Prevention of failure in a distillation unit exhibiting extensive scale formation

Over a period of 2 years in service, a heavy scale buildup was observed at the outer surfaces of heating tube bundles that make up the evaporator shell of a multi-effects distillation vapor compression (MED-VC) unit employed in an onshore oil producing facility. The unit production rate dropped by 30% below design capacity due to scale formation. A series of acid cleaning procedures, including: 17 batches of 10% sulfamic acid and 12 batches of ethylenediaminetetraacetic acid (EDTA), were used to clean the scale deposit at 68°C. However, these efforts met with limited success leading to an investigation into the cause of cleaning failure. Selected samples of the scale were removed and characterized using scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS), as well as X-ray diffraction (XRD) and wet analysis techniques. The scale was approximately 14 mm in thickness and primarily constituted of soft CaCO₃ closer to the tube metal surface and hard CaSO₄ deposit away from it. In addition to CaCO₃ and CaSO₄, significant proportion of the deposit comprised of organic matter. It is believed that the presence of organic matter within the scale was primarily responsible for the failure of EDTA and acid chemical cleaning procedures. To avoid MED unit failure due to continuous scale buildup, it was suggested that an appropriate hot alkaline cleaning agent (instead of acid cleaning) is used in order to remove the scale deposit rich in organic matter.