Cambridge Chemical Engineering Interview Questions and Mock Interviews

A plant turns reactant A into product B in a continuous loop. Each mole of A that reacts gives one mole of B. The fresh feed is 100 mol per hour, made up of 96 mol per hour of A and 4 mol per hour of an inert gas I that does not react. In the reactor, 40% of the A that enters is converted to B on each pass. The reactor outlet goes to a separator that removes all of the B as product. Everything else, the unreacted A and all of the I, forms a recycle stream. A fraction of this recycle stream is purged (sent away as waste, with the same composition as the recycle) and the rest is mixed with the fresh feed to make the reactor feed. The purge is set so that the reactor feed is always 20% I by moles, and the plant runs at steady state. (a) Explain what would happen to the amount of I in the loop if there were no purge. (b) Find the molar flow rate of the reactor feed, the flow rate of B produced, and the flow rate of the purge. (c) What percentage of the fresh A is lost in the purge, and what is the compromise in choosing a lower or a higher inert fraction in the reactor feed?

In a batch reactor, a reactant A is converted into a wanted product B, which then slowly breaks down into a waste product C. Both reactions are first order, with rate constant 0.30 per minute for A turning into B and 0.10 per minute for B turning into C. At time zero only A is present, at a starting concentration written A0 below. The reaction scheme and the resulting concentration of B at time t (in minutes) are given below. (a) Sketch how the concentrations of A, B and C change with time, and explain why the concentration of B rises and then falls. (b) At the moment when B is at its highest concentration, what must be true about the rate at which B forms and the rate at which it breaks down? (c) Check that the formula for B gives sensible values at t = 0 and for very long times, then find the time at which B is highest and the fraction of A0 that is present as B at that moment. (d) Why might a plant stop the reaction near that time rather than wait until all of the A has been used?

A gas-phase reaction X + 2Y to Z is reversible, and the forward reaction releases heat. Z is the product a plant wants as much of as possible each day, and all three substances are gases. (a) Discuss what raising the temperature does to the proportion of reactants converted at equilibrium and to the speed at which equilibrium is approached. (b) Discuss the same for raising the pressure. (c) Why might the plant choose a temperature that is not the one giving the highest equilibrium conversion? (d) What does a catalyst change, and what does it leave unchanged?

A reaction that releases heat is run safely in a spherical glass flask of radius 5.0 cm. The flask is kept at a steady temperature by standing it in a water bath. A company wants to scale the process up to a spherical vessel of radius 50 cm, with the same liquid, the same reaction rate in each cubic centimetre of liquid, and the same temperature difference across the wall. (a) By what factors do the volume and the wall area change? (b) What does that do to the heat produced each second compared with the heat that can be removed each second through the wall? (c) Suggest two ways in which the design of the large vessel could cope.

An enzyme is used to break down a sugar in a bioreactor. In experiment 1, the temperature is held fixed and the starting rate of reaction is measured with different amounts of sugar in the liquid: the rate rises steeply at first, then flattens off to a maximum value. In experiment 2, a large amount of sugar is used and the starting rate is measured at different temperatures: the rate rises, reaches a peak, then falls sharply. (a) Explain the shape of each graph in terms of what the enzyme molecules and the sugar molecules are doing. (b) A colleague runs the bioreactor at a temperature a little above the peak of the experiment 2 curve, saying that the rate is still high. Why might this be acceptable for a test lasting one minute but a poor choice for a week-long production run?

Two gas cylinders at the same temperature and pressure are joined by a pipe with a closed valve. One holds nitrogen and the other holds oxygen. The valve is opened. The gases mix completely, no heat flows in or out, and the temperature does not change (treat both gases as ideal). (a) Explain why the mixing happens by itself, but you would never expect to see the gases separate again by themselves. Try to argue in terms of probability and numbers of arrangements. (b) No energy was needed to mix them. Does it follow that no energy is needed to separate them? What does your answer suggest about obtaining pure oxygen from air? (c) Per mole of a minor component, would you expect the least possible energy for separating it out to be larger when it makes up half of the mixture or when it makes up one part in a thousand? Give a reason without calculation.

A key intermediate for a medicine is made in five reactions in a row. Each reaction turns 1 mole of its starting material into at most 1 mole of product, and each one actually gives 85% of that maximum. The product of one reaction is the starting material of the next. (a) What is the overall yield, and how many moles of the very first material are needed to make 100 moles of the final product? (b) Chemists can raise the yield of just one of the five reactions from 85% to 95%. Does the overall yield depend on which one they choose? Which would you improve first, thinking about cost, waste and safety?

Think of a product you have read about being made on a large scale, such as a fertiliser, a fuel, a medicine, a food ingredient or a plastic. Explain in plain language what goes in and what comes out, which part of the process you think is hardest to engineer, and why.

Think about an experiment, investigation or project you have carried out in science or engineering. Which single measurement contributed most to the uncertainty in your final result, and how would you change the method to reduce it? What new problems might appear if you had to repeat it at ten times the scale?

Think of a claim you have read about the environmental benefit of a technology, such as using hydrogen as a fuel, recycling a type of plastic or capturing carbon dioxide from a chimney. How would you test whether the claim holds? Say what you would measure, where you would draw the boundary around the system you are studying, and what could make the claim look better or worse than it really is.

A small car travelling at a steady speed uses 6.0 litres of petrol per 100 km. Treat petrol as pure octane, C8H18, with a density of 0.75 kg per litre. Estimate the mass of carbon dioxide that leaves the exhaust for every kilometre travelled. Is that mass larger or smaller than the mass of petrol burned, and how can that be? (Relative atomic masses: H 1, C 12, O 16.)

A porous spherical pellet of radius R soaks up a dye by diffusion. The diffusion coefficient D of the dye in the pellet has units of metres squared per second, and the soaking time t is assumed to depend only on R and D. Suppose you know no theory of diffusion. Using only the dimensions of these quantities, work out how t must depend on R and D, apart from a pure number. A pellet of radius 2.0 mm takes 40 seconds to soak through. Estimate the time for a pellet of radius 6.0 mm of the same material under the same conditions, and say why someone designing a catalyst might care.

A shallow circular dish, 10 cm across, holds water to a depth of 1.0 cm. It stands on a table in a still room. Estimate how long it takes for all of the water to evaporate, giving your answer in days. Make these simplifying assumptions: the water stays at 20 °C, and the vapour moves away only by diffusing upwards through a still layer of air 3.0 cm thick. The air touching the water is saturated with vapour, and the air at the top of the layer holds none. Use these values: air saturated with water vapour at 20 °C holds about 17 g of vapour per m³, and water vapour diffuses through air with a diffusion coefficient of about 0.25 cm² per second. Afterwards, say which of your assumptions would make the real evaporation faster or slower in an ordinary room.