The dry point of gasoline in atmospheric pressure towers is high, while the flash point of diesel is low
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The dry point of the gasoline at the top of the atmospheric pressure tower is too high and cannot be reduced, while the flash point of the diesel is too low; even by increasing the steam supply, it cannot be raised. What is causing this?
Excessively high temperature at the bottom of the tower or insufficient reflux causes certain components of diesel to end up in the gasoline, resulting naturally in a too high dry point for the gasoline and a too low flash point for the diesel.
2# jiangfeng517: Some of the diesel has mixed into the light oil fraction; therefore, the flash point of the diesel should increase, right?
The situation you described is relatively rare. For diesel: 1. Check whether there are any issues with the display of the superheated steam flow rate in the stripping tower, as well as with the temperature of the stripping steam. Generally, increasing the amount of stripping steam raises the flash point of diesel. 2. Check whether the diesel extraction temperature is too low; it is possible to increase the extraction temperature appropriately, as long as the dry point remains within acceptable limits.
If the furnace outlet temperature remains stable, reducing the steam volume used for stripping at the bottom of the tower and increasing the reflux at the middle section of the line downstream of the gasoline stream can alter the dry point of the gasoline. As for the flash point of diesel, it is generally necessary to increase the steam volume supplied to the stripping tower, or, while ensuring that the dry point of the diesel meets the required standards, raise the temperature at which the diesel is extracted.
The vapor-liquid phase imbalance leads to a decrease in distillation accuracy and overlapping of components. In this case, the dry point of gasoline should be adjusted first, followed by the flash point of diesel. A high dry point of gasoline indicates an excessive amount of gasoline in the distillate; it is necessary to increase the reflux flow at the top of the tower and lower the temperature there, in order to keep the gasoline dry point within the specified range. A low flash point of diesel suggests the presence of light components from the upper fraction of the distillate. Generally, increasing the amount of stripping steam can help achieve a satisfactory flash point. If this approach does not work, it may be due to an incorrect liquid level in the diesel stripping tower – when the tower is at full capacity, increasing the stripping steam further will result in a high vapor load inside the tower, leading to foam formation and a high gasoline dry point.
Gasoline has a higher dry point, which is due to the presence of more heavy components; diesel, on the other hand, has a lower flash point because of the presence of more light components. This seems somewhat contradictory. In such cases, there is no need to increase the steam used for stripping. Increasing the stripping steam can help maintain the flash point of diesel, but it cannot affect the dry point of gasoline. Increasing the top reflux can help maintain the dry point of gasoline, yet it cannot affect the flash point of diesel. If both the gas steam and the top reflux are increased, as mentioned earlier, I believe this is not advisable. Top reflux is meant to be the components that leave the tower; increasing it can lower the top temperature, but it also increases the load on the top condenser, leading to an increase in tower top pressure. If the stripping steam is then increased further, the tower top pressure will rise even more. It would be better to increase the mid-stage reflux and reduce the top reflux, while adjusting the amount of diesel output based on the test results! Additionally, I would like to clarify: are you referring to the stripping steam from the atmospheric pressure tower or the steam from the stripping tower? If it is the gas steam from the atmospheric pressure tower, then increasing the amount of such steam appropriately can also help raise the flash point of diesel!
This phenomenon occurs due to excessive overlap between the products; components that should be in diesel end up in gasoline, and vice versa. Such a situation is not very common, but overlap does exist between every few adjacent side-stream products. To address this issue, it is possible to reduce or even eliminate the overlapping portions between these products. 1. Lower the reflux temperature – too high a reflux rate may increase the vapor load at the top of the tower; in some cases, this can fail to lower the temperature at the top of the tower and may even have the opposite effect. Therefore, it is better to lower the reflux temperature in order to reduce the temperature at the tower’s top. 2. In the case of wet distillation (i.e., when steam is used at the bottom of the tower), the amount of steam used can be reduced, so as to prevent the vapor pressure inside the tower from becoming too low, as this can lead to a high dry point. 3. The stripping volume in the diesel stream can be increased, as the overlap between the products there is quite significant; this allows for the removal of the lighter components.
If possible, I can help raise the flash point for you. Our own diesel does not meet the standards; generally, a value of 35° can be increased to 50–55°.
Who is right after all? I’m confused
I wonder how long the device has been running? Is there any corrosion or detachment of the top tray?