Residence Time Distribution Questions and Answers

Answer: A

Correct answer: True

The tracer must record the flow without disturbing it or being disturbed by it. If it reacts, some is lost and the curve understates the long-residence material; if it adsorbs on a packing or a wall, it is released later and produces a spurious tail indistinguishable from dead volume. It must also be detectable at low concentration and not alter the density or viscosity of the fluid.

Answer: B

The expected mean is ten minutes and only eight is observed, so about a fifth of the volume is not participating. This is the standard way a dead zone is quantified from a tracer test. Twenty per cent is a substantial fault.

Answer: C

Nothing can be interpreted until the response is normalised, and comparison with an ideal model is the last step. Choosing the tracer badly invalidates everything that follows. Normalisation must precede interpretation.

Answer: D

A large number indicates near plug flow and a value near one indicates a well mixed vessel. It is a one parameter model fitted to the measured distribution. A large value approaches plug flow. One tank means a well mixed vessel.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: \( N \) is a fitted parameter obtained from the variance of the curve and need not be an integer, and a large value means a narrow distribution and behaviour approaching plug flow, with \( N=1 \) being a single stirred tank. Statement 3 confuses the model with the plant - a single vessel may fit as five tanks, and five vessels in series may fit as three.

Answer: B

Both are true and the reason gives its meaning, and a fractional value is perfectly acceptable since the model is a description rather than a physical picture. Variance is what the fit reproduces. Fractional values are perfectly acceptable.

Answer: C

They do for a first order reaction, but for other orders the degree of micromixing matters as well. The distribution fixes the range of possible conversions rather than a single value. Micromixing supplies the missing information.