Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Microplastics, which are plastic debris with a particle diameter of less than 5 mm, have attracted growing attention in recent years. Its widespread distributions in a variety of habitats have urged scientists to understand deeper regarding their potential impact on the marine living resources. Most studies on microplastics hitherto are focused on the marine environment, and research on risk assessment methodology is still limited. To understand the distribution of microplastics in urban rivers, this study investigated river sediments in Shanghai, the largest urban area in China. Seven sites were sampled to ensure maximum coverage of the city's central districts, and a tidal flat was also included to compare with river samples. Density separation, microscopic inspection and μ-FT-IR analysis were conducted to analyze the characteristics of microplastics and the type of polymers. The average abundance of microplastics in six river sediment samples was 802 items per kilogram of dry weight. The abundance in rivers was one to two orders of magnitude higher than in the tidal flat. White microplastic spheres were most commonly distributed in river sediments. Seven types of microplastics were identified, of which polypropylene was the most prevailing polymers presented. The study then conducted risk assessment of microplastics in sediments based on the observed results, and proposed a framework of environmental risk assessment. After reviewing waste disposal related legislation and regulations in China, this study conclude that in situ data and legitimate estimations should be incorporated as part of the practice when developing environmental policies aiming to tackle microplastic pollution.

Microplastics in Freshwater River Sediments in Shanghai, China: A Case Study of Risk Assessment in Mega Cities

The chemistry of chemical recycling of solid plastic waste via pyrolysis and gasification: State-of-the-art, challenges, and future directions

https://par.nsf.gov/servlets/purl/10172004

Valorization of hydrothermal liquefaction aqueous phase: pathways towards commercial viability

Lignocellulosic feedstocks such as forestry biomass and agricultural crop residues can be utilized to generate biofuels and biochemicals. In addition, a large amount of non-plant residues or biogenic wastes is also generated worldwide that has huge potentials but remains underutilized. Converting these organic waste materials through thermochemical and biochemical processes into biofuels is widely regarded as a remedial approach to address waste management and clean energy problems. One of such thermochemical biomass-to-gas technologies is hydrothermal gasification in the presence of subcritical or supercritical water, which utilizes the unique fluid properties of water to disintegrate effectively the organic wastes into hydrogen-rich syngas. This paper reviews the thermophysical chemistry of subcritical and supercritical water as well as their role in generating syngas from the hydrothermal decomposition of biogenic, polymeric and organic wastes such as municipal solid waste, animal manure, food waste, industrial effluents, sewage sludge, mixed plastics, waste tires and petrochemical wastes. The paper also describes different technologies used for syngas cleaning and conditioning together with gas-to-liquid technology such as Fischer-Tropsch synthesis to produce hydrocarbon fuels. The current progress, challenges and knowledge gaps in the research and development of hydrothermal gasification of biogenic wastes are also discussed.

A review on subcritical and supercritical water gasification of biogenic, polymeric and petroleum wastes to hydrogen-rich synthesis gas

Experimental investigation on gasification characteristics of plastic wastes in supercritical water

Experimental investigation on gasification characteristics of polyethylene terephthalate (PET) microplastics in supercritical water

In order to solve the problem of marine microplastics and realize the harmless resource utilization of plastics, the gasification experiments of polycarbonate (PC) microplastics were carried out in supercritical water and a novel seawater gasification of microplastic experiment was investigated. In this paper, the effects of different operating conditions (temperature, time, feedstock concentration, pressure) on gasification performance were discussed. The gasification kinetic of microplastics in supercritical water was calculated. The experimental results showed that the increase in gasification temperature and time enhanced the cracking reaction and free radical reaction of the microplastics to increase the gasification efficiency, while the reduction in feedstock concentration improved the gasification efficiency by increasing the gasification level of unit feedstock. The change in pressure had no significant effect on gasification due to the fact that the properties of the supercritical water were not significantly changed. It was found that the valuable results that all alkali metal salts in seawater promote hydrogen conversion, while in terms of carbon conversion, only KCl, CaCl2, NaHCO3 and seawater had a significant catalytic effect on the gasification. Seawater gasification of microplastics was a potential resource utilization method. Finally, it was considered that the PC plastic gasification conformed to the random nucleation and subsequent growth model (n = 3), and the reaction activation energy was 230.45 kJ/mol, which was smaller than that of traditional pyrolysis.

Experimental investigation on gasification characteristics of polycarbonate (PC) microplastics in supercritical water

The resource utilization of ABS plastic waste with subcritical and supercritical water treatment

The emulsification of heavy oil and water into a W/O emulsion would increase the viscosity of the W/O emulsion and reduce the mobility of heavy oil. As we know, addition of a viscosity reducer such...

Hua Zhang

Papers

Experimental investigation on gasification characteristics of plastic wastes in supercritical water

Experimental investigation on gasification characteristics of polyethylene terephthalate (PET) microplastics in supercritical water

Experimental investigation on gasification characteristics of polycarbonate (PC) microplastics in supercritical water

The resource utilization of ABS plastic waste with subcritical and supercritical water treatment

Study on the Re-emulsification Process of Water in Heavy Oil Emulsion with Addition of Water-Soluble Viscosity Reducer Solution