Process steps for PLA generation

The process of converting sugar into PLA plastic involves several steps (Fig. 3). First, sugar is fermented with robust and efficient non-genetically modified (GMO) strains to generate lactic acid (LA). Second, the LA is polymerized through a polycondensation reaction to a prepolymer with low molecular weight (Fig. 5). The polycondensation reactors, thanks to their flexible operating conditions, help to remove by-products and maximize the yield. The low molecular weight prepolymer is then converted, through a catalytic reaction, to lactide. The lactide is then purified with Sulzer distillation and crystallization equipment before being polymerized.

After being fed to a series of loop and plug-flow reactors, the lactide is converted to PLA. Thanks to the proprietary devolatilization devices of Sulzer, the remaining volatile components are separated by degassing the PLA melt. Depending on the final product application, the last mixer is used to add color or additives to the melt. As the last step, PLA is converted into solid pellets in order to be transported or stored.

Integrated PLA technology

With over 25 years of experience in lactic acid and lactide purification and polymerization, Sulzer and its partners can deliver single, key equipment solutions for PLA manufacturing. In addition, Sulzer designs and implements key equipment and integrated modular solutions (Fig. 4) for individual steps of the polymerization. Sulzer Chemtech also offers several additional services to ensure that customers in the agricultural, chemical and fiber sectors can continuously benefit from their integrated PLA technology. More precisely, installation supervisors and start-up engineers provide support in the assembly, installation, commissioning and start-up of the equipment.

Additionally, the operation and maintenance training offered by Sulzer helps plant operators to thoroughly understand the system. Finally, to assist customers in maximizing the performance and service life of their equipment, the company also provides recommendations and guidelines on the best maintenance practices.

Scale-up proven in industrial use

With the support of Sulzer, one of the world’s top two PLA manufacturers developed a large plant to produce 75’000 tons per year of PLA plastics, including heat-resistant PLA composites, for a wide range of applications. Specialized and highly qualified teams performed extensive laboratory and pilot testing and proposed a customized design that met the plant and manufacturing requirements. Thanks to the support provided by Sulzer Chemtech’s experts, the PLA manufacturer has been able to create one of the largest PLA plants in the world, and deliver high-quality bioplastic solutions for a greener future.

Controlled biodegradability

One of the main benefits of the Sulzer technology is that with the same installation, the properties of the polymer can be adapted easily. There is no need for different process lines. With the Sulzer technology, the monomer ratio and the molecular weight of the PLA can be set precisely according to the requirements of the market applications.

This flexibility of the process lines is important because the specific setting influences the speed of biodegradability of PLA-based products. The biodegradability is controlled by adjusting the relative amounts of D- and L-lactides. In fact, PLA polymers with high amounts of the D-configuration are easily biocompostable; PLA polymers with high amounts of L-lactides are not. As a result, PLA-based materials with higher amounts of D-lactides are suitable for disposable applications, e.g., food packaging with a short shelf life, whereas PLA-based materials with higher concentrations of L-lactides are required for more durable solutions, e.g., electronic components. Thanks to Sulzer’s smart measurement control and precise heat settings, the amount of L- and D-lactides in the PLA pellets can be easily adjusted.

Key equipment for PLA

Some of the key equipment developed by Sulzer as part of its PLA production offering includes falling film crystallizers, the loop and plug-flow reactor (SMX^TM), Sulzer Mixer Reactors (SMR^TM) distillation and degassing technologies (Fig. 5). In addition, these solutions are fully scalable and ideal to accommodate variable production volumes, allowing manufacturers to expand their business up to 100’000 tons per year.

Main benefits of Sulzer’s key equipment

The most important factor for good polymerization is that the process temperature is maintained at a controlled level and that the feed material is thoroughly mixed. This is more difficult in conventional polymerization, where the dimerization process and polymer reaction take place in a continuously stirred tank.

Sulzer can offer special mixer reactors that combine these two processes: mixing and heating or cooling. These reactors have a smaller diameter than the tank and provide uniform temperature distribution. The tubes inside the mixer heat exchanger are filled with a transfer media, depending on the application, and allow a regular temperature transfer. The transfer tubes are shaped in a special form which ensures the material passing through the reactor is mixed (Fig. 6).