distillation coulmn internals, principles and operation

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Introduction to Petroleum Refinery (Static Equipment)Click to edit Master subtitle style

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Contentsn n n n n

Towers (Distillation) Heat Exchangers (Shell & Tube) Boilers & Furnace Separators Valves

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ATMOSPHERIC DISTILLATIONEng. Ahmed Deyab Fares3 /14

1. Fundamentals of Separation Conten 2. Tower Internal ts

Crude Distillation4.

3.

Operatio n

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1.FUNDAMENTALS OF SEPARATION IN TOWERS

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Distillati onSeparation by distillation implies a difference in boiling points of two or more materials We separate many things by detecting a difference in a physical properties color, size, weight, shape5/22/12 666 /14

The components or compounds making up crude oil are numbered in thousands Many of these components have similar physical properties including boiling points that may differ by only a few degrees. Therefore, it is difficult to separate some pure compounds from the complex mixture of components in crude oil by distillation alone There are other methods of separation used in a refinery for example, extraction with a solvent, crystallization, and absorption Fortunately, rarely need pure compounds and it is often enough to separate groups of compounds 77 5/22/12 7 /14 from each other by boiling range

If we separate many compounds in crude oil into groups we find that these groups have characteristics that make them considerably more valuable than the whole crude oil Some of these groups are products Some may be feedstock to other processing units where they are chemically changed into more valuable products These products, in turn, are separated or purified by distillation5/22/12 888 /14

usually

Principles Of DistillationThe basic principle of distillation is simpleq

When a solution of two or more components is boiled The lighter component ( the one most volatile or the one with the greatest tendency to vaporize ) vaporizes preferentially5/22/129 /14

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Tow component mixture is contained in a vessel When heat is add until the more volatile material ( red dotes ) start to vaporize. Now the vapor contains a higher proportion of red dots than dose the original liquid

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It is important to note that an equilibrium in composition will be established

at a given temperature and pressureBy equilibrium we mean there is a given

concentration as red dots" in the vapor and in the liquid depending upon the original concentration of each component in the liquid and their respective 1111 5/22/1211 /

This results in the vapor above the liquid being relatively rich in the lighter ( more volatile material ) And the liquid is left with proportionately more of the less volatile ( heavier liquid ) Now, let's develop this simple distillation concept somea practical Thus a separation, to into degree, operation as it has taken place is used in the refinery5/22/1212 /14

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By cooling the over head vapor, we condense and remove it from the original mixture Thus to have made a partial separation, partial because you will note that there are a few 5/22/12 blue dotes"

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This

has

occurred and

because pressure

at we

the are

temperature

conducting the distillation, the heavier component extent This is because the components of interest in a given distillation usually14 /14

still

vaporizes

to

some

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Therefore, to purify the distillate product, we may have to conduct a second distillation as shown Obviously, we can continue to cascade these simple 5/22/12 distillations until we achieve

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The distillations depicted so far are those we call patch, and normally practical in the refinery, although it is done frequently in the laboratory Let us make our distillation equipment look more like refinery pieces of equipment and let us make continuous1616 5/22/12 instead of patch operation 16 /14

This is called Flash Vaporization As shown. The liquid is pumped continuously through a heater and into a drum where the pressure is lower The lighter material flashes instantaneously (vapor and liquid flow from the drum continuously) The same system is shown 5/22/12 diagrammatically in the in the17 /

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Suppose we have 50% of the charge taken overhead That is, we set the temperature and the pressure of the system in such a way that half the charge is boiled off And further, suppose the resulting

overhead product does not contain the desired concentration of the lighter product5/22/1218 /

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Suppose we add two more stages of distillation Although this is accomplishing our goal of increasing the purity of the light friction, we are also making large amounts of the intermediate product, each of which 5/22/12 contains the same /14 19

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An obvious simplification in equipment can be made if we allow the hot vapor from the stage above the next higher (the intermediate product) This Now eliminates we5/22/1220 /14

the the

need

for

the

intermediate condensers and heaters have continuous, multi-stage distillation2020

Tower We have described Sections staging for the purpose ofconcentrating the lighter component in the overhead The same principles apply to concentrating the heavier component in the bottom product The upper stages are called rectifying stages5/22/1221 /14

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q

The upper rectifying section increases the The lower stripping section increases the In many cases, the bottom product is the one For the bottom, or heavy, product the

purity of the overhead product.q

recovery of the overhead product.q

of primary interestq

rectifying section improves recovery5/22/1222 /14

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A stage, is defined as :

Equilibrium stage, or more specifically, Stage

an equilibrium

Any portion of the distillation column such that the liquid and vapor leaving it have composition in equilibrium with each other. By definition, then, a stage should be designed in such a way as to provide intimate contact, or mixing, of the rising vapor and the descending liquid. The concept of an equilibrium stage is converted to an actual 2323 5/22/1223 /14

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Column Internals :The plates or trays are contacting devices and used to hold up the liquid to provide better contact between vapour and liquid, hence better separation .1. Trays /Plates 2. Packings

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Sieves

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Sieve Trayn

This tray is a sheet of light metal with a large number of holes drilled through it. Vapor rising through the holes keeps the liquid on the tray and bubbles up through it. The overflow weir keeps a constant depth of liquid on the tray.28 /14

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A sieve tray is: Inexpensive, Easy to clean, and Maintains good liquid and vapor contact as long as it is operated at its design load. Because the sieve tray has fixed openings and does not have covers over the holes, it does not perform well if tower loads are constantly changing. 292929 /17 /14

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Bubble Cap / Valve traysIn valve trays / Bubble Cap trays, perforations are covered by liftable caps. Vapour flows lifts the caps, thus self creating a flow area for the passage of vapour. The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing

Valve Trays Sieve Trays Bubble Cap33 /14

TraysBubble Cap TrayThe vapor is broken into small bubbles which increases the surface area for vapor-liquid contact.

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The bubble cap sits on top of a riser. The riser channels vapors bubble3434 cap. 34 /17 /14 into the

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Valve TrayA valve tray has a variable opening for vapors to flow through. The hole has a cover that consists of a cap held in place by guides which go down through Master subtitle style Click to edit the plate, or tray and hook underneath it.

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When there is no vapor flow, the caps sits over the hole and close it. Under low pressure the cap start to rise. As the flow of vapors edit Master subtitle style Click to increases, the cap rise until it is stopped by the guide tabs.

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The valve tray is similar to the babble cap tray. Both are more adaptable to variations in tower loads than a sieve tray. The valve trays Click to edit Master subtitle style and bubble cap trays are designed to perform well with variable tower loads.

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Valve Tray

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Selection of Tray TypeThe principal factors to consider when comparing the performance of bubblecap, sieve and valve trays are: Cost, Capacity, Operating range, Maintenance and Pressure drop.

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Cost:Bubble-cap trays are appreciably more expensive than sieve or valve trays. The relative cost will depend on the material of construction used; For mild steel the ratios, bubble-cap: valve: sieve, are approximately

3.0 : 1.5 : 1.0

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Capacity:There is little difference in the capacity rating for the three types (the diam

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