Why does the lean methanol coming out of the regeneration tower need to go to the methanol-water separation tower?

Can you draw out the process? According to what you said, it should be the control of the reflux ratio in the water separation tower. In other words, it controls the purity of methanol exiting the water separation tower.

The lean methanol coming out of the regeneration tower is sent to the methanol-water separation tower to serve as a reflux stream; it helps to condense the water present in the mixed gas, thereby reducing the concentration of water in the methanol at the outlet at the top of the methanol/water separation tower. According to the design by Lind and Luchi, the methanol (with a purity of over 99.5%) coming out of the methanol/water separation tower enters the regeneration tower directly, where it is carried upward together with the methanol vapor rising from the lower part of the heat regeneration tower, into the methanol-rich stream in that tower. It is not clear whether a condenser is installed domestically. A small amount of water entering the thermal regeneration tower increases the load on the tower, but this is unavoidable; some water will always enter to some extent, though the impact is minimal.

A small portion of the methanol-depleted stream coming from the bottom of the thermal regeneration tower is sent to the methanol-water separation tower in order to keep the water content in the circulating methanol below the designed value (which is 1.5% wt here)； Methanol vapor from the top of the methanol-water separation tower is directly fed into the middle section of the heat regeneration tower, thereby recovering methanol while also providing heat. At that time, it is necessary to keep the water content below 1% by weight, and this can be achieved by controlling the temperature of the methanol vapor at the outlet of the methanol-water separation tower. In some processes, cold methanol is introduced to the top of the methanol-water separation tower from the outlet of the reflux pump at the top of the heat regeneration tower.

In my opinion, there are four functions: 1. Removing water from the system methanol, and by controlling this amount, regulating the water content in the system methanol. 2. As a top reflux stream in the methanol-water separation tower, it is used to capture the heavier components in the methanol vapor. 3. Preheating the feed to the methanol-water separation tower (the stream coming from the separation tank in the raw gas plant – Linde process). Regarding the methanol vapor at the top of the tower, in earlier processes a condensate was added, but nowadays this vapor is directly sent to the regeneration tower, where it is used as a heat source and for stripping purposes. As long as the temperature of the methanol-water separation tower is kept stable, much less water ends up in the regeneration tower compared to the amount that enters it

A small portion of the methanol-depleted stream coming from the bottom of the thermal regeneration tower is sent to the methanol-water separation tower in order to keep the water content in the circulating methanol below the specified level! The methanol vapor coming out of the top of the separation tower enters the regeneration tower directly, and it should not contain excessive water; if it does, it indicates that there is a problem with the operation of the separation tower. In my opinion, in addition to serving as a heat source and stripping steam for the methanol vapor at the top of the tower, using the same condensation system for both the separation tower and the regeneration tower can also help save on investment costs!

The methanol-water separation tower is used as a distillation tower to separate methanol from water, thereby reducing the water content in the system! The methanol that moves from the bottom of the regeneration tower to the top of the methanol-water separation tower serves as a reflux stream, thereby reducing the heavy components (mainly water) in the methanol vapor at the tower top! The methanol vapor from the tower top is directly fed into the middle section of the regeneration tower as a heat stripping medium, thereby reducing the load on the heat regeneration steam and saving energy! As long as the temperature of this methanol vapor is kept within the specified range, it will definitely help to reduce the water content in the system; it will not increase it! The use of a condenser for methanol vapor at the top of the tower, as mentioned by the original poster, is part of the old Lurgi process design; it is no longer used nowadays, and such designs basically do not exist any more!

Thank you all, it’s been great to learn from you

Everything mentioned above is great; let me add something: The flow from the regeneration tower to the water separation tower is determined by the water content. If the water content in methanol is greater than 0.5%, the flow enters at plate 38 of the water separation tower; if it is less than 0.5%, it enters at plate 48 of the same tower. This approach does not increase the water content in methanol, as the boiling point of methanol allows for the separation of water and methanol through temperature control.