Page 68 - Petroleum and Gas Field Processing

known as an ‘‘assay.’’ Crude assays are the systematic compilation of data
for the physical properties of the crude and its fractions, as well as the yield.
In other words, a crude assay involves the determination of the following:
1. The properties of crude oil
2. The fractions obtained: (a) their percentage yield and (b)
properties
Analytical testing only without carrying out distillation may be considered
an assay. However, the most common assay is a comprehensive one that
involves all of the above-stated parameters.
The basis of the assay is the distillation of a crude oil under specified
conditions in a batch laboratory distillation column, operated at high
efficiency [column with 14 plates and reflux ratio (RR)]. Pressure in
column is reduced in stages to avoid thermal degradation of high boiling
components.
A comparison of the characteristics of different types of crude oil
over the distillation range could be made via a graph that relates the
following:

The density of distillate fractions
Their mid-boiling points
Such a comparison is illustrated in Figure 1 [3]. The density level of a
crude at given boiling point on the curve is a function of the relative
proportions of the main three hydrocarbon series: aromatics, cyclopar-
afins, paraffins; their densities decrease in that order.
In order to show how the properties of crude oils affect strongly
processing requirements, product expectations, storage and transportation,
and others, a comparison is presented as given in Table 2.
Table 3, on the other hand, gives the percent yield and other
characteristics of the fractions obtained by the distillation of a typical
Arabian crude oil having an API gravity of about 34–37.

2.4 CRUDE OIL CLASSIFICATIONS AND
CHARACTERIZATION
2.4.1 Classifications
Although there is no specific method for classifying crude oils, it would be
useful to establish simple criteria to quantify the quality of a crude.
Numerous attempts have been made to devise a system to classify crude
oils into types based on the predominant hydrocarbon series present in the

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.