Ch E 452: Process Design, Analysis, and Simulation Flowsheet Recycle Structure

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Ch E 452 Process Design,Analysis, and
SimulationFlowsheet Recycle Structure

• David A. Rockstraw, Ph.D., P.E.
• New Mexico State University
• Chemical Engineering

2
Recycle Questions

• How many reactor systems are required? Is there
  any separation between reactor systems?
• How many recycle streams are required?
• Do we want to use an excess of one reactant?
• Is a gas compressor required? Costs?
• Should reactor be operated adiabatically, with
  direct heating/cooling, or diluent heat carrier?
• Do we want to shift the equilibrium conversion?
• How do the reactor costs affect the EP?

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Number of Reactor Systems

• If reaction sets
• use different temperatures and/or pressures,
• require different catalysts,
• produce interfering species, or
• require different solvents,
• use different reactor systems for each set.

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Number of Reactor Systems
1 reactor system

• toluene H2 ? benzene CH4
• 2 benzene ? diphenyl H2
• acetone ? ketene CH4
• ketene ? CO ½C2H4
• ketene acetic acid ? acetic anhydride

1300F 500 psia
2 reactor system
700C 1 atm
80C 1 atm
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Number of Recycle Streams

• Reaction steps are associated with Reactor
  numbers.
• Feed streams are also associated with Reactor
  numbers (the reactor in which the feed is a
  reactant).
• Recycle streams are associated with Reactor
  numbers in which a component of the recycle acts
  as a reactant.

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Number of Recycle Streams
products
acetic acid feed
Reactor 1
Reactor 2
separation train
acetone feed
acetic acid recycle
acetone recycle
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Number of Recycle Streams

• Order components by boiling point destination

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Number of Recycle Streams

• Analysis indicates two recycles, one gas and one
  liquid

H2, CH4
process
H2, CH4
benzene
diphenyl
toluene
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Number of Recycle Streams

• By unit operations, flowsheet becomes

compressor
purge H2, CH4
H2 feed
reactor
separator
benzene
diphenyl
toluene feed
toluene recycle
10
Number of Recycle Streams
11
Number of Recycle Streams
acetic acid feed
acetone feed
CO, CH4, C2H4
separator
Reactor 1
Reactor 2
acetic acid recycle
acetone recycle
anhyd.
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Excess Reactant

• Use of an excess of a reactant can shift the
  product distribution.
• Use of excess isobutene leads to improved
  selectivity to isooctane.
• The larger the excess, the larger the selectivity
  improvement, but the larger the cost penalty in
  recovering the excess isobutene.

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Excess Reactant

• Use of an excess of a reactant can force complete
  (or nearly complete) conversion of another
  reactant.
• CO Cl2 ? COCl2
• In the production phosgene for use in
  di-isocyanate manufacture, the product must be
  chlorine-free. Use of excess CO forces a high
  conversion of Cl2.

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Excess Reactant

• The use of an excess reactant can shift
  equilibrium conversion.
• Use of high excesses of hydrogen in the reaction
  of benzene to cyclohexane forces equilibrium
  conversion toward the saturated product, avoiding
  the close-boiling distillation of the two
  materials.

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Recycle Material Balances

• Material Balance on Limiting Reactant
• assuming complete recovery

compressor
purge H2, CH4
H2 feed
reactor
separator
benzene
diphenyl
FFT
FT
toluene feed
FT(1-x)
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Recycle Material Balances

• Material Balance on Limiting Reactant
• not assuming complete recovery

FP FR
reactor
separator
F
F(1-x)
FP
F(1-x) - FP
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Recycle Material Balances

• Material Balance on other Reactants
• Based on needed Mole Ratio (MR) in the reactor

FG, yFH
PG
RG, yPH
MR
reactor
separator
PB
PD
FT
FFT
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Recycle Material Balances
FG, yFH
PG
RG, yPH
MR
reactor
separator
PB
PD
FT
FFT
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By-products formed by secondary reversible
reactions

• If we recycle a by-product formed by a secondary
  reversible reaction, and let the component
  build-up to its equilibrium level, we find the
  recycle flow by using the equilibrium
  relationship at the reactor exit
• Where hydrogen and benzene concentrations can be
  found by neglecting the reversible reaction in
  the balance.

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By-products formed by secondary reversible
reactions

• Overall Balances
• Production of cyclohexane PC fixed
• Benzene fresh feed FFB PC
• Excess Hydrogen FE design variable
• Total Hydrogen feed FG 3PC FE yFHFG

PG, yPH
RG, yPH
yFH, yFCH4, yFN2
MR
FG
C6H6 3H2 ? C6H12
separator
PC
FFB
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By-products formed by secondary reversible
reactions

• Overall Balances
• Purge gas composition yPH FE / FE (1
  yFH)FG
• Makeup gas rate FG 3PC (1 yPH) / (yFH
  yPH)
• Purge rate PG FE (1 yFH) FG

PG, yPH
RG, yPH
yFH, yFCH4, yFN2
MR
FG
C6H6 3H2 ? C6H12
separator
PC
FFB
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By-products formed by secondary reversible
reactions

• Recycle Balances
• Benzene fed to reactor FB PC / x
• Recycle gas rate RG (1 / yPH) (MR PC) / x
  yFHFG

PG, yPH
RG, yPH
yFH, yFCH4, yFN2
MR
FG
C6H6 3H2 ? C6H12
separator
PC
FFB
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By-products formed by secondary reversible
reactions

• Reactor effluent
• Cyclohexane PC
• Benzene FB PC (1 x) / x
• Hydrogen MRB 3PC (MR/x 3) PC
• Inerts (1 yFH)FG (1 yPH)RG
  (1 yPH)(MR/x 3)PC / yPH

PG, yPH
RG, yPH
yFH, yFCH4, yFN2
MR
FG
C6H6 3H2 ? C6H12
separator
PC
FFB
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By-products formed by secondary reversible
reactions

• Equilibrium relationship

PG, yPH
RG, yPH
yFH, yFCH4, yFN2
MR
FG
C6H6 3H2 ? C6H12
separator
PC
FFB
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By-products formed by secondary reversible
reactions

• Since cyclohexane and benzene are close-boilers,
  we would like to avoid a distillation of these
  components.
• This can be accomplished by operating the reactor
  at a sufficiently high conversion subject to the
  expression developed for Ke f(xe, MR, yPH) that
  we can leave any unconverted benzene as a product
  impurity.

PG, yPH
RG, yPH
yFH, yFCH4, yFN2
MR
FG
C6H6 3H2 ? C6H12
separator
PC
FFB
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Separator Reactors

• If a product can be removed during the reaction,
  an apparent equilibrium-limited reaction can be
  forced to go to complete conversion.
• xe 0.32
• solid
• catalyst

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Separator Reactors

• Suspend catalyst in a high boiling solvent
  operating above the boiling point of the acetone.
  Both H2 and acetone are removed in the vapor,
  driving equilibrium to the right.
• xe 0.32
• solid
• catalyst

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Separator Reactors

• Both acrylic acid and ethyl acrylate are
  monomers, which tend to polymerize in reboilers
  of distillation columns.

• Operation
• Excess ethanol
• Operate just below TBP of acrylic acid
• ethanol, water, ethyl acrylate are taken overhead
• If a reboiler is used as the reactor, product can
  beisolated, and ethanol recovered

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Gas-Phase Diluents

• Temperature, Pressure, and molar ratio can be
  used to shift equilibrium conversions (already
  shown).
• An extraneous component (a diluent) can in some
  cases be added which also causes a shift in the
  xe.

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Gas-Phase Diluents

• 1100F, 20 psia
• Steam addition (or CH4) at inlet lowers the
  partial pressures of styrene and hydrogen,
  decreasing the reverse reaction rate.
• Steam also supplies heat needed to drive this
  endothermic reaction
• Water-hydrocarbon immiscibility simplifies
  separation of components

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Reactor Heat Effects

• Should reactor be operated adiabatically, with
  direct heating/cooling, or with a diluent heat
  carrier?
• If we use a heat carrier, the material balances
  will need to be changed, thus decide before
  proceeding further.
• To aid in the decision, estimate the reactor heat
  load and the adiabatic temperature change.

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Reactor Heat Load

• For single reactions, nearly all the fresh feed
  of the limiting reactant gets converted in the
  process, thus,
• Reactor heat load, QR DHrxn x FFT
• Adabatic T change can be found from
• QR FCp (TR,in TR,out)

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Heat Load Heuristics

• If adiabatic operation is not feasible, attempt
  direct heating/cooling.
• The maximum heat transfer area that fits into a
  shell-and-tube floating-heat heat exchanger is in
  the range of 6000 8000 ft2.
• Assuming an overall heat transfer coefficient of
  20 Btu/(hrft2F), a DT of 50F, a heat load of
  106 Btu/hr
• Therefore, when attempting to control heat by
  direct heating/cooling, 6-8 million Btu/hr is the
  maximum that can be handled by a single heat
  exchanger.

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Heat Carrier Diluents

• If we desire to moderate the temperature change
  in a reactor, increase flowrate (lower conversion
  per pass) can be used. Recycling a portion of
  the product will often result in an increased
  heat capacity of the reactor feed.
• Adding an extraneous component can be used when
  recycle of reactants/products is not
  possible/feasible. However, the separation
  system becomes more complex.

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Heat Carrier Diluents

• Methane recycle will act as a diluent

purge H2, CH4
H2 feed
reactor
separator
benzene
diphenyl
toluene feed
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Reactor Design

• Single, irreversible reactions (not
  autocatalytic)
• Isothermal always use a PFR
• Adiabatic
• PFR if reaction rate monotonically decreases with
  x
• CSTR operating at max rate followed by PFR
  section
• Single reversible reaction adiabatic
• Maximum temperature if endothermic
• A series of adiabatic beds with decreasing
  temperature profile if exothermic

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Reactor Design

• Parallel Reactions, composition effects
• A?R (desired) A ?S (waste)
• a1gta2, keep CA high
• Use batch or PFR
• High pressure, eliminate inerts
• Avoid recycle of products
• Can use a small reactor
• a1lta2, keep CA low
• Use CSTR with high x
• Large recycle of products
• Low pressure, add inerts
• Need a large reactor

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Reactor Design

• Parallel Reactions, composition effects
• AB ? R (desired) AB ? S (waste)
• a1gta2 and b1gtb2, both CA and CB high
• a1lta2 and b1gtb2, CA low and CB high
• a1gta2 and b1ltb2, CA high and CB low
• a1lta2 and b1ltb2, both CA and CB low

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Reactor Design

• Parallel Reactions, temperature effects
• E1gtE2, high temperature
• E1ltE2, increasing temperature profile

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Reactor Design

• Consecutive Reactions, composition effects
• Minimize mixing of streams with different
  compositions
• Low conversion and recycle A
• Consecutive Reactions, temperature effects
• Minimize mixing of streams with different
  compositions
• E1gtE2, decreasing temperature profile
• E1ltE2, low temperature

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Recycle Compressors

• Whenever a gas recycle stream is present, we need
  a recycle compressor.
• Compressors are specified by work duty.
• g (CP/CV 1) / (CP/CV)
• Q volumetric flowrate (ft3/min)
• P pressure (lbf/ft2)

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Recycle Compressors

• Exit temperature (in absolute units)
• Initially, assume an efficiency of 90 to account
  for fluid friction in the suction and discharge
  valves ports, friction of moving metal surfaces,
  fluid turbulence, etc.
• Assume a driver efficiency of 90 to account for
  conversion of input energy to shaft work.

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Recycle Compressors

• Redundancy
• Redundant compressors are seldom specified,
  particularly centrifugal compressors.
  Reciprocating compressors have a lower service
  factor, and a redundant may be included.
• In some cases, install two compressors, each
  operating at 60 of the load. In this manner,
  partial operation of the plant can be maintained
  if one of the units goes down, while also
  providing some flexibility in responding to
  changes in operating conditions.

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Recycle Compressors

• Multistage Compressors
• Gas is cooled to cooling water temperature
  (100F) between stages.
• Knockout drums are installed between stages to
  remove condensates.
• Condensation occurring inside the compressor will
  lead to liquid droplets contacting the high speed
  vanes, resulting in imbalances, vibrations, and
  ultimately, damage.

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Recycle Compressors

• Multistage Compressors
• Work-load required for a three-stage compressor
• Intermediate Ps that minimize work requirements

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Recycle Structure Decisions

• How many reactors are required?
• How many recycle streams are required?
• Use an excess of a reactant?
• Is a gas-recycle compressor required?
• Adiabatic, direct heating/cooling, diluent?
• Does an xe need to be shifted?
• How do reactor costs affect EP?

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Design Guidelines for First Design Recycle
Decisions

• If reactions take place at different temperatures
  and pressures and/or they require different
  catalysts, then a separate reactor system is
  required for each.
• Components recycled to the same reactor that have
  neighboring boiling points should be recycled in
  the same stream.
• A gas-recycle compressor is required if the
  recycled components boil at a temperature lower
  than propylene.
• If an excess reactant is desirable, there is an
  optimum amount of excess.

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Design Guidelines for First Design Recycle
Decisions

• If the reactor temperature, pressure, and/or
  molar ratio are changed to shift the equilibrium
  conversion, there must be an optimum value of
  these variables.
• For endothermic processes with a heat load of
  less than 6-8x106 Btu/hr, use an isothermal
  reactor with direct heating. For larger heat
  loads, consider diluent.
• For exothermic reactions, use an adiabatic
  reactor if DTrxtr is less than 10-15 of the
  inlet temperature. Otherwise, use direct cooling
  if the heat load is less than 6-8x106 Btu/hr.

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Design Guidelines for First Design Recycle
Decisions

• For single reactions, choose a conversion of 0.96
  0.98 of xe.
• The most expensive (or the heaviest) reactant is
  usually the limiting reactant.
• Recycle reactant if equilibrium conversion of a
  reversible by-product is small.
• The recycle flow of the limiting reactant is F
  FR (1-x)/x, where FR is the amount of limiting
  reactant needed for the reaction.
• Recycle flows of other reactants can be
  determined by specifying inlet molar ratios.