Variable Selection in Regression using Multilayer Feedforward Network

The selection of relevant variables in the model is one of the important problems in regression analysis. Recently, a few methods were developed based on a model free approach. A multilayer feedforward neural network model was proposed for developing variable selection in regression. A simulation study and real data were used for evaluating the performance of proposed method in the presence of outliers, and multicollinearity

Robust Winsorized Shrinkage Estimators for Linear Regression Model

In multiple linear regression, the ordinary least squares estimator is very sensitive to the presence of multicollinearity and outliers in the response variable. To handle these problems in the data, Winsorized shrinkage estimators are proposed and the performance of these estimators is evaluated through mean square error sense

Comparison of Estimators in GLM with Binary Data

Maximum likelihood estimates (MLE) of regression parameters in the generalized linear models (GLM) are biased and their bias is non negligible when sample size is small. This study focuses on the GLM with binary data with multiple observations on response for each predictor value when sample size is small. The performance of the estimation methods in Cordeiro and McCullagh (1991), Firth (1993) and Pardo et al. (2005) are compared for GLM with binary data using an extensive Monte Carlo simulation study. Performance of these methods for three real data sets is also compared

Support Vector Machine-based Modified Sp Statistic for Subset Selection with Non-Normal Error Terms

Support vector machine (SVM) is used for estimation of regression parameters to modify the sum of cross products (Sp). It works well for some nonnormal error distributions. The performance of existing robust methods and the modified Sp is evaluated through simulated and real data. The results show the performance of the modified Sp is good

Experimental and modelling studies on liquid-liquid slug flow capillary microreactors

Microreactor technology, an important method of process intensification, offers potential benefits to the chemical process industries due to the well-defined high specific interfacial area available for heat and mass transfer, which increases transfer rates, and enhances safety resulting from low hold-ups. The liquid-liquid slug flow capillary microreactor intensifies the mass transfer in biphasic systems through internal circulation caused by the shear between continuous phase/wall surface and slug axis, which enhances diffusive penetration and consequently increases the reaction rates observed. This reactor concept has been exploited for mass transfer and chemical reaction without detailed hydrodynamics involved and it is this aspect which is the subject of the analysis of the research presented. The present work highlights the hydrodynamics and mass transfer characteristics of the liquid-liquid slug flow capillary microreactors, which were investigated using complementary state-of-the-art experimental and computational techniques. Experiments were carried out to ascertain the extent of slug flow regime, to measure slug size and pressure drop and to investigate the mass transfer coefficients and effective interfacial areas. The results were compared with conventional contactors and it was found that such liquid-liquid microextractor-reactor offers superior performance and greater efficiency. A flow splitting element based on the preferential wettability of liquids with different solid materials was developed for the downstream separation of liquids and showed very good flow splitting providing precise contact time of biphasic mixture and thus accurate determination of mass transfer rates. Furthermore, CFD methodologies, developed to characterise internal circulations, to capture the slug flow generation mechanism and to study mass transfer with and without chemical reaction, improved the understanding and generated results showing that the processes relevant to this reactor concept can be studied using these methodologies. Thus precise tuning of hydrodynamics and mass transfer together with easy downstream separation of liquids and the excellent accessibility for process modelling suggest that liquid-liquid slug flow in capillaries is a powerful laboratory technique for elucidating the processes involving biphasic mixture and for identifying asymptotic performance limits in such processes.Die Mikroreaktortechnik, eine wichtige Methode zur Prozessintensivierung, bietet die Möglichkeit, chemische Prozesse sicher und mit hohen Ausbeuten zu betreiben. In einem flüssig-flüssig Kapillarmikroreaktor liegt eine gut definierte Strömung mit alternierenden Pfropfen vor. Innerhalb der Flüssigkeits-Pfropfen befinden sich Rezirkulationsströmungen, die den Stoffaustausch an den Phasengrenzen erhöhen und damit zu höheren effektiven Reaktionsraten führen. Bisherige Arbeiten, die Stofftransport und Reaktionen im Kapillarmikroreaktor untersuchten, berücksichtigten keine Detaillberechnung der Strömung. Die Hydrodynamik wird in der vorliegenden Arbeit vertieft untersucht. Schwerpunkt der Forschungsarbeit war die Untersuchung der Hydrodynamik und der Stofftransportcharakteristik der Pfropfenströmung in einem flüssig-flüssig Kapillarmikroreaktor. Dies wurde mit Hilfe moderner experimenteller und simulationsbasierter Methoden erzielt. Die Experimente konzentrierten sich auf die Abgrenzung der Strömungsregime sowie auf die Ermittlung der Pfropfengröße, des Druckverlustes und Stoffübergangskoeffizienten und der effektiven Phasengrenzfläche. Beim Vergleich des Kapillarmikroreaktors mit herkömmlichen Apparaten wurde sowohl eine überlegene Leistung als auch eine größere Effizienz festgestellt. Weiterhin konnte eine Vorrichtung zur Trennung des zweiphasigen Gemisches entwickelt werden. Das Trennungselement befindet sich am Ende der Misch- und Reaktionsstrecke und nutzt gezielt die unterschiedlichen Benetzungseigenschaften der beiden Flüssigkeiten aus und hat sich in Versuchen sehr gut bewährt. CFD Simulationen wurden durchgeführt, um die Rezirkulationsströmung zu berechnen, und die Entstehung und Stabilität der Pfropfenströmung besser verstehen zu können. Bei der Untersuchung des Stoffübergangs wurde mit und ohne reaktiven Anteilen gerechnet. Die angewendeten CFD-Methoden haben sich insgesamt als gut geeignet erwiesen. Zusammenfassend lässt sich sagen, dass mit dem Kapillarmikroreaktor eine genaue Einstellung der Hydrodynamik und des Stoffübergangs in Verbindung mit einer einfachen Phasentrennung und der ausgezeichneten Zugänglichkeit für Prozessmodellierung zur Verfügung steht. Damit können nun im Labor die limitierenden Einflüsse bzw. asymptotischen Leistungen von flüssig-flüssig Systemen aufgeklärt werden

Microstructured Reactors for Multiphase Reactions: State of the Art

The manufacture of chemicals in microstructured reactors (MSR) has become recently a new branch of chemical reaction engineering focusing on process intensification and safety. MSR have an equivalent hydraulic diameter up to a few hundreds of micrometers and, therefore, provide high mass- and heat-transfer efficiency increasing the reactor performance drastically, compared to the conventional one. This article provides a comprehensive overview of the state of the art of the MSR applied for multiphase reactions. The reactions are classified based on the number of phases involved: fluid-fluid, fluid-solid, and three phase reactions. In the first part of the review, limitations of conventional reactors are discussed in brief. Furthermore, different types of MSR and their advantages with respect to their conventional counterparts are described. Particular attention is given to the identification of the parameters that control the flow pattern formed in microcapillaries regarding the mass-transfer efficiency. Case studies of various multiphase reactions carried out in MSR are discussed in detail

Effect of substrate thermal resistance on space-domain microchannel

In recent years, Fluorescent Melting Curve Analysis (FMCA) has become an almost ubiquitous feature of commercial quantitative PCR (qPCR) thermal cyclers. Here a micro-fluidic device is presented capable of performing FMCA within a microchannel. The device consists of modular thermally conductive blocks which can sandwich a microfluidic substrate. Opposing ends of the blocks are held at differing temperatures and a linear thermal gradient is generated along the microfluidic channel. Fluorescent measurements taken from a sample as it passes along the micro-fluidic channel permits fluorescent melting curves to be generated. In this study we measure DNA melting temperature from two plasmid fragments. The effects of flow velocity and ramp-rate are investigated, and measured melting curves are compared to those acquired from a commercially available PCR thermocycler

Numerical studies of shear-thinning droplet formation in a microfluidic T-junction using two-phase level-set method

A conservative level-set method (LSM) embedded in a computational fluid dynamics (CFD) simulation provides a useful approach for the studying the physics and underlying mechanism in two-phase flow. Detailed two-dimensional (2D) computational microfluidics flow simulations have been carried out to examine systematically the influence of different controlling parameters such as flow rates, viscosities, surface wettability, and interfacial tensions between two immiscible fluids on the non-Newtonian shear-thinning microdroplets generation process. For the two-phase flow system that neglects the Marangoni effect, the breakup process of shear-thinning microdroplets in cross-flowing immiscible liquids in a microfluidic device with a T-shaped geometry was predicted. Data for the rheological and physical properties of fluids obeying Carreau-Yasuda stress model were empirically obtained to support the computational work. The simulation results show that the relevant control parameters mentioned above have a strong impact on the size of shear-thinning droplets generated. Present computational studies on the role and relative importance of controlling parameters can be established as a conceptual framework of the non-Newtonian droplet generation process and relevant phenomena for future studies