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Ind. Eng. Chem. Res. 1997, 36, 534-541
Kinetics of the Gas-Solid Hydroxyethylation of Potato Starch Norbert J. M. Kuipers and Antonie A. C. M. Beenackers* Department of Chemical Engineering, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
The kinetics of the reaction between gaseous ethylene oxide and semidry granular potato starch was studied in a pressure-controlled semibatch reactor with and without impregnation of the starch with the catalyst sodium hydroxide. Four parallel reactions are involved: the catalyzed (with reaction rate RRO-) and uncatalyzed hydroxyethylation of starch (rate RROH) and the catalyzed (rate ROH-) and uncatalyzed hydrolysis of ethylene oxide (rate RH2O). For 313 < temperature T < 368 K, 9.9 < moisture content W < 22.5 wt % dry basis, 0 < catalyst concentration cNaOH < 40 mmol/mol of anhydroglucose units, and 0 < ethylene oxide partial pressure pEO < 220 kPa, the reaction rates in units could be fitted by the following: uncatalyzed hydroxyethylation, eq 9; uncatalyzed hydrolysis, eq 10; catalyzed hydroxyethylation, eq 11; catalyzed hydrolysis, eq 12. The activity aEO ) pEO/pEO0 and pEO0 is the saturation pressure of ethylene oxide. High selectivities, approaching 80% toward the desired product hydroxyethyl starch, could be obtained at high T, low W, and high cNaOH. Introduction In hydroxyethyl starch, part of the hydroxyl groups of the glucose monomers have been converted into -O(2-hydroxyethyl) groups. The molar substitution (MS) is defined as the number of moles of ethylene oxide substituted per anhydroglucose unit (AGU) of starch. Hydroxyethyl starch with an MS below 0.1 is normally produced by reaction of starch with ethylene oxide in aqueous slurries of starch granules at temperatures below 320 K. NaCl or Na2SO4 is added to reduce swelling, whereas NaOH is used as a catalyst. The reaction product is easily separated from the reaction mixture by filtration. The product can be purified by washing with water. Hydroxyethyl starches with a low MS are applied in paper making, sizing, and coating and are used to size yarn in textile manufacturing (Moser, 1986; Wurzburg, 1986). Producing hydroxyethyl starch in an aqueous slurry becomes increasingly difficult for molar substitutions above 0.1 (Rutenberg and Solarek, 1984). The gelatinization temperature of the starch granules decreases with increasing MS. Highly substituted hydroxyethyl starch can be produced by contacting gaseous ethylene oxide directly with semidry starch granules in a gassolid reactor. Such products are used as a blood plasma volume extender and as a medium for protecting blood cells during freezing and thawing (Moser, 1986; Sputtek, 1990). The reaction temperatures can be higher than those in the slurry process because of an increasing gelatinization temperature with decreasing water content (Van den Berg, 1981). Apart from production methods described in patents of Kesler and Hjermstad (1950a-c) and limited introductory kinetic measurements of Van Warners et al. (1990) and Van Warners (1992), no information is available on this process. To the best knowledge of the authors, no data have been published concerning the concentration dependency of the kinetics of the hydroxyethylation reaction in a gas-solid system, whereas also no data are available on the kinetics of the uncatalyzed reactions in this system. Therefore, we investigated the * Author to whom correspondence is addressed. Phone: +31 50 634486. Fax: +31 50 634479. E-mail: G.Naber@ chem.rug.nl. S0888-5885(95)00570-7 CCC: $14.00
kinetics of both the uncatalyzed and catalyzed hydroxyethylation of potato starch in a gas-solid system. Reactions The main reaction in the OH--catalyzed starch hydroxyethylation, both in aqueous slurry and in solution, consists of three steps (Wurzburg, 1986; Lammers et al., 1993): c
K1
(1a) ROH + OH–
RO– + H2O
(1b) RO– + (CH2)2O
RO C C O–
(1c) RO C C O– + H2O
K2
c H2O RO–
with K1 = c c ROH OH– rate-determining step
RO C C OH + OH– c RO C C OH OH–
c
with K2 = c
(1)
ROH + (CH2)2O
c H2O RO C C O–
RO C C OH
The starchate ion produced in the weak acid-base reaction (1a) reacts with ethylene oxide [(CH2)2O], causing ring opening and simultaneous production of a new alkoxide ion (eq 1b). The latter ion picks up a proton from the water, thus liberating a hydroxide ion for further catalysis of the reaction, thereby producing hydroxyethyl starch, HES (reaction (1c)). The total amount of hydroxyl anions remains constant during the reaction. Acid-base reaction steps 1a and 1c are relatively rapid (Chlebicki, 1975). Reaction (1b), which proceeds according to an SN2 mechanism (Parker and Isaacs, 1959), is the rate-limiting step in the slurry process (Lammers et al., 1993). Therefore, this reaction has first-order kinetics in both ethylene oxide, EO, and the alkoxide ion, RO- (Van Warners, 1992):
RRO- ) kRO-cRO-cEO
(1)
Reaction (1) also occurs without a catalyst according to:
RROH ) kROHcROHcEO © 1997 American Chemical Society
(2)
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 535
As pointed out by Van Warners (1992) for the slurry process, RRO- increases with increasing water content. However, the presence of water also causes side reactions. In our case, ethylene glycol (EG) can be simultaneously produced via (Van Warners et al., 1990): HO C C O–
(2a) OH– + (CH2)2O
(2b) HO C C O– + H2O
K3
rate-determining step
HO C C OH + OH– c HO C C OH OH–
c
with K3 = c
(2)
H2O + (CH2)2O
c H2O HO C C O–
HO C C OH
The catalytic production rate of ethylene glycol can be described by (Van Warners, 1992):
ROH- ) kOH-cOH-cEO
Experiments
(4)
In principle, the HES and EG produced in reactions (1) and (2), respectively, can again react with ethylene oxide. However, the contribution of these reactions to the overall conversion rate of ethylene oxide can be neglected under the conditions applied in our experiments, i.e., MS < 0.2 (Merkus et al., 1977; Van Warners, 1992). For no catalyst added, the overall reaction rate of EO, REO, is the sum of the production rate of HES, according to eq 2, and the production rate of ethylene glycol, according to eq 4:
REO ) [kROHcROH + kH2OcH2O]cEO
(5)
The individual values of kROHcROH and kH2OcH2O follow from simultaneously measuring REO and the (differential) selectivity σ of ethylene oxide toward HES:
σ)
kROHcROH RROH ) RROH + RH2O kROHcROH + kH2OcH2O
(6)
If sodium hydroxide is added, the overall reaction rate of ethylene oxide follows from the sum of the catalyzed and uncatalyzed production rates of HES and ethylene glycol as described by eqs 1-4:
REO ) [kRO-cRO- + kROHcROH + kOH-cOH- + kH2OcH2O]cEO (7) From the mechanisms of reactions (1) and (2), it follows that cRO- and cOH- remain constant during the reaction. The overall selectivity σ now follows from:
σ)
[kROHcROH + kRO-cRO-] [kROHcROH + kRO-cRO- + kH2OcH2O + kOH-cOH-]
simultaneously solving eqs 5-8. The above rate equations have been applied for hydroxyethylation of both starch slurries and solutions (Van Warners, 1992; Lammers et al., 1993). However, no a priori information is available on the validity of these rate equations for the hydroxyethylation of semidry starch.
(3)
Also reaction (2) occurs without a catalyst according to:
RH2O ) kH2OcH2OcEO
Figure 1. Scheme of the experimental setup.
(8)
Assuming the uncatalytic reaction rates, i.e., kROHcROH and kH2OcH2O, to be independent of the amount of hydroxide added, kRO-cRO- and kOH-cOH- follow from
The gas-solid hydroxyethylation was studied in a stainless steel, pressure-controlled, semibatch reactor (inner diameter ) 49.4 mm, depth ) 100 mm); see Figure 1. Where possible, polymers were avoided, because of permeability problems and poor ethylene oxide resistance. However, Kalrez (a perfluorelastomer of Du Pont, compound number 4079) was used for sealing both the reactor and the stainless steel plungers of the magnetic valves. The latter seals had to be replaced every few months. Both solids and gas were mechanically stirred, usually at about 6.7 revolutions s-1. All gases used were at least of 99.9% purity. The reactor pressure and temperature were measured with a thermostated sensor (Barocel type 621, range 0-5000 mbar) and a Pt-100 (range 0-200 °C), respectively. The reactor, valves, and piping were thermostated. The weight of the ethylene oxide container was continuously recorded. The temperature of this container was kept at about 314 K, giving pEO ) 2900 mbar, which is sufficient to operate the reactor up to about 2500 mbar. All experiments were carried out with potato starch of AVEBE (Foxhol, The Netherlands). Before use, it was sieved and washed three times with deionized water to remove salts. In the case of alkalinization, a desired amount of NaOH solution was slowly added (to prevent gelatinization) to a stirred aqueous starch suspension under strict exclusion of atmospheric carbon dioxide. After this addition, the mass ratio of starch and water was 1:3. Sodium hydroxide additions of 0, 20, 40, and 60 mmol/mol of AGU (anhydroglucose unit of starch) were applied. To prevent gelatinization, 40 mmol of Na2SO4/mol of AGU was added to slurries containing 60 mmol of NaOH/mol of AGU. The alkalied starch slurry thus obtained was filtered with a Buchner filter. The filtrate was weighted and its NaOH content measured by titration with HCl. The starch was dried to the desired moisture content in a vacuum stove at 323 K. The value of cRO- was obtained by subtracting both the NaOH in the filtrate and the NaOH that reacted with the starch phosphate groups from the sodium hydroxide initially added (see reaction (1a)). From the titration of the alkalied starch in aqueous suspension with hydrochloric acid, the latter amount of phosphate
536 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 Table 1. cRO- and cOH- in the Semidry Starches Applied NaOH added (mmol/mol of AGU) 0 20 40 60b
W (wt % d.b.)
cRO(mol m-3 dry starch)a
cOH(mol m-3 dry starch)a
9.9, 14.2, 18.3, 22.5 14.2 14.2 9.9
0 63.1 132.9 429.5
0 19.3 31.6 21.4
a Based on F ) 1500 kg m-3 for starch with 9.9 e W e 22.5 wt p % d.b.; see Van den Berg (1981). b Also 40 mmol of Na2SO4/mol of AGU was added to the starch.
was measured as 4.37 mmol/mol of AGU. This agrees with the value of 4.14 mmol/mol of AGU reported by Van Warners (1992). cOH-, in mol m-3 starch dry basis (d.b.), could be determined by assuming equal OH- concentrations in the water phase of the starch and in the filtrate. Table 1 shows the compositions of the starches thus obtained. Note that the ratio cRO-/cOH- is substantially increased by the addition of Na2SO4 (Leach et al., 1961; De Willigen and De Groot, 1971). After drying the starch, 0.1 wt % Sipernat 22 (L.S., Degussa AG, Frankfurt) was added to make the powder free flowing. This improves gas-solid contact and prevents caking of the product on the walls of the reactor or on the blades of the agitator during the reaction. Also a more uniformly substituted hydroxyethylated product may be obtained this way (Tuschoff and Hanson, 1972). During the preparation of the alkalied starch powder, special care was taken to prevent contact of the starch (suspension) with carbon dioxide from the air. The CO2 was removed by bubbling the air through a sodium hydroxide solution. NaOH (Titrisol, Merck) and Na2SO4 (Merck) were of analytical grade. The volumetric mean diameter of the granules, as measured by a Coulter counter, was 34.5 µm (95% confidence limits 32.8-36.2 µm). After adding about 20 g of potato starch (dry basis) to the reactor, it was flushed with nitrogen. Then, while stirring, the reactor was heated to its operating temperature and, subsequently, ethylene oxide was added up to a desired pressure. The mass uptake of ethylene oxide by the granules resulted in a decrease of the pressure in the reactor below its setpoint, after which the magnetic valves (Herion) opened and additional ethylene oxide was added up to the setpoint (deviations from this setpoint were never more than 20 mbar). The weight of ethylene oxide sorbed therefore equals the weight loss of the reservoir. From each experiment an ethylene oxide mass uptake vs time graph was obtained. If no diffusion limitation occurred, REO was obtained directly from the resulting slope, once the starch was saturated with ethylene oxide. From the mass-uptake graph, both the mass uptake due to sorption and that due to reaction could be obtained. The slope is linear only if cROH and cH2O remain almost constant during reaction, i.e., for a low MS. This was always the case in our experiments. If diffusion limitation occurred, the mass-uptake graphs were fitted using the model of Kuipers and Beenackers (1993) to determine REO and D. The total amount of ethylene oxide reacted with both starch and water was measured by AVEBE (The Netherlands), using the methods of Morgan (1946) and Lortz (1956). After removing the ethylene glycol by washing with water, the MS of the starch product was measured in a similar way. The selectivity, σ, of the hydroxy-
Figure 2. REO/cEO,s as a function of cEO,s and REO/aEO as a function of aEO at T ) 313 K for potato starch particles of W ) 9.9 wt % d.b.
ethylation was calculated from the ratio of the converted ethylene oxide found in the washed and unwashed product, respectively. Results Uncatalyzed Gas-Solid Hydroxyethylation of Potato Starch. Figure 2 shows both REO/cEO,s as a function of cEO,s and REO/aEO as a function of activity aEO at T ) 313 K for starch of W ) 9.9 wt % d.b. Figure 2 shows no first-order behavior in the concentration of ethylene oxide, cEO,s. From the plot of REO/cEO,s vs cEO,s, again, the three regions of different penetrant behavior are found depending on the value of cEO,s (Kuipers, 1995). In region I, swelling is small because of no plasticization. Even then, REO/cEO,s is a function of cEO,s. In region II, substantial swelling occurs with a rapid rise of REO/cEO,s, with increasing cEO,s as a consequence. In region III, REO/cEO,s decreases with increasing cEO,s. In this region clustering occurs (Kuipers, 1995) which causes the ethylene oxide-ethylene oxide interactions to dominate relative to the interaction between ethylene oxide and the starch chains. So, clustering prevents part of the ethylene oxide molecules to contact, and therefore to react with, the starch chains. This means that the concentration of reactive ethylene oxide molecules is not represented by cEO,s for high cEO,s. The transition between regions II and III is not observed when nonideality effects are taken into account by plotting REO/aEO as a function of aEO, in which aEO ) pEO/pEO0 (Moore, 1985) with pEO0 the saturation pressure of ethylene oxide (Kirk and Othmer, 1985). Besides, first-order kinetics in ethylene oxide are observed in region I because here REO is proportional to aEO. Only ethylene oxide dissolved in the polymer matrix according to Henry’s law appears to contribute to reaction. In contrast, the ethylene oxide sorbed in the microvoids of the glassy starch contributes to neither swelling nor reaction, which causes REO to increase more than proportional with increasing cEO,s. Therefore, we describe the kinetics of hydroxyethylation in terms of aEO instead of cEO,s. Parts a and b of Figure 3 show the rate of the desired reaction, RROH, and the side reaction, RH2O, respectively. These reaction rates were obtained from the observed overall reaction rate REO and the selectivity: RROH ) σREO and RH2O ) (1 - σ)REO. The figures show the reaction rates to be almost proportional to aEO (for low aEO), but a quadratic relation is preferred to cover the total activity range. This is taken into account by the
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 537
Figure 4. Fits and experimental values of σ as a function of aEO for the uncatalyzed hydroxyethylation at various W and T. The fitted curves were obtained from eqs 9 and 10: σ ) RROH/(RROH + RH2O). For simplicity the fits are shown for the minimum and maximum W only: W ) 9.9 and 22.5 wt % d.b. (18.3 wt % d.b. for T ) 368 K), respectively.
Figure 3. Uncatalyzed hydroxyethylation rate of starch, RROH (a) fitted with eq 9, and the uncatalyzed hydrolysis rate, RH2O (b) fitted with eq 10, as a function of aEO.
fit equations for 9.9 < W < 22.5 wt % d.b. and 313 < T < 368 K:
[
RROH )
( (
aEO2 (1.3 ( 0.4) × 108 exp (7.0 ( 3.1) × 1012 exp -
[
) )
(59 ( 8) × 103 + RT
(94 ( 11) × 103 RT
aEO
RH2O ) aEO2 (0.018 ( 0.004) +
]
]
(9)
(10)
Figure 5. (a) REORp/(ucROH,0) as a function of aEO for various combinations of W and T where Case II diffusion was observed (uncatalyzed hydroxyethylation). (b) φ as a function of aEO for various combinations of T and W where Fickian diffusion in rubbery starch was observed (uncatalyzed hydroxyethylation).
RH2O is proportional to W which agrees with the hydroxyethylation in aqueous solution or suspension; see eq 4. Lammers et al. (1993) reported the activation energy for the uncatalyzed hydroxypropylation of starch in aqueous solution to be 71.2 kJ mol-1, whereas they observed Ea ) 77.2 kJ mol-1 for the uncatalyzed hydrolysis of propylene oxide. So, these values agree reasonably well with those of the uncatalyzed gas-solid hydroxyethylation of starch. The selectivity, as defined by eq 6, can be calculated from eq 9. Figure 4 compares the values of σ thus obtained to the experimental data for various aEO, T, and W. Usually, σ appears to increase with increasing aEO and increasing T. The fits also show such an increase of σ with increasing aEO. The rather poor fit is particularly due to the deviations in the fits of RROH
and RH2O, eqs 9 and 10, respectively. For a particular aEO, σ usually appears to decrease with increasing W and decreasing T. So, the highest selectivity is observed for relatively low W and high T. Assuming the overall reaction rate to be described by REO ) -k1,1cEO,scROH, Kuipers and Beenackers (1993) derived that a shrinking-core type of reaction may occur for k1,1cEO,sRp/u > 100, with u the velocity of the swelling front for anomalous and Case II diffusion and Rp the particle radius. Homogeneous substitution throughout the granule may be expected for k1,1cEO,sRp/u 0, whereas almost no dependency of σ on T is observed. Generally, a good agreement between fit and experimental data is obtained. Selectivities σ > 0.9 appear to be possible for starch that gelatinized during sorption and reaction (60 mmol of NaOH/mol of AGU, 40 mmol of Na2SO4/mol of AGU). Gelatinization of the starch caused the reaction rate to increase with time, as was observed for T > 353 K. Conclusions The reaction kinetics of the hydroxyethylation of granular potato starch was studied with and without
540 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997
the addition of sodium hydroxide as a catalyst. Four parallel reactions appear to be involved: the catalyzed and uncatalyzed hydroxyethylation of starch and the catalyzed and uncatalyzed hydrolysis of ethylene oxide. High selectivities toward hydroxyethyl starch of up to 80% could be obtained at higher T (g353 K), lower W (e 14.2 wt % d.b.), and higher cNaOH (g40 mmol/mol of AGU). The addition of NaOH results not only in higher reaction rates but also in a lower diffusivity of ethylene oxide; i.e., diffusion limitation is promoted by NaOH. Above 333 K, gelatinization of the starch was observed for NaOH contents > 60 mmol/mol of AGU.
RO- ) dissociated starch hydroxyl group ROH ) starch hydroxyl group ROH- ) catalyzed reaction rate to EG, mol m-3 starch d.b. s-1 RRO- ) catalyzed reaction rate to HES, mol m-3 starch d.b. s-1 RROH ) uncatalyzed reaction rate to HES, mol m-3 starch d.b. s-1 T ) temperature, K t ) time, s u ) Case II front velocity, m s-1 W ) moisture content of (alkalied) starch in weight percent on dry basis, wt % d.b. WEO ) specific mass uptake, kg of EO kg-1 of starch d.b.
Acknowledgment The investigations were financially supported by AVEBE, b.a., Veendam, the Dutch Carbohydrate Research Foundation (IOP-K, ′s Gravenhage), The Netherlands Foundation for Chemical Research (SON), and the Technology Foundation (STW, Utrecht). We thank Mr. J. van de Meer and Mrs. R. Ziengs of AVEBE Research Laboratory, Foxhol, for advice and for performing part of the analytical work.
Greek Symbols
Nomenclature aEO ) activity of ethylene oxide in the gas phase, aEO ) pEO/pEO0 AGU ) anhydroglucose unit, the monomer unit of starch cEO ) concentration of ethylene oxide, mol m-3 starch d.b. cEO,s ) equilibrium concentration of ethylene oxide, mol m-3 starch d.b. cNaOH ) amount of NaOH per mol of AGU, mmol (mol AGU)-1 cOH- ) concentration of OH-, mol m-3 starch d.b. cRO- ) concentration of RO-, mol m-3 starch d.b. cROH ) concentration of ROH, mol m-3 starch d.b. cROH,0 ) initial concentration of reactive hydroxyl groups of starch, mol m-3 starch d.b. D ) diffusion coefficient of ethylene oxide in starch, m2 s-1 Ea ) activation energy, J mol-1 EG ) ethylene glycol EO ) ethylene oxide HES ) hydroxyethyl starch K1 ) equilibrium constant for first step in hydroxyethylation, see reaction (1a) k1,1 ) rate constant of a (1,1) order reaction, m3 mol-1 s-1 K2 ) equilibrium constant for the final step in hydroxyethylation, see reaction (1c) K3 ) equilibrium constant for the final step in catalyzed hydrolysis of glycol, see reaction (2b) kH2O ) rate constant for uncatalyzed hydrolysis of glycol, m3 starch d.b. mol-1 s-1 kOH- ) rate constant for catalyzed hydrolysis of glycol, m3 starch d.b. mol-1 s-1 kov ) overall first-order reaction rate constant, s-1 kRO- ) rate constant for catalyzed hydroxyethylation, m3 starch d.b. mol-1 s-1 kROH ) rate constant for uncatalyzed hydroxyethylation, m3 starch d.b. mol-1 s-1 MS ) molar substitution, moles of ethylene oxide bound per mol of AGU pEO ) partial pressure of ethylene oxide, Pa pEO0 ) saturation pressure of ethylene oxide, Pa R ) gas constant: 8.314, J mol-1 K-1 Ra ) reaction rate of A, mol m-3 s-1 REO ) overall conversion rate of EO, mol m-3 starch d.b. s-1 RH2O ) uncatalyzed reaction rate to EG, mol m-3 starch d.b. s-1 Rp ) radius of starch particle, m
φ ) Weisz modulus, φ ) REORp2/(cEO,sD) γEO ) activity coefficient of EO in starch Fp ) density of starch, kg m-3 starch d.b. σ ) selectivity, defined by eqs 6 and 8
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ethylene oxide in starch granules in a gas-solid system. Starch/Sta¨rke 1990, 42, 427. Virtanen, P. O. I. Kinetics of the reactions of ethylene oxide with nucleophiles. Ann. Acad. Sci. Fenn. Ser. 1963, AII, 124. Westerterp, K. R.; Van Swaaij, W. P. M.; Beenackers, A. A. C. M. Chemical reactor design and operation, st. ed.; John Wiley & Sons: New York, 1987. Wurzburg, O. B. Modified starches: properties and uses; CRC Press Inc.: Boca Raton, FL, 1986.
Received for review September 14, 1995 Revised manuscript received September 13, 1996 Accepted September 19, 1996X IE950570L
X Abstract published in Advance ACS Abstracts, January 1, 1997.
