Mineral oils: processes and products – Chemical conversion of hydrocarbons – With prevention or removal of deleterious carbon...
Reexamination Certificate
2002-12-03
2004-08-10
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
With prevention or removal of deleterious carbon...
C208S050000, C208S0520CT, C208S106000, C208S131000
Reexamination Certificate
active
06773579
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method for reducing fouling in refinery units. More specifically, the invention relates to a method for reducing fouling in coking processes and fluid catalytic cracking (FCC).
BACKGROUND OF THE INVENTION
Fluidized bed coking (fluid coking) and FLEXICOKING are petroleum refining processes in which mixtures of heavy petroleum fractions, typically the non-distillable residue (resid) from fractionation, are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 900° F. to 1100° F. (about 480° C. to 590° C.) by heat supplied by fluidized coke particles.
Fouling in the stripper and scrubber sections of a Coker vessel results in decreased capacity and Run length of the unit, culminating in costly unplanned shutdowns.
Fluid Catalytic Cracking (FCC) is another petroleum refining conversion process in which heavy oil, typically the highest boiling distillable fraction, is converted to gasoline, diesel and jet fuel, heating oil, liquefied petroleum gas (LPG), chemical feedstocks, and refinery fuel gas by catalytic decomposition at similarly elevated temperatures of about 900° F. to 1100° F. (about 480° C. to 590° C.). Run length or capacity of an FCCU may likewise be limited by deposition of coke in the stripper, reactor overhead, plenum, nozzle, transfer line, or inlet to the fractionator.
What is needed in the art is an efficient, predictable, and effective way to mitigate the fouling tendency in the stripper, scrubber, surge drum and other sections of coking units.
SUMMARY OF THE INVENTION
An aspect of the invention is directed to a process for decreasing fouling in refinery unit reactor zones. Said refinery units operate at temperatures of at least about 300° C. as measured in the reactor zone. The method comprises preheating a feed to the refinery unit reactor zone and thereafter introducing the feed into the refinery unit reactor zone for reaction. The feed contains polymers and/or oligomers. The feed is preheated for a time and at a temperature sufficient to unzip an effective amount of said polymers and/or oligomers contained in said feed during the preheating step. The preheating step substantially reduces fouling in the refinery unit. Also, the upper limit for the temperature of the preheating is determined to prevent coke formation of more than about 5 percent by weight (wt %).
The invention is particularly useful in coking and cracking refinery units.
DETAILED DESCRIPTION OF THE INVENTION
Typically, hydrocarbon feeds to refinery coking units, may contain polymers and/or oligomers. These compounds may form in petroleum feeds by thermally initiated oxidative oligomerization reactions with certain feed components and are typically present in amounts of from about 0.02 wt % to about 5 wt % (200 to 50,000 wppm) in the feed. It is believed that these polymers and/or oligomers increase fouling in refinery reactors and equipment. Without wishing to limit the invention in any way it is believed that upon entering the reactor these compounds coat and/or thermally alkylate metal surfaces and the surfaces of any formed coke particles, making them sticky. Sticky coke particles can then agglomerate and also stick to the reactor and related equipment metallic and coke covered surfaces. The feed coating a coke particle surface can undergo further thermal conversion and a mesophase can form at the coke particle contact point, binding the coke particles together. The mesophase formation of the feed between the contact points of agglomeration is believed to be one mechanism leading to cementing together coke particles that were previously held together by weaker adhesive forces.
Typical feeds to coking units may comprise many different polymers, oligomers and mixed polymers and oligomers formed with styrenes, methylstyrenes, indenes, methylindenes and conjugated dienes with minor amounts of indoles, carbazoles, phenols, naphthols, thiophenols, thionaphthols, and the like components. The oligomers/polymers, depending on their molecular weight and the refinery unit reaction temperature, can become sticky and coat or alkylate coke particles with a sticky layer. It is believed that this sticky layer can lead to agglomeration of the coke particles and further coke formation at the contact points of agglomeration before they undergo sufficient thermal cracking conversion to reduce their stickiness via unzipping of the polymer chain.
In typical refinery unit operations, preheat treatments are typically used to raise the temperature of a feed to a desired temperature for energy balance purposes. Once the feed reaches the desired temperature, it is injected through feed nozzles into the reactor, or introduced by other means. It has been discovered that such a preheat scheme is inadequate to unzip any substantial amount of these sticky polymers and/or oligomers found in a feed. Upon entering the reactor the polymers and/or oligomers can lead to increased fouling. For example, thermal alkylation or coating of coke particles by long sticky oligomers and/or polymers may cause coke particle agglomeration. The heavier agglomerated sticky coke particles carry under to the stripper sheds and stick to the sheds causing fouling and buildup of foulant material. Applicants have unexpectedly discovered that preheating can be used to substantially reduce fouling in a refinery unit. Specifically, preheating a refinery feed containing a substantial amount of polymer and/or oligomers, for an amount of time and at a preheat temperature sufficient to unzip an effective amount of said polymers and/or oligomers substantially reduces fouling in the reactor and related reaction zone equipment such as stripper sheds. Without wishing to limit the invention in any way it is postulated that this additional preheating may prevent thermal alkylation and/or coating of coke particles by the most sticky polymeric materials. Alkylation and coating by smaller polymeric units and/or oligomers is then more easily overcome by thermal decomposition reactions at the higher temperature of the coke particles inside the reactor. For example, typically a reactor operating temperature may be about 530° C. while the feed may be preheated to about 345° C. in the preheat section of the reactor.
Thus, an embodiment of the invention includes preheating the feed to the refinery unit to a bulk temperature compatible with the economics of the unit and additionally continuing to preheat and thereafter introducing the feed into the reaction zone such that the combination of additional preheating and the heating which occurs due to the heating of the feed in the reaction zone causes at least 30%, preferably 50%, more preferably at least about 60%, and most preferably at least about 85% by weight of the polymers and oligomers contained in the feed to unzip. The preheating prior to introduction into the reaction zone is conducted such that no more than about 5 wt % coke is formed in the feed prior to introducing the feed into the reaction zone. The amount of polymers and/or oligomers that unzip is determined by a skilled artisan by running a Thermal Gravimetric Analysis (TGA) on the feed and determining the activation parameters; the preexponential factor and activation energy. The amount of unzipping many include both the amount of unzipping that occurs during preheating in addition to the amount of additional unzipping that would occur during the residence time of the feed in the reactor. For example, to determine if at least about 85% unzipping has occurred, in both the preheating and heating steps, the minimum residence time instead of the average residence time of the feed in the reaction zone would be utilized in the calculation for determining the amount of unzipping. This is because the amount of unzipping in the reactor should only be calculated for the minimum time that the coated coke particle could experience in the reactor section to be certain that it will be dry and not sticky when it enters other areas
Kelemen Simon Robert
Siskin Michael
ExxonMobil Research and Engineering Company
Georgellis George B.
Griffin Walter D.
Kliebert Jeremy J.
Nguyen Tam M
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