Question

Why is there a maximum in the rate of reaction with respect to conversion(hene with respect to temperature and reactor volume) for an adiabatic reactor?

Answer 1

Algorithm Adiabatic Reactions:

Suppose we have the Gas Phase Reaction

that follows an elementary rate law. To generate a Levenspiel plot to size CSTRs and PFRs we use the following steps or as we will see later use POLYMATH.

1.   Choose X
Calculate T	RK T- To-
Calculate k	E1 1 2
Calculate KC
Calculate To/T
Calculate CA
Calculate CB
Calculate -rA
2. Increment X and then repeat calculations.
3. When finished, plotvs. X or use some numerical technique to find V.
Levenspiel Plot for an exothermic, adiabatic reaction. Reactor Sizing We can now use the techniques developed in Chapter 2 to size reactors and reactors in series to compare and size CSTRs and PFRs.
Consider:

PFR Shaded area is the volume.
For an exit conversion of 40%	For an exit conversion of 70%
AO 4
CSTRShaded area is the reactor volume.
For an exit conversion of 40%	For an exit conversion of 70%
We see for 40% conversion very little volume is required. CSTR+PFR
(a)	(b)
For an intermediate conversion of 40% and exit conversion of 70%
	F, 「AD 0 0.4 x 7
(a)	(b)

Looks like the best arrangement is a CSTR with a 40% conversion followed by a PFR up to 70% conversion.

Adiabatic Equilibrium :

Conversion on Temperature

Exothermic ΔH is negative

Adiabatic Equilibrium temperature (T_adia) and conversion (X_eadia

Endothermic ΔH is positive

Reversible Reactions :

Consider the reversible gas phase elementary reaction.

The rate law for this gas phase reaction will follow an elementary rate law.

Where K_c is the concentration equilibrium constant. We know from Le Chaltlier's Law that if the reaction is exothermic, K_c will decrease as the temperature is increased and the reaction will be shifted back to the left. If the reaction is endothermic and the temperature is increased, K_cwill increase and the reaction will shift to the right.

at equilibrium

	Van't Hoff Equation
For the special case of: Integrating the Van't Hoff Equation gives:

GLOSSARY :

Adiabatic reactor: Vessel that is well insulated to minimize heat transfer and has an

increase or decrease in temperature from the initial inlet conditions due solely to the

heats of reaction.

Batch reactor: Vessel used for chemical reaction that has no feed or effluent streams.

The reactor is well stirred and usually run either isothermally or adiabatically. The

main design variable is how much time the reactants are allowed to remain in the

reactor to achieve the desired level of conversion.

Catalyst: Substance that increases the rate of a chemical reaction without being

consumed in the reaction.

Continuous stirred tank reactor: Sometimes called a continuous-flow stirred-tank

reactor, ideal mixer, or mixed-flow reactor, all describing reactors with continuous

input and output of material. The outlet concentration is assumed to be the same as

the concentration at any point in the reactor.

Conversion: Fraction or percentage that describes the extent of a chemical reaction.

Conversion is calculated by dividing the number of moles of a reactant that reacted

by the initial moles of reactant. Conversion is defined only in terms of a reactant.

Elementary reaction: Reaction that has a rate equation that can be written directly

from a knowledge of the stoichiometry.

Isothermal reactor: Any type of chemical reactor operated at constant temperature.

Mean residence time: Average time molecules remain in the reactor. Note that this is

different from space time.

Multiple reactions: Series or parallel reactions that take place simultaneously in a

reactor. For example, A + B → C and A + D → E are parallel reactions, and A + B

→ C + D → E + F are series reactions.

Plug flow reactor: Sometimes called a piston flow or a perfect flow reactor. The plug

flow reactor has continuous input and output of material. The plug flow assumption

generally requires turbulent flow. No radial concentration gradients are assumed.

Product distribution: Fraction or percent of products in the reactor effluent.

Rate constant: Constant that allows the proportionality between rate and concentration

to be written as a mathematical relationship.

Reactor Classifications :

Reactors may be classified by several different methods depending on the variables of

interest. There is no single clear cut procedure for reactor classification. As a result,

several of the more common classification schemes are presented here.

A. OPERATION TYPE

The operational configuration for the reactor can be a primary method of classification.

1. Batch
Batch reactors are operated with all the material placed in the reactor prior to the start of

reaction, and all the material is removed after the reaction has been completed. There is

no addition or withdrawal of material during the reaction process.

Semibatch

The semibatch reactor combines attributes of the batch and the continuous-stirred tank.

The reactor is essentially batch but has either a continuous input or output stream during

operation.

3. Continuous Flow Reactors

Continuous flow reactors represent the largest group of reactor types by operational

classification. Several continuous flow reactors are used industrially.

a. The continuous-stirred tank reactor (CSTR) involves feeding reactants into a

wellmixed tank with simultaneous product removal.

b. The plug flow reactor (PFR) consists of a long pipe or tube. The reacting mixture

moves down the tube resulting in a change in concentration down the length of the

reactor.

c. In the recycle reactor part of the outlet stream is returned to the inlet of the reactor.

Although not a typical reactor classification by type, the recycle reactor allows for

continuous operation in regimes between CSTR and PFR conditions.