Mineral oils: processes and products – Chemical conversion of hydrocarbons – With prevention or removal of deleterious carbon...
Reexamination Certificate
1999-06-11
2001-04-03
Yildirim, Bekir L. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
With prevention or removal of deleterious carbon...
C208S04800Q, C208S130000, C208S131000, C585S648000
Reexamination Certificate
active
06210560
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
The present invention relates to a method for reducing the tendency of thermal cracked oils to form solids that coat and plug process equipment. This method consists of reducing the time that the thermally cracked oil spends in the temperature range of 450 to 615° F. (232 to 324° C.), particularly 500 to 580° F. (260 to 304° C.). This is often most conveniently achieved by raising the temperature in the part of the process of highest residence time above 580° F. and preferably higher than 615° F.
It is well known that during the refining of petroleum oils that when the oil has been thermally cracked, the oil has a tendency to form carbonaceous, insoluble solids that coat and plug process equipment. The deposition of solids on process equipment is called fouling and the solids are called foulant. Example thermal cracking processes in a refinery include delayed coking, Fluid Coking, flexicoking, visbreaking, and gas oil thermal cracking. Examples of process equipment downstream of these thermal cracking processes are heat exchangers, reboilers, fractionators, and hydrotreater reactors. Often the carbonaceous solid has a puffed appearance that is called “popcorn coke”. Even small amounts of carbonaceous solids deposited on the surface of process equipment can greatly reduce the efficiency of refinery process equipment by reducing heat transfer. Large amounts of carbonaceous solids can result in high-pressure drop which reduces throughput so much that the unit has to be shut down for cleaning. Not only does this result in high cleaning cost but the even higher loss in revenue when the operation of the unit is interrupted and cannot process oil.
SUMMARY OF THE INVENTION
The thermal cracking of petroleum or petroleum derived products produces high concentrations (often 0.1 to 1% but as high as 10 wt. %) of molecules that contains at least one olefin (double bonded carbons) conjugated to an aromatic or another olefin. An olefin is conjugated to another olefin when the double bond is one carbon bond away from a carbon connected to a double bond:
These are called diolefins or dienes and R and R′ represent any hydrocarbon structure or hydrogen. Some double bonds are one carbon away from an aromatic, &phgr;, that could contain one or more fused aromatic rings:
but more likely
These olefins are conjugated to an aromatic. The term, conjugated olefins, includes both diolefins and olefins conjugated to an aromatic.
The concentration of conjugated olefins in an oil can most conveniently be measured by the Universal Oil Products (UOP) method number 326-82, which is well-known in the art. Although this test is said to measure the diene value, it is also known to measure olefins conjugated to aromatics. Thus, as defined here, this test measures the concentration of conjugated olefins. However, this test is not always accurate because not all conjugated olefin will be detected by the test. Examples are some cyclic dienes, such as acenapthalene. In addition some compounds that are not conjugated olefins are detected by the test, such as anthracene. (Anthracene is not typically found in petroleum derived oils.) Nevertheless, UOP method number 326-82 has been found to be accurate enough for the present invention. With the wide variety of hydrocarbon structures in petroleum oils, the conjugated olefins, when present, exist as a wide variety of structures and most are measured by this test.
According to UOP method number 326-82, the oil sample is dissolved in toluene with a known amount of maleic anhydride and refluxed for 3-4 hours. This allows the anhydride adduct to form from the reaction between maleic anhydride and conjugated olefins in the oil (illustrated in the figure below for the conjugated olefin, 2-vinyl naphthalene). Water is then added to the mixture and refluxed to convert remaining, unreacted maleic anhydride to maleic acid. The maleic acid is then isolated (water-soluble) and quantified by titration with sodium hydroxide. The amount of maleic anhydride that reacted with the oil is determined by difference.
The diene value is calculated by:
Diene
Value
=
(
B
-
A
)
(
1000
ml
/
l
)
(
M
)
(
126.9
)
100
W
=
gI
/
100
g
oil
Where, A=volume of NaOH solution required to titrate sample, mL
B=volume of NaOH solution required to titrate maleic anhydride without the oil
sample, mL
M=molarity of the NaOH solution (mol/L)
W=weight of oil sample, grams
Without knowing the exact molecular weight of the reacted dienes in the oil, the weight concentration cannot be determined. Hence, the reported units for diene value are on a molar basis. The molecular weight of iodine, 126.9 is standardly used and the reported diene value units are grams iodine/100 grams oil.
It is known that thermally cracked oils often have high diene values as measured by UOP method number 326-82 and that these oils are the ones that are most prone to fouling. The most common mitigating action is to add low concentrations of chemicals, such as free radical traps and/or dispersants. These can be expensive and often are not effective at temperatures above 530° F. Another mitigating action commonly used is to lower the temperature. This is not always possible within the constraints of the process and depending on the initial temperature, it could actually increase the rate of fouling.
The present invention is a method for mitigating the fouling from thermally cracked petroleum oils by minimizing the residence time at the temperature when the fouling is highest. It has been found that for petroleum oils having a diene value of 4 grams iodine/100 grams oil or greater, raising the temperature of the oil in the process above 580° F. reduces fouling.
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Brons Glen B
Cronin Linda S.
Wiehe Irwin A.
Exxon Research and Engineering Company
Hantman Ronald D.
Yildirim Bekir L.
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