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High-pressure experimental and theoretical study of CO2 solubility in aqueous blends of lysine salts with piperazine as new absorbents
Fluid Phase Equilibria 507 (2020) 112429
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
High-pressure experimental and theoretical study of CO2 solubility in aqueous blends of lysine salts with piperazine as new absorbents Humbul Suleman a, Abdulhalim Shah Maulud b, c, *, Afaf Syalsabila c, Muhammad Zubair Shahid c, Philip Loldrup Fosbøl a a
Center for Energy Resources Engineering, Department of Chemical and Biochemical Engineering, Technical University of Denmark, Kongens Lyngby, 2800, Denmark Centre of Contaminant Control and Utilisation, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, 32610, Perak, Malaysia c Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak, Malaysia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2019 Received in revised form 15 November 2019 Accepted 29 November 2019 Available online 30 November 2019
Vapour-liquid equilibrium of carbon dioxide loaded potassium and sodium L-lysine salts have been investigated in the region of high-pressure and high loadings. The equilibrium solubility of carbon dioxide was measured in separate aqueous L-lysine alkaline salts for a range of temperature (303.15 e363.15 K), solution concentrations (1.0e3.0 M) and pressure (95e4204 kPa). In addition to that, solubility of carbon dioxide in blends of both aforementioned amino acid salts with piperazine were investigated. All the studied solutions exhibited an increase in the carbon dioxide loading values with increase in the pressure and had a negative relationship with increase in the temperature and solvent concentration. Furthermore, the experimental data was correlated by the Kent-Eisenberg model. The correlated values shows a good agreement (AAD% of 5.15%) with experimental values. The regressed parameters of the model allows satisfactory estimation of the loadings of carbon dioxide in all studied solutions, for parametric studies. The study shows that L-lysine salts are potential green solvents for the carbon dioxide capture at high pressure. © 2019 Elsevier B.V. All rights reserved.
Keywords: Amino acids Blends Carbon capture Piperazine Vapour Liquid equilibrium
1. Introduction Separation of carbon dioxide is now considered as a mandatory step in gas cleaning processes. A number of separation techniques are applied and essentially operate at stringent process conditions, usually high pressure [1]. This includes absorption, adsorption, membranes and cryogenic methods [2]. Of which, absorption is the most commercially accepted method. Both physical and chemical solvent based absorption is used to separate carbon dioxide. Polyethylene glycols, methanol and organic carbonates are some common examples of physical solvents, while chemical solvents include ammonia, potassium carbonate and alkanolamines [3]. The solvent (either physical or chemical) carries the captured carbon dioxide to the stripper section for desorption at relatively high temperature and lower pressure than the absorber. Vast research efforts are underway to develop new techniques
and chemicals for an efficient carbon dioxide capture from the natural gas [4]. One of the research emphasis is the development of unique solvents and their blends for providing similar or better efficiency for the separation and energy savings with a reduction in the hazardous emissions [5,6]. Alkaline salts of amino acids are a potential option, as they are inherently environmental friendly and operate on similar absorption principle as of conventional alkanolamines [7]. This is possible due to their zwitterion structure, which allows the amino acids to behave both as a weak acid and weak base, when introduced to an acidic and basic medium, respectively. Equation (1) presents the zwitterion structure and its behaviour in acidic and basic mediums. þH þ
! NHþ NHþ 3 R COOH 3 R COO H þ
! NH2 R COO
* Corresponding author. Centre of Contaminant Control and Utilisation, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, 32610, Perak, Malaysia. E-mail address: [email protected] (A.S. Maulud). https://doi.org/10.1016/j.fluid.2019.112429 0378-3812/© 2019 Elsevier B.V. All rights reserved.
Low pH
Neutral
High pH
(1)
In a basic medium, the amine group of an amino acid becomes activated and behaves similar to an alkanolamine. Thus, these
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
Nomenclature
Abbreviations L-lysine salt AA AAD% average absolute percentage deviation ALK piperazine Bþ ion of the base BOH base (either sodium- or potassium hydroxide) CO2 carbon dioxide CO2 carbonate 3 HCO2 Henry's constant, atm.litre/mole HCO bicarbonate ion 3 Hþ ion of hydrogen H2O water k equilibrium constant Subscript a related to protonation of L-lysine salt Subscript b related to carbamate hydrolysis of L-lysine salt Subscript c related to bicarbonate Subscript d related to carbonate Subscript e related to water Subscript f related to base Subscript g related to piperazine MDEA Methyldiethanolamine MEA Monoethanolamine n number of moles of Subscript transfer transferred from feed to solubility cell Subscript 1 initial value before transfer of gas Subscript 2 final value after transfer of gas
activated structures can chemically react with carbon dioxide. A recent review [8] provides detailed information about the carbon dioxide solubility in various amino acids. Of which, L-lysine alkaline salts are potential candidates for a solvent in carbon dioxide capture. As its structure has two amine bonds, each amino acid molecule is theoretically capable of absorbing two carbon dioxide molecules. However, the behaviour of the second amine group on the lysine's R-group in capturing a carbon dioxide molecule is interesting. Mazinani et al. [9] investigated the performance of potassium lysinate as a potential carbon capture solvent. They studied the equilibrium carbon dioxide solubility at near-ambient temperatures (298.15e313.15 K), low pressure (5.40e41.47 kPa) and potassium lysinate concentrations (0.5e2.50 M). The group of Shen [10,11] have performed an elaborate study on the low pressure equilibrium CO2 solubility in aqueous potassium salt of L-lysine and their physicochemical properties. They have experimentally measured the solubility in various concentrations of aqueous potassium lysinate solutions (0.5e2.50 M) for low pressure conditions (0.07e115.26 kPa) and temperature (298.15e353.15 K). Our previous study [12] investigated the carbon dioxide solubility in the same salt at high partial pressures of carbon dixoide (150e4040 kPa) and absorber's temperature conditions. All aforementioned research works have noted a good absorption capacity of potassium lysinate but no study is available for sodium lysinate solutions, a sister salt. In our recent publication [13], we have also studied the effect of activator/co-promoter on equilibrium carbon dioxide solubility in equimolar (i.e. 1 þ 1 and 2 þ 2 M) concentrations of piperazine and 2-amino-2-methyl-1-propanolol, blended with potassium salt of Llysine. The research work concluded that the piperazine has substantial effect on carbon dioxide solubility. However, the study did
Subscript absorbed carbon dioxide absorbed by the solvent Subscript amino acid amino acid in the liquid solvent placed initially Subscript residue unabsorbed moles of CO2 (in gaseous form) at equilibrium OH e hydroxide ion Roman Symbols ANH unreacted piperazine ANH þ protonated piperazine 2 AA NH unreacted L-lysine AA NH þ protonated L-lysine 2 AA NCOO carbamate of L-lysine fax, fbx, fam, fbm correction parameters for the model kax regressed equilibrium constant for AA NH þ 2 kbx regressed equilibrium constant for AA NCOO P pressure, kPa unless mentioned otherwise Subscript Tot total pressure Subscript Amine L-lysine or L-lysine blended with piperazine Subscript N2 nitrogen Subscript H2O water s1 e s6 correction parameters for the models T temperature, K ½ concentration of any specie, mole/litre, unless mentioned otherwise Greek Symbols loading value of CO2, mole of CO2/mole of solvent
a
not study the effect of smaller concentrations of piperazine on the CO2 solubility in aqueous solutions of potassium lysinate. None of the experimental information presented in Refs. [12,13] has been included in this work. In this study, the equilibrium carbon dioxide solubility was experimentally measured in alkaline salts (both potassium and sodium) and their blends with piperazine. The study was conducted in the high pressure region (95e4204 kPa) and temperatures (303.15e363.15 K). The Kent-Eisenberg model (details in Ref. [14]) for the amino acid solutions [15] was applied to correlate the experimental findings for single potassium and sodium salt solutions of L-lysine and their separate blends with piperazine. The model shows good agreement with the experimental values. 2. Materials and methods Merck Malaysia provided all the solid chemicals for this study. Gas Walker supplied gases (carbon dioxide and nitrogen). Table 1 presents the information about the materials used in this study. No further purification of any chemicals was carried out. The preparation for the test solutions started with the chemicals being separately weighed on a Sartorius BSA224S-CW mass balance (u ¼ 0.1 mg). Weighed samples of chemicals were then conjointly added to a 250 ml volumetric flask (u ¼ 0.2 K) at 298.15 K. Deionized water was added to make the solution and ample time was allowed for the degassing. Fresh solutions were prepared for each run. The experimental equilibrium solubility in L-lysine alkaline salts and their blends with piperazine were determined by the highpressure vapour-liquid equilibrium apparatus. Fig. 1 shows an illustration of the apparatus. Information about the dimensions and fabrication of the equipment apparatus can be seen in Ref. [16].
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
3
Table 1 Sample provenance table. IUPAC Name
Abbreviation
CAS Registry Number
Puritya
Purification Method
Carbon dioxide Nitrogen L-lysine monohydrate Piperazine Sodium hydroxide Potassium hydroxide
CO2 N2 Lys PZ NaOH KOH
124-38-9 7727-37-9 39665-12-8 110-85-0 1310-73-2 1310-58-3
99.95 vol% 99.9 vol% 99.0 wt% 99.0 wt% 97.0 wt% 85.0 wt%
None None None None None None
a
Vol% ¼ by volume percentage, wt% ¼ by weight percentage.
Fig. 1. The apparatus used in this study.
Table 2 presents some of the reproduced experimental data taken from the open literature to validate the equipment performance. The feed cell was filled with carbon dioxide through valve V1 up to a pressure around 5000 kPa at 303.15 K. Two hours were allowed to the carbon dioxide gas in order to equilibrate with the feed tank's conditions. After careful degassing, the cell was filled with 100 ml of fresh amino acid salt solution. Nitrogen gas was passed through the solubility cell for 5 min. The temperature was then increased slowly (0.2 C/min) to the desired temperature and given ample time to
stabilize (usually 2 h). The CO2 gas was fed slowly into the solubility cell. Hence, ntransfer was calculated as follows.
ntransfer ¼ n1 n2
(2)
The pressure in the solubility cell decreased because of the physical dissolution and reaction of carbon dioxide with the solvent. The rate of pressure drop was quite fast in the start but slowed later, as it neared equilibrium. The temperature and pressure values were noted at the equilibrium. Upon removal, the contents of the cell were physically examined for formation of precipitates, if any
Table 2 Reproduced experimental information to validate equipment's performance for carbon dioxide loadings (a) in separate solutions of N-methyldiethanolamine (1) þ ‘water (2) and monoethanolamine (1) þ water (2) at different values of pressure (P), temperature (T) and molar concentrations of each alkanolamine (M1)a. Researcher
M1/Mole of alkanolamine. (litre of aqueous solution)1
N-methyldiethanolamine (1) þ water (2) Jou et al. [17] 2.00 2.00 4.28 4.28 monoethanolamine (1) þ water (2) Lee et al. [18] 2.50 2.50 5.00 5.00
T/K
Reported
This Study
Deviation (%)
P/kPa
a/mole of CO2. (mole of alkanolamine)1
P/kPa
a/mole CO2. (mole of alkanolamine)1
313.15 343.15 313.15 343.15
640.0 2320 2800 705.0
1.0830 1.1470 1.1700 0.740
647.5 2325 2795 700.0
1.0748 1.1612 1.1925 0.7658
0.75 þ1.23 þ1.92 þ3.48
313.15 353.15 313.15 353.15
100 3160 1000 316
0.6730 0.9160 0.7800 0.5550
102.5 3175 995.0 315
0.6610 0.9254 0.7695 0.5326
1.78 þ1.03 1.34 4.03
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.058 mol of CO2 per mole of alkanolamine (either N-methyldiethanolamine or monoethanolamine. Both at a confidence level of 0.95.
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
and the contents of the cell were washed and placed in special waste containers. The equilibrium pressure of carbon dioxide, PCO2, was calculated by equation (3).
PCO2 ¼ PTot PN2 PH2O PAmine
(3)
The values and PAmine, PN2 and PH2O are determined by Ref. [19]. The experimental loading value of CO2 in the salt of L-lysine and their blends with piperazine was calculated using equation (4).
a¼
nabsorbed namino acid
(4)
nabsorbed is calculated by equation (5).
nabsorbed ¼ ntransferred nresidue
(5)
Peng and Robinson equation of state was used for calculating the moles of carbon dioxide in equation (2)e5 [20]. 3. Equilibrium modelling The equilibrium model used in this work is a modified form of the model presented by Kent and Eisenberg [21]. The original model is based on first-principles chemistry and uses an iterative set of equations to determine the acid gas solubility in aqueous alkanolamines. The reactions are based on equilibrium constants. The value of activity coefficient for all species is taken as one and the non-ideal effects are lumped in correction parameters. These presumptions are somewhat valid for aqueous solutions with low concentrations, containing small number of ionic species [14]. The model was mathematically modified by Haji-Sulaiman et al. [22] to its present form, where the set of iterative equations in the original work were replaced by a polynomial equation to determine the hydrogen ion concentrations. However, the model basis and method of solution remains unchanged to the date. The model has been extended to correlate the carbon dioxide solubility in amino acid salt solutions with a minor modification. An equation for complete ionization of alkali in the mechanism of the model was added by Mondal et al. [23]. Recently, Suleman et al. [15] have provided a standard set of correction parameters for the correlation of carbon dioxide solubility in various amino acid salt solutions. However, to credit the original developers the model is acknowledged as Kent-Eisenberg model in this publication. 3.1. Kent-Eisenberg model for CO2 solubility in aqueous of L-lysine alkaline salts Equations (6)e(11) provide the detail about the reactions that occur in the solubility cell [24].
kf
BOH ! Bþ þ OH
ka e kf are the equilibrium constants as given above in equation (6)e11. These are described further below;
½Hþ HCO HCO ½AA NH½H þ 3 ½AA NH 3 ; k ; kd ka ¼ ¼ ¼ ; k c b ½AA NCOO ½CO2 AA NHþ 2 i h ½Hþ CO2 3 ; ¼ HCO 3 ke ¼ ½Hþ ½OH ; kf ¼
½AA ¼ ½AA NH þ AA NHþ 2 þ ½AA NCOO
(13)
Charge balance for the reaction:
i h þ 2½AA NCOO þ ½OH þ HCO 2 CO2 3 3 þ ½AA NH ¼ ½Hþ þ ½Bþ
(14)
Equation (15) is obtained by solving the above equations, which is used to determine the value of hydrogen ion concentration in the solvent mixture. 5
4
3
2
A1 ½Hþ þ B1 ½Hþ þ C1 ½Hþ þ D1 ½Hþ þ E1 ½H þ þ F1 ¼ 0
(15)
A1 ¼ kbx B1 ¼ kbx ½Bþ þ kax kbx C1 ¼ kax kbx ½Bþ þ kax kc ½CO2 kbx kc ½CO2 kbx ke kax kbx ½AA D1 ¼ kax kc ½CO2 ½Bþ 2kax kc ½CO2 ½AA kax kbx kc ½CO2 kax kbx ke 2kbx kc kd ½CO2 2
F1 ¼ 2kax k2c kd ½CO2
2
CO2 loading in the solvent is determined by equation (16).
(8)
(9)
ke
! H2 O OH þ Hþ
i
h
(7)
kd
þ ! HCO CO2 3 3 þH
2 þ ½CO2 a½AA ½AA NCOO ¼ HCO 3 þ CO3
E1 ¼ kax k2c ½CO2 kax kc ke ½CO2 2kax kbx kc kd ½CO2
kc
þ CO2 þ H2 O ! HCO 3 þH
(12)
Material balance for carbon dioxide:
(6)
kb
! AA NCOO þ H2 O AA NH þ HCO 3
½Bþ ½OH ½BOH
In addition to that, the material balance for the written equations are described here. Material balance for L-lysine salt:
ka
! AA NHþ AA NH þ Hþ 2
(11)
(10)
a¼
½CO2 kc kd ½CO2 þ ½AA NCOO ½CO2 þ kc½H þ þ þ 2 ½H
½AA
(16)
The values of ½AA NCOO and [CO2] is determined by equations (17) and (18).
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
½AA NCOO ¼
½CO2 ¼
5
8 9 < k ½Bþ ½CO ½Hþ 2 þ k ½CO ½Hþ 3 k2 ½CO 2 ½Hþ ::: = c c 2 2 2 c : ::: k k ½CO ½H þ 2k2 k ½CO 2 ; c e
2
c d
2
(17)
kbx ½Hþ 3 þ 2kc ½CO2 ½Hþ 2
PCO2 HCO2
½ANH½Hþ kg ¼ ANH þ 2
(18)
Equation (19) determines the value of Henry's constant [25].
Material balances in addition of amino acids is given as: Material balance for piperazine:
HCO2 ¼ expð 6789:04 = T 11:4519 ln T 0:010454T þ94:4914Þ
½ALK ¼ ½ANH þ ANH þ 2
(19)
(26)
Revised material balance for carbon dioxide with addition of piperazine:
ki ¼ expðai = T þ bi ln T þ ci Þ
Revised charge balance after addition of piperazine:
i h þ 2½AA NCOO þ ½OH þ HCO 2 CO2 3 3 þ ½AA NHþ þ þ ¼ ANH þ 2 þ ½H þ ½B
Parameters, ai to ci for the above equation are given in Table 3. The apparent equilibrium constants were regressed as given in equations (21) and (22).
6
fax ¼ expðs1 ½AMT þ s2 PCO2 þ s3 ½AMT 2 fbx ¼ expðs4 ½AMT þ s5 PCO2 þ s6 ½AMT 2
5
4
3
2
A2 ½Hþ þ B2 ½Hþ þ C2 ½H þ þ D2 ½Hþ þ E2 ½Hþ þ F2 ½Hþ þ G2
(22)
(28)
The equations are solved in same manner as equation (15).
(21)
kbx ¼ kb fbx
i (27)
(20)
kax ¼ ka fax
h
2 að½AA þ ½ALKÞ ½AA NCOO ¼ ½CO2 þ HCO 3 þ CO3
3.1.1. Regressed parameters for the model Equation (20) allows the determination of the values for reaction equilibrium constants given in equations (8)e(10).
¼0 (29)
(23) A2 ¼ kbx
(24) B2 ¼ kbx kg þ kbx ½Bþ þ kax kbx þ kbx ½ALK
The values of these parameters are given in Tables 3 and 4.
C2 ¼ kg kbx ½Bþ þ kax kbx kg þ kax kbx ½ALK þ kax kbx ½Bþ kbx kc ½CO2 kbx ke þkax kc ½CO2 kax kbx ½AA
3.2. Kent-Eisenrberg model for CO2 absorption in aqueous blends of lysine salts and piperazine With the addition of piperazine, its deprotonation (as given in equation (25)) is added to the reaction mechanism given in equations (6)e(11).
Table 4 Parameters for fax and fbx.
kg
! ANH þ ANH þ H þ 2
Type
(25)
fbx
fax s1
s2
s3
s4
s5
s6
Potassium Lysinate 0.0040 0.0117 0.0078 0.0273 0.0036 0.0054 Sodium Lysinate 0.1514 0.0003 0.0167 1.4779 0.0333 0.1637
kg is hence defined as below.
Table 3 Parameters for equation (20). Type of equilibrium constant
A
B
C
Reference
kc kd ke kg ka kb
12092.1 12431.7 13445.9 9103.5 7907.7 16636.9
36.7816 35.4819 22.4773 0 0 0
235.482 220.067 140.932 10.346 7.815 1.015
Edwards et al. [25] Edwards et al. [25] Edwards et al. [25] Regressed in this study Regressed in this study Regressed in this study
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
Table 5 Parameters for fam and fbm. Type
Potassium Lysinate Sodium Lysinate
fbm
fam
2
F3 ¼ kax k2c kg ½CO2 kax kc ke kg ½CO2 2kax kbx kc kd kg ½CO2
s1
s2
s3
s4
s5
s6
0.1426 0.1461
0.0268 0.0272
0.2707 0.2678
0.0121 0.0422
0.0831 0.1928
0.5649 0.3201
D2 ¼ kax kbx kg ½Bþ kbx kc kg ½CO2 kbx ke kg þ kax kc kg ½CO2 kax kbx kg ½AA þ kax kc ½CO2 ½ALK þkax kc ½Bþ ½CO2 2kax kc ½CO2 ½AA kax kbx kc ½CO2 kax kbx ke 2kbx kc kd ½CO2
2kax k2c kd ½CO2 2
G3 ¼ 2kax k2c kd kg ½CO2
2
The carbon dioxide loading is calculated by equation (30).
E3 ¼ kax kc kg ½Bþ ½CO2 2kax kc kg ½CO2 ½AA kax kbx kc kg ½CO2 kax kbx ke kg
a¼
2
2kbx kc kd kg ½CO2 kax k2c ½CO2 kax kc ke ½CO2 2kax kbx kc kd ½CO2
½CO2 kc kd ½CO2 ½CO2 þ kc½H þ ½AA NCOO þ þ þ 2
½H
(30)
ð½AA þ ½ALKÞ
The value of ½RR NCOO is determined by the following equation.
Table 6 Experimental equilibrium carbon dioxide loadings (a) in solutions of L-lysine (1) þ potassium hydroxide (2) þ water (3) for a range of temperature (T) and pressure (P)a. T/K
P/kPa
L-lysine
303.15
323.15
343.15
363.15
a/mole of CO2. (mole of potassium lysinate)1
P/kPa
a/mole of CO2. (mole of potassium lysinate)1
P/kPa
a/mole of CO2. (mole of potassium lysinate)1
(1) þ potassium hydroxide (2) þ water (3)
w1 ¼ 0.145, w2 ¼ 0.056/mass fractionb
w1 ¼ 0.280, w2 ¼ 0.107/mass fractionc
w1 ¼ 0.405, w2 ¼ 0.155/mass fractiond
206.25 602.50 1181.25 1535.00 2062.50 2438.75 3080.00 3408.75 4058.75 181.25 578.75 1210.00 1620.00 2080.00 2576.25 3088.75 3558.75 4106.25 106.25 573.75 1061.25 1721.25 2090.00 2515.00 3092.50 3611.25 4203.75 156.25 542.50 1097.50 1578.75 2016.25 2515.00 2986.25 3541.25 4035.00
181.50 536.25 1082.50 1531.25 2032.50 2547.50 2996.25 3506.25 4081.25 150.00 616.25 1202.50 1652.50 2083.75 2578.75 2948.50 3311.25 3836.25 117.50 612.50 1157.50 1698.75 2202.50 2611.25 3118.75 3513.75 4045.00 122.50 483.75 1013.75 1511.25 2097.50 2548.75 3033.75 3495.00 4007.50
117.50 510.00 1146.25 1415.00 2118.75 2536.25 3030.00 3541.25 3963.75 192.50 506.25 1087.50 1370.00 2265.00 2753.75 3225.00 3627.50 4016.25 112.50 486.25 1126.25 1561.25 2221.25 2673.75 3163.75 3502.50 3996.25 97.50 441.25 997.50 1511.25 2003.75 2583.75 3157.50 3520.00 3995.00
0.9055 1.1215 1.2968 1.3954 1.5325 1.6700 1.9061 2.0077 2.1455 0.5348 0.7625 0.9861 1.1081 1.2307 1.3922 1.4725 1.6048 1.6975 0.2355 0.4690 0.5882 0.7555 0.8215 0.9296 1.0460 1.1457 1.2554 0.1548 0.2606 0.3817 0.4710 0.5215 0.6271 0.7366 0.7958 0.8572
0.7364 0.9245 1.0825 1.1467 1.2409 1.3310 1.4064 1.5214 1.6248 0.3825 0.5949 0.7627 0.8587 0.9375 1.0183 1.1254 1.1352 1.1969 0.1855 0.3563 0.4487 0.5377 0.6105 0.6650 0.7287 0.7759 0.8369 0.1255 0.1906 0.2691 0.3277 0.3885 0.4318 0.4761 0.5166 0.5896
0.5826 0.8123 0.9743 1.0219 1.1264 1.1810 1.2418 1.3020 1.3502 0.3653 0.4843 0.6423 0.6986 0.8393 0.9024 0.9579 1.0021 1.0426 0.1525 0.2670 0.3809 0.4395 0.5158 0.5626 0.6100 0.6412 0.6849 0.1103 0.1667 0.2352 0.2828 0.3223 0.3645 0.4034 0.4269 0.4567
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.072 mol of CO2 per mole of aqueous blend of potassium lysinate. Both at a confidence level of 0.95. b The reported mass fraction corresponds to 1 mol l1 concentration of potassium lysinate solution. c The reported mass fraction corresponds to 2 mol l1 concentration of potassium lysinate solution. d The reported mass fraction corresponds to 3 mol l1 concentration of potassium lysinate solution.
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
½AA NCOO ¼
9 8 ½ALK > þ 3 þ þ 2 þ 3 > = < ½H þ ½B ½H þ ½H ::: þ kc ½CO2 kg þ ½H > > ; : ::: kc ½CO2 ½Hþ ke ½Hþ 2kc kd ½CO2 kbx ½Hþ 3 þ 2kc ½CO2 ½Hþ 2 (31)
The value for kg is provided in Table 3.
3.2.1. Regressed parameters of the model for blends The values of ka and kb used for single L-lysine salts are extendable for the blends. However, different values for the correction factors, fam and fbm were determined by equations (32) and (33).
fam ¼ expðs1 ½AMT þ s2 PCO2 þ s3 ½ALK Þ
(32)
fbm ¼ expðs4 ½AMT þ s5 PCO2 þ s6 ½ALK Þ
(33)
The regressed values are given in Table 5.
7
4. Results and discussion 4.1. Experimental carbon dioxide solubility in aqueous alkaline salt solutions of lysine The experimental equilibrium CO2 loadings (mole of CO2/mole of aqueous alkaline salt of L-lysine) in aqueous solutions of potassium lysinate and sodium lysinate are presented in Tables 6 and 7, respectively. Our experimental findings for the single aqueous potassium and sodium salts of L-lysine exhibit a analogous behaviour as with other alkanolamines. The CO2 loadings increase with increase in the pressure values. The solubility curves of all known solvents and the ones studied in this work exhibit a distinctive “kink” in values of CO2 loadings with respect to pressure [26] and can be observed in Figs. 2 to 11. At low pressure values, the free solvent rapidly reacts with the carbon dioxide. Hence, the loading value increases rapidly with a slight increase in pressure [27]. But as the solvent is spent, the loading value drops, where the physical absorption takes over at the high pressure values. At high pressure, the loading value increases slightly with nominal increase in the pressure of the gas [28].
Table 7 Experimental equilibrium carbon dioxide loadings (a) in mixture of L-lysine (1) þ sodium hydroxide (2) þ water (3) for a range of temperature (T) and pressure (P)a. T/K
(mole of sodium lysinate)1
a/mole of CO2. (mole of sodium lysinate)1
w1 ¼ 0.142, w2 ¼ 0.039/mass fractionb
w1 ¼ 0.278, w2 ¼ 0.075/mass fractionc
w1 ¼ 0.401, w2 ¼ 0.110/mass fractiond
217.50 613.75 1192.50 1548.75 2071.25 2450.00 3110.00 3423.75 4070.00 175.00 573.75 1203.75 1628.75 2068.75 2570.00 3080.00 3557.50 4105.00 110.00 578.75 1063.75 1712.50 2093.75 2518.75 3078.75 3605.00 4145.00 162.50 531.25 1095.00 1575.00 2027.50 2541.25 2990.00 3548.75 4011.25
175.00 528.75 1077.50 1523.75 2022.50 2546.25 2981.25 3513.75 4040.00 140.00 606.25 1110.00 1557.50 2085.00 2550.00 2980.00 3357.50 3963.75 105.00 605.00 1155.00 1692.50 2183.75 2622.50 3118.75 3518.75 4032.50 120.00 485.00 1010.00 1521.25 2136.25 2575.00 3152.50 3585.00 4010.00
106.25 505.00 1147.50 1398.75 2108.75 2572.50 3105.00 3513.75 3965.00 155.00 512.50 1080.00 1383.75 2273.75 2785.00 3222.50 3675.00 4005.00 108.75 495.00 1041.25 1475.00 2217.50 2662.50 3057.50 3596.25 3946.25 95.00 447.50 1031.25 1506.25 2015.00 2496.25 3160.00 3518.75 3955.50
P/kPa
L-lysine
303.15
323.15
343.15
363.15
a/mole of CO2. (mole of sodium lysinate)1
P/kPa
a/mole of CO2.
P/kPa
(1) þ sodium hydroxide (2) þ water (3)
0.8943 1.0755 1.2866 1.3634 1.5005 1.6380 1.8741 1.9757 2.2014 0.5412 0.7355 0.9541 1.0761 1.1987 1.3642 1.4488 1.5242 1.6748 0.2365 0.4522 0.5215 0.7355 0.7963 0.8915 1.0652 1.1137 1.2175 0.1612 0.2366 0.3965 0.4785 0.4937 0.6325 0.7145 0.7885 0.8363
0.7044 0.8925 1.0652 1.1754 1.2365 1.2990 1.3814 1.4933 1.6325 0.3785 0.5742 0.7742 0.8264 0.9255 0.9911 1.0934 1.1215 1.2549 0.1485 0.3255 0.4215 0.5311 0.5863 0.6342 0.7014 0.7658 0.8211 0.1315 0.1845 0.2484 0.3146 0.3742 0.3997 0.4441 0.5366 0.5896
0.5515 0.7742 0.9953 1.0355 1.1015 1.1547 1.2346 1.2741 1.3189 0.3333 0.4795 0.6325 0.6690 0.8217 0.9125 0.9415 0.9842 1.0266 0.1625 0.2715 0.3696 0.4175 0.5215 0.5897 0.5742 0.6155 0.6615 0.1125 0.1548 0.2486 0.2696 0.3105 0.3695 0.3854 0.4216 0.4615
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.072 mol of CO2 per mole of aqueous blend of sodium lysinate. Both at a confidence level of 0.95. b The reported mass fraction corresponds to 1 mol l1 concentration of sodium lysinate solution. c The reported mass fraction corresponds to 2 mol l1 concentration of sodium lysinate solution. d The reported mass fraction corresponds to 3 mol l1 concentration of sodium lysinate solution.
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
Table 8 Experimental equilibrium carbon dioxide loadings (a) in blended solutions of L-lysine (1) þ potassium hydroxide (2) þ piperazine (3) þ water (4) of for a range of temperature (T) and pressure (P)a. T/K
P/kPa
a/mole of CO2.
P/kPa
(mole of aqueous blend of potassium lysinate and piperazine)1 L-lysine
303.15
323.15
343.15
363.15
a/mole of CO2. (mole of aqueous blend of potassium lysinate and piperazine)1
(1) þ potassium hydroxide (2) þ piperazine (3) þ water (4)
w1 ¼ 0.247, w2 ¼ 0.095, w3 ¼ 0.146/mass fractionb
w1 ¼ 0.221, w2 ¼ 0.085, w3 ¼ 0.260/mass fractionc
185.00 512.50 1143.75 1551.25 2105.00 2508.75 3017.50 3492.50 4021.25 175.00 536.25 1247.50 1512.50 1996.25 2481.25 2945.00 3525.00 4106.25 157.50 506.25 1112.50 1502.50 2052.50 2581.25 3105.00 3675.00 4203.75 105.00 497.50 1000.00 1611.25 2015.00 2587.50 3011.25 3497.50 4035.00
152.50 468.75 996.25 1495.00 2006.25 2501.25 3045.00 3492.50 4081.25 166.25 586.25 1107.50 1517.50 2027.50 2537.50 3048.75 3562.50 3913.75 125.00 596.25 1035.00 1503.72 2002.50 2532.50 3008.75 3631.25 4012.50 103.75 513.75 990.00 1505.00 2103.75 2498.75 3108.75 3560.00 3965.00
0.6502 0.8612 1.0465 1.1301 1.2286 1.2947 1.3742 1.4460 1.5241 0.3275 0.5306 0.7503 0.8109 0.9068 0.9902 1.0619 1.1438 1.2195 0.1460 0.2664 0.4056 0.4788 0.5708 0.6512 0.7254 0.8015 0.8685 0.0897 0.1350 0.2066 0.2811 0.3267 0.3884 0.4325 0.4819 0.5354
0.5956 0.8245 0.9921 1.0959 1.1840 1.2610 1.3405 1.4034 1.4841 0.3084 0.5322 0.6923 0.7867 0.8844 0.9684 1.0435 1.1125 1.1567 0.1244 0.2781 0.3735 0.4587 0.5389 0.6167 0.6819 0.7620 0.8088 0.0855 0.1855 0.2145 0.2552 0.3192 0.3594 0.4194 0.4626 0.5007
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.088 mol of CO2 per mole of aqueous blend of potassium lysinate, and piperazine. Both at a confidence level of 0.95. b The reported mass fraction corresponds to aqueous blend of 0.5 mol l1 concentration of piperazine and 2 mol l1 concentration of potassium lysinate solution. c The reported mass fraction corresponds to aqueous blend of 1 mol l1 concentration of piperazine and 2 mol l1 concentration of potassium lysinate solution.
The carbon dioxide solubility decreases with an increase in the temperature. This phenomenon is well understood and coherent with the other absorption solvents and explained by the increase in the mean molecular energies with the increase in temperature. This results in reduction of physical solubility of the gas in the liquid solvent [29]. Similarly, the carbon dioxide loadings decrease with the increase in the value of solvent concentrations. This can be understood by equation (4), where large denominator value on right hand side of the equation will certainly result in smaller values on left hand side. However, this must not be construed with the solubility values as the CO2 solubility (moles of carbon dioxide/kg of aqueous solvent) increases with increase in solvent concentration [30]. A direct comparison of sodium and potassium lysinate reveals that the CO2 loadings in sodium lysinate are slightly lower than potassium lysinate solutions. Quantitatively, the sodium lysinate has an average difference of 0.038 (mol CO2/mol solvent) than potassium lysinate when compared at similar parametric conditions. It is postulated that the non-idealities, presence of potassium
carbonate as an impurity in KOH, and the difference in molecular interactions of potassium and sodium ions in the amino acid salt solutions caused the observed minimal difference in the CO2 loading values [31]. The physical examination of sodium lysinate and potassium lysinate solutions was interesting. Small yet visible quantities of white precipitates formed for 2 M solutions of sodium lysinate at temperatures of 303.15 K and 323.15 K and higher pressures (>3000 kPa). Similarly, 3 M solutions of sodium lysinate formed precipitates at all temperatures (303.15e36315 K) and high pressures (>3000 kPa). The amount of precipitate reduced with increase in temperature [32]. A similar formation of precipitate was seen in 3 M solutions of potassium lysinate at 303.15 K and pressures above 3000 kPa, only. Other potassium lysinate solutions of lower concentrations were clear.
4.2. Experimental carbon dioxide solubility in aqueous blends of piperazine and alkaline salt solutions of lysine Tables 8 and 9 present the values of carbon dioxide loadings in
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
9
Table 9 Experimental equilibrium carbon dioxide loadings (a) in blended solutions L-lysine (1) þ sodium hydroxide (2) þ piperazine (3) þ water (4) for a range of temperature (T) and pressure (P)a. T/K
P/kPa
a/mole of CO2.
P/kPa
(mole of aqueous blend of sodium lysinate and piperazine)1 L-lysine
303.15
323.15
343.15
363.15
a/mole of CO2. (mole of aqueous blend of sodium lysinate and piperazine)1
(1) þ sodium hydroxide (2) þ piperazine (3) þ water (4)
w1 ¼ 0.245, w2 ¼ 0.067, w3 ¼ 0.144/mass fractionb
w1 ¼ 0.221, w2 ¼ 0.060, w3 ¼ 0.259/mass fractionc
162.50 602.50 1235.00 1520.00 2102.50 2442.50 3165.00 3502.50 4011.25 167.50 607.50 1095.00 1521.25 2035.00 2487.50 2995.00 3542.50 4105.00 185.00 486.75 1007.50 1511.25 2106.25 2495.00 2991.25 3512.50 4145.00 190.00 431.25 1035.00 1605.00 1948.75 2613.75 3017.50 3510.00 4011.25
156.00 622.50 1015.00 1487.50 2005.00 2413.75 3013.75 3513.75 4022.50 187.50 488.75 937.50 1477.50 2033.75 2513.75 3152.50 3546.25 4021.25 145.00 511.25 1013.75 1437.50 2017.50 2543.75 3041.25 3518.75 3912.50 186.25 518.75 1087.50 1521.25 2110.00 2582.50 3041.25 3607.50 3995.00
0.6245 0.8963 1.0665 1.1242 1.2282 1.2841 1.3967 1.4475 1.5226 0.3212 0.5589 0.7117 0.8128 0.9139 0.9912 1.0693 1.1461 1.2193 0.1585 0.2610 0.3843 0.4803 0.5793 0.6385 0.7097 0.7802 0.8612 0.0875 0.1241 0.2111 0.2804 0.3193 0.3911 0.4331 0.4832 0.5331
0.6000 0.8855 0.9966 1.0945 1.1838 1.2478 1.3360 1.4064 1.4761 0.3253 0.4929 0.6466 0.7782 0.8855 0.9647 1.0579 1.1104 1.1699 0.1342 0.2565 0.3693 0.4473 0.5412 0.6183 0.6862 0.7479 0.7967 0.0895 0.1915 0.2070 0.2570 0.3198 0.3678 0.4129 0.4671 0.5035
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.088 mol of CO2 per mole of aqueous blend of sodium lysinate and piperazine. Both at a confidence level of 0.95. b The reported mass fraction corresponds to aqueous blend of 0.5 mol l1 concentration of piperazine and 2 mol l1 concentration of sodium lysinate solution. c The reported mass fraction corresponds to aqueous blend of 1 mol l1 concentration of piperazine and 2 mol l1 concentration of sodium lysinate solution.
the aqueous blends of piperazine with potassium lysinate and sodium lysinate, respectively. The piperazine blended solutions of both sodium lysinate and potassium lysinate showed formation of a precipitate at 303.15 K and 323.15 K and at pressures greater than 3000 kPa. However, the contents formed a murky solution upon shaking rather than settling down as clumps of precipitate, as seen in single solutions of sodium and potassium lysinate [33]. This situation warrants further study and invites exploiting their potential as a three phase (solidliquid-gas) carbon capture solvent for natural gas processing [34]. 4.3. Modelling results Table 10 reports the average absolute percentage deviation (AAD%) of the correlated results for the Kent-Eisenberg model upon comparison to the experimental values. Fig. 2 presents the modelling results against the experimental findings of this study for 1 M aqueous solutions of potassium salt of L-lysine. At low pressures, the model shows slight underprediction than the experimental values. This can be attributed
Table 10 Average absolute deviation (AAD%) for the K-E model correlation. Solvent
AAD%
Potassium lysinate Sodium lysinate Piperazine and potassium lysinate Piperazine and sodium lysinate
6.92 6.26 3.33 2.63
to the high carbon dioxide loadings observed at low pressure than correlated by a model that has been regressed to a majority of high pressure data, a region where the physical solubility dominates [14]. For the temperature of 363.15 K, the data is slightly under predicted, where the major errors arise in the low pressure region and are attributed to the same reason, discussed above. Nevertheless, the correlated values showed overall low errors for all solvent systems and exhibited correlation behaviour usually associated with Kent-Eisenberg type semi-empirical models [23]. The correlation trends for 2 M and 3 M solutions are identical to described in Fig. 2.
10
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
Fig. 2. Results of model against experimental equilibrium carbon dioxide loadings in 1 M potassium lysinate solutions at different temperatures and carbon dioxide pressures.
Fig. 4. Results of model against experimental equilibrium carbon dioxide loadings in aqueous blend of 0.5 M piperazine and 2 M potassium lysinate solutions at different temperatures and carbon dioxide pressures.
Fig. 3 presents the correlative results of the model for the equilibrium carbon dioxide loading in 2 M aqueous solution of sodium salt of L-lysine. The model results are exactly similar to the trends seen in the potassium lysinate solutions. At 303.15 K, the model values are slightly under-predicted at the pressures less than 100 kPa, whereas the trend at higher pressure (>1000 kPa) shows a slight over-prediction. This also means that the model parameters are well-regressed at 303.15 K, as errors are well distributed [14]. The model trends at 363.15 K show a slight underprediction at all pressure values. However, the errors are well below the experimental errors within the study and are unaffected by the increasing solvent concentration. Overall, the model results are in good agreement with the experimental values, determined in this study.
Fig. 4 illustrate the modelling results against the experimental values of carbon dioxide loadings in aqueous blends of 0.5 M piperazine with 2 M potassium lysinate, at equilibrium. Fig. 5 shows the correlation for the experimental values of equilibrium carbon dioxide loadings in aqueous blends of 1 M piperazine with 2 M sodium lysinate. For all studied blends, a slight over-prediction in model results is seen for all temperatures except 303.15 K. The error was induced due to the modelling effort to compensate for the error in low pressure region and contributes mainly to the error values [35]. On the contrary, the change in pressure and concentration do not have a decipherable effect on the error values of carbon dioxide loadings, which means that the both parameters are well-regressed [36]. The model results are in excellent agreement with our experimental data.
Fig. 3. Results of model against experimental equilibrium carbon dioxide loadings in 2 M sodium lysinate solutions at different temperatures and carbon dioxide pressures.
Fig. 5. Results of model against experimental equilibrium carbon dioxide loadings in aqueous blend of 1 M piperazine and 2 M sodium lysinate solutions at different temperatures and carbon dioxide pressures.
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
5. Conclusions In this study, experimental measurements of carbon dioxide solubility are reported in the sodium and potassium salts of L-lysine and their separate blends with piperazine in the high pressure e high loading region. The solubility has been studied by using a high pressure VLE apparatus for a wide range of temperature (303.15e363.15 K), amino acid salt concentrations (1e3 M) and pressure (95e4204 kPa). Piperazine in strengths of 0.5 and 1.0 M was added to investigate its effect on the overall absorption capacity. It was noted that CO2 dioxide loadings have a positive relationship with increase in pressure and addition of piperazine for all solvents studied. On the other hand, increase in temperature and solvent concentrations results in decrease of the CO2 loading values. The experimental results were correlated by the Kent-Eisenberg model. Correlated results exhibited an AAD% of 6.92% (potassium lysinate), 6.26% (sodium lysinate), 3.33% (PZ-potassium lysinate) and 2.63% (PZ-sodium lysinate) for the values of carbon dioxide loadings. The findings of this study indicate that the alkaline salt of L-lysine can be suitably used as a singular carbon dioxide solvent for a range of process conditions. Moreover, they can also be blended with piperazine for better carbon dioxide absorption capacity and probably better kinetic rates, which warrants further study.
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
Author contributions Humbul Suleman: Investigation, Formal Analysis, Conceptualization, Methodology, Software, Writing e Original Draft. Abdulhalim Shah Maulud: Supervision, Project Administration, Resources, Funding Acquisition. Afaf Syalsabila: Validation, Writing e Original Draft. Muhammad Zubair Shahid: Visualization, Writing e Review and Editing. Philip Loldrup Fosbøl: Writing e Review and Editing.
[20]
Declaration of competing interest
[21]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[22]
[17]
[18]
[19]
[23]
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
High-pressure experimental and theoretical study of CO2 solubility in aqueous blends of lysine salts with piperazine as new absorbents Humbul Suleman a, Abdulhalim Shah Maulud b, c, *, Afaf Syalsabila c, Muhammad Zubair Shahid c, Philip Loldrup Fosbøl a a
Center for Energy Resources Engineering, Department of Chemical and Biochemical Engineering, Technical University of Denmark, Kongens Lyngby, 2800, Denmark Centre of Contaminant Control and Utilisation, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, 32610, Perak, Malaysia c Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak, Malaysia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2019 Received in revised form 15 November 2019 Accepted 29 November 2019 Available online 30 November 2019
Vapour-liquid equilibrium of carbon dioxide loaded potassium and sodium L-lysine salts have been investigated in the region of high-pressure and high loadings. The equilibrium solubility of carbon dioxide was measured in separate aqueous L-lysine alkaline salts for a range of temperature (303.15 e363.15 K), solution concentrations (1.0e3.0 M) and pressure (95e4204 kPa). In addition to that, solubility of carbon dioxide in blends of both aforementioned amino acid salts with piperazine were investigated. All the studied solutions exhibited an increase in the carbon dioxide loading values with increase in the pressure and had a negative relationship with increase in the temperature and solvent concentration. Furthermore, the experimental data was correlated by the Kent-Eisenberg model. The correlated values shows a good agreement (AAD% of 5.15%) with experimental values. The regressed parameters of the model allows satisfactory estimation of the loadings of carbon dioxide in all studied solutions, for parametric studies. The study shows that L-lysine salts are potential green solvents for the carbon dioxide capture at high pressure. © 2019 Elsevier B.V. All rights reserved.
Keywords: Amino acids Blends Carbon capture Piperazine Vapour Liquid equilibrium
1. Introduction Separation of carbon dioxide is now considered as a mandatory step in gas cleaning processes. A number of separation techniques are applied and essentially operate at stringent process conditions, usually high pressure [1]. This includes absorption, adsorption, membranes and cryogenic methods [2]. Of which, absorption is the most commercially accepted method. Both physical and chemical solvent based absorption is used to separate carbon dioxide. Polyethylene glycols, methanol and organic carbonates are some common examples of physical solvents, while chemical solvents include ammonia, potassium carbonate and alkanolamines [3]. The solvent (either physical or chemical) carries the captured carbon dioxide to the stripper section for desorption at relatively high temperature and lower pressure than the absorber. Vast research efforts are underway to develop new techniques
and chemicals for an efficient carbon dioxide capture from the natural gas [4]. One of the research emphasis is the development of unique solvents and their blends for providing similar or better efficiency for the separation and energy savings with a reduction in the hazardous emissions [5,6]. Alkaline salts of amino acids are a potential option, as they are inherently environmental friendly and operate on similar absorption principle as of conventional alkanolamines [7]. This is possible due to their zwitterion structure, which allows the amino acids to behave both as a weak acid and weak base, when introduced to an acidic and basic medium, respectively. Equation (1) presents the zwitterion structure and its behaviour in acidic and basic mediums. þH þ
! NHþ NHþ 3 R COOH 3 R COO H þ
! NH2 R COO
* Corresponding author. Centre of Contaminant Control and Utilisation, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, 32610, Perak, Malaysia. E-mail address: [email protected] (A.S. Maulud). https://doi.org/10.1016/j.fluid.2019.112429 0378-3812/© 2019 Elsevier B.V. All rights reserved.
Low pH
Neutral
High pH
(1)
In a basic medium, the amine group of an amino acid becomes activated and behaves similar to an alkanolamine. Thus, these
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
Nomenclature
Abbreviations L-lysine salt AA AAD% average absolute percentage deviation ALK piperazine Bþ ion of the base BOH base (either sodium- or potassium hydroxide) CO2 carbon dioxide CO2 carbonate 3 HCO2 Henry's constant, atm.litre/mole HCO bicarbonate ion 3 Hþ ion of hydrogen H2O water k equilibrium constant Subscript a related to protonation of L-lysine salt Subscript b related to carbamate hydrolysis of L-lysine salt Subscript c related to bicarbonate Subscript d related to carbonate Subscript e related to water Subscript f related to base Subscript g related to piperazine MDEA Methyldiethanolamine MEA Monoethanolamine n number of moles of Subscript transfer transferred from feed to solubility cell Subscript 1 initial value before transfer of gas Subscript 2 final value after transfer of gas
activated structures can chemically react with carbon dioxide. A recent review [8] provides detailed information about the carbon dioxide solubility in various amino acids. Of which, L-lysine alkaline salts are potential candidates for a solvent in carbon dioxide capture. As its structure has two amine bonds, each amino acid molecule is theoretically capable of absorbing two carbon dioxide molecules. However, the behaviour of the second amine group on the lysine's R-group in capturing a carbon dioxide molecule is interesting. Mazinani et al. [9] investigated the performance of potassium lysinate as a potential carbon capture solvent. They studied the equilibrium carbon dioxide solubility at near-ambient temperatures (298.15e313.15 K), low pressure (5.40e41.47 kPa) and potassium lysinate concentrations (0.5e2.50 M). The group of Shen [10,11] have performed an elaborate study on the low pressure equilibrium CO2 solubility in aqueous potassium salt of L-lysine and their physicochemical properties. They have experimentally measured the solubility in various concentrations of aqueous potassium lysinate solutions (0.5e2.50 M) for low pressure conditions (0.07e115.26 kPa) and temperature (298.15e353.15 K). Our previous study [12] investigated the carbon dioxide solubility in the same salt at high partial pressures of carbon dixoide (150e4040 kPa) and absorber's temperature conditions. All aforementioned research works have noted a good absorption capacity of potassium lysinate but no study is available for sodium lysinate solutions, a sister salt. In our recent publication [13], we have also studied the effect of activator/co-promoter on equilibrium carbon dioxide solubility in equimolar (i.e. 1 þ 1 and 2 þ 2 M) concentrations of piperazine and 2-amino-2-methyl-1-propanolol, blended with potassium salt of Llysine. The research work concluded that the piperazine has substantial effect on carbon dioxide solubility. However, the study did
Subscript absorbed carbon dioxide absorbed by the solvent Subscript amino acid amino acid in the liquid solvent placed initially Subscript residue unabsorbed moles of CO2 (in gaseous form) at equilibrium OH e hydroxide ion Roman Symbols ANH unreacted piperazine ANH þ protonated piperazine 2 AA NH unreacted L-lysine AA NH þ protonated L-lysine 2 AA NCOO carbamate of L-lysine fax, fbx, fam, fbm correction parameters for the model kax regressed equilibrium constant for AA NH þ 2 kbx regressed equilibrium constant for AA NCOO P pressure, kPa unless mentioned otherwise Subscript Tot total pressure Subscript Amine L-lysine or L-lysine blended with piperazine Subscript N2 nitrogen Subscript H2O water s1 e s6 correction parameters for the models T temperature, K ½ concentration of any specie, mole/litre, unless mentioned otherwise Greek Symbols loading value of CO2, mole of CO2/mole of solvent
a
not study the effect of smaller concentrations of piperazine on the CO2 solubility in aqueous solutions of potassium lysinate. None of the experimental information presented in Refs. [12,13] has been included in this work. In this study, the equilibrium carbon dioxide solubility was experimentally measured in alkaline salts (both potassium and sodium) and their blends with piperazine. The study was conducted in the high pressure region (95e4204 kPa) and temperatures (303.15e363.15 K). The Kent-Eisenberg model (details in Ref. [14]) for the amino acid solutions [15] was applied to correlate the experimental findings for single potassium and sodium salt solutions of L-lysine and their separate blends with piperazine. The model shows good agreement with the experimental values. 2. Materials and methods Merck Malaysia provided all the solid chemicals for this study. Gas Walker supplied gases (carbon dioxide and nitrogen). Table 1 presents the information about the materials used in this study. No further purification of any chemicals was carried out. The preparation for the test solutions started with the chemicals being separately weighed on a Sartorius BSA224S-CW mass balance (u ¼ 0.1 mg). Weighed samples of chemicals were then conjointly added to a 250 ml volumetric flask (u ¼ 0.2 K) at 298.15 K. Deionized water was added to make the solution and ample time was allowed for the degassing. Fresh solutions were prepared for each run. The experimental equilibrium solubility in L-lysine alkaline salts and their blends with piperazine were determined by the highpressure vapour-liquid equilibrium apparatus. Fig. 1 shows an illustration of the apparatus. Information about the dimensions and fabrication of the equipment apparatus can be seen in Ref. [16].
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
3
Table 1 Sample provenance table. IUPAC Name
Abbreviation
CAS Registry Number
Puritya
Purification Method
Carbon dioxide Nitrogen L-lysine monohydrate Piperazine Sodium hydroxide Potassium hydroxide
CO2 N2 Lys PZ NaOH KOH
124-38-9 7727-37-9 39665-12-8 110-85-0 1310-73-2 1310-58-3
99.95 vol% 99.9 vol% 99.0 wt% 99.0 wt% 97.0 wt% 85.0 wt%
None None None None None None
a
Vol% ¼ by volume percentage, wt% ¼ by weight percentage.
Fig. 1. The apparatus used in this study.
Table 2 presents some of the reproduced experimental data taken from the open literature to validate the equipment performance. The feed cell was filled with carbon dioxide through valve V1 up to a pressure around 5000 kPa at 303.15 K. Two hours were allowed to the carbon dioxide gas in order to equilibrate with the feed tank's conditions. After careful degassing, the cell was filled with 100 ml of fresh amino acid salt solution. Nitrogen gas was passed through the solubility cell for 5 min. The temperature was then increased slowly (0.2 C/min) to the desired temperature and given ample time to
stabilize (usually 2 h). The CO2 gas was fed slowly into the solubility cell. Hence, ntransfer was calculated as follows.
ntransfer ¼ n1 n2
(2)
The pressure in the solubility cell decreased because of the physical dissolution and reaction of carbon dioxide with the solvent. The rate of pressure drop was quite fast in the start but slowed later, as it neared equilibrium. The temperature and pressure values were noted at the equilibrium. Upon removal, the contents of the cell were physically examined for formation of precipitates, if any
Table 2 Reproduced experimental information to validate equipment's performance for carbon dioxide loadings (a) in separate solutions of N-methyldiethanolamine (1) þ ‘water (2) and monoethanolamine (1) þ water (2) at different values of pressure (P), temperature (T) and molar concentrations of each alkanolamine (M1)a. Researcher
M1/Mole of alkanolamine. (litre of aqueous solution)1
N-methyldiethanolamine (1) þ water (2) Jou et al. [17] 2.00 2.00 4.28 4.28 monoethanolamine (1) þ water (2) Lee et al. [18] 2.50 2.50 5.00 5.00
T/K
Reported
This Study
Deviation (%)
P/kPa
a/mole of CO2. (mole of alkanolamine)1
P/kPa
a/mole CO2. (mole of alkanolamine)1
313.15 343.15 313.15 343.15
640.0 2320 2800 705.0
1.0830 1.1470 1.1700 0.740
647.5 2325 2795 700.0
1.0748 1.1612 1.1925 0.7658
0.75 þ1.23 þ1.92 þ3.48
313.15 353.15 313.15 353.15
100 3160 1000 316
0.6730 0.9160 0.7800 0.5550
102.5 3175 995.0 315
0.6610 0.9254 0.7695 0.5326
1.78 þ1.03 1.34 4.03
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.058 mol of CO2 per mole of alkanolamine (either N-methyldiethanolamine or monoethanolamine. Both at a confidence level of 0.95.
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
and the contents of the cell were washed and placed in special waste containers. The equilibrium pressure of carbon dioxide, PCO2, was calculated by equation (3).
PCO2 ¼ PTot PN2 PH2O PAmine
(3)
The values and PAmine, PN2 and PH2O are determined by Ref. [19]. The experimental loading value of CO2 in the salt of L-lysine and their blends with piperazine was calculated using equation (4).
a¼
nabsorbed namino acid
(4)
nabsorbed is calculated by equation (5).
nabsorbed ¼ ntransferred nresidue
(5)
Peng and Robinson equation of state was used for calculating the moles of carbon dioxide in equation (2)e5 [20]. 3. Equilibrium modelling The equilibrium model used in this work is a modified form of the model presented by Kent and Eisenberg [21]. The original model is based on first-principles chemistry and uses an iterative set of equations to determine the acid gas solubility in aqueous alkanolamines. The reactions are based on equilibrium constants. The value of activity coefficient for all species is taken as one and the non-ideal effects are lumped in correction parameters. These presumptions are somewhat valid for aqueous solutions with low concentrations, containing small number of ionic species [14]. The model was mathematically modified by Haji-Sulaiman et al. [22] to its present form, where the set of iterative equations in the original work were replaced by a polynomial equation to determine the hydrogen ion concentrations. However, the model basis and method of solution remains unchanged to the date. The model has been extended to correlate the carbon dioxide solubility in amino acid salt solutions with a minor modification. An equation for complete ionization of alkali in the mechanism of the model was added by Mondal et al. [23]. Recently, Suleman et al. [15] have provided a standard set of correction parameters for the correlation of carbon dioxide solubility in various amino acid salt solutions. However, to credit the original developers the model is acknowledged as Kent-Eisenberg model in this publication. 3.1. Kent-Eisenberg model for CO2 solubility in aqueous of L-lysine alkaline salts Equations (6)e(11) provide the detail about the reactions that occur in the solubility cell [24].
kf
BOH ! Bþ þ OH
ka e kf are the equilibrium constants as given above in equation (6)e11. These are described further below;
½Hþ HCO HCO ½AA NH½H þ 3 ½AA NH 3 ; k ; kd ka ¼ ¼ ¼ ; k c b ½AA NCOO ½CO2 AA NHþ 2 i h ½Hþ CO2 3 ; ¼ HCO 3 ke ¼ ½Hþ ½OH ; kf ¼
½AA ¼ ½AA NH þ AA NHþ 2 þ ½AA NCOO
(13)
Charge balance for the reaction:
i h þ 2½AA NCOO þ ½OH þ HCO 2 CO2 3 3 þ ½AA NH ¼ ½Hþ þ ½Bþ
(14)
Equation (15) is obtained by solving the above equations, which is used to determine the value of hydrogen ion concentration in the solvent mixture. 5
4
3
2
A1 ½Hþ þ B1 ½Hþ þ C1 ½Hþ þ D1 ½Hþ þ E1 ½H þ þ F1 ¼ 0
(15)
A1 ¼ kbx B1 ¼ kbx ½Bþ þ kax kbx C1 ¼ kax kbx ½Bþ þ kax kc ½CO2 kbx kc ½CO2 kbx ke kax kbx ½AA D1 ¼ kax kc ½CO2 ½Bþ 2kax kc ½CO2 ½AA kax kbx kc ½CO2 kax kbx ke 2kbx kc kd ½CO2 2
F1 ¼ 2kax k2c kd ½CO2
2
CO2 loading in the solvent is determined by equation (16).
(8)
(9)
ke
! H2 O OH þ Hþ
i
h
(7)
kd
þ ! HCO CO2 3 3 þH
2 þ ½CO2 a½AA ½AA NCOO ¼ HCO 3 þ CO3
E1 ¼ kax k2c ½CO2 kax kc ke ½CO2 2kax kbx kc kd ½CO2
kc
þ CO2 þ H2 O ! HCO 3 þH
(12)
Material balance for carbon dioxide:
(6)
kb
! AA NCOO þ H2 O AA NH þ HCO 3
½Bþ ½OH ½BOH
In addition to that, the material balance for the written equations are described here. Material balance for L-lysine salt:
ka
! AA NHþ AA NH þ Hþ 2
(11)
(10)
a¼
½CO2 kc kd ½CO2 þ ½AA NCOO ½CO2 þ kc½H þ þ þ 2 ½H
½AA
(16)
The values of ½AA NCOO and [CO2] is determined by equations (17) and (18).
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
½AA NCOO ¼
½CO2 ¼
5
8 9 < k ½Bþ ½CO ½Hþ 2 þ k ½CO ½Hþ 3 k2 ½CO 2 ½Hþ ::: = c c 2 2 2 c : ::: k k ½CO ½H þ 2k2 k ½CO 2 ; c e
2
c d
2
(17)
kbx ½Hþ 3 þ 2kc ½CO2 ½Hþ 2
PCO2 HCO2
½ANH½Hþ kg ¼ ANH þ 2
(18)
Equation (19) determines the value of Henry's constant [25].
Material balances in addition of amino acids is given as: Material balance for piperazine:
HCO2 ¼ expð 6789:04 = T 11:4519 ln T 0:010454T þ94:4914Þ
½ALK ¼ ½ANH þ ANH þ 2
(19)
(26)
Revised material balance for carbon dioxide with addition of piperazine:
ki ¼ expðai = T þ bi ln T þ ci Þ
Revised charge balance after addition of piperazine:
i h þ 2½AA NCOO þ ½OH þ HCO 2 CO2 3 3 þ ½AA NHþ þ þ ¼ ANH þ 2 þ ½H þ ½B
Parameters, ai to ci for the above equation are given in Table 3. The apparent equilibrium constants were regressed as given in equations (21) and (22).
6
fax ¼ expðs1 ½AMT þ s2 PCO2 þ s3 ½AMT 2 fbx ¼ expðs4 ½AMT þ s5 PCO2 þ s6 ½AMT 2
5
4
3
2
A2 ½Hþ þ B2 ½Hþ þ C2 ½H þ þ D2 ½Hþ þ E2 ½Hþ þ F2 ½Hþ þ G2
(22)
(28)
The equations are solved in same manner as equation (15).
(21)
kbx ¼ kb fbx
i (27)
(20)
kax ¼ ka fax
h
2 að½AA þ ½ALKÞ ½AA NCOO ¼ ½CO2 þ HCO 3 þ CO3
3.1.1. Regressed parameters for the model Equation (20) allows the determination of the values for reaction equilibrium constants given in equations (8)e(10).
¼0 (29)
(23) A2 ¼ kbx
(24) B2 ¼ kbx kg þ kbx ½Bþ þ kax kbx þ kbx ½ALK
The values of these parameters are given in Tables 3 and 4.
C2 ¼ kg kbx ½Bþ þ kax kbx kg þ kax kbx ½ALK þ kax kbx ½Bþ kbx kc ½CO2 kbx ke þkax kc ½CO2 kax kbx ½AA
3.2. Kent-Eisenrberg model for CO2 absorption in aqueous blends of lysine salts and piperazine With the addition of piperazine, its deprotonation (as given in equation (25)) is added to the reaction mechanism given in equations (6)e(11).
Table 4 Parameters for fax and fbx.
kg
! ANH þ ANH þ H þ 2
Type
(25)
fbx
fax s1
s2
s3
s4
s5
s6
Potassium Lysinate 0.0040 0.0117 0.0078 0.0273 0.0036 0.0054 Sodium Lysinate 0.1514 0.0003 0.0167 1.4779 0.0333 0.1637
kg is hence defined as below.
Table 3 Parameters for equation (20). Type of equilibrium constant
A
B
C
Reference
kc kd ke kg ka kb
12092.1 12431.7 13445.9 9103.5 7907.7 16636.9
36.7816 35.4819 22.4773 0 0 0
235.482 220.067 140.932 10.346 7.815 1.015
Edwards et al. [25] Edwards et al. [25] Edwards et al. [25] Regressed in this study Regressed in this study Regressed in this study
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
Table 5 Parameters for fam and fbm. Type
Potassium Lysinate Sodium Lysinate
fbm
fam
2
F3 ¼ kax k2c kg ½CO2 kax kc ke kg ½CO2 2kax kbx kc kd kg ½CO2
s1
s2
s3
s4
s5
s6
0.1426 0.1461
0.0268 0.0272
0.2707 0.2678
0.0121 0.0422
0.0831 0.1928
0.5649 0.3201
D2 ¼ kax kbx kg ½Bþ kbx kc kg ½CO2 kbx ke kg þ kax kc kg ½CO2 kax kbx kg ½AA þ kax kc ½CO2 ½ALK þkax kc ½Bþ ½CO2 2kax kc ½CO2 ½AA kax kbx kc ½CO2 kax kbx ke 2kbx kc kd ½CO2
2kax k2c kd ½CO2 2
G3 ¼ 2kax k2c kd kg ½CO2
2
The carbon dioxide loading is calculated by equation (30).
E3 ¼ kax kc kg ½Bþ ½CO2 2kax kc kg ½CO2 ½AA kax kbx kc kg ½CO2 kax kbx ke kg
a¼
2
2kbx kc kd kg ½CO2 kax k2c ½CO2 kax kc ke ½CO2 2kax kbx kc kd ½CO2
½CO2 kc kd ½CO2 ½CO2 þ kc½H þ ½AA NCOO þ þ þ 2
½H
(30)
ð½AA þ ½ALKÞ
The value of ½RR NCOO is determined by the following equation.
Table 6 Experimental equilibrium carbon dioxide loadings (a) in solutions of L-lysine (1) þ potassium hydroxide (2) þ water (3) for a range of temperature (T) and pressure (P)a. T/K
P/kPa
L-lysine
303.15
323.15
343.15
363.15
a/mole of CO2. (mole of potassium lysinate)1
P/kPa
a/mole of CO2. (mole of potassium lysinate)1
P/kPa
a/mole of CO2. (mole of potassium lysinate)1
(1) þ potassium hydroxide (2) þ water (3)
w1 ¼ 0.145, w2 ¼ 0.056/mass fractionb
w1 ¼ 0.280, w2 ¼ 0.107/mass fractionc
w1 ¼ 0.405, w2 ¼ 0.155/mass fractiond
206.25 602.50 1181.25 1535.00 2062.50 2438.75 3080.00 3408.75 4058.75 181.25 578.75 1210.00 1620.00 2080.00 2576.25 3088.75 3558.75 4106.25 106.25 573.75 1061.25 1721.25 2090.00 2515.00 3092.50 3611.25 4203.75 156.25 542.50 1097.50 1578.75 2016.25 2515.00 2986.25 3541.25 4035.00
181.50 536.25 1082.50 1531.25 2032.50 2547.50 2996.25 3506.25 4081.25 150.00 616.25 1202.50 1652.50 2083.75 2578.75 2948.50 3311.25 3836.25 117.50 612.50 1157.50 1698.75 2202.50 2611.25 3118.75 3513.75 4045.00 122.50 483.75 1013.75 1511.25 2097.50 2548.75 3033.75 3495.00 4007.50
117.50 510.00 1146.25 1415.00 2118.75 2536.25 3030.00 3541.25 3963.75 192.50 506.25 1087.50 1370.00 2265.00 2753.75 3225.00 3627.50 4016.25 112.50 486.25 1126.25 1561.25 2221.25 2673.75 3163.75 3502.50 3996.25 97.50 441.25 997.50 1511.25 2003.75 2583.75 3157.50 3520.00 3995.00
0.9055 1.1215 1.2968 1.3954 1.5325 1.6700 1.9061 2.0077 2.1455 0.5348 0.7625 0.9861 1.1081 1.2307 1.3922 1.4725 1.6048 1.6975 0.2355 0.4690 0.5882 0.7555 0.8215 0.9296 1.0460 1.1457 1.2554 0.1548 0.2606 0.3817 0.4710 0.5215 0.6271 0.7366 0.7958 0.8572
0.7364 0.9245 1.0825 1.1467 1.2409 1.3310 1.4064 1.5214 1.6248 0.3825 0.5949 0.7627 0.8587 0.9375 1.0183 1.1254 1.1352 1.1969 0.1855 0.3563 0.4487 0.5377 0.6105 0.6650 0.7287 0.7759 0.8369 0.1255 0.1906 0.2691 0.3277 0.3885 0.4318 0.4761 0.5166 0.5896
0.5826 0.8123 0.9743 1.0219 1.1264 1.1810 1.2418 1.3020 1.3502 0.3653 0.4843 0.6423 0.6986 0.8393 0.9024 0.9579 1.0021 1.0426 0.1525 0.2670 0.3809 0.4395 0.5158 0.5626 0.6100 0.6412 0.6849 0.1103 0.1667 0.2352 0.2828 0.3223 0.3645 0.4034 0.4269 0.4567
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.072 mol of CO2 per mole of aqueous blend of potassium lysinate. Both at a confidence level of 0.95. b The reported mass fraction corresponds to 1 mol l1 concentration of potassium lysinate solution. c The reported mass fraction corresponds to 2 mol l1 concentration of potassium lysinate solution. d The reported mass fraction corresponds to 3 mol l1 concentration of potassium lysinate solution.
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
½AA NCOO ¼
9 8 ½ALK > þ 3 þ þ 2 þ 3 > = < ½H þ ½B ½H þ ½H ::: þ kc ½CO2 kg þ ½H > > ; : ::: kc ½CO2 ½Hþ ke ½Hþ 2kc kd ½CO2 kbx ½Hþ 3 þ 2kc ½CO2 ½Hþ 2 (31)
The value for kg is provided in Table 3.
3.2.1. Regressed parameters of the model for blends The values of ka and kb used for single L-lysine salts are extendable for the blends. However, different values for the correction factors, fam and fbm were determined by equations (32) and (33).
fam ¼ expðs1 ½AMT þ s2 PCO2 þ s3 ½ALK Þ
(32)
fbm ¼ expðs4 ½AMT þ s5 PCO2 þ s6 ½ALK Þ
(33)
The regressed values are given in Table 5.
7
4. Results and discussion 4.1. Experimental carbon dioxide solubility in aqueous alkaline salt solutions of lysine The experimental equilibrium CO2 loadings (mole of CO2/mole of aqueous alkaline salt of L-lysine) in aqueous solutions of potassium lysinate and sodium lysinate are presented in Tables 6 and 7, respectively. Our experimental findings for the single aqueous potassium and sodium salts of L-lysine exhibit a analogous behaviour as with other alkanolamines. The CO2 loadings increase with increase in the pressure values. The solubility curves of all known solvents and the ones studied in this work exhibit a distinctive “kink” in values of CO2 loadings with respect to pressure [26] and can be observed in Figs. 2 to 11. At low pressure values, the free solvent rapidly reacts with the carbon dioxide. Hence, the loading value increases rapidly with a slight increase in pressure [27]. But as the solvent is spent, the loading value drops, where the physical absorption takes over at the high pressure values. At high pressure, the loading value increases slightly with nominal increase in the pressure of the gas [28].
Table 7 Experimental equilibrium carbon dioxide loadings (a) in mixture of L-lysine (1) þ sodium hydroxide (2) þ water (3) for a range of temperature (T) and pressure (P)a. T/K
(mole of sodium lysinate)1
a/mole of CO2. (mole of sodium lysinate)1
w1 ¼ 0.142, w2 ¼ 0.039/mass fractionb
w1 ¼ 0.278, w2 ¼ 0.075/mass fractionc
w1 ¼ 0.401, w2 ¼ 0.110/mass fractiond
217.50 613.75 1192.50 1548.75 2071.25 2450.00 3110.00 3423.75 4070.00 175.00 573.75 1203.75 1628.75 2068.75 2570.00 3080.00 3557.50 4105.00 110.00 578.75 1063.75 1712.50 2093.75 2518.75 3078.75 3605.00 4145.00 162.50 531.25 1095.00 1575.00 2027.50 2541.25 2990.00 3548.75 4011.25
175.00 528.75 1077.50 1523.75 2022.50 2546.25 2981.25 3513.75 4040.00 140.00 606.25 1110.00 1557.50 2085.00 2550.00 2980.00 3357.50 3963.75 105.00 605.00 1155.00 1692.50 2183.75 2622.50 3118.75 3518.75 4032.50 120.00 485.00 1010.00 1521.25 2136.25 2575.00 3152.50 3585.00 4010.00
106.25 505.00 1147.50 1398.75 2108.75 2572.50 3105.00 3513.75 3965.00 155.00 512.50 1080.00 1383.75 2273.75 2785.00 3222.50 3675.00 4005.00 108.75 495.00 1041.25 1475.00 2217.50 2662.50 3057.50 3596.25 3946.25 95.00 447.50 1031.25 1506.25 2015.00 2496.25 3160.00 3518.75 3955.50
P/kPa
L-lysine
303.15
323.15
343.15
363.15
a/mole of CO2. (mole of sodium lysinate)1
P/kPa
a/mole of CO2.
P/kPa
(1) þ sodium hydroxide (2) þ water (3)
0.8943 1.0755 1.2866 1.3634 1.5005 1.6380 1.8741 1.9757 2.2014 0.5412 0.7355 0.9541 1.0761 1.1987 1.3642 1.4488 1.5242 1.6748 0.2365 0.4522 0.5215 0.7355 0.7963 0.8915 1.0652 1.1137 1.2175 0.1612 0.2366 0.3965 0.4785 0.4937 0.6325 0.7145 0.7885 0.8363
0.7044 0.8925 1.0652 1.1754 1.2365 1.2990 1.3814 1.4933 1.6325 0.3785 0.5742 0.7742 0.8264 0.9255 0.9911 1.0934 1.1215 1.2549 0.1485 0.3255 0.4215 0.5311 0.5863 0.6342 0.7014 0.7658 0.8211 0.1315 0.1845 0.2484 0.3146 0.3742 0.3997 0.4441 0.5366 0.5896
0.5515 0.7742 0.9953 1.0355 1.1015 1.1547 1.2346 1.2741 1.3189 0.3333 0.4795 0.6325 0.6690 0.8217 0.9125 0.9415 0.9842 1.0266 0.1625 0.2715 0.3696 0.4175 0.5215 0.5897 0.5742 0.6155 0.6615 0.1125 0.1548 0.2486 0.2696 0.3105 0.3695 0.3854 0.4216 0.4615
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.072 mol of CO2 per mole of aqueous blend of sodium lysinate. Both at a confidence level of 0.95. b The reported mass fraction corresponds to 1 mol l1 concentration of sodium lysinate solution. c The reported mass fraction corresponds to 2 mol l1 concentration of sodium lysinate solution. d The reported mass fraction corresponds to 3 mol l1 concentration of sodium lysinate solution.
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
Table 8 Experimental equilibrium carbon dioxide loadings (a) in blended solutions of L-lysine (1) þ potassium hydroxide (2) þ piperazine (3) þ water (4) of for a range of temperature (T) and pressure (P)a. T/K
P/kPa
a/mole of CO2.
P/kPa
(mole of aqueous blend of potassium lysinate and piperazine)1 L-lysine
303.15
323.15
343.15
363.15
a/mole of CO2. (mole of aqueous blend of potassium lysinate and piperazine)1
(1) þ potassium hydroxide (2) þ piperazine (3) þ water (4)
w1 ¼ 0.247, w2 ¼ 0.095, w3 ¼ 0.146/mass fractionb
w1 ¼ 0.221, w2 ¼ 0.085, w3 ¼ 0.260/mass fractionc
185.00 512.50 1143.75 1551.25 2105.00 2508.75 3017.50 3492.50 4021.25 175.00 536.25 1247.50 1512.50 1996.25 2481.25 2945.00 3525.00 4106.25 157.50 506.25 1112.50 1502.50 2052.50 2581.25 3105.00 3675.00 4203.75 105.00 497.50 1000.00 1611.25 2015.00 2587.50 3011.25 3497.50 4035.00
152.50 468.75 996.25 1495.00 2006.25 2501.25 3045.00 3492.50 4081.25 166.25 586.25 1107.50 1517.50 2027.50 2537.50 3048.75 3562.50 3913.75 125.00 596.25 1035.00 1503.72 2002.50 2532.50 3008.75 3631.25 4012.50 103.75 513.75 990.00 1505.00 2103.75 2498.75 3108.75 3560.00 3965.00
0.6502 0.8612 1.0465 1.1301 1.2286 1.2947 1.3742 1.4460 1.5241 0.3275 0.5306 0.7503 0.8109 0.9068 0.9902 1.0619 1.1438 1.2195 0.1460 0.2664 0.4056 0.4788 0.5708 0.6512 0.7254 0.8015 0.8685 0.0897 0.1350 0.2066 0.2811 0.3267 0.3884 0.4325 0.4819 0.5354
0.5956 0.8245 0.9921 1.0959 1.1840 1.2610 1.3405 1.4034 1.4841 0.3084 0.5322 0.6923 0.7867 0.8844 0.9684 1.0435 1.1125 1.1567 0.1244 0.2781 0.3735 0.4587 0.5389 0.6167 0.6819 0.7620 0.8088 0.0855 0.1855 0.2145 0.2552 0.3192 0.3594 0.4194 0.4626 0.5007
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.088 mol of CO2 per mole of aqueous blend of potassium lysinate, and piperazine. Both at a confidence level of 0.95. b The reported mass fraction corresponds to aqueous blend of 0.5 mol l1 concentration of piperazine and 2 mol l1 concentration of potassium lysinate solution. c The reported mass fraction corresponds to aqueous blend of 1 mol l1 concentration of piperazine and 2 mol l1 concentration of potassium lysinate solution.
The carbon dioxide solubility decreases with an increase in the temperature. This phenomenon is well understood and coherent with the other absorption solvents and explained by the increase in the mean molecular energies with the increase in temperature. This results in reduction of physical solubility of the gas in the liquid solvent [29]. Similarly, the carbon dioxide loadings decrease with the increase in the value of solvent concentrations. This can be understood by equation (4), where large denominator value on right hand side of the equation will certainly result in smaller values on left hand side. However, this must not be construed with the solubility values as the CO2 solubility (moles of carbon dioxide/kg of aqueous solvent) increases with increase in solvent concentration [30]. A direct comparison of sodium and potassium lysinate reveals that the CO2 loadings in sodium lysinate are slightly lower than potassium lysinate solutions. Quantitatively, the sodium lysinate has an average difference of 0.038 (mol CO2/mol solvent) than potassium lysinate when compared at similar parametric conditions. It is postulated that the non-idealities, presence of potassium
carbonate as an impurity in KOH, and the difference in molecular interactions of potassium and sodium ions in the amino acid salt solutions caused the observed minimal difference in the CO2 loading values [31]. The physical examination of sodium lysinate and potassium lysinate solutions was interesting. Small yet visible quantities of white precipitates formed for 2 M solutions of sodium lysinate at temperatures of 303.15 K and 323.15 K and higher pressures (>3000 kPa). Similarly, 3 M solutions of sodium lysinate formed precipitates at all temperatures (303.15e36315 K) and high pressures (>3000 kPa). The amount of precipitate reduced with increase in temperature [32]. A similar formation of precipitate was seen in 3 M solutions of potassium lysinate at 303.15 K and pressures above 3000 kPa, only. Other potassium lysinate solutions of lower concentrations were clear.
4.2. Experimental carbon dioxide solubility in aqueous blends of piperazine and alkaline salt solutions of lysine Tables 8 and 9 present the values of carbon dioxide loadings in
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
9
Table 9 Experimental equilibrium carbon dioxide loadings (a) in blended solutions L-lysine (1) þ sodium hydroxide (2) þ piperazine (3) þ water (4) for a range of temperature (T) and pressure (P)a. T/K
P/kPa
a/mole of CO2.
P/kPa
(mole of aqueous blend of sodium lysinate and piperazine)1 L-lysine
303.15
323.15
343.15
363.15
a/mole of CO2. (mole of aqueous blend of sodium lysinate and piperazine)1
(1) þ sodium hydroxide (2) þ piperazine (3) þ water (4)
w1 ¼ 0.245, w2 ¼ 0.067, w3 ¼ 0.144/mass fractionb
w1 ¼ 0.221, w2 ¼ 0.060, w3 ¼ 0.259/mass fractionc
162.50 602.50 1235.00 1520.00 2102.50 2442.50 3165.00 3502.50 4011.25 167.50 607.50 1095.00 1521.25 2035.00 2487.50 2995.00 3542.50 4105.00 185.00 486.75 1007.50 1511.25 2106.25 2495.00 2991.25 3512.50 4145.00 190.00 431.25 1035.00 1605.00 1948.75 2613.75 3017.50 3510.00 4011.25
156.00 622.50 1015.00 1487.50 2005.00 2413.75 3013.75 3513.75 4022.50 187.50 488.75 937.50 1477.50 2033.75 2513.75 3152.50 3546.25 4021.25 145.00 511.25 1013.75 1437.50 2017.50 2543.75 3041.25 3518.75 3912.50 186.25 518.75 1087.50 1521.25 2110.00 2582.50 3041.25 3607.50 3995.00
0.6245 0.8963 1.0665 1.1242 1.2282 1.2841 1.3967 1.4475 1.5226 0.3212 0.5589 0.7117 0.8128 0.9139 0.9912 1.0693 1.1461 1.2193 0.1585 0.2610 0.3843 0.4803 0.5793 0.6385 0.7097 0.7802 0.8612 0.0875 0.1241 0.2111 0.2804 0.3193 0.3911 0.4331 0.4832 0.5331
0.6000 0.8855 0.9966 1.0945 1.1838 1.2478 1.3360 1.4064 1.4761 0.3253 0.4929 0.6466 0.7782 0.8855 0.9647 1.0579 1.1104 1.1699 0.1342 0.2565 0.3693 0.4473 0.5412 0.6183 0.6862 0.7479 0.7967 0.0895 0.1915 0.2070 0.2570 0.3198 0.3678 0.4129 0.4671 0.5035
a The standard uncertainty for the table are u(T) ¼ 0.1 K, u(P) ¼ 1.25 kPa, u(w1) ¼ 0.001 mass fraction. The combined expanded uncertainty for loading is Uc(a) ¼ 0.088 mol of CO2 per mole of aqueous blend of sodium lysinate and piperazine. Both at a confidence level of 0.95. b The reported mass fraction corresponds to aqueous blend of 0.5 mol l1 concentration of piperazine and 2 mol l1 concentration of sodium lysinate solution. c The reported mass fraction corresponds to aqueous blend of 1 mol l1 concentration of piperazine and 2 mol l1 concentration of sodium lysinate solution.
the aqueous blends of piperazine with potassium lysinate and sodium lysinate, respectively. The piperazine blended solutions of both sodium lysinate and potassium lysinate showed formation of a precipitate at 303.15 K and 323.15 K and at pressures greater than 3000 kPa. However, the contents formed a murky solution upon shaking rather than settling down as clumps of precipitate, as seen in single solutions of sodium and potassium lysinate [33]. This situation warrants further study and invites exploiting their potential as a three phase (solidliquid-gas) carbon capture solvent for natural gas processing [34]. 4.3. Modelling results Table 10 reports the average absolute percentage deviation (AAD%) of the correlated results for the Kent-Eisenberg model upon comparison to the experimental values. Fig. 2 presents the modelling results against the experimental findings of this study for 1 M aqueous solutions of potassium salt of L-lysine. At low pressures, the model shows slight underprediction than the experimental values. This can be attributed
Table 10 Average absolute deviation (AAD%) for the K-E model correlation. Solvent
AAD%
Potassium lysinate Sodium lysinate Piperazine and potassium lysinate Piperazine and sodium lysinate
6.92 6.26 3.33 2.63
to the high carbon dioxide loadings observed at low pressure than correlated by a model that has been regressed to a majority of high pressure data, a region where the physical solubility dominates [14]. For the temperature of 363.15 K, the data is slightly under predicted, where the major errors arise in the low pressure region and are attributed to the same reason, discussed above. Nevertheless, the correlated values showed overall low errors for all solvent systems and exhibited correlation behaviour usually associated with Kent-Eisenberg type semi-empirical models [23]. The correlation trends for 2 M and 3 M solutions are identical to described in Fig. 2.
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H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
Fig. 2. Results of model against experimental equilibrium carbon dioxide loadings in 1 M potassium lysinate solutions at different temperatures and carbon dioxide pressures.
Fig. 4. Results of model against experimental equilibrium carbon dioxide loadings in aqueous blend of 0.5 M piperazine and 2 M potassium lysinate solutions at different temperatures and carbon dioxide pressures.
Fig. 3 presents the correlative results of the model for the equilibrium carbon dioxide loading in 2 M aqueous solution of sodium salt of L-lysine. The model results are exactly similar to the trends seen in the potassium lysinate solutions. At 303.15 K, the model values are slightly under-predicted at the pressures less than 100 kPa, whereas the trend at higher pressure (>1000 kPa) shows a slight over-prediction. This also means that the model parameters are well-regressed at 303.15 K, as errors are well distributed [14]. The model trends at 363.15 K show a slight underprediction at all pressure values. However, the errors are well below the experimental errors within the study and are unaffected by the increasing solvent concentration. Overall, the model results are in good agreement with the experimental values, determined in this study.
Fig. 4 illustrate the modelling results against the experimental values of carbon dioxide loadings in aqueous blends of 0.5 M piperazine with 2 M potassium lysinate, at equilibrium. Fig. 5 shows the correlation for the experimental values of equilibrium carbon dioxide loadings in aqueous blends of 1 M piperazine with 2 M sodium lysinate. For all studied blends, a slight over-prediction in model results is seen for all temperatures except 303.15 K. The error was induced due to the modelling effort to compensate for the error in low pressure region and contributes mainly to the error values [35]. On the contrary, the change in pressure and concentration do not have a decipherable effect on the error values of carbon dioxide loadings, which means that the both parameters are well-regressed [36]. The model results are in excellent agreement with our experimental data.
Fig. 3. Results of model against experimental equilibrium carbon dioxide loadings in 2 M sodium lysinate solutions at different temperatures and carbon dioxide pressures.
Fig. 5. Results of model against experimental equilibrium carbon dioxide loadings in aqueous blend of 1 M piperazine and 2 M sodium lysinate solutions at different temperatures and carbon dioxide pressures.
H. Suleman et al. / Fluid Phase Equilibria 507 (2020) 112429
5. Conclusions In this study, experimental measurements of carbon dioxide solubility are reported in the sodium and potassium salts of L-lysine and their separate blends with piperazine in the high pressure e high loading region. The solubility has been studied by using a high pressure VLE apparatus for a wide range of temperature (303.15e363.15 K), amino acid salt concentrations (1e3 M) and pressure (95e4204 kPa). Piperazine in strengths of 0.5 and 1.0 M was added to investigate its effect on the overall absorption capacity. It was noted that CO2 dioxide loadings have a positive relationship with increase in pressure and addition of piperazine for all solvents studied. On the other hand, increase in temperature and solvent concentrations results in decrease of the CO2 loading values. The experimental results were correlated by the Kent-Eisenberg model. Correlated results exhibited an AAD% of 6.92% (potassium lysinate), 6.26% (sodium lysinate), 3.33% (PZ-potassium lysinate) and 2.63% (PZ-sodium lysinate) for the values of carbon dioxide loadings. The findings of this study indicate that the alkaline salt of L-lysine can be suitably used as a singular carbon dioxide solvent for a range of process conditions. Moreover, they can also be blended with piperazine for better carbon dioxide absorption capacity and probably better kinetic rates, which warrants further study.
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
Author contributions Humbul Suleman: Investigation, Formal Analysis, Conceptualization, Methodology, Software, Writing e Original Draft. Abdulhalim Shah Maulud: Supervision, Project Administration, Resources, Funding Acquisition. Afaf Syalsabila: Validation, Writing e Original Draft. Muhammad Zubair Shahid: Visualization, Writing e Review and Editing. Philip Loldrup Fosbøl: Writing e Review and Editing.
[20]
Declaration of competing interest
[21]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[22]
[17]
[18]
[19]
[23]
Acknowledgements [24]
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