Energy Integration of Production Systems with Batch Processes in Chemical Engineering

Batch processes have been extensively used in the process industries due to their appropriateness for small volume scale productions, high value-added products, as well as their adjustability to rapid market changes. Heat integration in a batch process can be carried out in two ways directly or indirectly. Direct heat integration is straight transfer of heat from hot to cold stream through heat exchanger, while for indirect heat integration, an intermediate fluid is involved which carries heat from hot to cold stream. In batch process, transfer of heat from one interval to another interval requires intermediate fluid. The usage of intermediate fluid needs pumping, piping, capital investment, and space. These are important aspects for any process while considering space limitation and capital investment. Minimizing intermediate fluid provides reduction in all these important fronts. In this paper, a graphical algorithm is developed to determine minimum requirement of intermediate fluid flow among time intervals while satisfying the minimum utility targets. The proposed algorithm is proved to be optimum rigorously and illustrated via examples.

This work addresses the indirect heat integration (i.e. resorting to intermediate heat storage) and the direct heat integration (i.e. heat exchanges between coexisting process streams) of batch processes. Tools and methods for the targeting of these two limiting cases of heat integration are proposed, and completed by the development and the application of an automatic design & optimization methodology using the Struggle genetic algorithm (GA). A brewery process is used to demonstrate the feasibility and the practical relevance of the proposed indirect heat integration models. The fluctuations of the process schedule and their effects on the optimal solutions are not modelled (the indirect heat integration is known to feature an inherently low sensitivity). The rescheduling opportunities are not searched for. For the indirect heat integration, fixed temperature/variable mass inventory heat storage units (HSUs) are applied. Two models of indirect heat recovery schemes (IHRSs) are proposed : one based on a closed heat storage system, another built around an open storage system suitable e.g. for food and beverages industries. The inequality constraints of the IHRS models are automatically met owing to an appropriate definition of the decision variables managed by the GA. A rough preadjustment of the mass balance equality constraints on HSUs is achieved by a preliminary stage of heat recovery (HR) maximization before actually minimizing the total batch costs (TBCs). Optimization runs and theoretical considerations on the generation & replacement strategy of Struggle demonstrate that the structural and the parametric variables cannot be efficiently optimized within a single level, resulting in a two-levels optimization scheme. The automatically designed closed storage IHRS solutions for the brewing process are as good as the solutions obtained by another author using a combinatorial method followed by a post-optimization stage. The open storage IHRS is 13 % cheaper while the HR increases by 12 %. Optimizing the IHRS on a one-week period (including the non-periodic start-up & shut-down phases) results in an even more realistic solution, featuring a significantly different trade-off between energy, HSU capacities and HEX areas. A GA based, two-levels optimization scheme is proposed for the design of direct batch heat exchanger networks (HENs). The HEN structures, managed by the upper-level GA, do not include stream splitting. The re-use of HEX units across time slices is a key issue and a methodology to specify the actually possible structural changes by repipe or resequence is proposed, accounting for the thermo-physical compatibility, chemical compatibility, and process schedule constraints. The optimum operation of an existing HEN during each time slice has been analysed and a sound solution procedure is proposed.

Energy minimised v. cost minimised alternatives for the US beef production system

The objective of this paper was to design energy integrated solid oxide fuel cell (SOFC) based microcombined heat and power (micro-CHP) systems using syngas derived from lignocellulosic biomass waste. The novel contributions of this work include (1) integration of syngas between a community-scale biomass gasification plant and SOFC-based micro-CHP systems in buildings; (2) heat integrated designs of SOFC-based micro-CHP systems; and (3) integration between SOFC and other heat-led renewable technologies, such as, syngas boilers, ground source heat pump (GSHP), and air source heat pump (ASHP). Conceptual process design frameworks including detailed heat recovery strategies were developed using Aspen plus. It is demonstrated that increases in heat integration between the SOFC inlet and outlet gases enhance the power-to-heat generation ratio from the SOFC, albeit at a higher cost of heat exchanger area. A straw (14.6 MJ/kg LHV) based community-scale gasification plant can generate 50−100 kWe of electricity at an overall CHP generation efficiency of ∼42%, while if integrated to SOFC-based micro-CHP systems (1 kWe) in individual dwellings via syngas, the overall energy efficiency can be greatly enhanced to ∼85%. It is envisaged that the SOFC should be operated at the highest electrical efficiency based on optimal heat integration; however, this needs to be coupled to other heat-led renewable technologies in order to meet the high residential heat to power demand ratio in the UK. Around 2.2 times more syngas may be required in boilers for supplementing residential heating. In integration with a GSHP loop, the flue gas from SOFC can itself be used as a refrigerant, and energy integration between the two systems can achieve an overall efficiency of ∼90%.

Forty years of Heat Integration: Pinch Analysis (PA) and Mathematical Programming (MP)

Heat integration of regenerative Rankine cycle and process surplus heat through graphical targeting and mathematical modeling technique

Energy Reduction Potential in Natural Gas Processing through Heat and Process Integration

Optimal scheduling of tasks to be performed in different unit operations is of paramount importance in the batch processes. The pressure toward sustainable operation has warranted more attentions to be paid to the issues of heat integration to reduce the utility consumption. This paper presents a new approach for incorporating heat integration in batch process scheduling. The method is based on a sequential framework, where the overall problem is decomposed into two sequentially solved problems of scheduling and heat integration. First, the schedule is optimized to meet the economic objective such as makespan or profit. Next, alternate schedules are generated through a stochastic search-based integer cut procedure that adds further constraints to the scheduling formulation. Such schedules may be the alternate optima of the scheduling problem. Finally, heat integration analysis coupled with the time average model (TAM) and time slice model (TSM) is applied to each of the resulting schedules to establish the minimum utility targets. The approach is based on the precept that opportunities for heat integration between tasks are higher in near-optimal schedules where tasks are temporally clustered together. The method differs from other sequential approaches published in the literature in that the heat integration problem is solved with the intent to retain the optimality of the scheduling solution. The proposed method has the ability to handle problems too complex to be solved using simultaneous optimization approaches. It has been applied to two literature case studies. The first one involves profit maximization as the objective function. The second one entails a more complex scenario involving makespan and utilities. The results from both case studies demonstrate the effectiveness of the approach.

Design and control for recycle plants with heat-integrated separators

An H2 Polymer Electrolyte Membrane (PEM) Fuel Cell power generation system composed of three subsystems: fuel reforming, fuel cell stack, and post combustion is examined. The system is simulated and optimized with a fuel cell model integrated within the process flow sheet. A case study optimization of the entire power generation system is presented and the influence of heat integration on the optimization is demonstrated. Our objectives are to maximize the energy and system efficiencies as well as the system profit. Here, optimization formulations with heat integration are used to realize our objectives, which explore the synergy between optimization and heat integration. Our results showed that we can achieve an energy efficiency as high as 58.30% and a system efficiency as high as 28.35%. For comparison, we provide results obtained from sequential process optimization followed by heat integration. The gain from the synergy is mostly exemplified in the profit maximization study, where the profit increases by almost 27.15% in the best case. Finally, we obtain optimization results that include feed vaporization of water to steam as a comparison for the heat integration. These results demonstrate the importance of simultaneous optimal heat integration for fuel cell‐based processes. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Heat integration including heat exchangers, combined heat and power, heat pumps, separation processes and process control

The green brewery concept – Energy efficiency and the use of renewable energy sources in breweries

Environmental and legislative constraints regarding sludge disposal, such as its application on agricultural land, stimulate research in this area. Since municipalities are responsible for sludge disposal and finance it from taxpayers’ money, cost is the major constraint in providing this service. Therefore, research in this area has to provide economically viable solutions. The Autothermal Thermophilic Aerobic Digestion (ATAD) process is one practical solution that enables the reuse of sludge in agriculture, though its high energy demand reduces its appeal. One of the main attractiveness of ATAD process is its stability and low sensitivity to changeable operating conditions. Our theory is that the cost efficiency of ATAD process could be enhanced by improving its energy performance and therefore operating cost attractiveness. Thus, the main objective of the research was to improve the energy efficiency of the ATAD sludge treatment process. The hypothesis of certain modifications applied to the ATAD process to increase its operational temperature can enhance its energy efficiency was investigated. This thesis answered the question whether the energy efficiency of the ATAD process can be improved by increasing the operating temperature in the reactors, e.g. reducing and/or eliminating the thermal shock. This research further deepened the understanding of the process and indicated areas of improvements for design and operation. It established, through analysis, redesign options and best operation conditions leading to a more energy efficient ATAD process. The dynamic mathematical model used is capable of predicting the rate of volatile solids degradation and the temperature of the sludge in the reactors, therefore the compliance with legislation (pasteurisation and stabilisation criteria) can be assessed. The applicability of the ATAD dynamic mathematical model on a full-scale was demonstrated. It was proven to give a more realistic picture of process performance than the current design guidelines. Recommendations for improving the existing process were proposed and verified by the ATAD mathematical model. Modifications made to the operational circumstances, such as changing the feeding pattern (feeding volume and frequency), changing the characteristics of the feed sludge, such as its concentration or temperature (achieved by heat integration) and changing the characteristics of the air supplied, such as its temperature; and combination of the above listed solutions were analysed. This project has proven that the design of the ATAD system could be further improved with the recommendation such as heat integration and changed feeding pattern. The results include: • More stable and steady operation. • The proposed improvements lead to reduced retention time, which also reduces the need for aeration and mixing energy – ultimately, this improves energy efficiency as it reduces the electricity needs by 25% for running the system for a certain throughput. A tool which can be utilized to evaluate the different ATAD systems has been provided. The model’s potential is not restricted to a tool for evaluation of process modifications from an energy efficiency perspective. It can be used along different objectives and at different stages of ATAD systems’ life span, such as design, operation and control; and even as a teaching tool. This research further deepened the understanding of the process and indicated areas of improvements for design and operation and also opens the doors for further research in this area. Improvements can lead to ATAD’s wider acceptance in the wastewater treatment field and its more extensive use in Ireland and around the globe.

Understanding how net primary production (NPP) and its allocation respond to climate warming is of fundamental importance in predicting ecosystem carbon (C) cycle and C-climate feedback. Especially, the optimal partitioning theory suggests that plants preferentially allocate photosynthates toward the above- or below-ground parts to acquire the limiting resources to maximize their growth rate and optimize resource use under environmental change. However, it remains elusive on how NPP allocation changes and regulates community water-use efficiency (WUEc) under climate warming. In this study, we conducted a manipulative warming experiment with three levels of warming treatments (control, + 1.5 °C and + 2.5 °C) to explore the response of NPP allocation and its regulation on WUEc in an alpine meadow. Results showed that above-ground NPP (ANPP) and below-ground NPP (BNPP) responded differently to warming. On average, W1.5 (+ 1.5 °C) and W2.5 (+ 2.5 °C) treatments increased BNPP by 28.98% and 33.28% and increased NPP by 20.05% and 38.70%, respectively, across 4 years. Whereas no consistent warming effect on ANPP was observed across years. The fraction of BNPP to total NPP (fBNPP) responded positively to warming under low ambient temperature and community biomass while it responded negatively under high ambient temperature and community biomass. Notably warming-induced changes in fBNPP negatively correlated with warming-induced changes in WUEc. These results suggested that warming effect on NPP allocation was largely contextually dependent and implied important biological regulation on WUEc. The emerging trade-offs between NPP allocation and WUEc reflect adaptation strategy of plant community under climate change.

Heat integration of multipurpose batch plants through multiple heat storage vessels