9/20/99 Chemical Reactor Characterization 1
CHEMICAL REACTOR
CHARATERIZATION
GOALS:
u Outline basic concepts
u Review reaction chemistry
u Develop basic definitions of performance
u Identify important reactor variables
u Review reaction thermodynamics
9/20/99 Chemical Reactor Characterization 2
PURPOSE OF A CHEMICAL REACTOR
The purpose of a chemical reactor is to convert
less desirable feed components
into
more desirable output components.
uConvert a cheap raw material into a more valuable
products,e.g.,natural gas into ammonia,ethylene into
polyethylene,etc.
uConvert hazardous compounds into ones that are
environmentally benign,e.g.,hydrogen sulfide into sulfur,
carbon monoxide into carbon dioxide,PCB’s into CO2,
H2O,and HCl.
9/20/99 Chemical Reactor Characterization 3
CHEMICAL REACTIONS
u A chemical reaction is an exact recipe for converting
raw materials into products or byproducts
u The key concept is that of the stoichiometric coefficient
u The stoichiometric coefficient aij specifies the mols of
the ith component produced per mol of the jth reaction.
u Reactions take place in mols,not mass!!
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EXAMPLE #1
1) CH4 + 2 O2 --> CO2 + 2 H2O
2) CH4 + 1 1/2 O2 --> CO + 2 H2O
i Component ai1 ai2
1 CH4 -1 -1
2 O2 -2 -1.5
3 CO2 1 0
4 CO 0 1
5 H2O 2 2
9/20/99 Chemical Reactor Characterization 5
CONVERSION AND SELECTIVITY
u Conversion is that fraction of component entering a
reactor that is converted to products and byproducts
u Conversion is generally based on one or more
independent key components of primary interest in the
reaction system
u The selectivity of a key component with respect to a
specified product is the fraction of that component
converted to the specified product divided by the total
conversion of the key component.
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EXAMPLE #2
Reactor Material Balance:
Mols/hr in
Component Feed Effluent Delta
CH4 100 20 -80
O2 200 50 -150
N2 400 400 0
CO2 0 60 60
CO 10 30 20
H2O 0 160 160
9/20/99 Chemical Reactor Characterization 7
EXAMPLE #2 (Cont’d)
CONVERSIONS
CH4,(60 + 20)/100 = 0.80 or 80%
O2,(120 + 30)/200 = 0.75 or 75%
SELECTIVITIES
CH4 to CO2,60/80 = 0.75 or 75%
CH4 to CO,20/80 = 0.25 or 25%
Note,The sum of the selectivities for a given key component
must sum to 1.0 or 100%.
CH4 to H2O,(60 + 20)/80 = 1.0 or 100%
O2 to CO2,120/150 = 0.8 or 80%
O2 to CO,30/150 = 0.2 or 20%
9/20/99 Chemical Reactor Characterization 8
TYPICAL CONVERSION VS SELECTIVITY
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
Conversion
Selectivity
9/20/99 Chemical Reactor Characterization 9
SERIES REACTIONS
Ethylene Glycols from Ethylene Oxide (EO) and H2O:
Series form:
1) C2H4O (EO) + H2O --> C2H6O2 (mono ethylene glycol)
2) C2H6O2 + C2H4O --> C4H10O3 (di-ethylene glycol)
3) C4H10O3 + C2H4O --> C6H14O4 (tri-ethylene glycol)
Convert to parallel form:
1) C2H4O + H2O --> C2H6O2 (MEG)
1)+2) 2 C2H4O + H2O --> C4H10O3 (DEG)
1)+2)+3) 3 C2H4O + H2O --> C6H14O3 (TEG)
9/20/99 Chemical Reactor Characterization 10
RECOMMENDED PROCEDURE
u Convert reactor requirements from a mass basis to a
mol basis
u Convert any series reactions to parallel reactions
u Divide all reactions through by the stoichiometric
coefficient of the key component
u Calculate the reactor based on the conversion and
selectivity specifications
u Convert from a mol basis back to a mass basis
u Check the reactor material balance (lbs in = lbs out)
9/20/99 Chemical Reactor Characterization 11
EXAMPLE #3
Nitration of toluene to form nitrotoluenes:
Reaction chemistry:
1) C7H8 + HNO3 --> C7H7NO2 (mono) + H2O
2) C7H7NO2 + HNO3 --> C7H6(NO2)2 (di) + H2O
3) C7H6(NO2)2 + HNO3 --> C7H5(NO2)3 (tri) + H2O
Calculate the reactor output for a feed of 1000 kg/hr of
toluene and 1500 kg/hr of HNO3 for a conversion of
toluene of 80%,selectivity to mono of 75%,to di of 20%,
and to tri of 5%.
9/20/99 Chemical Reactor Characterization 12
EXAMPLE #3 (Cont’d.)
Step #1,Convert reactor requirements from a mass
to a mol basis
Toluene feed,1000 kg/hr
Molecular weight,92,
Mol/hr,1000/92 = 10.87
Nitric acid feed,1500 kg/hr
Molecular weight,63.
Mol/hr,1500/63 = 86.81
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EXAMPLE #3 (Cont’d.)
Step #2,Convert any series reactions to parallel reactions:
1) C7H8 + HNO3 --> C7H7NO2 + H2O
1)+2) C7H8 + 2 HNO3 --> C7H6(NO2)2 + H2O
1)+2+3) C7H8 + 3 HNO3 --> C7H5(NO2)3 + H2O
Step#3,Divide all reactions through by the stoichiometric]
coefficient of the the key component
If toluene is the key component,all reactions are already
in this form.
9/20/99 Chemical Reactor Characterization 14
WHAT WE CAN EXPECT TO KNOW ABOUT A
NEW REACTION SYSTEM
What we can expect to know about a reaction system is learned in the
laboratory and the pilot plant,It generally includes:
The reaction chemistry,at least for the major reactions
The type of reactor used to produce the data and its performance
for at least one set of operating conditions:
Feed composition
Operating pressure and temperature
Phase (gas,liquid)
Type of catalyst
Reactor size and geometry
Residence time or space velocity
Conversion and selectivity
9/20/99 Chemical Reactor Characterization 15
REACTION THERMODYNAMICS
u Equilibrium
e Is reactor to be operated near equilibrium?
e If so,how can the operating conditions be modified
to improve this equilibrium?
u Energy Balance Considerations
e Can the reactor be operated adiabatically?
e If not,what provisions need to be made to add or
remove heat of reaction?