Chemical Engineering Advance Process Control - Coursework Example

 CONTROL SYSTEMS Introduction Control is the term used when referring to the methods or means utilised in governing performance of system, apparatus or machine. Control system are used in controlling processes in order to meet the desired end; this high quality, improved safety, high production rate or repeatability in production. Feedback control entails regulation of physical processes via the comparisons of value physical quantity with desired value; hence make adjustments accordingly.5 The combined feedforward and feedback control is used when an improved performance is required, this control mode is better than the simple feedback control because major disturbance are measured before affecting the process output. In ideal situations, the feedforward controls eliminate the effects of the measured disturbances appearing on the process output. In case of modelling errors, the feedforward controls reduce the effects associated with measured the disturbances on the output when compared to the feedback control working alone.1 Open-loop Control System An open- loop control systems exist when the process variables are not compared, and usually actions are taken not in accordance to response to the feedback from the variable process condition, instead action is taken disregard to the variable process conditions. An open loop control is also known as the feed forward control. Figure 1: Block diagram for an open-loop control system in the position control Figure 2: Open-loop for positioning system in chemical industry In figure 1, an open-loop control system, a robot arm is driven by an actuator; hence the process is associated with the movement of arm, while the controlled variable in this case is the arm’s angular movement position. The controller does not know an error, and also does not correct it. Figure 3: Maintaining the constant liquid level in a tank in constant value via the control of control of an incoming flow rate. Figure 4: Open-loop for water level system The liquid enters the reservoir tank from the top, the exits via an exit pipe found at the bottom of the tank. The process does not consider the external disturbances. Figure 5: Open control for valve position. 6 Figure 6: Open-loop for temperature level system The processes in figure 5 and 6 are stable process, additionally when there are no disturbances , the process find that the process variable stabilizes at availed values for the valve position, for example 110°F for the valve has 50% open. Additionally, temperature is maintained at 110°F given that there are no disturbances on the process.4 An open-loop control system includes the water valve used in opening cooling tower in order to add cooling water to chemical process in order to prevent the chemical process fluid from becoming too hot based on the pre-set time intervals, without the consideration of the actual temperature of the chemical process fluid. Feed Backward Control A feed backward control entails the measurement of process variables, comparing them to set points, and taking action in order to correct deviations from the set points. Therefore the feedback loop is utilised the measurement of the process variables and then sending the said measurement to the controller with the aim of comparison with set point. In case the process variables are not at the setpoint, then control takes action towards the returning of the process variable to their setpoint. From figure 7 below, illustrating the feedback loop that has the transmitter for the measurements of the temperature of the fluid, in case anything positive or negative, there is opening or necessary closing of the hot steam valve in order to adjust the fluid’s temperature.3 Figure 7: Feedback loop for temperature control Figure 8: Block Diagram for the Feedback loop for temperature control One of the advantages of the feedback loop is characterized with the direct controls of the desired processes variables. The major disadvantage associated with feedback loops is that the process variables have to leave setpoint in order for the action to be taken. Figure 9 below is an industrial process control system used for controlling the temperature of the pre-heated process fluid found in the jacketed kettle. The temperatures of the process fluid are taken or sensed via thermocouple transducers immersed in process fluid. Fluid temperature is represented by the thermocouple’s voltage, usually in milli-volts; the voltage is amplified via the amplifier in order to produce a conditioned voltage Vb. The potentiometer and the battery are the set point, with voltage Vr. The input voltage, represented by Ve is channelled to the amplifier, denoted by Ae and represents the difference the feedback voltage Vb and the reference voltage. The voltage output, Vo from the amplifier is directed at the solenoid coil in order to produce force, fs; the force proportional to the current via the coil i.e. ia. The solenoid is used when pulling the valve plug that in return makes the valve plug move the distance denoted by x. The valve opening is directly proportional to the steam flow rate qn. The temperature, represented by θ for the process fluid is proportional to the valve opening. Figure 9: The Feedback control system for the temperature control in the process fluid The flow rate, i.e. f, for the fluid the process fluid gradually increases while that of the temperature in the kettle decreases; this because the steam flow is not increased. The action for the control system is, usually, to increase the flow rate of steam to the jacketed kettle; this is carried on until the temperature for the process fluid become equal to the required or desired value set using the reference potentiometer. When the temperature of the kettle falls below, i.e. of the set point, the amplified value Vb and the thermocouple output voltage decrease in the magnitude. Vr and Ve = Vr – Vb are fixed in the case of the set point potentiometer, hence any decrease in Vb leads to Ve to increase. The amplifier’s output voltage also Vo increases, hence increasing the travel x for the valve plug. The closed loop control system in figure 9 is represented by the block diagrams as shown in figure 10. The required or the desired value for the system state is usually converted into the reference input via the transducers. Figure 10: The Block diagram for the temperature control closed loop system From figure 10, the controlled variable is then converted into an appropriate feedback signal through the feedback element; this is also converted into the indirectly controlled variable which represents an actual output for the entire feedback control system. There is subtraction of the reference input and feedback signal in order to get an actuating signal is represented via the small circle having signs for the representation of the arithmetic operation. The enclosed dotted line in the diagram is the controller of the system. Figure 11: Feed backward temperature control Figure 12: The Block diagram for the heater temperature control closed loop system From Figures 11 and 12, the controlled variable is the temperature, which is represented by Өo; this is an output value. The set value, Өi or the input value is the compared to the amplified value of Өo, whereby the resulting error is utilised when making correction action. Feed Forward Control The feedforward control is the control system which anticipates the load disturbances hence controlling them prior to their impact on the process variable. The control mode is hard to design since the user has to have the mathematical understanding of the manipulated variables that impact the process variable. In figure 13 is the feedforward loop that the flow transmitter closes or opens the hot steam valve on the basis of how much the cold fluid passes via the flow sensor. Figure 13 (a): Feedforward Control Figure 13 (a): Feedforward Control One advantage of the feedforward control is the prevention of the error instead of being corrected. Figure 14 (a): The Feedforward control for the feed in the continuously stirred tank reactor operation Figure 14 (b): The Feedforward control for the feed in the continuously stirred tank reactor operation block diagram The above continuously stirred tank reactor is representing the feedback temperature control. The feedforward control is utilised in the rapid suppression of the feed flow rate disturbances. The disturbances present in flow rate are dependent on the level of the fluid found in the tower, as depicted in figure 15, and can be easily measured via a flow meter. Immediately there is change in the flow rate within the input fluid, a corrective action is taken simultaneously through the adjustment of the steam input towards the heat exchanger. The process is possible via feeding of both the signals from the thermocouple to the temperature’s controller and the flow meter signal. Figure 15 (a): The feedforward temperature control system Figure 15 (b): The feedforward temperature control system block diagram Feed-forward control in chemical engineering applications also includes the heat exchangers, distillation columns and the CSTRs; below is a furnace used when heating up the input fluid by the fuel gas. Figure 16 (a): The feedforward furnace control system Figure 16 (b): The feedforward and feedback furnace control system block diagram The most anticipated disturbance in figure 16 is the flow rate of the incoming fluid. When, for example, the fluid input rate is increased by 30%, the needed heat duty is increased by approximately 30%. The feed-forward control strategy alters the fuel gas flow rate, hence the heat duty. The model’s performance, i.e. feed-forward controls, is normally limited by the model uncertainty, hence should be combined with the feedback control for better control of the system. In the incorporation of two, the feed-forward control compensates for the fluid input disturbances, on the other hand the feedback control compensates for the disturbances, for example fuel gas flow rate and the furnace’s temperature, with the model’s uncertainties. Combined Feed Forward Feed Backward Control Due to the existing difficulty in accounting for every anticipated or possible load or input disturbance in the feedforward system, there is need for the incorporation of the feedforward systems to the feedback systems. Therefore, the controllers utilising the summing functions are applied when the systems are combined, hence both the total the inputs emanating from the feedback loop and the feedforward loop send unified signals to the control element. The Figure 17, below, illustrates the feedforward-with-feedback loop using both a temperature transmitter and flow transmitter in providing information for the control of the hot steam valve. Figure 17 (a): Feedforward with Feedback Control Figure 17 (b): Feedforward with Feedback Control block diagram Figure 18 (a): The feed-forward control loop with the feedback control loop for the heat exchanger Figure 18 (b): The feed-forward control loop with the feedback control loop for the heat exchanger block diagram Figure 18 is the heat exchanger system utilises both the feedback control and feed-forward control systems. The controller is used in receiving input signals from the two sensors. Sensor along the inlet line avail the feed-forward signals, while the sensor at the outlet avails the feedback signal. The feed-forward control is used when adjusting the operations of the actuator in order to prevent changes at the controlled variable. Moreover, the feed-forward controllers are used in the making of sophisticated calculations for the computation in the changes of the actuator required for compensation for the variations due to disturbances. Figure 19 (a): Feed-forward with feedback control strategy for flow rate. Figure 19 (b): Feed-forward with feedback control schematic block diagram. From figure 19, the inlet flow rate is the measured as the disturbance, while the tank height is taken as the measured output taken for comparisons in order to make manipulations; outlet flow rate is the required manipulation. Conclusions The feedforward systems are utilised in cases characterised with the controlled variable having the potential of being huge or major load disturbances on the controlled process variable. Incorporation of the feedforward systems to the feedback systems ensure there are shared merits of the two systems1. References 1. de Silva, C. (2009). Modeling and Control of Engineering Systems. Hoboken: CRC Press. 2. Dong, J. (2009). Introduction to control engineering. Australia: Cengage. 3. Friedland, B. (2005). Control system design. Mineola, NY: Dover Publications. 4. Levine, W. (2011). The Control Handbook. Hoboken: CRC Press. 5. Liu, Z. (2015). Control Engineering and Information Systems. Boca Raton [u.a.]: CRC Press. 6. Morris, A. and Langari, R. (2012). Measurement and instrumentation. Waltham, MA: Academic Press. Read More