Sorption and activation of hydrocarbons by molecular sieves

Substantial progress has been made recently in the understanding of sorption and activation of alkanes. This progress reflects the emergence of new theoretical and experimental results, leading to a more quantitative picture of the elementary steps involved in the ordering of alkanes in molecular sieves and their chemical interaction with the acid site. Conversion of n-alkanes over various zeolites is now well understood to depend mainly upon the concentration of reactants sorbed. The sorption enthalpy and entropy of these molecules are linearly related and this relationship is characteristic of a particular molecular sieve (compensation effect). The interfacial chemistry that alkanes and alkenes undergo involves ionic species only in their transition state, whereas the stable intermediates are covalently bound. This leads to a description of the chemical transformations that resemble nucleophilic and electrophilic substitutions

Fiscal and social impact of a nominal exchange rate devaluation in Djibouti

Limited fiscal space limits Djibouti's ability to meet the Millennium Development Goals and improve the living conditions of its population. Djibouti's fiscal structure is unique in that almost 70 percent of government revenue is denominated in foreign currency (import taxes, foreign aid grants, and military revenue) while over 50 percent of government expenditure is denominated in local currency (wages, salaries, and social transfers). Djibouti's economic structure is also unusual in that merchandise exports of local origin are insignificant, and the country relies heavily on imported goods (food, medicines, consumer and capital goods). A currency devaluation, by reducing real wages, could potentially generate additional fiscal space that would help meet Djibouti's fundamental development goals. Using macroeconomic and household level data, the authors quantify the impact of a devaluation of the nominal exchange rate on fiscal savings, real public sector wages, real income, and poverty under various hypothetical scenarios of exchange-rate pass-through and magnitude of devaluation. They find that a currency devaluation could generate fiscal savings in the short-term, but it would have an adverse effect on poverty and income distribution. A 30 percent nominal exchange rate devaluation could generate fiscal savings amounting between 3 and 7 percent of GDP. At the same time, a 30 percent nominal devaluation could cause nearly a fifth of the poorest households to fall below the extreme poverty line and pull the same fraction of upper middle-income households below the national poverty line. The authors also find that currency devaluation could generate net fiscal savings even after accounting for the additional social transfers needed to compensate the poor for their real income loss. However, the absence of formal social safety nets limits the government's readiness to provide well-targeted and timely social transfers to the poor.Economic Theory&Research,Economic Stabilization,Rural Poverty Reduction,Fiscal&Monetary Policy,Macroeconomic Management

Interfacing a high performance disk array file server to a Gigabit LAN

Our previous prototype, RAID-1, identified several bottlenecks in typical file server architectures. The most important bottleneck was the lack of a high-bandwidth path between disk, memory, and the network. Workstation servers, such as the Sun-4/280, have very slow access to peripherals on busses far from the CPU. For the RAID-2 system, we addressed this problem by designing a crossbar interconnect, Xbus board, that provides a 40MB/s path between disk, memory, and the network interfaces. However, this interconnect does not provide the system CPU with low latency access to control the various interfaces. To provide a high data rate to clients on the network, we were forced to carefully and efficiently design the network software. A block diagram of the system hardware architecture is given. In the following subsections, we describe pieces of the RAID-2 file server hardware that had a significant impact on the design of the network interface

Design challenges for space bioreactors

The design of bioreactors for operation under conditions of microgravity presents problems and challenges. Absence of a significant body force such as gravity can have profound consequences for interfacial phenomena. Marangoni convection can no longer be overlooked. Many speculations on the advantages and benefits of microgravity can be found in the literature. Initial bioreactor research considerations for space applications had little regard for the suitability of the designs for conditions of microgravity. Bioreactors can be classified in terms of their function and type of operation. The complex interaction of parameters leading to optimal design and operation of a bioreactor is illustrated by the JSC mammalian cell culture system. The design of a bioreactor is strongly dependent upon its intended use as a production unit for cell mass and/or biologicals or as a research reactor for the study of cell growth and function. Therefore a variety of bioreactor configurations are presented in rapid summary. Following this, a rationale is presented for not attempting to derive key design parameters such as the oxygen transfer coefficient from ground-based data. A set of themes/objectives for flight experiments to develop the expertise for design of space bioreactors is then proposed for discussion. These experiments, carried out systematically, will provide a database from which engineering tools for space bioreactor design will be derived

The use of niobia in oxidation catalysis

This paper summarises the background to work carried out at the University of Twente on the use of niobia as a catalyst for the oxidative dehydrogenation of propane to propylene and discusses the development of promoted niobia catalysts for this reaction. Results are also presented which illustrate the use of niobia in catalysts for other reactions such as the oxidative coupling of methane, the oxidative dehydrogenation of ethane and the oxidative dehydrogenation of methanol. It appears that niobia and niobia-modified catalysts, when used in high-temperature oxidation processes, can exhibit relatively high selectivities compared with more conventional catalysts

Influence of preparation method on the performance of vanadia-niobia catalysts for the oxidative dehydrogenation of propane

The influence of various preparation methods on the performance of V-Nb-0 catalysts has been investigated. It was found that the activity and selectivity of a vanadium site depend on the nature of the neighbouring atoms. Vanadium neighbours provide activity, while niobium neighbours provide selectivity. Careful preparation of these catalysts ensures a homogeneous distribution and good mixing of the vanadium and niobium. It was also found that the vanadium becomes mobile upon reduction and this results in better distribution of vanadium in used catalysts

TAP investigations of the CO2 reforming of CH4 over Pt/ZrO2

The adsorption and reaction characteristics of methane, carbon dioxide, carbon monoxide, and hydrogen have been investigated over a ZrO2support and a Pt/ZrO2catalyst by using a temporal analysis of products reactor system. It was observed that on Pt/ZrO2both methane and carbon dioxide dissociate independently of one another. The dissociation of carbon dioxide acts as an oxygen supplier, while the decomposition products of methane scavenge the oxygen from the catalyst. When an abundance of oxygen is present, pulsing of methane leads to the production of carbon dioxide. It is concluded that both the selectivity with which methane produces carbon monoxide or carbon dioxide and the carbon dioxide conversion is determined by the same reaction: COads+Oads CO2,ads