Los viajes: voces del presente y el pasado

If the studied catalyst recovery and product concentration techniques are to be applied successfully in industry then the issues o f scale up must be addressed.

The scale-up o f a packed bed column has been described previously (section 5.1). The simplest and most accurate technique is to use design parameters measured on a small lab scale column (Collins, 1967), (Cooney, 1990). The increased scale for a new column can then be estimated by calculating the length o f equilibrium section (LES), and adding to this the length o f unused bed (LUB). Using this method the mass o f required resin to extract product from a given bioeonversion volume was calculated (Figure 7.1). B y applying, the linear flowrate o f 0.04 cms"^ as set out in chapter 5 it was also possible to estimate the process time for this operation.

For the product recovery and concentration o f lactone from a 500 m^ volume o f bioeonversion media then the volume o f resin required is 25.6 m^. This assumes the resin is used only once in a single packed colunm and takes a processing time

G eneral Discussion

o f 408 hrs (Figure 7.1). However, it has been shown that this resin can be recycled without loss in performance (section 5.3.3). A dramatic improvement in resin optimisation can be achieved by both reducing the volum e o f resin by recycling and reducing time by running multiple columns. In this way it is normal in industry to run a series o f columns in parallel with one set being loaded while another are eluted and regenerated, thus operating as a continuous system.

If this approach is applied to the product recovery process described for 500 m^ o f bioeonversion media it is possible to dramatically reduce the volume and process time. For example, the process may be operated as 8 columns with 5 recycles o f each, reducing the volume o f bed required to 1/5 o f the original. If the columns are then run as two lots o f four with one loading cycle and one elution and regeneration cycle then the processing time may be reduced to 1 0 1 hrs. This produces a greatly improved process in terms o f reduced resin cost and processing time.

In this manner the scaling o f the packed bed column can be achieved using the operating parameters established in the small-scale experiments performed in chapter 5. Issues o f pressure drop across the column are also important and for this reason packed columns are scaled by increasing diameter as opposed to length. This presents a further advantage in the operation o f multiple columns where shorter lengths can be applied, reducing the potential pressure drop across the system.

G eneral Discussion 50 0 3 0 0 0 0 2 5 0 0 0 4 0 0 20000 c w 2 15 0 0 0 200 w V) g 10000 100 5 0 0 0 - 50 0 3 0 0 4 0 0 0 100 200 F e rm e n te r vo lu m e (m )

Figure 7.1 Scale-up o f the packed bed adsorption using broth containing 4.5g/l o f product with single use o f the resin (calculated at 1% product breakthrough).

G eneral Discussion

In conventional solvent extraction several types o f contactor have been identified that have been reported as suitable for feeds containing emulsifying components (Liddell, 1994):

• Non-mechanical agitated: - Spray column

- Baffle plate column - Packed column • Mechanically agitated:

- Raining bucket contactor - Rotary film contactor - Centrifugal contactor

However, the use o f all o f these systems requires large volum es o f solvent and the use o f an expensive distillation step in order to recover and recycle the solvent. The application o f a technique that reduces the volume o f solvent and simplifies distillation is desirable.

In many bioconversions the product molecules tend to be o f a hydrophilic nature. This is especially the case in Baeyer-Villiger oxygenation where an oxygen atom is inserted into the substrate resulting in a more hydrophilic product. The use o f hydrophobic solvents that w ill form an immiscible layer is limited due to an extremely low partitioning o f the product into the solvent phase. This research has investigated the use o f an extraction technique that w ill allow continued recycling o f pure solvent and optimise the extraction process by maintaining maximum mass transfer rates. Small volumes o f solvent may be applied making the process safer in terms o f large-scale operation. The use o f reduced volumes o f solvents also allows concentration o f the product during extraction.

The liquid-liquid continuous extraction column has been applied to the concentration o f trace organic chemicals from large masses o f water (Umano et al, 1996). The work o f chapter 6 has highlighted that this system may also be successfully applied to the product recovery and concentration o f lactones from

G eneral Discussion

the Baeyer-Villiger monooxygenase reactions. Operation o f the liquid-liquid continuous extractor at scale has not been investigated. However, the system has been shown to handle crude bioconversion material without the formation o f emulsions. Improvement o f extraction times may be achieved by mixing the system. The application o f agitation must consider the formation o f an emulsion and level o f primary recovery required.