Mats Larsson

Design and operation of a bench-scale process development unit for the production of ethanol from lignocellulosics

A bench-scale unit for the development of an enzymatic process for the bioconversion of lignocellulosics to ethanol has been used to study the recycling of waste-water streams to minimize fresh-water requirements and reduce effluent streams. Willow, after impregnation with sulphur dioxide, was steam-pretreated, enzymatically hydrolysed, and the sugars produced were fermented using S. cerevisiae. The fermentation broth was distilled and the stillage was fractionated by evaporation into six separate condensate fractions and a residue. The overall yield of ethanol from willow was 65% of the theoretical yield based on total fermentable sugars. The inhibitory effect of the evaporation condensates was assessed by fermentation using S. cerevisiae. The non-volatile residue of the stillage was found to be inhibitory to fermentation. The ethanol yield decreased from 0.37 g/g in a pure sugar reference to 0.31 g/g in the residue and the average ethanol fermentation rate decreased fi om 6.3 g/(l h) to 2.7 g/(l h), respectively. The evaporation condensates, containing the volatile components, showed no negative effects on fermentation. The intermediate evaporation condensate fractions, fractions 4 and 5, had the lowest chemical oxygen demand (GOD), 1560 and 1120 mg/l, compared with 33 300 mg/l for the stillage. Therefore, these fractions can be released directly into the effluent without further treatment. Copyright (C) 1997 Elsevier Science Ltd. (Less)

Emergency voltage control using search and predictive control

This paper presents a method of optimal coordination of load shedding, capacitor switching and tap changer operation using a dynamic system model in order to preserve or retain long-term voltage stability. The method is based on model predictive control and tree search. A model of the controlled system, including the network and load dynamics, is used to predict the future system behaviour based on the current state and applied control actions. The optimal control state according to these predictions is then obtained using a search method similar to those used in chess computers. A detailed and three simplified predictors are presented, along with a tuning procedure that aims to ensure that a minimum amount of emergency control is scheduled by the controller.

Adsorbents for extractive bioconversion applied to the acetone-butanol fermentation

SummaryFour different polymeric resins were tested as adsorbents in extractive bioconversion applied to the fermentative production of acetone and butanol by Clostridium acetobutylicum. The polymers were tested for their ability to adsorb butanol from pure solutions, and fermentation broths. Furthermore, the effect on the fermentability of the media was tested. The pH was increased to prevent adsorption of intermediates such as acetic and butyric acids. Bonopore, the polymer giving the best adsorption pattern with no undesirable effects, was tested in repeated batch cultures with C. acetobutylicum.

Comparison of Three Electromechanical Oscillation Damping Estimation Methods

This paper describes three data driven methods to monitor electromechanical oscillations in interconnected power system operation. The objective is to compare and contrast the performance of the methods. The accuracy of damping ratio and frequency of oscillations are the measures of the performance of the algorithms. The advantages and disadvantages of various techniques and their limitations to measurement noise have been considered while assessing performance. The target frequency and damping are computed using the Nordic power system simulation model.

ObjectStab-an educational tool for power system stability studies

Traditionally, the simulation of transient and voltage stability in power systems has been constrained to domain-specific tools such as Simpow, PSS/E, ETMSP, and EuroStag. While being efficient and thereby able to simulate large systems, their component models are often encapsulated and difficult or impossible to examine and modify. Also, these simulators often require substantial training and are therefore not ideal for normal classroom use. For academic and educational use, it is more important that the component modeling be transparent and flexible, and that students quickly get started with their simulations. This paper describes a freely available power system library called ObjectStab intended for power system stability simulations written in Modelica, a general-purpose object-oriented modeling language. All component models are transparent and can easily be modified or extended. Power system topology and parameter data are entered in one-line diagram form using a graphical editor. The component library has been validated using comparative simulations with EuroStag.

Recirculation of process water in the production of ethanol from softwood

In the production of ethanol from lignocellulosics, large quantities of water are needed in various process steps. In industrial processes, recycling of process streams is necessary to minimise fresh water requirements and decrease the amount of wastewater produced. This leads to increases in various substances, both volatile and non-volatile, which inhibit fermentation. In the present study ethanol was produced from softwood in a bench-scale process development unit. The stillage stream from the distillation step was fractionated by evaporation and various evaporation fractions characterised and their inhibitory effect on fermentation investigated. It was found that the volatile fractions have no negative effect on the fermentation, while the non-volatile fraction inhibited fermentation and resulted in a decreased yield. Simulation of a six-effect evaporation unit based on the experimental data shows that the intermediate evaporation condensate fractions are most suited for disposal. The chemical and biological oxygen demands are only about 10% of the value in the stillage stream. Evaporation has been shown to be an effective way to drastically diminish the build-up of inhibitory compounds when process streams are recirculated, but the energy demand is high. Various process configurations to reduce the energy demand are suggested.

Extractive Bioconversions with Emphasis on Solvent Production

Necessite de lextraction du produit (inhibition de la fermentation par le produit, risque de denaturation du produit dans le milieu de fermentation). Fermentation en milieu biphasique: phase aqueuse plus phase organique contenant un solvant, ou deux phases aqueuses. Utilisation de CO 2 a letat supercritique. Adsorption en ligne. Fermentation sous pression reduite. Separation par membrane. Separation par gravite (floculation, centrifugation). Cellules immobilisees. Permeabilisation des cellules

Metabolic Behavior of Immobilized Cells—Effects of Some Microenvironmental Factorsa

Variations in microenvironmental conditions have been demonstrated to be important on the subcellular level. Model studies on immobilized enzymes have helped to clarify the picture. However, when cells have been immobilized, changed metabolic behavior has been reported in several cases, but very little is known about the mechanisms behind these changes. The new environment created in the immobilization process differs from that in free solution, for example, by the presence of high concentrations of polymers and by impeded diffusion of reactants (nutrients). The presence of polymer can be important in the sense that these molecules can directly interact with the cells, but probably the most overlooked effect is that polymers cause a change in water activity of the medium. When setting up a system to study the effects of the microenvironment on the metabolic behavior of cells, it may be useful to simplify the system. Suspensions of cells in water solutions of polymers represent such a simplified model. In the present study, polyethylene glycol (PEG) was used to study the effects of a decreased water activity on cell metabolism, thus simulating the behavior of immobilized cells. In order to prepare suspensions with a defined water activity, methods for calculating the reduction in water activity for buffered polymer-rich media have been developed.’ The water activity is defined as:

Global Control of Complex Power Systems

This chapter presents an overview of recent results on an approach to total control of power systems. It is upwards compatible from any conventional or prior advanced control and provides a framework for coordinated development of control to address all major dynamical problems. The approach appears applicable to complex systems generally where behaviour is influenced by nonlinearity, large-scale, uncertainty and hybrid nature. In particular, the presence of bifurcations of various kinds is allowed for while using ideas from switching and optimal control. The approach will be illustrated by consideration of coordinated control for transient stability, voltage regulation and emergency voltage control of power systems.

Limit cycles in power systems due to OLTC deadbands and load-voltage dynamics

Abstract In this paper, the phenomenon of limit cycle due to load–tap changer interaction is investigated. The role of OLTC deadband in creating a limit cycle phenomenon is addressed. The impact of tap dynamics modelling on system/voltage behaviour is carefully considered. Analytical insights into observed behaviour of power systems with different tap models are given using eigenvalue analysis, the describing function method and time simulation. The conditions for occurrence of limit cycles and the identification of key parameters influencing their characteristics are another important aspect of this investigation.