Mass Balances on co2 Absorption/Stripping Process Module 1




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Mass Balances on CO2 Absorption/Stripping Process

Module 1: Material Balances on Absorption/Stripping Process

Module Author: Nick Schroeder & Jason M. Keith

Author Affiliation: Michigan Technological University
Course: Material and Energy Balances

Concepts Illustrated: Mole balances

Text Referenced: Felder and Rousseau
Problem Motivation: Earth’s atmosphere is composed of a variety of chemical compounds that act as greenhouse gases. Two general types of greenhouse gases are those that naturally occur and those synthesized by humans. Naturally occurring gases include methane, nitrous oxide, water vapor, and carbon dioxide while gases used for aerosols are human made. Through these gases sunlight passes undisturbed and enters our atmosphere freely. The sunlight than reflects off of the earth’s surface back towards space in the form of infrared radiation. This infrared radiation (heat) is than trapped when absorbed by greenhouse gases. For earth to maintain consistent temperatures, a cycle involving the reemission of energy to space and the absorption of light radiated by the sun must be maintained [1].
Carbon dioxide (CO2) is of primary concern due to growing global emissions in recent history. Since the expansion of coal use during the industrial revolution and the expansion of petroleum use during the 20th century, CO2 emissions have risen steadily. This creates an increasing dependency on fossil fuels to maintain society’s standard of living which creates problems. The combustion of carbon-based fossil fuels over the last couple hundred years has created a significant increase in atmospheric CO2 concentrations. CO2 concentrations are regulated by a number of processes which collectively are known as the “carbon cycle”, however, this cycle only has the capacity to absorb a portion of total emissions which creates a positive imbalance. This increase in CO2 concentrations effectively increases global surface temperatures by preventing the reemission of infrared radiation back into space, a concept known as climate change [2]. Recent data collected in the South Pole showed that the average annual atmospheric CO2 concentrations increased 20.8% from 1958 to 2007 [3]. The dangers associated with climate change are that no one knows how the Earth will respond to the changes. Increased temperatures have the ability to affect the weather, sea levels, and land use patterns.
With growing populations and industrialization of countries the world energy use is steadily increasing which has created an increase in dependency on fossil fuels. This dependency is due to 86.2% of world energy produced being obtained from fossil fuels, although renewable energy sources such as nuclear, hydroelectric, and other renewable are making major improvements [4].
Petroleum, natural gas, and coal all emit CO2; however, they do not contribute equal emissions on an energy basis. Coal has higher contribution to CO2 emissions than its energy contribution. Petroleum which shows slightly higher CO2 emissions than coal is primarily used for transportation fuel which results in a large number of small CO2 emission sources. Coal, which is used for electricity generation, creates a unique scenario due to coal-fired power plants being a large CO2 emitter and having relatively few point sources. These point sources emit large quantities of CO2, which provides an opportunity to capture CO2 before it enters the atmosphere.

Emissions from coal fired power plants can be captured by an absorption/stripping process with a circulating chemical solvent as shown in Figure 1.




Figure 1: Typical absorber/stripper flow sheet for CO2 capture

The absorption/stripping system can be placed at the end of new or existing coal fired power plants with NOx and SOx controls. Amine solvents are used since they react with CO2 under certain conditions. A lean solvent (low CO2 concentration) is fed into the top of the absorber. The solvent is counter currently contacted by flue gas containing CO2. The CO2 chemically reacts with the amine solvent and the treated gas exits the top of the absorber. This rich amine (high CO2 concentrations) leaves the bottom of the absorber and is preheated by a cross exchanger before entering the top of the stripper. In the stripper, typically at a higher temperature and pressure than the absorber, the reaction between the amine and CO2 is reversed which liberates CO2 into a concentrated stream obtained at the top of the stripper. The lean solvent from the stripper undergoes heat exchange and goes back to the absorber to repeat the process [5].



Problem Information
Example Problem Statement: An absorption/stripping process is operating at constant temperature and pressure. This system is a proposed addition to a coal fired power plant in an attempt to minimize the amount of CO2 released by the power plant.

  1. Determine the molar flow rates of the treated gas and the concentrated gas; nTG & ncc respectively. Use the ideal gas law, PV=nRT.

  2. The lean amine used in this process is monoethanolamine (MEA); calculate the molar flow rate of the amine running counter currently to flue gas entering the absorbance column.

  3. Use parts a&b to calculate the loading of the absorption column; Absorber loading = (mol CO2/mol MEA).

  4. Calculate the percent recovery of CO2 for the absorption/stripping process.


Additional Information:
Gas Streams

Stream

Variable

Actual Volumetric

Flow rate (m3/min)



[CO2]

T (K)

P (atm)

Flue Gas

nfeed

14.16

12.3 %

324.25

1.0054

Treated Gas

nTG

?

2.76 %

324.25

1.0054

Concentrated CO2

ncc

?

100 %

324.25

1.0054


Liquid Stream

Liquid Amine

Density (lbm/gal)

Flow rate (gal/min)

MMEA (g/mol)

[MEA]

MEA

8.69

7.8

61

32.50%



Part a. Calculating the molar flow rate of nTG and ncc

Step 1- Using the ideal gas law we can calculate the molar flow rate of the entering flue gas (Feed) by solving the equation in terms of moles. From the information provided we know that P=1.0054 & T=324.25 K. This allows for us to solve the molar flow rate of the flue gas (Feed).




Step 2- Using the molar flow rate of the flue gas we can calculate the total molar flow rate of the CO2 it contains. This is done by simply multiplying the molar composition fraction by the flue gas (Feed).
(Feed Stream)

Step 3- From this we can calculate the total flow rate of the other gases (O.G.) in the feed stream which are not absorbed by the liquid amine. This is done by subtracting the total CO2 in the flue gas from the total molar flow rate of the flue gas. (O.G. = other gases)

Step 4- Since we know that the other gases (O.G.) in the flue gas do not react with the liquid amine, than we can use this information to calculate the flow rate of the treated gases.

First we know the concentration of the CO2 in the treated gas and therefore we can calculate the concentration of the other gases (O.G.) in the treated gas stream. This is done by subtracting the CO2 from 100% since these are the streams only two components.
Since,

(nTG)
The


This means that the stream nTG is 97.24% other gases beside CO2. Therefore we can calculate the total flow rate of nTG by dividing the molar flow rate of the other gases (O.G.) by the molar composition fraction.



Since the molar flow rate of ncc is equivalent to the molar flow rate of the CO2 in this stream due to 100% composition, we can calculate the flow rate of ncc simply by finding how much CO2 was absorbed into the liquid amine stream before escaping in nTG. For our sake, we will assume that there is 100% CO2 stripped from the liquid amine in the stripper column and this exits in the concentrated CO2 stream.

Since,

The ncc flow rate equals,




Part b. Calculating the molar flow rate of the monoethanolamine (MEA)

Step 1- Now we will calculate the molar flow rate of the monoethanolamine (MEA). We know the density of the Liquid amine and the volumetric flow rate. From this we can calculate the mass flow rate of the stream by multiplying these two values together.


Mass flow-rate of Liquid Amine


Step 2- Using the mass flow rate of the liquid amine we can calculate the mass flow rate of the MEA by multiplying the mass flow rate by the molar composition fraction of MEA.

Step 3- From there we can divide the total mass flow rate of the MEA by its molar mass to calculate the molar flow rate of the monoethanolamine (MEA).

Part c. Absorber Loading of Monoethanolamine

Now we can use the molar flow rate of the CO2 exiting the stripper column; because this is the value equivalent to the total CO2 absorbed by the stripper, and the total molar flow rate of the monoethanolamine to calculate the loading of the absorber column. This is done simply by dividing the molar flow rate of the CO2 by the total molar flow rate of the MEA.





Part d. Calculate the percent recovery of CO2 for the absorption/stripping process
For calculating the percent recovery of the CO2 you need two of the values previously obtained in this problem. Divide the CO2 contained in the concentrated stream by the total CO2 contained in the feed stream.





Home Problem Statement:
An absorption/stripping process is operating at constant temperature and atmospheric pressure. This system is a proposed addition to a coal fired power plant in an attempt to minimize the amount of CO2 released by the power plant. The process has a 90% recovery rate of CO2 and it utilizes a 32.5 weight% monoethanolamine solution as the lean amine absorbent in the system. The absorption column used in the process has a loading of 0.17.


  1. Determine the volumetric flow rate of ncc (concentrated CO2). (Hint: Use the ideal gas law to calculate this value.)




  1. Calculate the percent moles of CO2 entering in the flue gas.




  1. Calculate the percent moles of CO2 and volumetric flow rate of the treated gas leaving the absorption/stripping process.



Additional Information:
Gas Stream

Streams

Variable

Actual Volumetric Flow-rate (m3/min)

[CO2]

T(K)

P(atm)

Flue Gas

nfeed

22

?

324.25

2

Treated Gas

nTG

?

?

324.25

2

Concentrated CO2

ncc

?

?

324.25

2


Liquid Stream

Liquid Amine

Density (lbm/gal)

Flow rate (gal/min)

MMEA(g/mol)

[MEA]

MEA

8.69

14.5

61

32.50%



References
[1] Greenhouse Gases, Climate Change, and Energy. Energy Information Administration, 2 Apr. 2004. Web. 31 Mar. 2011. .
[2] "Data.GISS: GISS Surface Temperature Analysis: Analysis Graphs and Plots." National Aeronautics and Space Administration. Goddard Institute for Space Studies, 14 Mar. 2011. Web. 22 Mar. 2011. .
[3] Keeling, R. F., S. C. Piper, and A. F. Bollenbacher. Atmospheric Carbon Dioxide Record from the South Pole. May 2008. Raw data. University of California, La Jolla, California.
[4] United States. Energy Information Administration. Table 11.1 World Primary Energy Production by Source, 1970-2007. Energy Information Administration, 9 Mar. 2010. Web. 31 Mar. 2011. .
[5] Dugas, Robert E. "Pilot Plant Study of Carbon Dioxide Capture by Aqueous Monoethanolamine." Thesis. The University of Texas at Austin, 2006. Print.


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