Biochemical engineering

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INTRODUCTION

The 20th century has been considered as the age of biotechnology. Though it may be a relatively new term, biotechnological processes are a culmination of more than 8000 years of human understanding involving the use of organisms and fermentation processes have been used to make products such as beer, cheese, bread among many others. Today biotechnology has also been incorporated into manufacturing processes of health care, food and agriculture and many other applications. Biotechnology has been commonly defined as an application of scientific and engineering principles which can be used for processing of materials using biological agents in order to supply with products and services. Biological agents can be anything from cells, enzymes, microorganisms, plant and animal cells.

Implementation of biotechnological processes on a commercial scale is generally done in some form of bioreactor or fermenter for the production of enzymes, antibodies, vaccines, food or any other kind of product useful for mankind. The type of bioreactor used for this process is very important and is based on the type of product to be produced. Many different bioreactors are available such as (i)Stirred Fermenter (ii) Air-Lift Fermenter (iii) Fluidised Bed Bioreactor (iv) Packed Bed Bioreactor (v) Bubble Column Fermenter. Out of the following Stirred and Air-lift Fermenters are the most important having drastic differences in the mixing and agitation of the subtrates. The relative merits and demerits of using the two different systems are a point of interest for the understanding of which method to use for different products.

FERMENTERS/BIOREACTORS

Bioreactors can be described as a system in which a biological process takes place and is effected. They are basically mechanical vessels in which organisms are cultivated in a controlled and regulated manner, and also where materials are transformed by specific reactions. Bioreactors are specifically made to influence metabolic pathways of microorganisms. Though the term bioreactor is commonly used synonymously with fermenter in the strictest definition a fermenter is a system which effects the conversion of sugar to alcohol.

The products of bioreactions can be formed by three processes-

  • In which the cells produce a product which is extracellular eg. Alcohols or intracellular eg. Enzymes.
  • In which a cell mass is formed eg. Baker's yeast.
  • Biotransformation eg. Antibiotics

DESIGN AND OPERATION OF A FERMENTER (BIOREACTOR)

The main goal of a bioreactor is to control, contain and influence biological reaction in the positive direction. Two important parameters have to be considered in the design of the bioreactor. The first one includes parameters for the chemical, biological and the physical system. The physical or the macrokinetic system includes metabolite production and microbial growth. The second area to be considered includes several parameters such as temperature control, optimum pH, enough substrate, availability of water, salts, vitamins, oxygen, removal of product and by product. Apart from controlling the following parameters the bioreactor must also be able to promote formation of the microorganism and also be able to eliminate contamination of the organism.

CHEMICAL ENGINEERING ASPECTS OF FERMENTER DESIGN

MASS TRANSFER

A fundamental concept while designing bioreactors is the transfer of mass. This process deals with two aspects, firstly the distribution of the substrate and product molecules within the fluid and the secondly the transfer between the microbial cells and fluid bulk. The first aspect is generally dependant on the force of turbulence and convection produced by the agitation and the flow of gas while the second aspect is governed by diffusional forces. There doesn't have to be uniform concentration of an of the components in the fluid though it should not fall below or rise above the level which allows maximum microbial activity.

In cultures which are actively metabolising the concentration of any substrate in the bulk of the fluid as suspended particles, drops or bubbles will be below the equilibrium concentration in a sterile medium. Hence a steady state concentration will be present and can be found out by the balance between supply and demand.

HEAT TRANSFER

There is generation of heat in submerged microbial systems, mainly due to the metabolic activity of various microorganisms and in some cases as a result of mechanical work due to the agitator and by the gas bubbles while they expand moving from the sparger to the space above the culture.

Heat transfer between the microorganisms and the vessel is pretty much similar to that of mass transfer and involves interchange between cells and the fluid and the interchange between the fluid and the cooling surfaces. Similar to in mass transfer the temperature in the fluid is based on a dynamic balance between the heat evolution by cells and agitator and removal of heat by the vessel walls.

Heat transfer differs from mass transfer in one important respect. Mass transfer happens throughout the dispersion where as heat interchange is well defined and has fixed boundaries, and hence the hydrodynamic effects are better defined.

MIXING EFFECTS

Mixing helps to reduce variations in temperature and concentration. It is basically due to the random redistribution of components in the culture suspension at the impeller and also from interactions with the suspension bulk or by streams leaving the impeller. In most cases the rate of mixing is generally direct to the function of mass flow rate at the impeller. Circulation rate can be defined as "number of times a volume of suspension the same as the total volume passes through the impeller zone in unit time".

SCALING UP

Regardless of which criteria is used for scaling up it is practically impossible to create conditions which will give results in all scales of operations. Important criteria while scaling up are power input per unit volume, geometry and superficial air velocity. The importance of the superficial air velocity is to reduce air flow based on volume as the scale increases. This factor becomes important only if the overall rate of supply is able to meet the oxygen supply, purging requirements, as well as maintaining the necessary rates of mass transfer. This criterion is probably most dependent on the impeller. It may also be seen that the efficiency of oxygen utilization may be greater in deeper vessels mainly due to the fact that the increase in contact time between the nutrient medium and the bubbles.

TYPES OF BIOREACTORS

STIRRED TANK FERMENTER

In the 1940's microbial fermentations gained popularity mainly due to the production of antibiotics. Stirred tank fermenters were the vessel of choice for many of these processes. Functions of these fermenters are - homogenisation, suspension of solids, dispersion of gas liquid mixtures, heat exchange and aeration of liquid. These fermenters have a baffle and a rotating stirrer which is attached to either side either at the top or the bottom. These bioreactors are being used commonly in industrial processes though advancements have been made in bioreactor technology, these are sometimes preffered due to some characteristics.

AIR LIFT FERMENTER

These are fermenters which do not use any mechanical stirring for mixing and are generally known as pneumatic reactors. Adequate mixing of the liquid is brought about by the turbulence of the fluid flow. In the central section of the reactor is a draft tube. Circulatory flow int he reactor is caused by the upward motion of the introduces fluid (generally air/liquid). Air lift fermenters have several advantages which will be discussed in a later section.

FLUIDISED BED BIOREACTOR(FBB)

Fluidised bed reactors have increased in popularity in recent years due to their advantages over other reactors. Most of the fluidised bed bioreactors are three phase systems. These are operated in co-current upflow in which the liquid acts like the continuous phase.a big difference between the FBB and the air lift bioreactor is the presence of a physical draft tube inside which provides aerating and non aerating zones. The productivity produced in FBB is greater than in stirred or packed bed bioreactors. Many successful applications of bioprocesses are used using the FBB.

PACKED BED BIOREACTOR

Packed bed bioreactors are also known as fixed bed bioreactors are useful in wastewater treatment using biofilms. The concept of immobilisation increased popularity of this kind of bioreactor. The biocatalyst is immobilised and packed in a column and nutrients are fed from either the top or the bottom. These bioreactors are generally used when the substrate inhibition governs the rate of the reaction.

BUBBLE COLUMN FERMENTER

These are the simplest form of tower fermenters which consists of a tube through which air is sparged at the base. It is basically an elongated mechanical fermenter with an aspect ratio of 6:1. An important example has been in the production of citric acid.

STIRRED AND AIR LIFT BIOREACTORS: COMPARISION AND CONTRAST

Air Lift Bioreactors (ALB's) are a relatively new system of bioreactors which provide with certain advantages over the traditional system namely Stirred (mechanical) bioreactors especially for plant and animal cell culture.

In ALB's the contents are agitated pneumatically by some gas or air in some cases. Apart from agitating the contents the air also helps in mediating exchange between the medium and the gas phase, oxygen is provided to the liquid and also in some cases the metabolic products are removed. The bioreactor design determines fluid circulation by providing with channels for gas/liquid upflow(riser) as well as a channel for downflow. Both channels are linked at the top providing with a closed loop, the gas being injected at the bottom of the riser.

ALB's are of different types based on their structure.(i) external loop vessels are those in which the circulation takes place in different and distinct conduits. (ii) baffled vessels are those in which there is addition of additional baffles helping in the circulation. All ALB's regardless of configuration consist of 4 different parts having different flow characteristics-

  • Riser: gas is injected at the base which flows upwards
  • Downcomer: present in parallel to the riser connected at the bottom having downward flow. Driving force as a result of difference in mean density between the downcomer and the riser.
  • Base: it's the connection zone between the riser and the downcomer. Can influence gas holdup, liquid velocity and solidphase flow.
  • Gas separator: present at the top of the bioreactor connecting the riser and the downcomer allowing liquid recirculation.

ADVANTAGES OF AIR-LIFT BIOREACTORS

Traditional mechanical bioreactors(stirred) have all the requirements for providing suitable conditions for a bioprocess to go on smoothly but even then air lift bioreactors have reported more successful growth.

The main reason for this is in the difference in the fluid dynamics of air lift bioreactors compared to stirred bioreactors.

SHEAR STRESS

In Stirred Bioreactors the energy necessary for the movement of fluids is forced focally using a mechanical stirrer. This results in energy dissipation to be very high in the region of the stirrer. In the same way the shear is greater in the surrounding of the stirrer because of the momentum being directly sent to the fluid. It has been seen that the maximum shear gradient in a stirred reactor using a flat blade turbine has been found to be 14 times the mean shear gradient. These contrasting changes in the surroundings may result in endangering the integrity of cultured cells and even perhaps on the metabolism or morphology. Changes in morphology as a result of high shear forces were reported by Dion et al.

This pressure difference forces the fluid from the downcomer to the riser. There is no possibility of having focal points of energy dissipation and the shear force is homogenous throughout resulting in an almost constant environment.

Work done by Gavrilescu et al (1998) on "performance of air lift bioreactors in the cultivation of some antibiotic producing microorganisms" has shown several advantages of using air lift bioreactors in terms of cell mass production, Oxygen Transfer Rate (OTR) as well as energy expense over traditional Stirred (Mechanical) Bioreactors.

Risk of contamination and energy demand are reduced considerably because of the fact that air lift bioreactors do not use an external mechanical stirrer. They have also been shown as capable as stirred systems in terms of conversion of susbstrates, antibiotic production as well as biomass formation. They are capable in handling vicious broth as well as being flexible. The turbulence in ALB's have allowed it possible to produce conditions for mass transfer and also suspension/mixing of solids in non-Newtonian liquids.

RESULTS ON THE COMPARITIVE STUDIES

ALB's show interesting properties such as even distribution of microorganisms and nutrients over the entire volume and ability to handle vicious media. Appropriate supply of nutrients and o2 by gas-liquid mass transfer. Uniform dissipation of energy for achieving mass transfer and mixing.

It has been seen that the efficiency of power consumption in ALB's is 30-40% more than Stirred Bioreactors.

Productivity of Cell mass and Biomass are comparable for both the Bioreactor systems. Though the Oxygen Transfer Efficiency was around 50% higher in ALB's while thespecific power consumption was 2/3rds of the Stirred Bioreactor. Thus this showed that the ratio of the product concentration and the specific input was greater by a factor of 1.52 in the ALB.

For conditions involving aeration and agitation comparable kLa values were shown for non-biological components but biological media the OTR was 25% greater while it was 40% more efficient in terms of net energy consumption.

Another interesting point of observance is that the absence of having a mechanical agitator in ALB's it provides a gentler and safer environment for cultivation of microorganisms.

CONCLUSION

Air lift bioreactors are bioreactors which do not utilise any mechanical stirrer for agitation of the growth medium and instead have air injected at the bottom of the tank. They have been found to be more advantageous than the conventional stirred bioreactors which have been put to use for a long time now. Experiments show that ALB's are more power and energy efficient, have better mixing and the OTR is greater. Thus the potential to produce highly valuable bioprocess products is greater at lower costs and larger yield. Though close examination of ALB show that conventional stirred bioreactors are more useful to use in reality for large scale processes due to large investment costs. It is to be seen whether the special characteristics of ALB's allow it to be introduced into the complex biosynthesis industry and to be used for large scale production of products.

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