PTQ Q4 2024 Issue

hygroscopic nature, CaCl₂ cannot remain in stagnant ULSD for more than approximately a day without risking crystal- lisation or lumping within the dryer. This lumping is an irre- versible process that can cause channelling, subsequently impacting the performance of the salt dryer. Some refiners have reported negative experiences with CaCl₂ lumping. Hence, rock salt is more commonly used. Salt dryers also present several operational challenges, including problems with salt bridging, high salt consump- tion rates, difficulties in disposing of brine, and the need to continually monitor salt levels in the dryer. Additionally, there is a risk of corrosive chloride carryover into storage tanks. Coalescer cartridges, which are typically guaranteed for one year of operation, require replacement at a cost of about $2,500 each. This leads to an annual cartridge cost of $375,000, considering a typical number of 150 car- tridges. During periods when the coalescer is out of service for cartridge replacement, typically about one week, the salt dryer’s salt consumption can increase by up to 25 times the normal rate. Given these considerations, the salt drying at coalescer downstream option has become an increasingly unfavourable option among refiners. The use of vacuum dryers (VD) for drying ULSD has recently increased. VDs can vary significantly in their con - figurations, including single-stage systems with one vacuum column and two-stage systems featuring two vacuum col- umns. Additionally, vacuum system ejectors employed can range from single to multi-stage, each variant affecting over- all system efficiency and performance. This study examines various VD configurations, with objectives centred around reducing utility consumption, opti- mising equipment size, and minimising both capital expendi- tures (Capex) and operational expenditures (Opex). Except for higher utility costs, the operation of VDs is noted for its reliability and its ability to handle a wide range of feed water contents effectively. Detailed discussions of different VD configurations are provided in subsequent sections, outlining their specific advantages, limitations, and suitability under varying oper- ational conditions. Some refineries have adopted innovative processes such as chiller operations, where incoming ULSD is cooled to sufficiently low temperatures to convert dis - solved water into free water. A coalescer positioned down- stream of the chiller can effectively remove this free water, significantly reducing the total water content in the ULSD. In scenarios where ULSD water content of less than 10 wppm is required, molecular sieve dryers have been sug- gested as a viable option. Molecular sieves offer highly effective moisture removal capabilities, ideal for achieving stringent water content specifications. However, it should be noted that the authors have not encountered any instal- lations where molecular sieves are used specifically to meet the ‘bright and clear’ specifications for ULSD. Study of vacuum dryer configurations A comprehensive study was conducted to evaluate the pros and cons of various VD configurations. This assessment utilised Aspen Hysys for simulation purposes, selecting the

Solubility CaCl wt% Solubility NaCl wt%

30 35

Temperature (˚C)

Figure 2 Solubility of NaCl and CaCl₂

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 availa- ble 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 effective- ness of a coalescer is influenced by its operating tempera - ture; 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 tempera- ture of 22°C. At this temperature, the dissolved water esti- mated 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’ specifica - tion. However, not meeting bright and clear becomes more prominent in the geographic region where day-night tem- perature variation and summer-winter temperature varia- tion 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 . CaCl₂ has a significantly higher solubility than rock salt, making it more effective in removing dissolved water from ULSD, especially when the fuel contains higher levels of aromatics or polar components. However, due to its highly

PTQ Q4 2024