The processing of wax and wax additives with supercritical fluids By Cara Elsbeth Schwarz Dissertation presented for the Degree of DOCTOR OF PHILISOPHY (Chemical Engineering) In the Department of Process Engineering At the University of Stellenbosch Promoted by Prof. I. Nieuwoudt Prof. J.H. Knoetze Stellenbosch December 2005 Declaration I, the undersigned, hereby declare that the work contained in this thesis is my own original work and that I have not previously in its entirety or in part submitted it at any university for a degree. Cara Elsbeth Schwarz 16 September 2005 i ii Abstract Waxes have many potential uses but large-scale application is hampered by their virtual insolubility. By grafting the wax with a polyethylene glycol segment to form an alcohol ethoxylate, the solubility of the wax in commercial solvents is significantly increased. Alcohol ethoxylates are produced by the polymerisation addition of ethylene oxide onto an oxidised wax. Current methods of alcohol ethoxylate production from alcohols lead to wide ethylene oxide addition distribution and large quantities of residual alcohol. The objective of this study is to provide a method for narrowing the ethylene oxide distribution and to reduce the residual alcohol content. It is proposed to concentrate the alcohol ethoxylate in a post-production separation process using supercritical fluid extraction. The system is modelled to contain three pseudo-components: an alkane, an alcohol and an alcohol ethoxylate. Propane is selected as the supercritical solvent of choice due to the large solubility difference between the alkane and polyethylene glycol. Lower molecular weight alkane phase equilibrium data with propane is abundant but extrapolation to higher molecular weights requires further investigation as it may be complicated by molecular folding. Molecular folding occurs in crystalline polyethylene and high molecular weight normal alkanes but information regarding molecular folding in solution is inconclusive. A model is proposed for molecular folding of normal alkanes in supercritical solution. A high molecular weight alkane mixture is synthesised and phase equilibrium measurement with propane are conducted. A lower molecular weight alkane mixture is used to prove the application of the principle of congruency to high-pressure phase equilibria. In the high wax mass fraction region the measurements are between the no-folding and once-folded relationship, indicating the possibility of partial molecular folding. In the mixture critical and low wax mass fraction region the measurements are similar to the non-folding relationship. Molecular folding in solution is thus dependent on the solution concentration. No phase equilibria measurements exist for propane with either high molecular weight alcohols or alcohol ethoxylates. Measurements of propane with an alcohol mixture show total solubility below 140barA for temperatures up to 408K. Measurements of propane with an alcohol ethoxylate at temperatures between 378 and 408K shows that for an alcohol ethoxylate mass fraction between 0.025 and 0.5 pressures greater than 275barA are required for solubilisation. iii When comparing the solubility of the three pseudo-components, the alkane is the most soluble followed by the alcohol. The alcohol ethoxylate is the least soluble. A counter-current supercritical extraction process is proposed for the concentration of the alcohol ethoxylate. Pilot plant tests were conducted and the proposed set-up shows good separation. An estimate of the energy requirements shows that heating and cooling constitute the majority of the energy required but with the use of heat integration it can be reduced by approximately 33%. This work thus shows that the proposed process is both technically and economically viable. Although this work has provided a method for concentrating the alcohol ethoxylate, the process has not been optimised yet and future work includes the fine-tuning of this process. iv Opsomming Sintetiese wasse het baie potensiële gebruike maar die feit dat hulle byna onoplosbaar is belemmer grootskaalse aanwendings. Die oplosbaarheid van die was kan beduidend verhoog word deur ‘n poliëtileenglikol segment aan die was molekule te bind om sodoende ‘n alkohol etoksilaat te vorm. Alkohol etoksilate word produseer deur die polimerisasie addisie van etileenoksied aan geoksideerde was. Huidige alkohol etoksilaat produksiemetodes lei tot ‘n wye etileenoksied verspreiding en ‘n groot hoeveelheid residuele alkohol. Die doel van hierdie studie is die ontwikkeling van ‘n metode om die etileenoksiedverspreiding te vernou en om die residuele alkohol te verlaag. Daar word voorgestel om die alkohol etoksilaat te konsentreer in ‘n stroomaf skeidingsproses deur van superkritiese ekstraksie gebruik te maak. Die sisteem word gemodelleer as drie pseudokomponente naamlik, ‘n alkaan, ‘n alkohol en ‘n alkoholetoksilaat. Propaan is gekies as die mees geskikte oplosmiddel as gevolg van die groot oplosbaarheidsverskil tussen die alkaan en polietileenglikol in superkritiese propaan. Fase-ewewigs data van propaan met lae molekulêre massa alkane is volledig verkry, maar verdere ondersoek word benodig voor ekstrapolasie na hoër molekulêre massas kan plaavind aangesien dié fase-ewewig moontlik deur molekulêre vou beïnvloed kan word. Molekulêre vou kom voor in kristallyne polietileen and hoë molekulêre massa alkane maar inligting aangaande molekulêre vou van dié komponente in oplossing is onbeslissend. ‘n Model word voorgestel vir molekulêre vou in alkane in superkritiese oplossing. ‘n Hoë molekulêre massa alkaanmengsel is gesintetiseer en fase-ewewigsmetings met propaan is gedoen. ‘n Lae molekulêre massa alkaanmengsel is gebruik om die toepassing van die beginsel van kongruensie op hoëdrukfase-ewewigte te bewys. In die hoë was massafraksiegebied lê die metings tussen die geen-vou- en enkel-vouverhouding en dui op die moontlikheid van gedeeltelike molekulêre vou. In die mengsel kritiese- en lae was massafraksiegebied is die metings soortgelyk aan die geen-vouverhouding. Molekulêre vou is dus afhangklik van die oplosmiddelkonsentrasie. Geen fase-ewewigsmetings bestaan vir propaan met of ‘n hoë molekulêre massa alkohol of alkoholetoksilaat nie. Metings met propaan en ‘n alkoholmengsel wys totale oplosbaarheid onder 140barA vir temperature tot en met 408K. Metings van propaan met ‘n alkoholetoksilaat by temperature tussen 378 en 408K toon dat tussen massafraksies tussen 0.025 en 0.5 drukke groter as 275barA benodig word vir totale oplosbaarheid. v Wanneer die oplosbaarheid van die drie pseudokomponente vergelyk word, is die alkaan die mees oplosbare, gevolg deur die alkohol. Die alkoholetoksilaat is die minste oplosbaar. ‘n Teenstroom superkritiese eksktraksieproses word voorgestel om die alkoholetoksilaat te konsentreer. Proefaanlegskaal toetse is gedoen en die opstelling gee ‘n goeie skeiding. ‘n Beraming van die energie benodig wys dat verhitting en verkoellings energie die grootste komponente is en dat met die gebruik van energie integrasie die benodigde energie met ongeveer 33% verlaag kan word. Die werk wys dus dat die voorgestelde proses beide tegnies en ekonomies lewensvatbaar is. Dié werk het ‘n metode vir die konsentrering van alkoholetoksilaate ontwikkel en verdere werk sal die optimeering van die proses insluit. vi Acknowledgements The financial assitance of the Department of Labour (DoL) towards this research is hereby acknowledged. I thank the Harry Croxley Foundation for providing finance for my stay in at the TUHH, Germany in 2003 and SASOL for providing the pilot plant for the extraction experiments. Many people have helped me during the past four years while undertaking this project. My sincere thanks to you all, yet the following deserve special mention: • To my promotor, Prof. Nieuwoudt, for introducing me to the topic and all the guidance you have provided. • To my co-promotor, Prof. Knoetze, for all the support and guidance you have provided. • Prof. Brunner and the Thermal Separations Group at the Technical University of Hamburg- Harburg for a fulfilling stay at your group from January to May 2003. • Jannie Barnard, Anton Cordier and Howard Koopman from the mechanical workshop who dealt with my sometimes-impossible requests and for everything they constructed, fixed or modified for me. • Fransien Kamper for all the orders she placed and all the running after the suppliers to get what I wanted within a reasonable time. • To my second pair of hands, Vincent Carolissen. Thank you for going beyond the call of duty in all the assistance you gave me both during the synthesis of my alkane mixture and the construction and operation of my pilot plant. • To my parents, for always believing in me, for always being there and for the endless support, encouragement and love they have always shown towards me. vii viii The journey of a thousand miles begins with one step. Lao-Tsze x Contents DECLARATION I ABSTRACT III OPSOMMING V ACKNOWLEDGEMENTS VII CONTENTS XI 1 INTRODUCING SYNTHETIC WAX AND ITS DERIVATIVES 1 1.1 Synthetic waxes .......................................................................................................... 1 1.2 Wax derivatives........................................................................................................... 1 1.3 Problem statement and ultimate target....................................................................... 3 1.4 Aims of this work......................................................................................................... 3