Thesis (MEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: Supercritical fluids are enjoying ever increasing popularity as a solvent medium for extraction,
stripping and absorption processes. Being readily tuneable and able to achieve sharp, highly
efficient separations, supercritical fluids present an attractive alternative to traditional
solvents, while using less intrinsically harmful compounds. Although the potential of
supercritical fluids as solvents have been known for more than a century, there are still several
areas of uncertainty, one being the hydrodynamics of extraction columns operating under
supercritical conditions. This shortcoming can be attributed to the satisfactory performance of
modified standard hydrodynamics to approximate column design, along with a predominant
culture of overdesign in process engineering. Even though modified subcritical hydrodynamic
models provide a good approximation they do not successfully predict the effect of changes in
density, viscosity and surface tension of a supercritical fluid, leading to inaccuracies in column
design.
This study investigates the state of hydrodynamics under supercritical conditions in counter
current packed columns discussed in literature, identifies shortcomings in existing literature
and devises a way of addressing the said shortcomings. The primary objective of this study is to
establish a multipurpose supercritical pilot plant capable of measuring hydrodynamics under
supercritical conditions, followed by the secondary objective of measuring preliminary
hydrodynamic data to prove the plant can deliver on its design requirements in measuring
reliable hydrodynamic data. During a survey of available literature it was found that very little experimental work has been
performed on hydrodynamics under supercritical conditions and especially on random
packings. Further it is found that the systems investigated in literature were conducted under
conditions of significant mass transfer. As mass transfer directly affects flow rates and fluid
properties of the fluids in the column, it is vital to use systems with very little to no mass
transfer. This ensures the most accurate approach possible when investigating fundamental
hydrodynamic behaviour. Finally it was found that there are no well-defined correlations
available for a wide range of packings, fluid properties and hydrodynamic phenomena for
columns under supercritical conditions.
To remedy the shortcomings in hydrodynamic data it was decided that more pilot plant work is
required. It was found that no pilot plants available can measure hydrodynamic data. An
investigation was performed into retrofitting available pilot plants, plants used by other
research groups and commercially available plants. It was concluded that the best option was to salvage the major parts of an existing old pilot plant and use them to construct a new,
customized pilot plant. This provides the opportunity of constructing a custom, multipurpose
pilot plant capable of use in future research.
After an initial concept design a full design of the new pilot plant was performed. The plant
consists of two columns of 17 mm and 38 mm inside diameter and 3.5 m and 1.5 m packed
height, respectively, and is capable of pressures and temperatures of up to 300 bar and 200°C.
Furthermore the pilot plant can measure liquid hold-up, pressure drop, flooding and
entrainment in accordance with the objective of measuring supercritical hydrodynamic data.
Liquid hold-up was determined by stopping the process and allowing the column to drain, after
which the volume drained was measured. To measure the pressure drop an Endress+Hauser
Deltabar S PMD75 DP cell was used. Flooding was determined using the measured pressure
drop and volumetric rate of column overheads, from where a hydrodynamically inoperable
state is defined. Overall entrainment, although unlikely due to the presence of a demister in the
column, was investigated by comparing the column overheads to literature phase equilibria.
Preliminary hydrodynamic testing was performed using the 38mm diameter column packed
with 1/4” Dixon rings. Testing is performed with at 120 bar and 40°C with a CO2 supercritical
phase and polyethylene glycol liquid phase with an average molar mass of 400 (PEG 400). The
hydrodynamic data gathered showed expected trends, but showed discrepancy with literature
due to differences in liquids used, column packing and experimental system between the
respective studies. / AFRIKAANSE OPSOMMING: Superkritiese vloeistowwe is besig om toenemende gewildheid as 'n oplosmiddel vir ekstraksie,
stroping en absorpsie prosesse te geniet. Hierdie gewildheid is as gevolg van ʼn vermoë om
skerp, hoogs effektiewe skeidings te bewerkstellig deur gebruik te maak van ʼn maklik
aanpasbare oplosmiddel wat minder intrinsiek skadelik is as tradisionele oplosmiddels.
Hierdie voordele lei daartoe dat superkritiese vloeiers as ʼn aantreklike alternatief tot
tradisionele oplosmiddels gesien kan word. Alhoewel die potensiaal van superkritiese
vloeistowwe as oplosmiddels al vir meer as ʼn eeu bekend is, is nog weinig eksperimentele werk
al gedoen oor die hidrodinamiese gedrag van superkritiese gepakte kolomme. Hierdie
tekortkoming kan toegeskryf word tot die bevredigende prestasie van aangepaste standaard
hidrodinamiese korrelasies gedurende superkritiese kolomontwerp en ʼn oorheersende
kultuur van oorontwerp in proses-ingenieurswese. Alhoewel aangepaste standaard
hidrodinamiese korrelasies ʼn aanvaarbare benadering bied, beeld dit nie die effek van die
veranderde digtheid, viskositeit en oppervlakspanning van ʼn superkritiese vloeistof uit nie, wat
lei tot foute in kolomontwerp.
Hierdie studie ondersoek die stand van superkritiese hidrodinamika in literatuur, spesifiek in
teenstroom gepakte kolomme. Tekortkominge in die bestaande literatuur is geïdentifiseer en
'n metode om die genoemde tekortkominge reg te stel is bedink. Die primêre doel van hierdie
studie is om 'n veeldoelige superkritiese loodsaanleg te bou wat tot staat is om superkritiese
hidrodinamika te meet, gevolg deur die sekondêre doelwit wat die meet van voorlopige
hidrodinamiese data behels, wat sal bewys dat die loodsaanleg voldoen aan ontwerpsvereistes. Tydens 'n opname van beskikbare literatuur was daar gevind dat weinig eksperimentele werk
al gedoen is in die veld van superkritiese hidrodinamika, en nog minder oor sogenoemde
ongeordende of ‘random’ kolompakkings. Verder is daar gevind dat eksperimente uitgevoer in
literatuur slegs bestaan uit stelsels waar beduidende massa-oordrag plaasvind. Aangesien
massa-oordrag die vloeitempo en fisiese eienskappe van die vloeiers in ʼn kolom direk
beïnvloed, is dit noodsaaklik om gebruik te maak van stelsels met baie min of geen massaoordrag.
Dit verseker ʼn akkurate benadering tot die meet van fundamentele hidrodinamiese
gedrag. Laastens is gevind dat daar geen hidrodinamiese korrelasies beskikbaar is wat ʼn wye
verskeidenheid van kolompakkings, vloeier eienskappe en hidrodinamiese verskynsels onder
superkritiese toestande dek nie. Om die tekortkominge in superkritiese hidrodinamika in literatuur aan te spreek, word meer
eksperimentele loodsaanlegwerk vereis. Daar is gevind dat geen van die beskikbare
loodsaanlegte hidrodinamiese data kan meet nie. Ondersoek is ingestel tot die ombouing van
bestaande loodsaanlegte, aanlegte wat gebruik is deur ander navorsingsgroepe en
kommersieel beskikbare aanlegte. Daar is tot die gevolgtrekking gekom dat die beste opsie is
om ʼn nuwe loodsaanleg self te bou en gebruik te maak van parte uit een van die ou bestaande
aanlegte om kostes laag te hou. Sodoende kan ʼn veeldoelige, pasgemaakte loodsaanleg gebou
word wat ook vir toekomstige navorsing gebruik kan word.
Na ʼn aanvanklike konsep ontwerp vir die nuwe loodsaanleg, is ʼn volledige ontwerp gedoen. Die
aanleg bestaan uit twee kolomme van onderskeidelik 17 mm en 38 mm binnedeursnee en
3,5 m en 1,5 m gepakte hoogte, en is in staat om eksperimente by ʼn maksimum druk en
temperatuur van tot 300 bar en 200°C uit te voer. Verder is die loodsaanleg in staat daartoe
hidrodinamiese data te meet, naamlik die vloeistofophoud in die kolom, drukval oor die
kolompakking, kolomvloed en druppel meesleuring. Die vloeistofophoud in die kolom is
bepaal deur alle voer tot die kolom te stop en tyd toe te laat vir die vloeistof om te dreineer,
waarna die gedreineerde afgetap en gemeet is. Om die drukval te meet word ʼn Endress+Hauser
Deltabar S PMD75 DP sel gebruik. Kolomvloed is bepaal met behulp van die drukval oor die
kolom en die vloeitempo van die kolom se boonste produkstroom, van waar ʼn hidrodinamies
onbruikbare toestand gedefinieer word. Algehele druppel meesleuring, alhoewel
onwaarskynlik as gevolg van die teenwoordigheid ʼn ontwasemer in die kolom, is wel ondersoek
deur die vloeistofinhoud in die kolom se boonste produkstroom te vergelyk met fase
ewewigsdata in literatuur. Voorlopige hidrodinamiese eksperimente is uitgevoer met behulp van die 38mm deursnee
kolom gepak met 1/4 " Dixon ringe. Eksperimente is uitgevoer by 120 bar en 40 ° C met 'n CO2
kritiese fase en 'n poliëtileenglikol vloeistof fase met ʼn gemiddelde molêre massa van 400 (PEG
400). Die hidrodinamiese data het verwagte tendense getoon, maar diskrepansies met
literatuur waardes. Die verskille tussen die eksperimentele en literatuur data word geregverdig
deur die verskille tussen die vloeistowwe, pakking en eksperimentele stelsels wat in die
onderskeie studies gebruik is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/95961 |
| Date | 12 1900 |
| Creators | Franken, Hendrik Hermanus |
| Contributors | Schwarz, C. E., Knoetze, J. H., Stellenbosch University. Faculty of Engineering. Department of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xvii, 223 p. : ill. |
| Rights | Stellenbosch University |
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