plant design 5 s

8/13/2019 Plant Design 5 s

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CHAPTER 5INPUT-OUTPUT STRUCTURE

OF THE FLOWSHEET


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5.1 DECISIONS FOR THE

INPUT-OUTPUT STRUCTURE


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ProcessFeedstreamsProduct

By-Product

ProcessFeed

streamsProduct

By-Product

Purge

Flowsheet Alternative


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TABLE 5.1-1 Hierarchy of decisions1. Batch versus continuous2. Input-output structure of the flowsheet

3. Recycle structure of flowsheet4. General structure of the separation system

a. Vapor recovery system

b. Liquid separation system5. Heat-exchanger network


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TABLE 5.1-2 Level-2 decisions

1. Should we purify the feed streams before they

enter the process?

2. Should we remove or recycle a reversible by-product

3. Should we use a a gas recycle and purge

stream?

4. Should we not bother to recover and recycle

some reactants?

5. How many product streams will there be?

6. What are the design variables for the input-

output structure, and what economic trade-offsare associated with these variables?

Level 2 Decisions


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Guideline

If feed impurity is not inert and it present in significantquantities, remove it

If a feed impurity is present in a gas feed, as a first guessprocess the impurity

If a feed in the a liquid feed stream is also a by-product orproduct component, usually it is better to feed the processthrough the separation system.

If a feed impurity is present in large amounts, remove it

Purification of Feed


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If feed impurity is present as azeotrope with a reactant,often it is better to process the impurity.

If a feed impurity is inert but is easier to separate fromthe product than the feed, it is better to process theimpurity.

If a feed impurity is a catalyst poison, remove it.


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PROCESS ALTERNATIVE

If we not certain that our decision is correct, we list theopposite decision as a process alternative.


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ECONOMIC TRADE-OFFS FOR FEEDPURIFICATION.

Our decision of purifying the feed streams before they

are processed involves an economics trade-off between

building a preprocess separation system and increase the

cost of process be cause we handling the increased flowrate of inert materials. Ofcourse, the amount of inert

materials present and where they will enter and leave the

process may have a great impact on the processing costs.

Therefore, it is not surprising that there is no simpledesign criterion that always indicates the correct decision.


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Recover or Recycle Reversible By-

products

Toluene+H2Benzene+CH4

(4.1-3)

2Benzene Diphenyl+H2

The reactions to produce benzene fromtoluene are

The second reactions is reversible, we couldrecycle diphenyl black to the reactor and let

it build up in recycle loop until it eventuallyreached an equilibrium level .


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Gas Recycle and Purge

Whenever a light reactant and either a

light feed impurity or a light by-

product boil lower than propylene(-55

F, -48 C), use a gas recycle and

purge stream

A membrane separation process also should alwaysbe considered


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Do Not Recover and Recycle

Some Reactant

We should recover more than 99% all valuablematerials

Since some materials, such as air and water,

normally do not bother to recover and recycleunconverted amount of these component.


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Number of ProductStreams

It is never advantageous to separate two streams and then mixthem together.

The common sense design guideline


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TABLE 5.1-3 Destination codes and componentclassifications

Destination code Component classification1. Vent Gaseous by-products and feed

impurities2. Recycle and purge Gaseous reactants plus inert gases

and/or gaseous by products3. Recycle Reactants

Azeotropes with reactants (sometimes)

Reversible by-products (sometimes)4. None Reactant-if complete conversion or

unstable reaction intermediates


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TABLE 5.1-3 Destination codes and componentclassifications

Destination code Component classification5. Excess-vent Gaseous reactant not recovered and

recycled6. Excess-waste Liquid reactant not recovered or

recycled7. Primary product Primary product

8. Valuable by-product (I) Separate destinationfor different by-products9. Fuel By-products to fuel10. Waste By-products to fuel waste treatment


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Example 5.1-1 Suppose we have the 10 componentslisted in order of their boiling points and with destinationcodes indicated. How many product streams will therebe.

A Waste F Primary productB Waste G RecycleC Recycle H Recycle

D Fuel I Valuable by-product 1E Fuel J Fuel

Component Destination Component Destination


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Solution. The product stream are

1. A+B to waste(do not separate them andthen mix them in the sewer)2. D+E to fuel (do not separate them and then

mix them to burn)3. F-primary product (to storage for sale)

4. I-valuable by product I (to storage for sale)

5. J to fuel (j must be separated from D and E

to recover components F,G,H and I, so wetreat J as a separate product stream)


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Example 5.1-2 Hydroalkylation of toluene to

produce benzene. Find the number of product

stream for the HAD process; i.e, see Example 4.1-

1.

Solution.- List all component

-arrange these components in order of

their normal boiling point

-Destination code


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Example 5.1-4 Toluene to benzene

H2 -253 C Recycle and purge

CH4 -161 C Recycle and purgeBenzene 80 C Primary productToluene 111 C RecycleDiphenyl 253 C Fuel

Component Boiling point Destination Code


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The initial flowsheet

ProcessH2, CH4

Benzene

Diphenyl

Purge H2, CH4

Toluene

Fig. 5.1-2 Input-output structure of HDA process.


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Evaluation of the Flowsheet

Be certain that all products, by products

and impurities leave the process


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5.2 DESIGN VARIABLES, OVERALLMATERIAL BALANCES, AND STREAM

COST


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TABLE 5.2-1 Possible design variables for level 2Complex reactions: Reaction conversion

molar ratio of reactant

reaction temperature and/or pressureExcess reactions: Reactants not recovered or gas

recycle and purge

Design Variables


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TABLE 5.2-2 Procedures for developing overall materialbalances1. Start with the specified production rate.

2. From the stoichiometry (and, for complex reactions, the

correlation for product distribution) find the by-product

flows and reactant requirements (in terms of the designvariables)

3. Calculate the impurity inlet and outlet flows for the feed

streams where reactants are completely removed and

recycle

Material Balances Procedure


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TABLE 5.2-2 Procedures for developing overall materialbalances4. Calculate the outlet flows of in terms of a specified

amount of excess (above the reaction requirements) for

streams where the reactants are not recovered and

recycled

5. Calculate the and outlet flows for the impurities enteringwith the reactant stream in step 4.


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Example 5.2-1 Toluene to benzene. Develop the

overall material balances for HDA process.

Solution. The reactions of interest are


2Benzene Diphenyl+H2 (4.1-3)

From Ex. 4.1-1 The desired production rate of

benzene is PB=265 mol/hr.

If use a gas recycle and purge stream for the H2andCH4 and remove diphenyl, then there are three

product stream


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ProcessH2, CH4Benzene

Diphenyl

Purge H2, CH4

TolueneFig. 5.1-2 Input-output structure of HDA process.


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SELECTIVITY AND REACTION STOICHIOMETRY

SconvertedTolueneMoles

OutletReactoratBenzeneMolesySelectivit

S

P

F B

FT

Recover and remove all this benzene. Hencefor the production PBmol/hr, the toluene fed tothe process FFTmust be

(5.2-1)


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From Eq. 4.1-3

The amount of methane produced PR,CH4 must be

S

P

P B

CHR 4

,



(5.2-2)


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From Eq. 4.1-3

The amount of diphenyl produced PD must be

2

1

2

1 S

S

PSFP BFTD



(5.2-3)


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RECYCLE AND PURGEIf we feed an excess amount of H

2

, FE

, into

the process,.

The total amount of H2fed to the process will

be

GFHB

E FySS

PF )1(

2(5.2-4)

yFHFG: The amount of H2in the makeup gasstream


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The methane flow rate leaving the process

S

PFyP BGHFCH )1(4 (5.2-5)

Methane ProducedMethane entering the

process


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The total purge flow rate PGwill then be the

excess H2, FE, plus the total methane PCH4or

S

PFyFP BGHFEG )1( (5.2-6)


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Using FEas a design variable, we nornally

use the purge composition of the reactantyPH, where

G

EPH

P

Fy (5.2-7)

0


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Adding these expressions give

2

1 S

S

PFP BGG (5.2-10)

Then solve for FG

)(

1)1(1

PHFH

PHB

G yyS

S

SyP

F

(5.2-11)


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MATERIAL BALANCE IN TERMS OF EXTENT

OF REACTION. (in term of the extent of reaction)

222

2

2

2

HDiphenylBenzene

1111

42

- CHBenzeneHToluene

16)-(5.2-comsumedHydrogen

15)-(5.2comsumedToluene

14)-(5.2producedDiphenyl

13)-(5.2producedMethane

12)-(5.22-producedbenzeneNet

21

1

2

1

21


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EXTENT VERSUS SELECTIVITY.

(5.2-18)

aSconvertedreactantlimitingtheofamountthe

componentdesiredofproductiontheySelectivit

bScompronentundesiredtheofproductionthe

componentdesiredofproductiontheySelectivit

1

21a

2-S

2

21b

2-S

(5.2-19)


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Example 5.2-2 Toluene to benzene. Develop the

expressions relating the extents of reaction to

production rate and selectivity for the HDA process.

(5.2-20)S

PB

1

BP

212-

S

S1

2

P)P-(

2

1 BB12

(5.2-21)

From Eq. 5.2-15 and 5.2-1 we find that

Also from Eq. 5.2-12 , we find that

(5.2-22)


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Stream Tables.

ProcessH2, CH4 Benzene

Diphenyl

Purge H2, CH4

TolueneProduction rate =265

Design variable: FEand x

534

12


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Compo-nent 1 2 3 4 5H2 FH2 0 0 0 FECH4 FM 0 0 0 FM+PB/

SBenzene 0 0 PB 0 0Toluene 0 PB/S 0 0 0Diphenyl 0 0 0 PB(1-S)/(2S) 0Temp. 100 100 100 100 100Pressure 550 15 15 15 465

H2 E B1.544

G H2 M

0.0036 (1 S)S 1- F F P

(1-x) 2

(1 )(1 ) F F HM FH E B HM

whereS

SF y F P y

S


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Stream Cost: Economic Potential

For HDA process

23)-(5.2($/yr),CostMat.Raw-

Valueproduct-By-ValueProductP2 E

24)-(5.2CostGasMakeup-

costToluene-PurgeofValueFuel

DiphenylofValueFuelValueBP

enzeneE

plant design 5 s

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