Extractive Distillation Approach to the Removal of Dimethyl Disulfide

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/IECR

Extractive Distillation Approach to the Removal of Dimethyl Disulfide from Methyl Tert-Butyl Ether: Combined Computational Solvent Screening and Experimental Process Investigation Guo-xiong Zhan, Ben-xian Shen, Hui Sun,* and Xi Chen Petroleum Processing Research Center, and State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

ABSTRACT: Our present work focused on the extractive distillation desulfurization from methyl tert-butyl ether (MTBE). By using dimethyl disulfide (DMDS) as the model compound contained in MTBE, the intermolecular interactions that happen among solvent, DMDS, and MTBE were analyzed and defined, and the interaction energies were calculated by applying density functional theory method. The interactions between solvents and DMDS are found to be van der Waals and weak hydrogen bond forces. The distributions of DMDS in solvents and MTBE also are calculated using dissipative particle dynamics. According to the computational results coupled with batch extractive distillation experiment using constant reflux ratio, dimethyl sulfoxide has the strongest interaction with DMDS and, therefore, exhibits the highest efficiency for DMDS removal. In addition, the reflux ratio-programmed batch extractive distillation (RRPED) process was developed. Under the suitable operation conditions of RRPED process, the sulfur content of product can be reduced from 2000 μg/g to less than 5 μg/g. The RRPED process was observed to achieve the largely enhanced desulfurization efficiency.

1. INTRODUCTION

Removing sulfide from MTBE is, therefore, becoming a crucial issue to produce low-sulfur gasoline products. Various techniques including distillation,15,16 adsorption, oxidation, and extraction13,17−19 have been studied and applied for the desulfurization of MTBE. Specifically, process intensified distillation techniques like reactive distillation20 and extractive distillation21 were also investigated to obtain high desulfurization efficiency. Extractive distillation is a promising separation method and it expanded the range of applications in desulfurization of petrochemical products recently.22−27 For example, extractive distillation utilizes strong interactions between extractants and sulfides to enhance the desulfurization efficiency of gasoline.3,4,28 Meanwhile, it can also reduce process energy consumption. In addition, extractive distillation technology were also used for the production of low sulfur diesel, liquid

Around the world, more and more stringent regulations on vehicle emissions were built to cope with deteriorating environmental problems. Meanwhile, the limit of sulfur content in gasoline has been updated for several times in the past decades. Several industrial processes, including hydrodesulfurization, extractive distillation, adsorption, membrane separation, extraction, alkylation, and oxidation have been developed and employed in the sulfur removal from gasoline products.1−12 Hydrodesulfurization can achieve the high desulfurization efficiency but often is accomplished with loss of octane number. A significant advancement on reducing sulfur content of gasoline can be attributed to successful development of SZorb adsorption desulfurization technology, which can produce lower-sulfur gasoline blends.12 Meanwhile, because of its high octane number, methyl tert-butyl ether (MTBE) is widely used as a blending component to increase the octane number of gasoline and, therefore, is also expected to be of lower sulfur content.13,14 As a result, the sulfur contents of gasoline products are largely determined by the sulfur contents in MTBE. © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

April 26, 2017 December 29, 2017 February 20, 2018 February 20, 2018 DOI: 10.1021/acs.iecr.7b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research paraffin and so on.29 Molecular simulation studies about explaining the molecular interaction mechanism have also been carried out extensively.30−37 Johnson and his co-workers reported that using the reduced density gradient (RDG = 1/ (2(3π2)1/3)|∇ρ|/ρ4/3), the electron density (ρ(r)), and the sign of the second Hessian eigenvalue (λ2) can reveal noncovalent interactions between two molecules.30 The liquefied petroleum gas (LPG) is an important raw material to produce MTBE and contains dimethyl disulfide (DMDS), which is observed to be the main sulfide in MTBE product.13,16 However, neither detailed experimental studies on extractive distillation desulfurization of MTBE nor theoretical studies on revealing the interactions between extractive solvents and DMDS have been reported. Our present work focused on the extractive distillation for the desulfurization of MTBE through using DMDS as the model compound. With computational studies, the intermolecular interactions among solvents, DMDS, and MTBE were defined and mapped by using atoms in molecules (AIM) and reduced density gradient (RDG) analysis. Furthermore, the interaction energies and the distribution of DMDS in solvents were calculated by using density functional theory (DFT) and dissipative particle dynamics (DPD) to assist the solvent screening. In addition, the reflux ratio-programmed batch extractive distillation (RRPED) process was developed and investigated to achieve the largely enhanced desulfurization efficiency.

Figure 1. Flowchart of the batch extractive distillation experiment. (1manometers, 2-heating jacket, 3-column kettle, 4,10-thermometers, 5,9-column sections, 6-solvent reservoir, 7-solvent pump, 8-feed tube, 11-condenser, 12-product reservoir, 13-temperature controller)

The batch extractive distillation was first carried out at constant reflux ratio (R) to find the suitable flow rate of solvent. Moreover, to achieve higher efficiency for DMDS removal from MTBE, the batch extractive distillation was investigated by applying several variable reflux ratio programs (i.e., VRR1, VRR2, VRR3, VRR4, and VRR5). The yield of MTBE product can be given as Yield (%) = Vdistillate/Vfeed × 100, where Vdistillate and Vfeed represent the volumes of distillate and feed, respectively, in mL. 2.3. Analytical Methods. The contents of DMDS in all samples were analyzed by using a GC-9560 gas chromatograph (Shanghai Huaai Chromatography Analysis Co., Ltd., Shanghai, China) equipped with a flame photometric detector (FPD) and a SE-30 capillary column (30 m × 0.32 mm × 1.0 μm). Temperatures for the oven, injector, and detector were fixed at 180, 200, and 200 °C, respectively. Each sample was analyzed at least twice to ensure reproducibility and the error is confirmed to be less than 2%. 2.4. Computational. The molecular structures of different compounds and their complexes were optimized by using an approximate exchange-correlation energy function M06-2X at the 6-31++G (d, p) theory level.38,39 The same computational method has been previously confirmed accurate to evaluate the noncovalent interactions and thermochemistry properties of distinct organic solvent-involved systems,39−41 and the DFT calculation was performed using Gaussian 09w program package.42 The interaction energy (ΔE) of the complex can be observed from the total energies of the complex and its corresponding monomers.31 The basis set superposition error (BSSE) was calculated by applying the counterpoise method of Boys and Bernardi.31,43 Then the RDG (equal to 1/ (2(3π2)1/3)|∇ρ|/ρ4/3), the electron density (ρ(r)), and the sign of the second Hessian eigenvalue (λ2), which revealed the regions and identified the molecular interactions of the complex,30 were calculated using Multiwfn44,45 based on the results of DFT calculation at M06-2X/6-31++G(d, p) level. Furthermore, the distributions of DMDS in MTBE and solvents were calculated by using dissipative particle dynamics (DPD) in Materials Studio 6.1 package (Accelrys Ltd.). DPD is

2. MATERIALS AND METHODS 2.1. Materials. MTBE was purchased from Shanghai Titan Scientific Co., LTD (Shanghai, China). Dimethyl disulfide (DMDS) was provided by Maya Reagent Company (Shanghai, China). Dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and diethylene glycol (DEG) were obtained from Sinopharm Chemical Reagent Co., LTD (Shanghai, China). All the chemicals were of analytical purity and were used without further purification. 2.2. Procedure for Batch Extractive Distillation. The flowchart for the batch extractive distillation experiment is shown in Figure 1. The distillation column manufactured by Changshun Fine Chemical Co., Ltd. (Changzhou, China) was filled with multilayer rectangular wire mesh stainless steel packing (3 mm × 3 mm × 0.1 mm, the specific surface area is 1380 m2/m3). The height equivalent of theoretical plate (HETP) was about 0.0182−0.0192 m. The diameter of the column was 2 cm. The packing heights of the rectifying section and the solvent recovery section were, respectively, 20 and 8 cm, which were approximately equivalent to 10 and 4 column trays. During all the batch extractive distillation experiments, the MTBE containing 2000 μg/g DMDS (on element S weight basis) was used as the raw material. In each case, 500 mL of raw material was fed into the column kettle. An electric heating jacket was used to heat the column kettle and its temperature was fixed at 180 °C. Tap water was used as the cooling medium of the condenser. Reflux ratio was controlled by electromagnetic relay. Prior to extractive distillation, the experiment was first operated at total reflux to achieve the stable temperature gradient and concentration gradient, which contributed to mass transfer. Then various solvents were added into the system to evaluate the influence of each solvent on the separation of DMDS from MTBE. B

DOI: 10.1021/acs.iecr.7b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 2. Geometries of the DMDS···MTBE and DMDS···solvent complexes optimized at M06-2X/6-31++G (d, p).

Table 1. BCP Analysis of Different Complexes at M06-2X/6-31++G (d, p) Levela complexes

BCP

bond

ρ (a.u.)

∇2ρ (a.u.)

Vb × 100 (a.u.)

Gb × 100 (a.u.)

Hb × 100 (a.u.)

|Vb|/Gb

BD

DMDS···DMSO

27 28 34 38 35 44 45 52 54 34 39 41 46 48 53 63

C−S15···H4−C C−H20···O10S C−S15···S9O C−H12···O10S C−S10···N19−C C−H8···O20C C−S13···H22−C C−S13···O26−C C−H10···O26−C S−C5···H25−C C−H6···O28−C C−S10···H25−C C−H12···H25−C C−H6···H13−C C−S10···H12−C C−H3···H13−C

0.00758 0.01437 0.00568 0.01250 0.01055 0.01212 0.00818 0.01002 0.01086 0.00591 0.00443 0.00661 0.01006 0.00606 0.00540 0.00535

0.0240 0.0466 0.0189 0.0395 0.0300 0.0386 0.0262 0.0329 0.0376 0.0158 0.0173 0.0212 0.0419 0.0216 0.0158 0.0190

−0.360 −1.113 −0.274 −0.928 −0.672 −0.887 −0.419 −0.722 −0.779 −0.226 −0.269 −0.321 −0.588 −0.287 −0.226 −0.245

0.481 1.139 0.373 0.958 0.712 0.926 0.537 0.772 0.859 0.311 0.350 0.426 0.818 0.414 0.311 0.360

0.1205 0.0263 0.0987 0.0295 0.0400 0.0394 0.1186 0.0500 0.0798 0.0844 0.0815 0.1048 0.2306 0.1264 0.0844 0.1150

0.748 0.977 0.735 0.969 0.944 0.958 0.780 0.935 0.907 0.727 0.769 0.754 0.719 0.693 0.727 0.681

0.159 0.018 0.174 0.024 0.038 0.033 0.145 0.050 0.073 0.143 0.184 0.159 0.229 0.209 0.156 0.215

DMDS···DMF DMDS···DEG

DMDS···MTBE

a

“a.u.” is the abbreviation of atomic units; 1 a.u. (energy) = 2625.5 kJ/mol, 1 a.u. (electron density) = 1 electron/Bohr3.

3. RESULTS AND DISCUSSION

used for simulating the dynamic properties of simple and complex components.46,47 The simulation system (100 × 100 × 100 Å3) contained two equal layers. The first layer contained solvents and the second one contained DMDS and MTBE. The van Krevelen solubility parameters (δ) were calculated by Forcite module.46 χij was Flory−Huggins interaction parameter and it was calculated as χij = Vm × (δA − δB)2/RT. Vm was the average molar volume of the species (Vm = (VA + VB)/2). The repulsion parameters αij were calculated and given as αij = 25 + 3.5χij. The computational accuracy was selected as fine. The calculated error was no larger than 3%.

3.1. Intermolecular Interaction between Solvent and DMDS. 3.1.1. Geometries and Interaction Energies of Complexes. In the presence of intermolecular interactions among solvents, DMDS, and MTBE, binary complexes of DMDS···solvent and DMDS···MTBE can be formed. The structures of various complexes optimized at the M06-2X/631++G (d, p) level are displayed in Figure 2. 3.1.2. Intermolecular Interaction. The quantum theory of atoms in molecules (QTAIM) and the reduce density gradient (RDG) analysis were used to reveal the mechanism of interactions among solvents, DMDS, and MTBE molecules. C


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Figure 3. BCPs distribution of those complexes by AIM analysis. (A) DMDS···DMSO; (B) DMDS···DMF; (C) DMDS···DEG; (D) DMDS··· MTBE.

The bond critical point (BCP) existing between two molecules indicates the intermolecular interaction. BCP can be featured by the electronic density (ρ), laplacian values (∇2ρ), and energy densities. On the basis of QTAIM calculation, the BCP results for those complexes are shown in Table 1 and Figure 3. The parameters ρb, ∇2ρb, and energy densities (i.e., kinetic energy density Gb, potential energy density Vb and energy density Hb) can distinguish the supramolecular interactions. It was found that the ρ was below 0.05, ∇2ρ was positive, and Hb was negative, suggesting the week intermolecular interactions. The parameter |Vb|/Gb of below 1 represents the characteristic of closed interactions.48 Furthermore, bonding degree (BD, equals to Hb/ρ) was employed to describe the complementary index of noncovalent interactions and quantify the interaction strength of those complexes. BD values of those complexes are over zero, indicating the type of noncovalent interactions.49 The value of BD can be used to distinguish the interaction strength. The smaller BD indicates the stronger interaction strength. For DMDS···DMSO complex, the BCP(28) displays the lowest BD value (see Table 1), suggesting that the C−H20···O10S has

the most strongest interaction in the complex. The complexes of DMDS···DMSO, DMDS···DMF, and DMDS···DEG have two BCPs whose BD values are below 0.1, respectively. Those features indicate that the stronger interactions exist between solvent and DMDS. From Table 1, BD values of BCPs for DMDS···MTBE complex are all over 0.1, which indicates that the intermolecular interaction strength of DMDS···MTBE is weaker than those of other complexes. The minimum and second minimum BDs of the BCPs in those complexes are in the following order: C−H20···O10S (DMDS···DMSO) < C−H12···O10S (DMDS···DMSO) < C−S10···N19−C (DMDS···DMF) < C−H8···O20C (DMDS···DMF) < C− S13···O26−C (DMDS···DEG) < C−H10···O26−C (DMDS··· DEG) < S−C5···H25−C (DMDS···MTBE) < C−S10···H12− C (DMDS···MTBE). According to the judgment of interaction type, some BCPs between DMDS···DMSO, DMDS···DMF, and DMDS···DEG meet the standard of weak hydrogen bond, but it is weaker than traditional hydrogen bond.50 Therefore, it is called as similar weak hydrogen bond in this paper. From this analysis, all three complexes, DMDS···DMSO, DMDS···DMF, D


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Figure 4. Relationships of RDG versus ρ(r) for different systems. (A) DMDS···DMSO; (B) DMDS···DMF; (C) DMDS···DEG; (D) DMDS··· MTBE.

indicating that they all have attractive interactions. This is consistent with AIM result. Those spikes in the sign(λ2)ρ range of van der Waals (from −0.005 to 0.005) indicate the van der Waals forces involved in these binary complexes. Besides, several spikes can be observed in the region of sign(λ2)ρ < −0.01, suggesting that the stronger interactions happen. In addition, the gradient iso-surfaces (RDG = 0.5 au, dimensionless parameter) of those complexes are displayed in the 3D space (see Figure 6) to depict the distribution and variation of intermolecular interactions based on the analysis of RDG and sign(λ2)ρ. The blue−green−red scale, according to values of sign(λ2)ρ, is used to color the gradient iso-surfaces. It is clearly indicated that the complexes of DMDS···DMF and DMDS···MTBE have the strong repulsive interactions. All the complexes show green surfaces, suggesting that the weak attractive interactions occur between DMDS and solvent molecules. The absence of blue color in the 3D space means that these forces result from neither covalent interaction nor strong hydrogen bond interaction. As a result, these weak attractive interactions can be attributed to van der Waal force and similar weak hydrogen bond.50 On the basis of the geometry optimization, the calculated interaction energies are listed in Table 2. By using the BSSE correction, the energy of the intermolecular interaction (ΔE + BSSE) between DMDS and MTBE is calculated to be −12.12 kJ/mol, while the interaction energies increase from −18.17 kJ/

and DMDS···DEG, have stronger intermolecular attraction than DMDS···MTBE. Through the RDG method, the nonbonded interaction including repulsive and attractive interaction can been assured by associating with the electron density redistribution.51 The noncovalent interactions for various binary systems were also computed based on the same theoretical level to reveal the intermolecular interaction mechanism. Figure 4 displays the relationships of RDG versus ρ(r) for different systems. The interaction strength can be indicated according to the density value.30 The weak interaction can be recognized by finding several spikes appear at ρ < 0.05 and the corresponding RDG values of near zero. Specifically, ρ of less than 0.05 indicates the noncovalent interaction, while ρ of larger than 0.05 means the covalent interaction.30 From Figure 4, several spikes are found in the low-density (ρ < 0.05) and low RDG area, indicating the weak interactions existing in all of these binary complexes.52 Further differentiating the interaction type, the second Hessian eigenvalue (λ2) can distinguish attractive (λ2 < 0) interactions and repulsive (λ2 > 0) interactions,30 while the combined item sign(λ2)ρ can be used to judge the type and strength of interaction involved in certain region. Figure 5 displays the relationship between sign(λ2)ρ and RDG for different complexes. In the region of sign(λ2)ρ < 0, the complexes of DMDS···DMSO, DMDS···DMF, DMDS··· DEG, and DMDS···MTBE have similar spike distribution, E


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Figure 5. Relationship of RDG versus ρ(r) multiplied by the sign of the second Hessian eigenvalue (λ2). (A) DMDS···DMSO; (B) DMDS···DMF; (C) DMDS···DEG; (D) DMDS···MTBE.

mol for DMDS···DEG to −27.89 kJ/mol for DMDS···DMSO. It is indicated that all the three solvents have stronger interactions with DMDS in comparison with MTBE. Specifically, DMSO shows the strongest affinity with DMDS, therefore, can be expected to play the role of a promising solvent in extractive distillation process for DMDS removal from MTBE. The interaction energies of those complexes in Table 2 are all negative, which indicates that the attractive interactions are stronger than repulsive interactions. The computational results indicate that the main types of intermolecular attractive interactions involved in those complexes are van der Waals and weak hydrogen bond forces. 3.1.3. DPD Simulation. The distributions of DMDS in MTBE and extractants were calculated by DPD simulation. According the DPD simulation results, the parameters for DPD force field are shown in Tables 3 and 4. The values of repulsion parameter αij in different solvents and MTBE were set as 100 to avoid miscibility and it can form two layers in solvent− DMDS−MTBE system. In fact, the solvents can be miscible with MTBE. Figure 7 shows that the distribution of DMDS in MTBE and solvents derived from DPD simulation and the statistics results

of DPD simulation are listed in Table 5. The larger value of NSolvent/NTotal indicates the stronger affinity of extractant with DMDS. From Table 5, the NSolvent/NTotal for three systems ranks in the order: MTBE···DMDS···DMSO > MTBE··· DMDS···DMF > MTBE···DMDS···DEG. All the simulation results indicate that DMSO has stronger affinity with DMDS than the other two solvents. Furthermore, such conclusion will be confirmed by the batch extractive distillation experiments. 3.2. Batch Extractive Distillation with Constant Reflux Ratio. A series of batch extractive distillation experiments were first operated at a fixed solvent feeding flow (S) of 1 mL/min but without reflux to screen the optimal solvent. All of three solvent-adding systems exhibit distinct relationships of product yield versus sulfur content (see Figure 8). It is indicated that DEG, DMF, and DMSO can reduce the sulfide content largely, especially on the high-yield stage. The desulfurization efficiencies for different solvents are in the following order: DMSO > DMF > DEG. The experiment findings support our simulation results of both the solvent−solute interaction energies and DPD simulation. Among three solvents, DMSO can be considered the potentially suitable solvent in extractive distillation for the removal of DMDS from MTBE. F


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Figure 6. Gradient iso-surfaces (RDG = 0.5 au) for different complexes.

Table 2. Interaction Energies Involved in the Complexes with and without BSSE Correction interactions

BSSE (kJ/mol)

ΔE (kJ/mol)

ΔE + BSSE (kJ/mol)

DMDS···MTBE DMDS···DMSO DMDS···DMF DMDS···DEG

1.87 3.17 2.24 3.07

−13.99 −31.05 −24.98 −21.25

−12.12 −27.89 −22.74 −18.17

Table 3. Parameters for Calculation of Repulsion Parameter αij component

density, ρ (g/cm3)

molar volume, Vm (cm3/mol)

solubility parameter, δ (J/cm3)1/2

DMDS MTBE DMSO DMF DEG

1.06 0.74 1.10 0.95 1.12

88.9 119.0 71.00 77.4 94.9

20.02 15.68 21.49 24.35 29.89

Figure 7. Distributions of DMDS in different solvent systems. (A) MTBE···DMDS···DMSO; (B) MTBE···DMDS···DMF; (C) MTBE··· DMDS···DEG.

Table 5. Distribution Number N of DMDS in Different MTBE−Solvent Systemsa

Table 4. Repulsion Parameter αij of DPD Force Field

a

component

DMDS

MTBE

DMSO

DMF

DEG

DMDS MTBE DMSO DMF DEG

25.00 27.76 25.24 27.20 37.65

27.76 25.00 100 100 100

25.24 100 25.00

27.20 100

37.65 100

system

NSolvent

NTotal

NSolvent/NTotal

MTBE···DMDS···DMSO MTBE···DMDS···DMF MTBE···DMDS···DEG

108 53 24

320 320 320

0.338 0.166 0.075

Statistics scope of NSolvent: the number of DMDS in solvent.

3.2.1. Influence of Reflux Ratio. Reflux ratio is considered an important factor that determines both the performance and economy of an extractive distillation process. The batch extractive distillation experiments with and without solvent (S = 1 and 0 mL/min) were carried out to evaluate the influence

25.00 25.00 G


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concentrated in the rest of the raw material leaved in the kettle. Consequently, the increased separation difficulty at higher yield results in the varying maximum values of DMDS contents. 3.2.2. Influence of Solvent Flow Rate. For different reflux ratios, the influences of solvent flow rate on sulfur contents of overhead products are shown in Figure 10. With increasing solvent flow rates, the DMDS contents in distillate products decrease gradually, indicating the enhanced desulfurization efficiency of the extractive distillation process. One reason is that too low flow rate can enable the DMSO solvent to neither sufficiently disperse on the packing nor well contact with DMDS component. It is clear that both large reflux ratio and high solvent flow rate can benefit the removal of DMDS from MTBE based on an extractive distillation process. Considering the process economy, which can be largely determined by the solvent flow rate, 2 mL/min can be considered suitable. It is indicated that the sulfur contents in MTBE products can be reduced from 2000 to less than 30 μg/g with the solvent flow rate of 2 mL/min and reflux ratio greater than 0.2. Lower sulfur content (no more than 5 μg/g) is needed to meet the requirement of MTBE product, which can be used in clean gasoline blending. However, the batch extractive distillation operated at a constant reflux ratio and fixed solvent flow rate fail to provide the qualified MTBE products (sulfur content less than 5 μg/g) with higher than 50% product yield because of aforementioned reasons. 3.3. Reflux Ratio-Programmed Batch Extractive Distillation (RRPED). Furthermore, the extractive distillation operated at varying reflux ratios was found to be of an enhanced DMDS removing efficiency and able to produce superlow sulfur MTBE blends. The reflux ratio-programmed batch extractive distillation (RRPED) was conducted to upgrade the MTBE products to meet the required low-sulfur standard. Three sets of experiments were carried out with the different reflux ratioprogrammed runs, VRR1, VRR2, and VRR3 (see Figure 11). For VRR1, R was increased from 0.4 up to 2.2 at an interval of 0.2 within each 4% increment in yield of MTBE. VRR2 was run by continuing to raise R from 2.2 to 3.0 at an interval of 0.4 within every 8% increment of yield. Lastly, VRR3 was run by

Figure 8. Relationship between product yield and sulfur content with solvent feeding flow of 1 mL/min and without reflux.

of reflux ratio on sulfur contents of products at different distillation stages (see Figure 9). From the solvent-free experiments (results being presented in Figure 9A), the contents of DMDS in overhead products increase with the yield increasing. Figure 9 also indicates that higher efficiency for the separation of DMDS from overhead MTBE products can be achieved at higher reflux ratio. When the reflux ratio is increased over 0.3, the sulfur contents of MTBE products decrease slowly, which makes us consider 0.3 as the suitable reflux ratio. During further extractive distillation experiments, DMSO was fed at a flow rate of 1 mL/min. The results presented in Figure 9B reveal that DMSO can largely reduce the contents of DMDS in overhead products at various reflux ratios. Furthermore, the DMDS contents reach the maximum values at different product yields when using different reflux ratios. When the product yield is low, the column reflux flow plays crucial role in determining the product quality. Once the experiment reaches high yield, column reflux flow becomes little and the load of MTBE on packing decrease. With the same solvent flow rate, the load of the solvent on packing is relatively increased, therefore, can lead to the low content of DMDS. However, when the product with lower DMDS content was separated on high reflux ratio, DMDS are

Figure 9. Sulfur content versus yield for different systems with different reflux ratios (flow rate S = 1 mL/min). (A) Solvent-free; (B) DMSO-adding. H


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Figure 10. Sulfur content versus product yield at different solvent flow ratio S and reflux ratio R.

Figure 11. Reflux ratio-programmed runs.

Figure 12. Sulfur content in distillate products versus product yield for different RRPED runs.

continuing to raise R from 3.0 to 4.0 at an interval of 0.2 within every 4% increment of yield. From Figure 12, again the MTBE products from DMSOinvolved distillation process show lower sulfur content as compared to solvent-free distillation process. The solvent-free batch distillation process can only provide the MTBE products with less than 10 μg/g sulfur content before the product yield reaches 70%, suggesting that the larger reflux ratio should be maintained to achieve high desulfurization efficiency. The RRPED processes, by contrast, can produce low-sulfur MTBE up to much higher product yield at the cost of lower energy consumption. Among three RRPED experiments, VRR3 contributes the best performance for extractive distillation desulfurization (exhibiting lower sulfur content as compared to VRR1 or VRR2 in the yield range of 50−80%) because it reaches higher reflux ratio (until 4.0). However, too frequent adjustment to the reflux ratio is practically difficult in an

industrial extractive plant. Two sets of RRPED experiments were, hence, performed with more reasonable reflux ratioprogrammed runs, VRR4 and VRR5 (see Figure 13). From Figure 13, the sulfur contents of MTBE products obtained from VRR4 and VRR5 runs are observed in the ranges of 1.6 to 3.3 and 3.5 to 5.6 μg/g. Here, high R can still benefit the reduction of sulfur content in products. There exists a balance between product quality and operation cost of an industrial plant. The RRPED program should be made in such balance basis.

4. CONCLUSIONS From molecular simulation, the promising solvent for removing DMDS has been screened and the result of interaction energies calculation indicated that DMSO has a stronger interaction with DMDS. The order of efficiency for removing DMDS is I


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Figure 13. Sulfur content versus product yield with the reflux ratioprogrammed runs VRR4 and VRR5 at S = 2 mL/min.

DMSO > DMF > DEG. The interactions between solvents and DMDS are van der Waals and weak hydrogen bond forces. The interaction regions of those complexes are revealed in stereoscopic space. The distributions of DMDS in solvents and MTBE also are calculated using DPD. According to the computational results coupled with batch extractive distillation experiment using constant reflux ratio, dimethyl sulfoxide (DMSO) has the strongest interaction with DMDS and, therefore, the highest efficiency for DMDS removal. By employing the suitable RRPED process, the DMDS content in MTBE can be reduced from 2000 μg/g to less than 5 μg/g. The RRPED process was observed to achieve the largely enhanced desulfurization efficiency.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hui Sun: 0000-0002-8544-756X Xi Chen: 0000-0003-4274-7578 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Training Program of the Major Research Plan of the National Natural Science Foundation of China (Grant No. 91634112), the Natural Science Foundation of Shanghai (Grant No. 16ZR1408100), the Fundamental Research Funds for the Central Universities of China (Grant No. 22A201514010), and the Open Project of State Key Laboratory of Chemical Engineering (SKL-ChE16C01).

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Extractive Distillation Approach to the Removal of Dimethyl Disulfide

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