by: Souad LOUSDAD
The Crude Oil, after being extracted, desalted and separated into valuable products via the two separation processes, “Atmospheric Distillation” and “Vacuum Distillation”, subsequent processes are used to alter the molecular structures of the fractions to create more desirable products labeled in the name of “Conversion Processes”.
One of the most common conversion processes is cracking; it breaks or cracks heavier, higher-boiling point petroleum fractions into more valuable products.
The basic types of cracking: Thermal cracking, catalytic cracking and hydrocracking.
Let’s discuss thermal cracking.
Thermal cracking processes (back in 1913) heat distillate fuels and heavy oils under pressure and very high temperatures in large drums until they crack and divide into smaller molecules, and it does not require the addition of a catalyst. This approach is the oldest technology available for residue conversion, and the severity of thermal processing determines the conversion and the product characteristics. As the temperature and residence time are increased, the primary products undergo further reaction to produce various secondary products and so on, with the ultimate products (coke and others) being formed at extreme temperature of approximately 1000 °C.
This early method, which produced large amounts of solid, unwanted coke, has evolved into modern thermal cracking processes including visbreaking, coking and steam cracking.
First, what is visbreaking?
Visbreaking is an abbreviated term for “viscosity breaking” or “viscosity lowering”; a relatively mild liquid phase form of thermal cracking.
The main purpose of visbreaking process is to produce a fuel with a lower viscosity than that of the feed, a vacuum residue.
Residual from the vacuum distillation tower is mildly cracked in a heater at atmospheric pressure, the charge stream to visbreaker where it heated.
A visbreaker is a controlled-gradient, box-type heater with a preheater and two soaking coils.
The effluent from the heater is quenched with cool gas oil to control over cracking, and flashed in a distillation tower. The vapor from the fractionator is flashed off and separated into light distillate products, such as gases, gases, cracked naphtha, and a gas oil cut. The overhead vapor condensed in a condenser accumulates in unstabilized naphtha drum which provides reflux to the main fractionator column. The uncondensed vapor is further cooled and accumulates in stabilizer feed drum. The unstabilized naphtha flows to a stabilizer or a debutanizer column, which removes any C₄ gases.
The thermally cracked residue tar, which accumulates in the bottom of the fractionation tower, is vacuum flashed in a stripper and the distillate is recycled.
During the course of the run, coke forms in the tube. Therefore, permanent steam/air decoking facilities are provided.
The important process variables are charge stock quality, cracking temperature, and residence time of oil in the coil. Feed to visbreaker units is either reduced crude or vacuum residues.
The coil temperature profile and residence time are controlled to monitor the severity of operation, usually measured by the amount of gasoline produced.
The visbreaking produces a small quantity of light products and a large amount of gasoline.
Between brackets, the term “coil” refers to units where the cracking process occurs in the furnace tubes or “coils” and the cracking reaction actually occurs not in the furnace but in a drum located after the furnace and before the quench called the “soaker”.
Second, what about “coking”?
Coking is a severe form of thermal cracking used to obtain straight-run gasoline and various middle distillate fractions used as catalytic cracking feedstocks.
This process so completely reduces hydrogen from hydrocarbon molecules, that the residue is a form of almost pure carbon called coke.
The two most common coking processes are delayed coking and continuous (contact or fluid) coking which; depending upon the reaction mechanism, time, temperature and the crude feedstock.
In delayed coking, the feedstock is first charged to a fractionator to separate lighter hydrocarbons, and then combined with heavy recycle oil. The heavy feedstock is fed to the coker furnace to be heated to high temperatures at low pressures to prevent premature coking in the heater tubes, producing partial vaporization and mild cracking. The liquid/vapor mixture is pumped from the heater to one or more coker drums where the hot materials are held approximately 24 hours (delayed) at low pressures until it cracks into lighter products.
After the coke reaches a predetermined level in one drum, the flow is diverted to another drum to maintain continuous operation.
Vapor from the drum is returned to the fractionator to separate out gas, naphtha and gas oils, and to recycle heavier hydrocarbons through the furnace. The full drum is steamed to strip out uncracked hydrocarbons, cooled by water injection and decoked mechanically by rising from the bottom of the drum, or hydraulically by fracturing the coke bed with high-pressure water ejected from a roating cutter.
The process minimizes residence time in the furnace, while sufficient time is allowed in the drums where coking takes place.
The feed to the delayed coker can be any undesirable heavy stream containing high metal content.
A common feed is vacuum residue but it can also accept fluid catalytic cracking slurry and visbreaking tar (residues).
Coke amount can be up to 30% in delayed coking. It is produced as green coke which requires calcinations to remove the volatiles as fuel product.
The most common types of coke are: First, sponge Coke is named for its sponge-like appearance. It is produced from feeds having low to moderate asphaltene content (in the form of asphalt or bitumen products).
Second, needle coke has a needle-like structure and is made from feed having no asphaltene content such as decant oils from FCC.
And last, shot coke is an undesirable product and is produced when feedstock asphaltene content is high and/or when the temperature is too high.
In the other hand, continuous coking, also called fluid coking is a moving-bed process for which the operating temperature is higher than the temperatures used for the delayed coking.
In continuous coking, hot recycled coke particles are combined with liquid feed in a radial reactor. Vapors are taken from the reactor, quenched to stop any further reaction, and fractionated. The coke goes to a surge drum than to a classifier where the larger particles are removed as product. The smaller coke particles are recycled to a preheater where they are mixed with fresh feed.
Fluid coking makes more fuel gas than delayed coking.
