Front Matter

11.6.3 Membrane reactors

Membranes are thin barriers that can perform various degrees of separation using

differences in concentration, pressure, and electrical potential gradients between

the two compartments they separate. Membrane separations are achieved as a result

of combined actions among membrane, extractant, and molecules to be separated.

Membrane reactors are defined as the reaction systems in which membranes are

implemented. Membrane reactors have a very handyin situseparation capability

lacking in other types of reactors. Combining various membrane separations and

enzymatic reactions can generate many membrane reactor systems. The selective

removal of inhibitory byproducts better utilizes enzyme catalytic activity, and leads

to high productivity of the reactor. For example, in the lipase-catalyzed acidolysis

between medium-chain triacylglycerols (MCT) and long-chain polyunsaturated fatty

acids (PUFA), the released medium-chain fatty acids (MCFA) can be selectively

removed from the system, and this will change the equilibrium of the reversible

reaction and force the balance towards the products. The other characteristics of

membrane reactors include phase separation in the presence of solvent that makes

biphasic reactions possible and allows for an easy extraction; this has led to the study

of extractive bioconversions.

Many reports have been published regarding the application of membrane reactors

to different reaction systems, especially the applications in water phase, such as the

hydrolysis of protein, starch, cellulose, and other macromolecules. These applica-

tions have been reviewed (Chang and Furusaki, 1991; Prazeres and Cabral,

1994; Kragl, 1996). The application of membrane reactors in lipid processing is

still in its infancy, but many reports have focused on using membranes as the separa-

tion media (Snape and Nakajima, 1996). Membrane reactors for enzyme-catalyzed

lipid reaction systems have been mainly used in the hydrolysis of oils and fats (Hoq et

al., 1985; Prond et al., 1988; Taylor and Craig, 1991; Garcia et al., 1992; Goto et al.,

1992; Cuperus et al., 1993; Prazeres et al., 1993) and the esterification between

alcohol and fatty acids (van der Padt et al., 1990; 1992). In few cases of lipase-cat-

alyzed interesterification, membranes were used to retain the lipases when the reac-

tion mixture was circulated (Basheer et al., 1995c) or as the carrier for lipase im-

mobilization (Balcao and Malcata, 1998).In situseparations in the membrane-im-

plemented diffusion cell or in the ultrafiltration reactor have been performed during

the enzymatic acidolysis between MCT and EPA/DHA concentrates (X. Xu et al.,

unpublished results) and the membrane selectivity of MCFA over EPA or DHA has

been observed. A further incorporation of EPA and DHA into MCT in both mem-

brane reactors has been obtained beyond the reaction equilibrium level defined when

no membrane was installed (Xu et al., 2000).

Many primary factors affect the separations and unit operations used in biopro-

cessing, including molecule size, diffusivity, volatility, solubility, surface activity,

and hydrophobicity (Chang and Furusaki, 1991). Bioseparation can be successful

only if membrane, extractant, and molecules of interest are interacted in har-

mony. Ultrafiltration and reverse osmosis are modulated by pressure, whereas dia-

lysis and membrane extraction are carried out using the concentration differences. In

many cases membrane separations are based on a single separation. However, there

is a possibility of using two or more separation mechanisms for membrane separa-

11.6 Bioreactors for the lipase-catalyzed interesterification 207