I n d . E n g . Chem. Res. 1989, 28, 237-243
237
Gravimetric Analysis of Hydrogen Fluoride Vapor Absorption by Bigtooth Aspen Wood William R. Mohring and Martin C. Hawley* Department of Chemical Engineering, Michigan State University, East Lansing, Michigan 48824
To obtain data necessary for evaluation of hydrogen fluoride (HF) saccharification processes, a gravimetric techinque for measuring the quantity of HF vapor absorbed by lignocellulose was developed and used to measure absorption and desorption isotherms for dry bigtooth aspen wood (Populus grandidentutu) for temperatures from 30 to 80 "C and HF partial pressures from 0 to 1 atm. Hysteresis was observed in the absorption-desorption loop, apparently due t o cellulose decrystallization and saccharification. By use of the sugar yield data obtained elsewhere, the minimum HF requirement t o achieve the maximum sugar yield was determined t o be between 0.32 and 0.60 g of H F / g of wood. T h e heat of vaporization of HF from wood was calculated from the isotherm data using the Clausius-Clapeyron equation. Kinetic data for absorption and desorption were shown t o be influenced by heat-transfer limitations a t the conditions studied. The use of lignocellulose (wood) as a raw material for the production of fuels and chemicals is desirable as a way of reducing dependence on nonrenewable resources and also utilizing large amounts of wood waste produced by industry. One of the major difficulties in making woodbased chemical processes economically attractive is the extensive pretreatment required to break down the cellulose in wood to simple sugars (saccharification) which can then be processed by using fermentation. Dilute acid hydrolysis of cellulose is the most highly developed of the processes available, but yields of sugar are low due to sugar degradation. Concentrated acid processes, on the other hand, produce high yields, but the costs associated with acid consumption or recycle can be prohibitive. This study is part of an ongoing research program investigating anhydrous hydrogen fluoride (HF) saccharification of lignocellulose. This process produces the high sugar yields characteristic of other concentrated acid processes. In addition, because HF can be easily desorbed after reaction, it may be possible to develop a process with more economical acid recycle. Acid losses are, however, more critical to the economics of the HF process because HF is more expensive than other acids, such as hydrochloric and sulfuric. HF saccharification technology is less highly developed in comparison with other saccharification technologies, probably due in no small part to the difficulties in handling HF and its toxicity. However, HF is used in other industries, such as petroleum processing, and its adaptation to wood processing should also be possible.
Literature Survey of HF Saccharification The first major studies of hydrogen fluoride saccharification were done in Germany (Helferich and Bottger, 1929; Fredenhagen and Cadenbach, 1933). Fredenhagen and Cadenbach showed that HF saccharification of cellulose proceeds through cellulose dissolution (facilitated by the strong tendency of HF to form hydrogen bonds with the hydroxyl groups on the cellulose molecule) and the formation of a glucosyl fluoride intermediate (CGHI,O5F). The reaction rate was found to decrease with increasng water content of the system. In the presence of HF and H20, glucosyl fluorides were found to revert to easily hydrolyzed sugar oligomers. They also showed that saccharification of spruce wood is feasible using liquid or gaseous HF (normal boiling point of HF is 19.5 "C). In the gas-phase process, a 95% yield of sugar was obtained using an HF:wood ratio of 1:l by mass. Almost 99% of the HF could be removed after reaction by evaporation at
100 "C. The sugar oligomers were then washed from the lignin using H 2 0 or dilute acid. As a result of the work of Fredenhagen and Cadenbach, HF saccharification processes were investigated at the pilot scale. In one process (Luers, 19381, gaseous HF at a reduced pressure of 30 mmHg was exposed to wood chips at low temperature to cause HF absorption. After reaction, the HF was removed at low pressure by increasing the temperature to a maximum of 62 "C. The HF was then recycled after removing acetic acid that was produced as a byproduct. Apparently, work on HF saccharification was discontinued due to World War 11. HF saccharification was reinvestigated starting in 1979 here at Michigan State University (MSU) and also at approximately the same time by several researchers in Europe. At MSU, the first investigations used liquid HF with a water content of less than 10% and pure cellulose or bigtooth aspen (Populus grudidentata) chips. Yields of D-glucose (after posthydrolysis of the reversion oligomers) from pure cellulose approaching 100% were attained after 1h a t temperatures from 0 to 23 "C. Yields of D-glucose and D-xylose from wood were around 80%. These sugars were also found to ferment to ethanol. After extended evacuation at 100 "C, the fluoride content of the reacted aspen wood could be reduced to as low as 0.4% by weight, with the residual fluoride apparently bound by metal ions in the wood (Selke et al., 1982). Also, the glucosyl fluoride intermediate was isolated and identified (Hardt and Lamport, 1982). The gas-phase HF process has also been investigated at MSU (Rorrer et al., 1986, 1987, 1988). From the standpoint of commercialization, it is probably superior to the liquid-phase process because less HF is needed to achieve the same conversions, making HF recycle more economical. Also, the temperature in the gas-phase process can be varied above the normal boiling point of HF while still keeping the system a t atmospheric pressure. Results of kinetics studies using aspen wood showed that an 80-90% yield of D-glucose and a 7 0 4 0 % yield of D-xylose could be obtained (after posthydrolysis) in only a few minutes using pure HF vapor a t atmospheric pressure and 30 "C. The rate of saccharification was found to decrease with decreasing HF pressure and increasing temperature. Also, maximum sugar yields decreased at low HF pressures and high temperatures. Several investigators have recently studied gas-phase processes at the pilot scale (Franz et al., 1982; Ostrovski et al., 1984; Reffstrup and Kau, 1985). In each case, a
0888-588518912628-0237$01.50/0 0 1989 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989
glucose yield of 80-90 70and a xylose yield of 70-80 70were obtained, and also at least 99% of the H F was recovered after reaction. Objectives of T h i s S t u d y In order to carry out detailed reaction modeling of gas-phase HF saccharification, the H F absorption phenomenon needs to be characterized. Thus, this study was initiated to quantify the absorption equilibria and kinetics by measuring the H F loading, which will be defined as the mass of H F absorbed as a percent of the lignocellulose mass. Results from the work to be described have already been used to develop a reaction model for H F saccharification of lignocellulose (Rorrer et al., 1988). The model assumed that the rate-determining steps are dissolution of cellulose by H F and reaction of the solvated cellulose with H F to form glucosyl fluoride. The H F loading data were used to supply the H F concentration term in the rate equations. H F loading information is also necessary in order to develop and analyze gas-phase H F processes. It can be used to calculate HF flows, heat loads, and times necessary to carry out the absorption-reaction-desorption sequence. H F loading data a t different H F partial pressures and temperatures are needed to analyze schemes for absorbing and desorbing the H F in a continuous fashion. For this study, a gravimetric method for measuring H F loading under controlled conditions of H F partial pressure and temperature was developed. Because no gas-phase species are formed during H F saccharification (except for possible minor amounts of degradation products) that would affect the sample weight, the amount of H F absorbed is the same as the weight change of the sample during H F exposure. The validity of this assumption was tested by comparing the initial weight with the weight after H F desorption. A sensitive balance was used to continuously monitor the sample weight during H F exposure. This method was then used to obtain H F absorption and desorption isotherms (HF loading versus H F partial pressure at constant temperature) and kinetic data using dry bigtooth aspen wood a t H F partial pressures from 0 to 1 atm and temperatures from 30 to 80 "C. These conditions were chosen as the range likely to be of interest for process design. Low temperature and high H F pressure are desirable for absorbing HF. Temperatures much below 30 O C are undesirable for a process because of the need for a refrigeration system, and pressures above 1 atm are unnecessary and could cause the H F to condense in the equipment. High temperatures and low HF pressures are desirable for desorbing the HF, and these are represented in the range of variables. As in other studies a t MSU, bigtooth aspen was used because it is representative of species being developed for possible use in "energy plantations". It is a fast-growing species and has the ability to grow out from a stump after harvesting, thus eliminating the need for replanting. The H F loading data obtained were analyzed both to gain insight into the chemical and physical processes which occur during H F absorption and also to determine parameters expected to be useful in reaction modeling and process design studies. Actual reaction modeling and process development is, however, beyond the scope of this paper. Experimental Section Description of Apparatus. The flow diagram for the experimental apparatus is presented in Figure 1. A description of the system, which has been described previously (Rorrer et al., 19881, follows:
Figure 1. Flow diagram of apparatus. Legend: F1, nitrogen mass flowmeter; F2, HF mass flowmeter; V2, HF control valve; R, rotameter; P, pressure gauge; T, temperature probe; D, drying bed; CV, check valve; RV, relief valve.
HF vapor was removed from the top of a tank containing anhydrous HF liquid (minimum 99.970 purity). The tank was held in a water bath 2-4 "C above the normal boiling point of HF (19.5 "C) in order to cause HF flow. The flow rate of H F was controlled by using a mass flowmeter, electronic control valve, and controller purchased from Matheson Company. Calibration of the flowmeter was done by measuring the change in weight of the H F tank for a given time period at a given flow rate. Because the flowmeter output was found to be nonlinear, only two flow rates (0.23 f 0.01 SLPM and 0.98 f 0.03 SLPM; SLPM is standard liter per minute; standard conditions were 21 "C and 1 atm) were used. Extra-dry nitrogen (minimum 99.7% purity) was removed from a high-pressure tank and was further dried by passing over a molecular-sieve bed before being used to dilute the HF. The flow rate was controlled by using a mass flowmeter from Matheson (0-3 SLPM range, 1% accuracy) and a manual metering valve. In order to prevent HF condensation which can occur during mixing with N2, the HF stream was heated prior to mixing using heater cord wrapped around the tubing. The diluted stream was preheated to the temperature of the reactor and then was fed to the reactor to contact the sample. Temperature control for this stream was provided by a PID controller and a thermocouple located in the feed tube to the reactor. The partial pressure of H F in the reactor was determined from the H F and Nzflow rates, and all the experiments were conducted under flow conditions a t atmospheric pressure to maintain a constant concentration of H F in the reactor. The thermobalance (reactor and balance) used a novel design adapted from an earlier study (Costa and Smith, 1971) whereby the balance mechanism and the sample are put in separate enclosures without a direct connection to avoid contacting the balance with corrosive H F vapor. A schematic diagram of the reactor is given in Figure 2. The reactor was made from a 304 stainless steel pipe (1.5 in., schedule 40). The top and bottom halves were flanged to permit sample replacement. A thin sample of bigtooth aspen wood (20-100 mg) was held in the bottom half of the reactor by a O.OOl-in.-diameter chrome1 hang-down wire which was connected to a Cahn 2000 electrobalance located above the reactor. The wire passed through two orifices (0.015-in. and 0.03-in. diameter) located in the top half of the reactor. A specially designed stand kept the balance and reactor aligned so that the wire did not rub on the walls of the orifices. The weight of the sample was measured to within 0.1 mg, which
Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 239 001 DlA W I R E
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Figure 2. Reactor schematic.
was the highest resolution that could be obtained given the amount of noise produced in the weight signal by this configuration. The sample was placed inside a doubledover piece of monel screen, with a piece of Teflon film covering the bottom of the sample to prevent particles from dropping off the sample, which tended to become brittle after exposure to H F vapor. The HF-N2 mixture entered the bottom of the reactor, flowed up past the sample, and then exited through a baffle which was inserted between the flanges. A low flow rate of N2 ("containment N2") was metered to the top of the reactor using a rotameter and leaked out through the two orifices, creating a slight pressure increase which prevented the gases in the bottom of the reactor from escaping through the top of the reactor and into the atmosphere. The baffle between the flanges prevented the N2 that was flowing down from the top of the reactor from affecting the H F concentration. The N2 flowing out through the orifices created a small drag force on the hang-down wire, which affected the sample weight. A correction factor for this drag and any other systematic errors in the weight was determined by running the system under the same conditions as for the absorption experiments but with no sample present. The magnitude of this correction was less than 2 mg for H F pressures less than 0.1 atm and was less than 1mg for all other HF pressures. The temperature in the bottom half of the reactor was maintained to within 1 "C of the desired value by an electrical resistance heater wrapped around the reactor. Power input was controlled by using a proportional controller and a thermistor probe located near the sample. The reactor was insulated for experiments above 40 "C. Steel balls placed in the bottom of the reactor helped to heat the incoming feed and stabilize the reactor temperature. Because of the high toxicity of HF, the effluent from the reactor was passed through a bed containing calcium carbonate chips to neutralize the H F before releasing the gas stream into the atmosphere. Also, protective clothing was worn while operating the equipment, which was situated in a laboratory fume hood. All of the process variables were printed out during the course of the experiment and saved on a floppy disk using an IBM PC-XT microcomputer equipped with an analog-to-digital converter. Experimental Procedure. The following is a summary of the procedure used to measure the absorption and desorption isotherms:
A thin chip of bigtooth aspen (weighing 20-100 mg, with dimensions of 1 cm X 1 cm X 0.05 cm to 1cm X 1 cm X 0.25 cm) was placed in the tared sample holder, which was then enclosed in the reactor. The chip was heated to 101-105 "C in dry, flowing N2 for about 30 min in order to completely dry the chip (dryness achieved when chip weight stabilized). The reactor temperature was then adjusted to the value to be used for the experiment, and the weight of the sample was recorded as the initial sample weight. The HF partial pressure was stepped up and then down a predetermined set of values. Before the H F pressure was changed, the chip weight was allowed to come to steady state. Steady state was assumed to be achieved after the weight remained steady for at least 5 min. Some points were run for longer times (>1h) and did not show appreciable weight changes after the 5-min criterion was met. For the experiment in which the temperature of the chip was measured, a 0.07-cm-diameter hole was drilled 0.3-cm deep into the edge of a 1-cm X 1-cm X 0.2-cm chip, and a small, bare Type T thermocouple probe was lodged in the hole. Additional Experimental Information. The average chemical composition of bigtooth aspen wood is (by mass) 50% cellulose, a poly(D-glucosan) ((CsHloOs),); 29% hemicellulose, a polysaccharide giving on hydrolysis 75% Dxylose, 20% D-glucose, and 5% other sugars; 16.6% lignin; 4.1% extractives; and 0.3% ash (Selke, 1983). The variables used in the experiment are given below: HF loading = (weight change of sample due to H F exposure/weight of dry sample)100% (1) The resolution of the balance limited the precision of the HF loading to f0.5% (20-mg sample) or better, depending on sample size. The H F partial pressure in the reactor will be denoted as P H F and calculated from
where P is the system pressure (atm), f is the association factor (actual vapor density/ideal vapor density) of H F vapor at P H F and the system temperature, and y is the mole fraction of HF in the reactor. The association factor was calculated from the equations developed by Smith (1958). The HF partial pressure was accurate to fO.O1 atm below 0.5 atm and f0.02 atm above 0.5 atm.
Results and Discussion Absorption and Desorption Isotherms. The absorption isotherms at 30,40, and 50 "C are shown in Figure 3 and the isotherms for 60, 70, and 80 "C are shown in Figure 4. Each curve represents the results of a single experiment where the partial pressure of HF started at 0 and gradually was increased to 1 atm. The typical experiment took between 1 and 2 h. The isotherms include HF which is physically absorbed by the carbohydrates and lignin in wood as well as the HF which has reacted to form sugar fluorides. A small amount of water, which is present in H F in trace quantities, may also be absorbed. Stoichiometrically, 10 g of H F is required per 100 g of wood to react with the cellulose and hemicellulose. The amount of H F in the form of sugar fluorides may be less than the stoichiometric requirement, because of incomplete reaction and because H F may be regenerated by sugar reversion. Both saccharification and reversion are believed to take place through a carbonium ion intermediate (Defaye et al., 1983), same as the accepted mechanism for acid-catalyzed hydrolysis. It is generally held (Harris, 1975) that rapid
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PHF(ATM) Figure 4. Absorption isotherms for 60, 70, and 80 OC.
cellulose hydrolysis by acids requires that the cellulose be decrystallized so that the glucose rings are free to flex and form the planar carbonium ion. If the assumed mechanism for H F saccharification is correct, then cellulose decrystallization by HF must occur prior to reaction. H F can decrystallize cellulose by forming hydrogen bonds with the hydroxyl groups in cellulose which bond the molecules together in the crystal. The reaction sequence is cellulose or hemicellulose + H F sugar fluorides ~ r ! reversion oligomers + H F
-
Apparently, significant reversion of glucose can occur under the conditions of the isotherm experiments. This is shown by the work of Rorrer et al. (1987), who analyzed the products from vapor-phase HF solvolysis of pure cellulose. At 50 " C and 1 atm of HF, for example, reversion products with a degree of polymerization of >8 were found to be the predominant product. There is also some evidence that these sugars can react with the lignin in the presence of H F to form condensation products (Defaye et al., 1983). H F lignin does appear to be somewhat condensed. However, methoxyl analyses indicate that the ether linkages in the lignin remain intact (Defaye et al., 1983). The absorption isotherms exhibit a characteristic shape which will be described here. The slopes appear to be relatively large near P H F = 0 but decrease rapidly with increasing P H F . No data were obtained at P H F less than 0.07 atm because of equipment limitations which made it difficult to get very low, stable flow rates of H F vapor. Starting at an H F loading of about 15% and extending to 25 % , the slopes of the isotherms increase rapidly, forming "bumps" in the curves. At around 2570,the slopes decrease
somewhat and the curves continue up smoothly as PHF increases to 1 atm. When the dried aspen was immediately exposed to HF vapor at 1 atm, the HF loading obtained is greater than that obtained from the isotherm experiment (where the H F pressure was increased gradually). At 50 "C, the difference in H F loading is 8% (0.08 g of HF/g of wood); at 30 "C, the difference is 16%. Desorption isotherms were measured after completing the absorption measurements. These are shown along with their associated absorption isotherms for 40 and 60 "C in Figures 5 and 6, respectively. For both temperatures, the absorption and desorption isotherms are coincident above an H F loading of approximately 25%. Below this value, the desorption isotherms lie above the absorption isotherms, thus exhibiting hysteresis. The desorption isotherm for 80 "C was also measured (but not shown) and found to be coincident with the absorption isotherm over the entire range of P H F . In all the experiments, it was possible to return the wood sample to, or slightly less than, its initial weight by desorbing the HF into pure N2. This indicates that most of the sugar fluorides reverted to oligomers, releasing the HF. It is expected that the residual fluoride content after desorption was near 0.4%, which was determined in earlier studies at MSU (Selke et al., 1982). Referring to Figures 5 and 6, the H F loadings a t P H F = 0 on the desorption isotherms are negative, which is a result of the final desorbed weight of the sample being less than the initial weight. This may be due to the formation of volatile degradation products such as acetic acid. It also may be due to some error in the measurement system, since the weight losses were not consistently observed. In all experiments, however, the final desorbed weight was within 2.5% of the initial weight. The precision of the isotherm measurements was examined by repeating a number of them. Between HF loadings of 15% and 25% (approximately), where the isotherms are steep, the difference in measured H F loading between
Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 241
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Table I. Effect of HF Loading on Maximum Sugar Yield glucose temp, "C HF loading, % yield, 70 xylose yield, % 30 160 83 70 40 89 85 60 50 60 85 63 66
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identical runs was found to be as large as 10% (0.10 g of HF/g of wood). In other areas of the isotherms, the difference was typically less than 2% ; the largest observed difference was 3%. Some of the discrepancies can probably be attributed to slight differences in sample composition. Recently, H F desorption isotherms for prehydrolyzed (hemicellulose removed) wood appeared in the literature (Franz et al., 1987). For these experiments, the weight of a reactor filled with lignocellulose was measured. Because the lignocellulose used was different from that used here, the accuracy of the data from either experiment cannot be confirmed. It is interesting, however, that the desorption isotherms for the prehydrolyzed wood were higher (by as much as 0.2 g of HF/g of wood) than those measured here for untreated aspen wood. Relationship between Absorption and Yield Data. Previous studies have found that there is a minimum HF loading which is necessary to obtain the maximum sugar yield from lignocellulose. This behavior is probably because a certain amount of H F is necessary to decrystallize the cellulose so it can react, as discussed previously. The minimum H F loading required for maximum sugar yield is an important quantity from a process standpoint. Table I shows maximum D-glucose and D-xylose yield data obtained (Rorrer et al., 1988) for bigtooth aspen wood at various temperatures using H F vapor at 1 atm and also the H F loading data from this study under the same conditions. From this data, the minimum HF loading required to obtain the maximum yield of D-glucose is between 32% and 60%. For D-xylose, the minimum H F loading is between 13% and 32%. The slight increase in xylose yield at 30 "C is probably not significant, given the experimental uncertainties. The lower requirement of HF for maximum xylose yield is consistent with the fact that it is in the amorphous hemicellulose fraction of wood. It is possible that, by varying HF partial pressure instead of temperature (as is done in Table I) to change the H F loading, somewhat different results for the minimum H F loading could be obtained. These results for minimum H F loading are in the range of values determined by other investigators using different types of lignocellulose. Fredenhagen and Cadenbach (1933) found that an H F loading of 100% is necessary for spruce wood. Franz et al. (1982) reported between 40% and 80% is necessary, depending on the starting material and whether it was subjected to prehydrolysis. The yield data are also helpful in trying to explain the unusual bump observed in the absorption isotherms at H F loadings of 15-25%. Because the yields of glucose and xylose are increasing toward their maximum values at HF loadings of 15-2570, the bump in the isotherms may be due to chemical or physical changes related to saccharification. Most apparently, the disruption of the crystal structure of cellulose may make the hydroxyl groups on the cellulose chain more accessible and cause an increase in H F absorption. The hysteresis observed in the absorption-desorption loop below an HF loading of 25% is consistent with this explanation since the material does not recrystallize as H F is removed.
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Figure 7. Clausius-Clapeyron plots at HF loadings of lo%, 35%, 6O%, and 90% . Table 11. Heat of Vaoorization Data HF loading, 70 f 10 1.01 35 1.03 1.13 60 1.42 90 m 3.43
AHv, cal/mol of HF 10900 8 200 6 800 5 800
1800
Thermodynamics of Absorption. From the absorption isotherms, it is possible to calculate the heat of vaporization of H F from aspen wood at various levels of H F loading using the Clausius-Clapeyron equation. This approach was originally suggested by Franz et al. (1987). These heats of vaporization are expected to be useful in process design studies for calculating heat loads and also for studying the effects of heat transfer on the kinetics of the H F saccharification process. The Clausius-Clapeyron equation is (for constant AHvf) - M v f
In P H F = -+ C (3) RT where P H F is the partial pressure of H F (atm); AHvis the heat of vaporization of H F from wood (cal/mol of HF); f is the association factor for H F vapor; T is temperature (K); R is the ideal gas constant, 1.987 cal/(mol K); and c is a constant. This equation can be applied to P H F versus T equilibrium data where the H F loading is constant. Under the conditions of these experiments, H F vapor is slightly polymerized. Thus, the equation was derived without using the usual ideal gas assumption for the vapor. Instead, vapor nonideality is taken into account using the association factor, which is defined as the ratio of the actual vapor density to its ideal gas value. The semilog plots of PHF versus 1/T are shown in Figure 7 for HF loadings of lo%, 35%, 60%, and 90%. The lines in Figure 7 were fitted to the data by using linear regression. From the slopes of the lines and the calculated association factors, AHv's were obtained by using the Clausius-Clapeyron equation. The association factor was calculated at each point from the equations of Smith (1958), who fitted HF vapor density data by assuming a monomer-tetramer-hexamer equilibrium exists in H F vapor. Fortunately, the association factors are relatively constant, varying by less than 2% at each H F loading. This observation combined with the linearity of the Clausius-Clapeyron plot leads to the conclusion that AHv at a given HF loading is relatively constant over the range
242 Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989
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of variables studied. These data are presented in Table 11. For comparison, the heat of vaporization of pure H F (a HF loading) at its normal boiling point is also included in Table 11. The heats of vaporization show a definite decreasing trend with increased loading. As would be expected, the heat of vaporization gets closer to the heat of vaporization for pure H F as the H F loading increases. The AH,'s calculated here tend to be 2-3 times larger than those reported by Franz et al. (1987) using the same type of analysis of isotherm data for prehydrolyzed wood. Some, but probably not all, of the differences in AH, between the two studies could be because the lignocellulose in each was slightly different. However, the error in Franz et al.'s values could be quite large because they are based on only two closely spaced (10 "C) equilibrium points at each H F loading. Also, some of their calculations are incorrect due to their assumption that the association factor is independent of H F partial pressure. Kinetics of HF Absorption and Desorption. Kinetic data for HF absorption and desorption are necessary for reaction kinetics studies as well as design studies. Based on the previous discussion of thermodynamics, one might expect heat-transfer limitations (internal or external to the chip) to influence the kinetics. In order to examine H F absorption kinetics and the possible influence of heat-transfer limitations, two experiments will be described. In the first, a 1-cm x 1-cm X 0.2-cm dried aspen chip (85-mg weight) was exposed to H F vapor a t 0.5 atm (1SLPM of H F and 1 SLPM of N,) and 30 O C , and the HF loading was measured as a function of time. This transient curve is presented in Figure 8. As can be seen, HF absorption is quite rapid; the equilibrium loading was achieved within 30 min. The second experiment was identical with the first except that a thermocouple was inserted into the edge of the chip and the gravimetric system was not used. From this experiment, the temperature of the chip was found to rise to 75 "C within 10 s after HF exposure and then gradually decrease to the gas temperature of 30 "C. For comparison, the adiabatic temperature rise for the entire transition was calculated from the heat of vaporization data described in the previous section and found to be 200-300 "C. The actual temperature data obtained and the isotherm data were used to construct an equilibrium curve, which represents the maximum HF loading which could occur given the temperature of the chip. This curve is also shown in Figure 8. If heat-transfer resistance completely controlled the kinetics, one would expect the two curves in Figure 8
to be coincident. This is not quite the case, and some of the discrepancies may have been caused by experimental error. If, for example, the center of the chip (where the temperature was measured) was hotter than the edges, the measured temperature would be higher than the actual average temperature and could result in the equilibrium curve being below the measured H F loading curve, which occurs in Figure 8. In any case, the results of these two experiments show that heat-transfer resistance (either external or internal) affects the kinetics at the conditions of the experiment and probably for most of the experiments that were done. Because of this, the acquisition of kinetic data was deemphasized for this study. HF desorption proceeds at rates comparable to absorption except at low levels of H F loading. For the experiment described above where the chip temperature was measured, the chip temperature decreased to as low as 13 "C when the H F was desorbed into pure N2 (1SLPM) at 30 "C. This temperature decrease, due to evaporative cooling, shows that heat-transfer limitations will affect desorption kinetics at the conditions of the experiment. When desorbing H F into pure N2,the rate of desorption was found to slow down markedly at H F loadings of less than about 10%. This may be due to the fact that sugar fluorides must revert to oligomers to release all of the H F from the sample, and this reversion reaction probably occurs more slowly than desorption of physically absorbed HF. Temperature has a significant effect on the rate of desorption of HF, especially a t low levels of H F loading. For a chip initially exposed to HF vapor at 30 "C and 0.5 atm, it took 1 h to return the chip to a constant weight (0.2% resolution) by desorbing the H F into pure N2 at 80 "C. By contrast, if the same chip was desorbed at 30 O C , the H F loading would still be about 5% after 1 h and a t least 8 h would be required to attain a constant weight.
Conclusions Gravimetric analysis was found to be a useful method for studying H F absorption by lignocellulose. Although the sample weight was found to decrease by as much as 2.5% after H F desorption (possibly due to degradation), this error is small in comparison with the H F loading observed under most conditions. Because of the possibility of weight loss by the sample, however, gravimetric methods do not appear to be useful for measuring the residual fluoride content of materials after H F saccharification. The H F absorption isotherms are very steep between HF loadings of approximately 15% and 25%. This is most likely due to the decrystallization of cellulose making it easier for HF to interact with the hydroxyl groups on the cellulose chain. The phenomenon does not reverse itself during desorption; thus, hysteresis is observed in the absorption-desorption loop. Using the HF loading data and the heats of vaporization that were calculated in this study along with data on reaction yield, it will be possible to perform material and energy balances necessary for process design and analysis. The effects of using different types of lignocellulose with varying moisture content on H F loading still need to be determined so that the process can be evaluated more completely. HF absorption by lignocellulose is a rapid process at the conditions studied. H F desorption proceeds at similar rates except when the HF loading is less than 10% and the temperature is low. The data suggested that the absorption kinetics are controlled primarily by the rate of heat transfer away from the wood, but this needs to be confirmed with more experimentation. Transport limitations will cause difficulties in modeling H F saccharifi-
Ind. Eng. C h e m . Res. 1989, 28, 243-245
cation kinetics because the process may not be isothermal and because the absorption kinetics will be difficult to describe. For commercial processes where the wood chips will be larger than used here, transport resistances may be even more important and significantly influence reaction rates.
Acknowledgment Portions of this work were presented at the Summer National Meeting of the American Institute of Chemical Engineers, Minneapolis, MN, August 1987. This work was supported by US Department of Agriculture Grant 84CRSR-2-2489. Registry No. HF, 7664-39-3;glucose, 50-99-7;xylose, 58-86-6.
Literature Cited Costa, E. C.; Smith, J. M. AZChE J. 1971, 17, 947. Defaye, J.; Gadelle, A.; Papadopoulos, J.; Pedersen, C. J . Appl. Polym. Sci.: Appl. Polym. Symp. 1983, 37, 653. Franz, R.; Erckel, R.; Riehm, T.; Woernle, R. In Energy from Biomass, Second E. C. Conference;Strub, A., Chartier, P., Schleser, G., Eds.; Applied Science: London, 1982; p 873.
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Franz, R.; Fritsche-Lang, W.; Deger, H. M.; Erckel, R.; Schlingmann, M. J . Appl. Polym. Sci. 1987, 33, 1291. Fredenhagen, K.; Cadenbach, G. Angew. Chem. 1933,46, 113. Hardt, H.; Lamport, D. T. A. Biotech. Bioeng. 1982, 24, 903. Harris, J. F. J . Appl. Polym. Sci.: Appl. Polym. Symp. 1975,28, 131. Helferich, R.;Bottger, S. Liebigs Ann. Chem. 1929, 476, 150. Luers, H. Holz Roh Werkst. 1938, 1, 342. Ostrovski, C. M.; Duckworth, H. E.; Aitken, J. C., paper presented at the IV International Symposium on Alcohol Fuels Technology, Ottawa, Canada, May 1984. Reffstrup, T.; Kau, M. In New Approaches t o Research in Cereal Carbohydrates; Hill, R., Munck, L., Eds.; Elsevier Science: Amsterdam, 1985; p 313. Rorrer, G. L.; Hawley, M. C.; Lamport, D. T. A. Ind. Eng. Chem. Product Res. Deu. 1986,25, 589. Rorrer, G. L.; Hawley, M. C.; Lamport, D. T. A. Biomass 1987, 12, 227. Rorrer, G. L.; Mohring, W. R.; Hawley, M. C.; Lamport, D. T. A. Energy Fuels 1988, 2, 556. Selke, S. M. Doctoral Dissertation, Michigan State University, 1983. Selke, S. M.; Hawley, M. C.; Hardt, H.; Lamport, D. T. A.; Smith, G.; Smith, J. Ind. Eng. Chem. Product Res. Deu. 1982, 21, 11. Smith, D. F. J . Chem. Phys. 1958, 28, 1040.
Received for review January 22, 1988 Accepted September 6, 1988
COMMUNICATIONS Mass Transfer in a Pilot Plant Scale Airlift Column with Non-Newtonian Fluids Gas holdups and volumetric mass-transfer coefficients were measured in a 1000-L pilot plant scale fermentor (0.76 m in diameter) with non-Newtonian liquids ((carboxymethy1)cellulose). T h e data are in reasonable agreement with the correlations obtained for small bubble columns (up t o 0.305 m in diameter). It is found t h a t the gas holdups and the volumetric mass-transfer coefficients in non-Newtonian fluids are almost independent of column size, as in Newtonian fluids. Bubble columns or airlift columns have been widely used in the biochemical, pharmaceutical, and chemical industries. According to the demand of high-production capacity, some large-scale bubble column or airlift column bioreactors have been recently built. Examples include a single-cell protein unit in Billingham (a total reactor volume of about 2600 m3) and a wastewater treatment plant in Leverkusen Bayer (a total reactor volume of about 20 000 m3). Since microbiological cultures often behave as non-Newtonian fluids (Charles, 19851, much attention has been directed toward the performance of bubble columns or airlift columns with non-Newtonian fluids. However, all previous publications in this area reported on columns of up to only 0.305-m diameter (Godbole et al., 1984). For the discussion of scale-up problems, data for larger columns are clearly required. In this study, we measured gas holdups and volumetric mass-transfer coefficients in a 1000-L pilot plant scale airlift column with (carboxymethy1)cellulosesolutions which are usually used to simulate the rheological properties of pseudoplastic fermentation broths (Godbole et al., 1984).
Experimental Section The measurements were carried out in a 1000-L pilot plant scale fermentor (Figure 1). The column made of 0888-5885/89/2628-0243$01.50/0
stainless steel was 3.2 m high and 0.76 m in diameter. The inside diameter of the draft tube was 0.35 m and the outside diameter 0.356 m, yielding a Ad/Ar ratio of about 0.27. The draft tube had two 0.076-m slits to allow liquid circulation when the dispersion height was below the top of the draft tube during actual fermentations. Three ring spargers of diameters 0.70, 0.58, and 0.43 m having 53 orifices of 0.0018-m diameter, 43 orifices of 0.002-m diameter, and 32 orifices of 0.0023-m diameter, respectively, were located at the bottom of the fermentor. Gas was sparged in the annular region outside the draft tube. Thus, liquid circulation induced by the introduction of gas was upward in the annular region and downward in the draft tube. Since, as described above, the draft tube had two slits, secondary liquid circulations also developed with higher liquid levels. All experiments were done at a H L / D , ratio of 3.2. The range of superficial gas flow rate used in this work was from 1.3 x to 5.3 X m/s. In other words, all measurements were carried out in the bubbly flow and the churn-turbulent flow regimes, which are the most commonly occurring flow regimes in industrial bubble columns. The column was operated in a batch fashion. The gas holdup was determined by the difference between the clear liquid and dispersion heights. The volumetric mass0 1989 American Chemical Society
