The increasing concern over greenhouse gas emissions and global warming makes our CO2 absorption experiment more relevant now than ever. Our old steel absorption tower had become clogged with rust and residue from years of use with sodium carbonate solution as absorbent, however. To continue offering a CO2 absorption experience but to reduce cost and avoid column fouling in the future, we chose to use pure water as absorbent in our new smaller glass column packed with glass Raschig rings. Although using water does focus the lab on mass transfer concepts without the added complexity of reactions, the limited solubility of CO2 in water makes it necessary to have accurate analysis of the gas phase and to work with concentrated gas streams to get good results. A Rosemount Analytical, Inc. infrared analyzer provides accurate and reliable gas phase analysis and although determination of mass transfer coefficients is more complicated for concentrated systems than for dilute systems we will show that modern computing environments like MATLAB® can be used to integrate complex functions and easily back out mass transfer coefficients from laboratory data.
We will also show that a simple simulation of our column using COMSOL Multiphysics® finite element software can be used to gain insight into mass transfer fundamentals. In our simple convection and diffusion model, CO2 disappears from the gas phase and appears in the liquid phase at the same rate according to a “reaction” described by an overall mass transfer coefficient and appropriate driving force. The parametric solver feature of COMSOL allows straightforward determination of mass transfer coefficients that describe the experimental results and agree with those obtained by traditional analysis methods and published correlations. Using built-in plotting capabilities, the composition of the vapor and liquid phases and the driving force for mass transfer can be observed as a function of position in column. Further post processing features allow determination of fluxes and flow rates in and out of the column by boundary integration. A time dependent analysis can also be made to illustrate the column performance as a function of time. With this tool, students can perform “what if” analysis and design experiments and/or scaled up columns with ease.
A more detailed simulation of the absorption process gives even more insight into mass transfer fundamentals. In a more detailed convection and diffusion model, the column is assumed to contain a large number of glass rods with a thin layer of liquid flowing down surrounded by a gas stream flowing up. Equilibrium is assumed according to Henry's law at the gas liquid interface and realistic values are input for liquid and gas phase diffusion coefficients. The two film theory of mass transfer is illustrated by including a non-flowing layer of finite thickness on either side of the interface. By adjusting the effective diffusivity in each of these stagnant layers the experimental results can be reproduced by the model. Moreover, it can be shown that the effective diffusivities divided by the film thicknesses correspond to the individual gas and liquid side mass transfer coefficients that describe the process via traditional analysis methods. Using built-in plotting capabilities, students can plot the CO2 concentration in mol/m3 at any point in the column and observe the relative magnitude of the resistances to mass transfer given by the gas film, the interface, and the liquid film.
In future implementations of our unit operations laboratory course, we plan to evaluate any improvement in student learning and attitudes afforded by these simulations as we have already done with simulations of a heat exchanger, a gas permeation membrane, and a fluid flow experiment. We anticipate that use of these simulations will provide students with a clear understanding of the often confusing concepts of mass transfer fundamentals.