Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Nitrogen Sorption in a Transition Metal Complex Solution for N2 Rejection from Methane Zhikao Li, Gongkui Xiao, Brendan Graham, Gang Li, and Eric F. May*
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ARC Training Centre for LNG Futures, Fluid Science & Resources Division, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia ABSTRACT: Nitrogen is a ubiquitous impurity in natural gas that has to be removed for the production of liquefied natural gas (LNG). The most widely used technology for N2 rejection, cryogenic distillation, is a capital and energy intensive process. In this work, a nitrogen selective K[RuII(EDTA)] aqueous solution was prepared and tested for nitrogen sorption with the aim of reducing the N2 rejection costs by using a continuous recirculation absorption process analogous to the acid gas removal process in LNG production. The overall equilibrium amount of N2 sorption in the K[RuII(EDTA)] solution was obtained at 20 °C (0.075 mol N2/L solution at 2860 kPa), 30 °C (0.061 mol N2/L solution at 2873 kPa), and 40 °C (0.052 mol N2/L solution at 3049 kPa) using a custom-built volumetric sorption measurement apparatus. The corresponding specific N2 sorption amounts were 0.54 mol N2/mol Ru at 20 °C and 2860 kPa; 0.43 mol N2/ mol Ru at 30 °C and 2873 kPa; and 0.34 mol N2/mol Ru at 40 °C and 3049 kPa. These specific N2 sorption amounts exhibited similar values to the specific loading of carbon dioxide in monoethanolamine (MEA) which is one of the most widely used chemicals for acid gas removal in LNG production industry. The heat of N2 absorption in the K[RuII(EDTA)] solution was in the range of 30−60 kJ/mol N2, suggesting the regeneration of the K[RuII(EDTA)] solution would require less energy than that required for CO2 scrubbing using MEA aqueous solution. The N2/CH4 selectivity in the K[RuII(EDTA)] aqueous solution is in the range of 1.7 to 2.4 depending on the pressure of the gas, which is the highest N2/CH4 selectivity known for a liquid based N2 and CH4 separation system. The comparable specific N2 sorption capacity to CO2-amine system and the high N2/CH4 selectivity of the K[RuII(EDTA)] aqueous solution exhibit a great potential of the solution for nitrogen rejection from natural gas.
1. INTRODUCTION Nitrogen is a ubiquitous impurity in natural gas that has no heating value and poses a safety concern in the transportation and storage of liquefied natural gas (LNG).1 Many natural gas reserves contain less than 4 mol % nitrogen, while some subquality natural gas reserves contains more than 4 mol % nitrogen.2,3 The product specification for LNG is no more than 1 mol % nitrogen.2 The inert nature and the similarity of its physical properties to methane (CH4) make the separation of nitrogen from natural gas very challenging.3 Currently, cryogenic distillation is the most widely used commercial technology for separating nitrogen from natural gas in LNG production.2 This technology utilizes the difference in the boiling points of nitrogen and methane to separate this binary mixture at cryogenic temperatures and has a track record of being economically viable (95−98% methane recovery) for large gas flow rates (>15 MMscfd).2,3 However, the cryogenic process is energetically parasitic because a significant amount of nitrogen gas needs to be cooled to cryogenic temperatures unnecessarily. Furthermore, the cryogenic Nitrogen Rejection Unit (NRU) is located at the end of the LNG production process, thus the low-temperature gas processing facilities have to be designed to handle a greater volume of gas than © XXXX American Chemical Society
necessary if the nitrogen had been removed at ambient temperature. Nevertheless, no other commercial technology at present can economically separate nitrogen at moderate temperature on a gas flow scale comparable to the cryogenic separation. Potential alternative technologies include pressure swing adsorption (PSA), membrane technology, and absorption processes. Using PSA processes for CO2 capture from natural gas has been intensively studied and high performance materials have been explored. However, using PSA processes to reject nitrogen from natural gas is relatively less explored.3−5 PSA using molecular gate adsorbents has been demonstrated to be effective for nitrogen rejection (with 95−98% methane recovery) at low flow rates (2−15 MMscfd).6 However, this technology is still at its early stage of commercialization and struggles to accommodate higher gas flow rates (>15 MMscfd) because of the high capital and operation cost for large flow rate systems.6 Membrane technology is a prospective gas Received: Revised: Accepted: Published: A
March 11, 2019 June 5, 2019 July 1, 2019 July 1, 2019 DOI: 10.1021/acs.iecr.9b01356 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
2. MATERIALS AND APPARATUS 2.1. Materials Preparation. Magnesium metal (chips, 4− 30 mesh, 99.98 wt %), ethylenediaminetetraacetic acid tripotassium salt dihydrate (K3EDTA·2H2O, 98 wt %) and ruthenium(III) chloride hydrate (RuCl3·xH2O) were purchased from Sigma-Aldrich (Australia) and used without further treatment. Hydrochloric acid aqueous solution (HCl, 37 vol %) and potassium hydroxide pellets (KOH, 85 wt %) were purchased from Sigma-Aldrich as well and diluted or dissolved in deionized (DI) water. The pH values of the solutions were measured by an Oakton pH 5+ Hand-held Meter with a pH Probe (John Morris Scientific Pty Ltd.). The nitrogen (N2), methane (CH4), and argon (Ar) were purchased from Coregas with the following claimed purities: N2 99.999 mol %, CH4 99.995 mol %, and Ar 99.995 mol %. The preparation of the K[RuII (EDTA)] solution for nitrogen absorption measuremnts involved three steps: (1) the synthesis of K[RuIII(EDTA)(Cl)]·2H2O, (2) the reduction of K[RuIII(EDTA)(Cl)]·2H2O to K[RuII (EDTA)] in aqueous solution, and (3) the transfer of the K[RuII(EDTA)] aqueous solution into an absorption cell. The synthesis of K[RuIII(EDTA)(Cl)]·2H2O was carried out according to a method reported in literature.28,29 A mass of 4.43 g (∼0.01 mol) of K3EDTA·2H2O was first put in 20.00 mL of DI water in a 100 mL glass beaker, and the resultant mixture was gently heated up on a hot plate until all solid dissolved to give a clear solution. A ruthenium(III) chloride water solution composed of 2.08 g (∼0.01 mol) of RuCl3·xH2O, and 20.00 mL of DI water was added to the K3EDTA solution while swirling the beaker. The pH value of this solution was adjusted and maintained in the range of 4−5 by adding 0.10 mol/L KOH aqueous solution or 0.10 mol/L HCl aqueous solution when necessary. The resulted mixture solution was gently boiled off at 140−145 °C until no more solid precipitating out. Then, another 40.00 mL of DI water was added to the beaker to completely dissolve the solid precipitate. Following that, the water in the solution was evaporated again. This dissolvingprecipitating process was carried out several times until the color of the solid precipitate became light yellow. The collected solid product was washed thoroughly with ice water until it was free of RuIII ions (transparent residual solution without the brown color). Afterward, the product was washed with ethanol twice, predried using filter paper, and thoroughly dried in an oven at 80 °C for 10 h. A mass of 3.52 g of dry K[RuIII(EDTA)(Cl)]·2H2O was obtained with an overall yield of ∼70% based on the mass balance calculation of ruthenium. A Spectrum One FT-IR Spectrometer (PerkinElmer) equipped with an attenuated total reflection (ATR) sampler and a deuterated triglycine sulfate (DTGS) detector was used to record the IR spectra of both the raw K3EDTA· 2H2O and the synthesized K[RuIII(EDTA)(Cl)]·2H2O. The latter gave IR peaks at 3410 (−OH), 1724 (free −COOH), and 1610 (coordinated − COO−) cm−1 that were consistent with literature data;30−32 while the raw K3EDTA·2H2O gave IR peaks at 3410, 1634, and 1595 cm−1 as shown in Figure 1. An Elementary Vario Macro was used to analyze the total carbon and nitrogen contents. The theoretical weight percentage (wt %) of carbon (C) and nitrogen (N) for K[RuIII(EDTA)Cl]·2H2O were C 23.98% and N 5.59%; the measured values were C 24.90%, N 5.82%, which matched well with the theoretical values.
separation technique because of its intrinsic features of no phase change, low energy input, and the requirement of fewer pieces of moving parts.3,7 The most successful application of membrane gas separation is in the hydrogen (H2) recovery and purification industry, mainly attributed to the development of membranes with high H2 selectivity8 over other gas species such as carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and nitrogen (N2). However, there has been no commercial membranes that have high enough nitrogen over methane selectivity to enable cost-effective rejection of nitrogen on a large scale. It has been reported that a N2/CH4 selectivity of 15 is required to make the membrane process economically viable,7 but the highest N2/CH4 selectivity reported up to date is only around 8 from carbon molecular sieve membranes9,10 and 2−3 from polymer membranes.7 Absorption process is another technology that has been studied for the removal of nitrogen from natural gas with the same concept as in carbon dioxide removal using amine solutions. The core part of any gas absorption technology is an effective and affordable solvent and/or chemical that can selectively dissolve the gas with a reasonable capacity and rate. For CO2 absorption process amine is such a chemical, but for N2 absorption such a nitrogen selective chemical/solvent has yet to be established because of the inert nature of nitrogen gas at moderate temperature. However, because of the industry’s familarity with gas absorption processes and the potential of such a process to remove N2 from a large-scale natural gas flow (>30 MMscfd) with low capital and operational cost, the search for potential N2 absorbent has attracted much attention.11−21 Previous research has reported that certain transition metal complex (TMC) solutions could absorb nitrogen at various conditions depending on the type of the transition metal centers and the supporting ligands.11,12,16,17,22−27 However, most of these TMC solutions used organic solvents which inherently have higher solubilities for methane than for nitrogen, rendering the solutions unsuitable for nitrogenselective absorption. Therefore, a task-specific TMC in a proper solvent that has the ability to selectively absorb nitrogen under moderate conditions (T = ∼30 °C, P = 0−3000 kPa) is desirable. Here, a “proper solvent” should have (1) a high solubility for the TMC to achieve higher absorption capacity of nitrogen than that of methane, (2) a low viscosity to allow efficient liquid circulation, and (3) a low volatility to avoid solvent loss. As K[RuII(EDTA)] aqueous solution is one of the reported liquid solutions that can reversiblely absorb nitrogen, it was selected and studied in this work. Water was used as the solvent for K[RuII(EDTA)] because of its advantage of being both economical and environmentally friendly. A customdesigned static solubility apparatus was constructed to measure the nitrogen absorption amount in TMC solution, and the reliability of the apparatus was confirmed by measuring nitrogen solubility in deionized water and comparing with literature data. Nitrogen absorption in the TMC solution was conducted at three moderate temperatures of 20, 30, and 40 °C, in the presssure range of 100 to 3000 kPa covering the nitrogen partial pressure range in a typical natural gas processing plant. Nitrogen desorption experiments were also performed at 30 °C to characterize the reversibility of nitrogen sorption in this TMC solution. Moreover, a quantitative method has been presented to estimate the uncertainties of these absorption capacities. B
DOI: 10.1021/acs.iecr.9b01356 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 2. Schematic of the air-bath static gas solubility apparatus. V: valve; AC: accumulation cell; SC: solubility cell. An ISCO pump was used to inject the gas component into the AC to a specific pressure. An air oven (shown as the blue rectangle) was used to control and maintain the experimental temperatures. Four high pressure needle valves were used to connect the two cells and the associated line, which were extended to the outside of the oven. A valve (Ve) was used to depressurize the system and a vacuum pump was used to evacuate the system.
Figure 1. IR spectra of K3EDTA·2H2O and K[RuIII(EDTA)(Cl)]· 2H2O.
To reduce K[RuIII(EDTA)(Cl)]·2H2O to K[RuII(EDTA)], 2.93 g of dry K[RuIII(EDTA)(Cl)]·2H2O was first dissolved in 58.86 g of degassed DI water; the pH of the solution was adjusted to 7 by adding 0.10 mol/L KOH solution and 0.10 mol/L HCl solution as necessary with a total added mass of 0.95 g. This procedure was conducted in a glovebox filled with argon. Next, 0.42 g of magnesium chips (an excess amount of magnesium was required because of the slow reaction rate of magnesium with RuIII and the side reaction with water) was added to the resultant solution to reduce the RuIII to RuII.18 The solution with the added magnesium was kept in the glovebox for 10 h before the nitrogen absorption measurements. The overall ruthenium element concentration in the solution was calculated to be 0.10 mol/L by mass balance. The density of K[RuII(EDTA)] aqueous solution was measured independently with volumetric methods using 5 and 10 mL volumetric flasks, which gave an averaged density of 1.02 g/ mL. To transfer the K[RuII(EDTA)] aqueous solution into an absorption cell for the nitrogen absorption measurements, 10− 15 g of the solution was weighed using a digital balance in the glovebox. The absorption cell was then sealed with a stainlesssteel lid on which a 1/8 in. VCR gland was sealed with Parafilm (purchased from Sigma-Aldrich). The completely sealed absorption cell was transferred out from the glovebox and quickly connected to the absorption apparatus through VCR fittings. 2.2. Apparatus. A schematic of the custom-designed apparatus for absorption measurements is shown in Figure 2. Two cells (the accumulation cell, AC, and the solubility cell, SC) and their associated lines were placed in an oven which can control the temperature from 20 to 70 °C. Two pressure transducers were connected to the two cells, respectively. The Digiquartz pressure transmitter for AC has a full scale of 13 790 kPa and 0.01% uncertainty of reading over this range. The Digiquartz pressure transmitter for SC has a full scale of 6,895 kPa and 0.01% uncertainty of reading over this range. The four 100 Ω platinum resistance thermometers were installed at the top and the bottom of the two cells to monitor the cell temperatures, which were calibrated over the range of 0 to 70 °C with an uncertainty of 0.1 °C according to the International Temperature Scale of 1990 (ITS-90). Four high pressure needle valves were used to control gas flow between cells and pumps. The handles of these valves were extended to
the outside of the oven to minimize temperature interruptions when closing or opening the valves without opening the oven. A vacuum pump is attached to the solubility measurement system to evacuate the system before introducing gas. Nitrogen was first transferred to and stored in a syringe pump (ISCO 260 D) which has a maximum volume of 266.05 mL and is capable of delivering gas at constant pressure (up to 51710 kPa) by volume displacement. Then, a required amount of gas was transferred from the syringe pump to the accumulation cell (AC). When the temperature and pressure of the gas in the accumulation cell became steady, the valve between the accumulation cell and solubility cell (SC) was opened to introduce gas to the SC to start the sorption measurements. The solubility cell was equipped with a mini magnetic stirrer to ensure excellent mixing of the solution and gas species after they were introduced into the cell.
3. ANALYSIS AND UNCERTAINTY ESTIMATION The volumetric method for measuring gas absorption is based on the transfer of a known amount of gas from the accumulation cell (AC) to the solubility cell (SC) that contains the TMC solution. Because the two cells and the associated lines constitute a closed system, the mass balance of the gas in the system can be expressed by eq 1 as follows. G G nabs = V AC × ρAC (Pi(AC) , Ti(AC))
ij yz mL G zzρ G (P , T ) + jjjjV SC − L zz SC i(SC) i(SC) j z ρ ( P , T ) (SC) (SC) i i SC k { G G − V AC × ρAC (Pf (AC) , Tf (AC))
ij yz mL zzρ G (P G − jjjjV SC − L z f (SC) , Tf (SC)) j ρSC (Pf (SC) , Tf (SC)) zz SC k { (1)
Subscripts “abs”, “AC”, “SC”, “i”, and “f ” refer to absorbed gas, accumulation cell, solubility cell, initial value, and final value, respectively; the superscripts “G” and “L” refer to gas phase and liquid phase, respectively; V is the measured volume; P is the measured pressure; T is the measured temperature; ρG is the molar gas density determined using an equation of state at the measured P and T; ρL is the density of the solution C
DOI: 10.1021/acs.iecr.9b01356 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
ÄÅ ÉÑ2 ÅÅi ∂n y ÑÑ ÅÅjj abs zz G Ñ (u(nabs)) = ÅÅjj G zzu(V AC)ÑÑÑ ÅÅj ∂V z ÑÑ ÅÇk AC { ÑÖ ÅÄÅi ÑÉÑ2 ÅÅj ÑÑ zyz ∂nabs j G Å zzu(Pρ (Pi(AC), Ti(AC)))ÑÑÑ + ÅÅÅjjj G AC z ÑÑ ÅÅj ∂ρAC (Pi(AC), Ti(AC)) z ÑÑÖ ÅÇk { ÅÄÅi ÑÉÑ2 yz ÅÅj ÑÑ ∂nabs j z G Å Ñ +ÅÅÅjjj G zzzu(PρAC (Pi(AC), Ti(AC)))ÑÑÑ ÅÅj ∂ρAC (Pi(AC), Ti(AC)) z ÑÑ ÅÇk ÑÖ { É2 ÄÅ ÉÑ2 ÅÄÅi Ñ Ñ y Ñ Ñ ÅÅÅij ∂nabs yz ÅÅÅjj ∂nabs zz G Ñ L Ñ Ñ j z + ÅÅjj G zzu(V SC)ÑÑ + ÅÅÅj L zu(m )ÑÑÑÑ ÅÅj ∂V z ÑÑ ÅÅÇk ∂m { ÑÑÖ ÅÇk SC { ÑÖ ÑÉÑ2 ÅÄÅi ÑÑ ÅÅj zzy L ∂nabs j Ñ Å +ÅÅÅjjj L zzzu(ρSC (Pi(SC), Ti(SC)))ÑÑÑ ÑÑ ÅÅj ∂ρSC (Pi(SC), Ti(SC)) z ÑÖ ÅÇk {
Industrial & Engineering Chemistry Research determined by a volumetric method described in the material and apparatus section, mL is the mass of the solvent; and nabs is the moles of gas absorbed in the solvent. From eq 1, it is evident that the accuracy of the measured capacity relies on three factors: (1) the volume of the accumulation section (VAC) and the solubility section (VSC), (2) the volume of the transition metal complex solution, and (3) the density of the gas phase at corresponding temperature and pressures. The VAC and VSC were measured via the displaced volume in the ISCO pump (Figure 2) at a constant pressure mode. Specifically, the ISCO pump and the associated line until V1 was filled with nitrogen at 5000 kPa and operated in constant pressure mode at room temperature. A vacuum pump was used to evacuate the absorption apparatus, and then V2, V3, and V4 were closed to isolate AC from SC and the atmosphere. Certain amount of nitrogen was then transferred from the ISCO pump into the AC by opening V1. After a few minutes, the equilibrium of gas between the pump and the AC was reached and the decrease of volume in the ISCO pump was recorded as VAC. Ten repeated measurements of the volume of AC were performed, and the average volume of VAC was 33.10 ± 0.02 mL. The same method was used to measure VSC, and the averaged volume was 32.59 ± 0.02 mL. The density of the TMC solution was assumed to remain constant throughout the experiment conditions. The densities of the gas phase in the AC and SC at different pressures and temperatures were calculated by using equation of state (0.01% uncertainty at 0−77 °C and
