T5 FILTRATION

BIOSEPARATION

Filtration

Overview of Filtration

  • Definition: Filtration is defined as the separation of solids from liquids by passing a suspension through a permeable medium that retains the particles.

  • Outcomes of Filtration:

    • Waste or Cake: The particles retained during the filtration process are referred to as waste or filter cake.

    • Filtrate: The filtered fluid that passes through is called filtrate.

    • Fluid State: The fluid can exist in either liquid or gas form.

Strategies in Product Recovery

  • Importance: Product recovery is one of the most critical aspects of industrial bioprocessing.

  • Scales of Operation:

    • Laboratory scale: 1-100 Litres.

    • Pilot scale: 100-10,000 Litres.

    • Industrial scale: 10,000-100,000 Litres.

  • Approach: There is no absolute rule for product recovery strategies; it depends on various factors.

Downstream Processing

  • Characteristics of Bioproducts:

    • Bioproducts are usually labile or sensitive, thus requiring controlled conditions to maintain their activity.

    • Conditions to consider:

      • pH

      • Temperature

      • Ionic strength

  • Operational Nuance: There is no unique, ideal operation or sequence that can be universally recommended. Each operation must be tailored to meet specific challenges and problems effectively.

Factors in Downstream Processing

  • Nature of the Product: Physical and chemical properties influence processing choices.

  • Concentration: Higher concentrations may lead to processing difficulties such as clogging and increased viscosity.

  • Stability: Bioproducts may be labile, and stability is crucial; this can encompass conditions like pH, temperature, etc.

  • Desire for Purity: Higher purity leads to increased costs.

  • End Use Considerations:

    • For medical applications, products must be free of pyrogenic materials.

    • Adherence to current good manufacturing practices (cGMP) is essential, along with addressing biosafety during production.

Filtration System Components

  • Different parts involved in a filtration process include:

    • Suspension: The mixture of solids and liquids before filtration.

    • Medium: The permeable substance through which the suspension is filtered.

    • Filtrate Flow (Q): The flow rate of the filtrate.

    • Driving Force: Pressure drop ($ riangle P$) is required to initiate the filtration process.

Filtration Examples and Descriptions

  • Slurry Composition Example:

    • 10% solids, 90% liquid by weight.

  • Typical Outcomes of Filtration:

    • Filter-Cake: Composed of 25% liquid, 75% solids.

    • Filtrate: Contains approximately 0.01% solids.

Particle Size Comparison

  • Comparison Table (in µm and in. with US Mesh equivalent):

    • Beach Sand: >150 µm, >7 in., US Mesh: N/A

    • Granular Salt: 100 µm, 4 in., US Mesh: <80

    • Human Hair Diameter: 45-70 µm, 2-3 in., US Mesh: 325-200

    • White Blood Cells: 25 µm, 1 in., US Mesh: 550

    • Red Blood Cells: 8 µm, 0.3 in., US Mesh: 1750

    • Viruses: 0.008-0.1 µm, 0.0003-0.004 in., N/A

    • Aqueous Salt: <0.002 µm, <0.00008 in., N/A

Filtration Applications

  • Beyond Product Recovery: Filtration is used for handling heat-sensitive materials like antibiotics, antifungals, and vitamins, where high temperatures would cause denaturation.

  • Sterilization Methods:

    • Syringe filters for small volumes.

    • Sterilization of air involves pumping air through a sterilized filter to prevent contamination in aerobic cultures.

Filtration Equipment Classifications

  • Classification Criteria:

    • By Driving Force:

      • Gravity Filtration: Flow induced by hydrostatic head, e.g., water purification through sand.

      • Applied Forces: High pressure or vacuum to enhance filtration.

Classification by Filtration Mechanism

  • Cake Filtration: Solids accumulate on the filter’s surface, forming a cake that provides further filtration.

  • Clarifying Filtration: Solids are trapped within the pores of the filter medium.

Objective and Operating Cycles of Filtration

  • By Objective:

    • Dry Solids: Best achieved with cake filtration.

    • Filtrate: Best achieved with depth filtration.

  • By Operating Cycle:

    • Batch Filtration: Intermittent process.

    • Continuous Filtration: Ongoing; allows for cake removal continuously.

Nature of Solids in Filtration

  • Compressible vs Non-compressible: Different properties affecting filtration efficacy.

  • Examples include yeast for compressible cakes and sand or activated carbon for non-compressible cakes.

Types of Filtration

  • Depth Filtration: Constructed with a matrix (e.g., glass wool, filter paper) that traps particles within its structure.

  • Membrane/Absolute Filtration: Utilizes pores smaller than the filtered particles, applicable in ultrafiltration and reverse osmosis. This method is often more expensive.

Filtration Aids

  • Role and Functionality:

    • Aids in processes needing fine solid removal (<5 µm).

    • Applications include catalyst recovery, active pharmaceutical ingredients, and clarification of gray water.

  • Challenges with Fine Solids: Fine solids can lead to a rapid decline in filtration rates due to impermeable coatings on filters; pre-treatment with filter aids can address this issue.

  • Types of Filter Aids: Commonly, diatomite, perlite, and cellulose are used, requiring specific properties like rigidity and inertness.

Diatomite as a Filter Aid

  • Description: Contains microscopic silica skeletons of diatoms.

  • Processing: Mined, calcined, and milled to create various grades.

  • Advantages: Highly porous structure, inert in diverse applications.

Perlite as a Filter Aid

  • Description: Derived from volcanic rock, expanded through heating.

  • Advantages: Lower specific weight than diatomite, which reduces the amount needed for filtration.

Cellulose as a Filter Aid

  • Usage Characteristics: Less common due to higher costs and less filtration efficiency, but valuable for recovery processes and compatibility with hot caustics.

Relationship Between Particle Size and Filtration Efficiency

  • Particle size influences the ability to capture fine solids; smaller particles lead to improved clarity but increase resistance.

  • It’s critical to balance clarity with filtration efficiency, dictating optimal filter aid selection.

Filter Medium Characteristics

  • The composition of the filter medium is critical for functionality; it must effectively retain the filter aid while allowing liquid flow.

Types of Utilization for Filter Aids

  • Precoat: Involves establishing a layer of solids on the filter medium for improved filtration.

  • Body Feed: Involves mixing filter aids with the slurry feed to help develop a porous filter cake that promotes higher flow rates and improved clarity.

Filtration Systems with Filter Aids

  • Types of Systems:

    • Plate-and-frame filter presses.

    • Pressure leaf filters (horizontal and vertical).

    • Candle or tubular filters.

    • Nutsche filters.

    • Rotary vacuum drum filters.

Filter Press Functionality

  • Components: Consists of a frame, filter plates covered with cloth, and mechanisms for operations like opening and cleaning.

  • Operational Principle: Slurry is introduced and filtrate is extracted through cloths positioned within alternating plates, creating filter cakes that join when the framework fills up.

Advantages and Disadvantages of Filter Presses

  • Advantages:

    • Versatile for different materials and conditions.

    • Low maintenance costs with a compact footprint.

    • Effective in both cake and filtrate production.

  • Disadvantages:

    • Intermittent operation can lead to higher wear on cloths.

    • Labor-intensive processes may be required for maintenance.