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Another important relationship that is noticed occurs between the liquid flow rate and the mass transfer coefficient. One other justification is that the liquid boundary layer 22 resistance to mass transfer is reduced with higher flow rates, for similar reasons that absorption increases.
On the other hand, at slower liquid rates, the boundary resistance is higher due to a decrease in liquid-gas stream contact and interaction. Kya Illustrated in Figures 4. By analyzing the concentration as a function of position in the absorber, we were able to obtain the following graphs using post-processing methods in COMSOL Input Summaries are included in Appendix E.
With further study and experimentation with COMSOL, these trends can be used to analyze carbon dioxide composition in the gas phase as a function of time inside the packed tower. In the post-processing used to compare CO2 absorbed in the gas phase, we can perform the same analysis for the liquid phase. We can see that, not lonely does COMSOL accurately model the interactions and transport in the gas phase of the absorber, but also in liquid phase. For all four water flow rates used in the absorption lab, COMSOL approximated very well, almost matching the change in composition at both 1.
Given such successful results, it can be a useful learning tool for students to use COMSOL before performing experimental analysis. This can assist in providing an understanding of gas-liquid interactions within a packed tower, reinforcing important concepts, and providing students with a range of expected values for particular flow rates. If used after lab experimentation, modeling can be used as a set of guidelines to verify values collected in the lab.
COMSOL not only provides a quantitative analysis for the packed tower, in regards to predicting amount of CO2 removed, but also a qualitative analysis, which is essential in understanding the absorption process in its entirety. Gas Absorption Lab Experiment As this was the first year using the new gas absorber in the Unit Operations UO laboratory, there are several methods that can both be changed and implemented in the future.
In the UO lab, twenty minutes was allowed for the system to come to equilibrium, however, the maximum absorption for that particular liquid flow rate was not achieved. I believe that permitting an extra ten minutes would give more accurate data.
With this additional information, the CO2 in the liquid bottoms can be analyzed. One segment of the collection procedure that was omitted from this particular lab, was the analysis of carbon dioxide in the liquid outlet.
This can be tested using the 27 carbon electrode built specifically for the gas absorber. Techniques for assembling and using the electrode can be found in Appendix F. Another analysis tool that was not included in this years' lab was the use of equipment software. The absorber comes with a program that can be used to collect and record data without the use of the Rosemont Analyzer. Implementing these tools can help produce more accurate and reliable data.
Though we only used modeling for the simplest case, a dilute system, there are applications built for the analysis of concentrated vapors, such as the Maxwell-Stefan Diffusion and Convection application.
This particular mode allows for accurate modeling of a concentrated mixture by setting up the proper multi-component mass transport equations. It also permits the use of up to four species in the absorption column. Another important segment of gas absorption that can be modeled in the future is the mass transfer theories, specifically the two film theory.
In separation processes, materials must be diffused from one phase to another, which affect the overall mass transfer coefficient. In the two film theory, equilibrium is assumed at the interface, and the resistances to mass transfer in the two phases are added to an overall resistance .
Aspen Plus can be used for various chemical engineering applications. For example, it can execute tasks as simple as describing thermodynamic properties of an ethanol and water mixture, or as complex as predicting the steady-state behavior of a full-scale petrochemical plant . Aspen is also a useful tool for simulating reaction engineering scenarios, such as designing and sizing reactors, predicting reaction conversions, and understanding reaction equilibrium behavior.
Though this program does not create concentration profiles, it does allow for reasonable predictions for an absorber under certain conditions. However, in order to maximize its ability, the best way to model an absorption column would be to apply Rate-Based equations in the Rad-Frac mode. A similar test was tried for this project.
The input summary of the trial is included in Appendix F for future study. Retrieved October 23, , from http: Retrieved March 15, , from http: Mass Transfer in Fluid Systems 2nd ed. Unit Operations of Chemical Engineering 7th ed. Retrieved March 15, , from www.
Gas Purification 5th ed. Retrieved November 14, , from http: Retrieved April 22, , from http: Structure and activity relationships for amine based CO2 absorbents. International Journal of Greenhouse Gas Control, 1 1. Aspen Plus Conceptual design of chemical processes. Retrieved March 16, , from http: The removal of CO2 from industrial gas streams is becoming increasingly important due to the need to control greenhouse gas emissions to protect the environment. Carbon dioxide can be removed from an industrial effluent gas stream by absorption into a liquid solvent.
This separation process is normally achieved in a column packed with packing materials designed to promote direct contact between the solvent flowing downward over the packing and a continuous gas phase flowing upward. In industrial processes, the solvent is usually an aqueous potassium carbonate or amine solution that provides enhanced absorption through reaction with the CO2. In this experiment you will study the absorption of CO2 from air in a packed column using water as the solvent.
The main goal is to determine the effect of gas and liquid flow rates on the overall mass transfer coefficient for this absorption process. You will also be asked to use the information obtained for an absorption design calculation. The regulator pressure should be set at 20 psig for each gas. Flow rates of the gases are maintained at desired levels using flow control valves and rotameters. The gases are mixed using a specially designed mixing tube located after the flow meters and prior to entering the bottom of the tower.
It flows downward through the column and can be returned back to the sump tank or diverted to the drain using valves in the pipes below the column. If water is to be diverted to the drain, it is necessary to open valves to provide make-up tap water to the sump tank.
A float mechanism in the sump tank will maintain a constant level in the tank as long as the appropriate valves are opened. During column operation with gas flowing upward, a liquid seal must be maintained in the pipes 31 below the column by appropriate adjustment of the return or drain valves. That is, the rate of water flow returned to the sump or diverted to the drain must be maintained at a rate equal to the inlet water flow rate to maintain a constant height of water in the pipe below the column.
That way, water does not backup and flood the column and the gas entering the column at the bottom does not escape into the sump or out the drain. Calibration data is attached. Thermocouples at the column top and bottom provide temperature measurements that can be read on the column control panel.
Pressure drop across each of two sections of the column can be obtained from digital readings of differential pressure gages. A water-filled manometer provides a measure of the difference between the pressure at the column top and atmospheric pressure. The Rosemount analyzer provides a digital readout of the volume percent CO2 in the air. Procedure 1 Preliminary inspection of equipment It is necessary that each student understand the arrangement and operation of the equipment before any experimental work is undertaken.
A complete inspection of the equipment should be made and the function of each part of the apparatus should be determined. A detailed schematic should be drawn.
Each member of the lab group will be expected to answer questions about the equipment during the lab session. The standard gas cylinders have regulators that should be set at about psig. Sample valves on a panel above the analyzer can be opened one at a time to introduce the samples individually. A pressure of 1 inch of water at the manometer on the panel gives suitable flow rates for gases flowing into the analyzer. The flow control valve next to the manometer should be opened slowly to establish the flow that provides 1 inch of water.
The pure nitrogen gas is used as the zero point reference. Once nitrogen is introduced at the sample port and has been flowing for at least two minutes, press zero then enter on the Rosemount front panel.
After a minute or two, the instrument should read zero or nearly so. Close the flow control valve and the N2 sample valve. The calibration curves were obtained at 70 oF and 20 psig at the regulator.
Correction for other T and P conditions might need to be made. The water flow can be varied between 0. Inlet CO2 composition in the water entering the column can be assumed to be zero as long as the outlet water is completely diverted to the drain. It is suggested that you study four different water rates at a fixed gas rate during the first experimental period and that you study the same four water rates at a different gas rate for the second experimental period.
The CO2 composition of the outlet gas stream can be monitored continuously by opening the column top sample valve and opening the flow control valve on the panel above the instrument just enough to provide 1 inch of water at the manometer. It is important to wait long enough for steady state to be achieved.
It normally takes about 20 minutes for the outlet concentration to settle to a constant value. Theory The engineer who is required to design an absorption tower is interested in the rate of absorption of the material under the desired operating conditions. Considerable experimental work on a few systems has been reported in the literature that will enable the designer to predict the effect of certain operating variables on the rate of absorption for a given type of apparatus.
The absorption rate is generally expressed as an overall mass transfer coefficient, K, which may be based on either a gas or a liquid-phase driving force. In most cases it is impossible to determine the area of contact of the gas and liquid. Therefore, the coefficients are reported on a volume basis.
For dilute systems with straight operating and equilibrium lines, a design equation for the volume of a gas absorption tower may be written as: Under certain assumptions, a design equation for the column height is given by : The other part of Equation 2 has units of length and is called the height of a transfer unit based on the overall gas phase driving force, HOy.
Thus the height of the column is given by: This formulation is useful for design purposes because correlations are available for Hx and Hy. Alternatively, if the column height is given, and an estimate is obtained for HOy, the outlet compositions that will result for given inlet flows and compositions can be determined from Equation 3 together with a mass balance.
Equations 3 and 8 could also be used to evaluate HOy from experimental data obtained on a given column. Calculations The following calculations should be performed: The value of W to be used in this calculation should be obtained from a material balance on CO2 in the gas phase. Design Requirements Determine the outlet compositions vapor and liquid for an absorption process at 20 oC using our column with 2.
Results and Discussion A discussion of the errors in the results due to experimental uncertainty and their effect on the results through propagation of error should be included. How meaningful are your results when errors are considered. It is not sufficient to simply state your results in numerical form. They should be interpreted in terms of physical phenomena occurring within the process.
Do the trends in the data make sense? Do your results agree with published information or correlations? What is happening physically inside the column when the water rate is changed that can account for the observed dependence of the mass transfer coefficient on the water rate?
Report Requirements The pre-lab report should contain an introduction stating the objective of the experiments, including the rationale for expecting Kya to depend on the liquid flow rate, some background on gas absorption, a detailed derivation of the design equation from first principles, including the assumptions and simplifications made, a description of the equipment and purpose of each item, including a detailed schematic drawing, and a stepwise procedure, that would allow someone who is unfamiliar with the equipment to perform the experiment.
Following the first week of experiments, calculations of Kya for all liquid flow rates should be made and correlated against Gx. Error analysis is not required at this stage. The final report should contain the usual sections as specified in the course descriptions. In addition, an error analysis is required for all calculated values of Kya, and error bars are to be included on all plots.
Note that the measured water flow was 2. Figures 2 and 3 show calibration curves for air and CO2 rotameters, respectively. Note that the float travel is measured at the center of the float. Equations for best fit lines provided on these curves should not be extrapolated beyond the ranges shown. Calibration curve for absorption column water flow meter. Calibration curve for absorption column air rotameter. Calibration curve for absorption column CO2 rotameter. Mass Transfer in Fluid Systems, 2nd Ed.
Calculations for conversions into molar flow rate are only given for one species. Conversions for the other species were calculated using the identical formats. Initial concentration of CO2: As a result, the following conversions must be done. CO2 in gas and liquid phases: Axial symmetry 2D Independent variables: Convection and Diffusion chcd Application mode type: Convection and Diffusion chcd2 Application mode type: Linear system solver Parameter Value Pivot threshold 0.
None Flow basis for input: Mole Stream report composition: Download - Worcester Polytechnic Institute. Figure - Worcester Polytechnic Institute.
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