Solubility of Hydrogen Sulfide in Aqueous Blends of 2-Amino-2-methyl

Article pubs.acs.org/jced

Solubility of Hydrogen Sulfide in Aqueous Blends of 2‑Amino-2methyl-1-propanol and N‑Methyldiethanoleamine: Experimental Measurement and Modeling Mohammad Shokouhi,*,‡ Hamidreza Bozorgzade,‡ and Parisa Sattari‡‡ ‡

Gas Science Department, Gas Research Division, Research Institute of Petroleum Industry (RIPI), National Iranian Oil Company (NIOC), P.O. Box 14665-137, West Boulevard, Azadi Sport Complex, Tehran, Iran ‡‡ Northern Tehran Azad University, P.O. Box 1969633651, Valiasr Street, Ghobadian Ave. Num. 79., Tehran, Iran ABSTRACT: The solubility of H2S in aqueous solutions of 40 mass % MDEA + 5 mass % AMP, 30 mass % MDEA + 15 mass % AMP, and 22.5 mass % MDEA + 22.5 mass % AMP at (313.15, 333.15, and 353.15) K and pressure from the vapor pressure of the solutions up to 1.5 MPa using the isochoric saturation method were measured. The experimental data are presented as the partial pressure of H2S against acid gas loading (mole H2S/total moles of amine). The results showed that the H2S solubility decreases with an increase of temperature and increases with the rising of partial pressure of H2S, and also AMP concentration can improve absorption capacity of MDEA aqueous solution as [AMP]0/[MDEA]0 (ratio of AMP per MDEA concentration in fresh solutions) increases when total amine concentration is constant. The Deshmukh−Mather model and Peng−Robinson equation of state were used for liquid and vapor phase, respectively, to correlate the solubility data.

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INTRODUCTION The capture of acid gases such as CO2, H2S, COS, CS2, and some mercaptans is a very important task to control the injection of pollutants or even poison gases into the atmosphere. Aqueous alkanolamine solutions1 as chemical absorbents and tetramethylen sulfone2 as a physical one, are usually used in gas treating or gas capturing processes. Frazier and Kohl3 first described the use of methyldiethanolamine (MDEA) solution. MDEA is a tertiary amine which has a stable structure and is not prone readily to degradation reactions, and thereby it does not create corrosion problems for carbon steel. MDEA aqueous solutions show no carbamate formation in CO2 absorption processes, whereby they are thermodynamically and kinetically selective for H2S in the presence of CO2. Since the mid-1980s, to further improve the absorption performance of MDEA, blended MDEA-based alkanolamine solvents have been suggested. These blended solvents include MDEA−MEA (monoethanolamine), MDEA−DEA (diethanolamine), MDEA−DIPA (diisopropanolamine), MDEA−DGA (diglycolamine), and MDEA−AMP (2-amino-2-methyl-1-propanol). The blended solvents have the advantages of a single solvent in blended solvents. A blended solvent, which consists of a mixture of primary, secondary, or hindered amine with MDEA, combines the higher CO2 reaction rates of the primary or secondary amine with the higher CO2 loading capacity of the MDEA. Thus, the blended solvent providing both higher CO2 reaction rate and higher CO2 equilibrium capacity may result in substantially lower solvent circulation rates compared to a single solvent. So the development of a new solvent for © XXXX American Chemical Society

maximum CO2 absorption can be considered as a great invention for the sour gas sweetening process. A large amount of experimental work has been done over the past few years to characterize new solvents with respect to different properties such as solubility4−7 and as accelerator in amine blends as well.8−11 2-Amino-2-methyl-1-propanol (AMP) as a sterically hindered alkanolamine has been suggested recently as a commercially appropriate solvent for acid gas treatment over the usual amines such as MEA, DEA, DIPA, and MDEA.4−11 This is because the steric character reduces the stability of the formed carbamate; thus the carbamate can undergo hydrolysis to form bicarbonate and consequently enhance the CO2 equilibrium loading capacity to 1.0 mol of CO2 per mol of amine, as high as that of ternary amine. Degradation of alkanolamine especially carbamate polymerization is an important concern in natural gas sweetening and CO2 capture processes. Superior stripping qualities because of lower regeneration energy, higher degradation resistance because of the steric hindrance in AMP which causes CO2 induced degradation to slow down as compared to MEA,9 and a lower corrosion rate compared to those of conventional amines are some other advantageous of the AMP solution,12,13 and that is why tertiary formulated blends of MDEA/AMP/H2O may be a suitable solution to enhance the simultaneous absorption capacity and absorption rate of both CO2 and H2S. Received: March 3, 2015 Accepted: June 9, 2015

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DOI: 10.1021/acs.jced.5b00194 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Reported Data of the H2S Equilibrium Solubility in Aqueous MDEA and MDEA-Based Alkanolamine Solutions, Associated with Solvent Compositions, Temperature, and Pressure Ranges author/year

solution

temp range (K)

pressure range (kPa)

J. Xia/2003

PZ 4 molal and PZ 1.966 molal + MDEA 1.975 molal

313 to 393 and 353

F. Jou/1982 R. J. MacGregor/1991 R. J. MacGregor/1991 R. J. MacGregor/1991 R. Sidi-Boumedine/2004 A.M. Bhairi/1984 F.Y. Jou/1993

MDEA 1, 2, 4.28 molar 20.9 wt % MDEA/30.5 wt % Solfulane MDEA 2 molar 20.9 wt % MDEA/30.5 wt % Solfulane 46.78 wt % MDEA 23.4, 20, 11.8 wt % MDEA 48.9,23.4, 11.8 wt % MDEA

313.15 to 393.15 313.15, 373.15 313.15 313.15, 373.15 313, 373 298, 311, 323 298, 313, 343, 373, 393

F.Y. Jou/1986 B. Lemoine/2000 R.N. Maddox/1987

23.6, 11.8 wt % MDEA 23.4, 20, 11.8 wt % MDEA

0.04 to 1.5 13.2 to 1537.2

20 21 22

48.8 wt % MDEA

298, 313 298, 311, 323, 339, 389 313, 353, 393

147.9 to 2783

23

MDEA 1.929 molal and 3.987 molal MDEA 3.99 m + H2SO4 0.9862 molal 1.900 molal MDEA + 0.9566 molal Na2SO4 48.8 wt % MDEA

313.15 to 413.16 313.353,393 313.353,393 313, 353, 393

165.2 to 4895.9 95.8 to 3656 32.7 to 3866 150−2800.

24 25 25 26

MDEA/DEA/AMP

313,343,393

0.025 to 10.36

27

283,298 313.15 to 373.15 313.15 to 373.15

0.141 to 18.982 1.0−450 1.0 to 450

28 29 29

M. H. Li/1993 M. Dicko/2010

35, 50 wt % MDEA MDEA 2.57 molal MEA 3.97 molar/MDEA 0.51 molar and MEA 2.0 molar/MDEA 1.54 molar 30 wt % MDEA 50 wt % MDEA

313,333, 353,373 323.15

30 31

P.J.G. Huttenhuis/2009 F.-Y. Jou/1993 W. M. Qian/1995 S.H. Hung/1998 Haghtalab/2014

35, 50 wt % MDEA 35 wt % MDEA MDEA + sulfolane 50 wt % MDEA 25 wt % MDEA + (20,15,10,5)wt % AMP (0,5,10,15)wt % PZ

283.15,298.15 K 313.15, 373.15 313.15, 373.15 313,373 313.15, 328.15, 343.15

1.5 to 426.5 4.99 to 7.00 (mixed H2S/ CO2) mixed H2S/CO2 mixed H2S/CO2

B.S. Ho/1988 D. Speyer/2012

MDEA/DEA MDEA/piperazine

A. Pérez-Saladao Kamps/ 1996 G. Kuranov/1996 Y. Anoufrikov/2002 Y. Anoufrikov/2002 A. Pérez-Saladao Kamps/ 2001 M. E. Rebolledo-Libreros/ 2004 P. J. G. Huttenhuis/2007 M. H. Li/1993 M. H. Li/1993

313/353/393

158.6 to 8748 and 136.4 to 6207 0.001 to 6600 1.30 to 3210 0.52 to 1600 mixed H2S/CO2 6.21 to 1040.0 13.2 to 1537.2 0.002 to 5890

ref 14 15 16 16 16 17 18 19

mixed H2S/CO2 100 to 2100

32 33 34 35 36

mixed H2S/CO2 0.0064 to 1.257

37 38

solubilities vary systematically from 4.95 kmol m−3 MEA, to 3.97 kmol m−3 MEA + 0.51 kmol m−3 MDEA, to 2.0 kmol m−3 MEA + 1.54 kmol m−3 MDEA, and 2.57 kmol m−3 MDEA. For the same H2S partial pressure, the 4.95 kmol m−3 MEA aqueous solution yields a higher H2S solubility than the 2.57 kmol m−3 MDEA aqueous solution. Newest reported data belonged to Haghtalab et al.36 in which they reported H2S solubility in aqueous mixture of MDEA/AMP/PZ (PZ stand for piperazine) where total amine mass percent concentration was fixed at 45 %. They have presented that at the low gas loading regions (lower than 1), blending the PZ with the aqueous solution of MDEA/AMP leads to reduction of the solubility of H2S. In this work, following the line of research over absorption of acid gases in mixed hindered amine (especially AMP) and MDEA aqueous solution,39 the H2S solubility in aqueous solutions of 40 mass % MDEA + 5 mass % AMP, 30 mass % MDEA + 15 mass % AMP, and 22.5 mass % MDEA + 22.5 mass % AMP at (313.15, 333.15, and 353.15) K and pressures from vapor pressure of solutions up to 1.5 MPa using the isochoric saturation method have been measured, and the results were compared to those data obtained from H2S absorption in MDEA solution.17 The experimental data was correlated using the Deshmukh−Mather model as well.40,41

To achieve optimum design of an absorption process, we need the solubility data of acid gases in an aqueous solution of alkanolamine in a wide range of temperature and pressure and also in wide range of different amine compositions. The amount of experimental data of CO2 solubility in alkanolamine solution is much more than that of H2S. A literature survey on pure H2S and mixed H2S/CO2 solubilities in MDEA and mixed MDEA with the other alkanolamines aqueous solution has been done by our research group, and the results are summarized in Table 1.14−38 As may be seen from Table 1, H2S solubility in aqueous solutions of MDEA is more available than those measured for MDEA-based mixed alkanolamine solutions. Xia et al.14 reported the solubility of H2S in PZ and MDEA solution. MacGregor et al.16 investigated the solubility of H2S and CO2 and also their simultaneously solubility in mixedaqueous solvents consisting of MDEA and sulfolane that their results were compared with those data reported for the aqueous AMP and sulfolane. Li et al.29 measured the solubility of H2S in the aqueous mixture of MDEA and EMA. They have shown that, at T = 313.15 K, the loadings of H2S in 3.97 kmol m−3 MEA + 0.51 kmol m−3 MDEA are larger than those values in 2.0 kmol m−3 MEA + 1.54 kmol m−3 MDEA at the region of low partial pressures of H2S, and at T = 373.15 K, H2S B



Article

Table 2. Specifications and Sources of Chemicals Used in This Work

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chemical name

molecular formula

CAS registry number

purity

source

hydrogen sulfide N-methyldiethanoleamine 2-amino-2-methyl-1-propanol

H2S CH3N(CH2CH2OH)2 C4H11NO

7783-06-4 105-59-9 124-68-5

99.95 % (mol %) > 99 % (mass %) > 95 % (mass %)

Roham Gas Company Sigma-Aldrich Merck (Darmstadt, Germany)

EXPERIMENTAL SECTION Materials. Deionized water used for solution, was degassed in an ultrasonic bath (FUNGILAB, model UA10MFD) at a temperature of 353.15 K and wave frequency of 50 kHz about 30 min before use. The specifications and sources of the chemicals used in this work are summarized in Table 2. All the materials were reagent grade and used without further purification. All solvents were prepared by calibrated balance (Mettler model AE 200) with an uncertainty of ± 0.001 g. Apparatus and Procedure. The details of the experimental method for the measurement of gas solubility have previously been presented42 and only a short description will be provided here. The temperature of the double-wall equilibrium cell, which was connected to a water recirculation bath (PMT Tamson model T 2500), with a temperature stability within ± 0.02 K was measured using a model TM-917 Lutron digital thermometer with a 0.01 K resolution equipped with a Pt100 sensor inserted into the cell via a thermo-well. The equilibrium cell pressure was measured using a KELLER model PA-33X pressure transmitter sensor in the range of (0 to 3) MPa, which was accurate within 0.01 % of full scale and that of the gas container was measured using a Druck model PTX 1400 pressure transmitter sensor in the range of (0 to 4) MPa, which was accurate to within 0.1 % of full scale. The volumes of the equilibrium cell and gas container (gc) were obtained by performing pressure swing experiments. The pressure swing experiments consisted of measuring the pressure drop when a valve between an unknown and a reference volume opens, where prior to its opening, the reference volume was pressurized (atmospheric value) and the unknown volume was evacuated. Using the several measurements by this method both gas container and equilibrium cell volumes and their uncertainty were obtained. The operation of the apparatus was carried out in such a way that at first, a vacuum was applied to the equilibrium cell using vacuum pump BS 5000-11, type BS2208 A21042003 (up to 0.1 kPa). All solutions were degassed over an ultrasonic bath (FUNGILAB, model UA10MFD) for 20 min. Then, a certain amount of the solution of interest was determined by weight in an analytical balance with a precision of ± 0.001 g and introduced into the equilibrium cell. The temperature was then adjusted to the desired value, and after reaching the equilibrium state, the pressure sensor in this case shows the vapor pressure of the solution. An amount of H2S was introduced to the equilibrium cell from the gas container (gc) whose volume is well-known. The amount of H2S injected into the equilibrium cell was calculated with a procedure adopted by Park and Sandall,43 and also Hosseini Jenab et al.,42 in which accurate PVT data were obtained from the National Institute of Standards and Technology (NIST) for pure H2S.44 nCO2 =

Vgc ⎛ Pi P⎞ ⎜ − f⎟ RTa ⎝ Z i Zf ⎠

where Vgc denotes the volume of the gas container, Zi and Zf are the compressibility factors corresponding to the initial and final state in the gas container before and after transferring the H2S, and Ta is the ambient temperature. The equilibrium state inside the cell was normally achieved within 3 h by the operation of the mechanical header stirrer. The equilibrium partial pressure of H2S in the equilibrium cell was calculated by

PH2S = Pt − PVP

(2)

where Pt and PVP denote the total absolute pressure and vapor pressure of solution. The amount of remaining H2S in the gas phase nHg 2S was determined from n Hg 2S =

VgPH2S Z H2SRT

(3)

where Vg is the gas-phase volume, T is the equilibrium temperature of the cell, and ZH2S is the compressibility factor of carbon dioxide at PH2S and T. The amount of H2S in the liquid phase was then determined with n Hl 2S = n H2S − n Hg 2S

(4)

And finally, H2S loading and molality in the liquid phase was obtained as α H 2S =

m H 2S =

n Hl 2S(mole) namine n Hl 2S(mole) wsolvent(g)

(5)

·1000

(6)

in which, nHl 2S and namine are the amount of acid gas and amine in the liquid phase, respectively, wSolvent is the mass of solvent (H2O) in g. Equations 5 and 6 are related via eq 7, m H 2S α H 2S = namine(in 1000 g H 2O) mH S = 10·(wt % MDEA) 2 10·(wt % AMP) + M M (7) MDEA

AMP

where Mi is molar mass of pure i in g·mol−1, and wt % MDEA and wt % AMP stand for mass percent of MDEA and AMP in fresh alkanolamine solution, respectively. Using eqs 1 through 5 which are based on determining the mass balance, the loading of the gas is measured. The volume of gas phase in the equilibrium cell, Vg, is obtained from the difference between the cell volume and the volume of uncharged solvent. Temperature and composition dependency of the uncharged solvent density is obtained from reported density data in the open literature45,46 and correlated using the Setchenow type equation39,47 which has recently been used for correlating density, viscosity, and surface tension48 and also for thermal properties.49,50

(1) C



Article

Table 3. Parameters of Equilibrium Chemical Constants for Equations 8 through 11 in the Form of ln(Kr) = a/T + b ln(T) + cT + d, in which T is in terms of Kelvina a

b

c·102

d

−13445.9 −7261.78 −18034.72 −13445

−22.4773 −22.4773 −78.07186 −22.4773

−4.1447 0 9.19824 0

173.1912 142.58612 461.7162 140.932

reaction +

+

MDEAH + H2O = H3O + MDEA AMP + H2O = −OH + AMPH+ H2S + 2H2O = HS− + H3O+ 2H2O = H3O+ + OH−

(1) (2) (3) (4)

Numbers for reactions 1 and 2 are given on molarity scale and those for reaction 3 and 4 are given on molality base. Kc = Km(ρs(g·mL−1))Δυ and Kx = Km(Ms(kg mol−1))Δυ in which ρs and Ms are density and molar mass of solvent and Kc, Km, and Kx are molarity, molality, and mole fraction base of the equilibrium constant, respectively, and υ represents the stoichiometry number of species in each reaction. a

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THERMODYNAMIC MODEL The MDEA−AMP−H2O−H2S system includes the chemical reactions in the liquid phase and gas−liquid phase equilibria (H2O and H2S). The main chemical reactions in these systems include K1

MDEAH+ + H 2O ↔ H3O+ + MDEA K2 −

AMP + H 2O ↔ OH + AMPH+ K3

H 2S + 2H 2O ↔ HS− + H3O+ K4

+

2H 2O ↔ H3O + OH

−

specific interaction parameter of component i and j obtained with regression of the experimental data. The gas−liquid phase equilibria for H2S and water may be given by eqs 18 and 19:

(8) (9) (10) (11)

∏ aiυ ,r i

(12)

i

(18)

⎡ν ⎤ φw yw Pt = a w (φws Pws ) exp⎢ w (Pt − Pws )⎥ ⎣ RT ⎦

(19)

where aw is the activity of water which is approximately equal to mole fraction of water in the liquid phase. Pt is total pressure, T is temperature, R is universal gas constant, ϕsw and Psw are fugacity coefficient and vapor pressure of pure saturated water, respectively. ϕsi is fugacity coefficient of species i in the equilibrium state. mi and γi are molality and activity coefficients of species i in the liquid phase, respectively. HH2S is Henry’s law constant of H2S in pure water in the molality scale obtained by eq 20.55

The general form of the chemical equilibrium for the chemical reaction, r, is given as, K r(T /K) =

⎡ νH∞S ⎤ φH SyH S Pt = γH Sm H2SHH2S exp⎢ 2 (Pt − Pws )⎥ 2 2 2 ⎣ RT ⎦

where ai is the activity of component i and Kr is equilibrium constant of reaction r. The expression of K1 is taken from Li et al.,12 K2 is taken from Little et al.51 or Xu et al.,52 K3 is taken from Pérez-Saladao Kamps et al.,26,23 K4, K5 are taken from Bieling et al.,53 and K6 is taken from Edwards et al.54 The equation form as well as their coefficients of reactions mentioned in eqs 8 through 11 are reported in Table 3. The material balances can be expressed as follows:

⎛ HH S(T , Pws ) ⎞ 13236.8 2 ⎟⎟ = 340.305 − ln⎜⎜ T /K MPa kg/mol · ⎝ ⎠ − 55.0551ln(T /K) + 0.0595651(T /K)

In fact, both vapor pressure of pure saturated water and Henry’s law constant of H2S in pure water are equilibrium constants of two physical processes shown in eqs 21 and 22.

[MDEAH+] + [MDEA] = [MDEA]0

(13)

[AMPH+] + [AMP] = [AMP]0

(14)

H 2O (l) ↔ H 2O (g)

[H 2S] + [HS−] = αH2S([AMP]0 + [MDEA]0 )

(15)

H 2S (aq) ←⎯⎯⎯⎯⎯⎯⎯⎯→ H 2S (g)

Pws

(16)

Activity coefficients of both molecular and ionic species were calculated from the modified extended Debye−Hü ckel expression known as Deshmukh−Mather model for the electrolyte solution.39,41 1 + B ·I1/2

+ 2 ∑ βij ·mj j≠w

(22)

The vapor pressures of pure water calculated using the Saul− Wagner expression56 and the fugacities of H2S and H2O in the gas phase were calculated using the Peng−Robinson equation of state.57 Because of the low vapor pressure of MDEA and AMP in the temperature range considered here, the presence of MDEA and AMP in the vapor phase was neglected. νw is the molar volume of pure water and νH∞2S is the partial molar volume of H2S at infinite dilution in water estimated by using eq 23:58,31

[−OH] + [HS−] = [H3O+] + [AMPH+] + [MDEAH+]

Azi2I1/2

(21)

HH2S(T , Pws )

The electroneutrality equation is given by,

ln(γi) =

(20)

(17)

νH∞2S = 0.000599631(T − 273.15)2 + 0.002899997

where A depends on the density of solvent, temperature, and dielectric constant in which the dielectric constant is calculated with the Maryott and Smith formula.54 B is considered as a constant; in this work it assumed equal to 1.2. In the extended Debye−Hückel expression, zi is ionic charge of component i, I is ionic strength of solution, mj is molality of component j, βij is

(T − 273.15) + 34.825

(23)

Solving the set of nonlinear eqs 12 through 17 gives us the composition of solution. The method of Smith and Missen59 was applied to solve nonlinear equations. D



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Article

RESULTS AND DISCUSSION Validation of the experimental apparatus and the accuracy of the measurements have been checked in our previous papers.39,60,61

Table 5. Experimental Equilibrium Partial Pressure of H2S, ex PH and total pressure, Pt, at Each Given Loading Value, α, 2S and Also the Initial Pressures of Solutions Reported over 15 wt % AMP−30 wt % MDEA Solution at Different Temperaturesa

Table 4. Experimental Equilibrium Partial Pressure of H2S, ex PH and total pressure, Pt, at Each Given Loading Value, α, 2S and Also the Initial Pressures of Solutions Reported over 5 wt % AMP−40 wt % MDEA Solution at Different Temperaturesa PH2S/kPa (± 4.20)

T/K (± 0.1) 313.2

Pt/kPa (± 3.00)

loading (α)

18.50 24.90 109.0 115.4 408.5 414.9 677.0 683.4 991.0 997.4 AADP % = 6.80 MADP % = 9.54 49.50 65.20 215.0 230.7 568.5 584.2 862.5 878.2 1174. 1189.7 AADP % = 4.90 MADP % = 5.58 82.50 119.0 374.0 410.5 765.5 802.0 1090. 1126.5 1441.5 1478. AADP % = 0.96 MADP % = 1.40

333.2

353.2

T/K (± 0.1) 313.2

± Δα

0.468 0.819 1.023 1.114 1.206

± ± ± ± ±

0.023 0.023 0.023 0.022 0.025

0.461 0.794 0.989 1.078 1.177

± ± ± ± ±

0.023 0.023 0.023 0.022 0.025

0.454 0.760 0.951 1.037 1.130

± ± ± ± ±

333.2

353.2

0.023 0.023 0.023 0.023 0.025

N i=1

|Pical − Piexp| 1 = Piexp N

i=1

(24)

⎞ ⎛ |P cal − P exp| MRD% = maximum ⎜⎜ i exp i 100⎟⎟ Pi ⎠ ⎝ = maximum(PRD%)i

Pcal i

± Δα ± ± ± ± ±

0.021 0.022 0.021 0.021 0.022

0.572 0.714 0.873 0.968 1.058

± ± ± ± ±

0.021 0.023 0.021 0.021 0.022

0.560 0.718 0.834 0.926 0.979 1.082

± ± ± ± ± ±

0.021 0.021 0.021 0.021 0.022 0.023

(26)

The measured quantity q is dependent upon the variables r... u which fluctuate in a random and independent manner. The uncertainties of all of the instruments used in the measurements were considered for estimating the uncertainty of the solubility of H2S in the liquid phase. The main contributions to the uncertainty of the solubility are attributed to errors in the pressure sensor for equilibrium cell and gas container (± 0.003 and ± 0.004 MPa, respectively) and temperature sensors (± 0.1 K), volume of the gas sample, equilibrium cell (∼ ± 1.5 cm3) and amount of solvent in equilibrium cell (± 0.001 g). According to eq 5, the gas solubility is related to the amount of absorbed gas in the liquid phase, and the amount of amines in solvent. The uncertainty in this parameter is related to the l uncertainties of nag and namines; the average calculated uncertainties of these two parameters for T = 313.15 K were calculated to be δnlCO2 = ± 0.003 and δnamines = ± 0.0002, therefore the uncertainty in loading, ± δα according to eq 26 is equal to ± 0.02. With regards to the reported data in the open literature, the density of loaded aqueous MDEA/MEA/DEA solutions shows an increasing dependency with increasing carbon dioxide concentration.63 For estimating the error that might result from the volume change of solvent by the dissolved gas, we did a sensitivity test on the volume; when the ratio of volume of uncharged solution to equilibrium cell is lower than 1/3.5, a

N

∑ (PRD%)i

0.578 0.747 0.901 1.002 1.111

⎡⎛ ∂q ⎞ ⎤2 ⎡⎛ ∂q ⎞ ⎤2 δq = ± ⎢⎜ ⎟ dr ⎥ + ... + ⎢⎜ ⎟ du⎥ ⎣⎝ ∂u ⎠ ⎦ ⎣⎝ ∂r ⎠ ⎦

In Tables 4 to 6, the experimental data of H2S loading, modeling results, and percent relative deviation, PRD %, of each point for 5 mass % AMP−40 mass % MDEA, 15 mass % AMP−30 mass % MDEA, and 22.5 mass % AMP−22.5 mass % MDEA solutions are reported. The average relative deviations, ARD %, defined by eq 24 and maximum relative deviations, MRD %, defined by eq 25 for a number of N experimental points are reported in those tables as well.

∑

23.00 29.20 53.00 59.20 141.0 147.2 311.5 317.7 612.2 618.4 AADP % = 4.10 MADP % = 7.08 49.00 64.60 109.5 125.1 266.5 282.1 473.6 489.2 754.3 769.9 AADP % = 1.10 MADP % = 1.51 104.0 140.2 192.0 228.2 458.0 494.2 686.0 722.2 901.2 937.4 1368.5 1404.7 AADP % = 7.34 MADP % = 7.70

loading (α)

± δα, PRD %, ARD %, and MRD % stand for uncertainty of H2S loading, percent relative deviation, average relative deviation, and maximum relative deviation, respectively.


100 N

Pt/kPa (± 3.00)

a

a

ARD% =

PH2S/kPa (± 4.20)

(25)

Pexp i

and in eqs 24 and 25 stand for the calculated and experimental value of the partial pressure of H2S, respectively, PRD % is percent relative deviation for each point. The error propagation theory was used to estimate the uncertainties of the final results.62,37 In the base of this theory, the uncertainty δq of the interest variable q(r... u) is given by eq 26: E



Article

Table 6. Experimental Equilibrium Partial Pressure of H2S, ex PH and total pressure, Pt, at Each Given Loading Value, α, 2S and Also the Initial Pressures of Solutions Reported over 22.5 wt % AMP−22.5 wt % MDEA Solution at Different Temperaturesa T/K (± 0.1) 313.2

333.2

353.2

PH2S/kPa (± 4.20)

Pt/kPa (± 3.00)

0.00 5.20 15.50 20.70 28.50 33.70 82.00 87.20 207.0 212.2 412.5 417.7 656.5 661.7 901.0 906.2 1077. 1082.2 AADP%=10.04 MADP%=17.20 0.00 14.30 36.50 50.80 71.00 85.30 174.5 188.8 346.5 360.8 577.5 591.8 838.5 852.8 1152. 1166.3 AADP % = 4.43 MADP % = 6.30 0.00 34.10 57.00 91.10 88.50 122.6 155.0 189.1 321.5 355.6 543.0 577.1 794.0 828.1 1068. 1102.1 AADP % = 4.21 MADP % = 17.25

loading (α)

± Δα

0.000 0.543 0.666 0.852 0.975 1.067 1.139 1.207 1.253

± ± ± ± ± ± ± ±

0.020 0.021 0.021 0.021 0.022 0.022 0.023 0.023

0.000 0.539 0.657 0.832 0.947 1.036 1.108 1.194

± ± ± ± ± ± ±

0.020 0.021 0.021 0.021 0.022 0.022 0.023

0.000 0.405 0.529 0.641 0.804 0.911 0.997 1.070

± ± ± ± ± ± ±

0.020 0.021 0.021 0.021 0.022 0.022 0.023

Figure 1. H2S partial pressure−loading curves at three different temperatures for 5 wt % AMP−40 wt % MDEA solution. Points are experimental data and lines are from the model.


a

Figure 2. H2S partial pressure−loading curves at three different temperatures for 15 wt % AMP−30 wt % MDEA solution. Points are experimental data and lines are from the model.

Table 7. Interaction Parameters As a Linear Relation with Reciprocal of Temperature βi,j0

The experimental data show that solubility of H2S increases with increasing partial pressure and it decreases with increasing temperature. Besides, it is seen that increasing (AMP)0/ (MDEA)0 ratios increases the H2S loading. The specific interaction parameters are determined by fitting the model to experimental data via minimization of the sum of the percent relative deviation, ∑i N= 1(PRD %)i, of pressures as objecting function. In the system of AMP−MDEA−H2S−H2O, 10 species exist in liquid solution which contain H2O, OH−, H3O+, H2S, HS−, S2−, MDEA, MDEAH+, AMP, and AMPH+, and thereby in a general case there must be 100 interaction parameters. There are some constraints in the literature that reduce the number of interaction parameters, for instance, (1) all interactions related to species of solvent (H2O, H3O+, and − OH) are neglected; (2) all interactions between like-charge ions are discarded (such as AMPH+−MDEAH+); (3) all selfinteraction is not considered (except for amines used in this work); (4) interaction of species which are in low concentration can be neglected (such as S2−). To reduce

βi,j1

βi,j = βi,j0 + βi,j1/T (K) βMDEAH+,H2S βH2S,AMPH+ βMDEAH+,HS‑ βMDEA,HS‑ βAMPH+,HS‑ βAMP,HS‑ βMDEA,MDEA βMDEA,AMP βAMP,AMP

7.6305 0.7036 −0.0446 1.722 2.2765 −0.7433 3.1373 −1.9439 6.4244

−2252 −158.57 8.0334 −422.08 954.12 403.15 −773.11 988.56 −2261.1

fluctuation of 25 % in the uncharged solution volume may cause variations lower than 2 % in loading, which is on the same order of magnitude as the experimental uncertainty associated with the experimental loading values. F



■

Article

CONCLUSION In this work, the effect of the presence of AMP on the solubility of H2S in MDEA aqueous solutions were studied, and reported results have been shown that AMP affects efficiently the solubility of H2S in MDEA solution. All experimental measurements carried out using the isochoric saturation method and composition of uncharged solution contain 40 mass % MDEA + 5 mass % AMP, 30 mass % MDEA + 15 mass % AMP and 22.5 mass % MDEA + 22.5 mass % AMP, at (313.15, 333.15, and 353.15) K and pressure from the vapor pressure of solutions up to 1.5 MPa. As having reported in Tables 4 to 6 and shown in Figure 4, increasing the (AMP)0/ (MDEA)0 ratios, increases the H2S loading. The Deshmukh− Mather equation was employed to model experimental data. The quality of the model is reported using the average relative deviation ARD % and the maximum relative deviation MRD % as defined by eqs 24 and 25 in Tables 4 to 6, and graphically have been shown in Figures 1 to 3, as well.

■

Figure 3. H2S partial pressure−loading curves at three different temperatures for 22.5 wt % AMP − 22.5 wt % MDEA solution. Points are experimental data and lines are from model.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel.: +98 21 48252467. Fax: +98 21 44739716. Funding

We would like to thank to the Research Institute of Petroleum Industry (RIPI) for financial support and also to the research and development of National Iranian Oil Company (NIOC) for their support of this work. Notes

The authors declare no competing financial interest.

■ Figure 4. Effects of lean solution compositions on H2S loading at T = 313.15 K compared with the experimental data in aqueous MDEA solution (46.88 wt % MDEA) reported in literature.17

further the number of parameters, the sensitivity test was done on the values of the obtained parameters by data regression, to set to zero the value of some of the parameters having a negligible effect on the calculated total pressure. So, finally there were 9 interaction parameters left for data regression. In Table 7 all interaction parameters have been reported as a linear function of reciprocal of temperature. In Figures 1, 2, and 3, loadings−H2S partial pressure curves at temperatures 313.15, 333.15, and 353.15 K have schematically been shown for each solution and compared to values predicted by the model. In Figure 4, the effects of lean solution compositions were compared with each other at T = 313.15 K and different pressures, and they are compared with 46.8 mass % aqueous solution of MDEA.17 As may be seen from Figure 4, AMP can promote absorption capacity of MDEA aqueous solution. G

NOMENCLATURE NIST = National Institute of Standards and Technology R = universal gas constant MDEA = N-methyldiethanolamine AMP = 2-amino-2-methyl-1-propanol Vgc = volume of the gas container Vg = gas-phase volume in the equilibrium cell Zi and Zf = compressibility factors of the initial and final state in the gas container Ta = ambient temperature P0 = initial pressure of solution Pt = total absolute pressure PVP = vapor pressure of solution which is equal to P0 PH2S = partial pressure of H2S at equilibrium state nHg 2S = amount of H2S in the gas phase at equilibrium state nHl 2S = amount of H2S in the liquid phase at equilibrium state nAmine = amount of total amine in the liquid phase δ ni = uncertainty of amount of species i αH2S = loading of H2S in solution δα = uncertainty of loading Kr = equilibrium constant of reaction r B = constant, ∼1.2, in the extended Debye−Hückel expression A = Debye−Hückel limiting slope zi = ionic charge of component j mj = molality of component j, mol·kg−1 βij = binary interaction parameter of component i and j I = ionic strength of solution, mol. kg−1 D = dielectric constant of water, K−1 yi = mole fraction of species i in gas phase ϕw = fugacity coefficient of water at equilibrium state DOI: 10.1021/acs.jced.5b00194 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

ϕsw = fugacity coefficient of pure saturated water Psw = vapor pressure of pure saturated water aw = activity of water γi = activity coefficients of species i in liquid phase HH2S = Henry’s constant of H2S in pure water in molality scale νw = molar volume of pure liquid saturated water νH∞2S = molar volume of H2S at infinite dilution in water ARD = average of relative deviations MRD = maximum relative deviation PRD = percent relative deviation ρs = density of solution (g/cm3) ρw = density of pure water (g/cm3) WMDEA = weight percent of MDEA in uncharged solution WAMP = weight percent of AMP in uncharged solution

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Solubility of Hydrogen Sulfide in Aqueous Blends of 2-Amino-2-methyl

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