INVERSION OF CONCENTRATED SUCROSE
SOLUTIONS IN FIXED BEDS OF ION EXCHANGE RESIN NORMAN LIFSHUTZ AND JOSHUA S. DRANOFF Department of Chemical Engineering, Northwestern University, Evanston, Ill. 60201
The inversion of concentrated sucrose solutions by fixed beds of ion exchange resin catalyst was studied. Experiments were run with solutions containing up to 45 wt. % sucrose a t temperatures near 60" C. The reaction may b e carried out successfully under these conditions. However, it loses its apparent first-order character because of product degradation. Such degradation results in optically active products which may interfere with polarimeter determinations as well as in the formation of species which may cause fouling of the resin catalyst. No evidence of film mass transfer limitations was found for even the most concentrated feed solutions.
HE
catalytic inversion of sucrose solutions by fixed beds of
T ion exchange resin has received attention in the past few years for two principal reasons. O n one hand, the sucrose inversion reaction has been used as a convenient means for studying catalysis by ion exchange resins in general. I n addition, there is commercial interest in sucrose inversion by such catalytic beds. The two most recent studies of this process to be reported are the works of Reed and Dranoff (1964) and O'Connell (1964). Reed and Dranoff studied the inversion of 15% sucrose solutions by fixed beds of Amberlite IR-120 resin in the acid form. They determined that the reaction was first-order for temperatures between 50" and 75' C . for a wide range of Reynolds numbers. They concluded that the rate was strongly influenced by intraparticle diffusion, with no indication that external (film) mass transfer was significant for this system. They also found the activation energy for the catalyzed reaction to be approximately 16 kcal. per gram mole over the range of their experiments. O'Connell, in a more detailed study, investigated catalysis of approximately 20% sucrose solutions using Dowex 50W-X8 resins for most of his work. Over the range which he investigated, O'Connell also found the reaction to be first-order with respect to sucrose concentration, uninfluenced by external mass transfer effects, and strongly influenced by intraparticle diffusion. His measured activation energies were somewhat higher than those of Reed and Dranoff, running approximately 18 to 20 kcal. per gram mole. O'Connell studied other effects involved in this process, including the variation with experimental conditions of the sucrose absorption by the resin particles and its influence on the observed kinetics. Aside from the experimental details, these two studies have clearly demonstrated that in the 15 to 20% sucrose region, and for temperatures between 50" and 70" C., the reaction is first order, uninfluenced by external mass transfer effects, and strongly influenced by intraparticle diffusion. T h e present study was undertaken to investigate the behavior of this system under more concentrated conditions. I t was anticipated that the use of thicker sucrose solutions might influence the signifi266
I&EC PROCESS DESIGN A N D DEVELOPMENT
cance of external mass transfer effects as well as the basic kinetics of the inversion process. T h e relative significance of external and intraparticle mass transfer processes remains the same for concentrations up to 45% sucrose. However, the first-order characteristic of the inversion reaction is no longer valid because of apparent degradation reactions which become important a t these higher concentrations. Experimental
The experimental apparatus used in this work was similar to that described by Reed and Dranoff (1964). I t consisted essentially of a jacketed glass fixed-bed reactor through which sucrose solutions were pumped. The reactor had an inside diameter of 1 inch. The resin catalyst was supported within the reactor by fine-mesh stainless steel wire screens. Temperature control was maintained by constant temperature water circulated through the jacket. Reacted samples were collected and analyzed by polarimetry. T h e Dowex 50\Y-X8 resin used was separated into three size ranges by wet sieving in the hydrogen form. The actual sizes used and their measured acid capacities are indicated in Table I. The sucrose solutions were prepared initially from commercially available granulated sugar and subsequently from ACS reagent grade sucrose crystals. All of the data reported here were obtained with the reagent grade sucrose. There were no significant differences in the apparent kinetics for these two materials. However, the granulated sugar solutions were significantly colored at the high concentrations used in this work. This produced discoloration of the resin particles, which was felt to be undesirable. Reactant solutions were prepared from sucrose and deionized water at concentrations of 15, 30, and 45 wt. yo sucrose. Further details of the experimental apparatus and procedures are given by Lifshutz (1966).
Table 1.
Mesh Size 18-20 20-25 25-30
Resin Properties A v . Particle Radius, M m . 0.46 0.38 0.32
Exchange Capacity, Meq. /MI. 1.74 1.77 1.87
I .o
Results and Discussion
Several experiments were first carried out with 1501, sucrose solutions as a check on the previously reported work. T h e results obtained with 18- to 20-mesh particles for three temperatures are shown in Figure 1. This is a semilog plot of the fraction of sucrose remaining us. space time, which, for a first-order reaction, should be linear. I n all experiments the fraction of sucrose unconverted was calculated from measurements of optical rotation of product solutions, assuming that the sucrose hydrolyzes to form equal amounts of fructose and glucose. Clearly the data follow first-order kinetics, as reported earlier by Reed and Dranoff and O'Connell. A similar set of experiments \vas made using 30% sucrose feed solutions. I n this case, deviations from the linear plot were found a t high conversions. Although this might a t first be considered some evidence for an external mass transfer effect, it was also found that d a t a for three different bed sizes could be superimposed, as shown in Figure 2. This indicates that the deviation from linearity is due to the basic kinetics of the reactions. The line d r a u n on Figure 2 represents the best straight line through the early (low conversion) data and the origin and points up the increasing deviation as conversion increases. First-order rate constants. k, \vere calculated from the experimental data, and an Arrhenius plot was made for data obtained a t three temperatures with 15 and 30% sucrose solutions and 18- to 20-mesh particles. As shoivn on Figure 3, the data conform \vel1 to such a plot over the range of 50' to 70' C., with a n activation energy of 18.4 kcal. per gram mole for both sucrose concentrations. k values for the 30% solution correspond to the low conversion range in which the kinetics remain apparently first-order. The magnitude of the experimental activation energy corresponds exactly to that of O'Connell for particles of the same size range, although it is slightly higher than reported by Reed and Dranoff for Amberlite IR-120 resin. The differences may be due entirely to resin size and type. Following these results, attention was turned to experiments with 4570 sucrose feed solutions. A series of experimental runs was made with different bed sizes and with particles of all
1.0
0.5
( I - X,) 0.2
0.1
30 /'e Sucrose, 70°G 0-5Occ. Bed e-25 cc. Bed 0-IScc. Bed
0.05 0
IO
20
30
Space Time (min.) Figure 2.
First-order plot for 30% sucrose
three sizes a t 60' C. (Figure 4). Here again, straight lines have been drawn through the low conversion data to indicate the deviation of experimental points from first-order behavior as conversion progresses. Data for the 18- to 20-mesh particles obtained with two different bed sizes agreed, again indicating the absence of any film diffusional effect. Firstorder rate constants were fitted to the low conversion data and all of the results for 45% solutions compressed into a single curve, as shown in Figure 5. This method of correlating the data depends on first-order behavior in general, but is apparently satisfactory for the present results as well. No runs were made with 45% sucrose at other temperatures because of the difficulty of operation a t this high concentration. However, because of the trends observed with lower feed concentrations it is felt that intraparticle diffusion is as significant for 45% solutions as for the lower concentrations.
0.8
0.6
18-20 Mesh Resin
I5 O'/ Sucrose A-50°G, 25cc. Bed 0 - 6 O o C , 25cc. Bed
o-70°C, 25cc. Bed
0.3
0-70
OG,
35cc. Bed
0-30% Sucrose
0.2
0
,I0 20 Space Time (min.) Figure 1.
\
30
First-order plot for 15% sucrose V O L 7 NO. 2
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1.0 Table II.
0-18-20 Mesh, 5002.Bed
0.8
0-18-20 Mesh, I 0 c c . B e d 0-20-25Mesh, 50cc.Bed
0.6
a-25-30Mesh, 50cc. Bed
Effect of Sucrose Concentration on First-Order Rate Constants
Feed Con.cn., Wt. yo Sucrose 15 30.9 45.9
k, Min.-' 0,0482 0,0526
0.0603
All data for 60' C., particle radius = 0.46 mm.
(I-Xs)
0.4
0.3
Space l i m e (min.) Figure 4.
First-order plot for 45y0sucrose
T h e effect of solution concentration on the apparent firstorder rate constants is indicated in Table 11. The increase in the effective rate constant with concentration is undoubtedly due to a combination of two effects: the increase of the sorption coefficient for sucrose in the resin with higher concentration, and the increase in the homogeneous rate constant of the sucrose inversion reaction with sucrose concentration. Both effects have been noted i n the separate studies of O'Connell
k x (Space Time 1 Figure 5. 268
Correlation of 45% sucrose data
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
(1964), Jones and Lewis (1920), and Moran and Lewis (1922). Present data are not sufficiently comprehensive to permit a more detailed study of this point. Some consideration was given to the possible reasons for the deviation from first-order kinetics as concentration and temperature increased. I t has been concluded that this results from secondary decomposition reactions which set in under these conditions, especially as high conversions are approached. Moelwyn-Hughes (1934) has reported that the homogeneous reaction in 5% sucrose solutions begins to lose its first-order character to a slight extent above 50' C., owing to a secondary decomposition of the fructose produced in the sucrose inversion reaction. Fructose decomposition would increase the optical rotation of the solution, which was indeed observed in some of the present experiments. Furthermore, such secondary decomposition also results in the formation of colored degradation products. Qualitative experiments performed with 30% sucrose in homogeneous 3.V acid solutions (approximating the concentrations to be found within the resin particles) showed similar behavior. These sucrose solutions rapidly yellowed, darkened, and finally produced brownish precipitates. Some studies made with the heterogeneous system also showed similar behavior in which the clear reactant solution turned yellow during its passage through the reactor and deposits on the catalyst particles were formed. I n addition, catalyst activity was observed to decrease with time in these cases, undoubtedly because of the clogging of resin pores by degradation products. These experiments indicate clearly that a secondary reaction is taking place under these conditions. I t becomes significant as conversion increases for solutions of high sucrose concentrations and at high temperatures. No evidences of such degradation were found in the previous work with 15% sucrose solutions within the 50' to 70' C. range, even at very high conversions. This is not surprising, however, in view of the much lower fructose concentrations produced in the inversion of the more dilute feed. Since the present results indicate that external mass transfer effects are not significant even for solutions containing 4570 sucrose, it should be possible to predict the behavior of fixedbed reactors from experiments made in well stirred batch reactors. This simplification would permit design and scaleu p of such reactors with relative ease based on simple batch experiments. A test of this hypothesis was made by carrying out one experiment in a batch reactor using 4570 sucrose solution and 18- to 20-mesh resin particles. The apparent firstorder rate constant determined from these data was within 10% of that found for the fixed-bed experiments (Lifshutz, 1966). In view of the fact that truly first-order behavior is not observed in this system, this agreement is considered good. More quantitative agreement cannot be expected until one is able to specify in more detail the nature of the secondary reactions taking place and to determine thereby a more complete kinetic model for the over-all process.
This work has demonstrated that sucrose inversion may be carried out successfully in fixed beds a t concentrations up to 45% sucrose. However, considerable caution must be exercised in this range because of the imoortance and effects of secondary degravdation reactions. External mass transfer effects are not significant, even for such highly concentrated feed solutions. Rather, measured reaction rates are undoubtedly characteristic of the basic reaction kinetics under strong intraparticle diffusion influence.
Literature Cited
Jones, c. M., Lewis, lv. ‘2. M.2 J.C‘hem. sod. 1149 1120 (1920). Lifshutz, Norman, M. S. thesis, Northwestern University, Evanston, Ill., 1966. Moelwvn-Hughes. E. .4.. Z. Phw. Chem. B26.281 (1934’3. Moran; T., Lewis,’W. C.’ M., f.Chem. SOC.iil, 1613 (1922). O’Connell, J. E., D. Sc. thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1964. Reed, E. W., Dranoff, J. S., h d . Eng. Chem. Fundamentals 3, 304 (1964). RECEIVED for review April 14, 1967 ACCEPTED September 29, 1967
MIXED-GAS ADSORPTION AND VACUUM DESORPTION OF CARBON DIOXIDE
ON MOLECULAR SIEVE Thermo&amic and Rate Behauior PAUL FUKUNAGA AND K. C. HWANG AiResearch Manufacturing Co., Los Angeles, Calif. SAM H. DAVIS, JR., AND JACK WINNICK1 h T A S AManned Spacecraft Center, Houston, Tex:
A mathematical model for adsorption and desorption processes is given with experimental data. The data verify the model proposed and are used to determine several important operating and potential design parameters for the adsorption of CO2 on 5 A molecular sieve. The fundamental data reported include isotherm data for COZadsorption on Linde 5A molecular sieve. Empirically determined coefficients for mass transfer during adsorption and desorption cycles are given together with pressure drop coefficients for low pressures (during vacuum desorption). These empirical coefficients are compared in a numerical simulation for the small experimental bed. ESIGN
procedures for adsorption beds used in gas purifica-
D tion are well developed (Chemical Engineer’s Handbook, 1963). Special problems or limitations such as those encountered in space technology may require modifications of these procedures or a re-examination of some of the basic assumptions used in the development of the procedures. I n COS adsorption beds (with molecular sieve the primary adsorbent) most of the current designs require that the bed remain free of interfering contaminants (in particular water vapor), and bed regeneration be carried out a t a temperature high enough to remove essentially all COZ before the bed is re-used. I n space applications both of these requirements may be impossible (or too costly in power requirements) to achieve. In addition, critical weight requirements in space flights make a complete analysis of such adsorption beds worthwhile in order to determine optimum design parameters such as bed size, gas flow rate through the bed, thermal swing size between adsorption and desorption cycles, and cycle time. T h e need to determine the influence of many operating parameters in a rather complex system dictated a model approach for analysis of the molecular sieve bed. This analysis would include the determination of pertinent thermodynamic data such as equilibrium isotherms and heats of 1
Present address, AiResearch Manufacturing Co., Los Angeles, Calif.
adsorption, the evaluation of the relative importance of different transport resistances, and the determination of the most important rate coefficients, including those associated with mass transfer between the gas and the adsorbent sites and those associated with mass flow through the bed. Theory
Mass transport during adsorption must proceed by the following steps: Adsorbate is carried by bulk transfer in the gas stream, mass transfer occurs between the bulk gas and the exterior portion of the solid surfaces, mass transfer also occurs within the pores or over the interior surfaces of the solid, and time variation in the amount of adsorbate on the solid surfaces accounts for the accumulation of adsorbate. A Wth step is sometimes included to account for a “reaction” step a t the solid-fluid interface but is probably fast enough to be neglected. A mathematical model which accounts for each of these processes in a relatively simple manner was utilized in this study and found to agree well with the small scale dynamic adsorption data reported here. T h e mathematical model used in this analysis was based on the following assumptions.
1. T h e bed and gas flow are homogeneous over every bed cross section. 2. Mixing or dispersion in the direction of flow is negligible. VOL. 7
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