PTQ Q2 2026 Issue

Operational stability and lumping behaviour Rock salt remains chemically and structurally stable, with no tendency for lumping, even under stagnant condi- tions, due to the absence of hydrate formation in its oper- ating temperature range (see Figure 2 ). In contrast, CaCl₂ is prone to lumping if left stagnant and wet, especially when ambient cooling causes the bed to enter its crystalli- sation zone. Hydrate forma - tion, such as CaCl₂·6H₂O, can result in irreversible lumps. These hard lumps inhibit brine settling and promote formation of cracks, leading to channelling and prefer- ential flow through voids, which severely reduce drying

120

H O (g) + NaCl(s)

HO(g) + s l n

100

s l n + NaCl(s)

s l n

peritectic point

s l n + NaCl 2HO(s)

HO(s) + s l n

NaCl 2HO(s) + NaCl(s)

-20

H0(s) + NaCl 2HO(s

-40

100

Mass per cent NaCl

NaCl

Figure 2 Rock salt-water phase diagram7

Salt type and behaviour Different types of salt have been employed in salt dryers over the years, including sodium chloride (NaCl), calcium chloride (CaCl₂), sodium sulphate, potassium hydroxide, sodium hydroxide, lithium chloride, and lithium bromide. Lithium-based salts offer superior drying efficiency but are cost-prohibitive for large-scale use. However, due to cost and handling constraints, rock salt (NaCl) and CaCl₂ remain the two most widely used in refinery-scale applications. CaCl₂ briquettes usually have a nominal size of 30 x 26 x 14 mm, whereas rock salt is commonly supplied as irregular particles, typically 3.5-8 mm in size, produced by crushing naturally cubic halite crystals. A coarser grade of salt is pre - ferred as it is less prone to fusion and bridging upon contact with water, thereby reducing the likelihood of channelling within the salt bed. Additionally, extra coarse salt contains fewer fines, which helps minimise the risk of salt carrying over into downstream piping and equipment. Drying capability and saturation behaviour The final water content in ULSD after salt contact is deter - mined by the equilibrium between the salt solution and the hydrocarbon phase, which is strongly influenced by both the salt type and the operating temperature (see Figures 1 and 2 ). CaCl₂ is highly hygroscopic and effective in remov - ing both free and dissolved water, reducing the residual water content to approximately 50% of saturation. Rock salt, by comparison, typically reduces residual water content to about 70-75% of saturation. This means that if the target ULSD specification is 80 wppm total water after the salt dryer, the dissolved water content entering the dryer must not exceed about 110 wppm when using rock salt. Above this threshold, rock salt reaches its equi - librium limit and cannot achieve the required specification, making CaCl₂ necessary to meet bright and clear product quality.

efficiency. Although vendors recommend draining the CaCl₂ bed before shutdown, this is often avoided in practice due to slop generation and operational challenges. Locating the CaCl₂ dryer near the ULSD tank, where recirculation is pos - sible during downtime, can reduce this risk. Temperature-dependent solubility and salt consumption Solubility of salt in water plays a critical role in estimating the salt consumption for ULSD drying. The amount of water that can be absorbed per unit mass of salt is determined by the equilibrium solubility at operating temperature: • Rock salt (NaCl) : Solubility of rock salt can be determined using Equation 1 : Solubility_NaCl (wt%) = 0.0166 × T + 26.01 (T in °C) (Eq. 1) This equation, derived from solubility data,⁵ is valid in the temperature range 30-50°C. Using the equation, rock salt solubility at 38°C (≈100°F) is ≈26.6 wt%, meaning 26.6 g of NaCl is present in 100 g of saturated brine. Therefore, in every 100 g of saturated brine, about 73.4 g is water. At this temperature, the water- to-salt ratio is ≈73.4/26.6, or ≈2.76, meaning approximately 2.8 kg of water can be removed using 1 kg of rock salt. • Calcium chloride (CaCl₂) : Solubility of CaCl2 can be determined using Equation 2 : Solubility_CaCl₂ (wt%) = 0.269 × T + 40.61 (T in °C) (Eq. 2) This equation, derived from solubility data,5 is valid in the temperature range 30-50°C. Using the equation, CaCl2 solubility at 38°C is ≈50.83 wt%, meaning 50.83 g of CaCl₂ is present in 100 g of saturated brine. Therefore, in every 100 g of saturated brine, about 49.17 g is water. At this temperature, the

PTQ Q2 2026