Improving Temperature Measurement and Control Using the Exactus Optical Thermometers

Source: AIChE
  • Type:
    Conference Presentation
  • Conference Type:
    AIChE Spring Meeting and Global Congress on Process Safety
  • Presentation Date:
    April 11, 2016
  • Duration:
    30 minutes
  • Skill Level:
    Intermediate
  • PDHs:
    0.50

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Offering excellent accuracy, resolution, repeatability, and stability, EXACTUS® optical thermometry technology provides significant performance advantages in non-contact temperature measurement in a practical, rugged and user-friendly design.

To assess the performance of the BASF EXACTUS® optical pyrometer device for controlling the COT of steam cracking furnaces ethane steam cracking experiments have been carried out in the Pilot plant of the Laboratory for Chemical Technology (LCT), Gent, Belgium. The EXACTUS® device was used in two different configurations for providing the process value to the PI-controller of the natural gas burner valve. The control performance was compared with a similar setup in which the EXACTUS® device was replaced by a type K thermocouple, the latter being representative of the present state of the art in furnace control by a using wall temperature measurement. 

The BASF EXACTUS® measures the surface temperature of any given sample based on the emitted radiation of the surface at a wavelength of 900 nm or 1400 nm depending on the desired temperature range. Photons from a well-defined focal point are captured by a lens and transmitted through a fiber-optic cable to the detector. The latter converts the optical data to a digital signal. The known emissivity of the surface (by calibration) together with the measured radiation intensity allows to determine the temperature of the surface based on the Stefan‑Boltzmann law.

In each ethane steam cracking Pilot plant experiment the severity has been controlled by one of  the temperature measuring devices. EXACTUS COT refers to the measurement of the tube metal temperature of the reactor outlet just after exiting the furnace insulation. In TK COT, the temperature is measured at the same location but with a weld-on type K thermocouple. In EXACTUS TMT, the tube wall temperature is measured on the outlet part of the reactor coil right before passing through the furnace insulation, so inside the firebox. In TK PROCESS GAS, the process gas temperature is measured via a type K thermocouple inside a thermowell in the reactor close to the measurement point of EXACTUS TMT.

The experiments are compared in terms of temperature profile stability, product yield stability and coke formation. On-line gas chromatography analysis provided time-resolved data of the process gas composition. The standard deviations on the ethane conversion analysis and ethene yield measurement are an indication of the stability of the process with the given control system. For both the conversion analysis and the ethene yield analysis, the standard deviations are smallest for the EXACTUS® TMT control system.

Analysis of the amount of coke formed during the experiment can indicate process instability due to temperature overshoots in the reactor as shown in Table1. There is a slightly higher amount of coke formed in the reactor using EXACTUS® control on the reactor outlet compared to the Type K thermocouple in the same position. However, the difference is only 0.009 oz, which falls within the 5 % relative accuracy of the measurement. The coke yields for the EXACTUS® control on the tube metal temperature are lower compared to the control on the reactor outlet temperature.

Table1: Coke yield for the different control strategies.

 

Coke yield [oz]

EXACTUS COT

0.646 ± 0.032

TK COT

0.635 ± 0.032

EXACTUS TMT

0.565 ± 0.028



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