Question regarding excessive sulfur content in isooctane alkylated by the sulfuric acid method!
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As the title indicates, recently the isooctane content in the company’s 200,000-ton per year alkylation product produced via the sulfuric acid process (DuPont process) has been above the specified limits; moreover, the copper sheet corrosion test did not meet the 3B-4A standard (black-gray color). The boiling range is as follows: initial boiling point 40.50%-105.90%-165； Final boiling point 210, dry point 198. The purity of isooctane (excluding n-octane) is 63%. The acid wash did not yield satisfactory results, so it was removed; the alkali wash solution was yellow in color and contained turbidity, while the water wash worked normally. The reaction temperature in the alkylation reactor is 9°C, the pressure is 0.45 Mpa, and the alkene to alkane ratio is 9:1. Please help analyze the reasons – thank you very much! @qugd
The failure of copper sheets to resist corrosion is mainly caused by low-cost sulfides, typically hydrogen sulfide and thioethers. The raw materials for alkylation are mainly C4 and C5 olefins from liquefied gas components; these materials contain relatively high levels of sulfides, as the desulfurization process in the upstream stage is not up to standard. However, since sulfuric acid is used as a catalyst in the alkylation process, the presence of sulfides does not affect the catalytic efficiency, but it does affect the total sulfur content in the resulting gasoline. Conduct sulfide analysis on the upstream raw materials to determine the type of sulfur, and enhance the desulfurization efficiency of liquefied gas. Washing can only remove the sulfur part of hydrogen sulfide from sulfuric acid, but it has little effect on thiols.
I used raw material C4 measured in microcoulombs, and the test result was 1 ppm; the sulfur content in the raw material taken from the decarburization towers in the pretreatment unit was 0, so it seems that the problems with the raw material aren’t severe.
Measure the pH after alkali washing, and prepare a sample with acid.
It’s a bit strange if the total sulfur content in the raw material is within acceptable limits, but the copper sheets fail the corrosion test. Check whether the residual sulfuric acid has been properly washed away; if it is caused by residual sulfuric acid, it should be possible to determine the amount involved. Also check whether sulfur-containing materials from other locations have mixed into the alkylation feedstock or product pipelines.
There are many possible causes for this, and it has been recommended to check the raw materials in order to identify the cause step by step. Check again whether the sulfuric acid you added has a purity of 99% or higher; if the testing of your raw materials is accurate, then the parameters should be normal. It would be advisable to check the quality parameters of the sulfuric acid. Alkaline washing can only remove inorganic sulfur; organic sulfur is difficult to eliminate. Since your current product contains organic sulfur, it will gradually decompose in your isobutane tower, causing severe corrosion to the equipment. Also, pay attention to the current in your reactor and the amplitude of the periodic vibrations; use these to determine whether the impeller is severely corroded and needs to be replaced. Under normal flow conditions, if the current is below 21, there is a problem. Check the acid-to-hydrocarbon ratio as well
I encountered a similar situation 15 years ago: the copper corrosion test failed, with a sulfur content of around 7 or 8. Do something more meaningful. The method I used was to place the ether-treated material from a company in Shandong in a separate tank. After 5 repeated trials of switching between different options, we convinced the boss with facts – that material costing the least could account for up to 20% of the total, and although it seemed fine just by looking at the list of raw materials, it was actually okay.
After alkaline cleaning, the pH value is around 11; after acid cleaning, it is around 5-6. All the acid has been replaced with 98% fresh acid
Okay, I’ll check the sulfuric acid; it should not be a big problem.
Do you mean there are impurities in a certain raw material? It’s very difficult to conduct this routine analysis; we can only use gas chromatography
Everyone else has analyzed it; I’ll add something extra. Yellowing and turbidity after alkali cleaning indicate quite severe corrosion of the equipment. This device is being corroded; the current conditions are insufficient to determine which part of the device is the problem