【Chemical Engineering Theory】Etherified gasoline turns black in color
Thread Content
This post was last edited by Xiao Gong 1985 on 2016-3-15 09:12. Dear seniors, last September our company replaced the catalyst used in the gasoline etherification process; at the same time, we installed a new gasoline hydrogenation unit. The gasoline used in etherification is pre-hydrogenated light gasoline. Starting from January this year, the gasoline after etherification turns dark red while the temperature, pressure, and alcohol-to-gasoline ratio remain unchanged； Stop the methanol feed, lower the temperature, replace it with light gasoline, and then restart the operation. After operating normally for a while, the gasoline obtained after etherification changes color again. This occurred twice in January, four times in February, and three times in March; the cause has yet to be identified. I would like to ask those with more experience: what could be the possible reasons for the color change in the gasoline obtained after etherification?
What kind of gasoline hydrogenation process? Blackish-red; it is likely that gel has been formed.
I think it can be considered from the following aspects: 1: Is the selective hydrogenation to remove dienes complete? 2: Consider adding raw material washing to remove basic nitrogen compounds and metal ions.
After hydrogenation, as long as the operation is normal, it should not be a diene issue； The process design for the etherification of light gasoline is very important; it differs from the MTBE process. The color change is mainly due to the formation of gums ； Is there some issue with the reactor design?
The color of gasoline should be soy sauce-colored; a major reason for this is the loss of sulfonic acid groups on the catalyst. 1. The raw materials contain water, mainly water present in the raw materials or methanol. 2. The reaction temperature is too high. (Mainly present in the catalytic distillation column section) 3. When the bed is empty and the C4 content in the feed is too high, it can result in a gas phase forming at the top of the reactor, preventing contact with the catalyst; as a result, the reactive olefins enter the lower, high-temperature beds, causing localized overheating inside the reactor. (This problem does not exist in fluidized bed reactors.) 4. The metal ion content in the raw material is above the specified limit, requiring washing. 5. An inappropriate alcohol-to-olefin ratio can lead to an increase in side reactions, whether it is too high or too low. 6. An excessively high extraction rate of light gasoline leads to excessive sulfur content in the raw material. The sulfur content in methanol is above the standard.
The blackening and reddening of gasoline is caused by an excessive level of gums; it is likely that the amount of dienes after hydrogenation is too high. It would be better to use hydrogenated heavy gasoline for etherification, while light gasoline can be treated with desulfurization alone
Whether the diene content in the raw material exceeds the specified limit.
The hydrogenation is a selective hydrogenation process; that is, catalytic gasoline is fed into a gasoline hydrogenation unit, where it first undergoes selective hydrogenation in a pre-hydrogenation reactor to remove dienes, and then light gasoline is separated out in a pre-distillation tower to be used as a raw material for gasoline etherification. The reactor used is of the fixed expansion bed type. We do not have any methods to analyze dienes, but this kind of problem has not occurred despite the operation of this etherification unit for ten years. The gel content of the light gasoline before etherification was 1.4 mg/100 ml; after the problem arose, the gasoline became soybean-colored and its gel content increased to 13.8 mg/100 ml. Recently, this discoloration has occurred more frequently. We kindly ask our experienced colleagues to help analyze the reasons for this.
Yes, it’s a complex case that has never occurred before
The issue has been resolved, thank you to all the seniors
Where is the problem? How was it solved? Share it