March, 2010

Oil refining and Processing

CBE 432b

Coking

Coking Coking is a thermal process for the conversion of heavy low grade oil to

lighter products (mainly gasoline related). It can be considered as an improved version of thermal crackers.

As a result of thermal cracking, coke is formed in the reactor. The first delayed coker was built by Standard Oil of Indiana at Whiting,

Indiana in 1929. Removal of the coke deposit on the reactor wall was a major challenge. The development of hydraulic decoking was an important achievement

after which a number of commercial cokers were built. More than 50 cokers are in operation only in US.

Coking

The feed stock can be: Straight run residue Cracked residue Vacuum distillation bottom product

The products are:GasesNaphtaFuel oilGasoil (major product, feedstock for catalytic

cracking operations)Coke

Coking The coke obtained is usually used as fuel or further treated to

marketable products such as electrodes, metallurgical coke, etc.

Majority of sulfur in oil is concentrated on coke, providing products with less sulfur which facilitate the downstream operations.

The coke contains sulfur compounds and metal impurities, which must be removed before further treatment for environment consideration

Coking is a preferred process for oil from tar sand.

Coking

Coking is a semi-continuous operation as the solid coke should be removed from the reactor at certain times.

Coke forms a thick solid layer over the reactor inner wall. Coking reactors are relatively large in size due to

relatively large residence time. The reactor needs decoking upon completion of the

process. (to be explained in class).

Two or more reactors are used for continuous operation. One reactor is decoked while the other one is in operation.

Delayed Coking

Delayed coking is a process in which the feed is sent through a fired heater (480 – 515 C) with horizontal tubes.

With short residence time in the furnace, coking of the feed is thereby delayed until it reaches large coking drums which provides long residence time to allow the cracking reactions to proceed to completion.

The cracked products leave as overheads and coke deposits form on the inner surface of the drum.

The temperature in the drum ranges from 415 to 450 C at pressures from 15 – 90 psig.

Coking

Coking

Overhead products are sent to a fractionator, where naphta and heating oil fractions are recovered.

The bottom product of the fractionator is recycled back to the reactor after combined with the preheated feed.

The coke drum is usually on stream for 24 hours before becoming filled with coke.

Coke is removed in three stepsCooling the coke with water spray Drilling a hole through the center of the deposit

(opening the central top section of the drum)Cutting the deposit with hydraulic devices (high

pressure water jets)

Coking

Coke removal and cleaning operations usually takes less than 24 hours.

The drum should be heated before receiving the hot feed. Hot vapors from the online drum are recirculated in the cold cleaned drum, where the drum is heated (to about 350 C) and the vapors are condensed and removed continuously.

During the operation, vapors are continuously condensed, cracked and coke is gradually built up on the inner wall of the drum.

Different types of coke may be formed depending on the type of the feed and the operating conditions.

Coking Steam must be used before the switch and immediately after

the switch for stripping the unconverted liquid feed to avoid feed solidification during the cooling process which creates major problems in the operation.

Steam stripping also serves to transfer heat from the hot bottom to the unconverted feed on top of the coke drum.

The cracking reactions and the operation of cokers are very complex and requires advanced control systems for optimum results.

Coking furnace The furnace is the heart of the delayed coking process and provides all

the heat required for the process. The velocity inside the tubes is very high to maximize heat transfer The residence time is kept at the minimum, especially above the cracking

temperature to prevent coke formation. The temperature should be uniform (minimum maldistribution). Symmetrical piping arrangement within the furnace enclosure is critical to

achieve optimum results. Multiple steam injection points are installed for proper control and

maintenance of the heater tubes.

Coking Cutting High pressure water is used to cut the coke out of drum. Water pressure ranges from 1250 to 4000 psig and flow rates range from 750

GPM to 1250 GPM. Special structure is built on top of the drum to hold the drill stem and cables. The drill bit size is approximately 1 meter, making a hole at the center of the

drum (top to the bottom). Cutting nozzles are moving vertically and high pressure jets cut the coke

deposit. Presence of hot spots in the coke layer may result in excessive production of

steam, which may lead to explosion during the cutting operation.

Fluid coking Fluid coking is a continuous process in which coking occur is a fluidized

bed. The feed is continuously sprayed over a fluidized bed of hot coke particles. Due to the nature of Fluidized beds, coking will be carried out at a more

uniform and higher temperature and shorter contact time compared to delayed coking.

More valuable products and less coke are formed in Fluid coking. Fluid coking consists of two units, reactor and furnace. Coke particles are circulated between the reactor and the furnace to transfer

heat generated by burning a portion of the coke.

Fluid Coking Steam is used to fluidize coke particles inside the

reactor. The preheated feed is injected directly into the reactor at

250-350 C and atmospheric pressure. The temperature inside the reactor is between 480-570 C. The feed is partially vaporized, cracked or deposited on

the coke particles, which thermally cracks at high temperature leaving a residue that dries to form coke.

Cyclones are used to separate the entrained coke particles with the vapors (products).

Air added to the burner to combust coke and control the temperature.

The coke is withdrawn from the reactor to control the inventory.

Fexicoking

Flexicoking is a direct descendent of fluid coking, using the same configuration but has a gasification unit in which the excess coke can be gasified.

More valuable products will be obtained from gasification of coke.

Delayed coking is practiced by Suncor Canada plant, however Syncrude employs a fluid coking process.

In spite of advantages of fluid coking, majority of the coking plants have employed the delayed coking process.