T. Krull

Stress-field modeling and pressure drop prediction for slug-flow pneumatic conveying in an aerated radial stress chamber

Slug-flow pneumatic conveying is a full-bore mode of flow within the dense-phase flow regime where bulk materials are transported in the form of slugs at conveying speeds below saltation velocity. The mechanism of slug-flow pneumatic conveying consists of the particles being picked up from the stationary bed in front of a moving slug while the same amount of material is deposited behind the slug. Stress field modeling of the material slug is the first step in developing a prediction model for the pressure drop along a pneumatic conveying line. However, a reliable prediction strongly relies on an accurate assessment of several factors, including the particle properties, pipeline dimensions, and operating conditions. So far, the particle diameter has always been one of the crucial parameters, which is not desirable in regards to the limitations it imposes on the choice of bulk materials. This article focuses on one parameter, the stress transmission coefficient kw, which relates the lateral wall stress within a slug of material to the axial stress. To date, this parameter could not be measured directly in an aerated material bed and had to be estimated. Inaccuracies within the prediction were therefore unavoidable. A newly designed test chamber now enables the measurement of the lateral and axial stresses within a slug, which leads directly to this stress transmission coefficient. This article outlines the design of the test apparatus and reports on the experimental results. For the two materials tested, an exponential correlation between the pressure on top of the slug (frontal stress) and the stress transmission coefficient was obtained. Calculating the wall friction coefficient leads to a constant value above a certain material-specific air velocity.

Design of ship loading chutes to reduce dust emissions

This article presents an industrial case study to reduce dust emissions from a grain handling ship loader. The primary objective of the study was to reduce dust emissions to within acceptable environmental levels during ship loading. Several constraints were imposed on the solution as the result of time and budgetary restrictions, and the inability to add a dust suppression agent to the grain for quality reasons. Although this article specifically deals with grain, application of this technology is also equally suitable for reducing dust emissions while handling other particulate commodities.

Measurement of the Stress Transmission Coefficient of Material Slugs in an Aerated Radial Stress Chamber

In dense-phase pneumatic conveying, particles are transported along a pipeline at relatively low conveying speeds. Due to the relatively gentle handling characteristics of this mode of flow, it is suitable for conveying fragile and brittle bulk materials used in the food and chemical industries. The simulation of the stress field within a slug aims at developing an accurate prediction model for the pressure drop along a pneumatic conveying line. A reliable prediction of the pressure drop strongly depends on an accurate assessment of the particle properties, the pipeline dimensions, and the operating conditions. In past decades, a few models have been developed to serve this purpose, most of them including the mean particle diameter as a crucial parameter. This generally limits the selection of materials to those of nearly spherical particle shape, as it is extremely difficult to obtain a representative diameter for irregularly shaped particles or bulk commodities comprising differently sized and/or shaped particles. Another previously conflicting parameter is the so-called stress transmission coefficient k w , which relates the lateral wall stress within a slug of material to the axial stress. Previously, this parameter could not be measured directly in a test rig and had to be estimated; therefore, inaccuracies within the prediction were unavoidable. Consequently, a new test chamber was developed to measure the lateral and axial stresses within a slug, which leads directly to the stress transmission coefficient. The design of the test apparatus is outlined and the initial tests undertaken are reported. A strong dependence of the radial stress measurements on temperature changes of the test rig induced by the airflow was discovered. Possible solutions to compensate for this influence are addressed and further discussed.