CHE 480: Process Analysis and Design
Estimated study time: 8 minutes
Table of contents
Sources and References
Primary texts — Turton, R., Shaeiwitz, J.A., Bhattacharyya, D., and Whiting, W.B., Analysis, Synthesis, and Design of Chemical Processes, 5th ed., Prentice Hall, 2018; Seider, W.D. et al., Product and Process Design Principles: Synthesis, Analysis, and Evaluation, 4th ed., Wiley, 2017.
Supplementary texts — Peters, M.S., Timmerhaus, K.D., and West, R.E., Plant Design and Economics for Chemical Engineers, 5th ed., McGraw-Hill, 2003; Douglas, J.M., Conceptual Design of Chemical Processes, McGraw-Hill, 1988.
Online resources — MIT OCW 10.32 “Separation Processes”; NPTEL “Chemical Process Design” lectures; AIChE CCPS open guidance documents; NIST WebBook property data; DIPPR public summaries.
Chapter 1: Process Flowsheets and Conceptual Design
1.1 Douglas’s Hierarchy
Jim Douglas’s hierarchical decision procedure structures conceptual design:
- Batch vs. continuous.
- Input-output structure (feeds and products).
- Recycle structure and reactor considerations.
- Separation system (vapor recovery, liquid recovery).
- Heat exchanger network.
Each level resolves decisions before the next, with iteration when downstream constraints invalidate upstream choices. The approach prevents premature detail by forcing attention to economic drivers at each stage.
1.2 Flowsheet Diagrams
Block flow diagrams (BFDs) identify units and streams without geometry. Process flow diagrams (PFDs) add major equipment, principal streams with compositions/flows/T/P, and controllers of strategic importance. Piping and instrumentation diagrams (P&IDs) specify every pipe, valve, instrument, and interlock—detailed engineering’s contract with construction.
1.3 Process Synthesis Principles
A good flowsheet has few degrees of freedom, accessible startup/shutdown paths, inherent safety margins, and clear heat/mass integration potential. Minimizing recycle streams simplifies dynamics; maximizing heat integration lowers operating cost. These aims sometimes conflict.
Chapter 2: Heat Exchanger Networks
2.1 Pinch Analysis
Composite curves plot cumulative heat duty versus temperature for all hot streams (to be cooled) and cold streams (to be heated). The pinch point is where the curves approach within the minimum allowed \( \Delta T_{min} \); minimum hot and cold utility demands follow from the offsets at the extreme ends.
Three golden rules:
- Do not transfer heat across the pinch.
- Do not use cold utility above the pinch.
- Do not use hot utility below the pinch.
Violating any incurs double penalty—extra utility of both kinds.
2.2 Minimum Number of Units
The minimum number of exchangers equals \( N_{streams} + N_{utilities} - 1 \) for a single network region. A pinched problem has two regions; minimum units total across regions. Trade-offs between minimum units (fewer exchangers, higher utility) and minimum utility (more exchangers, lower utility) define a cost-optimal network.
2.3 Exchanger Design
Shell-and-tube designs per TEMA standards dominate at large scale. Design steps:
- Heat duty: \( Q = \dot m C_p \Delta T \) or \( \dot m \lambda \) for condensers/reboilers.
- Log-mean temperature difference with correction factor for multi-pass.
- Overall heat transfer coefficient estimate from tabulated values or detailed shell/tube-side correlations.
- Area \( A = Q/(UF\Delta T_{lm}) \).
- Tube count, shell diameter, baffle spacing, pressure drop check.
Air coolers, plate-and-frame exchangers, and compact (printed circuit, spiral) exchangers serve specific niches.
Chapter 3: Separation System Design
3.1 Distillation Column Design
Shortcut methods:
- Fenske: minimum stages at total reflux.
- Underwood: minimum reflux ratio for given split.
- Gilliland correlation: stages at operating reflux (\( 1.2-1.5 \times R_{min} \)).
Rigorous design uses tray-by-tray simulation (Aspen Plus, UniSim) with activity coefficient models. Column diameter from flooding correlations (Fair’s); tray spacing (typically 18–24 inches); downcomer sizing; packed vs. trayed selection (packed for vacuum service, low pressure drop, corrosive; trayed for high capacity, variable load).
3.2 Other Separation Technologies
Liquid-liquid extraction, adsorption (PSA, TSA), crystallization, membrane separations (reverse osmosis, gas permeation, pervaporation), centrifugation. Each has a distinguishing economic region. Hybrid systems (extraction + distillation, membrane + distillation) frequently outperform single-technology solutions.
3.3 Heuristics for Sequence Selection
For multicomponent mixtures, Douglas/Seider heuristics guide distillation train sequencing:
- Remove corrosive and hazardous components first.
- Perform the most difficult separation last.
- Favor the direct sequence (heaviest off bottom) when products are similar in volume.
- Do the 50/50 split first when feasible (even sizing).
Chapter 4: Fluid Movement
4.1 Pumps
Centrifugal pumps dominate (low viscosity, moderate head). Positive displacement pumps (gear, screw, piston) handle high-viscosity and metering applications. Design selection plots head vs. flow (characteristic curves) against system curves; the operating point is their intersection.
Net positive suction head:
\[ NPSH_a = \frac{P_{suction} - P_{vap}}{\rho g} + \frac{v^2}{2g} - H_{friction}. \]\( NPSH_a > NPSH_r \) prevents cavitation—typically with a safety margin of 1 m or more.
4.2 Compressors
Centrifugal (continuous, high flow), axial (high flow, aerospace), reciprocating (high pressure ratio, pulsating), screw (moderate). Polytropic or isentropic efficiencies relate ideal to actual work. Surge control requires anti-surge recycle; interstage cooling approaches isothermal compression and reduces total work.
4.3 Piping
Economic pipe diameter minimizes total annual cost (capital + pumping power). Rule-of-thumb velocities: liquid 1–3 m/s, gas 15–30 m/s, steam up to 60 m/s. Allowable pressure drops (\( \Delta P / L \)) correlate with these. Pipe schedules (thickness) follow ASME B31.3 for process piping.
Chapter 5: Staged Separations and Column Internals
5.1 Absorbers and Strippers
Absorber/stripper design mirrors distillation but without reboiler and condenser. Kremser equation approximates stages for dilute systems:
\[ N = \frac{\log\left[\frac{(y_{N+1} - m x_0)/(y_1 - m x_0)(1 - 1/A) + 1/A}{}\right]}{\log A}, \]with absorption factor \( A = L/(mV) \). Countercurrent arrangement maximizes driving force.
5.2 Column Hydraulics
Flooding, weeping, entrainment, and foaming limit tray operation. Sieve, valve, and bubble-cap trays have distinct pressure drop, turndown, and fouling characteristics. Packed columns use random (Raschig rings, Pall rings) or structured (Mellapak, Flexipac) packings; HETP (height equivalent to a theoretical plate) characterizes packing efficiency.
5.3 Control and Turndown
A column sized only for design-point feed struggles at reduced throughput. Good design accommodates ±30% turndown without operator intervention; flexibility often justifies extra capital.
Chapter 6: Economics and Project Execution
6.1 Capital Cost
Equipment costs scale as \( C = C_0 (S/S_0)^n \) with exponent \( n \) typically 0.6 (economy of scale). Correcting for year (CEPCI, Chemical Engineering Plant Cost Index), material (stainless steel, Hastelloy), and pressure (ASME Code factors). Bare module cost = equipment cost × module factor, accounting for installation, piping, instrumentation, and foundations.
Total direct costs + indirect (engineering, contingency) + land + working capital = total capital investment. Lang factors ranging from 3 to 5 relate purchased equipment to total capital.
6.2 Operating Cost
Utilities (steam 10–30 $/GJ, cooling water 0.1 $/GJ, electricity 50–150 $/MWh—regional and volatile), raw materials, catalyst/solvent makeup, waste disposal, labor (operators, supervisors, maintenance), and fixed overhead. Cost of manufacture typically decomposed as direct + fixed + general expenses.
6.3 Profitability Measures
NPV, IRR, payback period, ROI. The discount rate reflects risk; commodity chemicals often use 10–15%, specialty and bio-based higher.
\[ NPV = \sum_{k=0}^{N}\frac{CF_k}{(1+i)^k}. \]Sensitivity analysis on key inputs (price, raw material cost, capital, production rate) prioritizes attention.
6.4 Pollution Prevention and Safer Design
Every design step offers safety levers. Minimize inventory (small hold-up vessels), substitute hazardous solvents, prefer dilute feeds over neat streams for exothermic reactions, locate high-hazard units for isolation. A process with modest inherent safety margins often needs less active control than a theoretically efficient but aggressive alternative.
