Methanol Section Daily Question, April 12: The theoretical basis for the secondary condensation in the methanol distillation pre-column
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During the distillation of methanol, the vapor from the top of the pre-tower passes through the pre-tower condenser and the expansion gas cooler before the low-boiling-point components are removed. The first condenser condenses about 70–80% of the methanol at a temperature of 50–60 degrees, while the second condenser condenses 20–30% of the methanol at a temperature of 35–40 degrees. Doing this can fully condense methanol and discharge the non-condensable gases. But I don’t quite understand why doing this can achieve such an effect; what theory underpins it? This post was last edited by lanseqinghai on 2009-4-12 15:57.]
The gas phase discharged from the top of the pre-tower mainly consists of methanol and some light impurities. Methanol has a relatively high boiling point; therefore, at a temperature of around 60 degrees, a large amount of methanol condenses. However, the vast majority of the non-condensable gases, along with a small amount of uncooled methanol, proceed to stage 2 of cooling. The temperature in stage 2 is controlled at around 40 degrees, at which point methanol, with a boiling point of 64.7 degrees, can be almost completely cooled. Meanwhile, a small amount of methanol and non-condensable gases evaporate from the liquid phase in the reflux tank after stage 1 of cooling, and these gases can then enter stage 2 for further cooling, thereby minimizing methanol loss.
The function of the pre-tower is to remove the light components from methanol, preventing them from reaching subsequent stages and affecting product quality. However, the light components are not just syngas dissolved in methanol; there are also some organic compounds with low boiling points. If the top cooler of the pre-tower is of stage 1, then all low-boiling-point organic compounds are condensed. It then enters the pre-tower along with the return fluid and accumulates within the system. If two-stage cooling is used, methanol liquid (the heavier components) cools down first, followed by the low-boiling-point organic compounds. This portion can be extracted in proportion and fed into the fusel oil tank. Avoid accumulation in the system. It's just my personal opinion; please give your feedback.
The purpose of controlling the secondary cooling temperature is to maximize the removal of non-condensable gases and light components from the crude methanol, thereby improving the quality of the final product； Temperature control depends on the quality of the finished product, so it varies slightly from place to place
I thought about it carefully just now; I’m not sure if it’s correct, so please confirm. Most of the methanol condensed in the first cooling stage enters the reflux tank at a relatively high temperature (about 60 degrees), while the liquid resulting from the second cooling stage has a lower temperature (35–40 degrees). When this liquid from the second cooling stage enters the reflux tank, its low-boiling-point components evaporate again; the resulting gas then enters the second cooling stage for further condensation. This approach allows for maximum condensation of methanol, while simultaneously ensuring that the low-boiling-point components are effectively removed. It’s just my humble opinion; I’d appreciate everyone’s guidance.
There’s a misunderstanding, right? In the pre-tower, it operates in full reflux mode; after the liquid from the second cooling stage enters the reflux tank, the unremoved light components continue to flow back
It is recommended to recycle the liquid after secondary condensation to the fusel oil tank, which helps ensure the quality of the methanol product
This issue can be identified from the composition of crude methanol; there are some methanol azeotropes present in it. By adding extraction water for separation, these low-boiling components can be isolated and removed.
Isn’t there a gas pipeline from the pre-cooling return tank to the secondary cooling system installed there?
Set pre-cooling 2 to recover methanol; if there is no pre-cooling 2, the condensation temperature of pre-cooling 1 must be higher, resulting in more methanol loss. Controlling the temperature is beneficial for separating the light components within the separation tower to facilitate separation.
After the first cooling stage, it should be in the form of methanol liquid. The role of the second cooling stage is to cool the methanol completely; it’s also possible that some substances with boiling points only slightly lower than that of methanol are cooled as well. In our company, the liquid after the second cooling stage is analyzed for its methanol content, so as to decide whether it should be sent to the reflux tank or to the impurity tank as an impurity. Additionally, there is a pipeline from the top of the pre-reflux tank to the second cooling stage, which serves to prevent the loss of methanol. The control temperature of the secondary cooling should be adjusted accordingly based on changes in the composition of crude methanol.
Allow low-boiling substances to be separated in the pre-column, while minimizing methanol loss as much as possible.