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

5.1 DECISIONS FOR THE INPUT-OUTPUT STRUCTURE

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

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

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?

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

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 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 .

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 The common sense design guideline It is never advantageous to separate two streams and then mix them together.

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

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

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

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)

Example 5. 1-2 Hydroalkylation of toluene to produce benzene 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 Benzene 80 C Primary product Toluene 111 C Recycle Diphenyl 253 C Fuel

The initial flowsheet Purge H2, CH4 Benzene H2, CH4 Process Toluene Diphenyl 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

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

Material Balances Procedure TABLE 5.2-2 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

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.

Example 5. 2-1 Toluene to benzene 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

Purge H2, CH4 Benzene H2, CH4 Process Toluene Diphenyl Fig. 5.1-2 Input-output structure of HDA process.

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)

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

From Eq. 4.1-3 Toluene+H2  Benzene+CH4 2Benzene  Diphenyl+H2 (4.1-3) The amount of diphenyl produced PD must be (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 (5.2-4) yFHFG : The amount of H2 in the makeup gas stream

Methane entering the process The methane flow rate leaving the process (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 (5.2-6)

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

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)

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

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

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

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

Example 5. 2-2 Toluene to benzene Example 5.2-2 Toluene to benzene. Develop the expressions relating the extents of reaction to production rate and selectivity for the HDA process. From Eq. 5.2-15 and 5.2-1 we find that (5.2-20) Also from Eq. 5.2-12 , we find that (5.2-21) (5.2-22)

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

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

Stream Cost: Economic Potential For HDA process