Terrence Fu Yee

Simultaneous optimization models for heat integration. II, Heat exchanger network synthesis

Abstract In this paper, a mixed integer nonlinear programming (MINLP) model is presented which can generate networks where utility cost, exchanger areas and selection of matches are optimized simultaneously. The proposed model does not rely on the assumption of fixed temperature approaches (HRAT or EMAT), nor on the prediction of the pinch point for the partitioning into subnetworks. The model is based on the stage-wise representation introduced in Part I of this series of papers, where within each stage, potential exchanges between each hot and cold stream can occur. The simplifying assumption on isothermal mixing to calculate heat transfer area for stream splits allows the feasible space to be defined by a set of linear constraints. As a result, the model is robust and can be solved with relative ease. Constraints on the network design that simplify its structure, e.g. no stream splits, forbidden matches, required and restricted matches as well as the handling of multiple utilities can be easily included in the model. In addition, the model can consider matches between pairs of hot streams or pairs of cold streams, as well as variable inlet and outlet temperatures. Several examples are presented to illustrate the capabilities of the proposed simultaneous synthesis model. The results show that in many cases, heuristic rules such as subnetwork partitioning, no placement of exchangers across the pinch, number of units, fail to hold when the optimization is performed simultaneously.

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.

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.

Constant approach temperature model for HEN retrofit

This paper proposes a solution method based upon mathematical programming for Heat Exchanger Network (HEN) retrofit. This is a two-step approach. The first step uses a Constant Approach Temperature (CAT) model to optimize the structure of the final HEN. The CAT model simultaneously takes into account the cost of utilities, structural modifications and heat transfer areas by assuming the approach temperatures of all heat transfers inside the HEN to be a constant. The main advantage of this assumption is that area calculations are linearized, therefore, the model could be solved as a Mixed Integer Linear (MILP) problem. This shortens the solution time and removes the possibility of being trapped at local optimums. The CAT model does not guarantee feasible solutions; however, it determines a good network structure and drives the solution very close to the global optimum. Starting with this network structure, a Mixed Integer Nonlinear (MINLP) model is then used in the second step, which takes into account the actual approach temperatures, to finalize the design. Example problems from literature are used to demonstrate the effectiveness of the approach in terms of the solution quality and time.

Argon production from air distillation: Use of a heat pump in a ternary distillation with a side rectifier☆

Abstract A ternary feed mixture ABC can be separated into individual components through the use of a main distillation column with a thermally linked side rectifier. To enhance such a separation, a heat pump can be implemented to transfer heat from the condenser at the top of the side rectifier to the reboiler at the bottom of the main column. In this paper, one such heat pump is described and applied to an air distillation system separating the ternary mixture containing nitrogen, oxygen and argon. The separation is performed by a conventional double column with a crude argon side column. When this system is operated at an elevated pressure to obtain higher product pressures, the separation of oxygen and argon becomes very difficult and leads to reduced argon recovery. The proposed heat pump enhances the separation by providing a supplementary crude argon condensing duty through the vaporization of a liquid oxygen stream from the bottom of the low pressure (LP) column. This scheme improves the liquid/vapour ratio (L/V) in the bottom section of the LP column and, more importantly, increases the vapour feed to the crude argon column. This increased feed rate leads to a substantial increase in argon recovery for the elevated pressure air distillation process.