Application of microwave heating to pervaporation: A case study for separation of ethanol–water mixtures

Application of microwave heating to pervaporation: A case study for separation of ethanol–water mixtures

Chemical Engineering and Processing 81 (2014) 35–40 Contents lists available at ScienceDirect Chemical Engineering and Processing: Process Intensific...

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Chemical Engineering and Processing 81 (2014) 35–40

Contents lists available at ScienceDirect

Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Application of microwave heating to pervaporation: A case study for separation of ethanol–water mixtures Magdalena Komorowska-Durka, Reina van Houten, Georgios D. Stefanidis ∗ Process & Energy Department, Mechanical, Maritime & Materials Engineering Faculty, Delft University of Technology, Leeghwaterstraat 39, 2628 CB Delft, The Netherlands

a r t i c l e

i n f o

Article history: Received 18 July 2013 Received in revised form 19 March 2014 Accepted 22 April 2014 Available online 29 April 2014 Keywords: Pervaporation Membrane separation Microwaves Process intensification

a b s t r a c t Membrane pervaporation experiments for dewatering of water–ethanol mixtures were conducted, using a polymeric hydrophilic membrane, under microwave and conventional heating in a multimode microwave oven and a convection oven, respectively. Three feed temperatures (33.5, 45.5 and 51.5 ◦ C) and two feed compositions (5.5 wt% and 20 wt% water in the feed) were considered. At 20 wt% water content, higher water fluxes through the membrane were obtained in the convection oven. At lower water content in the feed (5.5 wt%), the opposite effect was observed; the water fluxes were higher under microwave heating over the considered temperature range. These differences may arise from the different dielectric properties and consequently thermal behaviour of the feed mixtures under microwave heating. Microwave coupling with ethanol is stronger than with water. Moreover, unlike water, the dielectric loss factor of ethanol increases with temperature, which makes microwave dissipation preponderant in hot areas. Hence, high ethanol concentrations in the feed can easily induce thermal gradients. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Membrane separation under pervaporation conditions is used in many applications, such as solvent dehydration and separation of organic mixtures. Pervaporation is advantageous over other techniques for separation of azeotropic and close boiling point mixtures [1]. Separation of azeotropic mixtures by traditional distillation, for example, is efficient with the addition of entrainers; it requires, however, supplementary separation steps [2]. Pervaporation is also applied to esterification reaction systems, where the water byproduct needs to be removed in order to shift equilibrium [3,4]. The pervaporation mechanism can be described by the solutiondiffusion mechanism and the process engages three steps: (1) selective sorption in the membrane on the feed side, (2) selective diffusion through the membrane and (3) desorption into a vapour phase on the permeate side [2]. Thus, the overall pervaporation selectivity is determined by solubility and diffusivity of the separated species in the membrane [5]. In a pervaporation process, the feed and retentate are in liquid form, whereas the permeate is in vapour phase on the other side of the membrane. The difference in

∗ Corresponding author. Tel.: +31 15 27 81 447. E-mail addresses: [email protected], [email protected] (G.D. Stefanidis). http://dx.doi.org/10.1016/j.cep.2014.04.009 0255-2701/© 2014 Elsevier B.V. All rights reserved.

partial pressures can be created either by reducing the total pressure on the permeate side using a vacuum pump or by sweeping an inert gas on the permeate side of the membrane. As a result, a chemical potential gradient is obtained, which makes transport through the membrane feasible [6,7]. Pervaporation is a non-isothermal process. Decrease in feed temperature occurs as a result of species vaporisation. Heat transfer in pervaporation involves the following steps: (1) heat transfer from the bulk flow to the membrane surface, (2) heat transfer through the membrane and (3) heat consumption due to species vaporisation followed by expansion at the permeate site of the membrane [8]. When the flow is laminar, considerable temperature drop is observed between the feed bulk temperature and the membrane surface [9]. Temperature is an important parameter due to its direct effect on mass transport through the membrane. Due to the phase change (and thus heat transfer) during pervaporation, the temperature in the liquid boundary layer decreases as the retentate stream flows through the unit. This temperature decrease can reduce mass transport in a pervaporation process [10]. Despite the energy requirements for pervaporation being lower compared to distillation, interstage heating of the retentate streams between membrane modules is necessary in order to maintain the desired temperature for separation. The cost of interstage heating is one of the major contributions to the total cost of a pervaporation process. The common way of energy delivery to the membrane is by

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heating up the liquid feed stream. As an alternative, direct electric heating of a membrane was presented by Wnuk and Chmiel [11]. Further, 33% heat recovery from a pervaporation process has been obtained by recuperating the condensation energy on the permeate side for heating of the liquid feed in a single module [12]. Rapid and volumetric heat transfer with microwave irradiation may be an alternative way of providing energy to a pervaporation process. The selective interaction of microwaves with a feed mixture component can induce thermal gradients that locally improve the permeate flux. It has been reported that in a pervaporation process for water–ethanol separation, the ethanol concentration on the permeate site was increased and the flux through a (hydrophobic) membrane was enhanced under microwave irradiation in comparison to the conventionally heated experiment [13,14]. Mass transfer enhancement under microwave irradiation in the event of a hydrophobic membrane was also reported in membrane distillation experiments [15] and in gas permeation experiments with several types of membranes (cellulose acetate, cellulose triacetate, hydroxypropyl cellulose, poly(methyl methacrylate) and polysterene) [16,17]. The reasoning put forward in the previous references to explain the observed gas permeability enhancement is that the direct interaction of microwaves with the polar functional groups of the membrane ( OH and/or CH3 COO ) increases their molecular motion. Furthermore, microwave heating may be an efficient way to enhance evaporation. A relevant extensive study was presented by Stuerga and Lallemant [18–21], where hydrodynamic instabilities during water and ethanol evaporation induced by microwaves were investigated. The effectiveness of microwave heating in relation to the temperature dependency of the dielectric loss of water and ethanol was studied. Higher evaporation rate of ethanol compared to water was observed over a range of applied microwave powers. These differences in the thermal behaviour of ethanol and water under microwave heating were explained in terms of the stronger coupling of microwaves with ethanol compared to water at the studied conditions and the in situ dissipation of the electromagnetic energy, which led to quicker induction of turbulence with ethanol than with water [18–21]. In this work, an experimental comparison of pervaporation processes conducted under microwave irradiation (MW) and conventional heating (CH) is presented. Pervaporation was applied to dewatering of ethanol–water mixtures at different temperatures. The aim is to compare the effect of the heating mode on the total fluxes through the membrane at different operating conditions.

in an injected sample. Density meter equipment has inboard calibration for binary water–ethanol mixtures. Additional validation of the calibration method was done by measurements of prepared ethanol–water mixtures with known components concentration. From the density measurements, the corresponding ethanol concentrations were obtained. For the water quantification by the Karl Fisher Coulometer technique, a sample size ranges between 0.003 and 0.02 g. The titrant was generated electrochemically in the titration cell by electrochemical oxidation of iodine. Water present in the injected sample reacts with the iodine present in a solution and this reaction changes the electrolytic properties of the sample. The amount of iodine – and though this, the amount of water – in the sample is determined through conductivity measurements. 2.2. Equipment description Microwave heating was performed in a multimode cavity (MARS, CEM Corp.) of 1600 W maximum power output and an inboard power control system based on temperature measurement. The size of the microwave cavity is ∼48 L. The microwave oven had two access ports, located on both sides of the oven, to allow for insertion of tubing into the applicator. The experiments under conventional heating were conducted in a convection oven (Binder, FD 115), which works over a temperature range up to 300 ◦ C. The oven has two access ports, with diameter of 50 mm, located on both sides of the oven, to allow for insertion of tubing into the chamber. The chamber has a size of 600 mm × 480 mm × 400 mm. In both types of experiment, a magnetic stirrer was used to ensure uniform temperature in the binary mixture in the feed vessel. A KNF SC-920 vacuum pump was used to apply vacuum conditions to the permeate side. The vacuum pump has a digital controller and a programmable pumping speed control unit (based on the set point of power or pressure). The desired pressure can be set and hold constant for a defined period of time. The hysteresis is 0.1 mbar and the lowest achievable vacuum is 2 mbar. A peristaltic pump (Watson Marlow, model 5040) was used to pump the feed mixture to the membrane module at constant rate. The membrane modules comprised a flat 9 cm diameter membrane, supported on glass support with thickness 6.5 mm and pore size 250–500 ␮m, housed in a (microwave transparent) PTFE case adapted with connections for the input and output of the feed, retentate and permeate streams. The membrane used, was a commercial hydrophilic PVA (poly vinyl alcohol) cross-linked with PAN (poly acrylonitrile) membrane. The membrane was operated in cross-flow mode.

2. Experimental 2.3. Temperature measurement and power control 2.1. Materials and analysis Ethanol with purity of 99.9+% was purchased from Merck. Demineralised water was used for preparation of the mixtures. Determination of water content in the samples was done using two independent analytical techniques, namely the density meter (digital density meter, model DMA 500 from Anton Paar) and the Karl Fisher Coulometer technique (KFC, 756 type from Metrohm). Each of the samples were analysed in triplicates by both analytical methods and the average value was calculated. Samples of the retentate and the feed mixture were analysed in pure form, whereas the permeate samples were diluted with ethanol at a ratio of 1–5 and analysed afterwards. The samples were binary mixtures of water and ethanol and determination of ethanol concentration was based on density measurements of the ethanol–water mixtures. The size of a sample required for a single injection to a density meter is 1 mL. The density measurements were conducted at isothermal conditions at 20 ◦ C. Special precaution was necessary to avoid gas bubbles

Temperature measurements in both ovens were performed by fibre optic sensors FOT-L-SD (FISO) connected to the Universal Multichannel Instrument (UMI FISO) equipped with four channels to connect up to 4 fibre optic temperature sensors. One of the fibre optic sensors was connected to the microwave oven. The fibre optic sensors are characterised by resolution of 0.1 ◦ C accuracy, response time less than 1.5 s and are immune to microwave irradiation. Both type of experiments (MW and CH) were conducted at the same upstream temperature of the liquid on the feed side. Temperature was measured by means of a fibre optic sensor attached perpendicularly to the membrane at position close to the membrane surface at the coordinates of the centre point. In the MW experiments, the power control system adjusts the microwave power input to the applicator upon reaching the set temperature in the feed stream and controls the amount of power over the duration of the process. It is noted that during the microwave experiments, power varied between 60 W and 400 W in order to keep the

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Fig. 1. Schematic view of the pervaporation set-up that was operated under microwave (MW) and conventional heating (CH).

temperature of the liquid film at the set temperature. Two additional fibre optic sensors were used to measure air temperature inside the oven (microwave or convection) and the feed mixture temperature in the feed vessel. 2.4. Experimental procedures A schematic view of the experimental set-up for membrane separation under pervaporation conditions is presented in Fig. 1. Experiments were run for 60 or 120 min in order to collect sufficient amount of permeate for quantitative analysis. The results comparing microwave and conventional heating modes refer to the same time span. During the experiments, the feed vessel (1) (numbered items refer to Fig. 1) and the membrane module (2) were both situated in the convection oven (3) or microwave oven (4), respectively. A PTFE 1.5 L beaker (1) is used as a feed tank containing ∼1 L of feed mixture (5.5 or 20 wt% H2 O in ethanol). Feed mixture from the feed vessel (1) is pumped by a peristaltic pump (5) with a speed of 10 rpm, which corresponds to volumetric flow equal to 3.7 mL/s, to the module inlet (L1, L2). The membrane module (2) is vertically orientated on the top of the feed vessel (1) and the feed flows from bottom to top along the membrane (5); thus, the membrane works in cross-flow conditions. The effective surface area of the membrane (5) is 44.2 cm2 . On the permeate side, at the middle (centre) of the membrane module (2), a temperature probe (TI) was placed to measure temperature of the liquid film close to the membrane surface (6) during both types of experiment (CH and MW). The fibre optic probes were inserted in glass capillaries. At the top of the module, the retentate leaves the module and is recycled back to the feed vessel via tube (L3). The tubing for fluid transport is made of PTFE. Considerable length of the transport tubing was inside the heating chamber and was heated during both sets of experiments, microwave and conventional. At the permeate side, vacuum is applied where permeate vapour is collected in one of the two cold traps. The downstream pressure was maintained by a vacuum pump and set constant at 20 mbar. This pressure enables processing without gasket leakage and is within the range of pressures at which the membrane was tested previously according to our literature study. The gaseous permeate is collected in either of the two cold traps (7a or 7b) and cooled down with liquid nitrogen by means of Dewar vessels (8a or 8b). The cold traps are connected with the membrane module with the line L4. The condensed permeate stays in the cold trap vessel until the end of the experiment, while the vacuum pump (9) keeps the permeate side

at vacuum conditions (L4, 7a or 7b, L5). Water concentration in the feed (retentate) vessel after a run was measured to be less than 0.5 wt% lower than the initial concentration in the feed stream. Therefore, pseudo-steady state conditions are assumed, i.e., there is no concentration change in the feed vessel during a run. In Fig. 2, the picture of the pervaporation set-up, placed in the MW cavity, is shown. Finally, the order of experiments was random and when a membrane was replaced, a reference experiment was performed as well.

3. Results and discussion The impact of MW and CH at different separation conditions (temperature and feed composition) on the permeation flux (kg/(m2 h)) of the binary components was studied. The driving force of pervaporation under the chosen process conditions was estimated. For the studied water–ethanol system, the variable parameters are as follows: temperature in the liquid film (near the centre of the membrane): 33.5, 45.5 and 51.5 ◦ C, and feed composition: 5.5 and 20 wt% water. An overview of the studied experimental conditions is given in Table 1.

Fig. 2. Picture of the pervaporation set-up placed inside the microwave cavity. The feed vessel and the membrane housing along with the temperature probe and the inlet/outlet streams are shown.

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Table 1 Overview of the studied experimental conditions.

70

Feed composition (wt% water)

Heating mode

33.5 33.5 45.5 45.5 51 51 33.5 33.5 45.5 45.5

5.5 5.5 5.5 5.5 5.5 5.5 20 20 20 20

Microwave Conventional Microwave Conventional Microwave Conventional Microwave Conventional Microwave Conventional

33.5oC 45.5oC

60 driving force [mbar]

Temperature (◦ C)

50 40 30 20 10

Each experiment at the given conditions was repeated at least three times (up to five times) and the order of experiments was random. The total pervaporation flux is calculated from Eq. (1) [2]: m A·t

(1)

where m [kg] is the mass of the permeate through the membrane with area A [m2 ] over process time t [h]. The partial pervaporation flux for each component (water or ethanol) through the membrane is calculated from Eq. (2) [2]: Ji = J · wip

(2)

[kg/(m2

h)] is the partial pervaporation flux of component where Ji i and wip is the mass fraction of component i in the permeate. In pervaporation, the driving forces for mass transport through the membrane is the vapour pressure difference between the components on the feed and permeate side of the membrane, as described by Eq. (3) [22]: Ji =

Pi  (p − pi ) l i

(3)

where Pi is the membrane permeability for component i [kg/(m2 h)*cm/mbar]; l is the thickness of the membrane [cm]; pi is the partial vapour pressure of component i on the feed side of the membrane [mbar]; pi is the partial vapour pressure of component i on the permeate side of the membrane [mbar]. The parenthetical term in Eq. (3) represents the difference between the feed fugacity and permeate fugacity; the permeate is assumed to be an ideal gas at low pressure. Assuming an ideal vapour phase, the fugacities for a pervaporation process can be expressed in terms of partial pressures on the permeate side and by saturation pressures of each component at the feed side (Eq. (3)). The driving forces for each component have been calculated via (Eq. (4)) [23]: (pi − pi ) = (i xi poi − yi p )

(4)

i activity coefficient, which is a function of temperature and composition of the solution (obtained from vapour–liquid equilibrium data by the UNIFAC method); xi is the mole fraction of component i; poi is the saturated vapour pressure of component i calculated by the Antoine equation [mbar]; yi is the mole fraction of the component i in permeate; p is the total vapour pressure on the permeate site of the membrane obtained directly from the vacuum meter [mbar]. Fig. 3 shows the pervaporation driving force (Eq. (4)) for water vs. the weight fraction of water in the feed at 33.5 ◦ C and 45.5 ◦ C. There is a major difference between the two studied heating systems. Under CH, the temperature in the feed vessel, in the feed stream inside the membrane module and in the air, outside the membrane module, was equal to one of the set temperatures (33.5, 45.5 or 51.5 ◦ C, depending on the experiment); namely, the environment in the convection oven is at thermal equilibrium. In contrast, under MW heating, only the feed (in the vessel and stream line) and possibly the membrane are heated, the latter due to its

0.05

0.10 0.15 0.20 weight fraction of water in the feed [g/g]

0.25

Fig. 3. Theoretical pervaporation driving force [mbar] for water at 33.5 ◦ C and 45.5 ◦ C and 20 mbar downstream pressure.

direct contact with the heated feed stream. The membrane housing is transparent to microwaves (PTFE) and is only heated via conduction. Temperature measurements in the case of MW show that when temperature in the feed stream, at the centre of the membrane, is at the set point, temperature in the feed vessel is a few degrees higher, whereas temperature in the MW oven is below 30 ◦ C. In Fig. 4, the influence of feed temperature on separation performance (total flux through the PVA membrane) under CH and MW at 20 wt% water in the feed is presented (downstream pressure was 20 mbar). Increase in feed temperature from 33.5 ◦ C to 45.5 ◦ C results in higher vapour pressure in the feed stream and in total flux increase through the membrane from ∼0.15 to ∼0.27 kg/(m2 h) under CH and from ∼0.09 to ∼0.22 kg/(m2 h) under MW heating. Moreover, at 20 wt% water content in the feed, the total flux under MW was lower than under CH. Further analysis of the total flux in Fig. 5 shows that the partial flux of ethanol remains comparable with the two heating systems. Rather, the water flux is higher under CH. It is also noted that the total fluxes reported here are in the same order of magnitude as that measured by Semenova (0.5 kg/(m2 h)) at 0.3 mbar downstream pressure and 50 ◦ C feed temperature using a similar type of membrane for ethanol–water mixtures [24]. In the MW experiments, it is likely that due to heat losses on the feed side and the non-uniform microwave field distribution, temperature over a part of the membrane surface is lower than the temperature measured near the centre of the membrane. This would result in reduction in the saturated vapour pressure on the feed solution at the gas–liquid interface and thereby in

0.30 CH MW 0.25 total flux [kg m-2 h -1]

J=

0 0.00

0.20 0.15 0.10 0.05 0.00 33.5

45.5 temperature [ºC]

Fig. 4. Effect of feed stream temperature on the total permeate flux in the case of 20 wt% water in the feed stream. Permeate pressure is 20 mbar.

M. Komorowska-Durka et al. / Chemical Engineering and Processing 81 (2014) 35–40 0.30 CH - ethanol

total flux [kg m-2 h -1]

0.25

MW - ethanol CH - water

0.20

MW - water

0.15 0.10 0.05 0.00 30

32

34

36

38

40

42

44

46

48

temperature [ºC]

Fig. 5. Effect of feed stream temperature on the partial permeate fluxes of water and ethanol in the case of 20 wt% water in the feed stream. Permeate pressure is 20 mbar. 0.12

39

the partial water flux is clearly higher under MW at 5.5 wt% water (Fig. 7) and lower at 20 wt% water (Fig. 4) in comparison to the CH experiments. Figs. 6 and 7 show that under conditions of lower water content in the feed (higher ethanol concentration), microwaves enhance mass transfer through the membrane. This should be attributed to the different dielectric properties of ethanol and water. As reported by Stuerga and Lallemant [19], microwave irradiation enhances thermal heterogeneities when the dielectric loss of the heated medium increases with temperature, which is the case for ethanol. On the contrary, the dielectric loss of water decreases with increasing temperature; therefore, microwave heating of water tends to alleviate thermal gradients and is quiescent in comparison to ethanol that is turbulent [19]. Hence, increase in the ratio of ethanol to water results in stronger coupling of microwaves with the heated feed that in turn may induce turbulent heating and, thereby, intense spatial temperature gradients owing to inhomogeneous field distribution.

total flux [kg m -2 h-1]

CH MW 0.10

4. Conclusions

0.08

Membrane pervaporation for dewatering of water–ethanol mixtures, using a hydrophilic membrane, were conducted under microwave and conventional heating in a multimode microwave oven and a convection oven, respectively. Experiments were conducted at three feed stream temperatures (33.5, 45.5 or 51.5 ◦ C), measured in the liquid film close to the centre of the membrane, and two feed compositions (5.5 wt% and 20 wt% water in ethanol). The following observations were made:

0.06 0.04 0.02 0.00 33.5

45.5

51.5

temperature [ºC]

Fig. 6. Effect of feed stream temperature on the total permeate flux in the case of 5.5 wt% water in the feed stream. Permeate pressure is 20 mbar.

reduction in the water flux through the membrane (Figs. 4 and 5, 20 wt% water in the feed). Fig. 6 shows the total flux under MW and CH in the case of 5.5 wt% water concentration in the feed. As the concentration of water in the feed decreases, the concentration gradient of water inside the membrane decreases as well, causing a decrease in both water and total flux irrespective of the heating mode. However, unlike the case of high water content (20 wt%) (Fig. 4) in the feed, at 5.5 wt% water, the total flux was higher under MW compared to CH over the temperature range 33.5 to 51.5 ◦ C (Fig. 6). The increase in total flux was 56%, 20% and 22% from the lowest to the highest temperature, respectively. Same as Fig. 5, Fig. 7 shows that the partial flux of ethanol remains comparable between the two heating modes with decreasing trend as temperature decreases. However, 0.12 CH - ethanol

total flux [kg m -2 h-1]

0.10

MW - ethanol CH - water

0.08

MW - water

0.06

• At 20 wt% water in the feed, the water flux through the membrane was higher under conventional heating. • At 5.5 wt% water in the feed, the opposite trend was found; the water flux through the membrane was higher under microwave heating. In the convection oven, temperature is uniform in the entire volume according to the temperature readings in the feed stream, the feed vessel and in the air outside the membrane volume. On the contrary, in the microwave oven, only the liquid binary mixture is heated and the temperature environment in the oven is strongly non-uniform. It is likely that due to heat losses in the microwave set-up, the temperature over a part of the membrane surface is lower than the temperature measured at the centre, which would in turn result in lower water driving forces in the case of microwaves and consequently in lower water fluxes as observed in the experiments with 20 wt% water in the feed. Enhancement in the permeate flux under microwaves, compared to conventional heating, at higher ethanol concentrations in the feed can be explained on the basis of stronger coupling of microwaves with ethanol than water. Contrary to water, the dielectric loss of ethanol increases with increasing temperature; therefore, microwave dissipation is preponderant in hot areas and can easily lead to local turbulent heating and spatial temperature gradients [18–21]. References

0.04 0.02 0.00 30

35

40

45

50

55

temperature [ºC]

Fig. 7. Effect of feed stream temperature on the partial permeate fluxes of water and ethanol in the case of 5.5 wt% water in the feed stream. Permeate pressure is 20 mbar.

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