Oct-2024

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

Article Summary

Water present in ultra-low sulphur diesel (ULSD) poses significant risks by corroding and plugging engine components. It is also a major factor contributing 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 stipulates that the total water content should be a maximum of 200 wppm. Additionally, international market specifications 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 spectroscopic 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 exhaustively 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 standards. 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 sufficient 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 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 component 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 lacking 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 precipitation 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 various 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 product stripper downstream of the reactor loop. However, it is crucial to limit the maximum outlet temperature of the reboiler to 385°C to prevent colouration issues in ULSD. In the absence of a reboiler, several drying options are available when using a steam stripper, including:
• Coalescer at ULSD rundown line
• Salt drying at coalescer downstream
• Vacuum dryer
• Molecular sieve and low-temperature coalescer.

Historically, conventional refinery units producing ULSD have been equipped with coalescers on the rundown line. Coalescers are designed to remove free water, achieving levels as low as 15-20 wppm of free water. The effectiveness of a coalescer is influenced by its operating temperature; at higher temperatures, ULSD can saturate with more water, which the coalescer cannot remove. For instance, when operating at 45°C, the ULSD may still appear bright and clear at the coalescer outlet with a total water content of 150 wppm, as indicated in Figure 1.

However, this clarity can be misleading, as the sample might appear hazy at the recommended testing temperature of 22°C. At this temperature, the dissolved water estimated to precipitate out, as per Figure 1, would amount to 70 wppm. This level of moisture potentially leads to haze, causing the ULSD to fail the ‘bright and clear’ specification. However, not meeting bright and clear becomes more prominent in the geographic region where day-night temperature variation and summer-winter temperature variation are very high.

Therefore, maintaining total water content below 100 wppm to meet the ‘bright and clear’ specification at the testing temperature can be challenging for a coalescer when the dissolved water content is high at the coalescer operating temperature.

A salt dryer functions as a large oil desiccator and can remove both free and dissolved water from ULSD. To reduce the operational load on the salt dryer, a coalescer is typically installed upstream. The type of salt used can vary between rock salt (NaCl) and calcium chloride (CaCl₂), each with differing solubilities, as depicted in Figure 2.