Patent Publication Number: US-2019169509-A1

Title: Method to mitigate fouling of a vacuum wash bed

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present patent application is a continuation application of non-provisional application Ser. No. 15/342,310, filed Nov. 3, 2016, which claims the benefit of provisional patent application No. 62/251,285, filed on Nov. 5, 2015. 
    
    
     TECHNICAL FIELD 
     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. 
     BACKGROUND OF THE INVENTION 
     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&#39;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 during operations. It has been observed that a thermoset polymer is forming in the wash bed. 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. 
     SUMMARY OF THE INVENTION 
     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 affected, 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 thermoset from ever forming. 
     In still another embodiment, a combination of an LCO and steam is effective in preventing the formation of a thermoset 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. 
    
    
     
       IN THE DRAWINGS 
         FIG. 1  is a schematic view of a typical vacuum distillation column. 
         FIG. 2  is a graphical representation of a tomography scan elevation of a first tower. 
         FIG. 3  is a graphical representation of slope density of the tower of  FIG. 2  over time. 
         FIG. 4  shows a scan of new packing and a scan of the new packing of the tower of  FIG. 2  three years later. 
         FIG. 5  is a graphical representation of a tomography scan elevation of a second tower. 
         FIG. 6  is a graphical representation of slope density of the tower of FIG. over time. 
         FIG. 7  shows a scan of new packing and a scan of the new packing of the tower of  FIG. 5  four and one half years later. 
         FIG. 8  shows vacuum tower mask bed coking. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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  FIGS. 2 and 5  yield extensive cross-sectional information and data to monitor fouling/coking in packed beds. Tomography scans can be used to monitor wash bed coking and to make decisions on operating conditions to target cycle lengths for the tower. 
       FIG. 1  is a schematic view of a typical vacuum distillation column.  FIG. 1  shows the introduction of a wash oil. The wash oil preferably is a hydrocarbon solvent, such as light cycle oil (LCO), to wash the polymer out of the packing before the curing process occurs. The injection of a solvent such as LCO before the temperature of the wash bed drops below 350° F. removes the polymer before it hardens. 
       FIG. 2  is a graphical representation of a tomography scan elevation of a first tower. The scan is a baseline scan with new packing. 
       FIG. 3  is a graphical representation of slope density of the tower of  FIG. 2  over time. The scans were measured over a period of 3 years. The bed density increased with time. The graphical representation shows the improved design and operation of this invention in refinery distillation. 
       FIG. 4  shows a scan of new packing and a scan of the new packing of the tower of  FIG. 2  three years later. The baseline scan with new packing shows no thermoset forming. The scan 3 years later shows some thermoset forming. 
       FIG. 5  is a graphical representation of a tomography scan elevation of a second tower. The scan is similar to the scan of  FIG. 1 . 
       FIG. 6  is a graphical representation of slope density of the tower of  FIG. 5  over time. The scans were measured over a period of time of about 4.5 years. The bed density increased with time. The graphical representation shows the improved design and operation of this invention in refinery distillation. 
       FIG. 7  shows a scan of new packing and a scan of the new packing of the tower of  FIG. 5  four and one half years later. The baseline scan with new packing shows no thermoset forming. The scan 4.5 years later shows some thermoset forming. However, the representation shows the improved design and operation of this invention in refinery distillation. 
       FIG. 8  shows vacuum tower wash bed coking. The thermoset cannot be melted after curing. Once the “hard candy” (thermoset) has setup in the packing, the packing eventually must be discarded. 
     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.