K

Aa

b

a

ARRA

KPSBRK

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ct

1d

Separation and Purification Technology 73 (2010) 106–113

Contents lists available at ScienceDirect

Separation and Purification Technology

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

inetics of reactive absorption of propylene in RTIL-Ag+ media

lfredo Ortiza, Lara María Galánb, Daniel Gorri a, André B. de Haanb,∗, Inmaculada Ortiza,∗∗

Advanced Separation Processes – Dep. Chemical Engineering & Inorganic Chemistry University of Cantabria, SpainProcess System Engineering Group- Dep. Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands

r t i c l e i n f o

rticle history:eceived 11 January 2010eceived in revised form 10 March 2010ccepted 11 March 2010

eywords:ropyleneilverMImBF4

eactive absorptioninetics

a b s t r a c t

In the present work the kinetics of absorption of propylene in Ag+-BMImBF4 medium have been analysed,discriminating the kinetic regime and determining the expression and parameters of the rate of reactiveabsorption as a function of the concentration of both reagents. A stirred cell reactor was used to obtain thekinetic information by loading pure absorption gas and recording the total pressure decrease at differentsilver salt concentrations [AgBF4 = 0–1 M] at 288–308 K. The experimental values of the enhancementfactors (EA) due to the presence of the chemical reaction indicate that for concentrations equal to orhigher than 0.25 M of Ag+ in BMImBF4 it is possible to assume instantaneous chemical reaction betweenpropylene and silver ions. Therefore, the rate of absorption in this case is governed by the rate at whichdissolved C3H6 and Ag+ diffuse to the reaction plane from the G–L interface and liquid bulk, respectively.

Physico-chemical parameters necessary to predict the absorption rate have been experimentally deter-

mined in this work. The liquid mass transfer coefficient (kL) was obtained in a non-reactive BMImBF4

medium as a function of stirring speed and temperature, showing a significant increase from 1.61 × 10−6

to 17.9 × 10−6 m s−1 when the temperature increased from 288 to 308 K at 500 rpm. Experimental vis-cosity measurements for the reactive system Ag+-BMImBF4 are reported in order to predict the diffusioncoefficient of propylene in the absorptive media. Diffusivity of the silver ions in BMImBF4 has been

oamp

determined by the chronAgBF4 = [0.05–0.25 M].

. Introduction

The separation of olefins and paraffins is of primary impor-ance in the chemical industry. Gas streams with extremely highurity (>99.9%) olefin-content are required in the production ofolymers and many other chemicals as propylene oxide. Tradi-ional systems, like low-temperature distillation, are expensive,nergy-consuming and voluminous process equipment due tohe similarity in boiling points of olefins and their correspond-ng paraffin [1–3]. Therefore, alternative, less-expensive separation

ethods are required. In a previous work, Ortiz et al. [4] pro-osed as an attractive alternative to the classical separationf gaseous mixtures propane/propylene the reactive absorptionombining the formation of reversible complexes olefin-silvern room temperature ionic liquids (RTILs) as efficient reaction

edia.The separation is based on the ability of silver ions to reversibly

omplexate olefins [2]. The advantage of chemical complexation ishat the bonds formed are stronger than those formed by van der

∗ Corresponding author. Tel.: +31 40 247 5259; fax: +31 40 246 3966.∗∗ Corresponding author. Tel.: +34 942201585; fax: +34 942201591.

E-mail addresses: a.b.dehaan@tue.nl (A.B. de Haan), ortizi@unican.es (I. Ortiz).

383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2010.03.008

erometry technique, showing a concentration dependency in the range of

© 2010 Elsevier B.V. All rights reserved.

Waals forces alone, so it is possible to achieve high selectivity andhigh capacity for the component to be bounded. At the same time,the bonds are still weak enough to be broken by using simple engi-neering operations such as raising the temperature or decreasingthe pressure [5].

Solvents with high olefin-capacity, selectivity and resistanceto contaminants and process conditions are still needed. Roomtemperature ionic liquids (RTILs) can be used as reactive absorp-tion solvents for olefin/paraffin separations because of their ionicand organic character together with its renowned and remarkableproperties. Among other properties, RTILs are non-volatile and canbe considered as designer solvents [6,7]. In this work, 1-butyl-3-methylimidazolium tetrafluoroborate, BMImBF4, has been selectedbecause of its absorption selectivity for propylene over propaneand its ability to dissolve a suitable silver salt (silver tetrafluorob-orate, AgBF4), allowing to absorb propylene due to the reversiblecomplexation of silver ions with unsaturated olefinic double bonds[4,8].

The knowledge of the physico-chemical characteristics of the

system, kinetic and equilibrium data of the complexation reac-tion between propylene and silver are required to design industrialseparation processes.

Several works have analysed the reaction of complexationbetween olefins and transition metals in aqueous and organic

A. Ortiz et al. / Separation and Purification Technology 73 (2010) 106–113 107

Nomenclature

[I] molar concentration component I (mol l−1)C concentration (mol l−1)D diffusivity (m2 s−1)EA enhancement factorEA∞ infinite enhancement factorHa Hatta numberH Henry’s law constants (mol l−1 bar−1)I current density (A)J molar flux (mol m−2 s−1)k1,1 kinetic constant of reaction 1, 1 order

(m3 mol−1 s−1)KEq complexation equilibrium constant (l mol−1)KG overall mass transfer coefficient

(mol m−2 s−1 bar−1)kG gas phase mass transfer coefficient

(mol m−2 s−1 bar−1)kL liquid mass transfer coefficient (m s−1)kLa volumetric mass transfer coefficient (s−1)P pressure (bar)R universal gas constant (J mol−1 K−1)T temperature (K)t time (s)V volume (l)�H activation energy (kJ mol−1)a specific surface area (m2 m−3)

Greek lettersı thickness of the reaction/diffusion film (m)� group of parameters in Eq. (7)� viscosity (cP)

Superscripts/subscriptAg+ silverC3H6 propyleneg gas phasei interfacialIL ionic liquido initial

staolrB

mtcaRrfis

2

R

T totalr reaction

olvents [7,9,10] but no information of any kinetic study onhe reactive absorption of propylene in Ag+-RTIL media is avail-ble in the literature. Galán et al. [6] reported a kinetic studyf the reaction between CO2 and the NH2-functionalized ioniciquid finding that the reaction took place in an intermediateegime and was limited by the diffusion of the reagents inMImBF4.

In a previous work, Ortiz et al. [4] developed a mathematicalodel to describe the propylene absorption equilibrium in the reac-

ion media under study, based on the formation of organometallicomplex species with different stoichiometry. This work aims atnalysing the kinetics of the reaction between C3H6 and Ag+ intoTIL media, and at determining the expression of the absorptionate with the characteristic kinetic parameters, mass transfer coef-cient of C3H6 in the ionic liquid, and diffusivity coefficients ofilver and propylene in the reactive media.

. Theoretical background

For the absorption of a gas solute, propylene, in a liquid phase,TIL in the present work, contained in a stirred tank the mass bal-

Fig. 1. Schematic of the film model used to describe mass transfer at the gas liquidinterface.

ance takes the form

q × CILC3H6in − q × CIL

C3H6out + KG · a × VIL × P ×(yC3H6out − CIL

C3H6 × 1HC3H6 · P

)− VIL × (−rC3H6) = VIL × ∂CIL

C3H6T

∂t(1)

In Eq. (1), the first two terms refer to convective flow into andout of the reactor, the third one to mass transfer between gas andliquid phases and the fourth term on the left side describes changesdue to the chemical reaction; the accumulation term is accountedfor in the right side of Eq. (1).

When the gas phase is assumed to be perfectly mixed, the propy-lene mass balance is given by

Fin × yC3H6in − Fout × yC3H6out − KG · a × VIL × P ×(yC3H6out − CIL

C3H6 × 1HC3H6 · P

)= VG

R · T× ∂(P · yC3H6out)

∂t(2)

Thus, Eqs. (1) and (2) represent the isothermal behaviour of atwo-phase stirred tank reactor where there is a chemical reactionbetween a solute coming from gas phase and a reagent alreadycontained in the liquid phase. For batch or semi-batch reactors theterms containing the input and output flow rate, q, are eliminated[11].

2.1. Mass transfer kinetics

2.1.1. Physical absorptionFor physical absorption of a pure gas into a RTIL medium, the film

model is a simplified model widely used to describe mass transferat the gas–liquid interface (Fig. 1). Mass transfer occurs within thefilm which separates the interface from the bulk of the liquid bymolecular diffusion alone. The liquid mass transfer coefficient (kL)represents the amount of solute transferred through the liquid filmper unit time, unit area, and unit of driving force in terms of liq-uid concentration [12]. The mass transfer coefficient in the liquidphase depends on the physical properties of the ionic liquid andthe hydrodynamics of the reactor (stirrer speed and geometry).

By considering ideal gas behaviour and including Henry’s law to

describe the gas–liquid equilibrium, for the batch stirred cell usedin this study Eqs. (1) and (2) transform into:

VILdCIL

C3H6dt

= kLa(CiC3H6

IL − CILC3H6)VIL; t = 0 → CIL

C3H6 = 0 (3)

108 A. Ortiz et al. / Separation and Purificatio

Table 1Kinetic regime as a function of Ha, EA and EA∞ for irreversible reactions of order (1,1).

Regime Ha EA

Very slow reactions <0.02 –

Slow reactions 0.02–0.3 –

b

C

ep

fi

k

i

J

tlftba

J

2

s

A

A

ado

ta

order of magnitude and the concentration of propylene in the bulk

EA∞ > 5Ha; EA ≈ Ha 1st order reactionFast reactions >2 5EA∞ > Ha > (EA/5) 2nd order reaction

Instantaneous >2 EA = EA∞; Ha > 5EA∞

Vg

R · T· dPC3H6

dt= −kLa(Ci

C3H6IL − CIL

C3H6)VIL; t = 0 → PC3H6 = PoC3H6

(4)

eing gas and liquid concentrations at equilibrium,

iC3H6

IL = HC3H6PC3H6 (5)

By simultaneous solution of the differential Eqs. (3) and (4) andxpressing the concentration in the gas phase as function of theartial pressure of C3H6, the following equation is obtained:

PC3H6 = PoC3H6(

Vg(HC3H6 · R · T) + VILe−((HC3H6·R·T)·kLa+kLa)(t/(HC3H6·R·T))

Vg(HC3H6 · R · T) + VIL

)(6)

After linearization of Eq. (6) the value of the mass transfer coef-cient of the liquid film (kL) can be determined:

La · t = �

1 + �· ln

(Po

C3H6(� + 1) · PC3H6 − � · Po

C3H6

)

with � = Vg

VIL · R · T · HC3H6(7)

The interfacial absorption flux for a system without reaction, ast is shown in Fig. 1, is given by

ILC3H6a = kLa(Ci

C3H6IL − Cb

C3H6IL

) (8)

For pure gas absorption, there is no mass transfer resistance inhe gas side. The gas liquid interface is considered to be at equi-ibrium so that the C3H6 concentration at the interface is obtainedrom the Henry constant. For maximum driving force, that is, whenhe concentration of C3H6 in the bulk of the ionic liquid is negligi-le, the flux of C3H6 absorbed into the ionic liquids is determineds:

ILC3H6a = kLaCi

C3H6IL = kLa · PC3H6 · HC3H6 (9)

.1.2. Reactive absorptionThe chemical reactions between C3H6 and the Ag+ cations dis-

olved in ionic liquid have been described as [4]

g+ + C3H6IL ↔ Ag+(C3H6)IL (10a)

g+(C3H6)IL + C3H6IL ↔ Ag+(C3H6)2

IL (10b)

Propylene dissolves from the gas phase into the liquid phaseccording to the Henry’s law, and subsequently it reacts with the

issolved silver ions to form the primary (reaction (10a)) and sec-ndary complexes (reaction (10b)).

In order to determine the influence of the chemical reactions onhe absorption kinetics, the enhancement factor (EA), that is defineds the ratio of the absorption flux of propylene in presence of Ag+

n Technology 73 (2010) 106–113

to the flux of pure physical absorption, can be used; fluxes must bebased on the same driving force [13].

EA = JC3H6 with reactionJC3H6 without reaction

(11)

For batch systems in the gas phase, the absorption rate of C3H6can be calculated from the change in the C3H6 concentration.

JAg+·ILC3H6 · a = dCIL

C3H6dt

= Vg

VIL · R · T

(−dPC3H6

dt

)(12)

Using the C3H6 mass balance given in Eq. (12), the enhance-ment factor for propylene absorption in presence of dissolved Ag+

is described by the expression

EA = JAg+·ILC3H6 a

JILC3H6a

= JAg+·ILC3H6 a

kLaCiC3H6

IL

= 1kLa

Vg

VIL · HC3H6 · PC3H6 · R · T

(dPC3H6

dt

)(13)

The enhancement factor for reversible reactions has been stud-ied previously by several authors. Analytical solutions proposedby Olander [14], and Secor and Beutler [15] for the asymptoticenhancement factor according to the film model have been adaptedin order to make more realistic with the surface renewal theorygiven by Danckwetrs [16], and Chang et al. [17].

EA∞ = 1 +(

DC3H6

DAg+

)0.5 KeqCILAg+T

(KeqCiC3H6

IL + 1)(14)

Danckwerts [16] suggested that reversible reactions of any kindand where reacting species had almost equal diffusivities couldprobably be represented approximately by the solution for irre-versible reactions with the parameter EA properly determined forthe particular situation. Thus, the asymptotic solution is given bythe following equation:

EA∞ =(

1 + DAg+CAg+

DC3H6CiC3H6

IL

)(DC3H6

DAg+

)0.5

(15)

The maximum conversion in the film and the maximum masstransport through the film are related through the Hatta number(Ha). For irreversible reactions, the dimensionless Hatta number isdefined as [13,18]

Ha2 =(2/(n + 1))km,n · (Ci

C3H6IL

)n−1

· CmAg+ · DC3H6

k2L

(16)

where kn,m is the kinetic constant of the chemical reaction, CiC3H6

ILis

the concentration of propylene at the interface, CmAg+ is the concen-

tration of silver ions in the liquid bulk phase, DC3H6 is the diffusioncoefficient of propylene in the liquid phase, and kL is the liquid sidemass transfer coefficient.

In this study, the Hatta number has been calculated using theapproximated solution of the mass balances that describes the con-centration of the reactants at the interface, proposed by DeCourseyfor absorption with irreversible second order (first-order in eachreactant) reaction [19]. The expression is more accurate when thediffusivities of the reactants in the liquid phase are of the same

of the liquid is zero.

EA = − Ha2

2 · (EA∞ − 1)+√

Ha4

4 · (EA∞ − 1)2+ EA∞ · Ha2

(EA∞ − 1)+ 1 (17)

ification Technology 73 (2010) 106–113 109

te

3

3

asnapSw

3

sIaot

2(wcpvtm

hGer2

tdc(ccwttaCcscoup[f

3

t

A. Ortiz et al. / Separation and Pur

Table 1 reports the discrimination criteria to determine the reac-ion regime for irreversible reactions of second order, first order toach reagent [18,20–22].

. Experimental

.1. Materials

Propylene and propane gases were purchased from Praxair withpurity of 99.5% and 99.5%, respectively. The RTIL used in this

tudy was 1-butyl-3-methylimidazolium tetrafluoroborate (CASumber: 174501-65-6) from Iolitec, with a minimum purity of 99%nd residual halide content less than 500 ppm. Reactive media wererepared using silver tetrafluoroborate AgBF4 of 99% purity (Apollocientific Ltd) dissolved in RTIL at room temperature. All chemicalsere used as received.

.2. Apparatus and measurements

The kinetic experiments were carried out in a stainless steeltirred cell reactor purchased from Büchi, model Ecoclave 075 TypeI of 1L. The temperature in the reactor is regulated (±0.1 K) usingn external water bath (Julabo MW) connected to the steel jacketf the reactor. The reactor is equipped with sensors to measureemperature (±0.1 K) and pressure (±0.001 bar).

The pressure, temperature and stirring speed are read out everys and the measurements are recorded digitally with a memo graph

Visual Data Manager, Endress&Hauser). The gas and liquid phasesere agitated by a turbine and a propeller. A line attached to the

over plate of the reactor was connected to an external vacuumump (Vacuubrand CVC 2). This pump was used to achieve theacuum pressure value (<2 mbar) needed during the preparation ofhe ionic liquid, and for regeneration of the Ag+-BMImBF4 reactive

edium after the C3H6 absorption.The dynamic viscosity oscillatory rheological measurements

ave been conducted using a Haake RS 300 rheometer (HaakembH, Karlsruhe, Germany) equipped with parallel plate geom-try (both 35 mm diameter and serrated). The temperature in theeactor was regulated (±0.1 K) using an external water bath at 288,98 and 308 K.

The chronoamperometric technique was adopted to determinehe values of diffusion coefficients based on electrochemical metaleposition. Chronoamperometric transient measurements werearried out on a VMP-3 potentiostat running ECLAB softwareBioLogic Science Instruments). A three-electrode electrochemicalonfiguration was employed in BMImBF4 media; that is, a glassyarbon disk working electrode, a Pt counter electrode, and an Agire as reference electrode. In order to compare, all potentials in

his paper were referred to the Ag/AgCl reference electrode. Prioro each experiment, the glassy carbon electrode was polished withlumina. The effective working electrode area was (0.071 cm2).hronoamperometry was performed at −0.5 V to ensure diffusionontrol of the silver reduction. The maximum concentration ofilver ions in the ionic liquid used was 0.25 M because at higher con-entrations the contribution of migration phenomena can enhancer suppress electric current at the microelectrodes. Diffusivity val-es of silver ions were calculated from the current transients by alot of I (A) vs. 1/sqrt (t) (s) in accordance with the Cottrell equation23]; Determinations were undertaken in a jacketed cell, protectedrom light, at temperatures of 288, 298 and 308 K.

.3. Procedure

The reactor was operated in batch mode with respect to bothhe gas and liquid phases having a flat and horizontal gas–liquid

Fig. 2. Decrease of the C3H6 pressure during the kinetics experiments at 298 K and500 rpm for different silver ion concentration in BMImBF4.

interface (4.88 × 10−3 m2). The measurements were carried out bythe decreasing pressure technique.

In order to start the kinetic experiments, 450 ml of the reac-tive Ag+-BMImBF4 mixture (0 1 M) were added to the autoclave.The reactor was closed and air was removed by a vacuum pump(<2 mbar). The equipment was allowed to stand undisturbed untilthe temperature was equilibrated (±0.1 K). The valve of the vac-uum line was closed and the C3H6 was supplied to the reactoruntil the pressure reached a selected value. The inlet gas valve wasclosed, then the stirrer was turned on and the absorption started.The C3H6 was introduced in the reactor in less than 4 s. The decreasein pressure was monitored and the data recorded every 2 s. Theexperiments were continued until the pressure reached a constantvalue (change < 0.1 bar for a period of 2 h). Once the final condi-tions were reached, the stirrer was stopped, and the solutions wereeither regenerated or kept for a subsequent absorption or desorp-tion experiment.

Experiments in BMImBF4 without silver salt were carried outusing the same procedure. Two replicates of each experimentwere performed and the experimental error was determined. Theweighted standard deviation of experimental data was calculatedleading to values of �w = 1.5% and concluding that the experimentswere replicable.

4. Results and discussion

This chapter collects kinetic data of the reactive absorption ofpropylene in Ag+-BMImBF4 together with the discrimination of thekinetic regime and determination of the absorption rate expression.For this purpose the values of the enhancement factors were calcu-lated from kinetic data obtained at different operational conditionsby using the integrated expression of Eq. (13).

Fig. 2 shows the evolution of propylene pressure with time asa function of the silver salt concentration from batch experimentsat 500 rpm and 298 K. A strong effect of silver salt concentrationis observed on the rate of disappearance of propylene from the gasphase. Fitting of kinetic data to the integrated expression of Eq. (13)leads to the enhancement factor as follows:

ln(PtC3H6) = ln(Po

C3H6) − EAkLa · R · T · VIL · HC3H6

Vg· t (18)

In order to apply Eq. (18) the value of liquid mass transfercoefficients kLa should be known. Finally, the experimental values

of enhancement factor together with the theoretical parameters,Hatta number (Eq. (17)) and infinite enhancement factor (Eqs. (14)and (15)), and according to the discrimination criteria established inTable 1, allow discrimination of the kinetic regime and thus providethe expression of the rate of absorption.

110 A. Ortiz et al. / Separation and Purification Technology 73 (2010) 106–113

4

ttsttTtokf

1vwv

dsiaobc

sdBifttp

TM

Table 3Experimental enhancement factor obtained for different silver ion concentrationand temperature in BMImBF4 medium at 500 rpm.

T (K) [AgBF4] (mol l−1) CC3H6 (×102 mol l−1) EA

0.1 11.3 1.81288 0.25 8.5 4.32

0.1 5.7 2.450.25 4.7 6.64

298 0.5 5.7 13.8

Fig. 3. Determination of volumetric mass transfer coefficient (kLa).

.1. Liquid mass transfer coefficient kL of propylene

To examine the role of the liquid mass transfer in the reac-ive absorption, silver-free RTIL was used as absorption medium. Inhis section, different non-reactive operational conditions, stirringpeed and temperature have been studied. The volumetric massransfer coefficient of the liquid phase (kLa) was determined fromhe straight slope of the regression of experimental data to Eq. (7).he right hand side of the equation was plotted versus time usinghe data from the experiments carried out for physical absorptionf C3H6 into BMimBF4 under the same reactor conditions as theinetic experiments were performed. Fig. 3 depicts a typical plotor determination of kLa at 308 K.

Knowing the volumetric interfacial area of the system0.85 m2 m−3, the mass transfer coefficient kL can be calculated. Thealue of the Henry coefficient HC3H6 of propylene in the ionic liquidas obtained in a previous work [4]. Table 2 reports the calculated

alues of kLa and kL.As expected, the mass transfer coefficient in the liquid phase

epends on the stirring speed and on temperature. Increasing thetirring speed improves the level of turbulence at the gas liquidnterface, resulting in a smaller effective boundary layer thickness,nd thus in a higher rate of gas–liquid mass transfer. The flatnessf the gas–liquid interface in the region of interest was confirmedy the linearity of data in a logarithmic plot of the mass transferoefficient versus the stirring speed.

It is well known that the viscosity of ionic liquids dependstrongly on temperature, and consequently on the moleculariffusivity (D) of the species dissolved in the ionic liquid. ForMImBF4, experimental measurements of the viscosity shows that

t decreases from 124 to 48.6 cP when the temperature increases

rom 288 to 308 K. Table 2 illustrates this dependency showinghat the mass transfer coefficient (kL) increased with an increase inemperature. Sharma et al. [24] reported similar effects of the tem-erature in the absorption of CO and H2 in BMImFP6 being much

able 2ass transfer parameters in the stirred cell reactor.

T (K) rpm KLa (×105 s−1) KL (×106 m s−1) HC3H6

(mol l−1 bar−1)

288 500 1.74 1.61 0.103

298400 4.29 3.96

0.069450 7.38 6.79500 13.1 12.09

308 500 19.4 17.9 0.055

1 4.8 38.3

0.1 3.8 3.05308 0.25 4.6 6.25

higher than in usual aqueous solutions. The very strong temper-ature effect on the liquid mass transfer coefficient was describedusing an Arrhenius type equation; in this work we have found anactivation energy value of �HkL = 89.4 kJ mol−l.

4.2. Kinetic regime

Discrimination of the reaction regime of the absorption of C3H6into the reactive solvent Ag+-BMImBF4 will be done by using thevalues of the experimental enhancement factor.

The calculated enhancement factors, Eq. (18) for different silverconcentrations in the range of (0.1–1 M) and temperatures between288 and 308 K of the reaction media (BMImBF4-Ag+) are collectedin Table 3. The experimental conditions, low initial concentrationof C3H6 at the interface, and silver concentrations between 0.1 and0.25 M are consistent with the assumption that the second chemi-cal reaction (Eq. (10b)) step is negligible as compared with the firststep (Eq. (10a)), because at the working pressures (Po

C3H6 < 1 bar)the primary complex is formed predominantly [4]. Thus, the sys-tem can be simplified and described by gas absorption accompaniedby a single second order chemical reaction. The value of the calcu-lated enhancement factor was higher than 1 for all the experiments.The enhancement factor exhibits a significant increment when thesilver concentration increases in the ionic liquid.

To analyse the experimental data obtained in the present workfrom the absorption experiments of propylene in Ag+-BMImBF4,the analytical solutions derived by Danckwerts [16] have beenused to calculate EA∞ (Eqs. (14) and (15)) and the expression pro-posed by DeCoursey [19] for the Hatta number (Eq. (17)), beingboth parameters necessary to assess the kinetic regime of the reac-tive absorption. The equilibrium parameters KEq,1 of the reaction ofpropylene with Ag+ solubilized in ionic liquid at different tempera-tures, which are constant for the range of silver ion concentrationsstudied were determined in a previous work [4]. Table 4 reportsthe experimentally obtained values of the enhancement factor byfitting of the kinetic data in (Eq. (18)) and the calculated values ofthe infinite enhancement factors together with the Hatta numberin the kinetic experiments carried out with silver concentrations of0.1 and 0.25 M and temperatures between 288 and 308 K.

The relation between both asymptotic enhancement factors(EA∞) obtained from (Eqs. (14) and (15)) is observed in Table 3.Thus, the hypothesis proposed by Danckwerts [16] that reversiblereactions of any kind and where reacting species had almost equaldiffusivities could probably be represented approximately by thesolution for irreversible reactions, can be satisfactorily applied.

According to the discrimination criteria established in Table 1,the kinetic regime of the absorption of C3H6 in Ag+-BMImBF4 has

been identified from the values of Hatta number and enhance-ment factors. In all cases the obtained experimental enhancementfactors are higher than 2. The Hatta numbers calculated by theDeCoursey [19] solution are higher than the experimental valuesof enhancement factor for all the experiments, but only the val-

A. Ortiz et al. / Separation and Purification Technology 73 (2010) 106–113 111

Table 4Experimental EA and calculated EA∞ and Ha for Ag+ [0.1–0.25 M]–BMImBF4 system at different temperature at 500 rpm.

T (K) [AgBF4] (mol l−1) CC3H6 (×102 mol l−1) EA KEq,1 (l mol−1) EA∞a EA∞b Hab

0.1 11.3 1.81 1.9 1.9 5.3288 0.25 8.5 4.32 285.8 4.5 4.5 24.9

0.1 5.7 2.45 2.8 2.6 7.3298 0.25 4.7 6.64 245.1 7.0 6.6 5876

0.1 3.8 3.05 3.5 3.2 11308 0.25 4.6 6.25

a Danckwerts’s equation for irreversible finite rate reactions.b Danckwersts’s equation for reversible reactions.

Table 5Viscosity of Ag+-BMImBF4 solutions at different silver salt concentration andtemperature.

[Ag+] (M) Viscosity (cP)

288 K 298 K 308 K

– 124 72.3 48.60.1 127 75 51.2

uictTfit

tveacwitvcifitm

4

romi

TEl

The diffusivities of the silver ions in BMImBF4 (DAg+) have beendetermined experimentally as a function of the silver salt concen-tration and temperature. The obtained DAg+ values for differentsilver salt concentrations ([AgBF4] = 0.05–0.25 M) and in the range

0.25 161 105 72.40.5 265 156 1021 353 202 123

es corresponding to a silver ion concentration of 0.25 M reach thenstantaneous regime condition of Ha > 5EA∞. For the experimentsarried out with 0.1 M of silver salt concentration the condi-ion of fast second order reaction, 5EA∞ > Ha > (EA∞/5), is fulfilled.herefore, comparing the values of the experimental enhancementactors with the values of the infinite enhancement factors at 0.1 M,t is observed that they are very similar being closer to the values ofhe experiments carried out with a silver concentration of 0.25 M.

Thus, these results indicate that the reaction takes place in theransition from the fast to the instantaneous regime for low sil-er ion concentrations in the medium, whereas for concentrationsqual to or higher than 0.25 M of Ag+ in BMImBF4 it is possible tossume that the reaction is instantaneous. Instantaneous chemi-al reaction between C3H6 and Ag+-BMImBF4 found in the presentork for concentrations of silver ions equal to or higher than 0.25 M

s in good agreement with the hypothesis of many authors thathe complexation reaction between olefins and transition metals isery fast or even instantaneous; Nymeijer et al. [9] reported that theomplexation between ethylene and silver ions in aqueous medias almost instantaneous, and Reine et al. [10] showed an excellenttting of the data assuming instantaneous reversible models forhe reactive absorption of ethylene in the system CuCl/aniline/n-

ethyl pyrrolidone.

.3. Absorption rate and gas solute diffusivities

In the separation of gaseous mixtures propane/propylene by

eactive absorption, when an instantaneous chemical reactionccurs, the kinetics of the system is controlled by the kinetics ofass transfer processes. Thus, the rate of absorption of propylene

n Ag+-BMImBF4 medium is given by expression (19), including the

able 6xperimentally determined diffusion coefficients of silver ions in BMImBF4 ioniciquid at different silver salt concentration and temperature.

AgBF4 (M) DAg+ (×106 cm2 s−1)

T = 288 K T = 298 K T = 308 K

0.05 1.5 1.68 1.710.1 1.59 1.79 2.060.25 1.78 1.99 2.15

212.2 6.7 6.2 6085

overall mass transfer coefficient.

JAg+·ILC3H6 a = KGa · PC3H6

(1+

DAg+ · CILAg+

DC3H6 · PC3H6 · HC3H6

)(DC3H6

DAg+

)0.5

(19)

where the overall coefficient is obtained from the individual masstransfer coefficients by means of the equation

1KG

= 1kG

+ 1kL · HC3H6

(20)

Therefore in order to obtain the values of the absorption rate as afunction of the partial pressure of propylene in the bulk gas and Ag+

concentration in liquid phase the diffusivity coefficients of propy-lene and silver cations in the liquid phase must be known togetherwith the individual mass transfer coefficients of propylene in thegas phase and liquid film, which can be calculated using suitablecorrelations depending on the hydrodynamics and geometry of theseparation system [24–26].

The diffusivity coefficient of C3H6 in the ionic liquid BMImBF4has been estimated using literature correlations. For imidazoliumbased ionic liquids, Morgan and co-workers developed a correlationwhich expresses the dependency of the diffusivity of solubilized gaswith the liquid viscosity as [27]:

D12 = 2.66 × 10−3 1

�0.66±0.032 V1.04±0.08

1

(21)

where 1 and 2 refer to solute and solvent respectively. �2 is viscos-ity in cP, V1 is the molar volume of propylene at the normal boilingpoint (68.95 cm3 mol−1) and diffusivity (D12) is obtained in cm2 s−1.For the reactive system formed by BMImBF4-Ag+, the effect of thetemperature and silver salt concentration has been included in theabove correlation through their implicit dependence on the vis-cosity. Table 5 reports the experimental dynamic viscosities usedto estimate the diffusivity of propylene in the reactive ionic liquidmedia as a function of temperature for different compositions ofthe reaction media.

of temperatures between 288 and 308 K are summarized in Table 6.

Table 7Summary of literature values of the diffusion coefficient of silver ions in differentionic liquids at room temperature.

Reference Ionic liquid [Ag+](×102 M)

Anion DAg+

(×106 cm2 s−1)

He et al. [28] BmimPF6 2 BF4− 0.03

BmimBF4 2 BF4− 0.99

Katayama et al. [29] EmimBF4 2.1 BF4− 0.6

Tai et al. [30] EMI-CI-BF4 10 Cl− 0.264Bhatt et al. [31] DIMCARB 0.56 CO3

−2 0.21–0.3Rogers et al. [32] C4mpyrrNTf2 0.08 NO3

− 0.5This work BmimBF4 10 BF4

− 1.79

112 A. Ortiz et al. / Separation and Purification Technology 73 (2010) 106–113

Table 8Rate of absorption of C3H6 in Ag+-BMImBF4 [Ag+ = 0.25 M] at 500 rpm assuming instantaneous reaction.

T (K) CiC3H6 (×105 mol m−3) kLa (×105 s−1) DAg+ (×106 cm2 s−1) DC3H6 (×106 cm2 s−1) JC3H6

Ag+IIsa (×103 mol m−3 s−1)

Tiddua

iowtitdr6fcb[

ldq(att

5

tmbgamkuv2tobbFtudrptt1c

B

[

[

[

[

[

[

[[

[

[

[[

[

[

288 8.5 1.74 1.78298 4.7 13.13 1.99308 4.6 19.4 2.15

he results presented indicate that the diffusion coefficient of silverons in BMImBF4 appears to be almost linearly concentration-ependent in the range studied in this work. The temperatureependence of the diffusion coefficients of silver was describedsing an Arrhenius type equation [33]; in this work we have foundn activation energy value of �HAg+ = 6.8 kJ mol−1.

Table 7 compares the values of the diffusion coefficient of silverons in different ionic liquids at room temperature with the databtained in this work. The silver ion concentration in this workas changed in the range between 0.1 and 0.25 M, which is higher

han the silver concentration used to determine the silver diffusiv-ty coefficient in BMImBF4 in the work reported by He et al. [28]. Onhe other hand, Eisele et al. [34] studied the concentration depen-ence of the diffusion coefficient of ferrocene in BMImBF4. Theyeported that the diffusion coefficient increased from 1.3 × 10−7 to.8 × 10−7 cm2 s−1 when the concentration of ferrocene increasedrom 4.3 to 32 mM. Nymeijer et al. reported values of the diffusionoefficients of the ethylene and Ag+ in aqueous solutions at 298 Keing two orders of magnitude smaller than obtained in this work9].

Finally, Table 8 shows the calculated absorption rates of propy-ene in the ionic liquid, BMImBF4, containing [AgBF4 = 0.25 M] atifferent temperatures with a stirring speed of 500 rpm. As a conse-uence of the increase in temperature, the mass transfer coefficientkL) increases and thus the absorption rates are higher. The C3H6bsorption rate is strongly dependant on the agitation rate, due tohe dependence of the volumetric mass transfer coefficient (kLa) onhe variable.

. Conclusions

After having checked in a previous work [4], the preferen-ial absorption capacity of propylene vs. propane in the reactive

edium Ag+-BMImBF4, the determination of the reaction kineticsecomes a crucial point in the design of a separation process ofaseous mixtures. Thus, in this work the kinetics of the reactivebsorption of C3H6 in Ag+-BMImBF4 medium has been experi-entally and theoretically analysed. First, discrimination of the

inetic regime was carried out after having determined the val-es of the absorption enhancement factor in the range of operatingariables. The values of the enhancement factors were higher thanunder the experimental conditions and strongly dependent on

he silver concentration; it was concluded that for concentrationsf Ag+ in BMImBF4 equal to or higher than 0.25 M the reaction cane considered instantaneous, being the rate of absorption limitedy the diffusion of C3H6 and of Ag+ in the ionic liquid BMImBF4.inally, the values of the mass transport parameters needed forhe design of absorption contactors have been determined. The liq-id side mass transfer coefficient of propylene in BmimBF4 wasetermined experimentally in a stirred cell obtaining values in theange of 1.61 × 10−6 m s−1 ≤ kL ≤ 17.9 × 10−6 m s−1 when the tem-erature increased from 288 to 308 K at 500 rpm. The diffusivity ofhe silver ions in the ionic liquid BMImBF4 was determined using

he chronoamperometric technique, leading to values between.68 × 10−6 and 1.99 × 10−6 cm2 s−1 at 298 K, with an almost linearoncentration dependency in the range 0.05 M < [AgBF4] < 0.25 M.

Thus, for instantaneous reactions of propylene in Ag+ inMImBF4 liquid media, absorption equipment would be designed

[

[

1.14 0.661.51 4.311.93 5.97

for high mass transfer coefficients with short contact times to selec-tively absorb more olefin than paraffin, this work providing thenecessary tools for simulation of the separation process.

Acknowledgements

This research has been funded by the Spanish Ministryof Education and Science (projects CTQ2008-00690/PPQ andCTM2006-00317). Alfredo Ortiz thanks MEC for the FPU fellowship.Also, we gratefully acknowledge Dr. Lathe Jones from CIDETEC-IK4for the electrochemical measurements.

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