What are the 6 factors involved in reactor design?
1) Feedstock composition
2) Scale of process, output of product
3) Process kinetics
4) Reactor type
5) Material of construction, instrumentation, safety
6) Economics
Why are bench scale batch reactors initially used?
- At small scale, heat and mass transfer gradients are minimised
- Better mixing can happen in smaller reactors
- More accurate determination of key chemical effects
Why are pilot plants used and some issues?
- Significantly larger than bench scale
- Can collect data and indicate potential difficulties in scale up to help design full scale reactor
- Works at a higher pressure
- Can be more difficult to separate reaction effects from heat&mass transfer e.g temperature gradient at the reactor walls
What are the features of batch reactors?
- No addition of reactants, or removal of products during the reaction
- Vessel is kept perfectly mixed, uniform concentrations throughout-composition will change with time
- Uniform temperature throughout-although may change with time
- Generally used for small scale processes, e.g. fine chemical and pharmaceutical manufacturing
What are some advantages and disadvantages of batch reactors?
- Can be repurposed and produce different products
- Low capital (startup) cost
- High labour cost
What is the material balance in a batch reactor?
Rate of accumulation of A in reactor = - Rate of A loss by reaction in reactor
-No flow in or out of reactor
What is the rate of disappearance of reactant, A?
-rA*Vr
rA=moles of A reacting/(unit volume*time)
Vr= volume of fluid in reactor:
- When liquid/solid=volume of fluid in reactor - When gas=total volume in reactor
What type of reactions are plug flow reactors used for?
Gas or liquid phase
What are the features of plug flow reactors?
- Steady movement of material along reactor
- No attempt to induce mixing, it is at steady state
Assume:
- At a given position perpendicular to flow there is no pressure, temp or composition change
- No diffusion between fluids
- All fluids have the same residence time
When do the assumptions in a plug flow reactor hold true?
- When there is good radial mixing (high flowates, Re > 10^4)
- When axial mixing can be neglected (length divided by diameter of reactor >50)
What are the 3 types of plug flow reactor?
Single tubes- used for minimal heat transfer and smal reaction volumes
Shell&tube- resembles a heat exchanger, often packed with granular material to improve turbulence & mixing, or packed with catalyst
Tubular furnaces- used for highly endothermic reactions with intense heating, burn fuel on outside of tubes, large quantities of feedstock used
What is the mass balance equation for a plug flow reactor?
Accumulation = input - output - disappearance by reaction
0 = Fa - (Fa+dFa) - (-ra)dVr
Where d is delta
- Fa,amount of a
- ra, disappearance of a
- Vr, reactor volume
What type of reactions are continous stirred tank reactors used for?
- Liquid phase reactions
- Gas phase in lab kinetic studies
What are the features of continuous stirred reactors?
- Perfect mixing occurs, everythings reacts before leaving, mixture in tank has identical properties to mixture leaving
- Continuous feed in and out of reactor
- Inlet stream instanteously mixes with bulk of reactor volume
What Vr do you use in continuous stirred reactir dependent on reactir contents?
Gas phase, Vr=whole reactor volume
Liquid phase, Vr= volume of liquid
What is the material balance in continuous stirred reactor?
Fa0 = Fa + (-ra)Vr
How and why are continuous stirred reactors used in industry
- Used extensively in process industries
- Mechanical and heat transfer sinular to stirred batch reactors
- Rate of reaction lower than plug flow as reactants are diluted, therefore tank needs to be bigger than equiv. PFR
- Easier to control than PFR
