Ivar J. Halvorsen

Dynamic Optimization of the LNG Value Chain

Abstract In operation of a large LNG processing plant there will be uncertainties related to planning and realization of an optimal operation strategy. Results show the importance of including the whole production chain in the optimization. For a test scenario where we look at the event of delayed ship arrival, this gives lower losses than a normal approach where the upstream part of the system and the LNG process plant are optimized individually. The system is modeled by simple models of each main component starting at the wells and near-well region and ending with the export tanks for LNG, LPG and condensate. These proxy models can be regarded as first order approximations of the real system. The individual models are nonlinear, some are static and others are dynamic models. The models have been coarsely validated using a high-fidelity simulator of the value chain at the Snohvit LNG plant. Use of these quite simple models combined with model-based optimization offers and interesting and feasible approach to optimize production in case of various events that require readjustment of the production planning.

Model predictive control of a Kaibel distillation column

Abstract This is a simulation study on controlling a Kaibel distillation column with model predictive control (MPC). A Kaibel distillation column has several advantages compared with conventional binary column setups. The Kaibel column separates a feed stream into four product streams using only a single column shell. The distillation process is a multivariable process which leads to a multivariable control problem. The objective for optimal operation of the column is chosen to be minimization of the total impurity flow. An off-line optimization on a mathematical model leads to temperature setpoints to be used by a controller. An MPC generally obtain less total impurity flow compared to conventional decentralized control when the distillation column is exposed to disturbances. It also counteract process interactions better than decentralized control.

On-Line PI Controller tuning Using Closed-Loop Setpoint Response

The proposed method is similar to the Ziegler-Nichols (1942) tuning method, but it is faster to use and does not require the system to approach instability with sustained oscillations. The method requires one closed-loop step setpoint response experiment using a proportional only controller with gain Kc0 . Based on simulations for a range of first-order with delay processes, simple correlations have been derived to give PI controller settings similar to those of the SIMC tuning rules (Skogestad, 2003). The controller gain (Kc/Kc0) is only a function of the overshoot observed in the setpoint experiment whereas the controller integral time (τ I) is mainly a function of the time to reach the peak (t p). Importantly, the method includes a detuning factor F that allows the user to adjust the final closed-loop response time and robustness. The proposed tuning method, originally derived for first-order with delay processes, has been tested on a wide range of other processes typical for process control applications and the results are comparable with the SIMC tunings using the open-loop model.

Multi-Product Dividing Wall Columns: A Simple and Effective Assessment and Conceptual Design Procedure

The objective of this paper is to further exploit and demonstrate the potential benefits of a V-min diagram based analysis, using a fifteen component feed mixture as base case. Detailed explanations are given on how to interpret properly a V-min diagram and use it for DWC assessment. Emphasis is on how to set the component or fraction splits to maximize potential gains as well as how to handle some variations in the feed with the same purpose. Interestingly, it appeared that depending on the choice and specification of interesting products (components or fractions), a simplified four-product DWC and even five-product DWC could be an interesting option for aromatics processing plants.

Optimal operation of a Petlyuk Distillation Column: Energy Savings by Over-fractionation

This work shows the unexpected result that over-fractionating one of the product streams in a Petlyuk distillation column may be optimal from a energy point of view. Analytic expressions for the potential energy savings are derived using the Underwood equations. The energy savings by over-fractionation may be further increased by bypassing some of the feed and mixing it with the over-fractionated product to meet product specications . Normally, the energy savings are small, so the main signicance of our results is to point out that over-fractionating is optimal in some cases.

Wireless communication in process control loop: Requirements analysis, industry practices and experimental evaluation

Wireless communication is already used in process automation for process monitoring. The next stage of implementation of wireless technology in industrial applications is for process control. The need for wireless networked control systems has evolved because of the necessity for extensibility, mobility, modularity, fast deployment, and reduced installation and maintenance cost. These benefits are only applicable given that the wireless network of choice can meet the strict requirements of process control applications, such as latency. In this regard, this paper is an effort towards identifying current industry practices related to implementing process control over a wireless link and evaluates the suitability of ISA100.11a network for use in process control through experiments.

Economically Optimal Control of Kaibel Distillation Column: Fixed boilup rate

Abstract In this study, a control structure is designed for a 4-product dividing wall (Kaibel) distillation column, based on the plant-wide control procedure by Skogestad [2000]. Unlike the common approach, vapour split is considered as a degree of freedom. The exact local method is used to find the best control variables as single measurements or combination of measurements.

A Shortcut Design for Kaibel Columns Based on Minimum Energy Diagrams

Abstract In this paper, a shortcut procedure is proposed to design a 4-product dividing-wall column. It is based on the information derived from V min diagram. This has the advantage of having more meaningful guesses for energy requirements and impurity flows in the column. An example is used for illustration.

Control Structure of a Micro Combined Heat and Power Fuel-Cell System for Lifetime Maximisation

The aim of this work is how to analyse controllability and design a suitable control structure for a commercial stationary micro combined heat and power system (\mchp) where the focus is to optimise the life time of the fuel cell, which is the central component in the system and the most critical with respect to durability. A three-layered control approach is proposed, building on the pre-existing low-level regulatory layer. A middle-level controller handles reversible degradation phenomena before they cause lasting damage, inducing temporary deviations from the steady state to recover from e.g. catalyst contamination or electrode flooding. A high-level optimising controller sets the nominal operating conditions to maximise a specific objective function, based on prognostics and health management, in which the lifetime of the stack is of pivotal importance.

Dynamic compensation of static estimators from Loss method

Abstract In this paper, we study different possibilities to overcome the inverse response problem which is caused by combining different measurements with fast and slow dynamics to form an estimator. Our goal is to obtain a response with “No inherent limitation”.