Christoph Josef Backi

Optimal boundary control for the heat equation with application to freezing with phase change

In this paper an approach for optimal boundary control of a parabolic partial differential equation (PDE) is presented. The parabolic PDE is the heat equation for thermal conduction. A technical application for this is the freezing of fish in a vertical plate freezer. As it is a dominant phenomenon in the process of freezing, the latent heat of fusion is included in the model. The aim of the optimization is to freeze the interior of a fish block below -18 °C in a predefined time horizon with an energy consumption that is as low as possible assuming that this corresponds to high freezing temperatures.

A comparative study of quality and safety of Atlantic cod (Gadus morhua) fillets during cold storage, as affected by different thawing methods of pre‐rigor frozen headed and gutted fish

Abstract BACKGROUND The catch of marine whitefish is typically seasonal, whereas the land‐based processing industry has a need for all‐year stable supply of raw materials. This challenge can be met by applying fish frozen at sea. When using frozen fish, the methods employed for thawing may influence the safety and quality of the final product. This study aimed to investigate the applicability of novel thawing strategies in order to provide an all‐year supply of high‐quality and safe cod products. RESULTS Comparative investigations of quality and safety factors after thawing in water, with and without air circulation, and contact thawing were performed. The parameters included water‐holding capacity, thawing loss, drip loss, cooking yield, sensory evaluation and microbiological analyses (including total volatile bases nitrogen). Water thawing with air circulation provided faster thawing than water thawing without air circulation and contact thawing. For all three methods, the quality of the thawed fish was acceptable and the shelf life of the fillets during chilled storage was between 10 and 14 days post‐filleting. CONCLUSION The results show that controlled freezing of cod, followed by appropriate thawing, may provide the processing industry with an all‐year delivery of raw materials, without compromising quality and safety of the final product.

Estimation of inner-domain temperatures for a freezing process

In this paper a state observer for a distributed parameter system (DPS) with nonconstant parameter functions is presented. The DPS describes the freezing of foodstuff in vertical plate freezers and is a nonlinear heat equation. The observer is based upon the Extended Kalman Filter, meaning that the nonlinear heat equation has been discretized in the spatial domain before designing the observer. We show that the observer is robust with respect to perturbations of parameter functions and noisy measurement signals and that the innerdomain temperatures can be correctly estimated.

Nonlinear observer design for a Greitzer compressor model

In this paper two different observers for a nonlinear compressor model have been developed and compared: A nonlinear observer based on a circle criterion design and an Extended Kalman Filter. Both of these observers were implemented together with linear control strategies in order to (surge-)control the nonlinear Greitzer compressor model. The newly developed nonlinear observer is a full state observer providing local asymptotic stability results. Compared to the Extended Kalman Filter, the nonlinear observer showed itself at least equivalent, even superior for open-loop estimation.

A simple dynamic gravity separator model for separation efficiency evaluation incorporating level and pressure control

In this work we introduce a simple gravity separator model for liquid level and pressure control as well as evaluation of separation efficiencies for the water and oil continuous phases, respectively. The model incorporates three dynamic state equations describing the levels of the overall liquid and water as well as the gas pressure subject to the in- and outflow dynamics. In addition, algebraic equations calculating simplified droplet distributions for each continuous phase are introduced in order to determine the exchange of water and oil between the continuous phases. The controllers are of PI-type and control the outflows of the separator.

Robust control of a two-state Greitzer compressor model by state-feedback linearization

In this paper we introduce an anti surge controller for a close-coupled valve (CCV) in a compression system. The controller is based upon feedback linearization in combination with linear state feedback. The mentioned CCV modifies the characteristics of the compressor, allowing it to be stably operated beyond the original surge line. It is shown that the original model consisting of the compressor in combination with the CCV can be transformed into the Byrnes-Isidori-Normalform, a necessity in order to enable feedback linearization. Furthermore, robustness of the controller is illustrated by circle criterion analysis with a sector nonlinearity feedback. Simulation examples illustrate the theoretical robustness investigation also for the case when the control signal is bounded and noise is added to the state derivatives and to the output.

Virtual inflow estimation with simplified tuning using cascaded and Kalman-like least squares observers

Abstract Multiphase metering is often unavailable, unreliable or expensive, which makes model-based, virtual monitoring a cheap and reliable alternative in order to obtain information about e.g. the inflows to a device. This paper presents an approach for inflow estimation to a gravity separator based on a standard Extended Kalman Filter. Potential improvements include a cascaded structure, a Kalman-like least squares observer with forgetting factor, and a combination of these.

A model for subsea oil-water gravity separator to estimate unmeasured disturbances

Abstract In subsea oil and gas industry, sensors for process monitoring are often unavailable, unreliable or expensive. Hence, state and parameter estimation can be a viable alternative. In this work, we developed a simplified 6-state model for gravity separator (often used as a first stage separator in subsea separation systems) with lumped fluid properties. We used the model in an Extended Kalman Filter estimator using measurements of levels and densities of fluids inside the separator in order to estimate unmeasured disturbances, namely inlet total flow rate, inlet oil cut and inlet droplet diameter. Results show that the estimated disturbances converge to their true values when process changes.

Virtual inflow monitoring for a three phase gravity separator

This paper presents a virtual monitoring approach for a gravity separator. It is used to separate the three phases oil, water and gas. The monitored variables are thereby the gaseous and liquid inflows as well as the effective split ratio of oil and water entering the continuous water phase. Measurements for these variables are either expensive, unreliable or even impossible to take (as for the split ratio). Therefore, an Extended Kalman Filter is designed, which can estimate the inflows and the split ratio based on a model of the dynamic system. The performance is shown in simulations, where the gravity separators states are controlled by PI controllers.