Automation in Crude Oil Distillation Process

Crude oil distillation process – Dr Marcio Wagner da Silva explains how the automation and control strategies can guarantee the profitability of the downstream industry.


  • Client

    Crude Oil Refining Industry

  • Services

    Industial Automation

  • Technologies

    Industrial automation

  • Dates




Despite the efforts to reduce the consumption, petroleum still represents the major part of the global energy matrix and has a strategic role in any nation’s desire to reach superior economic and technological development.


Crude oil, as found in the reservoirs, has few industrial uses. To become useful and economically viable, it is necessary to separate the fractions in products that have specific industrial interests like fuels (LPG, gasoline, jet fuel, diesel, etc), lubricants or petrochemical intermediates. To achieve this objective the crude oil is submitted to a series of physical and chemical processes aimed at adding value to the commodity, to the set of processes we called refining complex.


Process Arrangement

In the refining complex, the first and principal process applied to add value to the crude oil is distillation. This processing unit defines the processing capacity of the refinery and normally the other process units are sized on the basis of this yield.  Figure 1 shows a basic process flow diagram for a typical atmospheric crude distillation unit.


Figure 1 – Process Flow Diagram for a Typical Atmospheric Crude Oil Distillation Unit


The crude oil is pumped from the storage tanks and preheated by hot products that leave the unit in heat exchangers battery, then the crude oil stream receives an injection of water aimed to assist the desalting process. This process is necessary to remove the salts dissolved in the petroleum to avoid severe corrosion problems in the process equipment. The desalting process involves application of an electrical field to the crude oil-water mixture meant to raise the water droplets dispersed in the oil phase and accelerate the decanting. As the salt solubility is higher in the aqueous phase the major part of the salts is removed separated from the effluent desalter, called brine. Normally the petroleum desalting process is carried out at temperatures ranging from 120-160°C, higher temperatures raise the conductivity of oil phase and prejudice the phase separation, and this can lead to drag oil to the brine and result in process inefficiency.


In the desalter exit, the desalted oil is heated again by hot products or pumped around and fed into a flash drum. In this equipment the lighter fractions are separated and sent directly to the atmospheric tower, the main role of this vessel is to reduce the thermal duty needed in the furnace. Next, the stream from the bottom of the flash vessel is heated in the fired heater to temperatures close to 350-400°C (depending on the crude oil to be processed) and is fed to the atmospheric tower where the crude oil is fractionated according to the distillation range, like the example presented in Table 1.



Distillation Range (oC)

Gases (C1 – C4)

≤ 30

Light Naphtha (C5 – C7)


Heavy Naphtha (C8 – C11)


Kerosene (C11 – C12)


Light Diesel (C13 – C17)


Heavy Diesel (C18 – C25)


Atmospheric Residue (C25+)



Table 1 – Example of crude oil distillation cuts.


At the exit of the atmospheric tower, the products are rectified with steam in order to remove the lighter components. 


The gaseous fraction is normally directed to the LPG (C3-C4) pool of the refinery and the fuel gas system (C1-C2) where it will feed the furnaces and boilers. The light naphtha is normally commercialised as petrochemical intermediate or is directed to the gasoline pool of the refining complex; the heavy naphtha can be sent to the gasoline pool, and in some cases, this stream can be added to the diesel pool since it does not compromise the specification requirements of this product (Cetane number, density and flash point). Kerosene is normally commercialised as jet-fuel while the atmospheric residue is sent to the vacuum distillation tower. In some refining schemes it is possible to send this stream directly to the residue fluid catalytic process unit (RFCC), in this case, the contaminants content (mainly metals) of the residue need to be very low to protect the catalyst of the cracking unit. 


Nowadays, thanks to the necessity to reduce the environmental impact of the fossil fuels associated with restrictive legislations, the straight run products can be commercialised directly. The streams are normally directed to the hydrotreating units aimed to reduce the contaminants content (sulfur, nitrogen, etc), before being marketed.


In distillation units with higher processing capacity, normally the flash drum upstream of the atmospheric tower is substituted by a pre-fractionation tower. In this cases, the main advantage is the possibility of reduction of the atmospheric tower dimensions that results in cost reduction associated with the unit implementation and improved hydraulic behavior in the distillation tower, consequently with better fractionation. This arrangement is shown in Figure 2.


Figure 2 – Typical arrangement to Atmospheric Distillation with Pre-Fractionation Tower.


Like mentioned above, the residue from atmospheric distillation column is sent to the vacuum distillation tower; this strategy is adopted since under atmospheric column process conditions the continuity of heating leads to the thermal cracking of the residual fractions. In the vacuum distillation column, the atmospheric residue is submitted to reduced pressures with the aim to recover the lighter fractions that can be converted to high-value products.


Figure 3 shows a typical process scheme of vacuum distillation unit focusing on producing fuels. 


Figure 3 – Schematic Process Flow Diagram for Vacuum Distillation


The light vacuum gasoil (LVGO) is normally sent to the hydrotreating units to be incorporated into the diesel pool of the refinery while the heavy vacuum gasoil (HVGO) is directed to catalytic cracking units or hydrocracking. Depending on refining scheme adopted by the refiner, another possibility is to use this stream like dilutant to produce fuel oil. In some process configurations, there is still a withdrawal of the stream called residual vacuum gasoil aimed at keeping the quality of heavy gasoil in relation of carbon residue and metals content to avoid the rapid deactivation of the catalyst of this unit.


The vacuum residue is normally directed to produce asphalt and fuel oils, however, in most modern refineries this stream is sent to bottom barrel units as delayed coking and solvent deasphalting to produce higher-value products. 


The distillation unit design is strongly dependent by the characteristics of crude oil that will be processed by the refinery, for extra-heavy oils normally the crude is fed directly to the vacuum column. The design is generally defined based on a limited crude oil range that can be processed in the hardware (contaminant content, API grade, etc).        


Typical Control and Automation Strategies

To compensate these variations in the raw material, the control and automation system are fundamental to achieve the objectives to meet the derivatives specifications and maximise the profitability in the crude oil distillation process. Nowadays, it’s impossible to think of an efficient crude distillation process without an adequate control system.


The operating pressure is a fundamental variable to the distillation processes, the relative volatility varies according to the pressure and an adequate control leaves a good stability for the separation column. Figure 4 presents a classical strategy to control a distillation column when the distillate flow rate is sufficiently high to permit control the top drum level through this variable.


Figure 4 – Classical Control Strategy for Distillation Columns with high distillation flow rate.


In this case the top pressure is controlled through the action in the flow rate of gases leaving from the top drum. This condition is adequate when the light content in the crude oil is relatively high. The level in the bottom of column is controlled by the bottom product flow rate and the temperature is controlled by the flow rate of steam or another heat supplier to the column. Again, this strategy is adequate when the internal reflux of the distillation column is not much higher when compared with the flow rates withdrawn. The major part of crude oil distillation columns fall into this case. When the composition of side streams is relevant, normally analysers are installed in these streams to control the composition acting in the reflux flow rate.


For separation with high reflux flow rate, normally the hard separations, it is necessary change the control strategy, an example is presented in Figure 5.


Figure 5 – Example of Control Strategy for Distillation Columns to hard separations


In these cases, the composition is very sensitive to the temperature variation. To reduce these variations the bottom level is used to control the bottom temperature through the reboiler and the bottom flow rate is manipulated to control the composition. In the top of column the composition is controlled by the analyzer acting in the reflux flow rate.



The distillation process is the first step to which the crude oil is submitted, the others process units are strongly impacted by the quality of products from the distillation unit, mainly in relation to fractionation quality achieved in the distillation columns. A bad fractionation can lead to off-specification products (colour, sulfur content, corrosivity, etc), or irreversible damages to catalysts or process equipment, thus this unit requires special attention by the refiner. Since every chain of value generation to the crude oil processing depends on this process, the success of the crude distillation process is strongly linked with the adequate control strategy.  



  • Seader, J D; Henley, E J – Separation Process Principles, 2nd edition John Wiley & Sons, 2006

  • Fahim, M; Al-Sahhaf, T; Elkilani, A – Fundamentals of Petroleum Refining. Elsevier, 2009

  • Ogata, K – Modern Control Engineering. 5th edition Prentice Hall, 2010.


Dr Marcio Wagner da Silva

Dr Marcio Wagner da Silva is Process Engineer and Project Manager focusing on Crude Oil Refining Industry based in São José dos Campos, Brazil. A Bachelor in Chemical Engineering from University of Maringa (UEM), Brazil and PhD in Chemical Engineering from University of Campinas (UNICAMP), Brazil, he 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 has an MBA in Project Management from Federal University of Rio de Janeiro (UFRJ) and is certified in Business from Getulio Vargas Foundation (FGV).