Fine tuning the flow

Most industries in the UK use bulk particulate, but failure to ensure that a consistent material is being introduced into a process can result in an end-product that exhibits variations in chemical performance, flavour or texture, leading to it being rejected as out of specification.

The cause of many of these problems can often be traced back to the equipment used to store and discharge the powders. In many instances, the humble storage vessel is the main culprit, although poor interfacing to conveyors or processes can also be a significant contributory factor.

Two basic flow patterns are obtainable from hoppers: core flow and mass flow. The core flow discharge pattern occurs in the vast majority of installations. For materials that do not segregate, are not adversely affected by extended storage periods or for which a segregated discharge of material is not a problem, these types of vessel are usually quite satisfactory.

If a blend or a material with a wide range of particle properties is loaded into the vessel, then segregation of those particles is likely to occur. This is typified by the larger, more dense, or free-flowing, particles accumulating around the periphery of the heap within the hopper, while the finer (less free-flowing) particles will tend to reside in the central region.

Increased level of fines come through the process if material is drawn down through a central discharge channel. Later discharges from the same hopper will begin to show a lack of fines amongst the particles.

The combination of inconsistent particle size distribution and particle-on-particle shear generated during discharge will conspire to give an unreliable and erratic flow rate. This can be particularly troublesome for online process control equipment to keep track of.

In order to arrive at a design of plant equipment that will cause the minimum of degradation and segregation, it is essential that it is designed with these objectives in mind from the outset.

Hoppers designed for mass flow can deliver considerable improvements. Examination of the diagram demonstrating mass flow illustrates the first-in-first-out flow regime that facilitates the natural de-aeration — and hence densification — of the material prior to discharge.

In a core flow hopper the most recently introduced material (also the most aerated) will discharge first in preference to older and more dense material. The en-masse flow pattern of the mass flow hopper also ensures that an even cross-section of the inventory is drawn during discharge, equivalent to a lateral cross-section through the segregation pattern.

This flow pattern produces a more Homogeneous blend of particle sizes, which will exhibit a far more consistent bulk density and flow characteristics - considerably easing the job of on-line process control equipment. It follows that this Homogeneous material will also result in an end-product of much more consistent quality.

Having looked at the effects that the design of hopper can have on a process, the next major issue is how hoppers are interfaced into the process. If insufficient care is taken in the design of the hopper/process interface it is easy to revert a mass flow design of hopper back to core flow.

In essence, any interface to be used with a mass flow vessel must be designed and constructed such that the whole of the outlet area is fully activated during discharge. Failure to observe this basic rule will result in the development of a preferential draw from the hopper and the development of dead regions of non-flowing material, which will clearly be an issue for time-dependent products.

Typical examples of poor interfacing include: direct mounting of a rotary valve to the outlet flange of a hopper; parallel slot configuration on interfaces to conveying belts; use of constant pitch screws; use of screws in “V” troughs. All of the interfaces in this, by no means exhaustive, list will result in preferential draw or areas of static product and result, in consequence, in core flow.

The correct technique that should be applied is to remember that the whole of the outlet needs to be activated upon discharge.

Therefore, any device mounted to the outlet must be able to provide an increase in capacity through its travel across the outlet. Alternatively, it should permit the localised flow channel to expand to the extent that it can equal the diameter of the outlet to be activated at a higher level, as in the installation of a standpipe between the interface and the hopper.

Another aspect of handling materials that tend to degrade easily is to ensure that they are treated gently, so as not to generate even more fines that are likely to segregate out in the hopper.

Many powders are conveyed pneumatically, but this technique, whether using positive or negative pressure, can be very destructive to friable particles if the system is not set up correctly.

The gas used in a pneumatic conveying system must be introduced into the system at a given velocity that is sufficiently high to entrain the particles to be conveyed (this is the minimum conveying velocity).

As the gas is constrained by the diameter of the pipework, however, it will expand along the pipe as it travels. Hence, the exit velocities on some systems can be considerably higher than that at the start of the system, leading to the conveyed material travelling considerably quicker.

Higher velocities have a direct relationship to increased levels of particle breakage and, hence, fines generation. If a friable material is to be conveyed pneumatically it may be worthwhile considering the use of a stepped bore pipeline.

This type of pipeline features increases in pipe bore at calculated distances along its length that allow the gas to expand, thereby decreasing its velocity along the pipe. By having several of these changes in bore size along a pipeline it is possible to arrive at a terminal velocity that need not be substantially greater than the pick-up velocity at the start of the pipeline.

This design technique not only reduces the degradation of particles, but, as a result of the lower back pressure, also reduces pipe wear and energy consumption.