Author: Mr. S. Manikandan M.E., M.I.S.T.E. Assistant Professor & Head (i/c) Department of Civil Engineering, Shanmuganathan Engineering College, Arasampatti, Pudukkottai - 622507. E-Ma…Full description
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UNITS IN THIS COURSE UNIT 1
SEPARATORS SEPARATORS
UNIT 2
TOWERS AND COLUMNS
UNIT 3
FRACTIONATION AND DISTILLATION EQUIPMENT
UNIT 4
STORAGE TANKS
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TABLE OF CONTENTS Para Page 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 properly designed towers and trays are required for the efficient operation of -a fractionation tower. 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.
•
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Describe the advantages of various types of tray designs in a fractionation tower.
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2.1
COLUMNS AND TOWERS The chemical and petroleum refining industries use a type of equipment in various refining processes known as a "Column". Columns are also called "Towers". In this unit both words will be used and there is no difference in the meaning between them. Process towers / columns are vertical, circular vessels. They vary in diameter from a few inches to over thirty feet. Columns / Towers can be a few feet to two hundred feet high. The tower diameter depends on the volume of fluid which will be processed 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 wall thickness 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 upper part 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.
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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 and re-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 light vapour components are stripped (removed) out of heavier liquids by the hot vapours that rise up through the column from the reboiler. Page 4/13
4.
Accumulator Zone This is the part of the tower that is below the trays: The heavier liquid settles here before leaving the tower.
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1. Feed Inlet Nozzles These are used to direct the feed into the proper level of the tower in order to give the most efficient operation. 2. Trays The trays are used to give maximum fluid contact inside the tower. This produces 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. The rising vapours must pass through the liquid on the trays. This gives good contact between the liquids and the vapours. 2.2
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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 the column/tower with lumps of solids. These solids are called "packing". Packing provides a large surface area which gives more contact between the fluids being purified. A column filled with packing is called a "packed column": Two of the most commonly used packings are "Raschig Rings" (pronounced rash-cig) and "Berl Saddles".
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Figure 2-3 Packings Used in Columns
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These packings are usually made of solids that do not react with the liquids. The packing is supported near the bottom of the column by a metal grating. This grating is a strong steel screen. Another metal grating fits over the top of the packing to hold the packing in place.
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 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 the petroleum industry. The two most commonly used trays are the bubble cap tray and the valve cap tray.
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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 a bubble cap tray. Hot vapours flow up the column as liquid flows down the column.
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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 downcomer to the tray below. The downcomer must go below the surface of the liquid on the tray below. Page 9/13
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. You will learn more about these in later courses.
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Figure 2-6 Typical Bubble Cap Tray
2.
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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 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.
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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 the downcomer 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 learn more about what problems can happen in later courses.
