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Search Results: 1 - 10 of 5086 matches for " Vicente Costanza "
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Estimating pure diffusion contributions in alkaline pulping processes
Costanza,Vicente; Costanza,Pedro;
Latin American applied research , 2002,
Abstract: a model that predicts isothermal alkali diffusion and reaction with acetyl groups in moist wood chips was derived and approximated. system parameters were estimated from unsteady-state experimental data. simulation results reinforce the idea that the diffusion effect is not fully exploited in pulping processes. traditionally, digestion is conducted at high temperature, where delignification reaction kinetics is enhanced and the reaction effect is predominant. this approach is being reviewed by modern industry since energy and environmental savings associated with low temperature operation might compensate for high-yield productivity. the concentration of alkali at the center of the chip is a measure of the completeness of wood deacetylation, which translates into the aptitude of the final product for pulping purposes. this concentration is predicted here from the solution to a pair of coupled ode's. since alternatives combining both low and high-temperature processes are being studied, the results in this paper provide basic data for optimization analysis.
Estimating pure diffusion contributions in alkaline pulping processes
Vicente Costanza,Pedro Costanza
Latin American applied research , 2002,
Abstract: A model that predicts isothermal alkali diffusion and reaction with acetyl groups in moist wood chips was derived and approximated. System parameters were estimated from unsteady-state experimental data. Simulation results reinforce the idea that the diffusion effect is not fully exploited in pulping processes. Traditionally, digestion is conducted at high temperature, where delignification reaction kinetics is enhanced and the reaction effect is predominant. This approach is being reviewed by modern industry since energy and environmental savings associated with low temperature operation might compensate for high-yield productivity. The concentration of alkali at the center of the chip is a measure of the completeness of wood deacetylation, which translates into the aptitude of the final product for pulping purposes. This concentration is predicted here from the solution to a pair of coupled ODE's. Since alternatives combining both low and high-temperature processes are being studied, the results in this paper provide basic data for optimization analysis.
Optimal operating points in alkaline pulping
Costanza,Vicente; Zanuttini,Miguel A.;
Latin American applied research , 2004,
Abstract: the problem of optimizing the alkaline impregnation of wood chips is posed and solved under usual restrictions. the cost to be optimized balances opposite criteria by taking economics and product quality into account, and is conditioned by the system dynamics. evolution is modeled from typical transport phenomena equations. optimization is attacked in the lines of variational calculus, although the final treatment involves numerical methods. cost function design is provided for: alkali consumption, thermal energy consumption, product quality, and total production; each one affected by a preference-weighting coefficient. a new parameter, the "deacetylation index", is introduced as an observable quantity for tracking the end of the digestion stage in pulping processes. this index turns out to be significant even at low temperatures. cost terms depend essentially on three design variables: (i) alkaline bulk concentration, (ii) digester temperature, and (iii) total duration of the process. an algorithm to ascertain the optimal values of these variables is devised. numerical results provide insight in deciding changes on design variables in case they are allowed within a certain extent to be manipulated.
Initial values for Riccati ODEs from variational PDEs
Costanza, Vicente;Rivadeneira, Pablo S.;
Computational & Applied Mathematics , 2011, DOI: 10.1590/S1807-03022011000200005
Abstract: the recently discovered variational pdes (partial differential equations) for finding missing boundary conditions in hamilton equations of optimal control are applied to the extended-space transformation of time-variant linear-quadratic regulator (lqr) problems. these problems become autonomous but with nonlinear dynamics and costs. the numerical solutions to the pdes are checked against the analytical solutions to the original lqr problem. this is the first validation of the pdes in the literature for a nonlinear context. it is also found that the initial value of the riccati matrix can be obtained from the spatial derivative of the hamiltonian flow, which satisfies the variational equation. this last result has practical implications when implementing two-degrees-of freedom control strategies for nonlinear systems with generalized costs.
Optimal operating points in alkaline pulping
Vicente Costanza,Miguel A. Zanuttini
Latin American applied research , 2004,
Abstract: The problem of optimizing the alkaline impregnation of wood chips is posed and solved under usual restrictions. The cost to be optimized balances opposite criteria by taking economics and product quality into account, and is conditioned by the system dynamics. Evolution is modeled from typical transport phenomena equations. Optimization is attacked in the lines of variational calculus, although the final treatment involves numerical methods. Cost function design is provided for: alkali consumption, thermal energy consumption, product quality, and total production; each one affected by a preference-weighting coefficient. A new parameter, the "Deacetylation Index", is introduced as an observable quantity for tracking the end of the digestion stage in pulping processes. This index turns out to be significant even at low temperatures. Cost terms depend essentially on three design variables: (i) alkaline bulk concentration, (ii) digester temperature, and (iii) total duration of the process. An algorithm to ascertain the optimal values of these variables is devised. Numerical results provide insight in deciding changes on design variables in case they are allowed within a certain extent to be manipulated.
Numerical solution of the variational PDEs arising in optimal control theory
Costanza, Vicente;Troparevsky, Maria I.;Rivadeneira, Pablo S.;
Computational & Applied Mathematics , 2012, DOI: 10.1590/S1807-03022012000100003
Abstract: an iterative method based on picard's approach to odes' initial-value problems is proposed to solve first-order quasilinear pdes with matrix-valued unknowns, in particular, the recently discovered variational pdes for the missing boundary values in hamilton equations of optimal control. as illustrations the iterative numerical solutions are checked against the analytical solutions to some examples arising from optimal control problems for nonlinear systems and regular lagrangians in finite dimension, and against the numerical solution obtained through standard mathematical software. an application to the (n + 1)-dimensional variational pdes associated with the n-dimensional finite-horizon time-variant linear-quadratic problem is discussed, due to the key role the lqr plays in two-degrees-of freedom control strategies for nonlinear systems with generalized costs. mathematical subject classification: primary: 35f30; secondary: 93c10.
Optimal state-feedback regulation of the hydrogene evolution reactions
Costanza,V.;
Latin American applied research , 2005,
Abstract: a control strategy is developed in order to keep processes based on the hydrogen evolution reactions (her) near operational steady states. the problem is treated in the context of optimal control for nonlinear systems subject to quadratic cost objectives. the original dynamics is shown to be accurately approximated by a bilinear model without increasing the dimension, so the state variables retain their physical meaning. finite and infinite horizon optimal control strategies are developed, based on the hamiltonian formalism, and introducing a novel approach for working on-line with generalized riccati differential equations and the associated costate dynamics. when there exists a final penalty on the state deviation, then a first order quasi-linear partial differential equation is discovered and solved for the riccati matrix. the observability problem is also treated, since the natural state (electrode surface coverage) can not be measured continuously. the output variable (current density) is fed into a high-gain nonlinear observer based on lyapunov's stability considerations. the whole approach allows for (in general time-dependent) state-feedback control.
Visions of Alternative (Unpredictable) Futures and Their Use in Policy Analysis
Robert Costanza
Ecology and Society , 2000,
Abstract: The most critical task facing humanity today is the creation of a shared vision of a sustainable and desirable society, one that can provide permanent prosperity within the biophysical constraints of the real world in a way that is fair and equitable to all of humanity, to other species, and to future generations. Recent work with businesses and communities indicates that creating a shared vision is the most effective engine for change in the desired direction, yet most effort in "futures modeling" has focused on extrapolating past trends rather than envisioning alternative futures. Science and economics as applied to policy are in conflict more often over alternative visions of the world than purely "scientific" disagreements. Likewise, governance has gotten bogged down in mediating short term conflicts between special interests rather than its more basic role of creating broadly shared visions that can guide dispute resolution. This paper addresses the question of what policies are most appropriate for society now, given alternative visions of the future and the enormous uncertainty about the reality of the assumptions underlying these visions. Four specific visions are laid out as being representative of the major alternatives. For each vision the benefits of achieving the vision, the assumptions that would have to be true in order for it to be achieved, and the implications of it being attempted but not achieved are explored. It is argued that dealing with uncertainty about the nature of the world, its carrying capacity for humans, the impacts of climate change, and other aspects of its future can best be done at this level of future visions and assumptions, not at more detailed levels (like the parameter uncertainty in models). Application of this vision/uncertainty analysis can help us both to design the future society we want and to maximize the chances of our getting there safely.
Optimal state-feedback regulation of the hydrogene evolution reactions
V. Costanza
Latin American applied research , 2005,
Abstract: A control strategy is developed in order to keep processes based on the hydrogen evolution reactions (HER) near operational steady states. The problem is treated in the context of Optimal Control for nonlinear systems subject to quadratic cost objectives. The original dynamics is shown to be accurately approximated by a bilinear model without increasing the dimension, so the state variables retain their physical meaning. Finite and infinite horizon optimal control strategies are developed, based on the Hamiltonian formalism, and introducing a novel approach for working on-line with generalized Riccati differential equations and the associated costate dynamics. When there exists a final penalty on the state deviation, then a first order quasi-linear partial differential equation is discovered and solved for the Riccati matrix. The observability problem is also treated, since the natural state (electrode surface coverage) can not be measured continuously. The output variable (current density) is fed into a high-gain nonlinear observer based on Lyapunov's stability considerations. The whole approach allows for (in general time-dependent) state-feedback control.
MicroRNAs in Postischemic Vascular Repair
Andrea Caporali,Costanza Emanueli
Cardiology Research and Practice , 2012, DOI: 10.1155/2012/486702
Abstract: The term angiogenesis describes the growth of endothelial sprouts from preexisting postcapillary venules. More recently, this term has been used to generally indicate the growth and remodeling process of the primitive vascular network into a complex network during development. In adulthood, angiogenesis is activated as a reparative process during wound healing and following ischemia, and it plays a key role in tumor growth and metastasis as well as in inflammatory diseases and diabetic retinopathy. MicroRNAs (miRNAs) are endogenous, small, noncoding RNAs that negatively control gene expression of target mRNAs. In this paper, we aim at describing the role of miRNAs in postischemic angiogenesis. First, we will describe the regulation and the expression of miRNAs in endothelial cells. Then, we will analyze the role of miRNAs in postischemic vascular repair. Finally, we will discuss the role of circulating miRNAs as potential biomarkers in ischemic diseases. 1. Introduction Postnatal neovascularization occurs in pathological diseases, including diabetic retinopathy, arthritis, and tumor growth and metastasis, as well as during wound healing and postischemic repair [1]. Following occlusion of a major artery, two different types of vascular regrowth responses are activated to contrast the ischemic condition and salvage of injured ischaemic tissue: sprouting of capillaries (angiogenesis) and growth of collateral arteries from preexisting arterioles (arteriogenesis) [2]. Angiogenesis is the process of growth of endothelial sprouts from preexisting postcapillary venules. It is initiated by the vasodilation of venules leading to increased permeability, followed by the proliferation and migration of endothelial cells (ECs). Then, venules are divided by periendothelial cells (intussusception) or are separated by transendothelial cell bridges (bridging) to form capillaries [3, 4]. Arteriogenesis was originally defined as the process of collateral arteries formation which follows the occlusion of a major artery [5]. Arteriogenesis is partly driven by shear stress and involves the proliferation and migration of vascular endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). It leads to the production of mature arteries with a fully developed tunica media, which may provide alternative blood perfusion to ischemic areas [6]. microRNAs (miRNAs) are inhibitory regulators of gene expression which act by binding to complementary messenger RNA (mRNA) transcripts. Following initial studies in developmental biology and cancer, miRNAs have recently come into focus of
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