Background middle of the column near the outer wall

Background

 

Mass transfer operation gas
absorption is the driving principle behind the removal of soluble gas from a
gas flow entering the tower by flowing a liquid miscible liquid through the
tower. 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 bottom
respectively. The packing section is used artificially raise the wetted area
(or area of effective mass transfer), creating more interaction between the
liquid and gas, which will increase the absorption rate of the gas into the
liquid. This is important because the limiting step in absorbing the selected
gas is its ability to diffuse into the liquid. It is assumed that the reaction
occurs very quickly relative to this diffusion rate and acts as a sick to the
mass transfer of CO2.

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The inlet gas steam,
composed of air mixed with carbon dioxide (CO2), enters packed tower
at the bottom and NaOH solution is pumped to the top of the tower. The
stoichiometry of the reaction of CO2 and NaOH is as follows:

Assuming that nitrogen and oxygen are
insoluble 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 a
solid 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 tower
in Johnson Hall 214 was used to investigate the CO2 scrubbing
capabilities of a 0.05 N aqueous solution of sodium hydroxide. The packing
height of the pilot scale tower is 4 feet and 4 inches in diameter with
randomly packed with ½-in Raschig rings. A solution of 0.05 N aqueous sodium
hydroxide was prepared in Johnson 210D by dissolving 360 g of NaOH pellets in 2
L of deionized water and diluting the resulting solution in a total volume of
180 L with process water. The solution was thoroughly mixed using the pump and
column bypass line. Once the solution was well mixed the bypass line was
closed, and then it was passed to the tower and through the column. It was
noted that some channeling occurred in the middle of the column near the outer
wall at higher flow rates.

 

In experiment 1, pure CO2
from a gas cylinder was mixed with utility air at total volumetric flow rates
of VCO2
= 6.2 ml/s and VAir= 1101 ml/s respectively to reach a CO2 mole
fraction of  yA1 = 0.005
and it was assumed that the mole fraction and percent CO2 were
identical. The percent CO2 was verified by the Vernier Labquest
units equipped with CO2 sensors at the inlet and outlet of the
column and the SRI310 gas chromatograph (GC) for each trial. The average CO2
percent at the gas inlet over the 7 trials was 0.565 ± 0.0230 % and 0.543 ± 0.173 % with the Labquest units and
GC respectively.

 

The total volumetric liquid flow
rate, VLwas varied in 7 randomized trials from 0 to
approximately 1 GPM and the concentration of CO2 in the gas outlet
stream was observed with Labquest units. The maximum differential pressure drop
across the column measured by the water manometer was 1.1 mmH2O. For
each trial, the concentration of CO2 declined to a steady state
value associated with a given liquid flow rate. When the steady state value was
reached, a sample was taken from the gas outlet and measured with the GC.

 

This weeks experiments will focus two
separate things. The first will be keeping the liquid flow rate constant and
varying the CO2 and air flow rates proportionally to keep the inlet
concentration of CO2 at a constant 0.5 %. The outlet concentration
of CO2 will be measured with the Labquest units until a steady state
value is reached; the concentration will be verified with the GC. It is
anticipated that outlet CO2 concentration will increase associated
with higher total gas flow rates. This is because the gas will be flowing
through the column faster at higher volumetric flow rates, and will be in
contact with the fluid for less time.

The second will be to see the
magnitude of the effect that the reaction has on the mass transfer. This will
be done by removing the NaOH from the solution and running just water through
the column, and repeating the trials from before. This will show how much CO2
will be removed from the gas stream without the reaction as a sink. This will
show how much CO2 is removed by the reaction and how much is simply
removed by the diffusion of gas into the fluid. This is useful to accurately tune
the process such that all the NaOH is being used, and there is no remainder
left being put into the environment through the waste streams. Together, these
parameters will be used to assess the dependence of the overall mass transfer
coefficient for CO2 on the gas superficial molar velocity.

 

 

 

Results and Discussion:

 

The results of varying liquid flow
rates are depicted in Figure 1.  Outlet
concentrations of CO2 are plotted as a function of liquid flow
rates.  As expected, the outlet
concentrations decreased from about 1800 (ppm) to about 1500 (ppm) over the
tested range of liquid flow rates. As the liquid flow rate is increased the CO2
will be more likely to encounter water, and consequently any associated NaOH
within the water, causing it to be removed at a greater rate. It is also noted
that the concentration seems to asymptotically decay to some limit. This
indicates that there is some parameter of the mass transfer that operates far
slower than the others and is the “limiting parameter” of mass transfer in the
system. It is assumed in this experiment that this limiting parameter is the
diffusion of CO2 into the water, and not the reaction. The goal for
next lab meeting will be to show the limiting effect of CO2
diffusing into water, by attempting to remove CO2 by diffusing it
through water devoid of NaOH and comparing the results to the ones obtained
last week. Then the goal is to find the dependence of gas flow rate and carbon
dioxide concentration will be analyzed, and then further.  We are expecting similar behavior in
variability.

 

Acknowledgements:

 

Special thanks to Dr. Harding and (we love you Adam) the
entire CHE 415 lab team for setup and explanation of the pilot scale gas
absorption 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 gas
outlet = 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 the
concentration of CO2 in the exit stream as a function of liquid flow rate. It
is expected that the concentration of CO2 will go down as a function of
increasing liquid flowrate, and up as a function of gas flow rate.

 

Determination of
Sodium Hydroxide Mass:

 

The normality of the sodium
hydroxide solution must be 0.05. It is assumed that the equivalent factor of
water is 1.0 and the tank volume is known to be 180 L.  MNaOH
= 40.0 g/molNaOH.

 

 

 

 

 

 

 

 

 

 

Onda
Mass Transfer Correlation Parameters:

 

The Froude number,
Reynold’s number, and Weber number are calculated in order to determine the
wetted surface area of the packing.  All
properties are constant and evaluated at a temperature of 80°C.  Future goals include re-evaluating
relationship with constants taken at room temperature through more data
searching.  A complete list of these
constants are displayed in Table 1.  Note: The mass transfer coefficients will be
evaluated at liquid flow rate equal to 26.3 mL/s for the purpose of sample
calculations.

 

 

 

The wetted surface area calculation is
shown below in Equation 6.  Alpha is
first calculated in equation 5.

 

 

 

Onda
Correlation:

 

The onda mass transfer correlation, shown
below in Equations 7 and 8, has proved to be effective for estimating
individual film coeffiecients, kL
and kG.  The units for each variable must match that
provided in Table 1.

 

 

 

 

 

References: