A new multivariable control concept for the falling film evaporator process
Section snippets
Introduction to the falling film evaporator process
Falling film evaporators (FFEs) have a wide range of application in various industries. Especially, FFEs are very common in the food industry as part of a production line with downstream spray dryer to produce, e.g., milk or coffee powder. In this context, the FFE’s main task consists in increasing the dry matter content of the liquid. Due to hygienic constraints and to ensure high product quality, FFEs are often built up of multiple passes enabling slow and cautious evaporation of the liquid
State of the art on control of falling film evaporators
PI controllers are still standard in the industry [5]. However, it is commonly known that a PI controller is unable to adequately cope with dominant time delays since it typically leads to large-amplitude, long-lasting oscillations of . In particular, Winchester et al. [4] conclude that pure single-loop PI control is insufficient to reject disturbances due to .
Therefore, in the last decades, more advanced methods have been proposed and applied to enhance the -control loop. In [6], [7], a
Control-oriented model
Our control-oriented model was originally introduced in [3]. However, since this model is the basis for our control concept, we recall relevant details of the derivation in this section. Subsequently, we convert physical variables into control nomenclature, i.e., define manipulated variables , states , disturbances , and outputs . Thus, we specify the model’s state space representation, which reveals linear state equations and a nonlinear output equation. Finally, we transform the linear
New multivariable control concept
The design of our control concept is based on the Plant in Fig. 4 with transfer functions (21). In Section 4.1, a stepwise derivation of the concept is given. To avoid operation beyond the actuators’ limits, we derive a domain of feasible setpoints in Section 4.2.
Simulation
In this section, the control concept presented in Section 4 is tested w.r.t. parameter uncertainties, see Section 5.1, and validated via a digital plant twin in Section 5.2.
However, before the concept is simulated, we have to define a suitable test signal for the disturbance . To this end, the following scenario is considered: The liquid fed into the FFE comes from a feed tank, where the dry matter content at the bottom is larger than at the top due to imperfect mixing and sedimentation.
Conclusions
In this paper, a new multivariable control concept for the falling film evaporator process is developed. As single-loop control of only and manual ramp-up are common industrial practice, our concept additionally enables control of and an automated ramp-up process. Moreover, since our concept is represented in a block diagram structure, see Fig. 7, practical implementation into digital control systems is simplified. Consequently, it is readily accepted by plant operators and also cheaper
CRediT authorship contribution statement
Julian Hofmann: Methodology, Software, Validation, Investigation, Data curation, Writing – original draft. Anton Ponomarev: Methodology, Software, Formal analysis, Writing – review & editing. Veit Hagenmeyer: Funding acquisition, Project administration, Writing – review & editing. Lutz Gröll: Conceptualization, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Helmholtz Association under the program “Energy System Design”. Additionally, the authors thank GEA Wiegand GmbH for data and technical information regarding the falling film evaporator process.
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