Mineral oils: processes and products – Chemical conversion of hydrocarbons – Plural serial stages of chemical conversion
Reexamination Certificate
2000-03-02
2001-07-10
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Plural serial stages of chemical conversion
C208S072000, C208S077000, C585S324000, C585S329000, C585S330000
Reexamination Certificate
active
06258257
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing polypropylene from C
3
olefins produced in a two-stage process for selectively producing C
2
to C
4
olefins from a gas oil or resid.
BACKGROUND OF THE INVENTION
The need for low-emissions fuels has created an increased demand for light olefins used in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin production, particularly polypropylene production.
Fixed bed processes for light paraffin dehydrogenation have recently attracted renewed interest for increasing olefin production. However, these types of processes typically require relatively large capital investments and high operating costs. It is therefore advantageous to increase olefin yield using processes that require relatively small capital investment. It would be particularly advantageous to increase olefin yield in catalytic cracking processes.
While conventional fluid catalytic cracking processes are suitable for producing conventional transportation fuels, these fuels are generally unable to meet the more demanding requirements of low emissions fuels and chemical feedstock production. To augment the volume of low emission fuels, it is desirable to increase the amounts of light olefins, such as propylene, iso- and normal butylenes, and isoamylene. The propylene, isobutylene, and isoamylene can be reacted with methanol to form methyl-propyl-ethers, methyl tertiary butyl ether (MTBE), and tertiary amyl methyl ether (TAME). These are high octane blending components which can be added to gasoline to satisfy oxygen requirements mandated by legislation. In addition to enhancing the volume and octane number of gasoline, they also reduce emissions. It is particularly desirable to increase the yield of ethylene and propylene, which are valuable chemical raw materials. Conventional fluid catalytic cracking does not produce large enough quantities of these light olefins, particularly ethylene. Consequently, there exits a need in the art for methods of producing larger quantities of ethylene and propylene for chemicals raw materials, and other light olefins for low-emissions transportation fuels, such as gasoline and distillates.
A problem inherent in producing olefin products using FCC units is that the process depends upon a specific catalyst balance to maximize production. In addition, even if a specific catalyst balance can be maintained to maximize overall olefin production, olefin selectivity is generally low due to undesirable side reactions, such as cracking, isomerization, aromatization and hydrogen transfer reactions. Therefore, it is desirable to maximize olefin production in a process that allows a high degree of control over the selectivity of C
2
, C
3
, and C
4
olefins.
SUMMARY OF THE INVENTION
Gas oil or resid is reacted in a first stage comprising a fluid catalytic cracking unit wherein it is converted in the presence of conventional large pore zeolitic catalyst to reaction products, including a naphtha boiling range stream. The naphtha boiling range stream is introduced into a second stage comprising a process unit containing a reaction zone, a stripping zone, a catalyst regeneration zone, and a fractionation zone. The naphtha feed is contacted in the reaction zone with a catalyst containing from about 10 to 50 wt. % of a crystalline zeolite having an average pore diameter less than about 0.7 nanometers at reaction conditions which include temperatures ranging from about 500 to 650° C. and a hydrocarbon partial pressure from about 10 to 40 psia (about 70-about 280 kPa). Vapor products are collected overhead and the catalyst particles are passed through the stripping zone on the way to the catalyst regeneration zone. Volatiles are stripped with steam in the stripping zone and the catalyst particles are sent to the catalyst regeneration zone where coke is burned from the catalyst, which is then recycled to the reaction zone.
One embodiment of the present invention comprises a process for producing polypropylene from olefins produced in a two stage process for selectively producing C
2
to C
4
olefins from a heavy hydrocarbon feed, the process comprising the steps of (a) contacting a heavy hydrocarbon feed with a large-pore zeolitic catalytic cracking catalyst having an average pore diameter greater than about 0.7 nm in a first reaction stage comprising a fluid catalytic cracking unit to convert the heavy hydrocarbon feed to lower boiling reaction products; (b) fractionating said lower boiling reaction products into at least a naphtha boiling range fraction comprising between about 10 and about 30 wt. % paraffins and between about 15 and about 70 wt. % olefins; (c) contacting the naptha boiling range fraction with a second catalyst comprising between about 10 and about 50 wt. % of a crystalline zeolite having an average pore diameter less than about 0.7 nm in a second reaction stage comprising a process unit comprising a reaction zone, a stripping zone, a second catalyst regeneration zone, and a fractionation zone, wherein the naphtha boiling range fraction is contacted with the second catalyst in the reaction zone at reaction conditions which include temperatures ranging from about 500 to 650° C. and a hydrocarbon partial pressure from about 10 to about 40 psia (about 70-about 280 kPa) and a catalyst to second stage feed weight ratio of about 4 to about 10, and wherein propylene comprises at least about 90 mol. % of the total C
3
product; (d) passing catalyst particles through the stripping zone; (e) passing the stripped second catalyst particles to the regeneration zone where coke is combusted from the second catalyst; (f) recycling the hot catalyst particles to the second stage reaction zone; and, (g) separating the propylene and polymerizing the propylene to form polypropylene.
In another embodiment of the present invention the second stage catalyst is a ZSM-5 type catalyst.
In another embodiment of the present invention the second stage feed contains about 10 to 30 wt. % paraffins, and from about 20 to 70 wt. % olefins.
In yet another embodiment of the present invention the second stage reaction zone is operated at a temperature from about 525° C. to about 600° C.
DETAILED DESCRIPTION OF THE INVENTION
Catalytic cracking is an established and widely used process in the petroleum refining industry for converting petroleum oils of relatively high boiling point to more valuable lower boiling products, including gasoline and middle distillates, such as kerosene, jet fuel and heating oil. The pre-eminent catalytic cracking process now in use is the fluid catalytic cracking process (FCC) in which a pre-heated feed is brought into contact with a hot cracking catalyst in the form of a fine powder, typically having a particle size of about 10-300 microns, usually about 60-70 microns, for the desired cracking reactions to take place. During the cracking, coke and hydrocarbonaceous material are deposited on the catalyst particles. This results in a loss of catalyst activity and selectivity. The coked catalyst particles, and associated hydrocarbon material, are subjected to a stripping process, usually with steam, to remove as much of the hydrocarbon material as technically and economically feasible. The stripped particles containing non-strippable coke, are removed from the stripper and sent to a regenerator where the coked catalyst particles are regenerated by contact with air, or a mixture of air and oxygen, at an elevated temperature. This results in the combustion of the coke. The combustion is a strongly exothermic reaction that removes the coke and heats the catalyst to the temperatures appropriate for the endothermic cracking reaction. The process is carried out in an integrated unit comprising the cracking reactor, the stripper, the regenerator, and the appropriate ancillary equipment. The catalyst is continuously circulated from the reactor or reaction zone, to the stripper and th
Asplin John E.
Bedell Michael W.
Henry B. Erik
Ladwig Paul K.
Stuntz Gordon F.
Cromwell Michael A.
ExxonMobil Research and Engineering Company
Griffin Walter D.
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