Experimental Investigation of the Effect of Nano Heavy Metal Oxide

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Experimental investigation of the effect of nano heavy metal oxide particles in Piperazine solution on CO2 absorption using a stirrer bubble column Hassan Pashaei, Ahad Ghaemi, Masoud Nasiri, and Mohamad Heydarifaed Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03481 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Experimental investigation of the effect of nano heavy metal oxide particles in Piperazine solution on CO2 absorption using a stirrer bubble column Hassan Pashaeia, Ahad Ghaemib,*, Masoud Nasiria, Mohammad Heydarifardb a

Department of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iran

b

Department of Chemical Engineering, Iran University of Science and Technology, Tehran, Iran [email protected]

Abstract In this work, absorption of CO2 into nanofluid of TiO2, ZnO, and ZrO2 at Piperazine solution was investigated experimentally in a continuous stirrer bubble column. The dosage range of nanofluids was 0.01 to 0.1 wt% in the experiments. The process parameters such as nanoparticles type, solid loading, and stirrer speed were varied to the hydrodynamics and absorption performance including gas holdup, Sauter mean diameter, CO2 loading, CO2 removal efficiency, absorption rate, mass transfer flux and overall mass transfer coefficients. The results showed that the nanoparticle mass fraction and range of stirrer speed have an optimum value for the abovementioned performance. The optimum value of TiO2, ZnO, and ZrO2 nanoparticles were 0.05 wt%, 0.1 wt%, and 0.05 wt%, respectively. The maximum absorption rate of TiO2, ZnO, and ZrO2 in comparison with pure Pz solution were 14.7% (0.05 wt%), 16.6% (0.1 wt%), and 3.7% (0.05 wt%), respectively. And also, with an increase of the stirring speed, the absorption performance increased first up to 200 rpm and it starts to decrease after 200 rpm. The hydrodynamics studies indicate that the gas hold-ups increase and the Sauter mean diameter decrease in the bubble column with increasing nanoparticles to the base fluid. Keywords: Nanoparticles, Nanofluid, CO2 absorption, Solid loading, Stirrer speed, Enhancement factor

1. Introduction The bubble stirrer columns are widely applied in the chemical industries [1] due to noncomplex design [2], low investment and current cost, high gas–liquid interfacial area and high absorption performance [3]. On the other hand, the hydrodynamic behavior and scale-up of the bubble columns are difficult [4]. The significant parameters determining bubble stirrer columns performance are superficial gas velocity, operating pressure, and temperature [5]. There are

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important variables that affect bubble stirrer column performance such as gas holdup distribution [6], sparger design, superficial velocity, Sauter mean diameter, distribution of bubble size [7], bubble break up and coalescence [8], gas–liquid interfacial area, the back mixing amount of liquid phase and mass transfer performance [9]. With increasing greenhouse gasses in the atmosphere, various problems due to global climate change are creating [10-12]. Due to destructive role of CO2 [13, 14] in global warming [15], the contribution of CO2 to global warming is around 80% [16]. Therefore, its control is becoming more important nowadays [17]. Absorption, adsorption, membranes, hydrates, cryogenics and other technologies have been studied for CO2 removal [18, 19]. Each of the mentioned technologies in terms of cost, efficiency, corrosion of equipment, and regeneration has advantages and disadvantages [20, 21]. Meanwhile, the absorption of CO2 into an alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), and 2-amino- 2-methyl-1-propanol (AMP) are the most common methods [22-24]. Many researchers have tried to improve CO2 absorption performance by typical alkanolamines [25]. In this regard, Piperazine(Pz) could be an effective amine candidate for CO2 absorption as a new cyclic amine includes a primary, a secondary, and a tertiary amino group in a molecule [26, 27]. Pz molecule exhibits favorable physical properties, such as a low vapor pressure, high boiling point, and good solubility compared with MEA, DEA, MDEA, AMP and another amines. Pz has been reported to have many beneficial qualities such as low thermal and oxidative degradation rates, low regeneration energy, and low corrosivity relative to conventional alkanolamines [28]. For instance, Tan and Chen [29] studied results showed that the CO2 removal efficiency in MEA, AMP, and MDEA solutions is most lower than Pz solution. The CO2 absorption rate has been found to considerably increase when Pz is added to MDEA [30] and AMP solutions [31, 32]. Rowland et al. [33] reported that Pz had a greater absorption performance of CO2 into aqueous ammonia compared to inorganic acid and amino acid. Alstom [34] has been patented the use of Pz as a very effective promoter for chilled ammonia but little information was revealed in the report. In the recent years, various techniques have been used to enhance mass transfer and achieve better performance of absorption processes [35]. The technique for the improvements is commonly classified into the chemical treatment, mechanical treatment, and the fluid properties improvement [18, 19]. Recently, nanofluids are known as a high potential solvent [36] and one of the impressive mass transfer media [37]. A nanofluid is a suspension containing nanoparticles are dispersed constantly in a base fluid [38]. The nanoparticles sizes are smaller than 100 nanometers. The nanoparticles is made of metals such as copper, silver, nickel and or metal


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oxide such as aluminum oxide (Al2O3), copper oxide (CuO) and TiO2. Nanofluids needs lower pumping energy, low sedimentation speed, having higher surface area and high dispersion stability in compared to base fluids [39]. Therefore, nanofluids can potentially be applied in a wide range of applications. According to Krishnamurthy et al. [40] experiment, a color diffuses faster in a nanofluid than in water. They described that the Brownian motion of the nanoparticles causes to convection in the nanofluids. CO2 capture from the flue gaseous is one of the most important applications of nanofluids in gas absorption. Numerous studies have been done in order to enhance the CO2 absorption. Pineda et al. [41] studied the influence of SiO2 and Al2O3 nanoparticles using a tray column on the CO2 absorption in the methanol-based fluids. Their results showed that the absorption capacity of the SiO2 and Al2O3 nanofluids increased up to 9.7% and 9.4%, respectively, at the 0.05 vol% loading of the nanoparticles. In another research, Pineda et al. [42] studied the influence of Al2O3, SiO2 and TiO2 nanoparticles on CO2 absorption performance in a new annular contactor (AC) with trays absorber. Their report showed that the absorption rate of Al2O3, SiO2 and TiO2 enhanced up to 10%, 6% and 5%, respectively. Lee et al. [43] studied the absorption performance of CO2 increasing in a bubble column. The results indicate that the absorption rate of the SiO2 and Al2O3 nanofluids were enhanced up to 5.6% and 4.5%, respectively, at 0.01 vol% loading of the nanoparticles in comparison of methanol based fluids. Jung et al. [44] investigated the CO2 absorption rate in the nanofluid of Al2O3 nanoparticles and methanol-based fluids using a bubble column. Their research showed the absorption rate increased to 8.3% in comparison to the pure based fluids. Pang et al. [45] experimentally considered the NH3 absorption intensify in the silver nanofluid during the bubble absorber system. It was found the absorption rate was enhanced to 55% at 0.02 wt% loading of the silver nanoparticles in comparison to base water. Ma et al. [46] and Lee et al. [47] studied the NH3 mass and heat transfer rate in the absorption process using the Al2O3 and carbon nanotube (CNT) nanoparticles in a bubble absorption process. Their results demonstrated the CNT and Al2O3 lead to increasing the mass and heat transfer rates significantly. Sumin et al. [48] experimentally studied the influence of Al2O3 and CNT nanoparticles on CO2 absorption in a stirred thermostatic reactor. Samadi et al. [49] proposed a new wetted-wall column of the gas absorber with external magnets. They reported that the CO2 mass transfer flux and mass transfer coefficient of Fe3O4 nanofluids were enhanced to 22.35% and 59%, respectively. Salimi et al. [50] experimentally studied the Fe3O4 (at 0.005 vol % ) and NiO (at 0.01 vol % ) nanofluids on the CO2 absorption in the presence of magnetic field. They reported that the mass transfer rate of the Fe3O4 and NiO nanofluids were enhanced up to 12% and 9.5%, respectively, in comparison to water based fluids. Kim et al. [51] studied the CO2 absorption


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performance in the nanofluid of silica nanoparticle in a bubble absorber system. The results showed that the absorption rate of the silica nanofluids was enhanced 24% with 0.021 wt% of nanoparticles in comparison to the pure water based fluids. After absorption of gas in the nanofluid, the contact area was increased because of broken gas bubbles into small bubbles with the stable nanoparticles and finally leads to increasing absorption rate in the nanofluid. The objectives of this work are to examine the effect of TiO2, ZnO and ZrO2 nanoparticles on the hydrodynamic and absorption performance by experiments using a continuous stirrer bubble column. Another objective of this paper is to study the CO2 absorption rate enhancement and find the optimal conditions of nanofluids. As well as, the influence of some parameters on gas holdup, Sauter mean diameter, gas-liquid interfacial area, CO2 removal efficiency and overall mass transfer coefficients were investigated.

2. Theory 2.1. Hydrodynamics Hydrodynamic researches during the chemical absorption of CO2 into Pz aqueous solutions in the stirrer bubble column aimed at analyzing the variation in the gas-liquid interfacial area, gas holdup, bubble size distribution and Sauter mean diameter [52]. The gas holdup in stirrer bubble columns that varies with gas velocity, nanoparticle concentrations and bubble size [53] is measured using the liquid expansion method. This method allows us to determine the gas holdup values within 1.0% accuracy. In this method, the ratio of the initial height of liquid in the column and the liquid height after injecting gas are measured. The gas holdup with this method is indicated by the following equation [54-56]: εG =

∆L ∆L + L

( 1)

In equation 1, L is the initial height of nanofluid and ∆L is the height expansion of nanofluid after entering gas phase into the column. In order to study the dispersion of gas in the liquid, the notification of bubble size and bubble size distribution is necessary. The large gas bubbles rise quickly with the column than small bubbles. So the reduction of gas residence time decreases the gas holdup [7, 57, 58]. The images for the gas dispersion in the bubble column display that the bubbles have got an ellipsoid shape with Minor (e) and major (E) axes of the projected ellipsoid. The equivalent sphere diameter was considered as the representative bubble dimension as follow:


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d = 3 E 2e

( 2)

There are many authors recommend of bubble size that causes confusion. The most originally used definition is the Sauter mean diameter. It can be specified by the data calculated for the equivalent diameter through Eq. (3) with accuracy of ± 0.0 1% [59]. N

∑n d

3 i

∑n d

2 i

i

d 32 =

i =1 N

i

( 3)

i =1

Where di and ni are the diameter of a single bubble and bubble frequency with diameter di, respectively. The gas-liquid interfacial area is a main variable in the design of bubble columns, which is depending on the operating conditions, column geometry, and solution properties. The Sauter mean diameters (d32) and the gas holdup ( εG ) calculated values were used to specify the gas-liquid interfacial area as follows [60]:

a=6

εG

( 4)

d 32 (1 − εG )

The interfacial area was described by the specific interfacial area and the liquid volume used in the bubble column. The gas holdup (Eq. (1)) and Sauter mean diameters (Eq. (3)) were extracted by analyzing the images obtained, thus providing a calculation of the gas-liquid interfacial area.

2.2. CO2 Absorption After the contact occurred between mixed gas of CO2 and air fed from the bottom of the bubble stirrer column and nanofluid solution, the gas was out from the top of the column. The nanofluids were continuously fed from the column top. It was supposed that Air was not dissolved in the solution, and the liquid would not evaporate into the gas phase. Also, all the absorbents occurred in the experiments for one hour. The absorption rate was specified from the gas phase material balance using the measured inlet and outlet gas concentration. The CO2 inlet and outlet concentration were measured by the online analyzer. The rate of CO2 absorption into the nanofluids is calculated by the following equation: ∆ G = ρ g ,inQ g ,in (

y CO 2 ,in − y CO 2 ,out 1 − y CO 2 ,out

)


( 5)

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Where Qg,in, ρg ,in , y CO2 ,in and y CO2 ,out are mixed gas flow rate, inlet gas density, inlet CO2 mol fraction and outlet CO2 mol fraction, respectively. The removal efficiency is an significant parameter in the evaluation of nanofluids performance [61]. Removal efficiency is influenced by process operating parameters such as gasses partial pressure, temperature, type of solution, loading of materials, and circulation rate [62]. Meanwhile, the CO2 removal efficiency can be calculated using the following equation [63, 64]: CO2 removal efficiency=

y CO 2 ,in - y CO 2 ,out y CO 2 ,in

× 100

(6)

The enhancement factor is defined as the ratio of CO2 absorption rate by nanofluids to that by the base fluid to investigate the CO2 absorption enhancement by nanoparticles.

E=

CO 2 absorption rate in the presence of nanoparticles CO 2 absorption rate in the absence of nanoparticles

(7)

The value of mass transfer flux is obtained by dividing the number of CO2 molecules absorbed per the gas-liquid interfacial area in each time. The CO2 absorption flux into an aqueous Pz solution and nanofluids in the stirrer bubble column is regarding the driving force according to the following equation [65]: N

CO 2

=

C CO 2

a

= KG

(P

C O 2 ,b

− PC*O 2

)

( 8)

Where, NCO2 and C C O 2 are the CO2 mass transfer flux and CO2 concentration, respectively, KG * is the overall mass transfer coefficient, pCO2 ,b is the CO2 partial pressure of the gas bulk and pCO2

is the CO2 equilibrium partial pressure of the bulk solution. In equation 8, the term of pressure difference can be represented using the log mean average inlet and outlet CO2 partial pressure difference [66]:

∆pCO2 ,lm =

pCO2 ,in − pCO2 ,out ln (

* ( pCO2 ,in − pCO ) 2

( pCO2 ,out − p

* CO 2

)

)

( 9)

Where pCO2 ,in and pCO2 ,out are the inlet and outlet CO2 partial pressures, respectively, and since * pCO can be negligible in the case of CO2 unloaded absorbent, Because the rate of CO2 2

absorption is small relative to the total CO2 concentration of the liquid since the chemical reaction is fast [67-70]. Therefore, the log means average of the operational partial pressure of CO2 will be equal pCO2 ,b . Therefore the overall mass transfer coefficient, KG, can be defined by


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the plot slope of the CO2 absorption flux and driving force, ∆pCO2 ,lm , that can be expressed as below: KG =

N CO 2

(10)

∆pCO 2 ,lm

3. Experimental 3.1. Materials In this work, three TiO2, ZnO and ZrO2 were used as the nanoparticles in all nanofluids. Double distilled water and Pz were used for preparing the desirable nanofluids. The aqueous Pz solutions (0.1, 0.3 and 0.5 M) were prepared from Merk, Switzerland with a purity of ≥ 99 %. The gas phase consists of CO2 and Air supplied by Hamtagas Mehrabad Co. cylinder (99.99% purity) and air compressor (Mahak, AP-301, 300 lit capacity), respectively. The volume fraction of the residual CO2 at the outlet was measured using gas analyzer, Testo model 327-1. The gas and liquid flow rate were measured by calibrated rotameters separately and the speed of stirrer is controlled with a dimmer. The nanoparticles (TiO2, ZnO, and ZrO2) were obtained from US Research Nanomaterials, USA.

3.2. Preparation of nanofluids At first, specified dosages of nanoparticles are added into the Pz aqueous solutions, and then they are dispersed with the ultrasonic agitator for 1 h. Different types of nanoparticle are used in this work including Zinc dioxide (ZnO, 99.6%, 10-30 nm), Titanium oxide (TiO2, 99.6%, 80 vol% anatase +20 vol% rutile, 20 nm), and Zirconium oxide (ZrO2, 99.95%, 20 nm), and the solid loadings were 0.01 wt%, 0.05 wt% and 0.1 wt%. It has been proved that the provided nanofluids have good dispersion stability even after 24 h. and nanoparticles characterized by X-ray diffraction (XRD) (Philips) using1.5405Å for Cu Kα radiation and scanning electron microscope (SEM) (SEM JOEL 6400). 3.3. Experimental setup and procedures The experiments were carried out in a stirrer bubble column shown in Figure 1. The experimental setup consists of the cylindrical absorption section with 10 cm inner diameter and height equal 50 cm, a CO2 gas cylinder with a regulator, heater, and gas flow meter, an air


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compressor with a regulator, a sparger disc, and two solution and storage tanks. The cylindrical test section and sparger disc were made up of transparent glass with 78.5 cm2 specified surface area and stainless steel with six holes, respectively. The diameter of stirrer disc an each hole was 8cm and 1 mm, respectively. Prior to the main experiment, each type of nanoparticles with a specific concentration is mixed with the aqueous Pz solutions and stirred by the ultrasonic vibrator for 2 h. The solution concentration was 0.1 M of Pz aqueous solution, and nanoparticles were varied 0, 0.01, 0.5 and 0.1 wt%. Initially, the nanofluid is filled into the bubble column. A manometer was applied to control the level of the solution. After mixed gaseous, a gas flow meter and a Needle valve were used in the gas line. Then the gas was distributed into the bubble column through the sparger. In the experiments, the solution volume, pressure, CO2 partial pressure, and temperature were constant 3.0 lit, 80.0 kPa, 22.4 kPa, and 295.15 K, respectively. The gas flow rate, solution flow rate and range of stirrer speed were 5.0 lit/min, 1.5 lit/h, and 0300 rpm, respectively. The CO2 absorption rate was calculated as the difference between inlet and outlet of CO2 concentration due to the pressure and temperature constant. The average diameters of all nanoparticles were obtained under 30 nm by using scanning electron microscope (SEM). SEM images of the nanoparticles are shown in Figure 2. As it can be seen, the nanoparticles morphologies present a diameter of ranges 10-30 nm. The average crystallite size (B) for TiO2, ZrO2, and ZnO nanoparticles are calculated using Sherrer's Formula [71] as:

B(2θ ) =

0.9λ β cosθ

(11)

Where λ, β and, θ are the X-ray radiation wavelength, the full-width-at-half maximum (FWHM) of the diffracted peak, and the angle of diffraction, respectively. The maximum average crystallite size (B) for TiO2, ZrO2, and ZnO nanoparticle are calculated to be 27.9, 16.7 and 32.3 nm, respectively. The crystal structure calculation of XRD data is presented in Table 1.


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9 14

3

12

P.G

6

3

10

5

7 5 8 4

3

3

11

3 2

1

13

3

Figure 1: Experimental setup employed in the carbon dioxide absorption experiments: (1) CO2 cylindrical; (2) Air compressor; (3) Ball valve; (4) Gate valve; (5) Regulator; (6) Heater; (7) Needle valve; (8) Sparger; (9) Electromotor; (10) Rotameter; (11) Manometer; (12) Solution tank; (13) Storage tank; (14) Dimmer

Table 1: Structural information, Peak width and the average size of nanoparticles from XRD analyze ZnO Number

θ

64

0.27

53

Size

ZrO2

TiO2 FWHM

Number

θ

16.49

0.00872

100

0.22

0.29

27.67

0.00523

13

100

0.31

11.92

0.01221

27

0.41

28.85

38

0.49

29

Size

Size

FWHM

Number

θ

16.28

0.00872

32

0.27

10.99

0.01308

0.24

16.36

0.00872

100

0.32

16.75

0.00872

11

0.32

27.93

0.00523

14

0.38

11.42

0.01308

0.00523

22

0.33

16.80

0.00872

10

0.47

11.91

0.01308

25.68

0.00611

32

0.42

17.41

0.00872

8

0.50

12.11

0.01308

0.55

12.42

0.01308

22

0.47

17.84

0.00872

36

0.56

11.02

0.01483

27

0.59

19.19

0.00872

22

0.48

17.93

0.00872

6

0.66

13.44

0.01308

25

0.61

32.32

0.00523

15

0.55

11.63

0.01396

10

0.68

51.10

0.00349

7

0.60

16.06

0.01047

7

0.61

16.21

0.01047

10

0.66

14.34

0.01221

(nm)

(nm)


(nm)

FWHM

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In the following, the bubble diameters were measured by a photographic method based on the image analysis of bubbles taken along the column height. High-resolution video camera (CASIO, Exilim EX-ZR 1200) was used to take bubble images and numbers of about 400 bubbles were taken to measure this kind of distribution. Aiming at determining the bubble size, four pictures above the sparger surface (i.e., 9, 16, 23 and 30 cm) were periodically taken. Geometrical measurements of the bubbles were used by the image tool software package. The experimental conditions and the geometric details of the setup are summarized in Table 2. To evaluate the prepared nanofluids stability, the nanofluids dispersion stability was visualized in vials for 24 hours. Figure 3 showed the great stability of TiO2-Pz-H2O, ZnO-Pz-H2O and ZrO2-Pz-H2O nanofluids used in this work. To eliminate the influence of surfactant, the nanofluids were prepared without any surfactant addition.

(A)

(B)

(C)

Figure 2: The images of nanoparticles by scanning electron microscope (SEM), (A) ZnO, (B) TiO2, and (C) ZrO2


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Time (h)

TiO2 0.01 wt%

0.05 wt%

ZnO 0.1 wt%

0.01 wt%

0.05 wt%

ZrO2 0.1 wt%

0.01 wt%

0.05 wt%

0.1 wt%

0

24

Figure3. The photos of dispersion stability of nanofluids for 24 hours, CPz= 0.5 M, nanoparticles= 0.1 wt%

Table 2: Geometrical and experimental conditions

Parameter

Value

Parameter

Value

Diameter of test section

10 cm

DCO2 −gas

1.6×10-5 m2/s

Height of test section

50 cm

DCO2 −H2O

1.79×10-9 m2/s

Number of holes in sparger

6 number

DCO2 −Pz

1.60×10-9 m2/s

Diameter of sparger

7.5 cm

0.000975 Pa.s

Diameter of hole sparger

1.0 mm

µSol σSol

Mixer Disc diameter Pz Volume of solution Rotational speed Pressure Viscosity of Air Viscosity of CO2 Viscosity of water QCO2 QAir

Disc 80 mm 0.1M 3.0 lit 0- 300 rpm 80 kPa 1.983×10-5 Pa.s 1.48×10-5 Pa.s 0. 95×10-3 Pa.s 1.4 lit/min 3.6 lit/min

types of nanoparticles Average diameter of nanoparticle ZnO TiO2 ZrO2 Surface area of nanoparticles ZnO TiO2 ZrO2 Concentration of nanoparticles Ultrasonication

ZnO, TiO2 and ZrO2


0.07148 N/m

10-30 nm 20 nm 20 nm

20-60 m2/gr 10-45 m2/gr 30-60 m2/gr 0.01-0.1 wt%

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ρAir

1.22 kg/m3

Time

60 min

ρCO

1.98 kg/m3

Power

750 w

ρH O

997.2 kg/m3

Frequency

20 kHz

ρSol

998.2 kg/m3

2

2

3.4. Physicochemical properties In order to analyze the results of absorption studies, the knowledge of physicochemical properties such as density, viscosity of aqueous solutions and species diffusion coefficients in liquid phase is essential. For this propose, the density and dynamic viscosity of aqueous solution is obtained using equation (12) [72]:

ρ = ∑ i =0W i  Ai + B iT + C iT 2  2

ln µ Pz = φ1 +

φ2 T

(12)

+ φ3 ×T

(13)

Quantities of φi are presented as:

φi = A i + B i ×W + C i ×W 2 + D i ×W

3

(14)

Where W is the total mass fraction of Pz in the solution. The measured density and viscosity of aqueous Pz - H2O system is given in Table 3. Table 3: Parameters of Viscosity Correlations for Pz - H2O system [73] Index

ρ , (kg/m3) µ Pz

, Pa.s

i=1

Ai 0.7550

Bi 1.8866×10-3

Ci -3.6056×10-6

Di 0

i=2 i=3 i=1

3.1716×10-4 3.5437×10-5 -16.1096

7.0006×10-7 -1.7548×10-7 0.1599

-6.1337×10-10 2.2115×10-10 -2.1129×10-3

0 0 2.4405×10-4

i=2 i=3

3405.7166 0.01538

1.9515×10-3 -4.2813×10-4

1.3307×10-3 8.6945×10-6

-3.1447×10-3 -6.6882×10-7

The diffusion coefficient of CO2 in the gas phase is described below [74]: 1

4.36 ×10−5T 1.5 ( DCO2 − gas =

1

1 1 2 ) + M CO2 M Air 1

p [(ν CO2 ) 3 + (ν Air ) 3 ]


(15)

Page 13 of 41

The diffusion coefficient of CO2 in water derived from previously in several studies such as Versteeg and van Swaalj [75], and Kestin et al [76], as follows: DCO 2 − H 2O = 2.35 × 10 −6 exp(

−2119 ) T

(16)

The diffusion coefficient of CO2 in aqueous solution of Pz was estimated using the modified Stokes-Einstein relation [75] as given in Eq. (17):

DCO2 −Pz = DCO2 − H 2O (

µH O 0.8 ) µPz 2

(17)

4. Results and discussion 4.1. Bubble size distribution In Figure 4, a comparison of the bubble size distribution is presented for the nanofluid Pz-CO2H2O system. The measurements were carried out at a height of 5.0 cm above the sparger surface where the flow was found to be developed. The Pz solution concentration, CO2 and liquid flow rate and nanoparticles loading were 0.1 M, 1.4 lit/min and 1.5 lit/h and 0.05 wt%, respectively. The Sauter mean diameter values are in the range of 10-11 mm. It was found that with adding nanoparticles into the Pz solution, the Sauter mean diameter decreases. Moreover, there was a considerable increase in the number frequency for bubbles whose equivalent diameters were between 8.5 and 11.2 mm.

30 No nano, d32=10.73 mm Nano TiO2, d32=10.54 mm Nano ZnO, d32=10.60 mm Nano ZrO2, d32=10.66 mm

25 Number frequency, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

20 15 10 5 0 0

5

10 Bubble size (mm)

15

Figure 4: Effect of nanoparticles on bubble size distribution in the stirrer bubble column


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Page 14 of 41

The Sauter mean diameter quantified for TiO2, ZrO2, and ZnO nanofluid systems using the bubble size distribution data are listed in Table 4. The concentration of Pz solution, CO2 flow rate, and liquid flow rate were 0.1 M, 1.4 lit/min, and 1.5 lit/h, respectively. As a result, the gasliquid interfacial area increased with increasing in nanoparticles concentration up to 0.5 wt% for two TiO2 and ZrO2 nanofluids and then decreased with increasing nanoparticles concentration. But, for ZnO nanofluid, the gas-liquid interfacial area increased with increasing in nanoparticles concentration for all three dosages. The Sauter mean diameter decreased with increasing in nanoparticles loading for all three TiO2, ZrO2, and ZnO nanofluids. And also, it is clear that the Sauter mean diameter and interfacial area increased with increasing stirrer speed up to 200 rpm. But, both of them decreased with increasing the stirrer speed above 200 rpm due to creation coalescence in the center of the column.

Table 4: Sauter mean diameter and interfacial area for various nanoparticles Nanoparticles

TiO2

ZnO

ZrO2

Solid loading

Stirrer speed

(wt%)

( rpm )

0.01

d32 (mm)

a (m2/ m3)

-

10.62

51.3

-

10.54

53.9

100

10.51

54.7

200

10.53

56.3

300

10.51

54.5

0.10

-

10.43

52.9

0.01

-

10.63

49.0

-

10.61

50.6

100

10.60

51.4

200

10.59

52.9

300

10.52

52.5

0.10

-

10.57

51.5

0.01

-

10.70

47.2

-

10.65

51.8

100

10.64

53.4

200

10.61

53.6

300

10.53

55.0

-

10.63

50.5

0.05

0.05

0.05

0.10


Page 15 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4.2. Gas holdup Gas holdup is one of the most important factor in the describing of the stirrer bubble column operation. Table 5 displays the gas holdup values obtained using Eq. (1), for 0.1 M of Pz solution, 1.4 lit/min CO2 flow rate and 1.5 lit/h liquid flow rate in 0.05 wt% of different nanofluids.

Table 5: Gas holdup in mass fraction 0.05 wt% nanoparticles

Liquid phase Gas holdup

Base fluid 0.074

ZrO2 0.084

ZnO 0.082

TiO2 0.086

The results show that the gas holdup in the nanofluid is consistently higher than that in the base fluid. In the experiments, the difference in the gas holdup between the three nanofluids and Piperazine base fluid can be related to the differences in bubble size. Due to Table 3 that the bubble size in nanofluids is smaller than in Pz solution, and the smaller bubbles results in the larger gas holdup. The result showed that, 0.05wt% of TiO2 nanoparticles in the base fluid caused the maximum gas holdup. In the bubble columns, the influence factors on the gas holdup include density, gas distributor, CO2 loading, superficial gas velocity and liquid properties. Figure 5 indicate the effects of nanoparticles loading on the gas holdup at 0.1 M of Pz solution in the continuous operation. It can be seen that the gas holdup increases with increasing solid loading in nano-ZnO particles for all loading. But for high loading than 0.05 wt%, the rate of increase slows up. As it is seen, the gas holdup increased with the increase of solid loading up to 0.05 wt% and then decreased with increasing nanoparticles amount for nano-TiO2 and nano-ZrO2 particles. The increase in the gas holdup can lead to an increase in the area of the gas–liquid interface for the same nanoparticles loading. At higher loading, the increase in the apparent viscosity and density of the nanoparticles suspension is responsible for the decrease of the gas holdup. Because the aggregation of particles also increases that could affect negatively the gas holdup.


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0.087 0.086

Gas holdup

0.085 0.084 0.083 0.082 0.081 0.08 0.079 0.078

Nano ZnO

Nano ZrO2

Nano TiO2

0.077 0

0.02

0.04

0.06

0.08

0.1

0.12

Solid loading (wt%) Figure 5: Effect of solid loading on gas holdup

Figure 6 displays the gas holdup variation of TiO2, ZnO and ZrO2 nanofluids with stirrer speeds at 0.05 wt% of nanoparticles. It can be seen that the CO2 gas holdup increases up to 200 rpm of each three nanoparticles, and it starts to decrease after 200 rpm for all three nanofluids. The increasing phenomenon may be described by the Brownian motion, although, the bubble size decreases with an increasing the stirrer speed. The stirrer speed has negative influence on the gas holdup at the high speed than 200 rpm. With increasing the stirrer speed, the bubble coalescence and vortex increases in the fluid and channeling of the gas in the center of the column. Therefore, the bubble rises increasing and it causes to decreases in the gas holdup.

0.091 0.09 0.089 0.088 Gas holdup

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 41

0.087 0.086 0.085 0.084 0.083 Nano ZnO

0.082

Nano ZrO2

Nano TiO2

0.081 0

50

100

150

200

250

300

Stirrer speed (rpm) Figure 6: Gas holdup as a function of stirrer speed in different nanofluids

4.3. Interfacial area


350

Page 17 of 41

In Figure 7, the interfacial area dependencies on the solid loading are shown for different TiO2, ZnO and ZrO2 nanofluids at 0.1 M of Pz solution. The amounts of nanoparticles, which are used in the absorption experiments for the interfacial areas specification, are the same as in the experiments of the gas holdup. It can be observed from Figure 6 that the interfacial areas are also affected by the nanoparticles and increase with increasing solid loading up to 0.05 wt% in each three nanofluids. And it starts to decrease after 0.05 wt%. As well as, the decrease in the interfacial area with increasing nanoparticles loadings is larger for TiO2 and ZrO2 nanofluids. The increasing phenomenon may be explained by the hydrodynamic effect model 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. In addition, the particles collide with the gas-liquid interface, consequently the gas bubbles broken into small bubbles. And also, the increase in the bubble size can be related to an increase in the bubble coalescence rate or decrease rate of bubble breakup. 55 54 interfacial area (m2/m3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

53 52 51 50 Nano ZnO

49

Nano ZrO2

Nano TiO2

48 0

0.02

0.04

0.06

0.08

0.1

0.12

Solid loading (wt%) Figure 7: Effect of solid loading on interfacial area in different nanofluids

4.4. CO2 absorption in Pz solution and nanofluids Figure 8 indicates the variations in the real-time CO2 absorption amount with time for Pz solution as a base fluid and nanofluids of TiO2, ZnO and ZrO2 nanoparticles in the continuous system. During the process, the amount of CO2 absorption in Pz solution was 0.034 mol, which the total CO2 absorption of nanofluids TiO2, ZnO, and ZrO2 nanoparticles were 0.038, 0.037, and 0.035 mol, respectively. On the other hand, the nanofluids TiO2, ZnO, and ZrO2 nanoparticles in compare the base fluid increased the total CO2 absorption about 11.8%, 8.8%, and 2.9%, respectively. That means the CO2 absorption ability of nanofluids is larger than that of base fluid ACS Paragon Plus Environment

Energy & Fuels

Pz solution in all the time. Figure 8 shows that the nanoparticles had a markedly increasing influence on the real-time CO2 absorption. Nanoparticles can cause the micro-convection in nanofluids because of the Brownian motion, which can improve the gas-liquid mass transfer in the nanofluids. According to the bubble breaking effect, the nanoparticles contribute to crack the bubble size and increase the interfacial area. Nanoparticles diffuse to the gas-liquid interface and uptake the CO2 molecules, and then transfer through the liquid bulk and desorbing the gasses in it due to the grazing or shuttle effect. And also, nanoparticles enhance the interfacial area by covering the bubble surface which can prevent the bubbles coalescence according to the boundary mixing effect.

4.4 CO2 absorption amount×102 (mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

TiO2, 0.05 wt% ZnO, 0.05 wt% ZrO2, 0.05 wt% Base fluid

4.2

4

3.8

3.6

3.4 0

10

20

30 Time (min)

40

50

60

70

Figure 8: Variations in the CO2 absorption amount with time in Pz solution and nanofluids

4.5. Factors affecting mass transfer 4.5.1. Effect of nanoparticles on CO2 absorption rate Figure 9 (A), (B) and (C) display the alterations in the value of CO2 absorption rate with the time for the base fluid of Pz and nanofluids of TiO2, ZnO, and ZrO2. It can be seen clearly from Figure 9 (A) that the absorption rate for the ZnO nanofluid up to 0.01 wt% nanoparticles was higher than TiO2 and ZrO nanofluid and base fluid from 0 to 60 minutes. Also, by increasing the amount of nanoparticle from 0.01 wt% to 0.05 wt% according to Figure 9 (B), the absorption rate for TiO2 nanofluid was higher than ZnO and ZrO2 nanofluids. The addition of nanoparticles to the base fluid had a remarkable enhancement impression on the CO2 absorption rate in TiO2 and ZnO solutions due to the Brownian motion of nanoparticles.


Page 19 of 41

Absorption rate (gr/min)

(A) 1.8 1.75

Base fluid

Nano ZnO

1.7

Nano ZrO2

Nano TiO2

1.65 1.6 1.55 1.5 1.45 1.4 1.35 1.3 0

10

20

30 40 Time (min)

50

60

70


(B) 1.9 Base fluid

Nano ZnO

Nano ZrO2

Nano TiO2

1.8 1.7 1.6 1.5 1.4 1.3 0

10

20

30

40

50

60

70

Time (min)

(C)

1.9 1.8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Base fluid

Nano ZnO

Nano ZrO2

Nano TiO2

1.7 1.6 1.5 1.4 1.3 0

10

20

30

40

Time (min)


50

60

70

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9: The variations in the amount of CO2 absorption rate with the time in different nanofluid: (A) 0.01wt%; (B) 0.05 wt%; and (C) 0.1 wt% nanoparticles

4.5.2. Solid loading effect on CO2 absorption rate The CO2 absorption rate of nanofluids using TiO2, ZnO, and ZrO2 nanoparticles with the average size about 20 nm are presented in Figure 10. It is clear that the absorption rate increases from 1.45 gr/min to 1.68 gr/min by increasing the solid loading of TiO2 nanoparticle from 0.01 wt% to 0.05 wt%, respectively. But by increasing the nanoparticle loading up to 0.1 wt%, the absorption rate decreased to 1.54 gr/min. For nanofluids of TiO2 nanoparticles which have less absorption ability, with increase in the TiO2 nanoparticles loading, the reciprocal interference increase, which can increase absorption rate of CO2 according to the boundary mixing effect. Furthermore, nanoparticles around the bubble prevent bubble coalescence which leading to the increase of gas–liquid absorption area. However, when the solid loading is too large, since the gas–liquid interfacial area is finite, resulting is the decrease in the CO2 absorption rate. These two effects finally lead to the first increase and then decrease in the absorption rate of TiO2 nanoparticles with the solid loading. Besides the above-mentioned two effects, the shuttle effect may also play a role in the CO2 absorption rate, which leads to larger absorption rate compared to the ZnO and ZrO2 nanoparticles. As can be seen, the absorption rate of ZnO nanoparticles increases with increase in the solid loading according to the boundary mixing effect and the bubble breaking effect. As shown in Figure 9, the CO2 absorption rate in nanofluid of ZrO2 nanoparticles increased with increasing amount of solid loading to a maximum value (1.45 gr/min) and then decreased. On the other hand, the optimum solid loading of nanoparticle for increasing CO2 absorption rate was 0.5 wt%. This phenomenon could be probably described by three mechanisms: the boundary mixing effect, the bubble breaking effect and the shuttle or grazing effect.


Page 20 of 41

Page 21 of 41


1.80 1.70 1.60 1.50 1.40 1.30 TiO2

ZnO

ZrO2

1.20 0

0.02

0.04

0.06

0.08

0.1

0.12

Solid loading (wt%) Figure 10: Variations in the CO2 absorption rate with the solid loading nanoparticles

Figures 11-13 show the experimental data of CO2 absorption rate in the nanofluid during the time in the different dosage of TiO2, ZnO, and ZrO2 nanoparticles. All three figures illustrate that at the first, there is a high absorption. It was found that CO2 absorption rate was increased by increasing the ZnO nanoparticles mass fraction from 0.01 wt%- 0.1 wt%. But in TiO2 and ZrO2 nanoparticles, increase the solid loading above 0.05 wt% had a negative effect. From Figures 11 and 13, it can be understood that the 0.05 wt% ZrO2 nanoparticle has the maximum absorption rate. This phenomenon is could be probably explained by the boundary mixing and the Brownian motion effects. 1.9

TiO2 Absorption rate (gr/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.8

0.01 wt%

0.05 wt%

0.1 wt%

Base fluid

1.7 1.6 1.5 1.4 1.3 0

10

20

30 40 50 60 Time (min) Figure 11: Variation of CO2 absorption rate with time in different TiO2 dosage


70

Energy & Fuels

1.9 Absorption rate (gr/min)

ZnO 1.8

0.01 wt%

0.05 wt%

0.1 wt%

Base fluid

1.7 1.6 1.5 1.4 1.3 0

10

20

30

40

50

60

70

Time (min) Figure 12: Variation of CO2 absorption rate with time in different ZnO dosage

1.9

ZrO2

1.8 Absorption rate (gr/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 41

0.01 wt%

0.05 wt%

0.1 wt%

Base fluid

1.7 1.6 1.5 1.4 1.3 0

10

20

30

40

50

60

70

Time (min) Figure 13: Variation of CO2 absorption rate with time in different ZrO2 dosage

4.5.3. Effect of stirrer speed on CO2 mass transfer flux In this work, the effect of stirring speed on the CO2 mass transfer flux was investigated at different nanofluids. The CO2 mass transfer flux was measured using a 0.05 wt% of different nanoparticles in 0.1 M of Pz solution. Figures 14-16 indicate the experimental data of CO2 mass transfer flux in the nanofluids during the time at various stirrer speeds. The gas flow rate, CO2 concentration, solution flow rate and range of stirrer speed were 5.0 lit/min, 28.0 % (v/v), 1.5 lit/h and 0- 300 rpm, respectively. All figures indicate that at the first, there is a high mass transfer flux. Also, it was found that CO2 mass transfer flux was increased by increasing in stirring speed from 0-200 rpm because of broken bubbles to small bubbles and increasing contact area. But, by increasing in stirring speed at above 200 rpm due to the formation of coalescence


Page 23 of 41

phenomenon and accumulation bubbles in the center of the column, increasing in stirring speed had a reverse effect.

0.0045

TiO2

0.0044 NCO2 (mol/m2.s)

0.0043

no stirrer

100 RPM

200 RPM

300 RPM

0.0042 0.0041 0.004 0.0039 0.0038 0.0037 0

10

20

30 Time (min)

40

50

60

70

Figure 14: The effect of stirrer speed on mass transfer flux in nanofluid of TiO2 nanoparticles

0.0049

ZnO

0.0048 NCO2 (mol/m2.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.0047

no stirrer

100 RPM

200 RPM

300 RPM

0.0046 0.0045 0.0044 0.0043 0.0042 0.0041 0.004 0

10

20

30 Time (min)

40

50

60

70

Figure 15: The effect of stirrer speed on mass transfer flux in nanofluid of ZnO nanoparticles


Energy & Fuels

0.0047

ZrO2 NCO2 (mol/m2.s)

0.0045

no stirrer

100 RPM

200 RPM

300 RPM

0.0043 0.0041 0.0039 0.0037 0.0035 0

10

20

30 Time (min)

40

50

60

70

Figure 16: The effect of stirrer speed on mass transfer flux in nanofluid of ZrO2 nanoparticles

4.5.4. Effect solid loading on the overall mass transfer coefficient The experimental results of KG as a function of the different nanofluids of TiO2, ZnO, and ZrO2 nanoparticles versus the speed of stirrer in the continuous system are given in Figure 17. It can be observed from Figure 17, KG reduced with increasing stirrer speed. With raising the stirrer speed, the large bubbles were broken into the small bubbles and caused to increase the contact area and CO2 partial pressure in the liquid bulk. The increase in stirrer speed could increase the micro-convection as well as Brownian motion, which improved absorption performance and the number of CO2 molecules in the liquid phase. The resulting of above two reasons mentioned, the overall mass transfer coefficient decreases with increasing stirrer speed.

1.08 KG ×106 (kmol/m2.s.kPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 41

ZnO

1.06

ZrO2

TiO2

1.04 1.02 1 0.98 0.96 0.94 0

50

100

150

200

250

300

350

Stirrer speed (rpm) Figure 17: variation of overall mass transfer coefficient with stirrer speed in different nanofluids


Page 25 of 41

Figure 18 shows the overall mass transfer coefficient variation with nanofluids dosage of TiO2, ZnO, and ZrO2 nanoparticles for the 28.0 % (v/v) of CO2 concentration in the continuous system. In nanofluid of TiO2 and ZrO2 nanoparticles, the overall mass transfer coefficient decreases with the solid loading up to 0.05 wt% and then weak increases. With the adding in the weight percent of nanoparticles, the gas-liquid interfacial area increase which can increase absorption rate and CO2 partial pressure according to the boundary mixing, bubble breaking, and Brownian motion effect. When the weight percent of solid is too large, since the interfacial area is finite, resulting is the decrease in the CO2 partial pressure in the solution and decreasing KG. Besides the abovementioned three effects, the shuttle or grazing effect probably also perform a role in the CO2 absorption rate and CO2 partial pressure in the liquid bulk, which leads to decreases the KG in weight percent above 0.05 wt%. The evolution of Table 6 along the absorption time indicates that when the time increases, the value of CO2 loading, absorption rate, overall mass transfer coefficient and mass transfer flux decreases. Because during the time the concentration difference between gas and liquid bulk decrease and cause to decreases the driving force. 1.12 KG ×106 (kmol/m2.s.kPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

ZnO

1.1

ZrO2

TiO2

1.08 1.06 1.04 1.02 1 0.98 0

0.02

0.04

0.06

0.08

0.1

0.12

Solid loading (wt%) Figure 18: variation of overall mass transfer coefficient with solid dosage in different nanofluids

4.6. Factors affecting CO2 loading 4.6.1. Effect of nanoparticles on CO2 loading Figures 19 to 21 show the CO2 loading with respect to the weight percent of TiO2, ZnO and ZrO2 nanoparticles, respectively. In these figures, the CO2 loading with all amount nanoparticles is


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

higher than those without any nanoparticle in 0.1M of Pz solution. As the CO2 loading further, it means the amount of CO2 is more in the solution and increase the CO2 capacity. In the nanofluid of ZnO nanoparticles, the CO2 loading was increased by increasing in solid loading of nanoparticles from 0-0.1 wt%. But in TiO2 and ZrO2 nanoparticles, increase the percent of nanoparticles above 0.05 wt% caused the decrease the CO2 loading. This phenomenon is could be probably explained by the boundary mixing, bubble breaking, Brownian motion and the grazing effects. More nanoparticles can take more CO2 molecules, and thus more nanoparticles have a greater effect on CO2 loading. On the other hand, the empty activity site of Pz was finite in the experiments and the CO2 loading was limited. If the amount of nanoparticles was too high, there could be too many nanoparticles hindering the mass transfer and resulting in the decrease in the CO2 loading. Table 6: experimental data of mass transfer flux and overall mass transfer coefficients in TiO2 nanofluid time

0.01 wt%

(min)

0.05 wt%

KG

N

α

∆G

0

0.137

1.69

1.068

5

0.140

1.62

10

0.143

15

CO

KG

0. 1 wt% N

α

∆G

4.36

0.146

1.86

1.063

1.055

4.22

0.152

1.78

1.58

1.051

4.14

0.154

0.146

1.54

1.049

4.07

20

0.147

1.52

1.047

25

0.148

1.50

30

0.149

35

CO

α

∆G

4.49

0.144

1.34

4.33

1.73

1.020

0.157

1.68

4.02

0.159

1.046

3.98

1.49

1.046

0.149

1.48

40

0.150

45

6

2

2

N

CO

×106

1.840

1.069

4.49

0.149

1.747

1.041

4.32

4.23

0.153

1.678

1.028

4.19

1.012

4.15

0.156

1.631

1.023

4.11

1.64

1.006

4.07

0.158

1.608

1.020

4.07

0.161

1.62

1.003

4.02

0.159

1.590

1.018

4.04

3.96

0.162

1.61

1.002

4.00

0.160

1.576

1.017

4.01

1.045

3.94

0.162

1.60

1.001

3.98

0.160

1.571

1.017

3.99

1.47

1.045

3.93

0.162

1.60

1.001

3.98

0.161

1.562

1.016

3.98

0.150

1.47

1.045

3.92

0.162

1.60

1.001

3.97

0.161

1.558

1.016

3.97

50

0.151

1.46

1.045

3.91

0.163

1.59

1.000

3.96

0.161

1.553

1.016

3.96

55

0.151

1.46

1.044

3.89

0.163

1.59

1.000

3.96

0.162

1.548

1.015

3.95

60

0.152

1.45

1.044

3.88

0.163

1.59

1.000

3.95

0.162

1.544

1.015

3.94

×10

×10

6

×10


6

KG ×106

×10

6

2

Page 27 of 41

0.165

TiO2

CO2 loading (mnol/mol)

0.16 0.155 0.15 0.145 0.14 0.135

Base fluid

0.01 wt.%

0.05 wt.%

0.1 wt.%

0.13 0

10

20

30

40

50

60

70

Time (min) Figure 19: variation of CO2 loading at different weight percent for TiO2 nanoparticles

0.17

ZnO

CO2 loading (mnol/mol)

0.165 0.16 0.155 0.15 0.145 0.14

Base fluid

0.01 wt.%

0.135

0.05 wt.%

0.1 wt.%

0.13 0

10

20

30 40 Time (min)

50

60

70

Figure 20: variation of CO2 loading at different weight percent for ZnO nanoparticles

0.17

ZrO2

0.165 CO2 loading (mnol/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.16 0.155 0.15 0.145 0.14 0.135

Base fluid

0.01 wt.%

0.05 wt.%

0.1 wt.%

0.13 0

10

20

30 40 Time (min)

50

60

Figure 21: variation of CO2 loading at different weight percent for ZrO2 nanoparticles


70

Energy & Fuels

4.6.2. Effect of nanoparticles dosage on the CO2 loading Figure 22 indicate the variation of CO2 loading in different nanofluid for the 28.0 % (v/v) of CO2 concentration in the continuous system. It is clear that the slope of the loading curve increases with increasing solid dosage in ZnO nanoparticles for all dosage. But for high dosage than 0.05 wt%, the rate of increase slows up. And also, the experimental data in this figure indicate that the loading increased with the increase of solid dosage up to 0.05 wt% and then decreased with increasing nanoparticles amount for nano-TiO2 and nano-ZrO2 particles. The maximum CO2 loading is 0.167 in the case of nanofluid with 0.05 wt% ZrO2.

0.168 0.166 CO2 loading (mnol/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.164 0.162 0.16 0.158 0.156 0.154 0.152

ZnO

ZrO2

TiO2

0.15 0

0.02

0.04

0.06 0.08 Solid loading (wt%)

0.1

0.12

Figure 22: Variations of CO2 loading with the solid dosage in different nanofluid

4.6.3. Effect of stirrer speed on CO2 loading Experiments were carried out at 0.05 wt% of different nanoparticles in 0.1 M of Pz solution and stirrer speeds ranging from 0 to 300 rpm. The variations of experimental CO2 loading are presented in Figures 23 to 25 for the different stirrer speeds. The highest CO2 loading for this system was found to be 0.165, 0.171 and 0.175 for the TiO2, ZnO and ZrO2 nanoparticles, respectively, at a stirrer speed of 200 rpm. It can be seen that CO2 loading increases with the stirring speed in all nanoparticles up to a 200 rpm and then decreases by increasing stirrer speed above 200 rpm. Increase the stirrer speed caused to broken bubbles to small bubbles and increasing contact area. But increase the stirring speed at above 200 rpm cause to the formation of coalescence


Page 29 of 41

phenomenon in the center of the column and decreases the CO2 loading due to decreases of contact area and Brownian motion. 0.17 CO2 loading (mnol/mol)

TiO2 0.165 0.16 0.155 0.15 0.145

No stirrer

100 RPM

200 RPM

300 RPM

0.14 0

10

20

30

40

50

60

70

Time (min) Figure 23: The effect of stirrer speed on CO2 loading in nanofluid of TiO2 nanoparticles

0.175

ZnO


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Energy & Fuels

0.165 0.16 0.155 0.15 0.145

No stirrer

100 RPM

200 RPM

300 RPM

0.14 0

10

20

30

40

50

60

70

Time (min) Figure 24: The effect of stirrer speed on CO2 loading in nanofluid of ZnO nanoparticles


Energy & Fuels

0.18

ZrO2


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0.17 0.165 0.16 0.155 No stirrer

100 RPM

200 RPM

300 RPM

0.15 0.145 0.14 0

10

20

30

40

50

60

70

Time (min) Figure 25: The effect of stirrer speed on CO2 loading in nanofluid of ZrO2 nanoparticles

4.7. Factors affecting CO2 removal efficiency 4.7.1. Effect of nanoparticles on CO2 removal efficiency Figures 26 to 28 display the influence of TiO2, ZnO and ZrO2 nanoparticles on the CO2 removal efficiency. The solid loading of nanoparticles ranges in nanofluid was from 0-0.1 wt%. The results showed that adding nanoparticles to the base fluid increases removal efficiency and decreases of CO2 in the gas out from the bubble column. Also, the results showed that the injection 0.01, 0.05, and 0.1 wt% of TiO2 nanoparticles on the base fluid increased the removal efficiency 3.2%, 10.1%, and 7.8%, respectively in comparing the base fluid. But the injection of ZnO and ZrO2 nanoparticles in same weight percent increased the removal efficiency 5.8%, 8.1%, and 11.5%, for ZnO, and 1.1%, 2.5%, and 1.2%, for ZrO2, respectively. As it clear, increasing the concentration of nanoparticles in TiO2 and ZrO2 nanofluid above 0.05 wt%, the removal efficiency decreases. Because of the viscosity of nanofluid increases and cause to the decreasing of the CO2 diffusion coefficient and finally decreased the CO2 removal efficiency. And also, the Brownian motion could also be used to describe the above-mentioned phenomenon. When the amount of nanoparticles was too high, particles were exposed to accumulating together which caused to decreasing Brownian motion.


Page 31 of 41

CO2 removal efficiency (%)

95

TiO2

90

Base fluid

0.01 wt.%

0.05 wt.%

0.1 wt.%

85 80 75 70 65 0

10

20

30 40 Time (min)

50

60

70

Figure 26: The weight percent of TiO2 nanoparticles influence on CO2 removal efficiency

95 CO2 removal efficiency (%)

ZnO 90

Base fluid

0.01 wt.%

0.05 wt.%

0.1 wt.%

85 80 75 70 65 0

10

20

30

40

50

60

70

Time (min) Figure 27: The weight percent of ZnO nanoparticles influence on CO2 removal efficiency


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Energy & Fuels

ZrO2 90

Base fluid

0.01 wt.%

0.05 wt.%

0.1 wt.%

85 80 75 70 65 0

10

20

30

40

Time (min)


50

60

70

Energy & Fuels

Figure 28: The weight percent of ZrO2 nanoparticles influence on CO2 removal efficiency

4.7.2. Effect of stirrer speed on CO2 removal efficiency Figures 29 to 31 show the experimental variation of CO2 removal efficiency versus the time in the different stirrer speed in ranging from 0-300 rpm at 0.05 wt% of nanoparticles. The gas flow rate, CO2 concentration and solution flow rate were 5.0 lit/min, 28.0 % (v/v) and 1.5 lit/h, respectively. The results obviously demonstrate that the CO2 removal efficiency is depending on the stirrer speed. When the stirrer speed was 0 rpm in 60 min, absorption percent for TiO2, ZnO, and ZrO2 nanoparticles were about 79.2%, 77.2%, and 71.7%, respectively. The removal efficiency in all nanofluid increases with an increase in the stirrer speed up to 200 rpm and then decreased. The highest absorption percent for TiO2, ZnO, and ZrO2 nanoparticles were about 84.3%, 83.6%, and 78.1%, respectively at a stirrer speed of 200 rpm. Increase the stirrer speed caused to change the hydrodynamic condition and breaks bubbles to small bubbles and increasing the Brownian motion and contact area. While, by increasing the stirring speed at above 200 rpm, the bubbles accumulate in the center of the column and decreased the absorption performance. 100

TiO2 CO2 removal efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

No stirrer

100 RPM

200 RPM

300 RPM

95 90 85 80 75 0

10

20

30

40

50

60

Time (min)

Figure 29: Effect of stirrer speed on CO2 removal efficiency in TiO2 nanofluid


70

Page 33 of 41


ZnO 95 90

No stirrer

100 RPM

200 RPM

300 RPM

85 80 75 70 0

10

20

30

40

50

60

70

Time (min) Figure 30: Effect of stirrer speed on CO2 removal efficiency in ZnO nanofluid


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

ZrO2

95

No stirrer

100 RPM

200 RPM

300 RPM

90 85 80 75 70 0

10

20

30

40

50

60

70

Time (min) Figure 31: Effect of stirrer speed on CO2 removal efficiency in ZrO2 nanofluid

4.7.3. Effect of nanoparticles dosage on CO2 removal efficiency Figure 32 expresses the CO2 removal efficiency with respect to the dosage of TiO2, ZnO and ZrO2 nanoparticles in the continuous system. Since the CO2 removal efficiency of all three nanofluids is higher than 70%, it is surely implied that nanofluids can increase the absorption performance. It also can be concluded that the removal efficiency increases with the solid loading of TiO2 and ZrO2 nanofluids increasing at first, then gradually decreases. While the removal efficiency increases with the increasing of ZnO nanoparticles dosage in the all time. The maximum removal efficiency is 80.6 in the case of nanofluid with 0.1 wt% ZnO. The mechanisms of the nanofluid increasing CO2 removal efficiency have three maybe reasons as follows. Nanoparticles can cause the micro-convection in nanofluids because of the Brownian


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motion. This micro-convection can improve the CO2 absorption performance in the nanofluids. Nanoparticles in the nanofluids can cause the grazing or shuttle effect. As mentioned in Table 4 that the gas holdup in the nanofluid is higher than that of the base fluid. The increase in the gas holdup can increase the interfacial area.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 41

ZnO

ZrO2

TiO2

82 80 78 76 74 72 70 68 0

0.02

0.04

0.06

0.08

0.1

0.12

Solid loading (wt%) Figure 32: variations of CO2 removal efficiency with the nanoparticles dosage in different nanofluids

4.8. Effective parameters on Enhancement factor 4.8.1. Effect of solid loading on enhancement factor The variation of enhancement factor with the solid loading of nanofluids using TiO2, ZnO, and ZrO2 nanoparticles is presented in Figure 33. It can be seen that the enhancement factor increases with increasing solid dosage in ZnO nanofluid for all dosage. But for high dosage than 0.05 wt%, the rate of increase slows up. Because in the during absorption time, the bubbles in the column become more and more due to the presence of ZnO nanoparticles and the empty activity site in solution decreases. Therefore, the intensity of CO2 absorption decreases. The reason which cause to the ZnO particles behaves differently from the two TiO2 and ZrO2 nanoparticles is the boundary mixing effect, grazing, and the bubble breaking effect. While the enhancement factor in TiO2 and ZrO2 nanofluids increased up to a maximum value and then decreased. The result showed that enhancement factor of TiO2 and ZrO2 nanoparticles are higher and lower than the other types of nanoparticles, respectively. The maximum enhancement factors are 1.17, 1.15, and 1.04 for the nanofluids of ZnO, TiO2, and ZrO2 nanoparticles, respectively. There are several mechanisms to explain the above-mentioned phenomenon. The boundary mixing effect offers


Page 35 of 41

that the nanoparticles probably enhance the interfacial area by covering the bubble surface which can prevent the bubbles coalescence, resulting in smaller bubbles. However, when the solid loading is large since the interfacial area is limited which can decreases absorption performance. Also, according to the bubble breaking effect, the nanoparticles contribute to crack the bubble size and increase the interfacial area. After an optimum solid loading for TiO2 and ZrO2 nanoparticles, the nanoparticles become too dense and contribute to aggregation and sedimentation leading to a negative effect. Nanoparticles in the nanofluid can cause the grazing or shuttle effect. Nanoparticles diffuse to the gas-liquid interface and uptake the CO2 molecules, and then transfer through the liquid bulk and desorbing the gasses. If the value of nanoparticles was too high, there could be too many particles in the gas-liquid interface decreasing the absorption performance and resulting in reduce of enhancement factor. Another mechanism that can be described the above phenomenon is the Brownian motion induced micro-convection. The decrease in the solid loading smaller than the optimum value could increase the microconvection which improved absorption performance. But, when the weight percent of nanoparticles was too high, particles were contributed to aggregating leading to weakened Brownian motion.

1.18 1.16 Enhancement factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.14 1.12 1.1 1.08 1.06 1.04 1.02 ZnO

1

TiO2

ZrO2

0.98 0

0.02

0.04

0.06

0.08

0.1

0.12

Solid loading (wt%) Figure 33: Variations of enhancement factor with the solid loading of TiO2, ZnO and ZrO2 nanoparticles

4.8.2. Effect of stirrer speed on enhancement factor


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The experimental data of enhancement factors as a function of the stirrer speed in the range of 0300 rpm are shown in Figure 34. The gas flow rate, CO2 concentration, solution flow rate and weight percent of nanoparticles were 5.0 lit/min, 28.0 % (v/v), 1.5 lit/h and 0.05 wt%, respectively. Furthermore, the variation trends of enhancement factor with the stirring speed are same for all three the cases and they increased firstly, and then decreased. It can be found that enhancement factor increased by increasing in stirring speed from 0- 200 rpm because of broken bubbles to small bubbles and increasing contact area and prevents the agglomeration and precipitation of nanoparticles. While in agitation speed up to the 200 rpm the enhancement factors decreases with increasing stirrer speed due to the formation of coalescence phenomenon and accumulation bubbles in the center of the column. The maximum enhancement factors are obtained 1.09, 1.08, and 1.0 for ZrO2, ZnO, and TiO2 nanoparticles at 200 rpm, respectively. 1.1 1.08 Enhancement factor

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Page 36 of 41

1.06 1.04 1.02 1 ZnO

ZrO2

TiO2

0.98 0.96 0

50

100

150

200

250

300

350

Stirrer speed (rpm) Figure 34: Variations of enhancement factor with the stirrer speed of TiO2, ZnO and ZrO2 nanoparticles

5. Conclusion In this work, the effect of TiO2, ZnO, and ZrO2 nanoparticles in aqueous Pz solution on the hydrodynamic and CO2 absorption rate were experimentally investigated using a stirrer bubble column. The studied range of solid loading and stirrer speed were 0.01 wt% to 0.1 wt% and 0300 rpm, respectively. The effect of nanofluids parameters, nanoparticle concentration and the stirring speed on the gas holdup, Sauter mean diameters, interfacial area, CO2 loading, absorption flux, overall mass transfer coefficients, and CO2 absorption enhancement were investigated. The addition of nanoparticles in compare the pure Pz solution decreased the Sauter mean diameter and increased the gas holdup. The absorption performance increases with the


Page 37 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

solid loading of TiO2 and ZrO2 nanoparticle up to a maximum value and then decreases. The absorption rate of ZnO nanoparticles increased with the increase in the solid loading. But, overall mass transfer coefficients at first decreased up to a minimum value and then increased. And also, the result indicated that the addition of ZrO2 nanoparticles in comparing the TiO2 and ZnO nanoparticles was the lowest effect on the above-mentioned performance. It was observed that the effect of stirrer speed up to 200 rpm is positive on the CO2 enhancement factor.

Nomenclature

a

Specific gas-liquid area, m2/m3

pCO2 ,i

Interface partial pressure of CO2, kPa

A

specific interfacial area, m2

pCO2 ,in

Inlet CO2 partial pressure, kPa

B

Average crystallite size, nm

pCO2 ,out

Outlet CO2 partial pressure, kPa

CPz

Total Pz concentration, mol/lit

pCO2 ,in

Inlet CO2 partial pressure, kPa

CO2 concentration, mol/lit

pCO2 ,out

Outlet CO2 partial pressure, kPa

d

Equivalent sphere diameter, cm

∆pCO2 ,lm Log mean average CO2 partial pressure, kPa

D

Diameter of bubble column, m

S

Contact surface, m2

dB

Diameter of bubbles, cm

T

Temperature, K

di

Diameter of single bubble, mm

TC

Critical temperature, K

d32

Sauter mean diameter, mm

w

Total mass fraction

E

Enhancement factor

y CO2 −in

Mole fraction of inlet CO2

Gin

Gas molar flow rate, mol/s

y CO2 −in

Mole fraction of outlet CO2

KG

Overall mass transfer coefficient, kmol/m2.s.kPa

Greek Letters

L

Initial height of fluid in column, cm

αCO

CO2 loading, mol CO2/mol amine

∆L

Expansion height of fluid in column, cm

ρL

Density of liquid, kg/m3

ni

Bubble frequency

ρg,in

Inlet gas density, kg/m3

C

CO

2

2

P

Total system pressure, kPa

σ µl

Pz

Piperazine

β

Full-width-at-half maximum, rad

pCO2

Partial pressure of CO2, kPa

εG

Gas holdup

pCO2 ,b

Gas bulk partial pressure of CO2, kPa

λ

Wavelent of x-ray radiation, nm

PC

Critical pressure, MPa θ Equilibrium CO2 partial pressure of the bulk ∆G solution, kPa

N

CO

* pCO 2

2

Mass transfer flux, kmol/m2.s

Surface tension of liquid,, dyn/cm Solution viscosity, kg/m.s

Angle of diffraction, rad Absorption rate, gr/min


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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