Heuristics‐Chemical Reaction System

// Chemical Reaction System

• Reactor configurations
  • Batch reactor
  • Continuous stirred tank reactor (CSTR)
  • Plug flow reactor (PFR)
  • Fluidized bed reactor

• Reactor base conditions
  • Temperature
  • Pressure
  • Feed composition

• Production of desired product parameters
  • Conversion
  • Selectivity
  • Yield

• Reactor configuration selection
  • Batch reactor (stirred tank)
  • Continuous stirred tank reactor (CSTR)
  • Plug flow reactor (PFR)
  • Fluidized bed reactor
  • Packed bed reactor

Heuristics for reactors (Table 11.7, Turton et al.)

1. The rate of reaction in every instance must be established in the laboratory, and the residence time or space
   velocity and product distribution eventually must be found from a pilot plant.
2. Dimension of catalyst particles are 0.1 mm (0.004 in) in fluidized beds, 1 mm for slurry beds, and 2‐5 mm
   (0.078‐0.197 in) in fixed beds.
3. The optimum proportions of stirred tank reactors are with liquid level equal to the tank diameter, but at high
   pressures slimmer proportions are economical.
4. Power input to a homogeneous reaction stirred tank is 0.1‐‐0.3 kW /m3 (0.5‐1.5 hp/1000 gal), but three times
   this amount when heat is to be transferred.
5. Ideal CSTR (continuous stirred tank reactor) behavior is approached when the mean residence time is 5‐10
   times the length needed to achieve homogeneity, which is accomplished with 500‐2000 revolutions of a
   properly designed stirrer.
6. Batch reactions are conducted in stirred tanks for small daily production rates or when the reaction times are
   long or when some condition such as feed rate or temperature must be programmed in some way.
7. Relatively slow reactions of liquids and slurries are conducted in continuous stirred tanks. A battery of four or
   five in series is most economical.
8. Tubular flow reactors are suited to high production rates at short residence times (sec. or min.) and when substantial heat transfer is needed. Embedded tubes or shell‐and‐tube construction then are used.
9. In granular catalyst packed reactors, the residence time distribution is often no better than that of a five‐stage CSTR battery.
10. 10 For conversion under about 95% of equilibrium, the performance of a five‐stage CSTR battery approaches plug flow.
11. The effect of temperature on chemical reaction rate is to double the rate every 10 °C.
12. The rate of reaction in a heterogeneous system is more often controlled by the rate of heat or mass transfer than by the chemical reaction kinetics.
13. The value of a catalyst may be to improve selectivity more than to improve the overall reaction rate.

More heuristics for reactors from Seider et al.:
– When nearly pure products are required, eliminate inert species before
reactor when separations are easy and when the catalyst is adversely
affected by the inert, but not when large exothermic heat of reaction
must be removed.

// Example

‐ Consider the reaction and distillation operations for the isomerization
of n‐butane to isobutane according to the following reaction. The feed to the
process is a refinery stream which contains 20 mol% iso‐butane. Show the
alternatives for arrangement of the reaction and separation operations.

C₄H₁₀ ⟷ i-C₄H₁₀

More heuristics for reactors from Seider et al.:
– For reversible reactions, consider conducting them in a separator to
remove the products, thus driving the reactions to the right.

// Example

Considering the application of a reactive distillation
operation which is used in production of methyl acetate.
• Methanol is more volatile than acetic acid, it is fed to
the bottom of the reaction zone.
• Acetic acid is fed at the top of the reaction zone and
concentrates in the liquid phase.
• In this operation, the products are withdrawn from
the reaction section in vapour and liquid streams,
thus driving the reaction forward without excess
reactant or change in pressure.

More heuristics for reactors from Seider et al.:
• For competing reactions, both in series and parallel, adjust the
temperature, pressure, and catalyst to obtain high yields of the desired
products.

// Example:

Consider the series‐parallel reaction in manufacturing of allyl chloride. This system consist of three competing second‐order exothermic reactions.

What is the desirable reaction?

// Reaction parameters: conversion, selectivity and yield:

• Conversion: amount of reactant reacted (in terms of the limiting
  reactant)
  • Single – pass Conversion=  (reactant consumed in reactor)/(reactant fed to reactor)
• Overall conversion:
  • Overall Conversion = (reactant consumed in process)/reactant fed to process
• Selectivity:
  • Selectivity η = (rate of production of desired product)/(rate of production of undesired by – product(s))
• Yield:
  • Yield = (moles of reactant reacted to produce desired product)/(moles of limiting reactant reacted)

// Example: HDA Process

Calculate
a) Single‐pass conversion
b) Overall conversion
c) Yield of toluene