12.1 Filtration theory and equipment

Filtration is a crucial separation process that removes solids from fluids using a porous medium. It's essential in various industries, from water treatment to pharmaceutical production. Understanding the principles, equipment types, and cake formation is key to optimizing filtration processes.

Calculations play a vital role in analyzing filtration performance. Equations for filtration rate, constant pressure, and constant rate filtration help predict and optimize processes. Determining cake properties and considering scale-up factors are crucial for efficient industrial applications.

Filtration Fundamentals

Principles of filtration

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• Separation of solids from fluids using porous medium enables purification and concentration (water treatment, pharmaceutical production)
• Pressure difference across filter medium drives fluid flow and particle retention (vacuum filtration, pressure filtration)
• Filtration mechanisms trap particles:
  • Sieving blocks larger particles than pore size
  • Interception captures particles approaching filter surface
  • Inertial impaction collects particles deviating from fluid streamlines
  • Diffusion traps submicron particles through Brownian motion
• Filtration performance affected by multiple factors:
  • Particle size distribution influences capture efficiency (sand, clay, bacteria)
  • Fluid viscosity impacts flow resistance (water, oil, syrup)
  • Filter medium properties determine particle retention:
    • Pore size controls smallest particle captured
    • Porosity affects flow rate and cake formation
    • Thickness influences pressure drop and particle capture
  • Operating conditions optimize process:
    • Pressure drop drives filtration rate
    • Flow rate affects particle capture and cake formation
    • Temperature alters fluid viscosity and particle behavior
• Filtration efficiency measures particle removal effectiveness calculated as ratio of retained to total feed particles

Types of filtration equipment

• Plate and frame filter press handles high-pressure batch operations for slurry dewatering (mining tailings, chemical processing)
• Rotary vacuum filter provides continuous low-pressure filtration for suspension dewatering (food processing, wastewater treatment)
• Belt filter offers continuous gravity or vacuum-assisted dewatering of sludges (paper manufacturing, municipal wastewater)
• Cartridge filters use disposable or cleanable elements for liquid and gas filtration removing fine particles (hydraulic systems, HVAC)
• Bag filters employ fabric or synthetic media for dust collection and air pollution control (cement plants, power stations)
• Membrane filters separate particles based on size:
  • Microfiltration removes bacteria and large colloids
  • Ultrafiltration captures proteins and viruses
  • Reverse osmosis rejects dissolved salts and small molecules
• Depth filters utilize porous media for particle capture throughout filter volume:
  • Sand filters remove suspended solids from water
  • Multimedia filters combine materials for improved particle retention

Cake formation in filtration

• Cake filtration theory explains solid accumulation on filter medium creating growing cake layer over time
• Cake properties determine filtration performance:
  • Porosity affects fluid flow through cake
  • Specific cake resistance quantifies flow hindrance
  • Compressibility describes cake behavior under pressure
• Darcy's law for cake filtration relates flow rate to pressure drop across filter cake and medium
• Cake resistance represents intrinsic resistance of accumulated solids layer affected by:
  • Particle size and shape influence packing density
  • Cake thickness increases with filtration time
  • Applied pressure may compress cake altering structure
• Filter medium resistance constitutes initial clean filter resistance
• Total filtration resistance combines cake and medium resistances determining overall filtration rate

Filtration performance calculations

• Filtration rate equation describes flow behavior: $\frac{dV}{dt} = \frac{A \Delta P}{\mu (R_c + R_m)}$
  • V: filtrate volume, t: time, A: filtration area
  • ΔP: pressure drop, μ: fluid viscosity
  • R_c: cake resistance, R_m: medium resistance
• Constant pressure filtration analyzes time-volume relationship: $\frac{t}{V} = \frac{\alpha \mu c}{2A^2 \Delta P}V + \frac{\mu R_m}{A \Delta P}$
  • α: specific cake resistance, c: solids concentration in feed
• Constant rate filtration examines pressure-time relationship: $\Delta P = \frac{\mu q}{A}(R_m + \alpha c V/A)$
  • q: constant filtration rate
• Cake properties calculations determine:
  • Specific cake resistance from experimental data
  • Cake porosity using particle and cake densities
  • Cake compressibility index from pressure-resistance data
• Scale-up considerations transition from laboratory to industrial scale estimating required filtration area
• Optimization of filtration processes aims to:
  • Minimize filtration time increasing throughput
  • Maximize filtrate clarity improving product quality
  • Balance energy consumption and filtration efficiency reducing operational costs