Characterization of Novel Structured Packings for CO2 Capture

Available online at www.sciencedirect.com

Energy Procedia 37 (2013) 2145 – 2153

GHGT-11

Characterization of novel structured packings for CO2 capture Chao Wanga,b, Micah Perryb, Frank Seibertb, Gary T.Rochellea* a

The University of Texas at Austin, Department of Chemical Engineering, Luminant Carbon Management Program, Austin, TX 78712-1589, USA b Separations Research Program, Pickle Research Campus, The University of Texas at Austin, Austin, TX 78758, USA

Abstract Structured packing is widely being considered for post-combustion CO2 capture because of its high mass transfer per unit of pressure drop. The hydraulic and mass transfer performances of structured packings with different corrugation angles were measured. The effective mass transfer area was measured by absorption of CO2 with 0.1 gmol/L NaOH. The liquid film mass transfer coefficient was measured by desorption of toluene into air. The gas film mass transfer coefficient was measured by absorption of SO2 from air into 0.1 gmol/L NaOH. The effective area and the liquid film mass transfer coefficient are dependent on liquid velocity while the gas film mass transfer coefficient is a function of gas velocity. Increasing the corrugation angle from 45o to 60o (Mellapak 250Y to Mellapak 250X) resulted in (1) a 54% decrease in pressure drop, (2) a 6% decrease in effective area, (3) a 17% decrease in kG, and (4) a 18% decrease in kL. Increasing the corrugation angle from 45 o to 70 o, a larger difference was found in pressure drop, kG and kL. However, the impact of corrugation angle on effective area is not significant. In conclusion, increasing the corrugation angle will have a benefit on pressure drop with a negative impact on mass transfer properties. Future work will be focused on economic analysis between operation costs (related to pressure drop) and capital costs (related to mass transfer properties) to determine the optimum corrugation angle for a particular separation task.

© 2013 2013The TheAuthors. Authors.Published Published Elsevier © byby Elsevier Ltd.Ltd. Selectionand/or and/orpeer-review peer-review under responsibility of GHGT Selection under responsibility of GHGT

Keywords: CO2 capture; structured packing; corrugation angle; effective area; gas film mass transfer coefficient; liquid film mass transfer coefficient

* Corresponding author. Tel.: +1-512-471-7230 E-mail address: [email protected].

1876-6102 © 2013 The Authors. Published by Elsevier Ltd.

Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.06.093

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1. Introduction Structured packing is widely being considered for CO2 capture by amine scrubbing because of its low pressure drop, good mass transfer efficiency, and ease of installation. In the CO2 capture process, absorber and stripper performance are highly dependent on the effective mass transfer area of the packing (ae). The stripper performance also depends on the liquid film mass transfer coefficient (k L). The gas cooler and water wash performance depend on the gas film mass transfer coefficient (kG). The operating pressure drop will be reduced with a larger corrugation angle. However, the impact of the corrugation angle on mass transfer performance is still uncertain and needs to be explored. This paper is focused on the measurement of mass transfer characteristics for structured packing and the effect of corrugation angle on hydraulic and mass transfer characteristics. 2. Experimental Previous work has developed models that predict mass transfer properties for packing. However, these correlations were developed primarily with traditional packing designs. In the regression of the existing models, kG and kL have been determined by dividing measured ka values and proposed area models, which are not measured directly. The values for the mass transfer coefficient (kG and kL) obtained this way must be used with the corresponding area model to predict ka. Therefore, consistent and direct measurement of ae, kG and kL for novel packings is needed. 2.1. Packings To explore how packing corrugation angle influence the mass transfer properties, four different structured packings with different corrugation angle were measured. The characteristics of these packings are given in Table 1. Table 1. Packing information Packing name

MP250Y

MP250X

GT-PakTM350Y

GT-PakTM350Z

Type

Mellapak

Mellapak

GT-Pak

GT-Pak

Surface area,

250

250

350

350

Corrugation angle

45

60

45

70

Channel length, m

0.019

0.019

0.011

0.011

m2/m3

2.2. Apparatus All packings were measured in a column with an outside diameter of 0.46 m, an inside diameter of 0.427 m and packed height of 3 m [1, 2]. The operation was counter-current, with ambient air entering below the packing bed and flowing upward through the tower. The liquid was pumped in a closed loop and was distributed at the top of the column using a pressurized fractal distributor with 108 drip points.

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Samples were taken at the inlet and the outlet of the column. The experimental setup is shown in Figure 1.

Fig.1. Process flow diagram for the 427 mm ID packed column

2.3. Effective Mass Transfer Area (ae) Absorption of CO2 in ambient air with 0.1 gmol/L NaOH was used to measure effective mass transfer area [3, 4]. CO2 in the gas in and out of the column was measured with a Horiba VIA-510 infrared analyze. The system was liquid film controlled so the overall mass transfer coefficient KOG can be assumed to be equal to the liquid phase mass transfer coefficient kg ective area can be calculated as follows:

yCO2in ) yCO2out ZK KOG RT

uG ln( ae

yCO2in ) yCO2out Zkk g' RT

uG ln(

(1)

According to previous research [5, 6], kg was calculated by:

k g'

kOH [OH ]DCO2,L H CO2

(2)

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where the diffusivity of CO2, DCO2,L, and Henry's constant, HCO2, can be calculated. 2.4. Gas Film Mass Transfer Coefficient (kG) Absorption of sulfur dioxide with 0.1 gmol/L NaOH was applied to measure the gas film mass transfer coefficient [7, 8, 9]. The system was gas film controlled so KOG can be assumed to be equivalent to kG. The gas film mass transfer coefficient was calculated using Equation 3:

y SO2in ) y SO2out ZRTae

uG ln( kG

(3)

Two Thermo Scientific 43i SO2 analyzers were used to measure the inlet and outlet SO2 concentrations. A 0 100 ppb range analyzer was used to measure outlet SO2 concentration while a 0 100 ppm range analyzer was used to measure the inlet. Because of the high efficiency of SO2 absorption by NaOH, the bed height was reduced to 0.8 m to avoid operating below the lower limit of the outlet SO2 analyzer. 2.5. Liquid Film Mass Transfer Coefficient (kL) Air-stripping of toluene in water was applied to measure the liquid film mass transfer coefficient [6]. The system was liquid film controlled and there was no chemical reaction. Thus, the overall mass transfer coefficient, KL, can be assumed to be equivalent to the physical liquid film mass transfer coefficient kL and calculated using the following equation:

kL

C uL ln Toluene in Zae CToluene out

(4)

An inlet and an outlet toluene sample in water were taken and analyzed. A Hewlett Packed 5890A Gas Chromatograph was used for the analysis. 2.6. End effect For effective area and kL measurement, the end effect was calculated to be less than 5% and was neglected. However, for kG measurement, the end effect could not be ignored because a much shorter packed bed is used. Mass transfer coming from the upper end and lower end is measured to account for 20% of the overall mass transfer in kG measurement. Thus, mass transfer coming from the packing is obtained by subtracting the end effect mass transfer from the overall mass transfer. 3. Results and discussion 3.1. Effective Mass Transfer Area (ae) The influence of gas and liquid velocity on mass transfer area was measured. For all packings measured in this work, the effective mass transfer area is a function of liquid velocity and is relatively insensitive to gas velocity.

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1.2

Fractional area, ae/ap

Tsai's model GT-PAKTM350Z GT-PAKTM350Y 1.0

MP250Y MP250X

MP250Y > MP250X Overall deviation: 6% GT-PAKTM350Z > GT-PAKTM350Y Overall deviation: 7.5%

0.6 4.0E-6

8.0E-6

1.6E-5

3.2E-5

Generalized liquid velocity uL/aP

6.4E-5

,(m2/s)

Fig. 2. Effective Mass Transfer Area comparison

The influence of corrugation angle on mass transfer area is illustrated in Figure 2 as a comparison of packings with identical geometries except for the corrugation angle. According to Figure 2, the packing with 45o corrugation angle (MP250Y) has a 6% larger effective area than the packing with 60o angle (MP250X). Another comparison is made between packings with 45o corrugation angle and 70o angle (GT-PakTM 350Y and 350Z). The deviation between these two packings is 7.5%. Results show that changing corrugation angle does not have a significant influence on mass transfer area. The data measured in this work also shows consistency with the Tsai model [5, 6] expressed in a dashed line in Figure 2, with an average deviation of 10%. 3.2. Liquid Film Mass Transfer Coefficient (kL) Liquid film mass transfer coefficient result for one of the packing characterized in this work (MP250X) is shown in Figure 3. Liquid film mass transfer coefficient shows little dependence on gas velocity and large dependence on liquid velocity. This trend is also valid for all other packings measured in this work. The exponent of kL over uL is from 0.4-0.9 for the packings measured in this work. The effect of corrugation angle on liquid film mass transfer coefficient can be demonstrated by a comparison between two different pairs of packings with identical geometries except for corrugation angles (Figure 4). The kL with 45o packing (MP250Y) is 17% greater than the k L with 60o packing (MP250X). The kL of MP250Y has a weaker power law dependence (0.36) on the liquid rate than that of

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MP250X (0.76). The difference in k L is larger if the change in corrugation angle becomes larger. The kL for GT-PakTM 350Y (45o) is 28% greater than the kL for GT-PakTM 350Z (70o). 6E-5 uG = 0.59 m/s uG = 0.99 m/s uG = 1.48 m/s

kL = 0.0011*uL0.76

kL, (m/s)

4E-5

2E-5

0E+0 0

0.005

0.01

0.015

0.02

0.025

Liquid Velocity, m/s Fig.3. Liquid film mass transfer coefficient result for MP250X

5E-5

GT-PakTM350Y

uG=0.99 m/s

kL = 1.3E-3uL0.71

kL, (m/s)

MP250Y kL = 2E-4uL0.36

MP250X kL = 1.1E-3uL0.76

2E-5

GT-PakTM350Z kL = 1.9E-3uL0.86 1E-5

MP250Y > MP250X Deviation: 18% GT-PakTM350Y > GT-PakTM 350Z Deviation: 28%

6E-6 0.001

0.002

0.004

uL, (m/s) Fig.4. Liquid film mass transfer coefficient comparison

0.008

0.016

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0.06 uL=6.71E-3 m/s uL=3.36E-3 m/s

uL=1.01E-2 m/s

0.04

kG, m/s

kG = 0.035uG0.62

0.02

0 0.0

0.4

0.8

1.2

1.6

2.0

uG, m/s Fig.5. Liquid film mass transfer coefficient result for GT-PAKTM 350Y

0.06

L=36 m3/(m2*h)

MP250Y kG = 0.027uG0.61

GT-PakTM350Y kG = 0.035uG0.62

kG, (m/s)

17%

MP250X kG = 0.025uG0.43

0.03

34%

GT-PakTM350Y

kG = 0.023uG0.6

MP250Y > MP250X Overall deviation: 17% GT-PakTM350Y > GT-PakTM 350Z Deviation: 34%

0.015 0.45

0.90

1.80

uG, (m/s) Fig.6. Gas film mass transfer coefficient comparison

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3.3. Gas Film Mass Transfer Coefficient (kG) Results In contrast to kL, the gas film mass transfer coefficient (kG) increases with the gas flow rate and is essentially independent of the liquid flow rate (Figure 5). The other packings display similar behavior. The kG varied with the gas velocity to the n-power where n ranged from 0.4 to 0.7. The influence of corrugation angle on gas film mass transfer coefficient is explored by comparing packings with different corrugation angles in Figure.6. Results show that k G for packing with 45o corrugation angle (MP250Y) is 17% higher than packing with 60 o angle (MP250X). Similar to kL, larger changes in corrugation angle cause larger changes in kG: kG for packing with 45o angle (GT-PakTM 350Y) is 34% higher than packing with 70o angle (GT-PakTM 350Z). It can be concluded that increasing corrugation angle will also have a negative impact on gas film mass transfer coefficient. 4. Conclusions The effective mass transfer area (ae) increases with liquid velocity and is independent of gas velocities. All the data measured in this work shows consistency with the previous model (Tsai, 2010), with an average deviation of 10%. The liquid film mass transfer coefficient (kL) is dependent on liquid velocities and changes slightly with gas velocities. This dependence can be expressed as a power function, and the exponents regressed for the packings measured in this work varied from 0.4 to 0.9. Unlike kL, the gas film mass transfer coefficient (kG) is a function of gas velocities and independent of liquid velocities. The gas film mass transfer coefficient (kG) varies with superficial gas velocity (uG) to the 0.6 to 0.9 power for the packings studied in this work. The impact of corrugation angle on mass transfer properties is summarized in Table 2. The liquid film mass transfer coefficient, kL, and gas film mass transfer coefficient kG will decrease as corrugation angle increases. Reasons for this could be that increasing corrugation angle will reduce turbulence at the gas and liquid interface, and thus mitigate gas-liquid interaction. However, the impact of corrugation angle on effective area is not significant. In conclusion, increasing corrugation angle will obtain benefits from reduced pressure drop with a sacrifice of mass transfer at the same time. Future work will be focused on economic analysis between operation costs (related to pressure drop) and capital costs (related to mass transfer performance) to determine the optimum corrugation angle for a particular separation task. Table 2. Summary of corrugation angle impact on pressure drop and mass transfer. Impact of corrugation angle

Pressure drop

ae

kG

kL

Increase from 45° to 60°

-54%

-6%

-17%

-18%

-66%

+7.5%

-34%

-28%

(MP250X-MP250Y) Increase from 45° to 70° (GT-PAKTM350Z- GT-PAKTM350Y)

Chao Wang et al. / Energy Procedia 37 (2013) 2145 – 2153

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Acknowledgements This work was supported by the Luminant Carbon Management Program and the James R. Fair Process Science and Technology Center. We are grateful to GTC Technology and Sulzer Chemtech for providing the packing materials for this research. We also recognize the contributions of SRP staff members to this work.

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