cyclohexane mixtures by pervaporation using PEBA membranes

cyclohexane mixtures by pervaporation using PEBA membranes

Desalination 219 (2008) 14–25 Separation of benzene/cyclohexane mixtures by pervaporation using PEBA membranes Ahmet E. Yildirima, Nilufer Durmaz Hil...

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Desalination 219 (2008) 14–25

Separation of benzene/cyclohexane mixtures by pervaporation using PEBA membranes Ahmet E. Yildirima, Nilufer Durmaz Hilmioglub*, Sema Tulbentcic b

a Sem Ltd. Kadikoy, Istanbul, Turkey Chemical Engineering Department, Engineering Faculty, Kocaeli University, 41040, Kocaeli, Turkey Tel. +90 (262) 335-1148/1239; Fax: +90 (262) 335-5241; email: [email protected] c Chemical Engineering Department, Istanbul Technical University, Maslak, 34469, Istanbul, Turkey

Received 2 September 2005; Accepted 28 February 2007

Abstract Separation of the benzene/cyclohexane mixture is industrially significant. The aim of the study is to disrupt the azeotropic behaviour of the benzene/cyclohexane mixtures by pervaporation using Pebax membranes. To overcome the difficulty of the separation benzene/cyclohexane mixtures by conventional techniques, pervaporation should be used. For this purpose, sorption and pervaporation of benzene/cyclohexane mixtures was studied by using different grades of Pebax (poly(ether-block-amide)) polymer which has soft segments as ether and a hard segment as an amide. It was examined that increasing hardness of the polymeric membrane resulted decreasing pervaporation flux and increasing pervaporation selectivity. Swelling degrees of membranes are decreased from softer to harder grades. Benzene/cyclohexane mixture composition and temperature effects on sorption and pervaporation characteristics are determined at 30, 40, 50°C. Increasing concentration of benzene resulted increasing flux and decreasing selectivity. Fluxes were increased and selectivity decreased with increasing temperature. Keywords: Pervaporation; Sorption; Benzene/cyclohexane; Pebax (poly(ether-block-amide))

1. Introduction Pervaporation is becoming recognized as an energy efficient alternative to distillation and other separation methods of liquid mixtures, especially in cases in which the traditional *Corresponding author.

separation techniques are not efficient, such as the separation of azeotropic mixtures, close boiling mixtures, isomeric components and recovery or removal of trace organic substances from aqueous solutions. Pervaporation differs from all other membrane processes, except membrane distillation, because of the change of the permeate from liquid to vapor phase. The permeation

0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.02.031

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process consist of three steps, called the sorption– diffusion model: selective sorption of components into the membrane, molecular diffusion of the components through the membrane, and evaporation of the components at the membrane surface on the permeate side. Thus, the partial pressures on the permeate side are lower than the vapor pressures of the components at the same side [1]. In distillation, relative volatilities are indications of the ease of separation. In pervaporation, by contrast, the flux and selectivity of permeating components depend on the membrane sorption and membrane diffusion properties of the components. Sorption properties can be controlled by choosing the membrane material. Since diffusion through the membrane often is rate limiting, transport rate can be controlled by properly choosing the membrane thickness. The permselectivity in pervaporation for liquid mixtures is based on difference in the solubility and diffusivity of permeants in polymer membranes [2,3]. Separation of aromatic hydrocarbon from aliphatics is a most important target in the membrane separation process of organic-organic mixtures. Specifically, the mixture of benzene and cyclohexane is difficult to separate because they have close boiling points and molecular sizes. Therefore, the solubility difference should be a key to obtain higher selectivity [4]. Separation of the benzene/cyclohexane mixture is industrially significant. Cyclohexane is produced by addition of hydrogen to benzene under a Ni or Pt catalyst. The removal of the unreacted, benzene, from the reaction solution is a very important process. However, it is difficult to remove directly one component from benzene/ cyclohexane mixtures by distillation, which are close-boiling point chemicals. Therefore, many distillation processes such as azeotropic distillation process and an extractive distillation process to separate the benzene/cyclohexane mixtures are developed. Most of these process are carried out by adding a third component to the mixture. It is

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required to remove the third component from the distillate or distillation residue. Consequently, it is required to develop a more economical process to separate this mixture directly [5,6]. On this concept pervaporation experiments were carried out with many research groups [1,3,7–32]. A literature survey of the membranes for separating benzene/cyclohexane mixtures is represented in Table 1 [3,5,33–60]. A brief explanation of the cited works is represented in Table 1 also. In this study, the polymer used as a membrane material is a segment-elastomeric polyetheramide-block-copolymer, known to yield high pervaporation enrichment of aromatic hydrocarbons and high organic permeation rates. The polymer consist of two groups, polyether as soft segment and polyamide as hard segment. Three grades of polymer, commercially named Pebax, were used. Grades are named Pebax 2533, 3533, 4033, where the first two digits represent shore D hardness of the polymer and the last two digits are a code determining the density of the polymer — 1.01 g/cm3 each. The harder the grade, the greater the proportion of polyamide in the copolymer [27–32]. In this study the effect of hardness of polymer, composition of feed mixture and temperature dependence were examined on sorption and pervaporation characteristic.

2. Experimental 2.1. Materials The membrane materials Pebax 2533, 3533, 4033 were supplied from Atochem, France. The types of the Pebax used are commercially available. The first two digits represent the shore D hardness and last two digits represent the density codes (1.01 g/cm3). From 25 to 40 the shore D hardness of the polymer is increasing, which means increasing amide content. The chemicals

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Table 1 Separation of benzene/cyclohexane mixtures by pervaporation Membrane material

Separation factor

Feed conc. (wt%) benzene)

Reference

PMMA-EGDM PEMA-EGDM PMMA-PEMA-EGDM

5.98–3.99 9.35–6.7 21.5–6.71

10–90

[3] (“Permeation and separation of benzene/cyclohexane mixtures through crosslinked poly(alkylmetacrylate) membranes”)

LCP 80 LCP 100

3–2.5 5–1.7

10–60 40–70

[5] (“Permeation and separation of benzene/ cyclohexane mixtures through liquid crystalline polymer membranes”)

Poly ethylene

2.5

80–20

[33] (“Separation of liquid mixtures by using polymer membranes. 1. Permeation of binary organic liquid mixtures through polyethylene”)

Poly vinylidene fluoride

5.45–6.2

53

[34] (“Separation of aromatics and naphthens by permeation through modified vinylidene fluoride films”) [35] (“Solvent membrane separation of benzene and cyclohexane”)

Poly phosphonate alloys and cellulose acetate

8–12

20–80

[36] (“Polymeric alloys of polyphosphonates and acetyl cellulose I.Sorption and diffusion of benzene and cyclohexane”)

Poly (phenyleneoxidehydroquinone dimethyl phoshonate)

13.3

50

[37] (“Organic liquid mixtures separation by permselective polymer membranes I..Selection and characteristics of dense isotropic membranes employed in the pervaporation process”)

Poly(bis(2,2,2-trifluoroethoxy) phosphazane)

5–12

10–90

[38] (“Pervaporation of organic solvents by poly(bis(2,2,2-trifluoroethoxy) phosphazane) membrane”)

Poly phenylequinoxaline

5–4

20–80

[39] (“Pervaporation of organic liquid mixtures through polyphenylquinoxaline membranes”)

Cellulose acetate alloys and poly (bromophenyleneoxide dimethylphosphonate ester)

9.7

51

[40] (“Separation of liquid benzene/cyclohexane mixtures by pertraction and pervaporation”)

High density poly ethylene plasma grafted by poly methyl acrylate monomer

6

50

[41] (“Plasma graft filling polymerization: Preparation of a new type of pervaporation membrane for organic liquid mixtures”)

Methyl acrylate polymers

2

5

[42] (“Photo modification of a poly(acrylonitrileco-butadiene-co-styrene containing diaryltetrazolyl groups”)

10–90

[43] (“Separation of benzene/cyclohexane by pervaporation through poly(vinylalcohol/ poly(allylamime) blend membranes”)

Challate poly vinyl alcohol/poly- 26.4–2.5 ally amine blend

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Table 1, continued Membrane material

Separation factor

Feed conc. (wt%) benzene)

Reference

Poly ether imide based membranes

3.8–9.1

60

[44] (“Pervaporation of organic liquid mixtures through membranes of polyimides containing methyl-substituted phenylenediamine moieties”)

Benzyl chitosan

11–5

50

[45] (“Effect of permeation temperature on permeation and separation of a benzene/ cyclohexane mixture through liquid-crystalline membranes”)

Poly vinyl alcohol-AgNO3

60

80

[46] (“Faciliated transport separation of benzene and cyclohexane with poly (vinylalcohol-Ag NO3 membranes”)

Poly (AN-c-VAc) Poly (AN-c-MMA) Poly (AN-c-St)

36 28 22

50

[47] (“Development of new synthetic membranes for separation of benzene/cyclohexane mixtures by pervaporation: A solubility parameter approach”)

Poly [bis(phenoxy)phosphazenel] 3

50

[48] (“Pervaporation of aromatic/nonaromatic hydrocarbon mixtures through crosslinked membranes of polyimide with pendant phosphonate ester groups”)

PVC/EVA

7.8

50

[49] (“Compatibility of PVC/EVA blends and pervaporation of their blend membranes for benzene/cyclohexane mixtures”)

Poly amide

4

50

[50] (“Pervaporation of benzene/cyclohexane mixtures through aromatic polyamide membranes”

Carbon graphite-nylon 6 composite

20

45

[51] (“Specialty polymeric membranes:12. Pervaporation of benzene/ cyclohexane mixtures through carbon graphite-nylon6 composite membranes”)

Poly imide grafted by irradiation 10

50

[52] (“Preparation of polyimide composite membranes grafted by electron beam irradiation”)

Poly vinyl alcohol

1.7

20

[53] (“Modification of poly(vinyl alcohol) membranes for pervaporative separation of benzene/ cyclohexane mixtures”)

Poly urethane

9.97

53

[54] (“Polyurethane membrane preparation with and without hydroxypropyl-β-cylodextrin and their pervaporation characteristics”)

Poly acrylo nitrile

3.4

77.3

[55] (“Effects of preparation condition of photoinduced graft filling polymerized membranes on pervaporation performance”)

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Table 1, continued Membrane material

Separation factor

Feed conc. (wt%) benzene)

Reference

Grafted poly methyl acrylate

5

50

[56] (“Preparation of hollow fiber membranes by plasma graft filling polymerization for organicliqıid separation”)

6 FDA-based copolyimide (hexa- 6 fluoro-isopropylidene dianhyrid)

50

[57] (“Separation of aromatics/aliphatics with crosslinked 6FDA-based copolyimides”)

Commercial PERVAP 2200 (composite PVA on PAN)

64

25

[58] (“Studies on separation characteristics and pseudo-equilibrium relationship in pervaporation of benzene/ cyclohexane mixtures through composite PVA membranes on PAN supports”)

Poly(vinyl chloride)-graftpoly(buthyl methacrylate)

5.5

10

[59] (“Speciality polymeric membranes. 9. Separation of benzene/ cyclohexane mixtures through poly(vinyl chloride)-graft-poly(buthyl methacrylate”)

Cellulose acetate

65

50

[60] (“Pervaporative separation of organic mixtures using dinitrophenyl group containing cellulose acetate membrane”)

benzene, cyclohexane and formic acid were supplied from Merck. All the materials were used without further purification.

with filter paper to remove adherent solvent and weighed. The swelling ratio was calculated using Eq. (1):

2.2. Preparation of membranes

DS = (Ws!Wd)/Wd

The membranes were prepared by using the solution casting method. Polymer was dissolved in formic acid to give 12 wt.% solution at elevated temperature. Solution was casted on a rimmed glass plate at a thickness of 1 mm. Films were dried in nitrogen atmosphere for 1 week. The membranes were completely dried under reduced pressure for a few days. The final thickness of the membranes was about 150 µm. 2.3. Degree of swelling Initially, a piece of the dry membrane was weighed and immersed and left to swollen into the solution for 6 hours. The swollen membrane was taken out of the solution, wiped out quickly

(1)

where Wd is the weight of the dry membrane and Ws is the weight of the membrane swollen in the solution. Sorption measurements were derived for single components and known compositions of benzene/cyclohexane mixtures, repeated all types of membranes. 2.4. Pervaporation experiments A schematic representation of the pervaporation apparatus is presented in Fig. 1. The pervaporation cell was assembled from two half cells; all the system was constructed by stainless steel. The effective surface area of the membrane is 28.26 cm2. The membrane was left to swollen in the feed mixture before pervaporation experi-

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Fig. 1. Experimental pervaporation set-up.

ments. The membranes was placed on a porous material as a support material. The pressure on the permeate side was measured 0.3 mbar with a vacuum gauge. Permeated vapors were completely condensed in a trap placed in a dewar flask cooled by liquid nitrogen. Two serial traps were established for continuous work. The feed mixture recirculated by a pump and composition determined per hour. If required, feed composition was adjusted by adding benzene. The permeate and feed compositions were analyzed by gas chromatography. Performance of a membrane in pervaporation is characterized by flux (J) and selectivity (αij). The selectivity is calculated by using Eq. (2): αij = (Yi/Yj)/(Xi/Xj)

(2)

where i and j represent the components of benzene and cyclohexane respectively. Y and X represent the weight fractions in permeate and feed mixture, respectively.

The flux was obtained taking the ratio of the weight of the permeate collected to effective area of the membrane and time of collection of the sample in kg/m2h. Finally fluxes were normalized to the thickness of 150 µm to the proper membrane thickness. 3. Results and discussion 3.1. Sorption results Sorption experiments were carried out for single components and mixtures of benzene/ cyclohexane at 30, 40 and 50°C for all grades of Pebax. Figs. 2 and 3 are the plots of swelling degrees versus temperature, representing the pure component sorption characteristics of the Pebax membranes. Fig. 4 gives the plots of swelling degrees versus composition of the mixture at 30°C, representing the concentration dependence of the sorption characteristics of polymers. Fig. 5 is the temperature dependence of the swelling degrees for 20% benzene as volume.

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Fig. 2. Temperature dependence of sorption in benzene for Pebax membranes.

Fig. 3. Temperature dependence of sorption in cyclohexane for Pebax membranes.

Fig. 4. Concentration effect on sorption in the mixtures of benzene/cyclohexane at 30°C for Pebax membranes.

Fig. 5. Temperature dependence of sorption in the mixture containing 20 volume % benzene for Pebax membranes.

According to sorption results of the membranes, pure liquid sorption for all grades of Pebax, benzene sorption is higher than cyclohexane. All grades of Pebax had a higher swelling degree for benzene than cyclohexane. For all concentration ranges while the benzene content in the mixture increases, swelling degrees increase. These are the sign of higher affinity to benzene than cyclohexane. Increasing hardness of the Pebax resulted in a decrease of the swelling behavior of the polymer. As previously mentioned, a harder grade has a higher portion of polyamide, glassy and hard segment, in the polymer. This can be explained as a polyether segment has a higher affinity against

benzene. While a portion of polyamide increased, the penetration of the components decreased because the glassy polyamide segments hardly sorb the components. The temperature effect is determined as an increase of swelling degrees, while sorption properties of polymers are increasing as usual. Sorption characteristics of Pebax 2533 compared to Pebax 3533, 4033 show a much higher degree of swelling. This can be explained depending on the dissolution tendency of the polymer at 50°C. Dissolution of the polymer was not determined for the mixtures. This could be as a result of high interactions between benzene and cyclohexane.

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Fig. 6. Concentration effect on fluxes at 30°C for Pebax membranes.

Fig. 7. Concentration effect on selectivity at 30°C for Pebax membranes.

Fig. 8. Feed mixture-permeate, equilibrium diagram for pervaporation at 30°C for Pebax membranes.

Fig. 9. Concentration effect on total fluxes at 30, 40 and 50°C for Pebax 3533.

3.2. Pervaporation results Pervaporation experiments were carried out to the mixtures of benzene/cyclohexane ranging from 10 to 90 wt.% benzene for all grades of Pebax. Temperature effects were determined for Pebax 3533 and 4033 at 30, 40 and 50°C. Because of the dissolution tendency of Pebax 2533, pervaporation experiments were carried out only at 30°C. Figs. 6 and 7 are the plots of total flux and selectivity versus concentration of the mixture in weight % benzene, respectively at 30°C. Fig. 8 is the plot of benzene concentration in permeate versus benzene concentration in feed at 30°C, which means the equilibrium curve for per-

vaporation. Figs. 9 and 10 represent the effect of concentration on total fluxes at 30, 40 and 50°C for Pebax 3533 and 4033, respectively. Figs. 11 and 12 represent the effect of feed composition on selectivity at 30, 40 and 50°C for Pebax 3533 and 4033, respectively. Fig. 13 is the comparison of temperature dependence of selectivity for Pebax 2533 and Pebax 4033 for 40 wt.% benzene. Fig. 14 is the comparison of temperature dependence of total fluxes for Pebax 2533 and Pebax 4033 for 40 wt.% benzene. All grades of Pebax are selective to benzene. High fluxes were obtained because of the high swelling behavior of the membranes; as a result of this, the selectivity of the membranes is not high. Hardness of the polymer, related to the

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Fig. 10. Concentration effect on total fluxes at 30, 40 and 50 °C for Pebax 4033.

Fig. 11. Effect of feed composition on selectivity at 30, 40 and 50°C for Pebax 3533.

Fig. 12. Effect of feed composition on selectivity at 30, 40 and 50°C for Pebax 4033.

Fig. 13. Temperature dependence of selectivity for Pebax 2533 and Pebax 4033 for 40 wt.% benzene.

portion of polyamide, is effecting the separation characteristics. From Pebax 2533 to 3533, from softer to harder grade, while selectivity is increasing, fluxes are decreasing. Due to the close molecular sizes of benzene and cyclohexane, it was determined that the sorption selectivity was more effective than diffusion selectivity. Diffusion selectivity increases with the increase of polymer hardness. This is the result of the increasing polyamide ratio in the copolymer which has a crystalline structure. Due to the crystalline structure of the polyamide, sorption of the components through the membrane decreases and the difference between diffusion rates of the

components becomes more important. As a result of smaller molecular size of benzene compared to cyclohexane, the diffusion rate of benzene is higher than cyclohexane. Sorption characteristics of the membranes are increasing with the increasing benzene content in the mixture. Feed concentration effect is determined with increasing fluxes and decreasing selectivity. Also benzene was carrying more cyclohexane together while permeating through the membrane. Selectivities were decreasing with the increasing temperature because the interactions between polymer and components of the mixture became

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Fig. 14. Temperature dependence of total fluxes for Pebax 2533 and Pebax 4033, for 40 wt.% benzene.

weaker. Fluxes were increasing while the operation temperature increases. This was a result of increasing diffusion rates of the components with increasing temperature.

4. Conclusions It can be deduced that benzene/cyclohexane mixtures can be separated by pervaporation using Pebax membranes. Pebax 2533 can be used especially for this purpose. Pervaporation can be used to disrupt the azeotropic point only. The pervaporation technique cannot be replaced by the distillation technique entirely for the studied mixture. From the experiments it was observed that solubility and pervaporation separation characteristics of the membranes can be controlled by choosing the polymer hardness. In addition, a strong influence of the feed mixture composition and operating temperature was also observed. The tested membranes are benzene selective. Sorption and pervaporation results have the same tendency. As the polyamide ratio in the membrane increases, hardness of the polymer increases and swelling decreases. Because the membrane acts as a permselective medium, preferential sorption

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increases. Since polyamide segments are glassy, mobility decreases. The diffusion of penetrants through the membranes also decreases. Lower diffusion and lower selective sorption causes lower flux and higher selectivity. Because flux can be increased by changing some parameters such as temperature, membrane thickness, membrane modules, more selective membrane should be selected. It may be Pebax 2533 which is the softest one. Sorption may increase with higher benzene concentration and this causes higher permeability; on the other hand, because of decline of the selective sorption, pervaporation selectivity decreases also. As a conclusion, it can be deduced that higher swelling causes higher pervaporation flux and lower pervaporation selectivity. The temperature of the feed liquid showed a favorable effect on the separation performance of the systems studied. The flux increased with temperature, while the selectivity decreased. In the literature, there are no data similar to this study. Different membrane materials were used in the literature studies and lower selectivity values were obtained (except for commercial membranes). In this study lower selectivity was also obtained. In contrast, high permeation fluxes for the mixtures by Pebax membranes were obtained. Because the difficulty to separate benzene/ cyclohexane mixtures by conventional techniques, pervaporation should be used in combination with distillation as an economical hybrid system for breaking the azeotropy.

5. Symbols DS Ws Wd Xi

— Degree of swelling, % — Weight of the swollen membrane (g) — Weight of the dry membrane (g) — Weight fraction of benzene in feed

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Xj Yi Yj αij

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— Weight fraction of cyclohexane in feed — Weight fraction of benzene in permeate — Weight fraction of cyclohexane in permeate — Selectivity

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