Ethanol Based Biodiesel
OPTIMIZATION OF A BATCH TYPE ETHYL ESTER PROCESS
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ABSTRACT
Conversion of rapeseed oil into ethyl esters for use as Biodiesel
fuel involves transesterification of the oil triglycerides to mono-esters of
the component fatty acids. To accomplish this conversion, raw rapeseed oil
is treated at room temperature with ethyl alcohol in the presence of
potassium hydroxide as a catalyst. During the process, the glycerol which is
produced is insoluble in the ester product, and being heavier, settles out
carrying most of the dissolved KOH catalyst with it.
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Upon initial settling, some of the undesirable, emulsion-forming
by-products may remain in the ester layer, causing problems in the washing
stage. It was discovered (by tracking the process with a glycerol
determination) that most of these products could be removed by simply
restirring the glycerol into the ester, adding water and letting the mixture
settle out again. After draining off the glycerol/water layer, the product
(ethyl ester) can be easily water-washed to remove residual alcohol and
potassium.
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INTRODUCTION
Processing
Transesterification of rapeseed oil at the University of Idaho from 1980
to 1990 used methanol as the alcohol. Methanol is highly toxic, does not
produce a visible flame when burning, can be absorbed through the skin, and
is 100% miscible with water, so any kind of spill presents a serious
problem. Ethanol provides the advantage of making a Biodiesel fuel produced
entirely from renewable resources. The use of ethanol in Biodiesel
production has not been studied as extensively as has methanol.
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Water washing the ester was accomplished through sprinkling water into
the tank at an approximate rate of 100 gallons per hour. As the water
droplets travel through the ester, they remove the impurities. Washing would
continue for 20 to 30 hours consuming as much as 3,000 gallons of water.
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Oil Seed Press
Two commercially manufactured screw type oil expellers are used for
extracting oil from the rapeseed for this project. The seed is manually fed
into a 100 pound capacity tapered bin atop an auger. The seed is heated as
it moves up the auger to the screw type press. A retention time of
approximately 20 minutes is required for the seed to be heated before the
oil is extracted. These two presses with augers were mounted on a movable
base that were 12 feet in length, 4 feet wide, and 5 feet in height. Due to
the size of these platforms and the limitations of shop space only one press
could be feasibly operated at a time.
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Reactor
A 290 gallon cross-link polyethylene reaction tank and a 50-gallon
plastic rubbermaid barrel were used prior to this grant. The reactor tank
was capable of producing 200 gallons of ester per week. A sink type drain
was used with gaskets in the bottom of the reactor tank to drain the
glycerol and then the wash water from the ester layer. The drain gasket was
continually leaking and the cross-link polyethylene reaction tank was
warping due to material compatibility with ester and glycerol. Fluids were
pumped through the use of a centrifugal pump which required a hand primer
pump to transfer the alcohol from the plastic barrel to the reactor tank.
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MATERIAL AND METHODS
Processing
Reactants
The reactants for the transesterification process are used in the
following previously determined proportions in U.S. and metric units:
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Raw rapeseed oil 100 L 100 Kg 100 Gal
Anhydrous Ethanol 27.4 L 23.74 Kg 27.4 Gal
Potassium Hydroxide 1.30 Kg 1.43 Kg 10.83 lb
The input amount of raw rapeseed oil determines the batch size, and the
other components are calculated from the following formulas:
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EtOH = 0.2738 x RO
KOH = 0.013 x RO
where;
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EtOH = amount of ethanol required, in liiters
RO = the desired amount of oil to be processed, in liters
KOH = amount of KOH required, in kg
According to these formulas ethanol is added at a 65% stoichiometric
excess, or a molar ratio of 5.0: 1 EtOH to oil. The KOH is added at 1.43% of
the weight of input oil.
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Quality of Reactants
1. Rapeseed Oil Best when clear (filtered) because excess sediment
collects on the bottom of the reaction vessel during glycerol settling and
at the liquid interface during washing. This sediment interferes with the
separation of liquid phases and with the washout of catalyst, and may tend
to promote stable emulsion formation. Slight haziness of the oil probably
does no harm. The original oil must be water-free, because every
molecule of water destroys a molecule of catalyst, thus decreasing its
concentration.
2. Ethanol. The nearer to absolute (200 proof), the better.
Gasoline present in the alcohol as a denaturant appears to do no harm. The
reaction proceeds satisfactorily in mixtures of 200-proof ethanol with
10%(v/v) or more gasoline present. However, even small quantities of water
(less than 1%) can decrease the extent of the conversion reaction enough to
prevent the separation of glycerol from the reaction mixture.
3. Potassium Hydroxide Catalyst. Best if it has > 85%
KOH. Even the best grades of KOH have 14 to 15% water (which cannot be
removed). It should be low in carbonate, because potassium carbonate does
not serve as a satisfactory catalyst, and may cause cloudiness in the final
ester.
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Other catalysts which may be used are potassium ethoxide and sodium
ethoxide, but they are prohibitively expensive. Sodium hydroxide was not a
suitable catalyst because it was not aufficiently soluble in ethanol and it
tends to promote undesirable gel and emulsion formation during
transesterification.
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The Reactions
1. The first step is to activate the ethanol by dissolving the potassium
hydroxide to form potassium ethoxide. Stir vigorously in a covered container
until the KOH is dissolved, approximately 20 minutes. Protect as much as
possible from atmospheric CO(2) and moisture, both of which reduce the
activity ofthe catalyst. The entire portion of ethanol is used here. (There
is enough ethanol to accomplish the complete transesterification, with about
65% in excess.) Preparing this solution is, in effect, preparing a solution
of potassium ethoxide according to the following reaction. Reaction weights
are as follows:
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Mol. Wts.
56.1
46.07
84.15
18.02
KOH
C(2)H(5)OH —->
C(2)H(5)OK
H(2)O
For 100L oil
1.3 kg
x=1.07kg
y=1.95kg
0.42kg
Using a 100 liter batch of oil as an example, the KOH used reacts with
1.07kg of ethanol to produce 1.95kg of potassium ethoxide. This mixture now
contains (27.4×0.789)-1.07 = 20.55kg of free ethanol and 1.07kg of ethanol
as potassium ethoxide catalyst. Any water added to the entire system
reverses the above reaction and quenches a proportional amount of the
potassium ethoxide catalyst. One part of water can quench up to 84.15/18.02
= 4.67 parts of catalyst.
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The ethanol-KOH mixture is then poured into the rapeseed oil, and the
following transesterification reaction occurs: (a hypothetical formula for
the rapeseed oil, based on a typical oil analysis is used):
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Mol.Wts.
963.51
138.21
1009.02
92.09
(3×46.07)
(3×336.54)
O
H(2)C-O-C-C(19)H(35)
H(2)C-OH
O
O
H-C-O-C-C(19)H(35)
3C(2)H(5)OH ——
3C(19)H(35)COC(2)H(5)
H-C-OH
O
H(2)C-O-C-C(19)H(35)
H(2)C-OH
(Hypothetical Formula)
(Hypothetical Formula)
or,
1 Rapeseed Oil
3 Ethanol ——
3 Ethyl Ester
1 Glycerol
(100L or 91kg)
(13.1kg)
(95.3kg)
(8.70kg)
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From these relationships, 100 liters (91kg) of rapeseed oil reacts with
13.1kg of ethanol. The 21.62kg (or27.4L) of ethanol used in the batch
represents 21.62/13.1×100 = 165% of that required for complete
transesterification of 100 liters of rapeseed oil. (A 65% excess over the
theoretical requirement).
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The Mechanics of the Transesterification Process
1. Raw rapeseed oil is measured into the reactor.
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2. The required amount of ethanol is placed into a smaller covered
container.
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3. The required amount of potassium hydroxide is quickly weighed,
protecting it as much as possible from atmospheric moisture and carbon
dioxide.
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4. The solid potassium hydroxide is added to all of the ethanol which is
then vigorously stirred in the covered container until completely dissolved.
At this point the dissolved KOH is presumed to have been converted to
potassium ethoxide catalyst. Any undissolved pellets of KOH left in this
alcohol tend to remain undissolved during the entire subsequent
transesterification, essentially decreasing the amount of catalyst taking
part in the reaction.
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5. The ethanol-catalyst mixture is poured into the oil in the main
reactor and stirred rapidly. Mixing is continued for 6 hours at room
temperature. The reaction mixture usually changes to a turbid orange-brown
color within the first few minutes; then it changes to a clear transparent
brown color; finally, as the reaction is completed, the mixture again
becomes somewhat turbid and orange-brown colored due to the emulsified free
glycerol which has been formed.
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6. In a good completed reaction, the glycerol begins to separate
immediately upon cessation of stirring, and the settling mostly complete in
one hour. After initial settling, the entire contents of the reaction vessel
are again mixed together and stirred vigorously for 40 minutes. After the
first 20 minutes of restirring, water is added at 15% of the initial volume
of oil used in the reaction. Stirring should continue an additional 20
minutes after the water is added for a total of 40 minutes of restoring.
This mixture is then allowed to settle overnight or over a weekend. A longer
separation time facilitates the washing process. Remixing the glycerol layer
with the ester layer while adding water has the effect of collecting and
removing impurities and products of incomplete reaction from the ester. The
washing phase can then proceed at a more rapid pace than if the remixing
stage were left out.
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In batches where poorer quality (moisture-containing) ethanol is used the
reaction will not go to completion and requires much longer for the glycerol
to separate. If separation does not occur, the addition of a small amount
(perhaps 10% of the original volume) of alcoholic KOH with stirring may tip
the reaction balance in favor of separation. It is also sometimes possible
with the addition of a small amount of water (0.5% of the total volume)
after the reaction is supposedly completed, to effect the separation of the
glycerol from the ester. If the original ethanol contains as much as 1%
water, the reaction may be so incomplete that the glycerol may never
separate, and the entire batch must be discarded.
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7. After remixing the glycerol and 15% water addition, and completion of
the separation, the lower, heavier glycerol/water layer is drained off,
pumped into barrels and shipped to a recycler. A thin layer of gross
glycerol and sludge may adhere to the bottom of the reactor. It is advisable
to wash down the cone of the reactor to remove this adhering glycerol or
sediment by pouring a few gallons of cold water down and around the inside
circumference of the reactor. This should be done at least twice. Any
sediment probably consists of a host of minor components of the original
rapeseed oil (proteins, glycoproteins, waxes, sterols, carotenoids,
phosphatides, carbohydrates, etc.). Some of these constituents are
emulsifying agents, and others have affinity for water which causes hold-up
of undesirable impurities(e.g., potassium) and tend to prolong the washing
process.
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8. Finally, in order to remove the remaining alcohol and trace amounts of
potassium, glycerol or soap, the ester is washed with water at about 30% of
the ester volume or 30 gallons of water to a 100 gallon batch of ester. The
water is stirred into the ester with mechanical stirring and air agitation
as described in the next section. After a few hours the stirring/aeration is
stopped and the water is allowed to settle out for two to three days. At
this point the process is complete and the crystal clear product can be
pumped into fuel tanks for storage or immediate use.
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The Washing Process
Washing the ester product is necessary in order to improve its fuel
properties, largely by removing residual free glycerol and small amounts of
potassium remaining from the catalyst. The best method so far devised was
previously described. It is a combination of: 1. Mixing the glycerol layer
into the ester after the initial settling has occurred; adding 15% water;
stirring and settling. 2. A water wash with agitation and aeration after the
glycerol/water layer has been drained off.
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Soap Formation
Soaps, at least in trace amounts, can be formed by an accompanying
reaction during or subsequent to the transesterification process:
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O O
RC-OC(2)H(5) H(2)O——– RC-OH C(2)H(5)OH
Ethyl Ester Water Fatty Acid Ethanol
This is an equilibrium reaction, and any base will neutralize the acid
formed, removing it, and forcing the reaction to the right. Also the
reaction product of the base and acid is an undesired substance (a soap,
which is an emulsifying agent).
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O O
RC-OH KOH——– RC-OK H(2)O
Fatty Acid A Base Salt/Soap Water
These reactions have little tendency to occur during the
transesterification because of the small amount of water in the system. The
source of the interfering water for this reaction may be use of low-grade
water-containing ethanol, water in the other reactants at the beginning
(from atmospheric exposure), or even from the first stage of the water wash.
In any case, only a trace of soap needs be formed to promote emulsification
of the ester with the wash water.
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Wash Methodology
1. Agitation. During washing, many of the impurities in the ester
have a greater affinity for the water, and they are transferred by diffusion
across the phase boundary into the water. This process is greatly hastened
by agitation, which can increase the area of phase contact by emulsion
formation, or can promote transfer by maintaining the most effective
concentration gradient for transfer across the interface.
2. Mechanical Stirring. Best results have been obtained using a
mechanical stirrer whose rotation can be strictly controlled. The best speed
for the equipment used has been about 50 to 70 RPM. The stirrer shaft should
have two blades with one in the water phase and one in the ester phase
rotating to lift the solution upward. This orientation, along with aeration
develops maximum contact between ester and water.
3. Aeration Mixing. A unique method of aeration mixing was
discovered. If air is introduced deep into the water layer through a
sandstone, glass or stainless steel gas diffusion disk numerous air bubbles
are formed in the water phase. These numerous water coated bubbles rise
through the liquid interface into the ester, carrying large amounts of water
in the film, and accomplishing washing as they rise up through the ester.
Upon reaching the surface, the bubbles burst and form droplets of water
which fall back down through the ester, further washing it. These bubbles
and droplets seem to be of such size and nature that the droplets formed do
not remain emulsified when they reach the aqueous phase, but quickly
coalesce and disappear into the aqueous layer. This method greatly magnifies
the interface area, and at the proper aeration rate, half or more of the
ester phase volume seems to be filled with quite rapidly settling droplets
of aqueous phase.
4. Combination Aeration and Mechanical Stirring. By combining (2)
and (3) above very efficient and rapid washing can be achieved using minimum
amounts of water.
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Product Completeness
1. Bench Wash Test. Throughout the washing, a rough idea of the
completeness of the washing may be obtained by washing (in a 100 ml beaker
with magnetic stirring) 50 ml ester with 25 ml water for about 1/2 hour, and
then determining the pH of the wash water. If the ester is sufficiently
washed, the pH should be around pH 6 to 7. There is also a good way to
determine washing completeness by noting the emulsion-forming tendency
during this beaker wash-test. If the wash has been satisfactory, it is
possible to stir a sample rather vigorously in this test and form an
emulsion of large, clear, shiny droplets which will quickly separate,
settle, and disappear upon cessation of stirring.
2. Turbidity. Occasionally a batch of washed ester may end up with
turbidity caused by traces of condensed moisture. This moisture may be
conveniently removed (evaporated) by aeration with dry air, using the gas
diffusion disks from the washing step, to increase the surface contact
between the air and the ester. Slight warming along with the aeration also
hastens the removal of this trace of moisture.
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Process Optimization In the development of the above procedure for
producing high quality ethyl esters, A.O.C.S. method Ca 14-56 for
determining total, free and combined glycerol was used to follow the
process. Procedural methodology was determined on the basis of percent
glycerol left in the product after each step. Due to health and safety
concerns, heptane was used as the solvent in place of the recommended
chloroform. It was found to be very satisfactory. (n-Hexane and
2,2,4Trimethylpentane were not satisfactory)
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Five sets of tests were made, each with 6 small beaker batches of REE.
Batches were made according to the recipe previously described and allowed
to settle for 30 minutes (except for the first one) before further treatment
took place. Methods and inputs were generally kept constant except for the
one being studied. Glycerol determinations were made on samples settled for
at least 24 hours after treatment.
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Viscosity vs. Total Glycerol
At first, a direct correlation between viscosity and glycerol was determined
using A.O.C.S. Ca 14-15 (Figure 1). It was an effort to test the method for
accuracy and reliability by spiking some relatively pure REE with various
amounts of pure glycerol. The cluster of points at the lower end of the
graph represent production grade product. It was found that this correlation
only holds true for washed esters. REE at intermediate stages may contain
some alcohol which affects the viscosity reading.
Figure
1. Viscosity at 40C versus percent glycerol in Biodiesel.
Click
here to expand figure.
Addition of Water
1. Amount of Water. In this set all samples were allowed to settle
then remixed with glycerol for one minute before water was added with the
exception of the first one in which water was added at the end of the 2 hour
reaction time. All samples were stirred for 3 minutes after water was added.
At this stage of the process it was found that increasing water could be
tolerated to the point where a permanent emulsion was created, which was
approximately 30% by volume of the original amount of input oil. Figure 2
shows that water has very little effect on combined glycerol but that it
significantly reduces free glycerol in the ester. Notice, however, that the
free glycerol was plotted against the 2nd Y axis and that the numbers are
quite small. The amount of water added to the mixture that effected the
highest glycerol removal was found to be 20%. This figure will vary
according to the quality of the raw products used in the reaction. Poorer
quality reagents will produce a product higher in mono, di, and
triglycerides and thus will tolerate less water before a permanent emulsion
is formed. This was the reason that 15% water was recommended. Notice also
in Figure 3 that a point in the upper left hand comer shows the effect of
adding water at the end of the reaction without first letting the glycerol
settle out. It was higher in glycerol, which suggests that the action of
settling and remixing before adding water decreases combined glycerol. It
can be said that some amount of mono, di and/or triglycerides are removed or
converted by the glycerol only remix.
Figure
2. % Glycerol vs water added after one minute of remixing the glycerol
into the ester layer and mixing the water for three minutes.
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here to expand figure.
Figure
3. % Glycerol in Biodiesel with one minute remix of glycerol into the
ester and 20% water added to the reaction versus mixing time.
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here to expand figure.
2. Time of Water Remix. In this set the small beaker batches were
remixed with glycerol for one minute before the addition of water except for
the first one which was stirred for 10 minutes before water was added. Water
was added at 20% of the total for each sample. Stirring time varied from 1
to 30 minutes. In Figure 3 it can be seen that the glycerol decreased
steadily with time. Twenty minutes was chosen as the cutoff time for the
remixing. Data from the first point (10 min premix) confirmed the idea that
combined glycerol can be reduced and prompted the next set of tests in which
the glycerol only remix was lengthened.
Glycerol Remix
In this set of tests the glycerol only remixing time was varied from 5 to
30 minutes. All samples then had 20% water added and continued mixing for 10
min (first four) or 20 minutes (last two). Figure 4 shows that the combined
glycerol does in fact decrease, confirming again that some amount of mono,
di and/or triglycerides are removed or converted in this process. The free
or dissolved glycerol actually increased with time, however the final water
wash should effectively remove all but a trace of it. The lowest glycerol
readings were from the sample which was restirred for a total of 40 minutes,
20 minutes in glycerol only mode and 20 minutes after the 20% water was
added. These were the parameters chosen as a pretreatment to the final water
wash and can be referred to the 20/20/20 rule for treating batch processed
Biodiesel. As was stated earlier, the amount of water added should be
reduced to 15% if the reagents are not of the highest quality.
Figure
4. Varying glycerol remix into ester times with 20% water added after
the remix and then mixed an additional 10 minutes. The shorter graph lines
are with 20 and 30 minute glycerol remix times with 20% water added and
mixed an additional 20 minutes.
Click
here to expand figure.
Oil Seed Press
A seed press stand was designed and built for the seed to be above the
press and gravity fed. This eliminated the need for an auger and reduced the
size of the base to seven feet long, five feet wide, and a height of 11
feet. The seed bin was sized for a 2,000 pound capacity of seed and placed
on a digital readout scale. The meal bin was positioned beneath the seed bin
and was also placed on a digital readout scale for monitoring the press
efficiency. A seed heating chamber was sized for the seed to have a
retention time of twenty minutes before entering the screw-type press. The
second press was positioned so the gravity feed seed tube was connected to
the bottom of the existing funnel-type seed bin. This allowed for
simultaneous press operation and a minimal amount of monitoring the presses
and seed bins (due to the seed bin capacity of two days).
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Reactor
A 400 gallon capacity 316 stainless steel, 10 gage cone bottom, open top
tank was mounted on a 6 foot by 8 foot portable platform. An 80 gallon poly
tank with a cone bottom for the alcohol and catalyst mixing was mounted next
to the stainless steel reaction tank. The centrifugal pump was mounted below
the plastic alcohol tank eliminating the need for a hand primer pump. An air
driven transfer pump is used to transfer alcohol form barrels to the poly
alcohol/catalyst tank. A hydraulic motor with a shaft and mixing blades was
mounted to the top center of the reaction tank for the washing process.
Aeration tubes for the washing process are introduced to the reaction tank
during the washing process.
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CONCLUSIONS
Developments in processing rapeseed ethyl ester have been
dramatically improved during the course of the past two years. Remixing of
the glycerol layer with the ester layer for 20 minutes and then adding 20%
water and continuing mixing for an additional 20 minutes has reduced the
amount of water used from as much as 3,000 gallons to 150 gallons for a 200
gallon batch of ester. The remixing of glycerol and water also reduced the
possibility of forming an emulsion during the water washing process.
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The gravity feed system for the oil seed presses has allowed for two
presses to operate simultaneously with minimal amount of labor. The seed bin
requires filling once a day and the meal bins are emptied every other day.
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A larger reactor tank and alcohol/catalyst mixing tank mounted on the
same platform has increased production by 1.3 times that of the previous
reaction tank. Pumping procedures have been simplified along with handling
of the alcohol.
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