| 1. |
- Marton, Sofie, 1988, et al.
(author)
-
A steam utility network model for the evaluation of heat integration retrofits – A case study of an oil refinery
- 2016
-
In: Proceedings of the 2nd South East European Conference on Sustainable Development of Energy, Water and Environment Systems, SDEWES.SEE2016.0258.
-
Conference paper (peer-reviewed)abstract
- This paper presents a real industrial example in which the steam utility network of a refinery is modelled in order to evaluate potential heat integration retrofits proposed for the site. Total site heat integration techniques have previously been used for the refinery to identify a number of possibilities for heat saving retrofits. However, the profitability and actual primary energy saving will depend on the configuration and operation of the utility system.The supply of utilities such as fuel, steam and electric power represents a significant operating cost for many process industries. A reduced use of utilities can improve process economics and reduce the environmental footprint of the process. Heat integration, within and between the refinery process units, is one way to improve the energy efficiency and reduce the use of steam and fuels. Such heat integration measures will affect steam consumption and generation at different steam pressure levels within the network. A refinery, typically, has flexibility in their operation of the steam network from a number of switchable steam turbine-driven or motor-driven pumps and compressors. Furthermore, due to seasonal variations the amount of internally generated refinery fuel gas will vary, leading to periods of steam over-production as well as periods with a demand for additional purchased fuel. Consequently, the optimum operating strategy concerning steam production and the switchable mechanical drives is a trade-off between the cost of fuel and electricity.* Corresponding author 0258-12This paper presents a few examples of heat integration retrofit measures from a case study of a large oil refinery. In order to evaluate expected changes in fuel and electricity imports to the refinery after implementation of the proposed retrofits, a steam system model has been developed using commercial software. The model enables an analysis of how the operation of boilers and turbines could be changed in effect of a proposed change. The steam system model has been tested and validated with real steady state data from three different operating scenarios. It can be used for simulation or optimization to answer how changes to steam balances at different pressure levels will affect the overall steam balances, generation of shaft power in turbines and electricity demand for pumps and compressors, and the consumption of fuel gas.
|
|
| 2. |
- Marton, Sofie, 1988, et al.
(author)
-
A Steam Utility Network Model for the Evaluation of Heat Integration Retrofits – A Case Study of an Oil Refinery
- 2017
-
In: Journal of Sustainable Development of Energy, Water and Environment Systems. - : SDEWES Centre. - 1848-9257. ; 5:4, s. 560-578
-
Journal article (peer-reviewed)abstract
- This paper presents a real industrial example in which the steam utility network of a refinery is modelled in order to evaluate potential Heat Integration retrofits proposed for the site. A refinery, typically, has flexibility to optimize the operating strategy for the steam system depending on the operation of the main processes. This paper presents a few examples of Heat Integration retrofit measures from a case study of a large oil refinery. In order to evaluate expected changes in fuel and electricity imports to the refinery after implementation of the proposed retrofits, a steam system model has been developed. The steam system model has been tested and validated with steady state data from three different operating scenarios and can be used to evaluate how changes to steam balances at different pressure levels would affect overall steam balances, generation of shaft power in turbines, and the consumption of fuel gas.
|
|
| 3. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
A multiple input, multiple output model generator
- 2017
-
Reports (other academic/artistic)abstract
- When new methods of design and analysis are introduced in the control engineering field, it is scientificallyimportant to compare the new results with existing methods. Often this requires application of the methodson examples, and for this purpose benchmark processes are introduced. However, in many areas of controlengineering research the number of examples is limited to a relatively few examples, and in particular so whenmulti-input multi-output (MIMO) systems are considered. For a thorough assessment of a method, however, aslarge number of relevant models as possible should be used. As a remedy we propose a framework for generatinglinear multiple input, multiple output models based on predefined system properties, such as model type, size,stability, time constants, delays etc.
|
|
| 4. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
A new method of scaling the gramian based input-output pairing methods for improved results
- 2018
-
Conference paper (other academic/artistic)abstract
- A key problem in the application of process control systems is to decide which inputs should control which outputs. There are multiple ways to solve this problem, among them using gramian based measures, which include the Hankel interaction index array, the participation matrix and the Σ2 method. These methods take into account system dynamics as opposed to many other methods which only consider the steady-state system. However, the gramian based methods have issues with input and output scaling. Generally, this is resolved by scaling all inputs and outputs to have equal range. We will, however, demonstrate how this can result in an incorrect pairing. Further, we examine scaling of the gramian based measures, using either row or column sums, or by utilizing the Sinkhorn-Knopp algorithm instead. Then, to more systematically analyze the benefits of the scaling schemes, a multiple input multiple output model generator is used to test the different schemes on a large number of systems. This, along with implementation of automatic controller tuning, allows for a statistical comparison of the scaling methods. This assessment shows considerable benefits to be gained from the alternative scaling of the gramian based measures, especially when using the Sinkhorn-Knopp algorithm. The use of this method also has the advantage that the results are completely independent of the original scaling of the inputs and outputs.
|
|
| 5. |
|
|
| 6. |
- Bengtsson, Fredrik, 1989
(author)
-
Development and evaluation of methods for control and modelling of multiple-input multiple-output systems
- 2020
-
Doctoral thesis (other academic/artistic)abstract
- In control, a common type of system is the multiple-input multiple-output (MIMO) system, where the same input may affect multiple outputs, or conversely, the same output is affected by multiple inputs. In this thesis two methods for controlling MIMO systems are examined, namely linear quadratic Gaussian (LQG) control and decentralized control, and some of the difficulties associated with them. One difficulty when implementing decentralized control is to decide which inputs should control which outputs, also called the input-output pairing problem. There are multiple ways to solve this problem, among them using gramian based measures, which include the Hankel interaction index array, the participation matrix and the Σ2 method. These methods take into account system dynamics as opposed to many other methods which only consider the steady-state system. However, the gramian based methods have issues with input and output scaling. Generally, this is handled by scaling all inputs and outputs to have equal range. However, in this thesis it is demonstrated how this can cause an incorrect pairing. Furthermore, this thesis examines other methods of scaling the gramian based measures, using either row or column sums, or by utilizing the Sinkhorn-Knopp algorithm. It is shown that there are considerable benefits to be gained from the alternative scaling of the gramian based measures, especially when using the Sinkhorn-Knopp algorithm. The use of this method also has the advantage that the results are completely independent of the original scaling of the inputs and outputs. An expansion to the decentralized control structure is the sparse control, in which a decentralized controller is expanded to include feed-forward or MIMO blocks. In this thesis we explore how to best use the gramian based measures to find sparse control structures, and propose a method which demonstrates considerable improvement compared to existing methods of sparse control structure design. A prerequisite to implementing control configuration methods is an understanding of the processes in question. In this thesis we examine the pulp refining process and design both static and dynamic models for pulp and paper properties such as shives width, fiber length and tensile index, and various available inputs. We demonstrate that utilizing internal variables (primarily consistencies) estimated from temperature measurements yields improved results compared to using solely measured variables. The measurement data from the refiners is noisy, sometimes sparse and generally irregularly sampled. This thesis discusses the challenges posed by these constraints and how they can be resolved. An alternative way to control a MIMO system is to implement an LQG controller, which yields a single control structure for the entire system using a state based controller. It has been proposed that LQG control can be an effective control scheme to be used on networked control systems with wireless channels. These channels have a tendency to be unreliable with packet delays and packet losses. This thesis examines how to implement an LQG controller over such unreliable communication channels, and derives the optimal controller minimizing the cost function expressed in actuated controls. When new methods of control system design and analysis are introduced in the control engineering field, it is important to compare the new results with existing methods. Often this requires application of the methods on examples, and for this purpose benchmark processes are introduced. However, in many areas of control engineering research the number of examples are relatively few, in particular when MIMO systems are considered. For a thorough assessment of a method, however, as large number of relevant models as possible should be used. As a remedy, a framework has been developed for generating linear MIMO models based on predefined system properties, such as model type, size, stability, time constants, delays etc. This MIMO generator, which is presented in this thesis, is demonstrated by using it to evaluate the previously described scaling methods for the gramian based pairing methods.
|
|
| 7. |
- Bengtsson, Fredrik, 1989
(author)
-
Development and evaluation of methods for control of multiple-input multiple output systems
- 2018
-
Licentiate thesis (other academic/artistic)abstract
- In control, the most common type of system is the multiple-input multiple-output (MIMO) system, where the same input may affect multiple outputs, or conversely, the same output is affected by multiple inputs. In this thesis two methods for controlling MIMO systems are examined, namely linear quadratic Gaussian (LQG) control and decentralized control, and some of the difficulties associated with them. One difficulty when implementing decentralized control is to decide which inputs should control which outputs, that is the input-output pairing problem. There are multiple ways to solve this problem, among them using gramian based measures, which include the Hankel interaction index array, the participation matrix and the Sigma2 method. These methods take into account system dynamics as opposed to many other methods which only consider the steady-state system. However, the gramian based methods have issues with input and output scaling. Generally, this is resolved by scaling all inputs and outputs to have equal range. However, in this thesis it is demonstrated how this can result in an incorrect pairing. Furthermore this thesis examines other methods of scaling the gramian based measures, using either row or column sums, or by utilizing the Sinkhorn-Knopp algorithm. This thesis shows that there are considerable benefits to be gained from the alternative scaling of the gramian based measures, especially when using the Sinkhorn-Knopp algorithm. The use of this method also has the advantage that the results are completely independent of the original scaling of the inputs and outputs. An alternative way to control a MIMO system is to implement an LQG controller, which yields a single control structure for the entire system using a state based controller. It has been proposed that LQG control can be an effective control scheme to be used on networked control systems with wireless channels. These channels have a tendency to be unreliable with package delays and package losses. This licentiate thesis examines how to implement an LQG controller over such unreliable communication channels, and proposes an optimal controller which minimizes the cost function. When new methods of control system design and analysis are introduced in the control engineering field, it is important to compare the new results with existing methods. Often this requires application of the methods on examples, and for this purpose benchmark processes are introduced. However, in many areas of control engineering research the number of examples are relatively few, in particular when MIMO systems are considered. For a thorough assessment of a method, however, as large number of relevant models as possible should be used. As a remedy, a framework has been developed for generating linear MIMO models based on predefined system properties, such as model type, size, stability, time constants, delays etc. This MIMO generator, which is presented in this thesis, is demonstrated by using it to evaluate the previously described scaling methods for the gramian based pairing methods.
|
|
| 8. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
Finding feedforward configurations using gramian based interaction measures
- 2021
-
In: Modeling, Identification and Control. - : Norwegian Society of Automatic Control. - 1890-1328 .- 0332-7353. ; 42:1, s. 27-35
-
Journal article (peer-reviewed)abstract
- A sparse control structure can be seen as a decentralised controller that is expanded to include feedforward or MIMO blocks. Here, use of the gramian based interaction measures to determine a sparse control structure with feedforward is examined. A modification to the method used today is proposed and it is demonstrated that it results in a considerable improvement. Furthermore, recently proposed modifications to scaling gramian based measures are expanded to also cover sparse control structures. We show that the method that yields the best result is when two different scaling methods are combined, using one to design a decentralized controller and another to find feedforward connections.
|
|
| 9. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
LQG Control for Systems with Random Unbounded Communication Delay
- 2016
-
In: Proceedings of the 55th IEEE Conference on Decision and Control (CDC 2016); Las Vegas; United States; 12-14 December 2016. - 0743-1546. - 9781509018376 ; , s. Art no 7798406, Pages 1048-1055
-
Conference paper (peer-reviewed)abstract
- In this paper LQG control over unreliable communication links is examined. That is to say, the communication channels between the controller and the actuators and between the sensors and the controller are unreliable. This is of growing importance as networked control systems and use of wireless communication in control are becoming increasingly common. A proposed approach is to use tree codes to turn lossy channels into ones with a random delay. The problem of how to optimize LQG control in this case is examined, and it is found that to optimize LQG control previous control signals must also be used. Only the situation where communication between the components is done with acknowledgments is examined. An optimal solution is derived for finite horizon discrete hold-input LQG control for this case. The solution is compared with standard LQG control in simulations, which demonstrate that a significant improvement in the cost can be achieved when the probability of delay is high.
|
|
| 10. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
Modeling of tensile index using uncertain data sets
- 2020
-
In: Nordic Pulp and Paper Research Journal. - : Walter de Gruyter GmbH. - 2000-0669 .- 0283-2631. ; 35:2, s. 231-242
-
Journal article (peer-reviewed)abstract
- The objective of this investigation is to analyze and model tensile index. Two approaches are used, one based on training and validation data, while the other novel approach tests models using all possible combinations of data points. This approach is focused on small data sets which have here been obtained from nineteen pulp samples at different refining conditions in a full-scale TMP production line with a CD-76 refiner as a primary stage. From each pulp sample twenty handsheet strips for tensile index measurements were performed. Initially, specific energy and the external variables (dilution water feed rates and plate gaps) are used as predictors in a modeling approach based on an adjusted R 2 {R^{2}} approach. Thereafter, the resulting models are compared with a combination of specific energy and internal variables (primarily consistencies) obtained from temperature measurements inside the refining zones using a soft sensor concept. It is found that specific energy and internal variables as predictors outperform the external variables when estimating tensile index.
|
|
| 11. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
On the modeling of pulp properties in CTMP processes
- 2021
-
In: Nordic Pulp and Paper Research Journal. - : Walter de Gruyter GmbH. - 2000-0669 .- 0283-2631. ; 36:2, s. 234-248
-
Journal article (peer-reviewed)abstract
- The goal of this paper is to model the pulp properties fiber length, shives width and freeness. This will be done utilizing specific energy, flat zone inlet consistency and the internal variables, consistencies and fiber residence times estimated from refining zone soft sensors. The models are designed using more than 3600 hours of data from a RGP82CD refiner. The pulp properties are sampled using a measurement device positioned after the latency chest. Such measurements are noisy and irregularly sampled which opens for a number of challenges to overcome in modeling procedures. In this paper it is shown that the models for shives width and fiber length are capable of predicting most of the major dynamics. However, for freeness no reliable linear models can be derived. When estimating fiber length, the specific energy together with flat zone inlet consistency, fiber residence times and the consistency in the conical zone were the dominant inputs. For shives width it was found that a similar set of inputs resulted in the best models, except that the consistencies during normal process conditions did not significantly influence shives width. Furthermore, fiber residence times were shown to have considerably more pronounced impact on fiber length compared with shives width estimates.
|
|
| 12. |
|
|
| 13. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
Raw data for Tensile index estimations from a CD72-refiner
- 2019
-
Reports (other academic/artistic)abstract
- This report presents measurement from a CD72-refiner taken under a five day period. Moreover the data processing steps taken are discussed. The data sets contain measurements of tensile index, specific energy, maximum temperature, dilution water feed rates and plate gaps taken on 19 different occasions. Separate measurements are available from the flat and conical zones. Moreover the data sets included also contains estimated variables, namely consistencies and residence times in the different zones.
|
|
| 14. |
|
|
| 15. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
Resolving issues of scaling for gramian-based input–output pairing methods
- 2022
-
In: International Journal of Control. - : Informa UK Limited. - 0020-7179 .- 1366-5820. ; 95:3, s. 679-691
-
Journal article (peer-reviewed)abstract
- A key problem in process control is to decide which inputs should control which outputs. There are multiple ways to solve this problem, among them using gramian-based measures, which include the Hankel interaction index array, the participation matrix and the (Formula presented.) method. The gramian-based measures, however, have issues with input and output scaling. Generally, this is resolved by scaling all inputs and outputs to have equal range. However, we demonstrate how this can result in an incorrect pairing and examine alternative methods of scaling the gramian-based measures, using either row or column sums or by utilising the Sinkhorn-Knopp algorithm. To systematically analyse the benefits of the scaling schemes, a multiple-input multiple-output model generator is used to test the different schemes on a large number of systems. This assessment shows considerable benefits to be gained from the alternative scaling of the gramian-based measures, especially when using the Sinkhorn-Knopp algorithm.
|
|
| 16. |
- Bengtsson, Fredrik, 1989, et al.
(author)
-
Stochastic optimal control over unreliable communication links
- 2022
-
In: Automatica. - : Elsevier BV. - 0005-1098. ; 142
-
Journal article (peer-reviewed)abstract
- In this paper LQG control over unreliable communication links is derived. That is to say, the communication channels between the controller and the actuators and between the sensors and the controller are unreliable. This is of growing importance as networked control systems and use of wireless communication in control are becoming increasingly common. The problem of how to optimize LQG control in this case is examined in the situation where communication between the components is done with acknowledgments. Previous solutions to finite horizon discrete time hold-input LQG control for this case do not fully utilize the available information. Here a new solution is presented which resolves this limitation. The solution is linear and covers communication channels subject to both packet losses and delays. The new control scheme is compared with previous solutions for LQG control in simulations, which demonstrates that a significant improvement in the cost can be achieved by fully utilizing the available information.
|
|
| 17. |
- Svensson, Elin, 1980, et al.
(author)
-
Design of heat exchanger networks with good controllability
- 2018
-
Reports (other academic/artistic)abstract
- One important way to improve the energy efficiency of chemical process plants is to improve the heat integration within and between industrial processes. This is accomplished by recovering excess process heat at high temperatures and using it to replace primary heating at lower temperatures through a heat exchanger network. However, as a process becomes more heat integrated, process control may become more difficult. Poor control performance can, in turn, easily lead to increased costs that outweigh the predicted energy cost savings. It is therefore essential to model and analyze the effect of the process changes and address the identified potential control challenges. However, the majority of existing methods for controllability assessment of heat exchanger networks only consider steady-state properties, and not the dynamic aspects, which in reality can seriously affect process control characteristics. With better methods for controllability analysis alternative design options could be evaluated and compared more reliably at an earlier design stage. This report proposes the basic structure of a step-wise approach for integrating dynamic considerations into the design process for heat recovery improvements in process industry, and suggests suitable methods and tools to be used for the different steps of the proposed framework. As part of this, recent work that has been performed to evaluate and improve the methods used in controllability assessment is outlined. Additionally, a number of areas are identified in which significant further efforts are required before a complete controllability assessment framework can be specified and a toolbox for integrated design and controllability analysis can be developed. One central area requiring continued research and development is to define an adequate controllability index for use in heat exchanger network design. For example, it is relatively easy to argue that some of the commonly applied controllability measures are insufficient since they are based on steady-state system interactions only. However, as illustrated in the report, alternative measures of system interactions that take dynamics into account suffer from other drawbacks, of which one is scaling dependency. Nevertheless, these are interesting for further development of a new controllability assessment method, since the issues with scaling can possibly be dealt with using an approach evaluated in this project. Another area where further work is needed is to develop tools with some level of built-in support for formulation of dynamic models of heat exchanger networks. Model simplifications, or other means of handling the large model sizes typically resulting from dynamic modelling of heat exchanger networks may also be needed in order to overcome difficulties in model simulation and analysis. In addition to the development needs related to individual assessment steps, there is an apparent need for appropriate protocols for information transfer and conversion of models between different tools. This report gives an overview of insights revealed in recent research with respect to the controllability of heat exchanger networks. Through this research, the knowledge for continuing the effort to define a better controllability index for heat exchanger networks has been improved.
|
|
