ENADE Engenharia Química common mistakes
ENADE Engenharia Química · Understand the traps and how to avoid them.
Reading a general-formation text and answering from personal opinion or prior belief instead of locating the claim and the evidence the passage actually presents, so the chosen option reflects what the candidate thinks is true rather than what the author argued.
General-formation items are often about controversial social, scientific, or environmental issues where candidates already hold strong views; under time pressure they pattern-match the option that sounds most agreeable and skip the step of mapping the argument's premises to the conclusion.
What to do instead
Before looking at the options, write a one-line summary of the author's thesis and one line on what evidence supports it. Then eliminate any option that is true in the world but not stated or implied in the text, and any option that overstates (always, never, proves) beyond the passage. Practise with two texts on opposite sides of the same issue and force yourself to summarise each faithfully.
Treating an ethics or professional-responsibility scenario as a legal-compliance question, choosing the option that avoids formal penalty rather than the one that satisfies the engineer's duty to public safety, transparency, and the affected community.
Candidates study codes of ethics as lists of prohibitions and then reduce every dilemma to 'what is forbidden'. Scenarios are usually built so that the legally safe answer still leaves a foreseeable risk unaddressed, which is precisely the point being tested.
What to do instead
Apply a fixed three-question routine to every scenario: who can be harmed and how severe is the harm; what does the engineer know that affected parties do not; what is the least-harm action that still discloses the risk to those who need it. Rank options by protection of the public first, then disclosure, then loyalty to employer. Drill with cases where the employer's instruction is legal but unsafe.
Setting up a mass or energy balance without first drawing and labelling the system boundary and choosing a basis, then mixing units or forgetting a stream, so the resulting equation set is dimensionally inconsistent or underdetermined.
Candidates jump straight to algebra because the balance looks familiar. Without an explicit boundary, recycle and purge streams get double-counted or omitted, and without a declared basis (1 h, 1 kg feed, 100 mol) the degrees-of-freedom check is impossible.
What to do instead
Adopt a rigid template: sketch, draw the boundary, label every stream with composition and units, state the basis, list unknowns, count independent equations, and only then solve. Always convert to a single unit system (mass or molar, never mixed) before writing equations. Finish every problem with a degrees-of-freedom count, and if it is not zero, stop and find the missing relation before computing.
Using a single overall heat-transfer coefficient or a single log-mean temperature difference on a problem with phase change, fouling, or a non-constant stream heat capacity, and ignoring whether the flow arrangement is countercurrent or co-current.
Textbook drills repeat the plain concentric-tube formula, so candidates apply it by reflex. Phase change creates a pinch point and variable capacity rates, and fouling adds series resistances that cannot be folded into one guessed coefficient.
What to do instead
Before selecting a formula, classify the duty: sensible-only or with phase change; constant or variable capacity rate; clean or fouled. For phase change, compute the temperature profile in segments and check for a pinch. For fouling, add resistances in series explicitly. Verify the chosen mean temperature difference is valid for the arrangement, and sanity-check the area against typical overall coefficients.
Reporting a reaction-engineering result from the rate law at feed conditions while ignoring conversion-dependent volume change, mixing effects, or the fact that the reactor is not isothermal, so the predicted conversion or reactor size is wrong by a large factor.
Candidates memorise design equations for ideal batch, CSTR and PFR but treat the rate constant and volumetric flow as constants. Gas-phase reactions with mole change and exothermic reactions with heat exchange are the standard traps, and the algebra looks identical so the error goes unnoticed.
What to do instead
For every reactor problem, state explicitly: phase, isothermal or not, constant density or not, ideal or not. If gas phase with mole change, write volumetric flow as a function of conversion before integrating. If non-isothermal, write the energy balance alongside the mole balance and solve them together. Cross-check the answer by computing conversion at both inlet and outlet conditions and confirming the trend makes physical sense.
Choosing a separation sequence or solvent from a shortcut heuristic without checking the actual phase behaviour, azeotropes, or relative volatility across the whole composition range, then reporting a design that cannot meet the specified purity.
Shortcut methods such as relative-volatility rules and McCabe-Thiele constructions are taught first and are easy to apply, so candidates stop there. Real mixtures bend the equilibrium curve, form azeotropes, or need an entrainer, and the shortcut silently assumes constant volatility.
What to do instead
Always plot or inspect the equilibrium data over the full composition range before selecting a method. Flag any azeotrope or tangent pinch explicitly and state how it will be broken (pressure swing, entrainer, membrane). Check minimum reflux and stage count against a rigorous stage-by-stage calculation, and verify that the chosen solvent is recoverable and does not create a new azeotrope with the product.
Tuning or analysing a control loop by reading the controller output alone, without identifying the process gain, dead time, and time constant, and without distinguishing open-loop from closed-loop behaviour.
Candidates learn PID formulas as recipes and apply aggressive settings that look responsive on a plot but destabilise a process with significant dead time. They also confuse a change in setpoint with a change in disturbance when interpreting a response curve.
What to do instead
Before proposing any tuning, estimate process gain, dominant time constant, and dead time from the response data, and note the dead-time-to-time-constant ratio. Choose tuning rules appropriate to that ratio and state the resulting stability margins. When reading a trend, first label the disturbance, the setpoint change, and the manipulated variable, and check whether the loop is open or closed before drawing conclusions.
Selecting a material or specifying a corrosion allowance from a single property such as strength or cost, without matching the alloy to the specific environment (chlorides, pH, temperature, dissolved oxygen) and the relevant corrosion mechanism.
Candidates reduce materials selection to a table lookup of yield strength and price. Localised attack such as pitting, crevice, and stress-corrosion cracking depends on chemistry and temperature far more than on bulk strength, so a strong alloy can fail quickly in the wrong service.
What to do instead
For each materials question, write the environment first: composition, temperature, pH, presence of chlorides or oxygen, stagnant or flowing. Then name the plausible mechanisms (uniform, galvanic, pitting, crevice, SCC, erosion) and eliminate alloys known to be susceptible to each. Justify the corrosion allowance with an expected corrosion rate for that environment, and state the inspection or monitoring plan that would catch localised attack.
Proposing a process or product design with no hazard analysis and no quantified environmental burden, listing only the desired performance and cost, and omitting what happens in upset, start-up, and shutdown conditions.
Design questions reward creativity and candidates focus on the flowsheet that works at steady state. Inherent safety, relief requirements, and life-cycle emissions are treated as afterthoughts rather than design constraints, so the answer is technically feasible but professionally incomplete.
What to do instead
Structure every design answer with four explicit sections: performance, safety, environment, and operability. In the safety section, identify the worst credible event, the inventory of hazardous material, and the inherent-safety hierarchy (minimise, substitute, moderate, simplify). In the environment section, name the significant emissions or effluents and one metric (mass per unit product, energy per unit product). Add start-up, shutdown, and upset cases before concluding.
Treating a biochemical or alternative-energy route as automatically superior, quoting yield or emissions figures without stating the basis, the feedstock, the energy input, and whether the comparison is cradle-to-gate or full life cycle.
These topics attract strong prior enthusiasm, and candidates repeat headline claims without checking the functional unit. Comparing a biofuel with a fossil fuel on combustion emissions alone, or an enzymatic route with a chemical route without the downstream separation burden, gives a misleading answer.
What to do instead
State the functional unit and system boundary before any comparison, then list feedstock, energy and water inputs, co-products, and waste streams for each route. Include the separation and purification steps, which often dominate the biochemical process. Report yields per unit feedstock and per unit energy invested, and explicitly note where data are uncertain or missing rather than filling the gap with a favourable assumption.