1. Introduction
Benzene was one of the high-octane ingredients in gasoline and an essential chemical. However, it is strictly prohibited in various countries due to its carcinogenicity, high vapor pressure, and other drawbacks. The United States MSAT II specification, in particular, stipulates that the benzene content of gasoline shall not be greater than 0.62 vol% [
1]. According to the European Union’s European VI standard, there must be no more than 1.0 vol% benzene in gasoline [
2]. China VI vehicle emission standard [
3], which is stricter than China V vehicle emission standards [
4], was released in 2016 in response to the call for global green environmental protection. It stipulates that the amount of benzene in gasoline shall not surpass 0.8 vol%.
Catalytic cracking gasoline, reforming gasoline, and hydrocracking gasoline are the three significant gasoline sources. Most benzene derives from reforming gasoline. Hence, removing benzene and benzene precursors from reforming feedstock as well as benzene from gasoline reforming, along with choosing the suitable reforming feedstock and reforming operation plan [
5], is the primary way to reduce benzene in gasoline.
Alkylating [
6,
7,
8,
9,
10], hydrogenating [
11,
12,
13], and etherificating [
14] procedures are the principal processes used in gasoline benzene reduction technologies. Nevertheless, the three processes indirectly affect the octane number and yield of gasoline. At present, it is a challenge to balance energy, yield, and octane number during the benzene reduction process. We have to resolve this issue in a more elegant way. Catalytic cracking gasoline accounts for 80% of Chinese gasoline production [
15]. As a result, we suggested a novel benzene reduction procedure, as depicted in
Figure 1.
To be specific, distillation is used in this procedure to separate the catalytic cracking gasoline into a benzine-rich fraction and a benzine-low fraction. The aromatics extraction unit [
16] combines the reforming fraction with the benzene-rich fraction. The same procedure can be repeated by adding non-aromatic components to naphtha before going into the reformer. This method is consistent with most national circumstances accounted for by catalytic cracking gasoline, which can recover benzene, a substance vital to the catalytic breaking of gasoline. In addition, we can make ethanol gasoline by adding some ethanol to gasoline, which satisfies national standards for benzene content.
Ethanol gasoline can significantly lower emissions of carbon monoxide, hydrocarbons, and other key pollutants [
17] while also enhancing the performance and quality of oil products. The initial generation of bioethanol is mostly made from cane, corn, wheat, and other raw materials in an effort to find renewable energy, the second and third generations of which are primarily made from plants, trees, marine plants, and other raw materials. In recent years, biomass, which can be further broken down into lignocellulosic and vegetable types, has also been regarded as a significant source of bioethanol [
18]. The methods of ethanol refining mainly include pressure swing adsorption [
19,
20,
21] and extractive distillation [
18,
22].
In chemical industrial applications, most optimization problems are nonlinear. One of the best ways to resolve constrained nonlinear optimization issues is to use the SQP algorithm, which can be used to solve any objective function optimization problem with nonlinear equality and inequality constraints. The fundamental concept is to transform the SQP optimization model into a QP issue while minimising the constraints. Rotating shaft operation produces the positive constraint set, which converts the nonlinear issue into a collection of linear equations [
23]. The advanced correction direction is generated by using the positive constraint set, which is then iteratively updated in accordance with the correction direction. When the convergence requirements of the algorithm, the optimization model, and the chemical process constraint requirements are satisfied, the calculation is complete [
24].
Herein, we simulate the stringent rectification of catalytic cracking gasoline with minimal benzene by using Aspen Plus V9.0 process simulation software. First, the sensitivity analysis of key parameters of the model is carried out to ensure the continuity of values within the optimization range. Then, on the basis of sensitivity analysis, the SQP algorithm is used to optimize the key parameters to ensure the maximum yield of gasoline with benzene content less than or equal to 0.8%. The yield of qualified gasoline and energy consumption of the unit under different benzene content are analyzed. Finally, the economic analysis of the three processes and processes with different benzene content is carried out, and a direction for future development is identified.
4. Comprehensive Comparison
Industrialization is mainly determined by the catalytic cracking gasoline distillation’s economic effectiveness in reducing benzene. While the refinery primarily produces high-temperature and high-pressure steam through CFB boilers and sends it to the needed unit through the pressure reducer and attemperator, the unit’s reboiler uses medium-pressure steam as its heat source. Since low-quality coal serves as the primary fuel for CFB boilers, the heat load of a reboiler can be translated into the heat value of standard coal. The economic accounting is displayed in
Table 5 in accordance with the internal accounting in the refinery and the Chinese energy calculation standard [
36].
Therefore, the economic accountings of the divided wall distillation model, single-column distillation model, and double-column distillation model are carried out under the calculation criterion in
Table 5, as illustrated in
Figure 10.
The profit of recyclable benzene gradually rises as the benzene content in feedstocks grows, according to the information in the figure. When the benzene content exceeds 1.3 wt% for the divided wall distillation and single-column distillation processes, the benzene recovery profit even surpasses the public cost. On the other hand, the double-column distillation scheme grows with the public cost. In this scheme, benzene has the highest enrichment rate of the three models, but from an economic perspective, further optimization is required to achieve the prospect of industrialization.
5. Conclusions
Three distillation processes reduce the amount of benzene in catalytic cracking gasoline. The benzene enriched is recovered by extraction unit based on the benzene content in gasoline meeting the national norm, and the following conclusions are drawn.
(1) In comparison to the single-column distillation model, the divided wall distillation model and the double-column distillation model have more free variables. By regulating more factors, a higher benzene enrichment rate and gasoline yield can be achieved. However, double-column distillation needs higher energy consumption, and divided wall distillation needs rather complicated column equipment.
(2) Economic accountings reveal that the profit from benzene recovery compensates for or even exceeds the public cost for the divided wall distillation model and the single-column model as the benzene content in feedstock increases. Even with the single-column model, the qualified gasoline yield may reach a commendable 98.23 wt% when the benzene content in the feedstock is 1.5 wt%. Moreover, neither the overall ASTM D86 parameters nor the octane number of gasoline would be adversely impacted.
In other words, even though the optimization does not conduct a global search, there is a chance that the algorithm will locally converge to the maximum value. We can continue demonstrating the viability of distilling the benzene reduction process to produce the qualified gasoline. It offers theoretical guidance for businesses to perform pertinent optimization.