Tower Distillation

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preface


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A fractionating column or fractionation column is an essential item usedin the distillation of liquid mixtures so as to separate the mixtureinto its component parts, or fractions, based on the differences intheir volatilities. Fractionating columns are used in small-scalelaboratory distillations as well as for large-scale industrialdistillations.Laboratory fractionating columns

A laboratory fractionating column is a piece of glassware used toseparate vaporized mixtures of liquid compounds with closevolatilities. It can also be called a fractional column. Mostcommonly used is either a Vigreux column or a straight columnpacked with glass beads or metal pieces such as Raschig rings.Image 1: Fractional distillation apparatus using a Liebig condenser

Image 1: Fractional distillation apparatus using a Liebig condenser

Fractionating columns help to separate the mixture by allowing themixed vapors to cool, condense, and vaporize again in accordancewith Raoult's law. With each condensation-vaporization cycle, thevapors are enriched in a certain component. A larger surface areaallows more cycles, improving separation. This is the rationale fora Vigreux fractionating column or a packed fractionating column.Spinning band distillation achieves the same outcome by using arotating band within the column to force the rising vapors anddescending condensate into close contact, achieving equilibriummore quickly.

As shown in Image 1, as a liquid mixture in the round bottomedflask is boiled, vapor rises up the fractionating column. The vaporcondenses on the glass platforms (known as trays or plates) insidethe column, and runs back down into the liquid below and refluxesthe upflowing distillate vapor. The hottest tray is at the bottom ofthe column and the coolest tray is at the top. At steady state

conditions, the vapor and liquid on each tray is at equilibrium.Only the most volatile of the vapors stays in gas form all the way tothe top. The vapor at the top of the column, then flows through thewater-cooled condenser, which cools the vapor down until itcondenses into a liquid distillate. The separation may be enhancedby the addition of more trays (to a practical limitation of heat, flow,etc.)


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Image 2: Typical industrial fractionating columnsImage 2: Typical industrial fractionating columnsIndustrial fractionating columns

Fractional distillation is one of the unit operations of chemicalengineering.[1][2] Fractionating columns are widely used in thechemical process industries where large quantities of liquids haveto be distilled.[3][4][5] Such industries are the petroleumprocessing, petrochemical production, natural gas processing, coaltar processing, brewing, liquified air separation, and hydrocarbonsolvents production and similar industries but it finds its widestapplication in petroleum refineries. In such refineries, the crude oilfeedstock is a very complex multicomponent mixture that must beseparated and yields of pure chemical compounds are not expected,

only groups of compounds within a relatively small range of boilingpoints, also called fractions and that is the origin of the namefractional distillation or fractionation. It is often not worthwhileseparating the components in these fractions any further based onproduct requirements and economics.

Industrial distillation is typically performed in large, verticalcylindrical columns (as shown in image 2) known as "distillationtowers" or "distillation columns" with diameters ranging fromabout 65 centimeters to 6 meters and heights ranging from about 6meters to 60 meters or more.Image 3: Chemical engineering schematic of a continuousfractionating columnImage 3: Chemical engineering schematic of a continuousfractionating columnImage 4: Chemical engineering schematic of typical bubble-captrays in a fractionating columnImage 4: Chemical engineering schematic of typical bubble-captrays in a fractionating column

Industrial distillation towers are usually operated at a continuoussteady state. Unless disturbed by changes in feed, heat, ambienttemperature, or condensing, the amount of feed being addednormally equals the amount of product being removed.

It should also be noted that the amount of heat entering the column


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from the reboiler and with the feed must equal the amount heatremoved by the overhead condenser and with the products.

Image 3 depicts an industrial fractionating column separating afeed stream into one distillate fraction and one bottoms fraction.However, many industrial fractionating columns have outlets atintervals up the column so that multiple products having differentboiling ranges may be withdrawn from a column distilling a multi-component feed stream.

DISTILLATION COLUMNS (or TOWERS)

Distillation columns or towers are constructed to behave in the sameway as a series of separate stills as discussed earlier. Each 'still' sectionconsists of a number of 'TRAYS' or contacting devices arrangedvertically above one another in the column. These trays or contactorsbring liquid and vapour into intimate contact in order to obtain the

required separation of the mixture. The height of the tower and thenumber of trays or contacting devices it contains, depends upon thepurity of the 'Fractions' required.

Columns for the distillation process can be of the following types:

1. The 'PACKED' Tower2. The 'TRAY' Tower

1. THE PACKED TOWERAs its name implies, the packed tower is a vertical, steel column whichcontains 'Beds' of packing material which are used to bring the risingvapours into intimate contact with falling liquid within the tower. Theheat added to the mixture before entering the tower partially vaporises


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the mixture and the vapours rise up the tower and begin to cool.

The liquid falls towards the bottom of the tower. At the tower bottom, ingeneral, more heat is added to the liquid by a 'Reboiler' which may besteam heated or a fuel fired furnace type.

The addition of heat here causes more vapours to rise up the column. Asthe two phases of the mixture - falling liquid and rising vapour - cometogether, light components are stripped out of the liquid and enter thegas phase while heavy components in the vapour are condensed into theliquid phase.

In this way, as the vapour rises and gradually cools, it becomes lighterand, as the liquid falls, it becomes hotter and heavier.

With this type of distillation column there is generally only a top andbottom product. The quality of the products depends upon the height ofthe tower, the number of contacting devices, the tower temperature andpressure and their control, and the velocity of the rising vapours.

The type of packing materials used, also plays a part in the separationprocess. The packing can be of such types as:

Ceramic Raschig Rings, Stainless Steel Pall Rings or Ceramic Saddles ..etc


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2. THE TRAY TYPE TOWER

This is also a tall, cylindrical column. Inside, a series of trays are placed,one above the other. The trays are used to bring the rising vapour andfalling liquid into intimate contact. Tray towers do the same job aspacked towers but they are very much more efficient in the separationprocess than packed towers and, they are also more costly. There arevarious types of tray in use and the type selected depends upon thedegree of product purity required, the type of fluids, fluid velocity and

other process parameters of the system.

The types of tray used in distillation columns are as follows:

1. THE SIEVE TRAY is simply a metal plate containing drilledholes through which the rising vapour can pass into the liquidflowing across the tray. Figure: 11


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2. THE VALVE TRAY is similar to the sieve type but, each hole isfitted with a flapper valve which opens as vapour passes throughthe hole. This type is used where vapour velocity is not constantand the valves prevent liquid from dumping through the holes attimes of low gas velocity. Figure: 12

3. THE BUBBLE-CAP TRAY is the most efficient separation devicebut, is also the most costly. It consists of a number of 'Chimneys'or 'Risers' (small, short pipes set into the tray), through which thevapour can pass. Fitted over the riser is a 'Cap' which causes therising vapour to turn through 180 . This forces the gas to'Bubble' through the liquid flowing across the tray. The liquidlevel on the tray is maintained below the top of the riser toprevent dumping of liquid down the tower. Figure: 13

Each of the above trays also has a 'WEIR' that maintains the liquidlevel on the tray. As the liquid flows over the weir, it enters a'DOWNCOMER' - (a short pipe), that carries the liquid down to thetray below. The downcomer outlet is below the surface of the liquid onthe tray below, acting as a seal to prevent gas from bypassing the trayabove.


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The liquid is prevented from dumping through the perforations by the velocity of the up-

flowing gas passing through them. The 'WEIR' maintains the liquid level on the tray andthe gas is forced to bubble through the liquid. This gives intimate contact between the gasand liquid.

With the 'VALVE' tray, a non-return valve is fitted over each hole. This will close due tothe weight of liquid at times of low gas velocity.

OPERATION OF VALVE TRAYS


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OPERATION OF BUBBLE-CAP TRAYS


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SIMPLE CONTINUOUS DISTILLATION PROCESS

Refer to Figure: 14, as you read on. This represents a basic Crude Oildistillation column where the feed to, and the products from, the unit isa continuous operation.In the distillation process, the crude oil feed is first heated byexchanging heat with some of the hot products leaving the column. Thiscools the products and, at the same time reduces the fuel requirements

in the main heater - the fuel fired furnace.The hot feed now enters the tower into the 'Flash Zone'. At this point,due to the greatly increased volume of the column, the lightercomponents of the crude oil ' Flash Off ' (vaporise), and rise up thecolumn. The hot liquid will fall towards the column bottom.


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The bottom section of the column, below the Flash Zone, called the'Stripping Section', contains trays - generally Bubble-cap or Sieve type.The tower bottom liquid is re-circulated & re-heated in a steam or fired'Reboiler' which drives off vapours of light ends and some of the heavyends contained in the liquid. These vapours rise upwards through thetrays and contact the down-flowing liquid. This action further removes(strips out), light ends from the liquid.

The top section of the tower, above the flash zone, is called the'Rectifying Section'. Here again, the rising vapour passing through thetrays, contacts the liquid flowing across them.

Action of the Trays Each tray in the tower is acting like a single still asdiscussed in 'Batch Distillation'. As we rise above the flash zone, each

succeeding tray is slightly cooler than the tray below.

The down-flowing liquid, as it passes across the trays is becoming hotterand heavier as light ends boil off into the vapour phase. Conversely, therising vapour is becoming cooler and lighter as heavier ends condenseinto the liquid on the tray.

The down-flowing liquid is called 'Internal Reflux' and works in thesame way as in Figure: 7 (Page 14), where the liquid is returned to thepreceding still.

At pre-determined points in the column, the process conditions (mainlytemperature and pressure), are such that, the liquid components are atthe required purity to meet the specification desired as a product - like'Kerosene' for example. At these points, the tower will contain'Collecting Pans' from which the desired product can be drawn fromthe tower.

The lightest components of the crude oil mixture leave the top of the

tower as vapour. This is fed through condensers - generally water-cooled -and the condensate, usually Naphtha and water, passes into the'Overhead Receiver or Accumulator'.

In the receiver, light gases also build up. The control of these gases, (to afuel system or flare), also controls the pressure on the distillationprocess at the required level.


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The Naphtha liquid forms an interface above the water. The water isdrained away under control, to disposal. The Naphtha, also under levelcontrol, is divided into two - some is returned to the tower top tray as'External Reflux' which is used to control the tower top temperatureand thereby help to control the naphtha quality.

The remaining naphtha from the receiver is piped to storage and / or toother processes. The products leaving the side of the column -called'Side-streams', are usually passed through 'Stripping Towers' where aninjection of superheated steam removes final traces of light ends to meetthe specification required for the product. The light ends and steam arepassed back into the tower.

The control of the quality of the side-stream products is generallyhelped by a controlled flow of 'Intermediate Reflux' of some of theproduct into the column just above the section producing the product.

The side-stream products pass from the stripping towers through feed /product exchangers and water coolers to storage.

The tower bottom product as already mentioned, is reheated in areboiler to remove light ends and to provide stripping gases in thetower. The final bottom product, such as heavy fuel oil is pumped awayvia feed/product heat exchangers and water coolers to storage. In somedistillation systems, superheated steam may be injected into the towerbottom to assist the stripping process.

Crude oil distillation is often carried out under vacuum conditions. Thevacuum is produced by pulling the overhead vapour from the tower bysteam ejectors via surface condensers.

The explanation of crude oil separation given above, is that of a basic

system. Crude oils also produce chemicals, waxes, gasolines, lubricantsand many other products in everyday use.

DISTILLATION COLUMN CONTROL


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1. REFLUX RATIO2. TEMPERATURE GRADIENT

1. REFLUX RATIO

The reflux to a tower top is used to control the toptemperature thereby controls the purity of the overheadproduct.

The amount of reflux compared to the product is knownas the 'Reflux Ratio'.

An example of this is as follows:

The overhead liquid from a distillation column isdivided into 4 m3 per hour reflux and 2 m3 per hourproduct. Therefore:

When reflux ratio is increased, the amount of refluxincreases. Reflux represents cooled, condensed topproduct returned to the tower top and, as such it isbeing reprocessed. The top product will therefore bepurer. In general, the higher the reflux ratio, the fewerthe number of trays required for a given separation.

However, too high a ratio may cause flooding in thetower resulting in poor separation and causing 'off-spec' products throughout the system. The reflux rate isnormally controlled by a temperature controller in thevapour outlet which operates a control valve in the


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reflux pump discharge. An increase in tower toptemperature will cause the valve to open, increasing thereflux rate, and vice versa.

2. TEMPERATURE GRADIENT

Temperature (and pressure) control of a distillation tower will governthe purity of the products. The control of top temperature is asdiscussed above in 'reflux'. Control of the feed inlet temperature andthat of the reboiler are also very important.

Again, if feed and bottom temperatures are too high, too much heavyvapour will rise up the tower and put side-stream products off-spec.

This condition, combined with high reflux rate will again lead toflooding and poor separation. Opposite conditions can lead to liquidstarvation across the trays and again, a very upset process will result.

The careful control of top temperature, feed and reboiler temperatures,together with pressure control, will give the desired temperature profileacross the tower.

Remember, changes in pressure will affect the boiling points of thecomponents in the crude oil. The vapour pressures therefore, will alsobe affected and again, if the control parameters are incorrect, thesystem will be inefficient.

Examples:

High top temperature will result in heavy components in theoverhead product.

Low top temperature will result in a lighter top product. High feed temperature will give heavier side-streams and vice-

versa. High reboiler temperature will produce heavier bottoms product

and pass heavier vapours up the tower to affect the side-streams. Increased pressure in the system will give lighter components in

the overhead LIQUID product and decrease its Initial BoilingPoint, whereas the FBP is governed by the tower top temperature.


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It can be seen that careful, accurate control of the variables is veryimportant in order to achieve the required quality control of theproducts.

Also, with regard to the purity of the side-streams, control of thestripping towers' steam supply is very important.

A further point is, that high water content in the crude feed will causepressure surges as the water vaporises in the tower. The crude oilshould be as water free as possible.

Many modern distillation units are operated under high vacuum. Thismethod, due to the vacuum decreasing the BP's of the components of themixture to be separated, also reduces the amount of heat energy needed

to vaporise the components.

VACUUM DISTILLATION TOWER OVERHEADSSYSTEM


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In the above diagram, the surface condenser is a 'Total Condensing'

unit. This means that all fluids that can be condensed are changed toliquid. Due to this, a vacuum is formed in the tower the level of whichdepends upon the degree of condensation allowed to take place. This isgoverned by the level of distillate and how much of the condensingsurface is covered. The liquid level and therefore the amount ofcondensing surface available will decide the level of vacuum (AbsolutePressure) of the system.

The PRC is therefore controlling the available condensing area on the

cooling tubes. Increasing absolute pressure (decreasing vacuum), willopen the control valve, the liquid level in the condenser will fall thuspresenting more condensing area to the vapour. More vapour willcondense and therfore the pressure will drop back again - and vice-versa.

A small quantity of uncondensible gases will tend to build up in the


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tower and the surface condenser which, if allowed to build up, willslowly destroy the vacuum.

The ejector is used to remove the uncondensibles which are passed intothe separator after passing through the ejector condenser whichcondenses the ejector steam thus helping to maintain the vacuum,while the uncondensed gases are fed via a check-valve or control valveto atmosphere or flare system. The barometric seal loop holds a head ofliquid which will prevent the vacuum pulling gases back out of theseparator.

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Analysis of the 1.5 nm Sphalerite-CdS Core of[Cd2S14(SC6H5)36DMF4], NANO LETTERS, 2, 375-379 , 2002[5]. H. Inoue, H.; Ichiroku, N.; Torimoto, T.; Sakata, T.; Mori, H.;Yoneyama, H. Langmuir, 10, 4517, 1994[6]. Gaumet, J. J.; Strouse, G. F. J. Am. Soc. Mass Spectrom. 2000, 11, 338.[7]. Trager, J. C. Int. J. Mass Spectrom., 200, 387, 2000[8]. Plattner, D. A. Int. J. Mass Spectrom., 207, 125, 2001[9]. Pryzybylski, M.; Glocker, M. O. Angew. Chem., Int. Ed. Engl., 35, 806,1996[10]. H. Inoue, N. Ichiroku, T. Torimoto, T. Sakata, H. Mori, H. .

Yoneyama, Langmuir, 10, 4517, 1994[11]. M.A. Hines, P. Guyot-Sionnest, J. Phys. Chem. B, 102, 3655, 1998[12]. J.R. Sachleben, V.L. Colvin, L. Emsley, E.W. Wooten, A.P. .Alivisatos, J. Phys. Chem. B 10210117, 1998[13]. M. Tomaselli, J.L. Yarger, M. Bruchez, R.H. Halvin, D. DeGraw, .A. Pines, A.P. Alivisatos, J. Chem. Phys. 110 8861,1999[14]. J.R. Sachleben, E.W. Wooten, L. Emsley, A. Pines, V.L. Colvin, .


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A.P. Alivisatos, Chem. Phys. Lett. 198 431,1992[15]. X. Peng, J. Wickham, A.P. Alivisatos, J. Am. Chem. Soc. 120 5343,1998[16]. R.J. Arnold, J.P. Reilly, J. Am. Chem. Soc. 1201528, 1998[17]. N. Herron, J.C. Calabrese, W.E. Farneth, Y. Wang, Science 259,1426, 1993[18]. Jean-Jacques Gaumet and Geoffrey F. Strouse , Electrospray MassSpectrometry of Semiconductor Nanoclusters: Comparative Analysis ofPositive and Negative Ion Mode, J Am Soc Mass Spectrom, 11, 338344,2000

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Tower Distillation

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