Measurement and correlation of physical properties of aqueous solutions of tetrabutylammonium hydroxide,piperazine and their aqueous blends

Rizwan Safdar *,Abdul Aziz Omar Lukman B.Ismail,Arhama BariBhajan Lal

1 Chemical Engineering Department,Universiti Teknologi PETRONAS,Bandar Seri Iskandar,32610 Tronoh,Perak,Malaysia

2 School of Engineering and Physical Science,Heriot-Watt University Malaysia,Federal Territory of Putrajaya,62100 Putrajaya,Malaysia

Keywords:Density,ionic liquids Density Viscosity Refractive index

ABSTRACT The density,viscosity and refractive index of aqueous solutions of tetrabutylammonium hydroxide(TBAOH),piperazine(PZ)and their aqueous blends are determined at several temperatures(303.15 to 333.15 K).All these measured physicochemical properties decreases with an increase in temperature.The density data is used to calculate the coefficient of thermal expansion and excess molar volume of all aqueous binary and ternary solutions.The coefficient of thermal expansion increases with increase in temperatures and concentrations.The negativity of excess molar volume for all the aqueous solution decreased with increase in temperature.Each physical property is correlated with temperature by least square method and the corresponding coefficients for each property are presented.The prediction values from correlations for the physical properties are in good agreement with the experimental values.

1.Introduction

Global warming is major issue caused by greenhouse gases such as carbon dioxide,methane,nitrous oxide and water vapors[1].Among these greenhouse gases,CO2contributes increases in global temperature to 6°C[2-5].There is immense impact of global warming to the economic,social and environmental life.In this regard different techniques i.e.absorption through chemical solvent,physical absorption,separation by membrane,cryogenic process and microbial/algal systems have been developed,out of which most mature scientific and industrially adapted technique is absorption by liquid solvent[6-10].For a long time,different types of solvents have been employed to capture carbon dioxide from various gas streams[11-13].The most commonly used chemical solvents are alkanolamines,like monoethanolamine(MEA), diethanolamine(DEA)and methyldiethanolamine(MDEA) [14-16].Literature studies show that sterically hindered amines have high CO2loading capacity and easy regeneration of solvent[17-20].However,under certain conditions these sterically hindered amines have low CO2loading capacity than conventional alkanolamines[1].The blending of different alkanolamines to form new solvents is also a favorable approach as small addition of secondary alkanolamines to primary alkanolamines increases mass transfer coefficient of CO2two to three times without any change in properties of blends.Literature studies show that piperazine(PZ)is a good promoter for CO2absorption when it is used with alkanolamines[18,21].Despite the successful use of these solvents or their blends in industrial applications for CO2absorption,they still exhibit many drawbacks such as corrosion of pipe lines and equipments causing shutdowns of system,minimizing production,additional cost to restore the system,significant amount of energy requirement for regeneration of solvent,foaming and degradation of solvent[2,4,22-24].Due to the shortcoming of conventional solvents or blends,ionic liquids(ILs)as an alternative class of solvents appearasa promising choice to be used as absorbent for CO2separation.ILs from different groups including imidazolium,phosphonium,pyridinium and ammonium based have been synthesized and used for CO2separation[4,25-30].Among the ILs,ammonium based ILs appear as the most promising class as they are cheaper,easy to synthesized,water miscible and can absorb more CO2[31-33].The knowledge of physical properties like density,viscosity and refractive index are essential to design the acid gas removal systems[1,4,18,34-36].The thermophysical data for the tetrabutylammonium hydroxide and its blends with piperazine is unavailable,which makes a knowledge gap to use these types of ILs and their blends for different applications.In the present work physicochemical properties including density,viscosity and refractive index of aqueous TBAOH,PZ and their blended solutions over a wide range of temperatures(303.15 to 333.15 K)have been studied and reported systematically.The present study includes the analysis of the influence of temperature upon physical properties.Some other useful properties,coefficient of thermal expansion and excess molar volume are also derived by using density data.

2.Experimental Section

MDEA and PZ with a purity of≥99.00%are purchased from Merck.TBAOH with a purity of≥99.00%is purchased from Sigma Aldrich.All the chemicals are used without any further purification.The aqueous solutions of 55%and 10%TBAOH(by mass),10%PZ(by mass),aqueous blends of(11.11%+11.11%)TBAOH and PZ and(5%+5%)TBAOH and PZ are prepared in Ultrapure water(model Lab Tower EDI 15).The concentrations of the prepared solutions are measured by a weighing balance with an accuracy of±0.003.All prepared aqueous solutions are kept in airtight bottles before use.

2.1.Density measurement

The densities of binary and ternary aqueous solutions are measured at different temperatures ranging from 303.15 to 333.15 K using DMA 5000 with a measuring accuracy of±0.0006 g·cm-3.The measuring cell temperature is regulated with built-in solid state thermostat,maintaining a temperature within accuracy of±0.01 K.The reported densities are the average of three measurements.For better accuracy the equipment is calibrated with standard water of Millipore quality after each measurement.

2.2.Viscosity measurement

Electromagnetic viscosity meter(VINCI EV 1000)having a set of six calibrated pistons ranging from 0.02 mPa·s to 10000 mPa·s is used to measure the viscosities of all aqueous solutions at several temperatures(282.15 to 333.15 K).The complete system includes a viscosity measuring cell,a control station and a thermostatic bath.This viscometer directly senses the viscosity as associated software utilizes the variation of MacCoull formula.The reproducibility of the viscosity meter is±0.8%.The reported viscosities are the average of more than three readings measured after achieving the thermal equilibrium inside the measurement chamber.The thermostatic bath has a temperature control accuracy of±0.01 K.The equipment is calibrated repeatedly with standard water of Millipore quality after each measurement.

2.3.Refractive index measurement

The refractive indices of aqueous solutions of TBAOH,PZ and their aqueous blends are measured at various temperatures(303.15 to 333.15 K)using a digital Anton Parrefractometer(Abbemet)with an accuracy of±0.0005 nDand temperature control accuracy of±0.01 K.The reported data is the average of three measurements.For accuracy ofdata,the refractometer is calibrated repeatedly with water of Millipore quality after each measurement.

Table 1 Comparison of experimental results of pure MDEA and water with literature

3.Results and Discussion

All of the equipments used to measure the physical properties are calibrated by water of Millipore quality and pure MDEA of known properties.The experimental results are compared with literature and absolute relative deviation(ARD)values are presented in Table 1.Good agreement between literature and present work is found.The absolute relative deviation(ARD)is calculated by Eq.(1)[2].It is found that(ARD)for density ranges from 0.01 to 0.07,for the viscosity it ranges from 0.06 to 0.24 and for refractive index it ranges from 0.01 to 0.03.

Table 2 Densities(ρ,g·cm-3)of aqueous solutions of TBAOH,PZ and their aqueous blends at various temperatures

Fig.1.Densities of aqueous solutions of TBAOH,PZ and aqueous mixture ofTBAOH and PZ as a function of temperature:?,55%(by mass)TBAOH;?,10%(by mass)TBAOH;●,10%(by mass)PZ;▲,(11.11%+11.11%)TBAOH and PZ;■,(5%+5%)TBAOH and PZ.

Table 3 Correlation equation parameters and SD for densities(ρ,g·cm-3)of aqueous solutions of TBAOH,PZ and their aqueous blends

The reason for the deviations may be use of different apparatus,purity of chemicals and human errors.

The density measurements are performed for binary and ternary mixtures at different temperatures.With regard to influence of temperature on values of density for both types of mixtures,there is decrease in densities with the increase in temperature for all aqueous solutions as shown in Table 2 and Fig.1.The densities of aqueous blends of TBAOH and PZ increase with increase in compositions.

Fig.2.Comparison between the experimental and calculated values of density for aqueous solutions of:(a)TBAOH(55%,by mass);(b)TBAOH(10%,by mass);(c)PZ(10%,by mass);(d),TBAOH and PZ(11.11%+11.11%),by mass;and(e),TBAOH and PZ(5%+5%),by mass.

The experimental results show that density of aqueous solution of 10%(by mass)TBAOH is higher than 55%(by mass)aqueous solution of TBAOH for the same temperature and aqueous solution of 10%(by mass)PZ has higher density than aqueous solution of 55%(by mass)and 10%(by mass)TBAOH,aqueous blend of(11.11%+11.11%)(by mass)TBAOH and PZ and(5%+5%)(by mass)TBAOH and PZ for the same temperature.Overall it was found that density decreases with an increase in temperature.The same trend was found by Kavitha et al.and Kurnia et al.[37,38].Density is correlated with temperature by least square method by using Eq.(2)and the corresponding coefficients for this property are presented in Table 3.For density,the standard deviation(mean deviation)from the regression line and R2is calculated using Eq.(3).

where ρ represents density,A0and A1are correlation parameters and T is temperature in Kelvin.

The standard deviation(SD)for density data is calculated by using Eq.(3).

where SD represents standard deviation,XExpis experimental value,XCalis calculated value and n is number of data points.The comparison between experimental and predicted densities of all the aqueoussolutions is shown in Fig.2.The experimental density values are found to be in good agreement with calculated density values and their R2value is≥0.99.

The coefficient of thermal expansion αpis derived from density data and correlation parameters by using Eq.(4)[39].This property provides the information of how much a material can expand with respect to temperature.For designing of new acid gas removal systems,knowledge of the physical properties such as the coefficients of thermal expansion is also important.By considering this property,the behavior of mixtures a thigh temperature can be understandable which can result in more advancement of the acid gas removal system.

where αpstands for coefficient of thermal expansion,ρ is density,T is temperature,and A0and A1are correlation parameters of Eq.(2).The results are mentioned in Table 4.The standard uncertainty forαpis±0.02.When the temperature increases,the kinetic energy of the molecules increases and imparts them to larger distance which results in unwanted expansions.It can be seen from the results that there is only a slight change in coefficients of thermal expansions with increase in temperature for all of the aqueous solutions.The temperature did not appreciably affect the kinetic energy of the molecules and they slightly movedapart.The same trend was observed by the Kurnia et al.and Normawati et al.[38,40].This property also favors the tetrabutylammonium hydroxide and its blend with piperazine to be used as an absorbent for CO2separation systems at higher temperature so that all the unwanted thermal expansions in the absorbent will be avoided.

Table 4 Coefficient of thermal expansions of aqueous solutions of TBAOH,aqueous PZ and their aqueous blends

By seeing the slight change in coefficient of thermal expansion another useful property,excess molar volume()is also derived from the density data of TBAOH(aq),PZ solutions and their aqueous blends by using Eq.(5),same as used by Ayyaz Muhammad et al.[41].The calculatedvalues are presented in Table 5.The standard uncertainty foris±0.002.

Table 5 Excess molar volumes of aqueous solutions of TBAOH,aqueous PZ and their aqueous blends

Eq.(6)is used to calculate the excess molar volumes for ternary system where M1,M2,and M3are the molar masses of three components,ρ1,ρ2,and ρ3are the densities of pure component and ρ is the density of aqueous solution.

The experimental viscosity and refractive index of aqueous solutions of TBAOH,PZ and their aqueous blends are measured at different temperatures and are reported in Table 6.The obtained results of viscosity(η)and refractive index(nD)indicate that the values of both physicochemical properties decreased with increase in temperatures for binary and ternary solutions and their trends are shown in Figs.3 and 4.The same trend of decrease in viscosity and refractive index with increase in temperature was observed by the Kurnia et al.[38].The values of both physical properties of aqueous solution of 55%(by mass)TBAOHare higher than aqueous blend of(11.11%+11.11%)TBAOH and PZ,(5%+5%)TBAOH and PZ,10%(by mass)TBAOH and 10%(by mass)PZ.

Table 6 Viscosities(η,mPa·s)and refractive indices(n D)of aqueous solutions of TBAOH,PZ and their aqueous blends

The experimental results of(η)and(nD)are correlated by the leastsquares method as a function of temperature by using Eqs.(7)and(8)and coefficients are reported in Tables 7 and 8.

where A0,A1and A2are the correlation parameters,T is temperature in Kelvin.For all properties,the standard deviation(mean deviation)from the regression line and R2is calculated using Eq.(3).The correlation parameters and standard deviation values for all binary and ternary solutions are presented in Table 7.The experimental and calculated physicochemical properties are in good agreement and their R2values are≥0.93&≥0.99 respectively.The comparison between the experimental and calculated property viscosity and refractive index of all aqueous solutions is illustrated in Figs.5 and 6.

Fig.4.Refractive indices of aqueous solutions of TBAOH,PZ and aqueous mixture of TBAOH and PZ as a function of temperature:?,55%(by mass)TBAOH;?,10%(by mass)TBAOH;●,10%(by mass)PZ;▲,(11.11%+11.11%)TBAOH and PZ and■,(5%+5%)TBAOH and PZ.

Table 7 Correlation equation parameters and SDfor viscosities(η)of aqueous solutions of TBAOH,PZ and their aqueous blends

Table 8 Correlation equation parameters and SD for refractive index(n D)of aqueous solutions of TBAOH,PZ and their aqueous blends

Fig.3.Viscosities of aqueous solutions of TBAOH,PZ and aqueous mixture of TBAOH and PZ as a function of temperature:(a),?,55%(by mass)TBAOH;(b),?,10%(by mass)TBAOH;●,10%(by mass)PZ;▲,(11.11%+11.11%)TBAOH and PZ and■,(5%+5%)TBAOH and PZ.

Fig.5.Comparison between the experimental and calculated values of viscosity for aqueous solutions of:(a),TBAOH(55%,by mass);(b),TBAOH(10,by mass);(c),PZ(10%,by mass);(d),TBAOH and PZ(11.11%+11.11%)(e),TBAOH and PZ(5%+5%).

4.Conclusions

The physicochemical properties such as density,viscosity and refractive index of aqueous solutions of TBAOH,PZ and their aqueous blends are experimentally measured at different temperatures.All measured physicochemical properties decrease with an increase in temperatures.The results obtained from the coefficient of thermal expansion and excess molar volume for all the aqueous solutions indicate that these properties are influenced by the temperatures and concentrations.All the experimental values are correlated by least squares method and predicted values are estimated.The deviation calculations show a good agreement between the experimental and predicted values favoring the established correlations.The low density and high free volume of solvents and their blends can enhance the CO2absorption.Therefore the ionic liquid,TBAOH itself and with other solvents can be a good absorbent for CO2capturing.

Nomenclature

DEA diethanolamine

ILs ionic liquids

Fig.6.Comparison between the experimental and calculated values of refractive indices for aqueous solutions of:(a),TBAOH(55%,by mass);(b),TBAOH(10%,by mass);(c),PZ(10%,by mass);(d),TBAOH and PZ(11.11%+11.11%)(e),TBAOH and PZ(5%+5%).

MDEA methyldiethanolamine

MEA monoethanolamine

M1molar mass of water

M2molar mass of solvent

n number of data points

nDrefractive index

PZ piperazine

SD standard deviation

T temperature in Kelvin,K

TBAOH tetrabutylammonium hydroxide

XCalcalculated value

XExpexperimental value

x2,3mole fraction of solvent solvents

αpcoefficient of thermal expansion

η viscosity,mPa·s

ρ density of aqueous solution,g·cm-3

ρ2density of bidistilled water,g·cm-3

Acknowledgments

The authors are grateful to the CO2Management(MOR)research group of Universiti Teknologi PETRONAS for providing the financial support and facilities.