Lubricant Production Processes: Solvent Extraction x Hydro-Refining Route

Like others petroleum derivates, the economic and technology development have been required the production of lubricating oils with higher quality and performance, moreover with lower contaminants content. 

The main quality requirements for lubricating oils are viscosity, flash point, viscosity index (viscosity change with temperature), fluidity point, chemical stability, and volatility. 

According to American Petroleum Institute (API), the lubricating base oils can be classified like described in Table 1. 

Table 1 – Lubricating Base Oils Classification

The lube oils from groups II, III and IV have higher quality than base oils from the group I, the content of contaminants like sulfur and unsaturated compounds are significantly reduced, moreover, the viscosity index are superior for groups II, III, and IV.

The first step in the lubricant production process is vacuum distillation of atmospheric residue obtained like bottom product in the atmospheric distillation processes. For vacuum distillation units dedicated to producing lubricating fractions the fractionating need a better control than in the units dedicated to producing gasoils to fuels conversion, the objective is to avoid the thermal degradation and to control distillation curve of the side streams, a typical arrangement for vacuum distillation unit to produce lubricating fractions is presented in Figure 1. A secondary vacuum distillation column is necessary when is desired to separate the heavy neutral oil stream from vacuum residue. 

Figure 1 – Typical Arrangement for Vacuum Distillation Process to Lubricating Oil Production

In lubricating production units based on the solvent route the following steps are basically physical separation processes with the objective to remove from the process streams the components which can prejudice the desired properties of base oils, mainly the viscosity index, and chemical stability.  

Figure 2 shows a block diagram corresponding to the process steps to produce base lubricating oils through solvent extraction route.  

Figure 2 – Processing Scheme for Base Lubricating Oil Production through Solvent Route 

Like aforementioned in the vacuum distillation step, the fractionating quality obtained between the cuts is critical for these streams reach the quality requirements like flash point and viscosity. After vacuum distillation step the side cuts are pumped to aromatic extraction unit and the vacuum residue is sent to propane deasphalting unit.        

The Propane deasphalting process seeks to remove from vacuum residue the heavier fractions which can be applied as lubricating oil.  The Propane deasphalting units dedicated to producing lubricating oils apply pure propane like solvent because this solvent has higher selectivity to remove resins and asphaltenes from deasphalted oil.  

In the aromatic extraction step, the process streams are put in contact with solvents selective to remove aromatics compounds, mainly polyaromatics. The main objective in remove these compounds is the fact that they have low viscosity index and low chemical stability, this is strongly undesired in lubricating oils. As the nitrogen and sulfur compounds are normally present in the polyaromatic structures, in this step the major part of sulfur and nitrogen content of the process stream is removed. The solvents normally applied in the aromatics extraction process are phenol, furfural, and N-methyl pyrrolidone.

The subsequent step is to remove the linear paraffins with high molecular weight through solvent extraction. This step is important because these compounds prejudice the lubricating oils flow at low temperatures, a typical solvent employed in the solvent dewaxing units is the Methyl-Isobutyl-Ketone (MIK), but some process plants apply toluene and/or methyl ethylketone for this purpose.  

After paraffins removing, the lubricating oil is sent to the finishing process, in this step are removed heteroatom’s compounds (oxygen, sulfur, and nitrogen), these compounds can give color and chemical instability for the lube oil, furthermore are removed some remaining polyaromatic molecules. Some process plants with low investment and processing capacity apply a clay treatment in this step, however, modern plants and with higher processing capacity use mild hydrotreating units, this is especially important when the petroleum processed have higher contaminants content, in this case, the Clay bed saturates very quickly.  

The paraffins removed from lubricating oils are treated to removing the oil excess in the unit called wax deoiling unit, in this step, the process stream is submitted to reduced temperatures to remove the low branched paraffins which have low melting point. Like the lubricating oils, the subsequent step is a finishing process to remove heteroatoms (N,S,O) and to saturate polyaromatic compounds, in the paraffins case generally, is applied a hydrotreating process with sufficient severity to saturate the aromatic compounds that can allow to reaching the food grade in the final product.

As cited earlier, the solvent route is capable of producing group I lubricating oil, however, lube oils employed in severe work conditions (large temperature variation) need be had higher saturated compounds content and higher viscosity index, in this case, is necessary apply the hydro-refining route.   

In the lubricating oil production by hydro-refining, the physical processes are substituted by catalytic processes, basically hydrotreating processes. Figure 3 shows a block diagram of the processing sequence to produce base lube oils through hydro-refining route. 

Figure 3 - Processing Scheme for Base Lubricating Oil Production through Hydro-refining Route 

In this case the fractionating in the vacuum distillation step have more flexibility than in the solvent route, once that the streams will be cracked in the hydrocracking unit, so another distillation step is necessary.

After the vacuum distillation and propane deasphalting steps, the process streams are sent to a hydrotreating unit, this step seeks to saturate polyaromatic compounds and remove contaminants like sulfur and mainly nitrogen which is a strong deactivation agent for the hydrocracking catalyst. 

In the hydrocracking step, the feed stream is cracked under controlled conditions and chemical reactions like dehydrocyclization and aromatics saturation occur which give to the process stream the adequate characteristics to the application as lubricants. 

The following step, hydroisomerization, seeks to promote isomerization of linear paraffins (which can reduce de viscosity index) producing branched paraffins. 

After the hydroisomerization the process stream is pumped to hydrofinishing units to saturate remaining polyaromatic compounds and to remove heteroatoms, in the hydrofininshing step the water content in the lube oil is controlled to avoid turbidity in the final product.

Comparing the lubricant production routes can be observed that the hydro-refining route gives more flexibility in a relation of the petroleum to be processed. As the solvent route apply basically physical processes, is necessary to select crude oils with higher content of paraffins and low contaminants content (mainly nitrogen) to the processing, which can be a critical disadvantage in geopolitical instability scenario. The main disadvantage of the solvent route, when compared with the hydro-refining route, is that the solvent route can produce only group I lubricating oil, this can limit his application to restricted consumer markets, which can reflect in the economic viability.

Another solvent route disadvantage is the solvents applying which can cause environmental damage and needs specials security requirements during the processing, production of low value-added streams like aromatic extract is another disadvantage. 

As advantages of the solvent route over the hydro-refining route can be cited lower capital investment and the fact that the solvent route produces paraffins which can be directed to the consumer market like products with higher value-added. 

Read original by Dr. Marcio Wagner da Silva

Process Engineer and Project Manager at Petrobras