Agricultural Wastes 4 (1982) 97 116

HYDROLYSIS OF CELLULOSE USING HCl: A COMPARISON BETWEEN LIQUID PHASE AND GASEOUS

PHASE PROCESSES

F. J. HIGGINS & G. E. Ho

Schoal aj Environmental and Lije Sciences, Murdoch University, We,stern Australia 6150, Australia

ABSTRACT

Cellulose constitutes a sign![icant portion oJ a large amount o[ agricultural and/ores! residues, as well as urban waste derivedj?om Jbrest products (e.g. wastepaper'), aml represents a potential Jot" the production oJ ethanol, a liquidjuel. To realise this potential it is necessary to hydrolyse the celhdose toJermentable sugars. HydroO'sis using HCI in solution and as a gas was investigated using :t-cellulose. newspaper. wheat straw and wheat hulls as substrates, at room temperature and also when the reaction was heated. It was Jound that the use oJ HCI gas i'esulted in a more rapid hydrolysis o! crystalline cellulose and a sign(tic:ant inc,'ease was oh,wrl:ed in the hydrolysis rate when the reaction, which had proceeded at room temperature, wa.s heated to 50 C, when either HCl acid or gas was used. Similar ~'esults were obtained with whole newspaper and wheat straw as suhstrates.

INTRODUCTION

Cellulose is the major component of the cell walls of all plants and, as a naturally occurring fibre, is widely distributed in nature, mainly in woody tissues and annual plants. Many uses have been found for cellulose down through the course of human history, as Man has found uses for wood (comprised mainly of cellulose and lignin) and cotton (almost pure cellulose). Processes have been developed whereby cellulose may be transformed into modified physical and chemical forms, including such products as paper and paper products, rayon textiles, rayon lyre cords, cellophane paper, cellulose nitrate and cellulose acetate.

However, there still remains a vast volume of cellulose which, in either its native or processed form ---for example, waste paper--is not being effectively utilised. This

97 Agricultural Wastes 0141-4607/82/0004-0097/$02.75 i( ~, Appl ied Science Publishers Ltd, England, 1982 Printed in Great Britain

98 F. J. HIGGINS, G. E. HO

cellulose is mainly in the form of lignocellulosic agricultural waste materials, A recent estimate for Australia (Stewart et al., 1979) gave a figure of 24.6 million tonnes dry weight of these waste materials generated annually, with crop (cereals) residues contributing 15-8 million tonnes. With a projected shortage of liquid fuel from indigenous sources for transport there has been a great deal of interest in utilising the waste materials for the production of liquid fuel.

The gross energy contained in the waste materials is estimated at 390 PJ (1 PJ = 1015 J) per annum.

If 25 ~o of this energy can be realised in the form of liquid fuels, it will represent a substantial contribution to the requirement for liquid fuels for transport, estimated at 700 PJ for Australia. Western Australia, in particular, is well placed to utilise cereal residues because of the relatively large cereal crop production compared with its population. It is estimated that up to half its liquid transport fuel requirement could be produced from existing crop residues.

The energy input-output relationship for the conversion process and its economics must, however, be such that the system as a whole from the collection and storage of the raw material to the distribution and use of the fuel--is viable. As part of an attempt to show whether this is the case, a study was made of the conversion to ethanol of cellulose derived from wheat straw and waste paper. This paper reports the results of the hydrolysis of the cellulose into fermentable sugars-- the critical step in the conversion process--comparing, in particular, the hydrolysis using hydrochloric acid in solution and as a gas.

HYDROLYSIS OF CELLULOSE

Cellulose can be hydrolysed using either an acid or enzymes produced by microorganisms such as Tr ichoderma viride.

Wilke et al. (1976) proposed a scheme for the enzymatic hydrolysis of newsprint, with the process being designed on the basis of small-scale laboratory experiments, but up to this point there appears to have been no direct applications of the enzymatic hydrolysis of cellulose on a larger scale. Further work may be required as the results have not been encouraging when dealing with untreated cellulosic materials, especially those with a high degree ofcrystallinity, or where the cellulose is part of a lignocellulose complex. Some form of pretreatment of the raw material is required in order to reduce crystallinity, and also to increase the number of sites susceptible to hydrolysis, but the possible physical or chemical pretreatments are likely to be costly.

The enzymes themselves are costly to produce and much higher yields will be required if they are to be used on a commercial scale, Furthermore, it has been estimated, in the experiments carried out to date, that only 35 ~o of the enzymes used in the process are available for recovery and possible reuse (Wilke et al., 1976).

LIQUID AND GASEOUS CELLULOSE HYDROLYSIS 99

Grethlein (1978) prepared a comparison of the economics of acid and enzymatic hydrolysis of newsprint in which he presented a plant design with a capacity of 885 tonnes of newsprint per day, and compared the economic performance of this with an enzymatic hydrolysis process with a similar throughput, as proposed by Wilke et al. (1976). He concluded that the cost of producing glucose via the dilute sulphuric acid hydrolysis process was in the range of 3.9 to 5.4 cents (US) per kilogramme, compared with the estimate of 11.5 cents (US) per kilogramme for enzymatic hydrolysis with a 50 '~"o saccharification rate. It is claimed that the acid hydrolysis of cellulose is a viable and potentially economic process, but it must be borne in mind that these conclusions have been reached on the basis of untested extrapolations of laboratory data for both the acid and the enzymatic hydrolysis.

The technology for acid hydrolysis is more advanced and ethanol was produced from wood via the Scholler dilute sulphuric acid process before and during World War 2, a demonstration plant was built for the Madison dilute sulphuric acid process in 1946. while the Berguis Rheinau concentrated hydrochloric acid process was operated commercially for the production of xylose and crystalline glucose (Kent, 1974). In Japan the Noguchi Chisso hydrogen chloride gas process and the Hokkaido and Nihon- Mokuzai-Kagaku concentrated sulphuric acid processes have all reached the pilot-plant stage of development (Locke & Garnum, 1961).

A comparison of these processes (based on Siemon, 1975 and Kusama et a/., 1960) is shown in Table 1.

Other ways in which these processes could be compared would be the energy inputs which they require, the means by which acid is recovered and the materials required for the construction of the plant.

There is no evidence of any of the processes having been applied to newspaper or cereal straw substrates, although experiments have been carried out on the

TABLE 1 A COMPARISON OF A( ID HYDROLYSIS PROCESSES

Name Hydroh'sis conditions Yield o! Acid Acid Temperature Time glucose ~ consumed

(cC) (h) (kilogrammes (kilogramme~ per tonne per tonne oj wood) ~! wood)

Scholler 1 o H2SO 4 180 12 350 50 Udic Rheinau 41 9;_ HCI 20 About About 125

3 590 Nogucbi Chisso HCI gas 20 b < I About 100

590 Hokkaido 80 i~o H2SO4 40 < 1 About 200

550 Nihon Mokuzai Conc. ? ? About '~

Kagaku H2SO 4 570

Based on the cellulose content of eucalyptus hardwood. 5 Temperature increased to 50C to complete hydrolysis.

100 F. J. HIGGINS, G. E. HO

hydrolysis of waste paper by the dilute sulphuric acid process. In considering the possible use of acid hydrolysis as a process step in the recovery of energy from waste cellulose materials, the hydrogen chloride gas process seems to offer several advantages. The requirement for external energy is much lower than with, for example, the concentrated hydrochloric acid process; the gas recovery system should be relatively simple; the temperature and pressure regimes should be such that low-cost, non-corrosive polymers could be used in the construction of the reactor; the raw material should require little, if any, pretreatment and it may be possible to design a plant which could be operated economically on a smaller scale and which could be located in regional centres.

Itydrolysis utilising HCl The earliest attempts to hydrolyse cellulose were principally applied to wood and

wood wastes and can be traced back to almost a hundred years ago. Dangiville, in 1880, proposed the use of gaseous hydrochloric acid to hydrolyse wood (Prescott & Dunn, 1959). Willstfitter, in 1913 (Prescott & Dunn, 1959) demonstrated that a 40 ~,,, solution of hydrochloric acid was very different in action towards cellulose than 36 ~o commercial hydrochloric acid. He discovered that cellulose was transformed without waste to glucose within a few hours by a 40 oj/o acid solution at room temperature.

The hydrolysis of cellulose at room temperature in 36 '~0 acid proceeds very slowly and neither Willstfitter nor subsequent researchers appear to have been able to explain the precise reasons for the marked increase in hydrolysing capacity of the acid brought about by this 4}i; increase in hydrogen chloride concentration. However, this characteristic of hydrochloric acid is quite significant and has been the basis of subsequent hydrochloric acid and hydrogen chloride gas hydrolysis processes. By the time of the commencement of the second World War the Berguis Rheinau process had emerged (Kent, 1974).

Two hydrolysis processes utilising HC1 were reported to be developed in Japan in the early nineteen-sixties (Locke & Garnum, 1961).

The Udic Rheinau process is a modification of the Berguis-Rheinau process, with 41 ~o hydrochloric acid being used and recovered by distillation. In the Noguchi Chisso process anhydrous hydrochloric acid is used, and its character- istics are worth noting.

Raw material is sawdust and logs and waste-wood reduced to sawdust size. The wood particles are prehydrolysed by dilute acid to remove hemi- cellulose. The remaining cellulose and lignin is washed and dried. The prehydrolysed wood particles are treated with sufficient hydrochloric acid to give an equivalent concentration of water in the material of 50 to 60 ~i by weight and then placed in a fluidised-bed reactor through which a stream of hydrogen chloride gas is passing, and which provides for cooling of the

LIQUID AND GASEOUS CELLULOSE HYDROLYSIS 10l

wood particles. When sufficient hydrogen chloride gas has been absorbed by the particles, the material is heated to 45 to 50C to complete the hydrolysis. Further heating to a higher temperature is required to volatilise the hydrochloric acid and gas from the material. The gas is recycled after dehydration. The saccharified product is post-hydrolysed by heating it with water in the presence of the remaining acid.

Problems involved in developing the process were cooling and heating of the wood particles which have low heat conductivity, destruction of sugars during the acid recovery at high temperature and stickiness of the intermediate reaction product (hydrolysed material prior to acid removal). These problems were overcome by the use of a fluidised-bed reactor, the flash recovery process and the discovery of an anti-sticking agent.

Hydrolysis is achieved by using a relatively small quantity of hydrochloric acid and its recovery appears to be simple. The heat requirement for recovery is correspondingly small.

Hydrolysing, properties oJ hydrochloric acid~hydrogen chloride gas The properties of hydrogen chloride gas should be noted when considering the

hydrolysing capacity of hydrochloric acid. The solubility of hydrogen chloride in water is a function of temperature and varies from 45.1 i~0 at 0C to 35-9 Y~o at 60'~C on a weight/weight basis, at a pressure of one atmosphere. A constant boiling point azeotrope is formed at 108.6C, which contains 20.2 ?~ hydrogen chloride. The marked difference in hydrolysing capacity of 40 '}o acid compared with 36 '}~i acid has already been noted and 40 i~o or greater acid concentrations can only be achieved at temperatures below 30 C. There are thus two kinetic regimes to be considered with the hydrolysis of cellulose by hydrochloric acid or hydrogen chloride gas at atmospheric pressure:

(i) At temperatures below 30~'C and acid concentrations above 40'~,,, the reaction would be mainly a function of time, with temperature and acid concentration having an influence only if falls in temperature below 30~C were accompanied by increases in acid concentration, Maintaining acid concentrations greater than 40",i at temperatures above 30~'C would be possible only in a pressurised reactor.

(ii) At temperatures above 30 C, the rate of reaction would be a function of both temperature and acid concentration, bearing in mind that the constant boiling-point azeotrope will be reached at a temperature of 108-6('.

An additional factor which has an influence on reaction kinetics is that an increase in temperature above 30 C, applied after the substrate material has absorbed acid of above 40 ~/o concentration at a lower temperature, can result in a rapid increase in the rate of hydrolysis.

102 F. J. H IGGINS, G. E. HO

Kinetics o j acid hydrolysis o f cellulose The literature contains details of several studies dealing with the reaction kinetics

of acid hydrolysis, particularly relating to the dilute sulphuric acid processes. These studies indicate that the acid hydrolysis of both cellulose and hemi-celluloses follows a first order reaction mechanism (Kent, 1974, Guha et al., 1978, Andren & Nystrom, 1975). A kinetic study carried out by Saeman (1945) indicated that the hydrolysis of crystalline wood cellulose may be described by

kL k2 Cellulose---* sugars--* sugar decomposition products

and the reactions follow first order kinetics. Grethlein (1975) used a similar model to predict the isothermal yields of sugars

from the hydrolysis of newspaper, and from this predictive model he arrived at values for the rate constants:

k 1 =28 x 1019c 1'7s exp( -45 100/RT)min 1 (1)

k 2 = 4.9 x 101,~ c o. 55 exp( - 32 800/RT) min -- 1 (2)

where c = concentration of reactant. The rate constants, k 1 and k 2, have been used to set hydrolysis conditions which will give optimum yields of sugar. There appears to be little, if any, kinetic data in the literature on the hydrochloric acid processes. The use of dilute hydrochloric acid in cellulose hydrolysis has apparently not been investigated, whilst references to the kinetics of the concentrated hydrochloric acid and hydrogen chloride gas processes are qualitative in nature.

MATERIALS AND METHODS

Hydrolysis experiments were conducted using c~-cellulose derived from newspaper, cardboard, wheat straw and wheat hulls. The c~-cellulose was obtained as a result of analysing the raw materials for their contents of lignin, hemicelluloses and ~- cellulose.

Newspaper and cardboard were run through a Retsch KG (type SKI) hammer mill, fitted with a 5 mm screen and oven dried at 50 C. Wheat straw and wheat hulls were hammer milled to pass through a 2 mm screen and oven dried at 50 C; further screening was carried out and sizes between 710 and 500 p used for analysis (to avoid larger, and also very fine, particles).

Duplicate 25-g samples were then prepared for each of the materials. It was necessary for dust, water-soluble compounds and other organic compounds to he removed, so that only the ligno-cellulosic fraction remained for analysis, and for this purpose the following preliminary extractions were carried out.

Paper and wood samples." To each of the 25-g samples was added 400 ml of a 2:1 benzene/ethanol mixture, which was then placed in a stoppered 2-1itre

LIQUID AND GASEOUS CELLULOSE HYDROLYSIS 103

Erlenmeyer flask and allowed to stand overnight. The samples were then vacuum filtered, washed with 100 ml of ethanol and air dried. Wheat straw and wheat hulls: These samples were washed in cold water, hot water and ethanol. They were then vacuum filtered, dried with acetone and finally oven dried overnight at 50 C.

The analysis was then carried out according to the procedure outlined by Green (1963) for use with wood samples which, apart from the suggested sample size. is identical with that outlined by Whistler & BeMiller (1963).

Gas phase hydrolysis reactor Heat transfer plays an important part in the HCI gas phase hydrolysis process.

The absorption of HCI gas by the moistened material in the reactor generates considerable heat. This heat of absorption must be removed to maintain the temperature in the reactor at below 30C. to enable the equivalent HCI acid concentration to reach 40 , o or greater. As well as being cooled during this stage of the process, the material has to be heated to complete the hydrolysis.

Cellulose substrates are poor conductors of heat and in the Noguchi Chisso pro- cess this problem was overcome by using a small particle size and also a fluidised bed reactor. However, in preliminary experiments carried out in this study with moistened paper and cereal straw, neither of these materials fluidised very well, but were inclined to rise and fall as a plug in the reactor column. It was therefore decided to use a packed bed reactor for the experiments, into which the substrate material could be loosely packed and through which the gas could freely circulate. A conventional Liebig condenser proved to be suitable for this purpose, with either cooling water or heating water being circulated, as required, through the outer .jacket. The hydrolysis reactor was set up as shown in Fig. 1.

Hydrolysis using HCl in solution Table 2 summarises the experimental conditions for the hydrolysis using HC1 in

solution. Two runs (L1 and L2) were carried out at 20C using saturated HCI solution to

determine the reaction kinetics for the hydrolysis of ~-cellulose. In run 1 :~-cellulose tom newspaper was utilised and in run 2 ~-cellulose from wheat straw. Hydrogen chloride gas was passed through a 36 ~o (w/w) hydrochloric acid solution until it had reached its maximum concentration at 20 C of 41.7 o/ , /o (w/w). Ten samples of about 1 g of :~-cellulose were weighed and placed in 100 ml Erlenmeyer flasks, together with 20 ml of 41.7 '~ HCI acid. The flasks were stoppered and placed in a water bath set at 20 C. The residues in the flasks were then vacuum filtered and washed, in sequence, at time intervals of 5, 15, 30, 60, 90, 120, 180,240,300 and 360 rain. The samples were then oven dried at 50C overnight and weighed.

To investigate the effect of heating--and hence supersaturation of HC1 in solution--on the reaction, run L3 was carried out: 41-7 o~] HCI was prepared and five

104 r. J. HIGGINS, G. E. HO

HCI gas from cytinder

Gas dryer

CaCl z

Fig. l.

Water out~

Liebig

c ondensersampte 1 5erl t'i, I I

~'-~ T hermome ter

GGS f',ow meter

Apparatus for HCI gas phase hydrolysis.

ector

TABLE 2 EXPERIMENTAL CONDITIONS FOR HYDROLYSIS USING nCl IN SOLUTION

Run Raw HCI Temperature Temperature Time Objective No. material conc. ( C) variations (rain)

(w/w)

L1 s-Cellulose 41.7 % 20 Constant 5-360 Reaction from newspaper kinetics

L2 s-Cellulose 41.7 ~o 20 Constant 5 360 Reaction from wheat kinetics straw

L3 s-Cellulose 41.7 % 20 Kept at Then at Effect of from wheat + 20 C for 50 C for heating straw 50 (a) 15 min, (a) 5, 10min

(b) 30 min (b)5, 10, 15min L4 s-Cellulose 37-3 ~o 50 Constant 60-240 Effect of

from wheat temperature straw

L5 Newspaper 41.7% 18 Constant 5-180 Comparison (hammer-milled) with run 1

L6 Newspaper 37.3 % 50 Constant 60 240 Comparison (hammer-milled) with run 4

L7 Newspaper 41.7 ~o 20 Kept at Then at Comparison (hammer-milled) ,~ 20 C for 50 C for with run 3

50 (a) 15min, (a) 5, 10, 15min, (b) 30min (b)5, 10, 15min

L8 Newspaper 34.2% 18 Constant 5 300 Use of (hammer-milled) commercial

grade HCI L9 Wheat straw 34.2 % 18 Constant 5 300 Use of

(hammer-milled) commercial grade HCI

LIQUID AND GASEOUS CELLULOSE HYDROLYSIS 105

samples of about 1 g of c~-cellulose (extracted from wheat straw) were hydrolysed in the acid : however, after an initial period of hydrolysis at 20 C, heat was applied to the samples as follows. After hydrolysis for 15 min, one sample was heated to 50 C for 5min and another for 10rain in a water bath. After hydrolysis for 30min. one sample was heated to 50 C for 5 min. another for 10 min, and a third for 15 min. All five samples were then vacuum filtered, washed, oven dried and weighed.

Runs L5, L6 and L7 were similar to runs 2, 3 and 4 except that newspaper was used

rather than :~-cellulose, with the objective of investigating the effect on hydrolysis of using newspaper which was passed through a hammer-mil l fitted with a 5 mm screen.

Two runs (L8 and L9) were carried out using commercial grade HC1 to hydrolyse hammer-milled newspaper and wheat straw, respectively, to verify our belief that the extent of hydrolysis of the above substrates in commercial grade HCI at room temperature would be limited. Dried samples of about 1 g of milled newspaper and wheat straw were weighed and placed into 100 ml Erlenmeyer flasks, together with 20 ml of 34-2 0~i, HCI acid. Samples of both newspaper and straw were digested for times of 5, 15, 30, 60, 90, 120, 180,240 and 300 rain. after which the residues were vacuum filtered, washed, oven dried and weighed.

Hydro lys i s us ing HCI gas

Table 3 summarises the experimental condit ions for the hydrolysis using HCI gas. The first run was carried out at 20 C to determine the reaction kinetics using HCI

in gas form and corresponded to run El using HCI in solution.

TABLE 3 EXPERIMENTAL CONDIT IONS FOR HYDROLYSIS USING HCI GAS

Run Raw Temperature Temperature Time Objectit:e No. material ( C) variation ( rnin )

G 1 :~-Cellulose from 20 cardboard

G2 :~-Cellulose from 20 50 wheat straw

G3 :~-Cellulose from 50 wheat straw

G4 Newspaper Cooling for first control;

G5 Newspaper 50 G6 Newspaper 20 50

G7 ~-Cellulose from wheat hulls

G8 Whole newspaper and wheat straw

Constant 5 180 Reaction kinetics

Kept at Then at Effect of heating 20C for 50 C for on reaction la) 15 min, (a)5, 10.15 min, (b) 30rain (b)5, 10.15 rain Constant 5 120 Effect of temperature

reaction 3 rain, no further temperature Hydrolysis of temperature recorded

1 120 Constant 5 135 Kept at Then at 20C for 50C for {a) 15min, (a) 5, 10, 15 min, (b) 30 rain (b) 5, 10, 15min

Conditions as for run G2

Conditions as for run G6

whole newspaper

Effect of temperature

Analysis of sugars produced

Analysis of sugars produced

106 F. J. HIGGINS, G. E. HO

Nine samples of about 1 g of a-cellulose, which had been prepared from cardboard, were placed in stoppered 100 ml Erlenmeyer flasks, together with about 1-5 ml of water. After standing overnight to allow for even penetration of the water into the cellulose, the samples were introduced, in turn, into the hydrolysis reactor, where hydrolysis took place for reaction times of 5, 10, 15, 30, 60, 90, 120, 150 and 180 min.

Hydrogen chloride gas from a storage cylinder was passed continuously through the reactor at a pressure of 13.8 k Pa (0-136 atmospheres) and a flow rate of 2.7 litres min- 1. The ambient temperature was 20 C and for the first 3 min of the hydrolysis cold water was circulated in the outer jacket of the reactor to remove the heat of absorption of the HCI. At the completion of the hydrolysis time each sample was purged from the reactor using hot water, after which it was vacuum filtered, washed, oven dried at 50 C and weighed.

The effect of heating (run G2) was investigated in a similar manner to run L3.

Six 1 g samples were moistened and hydrolysed in the presence of HC1 gas. After a reaction time of 15 min, hot water heated to 50 C was circulated in the outer jacket of the reactor for 5 min with one of the samples, 10 min with another and 15 min with a third. These same heating times were then applied to three samples which had been hydrolysed for 30 min at 20 C. All samples were purged from the reactor using hot water, then were vacuum filtered, washed, oven dried overnight and weighed.

The effect of carrying out the reaction at a constant temperature of 50 C was investigated in run G3, and this corresponded to run L4. Six samples were prepared and hydrolysed in the presence of HC1 gas, but in this case hot water, at 50 C, was circulated through the outer jacket of the reactor for the whole course of the hydrolysis. The reaction times were 5, 15, 30, 60, 90, and 120min, respectively.

A further set of experiments (run G4) was conducted to provide intbrmation on the HCI gas phase hydrolysis of whole newspaper and also to note the influence of temperature on the hydrolysis kinetics. Measurements were also made of the extent of HC1 absorption by the samples during the hydrolysis. For these experiments, newspaper which had been oven dried was cut into 19.5 cm squares, each weighing approximately 2 g. The samples were saturated with water and then partially dried until the water content was about 2g, which was equivalent to 100//o of the dry weight of the sample. Accurate weighings were made of the dry weight and water content of the samples.

Samples which had been moistened in this way were folded "concertina' fashion and placed in the hydrolysis reactor, with HCI cylinder gas then passed continuously through the reactor at a pressure of 13.8 k Pa and a gas flow rate of 2.7 litres min 1 It was found that the gas circulated quite freely in the reactor with the paper folded in this manner. Cold water was circulated through the outer jacket of the reactor for the first 3 min of the hydrolysis. The reaction was carried out for time intervals of 1, 2, 3, 4, 5, 10, 15, 30, 60, 90 and 120min, respectively, for each sample. The

LIQUID AND GASEOUS CELLULOSE HYDROLYSIS 107

temperature within the reactor was recorded during the hydrolysis process. At the end of each hydrolysis, an air stream was passed through the reactor for 20 s to purge surplus HCI gas and the sample was then flushed from the reactor using hot water. Vacuum filtration followed, with the hydrolysate made up to 250 ml and analysed for HC1 concentration by titration with standard NaOH. A calculation could then be made of the equivalent HC1 concentration in the sample in the reactor. The residual material was oven dried overnight and weighed.

The same procedure was followed for an additional seven samples (run G5). excepting that the hydrolysis reaction was carried out at a constant temperature o1 50~'C. The reaction times were 5, 10. 15, 30, 60, 90, and 135min, respectively.

A further run (G6) was conducted to investigate the effect of applying heat to samples which had adsorbed the maximum level of HCI at ambient temperature. Six samples were prepared in all and, after a reaction time of 15 rain. hot water heated to 50 ~C was circulated through the outer jacket of the reactor for 5 rain for one sample. 10rain for a second and 15rain for a third. The three remaining samples were hydrolysed for 30rain and then heat was applied for 5, 10 and 15 rain, respectively. In each case air was passed through the reactor at the completion of" the hydrolysis. to remove surplus gas. The sample was then flushed from the reactor with hot water and vacuum filtration carried out. An analysis was carried out for HCI concentration in the hydrolysate, with the residual material being oven dried and then weighed.

As well as investigating the hydrolysis reaction itself, it was important that measurements were also made of the yield of reducing sugars resulting from this hydrolysis. A number of analyses were therefore carried out for this purpose (runs G7 and GS).

The gas phase hydrolysis of :~-cellulose from wheat hulls was carried out as described for run G2 over a range of reaction times and with the reaction being both unheated and also unheated initially followed by the application of heat to complete the hydrolysis. At the completion of each reaction, air heated to 40 C was passed through the reactor to drive offthe HCI gas and also HCI held as acid. Drying times of from 15 to 45 min were employed, which resulted in the samples reaching varying degrees of dryness. The residue was then flushed from the reactor with hot water and the hydrolysate analysed for reducing sugar concentration. The residue itself was oven dried and weighed.

The reducing sugar analysis was carried out using the 3.5-dinitrosalicylic acid method, based on Miller's modification of the procedure of Sumner Somers (Reese & Mandels. 1963). No attempt was made to analyse for particular sugars, but the concentration of reducing sugars in the samples was taken to be the glucose equivalent.

The remaining hydrolysate from each of the hydrolysis reactions was then given a post-hydrolysis boiling at 100C for 100min. A second set of dilutions was then prepared and analysed for reducing sugars.

108 F. J. HIGGINS, G. E. HO

TABLE 4 COMPOSITION OF RAW MATERIALS

Percentage of Newspaper Cardboard Wheat straw Wheat hulls original dry weight

Extractives removed in preliminary extraction 3-4 3.1 5.0 8-1

Lignin 19.8 18.9 19.2 22.2 Hemicelluloses 20.8 16-6 34-4 35.3 s-Cellulose 56.1 61.4 41-3 34.4

100.1 100.0 99.9 100.0

RESULTS

Composition of raw materials The results of the analysis are shown in Table 4.

Itydrolysis oJ~-cellulose m 41.7% HCI solution at 20C. (Runs L1 and L2) A plot of the weight remaining as a percentage of the original weight of 7-cellulose

as a function of reaction time on semi- logarithmic graph paper is shown in Fig. 2. Except for a short period after the commencement of hydrolysis and beyond 5 h, the points fall on straight lines which indicates that the hydrolysis follows first order reaction kinetics with a k 1 of 0.008 min - 1 for the hydrolysis of a-cellulose extracted from newspaper and a k l of 0.009 min 1 for the hydrolysis of a-cellulose extracted from wheat straw.

100-

c

"" ~ ~.Run L1

Run G1 \ \ \\, ~ ~Run L2

\\ i ,\

i i i i J r i

50 100 150 20 O 250 300 300 Time, rain.

Fig. 2. Hydrolysis of s-cellulose.

LIQUID AND GASEOUS CELLULOSE HYDROLYSIS 109

m o

100

[

BO

40 I

20 o,'O / ,

/ /

0

a

RunL3 ~ - - - RunL2

J J l~

/

/ /o - -

~ Run LL

30 ~'0 go 120 1so 180 2,0 2~0 2to 300 330 360

T ime, mln .

Fig. 3. Hydrolysis of ~-cellulose in HC1 acid solution.

Ejfi.'ct ol temperature and heating The results for the hydrolysis which was unheated initially, but was heated to

50'C to complete it (Run L3) and the results for the hydrolysis carried out at a constant temperature of 50 C (Run L4) are shown in Fig. 3. The results for Run L2 are also shown for comparison purposes.

Hvdroh'sis oj hanznzer-milled newspaper (Runs L3 to L7) The results of these runs are presented in Fig. 4.

1oo

o

80 g $

$601

z,0

20

Fig 4.

I Theorehca l max imum ce l lu lose(768%)

~~ f~Run L7

Run L5 ~- = ~ :=,~ Z : ~ . _ ~ - - - - = : : :4 -~ Run L6

.~p: : : : i ~ Run L9

~ . . . . . . . Run L8

310 ~ i ~ 60 90 120 !50 180

T me mqn

Hydrolysis of hammerimilled newspaper and wheat straw in HCi acid solution.

110 F . J . HIGGINS, G. E. HO

Hydrolysis using commercial grade HCI acid (Runs L7, L8 and L9) These are also presented in Fig. 4.

tlydrolysis using HCI gas Ilydrolysis of~-cellulose at 20 C (Run G1): Except for the first 30 min where little

change in weight took place after an initial rapid drop, the points fall on a straight line. The reaction rate constant, k 1, for the straight line portion is 0.017 min-1.

Effkct of heating and temperature (Runs G2 and G3) These are shown on Fig. 5. A comparison with the reaction at 20 C (run GI) is

also shown.

100-

80-

"7 o~

60

40-

20-

30

Fig. 5.

~ 'Run G2 un G1 / -

/ /

Run G3

9'0 ' f f ' 0 120 150 180 2i0 Time, rain

Hydrolysis of ~-cellulose in HCI gas.

TABLE 5 HYDROLYSIS OF CELLULOSE. RUN G4

Reaction time Reactor temperature Weight loss (min) (C) (as ~/o of original weight)

1 50 24.3 2 36 24.9 3 31 25.8 4 28 26 "4 5 26 24.6

10 20 27.3 15 19 27.5 30 18.5 31.2 60 17.5 34.7 90 18-0 44.7

120 17.5 50.3

LIQUID AND GASEOUS CELLULOSE HYDROLYSIS 1 1 1

100

8C

o

z:

~m L0

20

Theoretical maximum ce l lu lose [76 8%)

-71 : : . . . . I Run G6

/ /

,:; I' o Run GL

,l # ~ : Run G5

0 30

Fig. 6.

- - - - I - - I

60 gO 120 150 180

T~me,mm

Hydrolysis of whole newspaper in HC1 gas.

Hrdrolvsis o1 whole newspaper (Runs G4, G5 and G6) The results for Run G4 are shown in Table 5. The percentage of weight loss as a

function of time is compared with the reaction carried at 50 C, and where heating was introduced in Fig, 6.

Sugar yield The yield of reducing sugars from the hydrolysis of ~-cellulose extracted from

wheat hulls is shown in Table 6, whilst Table 7 shows sugar yield from hydrolysis of whole newspaper and wheat straw.

TABLE 6 SUGAR Y IELD FROM HYDROLYS IS OF d -CELLULOSE EXTRACTED FROM WHEAT HULLS

Sumple 1 2 3 4 5 6 7

Weight loss due to hydrolysis (g) 0-8418 0.6447 0"6211 0"9275 0.7981 0"6062 0.8513

Heating time for HCI removal (min) 45 30 30 Overnight 30 Nil 15

Heating temperature for HC1 removal (C) 40 40 40 45 40 Nil 40

Yield of reducing sugar as % of weight loss due to hydrolysis 42-2 75.2 87.7 78.3 89.1 57.0

112 F. J. HIGGINS, G. E. HO

TABLE 7 SUGAR YIELD FROM HYDROLYSIS OF NEWSPAPER AND WHEAT STRAW

Sample Newspaper Wheat straw

Weight loss due to hydrolysis (g) 1.2737

Glucose equivalent in sample (g) 0.933

Glucose equivalent in sample after post-hydrolysis heating (g) 1-083

Yield of reducing sugar as ~ of weight loss due to hydrolysis 85-0

I ' l l50

0'833

1 '067

95"7

DISCUSSION

Composition oJ samples In considering the a-cellulose content of the samples, it should firstly be noted that

the c~-celluloses extracted from newspaper and cardboard still contained some residual ink, which was not removed after completion of the kinetics experiments using a-cellulose from newspaper. It was estimated that the ink would comprise about 4 "//o of the a-cellulose by weight, which is equivalent to about 2.4 /o of the dry weight of the original samples. An enquiry made to the supplier of the ink for a local newspaper publisher (Pura, 1978) indicated that the major components of this ink are carbon black, mineral oil distillates and a bituminous wetting aid.

The overall results of the analysis are in close agreement with data published in the USA on the a-cellulose content of various cellulosic materials. The a-cellulose content of wheat straw was reported to be 50 ~o, of oat hulls 34 ~o, and of wood between 40 and 50 /o (Stephens & Heichel, 1975). Significant factors which may be noted from the analysis are the somewhat higher a-cellulose content of the paper and cardboard due no doubt to the pulping process which these materials would have experienced and the high hemicellulose content of the wheat straw and wheat hulls.

The important factor as far as ethanol production is concerned is the hexose sugars content of the materials. According to the hydrolysis equation:

(C6HloOs)n + nH20---~nC6H1206

the hydrolysis of cellulose should yield 111 /o by weight of glucose. The composition of the hemicellulose fraction of the materials would vary, depending on the nature of the substrate, but these would yield approximately two-thirds pentose sugar (xylose) and one-third hexose sugars (mainly mannose).

The samples which have been analysed would thus give the following theoretically maximum yields of hexose sugars (the figures for newspaper and cardboard have been reduced by 2.4 !Yo to allow for the estimated ink content): newspaper, 66-7 ~;

LIQUII) AND GASEOUS CELLULOSE ItYDR()LYSIS 1 13

cardboard, 71 0 '~;; wheat straw, 57.2 '~{i: wheat hulls, 50.0 I},,. From these figures it can be seen that newspaper and cardboard would be the best substrate materials for ethanol production, while the lowest yields would come from wheat hulls. The utilisation of by-products would be an important factor in any viable ethanol production process, and this would be particularly so with wheat straw and wheat hull substrates, where the pentose fraction of the hemicellulose could be used ['or the recovery of xylose or the production of SCP.

Reaction kinetic~ Reaction in 41.70~, acidsolution: From the graph of the hydrolysis of s-cellulose in

41.7 '~0 acid, at 20 ~C (Fig. 2) it can be seen that the regression line, if extended, would intersect the Y axis at a point corresponding to less than 100 ~}o of the dry weight of the cellulose. An explanation that appears reasonable is that the amorphous region of the cellulose would hydrolyse rapidly at the beginning of the hydrolysis hence the steep curve at this point (Mann, 1963). A deviation from linearity also occurs with the final two points of the hydrolysis, corresponding to the 300- and 360 rain time intervals. The hydrolysis has effectively ceased at this point, although the residue still comprises 10.5 I},~, of the original dry weight of the cellulose.

There are two reasons for this. First, residual ink would have comprised 4 "i, of the original dry weight and, secondly, side reactions which occur during the hydrolysis can result in insoluble humic substances being formed, which add to the residual weight (Mann, 1963).

The value for the rate constanL k~, arrived at experimentally was 0.008 Io 0.009 min - ~ and, by comparing this with the rate constant for the decomposition of cellulose which was arrived at by Grethlein (1975) it can be seen that the hydrolysis in 41-7 !},, hydrochloric acid at room temperature proceeds much more slowly than the dilute sulphuric acid high temperature reaction. Grethlein's constant kl at a temperature of 230 C and an acid concentration of 0.5 ifo is 2.1 min - ~. Temperature has, however, a marked influence on this latter reaction, and at a temperature of 180~'C and an acid concentration of 0.8 ~}o- the value for k 1 reduces to 0.033min 1

Reaction in HCI g, as. As with the 41.7 o; acid reaction, the hydrolysis of the amorphous region of the cellulose occurred rapidly during the early stages of the reaction. However, a definite lag phase then occurred and this would have been due to the time taken for the HC1 gas to diffuse into the crystalline cellulose. Experiments with newspaper indicated that it took about 15 min for the HCI concentration to reach an equivalent acid concentration of 40 30.

Once the critical HC1 concentration had been reached, the gas phase hydrolysis proceeded more rapidly than the 41 .7 acid reaction. The rate constant was 0.017min ~ compared with 0.009min ~ for the acid reaction. An explanation for this more rapid hydrolysis could be that, in addition to the HC1 which diffuses into the cellulose structure, some may also be directly adsorbed onto the outside of the particles, and this would result in additional H + ion activity.

114 V. J. HIGG1NS, G. E. HO

Influence rof temperature on the rate of hydrolysis Hydrolysis of a-cellulose in 41" 7 % acid and HCl gas: The reactions carried out at

50 C proceeded much slower and, after 120 min, only 34 to 37 ~o of the cellulose had been hydrolysed, compared with 71 to 74 ~o for the unheated reactions. These results confirm the marked influence that acid concentration has on the rate of hydrolysis of cellulose. An increase in temperature from 20C to 50C would be expected to increase the rate of hydrolysis to a great extent, if acid concentration did not affect the reaction, but this was not so and in fact there was a marked reduction in the rate as the acid concentration dropped from 41.7 ~,, to 37.3 ~o. Where the actual critical acid concentration lies was not determined in these experiments, and this could be an interesting matter for further research.

A dramatic increase in the rate of hydrolysis was noted with the reactions carried out initially at 18 C to 20 C, and then heated. With the acid reaction, heating for only 5 min, after an initial unheated period of 15 rain, resulted in 97.9 //o hydrolysis. This was not quite matched with the gas phase reaction, where an initial period of 30 min was required to give the best results. Heating for 5 min at this point resulted in 89.9 /o hydrolysis. An estimate of the rate constant (kl) for this heated portion of the reaction puts it in the range of 0.8 to 1.0 min 1 which compares favourably with the dilute sulphuric acid high temperature hydrolysis.

This characteristic of the hydrolysis using either HCI acid or HCI gas is probably the most significant factor observed during the experiments and the possibility of achieving a complete hydrolysis time of 20 min or less is an important point to consider in planning for HC1 gas phase hydrolysis to be applied on a larger scale. However, further research is required to determine the optimum operating conditions which will give the maximum sugar yield within the minimum time.

Hydrolysis oJ newspaper: The hydrolysis of whole newspaper in HCI gas and hammer-milled newspaper in HCI acid---shows a similar pattern of hydrolysis to the a-cellulose reactions and once again the gas phase reaction was slightly faster than the liquid phase, despite the fact that whole paper was used in the gas reaction, whereas the paper used in the acid reaction had been hammer milled.

The presence of hemicelluloses in the paper caused a rapid rate of hydrolysis during the early minutes of the reaction, but from then on the rate was slower than in the a-cellulose hydrolysis, particularly in the case of the completely unheated reaction.

There was again a very rapid increase in the rate of hydrolysis when the samples were heated to 50 C after having initially been unheated. Yields of from 94 to 96 ~ of the theoretical maximum were obtained in both cases when reactions which had been unheated for 30 min were heated to 50C for 5 rain.

During the unheated gas phase reactions, the heat of absorption of the HCI caused the temperature to rise to 50 C within the first minute of the reaction, despite the fact that the reactor was being cooled by passing water through the outer jacket. This temperature dropped to 20 C after 20 min. The acid concentration built up

LIQUID AND GASEOUS CELLULOSE HYDROLYSIS 1 1 5

rapidly to 34.6 /o during the first minute, but took 15 min to reach 40 ~o and 30 min to reach 43 ~0, This interrelationship between temperature and equivalent acid concentration would be an important factor to consider in designing a reactor for larger scale operation of the gas phase process.

Hydrolysis in 34.2 /o HCI acid at 18C. These experiments confirmed that very little cellulose is hydrolysed at room temperature with acid at this concentration. The weight loss after 5 h of hydrolysis would have been almost entirely due to hemicellulose decomposition.

Yield o/reducing sugars These yields were generally lower than would have been hoped for, and it appears

that varying degrees of decomposition of the sugars took place during the heating step to remove HCI from the hydrolysed material. There was a high rate of decomposition with sample 4, which was heated to dryness overnight, while sample 6, which had not been heated at all, gave a yield of 89.1 i"o- In general it appears that the sugars are subject to decomposition as they approach dryness in the presence of acid of 371!o to 38 . . . . . . and this is a /,, concentration, problem which would need to be overcome in any larger scale application of the process.

The post-hydrolysis step, using the acid remaining in the sample after the drying step, and carried out to convert oligosaccharides into monomer units, also has an influence on the yield of reducing sugars. It may be necessary for the heating time or temperature to be increased if yields are to be maximised.

CONf'LUSIONS

The hydrolysis of crystalline cellulose proceeded as a first order reaction using both HC1 in solution and HC1 gas, with the reaction proceeding at a more rapid rate using HCI gas. With hydrolysis using HCI gas at 20 C the reaction is slower than the dilute sulphuric acid high temperature hydrolysis. However, the reaction becomes more rapid when material which had adsorbed sufficient HC1 gas is heated to 50 C. with reaction rates comparing favourably with the dilute sulphuric acid high temperature hydrolysis. This similar technique applied to the hydrolysis of newspaper and wheat straw results in 94 o~, to 96 ~ o of the cellulose being hydrolysed within 35min and sugar yields which are higher than in the high temperature process.

REFERENCES

ANDREN, R. K. & NYSTROM, J. M. (1975). Cellulose -F rom solid waste to chemical resource. In: Energy j?om solid waste utilization. Proceedings of the Sixth Annual North-Eastern Regional Antipollution ConJerence, University oJ Rhode Island, p. 340.

116 F. J. HIGGINS, G. E. HO

GREEN, J. W. (1963). Wood cellulose. In: Methods in carbohydrate chemistry. Vol. 3. (Whistler, R. L. (Ed)), Academic Press, p. 9.

GRETIqLEm, H. E. (1978). Comparison of the economics of acid and enzymatic hydrolysis of newsprint. Biotechnology and Bioengineering, 20 (4), 503.

GUHA, B. K., FOWLER, G. E. & TITCHENER, A. L. (1978). Engineering evaluation of chemical conversion of wood to liquid fuel alcohol, Proceedings Alcohol Fuels Conference, Sydney. Inst. Chem. Eng., NSW Group.

KENT, J. A. (Ed.) (1974). Riegels handbook of industrial chemistry. (7th edn). Van Nostrand Reinhold Co., p.471.

KUSAMA, J. (1960). Wood saccharification by the hydrogen chloride gas process. Paper presented at FAO Technical Panel on Wood Chemistry Meeting, Tokyo.

LOCKE, E. G. & GARNUM, E. (1961). Working party on wood hydrolysis, Forests Products Journal, 11, 380.

MANN, J. (1963). Fine structure of cellulose. In: Methods in carbohydrate chemistry, Vol. 3. (Whistler, R. L. (Ed)), Academic Press, p. 112.

PRESCOTT, S. G. & DUNN, C. G. (1959). Industrial microbiology, McGraw Hill Book Co., p. 872. PURA, J. L. (1978). Personal communication from Collie and Company Limited, Melbourne. REESE, E. T. & MANDELS, M. (1963). Enzymatic hydrolysis of cellulose and its derivatives. In: Methods in

carbohydrate chemistry, Vol. 3. (Whistler, R. L. (Ed)), Academic Press, p. 141. SAEMAN, J. (1945). Kinetics of wood saccharification, Ind. Eng. Chem., 37, 43. SIEMON, J. R, (1975). The production oJ solar ethanol J?om Australian Jorests, CSIRO Solar Energy

Studies Report 75/5, p. 11. STEPHENS, G. R. & HEICHEL, G. H. (1975). Agricultural and forest products as sources of cellulose. In:

Cellulose as a chemical and energy' resource. (Wilke, C.R. (Ed)), Wiley Interscience. STEWART, G. A., GARTSIDE, G., GIFFORD, R. M., NIx, H. A., RAWLINS, W. H. M. & SIEMON, J. R. (1979).

Liquid fuel production from agriculture and forestry in Australia, Search, 10, 382 7. WHISTLER, R. L. & BEMILLER, J. N. (1963). Holocellulose from annual plants. In: Methods in

carbohydrate chemistry, Vol. 3. (Whistler, R. L. (Ed)), Academic Press, p. 21. WILKE, C. R., RENDER YANG 8. VON STOCKAR, U. (1976). Preliminary cost analysis for enzymatic

hydrolysis of newsprint. In: Enzymatic conversion o j cellulosic materials. (Gaden, E. L. (Ed)), Wiley lnterscience.