Optimal design and operation of molecular sieve gas dehydration units

Molecular sieve technology is widely used for the simultaneous removal of water and mercaptans from both gas and liquid feed streams. However, a better understanding of the design principles and the operation of molecular sieve units is needed. For economic reasons, it is important not to overdesign the molecular sieve unit. At the same time, it is essential to ensure that the unit does not become the bottleneck of the gas processing plant at the sieve’s end-of-run condition.
Molecular sieves background. When treating a gas or liquid stream so that it can be processed by a specific unit, one of the commonly used treating units is an adsorption unit. These units are commonly used to remove water from a feed stream, but they can also remove additional contaminants (e.g., mercaptans). Specifically, when deep removal is required (below 1 ppmv), molecular sieves—an adsorbent composed of a zeolite and, typically, a clay binder—are the preferred adsorbent.

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Adsorption units are capable of reaching extremely low specifications, which makes them viable pieces of equipment for incorporation into a process lineup. A major advantage of molecular sieves is that they can be regenerated, which reduces the required amount of molecular sieve to economically feasible quantities.

This article examines the use of molecular sieve units for natural gas dehydration, as they are critical components in the operation of an LNG or gas processing plant (typically combined with NGL extraction by cryogenic separation), and any limitation or loss in capacity of this unit can have a significant effect on overall plant economics.

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An adsorption unit used for water removal is called a dehydration unit (DHU). A DHU often consists of two or more vessels, filled with molecular sieves, that adsorb water during an adsorption period and are subsequently regenerated using a heated stream of treated gas. A sketch of a typical molecular sieve DHU is shown below.

Figure 1. Sketch Of Typical Molecular DHU

The high temperature during regeneration causes water to desorb from the molecular sieve, a process called temperature swing adsorption (TSA). Although TSA is a discontinuous process, the overall DHU behaves like a continuous process because one or more vessels are always in adsorption mode, while another vessel(s) is in regeneration mode.