Ethanol Evaporation Steam Furnace Water Raw ethylene Cruede ethylene Aqueous NaOH Chemical grade ethylene Light contaminants Polymer Grade Ethylene REACTION Aqueous effluent QUENCH Caustic effluent SCRUBBING DRYING Heavy contaminants DISTILLATION AND STRIPPING Representation of a generic process diagram of an ethanol-based ethylene plant. way, starch is obtained from corn by dry milling, then slurried with water, and hydrolyzed to glucose. The resulting solution of fermentable sugars obtained by both ways is fermented typically in batch or fed-batch by Saccharomyces cerevisiae yeast to produce a broth with 6 to 8% by weight of ethanol. The fermentation of the sugarcane juice is quite simple, because it can be fermented directly, and faster, taking in general less than 16 hours. By distilling the broth containing ethanol hydrated ethanol, about 93% by weight, is produced. The stillage, the bottom by-product stream of the distillation, is rich in nitrogen and potassium and is commonly recycled to the sugarcane crop by a practice called ferti-irrigation. Energy for the process A large amount of lignocellulosic material is also produced from the sugarcane feedstock. For an average yield of 80– 85 metric tons per hectare and 14% by weight of sugars, it produces, and in addition, 28% by weight of dry lignocelluloses fibers as bagasse and leaves. These fibers can be used to supply renewable heat and electricity to the ethanol process. Cosmetic Bottle (Courtesy Braskem) Its surplus of about 20–40% is used normally to co-generate renewable electricity to the grid and may also be used in other processes when integrated with the ethanol manufacture. If in the future the hydrolysis of hemicelluloses and celluloses would be economically competitive these fibers may be used as an additional source of sugars. As a consequence of these many aspects the energy balance of the sugarcane based ethanol is very favorable. This number is obtained dividing the fossil fuel energy input required by the entire manufacturing process, since the crop plantation, by the energy content of the biofuel output. For the Brazilian sugarcane ethanol the input/output energy balance is 1:9, while for the US corn ethanol this relationship is 1:1.5. Ethanol to Ethylene To generate ethylene from ethanol, you simple need to take the water out (dehydration). C 2 H 5 OH → C 2 H 4 + H 2 O Well, in real life, it is not that simple. The dehydration of alcohols, mainly ethanol, has been studied during the last centuries with different technologies and using a large variety of catalysts such as alumina, silica, silica-alumina, zeolites, clays, metal oxides, phosphoric acid, and phosphates. While older technologies were based on supported phosphoric acid, later activated alumina became predominant as a catalyst. The dehydration reaction is endothermic which means that energy has to be put into the process. The most accepted mechanism for the ethanol dehydration considers a simultaneous reaction: 2 CH 3 CH 2 OH → CH 3 CH 2 OCH 2 CH 3 + H 2 O → 2 H 2 C=CH 2 + H 2 O Ethanol Ether Ethylene Water 2 CH 3 CH 2 OH ─────────────── → 2 H 2 C=CH 2 + 2 H 2 O Ethanol Ethylene Water Diethyl ether is considered an intermediate and not a byproduct. Its formation is favored mainly between 150°C and 300°C, while ethylene formation is predominant between 320°C and 500°C. 54 bioplastics MAGAZINE [05/10] Vol. 5
A simplified generic process diagram of an ethanol-based ethylene plant, based on an isothermal or an adiabatic process, is represented by the schematic on the left. The first of the commercial plants to produce ethylene from ethanol was built and operated at Elektrochemische Werke G.m.b.H at Bitterfeld in Germany in 1913. It was a very small-scale plant that used alumina catalyst in isothermal conditions to produce ethylene for the preparation of pure ethane that was used in refrigeration. From 1930 until the Second World War, ethanol dehydration plants were the unique source of ethylene in Germany, Great Britain, and the United States. The process based on supported phosphoric acid was the basis for very primitive plants for all polyethylene production in England until 1951 Polypropylene While the production of biobased polyethylene is now starting on industrial scale, biobased polypropylene is still under development. Polypropylene is a plastic used in a wide range of everyday products, from food containers, drinking straws, and water bottles to washing machines, furniture, and car bumpers. It is the second most widely used thermoplastic with a global consumption in 2008 of 44 million metric tons. The market is estimated to be USD 66 billion, with an annual growth rate of 4%. Today, polypropylene is primarily derived from oil, but Braskem produced in 2008 in bench scale what is considered the first biobased polypropylene of the world. At the end of 2009, Braskem and the Danish company Novozymes started a partnership to develop a green alternative based on Novozymes’ core fermentation technology and Braskem’s expertise in chemical technology and thermoplastics. The initial development phase will run for at least five years. Conclusion Whilst biobased polypropylene is still a development project, biobased polyethylene is a reality and is already available on industrial scale in grades of high density (HDPE) and linear low density (LLDPE). To make it very clear: Biobased polyethylene (and, once available polypropylene) are NOT biodegradable. On the contrary: biobased PE and PP do not at all differ from petroleum based polyolefins. They have the same chemical structure and can be polymerized the same way. The same grades (film, injection or blow moulding etc) can be created and so on. The only difference is the origin of the carbon. Biobased polyolefins consist of renewable carbon. This can be tested and proven by the radio carbon method ( 12 C versus 14 C) as described in ASTM 6866. Sustainable Banco Imobiliario, a sustainable version of the Monopoly game (Courtesy Braskem) [1] www.wikipedia.org [2] Morschbacker, A. Bio-Ethanol Based Ethylene. Journal of Macromolecular Science®, Part C: Polymer Reviews, 49:79-84, 2009 [3] www.inbicon.com www.braskem.com First products from bio-PP shown at BioJapan 2008. Carpet made of PP homopolymer fibers and stretch blow moulded bottles made of bioPP random copolymer with bio ethylene (Courtesy Braskem) bioplastics MAGAZINE [05/10] Vol. 5 55
