| DENSE MEDIUM SEPARATION -> AGRIC: SUGAR | CARROTS POTATOES |
The Dominance of Billet Harvesters
Since 1995, there has been a
major shift in the Louisiana sugarcane industry away from
whole-stalk harvesting. Today more than 90% of the cane in
Louisiana is harvested as billets. A billet harvester cuts
the cane into pieces ranging from 6 to 14 inches in length
and discharges it into a trailer moving alongside.
The Problems
Created by Billet Harvester
The billet harvester knocks
the cane down and bites into the surface of the row, taking
along with the cane an abundance of trash such as tramp
iron, sand, clay, clay balls, stones, bricks, leaves and
tops. In the best case, a ton of cane contains 13% trash
(8% inorganic and 5% organic). Under wet conditions, a ton
of cane may contain up to 40% trash (30% inorganic and 10%
organic).
Huge Losses
of Recoverable Sucrose
Not only does this trash not
contain sugar, but as it leaves the mill in the form of
filter cake or bagasse, it carries away sugar at a rate of
roughly 0.75 lbs per percentage point of trash. Moreover,
each percentage point of trash represents a loss in
production of approximately 3 lbs of sugar.
Trash
Cripples Profitability
In the case of 13% trash,
the loss of sugar averages 9.75 lbs/TC, and production
losses exceed 16%. In the case of 40% trash, the loss of
sugar stands at 30 lbs/TC, and production losses exceed
50%. More trash means more maintenance, more down-time,
more flocculent, more lime, more natural gas, more unburned
bagasse, more filter cake, more front-end loaders, more
water treatment, more settling basins, more draglines to
clean out settling basins, more haulage, more inversion,
more molasses, less sugar and more cost. Every aspect of
this complex sugar-making process is negatively impacted by
the presence of trash. If growers and mills are intent on
making money, they can no longer tolerate the presence of
trash.
All mechanical harvesters
whether of potatoes, beets, carrots, salsifies and peas,
face the same problem of the intrusion of foreign matter
that varies widely in terms of grain size and density. For
over 40 years, the vegetable industry in Europe battled
this problem by means of vibratory screens, rotary
trommels, optical sorters, air separators and a variety of
dynamic effect separators employing water. None of these
technologies worked.
Failure of
All Attempts to Date to Deal with Trash
Likewise, we may try to
screen, rinse, wash or scrub the billet, but all of these
steps fall far short of producing an acceptable product.
The average washing efficiency for a single deck washing
table is about 53%, for a dual deck washing table, it is
about 59%, and for a cane washing drum, it is about 44%. In
a wet season, these efficiencies would decline
considerably. If the Louisiana sugar mill wants to make
money, produce a consistent product, eliminate the
randomness associated with weather, and aggressively
compete with the rest of the world, it must look for a
technology that can make a perfect separation of trash from
billets. Obviously the first question someone might ask:
does the density of the billet make it difficult to
separate from everything else?
The Specific
Gravity of Sugarcane
The density of a sugarcane
billet is no different from that of a sugar beet or potato,
as is readily seen in the results of density tests
conducted on January 17 and November 18 of 2003 at the USDA
laboratory in Houma, Louisiana.
In terms of size and shape,
the billet is no different from a large carrot or a
salsify. The salsify is a long root vegetable grown in a
soil that is predominately clay, and since it is harvested
in the middle of winter when it rains continuously, it
generally comes from the field with over 60% clay. Nothing
of this clay, not a single ounce, can ever be found in
salsifies exiting an ESR separator.
Since the size, shape and
density of the billet appear quite normal; since metal,
stone and clay all have densities at least twice that of
the billet; since Newton’s laws continue to operate at
sugar mills, nothing is unproven about ESR technology. This
technology has been around for over 20 years, and anyone
trained in the art of dense medium separation would find it
impossible to explain how it would not work in sugarcane.
What is dense medium separation?
The Dense
Medium Separation of Cane
Here we find the dynamic of
a quiescent bath where the density of water is changed by
means of fine particles in suspension. At first glance,
nothing could be simpler: one fraction floats, while the
other fraction sinks. But at high tonnages even the best
dense medium separators generate errors, and where should
one go to find the suspension materials needed to change
the density of water in separation of waste materials?
Twenty years ago, ESR
International invented a unique bi-directional dense medium
drum that has proved to be the most accurate separator ever
employed in the separation of a variety of root vegetables
such as carrots, potatoes, salsifies and beets. Not only
does this separator remove both high-and low-density trash,
but it is also able to distinguish good vegetables from bad
vegetables, vegetables that at times might differ in
density by only a few points to the third decimal place.
Suspension
Fines from Harvester Dirt
In this revolutionary
sorting process, the suspension fines needed to change the
density of water are obtained nowhere else but from the
dirt that mechanical harvesters extract along with the
vegetables. Two stages of classifying cyclones isolate
ultra-fine sand from the scrub and rinse water of the
pre-processing and cleaning line. In using these fines, one
avoids the dreadful health issues associated with the use
of clay or sand contaminated with dioxins or heavy metals,
and one avoids as well the thorny environmental and
maintenance issues associated with the use of salt and the
disposal of brine.
Eventually sixteen
bi-directional dense medium separators were sold in Belgium
and France. These separators have been in continuous
operation in Europe, some for almost 20 years. It is truly
remarkable that no one to this day has ever been able to
establish a single separation error in the finished
product.
Putting
Billets in Water
Perhaps the biggest
objection to the dense medium separation of sugarcane
billets concerns the loss of sugar associated with putting
them in water. The washing of whole stalk cane had been in
practice in Louisiana for about 50 years. But since the
harvesting of a whole stalk involves but one or two cuts,
the loss of sugar here does not exceed about one pound of
sugar/TC.
There are four factors that determine the loss of sugar
with respect to billeted cane:
1. the number of harvester cuts (billet length),
2. the precision of the harvester cut (clean or frayed),
3. the forces applied to the billet (agitation/tumbling),
4. the amount of time the billet is subjected to these
forces.
The Number of
Harvester Cuts
In general, it is fairly
safe to say that if all other factors are the same, twice
the number of harvester cuts will yield twice the loss of
sugar. In the study by Birkett and Stein, we see that short
billets of an average length of 5.25 inches lose twice as
much sugar as long billets of an average length of 9.55
inches (see Cane Washing Losses, pp. 3 & 4).
The Precision
of the Harvester Cut
If the billet harvester
cutting blades are not sharp, they will damage and fray the
ends of the billet. The same study by Birkett and Stein
shows that sugar losses increase almost threefold if the
billet is damaged (see p. 3). Therefore it is very
important to change the cutter blades on the combine
harvester at the frequency recommended by the harvester
manufacturer? (p.6).
The Force
Applied to the Billet
In vegetable separation, the
handling of the carrot or salsify within the dense medium
separation process is a very important issue. If, for
example, a carrot or salsify is broken, it has little or no
value. Several studies conducted in Europe have shown that
the carrot remains undamaged throughout the entire ESR
process.
Likewise, if a billet is
agitated or tumbled, either before or during the cleaning
process, it will lose sugar and drop in value. That is why
the scrubbing of the billet, as in a typical wash drum, is
far from ideal. It would take at least 5 minutes of violent
scrubbing to break down the large clay balls that often
accompany the billets. But a retention time of five minutes
would require a scrub drum of an enormous length, and
during this time, far too much sugar would be lost.
Time of
Agitation
The study by Birkett and
Stein shows that if billets are placed in a cane
preparation index apparatus and rotated at 19 RPM, a lot of
sugar is released by this tumbling action. The longer the
billet is tumbled, the greater the loss of sugar. In the
first 30 seconds, we see a loss of about one pound of
sugar/TC, while after 15 minutes, this loss increases to
almost 8 lbs/TC (see ibid., Figure 15).
Water Alone
Has Little Impact
It would appear, therefore,
that the simple act of putting the billet in water releases
very little sugar from within the billet. To determine the
density of the billet, ESR once spent an entire day at the
USDA lab in Houma weighing billets underwater. The billets
were left in a tranquil bath of water for several hours at
a time, and throughout the entire day, the water used in
this test was never changed. At the end of the day, the
amount of sugar in this water was indistinguishable from
the amount of sugar present in the original tap water used
for this density test. What can we learn from all of this?
Water does not diffuse sugar
out of a billet. Since the cell membrane of cane is
resistant to osmotic effects, the cell wall must be
ruptured and force must be applied to the wall of the
billet. Once the billet is cut, every effort should be made
to minimize all aspects of its handling. From field to
factory, it should be loaded and unloaded but one time. It
should never be dumped on a concrete slab or handled by a
wheel loader. In the cleaning process, it should not be
scrubbed, agitated or tumbled in any way, and it should not
remain in a separator for more than about 15 seconds.
An Estimation
of Sugar Losses
If we do all of the above,
then we have every reason to expect a sugar loss of about 3
or 4 lbs of sugar/TC. Keep in mind that a 3-lbs loss of
sugar/TC is made up for by removing only 4% trash/TC. Under
dry conditions, a mill would accept a loss of 3 lbs to
avoid a loss of 9.75 lbs, and under wet conditions, a mill
would accept a loss of 3 lbs to avoid a loss of 30 lbs.
Some people have put forward
the concept that the ESR dense medium separator would
involve a loss of sugar of about 20 lbs sugar/TC. But if
the average loss of sugar in the Patout wash drum was only
6.2 lbs sugar/TC, and if our separator effects only a
fraction of the tumbling action of the Patout wash drum,
how does one justify such a claim?
The Vegetable
Industry in Europe
The sugarcane industry in
Louisiana has a lot to learn from the vegetable industry in
Europe. If it handles billets with the same respect that
the vegetable industry in Europe handles carrots, and if it
separates billets with the same technology that Europe
employs to separate carrots, then most of the problems
created by the introduction of billet harvester in
Louisiana are easily solved.
How to Handle
a Billet
Putting a billet in water
will not cause sugar to come out of a billet. Putting a
billet in water simply shows us how much we have mishandled
it prior to putting it in water. The gentle action of a
dense medium separator can never be a major factor in sugar
loss. Perhaps we need to adjust the length of the billet,
change cutter blades more often, reduce the number of times
we load and unload the billet, eliminate the use of wheel
loaders, reduce the distance we allow a billet to fall, and
so forth. It has taken 8 years for the billet harvester to
dominate the sugarcane industry in Louisiana, but with the
advent of CAFTA, NAFTA and other free trade agreements, the
Louisiana sugarcane industry does not have the luxury of
waiting another 8 years to address the problems and
inefficiencies created by the billet harvester.
The Air
Separation of Leaves and Tops
The leaves and tops of the
sugarcane plant contain no sugar, and those that accompany
the billet into the mill can be easily removed by means of
an air separator. Since it is very important in air
separation to minimize particle interference, the billets
should be spread out over a broad two-dimensional plane,
and the blower should be situated in-line with the flow of
billets. The leaves and tops isolated by the air separator
are ideal for composting.
The
Elimination of Trash
Hopefully it is clear that
this dual strategy of air separation and dense medium
separation can eliminate virtually all organic and
inorganic trash. What mill in Louisiana has ever considered
the possibility of processing billets without extraneous
material? It is hard to imagine what this should mean. For
the first time ever, a mill will be able to operate under
constraints that are truly internal.
A mill would possess a level
of control in daily operations that it has never before
experienced. A lot more sugar would be recovered.
Production would stabilize at a high rate. Operating and
maintenance cost would decrease dramatically. Rain and even
hurricanes would have minimal impact on profitability.
Louisiana
Must Compete
The Louisiana sugarcane
industry will never compete with the rest of the world if,
during wet weather, it faces losses in production of over
50% and increases in operating and maintenance costs of
over 50%. The industry must learn to compete in an
aggressive marketplace, and no single technology offers
more hope of doing so than the technology briefly outlined
in this paper.
The elimination of trash
will greatly simplify every aspect of this complex
sugar-making process. It will offer growers and factories a
myriad of new options and new economic opportunities, and,
at the same time, it has the power to rescue and stabilize
one of Louisiana’s oldest and most important industries.