# Practice questions

Several practice questions have been covered in class time also, usually on Fridays. Please review your notes for extra problems.

Question 1

A sample of material was settled in a graduated lab cylinder 300mm tall. The interface dropped from 500mL to 215mL on the graduations during a 4 minute period.

- Give a preliminary estimate of the clarifier diameter required to treat a waste stream of 2100 L per minute. Over-design by a factor of 2, based on the settling rate, and account for about 7 \(\text{m}^2\) of entry area used to eliminate turbulence in the entering stream.
- If the feed concentration is 3.5 kg per \(\text{m}^3\) feed, what is the loading rate? Is it within the typical thickener range of 50 to 120 kg per day per square meter (Perry, 8ed, p22-79)?

*Solution*

Settling rate = 171 mm per 4 minutes = 42.8 mm/min.

Area = \(\displaystyle \frac{2.1~\text{m}^3.\text{min}^{-1}}{\left(\frac{1}{2}\right)\left(42.8 \times 10^{-3} \text{m}.\text{min}^{-1}\right)} = 98 + 7 \text{m}^2\)

\(\psi = C_0 v = 3.5 \displaystyle\frac{\text{kg}}{\text{m}^3} \cdot 0.022 \displaystyle \frac{\text{m}}{\text{min}} \cdot \displaystyle\frac{60 \times 24 \text{min}}{\text{day}} = 106 \displaystyle \frac{\text{kg}}{\text{day}.\text{m}^2}\)

Question 2

- Calculate the minimum area of a circular thickener to treat 720 \(\text{m}^3\) per hour of slurry containing 65micron particles of silica, whose density is about 2600 kg . \(\text{m}^{-3}\). The particles are suspended in water at a concentration of 0.650 kg . \(\text{m}^{-3}\). Use an over-design factor of 1.5 on the settling velocity. [Ans: \(v = 3.7/1.5~\text{mm.s}^{-1}\) and \(A=81.4~\text{m}^{2}\)]
- If it is desired to have an underflow density of 1560 kg solids per \(\text{m}^{3}\) underflow; what is the underflow volumetric flow rate if total separation of solids occurs? [Ans: \(Q_\text{under} = 0.3~\text{m}^3~\text{solids.hr}^{-1}\)]
- Calculate the separation factor. [Infinite]

Question 3

There are practice problems in the midterms from 2012 and 2013 on this website. The final exam from 2012 is also available. Use the "Search box" above to locate them. Also check out assignments (with full solutions) from 2012 and 2013.

Question 4

A pigment is being extracted from an aqueous feed stream to an organic solvent stream using a continuous counter-current liquid-liquid extractor. This device gives the equivalent of 3 theoretical equilibrium stages. The feed flow rate is 120 kg/min and has a pigment concentration of 0.18 kg/kg feed. The molecular weight of the pigment is 90 kg/kmole. The molecular weight of the organic solvent is 150 kg/kmol and enters the extractor at a flow rate of 55 kg/min. The equilibrium relationship is given by \(y = 45 x\) where \(x\) and \(y\) are the concentrations in the raffinate and the extract and respectively and are expressed in molar ratio. Calculate the concentration of the pigment in the extract and the raffinate. (Molar ratio = moles of solute/moles of solvent).

Question 5

50 litres of filtrate is collected in 30 minutes when a suspension is filtered through a membrane filter using a pressure drop of 50 kPa. How much filtrate will be collected if the pressure drop is doubled? Assume the membrane resistance is negligible and that the cake formed is incompressible.

Question 6

A protein is being adsorbed in a fixed bed 1.5 m long containing DEAE-cellulose. The breakthrough and exhaustion times are 8 hours and 10 hours respectively. What are the lengths of the equilibrium zone and adsorption zone at the point of breakthrough? What is the fraction of the bed capacity utilized?

Question 7

A particle size analysis was performed on the feed to a cyclone. The feed stream was 100 kg/hour, the overflow 40 kg/hour, and underflow was 60 kg/hour. The particle size results for each of these streams is shown in the table.

Screen mesh number | Feed mass fraction | Coarse mass fraction | Fines mass fraction |
---|---|---|---|

850 | 0 | 0 | 0 |

600 | 0.30 | 0.45 | 0.075 |

425 | 0.40 | 0.45 | 0.325 |

300 | 0.20 | 0.09 | 0.365 |

212 | 0.10 | 0.01 | 0.235 |

Total | 1.0 | 1.0 | 1.0 |

- Calculate the overall efficiency [Ans: 60%].
- Calculate the cut size [Ans: around 450 \(\mu \text{m}\)].

Question 8

There are many practice questions on drying in the Geankoplis textbook, and in Henly, Seader and Roper.