Descripción de las plantas seleccionadas para este proyecto

3.2.2.1 Control room and data acquisition

All on-line sensors in the pilot plant were wired back to a Programmable Logic Controller (PLC) housed in a control cabinet located in the control room. The PLC was connected to a computer which provided a user interface and along with the PLC was used to control various process variables in the plant. A photograph of the control room and cabinet is shown in Figure 3.7.

Figure 3.7 Pilot plant control room and cabinet showing PLC (right).

The PLC consisted of an Allen Bradley SLC 5/03 processor and two racks each with 12 module bays. The modules installed in the bays included: 2 analog current (32

channels), 2 counter (8 channels), 5 analog (20 channels), 2 relay output (16 channels), 5 thermocouple (20 channels) and 4 RTD input (16 channels) modules. A total of 112 channels were available to the plant for sensor input and delivery of control outputs. On occasion, a standalone Supervisory Control and Data Acquisition (SCADA) station was coupled to the PLC to provide additional sensor channels (e.g. used when experiments required the operation of all six MPHE modules and all six THE tubes at the same time).

The control computer was used to display and log data from the PLC and also provide PID feed back control to the pilot plant via the PLC. This was achieved using FIX DMACS Version 7.0 software installed on a Microsoft Windows NT 4.0 platform. Although the PLC was capable of PID control calculations, these were carried out by writing control loops in FIX DMACS to allow quick reprogramming of the control loops.

All sensor measurements were available to the plant operator via the control screens used to interface with the plant. Real-time trending of variables could be viewed using the historical display software which also ran while the plant was in operation. All data managed by the computer were logged to disk automatically. A sample of each sensor measurement was logged to disk every second. Control variables such as pump speeds were logged every 30 seconds. Data required for further analysis would be exported from the FIX DMACS software into comma-delimited text files so that they could be manipulated as a spreadsheet in Microsoft Excel.

The FIX DMACS software was also used to perform simple calculations in real-time. These included all sensor calibration calculations and important analytical data calculations like overall heat transfer coefficients. The calculated values were automatically logged to disk and could be displayed in real-time on the control screens and/or historical data display.

3.2.2.2 Temperature measurement

A combination of T-type thermocouples and resistance temperature devices (RTDs) were used to measure temperature across the preheating section of the pilot plant (T1-31). The majority of temperature measurements were made by thermocouples,

with RTDs only being used in less crucial areas (e.g. feed vat) where measurement accuracy was less important. Originally, RTDs were used for all temperature measurements but during the commissioning stage it was found they had a slow response time and could not accurately measure rapid changes in temperature. This function was important to the successful monitoring of fouling particularly when coupled with heat flux sensors. Bennett (2000) discusses in detail the issue of temperature measurement and gives a full account of the change from RTD to thermocouple sensors.

All temperature sensors were calibrated by recording the temperature of distilled

ice/water slurry (0°C) and boiling distilled water (100°C), designated as θ0 and θ100

respectively. Measurements were recorded at each temperature every second for five

minutes and averaged to obtain the recorded temperature (θr). The calibration

constants were obtained by assuming a linear relationship between the recorded and

correct (θc) values: c a r b θ = θ + (3.1) where θc θ = correct temperature r a = gradient constant = recorded temperature

b = y-axis intercept constant

For the freezing and boiling points of water these constants are:

100 0 100 a = θ - θ (3.2) 0 b = - aθ (3.3)

These equation constants were entered into the computer interface to display the calibrated values on-screen, not the raw recorded values. A sample calculation is provided in Appendix D.1.

3.2.2.3 Flow rate measurement

Several flow meters where used to monitor process fluid and cleaning flow rates throughout the plant. Two electromagnetic flow meters were used in the preheating

section, one for low flow rates (30 – 300 l/h: F1) and one for higher flows (300 – 10 000 l/h: F2). They were installed in parallel and the flow could be diverted through either one depending on how the pilot plant was being used at the time. The larger flow meter was generally only used to monitor CIP fluid flow through the plant but was sometimes used during visualisation experiments when large flow rates were required. The smaller flow meter was used the rest of the time.

Each flow meter was calibrated by operating at a range of flows, in which the flow meter was needed, and recording the actual flow rate with a measuring cylinder and stopwatch. Collected data were recorded in a spreadsheet and linear equations fitted to create calibration equations relating recorded flow to actual flow rate. As with the temperature calibration, the constants from these equations were entered into the computer interface so that the calibrated measurement was displayed on-line and in real time.

3.2.2.4 Pressure measurement

Pressure sensors were installed throughout the plant to monitor pressures in the process and steam lines. Locations of the pressure transmitters are given in the pilot plant’s piping and instrumentation diagrams of Appendix A. Depending on the pressure at each location either a 0 – 50 PSI.g or a 0 – 15 PSI.a sensor was used. On-line measurement of pressure was critical to the successful operation of the plant. In the preheating section the sensors were used to ensure good performance from the DSI units because when the process line pressure approached the steam pressure, the performance of the DSI units dropped. Also, the operating pressure of the heat exchangers was found to be very important during fouling experiments. Accurate measurements of the pressure in the process line were therefore required before and after the throttling valves leading to the research heat exchangers.

All pressure sensors were calibrated against a calibrated manual pressure gauge over a range of different pressures.