Journal of Molecular Liquids 180 (2013) 45–50

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Journal of Molecular Liquids

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Effect of CuO nanoparticle on dissolution of methane in water

Mostafa Keshavarz Moraveji ⁎, Milad Golkaram, Reza DavarnejadDepartment of Chemical Engineering, Faculty of Engineering, Arak University, Arak 38156-8-8349, Iran

⁎ Corresponding author. Tel.: +98 9363098063; fax:E-mail address: m-moraveji@araku.ac.ir (M.K. Mora

0167-7322/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.molliq.2012.12.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 July 2012Received in revised form 29 October 2012Accepted 9 December 2012Available online 21 December 2012

Keywords:CuO nanoparticleDissolutionMethaneHydrate

In this research, dissolution of methane in water with CuO nanoparticles was examined at temperatures of20, 15, 10, 8, 6 and 3 °C and at pressures of 10 and 15 bar. For this purpose, four samples including purewater, water with surfactant and two nanofluids with different concentrations were prepared. Each samplevolume was 200 cm3. Methane was contacted with each solution in a reactor with no mixer. Addition of0.05 wt.% SDS reduced surface and interfacial tensions by accumulating at the interface of immiscible fluids.This implies higher solubility of methane comparing with the pure water. The same procedure happened for0.1 wt.% CuO nanoparticles, as well. It was concluded that high effective surface area of nanoparticles im-proved the solubility. Furthermore, adding 1 wt.% nanoparticle, increased methane solubility by 144% and77.15% at pressures of 10 and 15 bar (compared with the pure water), respectively.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Recently natural gas hydrates (NGH) have been paidmuch attentionfor industrial utilization as a new method for storage and transport ofnatural gas in the form of clathrate-hydrates and crystalline solid com-pounds. The natural gas hydrates originally contain 150–180 volumes ofgas per volume of hydrate [1]. The industrial applications of NGH stor-age have been hindered by slow formation rate and volume of waterthat did not react withmethane gas that are results of very low solubil-ity ofmethane gas inwater (only a thin hydrate film is formed at the in-terface between water and methane gas [2]).

There have been a number of approaches proposed to increase sol-ubility of gas in the water that result enhancement of hydrate forma-tion, including chemical and mechanical methods. The mechanicalmethods use stirrer, liquid spraying by a nozzle and gas bubbling bya pipe or orifice plate [3,4] for solubility enhancement of gas inwater that result in hydrate formation. The agitation of water in-creases the water–gas interfacial area and improves gas solubility(hydrate formation).

In the chemical aspect, it is assumed that the hydrate formationrate can be increased efficiently by adding a certain quantity of ther-modynamic or kinetic promoters. According to the literature, hydratestorage properties can be improved by adding surfactants [5–7].

Surfactants are amphiphilic compounds which can reduce surfaceand interfacial tensions by accumulating at the interface of immisci-ble fluids. It can increase the solubility, mobility, bioavailability andsubsequent biodegradation of hydrophobic or insoluble organic com-pounds [6]. However, there are some limitations in the effective usesof the above methods in practice.

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Mandal and Laik studied effects of the anionic surfactants such asSDS on ethane hydrate formation, dissociation and storage capacity ina quiescent state system [7].

With the development of the synthesis technology of the nanometermaterials and thewide application of nanotechnology, the investigationof its effect on the heat and mass transport processes, in chemical andbiochemical technologies became more and more important.

Recently, Olle et al. measured the oxygen absorption rate in thepresence of dispersed colloidal nanoparticles of magnetite (Fe3O4)coated with oleic acid [8]. They found that the nanofluid improvedthe gas–liquid oxygen mass transfer coefficient more than 1.6-fold,while volumetric mass transfer coefficient increased up to 6-fold fora nanofluid (with volume fraction less than 1%) in an agitated andsparged reactor.

Krisnamurthy et al. measured the dye diffusion in liquids containingsuspended nanoparticles. They investigated that mass transfer in-creased in a nanofluid containing 20 nmAl2O3 nanoparticles (with vol-ume fraction of 0.5%) in comparison with the pure water [9].

Nagy et al. investigated mass transfer coefficient in the nanofluids[10]. They improved gas–liquid oxygen mass transfer coefficient to200% in a 65 nm suspension of n-hexadecane droplets (with volumefraction of 10%) while gas–liquid oxygen mass transfer coefficient in-creased to 600% in a suspension of 20 nm Fe3O4. The same trend wasobserved for the CO-water volumetric mass transfer coefficient whenmesoporous silica nanoparticles (with an average size of 250 nm)were used [11].

Li et al. used nano-copper suspension to increase the refrigerants—water mass and heat transfer [12]. They experimentally showed thatthe nano-copper addition increased heat and mass transfer duringHFC134a hydrate formation.

According to the literature, surface area enhancement and masstransfer coefficient increment increase the mass transfer [9,13,14].

46 M.K. Moraveji et al. / Journal of Molecular Liquids 180 (2013) 45–50

In the present work, the effects of nanofluids (CuO nanoparticleswith 40 nm diameter and weight fractions of 0.1% and 1% was dis-persed in water) on the methane gas dissolution were studied. Thesignificant goal of this research was to measure methane gas solubil-ity in the nanofluids under various operating conditions.

2. Experimental

2.1. Materials

Spherical copper oxide nanoparticles (with 40 nm nominal diam-eter, density of 6.32 g/cm3 and purity of 99%) were purchased fromMerck Co. Methane gas (with purity of 99.95%) was purchased fromTechnical Gas Services Co. (Iran) and deionized water (lab grade)was used for the hydrate formation. An anionic surfactant [SodiumDodecyl Sulfonate (SDS) with purity of 98%] as dispersant wasobtained from Acros Co. (Belgium).

2.2. Nanofluid preparation

Nanofluids with particle weight percentages of 0.1% and 1% wereprepared by dispersing the specified amounts of CuO nanoparticlesin a base fluid (deionized water). In order to stabilize the fluid, theprocedure demands the surfactant (SDS) dissolution. The samplewithout SDS was coagulated by 1 h. The sample coagulated within1 day when 0.01% SDS was applied. The fluid with 0.03% and 0.05%was coagulated within 5 days and a week, respectively. More thanthis amount of surfactant can influence the experiment to fully re-move the nanoparticles effect.

The samples were placed in an ultrasonic bath (Fritsch Ultrasonic,Laborette 17, with frequency of 50–60 Hz). It was used to break largeagglomerates of nanoparticles in the fluid and prepare a stable sus-pension. The uniformed and stable dispersed nanofluids were visuallyobtained.

Fig. 1. Schematic diagram of methan

2.3. Apparatus

The schematic diagram of the experimental set-upwhich is a customfabricated apparatus is shown in Fig. 1. The cell wasmounted on a pivot.The main part of set-up is a cylindrical high-pressure stainless steel cellwith an inner diameter of 8.4 cm, length of 21.4 cm and a total availablevolume of 1150 cm3 which was tested up to 150 bar. The cell wasequipped with a magnetically coupled rotary impeller system that is adisk impeller with six straight blades (Rushton turbine). The stirringspeed could vary between 0 and 1200 rpm which was kept at400 rpm in this study. A water bath (model: E200, manufacturedby Lauda Co.), able to maintain the temperature with accuracy of±0.01 K from the set point, was also used. There was a cooling jacketaround the reactor through which ethylene glycol–water solutioncould circulate out of the reactor. The high pressure cell was equippedwith a thermocouple (model: PT100 with accuracy of ±0.01 K) and apressure transducer (model: 5436 Worenlos with accuracy of ±0.1 barmanufactured by Huba Control Co.) was used to measure the celltemperature and pressure, respectively. Those measuring devices wereconnected to a data-logger and a personal computer to record the tem-perature and pressure versus time. Temperature and pressure wererecorded every second throughout the experiments and averagedevery 60 s. The reactor had some valves for injecting and venting gas.The other parts of the set-up were a vacuum pump to evacuate the celland a high pressure methane bottle to supply the methane.

2.4. Procedure

A required amount of nanofluid was weighed on a digital balance(up to 3 decimal points). The cell was rinsed with deionized watertwice. The solution was then poured into the cell. Air was removedfrom the cell by a vacuum pump and methane purging. The methanegas was then injected into the cell. Initial temperature of each testwas set at 15 °C. When the equilibrium was obtained (temperatureand pressure remained constant), the cooling process without mixingwas operated to decrease the system temperature down to 3 °C.

e hydrate formation apparatus.

47M.K. Moraveji et al. / Journal of Molecular Liquids 180 (2013) 45–50

There was a decrease in pressure due to dissolving methane gas in thesolution phase. Eight experiments (with pure water, mixture of waterand surfactant and two nanofluids) were carried out twice and eachone was done in two different pressures.

3. Results and discussion

In this research, the experiments were carried out to investigatethe effects of nanoparticle concentrations on the methane dissolution

Fig. 2. a. Methane pressure versus time at P=10 bar & T=3 °C. b. Methane pressure versusd. Methane pressure versus time at P=10 bar & T=10 °C. e. Methane pressure versus tim

process. The results were analyzed in terms of methane pressurereduction.

The cell reactor was filled up with the nanofluid and then saturat-ed with methane gas under the experimental conditions. The rate ofconsumed gas by the systems (trapped methane in hydrate cages)is equal to the methane pressure reduction. The experiments weredone in temperatures more than 3 °C to prevent hydrate formation.

Fig. 2(a–f) shows the effect of nanoparticles on dissolution ofmethane in water in various temperatures and at pressure of 10 bar.Fig. 2(a) and (c) shows a dramatic change in dissolution which was

time at P=10 bar & T=6 °C. c. Methane pressure versus time at P=10 bar & T=8 °C.e at P=10 bar & T=15 °C. f. Methane pressure versus time at P=10 bar & T=20 °C.

48 M.K. Moraveji et al. / Journal of Molecular Liquids 180 (2013) 45–50

due to existing SDS particles. However, nano-CuO particles can tangi-bly assist methane dissolution. There was no significant change intwo various concentrations of nano-CuO. Furthermore, all figuresshow that solubility reached the steady state condition. This wasdue to mass transfer and diffusion of methane through the nanofluid.In addition, the steady state condition is obtained during 15 min at8 °C while it was around 25 min at 3 °C. This is due to temperature ef-fect on methane mobility in the nanofluid. The mobility led to higherrate of mass transfer. As shown in Fig. 2(b), a significant change wasobserved by addition of different nanoparticle concentrations at

Fig. 3. a. Methane pressure versus time at P=15 bar & T=3 °C. b. Methane pressure versusd. Methane pressure versus time at P=15 bar & T=10 °C. e. Methane pressure versus tim

pressure of 10 bar and temperature of 6 °C. This showed that concen-trated solution highly causes methane dissolution at this pressure andtemperature. Fig. 2(d) and (e) shows the effect of nanoparticles atpressure of 10 bar and at temperatures of 10 and 15 °C, respectively.According to these figures, the higher temperature decreased meth-ane solubility. This output is also supported by Mohammadi et al.[15]. Fig. 2(f) illustrates that SDS and nanoparticles with higherconcentration (1 wt.%) increased methane solubility at pressure of10 bar and at temperature of 20 °C compared with the other solu-tions. Fig. 3(a–f) shows the effect of nanoparticles on dissolution of

time at P=15 bar & T=6 °C. c. Methane pressure versus time at P=15 bar & T=8 °C.e at P=15 bar & T=15 °C. f. Methane pressure versus time at P=15 bar & T=20 °C.

Fig. 4. Enhancement percentage of methane dissolution versus temperature at P0=10 bar in SDS, 0.1% and 1% nano-CuO solutions.

Fig. 6. Relative enhancement percentage of methane dissolution in 0.1% and 1%nano-CuO solutions at P0=10 bar.

49M.K. Moraveji et al. / Journal of Molecular Liquids 180 (2013) 45–50

methane in water in various temperatures and at pressure of 15 bar.According to Fig. 3(a–d), there was a little change in methane disso-lution compared with the SDS solution. Fig. 3(e) and (f) indicatesthat time is a significant parameter in the dissolution. Furthermore,the rate and capacity of methane dissolution increased in the aqueoussolutions with surfactant and nanoparticles in comparison with thepure water in particular at 3 °C (suitable for hydrate formation) andat pressures of 10 and 15 bar. The solubility at 15 bar was morethan that of 10 bar. Figs. 4 and 5 show the solubility reduction versustemperature enhancement at pressures of 10 and 15 bar, respective-ly. The best operating temperature for methane dissolution is 3 °C.Maximum percentages of methane dissolution are 144% and 142.1%for 0.1 wt.% and 1 wt.% nanofluid, respectively. As shown in Fig. 5,the best temperature was 3 °C that 77.15% enhancement in methanedissolution at 0.1 and 1 wt.% nanoparticles was obtained, although54.57% for SDS solution was found (in comparison with water).Fig. 6 illustrates that methane dissolution percentages were 22.2%and 24.1% for 0.1 and 1 wt.% nanoparticles respectively comparedwith the SDS solution. Fig. 7 illustrates that methane dissolution en-hancement is 22.58% for 0.1 and 1 wt.% nanofluid at pressure of10 bar. Therefore, nanoparticle addition into the SDS solution in-creased its capability although this was not as sharp as SDS addedinto the pure water. It was due to surface reduction and interfacialtension of SDS by accumulating at bigger surface and interface of im-miscible fluids. This implies higher solubility of methane comparedwith the pure water. According to the literature, 0.5 wt.% cyclodextrin

Fig. 5. Enhancement percentage of methane dissolution versus temperature at P0=15 bar in SDS, 0.1% and 1% nano-CuO solutions.

addition into the water increased the methane solubility around14.63% at temperature of 1 °C and at pressure of 3 bar [15].

The solubility enhancement percentage is calculated using the fol-lowing equation:

%Enhancement ¼2 Pi−Pj

� �� 100

Pi þ Pj

� � ð1Þ

where, Pi is the equilibrium pressure for the basic material (purewater). Pj is the methane pressure for SDS solution and nanofluids.

4. Conclusions

A dramatic enhancement inmethane solubilitywas observed in SDSsolution and nanofluid (with 0.1 wt.% CuO) in comparison with thepure water. This enhancement was more significant in low tempera-tures and at high pressure. The surface and interfacial tensions reducedwhen 0.05 wt.% SDSwas used. This caused higher solubility of methanecompared with the pure water. The same procedure happened when0.1 wt.% nanoparticles was used however highly effective surface areaof nanoparticles dramatically improved the solubility. The optimumconcentration was 1 wt.% CuO and 0.05 wt.% SDS. The best pressureand temperature were 10 bar and 3 °C, respectively. It led to 144% en-hancement in solubility compared with the pure water although itwas 24.1% compared with the SDS solution.

Fig. 7. Relative enhancement percentage of methane dissolution in 0.1% and 1%nano-CuO solutions at P0=15 bar.

50 M.K. Moraveji et al. / Journal of Molecular Liquids 180 (2013) 45–50

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