Dave Dickey Forum Moderator 292 Posts
Re: What is the best way to approach gas-gas mixing?31 January 2012 at 1:29pm
What you want to do and what is possible may be two different things. Surprisingly, gas-gas mixing is relatively difficult and normally done in turbulent flow. Laminar gas-gas mixing, especially at low flow rates may be limited by diffusivity, rather than flow path. I am not sure what the dimensions are on the gas rate you have provided, so actual calculation are impractical. A capital "N" represents Newtons, a measure of force. A lower case "n" represents nano 10^-9. What does 1 Nm3/hr mean?
If you can find structured packing to fit a 1/2 inch diameter pipe, you can find commercially available static mixers, especially the helical (twisted) element design. For laminar mixing with the twisted element design, 18 to 36 elements are recommended. With that guide, the mixing length needs to be 18 to 36 times the diameter, so a 36 inch long mixer should provide good mixing, whether you use static mixer elements or spherical beads. Better yet, reducing the tube diameter to 1/4 inch increases the velocity, turbulence and the availability of commercially available static mixers.
Axial dispersion will increase the axial mixing and deviation from plug flow. Increased axial dispersion might improve mixing, but laminar flow has more of a problem with a radial gradient. To reduce the radial gradient, you probably need 10 or more pebbles across the pipe diameter.
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I want to mix two low viscous gas streams in laminar regime. I am planning to mix them by passing through a pipe filled with spherical pebbles or some structured packing. I want to calculate the length required for the mixing expected. My required process stream details are as follows: Q = 1Nm3/hr He= 0.99 H2= 0.01 P= 1.2 bar Dia of pipe = 1/2 inch Temp= 25 degrees C I have gone through the text book "Handbook of Industrial Mixing." But that textbook has procedure for only commercially available static mixers. Please guide me through the design. Is dispersion model helpful in finding the length? If so, how?
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