2. These differences can be used to effect separations by the enrichment of one phase relative to the other, by differential transfer of mass of particular components across the phase boundary.
3. Rates of mass transfer across the phase boundaries are controlled by the differences between actual concentrations and equilibrium concentrations, which constitute the mass transfer driving force, and by resistances which impede transfer. Therefore the rate of mass transfer is very generally determined by a driving force and by a mass-transfer coefficient.
4. Analysis of mass transfer contact equilibrium systems is carried out by comparing the equilibrium conditions to the actual conditions in the system; and using the difference, together with material conservation relationships that describe the movements within and between the phases, to follow the transfer of mass.
5. The analysis can be carried out systematically, relating equilibrium conditions and material balance (or operating) conditions, and energy balances, over single and multistage systems.
1. The composition of air is 23 % oxygen, 77% nitrogen by weight, and
the Henry's Law constant for oxygen in water is 3.64 x 104
atm mole fraction-1 at 20°C. Calculate the solubility of
oxygen in water (a) as the mole fraction and (b) as a percentage by weight.
If in the deodorizer of worked Example
9.5, relative flow rates of cream and steam are altered to 1:1, what
will be the final concentration of the taint in the cream coming from
a plant with three contact stages?
Casein is to be washed, in a multistage system, by water. The casein curd
has initially a water content of 60% and between stages it is drained
on an inclined screen to 80% water (both on a wet basis). The initial
lactose content of the casein is 4.5 % on a wet basis , and it is necessary
to produce casein with a lactose content of less than 1 % on a dry basis.
How many washing steps would be needed if the wet casein is washed with
twice its own weight of fresh water in each step?
Estimate the osmotic pressure of a solution of sucrose in water containing
20% by weight of sucrose. The density of this solution is 1081 kg m-3,
and the temperature is 20°C
In a six-step sugar-boiling crystallization process, the proportions of
the sucrose present removed in the successive crystallizations are 66.7%,
60%, 60%, 50%, 50% and 33%. If the original sugar was associazted with
0.3% of its weight of non-sucrose solids, calculate (a0 the percentage
of non-sucrose material in the final molasses and (b) the proportion of
the original sugar that remains in the molasses. Assume that after each
crystallization, all of the impurities remain with the mother liquor.
6. For a particular ultrafiltration plant concentrating skim milk, for a concentration ratio of 7:1 of protein relative to lactose, the plant capacity is 570 kg m-2 h-1 of skim milk. Assume that this is the flow rate through the membrane. Estimate (a) the plant capacity at a concentration ratio of 2:1 and (b) the percentage of the water in the skim milk removed by the ultrafiltration.
[ (a) 1600 kgh-1 ; (b) 50% ]
CHAPTER 10: MECHANICAL SEPARATIONS
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