Adlard E.R. (ed.) Chromatography in the Petroleum Industry
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282
Chapter
10
of
liquids as the supercritical fluid can tend to pump the liquid through the sys-
tem rather than extract it. This
is
further complicated by the fact that in many
cases, two solutions are present during the extraction process; one is a solution
of
extract in the fluid and the other a solution of the fluid in the matrix. The sec-
ond effect can cause a significant increase
in
volume
of
the sample, particularly
on depressurization as dissolved gas
is
liberated, and if the cell is too full, the
contents can be carried through to the extract collection system. This is unfortu-
nate for the extraction of oil samples but several methods have been devised for
the extraction of such samples.
These include:
-
modified extraction cells
-
immobilization on a solid sorbent
-
inserts
for
conventional cells
Taylor
et
al.
have modified conventional extraction cells and described their
use
for
the extraction of liquids including the extraction of phenols
[26]
and
phosphonates
[27]
from water. The use of solid adsorbents has been employed
by many workers for the extraction of liquid samples. Here the liquid sample
is
placed on a solid substrate to immobilize it. This method has the advantage that
it
can be used to impart a greater degree of selectivity to the extraction process
as the sorbent can be chosen such that it selectively interacts with components in
the sample.
A
typical example
is
the analysis
of
PCB’s
in electrical oils
[28]
where the oil
is
adsorbed onto Florisil for the extraction process.
A
method
which we have successfully employed in our laboratories for the extraction of
END
SECTION
LO
S
G
IT
L
D
I
XA
L
S
E
C
T
I0
N
Fig.
10.5. Schematic
of
the boat in cell method
for
extracting liquids. (Reproduced courtesy
of
British Petroleum.)
Supercritical jluid extraction
283
oils employs glass boats which are inserted into a conventional extraction cell.
A
schematic representation
of
the system is shown in Fig.
10.5.
The boats are con-
structed to sit in the cell and the cell is maintained in a horizontal position
throughout the extraction. This type of arrangement has the disadvantage that
only the surface of the oil sample is exposed to the extraction fluid and as a re-
sult extraction times are generally longer. However, the method does have the
advantage when the residue after extraction is the component of interest as it is
contained in the sample boat and can be readily removed for further analysis.
This also minimizes contamination of the extraction vessel and can significantly
reduce the cleaning time required after extraction.
10.4.5
Reactive extraction
This is a relatively new concept where a reagent is added to the sample prior
to extraction. The reagent reacts with the analyte under supercritical conditions
to
form a species which is readily extracted by the supercritical fluid. The rea-
gent can also aid in releasing the analyte from the matrix by interacting with ac-
tive sites on the surface. This type of approach began with the application
of
common derivatization reactions used in
GC
and
HPLC
to improve the solubility
of
polar solutes in non-polar carbon dioxide. In most methods, the reagents are
added directly to the sample in the extraction cell and the derivatization
is
car-
ried out under supercritical conditions. This allows derivatization and extraction
to be performed in a single operation. Hawthorne
et
al.
[29-3 11
reported proce-
dures which produced and extracted methyl esters of components of interest.
Hills
et
al.
[32,33]
performed simultaneous silylation and extraction on a sample
of coffee which had already been exhaustively extracted with supercritical car-
bon dioxide and reported the extraction
of
many new components both silylated
and unsilylated from the matrix. This area of
SFE
will undoubtedly develop fur-
ther and one could envisage the development of selective extraction agents for
particular analytes and matrices which prove intractable by other techniques.
One area where this would be particularly applicable would be the development
of complexing agents for metals which would give SF-soluble metal complexes
which could then be extracted and determined.
10.4.6
Off-line extraction
Off-line extraction
is
the term given to methods where the extracted analytes
are collected and isolated in some way not directly coupled to the subsequent
analytical techniques which are to be employed. For example, they can be col-
References
pp.
301-303
284
Chapter
I0
lected
in
a
solvent or on a solid sorbent. This is the simplest way to perform SFE
and requires much
less
skill on the part of the analyst. It
is
therefore the most
common mode of SFE and should be quite adequate for most routine procedures.
In many cases, the most crucial decision in off-line SFE is the choice of the col-
lection methods described earlier.
10.4.7
On-line extraction
On-line extraction is the term given to methods where the extraction process
is coupled directly to the analytical technique which is to be used for the further
analysis of the extract. This
is
also sometimes called
a
“hyphenated technique”
and common examples include SFE-GC, SFE-SFC, SFE-HPLC and SFE-IR.
These methods are generally more difficult to perform than off-line extractions
as
it
is
vital to get the interface between the two techniques right
in
order to ob-
tain meaningful results. However, they do have several advantages including the
total transfer of the extract which results in maximum sensitivity. Sample han-
dling and exposure to the atmosphere can be eliminated between extraction and
analysis and this reduces the chances
of
analyte contamination and degradation.
Furthermore, when employed with combined chromatographic-spectroscopic
techniques such as GC-MS, complex multi-component data can be obtained from
a
single experiment.
10.5
PETROLEUM-BASED APPLICATIONS
The prime subject of this book
is
chromatography and therefore we shall con-
centrate on applications where SFE is employed as a pre-separation technique
for the various forms
of
chromatography and divide them into the categories of
off-line and on-line applications. The particular applications have been chosen to
cover exploration and production of the crude
oil,
through processing in the re-
finery, to its final use in automotive engines.
10.5.1 Off-line applications
IO.
5.
I.
I
Residual
oil
on drill cuttings
‘The use of oil-based drilling mud offers significant financial advantages over
water-based mud but there are disadvantages in that the drill cuttings produced
become contaminated with the oil from the mud. The levels of oil can be as high
as 10-1
5
weight
%
and as they can be produced at several tons per hour, the dis-
SupercriticalJuid extraction
285
PUMP
posal of these cuttings (for example dumped in the sea in an offshore operation)
could have
a
significant environmental impact. As
a
result, impending legislation
has fostered several initiatives to try to reduce the oil level of cuttings prior to
disposal. These have included roasting, washing with solvents and “super-
wetters”, and even by extraction with supercritical fluids
[34].
In order to assess
the efficiency
of
a method of cleaning oil from cuttings, it is necessary to be able
to measure the residual oil. The conventional method employs a “retort method”
where a weighed sample
of
cuttings are heated in
a
vessel and the desorbed oil
and water condensed and collected in a measuring cylinder. The resulting vol-
umes can be read and approximate levels determined. This provides acceptable
accuracy and precision for levels of
10-15%
but not at the proposed levels of
1%.
This
type
of application
is
ideally suited to off-line
SFE
with carbon dioxide
as the oils used (which are in the diesel oil range) are extremely soluble.
During trials
of
a clean up system, our laboratory was required to determine
the levels of oil in both the feed and cleaned cuttings at levels down to
0.1%.
A
method was developed where the cuttings were extracted with
SF
carbon dioxide
and the extracted oil quantified by gas chromatography. The schematic represen-
tation of the process
is
shown in Fig.
10.6.
A
weighed sample
of
cuttings is
placed into the extraction vessel and the oil extracted with carbon dioxide using
a dynamic extraction at
350
bar and
80°C
at a flow rate of
2
ml/min. On depres-
surization via a back pressure regulator, the expanded gas and extracted oil is
passed through a silica Sep-pak cartridge which collects the oil. The Joule
Thomson cooling of the expanding gas acts as a cryogenic coolant for the Sep-
pak cartridge and helps ensure efficient trapping. After the extraction is com-
r
SAMPLE
CELL
PRESSURE
REGULATOR
OVEN
U
CARBON
DIOXIDE FEED
Fig.
10.6.
Schematic manifold for the extraction of drill cuttings
to
determine residual oil.
(Reproduced courtesy of British Petroleum.)
References pp.
301-303
286
Chapter
10
TABLE
10.3
COMPARISON
OF
RESULTS OBTAINED
BY
SFE
AND SOXHLET EXTRACTION FOR THE
DETERMINATION OF RESIDUAL OIL
ON
DRILL CUTTINGS
(ALL
RESULTS
ARE
%
WEIGHTNEIGHT)
Sample
SFE
Soxhlet
A
2.06
B
I
.89
C
0.56
D
0.58
1.81
1.78
0.62
0.58
plete (approx.
30
min)
the Sep-pak is removed and the oil eluted from it using a
few millilitres of carbon disulphide.
A
known
weight of internal standard
is
then
added and the sample analysed by gas chromatography to quantify the oil. The
method was compared with an 8-h Soxhlet extraction on a number
of
samples
and the results are shown in Table
10.3.
These results show that the
SFE
method was comparable to the Soxhlet
method but it did not give any indication on the efficiency of extraction. This
was evaluated by submitting small samples (approx.
2
mg) of powdered drill
chips before and after extraction to thermal desorption gas chromatography
where they were rapidly heated such that all oil was desorbed into the
GC.
The
results obtained indicated that less than
2%
of the oil originally present on the
cuttings was left after the
SF
extraction, i.e. greater than
98%
extraction effi-
ciency was achieved. This method adequately shows the advantages of
SFE.
A
result could be obtained
in
less than an hour, nearly an order of magnitude
quicker than Soxhlet extraction. This can be important if the results are being
used to optimize or control a real process as performance variations can be de-
tected earlier and the necessary action taken. Each
SFE
sample required a few
millilitres of carbon disulphide, whereas each Soxhlet extraction produced a few
hundred millilitres of solvent which had to be stored and then disposed of. In
addition, the accuracy and precision of the method was more than adequate for
the customer requirements.
10.5.
I.
2
Drilling
mud
characterization
SFE
with carbon dioxide has been applied to the characterization of hydrocar-
bons
in
drilling mud
[35].
The method involves placing
0.5
g
of sample in an
extraction cell with “Hydromatrix”. The samples were
6045%
water and the
Hydromatrix is a drying agent which absorbs the water and prevents it interfer-
ing
with the extraction. The sample is extracted with carbon dioxide at
400
atm
and
100°C
with a compressed fluid flow rate of
2
ml/min. The extract
is
col-
lected in dichloromethane at
-3OOC
and then analysed by gas chromatography. A
Supercritical
fluid
extraction
287
c14
\
J
I
I
I
I
inject
\O
20
30
Retention Time (minutes)
k
Fig.
10.7.
Chromatogram illustrating a high resolution fingerprint
of
hydrocarbons obtained from
the extraction
of
a drilling mud. (Courtesy Suprex Corporation.)
chromatogram showing a typical
GC
fingerprint obtained from a mud is shown
in Fig.
10.7.
The
SFE
method took 30 min to carry out and replaced a Soxhlet
extraction with methylene chloride which typically took
6-8
h.
10.5.1.3
Petroleum core and rock extractions
Both off-line and on-line extraction techniques have been applied to the
analysis of petroleum-rich core and rock samples. Off-line methods have the ad-
vantage that they allow the use of modified solvents which cannot be used in
most on-line methods. For example a
50/50
mix of dichloromethane and metha-
nol has been employed as a modifier
(1
5%)
in carbon dioxide for the extraction
of
oil from cores
[36].
Methanol
(15%)
in carbon dioxide was also employed for the extraction of a
bitumen-rich core sample. Both extractions were performed at a pressure of
TABLE
10.4
COMPARISON
OF
SFE
AND
DEAN AND
STARK
FOR
THE EXTRACTION
OF
PETROLEUM CORE SAMPLES
The RSDs are determined upon triplicate extractions. (Data reproduced courtesy of Dionex
Corporation)
Sample Dean and Stark
wt%
SFE
wt%
SFE
%
RSD
Ground core
8-9 8.2 2.0
(700
feet deep)
Ground core
10-1
1
11.8 1.7
References
pp. 301-303
288
Chapter
I0
600
atm and a temperature of
100°C.
The collection solvent was toluene and the
extraction time was
60
min. The results obtained are given in Table
10.4.
Again
superior results were obtained in a shorter time than conventional methods.
10.5.
I.
4
Refinery
catalvsts, deposits
and
sludges
Refineries can produce a number of samples which are amenable to analysis
5y SFE including catalysts, deposits and sludges. The examination of micro-
porous catalysts can be important both in the
R&D
and operational environment
and SFE can be employed to extract compounds which are causing fouling
or
poisoning
of
the pore and active site of the catalyst. Subramaniam
et
al.
[37,38]
employed both reactions and extractions under supercritical conditions to study
the mechanisms of coke formation on a commercial isomerization catalyst. The
formation of deposits in pipework and vessels
is
also
a
problem in refinery
op-
erations and SFE can be employed in the characterization of such deposits both
in
terms
of
the extractable components and also as a means of removing hydro-
carbons to allow examination of the solid by other techniques such
as
thermal
analysis and spectroscopic methods where fluorescence
from
the hydrocarbons
interferes with the analysis. Refinery sludges are monitored for components such
as polycyclic aromatic hydrocarbons to meet environmental concerns and
Schmidt
et
al.
1391
have employed
SFE
with carbon dioxide to replace a tedious
solvent extraction procedure for this process. The extraction is performed on
-400
mg of sample in a 1.5-ml vessel using a pressure
of
350
bar, a temperature
of
40°C,
and a flow rate of
3.5
ml/min.
A
static extraction of
1
min
is
followed
by
a
dynamic extraction for
4.2
min. The extract
is
collected on depressurization
in a trap packed with
30pm
ODS
column packing which is maintained at
10°C
during the extraction. The extract was recovered from the trap by washing with
hexane
(2
ml) and then analysed by
GC-MS
with single ion monitoring to detect
the compounds of interest.
10.5.1.5
Automotive
engine
particidates
Automotive engine particulates include exhaust particulates, particulates
which have accumulated on filters
in
the engine, and particulates which have
agglomerated and formed deposits on engine components such as valves. They
are often only available
in
milligram quantities and are therefore difficult to
handle
in
solvent extraction procedures. However, they are well suited
to
SF
methods. The analysis of these particulates
is
important for several reasons in-
cluding the development of fuels and lubricants, and for environmental monitor-
ing.
Bartle
et
al.
[40]
have studied the use of SFE for the extraction of polycyclic
aromatic hydrocarbons from diesel exhaust particulates. It has also been applied
to
the
sequential extraction of fuel and lubricant components from engine filters
Supercritical
fluid
extraction
289
[41]
in an attempt to relate the results to the cause of engine failure. The extracts
were then characterized by GC. In this case one extraction yielded a
134%
re-
covery in comparison to Soxhlet extraction. The extraction with carbon dioxide
was carried out at
329
bar and 60°C (a density of
0.85
g/ml). Static extraction for
10
min was followed by dynamic extraction for 11 min at a flow rate of
3
ml/min. The extract was collected on a solid trap containing
ODS
and main-
tained at
20°C.
The extract was then recovered for GC analysis by rinsing the
trap with two 1.2-ml volumes of carbon disulphide.
The examination of engine deposits is another area where
SFE
can be em-
ployed particularly for the comparison of fuels and lubricants and their perform-
ance in minimising deposit formation. The yields obtained from the extraction of
deposits from
two
identical engines but using different lubricants are shown in
Table 10.5. The deposits were sampled from the piston crown, the cylinder head
and the inlet valve tulips. Sequential extractions were carried out on each sam-
ple. The first was with carbon dioxide only and the second was with carbon di-
oxide containing
10%
methanol as solvent modifier. The deposits were exam-
ined before and after extraction by elemental analysis and thermo-gravimetry to
give values for fixed and volatile carbon, hydrogen, oxygen, nitrogen and ash.
Solid state
I3C
NMR was also employed to give speciation information on the
carbon. The extracts were examined by GC-MS for the volatile components and
thin-layer chromatography (TLC) for the non-volatiles. GC-MS of the carbon
dioxide extract gave a typical lubricating base oil profiles with some substituted
and polycyclic aromatics. The methanol modified extracts contained mainly high
molecular weight aromatics up to six rings and the piston and head deposits
contained oxidized aromatics. TLC of the carbon dioxide extract showed mainly
non-polar material typical of a degraded oil and the valve deposit extract con-
tained antioxidant. The methanol-modified extract covered a wide polarity range
including mineral oil degradation products and additives. All the modified ex-
tracts contained sulphonates and other highly polar components.
TABLE
10.5
EXTRACTION YIELDS IN WEIGHT
Yo
OBTAINED BY THE SF EXTRACTION OF ENGINE
DEPOSITS FROM PISTON CROWNS, CYLINDER HEADS,
AND
VALVE TULIPS FROM
TWO IDENTICAL ENGINES USING TWO DIFFERENT OIL FORMULATIONS
X
AND
Y
~~
Extraction Oil X Oil Y
fluid
Piston Head Valve Piston Head Valve
co2
2
4
7
1
1
3
C02h4eOH
8
6
10
6
6
9
Total
10 10
17
7
7
12
References
pp.
301-303
290
Chapter
10
Thus, SFE was employed as a tool to help give detailed compositional analy-
sis of small amounts of sample and allowed studies to be carried out to try to
relate deposit formation to fuel and/or lubricant formulation. However, the SFE
results alone can give interesting information in that the deposits from oil
X
contain significantly higher carbon dioxide extractables than those with oil
Y.
The carbon dioxide extracts contain mainly low polarity base oil and the results
indicate that during this test there may have been a leaky valve seal allowing
more oil to ingress to the valve and therefore the combustion area. A study of the
engine test results confirmed that this was the case. The method could therefore
be
a useful tool
in
investigating the causes of engine failure and validating com-
pensation claims citing oil products as the cause of the failure. In a wider con-
text, the results demonstrate the power of modified solvents in that the addition
of methanol at a level of
10%
to carbon dioxide gave up to a sixfold increase in
the quantity of material extracted. It also shows how they can be employed to
gain fractionation of materials by polarity.
10.5.
I.
6
Environmental analysis
This is likely to be one of the biggest application areas for
SFE,
particu-
larly for the analysis of soil samples and there are a great many papers
in
the lit-
erature which deal with this topic. Therefore it is considered only briefly here
and readers wishing more detailed information are advised to consult the litera-
ture.
In
terms of petroleum industry applications, the
two
main areas are
in
the de-
termination of total petroleum hydrocarbons (TPH) and polycyclic aromatic hy-
drocarbons
(PAH).
Lopez-Aviia
et al.
[42]
have developed a method which em-
ploys
SFE
off-line followed by infrared (IR) spectroscopy for the determination
of petroleum hydrocarbons
in
soil. The most favoured method at present is
sol-
vent extraction with Freon-1 13 and they estimate that at present in the United
States alone, some
200
000
samples per annum are anaiysed in this way requir-
ing 30
000
1
of Freon-1 13. A similar approach using SFE with GC instead of
SFE-IR can also be employed
[43].
Campbell and Richards
[44]
compared SFE
with Soxhlet and sonication methods for the determination of priority pollutants
in
soil and concluded that SFE was,
in
the majority of cases, the most efficient
extraction method. In addition, significant reductions in organic solvent con-
sumption were achieved.
Hawthorne and co-workers have made extensive studies on the extraction of
PAHs
from
a variety of matrices, including soils, and using different supercriti-
cal fluids. They reported that chlorodifluoromethane yielded consistently higher
extraction efficiencies than either carbon dioxide
or
nitrous oxide and was par-
ticularly effective in removing water from wet matrix samples. However, it re-
mains to be seen whether such solvents would be universally accepted.
Supercritical jluid extraction
29
1
10.5.2
On-line applications
The advantages of coupling SFE on-line with chromatographic techniques has
stimulated a lot of work in this area. Much of the early work centred on coupling
SFE with SFC and this has been extensively studies particularly for the determi-
nation of low molecular weight components and additives in polymers
[45-47].
However, the limited applicability of SFC soon led researchers to investigate
coupling to GC and LC.
10.5.2.1
On-line
SFE-GC
Gas chromatography is probably the most widely applied analytical tool in the
petroleum industry and therefore on-line coupled SFE-GC is likely to be
of
in-
terest to the petroleum chemist. In this section, we briefly look at the history of
the technique and consider one way of coupling SFE to GC.
A
more detailed
discussion on the various approaches to coupling SFE and GC is given by Lee
and Markides
[4].
Smith
et al.
[48]
described a means of coupling SFE to GC
and described its use for the examination of polycyclic aromatics in a number
of
matrices. Monin
et al.
[49]
described the application
of
SFE-GC
for the extrac-
tion and examination of hydrocarbons from source rocks. Hawthorne
et al.
[50-
531
and Levy
et al.
[54-561
have published extensively on the different mecha-
nisms of coupling SFE with GC and have demonstrated the application of the
technique for a number
of
petroleum-related applications.
c02
I
GAS CHROMATOGR
H
Fig.
10.8.
Schematic representation
of
coupled
SFE-GC
with direct expansion into a split injector
port.
References
pp.
301-303