05-04-2012, 03:42 PM
Fuel ethanol production
Fuel ethanol production.pdf (Size: 125.07 KB / Downloads: 55)
History
Motor fuel grade ethanol (MFGE) is the fastest
growing market for ethanol worldwide; and
MFGE production dwarfs the combined total
production of all other grades of ethanol.
Fermentation ethanol, as fuel (and solvent), has
experienced several cycles of growth and decline
since the early 1800s. By 1860, production had
reached more than 90 million gallons per year.
In 1861 Congress imposed a tax of $2.08 per
gallon. About that time, oil was found in
Pennsylvania. Thus began the cycle of ‘control’
of fuel ethanol markets (and therefore
production) by taxation policy and oil industry
influence on government. Petroleum interests
dominated the world fuel industry in the post-
World War II era, until a major policy shift by
Brazil in the 1970s led to an ethanol-fueled motor
vehicle strategy, followed a decade later by the
US (Morris, 1994, personal communication). As
a result, the combined motor fuel ethanol
production from fermentation in the Western
Hemisphere exceeded 5.5 billion gallons per year
in 2002.
In Central and South America the dominant
MFGE feedstock is sugar, either in the form of
cane juice directly from crushed cane
(autonomous distilleries), or from molasses
(annexed distilleries). In North America, the
dominant feedstock is starch from grain, with
90% derived from corn. Feedstock choice
follows regional dominant agricultural output
(Katzen, 1987).
Since the technology for producing MFGE
from sugar sources is an abbreviated form of
ethanol production from starch, which is in turn
an abbreviated form of production from whole
grain, this chapter will focus on MFGE
production from whole grain as typically
practiced in North America. This technology is
generally known as dry milling (Raphael Katzen
Associates, 1978).
Introduction
A comparative evaluation of potable ethanol and
MFGE production processes reveals many
similarities. As the MFGE industry began to
develop, it looked to the distilled spirits industry
for technology. In the US, many early MFGE
plants copied beverage alcohol distillery
processes, differing primarily in the addition of
dehydration facilities copied from the industrialgrade
ethanol industry (Madson and Murtagh,
1991). This generally ensured a plant capable
of producing ethanol. This technology strategy,
coupled with a strong ethanol market during the
1980s, often resulted in positive cash flows.
This technology strategy continued until a
downward trend in ethanol pricing revealed the
Madson01.p65 1 28/08/2003, 15:57
2 P.W. Madson and D.A. Monceaux
critical difference between distillery and MFGE
economics. Distilleries are traditionally operated
for consistency in flavor and quality of product.
Other factors such as yield, energy efficiency,
labor cost, etc., while being important, did not
dominate the economics. For the beverage
distiller it is counterproductive to reduce cost of
production at the expense of flavor and quality
and, possibly, market share. Flavor and product
consistency are so important that any benefit
associated with a process change must be
extremely high to offset the inherent market risk,
which could be catastrophic if the product must
be aged for several years. This, combined with
the price differential between distilled potable
spirits and MFGE, caused a shift to new
technology development and differentiation of
the MFGE industry in order to survive periods
of high grain cost and low MFGE prices.
The MFGE producer has minimum product
quality-related constraints. MFGE specifications
for water content, acidity, solids, etc. (as defined
in ASTM D-4806) can be met while
concurrently minimizing operating costs. MFGE
producers have traditionally operated with
narrow profit margins. The drop in ethanol price
during the mid 1980s resulted in most of the
beverage distillery technology-based MFGE
producers ceasing operations. Many of these
operations were labor- and energy-intensive and
operated with poor yields (Madson, 1990).
The design of a successful MFGE facility
requires a clear understanding of the economic
sensitivities. Evaluation of dry milling operating
costs revealed that feedstock costs comprise over
60% of the total (Hill et al., 1986). Energy
consumption, at one time the central focus of
debate, has been reduced via a rapid
development of technology to less than 40,000
BTUs per gallon of product, which is
approaching the point of diminishing returns in
cost trade-offs (Hill, 1991; personal
communication). The key issues today are
feedstock conversion efficiency, capital
investment, environmental impact and userfriendliness.
Conversion efficiency (yield)
Most MFGE producers have little control over
feedstock pricing beyond hedging strategies
such as trading in futures. The producers’
primary edge is therefore to maximize yield.
Prior to the major growth of the MFGE industry,
the typical yield in the production of spirits and
industrial ethanol was five proof US gallons per
distiller’s bushel (56 lbs) equivalent to 2.375
undenatured gallons per bushel in MFGE terms.
By the early 1980s, the newly-developed MFGE
technologies had demonstrated (in topperforming
plants) achievements of 2.55
undenatured US gallons per distiller’s bushel
(gpb) in dry milling plants and 2.45 gpb in wet
milling plants. More recently, some dry milling
MFGE plants have achieved sustained yields of
2.8 gpb (undenatured basis).
Because of the different industry reporting
procedures, this discussion is based on the ‘payto-
pay’ analysis. That is, yields are presented on
the basis of unadjusted distiller’s bushels of grain
purchased and ethanol sold (undenatured basis)
over a time period exceeding three months. This
results in a market-based yield figure that refers
directly to profit.
What brought about this remarkable yield
increase? The major development in technology
has occurred in the dry milling industry,
primarily because of the variety of technologies
tested and the broad-based experience from
which to learn (Katzen et al., 1992).
Beginning with the cooking step, it has become
clear that the controlling factor in design of a
cooking system is not the cooking of starch, but
rather elimination of bacteria in order to achieve
and maintain sterility throughout the process
(Kemmerling, 1989). Because conditions
needed to achieve sterility are different from
conditions required to cook starch, other factors
must be considered. Cooking must be
conducted with minimum solubilization of
potential fermentables in order to minimize
adverse reactivity; yet all fermentables must be
released during the liquefaction, saccharification
and fermentation processes for complete
conversion to ethanol. This includes the
fermentable sugars embedded in the fiber matrix.
Premature solubilization of potential
fermentables risks side reactions that can result
in unfermentable starch and sugar complexes
because of high temperature and the presence
of water and other reactive components. These
reactions may be as simple as retrogradation of
starch or as complex as reactions between amino
Madson01.p65 2 28/08/2003, 15:57
Fuel ethanol production 3
acids and carbohydrates. An example of a
mashing and cooking system that has
demonstrated maximum yield is illustrated in
Figure 1.
Competing ‘cooking’ factors can be balanced
by selecting a grind that allows minimum
mobility of the sugars and starch within the grain
particle matrix, yet provides necessary
hydration. This is followed by instantaneous ‘jet
cooking’ in the absence of adverse catalysts. By
proper design of the cooking flash-down to
liquefaction temperature (including valve
selection), the ‘locked in’ fermentables can be
released for full access by liquefying and
saccharifying enzymes. Further, the non-starch
fermentables are released from the fiber matrix
to become available to the yeast. By keeping
the fermentables ‘locked in’ within the particle
matrix until the liquefaction tank has been
reached, maximum retention and availability of
fermentable value is achieved.
The next critical step is liquefaction. By
liquefying to minimum dextrose equivalence
(DE) at high temperatures for short time periods,
adverse reaction conditions that convert
fermentables to non-fermentables are
minimized. Little of the starch is converted, and
is therefore protected from adverse reaction
until fermentation conditions are reached.
The key to creating maximum availability of
fermentable carbohydrates is to reach the outlet
of the mash cooler with virtually sterile mash
while providing minimum exposure of
carbohydrate to adverse reactions. At the same
time, the system must maximize downstream
availability of fermentable carbohydrate
embedded in the fiber matrix. Upon reaching
the fermentation temperature, undesirable sidereactions
in the mixture are minimal.
From the mash cooler forward, yield is strictly
a function of enzymatic hydrolysis, fermentation
technology, sterility and completion. High
sustained yields, above 2.75 gpb (2.9 gpb as
denatured MFGE product), have been achieved
with simultaneous saccharification and
fermentation (SSF). SSF technology was
developed in the 1970s to solve a fundamental
problem in the conversion of cellulose to ethanol.
The MFGE industry has advanced this
technology to provide even higher yields by
incorporating simultaneous yeast propagation
(from active dry yeast) in the fermentor during
initial saccharification. Thus, SSYPF (simultaneous
saccharification, yeast propagation and
fermentation) has become the low-cost, highyielding
technology of choice (Figure 2).
Significant sugar concentrations do not develop
in the fermentor, thus avoiding sugar inhibition
of both enzymatic hydrolysis and yeast
metabolism. As a further consequence, bacterial
growth is inhibited due to lack of free sugar
substrate. Sugars are converted by yeast to
Figure 1. Mashing and cooking.
Recycled condensate
Steam
Cooker
Hot
well
Flash
tank
Steam Process
water
Liquefying
enzyme LIquefaction
tank Mash to
fermentation
Mash
coolers
Process
water heater
Recycled stillage
Meal
Backset
Madson01.p65 3 28/08/2003, 15:57
4 P.W. Madson and D.A. Monceaux
ethanol as rapidly as they are produced. By
proper maintenance of pH, nutrients and sterility,
full conversion of available starch and sugars to
ethanol is achieved. Any pH excursions below
4.2 at the end of fermentation correlate directly
with losses in yield (Bowman and Geiger, 1984).
This high-yield SSYPF technology has been
employed in four plants in North America for
which long-term technical results have been
reported (Katzen and Madson, 1991). South
Point Ethanol of South Point, Ohio (64+ million
gallons per year undenatured MFGE) was the
first plant known to have achieved the 2.75 gpb
sustained yield milestone with corn feedstock
for more than one year of operation (Hill, 1991;
personal communication). Reeve Agri-Energy
Corporation of Garden City, Kansas, operates
an 11 million gallons per year plant with yields
from milo and corn feedstock exceeding 2.75
gpb (Reeve and Conway, 1998; personal
communication). Pound-Maker Agventures LTD
of Saskatchewan, Canada, using wheat as
feedstock, has achieved sustained yields
equivalent to those of South Point Ethanol and
Reeve Agri-Energy on a raw material starch and
sugar basis (Wildeman and McCubbing, 1997;
personal communication). More recently,
Minnesota Energy Cooperative of Buffalo Lake,
MN, in a plant now producing 18 million gallons
per year, has achieved sustained yields of 2.8
gallons of MFGE per bushel of corn (Robideaux
and Johansen, 1999; personal communication).
On a ‘product sold’ basis, this yield is 2.95
gallons of denatured MFGE per bushel.
It has been suggested that lower yields result
in increased animal feed co-product production;
and since the market price of the co-product,
distillers dried grains with solubles (DDGS), is
generally greater than that of grain (per unit of
weight), the production facility generates DDGS
revenue to offset the losses in ethanol revenue.
However, several issues need to be considered:
1. Every pound of starch or sugar not converted
to ethanol must remain as starch, sugar, or
be converted to a compound that does not
involve the production of CO2 (or other
volatile by-product) in the requisite
metabolic pathways. If not, production of
one pound of DDGS will require
consumption of two pounds of sugar, thereby
negating the revenue trade-off.
2. If sugar or products of high-yield
stoichiometric reactions pass directly through
to DDGS, the soluble solids are recovered.
Frequently, however, this results in
complications in evaporator and dryer
operations due to carbohydrate fouling and
excess solubles syrup production. This can
necessitate disposal of concentrated solubles
syrup at significantly less than DDGS solids
equivalent pricing. It can also result in a
temporary decrease in production rate or
may cause shutdown for cleaning.
Figure 2. Simultaneous saccharification, yeast propogation and fermentation (SSYPF).
Yeast
Fermenters
Water Enzyme
Mash
Beer feed to
distillation (pH 4.5)
To carbon dioxide scrubber
(pH 5.2)
A B C D
Madson01.p65 4 28/08/2003, 15:57
Fuel ethanol production 5
3. Most plants are limited in centrifugation,
evaporation or dryhouse capacity because
these systems are the least productive from
a return-on-investment perspective.
Therefore higher ethanol yields maximize
both plant production and productivity of
the investment, since lower co-product
production reduces demand on associated
processing equipment.
4. For a plant of a given production capacity,
increasing yield reduces the required size of
most process equipment, with a
corresponding decrease in capital
investment. Higher yield, therefore, reduces
debt service per unit of production.
What is this yield worth? If corn is priced at
$2.50 per bushel, DDGS at $120.00 per ton and
ethanol at $1.12 per undenatured gallon, the net
result of a yield increase from 2.5 to 2.75 gpb is
$0.145 per purchased bushel of grain ($0.053
per gallon of ethanol) in additional profit. This
profit gain is after deduction of the $0.135 per
bushel decrease in DDGS sales owing to
reduction in carbohydrate pass-through to
DDGS. Actual profit, however, will exceed
$0.145 per bushel due to the efficiency value
of increased production with no increase in fixed
cost and little increase in variable cost.
What does it cost to achieve these yields?
Fortunately, the investment and operating costs
associated with this high-yield technology are
lower than those of the common technologies
available in the 1970s and early 1980s. Highyield
milling, mashing, cooking, liquefaction
and SSYPF technology represents one of those
pleasant, but rare, situations in which it costs
less to get more.
As the result of this extraordinary advance,
more than 90% of grain dry milling MFGE plants
operating in 2002 in North America have
adopted SSF, SSYPF or a similar fermenting
process. Further, more than 90% of plants under
construction have chosen this simultaneous
technology (Tetarenko, 2002; personal
communication).
Cascade fermentation
Although there are substantial variations in the
fermentation technologies applied in grain or
starch conversion to ethanol, current operations
can be described on a general basis. The broad
divisions of technology are wet milling and dry
milling. In wet milling, the major objective is to
separate corn into a number of products such as
starch, gluten, germ meal, germ oil, animal feed
residue (gluten feed), dextrose, fructose,
modified starches and a variety of specialty
products. Production of MFGE from the lower
grades of starch, or from all of the starch in an
MFGE-dedicated plant, is an established
conversion process in which starch slurry is
cooked, liquefied and saccharified prior to
fermentation.
All fermentation systems in use today are
continuous with respect to input and output.
Fermentation of saccharified starch in the wet
milling process is carried out by either
simultaneous (SSF or SSYPF) or cascade
processes. Figure 3 shows a typical system for
cascaded saccharification and yeast propagation.
Figure 4 shows the continuation of the cascade
with pre-fermentation and fermentation. This
cascade technology has been applied
successfully to wet-milled starch feedstock.
Application to whole corn dry milling operations
has been carried out on a large scale, but has not
yielded results comparable to SSYPF technology.
Not only is less equipment required for SSYPF
operation in dry milling plants, but the external
saccharification step of cascade systems, a major
source of infections, is eliminated. Also, SSYPF,
which starts at pH 5.2 and ends at pH 4.5, may
be carried out in carbon steel fermentors. On the
other hand, the cascade process requires
maintenance of low pH to minimize bacterial
infection. It operates at a pH near 3.5, thus
requiring stainless steel construction and
consequently higher investment.
Prior to the advent of fully-computerized
fermentor control, including automated cleaningin-
place (CIP) systems, labor costs favored
cascade operation. With today’s automation and
simplified design, labor costs associated with
operating either fermentation technology are
negligible.