Raúl Zarca, Alfredo Ortiz*, Daniel Gorri and Inmaculada Ortiz

Facilitated Transport of Propylene ThroughComposite Polymer-Ionic Liquid Membranes.Mass Transfer Analysis

DOI 10.1515/cppm-2015-0072Received December 22, 2015; accepted December 22, 2015

Abstract: Separation of light gaseous olefins from paraffin’sof the refinery process off-gasses has been traditionallyperformed by cryogenic distillation, which is a highly capi-tal and energy intensive operation. This handicap createsan incentive for the investigation of alternative olefin/par-affin separation technologies. In this regard, membranetechnology supposes a potential solution for process inten-sification. Previous works of our research group reportedthe use of facilitated transport composite membranes inte-grating the use of PVDF-HFPpolymer, BMImBF4 ionic liquidand AgBF4 silver salt. In this type of membranes, the silvercations react selectively and reversibly with the olefin,allowing the separation via mobile and fixed carriermechanisms. Ionic liquidswere selected asmembrane addi-tives because in addition to their negligible vapor pressurethat avoids solvent losses by evaporation, they providestability to the metallic cation dissolved inside, and modifythe structure improving the facilitated transport. This tech-nology offers a commercial attractive separation alternativethanks to their modular form of operation, high values ofselectivity and permeability and low operational costs. Inthe present work, propane/propylene permeation experi-ments involving the use ionic liquids and different mem-brane compositions were performed. Moreover, basing onthe transport and equilibrium parameters previouslyobtained, a mathematical model description of the systemwill be proposed fitting the remaining parameters andallowing the design and optimization of the propane/pro-pylene separation process at industrial levels.

Keywords: facilitated transport, propylene, ionic liquids,silver, membrane composite

1 Introduction

Light olefins such as ethylene and propylene are impor-tant petrochemical building blocks which are further pro-cessed to yield a wide range of final products such ascosmetics, textile products, paints, tools or plastics forinstance. Light olefins are usually obtained, as a mixturewith paraffins, by steam cracking processes, fluidizedcatalytic cracking or alkane dehydrogenation.

The separation of these streams is a key issuebecause it is one of the most difficult and also the mostcostly separation process in the petrochemical industry.Traditional separation processes like low-temperaturedistillation, require voluminous equipment operating athigh pressures or low temperatures and very large refluxratios due to the very small difference in the relativevolatilities between olefins and their corresponding par-affins [1]. In recognition of these costs alternative energy-saving separation processes are required.

In this sense, membrane technology presents a greatpotential for energy and capital saving and therefore theuse of membranes has been the focal point of research ofmany authors. Criteria for selecting the most suitablemembrane for a given application are complex; nonethe-less, durability, mechanical and thermal stability at theoperating conditions, productivity and separation effi-ciency and costs are important stipulations that must beconsidered [2]. Among these requirements selectivity andpermeation rate are clearly the most basic, while therelative importance of the rest of them varies with theapplication. Therefore a wide variety of different mem-brane alternatives can be considered [3]. The use of poly-meric, inorganic and supported liquid membranes forolefin/ paraffin separation has been studied rather exten-sively, however these membranes may suffer severe plas-ticization effect, low stability, poor mechanical resistanceand expensive and complex preparation methods andtherefore their performance has not met the requirementsfor commercial applications [4, 5].

Recently our research group proposed the use ofnovel polymer-ionic liquid-Ag+ composite as facilitatedtransport membranes to carry out the separation of

*Corresponding author: Alfredo Ortiz, Department of Chemical &Biomolecular Engineering, University of Cantabria, Av. Los Castross/n., 39005 Santander, Spain, E-mail: alfredo.ortizsainz@unican.esRaúl Zarca, Daniel Gorri, Inmaculada Ortiz, Department of Chemical& Biomolecular Engineering, University of Cantabria, Av. Los Castross/n., 39005 Santander, Spain

Chem. Prod. Process Model. 2016; 11(1): 77–81

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propane/propylene mixtures [6]. The separation perfor-mance is mostly associated to the ability of the olefins toreact selective and reversibly with silver cations Ag+ , byπ-complexation mechanism. Consequently silver ions actas carriers for the transport of olefins, thus facilitatingtheir selective permeation across the membrane [7, 8].Ionic liquids (IL) were selected as membrane additivesbecause in addition to their negligible vapor pressurethat avoids solvent losses by evaporation, they offermore affinity for the olefinic compounds compared tosaturated hydrocarbons and at the same time they pro-vide stability to the metallic cation dissolved inside act-ing as a medium for facilitated transport with mobilecarrier [9, 10]. Based on previous results the ionic liquidused in this work has been 1-butyl 3-methylimidazoliumtetrafluoroborate (BMImBF4) since it provided the bestresults in terms of separation selectivity and propylenesolubility [11–13]. On the other hand, the polymer used inthis work is poly(vinylidene fluoride-co-hexafluoropropy-lene) (PVDF-HFP) due to its well-known thermal, chemi-cal and mechanical properties. Furthermore it is partiallymiscible with BMImBF4 and compatible with hydrocar-bons which avoid plasticization effects. Once the mem-brane is fabricated the ionic liquid remains integratedwithin the polymer matrix, thereby increasing the stabi-lity of the membrane even at high transmembrane pres-sures. As the composite membrane is composed by apolymeric matrix and a liquid phase entrapped insidethe polymeric matrix, the silver salt added to the mem-brane is distributed between these two phases. Thereforeboth facilitated transport mechanisms (fixed carrier andmobile carrier) take place, leading to high permeabilitiescombined with high separation selectivities. Figure 1shows and schematic diagram of the transport mechan-isms of propane and propylene across polymer/ionicliquid composite membrane. The PVDF-HFP/BMImBF4–Ag+ facilitated transport membranes reported in a pre-vious work provided very promising results when tested

with 50/50%v/v C3H8/C3H6 mixtures obtaining C3H6 per-meabilities up to 6,630 barrer and C3H8/C3H6 selectivitiesover 700 combined with good long term stability [6].

In this work permeation experiments using constantvolume time-lag method were performed in order toobtain diffusivity and solubility parameters of propyleneand propane in PVDF-HFP polymer matrix and ionicliquid/polymer composite membranes. These resultscombined with facilitated transport experimental dataobtained from our previous works [6, 11] will allow thedevelopment of a mathematical model able to describethe behavior of the facilitated transport mechanisms ofpropylene through the composite membrane and esti-mate the unknown facilitated transport parameters.

2 Experimental section

2.1 Membrane preparation

The PVDF-HFP/BMImBF4 composite membranes were pre-pared by the solvent casting technique. First of all thepolymer is dissolved in 10 mL of THF at 40 °C for 8 h in aclosed vial to avoid the evaporation of the solvent. Once thepolymer has been dissolved, it is mixed with the ionicliquid and stirred for 5 min. Finally the membrane precur-sor mixture is placed in a Petri dish and the solvent isevaporated at 25 °C and 300 mbar under dark conditionsfor 24 h. The thickness of the membranes prepared in thiswork depends on the membrane composition, but in allcases the prepared membranes presented a range thicknessbetween 40–100 μm. Permeabilities have been calculatedtaking into account the real thickness of each membranewhich was measured using a digital micrometer MitutoyoDigmatic Series 369 (accuracy ± 0.001mm).

2.2 Time-lag experiments

Figure 2 shows the experimental setup used for conduct-ing permeation experiments using the time-lag techniqueat constant-volume and variable pressure. This apparatusconsists of two chambers separate by the membrane(47mm diameter). At time zero the gas of interest isintroduced into the upper chamber. The feed pressure ismaintained constant at the upper chamber while thepressure increase in the lower chamber, which occursdue to the passage of gas through the membrane, isrecorded. The appreciable increase in pressure in the

Figure 1: Proposed facilitated transport composite membranestructure.

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lower chamber occurs only after a period of time knownas time-lag (Ө). After the time-lag a diffusion processbegins in quasi-steady state until the pressure in bothchambers is equalized. The mathematical expression thatdescribes the increased pressure in the lower chambercan be obtained by applying Fick’s second law in thelimits of the membrane. By integrating Fick’s equationwith the boundary conditions by Laplace transforms,operating and neglecting terms, we reach the expressionfor the pressure in the permeate chamber versus time forthe quasi-steady flux:

PL tð Þ=AR � T � S � D � P0

V � L t −L2

6D

� �(1)

The above equation is a straight line, from which one canextract the term that subtracts the time, known as “time-lag” (Ө) and the slope:

θ=L2

6D(2)

After re-arranging terms, diffusivity, solubility and per-meability parameters can be obtained as:

D=L2

6θ(3)

S=V � L � Slopeð ÞA � D � R � T � P0

(4)

P =D � S (5)

Where, L is the membrane thickness, Ө is the “time-lag”,V is the permeate side volume, A is the permeation area,R is the gas constant, T is temperature and P0 is the feedpressure.

3 Results and discussion

The results of permeation experiments are discussed.Permeability of propylene, propane and carbon dioxide,as a reference gas, at different temperatures in PVDF-HFPand PVDF-HFP-BMImB4 (80/20%wt.) membranes areexperimentally obtained.

Figure 3 shows the propylene permeation time lagexperiment at 289 K in a PVDF-HFP membrane. Thecharacteristic initial stage corresponds to the time-lag(Ө) and the straight line corresponds to the quasi-steadystate permeation stage. We can observe the long durationrequired for the experiment to reach quasi-steady statepermeation flux and the slight increase in pressure in thepermeate chamber, which gives an idea of the low per-meability values expected in this polymer. In all cases thesupply pressure described here is 3.5 bar. The permeatechamber has a volume of 16.6 cm3.

Table 1 summarizes the experimental values of diffusivity,solubility and permeability of CO2, propane and propylene inPVDF-HFP membranes at different temperatures.

Permeability of CO2 in the dense polymer membraneis an order of magnitude higher than that of propyleneand propane. This effect is due to a significant increase indiffusivity because the smaller size of the CO2 molecule.

Furthermore, permeability values for propylene andpropane are very similar. So it is confirmed that this typeof dense membranes, in which the mass transfer takesplace only by the solution-diffusion mechanism, cannotbe used to carry out the separation of propane/propylene

Figure 2: Time-lag apparatus scheme.

Figure 3: Permeate side pressure increase for propylene in PVDF-HFPmembrane at 298 K.

R. Zarca et al.: Facilitated Transport of Propylene 79

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mixtures. For the three pure gases is observed that thediffusion coefficients are inversely proportional to theirkinetic diameters, as shown in Table 2.

Results in Table 1 show a significant increase inpermeability when temperature increases. The tempera-ture significantly increases diffusivity whereas the solu-bility decreases slightly. Activation energies for the threegases using Arrhenius equation were calculated in orderto know the influence of temperature on the permeationprocess. Activation energies are shown in Table 3, fromwhich it can be observed that the permeation of propaneis more sensitive to temperature than propylene andbeing the lowest for the CO2.

In order to study the influence of the addition of an ionicliquid to the polymer matrix permeation experiments in an

80% PVDF-HFP and 20% of BMImBF4 composite mem-brane were conducted. The addition of the ionic liquid(BMImBF4) increases the diffusivity between two andthree orders of magnitude and thus the permeability isenhanced between one and two orders given that thesolubility slightly decreases, as can be observed in Table 4.

This increase in diffusivity is a result of the presence ofthe ionic liquid that is entrapped in the polymer matrix,lowering the rigidity of the polymer chains and increas-ing the free volume, which facilitates the diffusion of thepermeant species.

4 Conclusions

Experimental diffusivity, solubility and permeabilityvalues of propane and propylene in PVDF-HFP andPVDF-HFP-BMImBF4 (80/20 wt%) membranes at differ-ent temperatures have been successfully obtained usingthe time-lag technique. These parameters along withother previously obtained and C3H6/C3H8 separationexperimental data will allow the development of a math-ematical model able to describe the behavior of the facili-tated transport mechanisms of propylene through thecomposite membrane considering all three mechanismsand design, sizing and optimization of industrial units.

Funding: This research was supported by the SpanishMinistry under the projects CTQ2012-31639 (MINECO,SPAIN-FEDER 2007–2013) and (CTM2013-44081-R).

References

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Table 1: Diffusivity, solubility and permeability of CO2, propane andpropylene PVDF-HFP 298, 308 and 318 K.

Temperature

(K)

D

( × cm/s)

S

( × mol/bar/cm)

P

(barrer)

CO . . .

. . .

. . .

Propylene . . .

. . .

. . .

Propane . . .

. . .

. . .

Table 2: Diffusivity of CO2, propane and propylene in PVDF-HFP at298 K and Lennard-Jones diameter [14].

D ( × cm/s) Lennard-Jones diameter (Å)

CO . .Propylene . .Propane . .

Table 3: Activation energies of CO2, propylene and propane inPVDF-HFP.

Gas Ea (kJ/mol)

CO

Propylene

Propane

Table 4: Diffusivity, solubility and permeability of propylene andpropane in HFP-PVDF and PVDF-HFP/BMImBF4 at 298 K.

Gas D

( × cm/s)

S

( × mol/bar/cm)

P

(barrer)

PVDF-HFP Propylene . . .

Propane . . .

PVDF-HFP/

BMImBF(/)

Propylene . . .

Propane . . .

80 R. Zarca et al.: Facilitated Transport of Propylene

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