The desalter is one of the first steps in preparing crude oil for distillation. As the name implies, the main purpose of the desalter is to remove salt impurities from the crude. Common salts found in crude oil include MgCl, CaCl, and NaCl. If left entrained in the oils, these salts cause corrosion issues, clogging of downstream piping, and problems with end product quality. The desalter also serves to separate out solids such as sand, drilling mud and paraffin waxes. Proper management of the desalter through chemical addition and pH control influences all other stages in the refinery. This paper will look at the challenges of this measurement.
pH Measurement Challenges
pH is typically measured on the wash water leading into the desalter as well as on the brine effluent. The wash water is acidifi ed using commodity acids such as sulfuric, acetic, or citric acid. The type of acid is dependent on the chemical profi le of the crude oil. Typical wash water values are 5.5 to 7.0 pH. The wash water is often recycled from other processes in refi nery thus it may contain ammonia or phenols. The water is pre-heated to 100 to 150°C. The combination of high temperatures and chemicals can be tough on the pH sensor.
pH measurement on the brine effluent is even more difficult. After the desalter, the water now contains a blend of salts, sulfides, heavy metals, and oil. Particulate material (mud) that settled out in the desalter can
also abrade and coat the electrode. pH can fluctuate depending on the crude oil source and the pre-treatment used in extraction and transport. Since the effluent water will be sent to the wastewater treatment plant pH will be measured to determine the treatment regime.