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Experiment 6 pre lab report


CHEMICAL AND BIOLOGICAL PROCESS LAB. 458.307 004

<Chemical and Biological Process Lab 6>

Reaction Rate Constant in CSTR and Tubular Reactor

Department: Chemical and Biological Engineering Student number: 2008-12078 Name: Chew Yueh Renn Group: Wednesday Group 8 Teacher Assistant: ??? Experiment Date: 2009. 10. 06 Submission Date: 2009. 10. 06

CHEMICAL AND BIOLOGICAL PROCESS LAB. 458.307 004

1. INTRODUCTION In the majority of industrial chemical processes, the reactor vessel in which the process takes place is the key component of the equipment. The design of chemical reactors is therefore crucial to the success of the industrial operation. In general, the aim is to produce a specified product at a given rate using known reactants. Various types of reactor are used to achieve these objectives namely, continuous stirred tank reactors (CSTRs), tubular (or plug flow) reactors, PBR and etc. In this experiment, we are going to measure the reaction rate constants at CSTR. The objectives of this experiment, as follow: i. To find the reaction rate constant in a CSTR and ii. reaction rate constant in a tubular reactor 2. THEORY A. Rate Law In this experiment, the reactants are sodium hydroxide and ethyl acetate. It is called saponification in organic chemistry. It is, a chemical reaction that using ester and the basic catalyst reaction to make carboxylic acid. However, carboxylic acid doesn’t exist in this case, only acetate ions and sodium ions are coexisting. The detail of the reaction is as following. CH2COOC2H5 + NaOH CH3COONa + C2H5OH

This reaction can be thought as the secondary reactions and its reaction rate can be expressed as follows. = N V k1,k2 CA CB CR CS 1 = = ?(1 ? 2 )

= mole number of CH3COOC2H5 = volume of the reactor = reaction rate law for forward and reverse reaction = molarity of CH3COOC2H5 = molarity of NaOH = molarity of CH3COONa = molarity of CH2Oh

If the concentration of sodium hydroxide and ethyl acetate are the same and forward reaction is dominant than reverse reaction, then the equation can be expressed as follows. = 2 = ?1

CHEMICAL AND BIOLOGICAL PROCESS LAB. 458.307 004

B. CSTR (Continuous-Stirred Tank Reactor) CSTR, also known as vat- or backmix reactor, is a common ideal reactor that the reactants and products are feeding continuously into the reactor tank. In general, flow rate of reactants and products in CSTR are the same and it is stirred continuously. CSTR could be assumed to have steady state. The behavior of a CSTR is often approximated or modeled by that of a Continuous Ideally Stirred-Tank Reactor (CISTR). All calculations performed with CISTRs assume perfect mixing. In a perfectly mixed reactor, the output composition is identical to composition of the material Figure 1: CSTR inside the reactor, which is a function of residence time and rate of symbol reaction. If the residence time is 5-10 times the mixing time, this approximation is valid for engineering purposes. Design equation could be obtain as following. 0 ? + =

By assuming steady-state and the perfect mixing in all the direction inside reactor, the above equation could be transformed as follows. By substituting equation as follows, the completed form of design equation can be obtained and from this we could find the k value. = 0 ? ?

= = Fj aj

0 ? (0 ? 1 ) = 2 2

(0 ? 1 ) , = + 2 1

= volumetric flow rate of unit of j = concentration of reactant of j

C. Tubular reactor Tubular reactor is a reactor that sent reactants through a long tube flow and at the end of the tube product is collected. Typically when setting up the design equation of tubular reactor, flow inside the tube is assumed to be the turbulent flow. That is, the fluid inside the pipe is flowing as plug flow. It is assumed to be no radial distribution once after the reactant enters the front of the tube as long as the reactant coming after does not mix with the former reactant. Also, steady-state is maintained at all location inside the inner tubes.

CHEMICAL AND BIOLOGICAL PROCESS LAB. 458.307 004

Figure 2: Tubular reactor

As the figure shown above, if the tube sub volume, ΔV, and Fj as quantity of j components enter the sub volume per unit time, and assume that reaction rate are not affected inside sub volume, the following equation could be obtained. ? + ? + ? = 0, ? = ?

Divide both sides with AΔy, doing some algebra and taken the limit the following design equation can be drawn. = = , = , →

Since Fa = Cav0, substitute this into the above equation and the following equation is obtained after the equation is solved. k= a0 ? a1 v0 a 0 a1 V

if Xa = (a0-a1)/a0 is used to substitute into above equation, the following equation could be presented. 1 = = , 0 1 ? D. Conductivity The electrical resistance of an object is a measure of its opposition to the passage of an electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material. Discovered by Georg Ohm in 1827, electrical resistance shares some conceptual parallels with the mechanical notion of friction. The SI unit of electrical resistance is the ohm (Ω). Resistance's reciprocal quantity is electrical conductance measured in Siemens. For a wide variety of materials and conditions, the electrical resistance does not depend on the amount of current through or the potential difference (voltage) across the object, : residence time

CHEMICAL AND BIOLOGICAL PROCESS LAB. 458.307 004

pmeaning that the resistance R is constant for the given temperature and material. Therefore, the resistance of an object can be defined as the ratio of voltage to current, in accordance with Ohm's law. By a derivation of Ohm's law, when the inverse of resistance R of a conductor of uniform cross section is called G, conductance, it can be computed as =

Where l is the length of sample, A is the cross-sectional area and k is the conductivity. In a solution, conductivity is rely on the concentration, thus concentration could be measured through this. E. Thermodynamic analysis of activated complex theory According to this theory, pre equilibrium is achieved very rapidly and fast between two reactant A, B, through the active complex, C*. A + B ? C? , ? = ? , = []

This activated complex, C* is vanished and forms the product during transition state with reaction rate of k*. C? ? P, Therefore the reaction velocity, v is v=kA B, = ? ? v = k ? [C? ]

If we take K* as the equilibrium constant, it is possible to have the following equation. Δ? G = ?RT ln K Using this into the reaction rate equation, following result could be yielded. k= ? ? ?Δ ?G = ?

CHEMICAL AND BIOLOGICAL PROCESS LAB. 458.307 004

3. PROCEDURE A. Reagent i. 2 liter of 0.1M of NaOH ? Molar mass: 40.00 ii. 2 liter of 0.1M of ethyl acetate ? Molar mass: 88.91 ? Density: 0.902g/cm3 B. Appartus a. Pipette b. 1L flask c. CET MK II d. CEM MK II e. CEX Service Unit f. CW-16 C. Procedure a. CSTR ① 0.1M sodium hydroxide and 0.1M ethyl acetate is filled into 5.0L batch. ② Lid of the reaction vessel gently opened and solution is filled until 50mm. Lid is
closed after this.

③ Temperature is adjusted to be 30?C. ④ Data record of conductivity is continued until the reactor reach steady-state. ⑤ ⑥ ⑦ ⑧
(Approximately 20 minutes). But it is better to obtain the data around 35 minutes in order to get good data. By using each calibration graph of the feed pump, the speed of the pump is adjusted to 40mL/min flow rate. (Calibration data is adjusted to 6.65 respectively) Stirrer speed is adjusted to 7.0. Each feed pump and mixer motor are activated. Then the data is read. After a few minutes, when the tip of sensor is covered with the solution (about 25mm of the liquid inside the reactor) hot water circulator is activated. Step ① ~ ⑦ are repeated for different temperature. (for example, 35 ℃, 40 ℃, 45 ℃)

b. Tubular reactor ① 0.1M sodium hydroxide and 0.1M ethyl acetate is filled into 5.0L batch. It has already ② ③ ④ ⑤ ⑥
been done in previous experiments. Temperature is adjusted to be 25?C. Data record of conductivity is continued until the reactor reach steady-state. (Approximately 20 minutes). But it is better to obtain the data around 25 minutes in order to get good data. By using each calibration graph of the feed pump, the speed of the pump is adjusted to 40mL/min flow rate. (Fa=Fb=40mL/min) Hot water circulator is activated and temperature of the water in the reaction vessel is begun to rise. Temperature is automatically set and kept the at 25 ℃. Two feed pump is activated and the data is read.

4. REFERENCES A. H. Scott Fogler "Elements of Chemical Reaction Engineering," 3rd Ed. Prentice-Hall,1999 B. J. B. Butt, "Reaction Kinetics and Reactor Design," Prentice-Hall, Engelwood Cliffs, N. J.,1980


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