(Vapour + liquid) equilibria (VLE) of CO₂ in aqueous solutions of 2-amino-2-methyl-1-propanol: New data and modelling using eNRTL-equation

Abstract

This work presents new experimental results for carbon dioxide (CO₂) solubility in aqueous 2-amino-2-methyl-1-propanol (AMP) over the temperature range of (298 to 328) K and CO₂ partial pressure of about (0.4 to 1500) kPa. The concentrations of the aqueous AMP lie within the range of (2.2 to 4.9) mol · dm⁻³. A thermodynamic model based on electrolyte non-random two-liquid (eNRTL) theory has been developed to correlate and predict the (vapour + liquid) equilibrium (VLE) of CO₂ in aqueous AMP. The model predictions have been in good agreement with the experimental data of CO₂ solubility in aqueous blends of this work as well as those reported in the literature. The current model can also predict speciation, heat of absorption, enthalpy of CO₂ loaded aqueous AMP, pH of the loaded solution, and AMP volatility.

Introduction

Removal of acid gas impurities, such as carbon dioxide (CO₂) and hydrogen sulphide (H₂S), from natural gas, refinery off-gases, hydrogen, and synthesis gas for ammonia production is an important operation in industrial gas processing. Besides, growing environmental concerns today for global warming and climate change have motivated extensive research activities towards developing more efficient and improved processes for CO₂ capture from large point sources of CO₂. The widely used processes for the removal of CO₂ from natural gas and industrial gas streams are the regenerative absorption of CO₂ into aqueous solutions of alkanolamines. Commercially important alkanolamines used for this purpose are monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP) [1]. The gas streams in these processes are usually at high pressures of about 3 to 10 MPa. But, the major challenges for CO₂ capture from fossil fuel based power plants are the large volumetric flow rates of flue gas at essentially atmospheric pressure with large amount of CO₂ at low partial pressures. The presence of SO_x, NO_x, and significant oxygen partial pressure in the flue gas from coal based power plants gives rise to further problems for implementation of the amine absorption process for CO₂ capture from power plant flue gas streams. The MEA is considered suitable for flue gas cleaning because of its high reaction rate at low CO₂ partial pressure and low raw material cost. However, the costs of absorption process using MEA are high because of the high energy consumption in regenerating and operation problems such as corrosion, solvent loss, and solvent degradation [2]. Furthermore, MEA can be loaded up to only 0.5 mol of CO₂ per mol MEA as a result of the stable carbamates formed [3].

Sartori and Savage [4] reported a new class of amines, sterically hindered amines (SHA), that can be commercially attractive new absorbent for CO₂ capture process. Sharma [5] observed that the steric effect influence the stability of the carbamates due to the amine-CO₂ reaction and proposed the use of highly branched amines such as AMP for higher cyclic absorption capacity for CO₂. As in MDEA, the CO₂ loading in AMP approaches a value of 1.0 mol of CO₂ per mole of amine, while the reaction rate constant for (CO₂ + AMP) is much higher than that for (CO₂ + MDEA). Since the sterically hindered amine does not form a stable carbamate, bicarbonate and carbonate ions may be present in the solution in larger amounts then carbamate ions [6]. Hence the cost of regeneration energy when aqueous solutions of AMP are used to absorb CO₂ may be lower than when aqueous MDEA solutions are used. Zhang et al. [7] observed that a number of heat-stable salts (HSS) could cause a reduction in CO₂ absorption capacity and regeneration efficiency. Their results indicate that AMP is easier to regenerate with less loss of absorption capacity than other amines, such as, MEA, diethanolamine (DEA), diethylenetriamine (DETA) and N-methyldiethanolamine (MDEA). According to them, the regeneration performance can be ranked in the following order: AMP > MDEA > DEA > MEA. Hence AMP is considered today as one of the most important sterically hindered amine for CO₂ capture from natural gas as well as from power plant flue gas [3], [8], [9].

The VLE data of the (CO₂ + amine + water) system at various temperatures and concentrations have a very important role in the design and optimisation of industrial gas treating process. In rate-based models, physical and chemical equilibria play an important role by defining the boundary conditions for the partial differential equations that describe mass transfer coupled with chemical reactions [10]. Besides, the equilibrium solubility of the CO₂ in aqueous amine solutions determines the minimum recirculation rate of the solution that would treat a CO₂-laden gas stream, and it determines the maximum concentration of CO₂ which can be left in the regenerated solution in order to meet the treated gas specifications. Although aqueous AMP and AMP blends are extensively used in the purification of gas streams contaminated with CO₂, a few results of CO₂ solubility in aqueous AMP solutions have been reported earlier [11], [12], [13], [14]. However, there are some discrepancies noted among the published data. Modified Kent and Eisenberg modelling approach [15] has been used by Tontiwachwuthikul et al. [12], Li and Chang [16], Park et al. [17] to correlate their experimental data. Teng and Mather [11], Jene and Li [18] adopted Deshmukh and Mather model [19]. Silkenbäumer et al. [13], Kundu et al. [20] used an approach based on Pitzer’s–G^E model to correlate the measured VLE data. However, in all the reported literature, the deviations between the measured and correlated results are quite high and lie in the range of 15% to 35%.

Reliable experimental data and a consistent thermodynamic model are still necessary to describe the thermodynamic equilibrium behaviour of CO₂ in concentrated aqueous AMP over wide ranges of temperature and pressure. In view of this, the objective of this present work is to provide additional solubility data of CO₂ in aqueous AMP over a wide range of concentration and at low to high CO₂ partial pressure. A thermodynamic VLE model based on electrolyte non-random two-liquid (eNRTL) theory [21], [22], [23], [24] has also been developed to represent the equilibrium solubility of CO₂ in aqueous AMP. Besides, this model is also needed to represent the VLE of CO₂ in activated and blended aqueous AMP solvents. In this work Aspen Plus® software, V-7.0 [25] has been used for data regression and model validation.