Reactors

Question 1

What is a reaction energy profile

Answer 1

A potential energy diagram that graphically represents the total energy of the molecule in the reaction as the reaction proceeds

Question 2

Explain the difference between exothermic and endothermic reactions

Answer 2

In endothermic reactions the activated complex-reactant energy difference is greater than the activated complex-product energy difference.
For exothermic reactions the opposite applies.

Question 3

Explain activation energy

Answer 3

For a forward reaction we define it as the energy difference between the reactants of the forward reaction and activated complex.
For a reverse reaction we define it as the energy difference between the reactants of the reverse reaction and activated complex.

Question 4

What are the requirements for molecules to successfully react

Answer 4

1. Collision
2. Molecular orientation
3. Energy of collision (sufficient energy for the bonds to be broken)

Question 5

Define reaction rate

Answer 5

The rate at which a reaction takes place

Question 6

Name the variables that can be measured to determine reaction rate

Answer 6

• Changes in pressure (indicates gas consumption or formation)
• Changes in pH (indicates acid/base formation or consumption)
• Changes in volume
• Changes in concentration of one or more of the molecules

Question 7

What factors affect reaction rates

Answer 7

• Components involved (bond strength of components)
• Concentration (improves the chance of molecular collision)
• Pressure (higher gas pressure, higher number of molecules in a given volume of gas, higher chance of collision)
• Reaction temperature (supplies the energy to break bonds)
• Contact
• Catalysts (increase reaction rate)

Question 8

Explain the role of a catalyst

Answer 8

A catalyst is added to a reaction in order to increase its reaction rate. The catalyst itself does not change in the reaction. It will enable a group of reactant molecules to react, come out of the process unchanged, and then be able to do the same job all over again on another group of reactant molecules.

Question 9

How does a catalyst increase the reaction rate

Answer 9

It lowers the activation energy

Question 10

Name the two types of catalysts

Answer 10

• Homogenous

- Heterogenous

Question 11

Define homogenous catalysts a describe how they lowers activation energy

Answer 11

Catalysts that are in the same phase as the reactants. In most cases this means that the reactants and catalyst are in solution. They lower the activation energy by inserting and extra reaction step

Question 12

Define heterogenous catalysts a describe how they lowers activation energy

Answer 12

Catalysts that are not in the same phase as reactants. They lower activation energy by:

• adsorbing reactants molecules on their surface, giving them higher local concentrations
• adsorption on surface weakens the bonds of the reactant molecule, giving a faster reaction

Question 13

How can we find the rate law

Answer 13

By doing experiments and measuring the changes in compound concentrations

Question 14

What is the rate determining step

Answer 14

The slowest step in a sequence of reaction steps - always has the highest activation energy

Question 15

Define a batch reactor

Answer 15

A reactor where there is no in or outflow of reactants or products throughout the time it takes for the reactions in the reactor to take place

Question 16

What two types of continuous flow reactors are there

Answer 16

• Continuous-stirred tank reactor (CSTR) or back mix reactor

- Tubular reactor

Question 17

If a CSTR is well-mixed, what can we assume

Answer 17

That there is no variation in the reaction rate, temperature or concentration throughout the reactor and that they are the same at the entrance and exit points

Question 18

What requirements must the design of an industrial chemical reactor satisfy

Answer 18

• Chemical factors (kinetics of a reaction - the design must provide sufficient residence time for the desired reaction to proceed to the required degree of conversion)
• Mass transfer factors (with heterogenous reactions the reaction rate may be controlled by the rates of diffusion of the reacting species rather than the chemical kinetics)
• Heat transfer factor (the removal or addition of the heat of reaction)
• Safety factors (the confinement of hazardous reactants and products and the control of the reaction and the process conditions)

Question 19

What characteristics are normally used to classify reactor designs

Answer 19

• Mode of operation (batch or continuous)
• Phases present (homogenous or heterogenous)
• Reactor geometry (flow pattern and manner of contacting phases)

Question 20

Describe the design procedure for reactors

Answer 20

• Collect together all the kinetic and thermodynamic data on the desired reaction and side reactions. Kinetic data will normally be obtained from lab and pilot plant studies. Values will be needed for rate of reaction over a range of operating conditions: pressure, temperature, flow-rate and catalyst concentration
• Collect the physical property data from literature, estimation or lab measurements if necessary
• Identify the predominant rate-controlling mechanism: kinetic, mass or heat transfer. Choose a suitable reactor type
• Make an initial selection of the reactor conditions to give the desired conversion and yield
• Size the reactor conditions to give the desired conversion and yield
• Size the reactor and estimate its performance
• Select suitable materials of construction
• Make a preliminary mechanical design for the reactor: the vessel design, heat transfer surfaces, internals and general arrangement
• Cost the proposed design and optimize the design as necessary