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Unsteady flows of granular media are ubiquitous yet remain largely unexplored. In this research, we apply unsteady flows to strongly segregating granular materials to control the segregation pattern and enhance overall mixing. Size-bidisperse granular mixtures with large size ratios flowing onto a quasi-2D bounded heap form stratified layers of large and small particles when the flow rate is modulated. These layers exhibit better average mixing than the segregated patterns generated by steady feed rates. The mechanisms of layer formation under modulated flow differ from those for spontaneous stratification and are related to changes in the composition of the flowing layer at different stages in each feed cycle. The thickness and length of the stratified layers can be controlled by changing the feed rates and feed cycle durations, which is potentially useful for reducing segregation in industrial processes.

The main objective of the herein described research was focused on performing satisfactory flow modulation (FM), in comprehensive two-dimensional gas chromatography-mass spectrometry (GC×GC-MS), using an MS-compatible second-dimension gas flow of approx. 4 mL min(-1). The FM model used was based on that initially proposed by Seeley et al. [3]. The use of limited gas flows was enabled through fine tuning of the FM parameters, in particular the duration of the re-injection (or flushing) process. Specifically, the application of a long re-injection period (i.e., 700 ms) enabled efficient accumulation-loop flushing with gas flows of about 4 mL min(-1). It was possible to apply such extended re-injection periods by using different restrictor lengths in the connections linking the modulator to the auxiliary pressure source. FM GC×GC-MS applications were performed on a mixture containing C9-10 alkanes, and on a sample of essential oil. GC×GC-MS sensitivity was compared with that attained by using conventional GC-MS analysis, in essential oil applications. It was observed that signal intensities were, in general, considerably higher in the FM GC×GC-MS experiments.

The possibility to use multicapillary gas-chromatography column (MCC) as the second-dimension column in comprehensive two-dimensional gas chromatography (GC × GC) has been demonstrated. Dependences of the efficiency on the carrier gas volumetric flow rate were studied for capillary columns (CC) HP-5 and ZB-WAX of length 5 m and multicapillary columns of length 0.25 m with stationary phases OV-73 and Carbowax 20 M. For standard capillary columns, the maximum efficiency is reached at a flow rate below 2 mL/min. In the GC × GC mode, a capillary column is operated at a flow rate strongly exceeding the optimal value. Therewith, the efficiency of standard capillary column decreases several-fold, to 1570 theoretical plates for HP-5 and 1000 theoretical plates for ZB-WAX. In the case of multicapillary columns that are used in this paper, the maximum efficiency is reached at the carrier gas velocities above 60 mL/min. This allows MCC to be used as the second-dimension column in GC × GC at the optimal flow rate, thus reaching a maximum efficiency of 5900 theoretical plates for MCC with the OV-73 stationary phase and 2800 theoretical plates with Carbowax 20 M. The GC × GC separation of aliphatic and aromatic compounds was used to demonstrate that high efficiency of MCC enhances peak resolution in the second dimension as compared to standard capillary column. MCC was shown to be superior in separation of the components with close retention times and substantially differing concentrations.

The present research is focused on the use and evaluation of a novel helium ionization detector, defined as barrier discharge ionization detector (BID), within the context of (low-)flow modulation comprehensive two-dimensional gas chromatography (FM GC × GC). The performance of the BID device was compared to that of a flame ionization detector (FID), under similar FM GC × GC conditions. Following development and optimization of the FM GC × GC method, the BID was subjected to fine tuning in relation to acquisition frequency and discharge flow. Moreover, the BID performance was measured and compared to that of the FID, in terms of extra-column band broadening, sensitivity and dynamic range. The comparative study was carried out by using standard compounds belonging to different chemical classes, along with a sample of diesel fuel. Advantages and disadvantages of the BID system, also within the context of FM GC × GC, are critically discussed. In general, the BID system was characterized by a more limited dynamic range and increased sensitivity, compared to the FID. Additionally, BID and FID contribution to band broadening was found to be similar under the operational conditions applied. Particular attention was devoted to the behaviour of the FM GC × GC-BID system toward saturated and aromatic hydrocarbons, for a possible future use in the field of mineral-oil food contamination research.

This paper presents the results of pulsed-GaCl flow modulation epitaxy (FME) on hydride vapor phase epitaxy (HVPE) at various interval times, supply times, and V/III ratios under a constant NH 3 flow. The best performance was observed at an interval time of 30 s, GaCl gas supply time of 10 s in one cycle and V/III ratio of 10. As compared to the conventional HVPE growth, the utilization efficiency of the GaCl gas improved by 2.5 times in the present method and the root-mean-square (RMS) values reduced to one-fifth of the original, without degradation of crystalline quality. We concluded that the dynamic change in the V/III ratio in one cycle of an FME sequence contributed to a change in the growth mode, resulting in an improved utilization efficiency of the GaCl gas.

Familiarized with the steady state behavior and advantages of the trickle bed reactors (TBR), for the past two decades, researchers and process engineers are continuously exploring the unsteady state hydrodynamics of periodically (or cyclic) operated trickle bed reactors to extract even more from its efficiency and performance. Despite its complicated nonlinear behavior that attributes to the process control safety at the commercial scale, cyclic operation of TBR is a promising process intensification technology that has immense potential for implementation on industrial TBR. In this paper, we have summarized and reviewed the research and progresses made in recent years on cyclic operation of TBR and potential applications of various mode of its operation. Several issues associated with its hydrodynamics scale-up and designs have been discussed with an emphasis on shock wave attenuation that arises during flow modulation.

The present investigation is focused on a simple flow modulator (FM), for comprehensive two-dimensional gas chromatography (GC×GC). The interface is stable at high temperatures, and consists of a metallic disc (located inside the GC oven) with seven ports, which are connected to an auxiliary pressure source via two branches, to the first and second dimension, to a waste branch (linked to a needle valve) and to an exchangeable modulation loop (2 ports). The ports are connected via micro-channels, etched on one of the inner surfaces of the disc. Modulation is achieved using a two-way electrovalve, connected on one side to the additional pressure source, and to the two metal branches, on the other. An FM enantio-GC×polar-GC method (using a flame ionization detector) was optimized (a 40-μL loop was employed), for the analysis of essential oils. As an example, an application on spearmint oil is shown; the method herein proposed was subjected to validation. Finally, an FM GC×GC diesel experiment was carried out, using an apolar-polar column combination, to demonstrate the effectiveness of the modulator in the analysis of a totally different sample-type.

The influence of periodic operation on trickle-bed reactor (TBR) hydrodynamics and gas–liquid mass transfer was investigated. Two-phase pressure drop, dynamic liquid hold-up and gas–liquid mass transfer coefficient (kLa) were determined at various liquid flow rates and for different modes of liquid flow variation (increasing and decreasing liquid flow rate). The results reveal the considerable influence of type of liquid flow rate modulation on kLa values (deviations of up to 80% in kLa). Simulation studies on gas-limited reaction in a periodically operated TBR indicate that an enhancement in conversion of about 14% can be expected from an appropriate selection of the operating mode, thus clearly demonstrating the quantitative process intensification feasible through increased gas–liquid mass transfer.

High-speed gas chromatographic (GC) separation of residual solvents in pharmaceutical preparations, using a flow-modulation technique, is described. These volatile compounds are separated on a series-coupled (tandem) column ensemble consisting of a polyethylene glycol column and a trifluoropropylmethyl/dimethylpolysiloxane column. This column ensemble is operated in stop-flow mode to enhance, or "tune", the separation. A valve between the junction point of the tandem column ensemble and a source of carrier gas at a pressure above the GC inlet pressure is opened for intervals of 2-8 s. This stops or slightly reverses the flow of carrier gas in the first column. Stop-flow pulses are used to increase the separation of target analytes that overlap in the total ensemble chromatogram, compared to non-stop-flow, or conventional, operation. All 36 target compounds, based on ICH Classes I and II residual solvent lists, are resolved in 12 min using the stop-flow technique and a single chromatographic analysis.

A new method to form compositionally graded multilayer films by using a single deposition source has been developed. In this method, the reactive gas flow rate has been changed periodically to form multilayers. As a result, the composition of the multilayer films obtained by this technique changes periodically without any interrupted interfaces. To form multilayers using this technique, a non-linear relationship between the reactive gas flow rate and the deposition parameters such as deposition rate and reactive gas consumption should be considered because they cause a non-linear relationship between the reactive gas flow rate and the composition distribution towards the film depth of deposited films. Without considering this, the film with a sinusoidal component distribution that is assumed to originate novel properties cannot be obtained. By comparing the oxygen distribution towards the film depth obtained by Auger electron spectroscopy with that calculated from O 2 consumption during film deposition, it was found that oxygen diffused towards the metal-rich underlayers during deposition of oxygen-rich layers. This diffusion makes it difficult to estimate and control film layer composition distribution from deposition parameters. By optimizing the gas flow rate change a TiO x multilayer film with a 5-layered sinusoidal component distribution has been obtained.

A review of flow modulation techniques is presented with a particular emphasis put on the EHD impedance. Due to the general formalism of impedances, a close relation is demonstrated between this EHD and the usual ac impedance. An analysis is hence presented about the general procedure for considering the effect of interfacial kinetics of finite rate. The technique is also demonstrated to be especially powerful when the interface is coated by a porous layer or is non-uniformly reactive (partial blocking). Finally, the concept of EHD impedance is also shown to be linked to the frequency response of microelectrodes used for investigating the properties of hydrodynamic fluctuations close to a wall.

This article presents a guide to the popular value types and how they are automated for remote on-off control and flow modulation.