-
Indian Journal of Chemical TechnologyVol 7. July 2000. pp.
172-177
Hydrocarbon class type distribution of crude oils by high
performanceliquid chromatography
S L S Sarowha, B K Sharma, C 0 Sharma, R L Sharma & 0 C
Madhwal.Indian Institute of Petroleum. Dehradun 248005. India
Received 3 February, 1999; accepted 15 February 2000.
High perrorrnance liquid chromatography (HPLC) provides a rapid
analytical technique for chemical group typeseparation for
evaluating crude oils. Seven crude oils of different origins and a
technical blend of these crudes were taken upfor this study. Crude
oils were separated into total saturates and total aromatics on
amino bonded silica column throughHPLC system using UV and RI
detectors. The quantitation of all class types were carried out by
applying the responsefactors of separated concentrates from the
technical blend of crude oils and calculating their response
factors. The precisionand the accuracy of the method was
demonstrated by determining the standard deviation for the peak
areas using purecompounds and comparison of HPLC procedure with
their gravimetric data obtained by open column chromatography
ontechnical blend of crude oils. The method is simple. accurate and
provides compound class type analysis with microgramsample and is
suitable for quick compositional surveys of crude oils. It uses a
single solvent elution, requires minimumsample preparation and the
time required for single analysis is less then an hour. Therefore.
the method can be adapted forroutine analysis of crude oils and
streams from various processes.
Qualitative and quantitative evaluation of crude oils isan
important consideration as it imparts detailedcharacterization for
all segments of petroleumindustry-exploration, production and
refining. Themajor compound group types present in a crude oilare
saturates, aromatics, polars and asphaltenes. Theamount of these
compound types depends on thenature of crude 1-2.
The hydrocarbon industry has been employing anumber of
traditional techniques for analyzing crudeoils and their products
in terms of their chemical andhydrocarbon type compositiorr'".
These techniques arelengthy, tedious, laborious as well as
uneconomical torun on a routine basis. These techniques for
suchanalysis were developed when similar crudes were incommon
supply. Today, not only the sources of crudestend to change more
frequently but also these crudesare different from those processed
in the past.Therefore, there is a need for techniques that are
faster,simpler, economical and free from interference
andinteraction of the materials being used with samplecomponents.
Several chromatographic procedures havebeen described in the
literature for the separation andcharacterization of crude oi Is
and other selectedproducts into hydrocarbon group types"!".
Amongthem, HPLC has been very successful especially whenseparation
of heavy oils and residues is to be achievedin minimum time.
Attempts have been made in the past by many
research workers to develop HPLC proceduresassaying petroleum
products. Suatoni and Sw:determined hydrocarbon group types
nansaturates, aromatics, polar and asphaltenes on aporasil column.
Dark et ({[12. described a methocfractionation of coal liquids by
HPLC accordingthe number of condensed ring structures. DarkMc
Gough,13 also separated asphalt using analytHPLC on
u-bondapak-Nl-I, column. The asphalt'pre-fractionated into maltenes
and asphaltenes andformer were then fractionated into
saturaaromatics and heteroarornaic aromatic fractiLiphard!"
reported the separation of coal liquidsorder of increasing double
bonds presenthydrocarbon molecules using stainless steel colupacked
with 10 micron lichrosorb NH: station.phase and n-hexane as eluting
solvent. VerkoczyKnmal" described the fractionation of heavy
amedium crude oils into saturates, aromatics and reson a 10 micron
PE amino - cyno packed column an-hexane and methyl tertiary butyl
ether fMTBE)solvent monitoring simultaneously the
eluantsrefractometer and UV (213 nrn) detector. colin et aseparated
nC7 - maltenes of petroleum residues Ifsaturates, aromatics and
resins using low press~liquid chromatography accompanied with furt
Iseparation of aromatics into mono-, di- and pOI'
'no-51 I'aromatics usi!lg HPLC on grafted ami HPLcolumn. Dark' f
has evolved a method based on
•••• S1
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SAROWHA ct al: HYDROCARBON CLASS TYPE DISTRIBUTION OF CRUDE OILS
BY HPLC 173
Table I-Physico-chemlcal properties of crude oils
Atomic Elemental Composition Density KUOP Mill 'IrHIC 'kC 'kH
'IeN %5 "C(OF)
Ncelam 1'J21 856S 1383 003 014 OX223 II'JO 2S7( 548)
BH 18
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174 [NOlAN J CHEM. TECHNOL JULY :2000
Table 3--Precision and repeatability of HPLC data generated on
standard pure compounds using refractive index detector
Cyclohcxanc O-Xylene o-Mcthyl Naphthalene Phenanthrene
6GH x 10" 168x I0" 293 x I Of, 357xIO"2 607 175 283 360
667 16~ 291 337.+ 632 162 293 3725 652 165 282 3626 051 171 291
3337 672 178 302 ',0J~_l
A veragc area 650 1700 2907 3.+8
Standard dcviationr 0".]) 023.+ 0556 0675 OlS7Standard error
008S 0210 02SS 007195%Contidencc Level 0216 051.+ 062.+ oln99%
Confidence Level 0327 0780 0946 0262Coefficient of Variation 359
328 232 537
Table 4--Repcatibility data of technically blended crude oil and
its comparison with open column liquid chromatography (LC) data
Actual (hy LC)Wt% by HPLC I
2345
Average(x)
SD (0 "-]lCV
% Relatively Difference
further separated into mono- aromatics, di-aromaticsand poly-
aromatics + polars on a dual packed silica-alumina column according
to the previously publishedprocedure". Physico-chemical
characteristics ofseven crude oi Is were determined to provide
aninsight into the nature of crude oils.
High performance Liquid Chromatograplvv+TtieHPLC analyses were
carried out with Waters (U.S.A.)modular chromatographic system
consisting of Pump(Model 510), differential refractometer (Model
410),UV Spectrometer (Dul'ont), U6k injector, back flushvalve and
maxima 820 data processing software.Hydrocarbon class type
separation was achieved ontwo ~l-BondaPack-NH2 columns (300 X 3.9
mm) usedin series. HPLC grade n-hexane run at a tlow rate
ofImLimin. gave satisfactory resolution. 40 ul. ofsamples of known
concentration ('" 1 wt'7c) in n-hexane were injected on to the
column.Results and Discussion
Table describes the elemental composition,.u omic H/e ratio
density and basic nature of thecrudes. The data reveals that Neelam
is the lightestcrude as shown by density (0.8223) and atomic
H/Cratio (1.9245), whereas Jodhpur is the heaviest crude
Total AromaticsTotal Saturates
67-+67556709661066-+1
6685668
0568085-09
32632.+53291339033593315332
0568171
+ 181
(density 0.9731). All the crudes are havingintermediate base as
indicated by characterizationfactor (KUOP). The quantity of
distillates availablefrom Bombay High crude is maximum as indicated
bylower mid % distillates temperature (270"C), whereasit is minimum
for Jodhpur crude in which only 20%of the crude is distilled at
370"C.
Normal phase HPLC has been used to separatewhole crude oils into
total saturates and totalaromatics as composite peaks of
constituents throughtwo energy analysis (amino) columns in series
using arefractive index (RI) detector and n-hexane as theeluting
solvent. The known amounts of crude oilswere ~dissolved in n-hexane
to obtain standardsolutions of nearly one-percent concentrations.
At thisdilution there was no apparent precipitation ofasphaltenes.
The peak of total aromatics consisting ofvarious aromatic
hydrocarbons and polars, waSobtained bv reversinz the tlow
direction of mobIlechase afte~ elution ~f saturates using a
siX-dPort, . at tobackflush valve. The two peaks were quantlta e
alobtain the crude oil composition in terms of tot
. d b n aroupsaturates and aromatics. Further hy rocar 0 ." T).
I . d f . tes (SA 'type separation was ac neve or satura
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SAROWHA et al: HYDROCARI30, CLASS TYPE DfSTRfRUTfO\,i OF CRCDE
OILS BY HPLC
250
/LU
'"zoQ.
'"..,a:2·00
'"•...-'o.>,2
1·50
1·00
0'50
(}OO 0·50
x t9 MINUTES RETENTION TIME
175
COIlOlTION
NOBIL£ ""'~ : .- HEX4NEFLOW RArE : I "'l/ml"
'"N ~6~+OJ ~I ~c~I'II,
I:I,I'
':I'
COLUNN . ENERGY 'NAlYSIS
(NH2 \
(300X3'IOIm\
OETECTOR . DIF. AI··
uv- 254 nrn _
1·00 1·5O 2·00
Fig. I-A typic.r: chromatogram "r a crude oil. Analytical
Conditions=-Mobih- phase: n-hcxane with !low r.ue I ()() mLirnin.
Culumn:Energy Analysj , I .unino bonded: Column (300d'! mrn),
Detector: Ditfcrenual RI and uY at 25~nl1l
mono aromatics (MA), di-urornarics (DA). and polyaromatics pills
polurs (PA +polars) using the same setof energy analysis (amino)
column with refractivendex (RI) and ultra-violet' (UV) detectors.
Thecomposite peak of saturates on RI contains all normal.ISOand
cycle paraffins. It has been shown that allalkanes at least throuzh
cholestone, clute beforemonoaromatics (dodec~1 benzene). The
elutionse'luence of arom.u ic-, is' stricti v based on the numberof
Condensed aromatic rinas. It 'Ins been observed thatan increase in
the alkvl chain lcnurh on aromatic ringleads to decrc;lse in tilt:
retention =I·olllllle. whcrca. ;t~Increase in naphihenic content
attached 10 arom.uic~:g tends.1O increase the retention \ olume.
Tllu". .u
parafflll-c!olllll1ated aromaucx clute beforerespective parent
rinu. while napluhene dominated~m . . - ..'" atlcs elUle
alll..'l"\\·ard, .. -\roma(k·' h;t\'lnC! ihl'c'c' "r"~re' . . -b
nngs h,I\'e been l'IUlL'd ,ti,'n;: with jlol,lr, tI'Ingackflush '.
I . I' . -I' . HllLC-Se . lei \1..' direr L' Il l lOfl ul
.u.rromuucs [
I Puration prnrile (If typical crude oil is ';[1O\\n In
I:·i~.eXhibit" S 'Ing .. '1.1. \1.-\.. D.-'\.. P.-\. +Polurx cluuon
order
on RI and UVD-254 11mdetectors.The qualitati ve idcnti fication
of \;InOUS
hydrocarbon types has been clone by measuring theretention
volumes of separated peaks In achromatographic run and comparing
the same withretention volumes of corresponding
calibratingstandards prepared from technical blend of crude
oils.These retention volumes have also been comparedwith those of
synthetic mixtures of SAT. '\'IA. D/\.and uiaromaucs (TA) obtained
by mixing purehydrocarbons. TI1I.'se retention data along with
relativeretention lime have been
-
176 INDIAN J CHEM. TECHNOL., JULY 2000
Crude Oil SampleTable 5- Compound type distribution (wt %) of
crude oils
RlD(128) RlD(l28)TcialS;.t Total arorn SAT MA DA PAtPtR
LingolaNeelamHeeraRawa&rrh~+litfITciliBknJAszoleJodhpur
753713709707674G68629567
247 746 82 27 l45287 705 123 34 13S291 G98 100 40 162293 685 118
42 155326 660 128 50 162332 6SS 108 2S 212371 GI6 116 21 247433 555
134 38 773
125115130131148204213240
detector signal proportionately vanes withconcentration, The
extent of linear dynamic range andthe noise of detector determine
the usableconcentration range for quantitative analysis.
The measurements of peak areas generated on RIdetector under
standardized experimental conditionsfor pure hydrocarbons have been
reported in Table 3.The Peak areas of seven injections obtained
from asynthetic mixture of cyclo-hexane, o-xylene, ci-methyl
naphthalene and phenanthcrene have showngood repeatability. The
standard deviations,coefficients of variations, confidence limits
at 95%and 99% level reported in Table 3 indicate theprecision and
repeatability of measurements carriedout using the above mentioned
HPLC instruments.The accuracy of quantitative data in HPLC is
afunction of precise determination of the detectorresponse factor
(RF) for identified class ofcompounds. Variations in retention data
within thesame class of compounds have also been observedwhich are
obviously due to the variations in themagnitude of compnsing
molecular species.Therefore, the response factors for hydrocarbon
groupwere determined for total saturates, total aromatics,MA, DA,
and PA +polars, separated from technicallyblended mixture of seven
crude oils. Theseconcentrates were considered to be
morerepresentati yes of compound types rather than thosefrom single
crude or synthetic mixture of purereference compounds. The response
factors arereported as peak area per unit concentrations for RIand
UV detector. The RF values determined for theseconcentrates are
presented in Table :: along withretention data. The ratio of
response factors do notpresent a definite trend of variations for
i\·L~,D"':\ andPA+polars concentrates, because these
aromaticconcentrates comprise various molecular moietieswith
varying amounts of alkyl and napthenic contentsand are not mixtures
of pure hydrocarbons.
Table 4 also provides the precision of HPLC data
UVD 254 rnm DuPontMA DA PA+A.
87117113III120100128147
3545485058283046
generated for total saturates and aromatic concentr:prepared
from technical blend and compares this \1that obtained by open
column chromatography usgravimetric method. Five repeated
rneasuremecarried out using HPLC exhibit a standard deviatof 0.568
for both saturates and aromatics, revealthe preciseness of
calculated values. The repeatabiof the quantitative data is
excellent for the technic;blended crude oil with a coefficient of
variat(C. V.) 0.85 for total saturates and 1.71 for tlaromatics.
The absolute concentration of ehydrocarbon class in the samples has
been calculaby dividing the peak area percentage vcorresponding RF
and nonnalising the values tobtained. The use of peak area percent
rather tpeak area is preferred for quantitation becauseformer
remains unaffected by dilution of sampleday to day fluctuation in
operating parameters, tproviding more reliable results.
Table 5 presents compound type distributionseven crude oils and
their technical blend. RI deterhas been used to quantitate total
saturates and tlaromatics in one run while SAT, MA, DA and+polars
have been determined in another run. The·detector at 254 nrn has
been used to provideidentical information to corroborate the RI
data. Cfirst run data of RI detector reveals that the tesaturate
content is highest (75.3 wt'7c) in Lingola 'lowest (56.7 wt'7c) 1I1
Jodhpur. The 10\\concentration of saturates in Joclhpur crude oils
artaccordance with the fact that it is the heaviest ofthe crude
oils. The Technical blend showed sligfhigher value of saturates
than the calculated aver3TI~e second run in which backflushinz is
used aelution of di-arornatics, provides data '-on SA.T, IvDA and
PA +polars and reveals the gradual increin saturates and decrease
in polars from Jodhpurwt to Linzola (75 wtSe). The data on
saturates ashow that N~elam, Heera, Rawa and BH. whichon-shore
satellite crudes of Bombay High regl
-
SAROWII.A et ct. HYDROC\RBOf\< CL\SS TYPE DISTRIBUTION OF
CRLDE OILS flY IIPLC
have almost identical amount of saturates andcategorized as
intermediate crudes. Jodhpur andAszole crudes belong to entirely
different regions andhave relatively lesser amount of saturates
showingtheir naphtheno-arornatic base. These facts are
againcorroborated by the KUOP values in the Table I. The,UTIount of
monoarornatics varies between 10 to I]wt % in other crude oils with
the exception of Lingola(8 wt 'Ie). The four satellite crude oils
have nearly thesame amount (12 wt';{.) of [\.'1A with exception
ofHeera having only 10 wt 'I". Technical blend of crudeoils
presents an average value (II wt %). The amountof DA ranges from 2
to :) wt '10 in all crude oils. ThePA +polars contents obtained
after backtlushingranges from 14 wt 'k to 16 wt % for other crude
oilswith exceptionally higher values for Aszole (24.7wt%) and
Jodhpur (27.] wt q,). The higher values ofPA+polars in all crude
oils might be due to thepresence of high molecular wt % resinous
compounddissolved in matrix. The dara.ximultaneously obtainedfrom
UV detector 254 urn for MA. DA andPA+polars corroborate the data
obtained by RID. Thearea percent distribution of hydrocarbon types
in anUV run has been divided by the corresponding RFsand normalized
the values thus obtained. These valueswere later recalculated by
distributing them in thetotal aromatics obtained by RID assuming
the lattermore close to correct value. The lower values of SAT.MA
and DA and relatively higher values ofPA+polars in case of Aszolc
and Jodhpur crude oilsaccount for their heavy nature and is in
keeping withthe values for density (Table I)
hj
h
.
II(
ConclusionFrom this study it can be concluded that HPLC
methods provide a quick survey on the compositionalparameters of
crude oils. The data reveals thatjodhpur crude is the heaviest one
followed by Aszoleas exhibited hv the larzext concentration of
arorn.uicsand polar het~roalOm;ic compounds. Bombay Highand its
satellite crudes are rich in saturates as also themediUm crudes
while Lingola crude oil is highlyparaffinic having highest saturate
content. The
177
repeatability of data is' good as calculated fortechnically
blended crude oil.
AcknowledgementThe Authors extend their appreciation to crude
oil
evaluation laboratory, lIP, Dchradun for providingsamples and
Director, lIP for his support andencouragement in this study.
ReferencesI Bland W F & Davidson R L. l'ctrolciun
I'I'OCCSSillg
hundbook. (Me Graw Hill. Ncw York). 19672 Corbett L W
8.:.Pcrroxsi U. lnd I:'llg C"(,111Prroil H,'s Del'. 17
(1