Dynamic control of flexible manufacturing systems

Flexible manufacturing systems (FMSs) are a relatively new technological and organisational approach to helping companies respond to real-time marketing conditions for their production. Under a proposal of the National Bureau of Standards the FMSs are subdivided into virtual manufacturing cells in a dynamic manner, on the basis of group technology.A method of dynamic optimisation for the design of manufacturing processes, capacity balancing and checking, and also production scheduling or rescheduling in virtual manufacturing cells is described. It can be used during real-time production control in FMSs.     

Optimisation of clarifier-thickeners processing stable suspensions for turn-up/turn-down

Solid-liquid separation equipment operating on wastewater duties seldom functions at steady state and may be expected to perform over a wide range of feed loads. Satisfactory control of the solids residence time in these clarifiers and thickeners is critical to their successful operation. High solids residence times can lead to hydrolysis and rising sludge. This paper addresses the design of clarifiers and thickeners with minimum solids residence times. A one-dimensional force balance theory of sedimentation is used as the basis for the optimal design. Turn-up and turn-down are considered directly within the optimisation. A robust algorithm is presented for the repeated solution of the design equations and the constrained optimisation of the solids residence time. Three types controlling phenomena are identified and illustrated with design cases. The designs are found to be taller with smaller plan areas than their point optimised equivalents.     

Availability and reliability considerations in the design and optimisation of flexible utility systems

Industrial utility plants are usually comprised of many interconnected units that must constitute a flexible and reliable system capable of meeting process energy requirements under different circumstances (e.g. varying prices, demands, or equipment shutdowns). Also, in order to avoid large economic penalties, the design and operation of a utility plant should consider that the equipment is not fully reliable and that each item needs to receive preventive and corrective maintenance. Conventionally, these issues are handled by installing additional units according to rules of thumb or heuristics, which usually imply excessive capital costs and might even result in designs that cannot satisfy the specified demands for certain situations. In contrast, during the present work a systematic methodology has been developed to address the design and operation of flexible utility plants incorporating reliability and availability considerations. The suggested method is based on a novel modelling and optimisation framework that can address grassroots design, retrofit, or (pure) operation problems in which design and operational parameters are optimised simultaneously throughout several scenarios. Thereafter, it is possible to define maintenance and failure situations in different operating periods to ensure that the plant will be able to cope with them, while meeting process requirements at minimum cost. Hence, for design cases, the most cost-effective elements of redundancy can be determined without pre-specifying any structural options in the final configuration (e.g. equipment sizes, types, and number of units). Furthermore, the proposed (multiperiod) MILP formulation is robust enough to tackle problems of the size and complexity commonly found in industry, and has the potential of yielding significant economic savings.     

A generic tool for cost estimating in aircraft design

A methodology to estimate the cost implications of design decisions by integrating cost as a design parameter at an early design stage is presented. The model is developed on a hierarchical basis, the manufacturing cost of aircraft fuselage panels being analysed in this paper. The manufacturing cost modelling is original and relies on a genetic-causal method where the drivers of each element of cost are identified relative to the process capability. The cost model is then extended to life cycle costing by computing the Direct Operating Cost as a function of acquisition cost and fuel burn, and coupled with a semi-empirical numerical analysis using Engineering Sciences Data Unit reference data to model the structural integrity of the fuselage shell with regard to material failure and various modes of buckling. The main finding of the paper is that the traditional minimum weight condition is a dated and sub-optimal approach to airframe structural design.     

Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part I: Modeling, simulation and optimization of two system solutions

Using CO₂ trans-critical system solutions in supermarket refrigeration is gaining interest with several installations already running in different European countries. Using a computer simulation model, this study investigates the performance of two main system solutions: centralized with accumulation tank at the medium temperature level and parallel with two separate circuits for low and medium temperature levels. Both system solutions are presented and the simulation model is described in details. Calculations have been performed to design the systems and optimize their performances where basic layout and size of each solution have been defined. For ambient temperature range of 10–40 °C, the reference centralized system solution shows higher COP of about 4–21% than the reference parallel solution. Using two-stage compression in the centralized system solution instead of single stage will result in total COP which is about 5–22% higher than that of the reference centralized system and 13–17% higher than that of the improved two-stage parallel system. The two-stage centralized system solution gives the highest COP for the selected ambient temperature range.     

Process optimisation for recovery of carotenoids from tomato waste

Carotenoids constitute an important component of waste originating from tomato processing plants. Studies were carried out to assess the extraction yield of tomato waste carotenoids in different solvents and solvent mixtures and to optimise the extraction conditions for maximum recovery. A mixture of ethyl acetate and hexane gave the highest carotenoid extraction yield among the others examined. Extraction conditions, such as percentage of hexane in the solvent mixture of ethyl acetate and hexane, ratio of solvent to waste and particle size were optimised using a statistically designed experiment. A regression equation for predicting the carotenoid yield as a function of three extraction variables was derived by statistical analysis and a model with predictive ability of 0.97 was obtained. The optimised conditions for maximum carotenoid yield (37.5 mg kg⁻¹ dry waste) were 45% hexane in solvent mixture, solvent mixture to waste ratio of 9.1:1 (v/w) and particle size 0.56 mm.     

Recent advances in engineering design optimisation: Challenges and future trends

Traditional engineering design optimisation which is the process of identifying the right combination of product parameters is often done manually, time consuming and involves a step by step approach. This paper identifies recent approaches to automating the manual optimisation process and the challenges that it presents to the engineering community. Engineering design optimisation is classified based on design evaluation effort and degrees of freedom viewpoints. An overview of different approaches for design optimisation is presented. The study identifies scalability as the major challenge for design optimisation techniques. Large-scale optimisation requires significant computing power and efficient algorithms such as swarm intelligence.     

Optimisation of interacting particle systems for rare event estimation

The interacting particle system (IPS) is a recent probabilistic model proposed to estimate rare event probabilities for Markov chains. The principle of IPS is to apply alternatively selection and mutation stages to a set of initial particles in order to estimate probabilities or quantiles more accurately than with usual estimation techniques. The practical issue of IPS is the tuning of a parameter in the selection stage. Kriging-based optimisation strategy with a low simulation cost is thus proposed in order to minimise the probability estimate relative error. The efficiency of the proposed strategy is demonstrated on different test cases.     

Optimisation based design of a district energy system for an eco-town in the United Kingdom

The reduction of CO₂ emissions linked with human activities (mainly energy services and transport), together with the increased use of renewable energies, remain high priorities on various political agendas. However, considering the increased consumption of energy services (especially electricity), and the stochastic nature of some of the most promising renewable energies (wind for instance), the challenge is to find the optimal mix of technologies that will provide the energy services, without increasing the CO₂ emissions, but nonetheless ensuring reliability of supply. The focus of this paper is to present the DESDOP tool, based on mixed integer linear optimisation technics, that helps giving insight in the optimal mix of technologies that will simultaneously help decrease the emissions while at the same time guarantee resilience of supply. The results show that while it is not yet possible to avoid electricity from the grid completely (hence nuclear or fossil fuel), CO₂ reductions up to 20%, at no extra costs compared to the business-as-usual case, are easily achievable.     

Fuel cell systems optimisation - Methods and strategies

This paper reviews the current state of modelling and optimisation with regard to fuel cell systems design. The existing models for portable, stationary and transportation fuel cell systems are identified and characterised by approach, state, system boundary, spatial dimension and complexity or detail. The different model-based design approaches such as parametric study, single-objective optimisation and multi-objective optimisation performed using fuel cell system models are summarised. A case study on the design of a fuel cell micro-cogeneration plant is presented to illustrate the use of modelling and optimisation in generating different design alternatives that contain trade-offs between competing objectives.