Hello, and welcome to Flow of Solids in Bins, Hoppers, Feeders and Chutes. My name is Eric Maynard; I'm a senior consultant and the
Director of Education at Jenike & Johanson. Jenike & Johanson is an engineering firm
specializing in bulk solids handling and pneumatic conveying, and we've been teaching
the public offering course Flow of Solids in Bins, Hoppers, Chutes and Feeders for approximately
thirty years, through the American Institute of Chemical Engineers.
Our motivation for the course is as follows. In many cases our bulk solids will not flow, or they may flow uncontrollably.
In the case of fine powders, we'll often see these materials flow with a liquid-like behavior.
making them very difficult to handle and hold with a typical screw or belt feeder.
Unfortunately, the theory of bulk solids handling is generally not often known. The training at
university level is sporadic at best, many rules of thumb are too generalized, and employee
turnover reduces effective ability to transfer valuable information and this experience to others.
As employees are retiring in the workforce, they are not able to effectively diffuse that knowledge,
those on-the-job learning experiences that are very valuable to a solids handling application,
such that new employees can learn from these valuable experiences and avoid the common pitfalls that occur.
A bulk granular solid, or some examples thereof, are shown in this slide. We see a collection of
different piles of material, whether it be plastic powder, brown sugar, corn meal, fiberglass,
glass cullet, coal. In all cases you can see discrete individual particles of a range of particle
size and shape. All of these materials are routinely handled in bulk form.
The characteristics of the material, whether it be size, shape, density, moisture content, can all
have a drastic influence on flow. Bulk solid is not one of the three phases of matter: a liquid,
a gas, or a solid material. It's actually falling somewhere in between, where at times it can
behave like a liquid. You take something like baby powder, It can flow like a liquid when
gas is entrained, and at other times you may take something like fertilizer you put on your
grass, and it can form a solid cake, a solid lump of material. So we can have solid behavior,
liquid behavior, and a wide range of behavior in between. But the bulk granular solid is where we
can notice discrete individual particles, even if we get down to the one micron or submicron
range, with titanium dioxide, talc, getting down to very fine cornstarch. Even though these
materials may appear liquid-like at times when they are aerated, they are discreet individual particles defining the structure.
Let's get into some of the solids flow problems that can occur with materials. The first one is
that of no flow, where it's obvious. You open the valve, you turn on the feeder beneath the hopper
and nothing comes out. If that is the case and the hopper has been completely filled and
nothing comes out at all, it's due to an arching or bridging problem. I'll demonstrate that in a minute.
Whereas if you open the valve, turn on the feeder, and you get flow established for a while
and then the flow stops, that's likely due to a ratholing problem, as manifest in the photo.
An open channel of material has opened up in this hoppper, that is, the material that is
discharged, and now you're left with a large stagnant zone of material called a stable rathole.
With a cohesive arch, we have a small chunk of material in this arch or dome shape discharging.
It's also known as bridging. Usually located over the hopper outlet, though in some cases as
operators beat on the side walls of the hopper, more and more material comes out, but you're
left with a larger arch that's propagating up into the hopper section. So under gravity flow
influences alone, this black material with interlayers of white material, kind of like frosting
in a multilayer cake, shows that very little material has discharged from the hopper,
and it's only the small dome shape chunk that has come out.
Dr. Andrew Janike recognized that that arch shape is very strong. We see that used in
structures, whether it be the St. Louis Arch in the United States, it could be the shape of an
egg, which is an arch, or it could even be a shape of a concrete dam holding back water.
That arch shape in nature is very strong and difficult to break. So of course, bulk solids want
to choose a strong shape and as an arch to prevent flow out of a hopper.
The cohesive arching, such as shown with this food powder in this picture, can be driven by
many factors: particle size, shape, density, moisture content, electrostatics,
Van der Waals forces; when you get down to submicron particle size, the intermolecular
bonding forces come into play. And this powder here is shown bridging over about a 9 inch
diameter outlet, which is 225 mm approximately, yet at the manufacturing plant, this powder
needed to flow through a 6 inch outlet, or 150 mm. Because of the bridging problem, the
packaging operation would routinely run out of material. Operators would open up the hoppers,
beat on them with sledge hammers and poke rods, and that would release a lot of the food
powder dust in the air. Well this food powder dust was highly combustible and explosive,
So it was a safety issue for the plant. They could have had an explosion at the plant that
would have leveled the plant, could have killed personnel, and it all comes back to the flow problem occurring in the packaging operation.
Here's an example of a stable rathole. We see large zones of stagnant material to the side
walls of the bin. The displacement field by the white and black marker layers show that very
little has moved along the walls. Only a central core of material has opened up, hence the
stable open rathole. Ratholes are very strong compared to arches. Arches are localized at the
outlet, the smallest cross-sectional area in the hopper. Tapping on the side walls of the hopper
may be able to break a bridge or an arch. You could use vibration, aeration, agitation, but yet
with a rathole, look at how much of the mass of material is dead or stagnant. So beating on the
side walls or vibrating it just tends to pack the material more tightly into the hopper rather than
really breaking the rathole into that central open channel and getting the material to discharge.
Furthermore, the material that collapses off a rathole, or the material that comes in down
through the center of the rathole and doesn't remain in the bin, can flood or flush out if it's a fine powder.
Here is a case where an operator is using a high pressure air lance to try and poke at material to
get it to flow with ore. This is very dangerous. If the operator were to fall in that hole he could be
killed, suffocated. If the operator were to receive particles blowing back in his face because of the
high pressure air, he could be very badly hurt. Some of those rocks are the size of 4 to 6 in.,
on the order of 100 to 150 mm. So it is an unsafe situation.
Of course trying to achieve the 1000-2000 ton per hour discharge rate from a stockpile is
certainly not possible by using an air-activated air lance such as what's being used in this picture.
You may have a pharmaceutical powder, as shown in the bottom of this slide. And though
you may only have 100 to 200 kg of this powder in a bin, its ratholing problem was directly
correlated to the high weight variability or mass ability in the tablets, as well as the content
uniformity issues. There was a chemistry difference in tablet to tablet being manufactured because of the powder separating.
Erratic flow is demonstrated on slide 29. We can see the ratholing material on the left and
then arching material are on the right. It may be going back and forth or it may just be an arching
problem that periodically occurs. Perhaps you have material sitting on a weekend, and when
you come in on Monday morning, turn on the feeder, open the gate, nothing comes out.
The operators may have to bang on the hopper to get the material moving. Or it could be you
start off with an arching problem and then eventually the material chews through and
develops a stable rathole. The rathole collapses and that can form another bridge.
Flooding is with regards to a fine material, where the average size is around 100 U.S. mesh or 150 micron.
We have two distinct regions, a compact de- aerated material the walls of the vessel, and
fluidized highly aerated powder in the center. If you have something like flour, or fly ash, or
cement, you know what I'm talking about by the fluid-like, fine nature of the material. It can be
deaerated at times, caking, and yet other times when air is introduced in that material, it
becomes very fluffy and fluidized. In this case, if we have a screw feeder or a belt feeder beneath
it, that fluidized material will spill out of the vessel, past that feeder, even it it's turned off, and flood or flush out of the vessel.