(2a) Improving Temperature Measurement and Control Using the Exactus Optical Thermometers

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.