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Chemical reaction engineering

Description of Plug Flow Reactor & Tubular Reactors, Chemical Reaction Engineering

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Chemical reaction engineering

  1. 1. Chemical Engineering PlugChemical Engineering Plug Flow & CSTR ReactorFlow & CSTR Reactor PRESENTED BYPRESENTED BY PREM BABOOPREM BABOO M.Sc.B.Tech(Chemical Engineering),M.Phil, M.B.A.M.Sc.B.Tech(Chemical Engineering),M.Phil, M.B.A. Fellow of Institution of Engineer (India)Fellow of Institution of Engineer (India) An Expert forAn Expert for www.ureaknowhow.comwww.ureaknowhow.com
  2. 2. ResourcesResources • Book – O.Levenspiel: “Chemical Reaction Engineering” – S.Fogler: “Elements of Chemical Reaction Engineering” – Internet
  3. 3. Reactor PerformanceReactor Performance Information needed to predict the reactor behaviour: KINETICS how fast things happen? input output CONTACTING PATTERNS how materials flow & contact each other? Output = f (input, kinetics, contacting)Performance equation • very fast - equilibrium • slow - rate, mass, heat• flowing patterns • contact • aggregation etc.
  4. 4. The Nature of the Reactor Design ProblemThe Nature of the Reactor Design Problem 1. What is the composition of the feedstock, conditions, and purification Procedures? 2. What is the scale and capacity of the process? 3. Is Catalyst needs? 4. What is operating condition? 5. Continuous or batch process? 6. What type of the reactor best meets the process requirement? 7. What size and shape reactor should be used? 8. How are the energy transfer?
  5. 5. How to choose the reactorHow to choose the reactor • Yield (should be large) • Cost (Should be economic) • Safety Consideration • Pollution How to Reactor Design Firstly; You have to know reaction rate expression Secondly; fluid velocity, temperature process, composition and characteristic of species
  6. 6. Source of the essential data for reactorSource of the essential data for reactor designdesign 1. Bench scale experiment (Laboratory Scale) The reactors is designed to operate at constant temperature, under condition (minimize heat transfer and mass transfer) 2. Pilot plant studies The reactors used is larger than bench scale 3. Operating data from commercial scale reactor The data come from another company and it can be used to design reactor. Unfortunately, data are often incomplete, inaccurate,
  7. 7. Reactor TypeReactor Type Batch Reactors (Stirred Tanks) 1. The Batch reactor is the generic term for a type of vessel (Cylinder Tank) widely used in the process industries. 2. A typical batch reactor consists of a tank with an agitator and integral heating/cooling system. Heating/cooling uses jacketed walls, internal coil, and internal tube. Batch reactor with single external cooling jacket Batch reactor with half coil jacket Batch reactor with constant flux (Coflux) jacket
  8. 8. AdvantagesAdvantages 1. Batch reactor Can be stopped between batches, so the production rate is flexible 2. Batch reactors are more flexible, in that one can easly use different compositions in different batches to produces product with different spesification 3. If the process degrades the reactor in some way, a batch reactor can be cleaned, relined, etc. between batches. Where continuous reactors must run a long time before that can be done. 4. If the reactant are stirred, a batche reactor can often achieve better quality than a plug flow reactor, and better productivity than a CSTR
  9. 9. Batch Reactor typesBatch Reactor types semi-batch reactor • flexible system but more difficult to analyse • good control of reaction speed • applications: • calorimetric titrations (lab) • open hearth furnaces for steel production (ind.)
  10. 10. Ideal Batch ReactorIdeal Batch Reactor - design equations -- design equations -               +               +               =               reactorthein reactantof onaccumulati ofrate reactorthein reactionchemical todueloss reactantofrate reactorof outflow reactant ofrate reactor intoflow reactant ofrate               −=               reactorthein reactantof onaccumulati ofrate reactorthein reactionchemical todueloss reactantofrate
  11. 11. Ideal Batch ReactorIdeal Batch Reactor - design equations -- design equations - ( )fluidofvolume fluid)ofume(time)(vol reactingAmoles       VrA )(− dt dNA − dt dN Vr A A −=− )(               −=               reactorthein reactantof onaccumulati ofrate reactorthein reactionchemical todueloss reactantofrate
  12. 12. Ideal Batch ReactorIdeal Batch Reactor - design equations -- design equations - dt dN Vr A A −=− )( dt dX N dt XNd dt dN A A AAA 0 0 )]1([ −= − = dt dX NVr A AA 0)( =− ∫ − = AX A A A Vr dX Nt 0 0 )( design equation = time required to achieve conversion XA 0AN t area =
  13. 13. Ideal Batch ReactorIdeal Batch Reactor - design equations / special cases -- design equations / special cases - ∫ − = AX A A A Vr dX Nt 0 0 )( Const. density ∫∫ − = − = AA X A A A X A AA r dX C r dX V N t 0 0 0 0 )()( ∫∫ − = − = A A A C C A A X A A A r dC r dX Ct 0 )()(0 0 0AC t area = tarea =
  14. 14. Continuous Stirred Tank ReactorContinuous Stirred Tank Reactor • In a CSTR, one or more fluid reagents are introduced into a tank reactor equipped with an impeller. The impeller stirs the reagents to ensure proper mixing Impeller
  15. 15. Some important aspects of the CSTRSome important aspects of the CSTR • At steady-state, the flow rate in must equal the mass flow rate out, otherwise the tank will overflow or go empty (transient state). • All calculations performed with CSTRs assume perect mixing. • The reaction proceeds at the reaction rate associated with the final (output) concentration. • Often, it is economically beneficial to operate several CSTR in series. This allows, for example, the first CSTR to operate at a higher reagent concentration and therefore a higher reaction rate. In these cases, the sizes of the reactors may be varied in order to minimize the total capital investment required to implement the process. • It can be seen that an infinite number of infinitely small CSTR operating in series would be equivalent to a PFR.
  16. 16. Advantages and DisadvantagesAdvantages and Disadvantages Kinds of Phases Present Usage Advantages Disadvantages 1. Liquid phase 2. Gas-liquid rxns 3. Solid-liquid rxns 1. When agitation is required 2. Series configurations for different concentration streams 1. Continuous operation 2. Good temperature control 3. Easily adapts to two phase runs 4. Good control 5. Simplicity of construction 6. Low operating (labor) cost 7. Easy to clean 1. Lowest conversion per unit volume 2. By-passing and channeling possible with poor agitation
  17. 17. CSTR ReactorCSTR Reactor - design equations -- design equations -               +               +               =               reactorthein reactantof onaccumulati ofrate reactorthein reactionchemical todueloss reactantofrate reactorof outflow reactant ofrate reactor intoflow reactant ofrate               +               =               reactorthein reactionchemical todueloss reactantofrate reactorof outflow reactant ofrate reactor intoflow reactant ofrate VrA )(−
  18. 18. CSTR ReactorCSTR Reactor - design equations -- design equations - 000 )1( AAA FXF =− 000 AA CvF = flowvolumetricv =0 flowmolarFA =0 ( )sm /3 ( )smol /       reactorintoflow reactantofrate ( )smol /       reactorofoutflow reactantofrate )1(0 AAA XFF −= VrXFF AAAA )()1(00 −+−= design equation FA0XA=(−rA)V ( )smol /
  19. 19. Ideal Flow ReactorIdeal Flow Reactor - space-time / space-velocity -- space-time / space-velocity - τ= 1 s = timerequiredtoprocessonereactorvolume offeedmeasuredatspecifiedconditions        Performance measures of flow reactors: 2 min – every 2 min one reactor volume of feed at specified conditions is treated by the reactor s= 1 τ = numberofreactorvolumesoffeedatspecified conditionswhichcanbetreatedinunittime       5 hr-1 – 5 reactor volumes of feed at specified conditions are fed into reactor per hour Ex. Ex.
  20. 20. Ideal Flow ReactorIdeal Flow Reactor - space-time / space-velocity -- space-time / space-velocity - τ= 1 s = CA0V FA0 = molesAentering volumeoffeed      volumeofreactor( ) molesofAentering time       = V v0 = reactorvolume volumetricfeedrate Residence time
  21. 21. CSTR ReactorCSTR Reactor - design equations -- design equations - V FA0 = τ CA0 = XA −rA FA0XA=(−rA)V τ= 1 s = CA0V FA0 = V v0 Design equation: Residence time: area= V FA0 = τ CA0 εA≠0 τ= V v0 = CA0V FA0 = CA0XA −rA
  22. 22. CSTR ReactorCSTR Reactor - design equations / general & special- design equations / general & special case -case - V FA0 = XA −rA = CA−CA0 CA0(−rA) XA =1− CA CA0 Special case - constant density: τ= V v0 = CA0XA −rA = CA−CA0 −rA Feed entering partially converted: V FA0 = XAf −XAi −rA( )f τ= VCA0 FA0 = CA0(XAf −XAi) −rA( )f εA=0
  23. 23. Plug Flow ReactorPlug Flow Reactor Definition. “Each and every particle having same residence time, back mixing not allowed.” The plug flow reactor (PFR) model is used to describe Chemical Reaction in continuous, flowing systems. One application of the PFR model is the estimation of key reactor variables, such as the dimensions of the reactor. PFRs are also sometimes called as Continuous Tubular Reactors (CTRs)
  24. 24. Plug Flow ReactorPlug Flow Reactor • The PFR model works well for many fluids: liquids, gases, and slurries. • Fluid Flow is sometimes turbulent flow or axial diffusion, it is sufficient to promote mixing in the axial direction, which undermines the required assumption of zero axial mixing. However if these effects are sufficiently small and can be subsequently ignored. • The PFR can be used to multiple reactions as well as reactions involving changing temperatures, pressures and densities of the flow.
  25. 25. Advantages and disadvantagesAdvantages and disadvantages • Plug flow reactors have a high volumetric unit conversion, run for long periods of time without labor, and can have excellent heat transfer due to the ability to customize the diameter to the desired value by using parallel reactors. • Disadvantages of plug flow reactors are that temperatures are hard to control and can result in undesirable temperature gradients. PFR maintenance is expensive. Shutdown and cleaning may be expensive. Applications Plug flow reactors are used for some of the following applications: •Large-scale reactions •Fast reactions •Homogeneous or heterogeneous reactions •Continuous production •High-temperature reactions
  26. 26. Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor - definition -- definition -  The composition of the fluid varies from point to point  No mixing or diffusion of the fluid along the flow path  Material balance – for a differential element of volume dV (not the whole reactor!) Characteristics: ( ) ( ) ( )onaccumulati reactionby ncedisappeara outputinput +      += Material balance: =0
  27. 27. Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor - material balance -- material balance - Input of A [moles/time] AF Output of A [moles/time] AA dFF + Disappearance of A by rxn. dVrA )(− dV
  28. 28. Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor - material balance -- material balance - ( ) dVrdFFF AAAA )(−++= dV ( ) ( ) ( )ncedisappearaoutputinput += [ ] AAAAA dXFXFddF 00 )1( −=−=)1(0 AAA XFF −= dVrdF AA )(−=− dVrdXF AAA )(0 −= ∫∫ − = AfX A A V A r dX F dV 00 0 design equation
  29. 29. Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor - design equations -- design equations - ∫∫ − = AfX A A V A r dX F dV 00 0 ∫ − == AfX A A AA r dX CF V 0 00 τ ∫ − === AfX A A A A A r dX C F VC v V 0 0 0 0 0 τ 000 AA CvF = flowvolumetricv =0 flowmolarFA =0 ( )sm /3 ( )smol / εA≠0  If the feed enters partially converted ∫ − == Af Ai X X A A AA r dX CF V 00 τ ∫ − === Af Ai X X A A A A A r dX C F VC v V 0 0 0 0 τ∫∫ → Af Ai Af X X X 0
  30. 30. Fixed Bed ReactorFixed Bed Reactor • Solids take part in reaction  unsteady state or semi-batch mode • Over some time, solids either replaced or regenerated 1 2 CA,in CA,out Regeneration
  31. 31. Fluidized bed reactorFluidized bed reactor • A fluidized bed reactor (FBR) is a type of reactor that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a granular solid material (usually a catalyst possibly shaped as tiny spheres) at high enough velocity to suspend the solid.
  32. 32. AdvantagesAdvantages • Uniform Particle Mixing: Due to the intrinsic fluid-like behavior of the solid material, fluidized beds do not experience poor mixing as in packed beds. This complete mixing allows for a uniform product that can often be hard to achieve in other reactor designs. The elimination of radial and axial concentration also allows for better fluid-solid contact, which is essential for reaction efficiency and quality. • Uniform Temperature: Many chemical reactions produce or require the addition of heat. Local hot or cold spots within the reaction bed, often a problem in packed beds, are avoided in a fluidized situation such as a FBR. In other reactor types, these local temperature differences, especially hotspots, can result in product degradation. Thus FBR are well suited to exothermic reactions. Researchers have also learned that the bed-to-surface heat transfer coefficients for FBR are high. • Ability to Operate Reactor in Continuous State: The fluidized bed nature of these reactors allows for the ability to continuously withdraw product and introduce new reactants into the reaction vessel. Operating at a continuous process state allows manufacturers to produce their various products more efficiently
  33. 33. DisadvantagesDisadvantages • Increased Reactor Vessel Size: Because of the expansion of the bed materials in the reactor, a larger vessel is often required than that for a packed bed reactor. This larger vessel means that more must be spent on initial startup costs. • Pumping Requirements and Pressure Drop: The requirement for the fluid to suspend the solid material necessitates that a higher fluid velocity is attained in the reactor. In order to achieve this, more pumping power and thus higher energy costs are needed. In addition, the pressure drop associated with deep beds also requires additional pumping power. • Particle Entrainment: The high gas velocities present in this style of reactor often result in fine particles becoming entrained in the fluid. These captured particles are then carried out of the reactor with the fluid, where they must be separated. This can be a very difficult and expensive problem to address depending on the design and function of the reactor. This may often continue to be a problem even with other entrainment reducing technologies. • Lack of Current Understanding: Current understanding of the actual behavior of the materials in a fluidized bed is rather limited. It is very difficult to predict and calculate the complex mass and heat flows within the bed. Due to this lack of understanding, a pilot plant for new processes is required. Even with pilot plants, the scale-up can be very difficult and may not reflect what was experienced in the pilot trial. • Erosion of Internal Components: The fluid-like behavior of the fine solid particles within the bed eventually results in the wear of the reactor vessel. This can require expensive maintenance and upkeep for the reaction vessel

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