Synthesis of optimal heat and mass exchange networks using a two-step hybrid approach including detailed unit designs

This PhD thesis develops a methodology for the synthesis of optimal heat and mass exchanger networks through a novel hybrid method. The two-step procedure makes use of simplified exchanger models in a network optimisation step, followed by a detailed design where the exchangers found in the network synthesis step are modelled in detail. Subsequent iterations of the network design step are then updated with information from the detailed network designs. The algorithm has certain advantages over previous methods in that the network optimisation is based on more realistic representations of the actual units therein and also that the method increases the likelihood of attaining a globally optimal solution through the generation and assessment of multiple candidate networks throughout the algorithm. The method can be used in a variety of applications and is demonstrated to be effective for large problems and multi-period scenarios. The thesis also shows that the method can be used in conjunction with multiple individual unit optimisation techniques including heuristics and fully explicit optimisation methods

Capital cost targets for the optimum synthesis of mass exchange networks

Includes bibliographical references.Pinch Technology is very well developed for heat exchanger network synthesis (HENS). It is possible to predict, on thermodynamic grounds, the minimum energy, capital and total costs for a network. These targets are set before any design and can also be optimised at this stage. Special design techniques exist which allow the targets to be met - or closely approached - in practice. The approach has recently been extended to mass exchange network synthesis (MENS). However, prior to this study, it was not as well developed for this field as it had been for HENS. Only targets for the minimum operating costs could be set and then achieved in design. Capital cost targets for MENS did not exist. The usual approach was to use the minimum number of mass exchange units - which could be targeted - as an attempt to minimise the capital cost of the network. However, this is not sufficient since the exchanger sizes are also important. This meant that there was no guarantee that the capital cost and hence total cost had been truly minimised. This thesis has developed a new method for targeting the minimum capital costs for mass exchange networks. The method is simple and based on insight, rather than relying on a mathematical 'black-box'. New graphical tools, the y-* composite curve plot and the y-y* composite curve plot have been introduced for this purpose and these allow the minimum exchanger sizes to be predicted before design. The new capital cost targets can be traded off against the established operating cost targets in order to optimise the total cost with no design being necessary

Simultaneous synthesis of flexible heat exchanger networks

In industry there is still lot of potential to make an energy system more efficient and thereby reduce the waste heat available. On the other hand there is an option to export the waste heat to another industry or to society. When the use of a heat exchanger network is considered for these tasks the optimization framework developed in this work can be implemented to calculate the cost of optimal investments. This thesis presents a framework for generating flexible heat exchanger networks (HEN) over a specified range of variations in the flow rates and temperatures of the streams, so that the total annual costs (TAC) as a result of utility charges, exchanger areas and selection of matches are minimized. The proposed framework includes (i) an initialization stage to reduce the problem size, (ii) a multiperiod simultaneous MINLP model to synthesize a flexible HEN configuration, (iii) a multiperiod LP feasibility test model to check the operability and identify critical conditions which are to be included in the possible resolve stage of the MINLP model, and (iv) an NLP improvement model for further optimization by partly removing simplifications related to the MINLP model. This framework results in a HEN which can work in varying conditions without losing stream temperature targets and can keep an economically optimal energy integration. This thesis also shows how the simplified superstructure presentation proposed by Yee and Grossmann (1990a) can be applied for generating flexible heat exchanger networks. Furthermore, this thesis presents a scheme which eliminates the modeling of bypasses, so that the nonlinear heat balances, binary variables, temperature variables and flow variables related to each bypass in the superstructure are no longer needed in the model. The elimination of bypass modeling, a stage-wise superstructure presentation and isothermal mixing assumption, make the MINLP model more robust and efficient to solve. Since this MINLP model is not solved until a problem is well prepared by the other parts of the developed optimization framework, the methodology presented in this thesis is applicable to solve industrial size grassroot design cases of flexible heat exchanger network problems. Lastly, the proposed HEN synthesis strategy has been successively applied to two industrial problems where the industrial waste heat streams have been cooled down, forming a local and site level energy integration to gain savings in steam consumption and to avoid cooling tower investment. Both these problems represent the special case of correlated uncertain parameters, which here means that there is a relationship between uncertain parameters given in the stream data.reviewe

Achievements and perspectives of process integration in cis countries

Due to the rapid growth in the world population, there has been an increase in energy consumption globally. The problem of efficient energy use becomes more relevant and stimulates research and development of new energy and resource-saving technologies. This task is becoming more complicated when the other factors are accounted for, resulting in multiple-factor trade-offs, such as the water-energy-food nexus. This paper highlights the main points for the development of Process Integration in the Commonwealth of Independent States (CIS) countries. It shows the main achievements in the field to date and demonstrates the scientific schools that are working on these problems. A comprehensive review of modern approaches and methods, which are now being developed or have been recently developed, was done. It shows a research gap in Process Integration in CIS and other leading countries. It demonstrates the significant research potential as well as practical applications. The main challenges in process systems engineering and for the sustainable development of industrial energy systems are also discussed. Industry digital transformation, energy transition, circular economy, and stronger energy and water integration are pointed out as priorities in analysis, design, and retrofit of society in the future. A state-of-the-art review in the area of integration of continuous and batch processes, mass integration technologies, and process intensification is presented to show the variety of existing approaches. The necessity of Process Integration development in the CIS is shown to be a necessary condition for building a more sustainable society and a resource-efficient economy

HEAT EXCHANGER NETWORK RETROFITTING

Ph.DDOCTOR OF PHILOSOPH

PATH ANALYSIS FOR THE RETROFIT OF HEAT EXCHANGER NETWORKS AND THE UTILITY SYSTEM

This study introduced a new approach for HEN retrofit that featuring area addition to the existing exchangers without massive topology changes in HEN. The approach has developed based on a combinatorial method to combine the available utility paths in HEN systematically to generate several alternatives for increasing the process-toprocess heat recovery. To ensure feasible heat transfer between hot and cold streams, the Heat Recovery Approach Temperature (HRAT) is maintained while increasing the heat recovery. The available exchangers’ pressure drop is considered in calculating the film heat transfer coefficients. A demonstrative example showed several retrofit options where the energy savings ranged from $150K/yr to$450K/yr with payback of less than 2 years to refund the investment rose from the mandatory area addition. The developed approach is termed as ‘Paths Combination Approach for HEN Retrofit’. Moreover, a concept of varying the process stream temperature has been established to further increase the heat recovery and make the infeasible solutions more competitive. This concept mainly depends on the process streams’ flexibility to changing the inlet and outlet temperature; and termed as the Temperature Flexibility concept (TF concept). Implementation alternatives are generated and integrated into the paths combination approach. Given that major changes in process conditions are rarely desired, the temperature changes has been kept within a small magnitude regardless of the usual process temperature oscillations. A user friendly computer programme has been developed for performing the approach in view of the significant number of iterations required. Most of the infeasible retrofit solutions have changed to the feasible zone where higher savings are featured along the temperature flexibility range. The energy savings derived from HEN retrofit have been further investigated to study the impact on the utility system. Through a case study integrating HEN retrofit and utility system, the most efficient way was found to redistribute the steam surplus among the utility system devices while considering the turbines flow constraints. Accordingly, the power production in the utility system has increased vi using one of the retrofit options from 4.1% to 10.5% when applying the full range of the TF in HEN