evaluation of reaction kinetics constants new

8/8/2019 Evaluation of Reaction Kinetics Constants New

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Instructors : TS.Mai Thanh Phong

Evaluation of Reaction Kinetics Constants 1

1. Nguyn Quc Khng Anh 608000432. L Vn Cng 60800222


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INTRODUCTION

SIMPLE REACTION SYSTEMS

COMPLEX REACTION SYSTEMS

NOTE IN REACTION CALORIMETRY WORK

CONCLUSION


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INTRODUCTION


Reaction kinetics has several applications in

the field of process optimization:

- minimize competing reactions

- favor the desired reaction

- maximize yield and minimize cycle times


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INTRODUCTIONInformation about the reaction kinetics of a

chemical process is vital for both process safety

and process optimization in the scale-up of batchand semibatch reactions.


This papers method is calorimetric reaction

and from the relationship between the heatoutput rate and the heat evolved, a reaction rate

constant was obtained


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SIMPLE REACTION SYSTEMSy The measurement made in a calorimetry

experiment is related to the rate of reaction as

expressed in the following equation:

dQ/dt = 7 (Qi d[R]i) / dt

- dQ/dt = rate of heat evolution, W/mol

- Qt = heat evolved at time t, J/mol

- Qn = heat of reaction of nth reaction of scheme,J/mol



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SIMPLE REACTION SYSTEMSThis method deals with isothermal

systems.The expressions for the relationship

between the heat evolution rate and the total

heat evolved may be illustrated by somecases:


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y Reaction (3): Ap B k1

,Q1A p C k2,Q2

[B]0, [C]0 = 0

Rate of reaction = k1[A] + k2[A]

dQ/dt = k1Q1 + k2Q2(k1 + k2) Qt


yReaction (2): Am B

k1 forward, k2 reverseRate of reaction = (k1[A]) (k2[B])

dQ/dt = k1Q (k1 + k2)Qt


y Reaction (1): Ap B

Rate of reaction = k[A]

dQ/dt = kQ kQt

Reaction (4): A + BpC[A]0 < [B]0Rate of reaction = k[A][B]dQ/dt = kQ[B]0 k([A]0 + [B]0)Qt+ k[A]0Qt2/Q

y Reaction (5): A + Bm C + D

k1 forward, k2 reverse

[A]0 < [B]0; [C]0, [D]0 = 0

Rate of reaction =k1[A][B] k2[C][D]

dQ/dt = k1Q [B]0 k1([A]0 + [B]0)Qt+ [A]0Qt2 / Q(k1 k2)


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Reaction (6): A + B m C

k1 forward, k2 reverse[A]0 < [B]0; [C]0 = 0

Rate of reaction = k1[A][B] - k2[C]

dQ/dt = k1Q[B]0(k1([A]0 + [B]0)+ k2)Qt + k1[A]0Qt2/Q



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Examples: Reaction of benzyl chloride with sodium methoxide

Figure 1: Heat evolution profile of benzyle chloride/sodium methoxide reaction


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COMPLEX REACTIONSYSTEMS

It is difficult or impossible to derive an analytical solution

for the relationship between the heat output rate and the heatevolved


For a more complex reaction system, more elaboratetechniques are required to establish a mechanism and to extract

reaction rate constants


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The observed data may be compared with those

predicted by computer simulation of the proposed reactionmechanism using trial values of rate constants.


However, a good fit to the experimental data is notconclusive proof of a reaction kinetic model andadditional corroborative evidence is required.



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Example 1: the case of a pair of simultaneous reversible reactions:Am BAm C

Rate of reaction = k1[A] k2[B] + k3[A] k4[C]


Figure 2: Heat evolution profile of simultaneous first-order reaction example


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COMPLEX REACTION

SYSTEMSExample 2: the case of an autocatalytic reaction:


Figure 3: Heat evolution profile of autocatalytic reaction example.


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Example 3: Reaction of acetic anhydride with methanol


Figure 4: Heat evolution profile of methanol/acetic anhydride reaction.


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NOTE IN REACTION CALORIMETRIC WORK

Experimental noise may also be more noticeablewhen the heat output rate is low, and this may be an

additional source of error.


Calorimetry calibrations need to be done verycarefully to avoid accumulative errors.


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CONCLUSIONSReaction calorimetry may be used to obtain

information about the kinetics of the reaction under study.

One of the significant advantages of using a reactioncalorimeter for reaction kinetics work is the facility forclose control of reaction mixture temperatures.

With simple reaction systems it is possible to obtainvalues for the reaction kinetic constants.



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For more complex systems, although reactioncalorimetry in isolation does give information about the

kinetics of the chemical process under study, it isnecessary to incorporate additional data to derive reactionkinectic constants.

Careful experimental technique is required to avoidsome of the errors that may be encountered in this method.


CONCLUSIONS


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evaluation of reaction kinetics constants new

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