How to Extract Profitability from Oil Residue Streams – Solvent Deasphalting Technologies

In the last decades, restrictive environmental regulations allies with the technological development of processes and equipment which require petroleum derivates more environmentally friendly and with better performance reduced drastically the consumer market for residue streams. Currently, even bunker oils suffer severe contaminants restriction, mainly related to sulfur content.

In this scenario, process units called bottom barrel, able to improve the quality of crude oil residue streams (Vacuum residue, Gas oils, etc.) or convert them to higher added value products gain strategic importance, mainly in countries that have large heavy crude oil reserves. These process units are fundamental for to comply the environmental and quality regulations, as well as to ensure profitability and competitivity of refiners through raising refining margin.

Available technologies to processing bottom barrel streams involve processes that aim to raise the H/C relation in the molecule, either through reducing the carbon quantity (processes based on carbon rejection) or through hydrogen addition. Technologies that involves hydrogen addition encompass hydrotreating and hydrocracking processes while technologies based on carbon rejection refers to thermal cracking processes like Visbreaking, Delayed Coking and Fluid Coking, catalytic cracking processes like Fluid Catalytic Cracking (FCC) and physical separation processes like Solvent Deasphalting units. 

The typical feedstock for deasphalting units is the residue from vacuum distillation that contains the heavier fractions of the crude oil. The residue stability depends on of equilibrium among resins and asphaltenes, once which they resins solubilize the asphaltenes, keeping a dispersed phase. 

The deasphalting process is based on liquid-liquid extraction operation where is applied light paraffin (propane, butane, pentane, etc.) to promotes resins solubilization inducing the asphaltenes precipitation, that correspond to the heavier fraction of the vacuum residue and concentrate the major part of the contaminants and heteroatoms (nitrogen, sulfur, metals, etc.). The process produces a heavy stream with low contaminants content called deasphalted oil (Extract phase) and a stream poor in the solvent containing the heavier compounds and with high contaminants content, mainly sulfur, nitrogen and metals called asphaltic residue (Raffinate phase). 

Figure 1 shows a basic process flow diagram for a typical process deasphalting unit. 

Figure 1 – Typical Arrangement for a Solvent Deasphalting Process Unit 

The vacuum residue is fed to the extracting tower where occurs the contact with the solvent leading to the saturated compounds solubilization, in the sequence, the mixture solvent/vacuum residue is sent to separation vessels where occurs the separation of asphaltic residue from deasphalted oil, as well as the solvent recovery. 

The choice of solvent employed have fundamental importance to the deasphalting process, solvents that have higher molar mass (higher carbon chain) presents higher solvency power and raise the yield of deasphalted oil, however, these solvents are less selective and the quality of the deasphalted oil is reduced once heavier resins are solubilized which leads to higher quantity of residual carbon in the deasphalted oil, consequently the contaminants content raises too. As normally the deasphalting unit aim to minimize the carbon residue, metals and heteroatoms in the deasphalted oil, propane are the usual solvent applied, mainly when the deasphalting process role in the refining scheme is to prepare feed streams for catalytic conversion processes. 

The main operational variables of the deasphalting process are feedstock quality, solvent composition, the relation solvent/feedstream, extraction temperature and temperature gradient in the extraction tower. Despite being a very important variable the extraction pressure is defined in the unit design step and is normally defined as the need pressure to keep the solvent in the liquid phase, in the propane case the pressure in the extraction tower is close to 40 bar. 

Feedstock quality depends on crude oil characteristics processed by the refinery, as well as vacuum distillation process. Depending on the fractionating produced in the vacuum distillation unit the vacuum residue can be heavier or lighter, affecting directly the deasphalting unit yield. Using propane as solvent the relation solvent/feedstream is close to 8 and the feed temperature in the extraction tower is close to 70 oC.

In refineries focused in fuels production (mainly LPG and gasoline), the deasphalted oil stream is normally sent to the Fluid Catalytic Cracking Unit (FCCU), in this case, the contaminants content and carbon residue needs to be severely controlled to avoid premature deactivation of the catalyst which is very sensitive to metals and nitrogen. In refineries dedicated to producing middle distillates, the deasphalted oil can be directed to hydrocracking units. 

When the deasphalting process is installed in refining units dedicated to producing lubricants, the quality of deasphalted oil tends to be superior in view that the crude oil processed is normally lighter and with lower contaminants content. In this case, the deasphalted oil is directed to aromatic extraction unit or to hydrotreatment/hydrocracking units, in the last case, the deasphalted oil quality is more critical because of the possibility of premature catalyst deactivation.

The asphaltic residue stream is sent to the fuel oil pool after dilution with lighter compounds (gas oils) or the stream can be used to produce asphalt. Another possibility is sent the asphaltic residue to a Delayed Coking Unit. As the aromatics content in the asphaltic residue is high, the coke produced presents a very good quality. 

The principal step in the solvent deasphalting process is the liquid-liquid extraction which depends on strongly of the solvent properties, in this sense, some licensors developed deasphalting processes based on the solvent in supercritical conditions. Above of critical point, the solvent properties are more favorable to the extraction process, mainly solvency power and the vaporization and compression facility, which reduce the power consumer in the process. 

The processes ROSE™ licensed by KBR Company, UOP-DEMEX™ licensed by UOP and the process SOLVAHL™ licensed by AXENS are examples of deasphalting technologies in supercritical conditions. Figure 2 presents a basic process scheme for a typical deasphalting unit under supercritical conditions. 

Figure 2 – Typical arrangement to solvent deasphalting unit under supercritical condition 

In addition to the cited processes, the FOSTER WHEELER Company in partnership with UOP developed the process UOP/FW-SDA™ which applies solvent in supercritical condition too. 

Like described earlier, the deasphalting process allows add value to residual streams as vacuum residue and, consequently, raise the refiners profitability furthermore the process can help in the production of higher quality and cleaner derivates.

As another residue upgrading technologies, the deasphalting process raises the refinery flexibility regarding the quality of crude oil processed, that can pass to process heavier crude oils that have normally lower cost, and this fact can improve the refining margin. 

Currently, the deasphalting technology has lost ground in the more modern refining schemes to Delayed Coking units since these units can process residual streams producing streams that can be converted into products with high added value (LPG, Gasoline, and Diesel), without the need of previous feed stream treatment to removal contaminants. However, the products from delayed coking units need hydrotreatment to be commercialized which raises significantly the operational and installation costs to the refinery. In some refining schemes, the deasphalting and delayed coking units can be complementary technologies, like aforementioned.

The choice of residue upgrading technology by the refiners normally involves an economic analysis which takes into account the refinery production focus (middle distillates, light products or lubricants), the market that will be served and the synergy among the processes that will be applied in the adopted refining scheme.

The process flow diagrams were produced based on information available in:

Encyclopaedia of Hydrocarbons, Volume II – Refining and Petrochemicals (2006). 
Read original by Dr.Marcio Wagner Da Silva