Distillation technology, adjustment of process parameters in distillation columns

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Extractive distillation is one of the most important separation methods in the chemical industry. As one of the special distillation processes with the highest selectivity, it is not well understood by many people. 01 Definition: A third component (referred to as an extractant or solvent) is added to the feed solution in order to alter the relative volatility of the existing components, thereby achieving separation. Unlike azeotropic distillation, the extractant does not form an azeotrope with any component of the feed solution. 02 Operational characteristics of extractive distillation: To increase the relative volatility of the components to be separated, it is necessary to maintain a sufficient concentration of additive in the liquid phase on each tray. When the feedstock and extractive solvent are introduced into the column in a certain ratio, there must exist an optimal reflux ratio. When the reflux without additives is too high, it not only fails to improve the composition of the distillate but also reduces the concentration of additives inside the tower, making separation more difficult. Similarly, when the reflux temperature at the top of the tower is too low or the temperature at which the additive is added is low, it causes partial condensation of the vapor inside the tower, thereby diluting the concentration of the additive on each tray. During design, in order to keep the additive concentrations in the distillation section and the stripping section roughly similar, the feed liquid for extractive distillation is often introduced into the tower under the thermal conditions of saturated steam. In the case of feeding at the bubble point, where the additive concentrations in the rectification and stripping sections are different, different phase equilibrium data should be used for calculations. In extractive distillation, a relatively large amount of additive is used, and its boiling point is high; a considerable portion of the heat energy required for distillation is spent on raising the temperature of this additive. 03 Typical process of the extractive distillation unit The main equipment is the extractive distillation column. Since the boiling point of the solvent is higher than that of the components in the original solution, it is always discharged from the bottom of the column. To maintain a high solvent concentration on the vast majority of the tray levels in the tower, the solvent inlet must be located above the raw material inlet. But under normal circumstances, it cannot be introduced from the top of the tower, because there must be several more trays above the solvent inlet to form a solvent recovery section, so that the solvent concentration in the distillate can be reduced to an negligible level before it is drawn off from the top of the tower. After being drawn out from the bottom of the extractive distillation column together with the heavy components, the solvent is fed into a solvent recovery unit. Generally, a distillation column is used to distill the heavy components from the solvent, which are then returned to the extractive distillation column for reuse. Generally, the loss of solvent throughout the entire process is minimal; only a small amount of fresh solvent needs to be added to compensate for it. For example, when separating butadiene from the C4 fraction of hydrocarbon cracking gas, it is difficult to separate butadiene from other components using conventional distillation methods. This is because the components in the C4 fraction have similar boiling points and relative volatilities; moreover, butadiene and n-butane can form an azeotrope. If extraction distillation is employed, by adding acetonitrile as an extractant to the C4 fraction, the relative volatility between the components can be increased, allowing the separation of butadiene, butane, and butylene – which have similar boiling points – through distillation. After being freed from C3 and C5 fractions, the C4 fraction enters the butadiene extractive distillation column. In the presence of the extractant acetonitrile, butadiene (including a small amount of alkynes), acetonitrile, and other components are separated from one another. The mixture is then withdrawn from the bottom of the column and fed into a stripper column, where butadiene and alkynes are stripped from the acetonitrile; the extractant is subsequently recycled. Butadiene and alkyne enter the alkyne extractive distillation column; butadiene emerges from the top of the column and, after being washed with water, yields the finished product butadiene. 04 Precautions for extractive distillation: Since a large amount of extractive agent is added (generally, xs > 0.6 is required), the amount of liquid flowing downward in the column is much greater than the amount of rising vapor. This results in poor vapor-liquid contact. Therefore, when designing the column, due consideration must be given to the trays and hydrodynamic conditions. Since the relative volatilities between components are adjusted by varying the amount of extractant added, when the overhead product does not meet specifications, it is not possible to make adjustments by increasing the reflux rate. The usual adjustment methods are: ① Increase the amount of extractant used ； ②Reduce the feed rate, while simultaneously decreasing the amount of product taken from the top of the tower ； When determining the tower diameter and designing the tray structure, in addition to calculations based on the steam volume, attention should also be paid to the large amount of extractant present in the liquid flow. 05 Selection of extractant: The selection principles for the additives used in extractive distillation (also known as extractants) are as follows: ① High selectivity, that is, a small amount of the additive is sufficient to significantly increase the relative volatility between the 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. 06 Scope of application: Extractive distillation is mainly used in situations where, after adding additives, the cost savings resulting from the increased relative volatility are sufficient to offset the costs of the additives themselves and the operations required for their recovery. 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. 07 Comparison of extractive distillation with azeotropic distillation: ① The extractant is easier to select than the entrainer. ②The extractant hardly vaporizes during the distillation process, and the energy consumption of extractive distillation is lower than that of azeotropic distillation. ③During extractive distillation, the amount of extractant added can vary widely, whereas in azeotropic distillation, an appropriate entrainment dosage is usually fixed. Therefore, the operation of extractive distillation is relatively flexible and easy to control. ④Extractive distillation is not suitable for batch operation, whereas azeotropic distillation can be carried out in batch mode. ⑤The operating temperature of azeotropic distillation is lower than that of extractive distillation; therefore, azeotropic distillation is suitable for separating heat-sensitive solutions. 08 Practical Applications of Extractive Distillation In fields such as chemistry and petrochemicals, extractive distillation is primarily used for two purposes: one is the separation of hydrocarbons with similar boiling points, such as the separation of butylene and butadiene, whose boiling points differ by only 2°C, with a relative volatility of 1.03 ； The second is the separation of azeotropes, such as aqueous solutions of organic substances like methanol-propanone, ethanol-ethyl acetate, and ethanol and acetic acid. The advantage of extractive distillation is that it increases the relative volatility between the components to be separated, thereby enabling the separation of difficult-to-separate mixtures ； The drawback is that a relatively large amount of extractant needs to be added, increasing the energy consumption of the separation process. Therefore, improving extractive distillation is of great significance for enhancing the separation process. ①In the process of aromatic hydrocarbon separation, liquid-liquid extraction has been in use for a long time in the recovery of aromatic hydrocarbons. This technique relies on the polarity of the components to influence their separation, with little effect on boiling points. Due to the limitations imposed by solvent selection, it is difficult to use extractive distillation for the separation of mixtures with a wide boiling range; previously, it could only be applied to materials with narrow boiling points, such as in the separation of C6 and C7 materials using N-methylpyrrolidone or N-formylmorpholine as solvents. However, with the development of extractive distillation technology, extractive distillation using mixed solvents has solved the above problems. The GT-BTX technology developed by the American company GTC Technology exemplifies the application of modern extractive distillation techniques in the separation of mixed aromatics (benzene, toluene, xylene). Compared to traditional mixed aromatic hydrocarbon separation processes, the GT-BTX process features low investment costs, a reduced number of required equipment units, excellent solvent performance, a low risk of product contamination, high product recovery rates and purity, as well as low energy consumption and great operational flexibility. After evaluating the technical and economic indicators of the industrialized process (1.2 million t/a), the purities of benzene and toluene reached 99.995% and 99.99%, respectively. The total aromatic hydrocarbon recovery rate is higher than 99.19%, the mass fractions of the raffinate and extract in the solvent are less than 10-6, and the energy consumption per kilogram of feed is 798 kJ. ②Desulfurization of catalytic cracking gasoline: 50%–60% (by mass) of the sulfides contained in catalytic cracking (FCC) gasoline are thiophene and its alkyl derivatives, with the remainder being thiol compounds and other sulfides. Under catalytic cracking conditions, thiophene compounds exhibit relatively high stability. Foreign companies generally employ hydrodesulfurization methods; in order to further reduce the sulfur content in gasoline, the current measure being taken is to enhance hydroprocessing capabilities. Hydrogenation is beneficial for desulfurization in fuels; however, it has drawbacks such as high operating costs and the fact that deep hydrogenation reduces the octane number of gasoline. Based on the characteristics of the sulfides present in oils, catalytic oxidation, complexation, catalytic adsorption, biological methods, solvent extraction, and alkaline washing are commonly used to remove sulfides from oils. Among these methods, extractive distillation has its own advantages. When processing FCC gasoline, this process uses a solvent that can alter the relative volatility of the non-aromatic components (including olefins) and thiophene compounds in the feed. While extracting thiophene compounds, it also extracts other aromatic sulfides (due to their high polarity), while the olefin-free components are sent to the hydrogenation system for further treatment. By using extraction distillation and alkaline washing, it features no loss of octane number, low hydrogenation load, the ability to process pyrolysis feedstocks with a wide range of sulfur contents, and high operational flexibility. By introducing extractive distillation before hydrogenation, the problems present in traditional processes were resolved; the thiophene sulfides in the aromatics were selectively extracted by a solvent, which reduced the olefin content in the raffinate. The raffinate with low sulfur content and high olefin content could be directly blended with gasoline containing 10×10-6 of thiophene sulfur. High levels of thiol can be treated in the feed or raffinate using conventional alkaline washing methods, allowing the total sulfur content to be easily reduced to (5–110)×10-6 without compromising the octane rating. ③Cracking gasoline recovery and styrene purification: Cracking gasoline by-products contain abundant petrochemical compounds; purifying and making full use of these compounds can yield significant economic benefits. Due to the similar boiling points of these components, complexes are formed, and it is difficult to separate them using conventional separation methods. The development of extractive distillation technology has made this possible; this technique is commonly used to purify butadiene and isoprene from the light components of pyrolysis gasoline, and it can also be employed to effectively separate styrene from C8 materials. The conventional cracking process involves a hydrogenation step in which, on the one hand, coking is a problem; on the other hand, the reaction requires a large amount of hydrogen. Recent studies have shown that styrene is one of the causes of coking, and reducing its content is an effective way to address coking issues. The extraction distillation technique using mixed solvents enables the extraction of styrene at lower costs; therefore, the application of this technique allows styrene to be transformed from a fuel product into a petrochemical product, thereby increasing its value. Additionally, hydrogen consumption during hydrotreatment is reduced, and the coking problem is resolved. The emergence of the Reactive Distillation process has completely transformed people’s traditional understanding of reaction and separation processes over the years. It combines chemical reaction processes with the physical processes of distillation separation, representing a new type of specialized distillation process that involves chemical reactions.