Consider the vacuum filtration system shown below. The process is part of a sludge treatment facility. The sludge enters the vacuum filter at about 5% solids and it is de

watered to about 25%solids during the filtration process. The filter ability of the sludge in the rotating filter is dependent on the pH of the sludge entering the filter. A method of controlling the moisture content of the sludge sent to the incinerator is to add ferric chloride to control the pH. The schematic drawing below shows the closed loop control system for this operation. The moisture transmitter has arange (span) of 55% to 95%. The transmitter gain can be deduced from the maximum reduction in sludge moisture content relative to the span. The accompanying data was obtained after an input step change of +12.5% delivered by the controller (MC101). (i) Identify two (2) main disturbance variables that can affect the moisture content of the sludge from the exit stream. (ii) Briefly describe the type of control mechanism used to control the moisture content of the sludge from the exit stream. Draw a block diagram to illustrate your control mechanism. (iii) Determine PID controller (MC101) tuning settings for the closed loop system using Cohen-Coon correlations. (iv) If the PID controller system designed in (iii) is made up of parallel PD and PI controller systems with the same tuning setting, determine the overall transfer function of the PID controller system.

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