All real processes generate entropy and the power/exergy loss is usually determined by means of the Gouy-Stodola law. If the system only exchanges heat at the environmental temperature, the Gouy-Stodola law gives the correct loss of power. However, most industrial processes exchange heat at higher or lower temperatures than the actual environmental temperature. When calculating the real loss of power in these cases, the Gouy-Stodola law does not give the correct loss if the actual environmental temperature is used. The first aim of this paper is to show through simple steam turbine examples that the previous statement is true. The second aim of the paper is to define the effective temperature to calculate the real power loss of the system with the Gouy-Stodola law, and to apply it to turbine examples. Example calculations also show that the correct power loss can be defined if the effective temperature is used instead of the real environmental temperature.
To cite this version:Pekka Ruohonen, Pekka Ahtila.Analysis of a mechanical pulp and paper mill using advanced composite curves.Applied Thermal Engineering, Elsevier, 2009, 30 (6- This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Analysis of a mechanical pulp and paper mill using advanced composite curvesPekka Ruohonen *), Pekka Ahtila
ACCEPTED MANUSCRIPT
AbstractIn this study a mechanical pulp and paper mill is analysed using advanced composite curves. It is a graphical pinch-based approach that takes into account the existing heat exchanger network and the utilities actually used at the mill. The possibilities of making a cost-effective heat exchanger retrofit in an operating mechanical pulp and paper mill are discussed. The study also shows that the advanced composite curves can be used to describe the amount of nonisothermal mixing taking place in the process.
International audienceIn this paper the potential for steam savings and excess heat levels is analysed for four Scandinavian thermo-mechanical (TMP) pulp and paper mills, using the Heat Load Model for Pulp and Paper (HLMPP). The results are compared with similar results from previous studies for two other TMP mills. Further, an analysis is made regarding the relationship between the steam consumption and temperature level of excess heat and mill-specific characteristics such as production rate and fresh warm water usage. Based on the results and the analysis, the potential for implementation of different biorefinery concepts is discussed. The results indicate that steam savings of 2-20% can be found in Scandinavian TMP mills. The pinch temperature is rather low, around 20-70°C for most of the studied mills, compared to the pinch temperature usually found at kraft pulp mills (100-140°C). Further, two of the mills show an un-pinched Grand Composite Curve (GCC) where the heat demand curve starts close to 0°C. Thus the potential utilization options for the excess heat are rather limited. The results also show that the level of heated fresh water affects both the steam consumption and the pinch temperature, and thus the potential for efficient integration of different biorefinery processe
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