Processing options

This work has shown that the extraction and concentration o f lactone products from the bioconversion media may be achieved using several process options.

It has been shown that in the application o f packed bed adsorption to the recovery o f product then the composition o f the feed stream has limited effect on the adsorption process. The only significant issue is that o f catalyst removal that must be applied in order to avoid column fouling and an increase in the pressure drop across the column. The operation o f this concentration step is relatively simple as shown in Figure 7.2. The process time displayed represents that for the column extraction only at the small scale operated in these experiments (processing 400 ml bioconversion broth plus elution step).

The choice o f catalyst recovery step has no effect on the operation o f the packed bed column. Selection must be made on existing equipment and the requirement for the biocatalyst post recovery. For example, continuous centrifugation processes present a high shear force in the feed zone that results in the damage and rupture o f cells. This may be overcome by the operation o f a filtration step that produces higher broth clarity with reduced catalyst damage. The cost and performance o f these processes in catalyst recovery has been studied and in the selection o f a new system then the use o f filtration is favoured for these reasons (Meindersma et al, 1997). However, the benefits are not great enough to justify the replacement o f an existing centrifugation system to be utilised.

G eneral Discussion Bioconversion Centrifugation_or Microfiltration Packed bed adsorption Process time (3hrs)*

Figure 7.2 Purification using packed bed adsorption * Time excluding primary recovery step.

The selection o f the optimum process route using the liquid-liquid continuous extraction column presents several options. These may be selected depending on the time required for processing, overall process costs or equipment available. The major advantage to this approach is the ability o f these systems to handle particulate in the feed stream.

The simplest process operation is direct extraction from the bioconversion media (Figure 7.3, Route A). This process route requires no catalyst recovery in the extraction and concentration o f the product. However, due to the rapid formation o f emulsion if agitation o f the system is used, then only gentle m ixing can take place. Therefore, the process time is long, as the partition distribution o f the product into solvent can not be fully exploited. The consideration that no catalyst recovery step is used must be taken into account in this process. The process option w ill not be suitable if biocatalyst recycle is required as cell lysis and decay w ill occur during the extraction.

Process times may be dramatically reduced if the catalyst is recovered from the bioconversion by either centrifugation or microfiltration (Figure 7.3, Route B). Once the whole cell catalyst has been removed the extraction process may be agitated and thus improve the degree to which the partition coefficient may be exploited. The degree o f agitation is still restricted by the formation o f an

General Discussion

emulsion within the system. The choice o f catalyst recovery has no influence on the rate o f extraction.

The shortest process time for the concentration step is achieved when the bioconversion media is ultrafiltered at a 10 kD cut-off (Figure 7.3, Route C). This allows the process to be agitated strongly so that the partition coefficient o f product into the solvent may be almost fully exploited. In this case, no emulsion forms during processing. The cost o f implementing an ultrafiltration system into a new process is not significantly different to that for a microfiltration system and the benefits in reduced time may be significant (Meindersma et al, 1997).

A

Liquid-Liquid Continuous

Extraction Bioconversion

Long process time (8hrs 45mins)

Bioconversion Centrifugation or Microfiltration Liquid-Liquid Continuous Extraction

Reduced process time (2hrs 30mins)"

Bioconversion _{Ultrafiltration}

Liquid-Liquid Continuous

Extraction

Short process time (Ihr SOmins)*

Figure 7.3 Purification using liquid-liquid continuous extraction * Time excluding primary recovery step.

General Discussion

The Optimum extraction flow sheet for liquid-liquid continuous extraction is the use o f ultrafiltration for catalyst recovery. This provides a system that can be run with maximum agitation and with no emulsion formation. The catalyst may also be easily recycled from this system. The final process selection w ill depend on the equipment available, the overall process and the suitability o f this technique to scale-up.

A consideration to all o f these processing options is how robust they are to process changes in the upstream stages. The major consideration for the upstream process is that o f catalyst damage during the bioconversion. This w ill result in the production o f fines generated from cell debris, cell strip o ff etc. These may have a major impact on the process flow sheet with the requirement o f further catalyst recovery in order to avoid column fouling or increased emulsion formation and stability.

Studies were performed on all systems in order to assess the impact o f resuspended biocatalyst. In this case, the samples were homogenised to produce particle fines and a poorly clarified process stream. In all experiments performed using these streams, similar process effects were seen as for those used to generate the process flow sheets developed. The process operations studied here are considered very robust with no significant alteration in the process flow sheet required for an alteration in media or particle fines during the upstream process.

An alteration in process options w ill be seen if solvents are introduced into the bioconversion. In the case o f a two phase system then the product may be expected to be present in the solvent phase. Product separation is then achieved by decanting the solvent and separating the emulsion by gravitation or physical contact. This may then be easily extracted by the use o f distillation or evaporation.

However, a m iscible solvent may be used in order to improve substrate solubility. For example, the presence o f glycerol may improve the solubility o f substrate in the model reaction used here (Simpson et al, 2001). The use o f these solvents has been shown to have no effect on either the adsorption process or the

General Discussion

solvent extraction step and as a result limited effects are expected in these situations.