Literatura académica sobre el tema "Pneumatic-tube transportation Bulk solids handling"

Dense-phase pneumatic conveying of powders is becoming increasingly popular in various industries such as power, pharmaceutical, cement, alumina, chemical, limestone, refinery, and so on. Some of the reasons include: minimum gas flows and power consumption; improved product quality; increased workplace safety. However, due to the highly concentrated and turbulent mode of the solids-gas flow, only limited progress has been be achieved so far in understanding the fundamental transport mechanisms and accurately predicting pipeline pressure drop, which is a key system design parameter. This thesis aims to overcome the present limitations and provide the industry with a new validated modelling procedure for the accurate prediction and scale-up of pressure drop and optimal operating conditions for fluidised dense-phase pneumatic conveying systems.Various popular/existing models (and model formats) for solids friction (for straight horizontal pipes) have been evaluated for scale-up accuracy and stability. It has been found that the models (and their use of parameter groupings) are generally not capable of accurately predicting pressure drop under scale-up conditions of pipeline diameter and/or length. Two new approaches and another method based on the parameters used by other researcher have been employed in this study as improved design techniques. One approach, derived by modifying an existing reliable dilute-phase model to make it suitable for dense-phase, has resulted in a substantial relative improvement in the overall accuracy of predictions under scale-up conditions for two types of fly ash, ESP dust, pulverised coal and fly ash/cement mixture. Another method has been derived using the concept of “two-layer” slurry flow modelling (i.e. suspension flow occurring on top of a non-suspension moving layer), and this has also resulted in similar improvements. The third method, using parameters that were mentioned by another researcher as providing better representation of the flow phenomenon, has also resulted in similar reliable predictions. Three different popular/existing bend models have been evaluated to select an optimal (bend loss) model for dense-phase powder conveying. It has been found that the estimation of bend pressure drop can have a considerable impact towards correctly predicting the total pressure loss in a pneumatic conveying system.An existing method of representing “minimum transport criteria” (based on superficial air velocity and solids loading ratio) has been found inadequate for predicting the unstable boundary, especially under diameter scale-up conditions. Based on the experimental data of various powders conveyed over a wide range of pipe lengths and diameters, it is found that with increase in pipe diameter, the requirement of minimum conveying air velocity increases. To capture the pipe diameter effect, a Froude number based approach has been introduced to reliably represent the minimum transport boundary.The thesis also investigates the suitability of using a direct differential pressure (DP) measurement technique across a straight length of pipe for fine powder conveying in dense-phase. Standard Deviations (SD) of the DP, as well as the static pressure signals are presented. The trend shows the SD values are increasing with increase in pipe length from pipe inlet to exit (i.e. a dependence on tapping location).

Pneumatic conveying is being selected for an increasing number of industrial applications and products and is playing a more vital and integral role in the transportation of solid materials such as plastic pellets, grain and chemicals. However, despite all the minimum conveying velocity research (one of the operating boundaries for pneumatic conveying) that has been undertaken for several decades, the wide scatter and contradictions in the predictions of the minimum conveying velocity for dilute phase pneumatic conveying exist yet, determination of the operating boundaries for pneumatic conveying (mainly maximum conveying velocity for dense phase and minimum conveying velocity for dilute phase) still has been one of the most important tasks to be solved for the design, optimising and upgrade of pneumatic conveying systems as a consequence of that the mechanisms involved in the formation of boundaries between dilute-phase and dense-phase pneumatic conveying through a horizontal pipeline have not been well explored.Saltation velocity was investigated initially in this thesis and then the emphasis was placed on the transition between dilute-phase and dense-phase. With careful observations, it is found that pneumatic conveying of granular solid materials through a horizontal pipeline can exhibit five different flow modes (as the air velocity is decreased): fully suspended flow; strand flow; stable or unstable strand flow over a stationary layer for low solid mass flow rates; stable or unstable strand flow over a slowly moving bed for high solid mass flow rates; low-velocity slug-flow. The pressure fluctuations within the unstable zone result from the flow mode alternation between a strand flow over a stationary layer (or slowly moving bed) and slug flow starting at the inlet due to a decrease in air velocity. The first slug moves quickly at a relatively high velocity and picks up a relatively thick stationary layer in front of it but only deposits a small amount of the material behind it. The increase in slug length and large increase in pressure cause severe pressure fluctuations and pipeline vibrations. Two different flow modes may exist simultaneously in the conveying pipeline: strand flow over a stationary layer or slowly moving bed near the feed point followed by the dilute-phase (suspension) flow of particles. For the latter, material erodes away from the end of the stationary layer or slowly moving bed and is conveyed in the form of small dunes (or pulsating strand flow).Based on the mass balance, force balance, momentum balance and the unstable flow forming mechanism, a theoretical three-layer model for the prediction of the transition zone boundaries has been established. With stability analysis, the boundaries of the transition zone in the state diagram have been identified, and have been found to agree very well with experimental data. According to the model established, the discussion on the influence of design parameters of particle and bulk properties of the material being conveyed and pipe wall properties on boundaries in the state diagram has been conducted.The discussion on the operating boundaries for pneumatic conveying of granular materials has been extended to conveying of powder materials and a principle for classification of granular materials and powder materials, which have different flow mode in PCC, has been proposed.The research also has been carried out on the pressure drop prediction for pneumatic conveying of granular materials in the form of low-velocity slug-flow in order to have a perfect PCC state diagram. A new approach for the direct measurement of stress transmission factor has been developed in this thesis. The effect of the weight of the granular material in the slug on pressure drop is taken in account according to the experimental test results. The model for pressure drop prediction also includes a modified equation for the frontal force of the moving slug - allowing for momentum balance of accelerating particles and the additional force from the stationary layer to resist the movement. The modelling predictions agree very well with test results obtained on poly pellets conveyed through 98 mm and 60.3 mm ID horizontal stainless steel pipelines, each 21 m in length.