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propane separation: Absorption equilibrium
Separation and Purification Technology 63 (2008) 311–318
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Room temperature ionic liquid with silver salt as efficient reaction media for propylene/propane separation: Absorption equilibrium Alfredo Ortiz, Alicia Ruiz, Daniel Gorri, Inmaculada Ortiz ∗ Departamento de Ingenier´ıa Qu´ımica y Qu´ımica Inorg´ anica, Universidad de Cantabria, Avda. Los Castros s/n, 39005 Santander, Spain
a r t i c l e
i n f o
Article history: Received 15 February 2008 Received in revised form 14 May 2008 Accepted 15 May 2008 Keywords: Gas separation Propylene Silver -Complexation Ionic liquids
a b s t r a c t In this study the selective absorption of propylene from their mixtures with propane by chemical complexation with silver ions in ionic liquid solutions has been performed. The solubilities of propylene and propane in the reactive medium, silver tetrafluoroborate dissolved in 1-butyl-3-methylimidazolium tetrafluoroborate (silver salt concentration = 0.25 M), were investigated as functions of temperature and pressure. The temperature range was between 278 and 318 K for the absorption equilibrium data obtained. A simple mathematical model has been developed to describe the total propylene absorption in the reaction media under study, based in the formation of complexes with different stoichiometry, and the parameter model were obtained. It was found that gas solubilities increased with pressure and decrease with temperature of the system; this indicates that complex formation is an exothermic process and consequently the equilibrium constants will decrease with increasing temperature. Complete regeneration of the reaction media can be carried out at room temperature, 800 rpm stirrer speed and 20 mbar vacuum for 3 h. As conclusion, the recovery of the olefin (propylene) via silver complexation has been successfully carried out in ionic liquid media and, the ionic liquid containing silver ions showed higher sorption capacity for propylene than the corresponding aqueous solution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years there has been much interest in the recovery of light olefins from the off-gas of catalytic crackers, driven largely by the escalating demand for feedstock for production of polyethylene and polypropylene [1]. Propylene is the key building block for the production of important petrochemicals, such as polypropylene, acrylonitrile, propylene oxide, cumene, phenol, isopropylic alcohol and many others [2]. Global propylene demand during 2006 saw a grow rate estimated at 5.5% bringing total propylene consumptions up to about 69 million metric tons. The 5-year outlook shows world propylene demand growth to average slightly less then 5% per year [3]. The production of polymers and other specialty chemicals from mono-olefins such propylene requires the olefin to be of extremely high purity (>99.9%), and since light olefins are commonly produced together with paraffin hydrocarbons (e.g., ethane and propane), the techniques for separating the both hydrocarbons are of primary importance in the petrochemical
∗ Corresponding author. Tel.: +34 942 20 15 85; fax: +34 942 20 15 91. E-mail addresses: [email protected] (A. Ortiz), [email protected] (A. Ruiz), [email protected] (D. Gorri), [email protected] (I. Ortiz). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.05.011
industry [4]. For over 60 years, propane/propylene separation is being performed by a highly energy-intensive distillation in a single or double column process with 150–200 trays at cryogenic temperatures between 233 and 183 K and pressures ranging from 16 to 20 bar. The reason for the extreme conditions used in such distillations is the similar boiling points of propylene and propane (ϑb, propylene = 225.3 K, ϑb, propane = 230.9 K at P = 1 bar) [5]. The method of olefin/paraffin separations certainly holds an enormous potential for capital and energy cost savings if a more efficient technique is developed. Numerous processes have been proposed and investigated as cost-efficient alternatives including extractive distillation, absorption, adsorption and membranes. For the case of the reactive absorption, complexation of the olefin with the transition metal (traditionally silver or copper) has been the most extensively studied approach [4]. One chemical characteristic of silver ions is their ability to bind specifically and reversibly with olefins. Olefin molecules donate electrons from their occupied 2p orbitals to the empty s orbitals of the silver ions to form -bonds. Back donation of electrons from the occupied d orbitals of the silver ions into the empty *–2p antibonding orbitals of the olefin molecules results in -bonding [6]. The advantage of chemical complexation is that the bonds formed are stronger than those formed by van der Waals forces alone, so it is possible to achieve high selectivity and high capacity
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Nomenclature C H Hr Hsol [I] KEq P R T V n
concentration (mol l−1 ) Henry’s law constants (mol (l bar)−1 ) enthalpy of reaction (kJ mol−1 ) standard solvation enthalpy (kJ mol−1 ) molar concentration component I (mol l−1 ) complexation equilibrium constant (l mol−1 ) pressure (bar) universal gas constant (J mol−1 K−1 ) temperature (K) volume (ml) number of moles (mol)
Greek letters ϑb boiling point w standard weighted deviation Superscripts/subscript 1,2 reaction identification Eq equilibrium exp experimental exp experimental replica i component identification r reaction sim simulated sol solvation T total upp upper chamber low lower chamber
for the component to be bound. At the same time, the bonds are still weak enough to be broken by using simple engineering operations such as raising the temperature or decreasing the pressure [7]. An olefin/paraffin separation process based upon reactive absorption has two major potential benefits. Utilization of a mass-separating rather than an energy-separating agent could substantially reduce energy requirements. Furthermore, a selective facilitator with a large olefin capacity and fast reaction rates would permit use of smaller contactors than are currently employed in distillation [8]. This work presents the potential use of room-temperature ionic liquids (RTILs) as reaction media in propylene/propane separation for reactive absorption. RTILs are salts composed exclusively of organic cations and inorganic anions that remain in the liquid phase at or below 423 K. RTILs are non-flammable compounds that have negligible vapor pressure, high chemical and thermal stability, and relatively large electrochemical windows. They can be water soluble depending on the hydrophilicity of the ionic moieties. Polarity and hydrophilicity/hydrophobicity can be tuned by a suitable combination of cation and anion; therefore they have been termed “designer solvents”. Hence, these aforementioned characteristics make RTILs potential substitutes for organic solvents as separating agents and media for reactions or electrochemical processes [9,10]. Their lack of volatility gives ionic liquids the feature that they can perform clean gas separations without any loss of solvent or contamination of the gas stream. [11]. The ions of RTILs can dissolve silver ions and posses minimal vapor pressure. RTILs can be used to control the interaction between the silver cation (Ag+ ) and its counteranion (X− ) in an RTILs/silver salt system, with the result that the silver cation becomes chemically more active in forming silver–olefin complexes [6]. To choose the most efficient RTIL for use as a reaction medium in gas separation, it is necessary to know the solubility of the gas in
the ionic liquid phase. Few data are available in the literature on the solubility of gases in ionic liquids [12]. Camper et al. [13] presented the Henry’s law constant of hydrocarbons gases such as ethane, ethane, propane, propene, isobutene, butane, 1-butene, and 1,3butadiene in BMImPF6 , BMImBF4 , EMImTf2 N, EMImCF3 SO3 , and EMImDca at 313.15 K. The five RTILs have higher solubility parameters than the polar solvents because ionic interactions are greater than polar interactions [13]. In this preliminary study, a RTIL composed of 1-butyl-3methylimidazolium tetrafluoroborate, BMImBF4 , was used as solvent for the silver salt, because it presents high physical solubility for propylene and high ratio between Henry’s constants from propane and propylene. In general, the olefin selectivity of a silver salt (AgX) in the system is dependent on the lattice energy of AgX. In this case, AgBF4 was chosen because it has low lattice energy and exhibits higher selectivity toward the studied separation than AgNO3 [6,14], and it is soluble in BMImBF4 because both contain the same anion. In general, when a gaseous component solubilizes in a liquid and complexes with its ions, the loading of the gas is greatly affected by its partial pressure, the temperature and the concentration of the complexing ions in the solution. In order to specify the process design conditions for absorption and desorption of the gas and thereby carry out an effective separation of the active gas from the inerts, a knowledge of the vapor-liquid equilibria is very important. The main objective of this work is to report and describe the equilibrium behaviour of the complexation reaction between propylene and silver ions into a selected ionic liquid at a temperature range between 278 and 318 K. 2. Mathematical modelling of the gas solubility in reaction media The objective of this section is to present a simple model that can adequately describe the propylene and propane solubility in a liquid media with and without reactive agent and can also be used for the calculation of the selectivity between propylene and propane. Equilibrium curves for propylene and propane gases generated with silver-free ionic liquid found both gases to exhibit ideal Henry’s law behaviour. For physical solubility, the Henry’s law constant can be defined as [4]: Hi =
ci Pi
(1)
where ci is concentration of the gas in the liquid phase, and Pi is the partial pressure of the gas. Since propane is not able to form complexes with silver ions, the only absorption is due to physical solubility and it can be described by the Henry’s law in reactive media too. To successfully describe the absorption process of propylene with physical and chemical effects, an equilibrium model was developed. Fig. 1 shows a schematic depiction for the distribution of propylene between a gas phase and a reactive liquid phase. The liquid phase contains the reactive metal that is capable of forming complexes with propylene in BMImBF4 as absorption media. In -complexation systems, various complexes of different stoichiometry can exist simultaneously. However, in most systems only complexes with a 1:1 stoichiometry (silver–olefin) are formed in significant amounts, and are referred to as the primary complex species; its reversible formation in a reactive phase proceeds according to [15,16]: Ag+ + C3 H6 ↔ Ag+ (C3 H6 )
primary complexation
(2)
Under certain conditions (high silver loading, high propylene partial pressure, and low temperature), complexes with
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For practical applications, the activities of the liquid phase species are assumed to be proportional to the concentration of the species, with the constant of proportionality (the nonidealities) taken up in the equilibrium constant. The concentration of dissolved propylene in the liquid phase in Eqs. (4a) and (4b) is obtained from Eq. (1). The total propylene solubility is the sum of physically dissolved propylene and the chemical complexation with silver: [C3 H6 ]T = [C3 H6 ] + [Ag+ (C3 H6 )] + 2[Ag+ (C3 H6 )2 ]
(5)
Combining Eq. (5) with Eqs. (1) and (4a) and (4b) gives [C3 H6 ]T = HC3 H6 PC3 H6 (1 + KEq,1 [Ag+ ] + 2KEq,1 KEq,2 (HC3 H6 PC3 H6 )[Ag+ ])
Fig. 1. Schematic depiction of propylene solubility in BMImBF4 with AgBF4.
different stoichiometries can exist: the secondary complex (1:2 silver–olefin) and the tertiary complex (2:1). The secondary and tertiary complexes species are formed as consecutive reactions of the primary complex with another propylene or silver ion, respectively [15,16]. After analysis of the experimental results, no indications were found that significant amounts of this tertiary complex are formed, in contrast to secondary complexes. The secondary complex is formed from a consecutive reaction of the primary complex with another propylene molecule: Ag+ (C3 H6 ) + C3 H6 ↔ Ag+ (C3 H6 )2
secondary complexation
(3)
This model is derived under the assumption that the silver tetrafluoroborate salt has no influence on the physical solubility of propylene, which therefore can be determined in the absence of metal salt [16]. In the present study this assumption has been verified, as shown in Section 4.2. The equilibrium constants are defined as KEq,1 =
[Ag+ (C3 H6 )] , [Ag+ ][C3 H6 ]
(4a)
KEq,2 =
[Ag+ (C3 H6 )2 ] [Ag+ (C3 H6 )][C3 H6 ]
(4b)
(6)
The concentration of free silver can be found by a total silver balance as the total concentration of the silver minus the silver concentrations in the primary and secondary complexes. T
[Ag+ ] = [Ag+ ] − ([Ag+ (C3 H6 )] + [Ag+ (C3 H6 )2 ])
(7)
Combining Eq. (7) with Eqs. (1) and (4a) and (4b) yields after rewriting: [Ag+ ] =
[Ag+ ]
T
(1 + KEq,1 PC3 H6 HC3 H6 + KEq,1 KEq,2 (PC3 H6 HC3 H6 )2 )
(8)
Combining Eqs. (6) and (8) gives the total propylene solubility as a function of the pressure and concentration of the silver metal. The equilibrium constants have been determined by parameter estimation with the software for integrated process engineering, Aspen Custom Modeler; calculated values of KEq were obtained by minimizing the weighted square error between simulated values with the model and experimental data. Selectivity of a solvent for propylene may be defined as the tendency for the ratio of C3 H6 to C3 H8 contents to be larger in the liquid phase than in the gas phase. The thermodynamic selectivity at equilibrium is expressed as [17]: Selectivity =
Fig. 2. Experimental setup.
([C3 H6 ]T /[C3 H8 ]T ) (PC3 H6 /PC3 H8 )
(9)
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Fig. 3. Equilibrium isotherms of propylene (a) and propane (b) in BMImBF4 : (䊉) experimental; (—) calculated.
3. Experimental methods 3.1. Materials Propylene and propane gases were purchased from Praxair with a purity of 99.5 and 99.5%, respectively. The RTIL used in this study was 1-butyl-3-methylimidazolium tetrafluoroborate (CAS number: 174501-65-6) from Solvent Innovation, with a minimum purity of 99%. The purity of an ionic liquid is a very important issue when measuring physical properties. BMImBF4 is water miscible; although water stable, this compound is hygroscopic, so the uptake of water vapor is an important issue. The ionic liquid used in the present work contained <0.1% water and residual halide content less than 500 ppm. Reactive media were prepared using silver tetrafluoroborate AgBF4 of 99% purity (Apollo Scientific Ltd.) dissolved in RTIL at room temperature. All the chemicals were used as received. 3.2. Apparatus and measurements The apparatus used in the measurements of the gases solubilities is shown in Fig. 2. The equipment consisted of a stainless steel vessel (upper chamber) used to transfer a known amount of gas into a stainless steel autoclave (lower chamber) for gas–liquid contacting. The volume of the lower chamber (Vlow ) (152 ml) was measured by filling the chamber with water and then measuring the volume of the water using a graduated cylinder. The volume of the upper chamber (Vupp ) (66 ml) was measured by the following process: the upper and lower chambers were evacuated, and then the valve separating the two chambers was closed. The upper chamber was pressurized and allowed to temperature equilibrate. The volume of the upper chamber, using the ideal gas law, was calculated from the pressure drop observed when the valve was opened communicating both chambers. The temperature of the system is controlled by fluid circulation in conjunction with computer controlled electric heating. The temperature was monitored with a type K thermocouple (Assy) placed inside the lower chamber and automatically maintained within 0.1 K of the setpoint. The pressure in the upper chamber is measured by a digital pressure gauge (Omega Engineering DPG1000B-30V100G with an accuracy of ±0.017 bar), and the autoclave pressure was measured with a pressure transducer (Hirsschmann12B-GDM0-25 bar with an accuracy of ±0.001 bar). 3.3. Procedure In order to begin the absorption experiments, 15 ml of the reactive silver–BMImBF4 mixture (0.25 M), were added to the autoclave.
Air was removed from both the upper and the lower chambers by a vacuum pump (<2 mbar). The valve was closed, separating the two chambers. Then, a pure gas sample was injected into the upper chamber to a desired pressure (Pupp ), which was the considered initial pressure. After the gas was charged, the stirrer was turn on (1500 rpm) and the silver–RTIL mixture was constantly stirred throughout the experiment. The equipment was allowed to stand undisturbed until the temperature was equilibrated. The equilibrium process began adding the solute gas to the autoclave when the valve was opened. The system pressure changed with time due to the system approaching equilibrium, the pressure and temperature readings were recorded in 1-min intervals until the pressure was consistent for 10 consecutive intervals, equilibration time varied between 30 and 90 min. The final pressure difference, after accounting for the increase in volume, was used to determine the number of moles of gas absorbed. Once the final equilibrium conditions were recorded, the stirrer was stopped, and the solutions were either regenerated or kept for a subsequent absorption or desorption experiment. Gas solubility was determined from the observed pressure changes on the basis of the following assumptions and experimental observations: (1) the absorption liquid was non-volatile at the absorption conditions. (2) The amount of absorbed gas in the initial liquid was negligible. (3) No volume change of the liquid was observed during the dissolution of gas. (4) Ideal gas behaviour is observed [4,18]. With these assumptions, a mass balance was used to relate the initial moles of gas in the upper chamber with moles of gas in the total volume at any given time during the experiment. nabsorbed = ninitial,Vupp − ntime,Vupp +Vlow
(10)
Experiments in BMImBF4 without silver salt were carried out using the same procedure. Two replicates of each experiment were performed and the experimental error was determined. The weighted standard deviation, defined by Eq. (11), was calculated leading to values of w = 2% and concluding that the experiments were replicable.
w =
n i=1
((Cexp − Cexp " )/Cexp ) N−1
2
(11)
4. Results and discussion The solubilities of propylene and propane in the reactive medium, BMImBF4 with silver salt [AgBF4 ] = 0.25 M, were investigated as functions of temperature and pressure. Typically in the literature, equilibrium data of olefin absorption are presented as moles of gas absorbed per liter of solution. The temperature range
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Table 1 Henry’s constants (mol/(1 bar)) from 278 to 318 K, and parameter values for Eq. (12) Temperature (K)
Propylene
Propane
278 288 298 308 318
0.145 0.103 0.069 0.055 0.039
0.059 0.045 0.028 0.022 0.011
Gas
Ho (mol/(l bar))
Hsol (kJ/mol)
Propylene Propane
4.28E−06 1.84E−07
−24.1 −29.5
Fig. 5. Propylene (total, chemical and physical equilibrium) and propane equilibrium isotherms with silver solution and silver-free BMImBF4 at 298 K.
Henry’s law parameter (Ho ) and the standard solvation enthalpy for gas-to-liquid transfer (Hsol ) of propylene and propane in BMImBF4 .
was between 278 and 318 K for the absorption equilibrium data obtained. 4.1. Physical solubility in RTIL To further examine the role of physical solubility in the absorption equilibrium curves, silver-free BMImBF4 was used as absorption media. Fig. 3 presents the experimental and calculated solubility values of propylene and propane from 278 to 318 K in RTIL without silver salt. Equilibrium curves for propylene and propane gases generated with this silver-free ionic liquid showed that both gases to exhibit ideal Henry’s law behaviour (evident by their linear trend with appropriate R2 values and intersection with the origin). The solubility line for propane had a lower slope than propylene due to the RTIL solution’s natural affinity for the olefin (intermolecular forces). As expected, in both cases the gas solubility increased with the system pressure and decrease with the system temperature. The experimental physical solubilities at different temperatures for propylene and propane in BMImBF4 are reported in Table 1. The results are given as Henry’s law constants. Those Henry’s constants were found by fitting the data to a straight line using a least squares method and taking the slope. The temperature dependencies of the Henry’s law constants and selectivity for gases were studied using a Van’t Hoff type equation [19], H(T ) = Ho e−Hsol /RT
(12)
The H values of propylene and propane, when plotted against the reverse of the temperature (Fig. 4), showed a linear relationship. Henry’s law constants (Ho ) and the standard solvation enthalpy for gas-to-liquid transfer (Hsol ) are listed in Table 1. For the silver-free ionic liquid, the ratio of the solubilities of the gases was 2.48, value that was independent of pressure.
Fig. 4. Propylene and propane Henry’s constant (H) as a function of the temperature in BMImBF4 .
4.2. Solubility in reactive RTIL media The absorption solution (BMImBF4 + [AgBF4 ] = 0.25 M) has much higher capacity for propylene than propane at 298 K, as clearly shown in Fig. 5. The trendline through the propane equilibrium data was fit with a y-intercept of zero, and the R2 value of the line was 0.99 with a slope of 0.0284 mol(l bar)−1 . The propylene equilibrium data in the reactive media was definitely not described by Henry’s law; the observed absorption in the experimental system was a combination of chemical effects and physical effects. Propane showed the same solubility in the silver-free ionic liquid as with the AgBF4 solution; therefore, a salting out effect was not observed. Because the presence of the salt did not decrease the solubility of propane, it would be correct to assume that the physical solubility of the propylene in the silver solution was the same as in the silver-free ionic liquid. A comparison of the physical and chemical effects is shown in Fig. 5. Examining the total propylene absorption at pressures over 3 bar revealed that the slope of the equilibrium data was nearly the same as the physical solubility. As a result, the calculated chemical absorption reached a plateau around 3 bar at a propylene concentration around 0.43 M in the reaction media. With the chemical absorption reaching an asymptotic limit, the silver solution is fully loaded chemically, and higher pressures result only in increased physical absorption. Because the silver concentration was 0.25 M, this would lead to the assumption that silver forms various complexes of different stoichiometry simultaneously. Selectivity defined by Eq. (9) has been plotted in Fig. 6 as a ratio of propylene/propane pure gases absorptions at the same pressure. For the [0.25] M silver solution, the selectivity reached the highest value at the lowest gas partial pressure because the physical solubility effects were dominated by the chemical complexation effects. At partial pressures over 3 bar, the selectivity starts to level
Fig. 6. Equilibrium selectivity at 298 K.
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Fig. 7. Comparative equilibrium isotherms for propane and propylene in BMImBF4 and aqueous media at 298 K.
out due to the fact that the silver sites were becoming saturated. A trade-off existed for propylene absorption and operating pressure such that higher pressures led to higher capacity but lower selectivity and lower pressures led to higher selectivity and lower capacity. Since the influence of the chemical effects was observed to be around 3 bar, this propylene pressure should be the maximum operating partial pressure. The minimum operating pressure should be around 1 bar in order not to compromise the absorption capacity. Based on the stated operating range of partial pressures from 1 to 3 bar, the mixed gas propylene/propane equilibrium selectivity for optimum process design should range between 16 and 7.5 from Fig. 6. For industrial application with the real mixed stream, total feed pressure will depend on the feed compositions and silver concentration. Higher silver salt concentrations will result in an increase on absorption capacity and selectivity values allowing work at higher pressures, but the regeneration stage would probably be more difficult. Desorption of propylene at high silver salt concentrations becomes more difficult because of the more stable complex; moreover, the higher the concentration of silver in the absorbing solution, the higher will be the density and viscosity of the mixture. This may present mass-transfer and pumping difficulties. Equilibrium isotherms for propane and propylene at 298 K in (BMImBF4 + [AgBF4 ] = 0.25 M) are compared in Fig. 7 with our previous experimental data obtained with aqueous silver solutions. It is shown that propylene solubility in BMImBF4 is higher than in aqueous media, being silver concentration four times smaller, indicating their greater efficiency in the reactive separation process. Complete propylene desorption was carried out at room temperature, 800 rpm stirrer speed and 20 mbar vacuum during 3 h. The reproducibility of the uptake experiments indicated that a consistent regeneration was obtained and the silver–RTIL medium can be reused many times without any loss of absorption capability and solvent. 4.3. Temperature effect and mathematical modelling in reactive RTIL media The propylene isotherms obtained in this work in the silver reactive media exhibited a nonlinear trend with pressure as shown in Fig. 8. The equilibrium constants for the complexation reactions (KEq,1 , KEq,2 ) are a function of temperature and can be described by the Van’t Hoff equation [20]. d ln KEq Hr =− R d(1/T )
(13)
where T is the temperature (K), Hr is the enthalpy of reaction (kJ/mol), and R is the gas constant (kJ/mol K). Eqs. (1), (6), (8), (9), (10), (12) and (13) constitute the proposed mathematical model for the description of the reactive gas absorption process. This set of differential and algebraic equations
Fig. 8. Equilibrium isotherms of propylene in AgBF4 [0.25 M]–BMImBF4 : (䊉) experimental; (—) calculated.
Table 2 Enthalpies of complexation and equilibrium constants for primary and secondary complexation Temperature (K)
KEq,1 (l/mol)a
KEq,2 (l/mol)b
278 288 298 308 318
337.0 285.8 245.1 212.2 185.5
18.2 10.2 5.9 3.6 2.2
a b
Hr ,1 = −11.0 (kJ/mol). Hr ,2 = −38.7 (kJ/mol).
was solved simultaneously using the equation-oriented simulation software Aspen Custom Modeler. As previously described some parameters are unknown, Hr,1 , Hr,2 , KEq,1 (278 K) and KEq,2 (278 K) from Eq. (13), and must be estimated. Hence, the unknown parameters are calculated employing the parameter estimation tool of the employed software together with experimental data series. Subsequently, the mathematical model is run using the values of the estimated parameters. The obtained values for Hr,1 , Hr,2 , KEq,1 f(T) and KEq,2 f(T) are shown in Table 2. The enthalpies of complexation of the propylene–silver complexes are −11.0 and −38.7 kJ/mol for primary and secondary complexes, respectively. This indicates that complex formation is an exothermic process and consequently the equilibrium constants will decrease with increasing temperature [8,15]. The obtained values for the enthalpies of complexation are close to the values of −23.5 and −50 kJ/mol that have been reported for propylene in aqueous silver nitrate by Cho et al. [18] and Chilukuri et al. [21]. The equilibrium constants determined from the experimental data are compared to the values reported in the literature for propylene and ethylene in aqueous media as shown in Table 3. A numerical solution of the mathematical model allows analyzing the distribution or speciation of the complexes. Fig. 9 shows the evolution of the primary and secondary complexes concentration dependence on pressure at 298 K, where it can be observed at pressures below 2.5 bar that the primary complex is formed predominantly. However, with increasing pressure the secondary complex formation is forced and the concentration of the primary complex decreased as the silver concentration must keep constant.
Table 3 Summary of equilibrium constants of olefins complexation in aqueous silver nitrate media at 298 K Reference
Propylene Nymeijer et al. [22] Baker [23] Trueblood et al. [24] Wasik and Tsang [25]
[Ag+ ] (mol/l)
KEq,1 (l/mol)
– – 96.8 79
Ethylene 99 116 98 85
1–3.5 0–0.7 0.5–1 <0.1
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Fig. 9. Primary and secondary complexes distribution dependence on pressure at 298 K.
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lower pressures creating a trade-off with capacity for the optimum absorption pressure. Complete regeneration of the reaction media can be carried out at room temperature, 800 rpm stirrer speed and 20 mbar vacuum for 3 h. Experimental results show that regeneration of the silver solution by stripping at vacuum conditions would be entirely feasible. Nonetheless, further studies at different conditions of temperature and/or pressure are indispensable in order to decide the optimal conditions to be used for the regeneration process. In all cases, gas solubilities increased with pressure and decrease with temperature of the system. The enthalpies of complexation for the primary and secondary propylene–silver complexes are −11.0 and −38.7 kJ/mol, respectively; this indicates that the complex formation is an exothermic process and consequently the equilibrium constants will decrease with increasing temperature. A simple mathematical model has been developed to describe the total propylene absorption in the reaction media under study, based on the formation of organometallic complexes with different stoichiometry. All the results of Csim fall within the interval Cexp ± 10% Cexp , proving that the proposed model offers a satisfactory description. The low values of the weighted standard deviation reinforce this conclusion. This work shows the viability of the separation of propylenepropane mixtures using AgBF4 dissolved in a suitable RTIL. Further work using membrane modules as contactors would be of great interest. Membrane processes are expected to have the potential to be less energy intensive and less voluminous. A hybrid membrane gas absorption process, combining the advantages of absorption (high selectivity) and membranes (modularity, small size) is a worthwhile alternative. Acknowledgements
Fig. 10. Parity graph of propylene for simulated and experimental equilibrium values in AgBF4 [0.25 M]–BMImBF4 .
A comparison was made between simulated and experimental values in order to validate the proposed mathematical model (see Fig. 8). The weighted standard deviation values of the reactive gas absorption process were calculated as w = 3%. The accuracy of the simulation results is presented in the parity graph shown in Fig. 10. Predicted concentration values, Csim , are shown versus experimental data, Cexp , under the conditions studied in this work. All the results of Csim fall within the interval Cexp ± 10% Cexp , proving that the proposed model offers a satisfactory description of the reactive absorption equilibrium. The values of the weighted standard deviation reinforce this conclusion. 5. Conclusions This study has shown that the use of RTILs as reaction media in propylene/propane separation improves its efficiency for propylene absorption because the silver cation becomes chemically more active in forming silver–olefin complexes. In relation to the process selectivity (expressed as separation factor), the studied process show a similar behaviour to other separation processes based on reactive absorption: the equilibrium showed that most part of the chemical complexation reaction took place at propylene partial pressures below 3 bar (for 0.25 M silver concentration and 298 K). Beyond this point, the equilibrium curve showed the same slope as predicted by physical solubility. As consequence, the selectivity of propylene to propane absorption was found to be higher at
The Ministry of Education and Science (Spain) financially supported this research under projects CTQ2005-02583/PPQ and CTM2006-00317. Alfredo Ortiz also thanks MEC for the FPU fellowship. References [1] D.M. Ruthven, S.C. Reyes, Adsorptive separation of light olefins from paraffins, Micropor. Mesopor. Mater. 104 (2007) 59–66. [2] A. Corma, F.V. Melo, L. Sauvanaud, F. Ortega, Light cracked naphtha processing: controlling chemistry for maximum propylene production, Catal. Today 107/108 (2005) 699–706. [3] 2007 World Light Olefins Analysis, Chemical Market Associates, Inc. (CMAI), Houston, 2006. [4] T.A. Reine, R.B. Eldridge, Absorption equilibrium and kinetics for ethylene–ethane separation with a novel solvent, Ind. Eng. Chem. Res. 44 (2005) 7505–7510. [5] F.F. Krull, M. Medved, T. Melin, Novel supported ionic liquid membranes for simultaneous homogeneously catalyzed reaction and vapor separation, Chem. Eng. Sci. 62 (2007) 5579–5585. [6] J. Won, D.B. Kim, Y.S. Kang, D.K. Choi, H.S. Kim, C.K. Kim, C.K. Kim, An ab initio study of ionic liquid silver complexes as carriers in facilitated olefin transport membranes, J. Membr. Sci. 260 (2005) 37–44. [7] C.J. King, Separation processes based on reversible chemical complexation, in: R.W. Rousseau (Ed.), Handbook of Separation Process Technology, Wiley, New York, 1987, pp. 760–774. [8] D.J. Safarik, R.B. Eldridge, Olefin/paraffin separations by reactive absorption: a review, Ind. Eng. Chem. Res. 37 (1998) 2571–2581. [9] L. Ferguson, P. Scovazzo, Solubility, diffusivity and permeability of gases in phosphonium-based room temperature ionic liquids: data and correlations, Ind. Eng. Chem. Res. 46 (2007) 1369–1374. [10] P. Luis, I. Ortiz, R. Aldaco, A. Irabien, A novel group contribution method in the development of a QSAR for predicting the toxicity (Vibrio fischeri EC50) of ionic liquids, Ecotoxicol. Environ. Safe. 67 (2007) 423–429. [11] J.L. Anthony, Gas Solubilities in Ionic Liquids: Experimental Measurements and Applications, Ph.D. Thesis, University of Notre Dame, 2004. [12] B.C. Lee, S.L. Outcalt, Solubilities of gases in the ionic liquid 1-n-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, J. Chem. Eng. Data 51 (2006) 892–897.
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[13] D. Camper, C. Becker, C. Koval, R. Noble, Low pressure hydrocarbon solubility in room temperature ionic liquids containing imidazolium rings using regular solution theory, Ind. Eng. Chem. Res. 44 (2005) 1928–1933. [14] H.W. Quinn, Hydrocarbon separations with silver (I) systems, in: E.S. Perry, C.J. van Oss (Eds.), Progress in Separation and Purification, vol. 4, WileyInterscience, New York, 1971, pp. 133–169. [15] A.E. Wentink, D. Kockmann, N.J.M. Kuipers, A.B. de Haan, J. Scholtz, H. Mulder, Effect of C6-olefin isomers on -complexation for purification of 1-hexene by reactive extractive distillation, Sep. Purif. Technol. 43 (2005) 149–162. [16] A.E. Wentink, N.J.M. Kuipers, A.B. de Haan, J. Scholtz, H. Mulder, Synthesis and evaluation of metal-ligand complexes for selective olefin solubilization in reactive solvents, Ind. Eng. Chem. Res. 44 (2005) 4726–4736. [17] D. Savage, E.W. Funk, W.C. Yu, G. Astarita, Selective absorption of H2 S and CO2 into aqueous solutions of methyldiethanolamine, Ind. Eng. Chem. Fundam. 25 (1986) 326–330. [18] I.H. Cho, D.L. Cho, H.K. Yasuda, T.R. Marrero, Solubility of propylene in aqueous silver nitrate, J. Chem. Eng. Data 40 (1995) 102–106.
[19] K.C. Hansen, Z. Zhou, C.L. Yaws, T.M. Aminabhavi, Determination of Henry’s law constants of organics in dilute aqueous solutions, J. Chem. Eng. Data 38 (1993) 546–550. [20] K. Denbigh, The Principles of Chemical Equilibrium, 4th ed., Cambridge University Press, Cambridge, 1981. [21] P. Chilukuri, K. Rademakers, K. Nymeijer, L. Van der Ham, H. Van der Berg, Propylene/propane separation with a gas/liquid membrane contactor using a silver salt solution, Ind. Eng. Chem. Res. 46 (2007) 8701–8709. [22] K. Nymeijer, T. Visser, W. Brilman, M. Wessling, Analysis of the complexation reaction between Ag+ and ethylene, Ind. Eng. Chem. Res. 43 (2004) 2627–2635. [23] B.B. Baker, The effect of metal fluoroborates on the absorption of ethylene by silver ion, Inorg. Chem. 3 (1964) 200–202. [24] K.N. Trueblood, H.J. Lucas, Coordination of silver ion with unsaturated compounds. V. Ethylene and propene, J. Am. Chem. Soc. 74 (1952) 1338–1339. [25] S.P. Wasik, W. Tsang, Gas chromatographic determination of partition coefficients of some unsaturated hydrocarbons and their deuterated isomers in aqueous silver nitrate solutions, J. Phys. Chem. 74 (1970) 2970–2976.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Room temperature ionic liquid with silver salt as efficient reaction media for propylene/propane separation: Absorption equilibrium Alfredo Ortiz, Alicia Ruiz, Daniel Gorri, Inmaculada Ortiz ∗ Departamento de Ingenier´ıa Qu´ımica y Qu´ımica Inorg´ anica, Universidad de Cantabria, Avda. Los Castros s/n, 39005 Santander, Spain
a r t i c l e
i n f o
Article history: Received 15 February 2008 Received in revised form 14 May 2008 Accepted 15 May 2008 Keywords: Gas separation Propylene Silver -Complexation Ionic liquids
a b s t r a c t In this study the selective absorption of propylene from their mixtures with propane by chemical complexation with silver ions in ionic liquid solutions has been performed. The solubilities of propylene and propane in the reactive medium, silver tetrafluoroborate dissolved in 1-butyl-3-methylimidazolium tetrafluoroborate (silver salt concentration = 0.25 M), were investigated as functions of temperature and pressure. The temperature range was between 278 and 318 K for the absorption equilibrium data obtained. A simple mathematical model has been developed to describe the total propylene absorption in the reaction media under study, based in the formation of complexes with different stoichiometry, and the parameter model were obtained. It was found that gas solubilities increased with pressure and decrease with temperature of the system; this indicates that complex formation is an exothermic process and consequently the equilibrium constants will decrease with increasing temperature. Complete regeneration of the reaction media can be carried out at room temperature, 800 rpm stirrer speed and 20 mbar vacuum for 3 h. As conclusion, the recovery of the olefin (propylene) via silver complexation has been successfully carried out in ionic liquid media and, the ionic liquid containing silver ions showed higher sorption capacity for propylene than the corresponding aqueous solution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years there has been much interest in the recovery of light olefins from the off-gas of catalytic crackers, driven largely by the escalating demand for feedstock for production of polyethylene and polypropylene [1]. Propylene is the key building block for the production of important petrochemicals, such as polypropylene, acrylonitrile, propylene oxide, cumene, phenol, isopropylic alcohol and many others [2]. Global propylene demand during 2006 saw a grow rate estimated at 5.5% bringing total propylene consumptions up to about 69 million metric tons. The 5-year outlook shows world propylene demand growth to average slightly less then 5% per year [3]. The production of polymers and other specialty chemicals from mono-olefins such propylene requires the olefin to be of extremely high purity (>99.9%), and since light olefins are commonly produced together with paraffin hydrocarbons (e.g., ethane and propane), the techniques for separating the both hydrocarbons are of primary importance in the petrochemical
∗ Corresponding author. Tel.: +34 942 20 15 85; fax: +34 942 20 15 91. E-mail addresses: [email protected] (A. Ortiz), [email protected] (A. Ruiz), [email protected] (D. Gorri), [email protected] (I. Ortiz). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.05.011
industry [4]. For over 60 years, propane/propylene separation is being performed by a highly energy-intensive distillation in a single or double column process with 150–200 trays at cryogenic temperatures between 233 and 183 K and pressures ranging from 16 to 20 bar. The reason for the extreme conditions used in such distillations is the similar boiling points of propylene and propane (ϑb, propylene = 225.3 K, ϑb, propane = 230.9 K at P = 1 bar) [5]. The method of olefin/paraffin separations certainly holds an enormous potential for capital and energy cost savings if a more efficient technique is developed. Numerous processes have been proposed and investigated as cost-efficient alternatives including extractive distillation, absorption, adsorption and membranes. For the case of the reactive absorption, complexation of the olefin with the transition metal (traditionally silver or copper) has been the most extensively studied approach [4]. One chemical characteristic of silver ions is their ability to bind specifically and reversibly with olefins. Olefin molecules donate electrons from their occupied 2p orbitals to the empty s orbitals of the silver ions to form -bonds. Back donation of electrons from the occupied d orbitals of the silver ions into the empty *–2p antibonding orbitals of the olefin molecules results in -bonding [6]. The advantage of chemical complexation is that the bonds formed are stronger than those formed by van der Waals forces alone, so it is possible to achieve high selectivity and high capacity
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Nomenclature C H Hr Hsol [I] KEq P R T V n
concentration (mol l−1 ) Henry’s law constants (mol (l bar)−1 ) enthalpy of reaction (kJ mol−1 ) standard solvation enthalpy (kJ mol−1 ) molar concentration component I (mol l−1 ) complexation equilibrium constant (l mol−1 ) pressure (bar) universal gas constant (J mol−1 K−1 ) temperature (K) volume (ml) number of moles (mol)
Greek letters ϑb boiling point w standard weighted deviation Superscripts/subscript 1,2 reaction identification Eq equilibrium exp experimental exp experimental replica i component identification r reaction sim simulated sol solvation T total upp upper chamber low lower chamber
for the component to be bound. At the same time, the bonds are still weak enough to be broken by using simple engineering operations such as raising the temperature or decreasing the pressure [7]. An olefin/paraffin separation process based upon reactive absorption has two major potential benefits. Utilization of a mass-separating rather than an energy-separating agent could substantially reduce energy requirements. Furthermore, a selective facilitator with a large olefin capacity and fast reaction rates would permit use of smaller contactors than are currently employed in distillation [8]. This work presents the potential use of room-temperature ionic liquids (RTILs) as reaction media in propylene/propane separation for reactive absorption. RTILs are salts composed exclusively of organic cations and inorganic anions that remain in the liquid phase at or below 423 K. RTILs are non-flammable compounds that have negligible vapor pressure, high chemical and thermal stability, and relatively large electrochemical windows. They can be water soluble depending on the hydrophilicity of the ionic moieties. Polarity and hydrophilicity/hydrophobicity can be tuned by a suitable combination of cation and anion; therefore they have been termed “designer solvents”. Hence, these aforementioned characteristics make RTILs potential substitutes for organic solvents as separating agents and media for reactions or electrochemical processes [9,10]. Their lack of volatility gives ionic liquids the feature that they can perform clean gas separations without any loss of solvent or contamination of the gas stream. [11]. The ions of RTILs can dissolve silver ions and posses minimal vapor pressure. RTILs can be used to control the interaction between the silver cation (Ag+ ) and its counteranion (X− ) in an RTILs/silver salt system, with the result that the silver cation becomes chemically more active in forming silver–olefin complexes [6]. To choose the most efficient RTIL for use as a reaction medium in gas separation, it is necessary to know the solubility of the gas in
the ionic liquid phase. Few data are available in the literature on the solubility of gases in ionic liquids [12]. Camper et al. [13] presented the Henry’s law constant of hydrocarbons gases such as ethane, ethane, propane, propene, isobutene, butane, 1-butene, and 1,3butadiene in BMImPF6 , BMImBF4 , EMImTf2 N, EMImCF3 SO3 , and EMImDca at 313.15 K. The five RTILs have higher solubility parameters than the polar solvents because ionic interactions are greater than polar interactions [13]. In this preliminary study, a RTIL composed of 1-butyl-3methylimidazolium tetrafluoroborate, BMImBF4 , was used as solvent for the silver salt, because it presents high physical solubility for propylene and high ratio between Henry’s constants from propane and propylene. In general, the olefin selectivity of a silver salt (AgX) in the system is dependent on the lattice energy of AgX. In this case, AgBF4 was chosen because it has low lattice energy and exhibits higher selectivity toward the studied separation than AgNO3 [6,14], and it is soluble in BMImBF4 because both contain the same anion. In general, when a gaseous component solubilizes in a liquid and complexes with its ions, the loading of the gas is greatly affected by its partial pressure, the temperature and the concentration of the complexing ions in the solution. In order to specify the process design conditions for absorption and desorption of the gas and thereby carry out an effective separation of the active gas from the inerts, a knowledge of the vapor-liquid equilibria is very important. The main objective of this work is to report and describe the equilibrium behaviour of the complexation reaction between propylene and silver ions into a selected ionic liquid at a temperature range between 278 and 318 K. 2. Mathematical modelling of the gas solubility in reaction media The objective of this section is to present a simple model that can adequately describe the propylene and propane solubility in a liquid media with and without reactive agent and can also be used for the calculation of the selectivity between propylene and propane. Equilibrium curves for propylene and propane gases generated with silver-free ionic liquid found both gases to exhibit ideal Henry’s law behaviour. For physical solubility, the Henry’s law constant can be defined as [4]: Hi =
ci Pi
(1)
where ci is concentration of the gas in the liquid phase, and Pi is the partial pressure of the gas. Since propane is not able to form complexes with silver ions, the only absorption is due to physical solubility and it can be described by the Henry’s law in reactive media too. To successfully describe the absorption process of propylene with physical and chemical effects, an equilibrium model was developed. Fig. 1 shows a schematic depiction for the distribution of propylene between a gas phase and a reactive liquid phase. The liquid phase contains the reactive metal that is capable of forming complexes with propylene in BMImBF4 as absorption media. In -complexation systems, various complexes of different stoichiometry can exist simultaneously. However, in most systems only complexes with a 1:1 stoichiometry (silver–olefin) are formed in significant amounts, and are referred to as the primary complex species; its reversible formation in a reactive phase proceeds according to [15,16]: Ag+ + C3 H6 ↔ Ag+ (C3 H6 )
primary complexation
(2)
Under certain conditions (high silver loading, high propylene partial pressure, and low temperature), complexes with
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For practical applications, the activities of the liquid phase species are assumed to be proportional to the concentration of the species, with the constant of proportionality (the nonidealities) taken up in the equilibrium constant. The concentration of dissolved propylene in the liquid phase in Eqs. (4a) and (4b) is obtained from Eq. (1). The total propylene solubility is the sum of physically dissolved propylene and the chemical complexation with silver: [C3 H6 ]T = [C3 H6 ] + [Ag+ (C3 H6 )] + 2[Ag+ (C3 H6 )2 ]
(5)
Combining Eq. (5) with Eqs. (1) and (4a) and (4b) gives [C3 H6 ]T = HC3 H6 PC3 H6 (1 + KEq,1 [Ag+ ] + 2KEq,1 KEq,2 (HC3 H6 PC3 H6 )[Ag+ ])
Fig. 1. Schematic depiction of propylene solubility in BMImBF4 with AgBF4.
different stoichiometries can exist: the secondary complex (1:2 silver–olefin) and the tertiary complex (2:1). The secondary and tertiary complexes species are formed as consecutive reactions of the primary complex with another propylene or silver ion, respectively [15,16]. After analysis of the experimental results, no indications were found that significant amounts of this tertiary complex are formed, in contrast to secondary complexes. The secondary complex is formed from a consecutive reaction of the primary complex with another propylene molecule: Ag+ (C3 H6 ) + C3 H6 ↔ Ag+ (C3 H6 )2
secondary complexation
(3)
This model is derived under the assumption that the silver tetrafluoroborate salt has no influence on the physical solubility of propylene, which therefore can be determined in the absence of metal salt [16]. In the present study this assumption has been verified, as shown in Section 4.2. The equilibrium constants are defined as KEq,1 =
[Ag+ (C3 H6 )] , [Ag+ ][C3 H6 ]
(4a)
KEq,2 =
[Ag+ (C3 H6 )2 ] [Ag+ (C3 H6 )][C3 H6 ]
(4b)
(6)
The concentration of free silver can be found by a total silver balance as the total concentration of the silver minus the silver concentrations in the primary and secondary complexes. T
[Ag+ ] = [Ag+ ] − ([Ag+ (C3 H6 )] + [Ag+ (C3 H6 )2 ])
(7)
Combining Eq. (7) with Eqs. (1) and (4a) and (4b) yields after rewriting: [Ag+ ] =
[Ag+ ]
T
(1 + KEq,1 PC3 H6 HC3 H6 + KEq,1 KEq,2 (PC3 H6 HC3 H6 )2 )
(8)
Combining Eqs. (6) and (8) gives the total propylene solubility as a function of the pressure and concentration of the silver metal. The equilibrium constants have been determined by parameter estimation with the software for integrated process engineering, Aspen Custom Modeler; calculated values of KEq were obtained by minimizing the weighted square error between simulated values with the model and experimental data. Selectivity of a solvent for propylene may be defined as the tendency for the ratio of C3 H6 to C3 H8 contents to be larger in the liquid phase than in the gas phase. The thermodynamic selectivity at equilibrium is expressed as [17]: Selectivity =
Fig. 2. Experimental setup.
([C3 H6 ]T /[C3 H8 ]T ) (PC3 H6 /PC3 H8 )
(9)
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Fig. 3. Equilibrium isotherms of propylene (a) and propane (b) in BMImBF4 : (䊉) experimental; (—) calculated.
3. Experimental methods 3.1. Materials Propylene and propane gases were purchased from Praxair with a purity of 99.5 and 99.5%, respectively. The RTIL used in this study was 1-butyl-3-methylimidazolium tetrafluoroborate (CAS number: 174501-65-6) from Solvent Innovation, with a minimum purity of 99%. The purity of an ionic liquid is a very important issue when measuring physical properties. BMImBF4 is water miscible; although water stable, this compound is hygroscopic, so the uptake of water vapor is an important issue. The ionic liquid used in the present work contained <0.1% water and residual halide content less than 500 ppm. Reactive media were prepared using silver tetrafluoroborate AgBF4 of 99% purity (Apollo Scientific Ltd.) dissolved in RTIL at room temperature. All the chemicals were used as received. 3.2. Apparatus and measurements The apparatus used in the measurements of the gases solubilities is shown in Fig. 2. The equipment consisted of a stainless steel vessel (upper chamber) used to transfer a known amount of gas into a stainless steel autoclave (lower chamber) for gas–liquid contacting. The volume of the lower chamber (Vlow ) (152 ml) was measured by filling the chamber with water and then measuring the volume of the water using a graduated cylinder. The volume of the upper chamber (Vupp ) (66 ml) was measured by the following process: the upper and lower chambers were evacuated, and then the valve separating the two chambers was closed. The upper chamber was pressurized and allowed to temperature equilibrate. The volume of the upper chamber, using the ideal gas law, was calculated from the pressure drop observed when the valve was opened communicating both chambers. The temperature of the system is controlled by fluid circulation in conjunction with computer controlled electric heating. The temperature was monitored with a type K thermocouple (Assy) placed inside the lower chamber and automatically maintained within 0.1 K of the setpoint. The pressure in the upper chamber is measured by a digital pressure gauge (Omega Engineering DPG1000B-30V100G with an accuracy of ±0.017 bar), and the autoclave pressure was measured with a pressure transducer (Hirsschmann12B-GDM0-25 bar with an accuracy of ±0.001 bar). 3.3. Procedure In order to begin the absorption experiments, 15 ml of the reactive silver–BMImBF4 mixture (0.25 M), were added to the autoclave.
Air was removed from both the upper and the lower chambers by a vacuum pump (<2 mbar). The valve was closed, separating the two chambers. Then, a pure gas sample was injected into the upper chamber to a desired pressure (Pupp ), which was the considered initial pressure. After the gas was charged, the stirrer was turn on (1500 rpm) and the silver–RTIL mixture was constantly stirred throughout the experiment. The equipment was allowed to stand undisturbed until the temperature was equilibrated. The equilibrium process began adding the solute gas to the autoclave when the valve was opened. The system pressure changed with time due to the system approaching equilibrium, the pressure and temperature readings were recorded in 1-min intervals until the pressure was consistent for 10 consecutive intervals, equilibration time varied between 30 and 90 min. The final pressure difference, after accounting for the increase in volume, was used to determine the number of moles of gas absorbed. Once the final equilibrium conditions were recorded, the stirrer was stopped, and the solutions were either regenerated or kept for a subsequent absorption or desorption experiment. Gas solubility was determined from the observed pressure changes on the basis of the following assumptions and experimental observations: (1) the absorption liquid was non-volatile at the absorption conditions. (2) The amount of absorbed gas in the initial liquid was negligible. (3) No volume change of the liquid was observed during the dissolution of gas. (4) Ideal gas behaviour is observed [4,18]. With these assumptions, a mass balance was used to relate the initial moles of gas in the upper chamber with moles of gas in the total volume at any given time during the experiment. nabsorbed = ninitial,Vupp − ntime,Vupp +Vlow
(10)
Experiments in BMImBF4 without silver salt were carried out using the same procedure. Two replicates of each experiment were performed and the experimental error was determined. The weighted standard deviation, defined by Eq. (11), was calculated leading to values of w = 2% and concluding that the experiments were replicable.
w =
n i=1
((Cexp − Cexp " )/Cexp ) N−1
2
(11)
4. Results and discussion The solubilities of propylene and propane in the reactive medium, BMImBF4 with silver salt [AgBF4 ] = 0.25 M, were investigated as functions of temperature and pressure. Typically in the literature, equilibrium data of olefin absorption are presented as moles of gas absorbed per liter of solution. The temperature range
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Table 1 Henry’s constants (mol/(1 bar)) from 278 to 318 K, and parameter values for Eq. (12) Temperature (K)
Propylene
Propane
278 288 298 308 318
0.145 0.103 0.069 0.055 0.039
0.059 0.045 0.028 0.022 0.011
Gas
Ho (mol/(l bar))
Hsol (kJ/mol)
Propylene Propane
4.28E−06 1.84E−07
−24.1 −29.5
Fig. 5. Propylene (total, chemical and physical equilibrium) and propane equilibrium isotherms with silver solution and silver-free BMImBF4 at 298 K.
Henry’s law parameter (Ho ) and the standard solvation enthalpy for gas-to-liquid transfer (Hsol ) of propylene and propane in BMImBF4 .
was between 278 and 318 K for the absorption equilibrium data obtained. 4.1. Physical solubility in RTIL To further examine the role of physical solubility in the absorption equilibrium curves, silver-free BMImBF4 was used as absorption media. Fig. 3 presents the experimental and calculated solubility values of propylene and propane from 278 to 318 K in RTIL without silver salt. Equilibrium curves for propylene and propane gases generated with this silver-free ionic liquid showed that both gases to exhibit ideal Henry’s law behaviour (evident by their linear trend with appropriate R2 values and intersection with the origin). The solubility line for propane had a lower slope than propylene due to the RTIL solution’s natural affinity for the olefin (intermolecular forces). As expected, in both cases the gas solubility increased with the system pressure and decrease with the system temperature. The experimental physical solubilities at different temperatures for propylene and propane in BMImBF4 are reported in Table 1. The results are given as Henry’s law constants. Those Henry’s constants were found by fitting the data to a straight line using a least squares method and taking the slope. The temperature dependencies of the Henry’s law constants and selectivity for gases were studied using a Van’t Hoff type equation [19], H(T ) = Ho e−Hsol /RT
(12)
The H values of propylene and propane, when plotted against the reverse of the temperature (Fig. 4), showed a linear relationship. Henry’s law constants (Ho ) and the standard solvation enthalpy for gas-to-liquid transfer (Hsol ) are listed in Table 1. For the silver-free ionic liquid, the ratio of the solubilities of the gases was 2.48, value that was independent of pressure.
Fig. 4. Propylene and propane Henry’s constant (H) as a function of the temperature in BMImBF4 .
4.2. Solubility in reactive RTIL media The absorption solution (BMImBF4 + [AgBF4 ] = 0.25 M) has much higher capacity for propylene than propane at 298 K, as clearly shown in Fig. 5. The trendline through the propane equilibrium data was fit with a y-intercept of zero, and the R2 value of the line was 0.99 with a slope of 0.0284 mol(l bar)−1 . The propylene equilibrium data in the reactive media was definitely not described by Henry’s law; the observed absorption in the experimental system was a combination of chemical effects and physical effects. Propane showed the same solubility in the silver-free ionic liquid as with the AgBF4 solution; therefore, a salting out effect was not observed. Because the presence of the salt did not decrease the solubility of propane, it would be correct to assume that the physical solubility of the propylene in the silver solution was the same as in the silver-free ionic liquid. A comparison of the physical and chemical effects is shown in Fig. 5. Examining the total propylene absorption at pressures over 3 bar revealed that the slope of the equilibrium data was nearly the same as the physical solubility. As a result, the calculated chemical absorption reached a plateau around 3 bar at a propylene concentration around 0.43 M in the reaction media. With the chemical absorption reaching an asymptotic limit, the silver solution is fully loaded chemically, and higher pressures result only in increased physical absorption. Because the silver concentration was 0.25 M, this would lead to the assumption that silver forms various complexes of different stoichiometry simultaneously. Selectivity defined by Eq. (9) has been plotted in Fig. 6 as a ratio of propylene/propane pure gases absorptions at the same pressure. For the [0.25] M silver solution, the selectivity reached the highest value at the lowest gas partial pressure because the physical solubility effects were dominated by the chemical complexation effects. At partial pressures over 3 bar, the selectivity starts to level
Fig. 6. Equilibrium selectivity at 298 K.
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Fig. 7. Comparative equilibrium isotherms for propane and propylene in BMImBF4 and aqueous media at 298 K.
out due to the fact that the silver sites were becoming saturated. A trade-off existed for propylene absorption and operating pressure such that higher pressures led to higher capacity but lower selectivity and lower pressures led to higher selectivity and lower capacity. Since the influence of the chemical effects was observed to be around 3 bar, this propylene pressure should be the maximum operating partial pressure. The minimum operating pressure should be around 1 bar in order not to compromise the absorption capacity. Based on the stated operating range of partial pressures from 1 to 3 bar, the mixed gas propylene/propane equilibrium selectivity for optimum process design should range between 16 and 7.5 from Fig. 6. For industrial application with the real mixed stream, total feed pressure will depend on the feed compositions and silver concentration. Higher silver salt concentrations will result in an increase on absorption capacity and selectivity values allowing work at higher pressures, but the regeneration stage would probably be more difficult. Desorption of propylene at high silver salt concentrations becomes more difficult because of the more stable complex; moreover, the higher the concentration of silver in the absorbing solution, the higher will be the density and viscosity of the mixture. This may present mass-transfer and pumping difficulties. Equilibrium isotherms for propane and propylene at 298 K in (BMImBF4 + [AgBF4 ] = 0.25 M) are compared in Fig. 7 with our previous experimental data obtained with aqueous silver solutions. It is shown that propylene solubility in BMImBF4 is higher than in aqueous media, being silver concentration four times smaller, indicating their greater efficiency in the reactive separation process. Complete propylene desorption was carried out at room temperature, 800 rpm stirrer speed and 20 mbar vacuum during 3 h. The reproducibility of the uptake experiments indicated that a consistent regeneration was obtained and the silver–RTIL medium can be reused many times without any loss of absorption capability and solvent. 4.3. Temperature effect and mathematical modelling in reactive RTIL media The propylene isotherms obtained in this work in the silver reactive media exhibited a nonlinear trend with pressure as shown in Fig. 8. The equilibrium constants for the complexation reactions (KEq,1 , KEq,2 ) are a function of temperature and can be described by the Van’t Hoff equation [20]. d ln KEq Hr =− R d(1/T )
(13)
where T is the temperature (K), Hr is the enthalpy of reaction (kJ/mol), and R is the gas constant (kJ/mol K). Eqs. (1), (6), (8), (9), (10), (12) and (13) constitute the proposed mathematical model for the description of the reactive gas absorption process. This set of differential and algebraic equations
Fig. 8. Equilibrium isotherms of propylene in AgBF4 [0.25 M]–BMImBF4 : (䊉) experimental; (—) calculated.
Table 2 Enthalpies of complexation and equilibrium constants for primary and secondary complexation Temperature (K)
KEq,1 (l/mol)a
KEq,2 (l/mol)b
278 288 298 308 318
337.0 285.8 245.1 212.2 185.5
18.2 10.2 5.9 3.6 2.2
a b
Hr ,1 = −11.0 (kJ/mol). Hr ,2 = −38.7 (kJ/mol).
was solved simultaneously using the equation-oriented simulation software Aspen Custom Modeler. As previously described some parameters are unknown, Hr,1 , Hr,2 , KEq,1 (278 K) and KEq,2 (278 K) from Eq. (13), and must be estimated. Hence, the unknown parameters are calculated employing the parameter estimation tool of the employed software together with experimental data series. Subsequently, the mathematical model is run using the values of the estimated parameters. The obtained values for Hr,1 , Hr,2 , KEq,1 f(T) and KEq,2 f(T) are shown in Table 2. The enthalpies of complexation of the propylene–silver complexes are −11.0 and −38.7 kJ/mol for primary and secondary complexes, respectively. This indicates that complex formation is an exothermic process and consequently the equilibrium constants will decrease with increasing temperature [8,15]. The obtained values for the enthalpies of complexation are close to the values of −23.5 and −50 kJ/mol that have been reported for propylene in aqueous silver nitrate by Cho et al. [18] and Chilukuri et al. [21]. The equilibrium constants determined from the experimental data are compared to the values reported in the literature for propylene and ethylene in aqueous media as shown in Table 3. A numerical solution of the mathematical model allows analyzing the distribution or speciation of the complexes. Fig. 9 shows the evolution of the primary and secondary complexes concentration dependence on pressure at 298 K, where it can be observed at pressures below 2.5 bar that the primary complex is formed predominantly. However, with increasing pressure the secondary complex formation is forced and the concentration of the primary complex decreased as the silver concentration must keep constant.
Table 3 Summary of equilibrium constants of olefins complexation in aqueous silver nitrate media at 298 K Reference
Propylene Nymeijer et al. [22] Baker [23] Trueblood et al. [24] Wasik and Tsang [25]
[Ag+ ] (mol/l)
KEq,1 (l/mol)
– – 96.8 79
Ethylene 99 116 98 85
1–3.5 0–0.7 0.5–1 <0.1
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Fig. 9. Primary and secondary complexes distribution dependence on pressure at 298 K.
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lower pressures creating a trade-off with capacity for the optimum absorption pressure. Complete regeneration of the reaction media can be carried out at room temperature, 800 rpm stirrer speed and 20 mbar vacuum for 3 h. Experimental results show that regeneration of the silver solution by stripping at vacuum conditions would be entirely feasible. Nonetheless, further studies at different conditions of temperature and/or pressure are indispensable in order to decide the optimal conditions to be used for the regeneration process. In all cases, gas solubilities increased with pressure and decrease with temperature of the system. The enthalpies of complexation for the primary and secondary propylene–silver complexes are −11.0 and −38.7 kJ/mol, respectively; this indicates that the complex formation is an exothermic process and consequently the equilibrium constants will decrease with increasing temperature. A simple mathematical model has been developed to describe the total propylene absorption in the reaction media under study, based on the formation of organometallic complexes with different stoichiometry. All the results of Csim fall within the interval Cexp ± 10% Cexp , proving that the proposed model offers a satisfactory description. The low values of the weighted standard deviation reinforce this conclusion. This work shows the viability of the separation of propylenepropane mixtures using AgBF4 dissolved in a suitable RTIL. Further work using membrane modules as contactors would be of great interest. Membrane processes are expected to have the potential to be less energy intensive and less voluminous. A hybrid membrane gas absorption process, combining the advantages of absorption (high selectivity) and membranes (modularity, small size) is a worthwhile alternative. Acknowledgements
Fig. 10. Parity graph of propylene for simulated and experimental equilibrium values in AgBF4 [0.25 M]–BMImBF4 .
A comparison was made between simulated and experimental values in order to validate the proposed mathematical model (see Fig. 8). The weighted standard deviation values of the reactive gas absorption process were calculated as w = 3%. The accuracy of the simulation results is presented in the parity graph shown in Fig. 10. Predicted concentration values, Csim , are shown versus experimental data, Cexp , under the conditions studied in this work. All the results of Csim fall within the interval Cexp ± 10% Cexp , proving that the proposed model offers a satisfactory description of the reactive absorption equilibrium. The values of the weighted standard deviation reinforce this conclusion. 5. Conclusions This study has shown that the use of RTILs as reaction media in propylene/propane separation improves its efficiency for propylene absorption because the silver cation becomes chemically more active in forming silver–olefin complexes. In relation to the process selectivity (expressed as separation factor), the studied process show a similar behaviour to other separation processes based on reactive absorption: the equilibrium showed that most part of the chemical complexation reaction took place at propylene partial pressures below 3 bar (for 0.25 M silver concentration and 298 K). Beyond this point, the equilibrium curve showed the same slope as predicted by physical solubility. As consequence, the selectivity of propylene to propane absorption was found to be higher at
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