Kinetics of Removal of Natural Cane Sugar Colorant with ton-Exchange Resin David F. Bagster Department of Chemical Engineering, Vniversity of Sydney, Sydney 2006, S.S.TT7., Australia
The performance of ion-exchange resin columns in decolorizing cane sugar liquors is examined in terms of well-known mass-transfer models. The ability to scale up results from columns of 15-cm length to predict performance of columns of industrial length is explained in terms of the Rosen model and similarity solutions. Diffusion within the resin beads limits decolorization rate a t low residence times, but when these are sufficiently long, plant scale performance can b e closely reproduced with small columns.
A
recent experimental study involving fixed beds of ioiiexchange rehill in decolorizing cane sugar liquor revealed that substantially the same performance could be achieved with a column as short as 15 cm as with a column four or eight, times longer (ljagster, 1970a) , provided the superficial residence time for columiis of different lengths was held constant,. The use of small columns results in a saving in the quantity of test liquor and a reduction in handling difficulties. The resin used in these test,s was Permutit Deacidite FFIP, a styrene divinylbenzene copolymer of the anion-exchange type, having active groups of trimethyl benzyl ammonium in the sodium form. The resiii was closely sized having been wetsieved to make a fraction between 22 and 30 British Standard mesh. The microscopically measured surface average diameter was 0.560 mm, standard deviation 0.051 mm. Using a reasonably small size range should eliminate concern over t,he correct mean diameter for the phenomenon being studied (Olney, 1964). The cane sugar liquors used in the decolorization trials were impure refinery syrups normally decolorized in the refinery process by passing over bone char. Such liquors are about 64% sucrose in xater solution, with less than 1% impurities, some of which are colored. The temperature used throughout was 70°C. The measure of color was the comtnon attenuation index (Iliet,z et al., 1952) multiplied by 1000 for convenient numbers. Thus, color was taken as measured by
A*
=
1000
~
-1ogioT bc
where
T
=
c
=
transmittance of the liquor coiicetitration of solids in g/cm3 of liquor in the cell of the spectrophotometer b = celllength A wave length of 420 nm was used, and the pII of the liquors was adjusted t o 9 for measurement. 1he mea,wre of color in this paper is treated as a conceiitratioii to be employed as aiiy other concent,ratioii used in masstransfer studies. T w o sets of columti trials are relevaiit to the analysis which follows. Figure 1 presents the results of 21 decolorization cycles oii the same re$iii on each of two columns. The same feed liquor was fed to each column, and a fresh liquor (of
nearly the same color) was used after the first 12 cycles. Each cycle corisisted of paisiiig 54 bed volumes (13.V.) over a column followed by the usual “sFyeeteiiing off , I ’ backwashing, brine regeneration, rinse, aiid then a further batch of liquor. The effluent liquor was collected in a single vessel for each columii, and the color of the batch was measured. The ratio of this bulk color to that of the feed is defined as Ubulk. The increase in Ubulk from cycle to cycle is due to irreversible adsorption of coloraiit or other impurities aiid has an important bearing on the economics of resin usage. Oiie resin coliirnii was of height z = 15 cm; the other mas GO cin high. The ,superficial residence times, OR, in each were
L x
I O X
x x
e
10
0
0
X
04f
00
x
x
O X O O O
0
x
0
0, 0
04
x
eo
i s OQ
X
0
Bed height z x IS em.
r 7
108 Ind. Eng. Chern. Process Des. Develop., Vol. 11, No. 1 , 1972
O
0 9 s x
o I
1
1
2 4
6
1
1
1
I
I
I
60 I
cm. I
8 10 12 14 16 18 20 22
II
Cycle Number Figure 1. Bulk effluent color ratio for different column lengths
0.2
J 0.1
/
0
0
1
10
I
I
20 30
1
1
I
l
40 50 60 70 Bed Volumes
1
80
0
I
90
Figure 2. Color ratio as function of residence time
riintie the same (20 niiii) by having tlie liquor volume flow rate from the loiiger colnnii! four time5 that froin the shorter. Siiice
where c 1 = superficial liquor velocity, the liquor velocity iii the loiiger eolunii! will be four time.; that iii tlie diorter one. 'i'lie values of zit,nlli achieveti i i i the columis of different leiigtli are clore, differiiig by an average of nine color uiiits iii a feed color of i!e:irly 1000, despite that the rare of color reriiovul i* velocity de1)eiideiit (Ihgster, 19iOa)-i.e., the ten1 is i i i the tlyiinniic regime (Joliiistoiie a i d Thriiig, 1957). The decolorized liquor was collected iii bulk from any oiie cycle hecause: is a single measure in a cycle giving a11 i d e s of performalice a:: a fuiictioii of resin age. 111 fwtories having buffer stomge before the vaciiiini paiis, the bulk color is a measure of the average feed to the pans. l\leasurenieiits of A* show considerable esperiineiital error. 'ti),,Ik
Hulking reduce-; the nuinher of samples mid reduces the effect of v:iriatioiis iii reriii perforiiiaiice during a cycle. Some trials rvei'e c:irrietl out, however, taking samples a t tiifferelit time- duriiig cycle.. I 0 will increase the value of u-Le, particle phase diffusion will become important.
0.2
3
/
Pretreated Liquor
1
Conclusion
=r
Although natural cane sugar colorant is a variable mixture of many different complex molecules, the use of a n attenuation index as a measure of colorant concentration allows the application of a film coefficient/diffusion model. 111 decolorization trials either in evaluating the life of a resin or in finding the ability of a liquor to decolorize, the fact t h a t a long column gives approximately the same effluent color as a short one allows more convenient experimentation and smaller quantities.
0.1
Raw Liquor ,
I
,
,
,
,
,
1 2 3 4 5 6 7 Cycle No.
Acknowledgment
The data on which this paper is based were obtained while the author was employed a t the Research Laboratories of the Colonial Sugar Refining Co. Ltd., Australia.
Figure 10. Effect of feed pretreatment 60 = B.V. OR = 20 min Z
= 15cm
Nomenclature
A*
The data of Table I correlate a t
kf
(17)
a vo.9
The film coefficient,s evaluated here involved UO,the value of 7 = 0 or y/x = 0, and this required a n extrapolation to the ordinate, Figure 2. Yo attempt was made t o take samples a t T = 0, because such samples are subject to errors owing to deiisity variations since the columns are “sweetening 011.” There may be ion-exclusion effects as well. Nonetheless, the almost linear dependence of Equation 17 is consistent with the linearity of the decolorization rate vs. velocity curve previously found (Bagst,er,1970a). The range of Reynolds numbers
u at
Re
=
d,G C!J
involved in Table I is 0.02 to 0.083, and film coefficients do not appear to have been reported a t such low Reynolds numbers. JWiamson et al. (1963) warn against extrapolating to below 0.08. With assumption of the dependence of Equation 17, the column which is four times longer will have u/x reduced from 0.278 for z = 15 cni to a value of 0.08. Thus at y/x = 0, u is reduced b y about 0.03, whereas a t y/x = 0.05 (about the end of an industrial cycle) and large X, u is reduced b y about’ 0.05. T h e experimental reduction in u b u l k for bulk effluent’ with a fourfold increase in bed height was about 0.01 (Figure I ) , a much closer scale-up than t h a t t o be expected from the Rosen model. This may, in part, be explained by assuming t h a t some colorant passes through effectively without a n y removal. Some evidence for this is available in Figure 10, which shows values of u b u l k for some raw liquor of the type used in the described decolorization trials and, under the same conditions, for liquor which had a previous partial color removal with activated carbon. The values of u b u l k for the pretreated liquor are higher so that relatively less color is removed.
attenuation index (color of sugar liquor), color units/cm3 Ao* = atteiiuatioii index of feed to column, color units/cm3 -4 = color conceiitratioii in resin bead material, color units/cma Ai* = average color concentration in resin bead, color units/icm3 a p = surface area of resin beads per unit volume of packed bed, cm2/cm3 B.V. = bed volumes based on empty columii (-) b = spectrophotometer cell length, cni c = concentration of sugar solids in solution, g/cm3 D , = empirical resin bead phase diffusivity, cm2/isec d, = resin bead diameter, em G = mass flow rate through columii based on empty column, g/’cm2 sec K = separat’ion constant, Equation 7 (-) kl = liquor side mass-transfer coefficient, cm/sec k , = resin side mass transfer coefficient, cm/sec m = ratio void volume to unit, volume of resin beads R, = liquor film resistance, sec d G Re = Reynolds number 2 (-) =
!J
r = radius a t some point in resin bead, cm rb = bead radius, c m S = empty columii cross-sectional area, cm2 s = surface area of resin beads per unit volume of
stirred batch, cm2/cm3 traiismittaiice of liquor (-) t = time, sec u = ratio effluent liquor color to feed liquor color a t a particular t’ime (-) ?&lk = ratio bulk effluent liquor color for a cycle to feed liquor color (- ) uo = ratio initial effluent liquor color to feed liquor color
T
=
(-1
Ti = liquor volume in batch stirred-tank experiments, ems Tin = resin volume in batch stirred-tank experiments, cm3 v = liquor velocity in direction of coluniii axis through intensities of bed, cm/sec = superficial liquor velocity, cm/sec T i ’ = volume flow rate of liquor to column, cm3/sec Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No, 1 , 1972
1 13
w = capacity ratio in batch stirred-tank experiments, VIVRK (-) z = dimensionless diffusivity parameter, Equation 9 (-) y = dimensionless time parameter, Equation 10 (-) 2
= column length, em
GRCCKLCTTCRS P = variable of Equation 13 (-) t =
bed voidage (-) time parameter, Equation 4 ; meclianism parameter (-) superficial residence time, Equation 2, see or mill
.r= dimensionless BR =
x= P = lJ= 7
=
4
liquor viscosity, g/cm/sec dimeiisionless resistance parameter, Equation 11 (-) dimensionless time parameter, Equation 5 (-)
literature Cited
Andrus, G. AI.,Sugar Azucar, 62 ( 5 ) ,54 (1967). Anzelius, A., A . Angew. Moth. Mech., 6 , 291 (1926). Bagster, D. F , Znt. Sugar J . , 72, 134 (1970a). Bagster, L). F., Znf. Sugar J . , 72, 200 (1970b). Bird, li. B., Stewart, W. E., Lightfoot, E. N., “Transport Phenomena,” Wiley, New York, X.Y.) 1960, p 707. Brown, R. L., Richards, J. C., “Principles of Powder Alechanics,” Pergamon, Oxford, England, 1970, p 16. Carberry, J. J., AZChE J., 6,460 (1960). Dietz, V. It., Pennington, N. L., Hoffman, II. L., J . Res. -Tat. Bur. Stand., 49 (6), 365 (1952). Farber, L., LlcDonald, E. J., Carpenter, F. G., Proc. Tech. Session Cane Sugar Refining Research, p 85, 1969.
Gaffney, B. J., Drew, T. B., Znd. Eng. Chon., 42, 1120 (1950). Hiester, N. K., €{adding, S.B., Nelson, 11. L., Vermeulen, T., AZChE J . , 2 , 404 (1956). IIelfferich, F., “Ion Exchange,” McGraw-Hill, New York, N.Y., 1962. n 385. Hougen,‘O. A , , Watson, K. A I . , “Chemical Process Principles,” Part 3, Wiley, Xew York, N.Y., 1966, p 1087. Johnstone, 11. E., Thriiig, 11. W., “Pilot Plants, Models and Scale-up Methods in Chemical ‘Engineering,” lIcGraw-Hill, S e w Tork, S.Y.,1957, p 65. Kuiiin, I Chcm. Eng. Progr. Sfyrnp. Scr., 5 5 124). 71 (19.59). Oliiey, I
