Øving 1

Oppgave 1

Hydrogen sulfide (0.1 %) in a carrier gas at 20 atm is to be absorbed at 20°C

by a solution containing 0.25 mol/liter mono-ethanolamine (MEA). Find the form of

rate equation which applies at these conditions and find how much faster is the

rate over straight physical absorption in pure water.

Data: H₂S and MEA react as follows

H₂S + RNH₂ = HS^- + RNH₃⁺

and since this is an acid base neutralization we can regard it as irreversible and

instantaneous. Also from Danckwerts (1970) we find

k_Ala = 0.030 s^-1

k_Aga = 6 10^-5 mol/(cm³ s atm)

D_Al = 1.5 10^-5 cm²/s

D_Bl = 10^-5 cm²/s

H_A = 0.115 atm-liter/mol—for H₂S in water

Component H₂S is A and MEA is B.

Oppgave 2

Consider the absorption of a base A by water in a packed column. At a

location where gas is being absorbed by pure water, the overall rate based on unit

volume of tower may be expressed as

where K_Aga is the overall mass transfer coefficient based on unit volume of tower.

(a) Now suppose that an acid B is added to the water to aid the absorption.

Assuming instantaneous reaction, show how K_Aga should vary with acid strength.

Show this on a plot of K_Aga versus acid strength. Also show how this plot should

allow estimation of the individual mass transfer coefficients for physical absorption.

(b) Sherwood and Holloway (1940) present the following data for the absorption

of ammonia in acid solution of various strengths at 25°C.

K_Aga, mol/(hr-liter-atm) 300 310 335 350 380 370

Acid normality 0.4 1.0 1.5 2.0 2.8 4.2

From this data find the gas film contribution (in percent) to the overall mass

transfer resistance for physical absorption of ammonia into water.