ABSTRACT
To explore the design of pervaporation membranes for ethanol recovery, zeolite nanosheets with different surface characteristics on the feed and permeate sides are investigated via molecular dynamics simulations. The results demonstrate the significant role of the permeate-side surface in the separation performance. By adopting an asymmetric membrane design with a hydrophobic feed-side surface and a hydrophilic one on the permeate side, the separation factor can be enhanced by nearly three-fold as compared to that of both hydrophobic surfaces, with an improved permeation flux.
ABSTRACT
As the demand for PET plastic products continues to grow, developing effective processes to reduce their pollution is of critical importance. Pyrolysis, a promising technology to produce lighter and recyclable components from wasted plastic products, has therefore received considerable attention. In this work, the rapid pyrolysis of PET was studied by using reactive molecular dynamics (MD) simulations. Mechanisms for yielding gas species were unraveled, which involve the generation of ethylene and TPA radicals from ester oxygen-alkyl carbon bond dissociation and condensation reactions to consume TPA radicals with the products of long chains containing a phenyl benzoate structure and CO2. As atomistic simulations are typically conducted at the time scale of a few nanoseconds, a high temperature (i.e., >1000 K) is adopted for accelerated reaction events. To apply the results from MD simulations to practical pyrolysis processes, a kinetic model based on a set of ordinary differential equations was established, which is capable of describing the key products of PET pyrolysis as a function of time and temperature. It was further exploited to determine the optimal reaction conditions for low environmental impact. Overall, this study conducted a detailed mechanism study of PET pyrolysis and established an effective kinetic model for the main species. The approach presented herein to extract kinetic information such as detailed kinetic constants and activation energies from atomistic MD simulations can also be applied to related systems such as the pyrolysis of other polymers.
ABSTRACT
Molecular dynamics simulations are employed to demonstrate the potential of zeolite nanosheets as pervaporation membranes for ethanol extraction. Our results show that zeolite nanosheets can provide orders of magnitude higher fluxes compared to currently available membranes and achieve outstanding separation factors. The dominant role of membrane surfaces in determining the separation performance is also identified and explored at an atomic level. Developing nanosheet membranes with hydrophobic surfaces and/or with a minimal surface silanol density represents the keys to enable highly selective separation processes.
