Analysis and prediction of input multiplicity for the reactive flash separation using reaction-invariant composition variables

In this study, we introduce a new approach for predicting and analyzing the input multiplicity in reactive flash separation processes. Specifically, we have identified necessary conditions to detect these multiple states in reactive flash separations using reaction-invariant composition variables. The presence of the input multiplicity is studied for the reactive systems of MTBE and TAME production to illustrate the capabilities of our methodology. For these reactive systems, we report the existence of multiple states for different operating conditions. In summary, our strategy can be applied with any reactive system and thermodynamic model, assuming that all reactions are reversible and in thermodynamic equilibrium and the operating conditions are away from the retrograde region. In general, our method is a robust procedure for the multiplicity analysis in flash separation of multi-reactive and multi-component systems.     

Rapid preparation of Bi0.5Na0.5TiO3 ceramics by reactive flash sintering of Bi2O3-NaCO3-TiO2 mixed powders

Lead-free Bi_0.5Na_0.5TiO₃ piezoelectric ceramics were successfully prepared by reactive flash sintering of Bi₂O₃-NaCO₃-TiO₂ mixed powders, where phase transformation and densification occurred simultaneously. The influence of electric field strength, current density and holding time at constant current state on the phase transformation and densification were investigated. The current density had a significant influence on the extent of phase transformation and densification. The holding time had no influence on the phase transformation, but had an important effect on crystallinity of sample. The sintered bulks exhibited the maximum polarization P_m of 16.8 μC/cm², remanent polarization P_r of 9.6 μC/cm², coercive field E_c of 29 kV/cmm, maximum electric-field-induced strain of 0.053 %, and piezoelectric coefficient d₃₃ of 85 pC/N. The reactive flash sintering can prepare the dense and single-phase ceramics from multiphase precursor powders in one step of flash, providing a new way for rapid production of ceramic materials.     

Ultrafast densification of high-entropy oxide (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 by reactive flash sintering

Pyrochlore-type high-entropy oxides (HEOs) are usually sintered at high temperatures for a long time to achieve full density. Herein, we synthesized pyrochlore-structured (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ HEOs with densities up to 99 % at a furnace temperature of 1200 °C in seconds via reactive flash sintering (RFS). The resultant HEOs achieved compositional uniformity at the atomic level and exhibited superior modulus, hardness and fracture toughness compared to the counterparts prepared by conventional solid-state sintering (at 1600 °C for 6 h). The underlying mechanisms for the ultrafast densification of the RFSed-HEOs were addressed in view of the roles of electric field, rapid heating, external pressure and internal reactions.     

The influence of the processing parameters on the reactive flash sintering of ZrO2-CeO2

Reactive flash sintering (RFS) is a method that was recently developed to produce dense single-phase bulk ceramic parts through solid-state reactions in a single-step that only takes a few minutes. The influence of the RFS parameters on the phase purity of a simple mixed oxide, (Zr_0.8,Ce_0.2)O₂, was investigated. Parameters such as furnace temperature, furnace atmosphere, electric current density, and alternating current (AC) or direct current (DC) were examined. It was found that (Zr_0.8,Ce_0.2)O₂ pellets with high densities, above 90% of its theoretical density, can be produced by RFS in a few minutes when RFS occurs under oxidizing atmospheres, AC fields with current densities of 100 mA·mm⁻², and at a furnace temperature of 1200°C. Reducing conditions such as Ar-H₂ atmosphere and DC fields, low furnace temperatures, and low current densities resulted in phase impurities and poor reactions between the ZrO₂ and the CeO₂ powders. These results show that RFS is a useful method to produce mixed oxides, but it is very sensitive to the processing parameters. This is the first time that the influence of most of the RFS processing parameters has been studied systematically. Thus, the present work aims to provide guidelines on selecting the right processing parameters when exploring RFS.     

A reactive molecular dynamics model of thermal decomposition in polymers: I. Poly(methyl methacrylate)

The theory and implementation of reactive molecular dynamics (RMD) are presented. The capabilities of RMD and its potential use as a tool for investigating the mechanisms of thermal transformations in materials are demonstrated by presenting results from simulations of the thermal degradation of poly(methyl methacrylate) (PMMA). While it is known that depolymerization must be the major decomposition channel for PMMA, there are unanswered questions about the nature of the initiation reaction and the relative reactivities of the tertiary and primary radicals formed in the degradation process. The results of our RMD simulations, performed directly in the condensed phase, are consistent with available experimental information. They also provide new insights into the mechanism of the thermally induced conversion of this polymer into its constituent monomers.     

Propagation of the contact-driven reduction of Mn2O3 during reactive flash sintering

As the field of flash sintering expands, more diverse flash processes are emerging that exhibit complex mechanisms and kinetics. Reactive flash sintering studies have been performed using precursor oxides and have yet to explore redox reactions. We show that Mn₂O₃ transforms into Mn₃O₄ during stage III of flash sintering via a moving reaction front, propagating from an electrode if sufficient energy is supplied. The power density and sample temperature increases as the transformation progresses due to the lower resistivity of Mn₂O₃ vs Mn₃O₄, a secondary thermal runaway effect, further confirming the presence of a transformation front. Additionally, in many studies, the contact resistance is accounted for, but not utilized. The energy for the transformation may either be supplied by the contact resistance–induced Joule heating or the furnace. Room-temperature impedance measurements demonstrate that Pt electrodes provide substantial contact resistance while Ag electrodes do not. The impedance study demonstrates that it is critical to select the appropriate electrode material to maximize or minimize contact resistance. The contact resistance may be used to create a hot spot and propagate a transformation front in any endothermic reduction reaction that occurs below 950°C in electronic conductors.     

Ultrafast synthesis and densification of ZrO2 doped KNN ceramics by reactive flash sintering

Fabrication of dense KNN-based lead-free piezoelectric ceramics at low temperatures in short time through a cost-effective way remains a challenge. Herein, this challenge could be addressed by using reactive flash sintering. It is demonstrated that the phase transformation of KNbO₃-NaNbO₃ into (K,Na)NbO₃ and densification occur simultaneously during the flash event. Most importantly, ZrO₂ doping can greatly decrease the onset flash temperature, which is ascribed to the increased conductivity of sample. In addition, the current limit has a significant effect on the phase transformation and densification. The flash-sintered KNN ceramics exhibit the good ferroelectric and piezoelectric properties. Furthermore, the ZrO₂ doped and undoped KNN ceramics show a comparable coercive field E_c, which may be related to the residual point defects after the flash. Besides the Joule heating, the avalanche generation of point defects is suggested to be responsible for the ultrafast solid-state reaction and densification rates.     

Coal flash pyrolysis: 2. Polymethylene compounds in low temperature flash pyrolysis tars

Pyrolysis of coals at low temperatures (< 600 °C) produces tars containing the precursors of the low molecular weight aliphatic hydrocarbons, such as ethylene and propylene, observed on flash pyrolysis of the coals at higher temperatures (700–800 °C). This is shown by further pyrolysis of these low temperature tars at high temperatures. Various methods, including isolation by h.p.l.c. were used to confirm the presence of straight chain paraffin and olefin pairs (C₁₄C₂₆ and above) in the low temperature tars. Pyrolysis of pure paraffins and olefins in this molecular weight range at temperatures > 700 °C produce ethylene, propylene and other cracking products similar to those obtained on flash pyrolysis of coal.     

Predicting transformations during reactive flash sintering in CuO and Mn2O3

Reactive flash sintering has been demonstrated as a method to rapidly densify and synthesize ceramic materials, but determining the extent of chemical reactions can be complex since the maximum temperature reached by the sample may be brief in time. The black body radiation (BBR) model has been shown to accurately predict the sample temperature during the steady state of flash (stage III). This work demonstrates situations where the BBR model alone does not accurately predict when a phase transformation will occur. We examine the model reactions of CuO reduction to Cu₂O during stage II and Mn₂O₃ reduction to Mn₃O₄ in stage III. In CuO, highly resistive samples result in initially localized current flow, a stochastic process resulting in inhomogeneous heating and error in the BBR model during stage II. CuO reduction does not occur in constant heating rate experiments with 6.25 V/mm fields, even though the sample temperature momentarily exceeds the phase transformation temperature. Increased furnace heating to 950°C before application of a field is required to drive the transition. In Mn₂O₃, the calculated sample temperature of the gauge is less than the transformation temperature, but localized heating at the contact will exceed the transformation temperature, causing the transformation to propagate away from the electrode during stage III. This work demonstrates two forms of inhomogeneity (local, stochastic current flow, and local contact resistance) that result in a complex thermal profile of the sample. This profile should be interrogated to understand reaction kinetics, and can be beneficial when engineered.     

Passive vacuum solar flash desalination

A model for a sustainable desalination process has been developed. The simulated process consists of pumping seawater through a solar heater before flashing it under a passively created vacuum in an elevated chamber. The vacuum enhances evaporation and is maintained by the balance between the hydrostatic pressure inside the elevated flash chamber and the atmospheric pressure. The developed model uses theoretical thermodynamic relations to describe the process setting it apart from previous empirical correlations. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

聚丙烯装置闪蒸过程的研究

通过调整聚丙烯装置闪蒸过程工艺参数,考察了生产负荷、聚丙烯颗粒平均直径,物料平均停留时间等因素对闪蒸丙烯汽化率的影响规律,工业试验结果表明,汽化率与生产负荷倒数,颗粒平均直径倒数,压差推动力,平均停留时间成线性关系,根据闪蒸过程数据进行数学关联,建立了闪蒸过程模型。     

二元有机混合液体闪点的实验研究

纯物质的闪点一般可通过查取文献获得,混合物的闪点则很难查到。由于混合液体在实际生产中有着广泛的应用,因此,掌握混合液体闪点的变化规律对于其在生产、储存和使用场所中的风险评估有重要意义。文中采用MINIFLASH FLPL全自动闪点测试仪,对由16种纯物质(包含醇类、酸类、酯类、酮类、醚类、烷烃类)组成的14组不同组成和配比的二元混合液体(含完全互溶和部分互溶)的闪点进行实验研究。用图示法分析了不同研究体系的二元有机混合物其闪点随配比和组分变化的经验规律,将二元混合液体闪点的变化规律大致划分为4类,并对每一类的闪点变化特征和机理进行了探讨。研究结果不仅为工程上提供了二元混合液体闪点的可靠实验数据,还为混合液体在生产、储存和使用过程中的火灾风险评估与安全管理提供了重要参考。     

Viscosity modification of flash pyrolysis tars

A study was made of the effect of a number of additives on the viscosity of a characteristic tar produced by the flash pyrolysis of indigenous Australian coals. The results indicate that aliphatic compounds are more effective than aromatic compounds in lowering the viscosity. This observation is of significance in the selection of suitable recycle solvents and the dissolution of aged tars.     

Multiphase equilibrium flash calculations

The determination of the correct number of equilibrium phases and their corresponding compositions at fixed temperature and pressure (TP flash) is studied. The novel aspects of this work center around unique initialization strategies and successive quadratic programming (SQP) enhancements that include the use of (1) only binary tangent plane analyses; (2) the determination of all partially miscible binary pairs and a dominant immiscible pair; (3) novel relative solubility calculations based on component activities and double tangency separation; (4) least squares solutions to compute phase fraction estimates; (5) a variety of algorithmic features that dynamically trap difficulties such as compositions well below machine accuracy, and trivial and collapsed solutions and (6) a posteriori testing of phase and solution stability. The overall algorithmic framework is one based on using a combination of binary tangent plane analyses, bubble point calculations and dimensionless Gibbs free energy minimizations. Binary tangent plane analyses are used to identify all immiscible or partially miscible binary pairs and to avoid dimensionality difficulties associated with locating all stationary points in the tangent plane distance function in the full composition space. The proposed approach consists of solving a sequence of subproblems (i.e. LE, LLE, VLLE,…) until the global minimum dimensionless Gibbs free energy (G/RT) is found. Maximum information from binary tangent plane analyses and previously solved subproblems are used to generate initial values for the next subproblem. The concept of relative solubilities is introduced and used to initialize phase compositions in all LLE calculations (i.e. phase split or flash). All completely miscible component relative solubilities are calculated using component activities while those for immiscible or partially miscible components are initialized using double tangency separation. Phase fractions are initialized using a least-square solution to the set of component mass balances. All subproblems are formulated in terms of component flows and solved using a full space SQP method using a modified Broyden–Fletcher–Goldfarb–Shanno (BFGS) update of the Lagrangian Hessian matrix. The proposed algorithm was tested within the Aspen Plus process simulator using a variety of physical properties options. Twenty six multicomponent mixtures including some four-phase (VLLLE) emulsion polymerization problems were used to test the proposed algorithm. All problems were easily solved and clearly demonstrate the capabilities of the present multiphase TP flash model.     

Power-controlled flash spark plasma sintering of gadolinia-doped ceria

Electric field-assisted sintering (FAST) is a rapidly growing scientific and engineering domain. In the present paper, we describe the process of flash sintering (FS) in a configuration of classical spark plasma sintering (SPS) (graphite punch and boron nitride (BN) die), also called flash spark plasma sintering (FSPS). The densification process of Gd_0.1Ce_0.9O_2-x powder is studied in detail with a focus on the transition from FAST to FS. We discuss the electrical, geometrical, and thermal evolution of the process and the characteristics of the final compacts. Low electrical fields are sufficient for the onset of FS. Ceria is a material difficult to sinter by FAST techniques due to its known mechanochemical transformations. We observed the disintegration of pellets after experiments with well-pronounced flash event.     

Inelastic relaxation in silica via reactive molecular dynamics

Silica glass exhibits rate-dependent and irreversible processes during deformation and failure, resulting in inelastic effects. To explore this phenomena, molecular dynamics simulations of structural relaxation surrounding a crack tip in silica glass were performed at four different temperatures (100, 300, 600, 900 K) using a reactive force field. Per-atom stresses were found to relax during the simulation, with the highest stress relaxation occurring at 900 K. Stress relaxation was radially dependent relative to the crack tip, with stress dissipation occurring primarily within a 25–30 Å inelastic region. Within 10 Å of the crack tip, the defect concentration decreased from 0.18 to 0.09 #/nm² during inelastic relaxation at 900 K. Conversely, the defect concentration 20 Å from the crack tip increased from 0.105 to 0.118 #/nm² at 300 K, and from 0.113 to 0.126 #/nm² at 600 K, which formed a defect-enriched region ahead of the crack tip. The difference in defect concentrations suggests the possibility of a stress mediated defect migration mechanism, where defects move away from the crack tip during inelastic relaxation. Additionally, defect speciation indicated that undercoordinated silica defects, such as non-bridging oxygen, were removed through the formation of higher coordination defects during relaxation. Overall, stress relaxation causes changes in the defect concentration profile near the crack tip, which has the potential to alter the properties of silica glass in the inelastic region during relaxation.     

Developments in the multi-stage flash desalting system

New developments in the design of the multi-stage flash (MSF) desalting system are outlined. The history of changing the flow sheet is given. The main design parameters as well as specifications of the equipment used are outlined.     

Steady state multiplicity in an UOP FCC unit with high-efficiency regenerator

The possibility of existence of multiple steady states in fluid catalytic cracking (FCC) units has a major impact in the supervision of these systems. The origins of these behaviours are usually due to the exothermicity of the catalyst regeneration reactions and to the strong interactions between the reactor and the regenerator system.Prior work has focused on modelling and control problems of different operating FCC units. However, none of these studies have considered a high-efficiency regenerator. This paper presents an analysis of the existence of output and input multiple steady states in an UOP FCC unit with a high-efficiency regenerator.The influence of unit disturbances and model uncertainties, such as coke composition and cracking enthalpy, in the output multiplicity, was studied and the results show that the high-efficiency regenerator exhibits at least three multiple output steady states and a maximum of five output steady states, in the operating range considered. Moreover, the state multiplicity analysis revealed that input multiplicity can be present in this FCC unit, depending on the choice of the control structure, and that operating the unit in full combustion mode can prevent instabilities due to input and output multiplicities. Therefore, these results can be used to guide the design of the most appropriate control structures in industrial applications. For the FCC unit with high-efficiency regenerator the most appropriate control structure corresponds to the control of the riser reactor temperature and the oxygen level in the flue gas, with the catalyst circulation rate and the combustion air flow rate, respectively.     

水膜闪蒸真空制冰的换热特性

引言当一定温度的液体突然暴露在低于其饱和压力的环境下时,液体由最初的平衡状态变成过热状态,液体中的过热量已经不能以显热的方式来包含能量,不能继续存在于液体中,会迅速转换为蒸发潜热,形成汽泡,同时液体温度迅速下降,这个过程被称为液体的闪蒸。由于闪蒸现象伴随着大量的水蒸气生成和剧烈的温度下降,在现代工业中,闪蒸被广泛应用在海