Harun Koku

Aspects of the metabolism of hydrogen production by Rhodobacter sphaeroides

Abstract Photosynthetic bacteria are favorable candidates for biological hydrogen production due to their high conversion efficiency and versatility in the substrates they can utilize. For large-scale hydrogen production, an integrated view of the overall metabolism is necessary in order to interpret results properly and facilitate experimental design. In this study, a summary of the hydrogen production metabolism of the photosynthetic purple non-sulfur (PNS) bacteria will be presented. Practically all hydrogen production by PNS bacteria occurs under a photoheterotrophic mode of metabolism. Yet results show that under certain conditions, alternative modes of metabolism—e.g. fermentation under light deficiency—are also possible and should be considered in experimental design. Two enzymes are especially critical for hydrogen production. Nitrogenase promotes hydrogen production and uptake hydrogenase consumes hydrogen. Though a wide variety of substrates can be used for growth, only a portion of these is suitable for hydrogen production. The efficiency of a certain substrate depends on factors such as the activity of the TCA cycle, the carbon-to-nitrogen ratio, the reduction-state of that material and the conversion potential of the substrate into alternative metabolites such as PHB. All these individual components of the hydrogen production interact and are subject to strict regulatory controls. An overall scheme for the hydrogen production metabolism is presented.

Kinetics of biological hydrogen production by the photosynthetic bacterium Rhodobacter sphaeroides O.U. 001

The kinetics and the effects of various parameters on hydrogen production by Rhodobacter sphaeroides O.U. 001 were investigated in a batch column photobioreactor. In particular, the effect of the inoculum age and the implementation of a light–dark cycle illumination scheme for emulating natural sunlight have been investigated in detail. The possibility of using yeast extract to replace the rather expensive vitamin mixture in the medium was also studied. The results show that hydrogen production is decreased when the initially inoculated bacteria have a high culture age. Exposure of the bacterial culture to light–dark cycles increased the amount of hydrogen compared to continuous illumination, all other parameters remaining the same. Similarly, the use of yeast extract to replace the vitamins increased the growth and hydrogen production rates, however, with a slight reduction in the total amount of gas produced and the hydrogen fraction in the evolved gas.

Dynamic modeling of temperature change in outdoor operated tubular photobioreactors

In this study, a one-dimensional transient model was developed to analyze the temperature variation of tubular photobioreactors operated outdoors and the validity of the model was tested by comparing the predictions of the model with the experimental data. The model included the effects of convection and radiative heat exchange on the reactor temperature throughout the day. The temperatures in the reactors increased with increasing solar radiation and air temperatures, and the predicted reactor temperatures corresponded well to the measured experimental values. The heat transferred to the reactor was mainly through radiation: the radiative heat absorbed by the reactor medium, ground radiation, air radiation, and solar (direct and diffuse) radiation, while heat loss was mainly through the heat transfer to the cooling water and forced convection. The amount of heat transferred by reflected radiation and metabolic activities of the bacteria and pump work was negligible. Counter-current cooling was more effective in controlling reactor temperature than co-current cooling. The model developed identifies major heat transfer mechanisms in outdoor operated tubular photobioreactors, and accurately predicts temperature changes in these systems. This is useful in determining cooling duty under transient conditions and scaling up photobioreactors. The photobioreactor design and the thermal modeling were carried out and experimental results obtained for the case study of photofermentative hydrogen production by Rhodobacter capsulatus, but the approach is applicable to photobiological systems that are to be operated under outdoor conditions with significant cooling demands.

Modelling of Diffusion in Random Packings of Core-Shell Particles

Çekirdek-Kabuk tipi parçacıklar kromatografide sıklıkla kullanılan malzemelerdir. Bu çalışmada, ÇekirdekKabuk parçacıklarının etrafında gerçekleşen difüzyonu taklit etmeye yönelik bir matematik modeli geliştirilmiştir. Difüzyonun simulasyonu için rastgele-yürüyüş temelli bir algoritma oluşturulmuş ve basit geometrik yapılar ve bağıntılardan oluşan bir Çekirdek-Kabuk geometrisi hesaplanmıştır. Bu parçacıklardan oluşan rastgele-istiflenmiş bir yapı üzerinde difüzyon simülasyonları gerçekleştirilmiştir. Model sonuçlarından elde edilen zamana bağlı difüzyon katsayısının davranışı, nükleer manyetik rezonans deneyleri vasıtasıyla gözenekli ortamlarda bir maddenin zamana bağlı öz-difüzyon katsayısının ölçüldüğü daha önceki bir çalışmanın sonuçlarıyla uyumludur.