Increasing the Refining Margin in Oil Residue Streams Processing - Delayed Coking Technologies
Despite the constant search for cleaner fuels and new technologies to minimize crude oil consumption and, consequently the environmental footprint of your value chain, the demand by oil derivates had raised in the last decades. In this sense, technologies which maximize the production of middle and light derivates against fuel oils gained strategic importance in the refining industry.
Process units aimed to upgrade residual streams as RFCC, hydrocracking, deasphalting and delayed coking had been the target of intense studies in an attempt to reduce operational and installation costs and raise the performance of these technologies in the processing of crude oil or residues increasingly heavier.
One of the most applied technologies by the refiners is the Delayed Coking process.
Delayed coking employs the thermal cracking concept under controlled conditions to produce light and middle streams (LPG, naphtha and gas oils) from residual streams which would normally be used as diluents in fuel oils production.
The typical feed stream for delayed coking units is the residue from vacuum distillation process that contains the heavier fractions of processed crude oil, however, streams like decanted oil from FCC unit and asphaltic residue produced in solvent deasphalting can compose the feed stream to the delayed coking unit, depending upon the refining scheme adopted by the refiner. Another possibility is sent the residue from atmospheric distillation directly to the delayed coking unit, in this case, the unit design is quite modified demanding greater robustness of the fractionating and gas compression section.
Due to the thermal cracking characteristics (low availability of hydrogen during the reactions), the streams produced by delayed coking unit have a high concentration in olefinic compounds, which are chemically unstable. Furthermore, due to the processing of residual streams that have high contaminants content like nitrogen, sulfur, and metals, therefore the refiners that apply delayed coking units need high hydrotreating capacity to convert these streams into added value products and that meets the contaminants level according to the environmental regulation.
Figure 1 presents the process flow scheme for a typical delayed coking unit.
Figure 1 – Typical Arrangement for Delayed Coking Unit
The feed stream is fed into the bottom of the main fractionating tower where is mixed with the heavier fraction of the thermal cracking products and then sent to the fired heater where thermal cracking reactions are initiated, the reaction conditions are controlled so that the reactions are completed in the coke drums, the residence time in the fired heater must be the lowest possible to minimize the coke precipitation in the fired heater tubes. A manner of minimizing the coke formation in the walls of tubes is the steam injection that raises the velocity and consequently reduces the residence time.
After the fired heater the feed stream is sent to the coke drum or reactor, where the thermal reactions are completed and the coke is deposited. The thermal cracking products are removed from the top of the reactor and receive an injection of quench with a cold process stream (normally heavy or middle gas oil) and directed to the main fractionators where the products are separated. The coke deposited in the reactor is removed through a cut with water under high pressure (about 250 bars).
Delayed coking is a process that occurs in batch, in order to make a semi-continuous process are always employed pairs numbers of reactors and each two reactors is applied one fired heater when one reactor is under reaction the other is in decoking step and so on. The delayed coking process occurs in cycles that can vary from 14 to 24 hours.
The main operational variables of the delayed coking unit are recycle ratio which is the quantity of the total feed stream which corresponds to the heavier fraction of the reaction products that are mixed with the fresh feed, reactor temperature, normally considered in the top of the coke drum, pressure in the top of reactor and the time of the reactor cycle.
The recycle ratio vary normally between 5 to 10% (to units dedicated to producing fuels) and the refiners seek to operate the unit with de lower recycle ratio possible in order to maximize the capacity of the plant in processing residual streams. The reactor temperature is close to 430 oC and is linked with the fired heater temperature, throughout the thermal cracking reactions the temperature fall due to the endothermic characteristics of the reactions.
The pressure in the reactor can vary between 1 to 3,5 bars, in units optimized to producing fuels the variable is maintained at lower levels, on the other hand, when the unit is dedicated to producing high-quality coke, the unit is operated at higher pressures.
Reactor cycle time is linked to the function performed by the delayed coking in the refining scheme. Units dedicated to producing fuels operate at shorter cycles and units optimized to producing high-quality coke operate under longer cycles.
The coke produced normally is seen as a by-product of the delayed coking unit, however, in some cases, the delayed coking process is optimized to producing high-quality coke and the coke becomes to the principal product of the process.
Depending on the feedstock quality that will be processed, three types of the coke can be produced:
· Shot coke – Poor quality coke produced from feedstock with high asphaltenes and contaminants (sulfur, nitrogen, and metals) content, normally this type of coke is commercialized as fuel;
· Sponge coke – In this case, the feedstock have a lower asphaltenes and contaminants content and the coke can be directed to raw material to anodes production process to the aluminum industry;
· Needle Coke – The production of this type of coke require the processing of feedstock with high aromatics content (decanted oil from FCC, for example) and these products are sent as raw material to producing anodes to the steel industry;
As mentioned above, production of high-quality coke requires a quality control of the feed stream that will be processed, in the most of the cases the refiners choose to install delayed coking units focusing in the production of middle and light distillates. Therefore, the unit optimization to produce needle coke occurs only in specific cases.
Figure 2 shows a delayed coking main fractionator scheme with the principal process streams.
Figure 2 – Main Fractionator Scheme for a Typical Delayed Coking Unit
The heavy gas oil stream is normally directed to the fluid catalytic cracking unit or can be utilized as fuel oil, in refining schemes that have deep hydrocracking units this stream can be used as feedstock to the unit. The sending of this stream to the fluid catalytic cracking unit needs be controlled to avoid the premature deactivation of the catalyst, face of the high level of contaminants, mainly nitrogen and metals.
Middle and light gas oils are normally sent to severe hydrotreating units to compose the diesel pool of the refinery. The heavy coker naphtha can be directed like feed stream to FCC units. When the flash point specification of diesel is not restricted this stream can be sent to the diesel pool, after deep hydrotreating process.
The lighter fraction of naphtha can be sent to the gasoline pool of the refinery after hydrotreatment or directed to FCC units, in this case, this stream contributes to raising de LPG production in the FCC unit. In some cases, the light coker naphtha can be sent to catalytic reforming units aiming to produce high octane gasoline or petrochemical precursors (benzene, toluene, and xylenes).
The overhead products from the main fractionator are still in the gaseous phase and are sent to the gas separation section. The fuel gas is sent to the refinery fuel gas ring, after treatment to remove H2S, where will be burned in fired heaters while the LPG is directed to treatment and further commercialization.
Figure 3 presents a typical scheme for a gas separation section for a delayed coking unit.
Figure 3 – Basic Process Flow Diagram for a Typical Gas Separation Section from Delayed Coking Unit
The main licensers for delayed coking technology nowadays are the companies FOSTER WHEELER™, CONOCOPHILLIPS ™, CB&I-LUMMUS™ and KBR™.
Delayed coking technology becomes especially attractive for refiners installed in countries with large heavy and extra-heavy crude oil reserves, like Brazil, Mexico, Canada, and Venezuela. The use of delayed coking in the refining scheme can minimize the production of low added value products like fuel oils and guarantees higher flexibility to the refinery in a relation of processed crude oil, minimizing the necessity to acquire light oils.
On the other hand, the delayed coking technology obliges the refiners the necessity of high hydrotreatment capacity once the streams produced by the unity needs severe treating process before being sent to the commercialization, this fact can raise the operational and installations costs.
Another delayed coking disadvantage is the necessity of handling, storage, and commercialization of coke which normally are not the focus of the refiners. Some variations of coking technology as FLUID COKING and FLEXICOKING ™, the last licensed by ExxonMobil ™ Company, can minimize or eliminate the coke formation during the oil residual streams upgrading.
Reference:
MYERS, R.A. Handbook of Petroleum Refining Processes. 3a ed. McGraw-Hill, 2004.
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