Process control: An overview and personal perspective

This paper presents an overview and personnal perspective on recent developments and current practice in computer process model. The basic concepts behind conventional process control are reviewed to provide a starting point for the non-control specialist and process engineer. The ensuing sections on Internal Model Control (IMC), Model Predictive Control (MPC) and adaptive control illustrate the evolution of control technology from the traditional multiloop strategies to the modern, multivariable, model-based, computer control systems now being used in industry. The paper does not provide a complete, critical review of process control. However, the discussion and the recommended references should provide a good starting point for anyone wanting to become familiar with current practice and some of the new directions of process control.     

Analysis and tuning of adaptive generalized predictive control

The adaptive Generalized Predictive Controller (Clarke et al., 1987a, b) is capable of controlling plants with variable dead-time, unknown model orders and unstable poles and zeros. This paper shows how the GPC control law can be written in an equivalent general linear transfer function form which simplifies closed-loop (eg. root locus) analysis. Three recommended strategies for selecting the design parameters during the commissioning stage allow the user to adjust the closed-loop speed of response on-line using only a single active tuning parameter. Experimental runs confirm the ability of adaptive GPC to provide a consistent closed-loop response in spite of large process changes.     

Modification of polyethylene surfaces with carbethoxy substituted carbenes and nitrenes

Carbethoxycarbene, carbethoxynitrene, and nitrene generated respectively by the pyrolysis of ethyl diazoacetate, ethyl azidoformate, chloramine and sodium hydroxyl amine-o-sulfonate are shown to irreversibly modify the surface of polyethylene as shown by wettability measurements. The nature of the modification is not fully understood; however, insertion of the carbene or nitrene into a carbon-hydrogen bond appears likely. The modified surfaces thus formed are shown to undergo several classical organic reactions as again determined by wettability measurements.     

A method for the design of flexible distillation sequence

A simple method has been developed to optimally design a distillation sequence with a fixed degree of flexibility in feed composition and flowrate, and accounts for uncertainty in stage efficiency, K-value and beat transfer coefficient calculations. The method minimizes a weighted cost function and ensures both optimal and feasible operation in the range of flexibility/uncertainty by using an approach termed “practical feasibility”. The method was tested on a 4 column distillation sequence separating a 5 component light alkane feed mixture. Results are presented to illustrate the effect of uncertainty on sequence cost and column overdesign.     

Origin of certain heterocyclic compounds from depolymerization of lignite

The origins of products observed in the acid-catalysed phenolic depolymerization of a lignite are discussed. The isolation of ¹⁴C-labelled xanthene and 9-arylxanthenes from the use of 1-¹⁴C-phenol in lignite depolymerization reactions demonstrates the solvent origins of aromatic portions of these products. Methylene-, methine-, and ethylene-bridged aromatic products were identified by gas chromatography—mass spectrometry (GCMS) techniques, demonstrating a close similarity to products from the depolymerization of brown coals and lignites. Reactions of model triarylmethane compounds under depolymerization conditions demonstrated the facile conversion of triarylmethine structures to diarylmethane structures, indicating ambiguity in the simple interpretation of diarylmethane products as having derived from diarylmethane functional groups in coal. Alternative mechanistic pathways to observed products are presented.     

The influence of molecular weight on the melt rheology of polypropylene

Melt viscosity and melt elasticity data were obtained over a broad range of temperatures and shear rates on a series of four polypropylenes of different molecular weight but approximately the same molecular weight distribution. The superposition technique was used with both temperature and molecular weight to shift flow curves for all four materials at three temperatures each along the shear rate axis to generate a master flow curve at a given temperature and molecular weight. For polypropylenes of this type, and molecular weight distribution shift, factors which can be used to extend the useful range of experimentally obtained flow data were determined. The dependency of apparent viscosity on weight average molecular weight at shear stresses as high as 10⁶ dynes/cm² is shown. The dependency of melt elasticity on molecular weight and temperature is discussed.     

Enniatins fromFusarium avenaceum isolated from balsam fir foliage and their toxicity to spruce budworm larvae,Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae)

Material extracted from hyphae ofFusarium avenaceum, isolated from foliage of balsam fir,Abies balsamea, was toxic to spruce budworm larvae when incorporated into insect diet. The major insecticidal component of the toxic fraction was identified by chemical and spectroscopic methods as enniatin complex, rich in enniatin A/A₁. Possible ecological implications of these observations are considered.     

Free-radical concentration in polymerizations

A method is described for deriving by digital computer a temperature program which, when applied to batch polymerization, will keep the supply of new free radicals constant throughout the main part of the reaction.     

The effect of ion implantation on the corrosion behaviour of pure iron—I. Argonion implantation

High purity iron has been implanted with relatively high (ca. 5 at.%) surface concentrations of argon by means of ion implantation. Using electrometric reduction it has been shown that at some stage during or after the implantation process the natural air-formed film on the iron is thickened, producing an oxide film comparable to a thermally grown film formed at low oxygen partial pressure (～ 30μN m^?2) at 475K. The factor responsible for this enhanced film growth has not been identified. Using a three sweep potentiokinetic polarization technique it has also been shown that once the thickened air formed film is removed by cathodic polarization there are no significant differences between the polarization behaviour of unimplanted and argon implanted iron that cannot be explained by changes in surface roughness associated with the implantation process.It is concluded that earlier reports that the implantation of metals with inert species such as argon confers enhanced corrosion resistance may be attributed to the thickening of the air-formed oxide film and not to the presence of the implant species per se. Furthermore, in the investigation of the effect of implantation of more active implant species it is necessary to take account of the film thickening that occurs during implantation.     

Enzymatic synthesis of monosaccharide fatty acid esters and their comparison with conventional products

5-O-Acyl-1,2-O-isopropylidene-D-xylofuranose and 6-O-acyl1,2∶3,4-di-O-isopropylidene-D-galactopyranose were enzymatically prepared from the corresponding monosaccharide acetals and commercial (crude) fatty acid mixtures. Subsequent acid-catalyzed hydrolysis of the isopropylidene group(s) gave monosaccharide esters with overall yields of 59–88%, where the monoester content was at least 80% (galactose oleate) and typically 90% for the other preparations. In contrast to sugar fatty acid esters prepared by conventional, high-temperature (trans)esterification, the enzymatically obtained monosaccharide esters contained no appreciable quantities of undersirable side products, and the only contaminants were monosaccharides and fatty acids.