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STUDY GUIDE · GKU 5

Separation processes study guide (Chemical Engineering)

Every chemical plant spends most of its capital and energy separating what the reactor made. This guide starts with how a molecule moves: by molecular diffusion down a concentration gradient, and by convection when a flowing fluid carries it to or from a surface. Fick's laws, diffusivities and their dependence on temperature and pressure, convective coefficients from Sherwood-number correlations, and the two-film picture of transfer between gas and liquid make up the first part. The second part turns those rates into equipment. It surveys the separation systems and the agent each one uses, then builds the equilibrium-stage methods: vapour–liquid equilibrium and flash, column material and energy balances, the McCabe–Thiele construction with minimum reflux and the Fenske equation, and absorbers sized with the Kremser equation. It ends with continuous-contact (packed) columns sized by HTU and NTU, and with the heat-and-mass operations of humidification, drying and evaporation.

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Structured by the official ETEC subject standard for this topic: every listed topic and learning outcome, with formulas, figures and worked examples. The full sections are available to members in the app.

SKU 5.1 · Modes of mass transfer

  1. Molecular diffusion and diffusivity

    Fick's law, the two classic steady cases (equimolar counter-diffusion and diffusion through a stagnant gas), and how temperature and pressure change the diffusion coefficient.

  2. Diffusion equations and unsteady diffusion

    The general diffusion equation from a shell balance, its steady and unsteady forms, and the error-function solution for a semi-infinite medium.

  3. Convective mass transfer coefficients

    The mass transfer coefficient, the Sherwood and Schmidt numbers, and the standard correlations for a sphere, a flat plate and a pipe, with the heat–mass analogy behind them.

  4. Two-resistance theory and overall coefficients

    How transfer between a gas and a liquid is split into two film resistances in series, how they add into an overall coefficient, and how to tell which phase controls.

SKU 5.2 · Equilibrium stage operations

  1. Separation systems

    The thermophysical separation processes, the property difference each one exploits and the separating agent it uses, and how to choose between them.

  2. Vapour–liquid equilibrium and flash

    Raoult's law, relative volatility and the y–x curve, bubble-point calculations, and the single equilibrium stage: the flash drum.

  3. Distillation column balances and energy duties

    Overall and component balances for distillate and bottoms, the internal flows set by the reflux ratio and the feed condition, and the condenser and reboiler duties.

  4. McCabe–Thiele method, minimum reflux and Fenske

    Operating lines, the q-line and graphical stage stepping for a binary column, with the two limits that bracket every design: minimum reflux and minimum stages.

  5. Gas absorption and stripping in stages

    Operating and equilibrium lines for a counter-current absorber, the minimum solvent rate, and the Kremser equation for the number of ideal stages.

  6. Packed columns: HTU, NTU and HETP

    Continuous-contact design: the packed height as height of a transfer unit times number of transfer units, the HETP link to stages, and the same method for chemical absorption and solvent extraction.

  7. Humidification, drying and evaporation

    The three operations in which heat and mass move together: air–water humidity, batch drying time from the rate curve, and the mass and energy balance of a single-effect evaporator.

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