Chapter1 - The Process DesignHeuristics for process synthesis
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Be familiar with types of reactor models available in simulators and their use in process simulation.
Be able to design a system for heat transfer in association with the reactor, to sustain an exothermic or endothermic reaction at the desired temperature, and study the design using simulation.
Overview of Ch. 7: Reactor Design
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Attempts to develop generalized reactor models have had limited success
Multiple phases (e.g. vapor, liquid, reacting solid, solid catalyst)
Different geometries (e.g. stirred tanks, tubular, nozzles, membranes)
Various regimes of momentum, heat, and mass transfer (e.g. viscous flow, turbulent flow, conduction, radiation, diffusion, dispersion)
Simulators provide ideal reactor models appropriate for early stages of process synthesis—give effluents and heat duties.
When details become important in later design stages, replace ideal models with custom-made models—can insert into simulators
Reactor Models
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1. Stoichiometric
2. Equilibrium
4. Plug-flow tubular reactor (PFR)
Aspen has 2 additional models:
1. Yield—models reactor by specifying reaction yields of each component. Useful when: a) stoichiometry and kinetics unknown, b) yield distribution data (or correlations) available
2. Batch—kinetic batch (or semi-batch) kinetic model; uses holding tanks to interface with steady-state operations
Ideal Reactor Models
Good for very early stages when conducting mass & energy balances; do not take kinetics into account!
Kinetic models useful for sizing reactors
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Stoichiometric Reactor Models
vij = stoichiometric coefficient for species j in rxn i
vij =
-2
-1
1
Aspen RStoich Block
Two methods can be used to calculate equilibrium:
1) Calculate equilibrium constants from Gibbs energy of reaction combined with material balance equations
Requires stoichiometry for all reactions
2) Determine minimum Gibbs energy for the reacting system
Only requires list of all possible products
Equilibrium Reactor Models
For equilibrium reactions:
Grxn = 0 equilibrium
Minimum Gibbs Energy
Ni = mole number of component i
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• Can restrict equilibrium by specifying one of:
– Molar extent of the reaction
– A temperature approach to chemical equilibrium
• Temperature approach is the number of degrees above the reactor temperature at which chemical equilibrium is determined: TEquil=TR + ΔT
Aspen REquil Block
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Handles simultaneous phase and chemical equilibrium by minimizing the Gibbs free energy with phase splitting
Does not require reaction stoichiometry
Can also restrict equilibrium
Aspen RGibbs Block
Can handle multiple phases and simultaneous phase equilibrium
Why not always use RGibbs instead of REquil?
Chemical reactions are usually known by the time the reactor is to be designed, thus can use REquil
RGibbs useful for preliminary exploration of thermodynamic possibilities
REquil vs. RGibbs
rate of formation j by reaction
+
-
=
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True for temperature and conversion, too
Concentration in exit stream is same as that in the tank
CSTRs
Assumptions:
PFRs
No axial mixing
Also referred to as “tubular” reactors
Plug flow
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To derive design equation, apply mole balance to a differential slice:
Assume:
Steady state
In terms of concentration:
PFRs
Fj
y
PFR:
FAo X = -rAV
Recall differential mole balance:
FA = FAo(1 – X)
Q: What CSTR volume is required to achieve 80% conversion?
Levenspiel Plot
Recall design equations:
Q: How about PFR volume? Will it be greater or less than for CSTR?
V ≈ 20 L
FA0
-rA
X
5
10
15
20
25
30
(L)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
CSTR
PFR
CSTR
PFR
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Consider 2 CSTRs in series: the 1st achieving 40% conversion, and the 2nd achieving 80%
V1 ≈ 3.6 L
CSTRs in series require less volume than single CSTR!
As # CSTRs in series ↑, the total volume approaches that of PFR
CSTRs in Series
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RCSTR performs a mass and energy balance around an ideal continuous stirred tank reactor with known reaction kinetics
Assumes perfect mixing on both macroscopic and microscopic levels
RPlug Performs a mass and energy balance around an ideal plug flow reactor
Assumes perfect mixing in the radial direction and no mixing occurs in the axial direction
Optional coolant stream can also be specified
Aspen RCSTR and RPlug Blocks
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Rate-controlled and non-electrolyte equilibrium reactions are specified as Reaction IDs that can be referenced in kinetic reactors, columns, and pressure relief calculations. These reactions can be used by:
Specifying Reactions for CSTRs and PFRs
RBatch, RCSTR, and RPlug, the kinetics-based reactor models
The reaction kinetics of rate-based reactions can be represented using any of the following expressions:
Power Law kinetic model
User-defined kinetic model written in Fortran or Aspen Custom Modeler
RadFrac for reactive distillation
Pressure Relief model for pressure relief calculations in reactive systems
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REquil
RGibbs
RCSTR
RPlug
RBatch
RYield
USER
RCSTR: adiabatic temperature rise and ΔHrxn
REQUIL: equilibrium, known stoichiometry
REGIBBS: equilibrium, unknown stoichiometry
RCSTR: kinetic CSTR reactor
RPLUG: kinetic PFR reactor
RBATCH: kinetic batch reactor
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For producing high-volume chemicals, flow reactors are usually preferred. Recall that an ideal PFR exactly duplicates the kinetic behavior of an ideal batch reactor.
Batch vs. Flow and Tank vs. Tube
Reasons for preferring one over the other involve secondary considerations such as heat and mass transfer, ease of scale-up, and materials handling logistics.
Flow reactors favored for large scale commodity chemicals.
Tube reactors (i.e. PFRs) are typically preferred over tank reactors; exceptions occur when plug flow is not possible and/or superior product results from the special reaction environment in stirred tanks.
Selectivity is often better in tube reactors than in stirred tanks (depends upon kinetics)
If high heat transfer rates are required small diameter tubes are used to increase the surface area to volume ratio. Several tubes may be arranged in parallel, connected to a manifold or fitted into a tube sheet in a similar arrangement to a shell and tube heat exchangers.
Tubular reactors are generally used for gaseous reactions, but are also suitable for some liquid phase reactions.
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Selectivity
Can have multiple reactions in which desired product, D, is formed and undesired product, U, is also formed
Parallel reactions:
Series reactions:
Instantaneous selectivity:
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continuous
batch
Case 1: α1 > α2
To maximize SD/U should keep CA as high as possible during reaction
Run gas-phase reactions w/o inerts at high P; run liquid-phase reactions with minimal diluents
Use batch or PFR reactor, NOT CSTR—in batch and PFR, CA starts high and drops progressively; in CSTR, CA always at lowest value (i.e. outlet concentration)
Case 2: α2 > α1
To maximize SD/U should keep CA as low as possible during reaction
CSTR preferred to keep CA at low value
Could use diluent and/or product recycle to dilute CA


Selectivity: Effect of Temperature
Case 3: ED > EU — kD increases more rapidly with increasing T than does kU maximize SD/U by operating at highest possible T
Case 4: EU > ED — kD increases less rapidly with increasing T than does kU maximize SD/U by operating at low T, but not so low that reaction rate becomes extremely slow
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Excess reactant
Inert diluent
Cold/hot shots
External heating/cooling
Inter-stage cooling/heating
Want design to maximize rate and conversion, and be safe!!
For highly exothermic or endothermic reactions, need temperature control:
Non-Isothermal Reactor Design
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Disadvantage: Introduce new component S to separate from product and recycle
Excess Inert Diluent S
Optimum temperature decreases as fractional conversion increases
Heat duty profile through PFR must be tailored to achieve optimal trajectory
Isothermal trajectory
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Recall ideal kinetic reactor design equations (CSTR, PFR)
Know rules of thumb for selecting batch vs. flow reactors, and tank vs. tube reactors
Design strategies for dealing with heat of reaction
Become familiar with Aspen Plus reactor models:
Stoichiometric (RSTOICH)
==ξ
k
outkink
k
v
nn
k
k ,,
==
C
A
k
D
rate of formation of D rate of formation of U
S
D/U
= ND
NU
=
!SD/U = FD FU
A
k
D
α1−α2
a
S
D/U