Sasol's automated pH cleaning cuts maintenance needs

London - Sasol Wax GmbH, based in Hamburg, Germany, currently produces around 350 kilotonnes per annum of paraffin and paraffin-related products. Derived from fossil raw material, paraffins are mainly made up of saturated hydrocarbons containing potentially toxic constituents such as hydrogen sulfide or aromatics which must be removed via processes such as high-pressure hydrogenation.

For Sasol's paraffin wax refining process, measurement of pH and ORP values must be optimised in association with an electrode cleaning and calibration system to improved electrode life and minimise maintenance. The poisonous hydrogen sulfide removed during the hydrogenation is converted to elementary sulfur in an absorber column via an oxidation stage.

To ensure consistent oxidation of sulfide, the redox potential values before and after the oxidizer are important control criteria for optimal removal of hydrogen sulfide in the column. Optimal process function is achieved with redox potential values of approx. -300 mV at the entry to the oxidizer and approx. -100 m Vat the exit. If the redox potential becomes too positive, salts that have a negative influence on the absorption of H2S can form. At the same time the pH value of the wash water also acts as a criterion for the absorption quality. If the value is too acidic, caustic neutralization is necessary.

Sasol, therefore, requires reliable analysis of the pH and redox potential values before and after the oxidizer. The main requirements of the process include:

• Measurement of the pH and ORP values at the different measuring points to be taken daily at different times with visualization via the PCS.
• Last measurement values to be held fixed throughout the period until the next measurements have been taken.
• Longest possible service life of the electrode.
• Ability to calibrate and monitor both parameters.
• Maintenance effort to be reduced to a minimum.
  

Originally the procedure was to be based on daily measurement of random samples, which resulted in a substantial amount of work and high indirect costs. To resolves these issues, Mettler Toledo proposed the use of  XEROYLT Plus type electrodes InPro 4250 (SG) in conjunction with fully automated, freely programmable EasyClean 400 X systems. The electrode is designed to measure pH, ORP, as well as temperature, and to operate over an extended period in applications involving H2S-bearing media.

According to the supplier, the EasyClean 400 X Systems installed were programmed such that the electrodes insert into the process every two hours for a defined period. For the rest of the time each electrode is stored wet, safely protected inside its own retractable housing. This guarantees extended life of the electrodes avoiding undue exposure to the process medium. The electrodes are automatically calibrated and adjusted weekly with a 2-point calibration for pH and a 1-point calibration for ORP.

Specific membrane resistivity values of the electrodes are stored in the transmitter so that any change in these values through e.g. junction fouling will prompt a warning. In addition, if the set tolerance values for redox are exceeded or understepped, this also triggers an alert.

According to Mettler Toledo, Sasol engineers have reported high system functionality and measurement availability following the introduction of the new system, with significantly lower maintenance effort and electrode life now extended to six months.