On the issue of catalytic fractionation
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I saw it on other forums, so I shared it here, hoping it might be useful as a reference. While I was on the 4-hour shift, there was a problem with distillation that I wasn’t able to figure out; I hope everyone can help me understand it. At 16:54, I received a notification from the control room stating that heavy contaminants were being mixed into the mixture. By 20:40, the liquid level in the diesel stripping tower dropped to 0 within two minutes; the on-site diesel pumps were unable to pump any fluid. The liquid level didn’t rise even by 24:00 when the shift changed, and the pumps still couldn’t deliver fluid. Upon investigating the trends, it was found that there were no significant changes in the pressure at the top of the tower or at the bottom of the distillation column. However, the pressure at the top of the tower increased from 32 kPa to 46 kPa, and the pressure fluctuations became more pronounced. During this time, increasing the reflux flow helped to raise the liquid level at the bottom of the distillation column somewhat, but it was necessary to increase the amount of slurry discharged in order to keep the liquid level under control. The backflow was reduced, and the liquid level at the bottom of the distillation tower returned to normal. However, the liquid level in the gasoline tank increased sharply. The temperature of the 16th tray rose from the normal level of 195 degrees to around 225 degrees; under normal conditions, the temperature of the 18th tray is about 20 degrees higher than that of the 16th tray. Now, there is no temperature difference between the 18th and 16th trays, with both having a temperature of around 225 degrees. Could someone help me analyze what could be causing this?
When heavy polluted oil is blended in, the reaction processes and product distribution inevitably change as a result. Regarding the phenomenon described by the poster, the countermeasure in terms of distillation should be to first reduce the amount of stripping steam used in the light diesel stripping tower, which should solve the problem.
I’m not sure whether your heavy residual oil is reprocessed through distillation or at the reaction unit Also, what is the reprocessing ratio? What are the properties of heavy crude oil? From the owner’s description, it can be seen that after reprocessing heavy contaminated oil, the light components at the top of the distillation tower increase, which leads to an increased gas load at the top and a significant rise in the pressure drop across the distillation tower; therefore, the proportion of oil that is reprocessed must be considerable.
It can be seen from the fact that both the bottom pressure and the pressure drop at the top of the distillation column increased from 32 kPa to 46 kPa that there is an abnormal liquid seal on the trays inside the distillation column. The cause is likely related to the heavily contaminated oil used in reprocessing; however, it is not clear whether this oil enters as a terminator, as part of the feed mixture, or separately. It is difficult to answer due to the uncertainty of the reprocessing location. But it’s certain that your gasoline is of poor quality; its octane rating is too high. This post was last edited by chengkang on 2009-3-6 18:23]
Reduce the amount of heavy residual oil and adjust the reaction process； Stop blowing steam; the backflow at layer 1 should not decrease too rapidly. Control the temperature at layer 16 as well as the top temperature. (The phenomenon mentioned by the poster may have caused slight tower damage; ensure proper gasoline delivery.)
Based on what the poster has described, the heavy hydrocarbons that were blended in appear to have come from the distillation tower; the amount blended in was significant, which resulted in an excessive liquid load in the lower sections of the tower. This led to an increase in the overall pressure drop across the tower, preventing diesel from flowing out； After reducing the medium load later on, the temperature in the middle of the tower rises; the liquid phase on the trays vaporizes, resulting in an increase in the amount of gasoline at the top of the tower. When fractionating and blending heavily polluted oil, due to its low temperature, the mid-stage reflux flow rate should be appropriately reduced to ensure satisfactory product quality.
The original poster should explain the question in more detail, especially the conditions, so it will be easier for everyone to analyze the reasons
This issue can be explained by the vapor pressure of hydrocarbons. When blending heavy dirty oil (similar to diesel components), it is necessary to increase the temperatures in the middle and top sections of the tower at the same time, as the vapor pressure of hydrocarbons in diesel increases under such conditions, resulting in a decrease in the 95% point of diesel. Similarly, when light contaminated oil is blended in, the vapor pressure of gasoline increases; therefore, the temperature of the top recycle tower should also be raised to prevent the dry point of gasoline from dropping too low, as well as to avoid other fluctuations.
Reprocessing of contaminated oil should, whenever possible, be done by blending it with crude oil in a certain proportion, provided that this does not affect operations. This phenomenon occurred because there were no detailed analysis data on the composition of the contaminated oil prior to reprocessing; as a result, there was an excess of light components and too little diesel content. The extraction pump became empty, and the stripping tower had no liquid level (in fact). It is appropriate to reduce the amount of steam used for stripping, primarily by decreasing the volume of contaminated oil sent for reprocessing, in order to adjust the load and pressure drops across the entire system.