Thread Content
1. How is the skirt height of the initial distillation tower, the atmospheric pressure tower, and the vacuum tower determined? Both the initial distillation tower and the atmospheric pressure tower operate under pressures slightly higher than atmospheric pressure. When the bottom pump is switched or restarted after evacuation, the oil at the bottom of the tower can flow directly into the pump due to the pressure inside the tower, thereby filling the pump. When starting the pump, its skirt height is primarily intended to ensure that the height difference between the product outlet at the bottom of the tower and the pump inlet is greater than the cavitation margin of the pump at the bottom of the tower, thereby preventing damage to the pump due to cavitation. The cavitation margin of domestically produced centrifugal oil pumps is generally around 4 m; therefore, the skirt height of the initial distillation tower and the atmospheric pressure tower is usually between 4 and 5 m. The vacuum distillation tower operates under negative pressure; if the liquid level at the bottom of the tower is not high enough, the pump located at the bottom of the tower cannot be started for operation once it is restarted. To provide sufficient priming pressure, the height difference between the liquid level at the bottom of the tower and the pump inlet is generally maintained between 7 and 10 meters. 2. What are the reasons for the decrease in vacuum level? In the operation of a vacuum distillation tower, maintaining a stable vacuum level plays a decisive role in ensuring the smooth operation of the tower, achieving satisfactory product quality, and obtaining high yields of products. As the vacuum level decreases, the vacuum gauge reading drops, and the liquid level at the bottom of the tower rises. This can lead to situations such as the intermediate reflux pump running dry, a decrease in the liquid level in the side-line stripping tower, and the side-line pump running dry. The main reasons for the decrease in vacuum level are: (1) Insufficient steam pressure used in the steam ejector, which affects the pumping capacity of the ejector; this is one of the common causes of a decline in vacuum level. The steam pressure should be adjusted in a timely manner; typically, it ranges from 0.8 to 1.1 MPa. Energy-saving ejectors use low-pressure steam for vacuum creation, and it is also necessary to maintain stable pressure. (2) High cooling water temperatures or low water pressure in the top condenser and the various stage condensation coolers can lead to an increase in the pressure at the inlet of each ejector, thereby reducing the vacuum level. In the case of condensation coolers equipped with air coolers, an increase in external temperature or a failure in the electrical system of the air cooling fans can also result in an increase in the pressure at the inlet of each ejector, leading to a decrease in the vacuum level. Efforts should be made to lower the water temperature, increase the water pressure, and improve the cooling effect. Industrial-style air blowing can also be used regularly to prevent water scaling, improve cooling efficiency, and reduce the inlet pressure at each stage of the ejectors. 3. What are the factors that affect the tightness of the labyrinth seal? (1) Excessive radial clearance, or too small a gap in the newly installed gas seal ring. (2) The sealing gasket or gas seal ring, as well as the areas between the teeth, become worn and rounded, or deform due to heat from prolonged friction, resulting in damage and rendering them unusable. (3) After long-term use, the spring becomes loose and deformed, preventing the gas seal ring from settling in place. Moreover, the accumulation of dust and dirt during operation can cause the pressure of the sealed medium to be lower than that of the working medium, or result in unstable pressures. 4. Purpose of water washing in air separation units: The main purposes and significance of water washing are as follows. (1) Reduce the temperature of the air entering the tower. The inlet air temperature of air separation equipment depends on the season, climate, location of installation, and the degree of pre-cooling of the air before it enters the equipment. High inlet air temperature can lead to: ① a decrease in the isothermal throttling effect, an increase in the amount of air that expands, and higher energy consumption for the product; ② an increased thermal load on the main heat exchanger; ③ worse operating conditions for the equipment in the air purification system; ④ an **increase in the moisture content in the air. Adopting measures to reduce the inlet temperature of air separation equipment has certain economic value. Especially in hot regions, it is technically and economically reasonable to use water cooling towers to reduce the intake air temperature, regardless of the season. (2) Reduce the content of moisture and carbon dioxide in the gas entering the tower. Water and carbon dioxide will precipitate from the air as its temperature drops, freezing and blocking the gas passages. Acetylene and other hydrocarbons accumulate in air separation equipment, and under certain conditions, they can also cause explosions. (3) Remove acidic substances from the air to reduce damage to the equipment and molecular sieves. 5. Factors affecting the nitrogen purity of the product: (1) Excessive amount of nitrogen removed. (2) An excessive amount of air is expanded at the top of the tower, resulting in a reduced reflux ratio at that level, a decrease in nitrogen purity, and a lower oxygen extraction rate. (3) The purity of liquid nitrogen at the bottom of the tower decreases. (4) The opening degree of the liquid nitrogen throttle valve at the lower tower and upper tower is too large or too small.