Lecture 3f

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Lecture 3

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A Hierarchical Decomposition for Process

Synthesis

To guide the selection of process alternatives, Douglas formalized aDecision Hierarchy as a set of levels, where more detail in theprocess flowsheet is successively added to the problem.

These levels are classified according to the following process

decisions: Level l: Batch versus continuous

Level 2: Input-output structure of the flow sheet

Level 3: Recycle structure of flow sheet

Level 4: Separation system synthesis 4a: Vapor recovery

4b: Liquid recovery

Level 5: Heat recovery network

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First level:

we consider batch processes only if at least one of the

following holds.

We must get the process operational in a few months.

The product is one where the first company to market

wins an enormous competitive advantage.

We need only a few days production for a year's supply.

We have little design information and the process is

sensitive to upsets and variations.

The product will likely have a total lifetime of one to twoyears before some other product will come out that

replaces it.

The value of the product overwhelms the cost to

manufacture it. 3


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level 2: we consider the number of raw materialand product streams and their overall relation tothe process.

We also consider the presence of by-products andinert components in the process and how they

participate in the reaction chemistry.An important question is the recovery of these

compounds. At this level, a process recycle maybe needed for the reactor, and the designer

needs to consider the addition of purge streamsto avoid the buildup of inert components or by-products.

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Level 3 Explores the recycle structure of theflow sheet and focuses more closely on the

reactor itself.

We consider the number of separate reactor

networks in the flowsheet and their

interactions through recycle streams.

We also consider the effects of reactor

conditions on the rest of the flow sheet. These

could include the effect of inerts as a diluent

in the reactor feed and the effects of

equilibrium in choosing pressure, excess

components, and adiabatic operation for the

reactor. 5


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Level 4 is divided into two decision stages: vaporand liquid recovery.Raw materials from this step will

be recycled to the reactor while products and by-products are generally processed further andremoved.

In vapor recovery, the more expensive stage, we also

need to consider the effect of purge streams and theremoval of components based on their value andtheir effect on the reactor if they are recycled.

In the liquid recovery stage, we prefer to use

distillation, as this is often the least expensiveseparation. Design decisions at this stage includesequencing of the separators and determining theiroperating conditions.

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Level 5 deals with the heat recoverynetwork (HEN) once all of the otherflowsheeting decisions have been made.

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Case study: Ethylene and Water to Ethanol

Water

Ethylene

Methane

Propylene

R

E

A

C

T

O

R

Methane(Waste)

Ethylene(Recycle)

Propylene (Waste)

Diethyl ether (Bi product)

Ethyl alcohol (Product)

Iso propyl alcohol (waste)

Water(Recycle)

535-575 K, 68 atm

5-7% conversion

W/E-4/1 but due to lowconversion per pass, we

choose small water ratio

0.6-1

10% M excess prevent coking

Excess water push

equilibrium towards ethanol

production and back to

reactant (ethanol)

Propylene to IPA 0.5-

0.7% conversion


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Maximum Potential Profit

Depending on the purity and composition of rawmaterial, product and market rate with inflations ,we calculate the maximum profit potential for thesetup.

Estimate the gross profit : Depending on themarket price of product and requirement andpurity

Calculate the cost of raw material

Profit =gross profit-cost of raw material

Equipment cost/3 + annual operating costprofit(assume payout time and depreciable life)


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BP (oC) Tcritical (oC) Pcritical (atm)

Water 100 374.4 217.6

Ethyl alcohol 78.4 243.1 63.1

Ethylene -103.7 9.6 50.7

Di ethyl ether 34.6 193.8 35.5

Methane -161.5 -82.1 45.8

Propylene -47.7 91.4 45.4

Isopropyl alcohol 82.4 235.16 47.0

Physical property data for speciesChemical formula, MW, Sp gravity, Melt Pt., enthalpy, VP (using antoine eq parameters)

M, E-M E, PL- E


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Ethylene + waste gases

Water

Diethyl

ether

Ethanol

Water+

waste

Water

Ethylene

Absorber

DC DC DCFlash

Reactor

Compressor

Compressor

4. Condensible and

non condensible

3. Recycle structure of

flow sheet

Purge

stream


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Sepn SepnREACTOR

SepnSepn

Sepn

REACTOR

REACTOR

W

ELPL

M

W

W

EL

PL

M

EL

PL

M

EL

PLM

EL

M

PL

EL

DEE

EAW

ELPL

M

DEE

EA

W

EL

MDEE

EA

W

EL

W

EL

W

EL

W

M

M PL IPA

EA

DEE

EA

DEE

EA

DEE

Alternative separation Schemes


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GENERAL PROCEDURE FOR

MATERIAL-BALANCE PROBLEMS

Procedure

Step 1. Draw a block diagram of the process.

Show each significant step as a block, linked by lines and

arrows to show the stream connections and flowdirection.

Step 2. List the available data.

Show on the block diagram the known flows (orquantities) and stream compositions.

Step 3. List all the information required from the balance.

Step 4. Decide the system boundaries.


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Step 5. Write out the chemical reactions involved for

the main products and byproducts.

Step 6. Note any other constraints, such as specifiedstream compositions, azeotropes, phase or reaction

equilibrium, tie substances

Step 7. Note any stream compositions and flows that

can be approximated.

Step 8. Check the number of conservation (and other)

equations that can be written, and compare with

the number of unknowns. Decide which variablesare to be design variables;

Step 9. Decide the basis of the calculation.


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calculate the stream flows for aproduction rate of 10,000 kg/h.


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Chemical Process Design

After economical and technical feasibility; mass and

energy balance are taken care.

Synthesis of process involves two steps

I. Individual steps are selected

II. These steps are interconnected

Leads to flowsheet structure of the process

Now the simulation of process is carried out usingmathematical models to predict the flow rates,

compositions, temperature and pressure of product

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Lecture 3f