Assay Of Crude Oils (Petroleum Refining)
It is vital to find out the physical and chemical characterizations of crude oil via a crude oil assay, since they are used in different areas within the petroleum refining trade. The most typical functions of petroleum assays are:
• To offer extensive detailed experimental data for refiners to establish the compatibility of a crude oil for a selected petroleum refinery
• To anticipate if the crude oil will fulfill the required product yield, high quality, and production
• To determine if during refining the crude oil will meet environmental and other requirements
• To assist refiners to make decisions about changes in plant operation, growth of product schedules, and examination of future processing ventures
• To produce engineering corporations with detailed crude oil analyses for their course of design of petroleum refining plants
• To facilitate companies’ crude oil pricing and to negotiate attainable penalties attributable to impurities and different nondesired properties
A crude oil assay is a compilation of laboratory (physical and chemical properties) and pilot-plant (distillation and product fractionation) knowledge that characterize a specific crude oil. Assay analyses of entire crude oils are carried out by combining atmospheric and vacuum distillation models, which when combined will provide a true boiling-point (TBP) distillation. These batch distillation methods, though taking between three and 5 days, allow the collection of a adequate amount of distillation fractions to be used in further testing. The values of the distillation ranges of the distilled fractions are normally defined on the idea of their refinery product classifications. The most common distillation ranges utilized in international assays of crude oils are reported in Table 1.5.
Table 1.5. Typical Distillation Range of Fractions in Petroleum Assays
Mild straight-run naphtha
Medium straight-run naphtha
Heavy straight-run naphtha
Mild vacuum gasoil
Heavy vacuum gasoil
There are various types of assays, which differ significantly in the quantity of experimental data decided. Some embody yields and properties of the streams used as feed for catalytic reforming (naphtha) and catalytic cracking (gasoline oils). Others give extra details for the potential production of lubricant oil and/or asphalt. At a minimum, the assay ought to comprise a distillation curve (usually, TBP distillation) for the crude oil and a particular gravity curve.
Probably the most complete assay consists of experimental characterization of the complete crude oil fraction and numerous boiling-vary fractions. Curves of TBP, specific gravity, and sulfur content material are regular data contained in a properly-produced assay. For example, assays of varied Mexican crude oils are offered in Table 1.6. The API gravity of these crude oils ranges from 10 to 33°API. API gravity is a measure of the relative density of a petroleum liquid and the density of water (i.e. how heavy or light a petroleum liquid is compared to water). Though, mathematically, API gravity has no units, it’s at all times known as being in “levels.” The correlation between specific gravity (sg) and degrees API is as follows (the precise gravity and petroleum 9 ch the API gravity are both at 60°F):
Viscosity should be provided at a minimum of three temperatures in order that one can calculate the sample viscosity at different temperatures. The commonest temperatures used to determine viscosity are 15.5, 21.1, and 25°C. If viscosities of the pattern cannot be measured at these temperatures, the pattern needs to be heated and better temperatures are used, resembling in the case of the 10 and 13°API crude oils reported in Desk 1.6. As soon as viscosities at three temperatures can be found, a plot of a double logarithm (log10) of viscosity against the temperature can be constructed, and viscosities at other temperatures might be obtained simply, as proven in Determine 1.1.
The characterization factor (KUOP or KWatson) of the Mexican crude oils reported in Desk 1.6 ranges from eleven.5 to 12.0. The K factor is just not decided experimentally; moderately, it’s calculated utilizing the following equation (for petroleum fractions):
where MeABP (in levels Rankine) is the mean average boiling point of the sample calculated with distillation curve knowledge.
Usually, if Ok > 12.5, the sample is predominantly paraffinic in nature, whereas Okay < 10.0 is indicative of highly aromatic material. The characterization factor thus provides a means for roughly identifying the general origin and nature of petroleum solely on the basis of two observable physical parameters, sg and MeABP. More detailed relationships of the K factor to the nature of the sample are given in Table 1.7 . The characterization factor has also been related to other properties (e.g. viscosity, aniline point, molecular weight, critical temperature, percentage of hydrocarbons), so it can be estimated using a number of petroleum properties.
Table 1.6. Assay of various Mexican Crude Oils
10 ° API
13 ° API
Particular gravity, 60°F/60°F
D – 287
Kinematic viscosity (cSt)
Characterization factor, ,KUOP
Pour level ( ° C)
D – ninety seven
Ramsbottom carbon (wt%)
Conradson carbon (wt%)
D – 189
Water and sediments (vol%)
D – 4007
Total sulfur (wt%)
D – 4294
Salt content material (PTB)
D – 3230
Hydrogen sulfide (mg/kg)
uOP – 163
Total acid number (mg KOH/g)
Total nitrogen (wppm)
Basic nitrogen (wppm)
uOP – 313
nC7 insolubles (wt%)
Toluene insolubles (wt%)
D – 4055
Chloride content material (wppm)
D – 808
Determine 1.1. Kinematic viscosities of several Mexican crude oils.
Table 1.7. Relationship of Kind of Hydrocarbon to the Characterization Issue
Type of Hydrocarbon
12.15 – 12.Ninety
Determine 1.2. True boiling-point curve of varied Mexican crude oils.
Asphaltenes, that are typically reported as n- heptane insolubles, are, strictly speaking, defined as the weight percentage of n- heptane insolubles (HIs) minus the burden proportion of toluene insolubles (TIs) in the sample (wt% of asphaltenes = wt% of Hello – wt% Universal hydraulic press of TI). For the crude oils given in Desk 1.6 , their asphaltene contents are 24.65, 17.83, 11.21, 1.Fifty six, and zero.57 wt% for the 10) API, 13°API, Maya, Isthmus, and Olmeca crude oils, respectively.
Figure 1.Three. API gravity of distillates versus common volume share.
Determine 1.4. Sulfur content of distillates versus common quantity share.
TBP distillations for Mexican crude oils are offered in Determine 1.2 . It is obvious that light crude oils that have excessive API gravity values current additionally the very best quantities of distillates [e.g. Olmeca crude oil (38.67°API) has 88.1 vol% distillates, whereas the 10° API has only 46vol% distillates]. Figures 1.Three and 1.4 illustrate plots of API gravity and the sulfur content of distillates towards the typical quantity share of distillates of the assorted crude oils. Distillates of heavier crude oils have lower API gravity and a better sulfur content than these obtained from mild crude oils.
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