Elisabeth Schröder

Study on Heat Transfer in Heat Exchangers for a New Supercritical Organic Rankine Cycle

Electrical power from geothermal heat is hardly efficient, if the temperature level of geothermal water is low. This situation is predominant in middle Europe, for example, in Germany, where even in hot-spot areas like Norddeutsches Becken, Oberrheingraben, and Molassebecken the temperature of geothermal water is less than 160°C. For this, efforts have to be made in optimizing the power cycle efficiency. Supercritical fluids provide a higher net energy output per unit mass than subcritical fluids but their physical properties strongly depend on temperature variations, especially close to the supercritical point. Based on the requirements of a new modular and mobile supercritical power cycle MONICA (modular low temperature power cycle Karlsruhe), which will be built within the next few years, different heat exchanger types are investigated within this study in order to determine the most compact design with respect to mobility, mountability, and efficiency. Changes in physical properties of propane, the working fluid of the new cycle, are taken into account by iterative, stepwise calculation of heat exchanger types like double-pipe, shell-and-tube, and plate heat exchangers. For this, common Nusselt number correlations are implemented in the stepwise iteration. Influence of geometry on flow conditions and analysis of part load sensitivity are provided.

Pyrolysis—gas chromatography of networks☆

Abstract Cis -1,4-polybutadiene was exposed to different doses of β-radiation (5–100 Mrad), and polybutadiene samples with different contents of 1,2- and 1,4-isomers were exposed to equal doses of radiation (50 Mrad). The cross-linked products were characterized by sol—gel analysis, distribution of the molar masses of the sol fraction, determination of the degree of swelling and compressibility, and pyrolysed by means of a Curie-point pyrolyser at 770°C in 4 s. Evaluation of the pyrograms taken with the help of a capillary gas chromatograph proved that pyrolysis—gas chromatography enables statements regarding the micro-structure of these cross-linked polymers to be made through the degradation products 1- cis -2- and 1- trans -2-dimethylcyclopropane, 2-methyl-1,3-butadiene and trans -1,3-pentadiene in a very short time without much preparation of the samples. The peak areas of 1,3-butadiene and 4-vinylcyclohexane can be used to characterize the network density.

Activated Carbon from Waste Biomass

As a result of environmental requirements in many countries and new areas of application the demand on activated carbon is still growing. Due to the unavailability of the main basic materials like hard coal, wood or coconut shells in many countries other biomass matters were tested for their appropriateness of activated carbon production. The objective of this experimental work is the conversion of waste biomass into activated carbon. Waste biomass like straw matters, olive stones, nut shells, coffee grounds and spent grain is converted thermally in two steps. First the biomass undergoes a pyrolysis process at 500°C–600°C in nitrogen atmosphere. The gaseous and liquid pyrolysis products can be used energetically either for heating the facilities or for electricity production. Second, the solid residue, the char, is treated in an activation process at 800°C–1000°C in steam atmosphere in order to enhance the char surface area which was analyzed by standard BET method. The increase of surface area depends on the type of biomass and on the activation parameters. The production methods were investigated in lab-scale facilities whereas a pilot scale reactor was designed for the transformation of the discontinuous activation process to a continuous production process. The use of agricultural by-products for activated carbon production as well as the influence of ash content, pyrolysis and activation conditions on the activated carbon quality is investigated by many authors. The high ash content of rice straw makes it difficult to achieve a sufficiently high surface area (Ahmedna et al., 2000). The influence of a one step and a two step thermal treatment of rice straw in CO2 atmosphere is discussed in (Yun et al., 2001). The two step treatment leads to higher surface areas than the one step treatment which correspond to the own results. Higher temperatures of physical activation in CO2 atmosphere leads to pore widening which causes an increase of mesopores. Physical activation by the use of an oxidizing gas like steam or CO2 results in carbons with low surface area whereas chemical activation enhances the carbon surface area (Ahmedna et al., 2000). Chemical activation of rice husks and rice straw is investigated in (Guo et al., 2002; Oh & Park, 2002). The impregnation of rice precursors with KOH or NaOH enhances the surface area. In addition the activation temperature can be lowered. Washing rice straw with alkaline solutions like NaOH allows to reduce the ash content as shown in Table 1 and (Huang et al., 2001). Carbonisation and activation of pretreated rice straw leads to higher surface areas than of non-treated straw