Sorption and Diffusion of Ethylene Oxide in Semidry Potato Starch Granules
The sorption and diffusion of gaseous ethylene oxide in semidry potato starch granules was studied by measuring the time-dependent mass-uptake rate of ethylene oxide by starch in an isothermal pressure-controlled semibatch reactor as a function of the ethylene oxide solubility (c EO,s < 8.7 kmol·...
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| Published in: | Industrial & engineering chemistry research 2003-11, Vol.42 (24), p.6068-6079 |
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| Main Authors: | , , , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | The sorption and diffusion of gaseous ethylene oxide in semidry potato starch granules was studied by measuring the time-dependent mass-uptake rate of ethylene oxide by starch in an isothermal pressure-controlled semibatch reactor as a function of the ethylene oxide solubility (c EO,s < 8.7 kmol·m-3), moisture content of the granules (5.9 ≤ W ≤ 22.5 wt % dry basis), and temperature (293 ≤ T ≤ 368 K). The data obtained, together with results of the chemical kinetics, are needed for the design of a reactor for the gas−solid hydroxyethylation of potato starch. The distribution coefficient of ethylene oxide, m EO, defined as the ratio of the concentrations of ethylene oxide in the starch and in the gas phase at equilibrium, could be fitted by the expression m EO = 1.69 × 10-4 exp(−0.65a EO + 6.1/W + 31.6 × 103/RT), with a EO = p EO/ as the ethylene oxide activity. Depending on W and T, the diffusion of ethylene oxide in potato starch is either Fickian, anomalous, or relaxation-controlled (case II diffusion). The latter is observed at T = 313 K for 5.9 ≤ W ≤ 9.9 wt % d.b. and c EO,s > , where is the threshold concentration for case II diffusion. Here, the case II front velocity u can be described by a power-law equation that takes into account the preswelling of the granules by water, namely, u = K w + K(c EO,s − ) n with K w = 2.14 × 10-10(W − 5.8)0.58, = 6.5 − 0.53W, K = 8.7 × 10-10 + 8.6 × 1011/W 24.6, and n = 7.0−0.33W. The Fickian diffusion coefficient D of ethylene oxide in rubbery potato starch appears to have a maximum, D max = 5.38 × 10-13 exp(0.22W − 15.9 × 103/RT), as a function of c EO,s at = 28.9 − 0.071T − 0.33W. For rubbery starch, the diffusion coefficient is fitted by D/D max = 0.19 exp(1.58c EO,s/ ) for c EO,s/ < 1 and by D/D max = 1.64 exp(−0.34c EO,s/ ) for c EO,s/ ≥ 1. These relations are valid above the glass transition temperature T g, i.e., for W ≥ 14.2 wt % d.b. and T ≥ 313 K or for W < 14.2 wt % d.b. and T ≥ 313 K, provided c EO,s ≥ . The diffusion coefficient for potato starch in the glassy state is described by D = 2.12 × 10-8 exp(0.49W + 0.51c EO,s − 57.2 × 103/RT) for 293 ≤ T ≤ 313 K, 5.9 ≤ W ≤ 9.9 wt % d.b., and c EO,s < . For temperatures very well below T g, so-called two-stage sorption was observed because of a slow increase in the solubility of ethylene oxide in potato starch with time. Based on sorption, diffusion and data concerning the chemical kinetics of the uncatalyzed and catalyzed hydroxyethylation of semidry potato st |
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| ISSN: | 0888-5885 1520-5045 |