Controller for the pH Control ApparatusWritten by Alma LincolnSpring 2007In the pH control experiment of the chemical engineering senior unit ops laboratory, streams of aqueous nitric acid (HNO3) and sodium hydroxide (NaOH) are combined in a continuous stirred tank reactor in order to neutralize the pH of the resulting mixture. The valves regulating flows are controlled electronically via a LabVIEW interface, which is equipped with PID controllers to regulate reactor pH and liquid level with respect to flows, as well as to control flows with respect to valve settings. The purpose of this experiment is to familiarize the student with the process of automated PID control on a system exhibiting nonlinear dynamics. This manual is intended to instruct students on the use of the LabVIEW interface for the experiment.LabVIEW User InterfaceThe following paragraph is taken from the document “Operation Manual” located in the pH control apparatus computer, which includes a detailed description of the apparatus as well as in-depth instructions.“The LabVIEW user interface allows the operator to manually, or automatically control three inlet valves and one outlet valve, while recording pH, flow, and vessel level. The interface is divided into five sections responsible for recording and sending the required data. The five sections are flow diagram, relative time plots, valves, PID controls, and data logging.”A diagram of the interface is shown on the following page. Sections of the diagram are numbered in red, and the functions of these sections are described subsequently. The interface is accessed from the desktop of the apparatus computer. There it appears as an icon labeled “Shortcut to pH Cascade ’04.”1. pH and level controllers – These sections contain the PID controllers for the reactor pH and liquid level. The user must select which variables will be usedto control pH and level. Level may be controlled by acid, base, or outlet flow,while pH may be controlled by acid or base flow. The same flow cannot be used to control both variables. Once the variables are selected, the user specifies setpoints and the controller parameters K, τI, and τD. The user can also activate data filtering to reduce measurement noise, and activate or deactivate cascade control. This will be explained in the next section.2. Flow controllers – These sections contain PID controllers for the flow rates of the acid, base, buffer, and outlet streams. These controllers are used during cascade control: a setpoint is specified for the reactor pH or level, and the controller maintains this setpoint by determining a necessary flow rate for a particular stream. At the same time, another controller regulates the flow of that stream by varying the valve setting, using the flow rate output of the first controller as a setpoint. This counteracts the effects of valve sticking and hysteresis. In these blocks the user may only specify setpoints and control parameters; the choice of variables is fixed. 3. Valves – This block displays the valve settings. These have units of percent open, ranging from 0% to 100%. These settings may be set manually by the user when control is inactive, or when a particular valve is not used by a controller. When a valve is involved in control, its setting is displayed here.123456784. Relative time plots – Data are plotted in this section. The top plot displays thesetpoint and measured value for the pH over time. Below is a similar plot for reactor liquid level. The user may adjust the axis scales of both plots.5. Data logging – In this section the user records and saves data. The user must specify a name for the file and a time interval for recording data. The default setting is one data point every ten seconds, though the user will likely prefer to collect data more often. Data may be collected as often as one point per second. Files are saved in the directory C:\pH data\ as .txt files, and may be imported to Microsoft Excel for analysis.6. Shut down – This button is used to shut down the interface in order to exit the program. The button needs to be held down for several seconds in order to end the program. Manual valves need to be closed before shutting down.7. Flow diagram – In this block, a flow diagram of the system is displayed. It shows schematically how the acid, base, and buffer tanks are connected to the reactor, and how the reactor is connected to the waste tank. The flows of the streams connecting all the vessels are displayed here, along with the liquid levels and pHs of the reactor and waste tank. Inside the picture of the reactor are buttons used to plot the reactor pH and level.The pH of the waste tank must remain within a certain range, between 5 and 9, for safe disposal. If the waste tank pH leaves this range, the waste pH indicator blinks red as a warning. In this event, the contents of the tank must be neutralized before disposal.8. Waste tank warning – This area of the interface is where the waste tank warning appears. This happens when the waste tank becomes too full, around 28 gallons. When this occurs, all flows are shut down until the tank is drained. The drain is located on the wooden platform, directly in front of the tank.LabVIEW User Interface with Frequency ResponseIn addition to the interface described previously, there is an alternative version of the interface which may be used to perform a frequency response analysis. In such an experiment, the setpoint of a controlled variable is not held constant. Instead it is varied as a sinusoidal function of time. This interface is also located on the desktop, where it appears as an icon labeled “Shortcut to pH Frequency Level 2.” This is not to be confused with “Shortcut to pH Frequency Level,” which is an earlier version of the interface that only allows frequency response analysis on the level controller and not the pH controller. It is displayed below.This interface has a layout similar to the one previously described. However, it includes an additional section, outlined here in red. In this section the user selects which setpoint to vary. The options are pH, tank level, or “NONE” (meaning frequency response is inactive.) Thus frequency response analysis may only be performed on one variable at a time. When frequency response is active, the setpoint value specified in Block 1 is used as the “zero” value around which the setpoint of