chemical reactors and controls
TRANSCRIPT
-
7/31/2019 Chemical reactors and controls
1/29
Reactor is an equipment or container intowhich the reactants are introduced, then mixed andheated until the reaction temperature is reached.
Flow sheet symbol:-
Definition:-
-
7/31/2019 Chemical reactors and controls
2/29
Reaction types carried out in Reactor:-Exothermic reactionEndothermic reactionIsothermal Reaction
Other types of reactions:-Irreversible reactionReversible reactionConsecutive reactionSimultaneous reaction
-
7/31/2019 Chemical reactors and controls
3/29
REACTORS
Continuous StirredTank Reactor
(CSTR)
Batch StirredTank Reactor
(BSTR)
Semi BatchReactor
Plug FlowReactor
-
7/31/2019 Chemical reactors and controls
4/29
In these reactors the reactant flow continuously into the reactor and the
products flow out continuously. Under ideal conditions, in well agitatedsystem, an uniform concentration is maintained throughout the vessel.
-
7/31/2019 Chemical reactors and controls
5/29
In a PFR, one or more fluid reagentsare pumped through a pipe or tube. Thechemical reaction proceeds as the reagentstravel through the PFR
-
7/31/2019 Chemical reactors and controls
6/29
Advantages:-1)Uniform partocle mixing2)Uniform temperature3)Ability to operate reactor in continuous state
Disadvantages:-1)Increase in reactor vessel size2)Pumping requirements and pressure drop
-
7/31/2019 Chemical reactors and controls
7/29
These are used exclusively for liquid phase reactions.The reactants are added to empty vessel and controlsare removed after the completion of reaction.
-
7/31/2019 Chemical reactors and controls
8/29
In these reactors one of the reactor is initially charged batch wise, while
the other reactant is fed into the reactor continuously.
-
7/31/2019 Chemical reactors and controls
9/29
Reaction rates and Kinematics.
Reactor time constant
Runaway reaction
-
7/31/2019 Chemical reactors and controls
10/29
K=Specific reaction rate coefficient (per min)
=Pre-exponential factor
E=activation energy (BTU/mole)
R=Perfect gas constant(1.99 BTU/(mole)(R))
T=absolute temperature (R)
The reaction rate coefficient exponentially increases with temperature.
-
7/31/2019 Chemical reactors and controls
11/29
Figure below illustrates the strong dependency of k on reaction temp for valuesof =e(29) and E/R=20000.
-
7/31/2019 Chemical reactors and controls
12/29
Fig below relate fractional conversion with time forcontinuous reactor and batch reactor.
-
7/31/2019 Chemical reactors and controls
13/29
Shinskey, suggested that if dead time can be keptunder 35% of the thermal time constant of thereactor, the process can be stabilized, where as ifit approaches 100% the reactor will beuncontrolled.
-
7/31/2019 Chemical reactors and controls
14/29
Runaway reaction occur in exothermic reaction , in which an increase in
temperature speeds up the reaction , which in turns release more heat andraises the temperature further.
In order to compensate this cycle , highly self-regulating cooling systems arerequired.
One of the most self regulating cooling system is a bath of boiling water.
Exothermic reactions produces heat and Endothermic reactions consumesheat.
Endothermic reactions are inherently self-regulating.
The reactor itself can be Isothermal ie. It is operated at constant temperatureor Adiabatic.
Runaway Reaction:-
-
7/31/2019 Chemical reactors and controls
15/29
Following graph shows the , relation of time with released , operatingtemperature and chemical concentration.
-
7/31/2019 Chemical reactors and controls
16/29
Following graphs shows the ,relation between time and reactor
temperature.
Following graphs shows the , relationbetween time and heat generated.
-
7/31/2019 Chemical reactors and controls
17/29
Temperature Control:-
1) Feedback Control (once through cooling)
2) Recirculated cooling water method.
3) Cascade temperature Control.4) Temperature control by boiling coolant.
-
7/31/2019 Chemical reactors and controls
18/29
-
7/31/2019 Chemical reactors and controls
19/29
Simple temperature control scheme.Reaction temperature is sensed and flow ofheat transfer medium to reactor jacket ismanipulated.
-
7/31/2019 Chemical reactors and controls
20/29
ADVANTAGES :-
1. Simplicity2. Low maintainence cost.
DISADVANTAGES:-
1. Non linearity and dynamic features.2. Formation fo cold spots and hot spots as the water
is not circulated uniformly.
3. Variable residence time of cooling water as the
jacket flow rate decreases.4. Varying dead time.
-
7/31/2019 Chemical reactors and controls
21/29
Temperature response of uncontrolled chemical reactor to step change in load
-
7/31/2019 Chemical reactors and controls
22/29
-
7/31/2019 Chemical reactors and controls
23/29
Inorder to overcome above disadvantagesrecirculated control scheme is used .
Here there is high rate of water circulationthrough jacket.
-
7/31/2019 Chemical reactors and controls
24/29
ADVANTAGES :-
Dead time is constant.
High heat transfer coefficient. Elimination of hot spots and cold spots.
DISADVANTAGE:-
As jacket has constant heat transfer area, at lowloads the process and process gain is large.
The above disadvantage can be overcome byusing variable gain control valve.
-
7/31/2019 Chemical reactors and controls
25/29
Process
variable
Manipulatedvariable
-
7/31/2019 Chemical reactors and controls
26/29
Purpose of slave loop is to correct outside disturbances without allowing
them to affect the reaction rate
Advantages:-
Removes non-linearity
Easily compensated with equql percentage valve whose gain increases asthe system gain drops
Eliminates deadtime from the system
Disadvantages
Continous change in varaibles to maintain value of process variable
-
7/31/2019 Chemical reactors and controls
27/29
Used in Gas phase Reactor usually athigh pres & high tempIn cases where both temp & presssensitive a vey stable heat removal s/m
is requiredReactor vessel is filled with boilingwater
-
7/31/2019 Chemical reactors and controls
28/29
Single cooling or heating media isinsufficient
-
7/31/2019 Chemical reactors and controls
29/29
Board questions appeared on this topic so far:-
1) Short note on temp control of reactor. 10M
2) Explain P and ID of reactor (Exothermic reaction). 10M
3) What are reactor characteristics? Draw the temperature Profiles. ExplainRunaway reaction. 10M
References:-
Bela G. Liptak