Michael Short

This article describes a study in which the various stagewise and interval-based superstructures for the mixed integer non-linear programming (MINLP) optimisation of heat exchanger networks were systematically compared, using exactly the same basis. The effect of using different logarithmic mean temperature difference (LMTD) approximations on the Total Annual Cost of the network and the network structure were examined. The accuracy of the approximations was also analyzed over a wide range of temperature difference ratios. It is concluded that the best approximation, which is the Underwood-Chen approximation, should be used in future mathematical optimisation of Heat Exchanger Network Synthesis problems.

This paper presents a methodology for integrating various energy sources such as solar, wind, fossil fuel and biomass for energy generation into the synthesis of multi-period heat exchanger networks (HEN). The integration solution is imperative due to the rising cost of fossil based energy sources and their attendant potential environmental impact. Furthermore, since chemical plant operations are in reality multi-period in nature, the demand for energy at various periods of operation can be efficiently satisfied not only in a cost-efficient manner but also in an environmentally sustainable way as well. This is accomplished in this paper by adopting existing multi-period superstructure model for heat exchanger networks and extending it to handle multiple options of utilities which may be available at various times of the day. The objective function entails a simultaneous minimisation of costs and environmental impact (EI). Both renewable and nonrenewable energy sources are included, however, since all renewable energy sources may not be available at all times in a day, a hybrid energy system (a system using two or more energy sources) was considered depending on the availability of the energy source. Pareto curves for these different hybrid systems were then generated. Based on the results obtained, it was found that the windbiomass hybrid energy source gave the most favourable HEN system with regards to cost and EI considerations. This was followed by wind-coal, solar-biomass and solar-coal respectively, also in terms of cost and EI considerations. There were several limitations in this study and recommendations have also been proposed for future work improvement.

In this study a novel methodology for multi-period heat exchanger network synthesis is presented. The new synthesis method aims to systematically generate many candidate networks and, through the use of more detailed individual heat exchanger designs and their evaluation over all periods, guide the network optimisation to more realistic designs. This is done by using the multi-period mixed integer non-linear programming (MINLP) stage-wise superstructure (SWS) model and modifying it to include correction factors. These correction factors enable the MINLP optimisation of the overall cost of the designed network, which uses only shortcut models of the individual exchangers, to be guided by more detailed models of the individual heat exchangers that comprise the network. The designs obtained at the topology optimisation stage thus more accurately represent an actual network. The correction factors take into account aspects of the real design, such as TEMA standards, FT correction factors, number of shells, and changes in overall heat transfer coefficients. Each exchanger is designed to function over all periods of operation, and if this is not possible, extra exchangers are designed for the periods that cannot be satisfied. The methodology is applied to a case study that demonstrates the benefits of the proposed approach.

Mass exchange network synthesis (MENS) can be used to reduce pollutant emissions into the environment as well as reduce the need for mass separating agents (MSAs). Designing an efficient network using mathematical programming is not a trivial task, due to the fact that the design equations involved are highly non-linear. Chemical plant process parameters also change from time to time due to issues such as changes in environmental conditions, plant start-up/shut-downs, changes in process feed quality, change in product quality demand, and some other disturbance in the system. Such changes in process parameters result in what is known as multi-period operations; hence the synthesised network has to be flexible to handle these changes. In order to circumvent the issues associated with solving non-linear equations in mathematical programming environment, most methods have simplified the cost functions for MENS. Solutions obtained from these methods may not be realistic because the simplified cost functions are based on the assumption that the diameter of a column is 1 m or 2 m and that the capital cost of packed columns are dependent only on the height of the column. The simplified models do not make provision to check whether the resulting designs are prone to flooding, operate at optimal pressure drops or even whether the ratio of column diameter to size of packing materials would enhance efficient operations. Furthermore the models assumed single period operations for mass exchange network systems. This paper presents a new synthesis method for single and multi-period MENS using detailed cost functions and correlations which check whether selected columns are prone to flooding or not and whether they have optimal pressure drops that ensure efficient separations as well as optimal use of MSAs. The solutions obtained are compared with an existing method that used pinch technology.

The synthesis of heat exchanger networks has received significant attention in the last four decades due to the rising cost of fossil based energy sources and their attendant greenhouse gas emissions potential. However, most of the methods presented in the literature for heat exchanger network synthesis (HENS) have assumed that plants’ process stream parameters, such as supply/target temperatures and stream flowrates, are fixed, hence having a single period of operation. In reality, process parameters vary within certain ranges due to changes in environmental conditions, changes in product quality demand, plant start-ups/shut-downs, and other disturbances which may upset the system. This implies that plants need to be designed to accommodate the aforementioned potential variations in operating parameters. This paper presents a new 3-step approach for the synthesis of flexible heat exchanger networks for multi-period operations with unequal period durations. The first step entails optimising a representative single period network of the multi-period problem. The solution to the representative network is then used to initialise the multi-period network in the second step. In the third step, the resulting network from the second step is redesigned/evaluated to handle unforeseen changes in lengths of periods. The solutions obtained from the newly presented method compare favourably with those in the literature.

Lignocellulosic biomass such as sugarcane bagasse is non-food biomass that can be used to produce ethanol. Lignocellulose is a complex network of cellulose, hemicellulose and lignin, which requires pre-treatment to improve access to cellulose for hydrolysis which produces glucose for fermentation. Lignin prevents access to cellulose thus delignification using alkaline is often included before hydrolysis. A variety of pre-treatment methods exist requiring different raw materials and operating conditions thus having different economics and environmental impacts. This paper aims to use computer modelling in an optimisation environment called GAMS (General Algebraic Modelling System) to screen a host of pre-treatment options of sugarcane bagasse for bio-ethanol production. The criteria to determine the best pre-treatment option evaluates both economic and environmental objectives. Pre-treatment options included steam explosion, with and without acid catalysis, and acid pre-treatment. Methane was produced from xylose formed in pre-treatment in all options. Delignification using NaOH was included in some investigated pre-treatment flowsheets. The delignification was included in these flowsheets prior to the hydrolysis stage which used either acid or enzymes. The solution space was used to evaluate possible flowsheets in terms of the two aforementioned objectives. For a scenario where methane is the only desired product, steam explosion would be recommended. Adding acid hydrolysis to steam explosion (SA) to produce bio-ethanol increases profitability and reduces environmental impact however the glucose flowrate from this flowsheet is low. For a scenario where higher glucose flowrate is desired, steam explosion with enzymatic hydrolysis pre-treatment flowsheet is recommended however the environmental impact of this flowsheet may be large depending on the energy efficiency of enzyme production.

This study makes use of a novel methodology for the synthesis of heat exchanger networks, which is aimed at overcoming the shortcomings associated with the use of shortcut models to represent individual exchangers in the synthesis network. The new approach entails the use of a number of correction factors to get networks which are based on the use of shortcut models, such as the mixed integer non-linear programming (MINLP) stage-wise superstructure (SWS) of Yee and Grossmann (1990) to more closely represent physically achievable heat exchangers and ensure that the MINLP network topology optimisation step of these models converge on a real design, rather than an approximated one. In this paper, the SWS formulation is used for the generation of an initial network after which its objective function is modified to include the correction factors that force its objective function towards the cost of a network whose individual exchangers are designed using methods such as Bell–Delaware and heuristics. The modified objective function includes parameters that modify the areas obtained by the shortcut based MINLP model so as to more closely represent the areas obtained by the detailed models and also includes a novel method for including the number of shells required for each exchanger duty. The correction factors account for pressure drops, Ft correction factors, number of shells, TEMA considerations, and changes to the overall heat transfer co-efficient of each stream match. The methodology is applied to two examples and the solutions are comparable with other solutions obtained in literature and were shown to produce good solutions. The reason that the method is effective is because many potential networks are evaluated during the iterative procedure and the best network, based on the detailed exchanger designs, is chosen. In this way it is possible to use the detailed exchanger designs to “guide” the MINLP optimisation towards more realistic networks and also to generate many different potential networks.