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Separation of Alcohol-Water Solutions by Distillation through Hollow Fibers Guoliang Zhang,*,†,‡ Lan Lin,‡ Qin Meng,† and Youyi Xu† College of Materials Science and Chemical Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and College of Biological and EnVironmental Engineering, Zhejiang UniVersity of Technology, Hangzhou 310014, People’s Republic of China
New results of an investigation on distillation in hollow fiber structured packing are discussed. The membranes used here for separation of methanol-, ethanol-, and 2-propanol-water solutions are nonselective coated hollow fibers, unlike those of membrane distillation. The fibers are nonselective with coating and have little resistance to mass transfer. Because liquid flows inside the lumens of the fibers and vapors flow countercurrently outside the fibers, this distillation process can avoid flooding effectively even at very high flows. As a result, when a mole fraction of alcohol equal to 0.04 was prepared as feed in all distillations, an attractive low value of the height of mass transfer unit (HTU) less than 0.1 m could be obtained at total reflux. Both the distillate concentration and the height of mass transfer unit more significantly decreased with heat rate in alcohols at low molecular weight than in those at high molecular weight. Compared with conventional structured packing, the ordinate data of the common capacity factor of hollow fiber column calculated with the Eckert version of the generalized pressure drop correlation (GPDC) were 3-5 times higher above flooding. The presented technology offered the possibility of distillation with better, more productive separations. 1. Introduction This paper continues to explore the distillation process with a new structured packing column, a new type of membrane contactors. As is well-known, conventional packed distillation columns always have some obstacles in selection of packing material and uniform distribution of fluids before they enter the packed bed. The interdependence of the two fluid phases that are contacted very closely often leads to operational problems such as emulsions, foaming, unloading, and flooding; see refs 1-3. Using a hollow fiber module is an alternative effective way to overcome these disadvantages and the relative experimental results have been proven to be promising, according to Zhang and Cussler.4,5 Unlike most packed towers, fluids that are contacted flow on opposite sides of membrane and the liquid/liquid interface forms at the mouth of each pore. Further, the hollow fibers are supposed to be nonselective by choosing suitable membranemaking and -coating materials and therefore to be different from the earlier method called “membrane distillation”.6-8 The hollow fiber distillation process shown in Figure 1 can a be potential route for faster separation, even if there is no direct commercial value now. Such a hollow fiber column has three advantages over conventional packed towers. First, it offers substantially more interfacial area than conventional approaches by nondispersive contacting via a microporous membrane. A typical interfacial area per volume is 201 ft2/ft3, almost triple the value of Sulzer Mellapak structured packing, which is only 76 ft2/ft3. Second, because liquid flows inside the fibers and vapor flows countercurrently outside the fibers, the two fluid flows are independent and can be easily kept to a uniform distribution to avoid flooding effectively even at very high flows. Moreover, the flows of liquid and vapor are now not around submerged objects and only a very small pressure drop across the membrane is required to ensure that the liquid/vapor interface * To whom correspondence should be addressed. Tel./Fax: +86(571)88320863. E-mail:
[email protected]. † Zhejiang University. ‡ Zhejiang University of Technology.
Figure 1. Schematic process for hollow fiber structured packing distillation.4 The hollow fiber module replaces the conventional packed tower used for differential distillation of alcohol-water systems. The structure of the new module tower in which liquid flows inside fibers and vapor rises countercurrently outside fibers leads to very different operational characteristics.
remains immobilized, which is derived from the sealed pores. Third, unlike traditional packed towers, no density difference between fluids is required and theoretically hollow fiber modules can be operated in any orientation. Our aim is to improve product purity and separation efficiency by using different nonselective hollow fiber membranes in
10.1021/ie061611o CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007
Ind. Eng. Chem. Res., Vol. 46, No. 23, 2007 7821 Table 1. Properties of Hollow Fiber Modules and Structured Packing module packing
size or type
void fraction,
area per volume, a (ft2/ft3)
packing factor, FP (a/3)
module 1 module 2 Sulzer Mellapak Sulzer BX Raschig rings Raschig rings
25 (no. of fibers) 50 (no. of fibers) 250Y BX 500 1 in. 1/2 in.
0.85 0.71 0.95 0.90 0.78 0.70
201 484 76 150 58 101
327 1352 89 206 179 580
different systems. We hope to maximize the mass transfer rate by optimizing our design and process operation. Here, the large interfacial area will be beneficial for making more efficient separations, according to refs 9 and 10, which means that the columns of hollow fiber structured packing will perform better, with the height of transfer unit (HTU) as small as possible. In order to do this, we will take three alcohol systems including methanol-, ethanol-, and 2-propanol-water solutions for separation experiments. We want to see whether the performance of the modules we worked with formerly can remain working well and determine how different alcohols affect the separation. 2. Theoretical Considerations The theoretical analysis is based on the mass balances and transfer unit theory basic to the operation of a conventional differential distillation column. We begin our work with the same data on estimates of vapor-liquid equilibrium as differential and staged distillation.3,11 For simplicity, the column of hollow fibers is operated at total reflux, which means the total vapor and liquid fluxes are equal in the column. The relative balances on the more volatile species in both vapor and liquid give the operating line y)x
(1)
Then we can obtain a balance on the more volatile species in the gas phase: 0 ) -VG
dy - Ka(y - y*) dz
3. Experimental Section (2)
Integrating this equation, we can easily find the number of transfer units (NTU) for our column: NTU )
∫
yl
y0
Kal dy ) y - y* VG
(3)
As usual, the NTU calculated by integration of the experimental data is a good measure of the difficulty of the distillation and shows how high the separation efficiency is. A larger value means an easier separation, and a smaller one signals a more difficult separation. Although transfer unit theory has been used for many years to design absorption columns in industry, the challenge has always been to design an appropriate height for the packed column. The height of an absorption column l is given by l ) (NTU)(HTU)
(4)
where HTU is the height of a transfer unit. In our experiment, the length of modules is set; therefore, the true value of HTU can be easily estimated by HTU ) l/NTU
be. HTU is normally a measure of an industrial tower’s or column’s separation efficiency. A still smaller HTU means a more efficient and more rapid separation process, and that is what we dream about. The technological route of a differential distillation tower structurally packed by hollow fibers is similar to that of a conventional one, as Kister described,3 but the process is very different. As noted above, we start our experiment with a very low feed concentration and the desired concentrations of distillate and bottom. The liquid and vapor flows are very independent because of the nonporous hollow fiber walls. The two-phase flow and maldistribution in a conventional process can then be effectively prevented. The liquid and vapor can be pumped very freely with almost no pressure drop coming from fluid friction, and still not be bothered by loading and flooding. Although we have little experience to guide our choice in column designing and material choosing, we think it important to choose the structure and dimension of the column’s internals, and we will specify the relative difficulty of the separation of different alcohol systems as NTU. We can analyze our experiment using the same equations as given above. After measuring the concentrations y0 and yl and calculating the NTU value, we can estimate mass transfer coefficients from the NTU and hollow fiber geometry and find if they are different in several separation systems or with empirical values of membrane contactors elsewhere.
(5)
Since the height of a transfer unit, HTU, can be defined as the height of a packed section required to accomplish a change in concentration equal to the average driving force in that section, the smaller the HTU is, the better the design should
The microporous polyether sulfone hollow fibers (Porous Media, USA, and RDCWTT, Hangzhou, China) used in this work have an inside diameter of 0.07 cm and an outside diameter of 0.11 cm. A thin layer of 5 µm of polydimethylsiloxane was coated on the membrane for stopping convection. Thus liquid flow could not pass through the wall to the vapor side, while vapor could flee out freely. From the earlier results of Zhang and Cussler,4,5 we chose 25 fibers in a glass shell with an inside diameter of 1.40 cm as the module design basis. Each module with two nozzles had an effective mass transfer length of 25 cm. The module properties are compared in Table 1 with commercial structured packingssSulzer Mellapak (Sulzer Chemtech).12 The void fractions of the hollow fiber modules are lower than those of the conventional structured packing, while the areas per volume are larger, according to Yang et al.13 and Kistler et al.14 The hollow fibers were potted with SG856 epoxy (Jinpeng Chem., Zhejiang, China) and HC 703 silicone (Xiguang, Wuxi, China). All distillation experiments were run at total reflux. In a typical experiment, all solvents including methanol, ethanol, and 2-propanol (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were of at least HPLC-UV grade and used as received, and an alcohol solution with a molar fraction of 0.04 was prepared as feed with doubly distilled water. About 800 cm3 of this solution was added to the reboiler and heated in a 350 W 98-1-C digital autocontrol heating mantle (Taisite Co., Tianjin, China). The heating rate was adjusted by a FATO SVC-500VA automatic voltage regulator and measured by a JL 4006B electric
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multimeter (Junling Co., Hangzhou, China). Then the vapor generated flew up the shell side with liquid reflux from the condenser dropping down inside the fibers concurrently (Figure 1). All flows were measured volumetrically. Solution concentrations were measured as 1 cm3 samples taken from the reboiler and from the distillate, respectively. The concentrations were determined by injecting 2 µL samples into a SF GC-1102 gas chromatograph equipped with a thermal conductivity detector (TCD) and a Super Porapak Q steel column. The carrier gas was hydrogen of 99.99% purity. Each concentration was measured at least in duplicate. To begin the experiment, we usually ran the column at a heating rate for about 1-2 h to approach steady state, when the first sample was taken. Subsequent samples were then taken at 15-30 min intervals to ensure steady state. The calculation of the number of transfer units from these data is complicated because the slope of each equilibrium line for the different alcohol systems is not constant, so m is not constant. Over the concentration range of 0.1 < x < 0.9, the vapor-liquid equilibrium fit the polynomials and a data relationship can be expressed mathematically:15 for methanol (no azeotropic point): 0.1 < x e 0.3
y* ) 0.1754 + 2.8545x - 4.0810x2
(6)
0.3 < x < 0.9
y* ) 0.4823 + 0.6679x - 0.1571x2
(7)
for ethanol (azeotropic concentration x ) 0.881): 0.17 < x e 0.33
y* ) 0.348 + 1.1821x - 1.4292x2
(8)
0.33 < x < 0.68
y* ) 0.4977 + 0.1791x - 0.2599x2
(9)
for 2-propanol (azeotropic concentration x ) 0.675): 0.13 < x e 0.33
y* ) 0.4438 + 0.5449x - 0.6984x2 (10)
0.33 < x < 0.79
y* ) 0.5678 - 0.2586x + 0.6278x2 (11)
Using these relations and eq 3, the NTUs and HTUs can be easily found for comparison with different separation systems operated in the same module. The results are given below. 4. Results and Discussion This section reports distillation separations using a structure packing of hollow fibers. We are especially interested in confirming its behavior in different alcohol systems, finding some regularity and achieving the lowest value of the height of a transfer unit as as we possibly can. As before, we can easily operate a hollow fiber at flows greater than that causing flooding, as shown in Figure 2. The relationship between the flooding gas velocity and other physical properties of the system is presented in the form of an empirical, experimental correlation for pressure drop in packed columns. This method was proposed by Sherwood in 1938 and improved by Eckert in 1970 to determine the flood point, which basically remains the standard procedure today.1,3,11 We plot the normal flow parameter on the abscissa and the common capacity factor on the ordinate. The solid curve is the empirically determined limit of flooding. For conventional packing, the gas velocity in an operating column must obviously be lower than the flooding velocity, and is often chosen as 1/2 to 80% of the predicted flooding velocity obtained from a generalized correlation. Therefore points above this solid curve cannot be reached to ensure safe operation. In Figure 2, we also show the data points of our experiments with hollow fibers in different alcohol
Figure 2. Hollow fiber distillation operate above flooding. Like conventional distillation process, the flow parameter on the abscissa is plotted vs the capacity factor on the ordinate. All the data of methanol-, ethanol-, and 2-propanol-water systems are at fluxes high above the normal flooding limit, shown as the solid line.
solutions. Without exception again, all the data points of methanol-, ethanol-, and 2-propanol-water systems are high above the limit solid curve where flooding normally occurs. The highest data are 3-5 times above the solid flooding line. Since we always ran at total reflux, these data cover only a narrow range of the abscissa in which they proceed in the order 2-propanol > ethanol > methanol. However, the results clearly show how easily hollow fiber structured packing can operate above the normal flooding point. In conventional cases, one must choose a velocity far from the flooding velocity but not so low as to require a much larger column which will consume more costly power, and lower gas-liquid velocities often lead to a nearly proportional reduction in the mass transfer rate. In our experiments, we could potentially operate at still higher or lower flow rates since vapor and liquid are in contact only across the holes of membrane walls. There is no resistance for a bubble rising through liquid, and the falling liquid is not slowed by the rising vapor, just as other membrane contactors do. That is where the hollow fiber columns always have an advantage. To compare the operational performance of a hollow fiber with normal structural packing in the distillation process, the F-factor is plotted vs the number of transfer units per meter (Figure 3). The data points here for the same hollow fiber module in different alcohol-water systems are compared with a type of Sulzer structured packing (Sulzer Chemtech), shown as the solid lines. The abscissa shows a flow parameter F, which tends to cover about the same range for different total pressures in conventional distillation, since the flooding velocity varies with FG1/2. The parameter F, generally used to characterize the performance of distillation packing and known as the F-factor (see Kister3 and the Sulzer Chemtech Product Bulletin12), is defined as F ) uGFG1/2
(12)
The data in Figure 3 show how the NTU increases as the F-factor gets smaller. This figure also shows some of the results
Ind. Eng. Chem. Res., Vol. 46, No. 23, 2007 7823
Figure 3. Operational performance of hollow fiber and normal structural packing in distillation process. The F-factor is plotted vs the number of transfer units. The data for the same hollow fiber module in different alcohol-water systems are compared with a Sulzer structured packing, shown as the solid line.
under conditions where the normal packing will flood, and the separation efficiency estimated from NTU can strongly compete with some excellent industrial structured packings such as Sulzer Mellapak. We next turn to finding out how the hollow fiber module works as flows change from high to low. The reciprocal of the number of transfer units (1/NTU) seems to be proportional to high vapor velocity for all three systems, which is similar to our former results elsewhere.4 While at low vapor velocity, the NTU keeps a relative high value for some period. This indicates that we can obtain faster separation and higher purity of alcohols in our hollow fiber column simply by reducing the flows. We can use slow flows without reducing the interfacial area just by making sure the the liquid inside fibers are full and can run continuously. Further, the NTU for lower molecular weight alcohols such as the methanol system separation increases faster than that for ethanol and 2-propanol, but at the same vapor velocity, the NTU for higher molecular weight alcohols such as ethanol and 2-propanol looks larger, which makes a more efficient separation. Since the vapor velocity in our experiments is calculated directly from the heat balance, as described by Poling et al.,16 and there is some insulation problem which may cause heat loss, especially when the countercurrent flow is slow, we use data read from a gauged cylinder and plot Figure 4 to show the change of the number of transfer units vs liquid flow more precisely. From eq 4, we have a similar form of the number of transfer units rewritten as NTU )
∫
xl
x0
[ ]
Kxa dx ) l x - x* L
(13)
Now, the results in Figure 4 meet our expectation from eq 13 very well; the reciprocal of the number of transfer units is proportional to the molar liquid flow. As above, a higher NTU value can be obtained when the liquid flow becomes slower, and the slope of the linear relationship is in the order methanol
Figure 4. Number of transfer units (NTU) vs liquid flow. The reciprocal of the number of transfer units is proportional to the molar liquid flow. Higher NTU value can be obtained when the liquid flow becomes slower. The slope of the linear relationship is in the order methanol > ethanol > 2-propanol.
Figure 5. Distillate concentration vs heating rate. As the heating rate increases, the distillate concentration at the top of the module decreases.
> ethanol > 2-propanol. This gives the overall mass transfer coefficient Kx for methanol, ethanol, and 2-propanol of 2.45 × 10-5, 5.73 × 10-5, and 5.95 × 10-5, respectively, in accordance with the same order as former results, according to Zhang and Cussler4,5 and Chung et al.17 The intercept on this plot may be dependent on the properties of flow, module configuration, and the heat loss of process. To reinforce our results, we show the variation of distillate concentration with heating rate in Figure 5. The higher distillate concentrations observed at the lower heating rate are a consequence of better mass transfer in the vapor/liquid interface, and the relative sensitivity of distillate concentration change with heating rate is methanol > ethanol > 2-propanol. As we know, there is an azeotropic point for both ethanol and 2-propanol solution, such as 2-propanol-water azeotrope forming at an 2-propanol mole fraction of 0.675, and at present we cannot break through it with our own modules in the experiments,
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operational range and to providing more significant energy savings in the near future. 5. Conclusions
Figure 6. Height of a transfer unit (HTU) vs heating rate. Because the heating rate is proportional to residence time in the column, the height of a transfer unit drops as the heating rate rises.
which means that the separation of methanol solution without azeotrope formation will be simpler and easier. Finally, the height of a transfer unit is plotted versus the heating rate in Figure 6. Because the heating rate is proportional to the residence time in the column, the height of a transfer unit drops obviously as the heating rate rises while the contacting time becomes shorter. In our experiments, a low mole fraction of alcohol as low as 0.04 was prepared as feed in all three solutions, and an attractive low value of the height of mass transfer unit (HTU) less than 0.1 m could be easily obtained at total reflux. The data in Figure 6 also show the same behavior as above: the decreasing rate of the height of mass transfer unit in the same hollow fiber module seems to change inversely with the molecular weight of alcohols. Although the above results have already shown a promising future for fast and efficient separation in hollow fibers, we hope to do better. In conventional distillation, packing always wins by high surface area per unit volume and high void fraction, so the hollow fiber can go farther with better characteristics. As before, the physical properties of the system, the liquid/gas flow, and the packing control the performance of conventional packed column. That means the geometry of packing, usually characterized as the “packing factor”, is critical for process design: see refs 3, 9, and 11. The packing factor FP is defined as FP )
a 3
(14)
To get a larger value of this packing factor, the packing in a set column should be chosen smaller because FP is roughly proportional to the packing size (see Table 1). However, even the smallest conventional packing cannot get a higher packing factor than hollow fibers whose interfacial area per volume is large. In our experiments, the same size module gives a packing factor of 327 ft-1 with 25 fibers and 1352 ft-1 with 50 fibers; the amplifier factor is very big. That means we still have a huge space to optimize our separation in hollow fibers. Although we might face some limitations and practical concerns such as materials of construction and solution handling with particulates, we look forward to more productive distillation over a wider
New results have been introduced which show the separation of methanol-, ethanol-, and 2-propanol-water solutions with distillation using hollow fiber structured packing. The membranes used here for are nonselective coated hollow fibers, unlike those of membrane distillation. The fibers are nonselective with coating and have little resistance to mass transfer. Because liquid flows inside the lumens of the fibers and vapors flows countercurrently outside the fibers, the distillation process can avoid flooding effectively even at 3-5 times higher flows than in conventional cases. With a mole fraction of alcohol as low as 0.04 prepared as feed in all three solutions, an attractive low value of the height of mass transfer unit (HTU) less than 0.1 m can be easily obtained at total reflux. Moreover, while the distillate concentration drops as the heating rate rises, just as noted before, the decreasing rate of distillate concentration and HTU in the same module seems to change inversely with the molecular weight of alcohols. Compared with conventional structured packing, the hollow fibers have a better geometry of packing and can realize a more productive distillation and will save more energy in separation. Further experiments will focus on the optimization of module structure and process design and on using more systems to validate our investigation. Acknowledgment The authors benefited from conversation with Dr. E. L. Cussler while visiting CEMS, UMN. This work was primarily funded by the Research Fund of the National Natural Science Foundation of China (Grant 20476096). Other financial support came from the Zhejiang Provincial Bureau of Science and Technology, China (Grants 2005C33040 and 2006C23067). Nomenclature a ) interfacial area per module volume (cm2/cm3) L ) molar flux of liquid (mol/s) l ) length of the hollow fiber module (m) K ) overall mass transfer coefficient based on vapor side (s/ cm) Kx ) overall mass transfer coefficient based on liquid side (s/ mol) uG ) vapor velocity (cm/s) VG ) gas velocity (cm/s) x ) mole fraction in liquid x* ) mole fraction in liquid at equilibrium y ) mole fraction in vapor y* ) mole fraction in vapor at equilibrium y0 ) vapor compositions at bottom (z ) 0) yl ) vapor compositions at top (z ) l) z ) distance from the bottom of column (m) FG ) vapor density (kg/cm3) ) void fraction of packing Literature Cited (1) Seader, J.-D.; Henley, E. J. Separation Process Principles; Wiley: New York, 1998. (2) Humphrey, J.-L.; Keller, G.-E. Separation Process Technology; McGraw-Hill: New York, 1997. (3) Kister, H.-Z. Distillation Design; McGraw-Hill: New York, 1992.
Ind. Eng. Chem. Res., Vol. 46, No. 23, 2007 7825 (4) Zhang, G.; Cussler, E.-L. Distillation in hollow fibers. AIChE J. 2003, 49, 2344-2351. (5) Zhang, G.; Cussler, E.-L. Hollow fibers as structured distillation packing. J. Membr. Sci. 2003, 215, 185-193. (6) Lawson, K.-W.; Lloyd, D.-R. Membrane Distillation. J. Membr. Sci. 2000, 124, 1-25. (7) Alan, G.; Hwang, S.-T. Hollow fiber membrane contactors. J. Membr. Sci. 1999, 159, 61-106. (8) Izquierdo-Gil, M.-A.; Jonsson, G. Factors affecting flux and ethanol separation performance in vacuum membrane distillation. J. Membr. Sci. 2003, 214, 113-130. (9) Bennet, D.-L.; Kovak, K.-W. Optimize distillation columns. Chem. Eng. Prog. 2000, 19, 84-96. (10) Kellehar, T.; Fair, J.-R. Distillation studies in a high gravity contactor. Ind. Eng. Chem. Res. 1996, 35, 4646-4655. (11) Cussler, E.-L. Diffusion; Cambridge University Press: Cambridge, 1997.
(12) Sulzer Chemtech. Structural packings for distillation and absorption. Product Bulletin 22.13.06.40-111, 00-70. (13) Yang, M.-C.; Cussler, E.-L. Designing hollow fiber contactors. AIChE J. 1986, 32, 1910-1920. (14) Kistler, K.-A.; Cussler, E.-L. Membrane modules for building ventilation. Trans. Inst. Chem. Eng. 2002, 80, 53-64. (15) Gmehling, J.; Onken, U. Vapor-Liquid Equilibrium Data Collection; DECHEMA: Frankfurt, 1977; Vol. 1, Part 1. (16) Poling, B.-E.; Prausnitz, J.-M.; O’Connell, J.-P. Gases and Liquids, 5th ed.; McGraw-Hill: New York, 2000. (17) Chung, J.-B.; Debrocher, J.-P.; Cussler, E.-L. Distillation with nanoporous or coated hollow fibers. J. Membr Sci. 2005, 257, 3-10.
ReceiVed for reView December 14, 2006 ReVised manuscript receiVed August 17, 2007 Accepted August 28, 2007 IE061611O
