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Heavy feed characterisation: a molecular approach

Characterising heavy feeds on a molecular basis, together with kinetic studies, accurately predicts the reactivity of a wide range of vacuum residues.

GLEN HAY and LANTE CARBOGNANI, Virtual Materials Group
HIDEKI NAGATA, Fuji Oil Company
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Article Summary
The characterisation and reactivity of heavy oil fractions (vacuum residue) were studied using a PIONA (paraffins, iso-paraffins, olefins, naphthenes, aromatics) molecular approach. Eleven crude oil assays along with vacuum residue pilot-scale thermal cracking information were provided. These feeds were characterised and their susceptibility to thermal cracking was evaluated all using a PIONA rigorous approach. A reactivity index related to the naphthenes-
aromatics-dehydrogenated aromatics ratio was found for each feed in order to match experimental results. This reactivity index was then correlated to the vacuum residue feed properties. The analysis of these results points to C7 asphaltenes (or equivalent), carbon residue and density of vacuum residue as key properties to be measured in order to capture the chemical nature of this fraction, and thus proper reactivity in a thermal cracking process.

Identifying and quantifying each component contained in oil fluids has proven to be both impractical and unfeasible. Characterisation of crude oils and their different cuts is commonly performed by measuring different properties. Traditionally, techniques such as gas chromatography (GC) for lighter fractions (light ends and C1-C5) and the use of distillation curves are commonly used for the liquid fractions. In this regard, the properties (densities, molecular weight, chemical family, and so on) of the light ends and lighter component ranges are usually well known, however the physical properties of the remainder fractions need to be determined separately and this comes at a price (both time and money). Organisations such as the American Society for Testing and Materials (ASTM) have developed routine tests for determining boiling ranges and properties of crude feedstock, distillation fractions and products, however the bulk of these techniques were originally intended to capture the properties and composition of conventional oils. Heavy oils pose a challenge since they contain a large, non-distillable fraction.1-3

Commonly measured properties in these non-distillable fractions, such as density and viscosity, can prove to be helpful when determining the physical and transport properties of these fractions. Other properties, for example carbon residue, SARA (saturates, aromatics, resins, asphaltenes) analysis and pour point also prove to be helpful when determining not only the quality of an oil product, but also its chemical nature and reactivity.2-3

Carbon residue is a particularly important characteristic of crude oil residues, since it not only can indicate the quality of the fraction, but also can be correlated to a number of properties such as hydrogen to carbon ratio (H/C), heteroatomic (S, N) content, asphaltenes content, or viscosity.2

The pour point of an oil fraction indicates the minimum temperature at which this material will flow. This property is affected by the presence of heavy molecules (which increase pour point), thus it can be correlated to molecular weight and density.2

SARA analysis proves to be useful for residue fractions, which usually contain large amounts of aromatics, resins, and asphaltenes. In particular, measurements of asphaltenes are important for heavy oils and residues to determine solid deposition probability, usually an issue in not only the production industry, but also in transportation and refining. Alternatively, the fractions of saturates and aromatics are of less importance due to their redundancy with other lighter range properties typically measured.2-3

Introduction to heavy oil kinetics
When dealing with reaction kinetics for thermal cracking of heavy oils, a considerable amount of material has been published on the subject in the last few decades. This material focuses more on the molecular structure approach to solving the problem.4 This approach is a more rigorous alternative to the lumped kinetic schemes that are being replaced due to lack of predictive accuracy across differing feedstocks. Moreover, this surge in molecular structure modelling comes at a time when personal computers have reached a point at which processors can keep up with the enormous computational demands of approaching the problem. It should be noted that molecular structure based kinetics, as well as physical property calculations, have been around since the 1960s and many influential and detailed papers on the topic were published as early as the 1980s.5-6

When looking at the thermal cracking of heavy oils, or even approaches to catalytic cracking and processing, there is an emphasis on aromatic groups with or without significant saturated branching.7 This distinction is of great interest due to the relationship of the saturated nature of these heavy aromatics to their ability to crack into highly desired liquid product, or alternatively propagating into larger molecules that can lead to solid precipitation or coking. Examples of such molecules can be seen in Figure 1. If these heavier cut molecules are characterised accurately, the approach to applying kinetics to these molecules would then depend upon the ability to break C-C bonds or the more challenging C-H scission at specific operating conditions where cracking occurs. Cracking reactions are usually proposed in three general steps: chain start, growth, and termination.8 At the same time, the propagation of such molecules will also occur at a calculated reaction rate and the balance of these reactive cracking and propagation pathways will ultimately lead to resulting product yields.

PIONA modelling approach
Different methods to describe a mixture of hydrocarbons and its chemical or physical properties have been attempted over time. The approach described within this work takes a middle ground within most simulation environments. The first simulation approach, a simplistic single property lumped approach, has dominated the software public domain since the 1900s. In this case, the crude oil feedstocks are divided into pseudo-components based on a range of a single focal property, which is usually the average boiling point or molecular weight. Although little information needs to be known about the feedstock to complete this approach, the drawback is the lack of predictive results due to over-generalisation of the molecular structure groups within the mixture. The properties predicted for mixtures also become direct correlations based on very general reference properties. An example of such a correlation was proposed for calculation of the aniline point of a mixture based on API gravity and mid boiling point alone (see Equation 1):2
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