Mass transfer in nanofluids: A review

International Journal of Thermal Sciences 82 (2014) 84e99

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Mass transfer in nanofluids: A review Seyedeh-Saba Ashrafmansouri, Mohsen Nasr Esfahany* Department of Chemical Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 September 2013 Received in revised form 20 March 2014 Accepted 26 March 2014 Available online

Growing attention has been recently paid to nanofluids because of their potential for augmenting transfer processes e i.e., heat and mass transfer. Conflicting results have been reported in the literature on mass transfer in nanofluids. The aim of this paper is to summarize the literature on mass transfer in nanofluids stating the conflicts and possible reasons. Literature on mass transfer in nanofluids has been reviewed in two sections. The first section concentrates on surveying mass diffusivity in nanofluids while the second section focuses on convective mass transfer in nanofluids. In each section, published articles, type of nanofluids used, size and concentration range of nanoparticles, measurement methods, maximum observed enhancement, and suggested mass transport mechanisms are summarized. Ó 2014 Elsevier Masson SAS. All rights reserved.

Keywords: Nanofluid Transfer processes Mass transfer Mass diffusivity

1. Introduction Nanoparticle colloids have particular physical properties that make them useful for a wide range of applications including paints and coatings, ceramics, drug delivery and food industries [1e5]. Colloids composed of ultrafine nanoparticles (w100 nm or smaller) are called nanofluids [5]. Growing attention has been recently paid to nanofluids because of their enhancement in heat transfer [5e11]. This desirable characteristic opens numerous applications of nanofluids as super-coolant in nuclear reactors, car engines, radiators, computers, X-rays and many other industrial products. Nanofluids are called super-coolant because they can absorb heat more than any traditional fluids, so they can reduce the size of system and increase its performance [5,12]. Oxide ceramics (Al2O3, CuO); nitride ceramics (AlN, SiN); carbide ceramics (SiC, TiC); metals (Ag, Al, Au, Cu, Fe); semiconductors (SiO2, TiO2); single, double, or multi wall carbon nanotubes (SWCNT, DWCNT, MWCNT); and composite materials such as nanoparticle coreepolymer shell composites are certain materials which are used to produce nanoparticles and are dispersed in a host liquid to make the nanofluid. Water is used as a traditional host liquid due to its high thermal conductivity, abundance, low cost, and friendliness to the environment [12]. Although by adding nanoparticles to the base fluid, the reduction in heat transfer coefficient has also been observed, most

* Corresponding author. Tel.: þ98 311 3915631; fax: þ98 3113912677. E-mail address: [email protected] (M. Nasr Esfahany). http://dx.doi.org/10.1016/j.ijthermalsci.2014.03.017 1290-0729/Ó 2014 Elsevier Masson SAS. All rights reserved.

studies show enhancement in heat transfer. To interpret enhancement, different mechanisms such as Brownian motion of nanoparticles and induced microconvection, thermal diffusion, increased conduction through aggregates, or particle-to-particle coupling through the interparticle potentials, liquid layering on the nanoparticleeliquid interface and reduction in thermal boundary layer thickness have been reported [11e13]. Since some researchers considered Brownian movement of nanoparticles as one of the major responsible factors in the enhancement of heat transfer, investigation of mass transfer enhancement in nanofluids with similar mechanism has been initiated [13e15]. Investigations on mass transfer in nanofluids can be divided into two main groups. The first group of studies deals with studying diffusion coefficients in nanofluids and the second group of studies focuses on studying convective mass transfer coefficients in nanofluids [16]. Mass diffusion is a molecular phenomenon refers to the diffusive transport of a species due to concentration gradients in a mixture. But, convective mass transfer occurs whenever fluid flows; that is, some mass is transferred from one place to another by the bulk fluid motion [17]. Most investigations on mass transfer in nanofluids have attempted to the second group and there are limited researches focusing on mass diffusivities in nanofluid systems. The greatest diffusivity enhancement has been reported by Fang et al. [18]. They found that the diffusion coefficient of Rhodamine B in Cuewater nanofluid with 0.5% Cu nanoparticle volume fraction is 26 times greater than that in the base fluid at 25
C [18]. Another group described observations of a 14-fold increase in diffusion coefficient of fluorescein dye in an aqueous

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suspension of Al2O3 nanoparticles with nanoparticle concentration of 0.5 vol% (volume fraction). If such mass diffusion enhancements are proved, it opens up numerous additional applications for nanofluids where mass diffusion is important. For example, many microfluidic devices, such as “lab-on-a-chip” type of systems, have limited mass transfer at low Reynolds numbers. If the mass transfer can be improved by passive, nonreacting nanoparticles, a convenient and inexpensive technique to improve the performance of microfluidic devices will be achieved [13]. Also, such results increase the possibility of manipulating the mass diffusion to accelerate micromixing processes [5,18e20]. The results of mass diffusion coefficients obtained by various researches are inconsistent. Some groups have reported enhancement in mass diffusion by addition of nanoparticles to the base fluid [13,18,21] while some observed reduction in diffusion coefficient [22,23] or no enhancement [5,24,25]. These conflicting results show that more researches are required to obtain reliable experimental data and understand the mechanism of mass transport in nanofluids [24]. For the second group studies of mass transfer in nanofluids (convective mass transfer), enhancements in nanofluids have been reported for a variety of nanoparticle types and nanoparticle concentrations. In this group of studies, by increasing nanoparticle concentration, mass transfer enhancement in the presence of different kinds of nanoparticles have been mostly reported, with the highest enhancement observed for carbon dioxide mass transfer, 48 times larger into water in the presence of 1% volume fraction of iron oxide nanoparticles [25,26]. The values of observed enhancements in mass diffusion coefficient and convective mass transfer coefficient in nanofluids are much greater than the reported values of enhancement in heat transfer studies. This shows the importance of dealing with studying the influence of nanoparticles on mass transfer and presenting mechanisms which are able to predict such enhancements. Although some review papers have been published on heat transfer in nanofluids [12,27e30], but mass transfer in nanofluids have not been reviewed yet. It is clear that such review paper provides the possibility of better comparison between the results of various researches in the field of mass transfer in nanofluids and can be very useful for future studies to achieve the reliable experimental data and to elucidate the reasons and mechanisms behind mass transport in nanofluids. So, this paper reviewed researches which have studied mass transport in nanofluids. In this work, researches on diffusion coefficient and convective mass transfer coefficient in nanofluids are reported in separate sections. In each section, performed studies, type of nanofluids, size and concentration range of nanoparticles, mass transfer measurement method, maximum observed enhancement and suggested mass transport mechanisms are pointed out.

2. Mass diffusion coefficient in nanofluids This section focuses on studying mass diffusion coefficient in nanofluids. In 2006, research in this field was initiated by Krishnamurthy et al. [13], who studied mass diffusion of fluorescein dye in nanofluids by taking time-dependent images [14]. They visualized dye diffusion in water-based nanofluids with 20-nm Al2O3. In their work, nanofluids were prepared by dispersing Al2O3 nanoparticles in deionized water for volume fractions from 0.1% to 1%. By postprocessing the images obtained from their experiments, they determined the diffusion coefficient of dye in nanofluids based on the mean displacement of the dye. They observed that the diffusion coefficient of fluorescein dye in the nanofluid was greater than that in deionized water, with a maximum 14 times enhancement in the

85

diffusion coefficient of dye at a nanoparticle volume fraction of 0.5% relative to in deionized water [13]. Based on an order-of-magnitude analysis, they suggested that the velocity disturbance field in the fluid, created by the Brownian motion of nanoparticles can be responsible for such enhancement. They assumed that with increasing nanoparticle volume fraction there is a greater possibility for particle aggregation, producing in effect larger, more massive particles with reduced capacity to increase localized convection and mass diffusion [13]. Similar to Krishnamurthy et al.’s observation [13], Fang et al. [18] also reported extraordinary enhancement in diffusion coefficient because of the presence of nanoparticles. They performed experiments on mass diffusion of fluorescent Rhodamine B in Cuewater nanofluids with different nanoparticle volume fractions (0.1e0.5%) and different temperatures (15, 20, and 25
C). They designed an optical experimental system to measure the diffusion coefficient of Rhodamine B in the nanofluid using Taylor dispersion method. By processing the fluorescence images of Rhodamine B diffusing over time and based on the mean square displacement of the dye, they obtained the diffusion coefficient of Rhodamine B in nanofluid [18]. Their results showed that Rhodamine B diffused faster in nanofluids compared to that in water and enhancement in diffusion coefficient increased with increasing nanoparticle volume fraction. They observed the maximum 26 times enhancement in the diffusion coefficient of Rhodamine B in Cuewater nanofluid with nanoparticle concentration of 0.5 vol% compared to that in the base fluid at 25
C. They interpreted that the presence of nanoparticles promotes mass transport inside the nanofluid similar to energy transport enhancement phenomenon as the result of created microconvection by Brownian motion of suspended nanoparticles. Based on their conclusion, increase in the nanoparticle volume fraction further enhances the intensity and frequency of the microdisturbance inside the suspension and leads to increasing the diffusion coefficient or thermal conductivity of nanofluids [18]. Moreover, Fang et al. [18] argued that higher suspension temperature leads to stronger Brownian motion of nanoparticles and more intense microconvection inside the nanofluid, which further enhances energy and mass transfer processes inside the suspension [18]. In 2009, contrarily to the mentioned observations, Turanov and Tolmachev [22] found reduction in mass diffusion coefficient by adding nanoparticles to the base fluid. Using the pulsed field gradient nuclear magnetic resonance (PFG NMR) method, they determined the solvent self-diffusion coefficient in aqueous suspensions of quasi-monodisperse spherical silica nanoparticles [22]. They measured the self-diffusion coefficient by means of StejskaleTanner sequence (90
ese180
es echo) with PFGs [31] for nanofluids with 3.8e23% nanoparticle volume fraction and at temperature of 20
C [22]. Turanov and Tolmachev [22] observed that self-diffusion coefficient in aqueous suspensions of spherical silica nanoparticles decreased with increasing nanoparticles volume concentration. They found that this decrease was faster than prediction by the effective medium theory and discussed that this deviation can be explained by interaction of water with the silica particles and water retention by the nanoparticles [22]. In agreement with Turanov and Tolmachev’s results [22], Gerardi et al. [23] also observed decrease in mass diffusion in the presence of nanoparticles. They measured the self-diffusion coefficient of water in the aqueous suspension of Al2O3 nanoparticles for concentrations up to 6% nanoparticle volume fraction at 25
C. In their work, the diffusion coefficient was measured using a pulsed gradient stimulated echo sequence with bipolar gradients, also known as the Cotts 13-interval pulse sequence [23,32]. Gerardi et al. [23] observed that the diffusion coefficient decreased with increasing nanoparticle concentration. They concluded that the reduction was due to two effects: first, the

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tortuosity of the diffusion path of water molecules was increased when solid particles stand in their way; second, water molecules in the ordered layer on the surface of the particles were “bound” to and move with the particles, which have a lower diffusion coefficient than the free molecules [23]. In 2010, new observation of mass diffusion in nanofluids was reported by Ozturk et al. [5]. They measured the tracer diffusion in nanofluids using a microfluidic approach. Under conditions matching Krishnamurthy et al.’s experiments [13] (aqueous suspension of 20-nm Al2O3 nanoparticles in nanoparticle volume fractions of 0.1e1%), they observed spontaneous formation of dyenanoparticle complexations. They argued that the anomalous enhancement for diffusivities determined from dye diffusion [13,18] is due to complexation interactions between the dye and nanoparticles [5]. By minimizing the interactions between dye and nanoparticles, Ozturk et al. [5] showed that Al2O3 nanoparticles did not enhance dye diffusion. In contrast, Veilleux and Coulombe [21] also minimized dye and nanoparticles interaction but still found a 10 times larger dye diffusion in Al2O3 nanofluids [25]. They measured the mass diffusion of Rhodamine 6G in water-based alumina nanofluids inside a millichannel geometry by means of total internal reflection fluorescence (TIRF) microscopy for nanoparticle volume fraction range of 0.1e4%. Veilleux and Coulombe’s experiments [21] showed that diffusion coefficient of Rhodamine 6G in nanofluids increased with nanoparticles concentration in dilute nanofluids and decreased with increasing nanoparticles concentration for higher concentrations [21]. They identified the maximum dye diffusivity enhancement in a 2 vol% nanofluid and postulated that for higher volume fractions, interactions among nanoparticles become significant and may affect the mechanism by which dye molecules diffusion is enhanced. They concluded that a single Brownian particle faces an increasing opposition to its free motion in the presence of an increasing number of similar particles. Consequently, a larger volume fraction leads to reduction in the particles’ root mean square velocity based on the StokeseEinstein relation and, in turn, to a lower contribution of the nanoparticles Brownian motion to mass dispersion effects in nanofluids [21]. In 2011, Subba-Rao et al. [24] reported the tracer diffusion of fluorescent dye in suspension of two different oxide nanoparticles using fluorescence correlation spectroscopy (FCS) technique. They performed FCS experiments by two-photon excitation of fluorescence [33]. They used fluorescent dye of Alexa-488 as the solute in silica-water nanofluid and Rhodamine-6G in the aqueous suspension of alumina. Since, both silica and alumina were charged (negative and positive, respectively), they used different dyes to prevent the formation of dye-nanoparticle complexations [24].

They investigated the effect of nanoparticles on the mass diffusion process in nanofluids with nanoparticle volume fractions up to 1% and 1.7% for alumina and silica, respectively. Their results showed no significant changes in diffusion of dyes. Their observed trend is consistent with the prediction of the excluded volume model [34] and also Ozturk et al.’s results [24]. Feng and Johnson [25] experimentally investigated effects of spherical SiO2 nanoparticles on oxygen and NaCl mass transfer by applying membrane diffusion cell. They found no mass transfer enhancements in the presence of nanoparticles and oxygen transfer was diminished at the highest nanoparticle volume fraction. They argued this observation was attributed to solution viscosity effects and the obstruction effects of impermeable nanoparticles. Based on their non-enhanced mass transfer results, Feng and Johnson [25] concluded that Brownian motion and micro-convection of nanoparticles cannot be the mechanism controlling heat and mass transfer in nanofluids [25]. Table 1 shows a summary of reported experimental works on mass diffusion coefficient in nanofluids. Fig. 1 reports the ratio of mass diffusion coefficient in the nanofluid to that in the base fluid versus nanoparticle concentration obtained by different researchers. As pointed, there are limited and controversial results for mass diffusion coefficients in nanofluids. Extensive measurements of mass diffusion coefficient in nanofluids are required to obtain consistent results and clarify the most critical factor affecting the mass diffusion in nanofluids [35]. Few researchers have presented theoretical models to predict mass diffusivity in nanofluids. By assuming that Brownian motion of suspended nanoparticles and induced microscopic convection of fluids around the nanoparticles are the most important factors for enhancement in mass transport process inside nanofluids, Xuan [36] applied two approaches for finding effective mass diffusivity in nanofluids. The first approach was application of GreeneKubo principle [37] and the second approach was based on heat and mass transfer analogy. Veilleux and Coulombe [21] presented the foundations of a Brownian motion-based dispersion model of mass diffusion in nanofluids, following an analysis of the velocity autocorrelation function (VACF) decay [21]. In another study [38], they followed their model and presented an explanation for the enhancement of mass diffusion in nanofluids using arguments based on dispersion in diluted fixed beds. Starting from the generalized Langevin equation, they showed that the velocity field established around a Brownian nanoparticle is similar to the velocity field predicted by Brinkman equations leading to the analogy between dispersion in diluted fixed beds and dispersion in nanofluids [38]. Their proposed model predicted the order of magnitude of mass diffusion enhancement that they had observed in their previous

Table 1 Experimental works on mass diffusion coefficient in nanofluids. Some data are from Ref. [25]. Investigator

Nanofluid type

Experimental method

Particle size (nm)

Particle volume fraction

Maximum enhancement ratio

Krishnamurthy et al. [13] Fang et al. [18]

Al2O3eH2O CueH2O

20a 25a

0.1e1% 0.1e0.5%

14 at 0.5% Al2O3 26 at 0.5% Cu

Turanov and Tolmachev [22] Gerardi et al. [23] Ozturk et al. [5] Veilleux and Coulombe [21] Subba-Rao et al. [24]

SiO2eH2O Al2O3eH2O Al2O3eH2O Al2O3eH2O SiO2eH2O Al2O3eH2O SiO2eH2O

Fluorescein dye diffusion by optical method Fluorescent Rhodamine B dye diffusion by Taylor dispersion method Self-diffusion coefficient of proton by PFG-NMR Self-diffusion coefficient of proton by PFG-NMR Fluorescent dye diffusion by microfluidic approach Rhodamine 6G dye diffusion by TIRF Alexa-488 and Rhodamine-6G dye diffusion by FCS

z10e30b 42.6b 20a 10b z22b

0.7 (reduction) at 23% SiO2 0.9 (reduction) at 6% Al2O3 1.0 (no enhancement) 10 at 2% Al2O3 1.0 (no enhancement)

Oxygen and NaCl mass transfer by diffusion cell measurements

z17e24b

3.8e23% 1e6% 0.25e1% 0.1e4% 0.1e1.7% 0.1e1% 0.5e5%

Feng and Johnson [25] a b

Size of nanoparticles in powder state before combining with base fluid to make nanofluid. Size of nanoparticles in suspension.

1.0 (no enhancement)

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diffusivity. Contrarily to the thermal conductivity, the mass diffusivity of a solute in the solid phase (the nanoparticles) is lower than in the liquid phase (host liquid). As such, nanoparticle clusters are not conducive to enhanced mass transport due to the solid phase, as opposed to heat conduction [21,38].  Increase in tortuosity of diffusion path in the presence of solid nanoparticles decrease the mass diffusivity [23]. 3. Convective mass transfer in nanofluids

Fig. 1. The ratio of mass diffusion coefficient in the nanofluid to that in deionized water versus nanoparticle concentration at room temperature (about 25
C).

experimental study (10-fold enhancement of Rhodamine 6G mass diffusivity under the optimum conditions for a suspension of 10nm alumina nanoparticles in deionized water) [38]. Based on Péclet numbers for heat and mass transport, their model also showed the order of magnitude differences between thermal conductivity and mass diffusivity enhancements reported [21]. Tang and Zhan [39] proposed a lattice Boltzmann model to simulate the diffusion processes of Rhodamine B in 25 nm Cue water nanofluids with different volume fractions and temperatures, and also calculated the effective mass diffusivity in nanofluids. They applied a lattice Boltzmann model with two dimensional nine speeds (D2Q9), coupled with double-distribution-function (DDF) model to analyze the forces between the nanoparticles and base fluid using two phase method [39]. Cui et al. [40] also simulated the mass diffusion in nanofluids by using the lattice Boltzmann method based on the finite volume of a particle method and based on the point source of particles method. They calculated mass diffusion coefficient of CO2 in nanofluids and compared their results with literature experimental data. Cui et al. [40] concluded that micro-perturbation of nanoparticles is responsible for mass transport enhancement in nanofluids [40]. 2.1. Controlling mechanisms of mass diffusion in nanofluids 2.1.1. Mass diffusion enhancement mechanisms  Brownian motion of nanoparticles and subsequent microconvection e Brownian motion is basically the random dynamic mode of particles in a liquid [12]. Small particles are expected to fluctuate about a mean path because of Brownian motion. The movement of particles subsequently causes momentum transfer and a continuous change in velocity with distance. Thus, the disturbance field created by the motion of the nanoparticles causes a microconvection which may be caused enhancing heat and mass transport in nanofluids [13,41].

2.1.2. Mass diffusion reduction mechanisms  Aggregation and clustering e Although aggregation may successfully explain the thermal conductivity enhancement in nanofluids, it fails to explain the enhancement of mass

In 2002, Moeser et al. [42] used colloidal dispersions of magnetic nanoparticles coated with different polymers for separation of organic contaminants from water. Then, Ditsch et al. [43] applied magnetic nanoparticles for ion-exchange purification of proteins and Li et al. [44] improved heat and mass transfer process of the formation and dissociation of HFC134a (CH2FCF3) hydrate by adding CuO nanoparticles into a refrigerant-water mixture [36]. To exactly investigate the influence of nanoparticles on the convective mass transfer coefficient, investigations have been performed which will be described in the following sections. In the present review, studies on convective mass transfer in nanofluids are presented in different systems including agitated absorption reactor, three-phase airlift reactor, bubble type absorption system, falling film absorption system, tray column absorption system, gaseliquid hollow fiber membrane system, direct measurements of mass transfer coefficient in nanofluid system and liquideliquid extraction system. 3.1. Agitated absorption reactor Olle et al. [45] experimentally investigated the effect of aqueous suspensions of 20e25 nm magnetic (Fe3O4) nanoparticles on the gaseliquid oxygen mass transfer in an agitated, sparged reactor. They characterized mass transfer by a dynamic method (surface aeration) and a steady-state method (sodium sulfite oxidation). Their results showed the gaseliquid oxygen transfer enhancement up to 6-fold (600%) at nanoparticle volume fractions below 1%. Based on their results, both the mass transfer coefficient and the gaseliquid interfacial area were enhanced in the presence of nanoparticles and the enhancement in volumetric mass transfer coefficient showed strong temperature dependence [45]. Park et al. [46] measured the chemical absorption rate of CO2 into an aqueous solution of nanometer-sized colloidal silica (0e 31 wt%) and 2-amino-2-methyl-1-propanol in a stirred vessel. They found that the volumetric liquid-side mass transfer coefficient and the absorption rate in the nanofluid decreased with increasing the nanoparticle concentration. Park et al. [46] compared the measured rate of CO2 absorption with the values estimated from a model based on film theory accompanied by chemical reaction and expressed an empirical correlation formula to present the relationship between volumetric mass transfer coefficient and the rheological behavior of the nanofluid [46]. Also, in another works, Park et al. [47e49], and Huang et al. [50] investigated the effect of SiO2 nanoparticles on the absorption rate of CO2 in aqueous solutions of diethanolamine, monoethanolamine, and diisopropanolamine using a stirred cell and again reported that the absorption rate decreased with increasing concentration of nanoparticles owing to elasticity of the solution [51]. Huang et al. [50] also found that volumetric mass transfer coefficient increased by increasing particle size up to 60 nm and at higher particle diameters it remained constant [50]. Nagy et al. [52], studied the transfer rate of oxygen, in the presence of nanometer size, organic n-hexadecane droplets, both experimentally and theoretically. In their work, the absorption of oxygen was measured in a stirred laboratory-scale reactor using the

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sodium sulfite oxidation method [52]. Nagy et al. [52], observed that mass transfer enhanced up to more than 200% by adding 10 vol % 65-nm n-hexadecane nanoparticles in the liquid phase [41]. Based on their results, enhancement in mass transfer rate increased more rapidly at low particle concentrations and rather slowly at larger concentrations (above about 6 vol%) [52]. They argued that the sudden and significant change in enhancement at a very low particle concentration may be caused by the Brownian motion of nanoparticles. Enhancement of the gase liquid mass transfer rate as a function of the particle concentration at different nanoparticle sizes was analyzed using homogeneous and heterogeneous mathematical models. They concluded that the effect of Brownian motion of nanoparticles is more dominant at a particle size range between 20 and 50 nm and this effect may be diminish at larger particle sizes (dp >about 50 nm). They also predicted the enhanced diffusion coefficient, due to the convective motion of the continuous liquid phase created by the nanoparticles and calculated its effect on the mass transfer enhancement using the homogeneous and heterogeneous mathematical models. Nagy et al. [52] concluded that for a better mass transfer model, the intensity of the convective motion of liquid elements in the nanofluid, created by Brownian motion of nanoparticles, should be measured or predicted [52]. Zhu et al. [53] indicated that mesoporous silica materials (MCM41) with average size of 250 nm increased volumetric mass transfer coefficient for absorption of CO in water in an agitated microreactor compared to micro-sized silica particles (1.4 and 7 mm) [16]. In their work, organic groups were grafted to w250nm-diameter mesoporous silica materials nanoparticles with a spherical morphology to enhance CO absorption for synthesis gas fermentation [53]. Zhu et al. [53] observed that COewater volumetric masstransfer enhancement was influenced by silica particle size and nanoparticle preparation procedures. In their work, when the surfactant for MCM41 synthesis was removed by extraction, enhancement increased from 1 to 1.55 as the nanoparticle concentration increased from 0 to 0.4 wt% (weight fraction). For large silica particles with diameters of 1.4 and 7 mm, the maximum enhancements were 1.29 and 1.01, respectively, when the particle concentration was 0.4 wt%. They found that the COewater masstransfer enhancement depended on the interaction between the nanoparticles and CO molecules, which was influenced by the hydrophobicity of the nanoparticles and the functional group on the nanoparticles [53]. Manikandan et al. [54] carried out experiments with Fe2O3e water nanofluids to study possible enhancement in oxygen transfer from air bubble to nanofluid, in an agitated, aerated bioreactor. The nanoparticles concentration studied was in the range of 0.022e 0.065 wt%. They observed an enhancement of 63% for 0.065 wt% Fe2O3ewater agitated at 200 rpm and 0.75 L min1 air flow and argued that nanoparticles contribute to enhance oxygen transfer through grazing effect. They also found that nanoparticles are more effective in enhancing volumetric mass transfer coefficient at lower air flow rate [54]. Lu et al. [55] experimentally studied the influence of nanoparticles on CO2 absorption in a stirred thermostatic reactor. They used nano-Al2O3 and carbon nanotube (CNT) particles which had different hydrophobic properties and compared the experimental results with that of micron-size activated carbon (AC) and Al2O3 particles. From their results, they found no enhancement by micron-size Al2O3. However, based on their observations, nanoAl2O3 showed a weak enhancement for the CO2 absorption. AC and CNT particles all intensified the gaseliquid mass transfer effectively, yet the trend of the enhancement factor with stirring speed for the two particles was different. With increasing stirring speed,

the enhancement factor of AC particles decreased, whereas in CNT suspensions it increased [55]. Lu et al. [55] found a difference in enhancement mechanism for particles with different sizes. They concluded that for nano-particles, besides the influence of adsorbability and hydrophobicity, the micro-convection caused by Brownian motion should be also taken into account. Considering the micro-convection effect, they developed a theoretical model based on the penetration theory to predict the variation of enhancement factor of nanoparticles [55]. Keshavarz Moraveji et al. [56] experimentally examined the dissolution of methane in water with CuO nanoparticles at temperatures of 20, 15, 10, 8, 6 and 3
C and at pressures of 10 and 15 bar in a stirrer cell. They observed higher solubility of methane comparing with the pure water in the presence of 0.1 wt% CuO nanoparticles and concluded that high effective surface area of nanoparticles have improved the solubility. They observed adding 1 wt% nanoparticle, increased methane solubility by 144% and 77.15% at pressures of 10 and 15 bar (compared with the pure water), respectively [56]. In Table 2, experimental works on convective mass transfer in nanofluids in agitated absorption reactors are reported. 3.2. Three-phase airlift reactor Wen et al. [57] observed the reduction in O2eH2O mass transfer coefficient in a three-phase internal loop airlift reactor in the presence of TiO2 nanoparticles with nanoparticle volume fractions of 1.1%, 2.2% and 3.3%. They blamed increased aggregation of nanoparticles due to increased particle concentration as the reason for adverse mass transport [14,57]. In another study, Feng et al. [58] also used water-based nanofluids with volume fraction of 1.1e3.3% hydrophobic TiO2 nanoparticles in a lab-scale gaseliquideTiO2 nanoparticles three-phase reactor to absorb oxygen and observed decreased gas holdup with increasing nanoparticle concentration [14,58]. They argued that the increase in the apparent density and viscosity of suspension is responsible for the decrease of gas holdup [58]. Table 3 indicates the experimental works on convective mass transfer in nanofluids in three-phase airlift reactor. 3.3. Bubble type absorption system Kim et al. [59] experimentally examined the effect of Cu, CuO and Al2O3 nanoparticles on the bubble type absorption in a compact NH3/H2O absorber system [35]. They considered initial concentrations of NH3/H2O solution (0e18.7 wt% NH3 solution) and the kinds and the concentrations of nanoparticles (0, 0.01, 0.05, 0.10 wt% weight percent) as key parameters. Their results showed that the absorption rate increased almost linearly with increasing the weight percent of nanoparticles in all cases. Based on their observations, the maximum effective absorption ratio (the ratio of absorption rate in the presence of nanoparticles to the absorption rate in the base fluid) was 3.21 in the case of 18.7 wt% concentration ammonia solution with 0.10 wt% Cu nanoparticles. They found that the addition of Cu nanoparticles was the most effective among considered nanoparticles. The trend of change in effective absorption ratio for different kinds of particles was very similar. They concluded that the differences among profiles of effective absorption ratio for each sort of particles are owing to the particle’s size and/or the number of particles in the same weight percent. They found that grazing effect was more dominant than the effect of nanoparticles’ type [59,60]. During the absorption process, Kim et al. [59] visualized the bubble behavior by using the shadowgraph method. Their experiments showed that the bubble size in the binary nanofluid

Investigator

Nanofluid type

Experimental method

Particle size (nm)

Particle mass fraction

Maximum enhancement ratio

Olle et al. [45]

Fe3O4eH2O

O2 mass transfer into H2O-based nanofluids in an agitated, sparged reactor CO2 absorption in H2O-based nanofluids in a flat-stirred vessel

20e25a z 20b

0.25e4%

6 at 4% Fe3O4

b

9e31%

z0.25 (reduction) at agitation speed of 50 rpm

CO2 absorption in H2O-based nanofluids in a flat-stirred vessel CO2 absorption in H2O-based nanofluids in a flat-stirred vessel CO2 absorption in H2O-based nanofluids in a flat-stirred vessel Absorption of O2 in H2O-based nanofluids in a stirred laboratory-scale reactor

12b

9e31%

z0.25 (reduction) at agitation speed of 50 rpm

12b

9e31%

z0.43 (reduction) at agitation speed of 50 rpm

b

9e31%

z0.38 (reduction) at agitation speed of 50 rpm

0.2e12 vol%c

2 at 10 vol% or more nanodroplets

0.05e4%

1.55 and 1.9 without and with mercaptan group surfactants at 4% SiO2 Reduction in mass enhancement and increase in volumetric mass transfer coefficient (kLa) by increasing particle size up to 60 nm, at higher particle diameters kLa remained constant 1.63 at 0.065% Fe2O3

Park et al. [46]

Park et al. [47] Park et al. [48] Park et al. [49] Nagy et al. [52]

Zhu et al. [53]

SiO2eH2O with 2-amino-2methyl-1-propanol SiO2eH2O with diethanolamine SiO2eH2O with monoethanolamine SiO2eH2O with diisopropanolamine Nanometer n-hexadecane droplets-H2O SiO2eH2O

Huang et al. [50]

SiO2eH2O with monoethanolamine

Manikandan et al. [54] Lu et al. [55]

Fe2O3eH2O

Keshavarz Moraveji et al. [56] a b c

Al2O3eH2O CNTeH2O CuOeH2O

12

12

65b

CO absorption in H2O-based nanofluids in an agitated microreactor filled CO2 absorption in H2O-based nanofluids in a flat-stirred vessel

250a

O2 absorption in H2O-based nanofluids in an agitated, aerated bioreactor CO2 absorption in H2O-based nanofluids in a stirred thermostatic reactor Methane dissolution in H2O-based nanofluids in a stirrer cell

20e50a, 120b

Size of nanoparticles in powder state before combining with base fluid to make nanofluid. Size of nanoparticles in suspension. Particle volume fraction.

17, 60, 111

a

b

20 , 10e20 40

a

a

9e31%

0.022e0.065% 0.2e1.6 kg/m3 0.1%

1.15 for Al2O3 and 1.83 for CNT at particle concentration 1.6 kg/m3 Increase in methane solubility by 144% and 77.15% at pressures of 10 and 15 bar, respectively

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Table 2 Experimental works on convective mass transfer in nanofluids in agitated absorption reactors.

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Table 3 Experimental works on convective mass transfer in nanofluids in three-phase airlift reactor. Investigator

Nanofluid type

Experimental method

Particle size (nm)

Particle volume fraction

Maximum mass enhancement ratio

Wen et al. [57]

TiO2eH2O

12a

1.1e3.3%

0.8 (reduction)

Feng et al. [58]

TiO2eH2O

O2 absorption in H2O-based nanofluids in a three-phase internal loop airlift reactor O2 absorption in H2O-based nanofluids in a lab-scale three-phase reactor

10a

1.1e3.3%

0.8 (reduction) and also decrease of gas holdup

a

Size of nanoparticles in powder state before combining with base fluid to make nanofluid.

(NH3/H2O solution with nanoparticles) was smaller than that in the binary mixture without nanoparticles. Besides, the bubble shape was sphere in the case with nanoparticles, while it became hemispherical shape in the case without nanoparticles. In addition, the residence time of a bubble in the binary nanofluid was shorter than that in the binary mixture [59]. Kim et al. [61] also reported that gas bubbles in a nanofluid are smaller than in pure water and applied this phenomenon directly to a gas absorption process in a bubble type absorber [62]. In another bubble observation, Fan et al. [63] studied the anomalous gas holdup and bubble behavior in a bubble column or a micro-channel with the presence of silicon dioxide nanoparticles in water. The nanoparticles were 20 nm in diameter and the nanoparticle weight percentages were in the range of 0.48e1.40 wt%. They observed reduction in bubble sizes and increase in the gas holdup in the bubble column in the presence of nanoparticles at superficial gas velocities above 7e10 cm/s [14,63]. Kim et al. [64] experimentally studied the effect of nanoparticles and surfactants on the NH3/H2O absorption process. In their work, Cu, CuO and Al2O3 nanoparticles were added into NH3/H2O solution to make the binary nanofluids, and 2-ethyl-1-hexanol, n-octanol and 2-octanol were used as the surfactants. The results showed that the absorption rate increased with increasing the weight percent of nanoparticles. They also found that the absorption rate for Cu binary nanofluid was slightly higher than those for CuO and Al2O3 binary nanofluids. Without adding the surfactant, the maximum effective absorption ratio was 3.21 in the case of ammonia 18.7 wt% binary nanofluid with 0.1 wt% Cu nanoparticles and this ratio was 5.32 with 2-ethyl-1-hexanol and again in the case of ammonia 18.7 wt%, Cu 0.1% binary nanofluid [64]. Ma et al. [65] studied the enhancement of heat and mass transfer process of bubble absorption using nanofluids as the working medium. They prepared carbon nanotubes (CNTs)-ammonia nanofluids with nanoparticle mass fractions up to 0.3%. Their results revealed that the absorption rates of binary nanofluids were higher than those without CNTs except those of the nanofluids with the initial ammonia concentration of 0% and the absorption rate increased with the mass fraction of CNTs. The effective absorption ratio increased with the mass fraction of CNTs in all cases studied. The maximum effective absorption ratio was 1.162 for the case of 23.29 wt% ammonia nanofluid with 0.23 wt% CNTs [65]. To discuss the mechanism of enhancement, Ma et al. [65] explained that the addition of CNTs can form localized convection because of Brownian motion. And the localized convection can promote ammonia diffusion within the binary nanofluid. Ma et al. [65] also believed that the grazing effect is another factor which enhances the performance of NH3/H2O bubble absorption [65]. In addition, Kang et al. [66] also investigated the influence of CNTs on ammonia absorption in binary nanofluid. They found that the absorption rate of ammoniaewater nanofluid with 0.001 wt% CNTs were 20% higher than that of the ammoniaewater without nanoparticles [66,67]. In a numerical study, Kim et al. [68] analyzed the combined heat and mass transfer process and theoretically studied the effects of binary nanofluids and surfactants on the absorber size. They

developed a differential mathematical model for designing the ammonia bubble absorbers based on ColburneDrew method. In order to express the effects of binary nanofluids and/or surfactants on the absorption performance, the effective absorption ratios for each case were applied in their numerical model. Kim et al. [68] considered 2-ethyl-1-hexanol, n-octanol, and 2-octanol as surfactants and Cu, CuO, and Al2O3 as nanoparticles. Their results showed that the application of binary nanofluids and surfactants can reduce the size of absorber significantly. Based on their results, in order to reach 16.5 wt% ammonia solution under the considered conditions, the application of binary nanofluids (Cu, 1000 ppm) can reduce the size of absorber up to 54.4%. They also found that the effect of mass transfer resistance is more dominant than that of heat transfer and thus, the enhancement of mass transfer by addition of nanoparticles is more effective than that of heat transfer [68]. Kim et al. [62] used silica-water nanofluid for CO2 absorption in a bubble type absorber. In their study, three kinds of nanofluids containing 30 nm, 70 nm, 120 nm particles were prepared and the absorption experiments were performed by changing the fraction of nanoparticles in the nanofluid from 0.01 wt% to 0.04 wt%. The absorption performance increased with increasing nanoparticle content. The average CO2 absorption rate during the first 1 min, and the total absorption in the 0.021 wt% nanofluid increased 76% and 24%, respectively, compared to water. They also used the solution containing K2CO3 and piperazine as the absorbent, and investigated the combined performances with the nanofluids. In the piperazine/ K2CO3 absorbent, the average CO2 absorption rate during the first 1 min and total absorption were increased 11% and 12% by the addition of 0.021 wt% nanoparticles, respectively [62]. Kim et al. [62] suggested possible mechanism for CO2 absorption in a nanofluid. They argued that the stable nanoparticles in a nanofluid provide additional energy to the solution and increase the total area of gas bubbles. In other words, the stable nanoparticles and flowing gas bubbles collide and gas bubbles are broken to small bubbles and the mass transfer area is increased [62]. The gas absorption graphs for three different sized nanoparticles of 30 nm, 70 nm, 120 nm almost coincided with each other and did not show significant changes depending on the particle size. To explain the reason of this observation, Kim et al. [62] concluded that the same content of nanofluid presents the same amount of additional energy to the solution and gas bubbles in the same contents of nanofluid are cracked to the same size even at the different sizes of the nanoparticles and result in the same absorption enhancement [62]. Sheng et al. [69] validated that the ammonia bubble absorption effect is enhanced by adding Al2O3 nanoparticles into the absorbent. They also suggested that the pressure difference between the inlet of absorber and the gas phase surface in the absorber is another factor to enhance absorption process [69]. Liu et al. [70] used FeO nanofluid to enhance the ammonia absorption. Their results showed that, at a constant flow rate of ammonia, the enhanced absorption effect was not observed until several minutes after the beginning of the absorption; under the condition of constant inlet pressure, the absorption enhancement

S.-S. Ashrafmansouri, M. Nasr Esfahany / International Journal of Thermal Sciences 82 (2014) 84e99

was observed immediately at the very beginning of the absorption process [67,70]. Ma et al. [71] experimentally investigated the enhancement of heat and mass transfer characteristics for NH3/H2O bubble absorption process using CNTseammonia binary nanofluid as a working medium. The effective absorption ratio increased with the mass fraction of CNTs up to 1.2 for initial ammonia concentration of 25 wt% and then decreased by increasing CNT concentration further. They considered micro-convection in aqueous ammonia because of the Brownian motion of nanoparticles and grazing effect as the possible mechanisms for enhancement [71]. To study the influence of nanoparticles on NH3/H2O bubble absorption process, Wu et al. [72] added Ag nanoparticles into NH3/ H2O solution to make binary nanofluid. Based on their results, in Ag nanoparticle concentration range of 0.005e0.020 wt%, the effective absorption ratio was always higher than 1.0 and got larger with the increase in the initial ammonia concentration, and both the absorption rate and effective absorption ratio increased with Ag nanoparticle concentration. The effective absorption ratio reached the maximum 1.55 for the initial ammonia concentration of 20% and Ag nanoparticle concentration of 0.020 wt% [72]. Lee et al. [73] produced binary nanofluids with Al2O3 nanoparticles and CNT particles and the binary mixture of NH3/H2O. They experimentally studied the effect of Al2O3 nanoparticles and CNTs on the absorption performance in a compact absorber for NH3/H2O absorption system. The experimental ranges of the key parameters were 20 wt% of NH3 concentration, 0e0.08 vol% of CNT particles, and 0e0.06 vol% of Al2O3 nanoparticles. They found that the heat transfer rate and absorption rate with 0.02 vol% Al2O3 nanoparticles were 29% and 18% higher than those without nanoparticles, respectively. Also, they observed that the heat transfer rate and absorption rate with 0.02 vol% CNT particles were 17% and 16% higher compared to the base fluid, respectively [73]. Based on their reported results, despite of greater thermal conductivity of CNT compared with alumina, the absorption performance of CNT nanofluids was not better than that of Al2O3 nanofluids. Lee et al. [73] interpreted that CNT particles with high aspect ratios have relatively smaller motion than Al2O3 nanoparticles which are in spherical shape. Also, the drag force on CNT is greater than that of Al2O3. CNT with a higher drag force will give a negative effect to the Brownian motion of nanoparticles compared with Al2O3 particles. In fact, they concluded that the effect of particles size and shape on the absorption is more dominant than that of particle type [73]. In another study, SiO2- and Al2O3emethanol nanofluids were used to augment CO2 absorption. They performed the absorption experiments in the bubble type absorber and carried out the parametric analysis on the effects of the particle type and concentrations on CO2 absorption rate. The nanoparticle concentration ranged from 0.005 to 0.5 vol%. They found that CO2 absorption was augmented up to 4.5% at 0.01 vol% of Al2O3emethanol nanofluids at 20
C, and 5.6% at 0.01 vol% of SiO2emethanol nanofluids at 20
C, respectively. Lee et al. [74] concluded that increase in CO2 absorption rate is the result of bubble breakage and reduction in velocity disturbance field. Besides, they argued that the nanoparticle aggregation and bonding eOH group to nanoparticles are the reasons for decreasing CO2 absorption with nanoparticle concentration [74]. Wu et al. [75] studied the effects of nanofluid on NH3/H2O bubble absorption performance under adiabatic and non-adiabatic conditions. In their work, NH3/H2O solution with 0.02 wt% Ag nanoparticles was used as the enhancement medium. The results showed that the absorption rate with Ag nanoparticles in bubble absorption process was higher than that of base liquid. They concluded that the heat transfer enhancement of nanofluid can

91

promote the NH3/H2O bubble absorption performance to a certain degree, and the enhancement of the absorption is not completely dependent on heat transfer [75]. In addition to Ag nanoparticles, Wu et al. [76] experimentally examined the enhancing mass transfer performance of FeO nanoparticles on NH3/H2O bubble absorption. The results showed that FeOeNH3/H2O binary nanofluid has an enhancing effect on mass transfer performance [76]. Jung et al. [77] prepared Al2O3emethanol nanofluids with nanoparticle volume fractions ranged between 0.005 and 0.1% and observed that the maximum CO2 absorption enhancement compared to the pure methanol was 8.3% at 0.01 vol% of alumina nanoparticles. Up to the 0.01 vol% alumina nanoparticles in the absorbents, the effective absorption ratio increased with increasing the concentration. While, after that, the effective absorption ratio decreased. Jung et al. [77] discussed that the enhanced CO2 absorption is the result of both breaking gas-bubbles and the mixing effect of nanoparticles due to the particle-laden flow created by Brownian motion and if the particle concentration increases over a critical value, there is less Brownian motion because of the interparticle interaction hindering this motion [77]. Tang et al. [78] tested CO2 absorption by ethanol-based nanofluids with Al2O3, MgO, SiO2 and TiO2 nanoparticles of 0.01e0.10 vol % to investigate the effects of volume fraction, type and size of nanoparticles. The results showed that the enhancement effect of absorption increased with the increase of the volume fraction of nanoparticles, and decreased with the increase of nanoparticles’ size [78]. Pang et al. [79] used Ag nanoparticles to prepare binary nanofluids (NH3/H2O binary mixture with Ag nanoparticles) for application in NH3/H2O absorption process in a bubble absorber. The NH3/H2O bubble absorption experiments were carried out in two cases, with and without the coolant (water). The results showed that the mass transfer in binary nanofluids with the coolant was enhanced more than that without the coolant. With the coolant, the highest absorption rate enhancement was observed to be 55% at 0.02 wt% Ag nanoparticles concentration. Pang et al. [79] concluded that the mass transfer enhancement by binary nanofluids was attributed to the enhanced heat transfer and gas bubble breakage [79]. Wu et al. [80] investigated the addition of Fe3O4 nanoparticles in combination with the application of an external magnetic field for enhancing absorption in an ammonia/water bubble-type absorber. The results showed that the combined effect of the nanoferrofluid and the external magnetic field significantly enhanced ammonia/ water bubble absorption, and the enhancement of the combined factors was greater than that by either factor alone. To obtain the greatest enhancement, they also determined the optimal concentration of nanoferrofluid within the investigated range and found that absorption was further enhanced with increasing magnetic field intensity at a constant nanoferrofluid concentration. At initial ammonia mass concentration of 20%, Fe3O4 nanoparticle mass concentration of 0.10% and external magnetic field intensity of 280 mT, the effective absorption ratio reached a maximum of 1.0812  0.0001 under adiabatic condition [80]. Wu et al. [80] concluded that the promotion effect on the enhancement of ammonia absorption may be due to various factors such as enhanced heat transfer, increase in bubble disruption and the disturbance of the absorption liquid, because of extrusion, distortion and elongation of bubbles along the direction of the magnetic field lines, reduction in surface tension and followed by it, creation of Marangoni convection effect which leads to sharp turbulence and a more nonuniform distribution of the solute at the gaseliquid interface [80]. Lee and Kang [81] studied Al2O3eNaCl aqueous nanofluids to enhance CO2 absorption in the bubble type absorber. The particle

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concentration ranged from 0.005 to 0.1 vol%. They found that the optimum concentration of Al2O3 nanoparticles was 0.01 vol% in the NaCl aqueous solution-based nanofluids and then CO2 solubility was enhanced to 11.0%, 12.5% and 8.7% for Al2O3eNaCl aqueous solution at 30
C, 20
C and 10
C, respectively. Lee and Kang [81] concluded that the effect of nanoparticles on the mass transfer enhancement is more dominant in the region of unsaturated state than that of the saturated state [81]. Amaris et al. [82] experimentally investigated the individual and simultaneous effects of CNTs (carbon nanotubes) and advanced surfaces on the performance of an NH3/LiNO3 tubular bubble absorber. Firstly, they performed experiments with the tubular absorber fitted with an inner smooth surface to analyze the effect of adding carbon nanotubes (0.01 wt%) to the base mixture NH3/ LiNO3. Then, they tested the tubular absorber using an inner advanced surface tube both with and without adding carbon nanotubes to the base mixture NH3/LiNO3. The advanced surface tube was made of aluminum. Their results showed that the maximum absorption mass flux achieved with the CNT binary nanofluid and the smooth tube was up to 1.64 and 1.48 times higher than reference values at cooling-water temperatures of 40 and 35
C, respectively [82]. Amaris et al. [82] worked with two CNT concentrations of 0.01 and 0.02 wt%. Absorber performance with CNT concentrations of 0.01 and 0.02 wt% showed no significant differences [82]. Experimental works on convective mass transfer in nanofluids in bubble type absorption systems are listed in Table 4. 3.4. Falling film absorption system Suresh and Bhalerao [83] showed increase in mass transfer in a wetted wall column by using nanoferrofluid under oscillating magnetic field. The magnetic field manipulated nanoparticles to get into the diffusion film by creating mixing in that region. When a periodic magnetic field at 50 Hz was employed, a 40e50% enhancement in mass transfer coefficient was observed, but little, if any, effect was found of the particles alone in the absence of the field [51]. Also, Komati and Suresh [51] performed some experiments on the CO2/MDEA system to investigate the influence of using nanoferrofluid/MDEA on the mass transfer coefficient in CO2 absorption process in a wetted wall column [51]. Their experimental results showed that the mass transfer coefficient increased with increasing volume fraction of suspended magnetic particles in the ferrofluid. The enhancement in mass transfer coefficient was 92.8% for a magnetite volume fraction of about 0.39%. Experiments were also carried out to further enhance the mass transfer rates by employing a periodic oscillating magnetic field and they observed no further impact of magnetic field on mass transfer rates. Under the conditions applied, their results were at variance with that reported by Suresh and Bhalerao [83]. To discuss the reason of this contradicting results, Komati and Suresh [51] discussed that it is possible the single-domain nature of the particles is lost in the process and hence the particles do not follow an external field. Based on their viewpoint, the magnetic nanoparticles need to be 15 nm or smaller in size to function as single domain magnetic particles [51]. Kang et al. [35] measured the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids. The binary nanofluids were H2O/LiBr solution with nanoparticles of Fe and CNTs with the concentrations of 0.0, 0.01 and 0.1 wt%. The vapor absorption rate increased with increasing the concentration of Fe and CNT nanoparticles. They found that the mass transfer enhancement from the CNTs (average 2.16 for 0.01 wt% and average 2.48 for 0.1 wt %) was higher than that from the Fe nanoparticles (average 1.71 for 0.01 wt% and average 1.90 for 0.1 wt%). From their results, the effect

of types of nanoparticles on the mass transfer enhancement seems to be more significant than that of the nanoparticle concentration. Kang et al. [35] also found that the mass transfer enhancement is much more significant than the heat transfer enhancement in the binary nanofluids with Fe and CNTs [35]. Komati and Suresh [26] examined the effect of magnetic iron oxide nanoparticles on gaseliquid mass transfer rates. In their work, carbon dioxide and oxygen were the gases absorbed, into a variety of reactive and nonreactive liquids. They performed experiments in a wetted wall column and in a capillary tube. Their experiments showed that the liquid phase mass transfer coefficients were significantly enhanced in the presence of nanoparticles in the region of concentration gradient, the extent of enhancement depending on the volume fraction of solid particles in the fluid, and on the particle size scaled with respect to the depth of penetration of the diffusing solute [26]. According to Komati and Suresh’s results [26], the enhancement effect, having been observed in the presence and absence of reaction and flow, points to the fundamental molecular-level transport processes being influenced by the nanoparticles. They also identified a modified Sherwood number based on the traditional theories of interphase mass transfer, as the dominant parameter which determines the magnitude of the mass transfer intensification effect at a given particle holdup, and derived a correlation for enhancement [26]. By preparation of Al2O3, Fe2O3 and ZnFe2O4 nanofluids, Yang et al. [67] carried out comparative experiments on the falling film absorption between ammoniaewater and ammoniaewater with various types of nanoparticles. The absorption of ammonia was weakened by only adding surfactants or adding poorly dispersed nanoparticles. Based on their results, the absorption rate increased with the increase of mass fraction of nanoparticles firstly, and then decreased. Particularly, this trend was more obvious for the nanofluid of Fe2O3 and ZnFe2O4. They believed that the increase of mass fraction of nanoparticles with matched surfactants can improve the absorption rate of ammonia under the condition that the viscosity of nanofluid does not increase remarkably, and there is an optimal mass fraction for each kind of nanoparticles and surfactant [67]. In their work, maximum absorption rate was occurred at nanoparticle mass fraction 0.1, 0.2 and 0.2% respectively for nanofluids with ZnFe2O4, Fe2O3 and Al2O3 nanoparticles mixed with optimal surfactants and the effective absorption ratio respectively increased by 50%, 70% and 30% for them at initial ammonia mass fraction of 15%. Yang et al. [67] concluded that the absorption enhancement by the nanofluid is attributable to factors such as decrease in viscosity of nanofluid (by adding suitable surfactant), heat transfer enhancement result of the microconvection and the grazing effect of nanoparticles. Since heat absorption process is a combined heat and mass transfer process, the improvement of heat transfer can decrease the temperature at the gaseliquid interface, heighten the absorption potential of aqueous ammonia and enhance the absorption rate of the ammonia vapor [67]. Kim et al. [84] measured the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids (H2O/LiBr with SiO2 nanoparticles) in H2O/LiBr absorption system. They found that the maximum improvements of heat and mass transfer rate reached 46.8% and 18%, respectively, when the concentration of SiO2 nanoparticle was 0.005 vol%. The performance enhancement with nanoparticles became higher than that of nanoparticles and the surfactant (2-Ethyl-1-Hexanol). They discussed that this is because the convective motion of nanoparticles such as Brownian movement gives a big impact on the absorption performance in SiO2 binary nanofluids and then the surfactant makes the convective motion of nanoparticles weak [84]. Table 5 provides a summary

Investigator

Nanofluid type

Experimental method

Kim et al. [59]

Cu, CuO or Al2O3eNH3/H2O solution

NH3 absorption by NH3/H2O based nanofluid

Kim et al. [64]

Cu, CuO or Al2O3eNH3/H2O solution

NH3 absorption by NH3/H2O based nanofluid

Ma et al. [65]

CNTseNH3 solution

NH3 absorption by NH3/H2O based nanofluid

Kim et al. [62]

SiO2eH2O SiO2eK2CO3/piperazine solution

CO2 absorption in H2O based or K2CO3/piperazie based nanofluids

Ma et al. [71]

Multi-wall CNTseNH3/H2O solution

NH3 absorption by NH3/H2O based nanofluid

Lee et al. [73]

Al2O3 or CNTseNH3/H2O solution

NH3 absorption by NH3/H2O based nanofluid

Lee et al. [74]

SiO2 or Al2O3emethanol

CO2 absorption in methanol based nanofluids

Jung et al. [77]

Al2O3emethanol

CO2 absorption in methanol based nanofluids

Pang et al. [79] Wu et al. [80]

mono AgeNH3/H2O solution Fe3O4eNH3/H2O solution

Lee and Kang [81]

Al2O3eNaCl aqueous solution

Amaris et al. [82]

CNTeNH3/LiNO3 solution

NH3 absorption by NH3/H2O based nanofluid NH3 absorption by NH3/H2O based nanofluid under external magnetic field CO2 absorption in nanofluid at temperatures of 10e30
C NH3 absorption in nanofluid in a tubular bubble absorber

a b c

Size of nanoparticles in powder state before combining with base fluid to make nanofluid. Size of nanoparticles in suspension. Particle volume fraction.

Particle size (nm)

Particle mass fraction

Maximum enhancement ratio

Cu: 50 CuO: 47a Al2O3: 33a <50a

0.01e0.1%

3.21 at 0.1% Cu

0.01e0.1%

d ¼ 10e20b L ¼ 5e15 mm 30, 70 and 120a

0.1e0.3%

5.32 at 0.1% Cu with 2-ethyl-1-hexanol as surfactant 1.162 at 0.23% CNTs

a

0.01e0.04%

d ¼ 20a L ¼ 5e10 mma Al2O3: 35a CNTs: 25a L ¼ 10 mma SiO2: 10e20a, 340b Al2O3: 40e50a, 200b 40e50a 200b 15a e

0.05e0.5%

1.24 in H2O based nanofluid with 0.021% SiO2 and 1.12 by the addition of 0.021% SiO2 into K2CO3/piperazine solution 1.2 at 0.2% CNTs

Al2O3: 0.01e0.06 vol%c CNTs: 0.01e0.08 vol%c

1.18 at 0.02 vol% Al2O3 1.16 at 0.02 vol% CNTs

0.005e0.5 vol%c

1.045 at 0.01 vol% of Al2O3 and 1.056 at 0.01 vol% of SiO2 1.083 at 0.01 vol% of Al2O3

0.005e0.02% 0.05e0.20%

1.55 with 0.02% Ag 1.0812 at 0.10% Fe3O4

40e50a, 190b at 10
C, 140b at 20
C and 135b at 30
C d ¼ 20e30a, L ¼ 10e30a mm

0.005e0.1 vol%c

1.125 at 0.01 vol% Al2O3 at 20
C

0.01, 0.02%

Up to 1.64 and 1.48 (absorption mass flux) at cooling-water temperatures of 40 and 35
C with the smooth tube

0.005e0.1 vol%

c

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Table 4 Experimental works on convective mass transfer in nanofluids in bubble type absorption systems.

93

0.02e0.39% 28e38b CO2 absorption in nanofluid in a wetted wall column

1.7, 1.5 and 1.3 in nanofluids with 0.2 wt% Fe2O3, 0.1 wt% ZnFe2O4 and 0.2 wt% Al2O3 1.18 at 0.005 vol% SiO2

48 at 1% Fe3O4

0.001e0.01%

0.1e0.3 wt%c

c

b

Vapor absorption by nanofluid in falling film absorber Kim et al. [84]

ZnFe2O4, Fe2O3 or Al2O3eNH3/H2O solution SiO2eLiBr/H2O solution Yang et al. [67]

a

Fe3O4eH2O Komati and Suresh [26]

Size of nanoparticles in powder state before combining with base fluid to make nanofluid. Size of nanoparticles in suspension. Particle mass fraction.

Al2O3: <20a, Fe2O3 and ZnFe2O4: <30a 10e20a

0.05e1%

Fe or CNTseLiBr/H2O solution Kang et al. [35]

O2 and CO2 absorption in reactive and nonreactive liquids in a wetted wall column and a capillary tube NH3 absorption in nanofluid falling film flow

Fe3O4eCO2/MDEA solution Komati and Suresh [51]

Vapor absorption in a falling film flow of binary nanofluids

a

Fe: 100 CNT: 25a L ¼ 5 mma z6e21a

0.01e0.1 wt%

c

Particle volume fraction Particle size (nm) Experimental method Nanofluid type Investigator

Table 5 Experimental works on convective mass transfer in nanofluids in falling film flow systems.

1.928 in the absence of magnetic field-no further enhancement in the presence of magnetic field 2.48 for 0.1 wt% of CNTs 1.90 for 0.1 wt% of Fe

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Maximum enhancement ratio

94

of experimental works on convective mass transfer in nanofluids in falling film flow systems. Ali et al. [85] theoretically investigated heat and mass transfer between air and falling desiccant film (CaCl2 solution) for co and counter flow configurations with the presence of Cu-ultrafine particles in the solution film. In their modeling, the conventional approach developed by Hamilton and Crosser and Xuan and Roetzel’s modified conventional approach [86] were used to estimate the thermal conductivity of the solideliquid mixture. Also, the extended Einstein’s equation by Brinkman [87] was applied for effective fluid viscosity [85,88]. The addition of Cu-ultrafine particles increased the thermal conductivity of the desiccant film. The results showed that the film thickness decreased with an increase in the volume fraction due to an increase in the effective density of the liquid solution. An increase in the volume fraction resulted in an increase in the heat transfer between air and desiccant film which resulted in lower exit air temperature. Their results revealed that the dehumidification and cooling rates of air were improved with an increase in the volume fraction of nanoparticles and the cocurrent flow arrangement provided better dehumidification and cooling for the air than the counter-flow channel [85,89]. In a similar work, Ali and Vafai [88] theoretically investigated heat and mass transfer between air and falling desiccant film for inclined co and counter flow configurations. Like their previous work, they examined the effect of Cu-ultrafine particles which were added to the desiccant film (CaCl2 solution) to study enhancements in dehumidification and cooling processes of the air and regeneration of liquid desiccant. Their results showed that only cooling process of air had a noticeable enhancement by addition of Cuultrafine particles volume fraction and the exit concentration of the desiccant film almost stayed constant with an increase in the volume fraction of Cu-ultrafine particles [88]. Moreover, Ali et al. [89] applied their theoretical method in a cross flow configuration [89]. Like their previous works [85,88], their results illustrated the enhancements in the stagnant thermal conductivity with an increase in the Cu-volume fraction as described by Hamilton and Crosser [90] formulation. The observed enhancement in the thermal conductivity caused not only to increase heat transfer but also to enhance rate of water evaporation. So, the exit air temperature and humidity decreased as Cu-volume fraction increased [89].

3.5. Tray column absorption system In Pineda et al.’s study [91], suspensions of Al2O3 and SiO2 nanoparticles in methanol (with nanoparticle volume fraction of 0.005e0.1%) were produced and analyzed for application in CO2 absorption in a tray column absorber. Based on their results, the effective absorption ratio increased as the particle concentration increased and both nanoparticles followed a similar trend. After reaching its optimum point at 0.05 vol%, both absorbents reduced their enhancement effect and an opposite effect took place. Their results showed maximum enhanced absorption rates of 9.4% and 9.7% for Al2O3 and SiO2 nanoparticles (compared to pure methanol), respectively. Pineda et al. [91] believed that in a dynamic flow system such as a tray column absorber, the forces induced by the movement of the liquid and vapor play more dominant roles in the mass transfer enhancement. They proposed bubble breaking model for the absorption based on the absorption experiments and the bubble visualization. Pineda et al. [91] concluded that at low concentrations the movement of the particles promotes mass transfer, however after the optimum point, the nanoparticles become too dense in the liquid phase and reduce the self-diffusion coefficient and the absorption of the gas phase [91].

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3.6. Gaseliquid hollow fiber membrane system Golkhar et al. [92] applied nanofluids of silica nanoparticle and carbon nanotube as absorbents in a gaseliquid hollow fiber membrane contactor for CO2 separation. In their study, a gas mixture of air and CO2 was fed into the shell side in contact with pure water or nanofluid which was fed into the fibers. Effects of different parameters including the type of nanofluid, nanoparticle concentration (0.25 and 0.5 wt%), liquid and gas flow rates, liquid temperature and CO2 inlet concentration were investigated experimentally on the removal efficiency of CO2. Their results showed that the injection of 0.5 wt% silica nanofluid increased the removal efficiency up to 20% at low liquid flow rates (compared to distilled water) while the increase was about 9% at high rates. Nanofluid of carbon nanotube showed much better separation than the silica and it enhanced the removal efficiency up to 40% and 20% at low and high liquid rates respectively [92]. 3.7. Direct measurements of mass transfer coefficients in nanofluids In 2011, Sara et al. [93] experimentally investigated the effect of suspended CuO nanoparticles on the mass transfer to a rotating disc electrode, using the electrochemical limiting diffusion current technique (ELDCT). They prepared nanofluids with the nanoparticle volume fractions from 0.39% to 1.94% and found that the addition of nanoparticles enhanced mass transfer, and this enhancement increased with the concentration of nanoparticles. The enhancement in mass transfer was up to 50% [93]. Sara et al. [93] discussed that the mass transfer enhancement can be the result of combined effect of decreasing diffusion boundary layer thickness and microconvective vortices, which are resulted from the rotating nanoparticles in the diffusion boundary layer. They also reported the Sherwood number correlation as the function of Reynolds number and the nanoparticle volume fraction [93]. Beiki et al. [16] also experimentally studied mass transfer of ferricyanide ions through electrolyte nanofluids using ELDCT. They investigated laminar convective mass transfer of ferricyanide ions in nanofluids consisted of 40 nm g-Al2O3 nanoparticles dispersed in ferrieferrocyanide and aqueous sodium hydroxide in a circular tube. Based on their results, for low nanoparticle volume fractions, below 0.01%, the mass transfer coefficient in electrolyte nanofluid was higher than that in electrolyte solution and for higher concentrations of nanoparticles from 0.015% to 0.025%, the mass transfer coefficient in electrolyte nanofluid decreased with the concentration of nanoparticles [16]. They found that the augmentation of mass transfer coefficient for 0.01% nanofluid, was about 16.8% at Re ¼ 1260 relative to the base liquid. Beiki et al. [16] argued that microconvections and disturbance fields created by the rotation and Brownian motion of the nanoparticles are responsible for the increased mass transfer for concentrations below 0.01%. Moreover, they believed that in greater concentrations, the formation of nanoparticle clusters decreases Brownian motion velocity and causes the reduction in mass transfer enhancement. They also observed that mass transfer enhancement slightly decreased with Reynolds number showing that enhancement effect of nanoparticles is more pronounced at smaller Reynolds numbers [16]. In another experimental work, Beiki et al. [94] investigated turbulent mass transfer in straight circular tube by applying ELDCT to measure the mass transfer coefficient in fully developed hydrodynamics and under developed mass transfer region. They added TiO2 and g-Al2O3 nanoparticles into the electrolyte solution (ferriand ferrocyanide solution with aqueous NaOH) to make electrolyte nanofluids. Their measurements revealed that enhancement in mass transfer reached 10% in a 0.01 vol% g-Al2O3-electrolyte

95

nanofluid while 18% in a 0.015 vol% TiO2-electrolyte nanofluid relative to the base electrolyte solution. Mass transfer coefficients increased with nanoparticles concentration up to an optimum concentration while decreased by increasing nanoparticles concentration further. They found that the mass enhancement ratio was a function of nanoparticle concentration and was independent of Reynolds number. Beiki et al. [94] suggested the mechanisms of nanoparticles Brownian motion and nanoparticles clustering to describe the behavior of the enhancement ratio in electrolyte nanofluids [94]. Recently, Keshishian et al. [95] reported the effect of silica nanoparticles on mass transfer coefficients in a circular tube by using ELDCT in both laminar and turbulent flow regimes. To prepare electrolyte nanofluid, they combined silica nanoparticles with the size range of 7e13 nm with potassium ferrieferrocyanide and sodium hydroxide solution. Their measurements for laminar regime indicated that mass transfer coefficient increased with nanofluid volume fraction up to 0.0057% and decreased with increasing the volume fraction of nanoparticles further. They observed that the maximum enhancement in mass transfer reached 21% at Reynolds number of 326 and in turbulent flow regime no enhancement was recognized due to the addition of silica nanoparticles to the base electrolyte solution [95]. Keshishian et al. [95] concluded that in laminar flow regime, Brownian motion of nanoparticles and disturbance fields of microconvections created by Brownian motion, mediation of electrophoresis near the wall and tendency of adsorbed electroactive ions onto nanoparticles to reach to the surface of electrode are responsible for enhancement in mass transfer. Also, they argued that hindering act of nanoparticles against diffusion of electroactive ions is the reason of decreasing trend in higher nanoparticle concentrations [95]. In addition, in turbulent flow regime, they observed no significant enhancement in mass transfer by adding silica nanoparticles. Keshishian et al. [95] believed that this observation is the result of the important role of eddies. Based on their discussion, as silica has low density, high shear rate near the wall in turbulent flow regime become more comfortable to take effect for inducing particles migration from wall to the core of tube therefore, due to this effect combined with thinner boundary layer thickness, silica nanofluid doesn’t demonstrate augmentation effect compared with laminar flow regime [95]. Table 6 shows a summary of experimental works on direct measurements of mass transfer coefficients in nanofluid systems. 3.8. Liquideliquid systems Despite the wide application of liquideliquid extraction process, there have been limited attempts to use nanofluids in liquideliquid extraction [41]. Bahmanyar et al. [14] experimentally studied the effect of nanofluids on the mass transfer performance and hydrodynamic characteristics of a pulsed liquideliquid extraction column and found that the mass-transfer coefficient increased by 4e60% using 0.01e0.1 vol% SiO2 nanoparticles in kerosene, which was used as the dispersed phase in an aqueous continuous phase [41]. They argued that Brownian motion of nanoparticles is liable of enhancement in mass transfer. They also observed that in the presence of the nanoparticles, static and dynamic dispersed phase hold-ups increased by 23e398%, and 23e257%, respectively [14]. Saien and Bamdadi [41] investigated the behavior of nanofluid single drops in the liquideliquid extraction process. In their study, the chemical system of tolueneeacetic acidewater was used, and the drops were organic nanofluids containing magnetite or alumina nanoparticles. They modified nanoparticles with fatty acids for hydrophobicity and ease of dispersion in the organic phase. Saien

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g-Al2O3: 40a

1.1 at 0.01% g-Al2O3 and 1.18 at 0.015% TiO2 Laminar flow: 1.21 at 0.0057% SiO2 Turbulent flow: 1 (no enhancement)

0.005e0.025% nanoparticles volume concentration g-Al2O3: 0.005e0.025% TiO2: 0.01e0.05% 0.0002e0.018%

3.9. Controlling mechanisms of convective mass transfer in nanofluids 3.9.1. Convective mass transfer enhancement mechanisms

Size of nanoparticles in powder state before combining with base fluid to make nanofluid. a

Keshishian et al. [95]

Beiki et al. [94]

40a

Laminar convective mass transfer in a circular tube using ELDCT Turbulent mass transfer in straight circular tube using ELDST Mass transfer in a circular tube by using ELDCT in laminar and turbulent flow regimes Beiki et al. [16]

and Bamdadi [41] examined drop sizes within the range 2.9e 4.3 mm, with magnetite and alumina nanoparticle concentrations of 0.0005e0.005 wt%. They achieved the maximum mass-transfer enhancement at a nanoparticle concentration of 0.002 wt% with the enhancement values of 157% and 121% for magnetite and alumina nanoparticles, respectively. They also found that small drops experienced greater mass enhancement but the size of the drops was not found to be influenced much by the nanoparticles. They argued that microconvection induced by Brownian motion and particle aggregation respectively cause increasing and decreasing trend in the rate of mass transfer with nanoparticle concentration. To predict the overall mass-transfer coefficient, Saien and Bamdadi [41] correlated the measured enhancement factors with an empirical expression together with Newman equation [41]. Table 7 indicates the conditions of reported studies on liquideliquid extraction process.

TiO2: 1e3a 7e13a

1.5 at 1.94% Cu 0.39e1.94% 30e50a Mass transfer to a rotating disc electrode using ELDCT

CuO/ferry- and ferro-cyanide solution with aqueous K2CO3 g-Al2O3/ferri- and ferrocyanide solution with aqueous NaOH g-Al2O3 or TiO2/ferri- and ferrocyanide solution with aqueous NaOH SiO2/potassium ferri-ferrocyanide and NaOH solution Sara et al. [93]

1.168 at 0.01% g-Al2O3

Maximum enhancement ratio Experimental method Nanofluid type Investigator

Table 6 Experimental studies on direct measurements of mass transfer coefficients in nanofluid systems.

Particle size (nm)

Particle volume fraction

96

 Brownian motion of nanoparticles and subsequent microconvection are reported to be responsible for augmentation of convective mass transfer [41].  Hydrodynamic effect model in gaseliquid systems e This model proposes that the particles may increase the specific interfacial area by covering the bubble surface and preventing the coalescence of the bubbles, resulting in smaller bubbles [96].  Grazing effect or shuttle effect mechanism in gaseliquid systems e By this mechanism, fine particles in suspension adsorb solute from the gas and rapidly transfer the same to the liquid [97,98].  Bubble breaking model in gaseliquid systems e In this model, the particles collide with the gaseliquid interface, consequently breaking the bubble into smaller bubbles. More bubbles mean a larger interfacial area which would promote the mass transfer rate from the gas to the liquid [91,99].  Decrease in film thickness due to shearing action by the particles is also proposed as an augmenting mechanism for convective mass transfer [100].  Reduction in surface tension and followed by it, creation of Marangoni convection effect which leads to sharp turbulence and a more nonuniform distribution of the solute at the gase liquid interface is another mechanism proposed for enhanced convective mass transfer [80].

3.9.2. Convective mass transfer degradation mechanisms  Agglomeration of particles has decreasing effect on convective mass transfer [57,58].  Increase in elasticity of suspension decrease convective mass transfer [46e49].  Lowering of diffusion coefficient in the liquid phase owing to increase in viscosity of the liquid decreases the convective mass transfer [100]. Although some mechanisms to describe diffusion and convective mass transfer phenomena in nanofluids have been presented, the general mechanism is still debating [74]. 4. Future aspects Some studies suggest that nanoparticles enhance mass transfer while others have observed no enhancement or degradation.

c

a

Size of nanoparticles in powder state before combining with base fluid to make nanofluid. Size of nanoparticles in suspension. Particle mass fraction.

5. Conclusion

b

Fe3O4 or Al2O3/toluene Saein and Bamdadi [41]

97

Because of limited and contradicting reported results for mass diffusion coefficient in nanofluids, experimental data are especially needed in this field to achieve reliable data and presenting reliable mechanism. Moreover, although most efforts in the field of convective mass transfer in nanofluids have proved that the suspended nanoparticles enhance the mass transfer process, but similar to mass diffusion, the mechanism leading to this enhancement is still unclear and more efforts are in urgent need for practical applications. Reported researches on liquideliquid extraction are very limited. Due to the extensive use of liquideliquid extraction in chemical industries, producing more reliable data and suggesting reliable mechanisms is needed to describe the effects of nanoparticles on the process. If mass transfer enhancement in the presence of nanoparticles is proved, then the question arises that when nanoparticles are used in the industry, how they can be removed and recycled finally. Although employing magnetic nanoparticles provides the possibility of particles separation at the end of the process using a magnetic field, extra investigations are still needed for separating and recycling other particles. In most reported works, only the influence of nanoparticles concentration on the mass transfer has been investigated. Type, size, shape, surface morphology and chemical nature of nanoparticles could be the focus of future studies.

Fe3O4: 1.157 Al2O3: 1.121 at 0.002 wt% of nanoparticles 0.0005e0.005 wt%c Fe3O4: 10a, 17b Al2O3: 25a, 30b

1.6 at 0.1% SiO2 0.01e0.1% 5e30a SiO2ekerosene Bahmanyar et al. [14]

Dispersion of nanofluid in an aqueous continuous phase in a pulsed liquideliquid extraction column with the chemical system of keroseneeacetic acidewater Nanofluid single drops in a liquideliquid extraction column with the chemical system of tolueneeacetic acidewater

Particle volume fraction Particle size (nm) Experimental method Nanofluid type Investigator

Table 7 Experimental studies on convective mass transfer in nanofluids in liquideliquid extraction systems.

Maximum enhancement ratio

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Published articles on mass transfer in nanofluids have been reviewed in two main sections in this review paper. In each section, performed studies, type of nanofluids, size and concentration range of nanoparticles, additives, mass transfer measurement, maximum observed enhancement, controlling mechanisms of mass transport and some other parameters were discussed. Limited and/or inconsistent experimental data and unclear dominant mechanism of mass transport in nanofluids show that much more efforts are needed to clarify the enhancements, identify the ambiguities and the reasons for inconsistent reported results and find the dominant mechanisms. This review also indicates that more studies are required to understand the influence of some parameters such as size, shape, surface morphology and chemical nature of nanoparticles on mass transfer in nanofluids. Also, it seems that finding suitable methods for separating and recycling nanoparticles after usage in industry and modeling of mass transport in nanofluids are other subjects for future studies.

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