Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The reactive absorption rate of CO2 into non-aqueous solvents containing the primary amine,
mono-ethanolamine (MEA) is recognised as a suitable method for measuring the effective
interfacial mass transfer area of separation column internals such as random and structured
packing. Currently, this method is used under conditions where the concentration of MEA in
the liquid film is unaffected by the reaction and the liquid phase reaction is, therefore, assumed
to obey pseudo first order kinetics with respect to CO2. Under pseudo first order conditions,
the effect of surface depletion and renewal rates are not accounted for. Previous research
indicated that the effective area available for mass transfer is also dependent upon the rate of
surface renewal achieved within the liquid film. In order to study the effect of surface depletion
and renewal rates on the effective area, a method utilising a fast reaction with appreciable
depletion of the liquid phase reagent is required.
The homogeneous liquid phase reaction kinetics of CO2 with MEA n-Propanol as alcoholic
solvent was investigated in this study. A novel, in-situ Fourier Transform Infra-Red (FTIR)
method of analysis was developed to collect real time concentration data from reaction
initiation to equilibrium. The reaction was studied in a semi-batch reactor set-up at ambient
conditions (T = 25°C, 30°C and 35°C, P = 1 atm (abs)). The concentration ranges investigated
were [MEA]:[CO2] = 5:1 and 10:1. The concentration range investigated represents conditions
of significant MEA conversion. The reaction kinetic study confirmed the findings of previous research that the reaction of CO2
with MEA is best described by the zwitterion reactive intermediate reaction mechanism. Power
rate law and pseudo steady state hypothesis kinetic models (proposed in literature) were found
to be insufficient at describing the reaction kinetics accurately. Two fundamentally derived rate
expressions (based on the zwitterion reaction mechanism) provided a good quality model fit of
the experimental data for the conditions investigated. The rate constants of the full
fundamental model were independent of concentration and showed an Arrhenius temperature dependence. The shortened fundamental model rate constants showed a possible
concentration dependence, which raises doubt about its applicability.
The specific absorption rates (mol/m2.s) of CO2 into solutions of MEA/n-Propanol (0.2 M and
0.08 M, T = 25°C and 30°C, P = ±103 kPa) were investigated on a wetted wall experimental setup.
The experimental conditions were designed for a fast reaction in the liquid film to occur
with a degree of depletion of MEA in the liquid film. Both interfacial depletion and renewal of
MEA may be considered to occur. The gas phase resistance to mass transfer was determined to
be negligible. An increase in liquid turbulence caused an increase in the specific absorption rate
of CO2 which indicated that an increase in liquid turbulence causes an increase in effective mass
transfer area. Image analysis of the wetted wall gas-liquid interface confirmed the increase in
wave motion on the surface with an increase in liquid turbulence. The increase in wave motion
causes an increase in both interfacial and effective area.
A numerical solution strategy based on a concentration diffusion equation incorporating the
fundamentally derived rate expressions of this study is proposed for calculating the effective
area under conditions where surface depletion and renewal rates are significant. It is
recommended that the reaction kinetics of CO2 with MEA in solvents of varying liquid
properties is determined and the numerical technique proposed in this study used to calculate
effective area from absorption rates into these liquids. From the absorption data an effective
area correlation as a function of liquid properties may be derived in future. / AFRIKAANSE OPSOMMING: Die reaktiewe absorpsie van CO2 in nie-waterige oplossings van die primêre amien, monoetanolamien
(MEA) word erken as ‘n geskikte metode om die effektiewe massaoordragsarea
van gepakte skeidingskolomme te bepaal. Tans word die metode gebruik onder vinnige pseudo
eerste orde reaksietoestande met betrekking tot CO2. Die pseudo eersteorde aanname beteken
dat die konsentrasie van MEA in die vloeistoffilm onbeduidend beïnvloed word deur die reaksie
en effektief konstant bly. Onder pseudo eerste orde toestande word oppervlakverarming- en
oppervlakvernuwingseffekte nie in ag geneem nie, juis as gevolg van die konstante konsentrasie
van MEA in die vloeistoffilm. Daar is voorheen bevind dat oppervlakverarming en
oppervlakvernuwing ‘n beduidende invloed het op die beskikbare effektiewe
massaoordragsarea. Hierdie invloed kan slegs bestudeer word met ‘n vinnige reaksie in die
vloeistoffilm wat gepaard gaan met beduidende oppervlakverarming van die vloeistoffase
reagens. Die homogene vloeistoffase reaksiekinetika van CO2 met MEA in die alkohol oplosmiddel, n-
Propanol, is in hierdie studie ondersoek. ‘n Nuwe, in-situ Fourier Transform Infra-Rooi (FTIR)
metode van analiese is ontwikkel in hierdie ondersoek. Die reaksie is ondersoek in ‘n semienkelladings
reaktor met MEA wat gevoer is tot die reaktor om met die opgeloste CO2 te
reageer. Die FTIR metode meet spesiekonsentrasie as ‘n funksie van tyd sodat die
konsentrasieprofiele van CO2, MEA en een van die soutprodukte van die reaksie gebruik kan
word om verskillende reaksiesnelheidsvergelykings te modelleer. Die reaksie is ondersoek
onder matige toestande (T = 25°C, 30°C and 35°C, P = 1 atm (abs)). Die konsentrasiebereik van
die ondersoek was [MEA]:[CO2] = 5:1 en 10:1. Hierdie bereik is spesifiek gebruik sodat daar
beduidende omsetting van MEA kon plaasvind. Die reaksiekinetieka studie het, ter
ondersteuning van bestaande teorie, bevind dat die reaksie van CO2 met MEA in nie-waterige
oplosmiddels soos alkohole, beskyf word deur ‘n zwitterioon reaksiemeganisme. Die bestaande
reaksiesnelheids modelle (eksponensiële wet en pseudo gestadigde toestand hipotese) kon nie
die eksperimentele data met genoegsame akuraatheid beskryf nie. Twee nuwe reaksiesnelheidsvergelykings, afgelei vanaf eerste beginsels en gebaseer op die zwitterioon
meganisme, word voorgestel. Hierdie volle fundamentele model het goeie passings op die
eksperimentele data getoon oor die volledige temperatuur en konsentrasiebereik van hierdie
studie. Die reaksiekonstantes van die fundamentele model was onafhanklik van konsentrasie en
tipe oplosmiddel en het ‘n Arrhenius temperatuurafhanklikheid. Die verkorte fundamentele
model se reaksiekonstantes het ‘n moontlike konsentrasieafhanlikheid gewys. Dit plaas
onsekerheid op die fundamentele basis van hierdie model en kan dus slegs as ‘n eerste
benadering beskou word.
Die spesifieke absorpsietempos (mol/m2.s) van CO2 in MEA/n-Propanol oplossings (0.2 M en
0.08 M MEA, T = 25°C and 30°C, P = ±103 kPa) is ondersoek met ‘n benatte wand (‘wetted wall’)
eksperimentele opstelling. Die eksperimentele toestande is gekies sodat daar ‘n vinnige reaksie
in die vloeistoffilm plaasgevind het, met beide beduidende en nie-beduidende MEA omsetting.
Die doel met hierdie eksperimentele ontwerp was om die invloed van intervlakverarming en
intervlakvernuwing op die spesifieke absorpsietempo te ondersoek. Gas fase weerstand was
nie-beduidend onder die eksperimentele toestande nie. Beide intervlakverarming en
intervlakvernuwing gebeur gelyktydig en is waargeneem vanuit die eksperimentele data. ‘n
Beeldverwerkingstudie van die gas-vloeistof intervlak van die benatte wand het bevind dat daar
‘n toename in golfaksie op die vloeistof oppervlak is vir ‘n toename in vloeistof turbulensie.
Hierdie golfaksie dra by tot oppervlakvernuwing en ‘n toename in effektiewe
massaoordragsarea. ‘n Numeriese metode word voorgestel om die effektiewe area van beide die benatte wand en
gepakte kolomme te bepaal vanaf reaktiewe absorpsietempos. Die metode gebruik die
fundamentele reaksiesnelheidsvergelykings, bepaal in hierdie studie, in a konsentrasie
diffusievergelyking sodat oppervlakverarming en vernuwing in ag geneem kan word. Daar word
voorgestel dat die reaksiekinetika van CO2 met MEA in oplossings met verskillende fisiese
eienskappe (digtheid, oppervlakspanning en viskositeit) bepaal word sodat die numeriese
metode gebruik kan word om ‘n effektiewe area korrelasie as ‘n funksie van hierdie eienskappe
te bepaal.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/86656 |
| Date | 04 1900 |
| Creators | Du Preez, Louis Jacobus |
| Contributors | Knoetze, J. H., Callanan, L. H., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xxv, 311 p. : ill. |
| Rights | Stellenbosch University |
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