Las concepciones de conflicto en el grupo de estudiantes de grado segundo

Millipore (MA., U.S.A.) have previously developed a scaleable filtration system in collaboration with Genentech (CA., U.S.A.) based on the theory that constant path length was the best way to scale effectively (van Reis et aZ., 1997a). The cartridges containing the membranes come in three sizes; 0.1 m^, 0.5 m^ and 2.5 m^. The larger membrane area between 0.1 to 0.5 m^ was achieved by using a wider membrane channel, whereas the 2.5 m^ cartridge was the same width as the 0.5 m^ cartridge but.

2. Materials and Methods D avies 2001

uses more channels through which the feed stream passes. The problems related to variable pressure drop across the cartridge, for different path lengths and the associated variations in flux and transmission were therefore avoided.

The cartridges come in different formats depending on the type of turbulence promoting screens that were employed. For the broth used here, the open channel V- Screen cartridge was used because of the low pressure drop. As shown in Section 3.2.3 the broth can get very viscous and the presence of mycelial clumps could block tighter channel designs. As in the Mini tan II experiments a hydrophillic Durapore

membrane was used with a pore size of 0.2 pm.

2.3.2.1. Pellicon Mini Rig

The Pellicon Mini cartridge holder is designed to take up to four Pellicon-2 0.1 m^ membrane cartridges. For these experiments however only one cartridge was used so that the maximum difference in scale could be achieved. The holder was connected to the Millipore Proflux M l2 self-contained microfiltration rig, which is represented diagrammatically in Figure 2.2. The system consisted of a peristaltic pump, three pressure transducers and a baffled reservoir. Cleaning was carried out as for the Minitan II rig (as described in Section 2.3.1) however the Pellicon Mini was flushed with 9 L of water prior to and after cleaning. The membrane was considered clean once the clean water flux test reached 80 % of the original value.

Flux was measured using a balance (model BB2400 Metler-Toledo Ltd., Leicester, U.K.) and stopwatch over 1 min intervals for 4 replicates after steady state permeate flux had been reached. Although the reservoir on the Proflux M12 was fitted with a baffle to aid mixing induced by the retentate flow, this was found to be inadequate

2. Materials and Methods D avies 2001

for such viscous broths. An impeller was thus added to the rig (as described in Section 2.3.1) to ensure thorough mixing of the feed stream. The process was run in concentration mode, and although the crossflow was not measured, the transcartridge pressure drop (TCP), an analogue of crossflow, was kept constant between the two ngs.

o-L-n

Balance

Drain

Figure 2.2. Diagrammatic representation of the Proflux M l2 rig in one pump format connected to the Pellicon Mini cassette holder. P I, P2 and P3 are the inlet, outlet and permeate pressure transducers respectively.

2.3.2.2. Pellicon Rig

As discussed previously the Pellicon holder can accept cartridges of area 0.5 m^ and upwards. The rig used to run the larger types of Pellicon cartridges is shown in Figure 2.3. The feed stream was re-circulated using a 0.75 h.p. 50 Hz Lafert motor (Venezia, Italy) and a Procon vane pump head controlled by an SKF Variable speed AC motor drive (SKF Automation Systems Ltd., U.K.). This gave a maximum flow

2. Materials and Methods D avies 2001

rate of approximately 15 L.min' . Flow was measured using a Altometer SC 80 AS magnetic flowmeter and converter (Sliederecht, The Netherlands). Flux was measured using the same equipment as used for the Minitan II experiments (Section 2.3.1). The experiments were carried out under exactly the same conditions as those conducted using the Pellicon Mini (Section 2.3.2.1). Maintaining the inlet, outlet and permeate pressure values identical to those used on the larger pumping rig leads to direct linear scale-up by scaling membrane area by process volume. Volumetric flow rates ranged from 0.48 to 3.84 L.min \ producing between 5,400 and 43,000 s'* shear rate at the membrane wall. The process was run in concentration mode and temperature was controlled at 21°C.

r—I x j

W

<5)

Figure 2.3. Diagrammatic representation of the Pellicon holder (0.5 m^ and upwards), P I, P2 and P3 are the inlet, outlet and permeate pressure gauges respectively and F is the flowmeter.

Cleaning was carried out by connecting the membrane holder to a model 605 Di Watson-Marlow peristaltic pump (Poole, U.K.). This was because the rig was made

2. Materials and Methods Davies 2001

of stainless steel and the cleaning solution (sodium hypochlorite) damaged the piping. Silicon tubing was used in the peristaltic pump and the membrane was cleaned by flushing with 40-50L of water (not returning the retentate to the reservoir) until the retentate was clear and then re-circulating warm (<50°C) sodium hypochlorite for 1 h. This was flushed out (40-50 L of water) and the membrane was rinsed with warm Redphos (<50°C) at 5 % v/v for 1 hour. Temperature was maintained using an MC 810 digital controller and a 500 W heater bar - BD 6933 Red Rod (Electrothermal Engineering Ltd., Essex U.K.). The membrane was then flushed with 40-50 L of RO water, if the clean water flux returned to 80 % that of the original clean water flux then cleaning was deemed successful, otherwise the process was repeated.