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### FLUID FLOW AND MIXING IN BIOREACTOR

1. Made by- Vikash Shashi Venishetty Vivek K. Srinivas Naik
2. Contents  Fluid Flow in Bioreactors.  Mixing in CSTR (Continuous Stir Reactor).  Mixing in Bubble Column Reactor.  Mixing in Airlift Reactor.  Mixing in Packed Bed Reactor.  Mixing in Trickle bed Reactor.  Mixing in Fluidised Bed Reactor.
3. INTRODUCTION  A fluid is a substance which undergoes continuous deformation when subjected to a shearing force .  A simple shearing force which causes thin parallel plates to slide over each other, as in a pack of cards.  Fluids in bioprocessing often contain suspended solids , consist of more than one phase , and have non Newtonian properties .  A shear force must be applied to produce fluid flow.
4.  Two physical properties are used to classify fluids . VISCOSITY and DENSITY.  DENSITY : Compressible fluids and Incompressible fluids .  VISCOCITY : an ideal or perfect fluid is a hypothetical liq. Or gas which is incompressible and has zero viscosity. Inviscid fluids and viscid fluids.  fluids can also be classified further as Newtonian and non Newtonian .  NEWTONIAN FLUIDS : which obeys the newton’s laws of viscosity i.e. t = mdv/dy ; where t = shear stress, m = viscosity of fluid, dv/dy = shear rate, rate of strain or velocity gradient.  NON NEWTONIAN FLUIDS : which do not obey the Newton's law of viscosity.
5. FLUIDS IN MOTION  When a fluid flows through pipe or over a solid object , the velocity of the fluid varies depending on position.  One way of representing variation in velocity is streamlines, which follow the flow path. Constant velocity is shown by equidistant spacing of parallel streamlines . As in fig. 1.  Where as in fig . 2 there is a reducing space between the streamlines indicates that velocity at top and bottom of the object is greater than at the front and back.  Therefore , slow fluid flow is called STREAMLINE or LAMINAR FLOW. And in fast motion, fluid particles cross and recross the streamline and the motion is called as TURBULENT FLOW.
6. REYNOLDS NUMBER  A parameter used to characterise fluid flow .  For full flow in pipes with cross section , Reynolds number Re is : Re = Duρ/μ ; where D is pipe diameter , u is the average linear velocity of the fluid,μ is fluid viscosity.  For a stirred vessel there is another definition of the Reynold no. Rei = Ni Di² ρ / μ ; where Rei is the impeller Reynolds no. , Ni is the stirrer speed , ρ is the fluid density , Di is the impeller diameter.  The Reynolds no. is a dimensionless variable .  Reynolds no. is named after OSBORNE Reynolds , who published in 1883 a classical series of papers on the nature of flow in pipes.
7. NON NEWTONIAN FLUIDS  Most slurries , suspensions and dispersions are non Newtonian .  Many fermentation processes involve materials which exhibit non Newtonian behaviour , such as starches, extracellular polysaccharides ,and culture broth containing cell suspensions or pellets.
8. HYDRODYNAMIC BOUNDARY LAYERS  The part of the fluid where flow is affected by the solid is called the ‘’ boundary layer ‘’.  Contact between moving fluid and the plate causes the formation of the boundary layer beginning at the leading edge and developing on both top and bottom of plate.  When fluid flows over a stationary object , a thin film of fluid in contact with the surface adheres it to prevent slippage over the surface. fluid velocity at the surface of the plate in fig 7.3 b is therefore zero.  When a part of flowing fluid has been brought to rest , the flow of adjacent fluid layers is slowed down by the action of ‘ viscous drag ‘ .  Compared with velocity uB in the bilk fluid , velocity in the boundary layer is zero at the plate surface but increases with distance from the plate to reach uB near the outer limit of boundary layer .
9. BOUNDARY LAYER SEPERATION  What happens when contact is broken between a fluid and a solid immersed in the flow path ?  when fluid reaches the top or bottom of the plate its momentum prevents it from making the sharp turn around the edge . As a result fluid separates from the plate and proceeds outwards into the bulk fluid .
10. Stir Tank Reactor The continuous stirred-tank reactor, also known back mix reactor, is a common ideal reactor type in chemical engineering. A CSTR often refers to a model used to estimate the key unit operation variables when using a continuous agitated-tank reactor to reach a specified output.
11. Mixing Method  Mixing method: Mechanical agitation • Baffles are usually used to reduce vortexing • Applications: free and immobilized enzyme reactions • High shear forces may damage cells • Require high energy input
12.  An ideal CSTR has complete back -mixing resulting in a minimisation of the substrate concentration, and a maximisation of the product concentration, relative to the final conversion, at every point within the reactor the effectiveness factor being uniform throughout. Thus, CSTRs are the preferred reactors, everything else being equal, for processes involving substrate inhibition or product activation. They are also useful where the substrate stream contains an enzyme inhibitor, as it is diluted within the reactor. This effect is most noticeable if the inhibitor concentration is greater than the inhibition constant and [S]0/Km is low for competitive inhibition or high for uncompetitive inhibition, when the inhibitor
13. Bubble column Bioreactor A bubble column reactor is an apparatus used for gas-liquid reactions first applied by Helmut Gerstenberg. It consists of vertically arranged cylindrical columns. The introduction of gas takes place at the bottom of the column and causes a turbulent stream to enable an optimum gas exchange. In BCR, gas & liquid reactants are compacted in presence of finely dispersed catalyst are used in different applications from fermentations to production of chemicals & pharmaceuticals. They have high volumetric productivity & excellent heat
14. Mixing  Bubble column reactors are widely used to carry out multi-phase reactions. Mixing and transport processes are the key issues in the design of bubble columns, especially for processes involving multiple reactions where selectivity to the desired product is important. Under such circumstances, liquid phase mixing often decides the reactor performance. The local flow field and turbulence governs the fluid mixing and is interrelated in a complex way with the design and operating parameters.
15. Mixing  Both axial and radial mixing are possible in bubble column reactor. Mixing in axial direction is a function of aeration rate, geometry of the column and the properties of the fluid. Rising gas bubbles carry elements of circulating fluid in bubble wakes produce axial mixing. Because bubble rises faster than the liquid, a certain amount of liquid is carried forward faster than the bulk flow of the liquid. This produces mixing in the axial direction.  For tubular reactors, axial mixing is usually several times higher than radial mixing. Thus, for most practical purposes, attention is focused only on axial mixing.  In case of radial mixing, bubbles may impinge on the walls of the reactor and break consequently with improvement of mass transfer.
16. Airlift reactor Air-lift bioreactors are similar to bubble column reactors, but differ by the fact that they contain a draft tube. The draft tube is always an inner tube or an external tube .This kind of air-lift bioreactor is called "air-lift bioreactor with an external loop” which improves circulation and oxygen transfer and equalizes shear forces in the reactor.
17. Mixing  Mixing method: Airlift • In these reactors mixing circulation and aeration is performed by gas injection and if needed by additional external liquid circulation to obtain the required mixing pattern. The figure, gives an example of a possible configuration. This usually results in less shear for a given quality of mixing than in stirred tanks. Air lifts give more vigorous recirculation for the same air flow, but often lower oxygen transfer rates than bubble columns. To limit shear, small bubbles can be used in aeration, but depending on conditions this may cause excessive foaming and requires more energy for their generation at porous distributors.
18. Packed-bed reactor  Packed-bed reactors are used with immobilized or particulate biocatalysts.  Medium can be fed either at the top or bottom and forms a continuous liquid phase. The advantage of using a packed bed reactor is the higher conversion per weight of catalyst than other catalytic reactors. The conversion is based on the amount of the solid catalyst rather than the volume of the reactor.
19. Mixing  In packed bed reactors, cells are immobilized on large particles. These particles do not move with the liquid. Packed bed reactors are simple to construct and operate but can suffer from blockages and from poor oxygen transfer. Continuous packed bed reactors are the most widely used reactors for immobilized enzymes eg. Amiloglucosidase 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.
20. Trickle-bed reactor The trickle-bed reactor is another variation of the packed bed reactors. Liquid is sprayed onto the top of the packing and trickles down through the bed in small rivulets. It is considered to be the simplest reactor type for performing catalytic reactions where a gas and liquid (normally both reagents) are present in the reactor and accordingly it is extensively used in processing plants.
21. Mixing In a trickle bed reactor the liquid and gas phases flow concurrently downwards through a fixed bed of catalyst particles while the reaction takes place. In certain cases, the two-phases also flow concurrently upwards. The concurrent upward flow operation provides better radial and axial mixing than the downward flow operation, thus facilitating better heat transfer between the liquid and solid phases. This is highly useful in exothermic reactions where heat is required to be removed continuously from the reactor. However, due to higher axial mixing in the upward flow operation, the degree of conversion, a crucial factor in the operation is preferred. Because of lower axial mixing, better mechanical stability and less flooding is achieved , thus facilitating processing of higher flow rates and increased reactor capacity.
22. Flow Regimes  Trickle bed reactors operate in a variety of flow regimes ranging from gas-continuous to liquid-continuous patterns. They usually fall into two broad categories referred to as low interaction regime (trickle flow regime) and high interaction regime (pulse, spray, bubble and dispersed bubble flow regimes). The low interaction regime is observed at low gas and liquid flow rates and is characterized by a weak gas-liquid interfacial activity and a gravity-driven liquid flow. High interaction regime is characterized by a moderate to intense gas- liquid shear due to moderate to high flow rate of one or both of the fluids. As a result, various flow patterns arise depending on the gas and liquid flow rates and the physical properties of the liquid. Schematic diagram of the trickle flow
23. Fluidized bed reactor Fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a granular solid material (usually a catalyst possibly shaped as tiny spheres) at high enough velocities to suspend the solid and cause it to behave as though it were a fluid. This process, known as fluidization, imparts many important advantages to the FBR.
24. Mixing  The solid substrate (the catalytic material upon which chemical species react) in the fluidized bed reactor is typically supported by a porous 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
25. References  NChE Journal (Vol. 21, No. 2)  Wikipedia  Braz. J. Chem. Eng. vol.31 no.1 São Paulo Jan./Mar. 2014  Trans IChemE, Part A, Chemical Engineering Research and Design, 2004, 82(A10): 1367– 1374  N. Kantarci et al. / Process Biochemistry 40 (2005) 2263–2283
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