I’m on my journey to Apple’s World Wide Developer’s Conference http://developer.apple.com/WWDC. It sold out in less than two minutes! Guess I’m not the only one that has a high interest in good user interface design. The Cocoa API’s and the Objective-C language have been a powerful combination for Jenike & Johanson to grow and deploy to all engineers, cutting edge tools for design and analysis of bulk solids handling systems including Discrete Element Modeling (DEM). Looking back in time, some of our solutions to the differential equations for solids flow that used to take longer than overnight, are now connected to slider controls which can change a boundary condition, recompute a solution, and display the new solution ten times per second. I can only hope this will be possible with DEM someday. This year my focus is on GPU computing to speed up our DEM models. We have been having a great year working on applying DEM technology to many interesting client problems in mining, steel, power and glass industries. Some movies of this work can be found here.

DEM technology has been around for over 30 years. Andrew Jenike spent the summer at the University of Minnesota in 1976, when I first joined Jenike & Johanson. His question to me was “when would computers be fast enough to work with millions of particles?”. After extrapolating Moore’s law, my reply was in about 30 years. His point was well taken and we continued to develop our continuum models, in conjunction with DEM. Those 30 years have now passed and our extrapolation was on target. GPU computing is ahead of CPU computing power by about 3 years in raw power (affordable teraflops), so hence our interest in GPU computing.

Many food products reach the consumer in powdered or granular form, and even more involve processing of this form of material, which we call a bulk solid, at some point in their manufacture.  These materials don’t always behave well in a manufacturing process, often finding their way to the root of problems that result in process downtime, product quality issues, excessive manpower or rework and reduced system capacity.  It is not uncommon for an otherwise viable new product to be abandoned when it becomes economically impractical to manufacture as a result of solids handling problems.

Commonly occurring solids handling problems in food manufacturing processes can include segregation of blended ingredients, flow interruptions or limitations, caking during storage or within packages, and loss of inventory control (and sometimes spoilage) in storage silos, to name a few.  These problems are often the result of equipment or system design that is based on a misunderstanding of how bulk solids behave.  Although the fundamentals of solids flow theory were established by Dr. Jenike in the 50’s, difficulties are still frequently encountered when processing powdered food products and ingredients.  Perhaps this is somewhat the result of the fact that the science behind solids flow typically isn’t taught in food science or engineering curricula.

All too often, solids storage vessels are designed based on the application of familiar methods.  Many times, what is familiar to the engineer is liquid storage, and since surge bins, hoppers and silos look something like liquid tanks, the misapplication of these design approaches often occurs.  For a liquid to drain reliably from a tank only requires that an outlet be placed at the low point and sized sufficiently large enough to provide the required discharge rate.

If you contemplate a flat-bottomed vessel with a hole in the bottom, it’s pretty easy to see that unlike liquid, most powders wouldn’t empty completely.  If a harder to handle material, like brown sugar, or cocoa powder were placed in the vessel, other problems, such as arching or ratholing, would likely occur.  The fine nature of some food products may also result in restricted discharge rates, or even flooding if the powder becomes aerated.  Not to mention that if a blended product is placed in the silo, mechanisms may be at work that cause the blend uniformity to degrade as the silo is filled or as it is discharged.

The good news is that vessels can be designed to reliably handle most bulk solids, even the difficult ones, and the approach is proven, reliable and based on material properties that are measured using ASTM test methods.  Jenike & Johanson, Inc. provides a wide range of testing and engineering services to help you ensure that your new process operates reliably.  If you have an existing process that is creating problems, we can help you correct the root cause of the problem, using proven methodologies.  Our engineers are available to discuss your process or solids handling problems without obligation and we’re eager to apply our understanding of solids flow to your process challenges.

In any bulk material handling system, it is critical to be able to convey material from point A to point B as well as feed material out of a bin or hopper.  Often times, the terms feeder and conveyor are used interchangeably without much thought given to their function and design requirements.

A feeder is the means by which the rate of material from a bin¹ or hopper² is controlled.  When a feeder stops, material flow should stop as well.  When a feeder is turned on, there should be a close correlation between its speed of operation and the rate of discharge of the bulk material.  Examples of feeders include a belt feeder for coal, a vibratory pan feeder for applying seasoning to chips, and an apron feeder for ore under a stockpile.

Mechanical conveyors are used to transport bulk materials but, unlike feeders, they are incapable of modulating the rate of material flow.  Examples of conveyors include a drag-chain conveyor for hot clinker, a screw conveyor for limestone, and a belt conveyor to take ore from a primary to a secondary crusher.

The main difference between a feeder and a conveyor is that feeders are flood-loaded while conveyors are not.  Also, while conveyors typically operate at a constant speed, feeders are always capable of varying the speed of operation.  As a result, feeders are capable of modulating the discharge rate from the vessel that is flood loading it. 

These differences are summarized in Table 1 below.

Dischargers are sometimes used to encourage material to flow out of a bin and, like feeders, they are flood-loaded.  However, a discharger is neither a feeder nor a conveyor since it cannot modulate the discharge rate from the vessel that is flood loading it.  An example of a discharger is a vibratory bin activator used for powdered soap.

Scale-up of Biomass Conversion Processes

Considerable efforts are underway to scale up new processes that convert biomass into biofuels, bioproducts, and biopower.  Most research process engineers have the training and experience to scale up processes that involve liquid and gaseous streams.  Biomass raw materials, however, are typically bulk solids (e.g., wood chips, sawdust, switch grass, and corn stover), and familiar scale-up methods usually cannot be applied.  Unfortunately, if scale-up is not performed properly, commercialization will suffer in terms of the time and expense it takes to correct flow problems, time to market, and in many cases, de-rating of the equipment used to receive and store biomass, reduce its particle size, dry the material, or convey the solids.

Retrofitting Systems to Handle Biomass

Twenty-nine states currently have regulations (a renewable portfolio standard) that required a certain portion of their state’s electricity to come from renewable energy sources. 

As a consequence, many utilities are modifying their coal-fired boilers to permit the use of biomass fuels such as wood chips, wood pellets, switch grass, and sawdust.  Using biomass to produce electricity also allows utilities to sell Renewable Energy Credits (ERCs).

To minimize handling problems and to be able to use existing solids handling equipment, co-firing coal and biomass is often preferable.  Unfortunately, adding even a small amount of biomass to coal may cause significant changes in the solids’ flow behavior.  The resultant blend may become more cohesive, resulting in flow stoppages in bunkers and silos due to arching.  In many cases, the blend may be more frictional and cause the silo to discharge in a funnel flow pattern.  In funnel flow, stable ratholes may form, and the silo will not empty completely.  In addition, sifting segregation can occur, resulting in an inconsistent feed to the boiler and making its operation difficult to control.  Many operations have taken cavalier or low-cost approaches to introducing biofuels into their existing facilities with terrible consequences to their maintenance and operating budgets.  Consideration must be given to setting incoming biofuel specifications that ensure no flow problems will be experienced.  In addition, the systems must be reviewed based on the flowability of the specified biofuel and strict acceptance/rejection criteria established.

Flowability studies are used to determine what modifications, if any, are required to ensure reliable, unobstructed flow of biofuels and their blends.Visit our Biomass page for more information.

]]> http://jenike.com/2013/05/22/handling-biomass/feed/ 0 http://jenike.com/2013/05/15/conveying/ http://jenike.com/2013/05/15/conveying/#comments Wed, 15 May 2013 16:30:40 +0000 David Morgan http://jenike.usmblogs.com/?p=9749 Continue reading →]]>

New, innovative solutions are being developed to transport more materials, faster, along routes and across terrains that would not be possible not long ago. Higher belt speeds and variability in materials make it even more important to get the fundamentals correct, like getting the load centered on the belt quickly and efficiently at each loading and transfer point. Not only does this make the conveying easier but it also reduces spillage, belt wear, dusting and material attrition.

Methods of transferring bulk solids still widely used, are to slam the material into a rock box to kill its momentum and then let it fall onto the receiving belt. The receiving belt is used to accelerate the material up to the belt speed. This does not work for high belt speeds and high tonnage rates and is not a good method even at lower rates.

Dropping the material vertically onto a belt requires an “acceleration zone” where large rocks bounce and roll, creating a belt wear problem, air is entrained in the fines to create a dust problem, and the particles exchanging energy can create a material attrition problem as well. These problems are compounded if the receiving belt is inclined.

Variability in the material, such as particle size or moisture content, will make the buildup in the rock box change and alter the flow through the rock box. If the material drops through the rock box off-center, the receiving belt will be loaded on one side. This will often result in spillage at the loading point, or if the belt is forced to ride up on the idlers eccentrically, spillage along the route. Trying to keep a large, high-speed belt centered with training idlers under these conditions is very difficult and will often result in more belt wear.

A correctly designed and efficient transfer point will eliminate these problems because the transfer chute handles the load relatively gently in spite of the high speeds and correspondingly high accelerations. By placing the material on the belt centerline at a speed close to the belt speed, the belt can easily convey it efficiently without spillage.

Instead of killing all the momentum of the material entering the chute, momentum can be used to keep the speed of the flowing material close to the belt speed all the way through the chute. Slowing the flowing stream of material down, requires that the cross sectional area increase. This is the beginning of a blocked chute. Also, slowing the material down and then accelerating it again so that the cross section changes rapidly, is an ideal way to entrain air and create dust.

Balancing the friction in the chute that slows the material down with gravity accelerating the material as it drops from the level of the incoming to the level of the receiving belt can be used to keep the speed through the chute more-or-less constant. Keeping the material in contact with the chute surfaces as much as possible reduces the possibility of entraining air. This also reduces points of impact and the wearing away of the chute at those points.

Placing the material on the receiving belt centerline at a speed close to the belt speed is also the best way to reduce belt wear and dusting at the interface between the material and the belt. If the chute is designed properly there should not be a need for an enclosed “acceleration zone” with a dust extraction port.

Finally, on April 16, 1953 at 3:15 in the afternoon — his 39th birthday — he made his decision. The topic he chose was the design of bins and hoppers for storage and flow of bulk solids. Up to that time, as most of us know, design of this equipment was mostly a black art. Most hoppers were either 45° or 60°, because those were the common triangles that all engineers carried around with them. No one gave much thought to the material being stored. After all, it’s “just a bin.”

Once Andy made his decision, he promptly went out to the garbage container at the apartment building in Salt Lake City where he and his wife were living and threw away box after box he had collected on every other topic that he considered. He wanted nothing to interfere with his decision, no looking back.

Jenike made that momentous decision 60 years ago — one that influenced and affected his life, the life of all of us who work at J&J, and indeed thousands of people around the world. It’s hard to imagine where each of us would be if he had decided to pursue a different subject that day!