chemical reactors and controls

7/31/2019 Chemical reactors and controls

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Reactor is an equipment or container intowhich the reactants are introduced, then mixed andheated until the reaction temperature is reached.

Flow sheet symbol:-

Definition:-


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Reaction types carried out in Reactor:-Exothermic reactionEndothermic reactionIsothermal Reaction

Other types of reactions:-Irreversible reactionReversible reactionConsecutive reactionSimultaneous reaction


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REACTORS

Continuous StirredTank Reactor

(CSTR)

Batch StirredTank Reactor

(BSTR)

Semi BatchReactor

Plug FlowReactor


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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.


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In a PFR, one or more fluid reagentsare pumped through a pipe or tube. Thechemical reaction proceeds as the reagentstravel through the PFR


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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


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These are used exclusively for liquid phase reactions.The reactants are added to empty vessel and controlsare removed after the completion of reaction.


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In these reactors one of the reactor is initially charged batch wise, while

the other reactant is fed into the reactor continuously.


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Reaction rates and Kinematics.

Reactor time constant

Runaway reaction


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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.


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Figure below illustrates the strong dependency of k on reaction temp for valuesof =e(29) and E/R=20000.


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Fig below relate fractional conversion with time forcontinuous reactor and batch reactor.


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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.


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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:-


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Following graph shows the , relation of time with released , operatingtemperature and chemical concentration.


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Following graphs shows the ,relation between time and reactor

temperature.

Following graphs shows the , relationbetween time and heat generated.


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Temperature Control:-

1) Feedback Control (once through cooling)

2) Recirculated cooling water method.

3) Cascade temperature Control.4) Temperature control by boiling coolant.


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Simple temperature control scheme.Reaction temperature is sensed and flow ofheat transfer medium to reactor jacket ismanipulated.


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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.


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Temperature response of uncontrolled chemical reactor to step change in load


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Inorder to overcome above disadvantagesrecirculated control scheme is used .

Here there is high rate of water circulationthrough jacket.


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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.


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Process

variable

Manipulatedvariable


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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


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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


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Single cooling or heating media isinsufficient


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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

chemical reactors and controls

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