How to address the high distillate yield at 500 degrees in vacuum residue
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The content of 500-degree residue in our unit often exceeds the limit, reaching 8–10 ml, whereas the specified control target is less than 5 ml. The main operating parameters are as follows: Vacuum degree at the top of the vacuum distillation tower: -98 KPa ； Vacuum degree in the vaporization section: -95 KPa; Top temperature of the tower: 60 degrees ； Vaporization section temperature: 388 degrees ； Outlet temperature of the vacuum furnace: 395 degrees. The lowest side stream in the vacuum system is vaporized oil, with full extraction. Some of the fuel oil returns to the bottom of the vacuum tower; the valves are almost fully closed (flow rate is very low). Strip steam temperature: 400 degrees. The flow rate is 0.5 t/h. The internal components use a combination of packing and trays; below the second stage, trays are used, with 3 trays in the stripping section. The open area ratio is 6.87. I’ve seen that in many atmospheric and vacuum distillation units, the distillate volume from the vacuum residue at 500 degrees is only 3 ml – why is there such a big difference? We tried many adjustment methods: such as increasing the stripping steam and raising the vacuum level (up to -99.5 kPa), but the effects were minimal.
Have you tried lowering the temperature at the bottom of the tower? Would it work if the outlet temperature of the heater is controlled at 385 degrees?
Adjust and optimize the operating parameters: furnace outlet temperature, vacuum level at the top of the vacuum distillation tower, and the flow rate of the side stream from the lower part of the vacuum distillation tower. If these adjustments do not yield any results, then it is likely a problem with the properties of the raw material. We also encounter such problems frequently; under the same operating conditions, there is a huge difference between different raw materials – the best ones yield 3.5, while the worst ones yield over 14.
1. Increase the outlet temperature of the vacuum furnace. 2. The trays in the stripping section were replaced with structured packing. 3. Today, regular packing is basically used in vacuum distillation towers; tray packing is used rarely, as it results in high pressure drops, which hinders an increase in the recovery rate. It is recommended to replace the entire tower with structured packing.
Check the vacuum level at the top of the vacuum distillation tower: if the vacuum level is not high, inspect the vacuum system to increase it； If the vacuum level is high, adjust the outlet temperature of the vacuum distillation furnace; too high a temperature may lead to thermal cracking of the light oil.
How is the 500 distillation temperature measured?