II Choice of Reactor

  I I Choice of Reactor Outline

  1. I ntroduction

  2. Reaction Path

  3. Types of Reaction System

  4. Reactor Performance

  5. Rate of Reaction

  6. I dealized Reactor Models

  7. Reactor Configuration

  8. Design Guideline for Reactor Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

  I I .1.

  I NTRODUCTI ON Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

  I ntroduction

  Choice of Reactor involves: 

  1. Type of Reactor

  2. Reaction Conditions (P, T, C, phase) Two Types of Reactor:

  

  1. Mixed-flow: CSTR, Fluidized

  2. Plug-flow: PFR, Fixed-Bed, Column Type of Reactor depends on:

  

  1. Type of reaction: single, parallel, series

  2. Heat effect (heat exchanger): adiabatic, direct/ indirect heating and/ or cooling

  3. Reaction conditions: T, P, phase, catalyst Most Reaction conditions are Limited by research’s results

   Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

  I ntroduction

  Temperature and Pressure affect to:

  

  1. Reaction rate: Arrhenius equation, concentration

  2. Reaction equilibrium: endothermic / exothermic (mole ratio of reactant) Reaction phase:

  

  1. Single phase ( gas, liquid, solid)

  2. Two phases or more (with or without catalyst) Catalyst:

  

  1. Homogen Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

  2. Heterogen I I .2.

REACTI ON PATH

  Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

  I ntroduction to choice of Reactor

  (Smith, R., 2005) Reactors can be broadly classified as chemical or biochemical.

  Most reactors, whether chemical or biochemical, are catalyzed. The strategy will be to choose the catalyst, if one is to be used, and the ideal characteristics and operating conditions needed for the reaction system. The issues that must be addressed for reactor design include:

   Reactor type Catalyst

    Size Operating conditions (temperature and pressure)

    Phase Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Feed conditions (concentration and temperature).

   Reactor Path

  (Smith, R., 2005)

  Preferred :

  Reaction paths that use the cheapest raw materials and produce the smallest quantities of byproducts are

  to be . preferred Avoided :

  Reaction paths that produce significant quantities of unwanted byproducts , since they can

  should especially be avoided create significant environmental problems.

  Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

  Example 2.2.1.

  Given that the objective is to manufacture vinyl chloride, there are at least three reaction paths that can be readily exploited.

  Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

  Molar masses and values of materials Oxygen is considered to be free at this stage, coming from the atmosphere.

  Which reaction path makes most sense on the basis of raw material costs, product and byproduct

  ?

  values Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

  Solution:

  potential of

  Decisions can be made on the basis of the economic

  the process. At this stage, the best that can be done is to define the economic potential (EP) :

  EP = (value of products) - (raw materials costs) Path 1

  EP = (62 × 0.46) - (26 × 1.0 + 36 × 0.39)

  • 1

  = – 11.52 $· kmol vinyl chloride product

  Path 2 EP = (62 × 0.46 + 36 × 0.39) - (28 × 0.58 + 71 × 0.23)

  • 1

  = 9.99 $· kmol vinyl chloride product This assumes the sale of the byproduct HCl. I f it cannot be sold, then:

  EP = (62 × 0.46) - (28 × 0.58 + 71 × 0.23)

  • 1
  • Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY = –4.05 $· kmol vinyl chloride product Path 3

      EP = (62 × 0.46) - (28 × 0.58 + 36 × 0.39)

      1

      = –1.76 $· kmol- vinyl chloride product Paths 1 and 3 are clearly not viable. Only Path 2 shows a positive economic potential when the byproduct HCl can be sold. I n practice, this might be quite difficult, since the market for HCl tends to be limited. I n general, projects should not be justified on the basis of the byproduct value. The preference is for a process based on ethylene rather than the more expensive acetylene, and chlorine rather than the more expensive hydrogen chloride. Electrolytic cells are a much more convenient and cheaper source of chlorine than hydrogen chloride. I n addition, it is preferred to produce no byproducts.

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Example 2.2.2.

      Devise a process from the three reaction paths in

      Example 2.2.1

      that uses ethylene and chlorine as raw materials and produces no byproducts other than water. Does the process look attractive economically?

      Solution:

      A study of the stoichiometry of the three paths shows that this can be achieved by combining Path 2 and Path 3 to obtain a fourth path.

      Path 2 and 3 Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      These three reactions can be added to obtain the overall stoichiometry.

      Path 4

      Now the economic potential is given by:

      EP = (62 × 0.46) - (28 × 0.58 + 1/ 2 × 71 × 0.23)

      1

      = 4.12 $· kmol- vinyl chloride product I n summary, Path 2 from Example 2.1 is the most attractive reaction path if there is a large market for hydrogen chloride. I n practice, it tends to be difficult to sell the large quantities of hydrogen chloride produced by such processes. Path 4 is the usual commercial route to vinyl chloride. Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      I I .3. TYPES OF REACTI ON SYSTEM Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Reaction systems can be classified into six

      (Smith, R., 2005):

      broad types

      1. Single Reaction 2. Multiple reactions in parallel producing byproducts.

      3. Multiple reactions in series producing byproducts.

      4. Mixed parallel and series reactions producing byproducts.

      5. Polymerization reactions.

      6. Biochemical reactions. Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      1. Single Reaction

      FEED PRODUCT  or FEED PRODUCT + BYPRODUCT

       or FEED1 + FEED2 PRODUCT

       Examples:

      Does not produce by product: Produce by product: Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      2. Multiple Reactions in Parallel Producing Byproducts

      FEED PRODUCT 

      FEED BYPRODUCT  or FEED PRODUCT + BYPRODUCT1

       FEED BYPRODUCT2 + BYPRODUCT3

       or FEED1 + FEED2 PRODUCT

       FEED1 + FEED2 BYPRODUCT

       Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY and so on

      Examples of a parallel reactions system occurs in the production of

      ethylene oxide

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    3. Multiple Reactions in Series Producing Byproducts

      FEED PRODUCT 

      PRODUCT BYPRODUCT  or FEED PRODUCT + BYPRODUCT1

       PRODUCT BYPRODUCT2 + BYPRODUCT3

       or FEED1 + FEED2 PRODUCT

       PRODUCT BYPRODUCT1 + BYPRODUCT2

       Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY and so on

      Examples of series reactions system occurs in the production of

      formaldehyde from methanol

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    4. Mixed Parallel and Series Reactions Producing Byproducts

      FEED PRODUCT 

      FEED BYPRODUCT 

      PRODUCT BYPRODUCT  or FEED PRODUCT

       FEED BYPRODUCT1

       PRODUCT BYPRODUCT2

       or FEED1 + FEED2 PRODUCT

       FEED1 + FEED2 BYPRODUCT1

       PRODUCT BYPRODUCT2 + BYPRODUCT3

       Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY and so on of mixed parallel and series reactions is the production of

      Examples

      Ethanolamines by reaction between Ethylene Oxide and Ammonia:

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    5. Polimerization Reactions

      

    monomer molecules are reacted together to produce a

    • high molar mass polymer.

      

    Depending on the mechanical properties required of the

    • polymer, a mixture of monomers might be reacted together to produce a high molar mass copolymer.

      Two broad types of polymerization reactions:

    • those that involve a termination step

        those that do not

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      An example of polimerization reaction that involves a termination

      step: Polymerization of Vinyl Chloride from a free-radical initiator • R I nitiation step:

      Propagation step: and so on, leading to molecules of the structure:

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      the chain is terminated by steps such as the union

      Eventually,

      of two radicals that consume but do not generate radicals:

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY a polymerization without a termination step is

      An example of polycondensation

      Here the polymer grows by successive esterification with elimination of water and no termination step. Polymers formed by linking monomers with carboxylic acid groups and those that have alcohol groups are known as polyesters. Polymers of this type are widely used for the manufacture of artificial fibers. For example, the esterification of terephthalic acid with ethylene glycol produces polyethy-lene terephthalate. Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    6. Biochemical Reaction

      often referred to as fermentations

      

      can be divided into two broad types, promoted by:

      

      1. microorganisms 2. enzymes the advantages

      

      1. operating under mild reaction conditions of temperature and pressure 2. usually carried out in an aqueous medium rather than using an organic solvent.

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY 

      an example of the reaction exploits the metabolic pathways in selected

      microorganisms  In such reactions, the microorganisms reproduce themselves.

       In addition to the feed material, it is likely that nutrients (e.g. a mixture containing phosphorus, magnesium, potassium, etc.) will need to be added for the survival of the microorganisms.

       Reactions involving microorganisms include:  hydrolysis  oxidation  esterification  reduction

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

       An example of an oxidation reaction is the production of citric acid from glucose:

      An example of the reaction that promoted by

      enzymes

      Enzymes are the catalyst proteins produced by

       microorganisms that accelerate chemical reactions in microorganisms.

       The biochemical reactions employing enzymes are of the general form:

       An example in the use of enzymes is the isomerization of glucose to fructose:

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY I I .4.

    REACTOR PERFORMANCE

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Reactor Performance

      (Smith, R., 2005) Three important parameters to describe reactor performance:

      The stoichiometric factor is the stoichiometric moles of reactant required per mole of

    product. When more than one reactant is required (or more than one desired product

    produced) three Equations above can be applied to each reactant (or product). Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Benzene is to be produced from toluene according

      Example 2.4.1:

      to the reaction

      Reactor feed and effluent streams: Calculate the conversion, selectivity and reactor yield with respect to the:

    a. Toluene feed

      b. Hydrogen feed Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Solution: Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      However, the principal concern is performance with respect to toluene, since it is more expensive Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY than hydrogen.

    RATE OF REACTI ON

      V r i i

      = time (s) … (2.5.1)

      3 ) t

      V = reaction volume (m

      = moles of Component i formed (kmol)

      ) N i

      ·s

      = rate of reaction of Component i (kmol·m

      1 where r i

         dt dN

         

      The rate of reaction is the number of moles formed with respect to time, per unit volume of reaction mixture:

      

      To define the rate of a reaction, one of the components must be selected and the rate defined in terms of that component.

      

      (Smith, R., 2005)

    • 1

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Rate of Reaction

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY I I .5.

    • 3
    • 3

    • cC + · · · → sS + tT + ···

      ]

      = reaction rate for component

      

    i

      (kmol· m

      · s

      )

      k i

      = reaction rate constant for component

      i

      ([ kmol· m

      NC – b – c- ... s

      where

      )

      NC = is the number of components in the rate expression

      C i

      = molar concentration of component

      i

      (kmol· m

      ) The exponent for the concentration ( b, c,...

      ) is known as the

      order of reaction .

      r i

      C C k r

      … (2.5.5) … (2.5.6) … (2.5.7) … (2.5.8)

      = molar concentration of Component i (kmol·m

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY 

      I f the volume of the reactor is constant (

      V

      = constant):

      

    dt

    dC

    dt V dN dt dN

      V r i i i i

          

         

      1 where C i

      ) 

      C C k r   c C b B T T

      The rate is negative if the component is a reactant and positive if it is a product. For example, for the general irreversible reaction:

      bB

      The rates of reaction are related by:

               

    t

    r

    s r c r b r

      T S C B … (2.5.2) … (2.5.4) … (2.5.3)

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY 

      I f the rate-controlling step in the reaction is the collision of the reacting molecules, then the equation to quantify the reaction rate will often follow the stoichiometry such that:

       c C b B B B

      C C k r    c C b B C C

      C C k r    c C b B S S

    • 3
    • 1
    • -1
    • 3
    • 3
    Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY 

      The reaction rate constant is a function of temperature, as will be discussed next.

      C C k C C k r '

      then the rate of reaction is the net rate of the forward and reverse reactions. I f the forward and reverse reactions are both elementary, then:

        t T s S B c C b B B B

      C C k C C k r '

           t T s S C c C b B C C

      C C k C C k r '

           t T s S S c C b B S S

          t T s S T c C b B T T

             tT sS cC bB

      C C k C C k r '

        where

      ' i k

      = reaction rate constant for Component i for the reverse reaction

      i k

      = reaction rate constant for Component i for the forward reaction

      

      I f the reaction is reversible, such that:

             t k s k c k b k

      ξ ε δ β T S C B B B

      T S C B

      Reactions for which the rate equations follow the stoichiometry are known as

      elementary reactions .

       I f there is no direct correspondence between the reaction

      stoichiometry and the reaction rate, these are known as

      non- elementary reactions and are often of the form:  

      C C C C k r    

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY 

      ξ ε δ β T S C B C C

      C C C C k r    

      ξ ε δ β T S C B S S

      C C C C k r   

      ξ ε δ β T S C B T T

      C C C C k r  where β , δ , ε , ξ = order of reaction

      … (2.5.9) … (2.5.13) … (2.5.10) … (2.5.11) … (2.5.12)

      … (2.5.14) … (2.5.15) … (2.5.16) … (2.5.17) … (2.5.18)

      I I .6.

      I DEALI ZED REACTOR MODELS Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      I dealized Reactor Models

      (Smith, R., 2005)

      I deal Batch Reactor

      the reactants are

      

      charged at the beginning of the operation.

      The contents are subjected to perfect mixing for a certain

       period, after which the products are discharged.

      Concentration changes with time, but the perfect mixing

      

      ensures that at any instant the composition and Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY temperature throughout the reactor are both uniform.

    1 I n term of reactor conversion (

      i C it

      X i i i

      V r dX N t

      … (2.6.4)

      I ntegration of (2.6.3): from the definition of reactor conversion, for the special case of a constant density reaction mixture:

      i it i i it i i

      C C C N N N

      X  

        C i

      = molar concentration of Component

      i C i0

      = initial molar concentration of Component

      = final molar concentration of Component

      

      i

      at time

      t

      Substitution of (2.6.5) into (2.6.3)

      i i r dt dC

         

      

      it C i C i i r dC t

      … (2.6.5)

      where

      … (2.6.6) … (2.6.7)

      I ntegration of (2.6.6):

        i

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY I deal Batch Model

      where

         

      t

      = batch time

      N i0

      = initial moles of Component

      i N it

      = final moles of Component

      i

      after time

      t dt dN

      V r

    i

    i

      1 converted reactant of moles   

       

      … (2.6.3) Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      

       it N i N

    i

    i

      V r

    dN

    t

      … (2.6.1)

      I ntegration of (2.6.1):

      … (2.6.2)    

      V r

    dt

    dX N dt

      X N d dt dN i

    i

    i i i i

          

      X i

      )

      I deal Batch Model

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY I dealized Reactor Models

      i

      i

      per unit time

      V r N N i in i out i

        , ,

      Rearrange (2.6.9): Substituting

      N i,out

      = N

      i,in

      (1-X

      ) into (2.6.10):

      N i,in

      i i in i r

      X N

      V ,

       … (2.6.8) … (2.6.9)

      where

      … (2.6.10)   in i i out i in i in i

      C r C C N

      

    V

    , , , ,

        

      … (2.6.11) … (2.6.12)

      = outlet moles of Component

      per unit time

      (Smith, R., 2005)

      for Component

      

      Feed and product takeoff are both continuous.

      Mixed- Flow or Continuous Well- Mixed or Continuous- Stirred- Tank Reactor ( CSTR)  The reactor contents are assumed to be perfectly mixed. 

      This leads to uniform composition and temperature throughout the reactor.

      

      Because of the perfect mixing, a fluid element can leave the instant it enters the reactor or stay for an extended period.

      

      The residence time of individual fluid elements in the reactor varies.

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Material Balance

      i

      i

      per unit time   

        

         

        

         

         unit time per product in reactant of moles unit time per converted reactant of moles unit time per feed in reactant of moles

        out i i in i

      N V r N , ,

         N i,in

      = inlet moles of Component

      For the special case of a constant density system, (2.6.5) can be substituted to give: Analogous to time as a measure of batch process performance,

      space–time ( ) can be defined for a continuous reactor: τ

      C

      V V i out

      , … (2.6.13)

        τ

      F N i in

      , 3 -1

      F

      where = volumetric flowrate of the feed (m .s )

      The reciprocal of space–time is space–velocity (s):

      … (2.6.14) s

       1  number of reactor vo lume processed in a unit time 

       τ

      Combining Equations (2.6.12) for the mixed-flow reactor with constant density and (2.6.13) gives:

      C Ci in i out

      , , 

      … (2.6.15) τ r

       i

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY C C

      This figure is a plot of (2.6.15), from to the rate of reaction

      i,in i,out C

      decreases to a minimum at . As the reactor is assumed to be

      i,out C

      perfectly mixed, is the concentration throughout the reactor, that

      i,out

      is, this gives the lowest rate throughout the reactor. The shaded area

      V /F in the figure represents the space–time ( ).

      Mixed-Flow Reactor Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Concentration vs Reaction Rate Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY I dealized Reactor Models

      (Smith, R., 2005)

      

      A steady uniform movement of reactant is assumed, with attempt to include mixing along the direction of flow

      

      Like the ideal-batch reactor, the residence time in a PFR is the same for all fluid elements.

      Plug- Flow Reactor Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      

      Plug-flow operation can be approached by using a number of mixed-flow reactors in series.

      

      The greater the number of mixed-flow reactors in series, the closer is the approach to plug-flow operation Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Plug- flow Model

          

      … (2.6.22)

      … (2.6.20) dV r dX dN i i in i

        ,

      I ntegration of (2.6.20):

      … (2.6.21)

        i

      

    X

    i i in i r dX

      N

      V ,

      Writing (2.6.21) in term of the space time:

       

    i

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      

    X

    i i in i r dX

      C ,

      τ

      For the special case of constant density systems, substitution (2.6.13) gives:

      … (2.6.23) 

         out i

    C

    in i

    C

    i i in i in i r dC

      C N

      V , ,

      , , … (2.6.24)

          out i C in i C i i r dC

      Rearrange (2.6.19):

      per unit time where

          

      = moles of Component

         

        

           

           unit time per volume l incrementa leaving reactant of moles unit time per converted reactant of moles unit time per volume l incrementa entering reactant of moles

      … (2.6.16)   i i i i

      N dN dV r N    

      … (2.6.17)

      (2.6.16) can be written per unit time as:

      N i

      

    i

      i

      per unit time where Rearrange (2.6.17):

      dV r dN i i

       … (2.6.18)

      Substituting reactor conversion into (2.6.17):

         

    dV r

      X N d dN i i in i i

        

      1 ,

      … (2.6.19) N i ,in

      = inlet moles of Component

      , , τ

      C

      This Figure is a plot of (2.6.24). The rate of reaction is high at

      i,in C

      and decreases to where it is the lowest. The area under the

      i,out curve now represents the space–time.

      Plug-Flow Reactor Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Concentration vs Reaction Rate Use of mixed- flow and plug- flow reactors.

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Example 2.6.1:

      Benzyl acetate is used in perfumes, soaps, cosmetics and household items where it produces a fruity, jasminelike aroma, and it is used to a minor extent as a flavor. I t can be manufactured by the reaction between benzyl chloride and sodium acetate in a solution of xylene in the presence of triethylamine as catalyst. or A + B C + D

       The reaction has been investigated experimentally by Huang and Dauerman in a batch reaction carried out with initial conditions given in Table as follows:

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    • 3

      3 The solution volume was 1.321 × 10 m and the temperature

      maintained to be 102 ◦C. The measured mole per cent benzyl chloride versus time in hours are given as follows:

      Experimental data for the production of benzyl acetate.

      Derive a kinetic model for the reaction on the basis of the experimental data! Assume the volume of the Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY reactor to be constant.

      Solution:

      Solution The equation for a batch reaction is given by (2.6.2):

      N At dN

      A t

      

      

      N r

      V A A

      I nitially, it could be postulated that the reaction could be zero order, first order or second order in the concentration of A and B. However, given that all the reaction stoichiometric coefficients are unity, and the initial reaction mixture has equimolar amounts of A and B, it seems sensible to first try to model the kinetics in terms of the concentration of A. This is because, in this case, the reaction proceeds with the same rate of change of moles for the two reactants. Thus, it could be postulated that the reaction could be zero order, first order or second order in the concentration of A. I n principle, there are many other possibilities. Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Substituting the appropriate kinetic expression into (2.6.11) and integrating gives the expressions in Table as follows:

      Expressions for a batch reaction with different kinetic models.

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY The experimental data have been substituted into the three models and presented graphically in Figure as follows: From Figure, all three models seem to give a reasonable representation of the data, as all three give a reasonable straight line. I t is difficult to tell from the graph which line gives the best fit. The fit can be better judged by carrying out a least squares fit to the data for the three models. Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      The difference between the values calculated from the model and the experimental values are summed according to:

      the best fit is given by a Results of a least squares fit for the three

      first order reaction model: kinetic models. r C A A A

      = k -1 with k = 0.01306 h . A Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Consider now which of the idealized models is preferred for the categories of reaction systems introduced in Section 2.3.

      1. Single reaction: Clearly, the highest rate of reaction is maintained by the highest

      

    • 3

      C concentration of feed ( , kmol· m ).

      FEED

      in the mixed-flow reactor the incoming feed is instantly diluted by  the product that has already been formed.

      The rate of reaction is thus lower in the mixed-flow reactor than  in the ideal-batch and plug-flow reactors, since it operates at the low reaction rate corresponding with the outlet concentration of feed.

      Thus, a mixed-flow reactor requires a greater volume than an  ideal-batch or plug-flow reactor. Consequently, for single Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY reactions, an ideal-batch or plug-flow reactor is preferred.

      2. Multiple reactions in parallel producing byproducts: The ratio of the rates:

       Maximum selectivity requires a minimum ratio r / r

      

      2

      1 A batch or plug-flow reactor maintains higher average

       concentrations of feed (C ) than a mixed-flow reactor, in which

      FEED

      the incoming feed is instantly diluted by the PRODUCT and BYPRODUCT.

      I f a > a : the primary reaction to PRODUCT is favored by a

      

      1

      2 high concentration of FEED: use batch or PFR I f a < a the primary reaction to PRODUCT is favored by a low

      

      1

    2 Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      concentration of FEED: use a mixed-flow reactor Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      3. Multiple reactions in series producing byproducts: For a certain reactor conversion, the FEED should have a

       corresponding residence time in the reactor.

      I n the mixed-flow reactor, FEED can leave the instant it enters or  remains for an extended period. Similarly, PRODUCT can remain for an extended period or leave immediately. Substantial fractions of both FEED and PRODUCT leave before and after what should be the specific residence time for a given conversion. Thus, the mixed-flow model would be expected to give a poorer selectivity or yield than a batch or plug-flow reactor for a given conversion.

       A batch or plug-flow reactor should be used for multiple . Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY reactions in series

      4. Mixed parallel and series reaction producing byproducts:

      a

      > : use a batch or plug-flow reactor  a

      1

      2 a

      < : use a mixed-flow reactor  a

      1

      2

       Series of mixed-flow reactors

       Plug-flow reactors with a recycle

       Series combination of plug-flow and mixed-flow Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY reactors

      Mixed parallel and series reactions producing byproducts

      As far as the parallel byproduct reaction is concerned, for high selectivity, if: > a , use a batch or plug-flow reactor

    • a

      1

      2

      < a , use a mixed-flow reactor

      1

      2

    • a

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      if a

      < a

      1

    2 Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY I I .7.

    REACTOR CONFI GURATI ON

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    1. Tubular Reactor there is steady movement only in one direction.

      

      I f heat needs to be added or removed as the reaction

      

      proceeds, the tubes may be arranged in parallel, in a construction similar to a shell-and-tube heat exchanger. Tubular reactors can be used for multiphase reactions.

      

      However, it is often difficult to achieve good mixing between phases, unless static mixer tube inserts are used. One mechanical advantage tubular devices have is when

      

      high pressure is required. Under high-pressure conditions, a small-diameter cylinder requires a thinner wall than a large-diameter cylinder.

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Tubular reactor

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    2. Stirred Tank Reactor

      Can be operated:

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Heat Transfer to and from Stirred Tank

      Continuous

      

      Semi batch

      

      Batch

      

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Application include:

      heterogeneous solid–liquid reactions

      

      heterogeneous liquid–liquid reactions

      

      heterogeneous gas–liquid reactions

      

      homogeneous liquid-phase reactions

      

       heterogeneous gas–solid–liquid reactions.

    3. Fixed- bed Catalytic Reactor the reactor is packed with particles of solid catalyst.

       Most designs approximate to plug-flow behavior. 

      I f the catalyst degrades (e.g. as a result of coke formation on the  surface), then a fixed-bed device will have to be taken off-line to regenerate the catalyst. This can either mean

      shutting dow n the or using . plant a standby reactor

      I f a standby reactor is to be used, two reactors are periodically  switched, keeping one online while the other is taken offline to regenerate the catalyst. Several reactors might be used in this way to maintain an overall operation that is close to steady state.

      However, if frequent regeneration is required, then fixed beds are  not suitable, and under these circumstances, a moving bed or a fluidized bed is preferred.

      Gas–liquid mixtures are sometimes reacted in catalytic packed  beds. Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Heat transfer arrangements for fixed- bed catalytic reactors.

      The simplest form of fixed-bed catalytic reactor uses an adiabatic arrangement Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Heat transfer arrangements for fixed- bed catalytic reactors If adiabatic operation is not acceptable because of a large temperature rise for an exothermic reaction or a large decrease for an endothermic reaction, then cold shot or hot shot can be used Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Heat transfer arrangements for fixed- bed catalytic reactors a series of adiabatic beds with intermediate cooling or heating can be used to maintain temperature control Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Heat transfer arrangements for fixed- bed catalytic reactors Tubular reactors similar to a shell-and-tube heat exchanger can be used, in which the tubes are packed with catalyst. The heating or cooling medium circulates around the outside Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY of the tubes.

    4. Fixed- bed Non- catalytic Reactor

      Fixed-bed noncatalytic reactors can be used to react a gas and a

       solid.

      For example, hydrogen sulfide can be removed from fuel gases by  reaction with ferric oxide:

      The ferric oxide is regenerated using air:

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      5. Moving- bed Catalytic Reactor I f a solid catalyst degrades in

       performance, the rate of degradation in a fixed bed might be unacceptable. I n this case, a moving-bed reactor can be used.

      Here, the catalyst is kept in  motion by the feed to the reactor and the product. This makes it possible to remove the catalyst continuously for regeneration.

      An example is a refinery  hydrocracker reactor

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      6. Fluidized- bed Catalytic Reactor

    I n fluidized-bed reactors, solid material in the form of fine

      

      particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat

      

      transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. The performance of fluidized-bed reactors is not

      

      approximated by either the mixed-flow or plug-flow idealized models. The solid phase tends to be in mixed-flow, but the bubbles

       lead to the gas phase behaving more like plugflow.

      Overall, the performance of a fluidized-bed reactor often lies

       somewhere between the mixed-flow and plugflow models. Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    7. Fluidized- bed Non- catalytic Reactor

      

    Fluidized beds are also suited to gas–solid noncatalytic

     reactions.

      All the advantages described earlier for gas–solid catalytic

       reactions apply here.

      As an example, limestone (principally, calcium carbonate)

      

      can be heated to produce calcium oxide in a fluidized-bed reactor according to the reaction Air and fuel fluidize the solid particles, which are fed to the

      

      bed and burnt to produce the high temperatures necessary for the reaction. Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

    8. Kiln

      

    Reactions involving free-flowing solid, paste and slurry materials

       can be carried out in kilns.

      I n a rotary kiln, a cylindrical shell is mounted with its axis making  a small angle to the horizontal and rotated slowly.

      The solid material to be reacted is fed to the elevated end of the  kiln and it tumbles down the kiln as a result of the rotation.

      Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Rotary Kiln The behavior of the reactor usually approximates plug-flow.

       High-temperature reactions demand refractory lined steel shells

       and are usually heated by direct firing.

      An example of a reaction carried out in such a device is the  production of hydrogen fluoride.

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