n2. MDEA reacts with only one of the acid species, the other reaction being kinetically blocked. Dat — Organic Chemistry Chemistry Question
Deacidification and desulfurization of natural gas
n2.
MDEA reacts with only one of the acid species, the other reaction being kinetically blocked.
Data at 298 K:
CO2(g) H2O(g) CH4(g) C5H12(l) CO(g) CH3CH3(g) H2(g)
ΔfH° (kJ mol‒1) ‒393.5 ‒241.8 ‒74.6 ‒178.4 ‒110.5 ‒84.0 0.0
Sm°(J K‒1 mol‒1) 213.8 188.8 186.3 260.4 197.7 229.2 130.7
pKa
Amines: MEAH+/MEA; MDEAH+/MDEA pKa = 9.5
CO2(aq) pKa1 = 6.4; pKa2 = 10.3
H2S pKa1 = 7.0; pKa2 = 13.0
CH3SH pKa = 10.3
[Question text missing in source]
Model Answer
6.1 CnH2n+2 + n H2O = n CO + (2n+1) H2
Calculate the equilibrium constant K° of reaction (1) at 900 °C.
Model Answer
ΔrH° = 205.9 kJ mol‒1; ΔrS° = 214.7 J K‒1 mol‒1;
ΔrG° = ‒46 kJ mol‒1 = ‒RT ln(K°) then K° = 112
Write down the thermodynamic quantitative (K° >> 1) reactions between the different gases and (i) the amine solution and (ii) the NaOH solution.
Model Answer
pH > 7: HCO3‒ and CO32‒ can be encountered in solution
(i) with amine:
A + CO2 + H2O = AH+ + HCO3‒ (10^2.9)
A + H2S = HS‒ + AH+ (10^2.5)
(ii) with NaOH:
If, for kinetic reasons, CO2 and/or H2S do not react with the amine:
CO2 + 2 HO‒ = CO2‒ + H2O (HO‒ in excess) (10^11.3)
H2S + 2 HO‒ = S2‒ + H2O (idem) (10^8.0)
in all cases:
CH3SH + HO‒ = CH3S‒ + H2O (10^3.7)
Determine the amount of each species in the solution (as a function of n1, n2 and n3) in the flask F1 before titration.
Model Answer
With MEA, case (i) of question 5:
HCO3‒: n1; HS‒: n2; MEA: n0 ‒ (n1 + n2); MEAH+: n1 + n2
Which chemical species is/are present in gas 2?
Model Answer
CH3SH
Using the curves A1F1 and A1F2, determine (i) n3 and (ii) a relation between n1 and n2.
Model Answer
A1F1, titration by H+ first:
Veq = 24 cm3 hence 24 mmol = n0 ‒ (n1 + n2) so: n1 + n2 = 26 mmol
A1F2:
between V1 and V2 (4 cm3): titration of CH3S‒ (n3) → n3 = 4 mmol
6.7 Determine the amount of the reacting species using curve A2F1.
6.8 Using curve A2F2, determine if MDEA selectively reacts with CO2 or with H2S. Calculate the two remaining n1 and n2.
Model Answer
CO2 – H2S – CH3S Gas 1 CO2 – H2S – CH3S
CO2 has reacted with MDEA hypothesis H2S has reacted with MDEA
MDEA – MDEAH+ – HCO3 ‒ F1 MDEA – MDEAH+ – HS‒
CH3SH (4 mmol) – H2S (12 mmol) Gas 2 CH3SH (4 mmol) – CO2 (12 mmol)
HO‒ (22 mmol) – CH3S ‒ (4 mmol) S2- (12 mmol) F2 HO‒ (22 mmol) – CH3S ‒ (4 mmol) CO3 2- (12 mmol)
pKa = 14.0; 13.0; 10.3; 7.0