Most industries view segregation [220K QuickTime video] as a major problem. Particle segregation is prevalent in industries that handle free-flowing bulk solids containing a wide particle size distribution. While there are other material characteristics that promote segregation, these two are most common.

Kraft Foods (KF) produces Stove Top® Stuffing Mix in its Dover, Delaware plant. To meet increased consumer demand, KF installed a new handling system. Unfortunately, during the startup process unacceptable variations in the particle size distribution of the bread crumbs occurred. KF contacted Jenike & Johanson, and one of our engineers immediately went to observe the operation. It was imperative that the problem be quickly resolved, so that the high cost associated with unnecessary down time and wasted product could be minimized.

It was apparent to our engineer that the problem was due to segregation in a new storage bin. For materials containing a wide range of particle sizes, such as the bread crumbs, sifting is the most prevalent segregation mechanism. Consider a pile being formed by a falling stream of material. If sifting segregation occurs, coarser material will tend to roll or slide to the periphery of the pile, whereas the finer material, sifting through the coarse material, will concentrate under the impact point. This commonly occurs in bins during filling.

The flow pattern of the material through a bin will determine the severity of segregation in the discharged material. Observations made on site and flow properties tests conducted by Jenike & Johanson confirmed that the existing bread crumb bin was funnel flow. Sifting segregation is made more severe by such a flow pattern. With funnel flow, the material flows primarily in a channel located above the discharge point.

Assuming a central fill and a central discharge, there are three possible results of sifting segregation in a funnel flow bin. If the fill rate is higher than the discharge rate, resulting in a rising level of material in the bin, the discharge will consist mostly of fines that are concentrated under the fill point. If the material level in the bin is falling, then the coarser material concentrated at the periphery starts falling into the flow channel, resulting in a mostly coarse discharge. If the fill and discharge rates are equal such that the material level in the bin remains constant, then what comes out will be similar to what goes into the bin.

Also, during discharge, fine particles can sift through the stationary coarse material, creating areas of high fines concentrations. When the level of material in the bin drops low enough for this region to fall into the flow channel, the fines content of the discharged material suddenly increases.

Since a funnel flow pattern results in a first-in, last-out sequence, material in the stagnant regions may remain there for an extended period if the bin is not routinely emptied. This becomes especially important when handling food products, where spoilage is a major concern.

We quickly determined that the solution to this segregation problem was to convert the flow pattern from funnel flow to mass flow. Mass flow is defined as the flow pattern where discharge of any material from the bin causes the entire contents to move. In such a bin, since the material moves down as a unit, if the fines and coarse segregate in a side-to-side pattern during filling, they are reunited at the outlet of the bin. In addition, there are no stagnant regions during discharge; therefore, the problem of spoilage is easily eliminated.

To maximize capacity, the existing bin had been designed to be rectangular with a pyramidal converging section. The critical angle, with a pyramidal geometry, is found in the valleys created by adjacent walls. Assuming a four-sided symmetrical hopper, these valleys can be almost 10 degrees shallower than the side walls, depending on the actual side wall angles.

To provide mass flow and solve the segregation problem, we considered three options. The first option was to line or coat the hopper walls to provide a significantly less frictional surface. Given the shallow angles, we felt that it was unlikely that a suitable liner or coating could be found. The second option was to replace the existing bin with a new one, properly designed for mass flow. This option would be the most costly and require the most time to implement. The third option, which was recommended by us as the most efficient solution, was to modify the existing bin with one of our patented BINSERT® geometries. A BINSERT® design provides mass flow with the use of a smaller, steeper hopper placed inside an existing hopper. In this case, a pyramidal inner hopper would be required. Due to the extremely shallow valley angles, we also recommended valley plates for this design, as shown in the figure.

Jenike & Johanson's BINSERT® is unique in that it is capable of providing mass flow with wall angles which would normally be too shallow for mass flow. It works by altering the pressures that are exerted on the walls of a conventional hopper so as to force material to slide along the walls, thus creating mass flow. We determine the proper design based on the flow properties of the material to be handled.

Another benefit of using the BINSERT® design is better control over the velocity profile in the hopper section. With a standard mass flow design, some segregation can occur when the level of material drops into the hopper section of a bin. With little head pressure acting on the material, it will flow faster in the center of the bin and slower along the walls. The BINSERT® is capable of maintaining a more uniform velocity profile throughout complete discharge of the bin.

A complete written report, with flow property test results and recommendations, was sent to KF approximately one week after the site visit. Shortly thereafter, during a weekend shutdown, KF installed our recommended inner pyramidal hopper and valley plates in the existing bread crumb bin. Since the installation, segregation has been noticeably reduced, the size distribution of discharged particles has been much more uniform, and, as a result, product quality has increased. According to Mario Suarez, Project Manager at KF, "The timing and quality of the work provided by Jenike & Johanson was exceptional. The recommended modification, which was implemented in an extremely short period, has proven very cost effective. We were able to utilize an existing bin, which did not provide good performance, and simply add an internal hopper to achieve excellent performance. The bottom line is product quality and that has definitely increased."