Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND SYSTEM FOR DETERMINING FREE
FATTY ACID CONTENT
BACKGROUN~ OF THE INVENTION
This invention relates to systems and
methods for determining the free fatty acid content
in oils, particularly edible oils.
Crude edible oils, such as soybean oil,
cottonseed oil, corn oil, fish oil and the like,
frequently contain undesirable amounts of free fatty
acids which affect their quality. Various techniques
are employed to remove free fatty acids and other
contaminants. In one technique commonly used, a base
is added to the oil to neutralize excessive amounts
of free fatty acids. The free fatty acid content for
many edible oils should be less than 0.05% and
preferably in the range of 0.02 to 0.03%.
A system capable of measuring the free
fatty acid content of oils during the refining
process is highly desirable for a closer process
control which can result in high yields, tighter
product specification and an increase in the
effectiveness of a subsequent bleaching step. A
system capable of continuously producing an output
signal representative of the free fatty acid content
on a continuous basis is particularly desirable
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because it can be conveniently incorporated into
completely automated process controls.
Conventional detectors employing an
electrode type sensor usually cannot be conveniently
used to determine the concentration of a component in
a non-aqueous medium without incorporating complex
auxiliary circultry or other equipment.
SUMMARY OF THE INVENTION
An object of the invention is to
provide a simple, colorimetric system and method for
continuously analyzing an oil for the free fatty acid
content.
Another object of the invention is to
provide such a system and method having the
capability of producing highly accurate measurements
under a relatively wide range of ambient temperature
conditions.
A further object of the invention is to
provide such a system and method which can be fully
automated.
A still further object of the invention
is to provide such a system which employs a
pressurized reservoir as the means for supplying a
continuous stream of the carrier solution and yet is
not subject to significant outgassing or gas bubble
formation.
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A yet further object of the invention
is to provide an automated system and method for
analyzing the free fatty acid content of edible oils
including a colorimetric detector and the capability
of introducing calibration solutions at programmable
intervals and generating an updated calibration curve
for the detector.
Other objects, aspects and advantages
of the invention will become apparent to those
skilled in the art upon reviewing the following
detailed description, the drawing and the appended
claims.
The invention provides a system and
method for analyzing an oil to determine free fatty
acid content. The system includes means for
supplying a continuous sample stream of the oil to be
analyzed, means for supplying a continuous stream of
a carrier solution containing a solvent for
dissolving the oil sample and a color indicating
reagent, a flow-through colorimetric detector, a
sample loop or conduit means, and a sample injection
valve which is movable between a load position
wherein the oil sample stream is routed to waste
through the sample loop and the carrier solution
stream lS routed through the detector and a measure
position where the sample stream is routed directly
to waste and the carrier solution stream is routed to
the detector via the sample loop so that an oil
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sample in the sample loop is mixed with the carrier
solution prior to passing through the detector. The
color indicating reagent in the carrier solution
reacts with the free fatty acid in the oil to produce
a color change, the intensity of which is measured by
the detector which produces a signal representative
of the free fatty acid content. The analyzing system
also includes means for heating the oil sample and
carrier solution streams and maintaining them at a
temperature above a level where the components of the
oil tends to separate and means for regulating liquid
flow through the detector.
In one embodiment, a computer operates
the sample injection valve between the load and
measure positions at programmable intervals.
In one embodiment, liquid flow through
the detector is regulated by a flow restrictor coil
connected to the detector outlet and this coil is
maintained at a predetermined, substantially constant
temperature.
In one embodiment, mi~ing of the oil in
carrier solution is enhanced by a diffusion coil
located between the sample injection valve and the
detector.
In one embodiment, separate supplies of
calibration solutions containing known concentrations
of a fatty acid are provided. The sample injection
valve and a selector valve manifold including valves
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for a controlling flow of the sample and calibration
solutions are operated at programmable intervals by
the computer which generates an updated calibration
curve for the detector.
In one embodiment, the carrier solution
is pressurized with a gas, such as air, and the
carrier solution is isolated from the pressurizing
gas by one or more layers of discrete, bubble-like
objects floating on the surface of the carrier
solution.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a schematic diagram of the
analyzing systeM of the invention shown with the
sample injection valve in the load position.
Fig~ 2 is a schematic diagram of the
sample injection valve in the measure position.
Fig. 3 is a diagrammatic representation
of an automatic control for the system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The analyzing system and method of the
invention can be used to measure the free fatty acid
content of a wide variety of hydrocarbon oils. The
system and method can be used for a wide range of
applications, such as for an on-line anaylsis of a
process stream or analysis of an oil used in
commercial fryers for free fatty acid content. The
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invention is particularly adaptable for analyzing
edible oils, such as soybean oil, cottonseed oil,
corn oil, fish oil, etc. and will be described in
connection with an on-line analysis of a process
stream for such an oil.
Referring to Fig. 1, the analyzing
system 12 includes a sample supply conduit 14
connected to a process line 16 carrying the edible
oil to be analyzed. Pressure in the process delivers
a continuous stream of the oil into the system
through a two-way, pneumatically-operated valve 17
connected to the supply conduit 14.
A carrier solution 18 is contained in a
reservoir 20. The carrier solution 18 includes an
organic solvent capable of dissolving substantially
all the components in the oil, particularly the free
fatty acids (e.g., oleic acid), and a color
indicating reagent capable of reacting with free
fatty acids to produce a measurable color change, the
intensity of which indicates the concentration of
free fatty acids.
The carrier solution 18 is pressurized
with a gas 22, such as air, supplied through a
conduit 24 from a suitable source 26, such as a small
compressor, and regulated to a desired pressure by a
regulator 28 in the conduit 24. The pressurized
carrier solution reservoir 20 delivers a continuous
stream of the carrier solution 18 through a carrier
solution supply conduit 30.
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~ he oil sample and carrier solution
streams are preheated to a temperature above the
level where components in the oil tend to settle out
or separate. This can be accomplished by passing
each through respective preheating coils 32 and 34
including a length o~ coiled tubing wrapped with a
thermostaticaly-controlled electric blanket (not
shown). This heating blanket maintains the
preheating coils 32 and 34 at a substantial constant
temperature (e.g., about 70C).
The oil supply conduit 14 and the
carrier solution supply conduit 30 are connected to a
sample injection valve, which in the illustrated the
preferred embodiment illustrated is a conventional,
pneumatically-operated, slider type valve 36 (e.g.,
CP Valve marketed by Bendix Corporation). Other
suitable type valves can be used, such as a rotary
valve.
When the slider valve 36 is in the load
position as illustrated in Fig. 1, the oil sample
stream enters the valve 36 through port 2, exits the
valve 36 through port 3, passes through an external
sample conduit or loop 38, reenters the valve 36
through port 8, exits again through port 7 and passes
through a sample waste conduit 40 into a sample waste
receptacle 42. At the same time, the carrier
solution stream enters the valve 36 through port 4,
exits the valve 36 through port 9, passes through a
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carrier solution conduit ~4 to a diffusion coil 46
including a length of coiled tubing wrapped with a
thermostatically-controlled, electric heating blanket
(not shown~. This heating blanket maintains the
diffusion coil 46 at a substantially constant
temperature (e.g., about 70C).
The carrier solution conduit 44 is
connected to the inlet 48 of a conventional, dual
beam, flow-through type colorimetric detector 50
capable of producing an electrical signal
representative of the free fatty acid content of a
liquid flowing therethrough in response to the color
intensity of that liquid (e.g., FIA-LITE 600 marketed
by FIAtron Systems, Inc.). The carrier solution
stream passing through the detector 50 is routed to a
carrier waste receptacle 52 by a carrier waste
conduit 54 connected to the outlet 56 of the detector
S0. The flow rate of this liquid is controlled by a
flow restrictor coil 58 including a length of coiled
tubing wrapped with a thermostatically-controlled
electric heating blanket (not shown). This heating
blanket maintains the flow restrictor coil at a
substantially constant temperature ~e.g., about 70C).
When the slider valve 36 is moved to a
measure position illustrated in Fig. 2, the oil
sample stream enters the valve 36 through port 2,
exits the valve 36 through port 7 and passes directl~
to the sample waste receptacle 42. At the same time,
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the carrier solution stream enters the valve 36
through port 4, exits the valve through port 3,
passes through the sample loop 38, reenters the valve
36 through port 8 and exits again through port 9.
Thus, a slug of the oil having a volume corresponding
to the internal volume of the sample loop (e.g., 55
microliters) is introduced into the carrier solution
conduit 44. As this slug of oil and the carrier
solution stream passes through the carrier solution
conduit 44, they are mixed together and the color
indicating reagent reacts with the free fatty acids
in the oil and produces a color change which can be
measured by the detector 50.
Preheating the oil sample and carrier
solution streams to a temperature above a level where
the oil components tend to separate by the preheating
coils 32 and 34 and maintaining the diffusion coil 46
at about the same temperature provides a mixture of
the oil and carrier solution for passage throuyh the
detector 50. This desired mixing is enhanced by the
diffusion coil 46 which imparts radial mixing.
As mentioned above, the solvent used in
the carrier solution should be miscible with
substantially all the components of the oil,
particularly the free fatty acids which usually are
long chain fatty acids such as oleic acid. This
ensures a more complete reaction between the fatty
acids and the color indicating reagent and,
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therefore, a more accurate measurement by the
detector. The solvent should be substantially
colorless, should not absorb light in the wave length
of the free fatty acids being analyzed, and should
have either weak acid or weak base properties so that
it does not act as an acid or base with respect to
the color indicating reagent or the free fatty acids
and interfer with the indicator reaction. In this
regard, the solvent should have a "pka" close to that
for the color indicating reagent and the free fatty
acids. It also should have a relatively high boiling
point so it does not volatize when heated during
passage through the various heating coils and a
relatively low freezing point so that it does not
solidify at lower ambient temperatures in colder
climates. For example, t-butanol generally is
undesirable because it tends to solidify at
relatively high ambient temperatures.
At present, n-butanol is the preferred
solvent. Other alcohols, polyols and chlorinated
hydrocarbons containing 3 to 8 carbon atoms can be
used.
When ambient air is used to pressurize
the carrier solution, moisture in the air is
introduced into the system. For many carrier
solutions, the addition of water up to a certain
point affects the pH at which the color indicating
agent changes color with a resulting effect on the
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accuracy of the measurement made by a colorimetric
detector. This potential problem is minimized by
adding sufficient water to the carrier solution to
raise the water content to a level where additional
water does not affect the pH. This level can be
determined empirically with a calibrated
colorimeter. For methyl red/n-butanol solutions,
this level is approximately 5 milliliters of water
per liter OL solution.
The pH of commercially available
organic solvents tends to vary from batch to batch
and, consequently, the point at which the color
indicating reagent changes color also can vary. To
obviate possible batch-to-batch variations in the
solvent pH, the solution is "neutralized" to a
predetermined pH prior to adding the color indicating
reagent. This can be accomplished by taking an
aliquot of solvent (95% butanol/5% water, adding an
equal aliquot of neutralized phenolpthalein/ethanol
indicator solution and manually titrating with either
0.lN NaOH or 0.lNHCl to permanent faint pink color of
phenolpthalein at 65C. Based upon this
determination, the pH of the solvent batch can then
be adjusted upward or downward by adding a calculated
amount of base or acid~ After the solvent has been
thus "neutralized", a sufficient amount of the color
indicating reagent is added. The solvent is then
additionally pH-adjusted to the optimum absorbance
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unit full scale (AUFS). As a guide, 0.4-0.5 AUFS
(measured at 520 nm) is optimum for a methyl
red/n-butanol/water carrier solution.
As used herein in connection with the
solvent, the ter;ms "pH" and "pka" mean a
representation of the acidity or alkalinity of a
reagent in an organic solvent and are not identical
to the pH or pka of that reagent in an aquous system.
As mentioned above, the color
indicating reagent must be capable of reacting with
the free fatty acids in the oil and producing a
measurable color change, the intensity of which
indicates a concentration of free fatty acids. The
chemical reaction between the free fatty acids and
the color indicating reagent can be represented as
follows:
RCOOH + INDICATOR RCOO + H-INDICATOR
(fatty acid) (base form) (acid form)
Thus, the color indicating reagent acts
as a base which reac~s with fatty acids as well as a
color indicator. The color indicating reagent should
have a "pKa" in thë range of but slightly higher than
that for the free fatty acids so there is a
stoichiometric reaction between the free fatty acids-
and the color indicating reagent. For that reason,
the carrier solution must contain at least a
stoichiometric amount of the color indicating reagent.
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At present, methyl red is the preferred
color indicating reagent. Other suitable color
indicating reagents include neutral red and azolitmin.
The presence of gas bubbles in the
system can adversely affect the accuracy of the
measurements made by the detector. The carrier
solution reservoir usually is pressurized to about 25
psig and, if fully exposed to the pressurizing gas,
the carrier solution would absorb a considerable
amount of gas within a few hours. To minimize
potential outgassing by gas absorbed into the carrier
solution, the carrier solution reservoir is provided
with means for substantially isolating the
pressurizing gas frorn the carrier solution or, stated
another way, for minimizing the effective surface
area of the carrier solution exposed to the
pressurizing gas.
In the preferred embodiment
illustrated, discrete, bubble-like objects or hollow
spheres 60 are floated on the surface of the carrier
solution for this purpose. The spheres 60 are made
from a material which is substantially inert with
respect to the carrier solution. Two or more layers
of the spheres 60 provide better isolation and,
therefore, are preferred. As a guide, it has been
found that three layers of 3/4-inch, hollow
polypropylene spheres about 1-1/2 to 2 inches deep
provides satisfactory protection against significant
absorption of air by the carrier solution.
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A quick-disconnect type filler
connection 62 can be used on the carrier solution
reservoir 20 so that carrier solution can be added to
the reservoir without introducing significant amounts
of air.
To further minimize outgassing, the
system is designed so that there is a minimum
pressure drop between the carrier solution reservoir
20 and the detector 50 under operating flow
conditions. As a guide, the flow rate of the carrier
solution stream usually is on the order of 1.5 to 2.0
ml/min, the inside diameter of the preheater coils 32
and 34, the sample loop 38 and the diffusion coil 46
is 0.030 inch, and the cross sectional areas of the
flow passages of the slider valve 36 and connections
between components in the system between the carrier
sclution reservoir 20 and the detector 50 are at ;
least as large.
By minimizing the pressure drop in the
system between the carrier solution reservoir and the
detector, the flow rate of the carrier solution
stream is governed by the pressure drop through the
flow restrictor coil 58. The use of relatively long,
coiled tubing (e.g., 3 meters) to provide the
pressure drop required to obtain the desired flow
rate permits the use of a cross sectional flow area
te.g., 0.02 inch inside diameter) which is large
enough to prevent plugging by particulate matter in
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the oil. A single plate orifice small enough to
provide the required pressure drop is subject to
becoming plugged by such particulate matter.
Maintaining the restrictor coil at a substantially
constant temperature minimizes ~ariations in flow due
to changes in ambient temperature.
The calibration of the detector 50
tends to change because of changes in ambient
temperature, normal instrumentation drift, etc. The
system includes means for introducing a plurality of
calibration solutions te.g., up to 3) at programmable
intervals and generating an updated calibration curve
for the detector 50. The illustrated preferred
embodiment includes a selector valve manifold 70
including the sample valve 17 and three separate
calibration solution reservoirs 72, 74 and 76
pressurized to about 5 psig and connected to the
sample conduit 14 via respective calibration conduits
78, 80 and 82. Each calibration conduit has a
two-way, pneumatically-operated valve 84 which is
selectively opened to introduce the corresponding
calibration solution into the sample supply conduit
14.
Each calibration solution contains a
known concentration of a free fatty acid, such as
oleic acid, dissolved in an organic solvent,
preferably the solvent as the one for the carrier
solution. The organic solvent is "neutralized" as
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described above before adding the free fatty acid.
As a guide, the free fatty acid concentration for the
three different calibration solutions can be 0.015,
0.030 and 0.05 weight %.
When the system is in the calibrating
mode, the sample valve 17 is closed and one valve 84
is opened with the slider valve 36 in the load
position. After the sample loop 38 is filled with a
first calibration solution, the slider valve 36 is
moved to the measure position and a slug of the
calibration solution is mixed with the carrier
solution and passes through the detector 50 as
described above. The selector valve 36 is returned
to the load position, another valve 84 is opened to
fill the sample loop 38 with a second calibration
solution and the above cycle repeated for the second
and third calibration solutions.
After the last calibration has been
completed, the sample valve 17 is reopened. The
sample waste conduit 40 preferably has no flow
restrictions so that the sample supply conduit 14,
the preheating coil 32 and the selector valve 36 can
be rapidly flushed out before, between and after
calibrations.
Fig. 3 is a diagrammatic representation
of an automatic control for the system. Operation of
the slider valve 36, the sample valve 17 and the
calibration solution valves 84 is controlled by the
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central processing unit (CPU) 90 of a microprocessor
(e.g., a Z80 microprocessor marketed by Zylog) which
transmits signals to a valve driver board 92
connected to those valves, The valve driver board
92, through on/off switches, controls the pilot
valves which pneumatically operate the slider valve
36, the sample valve 17 and the calibration valves 84.
The CPU 90 includes a real time
computer program which initiates electrical signals
for moving the slider valve 36 between the load and
measure positions and for opening and closing the
sample valve 17 and the calibrations solution valves
84. The time intervals for sampling and calibration
are programmable and can be changed by inputting the
appropriate data with a key pad 94 connected to the
CPU 90. The output signal of the detector 50 is sent
to a signal processing and analog/digital converter
(A/D) board 96. The A/D board 96 converts the analog
signals from the detector 50 to digitized signals
which are transmitted to the CPU 90.
The CPU 90 contains a computer program
which reads the signals from the signal processing
and A/D board 96 and initiates electrical signals
(represented by arrows 98~ which can be used to
trigger an alarm, drive a recorder and/or serve as an
input to an automated process control system
including a set point controller which operates a
control valve. A vacuum fluorescent display 100
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provides a visual readout of the current operating
conditions or program parameters. The CPU 90
contains a computer program which reads the signals
from the A/D board 96 during the calibration mode and
stores an updated calibration curve for the detector
50. This calibration curve becomes the standard to
which signals from the detector 50 during the measure
mode are compared to produce the output signals 98.
From the foregoing description, one
skilled in the art can easily ascertain the essential
characteristics of the invention, and without
departing from the spirit and scope thereof, make
various changes and modifications to adapt it to
various usages.
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