Michael Harasek

Design, simulation and application of a new micromixing device for time resolved infrared spectroscopy of chemical reactions in solution

We present a novel micromachined fast diffusion based mixing unit for the study of rapid chemical reactions in solution with stopped-flow time resolved Fourier transform infrared spectroscopy (TR-FTIR). The presented approach is based on a chip for achieving lamination of two liquid sheets of 10 microm thickness and approximately 1 mm width on top of each other and operation in the stopped-flow mode. The microstructure is made on infrared transmitting calcium fluoride discs and built up with two epoxy negative photoresist layers and one silver layer in between. Due to the highly laminar flow conditions and the short residence time in the mixer there is hardly any mixing when the two liquid streamlines pass through the mixing unit, which allows one to record a mid-IR transmission spectrum of the analytes prior to reaction. When the flow is stopped, the reactant streams are arrested in the flow-cell and rapidly mixed by diffusion due to the reduced interstream distances and the reaction can be directly followed with hardly any dead time. On the basis of two model reactions-neutralisation of acetic acid with sodium hydroxide as well as saponification of methyl monochloroacetate-the performance of the mixing device was tested revealing proper functioning of the device with a time for complete mixing of less than 100 ms. The experimental results were supported by numerical simulations using computational fluid dynamics (CFD), which allowed a reliable, quantitative analysis of concentration, pressure and flow profiles in the course of the mixing process.

Design and scale-up of an oxidative scrubbing process for the selective removal of hydrogen sulfide from biogas

Reliable and selective removal of hydrogen sulfide (H(2)S) is an essential part of the biogas upgrading procedure in order to obtain a marketable and competitive natural gas substitute for flexible utilization. A promising biogas desulfurization technology has to ensure high separation efficiency regardless of process conditions or H(2)S load without the use or production of toxic or ecologically harmful substances. Alkaline oxidative scrubbing is an interesting alternative to existing desulfurization technologies and is investigated in this work. In experiments on a stirred tank reactor and a continuous scrubbing column in laboratory-scale, H(2)S was absorbed from a gas stream containing large amounts of carbon dioxide (CO(2)) into an aqueous solution prepared from sodium hydroxide (NaOH), sodium bicarbonate (NaHCO(3)) and hydrogen peroxide (H(2)O(2)). The influence of pH, redox potential and solution aging on the absorption efficiency and the consumption of chemicals was investigated. Because of the irreversible oxidation reactions of dissolved H(2)S with H(2)O(2), high H(2)S removal efficiencies were achieved while the CO(2) absorption was kept low. At an existing biogas upgrading plant an industrial-scale pilot scrubber was constructed, which efficiently desulfurizes 180m(3)/h of raw biogas with an average removal efficiency of 97%, even at relatively high and strongly fluctuating H(2)S contents in the crude gas.

Time-Resolved FT-IR Spectroscopy of Chemical Reactions in Solution by Fast Diffusion-Based Mixing in a Micromachined Flow Cell

A new concept for the study of chemical reactions in solution by time-resolved Fourier transform infrared spectroscopy (TR/FT-IR) is presented. The key element of this concept is a micromachined mixing unit for fast and highly reproducible diffusion-based mixing that is incorporated in a flow cell for transmission measurements and operated in the stopped-flow mode. The mixing unit achieves multilamination of two liquid streamlines inside the flow cell. When the flow in both feeding channels is maintained, there is almost no mixing of the liquids, because of the short residence time inside the mixer, hence allowing for the recording of a reference spectrum of the reactants prior to reaction. When the flow is stopped by rapid switching of a dedicated injection valve, highly reproducible diffusion-controlled mixing takes place inside the flow cell so that spectral changes induced by the reaction under investigation can be directly followed. The total volume required for one experiment is ∼ 5 μL, and mixing times achieved so far are in the millisecond range. Factors governing time resolution in this new concept are the time required to stop the flow, the spacing of the individual streamlines, the diffusion coefficients of the reactants involved, and the signal strength of the spectral changes induced by the reaction under study. In this paper, the possibilities and limitations of the new concept are studied with the use of three model reactions, which are an acid-base neutralization reaction, the addition of sulfite to formaldehyde, and the basic hydrolysis of methyl monochloroacetate. In addition, the complete mixing process in the system was studied by computational fluid dynamics (CFD) simulations, which provided valuable insights into details of the mixing process itself as well as confirming the experimental results obtained.

Flow-Through Picoliter Dispenser: A New Approach for Solvent Elimination in FT-IR Spectroscopy

A new interface for FT-IR analysis of liquid samples on the basis of solvent elimination is presented. The approach is based on a piezoactuated flow-through microdispenser, a device built of two microstructured silicon wafers designed for micro-liquid handling. It could be verified during preliminary studies using a sequential injection (SI) system for automated liquid handling that the flow-through microdispenser as a possible interface for flow system–FT-IR analysis has the capability of meeting the demands of hyphenated miniaturized liquid handling systems (e.g., μ-HPLC, microhigh performance liquid chromatography), as it successfully provides highly stable, reliable and reproducible operating conditions for liquid handling in the picoliter range. Moreover, an increase in sensitivity for FT-IR measurements could be achieved, lowering the mass detection limit of sugars (such as the investigated sucrose) to 53 picograms. As is demonstrated on the example of an HPLC separation of a mixture of glucose and fructose, interfacing LC systems to FT-IR using a piezoactuated flow-through microdispenser is a feasible and promising approach.

Influence of Vortex-Finder Diameter on Axial Gas Flow in Simple Cyclone

Several types of flow patterns in cyclones were found in recent publications. As a means for structuring the findings the publications were split up into two classes. Each class represented a certain feature of the axial velocity profile of the gas flow inside the cyclone. Class V represents cyclones with a maximum of the axial velocity at the vortex core of the cyclone, whereas Class W represents cyclones with an axial velocity profile that resembles an upside down W. This class has a local minimum of axial velocity at the vortex core or even displays backflow.The geometries of the cyclones belonging either to Class V or to Class W showed only small relative differences. It was assumed that the diameter of the vortex finder had a great influence on the class membership. This influence was simulated and measured for different basic geometries. The goal of this work was to develop the basics for reliable simulation results using the preprocessor GAMBIT and the finite volume CFD code FLUENT (which also serves as a postprocessor). Laser Doppler Anemometry (LDA) experiments were carried out on a laboratory scale cyclone to verify simulation results.In addition the results of cyclones which were investigated in previously published papers were used as a basis for developing a method of simulation, to validate numerical results and class membership. The prediction of pressure drop over the cyclone is overestimated by FLUENT (regarding the test cases). It was proven experimentally that downstream geometry (pipes etc.) has a great influence on flow profiles inside the cyclone. The axial flow profile can change dramatically if the pipes downstream the vortex finder are removed. A Class V cyclone displaying a maximum of axial velocity at the vortex core can change to a Class W cyclone with a minimum of axial velocity at the vortex core.

Chemical-oxidative scrubbing for the removal of hydrogen sulphide from raw biogas: potentials and economics

In the present work chemical-oxidative scrubbing as a novel method for the desulphurisation of raw biogas is presented with a special focus on the process potentials and economics. The selective absorption of hydrogen sulphide from gas streams containing high amounts of carbon dioxide using caustic solutions is not trivial but has been treated in literature. However, the application of this method to biogas desulphurisation has not been established so far. Based on rigorous experimental work, an industrial-scale pilot plant has been designed, erected and commissioned at a biogas plant with biogas upgrading and gas grid injection in Austria. Data collected from the 12-month monitored operation has been used to elaborate performance as well as economic parameters for the novel desulphurisation method. The proposed technology offers significant operational advantages regarding the degree of automation and the flexibility towards fluctuations in process boundary conditions. Furthermore, the economic assessment revealed the high competitiveness of the chemical-oxidative scrubbing process compared with other desulphurisation technologies with the named advantageous operational behaviour.

CFD-simulation of mass transfer effects in gas and vapour permeation modules

Two main factors are important for the design of membrane modules used for gas and vapour permeation: concentration polarisation and flow distribution. The former causes a reduced driving force, significantly affecting membrane performance. A uniform flow distribution will ensure that the complete membrane area is utilised. In order to reduce the influence of concentration polarisation and to ensure an even flow distribution spacers located between two membrane surfaces or plates containing flow channels are employed. A comparison between these geometries using computational fluid dynamics (CFD) is presented. For CFD calculations, the commercial solver FLUENT™ has been used and the mass transfer through the membrane has been modelled by user defined functions.

Biogas desulfurization and biogas upgrading using a hybrid membrane system - modeling study.

Membrane gas permeation using glassy membranes proved to be a suitable method for biogas upgrading and natural gas substitute production on account of low energy consumption and high compactness. Glassy membranes are very effective in the separation of bulk carbon dioxide and water from a methane-containing stream. However, the content of hydrogen sulfide can be lowered only partially. This work employs process modeling based upon the finite difference method to evaluate a hybrid membrane system built of a combination of rubbery and glassy membranes. The former are responsible for the separation of hydrogen sulfide and the latter separate carbon dioxide to produce standard-conform natural gas substitute. The evaluation focuses on the most critical upgrading parameters like achievable gas purity, methane recovery and specific energy consumption. The obtained results indicate that the evaluated hybrid membrane configuration is a potentially efficient system for the biogas processing tasks that do not require high methane recoveries, and allows effective desulfurization for medium and high hydrogen sulfide concentrations without additional process steps.

Improvement of a Combustion Unit Based on a Grate Furnace for Granular Dry Solid Biofuels Using CFD Methods

The steadily increasing domestic and industrial energy demand can only be satisfied by optimal utilization of all available resources. The aim of this project is the design and construction of an improved small-scale combustion unit for biofuels like wood, straw pellets, and especially grain. Using computational fluid dynamics (CFD) methods and measurement data from a pilot unit, this study contributes to the continuous enhancement of the biomass firing technology by addressing the commonly known problems regarding emissions and ash melting. Based on the calculated results improvements for the existing prototype, new geometry has been suggested and was planned to be included in the design of a new 1.5-MW pilot-scale grate firing unit that was to start operation in early 2009.

A knowledge based system to support the process selection during waste water treatment

For the treatment of industrial wastewater a multitude of processes is available. It is the task of the chemical engineer to choose processes and to build up a treatment sequence suitable for the actual problem. Because of the large number of processes and process combinations, a knowledge-based system may be helpful during process selection and sequencing. The introduced knowledge based system delivers a treatment sequence based on heuristics and cost estimation. Depending on the supplied information a more or less detailed treatment sequence is suggested by the program. The article describes the aim and development of the knowledge based system, gives an overview on the program functions and presents the result of an example program consultation.