Seeking advice regarding the demethanization tower in an olefin separation unit
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This post was last edited by liaifeng on 2018-8-9 at 09:49. The image shown is a view from the DCS system of the olefin separation unit, specifically the demethanization tower. I have a few questions regarding this screen, and I hope those with more experience can offer their guidance: 1. This is a Rumus process flow; propane used for flushing is injected into the top of the demethanization tower from the bottom of the propylene distillation tower, in order to reduce the loss of ethylene. The temperature of the methane used for flushing is -24 degrees. I would like to know what the principle behind this is 2. As shown in the figure, the temperature at the bottom of the tower is around 14 degrees. Why is there still methane in the material at the bottom of the tower? (The critical temperature of methane is -82.1 degrees.) 3. As shown in the figure, there is no pattern in the temperature distribution along the tower body. What is the reason for this?
It might be related to the boiling points of C1, C2, and C3.
Natural gas dehydrogenation and dehydration~~ Here’s a picture of the DCS for the propane compressor, haha~
1. This is different from other distillation towers; it is an absorption tower that uses propane as an absorbent to absorb ethylene from the gas phase at the top of the tower, thereby reducing ethylene losses. 2. The methane content in the tower bottom should be quite low when it is normal; methane is present there mainly because it is dissolved in C2, and with such a high total flow rate, it is normal for a small amount of methane to be dissolved. 3. This is the absorption tower; the absorption process itself is exothermic, so an intercooler is required for cooling, and the uneven temperature distribution is related to this.
If the team leader is not aware of this, it is recommended to consult the plant manager, engineers, and other relevant personnel; DCS screens should not be uploaded to public networks.
MTO is not exactly a secret in China either; Lums has already set up 9 units there. In my humble opinion: 1. Since the ratio of methane to hydrogen in the gas used in MTO reactions is much lower than that in ethylene cracking, there is no need for expensive cryogenic separation systems. A simple absorption tower is sufficient to remove methane and ensure high recovery rates of ethylene. The absorption principle is the principle of similarity and compatibility; ethylene dissolves more easily in C3/C4 alkanes than methane, so it is feasible to recover ethylene using propane. The temperature of -24°C is chosen because it is close to the temperature at the location where the material is injected into the tray ； Another point is that low temperature and high pressure are favorable for absorption. 2. The critical temperature of methane refers to that of a pure substance; the critical temperature of mixtures varies, so mixtures do not necessarily enable methane to vaporize just by having a temperature higher than that of the pure substance. 3. I also can’t understand the temperature distribution; please help me with this
1) Absorption tower, similar to the stable absorption of gasoline in FCC； 2) The relative amount of methane at the bottom of the tower is very low; it is normal to have a small amount present ； 3) As for the tower temperature, you can consider this tower as two separate towers: the upper tower is an absorption tower, and the lower tower is a distillation tower; this makes it easier to understand.
1. The demethanization tower is a packed tower, and its main function is to separate hydrogen, methane, carbon monoxide, nitrogen from the C2 and C3 components. The Rums process uses propane from the bottom of Column 1 to wash the ethylene in the demethanization overhead vapor, thereby reducing ethylene losses. Propane at low temperatures has a better effect on dissolving ethylene, and the principle behind this is similar substances dissolve in each other. At the same time, this propane washing also serves as a supplementary reflux for the demethanization tower. 2. According to university theory, methane should not be present in the bottom of the tower. However, the alkenes and alkynes present in the components at the bottom of the tower can also dissolve methane; it’s just that under these conditions the amount of methane that can dissolve is artificially reduced. Therefore, a small amount of methane is acceptable, but high levels of methane at the bottom of the tower can affect the reboiling process there, as well as the subsequent ethylene column. This indicator is quite important. 3. At first glance, the temperature distribution in this tower appears irregular; upon closer analysis, it can be seen that the dissolution of ethylene by propane releases heat, and this heat needs to be removed, otherwise it will affect the distillation efficiency of the demethanization tower. Therefore, Lums has installed a propylene cooler set at -40°C. By analyzing the temperature distribution throughout the tower, it is possible to determine the location where the dissolution heat is highest, which in turn indicates the area with the highest dissolution efficiency. Additionally, a small amount of flashing also occurs in the tower for the liquid-phase feed. This temperature distribution is the standard for determining whether a tower is frozen. I hope my answer is helpful to you; please let me know if there are any mistakes.