6 replies to this topic

[doaa Kandil]

hi all

i am working in designing a packed column absorber for co2 using MDEA i need VLE data when i search for it i only found solubility data which includes only (moles co2/ mole mdea) and partial pressure and what i need is the mole fraction of co2 in liquid (MDEA) and vapor so i can design the absorber 

any one can help???

[MrShorty]

Assuming this is an aqueous solution, there is not enough information to go from n(CO2)/n(MDEA) to mole fraction. You will also need some information about the amount of MDEA relative to water.

 

x(CO2)=n(CO2)/[n(CO2)+n(MDEA)+n(H2O)] = [n(CO2)/n(MDEA)] / [n(CO2)/n(MDEA) + 1 + n(H2O)/n(MDEA)]

 

calculate mole number for each component and then you can calculate x

 

If I am wrong and this is an anhydrous solution, a little algebra should allow you to go from given solubility data on a mole ratio basis to solubility on a mole fraction basis. Let us know if you need help with the algebra.

Edited by MrShorty, 22 July 2014 - 12:15 PM.

[AlbertHahn]

basically this is correct.

 

you usually can get the molar concentration of amine in the literature data, say m_MDEA (mol/l).  You need to find out the molar density of MDEA (or convert it from mass density), say v_MDEA (mol/l),

 

neglecting volume changing caused by CO2, and assuming the total volume of MDEA solution is the sum of MDEA volume and H2O volume, so the volume of H2O in the 1 L solution is:

 

v_H2O =1  - m_MDEA / v_MDEA;

 

and the molar amount in the 1 L solution will be:

m_H2O=v_H2O * 55.5 mol/L  ( assuming molar density of water is 55.5 mol/L)

 

so now you got molar amounts in 1 L of solution:

MDEA : m_MDEA;

H2O: m_H2O;

CO2: m_MDEA * a_co2.;  (a_co2 is the CO2 loading: m_co2/m_MDEA)

 

so x_co2 = (m_MDEA * a_co2) / (m_MDEA + m_ H2O + m_MDEA * a_co2).

 

This is one way to calculate x_co2. There are other ways to calculate it too.

for molar fraction in vapor, just use:

 

y_co2= P_co2/ P_total  (the partial pressure of co2 over the total pressure)

Hope this helps.

[doaa Kandil]

thanks for your replies MrShorty and AlbertHahn

 

AlbertHahn i am a little confused so tell my if what i understand is right or wrong 

 

m_H2O is the molar density of water = 55.5 mol/l

m_MDEA is the the molar density of MDEA and i have calculated it = 8.71 mol/l

i know that the total solution volume is 0.23 l 

how i know the v_MDEA ??

 

is there any other way to calculate the VLE data??

[MrShorty]

v_MDEA will come from the given molarity of MDEA and the assumed molar volume of MDEA. Which, of course, is only useful if the MDEA concentration is given by the authors as molarity (mol/L). If it is given in other units, then this may not have any meaning at all. Since you haven't told us what units the MDEA composition is reported in, I would suggest that you first study your literature, discover what units are used, then report back here with that information. From there will either be able to clarify AlberHahn's unit conversion, or help you develop the unit conversion from the actual units that are given.

[doaa Kandil]

thank you MrShorty from your reply 

i got the information from (A model of Vapor-liquid equilibria for acid Gas-Alkanolamine-Water system by Austgen ) page 157

tell me if what i calculated is right:

m_mdea = 2 mol/l

v_mdea = 8.71 mol/l

v_h2o = 1- (m_mdea/v_mdea) = 1 - (2/8.71) =0.77

m_H2O=v_H2O * 55.5 mol/L  ( assuming molar density of water is 55.5 mol/L)

           = 0.77 * 55.5 = 42.75

x_co2 = (m_MDEA * a_co2) / (m_MDEA + m_ H2O + m_MDEA * a_co2).

for a_co2 =0.00603       x_co2 =0.000269

for a_co2 =0.0117         x_co2 =0.000522

for a_co2 =0.0215          x_co2 =0.000959

for a_co2 =0.0444        x_co2 =0.00198

for a_co2 =0.074            x_co2 =0.00329

for a_co2 =0.113             x_co2 =0.005

for a_co2 =0.362             x_co2 =0.0159

for a_co2 =0.842             x_co2 =0.0362

 

 

y_co2= P_co2/ P_total  (the partial pressure of co2 over the total pressure)

or p_co2 =0.0056       y_co2 =0.000056

for p_co2 =0.0151         y_co2 =0.000151

for p_co2 =0.0452        y_co2 =0.000452

for p_co2 =0.177       y_co2 =0.00177

for p_co2 =0.419            y_co2 =0.00419

for p_co2 =0.887            y_co2 =0.00887

for p_co2 =6.95             y_co2 =0.0695

 

for p_co2 =9.28             y_co2 =0.928

[MrShorty]

If I follow your notation it all looks correct.

 

I note that you do not indicate pressure units or assumptions. It appears that your conversion from partial pressure to y is assuming a total pressure of 100 (kPa???). It isn't clear to me what the rest of the vapor phase consists of. Is the rest water/MEA and the temperature is varied so the total system pressure is 100?