Liisa Rihko-Struckmann

Electrochemical Oxidation of Carbon-Containing Fuels and Their Dynamics in Low-Temperature Fuel Cells

Fuel cells can convert the energy that is chemically stored in a compound into electrical energy with high efficiency. Hydrogen could be the first choice for chemical energy storage, but its utilization is limited due to storage and transport difficulties. Carbon-containing fuels store chemical energy with significantly higher energy density, which makes them excellent energy carriers. The electro-oxidation of carbon-containing fuels without prior reforming is a more challenging and complex process than anodic hydrogen oxidation. The current understanding of the direct electro-oxidation of carbon-containing fuels in low-temperature fuel cells is reviewed. Furthermore, this review covers various aspects of electro-oxidation for carbon-containing fuels in non-steady-state reaction conditions. Such dynamic investigations open possibilities to elucidate detailed reaction kinetics, to sense fuel concentration, or to diagnose the fuel-cell state during operation. Motivated by the challenge to decrease the consumption of fossil fuel, the production routes of the fuels from renewable resources also are reviewed.

Operating Behavior and Scale-Up of an EXPrOx Unit for CO Removal from Reformate for PEM Fuel Cell Application

Recently, an approach involving electrochemical preferential oxidation (ECPrOx) of CO was suggested as having the potential to replace the PrOx concept for deep CO removal from reformate gas in proton exchange membrane (PEM) fuel cells. The first part of this paper deals with the characterization of such an ECPrOx unit from a reaction engineering point of view. Based on a spatially lumped, isothermal model, the qualitative selectivity-conversion behavior is discussed for varying feed flow rates and CO inlet mole fractions. A simple two-phase mechanism is suggested that explains the findings. The second part of the contribution considers qualitative questions on cascading of two ECPrOx reactors. The crucial importance of the configuration of their electrical connection is demonstrated and explained. While two cells connected electrically in parallel exhibit almost the same selectivity-conversion behavior in comparison with a single cell, an electrical series connection enables a considerable increase in the selectivity at the same CO conversion.

Comparison of flocculation methods for harvesting Dunaliella

Low cell concentrations of Dunaliella salina in production scale cultivations require high energy input for biomass harvesting. Flocculation is a potential preconcentration method to lower the dewatering costs for the β-carotene production. In the present study, optimal flocculant dosages were determined for several metal salts, NaOH, Ca(OH)2 and Al-electrolysis. Beside harvesting efficiency ηH and concentration factor CF, also the recyclability of the separated medium as well as the influence of the cell physiology on the harvesting performance were analyzed for selected flocculants. To assess the possible recycle of non-sedimented cells for the inoculation of new cultivations, cell vitality and the photosynthetic activity of D. salina were analyzed after the flocculation. As a result, the flocculation with NaOH led to a clear inhibition of both, the algal growth on recycled medium and the algal photosynthetic activity. The addition of FeCl3 seems most promising to flocculate D. salina.

Process configurations for the production of the 2-methoxy-2,4,4-trimethylpentane—a novel gasoline oxygenate

Abstract A comparison of various process configurations was made for the production of 2-methoxy-2,4,4-trimethylpentane, which is a promising gasoline component owing low solubility to water and high octane value. The ether is synthesised by reacting methanol with a mixture of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, which are obtained in large scale from the dimerisation of isobutene. The reaction equilibria were determined experimentally at temperature range 323–383 K in liquid phase using a commercial cation exchange resin Amberlyst 35 as catalyst. A first order kinetic model was developed and utilised in the process simulations. The production of the ether was found to be inefficient in a once through tubular reactor, because the conversion is strongly limited by the reaction equilibrium. Considerably higher conversion and more economical process are obtained by using reactive distillation in the process schema.

A dynamic growth model of Dunaliella salina: Parameter identification and profile likelihood analysis

In this work, a photoautotrophic growth model incorporating light and nutrient effects on growth and pigmentation of Dunaliella salina was formulated. The model equations were taken from literature and modified according to the experimental setup with special emphasis on model reduction. The proposed model has been evaluated with experimental data of D. salina cultivated in a flat-plate photobioreactor under stressed and non-stressed conditions. Simulation results show that the model can represent the experimental data accurately. The identifiability of the model parameters was studied using the profile likelihood method. This analysis revealed that three model parameters are practically non-identifiable. However, some of these non-identifiabilities can be resolved by model reduction and additional measurements. As a conclusion, our results suggest that the proposed model equations result in a predictive growth model for D. salina.

Spontaneous oscillations of cell voltage, power density, and anode exit CO concentration in a PEM fuel cell

The spontaneous oscillations of the cell voltage and output power density of a PEMFC (with PtRu/C anode) using CO-containing H(2) streams as anodic fuels have been observed during galvanostatic operating. It is ascribed to the dynamic coupling of the CO adsorption (poisoning) and the electrochemical CO oxidation (reactivating) processes in the anode chamber of the single PEMFC. Accompanying the cell voltage and power density oscillations, the discrete CO concentration oscillations at the anode outlet of the PEMFC were also detected, which directly confirms the electrochemical CO oxidation taking place in the anode chamber during galvanostatic operating.

UPLC-MS analysis of Chlamydomonas reinhardtii and Scenedesmus obliquus lipid extracts and their possible metabolic roles

The paper presents the ultra-performance liquid chromatography (UPLC) and high-resolution mass spectrometric analysis and comparison of total lipid profiles of two green algal species, Chlamydomonas reinhardtii and Scenedesmus (Acutodesmus) obliquus. The targeted UPLC-mass spectroscopy (MS) analysis revealed that both the green algae showed the presence of almost similar types of lipids. However, there were differences in the presence of three triacylglycerol (TAG) species (TAG 54:4, TAG 54:5 and TAG 54:8) and two diacylglycerol (DAG) species (DAG 36:3 and DAG 36:4) in C. reinhardtii that were found to be completely absent in Scenedesmus obliquus. The triacylglycerol content in S. obliquus was five times more than that in C. reinhardtii. In addition, amount of diacylglycerol-O-(N,N,N-trimethyl) homoserine, a characteristic algal lipid, in S. obliquus was only half of that in C. reinhardtii. The paper also discusses the metabolic roles of the lipids produced by these algal species with reference to the lipids identified by UPLC-MS analysis.

Partial Oxidation of n-Butane in a Solid Electrolyte Membrane Reactor Influence of Electrochemical Oxygen Pumping

The partial oxidation of n-butane to maleic anhydride (MA) in a solid electrolyte membrane reactor was studied in three different operation modes: electrochemical membrane reactor (EMR), cofeed membrane reactor (CR), and mixed-feed membrane reactor (MMR). The EMR operation exhibited lower selectivity to MA but higher conversion of oxygen than the CR operation. The electrochemical oxygen pumping in the MMR operation led to decrease in the MA selectivity compared to the CR operation. By means of comparing the three operation modes, the electrochemically pumped oxygen was found to be more reactive but less selective to MA than gas-phase oxygen, which is directly related to the different reaction mechanisms between the CR and EMR operations. The butane oxidation occurred in the EMR involved not only the same catalytic reaction mechanism as in the CR but also an electrocatalytic reaction. A simplified catalytic and electrocatalytic reaction-mechanism scheme was proposed for the butane oxidation carried out in the EMR.

A Dynamic Method for Computing Thermodynamic Equilibria in Process Simulation

Abstract A generalized approach for the calculation of complex chemical and phase equilibria is presented which is based on the simulation of the dynamic evolution of a mixture from arbitrary nonequilibrium initial composition towards the final equilibrium composition. The proposed method is able to deal with pure chemical or pure phase equilibria as well as with simultaneous chemical/phase equilibria. The unique advantage of our approach, compared to conventional equilibrium calculations, is the fact that the simulation algorithm always converges towards the thermodynamic equilibrium state regardless which initial composition is chosen.

Valorization of the aqueous phase obtained from hydrothermally treated Dunaliella salina remnant biomass.

Up to 90% of Dunaliella salina biomass remains unused after extraction of the main product β-carotene. The potential of mild hydrothermal liquefaction (HTL) to exploit this biomass as a source of valuable by-products was assessed. The results indicate that 80% of the remnant was converted into glucose by mild HTL (100°C, 0min). The recovered glucose was successfully used as a carbon source to cultivate biotechnologically relevant microorganisms, namely Chlorella vulgaris, Escherichia coli and Saccharomyces cerevisiae. Furthermore, the analysis of energy demand and operating costs confirms the beneficial effect of mild liquefaction on the overall process economics of algal β-carotene production.