Lubricating production

Figure 1. Growing trend in the demand for lubricants.¹

Like other crude oil derivatives, economic and technological development has necessitated the production of lubricating oils with higher quality and performance, as well as, importantly, 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 the American Petroleum Institute (API), the lubricating base oils can be classified as shown in Table 1. The lube oils from groups II, III, and IV have higher quality than base oils from group I; the content of contaminants such as sulfur and unsaturated compounds is significantly reduced and, moreover, the viscosity index is superior for these groups.

Table 1.

Lubricant production routes

The first step in the lubricant production process is vacuum distillation of the atmospheric residue obtained, such as bottom product in the atmospheric distillation processes. For vacuum distillation units dedicated to producing lubricating fractions, the fractionating requires greater control than in the units dedicated to producing gasoils to fuels conversion.² The objective is to avoid the thermal degradation and to control the distillation curve of the side streams. A secondary vacuum distillation column is necessary when it is desirable to separate the heavy neutral oil stream from vacuum residue.

In lubricating production units based on the solvent route, the following steps are in essence physical separation processes, with the objective to remove from the process streams the components that 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 the solvent extraction route.

Figure 2. Processing scheme for base lubricating oil production through solvent route.

As previously mentioned in the vacuum distillation step, the fractionating quality obtained between the cuts is critical for these streams to reach the quality requirements such as flash point and viscosity. After the vacuum distillation step, the side cuts are pumped to the aromatic extraction unit and the vacuum residue is sent to the propane deasphalting unit. The propane deasphalting process seeks to remove from vacuum residue the heavier fractions that can be applied as lubricating oil. The propane deasphalting units dedicated to producing lubricating oils apply pure propane such as 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 a solvents selective to remove aromatics compounds, mainly polyaromatics. The main objective in removing these compounds is the fact that they have a low viscosity index and low chemical stability – this is strongly undesirable 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 paraffin with high molecular weight through solvent extraction. This step is important because these compounds prejudice the lubricating oil flow at low temperatures. A typical solvent employed in the solvent dewaxing unit is the Methyl-Isobutyl-Ketone (MIK), but some process plants apply toluene and/or methylethylketone (MEK) for this purpose.

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

The paraffin removed from lubricating oils are treated to remove the excess oil in the unit, called the wax deoiling unit. In this step, the process stream is submitted to reduced temperatures to remove the low branched paraffin, which has a 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 paraffin case, in general, a hydrotreating process is applied with sufficient severity to saturate the aromatic compounds that can enable the food grade in the final product to be reached. As cited earlier, the solvent route is capable of producing group I lubricating oils. However, lube oils employed in severe work conditions (large temperature variation) need be have higher saturated compounds content and a higher viscosity index. In this case, it is necessary to apply the hydrorefining route. A great limitation in the lubricant production by the solvent route is the necessity of paraffinic crude oils, which tends to present higher costs and reduce the operational flexibility of the refiners, especially related to the crude oil supply in the event of a geopolitical crisis.

Despite the relevant strategic questions such as crude oil prices and supply, the Group I lubricating oils tend to lose market quickly due to their poor performance in comparison with the remaining groups, especially considering the growing technology development of the automotive industry.³ This fact is one of the most relevant driving forces to capital investments to improve the refining hardware capacity to produce high quality lubricating oils through the hydrorefining route. Another relevant factor which negatively impacts the competitiveness of refiners relying exclusively on the solvent route is that the Group I and II lubricating oils have lost market ground in recent years, mainly in relation to the technology requirements of the newest automotive engines, Figure 3 presents a forecast of the market share evolution to different kinds of base oils in the market.

Figure 3. Base oils market distribution.⁵

According to the data from Figure 3, a significant reduction in the demand for Group I base oils is expected, leading to a great competitive loss to refiners relying on base oil production exclusively through solvent routes.

The hydrorefining route

In the production of lubricating oil by hydrorefining, the physical processes are substituted for catalytic processes, essentially hydroprocesing processes. Figure 4 shows a block diagram of the processing sequence to produce base lube oils through the hydrorefining route.

Figure 4. Processing scheme for base lubricating oil production through hydrorefining route.

In this case, the fractionating in the vacuum distillation step has more flexibility than in the solvent route, once 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 such as sulfur and, above all, nitrogen, which is a strong deactivation agent for the hydrocracking catalyst.⁴

In the hydrocracking step, the feed stream is cracked under controlled conditions, and chemical reactions such as dehydrocyclisation and aromatics saturation occur, which gives to the process stream the adequate characteristics for their application as lubricants.

The following step, hydroisomerisation, seeks to promote isomerisation of linear paraffin (which can reduce the viscosity index) by producing branched paraffin.

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

When comparing the lubricant production routes, it can be observed that the hydrorefining route gives more flexibility in relation to the petroleum to be processed. As mentioned earlier, as the solvent route applies essentially physical processes, it is necessary to select crude oils with a higher content of paraffin and low contaminants content (mainly nitrogen) to the processing. Another solvent route disadvantage is the application of solvents, which can cause environmental damage and needs special security requirements during processing. Production of low value-added streams, such as aromatic extract, is another disadvantage.

Brazilian lubrication market

The Brazilian domestic market for paraffinic oils is supplied by refineries that apply the solvent route with the hydrofinishing step to produce lubricating oils and waxes to a variety of consumers, including the food and cosmetic industries among others. The country’s lubricating production in 2019 was 3.5 million bbl. Furthermore, the internal market is also supplied by some importers. According to data from the Brazilian Petroleum Agency (ANP), the internal consumption of lubricating oil reached 7.7 million bbl in 2019. Figure 5 shows the composition of the Brazilian lubricating oil market in 2019.

Figure 5. Balance of the Brazilian market of lubricating oils in 2019 (based on ANP data).

In the Brazilian case, a significant part of the market is supplied to recycled lubricating oil. Recycling or re-refining of used lubricating oil meets a double role: elimination of a hazardous residue and reducing the necessity of the extraction of higher quantities of petroleum to produce base lubricating oils. The industrial processes applied to recover the used lubricating oil are called: the acid-clay process (Meiken process), the deasphalting process through wiped film evaporators, and the hydroprocessing route that is capable of producing higher quality base oils (e.g. Groups II and III). Some researchers have dedicated their efforts to the development of new re-refining technologies for treating used lubricating oils, and some of these technologies have shown promise, such as ultrafiltration in membranes. However, the technology is still in the initial stage of development.

In Brazil, close to 40% of the lubricating oil consumption is recovered and sent to processing by re-refiners, according to the ANP. Despite this significant data, the Brazilian production of lubricating oils is concentrated in Group I and II oils.

Conclusion

As has been discussed, despite the high capital investment of the hydroprocessing units, the higher added value of the Groups II and III lubricants and the growing market can justify the investment, particularly when considering the trend of reduction in transportation fuels demand at a global level in the medium-term. This has been leading refiners to look at ways to ensure market share and revenues in the downstream industry through the maximisation of high added value derivatives, with petrochemicals and lubricating oils growing markets. Due to accelerated technological development, especially in the automotive market, the Group I lubricating oil is anticipated to lose market share in the coming years. This is leading refiners to look for capital investments in order to sustain their competitiveness in the lubricating market. Another side effect for lubricating producers based on solvent routes is increasing imports to supply the internal market, leading to an external dependence of critical production input as well as negative effects on the balance of payments.

References

1. BAU, A., BRUNI, G., HUSSIN, L., KIEWELL, D, KOHLER, B., and VERITY, R., ‘Lubes growth opportunities remain despite switch to electric vehicles’, McKinsey & Company, (7 December 2018).
2. GARY, J. H.; HANDWERK, G. E., ‘Petroleum Refining – Technology and Economics’, 4^th edition, Marcel Dekker, (2001).
3. ‘Lubricants Market Size, Share & Trends Analysis Report By Product (Industrial, Automotive, Marine, Aerospace), By Region, And Segment Forecast, 2019 - 2025’, Gran View Research, (2019).
4. ROBINSON, P.R.; HSU, C.S, ‘Handbook of Petroleum Technology’, 1^st edition, Springer, (2017).
5. STATISTA, https://www.statista.com/statistics/547119/projected-base-oil-demand-worldwide-distribution-by-type/

Written by Dr Marcio Wagner da Silva, Petrobras, Brazil.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/13042021/lubricating-production/