How to calculate the gas flow rate in an empty tower

Calculation of the gas velocity in an empty tower: 1. First, determine the flooding gas velocity = C × [（ρL – ρG）/ρG]0.5 (m/s); the 0.5 is a superscript. C is the gas load factor, given by C20/C = (20/σ)0.2. C20 represents the value of C at a surface tension of 20 mN/m, and this value can be found in tables. σ is the surface tension of the liquid phase in the system; it can be determined based on the properties of the material, and its value is in units of mN/m. ρL and ρG represent the densities of the gas phase and the liquid phase respectively. 2. To determine the empty tower gas velocity, u is generally taken as (0.6–0.8)uf

The so-called gas velocity in an empty tower refers to the linear gas velocity calculated based on an empty tower. As the fluid flow rate changes, the frictional resistance between the two fluids also changes, which in turn leads to a series of changes in the fluid flow pattern within the tower. Graph showing the relationship between the pressure drop p/H per unit height of the packing layer and the empty tower gas velocity u, at different spray densities (the amount of liquid sprayed per unit time per unit cross-sectional area of the empty tower), L. When L=0, that is, as the gas passes through the dry packing layer, there is a linear relationship between p/H and u; the slope of this line is 1.8–2.0, indicating that p/H is proportional to the 1.8–2nd power of u, and the flow regime is turbulent. When liquid is sprayed, some of the voids in the packing layer are occupied by the liquid. At the same empty tower gas velocity, as the liquid spray density increases, the liquid holding capacity of the packing increases, the free path for the gas flow decreases, and the pressure drop across the flow increases; as a result, the p/H–u relationship becomes curved. As the gas velocity increases, the frictional force between the two-phase fluid increases. When the gas velocity increases to a certain value, the flow of the liquid begins to be hindered by the frictional force between the two-phase fluid, causing the liquid holdup in the packing layer to increase as the gas velocity rises. This phenomenon is known as liquid retention. The turning point at which liquid retention begins is usually referred to as the load point, as shown in Figures 5–3 at , , and . The gas velocity in the empty tower corresponding to the load point is called the load point gas velocity. Beyond the critical gas velocity, the slope of the p/H–u relationship line increases, which facilitates an increase in the mass transfer rate. If the gas velocity is increased further to the next turning point (, , ), the increasing liquid holdup within the packing layer will cause the liquid to fill the entire free space in the packing layer, resulting in a sharp increase in pressure drop. At this point, the liquid begins to change from a dispersed phase to a continuous phase, while the gas begins to change from a continuous phase to a dispersed phase; it passes through the liquid layer in the form of bubbles and carries a large amount of liquid out of the top of the tower. The operation of the tower becomes extremely unstable, or even completely disrupted – this phenomenon is known as flooding. The turning points at which flooding begins (, , ) are known as the flooding points, and the corresponding gas velocity in the empty tower is referred to as the flooding gas velocity or the velocity at the flooding point. The factors affecting the gas velocity in a spargered system mainly include the properties of the filler (specific surface area, porosity, geometric shape, etc.), the physical properties of the fluid (density, viscosity, etc.), and the flow rates of the gas and liquid phases. The critical gas velocity is the upper limit for the gas velocity required for the normal operation of a packed tower; in practice, the actual gas velocity is usually set at 50% to 85% of the critical gas velocity.

I’ve learned about content studies*, thanks for sharing. . ，。 . . .

Reply to 3# wilsionchen: The explanation is very detailed, thank you.

It’s so complicated; I don’t know where to start Could you recommend some books to learn *it?

It’s very complex. If no engineering design is involved, a rough process estimation can be made, which is much more convenient.