- In this lecture, we present two important processes that are used to upgrade the residue product obtained from the vacuum distillation unit.
Significant amounts of vacuum residue is obtained from various crude oils. For instance Arabian heavy oil produces 23.2 vol% vacuum residue product.
- The residue consists of heavier hydrocarbons with molecular weights ranging from 5000 – 10000.
- Thermal cracking is most preferable for the vacuum residue.
- The vacuum residue also consists of other metals such as vanadium and nickel.
- Typically, vacuum residue is subjected to six different operations namely
- Vacuum residue desulphurization (VRDS)
- Residue fluid catalytic cracking (RFCC)
- Amongst these, Viskbreaking and coking are prominent operations that exist in many refineries and we restrict our discussion to these processes only.
- In Visbreaking operation, a mild thermal cracking is carried out.
- Visbreaking produces Naphtha, Gasoil and a residue. The residue has lower viscosity and pour point and hence can easily meet the requirements of the fuel oil specifications in the fuel oil blending pool.
- Visbreaking basically breaks the long paraffinic side chains attached to aromatic structures. Due to this the residue pour point and viscosities are considerably reduced.
- Two classes of reactions occur during visbreaking
- Cracking of side chained aromatic compounds to produce short chained aromatics and paraffins
- Cracking of large molecules to form light hydrocarbons
10.3 Process technology (Figure 10.1)
Figure 10.1 Coil Visbreaker
- Visbreaking is carried out either in a coil or in a soaker.
- When coil technology is used, the mild thermal cracking is carried out in the furnace coils
- When soaker technology is adopted, the cracking is carried out in a soaker unit that is kept immediately after the furnace.
- After cracking, the products are at high temperatures (480 oC for coiled furnace case or 430 oC for the soaker)
- After cracking, the products are cooled using quenching operation.
- Quenching is a direct heat transfer mechanism in which a hot stream is cooled with a hydrocarbon or water to reduce the temperature of the system drastically.
- After quenching, the mixture is fed to a distillation column supplemented with a side column and a partial condenser
- From partial condenser, water, naphtha and gas are produced. Naphtha liquid stream is sent as a reflux for the column.
- The side column is fed with steam and produces gas oil. Steam enhances hydrocarbon volatility as explained before in the lecture devoted towards crude distillation process.
10.4 Coking ( Figure 10.2 ,10.3 )
- Coking refers to extreme thermal cracking process
- Coking is a batch reaction. Feed is heated to 480 – 510 oC and left for some time so that coke and lighter products form.
- Since coking is a batch reaction, there can be different ways to carry out coking.
- There are two types of refinery coking processes namely delayed coking and fluid coking.
- Delayed coking: The heated feed is fed to a coke drum and coke forms in the drum. The lighter products are withdrawn from the top section of the delayed coker. After coking,the coke drum is full of coke and it is then removed using water jets. The process is called delayed coking because in this process the coal is heated using a much more complex system , and it consists of multiple furnaces or coke drums. Usually the coal is heated at a lower temperature for shorter periods of time , which removes water and other by – products more slowly than the conventional process, thereby increasing the overall time span.
- Fluid coking: The heated feed is fed to a fluidized bed where coke particles with finer particle sizes would aid fluidization. After coking, lighter products are withdrawn as overhead vapour and coke thus formed is removed continuously. The fluid coker also has an additional scrubber which will remove heavier compounds from the vapors (if any) and send them back with the feed stream. Here, the feed stream absorbs heavier hydrocarbons from the vapors generated. This is required as it is difficult to keep heavier hydrocarbons in the feed phase only due to pertinent high temperatures. The coke after coking reaction is cold coke. Therefore, to generate hot coke, a burner unit is used to heat the coke using exothermic CO2 reaction. The offgases from the burner are sent to cyclones, scrubbing and then to the vent. The hot coke thus obtained is recycled back to the fluidized bed or taken out as a net coke product.
- The coker products are fed to a complex distillation column i.e., main column supplemented with side columns. From the complex distillation column, naphtha, water, light gas oil and heavy gas oil are obtained.
- Additional complexities in the distillation unit are
- o Feed entering the distillation column but not the coker unit: This is to facilitate the removal of light ends from the feed (if any) and don’t subject them to cracking. This is also due to the reason that light ends are valuable commodities and we don’t want to loose them to produce cheap coke product. In this case, the bottom product from the distillation column is fed to the furnace for pre-heating and subsequent coking operation.
- Live steam in distillation: This is to facilitate easy removal of lighter hydrocarbons in various sections.
- Circulatory reflux (Pump around units): This is to facilitate good amount of liquid reflux in various sections of the main column. For Further details, of the above two issues, please refer to the crude distillation lecture notes.
10.5 Technical questions
1. Why thermal cracking is most preferable for vacuum residue than catalytic cracking?
Ans: Since heavier compounds are present in the vacuum residue, they can totally poison the catalyst with coke deposition. Therefore, thermal but not catalytic cracking is preferred.
2. Provide technical insight for quenching?
Ans: Quenching is one of the oldest technique to control temperatures of a stream without involving indirect heat transfer. The basic advantage of the quenching is that if a cold fluid with similar chemical constitution is mixed, then it reduces the temperature of the cold fluid and increases the temperature of the quenching fluid. Quenching is also an instantaneous operation which is not the case of indirect heat transfer. Also, in indirect heat transfer, losses could be significant due to additional process resistance for heat transfer. This is not the case in quenching where the hot fluid directly comes in contact with the cold fluid. The basic disadvantage in quenching is that if a cold fluid of different chemical constitution is used then additional separation (downstream) is required. If that can be avoided, quenching is an excellent operation.
3. Are there any heat integration opportunities existent for the visbreaking operation?
Ans: No, the reason is that the vapour from the fractionators is a hot stream and the quenching stream is also a hot stream. We need atleast one cold stream and hot stream to enable heat integration.
4. Explain the reason behind recycling of intermediate process streams in fluid coker?
Ans: An essential issue in fluidization is to maintain a uniform temperature throughout the fluidized bed. To do so, partial recycle of intermediate vapour streams is carried out so that uniform temperatures are maintained.
5. Do opportunities exist in coking operation for heat integration?
Ans: Yes, they exist very much. The hot products from the complex distillation unit as well as the vapors generated from the coke drums are the hot streams. The cold stream is the feed which is subjected to pre-heated. Since we have a cold stream and good number of hot streams, we can have a good heat integration opportunity in the coking operation.
6. What is flexicoking?
Ans: In flexi coking, coke is fed to a gasifier to react with air and steam. Eventually, the coke produces hydrogen, CO, CO2, N2, H2O and H2S. The gas thus obtained is used as a fuel gas in the refinery itself for various furnace operations. Therefore, flexicoking refers to flexibly alter the coking process to produce additionally gas using an extra gasification unit.