PRODUCTION OF LEVOGLUCOSAN BY PYROLYSIS O F CARBOHYDRATES Pyrolysis in Hot Inert Gas Stream C H A M B R A
M . L A K S H M A N A N
A N D
H A R O L D
E . H O E L S C H E R
School of Engineering, University of Pittsburgh, Pittsburgh, P a . 15213
Starch pyrolysis w a s carried out in a hot gas stream to establish optimum coriditions for the production of levoglucosan. Helium, nitrogen, a n d steam were used. Steam w a s most effective a n d convenient because of its high heat capacity anjd because it is easily condensed. Yields of levoglucosan increased as pressures decreased. The effect of temperature, feed weight, gas flow rate, reduced air pressure, a n d catalyst on the yield of levoglucosan is reported. Optimum operating conditions for the acid-catalyzed process w e r e established. A maximum yield
of 44.5% levoglucosan w a s obtained from cornstarch.
THEindustrial potential of a process for the production of levoglucosan, 1.,6-anhydro-/3~~-glucopyranose,by pyrolysis of carbohydrates such as starch or cellulose is indicated in a previous paper (Lakshmanan et al , 1969). Properties of levoglucosan which make it industrially attractive and the complex nature of the pyrolysis reaction were described. Pyrolysis studies conducted in vacuum, using batch and continuous type reactors, indicated that high yields of levoglucosan may be obtained from starch pyrolysis only if the product vapors are removed immediately from the high-temperature reaction zone. An efficient heating method is essential. Starch was pyrolyzed in a hot inert gas stream. This provides uniform heat transfer to the bulk of the material and reduces the residence time of the product vapors in the hot zone. New results using this method are presented. Experimental
Material. Commercial cornstarch, received in finely powdered form, was converted into lumps as described by Lakshmanan et al (1969). Various sieve fractions of this lump starch were then used for the study. Apparatus. The equipment used for pyrolysis in a gas Stream is shown in Figure 1. The reactor was a 2-liter stainless steel kettle, heated by a 650-watt Glas-col electrical heater. The gas was heated in a 1-inch x %foot electrically heated stainless steel preheater. Inert gases could be fed to the preheater from gas cylinders or steam could be fed from an automatic steam generator. Gas flow rates were measured by a rotameter, and the steam flow rate by condensation and measurement of the condensate volume. Gas inlet temperatures were measured, as were both inside and wall temperatures of the reactor. The system could be operated a t various pressures. Procedure. The reactor and the preheater were first brought to the required temperature. The gas was started through the reactor t o displace all air in the reactor and to reach steady-state temperatures throughout. Then a weighed quantity of starch granules was fed into the reac-
CY
Figure 1. Experimental setup
tor. For experiments a t subatmospheric pressures, the system pressure was reduced to the desired level immediately after feeding the starch. The gas was passed through the reactor until no further product vapors could be observed, before the heaters were turned off. Product was collected as a viscous sirup; when steam was used as the inert gas, the product was collected in a solution of condensate in the product receiver and traps. All product sirup (collected “as is” when helium and nitrogen were used as inert gases or recovered from the condensate by vaporization of water on a steam bath) was dissolved in methanol. The weight of the sirup produced was determined after removing all methanol and water. The percentage yield of levoglucosan was estimated by the liquid-gas chromatographic method developed by Sawardeker et al. (1965). Results and Discussion
The data obtained are presented in Figure 2 and Tables I and 11. The yield of levoglucosan was higher when steam was used as the gas medium rather than helium. Higher flow rates of helium are required to obtain the same yields as with steam at the same temperature. Maintaining the lowered pressure a t high flows was difficult, but with the low steam flow rates presented no problems. This factor, the relatively low cost of steam, and the ease Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 1, March 1970
57
A 0
Y
O
t
,
1
o
1
,
1
IO
5 FLOW R A T E , C C
20
15
CONDENSED S T E A M / M I N U T E
Figure 2. Effect of flow rate
of product recovery by condensation of the exit stream argue strongly in its favor for a commercial process. Results obtained at reduced pressures-without a gas flow-are presented for comparison in Table 111. The advantages of an inert gas flow us. air is readily apparent a t 0.25 atm (190 mm). Air pressure must be reduced to 8 to 14 mm to obtain yields of levoglucosan equivalent to those obtained in inert gas streams a t 190 mm. An
independent study of the oxidative degradation of starch in a fluid bed substantiates these results (Jariwala and Hoelscher, 1970). The observed effects of the other independent variables are about as expected-that is, higher temperatures give higher pyrolysis rates, but lower the yields by subsequent decomposition of product vapor before removal from the reaction zone and condensation (Table I V ) . This is consistent with results reported earlier (Lakshmanan et al., 1969). The yield tends to decrease with increased initial starch charged to the reactor used in this study (Table V ) , probably because the higher initial charge yields more product vapors and a higher mean residence time of product vapor in the hot reaction zone. The higher the flow rate, the more difficult it is to maintain the low pressure and the required temperature, but the lower is the mean residence time of product vapor in the reactor. An optimum flow rate is reported in Figure 2 and Table
VII. Studies using “catalysts” for the pyrolysis yielded some
Table I. Pyrolysis of Starch in Inert Gas Medium at Atmospheric Pressure
Starch. Corn
Reactor. Stainless steel kettle
Temp., C Reactor Gas
Inert Gas Nitrogen Helium Steam
400 400 400
Gas Flou per Minute, S T P
400 350 500 400 350 350
350
Feed weight. 25 grams
5 cc 5 cc 5 cc 5 cc
Sirup Weight, Gram
Fraction of LG in Sirup
kwglucosan, Yield %;c
7.9 9.7 10.7 10.5 10.7 8.8
0.26 0.31 0.22 0.30 0.30 0.32
8.2 12.7 9.4 12.6 12.8 11.4
2 liters 12 liters“ (condensate) (condensate) (condensateIb (condensate)
Equiualent to: “ 2 4 liters of helium per minute at 350” C. 14 liters of steam per minute at 350” C. Table II. Pyrolysis of Starch in Inert Gas Medium at Subatmospheric Pressures
Reactor. Stainless steel kettle
Starch. Corn
Temp., C
Pressure,
Feed weight. 25 grams
M m Hg Abs.
Inert Gas
Reactor
Gas
Gas Flow, Vol.iMin., S T P
760 190 760 760 570 380 380 190 100
Helium Helium Steam Steam Steam Steam Steam Steam Steam
400 400 400 400 400 400 400 400 400
350 350 400 350 400 400 350 350 350
12 liters 12 liters 5 cc (condensed) 5 cc (condensed) 5 cc (condensed) 5 cc (condensed) 5 cc (condensed) 5 cc (condensed) 5 cc (condensed)
Table Ill. Pyrolysis of Starch at l o w Pressure without Inert Gas
Reactor. Starch. Feed weight.
Reactor Pressure, Temp., C M m Hg Abs. O
Sirup Weight, Fration of Grams LG in Sirup
Fraction of LG in Sirup
Reactor. Starch. Feed weight. Inert gas. Flow rate. Pressure.
Lewglucosan, Yield 5%
Temp., C Reactor Gas O
350 400
58
7-10 8-14 6-18 190
15.8 17.4 17.7 7.6 7.0
0.46 0.46 0.44 0.22 0.22
12.7 27.6 12.6 12.8 17.7 28.3 29.0 32.9 37.0
Table IV. Effect of Temperature on Pyrolysis of Starch in Inert Gas Medium at l o w Pressure
Stainless steel kettle Corn 25 grams
Sirup Weight, Grams
0.31 0.45 0.30 0.30 0.34 0.42 0.43 0.46 0.49
9.7 15.4 10.5 10.7 13.0 16.8 16.9 18.1 18.7
Lewglucosan, Yield %c
29.5 32.0 31.2 6.7 6.2
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
350 400 400 450
350 350 400 450
Stainless steel kettle Corn 25.0 grams Steam 5 cc condensed steam/minute 190 mm Hg absolute
Sirup Weight, Grams
Fraction of LG in Sirup
Leuoglucosan, %c Yield
17.0 18.1 17.7 18.2
0.45 0.46 0.46 0.43
31.0 32.9 32.4 30.8
Table VI. Effect of Catalysts on Pyrolysis of Starch in Inert Gas Medium at low Pressure Table V. Effect of Feed Weight on Pyrolysis of Starch in Inert Gas Medium at low Pressure
Reactor. Inert gas. Pressure. Temperature. Starch. Flow rate.
Reactor. Feed weight. Flow rate.
Stainless steel kettle 25 grams 5 cc condensed steam/minute Pressure. 190 mm Hg absolute Temperature. Reactor 400" C, gas 350" C Starch. Corn Inert gas. Steam
Stainless steel kettle Steam 190 mm Hg absolute Gas 350" C, reactor 400" C Corn 5 cc condensed steamiminute
Catalyst Feed Weight, Grams
Sirup Weight, Grams
Fraction of LG in Sirup
Lewglucosan, 9; Yield
25 50 75 100
18.1 33.16 48.8 71.4
0.46 0.48 0.47 0.39
32.9 32.6 30.6 28.0
Calcium acetate, 1 0 7 ~ Ammonium acetate, 10% Ammonium oxatate, 10% Ammonium sulfite, 10% Ammonia gas 100 ccimin. Acetic acid, 10%
Sirup Weight, Grams
Fraction LG in Sirup
Leuoglucosan, 9; Yield
0.45 0.32 0.49 0.47 0.51 0.45
32.1 15.2 34.6 34.1 33.1 31.8
0.42 0.50
30.0 38.2
of
18.6 11.8 17.6 18.2 16.3 17.6 2.6 17.8 19.1
...
~
~
...
~~
Table Vll. Optimum Conditions for Pyrolysis of Acetic Acid Treated Starch in Inert Gas Medium
Reactor. Starch. Feed weight.
Stainless steel kettle Corn 25 grams
Temp , C Reactor GaS
Acetic Acid, ' r
S t e m F l o ~Rate, Cc C o d Min
5 10
20 50 10"
15 15 15 15 20 15 15 15
400 350 400 450 400 400 400 400
Wet starch 10
15
400
Wet starch u t h 4 3 O , mmture (weight of dry starch fed
=
PressuE M m Hg, Abs.
Sirup Weight, Grams
Fraction of LG in Sirup
Lewglucosan, % ' Yield
350 350 350 400 350 350 350 350
190 190 190 190 190 190 190 190
20.3 17.8 21.0 20.3 20.7 20.6 20.0 12.7
0.48 0.51 0.53 0.53 0.53 0.50 0.48 0.50
39.0 36.3 44.5 43.5 43.9 41.2 38.7 38.1
350
760
14.5
0.39
22.6
16 5 g)
unexpected results (Table VI). A variety of inorganic metal-acid salts were studied first, without encouraging results. Ammonium acetate seemed to increase the yield by only 2 7 . Low concentrations of ammonia in any one of the gases used decreased the yield by significant amounts. Pretreatment of the starch with 10% acetic acid solution, followed by drying before pyrolysis, gave significantly increased yields. This system was then studied in some detail. The results-thought to be the optimum obtainable from this study-are presented in Table VII. The highest yield was 44.5% of levoglucosan from the acetic acid-pretreated 5,tarch. These results were obtained a t 400" C reactor temperature, 350" C steam temperature, a steam flow rate corresponding to 15 cc of condensate per minute, and 190 mm of Hg absolute pressure. A single run using starch which had been treated with the l o L Cacetic acid solution but not dried-it contained 43'c moisture when charged to the reactor-gave a significantly lowered yield (38.1%) a substantially longer reaction time, as expected, and no evidence of foaming. The catalytic effect of acetic acid seems to support the suggestion of Byrne et ul (1966) that the reaction is acid-catalyzed. Conclusions
Starch pyrolysis in a hot inert gas stream, preferably
in superheated steam, gives higher yields of levoglucosan a t higher pressures than the vacuum pyrolysis process previously reported. Pretreatment of the starch with an aqueous solution of acetic acid followed by drying before charging to the reactor likewise significantly improves the yield. The acetic acid seems to be a catalyst for the desired reaction. This process has considerable potential for the commercial production of levoglucosan by pyrolysis of starch. Acknowledgment
The assistance of Jyoti S. Pandya in the experimental work is gratefully acknowledged. literature Cited
Byrne, G. A., Gardiner, D., Holmes, F. H., J . A p p l . Chem. 16, 81 (1966). Jariwala, S., Hoelscher, H. E., Ind. Eng. Chem. Process Design Deuelop. 9 , in press (1970). Lakshmanan, C . M., Gal-Or, B., Hoelscher, H. E., IND. ENG.CHEM.PROD. RES. DEVELOP. 8, 261 (1969). Sawardeker, J. S., Sloneker, J. H., Dimier, R. J., J . Chromatog. 20, 260 (1965). RECEIVED for review October 9. 1969 ACCEPTED December 22, 1969 Project supported by the Agricultural Research Service, U.S.D.A., Grant 12-14-100-8043 (71) administered by the Northern Utilization Research and Development Division, Peoria, Ill. Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 1, March 1970
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