NEUTRALIZATION PROCESS PILOT PLANT 59
4.3 Experimental Results from the Enhanced Data Acquisition System
As explained in the previous chapter, the software environment for the enhanced data acquisition system is based on MATLAB/Simulink software. At this stage in the research (i.e. external model validation) it was appropriate to use the capability of the software available in the new combined data acquisition and simulation system to further analyse and investigate the dynamic response of the pilot plant and the corresponding behaviour of the simulation model. Further investigations involving the developed pH model and comparisons with experimental results were all based on data obtained using the enhanced data acquisition system.
For these tests, which involve a continuous process, the reactor tank is filled with solution up to the maximum level (i.e. 80L) and the level will be constant as there is flow going out from the tank at this point. The initial pH value of the solution in the reactor tank may be set to a desired value by controlling the two valves for the acid and alkaline streams manually. In order to represent this experiment in terms of a computer simulation it is appropriate to use the pH model for the continuous process of Figure 4.3.
Two experiments were carried out to provide more information about the dynamic behaviour of the pH neutralization process. The results from these experiments were used to validate, in a more quantitative way, the developed pH model described in the previous section and led to important refinements of the model. The first of these experiments involves a step change of flow in the alkaline stream. During the experiment the control valve for the acid stream was set to the fully closed position.
Figure 4:4 shows the dynamic response of the pH neutralization process for the pilot plant for the first experiment. In principle, in this experiment the initial pH should be set to the lowest possible value. However the process to achieve the lowest pH value is quite time consuming as the reaction process in this region is very slow. In
lowest value possible. Thus it is a rather impractical and expensive procedure. Based on several trials, a pH value of 3 was chosen as a reasonable initial pH value for this experiment.
(a) Process reaction curve of the experiment
0 100 200 300 400 500 600 700 800 900 1000
(b) Flowrate of the acid and alkaline streams
Acid Alkaline
Figure 4:4: Experimental results obtained using the enhanced data acquisition system during a test involving a step change of the flow rate for the alkaline
stream.
As shown in Figure 4:4, before the experiment started the pH value had been brought down, approximately, to the specified initial pH value of 3. At t = 150 sec. the process continues with the average flowrate for alkaline stream in the reactor tank being suddenly increased from zero to a steady value of 135.92L/h. The conductivity meters provided average readings for the acid and alkaline solutions of 23.68mS and 10.29mS respectively. Based on these values for the conductivities of the solutions in the two tanks, the results indicate that the concentrations of both of the solutions were slightly below the expected concentration value of 0.05M, with a concentration value for the acid solution of 0.0485M and a value of 0.0489M for the alkaline solution.
Figure 4:4 shows clearly the nonlinearity of the process with various different reaction rates as the response moves through the operating range in terms of pH values. The dynamic response can be divided into five different regions with three different reaction rates as given in Table 4.1.
Table 4.1: Process reaction rate of the dynamic response Range of pH Value Reaction rate
3-4 Low
4-6 High
6-8 Moderate
8-10 High
10-12 Low
The dynamic behaviour shows two different equilibrium points. The first equilibrium point, pK1 is approximately at a pH value of 3.2 and the second point, pK2 is at a pH value of 6.8. As explained previously, the equilibrium (or break point) depends on the value of the dissociation constants. Theoretically, sulphuric acid will have two break points as it is categorised as a diprotic acid. However, due to the first dissociation constant being fairly large, the first break point cannot be seen on the titration curve.
As shown in Figure 4:4, it is believed that the dissociation constant for the acid solution has been decreased due to some other reaction in the acid tank. In this experiment the sulphuric acid was added to water and it is believed that an additional and unknown source of hydrogen and hydroxyl ions existed. Such a situation will make the ionic strength of the solution decrease and as a result the dissociation constant will also decrease.
The results from the second experiment are presented in Figure 4:5. It shows another interesting dynamic response of the pH neutralization pilot plant. The idea of this experiment is not exactly the same as the previous experiment where the main aim was to obtain the process reaction curve of the neutralization process.
0 50 100 150 200 250 300 350 400 450 500
(a) Dynamic response from the experiment
0 50 100 150 200 250 300 350 400 450 500
(b) Flowrate of the acid and alkaline streams
Acid Alkaline
Figure 4:5: The dynamic response from the neutralization pilot plant for square -wave variation of alkaline flowrate with constant flowrate of acid
solution.
The objective for this second experiment was to obtain further ins ight about how to control the alkaline stream and the pH value in the reactor tank with a constant flow in the acid stream. The initial pH value was set to a pH value of 7. The valve that is controlling the acid stream was set to an opening which provided an average flow value of the acid stream of 61.5L/h. The alkaline stream was set to behave as a square wave signal with a period of 50s. The average flowrate value of the alkaline stream at the peak was 273.68L/h.
The average concentration value for the acid was slightly higher (0.0475M) than for the alkaline solution (0.0465M). As shown in the figure, the initial pH value is at a pH value of 7 and the dynamic response of the pH value increases towards the upper range of the pH scale as the experiment continues. This is an expected dynamic response as the flowrate of the alkaline stream is three times larger than the flowrate of the acid stream. Thus there will be more sodium hydroxide than sulphuric acid at the end of the experiment.
Computational work was carried out to simulate the two experiments outlined above.
The simulated experiments were based on the actual settings and configuration as given in Table 4.2 and described in the previous paragraph. The simulation results in terms of the dynamic responses obtained from the developed model for both experiments are shown in Figure 4:6 and Figure 4:7.
Table 4.2: Parameter settings for the simulation work
Concentration Flowrate
0.0475M 0.0465M Constant flowrate at 61.5L/h
Square wave signal with period of 50s and peak flowrate
of 273.68L/h
Figure 4:6 shows the dynamic response for the first experiment. This simulated dynamic response should be similar to the actual response obtained experimentally from the pilot plant during the step response test, as shown in Figure 4:4. The simulated dynamic response that represents the second experiment (i.e. using a square-wave signal to vary the flowrate of the alkaline stream) is shown in Figure 4:7. The dynamic response obtained from this simulation experiment should be similar to the corresponding experimental result obtained from the test carried out on the pilot plant and shown in Figure 4:5. The simulation results shown in Figure 4:6 and Figure 4:7 show clearly that the dynamic responses from the simulation model are inadequate and do not properly represent the responses from the actual pilot plant in a number of ways. These results suggest that there are further investigations and modifications of the pH process model that need to be carried out.
10000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2
(a) Process reaction curve for simulation Experiment 1
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 -50
(b) Step change at alkaline stream
Time (s)
Flowrate (L/h)
Acid Alkaline
Figure 4:6: Dynamic response – simulation of Experiment 1
0 50 100 150 200 250 300 350 400 450 500 4
6 8 10
pH Value
(a) Dynamic response from simulation for Experiment 2
0 50 100 150 200 250 300 350 400 450 500
0 100 200 300
Time (s)
Flowrate (L/h)
(b) Flowrate of the streams for simulation
Figure 4:7: Dynamic response – Simulation of Experiment 2
As mentioned earlier the pH process model was developed following some assumptions introduced to make the model simpler in structure and computationally more convenient to translate into a simulation. The experimental results and the findings from the simulation model suggest that some of the assumptions should be reconsidered. The next section describes some modifications to the pH process model introduced in order to make the pH dynamic model more realistic. Issues such as imperfect mixing, dissociation of acid and base reaction and time constants for control valves are highlighted.