Chemical Process Perspective – Detailed Study Notes

Page 1

Chapter Scope & Pedagogical Thread

• Objective – establish a qualitative “chemical‐process perspective” before quantitative methods in later chapters.

• Two threaded case studies supply continuity:

  • A full, well-specified acrylic-acid production train (Turton et al., 2003).
    – Includes complete PFD, equipment list, and stream table.
    – Examples/exercises address material & energy balances, equipment sizing, operating-condition analysis (see Table 1.7 in text).

  • A proposed, information-poor biocatalytic route to hexyl-glucoside.
    – Exercises emphasise scale-up & process development (Table 1.8).

Fundamental Thermodynamic Concepts

• Equilibrium – state variables uniform (or appropriately related) so net driving force $=0$.

• Steady state – variables at fixed locations are time-invariant but may differ spatially; maintained by continual inflow/outflow.

• Driving force – intensive difference that provokes a flux; proportionality often assumed linear near equilibrium.

• Thermodynamic categories discussed:
  • Thermal
  • Chemical
  • Mechanical (left for Problem 1.1).

Thermal Illustration (Fig. 1.1)

1. Two fluids separated by diathermal, non-mass-transfer wall.

2. Batch case – spontaneous temperature equalisation $T<em>A \to T</em>{eq},\;T<em>B \to T</em>{eq}$.

3. Flow case – constant inflow/outflow fixes a temperature gradient; system operates at steady state.

4. Flux $q$ is proportional to $\Delta T$ close to equilibrium – linear driving-force model.

Page 2

Chemical (Un-reactive) Mass Transfer (Fig. 1.2)

• Two immiscible liquids, components partly soluble in each phase.

• Assumptions – isothermal, no reaction, rigid adiabatic walls.

• Each species diffuses until inter-phase chemical equilibrium is reached: chemical potential $\mu<em>i^{\alpha}=\mu</em>i^{\beta}$.

• Unlike heat transfer, equilibrium compositions usually different in the two phases.
  – Overall driving force must account for phase-equilibrium distribution $x<em>i^{\alpha}-x</em>i^{\beta*}$.

• Linear rate law utilised: $N_i \propto (\text{distance from equilibrium})$.

Terminology Links

• Phase-specific driving force vs overall driving force.

• Same conceptual structure as thermal example but with mass instead of energy flux.

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Chemical – Single Ideal Gas Reaction (Fig. 1.3)

• Reaction progress described by mass-action ratio $K=\prod y<em>i^{\nu</em>i} P^{\sum\nu_i}$.

• Batch system: $K\uparrow$ until $K=K_{eq}(T)$ – reaction equilibrium (chemical affinity $=0$).

• Continuous flow reactor: withdrawal prevents build-up; steady state persists at K<K_{eq}.

• Linear driving-force notion produces rate law: $r \propto (K<em>{eq}-K)$ → recovers classical law of mass action $r = k\prod C</em>i^{\nu_i}$ for elementary step.

Acrylic-Acid Case Introduction

• PFD (Fig. 1.4) – 50 000 t a⁻¹, $99.9\,\mathrm{mol\%}$ purity, single-step propylene oxidation.
  $\mathrm{C<em>3H</em>6 + \tfrac32 \,O<em>2 \;\to\; C</em>3H<em>4O</em>2 + H_2O}$

• Turton et al. used CHEMCAD → preliminary design taken as “plant data” for teaching.

• Continuous vs batch vs semi-continuous definitions; acrylic-acid plant represents bulk/commodity operation.

• PFD symbolism – pumps, compressors, mixers, heat exchangers, towers, etc. (see legend in Fig. 1.4).

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Process Streams & Utilities

• Streams link unit operations; utilities (steam, cooling water, etc.) act as thermal sources/sinks.

• Alternative synthetic routes normally screened by tech/economic/safety criteria.
  – Industrially common: two-step propylene → acrolein → acrylic acid.
  – Present chapter explores single-step alternative.

Preliminary Economic ‘Sanity Check’

• Approximate raw-material vs product value:

  • Propylene price tracked to crude-oil index (Fig. 1.5).

  • Conservative 1990s prices adopted: $\$0.225\,\text{/lb (C₃H₆)}$, $\$0.0175\,\text{/lb (O₂)}$, $\$0.41\,\text{/lb (acrylic acid)}$.

• Per lb-mole balance (Table 1.1): gross margin $\approx\$19.94$ → $\$0.473\,\text{/lb C₃H₆}$.
  – For 50 000 t a⁻¹ this is $13\,780\,$ h⁻¹ available for OPEX, CAPEX service, profit, etc.

Side-Reactions & Catalysis Need

\begin{aligned}
\text{Main: } &\mathrm{C3H6+\tfrac52 O2 \to C3H4O2 + H2O + CO2}\
\text{Over-oxidation: } &\mathrm{C3H6+\tfrac92 O2 \to 3CO2+3H_2O}
\end{aligned}

• Selectivity critical – catalyst coated on porous support walls.

Page 5

Catalysis Fundamentals

• Catalyst lowers potential-energy barrier between reactant & product troughs (Denbigh representation, Fig. 1.6).

• Heterogeneous vs homogeneous; solid supports offer >250\,\text{m}^2\,\text{g}⁻¹ surface area.

• Industrial relevance: 90 % of processes, 60 % of products involve catalysis (Vision 2020).

Illustrative Non-Biological Catalysts

• Hydrodesulfurisation: Co–Mo/Al₂O₃.

• Benzothiophene hydrogenolysis over Rh-polystyrene (Figs 1.7–1.8); requires adjacent Rh centres.

• TEM image (Fig. 1.9) of Pt/Al₂O₃ – particle dispersion crucial.

• Table 1.2 lists numerous commercial catalytic systems (acidic, oxidative, polymerisation, etc.) and typical poisons (S, Cl, coke).

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Feed Section Hardware (Fig. 1.10)

• Safety motive: steam dilution added to propylene + air to avoid flammability.

• Propylene (stream 3) pumped; air (stream 1) compressed (C-301 A/B) → combined with live steam (stream 2) → reactor feed (stream 4).

Pumps

• Centrifugal pump anatomy (Fig. 1.11): impeller, volute, suction eye; imparts kinetic → pressure energy.

Compressors / Blowers

• Reciprocating compressor cycle (Fig. 1.12) – suction, compression, discharge; multistage, inter-cooling at high ratios.

• Centrifugal blower (Fig. 1.13) used for $P<em>{out}/P</em>{in}\le2$.

Stream Table Snapshot (Table 1.3)

• State variables: $T,\;P,\;{\dot n_i}$ – underpin mass, energy, momentum balances.

• Example check: $\sum<em>i \dot n</em>{1+2+3}=\dot n_4$ (no generation/consumption in feed section).

• Temperature of stream 4 higher than inlets due to compressor work $W$ (1st-Law connection).

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Reactor Section (Fig. 1.14)

• Fluidised-bed catalytic reactor R-301; molten-salt jacket removes exotherm, circulated by pump P-301A/B to exchanger E-301.

• Salt: Hitec® (60 wt % NaNO₃ / 40 wt % KNO₃); heat capacity ≈ $42\,\text{Btu ft⁻³ °F⁻¹}$.

• Alternative: multitubular fixed-bed within shell-and-tube HX (Fig. 1.16).

Macroscopic Reaction Rates (Table 1.4)

Example derivation:
$R<em>1^{(V)}=+87.9\;\text{kmol h}^{-1}\;(\text{via C₃H₄O₂})$ $R</em>3^{(V)}=\frac{148.3-87.9-6.54}{3}=17.95\;\text{kmol h}^{-1}$ (using water balance).

• Law of stoichiometry: $R<em>j^{(V)}=\frac{\dot n</em>{out,i}-\dot n<em>{in,i}}{\nu</em>{i,j}}$ independent of $i$.

Energy Picture

• Overall reactor + salt loop: $\sum \dot H<em>{in}=\sum \dot H</em>{out}$.

• Cooling water in E-301 removes $Q$; constraints: CW in ≈ 90 °F, out ≤ 130 °F.

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Quench & Gas Absorption Pre-Separation (Fig. 1.17)

• Direct-contact quench (T-301) uses cooled recycle liquor to lower gas temp.

• Packed absorber T-302 – counter-current water wash of gas to pick up acrylic & acetic acids.
  – Design depends on interfacial area $a$, mass-transfer coefficients $k<em>L,k</em>G$, and driving force $\Delta C$.

• Water feed (stream 10) chosen such that overhead gas ~saturated with $\mathrm{H<em>2O}$: $y</em>{H<em>2O,eq}=\tfrac{P^{sat}</em>{H<em>2O}(48\,°C)}{P}\approx0.111\Rightarrow\dot n</em>{H_2O}=148\,\text{kmol h}^{-1}$, matching Table 1.3 value 150.2.

Loss Analysis (Table 1.5)

• Product losses with purge gas (stream 11):
  – Acrylic acid $12.2 \%$ of total produced.
  – Acetic acid $41.4 \%$.

• Trade-off: solvent rate vs recovery vs incineration cost.

System Boundaries Note

• T-301 + T-302 + E-302 + P-302 considered as one macro-unit.
  – Internal streams cancel in overall balances.

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Down-Stream Separation Logic (Fig. 1.18–1.22)

Solvent Extraction Loop

• Extractor T-303 contacts aqueous acid mixture (stream 9) with DIPE solvent (stream 23) – leverages preferential solubility.

• DIPE lighter → forms dispersed rising phase; column often Scheibel or Karr reciprocating plate (Fig. 1.19).

• Equilibrium-stage or rate-based design; residence time = $V/\dot V$.

• Heat exchanger E-309 uses low-pressure steam (latent heat) to pre-heat recycle solvent.

Solvent Recovery Column T-304

• DIPE (bp 156 °F) far more volatile than acids.

• Operated s.t. condenser T ≲130 °F → CW service.

• Overhead distillate: DIPE + trace H₂O (stream 16) → recycle.

• Bottoms (stream 14): acid products ≈93 mol % acrylic.

Waste-Water Column T-306

• Strips traces of DIPE & acid from aqueous stream 12.

• Ensures environmental compliance; returns DIPE to process.

Acid Purification T-305

• Vacuum operation to keep reboiler ≪54 °C (flash-point of glacial acetic).

• Produces acrylic-acid bottoms (product) and acetic-acid overhead (by-product).

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Storage & Tank-Farm Design (Fig. 1.23–1.25)

• Roles: buffer logistics, accommodate shutdowns, off-grade segregation.

• Fixed roofs for P_v<1.5 psia; external or internal floating roofs up to 11 psia limit emissions.

• Design capacities:
  – Net working volume (between LIC alarms).
  – Heel volume below low-level sensor.
  – Overfill protection volume above high-level sensor.

• Ancillary design: wind/seismic loads, foundations, materials, vent control, heat tracing for high-mp materials.

Safety Remark

• Process-safety engineering (relief, containment, HAZOP) not detailed herein; AIChE CCPS resources recommended.

Generic Chemical-Plant Block Diagram (Fig. 1.26)

• Feed prep → Reactor(s) with possible heat-integration → Separation train → Utilities & Waste treatment → Product storage.

• Recycling improves conversion but raises separation duty; economic optimisation required.

Tables of Embedded Practice Problems

• Table 1.7 – Acrylic-acid numerical exercises across later chapters (mass, energy, design).

• Table 1.8 – Hexyl-glucoside exercise roadmap.

Section 1.3 – Biocatalysis vs Chemocatalysis

• Enzymes: protein catalysts; high chemo-, regio-, stereoselectivity; rate accelerations up to $10^{17}$.

• Pure enzyme vs whole-cell systems.
  – Whole cell enables cofactor self-recycle (e.g., baker’s yeast NADH cases).

• Subtilisin Carlsberg (Fig. 1.27) – serine protease catalytic-triad archetype.

Industrial Biotransformations (Fig. 1.28)

• ~150 commercial processes; >50 % pharma, remainder food/agro.

• Chirality: enantiomeric excess $ee=\frac{|n<em>R-n</em>S|}{n<em>R+n</em>S}$ key KPI.

• Methods: chiral precursors, asymmetric synthesis (e.g., fumarate → L-aspartate), dynamic kinetic resolution (Fig. 1.31), whole-cell cascades.

Commodity & Fine-Chemical Enzyme Processes

• Continuous immobilised E. coli column → L-aspartic acid (Fig. 1.32).

• Nitto’s nitrile hydratase route to 50 wt % acrylamide (Fig. 1.33); impurity <5 ppm acrylic acid.

Alkyl-Glucoside Case for Future Exercises

• Enzymatic condensation of glucose + n-hexanol $\xrightarrow{\beta\text{-glucosidase}}$ hexyl-β-D-glucoside $+H_2O$ (Fig. 1.34).

• Advantages vs chemical route: regio/stereo specificity, no anomer separation, milder conditions, biodegradable surfactant.

Data Resources (Sec 1.4)

• Engineering databases: DIPPR 801/882, Perry, Yaws, Knovel tables, etc.

• Journal sources: AIChE J, Chem Eng J, Biotechnol J, Fluid Phase Equilibria.

• Importance of accurate property data for safe design & realistic economics.

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