Separations

Separations

different separations
1. by phase creation
2. by phase addition
3. by solid agent
4. by force field or gradient
5. energy addition
  1. distillation
6. mass addition
  1. liquid-liquid extraction
phase rule
feed line
1. slope
  1. -q/(1-q)
    1. also equal to -L/V
2. bubble point
  1. q is infinite (vertical line)
  2. all feed descends (sat)
3. dew point
  1. q is zero (horizontal line)
  2. all feed ascends (sat)
4. partiallty vaporized (when sat)
  1. feed liquid descends
  2. feed vapour ascends
5. single phase
  1. subcooled liquid
    1. all of feed descends
  2. superheated vapour
    1. all of feed ascends
Raoult's Law
Antoine's Equation
McCabe-Thiele
1. assumptions
  1. constant molar overflow
    1. molar flow rates do not change from stage to stage in each of two sections
  2. pressure uniform throughout column
  3. heat loss is negligible
  4. components have equal and constant molar enthalpies of vap
2. operating lines
  1. rectifying
    1. slope
      1. R/(R+1)
        R is reflux ratio = L/D (reflux/distillate)
        reflux ratio: R/Rmin
  2. stripping
    1. slope
      1. (Vb+1)/Vb
        Vb boilup ratio= V(bar)/B (boilup/bottoms)
  3. both operating lines intersect at same point with q-line
    1. equilibrium stages can be drawn after
3. minimum stages Nmin
  1. R goes to infinite
  2. both operating lines coincide with 45 line
  3. 'shut in' - no feed, distillate or bottoms
    1. increasing relative volatility reduces staging
4. minimum reflux
  1. pinch point
    1. infinite stages
  2. feed line touches equilibrium line
  3. optimal reflux ratio
    1. mulitple of Rmin
      1. dictated by economic considerations
5. simple basis for design of binary distillation proces
6. two different cases
  1. two feeds
    1. two q lines and intermediate operating line
  2. feed plus side stream
    1. side stream treated as feed but negative flow
7. steam injection
  1. sat steam injected at bottom
    1. instead of reboiler
  2. (xb, 0) becomes a point on lower operating line
8. Efficiency
  1. the steam leaving a stage in a plate distillation is not in complete thermodynamic equilibrium
    1. the actual no of plates needed is greater than theoretical stages
      1. accounted for by overall efficiency factor
  2. overall column efficiency
    1. N/E0
  3. partial reboiler treated as full theoretical stage
    1. so the no of theoretical stages required in column is one less than determined in McCabe-Thiele construction
  4. O'Connell Method
    1. correlation between overall column efficiency average viscosity and relative volatility
9. Columns
  1. Rectifying Column with Reflux
    1. feed at bottom - superheated vapour
    2. reboils some descending liquid
  2. Stripping Column with Reboiler
    1. feed at top - subcooled liquid
    2. creates an internal reflux of raising vapour
  3. these two columns may be operated independently
    1. both one operating line on the McCabe-Thiele diagram
10. non-ideal systems
  1. including with azeotropes
  2. if assumption is not adequate
    1. material and energy balances carried out for each stage
    2. eg pressure drop in column and constant molar overflow
Condenser Type
1. total condenser
  1. low pressure
  2. many trays
2. partial condensation
  1. if vapour top product is desired
  2. avoids requirement of chilled condenser water
condenser and reboiler duty
1. energy balance for entire column
2. used to estimate operating costs
Batch Distillation
1. differential distillation
  1. Rayleigh equation
    1. S - amount of liquid residue in the still
2. preferred for smaller quantities, multi-use plant and periodic demand
3. McCabe-Thiele
  1. additional assumption of negligible hold up

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