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How would you derive the mass transfer model for a device that exhibits both adsorption and absorption?

We are designing a device that will adsorb the CO2(g) from the gas stream that leaves a house through a chimney. Our device is a cylindrical canister that is filled with stacked activated charcoal discs that will have drilled holes through it. Every other disc will be rotated 90 degrees so that the gas stream will go through a hole in 1 disc and hit a solid surface at the next disc, which will then bounce back on the other solid surfaces and eventually will manage to diffuse through the system. We do not have any experimental data since this is a theoretical design, so we would like to know how to derive a mass transfer model for our system. If possible, if there is a way to derive the mass transfer coefficient/correlation, dimensions of our device, and if the potential resistance of the device is too strong so that the flue gas will end up being unable to escape through the chimney, we would greatly appreciate your help.

Public Comments

1. I would look at how power plant scrubbers are designed, since this what you are making. You will most likely need a fan to push the gas through the scrubber, since the heat alone from the fire may not have enough energy to do the job (since the hot rising air carries particulates through the chimney, and as the gas cools it loses energy). That should be relatively easy given the approximate flow rate of the gas through the chimney, height of the chimney, and some fluid dynamics work through the scrubber. The scrubber will cause a pressure loss due to the intricate pathways inside of it (as well as the pressure loss from height of the scrubber). As long as you have sufficient area for the gas to pass through each disc, then the air should flow relatively easily with a fan. To select a fan, you need to know the flow rate and the delta pressure you need to overcome. Most of this can be calculated with some effort and time, or it could be found experimentally using a variable rate fan and your completed system. I am not sure about the chemistry behind the reactions in the scrubber. Since you are designing the system, you should know what's going on inside the scrubber. Knowing what happens with the chemistry, you should be able to derive a mass balance equation for your system. Mass of gas in = mass of gas out - absorption+adsorption. You should be able to find out the mass xfer coefficient using the properties of the charcoal discs which should be known if this disc or material is used in industry (research). It is a complex process, but it just takes good research (theory and industry knowledge), planning, and compiling of several different subject matters into one project. I would break the project up into sections and split the work. 1) Research - read everything you can get your hands on dealing with power plant technology, scrubbers, etc. Also research the components that you will be using off the shelf, if any. Research the materials used in the scrubber. 2) Chemistry - figure out what's going on in the scrubber and figure out the reaction. If you are using a material for absorbtion that is readily available in industry, you should be able to find out the rate of absorbtion and other properties. If not, then you may need to design an experiment (although this will take much time and effort). 3) Fluid Dynamics/Thermodynamics - figure out the delta pressure through the whole system. Need: flow rates, chimney dimensions, and analysis of scrubber to determine delta pressure across it. This should also include choosing a fan, if necessary. 3) Compile all data into an easily manipulated system (I like using excel for all the equations, since you can change parameters and not have to calculate everything by hand). Design is an iterative process. So it may take you several iterations to come up with a final design. Hope this helps!