AMIE Section B Chemical common mistakes
AMIE Section B Chemical · Understand the traps and how to avoid them.
Using gauge pressure where absolute pressure is required (or vice versa) in ideal-gas, Clausius-Clapeyron, or equilibrium-constant calculations, producing answers off by ~1 bar of error.
Plant instruments and problem statements habitually quote gauge readings, and candidates substitute the number directly without adding atmospheric pressure; conversely they subtract atmospheric pressure where it was already absolute.
What to do instead
Before substituting any pressure, write 'P_abs = P_gauge + P_atm' explicitly in the working and state the assumed P_atm (101.325 kPa). Convert every quoted pressure to absolute at the point of entry and keep a units/pressure-basis line at the top of each solution.
Applying a single overall heat-transfer coefficient or a single LMTD to a heat exchanger without checking whether the flow arrangement (counterflow, multipass, crossflow) requires an F-correction factor or area correction.
The counterflow LMTD formula is the one most practised, so candidates default to it even when the problem specifies a 1-2 shell-and-tube or crossflow unit.
What to do instead
Identify the flow configuration first and write it down; if it is not pure counterflow, compute F from the appropriate chart/correlation and use Q = U·A·F·LMTD. If F < ~0.75, flag that the configuration is thermally inefficient and reconsider.
Mixing mass and molar bases in material balances and stoichiometry - e.g., feeding kg/h into a mole-based conversion expression, or reporting selectivity on the wrong basis.
Reaction data (conversion, selectivity, yield) are conventionally molar while plant flows are usually mass; candidates switch between them mid-calculation without a consistent basis.
What to do instead
Choose a basis (e.g., 100 kmol/h or 100 kg/h) and state it; convert all streams to that basis before balancing. Track conversion on the limiting reactant in moles and check that the sum of component balances closes to zero.
Designing a reactor from the wrong ideal model - using CSTR design equation for a plug-flow tubular reactor (or vice versa) - and ignoring that the correct volume depends strongly on reaction order and conversion.
Both equations look similar in form and candidates memorise 'V = FA0·X/(-rA)' without the CSTR's rate evaluated at exit conditions versus the PFR's integral form.
What to do instead
Write the design equation from first principles for the stated reactor type: CSTR uses the rate at outlet composition; PFR/Batch uses the integral over conversion. For multiple reactions or non-first-order kinetics, sketch 1/(-rA) vs X and confirm which model gives the smaller/larger volume.
Omitting or misplacing the mechanical-design allowances in pressure-vessel shell thickness - forgetting corrosion allowance, or using the wrong joint efficiency or design-code formula.
Candidates focus on the hoop-stress formula and treat corrosion allowance and weld efficiency as afterthoughts, or apply the thin-wall formula where the code specifies a different expression.
What to do instead
Use the code form t = P·D/(2·S·E − 1.2P) + CA (or the applicable ASME/IS clause), state each term's source (S from material table, E from radiography class, CA from service), and check t/D < 0.5 for thin-wall validity. Always add corrosion allowance last and round up to standard plate.
Skipping the economics step - presenting a design without capital cost, operating cost, or a profitability criterion, or comparing alternatives on equipment cost alone.
Candidates treat cost as outside the technical scope, or use a single purchased-cost figure without installation factor, depreciation, or time value of money.
What to do instead
For every design comparison, apply an installation/lang factor to purchased cost, annualise capital via depreciation or capital-recovery factor, and report a decision metric (payback, NPV, or equivalent annual cost). State the basis year and cost index.
Ignoring the controlling resistance in mass transfer - assuming gas-film or liquid-film control without computing the relative film resistances, and applying the wrong driving force (mole fraction vs partial pressure vs concentration).
Textbook problems often pre-specify the controlling phase; candidates carry that habit into problems where both resistances matter or where the equilibrium relation must be linearised.
What to do instead
Compute the gas-film and liquid-film resistances (1/kG and m/kL in consistent units), identify the larger, and justify ignoring the smaller. Convert all driving forces to one basis using Henry's law before combining resistances.
Treating a corrosion or materials-selection question as a single-property lookup (e.g., 'use stainless steel') without matching the alloy to the specific environment, temperature, and failure mode (pitting, SCC, galvanic, erosion-corrosion).
Candidates recall that stainless resists corrosion generally and stop there, missing that chloride service causes pitting/SCC and that dissimilar-metal joints create galvanic cells.
What to do instead
For each materials answer, name the environment (chlorides, acids, temperature, flow), the dominant corrosion mechanism, and why the chosen alloy resists it. Flag incompatible couples and specify isolation or compatible pairing.
In pollution-control and safety problems, calculating a single emission or hazard number without converting to the regulatory or consequence basis actually asked for (e.g., concentration at stack exit vs ground-level concentration, or LEL percentage vs ppm).
Candidates stop at the intermediate quantity they can compute and do not carry it through the dispersion/regulatory model the question requires.
What to do instead
Read what the question actually asks for (ground-level concentration, stack height, %LEL, TLV comparison) and work backwards to the required model (Gaussian dispersion, dilution, mass balance). Report in the requested units and compare against the standard to state compliance.
In polymer and biochemical problems, using a monomer- or cell-mass basis interchangeably with the polymer/product basis, or ignoring that conversion, molecular weight, and yield are defined differently in each field.
Carry-over of standard chemical-engineering conversion definitions into chain-growth polymerisation (where number-average vs weight-average matter) and fermentation (where yield is g product/g substrate, not molar).
What to do instead
State the definition being used before calculating: for polymers distinguish Mn, Mw and polydispersity and use the correct extent-of-reaction relation; for bioprocesses use mass yields (Yx/s, Yp/s) and check carbon/degree-of-polymerisation balances explicitly.