COMPUTATIONAL FLUID DYNAMICS-NEW TOOL IN FERMENTATION TECHNOLOGY

In the earlier posts of this blog, we did cover briefly on the importance of the use of coloured dyes to visualize the flow pattern of the fermentation broth in fermentors. This experiment might be suitable for demonstrations to students taking fermentation course only .In that article we also discussed the various limitations of using selected tracers in visualizing the mixing or flow of the broth.
The importance of visualizing the flow is much more than like appreciating the fish in the aquarium. The ability to visualize really help understanding and highlighting problem areas in the fermentation process such as the efficiency of mixing.
Mixing could be improved in order to enhance the fermentation process to the optimum level. After all mixing of the fermentation broth is one of the most important parameters which affect the fermentation process. Mixing also affects other parameters such as mass transfers and heat transfers. Any change in the mixing regime will cause adjustment of the values of other parameters
The most powerful tool now in studying mixing in fermentors is no longer the use of coloured tracers but by the use of computational fluid dynamics (CFD). CFD is more a computer simulation program used to visualize the happenings during mixings It will become a powerful tool in the design and operation of the fermentors
CFD is important especially in trouble shooting the fermentation process and even in the scaling up and scaling down exercise . Areas that could be studied and improved includes fluid dynamics, transport phenomenon, heat transfer, phase transition, and rheology
So how can we visualize the fluid dynamics of a fermentation system? Well we are now aware of various computer programs that execute computational fluid dynamics. In these specialized programs numerical methods and algorithms are applied to solve and analyze problems that involve fluid flows and associated phenomena.
The key to the success of applying CFD depends not only having the right computer programs and computers but the various sensors and interphase that allow the input of the data from the fermentor to the computer. CFD might not be useful if we do not have the proper sensors to detect measure and transmit to the computer
The more important role of CFD in fermentation technology is in prediction of the behavior based on whatever data we have. But the process is not simple as:
1 There is the need to vary the boreactor operating regimes. This is especially so when stirring, volume and pumping have to be changed
2 Effect of a change in one parameter affecting other parameters
3 Secondary effects generated which could be critical such as bubble breakup, and coalescence mechanisms and even effect of shears on microbes
4 Immense data storage is essential when using CFD
Whatever problems faced computer simulation studies using CFD is the future answer in fermentation technology


Type rest of the post here. Read more!

CHOOSING THE RIGHT STIRRER

One of the most fundamental requirements in any type of fermentor is the provision of a device or mechanism to mix the contents of the fermentor. Good mixing will result in homogenous conditions of the fermentation broth which will help improve good mass transfer leading to an efficient fermentation process.
It is therefore not surprising to see in fermentors the provision of a stirrer or some sort of mixing device to execute this important function. Good mixing is not only restricted in liquid fermentation but also in solid substrate fermentation such as cocoa fermentation and even composting.
Many of us are so ingrained by the ‘brainwashing’ of fermentation literature that a standard fermentor is and always must be accompanied by a standard mixing device such as the motor, shaft and impeller combination. To this influence we are also brainwashed that Rushton turbine is the ‘best’ impeller system. Due to this also we dare not think ‘outside the box’ and are happily satisfied with the standard Rushton turbine despite the fact that the kind of fermentation we are using is not the same as those reported for the standard stirrer. Part of the blame for this ignorance is to be blamed on the users for failing to understand themselves the kind of fermentation they are carrying out, their limitations and failing to appreciate the rheology of their own broth.
The only exceptions to this poor thinking which I observed are those involved in disposable bag reactors and those dealing with the cultivation of plant and mammalian cells. In plant cell cultivation the mixing is brought about by the circulation of the fine air bubbles. In the disposable bag fermentation they use gentle waves which help in the mixing of the fermentation broth.
In the market there are various types of stirrers, shafts and impellers. You must be able to choose the right combination for your purpose.
In this aspects it is important that we need to know :
1 What sort of fermentation are we carrying out?
2 What is the size and geometry of the fermentor
3 The rheology of the fermentation broth
4 The kind of flow we expect for the fermentation
It is only after we have considered the above we custom fit the stirring configurations.
The type of flow generated by the stirrer is important. It is easy to get laminar flow or even non laminar flow for Newtonian rheology. Things do becomes complex if the fermentation broth which we are dealing are of the Non Newtonian type. In certain cases, the types of Non Newtonian transformations become even more complex.

The choice of the proper stirrer will have to be seen from:
1 The shaft component
2 The blades or the impellers
If we are dealing with viscous fermentation broth it is advisable we use shafts with bigger diameter. The length of the shaft too is critical to the efficiency of the shaft
The choice of the type of blades will determine if the flow produced will be radial or axial. Propeller stirrer shaft will produce axial flows. Which will result the flow moving away from the shaft? The inclination or direction in change of rotation will have impact on the direction of the flow
In cases where the blades are arranged on the disc will have flow attributes which will show strong shearing properties. The flow will be radial and directed outwards. Such flow will generate strong axial suction ia top and bottom axis
Impeller stirrer shaft will provide strong radial flows .
There are the so called anchor design stirrers with the shaft fitting in the centre of a “U” shape structure. This effective in generating tangential flows but poor axial forces or movement of the fermentation broth
There are also hybrid stirrers where both type of propeller blades and impeller blades share the same shaft. Usually the propeller blade is located at the top to induce downward flow to the blades with radial flows
It is important therefore in decoding the choice of stirrers the user must understand the rheology and viscosity of their fermentation broth before rushing like a fool to carry out just any fermentation using the ‘standard stirrer’ The best part of these fermentation studies not only they will not be able to determine the optimal fermentation conditions for their fermentation study but they have faith in “ stupid data’ their studies generated.
I just love the way the fools rushed in where angels fear to tread.. Read more!

RELEVANCE OF REYNOLD’S NUMBER IN FERMENTATION TECHNOLOGY

I personally think that there is too much of irrelevant engineering in fermentation technology. The obsession with mathematical equations, models by engineers into fermentation technology just add further to the mysticism of fermentation. It only helps in making the subject of fermentation technology so esoteric and beyond the reach and understanding of most people including the engineers themselves. After all, the abuse of irrelevant engineering and mathematics only serve in trying to make “sense” out of “non sense”. The logic is simple enough. How can you ever claim to control and optimize the fermentation process if you have very little understanding of the variability of the process?
Yes! Such mathematical and engineering studies and experiments will still data or rather “erroneous data” which one can still try to make some sense out of it. The end product will yield wrong and erroneous conclusions. At this point I am not saying that all the engineering or mathematical input are useless or irrelevant.
The trouble is there is a persistent and irritating trend by chemical engineers and biochemical engineers trying to justify their intrusion into the field of fermentation technology and not vice versa! Good examples are the adoption of unit processes in downstream processing. In a feeble bid trying to improve the engineering or mathematical input into fermentation technology they even try to apply Monod’s equation, Lineweaver Burke into the fermentation modeling! How relevant or significant input these contributions are is another question.
We are thankful for the engineers in helping build or design the fermentors we have on the market now. But looking with perspective over time we see there is no real significant development or advancement in the design and structure of fermentors from the days of Fleming industrial production of penicillin. What we are having now are in fact “living fossils” of the old antiquated fermentors. Nothing much has really changed!
Let us look at the application and relevance of Reynold’s Number to fermentation technology.
Historically, the exposition of Reynold’s Number is the keystone to fluid mechanics. It is more and observation on the nature of fluid flow which includes air and liquid such as observed in wind tunnel, aerodynamics and in trying to explain the transformation of simple laminar flow to complex flow turbulence
The application of Reynolds number might be of relevance in aerodynamics of flight where it is necessary to understand the behavior of objects exposed to high speed wind velocity and turbulence. But I cannot really see the relevance of Reynolds Number at the level of fermentor where we are really dealing at low rpm. In fact, laminar flow is not really sought after in fermentor as turbulence is needed to improve the various mass transfer processes during fermentation
To complicate things further the behavior of the fermentation broth is complex in terms of its phases and changes that occur as a function of time. The rheology of the fermentation broth is complex and almost no two fermentation broth are the same. In fact there are so many variables involve which will influence the Reynolds number such as size, geometry, broth type, media composition and even the composition of microorganisms used in the fermentation process
There for do we really need to apply Reynolds Number in the study of fermentation process or is it just a vestigial reminder left by the engineers for us just like the mysterious smile of the sphinx?

Type rest of the post here. Read more!

RHEOLOGY PART 5: LESSONS FROM A DROP OF HONEY!

Sometimes we can learn more about the problems of rheology and mixing of fermentation broth by observing simple examples such as a drop of honey. A drop of honey as a rheological model is not truly reflective of the behavior of the fermentation broth but its behavior will allow you insights of mixing non Newtonian fluid.
If we try to stir the drop of honey on a surface using a tooth pick, we will see that it is very difficult to mix the drop of honey homogenously. There is movements by the tooth pick, but in most cases the honey will try to resist the movement and retract elastically back to its mass. Even if mixing occurs temporarily, it only occurs within the close proximity of the stirrer. Increasing the speed of mixing at most times does not increase the mixing of the honey.
Are we trying to say that in fermentors with very viscous broth mixing comes to nothing? Or better still have we come with properly designed stirrers that can really effectively stir the fermentation broth?
The biochemical engineers need to understand more about the properties of the non Newtonian broth and designed new stirrer configurations or even new modes of mixings to overcome this problem. Maybe it is high time or over time that they should start to look at the micromixing aspects rather than be over whelmed by macro mixing properties




Type rest of the post here.
Read more!

RHEOLOGY PART FOUR: APPLYING RHEOLOGY TO FERMENTATION

In most, if not all of fermentation broth, we are dealing with Non Newtonian fluid. The Non Newtonian nature is due to the composition of a fermentation broth which is not uniform and complex. The fermentation broth often show complex interactions of solid, liquid and gas phases.
To make things worst the rheology of the fermentation broth is always changing as a function of time and with the progress of the fermentation process.
It is more difficult to control and optimize a fermentation process if it is a Non Newtonian fluid! Things would definitely be easier if the fermentation broth is a Newtonian fluid. (But then again there would be no bread, cheese, yogurt, fish sauce and many more fermentation products!)
The main impact of Non Newtonian rheology is that it affect mixings and mass transfers of heat and oxygen and prevent efficient homogenous composition to occur.
We all know that in rheology it is the study of fluid deformation and flow under pressure and the relationship between stress and strain. Through simple observations we can see how difficult it is to mix and aerate viscous fluid. Each rheological type will give different mixing profile.
This has led to the classification of various classes of Non Newtonian fluids such as
1-viscoplastic fluid,
2-bingham fluid,
3-pseudoplastic fluid,
4-dilatant fluid
Non Newtonian rheology curves can be made up of various types. Most of these rhelogical curves are graphs where the x- axis is shear stress and the y- axis is shear rate
The rheological graphs are interesting not only in comparing between the Newtonian and the Non Newtonian but also the varying properties even among the various Non Newtonian fluid
It is interesting to note generally that all Non Newtonian fluids show some similarity in relationship with Newtonian fluid reflecting the effect of shear stress on shear rate. There is roughly a direct linear relationship (with variations) between shear stress and shear rate.
Differences only that Newtonian fluid adhere strictly and very linear and start at point zero of the axis.
1 Viscoplastic and Bingham starts only after certain level of shear stress. This means that the fluid will NOT respond immediately to applied stress and will only react after reaching the critical power point
2 Pseudoplastic and dilatants start at zero point but are curved in their shape. This mean that the fluid will respond immediately to the power or energy input. This is similar to Non Newtonian. However their response will be different in that it is not a linear relationship between stress and shear rate
3 Viscoplastic, pseudoplastic and dilatants are curved in their shapes This means that these fluids react in their yield behavior under stress differently.
So what does these observations mean in fermentation?
These rheological graphs will tell us how to respond efficiently with the type of broth being fermented.By understanding the various rheological changes that occur in the fermentation broth we can:
1 Try to achieve uniform homogenization and optimum mass transfer
2 Try to optimize energy usage in mixing of the fermentation broth
In carrying out the fermentation, we are using the impeller to mix the broth. Energy is transferred and dissipated to the broth by the impeller system. The impeller is in simplicity the shear stress being enforced upon the broth.
The effect of the impeller or mixing on the broth will result in the flow or turbulence of the broth. The broth will respond by exhibiting stress yield properties such as thinning out of the broth to improve mass transfer processes.
So if we know the rheology of the fermentation broth it will help us to adapt to obtain very efficient fermentation by adjusting our mixing regimes. This is especially so when the rheology changes with time and conditions.




Type rest of the post here.
Read more!

RHEOLOGY PART THREE: DISCUSSIONS

One of my students used to ask me this question,
“What is the point of trying to understand the complexities of fermentation rheology, when rheology itself is not one of the common parameters used in monitoring the fermentation process?”
I threw this question to the class for deeper discussions..
The general consensus is that:
1 Rheology is an important and relevant parameter of understanding and controlling the fermentation process.
2The use of rheology part could contribute significantly in understanding the changes that occur during the fermentation process.
3 Rheological data could be used in trying to optimize the conditions towards optimum fermentation process.
4 Rheological data could be used to extrapolate the likely event of impending fermentation failure
5 The use of rheology data such as the type of non Newtonian fluid could indicate the right opportunity to change the mixing regime and save energy
6 Operating parameters could be adapted such as mode of feeding to control the rheology of the fermentation broth
7 The failure to use rheology as the online parameter is because there are no sensors that can measure rheology on lines for now, and not because rheology is not an important parameter!


Type rest of the post here.
Read more!

MIXING AND RHEOLOGY PART TWO

We can say that in the design of fermentors, the choice of impellers to execute the mixing regime has often been too over simplified. It is strange that despite the complex rheology of various fermentation broths, nost models of impellers are based on two main classes of mixings that is euther radial or axial.
Radial mixings are generated by impellers which are often based on simple flat blade design or variations of the Rushton turbine design. In radial flows all the mixing particles are brought out to the walls of the fermentor subject to tangentianal and centrifugal forces leading to formation of the common close loop pattern too commonly encountered in most standard text books in fermentation technology
On the other hand, the axial mixing are often brought about by the propeller design inpeller leading to the axial flow commonly visualised in ceiling fans
There are variations of mixings which integrate both the radial and axial flows io create a more effective hybrid flows
The important point here is that in the design of these main types of impellers, too much emphasis is given on the following:
1The fermentation broth subjected to mixing is Newtonian by nature
2 The properties of the fermentation broth do not change or remain constant
These kinds of conditions are not commonly found in the industrial fermentation industries. The fermentation broth is often more complex than expected to adhere to the Newtonian characteristics. Second, the volume of fermentation broth or fermentor is huge. This will make it very difficult for radial or even axial mixing to be efficient.
If we learn from blendings carried out in various chemical or even food industries, the type of mixers are often simpler. However in their cases their main requirements are just homogenization and that aseptic requirements are often not carried out during the mixing process itself
It is therefore very important that we need to come up with a very specific design for the mixing of specific fermentation broth and not have the “inbox mentality” of using the same standard impellers


Type rest of the post here.
Read more!

RHEOLOGY OF FERMENTATION BROTH

The fermentation broth is a very complex soup or solution. Fundamentally, the fermentation broth is the sea of nutrients in which the microorganisms grow, reproduce and 'swim' . The fermentation broth supply the microorganisms with all the nutrients the microorganisms need to grow and produce the various fermentation products.

The fermentation broth too act as the medium for various physical, biochemical and physical reactions to take place. The fermentation broth will be implicated in all the mass and heat transfers that occur within the fermentor, and it will be the medium that holds the fermentation products formed.

The nature and composition of the fermentation broth temporally and spatially will affect the efficiency of the fermentation process. The interactions between the fermentation broth and the various components is complex and affect both directions

WHAT IS IN THE FERMENTATION BROTH?
-------------------------------------------------
At any time the composition of the fermentation broth is complex consisting of anything that ends up in the fermentation broth. This includes:

1 Raw substrates
2 Fermentation products
3 Microorganisms and its derivative components
4 Chemical additives added to the fermentor
5 Gases such as oxygen and other metabolic gases

All three main phases; solid, liquid and gases are present in the fermentation broth and their possible interactions


RHEOLOGICAL PROPERTIES OF FERMENTATION BROTH
----------------------------------------------------------------
One of the most important singular properties of the fermentation broth will be its rheological or viscosity characteristics. Let us not stick to strictly to the definition and quantification of rheology but rather try to appreciate it from its behaviour and its impact.

We always 'picture' fermentation broth as a thick gooey sticky mixture that is thick and viscous compounded by rising bubbles of gas exploding at the broth surface. Maybe this picture is too dramatic but in a way it is true!

The viscous nature or the rheological properties will affect the mixing regimes of the fermentor.
Viscosity is not a simple but a complex phenomena that is always changing and responding to various parameters. Very rarely can we describe a fermentation broth as following a Newtonian behaviour. In most cases it is a complex combinations of various Non Newtonian behaviour.

This poor understanding of the fluid behaviour of the fermentation broth will affect the efficiency of mixing and liquid circulations resulting in poorly controlled or less economical fermentation process


WHAT CAUSES THE BROTH TO BE VISCOUS?
--------------------------------------------------
The viscousness of the fermentation broth is caused by the interactions of the various components in the fermentation broth. The interactions may occur between the components of the broth and the water or it could result from the interactions between the components themselves. Such interactions in the viscosity of the broth could occur at the level of the ions and molecules which involve the various ionic forces or it could involve at the macrolevel such as between the various biopolymers tangling and sliding with each other. The overall result will be that the fermentation broth will be viscous.

Now let us look at one of the components which make the fermentation broth viscous, that is the contribution of sugars to the viscosity. Sugar or the carbohydrates are the main carbon source in any fermentation media and supply the carbon needed for energy and skeleton structures of the cells and organic compounds

Experience have shown to us that sugar in solution is sticky, but dry sugar is not sticky. The stickiness or viscousness of the sugar in solution is caused by hydrogen bondings which develop between the sugar molecules and water. During the interactions of sugar and water the hydrogens in the water molecules and the hydrogen in the sugar molecules have an attraction for each other. Thus it is the hydrogen bondings that make the sugar sticky!

Thus we see that most of the viscosity in the fermentation broth is caused by the various hydrogen and other ionic bondings


IMPACT OF VISCOSITY ON FERMENTATION
-----------------------------------------------
The most crucial effect of viscosity is that it makes the situation very difficult to achieve proper and complete mixings. This will affect the various mass transfer processes that occur in the fermentor. poor mixings due to high viscosity will also result in the formation of various physical and chemical gradients

Viscosity makes scaling up studies difficult due to the change in behaviour of the fermentation broth such as difficulty in mass heat transfers, solubility of components and gases and mixings at the upper scale of fermentation process Read more!