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Automated Continuous Process Control Part 10

Tham khảo tài liệu 'automated continuous process control part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | c07.qxd 7 3 2003 8 26 PM Page 170 170 FEEDFORWARD CONTROL Figure 7-7.4 Temperature control in a distillation column. mented. This column uses two reboilers. One of the reboilers R10B uses a condensing process stream as a heating medium and the other reboiler R10A uses condensing steam. For efficient energy operation the operating procedure calls for using as much of the process stream as possible. This stream must be condensed anyway and thus serves as a free energy source. Steam flow is used to control the temperature in the column. After startup of this column it was noticed that the process stream serving as heating medium experienced changes in flow and in pressure. These changes acted as disturbances to the column and consequently the temperature controller needed to compensate continually for these disturbances. The time constants and dead time in the column and reboilers complicated the temperature control. After the problem was studied it was decided to use feedforward control. A pressure transmitter and a differential pressure transmitter had been installed in the process stream and from them the amount of energy given off by the stream in condensing could be calculated. Using this information the amount of steam required to maintain the temperature at set point could also be calculated and thus corrective action could be taken before the temperature deviated from the set point. This is a perfect application of feedforward control. Specifically the procedure implemented was as follows. Because the process stream is pure and saturated the density p is a function of pressure only. Therefore using a thermodynamic correlation the density of the stream can be obtained c07.qxd 7 3 2003 8 26 PM Page 171 ADDITIONAL DESIGN EXAMPLES 171 p f P 7-7.1 Using this density and the differential pressure h obtained from the transmitter DPT the mass flow of the stream can be calculated from the orifice equation w KoJhp 7-7.2 where Ko is the orifice coefficient. Also knowing