What type of control is appropriate for the mixed xylene column in the aromatic extraction unit?

It’s the same as a regular tower; it depends on what requirements you need to meet. The xylene column in the extraction unit is the same as the xylene column in the adsorption separation circuit. There are two types: pressurized and atmospheric pressure. Analyze each specific problem individually!

The column pressure can be controlled using split-range control, while the temperature of the sensitive plate can be regulated through cascade control based on the flow rate of the heating medium in the bottom reboiler; the rest, such as feed and reflux, can be controlled using conventional single-loop control.

It is correct that tower pressure can be controlled using split-range control (but the pressurized xylene tower is somewhat special). The temperature of the sensitive plate can be regulated through cascade control based on the flow rate of the heating medium in the reboiler at the bottom of the tower; however, this control method is not generally used in xylene towers. It is also rarely employed in other types of towers

These are my notes on learning *aromatic hydrocarbon extraction (xylene fraction), for reference. Why is the range of allowed file extensions so limited (allowed extensions: gif, jpg, jpeg, png, gif)? This makes it impossible to upload attachments. Last edited by Yan Dian Qing Tian on 2007-1-29 18:08.]

For distillation columns, there are generally three control schemes: sensitive plate temperature control and top product flow rate control； Sensitivity plate temperature control tower top reflux flow ； There are also those that control the flow rate of production at the bottom of the tower. However, for the xylene column in an aromatic extraction unit, there should be no heat integration process; therefore, it is likely to be a atmospheric pressure column. The conventional approach is to control the flow rate of the product at the top of the column using the temperature of the sensitive plate (the temperature difference), which allows for very precise control over the quality of the mixed xylene at the column top. Could the poster provide a detailed description of the feed and discharge conditions of the xylene tower, so that we can make an assessment?

It is better to use direct material balance control (MBC), that is, temperature or temperature difference is controlled in cascade with the amount of top product withdrawn, while the reflux level is controlled in cascade with the reflux flow rate. The control of heat balance has relatively large fluctuations.

Normal temperature difference control is sufficient. The key is to ensure adequate reflux in order to achieve separation.

It’s the same as a regular tower; it depends on what requirements you need to meet. The xylene column in the extraction unit is the same as the xylene column in the adsorption separation circuit. There are two types: pressurized and atmospheric pressure. Analyze specific issues specifically

Xylene towers come in pressurized and atmospheric types. Xylene towers that employ thermal integration all use pressure boosting ； Because it is more energy-efficient, pressurization operations are now basically used in design ； In the case of pressure adjustment operations and heat recovery, the tower pressure is generally regulated using a reboiler that generates steam. To ensure stability in the heat source at the top of the tower, that is, to maintain stability in the heat output from the heat exchange equipment, a differential pressure controller needs to be installed between the top of the xylene tower and the reflux tank; this helps to keep the flow rate of heat utilization equipment stable ； For xylene separated by adsorption, the tower pressure is generally controlled using the reboiling capacity at the bottom of the stripping tower. Due to the frequent fluctuations in this reboiling capacity, an additional steam reboiler is needed to supply heat; thus, this tower has two sources of heating energy, which makes it difficult to maintain stable pressure control. The pressure control in the xylene tower is also unstable. In some design concepts or production units, cascade control is applied between the tower pressure and the reboiling at the bottom of the tower, but this introduces another problem: control lag occurs, and the heating furnace is constantly under adjustment, meaning that the reboiling at the bottom of the tower is also constantly being adjusted. While this approach can be used in distillation towers, it still tends to lead to fluctuations, and the quality of the product obtained from the bottom of the tower is not stable. The top product is generally controlled using a sensitive plate temperature, with adjustments made based on the external feed rate.