Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Pulp and paper mills are being placed under increasing pressure to maximise the use of the
biomass being processed for pulp, and move towards integrated biorefineries (IFBRs), where a
diverse range of products can be produced and not just pulp exclusively. Extracting
hemicelluloses prior to the pulping process could increase the profitability of the mills as the
hemicelluloses could be used to produce a number of additional products. Hemicelluloses are a
plant polysaccharides with the most abundant hemicellulose in hardwoods being xylan, with
xylose being the primary monosaccharide constituent of xylan. The majority of pulps produced
in the Southern Hemisphere are done with hardwoods as feedstock, typically with the Kraft
process. The attraction of the concept of extracting hemicellulose prior to pulping is further
augmented by the fact that hemicellulose is underutilised in the Kraft process. In the Kraft
process the hemicellulose is dissolved during pulping and burned along with lignin for the
production of energy, however, hemicellulose has about half the heating value when compared
to that of lignin.
The main objective of this study was to find a pre-extraction method that is effective in
releasing xylan from Eucalyptus grandis, the most important hardwood feedstock used for
pulping in the Southern Hemisphere. The method also needs to be practical in terms of
integrating it into the Kraft process and should have a minimal effect on pulp yield and
subsequent paper qualities.
Xylan extractions from E. grandis as feedstock were carried out with white liquor, green liquor
and NaOH. Green liquor is the dissolved smelt originating from the recovery boiler in the Kraft
process and consists mainly sodium carbonate and sodium sulphide. White liquor’s principal
components include sodium hydroxide and sodium sulphide and is used in the digesters during
the pulping stage of the Kraft process. NaOH is a make up chemical used in the Kraft process.
These chemicals were chosen since they are all already present within the Kraft process. The
suitability of these chemicals as xylan extraction methods is further bolstered by the fact that
their alkalinity may actually reduce chemical usage in pulping. This provides scope for integration of hemicellulose extraction into Kraft pulping without implementing major changes
to the existing industrial process. Moreover, alkali chemicals for pre-extraction allow for
minimal effect on resulting pulp and paper. In terms of the extracted product, the alkaline
conditions provided by these chemicals create conditions that are suitable for a high degree of
polymerisation of hemicelluloses.
Xylan pre-extracted chips from selected extraction conditions were subjected to varying
pulping conditions, to replicate pulp yields and properties obtained with untreated E. grandis
chips when using conventional pulping. Handsheets were also produced from the pulps
produced under the highest pulp yield conditions, and these were tested for pulp quality
properties. Furthermore, mass balances were performed to gauge the impact that
hemicellulose pre-extraction would have using green liquor, white liquor and NaOH on the
sodium and sulphur balances of the mill.
From the extractions performed, the highest fraction of xylan recovered was 15.15% w/w
utilising 2M NaOH, at 120°C for 90 minutes extraction time. This was followed by white liquor
extraction at 13.27% w/w utilising 20% AA at 140°C for 90 minutes. Green liquor extraction
produced the lowest xylan recovery at 7.83% w/w with 2% TTA and 160°C with an h-factor of
800. The residues from selected extraction conditions were utilised for these pulping
optimisation experiments.
Selected extraction conditions used for further pulping included 2% TTA and 160°C for green
liquor, 20% AA and 120°C and 140°C extraction temperature for white liquor, as well as 2M
concentration and 120°C for NaOH.
The highest yielding pulping conditions were achieved with a 35% reduction in pulping
chemicals and 45 minutes pulping time in combination with green liquor pre-extraction, while
for pulping combined with white liquor pre-extraction a 50% reduction in chemicals and 30
minute pulping time was preferred. For pulping subsequent to NaOH pre-extraction a 75%
reduction in the NaOH dosage and a 45 minute pulp time was preferred. All pulp steps were
performed at 170°C.
Unbeaten handsheets produced from the selected pulping conditions for white liquor and
green liquor extracted chips showed similar physical properties (burst, tear, tensile indices)
when compared to the control (pulps from non-extracted chips). However the greater quantity
of xylan removed from cellulose fibres with NaOH extraction, resulted in pulps with lower xylan
contents, which affected the burst and tear indices of the handsheets formed from these pulps.
An increase in tear, while a reduction in the burst index, was observed for the pulp produced
from NaOH extracted chips.
It was concluded that although white liquor and NaOH extraction allows for greater xylan
recovery, the large chemical expenditure associated with these methods will impose significant
cost impacts on the existing Kraft process. From mass balances performed, green liquor xylan
extraction due to its lower alkalinity, will be more forgiving in terms of additional make up
chemical costs. It also allowed for minimum effect on both the pulp and paper quality, thus
making it the most practical of the pre-extraction methods. However, whether the additional
make chemicals required for the green liquor extraction method will be justified by the quantity
of xylan extracted will only be answered by a thorough economic assessment, which was not in
the scope of this project. / AFRIKAANSE OPSOMMING: Pulp- en papiermeule word onder toenemende druk geplaas om die gebruik van die biomassa
wat vir pulp verwerk word, te maksimaliseer, en om te beweeg na geïntegreerde
bioraffinaderye, waar ʼn groot verskeidenheid produkte vervaardig kan word, en nie slegs
uitsluitlik pulp nie. Die ekstraksie van hemisellulose voor die verpulpingsproses kan die
winsgewendheid van die meule verhoog, aangesien die hemisellulose gebruik kan word vir die
vervaardiging van verskeie bykomende produkte. Hemisellulose is ʼn plantpolisakkaried, met
xilaan as die hemisellulose wat die oorvloedigste in hardehout gevind word, en met xilose as die
primêre monosakkaried-bestanddeel van xilaan. Die meerderheid van die pulp wat in die
Suidelike Halfrond geproduseer word, word met hardehout as voerstof gedoen, tipies met
behulp van die Kraft-proses. Die aanloklikheid van die konsep om hemisellulose voor verpulping
te win, word verder versterk deur die feit dat hemisellulose in die Kraft-proses onderbenut
word. In die Kraft-proses word die hemisellulose tydens verpulping opgelos en saam met lignien
verbrand vir die opwekking van energie, maar hemisellulose het egter ongeveer die helfte van
die verhittingswaarde van dié van lignien.
Die vernaamste doelstelling van hierdie studie was om ʼn pre-ekstraksiemetode te vind wat
xilaan doeltreffend van Eucalyptus grandis, die belangrikste hardhout-voerstof wat in die
Suidelike Halfrond vir verpulping gebruik word, kan vrystel. Die metode moet ook prakties wees
met betrekking tot integrering met die Kraft-proses, en dit moet ʼn minimale uitwerking op
pulpopbrengs en gevolglike papiergehalte hê.
Xilaan-ekstraksie uit E. grandis as voerstof is uitgevoer met wit loog, groen loog en NaOH.
Hierdie chemikalieë is gekies omdat hulle reeds in die Kraft-proses teenwoordig is. Die
geskiktheid van hierdie chemikalieë as xilaan-ekstraksiemetodes is verder ondersteun deur die
feit dat hul alkaliniteit moontlik chemiese verbruik in verpulping kan verlaag, wat ruimte vir die
integrasie van hemisellulose-ekstraksie in Kraft-verpulping laat sonder om grootskaalse
veranderinge aan bestaande nywerheidsprosesse te implementeer. Alkali-chemikalieë vir preekstraksie
lei boonop tot ʼn minimale uitwerking op resultante pulp en papier, terwyl die alkalitoestande, met betrekking tot die geëkstraheerde produk, toestande skep wat geskik is vir
ʼn hoë mate van polimerisasie van hemisellulose.
Uit die ekstraksies wat uitgevoer is, is die hoogste fraksie xilaan gewin deur die gebruik van
NaOH teen 15.15% w/w met 2M NaOH, teen 120 °C vir 90 minute ekstraksietyd. Dit is gevolg
deur witloog-ekstraksie teen 13.27% w/w met die gebruik van 20% AA teen 140 °C vir 90
minute. Groenloog-ekstraksie het die laagste xilaan-winning teen 7.83% w/w met 2% TTA en
160°C met ʼn h-faktor van 800 voortgebring.
Houtspaanders wat aan xilaan-pre-ekstraksie met groen loog onderwerp is, het pulp met
kappanommers en opbrengste soortgelyk aan dié van nie-geëkstraheerde spaanders
voortgebring toe die chemiese lading met 35% verlaag is, in verhouding tot dít wat vir niegeëkstraheerde
spaanders gebruik is. Xilaan-pre-geëkstraheerde spaanders met wit loog het ʼn
50%-vermindering in verpulpingschemikalieë gelewer in verhouding tot houtspaanders wat aan
konvensionele verpulping onderwerp is. Die chemiese reduksie van groen loog was minder as
dié van wit loog weens die laer alkalilading wat tydens hemisellulose-ekstraksie voor verpulping
gebruik is. Vir witloog-ekstraksie kon pulpopbrengste gehandhaaf word, alhoewel pregeëkstraheerde
spaanders met wit loog ʼn neiging getoon het om pulp met laer kappanommers
voort te bring. Alhoewel pulp wat uit houtspaanders gemaak is wat aan NaOH-ekstraksie
onderwerp is, gelei het tot ʼn 75%-vermindering van NaOH gebruik in verhouding tot dié van
konvensionele verpulping, is verwag dat geen NaOH benodig sou word nie, aangesien die
houtspaanders reeds tydens xilaan-ekstraksie aan 2M NaOH blootgestel is. Voorts, in die
literatuur is verpulping uitgevoer ná 2M NaOH-ekstraksie sonder dat die toevoeging van NaOH
tydens verpulping nodig was [61].
Handvelle is vervaardig uit die pulp wat in die hoogste pulpopbrengs-toestande vervaardig is,
en dit is vir pulpgehalte-eienskappe getoets. Die verpulpingstoestande met die hoogste
opbrengs is bereik met ʼn 35%-vermindering van verpulpingschemikalieë en 45 minute
verpulpingstyd in kombinasie met groenloog-pre-ekstraksie, terwyl vir verpulping met witloogpre-
ekstraksie ʼn 50%-vermindering van chemikalieë en 30 minute verpulpingstyd verkies is. Vir verpulping ná NaOH-pre-ekstraksie is ʼn 75%-vermindering van die NaOH-dosis en 45 minute
verpulpingstyd verkies. Alle verpulpingstappe is teen 170°C uitgevoer.
Ongeklopte handvelle vervaardig uit die gekose verpulpingstoestande vir witloog- en
groenloog- geëkstraheerde spaanders het soortgelyke fisiese eienskappe getoon (bars-, skeuren
trek-indeks) in vergelyking met die kontrole (pulp uit nie-geëkstraheerde spaanders). Die
grootste hoeveelheid xilaan is egter uit sellulose vesel met NaOH-ekstraksie verkry, wat gelei
het tot pulp met laer xilaaninhoud, wat die bars- en skeur-indeks van die handvelle wat uit
hierdie pulp vervaardig is, beïnvloed het. ʼn Toename in die skeur-indeks, met ʼn afname in die
bars-indeks, is waargeneem vir die pulp wat uit NaOH-geëkstraheerde spaanders vervaardig is.
Die gevolgtrekking is gemaak dat alhoewel witloog- en NaOH-ekstraksie groter xilaanwinning
moontlik maak, die groot chemiese uitgawe geassosieer met hierdie metode ʼn aanmerklike
koste-impak vir die bestaande Kraft-proses inhou. Groenloog-xilaanekstraksie sal, weens die
laer alkaliniteit, meer geskik wees met betrekking tot die koste van bykomende aanvullende
chemikalieë. Dit hou ook ʼn kleiner uitwerking op die pulp- en papiergehalte in, wat dit dus die
praktieste van die pre-ekstraksiemetodes maak. Of die bykomende chemikalieë nodig vir die
witloog- en NaOH-ekstraksies egter geregverdig kan word deur die hoeveelheid xilaan wat
gewin is, kan slegs deur ʼn deeglike ekonomiese assessering beantwoord word, wat nie binne
die omvang van hierdie projek geval het nie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/97003 |
| Date | 03 1900 |
| Creators | Joubert, Andre Jacobus |
| Contributors | Chimphango, Annie F. A., Gorgens, Johann F., 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 | 178 pages : illustrations |
| Rights | Stellenbosch University |
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