Biomedical Engineering Reference
In-Depth Information
These reactors can also be classified according to the different characteristics
of membrane material, such as the nature, structure, shape, and so on, giving the
enzyme-membrane reactor the following characteristics: (1) Continuous operation
makes it possible to greatly limit the use of enzymes, which not only can improve
the yield but also reduce costs. (2) The products can be continuously separated
from the reaction system, resulting in improved reaction speed in the enzyme-
membrane reactor and the conversion rate of the substrate. (3) The process is
without emulsification, so the reaction system avoids enzyme inactivation and power
caused by high-speed stirring emulsification. (4) Convection diffusion replaces the
molecular diffusion way of mass transfer.
The enzyme-membrane reactor also has its drawbacks. For example, the deac-
tivation of the catalyst requires additional fresh enzyme at appropriate times. The
reduction of reactor efficiency caused by the decrease of transfer mass efficiency
makes improvement of operating conditions necessary to effectively control the
concentration differential polarization and blockage of membrane.
When choosing the film in the design of an enzyme-membrane reactor, the size
of the enzyme molecules should be considered, as should the solution characteristics
of the substrate and the product and the characteristics of the film itself, such
as materials, flux, rejection rate, and other factors. The ideal film has a product
rejection coefficient of zero, ensuring that the product can be completely penetrated
through; the rejection coefficient of the enzyme should be 100 % to ensure that
the enzyme is completely retained in the reaction system. But, it does not mean that
when the substance molecules are smaller than the pores of membrane the molecules
must not be trapped. In addition to the size factor, although some molecules are
slightly smaller than the pores of membrane material, there may be electrostatic
repulsion. And, membrane chemistry can affect solute permeability because of
nonspecific interactions (electrostatic, hydrophobic, and hydrophilic interactions),
causing an additional layer (gel layer) to be formed on the surface of solute to reduce
the permeability (the concentration polarization phenomenon). When the separation
of enzyme and chemical reaction occur, the enzyme concentration polarization is
particularly evident.
A UF membrane is used most often in cellulose hydrolysis. Generally, the
molecular weight cutoff (MWCO) of the UF membrane is between 500 Da and
100,000 Da, and the size of the cellulase is between 40,000 and 60,000 Da. So, a
membrane whose MWCO is nearly 10,000 Da is usually chosen.
Membrane fouling and cleaning in the use process require attention. To reduce
film pollution, film material should be selected that has less adsorption on cellulase,
such as polysulfone, polyethersulfone, and the like. Yang et al. took a polyethersul-
fone membrane with a molecular weight of 10,000 Da as the research object, and
they studied the effects of membrane fouling. This showed that under the proper
cleaning conditions, the membrane flux of pure water recoverable rate could still be
90-95 % after use for more than 6 months.
Enzymatic hydrolysis coupled with membrane separation technology could dis-
charge the suppression of the product completely, improving enzymatic hydrolysis
efficiency and yield by isolating online small molecules obtained by enzymatic
