Mikhail Sorin

Analysis of oxygen-enriched combustion for steam methane reforming (SMR)

The technical feasibility of oxygen-enriched combustion (OEC) to enhance the performance of SMR was investigated. For a small-capacity unit, membrane separation is an appropriate air-enrichment technique. Four production scenarios were considered for the retrofit of an existing hydrogen-production unit. The scenarios were compared, using the new intrinsic exergy efficiency coefficient. The exergy analysis shows that reduced fuel consumption at a constant hydrogen-production rate is the most appropriate utilisation of OEC and that the optimum enrichment level is 29%.

Exergy efficiency and conversion of chemical reactions

An exergy analysis of the ammonia oxidation in nitrous oxide has been performed using alternatively the conventional second law efficiency and the more recently proposed intrinsic exergy efficiency. It is shown that the second law efficiency does not account for the conversion effect on the overall performance of a chemical system while the intrinsic exergy efficiency does. It is also shown that the results apply to any single reaction system.

Equivalent Temperature-Enthalpy Diagram for the Study of Ejector Refrigeration Systems

The Carnot factor versus enthalpy variation (heat) diagram has been used extensively for the second law analysis of heat transfer processes. With enthalpy variation (heat) as the abscissa and the Carnot factor as the ordinate the area between the curves representing the heat exchanging media on this diagram illustrates the exergy losses due to the transfer. It is also possible to draw the paths of working fluids in steady-state, steady-flow thermodynamic cycles on this diagram using the definition of “the equivalent temperature” as the ratio between the variations of enthalpy and entropy in an analyzed process. Despite the usefulness of this approach two important shortcomings should be emphasized. First, the approach is not applicable for the processes of expansion and compression particularly for the isenthalpic processes taking place in expansion valves. Second, from the point of view of rigorous thermodynamics, the proposed ratio gives the temperature dimension for the isobaric processes only. The present paper proposes to overcome these shortcomings by replacing the actual processes of expansion and compression by combinations of two thermodynamic paths: isentropic and isobaric. As a result the actual (not ideal) refrigeration and power cycles can be presented on equivalent temperature versus enthalpy variation diagrams. All the exergy losses, taking place in different equipments like pumps, turbines, compressors, expansion valves, condensers and evaporators are then clearly visualized. Moreover the exergies consumed and produced in each component of these cycles are also presented. The latter give the opportunity to also analyze the exergy efficiencies of the components. The proposed diagram is finally applied for the second law analysis of an ejector based refrigeration system.

Performance of Cogeneration Systems Using Waste Heat

The performance of two systems using a low-temperature heat source (100 and 125 °C) and an ORC with R245fa as the working fluid for combined heat and power production has been modeled. The first system supplies the heating load with a heat exchanger in series with the ORC boiler. In the second one this function is fulfilled by the ORC condenser. The results show that the effects of the heating load on the performance of the two systems are very different. The net power output decreases monotonically with increasing heating load for the first system while it exhibits a maximum in the case of the second one. The impact of the heating load and the source temperature on the turbine size, on the total thermal conductance of the heat exchangers, on the total exergy destruction and on several other parameters is also presented and discussed.Copyright