CHAPTER 5INPUT-OUTPUT STRUCTURE

OF THE FLOWSHEET

5.1 DECISIONS FOR THE INPUT-OUTPUT STRUCTURE

ProcessFeed streams

Product

By-Product

ProcessFeed streams

Product

By-Product

Purge

Flowsheet Alternative

TABLE 5.1-1 Hierarchy of decisions

1. Batch versus continuous

2. Input-output structure of the flowsheet

3. Recycle structure of flowsheet

4. General structure of the separation system

a. Vapor recovery system

b. Liquid separation system

5. Heat-exchanger network

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-offs are associated with these variables?

Level 2 Decisions

Guideline If feed impurity is not inert and it present in

significant quantities, remove it If a feed impurity is present in a gas feed, as a

first guess process the impurity If a feed in the a liquid feed stream is also a

by-product or product component, usually it is better to feed the process through the separation system.

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

Purification of Feed

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 from the product than the feed, it is better to process the impurity.

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

PROCESS ALTERNATIVE

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

ECONOMIC TRADE-OFFS FOR FEED PURIFICATION.

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 flow rate 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 simple design criterion that always indicates the correct decision.

Recover or Recycle Reversible By-products

Toluene+H2 Benzene+CH4

(4.1-3)

2Benzene Diphenyl+H2

The reactions to produce benzene from toluene are

The second reactions is reversible, we could recycle diphenyl black to the reactor and let it build up in recycle loop until it eventually reached an equilibrium level .

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 always be considered

Do Not Recover and Recycle Some Reactant

We should recover more than 99% all valuable materials

Since some materials, such as air and water, normally do not bother to recover and recycle unconverted amount of these component.

Number of Product Streams

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

The common sense design guideline

TABLE 5.1-3 Destination codes and component classifications

Destination code Component classification

1. Vent Gaseous by-products and feed impurities

2. Recycle and purge Gaseous reactants plus inert gases and/or gaseous by products

3. Recycle Reactants

Azeotropes with reactants (sometimes)

Reversible by-products (sometimes)

4. None Reactant-if complete conversion or unstable reaction intermediates

TABLE 5.1-3 Destination codes and component classifications

Destination code Component classification

5. Excess-vent Gaseous reactant not recovered and recycled

6. Excess-waste Liquid reactant not recovered or recycled

7. Primary product Primary product

8. Valuable by-product (I)

Separate destination for different by-products

9. Fuel By-products to fuel

10. Waste By-products to fuel waste treatment

Example 5.1-1 Suppose we have the 10 components listed in order of their boiling points and with destination codes indicated. How many product streams will there be.

Component Destination Component Destination

A Waste F Primary product

B Waste G Recycle

C Recycle H Recycle

D Fuel I Valuable by-product 1

E Fuel J Fuel

Solution. The product stream are

1. A+B to waste (do not separate them and then 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 we treat J as a separate product stream)

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

Example 5.1-4 Toluene to benzene

Component Boiling point Destination Code

H2 -253 C Recycle and purge

CH4 -161 C Recycle and purge

Benzene 80 C Primary product

Toluene 111 C Recycle

Diphenyl 253 C Fuel

The initial flowsheet

ProcessH2, CH4

Benzene

Diphenyl

Purge H2, CH4

Toluene

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

Evaluation of the Flowsheet

Be certain that all products, by products and impurities leave the process

5.2 DESIGN VARIABLES, OVERALL MATERIAL BALANCES, AND STREAM COST

TABLE 5.2-1 Possible design variables for level 2

Complex reactions: Reaction conversion

molar ratio of reactant

reaction temperature and/or pressure

Excess reactions: Reactants not recovered or gas recycle and purge

Design Variables

TABLE 5.2-2 Procedures for developing overall material balances

1. 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 design variables)

3. Calculate the impurity inlet and outlet flows for the feed streams where reactants are completely removed and recycle

Material Balances Procedure

TABLE 5.2-2 Procedures for developing overall material balances

4. 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 entering with the reactant stream in step 4.

Example 5.2-1 Toluene to benzene. Develop the overall material balances for HDA process.

Solution. The reactions of interest are

Toluene+H2 Benzene+CH4

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 H2 and CH4 and remove diphenyl, then there are three product stream

ProcessH2, CH4

Benzene

Diphenyl

Purge H2, CH4

Toluene

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

SELECTIVITY AND REACTION STOICHIOMETRY

Sconverted Toluene Moles

OutletReactor at Benzene MolesySelectivit

Recover and remove all this benzene. Hence for the production PB mol/hr, the toluene fed to the process FFT must be

S

PF BFT (5.2-1)

From Eq. 4.1-3

The amount of methane produced PR,CH4 must be

S

PP B

CHR 4,

Toluene+H2 Benzene+CH4

2Benzene Diphenyl+H2 (4.1-3)

(5.2-2)

From Eq. 4.1-3

The amount of diphenyl produced PD must be

2

1

2

1 S

S

PSFP BFTD

Toluene+H2 Benzene+CH4

2Benzene Diphenyl+H2 (4.1-3)

(5.2-3)

RECYCLE AND PURGE

If we feed an excess amount of H2 , FE, into

the process,.

The total amount of H2 fed to the process will

be

GFHB

E FySS

PF )1(

2(5.2-4)

yFHFG : The amount of H2 in the makeup gas stream

The methane flow rate leaving the process

S

PFyP BGHFCH )1(

4(5.2-5)

Methane Produced

Methane entering the process

The total purge flow rate PG will then be the

excess H2, FE, plus the total methane PCH4 or

S

PFyFP BGHFEG )1( (5.2-6)

Using FE as a design variable, we nornally

use the purge composition of the reactant yPH, where

G

EPH P

Fy (5.2-7)

0<yPH<1

yPH depends on the feed composition of reactant and the

conversion

Expressions for the makeup gas rate, FG and purge

rate PG explicitly in terms of the purge composition

of reactant yPH

GPHBB

GFH PyS

S

P

S

PFy

2

1(5.2-8)

And the methane in the feed plus the methane produced must all leave with the purge

GPHB

GFH PyS

PFy )1()1( (5.2-9)

Adding these expressions give

2

1 S

S

PFP BGG

(5.2-10)

Then solve for FG

)(

1)1(1

PHFH

PHB

G yySSS

yPF

(5.2-11)

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 ξ-ξcomsumedHydrogen

15)-(5.2 ξcomsumed Toluene

14)-(5.2 ξproduced Diphenyl

13)-(5.2 ξproduced Methane

12)-(5.2 2ξ -ξproduced benzeneNet

21

1

2

1

21

Generalize expressions

The number of moles(moles/hr) of any component is

Given by;

)reactant(- ),product(

tscoefficien tricstoichiome the:

ijv

(5.2-17)10 ijjj vnn

EXTENT VERSUS SELECTIVITY.

(5.2-18)

aSconvertedreactant limiting theofamount the

component desired of production theySelectivit

bS compronent undesired theof production the

component desired of production theySelectivit

1

21a

2-S

2

21b

2-S

(5.2-19)

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

PB1

BP21 2-

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)

Stream Tables.

Process

H2, CH4Benzene

Diphenyl

Purge H2, CH4

Toluene

Production rate =265

Design variable: FE and x

5

3

4

1

2

Compo-nent

1 2 3 4 5

H2 FH2 0 0 0 FE

CH4 FM 0 0 0 FM+PB/S

Benzene 0 0 PB 0 0

Toluene 0 PB/S 0 0 0

Diphenyl 0 0 0 PB(1-S)/(2S) 0

Temp. 100 100 100 100 100

Pressure 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

Stream Cost: Economic Potential

For HDA process

23)-(5.2 ($/yr) ,Cost Mat. Raw-

Valueproduct -By -ValueProduct P2 E

24)-(5.2 Cost Gas Makeup-

cost Toluene-Purge of Value Fuel

Diphenyl of Value Fuel Value BP

enzeneE