Product control of the propylene column in the gas phase section

When adjusting distillation, pressure and the top temperature should be adjusted together, right? Adjusting only the pressure will affect the evaporation efficiency.

(1) Under permissible conditions, the lower the pressure, the higher the relative volatility, and the easier it is to separate； (2) Increasing the reflux ratio and the tower pressure are two independent variables; they influence each other when the cooling load at the top of the tower is insufficient ； (3) Increasing the reflux rate helps improve the purity at the tower top, but this purity is also constrained by material balance

Reply to 3# fang_yanchen: Well, can it be understood in that way then? To improve purity, I can reduce the top pressure without increasing the reflux ratio by increasing the amount of condensation cooling at the tower top. Can it be understood in this way as well that the highest purity is achieved at lower pressure and a higher reflux ratio? Thank you. Also, I was wondering: to what extent can my stress levels be reduced?

You can’t think that way; without a heat pump, the propylene column can function at around 1.8 MPa. The pressure needs to be stable, and a lower pressure doesn’t necessarily guarantee better separation, while a higher pressure doesn’t necessarily result in higher purity of propylene at the column top. High pressure and increased reflux both achieve propylene purity at the cost of energy consumption.

This post was last edited by *qingzhuo on 2010-10-23 11:19. The impact of pressure on distillation: At low pressures, the distillation process works better, but this does not mean that the purity of propylene is higher; it simply means that separation is easier. The common methods to improve the purity of propylene are to increase the reflux ratio; both an increased reflux ratio and a higher operating pressure can lead to higher propylene purity. However, if the operating pressure is too high, the propylene component will be pushed to the bottom of the tower, which affects the purity of propane and reduces the yield of propylene. In our case, this pressure is usually controlled by the bottom temperature and the air cooler; by maintaining an appropriate bottom temperature, we ensure effective distillation of propylene and propane. Increasing the reflux rate and using the air cooler helps to improve the purity of propylene.

For the separation of propylene from propane, as long as the tower diameter permits (so that the gas phase does not exceed the critical velocity), the lower the pressure, the better. In reality, the tower pressure is determined by the cooling method used at the top of the tower; for example, with wet air cooling, the lowest temperature at the tower top in summer is not below 38 degrees, and the pressure cannot be lower than the saturated vapor pressure of propylene at that temperature

Generally, the precision of propylene is controlled by the reflux ratio, while pressure is controlled by the thermal bypass; however, it should be regulated as a single system, rather than treating them as independent variables

Increasing the reflux ratio can improve purity, but it leads to higher energy consumption. It is possible to take into account various factors such as changes in feed, changes in the heat source, or changes in cooling load, and implement corresponding measures – it’s not just a matter of adjusting pressure

The factor that affects the quality of propylene is actually the vapor pressure of oil and gas. 1. Under the same pressure, increasing the reflux ratio actually raises the vapor pressure of propylene at the top of the tower, reducing the volatilization of heavier components and thus improving the quality of propylene. 2. The pressure at the top of the tower is also limited. The lower limit for the pressure at the top of the tower is that the tower pressure must be greater than the saturated vapor pressure of propylene at the cooled temperature; otherwise, all of the propylene will vaporize. Excessively high tower pressure leads to high energy consumption; when the tower pressure is high, the bottom temperature must be increased. As long as the quality is good, it’s not necessary. 3. In addition, the linear velocity of the tray must also be considered.

In practice, the bottom temperature of the tower is usually increased in order to push the propylene at the bottom of the tower upward as much as possible. At this point, the tower pressure is proportional to the purity, but the degree of this proportionality is very small.