Background Mass transfer operation gasabsorption is the driving principle behind the removal of soluble gas from agas flow entering the tower by flowing a liquid miscible liquid through thetower. The packed bed tower is equipped with an inlet gas from the bottom,inlet liquid from the top, and gas and liquid outlets at top and bottomrespectively. The packing section is used artificially raise the wetted area(or area of effective mass transfer), creating more interaction between theliquid and gas, which will increase the absorption rate of the gas into theliquid.
This is important because the limiting step in absorbing the selectedgas is its ability to diffuse into the liquid. It is assumed that the reactionoccurs very quickly relative to this diffusion rate and acts as a sick to themass transfer of CO2. The inlet gas steam,composed of air mixed with carbon dioxide (CO2), enters packed towerat the bottom and NaOH solution is pumped to the top of the tower. Thestoichiometry of the reaction of CO2 and NaOH is as follows: Assuming that nitrogen and oxygen areinsoluble in water, this will be the driving force for the removal of CO2.Since the products of this reaction are water and sodium carbonate, which is asolid that falls out of the gas stream and is removed by the fluid flow. The overall mass-transfer coefficient defined by the equation: Materials and Methods: The pilot scale gas absorption towerin Johnson Hall 214 was used to investigate the CO2 scrubbingcapabilities of a 0.05 N aqueous solution of sodium hydroxide.
The packingheight of the pilot scale tower is 4 feet and 4 inches in diameter withrandomly packed with ½-in Raschig rings. A solution of 0.05 N aqueous sodiumhydroxide was prepared in Johnson 210D by dissolving 360 g of NaOH pellets in 2L of deionized water and diluting the resulting solution in a total volume of180 L with process water. The solution was thoroughly mixed using the pump andcolumn bypass line.
Once the solution was well mixed the bypass line wasclosed, and then it was passed to the tower and through the column. It wasnoted that some channeling occurred in the middle of the column near the outerwall at higher flow rates. In experiment 1, pure CO2from a gas cylinder was mixed with utility air at total volumetric flow ratesof VCO2= 6.2 ml/s and VAir= 1101 ml/s respectively to reach a CO2 molefraction of yA1 = 0.005and it was assumed that the mole fraction and percent CO2 wereidentical. The percent CO2 was verified by the Vernier Labquestunits equipped with CO2 sensors at the inlet and outlet of thecolumn and the SRI310 gas chromatograph (GC) for each trial. The average CO2percent at the gas inlet over the 7 trials was 0.565 ± 0.
0230 % and 0.543 ± 0.173 % with the Labquest units andGC respectively. The total volumetric liquid flowrate, VLwas varied in 7 randomized trials from 0 toapproximately 1 GPM and the concentration of CO2 in the gas outletstream was observed with Labquest units. The maximum differential pressure dropacross the column measured by the water manometer was 1.1 mmH2O.
Foreach trial, the concentration of CO2 declined to a steady statevalue associated with a given liquid flow rate. When the steady state value wasreached, a sample was taken from the gas outlet and measured with the GC. This weeks experiments will focus twoseparate things. The first will be keeping the liquid flow rate constant andvarying the CO2 and air flow rates proportionally to keep the inletconcentration of CO2 at a constant 0.5 %. The outlet concentrationof CO2 will be measured with the Labquest units until a steady statevalue is reached; the concentration will be verified with the GC.
It isanticipated that outlet CO2 concentration will increase associatedwith higher total gas flow rates. This is because the gas will be flowingthrough the column faster at higher volumetric flow rates, and will be incontact with the fluid for less time. The second will be to see themagnitude of the effect that the reaction has on the mass transfer. This willbe done by removing the NaOH from the solution and running just water throughthe column, and repeating the trials from before. This will show how much CO2will be removed from the gas stream without the reaction as a sink.
This willshow how much CO2 is removed by the reaction and how much is simplyremoved by the diffusion of gas into the fluid. This is useful to accurately tunethe process such that all the NaOH is being used, and there is no remainderleft being put into the environment through the waste streams. Together, theseparameters will be used to assess the dependence of the overall mass transfercoefficient for CO2 on the gas superficial molar velocity. Results and Discussion: The results of varying liquid flowrates are depicted in Figure 1. Outletconcentrations of CO2 are plotted as a function of liquid flowrates.
As expected, the outletconcentrations decreased from about 1800 (ppm) to about 1500 (ppm) over thetested range of liquid flow rates. As the liquid flow rate is increased the CO2will be more likely to encounter water, and consequently any associated NaOHwithin the water, causing it to be removed at a greater rate. It is also notedthat the concentration seems to asymptotically decay to some limit. Thisindicates that there is some parameter of the mass transfer that operates farslower than the others and is the “limiting parameter” of mass transfer in thesystem. It is assumed in this experiment that this limiting parameter is thediffusion of CO2 into the water, and not the reaction. The goal fornext lab meeting will be to show the limiting effect of CO2diffusing into water, by attempting to remove CO2 by diffusing itthrough water devoid of NaOH and comparing the results to the ones obtainedlast week.
Then the goal is to find the dependence of gas flow rate and carbondioxide concentration will be analyzed, and then further. We are expecting similar behavior invariability. Acknowledgements: Special thanks to Dr. Harding and (we love you Adam) theentire CHE 415 lab team for setup and explanation of the pilot scale gasabsorption unit and operating the GC.
Appendix 1: Independent Variables Total volumetric CO2 flow rate = VCO2= L/min Total volumetric air flow rate = VA = L/min Total volumetric liquid flow rate = VL = L/min Column Packed Height =Z = 4 feet = 1.22 m Column Diameter =D= 4 inches = 0.10 m Total gas superficial molar velocity rate (air + CO2)= G= mol/(ft2 × hr) Total liquid superficial molar velocity rate (solvent + CO2)= L= mol/(ft2 × hr) Mole fraction of CO2 in the inlet gas stream = yA1= 0.005 Saturated pressure of CO2 at 21oC = = atm Partial Pressure of CO2 in the outlet gas stream == atm Concentration of CO2 in the inlet liquid stream = = 0 mol/m3 Henry’s law constant = = (m3 × atm)/mol Appendix 2: Dependent Variables Mole fraction of CO2 in the outlet gas stream = yA2= N/A Moles of CO2/moles of carrier gas (air) at gasoutlet = YA2 = N/A Overall gas-capacity coefficient based on = Kya = mol/(s × m3) Molar flux of CO2 = = mol/(s × m2) Overall mass-transfer coefficient in the liquid = = mol/(s × m2 × mol/m3) Overall mass-transfer coefficient in the gas= = mol/(s × m2 × Pa) Convective mass-transfer coefficient in the liquid = = mol/(s × m2 × mol/m3) Convective mass-transfer coefficient in the gas= = mol/(s × m2 × Pa) Figure 1: Graph showing theconcentration of CO2 in the exit stream as a function of liquid flow rate. Itis expected that the concentration of CO2 will go down as a function ofincreasing liquid flowrate, and up as a function of gas flow rate.
Determination ofSodium Hydroxide Mass: The normality of the sodiumhydroxide solution must be 0.05. It is assumed that the equivalent factor ofwater is 1.0 and the tank volume is known to be 180 L. MNaOH= 40.0 g/molNaOH. OndaMass Transfer Correlation Parameters: The Froude number,Reynold’s number, and Weber number are calculated in order to determine thewetted surface area of the packing.
Allproperties are constant and evaluated at a temperature of 80°C. Future goals include re-evaluatingrelationship with constants taken at room temperature through more datasearching. A complete list of theseconstants are displayed in Table 1. Note: The mass transfer coefficients will beevaluated at liquid flow rate equal to 26.
3 mL/s for the purpose of samplecalculations. The wetted surface area calculation isshown below in Equation 6. Alpha isfirst calculated in equation 5.
OndaCorrelation: The onda mass transfer correlation, shownbelow in Equations 7 and 8, has proved to be effective for estimatingindividual film coeffiecients, kLand kG. The units for each variable must match thatprovided in Table 1. References: