Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
United States
Environmental Protection
Agency
Research and Development
EPA/600/S2-85/137 July 1986
Project Summary
High Temperature Dilute Acid
Hydrolysis of Waste Cellulose:
Batch and Continuous
Processes
Walter Brenner and Barry Rugg
The full report describes a 5-year
investigation on the conversion of cel-
lulosic materials and wastes to glucose,
potentially an enormous source of eth-
anol and a wide range of petrochem-
icals. Employing a co-rotating twin-
screw extruder as reaction vessel, the
process achieves a fully continuous
dilute-acid hydrolysis at temperatures
around 240°C and residence times of 5
to 10 seconds. It handles feedstocks
ranging from waste paper pulp at 10%
solids to corn bran to dry hardwood
sawdust at 95% solids without pretreat-
ment and gives good conversion yields,
around 60% of the available cellulose,
with low energy consumption.
Using a feasibility study based on
accurately measured material and en-
ergy balances, economically attractive
projections are given for scale-up from
the 2 ton per day pilot plant, which has
operated for three years, to a full-scale
commercial plant producing 25 million
gallons of ethanol per year.
Also given is a description of work on
separation of the product, analytical
techniques, studies on fermentation and
bioconversion to methane and utiliza-
tion of the hemicellulose and lignin
fractions of the plant material. Envi-
ronmental considerations are discussed,
as well as a proposal for a mobile
version.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The potential quantities of chemical
feedstocks that could be obtained from
waste cellulose are substantial. Assum-
ing a realistic 12% available cellulose in
the wastes and only 16% conversion to
ethylene, the estimated 1550 million tons
of farm waste alone would yield more
than 30 million tons of ethylene annually—
twice the U.S. consumption of this basic
chemical building block in 1978. The
economic feasibility of using waste cel-
lulose depends greatly, of course, on both
waste collection and processing conver-
sion costs, as well as on the quantities
and*values of the various end products
obtained.
Acid hydrolysis of cellulose has been
extensively studied, particularly in con-
nection with manufacturing ethanol from
wood wastes. The discovery that cellu-
lose can be hydrolyzed in acid solutions
and converted to its monomer, glucose,
was first reported by Bracconnot in 1819.
The reaction has been experimentally
investigated ever since then, mostly on a
strictly empirical basis, in order to de-
velop a cost-effective process for produc-
ing sugar from wood wastes and other
sources of waste cellulose.
Whilethe acid hydrolysis of cellulose is
heterogeneous, it can be regarded as a
homogeneous reaction provided that the
cellulose reactant is dispersed in the form
of fine particles, e.g., 200 mesh or less.
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A very extensive amount of research
and development has been aimed at the
development of cellulose pretreatments
which are both technically effective and
economically viable. The basic approach
has been to reduce the crystallinity and
disrupt the hydrogen bonding, thus ren-
dering the cellulose more accessible to
hydrolytic depolymerization reactions.
This should make it possible to approach
the predicted glucose yields more closely.
Employment of high energy ionizing
radiation has been shown to be at least
equally as effective as widely used grind-
ing pretreatments when the cellulose is
exposed to dosages on the order of 100
megarads. Sugar yields as high as 70%
based on the available cellulose content
have been reported. Such large dosages
of ionizing radiation are, however, too
high for industrial usage. Thermal and
radiation treatments have been combined
to allow lower radiation dosage levels,
which are more acceptable economically.
Various chemical treatments have been
investigated as well, with relatively little
success.
Development of lower cost pretreat-
ment technology, such as would produce
substantial crystallinity reductions for
improving the accessibility of cellulose, is
recognized as a prime consideration for
an economically as well as technically
effective waste cellulose to glucose con-
version process. This pretreatment must
furthermore be combined with a high
productivity, high yield acid hydrolysis
process so as to optimize the conversion
of the pretreated waste cellulose to
glucose.
Experiments carried out at New York
University's (NYU) laboratories have re-
sulted in the development of a new high
temperature acid hydrolysis process that
overcomes the major problems that have
been associated with this reaction. The
important features of this process include
the development of a cost-effective pre-
treatment for the waste cellulose that
enhances its reactivity in an acid medium,
and the establishment of reaction condi-
tions that can produce yields on the order
of 60% (based on the available cellulose
values) in very short periods of time.
Dilute sulfuric acid was used in the
aqueous hydrolysis medium.
This rapid high temperature acid hydrol-
ysis process has been successfully scaled
up, first to a 1-liter and then to a 5-liter
stirred autoclave reactor. The larger reac-
tor and associated equipment were oper-
ated on a batch basis very satisfactorily
for prolonged periods, and the glucose
yields obtained were reproducibly con-
sistent. Designing a continuous 1-ton/
day waste-cellulose-to-glucose pilot plant
facility was the next logical step using a
screw-type conveyor, mixer, and reactor
device for hydrolyzing waste cellulose in
a slurry form.
A three-year program was implement-
ed by an EPA Cooperative Agreement No.
EPA-R-805239-030 with NYU. Activities
were concentrated on the design of the
1-ton/day pilot plant unit, the necessary
detailed specification of equipment and
materials, their procurement, preparatory
work necessary for equipment installa-
tion and equipment shakedown.
Two main types of feedstock were
hydrolyzed, namely used newspaper and
sawdust. The effects of pertinent reaction
variables on the glucose yield obtainable
from these wastes were studied. Separa-
tion of the glucose from unconverted
material and other reaction products was
also investigated. Exploratory studies
were conducted on the acid hydrolysis of
alternative waste cellulose sources, espe-
cially agricultural residues. The results of
these investigations are the subject of the
full report. This work was carried out at
the Antonio Ferri Laboratories of NYU in
Westbury, Long Island.
Description of Study
In the reactor itself, the following
aspects of operation were addressed:
• feeding of cellulosic waste into high
pressure reaction zone,
• precise control of reaction pressure,
temperature, and residence time,
• addition of acid catalyst,
• addition of high pressure steam,
• minimization of moisture content pas-
sing through the reaction zone, and
• discharge of product from the high
pressure reaction zone.
A vessel was designed and constructed
for collecting and sampling the product as
it flashed from the reactor, and thus a
continuous conversion system that could
hydrolyze paper pulp was accomplished.
In addition to reactor design, the prob-
lems of product (glucose) separation from
the reaction mass as well as its analysis
were studied using on-site analytical
capability including semi-continuous cen-
trifugation with the aim of obtaining
additional information on separation ef-
ficiency.
By the onset of the third year, sawdust
had been successfully hydrolyzed. This
was seen as a major advance, since the
ability to handle a low-moisture cellulosic
feedstock without first forming a slurry
meant that the level of water carried with
the reaction mass was significantly de-
creased, and thus the energy requirement
was reduced substantially. Sawdust as a
feedstock naturally required redesign of
the feeding section of the process. Also,
the product was a more complex mixture
of sugars requiring improved analytical
capabilities.
Work during the final year of this study
was aimed at improving the versatility,
reliability, and efficiency of the extruder
system. Bacterial fermentation studies
were carried out at Louisiana State
University for conversion of glucose to
methane; yeast fermentation studies
were commenced at NYU for conversion
of glucose to ethanol. Materials and
energy balances were developed and
used as a basis for economic analyses
and projections, and environmental im-
pacts were studied.
Methods and Results
Several pretreatments for wastepaper
prior to hydrolysis were evaluated. These
included: (1) Wiley Mill grinding, (2)
industrial grinding, (3) hydropulping (lab-
oratory simulation), (4) electron beam
irradiation, and (5) hydrogen peroxide
degradation. A description of the proce-
dures follows:
Wiley Mill Grinding: Newspapers were
hand torn into 2" rectangles, then fed into
a standard #3 laboratory size Wiley Mill
fitted with a welded brass screen plate
with Va" round perforations. The paper
was passed through the machine once
and separated according to pore size.
Industrial Grinding: Newsprint was
passed once through a standard indus-
trial Williams Hammer Mill Shredder,
equipped with a Va" round hole perforated
screen plate.
Hydropulping: Fifty gram samples of
paper were weighed out and hand torn
into one-gallon Waring Blender with
2450 gms of water to make a slurry of
approximately 2% solids. The blender was
run at high speed for 15 seconds. The
excess water was then filtered off with a
Buchner funnel to make a higher solids
slurry prior to hydrolysis.
Irradiation: Fifty gram samples of paper
were hydropulped and filtered to a preset
slurry concentration, then sealed in plas-
tic bags. These samples were exposed to
high energy electrons at a specified
dosage (10 and 20 megarads).
Hydrogen Peroxide: Fifty gram samples
of paper were hydropulped with 2450
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gms of 0.1 M acetate buffer (pH - 4.2)
containing 2.31 gm FeS04. The slurry
was secured in a plastic bucket and
equilibrated in a 30°C water bath. Then
82.5 gm of 30% H202 was added and the
reaction begun with constant agitation.
The runs were 16-18 hrs in duration,
after which the peroxide was dissipated
and the treated newspaper was filtered,
washed, and refiltered before being sub-
jected to acid hydrolysis.
Hydrolysis batch experiments were
initially run in a 1 -liter reactor and in due
course scaled up to a 5-liter reactor.
These reactors were both stainless steel,
stirred autoclaves which were instru-
mented for reading temperatures and
pressures. Acid solutions were injected
from an external bomb after the reaction
mass was heated to the appropriate
temperature. The reactors were fitted
with quick-acting discharge valves and
properly-sized collection vessels. In the
early experiments using the 1-liter reac-
tor, a known amount of ice was put in the
collection vessel to quench the reaction.
With scale-up to the 5-liter reactor, a 55-
gallon drum was adapted with baffles to
absorb the thermal and kinetic shock of
the discharge. In this case, flashing to
atmospheric pressure was sufficient to
quench the reaction, thus dilution of the
product with ice was eliminated.
A typical procedure was as follows: the
waste cellulose slurry, approximately
10% solids, was poured into the auto-
clave, a typical charge being 1000 gm for
the 1-liter reactor and 3000 gm for the
5-liter reactor. The autoclave was then
electrically heated with stirring to various
reaction temperatures, with 220-240°C
found most desirable for optimum glucose
conversions. The pressures required to
obtain these temperatures were in excess
of 5OO psi. When the cellulose slurry
reached the desired temperature, a pre-
determined quantity of acid was injected
into the autoclave. This was accomplished
using the inert gas from the nitrogen
cylinder as the pressure medium to force
the acid into the autoclave. The acid
hydrolysis reaction was then run with
continuing autoclave agitation for pre-
determined reaction periods, ranging
from 5 seconds to more than 2 minutes.
After reaction, the "quick release" ball
valve was opened and the contents of the
reactor were discharged rapidly so as to
minimize hydrolytic degradation reactions
of the glucose formed. The reaction mass
was filtered to separate the glucose in the
liquor for subsequent analysis.
Specific experimental data of typical
acid hydrolysis runs which were carried
out at 232°C in the autoclave shows the
glucose yields as a function of reaction
time for a range of acid strengths, e.g.,
0.58% to 2.24%. The 232°C temperature
was employed for all the hydrolysis
experiments because the results of prev-
ious experimental work had shown it to
be optimal for glucose yield. The majority
of the acid hydrolysis experiments were
carried out with Eiley milled newspapers
as the waste cellulosic feedstock in order
to ascertain maximum glucose yields with
an "accessible" feed as a function of acid
content at 232°C hydrolysis temperature.
These experiments established the range
of acid content which gave the highest
glucose yields and the reaction times
necessary to achieve them. Experiments
with hydropulped cellulose feedstock
were then run at three acid concentra-
tions, 0.87%, 1.29%, and 2.24% with
glucose yields realized comparable with
those obtained from Wiley milled mater-
ial.
Conclusions
Maximum glucose yields increase with
acid content while the reaction times
necessary to achieve them decrease; the
experiments carried out show that glu-
cose yields on the order of 35% and
higher can be obtained with acid strengths
in the range of 1.30% to 2.25%. Reaction
times for maximum glucose yields are 20
seconds or less at the preferred range of
acid concentrations.
Hydropulped waste newspapers give
glucose yields which approach rather
closely those obtained from the more
expensive Wiley milled paper; also, reac-
tion times are similar. Thus, glucose
conversions on the order of 35% can be
obtained in less than 20 seconds at the
preferred range of acid concentrations,
with cellulose concentrations of 10% in
the slurries. i
Once it was shown that the rapid dilute
acid hydrolysis concept could be scaled
up through the NYU 1-liter alnd 5-liter
batch reactors, serious consideration was
given to devising a method for making the
process continuous, and scaling up fur-
ther to a pilot plant, and ultimately
industrial scale. Requirements are that a
reaction vessel must be charged with
feedstock and brought to the conditions of
temperature, pressure, and acid content,
then discharged within the few seconds
required for optimum conversion.
Summary
The principal result of this study was
the successful development of a fully
continuous dilute acid hydrolysis process
for converting cellulose to glucose. Batch
hydrolysis studies carried out for two
years gave extensive data on which the
continuous process was based.
Pretreatment studies led to the belief
that to gain cellulose accessibility, one
needed to impart energy in some form to
the cellulose. A combination treatment of
high energy electron beam irradiation in
conjunction with hydropulping was seen
to be more effective while less costly than
mechanical treatments. Results of chem-
ical pretreatments were inconclusive.
Hydrolysis experiments made with organ-
ic acids in place of sulf uric acid to reduce
corrosion proved effective, but too ex-
pensive for serious commercial consid-
eration. The emphasis on pretreatments
diminished as development of the con-
tinuous reactor proceeded.
Based on planttrials, a decision to use a
commercially available co-rotating twin-
screw extruder was made because of its
modular design and inherent versatility.
The problem of forming a high pressure
reaction zone in a flow system was the
key issue. This involved creating an
upstream seal while material is fed in,
and a downstream seal while material is
discharged.
The feasibility of continuously hydrolyz-
ing various cellulosic feeds at high glu-
cose yields using the extrusion technique
was proven. Additionally, the capability of
handling high solids feeds resulting in a
product with high glucose content was
established for the extruder.
The studies on process development
branched out to issues of handling the
product once out of the reactor. These
issues included separation, fermentation,
and analysis; further subjects included
environmental impact and overall process
economics.
Trial separation studies were done
using a semi-continuous centrifuge. It
was soon evident that hydrolysis reaction
conditions played an important role in
product separability. If the cellulose were
"under-reacted," the product contained a
high fiber content and subsequently the
water-soluble fraction was difficult to
remove. At best, a filter cake with 30%
solids was produced in the centrifuge.
Further washing of the cake to extract
residual sugars caused excessive dilution.
However, under more severe reaction
conditions where more of the fiber prop-
erties of the cellulose were destroyed, it
was shown that the product could be
separated more effectively and centrifuge
cakes of 50-70% solids content could be
produced.
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Fermentation studies on the glucose
initially concentrated on the generation
of methane. Subsequent studies on yeast
fermentation of the glucose to ethanol,
using a gradual acclimation technique,
were successful.
High pressure liquid chromatography
was the method used for separating and
analyzing carbohydrate mixtures includ-
ing monosaccharides, oligosaccharides,
and breakdown products allowing de-
tailed evaluation of the efficiency of the
process.
Environmental studies indicate no sig-
nificant adverse impact. Conventional
water and air treatments are adequate.
Economic analyses and a full engineering
feasibility study have been carried out for
the production of fuel grade ethanol for
gasohol from glucose, and indications are
that such a process would be competitive
with ethanol from grain under current
market conditions.
Walter Brenner and Barry Rugg are with New York University, Department of
Applied Sciences, New York, NY 10003.
Charles Rogers is the EPA Project Officer (see below).
The complete report, entitled "High Temperature Dilute Acid Hydrolysis of Waste
Cellulose: Batch and Continuous Processes," (Order No. PB 86-143 484/AS;
Cost: $16.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAII
EPA
PERMIT No G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-85/137
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