Frequency response of a feed preheater

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Virginia Polytechnic Institute

The purpose of this investigation was to measure the frequency response of a shell and tube heat exchanger when the input is the pressure to the steam valve and the output is the feed temperature. During this investigation the frequency response of the feed flow loop was also determined. Controller settings were determined for the feed rate controller.

The heat-exchanger under investigation was the food preheater connected to the ten place, bubble-cap, distillation column located in the Unit Operations Laboratory, Department of Chemical Engineering, Virginia Polytechnic Institute. This heat-exchanger was a single-pass, shell and tube type having a contact surface of nineteen 5/8-inch copper tubes 45.5 inches long.

The flow rate during the investigation of temperature and pressure frequency response was maintained at 3.70 gallons per minute. The feed mixture was composed of benzene, n-hepane, xylene, and toluene.

Frequency response tests were conducted on the steam pressure and hydrocarbon temperature to upsets in steam flow rates. Two tests were conducted on each the temperature and pressure. The first test was conducted at a mean temperature of 161°F and the second at 194°F. The frequency range for each of these tests was 0.008 - 0.05 cycles per second.

The ultimate frequency of the hydrocarbon temperature to steam flow upsets was determined to be 0.021 cycles per second.

Frequency response test performed on the feed flow loop was conducted at a mean hydrocarbon flow rate of 2.75 gallons per minute and room temperature. A fifteen percent disturbance was used and the unit was tested over the frequency range of 0.008 - 0.6 cycles per second.

The zero frequency gain of the feed flow loop was found to be 2.10. The ultimate frequency for upsets in flow was determined to be 0.53 cycles per second.

The controller settings determined for the two mode controller which controlled the hydrocarbon flow were, proportional band of 63.5 percent and reset of 0.262 minutes.