Exploring the Green Synthesis Process of 2-Mercaptobenzothiazole for Industrial Production

Abstract

This study outlines a high-yield green method for synthesizing MBT using aniline, carbon disulfide and sulfur as raw materials via a one-step reaction combined with high–low-temperature extraction. The process is supported by experimental results and lab-scale tests, and the operating conditions of the amplification process are evaluated using Aspen Plus simulation software, supplemented with Gaussian09 calculations. The sensitivity analysis results indicate that the MBT yield reaches its maximum value when the feed mass ratio of S:CS₂:C₆H₇N:C₇H₈ is 6:17:20:90. Additionally, setting the reaction temperature to 240 °C and pressure to 10 MPa improves the MBT synthesis yield from 58% to 82.5%. Optimal condensation and extraction conditions are achieved at −30 °C and 1 atm, followed by a separation step at 40 °C. The simulation results provide valuable guidance for the industrial production of MBT.

1. Introduction

2-mercaptobenzothiazole (abbreviated as MBT) was first discovered by A. W. Hofmann when he prepared a disulfhydryl derivative of thiocarbanilide. As an early application, MBT was used as a vulcanization accelerator in the rubber industry by Bedford, Sebrell, Bruni and Romani [1]. Vulcanization accelerators can significantly shorten the vulcanization time of raw rubber at a moderate temperature. Considering the aging resistance of rubber articles, an accelerator (such as MBT) can ensure sufficient cross-linking of sulfur and rubber and reduce undesirable side reactions [2,3,4,5,6]. As studies focusing on MBT have increased, other scholars have discovered that MBT displays a high degree of corrosion inhibition by forming a thin protective film on metal surfaces, particularly copper, lead, aluminum and its alloys [7,8,9,10,11,12]. Meanwhile, the formation of specific-colored metal–MBT complexes in aqueous media has provided an important indicator for the spectrophotometric determination of copper, selenium, osmium, palladium and cadmium [13,14,15]. In addition to its previously mentioned applications, MBT is an important scaffold that is known to be associated with a wide range of biological activities. Derivatives of MBT are known to possess, among other traits, antimicrobial, antimycobacterial, antioxidant, radioprotective, fungicidal and antitumor qualities [16,17]. Although varied aspects of MBT have been presented in the literature, the synthesis mechanism of MBT and how to realize the industrial production of MBT through green and clean methods has rarely been reported.

At present, thanks to the research conducted by scholars from various countries, there are numerous methods available for synthesizing MBT. In 1887, A. W. Hofmann found that 2-aminothiophenol or its disulfide can produce MBT when mixed with carbon disulfide for 12~14 h, which can allow about a 45% yield of MBT to be acquired [1,13]. Through this method, as high as 60% yields were obtained by L. B. Sebrell and C. E. Boor [1]. Based on Hofmann’s research, many methods for MBT synthesis from 2-aminothiophenol under certain conditions were reported [18,19,20]. Later, an MBT preparation method with a high yield and shorter reaction time was discovered by Dunbrook and Zimmermann, which was similar to the research of Jan Teppema and L. B. Sebrell. In the experiment of Dunbrook and Zimmermann, MBT was processed via the one-pot reduction and cyclization reaction of 2-chloronitrobenzene using sodium polysulfide and carbon disulfide into an aqueous solution of 2-cholonitrobenzene and sodium sulfide. An MBT yield of up to 87.5% can be achieved, and o-chloroaniline can also be generated as a by-product in the reaction [21,22]. In Jan Teppema and L. B. Sebrell’s study, the synthesis method instead produced sodium polysulfide via a stream of saturated H₂S [23]. Sebrell and Boord also outlined a reaction method to prepare MBT that utilized aniline and sulfur and carbon disulfide reagents. Under high temperature and pressures, these reagents can interact to directly obtain MBT, with a yield of about 77% [13]. When a catalyst (e.g., iodine, hydrochloric acid, aluminum chloride, etc.) is added to this reaction, an MBT yield of up to 90% can be achieved [24,25,26].

In spite of MBT’s various synthesis pathways, only the aniline method (using aniline as the main raw material) has been utilized in industrial production. The reaction principle is shown in Scheme 1. In this reaction, more than 50 by-products were identified by Isaacs and colleagues [27]. The tautomeric form of MBT (benzothiazoline-2-thione) is one of the by-products that can accompany the production of MBT. Koyama D. and Orr-Ewing A. J. demonstrated that the thione compounds’ form is strongly favored in both the gas and liquid (such as water) phases at room temperature [28]. In the presence of sulfur, benzothiazole (C₇H₅NS) was obtained as a by-product in the thermal degradation process of MBT; the mechanism of this is shown in Scheme 2 [27]. Louis Mina and colleagues thoroughly investigated C₇H₅NS as a by-product and found a likely equilibrium between C₇H₅NS and MBT [29]. In accordance with this discovery, MBT aniline method by-products may be incorporated as a starting material or recycled to enhance the yield of MBT. Moreover, in the main reaction, because of the incorporation of aniline and disulfide, phenylcarbamic acid and a tetrahedral substance are required as intermediates for the formation of MBT, which leads to the formation of a 2-anilinobenzothiazole (AnBTh) by-product (Scheme 3) [27]. This clearly shows that the excess hydrogen sulfide lies heavily in favor of MBT. Notably, the reaction can be conducted in the opposite direction when the nucleophilic power of the amine is increased (Scheme 4). In addition to the aforementioned by-products, other by-products, such as raw material residues and minor impurities, have not yet been described in detail.

In the currently used methods for industrial production of MBT, the generation of toxic substances and residues (such as MBT [30]) are inevitable. This is especially significant given the important roles of MBT industrial processes and in the new context of the green economy. Nowadays, mechanical–biological waste treatment technology, focused on the biological stabilization of the organic fraction of municipal solid waste, plays a key role in municipal solid waste treatment [31]. Scholars have surveyed the disadvantages of traditional technologies and, through this, have presented new, green methods. Umamaheswari B and Rama Rajaram presented a method of microaerobic degradation using the Alcaligenes sp. MH146 strain CSMB1 to treat MBT in industrial wastewater [32]. In industrial tar residue, MBT could convert to C₇H₅NS and aniline to treat the toxic tar residue formed through the hydrogenation procedure presented by Jianhong Zhao and colleagues [33].

In this article, through the traditional aniline and 2-chloronitrobenzenes method, we explore the green process of MBT synthesis and apply our findings to industrial applications [34]. Before applying to the industrial scale, the Aspen Plus simulation software is used to explore the optimal condition parameters with the help of the results of the experiment and theoretical study. The theoretical study aims to acquire the dynamics and thermodynamics data of each reaction by employing Gaussian software and thus obtain the rate-determining step. Finally, the study is applied to build the MBT industry up to the 10,000-ton scale of Shandong Yanggu Huatai Co., Ltd. (Liaocheng, China); this has functioned successfully to date.

2. Exploration of MBT Green Synthesis Process

2.1. Quantum Mechanical Research

Based on the MBT synthesis reaction mechanism, we use the quantum mechanics calculation method to study the feasibility of the MBT catalytic green synthesis process. The synthesis of MBT is divided into three continuous reaction processes. Among these, the content of the by-product C₇H₅NS and the mid-product 1,3-diphenyl-2-thiourea (abbreviated as DPTU) directly affects the MBT yield.

The quantum mechanics calculation utilized Gaussian09 software, and molecular optimization, including the reaction, mid-product and transition state, was calculated using the B3LYP/6-31++G(d,p) level [35]. By means of the calculation frequency and reaction coordinates, we obtained the correctness of the transition state. Furthermore, we obtained the kinetic data of three continuous reaction processes by analyzing the natural orbits of the mid-product and transition states. The results demonstrate that, on the one hand, an MBT catalytic synthesis reaction can improve the activation energy of the transition state and inhibit by-product formation. On the other hand, the activation energy of the transition state can be reduced in the DPTU conversion stage and, by taking advantage of DPTU, this transforms the MBT.

GROMCS 4.0.5 software is used to calculate the molecular simulation of MBT synthesis reactions, and the potential energy function is constructed using a GROMOS 53a6 force field (Equation (1)).

$$\begin{array}{cc}\hfill U=& \sum k_b {(r-r_0)}^2+\sum k_{\theta } {(\theta -{\theta }_0)}^2+\sum \left[{|k}_{\varnothing }\right|-k_{\phi }\cos (n\varnothing )]+\sum k_{\chi } {(\chi -{\chi }_0)}^2+{\displaystyle \sum _{i,j}}{\displaystyle \frac{q_i{q}_j}{4\pi {ᶊ}_{0r_{ij}}}}\hfill \\ & \hspace{1em}\hspace{1em}\hspace{1em} +{\displaystyle \sum _{i,j}}({\displaystyle \frac{A_{IJ}}{r_{ij}^{12}}}-{\displaystyle \frac{B_{ij}}{r_{ij}^6}})sw(r_{ij}^2,r_{on}^2,r_{off}^2) \hfill \end{array}$$

(1)

Non-bonded interaction is computed with a cutoff radius of 1.2 nm, while long-range electrostatic interaction and bond length are accounted for using the particle-mesh Ewald summation method and the LINCS method, respectively. Meanwhile, the Verlet integration algorithm and NPT ensemble simulation calculation are applied to solve the Newtonian equation of motion that eliminates high energy overlap to ensure energy minimization and achieve suitable density. Finally, the simulation is implemented in the NVT ensemble. The interaction energy is weakest between C₇H₈ and MBT (combined energy: −1.43 Kcal/mol), while the interaction ability is stronger in C₇H₈ and other by-products. For example, the combined energy of C₇H₈ and C₇H₅NS is −2.56 Kcal/mol. The interaction parameter X_ij of DPTU and C₇H₅NS with C₇H₈ is clearly below that of the interaction parameter X_ij of MBT with C₇H₈. In other words, this result can explain why DPTU and C₇H₅NS are easily dissolved in C₇H₈. The dynamics and thermodynamics of the reaction are also calculated, and the results show that the rate-determining step of the main reaction rate constant is 204 C⁻¹.

2.2. Experimental Methods

Through the molecular simulation described above, we explore a novel process for synthesizing MBT, described in the following reaction equations (Scheme 5 and Scheme 6).

The experimental methods used to perform MBT synthesis are typically divided into two steps. Firstly, a synthesis reaction is conducted in an autoclave that is made of stainless steel [36] and which is filled with CS₂ (315 L, 99%), S (140 kg) and C₆H₇N (378 L, 99%). This autoclave is able to withstand considerable pressure and temperature as it is equipped with a safety pressure relief valve and emergency cooling devices. Under the conditions of 255 °C and 9 MPa, about 98.5% crude MBT can be acquired after reacting for around 5 h. At the same time, H₂S is also produced, and needs to be absorbed by a sodium hydroxide solution. Subsequently, crude MBT is extracted by C₇H₈ (2500 L, 99%) in an extraction reactor after about 4 h. The extraction reactor is composed of glass and steel equipment, and the temperature should be controlled within 65 °C. Via the two main steps described above, an MBT purity and yield of 99.1% and 87.5%, respectively, can be achieved. For the calculation, the E-factor is 1.7, the energy consumption is 0.47 t (standard coal)/t (production) and the reaction mass efficiency (RME) is 69%.

This synthesis method possesses the ability to achieve high yield in an environment without H₂S. Meanwhile, this separation method abandons the traditional acid–base refining process and replaces it with the high–low-temperature liquid–liquid extraction green synthesis process. The production process has also been upgraded from traditional batch production to a fully automated production process, reducing the possibility of hazardous material leakage and greatly ensuring safety and occupational health. The experimental results are summarized in

Table 1. The effect on MBT yield of feed ratio.

Experiment Number	C6H7N:CS2:S	MBT Yield, %
1	1:1.25:1.05	91.28
2	1:1.25:1.10	91.84
3	1:1.25:1.12	92.25
4	1:1.25:1.14	92.88
5	1:1.25:1.16	93.12
6	1:1.25:1.18	93.06
7	1:1.25:1.20	92.60
8	1:1.25:1.22	92.28
9	1:1.25:1.25	92.26

,

Table 2. The effect on MBT yield and purity of extraction agents.

Extraction Agent	MBT, 20 °C (g/100 g)	By-Products, 20 °C (g/100 g)
Isopropanol	7.2	7.5
1-butanol	12.4	22
Cyclohexanol	18	20
Ethyl acetate	11.4	18
Acetic acid	15.82	18
Carbon disulfide	0	18
Aniline	26	0
Toluene	0.7	4.6
1,2-xylene	0.007	2.2

,

Table 3. The effect on MBT purity of extraction method.

Extraction Method	Mass of Crude MBT, kg	Volume of Toluene, L	Purity of MBT, %
Solid–liquid	500	2000	88.3
	500	2000	87.5
Liquid–liquid	500	2000	97.6
	500	2000	98.2

,

Table 4. The effect on MBT yield and purity of extraction agent volume.

Crude MBT:C7H8	MBT Purity, %	MBT Yield, %
5:1	96.54	88.6
6:1	97.22	88.05
7:1	98.95	87.65
8:1	99.15	87.0
9:1	99.28	86.0

and

Table 5. The effect on MBT yield and purity of feed and extraction temperature.

Feed Temperature, °C	Extraction Temperature, °C	MBT Purity, %	MBT Yield, %
10	20	96.2	89.1
10	40	97.2	88.4
10	60	99.0	85
15	20	96.6	88.9
15	40	97.9	88.2
15	60	99.3	84.9
20	20	97.8	88.1
20	40	98.0	87.8
20	60	99.5	84.5
30	20	97.8	88.0
30	40	98.1	87.6
30	60	99.4	84.4
40	20	98.0	87.9
40	40	98.1	87.6
40	60	99.9	84.4

.

3. Simulation of Preliminary Work

To achieve industrial implementation using the traditional industrial enlargement method, three stages must be performed: the experiment, a small test and a pilot test. Although the traditional method can ensure industrial success (such as a higher optimization of process conditions and a higher product yield and purity) to a large extent, its most observable disadvantages include the unavoidable long cycle and large investment, which can also be needed for the process described above. This problem has a particular impact on fine chemical engineering. In addition, if the experimental or synthesis process is extremely complex, the correlation of every process condition is closer, which also leads to difficulty in deciding the process parameters.

Compared to the traditional industrial enlargement method, advanced chemical engineering simulation software has been widely employed for process simulation purposes by industrial entities [37]. Among these simulation software, Aspen Plus (v11), released by the ASPEN TECH company, is the most representative. Aspen Plus is usually applied in fields including chemical engineering, oil refining, the coal chemical industry and oil and gas processing. Conversely, few studies have reported the use of Aspen Plus in the rubber additives field; therefore, using Aspen Plus and the results of the molecular theory simulation and experiment, we studied the possibility of the industrial production of rubber additives (MBT) in detail. While ensuring that the study was feasible, we aimed to achieve a product with the highest yield and purity and the highest optimization levels of all operating parameters.

3.1. Physical Property Data Acquisition

Generally, all chemical engineering simulation software relies on complete and precise physical property data. Aspen Plus is currently ahead of the curve on this point because it is famous for being the most complete physical property system acquired from studies and companies [37]. In this work, the pure-component physical property data of C₆H₇N, CS₂, S, MBT, H₂S, H₂SO₄, Na₂SO₄ and Na₂S are obtained from the in-house PURE37 database of Aspen Plus. Similarly, the binary parameters of mixtures, such as CS₂ and S, C₆H₇N and CS₂, CS₂ and H₂S, C₇H₈ and C₇H₅NS, C₇H₈ and C₆H₇N, and C₇H₈ and CS₂, are obtained from in-house NRTL database of Aspen plus [38]. However, for the special MBT and DPTU, we need to use a physical property estimate system belonging to an Aspen Plus module to solve the problem of the shortage of physical property data, since MBT and DPTU cannot be found in Aspen Plus’ in-house physical property databases. Thus, in this system, the physical properties of MBT and DPTU are estimated via the UNIFAC property method, which requires the molecular mass, molecular structure, normal boiling point, vapor pressure and molar enthalpy of combustion, sublimation and formation, etc., to be input [39].

3.2. Module Process Construction in Aspen Plus

In the actual industrial process of producing MBT, a series of complex undesirable side reactions must be performed underground in order to form the raw materials that react with MBT. Simultaneously, there are many unexpected issues that may occur. Thus, to satisfy the conditions that guarantee the calculation result is correct, we simplify the production process of MBT according to the actual conditions and, before performing the Aspen Plus simulation, we propose the following five suggestions:

• The process of MBT should occur in a steady state [40].
• Temperature should be distributed evenly in the reactor; there should be no temperature gradient.
• The influence of H₂S in the reaction should be ignored.
• Undesirable side reactions should be omitted.
• A reactor and an exchanger module should be used to simulate the cooling of the kettle.

According to the production process of MBT, we programed the module process of MBT in Aspen Plus, as is shown in Figure 1 below. Firstly, the CS2 stream and S stream are mixed in the MIX1 model and then entered into the MIX2 model with the C6H7NS stream. After the three streams are combined, these materials turn into the RC-1 model to react. Secondly, we introduced the FLASH1 model to achieve initial vapor–liquid separation because the streams, after reacting, are not divided into a vapor phase and a liquid phase in the RC-1 model. Vapor-phase streams are cooled in the FLASH2 model through stream 4. After cooling, uncondensed H₂S is absorbed in the RC-2 model using 32% NaOH solution, and the condensed S supplementary stream is mixed with the CS2 stream in the MIX4 model. The liquid-phase stream from the FLASH1 model with the C7H8 stream enter into the DE model (extraction kettle). Through extraction and purification, the product is sent to the next separation process through stream P, and waste liquid is disposed through stream 9.

4. Simulation Results and Analysis

In order to study the chemical industrial process, factors including reactant concentration, reaction temperature and pressure, reaction time, material ratio and conversion rate, and product yield and purity are regarded as key. However, since we are particularly focused on the MBT green synthesis process, we mainly studied the material ratio and reaction temperature, which directly affect the product and by-product yield. Thus, reactor modules supplemented by separation modules are mainly discussed in this paper; then, for every model selected in Aspen Plus, we perform an individual analysis and optimization to maximize the MBT yield.

4.1. Reaction Model

4.1.1. Influence of Material Feed Ratio

As shown in Figure 2, when the CS₂: C₆H₇N ratio is constant, the mass fraction of MBT linearly increases at the beginning and gradually decreases after reaching the maximum level as the feed amount of S increases. Compared to MBT, the change in the C₇H₅NS mass fraction is completely converse before reaching the minimum level and, subsequently, C₇H₅NS is kept at a low amount. This phenomenon can be explained by the chemical equilibrium between MBT and C₇H₅NS [29]. At the beginning of the reaction, the hydrosulfonyl content in MBT is likely destroyed by the abundant S due to the large difference between the electronegativity of the S and C atoms [39]. When C₇H₅NS reaches the peak, the chemical equilibrium moves rightwards to promote the generation of MBT due to S. The other mass fractions, including those of H₂S and C₆H₇N, decline slowly, while the mass fraction of DPTU, CS₂ and S drop asymptotically to zero during this process. This is attributed to the fact that C₆H₇N and H₂S are both by-products and reactants.

By the same means, the results show that MBT and C₇H₅NS, at the start, coexist in the system when S: C₆H₇N is controlled at a constant level and CS₂ is less than this (see Figure 2a). As expected, the development of the reaction is weak due to the limited production of MBT and greater possibility of collision between S and C₇H₅NS according to collision theory. It is evident that the maximal mass fraction of MBT increased by 9%, from 72% to 81%, thanks to the increase in CS₂. The rest of the H₂S and C₆H₇N appears to decrease, and the mass fraction of CS₂, S and DPTU remains almost unchanged and roughly stable at zero.

Based on the above analysis, the products and by-products are directly determined by the quality ratio of S, CS₂ and C₆H₇N, which affected the product quality under unreasonable conditions. Meanwhile, in order to accelerate the translation from C₇H₅NS to MBT, the initial magnitudes of S should maintain superfluous. Thus, in this optimization step, the quality ratio of S, CS₂ and C₆H₇N is determined at approximately 6:17:20 to ensure that the yield of MBT reaches its maximum and, for other materials, it is lower.

4.1.2. Influence of Pressure and Temperature

Reaction pressure and temperature also played an important role in the synthesis of MBT. We can draw the conclusion that the peak mass percentage achieved for MBT is 82.5% when the pressure and temperature of the reactor are 1 MPa and 240 °C, respectively, as shown in Figure 3. Conversely, the mass percentage of C₇H₅NS and S fall to their lowest at an identical temperature and pressure. This reaction pressure and temperature (1 MPa and 240 °C) are regarded as the optimal conditions. Below these optimal conditions, the MBT mass percentage gradually increases with varied pressure and enhanced temperature, and the value range (mass percentage of MBT) is clearly different. The MBT mass percentage increased from 40% to 82.5% under varying temperatures and improved from 58% to 82.5% under varying pressures. On the one hand, according to Le Chatelier’s principle, the main reaction shifted to the right due to the exothermic effects of the reaction. Furthermore, the impact of pressure depends on the presence of H₂S in side reactions (as shown in Scheme 6), which restrain side reactions as pressure increases. On the other hand, the phenomenon that high temperature counteracts the maintenance of the -SH bond and that high pressure promotes side reactions producing C₇H₅NS when the temperature and pressure exceed the optimum conditions, is accounted for. Otherwise, the mass percentage of H₂S is kept at about 18% and other materials do not clearly change at approximately 0.

On the basis of the curve of mass percentage obtained in Figure 3a,b, we reach the conclusion that these optimal conditions (1 MPa and 240 °C) should be investigated in a follow-up study.

4.2. Condensation Model

4.2.1. Influence of Pressure

Figure 4a presents clearly that CS₂ and H₂S display a converse change as the condenser pressure rises, as was expected. Compared to the mass fraction of CS₂ (right ordinate), H₂S occupies over 95% of the volume in stream 5 (left ordinate). It is worth noting that the content curve, as shown in Figure 4b, has a significant effect, as shown in Figure 4a. Below 120 kPa pressure, the mass percentage of both CS₂ and H₂S appears to show an upward trajectory, apart from the difference in the slope of the curve. Then, the mass percentage of CS₂ declines, while the mass percentage of H₂S keeps increasing. In stream 10, the mass percentage of CS₂ peaks at about 70%. The physical characteristics of CS₂ show that the boiling point and saturated vapor pressure are 46.2 °C and 53.32 kPa (28 °C), respectively. Thus, the gaseous state of CS₂ is subjected to pressure, which readily turns it into a liquid. When CS₂ almost enters into a liquid stream (10) in the condenser, some of the H₂S transforms into a liquid state, and this results in the proportion of CS₂ reducing as the pressure increases. The reasons for this phenomenon will not be discussed in this paper.

Considering the maneuverability and cost, we analyze the mass percentage of CS₂ and H₂S in streams 5 and 10 under 100 kPa and 120 kPa condenser pressure, respectively. The results are shown in

Table 6. The mass percentage of CS₂ and H₂S at 100 and 120 kPa in streams 5 and 10, respectively.

Pressure	Mass Fraction of CS2, %		Mass Fraction of H2S, %
	Stream 5	Stream 10	Stream 5	Stream 10
100 kPa	4.6%	62%	95%	13%
120 kPa	3.9%	70%	96%	12%

and further demonstrate that the separation efficiency of the condenser is not significantly different. After comprehensive consideration, a condenser pressure of 100 kPa is chosen.

4.2.2. Influence of Temperature

The graph showing the calculation results demonstrates the outstanding change in regard to CS₂ and H₂S in stream 5 and 10 in the outlet under the influence of temperature, which is different to the effect of pressure, as shown in Figure 5. In stream 5, the variation in the mass fraction of CS₂ and H₂S is exactly the opposite, which is similar to the results shown in Figure 5a. Nevertheless, in the mass fraction of CS₂, the inflection point occurs when the condenser temperature is at −29.2 °C, as shown in Figure 5a. When choosing a different temperature and then making a straight line parallel to the y-axis in Figure 5b for further analysis, it is clearly observed that the maximum difference value appears at the inflection point. The consequences of the results shown in Figure 5b can be explained by the dipole–dipole attraction of polar molecules. Because both H₂S and CS₂ are polar molecules, H₂S is attracted by CS₂ and fixed in stream 10, as the system temperature tends to move towards the boiling point of H₂S (−60.4 °C). For convenience, the optimal separation temperature was set at −30 °C.

4.3. Extraction Model

4.3.1. Influence of Toluene Feed

As is known, a significant extraction occurs as the quantity of the extraction agent C₇H₈ increases. The experimental consequences suggest that MBT is more easily soluble in C₇H₈ than the other materials (such as S, CS₂, DPTU, etc.). The “like dissolves like” rule is frequently used to argued that MBT and C₇H₈ belong to a polar molecule with a benzene ring. The results in Figure 6a show that the mass percentage of MBT rises gradually in stream P (left ordinate), and impurities (mainly S and CS₂ analyzed; the others are ignored) are isolated from MBT with the feed amount of C₇H₈. The mass percentages are exactly the same, and that of the C₇H₈ is very high in stream 9 (left ordinate), which is presented in Figure 6b. When the ratio of C₇H₈ and stream 7 values is roughly 2.8, a mass percentage of MBT of about 99.5% is achieved in stream P, and the recycled fraction of C₇H₈ reaches about 99.0%.

4.3.2. Influence of Temperature

Through our experimental study, we have demonstrated that the pressure does not play an obvious role in the extraction process. Therefore, we only analyze the influence of temperature in our work.

Compared to Figure 7a,b, when the extracting temperature is higher than 40 °C, a 99.6% mass percentage of MBT can be achieved (left ordinate), while the mass percentage of C₇H₈ is below 0.2% (right ordinate) in stream P. Notably, the variation in S, CS₂ and MBT is moderate or considered as immovable in stream 9. The mass percentage of C₇H₈ (left ordinate) decreases, while the mass percentage of MBT (right ordinate) rises as temperature increases. The abovementioned trend reflects that the extraction effect is nearly fixed when the extraction temperature is higher. In conclusion, we chose an extraction temperature of 40 °C.

5. Design of Green Synthesis Process

In summary, in this study, we provide an effective and green approach for MBT production through utilizing chemical engineering simulation technology. As can be seen in the flow block diagram of industrial MBT production shown in Figure 8, in which the parameters of the production process conditions are described, the MBT industrial process consisted of reaction, separation, and purification; the S cycle; and an auxiliary process. Through the application of the green process and the simulation results, a 10,000-ton-scale MBT industrial production process was established by Shandong Yanggu Huatai Co., Ltd. The operational result for MBT yield, at 93%, is in sharp contrast to that of the traditional process (2-chloronitrobenzene method), which achieved 84%. Meanwhile, the toxicity of raw materials and intermediates is reduced and the quantity of wastewater is notably decreased. In addition, because of this simple reaction process, plant-wide automation can be implemented. Therefore, the green process is superior to the traditional process, in terms of both security and cost.

However, the green process requires high temperature and pressure, which increase its danger, despite adopting automation. The associated hazards include reaction vessel explosion, fires and intoxication. To avoid these, we adopted a fully automated process from feeding to packaging. By implementing real-time personnel positioning, the number of on-site operators is maintained at no more than three, thereby maximizing personnel safety. For safety instrumentation, the DCS and SIS control systems enable the real-time monitoring of parameters such as temperature, pressure, liquid level, flow rate and combustible/toxic gas concentrations. In the event of abnormal conditions, alarms are triggered promptly, along with material cut-off and emergency relief operations within seconds. In case of the release of toxic and combustible steam from CS₂, C₆H₇N and C₇H₈ explosions, emergency equipment stations are installed across the site, equipped with personal protective gear including heavy-duty chemical protective suits, fire-resistant clothing, self-contained breathing apparatus (SCBA), gas filter canisters and protective goggles to ensure a rapid response is possible.

6. Conclusions

The traditional MBT synthesis and purification process has been replaced by catalytic synthesis and high–low-temperature liquid–liquid extraction. For all simulation modules, we use Aspen Plus to analyze the optimal condition parameters. The results indicate that the optimal feed ratio is 6 (S): 17 (CS₂): 20 (C₆H₇N): 90 (C₇H₈), and the optimal reaction, condensation and extraction temperatures and (or) pressures are 80 °C and 1 atm, −30 °C and 1 atm and 40 °C, respectively. Through practical implementation in Shandong Yanggu Huatai Co., Ltd., this process was applied to a 10,000-ton-scale MBT production plant. The simulation results had reasonable accuracy for predicting the green synthesis of MBT, and partial industrial production data is shown in

Table 7. Comparison of simulation results and actual parameters.

Key Condition Parameters	Materials and Equipment	Simulation Results	Actual Parameters
Quality ratio	S:CS2:C6H7N	6:17:20	6:17:17
Temperature °C	Reaction kettle	240	<265
	Condensation	−30	−27
	Extraction kettle	40	<60
Pressure MPa	Reaction kettle	98.69	96.7
	Condensation	1	1

.

7. Discussion

The main achievement of this study lies in the application of Aspen Plus simulation software to optimize and design an industrial-scale production process for MBT, based on the high-yield, green synthesis and purification route developed in the laboratory. The simulation served as a surrogate for the conventional scale-up process, allowing for detailed exploration of key operating parameters under controlled conditions. These parameters—including production route, separation sequence, reaction conditions and product recovery—were established through theoretical analysis and validated industrial experience [1,13,23,26,27,31]. As a result, several existing issues in MBT production were successfully addressed, and the overall process was significantly improved.

Importantly, the simulation results demonstrated strong alignment with actual industrial operating conditions, reinforcing the predictive and guiding value of this work. Compared with previously reported industrial methods [13,23,26,27], the optimized process developed here adheres to modern principles of circular economy and green manufacturing. It offers advantages such as high product purity, low environmental emissions and improved operational safety. These benefits can be attributed to the improved one-step synthesis strategy and the independently developed high-/low-temperature liquid–liquid extraction technique. Furthermore, by replacing the traditional batch production method with a fully automated, continuous process, this study paves the way toward more efficient and scalable MBT production.

Nonetheless, several limitations should be noted. The simulations were performed under steady-state assumptions, which do not fully capture dynamic factors such as startup convergence and stabilization time. Additionally, while Aspen Plus offers reasonably accurate predictions, deviations may occur due to limitations in mass and heat transfer in real industrial reactors. These factors could contribute to minor discrepancies between simulated and actual performance.

Future work should aim to enhance the consistency between the simulation and actual production processes. Specifically, the optimal conditions identified from the steady-state simulations can be used as initial inputs for dynamic simulation modeling. By analyzing system stability and response over time, feedback from dynamic simulations can be iteratively integrated into the steady-state optimization loop. This closed-loop approach will enable fine-tuning of operating parameters and further bridge the gap between theoretical simulation and industrial practice.