Pressureadded Centrifugation

It is more efficient to mechanically dewater solids than thermally, due to costly energy requirements. Filters such as pressure or vacuum units are used for solids/liquid separation, providing high forces to drive the liquid through the cake. Recently, equipment designed to combine both centrifuga-tion and pressurization has lead to increased dewatering of solids beyond what either process would do alone. (See Fig. 9.)

Figure 9. Inverting filter centrifuge with pressurization. (Courtesy of Heinkel Filtering Systems, Inc.)

A centrifugal field achieves mechanical separation of slurries by emptying the liquid in the capillaries between the solids. Larger particles will exhibit faster drainage of these capillaries.

Liquid in the interstices of the solids is retained due to high capillary forces in the micron pores and cracks in the particle. These capillary forces are so high that they can only be removed thermally. This contributes to a certain capillary height of liquid that is independent of the packed bed weight. After dewatering for an extended time, an equilibrium point is reached. Only changing the driving force by increasing centrifugal force will overcome and reduce this equilibrium saturation point. Product with a smaller particle size distribution will have higher capillary forces and thus a higher equilibrium saturation point or residual moisture.

By addition of the driving force pressure, or pressure differential across the packed bed, additional liquid is forced through the capillaries below the equilibrium saturation point, thus reducing the residual moisture.

Initial pressurization of the basket alone, thus avoiding pressurizing the entire centrifuge, can decrease final dewatering steps by as much as 80%. By blowing through the cake at a certain temperature, volume of gas, and pressure, drying will be achieved. Products that are crystalline and easy to filter can be dried in a relatively short period of time, not adding significantly to the overall cycle. Difficult filtering, amorphous materials may see overall cycle times reduced or products previously wet and sticky now easily handled at lower moisture levels going into a dryer. (See Table 3.) Downstream drying equipment can then be reduced in size, or possible eliminated.

The Inverting Filter with Pressure-Added Centrifugation has proved to dry products to 0.008% residual moisture using hot gas.

With heated gas, it is possible to break the upper surface of the moisture film, aiding in dewatering, or to dry or strip solvents. Steam washing can reduce wash quantities required.

Table 3. Pressure-Added Centrifugation

Product A-Unnamed pharmaceutical intermediate

Table 3. Pressure-Added Centrifugation

Product A-Unnamed pharmaceutical intermediate

LOD% under centrifugal force alone (1,200 g's)

9%

LOD% under nitrogen at 35°C (2.5 bar g)

<0.1%

Extra processing time for drying (% of filtration cycle)

0%

Mother liquor

Isopropyl alcohol

Product B-unnamed pharmaceutical intermediate

LOD% under centrifugal force alone (1,200 g's)

12%

LOD% under nitrogen at 50°C (6 bar g)

0.05%

Extra processing time for drying (% of filtration cycle)

■ 50%

Mother liquor

Toluene

Product c -unnamed pharmaceutical intermediate *

LOD% under centrifugal force alone (1,200 g's)

45%

LOD% under nitrogen at ZYC (2.5 bar g)

15%

Extra processing time for drying (% of filtration cycle)

75%

Mother liquor

Methanol

• Note: This extremely difficult filtering product went from behaving as a very sticky solid at 45% LOD, to a free flowing friable powder at 15%

• Note: This extremely difficult filtering product went from behaving as a very sticky solid at 45% LOD, to a free flowing friable powder at 15%

17.0 MANUFACTURERS

17.1 Filtering Centrifuges Inverting Filter Centrifuges

Heinkel Filtering Systems, Inc. Heinkel Industriezentrifugen GmbH+CO Bridgeport, New Jersey Bietigheim-Bissingen, Germany

Comi-Condur SpA Italy

Perforated Basket Centrifuges/ Vertical or Horizontal

Bir Machine Co., Inc./Ketema Walpole, MA

Dorr-Oliver (Acquired by Krauss-Maffei) Stamford, CT

Broadbent England

Krauss-Maffei, Inc. Krauss-Maffei Verfahrenstechnik GmbH

Florence, KY Munich, Germany

Robatel Robatel

Pittsfield, MA France

Western States Machine Company, Inc. Hamilton, OH

17.2 Sedimentation Centrifuges

Baker Perkins, Inc. Saginaw, MI

Bird Machine Company, Inc. South Walpole, MA

Centrico, Inc. (Westfalia) Northvale, NJ

DeLaval (Sharples/Pennwalt) Warminster, PA

17.3 Oxygen Analyzers

Neutronics Exton, PA

Orbisphere Laboratories, Inc. Emerson, NJ

Servomex Company, Inc. Norwood, MA

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    What is lod centrifugation?
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