Direct Sequence vs Indirect Sequence

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I V

Distillation Sequencing

Outline

1. Basic Concepts of Distillation

Sequence Design

2. Choice of Sequence and its

Operating Pressure.

3. Performance of Distillation

Column (Sieve tray and packed

tower)

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Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

I V.1.

BASI C CONCEPT OF

DI STI LLATI ON SEQUENCI NG

I V.1.1. I ntroduction

 Consider: Separation of a homogeneous multi-component fluid mixture into a number of products, whereas all separations are carried out using distillation only.

 I f this is the case, how to choice the distillation sequence?

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Direct Sequence vs I ndirect Sequence

direct sequence

indirect sequence

the lightest component is taken overhead in each column

the heaviest component is taken as bottom product in each column

requires less energy for both reboiling and condensation supplied by utilities

requires more energy for both reboiling and condensation supplied by utilities

component A (light material) is only vaporized once

Component A (light material) is vaporized twice

can be more energy-efficient if the feed to the sequence has a low flowrate of the light material (A) and a high flowrate of heavy material (C)

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Number of possible distillation sequences using simple columns

 The problem is that there may be signiﬁcant differences in the capital and operating costs between different distillation sequences that can produce the same products.

 I n addition, heat integration may have a signiﬁcant effect on operating costs (would be discussed next).

I V.1.2. Practical Constraints

1. Remove as early as possible:

a. A particularly hazardous component

_

safety consideration

b. Reactive or heat-sensitive component

_

to avoid problems of product degradation

c. Corrosive component

_

to minimize the use expensive material of construction

2. The main component that difficult to be condensed should be removed as early as possible

_

using refrigeration system or high pressure system

3. Don’t take the final product from the bottom of column if:

a. The component is decomposed in the reboilers (it can contaminates the product)

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I V.2.

CHOI CE OF SEQUENCE AND

I TS OPERATI NG PRESSURE

I V.2.1. Heuristics of Choice of Sequence

(Smith, R., 2005)

1. Component with

its relative volatility

close to unity or that

exhibit

azeotropic behavior

should be removed last

.

2. The lightest components

should be removed alone one by

one in column overheads (use direct sequence).

3. A component composing a large fraction of

the feed

should be removed first

.

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Example 4.2.1:

Data for a mixture of alkanes to be separated by

distillation are as follows:

 Use the heuristics to identify potentially good sequences that are candidates for further evaluation!

 The relative volatilities have been calculated on the basis of the feed composition to the sequence, assuming a pressure of 6 barg using

the Peng–Robinson

Equation of State with interaction parameters set to zero.

 Different pressures can, in practice, be used for different columns in the sequence

Solution:

Alternative- 1

Heuristic 1

: Do D/ E split last since this separation has the

smallest relative volatility.

Heuristic 2

: Favor the direct sequence:

Heuristic 3

: Remove the most plentiful

component ﬁrst:

Heuristic 4

: Favor near-equimolar

splits between top and

bottom products:

All four heuristics are in conﬂict here: Heuristic 1

suggests doing the D/E split last, and Heuristic 3

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Solution:

Alternative- 2

:

Take one of the candidates and accept, say, the A/ B split ﬁrst.

Heuristic 1

: Do D/ E split last

.

Heuristic 2

:

Heuristic 3

:

Heuristic 4:

Again the heuristics are in conﬂict

• Heuristic 1 again suggests doing the D/E split last, whereas again Heuristic 3 suggests it should be done ﬁrst.

• Heuristic 2 suggests the B/C split ﬁrst and Heuristic 4 the C/D split ﬁrst.

• There are 14 posible sequence

• This process could be continued and possible sequences identiﬁed for further consideration.

• Some possible sequences would be eliminated

Quantitative measure as other consideration



Since heuristics (qualitative procedure) can be in conflict, a

quantitative measure of the relative performance of different

sequences would be preferred



The vapor flow up the column as a physical measure can be readily

calculated. This provides an indication of both capital and operating

cost.



More vapor flow up the column, more heat duty required for

reboiler and condenser, increase the operating cost of hot utility

(steam) and cold utility (water or refrigerant)



A high vapor rate leads to a large diameter column, and also

requires large reboilers and condensers, therefore the capital cost

increases



Consequently, sequences with lower total vapor load would be

preferred to those with a high total vapor load.

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Prediction of the total vapor load



min



min

D

1 R

V





Underwood … (4.2.1)

Eq. (4.2.1) can also be written at ﬁnite reﬂux.

Deﬁning R_F _{to be the ratio} R/R_min _(typicaly R/R_min_{= 1.1):}



1 R

R

min



D

V





_F _{… (4.2.2)}

R_min can be calculated:

_



α=_{relative volatility between the key components} x_DLK=_{mole fraction of light key in the distillate} x_FLK=_{mole fraction of light key in the feed} x_DHK=_{mole fraction of heavy key in the distillate} x_FHK=_{mole fraction of heavy key in the feed} where:

Assuming a sharp separation:

• only the light key and lighter than LK components in the overhead • only the heavy key and heavier than HK components in the bottoms



D_{= distillate flow rate} where:

Combining (4.2.4) and (4.2.2):



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Example 4.2.2:

Using the data (below) for a ternary separation of benzene, toluene,

and ethyl benzene. Based on the vapor flow-up, determine whether

the direct or indirect sequence should be used!

Symbol Component

Flowrate

A Benzene 269 3.53

1.96

B Toluene 282 1.80

1.80 C Ethyl Benzene 57 1.00

Solution of Example 4.2.2:

Direct Sequence: A/ BC and B/ C



 





 





I ndirect Sequence: AB/ C and A/ B



 





 





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Direct and I ndirect Sequence

( example 4.2.2)

269 kmol/h

282 kmol/h

57 kmol/h 57 kmol/h 269 kmol/h 282 kmol/h

269 kmol/h

282 kmol/h

∑V_{=1713.8 kmol/h} ∑V=2287.4 kmol/h

Example 4.2.3:

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Solution of Example 4.2.3:

I V.3.

PERFORMANCE OF

DI STI LLATI ON COLUMN

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Distillation Tray and Packing

Distillation Tray Distillation Packing

Plate/ Tray Column vs Packed Column

Plate/ Tray Column Packed Column

Contact of vapor-liquid relatively good

chanelling and backmixing could be happen

More liquid hold-up

---Easy to be cleaned

--- Small Pressure drop, prefer to

vacuum operation

--- Cheaper for corrosive fluid

--- Prefer to small diameter

Can be used for liquid that contains solid particles

Solid particle plugs the packed

--- Foaming liquid

--- Lighter

Products can be taken from the side-stream

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---Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

I V.4.

SEPARATI ON AND RECYCLE SYSTEM

FOR CONTI NUES PROCESS

I V.4.1. I ntroduction



Do separation for some reasons:

1. to achieve product specification

2. to meet environment law



Material to be separated:

1. reactants

2. main product

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I V.4.2. Function of Process Recycles

1. Reactor conversion

:

Consider FEED

_

PRODUCT with conversion of about 95%

I ncomplete conversion in the reactor requires a recycle for

unconverted feed material.

2. Byproduct formation

Consider:

1

st

_:

_FEED

PRODUCT + BYPRODUCT

or

1

st

_:

_FEED

PRODUCT

2

nd

_:

_PRODUCT

BYPRODUCT

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FEED-Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

3. Recycling byproducts for improved selectivity

Consider:

_{If a byproduct is formed via a}

reversible secondary reaction then recycling the byproduct can inhibit its formation at source.

4. Recycling byproducts or contaminants that damage

the reactor



When recycling unconverted feed material, it is possible

that some byproducts or contaminants, such as products of

corrosion, can poison the catalyst in the reactor.

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5. Feed impurities

If the impurity has an adverse effect on the reaction or poisons the catalyst

5. Feed impurities (

continued

)

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5. Feed impurities (

continued

)



As with its use to

separate byproducts,

the purge saves the

cost of a separation,

but incurs raw

material losses.



This might be

worthwhile if the

FEED-I MPURI TY

separation

is

expensive.



Care should be taken to ensure that the resulting increase in

concentration of

I MPURI TY in the reactor does not have an

adverse effect

on reactor performance.

6. Reactor diluents and solvents.



An inert diluent such as steam is sometimes needed in the

reactor to lower the partial pressure of reactants in the vapor

phase.

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7. Reactor heat carrier.

The introduction of an extraneous component as a heat

carrier effects the recycle structure of the ﬂowsheet.

7. Reactor heat carrier (

continued

)

 This figure illustrates the use of the product as the heat carrier.  This simpliﬁes the recycle structure of the ﬂowsheet and removes the

need for one of the separators.