Heat transfer mechanisms in poplar wood undergoing torrefaction

Torrefaction, a thermal treatment process of biomass, has been proved to improve biomass combustible properties. Torrefaction is defined as a thermochemical process in reduced oxygen condition and at temperature range from 200 to 300 °C for shorter residence time whereby energy yield is maximized, can be a bridging technology that can lead the conventional system (e.g. coal-fired plants) towards a sustainable energy system. In efforts to develop a commercial operable torrefaction reactor, the present study examines the minimum input condition at which biomass is torrefied and explores the heat transfer mechanisms during torrefaction in poplar wood samples. The heat transfer through the wood sample is numerically modeled and analyzed. Each poplar wood is torrefied at temperature of 250, 270, and 300 °C. The experimental study shows that the 270 °C-treatment can be deduced as the optimal input condition for torrefaction of poplar wood. A good understanding of heat transfer mechanisms can facilitate the upscaling and downscaling of torrefaction process equipment to fit the feedstock input criteria and can help to develop treatment input specifications that can maximize process efficiency.     

Selecting batch size in discrete-time two-phase queuing system

This paper discusses the modeling and analysis of a discrete-time two-phase queuing system. Packets arrive at the system according to a Bernoulli process and receive batch service in the first queue and individual service in the second queue. We study the queue length, average waiting time of packets in the first queue and the effect of batch size on the waiting time.     

Response time in a tandem queue with blocking, Markovian arrivals and phase-type services

A novel approach for obtaining the response time in a discrete-time tandem-queue with blocking is presented. The approach constructs a Markov chain based on the age of the leading customer in the first queue. We also provide a stability condition and carry out several numerical examples.     

Vacation models in discrete time

A class of single server vacation queues which have single arrivals and non-batch service is considered in discrete time. It is shown that provided the interarrival, service, vacation, and server operational times can be cast with Markov-based representation then this class of vacation model can be studied as a matrix–geometric or a matrix-product problem – both in the matrix–analytic family – thereby allowing us to use well established results from Neuts (1981). Most importantly it is shown that using discrete time approach to study some vacation models is more appropriate and makes the models much more algorithmically tractable. An example is a vacation model in which the server visits the queue for a limited duration. The paper focuses mainly on single arrival and single unit service systems which result in quasi-birth-and-death processes. The results presented in this paper are applicable to all this class of vacation queues provided the interarrival, service, vacation, and operational times can be represented by a finite state Markov chain.An erratum to this article can be found at     

Rapid detection of nitroaromatic and nitramine explosives on chromatographic paper and their reflectometric sensing on PVC tablets

Rapid and inexpensive sensing of explosive traces in soil and post-blast debris for environmental and criminological purposes with optical sensors has recently gained importance. The developed sensing method for nitro-aromatic and nitramine-based explosives is based on dropping an acetone solution of the analyte to an adsorbent surface, letting the solvent to dry, spraying an analytical reagent to produce a persistent spot, and indirectly measuring its reflectance by means of a miniature spectrometer. This method proved to be useful for on-site determination of nitro-aromatics (trinitrotoluene (TNT), 2,4,6-trinitrophenylmethylnitramine (tetryl) and dinitrotoluene (DNT)) and nitramines (1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)) pre-adsorbed on a poly vinyl chloride (PVC) surface, with the use of different spray reagents for each group of explosives producing different colors. The calibration equations of the tested compounds as reflectance vs. concentration showed excellent linearity (correlation coefficient: 0.998-0.999). The linear quantification interval in terms of absolute quantity of analyte was 0.1-0.5 μg. The developed method was successfully tested for the analysis of military explosives Comp B and Octol, and was validated against high performance liquid chromatography (HPLC). The reflectometric sensing method could also be used for qualitative identification of the nitrated explosives on a chromatographic paper. The reagent-impregnated paper could also serve as sensor, enabling semi-quantitative determinations of TNT and tetryl.     

Improved organocatalytic electrophilic α-cyanation of β-keto amides with 1-cyanato-4-nitrobenzene

By using a readily accessible, new and safe cyano-transfer reagent, 1-cyanato-4-nitrobenzene, the enantioselectivity of the direct electrophilic α-cyanation of 1-indanone-derived β-keto amides was greatly improved as a result of an enhanced double-hydrogen bonding. Thus, in the presence of cinchonine as the bifunctional organocatalyst, a series of α-cyano β-keto amides were produced in excellent yields (73–97%) and good to high enantioselectivities (75–91% ee) under mild reaction conditions.     

Synthesis and Photophysical Characterizations of Thermal -Stable Naphthalene Benzimidazoles

Microwave-assisted synthesis, photophysical and electrochemical properties of thermal-stable naphthalene benzimidazoles and naphthalimides are studied in this paper. Microwave-assisted synthesis of naphthalene benzimidazoles provide higher yields than the conventional thermal synthesis. Comparative photophysical properties of naphthalene benzimidazoles and naphthalimides are revealed that conjugation of electron-donating group onto naphthalimide moiety increases fluorescence quantum yields. Fluorophore-solvent interactions are also investigated using Lippert-Mataga equation for naphthalimides and naphthalene benzimidazoles. Thermal stabilities of naphthalene benzimidazoles are better than naphthalimides due to increased aromaticity. The experimental E_LUMO levels of naphthalene benzimidazoles are found to be between 3.15 and 3.28 eV. Therefore, naphthalene benzimidazole derivatives consisting of anchoring groups are promising materials in organic dye sensitized solar cells.     

Discrete time analysis of MAP/PH/1 vacation queue with gated time‐limited service

We analyse a single‐server queue in which the server goes through alternating periods of vacation and work. In each work period, the server attends to the queue for no more than a fixed length of time, T. The system is a gated one in which the server, during any visit, does not attend to customers which were not in the system before its visit. As soon as all the customers within the gate have been served or the time limit has been reached (whichever occurs first) the server goes on a vacation. The server does not wait in the queue if the system is empty at its arrival for a visit. For this system the resulting Markov chain, of the queue length and some auxiliary variables, is level‐dependent. We use special techniques to carry out the steady state analysis of the system and show that when the information regarding the number of customers in the gate is not critical we are able to reduce this problem to a level‐independent Markov chain problem with large number of boundary states. For this modified system we use a hybrid method which combines matrix‐geometric method for the level‐independent part of the system with special solution method for the large complex boundary which is level‐dependent. This revised version was published online in June 2006 with corrections to the Cover Date.