Chemical engineering assignment help starts with one habit: draw the process, then balance it. Almost every problem, from a mixer to a full plant, rests on conservation of mass and energy.
This guide covers the core topics in chemical engineering coursework, a step-by-step routine for balance problems, worked reactor sizing examples and the places where marks usually slip. It closes with how STEM Donkey can prepare a custom solution for study and reference.
The Routine Behind Most Chemical Engineering Problems
Draw and label a flow diagram, choose a basis, list the unknowns and the independent equations, write the mass balances, add the energy balance if heat or work is involved, solve, then check units and sanity.
Most lost marks come from skipping the diagram, mixing mass and mole units, or never stating assumptions such as steady state.
Chemical Engineering Topics We Cover
Coursework spans the whole degree. Each area has its own tools, but they share the same discipline of balances, units and assumptions.
| Area | Typical assignment tasks |
|---|---|
| Material and energy balances | Mixers, splitters, recycle and purge streams, combustion, humidity |
| Thermodynamics | Equations of state, phase equilibrium, Raoult's law, fugacity |
| Fluid mechanics | Pipe flow, pressure drop, pump sizing, Bernoulli with losses |
| Heat and mass transfer | Heat exchanger design, LMTD and NTU methods, diffusion, absorption |
| Reaction engineering | Rate laws, Arrhenius analysis, batch, CSTR and PFR sizing |
| Separation processes | Distillation with McCabe-Thiele, flash calculations, extraction |
| Process control | Transfer functions, PID tuning, stability |
| Process design | Flowsheets, equipment sizing, costing, safety and HAZOP |
Mass and Energy Balances, Step by Step
The general balance is accumulation = in - out + generation - consumption. At steady state with no reaction, it reduces to in = out for every component and for total mass.
- Draw the flowsheet and label every stream with flow rate and composition.
- Choose a basis, such as 100 kg/h of feed or 100 mol.
- Do a degrees-of-freedom count: unknowns minus independent equations should be zero.
- Write component balances, starting where only one unknown appears.
- Solve, then scale to the real flow rate if you used a convenient basis.
Worked example: mixing two streams. Stream 1 is 100 kg/h of 20 wt% ethanol in water. Stream 2 is 50 kg/h of 50 wt% ethanol. Total out = 150 kg/h. Ethanol out = 0.20 × 100 + 0.50 × 50 = 20 + 25 = 45 kg/h, so the product is 45 / 150 = 0.30, or 30 wt% ethanol.
For energy, the steady-flow balance with negligible kinetic and potential energy is Q - Ws = ΔH. For sensible heating with constant heat capacity, Q = m cp ΔT. Heating 2.0 kg/s of water (cp = 4.18 kJ/(kg·K)) by 30 K needs 2.0 × 4.18 × 30 = 251 kW.
Thermodynamics and Phase Equilibrium
Thermodynamics questions test whether you pick the right model for the conditions. The ideal gas law, pV = nRT with R = 8.314 J/(mol·K), works well at low pressure but not near condensation.
- Raoult's law: yip = xipisat, for ideal liquid mixtures of similar molecules.
- Modified Raoult's law: adds an activity coefficient γi for non-ideal liquids.
- Cubic equations of state: Peng-Robinson or Soave-Redlich-Kwong for high-pressure gases and hydrocarbons.
- Antoine equation: estimates vapor pressure from temperature; check the units your constants assume.
State why you chose each model. A marker will accept a simplifying assumption if you justify it and comment on its effect.
Reactor Design: Batch, CSTR and PFR
Reactor sizing links a rate law to a design equation. Conversion X is the fraction of the limiting reactant that has reacted.
| Reactor | Design equation (in terms of conversion) | First-order, constant density |
|---|---|---|
| Batch | t = NA0 ∫ dX / (-rAV) | t = (1/k) ln[1/(1 - X)] |
| CSTR | V = FA0X / (-rA) | V = v0X / [k(1 - X)] |
| PFR | V = FA0 ∫ dX / (-rA) | V = (v0/k) ln[1/(1 - X)] |
Worked example: CSTR versus PFR. A liquid-phase first-order reaction has k = 0.20 min-1. The feed is v0 = 10 L/min and the target is X = 0.80.
CSTR: V = 10 × 0.80 / (0.20 × 0.20) = 8 / 0.04 = 200 L. PFR: V = (10 / 0.20) × ln(1 / 0.20) = 50 × 1.609 = 80.5 L. For a positive-order reaction the PFR needs less volume, because the CSTR runs entirely at the low outlet concentration.
Temperature Dependence
The Arrhenius equation, k = A exp(-Ea/RT), links rate to temperature. Plot ln k against 1/T: the slope is -Ea/R, so multiply the slope by -8.314 J/(mol·K) to get the activation energy. Use absolute temperature in kelvin.
Heat Exchangers and Separations
Heat transfer and separation problems appear in almost every second-year module. Both reward a clear diagram and careful bookkeeping of temperatures or compositions.
Sizing a Heat Exchanger with LMTD
The basic design equation is Q = U A ΔTlm, where the log mean temperature difference is ΔTlm = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2). Take ΔT1 and ΔT2 at the two ends of the exchanger.
Worked example: counter-current exchanger. Hot oil cools from 150 °C to 90 °C; water heats from 30 °C to 70 °C. In counter-current flow, ΔT1 = 150 - 70 = 80 K and ΔT2 = 90 - 30 = 60 K.
ΔTlm = (80 - 60) / ln(80/60) = 20 / 0.2877 = 69.5 K. For a duty of Q = 200 kW and U = 500 W/(m²·K), the area is A = 200,000 / (500 × 69.5) = 5.75 m².
If the exchanger is multi-pass, apply a correction factor F to the LMTD. When outlet temperatures are unknown, the effectiveness-NTU method is usually quicker.
Distillation and Flash Calculations
McCabe-Thiele questions need the equilibrium curve, the operating lines and the q-line drawn accurately. Step off stages from the top, and state whether the reboiler counts as a stage.
For flash calculations, solve the Rachford-Rice equation for the vapor fraction, then find each component's liquid and vapor composition. Check that both sets of mole fractions sum to 1.
Process Design Reports
Design projects add judgment to calculation. Markers look for a justified flowsheet, sized equipment, a cost estimate and a safety review, all tied to stated design criteria.
- Start from the design basis: capacity, product purity, feed composition and operating hours per year.
- Explain each unit choice and the alternatives you rejected.
- Show sizing calculations in an appendix, with key results in the main text.
- Include environmental, safety and HAZOP considerations, not as an afterthought.
Use simulation output from Aspen Plus, HYSYS or DWSIM as support for your reasoning, not a substitute for it. Hand checks on key balances show you understand the model.
Common Chemical Engineering Assignment Challenges
| Problem | How to avoid it |
|---|---|
| Mixing mass and mole fractions | Convert everything to one basis before balancing |
| Wrong degrees of freedom | Count unknowns and independent equations before solving |
| Celsius in Arrhenius or gas laws | Always use kelvin for absolute temperature |
| Recycle loops solved by guesswork | Balance around the overall process first, then the mixing point |
| Unstated assumptions | List steady state, ideal behavior or negligible heat loss explicitly |
A quick sanity check catches many errors. If your outlet temperature is below both inlet temperatures in an adiabatic mixer, something is wrong.
How STEM Donkey Helps with Chemical Engineering Assignments
Send the problem sheet or design brief, data tables your course uses, any required software and your marking rubric. Mention the textbook if your lecturer follows a particular notation.
You receive a custom solution written from scratch, with labeled flow diagrams, balances, design equations, units on every quantity and stated assumptions. It is for study and reference, so you can follow the method and apply it yourself. Free revisions within the scope of your original request are included.
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Send your problem sheet or design brief. You get a custom solution with clear diagrams, balances, units and stated assumptions.
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Frequently Asked Questions
Material and energy balances, thermodynamics, fluid flow, heat and mass transfer, reaction engineering, separations, process control and design reports, with full working shown.
It tells you whether a problem can be solved before you start. If unknowns exceed independent equations, you need more data or an assumption.
A CSTR is easier to control for temperature and suits liquid reactions needing good mixing. For simple positive-order kinetics, a PFR reaches the same conversion in a smaller volume.
Yes. Share your simulation files or outputs and the brief. The solution interprets the results and adds hand checks on key balances.
Yes. Units are carried through each calculation, because unit errors are one of the most common reasons chemical engineering answers lose marks.
That is exactly what it is for. Compare your balances and assumptions step by step and see where your method differs.
Deadlines run from 3 hours to 20 days. Full design reports with costing and HAZOP need more time than a single problem set.