What are the reasons for the high total sulfur content in naphtha in diesel hydrogenation units? Why is the total sulfur content in naphtha higher than that in refined diesel?
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This post was last edited by 328104062 on 2016-2-24 at 11:24. As the title suggests, it deals with a diesel hydrogenation unit equipped with a dew point reduction system. The main parameters are as follows: reactor inlet temperature of 320–350°C (varies depending on the season and required dew point); high-pressure side pressure of 3.8 MPa, low-pressure side pressure of 2.5 MPa; stripping tower bottom temperature of 160°C, top temperature of 90–120°C (varies depending on the season and product specifications); distillation tower bottom temperature of 285°C, top temperature of 100°C. The total sulfur content in the feedstock is approximately 400 ppm, while the design value is 580 ppm. The original design specified that the total sulfur content in the naphtha should be less than 5 ppm, but in reality it is 150 ppm. For refined diesel, the designed total sulfur content should be less than 10 ppm in winter, but in practice it ranges from 10–50 ppm (depending on whether National IV or National V standards apply; National V standards have not been fully implemented yet). Questions: 1. What are the design specifications for the naphtha used in diesel hydrogenation? Can it meet the design specifications? 2. Why isn’t the designed value achieved? Is it due to an error on the part of the designers or operational issues, such as the designed flow rate of the stripping steam being 3 t/h while the actual value is 2 t/h, or issues related to temperature? 3. Generally, the sulfur content in the heavy fractions at the bottom of the tower is higher than that in the light fractions at the top. In atmospheric and catalytic processes, the total sulfur content in diesel is greater than that in gasoline (naphtha). Why is it the opposite in hydrocracking distillation towers, where the total sulfur content in diesel is lower than that in gasoline (naphtha)? ? Is it the increase in small molecular sulfides after hydrogenation that leads to a higher sulfur content in the light components? ? LPG, oil, coal, and diesel hydrogenation design,
Personally, I believe it is due to the presence of antidetonants; insufficient supplementary refining agents being used after these antidetonants results in the olefins in the product oil reacting with H2S produced by enhanced desulfurization to form sulfides. Some of these molecules may be relatively large and fall within the diesel range, which leads to the diesel not meeting the required standards. I encountered a similar situation before; the reason back then was a shortage of supplementary materials.
A inlet temperature of 350 may indicate an advanced stage of the catalyst usage; will the reaction depth meet the requirements, and is the designed reaction pressure within the appropriate range? Additionally, by analyzing the composition of total sulfur, it was determined that either increasing the amount of stripping steam or enhancing the reaction depth is necessary
1. The design specification for naphtha in our company is 10 PPm, and under normal circumstances this requirement is met. 2. The amount of stripping steam used is determined based on the volume of material being processed; it is generally believed that this amount is around 3% of the feed volume, with adjustments made taking into account the quality of the product. 3. Generally speaking, naphtha consists of saturated hydrocarbons and does not undergo condensation reactions with substances such as hydrogen sulfide; therefore, the focus is mainly on the desulfurization rate. However, the sulfur content in diesel is 10 PPm, so desulfurization is not a concern. According to LZ’s description, (1) copper sheet corrosion tests can be conducted on naphtha to determine whether excessive hydrogen sulfide levels are causing a high sulfur content ； (2) Appropriately reduce the amount of naphtha added, and observe the sulfur content in the naphtha ； (3) Naphtha from the raw materials ends up in the finished product. Note: Please share when you find the cause, so we can learn together*. Thank you
Try adding an electric heater to the stripping steam
Based on the description, there is no issue with diesel desulfurization; in my opinion, the hydrogen sulfide stripping tower is not operating properly. Try adjusting various operational parameters, including those related to the stripping steam, to match the design values. I suspect that hydrogen sulfide or polysulfides are causing the problem. What about the doctoral experiment? I’m not sure If the pilot test is successful, it can basically be determined that the issue lies in the operation of the hydrogen sulfide stripping tower.
It is mainly due to the stripping tower: (for reference only) First, operational reasons: 1. Gradually increase the steam injection volume into the stripping tower to 3%； 2. Gradually increase the bottom temperature by 180-200 slowly℃ ； If the above steps do not work, the equipment issues in the stripping tower can be analyzed.
What is the feed temperature, and is it sufficient to vaporize most of the naphtha?