How are the pressure and temperature in the pressurized tower determined?

Pay attention to the following two points: 1. The pressure-boiling point relationship for methanol. 2. Understanding and application of Raoult’s and Henry’s laws.

I don’t understand; can someone tell me the table showing the pressure and boiling point of methanol?

Why is pressure applied to the pressure tower? To save thermal energy through the triple-column double-effect system, the condenser of the pressurized column is used as the reboiler for the atmospheric pressure column, and the methanol vapor from the top of the pressurized column serves as the heat source at the bottom of the atmospheric pressure column. Therefore, the temperature at the top of the pressurized column is determined by the requirements of the atmospheric pressure column (usually 0.5–0.57 Mpa; specifically, it is determined by the feed rates to both columns), while the pressure at the top of the pressurized column is determined by its temperature (112–121°C) ； Similarly, the temperature and pressure at the bottom of the pressurized tower are determined by the pressure and temperature at the top of the pressurized tower.

The overhead gas from the pressurized tower is used to heat the bottom of the atmospheric pressure tower, and there needs to be a certain temperature difference between it and the bottom of the atmospheric pressure tower. Since the bottom temperature of the atmospheric pressure column changes little, and the top of the pressurized column is close to a pure component, the pressure in the pressurized column depends on the temperature difference required by the double-effect reboiler. From a design perspective, if a higher pressure is chosen for the pressurized tower, its diameter can be smaller, and the area of the double-effect reboiler will also be reduced, which allows for savings in the capital investment for these elements. However, this will result in an increase in the area of the reboiler in the pressurized tower, leading to higher investment costs. At the same time, as the pressure increases, the relative volatility decreases, which causes the reflux ratio required for the pressurized tower to rise slightly, thereby increasing energy consumption as well. Design is essentially about finding a better point in between. As for operation, if it runs at the designed capacity, and since the equipment comes with some margin, the pressure tower should be able to operate at a pressure slightly lower than the design value. If the goal is to maximize the processing capacity, increasing the pressure in the pressurized tower helps to overcome the bottlenecks associated with the components inside that tower as well as the double-effect heat exchanger; whereas reducing the operating pressure helps to eliminate the bottlenecks related to the reboiler in the pressurized tower.

What everyone has said is correct, but how can we determine the pressure in the pressure tower that corresponds to the boiling point in order to ensure product quality? For example, when the pressure at the top of the pressure tower is 0.38 MPa, the temperature at the top is 105 degrees – is this considered a qualified product? How can we find out the relationship between pressure and temperature (boiling point) so as to ensure that the products produced are of good quality?

The vapor from the top of the pressurized tower is used to heat the bottom of the atmospheric pressure tower; the temperature at the bottom of the atmospheric pressure tower is usually 110 degrees, and a temperature difference of 30 to 20 degrees is required. Generally, the temperature at the top of the pressurized tower is controlled at 130 degrees. Since the temperature at the bottom of the atmospheric pressure column changes little, and the vapor at the top of the pressurized column is close to being a pure component, the vapor at the top of the pressurized column condenses when the temperature at the bottom of the atmospheric pressure column is 110 degrees. Based on the gas-liquid equilibrium data, the pressure in the pressurized column is determined by the temperature difference required by the double-effect reboiler. From a design perspective, if a higher pressure is chosen for the pressurized tower, its diameter can be smaller, and the area of the double-effect reboiler will also be reduced, which allows for savings in the capital investment for these elements. However, this will result in an increase in the area of the reboiler in the pressurized tower, leading to higher investment costs. At the same time, as the pressure increases, the relative volatility decreases, which causes the reflux ratio required for the pressurized tower to rise slightly, thereby increasing energy consumption as well. Design is essentially about finding a better point in between. As for operation, if it runs at the designed capacity, and since the equipment comes with some margin, the pressure tower should be able to operate at a pressure slightly lower than the design value. If the goal is to maximize the processing capacity, increasing the pressure in the pressurized tower helps to overcome the bottlenecks associated with the components inside that tower as well as the double-effect heat exchanger; whereas reducing the operating pressure helps to eliminate the bottlenecks related to the reboiler in the pressurized tower.

The methanol content at the top of the pressure tower is over 99.9%, and a change of 0.1 percentage point in this methanol content does not indicate any difference in temperature; therefore, relying on the relationship between tower top pressure and temperature to control product quality is akin to using a scale to weigh a grasshopper – it’s not reliable at all.

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