The paper discusses the results of a numerical analysis of the properties and regimes of the adsorption air separation and oxygen concentration process with a purity of ∼ 40-60%, carried out in a ...2-adsorption vacuum-pressure plant with a granular zeolite adsorbent 13X with a productivity of 1.6 · 10-5 m3/s. Computational experiments were carried out using the developed mathematical model and the influence of temperature, pressure, reflux ratio, the duration of the adsorption and desorption stages, the harmonic fluctuations of the inlet pressure during the adsorption stage and the outlet pressure during the desorption stage on the kinetics, and the efficiency of the air separation process by the PSA method were investigated. It is established that the specially organized harmonic fluctuations of the inlet pressure at the stage of adsorption and outlet pressure during the desorption stage lead to an increase in the purity of product oxygen by 4% (vol.).
The paper discusses the numerical analysis results of the aerodynamic structure of a flow in the granular adsorbent layer in a pressure swing adsorption unit adsorber during atmospheric air ...separation and oxygen concentration. The computational experiments were carried out using one-dimensional and two-dimensional mathematical models to calculate the velocity field in the adsorbent bulk layer. While assessing the accuracy of calculating the aerodynamic structure of the gas flow in the adsorbent, it was found that the use of a two-dimensional mathematical model provides an increase in the accuracy of calculations by an average of ~1-2% compared with the one-dimensional model.
The paper discusses the results of numerical analysis of the properties and modes of the adsorption air separation process and oxygen concentration with purity of ∼90% carried out in a ...double-adsorber PSA unit with a 13X granular zeolite adsorbent with capacity of up to 2 l/min. Calculation experiments have been performed using the developed mathematical model and the influence of perturbations (temperature, composition and pressure of atmospheric air) on the dynamics of the process, the unit performance, the extraction degree and oxygen purity has been investigated. It has been established that the most effective control action is the pressure at the compressor outlet, the increase in which from 2 to 6×105 Pa leads to the increase in the extraction degree and oxygen concentration on average by ∼57% and ∼12 vol.% respectively.
We report on the observation of a circular photogalvanic current excited by terahertz laser radiation in helical edge channels of two-dimensional (2D) HgTe topological insulators (TIs). The direction ...of the photocurrent reverses by switching the radiation polarization from a right-handed to a left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a ≈3μm distance at CNP approaches 2e2/h, the value characteristic for ballistic transport in 2D TIs. The data reveal that the photocurrent is caused by photoionization of helical edge electrons to the conduction band. We discuss the microscopic model of this phenomenon and compare calculations with experimental data.
A mathematical model of the dynamics of pressure swing adsorption was developed. The adsorption, aimed at separating multicomponent gas mixtures for hydrogen concentration, is performed in a ...four-adsorber unit with a CaA zeolite adsorbent. An original description of the mass transfer of the adsorbate (H
2
, CO
2
, CO) from the gas phase to the solid adsorbent for the mixed-diffusion region depending on the gas flow rate in the adsorbent bed was proposed. A computational treatment of the effects of the raw materials load and control variables on the dynamics of the cyclic adsorption process of gas mixture enrichment with hydrogen was performed. The problem of adaptive optimization of process variables was formulated and studied by modeling.
We report on the observation of the giant photocurrents in HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum emerges. At an exciting QW of 6.6 nm width by terahertz (THz) ...radiation and sweeping magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative (-0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical doping. The photocurrent data, accompanied by measurements of radiation transmission as well as Shubnikov-de Haas and quantum Hall effects, prove that the photocurrent is caused by cyclotron resonance in a Dirac fermion system, which allows us to obtain the effective electron velocity nu = 7.2 x 10 super(5) m/s. We develop a microscopic theory of the effect and show that the inherent spin-dependent asymmetry of light-matter coupling in the system of Dirac fermions causes the electric current to flow.
We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear ...energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency
ω
and momentum relaxation time
τ
l
larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light-induced impact ionization is proportional to
exp
(
−
E
0
2
/
E
2
)
, with the radiation electric field amplitude
E
and the characteristic field parameter
E
0
. As observed in experiment, it exhibits a strong frequency dependence for
ω
τ
≫ 1 characterized by the characteristic field
E
0
linearly increasing with the radiation frequency
ω
.
We report on the observation of magneto-oscillations of terahertz radiation induced photocurrent in HgTe/HgCdTe quantum wells of different widths, which are characterized by a Dirac-like, inverted, ...and normal parabolic band structure. The photocurrent data are accompanied by measurements of photoresistance (photoconductivity), radiation transmission, as well as magneto transport. We develop a microscopic model of a cyclotron-resonance assisted photogalvanic effect, which describes main experimental findings. We demonstrate that the quantum oscillations of the photocurrent are caused by the crossing of Fermi level by Landau levels resulting in the oscillations of spin polarization and electron mobilities in spin subbands. Theory explains a photocurrent direction reversal with the variation of magnetic field observed in experiment. We describe the photoconductivity oscillations related with the thermal suppression of the Shubnikov-de Haas effect.
In recent decades, active research has been under way to find alternative sources of renewable raw materials for the production of valuable components. Microalgae, characterized by flexible ...metabolism, have great potential for organization of production of useful substances for various applications: pharmaceuticals, production of food additives, feed additives for animals and fish, materials, biofertilizers, and biofuels. Despite the large number of scientific publications dealing with separate stages of processes for the production of useful substances from microalgae, a systematic analysis of trends in the development of this sector of bioeconomics remains an important problem. The review presents an analysis of key stages in the production of useful substances from microalgae and the peculiarities of their implementation. Some problems concerning the improvement of performance of such productions using computer modeling tools were discussed.
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