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 (cEO,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, mEO, 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 mEO = 1.69 × 10-4 exp(−0.65aEO + 6.1/W + 31.6 × 103/RT), with aEO = pEO/ 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 cEO,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 = Kw + K(cEO,s − )n with Kw = 2.14 × 10-10(W − 5.8)0.58, = 6.5 − 0.53W, K = 8.7 × 10-10 + 8.6 × 1011/W24.6, and n = 7.0−0.33W. The Fickian diffusion coefficient D of ethylene oxide in rubbery potato starch appears to have a maximum, Dmax = 5.38 × 10-13 exp(0.22W − 15.9 × 103/RT), as a function of cEO,s at = 28.9 − 0.071T − 0.33W. For rubbery starch, the diffusion coefficient is fitted by D/Dmax = 0.19 exp(1.58cEO,s/ ) for cEO,s/ < 1 and by D/Dmax = 1.64 exp(−0.34cEO,s/ ) for cEO,s/ ≥ 1. These relations are valid above the glass transition temperature Tg, 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 cEO,s ≥ . The diffusion coefficient for potato starch in the glassy state is described by D = 2.12 × 10-8 exp(0.49W + 0.51cEO,s − 57.2 × 103/RT) for 293 ≤ T ≤ 313 K, 5.9 ≤ W ≤ 9.9 wt % d.b., and cEO,s < . For temperatures very well below Tg, 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 starch particles, a schematic summary of the physical behavior of ethylene oxide in semi-dry glassy potato starch is proposed as function of aEO.