Why does the flowrate of products out of the reactor increase with increasing volume in a CSTR when the inlet flow rate remains constant
Larger volumes
... [Show More] increase the amount if time the fluid spends in the reactor on average
in a plug flow reactor the flow is
turbulent
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Batch reactors process
Reactants charged into reactor only at start
Contents heated to reaction temperature
Reactor is emptied and products purified reactor is cleaned
Takes longer than continuous due to heating time, cooling time, discharging time and cleaning time
When is the constant volume batch reactor equation valid
for most liq phase reactions
valid for gas phase reactions in which V is constant
General recipe for reactor design
Mole balance
Rate law
Stoichiometry
Combine
Evaluate
CSTRs
almost always operated in steady state
Assumed to be perfectly mixed
Conc temp and reaction rate in reactor is the same as at outlet
done 3
...
What can increase the rate constant k
- increasing temperature
- using a better catalyst
- switching to a better reactor type
-change the solvent in a liquid phase reaction
Sign of heat added to the system Q
+
Sign of heat removed from the system
-
Sign of work done by the system
+
Sign of work done on the system
-
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Adiabatic
no heat transfer with surroundings Q=0
Assumptions for an adiabatic batch reactor to get an analytical solution to the system of ODEs for E balance
- Shaft work can be neglected Ws=0
-Heat of reaction is constant
-Heat capacity is constant dCp=0
Thermal Runaway
When heat production is larger than heat removal for an exothermic reaction
Once control is lost, temperature can rise rapidly, leaving little time for correction
Elevated temperature may initiate secondary more hazardous runaways or decomposition
Thermal runaway may result in
Boiling over of reactor content
Large increase of temperature and pressure that can lead to explosion
. Release of flammable materials may cause fire or secondary explosions
' Hot liquids and toxic substances may contaminate the workplace and beyond
Causes for Thermal runaway
-Mischarging of reactants
During the manufacture of sodium lactate solution, solid caustic soda was added through an open manhole over 15 hours to aqueous lactic acid in a steel reactor. The mix erupted when 13 of the 30 bags had been added in 1.7 hours. The caustic soda had been added at too fast a rate and this was coupled with inadequate cooling capacity of the reactor to give rise to localised boiling of the mix.
• Insufficient knowledge of reaction chemistry or thermochemistry
In the manufacture of tetrachloroethane excess chlorine was reacted with acetylene at 373 K in the presence of a ferric chloride catalyst. The temperature of the mix dropped and an explosion ruptured the bursting disc and also cracked the reactor. It was suggested that monochloroacetylene had decomposed initiating the explosion.
• Failure in temperature control
In the manufacture of polyvinyl acetate a small quantity of monomer and catalyst were added to a stainless steel reactor, which was heated to initiate the reaction. The monomer was then progressively fed into the vessel, which was kept at 348K and the pressure of the reaction was about 0.14 bar (g). On the day of the incident, the progressive monomer additions were nearing completion and the temperature rose to 373 K accompanied by a rise in pressure. A bursting disc rated at 1.4 bar (g) ruptured and 6 m3 of reactant mass was discharged through the vent. The investigation revealed that the transmitter attached to the thermocouple had failed and caused the coolant water supply to stop.
• Inadequate agitation
Monoethanol amine was added slowly with stirring to 98% sulphuric acid which was maintained at 383 K in a glass-lined reactor. The monoethanol amine and sulphuric acid were immiscible. When the reaction was complete the mix was cooled and iso-propyl alcohol was added to precipitate the product. On the day of the incident, the reactor was charged with sulphuric acid, there was then a shift change and the oncoming shift not realising that the stirrer had not been switched on proceeded to add the mono-ethanol amine. Later it was seen that the temperature was not rising and investigation showed the stirrer was not switched on. This was done and the two liquids were mixed causing an instantaneous chemical reaction and explosion.
• Problems with maintenance
During the day of an incident, there had been problems with leakages necessitating the shut-down of a mononitrochlorbenzene plant. Repairs to the plant were followed by the high temperature alarm sounding in second, third and fourth reactors consecutively after the previous reactor had been cooled down. The fourth reactor could not be cooled due to a blockage in the monochlorbenzene return line to the first reactor. This blockage was cleared using live steam, shortly afterwards an explosion occurred. The plant shut-downs and particularly the blockage in the monochlorbenzene return line led to an excess of nitric acid instead of the normal excess of the monochlorbenzene. This resulted in the oxidation of monochlorbenzene instead of nitration, the oxidation proceeding as an uncontrolled exothermic reaction.
• Problems with raw material
An initiating mix of ether, butyl chloride, cyclohexane and butyl bromide for the preparation of a Grignard reagent was added to a reactor containing the magnesium. Cyclohexane was added and immediately vapours emerged from the condenser vent and the bursting disc ruptured. The investigation revealed that the cyclohexane transfer line was wet and the Grignard reagent had reacted with the water to produce hydrogen and ethane.
• Other human errors
A 1st-order, exothermic reaction is being carried out in an adiabatic CSTR. Suppose the feed rate to the reactor is doubled, the conversion will
Decrease
If feed is doubled and everything else remains the same, residence time is halved so conversion will be lower
An endothermic reaction takes place in an adiabatic reactor. The reaction R → P goes to completion. Which condition has the highest temperature?
a. Pure R is fed to the reactor.
b. A mixture of R and an inert is fed to the reactor.
c. Same temperature for both conditions.
b
If inert added temp doesn't decrease as much as must cool both product and inert
An endothermic, 1st-order reaction takes place in an adiabatic CSTR and conversion is 50%. If the inlet flow rate is decreased by a factor of two, the exit temperature will
Decrease
Residence time would double so more reaction takes place
Assume an adiabatic CSTR is large enough that the reversible, exothermic reaction R + SP + Q always reaches equilibrium. What will increase the conversion?
a. increase concentration of an inert
b. increase the inlet temperature
c. change feed conditions closer to stoichiometric
d. none of the above
A
If the inert is added exit temp will not be as high
lower temp increases equilibrium conversion [Show Less]