ENZYME REACTORSPunima Puri

CHOOSING SUITABLE ENZYME REACTOR

The choice of a reactor type depends on the form of enzyme (free or immobilized) to be used kinetics of reaction the scale of operation.  In an enzyme reactor, the highest specific enzyme

activity is desirable. 

It is considered an added bonus if the support that is used also aides in separation.

One approach is to use a molecular sieve as the support and pulse the reactor bed with the alternating passage of substrate solution and water

FOR AN INDUSTRIAL REACTOR

It is preferable to use supports that are

Non-biodegradable such as glass, silica, Celite, Bentonite, alumina, or titanium oxide, if possible.

Even the linkages between enzyme and support can be non-biodegradable, as they are in the case of titanium.

In some of these supports the physical nature of the surface becomes a major problem.

some supports that form excellent packed beds fail to do so when coated with enzyme.

Particles which ideally self-suspend in a fluid bed may form aggregates during use which will require more power to pump through substrate.

Problems were encountered using porous glass supports even that the glass itself could dissolve.

This problem can be been eliminated by treatment of the glass surface with zirconium.

.  The result is that bands of unused substrate and product progress down the column.

It so happens that the enzymes for which this technique would be useful are also those which in some cases benefit in having the enzyme immobilized on a porous support.

ENZYME REACTORS A vessel employed to carry out the desired

conversion using an enzyme is called enzyme reactor.

Several different types of reactors are available,

The different types of reactors used for enzyme-mediated .conversions are as follows:

(i) stirred tank reactors, 

(ii) membrane reactors, 

(iii) continuous flow reactors, e.g.,

(a) packed bed reactors,

BATCH REACTOR  It is the simplest type of reactor.

The immobilized enzyme is placed in a container with the reactants, and the reaction is allowed to proceed until the desired level of conversion is reached.

Some stirring or agitation of reaction mixture is also required.

Many modifications of these reactors have been designed to simplify recovery and reuse of the enzyme composite. 

STIRRED TANK REACTORS  These reactors are simple consist of a tank containing a stirrer and,

usually, fixed baffles to improve mixing. These reactors are used in batch mode, and

free enzymes can be employed. Efficient arrangements are provided to

maintain temperature, pH, etc. of the reaction mix.

BATCH REACTORS MAY INCLUDE:

Stirred Tank for Soluble Enzymes Stirred Tank for Immobilized Enzymes Stirred Tank with Immobilized Enzyme

Basket Paddles Stirred Tank with Immobilized Enzyme

Basket Baffles Total Recycle Packed Bed Reactor Total Recycle Fluidized Bed Reactor

BATCH STR   A batch stirred tank reactor is the

simplest type of reactor.  It is composed of a reactor and a mixer such as a stirrer, a turbine wing or a propeller. The batch stirred tank reactor is illustrated below: 

       

This reactor is useful for substrate solutions of high viscosity and for immobilized enzymes with relatively low activity.

However, a problem that arises is that an immobilized enzyme tends to decompose upon physical stirring.

The batch system is generally suitable for the production of rather small amounts of chemicals.

BATCH REACTOR

CONTINUOUS REACTORS MAY INCLUDE:

Stirred Tank Reactor with Filtration Recovery Stirred Tank Reactor with Settling Tank Recovery Stirred Tank Reactor with Immobilized Enzyme

Basket Paddles Stirred Tank Reactor with Ultra filtration

Recovery Packed Bed Reactor Packed bed with recycle Flat Bed Reactor Filter Bed Reactor Fluidized Bed Reactor, Membrane Reactor using hollow fibers

CSTR AND PACKED BED REACTOR

CSTR

FIXED BED REACTORS

 These are most widely used for large scale commercial operations. Different companies use these reactors for enzymes like 

(i) aminocylase, (ii) glucose isomerase and (iii) lactase.These reactors will keep on dominating the large scale commercial application due to their (i) high efficiency and (ii) ease and simplicity of operation.There are many types of fixed bed reactors including those with packed bed of particular material to which enzyme is coupled.

FLUIDIZED-BED REACTOR

 In these reactors, enzyme system is fluidized by the upward flowing of substrate solution.

This helps in eliminating any plugging of enzyme system, although small duration of contact may be insufficient for the desired conversion.

This can be overcome by decreasing velocity of solution by different methods.

Glucose isomerase and lactase have been shown to double their efficiency due to fluidization when compared with fixed beds.

FLUIDIZED BED REACTOR

INDUSTRIAL SCALE FBR

PACKED BED REACTOR

ADVANTAGES  A continuous packed bed reactor has the

following advantages over a batch packed bed reactor:

Easy, automatic control and operation Reduction of labor costs Stabilization of operating conditions Easy quality control of products

CONTINUOUS PACKED BED REACTORS  most widely used reactors for

immobilized enzymes and immobilized microbial cells.

In these systems, it is necessary to consider the pressure drop across the packed bed or column, and the effect of the column dimensions on the reaction rate.

There are three substrate flow possibilities in a packed bed and they are illustrated below:

Downward flow method Upward flow method Recycling method

MEMBRANE REACTORS

A membrane reactor uses a membrane, e.g., dialysis membrane, to contain the

enzyme in a chamber into which the substrate moves and the product moves out.

Generally, membrane reactors use a hollow fiber of 200 /lm diameter with 50 /lm thick membranes. 

Each reactor contains hundreds of such fibers into which the enzyme is retained; usually these reactors use soluble enzymes.

The substrate is kept in the main chamber of the reactor.

These reactors can also be used in a continuous mode and the substrate flow rate is adjusted to achieve the desired level of reaction.

ADVANTAGES AND LIMITATIONS

(i) easy to establish, 

(ii) permit the use of more than one enzyme to catalyze a chain of reactions,

(iii) allow easy replacement of enzymes, and 

(iv) are useful in producing small scale (g to kg) quantities. 

The chief limitations of these systems are:

(i) regular replacement of membranes adds to cost  

(ii) the need for substrate diffusion through the membrane often limits its applications

MEMBRANE FILTER REACTORS

COMBINED CSTR/UF REACTOR A combined CSTR/UF reactor is a

combination of a continuously stirred tank reactor and an Ultra Filtration Unit.

This type of reactor begins as a typical CSTR.

However, the product passes through an Ultra Filtration Unit where the enzyme is removed and recycled back into the reactor.

CSTR WITH UF

In a combined CSTR/UF reactor the enzymes are immobilized in that they can not leave the reactor because of the filtration unit.

This allows continuous processing with free enzymes in the CSTR.

The Ultra Filtration Unit contains a membrane which provides a semipermeable barrier which allows products and unreacted substrate, if there is any, to pass through while holding back the enzyme.

There are other possibilities for similar reactors, such as combined reactor-separators.

However, the combined CSTR/UF reactor has proven useful for several types of reactions where a typical immobilized enzyme would not be as effective.

One example of this is for the conversion of benzylpenicillin to 6-aminopenicillanic acid by penicillin amidase.

RECYCLE REACTOR A recycle reactor is a reactor that is not

seen very often, but is very important to consider when studying immobilized enzymes.

It is very important to chemical engineering because it allows some substrates to be processed, which could not be processed using other reactor types. 

The recycling method is advantageous when the linear velocity of the substrate solution affects the reaction flow rate.

This is because the recycling method allows the substrate solution to be passed through the column at a desired velocity.

For industrial applications, upward flow is generally preferred over downward flow because it does not compress the beds in enzyme columns as downward flow does.

When gas is produced during an enzyme reaction, upward flow is preferred.

In a recycle reactor a portion of the product stream is recycled and mixed with the inlet flow to the reactor

If the entire product stream is recycled back to the inlet stream, then it is called a total recycle reactor.

This can obviously only be used in a batch process, because if the entire product stream is recycled back into the reactor in a continuous reactor, the volume of the reactor would increase to infinity. 

This type of rector is used when you have a substrate that cannot be completely processed on a single pass, such as with an insoluble substrate.

These reactors continue to move the same substrate through the reactor so that the effective contact time is high enough to allow the substrate to be processed.

Recycle reactors also allow the reactor to operate at high fluid velocities.

This is important because it minimizes the bulk mass transfer resistance to the transport of the substrate.

A recycle reactor is simply a reactor, such as a CSTR or fluidized-bed reactor, with a recycle stream.       .

ULTRAFILTRATION MEMBRANE DEVICES

This device is suitable for a substrate of high molecular weight and a product of low molecular weight.

The enzyme used here is soluble, no improvement in the stability of the enzyme can be expected.

A hollow fiber device can also be used and its characteristics are essentially the same as those of an ultrafiltration membrane