Liquid drying by solid desiccant materials

The use of solid desiccant is an easy way to dry liquid hydrocarbon (kerosene, gasoil, LPG). Basically Newton’s technology allows going over rock salt, pearls or flakes limits.

Case study: Water Trap technology improves kerosene water specification

Background  A European refinery has difficulties to meet the Kerosene Haze specification during warm periods (spring and autumn)

The refinery decided to reach the technical target. Newton's was contacted to propose Water Trap technology and compare with other technical solutions (Kerosene coo-ling)./

Solution, The first step was to run the DRY MODEL® with the data Dryer sheet provided by the customer (Diameter, High of the bed, flow rate, water in, temperature, pressure)First Model output was presented to the refinery and a Water Trap Program was proposed for a test. Water Trap Ca associated with rock salt was the must appropriated combination to be implemented in this refinery.

Results It was observed that the results are more consistent when using Water Trap program than cooling the kerosene temperature. In other hand, cooling water temperature has nearly no effect on haze result. The target, 70 ppm water in the kerosene cut, was reached by implementing Water Trap program.

The use of consumable solid desiccant materials for organic liquid dehydration is very current in refinery or chemistry processes, especially when the water concentration is very low (<1000 ppm). In spite of its common use, the kinetics of liquid drying by solid desiccants is not well known, and scale-up of industrial drying processes using such materials is always problematic.

A current application of solid desiccants is the dehydration and sweetening of fuels or gas oils in the refinery. Dehydration of fuel is important because water phase demixion (called haze) is prohibited.

Industrial dryers are fixed-bed columns filled with solid desiccant.

The liquid phase flows through the fixed bed in an upward flow direction [4]. A brine phase is formed and settles at the bottom of the columns. The brine can also enhance the solvent dehydration by further extraction in counter-current flow.

Different kinds of solid desiccants can be used as Water Trap ® briquettes (based on desiccant properties of calcium chloride or sodium hydroxide or caustic potash). In deliquescing desiccant systems, the particular shape of the drying product is as significant as its desiccant capacity.

The interest in the moulded pieces used here is a higher specific contact area between solid and liquid compared with pearls of the same volume: the gain of specific area is around 20%, with a similar pressure drop. When very high specific areas are needed, other shapes and sizes can be used, such as small pellets or flakes, but in these cases, pressure drop through the bed can be much higher, and during operation the formation of chimneys or preferential paths is possible. This phenomenon can dramatically shorten the run time between two unit shutdowns and force the unloading of the desiccant charge, even if the residual volume of solid is theoretically still sufficient to treat the solvent. The moulded pieces have the ability to reorganise themselves when the bed is consumed, in a way that prevents the formation of chimneys. This phenomenon is illustrated in Figure 2.

Figure 2: pearls (left) and moulded pieces (right) bed consumption

The outline of Newton's research is the development of design procedures for hydrocarbon dryers. It focuses mainly on the measurement and analysis of dehydration kinetics, essential for industrial contactor modelling and scale-up

The study follows three steps:

-       batch experiments for the analysis of dehydration limitations and mechanisms,

-       small pilot experiments for mass transfer measurements,

-       big pilot experiments for desiccant consumption measurements on a large scale and long-term tests.

Results

Dehydration operations with solid desiccant can follow very different behaviours depending on the solvent to be treated, water concentration, and also depending shape of desiccant. It was observed during this study that dehydration with caustic desiccant is limited by external mass transfer, and this phenomenon was successfully characterised through a correlation between adimensional numbers. This result is important for the design of industrial columns. Dehydration kinetics observed with calcium chloride is different, and is not only limited by external mass transfer, but also by the internal diffusion of water inside the desiccant.

When desiccants are used with a highly wetted solvent, dehydration generates formation of a brine phase, which can increase the dehydration efficiency at the bottom of the column, especially if the liquid-liquid contact surface is optimised.

A very simple model was developed to estimate the evolution of the performances in columns over time and to predict bed ageing. The model is in good agreement with experimental results in the case of the use of moulded pieces.

Case study: Water Trap Newton's technology improves LCCO dryer run length. in a south Europe refinery

Background:

The refinery HDS unit was shut down for maintenance. The refinery found a market for the untreated LCCO produced during this period. The LCCO sales specification was less than 300 ppm water.

The operational staff was using normal "Nacl rocksalt" in a vessel dryer to remove the water from LCCO before storage and sale. The refinery never reached the goal.

Newton's was contacted to run his Dry-Model to improve the LCCO dryer run length and at the same time improve LCCO quality produced.

Solution,

The first step was to run the model with the data Dryer sheet provided by the customer (Diameter, High of the bed, flow rate, water in, temperature, pressure)

First Model output was presented to the refinery and a Water Trap Program was proposed for a test.

Water Trap Ca was the must appropriated program to be use in this refinery.

Results,

After load of Water Tarp Ca into the dryer, the refinery could process the LCCO produced with the right sales specification during the run (1150 h).

During the run, the coaleser set up before the dryer was shut down for filter cleaning. Even during this period the LCCO specification was respected in the final storage tank.

Newton's Dry-Model helps predicting the dryer run length with the remaining Water Trap Ca storage into the dryer.