PTQ Q3 2026 Issue

Design and operation of salt dryer for ULSD: Part 2

Practical guidance on salt selection, vessel configuration, process control, and reliability for effective application in ULSD service

Prabhas K Mandal and Rajib Talukder Aramco

A s with Part 1 published in PTQ Q2 2026, Part 2 has been prepared because significant performance losses have been experienced in refinery service due to design gaps in salt dryers. Beginning with dryer vessels and internals, the following documented learnings have been converted into clear design and operating guidance. Dryer vessel and internals Vessel The vessel is constructed from carbon steel. Its internal sur - face is epoxy-lined or coated with specialised paint to pre - vent pitting corrosion caused by salt exposure. As elevated temperatures can damage the internal coating, the vessel must not be subjected to steam-out conditions. Salt dryers are configured in an n+1 arrangement, where ‘n’ represents the number of dryers required in operation. The additional (spare) dryer is designed with the same diameter as the operating units and a height sufficient to accommodate one month’s salt replacement volume, sup - porting routine loading of dryers operating on a three to six-month cycle. All internal components are designed to be dismantled and modular, allowing their introduction through the ves - sel manway, typically limited to 610 mm in diameter, and assembled inside the vessel. Inlet distributor An ‘H’-shaped feed distributor is typically employed. The ultra low sulphur diesel (ULSD) flows along a horizontal plane through perforated holes, typically 14-15 mm in diameter, located on both sides of each distributor branch. This arrangement ensures uniform flow distribution across the salt bed. The holes are oriented horizontally to reduce the risk of smaller ceramic balls entering the distributor branches. Upward-facing holes may allow ingress of ceramic balls, especially if undersized or broken balls are present. Downward-facing holes are avoided as they can disturb the settling of brine and oil in the calming section below the distributor. To maintain uniform flow, the pressure drop across the holes must be adequate. The length of uncut metal between adjacent holes should exceed the length of cut metal to

prevent the holes from acting as a continuous slot, which can lead to maldistribution. Perforations are preferred over slots, as they are easier to fabricate and provide more con - sistent flow. All holes must be sharp-edged and free from burrs. The distributor must be properly supported to pre - vent vibration. Support grid The support grid rests on structural beams and consists of multiple panels. Each panel is slotted, with slot lengths restricted to a maximum of 15 mm and slot widths typically 3-5 mm. These dimensions are selected to minimise the risk of ceramic ball migration into the lower sections. Grid panels are joined using bolts, with bolt size and spac - ing defined by the manufacturer. Any clearance between the grid panels and the support beams, as well as between the grid panels and the vessel wall, is packed with com - pressed ceramic rope of appropriate diameter. This packing ensures tight sealing and prevents any leakage of salt or ceramic balls through the gaps. The support structure, including beams and grid, must provide an open area of not less than 50% of the vessel cross-section. In some designs, this is increased to 70% to reduce flow restriction. The mechanical strength of the sup - port grid is designed to withstand at least 1 kg/cm² pres - sure, in addition to the combined weight of salt, ceramic balls, and ULSD present within the void space. Ceramic ball layer A minimum 300 mm thick layer of ceramic balls is placed at the base of the salt bed to form a seal over the support grid and prevent salt migration. The balls also serve as a transition layer to support the salt above. This layer also acts as a mass transfer zone between the draining brine and the incoming ULSD, where the brine absorbs free water from the fresh feed before it enters the salt bed. To prevent migration of smaller particles into the larger support layer, the ratio of diameters between adjacent layers is limited to a maximum of 3:1. For example, if the base layer is made up of 19 mm ceramic balls, the next layer above should have particles no smaller than about 6.4 mm.

PTQ Q3 2026