Siloxanes capture by ionic liquids: Solvent selection and process evaluation
Graphical abstract
Introduction
Biogas technology has emerged as an alternative to conventional energy resources due to the intensification of global warming [1]. Biogases are considered green, environmental, and valuable renewable fuel, including anaerobic digestion biogas (AD) and landfill gas (LFG), whose major constituent is methane (CH4) [2]. Biogas also contains other undesirable chemicals such as CO2, ammonia, H2S, halogenated hydrocarbons, and siloxanes [3]. Siloxanes constitute the compounds that present most adverse effect on biogas utilization [1]. Siloxanes are volatile organic compounds that contains silicon-oxygen atoms joint (Si-O bonds) with organic groups such as methyl, or ethyl, among others [4]. They constitute a contaminant and an obstacle in biogas applications since they can severely damage equipment’s, especially turbines, heat exchangers or gas engines [5] due the formation of silica (SiO2) and microcrystalline quartz during combustion process that can be deposited at valves, spark plugs and cylinder heads [6]. For these reasons, it is necessary to remove traces of this contaminant from biogas before its application [7]. In fact, the maximum siloxane concentration in biogas (specified by engine manufacturers) is in the range 0.03–28 mg/m3 [8]. Thus, different separation methods have been applied for siloxanes removal, among which stand out adsorption and absorption processes. Adsorption technology using activated carbons [9], silica gel [5] or zeolites presents some advantages on its use such as easy operation, strong adsorption ability (high adsorbent capacity), wide raw materials and low costs [10]. The removal efficiencies of this technology can reach up to 90–99% values [1]. However, some disadvantages found are the high regeneration temperature required, loss of adsorption capacity after regeneration process and a limited thermal regeneration efficiency due to the formation of polymerization products, that in most cases make regeneration unfeasible or require several units (one for use and one for regeneration). Additionally adsorbents are very sensitivity to moisture conditions [11]. Absorption processes usually employ methanol or Selexol® as physical solvents. Also siloxanes are effective removed by chemical absorption, since they can be destroyed with strong acids and bases and their removal efficiency increase with the type of contacting phase [12]. Their efficiencies are higher than 97% when using an organic solvent and lower than 95% with an inorganic one [1]. The main drawbacks reported are the high investment and operation costs, environmental safety and corrosion, high energy required for removal process and absorbent regeneration. Moreover, siloxanes are difficult to be completely removed by physical absorption [8]. Other technologies that are being investigated for siloxanes removal are cryogenic condensation or membranes separation [1]. Cryogenic condensation efficiencies are governed by the operating temperature, reaching 99.3% for temperatures of −70 °C and decreasing to 15–50% at higher ones (–25 °C) [6]. It is a non-toxic technology with an easy operating way, but it implies high investment and operation costs, high energy required (to reach low temperatures) and it is economically feasible only when using high flow rate and high siloxanes loadings [13]. Membranes separations are simple and easy-operating technologies but they imply high investment costs, presenting risk of fouling, blocking and pollution [6]. In general, they present efficiencies higher than 80% [1].
It is well known that biogas contains CH4 (60%) and carbon dioxide (40%) as well as other impurities described before. Therefore, it is important to remove not only siloxanes but also CO2 present on biogas streams to increase the quality for its use [14]. Biogas upgrading concept was created focused on CO2 elimination. It aims to increase the calorific value of the biogas to convert it to a higher fuel standard [15]. Regarding CO2 removal methods, some disadvantages are found on the different technologies. Thus, physical absorption requires a huge amount of solvent to completely remove CO2 [16]. Chemical absorption is a high energy-demanding technology to regenerate the amine-based solvent [17]. Adsorption process usually requires a complex technology since it uses pressure swing adsorption process [18]. Membranes technology presents low CO2 selectivity and CH4 purity reached are not enough for the separation [19]. Cryogenic separation needs high investment and operating cost for cooling the mixtures [20]. In the last decade, the kind of solvents called ionic liquids (ILs) have been widely studied as CO2 absorbents due to their unique properties such as low vapor pressure, high thermal stability, high absorption capacity, among others [21]. It has been demonstrated competitive absorption capacities when compared with traditional solvents such as amine solutions [22], [23]. Recently, ILs have been applied on biogas upgrading due to the high CO2/CH4 selectivity values found [16], [24], [25]. To the best of our knowledge, no works involving siloxanes capture using ILs are reported up to date. In this context, our group has successfully used a multiscale research strategy -integrating both molecular and process simulation- to design new gas separation processes based on ILs, including CO2 [23], [26], [27], [28], NH3 [29], H2S [30], acetylene [31], toluene [31], among other solutes. As first step, a selection of the ILs able to absorb efficiently the solute can be done –among a huge database of cations and anions- by molecular simulation using COSMO-RS method [32]. Then, process simulations are applied to study the technical and economic viability of the gas capture operation based on the selected ILs at industrial scale, using the COSMO-based/Aspen Plus methodology [33]. For this purpose, ILUAM database was created allowing to use a collection of 100 ILs that did not exist on Aspen Properties databases [34].
The aim of this work is to evaluate the application of ILs as siloxanes absorbents based on molecular and process simulations. First, molecular simulation is employed to select ILs that can potentially capture the siloxanes among a wide sample of cations (35) and anions (35). COSMO-RS methodology is also used to analyze the solute–solvent intermolecular interactions, to understand the siloxane solubility in ILs. Then, Aspen Plus process simulator is applied to evaluate the behavior of selected ILs on siloxane absorption in commercial packing columns at process scale. Last, solvent regeneration process is studied to demonstrate the viability of IL reuse and to estimate related energy duty. A final concept test is performed by treating realistic multicomponent biogas flow with absorption technology based on ILs, to simultaneously capture siloxanes and CO2, filling the quality standards of biomethane.
Section snippets
COSMO-RS molecular simulation
ILs geometries were optimized using the independent counter ions (C + A) model at BP86/TZVP computational level using solvent effect by COSMO continuum solvation method as implicit in Turbomole. Siloxanes structures involved in this work (Table 1 collects the studied siloxane compounds, their physical properties, and the used nomenclature) were optimized at the same calculation level in gas phase. Once the geometries were optimized until their minimum energy level, a single point was performed
COSMO-RS molecular simulation
The prediction of thermodynamic properties of siloxane-ILs mixtures is possible thanks to COSMO-RS methodology [32]. It allows the quantification of interaction energy between surfaces with the polarized charge distribution (σ). σ-profile is the representation of polarized charge distribution in a histogram [40]. The chemical nature of the compounds can be rapidly analyzed by σ-profile representation. Thus, Fig. 1 shows the σ-profile of the siloxanes involved in this work, comprising linear and
Conclusions
COSMO-based/Aspen Plus methodology was successfully applied to reveal the promising application of ILs to siloxanes capture, centered on biogas upgrading technology. Molecular simulation results using COSMO-RS method suggested the high affinity of ILs for linear and cyclic siloxane gas compounds, obtaining Henry’s law constants in the range 0.001–1 bar. Intermolecular interaction analysis of siloxane-IL mixtures let us conclude that the absorption phenomenon is governed by excess enthalpy in
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are very grateful to Ministerio de Economía y Competitividad (MINECO) of Spain (project CTQ2017-89441-R) and Comunidad de Madrid (P2018/EMT4348) for financial support. We also thank Centro de Computación Científica de la Universidad Autónoma de Madrid for computational facilities.
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