School of Chemical Engineering & Advanced Materials

Module title: Engineering Practice (CME2121)Laboratory experiment: Continuous Stirred Tank Reactor

Group no.: 27Group members: Darren Tang (140661602)Jacqueline Tan (140661200)Tan Zi Xiu (140661222)Submission Date: January 2015

Summary

Safety

Hazard identification is important before starting any experiment. The hazards will be circled out and recorded down. These, along with the COSHH form and Risk Assessment form are to be assessed and verified by the lab technician, before deciding whether it is safe to carry on with the experiment. Hazard Identification and Mitigation S/NWork ActivityType of HazardPossible consequences to user if not mitigatedRisk Controls

1Preparation of solutionsSpill, Fire

Inhalation/Ingestion to userEnsure that proper PPE such as gloves are worn and avoid contact

2Operation of PC, equipmentElectricalElectric Shock to userNo liquids to be handled near them

3Ensure all drain points are closedSpills, electrical hazardElectrical shock, possible inhalation of vapor fumesCheck that drain points are closed before commencing experiment

4Adding/Refilling solution into CSTR tankSpillsIrritation to skin, electrical shock to userEnsure pouring and introduction of material into tank is done slowly

5Taking readings using the probesSpills, electrical hazardIrritation to skin, electrical shockEnsure no exposed wires and ensure gloves are worn.

Table 1.1: The identified hazards for the CSTR experiment.

Refer to Appendix G for detailed Risk Assessment table.

Risk Assessment The two solutions used for this experiment are Sodium Hydroxide and Ethyl Acetate. Taking them together will form sodium acetate. Sodium hydroxide is an irritant and hazardous in the case of skin contact. It is easily soluble in water however it is hygroscopic in nature, in which it generates heat upon contact with water. In the case of a spill, use appropriate tools such as acetic acid to neutralize and dispose of it, and avoid water contact at all costs. Ethyl Acetate is a clear, colourless liquid that is flammable in nature. Prolonged exposure to this chemical may cause irritation to the eyes and respiratory tract. The vapours given off are denser than air, hence it is at a risk of travelling to an ignition source and flashback. In the event of a spill, use a non-combustible absorbent to absorb the residue and dispose of it. The equipment was operated under 1atm (atmospheric pressure) and at room temperature (25degC). Risk Estimation MatrixSeverity of HarmHighMediumLowNegligible

SevereHighHighMediumEffectively Zero

ModerateHighMediumMedium/LowEffectively Zero

MinorMedium/LowLowLowEffectively Zero

NegligibleEffectively ZeroEffectively ZeroEffectively ZeroEffectively Zero

Table 1.2: Risk estimation matrix

Table of Contents1. Introduction and Theoretical Background ...............................................12. Experimental Design and Method .......23. Equipment........................................................................................................34. Results .........................................................................................................45. Discussion .......56. Conclusion67. Data and Error Analysis...78. References....89. Nomenclature ..910. Appendix10

1. Introduction and Theoretical Background

1.1 Reaction in CSTRContinuous Stirred Tank Reactor (CSTR) is the most fundamental continuous reactor used in chemical processes. During the operation, reactants are fed continuously into the reactor and the contents of the tank are assumed to be well mixed by the agitator. The products are then removed continuously from the reactor. The CSTR is commonly used in industrial processing, mainly in homogenous liquid-phase flow reactions where agitation is required. In this experiment, the reaction occurring within the reactor is saponification which is given by the following first order reaction: NaOH + H3C2OOC2H5 H2COONa + C2H5OHa + bc+dThe reactants are Sodium hydroxide (a) and Ethyl Acetate (b), react to form Sodium Acetate (c) and Ethyl Alcohol (d). The reaction was carried out at constant temperature, volume, flow rate and concentration of reagents under perfect mixing in CSTR to achieve steady state and optimal conversion.

1.2 Concentrations of each speciesThe conversion that is associated with the solution conductivity can be achieved through the use of following equations:The inlet concentration of each reactant is calculated by the following equations: (1) (2)The outlet concentration of the species is as follows: (3) (4)

(5) (6)

Note that the equations to use depends on which reactant is limiting reagent.

1.3 ConductivityThe equations (3), (4), (5) and (6) are combined to give an equation that solves for Xa from a measured value of conductivity. The conductivity of the solution is used to obtain conversion within the reactor. From the reaction, the conductivity of the mixture is contributed by sodium hydroxide (a) and sodium acetate (c). There is no conductivity for ethyl acetate and ethyl alcohol as the contents do not contain ions. The conductivities of pure solutions (a) and (c) with their concentrations are given by: (7) (8)

The conductivity of mixture is determined by the sum of equations 7 and 8: (9)

1.4 Conversion and rate constantUsing the following equations to obtained the optimal conversion: (10) Equation 10 is formed by manipulating the equations 7, 8 and 9. The concentration terms is written in terms of conversion, Xa and Xb depending on which reactant is the limiting reagent. (11)Since the continuous reactor runs under steady state condition, there is no change to the volume and hence, the equation 11 is rearrange to find the rate constant K. (12)

2. Experimental Design and Method

Continuous Stirred Tank Reactor (CSTR) was used to conduct pilot scale experiment on the reaction of sodium hydroxide and ethyl acetate. The flow rates were chosen to be 32ml/min, 52ml/min and 72ml/min for both the pump settings. At flow rate 32ml/min, the conductivity and temperature were recorded at every 3 minutes interval and at 52ml/min and 72ml/min, the readings were taken down at every 1 minute interval. Each run took about 30 minutes for at least 10 readings.

2.1 Preparation of stock solutionsAs the conductivity sensor should not be exposed to concentrations that exceed 0.05M of NaOH and 0.1M of H3C2OOC2H5, the reagent concentrations of 0.03M and 0.05M for NaOH and H3C2OOC2H5 are used respectively. The NaOH and H3C2OOC2H5 stock solutions were prepared by adding 2.4g of NaOH solids and 9.76ml of H3C2OOC2H5 solutions with the deionised water into 2L volumetric flask. Similarly, the H3C2OOC2H5 stock solution was made by transferring 9.76ml of H3C2OOC2H5 solution with deionised water into 2L volumetric flask.

2.2 Start-up experimentAt the start of the experiment, the taps for the feed tanks and CSTR were closed to prevent leakage of solutions. Then, 2L of each sodium hydroxide and ethyl acetate solutions were poured into the respective tanks. Next, the default readings for conductivity and temperature were recorded by placing the probe and thermocouple into the feed tanks for measurement. Once the readings were taken down, the probe and thermocouple was placed back into the reactor. The main power was switched on followed by the pumps A and B as well as the impeller. The temperature control was turned on only after the solution has filled up the thermocouple. Then, the pumps were adjusted to desired flow rates that had achieved a minimum of 10 readings per run as the temperature reached 30oC. Each reading was recorded at an interval of at least 1 minute interval for the system to reach steady-state. The procedure is repeated for other flow rates. Feed tanks were refilled with reactant solutions twice.

2.3 Shutdown EquipmentAfter collecting all the experiment data, the pumps and impeller switches were turned off follow by the main power switch. The recorded data will be used for discussion on the discrepancies between the published values and experimental values.

3. Equipment

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The reactor is made up of several components:i) Two feed tanks: To store the solutions.

ii) Two peristaltic pumps A and B: To pump the chemicals into the reactor with individual manual speed controls.

iii) Reactor vessel: Consist of impeller that ensure good mixing of reactants.

iv) Thermocouple: To control the reactor temperature.

v) Conductivity probe: To measure the solution conductivity.

vi) Hot/Cold Water circulation: Fluid that flows through heating/cooling coils.

vii) Conductivity meter: To display the conductivity measured by the probe

viii) Temperature meter: To display the temperature measured by the thermocouple

4. Results

5. Discussion

5.3 Rate constant kThe rate constants are calculated by the following equation: (5.3.1)No. of runsRun 1Run 2Run 3

Average Flow rate (L/s)0.0005330.0008590.00118

Average Temperature (oC)32.933.432.3

Rate Constant, k (L/mol. s)0.01650.02860.0507

Table 5.3.1: Experimental rate constant at various average flow ratesRefer to appendix C for detail calculation of rate constant k.SourceSmith et al. [Ref]

Rate Constant, k (L/mol. s)0.11

Table 5.3.2: Rate constant at steady state extracted from the literatureThe experimental rate constants are obtained through the calculation using the equation 5.3. From table 5.3.1, an increasing trend of experimental rate constants is observed as the average flow rates increase. During steady-state, the experimental rate constants are expected to be constant for the 3 runs however, the values deviate from each other due to several experiment errors. The fluctuation of flow rates could be due to the inconsistent of peristalsis pumps as it might affect the actual set point of flow rates. The inaccurate flow rates will indirectly affect the concentrations which in turn cause the rate constants to deviate. Besides, the settings for the pumps and impeller may have transportation lags in the system which occurs after every new setting is made. Therefore, the instrument readings will lag behind and fluctuate through a range of values. Due to these inevitable experimental errors, it can be used to explain the experimental rate constants are different from those found in literature. However, the theoretical rate constant only depends on the temperature therefore, the value is more reliable. According to the literature, the theoretical rate constant is abstracted to be 0.11 L/mol.s at steady state. The theoretical rate constant is correlated to temperature and the relationship is represented by Arrhenius law: (5.3.2)The exponential term that consists of the ratio of activation energy E to the average kinetic energy has significant influence on the rate constant. As the rate constant depends on the temperature, it can be infer that high temperature and low activation energy favour larger rate constant, and thus increase the rate of reaction. Theoretically, the rate constant can be derived by Arrhenius law. The deviation between the experimental k value and theoretical k value is due to the different equations (5.3.1 and 5.3.2) being used. The experimental k value is calculated based on the concentrations whereas the theoretical k value is derived based on temperature. Both dependencies are different which produce contrasting results.

5.1

Nomenclature

FaVolume feed rate of sodium hydroxide L/sFbVolume feed rate of ethyl acetateL/s[]Sodium hydroxide concentration in Feed Vesselmol/L[b] Ethyl acetate concentration in Feed Vesselmol/LTTemperatureKVVolume of the reactorLNiNumber of moles of species i in the reactormoles

viStoichiometric Coefficient-riReaction rate-Residence TimeskRate ConstantL/mol.sAPre-Exponential factor-EaActivation energykJ/molRGas ConstantJ/mol.K

11. Appendix A: Experimental Design Calculation

11.1 Calculation for the mass of NaOH pellet for stock solution

11.2 Calculation for the volume of H3C2OOC2H5 for stock solution

11. Appendix B: Results

11.1 Raw DataTime (min)Conductivity (mS/cm)Temperature (C)Time (min)Conductivity (mS/cm)Temperature (C)Time (min)Conductivity (mS/cm)Temperature (C)

Run 1Flowrate (ml/min)Pump A: 32Pump B: 32Run 2Flowrate (ml/min)Pump A: 52Pump B: 51Run 3Flowrate (ml/min)Pump A: 72Pump B: 70

03.7226.003.0829.803.0230.0

33.6331.113.1530.513.0529.9

63.3930.623.1630.723.0630.1

93.2630.133.1630.733.0530.5

123.2029.943.1530.643.0730.7

153.1730.553.1330.453.0430.5

183.1330.263.1030.363.0330.5

213.1030.073.0830.373.0130.3

243.1230.583.0730.282.9930.2

273.0730.293.0330.192.9730.1

303.0730.0103.0330.0102.9430.0

112.9329.8

122.9329.9

132.9629.6

142.9629.6

152.9529.5

Table 11.1: Raw Data

11.2 Concentration of A versus time

12. Appendix C: Sample Calculations

12.1 Calculate the concentration of species in feed vesselsInitial conductivity of sodium hydroxide (NaOH): 7.53mS/cm = 0.00753 S/cmInitial Temperature (T0) = 26.4C = 299.4KConcentration of NaOH:

Concentration of ethyl acetate in feed vessel: 0.05mol/L=0.05mol/dm3

Flow rates

Run 1ml/minL/sAverage (L/s)

Pump A (NaOH)320.0005330.000533

Pump B (Ethyl acetate)320.000533

Run 20.000859

Pump A (NaOH)520.000867

Pump B (Ethyl acetate)510.000850

Run 30.00119

Pump A (NaOH)720.00120

Pump B (Ethyl acetate)700.00117

Table 12.1: Average flow rates.

12.2 Calculation k value and conversion for run 1

0.364

12.3 Calculation k value and conversion for run 2

12.4 Calculation k value and conversion for run 3

The objectives are to understand the limitations in assuming ideal CSTR behaviour and how these can be minimised by controlling the operating conditions. Also, issues related to the scale up of a CSTR and deviations from the ideal CSTR will be investigate from the experiment.