UPDA Chemical common mistakes
UPDA Chemical · Understand the traps and how to avoid them.
Using gauge pressure where absolute pressure is required (or vice versa) when applying the ideal gas law, Bernoulli, or Antoine/Clausius-Clapeyron relations, producing answers off by ~1 atm.
Problem statements often give a gauge reading from a plant instrument while the correlation demands absolute pressure; candidates plug the number in directly without converting.
What to do instead
Before substituting, write a one-line unit/pressure audit: state whether each pressure is gauge or absolute, convert gauge to absolute by adding the local atmospheric pressure (typically 101.325 kPa), and carry units through the substitution.
Performing a material balance without first drawing and labeling the process flow diagram and choosing a basis, leading to an inconsistent number of equations and unknowns.
Candidates jump straight to algebra on the numbers in the text, lose track of recycle/purge streams, and double-count or omit a stream.
What to do instead
Always sketch the flowsheet, number every stream, list knowns/unknowns per unit, pick a convenient basis (e.g., 100 mol feed or 1 h), and only then write balances unit by unit.
Mixing mass and molar units inside a single balance or rate expression (e.g., kmol/h feed with kg/h product, or mol/L in a rate constant meant for kmol/m3).
Data tables in exam items deliberately mix units, and candidates convert only the obvious quantity, not every term in the equation.
What to do instead
Convert all quantities to one consistent set (SI: kmol, m3, s, K, Pa) before any calculation, and check dimensional consistency of the final expression term by term.
Selecting the wrong log-mean versus arithmetic mean temperature difference, or misapplying the correction factor F for multipass/crossflow exchangers.
Candidates memorize Q = UAΔTlm without checking whether flow is countercurrent, and forget that F must be looked up from the P-R chart or computed and that F = 1 only for pure countercurrent.
What to do instead
Identify the flow arrangement first; use ΔTlm only for countercurrent/co-current, multiply by F (from P, R) for multipass/crossflow, and verify F > 0.75 or reconsider the design.
Applying the design equation for the wrong reactor type (e.g., using the CSTR equation for a plug-flow system, or ignoring that conversion in a CSTR is evaluated at exit conditions).
The algebra looks similar across reactor types, and candidates pattern-match to the formula they remember rather than deriving from the mole balance for the stated reactor.
What to do instead
Start from the general mole balance, state the reactor type and assumptions (steady state, constant density or not), then reduce to the specific design equation before substituting kinetics.
Using the Reynolds number or friction factor with inconsistent diameter/velocity definitions, or applying the Moody chart without confirming laminar versus turbulent regime.
Candidates compute Re but then use the turbulent Colebrook/Churchill relation for Re < 2100, or use radius instead of diameter in Re and in the Darcy-Weisbach equation.
What to do instead
Compute Re first and explicitly branch: laminar f = 64/Re, turbulent use Colebrook/Churchill; confirm D is the inside pipe diameter and velocity is the mean velocity, and state the regime in the answer.
Treating a controller tuning or loop-stability question qualitatively (e.g., 'increase gain to reduce offset') without checking phase margin, ultimate gain, or the effect on stability.
Candidates recall that higher proportional gain reduces offset and stop there, ignoring that it also reduces stability margin and can cause oscillation.
What to do instead
For every tuning change, state both effects: offset/steady-state error and stability/phase margin; where applicable compute or bound the ultimate gain and describe the trade-off explicitly.
Computing an economic result (payback, NPV, equivalent annual cost) without consistent time basis, discount rate application, or treatment of depreciation and tax.
Candidates mix annual and total costs, apply the discount factor to the wrong year, or omit working capital and salvage value because the problem statement buries them.
What to do instead
Lay out a year-by-year cash flow table, state the discount convention (end-of-year), separate capital from operating cost, and handle depreciation/tax and salvage explicitly before computing NPV or payback.
Answering a safety or ethics item with the 'technically correct' process answer while ignoring the regulatory or professional-ethics obligation (e.g., proceeding with a relief valve sized to average rather than worst-case scenario, or not escalating a known hazard).
Candidates treat HSE/ethics items as soft add-ons and default to the calculation, missing that the exam expects recognition of codes, worst-case design basis, and duty to report.
What to do instead
For HSE/ethics items, first identify the governing code or professional obligation, size/design to the worst credible case (not average), and state the escalation/reporting action explicitly in the answer.
Skipping the sanity check on magnitude and sign of the final answer (e.g., negative absolute temperature, efficiency > 100%, conversion > 1, or a heat duty with the wrong sign).
Time pressure leads candidates to submit the first number obtained without verifying it against physical bounds and the problem's qualitative expectation.
What to do instead
Reserve the last minute to check: units, sign convention, physical bounds (0 ≤ X ≤ 1, 0 < η < 1, T > 0 K), and whether the magnitude is plausible relative to the given data.