Precipitation of asphaltene

The complexity of petroleum products raises the question of sample validity is the sample representative of the total flow The problem becomes that much more difficult when dealing with samples of heavy materials or samples coming from separations. The diverse chemical families in a petroleum cut can have very different physical characteristics and the homogeneous nature of the cut is often due to the delicate equilibrium between its components. The equilibrium can be upset by extraction or by addition of certain materials as in the case of the precipitation of asphaltenes by light paraffins. 

Solvent deasphalting. This is an extraction of the heaviest fractions of a vacuum residue or heavy distillate. The extract is used to produce the bitumen. The separation is based on the precipitation of asphaltenes and the dissolution of the oil in an alkane solvent. The solvents employed are butane or propane or a butane-propane mixture. By selecting the proper feedstock and by controlling the deasphalting parameters, notably temperature and pressure, it is possible to obtain different grades of bitumen by this process. 

Visbreaking severity is monitored to help minimize cracking and alteration of the nature of asphaltenes within the visbreaker feed. Paraffinic side chain cracking or destruction of the asphaltene-resin complex may occur during visbreaking operations and may result in precipitation of asphaltenes from solution. Asphaltene precipitation has been seen especially when visbroken material is blended with lighter-viscosity paraffinic fuels. 

Figure 1. Precipitation of asphaltenes using different n-alkanes (25)...

Figure 2. Separation scheme based on precipitation of asphaltenes followed by maltene separation on alumina...

The data in Table I are significant because they suggest that a one-to-one relationship of acids and bases does not exist for petroleum asphaltenes. The precipitation of asphaltenes may be attributed to a phenomenon other than precipitation of acid-base complexes or salts. The data strongly imply that the asphaltenes primarily consist of compounds capable of association through the hydrogen bonding mechanism. 

These fouling phenomena are believed to be due to the precipitation of asphaltenes from hydrocracked, effluent streams. Fragmentation reactions decrease the solubility power of effluent maltenes and the solubility of asphaltene micelles, thus facilitating precipitation [1]. 

Because asphaltenes are insoluble in light hydrocarbons (Cj—C8), the infiltration of gas or light oil into a fairly heavy oil, either in the reservoir or during migration, can result in precipitation of asphaltenes. 

Preheating (the preheating of fuel oils prior to their burning encourages the precipitation of asphaltenes and coking). 

Obvious depletion of n-alkanes and other paraffins, classically regarded as indicative of early biodegradation, is not observed in examined samples. However, Kuparuk viscous oils show slight to extreme selective depletion in long-chain alkyl aromatic (LCAA) hydrocarbon families (e.g. alkylbenzenes and alkyltoluenes). This is interpreted as indicative of an early stage of anaerobic microbial degradation that likely destabilized the oil to promote subsequent precipitation of asphaltenes as tar. 

In order to limit filtration blank contamination, a clean PFA filter unit was developed to separate the oil fi-actions by precipitation of asphaltenes (heavy fi"action) in n-Heptane. Membrane filters of size 0.45 pm in regenerated cellulose were used with the filter unit. One gram of crude oil was dissolved in 50 ml of n-Heptane (in a 250 ml PFA balloon). The solution was then heated for 20 min at 60°C under magnetic stirring and finally filtered under vacuum. Asphaltenes were accumulated on the filter and stocked in 50 ml polypropylene tubes at room temperature in the dark for further analysis. Maltenes, which are soluble in n-Heptane, were collected in a polyethylene Erlenmeyer and evaporated under pure nitrogen flow in 50 ml polypropylene tubes. 

It is well known that flocculation of asphaltenes in petroleum reservoirs, wells and surface separation-upgrading facilities pose technical problems and increase the cost of production and processing of crudes. Field conditions conducive to precipitation of asphaltenes include natural depletion, miscible flooding, caustic flooding, acid stimulation and gas-lift operations. Asphaltene precipitation is particularly important problem in miscible flooding since it can reduce permeability, affect well injectivities and productivities, alter rock wettability characteristics and even cause plugging of producing wells. ... 

In order to correlate peptizing efficiency with the concentration of amphiphiles, nonyl phenol was tested at different concentrations such as 0.5%, and 1% by weight. The results are shown in Figure 11. The precipitation of asphaltenes at 60% volume of pentane in solution has a 15 ppm difference between 1.0% nonyl phenol solution and standard solution, and the difference increases to 27 ppm when pentane volume reaches 80% in solution. It is clear that the peptizing efficiency increases with the increasing concentration of amphiphiles. 

The LMZ S-47 crude was first examined for asphaltene and resin concentration. Pentane precipitation of asphaltenes resulted in 10.5 (wt) % concentration of these particles in the crude. Resin concentration was determined to be 27.0 (wt) %. 

The precipitation of asphaltenes to form shot coke is a physical change involving no heat of reaction. Therefore, there is no temperature drop across the coke drum. 

When used, it is always ahead of extraction. The primary goal is to remove asphaltenes, which could be a possible byproduct and to make the viscosity specification that is required. This is accomplished by asphaltenes separation by solubility of non-asphaltenes in a solvent and precipitation of asphaltenes using e.g. propane as a solvent. Secondary effects include Conradson Carbon reduced, metals reduced, saturates increased, viscosity index increased, and color improved. 

Variations in temperature, pressure and chemical composition can cause precipitation of asphaltenes from crude oil. Rainfall and subsequent deposition of asphaltenes can cause problems in all stages of production, for example, transportation and processing, causing the loss of efficiency equipment in steps of production of crude oil. In the reservoir rock, seal can cause partial or complete its pores, resulting in the loss of oil recovery. 

The cost associated with the asphaltene deposition during production and refining operations is in the order of billions of dollars a year. For this reason, the prevention or minimization of precipitation of asphaltenes is an important goal for many oil companies (Rogel et al, 2010). 

The dispersion of asphaltenes is mainly attributed to the resins (polar aromatic). The resin molecules play a role of surfactants in stabilizing colloidal particles of asphaltenes in oil. There are concepts about precipitation of asphaltenes and the most widely accepted says that the dissolution of resins is followed by precipitation of asphaltenes (Shkalikov et al, 2010). On this basis, the stability of oil can be represented by three phase systems asphaltenes, aromatics (including resins) and saturated, which are delicately balanced (Speight, 1992). 

The presence of resins in oil prevents the precipitation of asphaltenes by keeping the same particles in colloidal suspension. When a solvent is added to oil, resins are dissolved in the liquid, leaving active areas of asphaltene particles, which allow the aggregation of the same and, consequently, precipitation (Andersen and Speight, 1999). 

It should be noted that during the process of purification and precipitation of asphaltenes due to the existence of clusters, always exists the possibility of a certain amount of resin precipitate with the asphaltenes. 

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