Fluid Catalytic Cracking

Fluid Catalytic Cracking (FCC, engl. ) Is the most significant mass conversion process in the petroleum-refining industry. This method is widely used for the implementation of heavy petroleum fractions into valuable olefins ( ethene, propene, butene ), cat cracker gasoline, gas oil (English Light Cycle Oil LCO ) and heavy oil components (Heavy Cycle Oil, HCO and slurry). As cracking processes of vacuum distillates, the thermal cracking was used originally, which has been almost completely replaced by the catalytic cracking process, since the latter produces more high-quality products (CC- gasoline olefins). Recently, the hydrocracking but steadily gains in importance.

The feed to an FCC unit consists usually of a fraction having a boiling range of ≈ 340 ° C to ≈ 560 ° C and an average molar mass of ≈ 200 to ≈ 600 g • mol -1. So technically is refinery heavy gas oil, vacuum gas oil (VGO ), deasphalted oil well (DAO ), or even furfural extract in question. In the FCC process, the high boiling petroleum fractions are evaporated and then the longer molecules in much smaller cleaved by bringing them at high temperature and moderate pressure with a fluidized, fixed shaped catalyst.

Process Description

Modern FCC units are continuous processes which are parked every 2 or 3 years of routine maintenance. There are different manufacturers each with their own style. The method can be licensed. There are two configurations: a vertical arrangement, in which the reactor through the regenerator in a single container is arranged and a horizontal arrangement in which both are arranged side by side.

The main providers and licensors are:

Horizontal arrangement:

• ABB Lummus Global
• Exxon Research and Engineering (ERE )
• Shell Global Solutions International
• Universal Oil Products (UOP )

Vertical arrangement:

• Kellogg Brown & Root ( KBR)

Reactor and regenerator

The pre-heated to 315 ° C to 430 ° C the starting material is fed with a partial stream from the bottom of the distillation column ( the slurry ) in the catalyst riser (6 ), where it is vaporized and cracked over the catalyst. All reactions take place in the catalyst riser. The hydrocarbon vapors fluidize the catalyst powder and promote it upward into the overlying separator in which amount the temperature about 535 ° C and the pressure is 1.7 barg.

In the separator, the spent catalyst is separated from the vapor product, with smaller particles in two hydrocyclones (4) are deposited. The catalyst then flows down through a Strippteil is injected into the steam to remove residual volatile hydrocarbons, which are also made of the product and are therefore valuable. The catalyst discharge is regulated by a valve (8).

Since during the cracking reactions is produced a larger amount of coke, which deposits on the catalyst surface, and these are rapidly deactivated, it is regenerated by oxidation. For this purpose it enters the regenerator, is injected into the air for combustion. The regenerator operates at a temperature of 700 ° C and a pressure of 2.4 bar. The heat of combustion is absorbed by the catalyst in part, and supplies the energy for the vaporization and for the endothermic reaction in the riser. For this reason, FCC units are referred to as "heat balanced ".

The hot catalyst from the regenerator is passed through a separator ( 5) can escape into the flue gases and recycled to the regenerator. The flow of regenerated catalyst in the lower part of the riser is controlled by means of a slide ( 7) in the catalyst line.

The hot flue gases leaving the separator through a series of two -stage hydrocyclones to separate the entrained catalyst particles.

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

Distillation column

The vapors from the reactor are passed into the distillation column, in which they are separated into gas, naphtha (CC- petrol), gas oil and heavy oil components. After further working-up and separation of sulfur components, the naphtha fraction, a mixing component for the gasoline represents the overhead vapors are directed to the continuous separation of the constituents of propane, propene, butane and butene, as well as the gases are hydrogen, methane, ethane and ethene. In some FCC units ethane and ethene is recovered.

Regenerator

Depends on the design of the FCC unit, the coke can be oxidized on the catalyst in the regenerator to be partially or fully CO to CO2. The amount of supply air to the regenerator is controlled depending on the FCC type that the desired ratio of CO/CO2 is established.

The design on the figure, the coke is only partially oxidized to CO2. The flue gas containing CO and CO2 is carried out at 715 ° C and 2.4 bar above atmospheric by means of a second swirl tubes containing separator conducted in which 70 to 90% of the fine particles are separated in the exhaust gas. This is necessary to avoid abrasion damage on subsequent turboexpander.

The exhaust gas is then expanded in turbo-expander ( 3), with a coupled compressor (2) compresses the feed air to the regenerator. The motor or generator may generate or consume electricity. If the relaxation is not enough generated power by the flue gas to drive the motor / generator provides the additional power needed. If the expansion provides more performance than is needed for the compression, it generates electric power, which is fed into the network of the refinery. The expanded flue gas is then passed into a steam generator (hereinafter CO boiler) in which the carbon is burned in the flue gas to generate steam for the recovery of refinery and to fulfill the demand permissible CO emission. Then the flue gas is passed through an electrostatic filter to be separated in the particles having a diameter of 2 to 20 microns.

The steam turbine (1) is used as the driving of the compressor at start-up until there is sufficient exhaust gas to operate the expansion turbine (2).

Catalysts

As catalysts, zeolites are mainly used. Preferably zeolite Y, is used. A subordinate role of alumina. The zeolite crystals are usually embedded in a matrix that is held together by a binder and a filler. The matrix can be made ​​of different materials and often also has a catalytic effect, so that larger molecules " precracked " so that they penetrate through the narrow pores of the zeolite and in the interior, where the active sites are, can react further.

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

• Good resistance to high temperatures in the presence of water vapor,
• High activity,
• Large macro-porosity of the matrix,
• Abrasion resistance,
• Low coking.

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

The binder is usually composed of silica particles and gives the necessary strength. Mullite as filler is often used which is obtained by calcination of kaolin.

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

The catalyst inventory of an FCC unit is always accompanied by a small amount of fresh catalyst, to replace the deactivated catalyst or abraded. The abrasive consists of particles which are to be separated is less than 2 microns and difficult. This abrasion is included in the heavy oil (English Marine Residual Fuel Oil), which is used in marine engines, where it can lead to wear when he is not separated before.

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

Chemistry

Crude oil consists of a plurality of different organic compounds that contain, inter alia, small amounts of sulfur, nitrogen and oxygen. Also copper, iron, nickel and vanadium are present in minor amounts. The hydrocarbons in the crude oil can be divided into three different types:

• Paraffins ( alkanes): Linear saturated structures without rings
• Naphthenes ( cycloalkanes ): Cyclic saturated compounds with or without paraffinic chains
• Aromatics: Cyclic unsaturated hydrocarbons such as benzene or naphthalene

Olefins ( alkenes) comprised of linear unsaturated compounds, are not included in the natural oil.

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

The cracking reactions arise from the long-chain saturated compounds shorter unsaturated compounds (olefins ) or aromatics. Depending on the degree of the reaction, components such as ethylene, propylene, butenes and isobutenes, which are valuable as petrochemical feedstocks form. Aromatics such as benzene, toluene or xylenes to increase the octane number of the gasoline.

Reactants

• Heavy vacuum gas oil ( standard feed)
• Light vacuum gas oil ( standard feed)
• Heavy gas oil (often when economic sense )
• Hydrowax (rarely, if available, if economically feasible )
• Visbreaker Flashed Distillate (often, if available)
• DAO ( rarely, if available)
• Coker heavy gas oil ( rarely, if available)
• Furfural extract ( rarely, if available)

Products

• Refinery gas (A and unsaturated compounds, optionally separating ethane / ethene )
• LPG ( unsaturated, optionally separated into propane / propylene and butane / butene )
• CC light naphtha (boiling range: ≈ 25 ° C to ≈ 100 ° C, after removal than Benzinblendingkomponente )
• CC- heavy naphtha (boiling range: ≈ 100 ° C to ≈ 170 ° C, after desulfurization as a reformer feed )
• Light Cycle Oil ( LCO ) ( range: ≈ 170 ° C to ≈ 340 ° C, after removal than HEL - blending component, Schwerölblendingkomponente, HCU Feed )
• Heavy Cycle Oil ( HCO ) ( range: ≈ 340 ° C to ≈ 470 ° C, Schwerölblendingkomponente, after purification as HCU Feed )
• Slurry ( range: ≈ 470 ° C to ≈ 580 ° C, Schwerölblendingkomponente, after purification as HCU Feed )