When designing bins, silos, and hoppers for handling bulk solids, the engineers at Jenike & Johanson often look to gravity discharge for a reliable, low maintenance solution. However, depending on the flow properties of the material to be handled and the operating conditions, gravity discharge is not always the most practical solution. One alternative to consider for fine powders is fluidized handling.

Handling a bulk solid in a fluidized condition can be beneficial for several reasons. These include the ability to obtain higher discharge rates with smaller outlets, and to maximize the use of available space.

If a fine powder is allowed to deaerate in a gravity discharged mass flow bin, the desired discharge rate may be possible only with a very large outlet. Arching may also be a concern with many materials that are somewhat cohesive. By keeping a material fluidized, much higher discharge rates can be achieved and arching conditions can be overcome, while using a reasonably sized outlet. Several factors determine whether a material can be handled in a fluidized state. Two examples of suitable applications follow.

Manufacturers of asphalt roofing shingles often use fine, hot limestone as a filler with liquid asphalt. In order to maintain a uniform mix viscosity and temperature, flow of limestone into the batch mixer must be controlled and reliable.

Before coming to Jenike & Johanson for assistance, one manufacturer tried various surge bin designs for fine limestone. Each of these designs was unacceptable for various reasons. Vibrating bin dischargers, for example, were most unsatisfactory since the discharge rate varied from no flow to uncontrolled flow, or flooding [524K QuickTime video].

To correct these problems, Jenike & Johanson engineers designed and supplied mass flow bins with mass flow screw feeders, after carefully considering the material flow properties and the required handling conditions. This approach was extremely successful, resulting in reliable flow at the required range of discharge rates.

At another facility, space constraints limited the size of the surge bin. The combination of a small bin and high required flow rates resulted in a low residence time for the material in the bin. Depending on the operating conditions, the material density could vary significantly, creating discharge rate control problems. This required that the bin operation be carefully monitored and controlled to provide acceptable discharge.

When a similar line was to be modified at this plant, our engineers ran a series of tests, from which they concluded that a fluidized bin was the best option. We designed, fabricated, and supplied the bin. Fluidization is accomplished through a 3 ft. diameter twin permeable membrane, the top portion of which is made from KEVLAR® because of high operating temperatures. Discharge is controlled by an existing rotary valve and weigh belt.

Since the bin was installed, several years ago, it has worked extremely well, providing reliable, controlled discharge at rates in excess of 20 tons/hr.

A major manufacturer of laundry detergent wanted to install a new line at a facility in Mexico. Jenike & Johanson was asked to design a surge bin to handle a material that would be ground prior to processing. As a first step, we tested the material in our laboratory to determine its flow properties. The company's concerns were well founded, as the tests confirmed and defined the poor handling characteristics of the material.

In addition to the design limitations created by the material's handling characteristics, the company also had several design requirements for the bin. Some of these requirements directly conflicted with the ultimate goal of reliably handling the material. Working with the client, we evaluated the necessity of each conflicting requirement and determined whether alternative solutions were available. By remaining flexible in our design approach, we were able to offer a unique design to satisfy our client's needs, while also ensuring that the material would still be reliably handled.

The major departure from what the client had envisioned as a solution was to use two smaller bins instead of one large bin. This dramatically improved the design by eliminating the need for a "pant-leg" arrangement to divide the discharge to two pneumatic transporters.

The selected design is shown in the figure. A fluidized discharger and a lower cone, lined with air pads, maintain a fluidized volume of material at the outlet of the bin. The capacity of the lower cone is equivalent to the capacity of the pneumatic transporter, which is filled on a batch basis. Fluidizing the material provides the high fill rates needed to make the transporters work efficiently (the transporters fill in less than 20 seconds). The bin also contains an insert, designed to ensure that mass flow is obtained when material is discharged from the bin.

Jenike & Johanson engineers performed a structural analysis and provided detailed fabrication drawings of the bins. We also sent an engineer to assist during startup. Our client anticipated handling problems because the material had changed and was now worse than originally expected.

These concerns were unfounded. The fluidizing surge bin worked as intended, reliably discharging the material to the pneumatic transporters.

For expert help with any discharge application, whether fluidized or by gravity, contact your nearest office of Jenike & Johanson.