Cstr lab report

2.0

OBJECTIVES

EXPERIMENT A: Batch Stirred Tank Reactor Experiments.

-

To determine the order of saponification reaction

-

To determine the reaction rate constant, k

EXPERIMENT B: Effect of Temperature on Reaction Rate Constant, k

-

To determine the effect of temperature on reaction rate constant, k for batch reaction

-

To determine the activation energy of saponification.

3.0

THEORY

EXPERIMENT A: Batch Stirred Tank Reactor Experiments. IDEAL STIRRED-TANK REACTOR

A stirred- tank reactor (STR) may be operated either as a batch reactor or as a steady-state flow reactor (better known as Continuous Stirred-Tank Reactor (CSTR)). The key or main feature of this reactor is that mixing is complete so that properties such as temperature and concentration of the reaction mixture are uniform in all parts of the vessel. Material balance of a general chemical reaction is described below. The conservation principle requires that the mass of species A in an element of reactor volume ∆V obey the following statement (Perry, 2008):

Rate of A Into volume

Rate of A _

out of volume

Rate of A +

produced

Rate of A =

accumulated

within volume

within volume

(1) element

element

element

element

BATCH STIRRED-TANK REACTOR (BSTR) The reaction studied in this is the saponification of ethyl acetate. This reaction is elementary and second-order. The reaction equation is NaOH + EtOAc  NaOAc + EtOH A

+

B

-->

C

+

D

In batch reactions, there are no feeds or exit streams and therefore equation (1) can be simplified into:

Rate of A produced

Rate of A =

accumulated

within volume

within volume

element

element

(2)

For a constant volume isothermal batch reactor, the design equation is: dC A  rA dt

For a bimolecular second order reaction, the rate equation is: -rA = k CACB

The rate of reaction of component A is defined as: moles  A  appear  by

rA 

1  dN A  =   V  dt by  reaction

reaction volume  time

(3)

By this definition, if A is a reaction product, the rate is positive; whereas if it is a reactant, which is consumed, the rate is negative (Perry, 2008).

Rearranging equation (3);

 rA  V

 N AO

dX A dt

(4)

Integrating equation (4);

t  N AO 

dX A  rA V

(5)

where t is the time required to achieve a conversion XA for either isothermal or non-isothermal operation.

Figure 1: Graphical representation of the performance equations for batch reactor, isothermal or non-isothermal at constant density.

STEADY STATE MIXED FLOW REACTOR The general material balance for this reactor is as equation (1) except no accumulation of the material A in rector. As shown in figure 2 below, if FAO = vOCAO is the molar feed rate of the component A to the reactor, then considering the reactor as a whole we have, Input of A (moles/time)

=

FAO (1 – XAO) = FAO

Output of A (moles/time)

=

FA = FAO (1 – XAO)

Disappearance of A by reaction (moles/time) =

(-rA) V

moles  A  reacting volume  of  reactor  (time )( volume)

=

Replacing equation (1) with mathematical formula above,

FAOXA =

(-rA) V

Which on arranging, will form the performance equation for mixed flow reactors,

V  X A   FAO C AO  rA

or,

 

1 V VC AO C AO X A    s vO FAO  rA

(7)

Figure 2: Components of Mixed Flow Reactor

In mixed flow reactors, XA = XAF and CA= CAF. In a constant density system, XA

=

1 – (CA/CAO)

The performance equation can be rewritten in terms of concentration, or

V X C  CA V C X  A  AO , or ,   AO A FAO  rA C AO (rA ) v (rA )

(8)

These expressions relate the four terms XA, -rA, V, FAO; thus, knowing any three allows the fourth to be found directly. In design, the size of reactor needed for a given duty or the extent of conversion in a reactor of given size is found directly. Each steady-state point in a mixed flow reactor gives the reaction rate for the conditions within the reactor (Green, 2008). The mixed flow reactor provides easier interpretation of reaction rate data and makes it very attractive in kinetic studies. Graphical Representation of the Design Equations for Steady State Mixed Flow Reactor.

Figure 3: Plot of 1 / (-rA) versus XA

Or in constant systems,

Figure 4: Plot of 1 / (-rA) versus CA

Irreversible Second-order Reaction (Bimolecular Type) Consider the reaction: A+B

Products

The rate equation can be written as:

 rA  

dCA dC   A  kCACB dt dt

(11)

Plot of ln (CB/CA) versus t (time) will produce a straight line with slope equals (CBO – CAO)k.

Figure 5: Plot of ln (CB/CA) versus t, CAO

CBO (for equation 11)

While for steady state mixed flow reactor, the plot of rA versus CACB will give a straight line (pass through the origin) with the slope equal to k.

Figure 6: Plot of –rA versus CACB, CAO

CBO (for equation 11)

For second order reaction with equal initial concentrations of A and B, the rate equation can be written based on only one component:  rA  

dCA 2  kCA2  kCAO (1  X A )2 dt

(12)

A plot of 1 / CA versus t will produce a straight line with slope equals to k.

Figure 7: Plot of –rA versus CACB, CAO

CBO (for equation 11)

In the case of steady state mixed flow reactor, the plot –rA versus CA2 will give the value of k and a straight-line pass through the origin.

Figure 8: Plot of –rA versus CACB, CAO

CBO (for equation 11)

EXPERIMENT B: Effect of Temperature on Reaction Rate Constant, k

In any single homogenous reaction, temperature, composition, and reaction rate are uniquely related. They can be represented graphically in one of three ways as shown below: r3 r2

C3

C2

r1 C1

Figure 9: Plot of C versus T, r versus T, and r versus C The effect of temperature on reaction rate constant can be demonstrated by performing a batch reaction run at different temperatures.

4.0

PROCEDURES

Experiment A: Batch and Continuous Stirred Tank Reactor Experiments.

By studying the saponification reaction of ethyl acetate and sodium hydroxide to form sodium acetate in a batch and in continuous stirred tank reactor, the students can evaluate the rate data needed to design a production scale reactor.

A process flow diagram of the reactor is shown in Figure 9. This reactor has a total volume of approximately 4 liters. A scale is provided to determine the reactor volume. Reactants Preparation Procedure:

1. 0.05 M NaOH and 0.05 M Ethyl acetate solutions are prepared in two separate 20-liter feed tanks. 2. The concentration of 0.1 M NaOH solution is confirmed by titrating a small amount of it with standard 0.1 M HCl using phenolphthalein as indicator. The concentration of ethyl acetate solution, on the other hand, is evaluated in the following manner. First, 0.1 M NaOH solution is added to a sample of feed solution such that the 0.1 M NaOH solution is in excess to ensure all the ethyl acetate reacted. The mixture is reacted overnight. On the following day, the amount of unreacted NaOH is determined by direct titration with standard 0.1 M HCl. The ethyl acetate real concentration is recorded. 3. 1 liter of quenching solution of 0.25 M HCl and 1 liter of 0.1 M NaOH is prepared for back titration.

Batch Reaction Procedure: 1. To begin a batch reactor experiment, the overflow tube in the reactor is adjusted to give a desired working volume, say 2.5 liters. The pump P1 is switch on and start pumping 1.25 liters of the 0.1 M ethyl acetate from the feed tank into the reactor. The pump P1 is stopped. 2. The pump P2 is switch on and starts pumping 1.25 liters of the 0.05 M NaOH into the reactor. The pump P2 is stopped when 2.5 liters volume is reached. The stirred is switched on and the speed is set in the mid range (say 180 rpm). Timing the reaction is started immediately and the time is recorded, tO. 3. 10 ml of the 0.25 M HCl is measured quickly in a flask. 4. After 1 minute of reaction, the sampling valve V7 is opened to collect a 50 ml sample and immediately the 10 ml of 0.25 M HCl prepared in step 3 is added, and mixed. The HCl will quench the reaction between ethyl acetate and sodium hydroxide. 5. The mixture is titrated with the 0.1 M NaOH to evaluate the amount of unreacted HCl; this will provide the information that the amount of NaOH in the feed solution, which has reacted, can be determined. 6. The steps 3 to 5 are repeated for reaction time of 5, 10, 15, 20 and 25 minutes.

Experiment B: Effect of Temperature on Rate Constant, k.

The effect of temperature on reaction rate constant can be demonstrated by performing a batch reaction run at different temperature.

1. To begin a batch reactor experiment, the overflow tube in the reactor is adjusted to give a desired working volume, say 2.5 liters. The pump P1 is switch on and start pumping 1.25 liters of the 0.1 M ethyl acetate from the feed tank into the reactor. The pump P1 is stopped. 2. The pump P2 is switch on and start pumping 1.25 liters of the 0.05 M NaOH into the reactor. The pump P2 is stopped when 2.5 liters volume is reached. The stirred is switched on and the speed is set at the lower range. Timing the reaction is started immediately and the time is recorded, tO. 3. 10 ml of the 0.25 M HCl is quickly measured in the flask. 4. After 1 minute of reaction, the sampling valve V7 is opened to collect a 50 ml sample and immediately the 10 ml of 0.25 M HCl prepared in step 3 is added, and mixed. The HCl will quench the reaction between ethyl acetate and sodium hydroxide. 5. The mixture is titrated with the 0.1 M NaOH to evaluate the amount of unreacted HCl; this will provide the information that the amount of NaOH in the feed solution, which has reacted, can be determined. 6. Steps 4 to 6 are repeated for reaction times 5, 10, 15, 20 and 25 minutes. 7. The experiment is repeated for reaction temperatures of 35, 40, and 45 oC 8. ln(CB/CA) versus t, lnK versus 1/T are plotted. 9. Activation energy is get from lnK versus 1/T plotted 10. All switches are switch off when the experiment is finished.

5.0

APPARATUS

EQUIPMENT DESCRIPTION

Before operating the unit, we have to be familiar with the unit. Figure at the appendices is using as reference.

a) Reactor: The reactor consists of a glass vessel with top and bottom plate made of stainless steel. The reactor comes with a cooling coil, a 1.0 kW heater, a temperature sensor, stirrer system, an overflow tube and a gas sparging unit.

b) Feed Inlet System (Liquid Reaction): For each liquid reactant, a 20-liter feed tank, a pump, a needle valve and a flow

meter are provided. Each

reactant is pumped from the feed tank to the appropriate inlet port at the top plate.

c) Gas Inlet System (Gas-Liquid Reactions: Optional): Gas from gas cylinder (not provided) flows through a needle valve, a flow meter and a gas sparger before reacting with the liquid inside the reactor.

d) Product/Waste Tank: A 50-liter rectangular tank made of stainless steel is provided for collecting lar tank made of stainless steel is provided for collecting lar tank made of stainless steel is provided for collecting the product or waste before being discharged.

e) Control Panel: The control panel consists of all the necessary electrical components for controlling the operations of the unit. Components mounted on the panel door are all labeled for convenience. The control panel also houses the necessary for data acquisition system.

f) Data Acquisition System (Optional): The data Acquisition System consists of a personal computer, ADC modules and instrumentations for measuring the process parameters. A flow meter with 4 to 20 mA output signal is provided for monitoring the changes in pH while the reaction is taking place. The ADC modules into digital signal convert all analog signals from the sensors before being sent to the personal computer for display and manipulation. REFERENCES 1. Green, P. W. (2008). Continuous Stirred Tank Reactors (CSTRs). Chemical and Biological Reaction Engineering, 5-6. 2. Perry, R. G. (2008). Perry Chemical Engineer's Handbook. New York: McGraw Hill.