CE20095 - Separation by Distillation and Evaporation

Question 1

How is relative volatility found?

Answer 1

By dividing the partial pressure of the component by its mole fraction

Question 2

How is liquid mole fraction of a found, considering vapour mole fraction and relative volatility?

Answer 2

x.a = y.a/(a-(a-1)*y.a)

Learn to derive

Question 3

What’s continuous fractionation?

Answer 3

When the distillation column cylinder is divided into sections by a series of perforated trays: the vapour passes through the tray as the liquid flows across it, over a weir and to the tray below.

There is a continual movement of vapour up, and liquid down through the column.

At each stage, an ideal column will attain equilibrium between liquid and vapour. This means that there is a transfer of more volatile components from the liquid to the vapour, and a transfer of less volatile components from the vapour to the liquid.

This results in a reduction in both temperature and pressure when moving up the column.

At the top of the column, the vapour is condensed and a portion returned as liquid reflux to the top plate. The rest goes as top product. At the bottom of the column, a portion of the liquid from the bottom plate is reboiled and the vapour sent up the column. The rest becomes bottom product.

There is thus a continual movement of vapour up, and liquid down the column.

Question 4

What nomenclature and subscripts are used within the McCabe Thiele method?

Answer 4

Nomenclature:
L – liquid flow rate
V-  vapour flow rate
W – bottom product rate
D – distillate (tops) product rate.
F – feed flow  rate
x- mol fraction of a component in the liquid phase
y- mol fraction of a component in the vapour phase

Subscripts:
m- plate below feed plate, n- plate above feed plate
d- distillate
q- intersection of operating line

Where q is the heat to vaporise 1 mole of feed / molar latent heat of feed

Question 5

What’s the q line in the McCabe Thiele method?

Answer 5

q is the heat to vaporise 1 mole of feed / molar latent heat of feed

The q line gradient depends upon the feed condition:

Boiling point feed q=1 (Slope of q line is vertical)

Cold liquid feed q>1 (slope of q line is positive)

Feed partly vapour 0

Question 6

What is the y-int of a McCabe Thiele diagram?

Answer 6

xd / (R + 1)

Where:
xd is the mole fraction of the distillate (top product)

R is the reflux ratio (equal to L/D)

Question 7

What’s reflux ratio and how is it found?

Answer 7

R = flow returned as reflux / flow of top product taken off

R= Ln/D (Vn = Ln + D)

nm - min # stages
Rm - min reflux ratio

Question 8

What’s nm and when does it occur?

Answer 8

Min number of stages - occurs at total reflux

Question 9

How can number of plates at total reflux be found?

Answer 9

Fenske method (can never get complete separation)

Or graphical

Question 10

What method is used to find min reflux ratio, Rm?

Answer 10

Underwood-Fenske

Or graphical

Question 11

What are typical reflux ratios used?

Answer 11

Practical reflux ratios lie between Rm and total reflux. A typical value for design purposes is 1.3 - 1.5 x Rm­.

Question 12

How is overall column efficiency found?

Answer 12

Eo = number ideal stages / number real stages

Question 13

How is murphree plate efficiency found?

Answer 13

En = (yn - yn-1)/(ye - yn-1)

Question 14

How are the actual number of trays in a column determined?

Answer 14

A ‘pseudo-equilibrium’ curve is drawn halfway (on a vertical line) between the operating lines and the true-equilibrium curve.
The true-equilibrium curve is used for the first stage (the partial reboiler is assumed to be an equilibrium stage), but for all other stages the vapour leaving each stage is assumed to approach the equilibrium value yn* only 50 % of the way. Consequently, the steps in the figure present actual trays.

Question 15

What’s flash vaporisation?

Answer 15

This is a single stage operation (usually continuous) the liquid mixture is partially vaporised, and allowed to contact the residual liquid. The system is at equilibrium.

The resulting vapour and liquid phases are separated and removed from the equipment. Since stage operations such as differential or flash distillation are limited in the maximum purity of the product possible.

This method, frequently carried out as a continuous process, consists of vaporising a definite fraction of the liquid feed in such a way that the vapour evolved is in equilibrium with the residual liquid.

The feed is usually pumped through a fired heater and enters the still through a valve where the pressure is reduced. The still is essentially a separator in which the liquid and vapour produced by the reduction in pressure have sufficient time to reach equilibrium. The vapour is removed from the top of the separator and is then usually condensed, while the liquid leaves from the bottom.

Question 16

What’s the purpose of weirs on tray plates?

Answer 16

A weir on the tray ensures that there is always some liquid (holdup) on the tray and is designed such that the the holdup is at a suitable height, e.g. such that the bubble caps are covered by liquid.

Being lighter, vapour flows up the column and is forced to pass through the liquid, via the openings on each tray. The area allowed for the passage of vapour on each tray is called the active tray area

Question 17

What’s the active tray area?

Answer 17

The area allowed for the passage of vapour on each tray

Question 18

How are DC trays/plates designed?

Answer 18

In any tray vapour rises through the liquid pool on the tray deck and then disengages from the liquid in the space above the deck.
Liquid enters the tray from a downcomer above and leaves via a downcomer below.

A tray has three functional zones:

(i) Active area for mixing vapour and liquid: This is the zone where mass transfer occurs.
(ii) Vapour space above the active area: This is the zone in which liquid is separated from vapour.
(iii) Downcomer between trays. This zone has two functions; moving liquid from one contacting tray to another and disengaging vapour from liquid.

Question 19

What do trays do within DCs?

Answer 19

A tray essentially acts as a mini-column, each accomplishing a fraction of the separation task.

The more trays there are, the better the degree of separation.
The overall separation efficiency will depend significantly on the design of the tray.

Trays are designed to maximise vapour-liquid contact by considering the liquid distribution and vapour distribution on the tray. This is because better vapour-liquid contact means better separation at each tray, translating to better column performance.
Less trays will be required to achieve the same degree of separation. Benefits include less energy usage and lower construction costs.

Question 20

What are the main types of DC trays?

Answer 20

Bubble cap trays
Sieve trays
Valve trays

Question 21

What are bubble cap trays?

Answer 21

Trays in DCs which have a riser or chimney fitted over each hole, and a cap that covers the riser.

The cap is mounted so that there is a space between riser and cap to allow the passage of vapour.

Vapour rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.

Question 22

What are sieve trays in DCs?

Answer 22

Sieve trays are simply metal plates with holes in them. Vapour passes straight upward through the liquid on the plate.
The arrangement, number and size of the holes are design parameters.

Question 23

What are valve trays in DCs?

Answer 23

Perforated sheet metal decks on which round, liftable valves are mounted.

The vapour flows through valves which are installed parallel to the outlet weir.

Valve trays combine high capacity and excellent efficiency with a wide operating range.

In valve trays, perforations are covered by liftable caps. Vapour flows lifts the caps, thus self creating a flow area for the passage of vapour.
The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays.

• Excellent liquid/ vapour contacting.
• Higher capacity.
• Higher flexibility than sieve trays.
• Can handle higher loadings.
• Low-pressure drop than bubble cap.

Question 24

What’s structured packing (in DCs)?

Answer 24

The term structured packing refers to a range of specially designed materials for use in absorption and distillation columns.

Structured packings typically consist of thin corrugated metal plates or gauzes arranged in a way that they force fluids to take complicated paths through the column, thereby creating a large surface area for contact between different phases.

Structured packing is formed from corrugated sheets of perforated embossed metal or wire gauze. The result is a very open honeycomb structure with inclined flow channels giving a relatively high surface area but with very low resistance to gas flow.

The surface enhancements have been chosen to maximize liquid spreading. These characteristics tend to show significant performance benefits in low pressure and low irrigation rate applications.

Question 25

What are benefits of using structured packing?

Answer 25

• Packings provide extra inter-facial area for liquid-vapour contact
• Efficiency of separation is increased for the same column height
• Packed columns are shorter than plate columns

Structured packing is formed from corrugated sheets of perforated embossed metal or wire gauze. The result is a very open honeycomb structure with inclined flow channels giving a relatively high surface area but with very low resistance to gas flow.

The surface enhancements have been chosen to maximize liquid spreading. These characteristics tend to show significant performance benefits in low pressure and low irrigation rate applications.

Question 26

What are the 2 flooding methods of plate columns?

Answer 26

Froth regime

Spray regime

Question 27

What occurs in froth regime (of flooding plate columns)?

Answer 27

In the froth regime, the liquid phase is continuous and the vapour is dispersed as bubbles in the liquid. The froth regime occurs at low to moderate vapour velocities and moderate to high liquid loads.

Froth entrainment flooding occurs at higher liquid rates. When vapour velocity is increased, the froth height increases. If the tray spacing is small, the froth may reach the tray above, causing liquid accumulation on the tray above.

Question 28

How does flooding occur (in DCs)?

Answer 28

Flooding can results from changes in feed and reflux conditions.

Flow regime refers to the way vapour and liquid are dispersed on the tray. The 2 different regimes are: (1) froth regime and (2) spray regime.

The regime that is encountered in a column depends on the tray geometry and operating conditions.

Question 29

What occurs in sptay regime (of flooding plate columns)?

Answer 29

In the spray regime, the vapour phase is continuous and the liquid is dispersed as droplets in the vapour. The spray regime usually occurs at high vapour velocities and low liquid loads.
Spray entrainment flooding occurs at low liquid flow rates. As vapour velocity is increased, a condition is reached where the liquid on the tray are “dispersed” by the rising vapour into droplets and are entrained into the tray above instead of flowing to the tray below.

Question 30

How can problems with flooding within distillation columns be reduced?

Answer 30

Reducing flow rates slightly may take a column out of flooding condition, and allow the specifications to be met.

Most towers are over-refluxed to ensure that the specs are met. Operators almost always increase the reflux when a spec. is not being met.

Decreasing reflux may reduce flows such that the tower becomes un-flooded.

Increasing pressure may reduce vapour velocity, and as pressure drop is a function of velocity squared, increasing the pressure may take the tower out of a flooding condition.

Load can be transferred from the rectification to the stripping section by altering the condition of the feed (reducing vapour load and increasing liquid load in feed).
This may allow unloading of the column’s problem section.

Question 31

What are key components?

Answer 31

Separation between top and bottom products is specified by setting limits on 2 key components, between which it is desired to make the separation.

The components that have their distillate and bottoms fractional recoveries specified are called key components.
The most volatile of the keys is called the light key (LK) and the least volatile is called the heavy key (HK).
The other components are called non-keys (NK).

The difficulty of the separation is measured by the number of trays required at a given reflux ratio, and is fixed by the concentrations of the key components in the top and bottom streams.

Question 32

What’s a sharp separation?

Answer 32

One where the key components are adjacent in the rank order of volatility, in sharp separations the keys are the only components that appear in both product streams in any appreciable concentrations.

Question 33

What are distributed and undistributed components (distillation)?

Answer 33

A distributed component is found in both the distillate and the bottoms products, whereas an undistributed component is found in only one product.
Light and heavy keys are always distributed.

Components more volatile than the light key are almost completely recovered in the distillate and those less volatile than the heavy key are recovered in the bottoms.
These are usually termed undistributed components.

Question 34

How is relative volatility of a compound found using K values and key components?

Answer 34

a = K.lk / K.hk

Lk - light key
Hk - heavy key

Or:
ai = Ki / K.hk

Question 35

What is the Fenske equation used for?

Answer 35

It can be used to find the number of theoretical plates, for multi component as well as binary systems, and makes use of the relative volatility of light and heavy keys.

Question 36

What’s the Underwood equation used for?

Answer 36

Predicted Rm, minimum reflux ratio

Question 37

What are Erbar-Maddox and Gilliland relationships used for?

Answer 37

They’re correlations examining how the number of stages changes as a function of reflux ratio.

They do not present a specific stage recommendation (as McCabe Thiele does for binary systems) but instead allow no. plates vs reflux ratio to be plotted, for which the optimum number of plates can be determined.

Both Erbar-Maddox and Gilliland relationships require a knowledge of nm and Rm

Question 38

What does the Kirkbride correlation help find?

Answer 38

The feed point position.

Where:
Nr - number of stages above the feed [including any partial condenser]

Ns - number of stages below the feed. [including the reboiler]

w - molar flow of bottom product

d - molar flow of distillate

Question 39

How is distillation efficiency different for binary and multi component systems?

Answer 39

In a binary system, the efficiency obtained for each component must be the same.

This is not so for a multicomponent system; the heavier components will usually exhibit lower efficiencies than the lighter components.

Question 40

What are suggested guidelines (from Toor and Burchard) which can be used to estimate the distillation efficiency for multi-component systems?

Answer 40

If the components are similar, the multicomponent efficiencies will be similar to the binary efficiency.

If the predicted efficiencies for the binary pairs are high, the multicomponent efficiency will be high.

If the resistance to mass transfer is mainly in the liquid phase, the difference between the binary and multicomponent efficiencies will be small.

If the resistance is mainly in the vapour phase, as it normally will be, the difference between the binary and multicomponent efficiencies can be substantial.

Question 41

What’s the O’Connell correlation used for?

Answer 41

A quick estimate of the overall column efficiency can be obtained from the correlation given by O’Connell (1946).

The overall column efficiency is correlated with the product of the relative volatility of the light key component (relative to the heavy key) and the molar average viscosity of the feed, estimated at the average column temperature.

The correlation was based mainly on data obtained with hydrocarbon systems, but includes some values for chlorinated solvents and water-alcohol mixtures.

It has been found to give reliable estimates of the overall column efficiency for hydrocarbon systems; and can be used to make an approximate estimate of the efficiency for other systems.
The method takes no account of the plate design parameters; and includes only two physical property variables.

Question 42

What’s the aim of distillation column control?

Answer 42

The primary objective of distillation column control is to maintain the specified composition of the top and bottom products, correcting for the disturbances in:

Feed flow-rate, composition and temperature.

Steam supply pressure.

Cooling water pressure and header temperature.

Ambient conditions, which cause changes in internal reflux.

Question 43

How are compositions controlled in distillation columns?

Answer 43

The compositions are controlled by regulating reflux flow and boil up. The column overall material balance must be controlled.

Column pressure is normally controlled at a constant value. The feed flow rate is normally set by a level controller on a preceding column.

Temperature is usually used as an indication of composition. The temperature sensor should be located at the position in the column where the rate of change of temperature with change in composition of the key component is a maximum; at the top and bottom of the column the change is of temperature with position is usually small.

Question 44

How is temperature used to control composition in distillation columns?

Answer 44

Temperature is usually used as an indication of composition.

The temperature sensor should be located at the position in the column where the rate of change of temperature with change in composition of the key component is a maximum; at the top and bottom of the column the change is of temperature with position is usually small.

Top temperatures are usually controlled by varying the reflux ratio, and the bottom temperatures by varying the boil-up rate.

Differential pressure control is often used on packed columns to ensure the packing operates at the correct loading.

Additional temperature indicating or recording points should be included up the column for monitoring column performance and for troubleshooting

Question 45

When is azeotropic distillation used?

Answer 45

Separation of components with similar boiling points is difficult by simple distillation, and maybe impossible if azeotropes are formed.

Such systems can often be improved by the addition of a third component which alters the relative volatility of the original components.

In azeotropic distillation, the third component sometimes called an entrainer, may be added to the binary mixture to form a new low boiling point azeotrope with one of the original constituents, with a volatility such that it can easily be separated from the original constituents.

Azeotropic distillation is thus a special case example of a multicomponent system.

Question 46

What’s extractive distillation?

Answer 46

This is a multicomponent rectification method similar in purpose to azeotropic distillation.

A binary mixture which is difficult to separate has a 3rd component (often called a solvent) added, which alters the relative volatility of the original constituents, enabling separation to occur.

Question 47

Steps in designing a distillation column:

Answer 47

The design of a distillation column can be considered as the following steps:

(i) Set product specifications
(ii) Select batch or continuous operation
(iii) Select plates or packing
(iv) Determine the number of theoretical plates and the reflux ratios required
(v) Size the column, specifying the diameter, and number of actual stages (needs an efficiency)
(vi) Design column internals (plates, packing etc)
(vii) Mechanical design, (vessel design).

After the number of real stages has been determined, we can consider the overall column size.

Column height:
Height of a column depends upon the number of plates and their spacing

Plate spacing:
Plate spacing is estimated on an empirical basis; using correlations, but for columns > 1.0 meter an initial value of 0.5m can be used. This can be revised in a more detailed design.
.

Question 48

What does distillation column diameter mainly depend on?

Answer 48

Vapour flow-rate.

The vapour velocity must be below that which would cause a high pressure drop, or too high a liquid hold up.

Correlations are used to determine the maximum allowable vapour velocity, and hence the minimum column diameter.

u.v can be calculated in terms of plate spacing and liquid and vapour densities

Question 49

What’s the main objective of distillation column control?

What disturbances must be corrected?

Answer 49

To maintain the specified composition of the top and bottom products.

Disturbances in:

• feed flow-rate, composition and temperature
• steam supply pressure
• cooling water pressure and heater temperature
• ambient conditions causing changes in internal reflux

Question 50

What are typical control requirements in distillation units?

Answer 50

Maintain constant pressure.

Operate over a range of feed flowrates.

Optimise for desired top product composition.

Optimise for desired bottom product composition.

Question 51

What are the properties of effective (packed tower) packing?

Answer 51

Low pressure drop

High capacity

Low weight and low liquid holdup

Large active surface area per unit volume

Large free volume per unit total volume

Question 52

What are the main types of packing?

Answer 52

Random

Stacked

Question 53

What considerations are made when choosing between plate towers and packed towers?

Answer 53

1) Stage efficiencies for packed towers must be based on experimental tests
2) Due to liquid-dispersion difficulties in packed towers, plate tower design is more reliable and requires less safety factors when the ratio of liquid mass velocity or gas mass velocity is low
3) Plate towers can be designed to handle wide ranges of liquid rates without flooding
4) Plate tower is preferred if liquids with dispersed solids are used as plates are more accessible for cleaning
5) Plate towers are preferred if interstate cooling is needed to remove heats of reaction or solution, since cooling coils can be installed on plates
6) Dry plate total weight is usually less than that of a packed tower for same duty
7) When there are large temperature changes, plate towers are preferred since thermal expansion/contraction may crush the packing
8) Design info for plate towers is more available
9) Random-packed towers are rarely designed with diameters larger than 4ft, and commercial plate towers are rarely less than 2ft
10) Packed towers are cheaper and easier to construct
11) Packed are preferred is the liquids have a large tendency to foam
12) Amount of liquid hold-up is less in packed towers
13) Pressure drop through packed towers may be less than the pressure drop through plate towers for same duty

Question 54

What’s the aim of evaporation?

Answer 54

To concentrate a solution of non-volatile solute in a volatile solvent.
Typically, the solvent is water.

Question 55

What liquid characteristics affect evaporation?

Answer 55

Concentration - the density and viscosity of a liquor will increase as solid fraction increases. During evaporation, boiling point can also rise.

Fouling – many solutions foul the heat exchange surface, causing loss of heat transfer efficiency.

Corrosiveness – many materials are attacked by some liquors.

Question 56

What are the five main fouling categories?

Answer 56

Crystallisation

Particulate fouling

Chemical reaction fouling

Corrosion fouling

Biological fouling

Question 57

How can fouling resistance be defined (considering distillation and evaporation, Bird unit)?

Answer 57

It can be defined as the deposit thickness divided by the disposition thermal conductivity.

R = δ/λ = Md / ρλAd

Where:
δ is deposit thickness
λ is thermal conductivity
Md is deposit mass
ρ is deposit density
Ad is the deposit area

Question 58

What are the main measures of performance for a steam heated tubular evaporator?

Answer 58

Capacity (kg water vaporised per hour)

Economy (kg water vaporised per kg steam fed into the unit)

Question 59

How do single and double effect evaporators differ?

Answer 59

In single effect, vapour from the boiling liquor is condensed and discarded.

The method is simple but used steam ineffectively.

If vapour from one evaporator is used to heat a second evaporator, the system is known to have a double effect. Having multiple effects has the same capacity, area and temperature difference as single effects, but makes better use of steam.