Rahul Anantharaman

The sequential framework for heat exchanger network synthesis—The minimum number of units sub-problem

Abstract An overview of an iterative and sequential methodology, called the Sequential Framework for heat exchanger network synthesis (HENS), is presented in the paper. The main objective of the Sequential Framework is to solve industrial size problems. The subtasks of the design process are solved sequentially using mathematical programming. There are two main advantages of the methodology. First, the design procedure is, to a large extent, automated while keeping significant user interaction. Second, the subtasks of the framework (MILP and NLP problems) are much easier to solve numerically than the MINLP models that have been suggested for HENS. One of the limiting factors in the methodology is related to the two MILP models where significant improvements are required to prevent combinatorial explosion. To ease this problem for the minimum number of units MILP sub-problem, two different approaches to the issue were studied. The minimum number of units problem is modified to reduce the gap using physical insights and heuristics. Another novel approach tested was to reformulate some parts of the model by use of some ideas from set partitioning problems. Results show that though both methods succeed in tightening the LP relaxation, the model solution times remain too long to be of interest in the Sequential Framework.

The Role of Process Synthesis in the Systematic Design of Energy Efficient Fossil Fuel Power Plants with CO2 Capture

CO2 capture and storage has a potential of reducing CO2 emissions from large point sources such as fossil fuel power plants. CO2 capture is associated with substantial capital expenditures, operational expenditures dominated by high energy use and potential operational restrictions on the underlying industrial processes. The main focus of significant research efforts worldwide is thus to reduce investment costs and improve efficiency of capture technologies. The systematic methodologies developed in our group at SINTEF/NTNU for design of energy efficient fossil fuel power plants with CO2 capture are presented and show the importance of utilizing process synthesis in the design of such plants. These methods range from targeting minimum capture work for different CO2 capture processes, optimization methods for process design of pre- and post-combustion capture processes, developing surrogate models for optimization.

Optimal integration of compression heat with regenerative steam Rankine cycles

Abstract The integration of process heat with regenerative steam Rankine cycles by preheating the boiler feedwater increases power generation from the steam turbines. In oxy- combustion coal based power plants, considerable compression heat from the air separation unit is available for such heat integration, however, there are at least two challenges: (1) how to integrate a heat stream with the steam cycle, and (2) how to optimize the compression scheme. This paper investigates the mentioned heat integration by building Mixed-Integer Nonlinear Programming (MINLP) models. Two special cases (adiabatic compression and “isothermal” compression) are also investigated to compare with the optimization approach. The results show that the thermal efficiency of the reference power plant increases by a maximum of 0.5–0.6 % points. The integration is less attractive when the temperature difference of the heat transfer between the compressed gas and the boiler feedwater is larger than 40°C.

Design of an IRCC with CO2 capture utilizing a mixed integer optimization method

Abstract Integrated Reforming Combined Cycles (IRCC) is a promising route for combined power generation and hydrogen production with CO 2 capture from natural gas. The design of an IRCC involves many parameters that interact in complex relationships. A methodological modeling and optimization approach to design the IRCC process is presented in this work that includes physical insight gained from engineering judgment. A superstructure of different possible configurations for an air blown IRCC is developed to be used in an optimization framework. A novel Mixed Integer Linear Programming (MILP) model for simultaneous optimization and heat integration of chemical processes is developed as part of this work to reduce the number of binary variables as compared to earlier formulations. The Mixed Integer Non-Linear Programming (MINLP) formulation to maximize net electric efficiency in an air blown IRCC process is modeled in GAMS and solved using BARON as the global solver. The optimized air blown IRCC process has a net electric efficiency of 49.4% with a novel integration scheme.

Developments in the sequential framework for heat exchanger network synthesis of industrial size problems

Abstract A Sequential Framework for Heat Exchanger Network Synthesis (HENS) is presented and the philosophy of this iterative methodology is explained. There are two main advantages of the proposed methodology. First, the design procedure is, to a large extent, automated while keeping significant user interaction. Second, the subtasks of the framework (MILP and NLP problems) are much easier to solve numerically than the MINLP models that have been suggested for HENS. The limiting factors of the methodology are the NLP and MILP models where enhanced convex estimators are required to reach global optimum in the former while significant improvements are required to prevent combinatorial explosion in the latter. This paper makes an attempt to address a few of the limiting elements of the framework.

Dual phase high-temperature membranes for CO2 separation – performance assessment in post- and pre-combustion processes

Dual phase membranes are highly CO2-selective membranes with an operating temperature above 400 °C. The focus of this work is to quantify the potential of dual phase membranes in pre- and post-combustion CO2 capture processes. The process evaluations show that the dual phase membranes integrated with an NGCC power plant for CO2 capture are not competitive with the MEA process for post-combustion capture. However, dual phase membrane concepts outperform the reference Selexol technology for pre-combustion CO2 capture in an IGCC process. The two processes evaluated in this work, post-combustion NGCC and pre-combustion IGCC, represent extremes in CO2 partial pressure fed to the separation unit. Based on the evaluations it is expected that dual phase membranes could be competitive for post-combustion capture from a pulverized coal fired power plant (PCC) and pre-combustion capture from an Integrated Reforming Cycle (IRCC).

Multi-Scale modelling of a membrane reforming power cycle with CO2 capture

Abstract This work presents the initial investigations of an Integrated Reforming Combined Cycle (IRCC) process with CO2 capture using a membrane reformer. A geometrically generic 1-dimensional model of a membrane reformer has been implemented in Matlab 7.9. This model includes detailed balance equations for energy, momentum and mass in all three sections of the membrane reformer. Widely accepted empirical relations have been used to take into account the mass and energy transport across the membrane as functions of the conditions inside the chemical reactor. The reactor model has been integrated into an overall steady state IRCC process simulation model developed in HYSYS and GTPro. The work shows that multi-scale modelling is necessary to capture the behaviour of the process. The overall cycle efficiency of the process was 46.83 % with 85 % CO2 capture.

A new approach to the identification of high-potential materials for cost-efficient membrane-based post-combustion CO2 capture

Developing “good” membrane modules and materials is a key step towards reducing the cost of membrane-based CO2 capture. While this is traditionally being done through incremental development of existing and new materials, this paper presents a new approach to identify membrane materials with a disruptive potential to reduce the cost of CO2 capture for six potential industrial and power generation cases. For each case, this approach first identifies the membrane properties targets required to reach cost-competitiveness and several cost-reduction levels compared to MEA-based CO2 capture, through the evaluation of a wide range of possible membrane properties. These properties targets are then compared to membrane module properties which can be theoretically achieved using 401 polymeric membrane materials, in order to highlight 73 high-potential materials which could be used by membrane development experts to select materials worth pushing towards further development once practical considerations have been taken into account. Beyond the identification of individual materials, the ranges of membrane properties targets also show the strong potential of membrane-based capture for industrial cases in which the CO2 content in the flue gas is greater than 11%, and that considering CO2 capture ratios lower than 90% would significantly improve the competitiveness of membrane-based capture and lead to potentially significant cost reduction. Finally, it is important to note that the approach discussed here is applicable to other separation technologies and applications beyond CO2 capture, and could help reduce both the cost and time required to develop cost-effective technologies.

A new paradigm in process synthesis focus on design of power plants and industrial processes integrated with CO2 capture

Abstract Process synthesis methods have only recently been applied for the design of CO 2 capture systems. At the crux of the CO 2 capture process is a separation technology (sorbent, membrane or phase separation) applied to CO 2 /N 2 , CO 2 /H 2 or O 2 /N 2 systems. Development of novel sorbents and membranes have been identified to be an important area of research to realize cost-competitive CO 2 capture technologies. Thus, there are different capture technologies at varying levels of maturity that need to be integrated with the power plant or industrial process. Traditional optimization based approaches to process synthesis of CO 2 capture systems, while important for mature process components, have a lesser role to play in this application wheer components are still in a constant state of development. Due to the multidisciplinary nature of CO 2 capture, integrating other sciences such as process chemistry with process synthesis should be emphasized. This work will present novel approaches to process synthesis that represent a new paradigm in process synthesis. The focus of process synthesis is modified from identifying an “optimum” process, to using the framework of the discipline to provide feedback on optimum operating parameters/conditions of the novel separation processes. Motivation for visual and hybrid methodologies based on attainable region approach as opposed to the more common optimization based methods are presented.

Revisiting the Simultaneous Process Optimization with Heat Integration Problem

Abstract Simultaneous process optimization and heat integration is essential in the optimal design and operation of process plants with high energy efficiency. This paper revisits the pinch location method and proposes a new compact formulation for simultaneous process optimization with heat integration. The developed simultaneous approach is applied in the process optimization of an Integrated Reforming Combined Cycle power plant that results in a highly efficient process with tight heat integration.