Selection of packed tower distillation

I) Comparison between fractional distillation columns and packed columns In industry, the performance indicators of column equipment are evaluated mainly in terms of: ① Production capacity； ⑦Separation efficiency ； ③Tower pressure drop ； ④Operational flexibility: ⑤ Structure, manufacturing, and cost, etc. 1. Production capacity – Therefore, per unit cross-sectional area of the tower, the production capacity of a packed tower is generally higher than that of a plate tower. 2 Separation efficiency Studies have shown that under normal and low-pressure conditions (pressure less than 0.3 MPa), the separation efficiency of packed towers is significantly higher than that of plate towers; however, under high-pressure conditions, the separation efficiency of plate towers is slightly better than that of packed towers. 3 Pressure drop: Typically, the pressure drop in a plate tower is about 5 times higher than that in a packed tower. Reducing pressure not only lowers operating costs and energy consumption, but also reduces the temperature at the bottom of the distillation column, which is beneficial for the separation of heat-sensitive substances. 4 Operating flexibility: Generally speaking, the packing itself has a high degree of adaptability to changes in gas and liquid loads; therefore, the operating flexibility of a packed tower depends on the design of its internal components, especially that of the liquid distributor. Thus, the operating flexibility of a packed tower can be determined according to actual requirements. In contrast, the operating flexibility of a plate tower is limited by factors such as liquid flooding on the plates, liquid mist entrainment, and the capacity of the downcomers, resulting in generally lower operating flexibility. 5. Structure, manufacturing, and cost, etc. Generally speaking, the structure of a packed tower is simpler than that of a plate tower, making it easier to manufacture and maintain; however, the cost of a packed tower is usually higher than that of a plate tower. It should be noted that the liquid holdup in a packed tower is lower than that in a plate tower. A higher liquid holdup enables smoother operation of the tower and reduces the likelihood of rapid precipitation of the product; therefore, plate towers are easier to operate than packed towers. Plate towers facilitate side feed and discharge, while packed towers are not very suitable for complex scenarios such as side feed and discharge. For local fillers with a large specific surface area, the filler layer is prone to clogging; therefore, packed towers are not suitable for directly treating materials containing suspended solids or those that tend to polymerize. (II) Selection of tower equipment In industry, tower equipment is primarily used in the unit operations of distillation and absorption mass transfer. In traditional designs, plate towers are commonly used for distillation processes, while packed towers are often used for absorption processes. In recent years, with the improvement in the design capabilities of tower equipment and the emergence of new types of tower components, these traditional practices have gradually been overturned. There are now some examples of the use of packed towers in distillation processes and of plate-type exchangers in absorption processes; in particular, packed towers are widely used in distillation operations. For the separation process, the choice of tower type should be based on production capacity, separation efficiency, and tower pressure drop. Requirements such as operational flexibility are taken into consideration, along with factors like the cost of dual manufacturing processes. For example, in the separation of thermosensitive systems, it is necessary to keep the tower pressure drop as low as possible, and a packed tower is a suitable choice in such cases ； For the process of feeding from the provincial side line and using mountain material, a plate tower is a more suitable choice ； For separations involving suspended solids or polymers that tend to aggregate, plate towers are preferable to prevent clogging. For process operations with an extremely low liquid spray density, if a packed bed is used, the packing layer cannot be adequately wetted, which significantly reduces its separation efficiency; therefore, a plate tower is the preferred choice ； For the separation of foamy systems, a packed tower is preferable, as the packing serves to break up the foam. Section 2: Design of Plate Columns. There are many types of plate columns, but their design principles are essentially the same. Generally, the design steps for a plate tower are as follows: ① Determine the design scheme based on the design requirements and process specifications ； ②Select the type of tray based on the design requirements and process specifications ； 3 Determine process dimensions such as tower diameter and tower height ； ④Design the plate cultivation setup, including the design and arrangement of the overflow mechanism ； ⑤Conduct fluid dynamics verification ； ⑥Draw the load performance diagram of the tray ； For the rising air passages (baffles, perforations, float valves, etc.), ⑦ an analysis of the design is carried out based on the load performance diagram; if the design is not ideal enough. Certain parameters can be adjusted, and the above design process can be repeated until satisfaction is achieved. I. Determination of the design scheme (1) Determination of the device process (2) Selection of operating pressure. The distillation process is classified into atmospheric distillation, vacuum distillation, and pressure distillation depending on the operating pressure. Generally, except for thermosensitive systems, normal pressure distillation should be used for any system in which the separation requirements can be met through normal pressure distillation, and whose distillates can be condensed using river water or recycled water. For thermosensitive systems or those with a high bubble point, vacuum distillation is advisable. For systems in which the condensation temperature of the atmospheric distillate is too low, it is necessary to increase the tower pressure or use deep well water or chilled brine as a coolant; systems that remain gaseous at atmospheric pressure must be subjected to pressurized distillation. (III) Selection of feed heat condition: There are five feed heat conditions in distillation operations. Different feed heat conditions affect the gas and liquid phase loads on each tray within the tower. In industry, liquid feed close to the bubble point and saturated liquid (bubble point) feed are commonly used. (IV) Selection of heating method: Indirect steam heating is commonly used in distillation, with two boilers being installed. (V) Selection of reflux ratio: The reflux ratio is an important process parameter in distillation operations. The principle for selecting it is to minimize the total cost of equipment and operating expenses. When designing the system, the reflux ratio should be determined based on actual conditions; experience from similar production processes can also be taken into consideration. II. Types of tray plates Tray plates are the main components of plate towers, and they can be divided into countercurrent tray plates and cross-flow tray plates. In industrial applications, cross-flow tray plates are more commonly used. The main types of cross-flow tray plates include the following: 1. Bubble cap tray plates. The main advantages of bubble cap tray plates are their large operational flexibility, wide range of liquid-to-vapor ratios, resistance to clogging, suitability for various materials, and stable and reliable operation. Its disadvantage is its complex structure and high cost ； The liquid layer on the plate is thick, resulting in high pressure drop across the plate, and thus lower production capacity and plate efficiency. In recent years, bubble trays have gradually been replaced by sieve trays and floating valve trays. In design, it is generally not selected unless there are special requirements (such as separating systems with high viscosity or those prone to coking). (II) Screen tray: The advantages of screen trays are their simple structure and low cost ； The height difference of the liquid r6 on the plate is small, resulting in lower gas pressure; thus, the production capacity is higher ； The gas is evenly dispersed, resulting in efficient mass transfer ； Higher sound level: Its drawback is that the sieve pores tend to clog, making it unsuitable for handling materials that are prone to coking or have high viscosity. (III) Floating valve trays: The advantages of floating valve trays are their simple structure, ease of manufacture, and low cost ； The tray has a high opening ratio, resulting in high production capacity ； Since the valve disc can rise and fall freely as the air volume changes, it offers great operational flexibility ； As the upward airflow enters the liquid layer horizontally, the gas-liquid contact time is longer, resulting in a higher tray efficiency. Its drawback is that when handling materials prone to coking and high viscosity, the valve disc tends to stick to the tray ； During operation, phenomena such as valve disc detachment or wear may occur, resulting in a decrease in the efficiency of the tray and its operational flexibility. III. Calculation of the process dimensions of the plate tower body The process dimensions of a plate tower body include its effective height and diameter. (1) Calculation of the effective height of the tower 3. Basic calculation formula The effective height of a plate tower refers to the height of the section where the tray is installed, and it is calculated using the following formula ； 2. Calculation of the number of theoretical plates: For a given design task, once the separation requirements and operating conditions are determined, the required number of theoretical plates can be obtained either by a step-by-step calculation method or by graphical methods. The relevant details are discussed in detail in the chapter on distillation in the textbook \"Chemical Mass Transfer and Separation Processes\", so they will not be repeated here. 3. Determination of tray spacing