Thesis (MEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: Distillation is one of the most widely used processes for the separation of fluids with different
volatilities. Due to the popularity of this process it is often assumed that the hydrodynamic
behaviour inside distillation columns is well-defined. However, this is not always the case and this
study therefore endeavoured to provide additional insight into the topic through a systematic
investigation into the hydrodynamics and the capacity limitations of a sieve tray distillation column.
The objective of the study was to measure and evaluate the effects of the following variables on
entrainment and weeping:
- Fluid flow rate (gas and liquid).
- Plate geometry (i.e. hole diameter and fractional hole area).
- Liquid properties (i.e. surface tension, viscosity and density).
- Gas properties (i.e. viscosity and density). The hydrodynamic effects were evaluated at zero mass transfer in a pilot-scale tray column, by
passing pure liquids and gases in counter current configuration. The pilot column was rectangular in
shape with internal dimensions of 175 mm by 635 mm. A chimney tray was used to capture the
weeping liquid, while a de-entrainment tray was used in combination with a mist eliminator pad to
capture the entrained liquid. The fractional hole areas for the sieve trays under investigation were
7%, 11% and 15% and the hole diameters were 3.2 mm (⅛ in.), 6.4 mm (¼ in.) and 12.7 mm (½ in.).
The experimental liquids were ethylene glycol, butanol, water and silicone oil, while the gases were
air and carbon dioxide (CO2).
These experimental measurements produced over 10 000 data points for entrainment and over 7
000 data points of weeping. The results were repeatable and the entrainment values compared
reasonably well with previous data produced by Nutter (1971) and Uys (2012).
The differences between entrainment for the different liquids were more significant in the spray
regime than in the froth regime, and butanol was entrained more readily than silicone oil, ethylene
glycol and water. Fluids that caused a larger spray layer in the dispersion zone produced more
entrainment. Entrainment increased with decreasing liquid density, decreasing liquid surface tension
and decreasing liquid viscosity. The more unstable the dispersion layer, the higher the entrainment.
The liquid density strongly influenced weeping, i.e. weeping increased with increasing liquid density.
On the other hand, gases with higher densities – and thus with a higher mass flow rates at similar
volumetric flow rates through the sieve tray – displayed less weeping and more entrainment than
less dense gases, because of an increased upward drag force on the fluids.
When considering tray geometry and when operating in the spray regime, the magnitude of
entrainment increased with decreasing fractional hole area, while the dependency of entrainment
on fractional hole area was more prominent at lower fractional hole areas. When operating in the
froth regime – typically above 23 m3/(h.m) – the fractional hole area had a relatively small influence
on the magnitude of entrainment, while the cross-flowing liquid rate dominated related effects. In the spray regime, i.e. typically below 23 m3/(h.m), the entrainment increased with increasing sieve
tray hole diameter, while hole diameter had a relatively small influence on entrainment at higher
liquid flow rates between 23 and 60 m3/(h.m). However, at even higher liquid flow rates in the froth
regime, i.e. above 60 m3/(h.m), the effect of hole diameter on the entrainment became more
prominent again, with increased entrainment for smaller hole diameters.
The effect of hole diameter on weeping differed with changing fluid combinations and the 12.7 mm
hole size caused notably less weeping than the 3.2 mm and 6.4 mm trays at higher liquid flow rates.
It is believed that weeping occurred preferentially at so-called localised high pressure zones on the
sieve tray. At high gas and liquid flow rates, the resultant extended dispersion layer allows minimal
intimate contact between the plate and the liquid (minimising such localized high-pressure zones). In
effect, the liquid ‘jumps’ over the entire flow path length in the test rig, thus resulting in low
weeping rates at high gas and liquid rates.
The effects of fractional hole area and hole diameter on entrainment and weeping can be correlated
with combinations of well-known hydrodynamic dimensionless numbers, such as the Weber number
(We), Froude number (Fr) and Reynolds number (Re). Within the limitations of this study, the
flow-Froude number was shown to be the most useful dimensionless number, since it displayed a
monotonic relationship with magnitude of entrainment for different combinations of fluid systems
and tray configurations. Furthermore, both the construction number and fluid density ratio could be
used in a sensible manner to correlate some of the effects of tray geometry on entrainment. / AFRIKAANSE OPSOMMING: Distillasie word wêreldwyd op groot en klein skaal toegepas as ʼn metode om chemiese komponente
van mekaar te skei, gebasseer op hul verskil in vlugtigheid. Die hidrodinamiese gedrag van
vloeistowwe en hul damp binne ʼn distillasiekolom beïnvloed die effektiwiteit van die
skeidingsproses. Hierdie studie beoog dus om bykomende insig te verskaf tot die hidrodinamika en
kapasiteitsbeperkings van ʼn plaat-distilleerkolom.
Die doelwit van die studie was om die invloed van die volgende veranderlikes op die meesleuring en
deurdripping van vloeistowwe te ondersoek:
- Gas- en vloeistof vloeitempo.
- Plaatgeometrie (i.e. gatdeursnit en fraksionele deurvloei-area).
- Vloeistofeienskappe (i.e. oppervlakspanning, viskositeit en digtheid).
- Gaseienskappe (i.e. viskositeit en digtheid). Die hidrodinamiese studie is uitgevoer in ʼn reghoekige plaatkolom met interne afmetings van
175 mm x 635 mm. Die vloeistof en gasfases is in kontak gebring op ʼn teenstroom basis, met geen
massa-oordrag wat plaasvind nie. ʼn Skoorsteenplaat het die vloeistof opgevang wat deurdrip terwyl
ʼn ekstra plaat aan die bokant van die kolom die meegesleurde vloeistof opgevang het. Hierdie
ekstra plaat is gebruik tesame met ʼn mis-elimineerder om al die meegesleurde vloeistof op te vang.
Plate met verskillende deurvloei-areas (7%, 11% en 15%) en gat deursnitte (3.2 mm, 6.4 mm en 12.7
mm) is gebruik in die ondersoek. Die vloeistowwe wat gebruik is, sluit in etileen glikol, butanol,
water en silikon olie. Lug en koolstofdioksied is as gasse gebruik.
Die eksperimentele data het goeie herhaalbaarheid getoon en is vergelykbaar met die gepubliseerde
data van Nutter (1971) en Uys (2012). Meer as 10 000 data punte is gemeet vir vloeistofmeesleuring
en meer as 7 000 vir deurdripping.
Die verskil in hoeveelheid meesleuring tussen die vloeistowwe, soos ondersoek in hierdie studie, was
mees beduidend in die spoei-regime. Butanol is die meeste meegesleur, gevolg deur silikon olie en
dan etileen glikol. Water is die minste meegesleur is. Vloeistowwe wat ʼn groter sproeivolume in die
dispersielaag bo die plaat gevorm het, is die meeste meegesleur. Meesleuring het toegeneem met ʼn
afname in digtheid, oppervlakspanning en viskositeit van die vloeistof. ʼn Onstabiele dispersielaag bo
die plaat het meer meesleuring tot gevolg gehad.
Vloeistofdeurdripping is sterk beïnvloed deur vloeistofdigtheid, i.e. deurdripping het sterk
toegeneem met digtheid. Gasse met ʼn hoër digtheid veroorsaak weer ʼn afname in deurdripping
a.g.v. die hoër opwaartse sleurkragte wat ʼn gas met hoë digtheid op die vloeistof uitoefen.
In die sproei-regime (tipies by vloeistofvloeitempos laer as 23 m3/(h.m) is gevind dat meesleuring
toeneem met ʼn afname in fraksionele deurvloei-area. Meesleuring se afhanklikheid van fraksionele
deurvloei-area was meer beduidend by laer fraksionele deurvloei-areas. In die skuim-regime (tipies
by vloeistofvloeitempos hoër as 23 m3/(h.m)) was die afhanklikheid van meesleuring op fraksionele
deurvloei-area relatief klein. In die sproei-regime is gevind dat meesleuring toeneem met ʼn toename in gat deursnit, terwyl
dieselfde veranderlike ʼn minder beduidende invloed op meesleuring getoon het by hoër
vloeistofvloeitempos (tussen 23 en 60 m3/(h.m)). By vloeitempos hoër as 60 m3/(h.m) het
meesleuring weer begin toeneem met ʼn afname in gat deursnit.
By hoë vloeistofvloeitempos het die plaat met 12.7 mm gat deursnit aansienlik minder deurdripping
getoon as plate met 3.2 mm en 6.4 mm deursnitte. Daar word vermoed dat deurdripping
hoofsaaklik plaasvind by lokale hoëdruk gebiede op die plaat. By hoër vloeistof- en gasvloeitempos
beslaan die dispersielaag ʼn groter volume en is daar dus minder gebiede van digte vloeistofkontak
met die plaat, wat ʼn afname in die lokale drukgebiede veroorsaak. Dit lei tot ʼn afname in
deurdripping by hoër gas- en vloeistofvloeitempos.
Die invloed van fraksionele deurvloei-area en gatdeursnit op meesleuring en deurdripping korreleer
goed met kombinasies van welbekende hidrodinamiese dimensielose getalle, i.e. die Webergetal
(We), die Froudegetal (Fr) en die Reynoldsgetal (Re). Die vloei-Froudegetal is mees bruikbaar om die
invloed van vloeistof-en-gas kombinasies en kolomuitleg op meesleuring te korreleer. Die
konstruksiegetal asook die digtheidsverhoudings tussen vloeistof en gas kan op ʼn sinvolle manier
aangewend word om van die invloede van plaatgeometrie op meesleuring te beskryf.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/95996 |
| Date | 12 1900 |
| Creators | Moses, Royston Kyle |
| Contributors | Burger, Andre J., Knoetze, J. 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 | English |
| Type | Thesis |
| Format | xxvi, 217 p. : ill |
| Rights | Stellenbosch University |
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