نشریه جداسازی و پدیده های انتقال
سال بیست و یکم شماره 2 (1389)

Direct hydrogen reduction of molybdenum disulphide in the presence of lime is a proper alternative method to produce molybdenum metal powder. This process has certain advantages including less operating stages and no pollution. A mathematical model for behavior prediction of the complex reaction is presented in this paper. The model has been formulated by incorporating the intrinsic kinetics of the individual reactions the parameters of which were determined from separate experiments. Finally, the data predicted from the model have been compared with those determined experimentally. Although there are a number of factors affecting the complex reactions; nevertheless, the model prediction was satisfactory with a mean absolute deviation of 18%. Reasons for such a deviation have been discussed in the current paper, and the effect of factors causing the discrepancies has been considered through adjusting the effective diffusivity and the reaction frequency factor.

TiO2 nanostructures can be synthesized via a single-stage hydrothermal reaction of TiO2 in an alkaline solution. Morphology of the synthesized nanostructures is affected by process parameters such as initial powder, NaOH concentration, reaction time and temperature, mixing time, etc. In this work, the effects of mixing, reaction time and temperature were investigated. The synthesized nanostructures were characterized by measuring specific surface area, Fourier transfer infrared spectroscopy (FT-IR), X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs. The obtained results showed that nanostructure products such as high and low surface area nanoparticles, nano tubes with internal diameter of 10-20 nm, wall thickness of 5-10 nm and length of 30-50 nm, and nano fibers with diameter of 50-200 nm and micrometric length can be synthesized by adjusting the investigated parameters. Also, it was confirmed that mixing in the synthesis reactor can be an effective factor in the synthesis of nano tubes and fibers.

The primary conclusions regarding the freeze drying process reveal that due to the long drying time, caused by slow moisture removal rate, the quasi-steady-state (QSS) formulation could be applied for performance analysis of freeze drying process. The verification of this idea is a basic necessity. May also the application of this idea, with some restrictions imposed on the operating conditions and drying solid properties is possible. In this study, using the conservation law, an un-steady-state model (USS) is developed for primary and secondary stages of freeze drying process of skimmilk. The model is solved numerically both for the QSS and USS conditions and the results of the QSS solution are compared with the corresponding results of the USS solution and the existing experimental datain the literature.A good agreement between the USS solution andthe experimental data is observed but the error related to the QSS solution is remarkable. The average error related to QSS solution for the amount of removed water during the primary stage is 35% and for the location of the moving interface is 29%.Moreover, the efficacy of operational conditions of drying solute is investigated based on the results of USS model.

In this research, Ultrafine particles of paracetamol were produced by Rapid Expansion of Supercritical Solution (RESS). The experiments were conducted to investigate the effects of extraction temperature, extraction pressure, pre-expansion temperature, post-expansion temperature, spraying distance and nozzle diameter on particles size and morphology of paracetamol particles. The characterization of the particles was determined by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Liquid Chromatography/Mass Spectrometry (LC–MS) analysis. The average particle size of the original paracetamol was 20.8 µm, while the average particle size of paracetamol after micronization via the RESS process was between 0.259 µm and 1.089 µm depending upon the experimental conditions used. Moreover, the morphology of the processed particles changed to spherical and regular while the virgin particles of paracetamol were needle-shape and irregular. The results showed that the size of the precipitated paracetamol particles was smaller than that of original particles and RESS parameters can significantly affect the particle size.

Silicon Carbide thin film deposition in a tubular hot reactor, using a polymeric source at atmospheric pressure has been studied numerically. A three dimensional model was considered based on the finite volume method. The effects of substrate position, concentration of the polymeric source and the substrate temperature on the deposition rate and on the quality of the deposited layer were investigated. It is shown that the deposition rate increased by decreasing the substrate distance, however the uniformity of deposited layer decreased. The rate of deposition increases with the polymeric source concentration while it decreases with the substrate temperature.

The advantages of membrane system, such as low initial investment, low space requirements and ease of scale up, has led to take into consideration the use of membranes for gas sweetening in recent decades. For this purpose, the membrane system, separately or in combination with amine unit is used. In the design of gas sweetening processes, economic evaluation and optimization are amongst important parts. The aim of this article is economic evaluation and optimization of the membrane systems for applications in gas refineries. In this regard, important optimization parameters (the ratio of membrane surfaces and permeate pressures of the stages) of two stage membrane system are determined firstly, and it will try to find their optimum values based on the inlet gas information of a refinery. Thereafter, the optimum content of the acid gases must be removed by membrane section of hybrid systems will be determined. Optimization results showed that at low to moderate concentrations of acid gases (less that 12%), for a feed gas rate equal to 70 MMSCFD, hybrid system containing two-stage membrane system presents lower cost than that of containing single-stage membrane system.

This paper presents numerical simulation of the turbulent flow in a Gas-Liquid Cylindrical Cyclone (GLCC) to understand the hydrodynamic flow behavior and so to assess the its separation efficiency. Despite the long history of cyclone technology and the seemingly simple design and operation of the GLCC, recently these cyclones have been widely used in petroleum industry. The turbulence flow through the cyclone has been simulated by Reynolds Stress Model (RSM) anisotropic turbulence model. The swirling flow creates helical path with upward axial velocity into cyclone due to the centrifugal forces. Comparison between obtained axial and tangential velocities from simulation with the reported experimental data shows good agreement. Simulation of swirling flow hydrodynamic in cyclone with utilizing of other turbulence models show the RNG k-ε model cannot predict the reverse flow zone and the Realizable k-ε model can calculate more accurately this back flow, but with appropriate computational grid, utilization of RSM model has minimum computational error.