Optimization of biodiesel production process from waste oil by using new guanidine catalysts

Abstract

Kako bi se unaprijedila proizvodnja biodizela s ekonomskog i ekološkog gledišta potrebno je istražiti primjenu novih katalizatora, ekološki prihvatljivijih i ekonomski isplativijih sirovina te optimirati proces s istima. Konvencionalni katalizatori poput NaOH i KOH kada se primjenjuju na biljna ulja s većim sadržajem slobodnih masnih kiselina i vode dovode do saponifikacije koja bitno otežava separaciju produkata i povećava proizvodnju otpadne vode. U isto vrijeme primjena biljnih ulja (posebice jestivih) negativno utječe na tržište hrane te smanjuje količinu iskoristivih poljoprivrednih površina. Osim toga, najčešće korišteni alkohol za transesterifikaciju, metanol, proizvodi se iz neobnovljivih izvora (povećava emisiju stakleničkih plinova) i toksičan je. Iz tih razloga u ovom radu ispitana je aktivnost novog gvanidinskog katalizatora N,N',N''-tris(3-dimetilamino)propil gvanidina (GV3) uz kojeg ne dolazi do saponifikacije, a kao sirovine korištene su otpadno restoransko ulje i niži alkoholi, metanol, etanol, n-butanol i i-butanol. Ovdje je posebice zanimljiva sirovina etanol kojeg je moguće proizvesti iz obnovljivih izvora (bioetanol). Shodno tome provedeno je modeliranje i optimizacija šaržnog procesa proizvodnje etilnih estera višemasnih kiselina (FAEE) transesterifikacijom otpadnih ulja s etanolom pomoću GV3 za najvažnije procesne parametre (maseni udio katalizatora, molarni omjer etanola i otpadnog ulja, temperatura i vrijeme provedbe reakcije) metodom planiranja pokusa s faktorskim centralno kompozitnim planom. Kao rezultat razvijen je vjerodostojan empirijski matematički model. Pomoću metode odzivnih površina (RSM) utvrđeni su optimalni uvjeti procesa za zadane ciljeve. Ciljevi optimiranja bili su postizanje maksimalne konverzije uz minimalan udio katalizatora i minimalnu temperaturu s omjerom reaktanata što bližim stehiometrijskom te što kraćem vremenu. Ovisno o zadanoj važnosti pojedinog cilja dobiveni su različiti optimalni uvjeti od kojih su odabrani: maseni udio katalizatora od 1,72 mas. %; molarni omjer etanola i otpadnog ulja od 5,35:1; temperatura od 40 °C i vrijeme od 40 min s predviđenom konverzijom od 91,44 %.In order to improve the production of biodiesel from an economic and environmental point of view, it is necessary to research the application of new catalysts, environmentally friendlier and cost-effective raw materials and their process optimization. Conventional catalysts, i.e. NaOH and KOH, when applied to vegetable oils with higher content of free fatty acids and water, usually lead to saponification. The resulting soap makes the separation of products difficult and thus increase waste water production. Furthermore, the use of edible vegetable oils has a negative impact on the food market by reducing the quantity of available farmland and increasing the price of food. Methanol is the most commonly used alcohol for transesterification. But it is toxic and produced from non-renewable sources, increasing greenhouse gas emissions. For this reason, the activity of the new guanidine catalyst N,N',N''-tris(3-dimethylaminopropyl)-guanidine (GV3) for the tansesterification of waste oil with various lower alcohols (methanol, ethanol, n-butanol and i-butanol) were examined. Ethanol is particularly interesting as a raw material because it can be produced from renewable sources, i.e. bioethanol. The process optimization with ethanol and waste oil was carried out for the most important process parameters (mass fraction of the catalyst, the molar ratio of ethanol and waste oil, reaction temperature and time) in accordance with the factorial central composite design of experiments. As result the significant empirical mathematical model was developed. Using Response Surface Methodology (RSM) optimal conditions were determined for the tested parameters. The goals of the optimization were to maximize the conversion, minimize the mass fraction of catalyst, at the lowest temperature and the shortest reaction time with the molar ratio of ethanol and waste oil closer to stechiometric. Depending on the importance given to a individual goal, different optimal conditions were calculated. The chosen optimal conditions were: the catalyst mass fraction of 1.72 wt.%; the molar ratio of ethanol and the waste oil 5.35:1; reaction time of 80 min at 40 °C with the calculated conversion of 91.44 %