Truls Gundersen

Numerical aspects of the implementation of cubic equations of state in flash calculation routines

Abstract It is shown that failures in flash calculations (K1 → 1.0) when cubic equations of state are used to estimate equilibrium K-values, result from two main deficiencies. One is the well-known problem of guessing good starting values of the phase compositions at high pressure. The other is that regular methods for solving the cubic equation in the compressibility often lead to a wrong “type” of root (i.e. vapour or liquid). An algorithm without these deficiencies is presented.

Annotated bibliography—Use of optimization in LNG process design and operation

Abstract This paper provides a review of the literature on applications of optimization for design and operation of processes for liquefaction of natural gas and a few related subjects. The review takes the form of an annotated bibliography. A short summary of each of the 186 published works published up to and including 2013 is combined with a classification of the different contributions. While the main focus is design and optimization of processes for liquefaction of natural gas, a selection of publications considering optimization of other parts of the LNG value chain and optimization of other sub-ambient refrigeration processes is also included.

An extended vertical MILP model for heat exchanger network synthesis

An extended vertical MILP model is proposed for selecting Heat Load Distributions (HLD=set of matches and corresponding duties) that will give networks with close to minimum area and total annual cost. This model is the crucial step in a sequential approach to Heat Exchanger Network Synthesis using Mathematical Programming. A previously published vertical model is based on the idea of selecting matches that to the largest possible extent transfer heat vertically between the composite curves, in order to utilize the available driving forces in an improved way. When process streams (and utilities) have significantly different heat transfer conditions, there are two effects that need to be addressed. The first effect (shifting) is that a hot stream with a high film heat transfer coefficient should be used to heat up a cold stream with higher temperatures than strict vertical heat transfer indicates. In the vertical model, this effect is implemented through modified temperatures using stream individual contributions to ΔT min based on film heat transfer coefficients. The second effect (pairing) is that it is often beneficial to isolate streams with poor film coefficients in separate heat exchangers and allow larger driving forces for these units. In the extended model, this effect is accounted for by a new penalty term in the objective function using data on film coefficients. The model is tested on a few examples and shows excellent ability to select heat load distributions that give networks with low area and low total annual cost.

Improved sequential strategy for the synthesis of near-optimal heat exchanger networks

Abstract A sequential franiework (LP, MILP, NLP) for Heat Exchanger Network Synthesis (HENS) is described that reduces significantly the numerical problems caused by non-linearities, discontinuities and combinatorial explosion in simultaneous MINLP models. The multiple trade-offs in HENS related to level of heat recovery, network topology and area distribution as well as the trade-off between total annual cost, network complexity and operability is dealt with by iteration, user interaction and a new MILP transportation model. This model, which is based on the same idea as our vertical MILP transshipment model, is crucial in the sequential approach, since it provides sets of matches that give designs with close to minimum area and cost when supplied to an NLP model for network generation and optimization. In addition to the framework itself, this paper will focus on specific issues related to the sequential approach such as: (1) improving the selection of matches in the MILP model by accounting for temperature driving forces, (2) reducing the combinatorial problem of the MILP model by a tighter formulation, and (3) improving the robustness and efficiency of the NLP model by increased use of information from the MILP model and the addition of convex estimators. Results will be presented for some of these problem areas.

Heat Integration: Targets and Heat Exchanger Network Design

Abstract: This chapter describes the basic steps of Pinch Analysis for heat recovery that made Process Integration a methodology employed by numerous designers and engineers worldwide and made industrial leaders in the 1980s claim these concepts to be the results of academic research that had the largest impact on industrial thinking in relation to design and operation in the process industries. Key elements of the chapter are the Heat Recovery Pinch, Performance Targets ahead of design, and a step-wise and systematic procedure for Heat Exchanger Network Design. One of the main characteristics of Pinch Analysis is the extensive use of graphical diagrams and representations, which give the designer a good overview of even the most complex processes. These tools provide insight and ease the communication between designers and engineers.

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.

Constraint handling in stochastic optimization algorithms for natural gas liquefaction processes

Abstract Liquefaction of natural gas requires energy intensive refrigeration. A fair comparison of different process concepts and energy efficient designs requires some use of optimization. In near optimal designs, the driving forces in heat transfer are small. Thus, rigorous thermodynamics providing accurate and reliable temperature profiles must be applied for the solution to have a practical value. Owing to the characteristics of the process and the thermodynamics, the optimization problem is non-convex. Furthermore, the optimal solution is expected to be located close to the boundary of the feasible region, suggesting the importance of constraint handling. In this paper, a single-mixed refrigerant process (PRICO®) has been optimized using adaptive simulated annealing. A constraint handling method utilizing process characteristics is proposed and compared with static penalty function formulations. The results indicate the importance of constraint handling, and the best solution found exceeds previously published results.

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.

A new graphical exergy targeting representation for processes operating above and below ambient temperature

The main purpose of this paper is to introduce an alternative graphical exergy targeting representation for heat recovery systems. The novel diagram uses a new energy quality parameter referred to in this text as exergetic temperature. The new diagram can be extended to systems with change in pressure; however, such systems will not be considered here. For energy and material recovery systems, a clear graphical representation of the sources, sinks, maximum recovery and minimum utility requirements is of great importance; i.e. for heat ( Linnhoff et al., 1982), mass (El-Halwagi and Manousiouthakis, 1989) and water (Wang and Smith, 1994). The proposed exergy diagram uses a composite representation of sources and sinks for the determination of exergy targets. The exergy targets include recovery, requirements, rejection and destruction of exergy. Two previous exergy representations are evaluated and compared with the proposed one.

New Graphical Representation of Exergy Applied to Low Temperature Process Design

This paper presents an alternative representation of exergy that can be used for process design in particular for low temperature processes such as the liquefaction of natural gas. The procedure combines Pinch and Exergy Analyses and uses a new graphical representation of exergy. One objective of this work is to illustrate the inclusion of exergy calculations in the early stages of design, such as in energy (and exergy) targeting. The paper introduces a novel diagram for exergy and energy targeting which utilizes a new energy quality parameter called exergetic temperature. This quality parameter can be used to manipulate exergy changes caused by pressure adjustments in the processes. The main objective of the pressure manipulations is to reduce both heat and power requirements. The Reverse Brayton process is used as the main case study for illustrating both the exergetic temperature and the novel diagram.