Mixing: Process, Equipment, and Measurement PDF

Mixing 1  What is mixing  Physical operation  Reduces non uniformities in fluid  Eliminates grad...

Mixing 1  What is mixing  Physical operation  Reduces non uniformities in fluid  Eliminates gradients of concentration, temperature and other properties  Mixing is accomplished by Interchanging material between different locations to produce mingling of components  Random homogeneous distribution of system properties in perfectly mixed system A standard tank with a working volume of 2 100 M3 and used for penicillin production  Mixing involves Blending Blending soluble components of medium e.g sugars Dispersing gases such as air through the liquid in form of small Dispersing bubbles Maintaining Maintaining suspension of solid particles such as cells Dispersing immiscible liquids to form emulsion or suspension of Dispersing fine drops Promoting Promoting heat transfer to or from the liquid 3  Importance of mixing  One of the most important operations in bioprocessing  Create optimal environment for fermentation i.e to provide cells access to all substrate  If mixing doesn’t maintain uniform suspension of biomass , then substrate concentration can quickly drop to zero in areas where cells settle out of suspension  Heat transfer; to maintain temperature at desired level ; rate of heat transfer from broth through the walls of vessel to cooling water depends on mixing conditions in vessel.  Effectiveness of mixing depends on 4 rheological properties of fluid  Mixing Equipment  Mixing usually carried out in a stirred tank, which is recommended to have the base is rounded at the edges rather than angled - will eliminate sharp corners and pockets into which fluid current may not penetrate and formation of stagnant region  Mixing is achieved by installation of impeller Typical configuration of a stirred 5 tank  Flow Patterns in Agitated Tanks  Flow pattern in agitated tank depends on: 1. Impeller design 2. Properties of fluid 3. Size and geometric proportions of the vessel, baffles and agitator. 6  Impeller designs  Many impeller design are available for mixing application.  Some impellers have flat blade, propeller and helical screw, which slope of individual blades varies continuously  Choice of impeller depends on several factors, including viscosity of the fluid and sensitivity of the culture system to mechanical shear  For low-to-medium-viscosity liquids – propellers and flat blade turbines  The most popular impeller used in industry is 6-flat-blade disc-mounted turbine or known as Rushton turbine 7   For Newtonian fluids – ratio of tank diameter Size and to impeller diameter is 3:1 geometric  The impeller usually installed overhead of stirrer shaft and at the bottom of the vessel proportions of the vessel  For efficient mixing with single impeller, the depth of liquid should not be more than 1.0- 1.25 times the tank diameter 8  Classification of impellers  There are three types of impeller which can be classified according to the predominant direction of flow leaving the impeller. 9 Axial-flow Radial-flow Low speed agitators  Radial-flow Devices  The flat-bladed (Rushton) turbines are essentially radial-flow devices, suitable for processes controlled by turbulent mixing (shear-controlled processes).  Liquid is driven radially from the impeller against the walls of the tank where it divides into two streams, one flowing up to the top of the tank and the other flowing down to the bottom.  These streams eventually reach the central axis of the tank and are drawn back to the impeller.  Radial-flow impellers also set up circular flow which must be reduced by baffles 10  Axial-flow Devices  The propeller and pitched-blade turbines are essentially axial-flow devices, suitable for bulk fluid mixing.  In general, axial-flow impellers have blades which make an angle of less than 90 to the plane of rotation and promote axial top to bottom motion.  Fluid leaving the impeller is driven downwards until it is deflected from the floor of the vessel.  It then spreads out over the floor and flows up along the wall before being drawn back to the impeller. 11  Low Speed Agitators  Low speed agitators with are used for effectively mix viscous liquids or semisolids.  There are several blades designs available but typically recommended is the helical blade design for maximum product turnover.  The heat transfer application can be organized into two broad categories.  One category is that in which open impellers operate at some distance from the vessel. The second category is a close clearance impeller such as an anchor impeller or helical impeller.  Paddle, anchor and helical ribbon agitators, and other special shapes, are used for more viscous fluids. 12  Mechanism of mixing  All these factors are important in mixing which can be described as combination of 3 physical processes: 1. Distribution (macro-mixing) often the slowest step of mixing 2. Dispersion (either micro or macro-mixing depending on the scale of fluid motion) is the process of breaking up bulk flow into smaller eddies. Facilitates rapid transfer of material throughout the vessel. the kinetic energy of turbulent fluid is directed into regions of rotational flow called eddies. 13 3. Diffusion (Micro-mixing) within eddies, flow is  Mechanism of Mixing  For effective mixing: 1. Fluid circulated by the impeller must sweep the entire vessel in reasonable time. 2. Velocity of the fluid leaving the impeller must be sufficient to carry material into remote parts of the tank 3. Turbulence must also be developed in the fluids 4. Circular flow also leads to vortex development Vortex : If a low viscosity liquid is stirred in an un-baffled tank by an axially mounted agitator, tangential flow follows a circular path around 14 the shaft & a swirling flow pattern is developed.  (a ) Baffles  They are vertical strips to reduce vertexing and swirling of liquid. (b )  Baffles are often fitted to the walls of the vessel.  To prevent vortex formation optimal baffle width depends on impeller design and fluid (c viscosity )  These take the form of thin strips about one- tenth of the tank diameter in width, and typically four equi-spaced baffles may be used. Baffle arrangements. (a) Baffles attached to the wall for low-viscosity liquids. (b) Baffles set away from the wall for moderate-viscosity liquids. (c) Baffles set away from the wall and at an angle for 15 high-viscosity liquids. Baffles 16  Mixing time  Useful parameter for assessing mixing effectiveness Assessing  The mixing time tm (Circulation time tc in Mixing stirred vessels)is the time required to achieve effectiveness by a given degree of homogeneity starting mixing time from the completely segregated state  Applied to characterize bulk flow in fermenters 17  Measurement of Mixing time  by injecting a tracer into the vessel and following its concentration at a fixed point in the tank. Tracers in common use include acids, bases and concentrated salt solutions; corresponding detectors are pH probes and conductivity cells. 18  Sparger  Sparging is a technical term for injecting gas into a liquid or for spraying a liquid onto a solid such as air or oxygen, into a bioreactor.  The sparger is located at the bottom of the bioreactor that allows gas to flow through it.  Spargers are porous disc or tube assemblies that are also referred to as Bubblers, Aerators, Porous Stones and Diffusers. 19  Sparger Design  The spargers are designed based on experience and are used based on convenience and availability.  The most common types of spargers used for process are;  Ring sparger ( >1mm)  Perforated Plate sparger (1-3mm)  Perforated pipe sparger (1-3mm)  Porous sparger (