Chapter 7
Pilot-Scale Membrane Filtration Process for the Recovery of an Extracellular Bacterial Protease 1
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Downloaded by CORNELL UNIV on September 16, 2016 | http://pubs.acs.org Publication Date: January 24, 1990 | doi: 10.1021/bk-1990-0419.ch007
John J . Sheehan , Bruce K. Hamilton , and Peter F. Levy 1
Washington Research Center, W. R. Grace & Company-Conn., 7379 Route 32, Columbia, MD 21044 Amicon Division, W. R. Grace & Company-Conn., 17 Cherry Hill Drive, Danvers, MA 01923 2
A two stage process consisting of crossflow microfiltration to remove bacterial cells at an initial dry weight concentration of 10-12 g/l (average flux of 25 liters/m -hr) followed by ultrafiltration for ten-fold concentration of an extracellular protease product at an initial broth concentration of 0.3-0.6 g/l (average flux 40-50 liters/m -hr) demonstrated consistently high recovery yield (>90%) of enzyme from 100 liter fermentation broths. High protease product yield (>90%) in the cell separation step, which involved transmission of the enzyme through the microfiltration membrane, was achieved only under conditions of low transmembrane pressure ( 1 meter/sec). To maintain the required low transmembrane pressure with high recirculation rate, it was necessary to pressurize the permeate chambers of the hollow fiber cartridges used for the cell separation step. In addition, use of a pump on the permeate outlet to maintain a constant permeate flow rate during the run resulted in increased flux performance and stability, while keeping transmembrane pressure low. For the subsequent enzyme concentration step, a regenerated cellulose spiral ultrafilter achieved 100% recovery of the protease. Economic analysis of the cell separation step indicates that the membrane process is twice as cost effective as a centrifuge and equivalent to a precoat filter, on a basis of unit cost of enzyme product recovered. 2
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A t w o stage pilot scale membrane process was developed to recover an extracellular protease from a bacterial fermentation. This process was first tested i n the laboratory to establish an alternative to the use of a semicontinuous disk centrifuge w h i c h h a d been used i n o u r pilot plant to remove cells from the fermentor broth. Centrifugation p r o v e d to be 0097-6156/90/0419-0130$07.50/0 © 1990 American Chemical Society
Hamel et al.; Downstream Processing and Bioseparation ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
7. SHEEHAN ET AL.
Recovery of an Extracellular Bacterial Protease
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inefficient because of product yield losses, and because of the need to repeat centrifugation a n d filtration steps downstream to remove cells a n d other solids not removed i n the primary separation step. The data presented i n this paper are from 100 liter scale fermentation recovery runs i n our pilot plant w h i c h were carried out to test the feasibility and economics of using membranes to remove the cells, as well as to produce enzyme for use i n i n house research and development activities.
Downloaded by CORNELL UNIV on September 16, 2016 | http://pubs.acs.org Publication Date: January 24, 1990 | doi: 10.1021/bk-1990-0419.ch007
DEFINITIONS Discussion of crossflow membrane filtration requires d e f i n i t i o n of a number of specific terms to describe operating conditions. Crossflow or tangential f l o w refers to the principal direction of process flow relative to the membrane surface (see Figure 1). W h e n the f l u i d to be filtered flows tangential to the surface of the membrane, shear forces along the membrane mitigate fouling by sweeping retained species from the membrane surface. Tangential f l o w m a y be quantified either as recirculation rate, or as the average linear velocity through the retentate channel, or as wall shear rate at the membrane surface. The retentate side of the filter contains the fluid which does not pass across the membrane, while the permeate side contains the fluid which passes through the membrane's filtration barrier. For such systems, the pressure d r i v i n g force across the filter is quantified as the average transmembrane pressure (TMP), defined (Figure 1) as: A v g . T M P = [(Pi + P )/2] - Pper
Ν C
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Drain
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Centrifugal Feed Pump
MF Permeate Pump
Permeate
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Retentate MF Retentate
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From Fermentor
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