FROM DATABASE TO OPERATION DESIGN USING A MULTIPLE MODEL APPROACH

We address the problem of estimating the input conditions (mixture compositions, operating temperature, pressure, etc.) that will result in manufacturing a product with the required properties using a data-based approach. A single-model type paradigm is shown to be unsuitable for all data sets or even for all output variables in a particular data set. A multiple-model type paradigm is therefore advocated. The input-output map is first obtained using one or more of the several modeling approaches. These models are then inverted to suggest operational strategies that will meet production goals. Several examples are presented to demonstrate the efficacy of the proposed methodology.     

一种约束一般模型控制新方法

在引入近似预测模型的基础上 ,应用基于二次规划的滚动优化算法 ,处理被控量、操作量及其变化速率的线性约束 .将此优化算法与经典的一般模型控制 (GMC)方法相结合 ,给出了一种基于二次规划的约束一般模型控制新方法 .     

SENSITIVITY AND UNCERTAINTY OF PROCESS DESIGNS TO THERMODYNAMIC MODEL PARAMETERS: A MONTE CARLO APPROACH

Process simultalion relies heavily on the calculation of physical properties through thermodynamic models. The parameters in these models are subject to uncertainties that may, at first, appear to be small. However, the designs developed through use of these models can be significantly sensitive to these parameter uncertainties. Previous studies have considered special cases of design error or have involved simple sensitivity analyses close to the optimum design case. In the present work, we use Monte Carlo simulation with Latin Hypercube Sampling to quantify the probability of design errors. An example of binary distillation with phase-equilibrium calculations from a cubic equation of state is used to illustrate the technique.     

MULTIPLE CATALYSTS PRODUCE A SYNTHETIC FUEL FROM COAL

A recent government study indicates that oil prices will be substantially higher within the next few years. Thus liquid fuels from coal, will be required. The study concludes, however, that the considerable effort, time and money spent to date in trying to make gasoline from coal will be of no help in reducing in the 1990's our almost entire dependence on foreign oil.

Any grade of coal may be gasified with oxygen. The syngas is separated and adjusted, to give substantially a mixture of hydrogen, carbon monoxide, carbon dioxide after sulfur removal, and adjustment of the carbon to hydrogen ratio for methanol. It is then passed at an elevated temperature and pressure through a catalyst bed. A small percentage is converted to methanol with the evolution of a large amount of heat. This heat is removed to prevent overheating the catalyst and, in the past, has been wasted to a large extent. The gas has been recycled to build up a maximum concentration of 3 to 4% methanol containing on condensation 10 to 20% water, which must be separated.

For the American (Wentworth) process, a second catalyst has been developed whose optimum reaction temperature is above that of the gas stream leaving the first catalyst. The gas stream is passed through this second bed of catalyst without cooling, and on through a third bed of previously developed catalyst, whose optimum reacting temperature is still higher. The accumulated heat of the gas stream leaving this third catalyst bed is at a high enough temperature to allow the recovery, along with waste heat from the gasifier steps of most of the reaction heat in a waste heat boiler together with that from subsequent passage of the gas through additional beds of the first two catalysts. This recovered heat supplies energy for the complete operation of the plant including coal handling and oxygen preparation, as well as, for some electric power for sale outside.

Flow sheets with temperatures and material and heat balances show the reasons of the advantages of the multiple catalyst process, including the considerable saving of energy accomplished in the much lower compression needs for recycle of the much lower amount of gas through the beds of catalysts.     

A UNIFIED APPROACH TO ARMA MODEL IDENTIFICATION AND PRELIMINARY ESTIMATION

Abstract. This paper reviews several different methods for identifying the orders of autoregressive-moving average models for time series data. The case is made that these have a common basis, and that a unified approach may be found in the analysis of a matrix G, defined to be the covariance matrix of forecast values.
The estimation of this matrix is considered, emphasis being placed on the use of high order autoregression to approximate the predictor coefficients. Statistical procedures are proposed for analysing G, and identifying the model orders.
A simulation example and three sets of real data are used to illustrate the procedure, which appears to be a very useful tool for order identification and preliminary model estimation.     

A SIMPLE INTEGRAL APPROACH TO STUDY VISCOUS FREE SURFACE FLOWS

A simple strategy, based on the integration of the components of the momentum balance equation, yields a small set of nonlinear algebraic equations that models viscous flow with free surfaces. This strategy is applied to two cases already studied by numerical analysis; they are (a) slot coating and (b) forward roll-coating.

The simple models presented in this work are employed to predict the behavior of both coating devices as a function of the relevant dimensionless parameters. The results obtained show that the simple models correctly estimate the general trends given by more realistic ones based on finite element analysis. The technique applied is particularly successful when the Reynolds number is small.     

STATE-DEPENDENT MODELS: A GENERAL APPROACH TO NON-LINEAR TIME SERIES ANALYSIS

Abstract. We construct a general class of non-linear models, called 'state-dependent models', which have a very flexible non-linear structure and which contain, as special cases, bilinear, threshold autoregressive, and exponential autoregressive models. We describe a sequential type of recursive algorithm for identifying state-dependent models, and show how such models may be used for forecasting and for indicating specific types of non-linear behaviour.     

应用正交试验解决螺杆挤压机不进料问题

论述了运用正交试验解决纺丝生产中的螺杆挤压机不进料问题。     

换热网络的同步综合设计

提出了换热网络的一种新的转运模型,以并串模式和非等温混合过程描述换热网络的热力学和经济性的总要求,不使用挟点分离、最小单元数等启发式规则,并允许交叉换热和不同物流间的膜传热系数与温差合理匹配.这一模型与优化模型相结合,自动产生换热网络,确定相应的最优公用工程费用、换热面积、换热器台数与冷热流间的匹配,其可行域由一集线性约束确定,鲁棒性很好,易于求解.该方法克服了换热网络的各种分步骤综合方法的缺陷(包括挟点技术、双温差法及相关数学规划方法).与文献上其它同步优化方法比较,具有模型简单、规模小、可算性强、初始化简易、计算结果可靠的优点.通过对文献中广泛使用的例题的检验,结果优于现有的其它方法.     

A NEW APPROACH TO DECENTRALISED PROCESS CONTROL USING PASSIVITY AND SECTOR STABILITY CONDITIONS

A new robust decentralised process control strategy based on the sector stability theorem is proposed in this paper. The interaction between sub-systems is measured by a revised passivity index, which is used in the development of a new stability condition for decentralised control. The controller synthesis method is developed to satisfy the stability condition and H_∞ performance specification. The decentralised control of a boiler furnace is studied as an application example.     

A NEW APPROACH TO DECENTRALISED PROCESS CONTROL USING PASSIVITY AND SECTOR STABILITY CONDITIONS

A new robust decentralised process control strategy based on the sector stability theorem is proposed in this paper. The interaction between sub-systems is measured by a revised passivity index, which is used in the development of a new stability condition for decentralised control. The controller synthesis method is developed to satisfy the stability condition and H_∞ performance specification. The decentralised control of a boiler furnace is studied as an application example.     

A NOTE ON NON-STATIONARITY AND CANONICAL ANALYSIS OF MULTIPLE TIME SERIES MODELS

Abstract. Box and Tiao (1977) established the correspondence between non-stationary roots and canonical correlations of an AR(1) process. In this paper, we give an alternative, more direct, proof of the correspondence and extend a special case of that result to AR( p ) processes. The usefulness of these results for multiple time series modelling is also briefly discussed.     

MACROPORE SIZE ANALYSIS USING DIGITAL IMAGE PROCESSING AND A STEREOLOGICAL MODEL

An analytical method employing digital image processing and a stereological model is developed and applied to macropore structure characterization. Digital images of porous particle cross-sections are collected and analyzed on an image processor to identify the two-dimensional pore profiles and to obtain the distribution for the equivalent radii of pore sections. A stereological model with improved numerical integration, approximation and data smoothing features is developed to unfold the size distribution of the three-dimensional pores from the two-dimensional data. The method is used to characterize the macropore structure of coal-derived chars. Presented results establish that the developed technique can quantify features of the macropore structure of chars that cannot be identified by indirect analytical methods.     

A KALMAN FILTER APPROACH TO THE FORECASTING OF MONTHLY TIME SERIES AFFECTED BY Morris Festivals

Abstract. Many economic time series are affected by the moving dates of festivals. In this paper a moving festival effect is defined and incorporated into a dynamic linear model which specifies how the parameters of several unobservable components of a time series evolve stochastically in time. The merits of this approach in comparison to other approaches are discussed and demonstrated using empirical data of three selected time series.     

A NUMERICAL STUDY OF TURBULENT HEAT TRANSFER IN A CHANNEL WITH BENDS USING REYNOLDS-STRESS MODEL

A numerical study is reported on the flow and heat transfer in a channel with two right-angle bends. The Reynolds-stress turbulence closure model is introduced for the computation of the flow field. The solution method of the governing transport equations is based on the finite volume method which incorporates the modified hybrid scheme. The computations are made for step ratios H/W = 1-4 and for Reynolds numbers Re = 3000-10,000. The complex patterns of the Reynolds stresses in the perpendicularly corrugated wall channel are analyzed and the mechanisms of heat transfer are explained with regard to the mean flow motions. The computed Nusselt numbers are compared with both experimental data in the literature and the computations which were obtained by using the κ-ε model.     

A THEORETICAL MODEL FOR PARTICLE SEGREGATION IN A FLUIDIZED BED DUE TO SIZE DIFFERENCE

An a priori model has been developed for the prediction of particle segregation in a fluidized bed of particles of same density. The model predicts the phase equilibrium of any two-component system by taking into account the Uml of both components and the minimum concentration of fines for minimum bed viscosity. An extension of the model to a multi-component system leads to an algorithm which allows the prediction of the quantity and the concentration profiles of particles in both phases of a segregated fluidized bed. The algorithm requires only the initial concentration profile or the mixture and the physical properties of each component and the fluidizing gas. A good agreement between the predicted and the previously measured concentration profiles of both a two-size and a wide size mixture shows the adequacy of the model.     

APPLICATION OF A DISSOLUTION-DIFFUSION MODEL TO THE RELEASE OF 5-FLUOROURACIL FROM POLYMER MICROSPHERES

A theoretical discussion of drug release from microspheres is provided and a model-based predictive algorithm developed. The model verification step includes literature data describing the release profile of 5-fluorouracil (5-FU) from poly(lactic acid) polymer. Material balance equations were written to describe drug transport from a porous sphere. The model included combined effects of dissolution, diffusion, and void fraction on the release of 5-FU and was validated against in vitro experimental data. Analyses, conducted on published 5-FU release test data, revealed that the process was governed by a dissolution-diffusion mechanism. Approximately 1.5 million microspheres were formed; the drug density, diffusivity, and dissolution rate constant were estimated at 1.110 g/cm³, 2.324 × 10^?15 m²/s, and 17.60 g/m³h, respectively. The dissolution rate was faster than the rate of diffusion by a ratio of 12.79 to 1. Manipulation of the microsphere porosity was an effective way to influence the diffusion-controlled process. The procedure, outlined in the study, for estimating process properties will help fabricate microspheres that meet specific requirements.     

OPTIMIZING THE FEED CONDITIONS IN A DIMETHYL ETHER PRODUCTION PROCESS TO MAXIMIZE METHANOL CONVERSION USING A HYBRID FIRST PRINCIPLE NEURAL NETWORK APPROACH

Modeling is a fundamental step in plant optimization and simulation. In this work, a new technique for modeling a gas-solid heterogeneous fixed-bed reactor is developed. Gas diffusion into the solid catalyst pellets requires solving the mass balance equations inside the catalyst. The computational load needed can be quite time-consuming due to system complexities and nonlinearities. This bottleneck prevents on-line optimization of the process. In this work, a trained three-layer neural network model is used to replace major parts of these computations. The model is then incorporated within the overall model of an adiabatic fixed-bed reactor to produce dimethyl ether (DME) from methanol dehydration over solid acidic catalysts. The performance of the reactor simulated using this procedure indicated good agreement with its experimental operation. Then an optimizer is employed to determine the best feed conditions. The proposed strategy can be applied to any heterogeneous fixed-bed reactor.     

SOLIDS DISTRIBUTION IN STIRRED SLURRY REACTORS: INFLUENCE OF SOME MIXER CONFIGURATIONS AND LIMITS TO THE APPLICABILITY OF A SIMPLE MODEL FOR PREDICTIONS

The features of solids concentration distribution were investigated in baffled and unbaffled tanks of high aspect ratio, stirred with multiple radial, mixed-flow or axial impellers. In the baffled tanks the average profiles can be predicted with the sedimentation-dispersion model regardless of impeller type - in spite of slight systematic departures from the average trend at the impeller plane and/or midway between the impellers. This is hardly possible for the unbaffled tanks due to lack of physical foundation of this simple model. When the unbaffled tank is stirred with hydrofoil impellers, an inverted profile is even obtained. The mentioned departures and this last anomalous behaviour are qualitatively discussed with reference to particle-turbulence interaction.