/jpk/hgyl/wlkc/ 10/fenbu . gif
The structure of a typical packed tower is shown in the figure. The main components are tower body, packing and support, fluid distributor and redistributor, demister, etc. In operation, the liquid enters from the upper part of the tower, and the gas is evenly sprayed on the cross section of the tower through the liquid distribution, and flows down on the surface of the packing in the form of a film; The gas enters from the lower part of the tower and is discharged from the top of the tower through the gap of the packing layer. Gas-liquid two-phase mass transfer on the surface of liquid film.
2. Evaluation of filler characteristics
Packing not only provides gas-liquid contact surface, but also promotes gas-liquid dispersion and liquid film renewal. The performance of fillers can be evaluated from the following three aspects.
(1) Specific surface area A: The filler should provide as much surface area as possible. This characteristic of the filler is expressed by the surface area of the filler in unit filling volume, which is called specific surface area A, and the unit is m2/m3.
⑵ Void ratio ε: the void volume per unit volume of filler, which is called void ratio. The gas flows in the gaps between the fillers. In order to reduce the flow resistance of gas and improve the allowable gas velocity of packed tower, the packed layer should have as large porosity as possible.
⑶ Geometry of packing: There may be significant differences in hydrodynamics and mass transfer performance between two kinds of packing with similar specific surface area and porosity but different shapes, but there is no quantitative expression of packing geometry at present.
3. Several common fillers
Commonly used fillers include bulk fillers and structural fillers, and materials include solid materials and mesh materials.
10.2.2 gas-liquid two-phase flow in packed bed
1, liquid
The ideal flow state is from top to bottom, flowing along the surface of packing in the form of film, and the liquid film is constantly updated from one packing to another. The liquid is required to spread on the surface of the packing to form a film, the liquid distribution in the tower should be uniform, and the thickness of the liquid film should be appropriate.
Liquid has a certain self-distribution ability in random packaging. Therefore, for small towers, the self-distribution ability can be utilized, and the pre-distribution requirements are low; For a large tower, it is difficult to realize the uniform distribution of the whole tower section by using the self-distribution ability of packing, so it is necessary to calibrate the initial distribution. In addition, channel flow or wall flow may occur in the packing layer, and the liquid needs to be redistributed.
The thickness of liquid film in the tower is related to the liquid holdup, which is the amount of liquid per unit filling volume. When the spraying amount is large, the liquid holding capacity is also large, and the thickness of liquid film increases; In the normal gas velocity range, the increase of gas velocity has little effect on the thickness of liquid film.
2. Gas
Driven by the pressure difference in the packed tower, the gas rises from bottom to top through the packing gap and contacts with the liquid film for mass transfer. The pressure drop of gas passing through the packing layer is related to the gas velocity and the liquid velocity.
When the liquid volume is zero, the pressure drop Δ p of dry packing increases with the increase of gas velocity u.
When there is liquid injection, the liquid quantity is constant, the gas velocity u increases and the pressure drop Δ p increases. At the same gas velocity, the pressure drop Δ p is higher than that of dry packing. When the gas velocity u is small, the gas velocity u increases and the liquid film thickness changes little. When the gas velocity u increases to a certain value, the thickness of liquid film begins to increase, and the liquid holdup also increases, resulting in liquid plugging. At this time, the slope of the relationship curve between packing pressure drop and empty tower velocity increases, which is called loading point. After the loading point, when the gas velocity u continues to increase to a certain value, the liquid holdup greatly increases, and the liquid accumulation appears flooding phenomenon, which is called flooding gas velocity.
With the increase of liquid volume, the gas velocity at the flooding point decreases. At the same gas velocity, the liquid volume is large and the pressure drop is also large.
3. Flood:
Flooding is an abnormal operation of packed tower. When flooding occurs, the liquid cannot flow down smoothly and the gas-liquid mass transfer cannot be carried out normally. Before the flooding point, the gas is a continuous phase and the liquid is a dispersed phase; After the flooding point, the gas is the dispersed phase and the liquid is the continuous phase. The flooding point is also called the inversion point. At this time, the pressure drop Δ p increases sharply, and the entrainment of liquid droplets in liquid back-mixing gas is serious, and the mass transfer effect is extremely poor.
During design, the operating gas velocity is 50% ~ 80% of the flash point gas velocity. The displacement gas velocity can be estimated according to the displacement correlation diagram.
4. Operating range of packed tower
When the amount of liquid is constant, if the amount of gas is small, the mass transfer process mainly depends on diffusion, and the mass transfer effect is not good; A large amount of gas will cause floods.
When the amount of gas is constant, if the amount of liquid is small, some fillers will not be wetted, and the mass transfer effect is not good; If the amount of liquid is large, flooding will occur.
The maximum gas volume or maximum liquid volume can be estimated according to the gas velocity of the flood point; The minimum amount of gas and liquid must be determined by experience.
10.2.3 mass transfer in packed column
The mass transfer rate in packed bed is an extremely complicated problem, which has not been clarified so far. Effective contact area is the area that really participates in mass transfer. The effective contact area includes the effective wetting surface of the filler and the surface area of possible droplets and bubbles, and the effective contact surface