Zdravko Kravanja

SIMULTANEOUS OPTIMIZATION MODELS FOR HEAT INTEGRATION. I, AREA AND ENERGY TARGETING AND MODELING OF MULTI-STREAM EXCHANGERS

Abstract In this paper, a simple yet general superstructure for heat integration is presented. The superstructure is a stage-wise representation where within each stage exchanges of heat can occur between each hot and cold stream. The proposed representation does not rely on any heuristics that are based on the concept of the pinch point, and its simplicity enables a simultaneous consideration for design factors without the limitations of a sequential analysis. In Part 1 of this three-part series of papers, an NLP model is first introduced for the exchanger networks. As will be shown, the model can simultaneously target for area and energy cost while properly accounting for the differences in heat transfer coefficients between the streams. Constraints on matches can also be easily handled. Furthermore, if a fixed utility consumption is specified, the model reduces to an area targeting model. In the last section of the paper, the proposed representation is also applied to the modeling of multi-stream exchangers. Examples for all the applications are presented to illustrate the efficiency and effectiveness of the proposed model.

New developments and capabilities in prosyn—An automated topology and parameter process synthesizer

Abstract This paper describes an improved, equation oriented user friendly version of the modular computer package PROSYN—a mixed-integer nonlinear programming (MINLP) process synthesizer. A number of strategies are described and implemented for the outer approximation and equality relaxation algorithm (OA/ER) in order to reduce the undesirable effects of nonstructured nonconvexities in the master problem. These strategies include use of valid outer-approximations for splitters and mixers and various ways of handling the linearizations. An NLP initializer, model generator and a comprehensive library of models for basic process units and interconnection nodes, and a comprehensive library of basic physical properties have been developed and built in the PROSYN package. In this way PROSYN can be used to solve problems at different levels of complexity: from the simple NLP flowsheeting up to the MINLP synthesis through the modeling and decomposition (M/D) strategy and simultaneous heat integration including HEN costs. Applications with PROSYN are demonstrated with several example problems.

Mixed-integer nonlinear programming techniques for process systems engineering

Abstract This paper presents an overview of mixed-integer nonlinear programming techniques by first providing a unified treatment of the Branch and Bound, Outer-Approximation, Generalized Benders and Extended Cutting Plane methods as applied to nonlinear discrete optimization problems that are expressed in algebraic form. The extension of these methods is also considered for logic based representations. Finally, an overview of the applications in many areas in process engineering is presented.

Simultaneous optimization models for heat integration—III. Process and heat exchanger network optimization

Abstract In this paper, the proposed heat integration representation of Part I (Yee et al. , Computers & Chemical Engineering 14, 1151, 1990) is used for the simultaneous optimization or synthesis of the process and its heat exchanger network. The basic idea involves embedding the proposed representation into a given process flowsheet or superstructure and optimize the combined superstructure simultaneously, where flows and temperatures of the potential heat integrated streams are treated as variables. The proposed model accounts for capital and operating costs of the heat exchanger network. It does not require the specification of a heat recovery approach temperature (HRAT) and can easily handle constraints for heat integration. Synthesis examples of a distillation sequence and of a process flowsheet are presented to illustrate the capabilities of the simultaneous model which can be formulated as an NLP problem or as an MINLP problem when the structure of the network is to be determined explicitly.

Prosyn—an MINLP process synthesizer

Abstract This paper reports the development of the program PROSYN, an automated implementation of the modelling/decomposition (M/D) strategy by Kocis and Grossmann (Computers chem. Engng 13, 797–819, 1989) for the MINLP optimization of process flowsheets. A systematic procedure is first presented for the decomposition of general superstructures of process flowsheets that avoids the optimization of units with zero flow. A model for simultaneous optimization and heat integration is also proposed that can account for area and energy considerations. Finally, a computer implementation is described that is suitable for the complex logic involved in the M/D strategy for the synthesis and heat integration of process flowsheets. Examples are presented to illustrate each of these points, as well as the performance of PROSYN.

Mixed-Integer Nonlinear Programming: A Survey of Algorithms and Applications

This paper presents an overview of mixed-integer nonlinear programming techniques by first providing a unified treatment of the Branch and Bound, Outer-Approximation, Generalized Benders and Extended Cutting Plane methods as applied to nonlinear discrete optimization problems that are expressed in algebraic form. The extension of these methods is also considered for logic based representations. Finally, an overview of the applications in many areas in process engineering is presented.

A rigorous disjunctive optimization model for simultaneous flowsheet optimization and heat integration

This paper presents a new model for heat integration that overcomes difficulties experienced either with direct integration approaches, or with the Duran and Grossmann (1986) model when handling isothermal streams. The proposed model uses logic disjunctions to explicitly model the relative placement of streams for various potential pinch locations, and explicitly considers the non-isothermal and isothermal streams as separate cases. It is shown that the proposed model can be reformulated as a large but tight MINLP problem, even though it uses big-M constraints. It is also shown that the MINLP model can be reduced to a MILP problem when dealing only with isothermal streams. Several examples with up to 12 streams are shown, and the performance of the model compared to that of Duran and Grossmanns (1986).

Simultaneous synthesis of distillation sequences in overall process schemes using an improved minlp approach

This paper describes the synthesis of distillation sequences simultaneously with the synthesis of other process subsystems. An improved MINLP approach is proposed for the synthesis of complex problems. The improvements were made on three major MINLP levels. By postulating the superstructures we propose to use smaller and more compact superstructures of the distillation sequences included in the flowsheet superstructure rather than the usual tree and network representations. In the modelling step a higher degree of simultaneity has been accomplished by the use of simultaneous models of the distillation column and aggregated models of other process units. To facilitate the optimization phase the modified OA/ER algorithm [Kravanja and Grossmann, Ind. Engng. Chem. Res.26, 1869–1880 (1994)], implemented in the computerized synthesizer PROSYN-MINLP, has been used to reduce the effect of nonconvexities. Also, a special initialization and linearization scheme for simultaneous synthesis of HEN has been proposed. The improved method is illustrated by the synthesis of the separation subsystem followed by the simultaneous synthesis of the heat integrated separation system and its heat exchanger network (HEN). Finally, the optimization of the overall process and the simultaneous synthesis of its distillation system and heat integrated HEN is presented.

Carbon and nitrogen trade-offs in biomass energy production.

This contribution provides an overview of carbon (CFs) and nitrogen footprints (NFs) concerning their measures and impacts on the ecosystem and human health. The adversarial relationship between them is illustrated by the three biomass energy production applications, which substitute fossil energy production applications: (i) domestic wood combustion where different fossil energy sources (natural gas, coal, and fuel oil) are supplemented, (ii) bioethanol production from corn grain via the dry-grind process, where petrol is supplemented, and (iii) rape methyl ester production from rape seed oil via catalytic trans-esterification, where diesel is supplemented. The life cycle assessment is applied to assess the CFs and NFs resulting from different energy production applications from ‘cradle-to-grave’ span. The results highlighted that all biomass-derived energy generations have lower CFs and higher NFs whilst, on the other hand, fossil energies have higher CFs and lower NFs.

Energy, Water and Process Technologies Integration for the Simultaneous Production of Ethanol and Food from the entire Corn Plant

Abstract This contribution presents the simultaneous integration of different technologies for converting different parts of the corn plant, and integration of energy, water and feedstocks. The traditional dry-grind process is used for obtaining ethanol from corn grains, whilst for the conversion of corn stover to ethanol, gasification using two different paths of syngas conversion is taken into account: syngas fermentation (thermo-biochemical path) and catalytic mixed alcohol synthesis (thermo-chemical path). These processes are modelled in the Mixed-Integer Process Synthesizer (MIPSYN) using mass and energy balances and conversion constraints. Short cut models for the more complex process units are obtained from the detailed ones, as developed by Martin and Grossmann (2011) . The results show that the best integrated process when using the entire corn plant is a combination of the dry-grind process and thermo-chemical conversion. For the base case (18 kg/s of grain and 10.8 kg/s of stover), the optimal scheme is comprised of dry-grind and thermo-chemical conversion technologies. The best integrated process requires only 17 MW of energy, 50 MW of cooling and 1.56 L/L of freshwater, with an ethanol production cost of 0.41