Note: Descriptions are shown in the official language in which they were submitted.
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Production of Biodiesel from Triglycerides by Using Thermal Cracking
Technical Field
The present invention relates to a method of producing biodiesel from
triglycerides using thermal cracking. The present invention specifically
relates to
the production of biodiesel from waste triglycerides.
Background Art
In recent years, the area of biodiesels has drawn a great deal of attention.
Biodiesels are plant and animal based fuels produced from the esterification
of
biomass-derived oils with alcohol. Biodiesel can be produced from such sources
as canola, corn, soybean etc. Biodiesels are generally considered less
environmentally damaging than traditional fossil fuels.
Another potential source for biodiesels is the waste triglycerides of animal
rendering facilities and waste cooking oils, such as those found as restaurant
trap
greases. However, this potential is presently still under-explored and waste
triglycerides are most commonly dumped into landfills (`Biodiesel Production
Technology, August 2002 - January 2004"; Van Gerpen, J. et al., July 2004,
NREL/SR-510-36244). Waste triglycerides often have a high contaminants
content that must effectively be removed before processing. Furthermore, waste
triglycerides tend to have a high content of free fatty acid (FFA), anywhere
in the
range of from 50% to 100%. Mixtures of free fatty acids and triglycerides have
been found to be very difficult to convert to useful fuels by any traditional
methods.
Traditional methods of producing biodiesels include transesterification and
esterification with alcohol using either an acid or base catalyst. However,
the high
FFA content in waste triglycerides causes undesirable soap formation in base
catalyzed esterification processes, rendering this process inoperable.
Waste triglycerides are also often heavily contaminated by contaminants,
such as bacteria, detergents, silts and pesticides. These contaminants must be
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removed before esterification can take place, without adding significant
additional cost to the overall processes.
One known method of processing high FFA feedstocks involves adding
glycerol to the feedstock to convert FFA's to mono- and diglycerides, followed
by conventional alkali-catalyzed esterification. This method addresses the
high
FFA content of waste triglycerides, but does not treat or remove contaminants.
A second method involves performing both esterification and
transesterification of triglycerides using a strong acid such as H2SO4.
However, water formation by FFA esterification prevents this process from
going to completion. A third method involves pre-treating an FFA-rich
triglyceride feedstock with an acid catalyst to convert FFA to alkyl-esters
and
reduce FFA concentrations to less than about 0.5%, followed by traditional
base-catalyzed esterification. This method again, only deals with the FFA
content of waste triglycerides, and not the high contaminant levels.
Thermal cracking of clean triglycerides under typical cracking
conditions with and without catalyst has been attempted, but this process was
found to yield mainly naphtha, not diesel fuels. Furthermore, in typical
thermal cracking of clean or waste triglycerides in the presence of a
catalyst,
there is a tendency for coke formation to occur on the catalyst, resulting in
rapid deactivation.
It is therefore greatly desirable to find a method of converting waste
triglycerides feedstocks to biodiesel that is both efficient and economical.
It is
also desirable to find ways of dealing with contaminants and high FFA content
in waste triglyceride feedstocks so that they can be converted into usable
fuels.
Disclosure of Invention
The present invention provides a method of producing biodiesel from a
waste triglyceride feedstock. The method first involves pretreating a waste
triglyceride feedstock by a thermal cracking reaction at a temperature in a
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range of 390-460 C and a pressure in a range of 0 to 60 psig to remove
contaminants and convert triglycerides to form a middle distillate fraction
rich
in free fatty acids. A middle distillate fraction having a boiling point range
from 150 to 360 C is separated from a remainder of a reaction product of the
thermal cracking. The middle distillate fraction is esterified in the presence
of
an alcohol and a catalyst to produce a biodiesel stream, and then the
biodiesel
stream is treated with a basic solution to convert unesterified free fatty
acids to
non-foaming metallic soaps. Finally, the non-foaming metallic soaps are
removed by centrifugation, filtering or a combination thereof.
The present invention also provides a method of producing a
biodiesel/naphtha mixture from a triglyceride feedstock. The method involves
first pretreating a waste triglyceride feedstock by thermal cracking at a
temperature in a range of 390-460 C and a pressure in a range of 0 to 60 psig
to remove contaminants and convert triglycerides to produce a middle
distillate
fraction rich in free fatty acids, a naphtha stream and a gas stream. A middle
distillate fraction having a boiling point range from 150 to 360 C, a naphtha
stream and a gas stream are separated from a remainder of a reaction product
of the thermal cracking. The naphtha stream and middle distillate fraction are
esterified in the presence of an alcohol and a catalyst to produce a mixed
biodiesel/naphtha stream, and the mixed biodiesel/naphtha stream is then
treated with a basic solution to convert unesterified free fatty acids to non-
foaming metallic soap;. Finally, the non-foaming metallic soaps are separated
by centrifugation, filtering or a combination thereof.
Brief Description of the Drawings
The present invention will now be described in further detail with
reference to the following drawing, in which:
Fig. 1 is a flow sheet of a first preferred process for carrying out the
present
invention.
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Fig. I is a flow sheet of a first preferred process for carrying out the
present
invention.
Best Modes for Carrying Out the Invention
The present process employs a novel combination of thermal cracking
followed by esterification to convert low quality triglycerides feedstock into
usable biodiesel. In the present process, thermal cracking is used as a pre-
treatment step to break down the triglycerides into a broad range of free
fatty
acids and lower molecular weight components. Thermal cracking also serves
to remove contaminants found in waste triglycerides, which can cause
problems downstream. The resulting product from the cracking step can then
be esterified to convert fatty acids into alkyl esters (biodiesel).
For the purposes of the present invention, thermal cracking is
considered to loosely cover the process of breaking down large molecules into
smaller molecules at a predetermined temperature and pressure.
A flow diagram of the process steps and streams of one embodiment of
the present invention is shown in Fig. 1. A feedstock 12 of low quality or
waste triglycerides is fed to a thermal cracking unit 10. The feedstock 12 can
be any variety of waste triglyceride, including restaurant trap greases, waste
greases from animal rendering facilities and other forms of waste oils and
greases and low-quality vegetable oils. The feedstock stream 12 can be
heterogeneous in nature and can contain water and other contaminants. Waste
triglycerides used as the feedstock stream 12 can also have free fatty acid
(FFA) content as high as 50 to 100 wt.%. In an optional embodiment (not
shown), the triglyceride feedstock 12 may be filtered to remove any
macroscopic contaminant particles prior to thermal cracking.
In the thermal cracking unit 10, triglycerides in the feedstock stream 12
are significantly reduced since they are converted into free fatty acids, thus
forming a mixture of free fatty acids and conventional hydrocarbons, such as
paraffins, olefins and aromatics. Thermal cracking is preferably carried out
at
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mild cracking conditions which, for the purposes of the present invention, are
described as an operating temperature preferably in the range of from 390 to
460 C, more preferably from 410 to 430 C, and preferably at an operating
pressure of from 0 to 60 psig (6.9 to 515 kPa), more preferably from 30 to
40 psig (308 to 377 kPa). Thermal cracking produces various fractions
including gases 14, naphtha 16, middle distillate 22, and residue 18.
Contaminants from the feedstock 12 end up in the residue stream 18.
It was noted that the mild thermal cracking conditions used in the
present invention to crack waste triglycerides produces mainly diesel, having
a
boiling range of between 165 C and 345 C, rather than naphtha (IBP to
165 C), as was produced from thermal cracking of triglycerides at higher
temperatures and pressures.
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The middle distillate fraction 22 makes up more than half of the thermally
cracked product and has been found to have suitable characteristics for
further
treatment by esterification. The middle distillate fraction 22 comprises free
fatty
acids formed from thermal cracking of triglycerides, the original free fatty
acids
5 present in the feedstock and conventional hydrocarbons. Middle distillates
typically encompass a range of petroleum equivalent fractions from kerosene to
lubricating oil and include light fuel oils and diesel fuel. In one embodiment
of
the present invention the middle distillate fraction 22 was found to have a
boiling
point range of from 150 to 360 C, and more preferably from 165 to 345 T. The
middle distillate fraction 22 still has some fuel quality issues such as high
viscosity, high acid number, high cloud point and high concentrations of
nitrogen
and/or sulphur.
The middle distillate fraction 22 is next fed to an esterification unit 20,
where it is reacted with an alcohol stream 24 in the presence of a catalyst to
produce alkyl esters (biodiesel). The esterification process is carried out at
a
temperature preferably ranging from 70 to 120 C, more preferably in the range
of
from 90 to 110 C, and preferably at atmospheric pressure. The alcohol stream
24
can be any suitable alcohol known in the art, or mixtures thereof. The alcohol
stream 24 is preferably methanol.
It is surprisingly noted that esterification could be carried out well above
the boiling temperature of the reacting alcohol, despite low alcohol
concentration
in the liquid phase of the reaction mixture. The ability to conduct the
esterification
at higher temperatures is advantageous since this allows continuous water
stripping by the flashing alcohol stream. Since water is a co-product of acid
esterification, it can detrimentally quench the esterification reaction if not
removed
continuously.
The catalyst can be either an acidic solid or liquid catalyst. Preferably, the
acid catalyst is chosen from sulphuric acid (H2SO4(1)), sulphamic acid
(H2NSO3H(j)), formic acid (HCO2H(q), acetic acid (CH3CO2H(1)), propionic acid
(CH3CH2CO2H(p), hydrochloric acid (HCl(,)), phosphoric acid (H3PO4(l)),
sulphated metal oxides such as sulphated zirconia, and styrene divinylbenzene
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copolymers having SO3H functional groups, such as Amberlyst 36TM. Amberlyst 36
is most preferred for the esterification reaction, as this does not leave any
trace in
the esterification product, and further washing of the esterification product
is thus
not required.
Free fatty acids can be acid esterified by the following reaction, here shown
with the alcohol optionally being methanol:
H+
RCOOH + CH3OH RCOOCH3 +H20
The water byproduct can inhibit the reaction, and may prevent
esterification from going to completion. As mentioned above, esterification at
temperatures above the boiling temperature of the alcohol has been
surprisingly
found to alleviate this problem in the present invention.
Esterification produces a raw diesel stream 26 of approximately 50% alkyl
esters (biodiesel) and 50% hydrocarbons. These hydrocarbons can include
tetradecane, pentadecane, 1-hexadecene, hexadecane, heptadecane, 1-octadecene,
octadecane, nonadecane, 1-eicosene, eicosane, heneicosane, 1-docosene,
docosane,
tricosane, tetracosane, pentacosane, hexacosane, heptacosane, octacosane,
nonacosane, triacontane, untriacontane, dotriacontane, tritriacontane,
tetratriacontane, pentatriacontane, hexatriacontane, heptatriacontane, and
octatriacontane.
It should be noted that, in addition to esterifying only the middle
distillates
fraction 22 from thermal cracking, it is also possible to esterify both the
naphtha
stream 16 and middle distillates fraction 22 from the thermal cracking step.
This
optional method circumvents an extra step of separating naphtha 16 from the
middle distillates 22.
Depending on the type of catalyst used and the degree of esterification
achieved, the raw diesel stream 26 may exceed acidity limits allowed by ASTM
specifications for biodiesel, namely 0.8 mg KOH/g. To reduce acidity, the raw
diesel stream 26 can optionally be fed to a base treatment unit 30, together
with a
basic solution 28. The basic solution 28 reacts with any unreacted fatty acids
in
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the raw diesel stream 26 to produce non-foaming metallic soaps with low
solubility in biodiesel. These non-foaming metallic soaps can then be removed
by either centrifugation or filtering or a combination thereof. Base treatment
is
preferably carried out at temperatures of from 30 to 60 C, and more preferably
at
temperatures of from 40 to 50 C and preferably at atmospheric pressure. The
basic solution is preferably chosen from lithium hydroxide (LiOH), magnesium
hydroxide (Mg(OH)2), and calcium hydroxide (Ca(OH)2). Most preferred are
LiOH and Ca(OH)2-
The base treatment step results in a mixed biodiesel/diesel product 32
that has been found to have excellent fuel properties. The boiling point of
the
resultant biodiesel/diesel product 32 is found to be lighter and the boiling
point
distribution broader than that of biodiesel produced by conventional
transesterification alone. The mixed biodiesel/diesel product 32 can be used
both neat or can optionally be further blended with regular diesel.
The naphtha stream 16 from the thermal cracking unit 10 contains
oxygenates and can optionally be sold as a valuable by-product such as octane
improver. The residue stream 18 can be discarded by well known means in the
art.
The following examples serve to better illustrate the process of the
present invention, without limiting the scope thereof:
Example 1: Conversion of restaurant trap grease into mixed biodiesel/diesel
product
Restaurant trap grease having an average density of 0.925 g/mL was fed
to a thermal cracking unit where it was cracked at a temperature of 418.5 C
and
a pressure of 29 psig (301 kPa) for 40 minutes. Thermal cracking produced a
gas stream, a naphtha stream, a middle distillate stream with a maximum
boiling
point of approximately 343 C, as well as water and residue. The middle
distillates stream made up 63.0 wt.% of the total cracked product and had an
acid
number of 83.93 mg KOH/g.
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The middle distillate stream was then fed to an acid esterification unit,
where it was contacted with methanol in the presence of an Amberlyst 36
catalyst.
Esterification was conducted at a temperature of 90 C and at atmospheric
pressure
for 20 hours.
Esterification produced a raw diesel stream which was then treated with a
calcium hydroxide solution, Ca(OH)2(aq), to produce a final mixed
biodiesel/diesel
product having an acid number of 0.45 mg KOH/g. The final product was found to
have 0.22 wt.% nitrogen, 136 ppm sulphur and a viscosity of 5.02 cSt; the
sulphur
content and viscosity being well within ASTM 6751 standards for biodiesel
Example 2: Conversion of rendered animal fat into mixed biodiesel/diesel
product
Rendered animal fat, having an average density of 0.918 g/mL was fed to a
thermal cracking unit in which it was cracked at 411 C and atmospheric
pressure
for 40 minutes. The thermally cracked product contained 68.6 wt% middle
distillates having a maximum boiling point of 345 C, naphtha and the remainder
gas, water and residues.
The middle distillate stream, having a viscosity of 8.50 cSt, and an acid
number of 146.96 mg KOH/g, was then fed to an acid esterification unit, where
it
was contacted with methanol in the presence of an Amberlyst 36 catalyst.
Esterification was conducted at a temperature of 90 C and at atmospheric
pressure
for 20 hours.
The resultant raw diesel stream was then treated with a calcium hydroxide
solution, Ca(OH)2(,q), to produce a final mixed biodiesel/diesel product
having an
acid number of 0.75 mg KOH/g. The final product was found to have 18 ppm
sulphur and 158 ppm nitrogen, and a viscosity of 4.84 cSt.
This detailed description of the process and methods is used to illustrate
certain embodiments of the present invention. It will be apparent to those
skilled
in the art that various modifications can be made in the present process and
methods and that various alternative embodiments can be utilized.
