Lab CSTR in Series

ABSTRACT Our experiment involves a continuous stirred tank reactor (CSTR) in series. Our system consists of 3 agitated, glass reactor vessels in series. Although the concentration is uniform for each reactor but there is a change in concentration as fluids move over from reactor to reactor. Our objective in this experiment is to determine the concentration response to a step change and pulse input and also to determine the effect of residence time on the response curve. 1st the deionised water are filled in the both two tanks with the sodium chloride were diluted in the tank one. Then deionised water from the two tank will flow through to fill up the three reactors. The flow rate of the deionised water is set to 150 ml/min to prevent from over flow. The only readings were taken at time t o after we get the readings of the conductivity are stable enough where the readings of the conductivity are quite similar from one to another. After that, readings are continuously taken every 3 minutes until to the point that the conductivity values for the three reactors are closed to each other. Then the graph of the conductivity versus time was plotted The graph that has been plotted is accordingly to the theory. From the graph we can determine the effect of the step change and pulse input to the concentration.

INTRODUCTION In the majority of industrial chemical process, a reactor is the key item of equipment in which raw materials undergo a chemical change to form desired product. The design and operation of chemical reactors is thus crucial to the whole success of the industrial operation. Reactors can widely form, depending on the nature of the feed materials and the products. Understanding non-steady behaviour of process equipment is necessary for design and operation of automatic control systems. One particular type of process equipment is the continuous stirred tank reactor. In this reactor, it is important to determine the system response to a change in concentration. This response of concentration versus time is an indication of the ideality of the system.

OBJECTIVES There are two objectives in this experiment: 1) To determine the effect of pulse input to the concentration. 2) To determine the effect residence time on the response curve.

THEORY General Mole Balance Equation

Assumptions

1) Steady state therefore 2) Well mixed therefore rA is the same throughout the reactor

Rearranging the generation

In terms of conversion

Reactors in Series Given -rA as a function of conversion, , -rA = f(X), one can also design any sequence of reactors in series provided there are no side streams by defining the overall conversion at any point.

Mole Balance on Reactor 1

Mole Balance on Reactor 2

Given -rA = f(X) the Levenspiel Plot can be used to find the reactor volume

For a PFR between two CSTRs

\

Effect of Step Change in Input Concentration to the Concentration of Solute in Stirred Tank Reactors in Series. When a step change of solute concentration is introduced at the feed of tank 1, the tank in series will experience a transient behaviour as a Figure 7 below. The response will be dependent on the residence time of each reactor in series.

Concentration

Concentration

---------------------------------Reactor 1 Reactor 2 Reactor 3

Time Figure 8a. Step change input

Time Figure 8b. Transient response of tank in series to the step input.

Effect of Pulse in Input Concentration to the Concentration of Solute in Stirred Tank in Series.

Concentration

Concentration

When a pulse input of solute concentration is introduced at the feed of tank 1, the transient behaviour will be different than the step change input due to the diminishing concentration from the input after pulsing as described in Figure 8.

Reactor 1

Reactor

2

Reactor 3

Time Time Figure 8a: Pulse input APPARATUS

Figure 8b: Transient response of tank in series to the pulse input.

1. Continuous stirred tank reactor in series. (Model: BP107) 2. Sodium chloride 3. Distillation water 4. Stirrer system 5. Feed tanks 6. Waste tank 7. Computerize system 8. Stopwatch 9. Dead time coil

PROCEDURE

Experiment: The Effect of Pulse Input In this experiment a pulse input would be introduced and the progression of the tracer will be monitored via the conductivity measurements in all the three reactors. 1. Tank 1 and tank 2 was filled up with 20L feeds deionised water. 2. 300g of Sodium Chloride was dissolved in tank 1until the salts dissolve entirely and the solution is homogenous. 3. Three way valve (V3) was set to position 2 so that deionised water from tank 2 will flow into reactor 1. 4. Pump 2 was switched on to fill up all three reactors with deionised water. 5. The flow rate (Fl1) was set to 150 ml/min by adjusting the needles valve (V4). Do not use too high flow rate to avoid the over flow and make sure no air bubbles trapped in the piping. The stirrers 1, 2 and 3 were switched on. 6. The deionised water was continued pumped for about 10 minute until the conductivity readings for all three reactors were stable at low values. 7. The values of conductivity were recorded at t0. 8. The pump 2 was switched off after 5 minutes. The valve (V3) was switched to position 1 and the pump 1 was switched on. The timer was started. 9. Let the pump 1 to operate for 5 minute, and then switched off pump 1. Switched the three ways valve (V3) back to position 2. The pump 2 was switched on. 10. The conductivity values for each reactor were recorded every three minutes. 11. Record the conductivity values were continued until reading for reactor 3 closed to reactor 1. 12. Pump 2 was switched off and the valve (V4) was closed. 13. All liquids in reactors were drained by opening valves V5 and V6.

RESULTS

Experiment: The Effect of Pulse Input

Time (min) 0.0 3.0 6.5 9.0 12.0 15.0 18.0 21.7 24.0 27.0 30.3 33.2 36.0 39.0 42.0 45.0 48.0 51.6 54.0 57.0 60.0 63.0 66.0 69.0 72.3 75.0 78.0 81.0 84.0

QT1 (mS/cm) 8.1615 7.7993 5.4289 4.2362 2.9866 2.2899 1.5690 1.0930 0.8812 0.6583 0.4180 0.3494 0.3204 0.1629 0.1348 0.1157 0.0957 0.0314 0.0718 0.0500 0.0035 0.0288 0.0454 0.0244 0.0000 0.0260 0.0284 0.0209 0.0000

QT2 (mS/cm) 1.1189 3.0860 4.2713 4.2183 4.0574 3.4993 3.1398 2.7290 2.3236 1.9075 1.4895 1.2872 1.0583 0.8205 0.6798 0.4690 0.3644 0.3214 0.2761 0.2167 0.1821 0.0884 0.0431 0.0521 0.0297 0.0190 0.0605 0.0000 0.0388

QT3 (mS/cm) 0.4304 1.1229 1.5806 2.2323 2.9361 3.2178 3.3956 3.3502 3.1665 2.9860 2.6579 2.2978 2.0529 1.8054 1.5063 1.3228 1.0349 0.8350 0.7141 0.5894 0.4692 0.3632 0.2840 0.2593 0.1767 0.1167 0.0905 0.1189 0.0919

Concentration vs Time

QT1 (mS/cm)

QT2 (mS/cm)

QT3 (mS/cm)

Graph 1: Concentration versus Time for the effect of pulse input

CALCULATIONS Sample of calculations Vi = FA0 (XA,i - X A,i-1) / (-rA)i Where Vi = volume of reactor i F A,i = molal flow rate of A into the first reactor XA,i = fractional conversion of A in the reactor i XA,i-1 = fractional conversion of A in the reactor i-1 For first order reaction, -rA = k CA,I = kCA0 (1 – XA,i) v = volumetric flow rate of A = 150mil/min = 0.15 liter/min For the first reactor: (V = 20 lit) (-rA)1 = (kCA)1 = kCA,1 = k CA0 (1 – XA,1) CA0 = FA0 / v i.e., FA0 = vCA0 XA,i-1 = XA,0 = 0 Therefore, Tank 1 Vi = FA0 (XA,i – XA,i-1) / (-rA)i 20 = 0.15 (XA,1 – 0) / (0.158 x (1 – XA,1)) XA,1 = 0.95

Tank 2 Vi = FA0 (XA,i – XA,i-1) / (-rA)i 20 = 0.15 (XA,1 – 0.95) / (0.158 x (1 – XA,1)) XA,1 = 0.997

Tank 3 Vi = FA0 (XA,i – XA,i-1) / (-rA)i 20 = 0.15 (XA,1 – 0.997) / (0.158 x (1 – XA,1)) XA,1 = 0.998

DISCUSSIONS A stirred tank is the most fundamental of mixers and many common mixers from the mixer used in lab to a cup of coffee with a spoon can be considered a stirred tank under some set of approximation. Mixing in a stirred tank is complicated and not well described although the use of dimensionless numbers and comparison with literature accounts can lead to some predictive capabilities. Often stirred tanks are used as industrial reactors where a chemical component of a flow stream resides for some time in the tank and then proceeds on to other steps in a chemical process. The objectives of this experiment were to determine the effect of pulse input to the concentration and to determine the effect residence time on the response curve. In this experiment, we record the values of conductivity of the three reactors every three minutes until the values of conductivity of reactor 3 closed to reactor 1. For this experiment, the amount of time that needed for the values of conductivity of reactor 3 closed to reactor 1 was 84 minutes. As we know the concentration can be calculated using electrical conductivity measurements and calibration supplied. The concentration is directly proportional to the conductivity. So, a graph of concentration versus time was plotted to show the effect of pulse input in this experiment. The pulse input caused the concentration change differently for every reactor. From graph 1, we can see that the concentration for reactor 1 decreased constantly until it reaches zero. For reactor 2 the concentrations increase until at 6.5 minutes and then start decreasing until it reaches a constant value. For reactor 3 the concentrations also increase until at 18 minutes and start decreasing. There are only a small different for every readings. The concentration in the reactor keep on decreasing because deionised water were constantly supplied throughout the experiment. This is because we wanted to investigate the effect of pulse input in this experiment.

CONCLUSION Based on the results that we have obtained, it can be concluded that the experiment was successful. The first objective of this experiment was to determine the effect of pulse input to the concentration. Graph 1 was plotted to show the effect of pulse input. The second objective was to determine the effect residence time on the response curve. Every reactor has its own concentration, because of that we conclude that the residence time for each reactor is different. So, the experiment was considered successful.

RECOMMENDATION There are several recommendations must be performed to decrease the percentage of the error in order to increase the accuracy of the results such as; i.

The general start-up procedures need to be performed before starting each of the experiment. This is to ensure all the components of the unit are in good conditions and working smoothly. It will affect the reading of the experiment and reduce the performance of the unit if the components are not in satisfied conditions.

ii.

To get more accurate reading, the experiment must be repeated at least 3 times by calculating the average reading. This will reduce the deviation from the theoretical data.

iii.

The reactor needs to be ensured that no leakage at the valve on the unit. The leakage will cause the result to tremendously change which in turns alter the results for this

iv.

experiment. Set an alarm for every 3 minutes so that we do not missed the time to record the result.

REFERENCES I.

Perry, R.H. and D. Green, Perry’s Chemical Engineer’s Handbook, 6th edition,

II.

McGraw-Hill, Japan, 1984. Elementary Fluid Mechanics 7th Edition, Robert L.Street, Gary Z. Watters, John K.

III. IV.

Vennard, John Wiley & Sons Inc. Smith, J.M, Chemical Engineering Kinetics, McGraw Hill, 1981. McCabe, W. L. and J. C. Smith, Unit Operations of Chemical Engineering, 2nd edition, McGraw-Hill, 1967.

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