Fluid catalytic cracking

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Fluid Catalytic Cracking (FCC) is the most important material conversion process in the petroleum processing industry. This process is widely used for converting heavy petroleum fractions into valuable olefins ( ethene , propene , butene ), catcracker gasoline, gas oil ( Light Cycle Oil , LCO) and heavy oil components ( Heavy Cycle Oil , HCO, and slurry). Thermal cracking was originally usedas the cracking process for vacuum distillates , which was almost completely replaced by the catalytic cracking process, as the latterproducesmore high-quality products (CC gasoline, olefins ). In recent times,however, hydrocracking has become more and more important.

The feed to an FCC unit usually consists of a fraction with a boiling range of ≈ 340 ° C to ≈ 560 ° C and an average molar mass of ≈ 200 to ≈ 600 g • mol −1 . So refinery Technically comes heavy gas oil , vacuum gas oil (VGO) as well as deasphalted oil (DAO), or even furfural - extract in question. In the FCC process, the high-boiling petroleum fractions are evaporated and then the longer-chain molecules are split into much smaller ones by being brought into contact with a fluidized solid catalyst at high temperature and moderate pressure.

The process goes back to Eugene Houdry and Eugène A. Prudhomme in France. Houdry eventually developed it further in the USA (mineral catalysts, methods of maintaining catalytic activity). After delays caused by the Great Depression of the 1930s, the first Pennsylvania refinery was converted from thermal cracking to FCC in 1938. Back then, people were amazed that it could produce twice the amount of gasoline than the old factory.

raw materials

Procedure

Scheme of a typical FCC unit in a petroleum refinery

Modern FCC units are continuous procedures that are routinely turned off for maintenance every 2 or 3 years. There are different manufacturers, each with their own design. The process can be licensed. There are two configurations: a vertical arrangement in which the reactor is arranged above the regenerator in a single vessel and a horizontal arrangement in which the two are arranged side by side.

The most important providers and licensors are:

Horizontal arrangement:

  • ABB Lummus Global
  • Exxon Research and Engineering (ERE)
  • Shell Global Solutions International
  • Stone & Webster Engineering Corporation (SWECO) / Institut Francais Petrole (IFP)
  • Universal Oil Products (UOP)

Vertical arrangement:

  • Kellogg Brown & Root (KBR)

Reactor and regenerator

The starting material, which has been preheated to 315 ° C. to 430 ° C., is introduced with a substream from the bottom of the distillation column (slurry) into the catalyst riser (6), where it is evaporated and cracked on the catalyst. All reactions take place in the catalyst riser. The hydrocarbon vapors fluidize the catalyst powder and convey it upwardly into the overlying separator, in which the temperature of approximately 535 ° C and the pressure 1.7 bar above amount.

The used catalyst is separated from the vaporous product in the separator, whereby smaller particles are separated in two hydrocyclones (4). The catalyst then flows down through a stripping section into which steam is blown to separate residual volatile hydrocarbons, which are also made up of product and are therefore valuable. The catalyst outflow is regulated by a valve (8).

Since a larger amount of coke is produced during the cracking reaction, which is deposited on the catalyst surface and quickly deactivates it, it is regenerated by oxidation. For this purpose, the coked catalyst is fed into the regenerator, into which air is blown to burn the coke. The regenerator works at a temperature of 700 ° C and an overpressure of 2.4 bar. The heat of combustion is partly absorbed by the catalyst and supplies the energy for the evaporation of the educt and for the endothermic reaction in the riser. For this reason, FCC units are referred to as “thermally balanced”.

The hot catalyst from the regenerator is passed through a separator (5), in which flue gases can escape and are returned to the regenerator. The flow of regenerated catalyst into the lower part of the riser is regulated by means of a valve (7) in the catalyst line.

The hot flue gases leave the separator through a series of two-stage hydrocyclones that separate the catalyst particles that have been carried along.

The amount of circulating catalyst is five times the mass of VGO feed.

Distillation column

The vapors from the reactor are fed into the distillation column, in which they are separated into gas, naphtha (CC gasoline), gas oil and heavy oil components. After further processing and separation of the sulfur components, the naphtha fraction is an admixture component for the gasoline. The vapors passing overhead are passed on for separation into propane, propene, butane and butene and the gases hydrogen , methane , ethane and ethene. In some FCC units, ethane and ethene are recovered ...

regenerator

Depending on the design of the FCC unit, the coke on the catalyst in the regenerator can be partially oxidized to CO or completely to CO 2 . The amount of air supplied to the regenerator is controlled depending on the FCC type so that the desired CO / CO 2 ratio is achieved .

In the design in the picture, the coke is only partially oxidized to CO 2 . The CO and CO 2 smoke-containing gas is at 715 ° C and 2.4 bar above passed through containing a second swirl tubes separators, are separated in the 70 to 90% of the fine particles in the exhaust gas. This is necessary to avoid abrasion damage to the downstream turboexpander.

The exhaust gas is then expanded at the turboexpander (3), with a coupled compressor (2) compressing the supply air to the regenerator. The motor or generator can generate or consume electricity. If the expansion of the flue gas does not generate enough power for the drive, the motor / generator delivers the additional power required. If the expansion delivers more power than is required for compression, it generates electricity that is fed into the refinery network. The expanded flue gas is then fed into a steam generator (called a CO boiler ), in which the CO in the flue gas is burned in order to generate steam to cover the refinery requirements and to comply with the permitted CO emissions. The flue gas is then passed through an electrostatic precipitator, in which particles with a diameter of 2 to 20 μm are separated.

The steam turbine (1) is used to drive the compressor when the system is started up until there is enough flue gas to operate the expansion turbine (3).

Catalysts

The main catalysts used are zeolites . Zeolite Y is preferably used. Aluminum oxide plays a subordinate role . The zeolite crystals are usually embedded in a matrix that is held together by a binding agent and a filler. The matrix can be made of different materials and often also has a catalytic effect, so that larger molecules are “pre-cracked” so that they can penetrate through the narrow pores of the zeolite and react further inside, where the active centers are.

The catalyst consists of small spheres with a diameter of 60 to 100 μm. The bulk density is 800 to 960 kg / m³. It should be characterized by the following properties:

  • good resistance to high temperatures in the presence of water vapor,
  • high activity,
  • large macroporosity of the matrix,
  • Abrasion resistance,
  • low coking.

The catalytic sites in the zeolite are acidic centers on the aluminum. These centers are occupied by a sodium ion during the production of the zeolite. The sodium is later exchanged for ammonium , which is then removed by calcination, so that Lewis and Brønsted acids are formed. In some catalysts, the Brönsted acids are replaced by ion exchange with rare earth metals such as lanthanum or cerium in order to increase stability.

The binder usually consists of silicon dioxide and gives the particles the necessary strength. Mullite , which is obtained by calcining kaolin , is often used as a filler .

The residues used contain heavy metals such as nickel and vanadium, which act as catalyst poisons. Some catalysts contain matrices that trap these metals and protect the zeolite.

A small amount of fresh catalyst is continually added to the catalyst inventory of an FCC unit to replace deactivated or worn catalyst. The abrasion consists of particles that are smaller than 2 μm and are difficult to separate. This abrasion is contained in the heavy oil (English Marine Residual Fuel Oil ), which is used in ship engines, and can lead to wear there if it is not removed beforehand.

The main manufacturers of FCC catalysts are Albemarle , WR Grace and BASF (formerly Engelhard ).

mechanism

Crude oil consists of a large number of different organic components that may a. also contain small amounts of sulfur, nitrogen and oxygen. Copper, iron, nickel and vanadium are also present in small amounts. The hydrocarbons in crude oil can be divided into three different types:

Olefins (alkenes), which consist of unsaturated linear compounds, are not contained in natural crude oil.

In the FCC process, long-chain hydrocarbons are split into smaller chains by bringing them into contact with a catalyst at high temperature and moderate pressure. The liquid feed evaporates on contact with the hot catalyst. The cracking reactions take place in the riser in the gas phase.

In the cracking reactions, the longer-chain saturated compounds result in shorter unsaturated compounds (olefins) or aromatics. Depending on the degree of the reaction, components such as ethylene , propylene , butenes and isobutenes are formed , which are valuable as petrochemical feedstocks. Aromatics such as benzene , toluene or xylenes increase the octane number in gasoline.

Products

  • Refinery gas (also unsaturated compounds, if necessary separation of ethane / ethene )
  • LPG (also unsaturated, possibly separation into propane / propene and butane / butene )
  • CC light naphtha (boiling range: ≈ 25 ° C to ≈ 100 ° C, after desulfurization as a gasoline blending component )
  • CC heavy naphtha (boiling range: ≈ 100 ° C to ≈ 170 ° C, after desulfurization as reformer feed)
  • Light Cycle Oil (LCO) (boiling range: ≈ 170 ° C to ≈ 340 ° C, after desulfurization as HEL blending component, heavy oil blending component, HCU feed)
  • Heavy Cycle Oil (HCO) (boiling range: ≈ 340 ° C to ≈ 470 ° C, heavy oil blending component, after cleaning as HCU feed)
  • Slurry (boiling range: ≈ 470 ° C to ≈ 580 ° C, heavy oil blending component, after cleaning as HCU feed)

Web links

Individual evidence

  1. ↑ Derek Lowe, Das Chemiebuch, Librero 2017, p. 314