Seeking advice on the inverted temperature in the methanol pre-tower
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Our plant’s 150,000-ton methanol distillation system has been in operation for three years. Recently, the temperature at the top of the pre-distillation tower has become higher than that in the reactor. According to the process parameters: 25 tons per hour of feed are used, along with 4.2 tons of extraction water per hour. The density of the liquid is 0.84 grams per milliliter. The temperature of the liquid entering the tower is 53 degrees Celsius; the flow rate is 11 cubic meters per hour, and the temperature of the reflux liquid is also 53 degrees Celsius. The outlet temperature of the condenser ranges from 53 to 60 degrees Celsius. The density of the reflux liquid is 0.9 grams per milliliter. The temperature at the bottom of the tower is 77 degrees Celsius, while the temperature at the top of the tower fluctuates between 76 and 78 degrees Celsius. The temperature in the reactor remains stable, as does the liquid level. The methanol content in the liquid entering the tower is 94.5%. During maintenance work on the reboiler in the pre-distillation tower, the temperature at its top increased; previously, it was between 74 and 75 degrees Celsius. The amount of extraction water used increased from 3.6 tons to 4.3 tons. This increase may be due to a leak in the reboiler, which allowed steam to enter the reactor. After the maintenance, more extraction water was used. The pre-distillation tower is equipped with structured packing in the distillation section and floating valve trays in the stripping section. It’s possible that some of the trays have fallen off. We would appreciate it if experts could provide valuable suggestions
Are the data you provided (feed rate and return flow rate) stable? If so, it’s likely that the gas-liquid heat exchange between the trays is not effective; shutting down the tower for inspection will clarify this.
Both the feed rate and the reflux rate are stable. The top temperature drops a bit when trying to reduce the prognosis, but the prognosis remains around 0.84. Now that it has dropped to 0.83, the top temperature has fallen to around 74.8
First, check what the alcohol content is in the crude methanol you produce. It’s possible that the high alcohol content in the crude methanol causes the material at the bottom of the tower to evaporate too quickly, thereby raising the temperature at the top of the tower. Moreover, the temperature at the bottom of your tower is quite low – will this be sufficient to ensure that all the light components are removed? Generally, the temperature at the bottom of the tower should be maintained between 83–84 degrees, while the temperature at the top should be around 75 degrees. Another issue is that the reflux rate is too low; with a feed volume of 25 cubic meters, only 11 cubic meters of reflux occurs. It is recommended to increase the reflux rate and keep the alcohol content in the feed within the range of 75–80 percent. This will help save steam and facilitate the distillation process.
Another thing to check is whether the temperature reading at the top of the tower is accurate. In my opinion, when the temperature at the top of the tower reaches 78 degrees, the temperature at the bottom of the tower is likely already high–it’s just that the rapid rise of the material carries away the heat.
I believe the main reason for this phenomenon is an increase in the amount of water used for extraction; for a detailed explanation, please search this forum as similar questions have been asked before.
Thank you all for your help. After checking, it turned out that the connection at the temperature sensing point was poor; now it’s working properly
What is the alcohol content in crude methanol? It’s possible that the high alcohol content in crude methanol causes the material at the bottom of the tower to evaporate too quickly, thereby raising the temperature at the top of the tower. Moreover, the temperature at the bottom of your tower is quite low – will this be sufficient to ensure that all the light components are removed? Generally, the temperature at the bottom of the tower should be maintained between 83-84 degrees, while the temperature at the top should be around 75 degrees. Another issue is that the reflux rate is too low; with a feed volume of 25 cubic meters, only 11 cubic meters of reflux occurs. It is recommended to increase the reflux rate and keep the alcohol content in the feed between 75 and 80 degrees. This will help save steam and facilitate the distillation process. Also, check whether the temperature readings at the top of the tower are accurate. In my opinion, when the temperature at the top of the tower reaches 78 degrees, the temperature at the bottom of the tower is likely already high – it’s just that the rapid rise of the material carries away the heat. Check whether the connection at the temperature measurement point is good