chemistry and technology of petroleum

chemistry and technology of petroleum By Dr. Dang Saebea

Thermal Cracking and Coking

Delayed Coking • The heat required to complete the coking reactions is supplied by a furnace, while coking itself takes place in drums operating continuously on a 24 h filling and 24 h emptying cycles. • The process minimizes residence time in the furnace, while sufficient time is allowed in the drums where coking takes place. • Coke is rejected in the drums, thus increasing the H/C ratio in the rest of the products. • These products are still unstable and unsaturated, and require further hydrogenation.

Component of feed • The feed to coker is usually vacuum residue which is high on asphaltenes, resins, aromatics, sulphur and metals. • A common feed is vacuum residue but it can also accept fluid catalytic cracking slurry and visbreaking tar (residues).

Component of product • The deposited coke contains most of the asphaltenes, sulphur, and metals present in the feed, and the products are unsaturated gases (olefins) and highly aromatic liquids. • The products from the coker are unsaturated gases (C1–C4), olefins (C2=-C4= ) and iC4.

C3/C4 gases are sent to the LPG plant. - The highly aromatic naphtha does not need reforming and is sent to the gasoline pool. Light gas oil (LCO) is hydrotreated and sent to the kerosene pool. Heavy coker gas oil is sent to the FCC for further cracking Role of delayed coker in the refinery

Process Description Coke fractionator two coke drums Stripper a furnace A schematic flow diagram of the delayed coking

Process Description lighter than heavy gas oil • Vacuum residue enters the bottom of the flash zone in the distillation column. • Fractions lighter than heavy gas oil are flashed off and the remaining oil are fed to the coking furnace.

To quench by the liquid feed 482 ˚C To prevent premature coking. • The liquid–vapour mixture leaving the furnace passes to one of the coking drum. • Coke is deposited in this drum for 24 h period while the other drum is being decoked and cleaned. • Hot vapours are quenched and simultaneously condensing a portion of the heavy ends which are then recycled.

steam, gas, naphtha, gas oils gas oil • Vapours from the top of the coke drum are returned to the bottom of the fractionator. • These vapours consist of steam and the products of the thermal cracking reaction • The vapours flow up through the quench trays of the fractionator. • Above the fresh feed entry in the fractionator, there are usually two or three additional trays below the gas oil draw off tray.

Steam and vaporized light ends are returned from the top of the gas oil stripper to the fractionator. • A pumparound reflux system is provided at the draw tray to recover heat at a high temperature level and minimize the low-temperature level heat removed by the overhead condenser.

Coke drum • The drums, where the reactions take place and coke is deposited on the drum walls, and the products flow back to the distillation column. In this case, the drum is in the ‘‘filling’’ mode. • The other drum is cut off from the rest of the system while the coke is being removed. The drum in this case is in the ‘‘cutting’’ mode.

Delayed Coking Variables Temperature • Temperature is used to control the severity of coking. • In delayed coking, the temperature controls the quality of the coke produced. • High temperature will remove more volatile materials. • Coke yield decreases as temperature increases. This might lead to coke formation in the furnace. High temperature low temperature This might lead to incomplete coking.

Delayed Coking Variables Cycle time • Short cycle time will increase capacity but will give lower amounts of liquid products and will shorten drum lifetime.

Delayed Coking Variables Pressure • Increasing pressure will increase coke formation and slightly increase gas yield. However, refinery economics require operating at minimum coke formation. • New units are built to work at 1 bar gauge (15 psig), while existing units work at 2.4 bar gauge (35 psig).

Delayed Coking Variables Recycle ratio • Recycle ratio is used to control the endpoint of the coker gas oil. • It has the same effect as pressure. • Units are operating at a recycle ratio as low as 3%.

Delayed Coking Variables Feedstock • Feedstock variables are the characterization factor and the Conradson carbon which affect yield production. • Sulphur and metal content are usually retained in the coke produced. Engineering variables • Engineering variables also affect the process performance. These include mode of operation, capacity, coke removal and handling equipment.

Types of Coke and their Properties • Coke amount can be upto 30 wt% in delayed coking. The most common types of coke are: Sponge coke • Sponge coke is named for its sponge-like appearance. • It is produced from feeds having low to moderate asphaltene content.

Types of Coke and their Properties Needle coke • This coke has a needle-like structure and is made from feed having no asphaltene contents such as decant oils from FCC. • It is used to make expensive graphite electrodes for the steel industry. Needle Coke Texture

Types of Coke and their Properties Shot coke • This coke is an undesirable product and is produced when feedstock asphaltene content is high and/or when the drum temperature is too high. • There are some methods of eliminating shot coke formation including adding aromatic feed, such as FCC decant oil, decreasing temperature, increasing pressure and the recycle ratio.

Types of Coke and their Properties

Coking and Decoking Operation • The decoking operation involves drilling a vertical hole in coke after cooling using a mechanical boring tool. • Further coke removal is carried out by using a hydraulic cutting tool,where a jet of water is capable of the removal of the remaining coke from the drum. However, this requires using a great amount of water which has to be treated later on. • Mechanical first boring tool. • (B) Final hydraulic tool

Coking and Decoking Operation • The decoking cycle • switching the drums, • cooling by steam, • draining in the coke • warming up the drum • leaving spare time for contingency Meanwhile, the other drum is under coking reactions. Steps of decoking operation

Time cycle for delayed coking

Delayed Coker Yield Prediction • Estimation of product yields can be carried out using correlations based on the weight percent of Conradson carbon residue (wt% CCR) in the vacuum residue.

Delayed Coker Yield Prediction • The naphtha can be split in light naphtha (LN) and heavy naphtha (HN). The split in wt% is 33.22 and 66.78, respectively, assuming corresponding gravities of 65 API and 50 API, also respectively. • The gas oil (GO) can be split also into light cycle gas oil (LCO) and heavy cycle gas oil (HCO). The spilt in wt% is 64.5 and 35.5, respectively, and the corresponding gravities are 30 API and 13 API.

Delayed Coker Yield Prediction Typical sulphur distribution in the products for delayed coking

Example • A vacuum residue of Conradson carbon (wt% CCR=15)is fed into a delayed coker at a rate of 200,000 lb/h, of API=8.5 and with a sulphur content of 3.0 wt%. • Find the amount of yield (lb/h) and their sulphur content. • Calculate yield of liquid products in BPD.

Fluid Coking • Fluid coking is a thermal cracking process consisting of a fluidized bed reactor and a fluidized bed burner. Vacuum residue is heated to 260 ˚C and is fed into the scrubber and it operates at 370 ˚C. The heavy hydrocarbons in the feed are recycled with the fine particles to the reactor as slurry recycle. 510–566 ˚C

Fluid Coking • The heavy vacuum residue feed is injected through nozzles to a fluidized bed of coke particles. • The feed is cracked to vapour and lighter gases which pass through the scrubber to the distillation column. • Coke produced in the reactor is laid down on the coke bed particles • Steam is introduced at the bottom of the reactor, where a scrubber is also added to scrub any heavy hydrocarbons from the surface of the coke particles.

Fluid Coking • Part of the coke flows into the burner where 15–30% is combusted by the injection of air into the burner. 593–677 ˚C • The rest of the hot coke is recycled back to the reactor to provide the required heat.

Fluid Coking Block diagram of fluid coking

Fluid Coking Yield and end uses of fluid coker process

Flexicoking • The flexicoking process is a development of the fluid coking process where only 2 wt% of coke is produced, thus most of the coke is used to heat the feed. • A fluidized bed is added to the process which acts as a gasifier in which steam and air are injected to produce synthesis gas called Low Btu Gas (LBG). • The gasifier produces hot coke which remains after combustion. 816–982 ˚C

Flexicoking This coke flows into the middle vessel, which acts as a heat exchanger to heat cold coke coming from the reactor. The operation of the reactor is the same as fluid coker. 593 ˚C

Flexicoking A block diagram of the process

Flexicoking • In the oxidation zone of the gasifier, the following reactions take place very rapidly: In the reduction zone, the following reactions take place slowly:

Yield Correlations for Flexicoking The yield correlations for flexicoking are based on the Conradson carbon content of the vacuum residue (CCR, wt%), its API gravity and sulphur content (Sf). Data compiled by Maples (1993) are correlated to express weight percent yields. Gas composition:

Yield Correlations for Flexicoking Sulphur distribution in products: Gravity of flexicoker feed and gas oil

Example • A vacuum residue of Conradson carbon (wt% CCR=15)is fed into a flexicokingat a rate of 200,000 lb/h, of API=8.5 and with a sulphur content of 3.0 wt%. • Find the amount of yield (lb/h) and their sulphur content. • Calculate yield of liquid products in BPD.

The End

chemistry and technology of petroleum