On Mixing and Separation During Polyhydroxyalkanoates Extraction – Yield and Purity Optimization

abstract

English

Polyhydroxyalkanoates (PHAs) are biodegradable thermoplastic polymers naturally produced by various organisms. Cupriavidus necator, which biomass was extracted in this work, is one of the most promising sources. In conjunction with utilizing waste substrates (e.g. used cooking oil) it offers cheap and near-to-zero carbon emission concept. Unfortunately the most economic form of production, which enables PHAs to be competitive with oil-derived plastics, utilizes halogenated hydrocarbons like chloroform. This means full recycling of solvent used in technology to prevent environmental harm. In case of accident, apart from all precautions, the amount of such solvent kept in factory need to be minimized, which means perfect optimization of all processes involved. Optimizing the amount of chloroform needed for effective extraction of unit amount of biomass is also important for product purity. Polymer purity is very important for numerous applications, typical example are biomed applications. Unfortunately traditional polishing treatments (hydroxide or hypochlorite solution) tend to ruin molecular weight hand in hand with mechanical properties. In our work we try to optimize composition of extraction solution, extraction time and temperature, and mixing patterns in order to reach highest yield of really pure (>99,5 % PHB in dry mass) polymer, while maintaining the use of chloroform at its necessary minimum. We took 32 factorial design enhanced with two center point repetitions for assessment of optimal composition of extraction solution with respect to water:residual biomass ratio and chloroform:residual biomass ratio. Data was analyzed using standard response surface methodology. Experiments were carried in Biostat 2L fermentor using two 6 blades Ruston turbines. We found that the amount of chloroform is not limited from above, more chloroform added means higher yield and higher purity. The only limits are economical. In case of water addition there exist an optimum, at which residual biomass is not dispersed in such a way, that leads to unnecessary destruction of cell components with final result of phase inversion, i.e. hard-to-separate emulsion system. With three different compositions (near the optimum find above) we tried to find optimal temperature of extraction in the range from 45 to 70 °C. We found that best results were obtained near boiling point of each extraction mixture. Use of higher temperatures leads to higher yields but also to massive decrease in purity. Last influence, but really not least, is the type and number of impellers used, speed of agitation and time of extraction. When trying to shorten extraction time as possible it was found that simple Rushton turbines are inappropriate, leading to inadequate purity. Combination of turbines, with enough shear to break biomass clusters, and axial type impellers (e.g. hydrofoils), to create sufficient overall mixing pattern to prevent cavern formation in viscous mixture, is necessary to compromise between yield, purity and time of extraction

polyhydroxyalkanoates; extraction; mixing; optimization

VANĚK, M.; TOMALA, L.; SZOTKOWSKI, M.; NOVÁČKOVÁ, I.; OBRUČA, S.

14. 9. 2018

978-80-214-5488-0

7th Meeting on Chemistry and Life 2018. Book of abstracts

1