1. Introduction
Naphthenic acids are organic acids found in crude oil and are mainly responsible for the acidity of crude oil [
1]. Higher acidity causes corrosion to distillation columns, heat exchangers, storage tanks, and pipelines and creates the need for frequent maintenance and replacement of equipment and pipelines [
2]. Currently, the deacidification of crude oils can be achieved by two approaches: The first approach involves destroying the carboxyl group in the acid (referred to as a chemical method for naphthenic acid removal) via esterification, neutralization, and decarboxylation [
3,
4,
5,
6,
7]. The second approach involves separating the naphthenic acids from the oil for other uses (referred to as a physical method for naphthenic acid removal) via liquid–liquid extraction and adsorption [
8,
9,
10,
11,
12,
13]. Decarboxylation is the removal of carboxyl acid functional group reaction. The decarboxylation reaction follows two major steps: breaking the C-C bond and launching the carbon dioxide (CO
2). The cleavage of the C-C bond gives carbonic acid as an intermediate, which decomposes to release CO
2 [
14,
15,
16]. Through this catalytic reaction,
n-alkane of one less carbon number is produced [
17]. In our previous research, studies were carried out to determine whether the total acid number (TAN) of a typical acidic crude oil derived from Al-Fula blocks in Western Sudan might be reduced by using the HZSM-5 zeolite catalyst [
18]. The properties of Al-Fula crude oil are listed in
Table 1. The effectiveness of the catalyst at three parametric levels (reaction temperature: 250–270–300 °C, reaction time: 2–3–4 h, and oil: catalyst weight ratio: 20–22–25 g/g) was better understood using a full factorial design of the experiment (DOE) framework. The results show that the HZSM-5 zeolite catalyst removes up to 99% of naphthenic acids via the decarboxylation route. Finally, a Langmuir–Hinshelwood (LH) kinetic model was developed to allow efficient prediction of the performance of the HZSM-5 zeolite catalyst in the decarboxylation reaction [
18].
A reactor is the core of many chemical processes; it is considered a vessel whereby chemical reaction occurs as it has been designed in different shapes for this purpose. The chemical reaction converts reactants (raw material) into products. Standard reactors are classified into batch, semi-batch, and continuous according to operation type; however, other non-ideal reactors, such as recycle reactors, can also be used. If the reaction takes place in one phase, it is then classified as a homogeneous reaction, and if more than one exists, then it is classified as a heterogeneous reaction. The reactions themselves can be catalytic or non-catalytic [
19]. Heterogeneous catalytic reactions are one of the important chemical technologies used to manufacture products; they typically deal with fluid reactants (gas or liquid) in the presence of a solid catalyst [
19]. Ideal reactors are classified into three types: batch, continuous stirred tank reactor (CSTR), and plug flow reactor (PFR) [
20].
The selection and design of a catalytic reactor depend primarily on the type of process and basic process variables such as residence time, temperature, pressure, mass transfer between different phases, the properties of the reactants, and the available catalysts [
21,
22]. The most important industrial catalytic reactors are two-phase reactors for the system fluid/solid and three-phase reactors for the system gas/solid/liquid. Catalytic reactions are generally carried out in continuous fixed-bed reactors (referred to as packed bed reactors), which are modeled as plug flow reactors (PFR) and operate without back mixing. On the other hand, the ideal continuous stirred-tank reactor with complete back mixing of the reaction mass is modeled as CSTR [
23].
The practice of process simulation has developed over the last two decades in the field of chemical and engineering processes [
24]. Many simulators were used for simulating chemical processes, such as ASPEN Plus
®, ASPEN HYSYS
®, Design Pro, and CHEMCAD
TM. ASPEN Plus
® is a leading software that builds a model and simulates the model based on mass and energy and phase equilibrium; it handles several chemical processes such as chemical reactors, separation processes, recycling, bypass, and electrolytes [
25,
26].
Researchers have used ASPEN Plus
® extensively to simulate and optimize various types of process equipment and unit operations. ASPEN Plus
® Saidi et al. [
27] used the continuous stirred tank reactor (RCSTR) to simulate the copper recovery process. Using absorber models, Sampath et al. Field [
16] also developed a model to simulate CO
2 removal from coal and gas-fired power plants. Qing et al. [
28] developed a model to simulate the synthesis of nanocrystalline cellulose using the waste biomass using the RCSTR reactor model. Liu et al. [
29] developed a model to simulate preparing biochar by biomass pyrolysis using a yield reactor (RYield). To date, a limited number of studies have been carried out in catalytic decarboxylation simulations, which are summarized in
Table 2. Plazas et al. [
30] developed a model for catalytic reduction of fatty acid through hydrogenation, decarboxylation, and decarbonylation using NiMo/γAl
2O
3 catalyst ASPEN Plus
® 7.3 Software, where an equilibrium reactor was used to predict the reaction behavior. The impact of many factors affecting the process, such as reactor pressure, temperature, and H
2/oil molar ratio, was analyzed. According to their research results, the optimized conditions were obtained at a pressure of 30 bar, a temperature of 300 °C, and a H
2/oil molar ratio of 20:1.
Kin Wai Cheah et al. [
31] developed a comprehensive techno-economic assessment using ASPEN Hysys
® V8.0 software for hydro-processed diesel fuel production via catalytic decarboxylation of rubber seed oil in Malaysia. In this study, the facility was modeled to be capable of producing a total of 20 million liters of renewable diesel product per annum; moreover, sensitivity analysis was conducted to show which factors were affecting the overall profitability profile. Their results show that the production cost of renewable diesel is more sensitive to the price of rubber seed oil and the price of hydrogen gas. Natelson et al. [
17] developed a model for the techno-economic evaluation of the production of Camelina jet fuel. ASPEN Plus
® was used for material and energy balance, the evaluation of parameters that impact the process, and identifying the optimal conditions for the production process. On the other hand, ASPEN Icarus
® was used for the cost evaluation. The results illustrated the importance of the choice of refinery location near farms. Braden et al. [
32] developed a techno-economic evaluation based on the ASPEN Plus
® simulation model. Sensitivity analysis studies were conducted to determine the impact of changes in key economic and processing parameters on the minimum selling price (MSP). Results reveal feedstock and total installed equipment costs are the most sensitive process parameters. Onwudili et al. [
33] developed a hypothetical continuous process model to deliver 20,000 tonnes/year of bio propane in an ASPEN
® simulation.
However, there is still a need to develop an industrial process for the catalytic decarboxylation of naphthenic acids that can be integrated into either oil production fields or refinery plants. The reliability of refinery equipment during the processing of high-acid crude oils is critical. Blending, material upgrading, inhibition, and process control are current solutions for corrosion. Blending can be used to reduce the naphthenic acid content of the feed, resulting in acceptable corrosion. Blending heavy and light crudes can alter shear stress parameters and may aid in corrosion reduction [
34]. Upgrading materials of construction from carbon steel (CS) to metals and alloys containing chromium, nickel, and molybdenum (stainless steels), alloys based on nickel, cobalt, copper, titanium, and aluminum, polymers, and composite materials are complex tasks that necessitate a large capital investment as well as a longer execution time. Stainless steels and nickel alloys are also prone to pitting and crevice corrosion, intergranular corrosion, and stress corrosion cracking. A proper alloy selection method is based on information and experience in standards, procedures, reports, and articles. The disadvantage of this method is that environmental changes can affect alloy corrosion resistance. Corrosion mitigation with additives is an alternative to hardware changes. Corrosion at high temperatures is mitigated during naphthenic acid crude processing by injecting either phosphate-based ester additives or sulfur-based additives, which provide an adherent layer that does not corrode or erode due to the effect of naphthenic acids [
34,
35]. Therefore, the need for a permanent method for mitigating naphthenic acid corrosion includes destroying the naphthenic acids. This paper proposed a permanent method for the destruction of these acids through a catalytic decarboxylation process, in which the carboxyl group reacted to produce carbon dioxide.
Researchers have proposed many laboratory methods but scaling up the process remains challenging [
23,
24,
25,
26,
27,
28,
29,
30,
31,
32]. Moreover, the literature cited in
Table 2 did not include a simulation of potential processes for the catalytic decarboxylation of naphthenic acids, as the process is yet to be developed. The development of the simulation model for the catalytic decarboxylation of naphthenic acids has yet to be considered due to the complex nature of the process and limited information in the literature. The development of simulation models in this area will facilitate the improvement of reactor technology for the removal process and help it find its way to commercialization. This study aims to develop a simulation model of the catalytic decarboxylation of naphthenic acids over the HZSM-5 zeolite catalyst and validate the applicability of the developed model using experimental data under similar conditions to the typical process. The validated model will be used to design commercial-scale plants. The novelty of this study is that it includes the economic feasibility of the proposed technology compared with existing commercial technology for highly acidic crude oil in Sudan. The highly acidic crude oil properties are listed in
Table 1.
This paper’s first part focuses on converting the catalytic decarboxylation process pathway into the preliminary conceptual design flowsheet via the ASPEN Plus
® simulator. For the validation of the model, experimental results from a batch reactor have been used to develop a hypothetical process [
18], followed by investigating the possibility of scaling up the process and choosing the best industrial reactor for a potential commercial plant. The second part reported a comprehensive economic assessment for a feedstock of 11,000 bbl/day of acidic crude. A sensitivity analysis was then conducted to determine the dominant factors affecting the annual operating cost of the project. Non-technical investors or oil companies can use the results obtained in this study to consider the option of building a catalytic decarboxylation facility as part of the crude treatment before transportation and refining.