What is the process for desulfurizing alcohols using C4 alkali washing?

It’s not clear whether a mixer can ensure that liquefied gas meets the required standards; those that use fiber membranes exist, but they aren’t much larger than mixers. The extraction tower replaces your mixer; after the liquefied gas and alkali solution have settled, they usually still need to be washed with water. Without a catalyst, the alkali solution cannot be regenerated. It is also necessary to back-extract the disulfides from the alkaline solution after oxidation, and this is done using a component of oil with low total sulfur content for back-extraction.

In the pre-alkaline washing process, the alkaline solution is recycled; is the alkaline residue generated in this process a low-concentration alkaline solution? Then should this low-concentration alkaline solution be drained and replaced with fresh alkaline solution? Does the liquefied gas coming out of the extraction tower need to be washed and settled again? There is still a small amount of recombinant components in my C4; I plan to remove the thiol compounds first and then send it to the distillation column for removal of the heavier components. Also, is the alkali liquor regeneration tower a regular distillation tower? Is anything needed to be loaded? Does the lye coming out of the lye regeneration tower go directly to the top of the extraction tower? How is the alkaline slag from this extraction-regeneration process disposed of?

We keep the initial concentration of our pre-alkaline washing solution at around 18%; it is not recycled (we know that some other facilities use recycling). It is a vertical tank with some filler in the middle, and the alkaline solution is filled to a certain height. Liquefied gas enters from the bottom of the equipment and passes through the alkaline solution. I don’t remember what the concentration of the remaining alkaline solution was before it was replaced. It’s hard to say whether water washing is necessary in your case; after water washing of liquefied gas, the gas phase is removed, and MTBE is present in that gas phase. The alkali solution after extraction is first heated to 60 degrees, then air is introduced and it enters the oxidation tower (which contains Böhringer rings, each over one meter high, in four sections); subsequently it goes into a three-phase separator and a cooler, thereby completing the regeneration process to remove disulfides. The replacement agent alkali is mainly passed through a three-phase separator.

The efficiency of alkali-eluting methanethiol is quite high, and it is even higher when a fiber membrane is used. However, alkali washing is basically ineffective for disulfides; distillation is one method. Methanethiol can generally be reduced to the order of a few ppm ； As for the extraction tower, it generally refers to the extraction of disulfides from the regenerated alkaline solution; that is, the alkaline solution reacts with thiols through a catalytic (catalyst: sulfonated cobalt phthalocyanine) and oxidative (usually using air) process, and the sodium hydroxide produced as a result is reused. The disulfides are extracted from the alkaline solution using extraction equipment and solvents.

Isn’t the extraction tower the key equipment for removing methanethiol? It shouldn’t be used for extracting disulfides, right? The pre-alkaline wash earlier is intended to remove H2S, which poisons the catalysts used later on, as well as to remove a portion of methanethiol. I think there should be some kind of equipment for the alkali solution that has been regenerated in the alkali regeneration tower before it enters the extraction tower, right? Removing the disulfides – I’m not sure if that’s the right approach; I’ve seen supergravity separators being used to remove disulfides.

I didn’t explain it clearly enough; the extraction tower is a key device for desulfurization of alcohols. Nowadays, fiber liquid membrane devices are being used as alternatives to extraction towers, and there is a trend toward their gradual replacement. There are also several methods available for the separation of disulfides in the regenerated alkaline solution (after the oxidation tower), such as the supergravity separation you mentioned, as well as air lifting and extraction. It can therefore be said that in this type of operating condition, the key to C4 desulfurization lies in alkali solution regeneration, and the key to alkali solution regeneration is the separation of disulfides within it.

Pre-alkaline washing can remove some thiols, which is feasible; however, the only thing to note is that dealing with the alkaline sludge at present is very difficult, and there is significant pressure regarding the discharge of wastewater. Therefore, the pre-alkaline washing tank is primarily used to remove the small amount of hydrogen sulfide remaining in the liquefied gas; the actual removal of thiols takes place in the subsequent thiol removal unit, where the alkaline solution can be regenerated and reused, thereby reducing the generation of alkaline sludge. Whether a fiber membrane or a liquid-liquid extraction tower is used, the principle remains the same. Disulfides can be separated using a three-phase separator, or they can first be separated by sedimentation and then extracted using solvent oil; the outcome is identical. The goal is to reduce the amount of disulfides contained in the regenerated alkaline solution. There is also 40 ppm of dimethyl disulfide remaining; this can be removed to some extent by separating certain recombined components in C4. Therefore, the normal desulfurization process for C4 feedstock involves first removing some of the active sulfur through double desulfurization, and then removing some of the inactive sulfur through gas-phase desulfurization.