distillation fundamentals week3

Ch.E 304 05/03/23

1Department of Chemicals Engineering, WEC, Pakistan

Distillation Fundamentals

Prof.Dr.G.M.Mamoor

Department of Chemical EngineeringWah Engineering College, Wah University

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FEED

TOP PRODUCT

BOTTOM PRODUCT

DISTILATION

DISTILATION

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Simple Distillation Process

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the liquid is boiled in a still while the overhead vapor is condensed in a tube or condenser surrounded by cold water. The distillate (overhead product) is collected, after condensing, in a receiver. In this simple illustration we have endeavored to separate the more volatile (lower boiling point) components by vaporization from the less volatile (higher boiling point) component in the feed


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When such a mixture is boiled, the molecules of each component will vaporize and the composition of the vapor phase will depend on the vapor pressures and concentrations of the components in the liquid phase. Since the lower boiling point components are more volatile and have higher vapor pressures, the distillate, at first, will be richer in these components. The liquid in the bottom of the still will have a higher concentration of the higher boiling point components. As the distillation process proceeds, the composition of both distillate and residue will change until all the liquid has been distilled into the receiver


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DISTILLATION COLUMN

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A Distillation Column

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Continuous Columns In contrast, continuous columns process a continuous feed stream. No interruptions occur unless there is a problem with the column or surrounding process units. They are capable of handling high throughputs and are the most common of the two types. We shall concentrate only on this class of columns.

batch and continuous columns. Batch Columns In batch operation, the feed to the column is introduced batch-wise. That is, the column is charged with a 'batch' and then the distillation process is carried out. When the desired task is achieved, a next batch of feed is introduced.

Types of Continuous Columns Continuous columns can be further classified according to:

Column Classifications

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the nature of the feed that they are processing, binary column - feed contains only two components multi-component column - feed contains more than two components

the number of product streams they have multi-product column - column has more than two product streams

where the extra feed exits when it is used to help with the separation, extractive distillation - where the extra feed appears in the bottom product stream isotropic distillation - where the extra feed appears at the top product stream

coluthe type of column internals tray mn - where trays of various designs are used to hold up the liquid to provide better contact between vapour and liquid, hence better separation packed column - where instead of trays, 'packing' are used to enhance contact between vapour and liquid

Column Classifications

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TRAY COLUMNS

Another method of providing a close contact between the materials in a column is to install trays. A tray is a metal plate that is installed in a horizontal position. They are installed at different levels inside the column. The number of trays and the space between them depends on the design of the column.

Some columns have only two or three trays. Very tall columns can have several hundred trays. The trays are held in position by support rings. The support rings are welded to the walls of the column / tower .

There are several types of trays used in industry in . The two most commonly used trays are the bubble cap tray and the valve cap tray

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Tray column

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Design of a Tray-Type A tray-type fractionator is a vertical, steel

cylindrical vessel divided into a number of horizontal sections by metal trays. The top section contains the rectifying trays and is called the rectifying section. The center section is called the feeding section and the bottom is called the stripping section.

Most large fractionators use trays or plates because they are easier to maintain and are not as subject to fouling as are packed towers. Products can be taken off at different levels (trays) if specific products are required. Trays are the most efficient method of fractionating liquid mixtures.

Design of a Tray-Type

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layout of a typical tray inside tower.

This Figure ( next slide) shows the layout of a typical tray inside tower. The bolts around the circumference are used to install the tray to brackets welded to the vessel wall. The center section of the tray can be removed without disturbing the main part of the tray so the vessel can be inspected or maintained. The weir on the right side of the tray maintains a liquid level on the tray.

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layout of a typical tray inside tower.

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1. Strips the vapors from the descending liquid to produce an acceptable bottom product.

2. Condenses the liquids from the ascending vapor to produce an acceptable overhead product.

Function of a tray

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Trays action

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Vapor /liquid contact

Tow things happen when vapor and liquid come into contact on a tray :

Some of the latent heat in the vapor is transferred to the liquid causing heavier vapor components to condense back to the liquid phase .

The heat absorbed by the liquid causes lighter liquid components to flash off and join the vapor flowing to the next higher tray .

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Liquid and Vapour Flows in a Tray Column the direction of vapour and liquid flow across a tray, and across a column. Each tray has 2 conduits, one on each side, called ‘down comers’. Liquid falls through the down comers by gravity from one tray to the one below it.. 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.

Flow Patterns on Tray

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Flow Patterns on Tray

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Equilibrium stage

Vapor

Liquid

Heavy liquid molecules light liquid molecules

Lighter molecules concentrate in the vapor ,heavier molecules in liquid

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Trays and Plates The terms "trays" and "plates" are used interchangeably. There are many types of tray designs, but the most common ones are:

Bubble cap trays Valve trays Sieve trays

Trays and Plates

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Plat/Tray Column

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The bubble cap tray

The bubble cap tray has metal caps covering the holes in the tray. Hot vapours flow up the column as liquid flows down the column.

The hot vapours pass up through the bubble caps. There are slots in the bubble caps. These slots slow down the flow of the hot vapours and make them pass through the liquid on the tray. This increases the contact between the fluids.

The liquid held on the tray covers the slots. The level of the liquid on the tray is controlled by a dam or weir. The vapour passes out of the slots as small bubbles. Small bubbles contact more of the liquid on the tray.

The liquid on the top tray of the column flows through the downcomers to the tray below. The downcomers must go below the surface of the liquid on the tray below.

Bubble cap trays are a very efficient way of allowing the lighter vapours to filter through the heavier liquids as they flow up the tower. This contact between the different fluids in the tower is an important part of the distillation process.

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The bubble cap tray

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Variations in Bubble Cap Design

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Mechanical Structure of a Bubble Cap

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Fouled Bubble Cap Tray

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Bubble caps

Bubble caps are very efficient and provide uniform distribution and action on each tray. Bubble caps provide intimate contact between the descending liquid and the ascending vapor. Reasons for using bubble cap trays are:

1. They can handle a wide range of feed rates.2. They can be used with clean or dirty liquids3. They provide uniform contact between liquid and

vapor.4. They handle large volumes of liquids and vapors.5. They are easily installed. Although bubble cap trays provide effective

fractionation, they have been replaced to a great extent by valve trays.

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Valve Trays

Valve trays are essentially modified sieve trays that may have several types of valves installed in the holes in the trays.

The valves are about 50 mm in diameter and are spaced between 76 and 152 mm on center.

The valves act like check valves allowing the vapour to rise through the trays while preventing liquid from flowing downward.

This ensures that all liquid on the trays is thoroughly scrubbed and contacted by the rising vapour.

In operations where there are wide swings in the flow to the fractionator, it is common practice to put light and heavier valves in each tray.

During low flow operation, the light valves open and when the flow increases, the heavier valves open .

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Valve Trays

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Valve Tray in position

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A tray in final Check-up

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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. Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many application

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The sieve or perforated tray has a lot of small holes in it. The vapour flowing up the column and the liquid flowing down the column pass through the holes. The vapour and the liquid come into contact as they pass through the holes. These trays are not used in processes that are difficult to control.

Sieve trays

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Sieve trays

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Tray Spacing

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trays are named

trays are named according to their function and location in the tower:

1. Top tray is the highest tray in the tower.

2. Reflux tray is the tray where reflux enters the tower.

3. Intermediate trays are the trays below the reflux tray where the overhead product is enriched.

4. Feed trays are where the raw product enters the tower.

5. Stripping trays are the trays where the bottom product is stripped of overhead fractions.

6. Seal pan is used to seal the bottom tray.

7. Chimney tray is used for liquid draw off or to maintain a liquid head to the reboiler.

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distillation fundamentals week3

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