Improving the Yield of High-Added Value Derivatives – Crude Oil Vacuum Distillation

To achieve the condition of marketable products and useful to the society, the crude oil needs to pass by processing steps aiming to add value through separation and conversion processes. The entrance gate of the crude oil in a refinery is the crude oil distillation unit that aims to separate the crude into process streams which, after adequate treatment, will be commercialized like derivatives as transportation fuels, petrochemical intermediates, etc. Figure 1 presents a scheme of a typical atmospheric crude oil distillation unit. 

Figure 1 – Typical Atmospheric Crude Oil Distillation Unit 

The bottom stream of the atmospheric column (Atmospheric Residue) still contains recoverable products capable to be converted into high added value derivatives, however, under the process conditions of the atmospheric unit, the additional heating lead to thermal cracking and coke deposition.

           Aiming to minimize this effect, the atmospheric residue is pumped to the vacuum distillation column where the pressure reduction leads to a reduction in the boiling point of the heavy fractions allowing the recovery while minimizing the thermal cracking process. Figure 2 shows a typical process arrangement for a vacuum crude oil distillation unit dedicated to producing intermediates streams to transportation fuels. 

Figure 2 – Vacuum Crude Oil Distillation to Transportation Fuels Production 

The heavy and light gasoil streams are normally directed to conversion units like hydrocracking or fluid catalytic cracking (FCC), according to the adopted refining scheme. The fractionating quality achieved in the crude oil vacuum distillation column has a direct impact upon the reliability and conversion units operation lifecycle, once which in this step is controlled the metals content and the residual carbon (CCR) concentration in the feedstock to these processes, high values of these parameters lead to a quickly catalyst deactivation raising operational costs and reducing profitability. 

           The vacuum generated in the column can be humid, semi-humid and dry. Humid vacuum occurs when is applied steam injection in the fired heater and in the column aiming to reduce the partial pressure of the hydrocarbons improving the recovery while in the semi-humid vacuum the steam is injected only in the fired heater minimizing the residence time reducing the coke deposition. The dry vacuum does not involve the steam injection, in this case, is possible to achieve pressures between 20 to 8 mmHg while in the humid vacuum the column operates under pressures varying between 40 to 80 mmHg, however, it’s possible to achieve comparable yields through the injection of stripping steam. Figure 3 presents a process arrangement for a typical vacuum generation system in a vacuum crude oil distillation unit.

Figure 3 – Process Arrangement for a Typical Vacuum Generation System for a Vacuum Crude Oil Distillation

Some refiners include additional side withdraws in the vacuum distillation column. When the objective is to maximize the diesel production, it’s possible to add a withdraw of a stream lighter than light vacuum gasoil that can be directly added to the diesel pool or after hydrotreating, according to the sulfur content in the processed crude oil. When the crude oil presents high metals content, it’s possible to include a withdraw of fraction heavier than the heavy gasoil called residual gasoil or slop cut, this additional cut concentrates the metals in this stream and reduce the residual carbon in the heavy gasoil, minimizing the deactivation process of the conversion processes catalysts as aforementioned. Normally, the residual gasoil is applied as the diluent to produce asphalt or fuel oil. 

When the refinery is focused to produce lubricants, the vacuum column has better fractionating quality while in the column dedicated to producing fuels the internals are designed mainly to promote the heat exchange between the streams. The better fractionation in the case of lubricants is due to the necessity to produce the lubricants cuts, as presented in Figure 4. 

Figure 4 – Process Arrangement for a Vacuum Column to Produce Lubricants 

Vacuum residue is normally directed to the asphalt production or, in refineries with higher conversion capacity, to bottom barrel conversion units like delayed coking and solvent deasphalting aiming to improve the yield of high added value derivatives.    

According to the refining scheme, the installation of vacuum distillation units can be dispensed. Refiners that rely on residue fluid catalytic cracking units (RFCC) can sent the atmospheric residue directly to feed stream of these units, however, it’s necessary to control the contaminants content (metals, sulfur, nitrogen, etc.) and residual carbon (CCR) aiming to protect the catalyst, this fact restricts the crude oil slate that can be processed, reducing the refiner operational flexibility. On the other hand, in refineries that process extra heavy crudes, normally the crude oil distillation unit is restricted to the vacuum unit once the yields of the atmospheric column would be very low and the coking risk very high.   

The processing of residual streams and the residue upgrading have key role to the economical performance of the downstream industry and this protagonism trends to grow after 2020 due to the start of IMO 2020 that establishes the reduction of the sulfur content in the bunker (Marine Fuel Oil) from the current 3,5 % (m.m) to 0,5 % (m.m), this regulation should restrict the use of high contaminants content streams as diluents to the production of fuel oils like adopted nowadays, this fact would lead to apply high added value streams (diesel, as example) as diluent which can pressure the refiners profitability, in this scenario refineries with higher complexity should have competitive advantage over the competitors. This fact can lead these refiners to carry out capital investments aiming to improve his bottom barrel conversion capacity.

References:

SPEIGHT, J.G. Heavy and Extra-Heavy Oil Upgrading Technologies. 1st ed. Elsevier Press, 2013.

ROBINSON, P.R.; HSU, C.S. Handbook of Petroleum Technology. 1st ed. Springer, 2017.

GARY, J. H.; HANDWERK, G. E. Petroleum Refining – Technology and Economics.4th ed. Marcel Dekker., 2001. 

Dr. Marcio Wagner da Silva is Process Engineer and Project Manager focusing on Crude Oil Refining Industry based in S?o José dos Campos, Brazil. Bachelor in Chemical Engineering from University of Maringa (UEM), Brazil and PhD. in Chemical Engineering from University of Campinas (UNICAMP), Brazil. Has extensive experience in research, design and construction to oil and gas industry including developing and coordinating projects to operational improvements and debottlenecking to bottom barrel units, moreover Dr. Marcio Wagner have MBA in Project Management from Federal University of Rio de Janeiro (UFRJ) and is certified in Business from Getulio Vargas Foundation (FGV).