Chemical reactor selection and design Rajesh Kumar Bhagat

Chemical reactor selection and design Rajesh Kumar Bhagat B. E. (CHE) 4 th year

Introduction Almost all chemical engineering process contains three operations. Raw material Unit operation (cleaning ) Chemical reactor Unit operation (separation) What does chemical reactor design means ? Product

Reactor System Homogenous Heterogeneous

Types of reactors 1. Batch- uniform composition everywhere in reactor but changes with time 2. Semi batch- in semi-batch one reactant will be added when reaction will proceed 3. Continuous reactor a. Mixed flow- this is uniformly mixed , same composition everywhere, within the reactor and at exit b. Plug flow- flow of fluid through reactor with order so that only lateral mixing is possible.

Reactor design parameter Reactor design basically means which type and size of reactor and method of operation we should employ for a given conversation Parameters • • • Volume of reactor Flow rate Concentration of feed Reaction kinetic Temperature pressure

Isothermal reactor design algorithm

Plug flow and mixed flow reactor design Mixed flow reactor design Applying mass balance performance equation for mixed flow reactor Plug flow reactor design Performance equation for plug flow reactor

Performance equation

Plug flow vs CSTR • • For any particular duty and for all positive reaction order the volume of mixed flow reactor will always be grater then plug flow Area under curve in figure is very small for plug flow as compared to mixed flow so volume is small for plug flow. When conversion is small, the reactor performance is only slightly affected by flow type. the perforation ratio very rapidly at high conversion. Density variation during reaction affects design, however it is normally of secondary importance compared to the difference in flow type.

Multiple reactor system • Number of plug flow reactor in series are theoretically same as equivalent volume of a single plug flow reactor. • Number of mixed flow reactor of equal size in series may be used when we need high conversion and can’t perform in a single reactor. • From the given graph, for first order reaction, conversion for series of equal size reactor can be find

Mixed flow reactor of different size in series • • • From the fig it is clear that for plug flow reactor volume can be find by dashed area and for mixed flow whole area. When we are have to use mixed flow reactor, then we can use different size mixed flow reactor so, that over all volume would be small To optimized or to find how different size of mixed flow reactor should used we have to maximized lower dashed rectangle. This optimization gives the slope of diagonal of the rectangle should be equal to slope of curve at intersection of these two reactor. Levenspiel , has proved that after overall economic consideration equal size reactors in series are economical.

Autocatalytic reactor • • • When a product will act like a catalyst then it is called auto catalytic reaction. In mixed flow reactor at fixed product concentration for high yield, efficiency of reactor will be very low. For no recycle for low product concentration mixed flow reactor will be preferred and for high conversion plug flow. For optimum efficiency we can use a recycle or back mixing plug flow reactors. For a particular exit concentration a particular optimum recycle ratio should be used. Optimum recycle ratio introduced to the reactor feed’s 1/(-r) value should be equal to average 1/(-r) value for whole reactor. Fig-1 Plug flow reactor with recycle Fig-2 Fig-3 Fig-4

Design for parallel reaction • • • When a reactant gives two product (desired, and undesired)simultaneously with different rate constant then this is called a parallel reaction. To keep maximum amount of desired product we can take following steps. Ifa 1>a 2 or the desired reaction is of higher order then keep reactant concentration high for high product concentration. If a 1<a 2 than for desired reaction keep reactant concentration low. For a 1=a 2 change in reactant concentration will not affect the product then, because rate constant k 1 and k 2 are different at different temperature so, we can keep our temperature such that desired product will be high or use of catalyst would be a option which are selective in nature.

Reactor design for multiple reaction • • • In multiple reaction reactor design contacting pattern is most important factor to get a particular product. In irreversible reaction in series like the mixing of fluid of different composition is the key to formation of intermediate. The maximum possible amount of intermediate is obtained if fluid of different composition and different stage of conversation are not allowed to mixed. In series of reaction if intermediate reactant is our desired product than semi batch reactor will be used.

Irreversible series-parallel reaction • • Multiple reaction that consist of steps in series and steps in parallel reaction. In these reaction proper contacting pattern is very important. The general representation of these reaction are Here the reaction is parallel with respect to reactant B and in series with A. Halogenations of alkane is a example of this kind of reaction where reaction is parallel with respect to halogen

Case study Product distribution with respect to contacting pattern • We will discuss simpler example of CASE-1 Add A slowly to B • By contacting A slowly in a beaker containing B and stirring to consume all A added , the mixer with very high concentration in S can be find. CASE-2 Add B slowly to A • Now by contacting B slowly to a beaker containing A, the concentration of R will be build up inside then after reaching a maxima R will convert in to S and the process will be gradual. CASE-3 Add A and B rapidly • In this case it will give the behavior of series reaction , R will increase first and after reaching a maxima it will diminish and concentration of S will increase.

Residence Time Distribution • • • RTD is important factor from the point of view of real equipment. Element of fluid will take different route through the reactor and may take different length of time to pass through the reactor. Ideal reactor design are made by considering volume of reactor or time spend by all the reactant will be same inside reactor. Completion of reaction will depend on time of exposure inside the reactor. The distribution of time inside the reactor is called exit age distribution E, have unite time-1. According to RTD fraction of exit stream of age between t and t+dt is E dt.

Residence time distribution determination • • RTD can be determined by two experimental method. (Pulse input experiment, and step input experiment ) In pulse experimental method in a steady state system we will put a pulse input of tracer and will plot the graph of this tracer concentration with time at output. This graph will show time variation or age distribution of tracer concentration with time. Another method of determination of RTD is by putting a step input (Preferably unite step input) of tracer. Then we can plot the graph between the concentration versus time graph of tracer. The slope versus time graph of this system will give us residence time distribution. Step input method is more accurate than pulse input method although impulse input would give the perfect distribution.

Holding time and residence time • • • Holding time is defined as time needed to treat one reactor volume. Residence time or mean residence time space time is defined as mean residence time of flowing material in the reactor. From fig when inside popcorn popper, when popping occurs at back end of popper then holding time and residence time will be same. When popping occurs in midway or every where inside the popper then the two time will be different. For unchanging density system holding time and residence time will be equal.

Heterogeneous system heterogeneous systems are those which consist of two or more than two phase Apart from temperature pressure and concentration, heat and mass transfer are important Catalytic systems Non-catalytic system

Catalytic system plug flow Reactor Differential reactor Integral reactor mixed flow type (Fluidized bed reactor) Performance equation

Catalytic reactor selection parameter and design • • • Reaction type Reactor type Economics Rate of deactivation Other process requirement

Reaction type • Chemical kinetics of reaction can be known by knowing the type of reaction • For reactor selection reaction type will tell us about heat of reaction either reaction is endothermic or exothermic. • Selectivity is defined as reaction rate ratio for two parallel reaction. • Catalyst are used to increase reaction rate and selectivity for a specific reaction. • We can determine what type of catalyst will be used. • Reaction temperature range will be determined.

Reactor type • Reactor may be a plug flow or mixed flow or batch flow reactor or other. • Contacting pattern of reaction will be known. • In case of expensive catalyst and high heat transfer rate required, mixed flow(fludized bed) reactor are used. • For high mass transfer plug flow (packed bed) reactor will be used.

Economics • For reactor design overall economics should be considered. • Like instead of different size of mixed flow reactor in series, equal size mixed flow reactor are economically good. • If catalyst is not very expensive then we may opt to non-regeneration but for expensive regeneration must be considered.

Packed bed Solid fluid contact will be most efficient High amount of catalyst will be used Heat transfer will be difficult Pressure drop will be high Effective for mass transfer control system With increase in temperature side reaction will be a problem and less selectivity • Sintering of catalyst may happen • • •

Fluidized bed • • • Industrially most widely used Heat transfer are very good Pressure drop is low Catalyst can easily replaced for regeneration Amount of catalyst necessary is less Surface area per unite mass of catalyst will be large

Fluidized bed catalytic reactor design Types of fluidized bed catalytic reactor • Bubbling fluidized bed(BFB)- industrial solid catalyzed reactor generally works as bubbling fluidized bed reactor. Calculation of conversion for bubbling flow varies between plug flow to mixed flow. • Turbulent fluidized bed reactor(TFB)- at high gas velocity BFB transform in to TFB in this case no distinct bubble of gas will flow and solid movement will be violent. • Fast fluidized bed- transition from TFB with very high speed of gas this FFB will formed. • Pneumatic conveying bed- highest gas velocity for fluidization are choking velocity and after that it will converted into pneumatic bed and this reactor pneumatic conveying fluidized bed reactor. • In all three model TFB, BFB, PCB solid entrain out of bed regularly.

Bubbling fluidized bed Model for bubbling fluidization • Dispersion and tank series model • Hydrodynamic flow model • K-L model for BFB • RTD Model • Contact time distribution model Bubbling fluidized bed seems like boiling of liquid and gas bubbles are moving up with faster velocity then dispersed gas.

Contact time distribution model • In BFB faster gas stayed mainly in bubbles and slow moving gas in emulsion, according to this model effective rate constant depend on length of stay of element of gas in bed. K= K 0 tm here m is a parameter for first order constant density system concentration at exit will be

Non-catalytic system • Heterogeneous fluid-fluid or solid-gas system with two or more phase • heat transfer and mass transfer are important factor for this model • Heat may be a product of this model • Contacting scheme is very important • Equilibrium solubility (if liquid-liquid system) • Overall rate scheme • Many method like shrinking core method of analysis may be used

Reactor selection & design for burning of coal Reaction type Burning of coal is a exothermic reaction C + O 2 = CO 2 + heat Reactor selection For burning of coal contact of air and coal is very important Resistance to mass transfer will be 1. film above the coal 2. Ash layer with burning of coal 3. Resistance due to chemical reaction So, very high mass transfer resistance conti………

Ignition temperature For burning of coal minimum ignition required so, heat should be recycled Plug flow reactor with recycle will be most suitable reactor for this system Mass Transfer resistance and rate equation Total resistance = film resistance + ash resistance + reaction resistance Plug flow reactor

• Know as we know the rate of reaction by knowing all resistance • We know the flow type and reactor type is plug flow • We know feed rate from heat balance of burning of coal • From performance equation we will get the volume of reactor

References Chemical reaction engineering (octave levinspeil) Element of chemical reaction engineering (H. scott fogler) Chemical reaction design (Peter harriott) http: //highwire. stanford. edu http: //ocw. mit. com

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