PTQ Q4 2024 Issue

Selection of ULSD dryers: Key technical considerations

An evaluation of various drying options for ULSD aiming to pass bright and clear specifications or haze ratings

Rajib Talukder and Prabhas Mandal Aramco

W ater present in ultra-low sulphur diesel (ULSD) poses significant risks by corroding and plugging engine components. It is also a major factor con - tributing to corrosion at the bottom of ULSD storage tanks and promotes bacterial growth. Water in ULSD can exist either as dissolved or as tiny, suspended droplets ranging in size from 0.1 µm to 10 µm. Exceeding a water content of 100 wppm may cause haziness, leading to the product becoming off-specification due to a lack of brightness and clarity. According to the ASTM D975-23 standard, the bottom sediment and water (BS&W) content in diesel must be less than 0.05% by volume. The European Norm EN 590 stip - ulates that the total water content should be a maximum of 200 wppm. Additionally, international market specifica - tions have mandated clear and bright, in which the ULSD contains no visible water drops or particulates and is free of haze or cloudiness. The haze rating for ULSD should not to exceed a level of 2.0, as per the ASTM D4176-22 standard. The ASTM D8148-22 test method provides a spectro - scopic procedure for determining the level of suspended water and particulate contamination (haze) in liquid middle distillate fuels, including those blended with synthesised hydrocarbons or biofuels. This method assigns an ordinal Instrument Haze Rating (IHR) from 1.0 to 6.0 and a Haze Clarity Index (HCI) from 50.0 to 100.0, assessed on a test specimen at 22.0°C ± 2.0°C. Correlation of water content and haze This discussion does not encompass a study of all factors influencing haze formation in ULSD. Such factors, not exhaus - tively examined here, include temperature, additives, fuel type, interfacial tension, and water separation characteristics. The haze rating of ULSD is primarily dependent on its water content, including both dissolved and free water. Therefore, refineries aiming to export ULSD must reduce the total water content to meet international market stand - ards. Previous studies, notably referenced in sources 1 and 7, have established that maintaining total water content in ULSD below 100 wppm at storage temperature is suffi - cient to meet haze specifications at the test temperature. Moreover, some pipeline operators impose maximum water content limits in ULSD to maintain the efficacy of additives and to minimise water settling in pipelines. For

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Hysys API Databook

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Figure 1 Dissolved water in ULSD

pipelines exceeding 200 km in length, operators typically restrict water content to a maximum of 80 wppm. Nevertheless, variances in haze ratings have been observed when comparing individual ULSD samples with equivalent water content levels. These discrepancies may be attributed to variations in the ability of each sample to dissolve water at the test temperature, a characteristic largely influenced by the fuel’s aromatic and polar com - ponent levels, such as bio-diesel content. For instance, a ULSD sample with a high aromatic or biodiesel content may dissolve more water than a highly paraffinic fuel lack - ing biodiesel or other polar components. At elevated temperatures, ULSD saturated with water can contain significant levels of dissolved water. As the temperature decreases, the dissolved water transitions to a suspended state. The ‘haze point’ or ‘saturation point’ is defined as the temperature at which water dissolved in the ULSD begins to precipitate and form free water droplets. Experimental data indicates that a temperature reduction of 25°F (14°C) below the saturation point leads to the pre - cipitation of a substantial amount of water droplets within the ULSD, causing it to appear hazy. Figure 1 illustrates the dissolved water content in a specific ULSD sample at var - ious temperatures, estimated using Hysys simulation and the API Technical Data Book equation. ULSD drying methods Drying ULSD is unnecessary if a reboiler is used for the

PTQ Q4 2024