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 BalanceQ - 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+BCA 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 DistillationReactor 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

– Below ambient 10F– Ambient to 300F 20F– Higher Temps 50F– 250 to 350 for furnace

• 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

– 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