Tuesday, January 27, 2009

Catalytic Cracking

Catalytic Cracking


Already in the 30's it was found that when heavy oil fractions are heated over clay type materials, cracking reactions occur, which lead to significant yields of lighter hydrocarbons. While the search was going on for suitable cracking catalysts based on natural clays, some companies concentrated their efforts on the development of synthetic catalyst. This resulted in the synthetic amorphous silica-alumina catalyst, which was commonly used until 1960, when it was slightly modified by incorporation of some crystalline material (zeolite catalyst). When the success of the Houdry fixed bed process was announced in the late 1930s, the companies that had developed the synthetic catalyst decided to try to develop a process using finely powdered catalyst. Subsequent work finally led to the development of the fluidised bed catalytic cracking (FCC) process, which has become the most important catalytic cracking process.

Originally, the finely powdered catalyst was obtained by grinding the catalyst material, but nowadays, it is produced by spray-drying a slurry of silica gel and aluminium hydroxide in a stream of hot flue gases. Under the right conditions, the catalyst is obtained in the form of small spheres with particles in the range of 1-50 microns.

When heavy oil fractions are passed in gas phase through a bed of powdered catalyst at a suitable velocity (0.1-0.7m/s), the catalyst and the gas form a system that behaves like liquid, i.e. it can flow from one vessel to another under the influence of a hydrostatic pressure. If the gas velocity is too low, the powder does not fluidise and it behaves like a solid. If velocity is too high, the powder will just be carried away with the gas. When the catalyst is properly fluidised, it can be continously transported from a reactor vessel, where the carcking reactions take place and where it is fluidised by the hydrocarbon vapour, to a regenerator vessel, where it is fluidised by the air and the products of combustion, and then back to the reactor. In this way the proces is truly continous.

The first FCC unit went on stream in Standard Oil of New Jersey's refinery in Baton Rounge, Louisiana in May 1942. Since that time, many companies have developed their own FCC process and there are numerous varieties in unit configuration.

FCC Process Configuration

Hot feed, together with some steam, is introduced at the bottom of the riser via special distribution nozzles. Here it meets a stream of hot regenerated catalyst from the regenerator flowing down the inclined regenerator standpipe. The oil is heated and vaporised by the hot catalyst and the cracking reactions commence. The vapour, initially formed by vaporisation and successively by cracking, carries the catalyst up the riser at 10-20 m/s in a dilute phase. At the outlet of the riser the catalyst and hydrocarbons are quickly separated in a special device. The catalyst (now partly deactivated by deposited coke) and the vapour then enter the reactor. The vapour passes overhead via cyclone separator for removal of entrained catalyst before it enters the fractionator and further downstream equipment for product separation. The catalyst then descends into the stripper where entrained hydrocarbons are removed by injection of steam, before it flows via the inclined stripper standpipe into the fluidised catalyst bed in the regenerator.

Air is supplied to the regenerator by an air blower and distributed throughout the catalyst bed. The coke deposited is burnt off and the regenerated catalyst passes down the regenerator standpipe to the bottom of the riser, where it joins the fresh feed and the cycle recommences.

The flue gas (the combustion products) leaving the regenerator catalyst bed entrains catalyst particles. In particular, it entrains "fines", a fine dust formed by mechanical rubbing of catalyst particles taking place in the catalyst bed. Before leaving the regenerator, the flue gas therefore passes through cyclone separators where the bulk of this entrained catalyst is collected and returned to the catalyst bed.

Normally modern FCC is driven by an expansion turbine to mimimise energy consumption. In this expansion turbine, the current of flue gas at a pressure of about 2 barg drives a wheel by striking impellers fitted on this wheel. The power is then transferred to the air blower via a common shaft. This system is usually referred to as a "power recovery system". To reduce the wear caused by the impact of catalyst particles on the impellers (erosion), the flue gas must be virtually free of catalyst particles. The flue gas is therefore passed through a vessel containing a whole battery of small, highly efficient cyclone separators, where the remaining catalyst fines are collected for disposal.

Before being disposed of via a stack, the flue gas is passed through a waste heat boiler, where its remaining heat is recovered by steam generation.

In the version of the FCC process described here, the heat released by burning the coke in the regenerator is just sufficient to supply the heat required for the riser to heat up, vaporise and crack the hydrocarbon feed. The units where this balance occurs are called " heat balanced" units. Some feeds caused excessive amounts of coke to be deposited on the catalyst, i.e. much more than is required for burning in the regenerator and to have a "heat balanced" unit. In such cases, heat must be removed from the regenerator, e.g. by passing water through coils in the regenerator bed to generate steam. Some feeds cause so little coke to be deposited on the catalyst that heat has to be supplied to the system. This is done by preheating the hydrocarbon feed in a furnace before contacting it with the catalyst.

Main Characteristics

  • A special device in the bottom of the riser to enhance contacting of catalyst and hydrocarbon feed.
  • The cracking takes place during a short time (2-4 seconds) in a riser ("short-contact time riser") at high temperatures ( 500-540 0C at riser outlet).
  • The catalyst used is so active that a special device for quick separation of catalyst and hydrocarbons at the outlet of the riser is required to avoid undesirable cracking after the mixture has left the riser. Since, no cracking in thereactor is required, the reactor no longer functions as a reactor; it merely serves as a holding vessel for cyclones.
  • The regenerator takes place at 680-720 0C. With the use of special catalysts, all the carbon monoxide (CO) in the flue gas is combusted to carbon dioxide (CO2) in the regenerator.
  • Modern FCC includes a power recovery system for driving the air blower.

Equipment in FCC

  • Large storage vessels for catalyst (fresh and equilibrium)
  • Regenerator
  • Reactor
  • Main Fractionator
  • Product Work Up section (several distillation columns in series
  • Product treating facilities

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