Seperation Pro. Lect #1 Fractionation

omar jomaa /OJT-INST

Fractionation Principle

• Fractionation- • The process of separating soluble liquid

hydrocarbon components from one another.• Feed: The stream of hydrocarbon mixture that

enters the fractionator.• Overhead Product: The lighter hydrocarbon

component(s) distilled from the feed.• Bottom Product: The original feed mixture

with the overhead product removed.

TOP PRODUCT

BOTTOM PRODUCT

Simple Distillation Process

• 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) hydrocarbons in the feed

• mixture of several liquids having different boiling points, each component has its own characteristic vapor pressure. The total vapor pressure will be the sum of the partial vapor pressures of the components in the feed. When the total vapor pressure is equal to the pressure in the space above the liquid surface in the flask (or still) the mixture will start to boil.

• 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

• Separation of components from a liquid mixture via distillation depends on the differences in boiling points of the individual components.

• Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. Therefore, distillation processes depends on the vapour pressure characteristics of liquid mixtures.

• The dew-point is the temperature at which the saturated vapour starts to condense.

• The bubble-point is the temperature at which the liquid starts to boil.

• The "Initial Boiling Point" (IBP) is defined as the temperature of the liquid when the first drop of distillate condenses into the receiver. The temperature gradually increases during distillation and the more volatile components are the first to distill. The liquid becomes richer in the higher boiling components until the last

• The liquid becomes richer in the higher boiling components until the last drop of liquid boils at the highest boiling temperature which is the "Final Boiling Point" (FBP

• Relative Volatility Relative volatility is a measure of the differences in volatility between 2 components, and hence their boiling points. It indicates how easy or difficult a particular separation will be.

1. Light liquids have low boiling point temperatures and high vapor pressures

2. heavy liquids have high boiling point temperatures and low vapor pressures

Crude oil distillation

DISTILLATION COLUMN

Molecular Wight

Crude oil or gas it is mixtures of molecules Wight

Principle distillation mixtures of pentane and hexane

Hexane heavier from pentane

Heating the mixtures Pentane has rich vapors and condensed

Sensible heat Latent heat

SENSIBLE HEAT AND LATENT HEAT

water start boils

VAPOR PRESSURE

• Vapour Pressure and Boiling The vapour pressure of a liquid at a particular temperature is the equilibrium pressure exerted by molecules leaving and entering the liquid surface.

• Here are some important points regarding vapour pressure:• energy input raises vapour pressure vapour pressure is

related to boiling a liquid is said to ‘boil’• when its vapour pressure equals the surrounding pressure

the ease with which a liquid boils depends on its volatility liquids with high vapour pressures (volatile liquids) will boil at lower temperatures the vapour pressure and hence the boiling point of a liquid mixture depends on the relative amounts of the components in the mixture distillation occurs because of the differences in the volatility of the components in the liquid mixture

PARTIAL PRESSURE

Reflux flow

• Reflux limits the amount of heavily components in the over heat product .

• Reflux absorb the heavier vapor components as the liquid flow across the tray and contact with the rising vapor .

• Reflux is used to adjust the overhead product composition ,by directly controlling the reflux rate ,excessive reflux increases the load on the tower and its cost . operation .

• Components in the Feed• The quality of the feed to the tower determines

the quality of the overhead and bottom products. For example, if it is desired to split a mixture of propane and butane into two finished products and the feed contains appreciable amounts of ethane, the propane product will not meet the specifications

• Flow rate of the feed • Temperature of the feed

COLUMNS AND TOWERS• The chemical and petroleum 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.

• Section / 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.

• 1. 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 and re-absorbs any liquid droplets from the rising vapours

• . 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.

• 4. Accumulator Zone• This is the part of the tower that is below the trays:• The heavier liquid settles here before leaving the tower

• 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.

• 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.

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

• 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

• the type of column internals tray column - 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

• 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 in Abu Dhabi. The two most commonly used trays are the bubble cap tray and the valve cap tray

TRAY COLUMNS

Tray column

Design of a Tray-Type • 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 hydrocarbons.

the layout of a typical how put tray inside tower.

• This Figure shows the layout of a typical how put 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.

the layout of a typical how put tray inside tower.

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.

•The action on the trays performs the following:

Trays action

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 .

• 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.

how the weir and downcomers direct the liquid from tray to tray

• how the weir and down comer direct the liquid from tray to tray. In this example, the level is maintained at 76 mm on the tray by the weir while the clearance between the down comer and the next lower tray is maintained at 63 mm. It is common practice for the down comer clearance to be 13 mm less than the weir height

how the weir and downcomers direct the liquid from tray to tray

The major factors affecting the level of enrichment that

occurs on each tray

• The relative of liquid and vapor present .• The temperature and pressure on the tray .• The relative volatility of the lighter components

compared to the heavier components .• The efficiency of the tray design in promoting

the vapor/liquid contact and approach to equilibrium.

Equilibrium stages

Liquid

Heavy liquid molecules

light liquid molecules

Lighter molecules concentrate in the vapor ,heavier molecules in liquid

• The number of trays required for a vessel will depend on such factors as:

1. If the products have a high relative vapor pressure, the products will be easier to separate and fewer trays will be required.

2.The purity required of the overhead and bottom products.

3.The efficiency of the action on the trays.

• 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

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.

The bubble cap tray

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.

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 .

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

Sieve trays

• 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

Valve Trays

• Installation of Trays• Trays are generally bolted into the fractionator so they can be removed for

repairs, replacement or for inspection of the vessel. They must be firmly installed to prevent surges from lifting the trays from their supports.

• Regardless of which type of tray or packing is used, the purpose is to provide intimate contact between the descending liquid and the rising vapor. Economics, product purity, cleanliness of the feed, operating pressures and temperatures, as well as other considerations enter into the choice of contacting equipment used in fractionators.

• Tray Spacing• Tray spacing is based on the mechanical design of the trays, the viscosity

of the products, velocity of the rising vapor and accessibility. The spacing must allow enough time and height for the entrained liquid droplets to disengage from the rising vapor.

Tray Spacing

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.

packed columns

• 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 "Bell Saddles". 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.

• Packings are passive devices that are designed to increase the interfacial area for vapour-liquid contact. The following pictures show 3 different types of packings. These strangely shaped pieces are supposed to impart good vapour-liquid contact when a particular type is placed together in numbers, without causing excessive pressure-drop across a packed section. This is important because a high pressure drop would mean that more energy is required to drive the vapour up the distillation column

Packing

Saddle Packing

Structure Packing

Rasching packing

Support Plates

• The bottom of the tower contains a support plate to carry the packing and distribute the flow across the packing.

• There are many types of support plates, an example being that shown in this Figure

• Support Plates

Vortex breakers at outlet liquid

DISTILLATION COLUMN

Packings versus Trays

• Packing versus Trays A tray column that is facing throughput problems may be de-bottlenecked by replacing a section of trays with packing. This is because:

• 1.packings provide extra inter-facial area for liquid-vapour contact

• 2.efficiency of separation is increased for the same column height

• 3.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 vapour and liquid are contacted.

equipment utilized in the operation of a fractionating

Preheater

1. Preheated

2. Tower

3. Condenser

4. Reflux accumulator

5. Reflux pump

6. Reboiler

• The feed heater is a heat exchanger that utilizes the heat in the bottom product after it has left the fractionate or to heat the feed entering the fractionat or. If heat from this source is not available, steam or another heating medium may be used. The feed flows through the tubes of the heat exchanger and the heating medium passes through the shell side around the tubes of the exchanger. A typical feed heater is shown in Figure 1

Pre heated the feed

Heat Exchanger

PREHEATER

• The purpose of the feed exchanger is to supply sufficient heat to the feed stream to vaporize the top product. The feed temperature should enter the feed tray at the same temperature as the mixture on the tray. When the heat from the bottom product is utilized in the feed exchanger, a bypass is installed in the bottom product stream to control the feed temperature entering the tower. Utilizing the heat in the bottom product often serves a twofold purpose in heating the feed and cooling the bottom product before it goes to storage.

Fractionator Reboiler

• Fractionator Reboiler (kettle type )• The reboiler provides the heat to partially vaporize the

fractionator bottom product. On small fractionators, the reboiler may be a steam coil located in the bottom of the tower and on larger units it is usually a separate vessel such as the Kettle type reboiler shown before.

• In this reboiler, the bottom product from the tower flows to the bottom of the reboiler and comes in contact with the hot coils which are heated by steam or another heating medium. Part of the liquid is vaporized and returns back to the tower. It is this hot vapor that passes up through the trays to fractionate the product on each tray. Stated another way, the heat drives the tower. The liquid that is not vaporized passes over the weir plate behind the tube bundle and is level controlled out of the reboiler.

• Thermal Syphon Reboiler• The thermal syphon reboiler uses convection alone

to produce circulation. The bottom product flows to the bottom of the reboiler by gravity. The addition of heat causes some of the liquid in the reboiler to vaporize and the remaining heated liquid expands. The mixture of vapor and hot liquid in the reboiler has a much lower relative density than the bottom liquid and a thermal syphon flow is produced. The flow through a thermal syphon reboiler is shown in Figure 1

KEETEL TYPE REBOILER

H.C from reheater

stabilizer

Reboiler

Flash drum

Stabilizer bottom

Over head product

H.C condensate product

Thermal Syphon Reboiler Assisted Thermal Syphon Reboiler

• The assisted thermal syphon reboiler, as shown in Figure 2, receives all the flow from the bottom tray. Flow through the reboiler is produced by the difference in static heads of the liquid to the reboiler and the liquid vapor mixture to the tower. The vapor to drive the tower passes up through the chimney in the bottom tray.

Fired Heater ReboilerLarge fractionators utilize fired heaters for reboilers. Figures 1 and 2 are examples of horizontal and vertical fired heaters. The flow through the fired heater type reboiler must be positive to prevent overheating of the tubes in the heater. The fractionator bottom pump circulates all or nearly all, of the bottom product through the reboiler. A positive flow through all passes of the reboiler is very critical and the controls must be interlocked so flow failure will shut down the burners to the heater. In some operations the fired heater will supply heat to more than one fractionator.

Vertical Fired Heater

Horizontal Fired Heater

Crude and Vacuum Heaters

Vertical Cylinderical

Heater

Two Cell Vertical

Cylinderical

Convection Section

Radiant Coils

Inside a Cabin Heater

Radiant Section Panel with Pins Welded

Ready for Ceramic Blanket Refractory.

Inside a Vertical Cylinderical

Furnace.

Forced Draft Fans

BURNERS

Convection Section without

Burner

Sootblower

Fired Heater

• Liquid-vapor contact in the top of the tower is required to purify the overhead product and to condense any bottom product that is being driven overhead. The condensing of some or all, of the overhead product is accomplished by cooling the overhead product in a heat exchanger.

• The overhead condenser may use any of the following for a cooling medium:

• 1. "Fin Fan Cooler", which is a heat exchanger containing finned tubes to increase the heating surface. Air is forced across the tubes by fans, hence the name "Fin Fan". The speed of the fans or the starting and stopping of fans, may be incorporated into the control system to regulate the overhead product temperature. Shutters on the coolers are also used to control the overhead product temperature.

OVERHEAD PRODUCT CONDENSING EQUIPMENT

Fins on tube

Performance Control of ACHEs

•Or by pass

•By pass on ACHES

• Water Cooled Condensers", in which the overhead product temperature may be controlled by regulating the flow of cooling water through the condenser. This method may be employed to condense all or part of the overhead product. When the overhead product contains light ends having a high vapor pressure, such as methane and ethane, the gases will not be condensed but will need to be pressure controlled from the reflux accumulator and be recovered as sales gas or fuel gas.

Partial Condenser

• Total Condensers• Total condensers are used to condense all the vapor

product coming from the top of the fractionator. The reflux and the condensed product are essentially of the same composition and control is maintained by regulating the amount of cooling medium passing through the condenser. Total condensers are commonly used in condensing LPG and heavier products.

Total Condenser (Flooded

Condenser

• Condenser : Condensers remove heat from a gas ,changing it to a liquid .

Chiller

• Chiller :chillers cool a liquid or gas using a refrigerant medium like propane .

Propane Chiller

• The reflux accumulator or reflux drum is that part of the condensing equipment that receives the condensed overhead product before it is pumped back into the tower as reflux or to storage as product. The accumulator pressure is usually maintained slightly below the tower pressure. The accumulator pressure corresponds to the bubble point pressure of the liquid at its temperature in the vessel. The accumulator is usually equipped with a level control, but with the flooded design it is not required

Reflux Accumulator

Reflux pump

Centerfuigel pump

• REFERENCES:• GASCO • SAIT • ARCO • API

Seperation Pro. Lect #1 Fractionation

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