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Structure & Synthesis of Process Flow Diagram

Chapter 2

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The Structure & Synthesis of Process Flow

Diagram

2.1 Hierarchy of process Design

2.2 Step 1 : Batch process vs. Continuous

process

2.3 Step 2 : Input / output structure of the process

2.3.1 Process concept diagram

2.3.2 I/O structure of PFD

2.3.3 I/O structure & other features of generic BFD

2.3.4 Other consideration for I/O structure of PFD

2.3.5 Information determined using I/O diagram:

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2.4 Step 3 The recycle structure of the

process

2.4.1 Efficiency of raw materials usage

2.4.2 Identification & definition of recycle

structure of process

2.4.3 Other issues affecting recycle structure

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2.1 Hierarchy of process Design

• Choose between alternatives according to

( raw material, complexity, environment,

byproducts…..etc.)

• New process decide whether :

1.Batch or continuous ?

2.Identify I/O structure of process

3.Identify recycle structure of process

4.Identify & design structure of separation

system

5.Identify & design heat exchangers & heat

recovery system

• Old process to simplify PFD start from 5 to

1 .

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2.2 Step 1 : Batch process vs. Continuous process

1. Batch process finite quantity of product

( batch ) is produced during a time period

• Different products are made from the same

facility

2. Continuous process raw materials are fed

continuously & products , wastes , by products

are removed continuously.

• There is a batch process and batch unit

operation

3. semi-batch process both batch & continuous

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Batch vs Continuous

1.Size or economy of scale favors continuous

Batch < 500 t / y , Continuous > 5000 t /y

2. Product Quality: Batch is better for

verification of each batch of products ( as

required by FDA regulation)

3. If off quality product can be dumped , stored or

reprocessed then continuous is preferred

4. Flexibility : Batch can use same equipment

for many operations , while continuous is built for

a fixed product.

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5. Process efficiency : batch requires

scheduling , not optimized & utility usage is

higher

6. Maintenance / Labor : Cost of labor for

batch is higher

7. Feedstock availability : Batch favors

seasonal products such as wineries. Continuous

for those which require massive storage

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8. Rate of reaction : Batch favors reactions with

long reaction rate e.g. fermentation , aerobic ,

anaerobic

9.Fouling : Batch is favored for big fouling since

cleaning is easy . for continuous Process use

two units in parallel

10. Safety : batch safety is less , less

automation , training of workers is essential

11.Controllibility : less for batch

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2.3 Step 2 - Input /output structure of the process

2.3.1 Process concept diagram

• Balanced chemical reaction forms basis for

process concept diagram as in Fig 2.1

• Steps to create diagram are :

1. Cloud around chemical reaction with

stoichiometry

2. Reactants Streams with names entering cloud

from left

3. Products Streams leaving cloud to right

4. Side reactions are represented & by-products

are drawn

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2.3.2 I/O structure of PFD

• Inputs enters from left & products leaves at

right

• PFD contains stream No , utilities , recycle

streams

• Fig 2.1 & Fig 2.2 have same I/O structure

• Inert with feed and other chemicals are fed

from left

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• Products either enter with feed or produced

by reaction

• Utility streams rarely interact with process

stream Fig 2.3 .

• Utilities to add or remove thermal energy or

work

• Other utilities :

fuel gas , air , instrument air , nitrogen ,

electricity ,plant air , process water

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2.3.3 I/O structure & other features of generic BFD

• Generic BFD is intermediate between

concept diagram & BFD

• BFD contains I/O & all logical building blocks

of process

• Generic BFD in Fig 2.4a shows process broken

to 6 blocks:

1. Reactor feed preparation : change feed

from storage to reaction conditions (T , P,

concentration ..)

2. Reactor

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3. Separation feed preparation ( T , P , Conc )

for separation

4. Separator ( unit operation for separation )

5. Recycle ( unreacted inert , pump or heat

exchanger )

6. Environmental control (dispose of gas ,

liquid ,solid waste)

Environmental treatment is not included in

BFD even though they are there & important

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1.Reactor feed Preparation block :

Prepare feed from storage to reactor conditions

2. Reactor Block:

Reaction to give products , by-products ,

unreacted raw materials , inert

3. Separation Feed preparation Block:

Prepare for conditions required in effective

separation

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4. Separator Block:

Separation of products , by-products , unreacted

Raw materials , inert

5. Recycle Block:

recycle unreacted raw materials after separation

back to reactor

Environmental Control Block:

Gas , liquid , solid wastes treated before

discharge

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2.3.4 Other consideration for I/O structure of PFD

1. Feed purity and trace components :

a. If in small quantities & “ inert ” – do not

separate

e.g. H2 feed contains 5% CH4 ( inert) so do not

remove CH4

b. If separation of impurities is difficult – Do not

separate

Liquids form Azeotrope which are difficult to

separate

e.g. (water / ethanol)

• Gases require high P & low T to separate .

Both are expensive e.g. separation of CH4 from

H2

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c. If impurities foul or poison catalyst then

separate

e.g. Sulfur poison Group VIII Metals (Pt , Pd ,

Ru , Rh)

d. If impurity reacts to form difficult-to-

separate material or hazardous product then

separate

• E.g. Phosgene COCl2 : H2 in CO produce HCl

which is corrosive & affects isocyanate product

• If impurity is in large quantities then purify.

This reduce cost of oversized equipment .

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2- Addition of feeds to stabilize product or enable separation

• Product quality often met by single train of

operations

• If product reactive or unstable chemicals are

added to stabilize it.

• The same for feeding solvents & catalyst to

process.

• Feed Water to absorb gases is another feed

• These feeds are included in mass balance &

cost

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4- Inert feed material to control exothermic reaction

• High exothermic reactions such as partial

oxidation of H/C

• Steam is fed to absorb heat, dilute H/C &

prevent explosion in partial oxidation of

propylene

• Steam is regarded as a feed and leaves as a

product , a by-product or waste stream

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3- Inert feed to control equilibrium reaction

• e.g. ethyl benzene to styrene

C6H5CH2CH3 C6H5CH = CH2 + H2

• Reaction at equilibrium, T = 600-750 0C,

endothermic & at P < 1 bar .

• Super heated Steam is added to provide

heat , dilute feed

• It shifts equilibrium to the right by Le

Chatelier’s principle

2.3.5 Information determined from using I/O

diagram

1. Carry Basic economic analysis based on profit

margin

2. What chemicals enter with feed & what leave

with product

3. All reactions desired & undesired

Profit margin = value of products – cost of

raw materials

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• Profit margin should be averaged over many

years

• Profit margin for HDA process is negative

• Disproportionation reaction to produce

benzene & mix of xylenes from toluene is

unprofitable but better than HAD

• So cost of raw material decides which

process to use

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Example 2.1

Evaluate profit margin for HDA process. From

Tab 8.3 & 8.4, the following prices for raw

materials & products are found:

• Benzene = $ 0.919/ kg

• Toluene = $ 1.033/ kg ,

• Natural gas ( methane and ethane, MW = (18)

= $ 11.10/ GJ = $ 11.89/ 1000 std. ft3 = $

0.302/ kg,

• Hydrogen = $ 1.000/ kg ( based on the same

equivalent energy cost as natural gas)

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• 1 kmol of toluene feed as a basis Cost of Raw

Materials

• 92 kg of Toluene = ( 92 kg)($ 1.033/ kg) = $

95.04

• 2kg of Hydrogen = ( 2 kg)($ 1.000/ kg) = $

2.00

• Value of Products

• Benzene = ( 78 kg)($ 0.919/ kg) = $ 71.68

• Methane = ( 16 kg)($ 0.302/ kg) = $ 4.83

• So

Profit Margin = (71.68 + 4.83) – (95.04

+ .00)= –$ 20.53

Or = –$ 0.223/ kg toluene

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• Based on this result, it is concluded that further

investigation of this process is definitely not

warranted.

• Despite the results illustrated in Example 2.1,

benzene has been produced for the last 50

years and is a viable starting material for a

host of petrochemical products.

• So try another route by dispropotionation

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Example 2.2

Evaluate profit margin for toluene

disproportionation process2C7H8 = C6H6 + C8H10

Toluene Benzene Xylene

From Table 8.4: Price Mixed Xylenes = 0.820 $/kg

Using 2 kmols of toluene feed as a basis:

184 kg of Toluene = (184 kg)($1.033/kg)=

$190.07

78 kg of Benzene = (78 kg)($0.919/kg) = $71.68

106 kg of Xylene = (106 kg)($0.820/kg) = $86.92

Profit Margin = 86.92 +71.68 – 190.07=–

$31.47

= –$0.171/kg T

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• Benzene via disproportionation of toluene

unprofitable but better than by HDA process

• purified xylenes more valuable than mixed

xylene

• Addition of purification to disproportionation is

profitable

• Toluene disproportionation is the preferred

process for benzene production over the last

two decades

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2.4 Step 3 The recycle structure of the process

1 Efficiency of raw materials usage

• Cost of raw materials constitute 20-75% of

operating cost, so unreacted materials are

recovered and recycled

• Recycle if recovery & recycle is easy &

economic

1. Single pass conversion = reactant

consumed in reactor / reactant fed to reactor

2. Overall conversion = reactant consumed in

process/ reactant fed to process

3. Yield = mole of desired product / moles of

limiting reactant

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HDA : single pass=75% , overall= 99.3%, yield

= 99.95%

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1. High single pass conversion reduces recycle

and cost of equipment

2. Overall conversion is usually high in

chemical processes because of recycle

3. Yield is high if no side reactions exist

4. H2 conversion = 11.3% which is low because

Excess H2 is needed to prevent deposits of

coke on catalyst

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2. Identification & definition of recycle structure of process

3 possibilities of recycle structure :

1. Separate & purify unreacted raw materials

from product

2. Recycle feed & product together and use a

purge system

3.Recycle feed & product together but

without a purge

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1. Separate products & purify :

• Choice of Unit Operation to separate depends

on conditions such as T , P

• Also it depends on differing physical or

chemical properties of materials to be

separated

• E.g. benzene from toluene Economic to

choose distillation

• H2 from CH4 can be separated by distillation ,

absorption , adsorption & membrane

separation

• Economic consideration favors adsorption &

membrane

•

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Example 2.3

Separation of methane and hydrogen

1-distillation:

• Normal boiling point of CH4 = - 161° C , H2 = -

252° C Separation by distillation easy due to

large difference in boiling points of two

components

• However, in order to obtain a liquid phase, a

combination of high pressure & low

temperature must be used

• This is costly so distillation is not best for this

separation

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2- Absorption :

• Possible to absorb or scrub methane from

Streams 8 &17 into a hydrocarbon liquid

• In order to determine which liquids are

suitable for this process, solubility parameters

for CH4 & H2 in different liquids must be

determined

• Because of the low boiling point of CH4 it would

require a low temperature & high pressure

for effective absorption.

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3-Pressure Swing Adsorption :

• Adsorption affinity of mole to adhere to a solid

material

• In pressure- swing adsorption, preferential

adsorption of one species from gas phase at

given P & desorption is facilitated by reducing P

& allowing the solid to “ de- gas.”

• 2 beds operate in parallel, one adsorbing ,

other desorbing

• H2 in gaseous H/C separated by P- swing

adsorption

• CH4 is preferentially adsorbed while H2 leaves

• This separation could be applied to the

HDA process.

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4- Membrane Separation :

• Commercial membrane processes are available

to purify H2 from H/C

• H2 pass more readily through certain

membranes than CH4

• Process occurs at moderate P, consistent with

HDA process

• However, H2 recovered is at low P & it have to

be recompressed prior to recycling

• This separation could be applied to the

HDA process

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Example 2.4

• What process should be used in the separation

of toluene and benzene?

• Distillation Normal boiling point of benzene =

79.8° C Normal boiling point of toluene = 110°

C

• Separation should be easy using distillation,

and neither excessive temperatures nor

pressures will be needed

• This is a viable operation for this separation of

benzene & toluene in the HDA process.

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2. Recycle feed & product together with purge system

• Separation not easy: recycle product &

unreacted raw material

• Purge when product does not react to avoid

accumulation

• Recycles in HDA : CH4 product & unused H2

• There is a purge in this system

• This is represented by Fig 2.5

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3. Recycle Feed & Product Together without a Purge Stream

• Feasible only when product can react further in

a reactor

• If product does not react & leaves it would

accumulate

C7H8 + H2 C6H6 + CH4

1. Small amount of Toluene reacts to form light

H/C

• does not affect separation . They are flashed

with H2& CH4

2. Also small amount of diphenyl is formed as by

product

• If recycled without separation it builds up to

equilibrium then decompose to benzene by

equilibrium reaction

2C6H6 C12H10 + H2

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How does excess reactant affect recycle

structure ?

• Decide which reactant in excess in reaction for

recycle .

• In HAD : H2/ Toluene is 5:1 to prevent coking

• so H2 recycle is large requiring large circuit &

large turbine

How many reactors are required ?

1. Equilibrium :

Many reactors are required for the reaction

N2 + H2 NH3

• Stage packed bed with feed of reactants

• This lower T & increase concentration thus shift

equilibrium to right

2. Temperature control :

• Mild exothermic or endothermic

• Add cold or hot shots between stages to

control Temp 38

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3. Concentration control :

• If one reactant forms a by-products , keep its

concentration low by using multiple feeds

/reactants.

4. Optimization for multi-reactions:

• Series reactions A R S T

• If T is desired product use multi-stage

reactor

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Do unreacted RM need to be purified prior to recycling ?

• If in distillation relative volatilities are each

near to each other there is no need to separate

Is recycling of an inert warranted ?

1. Depends on process .

2. May be worth to recycle

3. E.g. water to absorber in acetone production

4. Water was purified and recycled to prevent

pollution

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2.4.3 Other issues affecting recycle

structure

1- How many potential recycle stream are

there ?

• All processes with single pass conversion < 90

- 99% are potential for recycling

This Depends on :

1. cost of raw materials

2. cost to separate & recycle

3. cost of disposal as waste

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2. Can recycle shift equilibrium to produce

less unwanted product ?

• Recycle unwanted product or inert to change

conversion or selectivity of equilibrium

reaction

• Recycle CO2 in water gas shift reaction to

produce synthesis gas ( CO + H2) shifts

equilibrium to left & produce more CO

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3. Recycle of unwanted product or inert to control reactor operation

• Inert added in partial oxidation of H/C to

absorb heat & move mixture outside explosive

limit .

• Steam is most often used material, so any

available inert

• Steam is added to moderate T in coal

gasification

• Recycle CO2 in partial oxidation of coal to

absorb heat & reduce steam

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4. What is the phase of the recycle stream ?

• Phase of recycle determine separation &

recycle

1. For liquids problem of azeotropic mixtures

2. For gas problem of high P or low T for

separation , both options are expensive

• Avoid gas separation unless necessary

because of cost & difficulty .

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Summary

• First choice is whether to use batch or continuous• Continuous chemical processes have a

general structure:1. Input-output2. Recycle3. Separation

• I/O structure of a continuous chemical process consists of:4. Reactor feed preparation5. Reactor6. Separation feed preparation7. Separation8. Recycle9. Environmental control

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SHORT ANSWER QUESTIONS

1. What are the five elements of the hierarchy of process design?

2. What are the three types of recycle structures possible in a chemical

process ? Explain when each is used.

3. Give three criteria for choosing a batch process as opposed to a

continuous process.

4. When would one purposely add an inert material to a feed stream?

Illustrate this strategy with an example, and explain the advantages (and

disadvantages) of doing this.

5. In general, when would one purify a material prior to feeding it to a

process unit ? Give at least one example for each case you state.

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Example 2.5

• Consider the following two process alternatives

for the HDA process when side reaction of

benzene to form diphenyl occurs.

Alternative B shown in Figure E2.5( b)

• Additional separator is required, as a second

distillation column T- 102 along with associated

equipment (not shown) & extra utilities to carry

out separation

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Alternative A Fig E2.5( a)

• Cost of additional equipment is avoided

• but recycle stream ( S 11) will be larger

because it now contains toluene & diphenyl &

utilities and equipment through which this

stream passes ( H- 101, E- 101, R- 101, E- 102,

V- 102, V- 103, T- 101, E- 106) will all be larger

• Which is the economically preferable

alternative?

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• Answer depends upon value of equilibrium

constant for benzene- diphenyl reaction

• If equilibrium conversion of benzene is high,

then there will be a large amount of diphenyl

in recycle & costs to recycle will be high & vice

versa

• Equilibrium constant :

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• Exit conditions of reactor estimated by

assuming that benzene - diphenyl reaction has

reached equilibrium

• Using this assumption & data from Table 1.5

for Stream 9, if x kmol/ h of diphenyl is present

in reactor effluent, then:

• Solving for the only unknown gives x = 1.36

kmol/ h

• Thus, toluene recycle, Stream 11, will be

increased from 35.7 to 37.06 kmol/ h, an

increase of 4%, while increases in S4 & S6 will

be approximately 0.1%

• Based on this result, A < expensive than B.