Design of a Novel Process for Continuous Lactide Synthesis from

Subscriber access provided by University of Sussex Library

Process Systems Engineering

Design of a Novel Process for Continuous Lactide Synthesis from Lactic Acid Jinwoo Park, Hyungtae Cho, Dong Won Hwang, Seungnam Kim, Il Moon, and Myungjun Kim Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02419 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Design of a Novel Process for Continuous Lactide Synthesis from Lactic Acid Jinwoo Parka, Hyungtae Choa, Dongwon Hwangb, Seungnam Kima, Il Moona, and Myungjun Kima,* a

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea

b

Green Carbon Catalysis Research Group, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea

KEYWORDS: Process simulation, Lactide, Process flow diagram, Process design, Process modeling

ABSTRACT: Lactide synthesis is an energy-intensive process used to produce polylactic acid (PLA). In this study, we propose a continuous lactide synthesis process developed on the basis of a recently developed one-step reaction using a SnO2–SiO2 nanocomposite catalyst. This process enables a rapid reaction time of 40 ms for the catalyst, which achieves 94% lactide yield. To design an efficient chemical process, reaction kinetics were developed and a heterogeneous reactor model was applied to the reactor. The proposed process has advantages over commercial processes, having both a rapid reaction rate and high yield. Moreover, its rapid reaction rate significantly increases the productivity. In addition, this process operates under atmospheric pressure, which makes it more energy efficient than commercial processes that operate under high vacuum pressure of 20 mm Hg. Therefore overall process is an effective alternative for lactide production.

ACS Paragon Plus Environment

1

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

1. Introduction Bioplastic is expected to gain importance in future because of the increasing demand for solutions that resolve or alleviate environmental issues.1, 2 Polylactic acid (PLA) is a natural, degradable plastic that is increasingly being used in biomedical applications.3-8 The global PLA market is expected to reach USD 2,169.6 million by 2020.9 NatureWorks is the largest global producer of PLA, with current production of 150,000 tons annually,10 and is the world leader in the field of lactic polymer technology.11 To produce high molar mass PLA from lactic acid (LA), commercial process uses a reaction route utilizing ring-opening polymerization (ROP) through a lactide intermediate.12 By rigorously controlling the ROP with a lactide, relatively high-quality PLA can be obtained.13 Figure 1 shows the PLA process currently used comprising (1) a prepolymer section, (2) a lactide section, and (3) an ROP section.14, 15 In the prepolymer section, low-molecular-weight prepolymers are produced by removing the water. Next, lactide is produced by reaction and distillation process in the lactide section. Finally, the lactide forms PLA in the ROP section.16


2

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic diagram of the polylactic acid (PLA) process in current usage

This process focuses on the prepolymer and lactide sections, which are referred to collectively as the energy-intensive section.17 The current commercial process consumes a relatively large amount of energy because it is conducted under a high vacuum pressure of 20 mm Hg.18, 19 Furthermore, lactide distillation creates impurities that lower the final quality of the PLA.20, 21 To resolve the drawbacks associated with the commercial process, Dusselier et al. developed an H-beta zeolite catalyst and an improved manufacturing process.22 They integrated the prepolymer and lactide reactions into a one-step reaction to produce an 85% reaction yield. The reaction was conducted in the liquid phase to reduce the energy needed for the reaction. However, the productivity of this liquid-phase reaction is not as substantial as that of the commercial process; the liquid-phase reaction requires more than 3 h, whereas the reaction used by commercial process is completed in less than 10 min. Moreover, Dusselier et al. used toluene as a solvent; the use of this material is generally avoided in medicinal chemistry.23 Upare et al. (2016) developed a catalytic system using a one-step gas-phase reaction at Korea Research Institute of Chemical Technology (KRICT).24 This system produced a 94% reaction yield of lactide with almost 100% enantioselectivity; the yield of the commercial process at reaction is only 75%.21, 24 The new catalyst, SSO-80, comprises nanocomposite SnO2–SiO2 and is stable for a long period of more than >2,500 h.24 Moreover, the time of the gas-phase reaction requires only about 40 ms. Because the reaction rate of this method is significantly faster than that of the commercial PLA process, at approximately 10 min, the catalytic system requires the development of a new process scheme that considers reaction kinetics. Therefore, Yonsei University and KRICT developed a novel process scheme for the rapid gas-phase catalytic system conducted under atmospheric conditions; the commercial process requires high vacuum pressure.


3


Page 4 of 26

This study is structured as follows. First, the reaction pathways and kinetics are introduced along with the calculated reaction rates and reactor modeling; then the entire process scheme and all equipment are described; and finally the proposed process is compared with the commercial process.


4

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. Reaction The reaction pathways that produce lactide from LA dehydration are shown in Figure 2.24 LA is polymerized to form a lactic acid dimer (LA2) having two different reaction routes. The first is polymerization, which forms a lactic acid trimer (LA3), and the second is dehydration, which results in the formation of lactide. Two types of lactide stereoisomer are formed after the reaction: L-lactide (L-LT) and meso-lactide (M-LT). L-LT is easily transformed into M-LT when heated.

Figure 2. Reaction pathways for lactide synthesis from lactic acid (LA)

The chirality of lactide is an important factor in the quality of a PLA.25, 26 L-LT is always preferred because PLA from L-LT has higher mechanical strength than that of M-LT.18, 27 However, L-LT is chemically unstable; therefore, it can react with H2O or LA. H2O causes a reverse reaction that re-forms the dimer, whereas LA builds a trimer that is continuously polymerized to form LAn (n > 3). Owing to these reaction pathways, it is difficult to produce optically pure L-LT. In order for the yield of L-LT to be


5


Page 6 of 26

maximized, high conversion rates of LA and LA2 are required while the production of undesirable byproducts is minimized. The SSO-80 catalyst makes it feasible for a high yield of L-LT to be obtained and enables a rapid reaction rate. The experiment data were acquired by conducting the measurements under atmospheric pressure and at 240 °C, which are the same conditions as those used in previous studies.24 Table S1 described the gas chromatography analysis results and experimental methods are described in supporting information. In the experiment, LA3 and LAn were not detected during short residence times of

Design of a Novel Process for Continuous Lactide Synthesis from

Recommend Documents