1. Abstract This experiment involves a continuous stirred tank reactor (CSTR) in series. The reactor system consists of three agitated, glass reactor vessels in series. The concentration is kept uniform for each reactor and it is observed that there is a change in concentration as fluids move from one reactor to the other reactor. This experiment is carried out to determine and observe the effect of step change input. CSTR is one kind of chemical reactor system with non-linear dynamics characteristics. The usage of this equipment is to study the reaction mechanism as well as the dynamics of reactor with various types of inputs. CSTR is widely used in water treatment and chemical and biological processes. The deionised water are filled in both tanks with the sodium chloride are diluted in one tank. Then the deionised water from the second tank will flow through to fill up the three reactors. The flow rate of the deionised water is set to 159.7 ml/min to prevent from over flow. The readings are taken at the time to after the conductivity readings showing stable enough. After that, the readings are continuously taken for every 3 minutes until to the point where the conductivity values for three reactors are equivalent. Based on the result obtained, the graph has been plotted between conductivity, Q (mS/cm) against time, t (min).
2. Aim To study the effect of step change input to the concentration. Introduction In the industrial chemical process, a reactor seems to be the most important equipment in which raw materials undergo a chemical change to form a desired product. The design and operation of chemical reactors are essential criteria responsible to the whole success of the industrial operation. The stirred tank reactor in the form of either single tank, or more often a series of tanks, particularly suitable for liquid phases reactions and widely used in chemical industry, i.e pharmaceutical for medium and large scale of production. It can form a unit in a continuous process, giving consistent product quality, easy to control automatically and low man power requirement. The mode of operation of reactors may be batch flow or continuous flow. In a batch flow reactor, the reactor is charge with reactant, the content are well mixed and left to react and then the mixture will be discharged. A continuous flow reactor, the feed to reactor and the discharge from it are continuous. The three types of continuous flow reactor are plug flow reactor, the dispersed plug flow reactor, and completely mixed or continuously stirred tank reactors (CSTRs). CSTR consists of a stirred tank that has a feed stream and discharge stream. Frequently, several CSTRs in series are operating to improve their conversion and performance (Reynolds and Richards 1996). Complete mixing in a CSTR reactor produces the tracer concentration throughout the reactor to be the same as the effluent concentration. In other words, in an ideal CSTR, at any travel time, the concentration down the reactor is identical to the composition within the CSTR (Hoboken et al., 2005). It is also important to notice that the mixing degree in a CSTR is an extremely important factor (Cholette, Blanchet et al. 1960), and it is assumed that the fluid in the reactor is perfectly mixed in this case, that is, the contents are uniform throughout the reactor volume. In practice, an ideal mixing would be obtained if the mixing is sufficient and the liquid is not too viscous. If the mixing is inadequate, there will be a bulk streaming between the inlet and the outlet, and the composition of the reactor contents will not be uniform. If the liquid is too viscous, dispersion phenomena will occur and this fact will affect the mixing extent.
3. Theory The continuous flow stirred-tank reactor (CSTR), also known as vat- or backmix reactor, is a common ideal reactor type in chemical engineering. A CSTR often refers to a model used to estimate the key unit operation variables when using a continuousagitated-tank reactor to reach a specified output. The mathematical model works for all fluids: liquids, gases, and slurries. 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 inside the reactor, which is a function of residence time and rate of reaction. If the residence time is 5-10 times the mixing time, this approximation is valid for engineering purposes. The CISTR model is often used to simplify engineering calculations and can be used to describe research reactors. In practice it can only be approached, in particular in industrial size reactors. Assume: perfect or ideal mixing, as stated above Integral mass balance on number of moles Ni of species i in a reactor of volume V. General mol balance equation.
4. Assumption 1) Steady state therefore, dNA/dt = 0 2) Well mixed therefore rA is the same throughout the reactor = 0 0 = Rearranging the generation = 0 In term if conversion = 0 0 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. =
5. Mol balance on Reactor 1 In out + generation = 0 FA0 FA1 + rA1V1 = 0 1 = 0 1 0 FA1 = FA0 FA0X1 1 = 0 1 1 Mol balance on Reactor 2 In out + generation = 0 FA1 FA2 + rA2V2 = 0 2 = 0 2 0 FA2 = FA0 FA0X2 2 = 0(2 1) 2 Apparatus 1. Distillation water 2. Sodium chloride 3. Continuous reactor in series 4. Stirrer system 5. Feed tanks 6. Waste tank 7. Dead time coil 8. Computerize system 9. Stop watch Procedure
6. Experiment 1 : The effect of step change input. 1. The general start up procedure was perfomed by following the instruction of the manual given at the instrument. 2. Tank 1 and tank 2 was filled up with 20 L feeds deionizer water. 3. 200g of Sodium Chloride was dissolved in tank 1until the salts dissolve entirely and the solution is homogenous. 4. Three way valve (V3) was set to position 2 so that deionizer water from tank 2 will flow into reactor 1. 5. Pump 2 was switched on to fill up all three reactors with deionizer water. 6. 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. 7. The stirrers 1, 2 and 3 were switched on. The deionizer water was continued pumped for about 10 minute until the conductivity readings for all three reactors were stable at low values. 8. The values of conductivity were recorded at t0. 9. 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. 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.
9. Graph result based on data 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 10 20 30 40 50 60 70 80 90 CONDUCTIVITY,Q(mS/cm) TIME (min) Conductivity change in time for each reactor in pulse change Reactor 1 Reactor 2 Reactor 3
10. Calculation Vi = FA0 (XAi XAi-1)/(-rA)i Where Vi = volume of reactor i FAi = molal flow rate of A into the first reactor XAi = fractional conversion of A in the reactor i XAi+1 = fractional conversion of A in the reactor i-1 For the first order reaction, -rA = kCA1 = kCA0(1-XAi) v = volumetric flow rate of A = 159.7 ml/min = 0.1597 liter/min For the first reactor: (V=20 liter) (-rA)1 = (kCA)1 = kCA1 = kCA0 (1-XA1) CA0 = FA0/v i.e. FA0 = vCA0 XAi+1 = XA0 = 0 Therefore, Tank 1 Vi = FA0 (XAi - XAi-1) / (-rA)i 20 = 0.1597 (XA1 - 0) / (0.158 x (1 XA1)) XA1 = 0.95 Tank 2
11. Vi = FA0 (XAi - XAi-1) / (-rA)i 20 = 0.1597 (XA2 0.95) / (0.158 x (1 XA2)) XA2 = 0.997 Tank 3 Vi = FA0 (XAi - XAi-1) / (-rA)i 20 = 0.1597 (XA3 0.997) / (0.158 x (1 XA3)) XA3 = 1
12. Discussion In this experiment, we carried out an experimental procedure to determine the effect of step change input on the concentration of the salt solution used in the experiment which is sodium chloride, NaCl. The first step in the experiment was filling the reactor tanks with 20L of deionized water. In the experiment of CSTR in series, there are two main objectives to observe; effect of step-change input and effect of pulse input. But in this discussion, we are only focusing on the effect of step-change input. The difference between these two methods are that step-change input means we are continuously feeding the salt solution NaCl into the reactor throughout the experiment and through the time the salt solution will fill all three r