Questions about the distillation of crude benzene!
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Crude benzene is generally first treated with acid washing to remove olefins, sulfur-containing compounds, and a small amount of saturated hydrocarbons. Then distillation is carried out to separate benzene, toluene, and xylene. For simplicity, the distillation feed composition in the design is: 75% benzene, 20% toluene, and 5% xylene (as o-xylene). Under the above conditions, which distillation scheme is the best? Some use two towers, while others use three; how should the specific process flow be arranged? Additionally, in what form is the comprehensive benefit of the tower best achieved? I hope experts can give me some advice! ! :) :)
In the long run, the three-tower design definitely has an advantage in terms of cost (lower energy consumption). But in terms of cost, three towers are a bit expensive. Based on experience, a 3-tower setup has a greater advantage when the cost is below 30,000; it has a slightly greater advantage in the range of 30,000 to 50,000; and it has a significant advantage when the cost is above 50,000.
Could you explain how the process flows for three-tower and two-tower systems should be arranged in the best way? Thank you! :)
Three towers are good if conditions permit. . This holds great potential for future work improvements and the like. The energy consumption remains more or less the same in continuous mode. With 2 towers, toluene and 2-methyltoluene also need to be distilled separately. . It’s just a two-step distillation using the same tower; if there are 3 towers, then two towers are used for distillation separately. With 3 towers, it’s simpler to distill one product each, and production volume can also be increased. It's still better to have 3 towers if conditions permit.
With 2 towers, there is also additional time required for tower cleaning
It is recommended to use a system with three towers; although this increases the initial investment, it facilitates future expansion and upgrades. Adding another tower with a diameter of 600 allows for the processing of 50,000 tons of crude benzene containing xylene, and the additional investment amounts to just over 100,000 yuan. If modern tray designs are used, the costs can be further reduced.
Aromatic distillation towers generally operate at atmospheric pressure. Compared with conventional petroleum distillation, they have the following characteristics: 1) The product purity is required to be over 99.9%, and the product fraction is narrow. 2) The top and bottom products of the distillation tower must both meet the required standards, and a single tower can produce only one type of product; overlap is not allowed, otherwise the top product of the tower may fail to meet the standards or the top product of the next tower will also fail to meet them. 3) The products distilled from aromatic hydrocarbons cannot be blended or mixed. 4) To ensure that the purity of the separated product meets the required standards, a temperature difference control method is employed to keep the fluctuation range of the tower top temperature at less than 0.02°C, thereby controlling the quality of the product. Based on the above characteristics, a three-column design is generally chosen; however, given your conditions, the composition of the feed for distillation is: 75% benzene, 20% toluene, and 5% xylene (calculated as m-xylene). Two towers will suffice. However, if either benzene or toluene is found to be substandard, it is advisable to send it back to the aromatic hydrocarbon feed tank for reprocessing, in order to avoid excessive changes in the composition of the feedstock; the substandard aromatic hydrocarbons can also be sent out of the plant as components of gasoline.