MB

Question 1

Two film theory aassumptions

Answer 1

• For equimolar counter diffusion (EMCD) there is no turbulence at the interface, and a laminar layer exists in two films. (In reality, the conc gradient is linear close to the surface and become less so as you move away from it).
• Resistance to mass transfer lies in 2 layers, one either side of the interface.
• Conc gradient inside each layer is linear, and outside it is zero.
• Equilib exists at the interface and therefore the relative positions of C and D are determined by the equilibrium between the two phases.
• Mass transfer is at steady state.

Question 2

Penetration theort

Answer 2

Assumes that eddies bring an element of the fluid to the interface, the fluid is exposed to the second phase for a set time interval, after which the surface element is then remixed with the bulk. Fluid of an initial composition equal to that of the bulk is therefore suddenly exposed to the second phase. Assumes that the depth of the liquid element is infinite.

Question 3

Higbie surface renewal theory

Answer 3

All the elements last the same time - te

Question 4

Danckwert’s model

Answer 4

Rather than each element of the surface being exposed for the same time, the elements would have a random distribution of ages. Assumed the probability of any surface becoming destroyed and mixed with the bulk of the fluid was independent of the age of the fluid. The age dist can be calculated by assuming the rate of production of each fresh surface/unit total surface area are = s (reciprocal of the mean residence time)

Question 5

Continuous counter-current contacting, what do large and small values of H mean?

Answer 5

If H is very large, the component being transferred is relatively insoluble in the liquid, then the term H/kL will be much larger than 1/kG so KG is approx kL/H and KL is approx kL.

If H is very low, therefore a high solubility in the liquid, KG approx HkG and KG approx kG. This is a gas phase controlled mass transfer process, the mass transfer rate is almost proportional to kG but independent of kL.

Question 6

Tower cross sectional area and tower height importance

Answer 6

(ii) Tower cross-sectional area, Good liquid/gas contacting only occurs in a narrow range of flows. This determines the tower diameter
(iii) Tower height, even if the flows are good. The solute must have enough time to diffuse from the gas to the liquid- this time depends upon the height of the tower.

Question 7

Main problems flow through packing

Answer 7

1) ‘channelling’ – occurs due to uneven flow distribution, leads to poor mass transfer.
2) ‘loading’
3) ‘flooding’.
However, at a high gas flow, the liquid flow will decrease- this is called ‘loading’. At a very high gas flow, the liquid stops flowing altogether and collects in the top of the column – this condition is called ‘flooding’. Flooding dramatically reduces mass transfer. It can be avoided by reducing the liquid and gas fluxes. The flux (flow/unit area) can be reduced at design stage by increasing the tower C.S.A. this area can usually be found by the use of an empirical correlation.

Question 8

Fouling categories

Answer 8

• Crystallisation fouling: Some salts display the phenomenon of inverse solubility, this causes precipitation to occur on to hit surfaces. Inverse solubility is the term given to the property some compounds exhibit of becoming less soluble with increasing temperatures
• Particulate fouling: Surface accumulation of fine particles originally suspended in the process fluid
• Chemical reaction fouling: Fouling deposits are formed by a chemical reaction, either in the bulk fluid or at the hot surface
• Corrosion fouling: The surface may react to produce corrosion products, which will both foul the surface or lead to particulate fouling downstream
• Biological fouling: Macro and/or microorganisms attaching themselves to a surface

Question 9

Main measures of performance of a steam heated tubular evaporator

Answer 9

• Capacity: kg of water vaporised per hour
• Economy: kg of water vaporised per kg of steam feed
• Rate of heat transfer, Q=UADT

Question 10

Boiling point rises and hydrostatic head

Answer 10

Boiling point rise occurs because a solution has a boiling point higher than pure water. Thus, when a solution is concentrated, the boiling point will not be the same as pure water, and will change as the solution becomes more concentrated

Hydrostatic head effects occur in vertical tube evaporators. The liquor at the bottom of the vertical tube is under a greater pressure than the liquid at the top, this will affect the boiling point of the liquid.

Question 11

Feedforward and feedbackward feeding

Answer 11

The usual method of feeding a multiple effect evaporator is to pump the thin liquid feed into the first effect and sent it in turn to the other effects. This is forward feed. The slurry concentration increases from the first effect to the last.

Another common method is backward feed. The dilute slurry is fed to the last effect and pumped through successive effects to the first. Backward feed often gives a higher capacity and a lower economy than forward feed.