Crude Assay - Lecture Notes

7/23/2019 Crude Assay - Lecture Notes

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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

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Crude Assay - Lecture Notes