AI, RI, CI, HI

Determination of angle of nip

Compressibility factor

K

Plunger-type consolidation bench

FRI

Determination of limiting flow rate; prediction of air entrainment,  nip angle, roll separating force and roll power requirements

Strength of agglomerates made by plunger press

S

Direct shear test with a special clamp for the sample

AI, RI

Determination of required briquetting pressure for various temperatures, moisture contents and binders

Spring back

 

Volume % recovery of consolidated sample upon release of pressure

SBI

Prediction of required critical hopper outlet and wall angle

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The table relates the basic bulk-solids material flow properties to Johanson Indices and to critical design parameters in roll press operations. These properties have an impact on the design of upstream sections including chutes, conveyors, surge hoppers, the press feed hopper and the roll press, as well as downstream material handling. Dr. Johanson has created a mathematical model for the relationships between material properties and roll-press dimensions and operating conditions [7].

The solids flow rate index (FRI) is useful in many aspects of agglomeration process design and troubleshooting. This index takes into account both the compressibility and air permeability of solids, the most important factors for press performance [5]. The FRI is the limiting rate from a conical hopper outlet when the solids have deaerated [2]. It allows companies that produce solids that others press to check the quality of product by comparing the values over time. For roll presses there is essentially a linear correlation between FRI and the roll speed for a given press feed correlation. As the FRI decreases the rate for a gravity-feed roll press system declines proportionately, so the index can help determine if a screw-feed system would be better. Tableting presses and other systems where the feed to a die surface is the limiting factor can use the FRI as it correlates well with die-fill limiting rates. The tablet capping phenomenon also correlates with the FRI because capping results from entrained air [5]. Segregation mechanisms and tendencies can be predicted by deliberately segregating a small pile of product and running FRI tests on different sections [2].

The arching index (AI) and rathole index (RI) measure the cohesive strength and help in determining critical factors of the press feed system. The RI can predict whether a scraper is needed in a single-screw feed system while the AI is used to size the inlet, outlet and screw diameter [2]. The two indices can assess binder effects on a material and process. Running AI and RI tests on different types and quantities of binders will quantitatively predict the maximum allowable binder content before the feed system hangs up [2].

The hopper index (HI) is a function of the coefficient of friction between the bulk solid and the hopper wall and can provide a guideline for determining if a centralized flow channel will develop in a press feed hopper or if flow at the hopper walls will occur [2]. The chute index (CI) is a measurement of the amount of adhesion a material will have on the walls and screw flights of a system. A CI of 80 or greater generally indicates the material will likely stick to screw flights and decrease the feeder capacity over time [2].

The spring back index (SBI) indicates how much elastic spring back a material has.

Kristin O’Quest is a consulting engineer and laboratory manager of Diamondback Technology, Inc., Atascadero, Calif., a firm that specializes in equipment and consulting related to solids handling.  E-mail her at koquest@diamondbacktechnology.com. Lee Dudley is president of Diamondback Technology, Inc. E-mail him at ldudley@diamondbacktechnology.com.

References

1. Johanson, J.R., “Feeding roll presses for stable operations,” Bulk Solids Handling, Vol. 4, No. 2, p. 43 (June 1984).
2. Johanson, J.R., “Roll press feed systems,” Powder Handling & Processing, Vol. 8, No. 2, p. 159 (April/June 1996).
3. Johanson, J.R., “Reducing air entrainment problems in your roll press,” Powder and Bulk Engineering, p. 43 (Feb. 1989).
4. Johanson, J.R., “Predicting limiting roll speeds for briquetting presses,” Proceedings, 13th Biennial Conf. of the Inst. of Briquetting and Agglomeration, Vol. 13, p. 88 (Aug. 1973).
5. Johanson, J.R., “New solids flow property indices for predicting roll press or extrusion press performance,” Proceedings, 22nd Biennial Conf. of the Inst. of Briquetting and Agglomeration, Vol. 22 (1991).
6. Johanson, J.R., “Flow indices in the prediction of powder behavior,” Pharma. Mfg. Intl., p. 159 (1995).
7. Johanson, J.R., “A rolling theory for granular solids,” A.S.M.E. J. of Applied Mechanics. Vol. 30, No. 4, Series E, p. 842 (Dec. 1965).
8. Johanson, J.R., “The use of laboratory tests in the design and operation of briquetting presses,” Proceedings, 11th Biennial Conf. of the Inst. of Briquetting and Agglomeration, Vol. 11, p. 135 (Sept. 1969).