CE 10167 - CE Principles (Chemical Reaction Engineering) Flashcards Preview

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1

What’s chemical reaction Engineering (CRE)?

The field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which chemical reactions take place.

2

What is the aim of chemical reaction engineering, CRE?

To conduct chemical reactions at controlled conditions to achieve
maximum production (conversion), selectivity and energy efficiency, while minimizing the cost and the consequences to the environment
i.e. air/water pollution, utilities used, waste disposal.

3

What’s a chemical reaction?

A process in which one or more substances (the reactants), are converted to one or more different substances (the products).

Hence, whenever a chemical reaction takes place, there is change in the /chemical identity (atomic content/configuration) of a molecule brought about by interaction with other chemical species.

4

In what ways can a species’ chemical identity change in a reaction?

By decomposition, combination or isomerisation.

5

How can the conversion of a limiting reactant be calculated/written?

X = moles 'A' reacted/ moles 'A' fed

Where X is conversion (no units)

6

How can conversion of substance 'A' in a reaction be written?

X = ( N (Ao) - N (A) ) / N (Ao)

Where:
X is conversion
N (Ao) is initial moles of substance A
N (A) is the final moles / output moles of substance A.

This can be written for flow rate and concentration too.

7

What does the yield of a reaction show?

Yield, Y, shows how much of a desired product was formed.

8

What is selectivity (of a reaction)?

How much desired product was formed in ratio to the undesired product.

9

What is chemical equilibrium?

A state in which both reactants and products are present and have no further tendency to change in their concentration with time.

Le Chatelier's principle is used to predict the effect of a change in conditions on a chemical equilibrium.

Le Chatelier's principle can be stated as: If a system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to counteract the effect of the applied change and a new equilibrium is established.

10

What are reaction kinetics?

The measurement of how quickly reactions (in reactors) occur, and how long reactants should remain in the reactor.

11

What is space time (aka holding time or mean residence time)?

The time necessary to process one reactor's volume of fluid based on a particular set of entrance conditions.

12

How is space time calculated?

τ = V / v 0 (= s)


Where:
τ is space time
V is reactor volume
v 0 is volumetric flow rate

13

What is space velocity?

The reciprocal of space time.

SV = 1/τ = v0/V (=s^-1)

However, since the flow rates of the fluids are generally very large in industrial systems, space velocity is used in terms of hours.

14

How can rate of consumption of species A be calculated?

-rA = moles of A consumed / (volume * time)


-rA = ΔN(A) / (V*Δt)

[Since n = cV]

rA = -(ΔC (A) / Δt)

This is an average rate obtained by taking the change in concentration over a time period, which is an approximation of the average reaction rate in that time interval.

15

What formulae are used to calculate the average and instantaneous rates of reaction?

Average rate is obtained by taking the change in concentration over a time period:

rate A = -(ΔC (A) / Δt)

Instantaneous rate of reaction is defined as the change in concentration of an infinitely small time interval, which expressed as the limit or derivative expression:

rate A = - lim Δt→0 (ΔC (A) / Δt)

16

How is the instantaneous rate of reaction calculated?

Instantaneous rate of reaction is defined as the change in concentration of an infinitely small time interval, which expressed as the limit or derivative expression:

rate A = - lim Δt→0 (ΔC (A) / Δt)

Taking the limit as Δt approaches to zero, gives the differential:

r(A) = -dC(A)/dt


Using moles instead of 'A',

r(A) = -(1/V)(dN (A)/dt)
[1/V * rate of moles used]

17

What’s rate of reaction?

The speed at which a chemical reaction proceeds in terms of disappearance of a reactant (how quickly they are used up) or as the rate of appearance of a product (how quickly they are formed) per unit time.

18

What is the rate law?

The Rate Law describes the relationship between reactant rates and reactant concentrations (or pressures) in terms of a mathematical equation which then describes the progress of the reaction.

19

What is the mathematical formula: 'power law' for describing the rate law?

-rA = k CᵐA CⁿB
(r = k[A][B])

Where:
- C is concentration of species
- k is reaction rate constant
- m and n are reaction orders

20

What factors affect the rate of reaction?

Temperature
Order of Reaction 
Concentration 


Pressure 
Catalyst

21

How does temperature affect the rate of a reaction?

There is an exponential relationship between temperature and rate constant.

k = Ae^(-E/RT)

Therefore, as temp' increases, rate increases.

22

How do reaction orders affect reaction rate?

Zero order - constant rate. Rate is independent of conc'.

First order: rate is proportional to conc'.

Second order: rate is proportional to square of the conc' of a reactant.

23

How can a zero order reaction be determined graphically?

A concentration-time graph will show a straight line with a negative gradient.
Conc' falls at a steady rate with time.

(This wouldn't work with 1st and 2nd order reactions, which have curved conc'-time graphs)

24

How can a first order reaction be determined graphically?

Plot the graph of ln(C₀/C) against time. [C is conc']

If it produces a straight line with positive gradient, it is first order.
(Gradient = k)

25

How can a second order reaction be determined graphically?

Plot the graph of 1/C against time. [C is conc']

If it produces a straight line with positive gradient (and non-zero origin), it is second order.
(Gradient = k)

26

What are batch reactors?

Reaction vessels varying in size from <1L to 15,000L

27

What are semi-batch reactors?

Reaction vessels where reactants enter and remain in the system until the reaction is complete. However, gases produced during the reaction can be removed, or additional reactants can be added throughout the reaction.

It doesn't reach steady state.

28

What are the applications of batch reactors?

Wide variety of uses.
Mainly used for liquid phase reactions requiring long period of time.

Used when a small amount of product is wanted; thus it's preferred during product testing phases.

29

What are the advantages and disadvantages of batch reactors?

Adv:
- High conversions can be obtained by leaving reactants in reactor for longer times
- Batch reactor jackets allow the system to change heating or cooling power at constant jacket heat flux.
- Can make many products consecutively
- Easy to clean
- Good for producing small quantities

Disadv:
- High cost of labour per unit production
- Hard to maintain
- Long downtime for cleaning = less production

30

What are continuous flow reactors?

Reaction vessels where the reactant is continuously flowing in and products are flowing out.
(Examples include tubular flow reactor and CSTR).

They continuously flow at steady state.