MODELLING ION TRANSPORT IN AN AMINE SOLUTION THROUGH A NANOFILTRATION MEMBRANE

Brazilian Journal of Chemical Engineering ISSN 0104-6632 Printed in Brazil www.abeq.org.br/bjche Vol. 36, No. 04, pp. 1667 - 1677, October - December, 2019 dx.doi.org/10.1590/0104-6632.20190364s20190068 MODELLING ION TRANSPORT IN AN AMINE SOLUTION THROUGH A NANOFILTRATION MEMBRANE Asma Ghorbani1, Behrouz Bayati1* and Tavan Kikhavani1 1 Ilam University, Department of Chemical Engineering, Ilam, Iran. ORCID: 0000-0003-2686-802X; E-mail: ORCID: 0000-0002-4239-9828; ORCID: 0000-0002-2764-5103 (Submitted: February 8, 2019 ; Revised: May 12, 2019 ; Accepted: June 6, 2019) Abstract - The performance of a flat sheet polyamide nanofiltration membrane in rejection of a binary mixture of heat stable salts (acetate and sulfate) from methyl-diethanolamine (MDEA) solution was investigated. The effects of pressure and concentration of MDEA solution on rejection of acetate and sulfate ions were studied. At MDEA concentration of 20% wt. and pressure of 70 bar, 80% and 98% rejection can be obtained for acetate and sulfate ions, respectively. Membrane performance and transport coefficients were investigated using the Spiegler-Kedem-Katchalsky (SKK) model, film theory and extended Nernst-Planck (FT-ENP) model. The results of the FT-ENP model show accurate agreement with experimental results. This result can be obtained due to considering the charge repulsion of sulfate ions. For sulfate ions, both models show errors less than 1% with >R2=0.98. In the case of acetate, errors less than 3% (>R2=0.75) and 2% (>R2=0.89) were obtained for the SKK and FT-ENP models, respectively. Keywords: Amine solution; Nanofiltration membrane; Spiegler-Kedem-Katchalsky model; Film theory; NernstPlanck equation. INTRODUCTION Natural gas often contains impurities such as hydrogen sulfide (H2S) and carbon dioxide (CO2) that are toxic and cause corrosion in the equipment and environmental pollution during burning. Thus, these impurities must be removed from the gas to reduce the harmful effects. Alkanol amine solutions including methyl-diethanolamine (MDEA), monoethanolamine (MEA), piperazine (PZ), diethanolamine (DEA) and diisopropanolamine (DIPA) are used in sweetening natural gas and removing impurities (Jaafari et al., 2018; Lu et al., 2017; Najibi and Maleki, 2013; Poormohammadian et al., 2015; Qeshta et al., 2015; Qiu et al., 2014; Rebolledo-Libreros and Trejo, 2004; Santaniello and Golemme, 2018). MDEA is usually used in the concentration range of 40 to 50% for the removal of acid gases in the gas sweetening unit (Keewan et al., 2018; Pal et al., 2015). The * Corresponding author: Behrouz Bayati - E-mail: reaction between MDEA and hydrogen sulfide leads to the production of MDEAH+ ions and anions such as acetate, nitrate, formate, glycolate, chloride, and sulfate, etc. The reaction between these anions and MDEAH+ forms a stable salt (HSS) that cannot be regenerated through heating (Meng et al., 2008; Pal et al., 2013). The presence of these salts in amines leads to operational problems such as corrosion, fouling, foaming, high viscosity, and capacity reduction (Cho et al., 2015; Pal et al., 2014; Verma and Verma, 2009; Wang et al., 2015). Therefore, the removal of HSS ions from the amine solution is very important. Various technologies can be used to remove stable salts, including ion exchange, distillation and electrodialysis (ED) (Bazhenov et al., 2015; Cho et al., 2015; Dumée et al., 2012; Gao et al., 2015). Another effective method for the removal of HSS ions is the nanofiltration (NF) membrane process that is rarely studied. Distillation has high-energy consumption, 1668 A. Ghorbani et al. low amine recovery and high probability for thermal degradation. The ion exchange technique is suitable for low salts concentrations. Resin fouling, thermal degradation, permanent poisoning and high rate of resin consumption can be mentioned as disadvantages of this technique. ED has more power consumption and waste products than ion exchange and vacuum distillation, respectively. Furthermore, selective membranes with high chemical stability should be used in ED (Dumée et al., 2012). NF is a pressure-driven membrane process that has characteristics between ultrafiltration (UF) and reverse osmosis (RO) (Roy et al., 2017; Ryzhkov and Minakov, 2016; Wei et al., 2016; Zhao et al., 2016). NF has important advantages including lower pressure, higher permeate flux, low rejection for monovalent ions, high rejection for multivalent ions, and low energy consumption (Wei et al., 2016). The rejection of ions in NF is mainly based on the size (steric), the charge (Donnan exclusion) mechanisms and dielectric exclusion (Epsztein et al., 2018; Fridman-Bishop et al., 2018; Saliha et al., 2009; Santafé-Moros et al., 2005; Wei et al., 2018). The removal of HSSs from amine solutions was studied by .Lim et al. (2014). However, the results were not extended. They used NF to concentrate HSSs in MEA solution and found that 80% of HSS ions with amine rejection less than 7% can be removed using NF. Investigation of the transport mechanisms of ions through membranes provides a better understanding of membrane performance in ion removal. Therefore, the study of HSS ion transport parameters in the amine solution by the NF membrane can be helpful in better understanding the NF membrane performance and rejection mechanism that has not been addressed in the literature. The transport mechanisms of ions in water through the NF membrane have been studied. Hu and Dickson (2006) investigated the performance of the NF membrane for fluoride removal from water using constant surface electrical potential (ψ), pore radius (rP) and pure water permeability (LP) parameters. They estimated the membrane parameters by the extended Nernst-Planck equation and the Grahame equation and observed a good agreement between model and experimental data. The effect of the concentration on the rejection of ions and transport parameters (Keff and Φ) was investigated by Garba et al. (2003). They found that the transport parameters increase with increasing concentration and the model presented a satisfactory agreement with experimental results. Predicting the rejection of ions by the pore transport model was investigated by Déon et al. (2012). They found that the proposed model showed a good estimate of ion rejection by Pérez et al. (2017) evaluated the differences in rejection of salt by the solutiondiffusion-film model (SDFM). They observed a good agreement between model and experimental data. Therefore, the SDFM model could be useful for evaluating the NF membrane performance. Fang and Deng (2014) investigated the effects of operating pressure, pH, arsenate initial concentration, ionic strength on the membrane performance. They studied the transport mechanism by concentration polarization film theory and the Donnan Steric Pore Models. Their results showed that electro-migration and convection transport dominated at high flux and diffusive transport dominated a (...truncated)