This invention relates to a method for mitigating fouling of a vacuum tower wash bed during the vacuum distillation of a petroleum product. The invention may be used in the petroleum refining industry for fractionating of petroleum base stock in a vacuum column.
Refinery fractionator wash beds are monitored for fouling/coking of the wash bed on a regular basis. The density of the fouling/coking in the wash beds increase over time, which allows for a prediction of the tower's ability to run efficiently over time.
When the fouling/coking of the wash bed becomes a serious impediment to operating efficiency, the entire process unit may be taken off-stream for an extended period for renewal of the wash beds. This is commonly referred to as a turn-around (TAR).
During a TAR cycle, after the vacuum heater trips and the wash bed temperature drops below 400° F., a density scans show an increase in the slope of the fouling/coking in the wash bed. It has been observed that a thermoset polymer is forming in the wash bed during operations. A thermoset polymer is a petrochemical in a soft-solid or viscous state that changes irreversibly when cured into an infusible, insoluble polymer network. Once the thermoset is cured, then the polymer can only be removed by physically changing the packing in the wash bed during a TAR.
A first embodiment of the invention utilizes a hydrocarbon solvent, such as light cycle oil (LCO), to wash the polymer out of the packing before the curing process occurs. The flushing of this soft-solid polymer is effected, upon shutdown of the heater, by introducing a large amount of the solvent to the bed at an elevated temperature (>500° F.) to move the material down the tower to be removed with the vacuum bottoms stream.
A second embodiment of the invention introduces the solvent continuously, during operations, at a much smaller injection rate, to hinder the buildup of the soft-solid or viscous polymer in the wash bed and thus prevent the therrnoset from ever forming.
In yet another embodiment, introduction of a fluid medium of steam can keep the wash bed temperature above 350° F.-400° F. to prevent the therrnoset from forming.
In still another embodiment, a combination of an LCO and steam is effective in preventing the formation of a therrnoset polymer in the wash beds.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon a review of the following detailed description of the preferred embodiments and the accompanying drawings.
The application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the office upon request and payment of the necessary fee.
The cooling of the vacuum tower wash bed below 350° F. causes a thermoset polymer to form, which then cannot be removed except by mechanical means. An injection of a solvent such as LCO before the temperature of the wash drops below 350° F. removes the polymer before it hardens.
The LCO contains petroleum distillates. In one embodiment, the LCO is a complex mixture of paraffinic, cycloparaffinic, olefinic and aromatic hydrocarbons. The LCO is predominately C9-C25 hydrocarbons produced by the distillation of products from a catalytic cracking process. This stream is likely to contain a relatively large portion of bicyclical aromatic hydrocarbons.
In another embodiment, the fluid medium may be an FCC slurry or decanted oil. Typically the FCC slurry consists of aromatic hydrocarbons from FCC slurry oil processing technologies including hydrotreating, solvent refining and other separation techniques. Decanted oil may be a fluid catalytic cracker decanted oil, a heavy cycle oil, or a filtered decanted oil.
As an alternative to LCO steam can be introduced into the tower to keep the wash bed temperature above 350° F. to prevent the onset of thermoset polymer formation, with no significant change in wash bed performance.
In another embodiment, saturated steam may be introduced over a long period of time. Even if the wash bed cools to 350° F., the thermoset polymer will be kept from forming. While the preference is to inject the steam into the heater during the period of downtime, the continued introduction of stripping steam into the bottom of the column is adequate to prevent formation of the thermoset polymer.
Tomography scans such as shown in
The above detailed description of the present invention is given for explanatory purposes. It will be apparent to those skilled in the art that numerous changes and modifications can be made without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense, the scope of the invention being defined solely by the appended claims.
The present patent application is based upon and claims the benefit of provisional patent application No. 62/251,285, tiled on Nov. 5, 2015.
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Entry |
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Stuart Fraser, Distillation in Refining, Distillation Operation and Applications (2014), pp. 155-190 (Year: 2014). |
Number | Date | Country | |
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20170327749 A1 | Nov 2017 | US |
Number | Date | Country | |
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62251285 | Nov 2015 | US |