Reactive Distillation Performance of Difunctional Hollow Fiber

Article pubs.acs.org/IECR

Reactive Distillation Performance of Difunctional Hollow Fiber Composite Membranes with Catalytic and Separative Properties as Structured Packing Xiao-Hua Ma, Xin Wen, and Zhen-Liang Xu* State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China S Supporting Information *

ABSTRACT: Difunctional hollow fiber composite membranes (DHFCMs) with catalytic and separative properties were used as structured packing in reactive distillation to separate ethanol−water and to catalyze ethanol−acetic acid esterification without other catalysts. These fibers showed more excellent properties over conventional packing in the ethanol−water distillation, which worked 10-fold higher above the flooding limit that usually occurred in conventional cases. Also they showed similar properties with a membrane contactor using hollow fibers as structured packing. A small height of transfer unit (HTU) of 3.6 cm and the overall transfer coefficient KG of 0.48 cm/s at the vapor velocity of 39.4 cm/s were obtained experimentally, which demonstrated a higher efficiency separating performance. The conversion of acetic acid reached 92% when DHFCMs with catalytic and separative properties were used as structured packing and without other catalysts in ethanol−acetic acid reactive distillation.

1. INTRODUCTION Structured packing has been widely used for separation processes1 with many advantages such as high capacity, high mass transfer surface area, high turn down ratio, low pressure drop, and low liquid hold up comparison with trays or random packing.2 In general, the specific surface of structured packing is between 100 and 750 m2/m3.2 Hollow fiber membrane has a large number of specific area due to its small diameter (approximately 1 mm) and has been widely used in membrane processes such as ultrafiltration,3,4 nanofiltration,5,6 pervaporation,7−9 and gas separation.10−12 Cussler and his co-workers have explored the feasibility of hollow fiber membrane as distillation structured packing in membrane contactor, exhibiting an extensive application prospect.13−15 They have found that hollow fiber membrane as a new type of distillation structured packing can decrease the height of transfer unit (HTU) and enhance separation efficiency; moreover, it can be operated well above the normal flooding limit and has the potential for saving energy, since the energy use is huge, approximately 3−5% of total energy use in the U.S., and is one of the major costs in oil products.13,16 Zhang et al. found that the distillation of methanol−water solution with nonporous hollow fibers as structured packing could easily work 3−10 times higher above the limit where flooding usually occurred in conventional cases and HTU was as low as 8 cm, implying the high efficiency of the system.17 A similar work to separate light hydrocarbon mixtures using a distillation column packed with microporous hollow fibers carried out by Yang et al.18 showed a wider and a more stable operational range. Koonaphapdeelert et al.16 employed a ceramic hollow fiber membrane for the distillation of benzene− toluene at 93−97 °C. The results showed that the membrane had excellent chemical and thermal stability and separation efficiency. © 2013 American Chemical Society

Reactive distillation, also called catalytic distillation, is an excellent example of process intensification,19−21 which is a field of growing interest defined as any chemical engineering development leading to smaller inventories of chemical materials and higher energy efficiency. Most industrial scale reactive distillations (presently more than 150) operate worldwide today at capacities of 100−3000 kt/y and have started up less than 20 years ago,19 since they combine the separation process with the catalytic reaction in a reactive distillation column by virtue of many advantages, such as low capital cost, high yield, high selectivity, and energy saving and are widely used in many chemical engineering processes such as esterification.22−24 Catalyst particles cannot be used directly as distillation packing because they form too compact a mass for the upward flow of the vapor and the downward flow of the liquid. Therefore, the catalyst particles are placed in various ways in the column such as bale packing25 in which cotton, polyester, nylon, fiberglass, and the like are used for the cloth containers for catalyst particles or catalyst particles sandwiched between corrugated sheets of wire gauze are licensed from Sulzer (KataPak)26 and Koch-Glitsch (Katamax).25 The design of reactive distillation is difficult due to the conflicting demands of good distillation and good catalytic reaction. The efficiency and selectivity of the catalyst in reactive distillation is limited due to the size of the catalysts containers. A significant improvement can be made if the catalyst material could be mounted in the column as a thin film on the structured packing that is commonly used in the distillation process.27 The structured distillation packing then ensures good distillation characteristics, Received: Revised: Accepted: Published: 5958

April 19, 2012 March 13, 2013 April 3, 2013 April 3, 2013 dx.doi.org/10.1021/ie301016w | Ind. Eng. Chem. Res. 2013, 52, 5958−5966

Industrial & Engineering Chemistry Research

Article

while the larger contact area between catalytic film and reaction mixture, combined with the short diffusion length, ensures a high overall efficiency and selectivity of the reaction. Difunctional hollow fiber composite membrane (DHFCM)28 with catalytic and separative properties is a good choice because it has two layers and can combine both reaction and separation processes and can be used as structured packing.13−15 The research on difunctional membranes has been intensively investigated in recent years. For example, Jeon et al. prepared dual-functional cermet membranes for hydrogen separation.29,30 To the author’s knowledge, application of DHFCMs with catalytic and separative properties as structured packing and catalyst in reactive distillation for esterification has not been investigated by other researchers. The aim of this paper is to characterize the distillation performances of DHFCMs with catalytic and separative properties as structured packing to separate the ethanol−water system and to catalyze the esterification reaction without other catalysts.

0.3525 < x ≤ 1 R2 = 0.999

1 1 V = y dy = l D NTU K ∫y y * −y ya 0

HTU =

1 1 H = + KG kG kL

dy = K ya(y* − y) dz

(2)

Sh =

2

R = 0.977

(9)

kLd0 (1 − φ)2.2 ⎛ d0 ⎞ 0.6 0.33 ⎜ ⎟ Re Sc = 5.8 DL φ0.6 ⎝ l ⎠

(10)

where Sh is the Sherwood number; DL is the diffusion coefficient in the liquid; φ = (d0/ds) 2 × n is the packing density; Re is the Reynolds number; and Sc is the Schmidt number. The vapor mass transfer coefficient kG can be calculated as below:34

(3)

(4)

Sh =

where z is the distance from the bottom of the column (m); Ky is the overall gas phase mass transfer coefficient based on the gas molar fraction driving force (mol/m2 s); a is the interfacial area per volume (m−1), a = (4nd0)/(ds2 − nd02) in which n is the number of hollow fiber membranes, d0 is the outside diameter of the hollow fiber (m) and ds is the inner diameter of the distillation column (m); and y* is the equilibrium vapor concentration of the liquid composition. For the ethanol−water binary system, the slope of equilibrium line is not constant. Over the ethanol concentration range of 0.0317 < x ≤1, the vapor−liquid equilibrium data described by Arce et al.31 fit the polynomials: 0.0317 < x ≤ 0.3525

(8)

where kG and kL are individual mass transfer coefficients in the vapor and liquid phases; KG is the overall mass transfer coefficient; and H is the partition coefficient. The individual mass transfer coefficients can be estimated separately as follows. The liquid mass transfer coefficient kL can be calculated from a well-known Lév ěque solution. 32,33

(1)

The differential mass balance equation of the vapor phase can be written as V

l V = NTU K ya

The mass transfer coefficient is a key to evaluate differential distillation, which is a function of diffusion in the vapor and liquid. In this work, the more volatile compound (ethanol) encounters two resistances: one is from the vapor phase boundary layer and the other is from the liquid phase boundary layer. This mass transfer coefficient is complicated and can be shown theoretically as

where V and L are the molar rate of the vapor and liquid flow (mol/s); y and x are the molar fractions of the more volatile compound (ethanol) in the vapor and the liquid phases. Therefore, the operating line is y=x

(7)

where l is the effective length of the hollow fiber (m); yD and y0 are the vapor molar fractions at the top and the bottom of the column, respectively. The number of transfer unit (NTU) is a measure of the difficulty of the separation, from which the value of experimental overall mass transfer coefficient can be found. HTU, which is a measure of the efficiency of column, can also be worked out by

and

V=L

(6)

Integrating eq 4 from the bottom of the column shows

2. THEORETICAL BASIS 2.1. Ethanol−Water Separation. To evaluate the performance of hollow fiber distillation column in the separation ethanol−water system, the theoretical analysis was carried out as follows. To simplify the process analysis, the module was run at total reflux and based on the following assumptions: (i) constant molar flows in both vapor and liquid phases; (ii) steady-state operation; and (iii) ideal gas law applied in both vapor and liquid phases. The mass balance equations are given by Vy = Lx

y* = 0.620x 2 − 0.248x + 0.622

⎛ Redh ⎞0.93 0.33 k Gdh ⎟ Sc = 1.25⎜ ⎝ l ⎠ DG

⎛ d 2u ρ ⎞0.93⎛ μ ⎞0.33 h G G ⎟⎟ ⎜⎜ G ⎟⎟ = 1.25⎜⎜ μ l ⎝ ⎠ ⎝ DGρG ⎠ G

(11)

where dh is the hydraulic diameter for the shell side of the module, which is 0.45 cm here; DG is diffusion coefficient in the vapor; uG is vapor velocity; ρG is vapor density; and μG is kinematic viscosity. 2.2. Ethanol−Acetic Acid Esterification. The esterification reaction of acetic acid (HAc) and ethanol (EtOH) to form ethyl acetate (EtAc) and water (H2O) is a reversible reaction as follows

y* = − 4.324x 2 + 2.606x + 0.207

K+

HAc + EtOH XooY EtAc + H 2O

(5)

K−

5959

(12)

dx.doi.org/10.1021/ie301016w | Ind. Eng. Chem. Res. 2013, 52, 5958−5966


Article

According to mass balance, (dCfi/dt) = ui(dyi/dZ) is obtained. The gas−solid interface concentration yiint of component i can be obtained based on eqs 18 and 19.

The reaction rate can be expressed as ⎛ C EtAcC H2O ⎞ ⎟ ri = K+C HAcC EtOH⎜⎜1 − KeqC HAcC EtOH ⎟⎠ ⎝

yi int = yi +

⎛ C EtAcC H2O ⎞ ⎟ = K+C HAcC EtOH⎜⎜1 − CeqEtAcCeqH2O ⎟⎠ ⎝

where Keq = (K+/K−) = (CeqEtAcCeqH2O)/(CeqHAcCeqEtOH). Actually, the esterification reaction on the surface of DHFCMs with catalytic and separative properties is a surface reaction and can be expressed as

ui

⎛ CfEtAcCfH2O ⎞ ⎟ = −K f +CfHAcCfEtOH⎜⎜1 − dZ CeqfEtAcCeqfH2O ⎟⎠ ⎝ dyi

dyi dZ

=−

⎞ C C K f +CfHAcCfEtOH ⎛ ⎜⎜1 − fEtAc fH2O ⎟⎟ ui CeqEtAcCeqH2O ⎠ ⎝

(22)

This is different from NTU based on the vapor side driving force relation through the measured concentrations as shown in eq 8. The conversion of reactant is a very important coefficient in the kinetics study, which is defined as

(14)

As shown in eq 14, the esterification reaction rate depends on the surface concentration of each component on the surface of DHFCMs. During the reactive distillation process, every component (HAc, EtOH, EtAc, and H2O) competitively adsorbs on the surface of DHFCMs. The adsorbance of component i (i represents HAc, EtOH, EtAc, and H2O) can be described by the Langmuir adsorption isotherm.

η=

C0i − Cfi C = 1 − fi C 0i C 0i

(23)

When the adsorbance reaches the equilibrium state, the gas−solid interface concentration of component i is equal to the surface adsorbance concentration. Substitution of eq 20 in eq 23 results in

Γi = Γ∞ iθi Γ∞ iK iyi int

η=1−

4

1 + ∑i = 1 K iyi int

(21)

NTU is expressed as

⎛ CfEtAcEtAcCfH2O ⎞ ⎟ rfi = K f +CfHAcCfEtOH⎜⎜1 − Keqf CfHAcCfEtOH ⎟⎠ ⎝

=

(20)

Therefore eq 19 can be written as

(13)

⎛ CfEtAcCfH2O ⎞ ⎟ = K f +CfHAcCfEtOH⎜⎜1 − CeqfEtAcCeqfH2O ⎟⎠ ⎝

CfEtAcCfH2O ⎞ K f +CfHAcCfEtOH ⎛ ⎜⎜1 − ⎟ kgi CeqfEtAcCeqfH2O ⎟⎠ ⎝

(15)

⎛ kgiyi + K f +CfHAcCfEtOH⎜1 − ⎝ kgiC0i

C fEtAcC fH2O

⎞ ⎟

CeqfEtAcCeqfH2O ⎠

(24)

The mass transfer flux of component i from the bulk gas phase to the gas−solid interface is expressed as Ni = kgi(yi − yi int )

(16)

The surface concentration change of component i on the DHFCMs can be expressed as dCfi = K aiyi int (1 − dt

4

⎡

i=1

⎢⎣

∑ θi)⎢1 −

⎤ ⎥ 4 K iyi int (1 − ∑i = 1 θi) ⎥⎦ θi

⎛ CfEtAcCfH2O ⎞ ⎟ − K f +CfHAcCfEtOH⎜⎜1 − CeqfEtAcCeqfH2O ⎟⎠ ⎝

(17)

where Ki = (Kai/Kdi). The adsorption rate is usually much faster than the esterification reaction rate, so the esterification reaction is the control step. It is obvious that the adsorption will reach balance quickly. Equation 17 can be expressed as ⎛ CfEtAcCfH2O ⎞ dCfi ⎟ = −K f +CfHAcCfEtOH⎜⎜1 − dt CeqfEtAcCeqfH2O ⎟⎠ ⎝

(18)

Therefore, the concentration distribution in the reactive distillation column is much worth studying. The concentration change rate of component i along the height of reactive distillation column can be expressed as ui

dyi dZ

= kgi(yi − yi int )

Figure 1. Hollow fiber distillation column apparatus. Both of the vapor and liquid flow on the shell side of the hollow fibers.

(19) 5960



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As shown in eq 24, the conversion of reactant is closely related to the bulk gas concentration, initial concentration, and surface concentration.

Table 1. Parameters of Hollow Fiber Distillation Column Module module type fiber types module diameter (ds, cm) module cross section (cm2) fiber inner diameter (di, cm) fiber outer diameter (d0, cm) no. of fibers (n) effective length (l, cm)a area per volume (a, m2/m3) module void fraction (ε) packing factor (a/ε3, cm−1) PFSA loading (% in mass) a

module 1 module 2 module 3 module 4

module 5

DMT 1.60

DMT 1.60

DMT 1.60

DMA 1.60

DMS 1.60

2.01

2.01

2.01

2.01

2.01

0.115

0.115

0.115

0.115

0.115

0.180

0.180

0.180

0.180

0.180

20 28.0

30 28.0

40 28.0

40 28.0

40 28.0

753

1360

2280

2280

2280

0.747

0.620

0.494

0.494

0.494

1810

5710

18900

18900

18900

3.77

5.65

7.54

7.54

7.54

3. EXPERIMENTAL SECTION 3.1. Materials and Modules. Acetic acid and ethanol were analytical reagents purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. Deionized water was used for the preparation of the ethanol−water solution whose molar fraction was set at 0.610 mol % (80 wt %). The main part of the apparatus, as shown in Figure 1, was a hollow fiber distillation column module packed with a desired number of DHFCMs28 with catalytic and separative properties made in the lab (see the Supporting Information, Preparation of DHFCMs with catalytic and separative properties). At both module ends, the gaps among the fibers were potted with epoxy resin. The modules were dried in the air and checked for leakage before being used. The parameters of each module are shown in Table 1. 3.2. Equipment and Process Conditions. At the experimental beginning, 300 mL of ethanol−water or quantitative acetic acid−ethanol solutions were added to the reboiler of the distillation apparatus shown in Figure 1. The

Length of mass transfer, total length is 35.0 cm.

Figure 2. As usual, the flow parameter on the abscissa is plotted vs the capacity factor on the ordinate. (a) Hollow fibers operate above the normal flooding limit, shown as the solid line. (b) Modules operate at different flows.

Figure 3. Data for hollow fibers are at lower flows than suggested for four conventional packings, shown as the solid lines: (a) HTU vs F factor and (b) HTU vs F factor at different flows. 5961



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line is the empirically determined flooding limit of conventional packing.17,37 As shown in Figure 2a, the flow parameters (X) all equal to 0.040 due to the set of the experiments are conducted in the same module (packed with 40 hollow fibers) and under total reflux conditions. The capacity factors (Y) are all above the flooding limit of conventional packing, the maximum value of which, obtained when the maximum heating rate is used, is approximately 10-fold higher beyond the flooding limit of conventional packing. The results show that DHFCMs as structured packing can be operated well above the normal flooding limit, and it has performance similar with the device reported in the literature.15−17,38 Meanwhile, different fiber types have little impact on the hollow fiber distillation module (Figure 2a). As shown in Figure 2b, some capacity factors (Y) are located above the solid flooding line as the number of fibers increases from 20 to 40, indicating that packing density is an important parameter for the hollow fiber distillation column. By controlling the packing density, the velocities of both vapor and liquid phases can be adjusted to maximize their residence time to ensure that the two phases have sufficient time to contact along the fibers. During the operation process, there was no flooding observed in the hollow fiber modules. It was not sufficient to rule out the possibility of flooding in hollow fiber distillation columns. In fact, the vapor inside the column is condensed on the shell side of the fibers after contacts with the cooling reflux liquid. When the force of the upward vapor is high enough to overcome the gravity force of the liquid, the liquid on the shell side will be pushed upward and, therefore, flooding occurs. 4.1.2. Effect of Vapor Load on Separation Efficiency. Separation efficiency is one of the most important parameters to be considered in a distillation column design. High separation efficiency can save energy and reduce the capital investment. Figure 3 shows HTU (a measure of separation efficiency) versus the vapor load (so-called F factor, which is defined as the vapor velocity times the square root of the vapor density). As shown in Figure 3, the values of the F factor and HTU of DHFCMs structured packing are smaller than those of conventional packing due to the large specific area supplied by hollow fibers, as detailed in Tables 1 and 2. It is obvious that the F factors show no significant change with different fiber types, implying that the fiber types have little impact on the hollow fiber module (Figure 3a) and they increase with the fiber numbers (Figure 3b).

Table 2. Several Parameters of Conventional Packings conventional packing 25 mm Raschig ring (random)39 25 mm Berl saddle (random)39 Mellapark (250Y)14,38 Gauze BX (Sulzer)14,38

area per volume (a, m2/m3)

void fraction (ε)

packing factor (a/(ε3), cm−1)

190

0.68

604

260

0.68

827

76 150

0.95 0.90

89 206

reboiler was heated with an oil bath. Vapor produced in the flask flowed up the shell side of the hollow fiber module, used as a distillation column, to a water-cooled condenser. There was a small reservoir to receive the distillate from the condenser. The condensate flowed back into the shell side of the hollow fiber module at the top of the column. To simplify the process analysis, the module was run at total reflux. The column was run for about 1 h to approach steady-state before the first sample was taken. 3.3. Analysis and Calculation. To evaluate the performance of the hollow fiber distillation column, samples were taken from the distillate reservoir and from the liquid running out of the bottom of the column at the same time. A gas chromatograph (Techcomp, GC7890, China) equipped with a thermal conductivity detector (TCD) and a GDX-102 packed column was used to measure the samples. The calculation of ethyl acetate’s yield is defined as n − nP0 Φ= P × 100% nA0 (25) where nP and nP0 are the final and the initial molar weights of ethyl acetate, respectively; nA0 is the initial molar weight of acetic acid.

4. RESULTS AND DISCUSSION 4.1. Packing Performance. 4.1.1. Hollow Fiber Distillation Column Capacity. In a conventional packed column, the operation largely depends on the interaction between vapor and liquid phases. If vapor and liquid velocities are not proportional, the system might encounter hydrodynamic problems such as flooding and loading. Thus, the distillation column is usually designed to operate in the preloading region and within the flooding ranges.35,36 Normally, flooding is described with a plot of flow parameter (X) versus capacity factor (Y). Figure 2 presents such plots of the ethanol−water system, where the solid

Figure 4. (a) Distillate concentration vs heating rate and (b) distillate concentration vs heating rate at different flows. 5962



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4.1.3. Changes of Distillate Concentration with Heating Rate. Figure 4 shows the variation of distillate concentration with heating rate. The ethanol concentration in the reboiler was kept almost the same and set at a molar fraction of 0.610. After reaching the steady state, samples were taken at each heating rate and analyzed by GC. As shown in Figure 4, the ethanol distillate concentration on the top of the hollow fiber distillation column increases with decreasing heating rate. Low heating rate means a small vapor flow and, therefore, increases the contact time between vapor and liquid, causing a complete mass transfer process.16,17 Even high distillate concentration can be obtained by decreasing the heating rate, and the vapor is difficult to raise to the top of the hollow fiber distillate column. The highest distillation concentration obtained is 0.700 mol % (40 fibers), which means approximately 0.09 mol % enrichment achieved in such a small hollow fiber distillation column of 28 cm effective length. However, the azeotropic concentration of ethanol−water 0.88 mol % is not a breakthrough. Among the three kinds of hollow fibers, the distillate concentration changes little (less than 0.002) at the same vapor velocity (Figure 4a), implying that the hollow fiber types have little impact on the column efficiency. 4.1.4. HTU and Separation Efficiency. As mentioned previously, HTU is a measure of the column’s efficiency and can be calculated from experimental data directly. Figure 5 shows HTU versus vapor velocity, which increases with vapor velocity, meaning lower mass transfer efficiency due to the fact that the vapor rises too fast to complete contact with the liquid dropping

Figure 7. Conversion vs time (test conditions: molar ratio of acetic acid to ethanol of 4:1, oil temperature of 145 °C, fiber numbers of 40, DMT, total reflux).

down along the shell side of the hollow fibers. The minimum HTU, which is only 3.6 cm at the vapor velocity of 39.4 cm/s, represents high separation efficiency. This enhanced separation efficiency is not the result of a high mass transfer coefficient but results from the plethora of interfacial area. Fiber types and fiber numbers also have an obvious impact on the HTU, which is the lowest when using DMT at the same vapor velocity (Figure 5a) and decreases with the increase of fiber numbers (Figure 5b).

Figure 5. (a) HTU vs vapor velocity and (b) HTU vs vapor velocity at different flows.

Figure 6. Relationship between the overall mass transfer coefficients and the vapor velocity (a) with different fiber types and (b) at different flows. 5963



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Figure 8. Effect of oil temperature on (a) conversion and (b) ethyl acetate yield (test conditions: molar ratio of acetic acid to ethanol of 4:1, fiber numbers of 40, DMT, total reflux).

Figure 9. Compositions of (a) the bottom and (b) the top (test conditions: molar ratio of acetic acid to ethanol of 4:1, oil temperature of 140 °C, fiber numbers of 40, DMT, total reflux).

increasing kG, since kL will increase as the fiber diameter gets smaller and the liquid velocity gets larger, which cannot be realized simultaneously. 4.2. Reactive Distillation Performance. To investigate the catalytic activity of DHFCMs with catalytic and separative properties in reactive distillation for the esterification reaction, an experiment in a batch reactive distillation is carried out and the results are presented in Figure 7. The blank experiment is accomplished with hollow fiber membranes with a separative layer but without a catalytic layer at the same conditions. Specifically, by using difunctional hollow fiber DMT, the conversion rate of HAc achieves 92% within 8 h, which is approximately 1.3 times higher than that without catalytic layer. This conversion is similar with and comparable to the result of conventional catalyst (98% H2SO4, 90% for HAc). A possible explanation is that DHFCMs as packing not only provide a huge specific surface area but also provide a great deal of catalytic activity centers. As shown in eq 24, the conversion is closely related to the bulk gas concentration, initial concentration, and surface concentration, namely, related to heating power, feed molar ratio, and fiber numbers. Figure 8 shows the effect of heating power on the HAc conversion and ethyl acetate yield. The former rises, while the latter has little change with high heating power. At high heating power, the temperature of the reactive zone increases, which leads to an increase in the rate of the reaction. The velocities of liquid and

The efficiency of conventional packing is usually described in the form of height equivalent of a theoretical plate (HETP), which links to the concept of distillation in a plate column. The HTU can be converted by using HETP = HTU

ln(mG /L) m−1

(26)

where m is the slope of the equilibrium curve which is around 0.89 of ethanol. Thus, the HETP is about 1.06 times of the HTU for ethanol. 4.1.5. Mass Transfer Coefficients. The overall mass transfer coefficient (KG) of each module, as shown in Figure 6, is directly calculated through eq 8 from the experimental data. They increase as the vapor velocity increases and are influenced by fiber types and fiber numbers. The individual mass transfer coefficients in the liquid and vapor phases are separately estimated by eqs 10 and 11. In our case, the values of kG of 40 hollow fibers are in a range of 0.48−0.74 cm/s, which decrease gradually with the increase of fiber numbers. The resistances in hollow fiber distillation column are represented in the vapor (1/kG) and in the liquid (H/kL). It is obvious that the resistance in the vapor side 1/kG holds a dominant place of the total resistance 1/KG. To increase the overall mass transfer coefficient KG, mass transfer of kG and kL must be increased. Increasing kG is most easily achieved with carefully designed baffles. Increasing kL is much harder than 5964



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Figure 10. Effect of molar ratio on (a) conversion and (b) ethyl acetate yield (test conditions: oil temperature of 140 °C, fiber numbers of 40, DMT, total reflux).

Figure 11. Effect of fiber numbers on (a) conversion and (b) ethyl acetate yield (test conditions: molar ratio of acetic acid to ethanol of 4:1, oil temperature of 140 °C, DMT, total reflux).

of HAc reaches 92% when DHFCMs with catalytic and separative properties is used as structured packing and without other catalysts in ethanol−acetic acid reactive distillation, indicating high efficiency and an extensive application prospect.

vapor are both accelerated with high heating power to shorten the diffusion length and the residence time of reactants, but the water content in the bottom products is not reduced by increasing the heating power, as shown in Figure 9. The HAc conversion increases with increasing molar ratio of acetic acid to ethanol because the esterification reaction is a selfcatalyzed reaction. Whereas the ethyl acetate yield decreases due to the excess of acetic acid (eq 25), as shown in Figure 10. Figure 11 shows the effect of fiber numbers on the HAc conversion and ethyl acetate yield, both of which decrease even as the PFSA catalyst loading increases from 3.77 to 7.54 (% in mass) with fiber numbers increasing from 20 to 40 (Table 1). Possible explanations are that the packing factor is in inverse proportion to the module void fraction, and the reaction is no longer controlled by kinetics.23

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ASSOCIATED CONTENT

S Supporting Information *

Details of the preparation of DHFCMs with catalytic and separative properties. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-21-64253061. Fax: 86-21-64252989.

5. CONCLUSION DHFCMs with catalytic and separative properties as structured packing show more excellent performances over conventional structured packings and have similar results compared with the membrane contactor reported in the literature with the separation of the ethanol−water system. It can be operated successfully above the flooding limits. Furthermore, the minimum HTU is only 3.6 cm at the vapor velocity of 39.4 cm/s, representing high separation efficiency. The overall transfer coefficient KG, which is 0.48 cm/s at the vapor velocity of 39.4 cm/s, is obtained experimentally. Specifically, the conversion

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 20076009 and 21176067) and the Chemistry & Chemical Technology Research Center Plan of Shanghai Huayi Group Company (Grants A200-8608 and A20080726) for giving financial support in this project. 5965



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Reactive Distillation Performance of Difunctional Hollow Fiber

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