Counter-Current Absorption of Heavy Hydrocarbons

Consider now the absorption extraction of heavy hydrocarbons under the conditions of counter-current flow in a column absorber presented schematically in Fig. 20.7. Gas of a given composition yo = (yoi, yo2, , yon), where yoi is the molar fraction of i-th component, with the flow rate Qgo enters the bottom part of the column. At the same time, an absorbent with composition xo = (xoi 5 02, , on) and flow rate qo enters the top part of the column. The number of contact stages is equal to N. Each stage is equipped with a perforated plate operating in the ablation regime. This means that the liquid is not collected on the plate, but exists in a dispersed state in the inter-plate space. Each contact stage contains a separation device, for example, a mesh droplet catcher, in which the exhausted absorbent is separated from the gas and directed toward the next plate. 

We assume, for simplicity, that the droplet catcher completely separates the liquid from the gas. 

The average velocity of gas motion in a contact stage is equal to 

Here Qgp and Qgh are, respectively, gas flow rates (million m /day) under operatibg and normal conditions z is the compressibility factor of the gas p and T are pressure and temperature D is the diameter of working section of the column. So, at Qgh = 10 mill, m /day D = 2.4 m p = S MPa T = 253 °K and z = 0.7 we have U = 0.2 m/s. Reynolds number is Re = pg UDjpG = 5 10. The appropriate internal scale of turbulence is = 5 p. Knowing the average velocity of ascending gas flow, it is possible to determine the maximum size of drops that are carried away with the flow, by using Stokes formula  

For values hq = 2 10 Pa-s, pi = 650 kg/m, = 100 kg/m and U = 0.2 m/s we have = 56 g. Thus, the radius of drops moving together with the flow in the inter-plate space does not exceed R - The size of these drops can change during motion because of the mass exchange with the gas and the drop coagulation. Each of these processes is characterized by its own time. Thus, the characteristic time of mass exchange is equal to  

---