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Liquid Phase Chemical Reactors

By Wastack2013 Mar 05, 2015 4575 Words
Summary
Researchers typically use a batch reactor to study reaction kinetics under ideal conditions. This experiment was carried out in other to study effect of temperature on reaction rate constant and also to study the effect of the reaction rate constant in a batch stirred tank reactor its important in chemical industry because it is used to determine the effect of temperature on reaction rate constant; it is also used in chemical and process industry for solids dissolution, product mixing, chemical reaction, batch distillation, crystallization, and polymerization. It is also used in the laboratories in obtaining design, interpretation of rate of reaction and numerical treatment of kinetics experimental data for different types of reacting systems.The use of a batch reactor for the most part eliminates the effects due to fluid flow on the resulting reaction rates. Consequently, the data reflect the intrinsic kinetics for the reaction being investigated. The objectives of this experiment1

To find the reaction rate constant in a batch stirred tank reactor for the saponification of ethyl acetate with dilute sodium hydroxide. To determine the effect of temperature on reaction rate constant. To find the values of rate constant and Arrhenius parameters. Equipment used

Arm field batch stirred tank reactor
A stop clock
A conductivity meter
Water bath (tank) with a thermostat
Funnel
A heat controller
Two flasks of one litre each and Stock solutions (0.1M sodium hydroxide and 0.1M ethyl acetate). The key results obtained include
Table showing rate constant, , , in a batch stirred tank reactor. T(
T

Rate constant/

25

30

40

To conclude
All the objectives where met as the results below shows and from those results there is a clear indication that conductivity is inversely proportional to the temperature also the reaction rate constant is directly proportional to the temperature . From the experiment it shows that in a batch reactor, since there is no inflow or outflow, the reactants concentration reduces with time. For an overall second order reaction (first order in both components) like the one that was done in the lab, the reaction rate is also dependent on the concentration of the reactants unlike a zero order reaction that is independent of the concentrations of the reactants. Conductivity of a solution decreases with time as the solution loses it ions, it will become less conductive. The things that I learnt doing this experiment are

What I have learnt from this experiment was that conductivity is used instead of concentration may be since they both directly proportional to each other and affected with the same parameters. Also I have learnt how the rate of a reaction could be increased or decreased by factors such as temperature. Introduction

A batch reactor is used in chemical processes for small scale operation, for testing new processes that have not been fully developed, for the manufacturer of expensive products and for processes that are difficult to convert to continuous operations. The advantage of batch reactor is high conversion which be obtained by leaving the reactant in the reactor for long periods of time but it also has the disadvantages of high labor costs per batch and the difficulty of large scale production. In a batch reactor, all the reactants are loaded at once, the concentration then varies with time, but at any one time it is uniform throughout. Agitation serves to mix separate feeds initially and to enhance heat transfer. Batch reactors are popular in practice because of their flexibility with respect to reaction time and to the kinds and quantities of reactions that can process. The characteristic of batch reactor such as the total mass of each batch is mixed, each batch is a closed system and the reaction (residence) time for all elements of fluid is the same. A chemical reactor is an equipment unit in a chemical process where chemical reactions take place to generate a desirable product at a specified production rate, using a given chemistry. The reactor configuration and its operating conditions are selected to achieve certain objectives such as maximizing the profit of the process, and minimizing the generation of pollutants, while satisfying several design and operating constraints (safety, controllability, availability of raw materials, etc.). Usually, the performance of the chemical reactor plays a pivotal role in the operation and economics of the entire process since its operation affects most other units in the process (separation units, utilities, etc.). Chemical reactors usually fulfill three main requirements:

1. Provide appropriate contacting of the reactants.
2. Provide the necessary reaction time for the formation of the desirable product. 3. Provide the heat-transfer capability required to maintain the specified temperature range. In many instances these three requirements are not complimentary, and achieving one of them comes at the expense of another. A batch experimental reactor is used for slow reactions since species compositions can be readily measured with time. Batch reactors are used widely in industry at all scales. Batch reactors are tanks , commonly provided with agitation and a method of heat transfer ( usually by coils or external jacket) . This type of reactor is primarily employed for relatively slow reactions of several hours duration, since the downtime for filling and emptying large equipment can be significant . Agitation is used to maintain homogeneity and to improve heat transfer. In a batch reactor it consists of a tank, integral heating and cooling system; one or more fluid reagents are introduced into a tank reactor equipped with an impeller which helps to stirs the reagents to ensure proper perfect mixing. Batch chemical reactors are used for a variety of process operations such as solids dissolution, product mixing, chemical reactions, batch distillation, crystallization, liquid/liquid extraction and polymerization. Chemical reactors vary widely in size, shape and method of operation; the simple types of reactor are: batch reactor (BR); based on complete mixing, plug flow reactor (PFR); based on plug flow, continues stirred tank reactors (CSTR); based on back-mix flow and laminar flow reactor (LFR); based on laminar flow. Batch reactors are used both in laboratories and industrial process for producing chemicals. In the laboratories, it is used in obtaining design, interpretation of rate of reaction and numerical treatment of kinetics experimental data for different types of reacting systems. In industries, it is used for small scale production; especially for situations whereby switching from one process or product to another are required; such as in manufacture of pharmaceuticals. It is also used in the production of polyvinyl chloride (involving suspension of polymerization) and emotion polymerization latex Generally several factors could influence the rate of a chemical reaction. Some of these factors include: Temperature

Concentration
State of reactants (that is if they are solid, liquid or gas) Order of reaction

Theory
The overall mass balance for the stirred reactor can be written as: Rate of change within the reactor = Input –output-Loss by reaction For batch operation, the overall mass balance can be rearranged to: Rate of change within the reactor = Loss by reaction

i.e. for a material A:

Where:
CA0 is input concentration of A in the input stream
CA = exit concentration of A in the exit stream
rA = rate of reaction of A.
In homogeneous reaction systems, reaction rates depend on the concentration of the reactants.

Collision theory indicates a rate increase if the concentration of one or both of the reactants is increased. Conversely, lowering the concentration should have the opposite effect. However, the specific effect of concentration changes in a reaction system has to be determined by experimental methods. Increasing the concentration of substance A in reaction with substance B could increase the reaction rate, decrease it or have no effect on it – depending on the particular reaction. It is important to recognise that the balanced equation for the net reaction does not indicate how the reaction rate is affected by a change in concentration of reactants. The general form of the rate law for a bimolecular reaction is:

Where:

The reaction to be studied in this experiment for the batch reactor was for the saponification of ethyl acetate with dilute sodium hydroxide. The equation is as shown below:

Saponification is the name given to the chemical reaction that occurs when a vegetable oil or animal fat is mixed with a strong alkali. The products of the reaction are two: soap and glycerin. Water is also present, but it does not enter into the chemical reaction. The water is only a vehicle for the alkali, which is otherwise a dry powder. It is commonly refers to the reaction of a metallic alkali such as Lye (A.K.A. Sodium Hydroxide or NaOH) with an animal or vegetable fat, or oil to produce soap. In this reaction, two products result: Soap and Glycerine. The equation is as shown below:

The structural formula is shown below:

Sodium Hydroxide (NaOH) is a caustic base or metallic alkali NaOH is used in this experiment which means it will result in a hard soap will result. In saponification, the metallic alkali, in this case sodium hydroxide (NaOH) breaks down the fat with which it is mixed. In soap making, fats used can either be vegetable oils like olive oil, or animal fats. When the oil or fat is mixed with the base the process takes place it can be endothermic reaction meaning it absorbs surrounding heat or exothermic reaction releasing heat. In the reaction the rate is expected to depend on the concentration of A and B. The order of the reaction in both components is first; therefore the reaction has an overall order of two, since the overall order is the sum of the power order of each of the components. Thus, from Equating equation (1) and (2) it gives:

However, if the two concentration are made equal from the start, then;

Integrating gives the equation 5 below:

From equation (5), a graph of against can be plotted which will give a slope that is equal to rates constant

Concentration of hydroxide,

is the concentration of reactant ‘A’ in the exit stream at time t is the initial concentration of NaOH
is the conductivity of the solution at time t
is the conductivity of the solution at the end of reaction
is the initial conductivity at time t = 0
The relationship between reaction rate and temperature is explained by Arrhenius equation which is given as:

Where:
is the reaction rate constant
is the universal gas constant (8.314 J mol-1K-1)
is temperature (K)
is the activation energy (kJ mol-1)
is Arrhenius constant or pre-exponential factor (s-1) for a first order reaction By applying natural logarithm to both sides, equation 7 above can be re-written as shown below:

A plot of against would yield a straight line graph from which ‘A’ can be obtained as the intercept of the line at. The activation energy (Ea) can then be obtained from the slope of the graph which is given as. Thus if the slope, S =, then the activation energy can be estimated as:

Reactions with low activation energy are relatively temperature-insensitive while those with high activation energies are very temperature sensitive. Therefore, any given reaction is much more temperature-sensitive at a low temperature than at a high temperature. Thus the batch stirred tank reactor allows to measure rates of reaction by observing the change of reactant concentration within the reactor with time. It can also easily change and control the temperature of the system. Experimental equipment 1

Figure 1: Layout of the experimental equipment

Description
As seen in figure 2, the experimental diagram shows the equipment used in the laboratory practical and the layout. These include: 1. Arm field batch reactor, which was the main equipment that was used for the experiment to take place where the reactants were mixed in; 2. A stop clock for measuring time (in seconds) after start up as the run proceed; 3. A portable conductivity meter, which measures the time dependent conductivity of the solution in the tank. The unit on the meter is Siemens m-1; 4. Water bath with a thermostat, where ethyl acetate and sodium hydroxide were place in other to maintain their temperature; 5. Funnel used to pour ethyl acetate and diluted sodium hydroxide into the batch reactor; 6. Heat controller used to control the temperature for the experiment at the different temperature; 7. Flasks used to measure out 1 litre of each reactant(ethyl acetate and sodium hydroxide); 8. Water solution used to put the conductivity probe inside while the reactants were poured into the reactor; 9. A thermometer for measuring the water temperature.

Experimental Procedure
1. The reaction temperature was initially set to 25 oC on the thermostatic bath. 2. Two 1 litre flasks were filled to the mark with sodium hydroxide solution and ethyl acetate solution respectively. They were then placed in the bath. 3. The reaction temperature was set on the reactor control panel. 4. The conductivity meter was set up and its probe end was placed in the bath to come reaction temperature. 5. The flasks were allowed to reach reaction temperature.

6. The solution of sodium hydroxide (1 litre) was added to the reactor, as well as the solution of ethyl acetate (1 litre) was added to the reactor and the clock was started when ca. 50% was added. 7. After 30 seconds, the ca. 200 ml sample was withdrawn from the reactor and straight after that, its conductivity was measured, noting time. The sample was then returned to the reactor and the probe to the bath. 8. This was repeated every 30 seconds for 10 minutes.

9. The experiment was allowed to run for another 20 minutes, this time taking readings every 3 minutes. 10. The experiment was then repeated at different temperatures of 30oC and 40oC

Observations
It was observed that the conductivity readings obtained at 25oC, 30oC and 40oC decreases with time. It was observed that during the experiment the conductivity meter was not stable because the readings keep fluctuating. It was also observed that as the temperature increases the conductivity readings increases. It was observed that during the experiment there was condensate on the inner wall of the batch reactor. During the experiment it was observed that the temperature of the reactor panel keeps fluctuating.

Results
Results obtained during the experiment
Time (sec)
Conductivity at 25ºC (Ms/cm)
Conductivity at 30ºC
(Ms/cm)
Conductivity at 40ºC
(Ms/cm)

30
27.6
8.56
17.51
60
26.4
7.72
15.58
90
25.3
7.07
14.4
120
24.4
6.57
13.59
150
23.8
6.19
13.05
180
23.4
5.83
12.6
210
23.1
5.56
12.28
240
22.9
5.33
11.99
270
22.7
5.14
11.75
300
22.5
4.92
11.56
330
22.4
4.82
11.42
360
22.4
4.72
11.29
390
22.3
4.61
11.22
420
22.2
4.51
11.14
450
22.2
4.43
11.06
480
22.2
4.36
10.99
510
22.2
4.3
10.92
540
22.1
4.24
10.86
570
22.1
4.17
10.81
600
22.1
4.15
10.76
780
22.1
3.88
10.47
960
22.1
3.81
10.31
1140
22.1
3.74
10.19
1320
22.1
3.71
10.33
1500
22.1
3.7
10.41
Table 1 :Calculated values for 25ºC
Conductivity vs Time and 1/Ca - 1/Cao vs Time at 25ºC

Table 2 :Calculated values for 30ºC

Conductivity vs Time and 1/Ca - 1/Cao vs Time at 30ºC

Table 3 :Calculated values for 40ºC

Conductivity vs Time and 1/Ca - 1/Cao vs Time at 40ºC

T(
T

Rate constant/

25

30

40

Table4 : showing rate constant, , , in a batch stirred tank reactor.

Graph showing vs 1/T

Calculations
Sample calculations
Calculation of concentration of hydroxide, using temperature of 25oC

Where

(From graph 1)

Note:

is the concentration of reactant ‘A’ in the exit stream at time t is the initial concentration of NaOH
is the conductivity of the solution at time t
is the conductivity of the solution at the end of reaction
is the initial conductivity at time t = 0
was found by plotting a graph of the measured conductivity (k) against time and extrapolating to t = 0 as graph 1 shows ( Conductivity vs Time and 1/Ca - 1/Cao vs Time at 25ºC ) was also be found from the same graph by extrapolating to large t, which was found to be 0.1 for all temperatures measured . The reason for calculating the concentration of the hydroxide by evaluating conductivity is that the degree of conversion of reactants affects the conductivity of the reactant contents, so that recording the conductivity with respect to time helps to calculate the amount of conversion. Calculation of

Where

Calculation of

Where

Calculation of kt

Where

=

Calculation of

Where

Calculation of

Where

Calculation of Arrhenius parameters; Activation energy and frequency factor

Using Arrhenius Equation:

By equating this to equation of straight line which is

Where

From graph 7 which shows against

By equating this to equation of straight line which is to Arrhenius equation:

Where

Thus

Discussion
From graph(1,2,3) and also from table (2,3,4) shows that the conductivity decreases as the temperature increases, for instance at 30 seconds, the conductivity at 25oC was ,at 30oC was and at 40oC was . This trend also agrees with theory and this was as a result of low rate of reaction (low product formed) at 25oC. The rate of reaction usually increases as temperature increases.

From table 2 at 25it shows that as the reaction time increases the conductivity decreases from to, also from graph 1shows a negative gradient and as the time increases the conductivity decreases and then become constant as the reaction proceeds, shows a negative gradient and this is because as the reaction proceeds, the hydroxide ions are consumed and acetate ions are produced in the reaction. Positive and negative ions take part in the conduction of electricity. In ionic conduction, the charge is carried by ions and this is the mechanism of electrical conduction. The conductance of the ions depends on the ionic mobility which is also determined by the size of the ion. The conductance of the acetate ion is very weak so will not affect conductivity but that of hydroxide ion which is very strong electrolyte and fully ionised and more mobile and affects conductivity, but because more hydroxide ion is consumed and more acetate ion is produced, thus causing the conductivity of the reacting solution decreases as the reaction mixture proceeds with time.

From table 3 at 30 it shows that as the reaction time increases the conductivity decreases from to also from graph 2 as the time increases the conductivity decreases and then become constant as the reaction proceeds also shows a negative gradient and this is because as the reaction proceeds, the hydroxide ions are consumed and acetate ions are produced in the reaction. Positive and negative ions take part in the conduction of electricity. In ionic conduction, the charge is carried by ions and this is the mechanism of electrical conduction. The conductance of the ions depends on the ionic mobility which is also determined by the size of the ion. The conductance of the acetate ion is very weak so will not affect conductivity but that of hydroxide ion which is very strong electrolyte and fully ionised and more mobile and affects conductivity, but because more hydroxide ion is consumed and more acetate ion is produced, thus causing the conductivity of the reacting solution decreases as the reaction mixture proceeds with time. .

From table 4 at 40 it shows that as the reaction time increases the conductivity decreases from to also from graph 3 as the time increases the conductivity decreases and then become constant as the reaction proceeds which shows a negative gradient and this is because as the reaction proceeds, the hydroxide ions are consumed and acetate ions are produced in the reaction. Positive and negative ions take part in the conduction of electricity. In ionic conduction, the charge is carried by ions and this is the mechanism of electrical conduction. The conductance of the ions depends on the ionic mobility which is also determined by the size of the ion. The conductance of the acetate ion is very weak so will not affect conductivity but that of hydroxide ion which is very strong electrolyte and fully ionised and more mobile and affects conductivity, but because more hydroxide ion is consumed and more acetate ion is produced, thus causing the conductivity of the reacting solution decreases as the reaction mixture proceeds with time. .

From table 5 it shows that at 2540 increases decreases.

From graph 4 which shows against at 25 shows a positive gradient and as the time increases the increases and the slope of the graph gave the rate constant of. Rate of reaction is a function of rate constant; thus, the trends from the experimental results confirm that the reaction rate constant increases as temperature increases. The increase in rate of reaction with increasing temperature was due an increase in the number of collision between the molecules of the reactants. Before a reaction can take place, the molecules must collide with enough energy (Activation energy Ea minimum amount of energy need for a reaction to occur), as the temperature is increased the average kinetic energy of the molecules increases, also the molecules with the minimum energy to react increase thus, increasing the rate of the reaction.

From graph 5 which shows against at 30 shows a positive gradient and as the time increases the increases but at the conductivity decreases to zero and the slope of the graph gave the rate constant of. Rate of reaction is a function of rate constant; thus, the trends from the experimental results confirm that the reaction rate constant increases as temperature increases. The increase in rate of reaction with increasing temperature was due an increase in the number of collision between the molecules of the reactants. Before a reaction can take place, the molecules must collide with enough energy (Activation energy Ea minimum amount of energy need for a reaction to occur), as the temperature is increased the average kinetic energy of the molecules increases, also the molecules with the minimum energy to react increase thus, increasing the rate of the reaction. Although there are some discrepancies in the result and this may be due to error when taking the readings and also the temperature keeps fluctuating during the experiment.

From graph 6 which shows against at 40 shows a positive gradient and as the time increases the increases and the slope of the graph gave the rate constant of. Rate of reaction is a function of rate constant; thus, the trend from the test results confirm that the reaction rate constant increases as temperature increases. The increase in rate of reaction with increasing temperature was due an increase in the number of collision between the molecules of the reactants. Before a reaction can take place, the molecules must collide with enough energy (Activation energy Ea minimum amount of energy need for a reaction to occur), as the temperature is increased the average kinetic energy of the molecules increases, also the molecules with the minimum energy to react increase thus, increasing the rate of the reaction.

From graph 7 which shows against at 2540 in accordance to Arrhenius law, the graph gave the numerical value of and the intercept, from which the activation energy Ea was calculated and the pre-exponential factor (A) was calculated from the y-intercept. Activation energy calculated was and Pre-exponential factor obtained from calculation was shows a positive gradient which is an indication that the reaction is exothermic which means the reaction is releasing heat to the environment. Recommendations

It would have been better if the experiment was conducted under more controlled factor without interfering with the conductivity probe. Also using a digital stop watch would have been better as this would save time and time error but because time was done manually using stop watch and by the time the stop watch was stopped it is after few seconds the conductivity needs to be read which causes anomalous result. Because of the limited time during the laboratory where the mistake was made could not be redone. Conclusion

A batch experimental reactor is used for slow reactions since species compositions can be readily measured with time. In a batch reactor, since there is no inflow or outflow, the reactants concentration reduces with time. Conductivity of the solution decrease as temperatures increases, and decreases with increase in time. From the experiment it shows that the rate of reaction which is a function of time increases as temperature increases and vice versa, due to increase in the number of collision between molecules; thus rate constant also increases as temperature increases From the experiment it shows that in a batch reactor, since there is no inflow or outflow, the reactants concentration reduces with time. For an overall second order reaction (first order in both components) like the one that was done in the lab, the reaction rate is also dependent on the concentration of the reactants unlike a zero order reaction that is independent of the concentrations of the reactants. Conductivity of a solution decreases with time as the solution loses it ions, it will become less conductive. From the experiment Pre-exponential factor (obtained for the reaction was From the experiment the Activation energy (Ea) for the reaction was From the experiment the graph of against showed a decreasing slope and positive gradient therefore it is an exothermic reaction and that equilibrium conversion drops. Change in temperature has effects on the rate of reaction and the conductivity, for instant in this experiment as the temperature increases the conductivity increases but conductivity reduces with time because as the temperature increases then the more the molecules in the substances are ready to react and they move faster and thus dissociating more ions at a faster rate.

Nomenclature
CA0 is input concentration of A in the input stream (only for equation 1.1) CA is exit concentration of A in the exit stream (only for equation 1.1) rA is the rate of reaction of A

is the reaction rate constant
is the universal gas constant (8.314 J mol-1K-1)
is temperature (K)
is the activation energy (kJ mol-1)
is Arrhenius constant or pre-exponential factor (s-1) for a first order reaction is the concentration of reactant ‘A’ in the exit stream at time t is the initial concentration of NaOH
is the conductivity of the solution at time t
is the conductivity of the solution at the end of reaction
is the initial conductivity at time t = 0
References
1 Dr .S. Larkai (2011 -2012 ) , Principles of separation and reaction laboratory booklet : continuous stirred tank reactors . pp 1 – 4. Faculty of engineering science and the built environment, London south bank university. F.A.Holland and F.S.Chapman "Liquid Mixing and Processing in Stirred Tanks", p.109 (Chapman & Hall, 1996). J.J Carberry. "Chemical & Catalytic Reaction Engineering", p.92 (McGraw-Hill, 1976). J M Coulson & J F Richardson, Chemical engineering 2005.

Atkins. P and Jones.L (2010) Chemical principles, the quest for insight.5th Ed. New York: Freeman and company. Missen. R. W. et al (1999) Introduction to chemical reaction engineering andkinetics. New York: John Wiley & Sons. Smith. M. J (1981) Chemical engineering kinetics 3rded. New York: McGraw-Hill. Smith, J.M, H.C Van Ness, M.M Abbott (19161). Introduction to Chemical Engineering Thermodynamics. Boston: McGraw-Hill, c2005 Holland D. C & Anthony G. R. Fundamental s of Chemical Reaction Engineering. 2nd ed. Englewood Cliffs, New Jersey: Prentice Hall.

http://www.neduet.edu.pk/Chemical/PDF/CHEMICAL%20REACTION%20ENGINEERING%20LAB.pdf accessed on the 16th of march

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