The reliability of the process units is fundamental to allow refiners to achieve the desired reliability and keep the competitiveness and the consumer market supply. Operational continuity of a refinery relies on some factors and a strong management system, however, the quality of the raw material (crude oil) is one of the main factors to ensure the reliability and integrity of the refining processes.
Normally, the crude oil that will be processed in the refineries must meet some quality requirements aiming to preserve the separation and conversion process units, mainly the atmospheric distillation unit. The maximum water and sediments content in the crude oil is controlled lower than 1,0 % in volume, other relevant parameters are diluted salt content and the Total Acid Number (TAN), which is defined as the quantity of KOH (Potassium Hydroxide) needed to neutralize one gram of the crude oil.
To achieve these requirements, the crude suffers a series of treatment called “primary treatment” aiming to ensure the cycle life of the downstream and midstream assets. These processes generally are focused to separate water, gas and oil phases still in the upstream assets. Figure 1 shows the basic steps of the crude oil primary treatment through a block diagram.
The crude oil is withdrawn from the reservoir and is carried out the separation between the gas and liquid phases through pressure reduction, in the next step, the liquid phase is pumped to a separator drum to promote the separation of oil and water phases by decantation, in this step only the free water is separated from the oil. As part of the water is emulsified, subsequently, the mixture suffers a new treatment step applying an electrical field, demulsifier adding beyond the heating that aims to reduce the viscosity and allow a better phase separation yield.
Figure 1 – Steps of the Crude Oil Primary Treatment
The water-oil phase separation is carried out in decantation vessels which can be biphasic, when is realized just the separation between gas and liquid (water +oil) phases, or three-phase when occurs the separation of free water from oil additionally. Due the high superficial area, the separation vessels have normally horizontal configuration, however, in upstream units with great production flow rate oscillations and large sediments content the vertical configuration is adopted. Figure 2 presents schematically the separation equipments normally used in upstream assets.
Figure 2 – Separation Vessels Configurations
In the oil-water separation step, the emulsion is broke through the application of a high-intensity electrical field that promotes the water droplets polarization and consequently, decantation. Unlike that occurs in the refineries during the crude desalting process, the electrical treaters used in the upstream assets are low speed, in this case, the emulsion is fed in the bottom and distributed under laminar regime to the internals of the separation vessel.
After the separation step, the water is directed to treatment system, a simplified configuration of a typical water treating unit is presented in Figure 3.
Figure 3 – Oily water treating Process
The brine coming from electrostatic treaters is pumped to degassing vessel to remove dissolved gases, after this step, the oily residue is directed to the tank where occur the phase separation, the aqueous phase is sent to a new treating cycle while the oily phase is pumped to storage. The oily water is directed to a water-oil separation treatment step which normally applies API separators, however, in modern sites, are applied hydrocyclones due to his higher efficiency, after a flotation step the treated water can be directed to disposal or to be reinjected in the reservoir to improve the recovery of crude oil.
The natural gas produced is directed to treatment steps aiming to reduce the humidity content and sour gases removing. The dehydrating process carries out through the absorption process with TEG (Triethylene Glycol), while the sour gases (H2S + CO2) are removed through amine treatment, as presented in Figure 4.
Figure 4 – Basic Process Scheme for a Typical Amine Treating Process Unit
The produced gas stream still suffers treatment steps aiming to remove heavier compounds (C3 to C5+) that are considered condensable in the natural gas. This process consists basically in the controlled refrigeration of the gas to condense the heavier fractions, the processes generally employed are the Joule-Thomson expansion, simple refrigeration, and turbo expansion. The obtained stream have a great added value and can be applied as petrochemical feed stream due to his high paraffin content or, according to the consumer market, be directed to the refineries to improve the yield of LPG and gasoline
As aforementioned, an adequate treatment of the crude oil is fundamental to ensure the reliability and availability of the downstream industry, high salt and water content in the crudes leaves to higher corrosion and deposition rates in the process units, reducing the life cycle and rising operational costs due to unplanned shutdowns. Other assets that suffer strong degradation due to the failures in the primary treatment steps are the storage tanks and pipelines, in this sense, the integration between upstream and downstream systems is a key factor to ensure the sustainability in the crude oil production chain.
When some of the controlled parameters are out of the specification, is necessary the blending of different crudes to keep the feed stream to the crude oil distillation unit under controlled conditions, this fact raises the operational costs related with unnecessary operational handling that could be avoided.
An adequate asset management is a important step in the currently transformation of the downstream industry, the management system need to be based on two driving engines, the first focused in to keep the current operations once they will sustain the planned future and the second focused in innovative actions to ensure the perenniality of the business, this is an important consideration related with the called “digital transformation” this phenomenon is not only related with technology, the technologic advance make possible the easy access to data, but we need a modern and strong management system able to ensure that the right questions will be done to transform these data in information, knowledge and finally in wisdom.
References:
ABDEL-AAL, H.K.; AGGOUR, M.; FAHIM, M.A. Petroleum and Gas Field Processing. 2nd ed. Marcel Dekker, 2003.
GARY, J. H.; HANDWERK, G. E. Petroleum Refining – Technology and Economics.4th ed. Marcel Dekker., 2001.
MOULIJN, J. A.; MAKKEE, M.; VAN-DIEPEN, A. E. Chemical Process Technology. 2nd ed. John Wiley & Sons Ltd., 2013.
Written by
Dr. Marcio Wagner da Silva is Process Engineer and Project Manager focusing on Crude Oil Refining Industry based in São 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).