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Experimental and Theoretical Study of CO2 Absorption with Piperazine-Promoted Potassium Carbonate Solution in Hollow Fiber Membrane Contactors Amir Izaddoust, and Peyman Keshavarz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01554 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017
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Experimental and Theoretical Study of CO2 Absorption with PiperazinePromoted Potassium Carbonate Solution in Hollow Fiber Membrane Contactors Amir Izaddoust, Peyman Keshavarz* Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran * Corresponding Author: Tel.: +987116133713, Fax: +987116473180, E-mail:
[email protected] Abstract Potassium carbonate is an economical absorbent for CO2 separation from flue gases. In this study, potassium carbonate as absorbent and piperazine as promoter were applied to absorb CO2 from a gas mixture using a hollow fiber membrane contactor in experimental scale. Also, a mathematical model has been presented to find CO2 concentration profile in different phases and to analyze the process. Effects of concentration of potassium carbonate and piperazine and other parameters such as liquid and gas flow rates were experimentally observed and studied. Effects of membrane wetting on absorption recovery and piperazine concentration on effective length of module have been analyzed using the mathematical model. Experimental results show that CO2 recovery can rise up to 1.6 times and CO2 recovery can reach to 90% by adding 0.03 molar piperazine to 5% wt aqueous solution of potassium carbonate. The results of mathematical model at high gas flow rates reveal that CO2 recoveries are considerably higher (up to 8 times) for promoted potassium carbonate solution compared to aqueous potassium carbonate in membrane contactors. Also, high reaction rate of piperazine can prevent sharp reduction of recovery at low wetting fractions.
Keywords: CO2 absorption; Potassium carbonate; Piperazine; Membrane contactors
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1. Introduction In recent years, global warming has become a more serious concern due to its devastating effects on environment1. Hence, emission of greenhouse gases such as CO2 has become an important issue that needs to be carefully considered. There are various techniques to separate CO2 from flue gases, however in recent decades membrane processes have seen increased use due to their advantages over traditional processes. Some of these advantages are 1- Providing high contact area for mass transfer between two phases 2- Unlike the conventional absorption columns there is no need to have density difference between two phases 3- Avoiding phenomena such as flooding, loading, foaming, weeping, etc. 4- Absorption capacity can be easily increased or decreased by adding or removing membrane modules2. However, membrane contactors have some disadvantages. For example, their pores can be filled with solvent and drastically increase resistance against mass transfer after a certain amount of operating time. This phenomenon is called membrane wetting3. Hydrophobic membranes like PP (polypropylene) or PTFE (polytetrafluoroethylene) are common materials to reduce the possibility of membrane wetting. Zhang and Cussler4 were the first pioneers to use hollow fiber membrane contactors (HFMC) to separate CO2 by aqueous sodium hydroxide solution. Since then, HFMCs have been used to separate various gases such as CO2, SO2 and NOx. Karoor and Sirkar5 used HFMCs to absorb CO2 and SO2 from their mixtures with N2 by water. Their investigations indicated that the mass transfer flux was much higher than those usually found in packed towers. Kim and Young6 used amines such as AMP, MEA and MDEA to absorb carbon dioxide in a PTFE (polytetrafluoroethylene) hollow fiber membrane contactor and showed that AMP has higher absorption capacity and moderate absorption rate. Masoumi et al.2 presented a mathematical model to simulate the absorption of carbon dioxide in hollow fiber membrane contactors. They
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also checked the effects of membrane wetting on separation performance. Their results showed that absorption rate would drastically decrease by membrane wetting. Faiz and Al-Marzouqi7 explored the separation of CO2 and H2S from natural gas using aqueous potassium carbonate as a solvent in membrane contactors. Aqueous solution of potassium carbonate was also studied by Mehdipour et al.8 in a HFMC. They showed that there was an optimum concentration of potassium carbonate at each solution temperature. Golkhar et al.9 applied nanofluids of nanosilica and carbon nanotube as absorbents in a gas-liquid membrane contactor for CO2 separation. There is a variety of solvents for CO2 absorption in membrane contactors. However, for selecting a proper solvent, one should consider different parameters such as operational cost (price of solvent and energy requirement for regeneration), effect of absorbents on membrane wetting and rate of solvent degradation10. Today, the most common solvents are aqueous alkanolamines such as MEA (monoethanolamine), DEA (diethanolamine), MDEA (methyldiethanolamine), AMP (2amino-2-methyl-1-propanol) and DIPA (diisopropanolamine)11. Because of low surface tension, amines usually have a high tendency to wet the membrane pores. Hence, to reduce this tendency, in this study it is suggested potassium carbonate solutions be used as the main solvent, and piperazine as promoter to improve low reaction rate of potassium carbonate with CO2 in membrane contactors. Therefore, it is possible to have both the advantages of potassium carbonate as a low cost solvent, and piperazine as a solvent with high reaction rate with carbon dioxide. In this work a HFMC has been applied to separate CO2 from a mixture of CO2/N2 by aqueous solution of potassium carbonate and piperazine as promoter. Effects of concentration of potassium carbonate and piperazine and other parameters such as liquid and gas flow rates were
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experimentally observed. Also, a mathematical model has been developed for this system and it was verified with obtained experimental data. The mathematical model was used to show CO2 concentration profile in gas, liquid and membrane and to analyze the results. Effects of some parameters such as membrane wetting and effective length of module have also been investigated using the model.
2. Experimental section For measuring CO2 absorption through a hollow fiber membrane contactor, an experimental setup was made according to Figure 1. Potassium carbonate with 99.0% purity was prepared from Duksan Company and piperazine with 99.0% purity was prepared from Merck KgaA Company. Hollow fiber membrane contactor was built by Parsian Pishro Polymer Company and its specifications are represented in Table 1. A CO2 analyzer from Geotech Company was used to measure CO2 percent in inlet and outlet gas. A Peristaltic pump was applied to pump absorbent through the membrane fibers and measure liquid flow rate. Gas and liquid flow direction was cocurrent in all of the experiments. Experiments were held in non-wetted mode condition and absolute pressure and temperature was 103.100 Kpa and 297.15 K, respectively. Composition of inlet gas was 21.3 % CO2 and 78.7 % N2.
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Gas flow meter Flow control valve
Absorbent out
Mixture of CO2 + N2
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HFMC
Pump
Gas outlet
Absorbent in
CO2 analyzer
Figure 1- Piping flow diagram of experimental setup
Table 1- Specification of membrane module contactor
Membrane
Fiber
Fiber
Shell
Length of
No. of
Type
o.d.,
i.d.,
diam,
module,
fibers
µm
µm
cm
cm
500
230
2
17
polypropylene
400
Tortuosity
Porosity
5.5
0.5
A mixture of nitrogen and CO2 flowed through shell side and aqueous solution of potassium carbonate and piperazine flowed co-currently inside the fibers. The concentration of CO2 in gas outlet was measured by CO2 analyzer and CO2 recovery based on the relation (1) was reported. CO Recovery % =
Q C − Q C × 100 Q C
(1)
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3. Model description For diffusion of CO2 into absorbent, molecules of CO2 should diffuse through 3 phases. The first phase is gas mixture, the second phase is membrane and the third is liquid phase2. The path of absorption is shown in the Figure 2. After the diffusion of CO2 into the absorbent, reactions will start and CO2 will be consumed. In a fiber with a high degree of hydrophobicity, the pores will remain dry and they are completely filled with gas.
ro
ri
N2 CO2
Gas
Membrane
Liquid
re
Figure 2- Schematic representation of CO2 diffusion into solvent
A couple of assumptions have been considered in order to model CO2 absorption through a hollow fiber membrane contactor. Due to atmospheric pressure, gas behavior has been considered as ideal. Isothermal condition has been assumed in all phases and physical properties of fluids have been assumed to be constant. Since module length is short, pressure drop has been neglected and fluid velocity only changes with radius of module2.
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3.1. Reaction scheme In an aqueous solution of potassium carbonate and amine as promoter, K2CO3 is ionized into K+ and CO32-, and amine is protonated partially12, then the carbonate reacts with water to form hydroxide and bicarbonate ions13. Reactions (2-5) have been applied to present reaction mechanism of CO2 in aqueous solution of potassium carbonate and overall reaction has been shown by reaction (6)12.
H O H O + OH !
(2)
" #$"
CO + H O HCO! +H
(3)
%
! ! HCO! + OH CO + H O
(4)
& #$&
CO + OH ! HCO !
(5)
! CO + CO! + H O ↔ 2HCO
(6)
Where Reaction rate of CO2 can be written as follows7:
R )*+ = −k -CO . +
K 0 K 1 k - HCO ! . k 2 -CO .-CO! k 2 -HCO ! . . − + K 0 -HCO! K2 K -CO! . .
(7)
Since by consumption of one mole CO2, one mole CO32- would be consumed and two moles HCO3- would be generated, the reaction rates of CO32- and HCO3- will be achieved as below12: R 3)*4 = 2R )*+ "
(8)
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R )*+4 = R )*+ "
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(9)
In presence of piperazine, reactions (10-11) were applied13. PZ + CO +
(10)
$78 – :;4
OH ! PZCOO! + H O
(10-1)
$784;+ :
H O PZCOO! + H O
(10-2)
$78 – 78
PZ PZCOO! + PZH CO!
(10-3)
$784
