Oldershaw column

Perfluorobutyric acid [375-22-4] M 214.0, m -17.5", b 120"/735mm, d 1.651, n 1.295, -0.17. Fractionally distd twice in an Oldershaw column with an automatic vapour-dividing head, the first distn in the presence of cone H2SO4 as a drying agent. 

Oldershaw column. An all-glass perforated-plate column. The plates are sealed into a tube, each plate being equipped with a baffle to direct the flow of reflux liquid, and a raised outlet which maintains a definite liquid level on the plate and also serves as a drain on to the next lower plate [see Oldershaw Ind Eng Chem (Anal Ed) 11 265 1941]. 

Fractionally distd twice in an Oldershaw column with an automatic vapour-dividing head, the first distn in the presence of cone H2SO4 as a drying agent. 

Scale-up from a Pilot- or Bench-Scale Column This is a very common scale-up. No reduction in efficiency on scale-up is expected as long as several precautions are observed. These precautions, generally relevant to pilot- or bench-scale columns, are spelled out with specific reference to the Oldershaw column. 

Scale-up from Oldershaw Columns One laboratory-scale device that found wide application in efficiency investigations is the Oldershaw column [Fig. 14-44, Oldershaw, Ind. Eng. Chem. Anal. Ed. 13, 265 (1941)]. This column is available from a number of laboratory supply houses and can be constructed from glass for atmospheric operation or from metal for higher pressures. Typical column diameters are 25 to 100 mm (1 to 4 in), with tray spacing the same as the column diameter. 

Fair, Null, and Bolles [Ind. Eng. Chem. Process Des. Dev. 22, 53 (1983)] found that efficiency measurements in Oldershaw columns closely approach the point efficiencies [Eq. (14-133)] measured in... 

FIG. 14-44 An Oldershaw column. (From H. Z. Kister, Distillation Design, copyright 1992 by McGraw-Hill reprinted by permission.)... 

A mixing model can be used to convert the Oldershaw point efficiencies to overall column efficiencies. This enhances the commercial column efficiency estimates. A conservative approach suggested by Fair et al. is to apply the Oldershaw column efficiency as the estimate for the overall column efficiency of the commercial column, taking no credit for the greater plug-flow character upon scale-up. The author prefers this conservative approach, considering the poor reliability of mixing models. 

Previous work with Oldershaw columns [Ellis, Barker, and Contractor, Trans. Instn. Chem. Engnrs. 38, 21 (I960)], spells an additional note of caution. Cellular (i.e., wall-supported) foam may form in pilot or Oldershaw columns, but is rare in commercial columns. For a... 

Heat losses are a major issue in pilot and Oldershaw columns and can lead to optimistic scale-up. Special precautions are needed to keep these at a minimum. Vacuum jackets with viewing ports are commonly used. 

Uses of Oldershaw columns to less conventional systems and applications were described by Fair, Reeves, and Seibert [Topical Conference on Distillation, AIChE Spring Meeting, New Orleans, p. 27 (March 10-14, 2002)]. The applications described include scale-up in the absence of good VLE, steam stripping efficiencies, individual component efficiencies in multicomponent distillation, determining component behavior in azeotropic separation, and foam testing. 

GC-Mass Spectrometry. The ether extracts were concentrated using an Oldershaw column with 30 theoretical plates followed by a 200-plate spinning band distillation apparatus (Kontes Glass Co., Vineland,... 

Previous work with Oldershaw columns (209-211), however, spells a note of caution to Fair et al. s conclusion. For a fixed system, higher Oldershaw column efficiencies were measured under cellular foam conditions than under froth conditions. For this reason, Gerster (212) warned that when cellular foam can form, scaleup from an Oldershaw column may be dangerous. The conclusions presented by Fair et al. (208) do not extend to Oldershaw columns operating in the Cellular foam regime. Other considerations for scaleup from pilot columns (above) may also be important. The scaleup procedure recommended by Fair et al. (208) is... 

Establish the Oldershaw column operation at 60 percent of flood. 

If good VLE data are available, run the Oldershaw column at total reflux and calculate efficiency, Obtain overall Oldershaw column efficiency and assume it is the same as the commercial column point efficiency. Use a mixing model to calculate the overall column efficiency for the commercial column, A conservative alternative is to assume that overall commercial column efficiency is the same as overall Oldershaw column efficiency,... 

If good VLE data are not available, vary reflux ratio and find by test the combination of reflux and stages that will give the desired separation. Assume that a commercial column with the same number of trays and operating at the Bame reflux ratio will give the same separation as the Oldershaw column. The number of plates thus calculated can sometimes be reduced by estimating the efficiency enhancement from point to column efficiency. 

Some limitations to this procedure are mentioned above and in Ref. 208. In addition, it has been reported that reflux ratio may have a marked effect on Oldershaw column efficiency (213) and this variation must be studied carefully, especially if measurements are not conducted at total reflux. Also note that the formation of the cellular flow regime is composition-dependent (116,206), and it may occur under some conditions, but not under others. 

Direct Scale-Up of iMboratory Distillation Efficiency Measurements It has been found by Fair, Null, and Bolles [Ind. Eng. Chem. Process Des. Dev., 22,53 (1983)] that efficiency measurements in 25- and 50-mm- (1- and 2-in-) diameter laboratory Oldershaw columns closely approach the point efficiencies [Eq. (14-129)] measured in large sieve-plate columns. A representative comparison of scales of operation is shown in Fig. 14-37. Note that in order to achieve agreement between efficiencies it is necessary to ensure that (1) the systems being distilled are the same, (2) comparison is made at the same relative approach to the flood point, (3) operation is at total reflux, and (4) a standard Oldershaw device (a small perforated-plate column with downcomers) is used in the laboratory experimentation. Fair et al. made careful comparisons for several systems, utilizing as large-scale information the published efficiency studies of Fractionation Research, Inc. 

The checkers used a 1-m. vacuum-jacketed Oldershaw column, with a 2 1 reflux ratio. 

J.R. Fair, B.E. Reeves and A.F. Seibert, The Oldershaw column Useful for solving distillation problems, Proc. AIChE Meeting, New Orleans, March... 

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