0.8 46.8 ± 0.7 48.5 ± 0.7

– 16.8–25.0 28.6–42.9

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4. Conclusions In the present paper it is shown that under pumping of SCW through bulk of bitumen at 400 ◦ C, 30 MPa almost half of hydrocarbons are removed from bitumen. SCW not only partially dissolves bitumen but it can also be a hydrogen and oxygen donor, i.e. it participates in the radical reactions with bitumen component. Addition of zinc or aluminum shavings into bitumen in the amount providing for increase in H/C atomic ratio to ≈2.0 for the products and conversion residue results in twofold decrease in the weight of the conversion residue, significant increase in the degree of hydrogenation of bitumen, and changes of composition of the products. A distinctive peculiarity of the bitumen conversion with addition of Zn is the maximum yield of the liquid products with a high content of resin and a higher content of oxygen in the conversion products. It turns out that ZnO, as distinct from Al2 O3 , more actively reacts with H2 S to form ZnS providing for removal of the part of sulfur from bitumen. A distinctive feature of the bitumen conversion with addition of Al is the absence of oxygen-containing substances in the liquid products and a higher amount of oil in their composition. It implies that the processes of the SCW conversion of bitumen conjugated with the reactions (1) and (2) on the surface of Al2 O3 /Al and ZnO/Zn nanoparticles are different. The reaction (1) is accompanied by oxygen-containing resin formation and as for the reaction (2) it, on the one hand, suppresses this process and, on the other, it promotes the hydrogenation of bitumen. Rise in temperature of the reactants owing to heat evolution by the reaction (2) initiates a more extensive thermolysis of the bitumen components responsible for both intensification of the HC hydrogenation and increase in the yield of volatile products. From the practical point of view, the techniques for the bitumen SCW conversion proposed in the present paper are of great interest. For a periodical filling of the reactor (the reactor assembly unit) with bitumen and metal, the yield of the target conversion products can be optimized by means of the appropriate temperature, pressure and flow rate of the SCW pumping through the reactor to achieve uniform exhaustion of the reagents. Since the conversion residue has a high H/C atomic ratio and remains liquid during the bitumen hydrogenation at the expense of the reactions of the oxidation of metals by SCW, the schemes for a continuous supply of bitumen with metals into the reactor and drain of the conversion residue can become perspective. On the basis of the assumption that the energy consumed [50] for the reduction of zinc from oxide is lower than that of aluminum and that the melting point of zinc (Tm = 419 ◦ C [34]) is lower than that of aluminum (Tm = 660 ◦ C [34]), it follows that the usage of zinc for the bitumen hydrogenation can appear more perspective, especially for a continuous supply of the reagents into the reactor. The power required for the reduction of metals can be generated by a partial or complete oxidation of bitumen by the SCW/O2 fluids in the reactor, and the mixture of SCW with gases at the outlet from the reactor can also be used as a working substance of a combine cycle gas turbine.

Acknowledgements The authors gratefully acknowledge V.I. Zaikovskii and A.G. Cherkov for the electron-microscopic analysis of the samples, S.V. Tsybulya for the X-ray diffraction analysis of the samples, N.I. Fedorova for the technical support in the group analysis of liquid products, A.V. Shishkin for the support in the analysis of massspectra of volatile products, and M.Y. Sokol for assistance during the experiments.

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This study was supported by the Russian Foundation for Basic Research (Grant nos. 11-03-00388 and 12-08-00033) and by the Presidium of Russian Academy of Sciences (Project no. 3).

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