Oil refineries receive well production products as feedstock. These are mainly oil and gas resources, which are extracted in the form of an emulsion with impurities and mineral salts. Without preliminary purification, such mixtures can harm technological equipment even in the early stages of processing raw materials, therefore, methods of dehydration and desalting of oil are used, which can be compared in terms of effects to filtration.
General principles of dehydration and desalination technologies
A mixture of oil and associated impurities, as a rule, is formed from several types of liquids, which may include solid particles. In the simplest emulsions, the aqueous component is mixed with crude oil in thin drops over the molecular structure. It should be noted that the processes of dehydration and desalination of oil can be associated not only with natural pollution and dilution of the target product in the well and during production. The technology of airlift operation of wells provides for the deliberate dilution of the resource with the aim of extracting it to the surface under downhole pressure. Air or hydrocarbon gases can act as active raising media; therefore, further refining of oil is an obligatory technological measure of resource preparation. Another thing is that the low oxygen content with an airlift technique facilitates the separation of raw materials.
The most common application of oil refining technologies is associated with the separation of salt and water at the molecular level. In particular, the influence of an electrostatic field created by electrodes with transformer power supply under a voltage of 12-25 kV can be attributed to the simplest technologies for oil desalination. An electrostatic field causes water molecules to move, collide, and stick together. As the volume of liquid accumulates, it becomes possible to sediment it, followed by separation from the oil phase. This is one of the general principles of the dehydration and desalination methods, but technologies are also widely used, involving the addition of various active components that accelerate and optimize separation processes.
Crude oil and its characteristics
Natural crude emulsifiers with dispersed impurities and mineralized chlorides are also present in crude oil. In some cases, depending on the well development technology, gas components — volatile and inorganic — can also be stored. All these components are active and can be considered as mandatory for preservation or undesirable - their status is determined by the requirements for the final product and at the stages of processing determines the list of acceptable methods of dehydration and desalination of oil, which will also affect the choice of equipment for oil refineries. That is, even some of the useful components can harm technological units, therefore, at certain stages of processing they are also excluded, and then introduced back.
The dehydration process is considered one of the basic. It is realized through the destruction of the water-oil medium with the connection of demulsifiers, which, during adsorption at the interface of the phase separation, exfoliate liquid droplets in the oil. As the active component, a composition should be used, which in itself will easily be separated from the target product. For example, demulsifiers used for dehydration and desalting of oil do not affect the properties of the raw materials to be purified and do not react with water. These are synthesized compounds that are also inert to equipment and environmentally friendly. Demulsifiers from the group of oil-soluble easily mix with oil-containing emulsions and at the same time are poorly washed with water. There are organic non-electrolyte demulsifiers, the features of which include the dissolving function with respect to oil emulsifiers. As a result of chemical exposure, the viscosity of the feed is also reduced.
Justification for the need for oil desalination
The appropriateness of the task of reducing the concentration of salts in crude oil goes far beyond the damage that corrosion processes cause to equipment. It should be borne in mind that in production processes and the supply of transport infrastructure, oil products with certain sets of physico-chemical properties established by strict regulations are used. Therefore, oil desalination is, in principle, a completely rational procedure - it’s another matter that different technologies can be used to accomplish this task, not to mention the differences in the degree of concentration reduction. For example, in areas where water conservation is planned, a two-stage desalination process may be introduced.
What methods vary approaches to the regulation of salt content? It depends on the basic technique. So, in electrical methods, the current parameters will matter, and in the framework of chemical treatment, a wide array of active substances are used to dehydrate and desalinate oil, which initially affect the content of certain elements in different ways. Mostly these are the same chemicals from the general group of demulsifiers that are introduced into the emulsion under certain conditions. For example, to ensure tight mixing of a substance with an oil-containing raw material, it should be directed upstream at a standard distance from the wash tank or separation zone.
Oil heating
One of the preparatory measures, the purpose of which is to create a sufficient temperature regime for the effective implementation of the desalination process. What is it for? Heating has two basic tasks:
- At high temperatures, water particles move at a higher speed, which makes the process of fusion of molecules into a single structure more active. Accordingly, the process of desalination of oil increases, from which large water compounds are removed.
- A decrease in viscosity is also a consequence of temperature regulation. Viscosity as such indicates the ability of a fluid to resist flow. If this indicator decreases, then foreign components are more easily excreted, since they are counteracted by lesser obstacle force.
But which temperature regime will be optimal for the oil emulsion in terms of a positive effect on further separation processes? A specific indicator is established taking into account the characteristics of a particular sample. For example, moderate light temperatures are used for light, low-viscosity emulsions in order to prevent boiling of the oil phase, and for hydrocarbon heavy mixtures, it makes sense to raise the bar of thermal exposure. In most cases, the optimum temperature for desalination is taken from the heating temperature from 100 to 120 ° C. Increased is considered to be up to 140 ° C.
Chemical processing of oil
Processing or destruction of the emulsion structure in this way also requires special preparation. In particular, chemical methods of dehydration and desalination of oil are performed subject to the following physical conditions:
- To ensure contact between the oil component and the active substance, the interfacial film must be previously destroyed. This will allow the demulsifier to be added to the emulsion for the further process.
- A sufficient number of collisions of dispersed particles of water over a certain period of time should be provided. In other words, by stirring or by rotating the contents of the emulsion, the activity of destabilized water particles is artificially increased.
- The settling time is sustained, during which large water particles form a precipitate against the background of coagulation.
From this moment, you can begin to prepare the emulsion for the process of desalination of oil by heating. All the positive properties of increasing the temperature of the oil phase act in the chemical separation method, but it is important to take into account the limitations, since an excessive increase in temperature can lead to negative consequences. In some separation plants, when the temperature is incorrectly estimated, oil evaporates against the background of a decrease in the density of the substance and loss of volume. In order to prevent such effects, many enterprises use lower heating temperatures for safety reasons. To compensate for the lack of thermal energy, a larger volume of demulsifier and equipment with higher power are used.
Electrodehydrators for desalting oil
In the simplest schemes for the implementation of electromechanical processes for separating salt and water from a petroleum product, electric dehydrators are used. This is a multifunctional equipment that performs several phased tasks, including heating, electrical exposure, separation and sedimentation tank. Horizontal electric dehydrators for dehydration and desalting of oil are based on a tank in which one or two-stage separation processes take place. Models with a heating function (thermal separators) also contain a container at the heart of the design, but supplemented by an inlet heating section.
Electromechanical dehydrators are designed with coalescing units, electrostatic lattices and the same heating equipment. A distinctive feature of this modification can be called the implementation of coalescing devices designed to work with phases in the liquid / liquid format. This type of electric dehydrator for oil desalination is used in the maintenance of problematic emulsions.
In the general technology for the use of electromechanical dehydrators, the final step is the precipitation procedure. Within its framework, a divided oil flow is serviced, during its movement gas evolution is provided and temperature indicators are normalized.
The principle of the electric dehydrator
When the crude oil component enters the electric field, water molecules with a negative charge begin to move, taking the pear-shaped shape of the droplets facing the positive electrode. On the path to striving for the latter, droplets collide and form a large fraction, ready for further precipitation and separation. The difficulty lies in the fact that one cycle of processing the emulsion will not be enough to separate water and salt. Although salts naturally dissolve in the aquatic environment, it is impossible to completely remove them at high concentrations. For more effective cleaning, fresh water can be added to the mixture, which will wash out the salt part after several cycles of electrical exposure. Along with electric processing, the oil desalination unit with a dehydrator produces sedimentation (the function of a sump). For this, optional equipment is used, which can have different shapes, dimensions and auxiliary tools for process control.
Although electric dehydrators are technologically sophisticated and expensive equipment, they are increasingly being used not only by large, but also small refineries. This demand is due to the following advantages of the units:
- Saving. As practice shows, both for the cost of consumables and energy consumption, electric dehydrators are the most profitable solution for oil separation in their class.
- Ergonomics. This is a relatively new equipment, so its design in the first generations was developed with an emphasis on modern forms of control with automation and electronic control panels.
- Processing quality. A well-thought-out design system, coupled with the wide possibilities of using chemical catalysts, ensures practically laboratory quality of oil preparation for a wide variety of technological processes in critical industries.
- High degree of equipment reliability. The structure includes protective devices with automation, which, according to the laid down algorithms, control technological operations with a slight risk of error. At the same time, staff functions are minimized, and in high-tech versions are replaced by intelligent control systems.
Integrated oil emulsion separation
If electric dehydrators are used specifically for the tasks of separating pure oil from water and salts, industrial separators in the complex realize the function of dividing the emulsion into components. For example, when testing a well, it is necessary to obtain a general analysis of the hard layer in the bottom from the extracted sample. In these measures, oil desalination can be considered as an indirect task along with determining the concentration of iron or magnesium, but the use of the separator does not decrease. The fact is that oil refineries in practice are themselves interested not so much in the point withdrawal of salt from the target product, but in its comprehensive preparation for further use. In this sense, elimination of solid impurities along with dehydration and desalination is only welcome.
High-performance separators also work with providing input-mud and gas sludge. Such plants are used to desalinate water at oil treatment facilities for consuming enterprises with a finite production cycle. That is, the output should be pure commercial oil, the characteristics of which allow it to be used as fuel or other materials. For example, a separator prepares an oil emulsion with characteristics that allow the production of bitumen, lubricants, synthetic rubber, etc. Such a high quality oil is obtained through several processing steps, including gas scrubbers, coagulators, flushing tanks, thermal separators and other functional units in different configurations.
Deep desalination technology
The lack of desalination of the oil emulsion also affects the state of technological equipment and the quality of the final product. Therefore, for demanding manufacturers, processing enterprises produce products that have undergone a deep separation. In this case, oil desalination equipment reduces the amount of salts to 3-5 mg / l. How is this result achieved? Different technologies can be used, but the combined electrothermochemical method is considered optimal.
Achieving high levels of deep separation can be achieved through complex cleaning with the use of diverse methods for the removal of salts in the aquatic environment. In this case, intensive deposition in the washing liquid should be ensured under the strong influence of electric current. As for the chemical method, it is connected in the form of adding active demulsifiers.
Another way to ensure deep desalination is hydromechanical. In this case, chemical and electrical effects are not applied. The emphasis is on the gravitational function, which contributes to the natural separation of the aquatic environment from oil. The desalination unit in this scheme is a cylindrical sump with a capacity of 100 - 150 m3. It provides zones for the separation of fractions in which liquids overflow under pressure up to 1.5 MPa. The temperature range from 120 to 140 ° is also supported, which contributes to the processes of medium separation.
AC-Direct field exposure technology
This method is also called AC / DC field. That is, it is completely based on the electrical effect provided by the rectifier in the transformer. Under conditions of direct current, the electrostatic lattice acquires polarity (negative or positive), which contributes to the movement of water molecules in the direction of the electrode. As a result of the mutual attraction of the molecules to each other, an aqueous layer is formed, which is removed according to the most convenient scheme.
The complexity of using an electrical installation for dehydration and desalting of oil lies in the fact that the process of coalescence of an aqueous medium involves the risks of a short circuit. , - . , . AC-Direct . , .
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At large oil refineries, where the cleaning and separation processes are carried out with raw materials moving in the stream, special units are used on flotation and centrifugal operating principles. The capacities of the UPON in-line desalination unit allow to process up to 500 m3 / h of raw material, providing a salinity level of up to 3 g / m3. However, in order to maintain high separation rates, an appropriate pressure in the oil supply circuit is necessary. For this, separate or integrated compressor units are used. So, the average pressure at the inlet to the processing line is 1.1-1.5 MPa.
Under the conditions of implementing a simplified scheme for single-stage mixing, the emulsion is pre-diluted with water, after which the mixture is sent to the mixing valve and enters the separation unit. By means of a receiving pipeline, a continuous oil desalination unit distributes the prepared solution along the entire length of the separation vessel, which allows efficient separation of fractions. During mechanical separation, electrostatic effects can occur. At the final stage, the already refined oil is discharged into the general circulation channel with a direction to the next technological stage of refining or temporary maintenance. It should be noted that the quality of in-line desalination is rather low due to the exclusion of the function of the sump, however, in some areas, the requirements for high productivity in the preparation of the oil product put processing speed in the first place.
Additional sludge treatment systems
Most dehydrator and separator plants by default provide for the stage of coarse filtration with drainage of the sludge component. This procedure should not be confused with purification from impurities, since sludge is a side effect of oil production and can harm systems for fine purification of raw materials at the first stages of processing. Therefore, even before the desalination of oil, heavy impurities are removed. In this case, sludge is understood as rock sediments, sand and other coarse particles trapped in the emulsion at different stages of operation of the field well.
How is sludge treatment performed? Several removal processes are foreseen, but all of them are based on mechanical filtration methods with drainage and flushing. In industrial installations for dehydration and desalting of oil, a pressure supercharger of 4 Bar or higher is connected to these processes. In rare cases, the sludge is subjected to heat and chemical treatment - this applies to special stable compositions, the drainage treatment of which is ineffective.
Conclusion
The tasks of preparing oil for the main processes of technological processing for subsequent use in the manufacturing sector are solved by various means and methods. Dehydration and desalination technologies perform far from the most important operations of this spectrum, but you cannot do without them. Modern industry is trying to apply more optimized and energy-efficient methods for solving separation problems, which is manifested in the connection of new high-tech plants. In particular, modern generations of devices for dehydration and desalination of oil are actively developing in the direction of increasing functionality and ergonomics. This is evidenced by the emergence of self-regulating transformers and high-precision measuring sensors, allowing you to control all the main parameters of the cleaning process. Security systems are not ignored either. Both in chemical separation methods and in the use of electric dehydrators, insulating and protective means of protection are used both for the equipment itself and for the operators involved in the technological processing of oil.