Thesis (MScIng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: The fine ash produced at the Sasol Secunda Petrochemical Plant is disposed
of through a wet ash disposal system. Other process waste streams with high
salt concentrations are co-disposed of in the Sasol Secunda ash disposal
system. This has led to a steady rise in the salt concentrations of the recycled
clear ash effluent (CAE) over the past 17 years. To combat this increase in
salt concentrations, the capability of the Sasol Secunda ash disposal system
to act as a salt sink, needs to be enhanced.
This investigation focussed on ways to enhance the salt removal/retention
capabilities of the Sasol Secunda ash disposal system and consisted of the
following:
• A literature survey of relevant information.
• The mixing of different combinations of fine ash, brine and CAE.
• Adding CO2 to the fine ash and CAE mixtures.
• Investigation to enhance salt precipitation in the CAE and Evaporation
dams.
• Salt balances and a residence time calculation over the CAE and
Evaporation dams.
From these investigations it were concluded that the Sasol Secunda ash
disposal system could be used as a salt sink for SO4 ions. Up to 43% of the
SO4 is removed from the brines after the initial ash/water contact. It was also
found that the tubular reverse osmosis (TRO) brine could be used as a carrier
medium for the ash. The large amounts of Ca that is leached into the ash
water during the mixing of the CAE and fine ash can be prevented by the
addition of CO2 to the mixing point. There is usually an increase of 240% in
the Ca concentration and this is reduced to only an 8% increase with the CO2
addition. The most feasible precipitation enhancement for the CAE and Evaporation
dams is an increase in evaporation. This enhances CaCO3 precipitation,
which is the main mechanism for salt removal in the CAE and Evaporation
dams.
Ca, Na and Cl are retained in the evaporation and CAE dams. SO4 is leached
from solid phases in the dams. There is however an overall decrease in the
total dissolved solids (TDS) of the ash water. The salt removal of the CAE
and Evaporation dams is approximately 57 tons per day.
The capability of the Sasol Secunda ash disposal system to act as a salt sink
can be enhanced by the addition of CO2 at the mixing point and by increasing
the evaporation rate in the CAE and Evaporation dams. Using the TRO brine
as carrier medium may also increase the SO4 precipitation capabilities of the
Sasol Secunda ash disposal system. / AFRIKAANSE OPSOMMING: Die fynas wat by die Sasol Secunda Petrochemiese Aanleg geproduseer
word, word verwyder deur ‘n geslote nat asstelsel. Ander afvalstrome wat hoë
konsentrasies soute bevat word ook in die Sasol Secunda asstelsel gestort.
Dit het tot gevolg dat daar oor die afgelope 17 jaar ‘n volgehoue styging in die
sout konsentrasies van die hergebruikte aswater (genoem CAE – “clear ash
effluent”) was. ‘n Manier om hierdie styging in die sout konsentrasies teen te
werk, is om die sout verwyderingsvermoë van die Sasol Secunda asstelsel te
verbeter.
Hierdie ondersoek het gefokus op maniere om die sout verwyderings-
/terughoudingsvermoë van die Sasol Secunda asstelsel te verbeter en het die
volgende ingesluit:
• ‘n Literatuur oorsig van toepaslike inligting.
• Die meng van verskillende kombinasies van fynas, soutstrome en CAE.
• Toediening van CO2 by die fynas en CAE mengsels.
• ‘n Ondersoek na metodes om die soutverwydering in die CAE en
Verdampingsdamme te verbeter.
• Soutbalanse en ‘n residensie tyd berekening vir die CAE en
Verdampingsdamme.
Na hierdie ondersoeke kon die gevolgtrekking gemaak word dat die Sasol
Secunda asstelsel ‘n sout sink vir SO4 ione is. Tot 43% van die SO4 word
verwyder na die aanvanklike as/water kontak. Daar is ook gevind dat die
TRO (“tubular reverse osmosis”) soutstroom gebruik kan word as ‘n draer vir
die fynas. Die groot hoeveelhede Ca wat in die aswater in loog, kan voorkom
word deur die toediening van CO2 by die mengpunt van die fynas en aswater.
Daar is normaalweg ‘n verhoging van 240% in die Ca konsentrasie van die
aswater en dit word verminder na ‘n skrale 8% met die toediening van CO2. Die mees praktiese metode om die soutverwydering in die CAE en
Verdampingsdamme te verbeter, is met die verhoging van die verdamping.
Dit sal die neerslag van CaCO3, wat die meeste soutverwydering tot gevolg
het, verhoog.
Ca, Na en Cl word teruggehou in die Verdampings en CAE damme. SO4 loog
uit soliede fases in die damme. Daar is wel ‘n afname in die algehele
opgeloste spesies (“TDS”) van die aswater. Die soutverwydering van die
Verdampings en CAE damme is ongeveer 57 ton per dag.
Die vermoë van die Sasol Secunda asstelsel om as ‘n sout sink gebruik te
word, kan verbeter word deur CO2 by die mengpunt by te voeg en die
verdampingstempo in die Verdampings en CAE damme te verhoog. Die
gebruik van die TRO pekelstroom as draer van die as kan die SO4 neerslag in
die Sasol Secunda asstelsel ook verhoog.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16450 |
| Date | 12 1900 |
| Creators | Van den Berg, Jacobus A. |
| Contributors | Lorenzen, L., Schutte, B., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xv, 150 leaves : ill. |
| Rights | University of Stellenbosch |
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