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

Study of the microscopic mechanism of lauryl glucoside wetting coal dust: Environmental pollution prevention and control.

Sulfur removal technologies from fuel oil for safe and sustainable environment

Reactivation of spent S-Zorb adsorbents for gasoline desulfurization

The reactive adsorption desulfurization (RADS) of a model gasoline n-hexane containing thiophene was carried out with a NiO/ZnO-Al2O3-SiO2 adsorbent in N2 and H2, respectively. A declining RADS trend has been observed in N2, without the presence of H2, indicating that NiO is sulfurized and exhibits activity for RADS. TPR and XPS results presented NiO in the adsorbent is hard to be reduced because of the powerful interaction between NiO and the support. The sulfurization of NiO into NiSx is a primary condition for the RADS process, the same as the presulfurization of hydrotreating catalyst, while metallic Ni is an intermediate reduction product of NiSx. Results of a low RADS temperature at 300 °C, much lower than the reduction temperature of NiO, suggest that NiO plays an important role. Based on assumption of NiO as the main active component, the RADS could reduce the reaction temperature and energy consumption significantly. The participation of hydrogen and n-hexane in pretreatment conducted at 420 °C contributes to the activation of adsorbent. Also, these methods of pretreatment improved the desulfurization performance under the reaction temperature of 300 °C.

The Role of NiO in Reactive Adsorption Desulfurization Over NiO/ZnO-Al2O3-SiO2 Adsorbent

Reactive adsorption desulfurization (RADS) of Fluidized Catalytically Cracked (FCC) gasoline on reduced and unreduced NiO/ZnO–Al2O3–SiO2 adsorbents was studied. Various characterizations such as powder X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR), the H2/O2 pulse titration (HOPT), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) are used to evaluate the effects of hydrogen pretreatment of the adsorbents. XRD and HOPT results indicate that NiO is hard to be reduced to Ni0 under the conditions of RADS. H2-TPR shows that NiO might be reduced to Ni0 at the temperature of 598 °C, much higher than the temperature of RADS. The Ni 2p3/2 spectrum of Ni0 is not observed for the reduced adsorbent, but the main peak of Ni 2p3/2 of NiS is found for the spent adsorbent. The unreduced NiO/ZnO–Al2O3–SiO2 adsorbent performs a better desulfurization than the reduced adsorbent at the beginning of desulfurization process. NiO and Ni0 are assumed as the main active components and present a good desulfurization ability in RADS. Finally, a change in the RADS mechanism is presented and discussed.

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Reactive adsorption desulfurization of NiO and Ni0 over NiO/ZnO–Al2O3–SiO2 adsorbents: role of hydrogen pretreatment

Humic acid (HA) and water play an important role in polycyclic aromatic hydrocarbons (PAHs) adsorption and biodegradation in soil. In this work, molecular dynamics (MD) and electrostatic potential surfaces (EPSs) simulations are conducted to research the contribution of quartz surface, leonardite humic acid (LHA), and water to PAH adsorption. The adsorption energies between PAHs and LHA are much higher than that between PAHs and quartz. Simulation shows that the hydroxyl and carboxyl groups’ attraction by LHA is the main adsorption force between PAHs and LHA. The π-π interaction between PAHs and LHA also contributes to the adsorption process. In addition, the mobility of water on quartz surface is much higher than that of LHA. Water should be regarded as an adsorbate in the system as well as PAHs. However, the presence of water has a remarkable negative effect on the adsorption of PAHs on LHA and quartz. The bridging effect of water could only enhance the stability of the aggregation system. The adsorption contribution of quartz and LHA to PAHs in the soil model tends to 0 if the water layer reaches 2.0 nm.

Molecular dynamics simulation of the interaction of water and humic acid in the adsorption of polycyclic aromatic hydrocarbons

Transformation mechanism of FCC flue gas pollutants

Fluid catalytic cracking (FCC) unit emits a large amount of flue gas, which is a major concern of environmental protection supervision. Wet flue gas desulfurization (WFGD) technologies have been widely used to control the emissions of SO2 in refineries. In this study, stack tests for pollutants emission of a typical FCC unit were conducted. The emission characteristics of the FCC unit indicated that WFGD would cause a large amount of water vapor in the flue gas, which indirectly leads to large quantities of salt pollutants entrained in the flue gas including ammonium sulfite ((NH4)2SO3) and ammonium sulfate ((NH4)2SO4). A strong correlation among the concentrations of SO2, NH3, and H2O was observed, and factor analysis shows that these concentrations are dominated by a common factor. It was also found that a mass quantity of NH4+ and SO32- existed in the condensate water of the flue gas. The TG-MS analysis shows that (NH4)2SO3 could be decomposed at 94.1 °C, and NH3, SO2, and H2O are released as reaction products in the form of gas. Therefore, a part of the NH3 and SO2 obtained by Fourier transform infrared spectroscopy (FTIR) monitoring may be derived from the decomposition of (NH4)2SO3 in the flue gas due to the high temperature during the sampling process, which was also confirmed in a lab experiment. The hot and wet sampling process will lead to overestimation of NH3 and SO2 emissions rather than FTIR method itself when monitoring the high-humidity FCC flue gas. Thus, the concentration of H2O in the flue gas and the type of sampling process need to be taken into consideration during the monitoring process.

Influence of wet flue gas desulfurization on the pollutants monitoring in FCC flue gas

pH neutralization reaction process plays a crucial role in Waste Water Treatment Process (WWTP). Traditional PID Proportion Integral Differential, (or even advanced PID control) algorithms have poor performance on WWTP due to the strong non-linearity, large time lag, and large inertia characteristics of pH neutralization. Therefore, finding a superior control method to maintain the pH value of wastewater within the normal range will greatly help to improve the efficiency and effectiveness of wastewater treatment. The chemical reaction mechanism of pH neutralization reaction process is first analyzed, and a mechanistic model of pH neutralization reaction process is developed based on the reaction of ions during acid-alkali neutralization and the electric balance equation. Then, combining the characteristics of generalized predictive control and Model-Free Adaptive Control (MFAC), a Model-Free Adaptive Predictive Control (MFAPC) method based on compact format dynamic linearization is introduced. An Improved Model Free Adaptive PI Predictive Control algorithm (IMFAPC) with proportional (P) and integral (I) algorithms is proposed to further improve the control performance. IMFAPC is proposed on the basis of MFAPC, combining the advantages of generalized predictive control, introducing a PI module consisting of error and error sum, and predicting the PI module, making it possible to produce more accurate constraints on the control inputs, avoiding increasing errors, and improving the control effect of delayed systems at the same time. pH neutralization process simulation experimental results show that compared with the ordinary Model-Free Adaptive Control (MFAC) and MFAPC, the IMFAPC control algorithms has the best performance in terms of accuracy, overshoot, and the robustness.

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Zhihe Tang

Papers

Reactive adsorption desulfurization of NiO and Ni0 over NiO/ZnO–Al2O3–SiO2 adsorbents: role of hydrogen pretreatment

Molecular dynamics simulation of the interaction of water and humic acid in the adsorption of polycyclic aromatic hydrocarbons

Transformation mechanism of FCC flue gas pollutants

Influence of wet flue gas desulfurization on the pollutants monitoring in FCC flue gas

An Improved Method of Model-Free Adaptive Predictive Control: A Case of pH Neutralization in WWTP