Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The discovery that the reaction of CO2 with primary amines in both aqueous and
non-aqueous media provides a viable chemical method for determining the effective
interfacial mass transfer area for separation column internals has lead to an increase
in the interest of studying the reaction kinetics and determining the governing
reaction rate expressions. For the absorption studies conducted on these systems,
many authors assumed that power rate law reaction kinetics govern the reaction
rate, which simplified the derivation of absorption correlations. This has already
been proven to be an over simplifying assumption, since many authors suggest a
non-elementary rate expression based on the pseudo-steady state hypothesis for the
reactive zwitterion intermediate to be valid.
An evaluation of the existing reaction rate expressions for the homogeneous liquid
phase reaction of CO2 and mono-ethanolamine (MEA) in a 2-propanol solvent system
was performed. The reaction rate profiles of CO2 and MEA at 25ºC, 30ºC and 35ºC,
and relative initial concentrations of [MEA]i = [CO2]i, [MEA]i = 2.5[CO2]i, [MEA]i =
4[CO2]i were determined by means of an isothermal CSTR set-up. Scavenging of the
unreacted MEA with benzoyl chloride provided the means to be able to stop the
reaction in the product stream. This in turn allowed for the construction of
concentration- and reaction rate profiles.
The reaction rate data was modelled on various rate expressions by means of a
MATLAB® non-linear estimation technique, employing the Levenberg-Marquard
algorithm for minimizing the loss function. It was concluded that the rate
expressions proposed in literature are insufficient and a rate expression derived
fundamentally from first principals is proposed:
[ ][ ] [ ] [ ][ ] [ ]2
MEA 1 2 2 -r = k CO RNH - k2 Z + k3 Z RNH2 - k4 S
where ki are the reaction rate constants, Z is the zwitterion reactive intermediate
and S the salt product of the overall reaction mechanism.
In order to be able to determine the effective interfacial mass transfer area, the
absorption rate per unit area or specific rate of absorption for the solute gas as a rate expression function of species concentration must firstly be determined. This is
achieved by performing experimental absorption runs on a gas-liquid contactor of
known surface area. This study incorporated the well known wetted wall
experimental set-up. The aim was to construct and implement a wetted wall set-up
and conduct absorption experiments for a gas side CO2 concentration range
stretching from pure CO2 to diluted gas mixtures absorbing into solutions of varying
MEA concentrations.
Validation of the set-up was done by performing experiments at similar conditions to
a previous study. The study then proceeded to determine the absolute and specific
absorption rates at CO2 mass percentages of 100%, 78%, 55% and 30% into solutions
of MEA concentrations of 0.25 and 0.3 mol/L. These runs were conducted at 25ºC
and 30ºC. The wetted wall was designed to facilitate absorption studies at column
heights of 60, 90 and 105mm. This allowed the investigation of the effect that
surface area and column height has on the absolute rate of absorption as well as the
CO2 and MEA concentrations in the liquid phase
It was found that the specific absorption rate is independent of contact time, which
is consistent with the rapid nature of the reaction. It was furthermore found that an
increase in MEA concentration caused an increase in the absorption rate. The effect
of temperature is linked with the solubility of CO2 in the solution. As the
temperature increases, the solubility of CO2 decreases, but the absorption rate
increases. The result is that it seems as if a change in temperature has no effect on
the absorption rate, when in actual fact it does. An increase in the amount of CO2
absorbed is noticed for an increase in wetted wall surface area. This is expected and
indicates that there is an increase in the amount of CO2 absorbed as the column
length increases.
Stopping the absorption reaction by means of MEA scavenging with benzoyl chloride
at various column heights will allow for the construction of a concentration profile
for both CO2 and MEA as a function of column height. These profiles will allow for
the derivation of a non-elementary rate expression governing the specific absorption
rate. This has been identified as ‘n area of great interest for future investigation. / AFRIKAANSE OPSOMMING: ‘n Groot navorsingsbelangstelling in die reaksiekinetika van CO2 en
monoethanolamien (MEA) het ontstaan sedert die ontdekking dat hierdie reaktiewe
sisteem ook ‘n goeie metode is vir die bepaling van die effektiewe
massaoordragsoppervlakte van gestruktureerde pakkingsmateriaal. Die klem val op
die bepaling van eerstens die mees geskikte en akkurate model om die
reaksiekinetika te beskryf wat dan gebruik kan word om die absorbsiekinetika
deeglik te karaktariseer. Sommige van die vorige navorsers het vereenvoudigende
aannames gemaak rakende die reaksiekinetika ten einde die bepaling van geskikte
absopsievergelykings te vergemaklik. Ander het gevind dat die nie-elementêre,
pseudo-gestadigde toestand hipotese gebasseer op die reaktiewe zwitterioon
tussenproduk van die reaksie ‘n meer verteenwoordigende kinetiese model is.
Hierdie studie is eerstens gemik op die evaluasie van die bestaande
reaksiekinetikavergelykings deur die homogene vloeistoffase reaksie van CO2 met
mono-etanolamien (MEA) in die oplosmiddel, 2-propanol te ondersoek. Die studie is
uitgevoer in ‘n isoterme CSTR sisteem by onderskeidelik 25ºC, 30ºC en 35ºC en MEA
konsentrasies van [MEA]i = [CO2]i, [MEA]i = 2.5[CO2]i en [MEA]i = 4[CO2]i.
Die voorgestelde reaksiekinetikavergelykings was gemodelleer met ‘n nie-lineêre
datapassingstegniek verskaf deur die sagtewarepakket, MATLAB® wat die Levenberg-
Marquard algoritme gebruik om die resfunksie te minimeer. Uit die teorie en
datapassing word die volgende vergelyking voorgestel:
[ ][ ] [ ] [ ][ ] [ ]2
MEA 1 2 2 -r = k CO RNH - k2 Z + k3 Z RNH2 - k4 S
waar ki die reaksietempokonstante voorstel, Z die zwitterioontussenproduk en S die
soutproduk.
Die eerste stap in die bepaling van die effektiewe massaoordragsarea van
gestruktureerde pakkingsmateriaal is om ‘n geskikte vergelyking of korrelasie vir die
spesifieke absorpsie van die gas te bepaal. Dit word gedoen deur absoprsie
eksperimente te doen op toerusting van bekende oppervlakarea. Hierdie studie het
die reeds bekende ‘wetted wall’ opstelling gebruik. Die hoof doelwit van hierdie absorpsiestudie was om ‘n werkende opstelling te bou en absorpsie eksperimente vir
CO2 konsentrasies wat strek van suiwer CO2 tot verdunde mengsels uit te voer. Die
konsentrasie MEA is ook gevarieër.
Die geskiktheid van die opstelling is eerstens getoets deur eksperimentele lopies uit
te voer by soorgelyke toestande as ‘n vorige studie. Die doel van die studie is om die
absolute en spesifieke absorpsietempos van CO2 by gasfase massapersentasies van
100%, 78%, 55% en 30% in MEA/2-propanol oplossings met MEA konsentrasies van
0.25 en 0.3 mol/L te bepaal. Die lopies is uigevoer by beide 25ºC en 30ºC. Die
opstelling is ook ontwerp om absorpsie eksperimente by verskillende kolomhoogtes
uit te voer. Hierdie hoogtes is 60, 90 en 105mm. Hierdie studie het tweedens
gefokus op die effek wat absorpsiearea en kolomhoogte op die absorpsietempo van
CO2 het.
Die resultate van die studie toon dat die absorpsietempo onafhanklik is van
kontaktyd. Dit stem saam met die vinnige reaksietempo. ‘n Toename in MEA
konsentrasie het ‘n toename in spesifieke absorpsietempo tot gevolg, terwyl die
effek van temperatuur gekoppel kan word aan die oplosbaarheid van CO2. Soos die
temperatuur toeneem, neem die absolute absorpsietempo toe, maar die
oplosbaarheid van CO2 neem af, dit het beide ‘n toenemende en afnemende effek op
die spesifieke absorpsietempo. Die hoeveelheid CO2 geabsorbeer neem toe met ‘n
toename in kolomhoogte.
Die konsentrasie MEA in die uitlaatvloeistof toon ‘n skynbare eksponensiële afname
met ‘n toename in kolomhoogte. ‘n Studie gemik om die konsentrasieprofiele van
CO2 en MEA as ‘n funksie van kolomhoogte te bepaal, word voorgestel.
Absorpsiemodelle en korrelasies kan dan afgelei word uit hierdie profiele, wat die
berekening van die effektiewe massaoordragsarea akkuraat sal maak. Dit sal deel
vorm van toekomstige navorsing.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4192 |
| Date | 03 1900 |
| Creators | Du Preez, Louis Jacobus |
| Contributors | Knoetze, J. H., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 213 p. : ill. |
| Rights | University of Stellenbosch |
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