INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1949
= electrical resistance, ohms = radius of hollow heater; re for inside,
BIBLIOGRAPHY
ro for outside,
feet
= cross section for fluid flow, sq. ft.; S = rr(D22
Di2)/4
= average temperature of bulk stream of water, tl at inlet; t z at oytlet; t = ( t ~ t 2)/ 2 = film temperature, F. (tw t)/2
= saturation temterature
steam tables,
F.
+ + of gas-free
O
-
F.,
water from
= temperature of outer wall of heater, ’F. = value of tw at transition from nonboiling conditions
t o surface boiling conditions, as defined by Figure 3
W
XU
*
Y
= average velocity of bulk stream of water, ft./sec.; Vaeo= w/(3600)(PL)8, neglecting any bubbles = water rate through test section, lb./hr.
= thickness of wall of hollow cylindrical heater, feet; xW = (Do - D i ) / 2 = product of dimensionless ratios in correlation for
nonboiling conditions:
At
= total t e m p e r a t y e difference, heated wall to subcooled liquid, F.; 1, - t =
temperature excess of heated wall over water, tw
-
O
F.;
t,at
surboiling conditions, as defined by Figure 3,
= At,., a t transition from nonboiling conditions to
fc:e
1953
F.; & t , t r t w t r - teat total temperature diherence at transitio: from nonboiling conditions to surface boiling, F. = drop in tempera:ure through wall of hollow cylindrical heater, F.; At, = tt - to, see Equation 5 = dimensionless constant, 3.1416 = latent heat of vaporization, B.t.u./lb. = viscosity of fluid, lb./(hr.) (ft.) = viscosity at tw = density of liquid, lb./cubic foot Dimensionless moduli Prandtl number, a t average bulk stream temperature Prandtl number at tf Reynolds number based on average bulk stream temperature and equivalent diameter, D, same as above but based on t j Nusselt number based on k a t t and De same as above but based on kr =
Addoms, J. N., Mass. Inst. Technol., Sc.D. thesis Chem. Eng. (1948).
Carl, R., and Picornell, P., Zbid., S. M. thesis Chem. Eng. (1948). Carpenter, F. G., Colburn, A. P., Schoenborn, E . M., and Wurster,A., Trans. Am. Inst. Chem. Engrs., 42, 165-87 (1947). Colburn, A. P., Ibid., 29, 174-210 (1933). Colburn, A. P., and Schoenborn, E. M., and Sutton, C. S., Natl. Advisory Comm. Aeronaut., Rept. UD-NI (March 1945) ; T N 1498, (March 1948). Colburn, A. P., and Sutton, C. S., Ibid., UD-N2 (February 1948) ; TN 1498, (March 1948). Dew, J. E., Mass. Inst. Teohnol., S. M. thesis Chem. Eng. (1948).
Dittus, F. W., and Boelter, L. M. K., Univ. CaZif. Pubs. Eng., 2 , 443 (1930).
Drew, T. B., and Mueller, A. C., Trans. Am. I n s t . Chem. Engrs., 33, 449-71 (1937).
Hayes, V. R., and Bartol, J. A., Mass. Inst. Technol., S.M. thesis in Naval Construction and Engineering (1944). Kennel, W. E., Ibid., DSc. thesis Chem. Eng., pp. 228-340 (October 1948). Knowles, J. W., Can. J.Research, 26, 268-70 (1948). Kreith, F., and Summerfield, M., Calif. Inst. Technol., Progress Rept. 4-68, JPL (April 1948); ASME Paper No. 48-A-38. McAdams, W. H., “Heat Transmission,” 2nd ed., Chap. IX, New York, McGraw-Hill Book Co., 1942. McAdams, W. H., Purdue Univ. Eng. Bull., Research Ser. 104, 32, 1-56 (March 1948). Minden, C. S., personal communication (1948). Mosciki, I., and Broder, J., Rocznicki Chem., 6,321-54 (1926). Nukiyama, S., J . SOC.Mech. Engrs. ( J a p a n ) , 37, 367-74; 553-4 (1934).
Redlien, W. H., and Wilcox, W. R., Mass. Inst. Technol., S.M. thesis Chem. Eng. (1947). Sieder, E. N., and Tate, G. E., IND. ENG.CHEM.,26, 1429-36 (1936).
Taylor, J., personal communication t o W. H. McAdams (1943). Tibbetts, E. G., and Cohen, J. B., informal report to A. R. Kaufmann, Mass. Inst. Technol. Woods, W. K., Mass. Inst. Technol., Sc.D. thesis Chem. Eng. (1940); McAdams, W. H., Woods W. K., and Bryan, R. L., Trans. Am, SOC. X e c h . Engrs., 63, 545-52 (1941). RECEIVED January 31, 1949. Presented before the Meeting of the Division of Industrial and Engineering Chemistry, North Jersey Section, AMERICAN CHEMICAL SOCIETY,Newark, N. J., January 17, 1949.
A New Distillation Packing Rf. R. CANNON The Pennsylvania State College, State College, Pa.
z
new highly efficient distillation packing is made from protruded metal. Test data are given on two sizes of the packing i n 2 and 4 inch diameter columns. Because of the nature of the surface, the packing is readily wetted, requires no preflood, and attains equilibrium quickly. Consequently, considerable time is saved i n getting started. Since i t is made from flat metal, a relatively inexpensive starting material, i t i s less expensive than paclsings made from wire gauze or fine wire. I t is installed by simply pouring through a funnel.
\ I
HE new distillation packing described here consists of small units made from thin metal which has 1024 holes per square inch and is shaped into half cylinders with corners or edges bent inward to prevent nesting of one piece within another. Figure 1 illustrates the appearance of an individual piece. The holes are not clean cut but a protrusion or burr extends from one side. This surface has the unique property of being automatically wetted by liquid hydrocarbons. For example, if a strip of this
metal is partially submerged in a beaker of liquid hydrocarbon the liquid will crawl quickly up the surface of the metal. If the metal is touched with a blotter about 0.5 inch above the liquid level in the beaker, one can see absorption occur. The holes in the packing are not sealed by the liquid but both sides are wet. TEST EQUIPMENT AND PROCEDURE CoLirMNS. The test columns were made of 2- and 4-inch inside diameter Pyrex pipe with packed sections of 8.5 and 9.5 feet, respectively. Each column was offset from the still so t h a t all of the reflux from the column passed through a calibrated tube in which reflux rate could be measured accurately by closing the valve in this line and noting the rate of filling of the tube with the aid of a stop watch. The 2-inch diameter column was surrounded by a glass jacket which carried a resistance wire for heating. This jacket in turn was surrounded by a larger glass jacket t o reduce heat flow t o the room. Thermometers in the air space between the column and the heating jacket provided a means of determining the correct
1954
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
Vol. 41, No. 9
1 hour of draining was taken RS a measure of the static holdup. The data given in Table I are the sum of these two quantities. I,IQWID RATE. Liquid velocities through the packing were measured with the aid of a nonvolatile red dye. A concentrated solution of this dye in a test Inistore close to didillate composition u a s made. Fifty milliliters of the dve mixture mere injected into the top of the packing and its time of flow through the column was measured with a stop watch. In order to detect its appearance a t the base of the column it was necessary to have the liquid drop into the glass reflux measuring tube while it n u s maintained approximatelv half full ai' liquid. EFFICIEKCY OF THE PACKING
Figure 1. Schematic Drawing of Protruded Packing
voltage to impose on the Kinding in order to approximate adiabatic conditions. The 4-inch column had no electric winding but was covered with a 2.5-inch laxer of magnesia lagging. A heat balance indicated a heat loss suffjcient to causc only 2.1 liters of liquid per hour to condense becaiise of nonadiabatic conditions. DISTRIBUTOR PUTE. Dist,ributor plates were not used on the 2-inch column but one was used a t the top of the 4-inch column. This was constructed by drilling five 0.25-inch diameter liquid holes in a brass orifice plat,e, one in the center of the plate and four concentric on a 1.25-inch diameter. Five brass tubes 0.75 inch long with an inside diameter of 0.14 inch were driven into these holes and extended 0.5 inch from the plate on the bottom. There were twelve vapor risers on the top of the plate; these consisted of 2-inch tubes (0.4 inch inside diameter) driven into 0.5inch diameter holes. Pour of these were on a 2-inch diameter circle, and the other eight were on a 3.25-inch diameter circle. This arrangement directed the liquid to the center of the packing with a minimum of pressurc drop. TESTMIXTURE. The binary mixture, n-heptane-methylcyclohexane was used in all tests under conditions of total reflux and atmospheric pressure. The rnethylcrelohexaue received from the manufacturer was found to be contaminated with a volatile constituent which n-as removed in a column wVjt,h 60 t h o retical plates operating at, a reflux ratio of 25 to 1. T h e effect of this contaminant if not removed would be t o increase the refractive index of the distillate and result in a fictitiously low number of plates. Analyses were made by refractive index at. 20" C. using the data of BroniiIey and Quiggle (4). The purified methylcyclohexane had an n z = 1.4232 where the n-heptane had an n y = 1.3877. As poirited out by Fenske (6),rnaximum accuracy in analytical results is obtained when the still composition is not too lean nor the distiilate too rich in n-ticptaiie. The composition of the nlisture charged i o the st,i!l r m s adjusted accordingly. A value of a = 1.07 was used in calculating t!icoretical plates as reconimended by Fenslie (6') and B e a t t y and Ca!jngnert, (e). HOLDUP. Operating holdup consizt.s of two quantitiesdynamic and static. Dynamic holdup as determined by isolating the column from the still and measuring the amount of liquid discharged froin the base of the co1ulr.n after draining for 1 hour. Static holdrap was determined by dumping a large measured quantity of test mixture int,o the top of the previously dried packing, while it TWS maintained a t operating temperamre, and measuring the total liquid discharged from the base a t 10minute intervals. The difference between the quantity dumped into the top of the paclrine and that recovered a t the base after
SO many variables enter into packing effiriencv that a fair comparison cannot be made unlcsa paclrings are tested Tyith the same test' mixture in the same column diameter and a t approximately the same packed height. considerable data have been reported (6) on a number of gackings in a 2-inch diameter column packed for 9.5 feet and tested with n-hepta-ne-methylcyclohexane. Comparable test, data show that no packing, except Stedman (9), is as efficient as protruded packing. An important characteristic of protruded packin(s is that it wets itself thoroughly when any part of a piew of packicg touches a drop of liquid. Consequently, starting time is reduced because i t is not necessary to preflood the paclring. All of the tests reported in Table TI were ITithout preflootiing. Severa! runs were made with preflooding but no appreciable change in efficiency was obtained, although the runs were continued for 24 hours to ensure that equilibrium was reached. The degree to which a liquid will crawl over the protruded surface of a packing is probably a function of interfacial tension. It is not, proportional to the surface tension of the liquid-for example, mater will not crawl vertically up the metal surface whereas liquid hydrocarbons F? ill do so rapidly.
AXD LIQUID \'ELOCITIES TARLI: 1. HOLDUP
IN P R O T R U D E D
RlETAL l-'.4CXISG
(Measured with n-heptane-methylcyclohcrane) Paoliing SIZQ,
In. 0.24
X 0.21
0 . 1 6 X 0.16 0.16 X 0.16
Total Packed Hold~ii>,% Liqiud Column ThrouphHeight, oi Packed Velority, Diameter, p u t , L. Ft. \JoL I.'&./SCQ. In. Liqiid'H~.. 2.0 0.0 9.5 0.28 6.75 9.5 14.9 0,020 9.5 16.1 0.048 (2.0 15.4 9.5 17.9 0.049 2.0 4.0 9.5 17.0 ... 7.6 Y.6 19.0 ... 9.5 24.0 . , 9.5 16.0 ... 9.5 22.0
...
1:;
{i::
2;:;
...
h g m n ~ I ~ T R I B U T I O X . Several experiments mere performed with a distributor plnts, simiIar to the one described, with the addition of eight 1i:;uid holes on a 3.5-inch diameter circle. Under these conditions it WE observed lliat none of the liquid ftowed doim %hefive central tubcs but always flowed do\$-n the eight tubes near the column wvall. T h m appeared to be a liquitl depth of 0.123 inch on the plate a t high boil-up rates, but even thcn no iiquid Hen-ed dov-n the five central tubes. For these runs the total niiniber of iheoretica.1 plates w t s reduced from 30 to 34%. The diameter of the five centra! tubes appeared to he larger than necessary because v;iirii t,he eight peripheral tubes were sealed, forcing the liquid i o return by the central tubes, it mts observed that practically all of the liquid was returned to the column by one, two, three, or €our tubes dipmding on the boil-up rate. It,appears that tube diamet,er is not critical but a central location is necessary. The importance of good liquid distribution increases as column diameter increaws.
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1949
NICKELPACKINQ IN 2TABLE 11. TESTDATAON PROTRUDED Packing Size, In. 0 16 X 0.16
0.16 X 0.16
Column Diameter, In. ‘2 2 ,2 2 2 \, 2
Packed Height,
Ft.
2.58 2.58 2.58 8.5 8.5 8.5 9.5 9.5 9.5
4
0 . 1 6 X 0.16 0 . 2 4 X 0 24 0.24 X 0.24 h
34 ,4 4 .4 ,4 2 2
9.5
9.5 9.5 8.4 8.4 8.4
a
-2
8.4
Boil-up LRate,
L.
lquid/Hr. 5.2 7.2 9.8 2.4 5.7 9.2 16.0 29.0 42.0 30.0 42.3 62.0 3.7 6.1 11.3 15.6
FrEction Top 0.627 0.604 0.596 0.960 0.948 0.90 0.892 0.864 0.860 0.892 0.846 0,832 0.853 0.826 0.790 0.758
Bottom 0.252 0.241 0.238 0.156 0.131 0.062 0.112 0.102 0.078 0.193 0.132 0.121 0.108 0.123 0.110 0.106
AND
PINCH DIAMETER COLUMNS 1I.E.T.P.n ,
Total Plates 23.7 23.1 22.9 71.5 70.6 72.3 61.6 59.0 63.3 52.0 53.0 53.0 57.2 51.2 50.0 48.8
1955
In. 1.31 1.35 1.35 1.43 1.46 1.41 1.85 1.93 1.80 2.19 2.17 2.17 1.77 1.97 2.02 2.06
Pressure Drop, In. Water 1.8 3.0 8.7 0.9 3.6 28.0 3.54 11.9 31.0 3.54 8.57 26.4 1.02 2.38 7.22 14.3
Pressure Drop per Plate, In. Water 0.076 0.13 0.38 0.013 0.051 0.387 0.058 0.202 0.49 0.068 0.162 0.498 0.018 0.044 0.144 0.293
Time t o Reach Equilibrium, Hr. 2.0 1.8 2.0 2.5 2.7 5.0 4.0 5.0 4.0 2.3 2.5 2.3 2.0 2.0 1.5 1, R
Height equivalent of a theoretical plate.
It is the opinion of the writer t h a t the VELOCITYPRESSURE. above phenomenon can be explained by the distribution of velocity pressure throughout the cross-sectional area of the column. Velocity pressure of t h e vapor (or impact pressure) is calculated as follows:
Pi
7 .
V2
= - p
2g
where Pi = velocity pressure in pounds per square foot; V = vapor velocity in feet per second; g = gravitational constant in feet per second per second; and p = vapor density in pounds per cubic foot. Many experiments with Pitot tubes have shown t h a t in a n open tube the velocity pressure is a maximum in the center of the tube and zero at the walls. When packing is placed i n a tube the velocity profile is undoubtedly changed, but if vapor velocity remains greatest at the center, then the velocity pressure of the rising vapor will also be greatest at the center and consequently there will be less resistance t o the downward flow of liquid near the column walls. The fact t h a t a 30 to 34% increase in efficiency was obtained, when the eight peripheral liquid tubes were sealed, thus forcing the liquid into some of the five central liquid tubes, supports this reasoning. HOLDUP.Many factors influence holdup. Some of these are: velocity pressure, pressure drop, liquid phase density, liquid phase viscosity, packing geometry, and interfacial tension. Other factors being equal, a liquid of high density would be expected to produce a lower holdup and a higher flooding velocity than a liquid of lower density. This appears logical since the downward pressure of a column of liquid is equal to the product of density and liquid height. The test mixture benzene-ethylene dichloride has a high liquid density compared to the system used here; reported data (3, 7 )show high flooding velocities for it. LIQUIDVELOCITY. The liquid velocities of Table I are not average velocities but a measure of the fastest moving liquid stream. Such data may be quite useful in the study of packings -for example, a packing in which the liquid is channeling would be expected to give a relatively high liquid velocity compared to one in which no channeling occurs. The liquid velocity in the 2-inch column was 0.28 foot per second when no vapor was ascending the column whereas i t was approximately one tenth this rate a t a boil-up rate of 6.75 liters per hour. Since the liquid velocity changed very little when boil-up was increased from 10 to 15.4 liters per hour, i t is apparent that the thickness of the liquid films had to increase in order to discharge the greater amount of liquid. More extensive data on liquid rates were obtained by Divilbiss (6) and Adams ( 1 ) by this technique on another packing. They found the liquid velocity t o increase up t o a vapor velocity of 0.85 foot per second and then t o decrease as the vapor velocity
was increased further to 1.1 feet per second. At the point of complete flooding the lineal liquid velocity down the column becomes zero. PHYSICAL DATA
Table 111 gives physical data on two sizes of protruded nickel packing. The packing density will vary unless the drop height is 3 feet or more. D a t a obtained by pouring from 1 to 28 feet show an increase in density up to a drop height of 3 feet and a constant density for greater drop heights. Therefore, it is recommended that the packing be installed by pouring through a funnel into a tube or pipe extending 3 feet or more above the top of t h r column. Protruded metal packing is made froni cheaper raw material than screen or wire packings and consequently i t is less expensive. It can be made from a variety of materials and is currently available in nickel and Type 316 stainless steel.
TABLE111.
PHYSICAL
Packing Size,
In. 0.16 X 0.16 0.24 X 0.24
DATAON
Packing Density Lb./Cu. Ft. 44 28
PROTRUDED NICKEL PACKING
Free Space,
7%
92 95
Pieces per Cu. Ft. 1,200,000 340,000
It is expected t h a t additional work will develop other shapes which will be efficient; other packing sizes and the number and diameter of holes are to be varied also. ACKNOWLEDGMENT
The author is indebted to George Palick for his assistance in obtaining most of the data in the 2-inch diameter column. LITERATURE CITED
(1) Adams, F. A., Master’s thesis, Penn. State College, (June 19481, (2) Beatty, H. A., and Calingaert, G., IND.E m . CHEM.,26, 504 (1934). (3) Bragg, L. B., Ibid., 33, 279 (1941); Foster Wheeler Gorp., New York, Bull.. ID-44-2. (4) Bromiley, E. C., and Quiggle, D., IND. Ewc. CEEM.,23, 1136 (1933). (5) Divilbiss, R. J., hfaster’s thesis, Penn. State College (June 1948). (6) Fenske, M. R., Lawroski, S., and Tongberg, C. O., IND. ENQ. CHEM.,30,299 (1938). (7) Forsythe, W. L., Jr., Stack, T. G., Wolf, J. E., and Conn, A. L., Ibid., 29,714 (1947). R~CEIVE October D 1, 1948.
