New graphical methods for diagnosis and pragmatic retrofit of heat exchanger network

Abstract

Manufacturing plants typically undergo retrofit several times throughout their lifetime to improve efficiency and profitability. Insight-based heat exchanger network (HEN) retrofit methodologies are preferred by industry because they typically provide clear visualised insights to systematically guide users to conduct plant retrofit. Over the years, research works have been done to improve the features of numerous graphical tools. However, there are still rooms for improvements. Conventional graphical visualisation tools such as composite curves contain very limited information about the properties and profiles of individual streams in an HEN that are important to be considered for generating practical retrofit solutions. Users need to undergo trial-and-error for stream matching and perform iterative calculations to check temperature feasibility and enthalpy balance before obtaining the network design that can achieve the maximum heat recovery. Capital-energy trade-off is usually considered in mathematical optimisation approaches and less so in graphical methodologies. Practical constraints especially the plant layout-based factors have not been a consideration in almost all of the insight-based methods. This research aimed to develop new graphical methods for HEN retrofit which incorporate systematic retrofit methodologies based on the individual stream concept that consider economics and various physical constraints. In this research, three retrofit methodologies were proposed. First, the individual stream temperature versus enthalpy plot (STEP) retrofit methodology that involves simultaneous diagnosis and retrofit of existing HEN was proposed. Second, the heat exchanger area versus enthalpy (A vs H) plot was then developed to be used together with STEP to enable capital-energy trade-off. Third, a three-dimensional coordinate representation was developed to incorporate plant layout-based factors that may hinder processes from achieving maximum heat recovery. Results of the first methodology applied on a fluid catalytic cracking plant demonstrated the advantages of STEP diagram in terms of the insights offered by the graphical tool, the flexibility to customise the methodology to achieve retrofit goals, and results comparability to those of established retrofit methods. Results of the second methodology applied on a sunflower oil production plant showed that the graphical tools and the cost screening technique can be used to perform capital-energy trade-off to result in comparable energy savings and 20% shorter payback period as compared to other established retrofit methodologies. Application of the third retrofit methodology on an illustrative industrial case study resulted in 18% higher in the total annualised costs for the retrofit design which does not consider plant layout-based factors and the one with plant layout-based factors. Implementation of all the new developed retrofit methodologies on literature and industrial case studies shows the applicability of the methodologies to cover different aspects of HEN retrofit, i.e. the simultaneous representation of the vital information, the economic aspect, and the practicability of HEN retrofit methodology.