Thermal Cracking Processes

Question 1

What is the range of temperature where cracking of heavy HC starts ?

Answer 1

400-420°C

Question 2

What is the minimum temperature to operate a thermal cracking unit ?

Answer 2

440-450°C

Question 3

What are the two controling parameters for thermal cracking ?

Answer 3

Temperature and residence time

Question 4

What are the operating conditions in a visbreaker ?

Answer 4

450°C and 20 minutes residence time -> low severity thermal cracking

Question 5

What are the two versions of the visbreaking unit ?

Answer 5

• Coil cracker

- Soaker cracker

Question 6

Where does the reaction occur in the two versions of the visbreaking unit ?

Answer 6

• Coil cracker -> in the heater tubes

- Soaker creacker -> mainly in the soaker drum

Question 7

How is the soaker drum designed in a visbreaking unit ?

Answer 7

It is designed to have a residence time of 20 minutes

Question 8

What are the products of a visbreaker ?

Answer 8

• Gas, Gasoline and Gasoil in small quantities
• Vacuum Gasoil -> fuel pool or feedstock for FCC or VGO HDT/HDC
• Visbroken vacuum residue (65-80%) -> base for fuel pool

Question 9

What is the quality of the products at the outlet of the visbreaker ?

Answer 9

Low quality : rich in sulfur and unsaturated HC (olefins and diolefins)

Question 10

What are the advantages of using a visbreaker (vs nothing) ?

Answer 10

• Decrease heavy fuel production (we don’t know what to do with it)
• Increase gas oil production (higher value product)

Question 11

What is the feed temeprature in a delayed coker ?

Answer 11

500°C

Question 12

What is the difference between Visbreaker and delayed coker ?

Answer 12

Visbreaking we stop before we make coke while delayed coker we stop when all the feed has been converted

Question 13

How is the decoking done in a delayed coker ?

Answer 13

Through hydraulic decoking

Question 14

What are the products of the delayed coker ?

Answer 14

• Gas, gasolines and gasoil that need to be hydrotreated
• Distillate used as feedstock for FCC or HCK
• Coke

Question 15

What are the three grades of coke that can be produced ?

Answer 15

• Electrode grade
• Metallurgical grade
• Fuel grade

Question 16

What is green coke ?

Answer 16

The coke produced in delayed coker

Question 17

What is the highest grade for coke ?

Answer 17

Electrode coke (low sulfur, metal and nitrogen required)

Question 18

What are the three reactors in a flexicoker ?

Answer 18

• Reactor
• Heater
• Gasifier

Question 19

What is the highest temperature in the flexicoker and in which part is it ?

Answer 19

980°C in the gasifier

Question 20

What happens to the feed in the flexicoker ?

Answer 20

It is totally transformed into liquid or gaseous products

Question 21

What is injected to gasify the coke ?

Answer 21

Air and steam

Question 22

What is gasification ?

Answer 22

Partial combustion

Question 23

What happens to the gas obtained in a flexicoker ?

Answer 23

The gas is hydrotreated and used as fuel gas (but lower calorific value than regular fuel gas)