A study on optimization of acid sites concentration versus pore dimensions in modified solid acid catalysts for biodiesel production

The acid sites concentration of silica sulfuric acid was optimized against pore dimensions depending on the intensity of silanol groups as a key for binding modifier molecules. The texturally modified rice husk silica, of determined intensity of silanol groups, was functionalized with different loadings of SO₃H groups to produce solid acid catalysts with different concentrations of acid sites and different sizes of the pore systems. The catalysts were employed in trans-esterification of waste-cocking oil. The obtained optimum catalytic activity was attributed to the proper compensation between acid sites intensity and pore dimensions. The estimated TOF for methyl ester production was found to decrease with the increase of SO₃H loading on the catalyst surface. The role of compatibility of the reactants phase with the catalyst nature could be validated by the observed much higher activity of sulfonated lignin in the studied esterification reaction than the modified RHS-acid catalysts under study.     

Numerical simulation of condensate removal from gas channels of PEM fuel cells using corrugated walls

The removal of condensate water droplets from gas channels is necessary for proper operation of proton exchange membrane fuel cells. In the current work, it is shown that corrugated wall gas channels can help in the removal of condensate water droplets formed on the channel walls. Removal of sessile droplets from channels having semicircle, rectangular dent, and saw‐tooth corrugation and at different gas velocities is modeled numerically. It is shown that the time of condensate removal is much shorter in a corrugated channel as compared with that in an uncorrugated channel. Three different droplet removal regimes are identified: droplet, film, and misty flow regimes. The transition from one to another regime is mapped based on the inlet flow velocity and the type of the channel corrugate.     

Molecular identification and pectate lyase production by Bacillus strains involved in cocoa fermentation

We have previously reported the implication of Bacillus in the production of pectinolytic enzymes during cocoa fermentation. The objective of this work was to identify the Bacillus strains isolated from cocoa fermentation and study their ability to produce pectate lyase (PL) in various growth conditions. Ninety-eight strains were analyzed by Amplified Ribosomal DNA Restriction Analysis (ARDRA). Four different banding patterns were obtained leading to the clustering of the bacterial isolates into 4 distinct ARDRA groups. A subset of representative isolates for each group was identified by 16S rRNA gene partial sequencing. Six species were identified: Bacillus subtilis, Bacillus pumilus, Bacillus sphaericus, Bacillus cereus, Bacillus thuringiensis, together with Bacillus fusiformis which was isolated for the first time from cocoa fermentation. The best PL producers, yielding at least 9 U/mg of bacterial dry weight, belonged to B. fusiformis, B. subtilis, and B. pumilus species while those belonging to B. sphaericus, B. cereus and B. thuringiensis generally showed a low level of activity. Two kinds of PL were produced, as revealed by isoelectrofocusing: one with a pI of 9.8 produced by B. subtilis and B. fusiformis, the other with a pI of 10.5 was produced by B. pumilus. Strains yielded about 2 fold more PL in a pectic compound medium than in glucose medium and maximum enzyme production occurred in the late stationary bacterial growth phase. Together all these results indicate that PL production in the bacilli studied is modulated by the growth phase and by the carbon source present in the medium.     

NixCo1−x alloy nanoparticle-doped carbon nanofibers as effective non-precious catalyst for ethanol oxidation

Alloy structure generates special characteristics for the nano-metallic compounds which make this interesting class of materials promising candidates for many application fields. Moreover, the performance of the nanostructural catalysts is strongly influenced by the morphology; nanofibers reveal distinct catalytic activity compared to the nanoparticles. In this study, non-precious electrocatalysts based on alloy structure and nanofibrous morphology are introduced. Briefly, Ni_xCo_1−x (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) alloy nanoparticles incorporated in carbon nanofibers are investigated as electrocatalysts for ethanol oxidation. Preparation of the introduced nanofibers could be achieved by calcination of electrospun nanofibers composed of nickel acetate tetrahydrate, cobalt acetate tetrahydrate and poly(vinyl alcohol) in argon atmosphere at 800 °C. Polycondensation characteristic of the utilized metals precursors led to produce good morphology electrospun nanofibers as well as preserved the nanofibrous morphology during the calcination process for all formulations. The catalytic activity of cobalt enhanced the carbonization of the utilized polymer which resulted in producing nickel/cobalt alloys nanoparticles embedded in carbon nanofibers. Electrochemical investigation of the introduced nanofibers toward ethanol oxidation indicated that the alloy structure has a strong influence. For instance, the corresponding current densities of Ni- and Ni_0.9Co_0.1-doped carbon nanofibers were 37 and 142 mA/cm², respectively. Moreover, very low onset potential (−50 mV vs. Ag/AgCl) was observed when Ni_0.1Co_0.9-doped carbon nanofibers were utilized. Furthermore, Ni_0.9Co_0.1-doped carbon nanofibers could oxidize ethanol solution up to 5 M due to the observed active layer regeneration. The introduced nanofibers have good stability because of the alloy structure. Overall, this study opens new avenue for the transition metals alloys and the nanofibrous morphology to produce novel and effective non-precious electrocatalysts.     

Weight function method for transient thermomechanical fracture analysis of a functionally graded hollow cylinder possessing a circumferential crack

This article introduces a weight function method for fracture analysis of a circumferentially cracked functionally graded hollow cylinder subjected to transient thermomechanical loading. Analytical solutions for transient temperature and stress distributions in the uncracked cylinder are derived by applying finite Hankel transformation. These solutions are utilized to determine stress acting on the faces of the circumferential crack in the local perturbation problem. Thermomechanical material properties are assumed to be power functions of the radial coordinate in the derivations. Coefficients of the weight function are found using reference stress intensity factors computed through the finite element method. Domain form of the J-integral is used in the finite element calculations. Comparisons of the numerical results calculated by the proposed weight function method to those generated by finite element analysis demonstrate the high level of accuracy attained by the application of the developed procedures. Further parametric analyses are presented to illustrate the influences of dimensionless time, crack depth to thickness ratio, power law index, and convection coefficient upon transient mode I thermomechanical stress intensity factors.     

Enabling seamless multimedia wireless access through QoS‐based bandwidth adaptation

Effective support of real‐time multimedia applications in wireless access networks, viz. cellular networks and wireless LANs, requires a dynamic bandwidth adaptation framework where the bandwidth of an ongoing call is continuously monitored and adjusted. Since bandwidth is a scarce resource in wireless networking, it needs to be carefully allocated amidst competing connections with different Quality of Service (QoS) requirements. In this paper, we propose a new framework called QoS‐adaptive multimedia wireless access (QoS‐AMWA) for supporting heterogeneous traffic with different QoS requirements in wireless cellular networks. The QoS‐AMWA framework combines the following components: (i) a threshold‐based bandwidth allocation policy that gives priority to handoff calls over new calls and prioritizes between different classes of handoff calls by assigning a threshold to each class, (ii) an efficient threshold‐type connection admission control algorithm, and (iii) a bandwidth adaptation algorithm that dynamically adjusts the bandwidth of an ongoing multimedia call to minimize the number of calls receiving lower bandwidth than the requested. The framework can be modeled as a multi‐dimensional Markov chain, and therefore, a product‐form solution is provided. The QoS metrics—new call blocking probability (NCBP), handoff call dropping probability (HCDB), and degradation probability (DP)—are derived. The analytical results are supported by simulation and show that this work improves the service quality by minimizing the handoff call dropping probability and maintaining the bandwidth utilization efficiently. Copyright © 2006 John Wiley & Sons, Ltd.     

Data-driven safe gain-scheduling control

Data-based safe gain-scheduling controllers are presented for discrete-time linear parameter-varying systems (LPV) with polytopic models. First, $\lambda$-contractivity conditions are provided under which the safety and stability of the LPV systems are unified through Minkowski functions of the safe sets. Then, a data-based representation of the closed-loop LPV system is provided, which requires less restrictive data richness conditions than identifying the system dynamics. This sample-efficient closed-loop data-based representation is leveraged to design data-driven gain-scheduling controllers that guarantee $\lambda$-contractivity and, thus, invariance of the safe sets. It is also shown that the problem of designing a data-driven gain-scheduling controller for a polyhedral (ellipsoidal) safe set amounts to a linear program (a semi-definite program). The motivation behind direct learning of a safe controller is that identifying an LPV system requires satisfying the persistence of the excitation (PE) condition. It is shown in this paper, however, that directly learning a safe controller and bypassing the system identification can be achieved without satisfying the PE condition. This data-richness reduction is of vital importance, especially for LPV systems that are open-loop unstable, and collecting rich samples to satisfy the PE condition can jeopardize their safety. A simulation example is provided to show the effectiveness of the presented approach.