Delayed coking as a sustainable refinery solution
Fuel grade coke is a serious environmental pollutant but the producer, the delayed coking process, can be a contributor to future sustainable development in refineries.
MARCIO WAGNER DA SILVA, Petrobras
JOHN CLARK, Coke Consulting Company
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The challenge facing the refining industry in the medium term will be to remain economically relevant in the face of increasing process costs (heavier, higher sulphur crudes and environmental emission restrictions of its final products). Refinery production of automotive fuels produces heavy petroleum residuals, for example vacuum residue (VR). The viability of refineries has historically been primarily based on revenues generated by lighter fuels (LPG, petrol, diesel, and jet fuel). However, as heavier crude oils are increasingly processed, the volumes of straight run VR are expected to increase. As a consequence, refineries will have to rely on hydrocracking and carbon rejection technologies to augment lighter hydrocarbon distillates, with associated costs. Nowadays, the capacity to add value to the bottom barrel streams represents great competitive advantage among refiners, especially considering such stricter regulations as IMO 2020. This imposes a significant reduction in the sulphur content of marine fuel oils, requiring even more capacity to treat bottom barrel streams, especially for refiners processing heavier crude oils.
In this scenario, process units called bottom barrel processing, which are able to improve the quality of crude oil residue streams (VR, gas oils, and so on) or convert them to higher added value products, gain strategic importance, mainly in countries that have large heavy crude oil reserves. These process units are fundamental to comply with environmental and quality regulations, as well as to ensure profitability and competitiveness for refiners through higher refining margin.
Delayed coking is a carbon rejection technology using heavy petroleum residues to produce lighter hydrocarbon distillates (inclusive of naphtha and coker gasoil) and green coke as a solid by-product. While production philosophy over the delayed coking process has historically maximised lighter distillate production, its potential role as both a business and environmental enabler has to date not fully been exploited. This article examines the role of delayed coking in the context of generating environmentally sustainable refinery solutions for heavy petroleum residues to address future market reforms, specifically in the case of fuel grade coke.
Delayed coking technologies — a general overview
Delayed coking employs the thermal cracking concept under controlled conditions to produce light and middle streams (LPG, naphtha, and gasoils) 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 the vacuum distillation process that contains the heavier fractions of processed crude oil. However, streams like decanted oil from the FCC unit and asphaltic residue produced in solvent deasphalting can be in the feed stream to the delayed coking unit, depending upon the refining scheme adopted by the refiner. Another possibility is to send the residue from atmospheric distillation directly to the delayed coking unit; in this case, the unit design is modified, demanding greater robustness in the fractionating and gas compression section.
Due to their thermal cracking characteristics (low availability of hydrogen during the reactions), the streams produced by a delayed coking unit have a high concentration of olefinic compounds which are chemically unstable. Furthermore, due to the processing of residual streams that have high contaminants content like nitrogen, sulphur, and metals, refiners that apply delayed coking units need high hydrotreating capacity to convert these streams into added value products which meet contaminants levels according to the environmental regulation. Figure 1 shows the process flow scheme for a typical delayed coking unit.
The feed stream is fed into the bottom of the main fractionating tower. Here it 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 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 minimise coke precipitation in the fired heater tubes. A way of minimising coke formation in the walls of tubes is steam injection to raise the velocity and consequently reduce 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 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 gasoil) and directed to the main fractionators, where the products are separated. Coke deposited in the reactor is removed through a cut with water under high pressure (about 250 bar).
Delayed coking is a process that occurs in batch. In order to make a semi-continuous process, it is always employed as pairs of reactors, wherein one reactor is under reaction, the other is in the 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 that corresponds to the heavier fraction of the reaction products mixed with the fresh feed; reactor temperature, normally considered in the top of the coke drum; pressure in the top of the reactor; and the time of the reactor cycle.
The recycle ratio varies normally between 5% and 10% (in units dedicated to producing fuels) and the refiner seeks to operate the unit with as low a recycle ratio as possible in order to maximise the capacity of the plant in processing residual streams. The reactor temperature is close to 430°C and is linked to the fired heater temperature. Throughout the thermal cracking reactions, the temperature falls due to the endothermic characteristics of the reactions.
The pressure in the reactor can vary from 1 bar to 3.5 bar. In units optimised for producing fuels, the variable is maintained at lower levels. When the unit is dedicated to producing high quality coke, the unit is operated at higher pressures.
The coke produced normally is seen as a by-product of the delayed coking unit; however, in some cases, the delayed coking process is optimised to produce high quality coke, and coke becomes the principal product of the process.
Depending on the feedstock quality to be processed, three types of coke can be produced:
- Shot coke/fuel grade coke is poor quality coke produced from feedstock with a high asphaltenes and contaminants (sulphur, nitrogen, and metals) content; normally, this type of coke is commercialised as fuel.
- Sponge coke: in this case, the feedstock has a lower asphaltenes and contaminants content, and the coke can be directed as raw material for anodes production in the aluminium industry.
- Needle coke production requires the processing of feedstock with a high aromatics content (decanted oil from the FCC, for example), and these products are sent as raw material for producing anodes in the steel industry.
Production of high quality coke requires quality control of the feed stream. In most cases, refiners choose to install delayed coking units focusing on the production of middle and light distillates. Therefore unit optimisation to produce needle coke occurs only in specific cases.
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