Prior sustainability models have taken exact basic indicator (BI) values of an entity's sustainability and computed an exact value of the entity's sustainability index. However, BI values are either uncertain or difficult to obtain. In this work, we propose a novel approach that transforms BI data in the form of intervals containing all possible BI values to an interval containing all possible values of the sustainability index. This interval is termed the “sustainability index interval” (SII). Computation of the SII is achieved through solution of a minimization and a maximization problem using global optimization techniques. Although the underlying global optimization problems are nonconvex, they are shown to possess a number of properties that can be utilized to reduce the burden associated with SII's computation. Based on these properties, a branch‐and‐bound algorithm is developed, which exactly quantifies the SII in a finite number of iterations. © 2011 American Institute of Chemical Engineers AIChE J, 2012
The minimum total annualized cost problem for a series of nonisentropic compressors and coolers that brings a gas with constant compressibility factor from a specified initial pressure and temperature to a specified final pressure and the same temperature is studied in this work. It is established analytically that at the global optimum, the cooler outlet temperatures are equal to the minimum allowable temperature. For constant heat capacity, constant compressibility factor gases, additional properties of the globally optimal compressor sequence are analytically established for the minimum operating cost case. The aforementioned properties permit development of a solution strategy that identifies the globally minimum operating cost. Several case studies are presented to illustrate the developed theorems and solution strategies.
In this work the attainable region (AR) concept for process networks with outlet flow rate specifications is introduced for the first time. For process unit models to which the infinite dimensional State‐space conceptual framework is applicable, it is shown that identification of AR boundary membership is equivalent to feasibility assessment of an infinite linear program (ILP). A number of important AR properties are then theoretically established, including AR convexity, and representation of the AR in a concentration state space of reduced dimension. Finite dimensional approximations of the aforementioned ILP are then employed in creating increasingly accurate approximations of the AR. A case study for the vapor‐liquid equilibrium‐based separation of a ternary azeotropic mixture is used to illustrate the proposed method. The quantified two‐ and four‐Dimensional ARs indicate that acetone mole fractions above 0.79 (acetone/methanol azeotrope) are attainable for the considered outlet flow rate ratios. © 2013 American Institute of Chemical Engineers AIChE J, 60: 193–212, 2014
This work focuses on the incorporation of renewable energy resources into the natural-gas-based production of hydrogen, via steam methane reforming (SMR). The novel concept of "energetically enhanced steam methane reforming" (EESMR) is introduced, which changes the endothermicity level of the SMR process through incorporation of carbon monoxide and steam into the SMR feed. Novel EESMR flow sheets are presented in which the aforementioned resources are internally generated and recycled, so as to create a natural-gas-based hydrogen production system with methane as raw material and a hybrid (methane/solar) energy supply. The current worldwide carbon tax legislative environment is briefly reviewed, and its potential impact on SMR-versus-EESMR economic viability is quantified. A novel method for solution of linear parametric programming problems is proposed based on the concept of dimensionality reductionthis allows the analytic quantification of the optimum objective function value and associated optimum variable vector. Regions in carbon/renewable utility cost coefficient ratio space are identified, in which one technology is superior over the other. EESMR is shown to be preferable in the presence of significant levels of taxation on the use of natural gas as fuel.
In this work, a novel hybrid method is proposed for the global solution of an infinite collection of instances of the minimum total annualized cost (TAC) problem for a series of gas compressors and coolers that brings a gas from a given pressure and temperature to a specified final pressure and the same temperature. The gas is considered to possess a constant compressibility factor, and a constant, ideal gas, constant pressure, heat capacity. The collection of TAC problem instances considered is parametrized by the overall compression ratio. The proposed method combines the TAC problem’s first and second order necessary conditions of optimality with interval analysis, to determine converging upper and lower bounds to the globally optimal solution and associated optimal solution vector components, for all considered instances of the TAC problem. Two case studies are presented to illustrate the novel solution method and the impact of economies of scale on the global optimum.
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