On Kinetics of Upgrading Reactions by Supercritical Water Technology of Highly Sour Qayara Crude Oil over an Activated Carbon-Based Catalyst

Currently clean energy and zero emission fuel is a priority as there is a growing level of pollutants in air, sea and soil. Also, the conventional Iraqi crude oil is going to deplete according to the statistical analysis. Thus, methods of upgrading should attract attention in Iraqi fuel sector. The method of supercritical fluids (SCF) is one of the advanced approaches to upgrade the unconventional crude oil and removal of high levels of sulfur compounds. The present work aims at developing a kinetic model for upgrading reactions by supercritical water technology of a sour Iraqi crude oil. This aim was achieved via conducting sets of experiments in a hydrothermal autoclave reactor over a cobalt/activated carbon catalyst. The AC was used as a support for Co as an active metal. A set of upgrading kinetics experiments were applied at different temperatures (290-350 °C) and reaction times (0-45 min). Upon evaluation of the prepared catalysts for kinetics of upgrading by supercritical water technology, it was found that the process flows pseudo first order mechanism. Also, the activation energy of the chemical reaction was found to be 204.1 kJ/mol which is much less compared to previous studies.

particularly heavy oils (bitumen, asphaltene, etc.) and residues, to fulfill the growing demand for high-quality motor fuels and petrochemical materials (coal tar, residual oil, etc.), which contain a high amount of heteroatoms, a low H/C atomic ratio, and a high viscosity (sulfur, nitrogen, nickel, etc.) [2]. Modern heavy-oil upgrading solutions are centered on increasing the hydrogen content or reducing the carbon content of the oil to improve the H/C ratio [3,4].
Furthermore, neither the products of the distillation column nor the products of other upgrading procedures may be employed directly for commercial reasons [5,6]. Heavy trace metals, particularly sulfur compounds, tend to concentrate in heavy bottom products, which are used as feedstock in upgrading operations. As a result, further operations such as hydrotreating are required before the fuels may be sold to fulfill environmental laws [7][8][9]. However, hydroconversion procedures need a lot of hydrogen, and hydrogen generating units are one of the refineries' most expensive activities [2,9,10]. As a result, new heavy oil upgrading procedures with minimal or no hydrogen consumption, enhanced heteroatom removal, greater conversion (i.e. higher light liquid oil output), and suppressed coke deposits are needed [11][12][13].
The use of supercritical water (SCW) in upgrading technologies might allow for the external addition of a large amount of hydrogen at a low cost. Supercritical water (at 375°C and 22.1MPa) is particularly successful in converting bitumen-derived asphaltenes to lighter end products in recent research [14]. The SCW has a high reaction activity and may be employed as an acid/base catalyst to regulate the pressure field and engage in chemical reactions [15]. Date stones might be regarded as one of the greatest candidates among agricultural wastes for the synthesis of activated carbon that can be used as a support of the catalyst as they are inexpensive and plentiful, especially in Mediterranean nations. The use of such a catalyst is determined by the possibility of application as a kinetic reaction [15,16].
Galarraga et al. [17] studied the catalytic upgrading of heavy fractions at extreme conditions (400 °C and 6 MPa). They determined the kinetics of the reactions using dispersed NiWMo catalysts. They aimed to determine the kinetics rate law within the range of 320-380 °C and prolonged reaction times up to 70 h. For this study, they used a batch reactor with stirring with hydrogenation at 3.45 MPa. They found that the upgrading reactions followed first-order kinetics and the activation energy was 200 kJ mol -1 . Kang et al. [18] conducted a study on upgrading crude oil upgrading using supercritical methanol in a batch reactor. To understand the reaction mechanism of the technology they worked on developing a kinetic model at conditions of 653-693 K and reaction time up to 120 min. They showed that the cracking of asphaltene is the highly determined reaction among the other upgrading reaction network components.
Chang et al. [19] conducted a study on evaluating upgrading kinetics. They used lumped parameters for the evaluation and especially marcokinetics. The feedstock of the study was coal tar and the upgrading was conducted at supercritical conditions for xylene. They evaluated the different study parameters such as yield that was studied at different reaction times, the pressure of hydrogen, and the solvent (xylene)-to-feedstock ratio. They obtained that the reaction is first order and can be conducted at 380°C with high productivity.
Tan et al. [20] studied the supercritical water (SCW) in terms of upgrading reaction kinetics that occurred between the upgrading in the SCW and oil phases. They concluded that the reaction of pyrolysis can be conducted much faster at the SCW conditions based on comparison with the upgrading of the heavy oil regularly in an oil phase. Despite the faster rate of reaction, it was found that the rate of reaction was retarded as mixing was increased. To obtain the kinetic parameters, the lumping approach with four nodes was used. The decomposition of asphaltenes was found to be influenced by the mass transfer promoted by the uniform heating and pressurizing conditions provided by the SCW method. The present work aims to develop a kinetic model of a supercritical water upgrading of a highly sour Iraqi heavy oil using an AC-based catalyst under mild operating conditions.

Materials
The heavy crude oil used in the present study was a highly sour Iraqi crude oil obtained from the Qayyarah reservoir. The characteristics of the feedstock are summarized in Table (1). For purging of the hydrothermal reactor, nitrogen gas was provided from a local supplier at 99.9999% purity. It was also used for providing an inert atmosphere for calcination of the prepared carbon from the seeds to obtain the desired porosity. This atmosphere was provided to avoid the combustion of the carbon inside the furnace. For the preparation of the AC support from the Iraqi date palm seeds, several chemicals were used. Table (2) summarises these chemicals.

Table (2) Ingredients of the AC preparation solutions
Material Assay % Iraqi date palm seeds Agricultural waste Solution of phosphoric acid 85% Double distilled water -To impregnate the prepared AC with cobalt metal element, a wet impregnation method was used and there were different chemicals required for the preparation, Table (3) shows these chemicals.

Catalyst preparation
For the preparation of the Co/AC catalyst several steps were conducted;

Preparation of support
The support used in the present study was a synthetic AC based on agricultural waste, Iraqi date palm seeds:  Iraqi date palm seeds (DPS) collected from a local supermarket were cleaned by washing with tap water and deionized water. The clean DPS were dried in an oven  for soaking 100 g of the DPS. It was previously found that the optimal ratio of impregnation of the seeds with phosphoric acid is 2: 1 (acid: seeds) to digest the cellulose in the agricultural seeds [21][22][23]. Soaking of the dried DPS in phosphoric acid solution occurred with agitation over a hot plate stirrer (JISICO, Korea) at 75°C for 4 h.
 The stirring was conducted inside a fuming hood to get rid of the impregnation gases that evolved during soaking.
 The soaked sample was then placed inside the oven for 24 h at 120 °C.
 The dried sample was moved to a tubular furnace (SafeTherm, China) for activation and generation of the porous texture. The activation was completed via heating from room temperature to 500 ºC with a heating ramp of 4 ºC min -1 . The sample was left at that temperature for 2 h with the continuous blowing of nitrogen gas to avoid combustion of the carbon. After the 2 hrs, the sample was left to return to room temperature at the same rate of 4 °C per min. It was then taken out of the furnace and rinsed with ultrapure water to neutralize the sample to a pH of 7.

Impregnation with cobalt
Cobalt was chosen as an active metal element for upgrading reaction in the present study. To load 6% Co over the AC, the basic Incipient Wetness Impregnation (IWI) method was utilized.
The step of impregnation was as follows: 1-A salt of cobalt chloride (CoCl2) was used as a precursor for the preparation of the impregnation solution. 7.5 g of the salt was completely dissolved in 50 ml of deionized water. The calcinated sample was then tagged as Co/AC catalyst sample and was placed in a laboratory desiccator for dry use.

Experimental setup
A hydrothermal autoclave reactor was used in the present study to evaluate the prepared catalysts for upgrading heavy oil. The specifications of the autoclave reactor are shown in Table   ( 4). It consists of a 316 stainless steel microreactor placed inside a heating chamber (furnace).
A picture and a schematic diagram are shown in Figure (2).

Running of experiments
The experimental runs planned in Table (5) were conducted to determine kinetic parameters of the upgrading reaction over the prepared Co/AC for upgrading of the heavy oil in the hydrothermal autoclave reactor. Also, these experimental runs aim to examine the lifetime of the prepared Co/AC catalysts against deactivation reaction. The experiments were conducted as follows: 1. 24 ml of the feedstock, 12 ml of distilled water, and 1.5 gm of the catalyst were poured into the reactor vessel shown in Figure (3. a). The reactor was sealed with screws and high-temperature and pressure gaskets. Then the reactor vessel was placed in the heating furnace chamber of the reactor system while maintaining all valves closed and the reactor furnace off.
2. The reactor furnace was set at the desired temperature via the temperature controller (Autonics TK4 s series, USA) for the desired time of reaction according to the matrix of the experiment shown in Table 6. 4. Due to the synergetic effect between temperature and pressure in the hydrothermal processes, the pressure was spontaneously raised with raising the reaction temperature and the operating pressure was observed on the pressure gauge shown in Figure (3) and recorded accordingly. The maximum operating pressure was obtained upon raising the operating temperature to 350°C as it approached 235 bar.

Journal of Petroleum Research and Studies
5. At the end of each run, the reactor furnace was set to OFF according to a controlling cycle that returns the reactor furnace temperature to room temperature at a rate of 10°C/min. 6. The reactor vessel was taken out of the furnace chamber and after approaching atmospheric pressure the vessel was unsealed with a screwdriver and the products were poured into a sample vial to prepare for analysis.

The reactor vessel was rinsed with ethanol solution and deionized water and left to dry
to prepare for the next run.

Product analysis
Sulfur contents are measured by X-Ray Sulfur Analyzer (the specifications are shown in Table ( 6). The method of sulfur concentration determination is based on energy-dispersive Xray fluorescence with selective filters according to the ASTM D2622 standard [24].

Catalyst characterization
Atomic absorption spectrophotometers (Shimadzu AA6200, Japan) were used to examine the Co content in the prepared catalyst. Scanning electron microscope Electron Diffraction X-ray (EDX) (SEM) (CHL-SEM60/150, China) was used to characterize the surface morphology. The multipoint surface area was determined using the nitrogen adsorption method in Quantachrome cooperation, Autosorb, Oxford, UK).

Kinetic upgrading reactions:
Due to the complexity of upgrading reaction, desulfurization will be considered here as the main reaction being analyzed for the kinetics parameters. Based on the power law model: For n= 1 ln (1-X) = -kt or -ln (1-X) = kt (3)

Catalyst characterization
The characterization of the Co/AC catalyst showed that the BET surface area was 409.385 m²/g.
For the pore volume, it was found that it was 0.521 cm 3 /g, 0.452 cm 3 /g for the microporous volume, and 0.069 cm 3 /g for the mesoporous volume. Thus, it can be observed that the prepared AC is mostly microporous. Also, the pore diameter was found to be 1.685 nm that confirming the dominance of the microporous size of the pore developed [25] upon activation of the seeds and calcination of the carbon prepared. Figure (3) shows the EDX spectrum of the Co/AC catalyst. The spectrum obtained shows the surface elements of the prepared catalyst. It can be seen that Co was found at 3.7%, and the other elements such as P (3.1%), Cl (0.9%), Fe (0.3%), and O (12.2%) are present in different percentages due to the use of solvents such as H3PO4 and metal oxides those were used for catalyst preparation. In addition to the components of the seeds [26,27].

Conclusions
An unconventional crude oil, highly sour crude oil, was upgraded using an agricultural wastebased catalyst in the present study. The prepared catalyst was cobalt supported on activated carbon which showed a high surface area, microporous texture, and uniform distribution of the metal element within the activated carbon matrix. Also, it was found that the particle size distribution of the prepared catalyst was narrow as most of the particles possess almost the same size. The crude oil was upgraded in a supercritical water environment at various temperatures