Surface tension effects in sieve-plate distillation

005 d

-+Fig. 1.

O-05

0 15

01

02

o-25

I min Tlitle rates (1IF) in Fig. 2. The values for any one nozzle are seen to fall on a straight line passing through the origin, indicating that a single recycle constant operates for each nozzle diameter, irrespective of the flow rate. Similar behaviour was found in the previous liquid-phase work. Here it was shown that the recycle constants were predictable by Eq. (2) below, which is based on simple jet entrainment theory

Fig. 2. Table 1

Dld 115 75 46 36 30.7 18.4 15.3

In this equation, d is the nozzle diameter, and I#Jis the halfangle of the free jet, having a value of 7” for water[3]. The entrainment distance x was taken to be twice the vessel diameter, since in a vessel of H/D = 1, this is the approximate distance the inlet jet can travel before its original direction is completely reversed, and its entrainment capability dissipated. Taking 4 = 10” for a free air jet[3], the table below shows that the same equation closely predicts air recycle constants for a range of D/d from 15.3 to 46. The possible use of this type of vessel as an autothermal process reactor has been suggested previously[ 11. The observed gas flow characteristics now lend weight to this suggestion.

K (measured)

K (calc’d from Eq. (2))

60.2 39.5 31.1 25.5 21.4 18.1 11.3

Department of Chemical Engineering University of Manchester Institute of Science and Technology Manchester, England

84.0 54.5 33.6 26.3 22.4 13.5 11.3

G. T. CLEGG S. G. HORSFIELD

REFERENCES

[l] CLEGG G. T. and COATES R., Chem. Engng Sci. 1967 22 1177. [2] HORSFIELD S. G., M.Sc. Thesis, U.M.I.S.T., 1967. [3] DONALD M. B. and SINGER H., Trans. Inst. Chem. Engrs 195937 255. Chemical Engineering Science, 1968, Vol. 23, pp. 943-945.

Pergamon Press.

Printed in Great Britain.

Surface tension e&cts in sieve-plate distillation (First received 17 November 1967; in revisedform 6 February 1968) IN A PREVIOUS communication[L]

results were presented which indicated that under spray operating conditions of a sieve-plate distillation column surface-tension negative 943

systems show higher plate efficiencies than positive systems. A possible explanation of this behaviour was given in terms of the influence of the Marangoni effect on the mechanism of

Shorter Communications spray formation. Unfortunately the experimental results were not entirely conclusive because the value of surface tension varied considerably from system to system and across the concentration range. The importance of surface tension cannot be underestimated. It was shown[l] on the basis of vaporization studies that plate performance changed linearly with the reciprocal of surface tension. This dependence has since been confirmed for spray operation of sieve plates by Goederen[Z], again on the basis of vaporization studies. Results obtained by Bainbridge[3,4] for sieve-plate distillation at reduced pressure produced a similar dependence and are thus probably also best explained in terms of variation of surface tension with temperature (Fig. 1). This proportionality between plate performance and the inverse of

22 r

2

i c

.e I

12 0

02

I

Mdo factIon

Fig. 2. Variation

II

I 0.4

I

0.6

of

M.V.C

0.6 In llqdd,

I I.0

x

of surface tension at boiling point with composition.

point: density 710 and 700 kg/n?, kinematic viscosity of liquid 3.3 x 10-r and 3.0 X 10-r m*/s and viscosity of vapour 8.3 x 10-O and 7.7 x 1O-6 Ns/mP. Estimated molecular 12 B 26 diffusivities in the two phases were also similar. Since Surfm tenskm at botlirg point ImNhd the relative volatility of the two systems was reasonably high (l-6 and 1.7) and the distillation was conducted near the Fig. 1. Variation of plate performance with surface tension middle of the concentration range, the separation obtained on the plate was quite substantial, This, coupled with the ease for distillation at reduced pressure (calculated from results of of analysis (large difference in refractive indices), decreased Bainbridae at x = 05; + methyl_ 131): _ _, 0 benzene/n-hentane considerably the experimental error. cyclohexane/toluene at x = 0.5; -A benzenelcyclohexane-at The column employed was rectangular in cross-section, x = 0.25; 0 benzene/cyclohexane at x = 0.75. The pressures 0.10 m long and 0.20 m wide, having 3 plates placed O-45 m were 760,540,340 and 200 mm Hg for the first two systems, apart. The holes, 3 mm dia., were placed on 9.5 mm equiand 760, 540,350 and 240 mm Hg for the azeotropic system lateral triangular pitch. The hole area formed 6.8 per cent benzenelcyclohexane. of the tray area and segmental downcomers were employed. Only two weir heights were used- 19 and 9-5 mm The reason surface tension follows indirectly from results of other behind the choice of such low weir heights is the belief that workers on interfacial area and drop sizes in sprays and dispersions. Thus, according to Vermeulen er a1.[5], Sd 0~ industrial operation is better simulated in a laboratory column (T-O.~ and. from the work of Hassod and Mizrahi[6], d 0~a+@53 by using a low weir height. This applies in particular to o&ration around the phase inversion point. It can be exwhere S is the interfacial area per unit volume of dispersion, pected that, by analogy to the operation of a packed column d the mean drop diameter and o the surface tension. On or the behaviour of liquid-liquid dispersions, phase inversion combining the two expressions S, and thus Not, the number will critically depend on the volume ratio of the two phases. of overall gas transfer units per plate, become proportional to o-0.85. Since vapour velocities in laboratory columns are usually smaller than in industrial units the weir heights, which control From the above considerations it follows that definite conthe liquid volume, must also be correspondingly lower. clusions about comparative plate efficiencies of positive and negative systems can only be obtained from experiments conThe results obtained with the positive system P are shown ducted at constant surface tension. For this reason the in Fig. 3. As expected[2, 71, plate efficiency decreased with increasing hole velocity. Also, as predicted e.g. from the strongly positive system n-heptaneltoluene and the strongly equation of Walter and Sherwood[B], the efficiency denegative system benzene/n-heptane were selected for excreased with decreasing weir height. On the basis of photoperimental purposes. By adjusting the operating conditions so graphic evidence and &al observation the decreasing porthat the tray operated at approximately 33 per cent of heptions of the curves correspond to the gradual breaking down tane in the positive system, and 58 per cent of benzene in of the cellular foam into froth, e.g. for the 19 mm weir cellular the negative system the surface tension had the same value foam was only well defined up to about 4.5 mlsec. This on the test tray in both systems (Fig. 2). Other important breakdown was accompanied by significant increase in the physical properties were also approximately equal at boiling

Oe

944

Shorter Communications 60

r

version). Figure 3 also shows the results of efficiency runs obtained for the negative system N. Here the efficiencies were found to depend little on vapour velocity. As predicted by Zuiderweg and HarmensD] no foam regime was observed for this system. The froth gradually gave way to an increased amount of spray as the velocity increased. On comparing the efficiencies for the two systems it is seen that the positive system shows a higher efficiency at low hole velocity but a lower efficiency at higher velocities. It would seem that, once the cellular foam starts breaking down, the unstable tendencies of the negative system outweigh the conservative tendencies of the positive system in the production of fresh interfacial area. The influence of the Marangoni effect on drop formation, postulated in the previous communication[ 11, is the most likely explanation of the fact; the more so because all the important physical properties, except for the difference in sign of the surface tension-concentration gradient, have similar values for the two systems.

ml-

301 0

I

I

I

I

I

I

I

I.5

3

45

6

75

9

lo5

wbcity,

v,

WI

Fig. 3. Variation weir height for heptane/toluene, weir height 95 19 mm; A

ImId

of plate efficiency E with hole velocity and the positive and negative systems: 0 nweir height 19 mm; 0 n-heptane/toluene, mm; A benzene/n-heptane, weir height benzene/n-heptane, weir height 95 mm.

Department of Chemical Engineering A. FANE and Chemical Technology H. SAWISTOWSKI Imperial College of Science and Technology London, S. W.7

amount of spray and corresponding decrease in the height of the bubbling zone. It represented the transition from a vapour-dispersed to a liquid-dispersed system (phase in[ 1] [2] [3] [4] [5] [6] [7] [8]

REFERENCES BAlNBRlDGE G. S. and SAWISTOWSKI H., Chem. Engng Sci. 1964 19 992. DE GOEDEREN C. W. J., Chem. Engng Sci. 1965 20 1115. BAINBRIDGE G. S., Ph.D. Thesis, University of London 1964. SAWISTOWSKI H., BAINBRIDGE G. S., STACEY M. J. andTHEOBALD A., Proc. Symp. on Distillation, Chemical Industries Association, London 1968. VERMEULEN T., WILLIAMS G. M. and LANGLOIS G. E., Chem. Engng Prog. 1955 5185F. HASSOD D. and MIZRAHI J., Trans. Instn Chem. Engrs 196139 415. ZUIDERWEG F. J. and HARMENS A., Chem. Engng Sci. 1958 9 89. WALTER J. F. and SHERWOOD T. K., Ind. Engng Chem. 194133 493.

Chemical Engineering Science, 1968, Vol. 23, pp. 945-947.

Pergamon Press.

Printed in Great Britain.

Some observations on the static hold-up of aqueous solutions and liquid metals in packed (First received 7 December 1967; in revisedform 28 February 1968) INTRODUCTION paper[ I] it was shown that dynamic hold-up of liquid metals and of aqueous and organic solutions can be correlated by a single expression if the differences in physical properties are taken into acc0unt.t This work, therefore, supported Young’s[3] hypothesis (at least for the dynamic hold-up) that the usual chemical engineering concepts ‘must apply’ to liquid hold-up in metallurgical systems. IN

A RECENT

tSimilar observations concerning property values have also been made by Szekely[Z] in the field of high temperature kinetics in pyrometahmgy.

Since liquid hold-up also involves the static hold-up, verification of Young’s hypothesis is incomplete. It is the purpose of this communication to consider the static hold-up of liquid metals and its relationship to the static hold-up of nonmetallic liquids used in conventional chemical engineering practice. To this end, static hold-up was measured for mercury, cerrobend (an alloy of Pb-Bi-Cd-Sn) and water, both under wetting and non-wetting flow& in various tin. packings. All *It should be noted that in general non-wetting to most solids.

945

liquid metals are