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LECTURE NOTES
ON
CRUDE ASSAY
J M NAGPAL, J K DIMRI
RELIANCE INDUSTRIES LTD
JAMNAGAR
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INTRODUCTION
Assay or evaluation of a crude oil involves assessing the crude oil for its characteristics including micro-
constituents present in it and yields and characteristics of the straight run petroleum products, which can be
obtained from the crude oil.
CRUDE OIL COMPOSITION (1,2)
Main bulk of a crude oil is hydrocarbons. Very simple structures (C3, C4, C5hydrocarbons) to most complex
structures such as resins, poly-aromatics, asphaltenes and porphyrines etc. are present in a crude oil. Besides the
hydrocarbons, it consists of traces of compounds containing sulfur, nitrogen, oxygen and metals. Three main types
of hydrocarbons found in crude oils with their generic formulae are paraffins (CnH2n+2), naphthenes (CnH2n) and
aromatics (Cn Hn).
Branched as well as straight chain paraffins up to the carbon number of C120 and higher occur in a crude oil.
Naphthenes (saturated cyclic compounds) generally five or six member rings (monocyclic) or fused rings (poly-
cyclic) structures are present in a crude oil. Aromatics are the unsaturated cyclic structures having lower hydrogen
to carbon ratios as compared to naphthenes. Aromatics are present as a single ring, biphenyl structures and fused
(multiple) ring structures. In the heavy ends of a crude oil, the fused rings are of very large structures with
molecular weights ranging from 1000 to 100000. Compounds containing sulphur, nitrogen, oxygen and metals are
present in traces but they have significant effect on the quality of products and feed stocks derived from a crude
oil. The elemental analysis of crude oils is in the following ranges:
Element %wt.
C 83-87
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The average elemental analysis of petroleum varies in a narrow range but the nature (quality and product potential)
of the crude oils from two different reservoirs is never the same. It is the relative difference in homologues series
of hydrocarbons (paraffin, naphthenes and aromatics) and the level of micro constituents, which are responsible for
the variations in the crude quality from different fields.
OBJECTIVE AND SCOPE OF CRUDE ASSAY
For an oil producer the crude assay data is important for its transportation and marketing/pricing of the crude oil in
the international market and a refiner needs the crude assay data for selection, grading and valorization. A detailed
crude assay is required for the design of a new refinery as well as for the expansion of an operating refinery. It is
not possible for a refinery to depend on a single crude supply these days. Refiners try several crudes and blend
them to their operating requirements. All such decisions are based on crude assays. Therefore, evaluation of the
crude oil becomes a priority to producers as well as refiners. However, depending on the objective of a crude
assay, its scope also may very (Table-1).
BASE AND CHARACTERISTICS OF CRUDE OIL
Characteristics of a crude oil, generally carried out, are given in Table-2. A crude oil consists of millions of
individual compounds. Although all the crudes contain substantially the same types hydrocarbons but their gross
properties and product potential are likely to change with the relative predominance of particular type of
hydrocarbons. Paraffinic base crudes consisting of predominantly paraffinic hydrocarbon are generally light
crudes with relatively higher distillate yields On the other hand crude oils classified as naphthenic contain
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Several empirical correlations and approaches have been developed to classify the crude oils (2). Most commonly
used is a characterization factor method developed by UOP.
3
Characterization factor (KUOP) = --------------------
SWhere TBis mean average boiling point and S is specific gravity at 60/60F
KUOP Base of crude oil
--------------- ------------------------
> 12.1 Paraffinic
11.5-12.1 Intermediate
< 11.5 Naphthenic
The expected quality of crude oils as such, their products potentials and quality of the products with the base of the
crude oils are compared in Table-3 (3).
For assessing the nature of lighter ends of the crude oil, Reid vapor pressure and (RVP) and LPG potential by gas
liquid chromatography are carried out. Flow behavior of the crude oil, which is important for its transportation
through pipelines, is studied by determining viscosity at different temperatures (dynamic or kinematics at above
TB
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Characteristics such as asphaltene content, carbon residue and ash content give an idea of the nature of heavy ends
of a crude oil. Sulfur, nitrogen, metals and acidity are considered important quality parameters of a crude oil as
they deteriorate the quality of products and feed-stocks for secondary conversion processes. Cost intensive
processing is required in refineries to treat the distillates to bring down the levels of sulfur, nitrogen and metals.
These days, environmental concerns are forcing refiners to produce distillate fuels with ultra low sulfur contents.
Salt content (associated brine), water and sediment are the impurities in a crude oil. Presence of high salt content
is the main cause of overhead corrosion in distillation units and deterioration of the bottom product quality as well
as it is responsible for fouling of heat exchangers. Sediment and water occupy precious space in crude oil tanks
and increase the sludge as well as cause operational problems. Besides conventional fuels a detailed assay covers
potential of pure chemicals and other value added products and lube base stocks (Table : 5)
TBP ASSAY
True boiling point (TBP) distillation, consisting of a fractionating column with minimum 15 theoretical plates and
a provision for a reflux (5:1) (ASTM D2892) is a main tool for carrying out the TBP assay. Broad cuts from the
crude oil for a qualitative assessment in atmospheric range up to a temperature of ~ 400oC are also prepared in
TBP set up The residue can be further distilled using a high vacuum still (Pot still ASTM D5236) to prepare cuts
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In a TBP assay the crude oil is fractionated to prepare narrow cuts (2-6 %volume or weight intervals or 10 to 25oC
vapor temperature intervals). Yields of these cuts are assigned on cumulative weight and volume basis to prepare
TBP distillation curves (Figure-1). The narrow cuts can be analyzed for some key characteristics to study the
variation of properties with cut temperatures (Table 7). The data can be represented in more useful manner on mid
vol. / wt. % - property curves (differential curves) (Figure-2). The representation of additive properties on mid %
curves is more accurate and these curves can be used to find out the property of any width of fraction of
commercial interest.
Key properties of the broad cuts (prepared directly from the crude or by back blending of narrow cuts) can be
represented on property yield curves also. These curves are very useful in product optimization.
Broad cuts of commercial interest prepared directly from a crude oil are characterized in detail to study the yields
and quality of these products (Table-8).
ANALYTICAL DEVLOPMENTS
Advances in analytical techniques (Fig-3) have been of valuable support to crude and product analysis. Capillary
Gas Liquid Chromatography (GLC), Mass Spectrometry (MS), Nuclear Magnetic Resonance (NMR), Near Infra
Red (NIR), X-Ray Fluorescence (XRF), High Performance Chromatography (HPLC), Atomic Absorption
Spectroscopy (AAS) and Gel Permeation Chromatography (GPC) and other dedicated instrumental analytical
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CONCLUSION
A crude assay essentially comprises of crude characteristics, TBP assay and yield and characteristics of several
broad cuts in atmospheric and vacuum ranges. However, the scope of the crude assay varies with its required
application. Use of advanced analytical techniques can help in generating a detailed component-wise analysis or
hydrocarbon type analysis of the various cuts which is very useful in design of the refinery units. The crude assay
data is of immense importance to the crude producer as well as to the refiner.
REFERENCES
1. Davies J.F. Crude Oilin Modern Petroleum Technology Edited by Alan G. Lucas John Wiley & Sons Ltd
2000.
2. Nelson W.L.Evaluation of Oil Stocksin Petroleum Refining Engineering, Mc Graw Hill Book Company
Inc. 1949.
3. Nelson W.L. Which Base of Crude Oil is Best? Part I, Oil & Gas Journal 1979, Jan8, p112
4. Nagpal J M, Sharma R LRole of IIP in the Area of Crude Oil Evaluation and Product Analysisin
Challenges in Crude Oil Evaluation Edited by J M Nagpal, Tata Mc Graw Hill Publishing Company Limited, New
Delhi, 1995.
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Table 1: Objective and Scope of Crude Assays
Type of Assay Objective Scope
Preliminary
Assay
Consistency of crude supply Basic crude characteristics
Short Assay - Absorption of a new crude in a fuel
refinery
- To study the change in crude quality
over a period of time
Crude characteristics, TBP distillation
data, key characteristics of straight run
cuts in atmospheric range and long
residue
Detailed Assay -Design data for a grassroot refinery
-Product optimization
-Selection and design of secondary
conversion units
-Expansion modification of units
-Value Addition feasibility
-Detailed characterization including
crude oil micro-constituents
-TBP Assay in atmospheric and vacuum
range. Yield and characteristics of
several broad cuts with variations in IBP
and FBP
-Characteristics and composition of
specific cuts for process simulation and
feed stock quality for secondary
processing, value addition and lubemanufacture
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Table2. Characteristics of Crude Oil
Property Type Significance
-Density /API
-Base of crude oil
Basic physico- chemical
properties
-Chemical nature of the crude reflected
-Density / API extensively used in weight /
Volume conversions
- Reid Vapor Pressure
- Dissolved Gas analysis
Light ends in crude oils -LPG potential
-Storage, handling of crude oil
-Pour point
-Viscosity
-Wax Content
Flow properties of crude oil -Transportation and handling crude oil
-Waxy crude oils not suitable for lube
and bitumen manufacture
-Asphaltene
-Carbon Residue
-Ash
Heavy End Properties -Reflect heavy hydracarbons in crude oils
- Ash reflects presence of inorganic matter
-Sulphur
-Nitrogen
-Metals
-Acids
Micro- constituents Significant effect on product and feedstock
quality
-Salt Content
Water Content
-Basic Sediment &
Water
Impurities - Process implications
-Need to be eliminated before processing
the crude oil
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Table3: Correlation of Base of Crude Oil with Quality of Crude and Products
Crude /Product Superiority Base of Crude Oil
P I N
Crude Sp. Gravity low Intermediate High
Petroleum Naphtha
Petrochem./Fert.
BTX
Yield Best
-
Good
Fair
-
Good
Gasoline
Straight Run
Reformer feed
Thermal Crackate
Cat Crackate
Hydrocrackate
Yield
Oct. No.
Oct. No.
Oct. No.
Oct. No
Oct. No.
High
Low
-
-
Little -----Little-
----
Intermediate
Good
Good
Fair
Effect-------
Effect------
Low
V. Good
Best
Good
------
-------
Kerosene Smoke Point V. Good Good Poor
Jet Fuel Freezing point - Fair Good
Diesel Fuel Ignition
Pour Point
Good
-
Fair
Fair
-
Good
Cat Cracking Gasoline Yield Good Fair -
Hydrocracking Distillate Yield Good Fair -
Carbon Black Yield - - Good
Lube oil, Automotive VI
Pour Point
Best
-
Good
Good
-
Best
Residual Fuel Pour Point - Good Best
Asphalt Ductility - Good Best
Coke Electrode Carbon - Fair Best
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Table 4: Flow Characteristics of Crude Oils with Low and High Wax
Crude Basrah Lt. Mumbai High
Wax, %wt 3.5 10.9
Pour Point -24 +30
Kin. Viscosity, cSt at 40C
at 50 C
6.18
4.84
4.30
3.32
Yield Value, Dynes / cm2, at
32 C
24 C
18 C
16C
2.0
5.0
10.0
12.5
45.0
85.0
222.0
330.0
Plastic Viscosity CP at
32 C
24 C
18 C
16 C
9.6
14.7
6.0
17.3
7.9
30.7
43.7
45.0
Table 5: Potential of Pure Chemicals and Other Value Addition Products
Stream Value Added Products Cut Temperature
(Approximate)
Naphtha - Petrochemicals through
Olefins
- Petrochemicals through
Olefins
- Pure Chemicals (Pentane,
Hexanes, Heptane
- SBP Solvents
IBP -105C
60-140C
Optomised cut point from light
naphtha
Specific cut points depending on
grade in the range of IBP to
300C
Kerosene n Paraffins for LAB 175 265C
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TABLE : 6
CRUDE CHARACTERISATIONRESULT
Sr No CRUDES ZAFIRO ARAB HY. MARIB LT EOCENE
CHARACTERISTICS UOM
1 Density,at 15C kg/m3
872.2 890.8 802.6 940.3
2 Specific Gravity @15.6/15.6C
-- 0.8727 0.8913 0.8030 0.9
3 Barrel factor bbl/T 7.2 7.1 7.8 6.70
4 API Gravity API 30.6 27.3 44.6 18.9
5 Pour Point C
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TABLE : 7 TBP DISTILLATION DATA AND CHARACTERISATION OF 25C TBP CUTS
Fractions,C
wt%
Cummwt%
Density
@15C,Kg/M3
vol%
Cummvol%
API
R.I@20C
T.S,%wt
Mercaptans,ppm
Doctortest
TotalNitrogen,ppm
K.
Viscosity,cSt@40
C
TAN,mgKOH/g
PourPoint,C
SmokePoint,mm
AnilinePoint,C
DieselIndex
Kuop
15 0.9 0.9 0.58001.4
1.4 112.5 - - - - - - - - - -
38 1.1 2.0 0.62751.5
2.9 94.0 1.35608 - - - - - - - - - -
75 1.9 3.9 0.68912.4
5.3 73.8 1.38490 - 4.8 - - - - - - - - 12.2
100 3.5 7.4 0.73724.1
9.4 60.4 1.40733 - 4.0 -ve - - - - - - - 11.7
125 3.0 10.4 0.75763.5
12.9 55.2 1.41886 0.010 2.8 -ve - - - - - - 11.7
150 5 15.4 0.77605.6
18.5 50.8 1.42922 0.015 2.2 -ve 2.3 - - - - - 11.6
175 4.3 19.7 0.7938 4.7 23.2 46.7 1.43870 0.03 3.7 -ve 3.5 - 0.01 24 50.0 56.9 11.6
200 4.1 23.8 0.81364.4
27.6 42.3 1.44873 0.04 5.7 -ve 6.4 - 0.03 - 22 51.2 52.5 11.5
225 4.2 28 0.83214.4
32.0 38.5 1.45833 0.06 6.1 -ve 8.1 - 0.04
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TABLE : 8 CHARACTERISATION OF BROAD CUTS
Fractions C15C C5-105C 105-150C 150-165C 165-227C 227-270C 270-370C 370C+ 370-390C 390-410C 410-565C 565C+
LPG L.N M.N H.N. L.K. H.K. L.D. RCO HD LVGO HVGO V.R.
Density @15C,Kg/L 0.5800 0.7064 0.7671 0.7910 0.8193 0.8506 0.8794 0.9683 0.9190 0.9215 0.9442 1.0337
Specific gravity 0.7066 0.7674 0.7913 0.8197 0.8510 0.8799 0.9689 0.9195 0.9220 0.9447 1.0343
API 112.5 68.8 52.9 47.3 41.1 34.8 29.3 14.5 22.4 22.0 18.3 5.3
Pour Point, C -6 36 15 24 33 84
Refractive Index - 1.3958 1.42429 1.43747 1.45176 1.46997 1.48645 - - - - -
Freezing Point,C - - -
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15 0.9 1.4
38 2.0 2.9
75 3.9 5.3
105 7.4 9.4
124 10.4 12.9
150 15.4 18.5
175 19.7 23.2
200 23.8 27.6
222 28.0 32.0
250 34.0 38.1
275 39.8 44.0
300 45.3 49.5
325 50.6 54.8
350 55.7 59.8
370 59.8 63.8
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275 2.58 41.1
300 3.99 46.8
325 5.9 52.2
350 9.45 57.3
370 15.60 61.8
275 41.1 -30
300 46.8 -21
325 52.2 -9
350 57.3 0
370 61.8 12
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275 2.58 41.1
300 3.99 46.8
325 5.9 52.2
350 9.45 57.3
370 15.60 61.8
275 41.1 -30
300 46.8 -21
325 52.2 -9
350 57.3 0370 61.8 12
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Fig :3 Application of Instrumental Analyt ical Technique
GLC (Composition)
GLC(Composition,PIONA,Bz, Oxygenates)
NMR (PONA, C/H Ratio, Av. Mol Wt )
IR (PONA )
SFC ( Olefins )
AAS GLC HPLC ( Olefins )
ICP LPG Pot
Metals HPLC ( Hydrocarbon Types )
GLC (n-Paraffins )
UV (Total Aromatics )
MASS(EI) (Naphthalenes)
HPLC ( Hydrocarbon types )
UV
NMR
MASS (EI ) (Hydrocarbon Types)
MASS(EI) (Hydrocarbon Types )
HPLC (Aromatics )
NMR (n -, iso - cyclo paraffins )
AAS/ICP (Metals )
CRUDE BLENDING (ONLINE) UV ( Aromatic distribution )
NMR / NIR IR ( Functional group )
MASS (FI/FD) (Av. Mol weight and distribution)
AAS/ ICP (Metals )
NMR (Carbon type distribution, Av. Structure )
CRUDE OIL
GAS / LPG
NAPHTHA /GASOLINE
KEROSENE
GAS OIL/ DIESEL
VGO / LOBS
RESIDUE