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 oppurtunities 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 oppurtunities 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.
References:
1. Gary J.H. , Handwerk G.E., Petroleum Refining: Technology and Economics, Taylor & Francis, 2005
2. Jones D.S.J., Elements of Petroleum Processing, John Wiley & Sons, 1995