processing L2

• Drying


  Removal of excess moisture for safe storage
• Delay in drying


  Leads to damage
• Too rapid drying


  Results in stress cracks
• Over-drying


  Results in economic loss
• Benefits from drying


  • Reduces quantity and quality losses
  • Expands farmers' opportunities to temporarily store and seek better markets without quality deterioration
  • Averts potential losses in quality and quantity
• Forms of losses due to delay or improper drying

  • Discoloration
  • Loss of seed viability (seed growers)
  • Reduced milling recovery
  • Altered texture/appearance and taste
  • Spillages/other physical losses
  • Price reduction
• Drying principle


  • Heat transfer
  • Mass transfer
• Absorption


  • The process by which one substance takes up another substance through pores or spaces between them
  • Involves the whole volume of material
• Adsorption


  • The process in which there is adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to surface
  • Involves the surface area of material
• Common methods of heat transfer

  • Radiation (heat energy from the sun)
  • Conduction (transfer of thermal energy through direct contact)
  • Convection (transfer of thermal energy through the movement of liquid or gas)

  
• Mass transfer


  The net movement of mass from one location to another. It includes desorption, absorption, evaporation, distillation and filtration
• If VPg > VPa

  Desorption
• If VPg < VPa

  Absorption
• If VPg = VPa

  Equilibrium Moisture Content (EMC)
• Grain drying requirements


  • Heat (to hasten evaporation of moisture)
  • Air (to transport heat to grain)
• Too much heat


  Will result in stress cracks
• Too low airflow


  Will result in moisture gradient
• Stress cracks


  Occur when: evaporation is faster than diffusion, there's uneven shrinkage between surface and inner grain, grain is rewetted and surface expands abruptly due to absorption of moisture
• Grain drying characteristics


  • Constant rate period (moisture from the grain surface is evaporating as free water)
  • Falling rate period (moisture evaporation rate is dictated by rate of diffusion of moisture from the inner portion to the outer portion of the grain)
• It requires more energy to remove moisture at the falling rate period than at the constant rate period
• During constant rate period


  Higher drying air temperature can be used such as 70-80°C as grain is protected by effect of evaporative cooling
• When the surface (rice husk) starts to dry to 18% moisture

  Effect of evaporative cooling is nil so that use of high drying air temperature is not recommended
• Use 50 to 60°C if intended for milling, use a maximum of 43°C for seeds
• Two-stage drying strategy


  1. High-speed or flash drying using high temperature to partially dry grain down to 18% moisture

  2. Drying to 14% using conventional method such as sundrying
• Types of mechanical dryers

  • Batch type (grain is stationary during drying)
  • Continuous flow (grain moves across the drying chamber continuously and are equipped with tempering bins)
  • Batch-type recirculating (grain is continuously recirculated during drying)
• Flatbed dryer


  • Blower
  • Burner
  • Bin or bed
• Too much heat in a flatbed dryer

  Results in moisture gradient (overdried and underdried grain)
• Too low airflow in a flatbed dryer

  Also results in moisture gradient (overdried and underdried grain)
• Moisture gradient


  Occurs when there is moisture difference between grain at the bottom and top of the flatbed dryer
• Moisture gradient can be minimized by lower drying temperature, lower grain depth, higher air flow rate, and stirring
• There should be adequate air pressure (1 inch water) in a flatbed dryer
• Maintain recommended grain thickness (18 inches depth) in a flatbed dryer
• Flatbed dryer with reversible air flow

  • Drying process with reversed flow
• Parts of the LSU-type grain dryer

  • Grain buffer module
  • Drying Module 1
  • Drying Module 2
  • Elevator
  • Unloader Module
  • Loading hopper
  • Biomass Furnace
• LSU drying module


  • Grain is recirculated several times
  • Batch operation
• Problems with imported recirculating dryers

  • Blower too small
  • Small drying column
  • Complicated electronics control system (ECS)
• Cylindrical type recirculating batch dryer

  • Drying air
  • Ambient air
  • Grain flow
• Types of burners


  • Direct-fired (flue gas and other products of combustion goes into the dryer)
  • Indirect-fired (a heat exchanger is used to prevent smoke and other products of combustion to go with the drying air)
• Direct-fired burner


  Cheaper, common for fossil fuel burners (pot burners), danger of fire due to fly ash (rice hull-fed furnace)
• Indirect-fired burner


  Twice as expensive, common for biomass furnace, dry type uses air as heat transfer medium, wet type uses water as heat transfer medium