Ch E 452: Process Design, Analysis, and Simulation Understanding Process Conditions

1
Ch E 452 Process Design,Analysis, and
SimulationUnderstanding Process Conditions

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

2
Understanding Process Conditions

• The ability to make an economic analysis of a
  chemical process based on a PFD is not proof that
  the process will actually work.

3
Understanding Process Conditions

• It is usually easier to adjust the temperature
  and/or pressure of a stream than it is to change
  its composition.
• In fact, often, the concentration of a compound
  in a stream (for a gas) is a dependent variable
  and is controlled by the temperature and pressure
  of the stream.

4
Understanding Process Conditions

• In general, pressures between 1 and 10 bar and
  temperatures between 40C and 250C do not cause
  severe processing difficulties.

5
Understanding Process Conditions

• Decisions that must be justified
• operation outside the range of 1 to 10 bar
• operation above 400C
• operation outside of the range 40-260C, thus
  requiring special heating/cooling media.

6
High Temperature Operation

• Reactors gt 250C
• Justification
• Favorable equilibrium conversion for endothermic
  reaction
• Increase reaction rates
• Maintain a gas phase
• Improve selectivity
• Penalties
• Use of special heaters
• Need for special materials of construction

7
Low Temperature Operation

• Reactors lt 40C
• Justification
• Favorable equilibrium conversion for exothermic
  reaction
• Temperature sensitivity of materials
• Maintain a liquid phase
• Improve selectivity
• Penalties
• Use of refrigeration
• Need for special materials of construction

8
High Temperature Operation

• Separators gt 250C
• Justification
• Obtain gas phase required for VLE separation
• Penalties
• Use of special heaters
• Need for special materials of construction

9
Low Temperature Operation

• Separators lt 40C
• Justification
• Obtain a liquid phase required for V/L or L/L
  equilibrium
• Obtain a solid phase for crystallization
• Temperature sensitive materials
• Penalties
• Use of refrigeration
• Need for special materials of construction

10
High Pressure Operation

• Reactors gt 10 bar
• Justification
• Favorable equilibrium conversion
• Increase reaction rates for gas phase reactions
• Maintain a liquid phase
• Penalties
• Requires thick-walled equipment
• Requires expensive compressors if gas streams
  must be compressed

11
Low Pressure Operation

• Reactors lt 1 bar
• Justification
• Favorable equilibrium conversion
• Maintain a gas phase
• Penalties
• Requires large equipment designed for vacuum
  operation
• Air leaks into equipment may that may be
  dangerous and expensive to prevent

12
High Pressure Operation

• Separators gt 10 bar
• Justification
• Obtain a liquid phase for V/L or L/L separation
• Penalties
• Requires thick-walled equipment
• Requires expensive compressors if gas streams
  must be compressed

13
Low Pressure Operation

• Separators lt 1 bar
• Justification
• Obtain a gas phase for V/L separation
• Temperature sensitive materials
• Penalties
• Requires large equipment designed for vacuum
  operation
• Air leaks into equipment may that may be
  dangerous and expensive to prevent

14
Nonstoichiometric feeds

• Inerts
• Justification
• diluents to control the rate of reaction
• ensure that the reaction mixture is outside the
  explosive limits (exothermic reactions)
• Penalties
• Causes reactor and downstream equipment to be
  larger
• Requires additional separation equipment
• May cause side reactions
• Decreases equilibrium conversion

15
Nonstoichiometric feeds

• Excess Reactant
• Justification
• Increase equilibrium conversion of the limiting
  reactant
• Inhibit unwanted side reactions
• Penalties
• Requires separation equipment
• Requires recycle
• Additional feed costs due to losses in separation
  or due to absence of recycle

16
Nonstoichiometric feeds

• Product in feed stream
• Justification
• Cannot easily be separated from recycled feed
  material
• Recycled product retards formation of unwanted
  by-products formed from side reactions
• Product acts as a diluent to control the rate of
  reaction
• ensure that a reaction mixture for an exothermic
  reaction is outside explosive limits
• Penalties
• Requires larger reactor and downstream equipment
• Requires larger recycle loop
• Decreases equilibrium conversion

17
Acrylic Acid production bycatalytic partial
oxidation of propylene
18
Acrylic Acid production bycatalytic partial
oxidation of propylene
19
Exothermic/Endothermic?
E-301 provides 1995 ton/h cooling
20
Why add steam?
To drive equilibrium reaction to products?
NO
Water is a product.
21
Why add steam?
To maintain concentration of C3H6 outside
explosive limits?
NO
Explosive limits for C3H6 are 2.1 to 12.1 in
air at ambient. Without Steam addition
C3H68.7, with steam C3H65.1
22
Why add steam?
To dilute component concentrations?
YES
Steam acts as a thermal ballast to control
reaction temperature if the Reaction rates
increase. Also prevents coking reactions.
23
Why operate at elevated temperature of 310C?
Reaction is controlled by kinetics, not
equilibrium
Since kinetics controls, we operate at high
temperature to obtain a high (enough) reaction
rate.
24
Why not operate above 310C?
Desire reaction has lowest Ea Therefore,
selectivity to this reaction is decreased as
temperature is increased.
25
Why not operate at higher pressure?
Reaction rate equations are first order w.r.t.
Oxygen and Propylene. The net effect would be
that all the reactions would increase
proportionally. Selectivity would not be effected.
26
Why are the products first absorbed into water,
then absorbed into an organic phase?
water
Reactor effluent contains propylene and oxygen
and is hot. Reaction will continue if not
quenched. Water quench is a rapid way of
accomplishing this. Disadvantage wasted
potential energy.
27
Why are the products first absorbed into water,
then absorbed into an organic phase?
organic
Use of the organic phase is to allow an easier
VLE separation. Note that the latent heat of
diisopropyl ether is ½ that of water.
28
Suggest alternative separation
Consider elimination of the solvent extraction
step.
Eliminates T-303 (extractor) and one of the
towers on this page. Stream 9 (quenched reactor
effluent) would be treated first, water (lower
b.p.) would be taken overhead. Second column
T-305 would remain essentially the same.
29
Suggest alternative separation
Consider elimination of the quench tower
Disadvantage potential for oxidation reactions
to occur in heat exchanger after reactor.
Acrylic acid fires in reactor effluents have been
reported in the exchanger for processes of this
option.
Would have to replace with a heat exchanger
capable of cooling the reactor effluent to 50C.
Two exchangers would allow use of a waste
heat boiler followed by a trim cooler
30
Suggest alternative separation
Adsorption, Crystallization, Membranes, etc?
Unlikely any of these will be economically
viable, but consideration doesnt hurt.
31
Recycle off-gas, reduce steam?
Advantage unreacted propylene may be recovered.
Disadvantage Need a recycle compressor
(typically a high cost). Acids may react further
to form oxidized products, leading to additional
problems for the separation train.
32
Conditions of Special Concern
acrylic acid in a concentrated form can
polymerize (violently) if the temperature is
above 90C. Both columns are thus operated under
vacuum to eliminate this threat by allowing the
bottoms of each to operate below 90C.
the small quantities of acids and solvent must be
removed prior to discharge.