Unit 2 Towers & Columns

7/29/2019 Unit 2 Towers & Columns

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UNITS IN THIS COURSE

UNIT 1 SEPARATORS

UNIT 2 TOWERS AND COLUMNS

UNIT 3 FRACTIONATION AND DISTILLATION EQUIPMENT

UNIT 4 STORAGE TANKS

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TABLE OF CONTENTS

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2.0 COURSE OBJECTIVE 3

2.1 COLUMNS AND TOWERS 4

2.1.1 Sections of the Tower and their Functions 5

2.1.2 Mechanical Parts of the Tower and their Functions 6

2.2 PACKED COLUMNS 7

2.3 TRAY COLUMNS 9

2.0 COURSE OBJECTIVE

Upon completion of this unit the trainee will be able to describe why properlydesigned towers and trays are required for the efficient operation of -a fractionationtower. The trainee will:

Describe the main components of a fractionation tower.

Describe the direction of flow of the liquids and vapours in a fractionation tower.

Describe the advantages of various types of tray designs in a fractionationtower.

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2.1 COLUMNS AND TOWERS

The chemical and petroleum refining industries use a type of equipment in variousrefining processes known as a "Column". Columns are also called "Towers". In thisunit both words will be used and there is no difference in the meaning betweenthem.

Process towers / columns are vertical, circular vessels. They vary in diameter from afew inches to over thirty feet. Columns / Towers can be a few feet to two hundredfeet high. The tower diameter depends on the volume of fluid which will beprocessed in the tower. The tower height is determined by:

How long the fluids in the tower need to stay together.

The time needed for the fluids to pass through the tower.

The final products leaving the tower.

The material that a column / tower is made of depends upon its use. Towers /columns are made of steel, copper alloys and stainless steel. The shell or wallthickness will depend on what pressure the vessel has to work at.

All these things are decided by design engineers.

2.1.1 Sections of the Tower and their Functions

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Figure 2-1 Typical Fractionation Tower / Column

1.Feed Section 1 Flash Zone

The tower usually has more than one inlet nozzle between the middle and the upperpart of the tower. These direct the inlet streams into the correct level of the tower.

2.Enrichment / Rectifying Section

These are the trays that are above the feed nozzle.

a. The light vapours flash out of the feed fluid and rise up the tower.

b. The reflux flows down across the trays. As the reflux flows down it traps andre-absorbs any liquid droplets from the rising vapours.

3. Stripping Section

These are the trays that are below the feed nozzle. in this section the lightvapour components are stripped (removed) out of heavier liquids by the hotvapours that rise up through the column from the reboiler.

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4. Accumulator Zone

This is the part of the tower that is below the trays:

The heavier liquid settles here before leaving the tower.

2.1.2 Mechanical Parts of the Tower and their Functions

1. Feed Inlet Nozzles

These are used to direct the feed into the proper level of the tower in order togive the most efficient operation.

2. Trays

The trays are used to give maximum fluid contact inside the tower. Thisproduces the most efficient end product.

3. Downcomers

The downcomers direct the fluid from one tray to the next lower tray in the tower.

4. Weirs

The weirs are "walls" which keep the correct level of liquid on each tray. Therising vapours must pass through the liquid on the trays. This gives goodcontact between the liquids and the vapours.

2.2 PACKED COLUMNS

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Figure 2-2 Typical Packed Column

A simple method of providing a close contact between the liquids is to fill thecolumn/tower with lumps of solids. These solids are called "packing".

Packing provides a large surface area which gives more contact between the fluidsbeing purified. A column filled with packing is called a "packed column": Two of themost commonly used packings are "Raschig Rings" (pronounced rash-cig) and"Berl Saddles".

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Figure 2-3 Packings Used in Columns

These packings are usually made of solids that do not react with the liquids. Thepacking is supported near the bottom of the column by a metal grating. This gratingis a strong steel screen. Another metal grating fits over the top of the packing tohold the packing in place.

2.3 TRAY COLUMNS

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Figure 2-4 Column Fitted With Trays

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

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

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

1. Bubble Cap Tray

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Figure 2-5 Bubble Cap Tray

The bubble cap tray has metal caps covering the holes in the tray. Fig 2-5 shows abubble cap 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 bubblecaps. These slots slow down the flow of the hot. vapours and make them passthrough 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 iscontrolled 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 downcomer to the traybelow. The downcomer must go below the surface of the liquid on the tray below.

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

You will learn more about these in later courses.

Figure 2-6 Typical Bubble Cap Tray

2. Sieve or Perforated Tray

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Figure 2-7 Sieve Tray

The sieve or perforated tray has a lot of small holes in it. The vapour flowing up thecolumn and the liquid flowing down the column pass through the holes. The vapourand the liquid come into contact as they pass through the holes. These trays arenot used in processes that are difficult to control.

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Figure 2-8 Weir and Downcomer

Figure 2-8 shows how the weir controls the liquid level on the tray. Note how thedowncomer reaches below the liquid level on the lower tray. This stops vapours

passing up the downcomer.

Design Engineers decide on the number of trays that are in a column. You will learnmore about what problems can happen in later courses.

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Unit 2 Towers & Columns

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