Exam 2

Question 1

What is flooding? What is the primary variable used to quantify the approach to flooding?

Answer 1

Flooding is when there is so much liquid that it starts going up the column instead of down.
Primary variable: gas velocity.

Question 2

What is the loading point in the operation of a packed column?

Answer 2

The gas velocity where it begins to affect the liquid flow.

Question 3

What equipment design parameter most directly affects flooding in a packed column?

Answer 3

Diameter

Question 4

Why does pressure drop matter?

Answer 4

Because it costs money for the compressor or fan to put energy in to the system.

Question 5

What are three types of trays? How are they different?

Answer 5

Sieve trays: tray with holes.
Valve trays: tray with holes covered by liftable caps, so gas is directed horizontally into the liquid.
Bubble-cap trays: tray with holes and a riser covered by a cap, so the gas is directed down downward into the liquid.

Question 6

What differentiates jet flooding of trays from downcomer flooding?

Answer 6

Jet flooding: Too high gas velocity.
Downcomer flooding: Too high liquid rate or liquid velocity.

Question 7

What features of packing determine mass transfer in the gas phase? In the liquid phase?

Answer 7

Greater packing specific area and reduced packing angle increases mass transfer in both phases.

Question 8

What features of trays determine mass transfer (provide turbulence) in the gas phase? In the liquid phase?

Answer 8

Smaller hole area and greater weir height increase turbulence in both phases.

Question 9

What provides gas/liquid contact area on a tray? What design and operating parameters increase this area?

Answer 9

Contact area created by bubbling the gas through the holes.
Can be increased by increasing the height of the weir.

Question 10

How are packing and trays different in determining mass transfer?

Answer 10

Packing provides area as metal area. Trays use gas pressure drop to create both area and turbulence.

Question 11

How is gas and liquid distributed in packing? On trays?

Answer 11

Packing: liquid is distributed through a distributor, a plate with holes. Gas is distributed through another distributor at the bottom of the column.
Trays: liquid is dumped on packing and gas is distributed through holes on trays
Packing needs separate distributing equipment.

Question 12

What features does a typical tray have?

Answer 12

A weir, a downcomer, and a plate with holes. The holes might be covered by a cap.

Question 13

What features and operating parameters determine pressure drop of packing? of trays?

Answer 13

Trays: Parameters: Velocity of the gas. The bubbling area. Open hole area. Liquid depth (determined by weir height). Properties: Density of vapor.

PACKING MISSING

Question 14

What features and operating parameters determine flooding/capacity in packing? With trays?

Answer 14

Packing: Liquid and vapor flow rates.
Trays: Vapor flow rate.

Diameter for both.

Question 15

What principle of fluid mechanics determines flooding?

Answer 15

Pressure drop

Question 16

What are other internals required in a packed column besides packing?

Answer 16

Plates/trays for distributing the gas and liquid. Liquid distributor and gas distributor.

Question 17

How is gas/liquid contact area created? What energy is required? With Packing? Trays? Sprays?

Answer 17

Packing: Contact area created by buying packing, which creates a surface area of steel. No energy is required to create the area.

Trays: Contact area created by bubbling the gas through the holes. Energy is required to overcome the pressure drop. Compressor or fan.

Sprays: Contact area created by forcing liquid through nozzles. Energy is required to overcome pressure drop in the liquid phase. Energy to pump.

Question 18

How is turbulence created for mass transfer in gas? In liquid? Where does the energy come from? Packing? Trays? Sprays?

Answer 18

Trays: Turbulence for the gas is created by the pressure drop in the gas. The energy comes from a compressor or fan.
Packing: Energy for the liquid is required to pump it from the bottom of one column to the top of the next. It then gets potential energy.
Spray, liquid: Pump energy. Spray, gas: low pressure drop. Energy comes from liquid pumps.

Question 19

How are gas and liquid separated? Packing? Trays? Sprays?

Answer 19

Packing: gravity (column diameter)
Trays: gravity (column diameter)
Spray: gravity (column diameter)

Question 20

Which type of gas/liquid contactor has greater gas DP? Greatest pumping cost? Greatest cost for metal? Largest column diameter? Greatest column height? Most area/volume?

Answer 20

Greatest deltaP: Trays
Greatest pumping cost: Sprays
Greatest cost for metal: Packing
Largest column diameter: Sprays
Greatest column height, area, volume: Sprays

Question 21

What is the most important factor determining pressure drop on a tray?

Answer 21

Liquid depth

Question 22

Define Murphree tray efficiency.

Answer 22

Efficiency = absolute(Y_in - Y_out) / max(Y_in - Y_out)

Question 23

Why are distillation columns arranged for countercurrent flow of liquid and vapor?

Answer 23

To conserve driving force

Question 24

Under what conditions does constant molal overflow hold?

Answer 24

When you’re trying to separate two components with almost similar heats of vaporization.

Question 25

Define reflux ratio. Define boilup ratio.

Answer 25

The reflux ratio is the ratio between the amount of product going back to the column and the amount of product collected. The boilup ratio is the ratio between the amount of liquid boiled back into the column and the amount of liquid leaving.

Question 26

Define the McCabe-Thiele method. What are the five “lines”?

Answer 26

The McCabe Thiele method is a method for graphically determining the number of (computational) stages in a distillation column. x = y line, equilibrium line, q-line, rectifying-section operating line, stripping-section operating line.

Question 27

What three methods can be used to optimize the feed point?

Answer 27

Step off stages, optimum spans the q-line (intersection of the operating lines). Match the feed or vapor composition to the respective stage composition. For constant product specs managed by column load (reflux or heat duty), vary the feed stage to minimize the column load.

Question 28

Define minimum reflux.

Answer 28

The minimum reflux is the lowest value of reflux at which separation can be achieved even with an infinite number of plates.

Question 29

Why is there a minimum packing height for distillation but not for absorption?

Answer 29

The limiting value of L/V in a distillation colum is 1.0, resulting in a minimum number of equlibrium stages for the stripping and absorption sections.

Question 30

Draw a typical distillation tray. Why are there weirs? Why are the holes in a sieve plate not 1 inch in diameter? Why are the tradeoffs in selecting tray spacing?

Answer 30

The weir ensures that there is always enough liquid on the tray. If the holes were 1in in diameter, weeping would occur. Greater spacing reduced flooding velocity and column diameter Reduced spacing decrease column height and capital cost.

Question 31

Why does the efficiency of a distillation column decrease with increasing liquid viscosity?

Answer 31

Reynolds number and thereby turbulence decreases which means less mass transfer.

Question 32

In general, why should the operating pressure be high and the operating temperature be low for an absorber and the opposite for a stripper?

Answer 32

Greater P and reduced T increases solute solubility and reduces solvent rate. Reduced stripper P and increased T reduces solute solubility and reduces stripping vapor (steam).

Question 33

Define relative volatility.

Answer 33

alpha = (Y_lt/Y_hv)/(X_lt/X_hv)

Question 34

What determines the operating pressure of a distillation column?

Answer 34

The distillate must condense at T greater than cooling water T.

Question 35

Define and explain the minimum number of stages.

Answer 35

The number of stages you get using an infinite reflux ratio.

Question 36

Describe three methods for estimating the minimum number of trays.

Answer 36

1. Stepping off a McCabe Thiele plot either numerically or manually 2. The Fenske equation 3. Brute force in Aspen: Keeping the reflux rate constant while decreasing the feed rate and having a design spec for the product.

Question 37

Why do we want to avoid a distillation column T less than 30C? How would we accommodate such a T?

Answer 37

Would have to use refrigeration. Accommodate by using refrigeration.

Question 38

Why do we want to adjust the column pressure so that it just condenses with cooling water? How does that minimize lost work?

Answer 38

Adjusting column pressure → means running at the lowest temperature possible which is more efficient as the relative volatility will be greater. Less reflux, less heat for the reboiler, and fewer stages/trays/height of the column.

Question 39

What are the difficulties if a distillation column is less than 14.7 psia?

Answer 39

When there’s vacuum, air will leak IN if there’s a leak. Air is dangerous because it can blow up when mixed with whatever is in the column. We need a vacuum pump to maintain the pressure which is inconvenient.

Question 40

Why do we want to avoid a column pressure greater than 300 psia?

Answer 40

As P approaches the critical P the relative volatility decreases requiring more equilibrium stages and reflux.

Question 41

What happens if the top P > critical P?

Answer 41

Relative volatility goes to 1 and the separation can’t be run.

Question 42

Why do we want to avoid excessively high distillation column bottoms T? What must we use if the bottoms T is greater than 500 F?

Answer 42

Excessively high bottoms T would cause the liquid product to be boiled into vapor which we don’t want. Thermal degradation would occur. Must use refrigeration to cool.