1 reply to this topic

[peaston]

Hi there i am currently trying to preform a mass balance for a chemical reactor i am designing.

if you have read any of my previous threads then you may be familiar with the concept, but if not here is a small insight...

I am studying the feasibility of carbon dioxide sequestration via the precipitation of magnesium carbonate from reverse osmosis retentate.

the reaction has CO2 gas entering a vessel through the bottom of a tanks, batch reactor.

i deduced that once CO2 has dissolved into the solution forming CO2(aq) followed by carbonic acid the following equation can be deduced:

Mg²⁺ + CO₃^2- <---> MgCO₃ - with k_f being the rate constant for the forward reaction and k_r the rate constant for the reversible reaction

So for the second order reversible reaction, the following mass transfer can be obtained:

accumulation = input - output + generation - consumption

saying that Mg ions are A, CO3 ions are B and MgCO3 solid is M and concentration of each part is C:

Taking the reaction with respect to A:

V_T. dC_A/dt = 0 - 0 + V_T.k_r.C_M - V_T.k_f.C_AC_B

simplifies to give:

dC_A/dt = k_r.C_M - k_f.C_AC<sub>B

I want to solve for dt. i was thinking about changing its form as it might simplify into a form where it is not integrable

so using extent of reaction,</sub> ε we can say C_A= C_AO - ε, C_B= C_BO - ε and C_M = C_MO + ε which you could then substitute into the equation:

dt = dε/( k_r.[C_MO + ε] - k_f.[(C_AO - ε)(C_MO + ε)])

from there though i get stuck... any ideas how to further...

since I'm claiming it is a reversible reaction (should i say that it is????), could i consider it at equilibrium meaning that the accumulation (left hand side of the mass balance equation) could be taken as 0?

any help on the matter would be very much appreciated!

many thanks!

andrew

Edited by peaston, 06 January 2012 - 12:01 PM.

[kkala]

To my understanding, situation is more complex concerning the retentate, being concentrated sea water (up to a factor of 2) rejected from reverse osmosis plants.
There are Ca²⁺, Mg²⁺, HCO3^-, CO3^2- at a pH about 8.5. There are also other anions and cations. Then CO2 gas is introduced. What will happen? Introduced CO2 will shift pH to lower values. Will some precipitation occur?
Better to begin with ionic equilibrium and solubility products to see what can happen, neglecting kinetics at present. Or just do a rough experiment (in available equipment) to see qualitative result, being a guide for future action.
Further problematization as developed in http://www.cheresour...hemical-rector/ can be useful. This is a continuation of that thread. Discuss problems with Instructor too, for help.
Hope of good proceeding.

Edited by kkala, 08 January 2012 - 01:14 PM.