Effect of the Tower Type on the Gas Sweetening Process

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles (2017) 72, 24 Ó A. Sharifi and E. Omidbakhsh Amiri, published by IFP Energies nouvelles, 2017 DOI: 10.2516/ogst/2017018 Dossier Second and Third Generation Biofuels: Towards Sustainability and Competitiveness Seconde et troisième génération de biocarburants : développement durable et compétitivité Effect of the Tower Type on the Gas Sweetening Process Abdolkarim Sharifi and Elham Omidbakhsh Amiri* Department of Chemical Engineering, University of Mazandaran, Babolsar - Iran e-mail: * Corresponding author Abstract — The removal of acid gases, CO2 and H2S from natural gas streams is essential for environmental and health reasons. In this work, the simulated gas sweetening unit with MethylDiEthanolAmine (MDEA) solvent was studied to improve quality. Firstly, the effect of trays types and then, the effect of various packing and the effect of the packing size were considered on the flow rate of CO2 and H2S in the main streams. Results show that with considering the different trays types in the regenerator tower, the flow rate of CO2 in the sweet gas stream with bubble cap tray is lower than other trays types. Also, with considering the different trays types in the absorber tower, the flow rate of CO2 in the sweet gas stream with bubble cap tray is lower than other trays types in the absorber tower. In considering with different types of packing, results show that the flow rate of CO2 with ballast ring packing and the flow rate of H2S with Raschig ring packing are lower than other types of packing. However, in some types such as cascade miniring, Intalox Saddles and pall ring, there is no difference for the flow rate of CO2 or H2S. In all cases, with increasing the size of the packing, the flow rate of CO2 and H2S in the sweet gas stream increases, however, this increasing in the metal packing is very small. INTRODUCTION The growing needs of the natural gas in many of the industry lead to improving the existing processes for welfare of the people, energy and pollution control. One of the problems in the natural gas processing is the presence of acid gases such as CO2 and H2S. According to the contracts, specified H2S and CO2 content in the natural gas stream would be about 4 ppm and 2%, respectively (Stewart and Arnold, 2011). The presence of these gases in the pipeline can cause technical problems (i.e. corrosion), so, before transferring of the natural gas through pipelines, these acid gases must be removed. Many processes have been developed for the removal of CO2 and H2S from the natural gas in various operating conditions and various CO2 and H2S concentrations in the sour gas (Kohl and Nielsen, 1997; Processors, 2004; Rufford et al., 2012; Abdulrahman and Sebastine, 2013; Muhammad and GadelHak, 2014; Tavan and Tavan, 2014). One of these processes is the gas purification process by amine. The amines are the most generally accepted of the many available solvents for removal acid gas from the natural gas streams. First use of the amines for sweetening of the natural gas was referred to Bottoms who was given a patent in 1930. The amines to be used in the natural gas sweetening include MEA (MonoEthanolAmine), DiEthanolAmine (DEA), Tri-EthanolAmine (TEA), DiGlycolAmine (DGA), Methyl-DiEthanolAmine (MDEA) (Maddox, 1982). MDEA is rapidly increasing in importance as a solvent for the removal of high concentrations of acid gas, particularly CO2, because of its low energy requirements in regenerator specially, high capacity, excellent stability, and other favorable attributes. On the other hand, capability of MDEA for selective reaction with H2S in the presence of CO2 is important. Actually, when CO2 amount to H2S amount be high, MDEA is used in this process (Maddox, 1982; Borhani et al., 2016). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Page 2 of 9 Oil & Gas Science and Technology – Rev. IFP Energies nouvelles (2017) 72, 24 The gas sweetening processes by amine were developed to meet this need for a high pressure, high volume H2S removal process. There are many cases that need to be considered to select a process for gas sweetening. These include: – the different impurities to be removed from the natural gas stream; – the concentration level of these impurities and the desired degree of separation would be achieved; – the needed volume of the natural gas and the operation conditions of gas are set; – the feasibility of sulfur recovery to be considered; – relative economics of the suitable processes to be considered. The level of acid gas concentration in the sour gas is an important issue for selecting the proper amine solvent and on the other influence on human health. Table 1 shows the allowable of H2S concentrations for human contact time (Maddox, 1982). Abdulrahman and Sebastine (2013) studied the effect of amine solvents types on the percent of CO2 and H2S in the sweet gas stream. In their simulation, the optimization of process studied by using several amine types and blends of them, for example, MEA and MDEA. Also, they examined some of the critical factors for amine solvent types, such as solvent circulation rate and solvent concentration. They found that the use of (DEA 35% w/w) is mostly recommended. Abu-Zahra et al. (2007a, b) studied a parametric study of the technical and economic performance based on MEA. They found that with increasing the MEA concentration in the absorption solution, energy savings can be achieved. Higher efficiency of the regeneration can be seen by increasing the pressure. The costs of CO2 avoided and cost of electricity were found to show a shallow minimum for specification lean solvent loading. The highest concentration of MEA, if allowable from the corrosion point of view, can decrease the costs. Cho et al. (2015) studied amine-based acid gas sweetening with an emphasis on energy minimization. For the implementation of design and optimization of process, the process simulator Aspen HYSYS was linked to a stochastic optimization algorithm within MATLAB. With this configuration and operating conditions, the reboiler duty was significantly reduced and the overall utility costs are decreased. Muhammad and GadelHak (2014) extracted the numerical output of the simulation by experimental design models for laboratory setups. The key variable and the total cost are related to the percent of CO2 and H2S in the feed flow rate. Accordingly, the obtained relation is checked against different operating conditions to use for ensure the validity of the results. Tavan and Tavan (2014) noted that the absorption of CO2 into DEA is one of the most promising technologies for CO2 capturing due to its cost effectiveness, and capability of handling large amounts of acid gases. Azeotro (...truncated)