The use of flow control devices to improve the flow pattern and throughput of sieve trays

Compared to packings trays are more cost effective column internals because they create a large interfacial area for mass transfer by the interaction of the vapour on the liquid. The tray supports a mass of froth or spray which on most trays (including the most widely used sieve trays) is not in any way controlled. The two important results of the gas/liquid interaction are the tray efficiency and the tray throughput or capacity. After many years of practical experience, both may be predicted by empirical correlations, despite the lack of understanding. It is known that the tray efficiency is in part determined by the liquid flow pattern and the throughput by the liquid froth height which in turn depends on the liquid hold-up and vapour velocity. This thesis describes experimental work on sieve trays in an air-water simulator, 2.44 m in diameter. The liquid flow pattern, for flow rates similar to those used in commercial scale distillation, was observed experimentally by direct observation; by water-cooling, to simulate mass transfer; use of potassium permanganate dye to observe areas of longer residence time; and by height of clear liquid measurements across the tray and in the downcomer using manometers. This work presents experiments designed to evaluate flow control devices proposed to improve the gas liquid interaction and hence improve the tray efficiency and throughput. These are (a) the use of intermediate weirs to redirect liquid to the sides of the tray so as to remove slow moving/stagnant liquid and (b) the use of vapour-directing slots designed to use the vapour to cause liquid to be directed towards the outlet weir thus reducing the liquid hold-up at a given rate i.e. increased throughput. This method also has the advantage of removing slow moving/stagnant liquid. In the experiments using intermediate weirs, which were placed in the centre of the tray. it was found that in general the effect of an intermediate weir depends on the depth of liquid downstream of the weir. If the weir is deeper than the downstream depth it will cause the upstream liquid to be deeper than the downstream liquid. If the weir is not as deep as deep as the downstream depth it may have little or no effect on the upstream depth. An intermediate weir placed at an angle to the direction of flow of liquid increases the liquid towards the sides of the tray without causing an increase in liquid hold-up/ froth height. The maximum proportion of liquid caused to flow sideways by the weir is between 5% and 10%. Experimental work using vapour-directing slots on a rectangular sieve tray has shown that the horizontal momentum that is imparted to the liquid is dependent upon the size of the slot. If too much momentum is transferred to the liquid it causes hydraulic jumps to occur at the mouth of the slot coupled with liquid being entrained, The use of slots also helps to eliminate the hydraulic gradient across sieve trays and provides a more uniform froth height on the tray. By comparing the results obtained of the tray and point efficiencies, it is shown that a slotted tray reduces both values by approximately 10%. This reduction is due to the fact that with a slotted tray the liquid has a reduced residence time Ion the tray coupled also with the fact that large size bubbles are passing through the slots. The effectiveness of using vapour-directing slots on a full circular tray was investigated by using dye to completely colour the biphase. The removal of the dye by clear liquid entering the tray was monitored using an overhead camera. Results obtained show that the slots are successful in their aim of reducing slow moving liquid from the sides of the tray, The net effect of this is an increase in tray efficiency. Measurements of slot vapour-velocity found it to be approximately equal to the hole velocity.

Active vibration control at machinery feet

The unmitigated transmission of undesirable vibration can result in problems by way of causing human discomfort, machinery and equipment failure, and affecting the quality of a manufacturing process. When identifiable transmission paths are discernible, vibrations from the source can be isolated from the rest of the system and this prevents or minimises the problems. The approach proposed here for vibration isolation is active force cancellation at points close to the vibration source. It uses force feedback for multiple-input and multiple-output control at the mounting locations. This is particularly attractive for rigid mounting of machine on relative flexible base where machine alignment and motions are to be restricted. The force transfer function matrix is used as a disturbance rejection performance specification for the design of MIMO controllers. For machine soft-mounted via flexible isolators, a model for this matrix has been derived. Under certain conditions, a simple multiplicative uncertainty model is obtained that shows the amount of perturbation a flexible base has on the machine-isolator-rigid base transmissibility matrix. Such a model is very suitable for use with robust control design paradigm. A different model is derived for the machine on hard-mounts without the flexible isolators. With this model, the level of force transmitted from a machine to a final mounting structure using the measurements for the machine running on another mounting structure can be determined. The two mounting structures have dissimilar dynamic characteristics. Experiments have verified the usefulness of the expression. The model compares well with other methods in the literature. The disadvantage lies with the large amount of data that has to be collected. Active force cancellation is demonstrated on an experimental rig using an AC industrial motor hard-mounted onto a relative flexible structure. The force transfer function matrix, determined from measurements, is used to design H and Static Output Feedback controllers. Both types of controllers are stable and robust to modelling errors within the identified frequency range. They reduce the RMS of transmitted force by between 30?80% at all mounting locations for machine running at 1340 rpm. At the rated speed of 1440 rpm only the static gain controller is able to provide 30?55% reduction at all locations. The H controllers on the other hand could only give a small reduction at one mount location. This is due in part to the deficient of the model used in the design. Higher frequency dynamics has been ignored in the model. This can be resolved by the use of a higher order model that can result in a high order controller. A low order static gain controller, with some tuning, performs better. But it lacks the analytical framework for analysis and design.

Fast response static var generator for improving the power system performance of an electric arc furnace (EAF)

Cascaded multilevel inverters-based Static Var Generators (SVGs) are FACTS equipment introduced for active and reactive power flow control. They eliminate the need for zigzag transformers and give a fast response. However, with regard to their application for flicker reduction in using Electric Arc Furnace (EAF), the existing multilevel inverter-based SVGs suffer from the following disadvantages. (1) To control the reactive power, an off-line calculation of Modulation Index (MI) is required to adjust the SVG output voltage. This slows down the transient response to the changes of reactive power; and (2) Random active power exchange may cause unbalance to the voltage of the d.c. link (HBI) capacitor when the reactive power control is done by adjusting the power angle d alone. To resolve these problems, a mathematical model of 11-level cascaded SVG, was developed. A new control strategy involving both MI (modulation index) and power angle (d) is proposed. A selected harmonics elimination method (SHEM) is taken for switching pattern calculations. To shorten the response time and simplify the controls system, feed forward neural networks are used for on-line computation of the switching patterns instead of using look-up tables. The proposed controller updates the MI and switching patterns once each line-cycle according to the sampled reactive power Qs. Meanwhile, the remainder reactive power (compensated by the MI) and the reactive power variations during the line-cycle will be continuously compensated by adjusting the power angles, d. The scheme senses both variables MI and d, and takes action through the inverter switching angle, qi. As a result, the proposed SVG is expected to give a faster and more accurate response than present designs allow. In support of the proposal there is a mathematical model for reactive powered distribution and a sensitivity matrix for voltage regulation assessment, MATLAB simulation results are provided to validate the proposed schemes. The performance with non-linear time varying loads is analysed and refers to a general review of flicker, of methods for measuring flickers due to arc furnace and means for mitigation.

Flow patterns and performance of distillation trays

It is known that distillation tray efficiency depends on the liquid flow pattern, particularly for large diameter trays. Scale·up failures due to liquid channelling have occurred, and it is known that fitting flow control devices to trays sometirr.es improves tray efficiency. Several theoretical models which explain these observations have been published. Further progress in understanding is at present blocked by lack of experimental measurements of the pattern of liquid concentration over the tray. Flow pattern effects are expected to be significant only on commercial size trays of a large diameter and the lack of data is a result of the costs, risks and difficulty of making these measurements on full scale production columns. This work presents a new experiment which simulates distillation by water cooling. and provides a means of testing commercial size trays in the laboratory. Hot water is fed on to the tray and cooled by air forced through the perforations. The analogy between heat and mass transfer shows that the water temperature at any point is analogous to liquid concentration and the enthalpy of the air is analogous to vapour concentration. The effect of the liquid flow pattern on mass transfer is revealed by the temperature field on the tray. The experiment was implemented and evaluated in a column of 1.2 m. dia. The water temperatures were measured by thennocouples interfaced to an electronic computerised data logging system. The "best surface" through the experimental temperature measurements was obtained by the mathematical technique of B. splines, and presented in tenos of lines of constant temperature. The results revealed that in general liquid channelling is more imponant in the bubbly "mixed" regime than in the spray regime. However, it was observed that severe channelling also occurred for intense spray at incipient flood conditions. This is an unexpected result. A computer program was written to calculate point efficiency as well as tray efficiency, and the results were compared with distillation efficiencies for similar loadings. The theoretical model of Porter and Lockett for predicting distillation was modified to predict water cooling and the theoretical predictions were shown to be similar to the experimental temperature profiles. A comparison of the repeatability of the experiments with an errors analysis revealed that accurate tray efficiency measurements require temperature measurements to better than ± 0.1 °c which is achievable with conventional techniques. This was not achieved in this work, and resulted in considerable scatter in the efficiency results. Nevertheless it is concluded that the new experiment is a valuable tool for investigating the effect of the liquid flow pattern on tray mass transfer.

Distributed control of computer communication systems

Flow control in Computer Communication systems is generally a multi-layered structure, consisting of several mechanisms operating independently at different levels. Evaluation of the performance of networks in which different flow control mechanisms act simultaneously is an important area of research, and is examined in depth in this thesis. This thesis presents the modelling of a finite resource computer communication network equipped with three levels of flow control, based on closed queueing network theory. The flow control mechanisms considered are: end-to-end control of virtual circuits, network access control of external messages at the entry nodes and the hop level control between nodes. The model is solved by a heuristic technique, based on an equivalent reduced network and the heuristic extensions to the mean value analysis algorithm. The method has significant computational advantages, and overcomes the limitations of the exact methods. It can be used to solve large network models with finite buffers and many virtual circuits. The model and its heuristic solution are validated by simulation. The interaction between the three levels of flow control are investigated. A queueing model is developed for the admission delay on virtual circuits with end-to-end control, in which messages arrive from independent Poisson sources. The selection of optimum window limit is considered. Several advanced network access schemes are postulated to improve the network performance as well as that of selected traffic streams, and numerical results are presented. A model for the dynamic control of input traffic is developed. Based on Markov decision theory, an optimal control policy is formulated. Numerical results are given and throughput-delay performance is shown to be better with dynamic control than with static control.

All optical networking beyond 10 Gbits/s OTDM networks based on electro-optic modulators and fibre ring lasers

This thesis examines options for high capacity all optical networks. Specifically optical time division multiplexed (OTDM) networks based on electro-optic modulators are investigated experimentally, whilst comparisons with alternative approaches are carried out. It is intended that the thesis will form the basis of comparison between optical time division multiplexed networks and the more mature approach of wavelength division multiplexed networks. Following an introduction to optical networking concepts, the required component technologies are discussed. In particular various optical pulse sources are described with the demanding restrictions of optical multiplexing in mind. This is followed by a discussion of the construction of multiplexers and demultiplexers, including favoured techniques for high speed clock recovery. Theoretical treatments of the performance of Mach Zehnder and electroabsorption modulators support the design criteria that are established for the construction of simple optical time division multiplexed systems. Having established appropriate end terminals for an optical network, the thesis examines transmission issues associated with high speed RZ data signals. Propagation of RZ signals over both installed (standard fibre) and newly commissioned fibre routes are considered in turn. In the case of standard fibre systems, the use of dispersion compensation is summarised, and the application of mid span spectral inversion experimentally investigated. For green field sites, soliton like propagation of high speed data signals is demonstrated. In this case the particular restrictions of high speed soliton systems are discussed and experimentally investigated, namely the increasing impact of timing jitter and the downward pressure on repeater spacings due to the constraint of the average soliton model. These issues are each addressed through investigations of active soliton control for OTDM systems and through investigations of novel fibre types respectively. Finally the particularly remarkable networking potential of optical time division multiplexed systems is established, and infinite node cascadability using soliton control is demonstrated. A final comparison of the various technologies for optical multiplexing is presented in the conclusions, where the relative merits of the technologies for optical networking emerges as the key differentiator between technologies.

Packet switching in wide area broadband private networks

B-ISDN is a universal network which supports diverse mixes of service, applications and traffic. ATM has been accepted world-wide as the transport technique for future use in B-ISDN. ATM, being a simple packet oriented transfer technique, provides a flexible means for supporting a continuum of transport rates and is efficient due to possible statistical sharing of network resources by multiple users. In order to fully exploit the potential statistical gain, while at the same time provide diverse service and traffic mixes, an efficient traffic control must be designed. Traffic controls which include congestion and flow control are a fundamental necessity to the success and viability of future B-ISDN. Congestion and flow control is difficult in the broadband environment due to the high speed link, the wide area distance, diverse service requirements and diverse traffic characteristics. Most congestion and flow control approaches in conventional packet switched networks are reactive in nature and are not applicable in the B-ISDN environment. In this research, traffic control procedures mainly based on preventive measures for a private ATM-based network are proposed and their performance evaluated. The various traffic controls include CAC, traffic flow enforcement, priority control and an explicit feedback mechanism. These functions operate at call level and cell level. They are carried out distributively by the end terminals, the network access points and the internal elements of the network. During the connection set-up phase, the CAC decides the acceptance or denial of a connection request and allocates bandwidth to the new connection according to three schemes; peak bit rate, statistical rate and average bit rate. The statistical multiplexing rate is based on a `bufferless fluid flow model' which is simple and robust. The allocation of an average bit rate to data traffic at the expense of delay obviously improves the network bandwidth utilisation.

The application of robotics to the turbine blade encapsulation process

A survey of the existing state-of-the-art of turbine blade manufacture highlights two operations that have not been automated namely that of loading of a turbine blade into an encapsulation die, and that of removing a machined blade from the encapsulation block. The automation of blade decapsulation has not been pursued. In order to develop a system to automate the loading of an encapsulation die a prototype mechanical handling robot has been designed together with a computer controlled encapsulation die. The robot has been designed as a mechanical handling robot of cylindrical geometry, suitable for use in a circular work cell. It is the prototype for a production model to be called `The Cybermate'. The prototype robot is mechanically complete but due to unforeseen circumstances the robot control system is not available (the development of the control system did not form a part of this project), hence it has not been possible to fully test and assess the robot mechanical design. Robot loading of the encapsulation die has thus been simulated. The research work with regard to the encapsulation die has focused on the development of computer controlled, hydraulically actuated, location pins. Such pins compensate for the inherent positional inaccuracy of the loading robot and reproduce the dexterity of the human operator. Each pin comprises a miniature hydraulic cylinder, controlled by a standard bidirectional flow control valve. The precision positional control is obtained through pulsing of the valves under software control, with positional feedback from an 8-bit transducer. A test-rig comprising one hydraulic location pin together with an opposing spring loaded pin has demonstrated that such a pin arrangement can be controlled with a repeatability of +/-.00045'. In addition this test-rig has demonstrated that such a pin arrangement can be used to gauge and compensate for the dimensional error of the component held between the pins, by offsetting the pin datum positions to allow for the component error. A gauging repeatability of +/- 0.00015' was demonstrated. This work has led to the design and manufacture of an encapsulation die comprising ten such pins and the associated computer software. All aspects of the control software except blade gauging and positional data storage have been demonstrated. Work is now required to achieve the accuracy of control demonstrated by the single pin test-rig, with each of the ten pins in the encapsulation die. This would allow trials of the complete loading cycle to take place.

Wireless power charger for light electric vehicles

Wireless power transmission technology is gaining more and more attentions in city transportation applications due to its commensurate power level and efficiency with conductive power transfer means. In this paper, an inductively coupled wireless charging system for 48V light electric vehicle is proposed. The power stages of the system is evaluated and designed, including the high frequency inverter, the resonant network, full bridge rectifier, and the load matching converter. Small signal modeling and linear control technology is applied to the load matching converter for input voltage control, which effectively controls the wireless power flow. The prototype is built with a dsPIC digital signal controller; the experiments are carried out, and the results reveal nature performances of a series-series resonant inductive power charger in terms of frequency, air-gap length, power flow control, and efficiency issues.