FE Chemical common mistakes
FE Chemical · Understand the traps and how to avoid them.
Working every problem in strict SI units and spending several minutes converting given data (psia, °F, lbm, Btu/hr) before any calculation can start, then running out of time on the afternoon session.
Most FE Chemical study material is written in SI, but a large share of exam problems are stated in US customary units, and candidates who drill only SI problems have no fluent conversion reflexes, so each conversion becomes a fresh, error-prone derivation.
What to do instead
Build a one-page conversion card (1 Btu = 1.055 kJ, 1 psia = 6.895 kPa, °R = °F + 460, 1 lbm = 0.4536 kg, 1 hp = 0.7457 kW, 1 cP = 0.001 Pa·s) and drill 20 mixed-unit problems per week until conversions are automatic. Practice reading the requested answer units first and converting only at the end, keeping intermediate work in one consistent system.
Setting up material and energy balances without first choosing a basis and drawing/labeling a flow diagram with every stream, component, and unknown numbered, then discovering mid-solution that the degrees of freedom do not close.
Under time pressure, candidates jump straight to equations, skip the basis selection, and lose track of which compositions are mole fractions versus mass fractions or which recycle stream was already counted. The result is either an unsolvable system or a silently double-counted stream.
What to do instead
Adopt a rigid three-step ritual: (1) sketch and label every stream with a variable name, (2) declare a basis (e.g., 100 mol feed or 1 hour of operation), (3) count unknowns and independent equations before writing anything. If degrees of freedom are not zero, stop and re-read the problem statement for a missing specification such as a recycle ratio or a purge composition.
Treating a reactor problem as a single-pass conversion problem when the flowsheet contains recycle, so the overall conversion, per-pass conversion, and fresh-feed requirement are all confused with one another.
Recycle and purge are conceptually simple but easy to misapply: candidates compute the reactor inlet composition using fresh feed, or use overall conversion where per-pass conversion was specified, and the arithmetic then looks plausible so the error goes undetected.
What to do instead
Always solve recycle flowsheets by writing balances around two envelopes separately - the overall process boundary (fresh feed to product plus purge) and the reactor-plus-separator subsystem - and label which conversion the problem gives. Do a sanity check that per-pass conversion is always greater than or equal to overall conversion for a positive recycle stream.
Applying a formula from memory - LMTD, Nusselt number correlation, pump power, or K-value - without checking the validity conditions (fully developed flow, turbulent versus laminar, constant wall temperature versus constant heat flux, ideal solution).
Exam questions are deliberately written so that the given numbers fit several formulas, and candidates under time pressure grab the most familiar one. The Graetz versus Dittus-Boelter choice, or LMTD versus effectiveness-NTU, depends on whether outlet temperatures are known, and that distinction is easy to miss.
What to do instead
Before substituting, write the applicability conditions in one line next to the formula (Re range, Pr range, geometry, boundary condition) and confirm each against the problem statement. For heat exchangers, decide first whether both outlet temperatures are known (LMTD with correction factor F) or only inlet temperatures and flow rates are known (effectiveness-NTU).
Using the arithmetic mean temperature difference instead of the log mean, or omitting the correction factor F for multipass and crossflow exchangers, producing a duty that is off by 10 to 30 percent.
The arithmetic mean is faster to compute and feels adequate when the terminal temperature differences are close, but exam problems often specify widely different terminal differences precisely to test whether the candidate recognizes the log mean is required.
What to do instead
Memorize the rule: use LMTD whenever the terminal temperature differences differ by more than roughly 10 percent, and always multiply by F for anything other than pure countercurrent or pure cocurrent flow. Practice computing LMTD both from terminal differences and from the equivalent form using the temperature approach, and verify the answer is always between the arithmetic mean and the smaller terminal difference.
Getting the sign convention wrong on a thermodynamics problem - reporting work as positive when the system does work, or mixing up ΔH and ΔU - and then selecting an answer choice that matches the wrong sign.
Textbook sign conventions vary between sources, and candidates who studied from multiple references carry an inconsistent convention into the exam. Because exam answer choices usually include the sign-flipped value as a distractor, the error is invisible during a quick review.
What to do instead
Fix one convention before exam day (work positive when done by the system, or the reverse) and write it at the top of the scratch paper. For closed-system energy balances, explicitly convert between ΔH and ΔU using ΔH = ΔU + Δ(PV), and for ideal gases use ΔH = ΔU + nRΔT, checking that the correction has the expected magnitude.
Calculating a distillation or absorption requirement with the wrong driving force - using a simple ratio of mole fractions instead of the equilibrium-relative volatility expression, or applying Kremser equations outside their assumed constant-flow, straight-equilibrium-line conditions.
Shortcut formulas are memorized without their assumptions, and candidates do not check whether the problem specifies constant molar overflow or a nonlinear equilibrium curve. Absorption problems in particular mix up solute-free versus total mole fractions.
What to do instead
For binary distillation, derive the minimum reflux from the q-line intersection rather than recalling a formula, and use the Fenske equation only for total reflux. For absorption and stripping, confirm constant molar overflow and a straight equilibrium line before using Kremser, and always work in solute-free mole ratios when the solute is dilute and the gas flow changes appreciably.
Skipping the stoichiometry check on a reaction-engineering problem - using the wrong rate expression order, or plugging a conversion directly into a CSTR design equation without converting to concentration or residence time.
Design equations for batch, CSTR, PFR, and PBR look similar, and candidates substitute conversion where concentration is needed or forget that the CSTR volume uses the exit rate while the PFR integral must be evaluated over the conversion range.
What to do instead
Write the design equation symbolically first, list every variable it contains with units, and only then substitute. For liquid-phase systems use C_A = C_A0(1 − X); for gas-phase systems add the volumetric expansion factor ε and pressure and temperature corrections. Check that the computed volume or residence time has physically reasonable magnitude before selecting an answer.
Guessing on quantitative problems instead of eliminating answer choices by order-of-magnitude or limiting-case reasoning, and leaving questions unanswered because the full calculation seemed too long.
Candidates trained only on full worked solutions do not develop estimation reflexes, so a problem with awkward numbers looks unsolvable within the time budget even though one or two answer choices are physically absurd (negative absolute temperature, efficiency above 100 percent, a pump head far beyond the stated pressure rise).
What to do instead
Practice a two-pass strategy: on the first pass, solve only problems with a clear path; on the second, eliminate choices using bounds and units. Drill estimation habits such as computing a rough answer in scientific notation and checking the exponent, and verifying that any efficiency, conversion, or recovery falls between 0 and 1.
Misreading the question stem - answering the complementary quantity (heat removed instead of heat added, gauge instead of absolute pressure, molar instead of mass flow) because the last sentence of a long problem statement was skimmed.
FE problems embed the actual question at the end of a paragraph of process description, and under time pressure candidates begin calculating from the first number they see. Distractors are built directly from these swaps, so the wrong answer feels confirmed.
What to do instead
Before computing, underline the requested quantity and its units in the stem, then restate it in one short phrase on scratch paper (e.g., 'find absolute pressure at 2, kPa'). Convert gauge to absolute explicitly by adding the stated atmospheric pressure, and after obtaining a result, re-read the stem once to confirm the quantity and unit match what was asked.