# Simulation and Sensitivity Analysis of Molybdenum Disulfide Nanoparticle Production Using Aspen Plus

Authors: Mouad Hachhach; Hanane Akram; Mounir Hanafi; Ouafae Achak; Tarik Chafik
Journal: International Journal of Chemical Engineering
 (2019)

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## Abstract

The sensitivity analysis of molybdenum disulfide nanoparticles synthesis process is studied using Aspen Plus with the aim of investigating the effect of reactants’ amounts on the production of molybdenum disulfide nanoparticles. The adopted approach consists in simulating the synthesis process based on experimental data, obtained at laboratory scale followed by sensitivity analysis with respect to the following precursors: ammonium heptamolybdate, elemental sulfur, and hydrazine used as a reducing agent. The sensitivity analysis revealed that the precursors have more significant impact on the obtained amount of molybdenum disulfide compared to hydrazine. The obtained result showed that the approach adopted in the study might be of interest for further investigation of the process design and scaling-up.


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## Body

## 1. Introduction

The solvothermal synthesis is among the widely used methods for nanomaterials production through chemical reactions in a closed system involving appropriate solvent at temperatures higher than its boiling point. The technique was inspired from a natural production of many minerals in the earth’s crust under the conditions of high temperature and pressure permitting the formation of many minerals [1]. This technique was used afterward for the production of innovative nanomaterials of interest for different fields, such as catalysis [2–8] and environment protection and energy [9–11], as well as medicine [12] and mechanics [13]. As known, nanomaterials are obtained using various methods following bottom-up and top-down approaches [14]. The top-down method consists in breaking down large pieces of material to generate the required nanostructures, whereas bottom-up approach implies assembling single atoms and molecules into larger nanostructures. The bottom-up approach is more advantageous than the top-down approach with respect to nanomaterial production, particularly, the solvothermal method. Its application was found to be efficient for obtaining a variety of structures, sizes, and morphologies that drew attention for applications in heterogeneous catalysis, electrochemistry, and tribology [15–17]. On the other hand and as reported in our previous work, the solvothermal approach proves to be more eco-friendly in comparison with other technics [18].Among the most studied nanomaterials, molybdenum disulfide has shown interesting properties regarding various applications, among which it is used as wind turbines lubricants additive [19]. As reported in our previous work, the preparation of molybdenum disulfide via the solvothermal method is quite easy and consists in mixing reactants in a Teflon-lined stainless steel autoclave filled with a solvent up to 40%–80% of its total volume [8]. The autoclave is maintained at an appropriate temperature and time needed to allow reaction process to occur following either one or two steps. The use of surfactant, phase transferring agent, or templates such as silicon dioxide was found to help the control of molybdenum disulfide nanomaterials size and shape [20–22]. Indeed, the micellar structures are formed from self-assembling of surfactant molecules and yield to nanometre size droplets with insoluble cores and soluble shells. The process provides an interesting template contribution as nanoreactors for the synthesis of nanoparticles with controllable diameter. Thus, selection of appropriate surfactant type and amount is required, considering their determining role with respect to the chemical composition of the final product [23, 24]. On the other hand, even though the surfactants have the ability to help in the growth of nanoparticles as well as their size and shape, the obtained materials might exhibit nonhomogeneity in their chemical composition and crystallinity [13]. Hence, solvothermal synthesis proceeds generally, via three steps: first, the formation of a supersaturated solution, then nucleation, and finally nuclei growth. Note that the powder formation is controlled also by fluctuations phenomena in the supersaturated solution occurring during synthesis process [11]. Thus, it is important to study the correlation of pressure and temperature as well as the nature of solvent that affects the dielectric constants permitting higher supersaturation degree, in addition to the role of enhanced nucleation sites and rate, occurring under supercritical conditions. So far less data are available to provide comprehensive understanding of molybdenum disulfide solvothermal synthesis mechanism. Also, further kinetic and thermodynamic data are still lacking so as to help the scaling-up study. Nevertheless, this issue can be addressed through process simulation making use of available data obtained at laboratory scale. The approach can be of interest with respect to cost estimation and evaluation of the potential of process transfer at industrial scale, as reported in several studies [25, 26].In this regard, the use of Aspen Plus V9 can be of interest considering its ability to model complex processes using simple graphical interface [27]. The software allows, also, the use of available broad database of chemicals and physical parameters that permit built-in operation units suitable for sensitivity analysis and modelling of various processes related to petrochemicals [28], catalysts [29], hydrogen production [30], and carbon dioxide capture [31]. The application of Aspen Plus was reported in the field of nanotechnology [32, 33]; however, it should be pointed out that, to the best of our knowledge, this approach has never been used for investigating such nanomaterials synthesis. Hence, the present work is devoted to sensitivity analysis and simulation of large-scale production of molybdenum disulfide nanoparticles, using Aspen Plus. The study is carried out based on our data concerning the solvothermal preparation of molybdenum disulfide nanoparticles under mild conditions at laboratory scale [17].


## 2. Materials and Methods

### 2.1. Experimental

For the synthesis technique adopted in this work, we have used the one-pot solvothermal process developed by Akram et al. allowing the preparation of well-dispersed molybdenum disulfide nanospheres [17]. The used reagents were purchased from Sigma-Aldrich and used without further purification (Table 1). The synthesis process is as follows: putting 0.14 g of ammonium heptamolybdate ((NH4)6Mo7O24, H2O), 0.05 g of elemental sulfur (S), 1.15 g of lithium hydroxide (LiOH·H2O), 0.06 g of ammonium carbonate ((NH4)2CO3), and 8 ml of hydrazine (N2H4·H2O) in a Teflon-lined stainless steel autoclave. Ethylenediamine was used (as solvent) to fill the autoclave up to 80% of its total volume (200 ml). The autoclave was kept at 180°C during 24 h. Finally, the solid phase (nanoparticles) is recovered by centrifugation, washed with water and acetone, and then dried under vacuum at 60°C for 3 h. The choice of Ethylenediamine as solvent is justified by its ability to remain liquid, under the solvothermal conditions allowing reactants solubilisation. This is due to its appropriate polarity enabling better nucleation and growth of molybdenum disulfide nanoparticles, helping better control of their morphology and their sizes [15, 34].Table 1
Summary of the amount and the role of all used reagents.
Reagent
Amount
Role
CAS number
Ammonium heptamolybdate ((NH4)6Mo7O24, H2O)
0.14 g
Precursor
12054-85-2
Sulfur (S)
0.05 g
Precursor
7704-34-9
Lithium hydroxide (LiOH·H2O)
1.15 g
Strong base
1310-65-2
Ammonium carbonate ((NH4)2CO3)
0.06 g
Electrolyte
506-87-6
Hydrazine (N2H4·H2O)
8 ml
Reducing agent
7803-57-8
Ethylenediamine
144 ml
Solvent
107-15-3Further details of the used equipment for the synthesis, drying, and recovery of molybdenum disulfide nanoparticles are presented in Table2.Table 2
Summary of brand and models of instruments used in the synthesis of molybdenum disulfide.
Instrument
Brand
Model
Teflon lined stainless steel autoclave
Parr instrument
4748A
Centrifuge
Hermal
Z356k
Synthesis oven
Binder
ED35
Vaccum dryer oven
Thermo scientific
Heraeus VT6025As reported, the synthesis of molybdenum disulfide nanoparticles is assumed to occur following three steps [15]:(i)
Formation of a supersaturated solution as a result of reactants decomposition in the solvent under the aforementioned operating conditions(ii)
Nucleation and production of stables nuclei(iii)
Growth of the nuclei


### 2.2. Process Simulation

The adopted approach for the process simulation consists in using purpose-built software to define a system with interconnected components that are solved in either steady state or dynamic mode. Note that the software uses thermodynamic models available in the database and processes flow sheet diagrams so that the various involved unit operations are appropriately positioned and connected. This allows obtaining stable operating point of system behaviour with respect to balance calculation.Generally, for process modelling, the adopted approach consists in choosing the property method before the creation of the process flowsheet and, then, solving it using Aspen Plus. Note that the choice of property method is crucial because of its influence on the accuracy of material and energy balances [35]. This choice depends on process parameters, such as reactants’ amounts, temperature, and pressure in addition to other relevant parameters available in the software database.In the present study, the ELECNRTL model and the Redlich–Kwong state equations were used together to calculate the involved mixture properties. The ELECNRTL model has been applied mainly in electrolyte systems to represent the solid-liquid equilibrium (SLE) of aqueous multicomponent [36] and phase equilibrium of the mixture of solvents in a wide temperature range [37], while Redlich–Kwong equation is used for vapour phase properties [38].Flow sheeting is a complete description of material and energy streams needed for process simulation of interest for sizing calculation and cost estimation of chemical process [39, 40]. The flowsheet creation starts by selecting involved operation units and related equipment interconnects integration within a working system. Accordingly, the flow diagram of the overall process describing the studied molybdenum disulfide nanoparticles synthesis is shown in Figure 1 while the corresponding flowsheet obtained using Aspen Plus is given in Figure 2.Figure 1
Flow diagram of the molybdenum disulfide nanoparticles solvothermal synthesis process at laboratory scale.Figure 2
Flowsheet created for molybdenum disulfide synthesis process using Aspen Plus (Snapshot).As shown, the process involves two main operation units corresponding to the reactor and the separator. Further descriptions of the major blocks of process flowsheet and the involved streams are given in Tables3 and 4.Table 3
Description of the blocks of the created flowsheet.
Block
Description
B1: separator
Separator of components into two streams: Product and Waste
B2: centrifuge
Separation of liquid phase of r-output stream from the solid phase
B3: stoichiometric reactor
The autoclave reactants input and r-output streamsTable 4
Description of the flow sheet streams.
Stream
Description
Input
Input stream containing the reagents
r-output
Output of the reactor, composed of solution containing the final product feeding the centrifugation
C-output
Output stream of centrifugation and input of separator. It contains the solid phase with small amount of liquid phase
C-waste
Stream of the liquid phase containing mainly the solvents
Waste
The stream contains the remaining from the separation process
Product
Stream containing the pure molybdenum disulfideSo far, it is assumed for the studied process that the reaction occurs in a stoichiometric reactor “RStoich,” chosen among the seven built-in models provided by Aspen Plus database. This reactor seems to fit better the used autoclave device and usually selected when relevant kinetic data are lacking [41]. The input flows correspond to reagents and solvent, whereas the output flows are associated with products and by-products, which are obtained following the separation steps that involve centrifugation, washing, and drying in vacuum in order to separate the solid phase (molybdenum disulfide nanoparticles) from liquid phase (Waste).In Aspen Plus, the centrifuge is modelled as CFuge block located at the reactor outlet. The output flow contains two flows corresponding to liquid phase (C-waste) and the output stream (C-output) of centrifugation and input of separator block SEP, simulating the washing and drying units. The pure molybdenum disulfide is contained in the Product stream, while all the other impurities are in the Waste stream.The sensitivity analysis was carried out to study the effect of the variation of the amount of some reactants (precursors and hydrazine) on the final molybdenum disulfide nanoparticle production. Thus, according to base-case requirement, the adopted approach consists in changing the amounts of each reactant (starting with ammonium heptamolybdate, sulfur, and then hydrazine) while keeping the amount of the other reactants and the experimental conditions unchanged, and, afterwards, the amounts of two reactants were changed simultaneously.


## 2.1. Experimental

For the synthesis technique adopted in this work, we have used the one-pot solvothermal process developed by Akram et al. allowing the preparation of well-dispersed molybdenum disulfide nanospheres [17]. The used reagents were purchased from Sigma-Aldrich and used without further purification (Table 1). The synthesis process is as follows: putting 0.14 g of ammonium heptamolybdate ((NH4)6Mo7O24, H2O), 0.05 g of elemental sulfur (S), 1.15 g of lithium hydroxide (LiOH·H2O), 0.06 g of ammonium carbonate ((NH4)2CO3), and 8 ml of hydrazine (N2H4·H2O) in a Teflon-lined stainless steel autoclave. Ethylenediamine was used (as solvent) to fill the autoclave up to 80% of its total volume (200 ml). The autoclave was kept at 180°C during 24 h. Finally, the solid phase (nanoparticles) is recovered by centrifugation, washed with water and acetone, and then dried under vacuum at 60°C for 3 h. The choice of Ethylenediamine as solvent is justified by its ability to remain liquid, under the solvothermal conditions allowing reactants solubilisation. This is due to its appropriate polarity enabling better nucleation and growth of molybdenum disulfide nanoparticles, helping better control of their morphology and their sizes [15, 34].Table 1
Summary of the amount and the role of all used reagents.
Reagent
Amount
Role
CAS number
Ammonium heptamolybdate ((NH4)6Mo7O24, H2O)
0.14 g
Precursor
12054-85-2
Sulfur (S)
0.05 g
Precursor
7704-34-9
Lithium hydroxide (LiOH·H2O)
1.15 g
Strong base
1310-65-2
Ammonium carbonate ((NH4)2CO3)
0.06 g
Electrolyte
506-87-6
Hydrazine (N2H4·H2O)
8 ml
Reducing agent
7803-57-8
Ethylenediamine
144 ml
Solvent
107-15-3Further details of the used equipment for the synthesis, drying, and recovery of molybdenum disulfide nanoparticles are presented in Table2.Table 2
Summary of brand and models of instruments used in the synthesis of molybdenum disulfide.
Instrument
Brand
Model
Teflon lined stainless steel autoclave
Parr instrument
4748A
Centrifuge
Hermal
Z356k
Synthesis oven
Binder
ED35
Vaccum dryer oven
Thermo scientific
Heraeus VT6025As reported, the synthesis of molybdenum disulfide nanoparticles is assumed to occur following three steps [15]:(i)
Formation of a supersaturated solution as a result of reactants decomposition in the solvent under the aforementioned operating conditions(ii)
Nucleation and production of stables nuclei(iii)
Growth of the nuclei


## 2.2. Process Simulation

The adopted approach for the process simulation consists in using purpose-built software to define a system with interconnected components that are solved in either steady state or dynamic mode. Note that the software uses thermodynamic models available in the database and processes flow sheet diagrams so that the various involved unit operations are appropriately positioned and connected. This allows obtaining stable operating point of system behaviour with respect to balance calculation.Generally, for process modelling, the adopted approach consists in choosing the property method before the creation of the process flowsheet and, then, solving it using Aspen Plus. Note that the choice of property method is crucial because of its influence on the accuracy of material and energy balances [35]. This choice depends on process parameters, such as reactants’ amounts, temperature, and pressure in addition to other relevant parameters available in the software database.In the present study, the ELECNRTL model and the Redlich–Kwong state equations were used together to calculate the involved mixture properties. The ELECNRTL model has been applied mainly in electrolyte systems to represent the solid-liquid equilibrium (SLE) of aqueous multicomponent [36] and phase equilibrium of the mixture of solvents in a wide temperature range [37], while Redlich–Kwong equation is used for vapour phase properties [38].Flow sheeting is a complete description of material and energy streams needed for process simulation of interest for sizing calculation and cost estimation of chemical process [39, 40]. The flowsheet creation starts by selecting involved operation units and related equipment interconnects integration within a working system. Accordingly, the flow diagram of the overall process describing the studied molybdenum disulfide nanoparticles synthesis is shown in Figure 1 while the corresponding flowsheet obtained using Aspen Plus is given in Figure 2.Figure 1
Flow diagram of the molybdenum disulfide nanoparticles solvothermal synthesis process at laboratory scale.Figure 2
Flowsheet created for molybdenum disulfide synthesis process using Aspen Plus (Snapshot).As shown, the process involves two main operation units corresponding to the reactor and the separator. Further descriptions of the major blocks of process flowsheet and the involved streams are given in Tables3 and 4.Table 3
Description of the blocks of the created flowsheet.
Block
Description
B1: separator
Separator of components into two streams: Product and Waste
B2: centrifuge
Separation of liquid phase of r-output stream from the solid phase
B3: stoichiometric reactor
The autoclave reactants input and r-output streamsTable 4
Description of the flow sheet streams.
Stream
Description
Input
Input stream containing the reagents
r-output
Output of the reactor, composed of solution containing the final product feeding the centrifugation
C-output
Output stream of centrifugation and input of separator. It contains the solid phase with small amount of liquid phase
C-waste
Stream of the liquid phase containing mainly the solvents
Waste
The stream contains the remaining from the separation process
Product
Stream containing the pure molybdenum disulfideSo far, it is assumed for the studied process that the reaction occurs in a stoichiometric reactor “RStoich,” chosen among the seven built-in models provided by Aspen Plus database. This reactor seems to fit better the used autoclave device and usually selected when relevant kinetic data are lacking [41]. The input flows correspond to reagents and solvent, whereas the output flows are associated with products and by-products, which are obtained following the separation steps that involve centrifugation, washing, and drying in vacuum in order to separate the solid phase (molybdenum disulfide nanoparticles) from liquid phase (Waste).In Aspen Plus, the centrifuge is modelled as CFuge block located at the reactor outlet. The output flow contains two flows corresponding to liquid phase (C-waste) and the output stream (C-output) of centrifugation and input of separator block SEP, simulating the washing and drying units. The pure molybdenum disulfide is contained in the Product stream, while all the other impurities are in the Waste stream.The sensitivity analysis was carried out to study the effect of the variation of the amount of some reactants (precursors and hydrazine) on the final molybdenum disulfide nanoparticle production. Thus, according to base-case requirement, the adopted approach consists in changing the amounts of each reactant (starting with ammonium heptamolybdate, sulfur, and then hydrazine) while keeping the amount of the other reactants and the experimental conditions unchanged, and, afterwards, the amounts of two reactants were changed simultaneously.


## 3. Results and Discussion

The composition of the involved different streams, as simulated by Aspen Plus, is summarized in Table5. The results show that the synthesized molybdenum disulfide with by-products is present in the reactor output. The centrifugation allows separation of almost all of the liquid phase containing ethylenediamine from the solid phase containing the synthesized nanoparticles, which are subsequently purified after multiple washing and drying steps.Table 5
Results of the simulation with Aspen Plus.
Involved weight (g)
Input
R-output
CF-waste
C-output
Waste
Product
Sulfur
0.05
0.01
0.01
1.89 × 10−4
0
0
Molybdenum disulfide
0
0.10
1.60 × 10−3
0.10
0
0.10
Hydrazine
6.19
6.16
6.06
0.10
0.10
0
Nitrogen
0
0.03
0.03
4.41 × 10−4
4.41 × 10−4
0
Ammonia
0
0.01
0.01
8.97 × 10−3
1.53 × 10−4
0
Ammonium heptamolybdate
0.14
0.03
4.98 × 10−4
0.03
0.03
0
Ethylenediamine
80.91
80.91
79.55
1.36
1.36
0
Water
0
0.05
0.04
7.56 × 10−4
7.56 × 10−4
0
Ammonium carbonate
0.14
0.14
0.14
2.35 × 10−3
2.35 × 10−3
0The amount of molybdenum disulfide estimated by simulated process was about 100 mg, which is in agreement with the experimentally obtained data at laboratory scale using similar reagents amount and operating conditions [17]. Moreover, the simulation revealed that about 78% of the initial amounts of both precursors (sulfur and ammonium heptamolybdate) reacted during the synthesis process. Also, it is shown that very small amount of hydrazine (0.5%) was involved in the simulated process, which is in agreement with its role as reducing agent. Indeed, under solvothermal conditions, the use of hydrazine generates gaseous species (ammonia, nitrogen) [42], which may lead to an autogenously pressure inside the reactor as a result of supersaturation phenomena. In this regard, it should be pointed out that specific attention must be given to the use of hydrazine because of its instability due to its low autoignition temperature and low explosive limits, in addition to its severe toxicity. On the other hand, the solvent ethylenediamine is almost completely separated by centrifugation process with recovery value about 98% (Table 5), which agrees with the experimental result. This is due to the fact that the used solvent remains liquid under the operating conditions making its recuperation easier for further recycling.The process sensitivity analysis, investigated using Aspen Plus, is considered as an important task usually carried out prior to the scaling-up study, so as to assess the effect of the amounts of precursors and reducing agent on the final product yield [43]. As mentioned earlier, the amount of each precursor was changed in the input stream, starting first with ammonium heptamolybdate, then sulfur, and finally hydrazine. As shown in Figures 3 and 4, the variation of the precursor and hydrazine amount yields to a linear increase of the molybdenum disulfide production until reaching a stable maximal value. This maximum amount of molybdenum disulfide obtained is equal to 0.13 g for 0.18 g ammonium heptamolybdate, 0.10 g for 0.04 g of sulfur, and 0.10 g for 0.03 g of hydrazine while maintaining the same amount of the other reagents.Figure 3
Effect of the precursor amount variation on the obtained molybdenum disulfide.
(a)
(b)Figure 4
Effect of the hydrazine amount variation on the obtained molybdenum disulfide.The simultaneous variation of both precursors follows a linear profile allowing the monitoring of the impact of each precursor on the final product amount as shown in Figure5. The variation of the sulfur amount and the production of molybdenum disulfide follow a general linear trend, while the variation of the ammonium heptamolybdate amount increases quickly to reach the maximum stable point. The production of increased molybdenum disulfide nanoparticles is related obviously to the increased amounts of the used starting precursors. Figure 5 indicates the presence of an optimal point corresponding to the maximal production of molybdenum disulfide of 0.25 g for 0.10 g of sulfur and 0.38 g of ammonium heptamolybdate. The obtained results revealed that both sulfur and ammonium heptamolybdate have similar influence on the production of molybdenum disulfide.Figure 5
Effect of the simultaneous variation of the precursor amount on the obtained amount of molybdenum disulfide nanoparticles.Figures6 and 7 show that, as compared with the precursors, the hydrazine does not have an important effect on the amount of produced molybdenum disulfide, due to its low reactivity in agreement with its role as a reducing agent. Indeed, the obtained result revealed that molybdenum disulfide production is proportional to the amount of ammonium heptamolybdate and sulfur, as shown in Figures 6 and 7, respectively. This finding proves the stability of the adopted approach that might be useful for simulation of the product yield by the process at a larger scale.Figure 6
Effect of the simultaneous variation of the hydrazine and ammonium heptamolybdate amount on the obtained amount of molybdenum disulfide.Figure 7
Effect of the simultaneous variation of the hydrazine and sulfur amount on the obtained amount of molybdenum disulfide.


## 4. Conclusions

In this paper, Aspen Plus software was used for simulation and sensitivity analysis of molybdenum disulfide synthesis process. The simulation was based on the result obtained by designed experiments carried at our laboratory, alongside the software’s database and property methods. Interestingly, simulated results were found to be in good agreement with our experimental finding. The sensitivity analysis revealed that sulfur, ammonium heptamolybdate, and hydrazine have comparable effects on molybdenum disulfide production, whereas the simultaneous variation of the involved components does not have similar impact. The adopted approach shows quite interesting modelling stability that might pave the way to the scaling-up investigation of molybdenum disulfide at larger scale, although more accurate simulation might require further thermodynamic and kinetic investigations.



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*Source: 3953862-2019-10-28.xml*