FE Chemical Practice Question Solution included after investigation
CASE E-061 · FE

The Mixer That Needed 100 kg/h of Concentrate

A 10 wt% stream and a 40 wt% stream had to produce 25 wt% product. The concentrate flow was 100 kg/h, not 25.

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Case E-061 engineering failure visual
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Follow the investigation process used in the field — in five guided steps.

1

Inspect

Review the problem, diagram, and evidence.

2

Commit

Choose your hypothesis.

3

Prove

Run calculations and test your idea.

4

Fix

Select and validate a safe correction.

5

Sign Off

See the full debrief and key takeaways.

Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A steady mixer receives 100 kg/h of a solution containing 10 wt% solute. A second stream contains 40 wt% solute. The product must contain 25 wt% solute. A quick note says the second stream can be 25 kg/h because the target composition is 25%. Determine the required second-stream flow rate and product flow.

Material Energy Balances Chemical Design Verification Root Cause Analysis Safe Correction
E-061
INVESTIGATION WORKSPACE WORK THROUGH THE CASE — YOUR CHOICES MATTER.
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

Target mass fraction was mistaken for a stream flow.

What was assumed

Feed compositions could be averaged without flow weighting.

Corrected model

Use total and component material balances.

QUICK ANSWER

Let F2 be the 40 wt% stream. The solute balance is 0.10(100)+0.40F2=0.25(100+F2). Solving gives F2=100 kg/h, so the product flow is 200 kg/h.

ENGINEERING TAKEAWAY

Write both the total mass balance and the solute component balance; composition is flow-weighted.

CASE QUESTIONS

Questions this case should settle.

Why is the required 40 wt% stream 100 kg/h?

Because the solute component balance requires equal feed flow rates to produce the 25 wt% midpoint composition.

Can stream compositions be averaged directly?

Only when the contributing flow rates are equal.

NEXT FE PRACTICE QUESTION

The Heater That Needed 418 kW, Not 209

Material Energy Balances · 5–10 min

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WHAT YOU’LL LEARN

Real failures. Lasting skills.

Each case is designed to build the judgment, analysis, and confidence you need for engineering exams — and beyond.

  • Apply core engineering concepts to real-world problems
  • Practice structured troubleshooting and analysis
  • Strengthen exam-ready thinking through realistic scenarios
COMMON QUESTIONS
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The sealed debrief unlocks with the root cause, corrected reasoning, fix, and takeaway.