Peter Bongers

Scope for industrial applications of production scheduling models and solution methods

Abstract This paper gives a review on existing scheduling methodologies developed for process industries. Above all, the aim of the paper is to focus on the industrial aspects of scheduling and discuss the main characteristics, including strengths and weaknesses of the presented approaches. It is claimed that optimization tools of today can effectively support the plant level production. However there is still clear potential for improvements, especially in transferring academic results into industry. For instance, usability, interfacing and integration are some aspects discussed in the paper. After the introduction and problem classification, the paper discusses some lessons learned from industry, provides an overview of models and methods and concludes with general guidelines and examples on the modeling and solution of industrial problems.

Application of multi-stage scheduling

Abstract A simplified food factory model has been derived from a medium size ice cream manufacturing plant. This is done by building a multi-stage scheduling model which describes the infra-structure of the factory, which products are being produced and how the plant is operated. Key has been to translate complexity of the plant (and the operations) into a simplified, but realistic, multi-stage scheduling model. This model has been implemented in commercial available software. The production schedule could not be derived automatically, but needed manual intervention. Scheduling the factory as a whole, the available overall capacity of the factory can be increased significantly. The integrated factory schedule squeezes 10–30% additional capacity out of the factory.

Product driven process synthesis methodology

Abstract In the last ten years much more processes are being reported to be designed through a process synthesis approach. It has been recognized during those years that (i) processes for structured products are more difficult to design through process synthesis; (ii) process synthesis is disconnected from product development. In a response to those shortfalls a number of authors have described that the gaps need to be filled, however no methodology extension has been proposed. In this work, we will present extensions to the conceptual process synthesis methodology to include (structured) product design. The whole design methodology spans from how the new product can enlighten the consumer, financial and supply chain boundary conditions, through an optimal flowsheet able to produce the desired product cost effectively. A real case study will be used to illustrate the applicability and scope of the proposed methodology

A heat transfer model of a scraped surface heat exchanger for ice cream

Abstract A mathematical model of an ice cream freezer was developed by considering the freezer barrel as a series of well mixed stages and employing heat and mass transfer equations. The model was solved using a commercial simulation package to give predictions of product temperature, mechanical dissipation and heat transfer rate. These predictions were found to agree closely with experimental measurements. The process model has the potential to predict local temperature and shear conditions within an ice cream freezer and therefore represents an important first step towards systematic freezer design and performance optimisation and scale-up based on product quality considerations.

Modelling and experimental validation of emulsification processes in continuous rotor–stator units

Despite the wide range of industrial applications of structured emulsions, current approaches toward process design and scale-up are commonly based on trial-and-error experimentation. As this design approach is foreseen to deliver most likely suboptimal process solutions, we propose in this contribution a model-based approach as the way forward to designing manufacturing processes of structured emulsions. In this context, process modelling and simulation techniques are applied to predict production rates and equipment sizing. Moreover, sensitivity analysis of the process model provides insight about potential bottlenecks in the process.

Dynamic modelling of the margarine production process

Abstract A mathematical model of the margarine production process was developed by considering the votator barrel as a series of well mixed stages and employing heat and mass transfer equations. The model was solved using a commercial simulation package to give predictions of product temperature, mechanical dissipation and heat transfer rate. These predictions were found to agree closely with experimental measurements. The process model has the potential to predict local temperature and shear conditions within an ice cream freezer and therefore represents an important first step towards systematic freezer design, and performance optimisation and scale-up based on product quality considerations.

Product Driven Process Design Method

Abstract In the last ten years much more processes are being reported to be designed through a process synthesis approach. It has been recognized during those years that (i) processes for structured products are more difficult to design through process synthesis; (ii) process synthesis is disconnected from product development. In a response to those shortfalls a number of authors have described that the gaps need to be filled, however no methodology extension has been proposed. In this work, we will present extensions to the conceptual process synthesis methodology to include (structured) product design. The whole design methodology spans from how the new product can enlighten the consumer, financial and supply chain boundary conditions, through an optimal flowsheet able to produce the desired product cost effectively. A real case study will be used to illustrate the applicability and scope of the proposed methodology

Mathematical investigation of the case hardening phenomenon explained by shrinkage and collapse mechanisms occurring during drying processes

Although drying is one of the oldest approaches that have been used by humans to preserve foods, this technology is still an active scientific topic for researchers in both industry and academia. Indeed, during drying processes, food products undergo several physical, chemical, and structural changes which have a direct impact on the quality of the final products and therefore on the consumer perception and the acceptance of dried foods. To translate the consumer needs, scientists use some measurable attributes, such as bulk density which affects the visual aspects. In addition, density plays a major role in heat/mass transfers, which are crucial for process optimization and hence the mathematical modeling. In the literature, several models are available to describe density during drying. The majority of these models are empirical and very few are theoretical. Recently, our group (Khalloufi et al. 2010) has developed a new fundamental approach for predicting bulk density as a function of moisture content. This approach included, for the first time, simultaneous variations of both initial and instantaneous porosities during drying processes. The aim of this contribution is to assess the ability of this new theoretical model to investigate the temperature effect on bulk density and therefore to support the theoretical background behind the case hardening phenomenon. Experimental data obtained by an independent group for apple dried at three different temperatures (50oC, 80oC or 105oC) were used. The model was implemented and solved in Matlab using the fmincon, and then applied to simulate the bulk density behaviors for the three temperatures. For all three temperatures, the average deviation between the present model and the experimental data was <10%. The investigation of the temperature effect on bulk density was performed by using the two physical mechanisms involved in this model, namely the collapse and shrinkage phenomena. The results of this assessment showed that both mechanisms have the same profiles for the three temperatures, and their values at the end of the drying process were found to be temperature dependant. Indeed, the increase in drying temperatures leads to: (i) more preservation of the initial air existing at the beginning of the process and (ii) more replacement by air of the water removed during drying. Therefore, drying at high temperatures (e.g. 105oC) results in low bulk density. In the literature, this behavior is hypothetically ascribed to the case hardening phenomenon. This phenomenon consists in an instantaneous drying of the external layer of food products at high temperature, resulting in crust formation (sort of protective shell) which in turns leads to less shrinkage and low density. The results obtained by the present mathematical model support the concept of case hardening via a theoretical explanation based on the shrinkage and collapse mechanisms.

Prediction of Supercritical Carbon Dioxide Drying of Food Products in Packed Beds

Drying assisted by supercritical carbon dioxide is foreseen to become a promising technology for sensitive food products. In this contribution, a mathematical model is derived to describe the changes in water concentration in both a solid food matrix and a fluid carrier during drying. Finite different element method is used to solve the set of mass balance equations. A remarkable agreement between simulated and experimental data was obtained. Moreover, the simulated changes in water concentration in the solid and fluid carrier gave a coherent description of the process. This model can be used as a tool for optimizing the operating conditions and process scale-up in supercritical carbon dioxide assisted drying.

Modelling and simulation of extensional-flow units in emulsion formation

Here we studied the emulsification process carried out in an extensional-flow unit. By means of rigorous population and momentum balances we captured the phenomenological description of the first principles occurring in such unit. The strong feature of our model approach resides in the fully mechanistic description of the governing phenomena. A population balance equation was formulated and solved to account for the disappearance and appearance of droplets at each size class. Coalescence mechanism was included to account for the instability of newly created droplets. We validated the accuracy of the results obtained from our equation-based model with experimental data obtained at pilot-plant scale. The results obtained by simulation showed that at a given set of operational conditions and pre-emulsion properties the product obtained was within the desired and narrow specifications space. As a concluding remark we suggest further exploring the design and development of extensional-flow units for structured emulsions.