Jobst Hapke

Measurement of the effective thermal conductivity of a Mg–MgH2 packed bed with oscillating heating

The magnesium–magnesium hydride–hydrogen-system (Mg–MgH2–H2) offers, because of its combined hydrogen and heat storage capacity, the possibility to design hydride heat pumps and heat stores. For such industrial application systems based on cylindrically formed reactors filled with an active magnesium powder, the effective thermal conductivity limits the time in which the metal hydride alloy is charged and discharged with hydrogen. Determination of this transport coefficient is of fundamental importance for the optimum design of magnesium hydride reactors. The complex interrelation of the different transport mechanisms in a metal hydride packed bed and the hitherto undefined rule that the solid effective thermal conductivity behaves as a function of the hydrogen concentration, requires a reliable and simple-to-realize measuring method so as to determine the effective thermal conductivity of a magnesium hydride bed. In the present study, a report is given for the first time on the initiation of a measuring technique with oscillating change of temperature in a non-permeated packed bed of fine-grained material. The measurement of the effective thermal conductivity can ensue by tailoring the problem-specific mathematical result to the experimentally recorded temperature-time function. The effective thermal conductivity of the magnesium hydride bed varies between 2 and 8 W/(m K) in a temperature range of 523–653 K.

A Comprehensive Characterization of Commercial Nanofiltration Membranes

Abstract This paper presents a comprehensive characterization study of NF 90 (Filmtec), NF 270 (Filmtec), NF 2 (Sepro), and NF PES 10 (Microdyn‐Nadir) membranes by their morphology, charge, and separation performance parameters. Extensive investigations of the relations between membrane performance and membrane charge/morphological properties were performed by various measurement methods and active layer mass transport modelling. For this purpose, a broad experimental program was exerted with saccharides and polyethylene glycol at various pressures and with single salt solutions of NaCl, CaCl2, Na2SO4, and MgSO4 at various concentrations. pH effect on membrane performances was investigated as well. A Fortran program code of a hydrodynamic model was developed in order to determine the pore sizes of the membranes using the experimental data with organics. In addition, an equation‐based software, COMSOL Multiphysics, was utilized for the modelling with salt solutions. This eliminated the effort demanding program code writing for concentration and potential differential equations derived from extended Nernst‐Planck (ENP) equation, which is a common method in many publications. By this model, transport phenomena inside the membrane, relation between the membrane performance and its charge, as well as performance prediction of the membranes could be quested in detail.

Measurement of the pressure-composition isotherms of high-temperature and low-temperature metal hydrides

Many interesting applications are based on the knowledge of the thermodynamic equilibrium of the metal–metal hydride–hydrogen system. Practical applications for this system include hydrogen storage for stationary and vehicular purposes, hydrogen compression, heat pumps, cooling apparatus as well as heat stores. One of the most advantageous metal hydrides for high-temperature heat stores is nickel-doped magnesium, a material with excellent kinetic properties, high hydrogen concentration, low density and high heat of reaction. A disadvantage of magnesium is the minimum reaction temperature of about 423 K. The objective of this research work is to find low-temperature metal hydrides whose reaction enthalpy ΔrH is able to heat up the magnesium, starting from ambient temperature. In this study the results of an isochoric measurement method for determining pressure–composition isotherms (PCI), reaction enthalpies and reaction entropies are presented on the basis of already investigated as well as hitherto unsurveyed metal hydrides. The measuring method considers the real gas behavior in addition to a test reactor, which employs an improved sealing system. Presented in this study are the results of five metal–metal hydride–hydrogen systems in thermodynamic equilibrium. The measuring method has proved to be reliable because of the test reactors sealing system.

Approach for the Determination of Heat Transfer Coefficients for Filling Processes of Pressure Vessels With Compressed Gaseous Media

For fast and effective simulation of filling processes of pressure vessels with compressed gaseous media, the governing equations are derived from a mass balance equation for the gas and from energy balance equations for the gas and the wall of the vessel. The gas is considered as a perfectly mixed phase and two heat transfer coefficients are introduced. The first one is the mean heat transfer coefficient between the gas and the inner surface of the pressure vessel, and the second one is the heat transfer coefficient between outer surface of the vessel and the surroundings. Because of the heat capacity of the wall of the pressure vessel, heat transfer from the compressed gas to the vessel wall strongly influences the temperature field of the gas. Until now no correlations have been available for the heat transfer coefficient between inflowing gas and inner surface of the vessel. To solve this problem, a computational fluid dynamics tool is used to determine the gas velocities at the vicinity of the inner surface of the vessel for a number of discrete surface elements. The results of a large amount of numerical experiments show that there exists a unique relationship between the gas velocity at the inlet and the tangential fluid velocities at the vicinity of the inner surface of the vessel for each vessel geometry. Once this unique relationship is known, the complete velocity distribution at the vicinity of the inner surface can be easily calculated from the inlet gas velocity. The near-wall velocities at the outer limit of the boundary layer are substituted into the heat transfer correlation for external flow over flat plates. The final heat transfer coefficient is the area-weighted mean of all local heat transfer coefficients. The method is applied to the special case of filling with hydrogen a 70-MPa composite vessel for fuel cell vehicles.

Mathematical modeling of a continuous aerobic membrane bioreactor for the treatment of different kinds of wastewater

In this paper, a mathematical model, which describes a continuous aerobic membrane bioreactor (MBR) for the treatment of different kinds of wastewater, is developed. Firstly, a black box model is developed and verified by comparing simulation results with experimental data. In a second step, two shortcut models of different complexity are developed and verified. Both shortcut models use the black box balancing as a basis. However, component balances of carbon and oxygen are modified in order to connect the degradation of pollutants to the supply of substrate and dissolved oxygen. Both models show an increasing degree of agreement with experimental data.

Steady-State and Transient Behavior of Two Heat Exchangers Coupled by a Circulating Flowstream

Systems consisting of two heat exchangers coupled by a circulating flowstream are studied. The systems differ in the flow configurations of the single heat exchangers. For steady-state operation, there exists a heat capacity rate of the circulating flowstream that maximizes the temperature changes of the external flowstreams. Until now, this optimum has been calculated assuming that the overall heat transfer coefficients of the heat exchangers do not depend on the mass flow rate of the circulating flowstream. In this paper, the dependence of the overall heat transfer coefficient on the mass flow rate of the circulating flowstream is taken into account. For transient operating conditions, the system response to perturbations of inlet temperatures and mass flow rates is calculated by the method of Laplace Transforms and an explicit finite difference method. The most significant features of the coupled system become apparent by considering outlet temperature transients induced by perturbations of the mass flow rate of the circulating flowstream.

Investigations and simulation of a membrane bioreactor system for application in remote areas

The increasing demands for wastewater treatment and the establishment of new processes and systems necessitate applied research. Especially in the field of membrane technology new areas for wastewater treatment can be opened for implementation. Membrane bioreactors usually find their application field either for the treatment of high strength wastewater or where only little installation space is available. As there are only dissatisfying solutions for wastewater treatment on board ships, a highly efficient plant was invented. The combination of a bioreactor and ultrafiltration membranes is an encouraging possibility to overcome the requirements and gain permeate with low COD values. The result was a compact and modularised system with only little space required which was called Bio-Filt®. With this system there were obtained good experiences concerning the operation and the biological decomposition of wastewater. The simulation of the system gave good results, especially for the growth of the microorganisms in the bioreactor. Due to the variability of the system, the chance for the application with alternating loads and hydraulic demands is given. The new plant system complies with the requirements in such a way so that there was a noticeable step forward for environmental protection.

Rigorous Approach to Modeling of a Continuous Aerobic Membrane Bioreactor

A rigorous approach to mathematical modeling of a continuous aerobic membrane bioreactor (MBR) for the treatment of wastewater is reported. The idea is to apply the activated sludge model ASM3 to the special configuration of a membrane bioreactor. Therefore, the biochemical processes modeled by the ASM3 were implemented together with mass balances typical of a MBR running at constant TSS. The model parameters were adapted to the properties of an artificial wastewater by using a global search algorithm. The model could be validated by comparing effluent chemical oxygen demand (COD), sludge production and CO2 concentration in the exhaust to the experimental data.