Calculation of the Rate Constants of Vacuum Residue Hydrogenation Reactions in the Presence of a Chrysotile/NiTi Nanocatalyst

Abstract

The paper presents the results of an investigation into the kinetics of catalytic hydrogenation of vacuum residue at temperatures of 380, 400 and 420 °C and different durations, ranging from 30 to 70 min, using a nanocatalyst containing the active metals nickel and titanium supported on chrysotile. It was found that the yield of oils from 30 to 50 wt.% and tars from 12 to 18 wt.% increased with increasing temperatures and reaction times. A slight increase in the proportion of solids in the range of 2.0 to 6.0 wt.% is explained by the activity of the nanocatalyst used. In the study of the kinetics of vacuum residue hydrogenation, using the nanocatalyst developed by the authors, we were able to achieve a low yield of solids with a short contact time as well as a high yield of low-molecular-weight compounds such as oils and tars. To determine the kinetic parameters (rate constants and activation energies), Simpson’s integral method and a random search engine optimization method were used. High values of rate constants are characteristic of reactions in the formation of oils k₁, tars k₂ and asphaltenes k₃ in the temperature range of 380–420 °C. The high values of the rate constants k₁, k₂ and k₃ in the catalytic hydrogenation of the vacuum residue indicate the high reaction rate and activity of the nanocatalyst used. With an increase in temperature from 380 to 420 °C, the rate constant of the formation of gas products from vacuum residue and the conversion of asphaltenes into oils significantly increase, which indicates the accumulation of low-molecular-weight compounds in oils. The activation energy for reactions leading to the formation of oils, tars, asphaltenes, gas and solid products was 75.7, 124.8, 40.7, 205.4 and 57.2 kJ/mol, respectively. These data indicate that the processes of vacuum residue hydrogenation with the formation of oils and asphaltenes require the lowest energy inputs. Reducing the process temperature to increase the selectivity of the vacuum residue hydrogenation process when using the prepared nanocatalyst is recommended. The formation of oils at the initial stage plays a key role in the technology of the heavy hydrocarbon feedstock (HHF) hydrogenation process. Perhaps the resulting oils can serve as an additional solvent for high-molecular-weight products such as asphaltenes, as evidenced by the low activation energy of the process.

1. Introduction

The vacuum residue is a heavy fraction that is formed after vacuum distillation of crude oil at temperatures of 500–520 °C. The share of vacuum residues can reach up to 30% of the total volume of oil refining, and this share is expected to grow [1]. Various methods are used to process heavy hydrocarbon feedstocks, including hydrocracking, thermal and catalytic cracking, and other technologies. Despite the many existing industrial methods of vacuum residue processing, the conversion depth of these processes remains insufficient. The main problem is the high concentration of high-molecular-weight components such as asphaltenes [2]. In oil and residual oil fractions, asphaltenes are in a solid state in the form of a dispersed phase, the precipitation of which is complicated by the presence of solvate shells of tars and polar heteroatomic compounds contained in the feedstock. In addition, as the viscosity of the produced oil increases, the content of asphaltenes and tars increases. These components contribute to the formation of a significant amount of solid coke-like products, deactivation of catalysts used in conversion processes, and reduction in distillate fraction yields [3]. Heteroatoms and metals mainly remain in asphaltenes and tars, which increases refining costs and leads to contamination and loss of activity of expensive catalysts [4]. In this regard, it is necessary to conduct an in-depth study of the chemical properties of asphaltenes, tars and oils in order to understand their behavior in the processes of processing heavy-oil residues. This will help to find a solution to eliminate the above-mentioned problem. There is a considerable amount of research devoted to analyzing the composition, properties and molecular structure of these components [5,6,7,8].

A large number of studies are devoted to the kinetics of the catalytic hydrogenation of vacuum residue, but the chemical aspects of these transformations remain insufficiently studied. This is due to the complexity of vacuum residue itself, the difficulty of comparing data obtained using different heterogeneous catalysts [9,10,11,12,13,14,15,16,17,18] and the need for a comprehensive analysis of the kinetics of hydrogenation of heavy hydrocarbon feedstock using the nanocatalysts. In the literature, there are few studies devoted to the kinetics of the process of vacuum residue hydrogenation with the use of nanocatalysts. In [19], the authors demonstrated the effectiveness of the nanocatalyst in the hydrocracking of vacuum residue. However, during hydrocracking, nanoparticles tend to agglomerate, forming spherical and agglomerated structures, which reduces the activity of the nanocatalyst. To reduce the agglomeration of nanoparticles in the works [20,21], chrysotile, which consists of nanotubes, was used as a catalyst carrier.

The study of the kinetics of vacuum residue hydrogenation is complicated by the following two parallel processes: the formation of low-molecular compounds (oils) due to degradation and condensation of asphaltenes and tars into solid products, which leads to the formation of coke. The solution of this problem will make it possible to acquire new knowledge about carrying out the process of catalytic hydrogenation of vacuum residue in industrial conditions. Therefore, the problem of obtaining new experimental data on the thermal destruction of asphaltenes, tars and oils in the presence of the nanocatalysts remains unsolved. Calculation of kinetic parameters (rate constants and activation energy) of catalytic hydrogenation of vacuum residue will significantly help to control the processes of formation of low-molecular compounds and reduce the formation of solid products (coke).

Differential equations are used to model the chemical kinetics of the hydrogenation of coal and coal tar as well as heavy-oil residues. In previous studies [22,23,24,25,26], when studying the kinetics of hydrogenation of heavy and solid hydrocarbon raw materials, rigid types of differential equations were used to calculate kinetic parameters. It is known that the chemical kinetics of complex reactions is characterized by the presence of rapidly and slowly changing variables. Due to the presence of reaction stages with different rates, the solution of direct kinetic problems is complicated by the rigidity of the differential equation systems describing the mechanism of these reactions [27,28,29,30]. In recent years, Runge–Kutta methods, previously considered reliable, began to cede their dominant position among algorithms for solving ordinary differential equations in favor of modified methods capable of coping with rigid problems [31,32]. To solve the problems of chemical kinetics, the method of random search optimization [33] and the Simpson’s integral method [34] were first used. Previously, these methods were used separately to solve systems of differential equations and were not used together to solve direct kinetic problems.

The purpose of this work is to establish the kinetic patterns of the vacuum residue hydrogenation in the presence of a chrysotile-based nanocatalyst modified with metals such as nickel and titanium.

2. Materials and Methods

The study was conducted on vacuum residue produced by the Pavlodar Petrochemical Plant (PPCHP). The main physical and chemical characteristics of this vacuum residue are presented in

Table 1. Physicochemical characteristics of vacuum residue (PPCHP).

Indicator	Content
Viscosity at 70 °C, cSt	262
Density, kg/m3	1.000
Atomic ratio H/C	1.95
Ash content, %	1.64
Hydrogen, %	11.8
Carbon, %	72.5
Nitrogen, %	0.9
Sulphur, %	2.6
Oxygen, %	2.3
Asphaltenes, %	9.8
Tars, %	12.5
Oils, %	78.2

. The kinematic viscosity of the initial material was determined using an automatic viscometer SYD-265B-1, and the density was measured in accordance with GOST 1481-81.

Preparation of the nanocatalyst. As a catalyst and carrier, mountain flax was used (also known as chrysotile), which is produced by Kostanay Minerals JSC, a mining company specializing in the extraction of chrysotile asbestos and the production of chrysotile fiber. The chemical formula of chrysotile is Mg₃Si₂O₅(OH)₄. Chrysotile fibers are formed from twisted layers of MgO and SiO₂ [35].

In this work, a chrysotile-based nanocatalyst with additives of active metals, such as nickel and titanium (Chrysotile/NiTi), was used. The nanocatalyst (Chrysotile/NiTi) was prepared by a wet-mixing method using drain-free technology, as follows: chrysotile was previously treated with a 20% hydrochloric acid solution to remove magnesium, potassium and other alkaline earth elements; then, a solution of nickel nitrate and titanyl sulfate was mixed with chrysotile at 100 ± 5 °C for 60 min; then, the precipitate was separated, dried at 100 ± 5 °C and calcined at 550 °C.

Physical and chemical characteristics of the prepared nanocatalyst.

X-ray spectral fluorescence, gravimetric analysis of the initial chrysotile. Using X-ray spectral fluorescence analysis on spectrometer CPM-25 (X-ray MULTICHANNEL SPECTROMETER, Laborant Industrial Group, Russia, ), the chemical composition of the initial chrysotile was determined (

Table 2. X-ray spectral fluorescence analysis of the initial chrysotile.

Defined Components	Content, %
SiO2	66.595
TiO2	0.027
Al2O3	<0.95
Fe2O3	3.641
CaO	0.270
MgO	25.555
MnO	0.033
P2O5	0.021
K2O	<0.1
Na2O	0.557
LDC	3.40

LDC—losses during calcination: H₂O, CO₂, carbon, hydrogen, partly SO₃, trace elements.

).

From Table 2, it follows that the main components of chrysotile include silicon, magnesium and iron oxides.

Transmission electron microscopy. The particle size of the deposited nickel and titanium on the surface of chrysotile nanotubes was determined using a Joel Jem-1400Plus transmission electron microscope (JEOL, Freising, Germany) at an accelerating voltage of 120 kV. Figure 1 shows that the average diameter of the nanotubes in the nanocatalyst sample is 40 nm.

In the prepared nanocatalyst, TEM revealed the presence of finely dispersed particles of nickel and titanium oxide (Figure 2a) and, based on statistical processing, a curve of the distribution of nickel particles by size was plotted (Figure 2b). Figure 2b shows that nickel particles of 6–9 nm and 29–35 nm are evenly distributed on the surface of chrysotile nanotubes, while the distribution peak falls at 12 nm, which is 28% of the total amount of adsorbed nickel and titanium metals.

Atomic emission spectral analysis of the nanocatalyst. Using an inductively coupled plasma Profile Plus atomic emission spectrometer (Teledyne Instruments Leeman Labs Inc., Hudson, NH, USA) under standard conditions (temperature 21 °C and atmospheric pressure 727 mmHg), the amount of supported metals on chrysotile was measured.

According to atomic emission spectral analysis, the content of nickel and titanium in the nanocatalyst was 5%.

Investigation of the textural characteristics of the nanocatalyst. The porous properties of the initial chrysotile and the nanocatalyst based on it, containing nickel and titanium nanolayers, were studied. For this purpose, a low-temperature nitrogen adsorption method at 77 K on an automated Digisorb-2600 Micromeritics unit was used. Prior to measurements, the sample was vacuum-treated at 350 °C for 5 h. The classical method of analysing the isotherms of polymolecular adsorption (BET method) using the instrument software was also applied. The BET analysis gave a surface area for the initial chrysotile and nanocatalyst of 39.4 and 54 m²/g, respectively.

Thermo-programmable desorption. To confirm the acidic properties of metal-supported chrysotile, a thermo-programmable ammonia desorption assay was performed. The ammonia desorption rate curve shows a pronounced peak at 200 °C, which indicates the presence of acid centers at a concentration of 267 μmol/g.

Vacuum residue hydrogenation. The vacuum residue hydrogenation experiments were carried out in a 0.3 L high-pressure reactor with an internal stirrer (HXCHEM, Shanghaiha, China). The starting materials (vacuum residue and nanocatalyst) were premixed and placed in the reactor. The mass fraction of the nanocatalyst was 1.0% of the initial fraction. The reactor was closed and filled with hydrogen at an overpressure of 4.0 MPa. The temperature in the reactor was then raised at a rate of 10 °C/min to a predetermined value and held there for the specified time. The reactor was then cooled to room temperature. After 24 h, the reactor was opened, the solid product was separated from the liquid and the solid hydrogenation product was thoroughly washed with benzene.

Determination of component composition. To isolate the asphaltenes, the sample was diluted with a 40-fold volume of hexane and allowed to stand for a day, and then the precipitate was filtered off. The resulting precipitate was placed in a paper cartridge and washed with hexane in a Soxhlet apparatus to remove oils and tars, after which the asphaltenes were removed from the cartridge with chloroform. Deasphalted samples were applied to activated coarse silica gel, hydrocarbon components (oils) were sequentially extracted using hexane in a Soxhlet apparatus and the tars were isolated with a 1:1 mixture of benzene and ethanol [36].

Method for determining kinetic parameters of vacuum residue hydrogenation. To determine the kinetic parameters (rate constant and activation energy), the Simpson’s integral method and the random search method were used [33,34]. The solution of the differential equations system is carried out using the Simpson’s method, which allows us to calculate rate constants with a calculation error of up to 12%, and further calculation using the random search optimization method allows us to reduce the discrepancy between the obtained calculated data and the experimental values of the concentrations of the obtained products with an accuracy of 5%. As part of the study, a formalized mechanism of mutual transformations of asphaltenes, tars, oils, gas, and vacuum residue product in the hydrogenation process was developed (Figure 3).

When developing the scheme, it was assumed that, during the hydrogenation of vacuum residue, direct and parallel reactions of decay and condensation of its main components occur. Based on this mechanism, the following kinetic model of the vacuum residue hydrogenation process was developed:

$${\displaystyle \frac{d{C}_1}{d\tau }}=-\left(k_1+k_1+k_3+k_4+k_5\right)C_1,$$

(1)

$${\displaystyle \frac{d{C}_2}{d\tau }}=k_1{C}_1+k_6{C}_3+k_7{C}_4,$$

(2)

$${\displaystyle \frac{d{C}_3}{d\tau }}=k_2{C}_1-k_6{C}_3-k_8{C}_3,$$

(3)

$${\displaystyle \frac{d{C}_4}{d\tau }}=k_3{C}_1-k_7{C}_4,$$

(4)

$${\displaystyle \frac{d{C}_5}{d\tau }}=k_4{C}_1+k_8{C}_3,$$

(5)

$${\displaystyle \frac{d{C}_6}{d\tau }}=k_5{C}_1.$$

(6)

where, C₁, C₂, C₃, C₄, C₅ and C₆—mass fractions of vacuum residue, oil, tar, asphaltene, gas and solid product (coke); τ—process duration, min; k₁, k₂, k₃, k₄ and k₅-transformation of vacuum residue into oils, tars, asphaltenes, gas and solid products; k₆ and k₇—reactions from vacuum residue and asphaltene transformations into oils; k₈—reaction of vacuum residue transformation into gas products.

3. Results and Discussion

To determine the kinetic parameters of the catalytic hydrogenation of vacuum residue, experiments were carried out at temperatures of 380, 400 and 420 °C with durations from 30 to 70 min (

Table 3. Component composition of catalytic vacuum residue hydrogenation products.

T, K	τ, min	Content, wt.%
		VR, C1	Oil, C2	Tar, C3	Asphaltene, C4	Gas, C5	Coke, C6
653	30	44.7	30.1	11.7	2.7	8.8	2.0
	40	37.5	34.5	12.3	3.3	9.0	3.4
	50	30.0	40.3	12.5	3.5	9.2	4.5
	60	26.0	41.6	14.7	3.7	9.4	4.6
	70	23.2	43.0	15.5	4.0	9.5	4.8
673	30	37.8	37.1	13.5	4.2	5.2	2.2
	40	28.6	41.1	14.2	4.6	7.4	4.1
	50	22.2	45.1	14.9	4.8	8.5	4.5
	60	17.7	46.0	16.2	5.2	9.8	5.1
	70	12.9	48.2	17.1	5.5	10.8	5.5
693	30	24.1	42.1	15.1	5.7	8.6	4.4
	40	10.2	47.1	16.0	7.9	13.3	5.5
	50	6.2	49.5	17.1	8.0	13.5	5.7
	60	4.6	50.2	17.4	8.3	13.7	5.8
	70	3.6	50.5	17.6	8.4	14.0	5.9

). During hydrogenation, the destruction of tars and asphaltenes is observed, which increases the yield of oils, gases and solids.

With increasing temperatures and durations, an increase in the yield of oils from 30 to 50 wt.% and tars from 12 to 18 wt.%, respectively, was observed. The slight increase in solid products under the given conditions from 2.0 to 6.0 wt.% seems to be due to the activity of the prepared nanocatalyst (Chrysotile/NiTi).

Comparison of experimental data on the hydrocracking kinetics of vacuum residues without a catalyst and in the presence of a catalyst obtained in [2], where vacuum residues from different refineries were used without a catalyst, the yield of solid product varied from 19 to 38 wt.%, and, when the nanocatalyst was used, the yield of solid product decreased by 3–3.5 times, which shows the high activity of the nanocatalyst.

However, analysis of the literature has shown that the low yield of solids is associated with the duration of the process (72 h) [9]. By investigating the kinetics of vacuum residue hydrogenation in the presence of the nanocatalyst we prepared, a low solid yield was achieved with a short contact time, as well as a high yield of low-molecular-weight compounds such as oils and tars (Table 3).

Similarly, an increase in gas yield from 9 to 14% and asphaltenes from 3 to 8.4% is observed at the given temperatures and contact time duration. Increasing the duration and temperature of catalytic hydrogenation of vacuum residue contributes to a slight increase in the yield of new asphalt-containing substances, which is associated with the condensation reaction of asphaltenes and leads to the formation of solid products. The increase in the yield of gaseous products is due to catalytic destruction of vacuum residue.

Based on the above experimental data, a scheme of the vacuum residue hydrogenation in the presence of the nanocatalyst is proposed (Figure 2). Based on the kinetic scheme, a kinetic model of vacuum residue hydrogenation was compiled and rate constants were calculated.

Table 4. Kinetic parameters of the vacuum residue hydrogenation process.

T, °C	Rate Constants, × 10−2 min−1
	k1	k2	k3	k4	k5	k6	k7	k8
380	0.53	0.38	0.48	0.06	0.19	0.10	0.07	0.03
400	0.84	0.67	0.57	0.29	0.27	0.22	0.19	0.13
420	1.20	1.47	0.74	0.60	0.36	0.33	1.10	0.95
E, kJ/mol	75.7	124.8	40.7	205.4	57.2	110.5	254.5	310.8

shows the calculated values of the rate constants and the activation energies of the vacuum residue hydrogenation reaction.

High values of rate constants (Table 4) are observed for the reaction of formation of oils (k₁ = 0.53·10⁻²), resins (k₂ = 0.38·10⁻²) and asphaltenes (k₃ = 0.48·10⁻²) at a temperature of 380 °C. Apparently, the high values of the rate constants k₁, k₂, k₃ during the catalytic hydrogenation of vacuum residue indicate a high rate of the hydrogenation reaction and the activity of the prepared nanocatalyst. With an increase in temperature to 420 °C, the values of the rates constants during the formation of gas products from vacuum residue (k₄ = 0.60·10⁻²) and the reaction of converting asphaltenes to oils (k₇ = 1.10·10⁻²) sharply increase, which indicates the accumulation of low-molecular-weight compounds in oils, which is consistent with the data presented in [36]. As the temperature increases, there is a slight increase in the rate constants from 0.10·10⁻² to 33·10⁻² min⁻¹ for the conversion of tars to oils (k₆) and a sharp increase in the rate constants for the conversion of tars to gas products from 0.03·10⁻² to 0.95·10⁻² min⁻¹ (k₈), which is apparently due to the formation of a low-molecular-weight part of the products. Low values of rate constants from 0.19·10⁻² to 0.36·10⁻² min⁻¹ are characteristic of the reaction of solid formation (k₅), which once again emphasizes the high activity and selectivity of the nanocatalyst. Furthermore, it should be noted that the nanocatalyst used has an advantage over known nanocatalyzers which undergo agglomeration. Figure 1 and Figure 2 show that the prepared nanocatalyst does not undergo agglomeration, since metal nanoparticles are adsorbed on the surface and inside nanotubes with dimensions of 5–40 nm, which complicates the process of agglomeration of nickel and titanium nanoparticles.

To determine the activation energy of the main chemical transformations in the process of the vacuum residue hydrogenation, the dependences of reaction constants in the Arrhenius coordinates were plotted (Figure 3).

Figure 4 shows a graphical display of the dependence of the logarithm of the constant rate of conversion of the vacuum residue to oils with a reverse in temperature. Using the method of least squares in the angle of inclination on the line in the Arrhenius coordinates, the values of the activation energy of the process of converting the vacuum residue into components were calculated. The activation energies for the oil, tar, asphaltene, gas and solid formation reactions are presented in Table 4 and are 75.7, 124.8, 40.7, 205.4 and 57.2 kJ/mol, respectively. From the data obtained, it follows that the processes of the vacuum residue hydrogenation with the formation of oils and asphaltenes require the lowest activation energy. High values of activation energy are characteristic of reactions during the formation of tar (124.8 kJ/mol) and gas products from vacuum residue (205.4 kJ/mol), as well as the formation of oils from tars and asphaltenes (110.5 and 254.5 kJ/mol) and the conversion of tars into gas products (310.8 kJ/mol). It follows from these data that the processes of destruction of the organic mass of the vacuum residue with the formation of oils and asphaltenes (75.7 and 40.7 kJ/mol) are the least difficult energetically. Therefore, to increase the selectivity of the catalytic vacuum residue hydrogenation process, the lower process temperature should be limited. It is believed that the preferential formation of oils at the initial stage is important for the technology of the HHF hydrogenation process, since the formation of oils is an additional solvent for high-molecular-weight products such as asphaltenes, which confirms the low activation energy.

4. Conclusions

• It was found that finely dispersed particles of nickel oxide and titanium were present in the prepared nanocatalyst, and a particle size distribution curve of these metals was also plotted. The photomicrograph shows that metal particles of 6–9 nm and 29–35 nm are uniformly distributed on the surface of chrysotile nanotubes, with the maximum distribution being observed at 12 nm, which is 28% of the total amount of adsorbed metals. The difference of this nanocatalyst from known ones lies in the fact that the deposited nickel and titanium oxides do not agglomerate, which is explained by the adsorption of nanoparticles both on the surface and inside the nanotubes.
• A study of the kinetics of vacuum residue hydrogenation showed that the composition of high-molecular-weight compounds such as tars and asphaltenes significantly affects the material balance of the process. It was found that, with increasing temperatures and reaction times, an increase in the yield of oils from 30 to 50 wt.% and tars from 12 to 18% by weight was observed, respectively. The slight increase in the proportion of solids from 2.0 to 6.0 wt.% under these conditions is probably due to the activity of the nanocatalyst used (Chrysotile/NiTi). In the early stages of vacuum residue hydrogenation, the preferential formation of oils is of key importance for heavy-oil feedstock processing technology, since these oils serve as additional hydrogen donors for the formation of low-molecular-weight compounds and prevent precipitation of high-molecular-weight compounds such as asphaltenes and tars on the reactor walls.
• Using the Simpson’s integral method and the random search optimization method, the rate constants of the catalytic hydrogenation of vacuum residue were calculated. High values of constants of speed are observed for the reaction in the formation of oils (k₁ = 0.53·10⁻²), tars (k₂ = 0.38·10⁻²) and asphaltenes (k₃ = 0.48·10⁻²) at a temperature of 380 °C. These high values of constants indicate the rapid progress of the hydrogenation reaction and the high activity of the nanocatalyst used. When the temperature rises to 420 °C, there is an increase in the rate constants for the formation of gas products from vacuum residue (k₄ = 0.60·10⁻²) and for the reaction of converting asphaltenes to oils (k₇ = 1.10·10⁻²), which indicates the accumulation of low-molecular-weight compounds in oils. The low values of speed constants for the reaction in the formation of solid products (k₅), varying from 0.19·10⁻² up to 0.36·10⁻² min⁻¹, demonstrate the high activity and selectivity of the nanocatalyst. Calculations have shown that the value of the rate constants for the formation of oils, tars and asphaltenes depends not only on the initial concentration of components of heavy petroleum residues but also on the activity and selectivity of the nanocatalyst used.
• Based on the obtained data on the kinetics of vacuum residue hydrogenation, activation energy values were calculated for the conversion of the vacuum residue into various components. The activation energy for the formation reactions of oils, tars, asphaltenes, gases and solids are 75.7, 124.8, 40.7, 205.4 and 57.2 kJ/mol, respectively. The highest values of activation energy are observed for the reactions during the formation of tars (124.8 kJ/mol) and gas products from vacuum residue (205.4 kJ/mol), as well as for the transformation of tars into gas products (310.8 kJ/mol) and the formation of oils from resins and asphaltenes (110.5 and 254.5 kJ/mol). The processes of destruction of the organic mass of vacuum residue with the formation of oils and asphaltenes (75.7 and 40.7 kJ/mol) are the least energy-consuming. Therefore, lower heating temperatures are recommended to increase the selectivity of the catalytic hydrogenation of the vacuum residue.
• The obtained kinetic data can be used in the design of the reactor and the selection of active and selective catalysts for the destructive hydrogenation process of HHF; in addition, it will enable researchers to study the mechanism of conversion of HHF to low-molecular-weight compounds.