Design of a plant for polylactic acid polymer manufacture

Abstract

Over the last decades, plastic demand has grown increasingly at a level such that it is unavoidable for current society. Plastics versatility and unique properties makes them suitable for all kind of industries, which currently suppose a threat to both environment and human health at every stage, right from their production to their end-of-life scenario. Bioplastics stand out in this way as a greener solution able to reduce carbon footprint, with high recycling value and complete biodegradability or compostability. In this regard, polylactic acid constitutes a biodegradable and bio-based bioplastic which is gaining attention in the recent years, as it possesses the potential to replace conventional fossil-based plastics in food packaging and other applications. Continuous research and development in this field has made possible the polylactic acid mass production at a competitive cost, promising a sustainable future and overcoming the accumulation of plastic waste in landfills. The purpose of this project is the development of the Master’s Degree Final Project with which it is intended to apply for the Master Degree in Chemical Engineering by the “Universidad de Barcelona” The project is focused on the design of a plant for polylactic acid polymer manufacture from lactic acid solutions. Based on a comparison among different polylactic acid market players and the increasing demand reflected in the market analysis, the plant has a fixed annual production capacity of 60,000 tonnes and operates 330 days per year. A selection of the most adequate synthesis of the manufacturing process, according to product necessities, is conducted through a bibliographic research. By means of mass and energy balances, stream flows and operating conditions of each equipment are determined in order to carry out the basic design of the plant, in which the main design parameters and dimensions are detailed. A falling film evaporator type of reactor is selected for the condensation polymerization of lactic acid into polylactic acid of low molecular weight, as it provides the advantages required to optimize the polymer production due to its high surface area to volumetric hold up ratio, which reduces evaporation residence time. A detailed design of this equipment, based on heat transfer and mechanical design, is carried out and reflected in the equipment lay-out. As a final point, the control system of the plant is studied in order to supress the influence of external disturbances, ensuring the stability of the chemical process and the optimization of the process performance. The control system stands for the discussion of the control philosophy and the establishment of the control loops of the plant, described through a P&I diagram which includes the engineering details of the equipment as well as instruments, piping, valves and fittings.