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Downstream Processes

Downstream Processes. BIE 5930/6930 Spring 2010. Filtration.

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Downstream Processes

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  1. Downstream Processes BIE 5930/6930Spring 2010

  2. Filtration

  3. Filtration is the use of a medium to separate solids from liquid. The solids can be from the size of cells or cellular tissue to individual ions. Depending on the filtration medium chosen as the filter material selective cut-off of particle sizes are achieved in the filtration process.

  4. Selection of the proper filtration method is often referred to as an art as well as science. There basic equations the help to predict how efficient a filtration process will be are understood, but many of the subtleties of each individual system make experimentation an important part of any filtration system design

  5. Filtration systems require at least a filtration membrane and can include additional layers of a filter cake that aids in increasing performance of the membrane

  6. Filtration in protein separation is usually utilized to concentrate the material prior to/or after a more selective affinity column. Filtration can provide selectivity based on size, but not on charges, etc.

  7. Types of filtration • Filtration is usually broken down into two primary techniques: • deadend • cross-flow filtration

  8. DeadEnd Filtration (Through flow) • Deadend filtration is when the feed material is forced through the membrane. The flow is only in the direction perpendicular to the membrane. All the suspended solids in the feed end up on the membrane in a filter cake

  9. Cross-Flow Filtration (Tangential flow) • In cross-flow filtration the feed material is allowed to flow parallel to the membrane, while the pressure gradient is across the membrane. The primary advantage of cross-flow filtration is that it allows the solids to be kept in suspension and minimizes the build up of a filter cake to plug or foul the membrane

  10. Filtration Classification • Classification of filtration falls into several categorizes depending on the size of the particles being excluded by the membrane

  11. Microfiltration generally refers to the filtration of suspension particle such as cells and cellular fragments • Ultafiltration is the filtration of macromolecules • Reverse Osmosis is the filtration of molecules such as salts and sugars

  12. Deadend filtration • Depth filtration • Cake filtration

  13. Depth filtration • Depth filtration is traditionally filtration with sand or a clarify cartridge. Solids are trapped in the void space in the medium. As solids accumulate the filtrate plugs and the flow rate approaches zero. The filter bed must be replaced or regenerated. Depth filtration is best used in a low solids material that requires clarifying because of the rapid degradation of performance with high solids loads

  14. Cake filtration • Cake filtration generally refers to the use a selectively porous material that traps a layer of solids above it. The layer is called the filter cake. The material used can be cloth or paper membranes. Often a layer of filter aid (diatomaceous earth) is placed on the membrane to improve the performance of the filtration process and to allow the filtration system to operate longer between regeneration by providing a removable layer in the system

  15. Flow theory for filtrationFlow through a cake is described by: • Flow through a cake is described by: • V = volume of filtrate • A = filter area • q = time • P = pressure across filter medium • a= average specific cake resistance • W= weight of the cake • r = resistance of the filter medium • m =viscosity

  16. In other words: • To optimize the flow rate • Increase the area of the filter • Increase the driving pressure • Reduce the viscosity of the fluid (usually by heating or addition of water) • Reduce the Cake resistance • Reduce the resistance of filter medium

  17. Filter Cake Thickness • The filter cake thickness will increase with time of operation. In a batch operation the a decision point must be reached as to how thick the cake can get before it begins to effect the efficiency of the operation. While a thin cake may have less resistance, if the time to shut down the filtration system and remove the build-up is greater than the lose of efficiency from the reduce flow it may be beneficial to operate with a thicker cake than optimal in each cycle

  18. Filter Aid • Filter aid such as diatomaceous earth can be added to the cake directly or mixed in to the product slurry. The filter aid will increase the porosity of the filter cake and therefore reduce the cake resistance. • In general filter aid is added at approximately 1-2 % of the overall slurry weight. An other general rule of thumb is to add twice the volume of filter aid as solids in the slurry

  19. Cake filtration equipment • Filter Press • Nutsche Filters • Rotary Vacuum Drum Filter

  20. Filter Press • The filter press is still one of the most commonly used filters in many operation

  21. The basic design is a series of filter screens that alternately allow the filtrate and permeate to flow through the screens and filter cake

  22. Nutsche Filters • The Nutsche is one of the simplest designs of batch filters. The tank is feed with a slurry and the bottom of the tank is comprised of the filter membrane

  23. Rotary Vacuum Drum Filter • The rotary drum filter is the most common of the continuous cake filters. It allows the cake to be continuously be removed

  24. Theory of Filtration • In filtration, solid particles are separated from solid-liquid mixtures by forcing the fluid through a filter medium or filter cloth that retains the particles. • The Filtration rate can be improved either by using a vacuum or pressure. • Filter aides such as Diatomaceous Earth which are highly porous also improve the filtration rate. • Filtration theory is used to estimate the rate of filtration.

  25. Theory of Filtration The rate of filtration is usually measured as the rate at which liquid filtrate is collected. Filtration rate depends upon: • Area of the filter cloth • Viscosity of the fluid • The pressure difference across the filter • The resistance to filtration offered by the cloth and deposited filter cake.

  26. Darcy’s Law • Describes the flow of liquid through a porous bed of solids:

  27. At any instant during filtration, the rate of filtration is given by the equation: • A = filter area • Vf = volume of the filtrate • P = pressure drop across the filter • f = filtrate viscosity • Mc = total mass of solids in the cake in the filtrate volume Vf •  = average specific cake resistance (LM-1) • Rm = filter medium resistance (L-1). This includes the resistance offered by the filter cloth.

  28. •  is the measure of resistance of the filter cake to flow. • Its value depends upon the shape and size of the particles and size of the interstitial spaces between them. • Resistance of the filter medium is negligible in comparison to that of the cake resistance (). • , the specific cake resistance can be related to DP empirically: s = 0 if the cake incompressible s = 1 if the cake is highly compressible • ’ is a constant and depends upon site and morphology of the particles in the cake

  29. Determining filtration time Integration of the rate equation will allow the calculation of the time required to obtain a given amount of filtrate (filtrate volume). The mass of solids in the cake will depend upon the volume of filtrate collected (for a given concentration of solids in the liquid-solid mixture)

  30. Theory of Filtration Let Mc = rcVf , the mass of solids deposited per unit of filtrate volume. Concentration of solids in the solid-liquid system = c, where c is the mass of solids per volume filtrate and is related to the concentration of solids in the material to be filtered.

  31. Theory of Filtration • Or and

  32. This can be rewritten simply as: What type of equation is this? Where

  33. Theory of Filtration If we maintain P as constant during filtration, K1 and K2 remain as constants during constant pressure filtration. The equation is an equation for a straight line, when is plotted against Vf. The slope K1 depends upon P and properties of the cake. The intercept K2 also depends upon the pressure drop, but is independent of cake properties.  is calculated from the slope and (Rm) is determined from the intercept. The above equation is the basic filtration equation.

  34. Batch Filtration

  35. Increasing filtration rate Consideration of the equation for filtration rate will indicate the various strategies that can be adopted for increasing the filtration rate. • increase the filtration area • increase the filtration pressure drop (vacuum filtration) P increases , which causes filtration rate to reduce. • reduce the cake mass (Mc) • reduce the liquid viscosity (by dilution) • reduce the specific cake resistance (). Factors affecting specific cake resistance • increasing porosity • reducing specific surface area of the particles (by increasing the average size of the particles and by reducing the particle size distribution)

  36. Increasing filtration rate Experiments should first be conducted to evaluate the properties of the cake such as compressibility, specific cake resistance, filtrate clarity, ease of washing, dryness of the final cake, ease of cake removal, the effects of filter aids. Fungal mycelia are filtered relatively early because mycelial filter cake has a large porosity. Yeast and bacteria are much more difficult to handle because of their small size.

  37. Review

  38. Example: Filtration of Mycelial Broth: A 30 ml sample of broth from penicillin fermentation is filtered in the laboratory on a 3 cm2 filter at a pressure drop of 5 psi. The filtration time is 4.5 min. Previous studies have shown that filter cake of Penicillium chrysogenium is significantly compressible with s = 0.5. If 500 liters of broth from a pilot-scale fermenter must be filtered in one hour, what size filter is required if the pressure drop is 10 psi.

  39. Cross-Flow Filtration

  40. Cross-Flow or Tangential Flow • In cross-flow filtration (CFF) the membrane does the primary work compared to the combination of cake and membrane in deadend cake filtration. The cross-flow allows the membrane to be swept free of solids allowing for a lower resistance to fluid flow through the membrane

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