Thermacracking Flashcards
Why are coking units a thing?
Because the “bottom of the barrel” stuff has become a problem in refineries, their market has been decreasing, so they need to be processed to useful products
Coking units cover these heavy hydrocarbon mixtures into solid coke and lower-boiling products which serve as feedstocks to other units
What is Coking
It is a severe thermal cracking process where one of the end products is primarily carbon (coke).
What has been happening in recent years w.r.t coking.
Benefits (2)
it has been used to prepare coker gas oils streams suitable for hydrocracker feedstocks and high-quality coke.
This reduces refinery yield for the heavy residual fuel. Also reduces metal content of the catalytic cracker feedstocks
What is thermal cracking
4 types of thermal cracking
Heat and pressure break down and rearrange the hydrocarbon molecules
- Delayed coking
- Flexicoking
- Fluid coking
- Visbreaking (not coking)
What happens in delayed coking
Heaters rasie the feedstock temp above the coking point. But the feedstock is at high velocities that coking happens later on. Coking happens on the coking drums (an insulated drum).
How is fresh liquid added in delayed coking
either directly to the furnace or to the fractionator
Effect of heater outlet temp.
Higher temp leads to shorter time before the furnace tubes have to be decoked. Usually takes 3-5 months
D.C
what is zero recycle operation
When the feed is charged directly to the furnace
Products of thermal cracking reaction (top of the coke drum vapours).
Where does it go?
gas, naptha, gas oils
Base of the fractionator
What does charging the fresh feed to the fractionating column do?
- Any remaining material lighter than the desired coke drum feed is stripped from the fresh liquid feed.
- Prevents coke formation in the fractionator by cooling the hot vapor from the coke drum. Also condenses the heavy parts in that vapour
- The fresh feed liquid is further preheated, making the process more energy efficient.
T/F If a naphtha side draw is employed, additional trays are not usually required above the naphtha draw tray.
False, additional trays are required
How is a coke drum changed out
It is isolated, steamed to remove the hydrocarbon vapors, cooled by filling with water, and drained. The coke is cut and removed from the drum where it is transferred for storage.
Another name for coke storage
A calciner
Typical time for a full coke drum cycle (from filling to when it is ready to be used again)
48 hours (24h to fill, 22h to get cleaned and ready +2h extra)
How does coker operators increase capacity.
Effects/Consequences? (2)
By operating on shorter cycle times.
Leads to lower yield of liquid products (due to high drum and frac. tower pressures).
Short cycle times = short drum life due to stresses caused by rapid temp cycles.
What is the main product in coker naptha (light or heavy naptha)?
Heavy naptha
What is the main product of coker gas oil (light or heavy gas oil?)
light gas oil
Feedstock for flexi coking
Any heavy oils: vacuum resid, shale oil, or oil sand bitumen
Flexicoking steps
Feed is preheated and sprayed into the reactor where it makes contact w/ a hot fluidized bed of coke.
The coke is recycled tpo the reactor from the coke heater
Important reactions of steam gasification
Water-gas reaction (turns C to CO and H2)
Water -gas shift reaction (urns CO to CO2 and H2)
Boudouard reaction (turns C & CO2 to CO)
Methanation (turns C to CH4)
How many beds are used in fluid coking (what are they called). And why?
Only 2 fluid beds are used: reactor and a burner (which replaces the heater)
Only 2 are needed because only a small amount of coke is burned,just enough to satisfy the heat reqs of the reactor and feed preheat. (20-25% of the coke produced in the reactor).
3 things that affect the type of coke produced
Process used, operating conditions, feedstock properties
3 types of coke
Sponge, needle, shot
sponge (which type of coking does it mainly come from?)
hard, porous lumps
Delayed coker
