تفکیک دی اکسید کربن و هیدروژن سولفید با استفاده از مایع یونی دمای اتاق [bmim] [PF6] Separation of CO2 and H2S using room-temperature ionic liquid [bmim][PF6]

۱٫ Introduction Hydrogen sulfide (H2S) and carbon dioxide (CO2) are commonly removed from natural and synthesis gases through chemical absorption using aqueous solutions of organic bases like single amines, amine mixtures, or mixtures of an amine and a salt of an amino acid [1,2]. Extensive research has been conducted by several groups on aqueous solutions of alkanolamines, especially monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA) for natural gas treating and sweetening [3–۱۳]. The typical process involves competitive chemical absorption of H2S and CO2 in a packed column at low temperature (preferably ambient temperature) and elevated pressures (up to about 4 MPa or more). The gas desorption or solvent regeneration occurs at elevated temperatures (typically around 350–۴۰۰ K) and low pressures using a stripping column. Disadvantages of aqueous solutions of alkanolamines include loss of the amine during regeneration, transfer of water into the gas stream, degradation of the amine to form corrosive byproducts, and low temperature and high pressure absorption, all of which makes this process economically expensive. Room-temperature ionic liquids (RTILs) have been proposed for the capture of gases such as CO2. Several solubility studies of CO2 in many RTILs have been reported [14–۲۷]; however only a few researchers have examined the binary phase PTx (pressure–temperature composition) behavior of H2S in ionic liquids [28–۳۱]. Jou and Mather [28] reported the first solubility data of H2S in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) at temperatures from 298.15 to 403.15 K and pressures up to 9.6 MPa. Pomelli et al. [29] measured the solubility of H2S in different imidazolium-based ionic liquids with various anions and in a series of bis(trifluoromethyl)sulfonylimide (Tf2N)- based ionic liquids with various cations at 298.15 K and 1400 kPa. Heintz et al. [30] attempt to measure the solubility of CO2 and a mixture of N2/H2S in an ionic liquid with an ammonium cation and chloride anion from 300 to 500 K and pressures up to 2.3 and 30 bar for H2S and CO2, respectively; however the structure of the ionic liquid was unknown and an approximate chemical formula was assumed for calculating the mole fraction solubilities. Recently, Jalili et al. [31] measured the solubility of H2S in three RTIL ([bmim][PF6], [bmim][BF4] and [bmim][Tf2N]) at temperatures from 303.15 to 343.15 K and pressures up to 1 MPa. In each case the H2S solubility is much higher compared with CO2. For example, Henry’s law constants are 51.7 and 1.43 bar at 298 K for CO2 and H2S in RTIL ([bmim][PF6]), respectively [17,31]. This large difference in the Henry’s law constant suggests the selective cap turing and separation of these gases may be possible using ionic liquids. In the present study, we construct for the first time the ternary phase diagrams of CO2/H2S/[bmim][PF6], using our cubic equation-of-state (EOS) method [14,32]. The ternary EOS is based on interaction parameters of each binary system, and the binary interaction parameters were determined from our previous VLE (vapor–liquid equilibrium) measurements for CO2/[bmim][PF6] [17], and literature data for H2S/[bmim][PF6] [28] and CO2/H2S [13]. We have also measured the VLLE for the CO2/[bmim][PF6] and H2S/[bmim][PF6] binary systems to validate the boundary of the immiscibility region. In order to check the validity of the ternary EOS, VLE experiments for the CO2/H2S/[bmim][PF6] system were performed under various T, P, and feed compositions, and the EOS validity was satisfactorily confirmed. Then, the CO2/H2S selectivity with and without RTIL [bmim][PF6] are calculated at several feed, T, and P conditions. The selectivity advantage using this RTIL is discussed based on the present ternary phase calculations. ۲٫ Experimental ۲٫۱٫ Materials Hydrogen sulfide (mole fraction purity >0.995, CAS no. 7783- 06-4) and carbon dioxide (mole fraction purity >0.9999, CAS no. 124-38-9) were purchased from MG Industries (Philadelphia, PA). The [bmim][PF6] (C8H15N2F6P, Lot and filling number 1242554 15005226, CAS no. 174501-64-5) was obtained from Fluka (Buchs, Switzerland). Fig. 1 provides the chemical structure. The [bmim][PF6] ionic liquid sample was dried and degassed by first placing the sample in a borosilicate glass tube and pulling a vacuum on the sample with a diaphragm pump (Pfeiffer, model MVP055-3) for about 3 h. Next, the sample was fully evacuated using a turbopump (Pfeiffer, model TSH-071) to a pressure of about 4 × ۱۰−۷ kPa while simultaneously heating and stirring the ionic liquid at a temperature of about 348 K for 5 days. The final water content was measured by Karl-Fischer titration (Aqua-Star C3000, solutions AquaStar Coulomat C and A) and the ionic liquid contained 482 ppm of water (mass basis). 2.2. Binary VLE measurements In our previous report, we measured the gas solubility of CO2 and [bmim][PF6] using a gravimetric microbalance [17] (Hiden Isochema Ltd., IGA 003). Detailed descriptions of the experimental equipment and procedures for the VLE are given in our previous reports [17,33]. Solubility data for H2S and [bmim][PF6] were taken from Jou and Mather [28]; however not all the data were used. We excluded from our analysis the last two data points at 298.15 K and the last data point at other temperatures (e.g. 313.15 and 343.15 K) shown in Ref. [28] because these were determined using the NIST REFPROP EOS program [34] to be in the VLLE region. The CO2/H2S data were taken from Bierlein and Kay [13].