Extractive distillation

A special distillation method that involves continuously adding a high-boiling-point additive to the top of the distillation column, thereby changing the relative volatility between the components in the feed mixture and making it easier to separate liquid mixtures that are difficult to separate using conventional distillation. 　　The amount of additive has a significant impact on the separation efficiency and economic viability of extractive distillation. Taking the phase equilibrium relationship between isooctane and toluene at different concentrations of phenol (an additive) (Figure 1) as an example, it can be seen that at higher additive concentrations, the relative volatility between the original components is greater, and fewer tray numbers are required for separation. However, a large amount of additive is required, increasing the recycling costs. Therefore, the optimal amount of additive must be determined through economic analysis. When the raw materials and additives are added in a certain ratio, there is also a corresponding optimal reflux ratio. If the reflux ratio is increased excessively during operation, the concentration of the additive is reduced, which in turn deteriorates the separation efficiency. 　　Typical process: For example, if the feed liquid is an isooctane-toluene mixture, its relative volatility is very low. Phenol (with a normal boiling point of 181°C) is added as an additive from near the top of the distillation column (Figure 2). Phenol has very low volatility and is discharged together with toluene from the bottom of the tower. The additive maintains a certain concentration on each tray, causing changes in the phase equilibrium that are favorable for separation. The additives discharged from the bottom of the tower can be recovered using another distillation tower and reused. To prevent a small amount of additive from escaping with the volatile components at the top of the tower, one or two trays can be placed above the additive inlet for recovery, which is known as the additive recovery section. 　　Selection of additives The selection principles for additives (also known as extractants) in extractive distillation are: ① High selectivity, that is, a small amount of the additive is sufficient to significantly increase the relative volatility between components ； ②Low volatility, meaning it has a boiling point much higher than that of the components in the liquid mixture ； ③It has sufficient miscibility with the feed liquid, so no liquid phase separation occurs on the tray ； ④They are readily available at low prices; water and certain polar organic compounds are the most commonly used additives. 　　Extractive distillation is mainly used in those cases where the cost savings resulting from the increased relative volatility after adding additives are sufficient to offset the costs of the additives themselves and their recovery processes. Extractive distillation was initially used for the separation of mixtures such as butane and butylene, as well as butylene and butadiene. Currently, extractive distillation is more widely used than azeotropic distillation for the separation of aldehydes, ketones, organic acids, and other hydrocarbon oxides.

Compared to conventional distillation, extractive distillation involves more operational parameters. The extractant is usually recycled, and to ensure effective extraction it is necessary to control the purity of the solvent as well as the solvent-to-feed ratio. The control of overhead reflux also differs from that in conventional distillation; the reflux ratio should not be too high, as this can lead to a decrease in solvent concentration and thus affect the extraction efficiency. Generally, the amount of solvent used is much larger than the amount of the medium to be pre-separated. Controlling the solvent temperature is also important, as it has a significant impact on both the extraction efficiency and the heat balance throughout the tower……

Extraction involves separating substances by taking advantage of their different solubilities in various solvents. Extractive distillation refers to the use of distillation to separate the product after extraction; the process is repeated with further extraction and distillation in order to purify the product. You can refer to the benzene extraction unit for information on this. Distillation is a commonly used separation method in chemical production. It achieves separation by utilizing mass and heat transfer between the gas and liquid phases. For example, the pre-distillation section of the unit utilizes the difference in volatility among the various phases to separate out the light components. As for pressure and reflux ratio, the pressures in both conventional distillation and extractive distillation are not very high; the reflux ratio is determined by the requirements of production and the design of the tower. This post was last edited by NerveKnifeWindSlash on 2009-1-13 at 00:45.]

In extractive distillation, the distribution of the feed is extremely important! The quality of the distributor’s design has an impact on whether the extractive tower operates successfully or not. Do you need help with the design and calculation of extractive distributors? You can turn to me.

Extractive distillation requires the addition of a third component (extractant) to change the relative volatility of the components

In fact, the proportion of the extractant used is the most important; otherwise, separation will not be achieved

:victory: :victory::victory: :victory: :victory: :victory: Distillation means the vaporization followed by condensation of a substance; it is a method for separating substances based on their different boiling points. Extraction involves using the difference in solubility of substances in different solvents to separate them ； Distillation means vaporization followed by condensation; it is a method for separating substances based on their different boiling points. Extractive distillation refers to the use of distillation to separate the product after extraction; the process is repeated with further extraction and distillation in order to purify the product. A special distillation method that involves continuously adding a high-boiling-point additive to the top of the distillation column, thereby changing the relative volatility between the components in the feed mixture and making it easier to separate liquid mixtures that are difficult to separate using conventional distillation.

Principles and calculations of extractive distillation 《Chemical Engineering Separation Technology》 http://jpk.dqpi.net/flgc/jiaoxueziyuan/wlzy/ziyuan_wlkc/ketang/ts2.htm

To increase the relative volatility of the components to be separated, it is necessary to maintain a sufficient concentration of the extractant in the liquid phase on each tray of the distillation column. When the feed and the extractant are added to the column in a certain ratio, there is an optimal reflux ratio. When the solvent-free reflux is too high, it not only fails to improve the butane composition of the distillate, but also reduces the concentration of the extractant in the column, making separation more difficult. When the reflux temperature at the top of the tower is too low or the temperature at which the additive is added is low, part of the steam in the tower will condense, thereby diluting the concentration of the mixed solvent on each tray. Furthermore, in order to keep the concentrations of the mixed solvent in the distillation section and the stripping section roughly equal, the heat condition for feeding the components to be separated into the extraction column is such that saturated steam is introduced into the column, i.e., q=0.

The biggest difference between extractive distillation and conventional distillation lies in the reflux ratio: in conventional distillation, the reflux ratio is increased to improve product quality, whereas in extractive distillation, it is decreased instead.