The control of bed height and solids circulation rate in the standpipe of a cold flow circulating fluidized bed

Circulating fluidized beds (CFBs) are used widely in the chemical industry. Knowing or estimating the bed height in the standpipe and the solids circulation rate are essential for effective control of the system. This paper incorporates a 2-region model to calculate the bed height in the standpipe with a Kalman filter algorithm to estimate the solids circulation rate (SCR). Simulations of both the standpipe bed height and SCR were compared with experimental data and shown to give good agreement.

In addition, a neural network method was applied to model the entire cold flow CFB system and measured data sets were used to train the neurons of the network. Finally, a linear controller was applied to control both the bed height and solids circulation rate to desired set points. Simulations were performed for both positive and negative step inputs for both variables and satisfactory control was demonstrated using this controller in combination with the neutral network and Kalman estimator.     

A virucidal face mask based on the reverse-flow reactor concept for thermal inactivation of SARS-CoV-2

While facial coverings reduce the spread of SARS-CoV-2 by viral filtration, masks capable of viral inactivation by heating can provide a complementary method to limit transmission. Inspired by reverse-flow chemical reactors, we introduce a new virucidal face mask concept driven by the oscillatory flow of human breath. The governing heat and mass transport equations are solved to evaluate virus and CO₂ transport. Given limits imposed by the kinetics of SARS-CoV-2 thermal inactivation, human breath, safety, and comfort, heated masks may inactivate SARS-CoV-2 to medical-grade sterility. We detail one design, with a volume of 300 ml at 90°C that achieves a 3-log reduction in viral load with minimal impedance within the mask mesh, with partition coefficient around 2. This is the first quantitative analysis of virucidal thermal inactivation within a protective face mask, and addresses a pressing need for new approaches for personal protective equipment during a global pandemic.     

Effects of cement size distribution on capillary pore structure of the simulated cement paste

The evolution of tricalcium silicate (C₃S) microstructure during hydration is tri-dimensionally simulated based on an “Integrated Particle Kinetics Model”. The hydration degree, the contact surfaces between the hydrated particles, the hydraulic radius and the capillary pore size distribution of the simulated cement paste at various degrees of hydration are calculated. Three examples of the C₃S microstructure development with different size distributions are presented. The effects of the cement size distribution on pore structure of cement paste are demonstrated and the results are discussed. In these examples, the cement size distribution varies between 3–40, 5–40 and 10–40 μm, respectively.     

The effect of milling energy input and molar ratio on the dehydrogenation and thermal conductivity of the (LiNH2 + nMgH2) (n = 0.5, 0.7, 0.9, 1.0, 1.5 and 2.0) nanocomposites

Hydride nanocomposites in the (LiNH₂ + nMgH₂) system have been synthesized by ball milling with varying input of milling energy injected into powder particles, Q_TR (kJ/g). The grain (crystallite) size of LiNH₂ and MgH₂ decreases rapidly with increasing Q_TR up to approximately 150–200 kJ/g and subsequently more or less saturates at the value of 10–20 nm. For the injected energy Q_TR ≈ 250–350 kJ/g the specific surface area (SSA) increases from the initial 2.4 m²/g for powder mixtures before milling to 30–37 m²/g for nanocomposites after milling. After injecting Q_TR ≈ 550 kJ/g there is a further increase of SSA to 52 m²/g which is over 20-fold increase of SSA from its initial value. That clearly indicates that a profound reduction of particle size has occurred. The hydride phases formed during ball milling with relatively low Q_TR are identified as a-Mg(NH₂)₂ (amorphous magnesium imide) and LiH. The ball milled (LiNH₂ + nMgH₂) nanocomposite system with n = 0.5–0.9 can effectively desorb about 4–5 wt.% H₂ with a reasonable rate at the temperature range close to 200 °C. Within a low temperature range up to ∼250 °C, regardless of the molar ratio n and the injected energy Q_TR the thermal desorption of the (LiNH₂ + nMgH₂) nanocomposites occurs without any release of ammonia, NH₃. For all molar ratios, n, the hydride nanocomposites are fully reversible at 175 °C under a relatively mild pressure of 50 bar H₂. The quantity of H₂ desorbed decreases with increasing molar ratio n, due to increasing fraction of inactive, retained MgH₂. However, at 125 °C the dehydrogenation rate is very sluggish and the quantity of released H₂ is minimal. At the temperature range lower than ∼250 °C dehydrogenation of ball milled nanocomposites occurs through formation of the Li₂Mg(NH)₂ hydride phase. The value of the measured dehydrogenation enthalpy change of 46.7 kJ/molH₂ is relatively low and apparently, it is not responsible for sluggish dehydrogenation at 125 °C. The measurements of thermal conductivity for non-milled powders and ball milled nanocomposites show a dramatic reduction of thermal conductivity after ball milling. It seems that this could be a principal factor responsible for such a low dehydrogenation rate at low temperatures.     

Effect of pressure on heat transfer coefficient at the metal/mold interface of A356 aluminum alloy

The aim of this paper is to correlate interfacial heat transfer coefficient (IHTC) to applied external pressure, in which IHTC at the interface between A356 aluminum alloy and metallic mold during the solidification of casting under different pressures were obtained using the inverse heat conduction problem (IHCP) method. The method covers the expedient of comparing theoretical and experimental thermal histories. Temperature profiles obtained from thermocouples were used in a finite difference heat flow program to estimate the transient heat transfer coefficients. The new simple formula was presented for correlation between external pressure and heat transfer coefficient. Acceptable agreement with data in literature shows the accuracy of the proposed formula.     

The effects of the nanometric interstitial compounds TiC,ZrC and TiN on the mechanical and thermal dehydrogenation and rehydrogenation of the nanocomposite lithium alanate (LiAlH4) hydride

Profuse mechanical dehydrogenation occurs during controlled high energy ball milling of LiAlH₄ containing 5 wt.% of the nanometric interstitial compounds such as n-TiC, n-TiN and n-ZrC which involves a gradual decomposition of LiAlH₄ to the mixture of Li₃AlH₆ and Al (Stage I) followed by a further decomposition of Li₃AlH₆ to the mixture of Al and LiH (Stage II). XRD reveals that the interstitial compounds remain stable in the hydride matrix during entire ball milling duration. The effectiveness of the nanometric interstitial compound additives for mechanical dehydrogenation increases on the order of n-TiN > n-TiC > n-ZrC. X-ray diffraction (XRD) reveals that there is no measurable change in a unit cell volume of LiAlH₄ after ball milling which indicates that an accelerated mechanical dehydrogenation of LiAlH₄ containing the nanometric interstitial compounds is unrelated to the lattice expansion as we have already reported for the nanometric metal Fe (n-Fe). In addition, the observed strong catalytic activity of the nanometric interstitial compounds for mechanical dehydrogenation is not related to their valence electron concentration (VEC) number. However, the n-TiN additive, which is the most effective one for mechanical dehydrogenation, has the smallest average particle size of 20 nm and the largest Specific Surface Area (SSA > 80 m²/g). For thermal dehydrogenation in Stage I the average apparent activation energy, E_A, for the interstitial compound additives is within the range of 87–96 kJ/mol whereas, for comparison, the nanometric metallic additives, n-Fe and n-Ni, exhibit drastically smaller apparent activation energy on the order of 55–70 kJ/mol. The average apparent activation energy for thermal dehydrogenation in Stage II is in the range of 63–80 kJ/mol in the order of E_A(n-ZrC) < E_A(n-Ti = n-TiC) and is lower than that for the nanometric metal additives n-Ni and n-Fe. In summary, the nanometric interstitial compounds do not substantially affect the apparent activation energy of Stage I but are able to reduce the apparent activation energy of thermal dehydrogenation in Stage II. XRD reveals that the interstitial compounds remain stable in the hydride matrix up to the dehydrogenation temperature of at least 165 °C. Ball milled LiAlH₄ containing 5 wt.% n-TiC, n-TiN and n-ZrC is able to slowly discharge large quantities of H₂ up to 5–6 wt.% during storage at 40 °C. Unfortunately, the results of rehydrogenation at 165 °C under 95 bar for 5 h indicate that LiAlH₄ containing the nanometric interstitial compounds exhibits no rehydrogenation.     

The effects of the micrometric and nanometric iron (Fe) additives on the mechanical and thermal dehydrogenation of lithium alanate (LiAlH4), its self-discharge at low temperatures and rehydrogenation

LiAlH₄ containing 5 wt.% of nanometric Fe (n-Fe) shows a profound mechanical dehydrogenation by continuously desorbing hydrogen (H₂) during high energy ball milling reaching ∼3.5 wt.% H₂ after 5 h of milling. In contrast, no H₂ desorption is observed during low energy milling of LiAlH₄ containing n-Fe. Similarly, no H₂ desorption occurs during high energy ball milling for LiAlH₄ containing micrometric Fe (μ-Fe) and, for comparison, both the micrometric and nanometric Ni (μ-Ni and n-Ni) additive. X-ray diffraction studies show that ball milling results in a varying degree of the lattice expansion of LiAlH₄ for both the Fe and Ni additives. A volumetric lattice expansion larger than 1% results in the profound destabilization of LiAlH₄ accompanied by continuous H₂ desorption during milling according to reaction: LiAlH₄ (solid) → 1/3Li₃AlH₆ + 2/3Al + H₂. It is hypothesized that the Fe ions are able to dissolve in the lattice of LiAlH₄ by the action of mechanical energy, replacing the Al ions and forming a substitutional solid solution. The quantity of dissolved metal ions depends primarily on the total energy of milling per unit mass of powder generated within a prescribed milling time, the type of additive ion e.g. Fe vs. Ni and on the particle size (micrometric vs. nanometric) of metal additive. For thermal dehydrogenation the average apparent activation energy of Stage I (LiAlH₄ (solid) → 1/3Li₃AlH₆ + 2/3Al + H₂) is reduced from the range 76 to 96 kJ/mol for the μ-Fe additive to about 60 kJ/mol for the n-Fe additive. For Stage II dehydrogenation (1/3Li₃AlH₆ → LiH+1/3Al + 0.5H₂) the average apparent activation energy is within the range 77–93 kJ/mol, regardless of the particle size of the Fe additive (μ-Fe vs. n-Fe). The n-Fe and n-Ni additives, the latter used for comparison, provide nearly identical enhancement of dehydrogenation rate during isothermal dehydrogenation at 100 °C. Ball milled (LiAlH₄ + 5 wt.% n-Fe) slowly self-discharges up to ∼5 wt.% H₂ during storage at room temperature (RT), 40 and 80 °C. Fully dehydrogenated (LiAlH₄ + 5 wt.% n-Fe) has been partially rehydrogenated up to 0.5 wt.% H₂ under 100 bar/160°C/24 h. However, the rehydrogenation parameters are not optimized yet.