Pollution Prevention for Unit Operations

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Chapter 6

• Pollution Prevention for Unit Operations Part 2

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Pollution Prevention for Chemical Reactors

• From an environmental perspective, reactors are
  the most important unit operation in a chemical
  process.
• The degree of conversion of feed to desired
  products influences all subsequent separation
  processes, recycle structure for reactors, waste
  treatment options, energy consumption, and
  ultimately pollution releases to the environment.
• Once a chemical reaction pathway has been chosen,
  the inherent product and byproduct (waste)
  distribution for the process are to a large
  extent established.

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REACTOR PERFORMANCE

• Conversion (x)
• (reactant consumed in the reactor)/(reactant
  fed to the reactor)
• Selectivity (S)
• (desired product produced)/(reactant consumed
  in the reactor)SF
• Reactor Yield (Y)
• (desired product produced)/(reactant fed to the
  reactor)SF

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STOICHIOMETRIC FACTOR (SF)

• The stoichiometric moles of reactant required per
  mole of product

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Gas recycle
Purge
H2 , CH4
Toluene Benzene Diphenyl
Benzene
Reactor system
Separation system
H2 , CH4
Toluene
Dipheny1
Toluene recycle
Material Balance of the Limiting Reactant
(Toluene)
Assumption completely recover and recycle the
limiting reactant.
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Design Considerations

• The raw materials, products, and byproducts
  should have a relatively low environmental and
  health impact potential.
• The yield and selectivity should both be high.
• Energy consumption should be low.
• The life-cycle impacts reactants, products and
  byproducts should be relatively low.

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Waste Reduction Methods in Reactor Design

• Changing process chemistry (precursors and/or
  catalysts)
• Avoiding storage of hazardous materials (in situ,
  on-demand generation)
• Maximizing selectivity
• Prolonging catalyst life
• Combining reactor and separator.

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Material Use and Selection

• Raw materials and feedstocks
• New process chemistry
• Purer raw material
• Solvents
• Substitute solvent
• Catalysts
• can allow the use of more environmentally benign
  chemicals as raw materials,
• can increase selectivity toward the desired
  product and away from the unwanted by product
  (waste),
• can convert waste chemicals to raw materials,
• can create environmentally acceptable products
  directly from the reactions.

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Parallel Reaction Networks

• Parallel reactions
• Rate expressions

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The reaction selectivity is constant and
independent of residence time for 1st-order,
irreversible, isothermal parallel reactions.
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Series Reaction Networks

• Series reactions
• Rate expressions

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To minimize waste generation in series reactions,
it is important to operate the reactor so that
the ratio is as large as possible and to control
the reaction residence time.
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Reversible Reactions
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Impact of Temperature on Selectivity
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Impact of Concentration on Selectivity

• The selectivity ratio for parallel reactions
• If then selectivity is improved by
  increasing the conc. of R If otherwise, then the
  conc. of R should be decreased.
• The analysis of series reactions is more complex.

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Impact of Mixing on Selectivity

• Improve physical mixing in the reactor, which
  will improve selectivity if the reaction order is
  greater than 1.
• Distribute feeds better to avoid
  short-circuiting.
• Premixing of reactants may result in better
  selectivity.
• Provide a separate reactor for recycle streams.
• Examine heating and cooling techniques to avoid
  cool spots and hot spots. Eliminate direct steam
  injection.

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Wastes Generated by Separation Devices

• Separation unit operations generate waste because
  
• the separation steps themselves are not 100
  efficient, and
• require
• additional energy input or
• waste treatment
• to deal with off-spec products.

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Choice of Mass Separating Agent

• A poor choice may result in exposure to toxic
  substances fro not only facility workers but also
  consumers who use the end product.
• A poor choice may lead to excessive energy
  consumption and the associated health impacts of
  the emitted criteria air pollutants.

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Pollution Prevention Approaches for Separation
Equipments

1. Minimize the wastes and emissions that are
   routinely generated
2. Control excursions in operating conditions
3. Improve the design efficiency.

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Process Wastes Generated from Distillation

• By allowing impurities to remain in a product,
• By forming waste within the column itself (in
  reboiler),
• By inadequate condensing of overhead product
  (through the condenser vent), and
• By excessive energy use.

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Pollution Prevention Methods for Distillation
Columns

1. Increase the reflux ratio, add a section to the
   column, retray/repack the column, or improve feed
   distribution to increase column efficiency.
2. Changing the feed location may increase product
   purity.
3. Insulate or preheat feed to reduce the load on
   the reboiler.
4. Reduce the pressure drop in column, which reduce
   the load on reboiler.
5. Vacuum distillation may reduce reboiler
   requirements.

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Separative Reactors

• The key feature allowing for the prevention of
  waste generation and maximizing product yield is
  the ability to control the addition of reactant
  and the removal of product more precisely than in
  traditional designs.
• Separation units that have been integrated with
  reaction include distillation, membrane
  separation, and adsorption.

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Combined Reactor/Separator Catalytic
Distillation

• The conventional MTBE (methyl tert-butyl ether)
  producing process (from methanol and isobutylene)
  is given in Figure 6-9a.
• The alternative process is to feed the raw
  materials to a distillation column in which some
  of the packing material has been replaced by
  catalyst.
• Fugitive and process emissions are reduced.
• Fewer heat exchangers are required.
• Water is not needed to separate the components.
• Reaction equilibrium can be shifted since MTBE is
  less volatile than the reactants. In other
  words, it moves down the distillation column and
  away from the reaction zone as it is formed.

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Combined Reactor/Separator Membrane Technology

• Applicable when the product molecules are smaller
  than the reactant molecules.
• Both types of membrane in Figure 6-10 hold
  particular promise for reversible reactions
  because the product is removed as it is formed.

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Applications of Membrane Separative Reactors

• Thermodynamically-limited reactions, e.g.,
  C6H12?C6H63H2
• Parallel reactions in which product formation has
  a lower reaction order than byproduct generation
• Series reactions such as selective
  dehydrogenations and partial oxidations
• Series-parallel reactions

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Sources of Waste from Heat Exchangers

• Heat exchangers can be a direct source of waste
  when high temperatures cause the fluids they
  contain to form sludges.
• Because it reduces efficiency and increase energy
  requirements, sludge buildup in heat exchangers
  is an indirect source of combustion-related
  emissions.

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Sludge Reduction Methods

1. Reduce the temperature used in the heat
   exchanger (a) thermocompressor (Figure 6-12)
   (b) staged heating (Figure 6-13).
2. Plate-and-frame exchangers
3. Scraped-wall exchangers
4. Noncorroding tubes
5. Antifoulants
6. On-line cleaning techniques

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Fugitive Air Emissions

• These releases include equipment leaks from
  valves, pump seals, piping connectors, pressure
  relief valves, flanges, compressor seals,
  sampling connections, open-ended lines, and air
  releases from building ventilation system, etc.
• They are not easily identifiable and relatively
  large in number.

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Methods to Reduce Fugitive Emissions

• Leak detection and repair (LDAR) of leaking
  equipment
• Equipment modification or replacement with
  emission-less technologies.

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Leak Detection and Repair

• In a LDAR program, equipment such as pumps and
  valves are monitored periodically using an
  organic vapor analyzer (OVA).
• If the source registers an OVA reading over a
  threshold value (gt10000ppm), the equipment is
  said to be leaking and repair is required.

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