A STUDY OF THE CONTROL OF A TUBULAR REACTOR WITH RECYCLE I. Theoretical Results for a Zero-order Exothermic Reaction

The topic of interest in this paper is the behavior of a plug-flow tubular reactor with recycle under feedback control of the rcgutalor type. Attention is devoted mainly to the problem of stabilizing those sicady-slatc profiles which are unstable in the absence of control, but the investigation also includes the controlled behavior of those steady states which arc naturally liable. The study reported in this paper is theoretical; it invokes methods of linear stability analysis and computer simulations in considering two separate cases involving different controlled and manipulated variables. The class of controller functions considered includes the conventional proporIionaI-integraderivative controller. The principal discovery is that the presence of any time delay, regardless of its magnitude, in the controller response prevents stabilization of a naturally unstable state by means of control Strategics of this type. In one of the cases considered, It Is shown that if time delay in the control action is totally absent, stabilization can be achieved; in the other case, stabilization is shown to be possible only if both the time delay in the controller and the transportation lag in the recycle line are zero. The study reported here is based on a hypothetical zero-order reaction, but the icsutls ate linked Io prior experimental observations. Also, other realistic aspects of the basic problem are discussed, and the need for further work is pointed out.     

OSCILLATIONS IN THE OXIDATION OF CARBON MONOXIDE ON A PLATINUM FOIL

Reported here are experimental observations of self-sustained oscillations in the rate of oxidation of carbon monoxide on a platinum foil in a gradientless reactor. The results particularly show the effects of catalyst activity and illustrate some unusual and complex dynamic behavior under nearly isothermal conditions.     

Stereological estimation of surface area and barrier thickness of fish gills in vertical sections

Previous morphometric methods for estimation of the volume of components, surface area and thickness of the diffusion barrier in fish gills have taken advantage of the highly ordered structure of these organs for sampling and surface area estimations, whereas the thickness of the diffusion barrier has been measured orthogonally on perpendicularly sectioned material at subjectively selected sites. Although intuitively logical, these procedures do not have a demonstrated mathematical basis, do not involve random sampling and measurement techniques, and are not applicable to the gills of all fish. The present stereological methods apply the principles of surface area estimation in vertical uniform random sections to the gills of the Brazilian teleost Arapaima gigas. The tissue was taken from the entire gill apparatus of the right‐hand or left‐hand side (selected at random) of the fish by systematic random sampling and embedded in glycol methacrylate for light microscopy. Arches from the other side were embedded in Epoxy resin. Reference volume was estimated by the Cavalieri method in the same vertical sections that were used for surface density and volume density measurements. The harmonic mean barrier thickness of the water‐blood diffusion barrier was calculated from measurements taken along randomly selected orientation lines that were sine‐weighted relative to the vertical axis. The values thus obtained for the anatomical diffusion factor (surface area divided by barrier thickness) compare favourably with those obtained for other sluggish fish using existing methods.     

Receptance Coupling for Tool Point Dynamics Prediction on Machine Tools

Chatter has been a primary obstacle to the successful implementation of high speed machining.The frequency response function(FRF) of the tool point is crucial for identification of chatter free cutting conditions.In order to quickly acquire the FRF of the different components combinations of machine tool,the assembly of machine tool was always decomposed into several parts,where the fluted portion of tool,however,was always treated as a uniform beam,and the associated discrepancy was ignored.This paper presents a new method to predict the dynamic response of the machine-spindle-holder-tool assembly using the receptance coupling substructure analysis technique,where the assembly is divided into three parts:machine-spindle,holder and tool shank,and tool’s fluted portion.Impact testing is used to measure the receptance of machine-spindle,the Timoshenko beam model is employed to analyze the dynamics of holder and tool shank,and the finite element method(FEM) is used to calculate the receptance of the tool’s fluted portion.The approximation of the fluted portion cross section using an equivalent diameter is also addressed.All the individual receptances are coupled by using substructure method.The predicted assembly receptance is experimentally verified for three different tool overhang lengths.The results also show that the equivalent diameter beam model reaches an acceptable accuracy.The proposed approach is helpful to predict the tool point dynamics rapidly in industry.     

Atomic resolution imaging of beryl: an investigation of the nano‐channel occupation

Beryl in different varieties (emerald, aquamarine, heliodor etc.) displays a wide range of colours that have fascinated humans throughout history. Beryl is a hexagonal cyclo‐silicate (ring‐silicate) with channels going through the crystal along the c‐axis. The channels are about 0.5 nm in diameter and can be occupied by water and alkali ions. Pure beryl (Be₃Al₂Si₆O₁₈) is colourless (variety goshenite). The characteristic colours are believed to be mainly generated through substitutions with metal atoms in the lattice. Which atoms that are substituted is still debated it has been proposed that metal ions may also be enclosed in the channels and that this can also contribute to the crystal colouring. So far spectroscopy studies have not been able to fully answer this. Here we present the first experiments using atomic resolution scanning transmission electron microscope imaging (STEM) to investigate the channel occupation in beryl. We present images of a natural beryl crystal (variety heliodor) from the Bin Thuan Province in Vietnam. The channel occupation can be visualized. Based on the image contrast in combination with ex situ element analysis we suggest that some or all of the atoms that are visible in the channels are Fe ions.     

球墨外围奥氏体壳的形成过程与对石墨畸变的影响

本文用彩色金相法研究了球墨外围奥氏体壳的生长过程及奥氏体对石墨畸变的影响。结果表明,球墨外围的奥氏体壳不是连续的奥氏体层,而是由若干个楔形晶体组成。由于球墨外围奥氏体成核的有利条件不同;故在不同位置先后成核并生长。之后,互相接触形成一个壳。 经电子探针分析,沟槽中正偏析元素及低熔点金属含量高,因而沟槽液体的熔点低、凝固慢。根据沟槽的凝固速度,奥氏体壳可分成快封闭、慢封闭及不封闭三种类型,与石墨球的畸变有密切关系。     

MODELING OF THICKNESS FOR SEMISOLID FOODS

We investigated the relationship between orally perceived thickness and calculated shear stress on the tongue for mayonnaise and custard. To this end, the applicability of the models of Kokini et al. (1977), describing the mechanical breakdown in the mouth, have been tested. Within a limited range of shear stresses (mayonnaise < 150 Pa; custard < 30 Pa), there was a linear relationship between shear stress and thickness, in accordance with the work of Kokini et al. (1977). Beyond this range, the linear relationship breaks down and the thickness levels off with shear stress for both mayonnaise and custard. The relationship over the entire range of shear stresses used in this paper can be satisfactorily described by a semilogarithmic (Fechner) relation. For both types of products, the quality of the thickness prediction by the decreasing‐height model and the constant‐height model of Kokini et al. (1977) is similar. For most mayonnaises, the contribution of the lateral movement of the tongue to the shear stress in the decreasing‐height model of Kokini et al. (1977) is orders of magnitude larger than the contribution of the squeezing or compression movement of the tongue towards the palate. This difference in magnitude is affected by the low value measured for the compression force and by the high values for material consistency K. The values for K are high because yield‐stress behavior has been neglected when the flow curves were analyzed. For custard, the models of Kokini et al. (1977) are found to be less adequate. It is proposed that this is because the models ignore interactions with saliva. Several routes to improve the modeling by incorporating viscoelastic behavior were unsuccessful. Elongational stress and yield stress were neglected in all tested models.     

FORMATION OF AUSTENITE SHELL AROUND SPHEROIDAL GRAPHITE AND ITS EFFECT ON DETERIORATION OF GRAPHITE

To study the growth of austenitic shells around the spheroidal graphite,color metallographyhas been employed.It is found that the shell is not a continuous layer of austenite,but a clus-ter of wedge-shaped crystals.Three types of shells have been observed.That may strongly af-fect the deterioration of the spheroidal graphite.The mechanism of the formation of theaustenitic shells has also been discussed.