On the importance of cascading moisture recycling in South America

Continental moisture recycling is a crucial process of the South American climate system. In particular, evapotranspiration from the Amazon basin contributes substantially to precipitation regionally as well as over other remote regions such as the La Plata basin. Here we present an in-depth analysis of South American moisture recycling mechanisms. In particular, we quantify the importance of cascading moisture recycling (CMR), which describes moisture transport between two locations on the continent that involves re-evaporation cycles along the way. Using an Eulerian atmospheric moisture tracking model forced by a combination of several historical climate data sets, we were able to construct a complex network of moisture recycling for South America. Our results show that CMR contributes about 9–10% to the total precipitation over South America and 17–18% over the La Plata basin. CMR increases the fraction of total precipitation over the La Plata basin that originates from the Amazon basin from 18–23 to 24–29% during the wet season. We also show that the south-western part of the Amazon basin is not only a direct source of rainfall over the La Plata basin, but also a key intermediary region that distributes moisture originating from the entire Amazon basin towards the La Plata basin during the wet season. Our results suggest that land use change in this region might have a stronger impact on downwind rainfall than previously thought. Using complex network analysis techniques, we find the eastern side of the sub-tropical Andes to be a key region where CMR pathways are channeled. This study offers a better understanding of the interactions between the vegetation and the atmosphere on the water cycle, which is needed in a context of land use and climate change in South America

Optimal Control Of Heat Exchanger Networks

Energy integration among chemical process streams can lead to quite complex heat exchanger networks (HEN) with difficulties in terms of control. In the HEN, the control system primary objective is to keep outlet stream temperatures in a specified range. This objective can be achieved by manipulating bypasses in the exchangers or heat loads in coolers or heaters, which results in a positive degree of freedom, since the number of possible manipulated variables is greater than the number of controlled variables. Thus, a secondary control objective can be set: minimization of utility consumption. In this work HEN dynamics and steady state aspects were addressed with the aim of proposing a control strategy that minimizes utility consumption and satisfies imposed restrictions. With the tools developed in this work it was proposed a feedforward optimal control strategy, which consisted in the minimization of a steady state objective function, connected to utility consumption and subjected to constraints in outlet stream temperatures. The suggested approach is an interesting one since rigid or flexible control objectives can be set. The solution of the minimization problem resulted in optimal bypasses positions in the HEN, which were dynamically implemented using a ramp function and a step function. It was verified that ramp implementation was better, with tolerable dynamic violations. © 2009 Elsevier B.V. All rights reserved.27C16471652Boyaci, C., Uzturk, D., Konukman, A.E.S., Akman, U., Dynamics and optimal control of flexible heat exchanger networks (1996) Computers and Chemical Engineering, 20, pp. S775-S780Giovanini, L.L., Marchetti, J.L., Low-level flexible structure control applied to heat exchanger networks (2003) Computers and Chemical Engineering, 21, pp. 1129-1142Glemmestad, B., Skogestad, S., Gundersen, T., Optimal operation of heat exchanger networks (1999) Computers and Chemical Engineering, 23, pp. 509-522González, A.H., Odloak, D., Marchetti, J.L., Predictive control applied to heat exchanger networks (2006) Chemical Engineering and Processing, 45, pp. 661-671Heggs, P.J., Vizcaino, F., A rigorous model for evaluation of disturbance propagation through heat exchanger networks (2002) IChemE, 80, pp. 301-308Linnhoff, B., Townsend, D.W., Boland, D., Hewitt, G.F., Thomas, B.E.A., (1982) A User Guide on Process Integration for the Efficient Use of Energy, , London: Warwick Printing Company LtdMathisen, K.W., (1994) Integrated Design and Control of Heat Exchanger Networks, p. 195. , University of Trondheim, Thesis (D)Novazzi, L.F., (2007) Dinâmica e Controle de Redes de Trocadores de Calor, p. 138. , Universidade Estadual de Campinas, Thesis (D

Exergy Balance In A Distillation Column

This work is concerned with the analysis of thermodynamic efficiency concepts for distillation columns, based on exergy analysis. A procedure that computes rigorously the minimum exergy requirement of multicomponents systems, which cannot be separated under completely reversible condition in distillation columns, is presented. Ternary systems and a five hydrocarbon system were studied. The results show that the real minimum exergy requirement for such a separation has a major impact on the thermodynamic efficiency of the column.314691476Annakou, O., Mizsey, P., Rigorous comparative study of energy-integrated distillation schemes (1996) Ind. Eng. Chem. Res., 35, pp. 1877-1885Fonyo, Z., Thermodynamic analysis of rectification I. Reversible model of rectification (1974) Int. Chem. Eng., 14, pp. 18-27Koehler, J., Aguirre, P., Blaß, E., Minimum reflux calculation for nonideal mixtures using the reversible distillation model (1991) Chemical Engineering Science, 46, pp. 3007-3021Rathore, R., Wormer, K.A.V., Powers, G.J., Synthesis strategies for multicomponent separation systems with energy integration (1974) AIChE Journal, 20 (3), pp. 491-502Henley, E.J., Seader, J.D., (1981) Equilibrium-Stage Separation Operations in Chemical Engineering, , John Wile

Retrofit Of Pressure Drop Constrained Heat Exchanger Networks

The study of retrofit of heat exchanger networks is usually restricted to constant heat transfer coefficients, and pressure drop constraints due to the additional exchanger area is neglected. In this work, a new approach considering the distribution of heat transfer area and pressure drop in retrofit is presented. The problem is described as a non-linear model, and the additional area required for the new network condition and available pressure drop are estimated based on economical optimization (or process requirements).201514691480Jegede, F.O., (1990) Power, Capital and Energy Cost Trade-offs in Heat Exchanger Networks, , PhD Thesis, UMIST, ManchesterPolley, G.T., Panjeh Shahi, M.H., Jegede, F.O., Pressure drop considerations in the retrofit of heat exchanger networks (1990) Trans. IChemE, 68, pp. 211-220Polley, G.T., Panjeh Shahi, M.H., Interfacing heat exchanger network synthesis and detailed heat exchanger design (1991) Trans IChemE, 69, pp. 445-457Shokoya, C.G., (1992) Retrofit of Heat Exchanger Network for Debottlenecking and Energy Saving, , PhD Thesis, UMIST, ManchesterTjoe, T.N., Linnhoff, B., Using pinch technology for process retrofit (1986) Chem. Eng., 93, pp. 47-6

Artificial Neural Networks For The Solution Of The Phase Stability Problem

The prediction of the thermodynamic properties of multiphase systems is complex, because, besides equilibrium calculations, it involves determination of the number and nature of the phases present in the system (phase stability tests). For a system exhibiting a heterogeneous azeotrope, for example, the problem is to develop methods that can tell whether, for a given overall composition, the system lies inside or outside the binodal surface (two liquid phases in equilibrium or a single stable liquid phase). In this work, the application of artificial neural networks (ANNs) for the solution of the phase stability problem, a classification problem, is proposed. The input-output patterns, required for training the networks, were obtained computationally for the range of temperatures that covers liquid-liquid equilibrium, vapor-liquid-liquid equilibrium, vapor-liquid equilibrium and homogenous liquid and vapor. Hence, the ANN must be able to decide between these five possible regions. Two types of ANNs were tested: feedforward neural networks (FNNs) and probabilistic neural networks (PNNs). The results indicate that each kind of ANN is better for different conditions, the developed ANNs were able to predict correctly the type of equilibrium in more than 99.9% of the cases. © 2006 Elsevier B.V. All rights reserved.24518387Sofyan, Y., Ghajar, A.J., Gasem, K.A.M., (2003) Ind. Eng. Chem. Res., 42, pp. 3786-3801Nelson, P.A., (1987) Comput. Chem. Eng., 11, pp. 581-591Jiang, Y., Smith, W.R., Chapman, G.R., (1995) SIAM J. Control. Optim., 5, pp. 813-834Mohanty, S., (2005) Fluid Phase Equilib., 235, pp. 92-98Baker, L.E., Pierce, A.C., Luks, K.D., (1982) Soc. Petrol. Eng. AIME, 22, pp. 731-742Michelsen, M.L., (1982) Fluid Phase Equilib., 9, pp. 1-19Heidemann, R.A., (1983) Fluid Phase Equilib., 14, pp. 55-78Wasylkiewicz, S.K., Sridhar, L.N., Doherty, M.F., Malone, M.F., (1996) Ind. Eng. Chem. Res., 35, pp. 1395-1408Tessier, S.R., Brennecke, J.F., Stadtherr, M.A., (2000) Chem. Eng. Sci., 55, pp. 1785-1796Pham, H.N., Doherty, M.F., (1990) Chem. Eng. Sci., 45, pp. 1823-1836Smith, J.M., Van Ness, H.C., Abbott, M.M., (1996) Introduction to Chemical Engineering Thermodynamics, , McGraw-Hill, New YorkWalas, S.M., (1985) Phase Equilibria in Chemical Engineering, , Butterworth-Heinemann, StonehamWasserman, P.D., (1993) Advanced Methods in Neural Computing, , Van Nostrand Reinhold, New YorkSoares, M.E., Medina, A.G., McDermott, C., Ashton, N., (1982) Chem. Eng. Sci., 37, pp. 521-52

Thermodynamic analysis of multicomponent distillation columns: identifying optimal feed conditions

A new methodology for the optimisation of feed conditions as well as the calculation of minimum reflux ratio of distillation columns is presented. The reversible profile approach used for saturated liquid feeds is extended to consider other feed conditions. For flashed feed, the liquid fraction of the feed stream is used to compute the column pinch conditions and the minimum reflux ratio. The modifications required for subcooled liquid and superheated vapor feed are discussed, and a procedure to estimate the minimum reflux for those conditions is proposed. The methodology presented allows the identification of the optimal feed condition, without having to resort to a full stage-by-stage procedure.751760Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq