Mass Transfer Rate Enhancement in Nanofluids: Packed Column

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Thermodynamics, Transport, and Fluid Mechanics

Mass Transfer Rate Enhancement in Nanofluids: Packed Column Studies and a design basis Tejeswi Ramprasad, Ratnesh Khanolkar, and Akkihebbal K. Suresh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00770 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Industrial & Engineering Chemistry Research

Mass Transfer Rate Enhancement in Nanofluids: Packed Column Studies and A Design Basis Tejeswi Ramprasad1, Ratnesh Khanolkar2, and Akkihebbal K. Suresh* Department of Chemical Engineering, IIT Bombay, Powai, Mumbai 400076, India. ABSTRACT Enhanced mass transfer rates in nanofluids have been demonstrated in recent studies. A semi-empirical theory for predicting mass transfer rates in nanofluids was proposed in our earlier work, based on data in model apparatuses. In the present work, the applicability of this theory to a more complex hydrodynamic environment, that of a packed bed, is studied. Experiments have been carried out with silica nanofluids on the absorption of carbon dioxide with and without chemical reaction have been carried out in a laboratory packed bed. Enhancements are, in general, larger in physical absorption than in absorption with reaction, as expected by the theory. The average error in the prediction of packed bed data using the theory is of the order of 21%, which suggests that the theory can form a basis for design. A phase plot has been presented to identify regions in the parametric space for significant enhancement.

1. INTRODUCTION The interest in using nanoparticles for enhancement of transfer processes initially began with heat transfer. As heat and mass transfer are analogous, and because of the importance of mass transfer processes in industry, there was a natural interest in examining mass transfer rates in nanofluids, and several studies reported1–4 enhancements in mass transfer rates in gas liquid absorption systems. While a variety of results have been reported on different types of absorbers, no attempt seems to have been made towards organising these results into a coherent body, so that some basis for design of equipment based on the phenomenon of enhancement can be arrived at. Some insights into the mechanism of enhancement have been gained in studies2-3 in well characterized model apparatuses and correlating principles identified, but since these correlations have not been adequately tested for the complex hydrodynamics such as are encountered in industrial contacting equipment, it is not clear whether they can be used with confidence to design such equipment. The literature on nanofluids-enhanced mass transfer in industrial type of equipment is also often conflicting. This study has been undertaken in this background, in order to bridge this gap between the

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state of understanding of the enhancement phenomenon in model apparatuses and the behaviour of industrial type equipment. Current address: Research Scientist, Pulp and Fibre Innovation Centre, Grasim Aditya Birla, Taloja, Panvel 410208, India. * Author for correspondence. Email: [email protected]. Authors 1 and 2 have equal contributions as first author. 2. LITERATURE Early studies on mass transfer with nanofluids were due to Bhalerao and Suresh5 who added a ferrofluid to the solvent in a carbon dioxide absorption system, and attempted to create mixing in the near-interface region by manipulating the Fe3O4 nanoparticles using an oscillating external magnetic field. Subsequently, the interest has shifted largely to the effect of nanoparticles per se and there have been a number of studies on various nanofluids in a variety of absorption equipment, but on a limited range of gas-liquid systems. Table 1 groups these studies on the basis of the equipment used, and provides a summary. As this table shows, absorption of carbon dioxide in various solvents has been by far the most studied, probably because of the industrial importance of these systems. Oxide nanoparticles have been the most commonly studied, but some metal nanoparticles and CNT have also received some attention. While most studies report an enhancement in the absorption rates, contradictory results also exist in the literature. For example, in a study by Samadi et.al.6, Al2O3 showed a positive effect whereas TiO2 and Fe3O4 showed a negative effect on the rate of mass transfer of CO2 in water in a wetted wall column setup. Since in most cases, the nanoparticles are stabilised using a surfactant or polymer coating, some studies have addressed the effect of surfactant. Kim et. al.7 observed a synergistic effect of nanoparticles and surfactant on absorption of ammonia in water -- a maximum enhancement of 3.21 times was achieved using 0.1% Cu nanofluids and addition of surfactant (2-ethyl-1-hexanol) increased the enhancement to 5.32 times. However, Yang et al8 found the surfactant effect to be antagonistic to that of the nanoparticles. Both reactive and non-reactive systems have been studied, and in some of the latter studies, the enhancement due to the reaction alone has not been adequately factored in Kim et.al.9 studied the effect of Silica nanoparticles on CO2 absorption in water and piperazine promoted potassium carbonate solution in a bubble absorber. A 24% enhancement was observed for physical absorption whereas only 12% enhancement was seen in a reacting solvent in presence of 0.021 wt% of 30 nm Silica nanoparticles. The effect of magnetic field in systems with magnetic nanoparticles, starting with the work of Suresh and Bhalerao5, has also been studied by other workers. In such a study, Salimi et. al.10 found enhancements in mass transfer coefficient of 9% and 5.5% in the presence of 0.005% Fe3O4 and 0.01% NiO respectively, without magnetic field; in the presence of a field the enhancements increased to 14% and 10.5% respectively. Since, as observed earlier, the interest in nanofluid mass transfer followed that in nanofluid heat transfer, a comparison of the two transport processes has attracted some attention. Lee 2 ACS Paragon Plus Environment

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et.al.11 found 0.02% Al2O3 and CNT to enhance the rate of mass transfer in ammonia absorption in water by 18% and 16% respectively; the enhancement in heat transfer were 29% and 17% respectively, for the same nanoparticles. Kim et.al.12 studied the effect of Silica nanoparticles in H2O/LiBr binary nanofluid on heat and mass transfer on a falling film absorption process. An 18% enhancement in mass transfer and a 46% enhancement in heat transfer were obtained in presence of 0.005% Silica nanoparticles. Similar results were obtained by Kang et.al.13 using Fe and Carbon nanotube (CNT) particles, also in H2O/LiBr binary nanofluid on heat and mass transfer for falling film absorption process. CNT particles were found to be better than Fe nanoparticles; in contrast to the studies quoted above, the mass transfer enhancement was much more significant than heat transfer enhancement. While, as seen above, the number of experimental studies that report on the effect of nanoparticles on the base mass transfer rate (i.e., in the absence of nanoparticles) is large, the result are mostly reported as measured enhancement of either mass transfer rates or mass transfer coefficients; the fundamental mechanisms involved, or even the important parameters which determine the magnitude of the effect, are rarely addressed. It is thus not clear how to translate the results from one type of equipment to another type of equipment with a different hydrodynamic situation, or indeed, for the same equipment across different scales. Table 2 gives a summary of the studies aimed at identifying the important parameters. Particle size and hold-up have been found to be important in most cases. In some studies comparing the effect of different materials, particle density was found to be important. While an ‘optimum’ concentration of nanoparticles has sometimes been reported, it is not clear whether there was agglomeration at the larger volume fractions used, and one is therefore actually looking at the effect of particle size. In general, situations in which the base case mass transfer rate is poor seem to show the maximum benefit for the nanofluid strategy. For example, in studies with chemical reaction where the base case mass transfer rate is already enhanced due to reaction, the further enhancement due to nanoparticles is generally smaller than in the corresponding cases without reaction. Ghanadi et. al.14 observed for a liquid-liquid system, that nanoparticles have a negligible effect on the mass transfer rate when the mass transfer rate of the system without nanoparticles is high enough. Some of the above aspects have been rationalised based on systematic studies in model apparatuses, in which the hydrodynamics is well understood. In one such study involving several particle sizes, gas-liquid systems, types of apparatus, flow--non-flow and reactive-non-reactive systems, Komati and Suresh2 found that it is not the particle size per se, but the particle size scaled with respect to the penetration depth of the solute, that is of importance. This explains why the same size particles are less effective in a reactive absorption scenario, since the penetration depth is reduced in the latter case by reaction. For Fe3O4 particles, a single correlation could explain all their data, as well as other data in the literature1. In an extension of these studies to explore the effect of the material of nanoparticles, CO2 absorption in water was studied by Khanolkar and Suresh3 using Silica and TiO2 nanoparticles in capillary tube apparatus for which the results on Fe3O4 particles were reported earlier2. A model based on convective diffusion was developed which was able to explain the observed enhancement in presence of Silica, TiO2 and Fe3O4 nanoparticles. These 3 ACS Paragon Plus Environment


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studies show that the particle hold-up, the particle size scaled with respect to the solute penetration depth (a modified Sherwood number), and a ‘solid’ Reynolds’ number (the particle Reynolds’ number multiplied by the ratio of particle to fluid density) are the most important correlating parameters which determine the extent of enhancement. While fundamental mechanisms of nanofluids mass transport remain ill-understood, a recent study by Dhuriya and Sunthar15 hypothesizes that the increased rate of mass transfer in nanofluids is due to the phenomenon of Diffusiophoresis. While the fundamentals continue to be researched, it would be interesting to see how the results from model equipment (which have been well correlated, and are also easy to generate) can be extended to the more complex hydrodynamic situations encountered in industrial contactors. If this is established, it would provide a basis for the design of industrial type equipment using existing correlations or with little experimental effort on model apparatuses.


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Table 1. Literature on systems studied in the context of mass transfer in gas-nanofluid systems Equipment Gas-liquid system

Nanoparticles

Size (nm)

Capillary tube

CO2 in water, amines

Fe3O4

13.2, 15, 23 0.0048-0.0097 (vol fraction)

1.64 (0.3 vol% Fe3O4 in 4000 gmol/m3MDEA)

2

Capillary tube

CO2 in water

SiO2, TiO

12, 25

0.2,0.4 vol %(SiO2)

1.74 (0.4% SiO2)

3

0.01 vol % (TiO2)

1.64 (0.0118% TiO2)

Wetted wall column

CO2 in water

0.01,0.05,0.1 vol%

1.79 (1 vol% Al2O3)

TiO2, Al2O3

0.84(0.05 vol% TiO2)

Wetted wall column


Fe3O4

Falling film

NH3 in water

Fe2O3, ZnFe2O4 (Sodium Dodecyl Benzene Sulfonate (SDBS) )

20,30,30

Fe3O4

20-25

Agitated reactor

O2 in Sodium sulphite

TiO2, Al2O3

8,15,25

15

Concentration of nanoparticles (%)

Maximum enhancement (in absorption) factor

0.02,0.05,0.1,0.2,0.25,0.39 1.928 (0.39 vol %)

Reference

6

16

vol % Fe3O4 0.1 vol% ZnFe2O4 (1.5% SDBS)

1.7 (0.2% Fe2O3 1.5% SDBS)

0.2 vol%Fe2O3 (1.5% SDBS)

1.5 (0.1% ZnFe2O41.5% SDBS)

0.01,0.02,0.03,0.04,0.05

6 (0.05 vol %)

17

1

(vol%)


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Absorber with distributor

NH3 in water

Tray column

CO2 in methanol

Al2O3 , CNT

35,25

0.01,0.02,0.04,0.06,0.08 (vol %)

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1.18 (Al2O3 at 0.02)

11

1.16 (CNT at 0.02) Al2O3, SiO2

10-20, 4050

0.005,0.01,0.05,0.1(vol%)

1.094 (0.05 vol% Al2O3)

18

1.097(0.05 vol% SiO2) Bubble column

NH3 in water

Cu, CuO, Al2O3

50

0.01,0.05,0.1 (wt %)

3.21 (Cu at 0.1)

19

Bubble column

CO2 in water, amine

SiO2

30,70,120

0.01,0.02,0.03,0.04 (wt%)

1.24(0.02 wt% 30nm SiO2 in water)

9

1.12(0.02 wt% 30nm SiO2 in amine) Bubble column

CO2 in methanol

Al2O3

200

0.005, 0.01,0.05,0.1 (vol%)

1.08 (0.01 Al2O3)

20

Bubble column

CO2 in water

TiO2, SiO2, CNT

21, 12, 1030

0.01, 0.05, 0.1,0.5 vol% (TiO2 &SiO2)

1.78 (0.07% CNT)

21

1.34 (0.6 g/l TiO2 in 30 wt %MDEA)

22

0.01, 0.05, 0.07, 0.1 vol% (CNT) Bubble column

CO2 in TiO2, SiO2, MEA&MDEA MgO

10,20,60 (TiO2)

0.2,0.4.0.6,0.8,1 (g/l)


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Table 2. Parametric studies in literature on the influence of nanoparticles on gas/liquid mass transfer Equipment

Gas-Liquid system

Parameters studied

Capillary tube


1. Effect of size of particles 2. Absence of flow

Capillary tube

CO2 in water

1. Effect of particle density 2. Effect of particle concentration

Wetted wall column

CO2 in water

1. Effect of particle used

Wetted wall column

CO2 in water & amines

1. Absorption in slow to fast regime

Falling film

NH3 in water

1. Effect of stability of nanoparticles (mechanical agitation and ultrasonic ACS Paragon Plus Environment

Main observations 1. Inverse relationship with enhancement 2. Enhancement due to increase in liquid side mass transfer coefficient 1. Denser particles gives much better enhancement 2. Increase in particle concentration increases rate of absorption 1. Enhancement observed in Al2O3 but decrease in absorption in Fe3O4 and TiO2 (due to increased viscosity) 1. Enhancements with increase in concentration of nanoparticles and in presence of flow 2. Interfacial area did not change with addition of nanoparticles. Significant increase in mass transfer coefficient was observed with addition of nanoparticles 1. Optimum ultrasonic vibration for 30 min after which the absorption rate decreases due to cavitation

Reference 2

3

6

16

17

7

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vibration) 2. Effect of concentration of ammonia Agitated Sparged reactor

O2 in Sodium sulphite

1. Reason for increase in absorption 2. Effect of particle concentration

Absorber with a vapour distributor

NH3 in water

1. Effect of particle concentration

Tray column

CO2in methanol

1. Effect of type of particle concentration

Bubble column

NH3 in water

1. Effect of particle concentration

Bubble column

CO2 in water, amine

Bubble column

CO2in methanol

1. Effect of size of nanoparticles 2. Effect of reacting solvent 1. Effect of solvent concentration 2. Effect of particle concentration

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2. Enhancements observed at 15 wt% of ammonia in water , above this concn absorption rate decreases attributed to increase in pH of the system 1. Cannot be distinguished separately whether enhancement is caused due to mass transfer coefficient of the interfacial area 1. Absorption rate increases with increase in concentration of particles 1. Absorption rate increases, reaches a maximum at 0.05 vol % and then decreases 1. Rate of absorption increases with increase in particle concentration 1. No effect of size was found 2. Less enhancement was observed 1. Increase in solvent concn decreases the absorption rate 2. Maximum enhancement at 0.01 Al2O3 above which rate decreases

1

11

18

19 9

20


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Bubble column

CO2 in water

1. Effect of particle concentration 2. Effect of gas velocity

Bubble column

CO2 in MEA, MDEA

1. Effect of particle size 2. Effect of solvent 3. Effect of nanoparticle species

1. Absorption increases with nanoparticles concn an d then becomes steady 2. Increased gas velocity increases mass transfer 1. Enhancement observed in the order 10SiO2 due to increased absorption capacities

21

22



3.

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EXPERIMENTAL

3.1. Apparatus The absorption apparatus consists of a glass column of 0.15 m height and 25 mm diameter. The column was packed with ceramic raschig ring of 3 mm diameter. The setup was arranged as shown in Figure 1. Liquid from the reservoir was pumped using a peristaltic pump. It flows through a pulsation dampener, a liquid rota meter and finally enters the top of the column. The liquid flows out through a liquid seal. The dampener eliminates the pulsations due to the peristaltic pump and ensures a smooth flow. A CO2 (99% purity) gas cylinder was used as the gas source, the gas flow being monitored and controlled using a mass flow controller. The inlet gas flow rate was fixed by using the mass flow controller and the flow rate of gas at the outlet was measured by a gas flow totalizer. The packed column operates in a counter current manner.

Figure 1. Packed column setup Some experiments were also conducted in the capillary tube apparatus described by Komati and Suresh2 and Khanolkar and Suresh3. Design Considerations In order to study the enhancement in absorption, it was decided that the height and flow rates should be chosen such that the concentration of CO2 in the leaving fluid is not more than about 60% of saturation, so that sufficient driving force for mass transfer is available in all parts of the column. The column height of 150 mm and operating gas and liquid flow rates of 460 ml/min and 227 ml/min respectively, were chosen based on preliminary experiments to satisfy this criterion, and variations in flow rate made, if necessary, around these. The resulting L/G ratio is well below the flooding limit of 0.15 (wt/wt) as estimated for our column and packing. 10 ACS Paragon Plus Environment

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Gas-Liquid system In principle, the phenomena of mass transfer rate enhancement by nanoparticles can be studied in any gas-liquid absorption system; however we have chosen carbon dioxide absorption as a model system, partly because this is a well studied system and partly because, with growing concerns about environment, efficient technologies for capturing carbon dioxide will be a need of the future. Experiments were conducted in physical absorption regime and different reactive regimes such as slow-to-fast transition and instantaneous reaction regime (as defined in the classical theories of mass transfer). Liquid solvents were chosen based on the reaction regime in which the column was operated; water was used for physical absorption, methyl diethanolamine (MDEA) for absorption in the slow-to-fast transition regime, and monoethanolamine (MEA) for absorption in the instantaneous reaction regime. The physical absorption experiments were carried out with different liquid flow rates in order to obtain a range of (base case) mass transfer coefficients. The reactive experiments were carried out with a liquid flow rate of 227 ml/min, but with different concentrations of the reactant (MDEA or MEA) in order to realise a range of Hatta numbers. 3.2. Experimental Procedure 99% pure CO2 (purchased from Med Gas N Equipment), Distilled Water, Methyl Diethanol amine (MDEA, purchased from Amine & Plasticisers Ltd.), Mono Ethanol Amine (MEA, purchased from Sigma Aldrich) and Silica HS 40 of size 12 nm (purchased from Sigma Aldrich) were used in the experiments. The liquid reservoir was filled with the appropriate solvent; the pump started and set to the desired flow rate. The liquid flow rate was measured using a liquid rotameter. Once the liquid level in the liquid seal was satisfactory, gas flow was started at the required flow rate through the mass flow controller. After the gas and liquid flows are set to the desired flow rates, we wait for a few minutes for the flow rates to adjust and for the column to attain steady state. The outlet gas flow rate is measured using the gas flow totaliser. After steady state is achieved (as indicated by the constancy of the measured exit gas flowrate), the flowrate is recorded and the amount of gas absorbed is calculated from the difference between inlet and outlet gas flow rates. Experiments were conducted at different concentrations of nanoparticles and different concentrations of reacting solvents. Experiments were also conducted at different liquid flow rates. All experiments were done at least twice and an average of the readings taken for further analysis.



3.3.

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Theory

Column performance equations Let us consider the liquid entering the packed column from the top at a volumetric flow rate LS. Let CAi and CAo be the concentration of dissolved gas (mol/m3) in the liquid at column inlet and outlet respectively. Gin and Gout are the inlet and outlet gas molar flow rates. Let RA be the absorption flux. Let ‘A’ be cross sectional area of column and “a” the interfacial area per unit volume. A balance for the solute in the liquid phase across a differential section of the packing gives: 1

𝐿𝑆 𝑑𝐶𝐴 = 𝑑𝐺 = (𝑅𝐴𝑎) 𝑑𝑉 = (𝑅𝐴𝑎) 𝐴 𝑑𝑧

Using the appropriate expression for RA Eq (1) may be integrated and the result used to calculate the volumetric mass transfer coefficient from measured absorption rates. On the other hand, if the mass transfer coefficient is known, Eq (1) is also the basic equation for process design using the appropriate equation for RA. Physical Absorption regime In this regime (for example the absorption of carbon dioxide in water) the rate of absorption per unit volume of the packing is given by: 𝑅𝐴𝑎 = 𝑘𝐿𝑎(𝐶𝐴 * ― 𝐶𝐴)

2

Substituting equation 2 in 1 and integrating over the column, we get, 𝐿′𝑠 𝐶𝐴∗ ― 𝐶𝐴𝑖 𝑘𝐿𝑎 = ln ( ∗ ) 𝑧 𝐶𝐴 ― 𝐶𝐴𝑜

3

This equation has been used for calculating the mass transfer coefficient in the packed column in physical absorption regime in the presence and absence of nanoparticles. In this equation, the concentration of the solute in the entering liquid is zero, and that in the leaving fluid is calculated from an overall solute balance: 4

𝐿𝑠𝐶𝐴0 = 𝐺𝑖𝑛 ― 𝐺𝑜𝑢𝑡 Transition regime from Slow to fast reaction

In general, two dimensionless parameters23 govern the regime in which an absorption process operates. These are the Hatta number, Ha, given by: 𝐻𝑎 =

1 𝑘0𝐿

2 (𝛾 ― 1) 𝛽 𝐷 𝑘 𝐶∗ 𝐶𝐵 (𝛾 + 1) 𝐴 𝛾,𝛽 𝐴

5


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and the parameter q, defined by 𝐷𝐵 𝐶𝐵

𝑞 = 𝜈𝐷

6

∗ 𝐴𝐶𝐴

where γ and β are respectively the order of the reaction with respect to the gaseous and the liquid phase reactant, and ν the stoichiometric coefficient of the liquid phase reactant in the reaction, i.e., the moles of B reacting per mole of A (other symbols are defined at the end). A process is said to operate in the slow to fast reaction regime, with part of the reaction occurring in the diffusion film and part in the liquid bulk (to choose a film-theoretic description), if 1 < Ha < 3 and Ha

Mass Transfer Rate Enhancement in Nanofluids: Packed Column

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