Evaporatorfinal.docx

I. Introduction Solutions often consist of a solvent and a dissolved solid substance. By simple thermal separation the solvent can be vaporized leaving the solution with an increased concentration, this process is evaporation. The purpose of thermal separation can vary, but most often it is a matter of concentrating the solution, recovering the solvent or recovering the dissolved substance (Davidsson & Hedenberg, 2015). Evaporator is one of the equipment used to concentrate solute in the industry by means of thermal separation. As one of the most energy intensive processes used in the dairy, food and chemical industries, it is essential that evaporation be approached from the viewpoint of economical energy utilization as well as process effectiveness. This can be done only if the equipment manufacturer is able to offer a full selection of evaporation technology and systems developed to accommodate various product characteristics, the percent of concentration required, and regional energy costs. With this statement, process control is usually the resort to optimize the equipment’s productivity and efficiency. Process control is important in evaporator to regulate the disturbances in the system and guarantee a higher quality product. Also with the presence of process control, hazards can also be controlled in the work place. In this paper, a manual and automatic control program was developed using MATLAB R2013a Simulink that can simulate the liquid level and the product concentration inside a single-effect evaporator. The later part of the paper discussed the process dynamics of the designed equipment, the Simulink methodology and the features of the developed program.

II. Process Dynamics A. System Disturbances and Variables The model of the system assumes that the only disturbances in the system were the feed flow rate and the feed concentration. Other assumptions include constant vapor output and equal concentration of the product and liquid in inside. In evaporator, the most important consideration in controlling the quality of concentrate from an evaporator is forcing the vapor rate to match the flow of excess solvent entering in the feed (Perry & Green, 1997), with this statement, varying the feed flow rate can significantly affect the product concentration. To control the flow rate, valves is installed in the stream. In this study, the allowable level of liquid inside the evaporator is set to control the flow rate.

B. Derivation of Process Dynamic Model

Material balance

d L L  F F  P P  V V dt Component balance

d L x L L  F x L F  P x p P  0 dt d L x L L  F x L F  P x p P dt Assuming constant cross sectional area, the equation will be A

dx L H   F x L F  P x p P dt dH  F x L F   P x p P  dt A P x p

III. Simulink Method 1. Open MATLAB and create a new Simulink Mode

2. After starting a Simulink Model, Open Library Browser where the blocks needed for the simulation can be found.

3. Insert a signal builder block by dragging the icon from the Library Browser to the Simulink Model window. This will be the source of the input data.

4. To create a signal, open Microsoft Excel and create random data for the flowrate and product composition.

5. Save the file then import the file in the Simulink model.

6. Based mathematical evaporator model that was developed, continue adding blocks from the library to the Simulink model. Use constant blocks for constants and mathematical operators for the calculations. Integral to determine the desired output and scope to display the output. For the manual control, slider gain will be connected on the input block that needs to be controlled.

7. For the automatic control, the slider gain and one of the flowrate signal will be removed and replaced with a PID controller which will compare the output with the set point and will make necessary adjustments to achieve the desired output.

8. After the diagram is finished, run the simulation then double click on scope to display the results.

IV. Program Features 1. Simulink Model

Figure 1: Manual Control Block Diagram

Figure 2: Automatic Control Block Diagram The main feature of the simulink model is to control the level of a single effect evaporator using manual and automatic controller in which the manipulated variable is the flowrate input. The flow rate in

the manual control is controlled using a slider gain in which the adjustment is done manually to reach the given setpoint by increasing the value in the slider gain, the level will also increase. While for the automatic controller, PID is used as a controlling device which compares the current output with the desired output then its makes necessary adjustments on the input depending on the difference. The development of its model is done using a material balance in which accumulation is present, since the level inside the evaporator is to be maintained. Also,varying concentration in the feed stream and in the accumulating fluid inside the evaporator was accounted while the product concentration is constant.

2. Model Simulation

Figure 3:Level Simulation Using Manual Control Figures above illustrates the effect of evaporator using manual control. Initially, the flow rate is in stable condition where the level is at 0. This evaporator model can be controlled using the Simulink’s program feature Slider Gain. The slider gain act as an opening valve of the evaporator. At some time, the height level of the equipment starts to increase as the value of the slider gain increases. The time for the system to reach the set point depends on the valve opening and is set by the operator. Since initially, the graph starts at zero, therefore the operator can increase the valve opening in a degree it reaches the desired level in a shortest time. Large deviation can be seen in the graph can be seen in the figure, this is because of the error cause by the operator to maintain the level of equipment. Therefore, using manual control requires periodic reading of the graph in real time and adjustments should be done to decrease the error.

Figure 4: Level Simulation Using Automatic Control The figure shows the graph of automatic control of evaporator. The system has its own controller and automatically regulate the system flow rate when it receives disturbance. The graph illustrates the shortest error as possible. Unlike the manual control, no operator is needed in order to maintain the set point. Initially, it starts in the higher level then automatically adjusted near set point by the controller. As the simulation goes, the graph became smooth with minimal deviation near the set point.