ADJUSTING FLOW PROPERTIES OF BULK PARTICULATES

Field of invention

This invention relates to a method and apparatus for adjusting the bulk flow characteristics of particulates in incipient or established flow conditions. The invention has application to controlled flow of bulk particulates when filling vessels and other containers used in industry or dosing into processes or equipment.

Background of the invention

It is known that the controlled discharge of non-free flowing bulk particulates can pose significant issues in terms of erratic flow (too much or too little), inconsistent bulk density, dust generation and spillage. The current practice for aiding the flow of non-free flowing bulk particulates is to introduce fixed volumes of air into the bulk particulates irrespective of the bulk condition of the particulates being handled.

The use of air is primarily aimed at obtaining flow of non-free flowing bulk particulates. Conversely other systems use the application of vacuum to a small convergent section to halt flow. Additionally some packaging systems use vacuum evacuation of the container to be filled, to cope with excessive dust generation associated with aeration of fine particulates.

Summary of the invention The present invention provides an approach for controlling the bulk characteristics of particulates in an incipient or flowing condition. The invention is based on the finding that controlled introduction (or withdrawal) of gas volumes based on the existing void gas pressures in the bulk particulates can effectively optimise the flow conditions.

In one aspect the present invention provides a method for improving the bulk flow of particulates through a conduit, which comprises measuring the

pressure of gas within the particulate flow, comparing the pressure to a predetermined value and injecting gas into, or withdrawing gas from, the flow, most suitably, at a point downstream of the gas measuring point, to adjust the gas pressure within the flow to approximate to the predetermined value.

Suitably the pressure sensor in the gas flow sends a signal based on the measured gas pressure to a comparator where the gas pressure signal is compared with a stored value for the gas pressure, and the comparator generates a difference signal which is used to control the injection or withdrawal of gas.

Preferably more than one pressure sensor is provided at spaced intervals along the particulate flow and the output of the sensors is averaged before comparing with the stored value.

Suitably pressure is measured at the or each location along the flow by locating one or more sensors in an annular plenum chamber surrounding the conduit and open to the interior of the conduit for access of gas but shielded from the particulates.

In another aspect, the present invention provides apparatus for the bulk flow of particulates comprising: a conduit for the particulate flow; a pressure sensor positioned in the conduit; means for injecting or withdrawing gas into or from the conduit, typically located at a position which in use is downstream of the pressure sensor; comparator means for receiving a gas pressure signal from the pressure sensor, comparing it with a stored pressure value and generating a difference signal; and control means operable in response to the difference signal to activate the means for injecting or withdrawing gas so that the gas pressure within the conduit approximates to the stored value.

Preferably more than one pressure sensor is provided at spaced intervals in the conduit and the comparator includes averaging means which generates an average value of the signals from the sensors before comparing the average value with the stored value.

At the location of pressure measurement, the conduit may be divided into sections, and the downstream end of one section located inside the upstream end of the adjacent section to form an annular plenum in which the pressure sensor(s) are located.

The predetermined or stored value is suitably a value that is considered to be an optimal value for the system.

Description of the drawings Figure 1 is a schematic general arrangement drawing of one embodiment of apparatus in accordance with the invention.

Figure 2 is a schematic general arrangement drawing on an enlarged scale relative to Figure 1 , showing the positioning of pressure sensors in an apparatus of Figure 1.

Detailed description of the invention

In the present invention, modification of the wall friction characteristics and internal strength of the bulk particulates (and hence its flow behaviour) is achieved through the introduction (or withdrawal) of controlled volumes of gas. For a given discharge rate (and hence rate of dilation) a given gas pressure will develop as the bulk particulate dilates.

The basis for the invention lies in the determination of bulk particulate void pressures and the subsequent introduction (or where necessary removal) of a metered and controlled gas volume such that void pressures can be manipulated (either positively or negatively) to modify the bulk flow characteristics of the bulk particulate. The ability to influence the void gas

pressure will influence the dilation rate and hence discharge rate (and turn down thereof) achievable.

In a preferred embodiment the invention is implemented by the measurement and comparison of void gas pressure at two discrete points for a bulk particulate in a stable flow channel. The comparison of the two values is undertaken by a computational device and a single value established over multiple readings. The single instantaneous value is compared to a flow mapping profile for the bulk particulate to establish the difference between the actual void pressure and the optimal value for the required flow rate. Any difference (positive or negative) signals an air supply means to deliver/withdraw the appropriate gas volume downstream from the initial void gas pressure measurement points. In this way the bulk condition of the flowing particulates can be modified to suit operational requirements.

The optimal value for a desired discharge behaviour can be assessed by characterisation trials for the type of powder to be handled. The trials suitably use an apparatus similar in design and construction to that of the device to be used, or a suitable equivalent or model of the device, if the specific configuration is likely to be a factor in the flow rate.

The invention is described in more detail by reference to the accompanying drawings, firstly in terms of Figure 1 which shows a flow stream of bulk particulates 1 flowing from a storage vessel (not shown) through a standpipe 10.

The first in a series of void gas pressure measurements is made by a transducer or similar function device 2 and the resultant signal 4 is transmitted to a computational apparatus 6. Suitable pressure sensors to deliver a signal for further processing are capacitive pressure transmitters (such as Series 41 X by Keller AG). A second in a series of void gas pressure measurements is made by a transducer or similar function device 3 and the resultant signal 5

is transmitted to a computational apparatus 6. The computational apparatus 6 processes signals 4 and 5 from which a single value of void gas pressure is determined.

The computational device is constructed on conventional principles using systems to perform the indicated functions which will be known as suitable to those familiar with such devices.

Within computational apparatus 6, an algorithm is stored as software or firmware so that the processed single value of void gas pressure can be compared to a pre-determined optimal value for the required flow rate of bulk particulate. The optimum value is previously entered and stored in the computational apparatus.

The difference in gas pressure between the measured value and the ideal value is transmitted as a difference signal to an air control device 7. Based on the value of the difference signal the air control device develops either a positive or negative pressure in pipe work 8 connected to one or more gas introduction/evacuation points 9 down stream 11 from the point at which the pressure measurements were taken. The air control device is suitably a positive pressure pump delivering or removing a metered volume of gas based on the value of the difference signal. A valve may be provide to switch the pipework between communication with two opposite acting pumps, or between the inlet or outlet flow of a single pump, based on an indication whether the difference signal has a positive or negative value.

If necessary or desirable, the injection of air may be replaced by injection of another gas, for example an inert gas such as nitrogen or carbon dioxide

To avoid damage to the sensor(s) and to avoid impeding the flow, it is desirable that the pressure sensors 2,3 are not in contact with the flow of particulates. This may be achieved by the arrangement shown in Figure 2.

At the point of pressure measurement, the standpipe 10 is divided into sections 10A and 10B. The upper end of the lower section 10B has an enlarged diameter portion in which the lower end of upper section 10A is located. This creates an annulus 20, which is closed by annular plate 21 to form a plenum chamber in which the pressure sensors 22 are located. The annulus 20 is shielded from the flow of particulates passing from section 10A to section 10B, but the sensor 22 are still able to measure the prevailing pressure of air or other gas entrained in the flow of particulates.

The annulus may be divided into more than one plenum by dividing plates 23, with a pressure sensor being located in each plenum section. The output from each sensor is averaged to improve the relevance of the measurement.

The invention allows the gas void pressure to be controlled in a dynamic mode, so that fluctuations in bulk condition (internal friction, wall friction, bulk density) can be minimised, allowing greater accuracy when establishing operational control set points.

Optionally, the control device 7 may alternatively or additionally control gas injection or removal by reference to a downstream pressure sensor. However, it is envisaged that in introduction of a suitable feedback loop would have an adverse affect on the overall responsiveness of the system to pressure changes in the flow of bulk particulate solids.

The invention may be applied in any situation where bulk particulates flow through a conduit or standpipe, for example transfer conduits used to transfer particulates between storage vessels, or from a storage vessel to a feed hopper for a process, or filling transportable containers or tankers from bulk storage vessels.