Comparison of several packings for CO2 chemical absorption in a packed column

International Journal of Greenhouse Gas Control 5 (2011) 1163–1169

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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc

Comparison of several packings for CO2 chemical absorption in a packed column Xinglei Zhao a , Kathryn H. Smith b , Michael A. Simioni b , Wendy Tao b , Sandra E. Kentish b , Weiyang Fei a , Geoffrey W. Stevens b,∗ a

The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Cooperative Research Centre for Greenhouse Gas Technologies (CO2 CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia b

a r t i c l e

i n f o

Article history: Received 31 January 2011 Received in revised form 6 July 2011 Accepted 7 July 2011 Available online 10 August 2011 Keywords: Super Mini Rings (SMRs) CO2 capture Flue gas K2 CO3 solution Packed column

a b s t r a c t CO2 capture and storage has gained widespread attention as an option for reducing greenhouse gas emissions. Chemical absorption and stripping of CO2 with hot potassium carbonate (K2 CO3 ) solutions has been used in the past, however potassium carbonate solutions have a low CO2 absorption efficiency. Various techniques can be used to improve the absorption efficiency of this system with one option being the addition of promoters to the solvent and another option being an improvement in the mass transfer efficiency of the equipment. This study has focused on improving the efficiency of the packed column by replacing traditional packings with newer types of packing which have been shown to have enhanced mass transfer performance. Three different packings (Super Mini Rings (SMRs), Pall Rings and Mellapak) have been studied under atmospheric conditions in a laboratory scale column for CO2 absorption using a 30 wt% K2 CO3 solution. It was found that SMR packing resulted in a mass transfer coefficient approximately 20% and 30% higher than that of Mellapak and Pall Rings, respectively. Therefore, the height of packed column with SMR packing would be substantially lower than with Pall Rings or Mellapak. Meanwhile, the pressure drop using SMR was comparable to other packings while the gas flooding velocity was higher when the liquid load was above 25 kg m−2 s−1 . Correlations for predicting flooding gas velocities and pressure drop were fitted to the experimental data, allowing the relevant parameters to be estimated for use in later design. © 2011 Published by Elsevier Ltd.

1. Introduction In the battle against global warming, CO2 capture and storage has gained widespread attention as an option for reducing greenhouse gas emissions. Chemical absorption and stripping of CO2 with aqueous solvents including alkanolamines and hot potassium carbonate (K2 CO3 ) solutions are well-known and effective processes for removing CO2 from power plant flue gases (Jassim and Rochelle, 2006). Compared with alkanolamines, potassium carbonate is less toxic, less prone to oxidative solvent degradation and has a low heat of regeneration (Cullinane and Rochelle, 2004; Uyanga and Idem, 2007). The reaction of K2 CO3 with CO2 has been studied extensively and occurs via the following overall chemical reaction (Savage et al., 1980): CO2 + K2 CO3 + H2 O ↔ 2KHCO3

(1)

The main disadvantage with using potassium carbonate is that it has a low CO2 absorption efficiency. Various techniques can be

∗ Corresponding author. Tel.: +61 3 8344 6621; fax: +61 3 8344 8824. E-mail address: [email protected] (G.W. Stevens). 1750-5836/$ – see front matter © 2011 Published by Elsevier Ltd. doi:10.1016/j.ijggc.2011.07.006

used to improve the absorption efficiency of this process including adding promoters to the K2 CO3 solvent system and/or enhancing the efficiency of the packed column. A variety of promoters including piperazine (PZ) (Cullinane and Rochelle, 2004), diethanolamine (Rahimpour and Kashkooli, 2004) and arsenic trioxide (Epp et al., 2007) have been studied with K2 CO3 solvent. However equipment efficiency has not received as much attention for this application. Packed columns are the most commonly used equipment for capture of CO2 from power plant flue gases using solvent absorption. One way of improving equipment efficiency with a packed column is to replace traditional packings with newer types of packings (random or structured) which have been specially designed to improve performance. Newer structured packings can be advantageous due to lower pressure drops and improved efficiencies however they are much more expensive than random packings. With both operating and capital costs being important considerations when implementing carbon capture and storage on a large scale it is imperative to improve the efficiency of the CO2 absorption process while keeping the cost of the equipment to a minimum. Therefore a randomly packed column with novel internals has the potential to achieve improved performance with lower operating costs while minimizing capital costs.

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X. Zhao et al. / International Journal of Greenhouse Gas Control 5 (2011) 1163–1169

Nomenclature a specific surface area of the packing (m2 m−3 ) C constant d diameter (m) A, B constants from Eq. (2) C1 , CFl , CP,0 Billet and Schultes coefficients dp diameter (or nominal size) of packing (m) gas or vapour capacity factor (kg1/2 m−1/2 s−1 ) Fv g gravitational acceleration (m s−2 ) G mass flowrate of liquid H Henry’s Law constant (kmol m2 m−3 N) hL column holdup K wall factor overall gas phase mass transfer coefficient Ky (kmol m−3 h−1 ) k parameter in the Takehashi pressure drop correlation ky gas film mass transfer coefficient (kmol m−3 h−1 ) liquid film mass transfer coefficient (kmol m−3 h−1 ) kl L mass flow of liquid (kg m−2 s−1 ) LW liquid load (m3 m−2 h−1 ) Mt molecular weight of water (kg kmol−1 ) P pressure (kPa) Re Reynolds number (Re = Du/) S column cross sectional area (m2 ) Sc Schmidt number (Sc = /D) T temperature (K) u superficial velocity (m s−1 ) V mass flow of gas (kg m−2 h−1 ) x equilibrium CO2 concentration in solution (kmol m−3 ) X mass fraction of K2 CO3 in solution Y CO2 mol% in gas y* CO2 mol% in gas at equilibrium Z column height (m) Greek letters ˇ enhancement factor ε packing void fraction (m3 m−3 ) D diffusion coefficient (m2 s−1 )  viscosity (Pa s)  stripping factor  density (kg m−3 )  surface tension (kg s−2 ) resistance coefficient Subscripts v gas phase Fl flooding point L liquid P particles S loading point V gas or vapour

Randomly packed columns have been widely used in industry, including applications for gas absorption with high liquid loads, solvent extraction and high pressure distillation (Kister et al., 1994). Significant efforts have been dedicated to developing new, highefficiency random packings for some time (Brierley, 1994; Cao, 2000; Fei, 1996; Li and Liu, 2000). It has been shown that the Super Mini Ring (SMR), a novel random packing, has enhanced performance due to the elaborate design of the twisting-inwards arc units (Fei, 1989) (refer to Fig. 1). The SMR packing has been widely used in carbon dioxide absorption from synthetic ammonia plants and in LPG purification with significant economic benefits (Fei and Wen, 1995; Ma and Fei, 2000). The objectives of the present study were to compare the efficiency of CO2 absorption into potassium carbonate solutions using a laboratory sized column filled with Super Mini Rings (SMRs, a novel random packing), Pall Rings (a commonly used random packing) and Mellapak (a structured packing). Hydrodynamic and mass transfer correlations for predicting the performance of these different packings have also been investigated. 2. Materials and methods Experiments were performed in a glass column of 0.074 m internal diameter with CO2 absorption into potassium carbonate solution at atmospheric pressure. The height of the packed bed was 0.9 m. A schematic diagram showing the experimental setup is shown in Fig. 2. A mixture of CO2 (14 or 85 mol%) and N2 was used for the gas phase and 30 wt% K2 CO3 solution was used as the liquid solvent. Both phases flowed through the column counter currently via distributors, and flow rates were measured with calibrated rotameters. The range of gas phase velocities studied was from 0.19 to 0.56 m s−1 and the range of liquid phase velocities studied was from 4.8 to 33 kg m−2 s−1 . The gas phase was continuous and the liquid phase dispersed. Oil baths were used to heat each phase to the desired operating temperature. The gas exiting the top of the column passed through a condenser coil before venting to the extraction system at atmospheric pressure. Loaded solvent exiting the column was returned to another tank and later stripped of CO2 using another packed column. The inlet and outlet pressures and temperatures of the packed bed were measured at steady state conditions using pressure transmitters and thermocouples connected to LabView software. When liquid holdup was studied, gas and liquid flow rates were stopped simultaneously by closing the inlet and outlet valves. Liquid was then collected into a measuring cylinder until the liquid interface returned to its original position. Each measurement was repeated three times to ensure accuracy of experimental data and the data obtained agreed to within ±10%. Mass transfer performance was studied by taking liquid samples at steady state conditions and determining the concentration of CO3 2− and HCO3 − in the solution via acid–base titration. The titration was performed using a Metrohm–Titrando 809 (Switzerland) and the data was collected via a computer using the Metrohm tiamo software package. Each sample was analysed three times and the data obtained agreed to within ±10%. Generally hydrodynamic and mass transfer performance studies were completed with a CO2 gas concentration of 14 mol% which is the concentration commonly found in power plant flue gases. However as unpromoted K2 CO3 solution has a low CO2 absorption

Table 1 Geometric characteristics of SMR, Pall Ring and Mellapak packings. Type of packing

Specific surface area, a (m2 m−3 )

Voidage, ε (m3 m−3 )

Diameter (or equivalent diameter), dp (m)

Ø 13 mm SMR Ø 13 mm Pall Ring Mellapak 700Y

419 360 700

0.980 0.928 0.936

0.0130 0.0130 0.0036

X. Zhao et al. / International Journal of Greenhouse Gas Control 5 (2011) 1163–1169

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Fig. 1. Different packings used in this study: (a) Ø 13 mm SMR; (b) Ø 13 mm Pall Ring; and (c) Mellapak 700Y.

viscosity was calculated using the correlation presented by Correia et al. (1980). 3. Results and discussion It is desirable to have low pressure drop and high mass transfer rates when operating a packed column. When designing a column for CO2 capture it is also particularly important to ensure that the capital and operating costs of the equipment is kept to a minimum, as the cost of capture is one of the barriers to implementing carbon capture and storage technologies on a large scale. This study has therefore looked at optimizing the hydrodynamic and mass transfer performance of three different types of packings in a laboratory scale column. 3.1. Flooding velocity

Fig. 2. Experimental set-up and flow scheme (1, solvent pump; 2, oil bath; 3, rotameter; 4, absorber; 5, liquid sample point; 6, cooling water; 7, condenser coil; 8, rich solvent tank; 9, lean solvent tank).

rate, mass transfer coefficients were determined using a higher concentration of 85 mol% CO2 and 15 mol% N2 in order to enhance the absorption performance. The geometric characteristics of Ø 13 mm SMR, Ø 13 mm Pall Ring and Mellapak 700Y are listed in Table 1. The equivalent diameter of Mellapak 700Y was estimated using the calculation procedures presented by Haan et al. (McCabe, 1967). The height of each Mellapak 700Y packing element was 10 cm and 9 elements were used in the packed section of the column. The physical properties of the experimental system are shown in Table 2. Gas viscosity was calculated using the method presented in Perry’s Chemical Engineer’s Handbook (Perry, 1984) and liquid

In a packed column, the gas and liquid rates are limited by the tendency of the column to flood. As either liquid or gas velocity is increased, the liquid holdup in the packing increases, the free area for gas flow decreases, and the pressure drop through the column increases. A point is finally reached when the gas bubbles violently through the liquid, the pressure drop rises extremely sharply with the slightest increase in gas velocity, and liquid is raised to the top of the column via the upflowing gas. This point is called the “flooding point” and is determined by both gas and liquid rates. Although the best operating conditions are determined by an economic balance, a knowledge of flooding velocities is extremely useful in firstly determining the limiting gas and liquid rates above which operation is not possible, and secondly in estimating the optimum liquid and gas rates where there are not enough data to make an exact economic balance of operating and fixed cost (Sherwood et al., 1938). As shown in Fig. 3, the gas flooding velocity of Ø 13 mm SMR is about 10% higher than that of the other packings at higher liquid loads (>25 kg m−2 s−1 ). Higher liquid loads are preferred for this application in order to improve CO2 absorption, as K2 CO3 solutions have relatively low absorption efficiency for CO2 capture. Under these conditions, a higher gas velocity can also be utilized with Ø 13 mm SMR. These characteristics suggest that SMR packing would be beneficial for the high throughputs required for CO2 capture from coal fired power stations. Bain and Hougen’s equation (Bain and Hougen, 1944) can be used to predict the flooding gas velocity of the different packings discussed in this study (refer to Table 4). In the present case, the

Table 2 Physical properties of the gas and liquid phases. System type

Temperature, T (K)

Density,  (kg m−3 )

Viscosity,  × 105 (kg m−1 s−1 )

Liquid (30 wt% K2 CO3 ) CO2 (14 mol%) + N2 (86 mol%) CO2 (85 mol%) + N2 (14 mol%)

343 343 343

1300 1.074 1.477

987.27 1.905 1.678

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X. Zhao et al. / International Journal of Greenhouse Gas Control 5 (2011) 1163–1169 Table 4 Flooding velocity models for packed columns used in this study.

Gas Phase Flooding Velocity, uv,Fl (m/s)

0.7 0.6

Author

Flooding velocity model



0.5

uv.Fl = Billet and Schultes (1999)

0.4

Fl

=

2g(ε−hL,Fl )3 hL,Fl L V Fl a

g

   0.2 −2nFl L v L

C2 ∗ Fl

V

L

V

L

≥ 0.4,

 v L

0.3

when

0.2

CFl∗ = 0.6244CFl

5

 L 0.1028 V

to conditions when

 u2

0.0 0

V

and nFl = −0.708

Refer to Billet and Schultes  (1999) for equations relevant

SMR 13mm Mellapak 700Y Pall 13 mm

0.1

1/2

10

15

20

25

30

35

Bain and Hougen (1944)

log

v,Fl a V g ε3 L

L V



0.16

v L

≤ 0.4.

 L 1/4  v 1/8

= b − 1.75

L

V

Liquid Phase Flowrate, L (kg.m-2.s-1) Fig. 3. Gas flooding velocities for Ø 13 mm SMR, Ø 13 mm Pall Ring and Mellapak 700Y.

experimental data can be reasonably fitted to this expression. However, the value of the constant b, is significantly different from the range suggested by these authors (0.022–0.26) with an average value across the three packings of −0.96 ± 0.04. This constant is different to that proposed by Bain and Hougen most likely due to the fact that this correlation was developed using only three types of random packing materials and for a limited range of operating conditions. Alternatively, the expressions of Billet and Schultes (1999) can be used to determine the flooding velocity, in combination with the flooding holdup (see Tables 3 and 4). Again, the fit to this expression is reasonable, but the values of the adjustable flooding constant (CFl ) differ from that suggested by these authors (Table 5). 3.2. Liquid holdup In a packed column, the liquid holdup refers to the liquid retained in the packed bed including liquid films or droplets on the surface of the packing or the liquid trapped in the interstitial space between the packing. There is a need to study the liquid holdup because it has a direct influence on liquid-phase mass transfer, loading behaviour, gas-phase pressure gradients and mass transfer (Skrbic and Cvejanov, 1994). It can be seen from Fig. 4 that Ø 13 mm Table 3 Pressure drop models for packed columns used in this study. Author

Pressure drop model

Table 5 Values of the Billet and Schultes (1999) flooding constant (CFl ) in comparison to the literature values.

SMR 13 mm Pall Ring 13 mm Mellapak 700Y a

This study

Billet and Schultes (1999)

1.48 1.41 1.89

2.92a

Extrapolated from data for 25, 35 and 50 mm Pall Rings.

SMR and Mellapak 700Y generally have a higher liquid holdup than the Ø 13 mm Pall Ring. 3.3. Pressure drop Pressure drop determines the operating reliability and cost of the packed column. A comparison of the experimental pressure drop in Ø 13 mm SMR, Ø 13 mm Pall Ring and Mellapak 700Y has been shown in Fig. 5. From these results it can be seen that all three packings have comparable pressure drops under the conditions studied. Several models for predicting the pressure drop in a packed column have been presented in the literature and the details of two such correlations are shown in Table 3. Nonlinear regression of the experimental data was used to evaluate the model parameters for the different packings (Table 6 and Fig. 6). The constant developed for Pall Ring packing using the Billet and Schultes correlation is completely consistent with the values suggested in this paper, and the parameters for the other two packings are well within the range of values presented for all packings in this paper (0.19–1.3). The Takehashi correlation was less successful but the

For irrigated packing:

Billet and Schultes (1999)

=

L

Fv2 1 a (ε−hl )3 2 K

where: L =  64 Rev

CP.0

+

1.8 Re0.08 v



ε−hl ε

1.5  h 0.3 L

hL,s

exp(C1



0.16

FrL )

u2 a

FrL =

L

g

1 = 1 + 23 1−ε √ Fv = uv v 1 K

hL =

0.18

13,300 ˛3/2

C1 =

dp =

0.20

Holdup hL

P Z

6(1−ε) a 12 L g L



dp ds

uL a2

P Z

h=

= 2f



V

ε−h

0.10

0.06

1/3

V dp

SMR MellaPak Pall

0.04

where uv ≤ uv,s

0.02

−0.742 ; ReG ≥ 200, f = 6.85 ReG −0.216 ReG < 200,  f = 114  ReG u2

0.12

0.08

Refer to Billet and Schultes (1999) for equations relevant to conditions when uv ≥ uv,s Takahashi et al. (1979)

0.14

 u 2 3

+ kh

V

ε−h

1.53 × 10−4 + 2.90 × 10−5 ε Re0.66 L

  0.75  L

W

dp−1.20

0.00 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Gas Load Factor, Fv (kg1/2m-1/2s-1)

Fig. 4. Comparison of liquid holdup for CO2 (14 mol%) absorption into K2 CO3 solution at 70 ◦ C using 3 packings: Ø 13 mm SMR, Ø 13 mm Pall Ring and Mellapak 700Y at a liquid phase flowrate of 14 kg m−2 s−1 .

X. Zhao et al. / International Journal of Greenhouse Gas Control 5 (2011) 1163–1169

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3.4. Mass transfer performance

200 180

Pressure Drop (Pa)

160 140 120 100

SMR Mellapak Pall

80 60 40 20 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Gas Load Factor, Fv (kg1/2m-1/2s-1)

Fig. 5. Comparison of pressure drop for CO2 (14 mol%) absorption into K2 CO3 solution at 70 ◦ C using 3 packings: Ø 13 mm SMR, Ø 13 mm Pall Ring and Mellapak 700Y at a liquid phase flowrate of 14 kg m−2 s−1 .

Table 6 Values of the Billet and Schultes (1999) pressure drop constant (CP,0 ) and the Takahashi et al. (1979) pressure drop parameter (k) in comparison to the literature values.

SMR 13 mm Pall Ring 13 mm Mellapak 700Y a

Billet and Schultes CP,0 parameter

Takehashi k parameter

This study

This study

0.84 1.00 0.56

Billet and Schultes (1999) 0.96a

8.33 × 105 3.65 × 105 –

Takahashi et al. (1979)

1 1 H = + Ky a ky a kl ˇa

5.22 × 104

Extrapolated from data for 25, 35 and 500 mm Pall Rings.

∗ NCO2 = Ky (yCO2 − yCO )

(3)

2

Considering an element of column with height Z, the mass balance can be given as follows: G L dYCO2 = dxCO2 S S

(4)

where S represents the column cross-sectional area. Combining Eq. (3) with Eq. (4) gives: ∗ Ky a(yCO2 − yCO )=− 2

L dxCO2 G dYCO2 = S dZ S dZ

(5)

which rearranges to the following: Ky a = −

dYCO2 dxCO2 L G 1 1 = ∗ ∗ S (yCO2 − yCO S (yCO2 − yCO ) dZ ) dZ 2

(6)

2

Eq. (6) is universally used in the literature to determine the overall mass transfer coefficient (Aroonwilas and Tontiwachwuthikul, 1997a,b, 1998; Aroonwilas and Veawab, 2004; Aroonwilas et al.,

b

200

(2)

where H and ˇ stand for the Henry constant and the enhancement factor for the chemical reaction, respectively. Ideally the overall mass transfer coefficient Ky a would be directly determined from Eq. (2). However, this approach is not extensively used because experimental determinations of the individual mass transfer coefficients ky and kl involve the use of extremely difficult techniques. A more practical approach for calculating the overall mass transfer coefficient is to perform absorption experiments where the concentration profile of absorbed component in the gas phase is measured along the tested column length (Aroonwilas and Veawab, 2004). In this case the flux of CO2 is calculated and characterized by the overall gas-phase mass transfer coefficient (Ky ):

NA adZ = −

values determined for SMR and Pall Ring packings, determined by regression of the experimental data are well within the range of values quoted by this paper which range from 1.65 × 104 for Berl Saddles to 9.30 × 105 for Raschig rings. The Takehashi correlation overpredicted the dry pressure drop for Mellapak 700Y so no value of k provided a realistic fit to the data for this packing, suggesting that the Takehashi correlation is only appropriate for random rather than structured packing.

a

To examine the mass transfer performance of the different packings, the overall mass transfer coefficient per unit volume (Ky a) was determined. Higher Ky a values indicate better mass transfer performance. The overall mass transfer coefficient can be determined from the following Eq. (2) (Lin et al., 2003):

250

160 140 120 100

Pall Mellapak SMR

80 60 40

Experimental Pressure Drop (Pa)

Experimental Pressure Drop (Pa)

180 200

150

Pall SMR

100

50

20 0

0 0

50

100

150

Predicted Pressure Drop (Pa)

200

0

50

100

150

200

250

Predicted Pressure Drop (Pa)

Fig. 6. Comparison of pressure drop (wet) as calculated from (a) the Billet and Schultes (1999) correlation and (b) the Takahashi et al. (1979) correlation versus the experimental data for the three packings. No appropriate fit could be provided for the Takehashi correlation and Mellapak700Y.

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X. Zhao et al. / International Journal of Greenhouse Gas Control 5 (2011) 1163–1169

packing can be attributed to the special geometric characteristics of the SMR packings. With twisting-inwards arc units, the dispersion, aggregation and re-dispersion of liquid drops are greatly promoted. This packing, by means of the small height-diameter ratio, enhances the mass transfer rate between the contacting phases, simultaneously increasing specific surface area and accelerating the surface renewal. This in turn results in a larger area for mass transfer and hence improved mass transfer performance and column efficiency.

4.5 4.0

kya, kmol/m3h

3.5 3.0 2.5 2.0 1.5

4. Conclusions

1.0

SMR MellaPak Pall

where xCO2 represents the equivalent CO2 concentration in 30 wt% K2 CO3 solution at equilibrium conditions. The mass balance between the gas and solvent in the column is given as follows:

Ø 13 mm SMR packing has been shown to have a 20% and 30% higher mass transfer coefficient when compared to Mellapak 700Y and Ø 13 mm Pall Ring, respectively. Thus the height of a packed column with SMR would be substantially lower than that with Pall Rings or Mellapak. Meanwhile, compared to other packings the flooding gas velocity for SMR packing increased when the liquid loads were above 25 kg m−2 s−1 . Thus Ø 13 mm SMR has a higher gas flux at higher liquid loads when compared to Ø 13 mm Pall Ring and Mellapak 700Y, which is advantageous for absorption of CO2 under high throughput conditions seen at power plants. Pressure drops were comparable for all three packings. Correlations for predicting the gas flooding velocity, pressure drop and mass transfer coefficient have been recommended for the different packings used in this study and will be important for the design of large scale packed columns for CO2 capture. Overall, laboratory scale results from this study have shown promising potential for CO2 absorption using SMR randomly packed columns. Due to the special geometric characteristics of the SMR packing which promote even wetting on the packing surface as well as recurrent turbulence, relatively low pressure drop but sustained mass transfer rates are observed. SMR packing also has a significant cost advantage over the structured packing Mellapak which is known to be significantly more expensive per unit volume than random packing (Seader and Henley, 2006).

G(YCO2 ,in − YCO2 ,out ) = L(xCO2 ,out − xCO2 ,in )

Acknowledgments

0.5 0.0 0

5

10

15

20

25

Liquid Phase Flowrate, L (kg.m-2.s-1) Fig. 7. Mass transfer coefficient for Ø 13 mm SMR, Ø 13 mm Pall Ring and Mellapak 700Y at a gas velocity of 0.34 m s−1 .

1999). In an ideal case, the CO2 concentration in the gas phase along the column is measured and interpreted in terms of mole ratio and subsequently plotted as a function of column height. Due to equipment limitations it was not possible to obtain this profile in the current study, so Eq. (6) was further simplified. This was done by substituting yCO2 and y∗ in Eq. (6) by the equations for the operating and equilibrium curves. The resulting equation was solved numerically between the liquid inlet and outlet concentrations. The data of Tosh et al. (1959) was used to determine the equilibrium equation for CO2 absorption at 70 ◦ C using 30 wt% K2 CO3 : y∗ = 0.00568 × exCO2 /0.00767

YCO2 ,out = YCO2 ,in −

(7)

(xCO2 ,out − xCO2 ,in )L G

(8) (9)

According to Eq. (9), the equation of the operating curve can be presented as: yCO2 =

˛xCO2 + ˇ

(10)

˛xCO2 +

References

where ˛=

−L ; G

ˇ=

Yin + L × xin ; G

=ˇ+1

(11)

Replacing yCO2 and y∗ in Eq. (6) by Eqs. (7) and (10) gives: Ky a = −

The authors acknowledge the Cooperative Research Centre for Greenhouse Gas Technologies (CO2 CRC) and the Particulate Fluid Processing ARC Special Research Centre (PFPC) for financial support and vacation student Moshe Ross for assistance in completing the experimental work for this study.

dYCO2 G 1 ∗ S (yCO2 − yCO ) dZ

L = SZ



2

1 dxCO2 (˛xCO2 −ˇ)/(˛xCO2 − )−0.00433 × exCO2 /0.00795 (26)

Eq. (12) was numerically solved using function Quad (adaptive Simpson quadrature) in MATLAB. A comparison of the resulting overall mass transfer coefficients, Ky a, for Ø 13 mm SMR, Ø 13 mm Pall Ring and Mellapak 700Y are shown in Fig. 7. Ky a increases with increasing liquid loads for all packings. The Ky a of Ø 13 mm SMR is 30% and 20% higher than that with Ø 13 mm Pall Ring and Mellapak 700Y, respectively, at most liquid loadings. This means that the height of a packed column with SMR would be substantially lower than that with Pall Ring and Mellapak. The improved mass transfer performance seen with the SMR

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