Chemical Engineering Department Government Engineering College MIXING & AGITATION B.E. (Chemical)- Sem- V Subject : MO Subject Code: 2150502 Guided By: Prof. S.J.Solanki Chemical Engineering Department Prepared By: 140153105007-Meet Somani Chemical Engineering Department Government Engineering College Bhuj-370001

Batching and Mixing Batch feed always involves mixing, objective: - high uniformity, high reliability (more judgment (experience) rather science) Before mixing: action of feeding; next few graphs showing some common equipments, goal: good powder flowability. Most ideal state of mixing: random homogeneous mixture RHM

Properties of Mass of Particles Pressure is not the same in all directions: one applied pressure will create some pressures in other directions, but always smaller; related to particle shape and packing; define K’ = normal pressure/applied pressure Shear applied at surface will be transmitted through a static mass of particles Density of mass will vary Before flow, mass of particles will increase its volume first (dilation) When angular solids are piled up on a flat surface, there will be an angle of repose; (free flowing solids: this angle is between 15 and 30o)

Angle of friction: influenced by particle size, shape, or even water content, it often increase cohesive force between particles  increase angle of friction  more difficult to free flow Abrasion: another possible problem with ceramic particles during transportation

Powder Mixing More art than science; Can never achieve perfect mixing like that in fluid phase; Complete mixing: often refers to specific structure, not attainable from a random process;  Characterization of mixture: I.e. degree of homogeneity (1) a statistical problem; (2) sample size (scale of scrutiny) consideration – need to be “proper”, too large or too small: little value; e.g. sintering after mixing – then consider diffusion distance during sintering, choose appropriate size for sampling，can be considered as single sample within that size.

Important Parameters Mixing effect affected by:  type of equipment  energy input  flowability and composition of sampe (size, shape, density, surface characteristics) For complete description of mixing, one need:  sample variance (intensity of segregation)  scale of segregation (to micro-scale)  long range structure

Microscale and Macroscale Mixedness Microscale analysis  provide information on microscale mixedness Macroscale mixedness  can be analyzed by many technqiues (chemical or physical) Sampling and analysis error should be kept to a minimum uncertainty in the standard deviation (s) become low when a large number of samples are taken

Taken from JS Reed, 1995 Commercial mixers: usually with two or more mixing elements to produce: high shear mixing in a local region & low shear bulk mixing

One example of two mixing elements To avoid vortex, we may add baffles Turbulence and cavitation – for diffusion (micro-scale mixing)

Mixing and EMI Performance * ABS + Ni powder or fiber  mixing (Barbender mixer or dry mixing) composite for EMI measurements

Dry mixing produced better shielding effect than Barbender mixer (in terms of low threshold value)

Barbender mixer produced perfect mixing, not necessary good for EMI purposes; dry mixing produced macro-scale uniformity, not micro-scale uniformity (a) Barbender mixer (powder; 20%); (b) dry mixing (powder 7%); (c) dry mixing (filament, 7%)

Agitator Bioreactor: device, usually a vessel, used to direct the activity of a biological catalyst to achieve a desired chemical transformation. Fermenter: type of bioreactor in which the biocatalyst is a living cell.

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

Batch operation with stirring A foam breaker may be installed to disperse foam A batch bioreactor is normally equipped with an agitator to mix the reactant, and the pH of the reactant is maintained by employing either buffer solution or a pH controller Batch operation with stirring Change of Cs with time, t

Continuous operation without stirring Plug-flow mode An ideal plug-flow reactor can approximate the long tube, packed-bed and hollow fiber or multistaged reactor In a plug-flow reactor, the substrate enters one end of a cylindrical tube with is packed with immobilized enzyme and the product steam leaves at the other end. F, Cs F, Cs0 V Residence time t = 0 Continuous operation without stirring

Temperature Control (Heat Load) Heat load: Heat load is determined by energy balances Heat production rate: Popular method : heat production rate, kcal/ls V: reactor liquid volume, l : specific growth rate, s-1 C: biomass concentration (g/l) Ykcal: a yield coefficient given as grams of cells formed per kcal energy released, g cells/kcal

Agitation (gas transfer) 1. Biological reactions almost invariably are three-phase reactions (gas-liquid-solid). Effective mass transfer between phases is often crucial. For example, for aerobic fermentation, the supply of oxygen is critical. The equation governing the oxygen transfer rate is: Agitation: Mechanical stirring (for small reactors, and/or viscous liquids, low reaction heat) Air-driven agitation (for large reactors and/or high reaction heat)