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Managing Steam and Concentration Disturbances in Multi-Effect Evaporators Via Nonlinear Modelling an
Abstract
Evaporators are core units in many industrial processes including sugar mills. The dynamics of these systems are complex and hence the systems have been frequently used to demonstrate unusual systems and control behaviour. In this paper we explore a particular control architecture commonly employed in industry. We show that the architecture can lead to poor performance due to steam and concentration disturbances. An alternative architecture is then proposed which overcomes the difficulties.
... Despite the energy benefits of the multiple effect evaporators, the steam economy of the evaporator unit is largely influenced by the variations in the dependent process units like the crystallisation unit. The coupling of the crystallisation unit with the evaporator unit through pan vapour requirement has been reported as the main cause of long oscillatory perturbations in the evaporator unit [21]. Therefore, the energy monitoring of the crystallisation unit is considered essential. ...
... In accomplishing this goal, the process inputs, outputs and disturbances need to be identified. The work by Ref. [21] aimed at improving the control architecture of evaporators in a sugarcane mill identified the pan vapour demand, inlet juice concentration and flow as the main disturbances influencing the evaporator unit steam economy. The amount of imbibition water used has an influence on the inlet clear juice flow and concentration. ...
... An increase in A-massecuite recycling and temperature result in an increase in the pan vapour requirement which affects the pressure profile and energy efficiency of the evaporator unit. Published data from an Australian sugar mill showed that the pan vapour demand disturbances are responsible for the longest sustained oscillatory perturbation in the evaporator unit [21]. Therefore, further analyses were conducted to evaluate the influence of A-massecuite temperature and dry substance concentration on the pan vapour demand as shown in Fig. 4. ...
The cogeneration system is the most inefficient unit a typical sugar mill. This paper evaluates the improvements in the exergy performance of a sugar-mill cogeneration system, through the integration of the absorption heat pump (AHP)and the organic Rankine cycle (ORC)technologies for waste-heat recovery. The cogeneration system was simulated in Aspen Plus® process simulator and its thermodynamic performance determined through exergy analysis. In addition to providing an alternative cooling for the mill, the AHP integration minimized the irreversibility of the deaerator, sugar-drying unit and the overall system by 51%, 76% and 0.14%, respectively, saving 0.83% of total bagasse combusted. The ORC increased the cogeneration system's exergy efficiency by 1.7% by generating “green” electricity up to 10% of the cogeneration capacity. The AHP and ORC were economically feasible with payback periods of 8.3 years and 6.3 years, respectively. This paper provides basis for waste-heat recovery based exergy improvement studies in other process industries.
... Despite the energy benefits of the multiple effect evaporators, the steam economy of the evaporator unit is largely influenced by the variations in the dependent process units like the crystallisation unit. The coupling of the crystallisation unit with the evaporator unit through pan vapour requirement has been reported as the main cause of long oscillatory perturbations in the evaporator unit [21]. Therefore, the energy monitoring of the crystallisation unit is considered essential. ...
... In accomplishing this goal, the process inputs, outputs and disturbances need to be identified. The work by Ref. [21] aimed at improving the control architecture of evaporators in a sugarcane mill identified the pan vapour demand, inlet juice concentration and flow as the main disturbances influencing the evaporator unit steam economy. The amount of imbibition water used has an influence on the inlet clear juice flow and concentration. ...
... An increase in A-massecuite recycling and temperature result in an increase in the pan vapour requirement which affects the pressure profile and energy efficiency of the evaporator unit. Published data from an Australian sugar mill showed that the pan vapour demand disturbances are responsible for the longest sustained oscillatory perturbation in the evaporator unit [21]. Therefore, further analyses were conducted to evaluate the influence of A-massecuite temperature and dry substance concentration on the pan vapour demand as shown in Fig. 4. ...
The growing emphasis in alternative revenue streams, from the conversion of sugarcane bagasse into energy co-products has prompted a desire by the sugar factories to achieve higher levels of energy efficiency. This study presents a comprehensive overview of the parameters whose steady-state deviation leads to excess energy use in sugar mills. The effect of eight operating parameters on the defined energy indicators was investigated,
using a MATLAB simulation of a 250-tonne per hour sugar mill. The captured energy trends were used to develop energy prediction models based on the parameters whose steady-state offsets resulted in excess energy use. High prediction accuracies of over 95% were obtained when the developed energy prediction models were validated for seasonal cane quality and evaporator heat transfer coefficient variations. Increased A-massecuite recycling had a more pronounced effect on the energy used per kilogramme of water in the
evaporator unit as compared to the crystallisation unit. Of the eight parameters, elevated imbibition water use and A-massecuite recycling had a cumulative percentage effect of 79% on the overall steam used per tonne of cane and sugar produced. Hence increasing the syrup concentration, decreasing imbibition water use and massecuite recycling are preferred operational strategies for improved energy-efficiency in a sugar mill. Overall,
this study strengthens the existing literature by illustrating an approach for developing suitable energy indicators for excess energy use monitoring in sugarcane mills.
... As it is a very versatile problem, diverse approaches are found in the literature. Adams et al. 11 enhanced the poor control structure commonly used in the sugarcane industry by developing a feedforward scheme. That paper focused on problems presented by a double-stage evaporator caused by brix and level disturbances. ...
The concept of self-optimizing control is applied in the context of sensor network design to support the simultaneous selection of optimal sensors and controlled variables. Optimality is defined as minimizing the capital cost of the sensor network as well as the revenue loss due to disturbances and measurement errors under a constant set-point policy. The method is applied to a sugarcane mill case study, where 41 controlled variables (CVs) are selected as linear combinations of the measured variables, and the optimal supporting sensor network is identified. The economically optimal sensor placement found in the present study reduced the average revenue loss due to disturbances and measurement errors by 39% compared to a mill operating with a typical sensor network. The substantial reduction in revenue loss for the optimal sensor placement is attributed to an improved precision in 19 CVs compared to the base case network. The case study demonstrates the natural extension of self-optimizing control to optimal sensor network design, as well as the potential benefits of the combined approach.
Increasing energy demand, high cost of energy and global warming issues across the globe require energyintensive industries, such as paper mills to improve energy efficiency. Multi-stage evaporators used to concentrate the black liquor in such mills form its most energy consuming unit and require a strong understanding of steady and unsteady state behavior to ensure energy savings. The modeling of nonlinear heptads’ effect system yielded a set of complex nonlinear algebraic and differential equations that are analyzed using Interior-point method and state space representation. Dynamic response of product concentration and system vapor temperatures along with system stability and controllability have been explored by disturbing the flow rate, concentration and temperature of feed, and fresh steam flow rate. Simulations predict that steam flow rate, feed flow rate and its concentration invariably are major controlling factors (in decreasing order) of vapor temperature and product concentration. The interactive behavior between different effects translates into slower responses of the effects with increasing separation from disturbance source. This steady state and transient study opens many new explanations to this relatively less explored area and helps to propose and implement industrial PID controllers to reduce steam consumption and control product quality.
This paper discusses the problem of controlling a multiple-stage industrial evaporator in a sugar factory. The control objectives are: syrup should have a sucrose concentration of 72%, the evaporator should produce the required amount of steam, and the steam consumption should be as low as possible. Achieving these objectives is difficult, due to the following plant features: strong disturbances, large time delays and many constraints. The control problem is solved by developing a physical model which is used as benchmark; then, a parametric identification is performed to obtain a control model. Finally, a successful real-time application of a multivariable GPC controller is presented.
In this article, a unified approach is proposed for the design of
dynamical discontinuous feedback controllers leading to the
chattering-free stabilization of nonlinear single-input single-output
systems describing chemical processes. The adopted framework is that of
a generalized state representation form of the given nonlinear plant.
Use is made of the associated generalized observability canonical form
of such representation. Unification of discontinuous feedback policies
is achieved by zeroing of an input-dependent auxiliary output function
using simple discontinuous feedback control paradigms of various kinds.
The zeroing of such scaler stabilizing function induces asymptotically
stable controlled dynamics on the given nonlinear minimum-phase plant.
Pulse-frequency-modulation, pulse-width-modulation and sampled sliding
mode control strategies are considered from this unified viewpoint.
Examples are provided including simulations
In this study, a mathematical model was developed for multiple-effect evaporators. These evaporators have cocurrent, countercurrent and parallel flow operation options. Each operation was investigated with and without pre-heaters. The effect of pre-heating on evaporation process was investigated from the point of energy economy. A sugar factory's data was used with the applied models as a case study. The results obtained for pre-heated and non-pre-heated situations were compared with each other. The maximum COP is found as 3.33 for countercurrent operation with pre-heating. The minimum COP is 2.57 for parallel flow operation without pre-heating.
A generalized approach to the modeling of multieffect evaporators is presented which separates the development of dynamic equations from the specification of evaporator configuration. The result is a modular approach which is effective and convenient to use. A tenth-order nonlinear, dynamic model of a double-effect pilot plant evaporator was derived using this approach and then simplified and linearized to produce a fifth-order state-space model which gave generally good comparisons with experimental open-loop responses. Lower-order linear models were also developed but gave satisfactory results only in specific applications. The performance of models in the design and experimental implementation of conventional, inferential, feedforward, and multivariable optimal regulatory, and state-driving control systems is also examined.
The handbook has included the description of cane sugar manufacture, mills, diffusers, boilers and other factory machinery, calculation methods of capacity for every piece of equipment, and process and manufacturing techniques. This new edition has been revised and information that is either obsolete or of little interest has been deleted or shortened. Additions have been made in chapters dealing with recently developed equipment and a completely new chapter covers automation and data processing. Numerous figures, graphs, drawings, photographs, tables and formulae are provided.
This paper proposes a control scheme to control a multiple-effect evaporator (MEE) in the sugar industry using fuzzy logic control (FLC). An algorithm using genetic algorithms (GAs) is proposed and used to automatically tune the scaling factors (SFs) and membership functions (MFs) of the FLCs. The development and tuning of the FLCs is done in simulation on a mathematical model of the MEE, which has been optimised using GA. The tuned model was then used in all simulations. Comparison of the performances of the FLCs with only SFs tuned and with both SFs and MFs tuned is given. Substantial improvement in control accuracy has been observed after tuning MFs. Compared to manual control, the proposed intelligent control scheme promises significantly better performances.
Sugar production contains a number of unit operations in series. The most energy consuming unit is the evaporation process. The aim of this paper is to describe the development of a concept to control the product quality and the energy consumption in the evaporation process. Despite extensive use in sugar production the multiple effect evaporator has been a difficult system to analyze and control. To investigate the control strategies a real time simulator has been developed. Using the simulator, a control scheme has been designed and implemented. The control system is based on the idiomatic concept. The problem is to derive proper strategies for decomposing the process into smaller independent functions, and to synthesize control actions which achieve and maintain those functional structures. These structures are controlled using well known strategies (idioms). The control concept has been tested on the real time simulator and with minor modifications it has been implemented in a Danish sugar plant.
The paper deals with the problem of finding the extension of a
well-established connection between sign and stability of nontrivial
equilibria in positive linear systems to the class of quasi-monotone
positive nonlinear systems. The mathematical framework to provide the
sufficient conditions guaranteeing such a relationship is the stability
theory for nonlinear systems, especially the comparison principles and
the method of vector Lyapunov functions. As an illustration of our
results we present an example of a double-effect evaporator to study the
sign-stability property of its only equilibrium point
The development of controllers for a multiple-effect evaporator in sugar industry
- S L Elhaq
- F Giri
- H Unbehauen
S.L. Elhaq, F. Giri, and H. Unbehauen. The development
of controllers for a multiple-effect evaporator in sugar
industry. In proc. of the 1997 European Control Conference, Brussels, Belgium, July 1997.
Design of a nonlinear control for a double effect evaporator
- A Montano
- G Silva
A. Montano and G. Silva. Design of a nonlinear control
for a double effect evaporator. In Proceedings of the
First European Control Conference, pages 2256-2261,
Grenoble, France, July 1991.