In this cyclic process, a bed of solid desiccant adsorbs water from wet feed gas. Once the bed is water-saturated, it is regenerated. Regeneration of the desiccant is typically done using a gas fraction (previously dried or not) which is heated to a temperature sufficient to desorb the water.
To provide continuous operation, these units use multiple adsorption beds in a “swing” configuration : One (or several) bed(s) operate under adsorption mode, while the other one(s) operate under regeneration mode. The number of driers in adsorption mode (up to 4) and in regeneration mode (1 or 2) is a result of a deep optimization.
Sequence valves controlled by a software route process flows in accordance with the operating mode of each dryer, and perform dryers' mode change over automatically at the end of each sequence.
Range of Processes
The more common desiccants used are:
- Molecular sieves: used to achieve very low water dew points, from -100°C for ethane extraction units up to -160°C for the liquefaction of natural gas
- Activated Alumina: compared with molecular sieve, it provides better adsorption capacity at high partial pressure of H2O but a lower capacity at low partial pressure. Activated alumina is used when the amounts of water to extract are important and dew points not too difficult to reach (about -60°C). A combination molecular sieve / activated alumina to minimize the amount of adsorbent can be an option when it is required to simultaneously extract large quantities of water and achieve very low dew point.
- Silica Gel: proven adsorbent which has a high water adsorption capacity, but which cannot be used to achieve very low dew points. Silica Gel are also used to remove heavy hydrocarbons (C4 +) together with water (combined hydrocarbon and water dew points).
- A very well known and proven technology
- A process reliable and easy to operate
- A process offering high flexibility
- A good selectivity (coadsorption can be avoided)
- Very low water dew point can be reached (below 0.1 ppm water content).