01 Process Design Fundamental of Distillation

7/30/2019 01 Process Design Fundamental of Distillation

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Distillation Column Design

1. Specify the degree of separationrequired: set product specifications.

2. Select the operating conditions: batch orcontinuous; operating pressure.

3. Select the type of contacting device:plates or packing.

4. Determine the stage and refluxrequirements: the number of equilibriumstages.

5. Size the column: diameter, number ofreal stages.

6. Design the column internals: plates,distributors, packing supports.

7. Mechanical design: vessel and internalfittings.


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Process Design Fundamental

Continuous distillation: processdescription

Continuous distillation: basicprinciples

Design variables in distillation

Design methods for binary systems

Multicomponent distillation: generalconsiderations

Multicomponent distillation: short-

cut methods for stage and refluxrequirements

Multicomponent systems: rigoroussolution procedures (computermethods)


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Other distillation systems

Plate efficiency

Approximate column sizing

Plate contactors

Plate hydraulic design

Packed columns

Process Control


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The easy of separation depends ondifferences in volatility between thecomponents

Vapour flows up the column and liquidcounter-currently down the column.

The vapour and liquid are brought intocontact on plates, or packing.

Part of the condensate from thecondenser is returned to the top of thecolumn to provide liquid flow above the

feed point (reflux), and part of the liquid from the base of the

column is vaporised in the reboiler andreturned to provide the vapour flow.

A

B

A

B

A

B


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In the section below the feed, the morevolatile components are stripped from theliquid and this is known as the strippingsection.

Above the feed, the concentration of the

more volatile components is increasedand this is called the enrichment, or morecommonly, the rectifying section.

Virtually pure top and bottom productscan be obtained in a single column from abinary feed, but where the feed contains

more than two components, only a singlepure product can be produced, eitherfrom the top or bottom of the column.


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Reflux considerations

Feed-point location

Selection of column

pressure


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Reflux Considerations

The number ofstages required

Total reflux

stages required is the minimum

a useful guide to the likely number of stages that will be needed

Columns are often started up and tested at total reflux.

Minimum reflux

Optimum reflux ratio

infinite number of stages

o a value at minimum cost

o lie between 1.2 to 1.5 times the minimum reflux ratio


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Feed-point location

The precise location of the feed point will affect the numberof stages required for a specified separation and the

subsequent operation of the column.

As a general rule, the feed should enter the column at the

point that gives the best match between the feed

composition (vapour and liquid if two phases) and the

vapour and liquid streams in the column.

In practice, it is wise to provide two or three feed-point

nozzles located round the predicted feed point to allow for

uncertainties in the design calculations and data, andpossible changes in the feed composition after start-up.


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Selection of column pressure

The main consideration when selecting the

column operating-pressure will be to ensurethat the dew point of the distillate is above

that which can be easily obtained with the

plant cooling water.


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Continuous distillation: basicprinciples

Stage equations

Dew points and bubblepoints

Equilibrium flash

calculations


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Stage equations

All flows are the total streamflows (mols/unit time)

The specific enthalpies arealso for the total stream(J/mol).

In terms of equilibriumconstants:


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Dew points and bubble points

For multicomponent mixtures the temperature that satisfiesthese equations, at a given system pressure, must be foundby trial and error.

For binary systems the equations can be solved more readilybecause the component compositions are not independent;

fixing one fixes the other.


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Design variables in distillation

to carry out a design calculation thedesigner must specify values for acertain number of independentvariables to define the problemcompletely,

the ease of calculation will oftendepend on the judicious choice ofthese design variables.

the number of independent variables which must beset (by the designer) will equal the number that areset in the construction of the column or that can becontrolled by external means in its operation.


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Design Variables in Diltillation

A column with one feed, no side streams, a total

condenser, and a reboiler.

The number of stages above and below the feed point(2 variables)

The feed composition and total enthalpy will be fixedby the processes upstream (1 + (n 1)) variables

The feed rate, column pressure and condenser &reboiler duties (cooling water and steam flows) will becontrolled (4 variables)

Komposisi hasil perhitungantidak sesuai spesifikasi


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Feed composition

Total enthalpy of feed

Feed rate,

Column pressure


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Design methods for binary systems


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Multicomponent distillation: generalconsiderations

fixing one component compositiondoes not uniquely determine the othercomponent compositions and thestage temperature

it is not possible to specify the

complete composition of the top andbottom products independently

The separation between the top andbottom products is specified by settinglimits on two key components


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Key components

The designer must select the two key componentsbetween which it is desired to make the separation

The light key will be the component that it is desired tokeep out of the bottom product, and the heavy key thecomponent to be kept out of the top product.

Specifications will be set on the maximum concentrationsof the keys in the top and bottom products.

The non-key components that appear in both top andbottom products are known as distributed components

Those that are not present, to any significant extent, inone or other product, are known as non-distributedcomponents


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Multicomponent distillation: short-cutmethods for stage and reflux requirements

Pseudo-binary systems

Smith-Brinkley method

Empirical correlations

Distribution of non-key components(graphical method)


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Multicomponent systems: rigorous solutionprocedures (computer methods)

Lewis-Matheson method

Thiele-Geddes method

Relaxation methods

Linear algebra methods


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Number and sequencing of columns


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Number and sequencing of columns


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Other distillation systems

Batch distillation

Steam distillation

Reactive distillation


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Plate efficiency

The concept of a stage efficiency is used to link the performance of

practical contacting stages to the theoretical equilibrium stage. Three principal definitions ofefficiency are used:

Murphree plate efficiency

Point efficiency (Murphree point efficiency)

Overall column efficiency


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Plate efficiency

Prediction of plate efficiency

OConnells correlation

Van Winkles correlation AIChE method

Entrainment


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Approximate column sizing

Plate spacing

The overall height of the column will depend on the platespacing

Plate spacings from 0.15 m (6 in.) to 1 m (36 in.) arenormally used.

The spacing chosen will depend on the column diameterand operating conditions.

Column diameter

The principal factor that determines the column diameter isthe vapour flow-rate.

The vapour velocity must be below that which would causeexcessive liquid entrainment or a high-pressure drop.


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Plate Contactor

The liquid flows across the plate andthe vapour up through the plate

The flowing liquid is transferred fromplate to plate through verticalchannels called downcomers

A pool of liquid is retained on theplate by an outlet weir.


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o Sieve Tray atau Perforated Tray

o Bubble cap tray

o Ballast atau Valve Tray

o

Counter flow Tray

Tipe Tray / Plate


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Sieve plate (perforated plate)

The liquid is retained on theplate by the vapour flow.

There is no positive vapourliquid seal,

At low flow-rates liquid willweep through the holes,reducing the plate efficiency.


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Bubble-cap plates

Vapour passes up through

short pipes, called risers,covered by a cap with aserrated edge, or slots.

The use of risers ensures that

a level of liquid is maintainedon the tray at all vapour flow-rates.

Standard cap designs wouldnow be specified for most

applications.


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Valve plates (floating cap plates)

Valve plates are proprietary

designs.

They are essentially sieveplates with large-diameterholes covered by movable

flaps, which lift as the vapourflow increases.

As the area for vapour flowvaries with the flow-rate,valve plates can operate

efficiently at lower flow-ratesthan sieve plates: the valvesclosing at low vapour rates.


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Fungsi Tempat berlangsungnya proses perpindahan

Tempat terjadinya keseimbangan

Alat pemisah dua fasa seimbang

Bubble Cap


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Plate hydraulic design

Plate-design procedure

Plate areas

Diameter

Liquid-flow arrangement

Entrainment

Weep point

Weir liquid crest

Weir dimensions

Perforated area

Hole size

Hole pitch

Hydraulic gradient

Liquid throw

Plate pressure drop

Downcomer design

Provide good vapour-liquid

contact. Provide sufficient liquid hold-up

for good mass transfer (highefficiency).

Have sufficient area and spacing

to keep the entrainment andpressure drop within acceptablelimits.

Have sufficient downcomer areafor the liquid to flow freely from

plate to plate.


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Operating Range

The upper limit to vapour flow

is set by the condition offlooding.

The lower limit of the vapourflow is set by the condition ofweeping.

Coning occurs at low liquid

rates


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Packed columns

Types of packing

Packed-bed height

Prediction of the height of atransfer unit (HTU)

Column diameter (capacity)

Column internals

Wetting rates


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Choice of plates or packing

The choice between a plate or packed columnfor a particular application can only be madewith complete assurance by costing each design.

The choice can usually be made on the basis of

experience by considering main advantages anddisadvantages of each type; which are listedbelow:

1. Plate columns can be designed to handle a wider range

of liquid and gas flow-rates than packed columns.

2. Packed columns are not suitable for very low liquid rates


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3. The efficiency of a plate can be predicted with more certaintythan the equivalent term for packing (HETP or HTU).

4. Plate columns can be designed with more assurance thanpacked columns.

5. It is easier to make provision for cooling in a plate column; coilscan be installed on the plates.

6. It is easier to make provision for the withdrawal ofside-streamsfrom plate columns.

7. If the liquid causes fouling, or contains solids, it is easier tomake provision for cleaning in a plate column; manways can beinstalled on the plates. With small-diameter columns it may becheaper to use packing and replace the packing when it

becomes fouled.

8. For corrosive liquids a packed column will usually be cheaperthan the equivalent plate column.


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9. The liquid hold-up is appreciably lower in a packed columnthan a plate column. This can be important when theinventory of toxic or flammable liquids needs to be kept as

small as possible for safety reasons.

10. Packed columns are more suitable for handling foamingsystems.

11. The pressure drop per equilibrium stage (HETP) can be lowerfor packing than plates; and packing should be considered forvacuum columns.

12. Packing should always be considered for small diametercolumns, say less than 0.6 m, where plates would be difficultto install, and expensive.


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Selection of plate type

The principal factors to consider when comparingthe performance of bubble-cap, sieve and valveplates are:

cost

capacity (Dia.)

operating range

The ranking is sieve, valve, bubble-cap

Bubble-cap : valve : sieve, are

approximately 3.0 : 1.5 : 1.0.

No real distinction can be made

between them

Sieve plates give the lowest pressure

drop

The most significant factor: allow for

changes in production rate, and to

cover start-up and shut-down

conditions. Bubble-cap plates

efficiency

pressure drop


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Selection of plate type

Bubble-cap plates have a positive liquid seal and can thereforeoperate efficiently at very low vapour rates.

Sieve plates rely on the flow of vapour through the holes to holdthe liquid on the plate, and cannot operate at very low vapourrates. But, with good design, sieve plates can be designed togive a satisfactory operating range; typically, from 50 per centto 120 percent of design capacity.

Valve plates are intended to give greater flexibility than sieveplates at a lower cost than bubble-caps.

Some flexibility will always be required in an operating

plant to allow for changes in production rate, and tocover start-up and shut-down conditions.

turn-down ratio


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Feed: FC = flow control

Bottom: TC = temp. control FC =

flow control

LC = level control

FI = flow indicator

Top: PC = pressure control FC = flow control

LC = level control

FI = flow indicator

Process Control


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01 Process Design Fundamental of Distillation

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