We know that a
Distillation Tower generally consists of a tower kettle, tower section,
Condenser, discharge pipeline, output pipeline, reflux pipeline, etc. Why do we need a return line? Let's first talk about the main role of reflux in distillation towers:
Firstly, provide cold reflux on the tray to remove excess heat inside the tower and maintain heat balance inside the tower;
Secondly, provide a cold fluid on the tray, where the gas-liquid phases come into reverse contact. The heavy components in the upward gas condense, while the light components in the downward liquid absorb heat and vaporize. This repeated condensation and vaporization process fills the entire tower section with light components
The components, the lower part is the recombinant component, further improving the separation purity of the product. Therefore, reflux liquid is a necessary condition for distillation purification.
1. Common reflux methods in distillation operations
Firstly, according to the different reflux methods, it can be divided into "natural reflux" and "forced reflux". Natural reflux refers to the condenser of the tower located above the distillation tower section, with a certain height. The liquid storage outlet of the condenser is higher than the reflux outlet of the tower section and has a certain distance. The reflux liquid flows back into the tower under the action of gravity. Natural reflux operation is simple and does not require a reflux pump, saving power consumption. However, the reflux rate varies with the pressure inside the tower, and the reflux ratio is not strict. When production is abnormal, the adjustment is relatively slow. Natural reflux is widely used in small distillation units, requiring sufficient height and space. Forced reflux is the installation of a pump on the reflux pipeline, and the reflux liquid is pumped into the tower for reflux. The reflux flow rate of forced reflux is stable and easy to adjust. When production is abnormal, it can be adjusted quickly. However, forced reflux requires a pump, which consumes a lot of power, especially for low boiling materials, which can cause pump failure and affect operation. However, the condenser with forced reflux is not limited by height and can be installed in a convenient location for installation and maintenance.
Secondly, according to the different installation positions of condensation at the top of the tower, it can be divided into internal reflux and external reflux. Internal reflux refers to the vertical connection between the condenser and the tower section, located directly above the tower section. In distillation, internal reflux generally refers to the reflux on the tray, which is composed of the liquid produced by the condensation of the descending liquid and the rising gas. The auxiliary condensing equipment of the distillation tower includes a splitter, a total condenser, and a condenser. The top of the tower can be designed with a condenser. The gas phase at the top of the tower passes through the condenser, and a portion of the condensation directly flows back into the tower, which is called internal reflux. The remaining gas with condensation enters another condenser for condensation. A full condenser can also be installed at the top of the tower, with a receiving tray set below the full condenser. Part of it is extracted, while the other part flows back, which is also called internal reflux. Under normal circumstances, high boiling points and high toxicity should be treated with this internal reflux method. Directly entering the condenser from the top of the tower, partial condensation is carried out here, and the condensate naturally flows down the tray. The amount of reflux is difficult to control and cannot be accurately adjusted. Due to the influence of heating, the reflux rate varies greatly. However, this reflux condenser is directly installed at the top of the tower and does not require any other supporting structures, making installation convenient. The external reflux in distillation is to extract a portion of liquid from the tower section, cool it down, and then pour it into the tower. The condenser at the top of the tower is installed separately, and a sight glass, flow meter, regulating valve, etc. can be installed on the reflux pipeline to adjust the amount of reflux.
2. The difference between internal reflux and external reflux
Internal reflux refers to the material not leaving the top of the tower, but directly flowing back into the distillation tower after condensation at the top. Measurement is difficult, and the ratio of fractionation and reflux cannot be accurately determined. It is a direct return to the tower after gas-phase condensation at the top of the reflux tower. During operation, attention should be paid to controlling the extraction amount to prevent product failure. Although the internal reflux lacks a reflux pump, a reflux distributor should be installed between the condenser and the top of the tower, It is generally required to have a rotating or moving device to distribute the reflux ratio, and facilities that rely on electric motors or other power drives are not suitable for enclosed installation in the tower. This regulation is "graded" and is a non-standard equipment.
External reflux refers to the material leaving the top of the tower, passing through external pipelines, flow meters, etc., and then flowing back into the distillation tower. It can be metered for diversion or forced reflux. After the gas phase at the top of the tower condenses and enters the reflux tank, it is adjusted by the reflux pump control valve and flow meter
The flow returns to the tower. Most distillation towers in industry use external reflux, which can automatically and steplessly adjust the reflux rate to meet production needs, especially when there are fluctuations in feed volume or composition.
3. Application of external and internal reflux
External reflux is beneficial for controlling process flow and temperature, with high operating costs and no utilization of liquid potential energy, resulting in high costs. If the condenser at the top of the tower cannot meet the condensation requirements, a forced condensation reflux system can be added to achieve forced operation of the distillation tower. In addition, the relative size of operating costs and infrastructure investment costs also needs to be considered when investing.
If the metering requirements for the reflux liquid are not high or the operating flexibility of the reflux ratio is large, internal reflux can be used. If an online flow measurement instrument can be developed along the internal reflux route, internal reflux can be achieved, and distillation is generally referred to as external reflux.
The advantage of external reflux is that it is easy to adjust, but it increases operating costs and increases leakage points. It may not be suitable for some high-risk media, and internal reflux is preferred for high-risk media that are not too high in the tower. So the choice of reflux method should be comprehensively considered from multiple aspects.
According to the temperature of reflux, it can be divided into "hot reflux" and "cold reflux".
Hot reflux refers to the temperature of the reflux liquid being at the bubble point temperature, while cold reflux refers to the temperature of the reflux liquid being below the bubble point temperature. The reflux of a distillation tower is generally saturated liquid reflux, which is to ensure the stable working state of the distillation section and slightly undercooled reflux of the reflux liquid. The theoretical reflux ratio can be increased without increasing the reflux flow rate, because the reflux liquid entering the distillation section will cause a large amount of condensation of the rising steam, improving the purity of the top output while ensuring the quantity of top output. However, one downside is to increase the heat load of the tower kettle, The heat consumption is relatively high, and if the output added value is high, it is still economically reasonable and much more cost-effective than reflux of saturated liquids.
For distillation units with a full condenser, most industrial reflux uses cold reflux mainly because:
1. The top gas phase of the tower can achieve complete condensation during the condensation process, reducing the loss of gas phase emissions.
2. It is difficult to control the top temperature of a fully condensed tower in a saturated liquid state.
3. Slightly undercooling reflux can increase the theoretical reflux ratio without increasing the reflux flow rate.
Total reflux is the operation in which the condensate extracted from the top of the tower is returned to the distillation kettle as reflux liquid. Total reflux is a necessary process during start-up to ensure that the product is qualified as soon as possible. In normal production, total reflux cannot be carried out arbitrarily unless there are process fluctuations, as the distillation tower loses its meaning of existence without product extraction. If waiting for the analysis results of the product to clean the entire tower, total reflux can be used.
4. How to control reflux during distillation operation?
There are generally two types of tower top reflux control: manual control and automatic control. When manually controlling the distillation operation, as long as there is no significant change in the quality of the product at the top of the tower, the change in the reflux rate of the tower is very small, and can even remain unchanged. In actual operation, the reflux rate is basically not affected by the feed amount. The reflux tank liquid level should be maintained, and there should be no phenomenon of full or empty tanks. Experienced operators should control the reflux rate according to the actual situation of the tower and adjust the efficiency of the tower.
During automatic control, the reflux rate is affected by the amount of material extracted from the top of the tower. When the feed rate remains constant, it is necessary to control the amount of material extracted from the top of the tower. As the amount of material extracted from the top of the tower increases, the reflux ratio decreases, the gas-liquid contact is poor, and the quality of the product at the top of the tower is unqualified. If the feeding amount increases, the amount of increase in top extraction should be calculated. If the extraction is too small, the reflux rate increases, the material inside the tower increases, the rising steam speed increases, and the pressure difference between the top and bottom of the tower increases. In severe cases, it can cause liquid flooding. If the extraction amount is too large, the reflux rate decreases, the gas-liquid contact is poor, and the quality of the output at the top of the tower is unqualified. Generally, an automatic reflux device should be installed in a distillation tower, and the main discharge pipeline and output pipeline should also be equipped with self-control, with a fixed reflux ratio. All three must be changed simultaneously to ensure the normal operation of the entire distillation tower.
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