Regarding pre-column condensation in the three-column distillation of methanol?
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In the distillation process of crude methanol, two condensers are generally installed at the top of the pre-tower. The main function of the pre-tower is to remove the non-condensable gases (dissolved gases, low-boiling substances) from crude methanol; however, since the temperature at the top of the tower is kept above the boiling point of methanol, a large amount of vapor is produced, which needs to be condensed. I’m wondering whether it’s possible to: 1. Appropriately control the temperature at the bottom and top of the tower, keeping the temperature at the top around 40°C (this value can be adjusted); in this way, there would be no need for a separate condenser, which could help save some costs. 2. Slightly increase the height of the tower while keeping the temperature at the bottom of the tower constant, so as to control the temperature at the top of the tower at around 40°C (adjustable), thereby eliminating the need for a condenser at the top of the tower. Although I think these two methods are not very feasible, I can’t convince myself of that. Here, I ask the passing master to offer some guidance. This post was last edited by mountain16 on 2008-8-31 17:29.]
UID 77672: Hello! I was wondering if you are a methanol design specialist? I would like to establish technical cooperation with you! I study *refrigeration processes, and I plan to work with you on the calculation of the heat load for condensers :handshake
It is necessary for the temperature at the top of the tower to be slightly higher; if the temperature is too low, it becomes difficult to separate the components in the light fractions that are less volatile, which can result in excessive impurities in the liquid in the reactor. By raising the tower temperature appropriately, the excess methanol produced can be cooled using a second cooler. Especially as methanol synthesis progresses to the middle and later stages, increases in synthesis temperature and pressure lead to an increase in impurities, so the temperature at the top of the tower needs to be raised further
Currently, the condensation temperature at the top of the tower is generally kept at 40°C. Is it possible to maintain this temperature at 50°C? By using a condenser in this way, it is possible to **reduce the amount of cooling water as well as the volume of liquid that needs to be returned. Thereby reducing energy consumption. I wonder if this will work? Under the current operating conditions, a large amount of methanol vapor evaporates from the top of the tower and is then condensed to be recycled back into the tower. Is this a waste of energy, or is there some need for it?
Regarding this issue, there is some discussion in the chapter on distillation in methanol engineering; the basic reasons are similar to those mentioned above. For a more detailed understanding, you can refer to that section.
The poster’s suggestion is theoretically sound, but has the poster considered the following points: 1. How to control the temperature at the top of the tower? 2. Is building a taller tower cost-effective? At lower temperatures, it is difficult to separate the less volatile components present in the light fractions, which leads to excessive impurities in the reactor liquid. By appropriately increasing the tower temperature, the excess methanol can be cooled using a second cooler, thereby ensuring the quality of the product. Increasing the height of the tower is too costly; adding a condenser is not economical, and a condenser also makes it easier to control the temperature
It seems that there are some systems in which only one condenser is installed at the top of the pre-tower; this indicates that it is sufficient to allow the non-condensable gases to evaporate. The purpose of installing two condensers is simply to achieve more precise and stable control. The idea put forward by the original poster is very good. I also hope everyone can delve into this further together
I disagree with this view; the level of temperature control at the top of the tower is related to your process operations (as long as the design capabilities allow it, you can control the temperature however much you want). If you set a relatively high temperature in the first condenser, the methanol still needs to be condensed in the second condenser. It’s nothing more than two condensers connected in series. There’s nothing else special about it. So it can be completely achieved with a condenser. Just be careful to keep it under control smoothly.
It is not possible to operate without a condenser; this has been explained in the principles of chemical engineering. In a distillation system, heat balance must be maintained. Without a condenser, how can the heat in the bottom of the tower be balanced? Relying on condensation in the upper part of the tower is not practical. A condenser can fully meet the requirements; it is possible to adjust the lower limit of the temperature control, as long as the methanol vapor phase is not allowed to escape. The non-condensable gases can be vented through pipes into the flare system.
A standard distillation column must have a reboiler at the bottom and a condenser at the top; without evaporation and the upward movement of steam, how can gas-liquid mass transfer take place? If the light components do not evaporate, how can a pure product be obtained?
The pre-tower is used to remove from crude methanol substances such as dissolved syngas and low-boiling-point dimethyl ether, whose boiling points are generally below 40°C – this corresponds to the control temperature of the second condenser in the pre-tower. After being washed, these gases are released into the atmosphere! The pre-column also removes some substances that can form an azeotrope with methanol! The boiling point of the azeotrope is close to that of methanol; after condensation at the top of the tower, water is added to the reflux tank to extract methanol. Impurities separate from the methanol aqueous solution in the reflux tank or a dedicated container, and the light fractions are collected! If these substances that can form an azeotrope with methanol are not removed in the pre-tower, they will evaporate together with pure methanol in the pressurized tower or atmospheric pressure tower, which will affect the water solubility parameters of pure methanol! I’m sorry; I forgot the specific substances that form an azeotrope. You can find this information in “Questions and Answers on Methanol Production”!*
Controlling the temperature in the pre-column at 66°–71° is primarily aimed at extracting the light components; too low a temperature is certainly not desirable. Additionally, if the temperature at the top of the pre-column is too low, methanol will not evaporate, and the reflux tank in the pre-column becomes pointless. The reason for installing a secondary cooler in the pre-column is that the first stage is used to condense methanol, while the second stage is used to condense the alkane oils (i.e., the light components). Therefore, it is not advisable to omit a condenser in the pre-column