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Evaluating adsorbent bed process design and regeneration procedures can provide opportunities for optimization, ultimately reducing operating costs and emissions.
Adsorbent beds, sometimes referred to as process dryers or treaters, use an adsorption mechanism to remove contaminants like water, hydrogen sulfide (H2S), carbon dioxide (C02), and other polar molecules from process streams. Drying/treating systems are very common in refineries and petrochemical manufacturing facilities and have widespread applications. They are used to dry instrument air, remove sulfur species and CO, to meet product specifications, remove poisons from reactor feed streams, and remove water and CO, from cryogenic fractionation trains. The beds contain an adsorptive desiccant, usually an activated alumina, a molecular sieve, or a combination of the two.
The choice of desiccant depends on the size and type of molecule to be removed, as well as the final product specification. Activated aluminas have high loading capacities for more-polar molecules, but they are not as selective as some of the molecular sieves. А ЗА molecular sieve, for instance, will adsorb water and ammonia, but will exclude H2S and C02, which require at least a 4A molecular sieve. Activated aluminas, on the other hand, simply adsorb molecules based on polarity, and the more-polar molecules will displace the less-polar molecules. Manufacturers produce hybrid adsorbents that offer some of the benefits of both activated alumina and molecular sieve, and can tailor some desiccants for a specific application.
In most process plants, adsorbent beds are very reliable and easy to operate, but they do require periodic regeneration. Over time, operating conditions may change and warrant a review of the treater design, operation, and regeneration procedures to improve efficiency. With changes to energy and utility costs and improvements in desiccant technology, treaters that have been in service for a while may benefit from a fresh evaluation.
This article discusses the operation of adsorbent beds and the typical costs associated with regenerating them. Using examples from an ethylene plant, the article identifies methods for reviewing operation and regeneration techniques to reduce operating costs and emissions.
An overview of dryer operation
Regardless of the material or application, process dryers/treaters operate in much the same way. Each process stream is routed through a set of two or more beds (Figure 1). While one bed is...