*4.1. Batch Reactors*

Batch reactors are simple and flexible in terms of manufacture. They can vary in size from small (mL) to large (m<sup>3</sup> ). Immobilised enzymes can be recovered and reused in batch reactors, while additional amounts of enzyme can be added if required [87,89]. Markle et al. developed a batch reactor in which deracemization, stereoinversion and the asymmetric synthesis of l-leucine were carried out by combining enzymatic oxidation using d-amino acid oxidase with an electrochemical reduction step [90]. Mazurenko et al. used membrane-bound (S)-mandelate dehydrogenase encapsulated in silica film on an electrode surface to prepare phenylglyoxylic acid from an (S)-mandelic acid in a batch reactor [91]. Ali et al. developed an electrochemical batch reactor for the regeneration of 1,4-NADH via platinum and nickel nano-particles deposited on the electrode surface [92]. While batch systems are in widespread use in industrial applications, the development of flow reactors has been the subject of significant research as outlined below [93].

#### *4.2. Flow Reactors*

The use of flow can increase rates of mass transfer with subsequent increases in the rates of reaction, decreasing the reaction time and making it feasible to perform reactions at a large-scale using relatively small scale equipment [89]. Kundu et al. constructed a packed microreactor to investigate the polymerization of polycaprolactone from ε-caprolactone in batch and continuous flow modes. Polymerization in continuous flow mode was faster than in batch mode and higher molecular mass polymers were obtained in continuous flow mode [94]. Using lipase, the time for the resolution of (±)-1,2-propanediol decreased significantly from 6 h (batch) to 7 min (flow) [95]. Using enzymes as a catalyst for the production of chemicals on a large scale is limited due to the requirement for high concentrations of enzyme, a limitation that does not necessarily apply to flow reactors [96]. For example, Cosgrove et al. used galactose oxidase to investigate the oxidation of lactose on a large scale in batch and flow systems. In the batch reactor rate of reaction of 0.74 mmol L−<sup>1</sup> h <sup>−</sup><sup>1</sup> was reported, a rate that increased 224-fold in the flow reactor (167 mmol L−<sup>1</sup> h −1 ). Moreover, the concentration of enzyme decreased considerably from 2.5 mg mL−<sup>1</sup> (batch reactor) to 0.5 mg mL−<sup>1</sup> (flow reactor) [96]. Flow reactors possess additional advantages over batch reactors such as better mixing and thermal control, improved stability and life time [23,97]. A range of studies have described the use of flow reactors to prepare intermediates for the preparation of pharmaceutical drugs [98], agrochemicals such as fluorinated and chlorinated organic compounds [99] and electronic materials such as iron silicide–carbon nanotubes used in Li-ion batteries [100,101]. A flow reactor is based on the principle of using a pump to pump the substrate into a reactor where a product is formed and is then pumped out of the reactor. Pumps can be divided into two groups, continuous and semi-continuous systems. Semi-continuous systems need to be refilled while continuous flow systems do not require refilling. Generally, reactors are fabricated from glass, polymers (e.g., polytetrafluoroethylene, polyfluoroacetate and polyether ether ketone), stainless steel and metals [23,102].

In general, enzyme immobilisation brings about a number of benefits. Lower amounts of enzyme are required, making the process more cost effective [103], while the removal of product from the reactor is simple and easy. In addition, immobilised enzymes show high stability, selectivity in comparison with free enzymes [23]. As described earlier, there are five general methods of immobilising enzymes. A number of challenges can arise when enzymes are immobilised in a microreactor. The enzymes need to be stable under conditions of fluid flow. In addition, during the immobilisation process, clogging and solid formation should be minimized [104]. Enzymes can be immobilised in flow reactors in a number of ways. Enzymes can be attached to beads, loaded into the reactor. This approach can result in high enzyme loadings but requires high back pressures [105]. The enzyme can be immobilised on the walls of reactors [89,106,107], attached to inorganic and polymeric monoliths in the channels [108–110] immobilised on a membrane (a method that is outside the scope of this paper) [111–114].

In order to select the optimal conditions for the immobilisation of an enzyme, the support needs to be selected. The type of support chosen will determine the immobilisation procedure [106]. The process of enzyme immobilisation should be cost-effective and technologically efficient. Flow reactors can be divided into three groups, packed-bed microreactors, monolithic microreactors and wall-coated microreactors (Figure 3).

**Figure 3.** A schematic of (**a**) packed-bed; (**b**) monolithic; and (**c**) wall-coated microreactors.
