Refining Process of Crude Oil

Crude oil is extracted using different technologies according to the geology and location of the oil field. The design of the extraction plant depends on many factors, but the most important ones are the porosity of the rocks and the sediments’ viscosity. The efficiency of oil extraction reaches significant values due to the application of modern methods with chemical, physical, and thermal principles. The following methods are widely implemented in the industry, and their combined usage is mandatory to acquire the highest extraction efficiency of the plant.

1 Crude Oil Extraction Methods

The primary recovery method represents the first step of crude oil extraction. The oil stored underneath the surface is under high pressure, while atmospheric pressure is around 1 bar. Consequently, the difference between atmospheric and oil pressure represents the driving force for the oil lifting process. The space within the hollow well shaft is at the atmospheric pressure. The oil enters the shaft from the surrounding area and starts to flow upwards. While the oil travels upwards, the pressure gradient drops after some time. When the pressure of oil becomes equal to the atmospheric pressure, the driving force of the oil flow vanishes, and the extraction process stops. At that point, the pump usage is beneficial to generate the pressure difference. The pump pushes the gas underneath the oil. At that point, the gas pushes the oil up to the surface. The efficiency of the primary recovery is in the range from 5 to 10%.

The secondary recovery extraction method requires energy input to extract an additional amount of oil. Namely, the various types of fluids are injected in the reservoirs to produce higher pressures. Two primary types of secondary recovery methods are water injection and natural gas injection. The water injection method uses water created and separated from the oil in the primary recovery phase. That water is injected back underneath the oil to push oil to the surface. When recovered water ceases to be useful, it is disposed of. In that way, the water usage is optimized.

The gas injection method implies the usage of miscible or immiscible gases. The miscible gasses are injected to dissolve carbon dioxide and light hydrocarbon gasses in the oil, to lower its viscosity and resistance to flow. The usage of the miscible gases is highly desirable during the extraction of heavy crude oils. The reduction of oils viscosity and flow resistance significantly reduces the operational costs of the drilling process. On the other side, immiscible gases do not contribute to the dissolution of any fraction of crude oil. Their primary purpose is to increase the pressure in the ”gas cap”, and drive more crude oil to the wellbore.

2 Transportation

The refineries are regularly placed far away from the oil field. There are various transportation options from the oil field to the refinery. The crude oil may be transported via rail cars, trucks, and tanker vessels and through pipelines. The suitable transportation type depends on the distance between the oil field and the refinery and the particular amount of crude oil that needs to be transported. The crude oil doesn’t represent a flammable substance, but it can catch fire, which may present a serious environmental issue. The crude oil is extremely toxic, which is another reason its handling is regulated by specialized transportation and storage policies.

3 Storage of the Crude Oil

Whichever type of transport is used, the crude oil has to be stored in an external roof tank, near the refinery. This type of tank possesses extraordinary dimensions, and it can accumulate vast quantities of crude oil. Unlike the fixed roof tank, this type of tank has a unique type of roof construction. It has a cylindrical shape with a fixed shell. The roof is made out of steel, and the roof position is adjustable. The roof consistently follows the level of the liquid in the tank by moving up and down. Therefore, the roof is in constant contact with crude oil, and it prevents the evaporation process. The prevention of breathing loss contributes to an increase in the overall system’s efficiency and the inability of the diffusion of crude oil gasses into the atmosphere. That is exceptionally significant regarding the high toxicity of crude oil gas and its impact on the environment. Furthermore, the rim evaporation is disabled due to the application of the sealed systems between the tank shell and the roof.

4 Crude Oil Desalting Process

4.1. The composition of present salts in the crude oil and their harmful effects

The crude oil must be pretreated before the distillation process. The oil contains many impurities from which the salts represent the most dangerous ones. The present salts may be hazardous in further processes by the following mechanisms. Firstly, the calcium and magnesium carbonates are formed as the scale at places where the liquid-vapor phase transformation of water occurs. Secondly, highly corrosive hydrochloric acid is produced by magnesium and calcium chlorides at high temperatures. Thirdly, sodium, arsenic, cadmium, lead, and other metals may poison the catalyst in various equipment. Lastly, removing some solids like arsenic is crucial for the absence of toxic substances in the flue gas. In that way, the ecological standards of the flue gas composure are satisfied. Hence, the desalting process has to be utilized before the separation process.

The present salts are primarily in the form of magnesium, calcium, and sodium chlorides, and sodium chloride, represent the most abundant one. The salts are not dissolved in the oil itself, but preferably in the emulsified water droplets. Also, the salts are found in the form of the crystalized and the non-dissolved solids.

4.2. Process description of crude oil desalting

Two types of desalting processes are presented below. The sole difference between them represents the number of used electrostatic reactors. It is the standard industrial praxis to use, whether one or two electrostatic reactors. The exact number of electrostatic reactors depends on salt concentration and the electrostatic reactor’s design. Some companies designed modified single-stage reactors that are more efficient than traditional two stages electrolytic reactors.

The first step of this process is not demonstrated at the diagram, which is the heating of the crude oil to a temperature of 100-150 °C. The freshwater presents another inlet stream in this process, and its volume flow represents the 4-10% of the crude oil volume flow. The third stream is composed of the demulsifying agent.

The demulsifying agent is a compound used to break oil/water emulsion into separate phases. The primary obstacle for the differentiation of phases in the emulsion is the oil film’s existence around the water droplets. The purpose of the demulsifying agent upon addition is to reach the oil/water interface and deteriorate the rigid film. After the formation of torn parts of the rigid film, the water droplets demonstrate a higher tendency to coalesce and ultimately constitute the separate phases. Many parameters must be taken into account during the design of this process, like temperature, type of crude oil, and droplet size. For complete phase separation, it is equally crucial to reach the proper mixing of the three streams. For mixture purposes, the mixing valve is frequently used. This valve is a regular globe valve, which provokes a pressure drop in the range from 0.7 to 3.5 bars. The pressure drop causes the appearance of large shear stress between separate layers of the fluid. In this particular situation, the most important manifestation of the enhanced shear stress is between the water droplet and the oil. The increase of shear stress produces the turbulence flow of the distinct parts of the mixture. This phenomenon forces interaction between water droplets.

Besides the usage of the globe valve, the spray nozzle is frequently used for water injection purposes. The spray nozzle transports the additional water in the form of droplets into the crude oils. The formation of droplets increases the contact area between phases, which enhances the process of mixing. On the other side, the flow of the water and the nozzle diameter must be appropriately designed to prevent the appearance of the small-sized droplets. In that case, the emulsion would be stabilized, and the efficient separation of phases would be disabled. Hence, it is crucial to properly design the whole mixing process and carry out an optimal decision regarding these two opposite effects.

After the mixing process, the mixture flows into the electrostatic separator. This separator is a horizontal cylindrical tank with a large volume. The large size of the tank provides the ability for a residence time sufficient to separate the inlet mixture into two distinct phases. The water droplets with insufficient diameter do not possess enough mass to be divided by the influence of the gravity force. Therefore, two electrodes are posted in this tank, and they are inducing the electric field between them. The water molecules are highly dipolar in contrary to other organic compounds present in crude oil. Hence, the electric field induces the attraction between the water molecules in the nearby droplets, promoting coalescence. Both alternative and direct current may be used to produce an electric field with a potential range from 12, 000 to 35, 000 volts. Under the influence of an electric field, the water droplets’ attraction force is formulated as follows:Where ε is voltage gradient, d is droplet diameter, s is the distance between drops centers, and Ks is a constant for the particular emulsion system. After coalescence of water droplets into larger ones, their settling under the influence of gravity may be described by the Stock law:

Where:

More than 95% of salts and water is removed from the crude oil during the desalting process. The optimally designed processes remove up to 99% of salts.

5 Atmospheric Distillation of Crude Oil

Distillation of crude oil is commonly executed in the following sequence. The distillation under atmospheric pressure comes first, and vacuum distillation comes after it. The crude oil obtained from the desalter passes through the furnace heated up to 400°C. Natural gas is frequently used as a fuel because it presents the refining process’s end product, and its usage reduces the heating costs. The low boiling fractions usually vaporize below 400°C at atmospheric pressure. It is crucial to maintain the temperature below this level to avoid the thermal decomposition or so-called cracking of the hydrocarbons.

The distillation column possesses many trays along the vertical axis, and there is a temperature gradient from the bottom to the top of the column. The various fractions of the crude oil are separated at different heights of the column, as it is presented in Figure 3. The most elevated temperature occurs at the bottom of the column, and its value decreases as the height of the column increases. Three prime factors contribute to the formation of the temperature gradient along the column. The steam at elevated temperature enters the column at the lowest tray. The saturated steam carries a lot of energy, and it transfers heat to the liquid fraction of oil. On the other side, the gas at the top is condensed in the reflux drum and moved back to the highest tray in the column. Therefore, the reflux stream contributes to the cooling of the upper section of the column. The last factor that affects the temperature distribution is the change of mixture composition along the column. Specifically, the volatile compounds are more abundant in the higher sections, and their evaporation temperatures are lesser than the one of the heavier compounds.

5.1 Importance of the steam in the distillation process

The water steam is another inlet stream in the distillation column. The steam is in the form of the saturated or reboiled steam. It enters the column in a temperature range from 90 to 120 °C, and a pressure range from 1 to 2 bars. Steam distillation is a complex process to control due to many manipulative variables of the process and the number of present compounds in crude oil.

The primary role of the steam is to carry the volatile compounds through the column. The steam entrance in the vapor phase causes a decrease in the partial pressures of all compounds in the oil. The reduction of vapor pressure causes a drop in the volatile compounds boiling point. In that way, the volatile compounds evaporate at the lower temperatures. It is exceptionally significant to mention that this process is only feasible if compounds possess higher vapor pressure than water, which is the case with the crude oil compounds.

5.2 Separation of the crude oil fractions

Content and temperatures of the outlet fractions:

1. The residue is composed of lubricating oil, paraffin wax, and asphalt. Temperature:400°C
2. Atmospheric oil gas. Temperature:370°C
3. Diesel gas. Temperature:300°C
4. Temperature:200°C
5. Heavy naphtha. Temperature:150°C
6. Medium naphtha and above. Temperature:20°C

The thermodynamical equilibrium between the vapor and liquid phase is reached in every tray. The change of the molar ratio of compounds, during the distillation process, in both phases is shown in the graph below. The compound’s molar ratio rises as the temperature of the mixture approach the boiling point of the compound.

The crude oil and steam enter the first tray, after which the equilibrium between the gas and liquid phase is reached. The temperature is around 400°C, which causes all compounds besides the residue to evaporate, while the residue travels down the column in the form of the liquid. With the increase of the column’s height, the molar ratios of the heavier fractions decrease, while the composure of volatile compounds increases.

Let’s take, for example, the tray where the kerosene is separated. The vapor phase, mostly composed of kerosene and all fractions of naphtha, is coming to that tray. At that particular tray, the thermodynamical equilibrium is reached, and most of the kerosene is converted into the liquid phase due to the appropriate temperature, while other compounds continue to flow upwards in the vapor phase. The outlet stream, mostly composed of the kerosene, exits out of the column in the liquid phase.

That liquid stream enters the vessel where the steam stripping occurs. Namely, the liquid stream enters at the top of the vessel, while steam enters at the bottom and flows upwards. The steam retains a role to sustain the appropriate temperature, cause evaporation of the residual volatile compounds, and transport them back to the distillation column. In that way, the clean kerosene is obtained in the liquid phase at the bottom of the vessel.

Analogically, all other fractions are separated by the usage of the steam stripping.

6 Vacuum Distillation

The residue from the atmospheric distillation column travels to the vacuum distillation column. The primary purpose of the vacuum distillation is the separation of the residue fraction without affecting its structure. The temperatures higher than 400°C would cause the decomposition of the hydrocarbons. Accordingly, the invention of the vacuum contributes to the separation process. The evaporation of any compound occurs when its vapor pressure is equal to the pressure of its surroundings. The vapor pressure of the compound in the liquid phase rises, with the increase in temperature, as presented in Figure 5. The decreased pressure in the column enables the evaporation of the compounds at significantly lower temperatures.

The vacuum distillation column usually operates at 0.03bar pressure to enable evaporation of the heavy hydrocarbon with a boiling point higher than 450 degrees Celsius. The vacuum distillation may be designed to operate with or without the presence of the steam. The regular distillation is commonly used for gas oil and fuel production.  The incorporation of stream enables the same separation efficiency, but it significantly reduces the operational temperatures. The steam stripping is widely used in the production of gas oil, lube oils, asphalt, or waxes.

6.1 Design of the vacuum column, vacuum generation, and products

Vacuum columns posses greater diameters in comparison to the ones of atmospheric distillation columns.  The vacuum may be generated in two following ways. Firstly, the usage of steam ejectors at the top of the column. The pressure drops in the throat of the ejector. That drop causes vapor suction, resulting in a vacuum generation. Secondly, the vacuum pump can be used at the top of the column.

The products from vacuum distillation are light vacuum gas oil, heavy vacuum gas oil, asphalt, and vacuum residue.

7 Formation of Asphalt and Petroleum Coke from Vacuum Residue

Different types of asphalt can be produced depending on the crude oil composure. The asphalt is a highly viscous liquid, and the addition of additives seriously affects that feature as well as other asphalt characteristics. The additives are a type of emulsification agent, and the most abundant substances in these agents are fatty amines. The contribution of the fatty amines is up to 25% of emulsifying agents’ content. Another important role of the fatty amines is the enhancement of the asphalt binding to the crushed rock’s surface.  The de-asphalting unit is not presented in figure 2, but it is very important for producing pure and high-quality asphalt.  The vacuum residue is treated with propane or butane in a supercritical phase to absorb the residual lighter molecules. The lighter molecules are transferred to the top of the column with the propane or butane, while purified asphalt is extracted at the bottom.

The essential difference between the asphalt and the petroleum coke is their structure. Namely, the asphalt has an aromatic structure while the coke has an aliphatic one. To convert the vacuum residue into coke, there is a need for a thermal cracking process.

The essential process, in the petroleum coke formation, is delayed cooking. In contrary to all previously described processes that are continuous, this process is batch continuous. The vacuum residue flows continuously through the furnace, heated to 480°C, which is sufficient temperature for the thermal cracking.  The cracking of the hydrocarbons takes place in the drum. The cocker naphtha and the cocker gas oils are produced from the vapor phase, while the bottom outlet stream is composed of the coke.

8 Hydrotreatment

The primary purpose of the hydrotreating process is the removal of sulfur and nitrogen compounds. The naphtha hydrotreating unit commonly uses a cobalt-molybdenum catalyst to remove sulfur by converting it to hydrogen sulfide. The unreacted hydrogen is removed along with unreacted hydrogen. The part of the hydrogen sulfide-hydrogen stream is recycled back to the reactor to transfer the unreacted hydrogen, using a compressor.

Reactor conditions for the naphtha hydrotreating process are temperature from 205 to 260˚C and pressure from 25 to 45 bar.  The coke precipitates around the walls blocking the catalyst, so reactor temperature must be raised to maintain the desired process efficiency.  At the moment, when the temperature in the reactor reaches 400 ˚C, the catalyst must be recovered.

9 Catalytic Reformer and Isomerization Units

Heavy naphtha is mostly composed of the C7-C10 hydrocarbons. Catalytic reforming represents the process of C7–C10 hydrocarbons conversion to aromatic compounds and isoparaffins. The main goal of catalytic reforming is to increase the octane number of the heavy naphtha. The increase of the octane number means greater power of the engine and the higher fuel quality.

The reduction of the nitrogen and sulfur below 1ppm is mandatory for the successful catalytic reforming process. That is why the hydrotreating process must be monitored continuously, and the catalyst in that process must be recovered promptly. The catalytic reforming process is endothermic, and it requires extensive energy input.

The light naphtha is mainly composed of the C4-C7 hydrocarbons. The isomerization of those hydrocarbons presents the process of their transformation into branched chains.

The isomerization and catalytic reforming have similar working conditions, and both reactions require the excessive inlet of the hydrogen stream. Regarding the naphtha’s content, the reaction conditions are in the following range: temperature from 495 to 525 °C and pressure from 5 to 45 bars.

The following reactions take place in these two processes:

1. 1. The dehydrogenation of naphthenes into aromatics
   2. The isomerization of normal paraffin to isoparaffins
   3. The dehydrogenation and aromatization of paraffin to aromatics

1. The hydrocracking of paraffin into smaller molecules

Conversion of naphthenes to aromatics and isomerization of n-paraffins to i-paraffins are two most important reactions of interest. With proper control of the working parameters, aromatics in the feed and those produced in the reactor should remain its structure.

The by-product of the reformation reaction is hydrogen. It is essential to mention that the dehydrogenation catalyst can also catalyze the hydrogenation and hydrocracking. Noticing reactions 1 and 3, the excessive amount of hydrogen moves equilibrium to the left side by Le Chatelier’s principle. Therefore, the kinetics effect is not favorable in this case, and the thermodynamics effect must compensate it and ensure avoidance of potential side reactions.