Biomass and Biofuels Lecture 33

Introduction

  • Transition from previous session on reactor types and systems.
  • Quick recap of high temperature filter and heat exchanger modeling.

Types of Reactors

  • Fluidized Bed Reactors: Utilizes particles suspended in fluid.
  • Ablative Reactors: Engages thermal decomposition of materials.
  • Auger Reactors: Employs screw mechanism for material movement.
  • Microwave Reactors: Harnesses microwave radiation for pyrolysis.

Overview of Heat Exchangers

  • Purpose: To condense pyrolysis vapors into bio-oil.
  • Components of the system:
    • Heating Fluid Side: Hot fluid side containing pyrolysis vapors.
    • Cooling Fluid Side: Fluid which absorbs heat from vapor.

System Modeling

  • Overview of a simple model for a pyrolysis condensation system, emphasizing:
    • Heating and cooling fluid dynamics.
    • Need for temperature control to ensure efficient condensation.

Heat Exchange Concepts

Flow Configurations

  1. Parallel Flow:
    • Significant driving force upon fluid contact.
    • Gradual decrease in driving force as temperature differences diminish.
  2. Counter Flow:
    • Maintains more uniform driving force across the system.
    • Enables enhanced heat transfer efficiency.

Equations and Calculations

  • Logarithmic Mean Temperature Difference (LMTD):
    • Used to derive heat exchanger effectiveness in systems with varying temperature gradients.
    • Formula involving four temperatures (two for cooling fluid, two for heating fluid).
  • Correction Factor:
    • Needed when different configurations exist compared to basic designs.
    • Impacts estimated correction factor for optimizing design based on flow passes.

Example of Heat Exchanger Design

  • Objective: To design a heat exchanger for condensing pyrolysis vapors with chilling water.
  • Constraints: Length should not exceed eight feet (2.4 meters).
  • Flow Rate: Hot fluid (pyrolysis vapors) rates at 15,000 pounds per hour.
  • Initial Temperature Conditions:
    • Water heated from 100°F to 130°F (with additional unit conversions to Celsius).
  • Geometric Consideration:
    • Initial schematic shows a critical evaluation of length; adjustments involve multi-pass designs to fit spatial constraints.

Assumptions in Analysis

  • Assumes heat capacities of water and pyrolysis vapors to be identical for simplification purposes; a critical understanding of real scenarios cautioned.
  • Emphasis on recalculating dimensions based on design constraints and thermal requirements.

The Two-Dimensional Scrubber

Fundamentals

  • Fluid Mechanics: Revisit Bernoulli’s Principle and its applications in flow systems.
    • Exploration of energy, velocity, and pressure relationships in ducts.

Venturi Design

  1. Principle: Compresses fluid flow through tapered areas to increase velocity and decrease pressure.
  2. Demonstration: Visual representation of how pressure behaves as fluid transitions through a venturi tube.
    • Demonstration with colored water to illustrate pressure effects.

Application in Pyrolysis Systems

Pyrolysis Vapors Collection

  • Collection predominantly involves water (80-90% water saturated with organics).
  • Water Spray Integration:
    • Aims to coalesce residual vapors for efficient removal.
    • Mechanism to eliminate unwanted water from electrostatic precipitators.

Electrostatic Precipitators (ESP)

  • Operation Principle:
    • Electrostatic charges are employed to separate particulates from vapors.
    • High-voltage systems polarize bio-oil molecules, allowing condensation and particle collection effectively.
  • Voltage Specifications for Scaling:
    • Lab-scale operations require around 10-20 kilowatts.
    • Larger systems may demand up to 100 kV for effective operation in industrial settings.

Application in Industry

  • Adaptation from water pollution control technologies, effectively applied in reducing dust and separation of light particles.

Conclusion and Practical Implications

  • Understanding the design and operational standards of heat exchangers and ESP systems enhances efficiency in bio-oil extraction while reducing environmental impacts.
  • Emphasizes the necessity of managing heat transfer and separating unwanted components through advanced systems.
  • Engage with practical design strategies that respect engineering constraints yet strive for stability and efficiency in pyrolysis processes.

Demonstration of ESP Assembly

  1. Materials Needed: Thick wire, metal mesh, plastic jar, etc.
  2. Assembly Steps:
    • Construction of a mesh supported by non-conductive materials for effective particle collection.
    • Demonstrate voltage application and visual results with smoke interaction.

Closing Remarks

  • Encouragement of hands-on experimentation to deepen understanding of electrostatic precipitator dynamics in relation to pyrolysis systems.