Transcript PowerPoint 簡報 - National Cheng Kung University

Chapter 6
Pollution Prevention for Unit
Operations – Part 2
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.
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
STOICHIOMETRIC FACTOR
(SF)
The stoichiometric moles of reactant
required per mole of product
Gas recycle
Purge
H2 , CH4
Toluene 1  x
Benzene
Sx
1
2
Benzene
Diphenyl (1  S) x
H2 , CH4
Reactor
system
Toluene
Separation
system
Sx
Dipheny1
x
1 x
Toluene recycle
Material Balance of the Limiting
Reactant (Toluene)
Assumption: completely recover and recycle the limiting reactant.
1
(1  S ) x
2
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.
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.
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.
Parallel Reaction Networks
• Parallel reactions
kP
R 
P
R W
kW
• Rate expressions
 EP  nP
rP  k exp  
CR

 RT 
0
P
 EW  nW
rW  k exp 
CR

 RT 
0
W
np  nw  1
The reaction selectivity is constant and independent of residence time for
1st-order, irreversible, isothermal parallel reactions.
Series Reaction Networks
• Series reactions
R  P W
kP
kW
• Rate expressions
 EP  nP
rP  k exp  
CR

 RT 
0
P
 EW  nW
rW  k exp 
CP

 RT 
0
W
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.
Reversible
Reactions
Impact of Temperature on
Selectivity
rp
rw


rp
rw

0  E p / RT
p
k e

if
0  Ew / RT 
w
k e

e
e
 EP / RT1 
 EP / RT0 
e
e
n p  nw  1
 EW / RT1 
 EW / RT0 
 ( E p  Ew ) / RT1
e
  ( E p  Ew ) / RT0
e
Impact of Concentration on
Selectivity
• The selectivity ratio for parallel reactions
rp
rw

k pCRnP
nW
R
kwC

kp
kw
CRnP nW
If nP  nW 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Applications of Membrane
Separative Reactors
• Thermodynamically-limited reactions, e.g.,
C6H12↔C6H6+3H2
• 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
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.
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
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.
Methods to Reduce Fugitive
Emissions
• Leak detection and repair (LDAR) of
leaking equipment
• Equipment modification or replacement
with emission-less technologies.
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 (>10000ppm), the
equipment is said to be leaking and repair is
required.