Obviously another important design parameter is therefore being able to predict the rate of deactivation, for that it is important to understand the main factors that contribute to the deactivation of the molecular sieve. In general, these contributing factors are a consequence of the thermal regeneration process.
During regeneration of the water-saturated molecular sieve, hot dry gas is passed upwards through the molecular sieve bed and water desorbs. If the beginning of the regeneration cycle were started with dry gas at the maximum temperature, water would first desorb from the bottom portion of the bed, thus using much of the heat of the gas. Subsequently, water that desorbed from the bottom portion of the bed would be carried to the top portion, which would not yet have been heated by the regeneration gas.
Because the top portion of the bed would still be cold and saturated with water, the desorbed water from the bottom portion of the bed could condense to form liquid water. The formation of hot liquid water in the bed can cause the clay binder of the molecular sieve to dissolve, subsequently forming a cake during the regeneration step.
Caking due to hot liquid water formation during regeneration can be avoided by using a well-designed heating profile during the regeneration cycle. If the temperature of the regeneration gas entering the bed is slowly increased, the top portion of the bed can be pre-heated before much water desorbs from the bottom portion of the bed.
Pre-heating of the top portion of the bed prevents the condensation of liquid water when the water from the bottom portion of the bed finally is desorbed. If a non-ideal temperature profile is used during regeneration, degradation of the molecular sieve might still occur without the formation of a large solid cake.
The characteristics of the temperature profile required to avoid liquid water formation depend on a number of factors. The flow rate and composition of the regeneration gas dictate how much heat is delivered to the molecular sieve bed.
The diameter and materials of the vessel determine how much heat, and at what rate, is transferred to the vessel, as opposed to the amount of heat that is transferred to the molecular sieve. Furthermore, the amount of water adsorbed on the molecular sieve, taking into account the mass transfer zone, dictates the gas-phase concentration profile of water moving upwards through the bed.
Another form of molecular sieve deactivation is through coke deposition on the molecular sieve. If the feed to the unit is heavy, the heavier hydrocarbons can be adsorbed onto the molecular sieve binder during adsorption. A portion will be desorbed during regeneration but the residual will remain on the binder, decompose and irreversibly form coke under hot regeneration conditions, which can lead to pore blocking that results in a lower water uptake capacity.
The least contributing factor is the effect that continual thermal cycling has on the molecular sieve. Some thermal degradation and the subsequent loss in water removal capacity of the molecular sieve will occur, the extent of which is dependent on the thermal stability of the molecular sieve. The regeneration process also results in the degradation of the binder material. The expansion and contraction experienced during thermal swings leads to dust production and a consequential increasing pressure drop that is detrimental to LNG production
As the capacity of an adsorbent is limited and the vessels become more expensive as the diameter increases (the required wall thickness increases) there is a significant economic driver to keep the vessels as small as possible. One way to achieve this is to operate at high pressures (typically 60 bara) as the mass flow to be treated is than compressed in a smaller volume.
Another way to achieve this is by reducing the amount of water that is deposited on the bed as much as possible by cooling down the feed stream and knocking out excess water in the feed KO drum. The temperature reduction is limited by the hydrate formation temperature and the operating temperature is chosen such that one can safely operate above that temperature. Another reason to operate at a low temperature is that the uptake capacity of the molecular sieve is higher.
Not discussed yet is the type of molecular sieve to be used. For dehydration, and especially for molecular sieve units used in LNG application, type 4A is used. As mentioned before, a molecular sieve is composed of a zeolite and a binder (typically clay). The binder is used as a glue i.e. to strengthen the particles. The type of zeolite typically employed for dehydration is de Linde Type A (LTA), whereby A refers to Angstrom, indicating the diameter of the zeolite channels where the water is adsorbed i.e. 4A refers to a zeolite with a channel diameter of 4 Angstrom.
Apart from 4A there are also 3A and 5A LTA sieves, whereby the type of cation determines the channel diameter (K+ for 3A, Na+ for 4A and Ca2+ for 5A). The effectiveness of zeolites for water removal is based on two important characteristics, the channel diameter acting as a filter, thereby limiting the number of species that can co-adsorb, and the polar environment created in the channels, thereby creating an environment whereby preferentially polar molecules are adsorbed. Water is the molecule that is adsorbed strongest (consequently, water will displace all other adsorbed molecules).
As mentioned before, for LNG applications typically a 4A sieve is used as this sieve is more stable and has higher water uptake capacity than 3A sieve, and less co-adsorption will occur when compared to a 5A sieve. However, in gas processing plants that process pipeline gas typically 3A sieve is used.
The reason for that is that if methanol is added to the feed gas to prevent hydrate formation, it will pass the 3A sieve bed (methanol does not fit into the pores) and can be recovered downstream the beds in the part of the line-up where heavier hydrocarbons (e.g. propane, butane) are separated from the natural gas. Furthermore, co-adsorption of methanol on larger pore sieves will adversely affect the effectiveness of the molecular sieve w.r.t. water removal.
The question why molecular sieves are best suited for deep dehydration is illustrated in Figure 4 (Part 1), where typical isotherms of adsorbents used for dehydration are shown. Although molecular sieves have a lower saturation capacity than alumina or silica gel, that capacity is much less dependent on the partial water pressure which enables reaching deeper specifications (< 0.1 ppmv H2O) as well as a shorter Mass Transfer Zone enabling a more efficient use of the adsorbent bed. In this context it should be noted that the length of the MTZ is also determined by the particle size.
For that reason, molecular sieve beds are typically layered, with the larger particles (e.g. 1 /8”) used in the top layers to minimize the overall bed pressure drop and the smaller particles (e.g. 1/16”) in the bottom layer. Another reason to select molecular sieves for dehydration is that the water uptake capacity is less feed temperature dependent in comparison to silica gel and alumina. Furthermore, in contrast to molecular sieves, alumina and silica gel are large pore adsorbents and therefore more co-adsorption of hydrocarbons will occur.
Similar to molecular sieves, water will displace the co-adsorbed components as it adsorbs stronger to the adsorbent, but co-adsorption will also influence the length of the MTZ. Maybe even more importantly, when regenerating the adsorbents these co-adsorbed species will leave the adsorbent as peaks and if the regeneration gas has to be treated, the treating unit will have to be sized such that it can process these peaks. This significantly increases the cost of such a unit.