Continuous sensing of bulk density and bulk density oscillations

The bulk density of a free flowing powder or granular solid can be measured using simple apparatus consisting of an open ended vessel attached to a weighing device such as a load cell or electronic balance. Solids are continuously sampled from the process, and flow into the vessel via a downcomer which terminates below the level of the vessel rim and so defines a constant volume of solids within the vessel. The downcomer also isolates the material in the vessel from the process. The flow rate of solids through the vessel is controlled by an orifice at the lower end of the vessel, and care is taken to ensure that sufficient solids are available to keep the vessel full at all times. The mass of the constant volume of solids within the vessel is obtained from the calibrated weighing device, enabling bulk density to be calculated at any instant. Mean values of bulk density found in this way agree within about 2 % with values of loose poured bulk density. It is well known that the flow of a granular material is characterized by successive episodes of dilation and contraction within the active granular mass. These changes in local voidagemore » or bulk density can affect the mean value of bulk density as measured by the apparatus described here. It has been found experimentally that for different materials there are differences in the fluctuating component of the signal recorded from the sensing device. Spectral density and phase space plots have been used to explore and analyze the weighing device signals for different materials including sand, wheat and polypropylene pellets. 9 refs., 4 figs., 1 tab.« less