An Automated Method for Testing Process Parameters
MonroeWeber-Shirk
220 Hollister Hall
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Fax-(607) 255-9004
mw24@cornell.edu
An Automated Method for Testing Process Parameters
Author:Monroe Weber-Shirk
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The Process Control software is available at http://ceeserver.cee.cornell.edu/mw24/Software
. The software checks for updates whenever it is started and if a newer version is available at the website it gives the user the option of downloading the latest version.
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pH sensors produce a voltage output in the range that would normally be easy to measure using standard data acquisition hardware. Unfortunately, the impedance requirement for a pH sensor is orders of magnitude higher than the inputs of standard data acquisition hardware and thus a signal conditioning circuit must be used to amplify the pH sensor output. The circuit consists of unity gain amplifiers that have less than 0.1 pA input leakage current (anonymous , 1993). A circuit diagram is available at http://ceeserver.cee.cornell.edu/mw24/Labdocumentation/pH%20Circuit.pdf. The Process Control software includes automatic pH buffer recognition and piecewise linear calibration between buffers.
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Dissolved oxygen probes produce a current in the pA range that is proportional to the oxygen concentration in the bulk solution. The signal conditioning circuit is designed to convert this very small current into a measurable voltage and to isolate the probe from the effects of fluctuations in the voltage level of the solution containing the probe. This isolation is critical if the solution is monitored with additional probes or if the solution is electrically connected to a building plumbing system or to any other voltage source. The circuit design is available at (http://ceeserver.cee.cornell.edu/mw24/Labdocumentation/Dissolved_oxygen/circuit.pdf)
Temperature
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The Stamp microprocessor control boxes have six 24 V outputs that can be used to control solenoid valves, pinch valves, relays, or other low current devices. The 24 V circuits include a pulse width modulation chip that reduces the voltage to the device to reduce power consumption and heating. This feature works well for solenoid valves since they can be held open with a relatively low voltage once open. The circuit design for the Stamp microprocessor controls is available at http://ceeserver.cee.cornell.edu/mw24/Labdocumentation/StampBox.pdf.
Process Control Software
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LabVIEW executables can be enabled to connect to external code. This capability makes it possible to easily extend the capabilities of the Process Control software. The external code must be designed to meet specific requirements for the data types of inputs and outputs. An external code interface has been created to take a variable number of numeric inputs and produce a single numeric output. The external code can be used for a wide variety of functions including simple math functions (Figure 5), a specialized function (such as one which sets a coagulant dose based on raw water turbidity, (Figure 6), proportional-integral-derivative control that can be used to force a controlled parameter to a desired set point, and data acquisition functions that acquire digital data from instruments.
Figure5. LabVIEW block diagram showing the external code that adds two variables or set points and returns the result.
Figure6. Screen shot from the Process Controller showing how inputs are sent to external code. In this case the external code estimates an alum dose based on measured raw water turbidity and a simple model that relates turbidity and alum dose.
The programming environment for creating rules that determine exit conditions for states and which state to go to readily facilitates setting up the algorithms for controlling simple repetitive processes such as a sequencing batch reactors or rapid sand filters. For experimental purposes it is desirable to have the capability to systematically vary a parameter to test the performance of the process over a range of input values. This is accomplished via an external code that compares the number of specified replicates to a parameter that increments when the process controller enters a specified state. The output parameter can be used to control pump speeds, times, or can be an input to subsequent calculations.
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A second computer running Process Control software is used to control the temperature of a tap water supply reservoir. The reservoir is instrumented with a temperature sensor and a pressure sensor. The temperature sensor is used to determine whether hot tap water or cold tap water should be added and the pressure sensor is used to determine if water needs to be added to keep the reservoir full. The reservoir contains an aeration stone and is constantly aerated to remove any supersaturated gases that are commonly found in cold surface waters and prevent evolution of gas bubbles in the filter column.
Figure8. Photo of the filter test apparatus.
The experimental objective of the FTA is to test the effect of various chemical treatments of the filter media on the particle removal efficiency. The Process Controller is used to cycle through the following states for each test.
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4) Backwash – The accumulated particles and the chemical from the pretreatment step are removed from the porous media. Additional Additional states can be used to flush the turbidimeter cells with clean water to remove any settled clay or to pump an acidic solution through the filter to ensure complete removal of the surface coatings created by pretreatment of the filter media.
Figure9. Schematic of the filter test apparatus showing the valve and pump configuration for pretreatment with aluminum sulfate.
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The filter performance is measured as pC* where C* is the turbidity of the effluent water normalized by the turbidity of the influent water and p is the --log function. Thus a value of zero for pC* would indicate no particle removal and a value of 1 indicates 90% particle removal. The x axis is time and data is shown between 600 and 1000 s. The data from the first residence time for the experimental apparatus are not shown since it takes several residence times to purge the filter column of the water from the previous state. The aluminum pretreatment is reported on the y axis as moles of aluminum per square meter of filter top surface area. The data clearly show that the aluminum pretreatment enhances particle capture and that at low doses the filter performance improves as the amount of aluminum applied to the filter increase. However, at the very highest dose the filter performance begins to deteriorate.
Figure10. Data Data surface showing the effects of pretreatment with aluminum sulfate. The black data points are measured values and include replicates at each aluminum treatment level. The mass of aluminum applied per filter area increases from left to right on the y axis. The ability of the filter to remove clay is measured as pC* where C* is the effluent turbidity normalized by the influent turbidity and "p" is the negative log function. The filter run time is on the x axis. There is a gradual deterioration in filter performance with run time.
We will compare aluminum sulfate with other filter aids and discuss the significance of these experimental results in a future paper.
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This work was funded by the National Science Foundation and CornellUniversity. The author would like to thank Leonard Lion for his thoughtful review of this paper, Po-Hsun Lin for the rapid sand filtration data, and Cameron Willkens for design and fabrication of the microprocessor control box and the sensor signal conditioning circuits.
References
anonymous anonymous (1993) MAXIM 1.2 mA Max, Single/Dual/Quad, Single-Supply Op Amps. Accessed 4/16 2007. http://www.ortodoxism.ro/datasheets/maxim/MAX406-MAX419.pdf
Braatz, R. D. and Johnson, M. R. (1998) Process control laboratory education using a graphical operator interface. Computer Applications in Engineering Education 6(3), 151.
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Rehg, J. A., Swain, W. H., Yangula, B. P., Wheatman, S. (1999) Fieldbus in the process control laboratory - its time has come. In: Proceedings - Frontiers in Education Conference, 13b4-12.