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Modelling of high pressure CO2 absorption using PZ+AMP blended solution in a packed absorption column
Accepted Manuscript Modelling of high pressure CO2 absorption using PZ+AMP blended solution in a packed absorption column N.A.H. Hairul, A.M. Shariff, W.H. Tay, A.M.A.v.d. Mortel, K.K. Lau, L.S. Tan PII: DOI: Reference:
S1383-5866(16)30177-0 http://dx.doi.org/10.1016/j.seppur.2016.04.002 SEPPUR 12944
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
7 January 2016 28 March 2016 2 April 2016
Please cite this article as: N.A.H. Hairul, A.M. Shariff, W.H. Tay, A.M.A.v.d. Mortel, K.K. Lau, L.S. Tan, Modelling of high pressure CO2 absorption using PZ+AMP blended solution in a packed absorption column, Separation and Purification Technology (2016), doi: http://dx.doi.org/10.1016/j.seppur.2016.04.002
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Modelling of high pressure CO2 absorption using PZ+AMP blended solution in a packed absorption column N. A. H. Hairula,b, A. M. Shariffa*, W. H. Taya, A. M. A. v. d. Mortelc, K. K. Laua and L. S. Tana,d a
Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia
b
School of Bioprocess Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia
c
Mechanical Engineering, Eindhoven University of Technology, De Rondom 70, 5612 AP Eindhoven, Netherlands
d
Department of Chemical & Petroleum Engineering, UCSI University, 56000 Kuala Lumpur, Malaysia
*
Corresponding author. Tel.: +60 5 368 7570; Fax: +60 5 365 6176. E-mail address: [email protected]
Abstract: A rate-based steady-state model for CO2 absorption into a PZ+AMP blended solution is presented by taking into account the column hydraulics, mass transfer resistances and chemical reactions. The simulation results were compared with the experimental results 1
of CO2 absorption by a PZ+AMP blended solution in a packed absorption column at low and high CO2 partial pressure conditions. The model predicts CO2 concentration, amine concentration, the chemical enhancement factor, and liquid temperature profiles along the column. The model was in good agreement to predict the CO2 concentration profiles along the column at low CO2 partial pressure. However, it was found that the model needs to be corrected by introducing a correction factor for overall volumetric mass transfer coefficient (Kgae) for the simulation of CO2 concentration profiles along the column at high CO2 partial pressure conditions in the range of 404 – 1616 kPa.
Keywords: CO2 capture, simulation, high CO2 partial pressure, amine solvent, structured packing
1.0
Introduction Natural gas (NG) rich in CO2 is an alternative resource of NG that remains unexplored
due to the existing high CO2 content in NG. As an impurity, CO2 becomes corrosive with the presence of water and causes damage in gas pipelines that transport NG from offshore platforms to onshore gas processing plants. Therefore, CO2 content in NG must be reduced to an acceptable level by CO2 capturing technology before transporting NG through subsea gas pipelines. Among these technologies, CO2 removal by chemical absorption was implemented for decades to be an established technology for natural gas processing and post-combustion CO2 capturing. However, the process was limited to the removal of low CO2 concentrations, which typically were conducted in the range of 1 – 20% CO2. Amine-based absorbents, such as monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA) and di-2-propanolamine (DIPA) were the most widely used absorbents due to their high reactivity
2
with CO2. However, amines have several limitations, such as a low CO2 absorption capacity, being highly corrosive, and require high energy for regeneration. Therefore, the absorbent used for the process needs to be improved in order to capture high CO2 content in NG effectively and economically. An alternative absorbent, such as piperazine (PZ) promoted 2-amino-2-methyl-1propanol (AMP), which has a high potential to be implemented for the purification of CO2 rich NG. This potential solvent offers high reactivity and a high CO2 loading capacity compared to MEA, which was reported by numerous researchers for CO2 absorption at atmospheric conditions [1-5]. A study at high pressure conditions also proved that the performance of this potential solvent was better than MEA [6]. A rate-based steady-state model for CO2 capture by chemical absorption at an atmospheric condition was developed in literatures and validated for low CO2 partial pressure in the range of 1 – 20 kPa [7-15]. However, the validation of the model for CO2 absorption at a high CO2 partial pressure has yet to be reported due to the unavailability/limitations of the performance data at high pressure conditions. In the case of CO2 absorption from CO2-rich NG, it is believed that a significant decrease of total gas flow rate along the column during CO2 absorption will give a significant effect to the process’s performance. Furthermore, some researchers suggested that an effective mass transfer area in the column decreases at high pressure operations due to severe back mixing in the column [16]. Therefore, in order to develop a reliable process model for separating high CO2 concentrations from NG in a packed absorption column, it is crucial to validate the existing model that was developed at a low CO2 partial pressure with the performance data at high CO2 partial pressure conditions. In this study, a rate-based steady-state model was developed using a similar mathematical approach proposed by Kahn et al. [10]. This model implemented the mass transfer correlations and effective mass transfer area for structure packing developed by
3
Rocha et al. [17, 18]. The model took into account the mass transfer resistances in the gas and liquid films and was based on the fast second-order kinetics for CO2-(PZ+AMP) reactions in the liquid film. The heat effects associated with chemical absorption as well as heat loss to the surroundings were taken into account through energy balances in the gas and liquid phases. In order to ensure reliable prediction, correlations for thermodynamics, physical properties, and hydraulic factors were incorporated into the model. The simulation results were compared with the experimental data produced at low CO2 partial pressure (atmospheric) as well as at high CO2 partial pressure in the range of 404 – 1616 kPa as reported in our previous publication [6]. This study validates the suitability of the existing model at a low pressure condition for predicting the process performance of CO2 absorption at high pressure conditions in a packed absorption column. There is the possibility of an inaccurate prediction of the simulated results at a high pressure condition due to several factors, which may lead to the implementation of a correction factor in the developed model.
2.0
Reactive absorption model formulation
2.1
Kinetic reaction of CO2 absorption into PZ+AMP blended solution Piperazine (PZ) is a diamine that reacts with CO2 to produce both single and
dicarbamate products as can be described in the following reaction scheme, R1 and R2 [19]:
CO2 PZ PZCOO- H
(R1)
CO2 PZCOO- PZCOO2 H 2
(R2)
The reaction between CO2 and 2-amino-2-methyl-1-propanol (AMP) in the presence of water forms bicarbonate ions that were usually described by second-order reaction [20, 21]. The overall reaction between AMP and CO2 can be explained through the following equation, R3 [22]:
4
CO2 AMP H 2O AMPH HCO-3
(R3)
The CO2 absorption into a PZ+AMP blended solution can be regarded to be gas absorption accompanied by a second order reaction. The overall reaction rate was expressed in terms of the molar concentration of CO2 and PZ+AMP as shown below:
r CO2 k 2, PZ PZ k 2, AMP AMP
(1)
The second order reaction rate constant, k2 for PZ reaction can be approximated from the correlation by Sun et al. [23], while the correlation reported Camacho et al. [21] was used for AMP reaction: 5712 k2,PZ 4.49 1012 exp T
(2)
8186.9 k2, AMP 4.80 1012 exp T
(3)
2.1.1 Two-film theory The absorption of CO2 in a column can be modelled using the two-film theory. The film is stagnant and the CO2 diffuses from the bulk gas through the gas film to the interface. The reaction would take place at the interface and the products would diffuse to the bulk liquid. In principle, the reaction between a dissolved gas and a solvent depends on the kinetics regime, such as an instantaneous reaction, fast reaction, intermediate rate, slow reaction and infinitely slow reaction [24]. For CO2 - (PZ+AMP) reactions, it is assumed that a fast reaction in the thin film occurred whereby the reaction would only take place within the liquid film only. The CO2 concentration at the interface is hard to predict and cannot be measured, therefore, an overall mass transfer coefficient has to be determined. The equation for an overall volumetric mass transfer coefficient in the gas phase, Kgae is given in Eq. (4): 1 H CO2 K g ae k a g e k l ae E
1
(4)
5
Where K g is the overall mass transfer coefficient in the gas phase, k g is the individual gas phase mass transfer coefficient, k l is the individual liquid phase mass transfer coefficient, ae is the effective mass transfer area of the packing, E is the chemical enhancement factor and
H CO2 is the Henry’s law constant.
2.2
Reaction Model
The source term, rCO2 represents the overall rate of CO2 absorption and is given in Eq. (5) to be the following equation: rCO2 K g ae ( pCO2 pCO ) 2
(5)
Where pCO is the CO2 partial pressure in the gas phase, pCO is the equilibrium CO2 partial 2
2
pressure and pCO2 pCO is the driving force for the gas phase. In this model, it is assumed 2
that the equilibrium CO2 partial pressure is equal to zero. This assumption is appropriated due to the fast chemical absorption whereby the equilibrium CO2 partial pressure is considered to be too small [25-28]. A substitution of Eq. (4) into Eq. (5) gives the following equation: rCO2
pCO2
1 / k a ) ( H g
e
CO2
/ kl ae E
(6)
The chemical enhancement factor (E) for the second-order chemical reaction can be assessed by using a set of equations [29]:
1.35
1 E 1
1.35
1 E 1
1.35
1 E1 1
(7)
C D E 1 Aml Aml bDco l Cco i 2 2
(8)
6
E1
Ha
Ha tanh Ha
(9)
Dco2l k2C Aml ( kl ) 2
(10)
Where C Aml is the molar concentration of amine in the liquid, DAml is the molecular diffusivity of amine in the liquid, b is the stoichiometric factor of the reaction, DCO l is the molecular 2
diffusivity of CO2 in liquid, CCO i is the molar concentration CO2 at the interface and Ha is 2
the chemical reaction parameter, which is also known as the Hatta number. k 2 is the reaction rate given by Eq. (2) and (3).
Eq. (11) to Eq. (22) for the calculation of the hydraulics, mass transfer coefficients and effective mass transfer area are presented in Table 1. The empirical mass transfer correlations for the structured packing developed by Rocha et al. [17, 18] were used in this study to calculate the mass transfer coefficients in the liquid and gas phases as shown in Eq. (11) and (12).
The liquid holdup affects the hydraulic and mass transfer in the process. In terms of the hydraulic aspect, an increase in the velocity of any phase increases the thickness and liquid holdup of the film, which consequently increases the pressure drop [16]. In addition, an increase in the liquid holdup resulted in an increase in interfacial area that gives higher mass transfer rates [16]. The liquid holdup can be calculated using the equations from Rocha et al. [17, 18] as shown in Eq. (15). The effective mass transfer area, ae can be estimated with the use of Eq. (22).
7
Table 1
Hydraulics, mass transfer coefficients and effective mass transfer area correlations for structured packing [17, 18] Correlations
kl 2 kg S DCO2 g
Equation no.
0.9 DCO2l ule
(11)
S
u u S 0.054 ge le g g
u ge
ug (1 hl ) sin
ule
ul hl sin
0.8
g DCO2 g g
0.33
(12)
(13)
(14)
2
3l ul F 3 hl 4 t S l sin( )g eff
Ft 29.12(WeFr )l0.15
1
3
(15)
S 0.359 Rel0.2 0.6 (1 0.93 cos( ))(sin )0.3
cos( ) 5.21110( 16.835 )
g eff
(17)
dp g 1 dz g l l dp dz flood
(18)
dpdry dp dz dz (1 (0.614 71.35S )hl )5
dpdry dz A
(19)
(20)
Au g2 Bu g 0.1775 g
S sin( ) 2
2
,
(16)
B
88.774 g
(21)
S 2 sin( )
ae 0.35Ft a
(22)
8
2.3 Physical and thermodynamic properties The molar concentration of CO2 at the interface, CCO2i can be approximated using Henry’s law as given in Eq. (23).
CCO2i
pyCO2 H CO2
(23)
Henry’s law describes the amount of CO2 that can dissolve in an amine solution. This is directly proportional to the partial pressure of the gas in equilibrium with the liquid. The Henry’s law constant of CO2 can be computed from the correlations using N2O analogy. In N2O analogy, the Henry’s law constant of CO2 in an aqueous amine solution can be calculated from a Henry’s law constant of N2O in the same solvent. The physical solubility can be measured directly because the N2O does not react with aqueous amine solutions. This analogy was based on the similarities in mass, molecular structure, and molecular interaction parameters existing between CO2 and N2O. The Henry’s law constant of CO2 in an aqueous amine solution, H CO2 can be calculated from the Henry’s law constant of N2O in an aqueous amine solution, H N2O using the following equation:
H CO2 H CO2 H N2O HN O 2 W
(24)
Table 2 shows the correlations used in this work for the calculation of the Henry’s law constant of CO2 in PZ+AMP blended solution. The correlations for physical properties used in the absorber model are presented in Table 3.
9
Table 2
Correlations used in this work for the calculation of the Henry’s law constant of CO2 in PZ+AMP blended solution Correlations
Ref. [30]
8172.355 H CO2 ,W exp 145.369 19.303 ln(T ) T 9048.596 H N 2 O ,W exp 158.245 20.860 ln(T ) 0.00252T T
[31]
H N 2O H N 2O , w xw H N 2O , AMP x AMP H N 2O , PZ xPZ T 2 exp C123( xPZ x AMP ) A123xw ( xPZ xAMP ) 1 B123
[32]
H N2O, AMP 8589.10 34.799T
[33]
ln( H N2O,PZ ) 4.7017
Table 3
4.9497 103 2.7623 103 T T
[34]
Correlations of physical properties used in the absorber model Correlations
Ref. [35]
Molecular diffusivity of CO2 in an amine aqueous solution
DCO2 ,l DCO2 ,w w l DCO2 ,w 2.35 106 exp( 2119 / T )
Viscosity of the PZ+AMP blended solution
l exp(34.2386 7121.8646 / T 0.036398T )
Diffusivity of amine molecules in the liquid solution
DAml CPZ DPZ C AMP DAMP
0.8
DPZ exp( 13.672 2160.9 / T 0.00019263CPZ ) DAMP exp(13.2922 2285.77 / T 0.000217CAMP )
[36]
[37] [38]
1/ 2
Binary gas diffusivities (Fuller Model)
1 1 10 T M CO M CH 2 4 1/ 3 1/ 3 P VCO2 VCH4 3
DCO2 CH4
[39]
1.75
3
3
VCO2 = 27.46 cm /gmol, VCH4 24.42 cm /gmol) Surface tension of the aqueous 7 wt% PZ + 23 wt% AMP solution
96.24 0.17T
[40]
10
2.3 Absorber model The model consists of a set of differential mass and energy balances with the following assumptions:
Steady-state operations prevail.
The reaction is fast and takes place in the liquid film of the gas-liquid interface system.
Plug-flow of liquid and gas present in absorption column.
The gas and liquid flow rates are constant throughout the column.
Heat loss to the surroundings is taken into account in the energy balance.
Vaporisation of water and AMP+PZ blended solution is not under consideration in the mass conservation equation.
2.4.1
Mass balance The CO2 concentration in the gas phase along the height of the column is calculated by
the following steady state, one-dimensional mass conservation equation:
dN CO2 , g dz
d (GYCO2 ) dz
rCO2
(25)
Where N CO2 is the molar flux of CO2, G is the total gas flow rate per unit cross-sectional area of the column, YCO2 is the mol ratio of CO2 in the gas phase and dz is the incremental height of the column. Fig. 1 shows the schematic of a packed absorption column in an infinitesimal element for mass and energy balances. Eq. (25) is discretised over an incremental height of the column, dz, using a forward difference scheme.
11
Interface Gas
XAm,i TLi
YCO2i TGi
Liquid Gas film
Gas bulk
NCO2
Liquid film
Liquid bulk
TG
NCO2
TL dz
NCO2
NCO2
YCO2
NCO2
XCO2
Gas
Liquid YCO2 i+1 TG i+1
XAm,i+1 TL i+1
Fig. 1 Schematic of a packed absorption column in an infinitesimal element for mass and energy balances.
The amines concentration in the solution can be calculated by performing a mass balance over the incremental height:
X Am ,i X Am ,i1
(YCO2i1 YCO2i )bG L
(26)
Where X Am is the mol ratio of amine in water and L is the molar flow rate of the liquid. 2.4.2
Energy balance
Due to an exothermic reaction between CO2 and PZ+AMP solution, the liquid temperature will increase, which results in heat transfer from the liquid to the gas phase. The energy balance for the gas and liquid phases are given in Eq. (27) and (28): GC pg
LC p l
dTg dz
hg ae (Tg Tl )
(27)
dYCO2 dT dTl GC pg g G C pCO2 Tl To H R H out (Tl 303) dz dz dz
12
(28)
Where hg is the gas phase heat transfer coefficient, C pl , C pg and C pCO are the heat capacities of 2
the liquid phase, gas phase and CO2, respectively. Tg, Tl and To are the temperature of the liquid phase, gas phase and reference temperature, respectively. YCO2 is the mol ratio of CO2. HR is the heat of chemical absorption between CO2 and PZ+AMP blended solution, which is equal to 86002 kJ/kmol [41]. Hout is the heat transfer constant of heat loss through the column wall to the atmosphere.
2.5 Numerical procedure Fig. 2 shows the simplified flowchart for the CO2-(PZ+AMP) simulation model. The physical properties, mass transfer correlations, mass balance and energy balance equations were discretised and implemented in a MATLAB script. A small step size, dz, was chosen in order for the values to be computed at each step. The inlet concentration of PZ+AMP at the top and CO2 at the bottom was known, therefore, an iterative procedure was implemented in order to compute the absorption of CO2 in the absorption column. The procedure started at the top of the column with the known inlet concentration of PZ+AMP and an initial guess of the outlet condition of CO2. The model solved for each step and the resulting CO2 concentration at the bottom was compared with the known inlet CO2 concentration. If the computed value was higher than the CO2 inlet concentration, the procedure would be repeated. If the computed value was too low, the initial CO2 guess was increased with a small step until the CO2 concentration at the bottom was achieved.
13
START
Declare input conditions G, L, CCO2,in, CAm,in, Tg,in, Tl,in
Fit CO2 concentration at the top of the column
Determine physical properties of the gas and liquid components
Calculate effective mass transfer area (ae) and individual mass transfer coefficients (kg and kl)
Calculate chemical enhancement factor (E).
Calculate absorption rate for the segment (rCO2).
Calculate composition of CO2 in the gas phase (yCO2) and composition of amine in the liquid phase (xAm) for the next column segment based on the absorption rate.
Calculate temperature of the gas phase (Tg) and temperature of the liquid phase (Tl) for the next column segment.
Final bottom segment of the column?
No
Yes Adjust CO2 concentration at the top of the column.
No
Compare the calculated CO2 concentration at the bottom with the input value. Yes STOP
Fig. 2 Flowchart for the CO2-amine simulation model.
14
Next segment.
2.6 Source of experimental data Experimental data for CO2 absorption from high concentration CO2 in NG into a PZ+AMP blended solution were taken from our previous work [6]. The inner diameter of the column was 0.046 m, which was packed with 2.04 m of Sulzer metal gauze packing. The specifications of the absorber column and packing material are shown in Table 4. The concentration of CO2 was 40% in NG and the amine composition was 7 wt% PZ + 23 wt% AMP (3.5 kmol/m3). The experiments were conducted at various operating pressures in the range of atmospheric up to 4.04 MPa in a packed absorption column. Therefore, the CO2 partial pressure was in the range of 40 to 1616 kPa. The details of the process flow diagram and experimental procedures can be found in Halim et al. [6, 42]. The CO2 concentration and temperature profiles along the column were recorded at a steady state condition. These experimental data were used to validate the simulation results generated from the model.
Table 4
Specifications of the absorber column and packing material Parameters Material packing Height of packing Diameter of packing Packing surface area per volume, a Cross section area of column Void fraction, ε Corrugation inclination angle, θ Corrugation side length, S
Specifications Stainless steel 2.04 (m) 0.046 (m) approx.500 (m2/m3) 1.67 x 10-3 (m2) approx. 0.90 60° approx. 8.90 (mm)
3.0 Results and Discussion 3.1
Validation of the existing model The model used in this study was adapted from Khan et al. [10] and was validated using
experimental results published by Tontiwachwuthikul et al. [43] for run number T22. The CO2 absorption study was conducted by Tontiwachwuthikul et al. [43] in a pilot-plant 15
absorption column with an internal column diameter of 0.1 m. The column was packed with 6.6 m of 12.7 mm ceramic Berl saddles. The solvent used in their study was 3.0 kmol/m 3 of MEA. The model developed by Khan et al. [10] used the mass transfer correlations and effective mass transfer area for random packing proposed by Onda et al. [44]. Fig. 3 (a) and (b) shows the comparison between the simulated and measured gas phase CO2 concentration as well as MEA concentration profiles along the column height. As reported by Khan et al. [10], the simulation and experimental results were in good agreement for both graphs. 3000
20 18
2500
MEA Concentration (mol/m 3)
14 12 10 8
2
CO concentration (mol%)
16
6 4
2000
1500
1000
500
2 0
0
0
1
2 3 4 5 Distance from the bottom of the column (m)
6
7
0
1
2 3 4 5 Distance from the bottom of the column (m)
6
(b)
(a)
Fig. 3 Comparison between the experimental and simulation results (a) CO2 concentration profile (b) MEA concentration profile for CO2-MEA system at atmospheric condition. (-) Simulation and (x) Experiment data. (Gas flow rate = 14.8 mol/m2.s; 19.1% CO2 in NG; [MEA] = 3.0 kmol/m3; L = 9.5 m3/m2.h; T = 19°C)
3.2
CO2 absorption model using PZ+AMP solution at atmospheric condition Khan et al. [10] developed a rate-based steady-state model for CO2 absorption using
MEA as the solvent. Since this study used a PZ+AMP blended solution, the rate of reaction as well as physical and thermodynamic properties needs to be changed accordingly. Furthermore, suitable mass transfer correlations and effective mass transfer area for the 16
7
structured packing needs to be introduced, which were taken from Rocha et al. [17, 18]. In addition, the model implemented in the current work also considers the heat loss through the wall of the column to the atmosphere. Fig. 4 shows the CO2 concentration profiles along the column height for experimental and simulation results for CO2 - PZ+AMP systems at atmospheric condition. From the figure, it can be seen that the overall agreement between the prediction and experiment is generally good. The mean average error (MAE) of the simulated concentration from the experimental concentration was 3.75%. 45
MAE = 3.75%
2
CO Concentration (mol%)
40
35
30
25
20
15
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
Fig. 4 A comparison between the experimental and simulated CO2 concentration profile for CO2 - PZ+AMP system at an atmospheric condition. (-) Prediction and (x) Experiment data. (Gas flow rate = 33 kmol/m2.h; 40% CO2 in NG; [PZ+AMP] = 30 wt%; L = 3.61 m3/m2.h; T = 30°C)
17
3.3 Comparison between the simulation and experimental results at high pressure conditions The model was further used to predict the performance of CO2 absorption using PZ+AMP blended solution at high pressure conditions in the range of 1.01 to 4.04 MPa by incorporating the physical properties of the gas mixture at high pressure conditions. Fig. 5 (a, b, c, d) shows the comparison between simulation and experimental results at various operating pressures in the range of 1.01 to 4.04 MPa. It can be seen that at high pressure operations, the simulation results using the low CO2 partial pressure model showed a large deviation from the experimental results. Since the low CO2 partial pressure model unable to predict the performance of CO2 absorption at high CO2 partial pressure conditions, the model need to be corrected by introducing a correction factor (F) for overall volumetric mass transfer coefficient (Kgae). The correction factor for each operating pressure was taken by curve fitting method. The same technique can be used to predict the correction factor for other solvents. Fig. 6 shows the correction factor versus CO2 partial pressure in the range of 404 to 1616 kPa. In order to achieve a close agreement between experimental and simulation results, a correction factor (F) for overall volumetric mass transfer coefficient (Kgae) to be a function of
pCO2 was added to the overall rate of CO2 absorption, rCO2 in Eq. (5) as follows: F 5 108 ( pCO2 )2 9 106 pCO2 0.0975
rCO2 FK g ae pCO2
(42) (43)
As shown in Fig. 5 (a, b, c, d), the addition of a correction factor (F) in the model gives a good agreement between the simulation and experimental results with a MAE of less than 7.70%. It was believed that the correction factor was needed to correct the overall rate of
18
reaction due to several factors that were contributed by the high CO2 partial pressure condition.
45
45
Simulation with correction factor, MAE = 3.39%
CO Concentration (mol%)
35
35
30 25 20
2
2
Simulation without correction factor
15
30 25 20
Simulation without correction factor
15 10
10 5
Simulation with correction factor, MAE = 3.35%
40
CO Concentration (mol%)
40
5
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
0
2.5
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
(a)
(b) 45
45
Simulation with correction factor, MAE = 7.13%
40
35
CO Concentration (mol%)
30 25 20
2
2
CO Concentration (mol%)
Simulation with correction factor, MAE = 7.65%
40
35
Simulation without correction factor
15 10 5 0
2.5
30 25 20
10 5 0
0
Simulation without correction factor
15
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
(d)
(c)
Fig. 5 Comparison between the experimental and simulated CO2 concentration profile for CO2 - (PZ+AMP) system at different operating pressure. (-) Prediction and (x) Experiment data. (Gas flow rate = 33 kmol/m2.h; 40% CO2 in NG; [PZ+AMP] = 30 wt%; L = 3.61 m3/m2.h; T = 30°C; (a) P = 1.01 MPa; (b) P = 2.02 MPa; (c) P = 3.03 MPa; (d) P = 4.04 MPa)
19
0.25
Correction Factor
0.2
y = 5E-08x2 + 9E-06x + 0.0975 R² = 0.9948
0.15
0.1
0.05
0 0
200
400
600
800
1000
1200
1400
1600
1800
Pco2 (kPa)
Fig. 6 Correction factor for CO2 absorption at high CO2 partial pressure condition for PZ+AMP blended solution
One of the possible reasons is the significant decrease of total gas flow rate along the column during the absorption process that was not accounted in the low CO2 partial pressure model. Besides that, it was expected that the assumption of the plug flow in the model was no longer valid due to severe back mixing in the column as reported by Gualito et al. [16] at high pressure conditions of up to 27.2 bar. They also suggested that the correction factor due to back mixing could be applied to the effective mass transfer area, ae. However, in this work, the correlation of ae introduced by Gualito does not fit well with our system. Therefore, the correction factor in this study was introduced to the Kgae in general, in order for further research to possibly be conducted in order to investigate the factors that might contribute to this behaviour in the absorption of high CO2 concentration from NG at high pressure conditions.
20
Fig. 7 (a) and (b) show good agreement between the experimental and simulation results for CO2 concentration and liquid temperature profiles along the column for CO2 absorption at an operating pressure of 4.04 MPa. The MAE of the simulated CO2 concentration from the experimental concentration was 7.65% whereas the MAE for the liquid temperature was 0.54%. The exothermic reactions between CO2 and amine resulted in rapid variation of the liquid temperature at the top of the column due to a significant amount of CO2 absorption in this region. The peak temperature that represents the location of the temperature bulge was closed to the top of the column due to a low L/G ratio [45]. The magnitude of the temperature bulge and its location depends on the heat of reaction, L/G ratio, and the location of CO2 absorption into amine occurring [46]. Fig. 7 (c) shows amine concentration profiles along the column height simulated from the model. The amine concentration decreased from the top to the bottom of the column due to continuous chemical absorption along the column until all amines depleted at about 1.25 m from the bottom of the column. Therefore, as can be observed in Fig. 7 (d), the chemical enhancement factor along the column also shows a similar trend with the amine concentration profiles. Thus, this behaviour indicates that the chemical enhancement factor is strongly influence by the availability of the amine molecules to react with CO2 in the column.
21
320
45
MAE = 7.65%
40
316
Liquid Temperature, T (K)
l
30 25 20
2
CO Concentration (mol%)
35
15
314 312 310 308
10
306
5
304
0
MAE = 0.54%
318
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
302 -0.5
0
0.5 1 1.5 Distance from the bottom of the column (m)
(a) 3500
70 Chemical Enhancement Factor, E
80
3000
Amine Concentration (mol/m 3)
2.5
(b)
4000
2500 2000 1500 1000 500 0
2
60 50 40 30 20 10
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
0
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
(c)
2.5
(d)
Fig. 7 (a) CO2 concentration profiles along the column height (b) Liquid temperature profile (c) Amine concentration profiles (d) Chemical enhancement factor along the column height. (-) Prediction and (x) Experiment data. (Gas flow rate = 33 kmol/m2.h; 40% CO2 in NG; [PZ + AMP] = 30 wt%; L = 3.61 m3/m2.h; T = 30°C; P = 4.04 MPa)
3.4
Mass transfer coefficients at various operating pressures
Fig. 8 presents the individual mass transfer coefficient for gas (kg) and liquid (kl) phases at various operating pressures calculated using a Rocha model [17, 18]. It can be observed that kl was almost constant at 6.4 x 10-5 m/s. In Eq. (11), the kl depends on liquid properties that 22
are not significantly affected by pressure. Moreover, the kg decreased with the increasing of operating pressure due to an increase of gas density and the decreased DCO2 g at a higher pressure as shown in Eq. (12). Significant decreases in kg can be observed from 1.12 x 10-3 to
Individual mass transfer coefficient in gas phase (kg)
1.2E-03
7.0E-05
1.1E-03
6.5E-05
1.0E-03 6.0E-05
9.0E-04
kg kl
8.0E-04
5.5E-05
7.0E-04
5.0E-05
6.0E-04 4.5E-05
5.0E-04 4.0E-04
Individual mass transfer coefficient in liquid phase (kl)
5.5 x 10-4 m/s because the operating pressure increased from 1.01 to 4.04 MPa.
4.0E-05 0
1
2
3
4
5
Operating Pressure (MPa)
Fig. 8 Individual mass transfer coefficients at various operating pressures
3.5
Overall rate of CO2 absorption at various operating pressures Fig. 9 shows the simulation results of the corrected overall rate of CO2 absorption
(rCO2) at various operating pressures. Based on this figure, it is apparent that the corrected overall rate of CO2 absorption increased with increasing operating pressures. This is due to a higher mole fraction driving force for separation at a higher pressure condition. Moreover, it can be observed that the upward trend is more obvious from 1.70 to 2.04 m for all pressures due a high concentration of PZ+AMP blended solution at the top of the column. 23
Overall rate of CO2 absorption (rCO2)
1.2E-02 P = 1.01 MPa
1.0E-02
P = 2.02 MPa P = 3.03 MPa
8.0E-03
P = 4.04 MPa
6.0E-03
4.0E-03
2.0E-03
0.0E+00 0
0.5
1
1.5
2
2.5
Distance from the bottom of the column (m)
Fig. 9 Corrected overall rate of CO2 absorption at various operating pressures
4.0 Conclusions A rate-based steady-state model for the absorption of high CO2 concentration from NG using PZ+AMP blended solution was developed and validated with experimental data at atmospheric up to 4.04 MPa operating pressure. A correction factor was introduced to the overall volumetric mass transfer coefficient for the prediction of process performance at high CO2 partial pressure conditions in the range of 404 to 1616 kPa. The simulated CO2 concentration and liquid temperature profiles along the column height were in good agreement with the experimental results.
5.0 Acknowledgement The authors would like to acknowledge the Research Centre for CO2 Capture, Universiti Teknologi PETRONAS (UTP) for research funding, the scholarship support from the Ministry 24
of Higher Education Malaysia and Universiti Malaysia Perlis awarded to HNAH and Sulzer Chemtech Pte Ltd, Winterthur, Switzerland for the sponsored Sulzer metal gauze packing.
Abbreviations a = total packing surface area per unit volume of packing ae = effective mass transfer area A123, B123 and C123 = the ternary fitted parameters
b = stoichiometric factor of the reaction C Aml = molar concentration of amine in the liquid
C AMP = concentration of AMP CCO2 i = molar concentration CO2 at the interface
C pg
= heat capacity of the gas phase
C pl = heat capacity of the liquid phase
C pCO2 = heat
capacity of CO2
C PZ = concentration of PZ dp = pressure drop when the gas is in counter-current flow with the liquid dz dp dry dz
= pressure drop when only gas is flowing in the column
25
dp = pressure drop at which flooding occurs dz flood dz = incremental height of the column DAml = molecular diffusivity of amine in the liquid
DCO2 g = molecular diffusivity of CO2 in gas
DCO2l = molecular diffusivity of
CO2 in liquid
DCO2 ,l = molecular diffusivity of CO2 in amine aqueous solution
DCO2 ,w
= diffusivity of CO2 in water
DCO2 CH4 = binary gas phase diffusivity of CO2 in CH4
DAml = diffusivity of amine molecules in the liquid solution
DAMP = diffusivity correlation of AMP DPZ = diffusivity correlation of PZ E = chemical enhancement factor F = correction factor for overall volumetric mass transfer coefficient
Fr = Froude number Ft = correction factor for total holdup
g eff = effective gravity G = total gas flow rate per unit cross-sectional area of the column
26
hg = gas phase heat transfer coefficient
hl = liquid holdup
Ha = Hatta number H CO2 = Henry’s law constant of CO2 in aqueous amine solution
H CO2 ,W = Henry’s law constant of CO2 in water
H N2O = Henry’s law constant of N2O in aqueous amine solution H N2O = Henry’s law constant of N2O in aqueous ternary amine solvents H N 2 O,W = Henry’s law constant of N2O in water H N2O, AMP Henry’s law constant of N2O in aqueous AMP solution H N2O ,PZ Henry’s law constant of N2O in aqueous PZ solution
HR = heat of chemical absorption between CO2 and PZ+AMP solution Hout = heat transfer constant of heat loss through the column wall to the atmosphere k2 = second order reaction rate constant K g = overall mass transfer coefficient in the gas phase
k g = individual gas phase mass transfer coefficient
k l = individual liquid phase mass transfer coefficient L = molar flow rate of the liquid
27
M = molecular weight N CO2 = molar flux of CO2
p = total pressure of gas phase pCO2 = CO2 partial pressure in the gas phase
pCO 2
= equilibrium CO2 partial pressure
rCO2 = overall rate of CO2 absorption
Re = Reynolds number S = corrugation side length of the packing Tg = temperature of the gas phase Tl = temperature of the liquid phase To = temperature of the reference temperature
u ge = effective gas velocity u le = effective liquid velocity ul = velocity of liquid
We = Weber number X Am = mol ratio of amine in water
x AMP = mol fraction of AMP
xPZ = mol fraction of PZ 28
xw = mol fraction of water yCO2 = mol fraction of CO2 in the gas
YCO2 = mol ratio of CO2 in the gas phase g = viscosity of gas
= void fraction of the packing, = corrugation inclination angle of the packing l = viscosity of PZ+AMP solution w = viscosity of water g = the gas density
l = liquid density
= contact angle between solid and liquid = surface tension
V = the sum of the diffusion volume
6.0 References [1] Y. Artanto, J. Jansen, P. Pearson, G. Puxty, A. Cottrell, E. Meuleman, P. Feron, Pilot-scale evaluation of AMP/PZ to capture CO2 from flue gas of an Australian brown coal–fired power station, Int. J. Greenhouse Gas Control, 20 (2014) 189-195. [2] S.K. Dash, A.N. Samanta, S.S. Bandyopadhyay, Experimental and theoretical investigation of solubility of carbon dioxide in concentrated aqueous solution of 2-amino-2-methyl-1-propanol and piperazine, J. Chem. Thermodyn., 51 (2012) 120-125. [3] S.K. Dash, A. Samanta, A. Nath Samanta, S.S. Bandyopadhyay, Absorption of carbon dioxide in piperazine activated concentrated aqueous 2-amino-2-methyl-1-propanol solvent, Chem. Eng. Sci., 66 (2011) 3223-3233. [4] Z.-Y. Yang, A.N. Soriano, A.R. Caparanga, M.-H. Li, Equilibrium solubility of carbon dioxide in (2-amino2-methyl-1-propanol + piperazine + water), J. Chem. Thermodyn., 42 (2010) 659-665. [5] A. Samanta, S.S. Bandyopadhyay, Absorption of carbon dioxide into aqueous solutions of piperazine activated 2-amino-2-methyl-1-propanol, Chem. Eng. Sci., 64 (2009) 1185-1194. [6] H.N.A. Halim, A.M. Shariff, M.A. Bustam, High pressure CO2 absorption from natural gas using piperazine promoted 2-amino-2-methyl-1-propanol in a packed absorption column, Sep. Purif. Technol., 152 (2015) 87-93. [7] J. Gabrielsen, M.L. Michelsen, E.H. Stenby, G.M. Kontogeorgis, Modeling of CO2 absorber using an AMP solution, AIChE J., 52 (2006) 3443-3451. [8] F.A. Tobiesen, H.F. Svendsen, O. Juliussen, Experimental validation of a rigorous absorber model for CO2 postcombustion capture, AIChE J., 53 (2007) 846-865.
29
[9] A. Aboudheir, P. Tontiwachwuthikul, R. Idem, Rigorous Model for Predicting the Behavior of CO2 Absorption into AMP in Packed-Bed Absorption Columns, Ind. Eng. Chem. Res., 45 (2006) 2553-2557. [10] F.M. Khan, V. Krishnamoorthi, T. Mahmud, Modelling reactive absorption of CO2 in packed columns for post-combustion carbon capture applications, Chem. Eng. Res. Des., 89 (2011) 1600-1608. [11] A. Aroonwilas, A. Chakma, P. Tontiwachwuthikul, A. Veawab, Mathematical modelling of mass-transfer and hydrodynamics in CO2 absorbers packed with structured packings, Chem. Eng. Sci., 58 (2003) 4037-4053. [12] A. Lawal, M. Wang, P. Stephenson, H. Yeung, Dynamic modelling of CO2 absorption for post combustion capture in coal-fired power plants, Fuel, 88 (2009) 2455-2462. [13] L.L. Simon, Y. Elias, G. Puxty, Y. Artanto, K. Hungerbuhler, Rate based modeling and validation of a carbon-dioxide pilot plant absorbtion column operating on monoethanolamine, Chem. Eng. Res. Des., 89 (2011) 1684-1692. [14] J. Gáspár, A.-M. Cormoş, Dynamic modeling and validation of absorber and desorber columns for postcombustion CO2 capture, Computers Chem. Eng., 35 (2011) 2044-2052. [15] G.B. Liu, K.T. Yu, X.G. Yuan, C.J. Liu, Q.C. Guo, Simulations of chemical absorption in pilot-scale and industrial-scale packed columns by computational mass transfer, Chem. Eng. Sci., 61 (2006) 6511-6529. [16] J.J. Gualito, F.J. Cerino, J.C. Cardenas, J.A. Rocha, Design Method for Distillation Columns Filled with Metallic, Ceramic, or Plastic Structured Packings, Ind. Eng. Chem. Res., 36 (1997) 1747-1757. [17] J.A. Rocha, J.L. Bravo, J.R. Fair, Distillation columns containing structured packings: a comprehensive model for their performance. 1. Hydraulic models, Ind. Eng. Chem. Res., 32 (1993) 641-651. [18] J.A. Rocha, J.L. Bravo, J.R. Fair, Distillation Columns Containing Structured Packings: A Comprehensive Model for Their Performance. 2. Mass-Transfer Model, Ind. Eng. Chem. Res., 35 (1996) 1660-1667. [19] G. Puxty, R. Rowland, Modeling CO2 Mass Transfer in Amine Mixtures: PZ-AMP and PZ-MDEA, Environ. Sci. Technol., 45 (2011) 2398-2405. [20] F. Bougie, M.C. Iliuta, Sterically Hindered Amine-Based Absorbents for the Removal of CO2 from Gas Streams, J. Chem. Eng. Data, 57 (2012) 635-669. [21] F. Camacho, S. Sánchez, R. Pacheco, A. Sánchez, M.D. La Rubia, Thermal effects of CO2 absorption in aqueous solutions of 2-amino-2-methyl-1-propanol, AIChE J., 51 (2005) 2769-2777. [22] M. Afkhamipour, M. Mofarahi, Sensitivity analysis of the rate-based CO2 absorber model using amine solutions (MEA, MDEA and AMP) in packed columns, Int. J. Greenhouse Gas Control, 25 (2014) 9-22. [23] W.-C. Sun, C.-B. Yong, M.-H. Li, Kinetics of the absorption of carbon dioxide into mixed aqueous solutions of 2-amino-2-methyl-l-propanol and piperazine, Chem. Eng. Sci., 60 (2005) 503-516. [24] O. Levenspiel, Chemical Reaction Engineering, third ed., John Wiley & Sons, Inc., Danvers, 1999. [25] W.-J. Choi, J.-B. Seo, S.-Y. Jang, J.-H. Jung, K.-J. Oh, Removal characteristics of CO2 using aqueous MEA/AMP solutions in the absorption and regeneration process, J. Environ. Sci., 21 (2009) 907-913. [26] Q. Zeng, Y. Guo, Z. Niu, W. Lin, Mass Transfer Coefficients for CO2 Absorption into Aqueous Ammonia Solution Using a Packed Column, Ind. Eng. Chem. Res., 50 (2011) 10168-10175. [27] A. Naami, M. Edali, T. Sema, R. Idem, P. Tontiwachwuthikul, Mass Transfer Performance of CO2 Absorption into Aqueous Solutions of 4-Diethylamino-2-butanol, Monoethanolamine, and NMethyldiethanolamine, Ind. Eng. Chem. Res., 51 (2012) 6470-6479. [28] A. Aroonwilas, P. Tontiwachwuthikul, High-efficiency structured packing for CO2 separation using 2amino-2-methyl-1-propanol (AMP), Sep. Purif. Techol., 12 (1997) 67-79. [29] R.M. Wellek, R.J. Brunson, F.H. Law, Enhancement factors for gas-absorption with second-order irreversible chemical reaction, Can. J. Chem. Eng., 56 (1978) 181-186. [30] J.J. Carroll, J.D. Slupsky, A.E. Mather, The solubility of carbon dioxide in water at low pressure, J. Phys. Chem. Ref. Data, 20 (1991) 1201-1209. [31] A.K. Saha, S.S. Bandyopadhyay, A.K. Biswas, Solubility and diffusivity of nitrous oxide and carbon dioxide in aqueous solutions of 2-amino-2-methyl-1-propanol, J. Chem. Eng. Data, 38 (1993) 78-82. [32] A. Penttilä, C. Dell’Era, P. Uusi-Kyyny, V. Alopaeus, The Henry's law constant of N2O and CO2 in aqueous binary and ternary amine solutions (MEA, DEA, DIPA, MDEA, and AMP), Fluid Phase Equilibr., 311 (2011) 59-66. [33] Y.W. Wang, S. Xu, F.D. Otto, A.E. Mather, Solubility of N2O in alkanolamines and in mixed solvents, Chem. Eng. J., 48 (1992) 31-40. [34] A. Samanta, S. Roy, S.S. Bandyopadhyay, Physical Solubility and Diffusivity of N2O and CO2 in Aqueous Solutions of Piperazine and (N-Methyldiethanolamine + Piperazine), J. Chem. Eng. Data, 52 (2007) 1381-1385. [35] G.F. Versteeg, W.P.M. van Swaaij, On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions—I. Primary and secondary amines, Chem. Eng. Sci., 43 (1988) 573-585. [36] A. Samanta, S.S. Bandyopadhyay, Density and Viscosity of Aqueous Solutions of Piperazine and (2Amino-2-methyl-1-propanol + Piperazine) from 298 to 333 K, J. Chem. Eng. Data, 51 (2006) 467-470.
30
[37] P.W.J. Derks, E.S. Hamborg, J.A. Hogendoorn, J.P.M. Niederer, G.F. Versteeg, Densities, Viscosities, and Liquid Diffusivities in Aqueous Piperazine and Aqueous (Piperazine + N-Methyldiethanolamine) Solutions, J. Chem. Eng. Data, 53 (2008) 1179-1185. [38] L.-C. Chang, T.-I. Lin, M.-H. Li, Mutual Diffusion Coefficients of Some Aqueous Alkanolamines Solutions, J. Chem. Eng. Data, 50 (2005) 77-84. [39] R.C. Reid, J.M. Prausnitz, T.K. Sherwood, The properties of gases and liquids, 3rd ed. ed., McGraw-Hill, New York, 1977. [40] G. Murshid, A.M. Shariff, L.K. Keong, M.A. Bustam, Physical Properties of Aqueous Solutions of Piperazine and (2-Amino-2-methyl-1-propanol + Piperazine) from (298.15 to 333.15) K, J. Chem. Eng. Data, 56 (2011) 2660-2663. [41] P. Brúder, A. Grimstvedt, T. Mejdell, H.F. Svendsen, CO2 capture into aqueous solutions of piperazine activated 2-amino-2-methyl-1-propanol, Chem. Eng. Sci., 66 (2011) 6193-6198. [42] H.N. Abdul Halim, A.M. Shariff, L.S. Tan, M.A. Bustam, Mass transfer performance of CO2 absorption from natural gas using monoethanolamine (MEA) in high pressure operations, Ind. Eng. Chem. Res., 54 (2015) 1675-1680. [43] P. Tontiwachwuthikul, A. Meisen, C.J. Lim, CO2 absorption by NaOH, monoethanolamine and 2-amino-2methyl-1-propanol solutions in a packed column, Chem. Eng. Sci., 47 (1992) 381-390. [44] K. Onda, H. Takeuchi, Y. Okumoto, Mass transfer coefficients between gas and liquid phases in packed column, J. Chem. Eng. Japan, 1 (1968) 56-61. [45] L. Faramarzi, G.M. Kontogeorgis, M.L. Michelsen, K. Thomsen, E.H. Stenby, Absorber Model for CO2 Capture by Monoethanolamine, Ind. Eng. Chem. Res., 49 (2010) 3751-3759. [46] H.M. Kvamsdal, G.T. Rochelle, Effects of the Temperature Bulge in CO2 Absorption from Flue Gas by Aqueous Monoethanolamine, Ind. Eng. Chem. Res., 47 (2008) 867-875.
31
Highlights A rate-based steady-state model at low CO2 partial pressure conditions was validated with the data at high pressure conditions The model needs to be corrected by introducing a correction factor for overall volumetric mass transfer coefficient (Kgae) at high CO2 partial pressure conditions.
32
S1383-5866(16)30177-0 http://dx.doi.org/10.1016/j.seppur.2016.04.002 SEPPUR 12944
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
7 January 2016 28 March 2016 2 April 2016
Please cite this article as: N.A.H. Hairul, A.M. Shariff, W.H. Tay, A.M.A.v.d. Mortel, K.K. Lau, L.S. Tan, Modelling of high pressure CO2 absorption using PZ+AMP blended solution in a packed absorption column, Separation and Purification Technology (2016), doi: http://dx.doi.org/10.1016/j.seppur.2016.04.002
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Modelling of high pressure CO2 absorption using PZ+AMP blended solution in a packed absorption column N. A. H. Hairula,b, A. M. Shariffa*, W. H. Taya, A. M. A. v. d. Mortelc, K. K. Laua and L. S. Tana,d a
Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia
b
School of Bioprocess Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia
c
Mechanical Engineering, Eindhoven University of Technology, De Rondom 70, 5612 AP Eindhoven, Netherlands
d
Department of Chemical & Petroleum Engineering, UCSI University, 56000 Kuala Lumpur, Malaysia
*
Corresponding author. Tel.: +60 5 368 7570; Fax: +60 5 365 6176. E-mail address: [email protected]
Abstract: A rate-based steady-state model for CO2 absorption into a PZ+AMP blended solution is presented by taking into account the column hydraulics, mass transfer resistances and chemical reactions. The simulation results were compared with the experimental results 1
of CO2 absorption by a PZ+AMP blended solution in a packed absorption column at low and high CO2 partial pressure conditions. The model predicts CO2 concentration, amine concentration, the chemical enhancement factor, and liquid temperature profiles along the column. The model was in good agreement to predict the CO2 concentration profiles along the column at low CO2 partial pressure. However, it was found that the model needs to be corrected by introducing a correction factor for overall volumetric mass transfer coefficient (Kgae) for the simulation of CO2 concentration profiles along the column at high CO2 partial pressure conditions in the range of 404 – 1616 kPa.
Keywords: CO2 capture, simulation, high CO2 partial pressure, amine solvent, structured packing
1.0
Introduction Natural gas (NG) rich in CO2 is an alternative resource of NG that remains unexplored
due to the existing high CO2 content in NG. As an impurity, CO2 becomes corrosive with the presence of water and causes damage in gas pipelines that transport NG from offshore platforms to onshore gas processing plants. Therefore, CO2 content in NG must be reduced to an acceptable level by CO2 capturing technology before transporting NG through subsea gas pipelines. Among these technologies, CO2 removal by chemical absorption was implemented for decades to be an established technology for natural gas processing and post-combustion CO2 capturing. However, the process was limited to the removal of low CO2 concentrations, which typically were conducted in the range of 1 – 20% CO2. Amine-based absorbents, such as monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA) and di-2-propanolamine (DIPA) were the most widely used absorbents due to their high reactivity
2
with CO2. However, amines have several limitations, such as a low CO2 absorption capacity, being highly corrosive, and require high energy for regeneration. Therefore, the absorbent used for the process needs to be improved in order to capture high CO2 content in NG effectively and economically. An alternative absorbent, such as piperazine (PZ) promoted 2-amino-2-methyl-1propanol (AMP), which has a high potential to be implemented for the purification of CO2 rich NG. This potential solvent offers high reactivity and a high CO2 loading capacity compared to MEA, which was reported by numerous researchers for CO2 absorption at atmospheric conditions [1-5]. A study at high pressure conditions also proved that the performance of this potential solvent was better than MEA [6]. A rate-based steady-state model for CO2 capture by chemical absorption at an atmospheric condition was developed in literatures and validated for low CO2 partial pressure in the range of 1 – 20 kPa [7-15]. However, the validation of the model for CO2 absorption at a high CO2 partial pressure has yet to be reported due to the unavailability/limitations of the performance data at high pressure conditions. In the case of CO2 absorption from CO2-rich NG, it is believed that a significant decrease of total gas flow rate along the column during CO2 absorption will give a significant effect to the process’s performance. Furthermore, some researchers suggested that an effective mass transfer area in the column decreases at high pressure operations due to severe back mixing in the column [16]. Therefore, in order to develop a reliable process model for separating high CO2 concentrations from NG in a packed absorption column, it is crucial to validate the existing model that was developed at a low CO2 partial pressure with the performance data at high CO2 partial pressure conditions. In this study, a rate-based steady-state model was developed using a similar mathematical approach proposed by Kahn et al. [10]. This model implemented the mass transfer correlations and effective mass transfer area for structure packing developed by
3
Rocha et al. [17, 18]. The model took into account the mass transfer resistances in the gas and liquid films and was based on the fast second-order kinetics for CO2-(PZ+AMP) reactions in the liquid film. The heat effects associated with chemical absorption as well as heat loss to the surroundings were taken into account through energy balances in the gas and liquid phases. In order to ensure reliable prediction, correlations for thermodynamics, physical properties, and hydraulic factors were incorporated into the model. The simulation results were compared with the experimental data produced at low CO2 partial pressure (atmospheric) as well as at high CO2 partial pressure in the range of 404 – 1616 kPa as reported in our previous publication [6]. This study validates the suitability of the existing model at a low pressure condition for predicting the process performance of CO2 absorption at high pressure conditions in a packed absorption column. There is the possibility of an inaccurate prediction of the simulated results at a high pressure condition due to several factors, which may lead to the implementation of a correction factor in the developed model.
2.0
Reactive absorption model formulation
2.1
Kinetic reaction of CO2 absorption into PZ+AMP blended solution Piperazine (PZ) is a diamine that reacts with CO2 to produce both single and
dicarbamate products as can be described in the following reaction scheme, R1 and R2 [19]:
CO2 PZ PZCOO- H
(R1)
CO2 PZCOO- PZCOO2 H 2
(R2)
The reaction between CO2 and 2-amino-2-methyl-1-propanol (AMP) in the presence of water forms bicarbonate ions that were usually described by second-order reaction [20, 21]. The overall reaction between AMP and CO2 can be explained through the following equation, R3 [22]:
4
CO2 AMP H 2O AMPH HCO-3
(R3)
The CO2 absorption into a PZ+AMP blended solution can be regarded to be gas absorption accompanied by a second order reaction. The overall reaction rate was expressed in terms of the molar concentration of CO2 and PZ+AMP as shown below:
r CO2 k 2, PZ PZ k 2, AMP AMP
(1)
The second order reaction rate constant, k2 for PZ reaction can be approximated from the correlation by Sun et al. [23], while the correlation reported Camacho et al. [21] was used for AMP reaction: 5712 k2,PZ 4.49 1012 exp T
(2)
8186.9 k2, AMP 4.80 1012 exp T
(3)
2.1.1 Two-film theory The absorption of CO2 in a column can be modelled using the two-film theory. The film is stagnant and the CO2 diffuses from the bulk gas through the gas film to the interface. The reaction would take place at the interface and the products would diffuse to the bulk liquid. In principle, the reaction between a dissolved gas and a solvent depends on the kinetics regime, such as an instantaneous reaction, fast reaction, intermediate rate, slow reaction and infinitely slow reaction [24]. For CO2 - (PZ+AMP) reactions, it is assumed that a fast reaction in the thin film occurred whereby the reaction would only take place within the liquid film only. The CO2 concentration at the interface is hard to predict and cannot be measured, therefore, an overall mass transfer coefficient has to be determined. The equation for an overall volumetric mass transfer coefficient in the gas phase, Kgae is given in Eq. (4): 1 H CO2 K g ae k a g e k l ae E
1
(4)
5
Where K g is the overall mass transfer coefficient in the gas phase, k g is the individual gas phase mass transfer coefficient, k l is the individual liquid phase mass transfer coefficient, ae is the effective mass transfer area of the packing, E is the chemical enhancement factor and
H CO2 is the Henry’s law constant.
2.2
Reaction Model
The source term, rCO2 represents the overall rate of CO2 absorption and is given in Eq. (5) to be the following equation: rCO2 K g ae ( pCO2 pCO ) 2
(5)
Where pCO is the CO2 partial pressure in the gas phase, pCO is the equilibrium CO2 partial 2
2
pressure and pCO2 pCO is the driving force for the gas phase. In this model, it is assumed 2
that the equilibrium CO2 partial pressure is equal to zero. This assumption is appropriated due to the fast chemical absorption whereby the equilibrium CO2 partial pressure is considered to be too small [25-28]. A substitution of Eq. (4) into Eq. (5) gives the following equation: rCO2
pCO2
1 / k a ) ( H g
e
CO2
/ kl ae E
(6)
The chemical enhancement factor (E) for the second-order chemical reaction can be assessed by using a set of equations [29]:
1.35
1 E 1
1.35
1 E 1
1.35
1 E1 1
(7)
C D E 1 Aml Aml bDco l Cco i 2 2
(8)
6
E1
Ha
Ha tanh Ha
(9)
Dco2l k2C Aml ( kl ) 2
(10)
Where C Aml is the molar concentration of amine in the liquid, DAml is the molecular diffusivity of amine in the liquid, b is the stoichiometric factor of the reaction, DCO l is the molecular 2
diffusivity of CO2 in liquid, CCO i is the molar concentration CO2 at the interface and Ha is 2
the chemical reaction parameter, which is also known as the Hatta number. k 2 is the reaction rate given by Eq. (2) and (3).
Eq. (11) to Eq. (22) for the calculation of the hydraulics, mass transfer coefficients and effective mass transfer area are presented in Table 1. The empirical mass transfer correlations for the structured packing developed by Rocha et al. [17, 18] were used in this study to calculate the mass transfer coefficients in the liquid and gas phases as shown in Eq. (11) and (12).
The liquid holdup affects the hydraulic and mass transfer in the process. In terms of the hydraulic aspect, an increase in the velocity of any phase increases the thickness and liquid holdup of the film, which consequently increases the pressure drop [16]. In addition, an increase in the liquid holdup resulted in an increase in interfacial area that gives higher mass transfer rates [16]. The liquid holdup can be calculated using the equations from Rocha et al. [17, 18] as shown in Eq. (15). The effective mass transfer area, ae can be estimated with the use of Eq. (22).
7
Table 1
Hydraulics, mass transfer coefficients and effective mass transfer area correlations for structured packing [17, 18] Correlations
kl 2 kg S DCO2 g
Equation no.
0.9 DCO2l ule
(11)
S
u u S 0.054 ge le g g
u ge
ug (1 hl ) sin
ule
ul hl sin
0.8
g DCO2 g g
0.33
(12)
(13)
(14)
2
3l ul F 3 hl 4 t S l sin( )g eff
Ft 29.12(WeFr )l0.15
1
3
(15)
S 0.359 Rel0.2 0.6 (1 0.93 cos( ))(sin )0.3
cos( ) 5.21110( 16.835 )
g eff
(17)
dp g 1 dz g l l dp dz flood
(18)
dpdry dp dz dz (1 (0.614 71.35S )hl )5
dpdry dz A
(19)
(20)
Au g2 Bu g 0.1775 g
S sin( ) 2
2
,
(16)
B
88.774 g
(21)
S 2 sin( )
ae 0.35Ft a
(22)
8
2.3 Physical and thermodynamic properties The molar concentration of CO2 at the interface, CCO2i can be approximated using Henry’s law as given in Eq. (23).
CCO2i
pyCO2 H CO2
(23)
Henry’s law describes the amount of CO2 that can dissolve in an amine solution. This is directly proportional to the partial pressure of the gas in equilibrium with the liquid. The Henry’s law constant of CO2 can be computed from the correlations using N2O analogy. In N2O analogy, the Henry’s law constant of CO2 in an aqueous amine solution can be calculated from a Henry’s law constant of N2O in the same solvent. The physical solubility can be measured directly because the N2O does not react with aqueous amine solutions. This analogy was based on the similarities in mass, molecular structure, and molecular interaction parameters existing between CO2 and N2O. The Henry’s law constant of CO2 in an aqueous amine solution, H CO2 can be calculated from the Henry’s law constant of N2O in an aqueous amine solution, H N2O using the following equation:
H CO2 H CO2 H N2O HN O 2 W
(24)
Table 2 shows the correlations used in this work for the calculation of the Henry’s law constant of CO2 in PZ+AMP blended solution. The correlations for physical properties used in the absorber model are presented in Table 3.
9
Table 2
Correlations used in this work for the calculation of the Henry’s law constant of CO2 in PZ+AMP blended solution Correlations
Ref. [30]
8172.355 H CO2 ,W exp 145.369 19.303 ln(T ) T 9048.596 H N 2 O ,W exp 158.245 20.860 ln(T ) 0.00252T T
[31]
H N 2O H N 2O , w xw H N 2O , AMP x AMP H N 2O , PZ xPZ T 2 exp C123( xPZ x AMP ) A123xw ( xPZ xAMP ) 1 B123
[32]
H N2O, AMP 8589.10 34.799T
[33]
ln( H N2O,PZ ) 4.7017
Table 3
4.9497 103 2.7623 103 T T
[34]
Correlations of physical properties used in the absorber model Correlations
Ref. [35]
Molecular diffusivity of CO2 in an amine aqueous solution
DCO2 ,l DCO2 ,w w l DCO2 ,w 2.35 106 exp( 2119 / T )
Viscosity of the PZ+AMP blended solution
l exp(34.2386 7121.8646 / T 0.036398T )
Diffusivity of amine molecules in the liquid solution
DAml CPZ DPZ C AMP DAMP
0.8
DPZ exp( 13.672 2160.9 / T 0.00019263CPZ ) DAMP exp(13.2922 2285.77 / T 0.000217CAMP )
[36]
[37] [38]
1/ 2
Binary gas diffusivities (Fuller Model)
1 1 10 T M CO M CH 2 4 1/ 3 1/ 3 P VCO2 VCH4 3
DCO2 CH4
[39]
1.75
3
3
VCO2 = 27.46 cm /gmol, VCH4 24.42 cm /gmol) Surface tension of the aqueous 7 wt% PZ + 23 wt% AMP solution
96.24 0.17T
[40]
10
2.3 Absorber model The model consists of a set of differential mass and energy balances with the following assumptions:
Steady-state operations prevail.
The reaction is fast and takes place in the liquid film of the gas-liquid interface system.
Plug-flow of liquid and gas present in absorption column.
The gas and liquid flow rates are constant throughout the column.
Heat loss to the surroundings is taken into account in the energy balance.
Vaporisation of water and AMP+PZ blended solution is not under consideration in the mass conservation equation.
2.4.1
Mass balance The CO2 concentration in the gas phase along the height of the column is calculated by
the following steady state, one-dimensional mass conservation equation:
dN CO2 , g dz
d (GYCO2 ) dz
rCO2
(25)
Where N CO2 is the molar flux of CO2, G is the total gas flow rate per unit cross-sectional area of the column, YCO2 is the mol ratio of CO2 in the gas phase and dz is the incremental height of the column. Fig. 1 shows the schematic of a packed absorption column in an infinitesimal element for mass and energy balances. Eq. (25) is discretised over an incremental height of the column, dz, using a forward difference scheme.
11
Interface Gas
XAm,i TLi
YCO2i TGi
Liquid Gas film
Gas bulk
NCO2
Liquid film
Liquid bulk
TG
NCO2
TL dz
NCO2
NCO2
YCO2
NCO2
XCO2
Gas
Liquid YCO2 i+1 TG i+1
XAm,i+1 TL i+1
Fig. 1 Schematic of a packed absorption column in an infinitesimal element for mass and energy balances.
The amines concentration in the solution can be calculated by performing a mass balance over the incremental height:
X Am ,i X Am ,i1
(YCO2i1 YCO2i )bG L
(26)
Where X Am is the mol ratio of amine in water and L is the molar flow rate of the liquid. 2.4.2
Energy balance
Due to an exothermic reaction between CO2 and PZ+AMP solution, the liquid temperature will increase, which results in heat transfer from the liquid to the gas phase. The energy balance for the gas and liquid phases are given in Eq. (27) and (28): GC pg
LC p l
dTg dz
hg ae (Tg Tl )
(27)
dYCO2 dT dTl GC pg g G C pCO2 Tl To H R H out (Tl 303) dz dz dz
12
(28)
Where hg is the gas phase heat transfer coefficient, C pl , C pg and C pCO are the heat capacities of 2
the liquid phase, gas phase and CO2, respectively. Tg, Tl and To are the temperature of the liquid phase, gas phase and reference temperature, respectively. YCO2 is the mol ratio of CO2. HR is the heat of chemical absorption between CO2 and PZ+AMP blended solution, which is equal to 86002 kJ/kmol [41]. Hout is the heat transfer constant of heat loss through the column wall to the atmosphere.
2.5 Numerical procedure Fig. 2 shows the simplified flowchart for the CO2-(PZ+AMP) simulation model. The physical properties, mass transfer correlations, mass balance and energy balance equations were discretised and implemented in a MATLAB script. A small step size, dz, was chosen in order for the values to be computed at each step. The inlet concentration of PZ+AMP at the top and CO2 at the bottom was known, therefore, an iterative procedure was implemented in order to compute the absorption of CO2 in the absorption column. The procedure started at the top of the column with the known inlet concentration of PZ+AMP and an initial guess of the outlet condition of CO2. The model solved for each step and the resulting CO2 concentration at the bottom was compared with the known inlet CO2 concentration. If the computed value was higher than the CO2 inlet concentration, the procedure would be repeated. If the computed value was too low, the initial CO2 guess was increased with a small step until the CO2 concentration at the bottom was achieved.
13
START
Declare input conditions G, L, CCO2,in, CAm,in, Tg,in, Tl,in
Fit CO2 concentration at the top of the column
Determine physical properties of the gas and liquid components
Calculate effective mass transfer area (ae) and individual mass transfer coefficients (kg and kl)
Calculate chemical enhancement factor (E).
Calculate absorption rate for the segment (rCO2).
Calculate composition of CO2 in the gas phase (yCO2) and composition of amine in the liquid phase (xAm) for the next column segment based on the absorption rate.
Calculate temperature of the gas phase (Tg) and temperature of the liquid phase (Tl) for the next column segment.
Final bottom segment of the column?
No
Yes Adjust CO2 concentration at the top of the column.
No
Compare the calculated CO2 concentration at the bottom with the input value. Yes STOP
Fig. 2 Flowchart for the CO2-amine simulation model.
14
Next segment.
2.6 Source of experimental data Experimental data for CO2 absorption from high concentration CO2 in NG into a PZ+AMP blended solution were taken from our previous work [6]. The inner diameter of the column was 0.046 m, which was packed with 2.04 m of Sulzer metal gauze packing. The specifications of the absorber column and packing material are shown in Table 4. The concentration of CO2 was 40% in NG and the amine composition was 7 wt% PZ + 23 wt% AMP (3.5 kmol/m3). The experiments were conducted at various operating pressures in the range of atmospheric up to 4.04 MPa in a packed absorption column. Therefore, the CO2 partial pressure was in the range of 40 to 1616 kPa. The details of the process flow diagram and experimental procedures can be found in Halim et al. [6, 42]. The CO2 concentration and temperature profiles along the column were recorded at a steady state condition. These experimental data were used to validate the simulation results generated from the model.
Table 4
Specifications of the absorber column and packing material Parameters Material packing Height of packing Diameter of packing Packing surface area per volume, a Cross section area of column Void fraction, ε Corrugation inclination angle, θ Corrugation side length, S
Specifications Stainless steel 2.04 (m) 0.046 (m) approx.500 (m2/m3) 1.67 x 10-3 (m2) approx. 0.90 60° approx. 8.90 (mm)
3.0 Results and Discussion 3.1
Validation of the existing model The model used in this study was adapted from Khan et al. [10] and was validated using
experimental results published by Tontiwachwuthikul et al. [43] for run number T22. The CO2 absorption study was conducted by Tontiwachwuthikul et al. [43] in a pilot-plant 15
absorption column with an internal column diameter of 0.1 m. The column was packed with 6.6 m of 12.7 mm ceramic Berl saddles. The solvent used in their study was 3.0 kmol/m 3 of MEA. The model developed by Khan et al. [10] used the mass transfer correlations and effective mass transfer area for random packing proposed by Onda et al. [44]. Fig. 3 (a) and (b) shows the comparison between the simulated and measured gas phase CO2 concentration as well as MEA concentration profiles along the column height. As reported by Khan et al. [10], the simulation and experimental results were in good agreement for both graphs. 3000
20 18
2500
MEA Concentration (mol/m 3)
14 12 10 8
2
CO concentration (mol%)
16
6 4
2000
1500
1000
500
2 0
0
0
1
2 3 4 5 Distance from the bottom of the column (m)
6
7
0
1
2 3 4 5 Distance from the bottom of the column (m)
6
(b)
(a)
Fig. 3 Comparison between the experimental and simulation results (a) CO2 concentration profile (b) MEA concentration profile for CO2-MEA system at atmospheric condition. (-) Simulation and (x) Experiment data. (Gas flow rate = 14.8 mol/m2.s; 19.1% CO2 in NG; [MEA] = 3.0 kmol/m3; L = 9.5 m3/m2.h; T = 19°C)
3.2
CO2 absorption model using PZ+AMP solution at atmospheric condition Khan et al. [10] developed a rate-based steady-state model for CO2 absorption using
MEA as the solvent. Since this study used a PZ+AMP blended solution, the rate of reaction as well as physical and thermodynamic properties needs to be changed accordingly. Furthermore, suitable mass transfer correlations and effective mass transfer area for the 16
7
structured packing needs to be introduced, which were taken from Rocha et al. [17, 18]. In addition, the model implemented in the current work also considers the heat loss through the wall of the column to the atmosphere. Fig. 4 shows the CO2 concentration profiles along the column height for experimental and simulation results for CO2 - PZ+AMP systems at atmospheric condition. From the figure, it can be seen that the overall agreement between the prediction and experiment is generally good. The mean average error (MAE) of the simulated concentration from the experimental concentration was 3.75%. 45
MAE = 3.75%
2
CO Concentration (mol%)
40
35
30
25
20
15
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
Fig. 4 A comparison between the experimental and simulated CO2 concentration profile for CO2 - PZ+AMP system at an atmospheric condition. (-) Prediction and (x) Experiment data. (Gas flow rate = 33 kmol/m2.h; 40% CO2 in NG; [PZ+AMP] = 30 wt%; L = 3.61 m3/m2.h; T = 30°C)
17
3.3 Comparison between the simulation and experimental results at high pressure conditions The model was further used to predict the performance of CO2 absorption using PZ+AMP blended solution at high pressure conditions in the range of 1.01 to 4.04 MPa by incorporating the physical properties of the gas mixture at high pressure conditions. Fig. 5 (a, b, c, d) shows the comparison between simulation and experimental results at various operating pressures in the range of 1.01 to 4.04 MPa. It can be seen that at high pressure operations, the simulation results using the low CO2 partial pressure model showed a large deviation from the experimental results. Since the low CO2 partial pressure model unable to predict the performance of CO2 absorption at high CO2 partial pressure conditions, the model need to be corrected by introducing a correction factor (F) for overall volumetric mass transfer coefficient (Kgae). The correction factor for each operating pressure was taken by curve fitting method. The same technique can be used to predict the correction factor for other solvents. Fig. 6 shows the correction factor versus CO2 partial pressure in the range of 404 to 1616 kPa. In order to achieve a close agreement between experimental and simulation results, a correction factor (F) for overall volumetric mass transfer coefficient (Kgae) to be a function of
pCO2 was added to the overall rate of CO2 absorption, rCO2 in Eq. (5) as follows: F 5 108 ( pCO2 )2 9 106 pCO2 0.0975
rCO2 FK g ae pCO2
(42) (43)
As shown in Fig. 5 (a, b, c, d), the addition of a correction factor (F) in the model gives a good agreement between the simulation and experimental results with a MAE of less than 7.70%. It was believed that the correction factor was needed to correct the overall rate of
18
reaction due to several factors that were contributed by the high CO2 partial pressure condition.
45
45
Simulation with correction factor, MAE = 3.39%
CO Concentration (mol%)
35
35
30 25 20
2
2
Simulation without correction factor
15
30 25 20
Simulation without correction factor
15 10
10 5
Simulation with correction factor, MAE = 3.35%
40
CO Concentration (mol%)
40
5
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
0
2.5
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
(a)
(b) 45
45
Simulation with correction factor, MAE = 7.13%
40
35
CO Concentration (mol%)
30 25 20
2
2
CO Concentration (mol%)
Simulation with correction factor, MAE = 7.65%
40
35
Simulation without correction factor
15 10 5 0
2.5
30 25 20
10 5 0
0
Simulation without correction factor
15
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
(d)
(c)
Fig. 5 Comparison between the experimental and simulated CO2 concentration profile for CO2 - (PZ+AMP) system at different operating pressure. (-) Prediction and (x) Experiment data. (Gas flow rate = 33 kmol/m2.h; 40% CO2 in NG; [PZ+AMP] = 30 wt%; L = 3.61 m3/m2.h; T = 30°C; (a) P = 1.01 MPa; (b) P = 2.02 MPa; (c) P = 3.03 MPa; (d) P = 4.04 MPa)
19
0.25
Correction Factor
0.2
y = 5E-08x2 + 9E-06x + 0.0975 R² = 0.9948
0.15
0.1
0.05
0 0
200
400
600
800
1000
1200
1400
1600
1800
Pco2 (kPa)
Fig. 6 Correction factor for CO2 absorption at high CO2 partial pressure condition for PZ+AMP blended solution
One of the possible reasons is the significant decrease of total gas flow rate along the column during the absorption process that was not accounted in the low CO2 partial pressure model. Besides that, it was expected that the assumption of the plug flow in the model was no longer valid due to severe back mixing in the column as reported by Gualito et al. [16] at high pressure conditions of up to 27.2 bar. They also suggested that the correction factor due to back mixing could be applied to the effective mass transfer area, ae. However, in this work, the correlation of ae introduced by Gualito does not fit well with our system. Therefore, the correction factor in this study was introduced to the Kgae in general, in order for further research to possibly be conducted in order to investigate the factors that might contribute to this behaviour in the absorption of high CO2 concentration from NG at high pressure conditions.
20
Fig. 7 (a) and (b) show good agreement between the experimental and simulation results for CO2 concentration and liquid temperature profiles along the column for CO2 absorption at an operating pressure of 4.04 MPa. The MAE of the simulated CO2 concentration from the experimental concentration was 7.65% whereas the MAE for the liquid temperature was 0.54%. The exothermic reactions between CO2 and amine resulted in rapid variation of the liquid temperature at the top of the column due to a significant amount of CO2 absorption in this region. The peak temperature that represents the location of the temperature bulge was closed to the top of the column due to a low L/G ratio [45]. The magnitude of the temperature bulge and its location depends on the heat of reaction, L/G ratio, and the location of CO2 absorption into amine occurring [46]. Fig. 7 (c) shows amine concentration profiles along the column height simulated from the model. The amine concentration decreased from the top to the bottom of the column due to continuous chemical absorption along the column until all amines depleted at about 1.25 m from the bottom of the column. Therefore, as can be observed in Fig. 7 (d), the chemical enhancement factor along the column also shows a similar trend with the amine concentration profiles. Thus, this behaviour indicates that the chemical enhancement factor is strongly influence by the availability of the amine molecules to react with CO2 in the column.
21
320
45
MAE = 7.65%
40
316
Liquid Temperature, T (K)
l
30 25 20
2
CO Concentration (mol%)
35
15
314 312 310 308
10
306
5
304
0
MAE = 0.54%
318
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
302 -0.5
0
0.5 1 1.5 Distance from the bottom of the column (m)
(a) 3500
70 Chemical Enhancement Factor, E
80
3000
Amine Concentration (mol/m 3)
2.5
(b)
4000
2500 2000 1500 1000 500 0
2
60 50 40 30 20 10
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
2.5
0
0
0.5 1 1.5 2 Distance from the bottom of the column (m)
(c)
2.5
(d)
Fig. 7 (a) CO2 concentration profiles along the column height (b) Liquid temperature profile (c) Amine concentration profiles (d) Chemical enhancement factor along the column height. (-) Prediction and (x) Experiment data. (Gas flow rate = 33 kmol/m2.h; 40% CO2 in NG; [PZ + AMP] = 30 wt%; L = 3.61 m3/m2.h; T = 30°C; P = 4.04 MPa)
3.4
Mass transfer coefficients at various operating pressures
Fig. 8 presents the individual mass transfer coefficient for gas (kg) and liquid (kl) phases at various operating pressures calculated using a Rocha model [17, 18]. It can be observed that kl was almost constant at 6.4 x 10-5 m/s. In Eq. (11), the kl depends on liquid properties that 22
are not significantly affected by pressure. Moreover, the kg decreased with the increasing of operating pressure due to an increase of gas density and the decreased DCO2 g at a higher pressure as shown in Eq. (12). Significant decreases in kg can be observed from 1.12 x 10-3 to
Individual mass transfer coefficient in gas phase (kg)
1.2E-03
7.0E-05
1.1E-03
6.5E-05
1.0E-03 6.0E-05
9.0E-04
kg kl
8.0E-04
5.5E-05
7.0E-04
5.0E-05
6.0E-04 4.5E-05
5.0E-04 4.0E-04
Individual mass transfer coefficient in liquid phase (kl)
5.5 x 10-4 m/s because the operating pressure increased from 1.01 to 4.04 MPa.
4.0E-05 0
1
2
3
4
5
Operating Pressure (MPa)
Fig. 8 Individual mass transfer coefficients at various operating pressures
3.5
Overall rate of CO2 absorption at various operating pressures Fig. 9 shows the simulation results of the corrected overall rate of CO2 absorption
(rCO2) at various operating pressures. Based on this figure, it is apparent that the corrected overall rate of CO2 absorption increased with increasing operating pressures. This is due to a higher mole fraction driving force for separation at a higher pressure condition. Moreover, it can be observed that the upward trend is more obvious from 1.70 to 2.04 m for all pressures due a high concentration of PZ+AMP blended solution at the top of the column. 23
Overall rate of CO2 absorption (rCO2)
1.2E-02 P = 1.01 MPa
1.0E-02
P = 2.02 MPa P = 3.03 MPa
8.0E-03
P = 4.04 MPa
6.0E-03
4.0E-03
2.0E-03
0.0E+00 0
0.5
1
1.5
2
2.5
Distance from the bottom of the column (m)
Fig. 9 Corrected overall rate of CO2 absorption at various operating pressures
4.0 Conclusions A rate-based steady-state model for the absorption of high CO2 concentration from NG using PZ+AMP blended solution was developed and validated with experimental data at atmospheric up to 4.04 MPa operating pressure. A correction factor was introduced to the overall volumetric mass transfer coefficient for the prediction of process performance at high CO2 partial pressure conditions in the range of 404 to 1616 kPa. The simulated CO2 concentration and liquid temperature profiles along the column height were in good agreement with the experimental results.
5.0 Acknowledgement The authors would like to acknowledge the Research Centre for CO2 Capture, Universiti Teknologi PETRONAS (UTP) for research funding, the scholarship support from the Ministry 24
of Higher Education Malaysia and Universiti Malaysia Perlis awarded to HNAH and Sulzer Chemtech Pte Ltd, Winterthur, Switzerland for the sponsored Sulzer metal gauze packing.
Abbreviations a = total packing surface area per unit volume of packing ae = effective mass transfer area A123, B123 and C123 = the ternary fitted parameters
b = stoichiometric factor of the reaction C Aml = molar concentration of amine in the liquid
C AMP = concentration of AMP CCO2 i = molar concentration CO2 at the interface
C pg
= heat capacity of the gas phase
C pl = heat capacity of the liquid phase
C pCO2 = heat
capacity of CO2
C PZ = concentration of PZ dp = pressure drop when the gas is in counter-current flow with the liquid dz dp dry dz
= pressure drop when only gas is flowing in the column
25
dp = pressure drop at which flooding occurs dz flood dz = incremental height of the column DAml = molecular diffusivity of amine in the liquid
DCO2 g = molecular diffusivity of CO2 in gas
DCO2l = molecular diffusivity of
CO2 in liquid
DCO2 ,l = molecular diffusivity of CO2 in amine aqueous solution
DCO2 ,w
= diffusivity of CO2 in water
DCO2 CH4 = binary gas phase diffusivity of CO2 in CH4
DAml = diffusivity of amine molecules in the liquid solution
DAMP = diffusivity correlation of AMP DPZ = diffusivity correlation of PZ E = chemical enhancement factor F = correction factor for overall volumetric mass transfer coefficient
Fr = Froude number Ft = correction factor for total holdup
g eff = effective gravity G = total gas flow rate per unit cross-sectional area of the column
26
hg = gas phase heat transfer coefficient
hl = liquid holdup
Ha = Hatta number H CO2 = Henry’s law constant of CO2 in aqueous amine solution
H CO2 ,W = Henry’s law constant of CO2 in water
H N2O = Henry’s law constant of N2O in aqueous amine solution H N2O = Henry’s law constant of N2O in aqueous ternary amine solvents H N 2 O,W = Henry’s law constant of N2O in water H N2O, AMP Henry’s law constant of N2O in aqueous AMP solution H N2O ,PZ Henry’s law constant of N2O in aqueous PZ solution
HR = heat of chemical absorption between CO2 and PZ+AMP solution Hout = heat transfer constant of heat loss through the column wall to the atmosphere k2 = second order reaction rate constant K g = overall mass transfer coefficient in the gas phase
k g = individual gas phase mass transfer coefficient
k l = individual liquid phase mass transfer coefficient L = molar flow rate of the liquid
27
M = molecular weight N CO2 = molar flux of CO2
p = total pressure of gas phase pCO2 = CO2 partial pressure in the gas phase
pCO 2
= equilibrium CO2 partial pressure
rCO2 = overall rate of CO2 absorption
Re = Reynolds number S = corrugation side length of the packing Tg = temperature of the gas phase Tl = temperature of the liquid phase To = temperature of the reference temperature
u ge = effective gas velocity u le = effective liquid velocity ul = velocity of liquid
We = Weber number X Am = mol ratio of amine in water
x AMP = mol fraction of AMP
xPZ = mol fraction of PZ 28
xw = mol fraction of water yCO2 = mol fraction of CO2 in the gas
YCO2 = mol ratio of CO2 in the gas phase g = viscosity of gas
= void fraction of the packing, = corrugation inclination angle of the packing l = viscosity of PZ+AMP solution w = viscosity of water g = the gas density
l = liquid density
= contact angle between solid and liquid = surface tension
V = the sum of the diffusion volume
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Highlights A rate-based steady-state model at low CO2 partial pressure conditions was validated with the data at high pressure conditions The model needs to be corrected by introducing a correction factor for overall volumetric mass transfer coefficient (Kgae) at high CO2 partial pressure conditions.
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