Reactors

Question 1

Why is liquid better than gas in designing a reactor?

Answer 1

1. Smaller volumes required
2. Higher concentrations maintained
3. Better heat transfer
4. Faster reaction as molecules closer together
5. Easier to transport, store and handle
6. Less dangerous
7. More predictable

Question 2

Considerations required for exothermic, reversible reaction

Answer 2

1. Equilibrium constant
2. Temperature
3. Pressure
4. Balance T and P so the equilibrium constant matches the design objectives

Question 3

Considerations required for exothermic, irreversible reaction

Answer 3

1. How to remove and control heat produced
2. Yield
3. Conversion

Question 4

If the aim of a design was to minimise reactor volume, in which order would you suggest reactor types?

Answer 4

1. PFR
2. Batch reactor
3. Multiple CSTRs in series
4. One CSTR

This would be in reverse order for a negative order reaction.

Question 5

How would you place CSTRs of different sizes in series?

Answer 5

+ve order: smallest first to take advantage of high concentrations for as long as possible
-ve order: largest first to take advantage of low concentrations for as long as possible

Question 6

What are multiple-bed PFRs used for?

Answer 6

For reactions with a complex temperature profile. It breaks the temperate profile into manageable chunks that make predicting the reaction rate at a given point easier. The temperature is much easier to control in a smaller bed, as there is less chance of hot spots occurring and they can be considered roughly isothermal if the temperature profile is broken into small enough chunks.

Question 7

What are tubular PFRs used for?

Answer 7

To maximise heat transfer. Can optimise heat recovery if the reaction is highly exothermic.

Question 8

Describe the two types of heating jacket

Answer 8

1. Simple jacket. Need to calculate if the area given by this jacket is sufficient for the heat transfer required.
2. Hemicoil jacket. This is a spiral of half pipes fixed to the vessel. It gives a higher velocity for heat transfer which means more energy can be transferred per unit area

Question 9

State 3 ways of increasing heat transfer rate/area to a reaction vessel

Answer 9

1. Use multiple, smaller vessels in parallel
2. Immerse a heating coil inside the fluid so less heat is lost to the surroundings
3. Add additional circulation to a heat exchanger and mix the discharge with the current contents of the vessel

Question 10

How full should a CSTR be?

Answer 10

No more than 80%