Environmental Considerations

1
Environmental Considerations

• Waste Reduction and Gas Treatment

2
Outline

• Environmental Considerations in Design
• Continuous vs. Fugitive Emissions
• Waste Avoidance and Minimization
• Treatment of Gaseous Emissions

3
Introduction

• A most important consideration in the design of
  chemical processes is their possible impact on
  the environment
  
• First Cannon of the Code of Engineering
  EthicsEngineers shall hold paramount the
  safety, health, and welfare of the public in the
  performance of their duties

4
Motivation

• Your main motivation for environmental concern
  must be your genuine concern for the environment,
  not the avoidance of fines and penalties
  
• You must also be familiar with and comply with
  environmental regulations (some are in Perrys,
  7th ed., Section 25)
  
• As chemical engineers you are the real
  environmentalists

5
Fugitive vs. Continuous Emissions

• The impact of a process on the environment is
  through the materials that come out of the
  process, its emissions
  
• Some emissions are unplanned, due to accidents or
  leaks, these are the fugitive emissions
  
• Other emissions are the waste streams coming out
  of the process, the continuous emissions
  
• At process design time we are more interested in
  handling the continuous emissions

6
Continuous Emissions

• Gas streams have the potential of polluting the
  air
  
• Liquid and waste water streams have the potential
  to pollute rivers and ground water
  
• Solid waste streams have the potential of
  polluting the landscape
  
• If it is not possible to eliminate these waste
  streams, they must be reduced and treated

7
Louis J. Thibodeaux, Ph.D., P.E., LSU Chemical
Engineering
8
Waste Avoidance Reduction

• Best block, not be there says the Karate Master
  to the kid in Karate Kid II.
  
• Best prevention, not make waste is an equivalent
  strategy
  
• Waste prevention techniques depend on the source
  of the waste

9
Waste is by-product of the main chemical reaction

• If the reaction that produces the product also
  produces the waste, there is no way to reduce the
  waste except by finding another
  reaction Reactants ? Product Waste
• Example Carbon dioxide is produced when we burn
  fossil fuels

10
Waste made by a side reaction

• If the waste is produced by a side reaction, it
  can be eliminated or reduced by changing the
  reaction conditions
  
• Temperature
• Pressure
• Excess reactant ratio
• Catalyst
• Redesign the reactorplug flow instead of stirred
  tank
  
• Example NOx can be reduced by redesigning the
  burners to avoid hot spots

11
Waste is un-reacted reactant

• If the waste is an un-reacted reactant it can be
  reduced by increasing the conversion through
  
• Changing reactor conditions or design
• Separating and recycling the un-reacted reactant
• If the reactant is a gas, the cost of the recycle
  compressor may be high
  
• This also reduces raw material costs
• Example recycle hydrogen in nitrobenzene
  hydrogenation process to produce aniline
  

12
Pollutant is a valuable chemical

• This happens when a product or by-product is not
  completely recovered from a waste stream
  
• Can be reduced by increasing recovery in existing
  separation equipment or installing new separation
  system
  
• Example recovery of aniline from waste water in
  aniline process

13
Waste Treatment

• After the rate of waste production is reduced,
  the remaining waste must be treated into
  earthenlike materials (LJT, Law 3)
  
• The method of treatment depends on the type of
  waste, gas, liquid, or solid

14
Gaseous Waste Treatment

• The separation methods are the same as used in
  the gas separation train in the process
  
• Absorption (Scrubbing) contact gas stream with
  liquid solvent, e.g., HCl from vent streams
  
• Condensation compress or refrigerate or both,
  e.g., cumene from oxidation reactor vent
  
• Reaction hydrocarbons can be harmlessly burned
  in a furnace or incinerator to recover the fuel
  value of the stream, e.g., hydrogen purge from
  hydrogenation processes

15
Gas Treatment (Continued)

• Adsorption contact gas with a solid sorbent.
  Used to remove the last traces of the pollutant
  after other operations have removed the bulk,
  e.g., cumene from reactor vent after
  condensation
• Membrane Separation probably would not be
  expected to reduce the concentration of the
  pollutant sufficiently
  

16
Particulates Removal

• Some gas streams carry solid particles that must
  be removed, e.g., bagasse dust from sugar mills
  
• Common operations are
• Bag Houses
• Electrostatic precipitators
• Cyclones
• Scrubbers

17
Summary

• The impact of the process on the environment is
  an important process design consideration
  
• First option should be the reduction or
  elimination of waste by changing design
  conditions
  
• The remaining waste must be properly treated so
  that it does not damage the environment
  
• Gas emissions can be treated by the same
  operations used for recovering gases in the
  process