1. INPUT-OUTPUT STRUCTURE OF THE FLOWSHEET
CHAPTER 5
INPUT-OUTPUT STRUCTURE
OF THE FLOWSHEET

2. 5.1 DECISIONS FOR THE INPUT-OUTPUT STRUCTURE

3. Flowsheet Alternative
Product
Feed streams
Process
By-Product
Purge
Product
Feed streams
Process
By-Product

4. TABLE 5.1-1 Hierarchy of decisions
Batch versus continuous
Input-output structure of the flowsheet
Recycle structure of flowsheet
General structure of the separation system
a. Vapor recovery system
b. Liquid separation system
Heat-exchanger network

5. TABLE 5.1-2 Level-2 decisions
Should we purify the feed streams before they enter the process?
Should we remove or recycle a reversible by- product
Should we use a a gas recycle and purge stream?
Should we not bother to recover and recycle some reactants?
How many product streams will there be?
What are the design variables for the input-output structure, and what economic trade-offs are associated with these variables?

6. Purification of Feed 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

7. 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.

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

9. 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.

10. Recover or Recycle Reversible By-products
The reactions to produce benzene from toluene are
Toluene+H2  Benzene+CH4
(4.1-3)
2Benzene  Diphenyl+H2
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 .

11. 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

12. 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.

13. Number of Product Streams
The common sense design guideline
It is never advantageous to separate two streams and then mix them together.

14. TABLE 5.1-3 Destination codes and component classifications
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

15. TABLE 5.1-3 Destination codes and component classifications
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

16. A Waste F B G Recycle C H D Fuel I E J Primary product
Example 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 A
Waste F
Primary product B G
Recycle C H D
Fuel I
Valuable by-product 1 E J

17. Solution. The product stream are
A+B to waste (do not separate them and then mix them in the sewer)
D+E to fuel (do not separate them and then mix them to burn)
F-primary product (to storage for sale)
I-valuable by product I (to storage for sale)
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)

18. Example 5. 1-2 Hydroalkylation of toluene to produce benzene
Example Hydroalkylation of toluene to produce benzene. Find the number of product stream for the HAD process; i.e, see Example
Solution.- List all component
-arrange these components in order of their normal boiling point
-Destination code

19. Example 5.1-4 Toluene to benzene
Component
Boiling point
Destination Code H2
-253 C
Recycle and purge
CH4
-161 C
Benzene
80 C
Primary product
Toluene
111 C
Recycle
Diphenyl
253 C
Fuel

20. The initial flowsheet Purge H2, CH4 Benzene H2, CH4 Process Toluene
Diphenyl
Fig Input-output structure of HDA process.

21. Evaluation of the Flowsheet
Be certain that all products, by products and impurities leave the process

22. 5.2 DESIGN VARIABLES, OVERALL MATERIAL BALANCES, AND STREAM COST

23. Design Variables 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

24. Material Balances Procedure
TABLE Procedures for developing overall material balances
Start with the specified production rate.
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)
Calculate the impurity inlet and outlet flows for the feed streams where reactants are completely removed and recycle

25. TABLE 5.2-2 Procedures for developing overall material balances
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
Calculate the and outlet flows for the impurities entering with the reactant stream in step 4.

26. Example 5. 2-1 Toluene to benzene
Example Toluene to benzene. Develop the overall material balances for HDA process.
Solution. The reactions of interest are
Toluene+H2  Benzene+CH4
2Benzene  Diphenyl+H (4.1-3)
From Ex 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

27. Purge H2, CH4
Benzene
H2, CH4
Process
Toluene
Diphenyl
Fig Input-output structure of HDA process.

28. SELECTIVITY AND REACTION STOICHIOMETRY
Recover and remove all this benzene. Hence for the production PB mol/hr, the toluene fed to the process FFT must be
(5.2-1)

29. From Eq
Toluene+H2  Benzene+CH4
2Benzene  Diphenyl+H (4.1-3)
The amount of methane produced PR,CH4 must be
(5.2-2)

30. From Eq
Toluene+H2  Benzene+CH4
2Benzene  Diphenyl+H (4.1-3)
The amount of diphenyl produced PD must be
(5.2-3)

31. 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
(5.2-4)
yFHFG : The amount of H2 in the makeup gas stream

32. Methane entering the process
The methane flow rate leaving the process
(5.2-5)
Methane Produced
Methane entering the process

33. The total purge flow rate PG will then be the excess H2, FE, plus the total methane PCH4 or
(5.2-6)

34. yPH depends on the feed composition of reactant and the conversion
Using FE as a design variable, we nornally use the purge composition of the reactant yPH, where
(5.2-7)
0<yPH<1
yPH depends on the feed composition of reactant and the conversion

35. Expressions for the makeup gas rate, FG and purge rate PG explicitly in terms of the purge composition of reactant yPH
(5.2-8)
And the methane in the feed plus the methane produced must all leave with the purge
(5.2-9)

36. Adding these expressions give
(5.2-10)
Then solve for FG
(5.2-11)

37. MATERIAL BALANCE IN TERMS OF EXTENT OF REACTION
MATERIAL BALANCE IN TERMS OF EXTENT OF REACTION. (in term of the extent of reaction)

38. Generalize expressions
The number of moles(moles/hr) of any component is
Given by;
(5.2-17)

39. EXTENT VERSUS SELECTIVITY.
(5.2-18)
(5.2-19)

40. Example 5. 2-2 Toluene to benzene
Example Toluene to benzene. Develop the expressions relating the extents of reaction to production rate and selectivity for the HDA process.
From Eq and we find that
(5.2-20)
Also from Eq , we find that
(5.2-21)
(5.2-22)

41. Stream Tables. 5
Purge H2, CH4 1 3
H2, CH4
Benzene 2
Process 4
Toluene
Diphenyl
Production rate =265
Design variable: FE and x

42. Compo-nent 1 2 3 4 5 H2
FH2 FE
CH4 FM
FM+PB/S
Benzene PB
Toluene
PB/S
Diphenyl
PB(1-S)/(2S)
Temp.
100
Pressure
550 15
465

43. Stream Cost: Economic Potential
For HDA process