Improved Performance in Perforated Plate Columns G . M. Cooke Research Department, Imperial Oil Enterprises, Ltd., Sarnia Ontario, Canada A new design of Oldershaw tray makes possible a laboratory fractionator of increased capacity and greater efficiency. Data obtained at the Sarnia laboratories are compared with data in the literature. These data place the perforated plate tower in a position above the particle type packing but below the recently introduced knitted wire mesh packing called “Goodloe” for which some confirmatory data from Sarnia labs are also included.
DATAIN THE LITERATURE on the subject of laboratory distillation are presented in a variety of ways making comparison of data inconvenient. In this report, the practical laboratory interest is being emphasized rather than the theoretical side, and where possible, multiple scales are included in the charts to facilitate comparison. It deals only with high speed packings able to carry a boilup of over 750 cc/hr/cm*. Emphasis is placed on significant distilling rates-Le., 75 or more of maximum boilup rate since time-saving in the laboratory is considered to be more important than a small gain in efficiency. Boilup refers to the carrying capacity of the packing in the tower, the maximum rate being the point of incipient flooding. The following relationships were studied using the conventional n-heptane-methylcyclohexane binary a t atmospheric pressure only. 1 . Efficiency in theoretical plates cs. boilup rate. HETP cs. G, the transfer rate in cc/’hr/cm* of tower cross section at total reflux and a t finite reflux ratios. 2. Pressure drop cs. boilup rate. A p in mm Hg per plate or per foot of packed height cs. G above. 3. Holdup cs. boilup rate. Dynamic holdup as per cent of tower volume cs. G above. 4. Maximum boilup, G,,,. EXPERIMENTAL
The efficiencies were measured using the method of Fenske ( I ) and the published data for this mixture ( 2 , 3). No attempt was made to evaluate efficiencies under vacuum, partly because perforated tray towers are not well suited to vacuum operation and partly because a comparison a t atmospheric pressure is satisfactory for most applications. Only the popular n-heptane methylcyclohexane binary was employed in approximately 50: 50 mixture because it is ideal for measurements of efficiency of this level. The test liquids were research grade and percolated through silica gel until free of aromatics. Compositions of the overhead and bottoms product were determined on a 5-place refractometer. Equilibrium was confirmed before samples were taken for the record and frequent checks were made of the test mixture by ultraviolet absorption a t 268 mp t o ensure that no significant concentration of aromatics was present. The effects of tray spacing, hole diameter, weir height, and
Figure 1. Single perforated plate other parameters were not investigated; see (4-6). Static holdup is not significant at viscosities corresponding to this test mixture. The new design of perforated plate was developed by H. S. Martin and Son, 1916 Greenleaf St., Evanston, Ill., using a technique for drilling that makes accurately sized holes, precisely positioned on the plate. This results in more uniformity from plate to plate and a greater number of holes per plate. Thus, boilup capacity is increased about 1 5 % with a n increase of about 10% in efficiency. Figure 1 is a photograph of a single plate made of borosilicate glass.
(4) C. F. Oldershaw, IND. ENG.CHEWANAL.ED.,13,265 (1941). ( 5 ) C. L. Ulmholtz and M. Van Winkle, Znd. Etig. Clzem., 49, 226 ( 1957). (6) D. P. Jones and M. Van Winkle, Zbid., p. 232.
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ANALYTICAL CHEMISTRY
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0
286
I
cc/hr/cmz
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(I) M. R. Fenske, Znd. Eng. Chem., 24, 482 (1932). (2) E. C. Bromiley and D. Quiggle, Ibid., 26, 1136 (1934). (3) H. A. Reatty and G. Calingaert, Ibid., p. 504.
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Figure 3. E:fficiency perforated plates
Figure 6 . Pressure drop perforated plates
Total reflux 24-tray, 100.mm diameter, hlK I1 tower See Figure 2 for legend
Total reflux 24-tray, 100-mm diameter, M K I1 tower See Figure 2 for legend
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Figure 4. EXiciency perforated plates At finite reflux ratios 24-tray, 50-mm diameter, M K I1 tower See Figure 2 for legend RESULTS AND DISCUSSION
BOILUP cc/hr/cm2
Figure 5. Prc:ssure drop perforated plates Total reflua 24-tray, 50.,mm diameter, MK I1 tower See Figure 2 for legend
Apparatus. The twc towers tested had an inside diameter of 2 inches and 4 inches, respectively, and contained 24 trays each. They were buili. into MK I1 fractionator design (7) which has no neck joint below the reflux divider in order to minimize heat losses iii the critical area. Heat transfer of this device was reported at 0.023 cal/cm2/min/"C without a heat compensating jacket. The liquid dividing valve was calibrated using the test mixture and reflux ratios were corrected where necessary. The comparative data for protruded stainless steel and knitted wire mesh p a d i n g s were obtained on towers of the same design and using the same technique. (7) G. M. Cooke and €1. G. Jameson, ANAL.CHEM.,27, 1798 (1955).
Observations. The measurements of efficiency at total reflux made on the 2-inch diameter perforated plate tower are shown in Figure 2; literature data (8) for a 1-inch tower are shown for comparison. There appear to be no data in the literature on larger diameter towers. The gain in efficiency is approximately 10 for the new design over the old. Note that the maximum efficiency occurs at about 50% of full boilup rather than at 70% as reported in (8) and shown in Figure 2. In Figure 3, the data for a 100-mm tower show a peak efficiency at about 80 % of full boilup. Figure 4 shows the data obtained to illustrate the effect of reflux ratio on efficiency at moderately high rates. The reflux ratios chosen are those useful with a tower rating about 15 theoretical plates at total reflux. Higher ratios gain little efficiency at a high cost in time consumption in such a tower and were not studied. The relationship between boilup and pressure drop as shown in Figures 5 and 6 indicates no change in pressure drop compared to earlier designs. The data on dynamic or operating holdup were obtained separately, and are illustrated in Figure 7. Holdup appears to behave erratically at rates below 50% of maximum boilup. This term refers to the total amount of reflux held in the tower and condenser while operating. It is not possible to make a categorical statement as to which type of packing is best because of the several variables involved and the many applications to which a still can be put. Therefore in order to compare the new perforated trays with other high speed packings, data were assembled from the ~
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(8) F. C. Collins and V. Lantz, Ind. Eng. Chem., 18,673 (1946). VOL. 39,
NO. 3 ,
MARCH 1967
e
287
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Figure 8. Efficiency-Goodloe packed MK I1 towers At total reflux. See Figure 2 for legend 200
400
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Figure 10. Pressure drop-Goodloe 6s. Cannon packing See Figure 2 for legend W I
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Figure 9. Efficiency-Cannon packed MK I1 towers At total reflux. See Figure 2 for legend
$ 108-
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literature for Cannon protruded stainless steel packing (9-12) and from our own recent experiments on Goodloe knitted wire mesh. Sarnia data are compared with literature data in Figures 8 and 9 to illustrate the performance of these packings. Table I lists a direct comparison of the major variables. It is evident from the table and figures that Goodloe packing is slightly inferior in HETP to Cannon packing but much superior in boilup capacity. While a 110-mm i.d. tower that would develop 15 theoretical plates at 800 cc/hr/cm2 (at total (9) A. C. Heinlein, R. E. Manning, and M. R. Cannon, Chem. Eng. Piogr., 41, 344 (1951). 110) Scientific Development Co., Bull. 23, P. 0. Box 795, State . college, Pa. (11) M. R. Cannon, Ind. Eng. Chem., 41,953 (1949). (12) L. B. Bragg, Zbid., 49, 1062 (1957).
Table I.
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reflux) would be about 64 inches high packed with Goodloe, it would only be 46 inches high packed with 0.24-inch Cannon packing. However, Goodloe packing could be operated a t over 50% higher boilup rate (1200 cc/hr/cm*) at a loss of only 20% in efficiency while Cannon packing cannot operate above 900 cc/hr/cm2. By comparison, the tray efficiency of a perforated plate tower would be about 67 % a t this rate and it would be about 90 inches high. Its flood point is likewise less than 1000 cc/hr/cm2.
High Speed Fractionation Packings
Pressure dropa mm Hg per cc/hr/cm2 HETP,b in. mm/ft theor plate 2.2 0.37 2.0 1300 Goodloe meshb 1.9 0.32 2.1 1300 Goodloe mesh (12) 0.40 2.0 2.4 1250 Goodloe mesh (12) 0.6 0.30 6.0 1600 Goodloe meshb 1,550 4.8 670 0.61 Perf’d plates (8) 1.20 3.0” 4.8 875 Perf’d platesb 1.66 3.5 5.P 950 Perf’d platesb 2.0 0.23 1.4 850 Cannon 0.16* 2.15 0.36 2.0 50 850 0.24 (10) 0.27 1.4 2.3 850 100 0.24 (10) a At 75 of G,,. b This report. Redistributor plates not used with Goodloe. c A pseudo-HETP assuming tray spacing equals diameter. Tower diam, mm 25 25 49 110 28 50 100 25
288
ANALYTICAL CHEMISTRY
Holdupa
Gmm,
Packed vol,
...
16.7 20.6 9.7 11.8 6.0 ... 17.5 17.5b 18.0h
ml per theor plate
... 4.4 26 100 2.8 9.0 ... 3.4 18.5h 86b
Goodloe packing appears to have a low pressure drop (Figure 10). Present observations on pressure drop, however, are scattered and unsatisfactory. The reason, in view of the very precise data published for this material (12), is not evident at present. The sizing and fitting of the packing bundles into the tower is known to present a problem, however, and even affects the efficiency. Holdup is surprisingly low for Goodloe packing on the basis of Figure 1 1 . It ranges from 8 to 12% of the packed volume at useful rates of operation (1000-1400 cc/hr/cm2). Cannon packing measures 17-21 % holdup over its range of 60&800 cc/hr/cm2. It is difficult to compare perforated plates o n this basis but a 15 theoretical plate tower (24 trays) ranges from a holdup cf 12 in 1-inch diameter to 7 for 2inch and 6 z for 4-inch. These towers are, however, physically larger than hire mesh or particle packed towers and so the total volume of holdup is slightly higher than it is for the others, based on a given charge-to-holdup ratio. Reference to Figure 12, will show the comparison on a different basis. Calculated from the foregoing data the dynamic holdup in milliliters per theoretical plate is shown to be lowest for plate type columns and highest for wire mesh columns.
z
LE
z
CONCLUSION Where the cost of doing many distillations of moderate efficiency is concerned, perforated plates have adequate capacity and are chemically inert but usually unsatisfactory at low pressures. Protruded stainless steel and especially knitted wire mesh are Bster and operate well under vacuum but, being made of m~:tal, are not inert and are somewhat
TOWER
ID
INCHES
Figure 12. Dynamic holdup Comparison of various packings at 7 5 z Gmax
difficult to clean. If physical height is a limitation, refer to Table I. Where the prime consideration is high efficiency and cost of operation is unimportant, one of the several more efficient packings should be chosen from the literature. Distilling times will be more than doubled.
RECEIVEDfor review September 21, December 16,1966.
1966.
Accepted
Gas-Sol id-Liiq uid Chromatography: Dispersed Metal Phthailocyanines as a Substrate Robert L. Pecsok and Eva M. Vary Department of Chemistry, University of California, Los Angeles, Calif. 90024
A dispersion of finely divided metal phthalocyanine i n silicone oil is an effective substrate for gas chromatography. The porlphyrin structure of these very stable chelates leaves positions available for additional ligands. The technique is reproducible and provides an additional variable for tailoring columns for particularly difficult separations. In this way, the advantages of gas-soliid and gas-liquid chromatography are combined. Supporting evidence for the nature of the separation is given for copper, zinc, iron(lI), cobalt, nickel, aluminum, and chromium(ll1) phthalocyanines. THEUNUSUALLY HIGH CHEMICAL and thermal stability of the metal phthalocyanines should lead to useful applications as substrates in gas chromatography. However, the chemical inertness and negligible solubility, which lead t o ideal properties as pigments, also hinder any investigations of their latent reactivities toward additional ligands. I n a spectrophotometric study Lagenback et al. ( I ) found that iron(I1) phthalocyanine can add two molecules of pyridine (1) W. Lagenback, H. Schubert, and H. Giesemann, Ann. Chem., 585,68 (1954).
o r imidazole in chloronaphthalene solution. I n a n infrared study, Terenin and Sidorov ( 2 ) noticed changes in the spectra of several metal phthalocyanines in the presence of electron donor molecules such as water, hydrogen sulfide, ammonia, and amines. Elvidge and Lever (3) reported a similar study. Owen and Kenney ( 4 ) found that hydrated AlPcOH (Pc = phthalocyanine) loses two of its three water molecules very easily, while the third is strongly bonded, apparently in the sixth coordination position of a n octahedral structure. The use of gas chromatography to study metal complexes is already well known (5-9). Because the phthalocyanines are (2) A. N. Terenin and A. N. Sidorov, Spectrochim. Acta, S~ippl.,
1957,573. (3) J. A. Elvidge and A. B. P. Lever, J . Chem. Soc., 1961,1257. ( 4 ) J . E. Owen and M. E. Kenney, Iiiorg. Chem., 1, 331 (1962). ( 5 ) G. P. Cartoni, R. S. Lowrie, C. S. G. Phillips, and L. M. Venanzi, in “Gas Chromatography 1960,” R. P. W. Scott, ed., Butterworths, London, 1960, p. 273. (6) L. A. duPlessis, J . Gas Chromatog., 1, 6 (1963). (7) E. Gil-Av and J. Herling, J . Phys. Chem., 66, 1208 (1962). (8) C. S . G. Phillips, D. W. Barber, G. F. Tusa, and A. Verdin, J. Chem. SOC.,1959, 18. (9) L. B. Rogers and A. G. Altenau, ANAL.CHEM.,36,1726 (1964). VOL. 39, NO. 3, MARCH 1967
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