Solubility and thermodynamic modeling of hydrogen sulfide in aqueous (diisopropanolamine + 2-amino-2-methyl-1-propanol + piperazine) solution at high pressure
Introduction
Presently over 85% of world energy demand is supplied by fossil fuels [1] such as natural gas that consists of a mixture of methane as a main component, ethane and propane. When natural gas is burning, it produces less harmful by-products than the other fossil fuels, but nevertheless it produces from reservoirs containing several contaminates such as water, mercaptanes and acid gases [2]. The removal of acid gas impurities such as CO2 and H2S from gas streams is a vital operation in natural gas separation and purification [3], [4], [5]. The CO2 reduces the heating value of natural gas and H2S is a poisonous, toxic gas so that both of CO2 and H2S are corrosive and damage pipeline, valves, etc. Thus, natural gas should be treated to remove these acid gases prior utilization [6], [7], [8]. The natural gas pipeline specifications usually limit the CO2 and H2S content to less than 2% and 4 ppm, respectively [9]. The several processes such as physical and chemical absorption methods have been used in which among them the chemical solvents such as alkanolamines are widely used in the gas separation and purification technologies, because of highly reactive nature and low cost of these solvents [3]. On the other hand, for optimum design and operation of natural gas purification unit, the equilibrium data on solubility of CO2 and H2S in the aqueous amine/alkanolamines solutions are necessary over the wide range of temperatures and pressures. There are several alkanolamine such as N-methyldiethanolamine (MDEA), diisopropanolamine (DIPA), sterically hindered amines such as 2-amino-2-methyl-1-propanol (AMP) and the chemical activators such as piperazine (Pz). MDEA is a tertiary amine with good selectivity towards H2S, but a slower reaction rate with CO2 and a lower absorption capacity at low concentrations of CO2 [10], [11]. DIPA as a secondary amine is blended with sulfolane (a physical solvent) that is applied in the Sulfinol process (ADIP), which is widely used as a mixed-solvent process licensed by Shell. Moreover, DIPA presents low regeneration steam requirements and thermal stability so that it is less corrosive in comparison to the other primary or secondary alkanolamines. The selectivity of DIPA is more towards H2S than CO2 that is used in the Claus plant tail gas so that it can remove the other sulfur compounds such as COS and CS2 without detrimental effects to the solution [12]. The first time AMP as a hindered amine was introduced by Sartori and Savage [13] which is selective towards H2S similar to DIPA. Since Pz is resistant to thermal and oxidative degradation, the alkanolamines are blended with Pz to improve the CO2 loading in aqueous amine solvents and reduce the volatility [14]. In recent years, the mixed amine/alkanolamines are widely studied to overcome the disadvantage of using a single alkanolamine. Thus, knowledge of the solubility of the acid gases in blended amines is necessary in the design of industrial gas absorption units. However, most of the equilibrium data is devoted to the solubility of CO2 [15], [16], [17], [18], and less data are available for H2S solubility particularly in blended alkanolamines. The several works [19], [20] for solubility of H2S in alkanolamines are given as follows. Isaacs et al. [19] measured solubility of H2S in aqueous solution of DIPA at T = (313 and 373) K and compared their data with those were obtained by MEA solution. They showed the solubility of H2S in DIPA is much lower than MEA at the both temperatures. Uusi-Kyyny et al. [21] investigated the solubility of H2S in aqueous DIPA solution at several temperatures ranging from (322 to 353) K using a static total pressure method. They also measured the vapor pressure of pure DIPA with a distillation method within a temperature range of (420 to 465) K. Jane and Li [22] obtained data on the solubility of H2S in the aqueous AMP solution at T = 313 K and Teng and Mather [23] measured the equilibrium solubility of H2S in AMP solution at T = 324 K. They concluded that solubility of H2S is significantly higher than that of CO2 in AMP solution at the same partial pressure. Moreover, Li and Chang [24], Roberts and Mather [25] measured the solubility of H2S in the aqueous of AMP at different temperatures. Xia et al. [26] obtained the solubility values for H2S in the Pz solution within the range T = (313 to 393) K. They illustrated that at the beginning only the total gas pressure changes slightly by chemical absorption, but at the end the pressure increases steeply through physical absorption. Also Speyer and Maurer [27] measured the solubility of H2S in the aqueous of Pz at T = (313.5 and 392.2) K in the low loading region. Mazloumi et al. [28] obtained solubility of H2S in aqueous solutions of DIPA and mixtures of DIPA and Pz at T = (313, 333 and 353) K. They observed that at acid gas loadings below the crossing point, the H2S solubility is enhanced with increasing Pz concentration, and above that point, the reverse phenomenon takes place. Finally, Haghtalab et al. [29] measured solubility of H2S in the aqueous (AMP + Pz) solution at T = (313, 328 and 343) K. They observed that at fixed partial pressure and constant mass percent of (AMP + Pz), with decreasing temperature the H2S loading is enhanced.
The objective of this work is to provide equilibrium values for the H2S solubility in the blended aqueous (DIPA + AMP) and (DIPA + AMP + Pz) systems that have not been presented to date. Moreover, we investigate the influence of Pz and AMP on the acid gas solubility, and we use the Electrolyte-NRTL activity coefficient equation for the correlation and prediction of the partial pressure of H2S versus the acid gas loading. Using a static high pressure system, the solubility values are determined at fixed 45 mass per cent of total amine so that the solubility of H2S in the present systems is investigated under isothermal conditions at T = (313.15, 328.15 and 343.15) K and in the pressure range of (0.1 to 2.1) MPa.
Section snippets
Materials
Diisopropanolamine and Pz with mass fraction purity > 0.98 and 0.99, respectively, were supplied from Sigma–Aldrich and 2-amino-2methyl-1-propanol with mass fraction purity > 0.97 was purchased from Fluke. The H2S gas cylinder was supplied by Technical Gas Service Company with the mole fraction purity > 0.99. The specifications and sources of the supplied chemicals used in this work are presented in table 1. All the materials were used without further purification. A digital balance with accuracy of
Thermodynamic modeling
In the previous work [29], the detailed thermodynamic framework for modeling acid gas solubility in aqueous amine/alkanolamines solutions was presented. Thus, here the basic thermodynamic framework is explained as follows. The two types of equilibrium calculations as chemical and physical are taking place for solubility of H2S in the aqueous mixtures of the amine/alkanolamines. In these aqueous systems, H2S reacts with amine/alkanolamines through an acid-base buffer mechanism so that the
Results and discussion
In the previous work [29], the validation of the density was carried out using a pycnometer through measuring the density of pure water from T = (300 to 360) K and comparing with literature values [38]. Also validation of the apparatus and procedure that is used for measuring the solubility of H2S in aqueous amine/alkanolamines solutions was presented in the previous works [29], [30], [32]. The density of the aqueous amine/alkanolamine systems at pressure of 0.1 MPa was measured at the given
Conclusions
In this work, the (gas + liquid) equilibrium solubility of H2S in the prepared aqueous amine/alkanolamine solutions was determined at T = (313.15, 328.15 and 343.15) K using a high pressure static apparatus through a volumetric method. The pressure range covered from (0.1 to 2.1) MPa and the mass fraction of the total amine was fixed at 0.45. The prepared aqueous amine/alkanolamines were as {DIPA + AMP (25 + 20) mass per cent}, {DIPA + AMP + Pz (25 + 15 + 5) mass per cent}, {DIPA + AMP + Pz (25 + 10 + 10) mass per cent}
Acknowledgement
The authors acknowledge for financial support of the research, development and technology division of the National Iranian Gas Company (NIGC), Grant No. 087162 particularly Dr. Saeid Pakseresht and Eng. Hamid Bonyad.
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