Bead mills are used in various industries to homogenize fluids and powders. In the biochemical industries they are used to disrupt (break-open) microorganisms and release intracellular products. The mechanism of cell disintegration by bead mill is based on the concussion of glass (or steel) beads on the cell surfaces. A bead mill has a horizontal chamber into which cell suspensions can be fed in either batch or continuous mode. The chamber is filled (up to 80-85% of the chamber volume) with glass beads of a fixed diameter which ranges from 0.1 to 3 mm. A rapidly rotating shaft (2,000 - 6,000 RPM) is located in the center of the chamber and is fitted with disks. The rotation of the disks causes the grinding beads to move in a circular manner in the chamber. The kinetic energy transferred from the beads creates impact and shear forces between the individual beads and between the beads and the microbial cells.
In Design Mode of calculation, the user specifies the residence time (tR) and the packing density. The working (liquid) volume (Vw) and the vessel volume (V) are calculated using the following equations:
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where F is the feed volumetric flowrate. If the calculated vessel volume exceeds its maximum possible value (specified through the Equipment tab), the program assumes multiple, identical units operating in parallel with a total vessel volume equal to the calculated.
In Rating Mode, the user specifies the vessel volume and the number of units and the program calculates the residence time.
If this unit operates in a batch plant, the feed flowrate F is calculated by dividing the volume of material that needs to be processed per cycle by the process time.
The rates of product release and denaturation are described by the following equations:
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eq. (A.158) |
where k is a kinetic constant, t is the average residence time of broth in the chamber (usually around 2 min), and f is the denaturation parameter.
To account for situations where the values of k and f are not known but instead actual experimental data are available, the user also has the option to specify the release and denaturation fractions.
The user may specify a reaction stoichiometry for the bead mill representing the disruption of biomass. If, for instance, the product of fermentation is an intracellular protein that forms inclusion bodies, the cell disruption reaction may be:
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with a stoichiometry which is also given above. Note that this is a mass stoichiometry and the sum of all coefficients must be equal to zero to have conservation of mass.
In this case, simply specify the residence time so that the program can estimate the required vessel volume and power requirement. There is no need to specify a reaction stoichiometry since no material transformation takes place.
The power requirement can either be calculated or set by the user. The percentage of power that dissipates to heat is also set. If the power is set by user, the program assumes that the user sets the total power, including the % of the power that dissipates to heat.
If the user chooses the power to be calculated by the program the power required for homogenization is estimated using the following equation, which was derived from experimental data for cell disruption of E. coli (Goldberg, 1987):
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eq. (A.159) |
where the holding volume (V) of the bead mill is in L and the resulting power is in kW. The total power requirement is then calculated based on the percentage of power that dissipates to heat (which contributes to the heating/cooling agent requirement).
1. Goldberg (1987). Personal Communication. GlenMills Inc. 203 Brookdale Street, Maywood, N.J. 07607.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Bead Milling: Oper. Conds Tab
● Mat. Balance, see Bead Milling: Mat. Balance Tab
● Labor, etc, see Operations Dialog: Labor etc. Tab
● Description, see Operations Dialog: Description Tab
● Batch Sheet, see Operations Dialog: Batch Sheet Tab
● Scheduling, see Operations Dialog: Scheduling Tab
