Distillation & Distillation Column

Distillation is a unit operation a physical separation process, and not a chemical reaction, in which two or more liquid components of a liquid solution into are separate by thermal mean.

The vapour phase is created by supplying heat as a separating agent.

Separation is achieved because of the difference in the vapour pressure of the component at a given temperature.

The more volatile component vaporizing and passing from the liquid to the vapour phase and less volatile component condensing.

The resulting vapour, enriched in one component and depleted in another is called distillate product.The liquid effluent is called the product.

Commercially, distillation has a number of applicationsIt is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating.

Water is distilled to remove impurities, such as salt from seawater.

Air is distilled to separate its components—oxygen, nitrogen, and argon—for industrial use.

Distillation of fermented solutions has been used since ancient times to produce distilled beverages with higher alcohol content.

TYPES OF DISTILLATION SIMPLE BATCH ORDIFFERENTIAL DISTILLATION

EQUILIBRIUM OR FLASH DISTILLATION

STEAM DISTILLATION

FRACTIONAL DISTILLATION

AZEOTROPIC DISTILLATION

SIMPLE BATCH OR DIFFERENTIAL DISTILLATION

In this distillation, a known quantity of a liquid mixture is charged into a still.

The still is provided for heating the liquid mass in the still with help of heating media such as steam.

The charge is provided slowly, vapours formed are withdrawn and fed to the condenser where they are liquefied and collected in a receiver as a distillate.

EQUILIBRIUM OR FLASH DISTILLATION

Flash distillation is carried out in a continuous manner.

In this distillation, a liquid mixture is partially vaporized, the vapour and liquid are allowed to attain equilibrium and finally and withdrawn separately.

Feed is heated in a tubular heat exchanger.

The hot liquid mixture is then fed to a separator via pressure reducing valve whereby pressure is reduced and the vapour is formed at expense of liquid adiabatically.

The liquid is withdrawn from the bottom from the separator and the equilibrium vapour leaves the separator which is then liquefied in a condenser.

Flash distillation is commonly used in petroleum industry.

STEAM DISTILLATION

Steam distillation is used;For separating a high boiling component from the non-volatile impurities

For separating a high boiling mixture into different fractions wherein the decomposition of material might occur if direct distillation were employed.

In cases where the vaporization temperature cannot be reached by steam heat

The necessary condition for employing steam distillation is that the solubility of steam in the liquid must be very low or immiscible.

Steam distillation is especially adopted in cases where substances involved cannot withstand temperature of distillation and decompose.

Substances of this kind can be separated by reducing the partial pressure of the volatile component.

FRACTIONAL DISTILLATION

Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound evaporates.

Fractional distillation is used in air separation, producing liquid oxygen, liquid nitrogen, and highly concentrated argon.

AZEOTROPIC DISTILLATION When the composition of a liquid mixture is the same as that of the vapour generated from it, the liquid mixture is called Azeotropic or constant boiling mixture.

Since the liquid and vapour composition are the same, the boiling point remain constant as vapour is generated.

Azeotropes may be two types homogeneous & heterogeneous

In the case of homogeneous azeotrope, one liquid phase is in equilibrium with the vapour, where as in the case of heterogeneous Azeotropes two liquid phases are in equilibrium with the vapour.

Since two liquid phases vary in composition at or below the boiling point, a heterogeneous azeotrope may be separate in two conventional columns.

A homogeneous azeotrope may be separated by adding a third component which will form a ternary mixture with no azeotrope.

The ternary mixture is now separated with one component in the distillate and the other in the bottom product. The third component may be formed in one or both products.

DISTILLATION COLUMNSThe gas-liquid contacting devices for mass transfer are the same – such as plate and packed columns It is a tall vertical cylindrical shell with number of nozzles. The internal of the columns consist of a series of plates or tray and a variety of packings.

Types of distillation columns

Plate or tray column

Packed column

Plate or Tray columnThe process of mass transfer is continuous.

Each plate in a vertical column represents a stage as on each plate, the phases are brought into intimate contact, mass transferred from one phase to another, and finally phases are separated and leaves the tray or plate.

A plate column is a vertical column which consist a number of plates. In this column, liquid enters from the top, flows over every plate and from plate to plate via down comers.

Various plates used for phase contacting are: 

BUBBLE CAP PLATE Bubble cap disperses the gas phase into liquid as fine bubbles; it prevents liquid drainage through the gas passage at low gas rates.

bubble caps columns can be operated with a wide range of vapour and flow rates.

Bubble cap plates are now rarely used for installation because of (1) their relatively high cost.

(2) High pressure drop.

SIEVE PLATEVery common plates in use in recent year are the perforated plates because of their simplicity and of low cost.

A sieve plate is perforated plate that employs perforation for dispersing a gas into a liquid on the plate.

It is metal sheet having hundreds of round perforations init of the ranging between 3 to 12mm.

The total area of holes range from 5 to 15% of the plate area.

VALVE PLATEIt is the recent development in perforated plates. A valve plate is nothing but a perforated metal sheet wherein the perforations are covered with lift able caps.

The valves are metal discs up to about 38 mm diameter and are held in the plate by means of legs which restrict the upward motion of the caps.

The valve provides variableorifices for gas flow.

PACKED COLUMNIn industry, a packed column is a type of packed bed used to perform separation process distillation. A packed column is a pressure vessel that has a packed section. The column can be filled with random dumped packing or structured packing sections, which are arranged or stacked. In the column, liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. Packing material can be used instead of trays to improve separation in distillation columns. Differently shaped packing materials have different surface areas and void space between the packing. Both of these factors affect packing performance.

Packing ObjectiveObjectives for maximizing efficiency To spread surface area uniformly.To promote uniform distribution of vapour & liquid throughout the bedTo maximize the void space per unit column volume.To minimize friction (good Aerodynamic characteristics).To ensure uniform resistance to vapour & liquid flow throughout the bed.

Random packingRandom packing are simply dumped into the tower during installation and allowed to fall at random.

Pall Rings

Primarily made of 304SS and 316L SS metal alloys for quick replacement in kind from stock materials. 

Carbon Steel and specialty alloys, such as Monel 400 and Hastelloy C276, made are used for specific applications

Pall Rings are available in various sizes such as (mm) 16, 25, 38, and 50.

Raschig Ring made from metals

like carbon steel or very high alloys such as Monel 400 or Hastelloy C276.

Special Carbon or Graphite made are used in specific applications

Resistant to most acids, alkalis and solvents at temperatures as high as 1500 C 

Good and thermal shock resistance.

Saddle Rings

In terms of performance, i.e., low-pressure drop and high efficiency.

Large effective interfacial area,

High mechanical strength

And lower cost due to less metal than previous generations of random packing.

Available in various sizes, which give different combinations of efficiency and pressure drop

BETA RING™High Performance Random Packing High vapour capacity

and low pressure drop

High efficiency

High liquid handling capacity

Fouling resistant

CASCADE MINI-RINGS® (CMRTM)High Performance Random Packing Open side facing

vapour flow, reduces friction.

High capacity and low pressure drop

High efficiency

Fouling resistant

HY-PAKRandom Packing

More internal tongues helps in spread in surface area

Higher capacity and lower pressure drop than Pall rings

Higher efficiency compared to Pall rings for the same capacity.

Higher mechanical strength than Pall rings

FLEXIRINGRandom Packing

Good capacity and low pressure drop

Higher liquid hold-up and residence time

Versatile standard packing

ULTRA Random Packing Lowest pressure drop

and highest capacity

Highest distillation, absorption, and stripping efficiency

High strength to weight ratio

High Performance Random Packing High void fraction

&well distributed surface area.

More open shape improves liquid spreading.

Low pressure drop and high capacity

High specific heat transfer coefficient.

High strength to weight ratio.

Structured PackingStructured Packing are arranged into the tower during installation.

Sulzer Gauze Packing BX and CY

Employed in industry since 40 years for gentle distillation

High separation efficiency with low pressure drop for low liquid loads/vacuum applications

Available made of wide pallet of stainless steel, alloys and thermoplastics

GOODLOE™ (structured wire gauze packing)

Multifilament of fine

diameter wire,

knitted together to

form tube type

structure.

Available in metal,

plastic,alloyes with

teflon coating

Hyperfil (Structured Packing)

Made of

multifilament of

fine diameter wire.

Rolling the knitted

wire structure in

parallel vertical

layer.

Made of stainless

steel, copper and

corrosion resistant

wire

Montz

Katapak™-SP

Corrugated sheet

spreads in a series of

parallel planes.

Packing for reactive

distillation and trickle-

bed reactors

High separation

efficiency and high

reaction capacity

Mellapak™

Universal packing type with surface area of 250m2 /m3

Suitable for a wide range of applications for low to very high liquid loads/ vacuum to gauge pressure.

Available made of wide pallet of stainless steel, alloys and thermoplastics