Catalytic cracking process

Details Catalytic cracking process Catalytic cracking process

2001-08-09

2003-12-09

Griffin, Walter D. (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

Cracking

C208S158000

Reexamination Certificate

active

06660158

ABSTRACT:

REFERENCE TO RELATED APPLICATIONS
This application is a National Phase filing under 35 U.S.C. §371 of PCT/NO00/00051 filed Feb. 10, 2000, which claims priority to Norwegian application #19990651, filed Feb. 11, 1999.
The present invention is related to a catalytic cracking and conversion process for upgrading of heavy oil in a hot fluidized bed in a vertical reactor whereby the gases leaving the reactor are be condensed or separated by distillation in a distillation tower.
The following general introduction to catalytic cracking, highlights present status and the outlined words and sentences focus on the difficulties/precautions which have to be met from case to case.
Catalytic cracker units (FCCU) processes are widely utilised in the petroleum industry in upgrading of oils. The ‘heart’ of such processes consists of a reactor vessel and a regenerator vessel interconnected to allow the transfer of spent catalyst from the reactor to the regenerator and of regenerated catalysts back to the reactor. The oil is cracked in the reactor section by exposing it to high temperatures and in contact with the catalyst. The heat for the oil cracking is supplied by the exothermic heat of reaction generated during the catalyst regeneration. This heat is transferred by the regenerated fluid catalyst stream itself. The oil streams (feed and recycle) are introduced into this hot catalyst stream en route to the reactor. Much of the cracking occurs in the dispersed catalysed phase along this transfer line or riser.
The final contact with the catalyst bed in the reactor completes the cracking mechanism. The vaporised cracked oil from the reactor is suitably separated from entrained catalyst particles by cyclones and routed to the recovery section of the unit. Here it is fractionated by conventional means to meet the product stream requirements. The spent catalyst is routed from the reactor to the regenerator after separation from the entrained oil. Air is introduced into the regenerator and the fluid bed of the catalyst. The air reacts with the carbon coating on the catalyst to form CO/CO
2
. The hot and essentially carbon-free catalyst completes the cycle by its return to the reactor. The flue gas leaving the regenerator is rich in CO. This stream is often routed to a specially designed steam generator where the CO is converted to CO
2
and the exothermic heat of reaction used for generating steam (the CO boiler).
Feedstocks to the FCCU are primarily in the heavy vacuum gas oil range. Typical boiling ranges are 340° (10%) to 525° C. (90%). This gas oil is limited in end point by maximum tolerable metals, although the new zeolite catalysts have demonstrated higher metals tolerance than the older silica-alumina catalyst. The processes have considerable flexibility. Apart from processing the more conventional waxy distillates to produce gasoline and other fuel components, feedstocks ranging from naphtha to suitably pre-treated residuum are successfully processed to meet specific product requirement.
The fluid catalytic cracker is usually a licensed process. Correlations and methodology are therefore proprietary to the licensor although certain data are divulged to clients under the licensor agreement. Such data are required by clients for proper operation of the unit, and may not be divulged to third parties without the licensor's express permission.
These and others, including operating instructions, are required for the proper operation of the units. Most of the proprietary data, however, concern the reactor/regenerator side of the process. The recovery side—that is, the equipment required to produce the product streams from the reactor effluent—utilises essentially conventional techniques in their design and operating evaluation.
Up to the late 1980s feedstocks to FCCU were limited by characteristics such as high Condradson carbon and metals. This excluded the processing of the ‘bottom of the barrel’ residues. Indeed, even the processing of vacuum gas oil feeds were limited to
Condradson carbon<10 wt %
Hydrogen content>11,2 wt %
Metals NI+V<50 ppm
During the late 1980s significant research and development breakthroughs produced a catalytic process that can handle these heavy feeds and indeed some residues. Feedstocks heavier than vacuum gas oil when fed to a conventional FCCU tend to increase the production of coke and this in turn deactivates the catalyst. This is mainly the result of:
A high portion of the feed that does not vaporise. The un-vaporised portion quickly cokes on the catalyst, choking its active area.
The presence of high concentrations of polar molecules such as polycyclic aromatics and nitrogen compounds. These are absorbed into the catalyst's active area causing instant (but temporary) deactivation.
Heavy metals contamination that poison the catalyst and affect the selectivity of the cracking process.
High concentration of polynaphthenes that dealkylate slowly.
In the FCCU process conventional feedstock cracking temperature is controlled by the circulation of hot regen catalyst. With the heavier feedstocks, with an increase in Condradson carbon there will be a larger coke formation. This in turn produces a high regen catalyst temperature and heat load. To maintain heat balance catalyst circulation is reduced, leading to poor or unsatisfactory performance. Catalyst cooling or feed cooling is used to overcome this high catalyst heat load and to maintain proper circulation.
The extended boiling range of the feed as in the case of residues tends to cause an uneven cracking severity. The lighter molecules in the feed are instantly vaporised on contact with the hot catalyst and cracking occurs. In the case of the heavier molecules vaporisation is not achieved as easily. This contributes to a higher coke deposition with a higher rate of catalyst deactivation. Ideally, the whole feed should be instantly vaporised so that a uniform cracking mechanism can commence. The mix temperature (which is defined as the theoretical equilibrium temperature between the uncracked vaporised feed and the regenerated catalyst) should be close to the feed dew point temperature. In conventional units this is about 20-30 'C above the riser outlet temperature. This can be approximated by the expression:
T
m
=T
R
+0
u
1
&Dgr;AH
c
T
m
=the mix temperature
T
R
=riser outlet temperature (° C.)
&Dgr;Ah
c
=heat of cracking (BTU/lb or kJ/kg)
This mix temperature is also slightly dependent on the catalyst temperature. Cracking severity is affected by polycyclic aromatics and nitrogen. This is so because these compounds tend to be absorbed into the catalyst. Raising the mix temperature by increasing the riser temperature reverses the absorption process. Unfortunately, a higher riser temperature leads to undesirable thermal cracking and production of dry gas.
The processing of heavy feedstocks therefore requires special techniques to overcome:
Feed vaporisation.
High concentration of polar molecules.
Presence of metals.
Some of the techniques developed to meet heavy oil cracking processing are as follows:
Two-stage regeneration.
Riser mixer design and mix temperature control (for rapid vaporisation).
New riser lift technology minimising the use of steam.
Regen catalyst temperature control (catalyst cooling).
Catalyst selection for:
Good conversion and yield pattern.
Metal resistance.
Thermal and hydrothermal resistance.
High-gasoline RON.
An important issue in the case of heavy oil fluid catalytic cracking is the handling of the high coke lay-down and the protection of the catalyst. One technique that limits the severe conditions in regeneration of the spent catalyst is a two-stage regenerator.
The spent catalyst from the reactor is delivered to the first regenerator. Here the catalyst undergoes a mild oxidation with a limited amount of air. Temperatures in this regenerator remain fairly low, around 700-750° C. From this first regenerator the catalyst is pneumatically conveyed to a second. Here excess air is used to complete the carbon burn-off and tempe

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