Freeze protection in gasholders

In cold weather, the water seals of gasholders need protection from freezing to avoid compromising the seal. These holders have a large reservoir of "tank water" at the base which is below ground. At present freeze-protection is achieved by external heating of the seal water which is in a slotted channel called a cup. Electrical heating or circulation of heated tank water to the cup are examples of systems presently used. The tank water has a large thermal capacity and National Grid wishes to investigate whether circulation of the tank water without external heating could provide sufficient energy input to avoid freezing. Only tanks in which the tank water is below ground are investigated in the report. The soil temperature under the reservoir at depth of 10m and lower is almost constant

Crushing and screening models for simulation

Accurate crushing and screening models are essential for the meaningful simulation of ore dressing plants such as the iron ore beneficiation plant studies in this work. Crushing and screening models were studies, developed and enhanced. The basic simulator used was Microsim, which was subsequently expanded to incorporate these new models. The models were then evaluated individually. The Whiten crushing model was modified for haematite, as this is a particularly hard ore. A new model was developed for the gyradisc crushing of haematite, with two interparticle breakage mechanisms taking place in the crushing chamber. The gyradisc model has only the crusher closed side setting as parameter. Both models showed good agreement with experimental data. The Karra screen model was enhanced to include the use of ‘poly’ decks and non-square screen apertures, as used on the plant. The Rose efficiency model was developed to use efficiencies from plant data or experiments. The Rose model showed a better relative accuracy than the entirely empirical enhanced Karra model. The screen models developed can be used for different ores but the crushing models are not transferable. Thereafter simulation was done for the quaternary sub-plant manufacturing two or three products respectively. The product ratios obtained by simulation were accurate in both cases. The optimization routine was then used to calculate an improved fine to lumpy ore ratio. It was found that only a small improvement could be achieved in the quaternary plant. Simulation of the preliminary comminution plant, however, failed to give accurate results. This plant does not operate in steady state, therefore the error could be ascribed to this. The accuracy of the Nordberg gyratory crushing model used in this sub-plant could not be established. The washing and screening plant was successfully simulated with the new screening models. The choice of size classes was found to be very important in the simulation of crushing and screening circuits. The maximum number must be used and should coincide with screen apertures. Microsim has proved to be a powerful simulator subject, to the quality of input data and evaluative skills of the user.Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2007.Chemical Engineeringunrestricte