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Jan-2009

Stabilisation 
of asphaltenes

Asphaltenes can plug refinery equipment and can decrease or stop production. Stabilisation of asphaltene particles is an efficient and cost-effective way to avoid undesired asphaltene agglomeration or precipitation

Berthold Otzisk and Hermann Kempen
Kurita Europe

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Article Summary

From the oil well through the refinery, fouling can interfere with production equipment. It is a common problem for oil production companies and refiners, and periodically process units have to be shut down for mechanical cleaning due to fouling, resulting in throughput losses or production shutdown. The incentives to reduce fouling are therefore significant, such as increased productivity and reduced operational costs.

The fouling phases can be coke, asphaltenes, wax, stable emulsions or inorganic solids. Refiners are currently converting more residuals into light fractions, which results in heavy fuels production that contains a higher concentration of asphaltenes. Most organic fouling is caused by insoluble asphaltenes, including coke formation. Asphaltenes are components of petroleum, and their behaviour and structure change with temperature, pressure and the composition of oil. Unwanted asphaltene precipitation can plug well bores and distillation equipment and, in many cases, decrease or stop production.

Asphaltenes and resins are responsible for adding most of the colour to crude oils. Asphaltene particles have a high affinity to associate with one another and a high tendency to adsorb resins. The wide range of size distribution indicates that asphaltenes are partly dissolved and partly in a colloidal and/or micellar form. Resins are not known to precipitate on their own, but they do deposit with asphaltenes. At a low temperature, waxy crudes containing small amounts of asphaltenes can behave differently to a clean, waxy crude containing no other heavy organics.

Asphaltene precipitation during oil production and processing is a serious problem. The economic implications are significant and can cost millions of euros. In some cases, fouling problems have been reported to be so drastic that many oil wells have been redrilled. The fouling material, similar to that shown in Figure 1, is often transported from the well tubing to the flow lines, separator, process pumps, strainers and other downstream equipment. Asphaltenes are sensitive to shearing forces and electrostatic interactions, and crude distillation and vacuum units often are affected by asphaltene fouling. Crude preheat train and vacuum bottom heat exchangers can plug, and as a result require chemical or mechanical cleaning; otherwise, throughput has to be reduced, leading to a loss of production.

FCCUs often report fouling problems in the slurry circulation system. Asphaltenes, catalyst slurry fines, coke particles and dehydrogenated polynuclear aromatics (PNAs) can all cause fouling.

The best strategy for reducing fouling is to use basic knowledge to eliminate its formation. Asphaltenes do not melt above 300–400°C; instead, they decompose, forming carbon and volatile components. Asphaltenes contain thermally stable PNA cores, and their pendant groups are connected to the core by thermally unstable bonds. These bonds break to form free radicals above 350°C. The dispersed asphaltenes abstract hydrogen from hydro-aromatics and terminate the free radicals. The loss of the pendant groups results in a lower solubility of the asphaltenes. The asphaltene free radicals combine to form high molecular weight and insoluble coke.

Crude oils are a mixture of many organic compounds, and their overall properties differ from well to well. Residual oils from the Middle East and Latin America are often rich in asphaltenes. A high asphaltene content means a high Conradson carbon residue (CCR) and a high metals content (nickel and vanadium). CCR is the amount of coke produced in a standard thermal cracking test, and is a measure for potential coke formation at high temperatures.

Analytical methods
It is essential for a refiner to know the asphaltene concentration in crude oil, residue or heavy fuel. There are a number of analytical instruments and methods on the market to analyse the structure of asphaltenes, including:
—  X-ray analysis
—  NMR spectroscopy
—  Mass spectral methods
—  Elemental analysis
—  Diffusion techniques.

The main diffusion techniques are:
—  Fluorescence correlation spectroscopy (FCS)
—  Time-resolved fluorescence depolarisation (TRFD)
—  Taylor dispersion.

Asphaltenes are very complex molecules. Their structure is often desorbed to a compact stack of many thin sheet-like layers (similar to that of a book). They are suspended by tiny colloidal particles in crude oils. The molecules are stabilised by natural resins and maintained in the oil due to its stabilisation.

The industry’s knowledge of asphaltenes is still limited and little is known about their chemical properties. Asphaltenes are lyophilic with respect to aromatics, in which they form colloidal solutions. Removal of the small colloidal particles or resins immediately results in instability of the oil. The consequence is the formation of an agglomeration, followed by precipitation.

ASTM D2007-80 is a widely recognised standard procedure for separating asphaltenes from crude oils. The solvent n-pentane is used for asphaltene precipitation, followed by filtration. The filtered asphaltenes must be dried under a hood for several days. If the drying process is too slow, the asphaltenes can be redissolved in a minimal amount of toluene. They can then be reprecipitated with an alkane of lower molecular weight and a higher volatility. The dried filtered asphaltenes can be weighted to determine the amount of asphaltenes in the oil.

Alkane titration
Operationally, asphaltenes are defined to be insoluble in alkanes like n-heptane and soluble in aromatic solvents like toluene. ASTM D7157 is a standard test method for determining the instability of asphaltene-containing residues, heavy fuels and crude oils. There are a number of suitable automated fuel stability analysers on the market that conform to ASTM D7157.


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