Integration of xylan extraction prior to kraft and sodaAQ pulping from South African grown Eucalyptus grandis, giant bamboo and sugarcane bagasse to produce paper pulps, value added biopolymers and fermentable sugars

Vena, Phumla Faith (2013-03)

Thesis (PhD)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: The extraction of hemicelluloses prior to pulping that would have been dissolved in black liquor during pulping process, is an attractive alternative for pulp and paper mills as they, in addition to their core products, can increase their revenue by producing biofuels, biopolymers, paper additives and other chemicals. However, the amount of hemicelluloses extracted will be limited by the requirement to maintain pulp yield and pulp quality in comparison to existing pulping processes. In the present study, mild alkaline (NaOH) and dilute sulphuric acid conditions were used to extract hemicelluloses from Eucalyptus grandis, giant bamboo (Bambusa balcooa) and sugarcane (Saccharum officinarum) bagasse (SCB) prior to kraft or sodaAQ pulping processes. The effects of catalyst concentration, temperature and reaction time on hemicelluloses pre-extraction were studied, using a statistical experimental design to investigate conditions under which hemicelluloses could be extracted prior to alkaline pulping with minimal interference on cellulose (glucan) content. Subsequently, selected pre-extracted materials were subjected to kraft or sodaAQ pulping to evaluate the effect of the hemicelluloses pre-extraction on cooking chemicals, pulp yield and properties. This study also included evaluation of hot water hemicelluloses pre-extraction of SCB as it was part of a dilute sulphuric acid experimental design. The pulp yield, cooking chemicals and handsheet strength properties were compared with those obtained from kraft or sodaAQ pulping of non extracted raw materials. The results showed that alkaline pre-extraction options investigated preserves the pulp yield with minimal effect on handsheet strength properties depending on the choice of the subsequent pulping method while a fraction of xylan was extracted in polymeric form. In addition, less active alkali was required to delignify the xylan extracted materials. The integration of hemicelluloses pre-extraction by alkaline methods into a kraft pulping process was preferred for giant bamboo and E. grandis since it maintained pulp yields at desired industrial levels of 50%, and pulps within a bleachable kappa number range. Another advantage observed was the reduction in total cooking active alkali required to delignify alkaline extracted giant bamboo or E. grandis by 8or 3 percentage points respectively. However, the compromise to maintain the pulp yield was obtained when only 13.6% or 12.4% polymeric xylan was solubilised from giant bamboo or E. grandis respectively. Slight improvement in burst index of the handsheet was observed for extracted giant bamboo. On the other hand, pulp viscosity was increased by 13% due to the removal of low molecular weight hemicelluloses, while the breaking strength of the handsheet was also increased by 8.9% for pulps produced from extracted E. grandis. In the case of sugarcane bagasse, hemicelluloses pre-extraction by alkaline methods integrated well with the sodaAQ pulping process. It enabled a xylan recovery of 69.1%, while providing pulp with higher screened pulp yield (45.0%), with an advantageous decrease in kappa number (15.5). The handsheet tear index was superior without reduction in viscosity compared to pulp produced from non extracted SCB. On the contrary, results obtained from optimised dilute sulphuric acid pre-extraction of all the tested feedstocks were found to negatively impact subsequent kraft or sodaAQ pulping processes resulting in lower pulp yields and poorer strengths properties. Nonetheless, the differences were better when sodaAQ pulping was used compared to kraft pulping. SodaAQ protects the carbohydrates against the peeling reaction under alkaline medium. Conversely, pre-extraction of SCB with hot water resulted in low concentration of xylooligomers (5.7%), while the subsequent sodaAQ pulping resulted in no pulp yield reduction. The tear index and optical brightness of the handsheet papers produced from hot water extracted SCB were slightly improved while the breaking length, tensile and burst indexes were similar to those of pulps produced from non extracted SCB fibres. Of equal importance were the observed higher tear and burst indexes of handsheets produced from giant bamboo compared to E. grandis for both extracted and non extracted materials prepared under similar pulping processes. The advantage of bamboo was due to the larger fibre length and different morphological properties to those of hardwoods. However, the pulps produced from giant bamboo showed higher kappa numbers than those pulps produced from E. grandis due to the high condensation behaviour of bamboo lignins under alkaline conditions. Higher kappa numbers explained the higher demand for subsequent bleaching chemicals. In conclusion, the pulp mill biorefinery concept through hemicelluloses pre-extraction with NaOH can be achieved with modified kraft pulping or the sodaAQ pulping processes, but it depends on the type of raw material, extraction method and quality and performance requirements of a particular paper. The low pulping chemicals demand, comparable pulp yields and the improvement in some physico-chemical properties of the pulps from preextracted materials were observed. Furthermore, owing to xylan pre-extraction a larger amount of (extracted) material could be loaded into the digester as when non-extracted materials were used.

AFRIKAANSE OPSOMMING: Ekstraksie van hemiselluloses wat tydens verpulping in die swartloog opgelos word, bied ‘n aantreklike alternatief aan pulp- en papiermeulens om, addisioneel tot hul hoofprodukte, hul inkomste deur die vervaardiging van biobrandstowwe, biopolimere, papierbymiddels en ander chemikalië, daardeur te kan verhoog. Die hoeveelheid hemiselluloses wat ge-ekstraheer kan word, sal egter beperk word deur die vereiste dat pulpopbrengs en –kwaliteit tydens bestaande verpulpingsprosesse gehandhaaf moet word. In hierdie ondersoek is matige alkaliese (NaOH) en verdunde swawelsuurtoestande gebruik om hemiselluloses vóór kraft- of natriumantrakinoonverpulping uit Eucalyptus grandis, reuse bamboes (Bambusa balcooa) en suikerriet (Saccharum officinarum) bagasse, mee te ekstraheer. Die invloed van katalisatorkonsentrasie, temperatuur en reaksietyd is mbv ‘n statistiese, eksperimentele ontwerp ondersoek om die toestande te bepaal waaronder hemiselluloses, met minimale effek op die sellulose (glukaan) –inhoud, vóór alkaliese verpulping ge-ekstraheer kan word. Die pre-ge-ekstraheerde materiale, met hoë glukaan- en voldoende hemisellulosesinhoud, is vervolgens aan kraft- en natriumantrakinoonverpulping onderwerp om die invloed van pre-ekstraksie van hemiselluloses op die verpulpingsreagense, pulpopbrengs en - eienskappe vas te stel. Hierdie studie het ook die evualering van warmwater hemisellulosespre-ekstraksie van suikerrietbagasse, wat deel is van ‘n verdunde swawelsuur eksperimentele uitleg, ingesluit. Pulpopbrengs, die hoeveelheid verpulpingsreagense en handveleienskappe van dieselfde materiale wat nie vooraf ge-ekstraheer is nie, is vergelyk. Die resultate toon dat alkaliese pre-ekstraksie metodes wat ondersoek is die pulpopbrengs met minimale effek op handvel sterkte-eienskappe afhangende van die keuse van daaropvolgende pulpmetode kon handhaaf terwyl ‘n fraksie van xilaan in polimeriese vorm ge-ekstraheer is. Addisioneel, is minder aktiewe alkali benodig om die xilaan ge-ekstraheerde materiale te delignifiseer. Die integrasie van hemisellulosespre-ekstraksie dmv alkaliese metodes tydens ‘n kraft verpulpingsproses is vir reuse bamboes en E. grandis verkies omdat pulpopbrengste op ideale industriële vlakke van 50% gehandhaaf en is en pulp in ‘n bleikbare kappa nommergebied interval kon lewer. ‘n Verdere voordeel wat waargeneem is was die vermindering in die totale gekookte aktiewe alkali benodig vir reuse bamboes of E. grandis met 8 of 3 persentasiepunte onderskeidelik. Die kompromie om die pulpopbrengs te handhaaf is verkry toe slegs 13.6% of 12.4% polimeriese xilaan opgelos is vanuit reuse bamboes of E. grandis onderskeidelik. ‘n Effense verbetering in bars-indeks van die handvelle is waargeneem vir ge-ekstraheerde reuse bamboes. Pulpviskositeit het met 13% gestyg agv die verwydering van die lae molekulêre massa hemiselluloses, terwyl breeksterkte van handvelle ook met 8.9% toegeneem het vir pulp verkry uit pre-gekstraheerde E. grandis. NaOH pre-ekstraksie van 69.1% xilaan (droë massa) uit suikerriet bagasse (SCB) het ‘n hoër natriumantrakinoon, gesifte pulpopbrengs gelewer (45.0%) met ‘n verbeterde afname in kappa-getal (15.5) en uitstekende skeursterkte sonder verlaging in viskositeit, soos vergelyk met nie-ge-ekstraheerde suikkerrietbagasse. Daarteenoor het die resultate verkry met die geoptimeerde verdunde swawelsuur preekstraksie van al die getoetste rumateriale getoon om‘n negatiewe effek te gehad het op die daaropvolgende kraft- of natriumantrakinoonverpulping dws het laer pulpopbrengste en swakker sterkte-eienskappe opgelewer. Die verskille was nietemin kleiner toe natriumantrakinoonverpulping ipv kraftverpulping gebruik is. Antrakinoon beskerm die koolhidrate teen die afskilreaksie in alkaliese medium. Daarteenoor het pre-ekstraksie van suikerrierbagasse met warm water tot 'n lae hoeveelheid (5.7%) xilaanoligomere gelei, terwyl die daaropvolgende natriumantrakinoonverpulping geen verlaging in pulpopbrengs veroorsaak het nie. Skeursterkte en optiese helderheid van handvelle wat uit warm water ge-ekstraheerde suikerrietbagasse vervaardig is, het ietwat verbeter terwyl breek-, trek- en barssterkte dieselfde was as van suikerrietbagasse pulp wat nie ge-ekstraheer is nie. Net so belangrik was die waargenome hoër skeur- en barsindekse van handvelle vervaardig van reuse bamboes in vergelyking met E. grandis van beide geekstraheerde en nie ge-ekstraheerde materiale voorberei onder dieselfde verpulpings toestande. Bamboes se sterker eienskappe was as gevolg van die hoër vesellengte en ander morfologiese eienskappe as diévan loofhout. Pulp wat vervaardig is van reuse bamboes het ‘n hoër kappanommer getoon as pulp van E. grandis as gevolg van die hoë kondensasiegedrag van bamboeslignien onder alkaliese toestande. Hoër kappanommers kon die gepaardgaande hoër aanvraag vir bleikchemikalieë verklaar. Ten slotte, die pulpmeul bio-raffinaderykonsep nl. deur hemisellulosesekstraksie met NaOH gekombineer met óf ‘n gemodifiseerde kraft verpulping óf ‘n gemodifiseerde natriumantrakinoon verpulping, is wel uitvoerbaar. Dit word egter sterk beïnvloed deur die tipe ru-materiaal en die ekstraksie-metode gebruik, asook deur die kwaliteits- en gebruiksvereistes van verskillende tipes papier. ‘n Lae aanvraag vir verpulpingschemikalieë, vergelykbare pulpopbrengste en die verbetering in fisies-chemiese eienskappe van pulp vanaf pre-ge-ekstraheerde materiale is waargeneem. Verder kon, as gevolg van xilaan ekstraksie, meer ge-ekstraheerde materiaal in die verteerder gelaai word as wanneer nie-ge-ekstraheerde materiaal gebruik is.

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