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Chemical Engineering Research and Design 1 3 1 ( 2 0 1 8 ) 375–384

Contents lists available at ScienceDirect

Chemical Engineering Research and Design

j ourna l h omepage: www.elsev ier .com/ locate /cherd

odeling of a CO2-piperazine-membranebsorption system

hien Zhanga,b,d,∗,1, Feng Chenb,∗∗, Mashallah Rezakazemic,1,enxiang Zhange, Cunfang Lua, Haixing Changa, Xuejun Quana

School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, ChinaFujian Province University Key Laboratory of Green Energy and Environment Catalysis, Fujian Provincial Keyaboratory of Featured Materials in Biochemical Industry, Ningde Normal University, Ningde 352100, ChinaDepartment of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, IranKey Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education of China,hongqing University, Chongqing 400044, ChinaSchool of Environmental Science and Engineering and Institute of Environmental Health and Pollution Control,uangdong University of Technology, Guangzhou 510006, China

r t i c l e i n f o

rticle history:

eceived 25 July 2017

eceived in revised form 14

ovember 2017

ccepted 16 November 2017

vailable online 6 December 2017

eywords:

FD

O2 absorption

ollow fiber membrane

a b s t r a c t

CO2 is a main greenhouse gas emission causing global warming and other environmental

issues, which is mainly emitted from the fossil fuels combustion or utilization. Membrane

absorption is a novel CO2 capture method that combines the advantages of chemical absorp-

tion and membrane separation. In this paper, a comprehensive 2D symmetric model for

a CO2-piperazine (PZ)-membrane absorption process was proposed. PZ solutions showed

high CO2 capture performance due to high chemical reaction rate constant. Decreasing the

gas flowrate and increasing the absorbent flowrate promoted the CO2 absorption efficiency.

On the other hand, varying the membrane contactor properties could also affect the cap-

ture of CO2. 0.28 m/s gas velocity, 0.08 m/s absorbent velocity, 20% CO2 in gas mixture, and

0.94 mol/L PZ were recommended as the optimum conditions after the parametric study.

This numerical model is reliable for potential use in CO2-absorbent-membrane absorption

iperazine

odeling

arametric study

systems.

© 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

provides high gas removal efficiency that is similar to the chemical

absorption. Furthermore, it solves the issues of channeling, foam-

. Introduction

n order to reduce the greenhouse gas emissions, CO2 capture from gas

ixture has attracted more and more attention and interests around

he world (Scott et al., 2012; Favre, 2007; Rezakazemi et al., 2014, 2017).

umerous techniques such as absorption (Yan et al., 2014a; Dave et al.,

011), vacuum pressure swing adsorption (VPSA) (Nikolaidis et al.,

016), cryogenic distillation (Tuinier et al., 2010), membrane separation

Rezakazemi et al., 2014, 2011) have been extensively reported in the

∗ Corresponding author at: School of Chemistry and Chemical Engineerin∗∗ Corresponding author.

E-mail addresses: zhienzhang@hotmail.com, zhienzhang@cquezakazemi@shahroodut.ac.ir (M. Rezakazemi), zhangwenxiang@hanghx@cqut.edu.cn (H. Chang), hengjunq@cqut.edu.cn (X. Quan).1 These authors share the first authorship of this paper.ttps://doi.org/10.1016/j.cherd.2017.11.024263-8762/© 2017 Institution of Chemical Engineers. Published by Elsev

gas separation fields (Zhang et al., 2018; Razavi et al., 2016; Rezakazemi

et al., 2012). Among them, a new method of membrane gas absorp-

tion combines the advantages of conventional chemical absorption

and membrane separation, which offers small installment space and

device, and separately control of the gas and liquid phases. It also

g, Chongqing University of Technology, Chongqing 400054, China.

t.edu.cn (Z. Zhang), chenfeng710828@163.com (F. Chen),gdut.edu.cn (W. Zhang), lucunfang@cqut.edu.cn (C. Lu),

ier B.V. All rights reserved.

376 Chemical Engineering Research and Design 1 3 1 ( 2 0 1 8 ) 375–384

Nomenclature

SymbolsC Concentration (mol/L)D Diffusion coefficient (m2/s)e Relative errorH Henry’s coefficient (mol/mol)k Reaction rate constant (m3/(mol s))L Effective membrane length (cm)m Number of experimental datan Number of hollow fibersr Radius (mm)Ri Reaction rate (mol/(m3 s))S Effective membrane area (m2)T Temperature (K)V Velocity (m/s)

Greek� Packing densityε Porosity (%)� Absorption efficiency (%)� Tortuosity

Subscriptab Absorbentg Gasin Gas inletout Gas outletp Membrane pores

AbbreviationsAMP 2-Amino-2-methyl-1-propanolENRTL Electrolyte nonrandom two-liquidMDEA MethyldiethanolamineMEA MonoethanolamineMNPZ N-NitrosopiperazinePP PolypropylenePZ PiperazineVPSA Vacuum pressure swing adsorption

in the tube side, the governing differential equation for any

ing, etc. existing in the absorption approach while using an absorber

(Zhang, 2016).

The choice of the suitable absorbents is a major factor for the mem-

brane gas absorption process. The common CO2 absorption solutions

include H2O, NaOH, Na2CO3, and amine solvents (Zhang et al., 2014a,e).

Table 1 demonstrates the usage of Piperazine (PZ) and its activated solu-

tions for CO2 absorption using membrane technologies. As illustrated

in this table, chemical absorption or membrane gas absorption method

is mainly used in the CO2-PZ system. PZ is extensively used as an acti-

vator into the solutions for CO2 absorption rather than a single solvent.

Especially, PZ activated aqueous solutions show better CO2 absorption

performance than the base solutions.

PZ (C4H10N2) is a cyclic diamine with two secondary amino groups

and has a high CO2 reaction rate constant. So far, a lot of works

focusing on its activated solutions performance for CO2 capture using

membrane techniques have been extensively reported. In terms of

alkanolamines, PZ was successfully applied in the improvements of the

single alkanolamine performance. Lin et al. (Lin et al., 2009) compared

the CO2 absorption performance of AMP/PZ and MDEA/PZ in a hol-

low fiber membrane contactor. When using AMP/PZ as the absorbent

and reducing the wetting ratio of the membrane, the CO2 absorption

flux and the membrane durability increased for the plasma-modified

membrane. Similar results have also been obtained in their previous

study (Lin et al., 2008). In comparison with using AMP as the activa-

tor, activator PZ showed better mass-transfer improvements into the

MDEA aqueous solutions (Lu et al., 2007). Razavi et al. (2013) numeri-

cally observed the CO2 transport process with AMP/PZ solutions inside

a membrane contactor. It was reported that an increment in PZ concen-

tration increased the CO2 absorption performance. For a base solution

of carbonates, Oexmann and Kather (2009) made a thermodynamic and

economic comparison of K2CO3/PZ and MEA/PZ capturing CO2 from

the gas mixture. Cullinane and Rochelle (2004) conducted a thermo-

dynamics study of the K2CO3-PZ-CO2 system based on the electrolyte

nonrandom two-liquid (ENRTL) theory. Thus, PZ used as an activator

showed an excellent performance of CO2 absorption enhancement.

The studies focusing on CO2-PZ system are helpful to understand

the CO2 absorption behaviors of the PZ activated solutions. However,

there are only a few studies on investigations of the solubility, kinet-

ics, and removal efficiency between CO2 and PZ (Bishnoi and Rochelle,

2000; Derks et al., 2006; Sun et al., 2005). Bishnoi and Rochelle (2000)

experimentally investigated the CO2 absorption process with PZ solu-

tions at temperatures from 298 to 333 K. The equilibrium solubility and

the mass-transfer and chemical kinetics characteristics were obtained.

Moreover, Derks et al. (2006) observed the CO2 absorption rates into PZ

solutions with various temperatures, PZ concentrations, and CO2 par-

tial pressures. It was indicated that the second-order rate constant was

0.28 m3/(mol s). So far, there is no reported experimental and theoret-

ical work using PZ as the stand-alone absorbent for CO2 absorption in

the membrane contactor.

The aim of this research is the investigations of CO2 absorption

into PZ solutions in a hollow fiber membrane contactor. The gas and

liquid process is simulated in a 2D model. The governing equations

in the calculations are solved by finite element method (FEM) method

at the given boundary conditions. Moreover, the modeling predictions

are validated with the experimental data to guarantee the accuracy

and reliability of the established model. The effects of fluid velocity,

CO2 content, absorbent concentration, the number of contactors, mem-

brane thickness, and fiber radius on CO2 absorption are systematically

investigated and analyzed. Finally, a parametric study of the system is

carried out to find out the most important influencing factor and the

optimum operating conditions.

2. Mathematical modeling

In this case, the simulated flue gas containing 16% CO2

and 84% N2 flows in the shell side. PZ solution used asthe absorbent enter counter currently into the tube side. Asdepicted in Fig. 1, under the non-wetted conditions the gasmixture enters into the shell side and passes through themembrane pores. Then, CO2 reacts with the absorbent withinthe hollow fibers and the rich absorbent (i.e., the absorbed CO2

solution) flows out at the outlet of the tube (z = L). In the mean-time, remaining N2 flows out through the shell at z = 0. Thefollowing assumptions are made in the developed model:

(1) The CO2-absorbent-membrane absorption system isisothermal and steady-state.

(2) The absorbent in the tube is a fully developed laminar flow.(3) The membrane has a uniform thickness and pore size.(4) The solute follows Henry’s Law in the gas and liquid inter-

face.(5) Ideal gas behavior.

Based on the assumptions above, the governing equationsfor all three domains are shown as follows.

2.1. Tube

As the chemical reaction between CO2 and absorbent occurs

species could be written as (Wang et al., 2017; Zhang et al.,

Ch

emica

l En

gin

eering

Resea

rch a

nd

Desig

n

1 3

1

( 2

0 1

8 )

375–384

377

Table 1 – Summary of use of PZ and its blended solvents for CO2 absorption.

Absorbent type Gas composition Method Note Ref.

PZ Pure CO2 Absorption/wetted wallcolumn

The solubility and kinetics of CO2 into PZaqueous solutions were observed.

Bishnoi and Rochelle (2000)

PZ Pure CO2 Absorption/stirred cell The second-order kinetic rate constant ofCO2-PZ was reported.

Derks et al. (2006)

PZ Pure CO2 Absorption/wetted wallcolumn

A coupled masstransfer-kinetics-equilibriummathematical model was proposed forcarbon capture.

Samanta and Bandyopadhyay (2007)

PZ 3.9% or 13.25% CO2 influe gas

Absorption/packedcolumn

8 mol/L PZ was proved to be a promisingalternative solvent for theenergy-efficient CO2 capture.

Gaspar et al. (2016)

PZ, DEA/PZ CO2:N2 = 14%:86% Membrane gasabsorption/membranecontactor

DEA/PZ showed the best CO2 absorptionperformance.

Zhang (2016)

2-Amino-2-methyl-1-propanol (AMP)/PZ,methyldiethanolamine(MDEA)/PZ

CO2:N2 = 1–15%:balance Membrane gasabsorption/membranecontactor

PZ and alkanolamines mixed solutionswere experimentally investigated in aplasma-treated polypropylene (PP)membrane contactor.

Lin et al. (2009)

MDEA/PZ CO2:CH4:othergases = 10%:84%:balance

Absorption/absorber The influence of PZ in the absorbents wasexamined for CO2 removal from naturalgas.

Ibrahim et al. (2014)

Monoethanolamine(MEA)/PZ

Pure CO2 Absorption/wetted wallcolumn

The CO2 absorption rate and solubility inMEA/PZ/H2O were measured.

Dang and Rochelle (2003)

N-Nitrosopiperazine(MNPZ)

Pure CO2 Absorption/absorber MNPZ was formed from the reaction of PZand nitrite in the process of CO2

absorption.

Goldman et al. (2013)

Potassium carbonate(K2CO3)/PZ

13% CO2 in flue gas Absorption/absorber A simple absorber system of CO2 andK2CO3/PZ was evaluated.

Plaza et al. (2010)

378 Chemical Engineering Research and Design 1 3 1 ( 2 0 1 8 ) 375–384

nspo

Fig. 1 – Schematic of gas tra

2014b):

Di-tube

[∂Ci-tube

∂r+ 1

r

∂Ci-tube

∂r+ ∂2Ci-tube

∂z2

]+ Ri = Vz-tube

∂Ci-tube

∂z

(1)

where i denotes CO2 or absorbent, D, C, and V are respec-tively the diffusion coefficient, concentration, and velocity ofthe species, and Ri is the reaction rate between the gas andliquid phases. The velocity distribution is assumed to followNewtonian laminar flow due to a low Reynolds number (Birdet al., 2002):

Vz-tube = 2V̄tube

[1 −

(r

r1

)2]

(2)

where V̄ and r1 represent the average velocity of the speciesand the inner fiber radius, respectively.

2.2. Membrane

Only gas phase is filled in the membrane pores at the non-wetted conditions. Thus, the governing differential equationin the membrane section could be expressed by:

DCO2-mem[∂CCO2-mem

∂r+

1r

∂CCO2-mem

∂r+

∂2CCO2-mem

∂z2]=0 (3)

The diffusion coefficient of CO2 in the membrane sec-tion considering the impacts of the membrane tortuosity andporosity is estimated by (Faiz et al., 2014):

DCO2−mem = ε

�DCO2−shell (4)

where ε and � denote respectively the porosity and tortuosityof the membrane.

rt inside the hollow fibers.

2.3. Shell

Flus gas containing N2 and CO2 flow in the shell domain, sothe governing differential equation could be expressed by:

DCO2-shell[∂CCO2-shell

∂r+

1r

∂CCO2-shell

∂r+

∂2CCO2-shell

∂z2]=Vz-shell

∂CCO2-shell

∂z(5)

The velocity profile in this section can be estimated by Hap-pel’s free-surface model (Happel, 1959):

Vz-shell=2V̄shell[1 − (r2

r3)2][

(r/r3)2 − (r2/r3)2+2ln(r2/r)

3+(r2/r3)4 − 4(r2/r3)2+4ln(r2/r3)] (6)

where r2 is the outer radius of fibers, and r3 denotes Happelradius which could be calculated as (Shirazian et al., 2012a):

r3 = r2

√1/� (7)

in which � is the packing density of the membrane module.The initial and boundary conditions for solving the differentialgoverning equations are given in Table 2.

3. Reaction scheme

For the absorption of CO2 into aqueous solutions, there arenormally two steps including the CO2 hydration and the bicar-bonate formation (Pinsent et al., 1956). Due to a very lowreaction rate, the first step is neglected. The second step ofthe bicarbonate formation could be expressed as:

CO2 + OH−↔HCO−3 (8)

The forward reaction can be calculated by:

RCO2−OH− = kOH− CCO2 COH− (9)

The reaction rate constant for CO2 hydration can be writtenas:

kOH− = 10(13.635− 2895T )

1000(10)

Chemical Engineering Research and Design 1 3 1 ( 2 0 1 8 ) 375–384 379

Table 2 – Initial and boundary conditions for the governing differential equations in three domains.

Position Tube Membrane (Non-wetted mode) Shell

z = 0 Cab-tube = Cab,0 Insulation∂CCO2-shell

∂r = 0z = L Convective flux Insulation CCO2-shell = CCO2 ,0

r = 0∂Cab-tube

∂r = 0 – –

r = r1 CCO2−tube = m ∗ CCO2−mem CCO2-mem = CCO2-tubem –

r = r2 – CCO2-mem = CCO2-shell CCO2−shell = CCO2−mem

Table 3 – The parameters and conditions of theCO2-PZ-membrane system.

Name Symbols Value UnitMembrane material Polypropylene (PP)

Effective membrane length L 920 cmMembrane contactor diameter D 90 mmNumber of hollow fibers n 7000Effective membrane area S 8.6 m2

Inner membrane diameter d1 0.3 mmOuter membrane diameter d2 0.4 mmMembrane pore size dp 0.1–0.2 �mMembrane porosity ε 45 %Membrane tortuosity � 2

w

s

R

wo

k

4

TswbSosliaSuw

5

5

Imw

�

Fig. 2 – Comparisons of experimental and modeling results

efficiency of the experimental and model results decreased

here T represents the system temperature.The overall reaction rate of CO2 absorption into PZ aqueous

olutions can be written as (Sun et al., 2005):

ov = RCO2−PZ + RCO2−OH− = k2,PZCCO2 CPZ + kOH− CCO2 COH− (11)

here k2,PZ denotes the second-order reaction rate constantf PZ and CO2 which can be determined by:

2,PZ = 4.49 × 109exp

(−5712

T

)(12)

. Numerical solution

he governing differential equations in three domains wereolved by FEM method (Li et al., 2017). The governing equationshich were derived for gas and PZ in three sections with theoundary conditions were solved using the software of COM-OL Multiphysics version 5.2. The parameters and conditionsf the CO2-PZ-membrane system are listed in Table 3. Table 4hows the physical and chemical properties of both gas andiquid phases. UMFPACK solver was used for adaptive mesh-ng and error control that was highly suited for the non-stiffnd stiff non-linear boundary issues (Rezakazemi et al., 2011;hirazian et al., 2012b). 23,211 grids for the mesh analysis weresed in this model. The accuracy of the simulation procedureas validated in our previous work (Zhang et al., 2014d).

. Results and discussion

.1. Model validation

n order to compare the simulation results and the experi-ental data, the CO2 absorption efficiency is used as an index,hich can be obtained from the equation below:

=(

1 − Cout)

× 100% (13)

Cin

at different absorbent velocities (Vg = 0.849 m/s, T = 293 K).

in which � is CO2 absorption efficiency. Cout is calculated byintegral at z = 0:

Cout =∫∫

z=0C (r) dA∫∫

z=0dA

(14)

Figs. 2 and 4 are the modeling verifications in comparisonwith the experimental results using 0.5 mol/L PZ solution asabsorbent. The experimental data was reported in previouswork (Zhang, 2016). The root mean-square error (rRMSE) canbe evaluated by:

rRMSE(%) = 100

√1m

∑e2

i(15)

where m is the number of experimental data used for theverifications, and ei denotes the relative error.

The effect of absorbent velocity (0.0142–0.0849 m/s) on CO2

absorption is illustrated in Fig. 2. The rRMSE value for this casewas 1.10%. As the absorbent velocity increased from 0.0142 to0.0849 m/s, the experimental and modeling results of the CO2

absorption efficiency increased 13.3% and 13.8%, respectively.This is due to that increasing the absorbent flow rate increasedthe mass-transfer driving force and reduced the membranemass-transfer resistance. Fig. 3 illustrates 3D and 2D plot of PZconcentration profile in the tube side at a given gas velocityof 0.849 m/s, an absorbent velocity of 0.0425 m/s and 293 K. Itwas obvious that the PZ concentration in the tube along theaxial direction decreased from 0.5 mol/L to around 0.05 mol/Ldue to the continuous chemical reaction in the tube side.

Fig. 4 indicates the effect of gas velocity (0.283–1.415 m/s)on the absorption of CO2. The rRMSE value for this case was8.86%. As illustrated in this figure, CO2 absorption significantlydecreased with an increase of gas velocity. The CO2 absorption

respectively 38.5% and 46.6%. This phenomenon is attributed

380 Chemical Engineering Research and Design 1 3 1 ( 2 0 1 8 ) 375–384

Table 4 – The physicochemical properties of CO2 and PZ used in this study (Bishnoi and Rochelle, 2000; Sun et al., 2005;Yan et al., 2014b).

Name Symbols Value Unit

CO2 diffusion coefficient in tube CCO2-tube 1.7 × 10−9 m2/sCO2 diffusion coefficient in shell CCO2-shell 1.8 × 10−5 m2/sPZ diffusion coefficient in tube CPZ-tube 1.05 × 10−9 m2/sDimensionless Henry’s coefficient of CO2 and PZ H 3.59 × 10−7 mol/mol

Fig. 3 – PZ concentration profile in the tube domain (Vg = 0.849 m/s, Vab = 0.0425 m/s, T = 293 K).

Fig. 4 – Comparisons of experimental and modeling resultsat different gas velocities (V = 0.0425 m/s, T = 293 K).

Fig. 5 – Effect of CO2 content in flue gas on CO2 absorption(Vg = 0.849 m/s, Vab = 0.0425 m/s, T = 293 K).

ab

to the decrease of the gas residence time inside the membranecontactor. Thus, CO2 did not fully react with the absorbent inthe tube and then flowed out at the outlet of the shell side.

5.2. Effect of CO2 content

Generally, CO2 content in flue gas is around 10–20%. Fig. 5 indi-cates the impacts of CO2 percent in the gas mixture on CO2

removal. It was obvious that CO2 absorption efficiency dra-matically decreased with increasing the CO2 content in themixed gases. As observed in this figure, when the CO2 con-tent was below 12%, the complete removal (� > 95%) of CO2

was achieved. In addition, the efficiency of CO2 absorptiondecreased to 61.9% with a 20% CO2 in the feed gas. This wasdue to that with a high CO2 content and constant liquid prop-erties, CO2 could not be fully absorbed by the absorbent, whichfinally deteriorated the absorption performance.

Chemical Engineering Research and Design 1 3 1 ( 2 0 1 8 ) 375–384 381

Fig. 6 – Effect of absorbent concentration on CO2 absorption(Vg = 0.849 m/s, Vab = 0.0425 m/s, T = 293 K).

Fig. 7 – (a) CO2 absorption efficiency and (b) outlet CO2

concentration with various numbers of membranecontactors (V = 1.415 m/s, V = 0.0425 m/s, T = 293 K).

5

T1FoWrteumabi

5

Ttt

Fig. 8 – Effect of inner hollow fiber radius on CO2

absorption (Vg = 0.849 m/s, Vab = 0.0425 m/s, T = 293 K).

Fig. 9 – Effect of effective membrane length on CO2

increased from 56.3 to 88.3%. This was due to that increasing

g ab

.3. Effect of initial absorbent concentration

he influence of PZ concentration in the range from 0.1 to.0 mol/L on the absorption process of CO2 is depicted inig. 6. As shown in this figure, CO2 absorption was obvi-usly enhanced with an increment in the concentration of PZ.hen the PZ concentration was low (below 0.5 mol/L), the CO2

emoval efficiency was poor (lower than 60%). As increasinghe absorbent concentration to 1.0 mol/L, the CO2 absorptionfficiency reached 100% for this case. However, a high liq-id concentration could result in irreversible damages to theembrane materials and corrosion problems. Thus, optimal

bsorbent concentrations should be decided by a compromiseetween the CO2 absorption efficiency and the mentioned

ssues.

.4. Effect of serial membrane contactors

he serial connection of membrane contactors is an alterna-ive way to improve the poor CO absorption. Fig. 7 compares

2

he CO2 absorption using a single membrane contactor or

absorption (Vg = 0.849 m/s, Vab = 0.0425 m/s, T = 293 K).

serial membrane contactors. As illustrated in Fig. 7, at theidentical operating conditions, serial membrane contactorsprovide better CO2 removal compared with a single contactor.The CO2 absorption efficiency of contactors in series sub-stantially increased 45.6%, and the outlet CO2 concentrationdecreased to 0.14% (two contactors) from 6.60% (one contac-tor). Because two membrane contactors in series offered moregas and liquid contact areas owing to the increase of the effec-tive membrane length.

5.5. Effect of inner fiber radius

The inner fiber radius employed in this section ranged from100 to 200 �m, whereas the membrane thickness was a con-stant. Fig. 8 shows the positive effect of the inner fiber radiuson CO2 absorption. The model prediction results confirmedthat the absorption efficiency of CO2 increased by increasingthe fiber radius. Its value substantially increased from 72.2 to90.2%. This is because a bigger inner fiber provided a biggermass-transfer area that promoted the CO2 absorption processin the membrane contactor. A similar phenomenon has beenreported in previous studies (Zhang et al., 2014c).

5.6. Effect of effective membrane length

The effective hollow fiber membrane lengths of 200, 400,600, 800 and 1000 cm were investigated. At the given operat-ing conditions, the CO2 absorption efficiency with differentmembrane lengths is depicted in Fig. 9. It was clearly foundthat an increase in the membrane length improved the CO2

absorption efficiency. As the membrane length increased from200 to 1000 cm, the CO2 absorption efficiency correspondingly

the length of the membrane increased the gas and liquid con-

382 Chemical Engineering Research and Design 1 3 1 ( 2 0 1 8 ) 375–384

Fig. 10 – Effect of membrane thickness on CO2 absorption(Vg = 0.849 m/s, Vab = 0.0425 m/s, T = 293 K).

tact area, which consequently accelerated the mass transferof CO2 in the membrane.

5.7. Effect of membrane thickness

The influence of the hollow fiber membrane thicknesses(10–80 �m) on CO2 removal from flue gas is shown in Fig. 10.As demonstrated in this figure, the CO2 absorption efficiencygradually reduced to 66.0 from 100.0% as the membrane thick-ness varied to 100 from 10 �m. A thicker membrane had aworse CO2 removal performance in comparison with a thinnermembrane. The results could be explained by the increase inthe mass-transfer resistance in the membrane domain with

increasing the thickness of the membrane. Therefore, the CO2

absorption process was deteriorated.

Fig. 11 – 3D response surface plot for the effect of gas and liquidand gas velocity, (b) absorbent concentration and velocity, and (c

conditions on CO2 absorption. (a) Absorbent concentration) absorbent and gas velocities.

Chemical Engineering Research and Design 1 3 1 ( 2 0 1 8 ) 375–384 383

5

Ii(cmcttsvaPvnpca

6

AidvtvTwdwsciapctrfsfm

A

WFn(UoaFaSM

R

B

.8. Parametric study

n this section, a parametric study of the operating conditionsncluding gas velocity (0.283–1.415 m/s), absorbent velocity0.0142–0.0849 m/s), CO2 content (10–20%), and absorbent con-entration (0.1–1.0 mol/L) was carried out to identify theost important influencing factor and the best operating

onditions. Fig. 11 shows a 3D response surface plot forhe effect of gas and absorbent parameters on CO2 absorp-ion efficiency. The order of the influencing factors for theystem can be sequenced as absorbent concentration > gaselocity > absorbent velocity > CO2 content. In this case, thebsorbent concentration was the topmost factor of the CO2-Z-membrane absorption system. As mentioned above, theariations of the absorbent concentration accelerated sig-ificantly the gas and absorbent reactions process. For theurpose of obtaining better absorption of CO2, the optimumonditions were identified, i.e., 0.28 m/s gas velocity, 0.08 m/sbsorbent velocity, 20% CO2 in gas mixture, and 0.94 mol/L PZ.

. Conclusions

2D model for the CO2-PZ-membrane absorption systemn the non-wetted mode is proposed in this study. The pre-icted results with different gas and absorbent velocities wereerified with the experimental data using PZ aqueous solu-ions as the absorbents. 2D and 3D concentration distributionsividly expressed the changes of PZ concentration in the tube.he simulation results revealed that CO2 absorption improvedhen increasing the absorbent velocity or concentration andecreasing the gas velocity or the CO2 content in flue gas. Itas also indicated that using two membrane contactors in

eries significantly improved the CO2 absorption efficiency asompared with use of one membrane contactor. Moreover, anncrease of the inner fiber radius or the membrane length,s well as, a reduction in the membrane thickness had aositive influence on CO2 absorption. The developed modelould provide more guidelines for the parametric study ofhe membrane gas absorption system. The parametric studyesults help to choose the more suitable conditions in theuture studies. Meanwhile, the CO2-PZ-membrane absorptionystem also helps to understand the mechanisms and the per-ormance of CO2 absorption into PZ-blended solutions in a

embrane contactor.

cknowledgments

e would like to acknowledge the financial support from Openund of Key Laboratory of Low-grade Energy Utilization Tech-ologies and Systems, China, Ministry of Education of China

No. LLEUTS-201708), Scientific Research Fund of Chongqingniversity of Technology, China (No. 2016ZD07), Open Fundf Fujian Province University Key Laboratory of Green Energynd Environment Catalysis, China (No. FJ-GEEC201702), Openund of Fujian Provincial Key Laboratory of Featured Materi-ls in Biochemical Industry, China (No. FJKL FMBI201704) andcientific and Technological Research Program of Chongqingunicipal Education Commission, China (No. KJ1709193).

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