FOOD AND INDUSTRIAL MICROBIOLOGY

Lesson 23. Lesson 23 FERMENTORS: TYPES, FUNCTIONS, DESIGN AND CONTROL

Module 7. Bioreactor configuration

Lesson 23
FERMENTORS: TYPES, FUNCTIONS, DESIGN AND CONTROL

23.1 Introduction

The reliable operation of a fermentation system to achieve process objectives depends on two factors: the fermentor design and the fermentation process. Therefore, rational design of a fermentor requires careful consideration of how it is integrated into the process design. The design should consider from the outset such factors as plant scheduling, space constraints, relationships between fermentor productivity and throughput rates of downstream equipment, containment and validation requirements, utilities requirements, potential interruptions in normal plant operation, overall labor requirements and operating versus capital costs. Consistency, safety, cost, and compliance with statutory requirements usually are of prime concern for production equipment.

23.2 Fermentor and Bioreactor

In general terms, a fermentor is something that, as its name would suggest, ferments. The process of fermentation has been known of for thousands of years, but has been mainly used over this time to the glucose found in various fruits, seeds and tubers into alcohol, later used for human consumption. In recent times, however, a fermentor is simply an optimal environment for bacteria and / or fungi to grow in, and the cultivation of said organisms will yield a desirable substance.

A bioreactor is a vessel in which is carried out a chemical process which involves organisms or biochemically active substances derived from such organisms. Bioreactors are commonly cylindrical, ranging in size from some liter to cube meters, and are often made of stainless steel. In brief bioreactor can be considered as a large scale operation whose volume/capacity ranges to several litres. Bioreactor is a system used for the growth and maintenance of a population of mammalian or insect cells whereas Fermentor is a system used for the growth and maintenance of a population of bacterial or fungal cells. There is also geometrical difference between Fermentor and Bioreactor taller vessels are used for bacterial processes to improve oxygen mass transfer whereas Shorter vessels for mammalian cel culture to improve mixing.

23.2.1 Bioreactor configuration

23.2.1.1 Design criteria

The objective of fermentor design is to produce fermentation systems necessary to build economical production facilities that satisfy well considered performance criteria. Reactor design and scale up considerations are driven by the need to provide the organism optimal conditions for producing the desired product uniformly in the reactor.

a ) Mechanical aspects

Mechanical design aspects are important for the successful operation of any fermentation

plant. Some practical aspects of vessel design are:

1. Space requirements: The vessel dimensions must be chosen to meet plant space limitations. Poor choice of equipment sizes can cause inordinately high building costs.

2. Transportation: Shop built vessels are usually less expensive and of higher quality than field built vessels.

3. Special heads: A hemispherical bottom gives better mixing and fewer shears than does a standard dished head.

b ) Process aspects

The fermentation process guidelines commonly employed without proper consideration of the process aspects do very little to promote good design. These include:

1. Aeration rate: The airflow rate to the fermentor must be generally one volume of air per liquid volume per minute (1 vvm). However, in practice, the aeration rates depend on oxygen solubility in media, and 1 vvm may be too high for a particular fermentation process when operated at a larger scale.
2. Impeller tip speed: The tip speed of a fermentor impeller must not exceed 7.6 m/s.
3. Arrest of fermentative metabolism: At the end of fermentation, the fermentor broth must be cooled immediately and stored at 4°C, to arrest the fermentative metabolism.
4. The maximum production rate cell mass theory: Process optimization is achieved by obtaining very high cell mass concentrations at very high growth rates.
5. Oxygen transfer rate: The consequences of increasing OTR by increasing air flow rate and agitation could lead to foaming, increased gas holdup, higher gas velocities, higher vessel pressure, and oxygen enrichment.
6. Heat transfer rate: Heat transfer usually is the limiting constraint for highly aerobic large scale fermentors.

c) Jackets and coils

Jackets are used for the circulation of steam and cooling water during the heating and cooling cycles of sterilization of the fermentor. Since microbial reactions are exothermic, the heat produced during fermentations leads to a rise in the temperature of the broth, necessitating the need to maintain the temperature at the optimal value. Normally, steam is used as the heating fluid; and water, chilled water, or chilled brine is used as cooling fluids. The contact surface area of the jacket with the fermentor should be maximal and the pressure drop of the circulating fluids in the jacket should be minimal for better process performance. The layout of the external jackets could be a double jacket, a full pipe, or a half pipe (limpet coil), depending on the required area for heat transfer, the heating or cooling medium, and the circulation velocity. If internal cooling coils are used, they can not only remove the excess heat from fermentation, but also act as baffles for better mixing.

d) Safety codes

The safety of the vessel should be the foremost consideration in vessel design. The vessel must be fabricated in accordance with the Standard Code for unfired pressure vessels and tested at design conditions to insure that the vessel can withstand all forces generated under the specified operating conditions. If operation is required at high operating pressures, one should consider ways to minimize the metal thickness to allow the use of cold rolled sheet rather than plate. This results in better heat transfer, a better interior finish, and a lower price.

e) Material of construction

Stainless steel is the more commonly employed material for the fabrication of biotechnology equipment. The selection of the right steel quality in biotechnology is based on a compromise between material costs, availability, and the physical and chemical requirements of the process. The low carbon steels, SS 304L and SS 316L, are known worldwide as standard steels. Generally, vessels used in biological processes are fabricated with 316 or 316L steel. The vessels widely used in food technology or harvest storage tanks are fabricated with a cheaper and less corrosion resistant steel of grade 304 or 304L.

The selection of a vessel material for fabrication should take into consideration:

1. Sensitivity of the organism, particularly eukaryotic cells

2. Extent of vessel corrosion on exposure to fermentation media and utilities

3. Aseptic operation requires use of SS316, SS316L, SS304, or SS304L.

f) Baffles

Baffles are usually welded to the interior of the vessel. Baffles usually take the form of metal strips, roughly one tenth of the vessel diameter in width, extending vertically down the height of the vessel and attached radially to the wall (Figure 23.1). Removable baffles mean unsealed joints. Baffles should be set off from the wall, to minimize solids build up and to simplify cleaning. Slots between the baffle and the vessel wall prevent the formation of dead spots. The provision of baffles increases the turbulence in the liquid and results in a more efficient utilization of power.

Fig. 23.1 Baffles (www.googleimages.com)

g) Sparger

A device for introducing air below the liquid level in the fermentor vessel is called a sparger (Figure 23.2). The use of spargers with very small orifices is more efficient than a single orifice delivering the same volume of air. However, aerobic fermentations, e.g., penicillin, produce large quantities of mycelium and mechanical agitation must be used in order to ensure adequate dispersion of air and other nutrients in large scale fermentations. As the degree of agitation is increased, the relative efficiencies of the various types of spargers tend to converge, and at high agitation levels, all spargers give approximately the same performance. The types of spargers in general use may be classified as nozzle spargers, orifice spargers, and porous spargers.

Fig. 23.2 Sparger (www. googleimages.com)

h) Nozzles and manways

Nozzle design must take into consideration the following:

1. To ensure aseptic connections to external piping,
2. In order to facilitate free draining
3. For proper and safe cleaning of the nozzles, protrusions inside the vessel should be as minimal as possible.
4. Addition of feed through nozzles to the fermentor to take care that the added liquids do not dribble down the interior surfaces.
5. The use of manways obviates the need for full opening heads on larger vessels.

i) Piping and valves

Piping materials: The most commonly used piping materials for biotechnology plants, in order of usage, are stainless steel, thermoplastics (polypropylene, polyethylene, polyvinylidine fluoride), carbon steel, copper, iron, glass, and lined pipe (glass and plastic liners). In sterile fermentation processes, a great deal of attention is necessary for the layout of lines and the construction of joints. Lines for sterile air and lines for transferring sterile mash are the subject of special care during installation to prevent the formation of pockets where liquid can collect. Adequate slopes, continuous in one direction, have to be given in what would be normally horizontal lines. Loops in lines are best excluded, but if unavoidable have to be provided with drain points so that residual mash and steam condensate can be removed. For purposes of sterilization, steam is introduced wherever possible at the highest point or points in the system, with the steam condensate removed from the lowest points. Valves used on sterile lines have given cause for thought for a considerable time. For robust industrial processes such as alcohol fermentation, or even for yeast cultivation, the use of the standard type of gate valve is normally acceptable. However, for fermentation systems more prone to contamination, the use of a standard valve has obvious shortcomings. The introduction of a diaphragm capable of standing up to steam for longer periods into the valve has encouraged a gradual changeover to the use of the diaphragm type of valve. Presently, with a diaphragm life of 3–4 months, the use of this type of valve for aseptic applications is justified.

j) Steam locks

A good steam lock assembly should have the following features:

1. Elimination of the chances of dead spots with inefficient sterilization
2. Thorough steaming of sterile lines to be done during non usage to avoid contamination
3. Lines from feed addition tanks should be able to be steamed through their entire lengths at any time after they are connected to the assembly
4. Steam locks to be checked for leakage of steam during steaming
5. Self draining of condensate through steam traps
6. Easy cleaning and maintenance of the steam lock assembly

k) Welds and joints

Vessel welds of high quality are required to adhere to code purposes, ensure maximum smoothness and cleanability, and to minimize corrosion problems. Welding for aseptic fermentors should be carried out under an inert gas shield to minimize oxidation and flux residue, and create smoother, pit free welds. A flanged joint for sterile fermentation processes should be constructed with smooth bore continuity throughout the joint. Smooth bore continuity is important, and can be compromised by such factors as tolerances in bore and circularity of commercial tubing, coupled with the tolerances in the bore of such things as slip on welding flanges, and the standard clearance holes for flange bolts. If in addition, the joint is badly cut or badly placed, focal points for lack of sterility with subsequent contamination can easily be produced.

l) Surface treatment and finish

The surface treatment of a vessel is required for any surface that comes in contact with the product. It is imperative that all stainless steel surfaces are treated and cleaned in a way that prevents corrosion under the operating conditions. Stainless steels are corrosion resistant due to the formation of a microscopically thin, invisible chromium oxide layer, which occurs on clean metal and polished surfaces only. The three main surface treatment methods used are mechanical, chemical, and electrochemical.

Mechanical Surface Treatment: It is possible to improve the surface quality of steel parts in contact with the product by means of mechanical treatment. The surface finish qualities of the vessel exterior are governed more by aesthetic rather than functional attributes.

Chemical surface treatment: The two main types of chemical surface treatment are cauterization and passivation. The impurities that require cauterization may be interior surfaces with oxidation tints, welding slag residues, and fine scales or overlaps generated

during the working process. Passivation is used either after cauterization or as a final treatment of ground, brushed, or polished steels with special surface structures. A natural passive layer is invariably formed when stainless steel is exposed to air.

Electrochemical surface treatment: Electropolishing, or mirror polishing, is an electrochemical treatment which smoothes and polishes rough, dull surfaces. The vessel is immersed in an electrolyte and serves as an anode.

The quality of the surface and welds after surface treatments are checked with following methods:

1. Visual inspection
2. Detection of ferric contaminations with the ferroxly test
3. Palladium testing to check the surface passivation
4. Detection detergent residues
5. Detection of chloride or sulfur contamination
6. Detection of pickling damage

m) Agitation system design

Typical agitation equipment consists of the prime mover (usually an electric motor) coupled to the shaft through a reduction gear. Impellers and baffles are fitted to the shaft and vessel, respectively, to give the desired liquid motion. The shaft may enter from the top, side or bottom, and is usually fitted with a mechanical seal at the vessel wall. The number and location of impeller units depends on the vessel. In a smooth walled tank, the liquid swirls round in the same general direction as the agitator. As the impeller speed is increased, a vortex is formed and the liquid level at the wall is raised above the average liquid level. This is normally undesirable for the following reasons:

1. Power is wasted in holding up the liquid at the wall

2. The relative speed of the impeller to the liquid is reduced

3. Slight radial movements of the vortex cause the liquid to swirl unevenly, and undesirable side thrusts are set up

23.3 Biofermentor Controls

Bioreactor or fermentation processes are the core manufacturing process in the biotech industry. Implementation delays and process upsets can result in the loss of millions of dollars in revenue through lost product and downtime. Because the bioreactor is such a critical component, getting it into production as quickly as possible and keeping it running, are essential to the profitability of a biotech operation. During implementation, many end users strive to reduce the I/O footprint of their control devices, since bioreactors use a wide-range of varied signals. Analog I/O points measure pressure, temperature and bring in flow rates for buffer and media. Discrete I/O controls peristalsic/ metering pumps and valves. Additionally, analytical probes control pH, dissolved oxygen and conductivity. Throughout the process, bioreactors need to maintain

precise control speed in the agitator to minimize shear. If the agitator creates too much turbulence, the microorganisms being grown may be torn apart. High rates of oxygen flowing through the sparge tube and improper agitator design can add to the problem of shear. The various types of biofermentor are shown in Figure 23.3. A comprehensive depiction of fermentor controls is shown in Figure 23.4.

23.3.1 Temperature control

The temperature control system is designed using PID control algorithms. The liquid temperature is sensed and compared it with the desired temperature to form the error signal. The error signal is processed using control algorithms to produce the desired output to the heater driver. The feed back control system operates on the heating system to maintain the temperature at the required set value by reducing the error towards zero. Continuously variable controllers are designed to produce power to the heater proportional to the error signal. As the measured value approaches its desired value, the power fed to the heater progressively reduced. PID controllers are most commonly used controllers in temperature control.

23.3.2 pH control

In real life, bioreactors actually use on-off control for pH. The two position control system is designed; so that the element controlling reagent addition is always set in one of two positions, either fully open or fully closed. The important consideration in pH control is hold time, which is required to provide time for neutralization reaction to go to completion.

Fig. 23.3 Biofermentor controls (www,googleimages.com)

Fig. 23.4 A comprehensive depiction of fermentor controls (Okofer et al., 2007)