Issues regarding the extraction of water using a methanol pre-tower
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I checked the discussions on this forum regarding the water used for pre-column extraction of methanol, but there are still some points I don’t understand. I would like to ask everyone: 1. Many people say that the amount of water added is intended to keep the specific gravity of the methanol after extraction between 0.84 and 0.87, and it generally should not exceed 20% of the feed volume. I would like to ask why the predicted methanol specific gravity is set at 0.84-0.87? If there is less water, the load on the pressure columns and the atmospheric column later on will be reduced, wouldn’t that be better? 2. Also, should the extracted water be added to the reflux tank or to the crude methanol storage tank? Based on the discussions, it seems that it is preferred to add it to the reflux tank; the reasons for this are as follows: (1) If water is added to the feed, there will be more water in the lower part of the tower, while very little water in the distillation section, which prevents effective extraction. (2) If it is added to the crude methanol tank, it is inconvenient when the production load changes or when the quality of pure methanol requires a change in the expected specific gravity, as the crude methanol tank has a large volume; therefore, it is better to add it to the pre-condensation reflux tank. But tang690803 once mentioned, “Let’s talk about the situation in our factory so that everyone can refer to it. In our factory, we use a 3+1 column for distillation; the feed temperature is 65 degrees, the temperature at the top of the column is 71.5 degrees, and the temperature at the bottom is 83 degrees. The reflux temperature is 64.5 degrees. The pre-column is a composite column with 28 layers for feeding and 12 layers for adding alkali. There was no extraction water included in the design. It was difficult to control the liquid level at the bottom of the pre-column. Moreover, the solubility of the product in water was not good. Later, extraction water was added to the reflux stream of the pre-column. As a result, the temperature at the top of the column became 83 degrees, the temperature at the bottom was 81.5 degrees, the feed temperature remained at 65 degrees, and the reflux temperature was 70 degrees. The discharge volume from the top of the column was unstable. Eventually, the extraction water was moved to the feed line; as a result, the temperature at the top of the column became 73 degrees, the temperature at the bottom was 84.5 degrees, and the reflux temperature was 67 degrees. The discharge from the top of the column became stable, and the product reached AA grade. The composition of the materials inside the column is the main factor affecting the column temperature. It’s normal for the methanol concentration at the top of the column to result in a higher boiling point, while a high methanol concentration at the bottom leads to a lower boiling point.” So I’m still confused about this issue. Could everyone please discuss it? Last edited by lanseqinghai on 2009-4-20 12:23
My understanding is that since some of the light components in the crude alcohol have boiling points similar to those of methanol, it is difficult to remove them in the pre-tower; by adding extraction water, these light components form an azeotrope with water, resulting in a lower boiling point and making it easier to remove them in the pre-tower. As for the prognostic data for controlling the methanol specific gravity, it should merely be an empirical value obtained by adding extraction water.
In a design document I saw, the predicted density of methanol was only 0.759; I’m not sure why there’s such a large difference from the value accepted by everyone else
Could any netizen help us? The material of the steel pipes used for methanol shift is OCR18NI9, with a temperature of 400 degrees and a pressure of 39 KG/CM2. Is this suitable?
The extraction water is generally added to the reflux tank. Last edited by ylbxh on 2009-4-20 at 10:08.]
We add it here to the return tank.
I agree with what was said on the 2nd floor; the so-called extractive distillation is just a term used for this process, and it isn’t true extractive distillation. Adding water to the feed increases the heat capacity at the bottom of the tower, which makes it easier to heat that area. Otherwise, before the temperature can rise, all the material would have vaporized, and it would be difficult to control the liquid level at the bottom of the tower. After adding water and forming a low-boiling azeotrope, it is easier for it to evaporate, resulting in the temperature gradually decreasing from bottom to top in the tower.