Hierarchy of Decisions

LEVEL 2 Separation Reactor System Purge H2 , CH4 H2 , CH4 Benzene Toluene Diphenyl LEVEL 2

LEVEL 3 DECISIONS How many reactors are required ? Is there any separation between the reactors ? How many recycle streams are required ? Do we want to use an excess of one reactant at the reactor inlet ? Is there a need to separate product partway or recycle byproduct ? Should the reactor be operated adiabatically or with direct heating or cooling ? Is a diluent or heat carrier required ? What are the proper operating temperature and pressure ? Is a gas compressor required ? costs ? Which reactor model should be used ? How do the reactor/compressor costs affect the economic potential ?

Q1: 程序中應該要有幾個反應系統？ 若決定好的反應途徑中，有個別反應須操作在不同的操作條件或使用不同觸媒，則必須分別設計反應系統，若無以上情況則可使用一個反應系統。 範例 　　產製苯的程序之反應路徑如下： 以上的反應合適操作條件為　　　　　 ，500 psia， 故設計一個反應系統即可。

Ketene + Acetic Acid  Acetic Anhydride Q1: Number of Reactor Systems If sets of reactions take place at different T and P, or if they require different catalysts, then we use different reactor systems for these reaction sets. For example, Acetone  Ketene + CH4 Ketene  CO + 1/2C2H4 700C, 1atm Ketene + Acetic Acid  Acetic Anhydride 80 C, 1atm

Destination Codes and Component Classifications Destination code 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 Reaction intermediates Azeotropes with reactants (sometimes) Reversible by-products (sometimes) 4.None Reactants-if complete conversion or unstable reaction intermediates 5.Excess - vent Gaseous reactant not recovered or recycles 6.Excess - vent Liquid reactant not recovered or recycled 7.Primary product Primary product 8.Fuel By-products to fuel 9.Waste By-products to waste treatment should be minimized A ) List all the components that are expected to leave the reactor. This list includes all the components in feed streams, and all reactants and products that appear in every reaction. B ) Classify each component in the list according to Table 5.1-3 and assign a destination code to each. C ) Order the components by their normal boiling points and group them with neighboring destinations. D ) The number of groups of all but the recycle streams is then considered to be the number of product streams.

Q2: Number of Recycle Streams EXAMPLE HDA Process Component NBP , C Destination H2 -253 Recycle + Purge Gas Recycle CH4 -161 Recycle + Purge Gas Recycle Benzene 80 Primary Product Toluene 111 Recycle Liquid Recycle Diphenyl 255 By-product Compressor CH4 , H2 (Purge) (Gas Recycle) Benezene (PrimaryProduct) Reactor Separator (Feed)H2 , CH4 (Feed) Toluene Diphenyl (By-product) Toluene (Liquid Recycle)

Q2: Number of Recycle Streams EXAMPLE Acetone  Ketene + CH4 700C Ketene  CO + 1/2C2H4 1atm Ketene + Acetic Acid  Acetic Anhydride 80 C, 1atm Component NBP, C Destination CO -312.6 Fuel By-product CH4 -258.6 “ C2H4 -154.8 “ Ketene -42.1 Unstable Acetone 133.2 Reactant Acetic Acid 244.3 Reactant Acetic Anhydride 281.9 Primary Product CO , CH4 , C2H4 (By-product) Acetic Acid (feed) Acetone (feed) R1 R2 Separation Acetic Anhydride (primary product) Acetic Acid (recycle to R2) Acetone (recycle to R1)

Q3: Reactor Inputs and Outputs 3.1 Feed Excess Reactants shift product distribution force another component to be close to complete conversion shift equilibrium Therefore, molar ratio of reactants entering reactor should be treated as a design variable!

 

O O O O O Type 3 Multiple reactions in series producing byproducts shift product distribution ex. CH3 + H2  + CH4 excess 5:1 2 + H2 Type 4 Mixed parallel and series reactions  byproducts ex. CH4 + Cl2  CH3Cl + HCl Primary excess 10:1 CH3Cl + Cl2  CH2Cl2+ HCl CH2Cl2+ Cl2  CHCl3 + HCl Secondary CHCl3 + Cl2  CCl4 + HCl O O O O O

 

 

Some of the decisions involve introducing a new component (inert) into the flowsheet, e.g. adding a new component to shift the product distribution, to shift the equilibrium conversion, or to act as a heat carrier. This will require that we also remove the component from the process and this may cause a waste treatment problem. Example Ethylene production C2H6 = C2H4 +H2 Steam is usually used as the C2H6 + H2 = 2CH4 diluent. Example Styrene Production EB = styrene +H2 EB  benzene +C2H4 Steam is also used. EB + H2  toluene + CH4

3.3 Remove Product during Reaction to shift equilibrium + product distribution Type 1(b): single reversible reaction: ex. 2SO2 + O2 = 2SO3 H2O H2O SO2 REACT ABSORB REACT ABSORB O2 + N2 H2SO4 H2SO4 Type 3: multiple reactions in series  byproduct FEED  PRODUCT remove PRODUCT = BYPRODUCT .

3.4 Recycle Byproduct to shift equilibrium + product distribution O O CH3 + H2  + CH4 2 = + H2 O O O O O

Q4: Reactor Operation 4.1 Setting Reactor Temperature T   k   V  Single Reaction : - endothermic AHAP ! - exothermic * irreversible AHAP ! * reversible continuously decreasing as conversion increases.  Multiple Reaction max. selectivity T  400C  Use of stainless steel is severely limited ! T  260C  High pressure steam ( 40~50 bar) provides heat at 250-265 C T  40C  Cooling water Temp 25-30C

 

Arrangements for fixed-bed reactor

Arrangements for fixed-bed reactor

Arrangements for fixed-bed reactor

Arrangements for fixed-bed reactor

  O decrease in the number of moles AHAP

Quick Calculations to establish recycle material balance - Example: HDA process in which toluene is limiting reactant (to be done first ) yPH RG Purge , PG FG , yFH H2 , CH4 Benzene , PB reactor separator   FT FT ( 1-X ) PD Toluene Diphenyl LEVEL 3 FT ( 1-X ) LEVEL 2 always valid for limiting reactant when there is complete recovery and recycle of the limiting reactant

Quick calculations to establish recycle material balance - Example: HDA process in which H2 is the other reactant (Next ) molar ratio extra design variable Note that details of separation system have not been specified at this level. Therefore, we assume that reactants one recovered completely.

Q5: Compressor Design and Cost Whenever a gas-recycle stream is present, we will need a gas- recycle compressor. Covered in “Unit Operation (I)”

Q7: Reactor Design and Costs Q6: Reactor Models Q7: Reactor Design and Costs Covered in “Reactor Design and Reaction Kinetics”

Economic Potential at Level 3 Note, $ $ EP3=EP2-annualized costs of reactors -annualized costs of compressors 2  106 1  106 0.2 0.4 0.6 $/year 0 0.1 0.3 0.5 0.7 -1  106 -2  106   does not include any separation or heating and cooling cost