Molecular dynamics simulation guiding the improvement of EVA-type pour point depressant

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1 Fuel 84 (2005) Molecular dynamics simulation guiding the improvement of EVA-type pour point depressant Chuanjie Wu a, Jin-li Zhang a, *, Wei Li a, Nan Wu b a School of Chemical Engineering & Technology, Tianjin University, Tianjin , People s Republic of China b Chinese Science & Technology Exchange Center, Ministry of Science & Technology, Beijing , People s Republic of China Received 5 November 2004; received in revised form 21 December 2004; accepted 21 December 2004 Available online 19 January 2005 Abstract Addition of pour point depressants (PPDs) has been proved to be an efficient way to inhibit wax deposition of diesel fuels. However, the complexity of the oil is far beyond current commercial PPD products. So far it mainly depends on syntheses of numerous candidate compounds followed by repeating experimental measurements in order to improve the efficiency of PPDs. In this article, molecular dynamic simulation was successfully used to investigate the interaction between crystal planes of wax and EVA, as well as its derivatives with different branches, based on the model of wax. Side chain effects on adsorption energy and equilibrium adsorption conformations were studied under different kind and number of branches. It was concluded that side chains introduced by propylene were benefit to the affinity between the EVA-type molecules and alkanes in the wax plane, comparing with those branches introduced by butylenes. MD simulation calculations indicated that EVAP with one branch adjacent to the VA group would be a better PPD additive than EVA in diesel fuels, which has been proved in our experimental measurements. Therefore, the MD simulation is a promising method not only for exploring the interaction mechanism in polymer system, but also for directing the design of new candidates of PPD. q 2005 Elsevier Ltd. All rights reserved. Keywords: Molecular dynamics; Diesel fuel; Pour point depressant; Wax crystal 1. Introduction Pour point depressants (PPDs) are used as chemical additives when transporting crude oils and operating diesel engines at temperatures below their wax appearance points. There have been developed many kinds of polymers that are used as PPDs to influence the behavior of the paraffin crystallites formation [1 3]. The copolymer of ethylene vinyl acetate (EVA) is one of the popular commercial PPD products. The performance of EVA used as PPDs is greatly affected by many factors such as the content of vinyl acetate group, the molecular weight, the solubility parameter, etc. [4,5]. On the other hand, owing to the complexity of oils and diesel fuels, it is demanded to increase the efficiency of * Corresponding author. Tel.: C ; fax: C address: xinxing@tju.edu.cn (J.-l. Zhang). PPDs and to improve their adaptability to oils produced from different oilfields. In order to improve the efficiency of the additives, there appear theoretical analyses to explain the interactive mechanism of PPDs in oils [6], e.g. the arguments of adsorption, co-crystallization, nucleation, or improved wax solubility. However, so far it mainly depends on producing a number of compounds and repeating experimental measurements so as to improve the efficiency of PPDs. Recently, molecular simulation methods have attracted much attention to understand the interaction mechanism of polymers. Duffy and Rodger studied the interaction of poly(octadecyl acrylate) molecules with an octacosane crystal [7,8]. The previous work of our group investigated the performance mechanism of the copolymer of ethylene vinyl acetate propylene (EVAP) additives by using DFT and MM calculation methods [9]. It is still intriguing whether or not the molecular dynamics (MD) simulation can /$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi: /j.fuel 转载

2 2040 C. Wu et al. / Fuel 84 (2005) be used to direct the design of PPD candidates for the complex oil system. In this article, MD simulation was used to investigate the interaction between crystal planes of wax and EVA, as well as its derivatives of EVAP and the copolymer of ethylene vinyl acetate butylene (EVAB) with different branches. Side chain effects on adsorption energy and equilibrium adsorption conformations were studied under different kind and number of branches. All the results illuminate that the MD simulation is a promising method not only for exploring the interaction mechanism in polymer system, but also for directing the design of new candidates of PPD. 2. Calculation details 2.1. Adsorption energy calculation The adsorption energy was used to reflect the binding probability and stability between the adsorbent substrates and PPD molecules. The adsorption energy of the PPD molecule on the wax crystal surfaces was defined as follows DE Z EðAdsorption ComplexÞ K EðAdsorbentÞ KEðGuest MoleculeÞ where DE was the adsorption energy, kj/mol; E(Adsorption Complex) was the energy of the equilibrium complex between the PPD molecule and wax crystal surfaces, E(Adsorbent) was the energy of the sole wax crystal surfaces before adsorption, and E(Guest Molecule) was the energy of the sole PPD molecule before adsorption. A positive DE indicates that the complex of the PPD molecule and wax crystal surfaces is not stable and the adsorption process cannot happen automatically. While a negative DE suggests an automatic adsorption of the PPD molecule on the substrate, and the stability of the complex increase as the increment of the absolute value of DE. Three different compounds were selected as the model of PPD molecules, i.e. EVA, EVAP, and EVAB. These selected PPD models have similar structures with molecular weight about 2000 except various side chains in the molecular backbone. Following assumptions were put forward in order to get the simulation easier. (1) Vinyl acetate (VA) groups were distributed uniformly in the chain of PPD molecules. Therefore, the main chains of PPD molecules were composed of some segments of methyl and/or methylene groups. (2) The side chains resulted from the copolymerizing of propylene or butylenes were considered in the simulation, and the introduced branches were also distributed uniformly in the main chains of the PPDs. However, the potential branches produced by the isomerization during the polymerizing process were neglected. Fig. 1. Unit cell of paraffin crystal. (3) As the mean carbon number distribution of the wax crystal is among C16 C24, the wax crystal was modeled with the normal alkane of C 18 H 38, and the crystal cell was shown in Fig. 1. No other components from the diesel fuels were considered in the simulation system. The solvent effect on the interaction between PPD molecules and wax crystal surface was also neglected [9] Modeling the morphology and cell structure of wax crystals The Bravais Friedel Donnay Harker model and the Growth Morphology method provided in Cerius 2 were used to calculate the wax crystal morphology and its growth surfaces. Fig. 2 showed the morphology of the wax crystal, which was further analyzed by using X-ray powder diffraction simulation with the parameters listed in Table 1. Based on the crystal surface analysis, it is easy to find that surface (001), (010), (101) and (111) are the mainly visible surfaces of the wax crystals, among which surface (001) has the lowest surface energy followed by the surface (010) [10]. Surface (001) consists mainly of the methyl groups at the end of alkane molecules, while surface (010), (101) and (111) comprises mainly the methylene groups of alkane molecules, as shown in Fig. 3. During the simulation, surfaces (001) and (010) were selected as the representative adsorbent substrates and the corresponding adsorption energies were calculated. All the calculations were performed in the force field of CVFF95 using the molecular simulation software package Cerius (Accelrys, Inc.) on the SGI server system with 2 GB internal memory and eight 500 MHz CPU. The Coulomb interaction was calculated according to Coulomb rules, and the VDW energy was derived from Lennard Jones 12 6 potential. Parameters of the MD simulation were listed in Table 2. The conformation with highest frequency was obtained through the MD simulation. Based on the equilibrated state, the molecular mechanics

3 C. Wu et al. / Fuel 84 (2005) Results and discussion 3.1. Adsorption behavior of EVA on surface (001) and (010) Fig. 2. The calculated morphology of paraffin crystal. (MM) was further used to minimize the energy of the total system. To simulate the complex formation between PPD molecules and the adsorbent surfaces, periodic boxes were built up to confine the movement of the atoms in the adsorbent surfaces. The surfaces were put on one side of each cell box. Over the surface there was a space large enough to avoid the effect of the upside surfaces on the other cell box. After the periodic boxes were established, the PPD molecules were put over the frozen wax surfaces with a distance about nm to assure the interaction between the PPD and the surface during the simulation. Table 1 Parameters used in XRD simulation for the wax crystal Items Values X-ray source COPPER Wave length (Å) Polarization fraction 0.5 Crystal monochromator SINGLE Monochromator angle (deg.) Monochromator d-spacing (Å) Table 3 showed the adsorption energy of EVA molecules on surface (001) and (010) of paraffin. The content of vinyl acetate (VA) in EVA molecules ranges from 24 to 34%. It was indicated that the lowest adsorption energy corresponded to EVA with 31% of VA, and that the adsorption of EVA on surface (010) was much stable compared with the adsorption on surface (001). These results suggested that EVA molecules, as the PPD additive in diesel fuels, mainly interacted with the alkane chain in surface (010) versus the minor interaction with the terminal methyl groups in surface (001). It was also investigated that the adsorption conformation of EVA molecules on surfaces of paraffin in the same range of VA content. There appeared no significant influence of the polar groups of VA on the adsorption conformation. Fig. 4 showed the representative conformations of the EVA molecules with the VA content of 31% on surface (001) and (010) before and after the adsorption. The alkane backbone of the EVA molecules showed good affinity to the surface of paraffin but the polar groups of VA in EVA molecules extended out of the surface. It has been indicated in our previous work on DFT and MM calculation of the PPDs that the electronegativity of the terminal methyl groups was higher than that of the methylene groups in alkane chains. Therefore, it was reasonable that the vinyl acetate groups of the EVA molecules were repulsed by the methyl groups in the surface (001) of the paraffin. At the equilibrium adsorption states, EVA molecules kept accordant orientation with the alkane molecules of surface (010), which made the non-polar groups of EVA approximated closely to the lattice of the wax crystal cells while the polar groups extended outside this plane. It reflected the fact that the function of EVA additives is to hinder the extension rate of plane (001), but relatively facilitate the growth in the perpendicular direction to plane (001), so that the EVA inhibits the wax crystal to form larger 2-dimension plates that deteriorate the flow property. Further it was suggested that the acetate groups of EVA molecules not only served as the polar groups that prevented the consequent deposition of the alkane molecules from diesel fuels, but also divided the backbone of EVA molecules into non-polar alkane segments with certain carbon numbers. The carbon bonds in the non-polar alkane chain segments were flexible to form irregular coils, while the local carbon bonds around the acetate groups possessed rigidity. The nonpolar parts can provide good affinity between EVA molecules and the alkane chains in the plane (010), whereas the rigid parts can ensure EVA molecules to maintain a ready-interaction conformation to the wax crystal units. If the VA content was too low, e.g. using long chain normal alkanes with the same carbon numbers as the substituted

4 2042 C. Wu et al. / Fuel 84 (2005) Fig. 3. Visible surface models of paraffin crystal. additive for EVA molecules in the simulation, there obtained worse adsorption stability because of the random coiling and bending of the long chain alkanes. If the VA percentage was too high, the excessive rigidity of EVA molecules resulted in poor affinity to the wax surface and poor solubility in diesel fuels at even higher VA content. Thus, both the rigidity and the flexibility of molecules were important to the performance of PPD additives.

5 C. Wu et al. / Fuel 84 (2005) Table 2 Parameters used in molecular dynamic simulation Items Values Ensemble Canonical ensemble, NVT Thermostat Nosé-Hoover Relaxation time 0.1 ps Required temperature 300K Dynamics time step ps Number of steps 40,000 Boundary condition PBC Table 3 The adsorption energy on wax surface of EVA with different VA% Content of VA, % Adsorption energy on surface (001) Adsorption energy on surface (010) 34 K K K K K K K K Adsorption energy differences between surface (001) and (010) a Energy unit: kj/mol. a It was calculated as: differenceszadsorption energy on surface (010)K adsorption energy on surface (001) Side chain effects on adsorption behavior of EVAtyped polymers During the simulation, propylene and butylenes were, respectively, used as the third monomer in order to introduce branches into EVA molecules, and the corresponding polymers were named as EVAP and EVAB. Table 4 reflected the side chain effects on the adsorption energy of EVA-type polymers with the same VA content of 31% on surface (001) and (010). Comparing with EVA, EVAP showed lower adsorption energy on both surface (001) and (010), and the difference of adsorption energy between these two surfaces were larger than that of EVA when the number of branches introduced by propylene were below 3. When the number of branches was larger than 3, the adsorption energy on surface (010) decreased as the increase of the number of branches. It can be concluded that there exists an optimal number of branches introduced by propylene to get good performance of this kind of PPD candidate. Moreover, the position of branches also influenced the adsorption energy. In the case of only one branch, the adsorption energy of EVAP on plane (010) lowered significantly as the side chain got closer to the polar group of VA, although its adsorption energy on plane (001) showed a small amount of increment. These results suggested that EVAP with the optimal number and position of branches was superior to adsorption on surface (010) and can be used as the PPD additive in diesel fuels. As for the EVAB, its adsorption energy on plane (010) increased as the increase of branches introduce by butylene, which suggested poor stability of EVAB adsorption on surface (010). The adsorption energy of EVAB on surface (001) was lower than that of EVA when the branch number was below 3, while at higher branch number its adsorption Fig. 4. Side-viewed adsorption configuration of EVA (VAZ31%) on surface (001) and surface (010), respectively.

6 2044 C. Wu et al. / Fuel 84 (2005) Table 4 Side chain effects on adsorption energy on wax surface (001) and (010) Species of the third monomer The number of introduced side chains Adsorption energy on surface (001) Adsorption energy on surface (010) Propylene 1 (in the middle) K K (adjacent to VA) K K K K K K K K K K Butylene 1 K K K K K K Energy unit: kj/mol. Adsorption energy differences between on (001) and (010) planes energy increased owing to the stereo-hindrance of too many side chains. In order to detect the reason of differences between these two kinds of side chains, the empirical method PM3 was used to simply calculate atom charges of one representative segment of EVA, EVAP, and EVAB, respectively, as shown in Fig. 5. Fig. 5(a) and (b) indicated that the branch introduced by propylene obviously affected the charge of carbon atom connected with the VA group, i.e. the atom C4 in EVA charged 0.075, while the C5 in EVAP charged Additionally, other carbon atoms that did not connect directly with the VA group charged negatively. Carbon atoms in alkane molecules of surface (010) were also charged negatively, they had good affinity for molecules with similar electro-negativity. Therefore, the decrease of positively charged tertiary carbon atom in EVAP reduced the repulsion to the polar groups of EVAP and benefited to the molecular adsorption on surface (010). However, the effect of side chain on the charge of tertiary carbon atom lessened as the position of the side chain went far away from the VA polar group. On the other hand, too many introduced branches resulted in great variations of the atom charge distribution of the carbon backbone of EVAP, which together with the effect of stereo-hindrance made the difference of polar distribution between the EVAP and the alkane in surface (010) and then reduced their affinity. The atom charge of EVAB molecule segments shown in Fig. 5(c) were different from that of EVAP, although EVAB also possessed the positively charged carbon atom that connected with the VA group together with other negatively charged carbon atoms. Further, the equilibrium adsorption conformations enlightened us on the difference Fig. 5. Atom charge distribution of EVA, EVAP and EVAB molecule segment calculated using PM3.

7 C. Wu et al. / Fuel 84 (2005) Fig. 5 (continued) of the intermolecular interactions on surface (010) for both EVAP and EVAB. Fig. 6(a) (d) displayed the adsorption conformations of EVAP with different number of branches introduced by propylene, while Fig. 6(a1) (d1) were related to EVAB with different number of branches introduced by butylenes. In the case of EVAP, because there existed no polarity difference between methyl groups of the branches and

8 2046 C. Wu et al. / Fuel 84 (2005) Fig. 6. Side-viewed adsorption conformations on surface (010) for EVAP and EVAB with different number of branches in each segment. (a), (b), (c) and (d) corresponded to EVAP with branches introduced by propylene; (a1), (b1), (c1) and (d1) corresponded to EVAB with branches introduced by butylenes.

9 C. Wu et al. / Fuel 84 (2005) the methylene groups of the alkane molecules in plane (010), the close adjacent of the molecular backbone to the plane indicated good affinity between the alkane chains in plane (010) and EVAP with no more than two branches in each molecular segment. When there existed six branches in each segment of EVAP, the stereo-hindrance of the side chains was adverse to the affinity between EVAP molecules and plane (010) and then made the distance between these two parts increased. Fig. 6(a1) (d1) indicated that the distance between the molecule and plane (010) increased and there appeared humps around branches as the number of branches increased in each EVAB molecular segment. It is reasonable because the stereo-hindrance of branches and the electronegativity difference induced the poor affinity between EVAB and surface (010). Combining the side chain effects on adsorption energy and adsorption conformations for these EVA-type polymers, it was concluded that EVAP with one branch adjacent to the VA group would have better performance than EVA if it was used as the PPD additive, which has been proved in our experimental measurements. All these results illuminate that the MD simulation is an available guiding method to produce new candidates for polymer additives beyond the conventional numerous experimental synthesis and measurements. In order to improve the performance of EVA, two different kinds of side chains were introduced to produce two derivatives, i.e. EVAP and EVAB. Side chain effects on adsorption energy and equilibrium adsorption conformations were studied under different number of branches. It was concluded that side chains introduced by propylene were benefit to the affinity between the EVA-type molecules and alkanes in the wax plane, comparing with those branches introduced by butylenes. MD simulation calculations indicated that EVAP with one branch adjacent to the VA group would be a better PPD additive than EVA in diesel fuels, which has been proved in our experimental measurements. Therefore, the MD simulation is a promising method not only for exploring the interaction mechanism in polymer system, but also for directing the design of new candidates of PPD. Acknowledgements We greatly appreciated the Research Institute of Petroleum Processing of SinoPec for their kindly help in molecular simulation calculation. The project was partially supported by the National Nature Science Foundation of China ( and ). 4. Conclusion Molecular dynamic simulation was successfully used to investigate the interaction between crystal planes in the wax and EVA, as well as its derivatives with different branches. Based on the wax model, adsorption energy and equilibrium adsorption conformations were studied for EVA. At the adsorption equilibrium states, the non-polar groups of EVA approximated closely to the lattice of the wax crystal cells, while the polar groups extended outside the plane (001), which suggested that the function of EVA additives was to hinder the extension rate of plane (001) but relatively facilitate the growth in the perpendicular direction to plane (001). References [1] Wang SL, Flamberg A, Kikabhai T. Hydrocarbon Process 1999; 78(2):59. [2] Zhang F, Xie H, Dong L. Proc SPE Int Symp Oilfield Chem 2001;637. [3] Chanda D, Sarmah A, Borthakur A, Rao KV, Subrahmanyam B, Das HC. Fuel 1998;77(11):1163. [4] Machado AL, Elizabete F, Gonzalez G. J Petrol Sci Eng 2001; 32(2 4):159. [5] Zhou GH, Yang WY, Xu YL. J Appl Polym Sci 2002;83(4):815. [6] Zhang JL, Wu CJ, Li W, et al. Fuel 2003;82:1419. [7] Duffy DM, Rodger PM. PCCP 2000;2: ;3: ;4:328. [8] Duffy DM, Rodger PM. J Am Chem Soc 2002;124:5206. [9] Zhang JL, Wu CJ, Li W, Wang YP, Han ZT. Fuel 2004;83:315. [10] Hartman P, Bennema P. J Cryst Growth 1980;49:145.