Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Reexamination Certificate
1999-09-02
2003-05-06
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
C208S153000, C208S157000
Reexamination Certificate
active
06558530
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the fluid catalytic cracking which is improved by the pre-vaporization of the hydrocarbon feed that is to be cracked in the catalytic cracking unit. More specifically, the present invention relates to a process for the catalytic cracking in a fluidized bed where the feed is completely and previously vaporized before it is made to contact the hot cracking catalyst from the regenerator vessel. In terms of the vaporization of the feed the pre-vaporization of the feed dispenses with the heat exchange with the catalyst, so that coke deposits on the catalyst particles are minimized.
BACKGROUND INFORMATION
Fluid catalytic cracking (FCC) is a process of paramount importance for obtaining valuable oil products such as gasoline, diesel oil and liquefied petroleum gas (LPG).
In spite of the fact that the FCC process has been known for more than 50 years, new, improved techniques are continuously sought which could improve the process, so as to increase the yield in more valuable products. Broadly speaking, it is common knowledge that the main object of the FCC processes is the maximization of more valuable products.
Generally, the feed stocks which are more usually submitted to the FCC process are those refinery streams from side cuts of vacuum towers, the so-called vacuum heavy gasoil (VHGO), or streams which are heavier than the former, from the bottom of atmospheric towers, the so-called atmospheric residue (AR), or still, mixtures of such streams. When submitted to the FCC process, such streams are made to contact a catalyst made up of fine particulate material in a conversion zone, in the absence of hydrogen, so as to be converted into lighter and more valuable hydrocarbon streams.
According to the state-of-the-art FCC process, it is only when the feed is made to contact the regenerated hot catalyst that heat transfer occurs so as to attain the required temperatures to vaporize the feed and effect the cracking reactions.
A well-known aspect of the process is the initial contact of the catalyst and the feed, which has a decisive influence on the conversion and selectivity to noble products.
Improvements in the contact between catalyst and feed have been tried, always aiming at promoting a quick vaporization of the feed and an intimate contact with the catalyst in the short time available in the reaction zone, which averages 2 seconds. In the reaction zone, the catalyst may flow upstream (riser) or downstream (downer).
In order that the catalytic reactions be processed, it is necessary that the vaporization of the feed and catalyst in the mixing area occurs in a few milliseconds, so that the molecules of vaporized hydrocarbons may contact the catalyst particles, permeate its micropores and be influenced by the acidic sites of the catalyst which cause the catalytic cracking.
In case a quick vaporization is not reached, the thermal cracking of the liquid portions of the feed is observed. It is well known that thermal cracking leads to by-products such as coke and fuel gas, mainly for residua-containing feeds. Therefore, thermal cracking on the basis of the reaction zone undesirably competes with the catalytic cracking aimed by the process.
A better vaporization will be obtained if the feed is properly vaporized so as to form a fine spray on the catalyst phase. In order to obtain such spray several models of feed atomizer in the reaction zone have been developed, for example, the ULTRAMIST atomizer developed by PETROBRAS. A really efficient system for atomizing the feed would yield a spray having particles fine enough so that 90% of the feed could be vaporized in less than 0.1 seconds, preferably in less than 0.05 seconds, considered from the moment of the injection of the feed in the reaction zone.
A further important parameter in the atomization of the feed is the temperature of the feed in the atomizer. The higher the temperature of the feed in the atomizer, the higher will be the surface area of the droplets in the spray, and therefore the higher will be the contact area between the feed and the catalyst, with significant impact on the ease of vaporization.
For the residual feeds used in the FCC process and for the temperature ranges practiced, it can be seen that the increase in the contact area by using higher temperatures of the feed may attain 30%.
In the industrial furnaces used for pre-heating the feed of the FCC unit, the temperature is limited to 300° C., which is insufficient to vaporize it. It is obvious that the feeds of the FCC unit cannot be indefinitely heated since the excessive heating would cause the thermal cracking and the coking of the furnaces.
Therefore, although desirable, the complete vaporization of the feed does not occur in an efficient way in the conventional FCC units, mainly in those, which operate with heavy gasoil or atmospheric residua, or even with a mixture of these feeds. The contact of the feed, which is not completely vaporized, with the hot catalyst eventually results in the deposition of liquid droplets of the feed on the surface of catalyst particles, so that coke production is increased.
Only the pre-vaporization of the feed to the FCC unit would prevent the deposition of liquid on the catalyst particles and eliminate the negative effects of such deposition. The specialized literature, however, has not focused this matter, the subject of choice being the atomization or the pre-treatment of the feed.
One point, which has not received much attention from the experts in the fluid catalytic area, is that if the heating of the feed up to the point of complete vaporization before contacting the regenerated catalyst could bring further benefits to the FCC process.
The complete vaporization of the feed seems to be actually necessary. A few studies show that, even at the high temperatures found in the reaction zone, the feed may “wet” the hot catalyst particles and be sucked by its pores through a capillary action, whereby coke is formed. The estimated time of suction, through capillary action of the liquid present in the surface into the interior of the pores is of the order of 0.1 seconds, this figure being frequently lower than the time required for the complete vaporization of the liquid.
G. Yang et al. in “Some Fundamental Aspects of Residuum Catalytic Cracking”, Acta Petrolei Sinica (Petroleum Processing Section), October, 1997, pages 12-16, collected catalyst samples at various heights of the reaction zone (riser) of an industrial FCC unit and assessed the carbon content and the microactivity (MAT) of such samples. The results indicate that at 1.0 meter height up to the point of feed injection, the carbon content rises steeply from 0.2 to 2.71 weight %, while MAT activity decreases from 65.9 to 28.1 percent. The coke content of samples from a higher point, at 3.3 meters in the reaction zone (riser) decreases up to 1.01 weight %, the MAT activity is partially recovered, and attains 45.9%. Above this point, carbon and MAT activity are only slightly altered.
To explain these results, the experts assumed that the catalyst coverage by carbon and the filling of the catalyst pores with droplets of liquid feed lower the catalyst activity, the portion of the feed, which is not vaporized being considered as coke. The reactions of the non-vaporized feed proceeding throughout the reaction zone (riser), the catalyst surface and the MAT activity are in part recovered.
Therefore, these observations have led the Applicant to infer that the complete vaporization of the feed of the FCC unit, before the contact with the regenerated catalyst may bring additional benefits to the FCC process, besides those obtained by just the increase in the feed temperature.
However, in spite of several clues as to the importance of the complete vaporization of the feed for the FCC units, the patent literature focuses the solution of the problem of heavy feed cracking on other issues.
U.S. Pat. No. 5,324,418 and U.S. Pat. No. 5,324,419 suggest to make closer the temperature of the regenerated catalyst and the f
Pereira Carlos Guerra
Pinho Andrea De Rezende
Schlosser Claudio Romeo
Griffin Walter D.
Petroleo Brasileiro S.A.-Petrobas
Sughrue & Mion, PLLC
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