Heuristics for Process Design
Chapter 6
Terry A. Ring
Heuristics
• 1-Select raw materials and chemical
reactions to avoid or reduce the handling
and storage of hazardous and toxic
chemicals.
• 2-Use an excess of one chemical reactant
in a reaction to consume completely a
valuable, toxic or hazardous chemical
reactant. (see MSDS for chemical
hazards)
Heuristics
• 3-For pure products, eliminate inert
species before reaction when separations
are easy and when the catalyst is
adversely affected by the inert, but not
when a large exothermic heat of reaction
must be removed.
Reactor Heat Balance
Q - Ws - FAoΣθiCp_i(T-To) – FAoX [ΔHºR(TR) + ΔCp(T-TR)=0
T=(X [- ΔHºR(TR)+ ΔCpTR] + ΣθiCp_iTo)/(ΣθiCp_i + X ΔCp)
Heuristic
• 4- Introduce purge streams to provide
exists for impurities when the impurities
are in trace quantities or difficult to
separate. Light species leave via vapor
purge and heavier species leave via liquid
purge streams.
Heuristic
• 5- Do not purge species that are valuable
or toxic or hazardous even in small
concentrations. Add separators to capture
valuable species. Add reactors to
eliminate toxic or hazardous species.
Heuristic
• 6- Byproducts that are produced in
reversible reactions, in small quantities,
are usually not recovered in separators or
purged. Instead they are recycled to
extinction
A+BC
A  D
Heuristic
• 7-For competing reactions, both in series
and parallel adjust T and P and catalyst to
obtain high yields of the desired products.
Check that there are no kinetic limits to
this assumption.
Heuristic
• 8-For reversible reactions, consider
conducting them in a separation device
capable of removing the products driving
the reaction to more products. This gives
a very different distribution of products.
Reactive Distillation
Reactor with selective membrane wall
Separation Heuristics
• 9- Separate liquid mixtures with distillation,
stripping, enhanced distillation, LL extraction,
crystallization and/or adsorption.
• 10-Condense vapor mixtures then separate via
Heuristic 9
• 11-Separate vapor mixtures using partial
condensation, cryo D, absorption, adsorption,
membrane sep. and/or desublimation.
• 12-20 Separations involving particles Heuristics
Reaction Heat Heuristics
• 21-High exothermic heat of reaction: Consider
using excess reactant, an inert diluents or cold
shots. Consider them early on in the design
• 22-Lower exothermic heat of reaction: Use heat
exchanger on/in reactor. Or use intercoolers
between adiabatic reaction stages.
• 23-High endothermic heat of reaction: Consider
use of excess reactant, inert diluents or hot
shots. Consider them early on in the design.
• 24-Lower endothermic heat of reaction: Use
heat exchanger on/in reactor. Or use
interheaters between adiabatic reaction stages.
Heat Exchanger Heuristics
•
•
25-Use shell and tube HX in counter current for process streams. For
T>750F use furnace.
26-Near optimal approach ΔT’s
–
–
–
–
•
•
•
•
27-CW temperature range used is 90 to 120 F
28-Boiling a liquid use 45 F approach ΔT
29-30 Other Heuristics
31- ΔP in HX
–
–
–
–
–
•
Below ambient
10F
Ambient to 300F
20F
Higher Temps
50F
250 to 350 for furnace
1.5 psi for boiling or condensation
3 psi for gas
5 psi for low viscosity liquid
7-9 psi for high viscosity liquid
20 psi for process fluid in furnace
32-33 Other Heuristics
Pressure Operation Heuristics
• 40-42- pressure decrease
• 43-pump a liquid rather than compress a
gas, unless refrigeration is needed.
• 44- Air leak rate into vacuum
• 45-47 Vacuum pumps
Solid Particle Processing Heuristics
• 48-53 Covering
– Conveying
• Pneumatic
• Conveyor belt
– Reaction
– Separations
• Cyclone
• Crystallization
• Agglomeration
– Heat transfer