INDUSTRIAL AND ENGINEERING CHEMISTRY
714 at
.In aveiage of many tests on the oxygen generato1 opeiatiiig I t s normal production rate showed the folloning steady state: Air charged, lb./hr. Flash, % Liquid rate, Ib /hr. Oxygen yield, lb./hr. Oxygen purity. wt. % Tower pressure, atm. abs.
115.0 12.5 100.6 12.4 99.5 1.5
I n order t o calculate the H.E.T.P. of the 18-inch high packed section, it was necessary t o est,imate the separation accomplished by the reboiler. T h e reboiler used was not of conventional design. T h e principle of progressive evaporation was used, and i t was estimated t h a t t,he total effect was equivalent t o 2.5 t,heoretical plates. T h e over-all effect of reboiler and packing is equivalent t o approximately 14.5 theoretical plates. T h u s the 18-inch packed section is equivalent, t o approximately 12.0 plates. This indicates a n H . E . T . P . of 1.5 inches. The vapor mass velocit,y for these conditions is G = 1960 pounds per hour per square foot, calculated on the basis of a n empty toner 2 inches in inside diameter. T h e vapor density is p = 0.420 pound per cubic foot. T h e liquid rate is 2231 pounds per hour per square foot. Therefore the rate is G/&- = 3025. CONCLUSIONS
A column packing is described suitable for pilot plant or laboratory use. It is distinguished b y lorn H.E.T.P., high rate, low
Vol. 39, No. 6
w i g h t , arid h w heat capacity. Fen- csact perforinalicc data are given because of the complexity of the liquid air system for which the packing was used and the difficulty of accurate interprttation of results in terms t,hat would be generally applicahlc. Thi, paper .by Forsythe, Stack, Wolf, and Conn (page 714: prewnt. accurate test data for benzene-ethylene dichloride mixture. ACKNOWLEDGMENT
This Lvorli \vas done in part under Contract S o . OElI-sr-260 ti(>tween Arthur D. Little, Inc., and the Office of Scientific Research and Development and in part under Contract S o . 11-265, XDC re-182 between llassachusetts Instit,ut,e of Technology and O.S.R.D., Tvhich assumes no responsibilit,y for the accuracy of the statements contained herein. T h e project was carried out under the auspices of t h e National Defense Research Committei,, Section 11.1, in connection with the general problem of developing portable oxygen-generating equipment, for the armed forces. This particular development, enjoyed the active interest and support of t,he . k r o Medical Laboratory of the -4rmy Air Forces a t Wright Field, Dayton, Ohio, and was conducted under a contract administered by S. C. Collins, Massachusetts Institute of Technology. PREBEXTED as part of the Thirteenth Annual Chemical Engineering Symposium of the Division of Industrial and Engineering Chemistry, AMERICAX CHEXICAL SOCIETY.
Performance of M c M a h o n Packing TESTS IN A 6-INCH-DIAMETER FRACTIONATING COLUMN W. L.
Forsythe, Jr.,
T. G. Stack, 1, E. Wolf,
and
A. L.
Conn
STANDARD OIL COMPANY (INDIANA), WHITING, IND.
M c M a h o n packing, a new type of column packing made of wire gauze shaped in the form of Berl saddles, has been tested with benzene-ethylene dichloride mixtures to determine its fractionation performance. In a 6-inch column, filled with 'a-inch packing to heights of approximately 5 and 9 feet, H.E.T.P. values of P to 4 inches were obtained. These may be compared with published results from Stedman packing in which H.E.T.P. values of 1 . 5 to P inches were obtained with a packed height of 3 feet. The pressure drop for M c M a h o n packing is lower than that for Stedman, which makes it particularly useful for vacuum fractionation. The maximum capacity and liquid holdup are essentially the same for the two packings. M c M a h o n packing has several mechanical advantages over Stedman since it does not require accurately machined column walls, is not restricted to one column diameter, and i s more readily installed and removed.
ECEST trends in laboratory fractionation work have required the use of highly efficient, columns having as many as one hundred theoretical plates. As a result, there has been developed avarietyof fractionating columns (4,7,10,11, I S ) , which range u p tmo2 inches in diameter and have H.E.T.P. values from 2 inches t o as low as 0.5 inch. Such fractionating columns make it possible t o obtain extremcly sharp separations with relatively short columns. For carrying out fract,ionation work on a larger scale, however, such as is required in pilot plant operations, t h e only highly efficient packing which has been available is Stedman packing. The reported H.E.T.P.'s for this packing range from 1.0 to 2.5 inchcss
R
in columns 3 to 12 inches in diameter ( 1 ) . Tests on other packing materials which are highly efficient in 2-inch and smaller sizes have shown a decrease in efficiency with increased diameter. This has discouraged their use in larger size columns. -4lthougli Stedman packing has the advantages of low H.E.T.P. and Ion- holdup per theoretical plate, its principal disadvantages are high cost and lack of flexibility because the packing can be used only in the column size for which it \vas fabricated. The high cost i.: primarily due to the careful machining which is necessary in order to meet the required t,olerances of *0.005 inch on column diameter. For these reasons the development of a dumped packing n-hich retains high efficiency in columns a s large as 6 inches in diameter is of special interest in pilot plant work. hlcRIahon (6) has described such a packing which c0nsist.s of small squares of wire cloth bent into shapes resembling Berl saddles. This typcl, termed "lIcMahon packing," has been obtained in the '/n. .ifter all leaks had been stopped, the assembly n-as cleanc’d by boiling with acetone until the liquid returning from the column was rater-\?hit?. Final traces of aceton(%irere rcmovcd hy c,vacuation. Crude ethylene dichloride \vas t h w charged t o the reboiler and overhrad material rtmioved until the refractive indices a t both the top and the bottom of the rolumn corresponded to that for pure ethylene dichloridc (n2$ = 1.4450). r2t this point the column was shut (down and drained, and then purified benzene-ethylene dichloride mixture was introduced into the ,system. Because of the cspericnces n.ith other packed volumns ( 1 , 4, T), it, vias considered ncccssary t o wet the packing in order to obtain maximum efficiency. K r t t i n g of the packing was carried out as follows: The boilup rate was increased by small increments a t 15minute intervals, and prrssure-drop readings acrors thp packed
716
Vol. 39, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
ing \vas made by collecting the liquid from the column after shutting doivn. This was accomplished b y simultaneously shutting off the valve on the liquid return t o the reboiler, opening the sample valve, and stopping the steam flow t o the reboiler. The column n as then allowed t o drain for several hours, and the total drainings \\-ere weighed. DATA ON BENZENE-ETHYLENE DICHLORIDE
I
I.o
I
I.5
2.0 &X
Figure 2.
The data obtained in the tests with benzene-thylene dichloride mixturrs are presented in Table I. K i t h the exception of the one holdup determination, each result reported was checked by a second set of data taken after a n interval of one hour or longer and is therefore believed to be a n equilibrium value. Theoretical plates were calculated from refractive indices determined on top and bottom samples by Abbe refractometer. The calculations n-ere made by means of the Fenske equation for total reflux ( 8 ) , together with the relative volatility for benzenct-ethylene dichloride calculated from vapor pressure data by Smith and l l a t h e s n (91, assuming Raoult’s la1T. This basis was adopted in order to give conservative results and to avoid the controversy 131 on the vapor-liquid equilibrium data for this mixture, n-hich has not yet been settled. If the experimental values ( 2 , 5 ) were used, 3IclIahon packing x~oulds h o x considerably improved performance, n.ith H.E.T.P. values reduced to 1.6-2.8 inchrs. Boilup rates were calculated from steam flowmeter readings, uming no heat lossrs. The calculated rates were checked by collection of steam condensate, and it !vas found that, the masimum error n’as 3cc at a low steam rate, decreasing t o ly0at high rates. In addition, reflux rates for the first two sets of tests were calculated from the condenser heat load. I n these calculations, the data used ivrrc: those obtained by Bragg and Richards ( 2 ) on composition and specific gravity os. refractive index for benzenecthylene dichloride mistures. It, was assumed t h a t latent heats were additive. The data calculated from steam flow and condenser water ineasurcmonts indicate that the reflux rate a t the top of the column was approximately 2 gallons per hour lower than the boilup ratr c.alculatcd from the steam meter. I t is believed that this difference was caused by heat losses. IIowcver, since only the hoilup rates were measured in all the tests, it was found desirable t o compare the tests on the basis of boilup rates. I n order t o facilitate comparisons, the boilup rate was used t o calculate the quant,ity G 2//;. This function, v-hich is the mass velocity divided by the square root of the vapor density, has been found ive fairly good correlation of pressure drop data with different ems. I n order t o dctermine holdup, it, was necessary t o estimate the quantity of liquid on the distributor plates and in the piping, and deduct this from the total volume collected t o give the net amount in the packing. Therefore the accuracy of the holdup determination is probably + 1.55, alloii-ing for time lag in shutting off the steam as well as possible errors in the cstirnated deductions.
2.5 10-3
Fractionation Efficiency of McMahon Packing
0 57 inches brass A 105 inch& brass
C 57.5 inch&, M o n o l
~
M c M a h o n packing Stedman packing
section were taken before each increase. This was continucd until there was evidence t h a t the column was loading’, as indieat’edby a sharp increase in pressure drop. At the same time the steam pressure in the reboiler tubes also increased, an indication t h a t almost all of the original 5-6 gallons charged t o the reboilrr had been boiled up into the column, This occurred at, a boilup r a t e of approximately 30 gallons per hour. Supplementary charges totaling 2-4 gallons of benzene-ethylene dichloride mist u r e ivere t,hen made t o the reboiler in order t o maintain opcration a t t,his high boilup rate for periods of 1-3 hours. The hoilup rate was then reduced below the load point, and samples were taken for the determination of the number of theoretical pla?es in t h e column. In all cases determinations of the number of plates were made successively a t rates of approximately 22, 17, and 12 gallons pcr hour in order to determine the effect of boilup rate on column performance. Before t h e first samples were taken for the determination of the theoretical plates, the column was lined out in steady operation at’ tot’al reflux for at least one hour. Top and bottom samples were than taken a t one- or two-hour intervals for determinat,ion of refractive index until identical values were obtained In at least two succeeding sets of samples. I n all cases at least 5 hours of steady operation elapsed between the set,tingof thr. steam rate and the withdrawal of t,he last sample. I n order to ensure that representative samples were obtained, the line was f l u s h d with approximately one ounce Qf liquid just prior t o the withdrawal of the one-ounce> sample. T A B L E I. EFFICIESCY TESTS \TITH BESZESE-ETHYLENE DICHLORIDE
A determination of the amount of liquid held up on t’he packing a t the end of t’he last run on the 105inch assembly of brass packT h e reboiler, aliich was not designed f o r the operation described here, did not h a r e sufficient liquid complete flooding oapacity t o 1
Assembly B ~ 57 in. ~
-
Brass, 105 i n . ; one redistributor
11.9 16.5 21.7
1340 1810 2320
Xlonel, 5 i . 5 in,
12,s 17.5 22.3
1330 1810 2290
ofthe column.
Hence, it was necesa a r y t o operate just beyond the point of incipient flooding, which has been t e r m e d the “loading point” ( 1 2 ) .
Boilup G Rate, Gal./Hr. dp~ 1 2 .~2 , 1320 16.8 1820 21.5 2340
a
By condenser heat balance.
Reflux Ap, R a t e , I n . HLJ Gal.,.”r Ft. 9 . ~ o 26 , . . 0.65 . , 1.31
7i
2Lp
Top 1.4935 1,4900 1.4869
Bottom 1.4591 1.4590 1.4598
Theoretical Plates 27.4 23.0 19.2
H.E.T.P , Remarks In. 2.1 Pretest boilup rate, 2.5 30.1 gal./hr. 3.0
9 0: 14,5
0.25 0.63 1.15
1 4973 1,4978 1.4958
1.4605
1 4505 1,4539
45.5 41.9 30 1
2 3 2.5 3.5
Pretest boilup rate, 28.0 gal./hr; holdu p a t end of r u n , 0.21 gal./ft.
.. .. ,,
0.25 0 50 0.97
1.4932 1.4924 1.4910
1,4718 1.4712 1.4716
17,3 16.6 14.6
3.3 3.5 3.9
Pretest boilup rate, 30.7galJhr.
20.0a
, ,
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1947
PACKING PERFORMANCE ~,
.
FRACTIO~ATION EFFICIESCY. D a t a on fractionation efficiency are presented in Table I and plotted in Figure 2. I n all cases efficiency has been expressed in terms of H . E . T . P . plotted against I t can he boilup rate, expressed in terms of the function G&. seen t,hat the H . E . T . P . values range from 2 to 4 inches, and in all cases hIclIahon packing shows the decrease in t,heoretical plates with increased boilup rate which is characteristic of many types of packed columns (1, 4, 7 ) . T h e brass packing s h o w a consistently lower H.E.T.P. than the Monel packing. This appears to be related to the pressure drop data presented in Figure 3 n-hich show that the pressure drop for the 1Ionel packing is from 10 t o 30% l o w r than for the corresponding length of brass packing. Factors such as differences in the n-etting characteristics of the metal surfaces or small variations in shape caused hy different bending characteristics of the two metals, both of n-hich n-ould affect the pressure drop, are apparently responsible for the variation in packing efficiency. I n the case of the brass packing, a comparison of the 57-inch and 105-inch installations shows a decrease in efficiency with height in spite of the presence of a redistributor plate. This appears to be due to more thorough wetting in the upper section, which could happen if loading started in the upper portion of the column and did not progress t o the lower portion due t o the higher pressure existing in that section.
IO
ecr w
IL
cr
W
5i 3
LL
0
cn w
I 0
z L"
0 E
n w
cr cn cn W cr
3
&
I
2
3 4
Figure 3. Pressure Drop Through M c M a h o n Packing
0 5 7 inches,
brass
A 1 0 5 inches brass 0 5 7 . 5 inch&, M o n e l
~
M c M a h o n packing Stedman packing
717
Figure 2 also shows data for 6-inch circulai~Stednian packing reported by Bragg (1). It, is evident, t h a t even the most favorable curve for h4cMahon packing still gives H.E.T.P. values which are about 30% greater than those reported for Stedman. It should be noted, hon-ever, that the packed height in the case of Stedman packing was only 36 inches as compared with 57 inches or more for LIcAIahon. -4s indicated previously, increase in height results in a n appreciable decrease in the efficiency obt,ained with RIcAIahon packing; hence it appears that the performance of hIchLahon packing in a 36-inch height would more closely approach Stedman than is indicat,ed by the curves of Figure 2. ridditional tests made at, this laboratory on a special mixture have indicated None1 McMahon packing t o give an H.E.T.P. of 3.5 to 4.0 inches in a 30-foot column equipped wit,h a redistributor plate every 5 feet, a n indication that the packing can be used to advantage in tall columns. PRESSCRE DROP. Figure 3 presents pressure drop data for 1IclIahon packing obtained a t equilibrium conditions plotted OD logarithmic coordinates w. G A . rllso included are data on 6inch circular Stedman packing ( 1 ) . Both of the curves show t h e sharp change in slope which is generally t,aken to be the loading point of the packing (12) and is not necessarily the flood point. T h e entire curve for McRIahon packing was defined by data obtained during the wetting of t,hepacking prior t o the steady operation for determining the number of theoretical plates. These data, although fairly well represented by t,he curve through t h e equilibrium data, were omitted since they were not obtained under equilibrium conditions. T h e loading point for 6-inch Stedman packing corresponds closcly to that, for the 6-inch diameter 1IclIahon packed column, which indicates t h a t their maximum capacities are approximately equal. It should also he noted that, the pressure drop for RIcUahon packing is less than t h a t for Stedman even when calculated as pressure drop per theoretical plate : this makes it especially advantageous for vacuum fract ionat ion. HOLDUP. T h e holdup value of 0.21 gallon per foot for McJIahon packing determined a t 20 gallons per hour reflux rate with benzene-ethylene dichloride (Table I) is comparable to a value of 0.27 gallon per foot determined by Bragg for 6-inch circular Stedman packing (1) a t the same boilup rate. Since the accuracy of the value for AIcAIahon packing is believed t,o be within .*l5%, it apppars that the holdup of McMahon packing is comparahle t o that, of Stednian packing for the same column diameter. From a comparison of the niechani~ I E C H A N FEATURES. ICAL cal features of the two types of packing, it is evident t h a t McMahon has several advantages over Stedman. McRIahon packing does not require accurate machining of the walls of t h e column, and its use is not limited to a single column diameter. When removal of the packing is required, McMahon is readily taken out and reinstalled, whereas special precautions are necessary in the case of Stedman, and whole sections of packing must sometimes be discarded because of damage during removal. On the other hand, RlchIahon packing requires support and holddown screens and is not rigid when in the column. It may, therefore, be subject to sett,ling, although none was observed in the tests made. .Loose wires resulting from fraying of the edges of the packing may collect on distributor plates and in piping and cause difficulties. It is therefore desirable t o avoid unnecessary handling of 3IclIahon packing just as in the case of Stedman packing. CONCLUSIONS
1. Tests on the performance of McMahon packing in a 6-inchdiameter column show H.E.T.P. values of 2 to 4 inches in a height of approximately 5 feet. These results are t o be compared with the H.E.T.P.'s of 1.5 t o 2 inches reported for 6-inch circular Stedman packing, packed t o a height of 3 feet. 2. As in the case of many other types of packing, McMahon packing shows a decrease in t,heoretical plates as the boilup rate is increased.
Vol. 39, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
718
3. McMahon packing gives less pressure drop than Ytedinari, which is of special advantage in vacuum fractionation. The maximum capacity and the liquid holdup are essentially the same for the two packings. 4. MclClahon packing has a number of mechanical advantage, over Stedman packing. It does not require accurately machined column walls, is not limited to a partirular column diameter, anti is more readily installed and removed. NOMENCLATURE
G = mass velocity, pounds,/hour,/square foot H.E.T.P. = height equivalent to a theoretical plate p = vapor density, pounds/cubic foot L I T E R A T U R E CITED
(1) Bragg, L. B., IXD.ESG.CHEX.,33, 279 (1941); E'ostei, C17hieler Bull. ID-44-2. (2) Biagg, L. B., and Richards, -4.l t . , ISD.Esc:. ('HEM., 34, 1088
(1942).
on, H. C., and Colburn, A. P.. I h i d . . 34, 1533 (194"). ke, 11. K., Lawroski, S.,and Tongberg. C . O., I M . , 30, 299
(1935). ( 5 ) Lettieri, V. J., master's thesis, Pa. State College, Jan. 194%. (6) McMahon, H. O., ISD. ESG. CHEM., ( 7 ) Podbielniak, W. J., ISD. ESG.CHEM.. ( 8 ) Robinson, C. S.,and Gilliland, E . T
