Chemical engineering basics: 50 core concepts

All 50 cards

1. 1
   What do chemical engineers do?
   Design, build, run and improve processes that turn raw materials into useful products on an industrial scale, safely and economically

   Fuels, plastics, medicines, fertilizers, food and clean water all come from such processes.
2. 2
   Unit operation
   A basic physical step that appears in many processes, such as distillation, filtration, heat exchange, drying or mixing

   The idea lets engineers study each step once and reuse it across very different industries.
3. 3
   Block flow diagram
   The simplest process diagram: boxes for the main process steps, joined by arrows for the main streams
4. 4
   Process flow diagram (PFD)
   Shows the major equipment and main process streams, usually with flow rates, compositions, temperatures and pressures

   It leaves out most piping details, small valves and instruments.
5. 5
   Piping and instrumentation diagram (P&ID)
   A detailed drawing of all the equipment, piping, valves, instruments and control loops in a plant section

   Used for construction, operation, maintenance and safety reviews.
6. 6
   Pilot plant
   A small-scale version of a process, built to test it and collect design data before a full-size plant is built
7. 7
   Batch process
   The feed is charged into a vessel, processed, and removed some time later; nothing flows in or out in between

   Suits small volumes and many products, such as pharmaceuticals and specialty chemicals.
8. 8
   Continuous process
   Feed and product streams flow in and out the whole time the process runs

   Suits large volumes, such as oil refining and ammonia production. Usually run at steady state.
9. 9
   Semibatch process
   Neither batch nor continuous: material enters or leaves while the process runs, such as a tank slowly fed a reactant with nothing withdrawn
10. 10
    Steady state
    No process variable (flow, temperature, pressure, composition) changes with time, so nothing accumulates

    Startup, shutdown and upsets are transient, or unsteady-state, operation.
11. 11
    General balance equation
    $$\text{input} - \text{output} + \text{generation} - \text{consumption} = \text{accumulation}$$

    Generation and consumption apply only to species formed or used up by reactions; total mass is neither generated nor consumed.
12. 12
    Mass balance at steady state with no reaction
    Input = output, for total mass and for each component
13. 13
    Are total mass and total moles conserved in a chemical reaction?
    Total mass is; total moles are not necessarily

    $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$ turns 4 mol into 2 mol, but the mass is unchanged.
14. 14
    Basis of calculation
    An amount or flow rate of one stream, such as 100 mol of feed, chosen so the balance can be solved; results are scaled afterwards

    With a feed composition in mole fractions, 100 mol is convenient; with mass fractions, 100 kg.
15. 15
    Degrees of freedom of a balance problem
    Unknowns minus independent equations; the problem can be solved only when this is zero

    A positive number means more information is needed; a negative number means the problem is overspecified.
16. 16
    Stoichiometric coefficients
    The numbers in a balanced equation that give the mole ratios in which species react and form

    In $\mathrm{2SO_2 + O_2 \rightarrow 2SO_3}$, 2 mol of $\mathrm{SO_2}$ react with 1 mol of $\mathrm{O_2}$.
17. 17
    Limiting reactant
    The reactant present in less than its stoichiometric proportion; it would run out first, so it sets the maximum amount of product
18. 18
    Percent excess of a reactant
    $\dfrac{n_{\text{fed}} - n_{\text{stoich}}}{n_{\text{stoich}}} \times 100\%$

    $n_{\text{stoich}}$ is the amount needed to react completely with the limiting reactant.
19. 19
    Fractional conversion
    $X = \dfrac{\text{moles of reactant that react}}{\text{moles of reactant fed}}$

    Usually based on the limiting reactant; it runs from 0 to 1.
20. 20
    Extent of reaction
    $\xi$ in $n_i = n_{i0} + \nu_i \xi$: one number that tracks the progress of a reaction for every species

    $\nu_i$ is the stoichiometric coefficient, negative for reactants and positive for products.
21. 21
    Yield of a reaction
    Moles of desired product formed divided by the moles that would form if the limiting reactant reacted completely with no side reactions

    Definitions vary: some books divide by the moles of reactant fed or consumed instead.
22. 22
    Selectivity
    Moles of desired product formed per mole of undesired product formed

    Side reactions lower it. Some books define it per mole of reactant consumed instead.
23. 23
    Composition of dry air
    About 21 mol% $\mathrm{O_2}$ and 79 mol% $\mathrm{N_2}$

    So each mole of $\mathrm{O_2}$ brings about $3.76\ \mathrm{mol}$ of $\mathrm{N_2}$ $(79/21)$.
24. 24
    Theoretical air
    Air containing just enough oxygen to burn all the fuel completely, with all carbon going to $\mathrm{CO_2}$ and all hydrogen to $\mathrm{H_2O}$

    Air fed beyond this is excess air: 20% excess air means 1.2 times the theoretical amount.
25. 25
    Wet basis vs dry basis
    A gas composition on a wet basis includes water vapor; on a dry basis it leaves the water out

    Flue gas analyzers such as the Orsat report a dry-basis composition.
26. 26
    Mole fraction
    $x_i = \dfrac{n_i}{n_{\text{total}}}$

    Mole fractions in a mixture sum to 1. By convention $x$ is used for liquids and $y$ for gases.
27. 27
    Mass fraction
    $w_i = \dfrac{m_i}{m_{\text{total}}}$

    Mass and mole fractions differ unless all the components have the same molar mass.
28. 28
    Average molar mass of a gas mixture
    $\bar{M} = \displaystyle\sum_i y_i M_i$

    For air, about $29\ \mathrm{g/mol}$.
29. 29
    Molar flow rate from mass flow rate
    $\dot{n} = \dfrac{\dot{m}}{M}$

    $M$ is the molar mass, so $36\ \mathrm{kg/h}$ of water is about $2\ \mathrm{kmol/h}$.
30. 30
    Density of liquid water
    About $1000\ \mathrm{kg/m^3}$, or $1\ \mathrm{g/cm^3}$, or $62.4\ \mathrm{lb_m/ft^3}$

    Density links mass and volumetric flow: $\dot{m} = \rho\dot{V}$.
31. 31
    Specific gravity
    $SG = \dfrac{\rho}{\rho_{\text{ref}}}$, usually with liquid water at 4 °C as the reference

    It is dimensionless, so a liquid with SG 0.8 has a density of about 800 kg/m³.
32. 32
    Parts per million (ppm)
    A fraction multiplied by $10^6$, used for trace amounts

    For gases it is normally by moles (volume); for liquids, by mass.
33. 33
    Ideal gas law for a flowing stream
    $P\dot{V} = \dot{n}RT$

    Use absolute pressure and absolute temperature. It works best at low pressure and high temperature.
34. 34
    Gas constant R in L·atm units
    $R = 0.08206\ \mathrm{L\,atm\,mol^{-1}\,K^{-1}}$

    The same as $8.314\ \mathrm{J\,mol^{-1}\,K^{-1}}$.
35. 35
    Molar volume of an ideal gas at 0 °C and 1 atm
    $22.4\ \mathrm{L/mol}$, or $22.4\ \mathrm{m^3/kmol}$

    Also 359 ft³ per lb-mol. At 0 °C and 1 bar it is 22.7 L/mol.
36. 36
    Gauge vs absolute pressure
    $P_{\text{abs}} = P_{\text{gauge}} + P_{\text{atm}}$

    Gauges read relative to the atmosphere, but gas laws need absolute pressure. Units such as psig and psia say which is meant.
37. 37
    One standard atmosphere in other units
    $101.325\ \mathrm{kPa} = 1.01325\ \mathrm{bar} = 14.696\ \mathrm{psi} = 760\ \mathrm{mmHg}$
38. 38
    Absolute temperature scales
    $T(\mathrm{K}) = T({}^\circ\mathrm{C}) + 273.15$ and $T({}^\circ\mathrm{R}) = T({}^\circ\mathrm{F}) + 459.67$

    A temperature change of 1 K (or 1 °C) equals 1.8 °F (or 1.8 °R).
39. 39
    Pound-mole
    The amount of a substance whose mass in pounds equals its molar mass: about $453.6\ \mathrm{mol}$

    So 1 lb-mol of water has a mass of about $18\ \mathrm{lb_m}$.
40. 40
    Pound-force in terms of pound-mass
    $1\ \mathrm{lb_f} = 32.174\ \mathrm{lb_m\,ft/s^2}$

    The factor $g_c = 32.174\ \mathrm{lb_m\,ft\,lb_f^{-1}\,s^{-2}}$ appears in equations written in US customary units.
41. 41
    Dimensional homogeneity
    Every term added or subtracted in a valid equation must have the same dimensions

    A quick check for errors in derived formulas and unit conversions.
42. 42
    Pump vs compressor
    A pump raises the pressure of a liquid; a compressor raises the pressure of a gas

    Fans and blowers move gases with only a small pressure rise.
43. 43
    Shell-and-tube heat exchanger
    One fluid flows through a bundle of tubes and the other around them inside a shell; heat passes through the tube walls
44. 44
    Countercurrent vs cocurrent flow in a heat exchanger
    Countercurrent: the fluids flow in opposite directions. Cocurrent (parallel): the same direction. Countercurrent transfers more heat for the same area

    Only countercurrent flow lets the cold fluid leave hotter than the hot fluid leaves.
45. 45
    Distillation column
    Separates a liquid mixture by differences in volatility: the more volatile components concentrate in the vapor rising to the top

    The top product is the distillate; the bottom product is the bottoms.
46. 46
    Reboiler and condenser
    The reboiler boils liquid at the bottom of a distillation column to make the rising vapor; the condenser liquefies the vapor at the top, and part of it returns as reflux
47. 47
    Flash drum
    A vessel where a feed partly vaporizes, often after a pressure drop, and the vapor and liquid are separated

    With the two phases at equilibrium, it acts as a single equilibrium stage.
48. 48
    Decanter
    A vessel that separates two immiscible liquids, such as oil and water, by letting them settle into layers by density
49. 49
    Three ideal reactor types
    Batch reactor, continuous stirred-tank reactor (CSTR) and plug flow reactor (PFR)

    A CSTR is perfectly mixed, so its outlet matches its contents; a PFR has no mixing along the flow direction.
50. 50
    Utilities
    Services a plant needs to run, such as steam, cooling water, electricity, fuel, compressed air and refrigeration