II Choice of Reactor

Transcription

1 II Choice of Reactor Outline Introduction Reaction Path Types of Reaction System Reactor Performance Rate of Reaction Idealized Reactor Models Reactor Configuration Design Guideline for Reactor Page 1 of 48

2 II.1. INTRODUCTION Introduction 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 Page 2 of 48

3 Introduction 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 2. Heterogen II.2. REACTION PATH Page 3 of 48

4 Introduction 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 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 should especially be avoided, since they can create significant environmental problems. Page 4 of 48

5 Example Given that the objective is to manufacture vinyl chloride, there are at least three reaction paths that can be readily exploited. 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? Page 5 of 48

6 Solution: Decisions can be made on the basis of the economic potential of 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 = ( ) - ( ) = $ kmol-1 vinyl chloride product Path 2 EP = ( ) - ( ) = 9.99 $ kmol-1 vinyl chloride product This assumes the sale of the byproduct HCl. If it cannot be sold, then: EP = ( ) - ( ) = 4.05 $ kmol-1 vinyl chloride product Path 3 EP = ( ) - ( ) = 1.76 $ kmol-1 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. In practice, this might be quite difficult, since the market for HCl tends to be limited. In 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. In addition, it is preferred to produce no byproducts. Page 6 of 48

7 Example Devise a process from the three reaction paths in Example 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 These three reactions can be added to obtain the overall stoichiometry. Path 4 Now the economic potential is given by: EP = ( ) - ( / ) = 4.12 $ kmol-1 vinyl chloride product In summary, Path 2 from Example 2.1 is the most attractive reaction path if there is a large market for hydrogen chloride. In 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. Page 7 of 48

8 II.3. TYPES OF REACTION SYSTEM Reaction systems can be classified into six broad types (Smith, R., 2005): 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. Page 8 of 48

9 1. Single Reaction FEED PRODUCT or FEED PRODUCT + BYPRODUCT or FEED1 + FEED2 PRODUCT Examples: Does not produce by product: Produce by product: 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 and so on Page 9 of 48

10 Examples of a parallel reactions system occurs in the production of ethylene oxide 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 and so on Page 10 of 48

11 Examples of series reactions system occurs in the production of formaldehyde from methanol 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 and so on Page 11 of 48

12 Examples of mixed parallel and series reactions is the production of Ethanolamines by reaction between Ethylene Oxide and Ammonia: 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 Page 12 of 48

13 An example of polimerization reaction that involves a termination step: Polymerization of Vinyl Chloride from a free-radical initiator R Initiation step: Propagation step: and so on, leading to molecules of the structure: Eventually, the chain is terminated by steps such as the union of two radicals that consume but do not generate radicals: Page 13 of 48

14 An example of a polymerization without a termination step is 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. 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. Page 14 of 48

15 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 An example of an oxidation reaction is the production of citric acid from glucose: Page 15 of 48

16 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: II.4. REACTOR PERFORMANCE Page 16 of 48

17 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). Example 2.4.1: Benzene is to be produced from toluene according 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 Page 17 of 48

18 Solution: However, the principal concern is performance with respect to toluene, since it is more expensive than hydrogen. Page 18 of 48

19 II.5. RATE OF REACTION Rate of Reaction (Smith, R., 2005) To define the rate of a reaction, one of the components must be selected and the rate defined in terms of that component. The rate of reaction is the number of moles formed with respect to time, per unit volume of reaction mixture: ri 1 dn i V dt (2.5.1) where ri = rate of reaction of Component i (kmol m -3 s-1) Ni = moles of Component i formed (kmol) V = reaction volume (m3) t = time (s) Page 19 of 48

20 If the volume of the reactor is constant (V = constant): ri 1 dn i dn i V dci V dt dt dt (2.5.2) where Ci = molar concentration of Component i (kmol m-3) 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 + cc + ss + tt + (2.5.3) The rates of reaction are related by: r r rb r C S T b c s t (2.5.4) If 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: rb k B C Bb CCc (2.5.5) rc kc C Bb CCc (2.5.6) rs k S C Bb CCc (2.5.7) rt kt C Bb CCc (2.5.8) where ri = reaction rate for component i (kmol m-3 s-1) ki = reaction rate constant for component i ([kmol m-3]nc b c-... s-1) NC = is the number of components in the rate expression Ci = molar concentration of component i (kmol m-3) The exponent for the concentration (b, c,...) is known as the order of reaction. Page 20 of 48

21 The reaction rate constant is a function of temperature, as will be discussed next. k k B kc k (2.5.9) S T b c s t Reactions for which the rate equations follow the stoichiometry are known as elementary reactions. If 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: rb k B C B CC CS CT (2.5.10) rc kc C B CC CS CT (2.5.11) rs k S C B CC CS CT (2.5.12) rt kt C B CC CS CT (2.5.13) where,,, = order of reaction If the reaction is reversible, such that: bb cc ss tt (2.5.14) then the rate of reaction is the net rate of the forward and reverse reactions. If the forward and reverse reactions are both elementary, then: rb k B C Bb CCc k B' CSs CTt (2.5.15) rc kc C Bb CCc kc' CSs CTt (2.5.16) rs k S C Bb CCc k S' CSs CTt (2.5.17) rt kt C Bb CCc kt' CSs CTt (2.5.18) where ki = reaction rate constant for Component i for the forward reaction ki' = reaction rate constant for Component i for the reverse reaction Page 21 of 48

22 II.6. IDEALIZED REACTOR MODELS Idealized Reactor Models (Smith, R., 2005) Ideal 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 temperature throughout the reactor are both uniform. Page 22 of 48

23 Ideal Batch Model moles of reactant 1 dn i r i converted V dt (2.6.1) where t = batch time Ni0 = initial moles of Component i Nit = final moles of Component i after time t Nit dn i Ni 0 riv t Integration of (2.6.1): (2.6.2) In term of reactor conversion (Xi) dn i d N i 0 1 X i dx i Ni0 riv dt dt dt (2.6.3) Ideal Batch Model Integration of (2.6.3): Xi t Ni0 0 dx i riv (2.6.4) from the definition of reactor conversion, for the special case of a constant density reaction mixture: Xi N i 0 N it Ci 0 Cit Ni0 Ci 0 (2.6.5) where Ci =molar concentration of Component i Ci0 = initial molar concentration of Component i Cit = final molar concentration of Component i at time t Substitution of (2.6.5) into (2.6.3) Integration of (2.6.6): dci ri dt Cit dci Ci 0 ri t (2.6.6) (2.6.7) Page 23 of 48

24 Idealized Reactor Models (Smith, R., 2005) Mixed-Flow or Continuous Well-Mixed or ContinuousStirred-Tank Reactor (CSTR) Feed and product takeoff are both continuous. 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. Material Balance for Component i per unit time moles of reactant in feed per unit time moles of reactant moles of reactant in converted per unit time product per unit time (2.6.8) N i,in riv N i,out (2.6.9) where Ni,in = inlet moles of Component i per unit time Ni,in = outlet moles of Component i per unit time Rearrange (2.6.9): N i,out N i,in riv (2.6.10) Substituting Ni,out = Ni,in (1-Xi) into (2.6.10): V For the special case of a constant density system, (2.6.5) can be substituted to give: V N i,in X i ri N i,in Ci,in Ci,out ri Ci,in (2.6.11) (2.6.12) Page 24 of 48

25 Analogous to time as a measure of batch process performance, space time ( ) can be defined for a continuous reactor: V Ci,outV F N i,in (2.6.13) where F = volumetric flowrate of the feed (m3.s-1) The reciprocal of space time is space velocity (s): s 1 number of reactor volume processed in a unit time (2.6.14) Combining Equations (2.6.12) for the mixed-flow reactor with constant density and (2.6.13) gives: Ci,in Ci,out (2.6.15) ri This figure is a plot of (2.6.15), from Ci,in to Ci,out the rate of reaction decreases to a minimum at Ci,out. As the reactor is assumed to be perfectly mixed, Ci,out is the concentration throughout the reactor, that is, this gives the lowest rate throughout the reactor. The shaded area in the figure represents the space time (V /F ). Mixed-Flow Reactor Concentration vs Reaction Rate Page 25 of 48

26 Idealized Reactor Models (Smith, R., 2005) Plug-Flow Reactor 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 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 Page 26 of 48

27 Plug-flow Model moles of reactant moles of reactant moles of reactant entering incremental leaving incremental converted per unit time volume per unit time volume per unit time (2.6.16) (2.6.16) can be written per unit time as: N i ri dv N i dn i (2.6.17) where Ni = moles of Component i per unit time dn i ri dv Rearrange (2.6.17): (2.6.18) Substituting reactor conversion into (2.6.17): dn i d N i,in 1 X i ri dv (2.6.19) where Ni,in= inlet moles of Component i per unit time dn i,in dx i ri dv Rearrange (2.6.19): Xi Integration of (2.6.20): V N i,in dx i ri (2.6.21) Xi dx i ri (2.6.22) 0 Writing (2.6.21) in term of the space time: For the special case of constant density systems, substitution (2.6.13) gives: (2.6.20) Ci,in 0 V N i,in Ci,out dci Ci,in Ci,in ri Ci,out dci Ci,in ri (2.6.23) (2.6.24) Page 27 of 48

28 This Figure is a plot of (2.6.24). The rate of reaction is high at Ci,in and decreases to Ci,out where it is the lowest. The area under the curve now represents the space time. Plug-Flow Reactor Concentration vs Reaction Rate Use of mixed-flow and plug-flow reactors. Page 28 of 48

29 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. It 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: The solution volume was m3 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 reactor to be constant. Page 29 of 48

30 Solution: Solution The equation for a batch reaction is given by (2.6.2): N At dn A N A0 rav t Initially, 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. In principle, there are many other possibilities. 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. Page 30 of 48

31 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. It 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. The difference between the values calculated from the model and the experimental values are summed according to: Results of a least squares fit for the three kinetic models. the best fit is given by a first order reaction model: ra = ka CA with ka = h-1. Page 31 of 48

32 Consider now which of the idealized models is preferred for the categories of reaction systems introduced in Section Single reaction: Clearly, the highest rate of reaction is maintained by the highest concentration of feed (CFEED, kmol m-3). 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 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 r2/r1 A batch or plug-flow reactor maintains higher average concentrations of feed (CFEED ) than a mixed-flow reactor, in which the incoming feed is instantly diluted by the PRODUCT and BYPRODUCT. If a1 > a2 : the primary reaction to PRODUCT is favored by a high concentration of FEED: use batch or PFR If a1 < a2 the primary reaction to PRODUCT is favored by a low concentration of FEED: use a mixed-flow reactor Page 32 of 48

33 3. Multiple reactions in series producing byproducts: For a certain reactor conversion, the FEED should have a corresponding residence time in the reactor. In 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 reactions in series. Page 33 of 48

34 4. Mixed parallel and series reaction producing byproducts: a1 > a2: use a batch or plug-flow reactor a1 < a2: use a mixed-flow reactor Series of mixed-flow reactors Plug-flow reactors with a recycle Series combination of plug-flow and mixed-flow reactors Mixed parallel and series reactions producing byproducts As far as the parallel byproduct reaction is concerned, for high selectivity, if: a1 > a2, use a batch or plug-flow reactor a1 < a2, use a mixed-flow reactor Page 34 of 48

35 if a1 < a2 II.7. REACTOR CONFIGURATION Page 35 of 48

36 1. Tubular Reactor there is steady movement only in one direction. If 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. Tubular reactor Page 36 of 48

37 2. Stirred Tank Reactor Application include: homogeneous liquid-phase reactions heterogeneous gas liquid reactions heterogeneous liquid liquid reactions heterogeneous solid liquid reactions heterogeneous gas solid liquid reactions. Can be operated: Batch Semi batch Continuous Heat Transfer to and from Stirred Tank Page 37 of 48

38 3. Fixed-bed Catalytic Reactor the reactor is packed with particles of solid catalyst. Most designs approximate to plug-flow behavior. If 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 down the plant or using a standby reactor. If 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. Heat transfer arrangements for fixed-bed catalytic reactors. The simplest form of fixed-bed catalytic reactor uses an adiabatic arrangement Page 38 of 48

39 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 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 Page 39 of 48

40 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 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: Page 40 of 48

41 5. Moving-bed Catalytic Reactor If a solid catalyst degrades in performance, the rate of degradation in a fixed bed might be unacceptable. In 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 6. Fluidized-bed Catalytic Reactor In 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. Page 41 of 48

42 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. Page 42 of 48

43 8. Kiln Reactions involving free-flowing solid, paste and slurry materials can be carried out in kilns. In 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. 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. Page 43 of 48

44 II.8. DESIGN GUIDELINE FOR REACTOR Design Guideline for Reactor: I. II. Single irreversible reaction (not autocatalytic) A. Isothermal always use a plug-flow reactor B. Adiabatic 1. Plug-flow if the reaction rate monotonically decrease with conversion 2. CSTR operating at the maximum reaction rate followed by a plug-flow section Single reversible reaction adiabatic A. Maximum temperature if endothermic B. A series of adiabatic beds with a decreasing temperature profile if exothermic Page 44 of 48

45 Design Guideline for Reactor: III. Parallel reactions composition effects A. for A B (desired) and A S (waste), where the ratio of the reaction rates is: 1. if a1 > a2, keep CA high a. Use batch or plug-flow b. High pressure, eliminate innerts c. Avoid recycle of products d. Can use a small reactor 2. if a1 < a2, keep CA low a. Use a CSTR with a high conversion b. Large recycle of product c. Low pressure, add innerts d. Need a large reactor Design Guideline for Reactor: B. for A +B R (desired) and A + B S (waste), where the ratio of the reaction rates is: IV. if a1 > a2 and b1 > b2, both CA and CB high if a1 < a2 and b1 > b2, then CA low and CB high if a1 > a2 and b1 < b2, then CA high and CB low if a1 < a2 and b1 < b2, both CA and CB low See fig below: Consecutive reactions composition effects: A R (desired); R S (waste) : minimize the mixing of streams with different compositions Page 45 of 48

46 Design Guideline for Reactor: V. Parallel reactions temperature effects: A. if E1 > E2, use a high temperature B. if E1 < E2, use an increasing temperature profile VI. Consecutive reactions temperature effects: A. if E1 > E2, use a decreasing temperature profile not very sensitive B. if E1 < E2, use a low temperature Sumber: bahan ajar PPK TK UGM Page 46 of 48

47 Choosing heat transfer in the reactor: Operating temperature for favorable product distribution Page 47 of 48

48 Operating temperature for favorable product distribution Operating temperature for favorable product distribution Note: See Denbigh (1958) for discussion of this reaction Sumber: bahan ajar PPK TK UGM Page 48 of 48