Article pubs.acs.org/IECR
Heat-Treated Polyacrylonitrile (PAN) Hollow Fiber Structured Packings in Isopropanol (IPA)/Water Distillation with Improved Thermal and Chemical Stability Wanbin Li,† Zhihong Yang,† Guoliang Zhang,*,† and Qin Meng§ †
College of Chemical Engineering and Material Science, Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China § College of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *
ABSTRACT: In this study, polyacrylonitrile (PAN) hollow fiber membrane (HFM) was heat-treated by muffle furnace to strengthen the thermal and chemical stability. Membrane morphology with different materials was characterized by scanning electron microscopy (SEM). It has shown that both porosity and pore size decreased with increasing heat treatment time (t = 0.5, 6, 12 h) and temperature (T = 200, 250, 300, and 350 °C). FTIR was used to explore the change of chemical bonds and found that dehydrogenation, cyclization, and cross-linking reactions occurred in thermal treatment. Compared with original PAN membrane, the hydrophobicity of heat-treated membranes was obviously improved. The heat-treated membrane PAN-250-6 (PAN−temperature−duration) was selected and immersed in various boiling solvents for 24 h to test material stability. PAN250-6 membrane presented excellent thermal and chemical stability especially in strong solvent, N,N-dimethylacetamide (166.1 °C), whereas original PAN membrane was dissolved completely. For comparison, PAN and PAN-250-6 HFMs were further chosen for packing modules, which were used for the distillation of isopropanol−water solution. During 10 days of operation, module PAN-250-6 showed high separation efficiency with comparatively low height of mass transfer unit (HTU) and larger overall mass transfer coefficients in the ranges of 0.1−0.18 m and 2.5−3.2 cm/s respectively. By analyzing the impact of wetting condition on mass transfer, it was found that membrane resistance should be sensitive and attributed more to the change of the overall resistance. The membrane with better hydrophobicity after heat treatment was more conducive to distillation with HFMs. With superior thermal and chemical stability in distillation, this kind of heat-treated hollow fiber structured packing will be promising in future distillation applictions.
1. INTRODUCTION Distillation is the most common method of chemical separation, which is the workhorse of the chemical and related process industries. As a key to production of most commodity chemicals, it is basic to the four most important separations of organics: aliphatic from aromatic hydrocarbons, linear from branched hydrocarbons, olefins from alkanes, and alcohols from water. In North America alone, distillation consumes over 1 million barrels of oil per day, or about 4% of the continent’s energy consumption.1 It is estimated to be only 11% efficient and, therefore, offers challenging opportunities for the increasing energy demand. The usage of hollow fiber membranes (HFMs) as structured packings in distillation2 was first proposed with a preliminary suggestion that retrofitting distillation columns with hollow fibers can make columns more productive. Because liquid and vapor are countercurrently contacted with the fibers serving as column packing, which is different from conventional distillation, distillation with HFM can effectively avoid flooding and loading even at extreme flows, and the separation efficiency has been proved to be sufficiently high.3−6 Distillation with HFM has nearly infinite turndown ratio and can achieve a mass transfer unit (HTU) as small as possible. It can provide a huge interfacial area per volume (normally 1000−3000 m2/m3) and keep the interface stable in distillation.7 In related studies, only © 2013 American Chemical Society
limited papers used ceramic membranesl the most commonly used membranes were organic materials such as polyether sulfone (PES), polypropylene (PP), polysulfone (PS), polyvinylidene fluoride (PVDF)8−10 because the material elasticity and flexility can easily be approached in module design and making. The organic hollow fiber membrane packings are limited to the separation of alcohols/water and olefins/alkanes systems, where conditions are comparatively modest. The utility of organic membranes is questionable because they would obviously not withstand the conditions routinely present in a refinery’s process. With the consideration of thermal and chemical stability, Koonaphapdeelert et al.11 once tried a ceramic HFM for the distillation of strong solvent pair benzene/toluene. Although the ceramic membranes showed some advantages, there are still many difficulties for hollow fiber structured packings with complicated structures in chemical industry, for example, their fragility and expense.12 Therefore, good candidates for highly efficient hollow fiber structured packings should take advantage of both organic and ceramic materials. Received: Revised: Accepted: Published: 6492
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With the above considerations for hollow fiber structured packings, thermally stabilized PAN HFM is an apparently good choice. As is known, PAN is a particular polymer with a chain of carbons connected to one another, which is a hard, horny, relatively insoluble, and a high-melting material. Currently 90% of all commercial carbon or graphite fibers are produced by the thermal conversion of a PAN precursor, which is a form of acrylic fiber.13 The processes that converts the precursor of PAN fiber to carbon are divided into three steps: oxidative stabilization, high-temperature carbonization, and graphitization.12 Stabilization of PAN fibers is usually carried out by heat treatment in the region of 180−300 °C, which can ensure both the molecules and the molecular orientation.14 The reaction between two nitrile groups (Figure 1) will occur in this
0 = −G
dy + K ya(y* − y) dz
(2)
where G is the mole vapor flux in column, z is the position in the column, Ky is the overall mass transfer coefficient based on vapor phase, a is the fiber area per volume, and y* is the mole fraction in vapor phase at equilibrium. Integrating the equation from the bottom to the top of the column gives l=
G K ya
∫y
y1
0
dy y* − y
(3)
where l is the length of the column. For simplicity, eq 3 is usually rewritten as l = HTU × NTU
(4)
in which NTU =
∫y
y1
0
HTU =
dy y* − y
u l G = = G NTU K ya K Ga
(4a)
(4b)
where KG is the overall mass transfer coefficient based on a vapor-side concentration driving force and KG = Ky/C0, in which C0 is the total vapor concentration, uG is the vapor velocity (the flow rate of the vapor in the shell side), and HTU and NTU are the height and number of mass transfer units, respectively. HTU reflects the efficiency of the column: the smaller the HTU is, the better the design should be. Further specifics are detailed in refs 2 and 5. The mass transfer coefficient KG also can be calculated by mass transfer rate, the diffusion of volatile constituent in the vapor, in the liquid, and in the membrane. Because the liquid phase and membrane phase diffusivity is much lower than vapor phase diffusivity, the mass transfer resistance through liquidfilled pores is much larger than that through vapor-filled pores. Therefore, the methods of calculation are different for vaporfilled (nonwetted) or liquid-filled (wetted).2,8 The overall mass transfer resistance can be expressed by eqs 5a and 5b, respectively.
Figure 1. Schematic of chemical reactions in heat treatment of PAN fibers.
processes;15 thus, the macromolecules cross-link together through the chemical bonds and make PAN membranes with good chemical stability and without obvious thermal distortion. Tsai et al.16,17 studied the influence of membrane heat treatment on the pervaporation performance. The results indicated that membrane permeation rate and selectivity reduced and increased, respectively, with temperature increase. David et al.18 investigated the influences of the thermal stabilization conditions and soaking time on carbon membrane performance. They found that a longer soaking time improved the membrane selectivity and reduced the permeability. In this study, to obtain hollow fiber structured packing with thermal and chemical stability, PAN HFMs were investigated by thermal treatment in a muffle furnace. Effects of different heat treatment conditions on the membrane morphology, material stability, chemical structure, and mechanical characters were studied. To validate the efficiency of this kind of novel material in distillation with HFM, the performance of different hollow fibers such as PAN and PAN-250-6 membranes was tested in the distillation of an isopropanol/water system.
for the nonwetted membrane: 1 1 1 do H do = + + KG kG kM dm kL d i
(5a)
for the wetted membrane: 1 1 H do H do = + + KG kG kM dm kL d i
2. THEORETICAL CONSIDERATIONS The theoretical analysis is based on mass balance and transfer unit theory basic to the operation of a conventional distillation column. For simplicity, the device is operated at total reflux. The operation line can be shown as y=x (1)
(5b)
where kG, kM, and kL are the individual mass transfer coefficients in vapor phase, membrane, and liquid, respectively; do and di are the outer and inner fiber diameters, respectively; and dm is the log mean diameter of the hollow fiber. The partition coefficient H is closely related to Henry’s law coefficient here. The individual mass transfer coefficients can be estimated separately as follows. The gas mass transfer coefficient kG can be estimated from the correlation1,19
where y and x are the mole fractions of isopropanol (IPA) in the vapor and liquid phases, respectively. From the mass balance of IPA on the vapor phase along the length of the column, we can find 6493
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Figure 2. Schematic and parameters of two kinds of hollow fiber modules used as structured packings.
k G = 1.25
0.93 0.33 DG ⎛ dh2uG ⎞ ⎛ υ ⎞ ⎜ ⎟ ⎜ ⎟ dh ⎝ lυ ⎠ ⎝ DG ⎠
treatment processes. PAN HFMs were heated to the target temperature at a 2 °C/min increment after the muffle furnace reached a temperature of 100 °C and was maintained for 0.5, 6, and 12 h, respectively. Subsequently, the membranes were cooled in natural conditions to room temperature. The membranes after heat treatment were labeled “PAN−temperature−duration”, for example, PAN-250-6. 3.3. Characterization. The morphology of the membranes was observed by using a scanning electron microscope (TM1000, Hitachi, Japan). Liquid nitrogen was used for the freeze fracture of cross-section samples of hollow fibers. For SEM characterization, all specimens were coated with a thin layer of platinum by an ion sputter coater (E-1045, Hitachi, Japan) to minimize surface charging. The change of PAN chemical groups was analyzed with FTIR spectroscopy (Nicolet 6700, Thermo Scientific, USA). The hydrophobicity was studied by observing the change of the deionized water droplet on the membrane surface, which was dropped with a microsyringe (1 μL).20 The tensile strength of membranes was measured by a tensile test instrument (HS-3000A, ShangHai Heson, China). More than three specimens were tested for each sample. Each sample with a gauge length of 50 mm was stretched directionally with a constant rate of 25 mm/min. The tensile stress at break (N), elongation at break (%), and strength at break (MPa) were determined. Thermal gravimetric analysis (TGA) (Perkin Elmer model TGA 7) was performed at a rate of 10 °C/min in the temperature range of 30−500 °C under air atmosphere. The overall porosity of each specimen was determined from the equation
(6)
where DG is the diffusion coefficient of vapor, dh is the hydraulic diameter of the shell side, and υ is the kinematic viscosity of vapor. The liquid mass transfer coefficient kL can be calculated from the Lévêque equation2 1/3 DL ⎛ d i 2uL ⎞ ⎟ kL = 1.64 ⎜ d i ⎝ lDL ⎠
(7)
where DL is the diffusion coefficient of liquid and uL is liquid velocity. The membrane mass transfer coefficient, kM, generally is affected by the pore structure of the membrane and depends on the operation mode. It can be correlated by an empirical correlation as kM =
DM ε δτ
(8)
where ε, δ, and τ are the porosity, thickness, and tortuosity of membrane, respectively; and DM is the solvent (such as isopropanol) diffusivity in the vapor or liquid phase.
3. EXPERIMENTAL SECTION 3.1. Materials. All solvents including ethanol (EtOH), isopropanol (IPA), hexamethylene, acetone, toluene (Tol), and N,N-dimethylacetamide (DMAc) are of A.R. grade and purchased from Sinopharm Chemical Reagent Co. Ltd., China. Ammonia−water (25−28 m/m %) is available from Changzheng Chemical Reagent, China. PAN HFMs were selfmade membranes with molecular weight cutoff of 60000 Da. All water used in this study is deionized water, which was analyzed by IRIS Intrepid ICP and Metrohm 861 Compact IC and controlled to meet the requirement of σ ≤ 0.5 μS/cm. 3.2. Heat-Treated PAN HFMs. The original PAN HFMs were thermally treated in air atmosphere by using a muffle furnace. To maintain the straightness of PAN HFM in the heating process, a stainless steel wire with a diameter of 0.5 mm was punctured through the tube side carefully. They were put together into the chamber of the muffle furnace. Table S1 in the Supporting Information shows the conditions of heat
ε=
Vtotal − Vpolymer lA − (m /ρ)polymer Vvoid = = Vtotal Vtotal lA
(9)
where Vvoid, Vtotal, and Vpolymer are the void volume, the total specimen volume, and the polymer volume, respectively. l and m are the length and weight of specimen, respectively. ρ is the polymer density determined by pycnometer methods. A is the cross-sectional area defined as A=
π (do2 − d i2) 4
(10)
where the do and di of each specimen were tested by SEM. 6494
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Table 1. Dimension, Porosity, and Mechanical Properties of PAN HFMsa inner diameter di (μm) outer diameter do (μm) porosity (%) strain at break (%) maximum load at break (N) tensile stress at break (MPa) a
1
2
3
4
5
6
7
1050 ± 18 1580 ± 19 75 ± 5 13.9 ± 4.5 5.7 ± 0.1 5.5 ± 0.2
780 ± 14 1177 ± 16 59 ± 2 6.5 ± 1.3 7.2 ± 0.6 11.1 ± 1.3
730 ± 6 1100 ± 16 50 ± 3 5.1 ± 0.3 2.1 ± 0.3 3.7 ± 0.3
667 ± 22 1038 ± 28 50 ± 3 3.5 ± 0.2 2.0 ± 0.4 3.2 ± 0.4
586 ± 14 808 ± 17 59 ± 4 3.7 ± 0.4 1.6 ± 0.3 3.1 ± 0.3
688 ± 11 1064 ± 18 52 ± 3 3.6 ± 0.3 1.3 ± 0.2 2.6 ± 0.2
656 ± 8 1025 ± 19 52 ± 2 3.5 ± 0.5 1.6 ± 0.2 3.1 ± 0.4
1, PAN; 2, PAN-200-6; 3, PAN-250-6; 4, PAN-300-6; 5, PAN-350-6; 6, PAN-300-0.5; 7, PAN-300-12.
Figure 3. SEM images and EDS data of different membranes: 1, 4, inner edge cross section; 2, 5, middle cross section; 3, 6, outer edge cross section.
3.4. Tests on Thermal and Solvent Stability of HFMs. PAN HFMs were cut into many pieces by 5 cm length and loaded into a round-bottom flask for immersion in various organic solvents, such as N,N-dimethylacetamide (DMAc), isopropanol (IPA), ethanol (EtOH), acetone, toluene (Tol), and hexamethylene. To maintain consistency with the distillation as practical as possible, all solvents were heated to the boiling point and refluxed with 24 h. Afterward, hollow fiber samples were taken out and the lengths measured immediately and then air-dried (80 °C, 24 h) prior to the weight measurement and morphology characterization. The weight loss fraction (WF) can be defined as WF = (Wafter treatment −Woriginal)/Woriginal. In the same way, the length shortening fraction (LF) is defined as LF = (Lafter treatmet − Loriginal)/Loriginal. 3.5. Distillation Examination. As presented in our previous work,8 original PAN and thermally treated PAN HFMs were installed in a glass shell with an inner diameter of 1 cm and glued together with epoxy resins (Jinpeng Chem, China). Each module with two nozzles had an effective mass transfer length of 6.5 cm. The geometric dimensions of hollow fibers and the parameters of modules are presented in Figure 2. In the experiment, IPA (A.R. grade, Sinopharm Chemical Reagent, China) was used as received and about 800 mL of alcohol solution with 20 v/v % was prepared as feed with doubly distilled water. Solution was added into the reboiler and heated in a 350 W digital autocontrol heating mantle (Taisite, China). The vapor produced in the flask flowed up to the shell side of the hollow fiber module and went to the condenser. The vapor was then condensed into a liquid and flowed back to the flask through the tube side of the module. All distillation
experiments were run at total reflux. Samples taken from distillation were analyzed by a GC-1102 gas chromatograph equipped with a thermal conductivity detector (TCD) and a Super Porapak Q steel column. The schematic diagram of the membrane modules is depicted as Figure 2, and more detailed description on hollow fiber structured packing distillation is described in refs 2 and 3.
4. RESULTS AND DISCUSSION 4.1. Membrane Characterization. 4.1.1. Properties of PAN HFM after Thermal Treatment. As shown in Figure S1 in the Supporting Information and Table 1, the change of membrane was sharp with heat treatment. The fraction of shrinkage and weight loss ranged from 23 to 35% and from 25 to 40%, respectively, when the temperature was increased from 200 to 350 °C. As reported,15 shrinkage was caused by physical and chemical changes normally. The physical referred to the contraction and rearrangement of polymer chains when environmental temperatures were above the range of the glass transition temperature21 (Tg, 80−100 °C). The chemical included intramolecular and intermolecular cyclization. Generally, the extent of the shrinkage of PAN fibers can vary from 13 to 35%, depending on the ratio of inter- to intramolecular reactions. For weight loss, this was due to the evaporation of molecules of solvent, which was added in membrane casting, and the loss of small molecules (CH4, N2, or O2) during heat treatment.18 There was no big difference of the variation of the shrinkage and weight with increasing treatment time. Moreover, dimensions of di and do, overall porosity, and the mechanical strength exhibited similar tendencies. Herein, the mechanical 6495
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Figure 4. Effect of heat treatment temperature on the morphology of PAN HFMs: (a) picture of PAN-250-6 membrane. The other SEM pictures are, from left to right, the middle cross section, inner edge cross section, and inner surfaces of membranes, respectively; the magnification is 10000 times.
preserved comparatively good flexibility (Figure 4a), which is affordable for distillation. 4.1.3. Chemical Structure. As shown in Figure 1, the decreases of −CH2− bonds at 2940 cm−1 and −CN bonds at 2240 cm−1 can be used as indicators for the dehydrogenation and cyclization reactions, respectively. The dehydrogenation reaction let the single bond of −CC− in the main chain of PAN change to the double bond of −CC−, and the cyclization reaction let the −CC− bond change to the cyclic structure. The FTIR spectra of the heated membranes are presented in Figure 5. Compared with the original membrane, the signal intensities of −CN and −CH2 bands at 2240, 2940, and 1450 cm−1 were significantly attenuated, indicating the reduction of these two groups. Meanwhile, the −CC− and
strength should be emphasized as it was crucial to practical application. As shown in Table 1, the elongation value decreased drastically with increasing temperature up to 250 °C and remained approximately constant at about 4% above 250 °C. Tensile stress also showed a similar tendency. PAN200-6 was special, giving the highest values of the heat-treated membranes for maximum load and tensile stress at break. That may be deduced from the intermolecular or cross-linking reaction. Details will be analyzed in the following section. 4.1.2. Membrane Morphology. Membrane structure is a crucial factor for the separation efficiency of HFM structured packing in distillation. SEM was used to characterize the PAN HFM morphology. To present the special character of PAN HFM during thermal treatment, an experiment of heat treatment for PVDF hollow fibers has also been carried out at 200 °C for 6 h. It was found that the membrane structure was completely destroyed and the porous membrane wall became a dense wall, which presented a very huge mass transfer resistance (Figure 3). In fact, PS and PES HFMs, which are not shown here, also presented similar phenomena. Different from the membranes mentioned above, when PAN fibers were heated to 200−400 °C in air atmosphere, cyclization happened and thus prevented melting. 22 Figure 4 presents the morphology of HFMs of different materials. It can be found that the configuration of the cross section after heat treatment was maintained commendably except in PAN-350-6, in which the deformation of finger-like macropores and cross section emerged. From the magnification images in Figure 4, the inner edges of cross section, inner surface, and sponge-like microporous structure exhibited that the morphology became denser with increasing thermal treatment temperature and time. Although the data shown in Table 1 imply that the mechanical properties weaken drastically, the heated PAN HFMs still
Figure 5. FT-IR spectra of HFMs: (A) PAN; (B) PAN-200-6; (C) PAN-250-6; (D) PAN-300-6; (E) PAN-350-6; (F) PAN-300-0.5; (G) PAN-300-12. 6496
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Figure 6. Shape of water droplets on membrane surfaces predicting changes of hydrophobicity.
−CN bands at 1600 cm−1 were observed to increase, which indicates the occurrence of the cyclization reaction of PAN. It is concluded that nitrile cyclization was activated first at temperatures around 175 °C23,24 and propagated at higher temperatures (around 215 °C). The cyclization was only activated in partial segments when the heating temperature reached 200 °C. Therefore, signal intensity at 2240, 2940, and 1450 cm−1 still exists obviously in the FTIR pattern of PAN200-6. When the temperature was >240 °C, cyclization extended to the whole molecular chains rapidly, so there is very little difference in FTIR patterns when the temperature is >250 °C. We also found that the cyclization preceded dehydrogenation according to the −CH2− bands at 2940 cm−1 persisting to higher temperature than −CN bands at 2240 cm−1. From observing Figure 5 carefully, it can be found that the absorption peak of the −CN− and −CC− bonds around 1600 cm−1 tended to shift to smaller wavenumber (from A to G, the value shifts from 1606 to 1579 cm−1) with increasing heating temperature. There are two reasons for red shift: first, the recession inductive effect as the decrease of polar nitrile groups with cyclization and cross-linking reaction; second, the enhancement of conjugative effect due to the generation of −CC− and −CN− bonds. Therefore, the red shift can demonstrate the occurrence of cyclization again. Besides cyclization, an oxidation reaction also took place, which could be identified by EDS data in Figure 3 where the oxygen content of the outer edges increased after heating. Figure 6 shows a great increase of the hydrophobicity of the membrane after heat treatment. Moreover, as observed in other membranes, it was found that the hydrophobicity increased with heat treatment temperature and time as a consequence of the decomposition of polar nitrile groups and densification of skin layer possibly. 4.2. Effects of Thermal Treatment on Thermal and Chemical Stability of PAN HFMs. Using a polymeric membrane as structured packing presents a thorny problem; that is, the carbon-based polymer membrane cannot withstand conditions routinely present in a refinery’s process. On the one hand, the commercially available polymers, such as PS, PES and PVDF, cannot withstand even strong solvents such as acetone and toluene, let alone DMF or DMAc. On the other hand, polymers cannot withstand high temperature, which is very common in distillation. For example, the PES and PS membranes will not be dissolved in acetic acid; however, porous morphology will change to nonporous structure (like the PVDF, Figure 3) when solvent is heated to the boiling point (118 °C). Huge mass transfer resistance sometimes makes membranes useless. Even worse, polymers will disintegrate at higher temperature. For this reason, DMAc
was selected first for tests of the thermal and chemical stability of membranes because it is a somewhat strong solvent applied in industry. In our experiments, the original PAN membrane was dissolved in boiling DMAc (166 °C) within only a few minutes, whereas the heat-treated membranes were not found to be dissolved even after 24 h. As shown in Figure 7, there is
Figure 7. Effect of boiling DMAc exposure on different heat-treated membranes: (a) heat treatment temperature; (b) heat treatment time.
only a little swelling of membrane (extension of length and weight, 250 °C for obtaining HFM packings with excellent thermal and chemical stability. In Table 2, Figure 10. Mechanism of HFM heat treatment predicting excellent thermal and solvent resistance except weak mechanical properties.
Table 2. Effect of Boiling Solvent Exposure on Membranes length (%)
a
solvent
PAN-250-6
DMAc IPA EtOH Ace Tol Cyh
5.4 0.0 0.5 1.7 −4.5 0.2
± ± ± ± ± ±
0.8 0.1 0.2 0.4 0.5 0.1
weight (%) PAN
×a −8.3 2.2 −5.9 −12.5 −2.2
± ± ± ± ±
PAN-250-6 0.5 0.3 0.9 0.4 0.4
−5.2 −5.6 −3.3 −1.9 −2.2 −2.0
± ± ± ± ± ±
0.3 0.3 0.2 0.2 0.3 0.3
understanding of the change of mechanical strength and enhancement of stability. As we know, polymer chains are randomly arranged and intertwined, which gives polymers good flexibility. When PAN membrane is heated, the intertwining chains will slacken first and then contract and rearrange to an ordered structure, which will lessen flexibility drastically. For PAN HFMs, when the heating temperature reaches 200 °C, the elevated load and tensile stress at break can be caused by the counterforce on strain, which is composed by intertwining chains and cross-linking bonds as the incomplete intramolecular and intermolecular reaction. As the temperature continues to rise (for example, >250 °C), reactions extend to the whole chain segment rapidly. Then the counterforce on strain could be provided only by the intermolecular band (−CNC−), which is not a stable fused ring sequence; meanwhile, the higher temperature treatment makes polymer structures with fewer single bonds. As a result, the membrane tends to be fragile and its mechanical properties are weakened drastically. However, the intramolecular and intermolecular reactions make polymer chains stiff and cross-linked, which brings excellent thermal and solvent resistance. The decreased elongation at breakage is the result of the higher cross-link density, fewer intertwining chains, and single bonds for higher temperature treatment. 4.4. Distillation in Hollow Fibers. 4.4.1. HFM Distillation Performance. The characteristics of heat-treated HFM structured packing were analyzed in the experiment of distillation. The IPA concentrations in the distillate at different vapor velocities are shown in Figure 11. Great enrichment of IPA concentrations can be achieved in a small column of 13 cm effective length in distillation. The highest distillation concentration obtained was 55% (mol/mol) for module PAN compared to 53% (mol/mol) for module PAN-250-6. In
PAN × −30.0 −36.5 −29.2 −26.7 −6.6
± ± ± ± ±
1.5 1.3 1.3 1.1 0.7
× indicates the membrane was dissolved by solvent.
another five types of solvents including ethanol (78 °C), isopropanol (82 °C), acetone (57 °C), toluene (111 °C), and hexamethylene (81 °C) were selected in the tests of stability for exploiting wider applications. Figure 9 reveals the TGA results of original and PAN-250-6 membranes. From these data, it can
Figure 9. TGA of PAN and PAN-250-6 membrane. 6498
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Figure 12. Experimental and predicted overall mass transfer coefficients of two kinds of modules: E, experimental overall mass transfer coefficients; N, predicted overall mass transfer coefficients according to nonwetted mode; W, predicted overall mass transfer coefficients according to wetted mode.
Figure 11. HTU and distillate concentration versus vapor velocity.
addition, as vapor velocity decreased, the distillate concentration increased; this result was consistent with previous studies.4,5 This is because the lower vapor velocity lengthened the contact time of the two fluids through the pores of the membrane, which caused a better and more complete mass transfer process. However, the impact of the vapor velocity on the distillate concentration was not as remarkable as in previous studies.5,6 Because two modules were employed in series, the vapor can flow from the top of the below module and then into the bottom of the above module, which was equivalent to the module with baffles, and can increase the extent of turbulence. Therefore, the slope of the linear relationship was not remarkable. It can also be found from this figure that the distillate concentrations of module PAN were slightly larger than that of the module PAN-250-6. Because the do and di of the PAN-250-6 membrane become smaller, the mass transfer interface decreased and then turned to lower separation efficiency. The height of a transfer unit is also plotted versus vapor velocity in Figure 11. The HTU increased with vapor velocity, and an attractive low value of HTU of
