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What is fractionation

1-What is fractionation?

Fractionation refers to separation of mixture of hydrocarbons without affecting their molecular structure.seperation of compounds in to their constituents' components (fractions) is called fractionation.

2-Differentiate between cracking and reforming?

Cracking means heating of higher boiling petroleum fractions like heavy fuel oil at higher temperature and pressure to produce lower boiling lighter fractions.

Reforming mean rearrangement of molecules without much affecting the averages molecular weight of feed. Reforming is carried out to produce high quality (octane number) gasoline by heating with or without catalyst.

3-What are light and heavy liquids?

Light liquids are those having low boiling point and high volatility, heavy liquids are those having high boiling point low volatility.

4-How can vapor pressure, boiling point & volatility relate each other?

Low volatile substance has high boiling point and low vapor pressure; vapor pressure is directly proportional to volatility and both are inversely proportional to boiling point.

5-What is unsteady state distillation and steady state distillation?

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When mixture to be vaporizes is put into the still batch by batch it is called unsteady state distillation. A continues distillation process is called as steady state.

6-One complete assembly of tray consists of which parts?

A complete assembly of tray consists of a weir, a down comer and perforated plates I;e active tray area., weir control the liquid level above plate and perforations are to pass the vapors of mvc.where as the down comer is to direct the liquid more than the requirement on tray. In bubble cap and valve tray perforations' are covered with a cap and valve respectively.

7-Which stream is more pure in of two on a tray liquid or vapor?

A vapor stream is more pure and is of desired product. As it contain mvc.

8-What are different fractions of crude oil which one is lightest and heaviest?

Crude oil has following fractions.

  • Gasoline (also known as petrol)
  • Naphtha
  • Kerosene and related jet aircraft fuels
  • Diesel fuel
  • Fuel oils
  • Lubricating oils
  • Paraffin wax
  • Asphalt and tar
  • Petroleum coke

Kerosene oil is lighter fraction. Lubricating oil and paraffin wax are heavier products.

9-What is entrainment how it can be controlled in distillation column?

When drops of liquid suspended in vapors are carried up into the vent or into the overhead it is called entrainment. Entrainment can be controlled by providing proper space between trays. And all trays should be perfectly leveled.

10-What are equilibrium flash calculations?

The equilibrium flash of a multi-component liquid may be visualized as a simple distillation process using a single equilibrium stage. It is very different and more complex than the flash evaporation of single-component liquid. For a multi-component liquid, calculating the amounts of flashed vapor and residual liquid in equilibrium with each other at a given temperature and pressure requires a trial-and-error iterative solution. Such a calculation is commonly referred to as an equilibrium flash calculation.


  • zi is the mole fraction of component i in the feed liquid (assumed to be known);
  • β is the fraction of feed that is vaporised;
  • Ki is the equilibrium constant of component i.

11-Why corrosion allowances are used in designing of column?

A tower is built with some extra thickness; it is called corrosion allowance. It is provided so that no leakage and heat loss occur in case of column get corrode. Designing a distillation column, corrosion allowance is used to compensate the corrosion damage for the inevitable effects of corrosion on the column exposed to environments.

12-Why we preheat the feed in separation of heavy liquids?

Preheating the low volatile liquids reduce the re-boiler duty and improve the column efficiency resulting in ease of separation.

13-What purpose risers and boot are used in tray columns?

A bubble cap tray has riser or chimney fitted over each hole from where the vapors rise up the plate and directed down by a cap. Down comer of tray is called as boot to direct the liquid reflux. The passage way for liquid down from one tray to next is called down comer.

14-if one introduce pressurized feed in column what is its effect on separation and on column parameters?

With increase of pressure relative volatility decreases and separation becomes difficult. Also with high pressure a tower may rupture.

15- Is the temperature of the column being controlled by amount of reflux?

Yes temperature of the distillation column is controlled by amount of reflux. Relux is actually condensed product , therefore, at a lower temperature than vapors rising from the reboiler. Without any reflux, difference between temperatures at top and bottom of the distillation column is small which is never good for good separation. Reflux lowers the temperature at the top of the column, hence creating a temperature gradient in the column. Variation in reflux also varies temperature of the column.

16-What is hydrostatic head on tray?

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It is the liquid level provided on tray using weir so that maximum mass transfer and efficient separation be a result.

17-What is equimolecular counter diffusion?

When number of moles of one component being diffuse to other is same and net rate of diffusion is zero it is called equimolar counter diffusion.throught the column this type of mass transfer take place. In equimolar counter-diffusion, the molar fluxes of A and B are equal, but opposite in direction, and the total pressure is constant throughout. Hence we can write:

N = NA + NB = 0

Na = -Nb

18-Define total reflux, minimum reflux, pinch point, optimum reflux ratio?

If no product is withdrawn from the still (D=0), the column is said to operate under conditions of total reflux and top operating line has its maximum slope of unity and concedes with the line y = x. minimum number of plates are required for a given separation at condition of total reflux

When all liquid enrich in more volatile liquid is introduce back into the tower from top plate it is called total reflux. Minimum reflux mean a small quantity of liquid enrich in mvc is transferred back to top plate, because of this large number of stages are required. Below minimum reflux ratio it is impossible to obtain the desired enrichment, however many plates are used. The minimum reflux condition represents the theoretical opposite of total reflux -- an infinite number of ideal separation stages. In this case, the intersection of the operating lines lies on the equilibrium curve itself. Thus, the distance between the equilibrium curve and the operating lines is at its minimum, the stepping triangles become very small, there is no gap between the equilibrium curve and the intersection point, so you cannot step past the feed point.

Pinch point: The intersection of an operating line and the equilibrium curve is called a pinch point. A simple column will have two pinch points (because there are two operating lines). The points change when the operating lines do. An existing column can "pinch" if its operating line is too close to its equilibrium curve.

Optimumreflux:ratio mean the required reflux is being use in distillation column and remaining is being cooled and stored in product tank. The reflux ratio is the ratio of the amount of moles returned as refluxed liquid to the fractionating column and the amount of moles of final product, both per unit time. Optimum reflux ratio reduces fixed and capital cost of column.

19-What is effect of reflux on economics of the process?

Due to reflux the purity of product increases and a constant composition of product is obtain, thus process efficiency increases and process become more economical. Reflux reduces the number of stages and thus controls the fixed and capital cost.

20-What are dew point & bubble point, for what purpose these are used in distillation column?

Bubble point is the temperature at which the first drop of liquid bubbles up and dew point first drop from vapors condenses at constant barometric pressure . Bubble point is use so that we can control the temper ature of re-boiler and dew point to know the condenser performance. The dew point is a saturation temperature.

21-What are selection criteria between plate and packed column?

Selection criteria is ease of maintenance, cost factors, contact area, surface area, efficiency i:e purity of product, and weight and length of column. .

A tray column that is facing through-out problems may be de-bottlenecked by replacing a section of trays with packing's. This is because:

packing's' provide extra inter-facial area for liquid-vapors contact

efficiency of separation is increased for the same column height

packed columns are shorter than trayed columns

Packed columns are called continuous-contact columns while trayed columns are called staged-contact columns because of the manner in which vapors and liquid are contacted.

22-What are different types of trays write their selection criteria?

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There are 3 types of trays

Bubble cap trays

A bubble cap tray has 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.

Valve trays

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.

Sieve trays

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

Selection criteria is ease of maintenance, cost factors, contact area, efficiency i:e purity of product, and weight and length of column.

23-What are different types of packing used for columns what is selection criteria of packing?

Packing increase mass transfer reduces cost and reduces pressure drop.2 types of packing used for column

  • Stacked packing (arranged)
  • Dumped packing (random)
  • These two have the following types
  • Raschig rings
  • Berl saddles
  • Intalox saddles
  • Pall rings
  • Metal intalox
  • Spiral rings
  • Lessing rings
  • Cross partition rings
  • Grid tile
  • Teller rosette
  • Spraypck
  • Panapak
  • Stedman packing
  • Wood grids
  • Poly grids
    • Cannon packing
    • Wire mesh packing

Selection of packing depends on cost, surface area and weight.

24-Height of column depends on what parameters?

Distillation columns are designed on the basis of the properties of the boiling point of the components in the mixtures being separated. This is done using the vapor-liquid equilibrium (VLE) data for the mixtures to be separated. The VLE characteristics of the mixture will determine the number of stages, and also the number of trays required for the separation. In other words, the sizes, particularly the height of distillation columns and the overall design are determined by the VLE data for the mixtures. Overall column height depends on plate Height of column depends on following parameters.

  1. Number of plates
  2. Tray spacing
  3. Amount of reflux
  4. Composition of feed
  5. spacing.

25-Define the following; entrainment, flooding, weeping, channeling, dumping & coning?


Entrainment refers to the liquid carried by vapour up to the tray above and is again caused by high vapour flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate.


Ideally there is a smooth flow of liquid across a tray in a distillation tower and down the downcomer to the tray below. At the same time there is a smooth flow of vapors from the tray below through the valve caps or bubble caps.

Flow of vapor in the wrong direction can lead to weeping/dumping, foaming, flooding etc. Liquid Entrainment - High vapour velocities blow liquid off the immediate tray onto the tray above it.When the relative flow rates of the vapor and liquid are such that the drag force is greater than or equal to the gravity force; then, the liquid stops flowing down the column. This condition is called flooding. Flooding can begin at any vertical location in the column.

Down coming flooding:

In a flooding condition there is too much liquid coming down the column and it swamps the downcomers and causes the liquid on the tray to build up. This puts exerts extra pressure on the vapor and it can not flow up through the valve caps. This cause the pressure from the vapors to build up until the pressure is high enough to overcome the extra liquid on the trays. It then blows through in much the same way the water in a coffee perculator does.


Entrainment flooding:

Flooding means that the gas velocity is very high, therefore, does not allow the flow of the liquid from the top of the tower, and flooding occurs on the top of it


Weeping is when liquid flows downward through the holes in a distillation tray. Normally vapor rises up through the holes and contacts the liquid on the tray. If the vapor rate is too low the liquid may be able to drop to the next stage through the holes, resulting in less than optimal vapor/liquid contact (and therefore less than optimal separation). Condensed liquid from above trays is generally distributed onto a distillation tray via a weir.


In buble cap tray, when level of liquid on the tray drops,the upcoming vapour donot mix properly with the liquid and splashes out to the condenser because of less cotact time between liquid and vapour known as coning.


it is the unequal wetting of the packings in the tower. If channeling occurs, poor mass transfer happens, and therefore, less efficiency of the tower happens.


When the liquid height at a tray's outlet is less than at its inlet, vapor flows

preferentially through the outlet region of the tray. A pressure imbalance is created above the froth

and vapor then flows horizontally in the disengagement space above the froth, in the direction

opposite to the liquid flow. The horizontal vapor traffic, in fact, increases the froth gradient.


During dumping, a substantial portion of liquid flowing down the column passes through a region of the perforated tray deck. Often, most of this liquid has not been exposed to the rising vapor; therefore, performance degrades significantly frequently resulting in overall tray efficiency being significantly less than local point efficiency.

Weeping and dumping are related but different phenomena. During weeping, a minor fraction of liquid flows to the tray below through the tray perforations rather than the downcomer. This downward-flowing liquid typically has been exposed to rising vapor; so, weeping only leads to a small reduction in overall tray efficiency, to a level rarely worse than the tray point efficiency.

26-What are different sections of column and why they are called so?

The part of the distillation column above the feed point is commonly referred to as the “Enriching Section” or the “Rectification Section”. The part of the distillation column

below the feed point is known as the “Stripping Section”. Enriching Section: That part of the column which enriches the Lower Boiling Component

Stripping Section: The part of the column which strips out the Higher Boiling Component. The vapors required for the stripping section may be generated at the column bottom by

either direct injection of steam or by the use of reboilers. The use of steam or reboiler depends on the components to be separated, degree of separation, heat loads, column design etc.

27-What is tray gradient?

As temperature in distillation column changes from point to point therefore there exists a temperature gradient throughout the column. Thus a tray gradient is defined as the temperature difference between two plates.

28-What are different flow arrangements in distillation column which one is the best?

There are different flow arrangement in DC

  • Cross flow
  • Reverse flow
  • Double pass flow
  • Double pass cascade flow

29-What are q-lines?

If the feed composition is such that it does not coincide with the intersection of the operating lines, this means that the feed is not a saturated liquid. The condition of the feed can be deduced by the slope of the feed line or q-line. The q-line is that drawn between the intersection of the operating lines, and where the feed composition lies on the diagonal line.

30-Is the temprerature of column being controlled by amount of reflux?
Yes temperature of the distillation column is controlled by amount of reflux. Relux is actually condensed product , therefore, at a lower temperature than vapors rising from the reboiler. Without any reflux, difference between temperatures at top and bottom of the distillation column is small which is never good for good separation. Reflux lowers the temperature at the top of the column, hence creating a temperature gradient in the column. Variation in reflux also varies temperature of the column.

31- What are conditions at which flooding occurs?

The column is flooded when

  • Forth and foam in the down comer back up to the tray
  • In a flooding condition there is too much liquid coming down the column and it swamps the down comers and causes the liquid on the tray to build up. This puts exerts extra pressure on the vapor and it cannot flow up through the valve caps. This cause the pressure from the vapors to build up until the pressure is high enough to overcome the extra liquid on the trays.

32-When flooding occurs why pressure gradient enhances?

Pressure gradient in distillation column is due to the pressure drop of vapor while moving from bottom to top. as in flooding height of liquid on the tray increases, the pressure drop is high because vapor found no space to go up hence pressure gradient is high while flooding.

33-How flooding can be recognized in distillation column write three parameters?

Flooding is recognized by sharp increases in column differential pressure and significant decrease in separation efficiency.

34- What are distributor weir & overflow weir?

Distributor weir

Weir Type Liquid Distributors effectively handle a wide variety of gases and liquids. They feature a series of closed end troughs that have slots through which the liquid is uniformly dispersed over the packed bed. Liquids feed evenly to one or more of the parting boxes, which in turn distribute the flow to the troughs. The parting boxes receive the liquids through sparger type feed pipes. The tower diameter and the liquid flow determine the number of parting boxes. Up to three parting boxes may be required depending upon tower size and flow rates.

Overflow weir

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.

The weirs maintain the liquid level on each tray in the column. Without a liquid lay on each column the vapor and liquid would have very little contact and the efficiency of the column would be greatly reduced.

35-What are inlet & outlet down comers?

Inlet downcomer

The downcomer through which the liquid move to the plat or tray.

Outlet downcomers

The downcomer through which liquid leaves the tray and goes to the next plate

36-What are strength test & tightness test of Distillation column?

The tightness test is used to find leak detection in pressure and vacuum systems (i.e., boilers, heat exchangers, condensers, chillers, distillation columns, vacuum furnaces, specialty gas systems), bearing inspection, steam trap inspection, valve blow-by, pump cavitations, detection of corona in switch gear, compressor valve analysis, integrity of seals and gaskets in tanks, pipe systems and large walk-in boxes.

Mechanical property evaluation is a powerful tool for estimating the residual life of in-service components and health monitoring. To find this we used strength test.

37-What are Normal Operation Pressure and Relief Pressure in distillation column operation?

A distillation column operating under pressurized conditions may not be designed for full vacuum. A vacuum relief valve on such column must, therefore, be sized to prevent negative pressures in the shell under the worst case scenario, i.e. when the feed, bottoms, distillate and reboiler steam are cut off, but the condenser still operates. The valve can be sized considering that the inflowing air (or other gas) first accumulates in the condenser, which is the vacuum source, and blankets the heat exchange area stopping the condensation

Normally distillation will carry at atmospheric pressure, this type known as Normal operation pressure but some situations will forced to do the vacuum distillation where the main feed which is handling may not capable to with stand that temperature means it will thermally decomposed at that temperature

38-What are manholes & weep holes?


Weep holes are used in tray drainage. When the column is shut down, some liquids is trapped on bobble-cap trays and certain valve trays, and at low points such as seal pans and inlet-weir areas. Weep holes allow this liquid to drain.

Weep holes are on plates for drainage of column before any internal maintenance

Can be started or before fluid services can be changed some weep holes are placed in the downcomer inlet area


Weep holes dia is equivalent to the area of all the weep holes/tray


Entry into the shell of a distillation tower is made via manholes. These are usually fitted in the column so that each serves 10 to 20 trays. When the service is clean and noncorrosive, up to 30 trays or more may be served by one manhole.

39-If we want to increase the plates in distillation column in which section we will add these?

If we want to increase the number of plates then we will add these on the top of the column easily, we can not add these at bottom this will change the placement of the each tray.

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