Fractional Distillation: McCabe-Thiele Method Flashcards
A series of flash-vaporization stages arranged in a series which the vapor and liquid products from each stage flow countercurrently to each other.
Rectification (Fractionation) or Stage Distillation with Reflux
True or False.
The liquid in a stage is conducted or flows to the stage above and the vapor from a stage flow downward to the stage below.
False. Liquid -> Below. Vapor -> Above.
It is referred to as sieve plates or trays.
Stages
True or False.
In distillation with reflux, the feed enters the column somewhere in the middle of the column.
True.
True or False.
If feed is liquid, it flows above to a sieve tray or stage.
False. Flows ‘down’.
True or False.
Vapor enter the tray and bubbles through the liquids on this tray as the entering liquids flows across.
True.
True or False.
In distillation with reflux, the vapor and liquid leaving the tray are not in equilibrium.
False. They are in equilibrium.
True or False.
The vapor continues up to the next tray or stage, where it is again contacted with a downflowing liquid.
True.
The concentration of the more volatile component is being ___________ in the vapor from each stage going ________ and ___________ in the liquid from each stage going ________.
Increased; Upward; Decreased; Downward
The final vapor product coming overhead is condensed in a condenser and a portion of the liquid product is removed, which contains _______________________.
High Concentration of A.
The remaining liquid from the condenser is _________ as liquid to the top tray.
Returned or Refluxed.
The liquid leaving the bottom tray enters a __________, where it _________ vaporized, and the remaining liquid is withdrawn as liquid product.
Reboiler; Partially
The liquid leaving the bottom tray is ______ in A or _____ in B.
Lean in A, Rich in B
True or False.
The vapor from the reboiler is sent back to the bottom stage or trays.
True.
A mathematical-graphical method for determining the number of theoretical trays or stages needed for a given separation of a binary mixture of A and B.
McCabe-Thiele Method of Calculation for Number of Theoretical Stages
This method uses material balances around certain parts of the tower, which give operating lines and the xy equilibrium curve for the system.
McCabe-Thiele Method
Enumeration.
2 Main Assumptions of McCabe-Thiele Method of Calculation.
- Equimolar overflow through the tow between the feed inlet and the top tray and the feed inlet and bottom tray.
- Liquid and vapor streams enter a tray are equilibrated, and leave.
What is the equation for the enriching-section operating line?
Yn+1= (Ln/Vn+1)*Xn + Dxd/Vn+1
Equation for Reflux Ratio.
R = Ln/D
Equation for the enriching-section operating line applying reflux ratio.
Yn+1= [R/(R+1)]*Xn + Xd/(R+1)
The slope formula in enriching section.
Ln/Vn+1 or R/(R+1)
The theoretical stages are determined by starting at __ and stepping of the first place to __. Then __ is the composition of the vapor passing the liquid __. In a similar manner, the other theoretical trays are stepped off down the tower in the enriching section to the feed tray.
XD, XL, Y2, XL
The equation for the stripping-section operating line.
Ym+1= (Lm/Vm+1)*Xm - (WXw/Vm+1)
True or False.
Since equimolar flow is assumed Lw=Lv= constant, and Vm+1=Vn= constant
True.
The theoretical stages for the stripping section are determined by starting at __ going up to __ and then across to the operating line
Xw, Yw
The condition of feed stream is represented by ______.
Quantity (Q)
The number of moles of saturated liquid produced on the feed plate by each mole of feed added to the tower.
Quantity (q)
The locus of the intersection of the two operating lines.
q-line equation
What is q value for saturated vapor?
Zero (0)
What is the q value for saturated liquid?
One (1)
What is the q value for subcooled liquid?
Greater than One (>1)
What is the value for superheated vapor?
Less than Zero (<0)
What is the q value for mix of liquid and vapor?
0 < q < 1; q is equal to the liquid fraction of the feed
True or False.
Minimum number of trays can be obtained by returning all the overhead condensed vapor Vs from the top of the tower back to the tower as total reflux.
True
True or False
In total flux, all the products distillate and bottoms are reduced to zero flow.
True
Columns are operated at total reflux during their __________, and product is not withdrawn until a separation close to that desired is achieved.
Initial Startup
True or False.
In total reflux, the slope of the enriching operating line becomes 1.0 and the operating lines of both sections of the column coincide with the 45 deg diagonal line.
True
One of the parameters a chemical engineer can manipulate upon the operation of a distillation column.
Reflux Ratio
The number of stages reaches a minimum value.
Infinite Reflux Ratio (Total Reflux)
As the reflux ratio increases, the number of stages _________ because the operating lines become farther to the equilibrium curve.
Decreases
Another method for determining the minimum number of trays required for a given separation.
Fenske Equation
This correspond to the minimum vapor flow in the tower, and hence the minimum reboiler and condenser sizes.
Minimum Reflux Ratio
It will require an infinite number of trays for the given separation desired.
Minimum Reflux Ratio
If R is decreased, the slope of the enriching operating line is ________, and the intersection of this line and the stripping line with the q line moves farther from the 45 deg line and closer to the equilibrium line.
Decreased
Two operating lines that touch the equilibrium line.
Pinch Point
Number of plates is minimum, but the tower diameter is infinite.
Total Reflux
This corresponds to an infinite cost of tower and steam and cooling water. This is the limit in the tower operation.
Total Reflux
Number of trays is infinite which again gives an infinite cost. These are the two limits in operation of the tower.
Minimum Reflux
Actual operating reflux ratio to use is in between these two limits.
Minimum Reflux
The optimum reflux ratio to use for the lowest total cost per year is between the minimum Rm and total reflux.
1.2Rm to 1.5Rm
In some cases the feed to be distilled is not supplied to an intermediate point in a column but is added to the top of the stripping column.
Stripping-column distillation
The feed is usually a saturated liquid at the boiling point and the overhead product Vd is the vapor rising from the top plate, which goes to a condenser with no reflux or liquid returned back to the tower.
Stripping-column Distillation
True or False.
In stripping-column distillation, the bottoms product usually has a low concentration of the less volatile component.
False. The bottoms product usually has a high concentration of LVC.
The column operates as a stripping tower with the vapor removing the more volatile from the liquid as it flows downward.
Stripping-column distillation
True or False.
In stripping-column distillation, if the feed is saturated liquid then Lm=F.
True
Enriching towers are also sometimes used, where the feed enters the bottom of the tower as a vapor.
Enriching-column distillation
True or False.
In enriching-column distillation, if the feed is saturated vapor, the vapor in the tower Vn=F.
True
Feed line: Total condenser, no reboiler, saturated vapor feed, liquid bottoms F and B are passing streams.
Rectifying Column
Feed line: Partial reboiler, no condenser, saturated liquid feed, vapor distillate F and V are passing streams.
Stripping Column
True or False.
Generally, the heat to a distillation tower is applied to one side of a heat exchanger (reboiler) and the steam does not directly contact the boiling solution.
True
When an aqueous solution of more volatile A and water B is being distilled, the heat required may be provided by the use of open steam injected directly at the bottom of the tower. The reboiler is then not needed.
Rectification with Direct Steam Injection
The ________ is injected as small bubbles into the liquid in the tower bottom.
Steam
True or False.
In direct steam injection, the vapor leaving the liquid is then in equilibrium with the liquid if sufficient contact is obtained.
True
True or False.
In the discussion of actual trays, it is always assumed that the vapor leaving a tray is in equilibrium with the liquid leaving.
False. This concept is discussed on theoretical trays.
It is defined as the ratio of the number of theoretical or ideal trays needed in an entire tower to the number of actual trays used.
Overall Tray Efficiency or Plate Efficiency
