Measuring SARA Fractions
Investigation Part I: Separation of the Maltenes and Asphaltenes
As a modern society, we depend on the combustion and use of fossil fuels.
We are also well aware that the reserves of oil are limited, and will eventually run
out. Alberta is home to the world’s largest known petroleum resource – 138 000
square kilometres of oil sands deposits situated in northern Alberta.1
Over millions of years, micro-organisms (bugs) have been eating away at the
hydrogen rich oil. What remains is bitumen, a thick tarry mixture of petroleum
hydrocarbons coating the sands. The composition of bitumen can vary, depending on
the amount of biodegradation that has occurred.
Much of the geochemical analysis of crude oil over the past decade has been
focused on hydrocarbons. As a result, little is known about the other molecules
that represent over half of the composition of bitumen. These include compounds
that contain oxygen, nitrogen and sulphur, as well as traces of nickel, iron and
vanadium.1
In order to minimize the costs of in situ technology, involving underground
refining and extraction of bitumen from the oil sands, and to access the impact on
the environment, a thorough knowledge of the molecular structure and behaviour
of the bitumen is needed.
SARA analysis is a method that uses solubility to characterize the
components of heavy oils by separating them into smaller fractions: Saturated
hydrocarbons (mostly made up of non-polar straight, branched chain, and cyclic
paraffins); Aromatics (including fused benzene rings compounds); Resins (polar
aromatic rings systems containing nitrogen, oxygen, or sulphur); Asphaltenes
(highly polar, complex aromatic ring compounds with varying composition, containing
nitrogen, oxygen, and or, sulphur, and having molecular weights averaging 700 amu).
Initially, the oil samples can be separated into two parts: the maltenes
(saturated hydrocarbons, aromatics and resins), and the asphaltenes. The
asphaltenes are not soluble in straight chain solvents (pentane, hexane and
heptane). The hydrocarbon solvent will disrupt the stability of the asphaltene
dispersion and cause it to precipitate.
Purpose
The purpose of this investigation is to separate light, medium and heavy oil samples
into their maltene and asphaltene fractions, and compare them.
Problem
What are the percentage compositions of asphaltenes in light, medium and heavy
oils?
Design
Precisely measured samples of light, medium and heavy oils are dissolved in hexane
and left over night. The solid precipitates are separated by filtration and dried.
The mass of asphaltene in each sample is measured to determine the percent
composition. The filtrates, containing the maltenes, are left in the fume hood to
allow the hexane solvent to evaporate. The maltenes will be dried in anhydrous
sodium sulphate, and analyzed in Part II of this investigation.
Materials
-
0.5 g of light, medium or heavy oil
two 25 mL glass sample vials with caps
1.5 mL and 2 mL glass Pasteur pipettes
30 mL of hexane
aluminum foil
vial labels
# 1 filter paper
funnel
ring stand
ring
glass stirring rod
- fume hood
- small wisp of cotton ball or
glass wool
- anhydrous sodium sulphate
- clamp to secure pipette
- small waste beaker
- 10 mL graduated cylinder
- pipette bulb
- 7 mL of dichloromethane
(DCM)
- 8 mL glass sample vial with cap
Safety Precautions
Wear safety goggles at all times. Hexane and dichloromethane are volatile, so only
use in a well-ventilated area. Wear gloves at all times when handling the oil samples
and organic solvents.
Procedure
Day 1:
1. Observe the samples of light, medium and heavy oil and note their physical
properties.
2. As directed by your teacher, select one sample of oil. You will share your
results with two other groups so that you have data for all three oil samples.
3. Tare a 25 mL sample vial (without the cap) on an analytical balance.
4. Carefully dip the tip of a Pasteur pipette into the oil sample and remove
without touching the sides.
5. Transfer the oil on the Pasteur pipette tip into the tared vial by touching
the tip to the bottom and smearing it against the wall, as necessary.
6. Fill the vial with hexane. Cover the top with aluminum foil and screw on the
cap. Securing the cap and the top of the vial in your hand, shake the vial
vigorously to dissolve the sample, and remove it from the sides of the vial.
7. Label the sample and leave overnight in the fume hood.
Day 2:
1. Measure and record the mass of a piece of #1 filter paper.
2. Record the mass of a new clean, dry, 25 mL vial, without the cap. This will be
used to collect the maltene fraction.
3. Shake the sample vial, containing the oil in hexane. Using filtration
techniques practiced in Chemistry 20, pour the contents of the vial down a
glass stirring rod into the filter paper. Collect the filtrate in the preweighed maltene sample vial.
4. Rinse the vial and the stirring rod with a small volume of hexane to
quantitatively transfer the sample into the filter paper. Be careful not to
overfill the maltene filtrate vial.
5. Place both the filter paper and the open sample vial, containing the maltene
fraction, in the fume hood to allow the hexane to evaporate over night.
Day 3:
1. Measure the mass of the filter paper containing the asphaltene sample.
2. Transfer a portion of the asphaltene sample to a labelled vial. Cover the top
with aluminum foil and secure the cap.
3. Dispose of the soiled filter paper as instructed by your teacher into an
organic waste container. Do not throw into the garbage.
4. Measure the mass of the maltene sample + the vial
5. Exchange measurements with two other groups so that each group has data
for all three oils.
In order to perform Part II of the investigation, excess water must be removed
from the oil sample, as it will interfere with the chromatography design. This is
accomplished by setting up a Pasteur pipette column.
6. Using the tip of a longer Pasteur pipette,
carefully push a small wisp from a cotton
ball, or a small wisp of glass wool, down the
neck of a 1.5 mL Pasteur pipette and plug
the bottom. Fill the pipette approximately
half way with anhydrous sodium sulphate by
dipping the top of the pipette into the
anhydrous sodium sulphate, and scooping it
in. Tap the column to settle the sodium
sulphate and remove any bubbles.
7. Secure the pipette into a clamp on a ring
stand and place a small waste beaker below
it.
8. Measure about 2 mL of dichloromethane
(DCM) into a clean and dry 10 mL graduated
cylinder, and use a Pasteur pipette to
carefully add DCM to the top of the column. Allow all of the DCM to run
through the column, and dispose of the organic waste into the waste
container in the fume hood. DO NOT POUR ANY ORGANIC WASTES
DOWN THE SINK!
9. Place a clean, dry 8 mL sample vial underneath the column. (It is not
necessary to measure the mass of the vial.)
10. Measure about 5 mL of DCM into the 10 mL graduated cylinder. Using your
Pasteur pipette, dissolve the oil sample in a minimum volume (less than 1 mL)
of DCM. Force the air from the pipette into the oil sample to help it mix.
Carefully, and slowly, add the mixture to the top of the column. Allow the
whole sample to sink into the column.
11. Again, use your pipette to rinse the 25 mL vial with a small volume of DCM
and add to the top of the column. Continue to rinse the vial with the DCM
until the whole sample has been transferred and passed through the column
into the 8 mL vial. Stop when only the colorless DCM is dripping out into the
vial.
12. Place the vial in the fume hood and allow the DCM to evaporate.
13. Place the columns in the fume hood to dry out. Once dry, the used sodium
sulphate can be disposed of in the garbage, and the glass pipettes can be
disposed of in the glass waste/recycling.
Cautionary note: The DCM evaporates quite quickly (within an hour or two) and
samples should not be left open overnight. The samples should be monitored by the
teacher. Once the DCM has evaporated, the vials need to be covered with aluminum
foil and capped. As the last of the DCM evaporates, some of the saturated
hydrocarbon fraction could also easily evaporate if the vial is not capped.
The maltene sample is now ready for Part II of the Investigation.
Complete the Evidence, Analysis and Evaluation for this lab.
Credits
1. Natalie St-Denis. Alberta Ingenuity Centre for In Situ Energy Pamphlet:
Unlocking Alberta’s Oil Sands Potential. Sundog Printing. 2007.