Full Length ArticleEstimation of CO2 equilibrium absorption in aqueous solutions of commonly used amines using different computational schemes
Graphical abstract
Introduction
Nowdays, reduction of CO2 emission into the atmosphere is an important issue [1], [2]. Among diverse greenhouse gases (GHGs), it is widely accepted that CO2 is the most eminent GHG which leads to the nearly 60% of the global climate change [2], [3]. Considering the before-mentioned implications of CO2 emission, mainly released by CO2 rich streams (e.g., flue gas), it is extremely important to remove CO2 by efficient separation and purification techniques.
Absorption by using aqueous amine solvent is one of the most prevalent techniques for post-combustion CO2 capture due to its privileges, such as high flexibility for industrial applications and great performance [4], [5]. Among a wide variety of influential parameters in the absorptive removal of CO2 by amine solvents, CO2 solubility or equilibrium CO2 capacity has been known as the most important parameter affecting the effectiveness and performance of amine solvents to a great extent [4].
Different separation and purification techniques have been extensively employed (e.g., physical and chemical absorption [5], cryogenic separation [6], adsorption by solid sorbents [7], membrane technology [8], [9], [10], and hybrid processes) for CO2 capture from natural gas and post-combustion CO2 capture. Although gas hydrates have attracted a great attention as a new technology [11], [12], chemical absorption of CO2 by alkanolamine solvents is the most preferred technique which has been used during past decades [13], [14]. The physical and chemical properties of commercial amine solvents, mass transfer mechanisms and kinetics of absorption can be found in the published literature [15], [16].
As well as typical measurements by experimental methods, a series of thermodynamic models have been proposed to calculate the equilibrium absorption capacity of CO2 at various operating conditions. The well known thermodynamic models developed on the basis of vapor-liquid equilibrium (VLE) theory, include Kent-Eisenberg [17], [18], electrolyte-NRTL [19], Deshmukh-Mather [20] and extended UNIQUAC [21], [22]. Nevertheless, thermodynamic models are suffering from some drawbacks, which make them inappropriate for accurate estimation of CO2 loading in a wide range of conditions and different amine solutions, and normally, physicochemical properties should be known for each amine solution. For instance, Benamor and Aroua [23] designed a number of experiments to assess the accuracy of modified Deshmukh-Mather model to estimate the CO2 loading in DEA, MDEA and DEA/MDEA solutions at different operating temperatures (30–50 °C), CO2 partial pressures (0.09–100 kPa) and amine concentrations (2–4 M). Though they reported an acceptable accuracy between experimental and model estimated values for both DEA and MDEA, the limited number of amines and narrow ranges of operating variables can seriously barricade its applicability.
Various methodologies of machine learning, including artificial neural network (ANN), adaptive neuro-fuzzy inference system (ANFIS) and support vector machine (SVM), have been presented for modeling and parametric estimation in different fields [24], [25], [26]. Similarly, many research works have investigated the application of machine learning methodologies in the area of aqueous alkanolamine solvents for CO2 capture. Koolivand Salooki et al. [27] succeeded to estimate the output parameters of stripper column, located at Hashemi Nejad Gas Refinery in Iran, by using ANN model and reported a good agreement between operational data and the result obtained from model. Adib et al. investigated the ability of SVM model to estimate output variables of regeneration column (i.e., temperature and reflux flow rate) of Hashemi Nejad Gas Refinery [28]. A simple comparison between ANN and SVM model revealed that SVM can estimate processes variables better than ANN model with minimum square correlation coefficient of 0.99. In 2014, Ghiasi and Mohammadi [29] utilized Least Squares Support Vector Machine (LSSVM) approach to model CO2 loading in different aqueous amine solvents with different concentrations, temperatures and CO2 partial pressures. In 2016, Ghiasi et al. [1] employed ANFIS technique to model a similar system and indicated that ANFIS soft computing approach substantially improves the accuracy of model to estimate the process variables, compared to the previously used LSSVM.
Saghatoleslami and his colleagues [30] performed similar experiments by genetic algorithm. Sipöcz et al. [31] used feed forward ANN to model steady-state CO2 capture process by aqueous MEA solvent in a power plant. In the published article by Zhou et al. [32], a combination of ANN with both adaptive-network-based fuzzy interface system (ANFIS) and sensitivity analysis was used to model post-combustion CO2 capture by amine solvents. Similar investigations can be found in the published research works [33], [34], [35].
ANN modeling was employed to estimate the experimental values of CO2 solubility in aqueous TIPA, TIPA/MEA and TIPA/PZ solutions by Daneshvar et al. [36]. Shahsavand and his coworkers [37] studied the capability of both radial basis function neural network and multi-layer perceptron ANN to calculate the equilibrium CO2 absorption capacity of aqueous DEA and MDEA solutions considering different concentrations and rates. A comparative study was performed by Pahlavanzadeh et al. [38] to evaluate the abilities of Deshmukh-Mather and ANN models to estimate CO2 solubility in 2-amino-2-methyl-1-propanol (AMP) at low partial pressures (7.47–69.87 kPa). ZareNezhad and Aminian used a feed-forward ANN model to estimate the H2S solubility in PZ solvent [39] and a fuzzy network model to anticipate the solubility of H2S in aqueous brine solutions [40]. In another research, Bastani et al. [41] employed feed forward ANN model to estimate CO2 absorption capacity of aqueous chemical absorbers in a wide range of operating parameters (i.e., temperature, pressure and concentration).
To the best of our knowledge, there is not any research work attempting to estimate the experimentally obtained values of CO2 absorption capacities of all kinds of aqueous amines by only a single comprehensive model.
In this work, the experimental data of CO2 solubility in three commercialized amines (including MEA, DEA, and MDEA) and nine recently developed amines (including 2-amino-2-methyl-1-propanol (AMP), piperazine (PZ), triisopropanolamine (TIPA), monoproanolamine(MPA), 1-amino-2-propanol (MIPA), 4-(diethylamino)-2-butanol(DEAB), methyl amino ethanol (MAE), 2-(Diethylamino)- ethanol(DEEA) and 3-(Methylamino)-propylamine (MAPA)) were used which can cover primary, secondary and tertiary amine classification. Hence, the accurate results of ANN models for anticipating the equilibrium CO2 absorption capacities of all before-mentioned amine solutions can ameliorate the limitations of typical theoretical models, which are only applicable for specific cases.
Almost all previously published research papers agree that data validation is the most crucial factor for developing a promising model.
Section snippets
Radial basis function neural network (RBF-NN)
RBF-ANN is constructed from three layers; namely input, output and hidden layers [42]. Among different ANNs, RBF-ANN can be categorized into the single hidden layer group which is more beneficial than multiple interconnected hidden layers, such as the multilayer perception neural network (MLP-NN) [42]. In detail, activation function and the maximum number of neurons are two important parts of RBF-NN, which can substantially affect the processing conditions. Gaussian function is normally used as
General step
In order to employ machine learning technique for modeling CO2 solubility in diverse aqueous amine solutions (e.g., MEA, DEA, and MDEA, AMP, PZ, TIPA, MPA, MIPA, DEAB, MAE, DEEA and MAPA), a comperhensive databank of experimental data was extracted from literature. The CO2 solubility is defined as the moles of CO2 per moles of amine and is a function of CO2 partial pressure (PCO2, kPa), temperature (T, K), amine concentration (Camine, mol/L), molecular weight, hydrogen bond donor/acceptor
Model development
The CSA approach was utilized to find the optimal values of the LSSVM parameters, including γ and σ2. Values of these parameters are 101.6330 and 0.389, respectively. The structure of RBF consists of couple of tuning parameters that are maximum number of neurons and spread. The trial and error were employed to find the optimum values, which were 95 and 260, respectively. The MSE values at different neurons are depicted in Fig. 1. PSO and GA methods were employed to determine optimum values of
Conclusion
In this work, different models (i.e. GA-ANFIS, PSO-ANFIS, RBF-ANN and LSSVM) were successfully developed to anticipate the equilibrium absorption capacity of CO2 in 12 aqueous alkanolamine solutions. The results indicate that the models are capable to acceptably estimate experimental values of CO2 loading over a wide range of independent variables (i.e. CO2 partial pressure, temperature and amine concentration). Furthermore, it is declared that the results of the developed model (in this study)
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