Synchronous and Sequential Strategies in the Process Design of Cascade Equipment

Cascade or multistage equipment is characterized by the repetition of similar equipment elements in series. Process design, resulting into the main geometric and kinematic dimensions of the equipment, makes use of different strategies. These strategies, based on a process description, the (equality- and inequality) constraints and the number of degrees of freedom of the mathematical system, which describes the process, can be divided in synchronous- and sequential procedures. In a synchronous strategy no a priori requirements are made as to the distribution of a given process variable over the stages, so that the equipment dimensions are obtained simultaneously. In contrast to this a sequential strategy makes use of a priori statements resulting into stage-to-stage calculations and a decreasing number of degrees of freedom. The general theory presented with detailed information on process description, constraints and degrees of freedom, has been applied to the process design of a multi-stage centrifugal compressor

Studia Litteraria

Németh G. Béla: Nagy Miklós hetvenedik évére p. 5-9. Tamás Attila: Petőfi Sándor időszemléletéről p. 9-19. D eb re czeni Attila: Remény nélküli boldogság (Petőfi költői versei) p. 19-33. Rácz István György: „Apokalipszis ma" (Vörösmarty Mihály: Előszó) p. 33-51. Csűrös Miklós: Pázmán lovag Gondolatok Arany „víg balladájáéról p. 51-59. Nyilasy Balázs: Arany János naiv regéje a teljességről: Rege a csodaszarvasról p. 59-67. S. Varga Pál: Egy új Komjáthy-kiadás körvonalai p. 67-101. Lőkös István: Látlelet a századvégről egy horvát regényben (J. Leskovar: Roskadozó kastélyok) p. 101-113. Imre László: „Műfajfejlődés" vagy „rendszercsere"? (Bevezetés egy irodalmi korszak vizsgálatához) p. 113-125. Szegedy-Maszák Mihály: Az összehasonlító irodalomtudomány időszerűsége p. 125-136. Resumé p. 136-139. Regiszter a Studia Litteraria megjelent számaihoz p. 139-145. Névmutató p. 145-147

Modeling bursts and heavy tails in human dynamics

Current models of human dynamics, used from risk assessment to communications, assume that human actions are randomly distributed in time and thus well approximated by Poisson processes. We provide direct evidence that for five human activity patterns the timing of individual human actions follow non-Poisson statistics, characterized by bursts of rapidly occurring events separated by long periods of inactivity. We show that the bursty nature of human behavior is a consequence of a decision based queuing process: when individuals execute tasks based on some perceived priority, the timing of the tasks will be heavy tailed, most tasks being rapidly executed, while a few experiencing very long waiting times. We discuss two queueing models that capture human activity. The first model assumes that there are no limitations on the number of tasks an individual can hadle at any time, predicting that the waiting time of the individual tasks follow a heavy tailed distribution with exponent alpha=3/2. The second model imposes limitations on the queue length, resulting in alpha=1. We provide empirical evidence supporting the relevance of these two models to human activity patterns. Finally, we discuss possible extension of the proposed queueing models and outline some future challenges in exploring the statistical mechanisms of human dynamics.Comment: RevTex, 19 pages, 8 figure

Determination of membrane properties for use in the modelling of a membrane distillation module

Membrane distillation carried out in a counter current flow module, in which latent heat is recovered by heating the incoming cold seawater with the condensing product water flow, is a promising technology for low cost seawater desalination. The membranes used in this module are hydrophobic (polypropylene, polyethylene) and highly permeable fibre membranes. For modelling purposes the Knudsen diffusion and viscous flow membrane characteristics (K0 and B0 respectively) of five fibre membranes are determined. This paper presents a new, specially developed method for the determination of K0 and B0 values of highly permeable fibre membranes with single gas permeation experiments through a short dead end fibre. In order to be able to make use of a reliable method to determine the values of K0 and B0, it is essential that the pressure inside the permeable part of the membrane is constant. To determine the conditions at which the pressure drop in the permeable part of the membrane fibre is negligible, this part is reduced in length until the values of K0 and B0 become constant. For all membranes the gases He, N2 and CO2 were used. The gases N2 and CO2 lead to consistent values of K0 and B0. Helium gives less accurate results due to its low molecular weight. The three polypropylene membranes have a similar structure and have therefore about the same values for K0 and B0. The same was found for the two polyethylene fibres

Modelling of a transmembrane evaporation module for desalination of seawater

Transmembrane evaporation (often called membrane distillation) carried out in a countercurrent flow module, in which incoming cold seawater is heated by the condensing product water flow, is a promising technology for low-cost seawater desalination. This paper presents a model for preliminary design calculations for such a module. The model calculates temperature profiles alongside and across the module, heat and vapour fluxes through the membrane and the total product flow. Mass transfer of the water vapour through the membrane and the air gap is described with molecular diffusion through stagnant air. Estimated properties and constants have been used to calculate the energy transport. Calculations with variable temperatures, membrane fibre diameter and diffusion distance have been performed. From the calculation results can be concluded that the model is able to describe the process of interest in a qualitative way. The calculations indicate that the highest productivity will be obtained with high temperatures, small membrane fibres and a small air gap