Effect of the benzene ring of the dispersant on the rheological characteristics of coal-water slurry:Experiments and theoretical calculations

Lin Li,Chundong M,,Shnpei Hu,,Meng He,Ho Yu,Qingio Wng,Xioqing Co,Xiofng You,*

a College of Chemical and Biological Engineering,Shandong University of Science and Technology,Qingdao 266590,China

b College of Safety and Environment Engineering,Shandong University of Science and Technology,Qingdao 266590,China

c National Engineering Laboratory for Coalmine Backfilling Mining,Shandong University of Science and Technology,Tai’an 271019,China

d Department of Resource and Civil Engineering,Shandong University of Science and Technology,Tai’an 271019,China

Keywords:Low rank coal Low-rank coal-water slurry Benzene ring Molecular dynamics simulation

ABSTRACT In this study,low-rank coal-water slurry (LCWS) was prepared using polyoxyethylene dodecylphenol ether (PDPE) and polyoxyethylene lauryl ether (PLE),respectively.A combination of experiments and simulations was used to investigate the pulping properties and microscopic mechanism of the LCWS samples prepared using the two agents,so as to explore the influence of benzene ring on the performance of dispersant.The results of the LCWS preparation experiments revealed that the pulp-forming performance of PDPE exceeded that of PLE.When LCWS concentration is 62%,64%,and 66%,the apparent viscosity corresponding to PDPE is 247.80,504.17,and 653.10 mPa·s,and the apparent viscosity corresponding to PLE is 548.10,1470.61,and 1549.98 mPa·s,respectively.The C1000(When the apparent viscosity is 1000 mPa·s,the corresponding concentration of LCWS is defined as C1000)values of PDPE and PLE are 67.60% and 62.95%,respectively.In addition to the van der Waals forces and hydrogen bonds between the PDPE and/or PLE molecules and coal,the benzene rings of PDPE present π-π stacking effect with the aromatic rings of coal.That could facilitate and strengthen the adsorption of PDPE on coal,which would be conducive to further improving the dispersion of coal particles.The two dispersants have no significant difference in effect on the pyrolysis of LCWS.The simulation results indicated that the times for PDPE and PLE molecules to reach flat adsorption state on coal are approximately 290 and 565 ps,respectively.The self-diffusion coefficient (D) of the PDPE and PLE on coal is 3.16 × 10-6 and 6.57 × 10-6 m2/s,respectively,and their interaction energies with coal are 785.71 and 648.60 kcal/mol,respectively.The results of the simulation calculations demonstrated that PDPE adsorbed on coal easier than PLE,and its binding is more stable than that of PLE owing to the π-π stacking effect,which is conducive to uniform dispersion of coal in solution.The simulation results confirmed the experimental results.

1.Introduction

Coal-water slurry (CWS) [1,2] is a high-efficiency and lowpollution coal-based liquid fuels developed in the 1980 s that feature both the original properties of coal and fluidity and stability of oil.The proportion of low-rank coal reserves and production in China account for more than 55%.The use of low rank coal is crucial for the current energy needs [3].The preparation of low-rank coal-water slurry (LCWS) from low-rank coal is a clean coal technology,which is suitable for China’s national conditions [4,5].In addition,low-rank coal is inexpensive,and therefore,the production cost of LCWS can be decreased by using low-rank coal.However,owing to the high intrinsic moisture,volatile content,and porosity of low-rank coal,it is difficult to prepare high concentration LCWS from it.

Dispersant [6,7] is the key factor in the preparation of CWS,so exploring high-efficiency LCWS dispersant is a simple and effective way to solve low-rank coal pulping.Studies have shown that dispersants containing benzene rings have better performance [8].Das and his co-workers[9-12]studied the influence of natural and mixed surfactant systems on the slurry ability of CWS.Kakui and Kamiya [13] studied the effect of sodium aromatic sulfonate on the viscosity of Tatung CWS;their study found that anionic polymers of sodium aromatic sulfonate,such as sodium benzenesulfonate and sodium naphthalenesulfonate,reduced the viscosity of high concentration CWS.Zhou et al.[14] compared the properties of CWS prepared by the modified wheat straw alkali lignin,lignosulphonate,and naphthalenesulfonate-formaldehyde condensate as dispersants.Phulkerd et al.[15] synthesized cardanol-formaldehyde sulfonate(CFS)by sulfomethylation of cardanol as an additive to CWS.It was found that the surface tension of CFS decreased with increasing concentration,indicating that it has good surface activity.But there is no special study on the mechanism of action of benzene ring.

The research on the properties of dispersants with the benzene ring mainly focuses on the experimental research,and the research on the interaction mechanism from the micro-level is lacking.Few studies on the microscopic mechanism of dispersants in CWS systems using molecular dynamics (MD) simulations [16-19],and dispersant molecules have been mainly studied as a whole.Wang et al.[20]studied the effects of ammonia nitrogen and low molecular weight organic compounds on the adsorption of sodium lignosulfonate in Shenhua coal using experiments and MD simulations.Wang et al.[21] used MD simulations to study the dynamic adsorption of coal by sodium methylene bisnaphthalenesulfonate in phenol solution and water,respectively.Lu et al.[22] used a three-dimensional body model to build a Jinjie coal (JC) surface model and investigated the interactions between naphthalenesulfonic formaldehyde condensates/water and the surface of JC.To date,there are few report on MD simulations to research the effect of a characteristic functional group of pharmaceutical molecules on the slurry ability of CWS.

In this study,two kinds of non-ionic dispersants were selected,namely polyoxyethylene dodecylphenol ether (PDPE) and polyoxyethylene lauryl ether (PLE).The effect of the presence or absence of benzene rings in the dispersant on the slurry properties of LCWS was analyzed.Moreover,the experimental results were confirmed using MD simulations.

2.Materials and methods

2.1.Materials

The coal quality analysis results of the low-rank (subbituminous) coal used in the experiment are summarized in Table 1.The particle size of the pulverized coal used in the experiment was analyzed using a BT-9300Z (Dandong Better Instrument Co.,Ltd.,China)laser particle size analyzer,and the results are depicted in Fig.1.Both PDPE (analytical grade,≥98.0% purity) and PLE (analytical grade,≥98.0% purity) were purchased from Jiangsu Haian Petrochemical Plant,China,and the number of ethylene oxide is 30.

Table 1 Analysis results of Shendong coal quality.

2.2.Preparation of LCWS

The total mass of each group of LCWS was 180 g,and the amount of coal and water was calculated according to the set concentration.The additive dosage is 1% of dry coal dosage.The coaladditive-water mixture was stirred for 10 min using a high-speed mixer (1000 r/min).

2.2.1.Rheological analysis

The rheological properties of the LCWS were analyzed using an NXS-11B(Chengdu Instrument Co.,Ltd.,China)rotational viscometer at room temperature,and the shear rate ranged from 0 to 120 s-1.To reduce the experimental error,the viscosity was recorded three times for each shear rate,and the viscosity was recorded once every 5 s,and the average value was finally taken.The Herschel-Bulkley(H-B)model was used to fit LCWS rheological data,as follows [23].

where τ is the shearing stress in Pa;τ0the yield stress in Pa;η the shearing rate in s-1;K the consistency index;and n the flow index(expansion plastic fluids n greater than 1,plastic fluids n=1,pseudoplastic fluids n ＜1).

2.2.2.Slurry ability

The apparent viscosity (ν100) is the viscosity measured at a shear rate of 100 s-1.To reduce the experimental error,the shear rate is uniformly increased from 0 to 100 s-1.When the shear rate reaches 100 s-1,the data is recorded every 5 s,a total of six times,the average of the data is the apparent viscosity of the CWS.The concentration at the time when ν100equal to 1000 mPa·s is defined as C1000.The higher C1000,the better the slurry ability of the CWS.In this study,C1000was obtained by preparing LCWS of different concentrations,determining their slurry ability indices,and interpolating the data.

2.2.3.Stability analysis

The glass rod penetration method was used to evaluate the stability of LCWS [24-26].After the preparation of LCWS,it was stored in the measuring cylinder for 24 h at room temperature(the diameter of measuring cylinder is 3 cm and the height of LCWS layer is 15 cm).A glass rod (20 g,5 mm in diameter) was allowed to drop freely from the surface of each LCWS,and then the data were recorded.The penetration ratio (P) was calculated as follows.

where d is the falling distance of the glass rod in cm;and dtthe maximum drop distances of the rod in cm.

2.3.Sedimentation experiment

A sedimentation experiment was conducted to evaluate the performance of the dispersants.Eight groups of coal slurry water were prepared,as illustrated in Fig.2.The concentration and volume of each coal slurry water were 1% and 100 mL,respectively,and the dispersant dosage was 1% of the dry coal powder dosage.The prepared samples were allowed to set for 0,0.5,1,3,5,7,12 and 24 h,respectively.Then,5 mL supernatant suspension was collected using a 5 mL syringe,each suspension was diluted 10 times,and the turbidity of each sample was measured using a 2100Q (Shanghai Shilu Instruments Co.,Ltd.,China) portable turbidimeter.

2.4.Pyrolysis experiment of LCWS

The test used a German NETZSCH STA 449F3 thermogravimetric(TG)analyzer.The heating rate was 20°C/min,the heating interval was 35-1000 °C.The purge gas was dry nitrogen at a flow rate of 20 uL/min.

2.5.MD simulation

The MD simulation was carried out using the Forcite module(Materials Studio 8.0).To save calculation time,the authors focused on the effect of the benzene ring of the dispersant molecule.The carbon chain length of the dispersant molecule and degree of polymerization of polyoxyethylene were simplified,asillustrated in Fig.3a.Because the benzene ring of PDPE possessed two carbon atoms attached to nonyl chains,two methylene groups were added to the PLE model to reduce experimental errors.Furthermore,a low-rank coal model was built based on the Hatcher coal model [27,28],as presented in Fig.3b.

Fig.1.Coal particle size.

Fig.2.Initial state of settlement experiment.

Fig.3.Simplified model of pharmaceutical molecules and low-rank coal molecular model.

The simulation was performed in a 35×35×100 Å3simulation cell with three-dimensional periodic boundary.The system contained 20 coal,9 agent,and 2000 water molecules(Fig.4).The simulation was carried out using the COMPASS force field [29,30].Detailed simulation process can refer to Lyu et al.[31].

The equation of COMPASS force field can be represented as follows.

Fig.4.Initial state of simulation system (the water molecules are hided in the simulation for easy observation).

where Etotal,Ebond,Eangle,Eoop,Etorsion,Ecross,Eelect,and Eljare the total potential energy,bond stretching energy,angle bending energy,out-of-plane interacting energy,bond torsion energy,cross-term interacting energy,coulomb energy,and van der Waals energy,respectively.

3.Results and discussion

3.1.Results of LCWS experiment

3.1.1.Rheology

The effects of two dispersants i.e.PDPE and PLE on the rheological properties of the LCWS were studied at room temperature and the same pH.

All rheological curves in Fig.5 present y-intercepts,which indicates that the prepared LCWS are non-Newtonian fluids.The LCWS of different concentrations obtained using PDPE as dispersant present shear thinning characteristics(Fig.5a).Only two of the LCWS obtained using PLE present shear thinning characteristics(Fig.5b):those with the concentration of 62% and 64%,whereas the shear stress depended linearly on the shear rate for the LCWS with the concentration of 66%.The H-B fitting results of the rheological data of the two dispersants are presented in Table 2.

Table 2 H-B model fitting results.

Fig.5.Rheological curves of LCWS at different concentrations.

For the same LCWS concentration,the yield stress of the LCWS obtained using PDPE is lower than that of the LCWS obtained using PLE (Table 2).The n values obtained by fitting the rheological curves of the LCWS obtained using PDPE are all lower than 1,which indicates that these LCWS are pseudoplastic fluids at the concentrations of 62%,64%,and 66%.The n values obtained after fitting the rheological curve of the LCWS obtained using PLE are lower than 1 for the LCWS concentrations of 62% and 64%,which indicates that these two LCWS are pseudoplastic fluids,and n=1 for the LCWS concentration of 66%,which indicates that this is an approximately plastic fluid.The fitting results reveal that R2is 0.99 for all the analyzed samples.This indicates that the fitting effect is good,and therefore,the H-B model is suitable for LCWS rheological data fitting.The fitting results of the rheological data in Table 2 reveal that the properties of the LCWS prepared using PDPE are superior to those of the LCWS prepared using PLE.This is mainly due to the fact that in addition to the alkyl chain binding to the hydrophobic group in the coal,the benzene ring in PDPE also has a π-π stack with the aromatic ring in the coal [32,33].The interaction between PLE and coal is mainly the interaction between the alkyl chain and the hydrophobic group in the coal.This indicates that the adsorption of PDPE on coal is easier and more stable than that of PLE.For LCWS,after the coal adsorbs the dispersant,the hydrophilic groups of the dispersant are still exposed to water,and therefore,the dispersant is bound to water via hydrogen bonds.Therefore,the stable adsorption configuration of PDPE on coal led to the increase in the number of hydrophilic groups on coal,which is conducive to forming a hydration film,and consequently,improves the pulping effect of PDPE.

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3.1.2.Slurry ability

Fig.6 depicts the relationships between the apparent viscosities and mass concentrations of the LCWS prepared using PDPE and PLE.The C1000values of the LCWS prepared using the two dispersants are obtained via interpolation (Fig.6).The C1000values of the LCWS prepared using PDPE and PLE are 67.60% and 62.95%,respectively.This indicates that PDPE is a better dispersant than PLE.In addition,the ν100values of the LCWS of different concentrations prepared using PDPE are not significantly different between two adjacent concentrations and gradually increase with the increase in concentration.Conversely,the ν100values of the LCWS prepared using PLE increase rapidly as the concentration increased from 62% to 64%.Moreover,the ν100values of the LCWS with the concentrations of 64% and 66% prepared using PLE do not change significantly,and are higher than 1000 mPa·s.This negligible change in ν100is attributed to the degree of particle agglomeration being already high at the LCWS concentration of 64%,and thus,further increasing the concentration does not significantly affect ν100.These results demonstrate that the rheological properties of the LCWS prepare using PDPE are superior to those of the LCWS prepare using PLE,which indicates that the presence of the benzene ring in the structure of PDPE is beneficial.

Fig.6.Relationship between apparent viscosity and concentration of different dispersants.

3.1.3.Stability

Fig.7 shows the stability of LCWS.The stability of the LCWS prepared using the same dispersant decreases as the concentration increases(Fig.7).This indicates that as the concentration increases,the degree of particle agglomeration increases,and the lubricating buffering effect of water decreases.For the same concentration,the stability of the LCWS prepared using PDPE exceeds that of the LCWS prepared using PLE.This indicates that the hydrophobic groups of PDPE combine with the hydrophobic groups of coal[29,34].Moreover,π-π stacking occurs between the benzene rings of PDPE and aromatic rings of coal,and that renders the adsorption of PDPE on coal more stable than that of PLE.The presence of the benzene ring in the structure of PDPE improves the adsorption capacity on coal,and therefore the coal particles that adsorbed PDPE can be more stably dispersed in water.

Fig.7.Stability of LCWS.

3.2.Settlement test results

The results of the settlement tests are illustrated in Fig.8.During the initial stage of the settlement test,the turbidity of the upper parts of the suspension obtained using the two dispersants is almost the same (Fig.8).This indicates that the suspensions were well shaken during the preparation stage of the test.The turbidities of the upper parts of the two suspensions greatly differ before 5 h,and the difference between turbidities is most noticeable during the first 3 h of the settlement test.As the settling time increases beyond 5 h,the turbidities of the two suspensions are almost the same.This is mainly attributed to the dispersion performance of the coal particles in the suspension prepare using PDPE being superior to that of the suspension prepare using PLE during the initial stage of the settlement test,and thus,the settling speed of the coal particles in the suspension prepare using PDPE is significantly lower than that of the particles in the suspension prepare using PLE.This could be attributed to the π-π stacking effect between the benzene rings of PDPE and aromatic rings of coal,which facilitate the binding of PDPE to coal,and therefore,facilitate the dispersion of coal particles in water.However,as the settlement time increases,the suspensions settle further,and only 5 mL of the suspension is collected from the upper layers for turbidity measurement,resulting in the same turbidity measured by the two.

3.3.Analysis of pyrolysis test results

Combining Fig.9a and b,it can be seen that the pyrolysis of LCWS prepared by the two dispersants includes three stages.The first stage is the initial temperature to 170 °C,which is mainly to remove a large amount of water from the sample.The second stage is 170 to 400°C,the sample loses less weight,and a small amount of weight loss is a slight pyrolysis of the coal sample.The third stage is from 400 °C to the final temperature.Combined with the DTG (the derivative of TG data with respect to time) curve,it can be found that there is a small weight loss peak at 700 °C,which is mainly formed by the rapid decomposition of carbonate.

It can be seen from the DTG curve that both samples correspond to three weight loss peaks,the weight loss peak of water at 128°C,the pyrolysis peak of coal at 460°C,and the decomposition of carbonate at 700°C.The initial precipitation temperature of pyrolysis products is 400 °C.

Combined with the above analysis,there is almost no difference in the effects of the two dispersants on the pyrolysis of LCWS.This may be because a single benzene ring has little effect on pyrolysis,and considering that coal itself contains a large amount of benzene rings,it is difficult to observe the effect of two dispersants on the pyrolysis of LCWS.

Fig.8.Settlement test results.

3.4.Simulation results

3.4.1.MD simulation of adsorption process

Simulations were run for 1000 ps,and the calculation process is presented in Fig.10.First,the dispersant molecules are evenly dispersed in water.In time,the dispersant molecules first diffuse freely in water,and then,they gradually converge toward the coal surface,and are further flat-adsorbed on coal.From the time evolution of the process,it can be concluded that at 290 ps,the PDPE molecules are flat-adsorbed on the surface of coal.Moreover,some of the PLE molecules are located close to the coal surface and formed angles with it,but are not effectively adsorbed on coal until the 565 ps mark.This demonstrates that PDPE can be adsorbed on coal stably in a shorter time than PLE.

3.4.2.Mean square displacement (MSD)

The migration and diffusion capacity of dispersant molecules on coal can be studied using the mean square displacement (MSD)and self-diffusion coefficient (D),which can be calculated using Eq.(4) and the Einstein’s equation (Eq.(5)) [35],respectively.

where N is the number of atoms;ri(0) the initial position;ri(t) the position after time t;i the i-th atom;j the j-th atom;and the angled brackets denote the population mean.

Furthermore,the calculation method of D could be expressed as follows.

where KMSDis the slope of the MSD curve.

Fig.11 presents the MSD curves of the dispersants in this study.The D values of PDPE and PLE are calculated to be 3.16×10-6and 6.57×10-6m2/s,respectively.The D value of PDPE is significantly lower than that of PLE,which indicates that PDPE is more likely to adhere to coal than PLE.This is probably due to that PDPE will be bound to coal by the same van der Waals force and hydrogen bonding as PLE.Besides,PDPE will also be affected by the π-π stacking effect of the benzene ring to improve the binding capacity with coal.Thus,the diffusion rate of PDPE on coal decreases.

3.4.3.Binding energies

The binding energy can be used to calculate the relative strength of the interaction between additives and coal.The total binding energy (Ebind) can be calculated as follows [36].

Fig.9.Pyrolysis curve of LCWS.(The point A is the peak point on the DTG curve when coal burns violently;point B is the intersection of the vertical line passing through point A and the TG curve;point C is the intersection of the parallel line of the corresponding point at the beginning of weightlessness and the tangent line of the TG curve at point B,and the temperature corresponding to point C is defined as the ignition point temperature.)

Fig.10.1000 ps simulations process (for the convenience of observation,set the color of PDPE to green and PLE to brown).

Fig.11.MSD of the agent molecules.

where Emediumis the potential energy of coal/water system;and Ead-ditivethe potential energy of additives.

The dispersants could be selected by quantitatively analyzing the interactions between the dispersants and mineral surfaces[37].The strength of the interactions can be estimated by comparing Ebindvalues.The larger the positive Ebindvalue,the stronger the adsorption.Table 3 indicates that the adsorption of the PDPE-coal system is stronger than that of the PLE-coal system.This is attributed to the π-π stacking interactions between the aromatic rings of the coal matrix and the benzene rings of PDPE.Therefore,these results demonstrate that PDPE is more effective than PLE as LCWS dispersant.

Table 3 Binding energy of different dispersant systems.

4.Conclusions

In this study,PDPE and PLE were selected as LCWS dispersants.A combination of experimental results and simulation data was used to analyze the performance and microscopic mechanism of the LCWS preparation using the two dispersants.

The experimental results indicate that both dispersants could be successfully used to prepare LCWS.However,for the same dispersant amount and LCWS concentration,the LCWS prepared using PDPE present better rheological properties,higher C1000value,and better stability than that prepared using PLE.The C1000values of the LCWS prepared using PDPE and PLE are 67.60% and 62.95%,respectively.The settlement test results demonstrate that the settling rate of the suspension prepared using PDPE is lower than that of the suspension prepared using PLE for the same settling time.This indicates that PDPE is more favorable for the dispersion of coal particles than PLE.Moreover,the authors conclude that both dispersants interact with coal via van der Waals forces and hydrogen bonds,which improves the dispersion of coal particles in water.In addition,the benzene rings of PDPE stacked with the benzene rings of coal,which further improves the stability of the binding of PDPE to coal,thus improving the performance of the LCWS prepare using PDPE.The two dispersants have no significant difference in effect on the pyrolysis of LCWS.

The calculation results demonstrate that both dispersants can be successfully adsorbed on coal and PDPE is more easily adsorbed to coal stably than PLE.According to the authors’ calculations,the D and Ebindvalues of PDPE are significantly lower and higher,respectively,than those of PLE.The D values of PDPE and PLE are calculated to be 3.16 × 10-6and 6.57 × 10-6m2/s,respectively.The Ebindof PDPE and PLE are 785.71 and 648.60 kcal/mol,respectively.These results indicate that PDPE is more easily adsorbed on coal than PLE,and that is attributed to their π-π stacking interactions between the aromatic rings of the coal matrix and the benzene rings of PDPE.Thus,the simulation results confirm the experimental results.

Acknowledgements

This work was supported by National Natural Science Foundation of China(Nos.51904174 and 52074175),Postdoctoral Science Foundation of China(No.2019M662403),Young Science and Technology Innovation Program of Shandong Province (No.2020KJD001),Key Research and Development Project of Shandong(No.2019GGX103035),SDUST Research Fund (No.2018TDJH101),and Project of Shandong Province Higher Educational Young Innovative Talent Introduction and Cultivation Team.