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

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

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

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

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