CE 10167 - CE Principles (Chemical Reaction Engineering)

Question 1

What’s chemical reaction Engineering (CRE)?

Answer 1

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

Question 2

What is the aim of chemical reaction engineering, CRE?

Answer 2

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.

Question 3

What’s a chemical reaction?

Answer 3

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.

Question 4

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

Answer 4

By decomposition, combination or isomerisation.

Question 5

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

Answer 5

X = moles ‘A’ reacted/ moles ‘A’ fed

Where X is conversion (no units)

Question 6

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

Answer 6

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.

Question 7

What does the yield of a reaction show?

Answer 7

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

Question 8

What is selectivity (of a reaction)?

Answer 8

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

Question 9

What is chemical equilibrium?

Answer 9

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.

Question 10

What are reaction kinetics?

Answer 10

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

Question 11

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

Answer 11

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

Question 12

How is space time calculated?

Answer 12

τ = V / v 0 (= s)

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

Question 13

What is space velocity?

Answer 13

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.

Question 14

How can rate of consumption of species A be calculated?

Answer 14

• 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.

Question 15

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

Answer 15

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)

Question 16

How is the instantaneous rate of reaction calculated?

Answer 16

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]

Question 17

What’s rate of reaction?

Answer 17

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.

Question 18

What is the rate law?

Answer 18

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.

Question 19

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

Answer 19

-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

Question 20

What factors affect the rate of reaction?

Answer 20

Temperature
Order of Reaction 
Concentration 


Pressure 
Catalyst

Question 21

How does temperature affect the rate of a reaction?

Answer 21

There is an exponential relationship between temperature and rate constant.

k = Ae^(-E/RT)

Therefore, as temp’ increases, rate increases.

Question 22

How do reaction orders affect reaction rate?

Answer 22

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.

Question 23

How can a zero order reaction be determined graphically?

Answer 23

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)

Question 24

How can a first order reaction be determined graphically?

Answer 24

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

Question 25

How can a second order reaction be determined graphically?

Answer 25

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

Question 26

What are batch reactors?

Answer 26

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

Question 27

What are semi-batch reactors?

Answer 27

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.

Question 28

What are the applications of batch reactors?

Answer 28

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.

Question 29

What are the advantages and disadvantages of batch reactors?

Answer 29

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

Question 30

What are continuous flow reactors?

Answer 30

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.

Question 31

What are tubular flow reactors:

Answer 31

A type of continuous flow reactor where reactants continuously flow at steady state e.g. plug flow reactors.

As the reactant travels along the tube, conc’ of reactant decreases.

Question 32

What are the advantages and disadvantages of tubular reactors?

Answer 32

Advs:

• Used large scale
• High conversion per unit volume
• Good heat transfer
• Low operating cost

Disadv:

• May have thermal gradients
• Poor temp’ control
• Channelling may occur
• Shutdown and cleaning is expensive

Question 33

What’s a CSTR, Continuous Stirred Tank Reactor?

Answer 33

A type of continuous flow reactor. reactants flow in and products flow out, and the system is continuously stirred throughout.

They’re open systems and steady state.

A steady state must be reached where the flow rate into the reactor equals the flow rate out and the conditions in the reactor do not change with time, otherwise the tank would empty or overflow.

CSTRs are very well mixed, so the contents have relatively uniform properties such as temperature, density, etc. throughout the operation.

Question 34

What are the advantages and disadvantages of CSTR?

Answer 34

Adv:
- Easy to maintain temp'
- Cheap to build
- Has large heat capacity
Interior of reactor is easily accessed

Disadv:
- Conversion of reactant to product per volume of reactor is small compared to other flow reactors

Question 35

How can reactors be classified?

Answer 35

Mode of operation e.g. batch, semi-batch, continuous

End use e.g. chemical, biological, electrochemical or nuclear

Catalyst (or phase) based e.g. homogenous (single phase) or heterogenous (multi-phase)

Question 36

What is the overall material balance on a system?

Answer 36

Mass flow in - mass flow out = accumulation

Question 37

What is the material balance for a certain species, A, on a system?

Answer 37

(Molar flow of species A in)-(Molar flow of species A out) + or - (rate of species A produced/consumed)

=

(Rate of accumulation of species A in time t)

Question 38

What’s the symbol for molar flowrate?

Answer 38

F A

Question 39

What are the assumptions in deriving a design equation for a batch reactor?

Answer 39

Closed system

Perfectly mixed / uniform composition

Conditions can vary over time

Question 40

What are the rate constant equations for batch reactors for zero, first and second order reactions?

(Should be derived from rate/-dC/dt and not memorised)

Answer 40

Zero:
k = 1/t (C₀ - C)

First:
k = 1/t ln(C₀/C)

Second:
k = 1/t (1/C - 1/C₀)

Question 41

How is the design equation for a CSTR reactor formulated?

Answer 41

Molar inflow - Molar outflow + rate of species produced - rate of species consumed = rate of accumulation.

However, steady state therefore no accumulation.

F in -F out - (-rV) = 0

V = (F in - F out) / (-rA)

In terms of conversion, X:
V = XF in / (-rA)

F in = v₀CA₀
-rA = kCA₀ (1 - X)

V = (Xv₀CA₀) / (kCA₀ (1 - X))

Question 42

What’s the Levenspiel plot?

Answer 42

A plot to determine the required volume of a chemical reactor using the experimental data on the reaction taking place in the reactor.

It is a plot of the inverse of the reaction rate (y axis) as a function of conversion (x axis). The area is therefore equal to V/F A₀.
Knowing F A₀ means that reactor volume can be estimated.

Question 43

What is the Levenspiel plot used for?

Answer 43

It is a plot of the inverse of the reaction rate (y axis) as a function of conversion (x axis), used to determine the required volume of a chemical reactor using the experimental data on the reaction taking place in the reactor.

The area is therefore equal to V/F A₀.
Knowing F A₀ means that reactor volume can be estimated.

Question 44

What is the Reynolds number?

Answer 44

Reynolds number, Re, is a dimensionless quantity used to predict flow patterns.
It is measured by inertia force / viscous force ( = pvd/mu)

Re < 2100 indicates laminar flow
Re > 2100 indicates turbulent flow

Question 45

What are the 2 types of flow in tubular reactors?

Answer 45

Laminar

Turbulent

Question 46

What is laminar flow?

+ properties

Answer 46

Flow with a parabolic velocity profile (fluid moves greater at centre of tube)
Occurs when Re < 2100
Flow rate / velocity is lower
Streamline
No eddies or mixing

Question 47

What is turbulent flow?

+ properties

Answer 47

Flow with a flat velocity profile (fluid appears to move as a line)
Occurs when Re > 2100
Flow rate / velocity is higher
Random eddies and well mixing
Perfect mixing in radial dimension
Plug flow

Question 48

How is a design equation for a PFR (plug flow reactor) determined?

Answer 48

(The reactor is divided into sub-volumes and a mass balance is carried out on the sub-volume rather than the whole volume, ΔV)

Therefore:
[molar flow of A into ΔV] - [molar flow of A out of ΔV] +/- [rate of formation/consumption in ΔV] = [Rate of accumulation in ΔV during Δt]

Fₐ₍ᵥ₎ - Fₐ₍ᵥ₊Δᵥ₎ - (-rₐΔV) = 0

Rearrange to find -rₐ and take the limit as ΔV approaches 0.
The design equation becomes: -dFₐ/dV = -rₐ

Integrate both sides:
V = ∫ dFₐ/rₐ from Fₐ to Fₐ,₀

Writing in terms of conversion, (Fₐ = Fₐ,₀ - Fₐ,₀X)
so Fₐ,₀*dX/dV = rₐ

Now, integrating from X = 0 to X = X:
V = Fₐ,₀*∫dX/-rₐ