Increasing the stream of recycled plastic necessitates an approach beyond the traditional recycling via melting and re‐extrusion. Various chemical recycling processes have great potential to enhance ...recycling rates. In this Review, a summary of the various chemical recycling routes and assessment via life‐cycle analysis is complemented by an extensive list of processes developed by companies active in chemical recycling. We show that each of the currently available processes is applicable for specific plastic waste streams. Thus, only a combination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams, while collection and sorting infrastructure will need to be improved, that is, by stricter regulation. This Review aims to inspire both science and innovation for the production of higher value and quality products from plastic recycling suitable for reuse or valorization to create the necessary economic and environmental push for a circular economy.
Plastic fantastic: Plastic can rise again and again as a new product. Researchers now know methods with which new plastics can be produced from 100 % recycled material, products that can even be used for food applications. This development is possible thanks to chemical recycling, through which polymer chains are first broken to then be reformed into new molecules, such as plastics but also other chemicals.
Co-crystallization is known as a product formulation technology, but it can also be used as a tool to solve crystallization problems. Product removal by co-crystallization in fermentations is used as ...a showcase to demonstrate the potential of co-crystallization as a separation technique. In fermentations, the product solubility is often higher than the limiting concentration. Hence, the solubility should be decreased to below this limiting concentration to enable product removal by crystallization. Co-crystallization has the potential to accomplish this goal since co-crystals can have solubilities lower than pure component solubilities. Cinnamic acid (CA) forms co-crystals with 3-nitrobenzamide (NBA). The phase diagram of CA, NBA, and water shows that CA-NBA co-crystals can be formed at CA mole fractions seven times below the solubility of pure CA, facilitating the removal of CA from the solution at concentrations well below the pure CA product solubility. Co-crystal fine-tuning may lead to a substantially extended decrease of the CA co-crystal product solubility, so that product removal by co-crystallization can take place at even lower product concentrations. Co-crystallization experiments in a simulated fed-batch fermentation were performed by slowly adding a saturated solution of CA, simulating the production of CA in the fermentation process, to a suspension of pure co-former NBA. In time, the co-former crystals completely transform to CA-NBA co-crystals. During the transformation, a stationary state is achieved in which the CA and NBA mole fractions are constant because the added CA, together with the NBA from the dissolving co-former crystals, is captured in the co-crystals. In the stationary state, the process is operated near the 3-phase equilibrium point with equilibrium between co-crystals, co-former crystals, and solution. Co-crystals thus not only offer a route toward fine-tuned crystal properties of active pharmaceutical ingredients. This paper shows that, moreover, co-crystals offer solutions to crystallization problems.
During the antisolvent crystallization of l-histidine from aqueous solution with the antisolvent ethanol, the polymorphic fraction of the metastable polymorph B increased with increasing ...supersaturation ratio. For supersaturation ratio S A ≤ 2.0, a mixture of the stable polymorph A and the metastable polymorph B was obtained, while for high supersaturation, S A ≥ 2.3, only form B was observed. The supersaturation ratio was increased either by increasing the l-histidine concentration in the aqueous solution or by increasing the ethanol volume fraction. The induction time for the antisolvent crystallization was one to several orders of magnitude smaller than the time to transform the metastable polymorph B into stable polymorph A in solution, indicating that both polymorphs formed simultaneously. Process simulation of the batch antisolvent crystallization of the polymorphs of l-histidine was applied to predict the outcome of the competition between heterogeneous nucleation and polynuclear growth rates for both polymorphs. When the interfacial energy was calculated as a function of the solubility, only the formation of the stable polymorph A was erroneously predicted. By modification of the relative interfacial energy and the relative step free energy, the predicted polymorphic fraction could be directed toward the observed polymorphic fraction.
Das Vergrößern des Recyclingstroms erfordert einen neuen Ansatz, der über das Schmelzen und Umformen hinausgeht. Es gibt einige Techniken des chemischen Recyclings mit dem Potential, bestehende ...Recyclingmöglichkeiten zu ergänzen. In diesem Aufsatz sind die Methoden des chemischen Recyclings dargestellt und anhand einer Ökobilanz bewertet, ergänzt durch eine Aufzählung von Prozessen und Firmen in diesem Bereich. Wir zeigen, dass bestimmte Techniken besonders für spezifische Müllströme geeignet sind und dass nur eine Kombination aus den vorhandenen Methoden geeignet ist, das Kunststoffmüllproblem zu lösen. Aktuelle Forschung sollte realistischeren und weniger reinen Mischströmen größere Aufmerksamkeit widmen, während Trenn‐ und Sortierprozesse z. B. durch effektivere Regularien verbessert werden müssen. Dieser Aufsatz soll zur Entwicklung von Verfahren inspirieren, mit denen hochwertige Produkte hergestellt werden können, die die Kreislaufwirtschaft antreiben, indem sie nötige Wirtschaftsanreize und Erleichterungen für die Umwelt bieten.
Kunststoffmüll als wertvoller Ausgangsstoff: Wissenschaftler kennen seit Jahren Methoden, mit denen sich neue Kunststoffe aus 100 % zurückgewonnenem Material herstellen lassen, die selbst für Lebensmittelanwendungen verwendbar sind. Dies ist möglich durch chemisches Recycling, bei dem Polymerketten erst aufgespalten werden, um zu neuen Verbindungen, wie Kunststoffen oder anderen Chemikalien, verwertet werden zu können.
For precipitation processes, measurement of the nucleation rate as a function of supersaturation is not so easy because the formation of the precipitate is fast and often hard to control due to the ...high supersaturation. For a number of studies on both ionic and molecular precipitating compounds, the experimental methods are discussed. From the experimentally determined nucleation rates, the probable nucleation mechanism, homogeneous or heterogeneous, is derived and compared to the theoretically expected mechanism. For all ionic compounds and for one molecular compound considered in this work, theoretically homogeneous nucleation is achievable. However, only for two ionic compounds, barium sulfate and boehmite, the measurements possibly indicated a homogeneous nucleation mechanism. For the other studies, including all molecular compounds, a heterogeneous mechanism was more likely. The difference between a theoretically expected and experimentally observed mechanism may be explained by not only the use of less suitable experimental methods and measurement techniques but also by secondary processes, such as agglomeration and transformation, taking place in the precipitated suspension. On the basis of these findings, an experimental procedure is suggested to standardize and to improve nucleation rate measurements in precipitation.
Measurement of the stationary nucleation rate as a function of supersaturation in reactive precipitation requires that nucleation and growth of the crystals to an observable size take place under ...well-controlled hydrodynamic conditions and at a constant supersaturation. An experimental method was chosen that is based on the measurement of the increase of the particle concentration with the increase of the residence time in a precipitation tube. This method was applied to measure the nucleation rate for the pH shift precipitation of the molecular compound H4EDTA. The parameters A and B in the classical nucleation theory equation were derived from the measured nucleation rates. A value of A = (5.7 ± 1.0) × 1015 m-3 s-1 was found for the intercept with the ordinate and a value of B = (3.3 ± 2.0) × 102 was found for the slope. The estimated value for parameter A is lower than the theoretical value of A HON = 1033 m-3 s-1 for homogeneous nucleation. Using the experimentally determined value for B the interfacial energy is found to be γ = 21 ± 9 mJ m-2 compared to a theoretical value of γHON = 38 mJ m-2 based on bulk properties. The experimentally obtained values for A and γ indicate heterogeneous nucleation.
Das Vergrößern des Recyclingstroms erfordert einen neuen Ansatz, der über das Schmelzen und Umformen hinausgeht. Es gibt einige Techniken des chemischen Recyclings mit dem Potential, bestehende ...Recyclingmöglichkeiten zu ergänzen. In diesem Aufsatz sind die Methoden des chemischen Recyclings dargestellt und anhand einer Ökobilanz bewertet, ergänzt durch eine Aufzählung von Prozessen und Firmen in diesem Bereich. Wir zeigen, dass bestimmte Techniken besonders für spezifische Müllströme geeignet sind und dass nur eine Kombination aus den vorhandenen Methoden geeignet ist, das Kunststoffmüllproblem zu lösen. Aktuelle Forschung sollte realistischeren und weniger reinen Mischströmen größere Aufmerksamkeit widmen, während Trenn‐ und Sortierprozesse z. B. durch effektivere Regularien verbessert werden müssen. Dieser Aufsatz soll zur Entwicklung von Verfahren inspirieren, mit denen hochwertige Produkte hergestellt werden können, die die Kreislaufwirtschaft antreiben, indem sie nötige Wirtschaftsanreize und Erleichterungen für die Umwelt bieten.
A relation between template properties and template performance has been established: template particles with a surface layer of cinnamic acid molecules, either adsorbed or covalently bonded, promote ...crystallization of cinnamic acid, while particles without such a layer do not. The BET surface area and dynamic light scattering (DLS) particle size were measured to gain control over the total surface area of added template particles (ZrO2, TiO2, TiO2−SiO2, and SiO2−CAH). Zeta-potential measurements indicated whether adsorption of cinnamate on particles occurred, while the loading of the surface layer with cinnamic acid molecules was determined by adsorption isotherm measurements and spectrophotometry. The template performance was evaluated using induction time measurements. Scanning electron microscopy (SEM) images revealed that effective templates not only improve nucleation rates, but may also affect the size and shape of crystals. Template induced crystallization (TIC), where template particles promote crystallization, can be used as an in-situ product removal (ISPR) tool in fermentation processes to alleviate inhibitory and/or toxic constraints which are due to high product titers. The established relation between template properties and template performance can be used to predict which materials are effective templates for a specific (fermentation) product.
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