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Separation and Purification Technology 63 (2008) 311–318

Contents lists available at ScienceDirect

Separation and Purification Technology

journa l homepage: www.e lsev ier .com/ locate /seppur

oom temperature ionic liquid with silver salt as efficient reaction media forropylene/propane separation: Absorption equilibrium

lfredo Ortiz, Alicia Ruiz, Daniel Gorri, Inmaculada Ortiz ∗

epartamento de Ingenierıa Quımica y Quımica Inorganica, Universidad de Cantabria, Avda. Los Castros s/n, 39005 Santander, Spain

r t i c l e i n f o

rticle history:eceived 15 February 2008eceived in revised form 14 May 2008ccepted 15 May 2008

eywords:as separationropyleneilver

a b s t r a c t

In this study the selective absorption of propylene from their mixtures with propane by chemical com-plexation with silver ions in ionic liquid solutions has been performed.

The solubilities of propylene and propane in the reactive medium, silver tetrafluoroborate dissolvedin 1-butyl-3-methylimidazolium tetrafluoroborate (silver salt concentration = 0.25 M), were investigatedas functions of temperature and pressure. The temperature range was between 278 and 318 K for theabsorption equilibrium data obtained. A simple mathematical model has been developed to describe thetotal propylene absorption in the reaction media under study, based in the formation of complexes withdifferent stoichiometry, and the parameter model were obtained.

-Complexationonic liquids

It was found that gas solubilities increased with pressure and decrease with temperature of the sys-tem; this indicates that complex formation is an exothermic process and consequently the equilibriumconstants will decrease with increasing temperature. Complete regeneration of the reaction media canbe 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 car-ried out in ionic liquid media and, the ionic liquid containing silver ions showed higher sorption capacity

respo

ibst1d(mptadote

for propylene than the cor

. Introduction

In recent years there has been much interest in the recovery ofight olefins from the off-gas of catalytic crackers, driven largely byhe escalating demand for feedstock for production of polyethylenend polypropylene [1]. Propylene is the key building block for theroduction of important petrochemicals, such as polypropylene,crylonitrile, propylene oxide, cumene, phenol, isopropylic alcoholnd many others [2]. Global propylene demand during 2006 saw arow rate estimated at 5.5% bringing total propylene consumptionsp to about 69 million metric tons. The 5-year outlook shows worldropylene demand growth to average slightly less then 5% per year3].

The production of polymers and other specialty chemicalsrom mono-olefins such propylene requires the olefin to be

f extremely high purity (>99.9%), and since light olefins areommonly produced together with paraffin hydrocarbons (e.g.,thane and propane), the techniques for separating the bothydrocarbons are of primary importance in the petrochemical

∗ Corresponding author. Tel.: +34 942 20 15 85; fax: +34 942 20 15 91.E-mail addresses: ortizal@unican.es (A. Ortiz), ruizpal@unican.es (A. Ruiz),

orrie@unican.es (D. Gorri), ortizi@unican.es (I. Ortiz).

seotb

fa

383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2008.05.011

nding aqueous solution.© 2008 Elsevier B.V. All rights reserved.

ndustry [4]. For over 60 years, propane/propylene separation iseing performed by a highly energy-intensive distillation in aingle or double column process with 150–200 trays at cryogenicemperatures between 233 and 183 K and pressures ranging from6 to 20 bar. The reason for the extreme conditions used in suchistillations is the similar boiling points of propylene and propaneϑb, propylene = 225.3 K, ϑb, propane = 230.9 K at P = 1 bar) [5]. The

ethod of olefin/paraffin separations certainly holds an enormousotential for capital and energy cost savings if a more efficientechnique is developed. Numerous processes have been proposednd investigated as cost-efficient alternatives including extractiveistillation, absorption, adsorption and membranes. For the casef the reactive absorption, complexation of the olefin with theransition metal (traditionally silver or copper) has been the mostxtensively studied approach [4].

One chemical characteristic of silver ions is their ability to bindpecifically and reversibly with olefins. Olefin molecules donate �lectrons from their occupied 2p orbitals to the empty s orbitalsf the silver ions to form �-bonds. Back donation of electrons from

he occupied d orbitals of the silver ions into the empty �*–2p anti-onding orbitals of the olefin molecules results in �-bonding [6].

The advantage of chemical complexation is that the bondsormed are stronger than those formed by van der Waals forceslone, so it is possible to achieve high selectivity and high capacity

312 A. Ortiz et al. / Separation and Purificatio

Nomenclature

C concentration (mol l−1)H Henry’s law constants (mol (l bar)−1)�Hr enthalpy of reaction (kJ mol−1)�Hsol standard solvation enthalpy (kJ mol−1)[I] molar concentration component I (mol l−1)KEq complexation equilibrium constant (l mol−1)P pressure (bar)R universal gas constant (J mol−1 K−1)T temperature (K)V volume (ml)n number of moles (mol)

Greek lettersϑb boiling point�w standard weighted deviation

Superscripts/subscript1,2 reaction identificationEq equilibriumexp experimentalexp′′ experimental replicai component identificationr reactionsim simulatedsol solvationT total

fwsohrels

lfoanrbab“mi[to

vtsc

i

tstebEet

msbpsIaAt

cbtctiTlab

2m

amtEsbd

H

wtcsm

wdoliIioipaccording to [15,16]:

upp upper chamberlow lower chamber

or the component to be bound. At the same time, the bonds are stilleak enough to be broken by using simple engineering operations

uch as raising the temperature or decreasing the pressure [7]. Anlefin/paraffin separation process based upon reactive absorptionas two major potential benefits. Utilization of a mass-separatingather than an energy-separating agent could substantially reducenergy requirements. Furthermore, a selective facilitator with aarge olefin capacity and fast reaction rates would permit use ofmaller contactors than are currently employed in distillation [8].

This work presents the potential use of room-temperature ioniciquids (RTILs) as reaction media in propylene/propane separationor reactive absorption. RTILs are salts composed exclusively ofrganic cations and inorganic anions that remain in the liquid phaset or below 423 K. RTILs are non-flammable compounds that haveegligible vapor pressure, high chemical and thermal stability, andelatively large electrochemical windows. They can be water solu-le depending on the hydrophilicity of the ionic moieties. Polaritynd hydrophilicity/hydrophobicity can be tuned by a suitable com-ination of cation and anion; therefore they have been termeddesigner solvents”. Hence, these aforementioned characteristicsake RTILs potential substitutes for organic solvents as separat-

ng agents and media for reactions or electrochemical processes9,10]. Their lack of volatility gives ionic liquids the feature thathey can perform clean gas separations without any loss of solventr contamination of the gas stream. [11].

The ions of RTILs can dissolve silver ions and posses minimalapor pressure. RTILs can be used to control the interaction between

+ −

he silver cation (Ag ) and its counteranion (X ) in an RTILs/silveralt system, with the result that the silver cation becomes chemi-ally more active in forming silver–olefin complexes [6].

To choose the most efficient RTIL for use as a reaction mediumn gas separation, it is necessary to know the solubility of the gas in

A

l

n Technology 63 (2008) 311–318

he ionic liquid phase. Few data are available in the literature on theolubility of gases in ionic liquids [12]. Camper et al. [13] presentedhe Henry’s law constant of hydrocarbons gases such as ethane,thane, propane, propene, isobutene, butane, 1-butene, and 1,3-utadiene in BMImPF6, BMImBF4, EMImTf2N, EMImCF3SO3, andMImDca at 313.15 K. The five RTILs have higher solubility param-ters than the polar solvents because ionic interactions are greaterhan polar interactions [13].

In this preliminary study, a RTIL composed of 1-butyl-3-ethylimidazolium tetrafluoroborate, BMImBF4, was used as

olvent for the silver salt, because it presents high physical solu-ility for propylene and high ratio between Henry’s constants fromropane and propylene. In general, the olefin selectivity of a silveralt (AgX) in the system is dependent on the lattice energy of AgX.n this case, AgBF4 was chosen because it has low lattice energynd exhibits higher selectivity toward the studied separation thangNO3 [6,14], and it is soluble in BMImBF4 because both contain

he same anion.In general, when a gaseous component solubilizes in a liquid and

omplexes with its ions, the loading of the gas is greatly affectedy its partial pressure, the temperature and the concentration ofhe complexing ions in the solution. In order to specify the pro-ess design conditions for absorption and desorption of the gas andhereby carry out an effective separation of the active gas from thenerts, a knowledge of the vapor-liquid equilibria is very important.he main objective of this work is to report and describe the equi-ibrium behaviour of the complexation reaction between propylenend silver ions into a selected ionic liquid at a temperature rangeetween 278 and 318 K.

. Mathematical modelling of the gas solubility in reactionedia

The objective of this section is to present a simple model that candequately describe the propylene and propane solubility in a liquidedia with and without reactive agent and can also be used for

he calculation of the selectivity between propylene and propane.quilibrium curves for propylene and propane gases generated withilver-free ionic liquid found both gases to exhibit ideal Henry’s lawehaviour. For physical solubility, the Henry’s law constant can beefined as [4]:

i = ci

Pi(1)

here ci is concentration of the gas in the liquid phase, and Pi ishe partial pressure of the gas. Since propane is not able to form �omplexes with silver ions, the only absorption is due to physicalolubility and it can be described by the Henry’s law in reactiveedia too.To successfully describe the absorption process of propylene

ith physical and chemical effects, an equilibrium model waseveloped. Fig. 1 shows a schematic depiction for the distributionf propylene between a gas phase and a reactive liquid phase. Theiquid phase contains the reactive metal that is capable of form-ng � complexes with propylene in BMImBF4 as absorption media.n �-complexation systems, various complexes of different sto-chiometry can exist simultaneously. However, in most systemsnly complexes with a 1:1 stoichiometry (silver–olefin) are formedn significant amounts, and are referred to as the primary com-lex species; its reversible formation in a reactive phase proceeds

g+ + C3H6 ↔ Ag+(C3H6) primary complexation (2)

Under certain conditions (high silver loading, high propy-ene partial pressure, and low temperature), complexes with

A. Ortiz et al. / Separation and Purificatio

dstp[wfpw

A

toov

K

K

stt

Ebc

[

[

bc

[

r

[

aetCmtl

Fig. 1. Schematic depiction of propylene solubility in BMImBF4 with AgBF4.

ifferent stoichiometries can exist: the secondary complex (1:2ilver–olefin) and the tertiary complex (2:1). The secondary and ter-iary complexes species are formed as consecutive reactions of therimary complex with another propylene or silver ion, respectively15,16]. After analysis of the experimental results, no indicationsere found that significant amounts of this tertiary complex are

ormed, in contrast to secondary complexes. The secondary com-lex is formed from a consecutive reaction of the primary complexith another propylene molecule:

g+(C3H6) + C3H6 ↔ Ag+(C3H6)2 secondary complexation (3)

This model is derived under the assumption that the silveretrafluoroborate salt has no influence on the physical solubilityf propylene, which therefore can be determined in the absencef metal salt [16]. In the present study this assumption has beenerified, as shown in Section 4.2.

The equilibrium constants are defined as

+

Eq,1 = [Ag (C3H6)]

[Ag+][C3H6], (4a)

Eq,2 = [Ag+(C3H6)2][Ag+(C3H6)][C3H6]

(4b)

ct

S

Fig. 2. Experime

n Technology 63 (2008) 311–318 313

For practical applications, the activities of the liquid phasepecies are assumed to be proportional to the concentration ofhe species, with the constant of proportionality (the nonidealities)aken up in the equilibrium constant.

The concentration of dissolved propylene in the liquid phase inqs. (4a) and (4b) is obtained from Eq. (1). The total propylene solu-ility is the sum of physically dissolved propylene and the chemicalomplexation with silver:

C3H6]T = [C3H6] + [Ag+(C3H6)] + 2[Ag+(C3H6)2] (5)

Combining Eq. (5) with Eqs. (1) and (4a) and (4b) gives

C3H6]T = HC3H6 PC3H6 (1 + KEq,1[Ag+]

+ 2KEq,1KEq,2(HC3H6 PC3H6 )[Ag+]) (6)

The concentration of free silver can be found by a total silveralance as the total concentration of the silver minus the silveroncentrations in the primary and secondary complexes.

Ag+] = [Ag+]T − ([Ag+(C3H6)] + [Ag+(C3H6)2]) (7)

Combining Eq. (7) with Eqs. (1) and (4a) and (4b) yields afterewriting:

Ag+] = [Ag+]T

(1 + KEq,1PC3H6 HC3H6 + KEq,1KEq,2(PC3H6 HC3H6 )2)(8)

Combining Eqs. (6) and (8) gives the total propylene solubility asfunction of the pressure and concentration of the silver metal. Thequilibrium constants have been determined by parameter estima-ion with the software for integrated process engineering, Aspenustom Modeler; calculated values of KEq were obtained by mini-izing the weighted square error between simulated values with

he model and experimental data. Selectivity of a solvent for propy-ene may be defined as the tendency for the ratio of C3H6 to C3H8

ontents to be larger in the liquid phase than in the gas phase. Thehermodynamic selectivity at equilibrium is expressed as [17]:

electivity = ([C3H6]T /[C3H8]T )(PC3H6 /PC3H8 )

(9)

ntal setup.

314 A. Ortiz et al. / Separation and Purification Technology 63 (2008) 311–318

ropan

3

3

aw1owbuittsr

3

tvaibuucvbvlc

lT(tcnato

3

t

Abtuststewttptatcto

ctatoo

it

n

u

eta

�

4

Fig. 3. Equilibrium isotherms of propylene (a) and p

. Experimental methods

.1. Materials

Propylene and propane gases were purchased from Praxair withpurity of 99.5 and 99.5%, respectively. The RTIL used in this studyas 1-butyl-3-methylimidazolium tetrafluoroborate (CAS number:

74501-65-6) from Solvent Innovation, with a minimum purityf 99%. The purity of an ionic liquid is a very important issuehen measuring physical properties. BMImBF4 is water misci-

le; although water stable, this compound is hygroscopic, so theptake of water vapor is an important issue. The ionic liquid used

n the present work contained <0.1% water and residual halide con-ent less than 500 ppm. Reactive media were prepared using silveretrafluoroborate AgBF4 of 99% purity (Apollo Scientific Ltd.) dis-olved in RTIL at room temperature. All the chemicals were used aseceived.

.2. Apparatus and measurements

The apparatus used in the measurements of the gases solubili-ies is shown in Fig. 2. The equipment consisted of a stainless steelessel (upper chamber) used to transfer a known amount of gas intostainless steel autoclave (lower chamber) for gas–liquid contact-

ng. The volume of the lower chamber (Vlow) (152 ml) was measuredy filling the chamber with water and then measuring the vol-me of the water using a graduated cylinder. The volume of thepper chamber (Vupp) (66 ml) was measured by the following pro-ess: the upper and lower chambers were evacuated, and then thealve separating the two chambers was closed. The upper cham-er was pressurized and allowed to temperature equilibrate. Theolume of the upper chamber, using the ideal gas law, was calcu-ated from the pressure drop observed when the valve was openedommunicating both chambers.

The temperature of the system is controlled by fluid circu-ation in conjunction with computer controlled electric heating.he temperature was monitored with a type K thermocoupleAssy) placed inside the lower chamber and automatically main-ained within 0.1 K of the setpoint. The pressure in the upperhamber is measured by a digital pressure gauge (Omega Engi-eering DPG1000B-30V100G with an accuracy of ±0.017 bar),nd the autoclave pressure was measured with a pressureransducer (Hirsschmann12B-GDM0-25 bar with an accuracyf ±0.001 bar).

.3. Procedure

In order to begin the absorption experiments, 15 ml of the reac-ive silver–BMImBF4 mixture (0.25 M), were added to the autoclave.

mtlm

e (b) in BMImBF4: (�) experimental; (—) calculated.

ir was removed from both the upper and the lower chambersy a vacuum pump (<2 mbar). The valve was closed, separatinghe two chambers. Then, a pure gas sample was injected into thepper chamber to a desired pressure (Pupp), which was the con-idered initial pressure. After the gas was charged, the stirrer wasurn on (1500 rpm) and the silver–RTIL mixture was constantlytirred throughout the experiment. The equipment was allowedo stand undisturbed until the temperature was equilibrated. Thequilibrium process began adding the solute gas to the autoclavehen the valve was opened. The system pressure changed with

ime due to the system approaching equilibrium, the pressure andemperature readings were recorded in 1-min intervals until theressure was consistent for 10 consecutive intervals, equilibrationime varied between 30 and 90 min. The final pressure difference,fter accounting for the increase in volume, was used to determinehe number of moles of gas absorbed. Once the final equilibriumonditions were recorded, the stirrer was stopped, and the solu-ions were either regenerated or kept for a subsequent absorptionr desorption experiment.

Gas solubility was determined from the observed pressurehanges on the basis of the following assumptions and experimen-al observations: (1) the absorption liquid was non-volatile at thebsorption conditions. (2) The amount of absorbed gas in the ini-ial liquid was negligible. (3) No volume change of the liquid wasbserved during the dissolution of gas. (4) Ideal gas behaviour isbserved [4,18].

With these assumptions, a mass balance was used to relate thenitial moles of gas in the upper chamber with moles of gas in theotal volume at any given time during the experiment.

absorbed = ninitial,Vupp − ntime,Vupp+Vlow(10)

Experiments in BMImBF4 without silver salt were carried outsing the same procedure.

Two replicates of each experiment were performed and thexperimental error was determined. The weighted standard devia-ion, defined by Eq. (11), was calculated leading to values of �w = 2%nd concluding that the experiments were replicable.

w =

√∑ni=1((Cexp − Cexp ")/Cexp)2

N − 1(11)

. Results and discussion

The solubilities of propylene and propane in the reactiveedium, BMImBF4 with silver salt [AgBF4] = 0.25 M, were inves-

igated as functions of temperature and pressure. Typically in theiterature, equilibrium data of olefin absorption are presented as

oles of gas absorbed per liter of solution. The temperature range

A. Ortiz et al. / Separation and Purification Technology 63 (2008) 311–318 315

Table 1Henry’s constants (mol/(1 bar)) from 278 to 318 K, and parameter values for Eq. (12)

Temperature (K) Propylene Propane

278 0.145 0.059288 0.103 0.045298 0.069 0.028308 0.055 0.022318 0.039 0.011

Gas Ho (mol/(l bar)) �Hsol (kJ/mol)

Propylene 4.28E−06 −24.1P

Ht

wo

4

aasw

wiautf

tTprwmHa

H

tHg

g

Fi

Fr

4

hsdlldie

aosst

F3satalptp

oFor the [0.25] M silver solution, the selectivity reached the high-est value at the lowest gas partial pressure because the physicalsolubility effects were dominated by the chemical complexationeffects. At partial pressures over 3 bar, the selectivity starts to level

ropane 1.84E−07 −29.5

enry’s law parameter (Ho) and the standard solvation enthalpy for gas-to-liquidransfer (�Hsol) of propylene and propane in BMImBF4.

as between 278 and 318 K for the absorption equilibrium databtained.

.1. Physical solubility in RTIL

To further examine the role of physical solubility in thebsorption equilibrium curves, silver-free BMImBF4 was used asbsorption media. Fig. 3 presents the experimental and calculatedolubility values of propylene and propane from 278 to 318 K in RTILithout silver salt.

Equilibrium curves for propylene and propane gases generatedith this silver-free ionic liquid showed that both gases to exhibit

deal Henry’s law behaviour (evident by their linear trend withppropriate R2 values and intersection with the origin). The sol-bility line for propane had a lower slope than propylene due tohe RTIL solution’s natural affinity for the olefin (intermolecularorces).

As expected, in both cases the gas solubility increased withhe system pressure and decrease with the system temperature.he experimental physical solubilities at different temperatures forropylene and propane in BMImBF4 are reported in Table 1. Theesults are given as Henry’s law constants. Those Henry’s constantsere found by fitting the data to a straight line using a least squaresethod and taking the slope. The temperature dependencies of theenry’s law constants and selectivity for gases were studied usingVan’t Hoff type equation [19],

(T) = Hoe−�Hsol/RT (12)

The H values of propylene and propane, when plotted againsthe reverse of the temperature (Fig. 4), showed a linear relationship.enry’s law constants (H ) and the standard solvation enthalpy for

o

as-to-liquid transfer (�Hsol) are listed in Table 1.For the silver-free ionic liquid, the ratio of the solubilities of the

ases was 2.48, value that was independent of pressure.

ig. 4. Propylene and propane Henry’s constant (H) as a function of the temperaturen BMImBF4.

ig. 5. Propylene (total, chemical and physical equilibrium) and propane equilib-ium isotherms with silver solution and silver-free BMImBF4 at 298 K.

.2. Solubility in reactive RTIL media

The absorption solution (BMImBF4 + [AgBF4] = 0.25 M) has muchigher capacity for propylene than propane at 298 K, as clearlyhown in Fig. 5. The trendline through the propane equilibriumata was fit with a y-intercept of zero, and the R2 value of the

ine was 0.99 with a slope of 0.0284 mol(l bar)−1. The propy-ene equilibrium data in the reactive media was definitely notescribed by Henry’s law; the observed absorption in the exper-

mental system was a combination of chemical effects and physicalffects.

Propane showed the same solubility in the silver-free ionic liquids with the AgBF4 solution; therefore, a salting out effect was notbserved. Because the presence of the salt did not decrease theolubility of propane, it would be correct to assume that the physicalolubility of the propylene in the silver solution was the same as inhe silver-free ionic liquid.

A comparison of the physical and chemical effects is shown inig. 5. Examining the total propylene absorption at pressures overbar revealed that the slope of the equilibrium data was nearly the

ame as the physical solubility. As a result, the calculated chemicalbsorption reached a plateau around 3 bar at a propylene concen-ration around 0.43 M in the reaction media. With the chemicalbsorption reaching an asymptotic limit, the silver solution is fullyoaded chemically, and higher pressures result only in increasedhysical absorption. Because the silver concentration was 0.25 M,his would lead to the assumption that silver forms various com-lexes of different stoichiometry simultaneously.

Selectivity defined by Eq. (9) has been plotted in Fig. 6 as a ratiof propylene/propane pure gases absorptions at the same pressure.

Fig. 6. Equilibrium selectivity at 298 K.

316 A. Ortiz et al. / Separation and Purification Technology 63 (2008) 311–318

Fa

oAssctmscftftvawacbtid

(osot

artrs

4R

tF(t

w(

pa

Fig. 8. Equilibrium isotherms of propylene in AgBF4 [0.25 M]–BMImBF4: (�) exper-imental; (—) calculated.

Table 2Enthalpies of complexation and equilibrium constants for primary and secondarycomplexation

Temperature (K) KEq,1 (l/mol)a KEq,2 (l/mol)b

278 337.0 18.2288 285.8 10.2298 245.1 5.9308 212.2 3.6

ws

�mtwmoiccawu−aTaa

ittapredominantly. However, with increasing pressure the secondarycomplex formation is forced and the concentration of the primarycomplex decreased as the silver concentration must keep constant.

Table 3Summary of equilibrium constants of olefins complexation in aqueous silver nitratemedia at 298 K

Reference KEq,1 (l/mol) [Ag+] (mol/l)

ig. 7. Comparative equilibrium isotherms for propane and propylene in BMImBF4

nd aqueous media at 298 K.

ut due to the fact that the silver sites were becoming saturated.trade-off existed for propylene absorption and operating pres-

ure such that higher pressures led to higher capacity but lowerelectivity and lower pressures led to higher selectivity and lowerapacity. Since the influence of the chemical effects was observedo be around 3 bar, this propylene pressure should be the maxi-

um operating partial pressure. The minimum operating pressurehould be around 1 bar in order not to compromise the absorptionapacity. Based on the stated operating range of partial pressuresrom 1 to 3 bar, the mixed gas propylene/propane equilibrium selec-ivity for optimum process design should range between 16 and 7.5rom Fig. 6. For industrial application with the real mixed stream,otal feed pressure will depend on the feed compositions and sil-er concentration. Higher silver salt concentrations will result inn increase on absorption capacity and selectivity values allowingork at higher pressures, but the regeneration stage would prob-

bly be more difficult. Desorption of propylene at high silver saltoncentrations becomes more difficult because of the more sta-le complex; moreover, the higher the concentration of silver inhe absorbing solution, the higher will be the density and viscos-ty of the mixture. This may present mass-transfer and pumpingifficulties.

Equilibrium isotherms for propane and propylene at 298 K inBMImBF4 + [AgBF4] = 0.25 M) are compared in Fig. 7 with our previ-us experimental data obtained with aqueous silver solutions. It ishown that propylene solubility in BMImBF4 is higher than in aque-us media, being silver concentration four times smaller, indicatingheir greater efficiency in the reactive separation process.

Complete propylene desorption was carried out at room temper-ture, 800 rpm stirrer speed and 20 mbar vacuum during 3 h. Theeproducibility of the uptake experiments indicated that a consis-ent regeneration was obtained and the silver–RTIL medium can beeused many times without any loss of absorption capability andolvent.

.3. Temperature effect and mathematical modelling in reactiveTIL media

The propylene isotherms obtained in this work in the silver reac-ive media exhibited a nonlinear trend with pressure as shown inig. 8. The equilibrium constants for the complexation reactionsKEq,1, KEq,2) are a function of temperature and can be described byhe Van’t Hoff equation [20].

d ln KEq

d(1/T)= −�Hr

R(13)

here T is the temperature (K), �Hr is the enthalpy of reactionkJ/mol), and R is the gas constant (kJ/mol K).

Eqs. (1), (6), (8), (9), (10), (12) and (13) constitute the pro-osed mathematical model for the description of the reactive gasbsorption process. This set of differential and algebraic equations

NBTW

318 185.5 2.2

a �Hr,1 = −11.0 (kJ/mol).b �Hr,2 = −38.7 (kJ/mol).

as solved simultaneously using the equation-oriented simulationoftware 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 esti-ated. Hence, the unknown parameters are calculated employing

he parameter estimation tool of the employed software togetherith experimental data series. Subsequently, the mathematicalodel is run using the values of the estimated parameters. The

btained values for �Hr,1, �Hr,2, KEq,1 f(T) and KEq,2 f(T) are shownn Table 2. The enthalpies of complexation of the propylene–silveromplexes are −11.0 and −38.7 kJ/mol for primary and secondaryomplexes, respectively. This indicates that complex formation isn exothermic process and consequently the equilibrium constantsill decrease with increasing temperature [8,15]. The obtained val-es for the enthalpies of complexation are close to the values of23.5 and −50 kJ/mol that have been reported for propylene inqueous silver nitrate by Cho et al. [18] and Chilukuri et al. [21].he equilibrium constants determined from the experimental datare compared to the values reported in the literature for propylenend ethylene in aqueous media as shown in Table 3.

A numerical solution of the mathematical model allows analyz-ng the distribution or speciation of the complexes. Fig. 9 showshe evolution of the primary and secondary complexes concentra-ion dependence on pressure at 298 K, where it can be observedt pressures below 2.5 bar that the primary complex is formed

Propylene Ethylene

ymeijer et al. [22] – 99 1–3.5aker [23] – 116 0–0.7rueblood et al. [24] 96.8 98 0.5–1asik and Tsang [25] 79 85 <0.1

A. Ortiz et al. / Separation and Purificatio

Fig. 9. Primary and secondary complexes distribution dependence on pressure at298 K.

Fu

vFasPtrttd

5

ilacsrcpBai

la

ousfpo

wtamc

tbeCtr

pwibg(w

A

pCs

R

[

ig. 10. Parity graph of propylene for simulated and experimental equilibrium val-es in AgBF4 [0.25 M]–BMImBF4.

A comparison was made between simulated and experimentalalues in order to validate the proposed mathematical model (seeig. 8). The weighted standard deviation values of the reactive gasbsorption process were calculated as �w = 3%. The accuracy of theimulation results is presented in the parity graph shown in Fig. 10.redicted concentration values, Csim, are shown versus experimen-al data, Cexp, under the conditions studied in this work. All theesults of Csim fall within the interval Cexp ± 10% Cexp, proving thathe proposed model offers a satisfactory description of the reac-ive absorption equilibrium. The values of the weighted standardeviation reinforce this conclusion.

. Conclusions

This study has shown that the use of RTILs as reaction median propylene/propane separation improves its efficiency for propy-ene absorption because the silver cation becomes chemically morective in forming silver–olefin complexes. In relation to the pro-ess selectivity (expressed as separation factor), the studied processhow a similar behaviour to other separation processes based oneactive absorption: the equilibrium showed that most part of thehemical complexation reaction took place at propylene partial

ressures below 3 bar (for 0.25 M silver concentration and 298 K).eyond this point, the equilibrium curve showed the same slopes predicted by physical solubility. As consequence, the selectiv-ty of propylene to propane absorption was found to be higher at

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ower pressures creating a trade-off with capacity for the optimumbsorption pressure.

Complete regeneration of the reaction media can be carriedut at room temperature, 800 rpm stirrer speed and 20 mbar vac-um for 3 h. Experimental results show that regeneration of theilver solution by stripping at vacuum conditions would be entirelyeasible. Nonetheless, further studies at different conditions of tem-erature and/or pressure are indispensable in order to decide theptimal conditions to be used for the regeneration process.

In all cases, gas solubilities increased with pressure and decreaseith temperature of the system. The enthalpies of complexation for

he primary and secondary propylene–silver complexes are −11.0nd −38.7 kJ/mol, respectively; this indicates that the complex for-ation is an exothermic process and consequently the equilibrium

onstants will decrease with increasing temperature.A simple mathematical model has been developed to describe

he total propylene absorption in the reaction media under study,ased on the formation of organometallic complexes with differ-nt stoichiometry. All the results of Csim fall within the intervalexp ± 10% Cexp, proving that the proposed model offers a satisfac-ory description. The low values of the weighted standard deviationeinforce this conclusion.

This work shows the viability of the separation of propylene-ropane mixtures using AgBF4 dissolved in a suitable RTIL. Furtherork using membrane modules as contactors would be of great

nterest. Membrane processes are expected to have the potential toe less energy intensive and less voluminous. A hybrid membraneas absorption process, combining the advantages of absorptionhigh selectivity) and membranes (modularity, small size) is aorthwhile alternative.

cknowledgements

The Ministry of Education and Science (Spain) financially sup-orted this research under projects CTQ2005-02583/PPQ andTM2006-00317. Alfredo Ortiz also thanks MEC for the FPU fellow-hip.

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