Communication pubs.acs.org/jchemeduc
An Efficient “Brush” Packing for Fractional Distillation J. Michael Robinson* and Debra L. Williams Chemistry Department, The University of Texas of the Permian Basin, Odessa, Texas 79762, United States ABSTRACT: A novel packing material has been developed for fractional distillation: a spiral-wound, high-surface-area brush. The “Texas brush” packing provides exceptionally easy use and consistency. Fractions collected for the classical boiling point versus volume plot were analyzed by 1H NMR for absolute identification of components and for quantitation. The 1/2 in. brush has a 2.3 in. HETP (height equivalent of a theoretical plate) at a reflux ratio of 22.
KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Hands-On Learning/Manipulatives, Instrumental Methods, Laboratory Equipment/Apparatus, Separation Science small improvement is very desirable because “the degree of separation of two components by fractional distillation under conditions of high reflux varies logarithmically, and not linearly, with the number of theoretical plates.”10 Therefore, more bristles/turn with more surface area was eventually achieved with an X-shaped Nylon 612 fiber. The insets in Figure 1 show the high density of bristles and the X-shaped tips. A stainless steel wire core precludes corrosion. For school color reasons, an orange colored fiber was chosen. The Texas brush is now available from the Justman Brush Co.11 The “brush” simply slides into a fractionating column. With standard taper 14/20 semi-micro ground glass kits, the columns are about 10 in. long and 1/2 in. i.d. with enough space for a 7 in. length of brush. The brush is held snugly since the brush’s o.d. is the same as the column i.d.; the slight tension on the fibers provides resistance to movement. With a larger diameter column, the periphery of a single brush may be too spacious and thus less efficient. However, in a 1 in. i.d. column, a parallel array of five 1/2 in. diameter Texas brushes is useful.
F
ractional distillation experiments are performed at many institutions in introductory organic chemistry labs. Fractions have been analyzed by index of refraction1−3 or gas chromatography,4−6 usually because of simplicity, speed, or availability. Fractional distillation efficiency has been studied with a variety of column sizes and types of packing materials.7 While the HETP (height equivalent of a theoretical plate) varies with column dimensions, mixture components, velocity, and several other factors, the surface area of the packing is the most important factor. Small diameter glass or wire helices gave the best results and the HETP averaged about 0.40−0.47 TP/ in. (2.1−2.5 in/TP). Other papers generally confirmed these findings, but did not discuss both the packing size and the column support.8 Interestingly, there is a dearth of discussion about how each packing type is supported, a factor of significant practical importance. A typical lab text states that “If a condenser is packed with glass beads, ..., the packing must be held in place by inserting a small plug of stainless steel sponge into the bottom of the condenser.” The impact of this “plug” was not discussed.9 Ironically, use of the word “plug” connotes the exact opposite impression desired by the experimenter. The “plug” often becomes too tight and causes flooding. Alternatively, the entire column may be packed with copper ribbon or steel sponge, but this is problematic because these are inserted randomly and lack consistency. There remained a need to develop a facile and consistent packing for fractional distillations that solves these problems; one that would provide high surface area and have sufficient free space with little chance of flooding.
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FRACTIONAL DISTILLATION The components of a cyclohexane/toluene mixture (20 mL each) may be tentatively identified by initial and final boiling point (bp) even if little separation occurs as shown in Figure 2 for a simple distillation. This mixture has a nominal 30 °C bp difference, 80.7 °C, and 110.6 °C, respectively. A typical lab text9 has a table or liquid−vapor diagram showing that ∼3 TP are needed to separate a 1/1 mixture that boils ∼30 °C apart. In distillation, separation means to obtain a substantial volume of each component with high purity, that is, >95%. The degree of separation actually achieved depends upon the reflux rate and
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BRUSH PACKING Initially, brush packing was used with solid cylindrical bristles with about 8−10 turns/in., analogous to a common brush used to wash glassware. It worked surprisingly well. However, even a © 2014 American Chemical Society and Division of Chemical Education, Inc.
Published: January 27, 2014 457
dx.doi.org/10.1021/ed400515n | J. Chem. Educ. 2014, 91, 457−460
Journal of Chemical Education
Communication
reasonably consistent volume and provides enough sample for several detailed analyses for comparisons of the simple versus fractional distillation. The relative volatility12 is considered ideal and is simply calculated using the mole fractions of this liquid sample. Since a flat bp plateau might be coincidental for certain mixtures,1 a separate analysis should always be performed. Fractions collected in these experiments were analyzed for both identity and purity by 1H NMR. The brush packing afforded a distillate that was ≥99% purity cyclohexane for 80% (16.1 mL) of the possible volume. The final portion of distillate was ≥99.8% toluene. With efficient packing, it is clear from mole fraction data shown in Figure 3 that a sizable volume of high purity, low bp component can indeed be separated.
Figure 1. Distillation apparatus with orange “brush packing”: (right inset) the brush that goes into the reflux column and (left inset) an enhanced image of the brush, showing the cross section of the individual bristles. Figure 3. Distillation results by NMR analyses: mole fraction (χ) of cyclohexane of distillation fractions from Figure 2.
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HETP COMPARISONS The number of TP was calculated using Fenske’s equation from the mole fractions of the first 0.5 mL sample of the distillates. How does the number of TP of the brush compare with other distillation columns and packing media? Pavia describes a Vigreaux column as “not very efficient”.9 Contrary to Fenske’s reports,7,10 Campbell8 stated that “hold-up becomes a serious problem with helices” and that “glass beads appear to be superior to other packing in most respects”. Therefore, 5 and 3 mm glass beads were included for comparison. The 3 mm beads were almost twice as effective as the 5 mm beads but with less interstitial space have an increased danger of flooding. An aluminum screen supported the beads so no “plug” could occur. Table 1 compares these column efficiencies. The 7 in. Nylon Texas Orange brush fractional distillation delivered ideal results with 4.11 TP total. An HETP of 2.3 in./
Figure 2. Temperature versus volume comparison of simple versus fractional distillation with a 7 in. Texas brush using 20 mL each of cyclohexane and toluene.
the number of TP in the column. A slow warm-up ensures a gradual climb of the reflux ring up through the column and provides the best results with a high reflux to distillate ratio. The reflux rate was ∼130 drops/min and distillate was collected at ∼6 drops/min for a ratio of 22. With temperature versus volume plots of distillations, as shown in Figure 2, the impact of the fractional distillation on separation is visually apparent. The initial 1/2 mL of distillate of both simple and fractional distillations were collected accurately to calculate the TP differences. Although the first few drops give a better number, this small volume (1.25%) of the total charge provides a
Table 1. HETP Comparisons
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Distillation Type
Packing/in.
TP System
TP Column
HETP/ (in./TP)
Simple Empty column Vigreaux 5 mm glass bead 3 mm glass bead Brush, “Texas”
none 7 5 5 5 7
1 1.42 1.60 1.74 2.35 4.11
none 0.42 0.60 0.74 1.35 3.11
16.9 8.4 6.7 3.7 2.3
dx.doi.org/10.1021/ed400515n | J. Chem. Educ. 2014, 91, 457−460
Journal of Chemical Education
Communication
equilibrium stages provided.”13 The Texas brush seems to preclude channeling compared to glass helices or small glass beads, which is particularly important in nonheated columns. The central drain (wire) may also assist a small holdup. It is known “that the sharpness of separation obtainable in a batch distillation is approximately a linear function of charge to holdup”.14 The small holdup by the Texas brush thus requires fewer TP to obtain a sharp separation. The results from the simple distillation clearly show that it is not sufficient to separate this mixture (Δ 30 °C). In contrast, this novel Texas brush packing for fractional distillation allows a successful separation of a binary mixture (1:1) of two organic compounds that differ in boiling point by ∼30 °C. The techniques and plots presented allow analysis of similar (near ideal) mixtures as unknowns, if so desired by an instructor. Suitable binary and ternary mixtures are mentioned in the references cited. By incorporating this simple, efficient, and inexpensive “Texas brush” packing for undergraduate laboratory fractional distillations, each student has an efficient column with a rapid equilibrium design that ensures a successful outcome and increased understanding.
TP is within the best range reported for other media. This unique packing is inexpensive, readily available, and fairly inert (Nylon and stainless steel). It achieves not only the most important goals of a spatially consistent and high-surface-area packing with sufficient free space to preclude flooding but also is self-supporting, which avoids the notorious “plug” support.
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CLASS RESULTS In fall 2012 and 2013, 7 in. and 5 in. Texas orange brushes were used as packing for the entire organic chemistry lab course (3 sections, 18−20 students each). The 7 in. columns performed well, as expected, giving students a sharp step-shaped bp plot and 12−17 mL fractions were collected with a 2−3 degree bp range of cyclohexane from a starting 20/20 mL mixture of cyclohexane/toluene. Students performed a simple distillation before recycling the entire distillate, adding the fractionation column and performing the second distillation to show the dramatic difference in the two-line plot of temperature versus volume data. High-quality results were achieved similar to the development data (Figures 2 and 3) even though students were hurried and most did not ensure a high reflux ratio. Shorter 5 in. columns must be started with a lower initial power setting and were not quite as efficient as our development work indicated. A few less successful experiences were when (1) water from wet glassware caused competing (cloudy) azeotropic distillation; (2) loose fitting heating mantles were problematic for good, consistent heat transfer; and (3) when one brush was pushed into the column indentions, rather than just above them, causing a tightly filled zone that resulting in flooding. Small mechanical and evaporative losses of components do afford differences in total volumes, mole fraction, and so forth, when the distillate is recycled. Recycling is an economic choice for large classroom use. Fresh mixtures give better results for the second fractional distillation. Recycled distillates may be used in subsequent laboratories for years but rebalanced to the 1/1 ratio. Both distillations were complete within the 4.5 h lab. A 30 mL total of the mixture was used for one lab in 2013 and did save some lab time. The only hazard is to remove the hot mantle between distillations so that any potential spill in placing a fresh 30−40 mL portion of mixture into the flask for the second distillation cannot contact the hot mantle and precludes any flash boil or fire. The brush also precluded any safety problem of spilled glass beads rolling around the lab floor. The detailed NMR analyses for mole fraction and HETP calculations are based on both the NMR of the initial mixture and upon the first 0.5 mL from both the simple and the fractional distillations. A plastic 1.5 mL snap-cap microcentrifuge tube is a convenient means to capture essentially equivolume 0.5 mL samples. The 100 mL distillation flask provides space to allow any foaming to not interfere with the column. This size mantle is usually never overpowered like the 50 mL mantles and thus lasts longer.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The “Texas brush” was developed through trials of several brush samples with the assistance of John Matthews of the Justman Brush Company. The data collected and graphs presented herein were done by a student (Debra Williams, coauthor) for a senior research project. Photography was provided by Roy Williams, an undergraduate at UTPB. This research was supported by a Robert A. Welch Foundation Chemistry Department Research Grant AW-0013.
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REFERENCES
(1) Tongberg, C. O.; Quiggle, D.; Fenske, M. R. Efficient Small-Scale Fractionating Equipment. Ind. Eng. Chem. 1934, 26, 1213−1217. (2) Buck, A. C. Efficiency of Fractional Distillation Columns. J. Chem. Educ. 1944, 21, 475−476. (3) Schwartz, M. H. Microscale distillation − Calculations and comparisons. J. Chem. Educ. 1992, 69, A127−A128. (4) Ault, A. Fractionating Column Efficiency. J. Chem. Educ. 1964, 41, 432−433. (5) Thompson, E. M.; Almy, J. Separation and IR Analysis of a Mixture of Organic Compounds. J. Chem. Educ. 1982, 59, 617. (6) Donahue, C. J. Fractional Distillation and GC Analysis of Hydrocarbon Mixtures. J. Chem. Educ. 2002, 79, 721−723. (7) Fenske, M. R.; Tongberg, C. O.; Quiggle, D. Compositrion of Straight-Run Pennsylvania Gasoline. Ind. Eng. Chem. 1932, 24, 408− 418; Packing Materials for Fractionating Columns. Ind Eng. Chem. 1934, 26, 1169−1177. (8) Campbell, R. D. Fractional Distillation. J. Chem. Educ. 1962, 39, 348−353. (9) Pavia, D. L., Lampman, G. M.; Kriz, G. S.; Engel, R. G. A Small Scale Approach to Organic Laboratory Technique, 3rd ed.; Cengage: New York, 2011; pp 719−748. (10) Fenske, M. R.; Tongberg, C. O.; Quiggle, D.; Cryder, D. S. Fractional Distillation Columns with One Hundred Theoretical Plates. Ind. Eng. Chem. 1936, 28, 644−645.
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CONCLUSION Many factors determine the degree of success in separating and identifying binary mixtures using a fractional distillation. Although discussing a continuous process, in Distillation Principles and Design Procedures, R. J. Hengstebeck’s statement is also applicable to a batch distillation: “The sharpness of any separation depends upon the relative volatilities of the feed components, the amount of reflux employed and the number of 459
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(11) Justman Brush Company, Omaha, NE; part number 29002072, http://www.justmanbrush.com (accessed Jan 2014). (12) Shinskey, F. G. Distillation Control: For Productivity and Energy Conservation, 2nd ed.; McGraw-Hill: New York, 1984; pp 70−76. (13) Hengstebeck, R. J. DistillationPrinciple and Design Procedures.: Reinhold Publishing Corp: New York, 1961; p 80. (14) Daniels, F.; Williams, J. W.; Bender, P.; Alberty, R. A.; Cornwell, C. D. Experimental Physical Chemistry, 6th ed.; McGraw-Hill: New York, 1962; p 450.
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