Andreja Nemet

Designing a Total Site for an entire lifetime under fluctuating utility prices

Abstract This paper describes a synthesis of Total Site in order to obtain additional energy savings by process-to-process heat integration. Enhanced Heat Integration and economically viable designs can be obtained by establishing an appropriate trade-off between the operating cost and the investment. The aim of this work was to improve the modeling of the Total Site by including proper pressure levels selection for intermediate utilities, preheating of intermediate utilities because of incomplete condensate recovery, pipeline layout design, and optimal pipe design with optimal pressure/temperature drops and optimal insulation thickness and heat losses during transportation along the pipes. Additionally, future utility prices are considered when synthesizing the Total Site as they are expected to influence the trade-off between investment and operating cost. A stochastic multi-period mixed-integer nonlinear programming model for the optimal synthesis of Total Site over its entire lifetime has been developed by including all the above-mentioned design aspects.

Integration of solar thermal energy into processes with heat demand

An integration of solar thermal energy can reduce the utility cost and the environmental impact. A proper integration of solar thermal energy is required in order to achieve it. The objective of this study is to maximise the solar thermal energy delivered to the process. It is a result of trade-off between the captured solar thermal energy and maximal energy delivered to the process (process demand). Two novel curves are introduced to present this trade-off: (i) The Captured Solar Energy Curve (CSEC), which represents the available amount of heat from solar source and (ii) The Minimal Capture Temperature Curve (MCTC), indicating the minimal temperature making the heat transfer feasible. The crossing point of these two curves presents the minimal temperature of the capture being still sufficiently high to be usable for processes. The suitability of these curves for using in combination with standard heat integration methods is analysed and evaluated. The capture potential is revealed in full when the CSEC and MCTC are used with the Grand Composite Curve. In Total Site Profiles, the heat recovery is first maximised and then the CSEC and MCTC tool is applied. The implementation of CSEC and MCTC approach is illustrated by two case studies.

Capital Cost Targeting of Total Site Heat Recovery

Exploiting heat recovery on Total Site level offers additional potential for energy saving through the central utility system. In the original Total Site Methodology (Klemes et al., 1997) a single uniform ΔTmin specification was used. It is unrealistic to expect uniform ΔTmin for heat exchange for all site processes and also between processes and the utility system. The current work deals with the evaluation of the capital cost for the generation and use of site utilities (e.g. steam, hot water, cooling water), which enables the evaluation of the trade-off between heat recovery and capital cost targets for Total Sites, thus allowing to set optimal ΔTmin values for the various processes. The procedure involves the construction of Total Site Profiles and Site Utility Composite Curves and the further identification of the various utility generation and use regions at the profile-utility interfaces. This is followed by the identification of the relevant Enthalpy Intervals in the Balanced Composite Curves. A preliminary result for evaluation of heat recovery rate and capital cost can be obtained.

Heat Integration retrofit analysis—an oil refinery case study by Retrofit Tracing Grid Diagram

Heat Integration has been established over the last decades as a proven chemical engineering methodology. Two design implementations are often used in the industry: grassroots and retrofit. Although various methods have been developed for retrofit, it still needs more development to ensure simultaneously thermodynamic feasibility and economic viability. In this paper, a novel graphical approach has been developed to facilitate the understanding of the current situation and scope of improvement. The Retrofit Tracing Grid Diagram presents all streams and heat exchangers in temperature scale and the heat exchangers are clearly separated from each other, enabling clear visualisation of the current state. The tool incorporates the previously developed Cross-Pinch Analysis as well as path approach for retrofit. Additionally, the non-vertical heat transfer can be evaluated. The application of the developed tool has been validated on an oil refinery case study. The applicability of the tool is evident as it can reveal additional options for modification that none of the previous methods considered.

Total Site Methodology as a Tool for Planning and Strategic Decisions

A Total Site (TS) is defined as a set of processes (industrial plants, residential, business and agriculture units) linked through the central utility system. The utility system incorporates a number of operating units such as boilers, steam turbines, gas turbines and letdown stations. Many sites are using the TS system representation. Heat Integration at TS level has been well developed and successfully implemented. However, sites typically develop with time and even minor changes/extensions can affect TS heat recovery significantly. It is beneficial to plan their strategic development in advance, to increase or at least not to decrease the rate of heat recovery when integration of additional processes takes place. Even when this has not been done at the initial stage, the TS methodology can still be used as a tool for the strategic planning decision making. This work illustrates how the TS methodology can contribute to the strategic development and the extension planning of already existing TS. The aim is to reveal the potentials for Heat Integration, when new units or processes are considered for the inclusion in the TS. Moreover, some operating parameters (e.g. temperature or capacity) of the unit can be proposed to achieve the best possible heat recovery. The degrees of freedom for TS changes can be on two levels: (i) only adding an operating unit to the current utility system (the Total Site Profiles remain the same) or (ii) changing of the TS by including more processes (the Total Site Profiles are changed). The first group of changes includes the integration of heat engines to produce electricity utilising heat at higher temperature and returning it to the system at lower temperature, which is still acceptable for the heat recovery and simultaneously for the electricity production. The second group of changes is more complex. For evaluating these changes a plus/ minus principle is developed allowing the most beneficial integration of new units to the TS. Combinations of both types of changes are also considered.

Cleaner production, Process Integration and intensification

A considerable number of studies have been performed and are ongoing for enhancing cleaner production, process integration and also process intensification. These topics reflect some of the most important challenges of our society and have been targeted in this journal. Considerable research effort has been devoted to addressing process integration and process intensification as well as environmentally friendlier production. This article has made an attempt to provide a short assessment of the current state-of-art covered in the recent publications.

Methodology for Maximising the Use of Renewables with Variable Availability

A problem in exploiting renewable energy sources, as wind and solar radiation, is their fluctuating availability. To deal with it, the Heat Integration methodology for batch processes based on Time Slices (TS) was extended. Two approaches to identify the size and number of TS for variable renewable supply are investigated for solar energy. The first specifies a threshold load within which the fluctuation can be neglected. The optimal threshold value is selected from the trade-off between the overall inaccuracy and number of TSs. The second approach is to partition the measured/forecasted heat load curve using a large number of candidate time boundaries and approximate it by a piecewise-constant profile with high precision. The profile is supplied to a MILP formulation to screen the candidate time boundaries using binary variables of acceptance/rejection. The TS definitions are completed by approximating heat loads. The solutions are compared using several criteria including inaccuracies and numbers of TSs. The number of TSs and their sizes are ready to be used in Total Site analysis.

Mathematical Programming Approach to Total Site Heat Integration

Abstract Heat Integration is one of frequently used methods for decreasing utility consumption. Originally it was developed for integration at the process level and gradually extended to heat recovery between various processes. In this current work a mixed-integer nonlinear programming (MINLP) model has been enhanced to evaluate the rate of heat recovery between those various processes and also to obtain the optimal temperature for the intermediate utility. This is achieved by considering: i) Heat losses during the transporting of steam through the pipeline, ii) Increased investment into heat exchangers and pipelines due to higher pressures and iii) Optimising the areas of heat the exchangers not only at the process level but also for those heat exchanger areas for heat transfer between process stream and intermediate utilities. A created superstructure considers that processes are connected to each other through various intermediate utilities, the temperature of which vary within their temperature intervals. Two different strategies for obtaining Total Site Heat Exchanger networks of a Total Site have been applied: a sequential and a simultaneous one. According to the first strategy Heat Integration is first obtained at the process level and then at the Total Site level from the resulting utility requirements obtained from the first step. In the second strategy the heat recovery is achieved simultaneously at the process and the Total Site levels. Both strategies were compared in order to evaluate, which one performs better.

Increasing solar energy utilisation by rescheduling operations with heat and electricity demand

a Centre for Process Integration and Intensification CPI 2 , Research Institute of Chemical and Process Engineering MUKKI, Faculty of Information Technology, University of Pannonia, Egyetem utca 10, 8200 Veszprem, Hungary b Department of Computer Science and Systems Technology, Faculty of Information Technology, University of Pannonia Egyetem utca 10, 8200 Veszprem, Hungary nemet@cpi.uni-pannon.hu

Safety Analysis Embedded in Heat Exchanger Network Synthesis

Abstract Optimization of Heat Exchanger Networks (HEN) has received considerable attention in last decades, but a few studies on inherent safety. In this paper, risk assessment is considered simultaneously during the synthesis of HENs. As risks depend on the equipment selected, a superstructure enabling selection of direct and indirect heat transfer between hot and cold streams and different types of heat exchangers (HEs) was tested. The individual heat transfer and the overall HEN risk were analyzed. Different individual risk limits have been introduced for certain types of heat transfer, e.g. between two process streams or between utility and process streams. The sensitivity analyses were performed first, considering only toxicity as a risk, but later flammability and explosiveness were also simultaneously tested, in order to consider the most important aspects of safety. The results obtained indicate that rather significant changes in HEN designs can increase safety, while still exhibiting similar economic efficiency.