Oscillations of two competing microbial populations in configurations of two interconnected chemostats

It is known that, when two microbial populations competing for a single rate-limiting nutrient are grown in a spatially uniform environment, such as a single chemostat, with competition being the only interaction between them, they cannot coexist, but eventually one of the two populations prevails and the other becomes extinct. Spatial heterogeneity has been suggested as a means of obtaining coexistence of the two populations. A configuration of two interconnected chemostats is a simple model of a spatially heterogeneous environment. It has been shown that, when Monod's model is used for the specific growth rates of the two populations, steady-state coexistence can be obtained in such systems for wide ranges of operating conditions. In the present work, we study a model of microbial competition in configurations of interconnected chemostats and we show that, if a substrate inhibition model is used for the specific growth rates of the two populations, coexistence in a periodic state is also possible. The analysis of the model is done by numerical bifurcation theory methods.     

Using distributed object technologies in telecommunications networkmanagement

The Telecommunications Management Network (TMN) has been developed as the framework to support administrations in managing telecommunications networks. It suggests the use of OSI Systems Management (OSI-SM) as the technology for management information exchanges. Distributed object technologies, such as the Common Object Request Broker architecture (CORBA), address the use of software application program interfaces (APIs) in addition to interoperable protocols. Their use in TMN has been the subject of intensive research in previous years, with most approaches focusing on interoperability aspects with OSI-SM. We examine the issues behind using distributed object technologies in TMN via a native fashion, with network elements supporting distributed objects directly, e.g., a “CORBA to the switch” approach. The proposed solution tries to maintain the full OSI-SM expressive power in a way that other solutions have not attempted before. Performance and scalability issues are considered, while the approach has been validated through implementation     

An overview of routing optimization for internet traffic engineering

Traffic engineering is an important mechanism for Internet network providers seeking to optimize network performance and traffic delivery. Routing optimization plays a key role in traffic engineering, finding efficient routes so as to achieve the desired network performance. In this survey we review Internet traffic engineering from the perspective of routing optimization. A taxonomy of routing algorithms in the literature is provided, dating from the advent of the TE concept in the late 1990s. We classify the algorithms into multiple dimensions: unicast/multicast, intra-/inter- domain, IP-/MPLS-based and offline/online TE schemes. In addition, we investigate some important traffic engineering issues, including robustness, TE interactions, and interoperability with overlay selfish routing. In addition to a review of existing solutions, we also point out some challenges in TE operation and important issues that are worthy of investigation in future research activities.     

Use of natural clinoptilolite for the removal of lead, copper and zinc in fixed bed column

This work deals with the removal of lead, copper and zinc from aqueous solutions by using natural zeolite (clinoptilolite). Fixed bed experiments were performed, using three different volumetric flow rates of 5, 7 and 10bed volume/h, under a total normality of 0.01N, at initial pH of 4 and ambient temperature (25 degrees C). The removal efficiency increased when decreasing the flow rate and the following selectivity series was found: Pb(2+)>Zn(2+)> or =Cu(2+). Conductivity measurements showed that lead removal follows mainly ion exchange mechanism, while copper and zinc removal follows ion exchange and sorption mechanism as well.     

Exploiting agent mobility for large-scale network monitoring

As networks become pervasive, the importance of efficient information gathering for purposes such as monitoring, fault diagnosis, and performance evaluation increases. Distributed monitoring systems based on either management protocols such as SNMP or distributed object technologies such as CORBA can cope with scalability problems only to a limited extent. They are not well suited to systems that are both very large and highly dynamic because the monitoring logic, although possibly distributed, is statically predefined at design time. This article presents an active distributed monitoring system based on mobile agents. Agents act as area monitors not bound to any particular network node that can "sense" the network, estimate better locations, and migrate in order to pursue location optimality. Simulations demonstrate the capability of this approach to cope with large-scale systems and changing network conditions     

Characterization of various cement grinding aids and their impact on grindability and cement performance

The aim of the present research work was the evaluation of six commercial grinding additives, which were used for the production of Portland cement (ground in a ball mill at a laboratory stage). For this purpose, a reference sample was also produced without using any admixture. The characterization of the grinding aids (GA) was carried out by Fourier transform infra-red spectroscopy (FT-IR) and gas chromatography/mass spectrometry (GC/MS). All the cement mixtures were tested for initial and final setting times, consistency of standard paste, flow of normal mortar and compressive strengths after 2, 7 and 28 days. In all cases the addition of grinding aids resulted in improvement of the specific surface and grindability index, a fact that was attributed to the additive ability not only to reduce resistance to comminution, but also to prevent agglomeration and powder coatings of ball and mill. The mortars made with cements ground with the addition of commercial additives, which contained triisopropanolamine (TIPA), exhibited higher strength at any age and higher setting times. On the other hand, the presence of triethanolamine (TEA) on the commercial grinding additives did not affect the mechanical properties of the produced cements but slightly decreased their setting times.     

ETHYLENE OXIDATION ON SILVER CATALYSTS: EFFECT OF ETHYLENE OXIDE AND OF EXTERNAL TRANSFER LIMITATIONS

Ethylene oxidation to ethylene oxide and to carbon dioxide over silver catalysts was studied in a CSTR. The effects of two factors on the catalyst performance were examined. The first was the presence of excess ethylene oxide in the feed. A kinetic model was introduced which assumed that ethylene and ethylene oxide compete for the same sites on the catalyst surface. This model provided reasonable quantitative agreement with kinetic and potentiometric measurements. The second factor that was studied was the presence of external heat and mass transfer limitations. It was found that such limitations cause a significant decrease of the selectivity to ethylene oxide. This decrease is a result of the temperature difference between the catalyst surface and the bulk of the gas phase and of the fact that the activation energy of ethylene combustion is greater than that of ethylene epoxidation. The contribution of other factors such as inhibition by CO₂ or possible incomplete mixing in the reactor is shown to be insignificant.