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TYPES OF BIOREACTORS

A Bioreactor refers to any manufactured device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out that involves organisms or biochemically active substances derived from such organisms. A typical bioreactor consists of an agitator used for the mixing of the contents of the reactor which keeps the cells in the perfect homogenous condition for better transport of nutrients and oxygen to the desired product. It has a Baffle used to break the vortex formation in the vessel which is usually highly undesirable as it changes the center of gravity of the system and consumes additional power. In aerobic cultivation process, a Sparger is used to supply adequate oxygen to the growing cells. It consists of a jacket that provides annular area for circulation of constant temperature of water which keeps the temperature of the bioreactor at a constant value.

Continuous Stirred Tank Bioreactor

Continuous Stirred Tank Bioreactor

The continuous stirred tank bioreactor consists of a cylindrical vessel with motor driven central shaft that supports one or more agitators. The continuous stirred tank bioreactor is also known as a mixed flow bioreactor which is a common model for a chemical reactor in chemical engineering and environmental engineering. It often refers to a model used to estimate the key unit operation variables when using a continuous stirred tank bioreactor to reach a specified output. The mathematical model works for all fluids: liquids, gases and Slurry. The number of impellers is variable and depends on the size of the bioreactor that is height to diameter ratio, referred to as aspect ratio. The aspect ratio of a stirred tank bioreactor is usually between 3-5. However, for animal cell culture applications, the aspect ratio s usually 1/3rd of the vessel diameter. The shaft is fitted at the bottom of the tank. The distance between two impellers is approximately 1.2 impeller diameter. Different types of impellers like rustom disc, concave bladed, marine Propeller are used. In Stirred tank bioreactors, the air is added to the culture medium under pressure through a device called sparger. The sparger may be a ring with many holes or a tube with single orifice. The sparger along with Impeller enables better gas distribution system throughout the vessel. The bubbles generated by sparger are broken down to smaller ones by impellers and dispersed throughout the medium. This enables the creation of a uniform and homogenous environment throughout the bioreactor.

Basic features of continuous stirred tank bioreactor

An Agitator system

• The function of agitation system is to provide good mixing and thus increase mass transfer rates through the bulk liquid and bubble boundary layers.
• It consists of an Agitator and the baffles.
• The baffles are used to break the liquid flow to increase turbulence and mixing efficiency.
• The agitator consists of an impeller attached to a long rod called shaft. The number of impellers will depend on the height of the liquid in the reactor.

Oxygen delivery system

• A compressor forces the air into the reactor. The compressor will need to generate sufficient pressure to force the air through the filter.
• Sterilization of the inlet air is undertaken to prevent contaminating organisms from entering the reactor.
• The exit air on the other hand is sterilized not only to keep contaminants from entering but also to prevent organisms in the reactor from contaminating the air.
• A disk shaped hydrophobic Teflon membranes of polypropylene are used.

Foam control system

• Excessive Foam formation lead to blocked air exit filters and to build up pressure in the reactor.
• Foam is typically controlled with aid if anti foaming agents based on silicone or on vegetable oils.
• Excessive anti foam addition can result in poor oxygen transfer rates.

Temperature control system

• It consists of temperature probes and heat transfer system.
• The heat transfer system will use a jacket to transfer heat in or out of the reactor.
• The jacket is a shell which surrounds part of the reactor.
• The liquid in the jacket does not come in direct contact with the fermentation fluid.

pH control system

• The pH control system consists of a pH Probe, an alkali delivery system and acid delivery system.
• The neutralizing agents used to control pH should be non corrosive and non toxic to cells when diluted in the medium.
• Hydrochloric acid should never be used as it is corrosive even to stainless steel.

Cleaning and sterilization facilities

• Small scale reactors are taken apart and then cleaned before being re-assembled, filled and then sterilized in an autoclave.
• It involves the cleaning of not only the fermentor but also all lines linked to the internal components of the reactor.
• Steam, cleaning and sterilizing chemicals, spray balls and high pressure pumps are used in these processes.

Applications

Continuous stirred tank bioreactors facilitate rapid dilution of reagents through mixing. Therefore, for non zero order reactions, the low concentration of reagent in the reactor means a CSTR will be less efficient at removing the reagent. However, one of the added benefits of dilution is the ability to neutralize shocks to the system.

Environmental engineering

• Activated Sludge process for waste water treatment.
• Lagoon treatment systems for natural waste water treatment.
• Anaerobic digesters for the stabilization of waste water biosolids.

Chemical engineering

• Loop reactor for pharmaceutical production.
• Fermentation.
• Biogas production.

Bubble Column Bioreactor

Bubble column Bioreactor

A bubble column bioreactor is an apparatus used to generate and control gas-liquid chemical reactions. It consists of a vertically-arranged cylindrical column filled with liquid, at the bottom of which gas is inserted. The introduction of gas takes place at the bottom of the column and causes a turbulent stream to enable an optimum gas exchange. Numerous forms of construction exist. The mixing is done by the gas sparging and it requires less energy than mechanical stirring. The liquid can be in parallel flow or counter-current. Bubble column reactors are characterized by a high liquid content and a moderate phase boundary surface. The bubble column is particularly useful in reactions where the gas-liquid reaction is slow in relation to the absorption rate.Bubble column reactors are used in various types of chemical reactions like wet oxidation, or as algae bioreactor Since the computerized design of bubble columns is restricted to a few partial processes, experience in the choice of a particular type column still plays an important role.

Applications

• Bubble column bioreactors are used in the production of Baker's yeast, beer and Vinegar.
• They are also used in aeration and treatment of waste water.
• In bubble columns, the hydrodynamics and mass transfer depend on the size of the bubbles and how they are released from the sparger.

Airlift Bioreactors

Airlift bioreactor

Airlift bioreactors are the reactors used for cell culturing, pallet form fermentation and immobilized enzyme reactions.Typically, airlift bioreactors are used when the desired reactants or final products are in a gaseous state and for aerobic cell cultures. It works by agitating the contents of the bioreactor using gas. The gas used for agitation can act to either introduce new molecules to the mixture or remove specific metabolic molecules produced by microorganisms. They have a built in bubble column designed to release gas into the bioreactor. Gas is usually injected into the bubble column at the bottom of the reactor. Mixing occurs as the bubbles rise through the bubble column to the top of the bioreactor. The pattern of fluid circulation inside the bioreactor can be customized through the design of it's bubble column and shape. There are two separate channels within an airlift bioreactor; one for gas/liquid up-flow and one channel for gas/liquid down-flow. Both channels create a closed circuit, and has a mechanism for removing gaseous substances at the top of the bioreactor called the gas separator.

Types of Airlift Bioreactors

Internal-loop Airlift bioreactor

It has a single container with a central draft tube that creates interior liquid circulation channels. These bioreactors are simple in design, with volume and circulation at a fixed rate for fermentation.

External-loop Airlift bioreactor

It possesses an external loop so that the liquid circulates through separate channels. These reactors can be suitably modified to suit the requirements of the different fermentations. In general, the airlift bioreactors are more efficient than bubble columns, particularly for more denser suspensions of microorganisms.

Applications

• Airlift bioreactors are commonly employed for aerobic bioprocessing technology.
• They ensure a controlled liquid flow in a recycle system by pumping.
• Due to high efficiency, airlift bioreactors are sometimes preferred in methanol production, waste water treatment.

Fluidized Bed Bioreactors

Fluidized Bed Reactor

A fluidized bed bioreactor is a type of reactor device that can be used to carry out variety of multiphase chemical reactions. In this type of reactor, a fluid is passed through a solid granular material at high enough speeds to suspend the solid and cause it to behave as though it were a fluid. The solid substrate (the catalytic material upon which chemical species react) material in the fluidized bed reactor is typically supported by a plate, known as a distributor. The fluid is then forced through the distributor up through the solid material. At lower fluid velocities, the solids remain in place as the fluid passes through the voids in the material. As the fluid velocity is increased, the reactor will reach a stage where the force of the fluid on the solids is enough to balance the weight of the solid material. This stage is known as incipient fluidization and occurs at this minimum fluidization velocity. Once this minimum velocity is surpassed, the contents of the reactor bed begin to expand and swirl around much like an agitated tank or boiling pot of water. The reactor is now a fluidized bed. Depending on the operating conditions and properties of solid phase various flow regimes can be observed in this reactor.

Advantages

• Uniform particle mixing.
• Uniform temperature gradients.
• Ability to operate reactor in continuous state.

Disadvantages

• Increased reactor vessel size.
• Pumping requirements and pressure drop.
• Lack of current understanding.
• Erosion of internal components.
• Pressure loss situations.

Packed Bed Bioreactors

A Packed bed reactor is very versatile and used in many chemical processing applications such as absorption, distillation, stripping, separation process and catalytic reactions. Across the diverse applications in which they are used, the physical dimensions of the beds vary greatly. Typical reactors consist of a chamber such as tube or channel that contains a catalyst particles or pellets and a liquid that flows through the catalyst. The liquid interacts with the catalyst across the length of the tube altering the chemical composition of the substance.

Design of a Packed Bed Bioreactor

When designing a packed bed reactor, the design must include mass transfer (or species transport) in the bed as well as heat transfer and chemical reactions. Understanding and optimizing the heat transfer through packed beds is important in order to decrease the cost of running the equipment. The packed catalyst is also critically important to the successful modeling of the device; the catalyst can be modeled as a porous structure, which leads to particle transport with different orders of magnitude, making the analysis of mass and energy transport a challenging task. Another challenge when designing these devices lies in the pressure drop that occurs across the length of the reactor. The pressure drop can be reduced by using larger catalyst particles, but this causes lower intraparticle diffusion, making the reaction progress slower. The trade-off here is to find a particle size that is large enough to limit the pressure drop and small enough to allow the reaction to proceed at a fast enough rate. A catalyst particle radius is typically in the order of magnitude of 1 millimeter. The space located between particles is described as macroporous structure of the bed, while pores inside the catalyst themselves form what is known as the microstructure.

Applications

• Synthesis of gas production.
• Methanol synthesis.
• Fischer-Tropsch synthesis.
• Valorization of food.
• To enhance the contact between vapour and liquid instead of trays packing.

Photobioreactors

Photobioreactor

A Photobioreactor is a device that utilizes a light source to cultivate phototropic organisms. These organisms use photosynthesis to generate biomass from light and carbondioxide and include plants, mosses, algae and purple bacteria. Within the artificial environment of a photobioreactor, specific conditions are carefully controlled for respective species. Thus, a photobioreactor allows much higher growth rates and purity levels than anywhere in nature.

Open systems

The first approach for the controlled production of phototrophic organisms was a natural open pond or artificial raceway pond. Therein, the culture suspension, which contains all necessary nutrients and carbon dioxide, is pumped around in a cycle, being directly illuminated from sunlight via the liquid’s surface. This construction principle is the simplest way of production for phototrophic organisms. But due to their depth (up to 0.3 m) and the related reduced average light supply, open systems only reach limited areal productivity rates. In addition, the consumption of energy is relatively high, as high amounts of water containing low product concentration have to be processed. Open space is expensive in areas with a dense population, while water is rare in others. Using open technologies causes high losses of water due to evaporation into the atmosphere.

Closed systems

Since the 1950s several approaches have been conducted to develop closed systems, which theoretically provide higher cell densities of phototrophic organisms and therefore a lower demand of water to be pumped than open systems. In addition, closed construction avoids system-related water losses and the risk of contamination through landing water birds or dust is minimized. All modern photobioreactors have tried to balance between a thin layer of culture suspension, optimized light application, low pumping energy consumption, and microbial purity. Many different systems have been tested, but only a few approaches were able to perform at an industrial scale.

Types of Photobioreactors

Plate photobioreactor

Plate photobioreactor

Another development approach can be seen with the construction based on plastic or glass plates. Plates with different technical design are mounted to form a small layer of culture suspension, which provides an optimized light supply. In addition, the simpler construction compared to tubular reactors allows the use of less expensive plastic materials. From the pool of different concepts e.g. meandering flow designs or bottom gassed systems have been realized and shown good output results. Some unsolved issues are material life time stability or the biofilm forming. Applications at industrial scale are limited by the scalability of plate systems.

Horizontal Photobioeactor

Horizontal Photobioreactor

This photobioreactor type consists of a plate-shaped basic geometry with peaks and valleys arranged in regular distance. This geometry causes the distribution of incident light over a larger surface which corresponds to a dilution effect. This also helps solving a basic problem in phototrophic cultivation, because most microalgae species react sensitively to high light intensities. Most microalgae experience light saturation already at light intensities, ranging substantially below the maximum daylight intensity of approximately 2000 W/m². Simultaneously, a larger light quantity can be exploited in order to improve photoconversion efficiency. The mixing is accomplished by a rotary pump, which causes a cylindrical rotation of the culture broth. In contrast to vertical designs, horizontal reactors contain only thin layers of media with a correspondingly low hydrodynamic pressure. This has a positive impact on the necessary energy input and reduces material costs at the same time.

Foil Photobioreactor

The pressure of market prices has led the development of foil-based photobioreactor types. Inexpensive PVC or PE foils are mounted to form bags or vessels which cover algae suspensions and expose them to light. The pricing ranges of photobioreactor types have been enlarged with the foil systems. It has to be kept in mind, that these systems have a limited sustainability as the foils have to be replaced from time to time. For full balances, the investment for required support systems has to be calculated as well.

Porous Substrate Bioreactor

Porous Stable Bioreactor is being developed at University of Colgne, also known as twin-layer system, uses a new principle to separate algae from a nutrient solution by means of a porous reactor surface on which the microalgae are trapped in biofilms. This new procedure reduces by a factor of up to one hundred the amount of liquid needed for operation compared to the current technology, which cultivates algae in suspensions.

Applications

Photobioreactors are used in commercial production of;

• Pigments
• Fatty Acids
• Cosmetics
• Bio-Fuels

Conclusion

High productivity, high product yield and high product concentration are the major objectives of plant tissue process development. A variety of bioreactor types providing growth and expression of bioactive substances are available today for plant cell and tissue cultures. Low biomass and product level can be achieved in any type of bioreactor. However, an improved understanding of the manifold interactions between bioreactor types will enhance and sustain high productivity and also reduce the process costs.